Internet DRAFT - draft-fox-tcpm-shared-memory-rdma
draft-fox-tcpm-shared-memory-rdma
TCPM working group M. Fox
Internet Draft C. Kassimis
Intended Status: Informational J. Stevens
Expires: 11/7/2015 IBM
May 7, 2015
IBM's Shared Memory Communications over RDMA
draft-fox-tcpm-shared-memory-rdma-07.txt
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Abstract
This document describes the IBM's Shared Memory Communications over
RDMA (SMC-R) protocol. This protocol provides RDMA communications to
TCP endpoints in a manner that is transparent to socket applications.
It further provides for dynamic discovery of partner RDMA
capabilities and dynamic setup of RDMA connections, transparent high
availability and load balancing when redundant RDMA network paths are
available, and it maintains many of the traditional TCP/IP qualities
of service such as filtering that enterprise users demand, as well as
TCP socket semantics such as urgent data.
Table of Contents
1. Introduction...................................................5
1.1. Summary of changes in this draft..........................6
1.2. Protocol overview.........................................6
1.2.1. Hardware requirements................................8
1.3. Definition of common terms................................8
2. Link Architecture.............................................10
2.1. Remote Memory Buffers (RMBs).............................12
2.2. SMC-R Link groups........................................16
2.2.1. Link group types....................................17
2.2.2. Maximum number of links in link group...............20
2.2.3. Forming and managing link groups....................21
2.2.4. SMC-R link identifiers..............................22
2.3. SMC-R resilience and load balancing......................23
3. SMC-R Rendezvous architecture.................................25
3.1. TCP options..............................................25
3.2. Connection Layer Control (CLC) messages..................26
3.3. LLC messages.............................................26
3.4. CDC Messages.............................................28
3.5. Rendezvous flows.........................................28
3.5.1. First contact.......................................28
3.5.1.1. TCP Options pre-negotiation....................28
3.5.1.2. Client Proposal................................29
3.5.1.3. Server acceptance..............................30
3.5.1.4. Client confirmation............................31
3.5.1.5. Link (QP) confirmation.........................31
3.5.1.6. Second SMC-R link setup........................34
3.5.1.6.1. Client processing of "Add Link" LLC message
from server..........................................34
3.5.1.6.2. Server processing of "Add Link" reply LLC
message from the client..............................35
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3.5.1.6.3. Exchange of Rkeys on second SMC-R link....37
3.5.1.6.4. Aborting SMC-R and falling back to IP.....37
3.5.2. Subsequent contact..................................37
3.5.2.1. SMC-R proposal.................................38
3.5.2.2. SMC-R acceptance...............................39
3.5.2.3. SMC-R confirmation.............................40
3.5.2.4. TCP data flow race with SMC Confirm CLC message40
3.5.3. First contact variation: creating a parallel link group
...........................................................41
3.5.4. Normal SMC-R link termination.......................42
3.5.5. Link group management flows.........................43
3.5.5.1. Adding and deleting links in an SMC-R link group43
3.5.5.1.1. Server initiated Add Link processing......43
3.5.5.1.2. Client initiated Add Link processing......44
3.5.5.1.3. Server initiated Delete Link Processing...44
3.5.5.1.4. Client initiated Delete Link request......46
3.5.5.2. Managing multiple Rkeys over multiple SMC-R links
in a link group.........................................48
3.5.5.2.1. Adding a new RMB to an SMC-R link group...49
3.5.5.2.2. Deleting an RMB from an SMC-R link group..52
3.5.5.2.3. Adding a new SMC-R link to a link group with
multiple RMBs........................................53
3.5.5.3. Serialization of LLC exchanges, and collisions.54
3.5.5.3.1. Collisions with ADD LINK / CONFIRM LINK
exchange.............................................56
3.5.5.3.2. Collisions during DELETE LINK exchange....57
3.5.5.3.3. Collisions during CONFIRM_RKEY exchange...57
4. SMC-R memory sharing architecture.............................59
4.1. RMB element allocation considerations....................59
4.2. RMB and RMBE format......................................59
4.3. RMBE control information.................................59
4.4. Use of RMBEs.............................................60
4.4.1. Initializing and accessing RMBEs....................60
4.4.2. RMB element reuse and conflict resolution...........61
4.5. SMC-R protocol considerations............................62
4.5.1. SMC-R protocol optimized window size updates........62
4.5.2. Small data sends....................................63
4.5.3. TCP Keepalive processing............................63
4.6. TCP connection failover between SMC-R links..............66
4.6.1. Validating data integrity...........................66
4.6.2. Resuming the TCP connection on a new SMCR link......67
4.7. RMB data flows...........................................67
4.7.1. Scenario 1: Send flow, window size unconstrained....68
4.7.2. Scenario 2: Send/Receive flow, window unconstrained.70
4.7.3. Scenario 3: Send Flow, window constrained...........71
4.7.4. Scenario 4: Large send, flow control, full window size
writes.....................................................73
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4.7.5. Scenario 5: Send flow, urgent data, window size
unconstrained..............................................76
4.7.6. Scenario 6: Send flow, urgent data, window size closed78
4.8. Connection termination...................................80
4.8.1. Normal SMC-R connection termination flows...........80
4.8.1.1. Abnormal SMC-R connection termination flows....85
4.8.1.2. Other SMC-R connection termination conditions..87
5. Security considerations.......................................88
5.1. VLAN considerations......................................88
5.2. Firewall considerations..................................88
5.3. Host-based IP Filters....................................89
5.4. Intrusion Detection Services.............................89
5.5. IP Security (IPSec)......................................89
5.6. TLS/SSL..................................................89
6. IANA considerations...........................................89
7. References....................................................90
7.1. Normative References.....................................90
7.2. Informative References...................................90
8. Acknowledgments...............................................90
9. Conventions used in this document.............................90
Appendix A. Formats..............................................91
A.1. TCP option...............................................91
A.2. CLC messages.............................................91
A.2.1. Peer ID format......................................91
A.2.2. SMC Proposal CLC message format.....................93
A.2.3. SMC Accept CLC message format.......................96
A.2.4. SMC Confirm CLC message format......................99
A.2.5. SMC Decline CLC message format.....................102
A.3. LLC messages............................................103
A.3.1. CONFIRM LINK LLC message format....................104
A.3.2. ADD LINK LLC message format........................106
A.3.3. ADD LINK CONTINUATION LLC message format...........108
A.3.4. DELETE LINK LLC message format.....................111
A.3.5. CONFIRM RKEY LLC message format....................113
A.3.6. CONFIRM RKEY CONTINUATION LLC message format.......116
A.3.7. DELETE RKEY LLC message format.....................118
A.3.8. TEST LINK LLC message format.......................120
Appendix B. Socket API considerations...........................126
Appendix C. Rendezvous Error scenarios..........................128
C.1. SMC Decline during CLC negotiation......................128
C.2. SMC Decline during LLC negotiation......................128
C.3. The SMC Decline window..................................130
C.4. Out of synch conditions during SMC-R negotiation........130
C.5. Timeouts during CLC negotiation.........................131
C.6. Protocol errors during CLC negotiation..................131
C.7. Timeouts during LLC negotiation.........................132
C.7.1. Recovery actions for LLC timeouts and failures.....133
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C.8. Failure to add second SMC-R link to a link group........140
1. Introduction
This document specificies IBM's Shared Memory Communications over
RDMA (SMC-R) protocol. SMC-R is a protocol for Remote Direct Memory
Access (RDMA) communication between TCP socket endpoints. SMC-R
runs over networks that support RDMA over Converged Ethernet (ROCE).
It is designed to permit existing TCP applications to benefit from
RDMA without requiring modifications to the applications or
predefinition of RDMA partners.
SMC-R provides dynamic discovery of the RDMA capabilities of TCP
peers and automatic setup of RDMA connections that those peers can
use. SMC-R also provides transparent high availability and load
balancing capabilities that are demanded by enterprise installations
but are missing from current RDMA protocols. If redundant RoCE
capable hardware such as RDMA NICs (RNICs)and RoCE capable switches
is present, SMC-R can load balance over that redundant hardware and
can also non-disruptively move TCP traffic from failed paths to
surviving paths, all seamlessly to the application and the sockets
layer. Because SMC-R preserves socket semantics and the TCP three-way
handshake, many TCP qualities of service such as filtering, load
balancing, and SSL encryption are preserved, as are TCP features such
as urgent data.
Because of the dynamic discovery and setup of SMC-R connectivity
between peers, no RDMA connection manager (RDMA-CM) is required. This
also means that support for UD queue pairs is also not required.
It is recommended that the SMC-R services be implemented in kernel
space, which enables optimizations such as resource sharing between
connections across multiple processes and also permits applications
using SMC-R to spawn multiple processes (e.g. fork) without losing
SMC-R functionality. A user space implementation is compatible with
this architecture, but it may not support spawned processes (i.e.
fork) which limits sharing and resource optimization to TCP
connections that originate from the same process. This might be an
appropriate design choice if the use case is a system that hosts a
large single process application that creates many TCP connections to
a peer host, or in implementations where a kernel space
implementation is not possible or introduces excessive overhead for
kernel space to user space context switches.
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1.1. Summary of changes in this draft
Changed the title to add "IBM's".
1.2. Protocol overview
SMC-R defines the concept of the SMC-R Link, which is a logical
point-to-point link using reliably connected queue pairs between
TCP/IP stack peers over a RoCE fabric. An SMC-R link is bound to a
specific hardware path, meaning a specific RNIC on each peer. SMC-R
links are created and maintained by an SMC-R layer, which may reside
in kernel or user space depending upon operating system and
implementation requirements. The SMC-R layer resides below the
sockets layer and directs data traffic for TCP connections between
connected peers over the RoCE fabric using RDMA rather than over a
TCP connection. The TCP/IP stack with its fragmentation,
packetization, etc. requirements is bypassed and the application data
is moved between peers using RDMA.
Multiple SMC-R links between the same two TCP/IP stack peers are also
supported. A set of SMC-R links called a link group can be logically
bonded together to provide redundant connectivity. If there is
redundant hardware, for example two RNICs on each peer, separate SMC-
R links are created between the peers to exploit that redundant
hardware. The link group architecture with redundant links provide
load balancing, increased bandwidth as well as seamless failover.
Each SMC-R link group is associated with an area of memory called
Remote Memory Buffers (RMBs), which are areas of memory that are
available for SMC-R peers to write into using RDMA writes. Multiple
TCP connections between peers may be multiplexed over a single SMC-R
link, in which case the SMC-R layer manages the partitioning of the
RMBs between the TCP connections. This multiplexing reduces the RDMA
resources such as queue pairs and RMBs that are required to support
multiple connections between peers, and also reduces the processing
and delays related to setting up queue pairs, pinning memory, and
other RDMA setup tasks when new TCP connections are created. In a
kernel space SMC-R implementation in which the RMBs reside in kernel
storage, this sharing and optimization works across multiple
processes executing on the same host. In a user space SMC-R
implementation in which the RMBs reside in user space, this sharing
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and optimization is limited to multiple TCP connections created by a
single process, as separate RMBs and QPs will be required for each
process.
SMC-R also introduces a rendezvous protocol that is used to
dynamically discover the RDMA capabilities of TCP connection partners
and exchange credentials necessary to exploit that capability if
present. TCP connections are set up using the normal TCP 3-way
handshake, with the addition of a new TCP option that indicates SMC-R
capability. If both partners indicate SMC-R capability then at the
completion of the 3-way TCP handshake the SMC-R layers in each peer
take control of the TCP connection and use it to exchange additional
connection level control (CLC) messages to negotiate SMC-R
credentials such as queue pair (QP) information, addressability over
the RoCE fabric, RMB buffer sizes, keys and addresses for accessing
RMBs over RDMA, etc. If at any time during this negotiation a
failure or decline occurs, the TCP connection falls back to using the
IP fabric.
If the SMC-R negotiation succeeds and either a new SMC-R link is set
up or an existing SMC-R link is chosen for the TCP connection, then
the SMC-R layers open the sockets to the applications and the
applications use the sockets as normal. The SMC-R layer intercepts
the socket reads and writes and moves the TCP connection data over
the SMC-R link, "out of band" to the TCP connection which remains
open and idle over the IP fabric, except for termination flows and
possible keepalive flows. Regular TCP sequence numbering methods are
used for the TCP flows that do occur; data flowing over RDMA does not
use or affect TCP sequence numbers.
This architecture does not support fallback of active SMC-R
connections to IP. Once connection data has completed the switch to
RDMA, a TCP connection cannot be switched back to IP and will reset
if RDMA becomes unusable.
The SMC-R protocol defines the format of the Remote Memory Buffers
that are used to receive TCP connection data written over RDMA, as
well as the semantics for managing and writing to these buffers using
Connection Data Control (CDC) messages.
Finally, SMC-R defines link level control (LLC) messages that are
exchanged over the RoCE fabric between peer SMC-R layers to manage
the SMC-R links and link groups. These include messages to test and
confirm connectivity over an SMC-R link, add and delete SMC-R links
to or from the link group, and exchange RMB addressability
information.
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1.2.1. Hardware requirements
SMC-R does not require full Converged Enhanced Ethernet switch
functionality. SMC-R functions over standard Ethernet fabrics
provided endpoint RNICs are provided and IEEE 802.3x Global Pause
Frame is supported and enabled in the switch fabric.
While SMC-R as specified in this document is designed to operate over
RoCE fabrics, adjustments to the rendezvous methods could enable it
to run over other RDMA fabrics such as Infiniband and iWARP.
1.3. Definition of common terms
This section provides definitions of terms that have a specific
meaning to the SMC-R protocol and are used throughout this document.
SMC-R link
An SMC-R Link is a logical point to point connection over the
RoCE fabric via specific physical adapters (MAC/GID). The Link
is formed during the first contact sequence of the TCP/IP 3 way
handshake sequence that occurs over the IP fabric. During this
handshake an RDMA RC-QP connection is formed between the two peer
SMC hosts and is defined as the SMC Link. The SMC Link can then
support multiple TCP connections between the two peers. An SMC
link is associated with a single LAN (or VLAN) segment and is not
routable.
SMC-R link group
An SMC-R Link Group is a group of SMC-R Links typically each over
unique RoCE adapters between the same two SMC-R peers. Each link
in the link group has equal characteristics such as the same VLAN
ID (if VLANs are in use), access to the same RMB(s) and the same
TCP server / client
SMC-R peer
The SMC-R Peer is the peer software stack within the peer
Operating System with respect the Shared Memory Communications
(messaging) protocol.
SMC-R Rendezvous
The SMC-R Rendezvous is the SMC-R peer discovery and handshake
sequence that occurs transparently over the IP (Ethernet) fabric
during and immediately after the TCP connection 3 way handshake
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by exchanging the SMC capabilities and credentials using
experimental TCP option and CLC messages.
TCP Client
The TCP socket-based peer that initiates a TCP connection
TCP Server
The TCP socket-based peer that accepts a TCP connection
CLC messages
The SMC-R protocol defines a set of Connection Layer Control
Messages that flow over the TCP connection that are used to
manage SMC link rendezvous at TCP connection setup time. This
mechanism is analogous to SSL setup messages
LLC Commands
The SMC-R protocol defines a set of RoCE Link Layer Control
Commands that flow over the RoCE fabric using RDMA sendmsg, that
are used to manage SMC Links, SMC Link Groups and SMC Link Group
RMB expansion and contraction.
CDC message
The SMC-R protocol defines a Connection Data Control message that
flows over the RoCE fabric using RDMA sendmsg that is used to
manage the SMC-R connection data. This message provides
information about data being transferred over the out of band
RDMA connection, such as data cursors, sequence numbers, and data
flags (for example urgent data). The receipt of this message
also provides an interrupt to inform the receiver that it has
received RDMA data.
RMB
A Remote (RDMA) Memory Buffer is a fixed or pinned buffer
allocated in each of the peer hosts for a TCP (via SMC-R)
connection. The RMB is registered to the RNIC and allows remote
access by the remote peer using RDMA semantics. Each host is
passed the peer's RMB specific access information (RKey and RMB
Element offset) during the SMC-R rendezvous process. The host
stores socket application user data directly into the peer's RMB
using RDMA over RoCE.
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Rtoken
The combination of an RMB's Rkey and RDMA virtual addressing, an
Rtoken provides addressability to an RMB to an RDMA peer
RMBE
The Remote Memory Buffer Element is an area of an RMB that is
allocated to a specific TCP connection. The RMBE contains data
for the TCP connection. The RMBE represents the TCP receive
buffer whereby the remote peer writes into the RMBE and the local
peer reads from the local RMBE. The alert token resolves to a
specific RMBE.
Alert Token
The SMC-R alert token is a four byte value that uniquely
identifies the TCP connection over an SMC-R connection. The
alert token allows the SMC peer to quickly identify the target
TCP connection that now has new work. The format of the token is
defined by the owning SMC-R end point and is considered opaque to
the remote peer. However the token should not simply be an index
to an RMBE element; it should reference a TCP connection and be
able to be validated to avoid reading data from stale
connections.
RNIC
The RDMA capable Network Interface Card (RNIC) is an Ethernet NIC
that supports RDMA semantics and verbs using RoCE.
First Contact
Describes an SMC-R negotiation to set up the first link in a link
group
Subsequent Contact
Describes an SMC-R negotiation between peers who are using an
already existing SMC-R link group
2. Link Architecture
An SMC-R link is based on reliably connected queue pairs (QPs) that
form a "logical point to point link" between the two SMC-R peers over
a RoCE fabric. An SMC-R link extends from SMC-R peer to SMC-R peer,
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where typically each peer would be a TCP/IP stack would reside on
separate hosts.
,,.--..,_
+----+ _-`` `-, +-----+
|QP 8| - RoCE ', |QP 64|
| | / VLAN M . | |
+----+--------+/ \+-------+-----+
| RNIC 1 | SMC-R Link | RNIC 2 |
| |<--------------------->| |
+------------+ , /+------------+
MAC A (GID A) MAC B (GID B)
. .`
`', ,-`
``''--''``
Figure 1 SMC-R Link Overview
Figure 1 illustrates an overview of the basic concepts of SMC-R peer
to peer connectivity which is called the SMC-R Link. The SMC-R Link
forms a logical point to point connection between two SMC-R peers via
RoCE. The SMC Link is defined and identified by the following
attributes:
SMC-R Link = RC QPs (source VMAC GID QP + target VMAC GID QP + VLAN
ID)
The SMC-R Link can optionally be associated with a VLAN ID. If VLANs
are in use for the associated IP (LAN) connection then the VLAN
attribute is carried over on the SMC-R link. When VLANs are in use
each SMC-R link group is associated with a single and specific VLAN.
The RoCE fabric is the same physical Ethernet LAN used for standard
TCP/IP over Ethernet communications, with switches as described in
1.2.1.
An SMC-R Link is designed to support multiple TCP connections between
the same two peers. An SMC Link is intended to be long lived while
the underlying TCP connections can dynamically come and go. The
associated RMBs can also be dynamically added and removed from the
link as needed. The first TCP connection between the peers
establishes the SMC-R link. Subsequent TCP connections then use the
previously established link. When the last TCP connection terminates
the link can then be terminated, typically after an implementation
defined idle time-out period has elapsed. The TCP server is
responsible for initiating and terminating the SMC Link.
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2.1. Remote Memory Buffers (RMBs)
Figure 2 shows the hosts X and Y and their associated RMBs within
each host. With the SMC-R link and the associated RMB keys (Rkeys)and
RDMA virtual addresses each SMC-R enabled TCP/IP stack can remotely
access its peer's RMBs using RDMA. The RKeys and virtual addresses
are exchanged during the rendezvous processing when the link is
established. The combination of the Rkey and the virtual address is
the Rtoken. Note that the SMC-R Link ends at the QP providing access
to the RMB (via the Link + RToken).
Host X Host Y
+-------------------+ ,.--.,_ +-------------------+
| | .'` '. | |
| Protection | ,' `, | Protection |
| Domain X | / \ | Domain Y |
| +------+ / \ +------+ |
| QP 8 |RNIC 1| | SMC-R Link | |RNIC 2| QP 64 |
| | | |<-------------------->| | | |
| | | || || | | |
| | +------+| VLAN A |+------+ | |
| | || || | |
| | | | RoCE | | | |
| |RTokenX) | \ / |RToken (Y)| |
| | | \ / | | |
| V | `. ,' | V |
| +--------+ | '._ ,' | +--------+ |
| | | | `''-'`` | | | |
| | RMB | | | | RMB | |
| | | | | | | |
| +--------+ | | +--------+ |
+-------------------+ +-------------------+
Figure 2 SMC link and RMBs
An SMC-R link can support multiple RMBs which are independently
managed by each peer. The number of and the size of RMBs are managed
by the peers based on host unique memory management requirements;
however the maximum number of RMBs that can be associated to a link
group on one peer is 255. The QP has a single protection domain, but
each RMB has a unique RToken. All RTokens must be exchanged with the
peer.
Each peer manages the RMBs in its local memory for its remote SMC-R
peer by sharing access to the RMBs via Rtokens with its peers. The
remote peer writes into the RMBs via RDMA and the local peer (RMB
owner) then reads from the RMBs.
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When two peers decide to use SMC-R for a given TCP connection, they
each allocate a local RMB Element for the TCP connection and
communicate the location of this local RMB Element during rendezvous
processing. To that end, RMB elements are created in pairs, with one
RMB element allocated locally on each peer of the SMC-R link.
--- +-----------+----------------+
/\ |Eyecatcher | |
| +-----------+ |
| | |
RMB Element 1 | |
| | Receive Buffer |
| | |
| | |
\/ | |
--- +-----------+----------------+
/\ |Eyecatcher | |
| +-----------+ |
| | |
RMB Element 2 | |
| | Receive Buffer |
| | |
| | |
\/ | |
--- +----------------------------+
| . |
| . |
| . |
| . |
| (up to 255 elements) |
+----------------------------+
Figure 3 RMB Format
Figure 3 illustrates the basic format of an RMB. The RMB is a virtual
memory buffer whose backing real memory is pinned, which can support
up to 255 TCP connections to exactly one remote SMC-R peer. Each RMB
is therefore associated with the SMC-R links within a link group for
the two peers and a specific RoCE Protection Domain. Other than the 2
peers identified by the SMC-R link no other SMC-R peers can have RDMA
access to an RMB; this requires a unique Protection Domain for every
SMC-R Link. This is critical to ensure integrity of SMC-R
communications.
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RMBs are subdivided into multiple elements for efficiency, with each
RMBE element (RMBE) is associated with a single TCP connection.
Therefore multiple TCP connections across an SMC link group can share
the same memory for RDMA purposes, reducing the overhead of having to
register additional memory with the RNIC for every new TCP
connection. The number of elements in an RMB and the size of each RMB
Element is entirely governed by the owning peer subject to the SMC-R
architecture rules, however, all RMB elements within a given RMB must
be the same size. Each peer can decide the level of resource sharing
that is desirable across TCP connections based on local constraints
such as available system memory, etc. An RMB Element is identified to
the remote SMC-R peer via an RMB Element Token which consists of the
following:
o RMB RToken: The combination of the Rkey and virtual address
provided by the RNIC that identifies the start of the RMB for RDMA
operations.
o RMB Index: Identifies the RMB element index in the RMB. Used to
locate a specific RMB element within an RMB. Valid value range is
1-255.
o RMB element length: The length of the RMB element's eyecatcher
plus the length of receive buffer. This length is equal for all
RMB elements in a given RMB. This length can be variable across
different RMBs.
Multiple RMBs can be associated to an SMC-R link group and each peer
in an SMC-R link group manages allocation of its RMBs. RMB allocation
can be asymmetric. For example, server X can allocate 2 RMBs to an
SMC-R link group while server Y allocates 5. This provides maximum
implementation flexibility to allow hosts optimize RMB management for
their own local requirements. The maximum number of RMBs that can be
allocated on one peer to a link group is 255. If more RMBs are
required, the peer may fall back to IP for subsequent connections or,
if the peer is the server, create a parallel link group.
One use case for multiple RMBs is multiple receive buffer sizes.
Since every element in an RMB must be the same size, multiple RMBs
with different element sizes can be allocated if varying receive
buffer sizes are required.
Also since the maximum number of TCP connections whose receive
buffers can be allocated to an RMB is 255, multiple RMBs may be
required to provide capacity for large numbers of TCP connections
between two peers.
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Separately from the RMB, the TCP/IP stack that owns each RMB
maintains control data for each RMB element within its local control
structures. The control data contains flags for maintaining the
state of the TCP data (for example, urgent indicator) and most
importantly, two cursors which are illustrated in Figure 4:
o The peer producer cursor: This is a wrapping offset into the RMB
element's receive buffer that points to the next byte of data to
be written by the remote peer. This cursor is provided by the
remote peer in a Connection Data Control (CDC message), which is
sent using RDMA sendmsg processing, and tells the local peer how
far it can consume data in the RMBE buffer.
o The peer consumer cursor: This is a wrapping offset into the
remote peer's RMB element's receive buffer that points to the next
byte of data to be consumed by the remote peer in its own RMBE.
The local cannot write into the remote peer's RMBE beyond this
point without causing data loss. This cursor is also provided by
the peer using a Connection Data Control message.
Each TCP connection peer maintains its cursors for a TCP connection's
RMBE in its local control structures. In other words, the peer who
writes into a remote peer's RMBE provides its producer cursor to the
peer whose RMBE it has written into. The peer who reads from its
RMBE provides its consumer cursor to the writing peer. In this
manner the reads and writes between peers are kept coordinated.
For example, referring to Figure 4, peer B writes the hashed data
into the receive buffer of peer A's RMBE. After that write
completes, peer B uses a CDC message to update its producer cursor to
peer A, to indicate to peer A how much data is available for peer A
to consume. The CDC message that peer B sends to peer A wakes up
peer A and notifies it that there is data to be consumed.
Similarly, when peer A consumes data written by peer B, it uses a CDC
message to update its consumer cursor to peer B to let peer B know
how much data it has consumed, so peer B knows how much space is
available for further writes. If peer B were to write enough data to
peer A that it would wrap the RMBE receive buffer and exceed the
consumer cursor, data loss would result.
Note that this is a simplistic description of the control flows and
they are optimized to minimize the number of CDC messages required,
as described in 4.7. RMB data flows.
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Peer A's RMBE Control Info Peer B's RMBE Control Info
+--------------------------+ +--------------------------+
| | | |
/----Peer producer cursor | +-----+-Peer consumer cursor |
/| | | | |
| +--------------------------+ | +--------------------------+
| Peer A's RMBE |
| +--------------------------+ |
| | +------------------+
| | | |
| | \/ |
| | +------------|
| |-------------+/////////// |
| |//RMA data written by /// |
| |/// peer B that is ////// |
| |/available to be consumed/|
| |///////////////////////// |
| |///////// +---------------|
| |----------+/\ |
| | | |
\| | |
\ / |
|\---------/ |
| |
| |
Figure 4 RMBE cursors
Additional flags and indicators are communicated between peers. In
all cases, these flags and indicators are updated by the peer using
CDC messages with the control information contained in inline data.
More details on these additional flags and indicators are described
in . 4.3. RMBE control information.
2.2. SMC-R Link groups
SMC-R links are logically grouped together to form an SMC-R Link
Group. The purpose of the Link Group is for supporting multiple links
between the same two peers to provide for:
o Resilience: Provides transparent and dynamic switching of the link
used by existing TCP connections during link failures, typically
hardware related. TCP traffic using the failing link can be
switched to an active link within the link group avoiding
disruptions to application workloads.
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o Link utilization: Provides an active/active link usage model
allowing TCP traffic to be balanced across the links, which
increases bandwidth and avoids hardware imbalances and
bottlenecks. Note that both adapter and switch utilization can
become potential resource constraint issues
SMC-R Link Group support is required. Resilience is not optional.
However, the user can elect to provision a single RNIC (on one or
both hosts).
Multiple links that are formed between the same two peers fall into
two distinct categories:
1. Equal Links: Links providing equal access to the same RMB(s) at
both endpoints whereby all TCP connections associated with the
links must have the same VLAN ID and have the same TCP server
and TCP client roles or relationship.
2. Unequal Links: Links providing access to unique, unrelated and
isolated RMB(s) (i.e. for unique VLANs or unique and isolated
application workloads, etc.) or have unique TCP server or client
roles.
Links that are logically grouped together forming an SMC Link Group
must be equal links.
2.2.1. Link group types
Equal links within a link group also have another "Link Group Type"
attribute based on the link's associated underlying physical path.
The following SMC-R link types are defined:
1. Single Link: the only active link within a link group
2. Parallel Link: not allowed - SMC Links having the same physical
RNIC at both hosts
3. Asymmetric Link: links that have unique RNIC adapters at one
host but share a single adapter at the peer host
4. Symmetric Link: links that have unique RNIC adapters at both
hosts
These link group types are further explained in the following figures
and descriptions.
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Figure 2 above shows the single link case. The single link
illustrated in Figure 2 also establishes the SMC-R Link Group. Link
groups are supposed to have multiple links, but when only one RNIC is
available at both hosts then only a single link can be created. This
is expected to be a transient case.
Figure 5 shows the symmetric link case. Both hosts have unique and
redundant RNIC adapters. This configuration meets the objectives for
providing full RoCE redundancy required to provide the level of
resilience required for high availability for SMC-R. While this
configuration is not required, it is a strongly recommended "best
practice" for the exploitation of SMC-R. Single and asymmetric links
must be supported but are intended to provide for short term
transient conditions, for example during a temporary outage or
recycle of a RNIC.
Host X Host Y
+-------------------+ +-------------------+
| | | |
| Protection | | Protection |
| Domain X | | Domain Y |
| +------+ +------+ |
| QP 8 |RNIC 1| SMC-R Link 1 |RNIC 2| QP 64 |
|RToken X| | |<-------------------->| | | |
| | | | | | |RToken Y|
| \/ +------+ +------+ \/ |
|+--------+ | | +--------+ |
|| | | | | | |
|| RMB | | | | RMB | |
|| | | | | | |
|+--------+ | | +--------+ |
| /\ +------+ +------+ /\ |
|RToken Z| | | SMC-R Link 2 | | |RToken W|
| | |RNIC 3|<-------------------->|RNIC 4| | |
| QP 9 | | | | QP 65 |
| +------+ +------+ |
+-------------------+ +-------------------+
Figure 5 Symmetric SMC-R links
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Host X Host Y
+-------------------+ +-------------------+
| | | |
| Protection | | Protection |
| Domain X | | Domain Y |
| +------+ +------+ |
| QP 8 |RNIC 1| SMC-R Link 1 |RNIC 2| QP 64 |
|RToken X| | |<-------------------->| | | |
| | | | .->| | |RToken Y|
| \/ +------+ .` +------+ \/ |
|+--------+ | .` | +--------+ |
|| | | .` | | | |
|| RMB | | .` | | RMB | |
|| | | .`SMC-R | | | |
|+--------+ | .` Link 2 | +--------+ |
| /\ +------+ .` +------+ |
|Rtoken Z| | | .` | |down or |
| | |RNIC 3|<-` |RNIC 4|unavailable |
| QP 9 | | | | |
| +------+ +------+ |
+-------------------+ +-------------------+
Figure 6 Asymmetric SMC-R links
In the example provided by Figure 6, host X has two RNICs but Host Y
only has one RNIC. This configuration allows for the creation of an
asymmetric link. While an asymmetric link will provide some
resilience (i.e. when RNIC 1 fails) ideally each host should provide
two redundant RNICs. This should be a transient case, and when RNIC
4 becomes available, this configuration must transition to a
symmetric link configuration. This transition is accomplished by
first creating the new symmetric link, then deleting the asymmetric
link with reason code "Asymmetric link no longer needed" specified in
the DELETE LINK LLC message.
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Host X Host Y
+-------------------+ +-------------------+
| | | |
| Protection | | Protection |
| Domain X | | Domain Y |
| +------+ SMC-R link 1 +------+ |
| QP 8 |RNIC 1|<-------------------->|RNIC 2| QP 64 |
|RToken X| | | | | | |
| | | |<-------------------->| | |Rtoken Y|
| \/ +------+ SMC-R link 2 +------+ \/ |
|+--------+ QP 9 | | QP 65 +--------+ |
|| | | | | | | | |
|| RMB |<-- + | | +---->| RMB | |
|| | | | | | |
|+--------+ | | +--------+ |
| +------+ +------+ |
| down or| | | |down or |
| unavailale|RNIC 3| |RNIC 4|unavailable |
| | | | | |
| +------+ +------+ |
+-------------------+ +-------------------+
Figure 7 SMC-R parallel links (not supported)
Figure 7 shows parallel links, which are two links in the link group
that use the same hardware. This configuration is not permitted.
Because SMC-R multiplexes multiple TCP connections over an SMC-R link
and both links are using the exact same hardware, there is no
additional redundancy or capacity benefit obtained from this
configuration. However this configuration does add unnecessary
overhead of additional queue pairs, generation of additional Rkeys,
etc.
2.2.2. Maximum number of links in link group
The SMC-R protocol defines a maximum of 8 symmetric SMC-R links
within a single SMC-R link group. This allows for support for up to
8 unique physical paths between peer hosts. However, in terms of
meeting the basic requirements for redundancy support for at least 2
symmetric links must be implemented. Supporting greater than 2
links also simplifies implementation for practical matters relating
to dynamically adding and removing links, for example starting a
third SMC-R link prior to taking down one of the two existing links.
Recall that all links within a link group must have equal access to
all associated RMBs.
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The SMC-R protocol allows an implementation to implement an
implementation specific and appropriate value for maximum symmetric
links. The implementation value must not exceed the architecture
limit of 8 and the implementation must not be lower than 2, because
the SMC-R protocol requires redundancy. This does not mean that two
RNICs are physically required to enable SMC-R connectivity, but at
least two RNICs for redundancy are strongly recommended.
The SMC-R peers exchange their implementation maximum link values
during the link group establishment using the defined maximum link
value in the CONFIRM LINK LLC command. Once the initial exchange
completes the value is set for the life of the link group. The
maximum link value can be provided by both the server and client. The
server must supply a value, whereas the client maximum link value is
optional. When the client does not supply a value, it indicates that
the client accepts the server supplied maximum value. If the client
provides a value it can not exceed the server maximum value. If the
client passes a lower value then this lower value then becomes the
final negotiated maximum number of symmetric links for this link
group. Again, the minimum value is 2.
During run time the client must never request that the server add a
symmetric link to a link group that would exceed the negotiated
maximum link value. Likewise the server must never attempt to add a
symmetric link to a link group that would exceed the negotiated
maximum value.
In terms of counting the active link count within a link group, the
initial link (or the only / last) link is always counted as 1. Then
as additional links are added they are either symmetric or asymmetric
links.
With regards to enforcing the maximum link rules, asymmetric links
are an exception having a unique set of rules:
o Asymmetric links are always limited to one asymmetric link allowed
per link group
o Asymmetric links must not be counted in the maximum symmetric link
count calculation. When tracking the current count or enforcing
the negotiated maximum number of links, an asymmetric link is not
to be counted
2.2.3. Forming and managing link groups
SMC-R link groups are self-defining. The first SMC-R link in a link
group is created using TCP option flows on the TCP three-way
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handshake followed by CLC message flows over the TCP connection.
Subsequent SMC-R links in the link group are created by sending LLC
messages over an SMC-R link that already exists in the link group.
Once an SMC-R link group is created, no additional SMC-R links in
that group are created using TCP and CLC negotiation. Because
subsequent SMC-R links are created exclusively by sending LLC
messages over an existing SMC-R link in a link group, the membership
of SMC-R links to a link group is self-defining.
This architecture does not define a specific identifier for an SMC-R
link group. This identification may be useful for network management
and may be assigned in a platform specific manner, or in an extension
to this architecture.
In each SMC-R link group, one peer is the server for all TCP
connections and the other peer is the client. If there are
additional TCP connections between the peers that use SMC-R and have
the client and server roles reversed, another SMC-R link group is set
up between them with the opposite client-server relationship.
This is required because there are specific responsibilities divided
between the client and server in the management of an SMC-R link
group.
In this architecture, the decision of whether or not to use an
existing SMC-R link group or create a new SMC-R link group for a TCP
connection is made exclusively by the server.
Management of the links in an SMC-R link group is also a server
responsibility. The server is responsible for adding and deleting
links in a link group. The client may request that the server take
certain actions but the final responsibility is the server's.
2.2.4. SMC-R link identifiers
This architecture defines multiple identifiers to identify SMC-R
links and peers.
o Link number: This is a one-byte value that identifies an SMC-R
link within a link group. Both the server and the client use this
number to distinguish an SMC-R link from other links within the
same link group. It is only unique within a link group. In order
to prevent timing windows that may occur when a server creates a
new link while the client is still cleaning up a previously
existing link, link numbers cannot be reused until the entire link
numbering space has been exhausted.
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o Link User ID: This is an architecturally opaque four byte value
that a peer uses to uniquely define an SMC-R link within its own
space. This means that a link user ID is unique within one peer
only. Each peer defines its own link user ID for a link. The
peers exchange this information once during link setup and it is
never used architecturally again. The purpose of this identifier
is for network management, display, and debugging purposes. For
example an operator on a client could provide the operator on the
server with the server's link user ID if he requires the server's
operator to check on the operation of a link that the client is
having trouble with.
o Peer ID: The SMC-R peer ID uniquely identifies a specific instance
of a specific TCP/IP stack. It is required because in clustered
and load balancing environments, an IP address does not uniquely
identify a TCP/IP stack. An RNIC's MAC/GID also doesn't uniquely
or reliably identify a TCP/IP stack because RNICs can go up and
down and even be redeployed to other TCP/IP stacks in a multiple
partitioned or virtualized environment. The peer ID is not only
unique per TCP/IP stack but is also unique per instance of a
TCP/IP stack, meaning that if a TCP/IP stack is restarted, its
peer ID changes.
2.3. SMC-R resilience and load balancing
The SMC-R multi-link architecture provides resilience for network
high availability via failover capability to an alternate RoCE
adapter.
The SMC-R multilink architecture does not define primary, secondary
or alternate roles to the links. Instead there are multiple active
links representing multiple redundant RoCE paths over the same LAN.
Assignment of TCP connections to links is unidirectional and
asymmetric. This means that the client and server may each choose a
separate link for their RDMA writes associated with a specific TCP
connection.
If a hardware failure occurs or a QP failure associated with an
individual link, then the TCP connections that were associated with
the failing link are dynamically and transparently switched to use
another available link. The server or the client can detect a
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failure and immediately move their TCP connections and then notify
their peer via the DELETE LINK LLC command. While the client can
notify the server of an apparent link failure with the DELETE LINK
LLC command, the server performs the actual link deletion.
The movement of TCP connections to another link can be accomplished
with minimal coordination between the peers. The TCP connection
movement is also transparent to and non disruptive to the TCP socket
application workloads for most failure scenarios. After a failure,
the surviving links and all associated hardware must handle the link
group's workload.
As each SMC-R peer begins to move active TCP connections to another
link all current RDMA write operations must be allowed to complete.
Then the moving peer sends a signal to verify receipt of the last
successful write by its peer. If this verification fails, the TCP
connection must be reset. Once this verification is complete, all
writes that failed may then be retried, in order, over the new link.
Any data writes or CDC messages for which the sender did not receive
write completion must be replayed before any subsequent data or CDC
write operations are sent. LLC messages are not retried over the new
link because they are dependent on a known link configuration, which
has just changed because of the failure. The initiator of an LLC
message exchange that fails will be responsible for retrying once the
link group configuration stabilizes.
When a new link becomes available and is re-added to the link group
then each peer is free to rebalance its current TCP connections as
needed or only assign new TCP connections to the newly added link.
Both the server and client are free to manage TCP connections across
the link group as needed. TCP connection movement does not have to
stimulated by a link failure.
The SMC-R architecture also defines orderly vs. disorderly failover.
The type is communicated in the LLC Delete Link command and is simply
a means to indicate that the link has terminated (disorderly) or link
termination is imminent (orderly). The orderly link deletion could
be initiated via operator command or programmatically to bring down
an idle link. For example an operator command could initiate orderly
shut down of an adapter for service. Implementation of the two types
is based on implementation requirements and is beyond the scope of
the SMC-R architecture.
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3. SMC-R Rendezvous architecture
Rendezvous is the process that SMC-R capable peers use to dynamically
discover each others' capabilities, negotiate SMC-R connections, set
up SMC-R links and link groups, and manage those link groups. A key
aspect of SMC-R rendezvous is that it occurs dynamically and
automatically, without requiring SMC link configuration to be defined
by an administrator.
SMC-R Rendezvous starts with the TCP/IP three-way handshake during
which connection peers use TCP options to announce their SMC-R
capabilities. If both endpoints are SMC-R capable, then Connection
Layer Control (CLC) messages are exchanged between the peers' SMC-R
layers over the newly established TCP connection to negotiate SMC-R
credentials. The CLC message mechanism is analogous to the messages
exchanged by SSL for its handshake processing.
If a new SMC-R link is being set up, Link Layer Control (LLC)
messages are used to confirm RDMA connectivity. LLC messages are
also used by the SMC-R layers at each peer to manage the links and
link groups.
Once an SMC-R link is set up or agreed to by the peers, the TCP
sockets are passed to the peer applications which use them as normal.
The SMC-R layer, which resides under the sockets layer, transmits the
socket data between peers over RDMA using the SMC-R protocol,
bypassing the TCP/IP stack.
3.1. TCP options
During the TCP/IP three-way handshake, the client and server indicate
their support for SMC-R by including experimental TCP option 254 on
the three-way handshake flows, in accordance with RFC 6994 "Shared
Use of Experimental TCP Options". The ExID value used is the string
'SMCR' in EBCDIC (IBM-1047) encoding (0xE2D4C3D9). This ExID has
been registered in the TCP ExIDs registry maintained by IANA.
After completion of the 3-way TCP handshake each peer queries its
peer's options. If both peers set the TCP option on the three-way
handshake, inline SMC-R negotiation occurs using CLC messages. If
neither peer or only one peer set the TCP option, SMC-R cannot be
used for the TCP connection, and the TCP connection completes setup
using the IP fabric.
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3.2. Connection Layer Control (CLC) messages
CLC messages are sent as data payload over the IP network using the
TCP connection between SMC-R layers at the peers. They are analogous
to the messages used to exchange parameters for SSL.
Use of CLC messages is detailed in the following sections. The
following list provides a summary of the defined CLC messages and
their purposes:
o SMC PROPOSAL: Sent from the client to propose that this TCP
connection is eligible to be moved to SMC-R. The client identifies
itself and its subnet to the server and passes the SMC-R elements
for a suggested RoCE path via the MAC and GID.
o SMC ACCEPT: Sent from the server to accept the client's TCP
connection SMC proposal. The server responds to the client's
proposal by identifying itself to the client and passing the
elements of a RoCE path that the client can use to to perform RDMA
writes to the server. This consists of SMC-R ink elements such as
RoCE MAC, GID, RMB information etc.
o SMC CONFIRM: Sent from the client to confirm the server's
acceptance of SMC connection. The client responds to the server's
acceptance by passing the elements of a RoCE path that the server
can use to to perform RDMA writes to the client. This consists of
SMC-R ink elements such as RoCE MAC, GID, RMB information etc.
o SMC DECLINE: Sent from either the server or the client to reject
the SMC connection, indicating the reason the peer must decline
the SMC proposal and allowing the TCP connection to revert back to
IP connectivity.
3.3. LLC messages
Link Layer Control (LLC) messages are sent between peer SMC-R layers
over an SMC-R link to manage the link or the link group. LLC
messages are sent using RoCE sendmsg with inline data and are 44
bytes long. The 44 bytes size is based on what can fit into a RoCE
Work Queue Element (WQE) without requiring the posting of receive
buffers.
LLC messages generally follow a request-reply semantic. Each message
has a request flavor and a reply flavor, and each request must be
confirmed with a reply, except where otherwise noted. Use of LLC
messages is detailed in the following sections. The following list
provides a summary of the defined LLC messages and their purposes:
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o ADD LINK: Add a new link to a link group. Sent from the server to
the client to initiate addition of a new link to the link group,
or from the client to the server to request that the server
initiate addition of a new link.
o ADD LINK CONTINUATION: This is a continuation of ADD link that
allows the ADD link to span multiple commands, because all the
link information cannot be contained in a single ADD LINK message
o CONFIRM LINK: Used to confirm that RoCE connectivity over a newly
created SMC-R link is working correctly. Initiated by the server,
and both this message and its reply must flow over the SMC-R link
being confirmed.
o DELETE LINK: When initiated by the server, deletes a specific link
from the link group or deletes the entire link group. When
initiated by the client, requests that the server delete a
specific link or the entire link group.
o CONFIRM RKEY: Informs the peer on the SMC-R link of the addition
of an RMB to the link group.
o CONFIRM RKEY CONTINUATION: This is a continuation of CONFIRM RKEY
that allows the ADD link to span multiple commands, in the event
that all of the information cannot be contained in a single
CONFIRM RKEY message.
o DELETE RKEY: Informs the peer on the SMC-R link of the deletion of
one or more RMBs from the link group
o TEST LINK: Verifies that an already-active SMC-R link is active
and healthy
o Optional LLC message: Any LLC message in which the two high order
bits of the opcode are b'10' is an optional message and must be
silently discarded by a receiving peer that does not support the
opcode. No such messages are defined in this version of the
architecture, however the concept is defined to allow for
toleration of possible advanced, optional functions.
CONFIRM LINK and TEST LINK are sensitive to which link they flow on
and must flow on the link being confirmed or tested. The other flows
may flow over any active link in the link group. When there are
multiple links in a link group, a response to an LLC message must
flow over the same link that the original message flowed over, with
the following exceptions:
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o ADD LINK request from a server in response to an ADD LINK from a
client
o DELETE LINK request from a server in response to a DELETE LINK
from a client
3.4. CDC Messages
Connection Data Control (CDC) messages are sent over the RoCE fabric
between peers using RoCE sendmsg with inline data, and are 44 bytes
long which is based on the size that can fit into a RoCE Work Queue
Element (WQE) without requiring the posting of receive buffers. CDC
messages are used to describe the socket application data passed via
RDMA write operations, and TCP connection state information including
producer and consumer cursors, RMBE state information, and failover
data validation.
3.5. Rendezvous flows
Rendezvous information for SMC-R is be exchanged as TCP options on
the TCP 3-way handshake flows to indicate capability, followed by in-
line TCP negotiation messages to actually do the SMC-R setup. Formats
of all rendezvous options and messages discussed in this section are
detailed in Appendix A.
3.5.1. First contact
First contact between RoCE peers occurs when a new SMC-R link group
is being set up. This could be because no SMC-R links already exist
between the peers, or the server decides to create a new SMC-R link
group in parallel with an existing one.
3.5.1.1. TCP Options pre-negotiation
The client and server indicate their SMC-R capability to each other
using TCP option 254 on the TCP 3-way handshake flows.
A client who wishes to do SMC-R will include TCP option 254 using an
ExID equal to the EBCDIC (codepage IBM-1047) encoding of "SMCR" on
its SYN flow.
A server that supports SMC-R will include TCP option 254 with the
ExID value of EBCDIC "SMCR" on its SYN-ACK flow. Because the server
is listening for connections and does not know where client
connections will come from, the server implementation may choose to
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unconditionally include this TCP option if it supports SMC-R. This
may be required for server implementations where extensions to the
TCP stack are not practical. For server implementations which can
add code to examine and react to packets during the three-way
handshake, the server should only include the SMC-R TCP option on
SYN-ACK if the client included it on its SYN packet.
A client who supports SMC-R and meets the three conditions outlined
above may optionally include the TCP option for SMC-R on its ACK
flow, regardless of whether or not the server included it on its SYN-
ACK flow. Some TCP/IP stacks may have to include it if the SMC-R
layer cannot modify the options on the socket until the 3-way
handshake completes. Proprietary servers should not include this
option on the ACK flow, since including it on the SYN flow was
sufficient to indicate the client's capabilities.
Once the initial three-way TCP handshake is completed, each peer
examines the socket options. SMC-R implementations may do this by
examining what was actually provided on the SYN and SYN-ACK packets
or by performing a getsockopt() operation to determine the options
set by the peer. If neither peer, or only one peer, specified the TCP
option for SMC-R, then SMC-R cannot be used on this connection and it
proceeds using normal IP flows and processing.
If both peers specified the TCP option for SMC-R, then the TCP
connection is not started yet and the peers proceed to SMC-R
negotiation using inline data flows. The socket is not yet turned
over to the applications; instead the respective SMC layers exchange
CLC messages over the newly formed TCP connection.
3.5.1.2. Client Proposal
If SMC-R is supported by both peers, the client sends an SMC Proposal
CLC message to the server. On this flow from client to server it is
not immediately apparent if this is a new or existing SMC-R link
because in clustered environments a single IP address may represent
multiple hosts. This type of cluster virtual IP address can be owned
by a network based or host based layer 4 load balancer that
distributes incoming TCP connections across a cluster of
servers/hosts. Other clustered environments may also support the
movement of a virtual IP address dynamically from one host in the
cluster to another for high availability purposes. In summary, the
client can not pre-determine that a connection is targeting the same
host simply by matching the destination IP address for outgoing TCP
connections. Therefore it cannot pre-determine the SMC-R link that
will be used for a new TCP connection. This information will be
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dynamically learned and the appropriate actions will be taken as the
SMC-R negotiation handshake unfolds.
On the SMC-R proposal message, the initiator (client) proposes use of
SMC-R by including its peer ID and GID and MAC addresses, as well as
the IP subnet number of the outgoing interface (if IPv4) or the IP
prefix list for the network that the proposal is sent over (if IPv6).
At this point in the flow, the client makes no local commitments of
resources for SMC-R.
When the server receives the SMC Proposal CLC message, it uses the
peer ID provided by the client plus subnet or prefix information
provided by the client, to determine if it already has a usable SMC-R
link with this SMC-R peer. If there is one or more existing SMC-R
links with this SMC-R peer, the server then decides which SMC link it
will use for this TCP connection. See subsequent sections for the
cases of reusing an existing SMC-R link or creating a parallel SMC
link group between SMC-R peers.
If this is a first contact between SMC-R peers the server must
validate that it is on the same LAN as the client before continuing.
For IPv4, the server does this by verifying that it has an interface
with an IP subnet number that matches the subnet number set by the
client on the SMC Proposal. For IPv6 it does this by verifying that
it is directly attached to at least one IP prefix that was listed by
the client in its SMC Proposal message.
If server agrees to use SMC-R, the server begins setup of a new SMC-R
link by allocating local QP and RMB resources (setting its QP state
to INIT) and providing its full SMC-R information in an SMC Accept
CLC message to the client over the TCP connection, along with a flag
set indicating that this is a first contact flow. While the SMC
Accept message could flow over any route back to the client depending
upon IP routing, the SMC-R credentials provided must be for the
common subnet or prefix between the server and client, as determined
above. If the server cannot or does not want to do SMC-R with the
client it sends an SMC Decline CLC message to the client and the
connection data may begin flowing using normal TCP/IP flows.
3.5.1.3. Server acceptance
When the client receives the SMC Accept from the server, it uses the
combination of the first contact flag, its GID/MAC and the GID/MAC
returned by the server plus the LAN that the connection is setting up
over and the QP number provided by the server to determine if this is
a new or existing SMC-R link.
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If it is an existing SMC-R link, and the client agrees to use that
link for the TCP connection, see 3.5.2. Subsequent contact below. If
it is a new SMC-R link between peers that already have an SMC link,
then the server is starting a new SMC link group.
Assuming this is either a first contact between peers or the server
is starting a new SMC link group, the client now allocates local QP
and RMB resources for the SMC-R link (setting the QP state to RTR or
"ready to receive"), associates them with the server QP as learned on
the SMC Accept CLC message, and sends an SMC Confirm CLC message to
the server over the TCP connection with its SMC-R link information
included. The client also starts a timer to wait for the server to
confirm the reliable connected QP as described below.
3.5.1.4. Client confirmation
Upon receipt of the client's SMC Confirm CLC message, the server
associates its QP for this SMC-R link with the client's QP as learned
on the SMC Confirm CLC message and sets its QP state to RTS (ready to
send). Now the client and the server have reliable connected QPs.
3.5.1.5. Link (QP) confirmation
Since setting up the SMC-R link and its QPs did not require any
network flows on the RoCE fabric, the client and server must now
confirm connectivity over the RoCE fabric. To accomplish this, the
server will send a "Confirm Link" Link Layer Control (LLC) message to
the client over the RoCE fabric. The "Confirm Link" LLC message will
provide the server's MAC, GID, and QP information for the connection,
allow each partner to communicate the maximum number of links it can
tolerate in this link group (the "link limit"), and will additionally
provide two link IDs:
o a one-byte server-assigned Link number that is used by both peers
to identify the link within the link group and is only unique
within a link group.
o a four byte link user id. This opaque value is assigned by the
server for the server's local use and is provided to the client
for management purposes, for example to use in network management
displays and products.
When the server sends this message, it will set a timer for receiving
confirmation from the client.
When the client receives the server's confirmation "Confirm Link" LLC
message it will cancel the confirmation timer it set when it sent the
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SMC Confirm message. It will also advance its QP state to RTS and
respond over the RoCE fabric with a "Confirm Link" response LLC
message, providing its MAC, GID, QP number, link limit, confirming
the one byte link number sent by the server, and providing its own
four byte link user id to the server.
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Host X -- Server Host Y -- Client
+-------------------+ +-------------------+
| PeerID = PS1 | | PeerID = PC1 |
| +------+ +------+ |
| QP 8 |RNIC 1| |RNIC 2| QP 64 |
|RToken X| |MAC MA| |MAC MB| | |
| | |GID GA| |GID GB| |Rtoken Y|
| \/ +------+ (Subnet S1) +------+ \/ |
|+--------+ | | +--------+ |
|| RMB | | | | RMB | |
|+--------+ | | +--------+ |
| +------+ +------+ |
| |RNIC 3| |RNIC 4| |
| |MAC MC| |MAC MD| |
| |GID GC| |GID GD| |
| +------+ +------+ |
+-------------------+ +-------------------+
SYN TCP options(254,"SMCR")
<---------------------------------------------------------
SYN-ACK TCP options(254, "SMCR")
--------------------------------------------------------->
ACK [TCP options(254, "SMCR")]
<--------------------------------------------------------
SMC Proposal(PC1,MB,GB,S1)
<--------------------------------------------------------
SMC Accept(PS1,first contact,MA,GA,MTU,QP8,RToken=X,RMB elem ndx)
--------------------------------------------------------->
SMC Confirm(PC1,MB,GB,MTU,QP64,RToken=Y, RMB element index)
<--------------------------------------------------------
Confirm Link (MA,GA,QP8, link lim, server's link userid, linknum)
.........................................................>
Confirm Link Rsp(MB,GB,QP64, link lim, client link userid, linknum)
<........................................................
Legend:
------------ TCP/IP and CLC flows
............ RoCE (LLC) flows
Figure 8 First contact rendezvous flows
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Technically, the data for the TCP connection could now flow over the
RoCE path. However if this is first contact, there is no alternate
for this recently established RoCE path. Since in the current
architecture there is no failover from RoCE to IP once connection
data starts flowing, this means that a failure of this path would
disrupt the TCP connection, meaning that the level of redundancy and
failover is less than that provided by IP. If the network has
alternate RoCE paths available, they would not be usable at this
point, which is an unacceptable condition
3.5.1.6. Second SMC-R link setup
Because of the unacceptable situation described above, TCP data will
not be allowed to flow on the newly established SMC-R link until a
second path has been set up, or at least attempted.
If the server has a second RNIC available on the same LAN, it
attempts to set up the second SMC-R link over that second RNIC. If
it only has one RNIC available on the LAN, it will attempt to set up
the second SMC-R link over that one RNIC. In the latter case, the
server is attempting to set up an asymmetric link, in case the client
does have a second RNIC on the LAN.
In either case the server allocates a new QP over the RNIC it is
attempting to use for the second link, assigns a link number to the
new link and also creates an RToken for the RMB over this second QP
(note that this means that the first and second QP each has its own
RToken to represent the same RMB). The server provides this
information, as well as the MAC and GID of the RNIC it is attempting
set up the second link over in an "Add Link" LLC message which it
sends to the client over the SMC-R link that is already set up.
3.5.1.6.1. Client processing of "Add Link" LLC message from server
When the client receives the server's "Add Link" LLC message, it
examines the GID and MAC provided by the server to determine if the
server is attempting to use the same server-side RNIC as the existing
SMC-R link, or a different one.
If the server is attempting to use the same server-side RNIC as the
existing SMC-R link, then the client verifies that it has a second
RNIC on the same LAN. If it does not, the client rejects the "Add
Link" request from the server, because the resulting link would be a
parallel link which is not supported within a link group. If the
client does have a second RNIC on the same LAN, it accepts the
request and an asymmetric link will be set up.
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If the server is using a different server-side RNIC from the existing
SMC-R link then the client will accept the request and a second SMC-R
link will set up in this SMC-R link group. If the client has a
second RNIC on the same LAN, that second RNIC will be used for the
second SMC-R link, creating symmetric links. If the client does not
have a second RNIC on the same LAN, it will use the same RNIC as was
used for the initial SMC-R link, resulting in the setup of an
asymmetric link in the SMC-R link group.
In either case, when the client accepts the server's "Add Link"
request, it allocates a new QP on the chosen RNIC and creates an Rkey
over that new QP for the client-side RMB for the SMC link group, then
sends an "Add Link" reply LLC message to the server providing that
information as well as echoing the Link number that was set by the
server.
If the client rejects the server's "Add Link" request, it sends an
"Add Link" reply LLC message to the server with the reason code for
the rejection.
3.5.1.6.2. Server processing of "Add Link" reply LLC message from the
client
If the client sends a negative response to the server or no reply is
received, the server frees the RoCE resources it had allocated for
the new link. Having a single link in an SMC-R link group is
undesirable and the server's recovery is detailed in C.8. Failure to
add second SMC-R link to a link group.
If the client sends a positive reply to the server with
MAC/GID/QP/Rkey information, the server associates its QP for the new
SMC-R link to the QP that the client provided. Now the new SMC-R
link is in the same situation that the first was in after the client
sent its ACK packet - there is a reliable connected QP over the new
RoCE path, but there have been no RoCE flows to confirm that it's
actually usable. So at this point the client and server will
exchange "Confirm Link" LLC messages just like they did on the first
SMC-R link.
If either peer receives a failure during this second "Confirm Link"
LLC exchange (either an immediate failure which implies that the
message did not reach the partner, or a timeout), it sends a "Delete
Link" LLC message to the partner over the first (and now only) link
in the link group which must be acknowledged before data can flow on
the single link in the link group.
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Host X -- Server Host Y -- Client
+-------------------+ +-------------------+
| PeerID = PS1 | | PeerID = PC1 |
| +------+ +------+ |
| QP 8 |RNIC 1| |RNIC 2| QP 64 |
|RToken X| |MAC MA| |MAC MB| | |
| | |GID GA| |GID GB| |RToken Y|
| \/ +------+ +------+ \/ |
|+--------+ | | +--------+ |
|| | | | | | |
|| RMB | | | | RMB | |
|| | | | | | |
|+--------+ | | +--------+ |
| /\ +------+ +------+ /\ |
| | |RNIC 3| |RNIC 4| | |
|RToken Z| |MAC MC| |MAC MD| |RToken W|
| QP 9 |GID GC| |GID GD| QP 65 |
| +------+ +------+ |
+-------------------+ +-------------------+
First SMC-R link setup as shown in Figure 8
<-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.->
ADD link request (QP9,MC,GC, link number=2)
............................................>
ADD link response (QP65,MD,GD, link number=2)
<............................................
ADD link continuation request (RToken=Z)
............................................>
ADD link continuation response(RToken=W)
<............................................
Confirm Link(MC,GC,QP9,link number=2, link userid)
.............................................>
Confirm Link response(MD,GD,QP65,link number=2, link userid)
<.............................................
Legend:
------------ TCP/IP and CLC flows
............ RoCE (LLC) flows
Figure 9 First contact, second link setup
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3.5.1.6.3. Exchange of Rkeys on second SMC-R link
Note that in the scenario described here, first contact, there is
only one RMB Rkey to exchange on the second SMC-R link and it is
exchanged in the Add Link Continuation request and reply. In
scenarios other than first contact, for example, adding a new SMC-R
link to a longstanding link group with multiple RMBs, additional
flows will be required to exchange additional RMB Rkeys. See
3.5.5.2.3. Adding a new SMC-R link to a link group with multiple RMBs
for more details on these flows
3.5.1.6.4. Aborting SMC-R and falling back to IP
If both partners don't provide the SMC-R TCP option during the 3 way
TCP handshake, the connection falls back to normal TCP/IP. During
the SMC-R negotiation that occurs after the 3 way TCP handshake,
either partner may break off SMC-R by sending an SMC Decline CLC
message. The SMC Decline CLC message may be sent in place of any
expected message, and may also be sent during the Confirm Link LLC
exchange if there is a failure before any application data has flowed
over the RoCE fabric. For more detail on exactly when an SMC Decline
can flow during link group setup, see C.1. SMC Decline during CLC
negotiation and C.2. SMC Decline during LLC negotiation
If this fallback to IP happens while setting up a new SMC-R link
group, the RoCE resources allocated for this SMC-R link group
relationship are torn down and it will be retried as a new SMC-R link
group next time a connection starts between these peers with SMC-R
proposed. Note that if this happens because one side doesn't support
SMC-R, there will be very little to tear down as the TCP option will
have failed to flow either on the initial SYN or the SYN-ACK, before
either side had reserved any local RoCE resources.
3.5.2. Subsequent contact
"Subsequent contact" means setting up a new TCP connection between
two peers that already have an SMC-R link group between them, and
reusing the existing SMC-R link group. In this case it is not
necessary to allocate new QPs. However it is possible that a new RMB
has been allocated for this TCP connection, if the previous TCP
connection used the last element available in the previously used
RMB, or for any other implementation-dependent reason. For this
reason, and for convenience and error checking, the same TCP option
254 followed by inline negotiation method described for initial
contact will be used for subsequent contact, but the processing
differs in some ways. That processing is described below.
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3.5.2.1. SMC-R proposal
When the client begins the inline negotiation with the server, it
does not know if this is a first contact or a subsequent contact.
The client cannot know this information until it sees the server's
peer ID to determine whether or not it already has an SMC-R link with
this peer that it can use. There are several reasons why it is not
sufficient to use the partner IP address, subnet, VLAN or other IP
information to make this determination. The most obvious reason is
distributed systems: if the server IP address is actually a virtual
IP address representing a distributed cluster, the actual host
serving this TCP connection may not be the same as the host that
served the last TCP connection to this same IP address.
After the TCP three way handshake, assuming both partners indicate
SMC-R capability, the client builds and sends the SMC Proposal CLC
message to the server in exactly the same manner as it does in the
first contact case, and in fact at this point doesn't know if it's
first contact or subsequent contact. As in the first contact case,
the client sends its Peer ID value, suggested RNIC GID/MAC, and IP
subnet or prefix information.
Upon receiving the client's proposal, the server looks up the peer ID
provided to determine if it already has a usable SMC-R link group
with this peer. If it does already have a usable SMC-R link group,
the server then needs to decide if it will use the existing SMC-R
link group, or create a new link group. For the new link group
case, see 3.5.3. First contact variation: creating a parallel link
group, below.
For this discussion assume the server decides to use the existing
SMC-R link group for the TCP connection, which is expected to be the
most common case. The server is responsible for making this decision.
Then the server needs to communicate that information to the client,
but it is not necessary to allocate, associate, and confirm QPs for
the chosen SMC-R link. All that remains to be done is to set up RMB
space for this TCP connection.
If one of the RMBs already in use for this SMC-R link group has an
available element that uses the appropriate buffer size, the server
merely chooses one for this TCP connection and then sends an SMC
Accept CLC message, providing the full RoCE information for the
chosen SMC-R link to the client, using the same format as the SMC
Accept CLC message described in the initial contact section above.
The server may choose to use the SMC-R link that matches the
suggested MAC/GID provided by the client on the SMC Proposal for its
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RDMA writes but is not obligated to. The final decision on which
specific SMC-R link to assign a TCP connection to is an independent
server and client decision.
It may be necessary for the server to allocate a new RMB for this
connection. The reasons for this are implementation dependent and
could include: no available space in existing RMB or RMBs, or desire
to allocate a new RMB that uses a different buffer size from the ones
already created, or any other implementation dependent reason. In
this case the server will allocate the new RMB and then perform the
flows described in 3.5.5.2.1. Adding a new RMB to an SMC-R link
group. Once that processing is complete, the server then provides the
full RoCE information, including the new Rkey, for this connection
on an SMC Confirm CLC message to the client.
3.5.2.2. SMC-R acceptance
Upon receiving the SMC Accept CLC message from the server, the client
examines the RoCE information provided by the server to determine if
this is a first contact for a new SMC link group, or subsequent
contact for an existing SMC-R link group. It is subsequent contact
if the server side peer ID, GID, MAC and QP number provided on the
packet match a known SMC-R link, and the "first contact" flag is not
set. If this is not the case, for example the GID and MAC match but
the QP is new, then the server is creating a new, parallel SMC-R link
group and this is treated as a first contact.
A different RMB RToken does not indicate a first contact as the
server may have allocated a new RMB, or be using several RMBs for
this SMC-R link. The client needs the server's RMB information only
for its RDMA writes to the server, and since there is no requirement
for symmetric RMBs, this information is simply control information
for the RDMA writes on this SMC-R link.
The client must validate that the RMB element being provided by the
server is not in use by another TCP connection on this SMC-R link
group. This validation must validate the new <rtoken, index> across
all known <rtoken, index> on this link group. See 4.4.2. RMB element
reuse and conflict resolution for the case in which the server tries
to use an RMB element that is already in use on this link group.
Once the client has determined that this TCP connection is a
subsequent contact over an existing SMC link, it performs a similar
RMB allocation process as the server did: it either allocates an
element from an RMB already associated with this SMC-R link, or it
allocates a new RMB and associates it with this SMC-R link and then
chooses an element out of it.
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If the client allocates a new RMB for this TCP connection, it
performs the processing described in 3.5.5.2.1. Adding a new RMB to
an SMC-R link group. Once that processing is complete, the client
provides its full RoCE information for this TCP connection on an SMC
Confirm CLC message.
Because an SMC-R link with a verified connected QP already exists and
is being reused, there is no need for verification or alternate QP
selection flows or timers.
3.5.2.3. SMC-R confirmation
When the server receives the client's SMC Confirm CLC message on a
subsequent contact, it verifies the following:
o the RMB element provided by the client is not already in use by
another TCP connection on this SMC-R link group (see section
4.4.2. RMB element reuse and conflict resolution for the case in
which it is).
o The MAC/GID/QP info provided by the client matches an active link
within the link group. The client is free to select any valid /
active link. The client is not required to select the same link as
the server.
If this validation passes, the server stores the client's RMB
information for this connection and the RoCE setup of the TCP
connection is complete.
3.5.2.4. TCP data flow race with SMC Confirm CLC message
On a subsequent contact TCP/IP connection, a peer may send data as
soon as it has received the peer RMB information for the connection.
There are no additional RoCE confirmation flows, since the QPs on the
SMC link are already reliably connected and verified.
In the majority of cases the first data will flow from the client to
the server. The client must send the SMC Confirm CLC message before
sending any connection data over the chosen SMC-R link, however the
client need not wait for confirmation of this message, and in fact
there will be no such confirmation. Since the server is required to
have the RMB fully set up and ready to receive data from the client
before sending SMC Accept CLC message, the client can begin sending
data over the SMC-R link immediately upon completing the send of the
SMC Confirm CLC message.
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It is possible that data from the client will arrive into the server
side RMB before the SMC Confirm CLC message from the client has been
processed. In this case the server must handle this race condition,
and not provide the arrived TCP data to the socket application until
the SMC Confirm CLC message has been received and fully processed,
opening the socket.
If the server has initial data to send to the client which is not a
response to the client (this case should be rare), it can send the
data immediately upon receiving and processing the SMC Confirm CLC
message from the client. The client must have opened the TCP socket
to the client application upon sending of SMC Confirm CLC message so
the client will be ready to process data from the server.
3.5.3. First contact variation: creating a parallel link group
Recall that parallel SMC-R links within an SMC-R link group are not
supported. These are multiple SMC-R links within a link group that
use the same network path. However, multiple SMC-R link groups
between the same peers are supported. This means that if multiple
SMC-R links over the same RoCE path are desired, it is necessary to
use multiple SMC-R link groups. While not a recommended practice,
this could be done for platform specific reasons, like QP separation
of different workloads. Only the server can drive the creation of
multiple SMC-R link groups between peers.
At a high level, when the server decides to create an additional SMC-
R link group with a client it already has an SMC-R link group with,
the flows are basically the same as the normal "first contact" case
described above. The following provides more detail and
clarification of processing in this case.
When the server receives the SMC Proposal CLC message from the client
and using the GID/MAC info determines that it already has an SMC-R
link group with this client, the server can either reuse the existing
SMC-R link group (detailed in 3.5.2. Subsequent contact above) or it
can create a new SMC-R link group in addition to the existing one.
If the server decides to create a new SMC-R link group, it does the
same processing it would have done for first contact: allocate QP and
RMB resources as well as alternate QP resources, and communicate the
QP and RMB information to the client on the SMC Accept CLC message
with the "first contact" flag set.
When the client receives the server's SMC Accept CLC message with the
new QP information and the "first contact" flag, it knows the server
is creating a new SMC-R link group even though it already has an SMC-
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R link group with the server. In this case the client will also
allocate a new QP for this new SMC link and allocate an RMB for this
link and generate an Rkey for it.
Note that multiple SMC-R link groups between the same peers must
access different RMB resources, so new RMBs will be required. Using
the same RMBs that are in use in another SMC-R link group is not
permitted.
The client then associates its new QP with the server's new QP and
sends its SMC Confirm CLC message back to the server providing the
new QP/RMB information and sets its confirmation timer for the new
SMC-R link.
When the server receives the client's SMC Confirm CLC message it
associates its QP with the client's QP as learned on the SMC Confirm
CLC message and sends a confirmation LLC message. The rest of the
flow, with the confirmation QP and setup of additional SMC-R links,
unfolds just like the first contact case.
3.5.4. Normal SMC-R link termination
The normal sockets API trigger points are used by the SMC-R layer to
initiate SMC-R connection termination flows. The main design point
for SMC-R normal connection flows is to use the SMC-R protocol to
first shutdown the SMC-R connection and free up any SMC-R RDMA
resources and then allow the normal TCP connection termination
protocol (i.e. FIN processing) to drive cleanup of the TCP connection
that exists on the IP fabric. This design point is very important in
ensuring that RDMA resources such as the RMBEs are only freed and
reused when both SMC-R end points are completely done with their RDMA
Write operations to the partner's RMBE.
When the last TCP connection over an SMC-R link group terminates, the
link group can be terminated. Similar to creation of SMC-R links and
link groups, the primary responsibility for determining that normal
termination is needed and initiating it lies with the server.
Implementations may opt to set timers to keep SMC-R link groups up
for a specified time after the last TCP connection ends, to avoid
churn in cases when TCP connections come and go regularly.
The link or link group may also be terminated as a result of an
operator initiated command. This command can be entered at either
the client or the server. If entered at the client, the client
requests that the server perform link or link group termination, and
the responsibility for doing so ultimately lies with the server.
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When the server determines that the SMC-R link group is to be
terminated, it sends a DELETE LINK LLC message to the client, with a
flag set indicating that all links in the link group are to be
terminated. After receiving confirmation from the adapter that the
DELETE LINK LLC message has been sent, the server can clean up its
end of the link group (QPs, RMBs, etc). Upon receipt of the DELETE
LINK message from the server, the client must immediately comply and
clean up its end of the link group. Any TCP connections that the
client believes to be active on the link group must be immediately
terminated.
The client can request that the server delete the link group as well.
The client does this by sending a DELETE LINK message to the server
indicating that cleanup of all links is requested. The server must
comply by sending a DELETE LINK to the client and processing as
described above. If there are TCP connections active on the link
group when the server receives this request, they are immediately
terminated by sending a RST flow over the IP fabric.
3.5.5. Link group management flows
3.5.5.1. Adding and deleting links in an SMC-R link group
The server has the lead role in managing the composition of the link
group. Links are added to link group by the server. The client may
notify the server of new conditions that may result in the server
adding a new link, but the server is ultimately responsible. In
general links are deleted from the link group by the server, however
in certain error cases the client may inform the server that a link
must be deleted and treat it as deleted without waiting for action
from the server. These flows are detailed in the following sections
3.5.5.1.1. Server initiated Add Link processing
As described in previous sections, the server initiates an Add Link
exchange to create redundancy in a newly created link group. Once a
link group is established the server may also initiate Add Link for
other reasons, including:
o Availability of additional resources on the server host to support
an additional SMC-R link. This may include the provisioning of an
additional RNIC, more storage becoming available to support
additional QP resources, operator command, or any other
implementation dependent reason. Note that, to be available for
an existing link group, a new RNIC must be attached to the same
RoCE LAN that the link group is using.
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o Receipt of notification from the client that additional resources
on the client are available to support an additional SMC-R link.
See 3.5.5.1.2. Client initiated Add Link processing.
Server initiated Add Link processing in an established SMC-R link
group is the same as the Add Link processing described in 3.5.1.6.
Second SMC-R link setup with the following changes:
o If an asymmetric SMC-R link already exists in the link group a
second asymmetric link will not be created. Only one asymmetric
link is permitted in a link group.
o TCP data flow on already existing link(s) in the link group is not
halted or otherwise affected during the process of setting up the
additional link.
In no case will the server initiate Add Link processing if the link
group already has the maximum number of links negotiated by the
partners.
3.5.5.1.2. Client initiated Add Link processing
If an additional RNIC becomes available for an existing SMC-R link
group on the client's side, the client notifies the server by sending
an Add Link request LLC message to the server. Unlike an Add Link
request sent by the server to the client, this Add Link request
merely informs the server that the client has a new RNIC. If the
link group lacks redundancy, or has redundancy only on an asymmetric
link with a single RNIC on the client side, the server must initiate
an Add Link exchange in response to this message, to create or
improve the link group's redundancy.
If the link group already has symmetric link redundancy but has fewer
than the negotiated maximum number of links, the server may respond
by initiating an Add Link exchange to create a new link using the
client's new resource but is not required to.
If the link group already has the negotiated maximum number of links,
the server must ignore the client's Add Link request LLC message.
Because the server is not required to respond to the client's Add
Link LLC message in all cases, the client must not wait for a
response or throw an error if one does not come.
3.5.5.1.3. Server initiated Delete Link Processing
Reasons that a server may delete a link include:
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o The link has not been used for TCP connections for an
implementation defined time interval, and deleting the link will
not cause the link group to lack redundancy
o An error in resources supporting the link. These may include but
are not limited to: RNIC errors, QP errors, software errors
o The RNIC supporting this SMC-R link is being taken down, either
because of an error case or because of an operator or software
command.
If a link being deleted is supporting TCP connections, and there are
one or more surviving links in the link group, the TCP connections
are moved to the surviving links. For more information on this
processing see 2.3. SMC-R resilience and load balancing.
The server deletes a link from the link group by sending a Delete
Link request LLC message to the client over any of the usable links
in the link group. Because the Delete Link LLC message specifies
which link is to be deleted, it may flow over any link in the link
group. The server must not clean up its RoCE resources for the link
until the client responds.
The client responds to the server's Delete Link request LLC message
by sending the server a Delete Link response LLC message. The client
must respond positively; it cannot decline to delete the link. Once
the server has received the client's Delete Link response, both sides
may clean up their resources for the link.
Positive write completion or other indication from the RNIC on the
client's side is sufficient to indicate to the client that the server
has received the Delete Link response.
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Host X Host Y
+-------------------+ +-------------------+
| +------+ +------+ |
| QP 8 |RNIC 1| SMC-R Link 1 |RNIC 2| QP 9 |
|RToken X| |Failed|<--X----X----X----X-->| | |
| | | | | | |
| \/ +------+ +------+ |
|+--------+ | | |
|| deleted| | | |
|| RMB | | | |
|| | | | |
|+--------+ | | |
| /\ +------+ +------+ |
|RToken Z| | | SMC-R Link 2 | | |
| | |RNIC 3|<-------------------->|RNIC 4| |
| QP 64| | | | QP 65 |
| +------+ +------+ |
+-------------------+ +-------------------+
DELETE LINK(Request, link number = 1,
................................................>
reason code = RNIC failure)
DELETE LINK(Response, link number = 1)
<................................................
(note, architecturally this exchange can flow over either
SMC-R link but most likely flows over link 2 since
the RNIC for link 1 has failed)
Figure 10 Server initiated Delete Link flow
3.5.5.1.4. Client initiated Delete Link request
The client may request that the server delete a link for the same
reasons that the server may delete a link, except for inactivity
timeout.
Because the client depends on the server to delete links, there are
two types of delete requests from client to server:
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o Orderly: the client is requesting that the server delete the link
when able. This would result from an operator command to bring
down the RNIC or some other nonfatal reason. In this case the
server is required to delete the link, but may not do it right
away.
o Disorderly: the server must delete the link right away, because
the client has experienced a fatal error with the link.
In either case the server responds by initiating a Delete Link
exchange with the client as described in the previous section. The
difference between the two is whether the server must do so
immediately or can delay for an opportunity to gracefully delete the
link.
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Host X Host Y
+-------------------+ +-------------------+
| +------+ +------+ |
| QP 8 |RNIC 1| SMC-R Link 1 |RNIC 2| QP 9 |
|RToken X| | |<---X--X--X--X--X--X->|Failed| |
| | | | | | |
| \/ +------+ +------+ |
|+--------+ | | |
|| deleted| | | |
|| RMB | | | |
|| | | | |
|+--------+ | | |
| /\ +------+ +------+ |
|RToken Z| | | SMC-R Link 2 | | |
| | |RNIC 3|<-------------------->|RNIC 4| |
| QP 64| | | | QP 65 |
| +------+ +------+ |
+-------------------+ +-------------------+
DELETE LINK(Request, link number = 1, disorderly,
<...............................................
reason code = RNIC failure)
DELETE LINK(Request, link number = 1,
................................................>
reason code = RNIC failure)
DELETE LINK(Response, link number = 1)
<................................................
(note, architecturally this exchange can flow over either
SMC-R link but most likely flows over link 2 since
the RNIC for link 1 has failed)
Figure 11 Client-initiated Delete Link
3.5.5.2. Managing multiple Rkeys over multiple SMC-R links in a link
group
After the initial contact sequence completes and the number of TCP
connections increases it is possible that the SMC peers could add
additional RMBs to the Link Group. Recall that each peer
independently manages its RMBs. Also recall that an RMB's RToken is
specific to a QP, which means that when there are multiple SMC-R
links in a link group, each RMB accessed with the link group requires
a separate RToken for each SMC-R link in the group.
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Each RMB that is added to a link must be added to all links within
the Link Group. The set of RMBs created for the Link is called the
"RToken Set". The RTokens must be exchanged with the peer. As RMBs
are added and deleted, the RToken Set must remain in sync.
3.5.5.2.1. Adding a new RMB to an SMC-R link group
A new RMB can be added to an SMC-R link group on either the client or
the server side. When an additional RMB is added to an existing SMC-
R link group, that RMB must be associated with the QPs for each link
in the link group. Therefore when an RMB is added to an SMC-R link
group, its RMB RToken for each SMC-R link's QP must be communicated
to the peer.
The tokens for a new RMB added to an existing SMC-R link group are
communicated using "Confirm Rkey" LLC messages, as shown in Figure
12. The RToken set is specified as pairs: an SMC link number, paired
with the new RMB's RToken over that SMC Link. To preserve failover
capability, any TCP connection that uses a newly added RMB cannot go
active until all RTokens for the RMB have been communicated for all
the links in the link group.
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Host X Host Y
+-------------------+ +-------------------+
| +------+ +------+ |
| QP 8 |RNIC 1| SMC-R Link 1 |RNIC 2| QP 9 |
|RToken X| | |<-------------------->| | |
| | | | | | |
| \/ +------+ +------+ |
|+--------+ | | |
|| new | | | |
|| RMB | | | |
|| | | | |
|+--------+ | | |
| /\ +------+ +------+ |
|RToken Z| | | SMC-R Link 2 | | |
| | |RNIC 3|<-------------------->|RNIC 4| |
| QP 64| | | | QP 65 |
| +------+ +------+ |
+-------------------+ +-------------------+
CONFIRM RKEY(Request, Add,
................................................>
RToken set((Link 1,RToken X),(Link2,RToken Z)))
CONFIRM RKEY(Response, Add,
<................................................
RToken set((Link 1,RToken X),(Link2,RToken Z)))
(note, this exchange can flow over either SMC-R link)
Figure 12 Add RMB to existing link group
Implementations may choose to proactively add RMBs to link groups in
anticipation of need. For example, an implementation may add a new
RMB when all of its existing RMBs are over a certain threshold
percentage used.
A new RMB may also be added to an existing link group on an as needed
basis. For example, when a new TCP connection is added to the link
group but there are no available RMB elements. In this case the CLC
exchange is paused while the peer that requires the new RMB adds it.
An example of this is illustrated in figure 13.
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Host X -- Server Host Y -- Client
+-------------------+ +-------------------+
| PeerID = PS1 | | PeerID = PC1 |
| +------+ +------+ |
| QP 8 |RNIC 1| SMC-R link 1 |RNIC 2| QP 64 |
|RToken X| |MAC MA|<-------------------->|MAC MB| | |
| | |GID GA| |GID GB| |RTokenY2|
| \/ +------+ +------+ \/ |
|+--------+ | | +--------+ |
|| | | SUBNET S1 | | New | |
|| RMB | | | | RMB | |
|+--------+ | | +--------+ |
| /\ +------+ +------+ /\ |
| | |RNIC 3| SMC-R link 2 |RNIC 4| |RTokenW2|
| | |MAC MC|<-------------------->|MAC MD| | |
| QP 9 |GID GC| |GID GD| QP65 |
| +------+ +------+ |
+-------------------+ +-------------------+
SYN / SYN-ACK / ACT TCP 3-way handshake with TCP option
<--------------------------------------------------------->
SMC Proposal(PC1,MB,GB,S1)
<--------------------------------------------------------
SMC Accept(PS1,not 1st contact,MA,GA,QP8,RToken=X,RMB elem index)
--------------------------------------------------------->
Confirm Rkey(Request, Add,
<........................................................
RToken set((Link1, RToken Y2),{Link2, RToken W2)))
Confirm Rkey(Response, Add,
........................................................>
RToken set((Link1, RToken Y2),{Link2, RToken W2)))
SMC Confirm(PC1,MB,GB,QP64,RToken=Y2, RMB element index)
<--------------------------------------------------------
Legend:
------------ TCP/IP and CLC flows
............ RoCE (LLC) flows
Figure 13 Client adds RMB during TCP connection setup
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3.5.5.2.2. Deleting an RMB from an SMC-R link group
Either peer can delete one or more of its RMBs as long as it is not
being used for any TCP connections. Ideally an SMC-R peer would use
a timer to avoid freeing an RMB immediately after the last TCP
connection stops using it, to keep the RMB available for later TCP
connections and avoid thrashing with addition and deletion of RMBs.
Once an SMC-R peer decides to delete an RMB, it sends a DELETE RKEY
LLC message to its peer. It can then free the RMB once it receives a
response from the peer. Multiple RMBs can be deleted in a DELETE
RKEY exchange.
Note that in a DELETE RKEY message, it is not necessary to specify
the full RToken for a deleted RMB. The RMB's Rkey over one link in
the link group is sufficient to specify which RMB is being deleted.
Host X Host Y
+-------------------+ +-------------------+
| +------+ +------+ |
| QP 8 |RNIC 1| SMC-R Link 1 |RNIC 2| QP 9 |
|RToken X| | |<-------------------->| | |
| | | | | | |
| \/ +------+ +------+ |
|+--------+ | | |
|| deleted| | | |
|| RMB | | | |
|| | | | |
|+--------+ | | |
| /\ +------+ +------+ |
|RToken Z| | | SMC-R Link 2 | | |
| | |RNIC 3|<-------------------->|RNIC 4| |
| QP 9 | | | | |
| +------+ +------+ |
+-------------------+ +-------------------+
DELETE RKEY(Request, Rkey list(Rkey X))
................................................>
DELETE RKEY(Response, Rkey list(Rkey X))
<................................................
(note, this exchange can flow over either SMC-R link)
Figure 14 Delete RMB from SMC-R link group
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3.5.5.2.3. Adding a new SMC-R link to a link group with multiple RMBs
When a new SMC-R link is added to an existing link group, there could
be multiple RMBs on each side already associated with the link group.
There could also be a different number of RMBs on one side as on the
other, because each peer manages its RMBs independently. Each of
these RMBs will require a new RToken to be used on the new SMC-R
link, and then those new RTokens must be communicated to the peer.
This requires two-way communication as the server will have to
communicate its RTokens to the client and vice versa.
RTokens are communicated between peers in pairs. Each RToken pair
consists of:
o The RToken for the RMB, as is already known on an existing SMC-R
link in the link group
o The RToken for the same RMB, to be used on the new SMC-R link.
These pairs are required to ensure that each peer knows which RTokens
across QPs are equivalent.
The "Add Link" request and response LLC messages do not have room to
contain any RToken pairs. "Add Link continuation" LLC messages are
used to communicate these pairs, as shown in Figure 15. The "Add
Link Continuation" LLC messages are sent on the same SMC-R link that
the "Add Link" LLC messages were sent over, and in both the "Add
Link" and the "Add Link Continuation" LLC messages, the first RToken
in each RToken pair will be the RToken for the RMB as known on the
SMC-R link that the LLC message is being sent over.
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Host X -- Server Host Y -- Client
+-------------------+ +-------------------+
| PeerID = PS1 | | PeerID = PC1 |
| +------+ +------+ |
| QP 8 |RNIC 1| |RNIC 2| QP 64 |
|Rkey Set| |MAC MA| |MAC MB| |Rkey set|
|X,Y,Z | |GID GA| |GID GB| |Q,R,S,T |
| \/ +------+ +------+ \/ |
|+--------+ | | +--------+ |
|| 3 RMBs | | | | 4 RMBs | |
|+--------+ | | +--------+ |
| /\ +------+ +------+ /\ |
|Rkey set| |RNIC 3| |RNIC 4| | Rkey set|
|U,V,W | |MAC MC| |MAC MD| | L,M,N,P |
| QP 9 |GID GC| |GID GD| QP 65 |
| +------+ +------+ |
+-------------------+ +-------------------+
ADD link request (QP9,MC,GC, link number=2)
............................................>
ADD link response (QP65,MD,GD, link number=2)
<............................................
ADD link continuation req(RToken Pairs=((X,U),(Y,V),(Z,W)))
............................................>
ADD link continuation rsp(RToken Pairs=((Q,L),(R,M),(S,N),(T,P)))
<.............................................
Confirm Link Req/Rsp exchange on link 2
<.............................................>
Legend:
------------ TCP/IP and CLC flows
............ RoCE (LLC) flows
Figure 15 Exchanging Rkeys when a new link is added to a link group
3.5.5.3. Serialization of LLC exchanges, and collisions
LLC flows can be divided into two main groups for serializaion
considerations.
The first group is LLC messages that are independent and can flow at
any time. These are one-time, unsolicited messages that either do
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not have a required response, or that have a simple response that
does not interfere with the operations of another group of messages.
These messages are:
o TEST LINK from either the client or the server: This message
requires a TEST LINK response to be returned, but does not affect
the configuration of the link group or the Rkeys.
o ADD LINK from the client to the server: This message is provided
as an "FYI" to the server to let it know that the client has an
additional RNIC available. The server is not required to act upon
or respond to this message.
o DELETE_LINK from the client to the server: This message informs
the server that the client has either experienced an error or
problem that requires a link or link group to be terminated, or
that an operator has commanded that a link or link group be
terminated. The server does not respond directly to the message,
rather it initiates a DELETE LINK exchange as a result of
receiving it.
o DELETE LINK from the server to the client with the "delete entire
link group" flag set: This message informs the client that the
entire link group is being deleted.
The second group is LLC messages that are part of an exchange of LLC
messages that affects link group configuration that must complete
before another exchange of LLC messages that affects link group
configuration can be processed. When a peer knows that one of these
exchanges is in progress, it must not start another exchange. These
exchanges are:
o ADD LINK / ADD LINK response / ADD LINK CONTINUATION / ADD LINK
CONTINUATION response / CONFIRM LINK / CONFIRM LINK RESPONSE:
This exchange, by adding a new link, changes the configuration of
the link group.
o DELETE LINK / DELETE LINK response initiated by the server: This
exchange, by deleting a link, changes the configuration of the
link group.
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o CONFIRM RKEY / CONFIRM RKEY response or DELETE RKEY / DELETE RKEY
response: This exchange changes the RMB configuration of the link
group. RKeys can not change while links are being added or
deleted (while ADD or DELETE LINK is in progress). However,
CONFIRM RKEY and DELETE RKEY are unique in that both the client
and server can independently manage (add or remove) their own
RMBs. This allows each peer to concurrently change their RKeys
and therefore concurrently send CONFIRM RKEY or DELETE RKEY
requests. The concurrent CONFIRM RKEY or DELETE RKEY requests can
be independently processed and do not represent a collision
Because the server is in control of the configuration of the link
group, many timing windows and collisions are avoided but there are
still some that must be handled.
3.5.5.3.1. Collisions with ADD LINK / CONFIRM LINK exchange
Colliding LLC message: TEST LINK
Action to resolve: Send immediate TEST LINK reply
Colliding LLC Message: ADD LINK from client to server
Action to resolve: Server ignores the ADD LINK message. When
client receives server's ADD LINK, client will consider that
message to be in response to its ADD LINK message and the flow
works. Since both client and server know not to start this
exchange if an ADD LINK operation is already underway, this can
only occur if the client sends this message before receiving the
server's ADD LINK and this message crosses with the server's ADD
LINK message, therefore the server's ADD LINK arrives at the
client immediately after the client sent this message.
Colliding LLC Message: DELETE LINK from client to server, specific
link specified
Action to resolve: Server queues the DELETE link message and
processes after the ADD LINK exchange completes. If it is an
orderly link termination, it can wait until after this exchange
continues. If it is disorderly and the link affected is the one
that the current exchange is using, the server will discover the
outage when a message in this exchange fails.
Colliding LLC Message: DELETE LINK from client to server, entire link
group to be deleted
Action to resolve: Immediately clean up the link group
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Colliding LLC message: CONFIRM RKEY from the client
Action to resolve: Send negative CONFIRM_RKEY response to the
client. Once the current exchange finishes, client will have to
recompute its Rkey set to include the new link, and start a new
CONFIRM RKEY exchange.
3.5.5.3.2. Collisions during DELETE LINK exchange
Colliding LLC Message: TEST LINK from either peer
Action to resolve: Send immediate TEST LINK response
Colliding LLC message: ADD LNK from client to server
Action to resolve: Server queues the ADD LINK and processes it
after the current exchange completes
Colliding LLC message: DELETE LINK from client to server (specific
link)
Action to resolve: Server queues the DELETE link message and
processes after the current exchange completes. If it is an
orderly link termination, it can wait until after this exchange
continues. If it is disorderly and the link affected is the one
that the current exchange is using, the server will discover the
outage when a message in this exchange fails
Colliding LLC message: DELETE LINK from either client or server,
deleting the entire link group
Action to resolve: immediately clean up the link group
Colliding LLC message: CONFIRM_RKEY from client to server
Action to resolve: Send negative CONFIRM_RKEY response to the
client. Once the current exchange finishes, client will have to
recompute its Rkey set to include the new link, and start a new
CONFIRM RKEY exchange
3.5.5.3.3. Collisions during CONFIRM_RKEY exchange
Colliding LLC Message: TEST LINK
Action to resolve: Send immediate TEST LINK reply
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Colliding LLC message: ADD LINK from client to server
Action to resolve: Queue the ADD LINK and process it after the
current exchange completes
Colliding LLC message: ADD LINK from server to client (CONFIRM RKEY
exchange was initiated by the client and it crossed with the server
initiating an ADD LINK exchange)
Action to resolve: Process the ADD LINK. Client will receive a
negative CONFIRM RKEY from the server and will have to redo this
CONFIRM RKEY exchange after the ADD LINK exchange completes.
Colliding LLC message: DELETE LINK from client to server, specific
link to be deleted (CONFIRM RKEY exchange was initiated by the server
and it crossed with the client's DELETE LINK request
Action to resolve: Server queues the DELETE link message and
processes after the ADD LINK exchange completes. If it is an
orderly link termination, it can wait until after this exchange
continues. If it is disorderly and the link affected is the one
that the current exchange is using, the server will discover the
outage when a message in this exchange fails.
Colliding LLC message: DELETE LINK from server to client, specific
link deleted (CONFIRM RKEY exchange was initiated by the client and
it crossed with the server's DELETE LINK)
Action to resolve: Process the DELETE LINK. Client will receive a
negative CONFIRM RKEY from the server and will have to redo this
CONFIRM RKEY exchange after the ADD LINK exchange completes.
Colliding LLC message: DELETE LINK from either client or server,
entire link group deleted
Action to resolve: immediately clean up the link group
Colliding LLC message: CONFIRM LINK from the peer that did not start
the current CONFIRM LINK exchange
Action to resolve: Queue the request and process it after the
current exchange completes.
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4. SMC-R memory sharing architecture
4.1. RMB element allocation considerations
Each TCP connection using SMC-R must be allocated a RMBE by each SMC-
R peer. This allocation is performed by each end point independently
to allow each end point to select an RMBE that best matches the
characteristics on its TCP socket end point. The RMBE associated with
a TCP socket endpoint must have a Receive buffer that is at least as
large as the TCP receive buffer size in effect for that connection.
The receive buffer size can be determined by what is specified
explicitly by the application using setsockopt() or implicitly via
the system configured default value. This will allow sufficient data
to be RDMA written by the SMC-R peer to fill an entire receive buffer
size worth of data on a given data flow. Given that each RMB must
have fixed length RMBEs this implies that an SMC-R end point may need
to maintain multiple RMBs of various sizes for SMC-R connections on a
given SMC link and can then select an RMBE that most closely fits a
connection.
4.2. RMB and RMBE format
An RMB is a virtual memory buffer whose backing real memory is
pinned, which is divided into a whole number of equal sized RMB
Elements (RMBEs). Each RMBE begins with a four byte eye catcher for
diagnostic and service purposes, followed by the receive data buffer.
The contents of this diagnostic eyecatcher are implementation
dependent and should be used by the local SMC-R peer to check for
overlay errors by verifying an intact eyecatcher with every RMBE
access.
The RMBE is a wrapping receive buffer for receiving RDMA writes from
the peer. Cursors, as described below, are exchanged between peers
to manage and track RDMA writes and local data reads from the RMBE
for a TCP connection.
4.3. RMBE control information
RMBE control information consists of consumer and producer cursors,
wrap counts, CDC message sequence numbers, control flags such as
urgent data and writer blocked indicators, and TCP connection
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information such as termination flags. This information is exchanged
between SMC-R peers using CDC messages, which are passed using RDMA
message passing with inline data, with the control information
contained in the inline data. A TCP/IP stack implementing SMC-R must
receive and store this information in its internal data structures as
it is used to manage the RMBE and its data buffer.
The format and contents of the CDC message is described in detail in
4.3. RMBE control information. The following is a high level
description of what this control information contains.
o Connection state flags such as sending done, connection closed,
failover data validation, and abnormal close
o A sequence number that is managed by the sender. This sequence
number starts at 1, is increased each send, and wraps to 0. This
sequence number tracks the CDC message sent and is not related to
the number of bytes sent. It is used for failover data
validation.
o Producer cursor: a wrapping offset into the receiver's RMBE data
area. Set by the peer that is writing into the RMBE, it points to
where the writing peer will write the next byte of data into an
RMBE. This cursor is accompanied by a wrap sequence number to help
the RMBE owner (the receiver) identify full window size wrapping
writes. Note that this cursor must account for (i.e., skip over)
the RMBE eyecatcher that is in the beginning of the data area.
o Consumer cursor: a wrapping offset into the receiver's RMBE data
area. Set by the owner of the RMBE (the peer that is reading from
it), this cursor points to the offset of the next byte of data to
be consumed by the peer in its own RMBE. The sender cannot write
beyond this cursor into the receiver's RMBE without causing data
loss. Like the producer cursor, this is accompanied by a wrap
count to help the writer identify full window size wrapping reads.
Note that this cursor must account for (i.e., skip over) the RMBE
eyecatcher that is in the beginning of the data area.
o Data flags such as urgent data, writer blocked indicator, and
cursor update requests.
4.4. Use of RMBEs
4.4.1. Initializing and accessing RMBEs
The RMBE eyecatcher is initialized by the RMB owner prior to
assigning it to a specific TCP connection and communicating its RMB
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index to the SMC-R partner. After an RMBE index is communicated to
the SMC-R partner the RMBE can only be referenced in "read only mode"
by the owner and all updates to it are performed by the remote SMC-R
partner via RDMA write operations.
Initialization of an RMBE must include the following:
o Zeroing out the entire RMBE receive buffer, which helps minimize
data integrity issues (e.g. data from a previous connection
somehow being presented to the current connection).
o Setting the beginning RMBE eye catcher. This eye catcher plays an
important role in helping detect accidental overlays of the RMBE.
The RMB owner must always validate these eye catchers before each
new reference to the RMBE. If the eye catchers are found to be
corrupted the local host must reset the TCP connection associated
with this RMBE and log the appropriate diagnostic information.
4.4.2. RMB element reuse and conflict resolution
RMB elements can be reused once their associated TCP and SMC-R
connections are terminated. Under normal and abnormal SMC-R
connection termination processing both SMC-R peers must explicitly
acknowledge that they are done using an RMBE before that element can
be freed and reassigned to another SMC-R connection instance. For
more details on SMC-R connection termination refer to section 4.8.
However, there are some error scenarios where this 2 way explicit
acknowledgement may not be completed. In these scenarios (mentioned
explicitly elsewhere in this document) an RMBE owner may chose to re-
assign this RMBE to a new SMC-R connection instance on this SMC link
group. When this occurs the partner SMC-R peer must detect this
condition during SMC-R rendezvous processing when presented with an
RMBE that it believes is already in use for a different SMC-R
connection. In this case, the SMC-R peer must abort the existing
SMC-R connection associated with this RMBE. The abort processing
Resets the TCP connection (if it is still active) but it must not
attempt to perform any RDMA writes to this RMBE and must also ignore
any data sitting in the local RMBE associated with the existing
connection. It then proceeds to free up the local RMBE and notify
the local application that the connection is being abnormally reset.
The remote SMC-R peer then proceeds to normal processing for this new
SMC-R connection.
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4.5. SMC-R protocol considerations
The following sections describe considerations for the SMC-R protocol
as compared to the TCP protocol.
4.5.1. SMC-R protocol optimized window size updates
An SMC-R receiver host sends its Consumer Cursor information to the
sender to convey the progress that the receiving application has made
in consuming the sent data. The difference between the writer's
Producer Cursor and the associated receiver's Consumer Cursor
indicates the window size available for the sender to write into.
This is somewhat similar to TCP window update processing and
therefore has some similar considerations, such as silly window
syndrome avoidance, whereby the TCP protocol has an optimization that
minimizes the overhead of very small, unproductive window size
updates associated with sub-optimal socket applications consuming
very small amount of data on every receive() invocation. For SMC-R,
the receiver only updates its Consumer Cursor via a unique CDC
message under the following conditions:
o The current window size (from a sender's perspective) is less than
half of the Receive Buffer space and the Consumer Cursor update
will result in a minimum increase in the window size of 10% of the
Receive buffer space. Some examples:
a. Receive Buffer size: 64K, Current window size (from a
sender's perspective): 50K. No need to update the Consumer
Cursor. Plenty of space is available for the sender.
b. Receive Buffer size: 64K, Current window size (from a
sender's perspective): 30K, Current window size from a
receiver's perspective: 31K. No need to update the Consumer
Cursor; even though the sender's window size < 1/2 of the
64K, the window update would only increase that by 1K which
is < 1/10th of the 64K buffer size.
c. Receive Buffer size: 64K, Current window size (from a
sender's perspective): 30K, Current window size from a
receiver's perspective: 64K. The receiver updates the
Consumer Cursor (sender's window size < 1/2 of the 64K, the
window update would increase that by > 6.4K).
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o The receiver must always include a Consumer Cursor update whenever
it sends a CDC message to the partner for another flow (i.e. send
flow in the opposite direction). This allows the window size
update to be delivered with no additional overhead. This is
somewhat similar to TCP DelayAck processing and quite effective
for request/response data patterns.
o If a peer has set the B-bit in a CDC message then any consumption
of data by the receiver causes a CDC message to be sent updating
the consumer cursor until that a CDC message with that bit cleared
is received from the peer.
o The optimized window size updates are overridden when the sender
sets the Consumer Cursor Update Requested flag in a CDC message to
the receiver. When this indicator is on the consumer must send a
Consumer Cursor update immediately when data is consumed by the
local application or if the cursor has not been updated for a
while (i.e. local copy consumer cursor does not match the last
consumer cursor value sent to the the partner). This allows the
sender to perform optional diagnostics for detecting a stalled
receiver application (data has been sent but not consumed). It is
recommended that the Consumer Cursor Update Requested flag only be
sent for diagnostic procedures as it may result in non-optimal
data path performance.
4.5.2. Small data sends
The SMC-R protocol makes no special provisions for handling small
data segments sent across a stream socket. Data is always sent if
sufficient window space is available. There are no special provisions
for coalescing small data segments, similar to the TCP Nagle
algorithm.
An implementation of SMC-R may optimize its sending processing by
coalescing outbound data for a given SMC-R connection so that it can
reduce the number of RDMA write operations it performed in a similar
fashion to Nagle's algorithm. However, any such coalescing would
require a timer on the sending host that would ensure that data was
eventually sent. And the sending host would have to opt out of this
processing if Nagle's algorithm had been disabled (programmatically
or via system configuration).
4.5.3. TCP Keepalive processing
TCP keepalive processing allows applications to direct the local
TCP/IP host to periodically "test" the viability of an idle TCP
connection. Since SMC-R connections have both a TCP representation
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along with an SMC-R representation there are unique keepalive
processing considerations:
o SMC-R layer keepalive processing: If keepalive is enabled for an
SMC-R connection the local host maintains a keepalive timer that
reflects how long an SMC-R connection has been idle. The local
host also maintains a timestamp of last activity for each SMC link
(for any SMC-R connection on that link). When it is determined
that an SMC-R connection has been idle longer than the keepalive
interval the host checks whether the SMC-R link has been idle for
a duration longer than the keepalive timeout. If both conditions
are met, the local host then performs a Test Link LLC command to
test the viability of the SMC link over the RoCE fabric (RC-QPs).
If a Test Link LLC command response is received within a
reasonable amount of time then the link is considered viable and
all connections using this link are considered viable as well. If
however a response is not received in a reasonable amount of time
or there's a failure in sending the Test Link LLC command then
this is considered a failure in the SMC link and failover
processing to an alternate SMC link must be triggered. If no
alternate SMC link exists in the SMC link group then all the SMC-R
connections on this link are abnormally terminated by resetting
the TCP connections represented by these SMC-R connections. Given
that multiple SMC-R connections can share the same SMC link,
implementing an SMC link level probe using the Test Link LLC
command will help reduce the amount of unproductive keepalive
traffic for SMC-R connections; as long as some SMC-R connections
on a given SMC link are active (i.e. have had I/O activity within
the keepalive interval) then there is no need to perform
additional link viability testing.
o TCP layer keepalives processing: Traditional TCP "keepalive"
packets are not as relevant for SMC-R connections given that the
TCP path is not used for these connections once the SMC-R
rendezvous processing is completed. All SMC-R connections by
default have associated TCP connections that are idle. Are TCP
keepalive probes still needed for these connections? There are
two main scenarios to consider:
1. TCP keepalives that are used determine whether the peer TCP
endpoint is still active. This is not needed for SMC-R
connections as the SMC-R level keepalives mentioned above will
determine whether the remote endpoint connections are still
active.
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2. TCP keepalives that are used to ensure that TCP connections
traversing an intermediate proxy maintain an active state. For
example, stateful firewalls typically maintain state
representing every valid TCP connection that traverses the
firewall. These types of firewalls are known to expire idle
connections by removing their state in the firewall to conserve
memory. TCP keepalives are often used in this scenario to
prevent firewalls from timing out otherwise idle connections.
When using SMC-R, both end points must reside in the same layer
2 network (i.e. the same subnet). As a result, firewalls can
not be injected in the path between two SMC-R endpoints.
However, other intermediate proxies, such as TCP/IP layer load
balancers may be injected in the path of two SMC-R endpoints.
These types of load balancers also maintain connection state so
that they can forward TCP connection traffic to the appropriate
cluster end point. When using SMC-R these TCP connections will
appear to be completely idle making them susceptible to
potential timeouts at the LB proxy. As a result, for this
scenario, TCP keepalives may still be relevant.
The following are the TCP level keepalive processing requirements for
SMC-R enabled hosts:
o SMC-R peers should allow TCP keepalives to flow on the TCP path of
SMC-R connections based on existing TCP keepalive configuration
and programming options. However, it is strongly recommended that
platforms provide the ability to specify very granular keepalive
timers (for example, single digit second timers) should consider
providing a configuration option that limits the minimum keepalive
timer that will be used for TCP layer keepalives on SMC-R
connections. This is important to minimize the amount of TCP
keepalive packets transmitted in the network for SMC-R
connections.
o SMC-R peers must always respond to inbound TCP layer keepalives
(by sending ACKs for these packets) even if the connection is
using SMC-R. Typically, once a TCP connection has completed the
SMC-R rendezvous processing and using SMC-R for data flows, no new
inbound TCP segments are expected on that TCP connection other
than TCP termination segments (FIN, RST, etc). TCP keepalives are
the one exception that must be supported. And since TCP keepalive
probes do not carry any application layer data this has no adverse
impact on the application's inbound data stream.
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4.6. TCP connection failover between SMC-R links
A peer may change which SMC-R link within a link group it sends its
writes over in the event of a link failure. Since each peer
independently chooses which link to send writes over for a specific
TCP connection, this process is done independently by each peer.
4.6.1. Validating data integrity
Even though RoCE is a reliable transport there is a small subset of
failure modes that could cause unrecoverable loss of data. When an
RNIC acknowledges receipt of an RDMA write to its peer, that creates
a write completion event to the sending peer, which allows the sender
to release any buffers it is holding for that write. In normal
operation and in most failures, this operation is reliable.
However there are failure modes possible in which a receiving RNIC
has acknowledged an RDMA write but then was not able to place the
received data into its host memory, for example a sudden, disorderly
failure of the interface between the RNIC and the host. While rare,
these types of events must be guarded against to ensure data
integrity. The process for switching SMC-R links during failover that
is described in this section guards against this possibility, and is
mandatory.
Each peer must track the current state of the CDC sequence numbers
for a TCP connection. The sender must keep track of SS, which is the
sequence number of the CDC message that described the last write
acknowledged by the peer RNIC. In other words, SS describes the last
write that the sender believes its peer has successfully received.
The receiver must keep track of SR, the sequence number of the CDC
message that described last write that it has successfully received,
i.e., the data has been successfully placed into an RMBE.
When an RNIC fails and the sender changes SMC-R links, the sender
must first send a CDC message with the 'F' flag set over the new SMC-
R link. This is the failover data validation message. The sequence
number in this CDC message is equal to SS. The CDC message key, the
length, and SMC-R alert token are the only other fields in this CDC
message that are significant. No reply is expected from this
validation message, and once the sender has sent it, the sender may
resume sending on the new SMC-R link as described in 4.6.2. below
Upon receipt of the failover validation message, the receiver must
verify that its SR value for the TCP connection is equal to or
greater than the sequence number in the failover validation message.
If so, no further action is required and the TCP connection resumes
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on the new SMC-R link. If SR is less than the sequence number value
in the validation message, data has been lost and the receiver must
immediately reset the TCP connection.
4.6.2. Resuming the TCP connection on a new SMCR link
When a connection is moved to a new SMC-R link and the failover
validation message has been sent, the sender can immediately resume
normal transmission. In order to preserve the application message
stream the sender must replay any RDMA writes (and their associated
CDC messages) that were in progress or failed when the previous SMC-R
link failed, before sending new data on the new SMC-R link. The
sender has two options for accomplishing this:
o Preserve the sequence numbers "as is": Retry all failed and
pending operations as they were originally done, including
reposting all associated RDMA write operations and their
associated CDC messages without making any changes. Then resume
sending new data using new sequence numbers.
o Combine pending messages and possibly add new data: Combine failed
and pending messages into a single new write with a new sequence
number. This allows the sender to combine pending messages into
fewer operations. As a further optimization this write can also
include new data, as long as all failed and pending data is also
included. If this approach is taken, the sequence number must be
increased beyond the last failed or pending sequence number.
4.7. RMB data flows
The following sections describe the RDMA wire flows for the SMC-R
protocol after a TCP connection has switched into SMC-R mode (i.e.
SMC-R rendezvous processing is complete and a pair of RMB elements
has been assigned and communicated by the SMC-R peers). The ladder
diagrams below include the following:
o RMBE control information kept by each peer. Only a subset of the
information is depicted, specifically only the fields that reflect
the stream of data written by Host A and read by Host B.
o Time line 0-x that shows the wire flows in a time relative fashion
o Note that RMBE control information is only shown in a time
interval if its value changed (otherwise assume the value is
unchanged from previously depicted value)
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o The local copy of the producer and consumer cursors that is
maintained by each host is not depicted in these figures. Note
that the cursor values in the diagram reflect the necessity of
skipping over the eyecatcher in the RMBE data area. They start
and wrap at 4, not 0.
4.7.1. Scenario 1: Send flow, window size unconstrained
SMC Host A SMC HostB
RMBE A Info RMBE B Info
(Consumer Cursors) (Producer Cursors)
Cursor Wrap Seq# Time Time Cursor Wrap Seq# Flags
4 0 0 0 4 0 0
0 0 1 ---------------> 1 0 0 0
RDMA-WR Data
(4:1003)
4 0 2 ...............> 2 1004 0 0
CDC Message
Figure 16 Scenario 1: Send flow, window size unconstrained
Scenario assumptions:
o Kernel implementation
o New SMC-R connection, no data has been sent on the connection
o Host A: Application issues send for 1,000 bytes to Host B
o Host B: RMBE receive buffer size is 10,000, application has issued
a recv for 10,000 bytes
Flow description:
1. Application issues send() for 1,000 bytes, SMC-R layer copies
data into a kernel send buffer. It then schedules an RDMA write
operation to move the data into the peer's RMBE receive buffer,
at relative position 4-1003 (to skip the four byte eyecatcher in
the RMBE data area). Note that no immediate data or alert (i.e.
interrupt) is provided to host B for this RDMA operation.
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2. Host A sends a CDC message to update the Producer Cursor to byte
1004. This CDC message will deliver an interrupt to Host B. At
this point, the SMC-R layer can return control back to the
application. Host B, once notified of the completion of the
previous RDMA operation, locates the RMBE associated with the
RMBE alert token that was included in the message and proceeds
to perform normal receive side processing, waking up the
suspended application read thread, copying the data into the
application's receive buffer, etc. It will use the Producer
Cursor as an indicator of how much data is available to be
delivered to the local application. After this processing is
complete, the SMC-R layer will also update its local Consumer
Cursor to match the Producer Cursor (i.e. indicating that all
data has been consumed). Note that a message to the peer
updating the Consumer Cursor is not needed at this time as the
window size if unconstrained (> 1/2 of the receive buffer size).
The window size is calculated using by taking the difference
between the Producer and the Consumer cursors in the RMBEs
(10,000-1,004=8,996).
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4.7.2. Scenario 2: Send/Receive flow, window unconstrained
SMC Host A SMC HostB
RMBE A Info RMBE B Info
(Consumer Cursors) (Producer Cursors)
Cursor Wrap Seq# Time Time Cursor Wrap Seq# Flags
4 0 0 0 4 0 0
0 0 1 ---------------> 1 0 0 0
RDMA-WR Data
(4:1003)
4 0 2 ...............> 2 1004 0 0
CDC Message
0 0 3 <-------------- 3 1004 0 0
RDMA-WR Data
(4:503)
1004 0 4 <.............. 4 1004 0 0
CDC Message
Figure 17 Scenario 2: Send/Recv flow, window size unconstrained
Scenario assumptions:
o New SMC-R connection, no data has been sent on the connection
o Host A: Application issues send for 1,000 bytes to Host B
o Host B: RMBE receive buffer size is 10,000, application has
already issued a recv for 10,000 bytes. Once the receive is
completed, the application sends a 500 byte response to Host A.
Flow description:
1. Application issues send() for 1,000 bytes, SMC-R layer copies
data into a kernel send buffer. It then schedules an RDMA write
operation to move the data into the peer's RMBE receive buffer,
at relative position 4-1003. Note that no immediate data or
alert (i.e. interrupt) is provided to host B for this RDMA
operation.
2. Host A sends a CDC message to update the Producer Cursor to
byte 1004. This CDC message will deliver an interrupt to Host B.
At this point, the SMC-R layer can return control back to the
application.
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3. Host B, once notified of the receipt of the previous CDC
message, locates the RMBE associated with the RMBE alert token
and proceeds to perform normal receive side processing, waking
up the suspended application read thread, copying the data into
the application's receive buffer, etc. After this processing is
complete, the SMC-R layer will also update its local Consumer
Cursor to match the Producer Cursor (i.e. indicating that all
data has been consumed). Note that an update of the Consumer
Cursor to the peer is not needed at this time as the window size
is unconstrained (> 1/2 of the receive buffer size). The
application then performs a send() for 500 bytes to Host A. The
SMC-R layer will copy the data into a kernel buffer and then
schedule an RDMA Write into the partner's RMBE receive buffer.
Note that this RDMA write operation includes no immediate data
or notification to Host A.
4. Host B sends a CDC message to update the partner's RMBE Control
information with the latest Producer Cursor (set to 503 and not
shown in the diagram above) and to also inform the peer that the
Consumer Cursor value is now 1004. It also updates the local
Current Consumer Cursor and Last Sent Consumer Cursor to 1004.
This CDC message includes notification since we are updating
our Producer Cursor which requires attention by the peer host.
4.7.3. Scenario 3: Send Flow, window constrained
SMC Host A SMC HostB
RMBE A Info RMBE B Info
(Consumer Cursors) (Producer Cursors)
Cursor Wrap Seq# Time Time Cursor Wrap Seq# Flags
4 0 0 0 4 0 0
4 0 1 ---------------> 1 4 0 0
RDMA-WR Data
(4:3003)
4 0 2 ...............> 2 3004 0 0
CDC Message
4 0 3 3 3004 0 0
4 0 4 ---------------> 4 3004 0 0
RDMA-WR Data
(3004:7003)
4 0 5 ................> 5 7004 0 0
CDC Message
7004 0 6 <................ 6 7004 0 0
CDC Message
Figure 18 Scenario 3: Send Flow, window size constrained
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Scenario assumptions:
o New SMC-R connection, no data has been sent on this connection
o Host A: Application issues send for 3,000 bytes to Host B and then
another send for 4,000
o Host B: RMBE receive buffer size is 10,000. Application has
already issued a recv for 10,000 bytes
Flow description:
1. Application issues send() for 3,000 bytes, SMC-R layer copies
data into a kernel send buffer. It then schedules an RDMA write
operation to move the data into the peer's RMBE receive buffer,
at relative position 4-3003. Note that no immediate data or
alert (i.e. interrupt) is provided to host B for this RDMA
operation.
2. Host A sends a CDC message to update its Producer Cursor to byte
3003. This CDC message will deliver an interrupt to Host B. At
this point, the SMC-R layer can return control back to the
application.
3. Host B, once notified of the receipt of the previous CDC
message, locates the RMBE associated with the RMBE alert token
and proceeds to perform normal receive side processing, waking
up the suspended application read thread, copying the data into
the application's receive buffer, etc. After this processing is
complete, the SMC-R layer will also update its local Consumer
Cursor to match the Producer Cursor (i.e. indicating that all
data has been consumed). It will not however update the partner
with this information as the window size is not constrained
(10000-3000=7000 of available space). The application on Host B
also issues a new recv() for 10,000.
4. On Host A, application issues a send() for 4,000 bytes. The SMC-
R layer copies the data into a kernel buffer and schedules an
async RDMA write into the peer's RMBE receive buffer at relative
position 3003-7004. Note that no alert is provided to host B for
this flow.
5. Host A sends a CDC message to update the Producer Cursor to
byte 7004. This CDC message will deliver an interrupt to Host B.
At this point, the SMC-R layer can return control back to the
application.
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6. Host B, once notified of the receipt of the previous CDC
message, locates the RMBE associated with the RMBE alert token
and proceeds to perform normal receive side processing, waking
up the suspended application read thread, copying the data into
the application's receive buffer, etc. After this processing is
complete, the SMC-R layer will also update its local Consumer
Cursor to match the Producer Cursor (i.e. indicating that all
data has been consumed). It will then determine whether it
needs to update the Consumer Cursor to the peer. The available
window size is now 3,000 (10,000 - (Producer Cursor - Last Sent
Consumer Cursor)) which is < 1/2 receive buffer size
(10,000/2=5,000) and the advance of the window size is > 10% of
the windows size (1,000). Therefore a CDC message is issued to
update the Consumer Cursor to peer A.
4.7.4. Scenario 4: Large send, flow control, full window size writes
SMC Host A SMC HostB
RMBE A Info RMBE B Info
(Consumer Cursors) (Producer Cursors)
Cursor Wrap Seq# Time Time Cursor Wrap Seq# Flags
1004 1 0 0 1004 1 0
1004 1 1 ---------------> 1 1004 1 0
RDMA-WR Data
(1004:9999)
1004 1 2 ---------------> 2 1004 1 0
RDMA-WR Data
(4:1003)
1004 1 3 ...............> 3 1004 2 Wrt
CDC Message
1004 2 4 <............... 4 1004 2 Wrt
CDC Message
1004 2 5 ---------------> 5 1004 2 Wrt
RDMA-WR Data Blk
(1004:9999)
1004 2 6 ---------------> 6 1004 2 Wrt
RDMA-WR Data Blk
(4:1003)
1004 2 7 ...............> 7 1004 3 Wrt
CDC Message
1004 3 8 <............... 8 1004 3 Wrt
CDC Message
Figure 19 Scenario 4: Large send, flow control, full window size
writes
Scenario assumptions:
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o Kernel implementation
o Existing SMC-R connection, Host B's receive window size is fully
open(Peer Consumer Cursor = Peer Producer Cursor).
o Host A: Application issues send for 20,000 bytes to Host B
o Host B: RMB receive buffer size is 10,000, application has issued
a recv for 10,000 bytes
Flow description:
1. Application issues send() for 20,000 bytes, SMC-R layer copies
data into a kernel send buffer (assumes send buffer space of
20,000 is available for this connection). It then schedules an
RDMA write operation to move the data into the peer's RMBE
receive buffer, at relative position 1004-9999. Note that no
immediate data or alert (i.e. interrupt) is provided to host B
for this RDMA operation.
2. Host A then schedules an RDMA write operation to fill the
remaining 1000 bytes of available space in the peer's RMBE
receive buffer, at relative position 4-1003. Note that no
immediate data or alert (i.e. interrupt) is provided to host B
for this RDMA operation. Also note that an implementation of
SMC-R may optimize this processing by combining step 1 and 2
into a single RDMA Write operation (with 2 different data
sources).
3. Host A sends CDC message to update the Producer Cursor to byte
1004. Since the entire receive buffer space is filled, the
Producer Writer Blocked flag (WrtBlk indicator above) is set and
the Producer Window Wrap Sequence Number (Producer WrapSeq#
above) is incremented. This CDC message will deliver an
interrupt to Host B. At this point, the SMC-R layer can return
control back to the application.
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4. Host B, once notified of the receipt of the previous CDC
message, locates the RMBE associated with the RMBE alert token
and proceeds to perform normal receive side processing, waking
up the suspended application read thread, copying the data into
the application's receive buffer, etc. In this scenario, Host B
notices that the Producer Cursor has not been advanced (same
value as Consumer Cursor), however, it notices that the Producer
Window Wrap Size Sequence number is different from its local
value (1) indicating that a full window of new data is
available. All the data in the receive buffer can be processed,
the first segment (1004-9999) followed by the second segment (4-
1003). Because the Producer Writer Blocked indicator was set,
Host B schedules a CDC message to update its latest information
to the peer: Consumer Cursor (1004), Consumer Window Wrap Size
Sequence Number (2: the current Producer Window Wrap Sequence
Number is used).
5. Host A, upon receipt of the CDC message locates the TCP
connection associated with the alert token, and upon examining
the control information provided notices that Host B has
consumed all of the data (based on the Consumer Cursor and the
Consumer Window Wrap Size Sequence number) and initiates the
next RDMA write to fill the receive buffer at offset 1003-9999.
6. Host A then moves the next 1000 bytes into the beginning of the
receive buffer (4-1003) by scheduling an RDMA write operation.
Note at this point there are still 8 bytes remaining to be
written.
7. Host A then sends a CDC message to set the Producer Writer
Blocked indicator and to increment the Producer Window Wrap Size
Sequence Number (3).
8. Host B, upon notification completes the same processing as step
4 above, including sending a CDC message to update the peer to
indicate that all data has been consumed. At this point Host A
can write the final 8 utes to host B's RMBE into positions 1004-
1011 (not shown).
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4.7.5. Scenario 5: Send flow, urgent data, window size unconstrained
SMC Host A SMC HostB
RMBE A Info RMBE B Info
(Consumer Cursors) (Producer Cursors)
Cursor Wrap Seq# Time Time Cursor Wrap Seq# Flag
1000 1 0 0 1000 1 0
1000 1 1 ---------------> 1 1000 1 0
RDMA-WR Data
(1000:1499)
1000 1 2 ...............> 2 1500 1 UrgP
CDC Message UrgA
1500 1 3 <............... 3 1500 1 UrgP
CDC Message UrgA
1500 1 4 ---------------> 4 1500 1 UrgP
RDMA-WR Data UrgA
(1500:2499)
1500 1 5 ...............> 5 2500 1 0
CDC Message
Figure 20 Scenario 5: send Flow, urgent data, window size open
Scenario assumptions:
o Kernel implementation
o Existing SMC-R connection, window size open, all data has been
consumed by receiver.
o Host A: Application issues send for 500 bytes with urgent data
indicator (OOB) to Host B, then sends 1000 of normal data
o Host B: RMBE Receive buffer size is 10,000, application has issued
a recv for 10,000 bytes and is also monitoring the socket for
urgent data
Flow description:
1. Application issues send() for 500 bytes of urgent data. SMC-R
layer copies data into a kernel send buffer. It then schedules
an RDMA write operation to move the data into the peer's RMBE
receive buffer, at relative position 1000-1499. Note that no
immediate data or alert (i.e. interrupt) is provided to host B
for this RDMA operation.
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2. Host A sends a CDC message to update its Producer Cursor to byte
1500 and to turn on the Producer Urgent Data Pending (UrgP) and
Urgent Data Present (UrgA) flags. This CDC message will deliver
an interrupt to Host B. At this point, the SMC-R layer can
return control back to the application.
3. Host B, once notified of the receipt of the previous CDC
message, locates the RMBE associated with the RMBE alert token,
notices that the Urgent Data Pending flag is on and proceeds
with Out of Band socket API notification. For example,
satisfying any outstanding select() or poll() requests on the
socket by indicating that urgent data is pending (i.e. by
setting the exception bit on). The Urgent Data Present indicator
allows Host B to also determine the position of the urgent data
(Producer cursor points one byte beyond the last byte of urgent
data). Host B can then perform normal receive side processing
(including specific urgent data processing), copying the data
into the application's receive buffer, etc. Host B then sends a
CDC message to update the partner's RMBE Control area with its
latest Consumer Cursor (1500). Note this CDC message must occur
regardless of the current local window size that is available.
The partner host (Host A) cannot initiate any additional RDMA
writes until acknowledgement that the urgent data has been
processed (or at least processed/remembered at the SMC-R layer).
4. Upon receipt of the message, Host A wakes up, sees that peer
consumed all data up to and including the last byte of Urgent
data and now resumes sending any pending data. In this case,
the application had previously issued a send for 1000 bytes of
normal data which would have been copied in the send buffer and
control would have been returned to the application. Host A now
initiates a RDMA write to move that data to the Peer's receive
buffer at position 1500-2499.
5. Host A then sends a CDC message with inline data update its
Producer Cursor value (2500) and turn off the Urgent Data
Pending and Urgent Data Present flags. Host B wakes up,
processes the new data (resumes application, copies data into
the application receive buffer) and then proceeds to update the
Local current consumer cursor (2500). Given that the window size
is unconstrained there is no need for Consumer Cursor update in
the peer's RMBE.
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4.7.6. Scenario 6: Send flow, urgent data, window size closed
SMC Host A SMC HostB
RMBE A Info RMBE B Info
(Consumer Cursors) (Producer Cursors)
Cursor Wrap Seq# Time Time Cursor Wrap Seq# Flag
1000 1 0 0 1000 2 Wrt
Blk
1000 1 1 ...............> 1 1000 2 Wrt
CDC Message Blk
UrgP
1000 2 2 <............... 2 1000 2 Wrt
CDC Message Blk
UrgP
1000 2 3 ---------------> 3 1000 2 Wrt
RDMA-WR data l Blk
(1000:1499) UrgP
1000 2 4 ...............> 4 1500 2 UrgP
CDC Message UrgA
1500 2 5 <............... 5 1500 2 UrgP
CDC Message UrgA
1500 2 6 ---------------> 6 1500 2 UrgP
RDMA-WR data l UrgA
(1500:2499)
1000 2 7 ...............> 7 2500 2 0
CDC Message
Figure 21 Scenario 6: Send flow, urgent data, window size closed
Scenario assumptions:
o Kernel implementation
o Existing SMC-R connection, window size closed, writer is blocked.
o Host A: Application issues send for 500 bytes with urgent data
indicator (OOB) to Host B, then sends 1000 of normal data.
o Host B: RMBE Receive buffer size is 10,000, application has no
outstanding recv() (for normal data) and is monitoring the socket
for urgent data.
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Flow description:
1. Application issues send() for 500 bytes of urgent data. SMC-R
layer copies data into a kernel send buffer (if available).
Since the writer is blocked (window size closed) it cannot send
the data immediately. It then sends a CDC message to notify the
peer of the Urgent Data Pending (UrgP)indicator (the Writer
Blocked indicator remains on as well). This serves as a signal
to Host B that urgent data is pending in the stream. Control is
also returned to the application at this point.
2. Host B, once notified of the receipt of the previous CDC
message, locates the RMBE associated with the RMBE alert token,
notices that the Urgent Data Pending flag is on and proceeds
with Out of Band socket API notification. For example,
satisfying any outstanding select() or poll() requests on the
socket by indicating that urgent data is pending (i.e. by
setting the exception bit on). At this point it is expected that
the application will enter urgent data mode processing,
expeditiously processing all normal data (by issuing recv API
calls) so that it can get to the urgent data byte. Whether the
application has this urgent mode processing or not, at some
point the application will consume some or all of the pending
data in the receive buffer. When this occurs, Host B will also
send a CDC message with inline data to update its Consumer
Cursor and Consumer Window Wrap Sequence Number to the peer. In
the example above, a full window worth of data was consumed.
3. Host A, once awakened by the message will notice that the window
size is now open on this connection (based on the Consumer
Cursor and the Consumer Window Wrap Sequence Number which now
matches the Producer Window Wrap Sequence Number) and resume
sending of the urgent data segment by scheduling an RDMA write
into relative position 1000-1499.
4. Host A the sends a CDC message to advance its Producer Cursor
(1500) and to also notify Host B of the Urgent Data Present
(UrgA) indicator (and turn off the Writer Blocked indicator).
This signals to Host B that the urgent data is now in the local
receive buffer and that the Producer Cursor points to the last
byte of urgent data.
5. Host B wakes up, processes the urgent data and once the urgent
data is consumed sends a CDC message with inline data to update
its Consumer Cursor (1500).
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6. Host A wakes up, sees that Host B has consumed the sequence
number associated with the urgent data and then initiates the
next RDMA write operation to move the 1000 bytes associated with
the next send() of normal data into the peer's receive buffer at
position (1500-2499). Note that send() API would have likely
completed earlier in the process by copying the 1000 bytes into
a send buffer and returning back to the application even though
we could not send any new data until the urgent data was
processed and acknowledged by Host B.
7. Host A sends a CDC message to advance its Producer Cursor to
2500 and to reset the Urgent Data Pending and Present flags.
Host B wakes up and processes the inbound data.
4.8. Connection termination
Just as SMC-R connections are established using a combination of TCP
connection establishment flows and SMC-R protocol flows, the
termination of SMC-R connections also uses a similar combination of
SMC-R protocol termination flows and normal TCP protocol connection
termination flows. The following sections describe the SMC-R protocol
normal and abnormal connection termination flows.
4.8.1. Normal SMC-R connection termination flows
Normal SMC-R connection flows are triggered via the normal stream
socket API semantics, namely by the application issuing a close() or
shutdown() API. Most applications, after consuming all incoming data
and after sending any outbound data will then issue a close() API to
indicate that they are done both sending and receiving data. Some
applications, typically a small percentage, make use of the
shutdown() API that allows then to indicate that the application is
done sending data, receiving data or both sending and receiving data.
The main use of this API is scenarios where a TCP application wants
to alert its partner end point that it is done sending data, yet is
still receiving data on its socket (shutdown for Write). Issuing
shutdown for both sending and receiving data is really no different
than issuing a close() and can therefore be treated in a similar
fashion. Shutdown for read is typically not a very useful operation
and in normal circumstances does not trigger any network flows to
notify the partner TCP end point of this operation.
These same trigger points will be used by the SMC-R layer to initiate
SMC-R connections termination flows. The main design point for SMC-R
normal connection flows is to use the SMC-R protocol to first
shutdown the SMC-R connection and free up any SMC-R RDMA resources
and then allow the normal TCP connection termination protocol (i.e.
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FIN processing) to drive cleanup of the TCP connection. This design
point is very important in ensuring that RDMA resources such as the
RMBEs are only freed and reused when both SMC-R end points are
completely done with their RDMA Write operations to the partner's
RMBE.
1
+-----------------+
|-------------->| CLOSED |<-------------|
3D | | | | 4D
| +-----------------+ |
| | |
| 2 | |
| V |
+----------------+ +-----------------+ +----------------+
|AppFinCloseWait | | ACTIVE | |PeerFinCloseWait|
| | | | | |
+----------------+ +-----------------+ +----------------+
| | | |
| Active Close | 3A | 4A | Passive Close |
| V | V |
| +--------------+ | +-------------+ |
|--<----|PeerCloseWait1| | |AppCloseWait1|--->----|
3C | | | | | | | 4C
| +--------------+ | +-------------+ |
| | | | |
| | 3B | 4B | |
| V | V |
| +--------------+ | +-------------+ |
|--<----|PeerCloseWait2| | |AppCloseWait2|--->----|
| | | | |
+--------------+ | +-------------+
|
|
Figure 22 SMC-R connection states
Figure 23 describes the states that an SMC-R connection typically
goes through. Note that there are variations to these states that can
occur when an SMC-R connection is abnormally terminated, similar in a
way to when a TCP connection is reset. The following are the high
level state transitions for an SMC-R connection:
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1. An SMC-R connection begins in the Closed state. This state is
meant to reflect an RMBE that is not currently in use (was
previously in use but no longer is or one that was never
allocated)
2. An SMC-R connection progresses to the Active state once the SMC-
R rendezvous processing has successfully completed, RMB element
indices have been exchanged and SMC-R links have been activated.
In this state, TCP connection is fully established, rendezvous
processing has been completed and SMC-R peers can begin exchange
of data via RDMA.
3. Active close processing (on SMC-R peer that is initiating the
connection termination)
A. When an application on one of the SMC-R connection peers issues
a close() or shutdown(write or both) the SMC-R layer on that host
will initiate SMC-R connection termination processing. First if
close() or shutdown(both) is issued it will check to see that
there's no data in the local RMB element that has not been read
by the application. If unread data is detected, the SMC-R
connection must be abnormally reset - for more detail on this
refer to "SMC-R connection reset". If no unread data is pending,
it then checks to see whether any outstanding data is waiting to
be written to the peer or if any outstanding RDMA writes for this
SMC-R connection have not yet completed. If either of these two
scenarios are true, an indicator that this connection is in a
pending close state is saved in internal data structures
representing this SMC-R connection and control is returned to the
application. If all data to be written to the partner has
completed this peer will send a CDC message to notify the peer of
either the PeerConnectionClosed indicator (close or shutdown for
both was issued) or the PeerDoneWriting indicator. This will
provide stimulus to the partner SMC-R peer that the connection is
terminating. At this point the local side of the SMC-R connection
transitions in the PeerCloseWait1 state and control can be
returned to the application. If this process could not be
completed synchronously (close pending condition mentioned above)
it is completed when all RDMA writes for data and control cursors
have been completed.
B. At some point the SMC-R peer application (passive close) will
consume all incoming data, realize that that partner is done
sending data on this connection and proceed to initiate its own
close of the connection once it has completed sending all data
from its end. The partner application can initiate this
connection termination processing via a close() or shutdown()
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APIs. If the application does so by issuing a shutdown() for
write, then the partner SMC-R layer will send a CDC message to
notify the peer (active close side) of the PeerDoneWriting
indicator. When the "active close" SMC-R peer wakes up as a
result of the previous CDC message, it will notice that the
PeerDoneWriting indicator is now on and transition to the
PeerCloseWait2 state. This state indicates that the peer is done
sending data and may still be reading data. The "active close"
peer will also at this point need to ensure that any outstanding
recv() calls for this socket are woken up and remember that that
no more data is forthcoming on this connection (in case the local
connection was shutdown() for write only)
C. This flow is a common transition from 3a or 3b above. When the
SMC-R peer (passive close) consumes all data, updates all
necessary cursors to the peer and the application closes its
socket (close or shutdown for both) it will send a CDC message to
the peer (the active close side) with the PeerConnectionClosed
indicator set. At this point the connection can transition back
to Closed state if the local application has already closed (or
issued shutdown for both) the socket. Once in the Closed state,
the RMBE can now be safely be reused for a new SMC-R connection.
When the PeerConnectionClosed indicator is turned on, the SMC-R
peer is indicating that it is done updating the partner's RMBE.
D. Conditional State: If the local application has not yet issued
a close() or shutdown(both) yet, we need to wait until the
application does so (ApplFinWaitState). Once it does, the local
host will send a CDC message to notify the peer of the
PeerConnectionClosed indicator and then transition to the Closed
state.
4. Passive close processing (on SMC-R peer that receives an
indication that the partner is closing the connection)
A. Upon receipt of an inbound RDMA write notice the SMC-R layer
will detect that the PeerConnectionClosed indicator or
PeerDoneWriting indicator is on. If any outstanding recv() calls
are pending they are completed with an indicator that the partner
has closed the connection (zero length data presented to
application). If any pending data to be written and
PeerConnectionClosed is on then an SMC-R connection reset must be
performed. The connection then enters the ApplCloseWait1 state on
the passive close side waiting for the local application to
initiate its own close processing
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B. If the local application issues a shutdown() for writing then
the SMC-R layer will send a CDC message to notify the partner of
the PeerDoneWriting indicator transition the local side of the
SMC-R connection to the ApplCloseWait2 state.
C. When the application issues a close() or shutdown() for both,
the local SMC-R peer will send a message informing the peer of
the PeerConnectionClosed indicator and transition to the Closed
state if the remote peer has also sent the local peer the
PeerConnectionClosed indicator. If the peer has not sent the
PeerConnectionClosed indicator, we transition into the
PeerFinalCloseWait state.
D. The local SMC-R connection stays in this state until the peer
sends the PeerConnectionClosed indicator in our RMBE. When the
indicator is sent we transition to the Closed state and are then
free to reuse this RMBE.
Note that each SMC-R peer needs to provide some logic that will
prevent being stranded in termination state indefinitely. For
example, if an Active Close SMC-R peer is in a PeerCloseWait (1 or 2)
state awaiting the remote SMC-R peer to update its connection
termination status it needs to provide a timer that will prevent it
from waiting in that state indefinitely should the remote SMC-R peer
not respond to this termination request. This could occur in error
scenarios; for example, if the remote SMC-R peer suffered a failure
prior to being able to respond to the termination request or the
remote application is not responding to this connection termination
request by closing its own socket. This latter scenario is similar
to the TCP FINWAIT2 state that has been known to sometimes cause
issues when remote TCP/IP hosts lose track of established connections
and neglect to close them. Even though the TCP standards do not
mandate a time out from the TCP FINWAIT2 state, most TCP/IP
implementations implement a timeout for this state. A similar
timeout will be required for SMC-R connections. When this timeout
occurs, the local SMC-R peer performs TCP reset processing for this
connection. However, no additional RDMA writes to the partner RMBE
can occur at this point (we have already indicated that we are done
updating the peer's RMBE). After the TCP connection is Reset the RMBE
can be returned to the free pool for reallocation. See section 3.2.5
for more details.
Also note that it is possible to have two SMC-R end points initiate
an Active close concurrently. In that scenario the flows above still
apply, however, both end points follow the active close path (path
3).
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4.8.1.1. Abnormal SMC-R connection termination flows
Abnormal SMC-R connection termination can occur for a variety of
reasons, including:
o The TCP connection associated with an SMC-R connection is reset.
In the TCP protocol either end point can send a RST segment to
abort an existing TCP connection when error conditions are
detected for the connection or the application overtly requests
that the connection be reset.
o Normal SMC-R connection termination processing has unexpectedly
stalled for a given connection. When the stall is detected
(connection termination timeout condition) an abnormal SMC-R
connection termination flow is initiated.
In these scenarios it is very important that resources associated
with the affected SMC-R connections are properly cleaned up to ensure
that there are no orphaned resources and that resources can reliably
be reused for new SMC-R connections. Given that SMC-R relies heavily
on the RDMA Write processing, special care needs to be taken to
ensure that an RMBE is no longer being used by a SMC-R peer before
logically reassigning that RMBE to a new SMC-R connection.
When an SMC-R peer initiates a TCP connection reset it also initiates
an SMC-R abnormal connection flow at the same time. The SMC-R peers
explicitly signal their intent to abnormally terminate an SMC-R
connection and await explicit acknowledgement that the peer has
received this notification and has also completed abnormal connection
termination on its end. Note that TCP connection reset processing can
occur in parallel to these flows.
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+-----------------+
|-------------->| CLOSED |<-------------|
| | | |
| +-----------------+ |
| |
| |
| |
| +-----------------+ |
| | Any State | |
|1B | (before setting | 2B|
| | PeerConnClosed | |
| | Indicator in | |
| | Peer's RMBE) | |
| +-----------------+ |
| 1A | | 2A |
| Active Abort | | Passive Abort |
| V V |
| +--------------+ +--------------+ |
|-------|PeerAbortWait | | Process Abort|------|
| | | |
+--------------+ +--------------+
Figure 23 SMC-R abnormal connection termination state diagram
Figure 24 above shows the SMC-R abnormal connection termination state
diagram:
1. Active abort designates the SMC-R peer that is initiating the
TCP RST processing. At the time that the TCP RST is sent the
active abort side must also
A. Send the PeerConnAbort indicator to the partner via RDMA
messaging with inline data and then transition to the
PeerAbortWait state. During this state it will monitor this SMC-
R connection waiting for the peer to send its corresponding
PeerConnAbort indicator but will ignore any other activity in
this connection (i.e. new incoming data). It will also surface an
appropriate error to any socket API calls issued against this
socket (e.g. ECONNABORTED, ECONNRESET, etc.)
B. Once the peer sends the PeerConnAbort indicator to the local
host, the local host can transition this SMC-R connection to the
Closed state and reuse this RMBE. Note that the SMC-R peer that
goes into the Active abort state must provide some protection
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against staying in that state indefinitely should the remote SMC-
R peer not respond by sending its own PeerConnAbort indicator to
the local host. While this should be a rare scenario it could
occur if the remote SMC-R peer (passive abort) suffered a failure
right after the local SMC-R peer (active abort) sent the
PeerConnAbort indicator. To protect against these types of
failures, a timer can be set after entering the PeerAbortWait
state and when if that timer pops before the peer has sent its
local PeerConnAbort indicator (to the active abort side) then
this RMBE can be returned to the free pool for possible re-
allocation. See section See section 3.2.5 for more details.
2. Passive abort designates the SMC-R peer that is the recipient of
an SMC-R abort from the peer designated by the PeerConnAbort
indicator being sent by the peer in a CDC message. Upon
receiving this request, the local peer must
A. Indicate to the socket application that this connection has
been aborted using the appropriate error codes, purge all in-
flight data for this connection that is waiting to be read or
waiting to be sent.
B. Send a CDC message to notify the peer of the PeerConnAbort
indicator and once that is completed transition this RMBE to the
Closed state.
If an SMC-R peer receives a TCP RST for a given SMC-R connection it
also initiates SMC-R abnormal connection termination processing if it
has not already been notified (via the PeerConnAbort indicator) that
the partner is severing the connection. It is possible to have two
SMC-R endpoints concurrently be in an Active abort role for a given
connection. In that scenario the flows above still apply but both
end points take the active abort path (path 1).
4.8.1.2. Other SMC-R connection termination conditions
The following are additional conditions that have implications of
SMC-R connection termination:
o A SMC-R peer being gracefully shut down. If an SMC-R peer supports
a graceful shutdown operation it should attempt to terminate all
SMC-R connections as part of shutdown processing. This could be
accomplished via LLC Delete Link requests on all active SMC Links.
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o Abnormal termination of an SMC-R peer. In this example, there may
be no opportunity for the host to perform any SMC-R cleanup
processing. In this scenario it is up to the remote peer to
detect a RoCE communications failure with the failing host. This
could trigger an SMC link switch but that would also surface RoCE
errors causing the remote host to eventually terminate all
existing SMC-R connections to this peer.
o Loss of RoCE connectivity between two SMC-R peers. If two peers
are no longer reachable across any links in their SMC Link group
then both peers perform a TCP reset for the connections, surface
an error to the local applications and free up all QP resources
associated with the link group.
5. Security considerations
5.1. VLAN considerations
The concepts and access control of virtual LANs (VLANs) must be
extended to also cover the RoCE network traffic flowing across the
ethernet.
The RoCE VLAN configuration and accesses must mirror the IP VLAN
configuration and accesses over the CEE fabric. This means that
hosts, routers and switches that have access to specific VLANs on the
IP fabric must also have the same VLAN access across the RoCE
fabric. In other words, the SMC-R connectivity will follow the same
virtual network access permissions as normal TCP/IP traffic.
5.2. Firewall considerations
As mentioned above, the RoCE fabric inherits the same VLAN
topology/access as the IP fabric. RoCE is a layer 2 protocol that
requires both end points to reside in the same layer 2 network (i.e.
VLAN). RoCE traffic can not traverse multiple VLANs as there is no
support for routing RoCE traffic beyond a single VLAN. As a result,
SMC-R communications will also be confined to peers that are members
of the same VLAN. IP based firewalls are typically inserted between
VLANs (or physical lans) and rely on normal IP routing to insert
themselves in the data path. Since RoCE (and by extension SMC-R) is
not routable beyond the local VLAN, there is no ability to insert a
firewall in the network path of two SMC-R peers.
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5.3. Host-based IP Filters
Because SMC-R maintains the TCP three-way handshake for connection
setup before switching to RoCE out of band, existing IP filters that
control connection setup flows remain effective in an SMC-R
environment. IP filters that operate on traffic flowing in an active
TCP connection are not supported, because the connection data does
not flow over IP.
5.4. Intrusion Detection Services
Similar to IP filters, intrusion detection services that operate on
TCP connection setups are compatible with SMC-R with no changes
required. However once the TCP connection has switched to RoCE out
of band, packets are not available for examination.
5.5. IP Security (IPSec)
IP Security is not compatible with SMC-R because there are no IP
packets to operate on. TCP connections that require IP security must
opt out of SMC-R.
5.6. TLS/SSL
TLS/SSL is preserved in an SMC-R environment. The TLS/SSL layer
resides above the SMC-R layer and outgoing connection data is
encrypted before being passed down to the SMC-R layer for RMDA write.
Similarly, incoming connection data goes through the SMC-R layer
encrypted and is decrypted by the TLS/SSL layer as it is today.
The TLS/SSL handshake messages flow over the TCP connection after the
connection has switched to SMC-R, so are exchanged using RDMA writes
by the SMC-R layer, transparently to the TLS/SSL layer.
6. IANA considerations
The scarcity of TCP option codes available for assignment is
understood and this architecture uses experimental TCP options
following the conventions of RFC 6994 "Shared Use of Experimental TCP
Options".
If this protocol achieves wide acceptance a discrete option code may
be requested by subsequent versions of this protocol.
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7. References
7.1. Normative References
[ROCE] RDMA over Converged Ethernet specification, URL,
http://members.infinibandta.org/kwspub/spec/Annex_RoCE_fina
l.pdf
[IBTA] Infiniband Architecture specification, URL,
http://www.infinibandta.org/specs
[RFC793] University of Southern California Information Services
Institute, "Transmission Control Protocol", RFC 793,
September 1981.
[RFC4727] Fenner B., "Experimental Values in IPv4, IPv6, ICMPv4,
ICMPv6, UDP, and TCP Headers", RFC 4727, November 2006.
7.2. Informative References
[RFC 6994] Touch, J., "Shared use of Experimental TCP Options",
draft URL, https://tools.ietf.org/html/rfc6994
8. Acknowledgments
This document was prepared using 2-Word-v2.0.template.dot.
9. Conventions used in this document
In the rendezvous flow diagrams, dashed lines (----) are used to
indicate flows over the TCP/IP fabric and dotted lines (....) are
used to indicate flows over the RoCE fabric.
In the data transfer ladder diagrams, dashed lines (----) are used to
indicate RDMA write operations and dotted lines (....) are used to
indicate CDC messages, which are RDMA messages with inline data that
contain control information for the connection.
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Appendix A. Formats
A.1. TCP option
The SMC-R TCP option is formatted in accordance with RFC 6994 "Shared
Use of Experimental TCP Options". The ExID value is IBM-1047
(EBCDIC) encoding for 'SMCR'
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Kind = 254 | Length = 6 | x'E2' | x'D4' |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| x'C3' | x'D9' |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 24 SMC-R TCP option format
A.2. CLC messages
The following rules apply to all CLC messages:
General rules on formats:
o Reserved fields must be set to zero and not validated
o Each message has an eyecatcher at the start and another eyecatcher
at the end. These must both be validated by the receiver.
o SMC version indicator: The only SMC-R version defined in this
architecture is version 1. In the future, if peers have a
mismatch of versions, the lowest common version number is used.
A.2.1. Peer ID format
All CLC messages contain a peer ID that uniquely identifies an
instance of a TCP/IP stack. This peer ID is required to be
universally unique across TCP/IP stacks and instances (including
restarts) of TCP/IP stacks.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Instance ID | RoCE MAC (first two bytes) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RoCE MAC (last four bytes) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 25 Peer ID format
Instance ID
A two-byte instance count that ensures that if the same RNIC MAC
is later used in the peer ID for a different TCP/IP stack, for
example if an RNIC is redeployed to another stack, the values are
unique. It also ensures that if a TCP/IP stack is restarted, the
instance ID changes. Value is implementation defined, with one
suggestion being two bytes of the system clock.
RoCE MAC
The RoCE MAC address for one of the peer's RNICs. Note that in a
virtualized environment this will be the virtual MAC of one of
the peer's RNICs.
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A.2.2. SMC Proposal CLC message format
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| x'E2' | x'D4' | x'C3' | x'D9' |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 1 | Length |Version| Rsrvd |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+- Client's Peer ID -+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+- -+
| |
+- Client's preferred GID -+
| |
+- -+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Client's preferred RoCE |
+- MAC address +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |Offset to mask/prefix area (0) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. Area for future growth .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv4 Subnet Mask |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv4 Mask Lgth| Reserved |Num IPv6 prfx |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: :
: (Variable length) array of IPv6 Prefixes :
: :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| x'E2' | x'D4' | x'C3' | x'D9' |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 26 SMC Proposal CLC message format
The fields present in the SMC Proposal CLC message are:
Eyecatchers
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Like all CLC messages, the SMC Proposal has beginning and ending
eyecatchers to aid with verification and parsing. The hex digits
spell 'SMCR' in IBM-1047 (EBCDIC)
Type
CLC message type 1 indicates SMC Proposal
Length
The length of this CLC message. If this an IPv4 flow, this
value is 52. Otherwise it is variable depending upon how many
prefixes are listed.
Version
Version of the SMC-R protocol. Version 1 is the only currently
defined value
Client's Peer ID
As described in A.2.1. above
Client's preferred RoCE GID
This is the IPv6 address of the client's preferred RNIC on the
RoCE fabric
Client's preferred RoCE MAC address
The MAC address of the client's preferred RNIC on the RoCE
fabric. It is required as some operating systems do not have
neighbor discovery or ARP support for RoCE RNICs.
Offset to mask/prefix area
Provides the number of bytes that must be skipped after this
field, to access the IPv4 Subnet Mask and the fields that follow
it. Allows for future growth of this signal. In this version of
the architecture, this value is always zero.
Area for future growth
In this version of the architecture, this field does not exist.
This indicates where additional information may be inserted into
the signal in the future. "The Offset to mask/prefix area" field
must be used to skip over this area.
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IPv4 Subnet mask
If this message is flowing over an IPv4 TCP connection, the value
of the subnet mask associated with the interface the client sent
this message over. If this an IPv6 flow this field is all
zeroes.
This field, along with all fields that follow it in this signal,
must be accessed by skipping the number of bytes listed in the
"Offset to mask/prefix area" field after the end of that field.
IPv4 Mask Lgth
If this message is flowing over an IPv4 TCP connection, the
number of significant bits in the IPv4 subnet mask. If this an
IPv6 flow, this field is zero.
Num IPv6 prfx
If this message is flowing over an IPv6 TCP connection, the
number of IPv6 prefixes that follow, with a maximum value of 8.
if this is an IPv4 flow this field is zero and is immediately
followed by the ending eyecatcher.
Array of IPv6 Prefixes
For IPv6 TCP connections, a list of the IPv6 prefixes associated
with the network the client sent this message over, up to a
maximum of 8 prefixes.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ IPv6 Prefix value +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix Length |
+-+-+-+-+-+-+-+-+
Figure 27 Format for IPv6 Prefix array element
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A.2.3. SMC Accept CLC message format
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| x'E2' | x'D4' | x'C3' | x'D9' |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 2 | Length = 68 |Version|F|Rsvd |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+- Server's Peer ID -+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+- -+
| |
+- Server's RoCE GID -+
| |
+- -+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Server's RoCE |
+- MAC address +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | Server QP (bytes 1-2) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+---+
|Srvr QP byte 3 | Server RMB Rkey (bytes 1-3) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Srvr RMB byte 4|Server RMB indx| Srvr RMB alert tkn (bytes 1-2)|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Srvr RMB alert tkn (bytes 3-4)|Bsize | MTU | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+- Server's RMB virtual address -+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Server's initial packet sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| x'E2' | x'D4' | x'C3' | x'D9' |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 28 SMC Accept CLC message format
The fields present on the SMC Accept CLC message are:
Eyecatchers
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Like all CLC messages, the SMC Accept has beginning and ending
eyecatchers to aid with verification and parsing. The hex digits
spell 'SMCR' in IBM-1047 (EBCDIC)
Type
CLC message type 2 indicates SMC Accept
Length
The SMC Accept CLC message is 68 bytes long
Version
Version of the SMC-R protocol. Version 1 is the only currently
defined value.
F-bit
First Contact flag: A 1-bit flag that indicates that the server
believes this TCP connection is the first SMC-R contact for this
link group
Server's Peer ID
As described in A.2.1. above
Server's RoCE GID
This is the IPv6 address of the RNIC that the server chose for
this SMC Link
Server's RoCE MAC address
The MAC address of the server's RNIC for the SMC link. It is
required as some operating systems do not have neighbor discovery
or ARP support for RoCE RNICs.
Server's QP number
The number for the reliably connected queue pair that the server
created for this SMC link
Server's RMB Rkey
The RDMA Rkey for the RMB that the server created or chose for
this TCP connection
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Server's RMB element index
This indexes which element within the server's RMB will represent
this TCP connection
Server's RMB element alert token
A platform defined, architecturally opaque token that identifies
this TCP connection. Added by the client as immediate data on
RDMA writes from the client to the server to inform the server
that there is data for this connection to retrieve from the RMB
element
Bsize:
Server's RMB element buffer size in four bits compressed
notation: x=4 bits. Actual buffer size value is (2^(x+4)) * 1K.
Smallest possible value is 16K. Largest size supported by this
architecture is 512K.
MTU
An enumerated value indicating this peer's QP MTU size. The two
peers exchange this value and the minimum of the peer's value
will be used for the QP. This field should only be validated on a
first contact exchange.
The enumerated MTU values are:
0: reserved
1: 256
2: 512
3: 1024
4: 2048
5: 4096
6-15: reserved
Server's RMB virtual address
The virtual address of the server's RMB as assigned by the
server's RNIC.
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Server's initial packet sequence number
The starting packet sequence number that this peer will use when
sending to the other peer, so that the other peer can prepare its
QP for the sequence number to expect.
A.2.4. SMC Confirm CLC message format
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| x'E2' | x'D4' | x'C3' | x'D9' |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 3 | Length = 68 |Version| Rsrvd |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+- Client's Peer ID -+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+- -+
| |
+- Client's RoCE GID -+
| |
+- -+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Client's RoCE |
+- MAC address +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | Client QP (bytes 1-2) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+---+
|Clnt QP byte 3 | Client RMB Rkey (bytes 1-3) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Clnt RMB byte 4|Client RMB indx| Clnt RMB alert tkn (bytes 1-2)|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Clnt RMB alert tkn (bytes 3-4)|Bsize | MTU | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+- Client's RMB Virtual Address -+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Client's initial packet sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| x'E2' | x'D4' | x'C3' | x'D9' |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 29 SMC Confirm CLC message format
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The SMC Confirm CLC message is nearly identical to the SMC Accept
except that it contains client information and lacks a first contact
flag.
The fields present on the SMC Confirm CLC message are:
Eyecatchers
Like all CLC messages, the SMC Confirm has beginning and ending
eyecatchers to aid with verification and parsing. The hex digits
spell 'SMCR' in IBM-1047 (EBCDIC)
Type
CLC message type 3 indicates SMC Confirm
Length
The SMC Confirm CLC message is 68 bytes long
Version
Version of the SMC-R protocol. Version 1 is the only currently
defined value.
Client's Peer ID
As described in A.2.1. above
Clients's RoCE GID
This is the IPv6 address of the RNIC that the client chose for
this SMC Link
Client's RoCE MAC address
The MAC address of the client's RNIC for the SMC link. It is
required as some operating systems do not have neighbor discovery
or ARP support for RoCE RNICs.
Client's QP number
The number for the reliably connected queue pair that the client
created for this SMC link
Client's RMB Rkey
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The RDMA Rkey for the RMB that the client created or chose for
this TCP connection
Client's RMB element index
This indexes which element within the client's RMB will represent
this TCP connection
Client's RMB element alert token
A platform defined, architecturally opaque token that identifies
this TCP connection. Added by the server as immediate data on
RDMA writes from the server to the client to inform the client
that there is data for this connection to retrieve from the RMB
element
Bsize:
Client's RMB element buffer size in four bits compressed
notation: x=4 bits. Actual buffer size value is (2^(x+4)) * 1K.
Smallest possible value is 16K. Largest size supported by this
architecture is 512K.
MTU
An enumerated value indicating this peer's QP MTU size. The two
peers exchange this value and the minimum of the peer's value
will be used for the QP. The values are enumerated in A.2.3. This
value should only be validated on the first contact exchange.
Client's RMB virtual address
The virtual address of the server's RMB as assigned by the
server's RNIC.
Client's initial packet sequence number
The starting packet sequence number that this peer will use when
sending to the other peer, so that the other peer can prepare its
QP for the sequence number to expect
.
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A.2.5. SMC Decline CLC message format
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| x'E2' | x'D4' | x'C3' | x'D9' |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 4 | Length = 28 |Version|S|Rsrvd|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+- Sender's Peer ID -+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Peer Diagnosis Information |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| x'E2' | x'D4' | x'C3' | x'D9' |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 30 SMC Decline CLC message format
The fields present on the SMC Decline CLC message are:
Eyecatchers
Like all CLC messages, the SMC Decline has beginning and ending
eyecatchers to aid with verification and parsing. The hex digits
spell 'SMCR' in IBM-1047 (EBCDIC)
Type
CLC message type 4 indicates SMC Decline
Length
The SMC Decline CLC message is 28 bytes long
Version
Version of the SMC-R protocol. Version 1 is the only currently
defined value.
S-bit
Synch Bit. Indicates that the link group is out of synch and
receiving peer must clean up its representation of the link group
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Sender's Peer ID
As described in A.2.1. above
Peer Diagnosis Information
Four bytes of diagnosis information provided by the peer. These
values are defined by the individual peers and it is necessary to
consult the peer's system documentation to interpret the results.
A.3. LLC messages
LLC messages are sent over an existing SMC-R link using RoCE message
passing and are always 44 bytes long so that they fit into the space
available in a single WQE without requiring the receiver to post
receive buffers. If all 44 bytes are not needed, they are padded out
with zeroes. LLC messages are in a request/response format. The
message type is the same for request and response, and a flag
indicates whether a message is flowing as a request or a response.
The two high order bits of an LLC message opcode indicate how it is
to be handled by a peer that does not support the opcode.
If the high order bits of the opcode are b'00' then the peer must
support the LLC message and indicate a protocol error if it does not.
If the high order bits of the opcode are b'10' then the peer must
silently discard the LLC message if does not support the opcode. This
requirement is inserted to allow for toleration of advanced, but
optional function.
High order bits of b'11' indicate a Connection Data Control (CDC)
message as described in A.4.
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A.3.1. CONFIRM LINK LLC message format
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| type = 1 | length = 44 | Reserved |R| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sender's RoCE |
+- MAC address +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| |
+- -+
| Sender's RoCE GID |
+- -+
| |
+- +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |Sender's QP number, bytes 1-2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Sender QP byte3| Link number |Sender's link userid, bytes 1-2|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Sender's link userid bytes, 3-4| Max links | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+- Reserved -+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 31 CONFIRM LINK LLC message format
The CONFIRM LINK LLC message is required to be exchanged between the
server and client over a newly created SMC-R link to complete the
setup of an SMC link. Its purpose is to confirm that the RoCE path
is actually usable.
On first contact this flows after the server receives the SMC Confirm
CLC message from the client over the IP connection. For additional
links added to an SMC link group, it flows after the ADD LINK and ADD
LINK CONTINUATION exchange. This flow provides confirmation that the
queue pair is in fact usable. Each peer echoes its RoCE information
back to the other.
Type
Type 1 indicates CONFIRM LINK
Length
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All LLC messages are 44 bytes long
R
Reply flag. When set indicates this is a CONFIRM LINK REPLY
Sender's RoCE MAC address
The MAC address of the sender's RNIC for the SMC link. It is
required as some operating systems do not have neighbor discovery
or ARP support for RoCE RNICs.
Sender's RoCE GID
This is the IPv6 address of the RNIC that the sender is using for
this SMC-R Link
Sender's QP number
The number for the reliably connected queue pair that the sender
created for this SMC-R link
Link number
An identifier assigned by the server that uniquely identifies the
link within the link group. This identifier is ONLY unique
within a link group. Provided by the server and echoed back by
the client
Link User ID
An opaque, implementation defined identifier assigned by the
sender and provided to the receiver solely for purposes of
display, diagnosis, network management, etc. The link user ID
should be unique across the sender's entire software space,
including all link other link groups.
Max Links
The maximum number of links the sender can support in a link
group. The maximum for this link group is the the smaller of the
values provided by the two peers.
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A.3.2. ADD LINK LLC message format
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| type = 2 | length = 44 | Rsrvd |RsnCode|R|Z| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sender's RoCE |
+- MAC address +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| |
+- -+
| Sender's RoCE GID |
+- -+
| |
+- +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |Sender's QP number, bytes 1-2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Sender QP byte3| Link number |Rsrvd | MTU |Initial PSN |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Initial PSN, continued | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ -+
| Reserved |
+- -+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 32 ADD LINK LLC message format
The ADD LINK LLC message is sent over an existing link in the link
group when a peer wishes to add an SMC-R link to an existing SMC-R
link group. It sent by the server to add a new SMC-R link to the
group, or by the client to request that the server add a new link,
for example when a new RNIC becomes active. When sent from the
client to the server, it represents a request that the server
initiate an ADD LINK exchange.
This message is sent immediately after the initial SMC link in the
group completes, as described in 3.5.1. First contact. It can also be
sent over an existing SMC-R link group at any time as new RNICs are
added and become available. Therefore there can be as few as 1 new
RMB RTokens to communicate, or several. Rtokens will be
communicated using ADD LINK CONTINUATION messages.
The contents of the ADD LINK LLC message are:
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Type
Type 2 indicates ADD LINK
Length
All LLC messages are 44 bytes long
RsnCode
If the Z (rejection) flag is set, this field provides the reason
code. Values can be:
X'1' - no alternate path available: set when the server provides
the same MAC/GID as an existing SMC-R link in the group, and the
client does not have any additional RNICs available (i.e., server
is attempting to set up an asymmetric link but none is available)
X'2' - Invalid MTU value specified
R
Reply flag. When set indicates this is an ADD LINK REPLY
Z
Rejection flag. When set on reply indicates that the server's
ADD LINK was rejected by the client. When this flag is set, the
reason code will also be set.
Sender's RoCE MAC address
The MAC address of the sender's RNIC for the new SMC-R link. It
is required as some operating systems do not have neighbor
discovery or ARP support for RoCE RNICs.
Sender's RoCE GID
The IPv6 address of the RNIC that the sender is using for the new
SMC-R Link
Sender's QP number
The number for the reliably connected queue pair that the sender
created for the new SMC-R link
Link number
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An identifier for the new SMC-R link. This is assigned by the
server and uniquely identifies the link within the link group.
This identifier is ONLY unique within a link group. Provided by
the server and echoed back by the client
MTU
An enumerated value indicating this peer's QP MTU size. The two
peers exchange this value and the minimum of the peer's value
will be used for the QP. The values are enumerated in A.2.3.
Initial PSN
The starting packet sequence number that this peer will use when
sending to the other peer, so that the other peer can prepare its
QP for the sequence number to expect.
A.3.3. ADD LINK CONTINUATION LLC message format
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| type = 3 | length = 44 | Reserved |R| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Linknum | NumRTokens | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+- -+
| |
+- Rkey/Rtoken Pair -+
| |
+- -+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+- -+
| |
+- Rkey/Rtoken Pair or zeroes -+
| |
+- -+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 33 ADD LINK CONTINUATION LLC message format
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When a new SMC-R link is added to an SMC-R link group, it is
necessary to communicate the new link's RTokens for the RMBs that the
SMC-r link group can access. This message follows the ADD LINK and
provides the RTokens.
The server kicks off this exchange by sending the first ADD LINK
CONTINUATION LLC message, and the server controls the exchange as
described below.
o If the client and the server require the same number of ADD LINK
CONTINUATION messages to communicate their RTokens, the server
starts the exchange by sending the client the first ADD LINK
CONTINUATION request to the client with its RTokens, then the
client responds with an ADD LINK CONTINUATION response with its
RTokens, and so on until the exchange is completed.
o If the server requires more ADD LINK CONTINUATION messages than
the client, then after the client has communicated all its
RTokens, the server continues to send ADD LINK CONTINUATION
request messages to the client. The client continues to respond,
using empty (number of RTokens to be communicated = 0) ADD LINK
CONTINUATION response messages.
o If the client requires more ADD LINK CONTINUATION messages than
the server, then after communicating all its RTokens the server
will continue to send empty ADD LINK CONTINUATION messages to the
client to solicit replies with the client's RTokens, until all
have been communicated.
The contents of this message are:
Type
Type 3 indicates ADD LINK CONTINUATION
Length
All LLC messages are 44 bytes long
R
Reply flag. When set indicates this is an ADD LINK CONTINUATION
REPLY
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LinkNum
The link number of the new link within the SMC link group that
Rkeys are being communicated for
NumRTokens
Number of RTokens remaining to be communicated (including the
ones in this message). If the value is less than or equal to 2,
this is the last message. If it is greater than 2, another
continuation message will be required, and its value will be the
value in this message minus 2, and so on until all Rkeys are
communicated. The maximum value for this field is 255.
Up to 2 Rkey/RToken pairs
These consist of an Rkey for an RMB that is known on the SMC-R
link that this message was sent over (the reference Rkey), paired
with the same RMB's RToken over the new SMC link. A full RToken
is not required for the reference because it is only being used
to distinguish which RMB it applies to, not address it.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reference Rkey |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| New Rkey |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+- New Virtual Address -+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 34 Rkey/Rtoken pair format
The contents of the RKey/RToken pair are:
Reference Rkey
The Rkey of the RMB as it is already known on the SMC-R link over
which this message is being sent. Required so that the peer knows
which RMB to associate the new Rtoken with.
New Rkey
The Rkey of this RMB as it is known over the new SMC-R link
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New Virtual Address
The virtual address of this RMB as it is known over the new SMC-R
link.
A.3.4. DELETE LINK LLC message format
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| type = 4 | length = 44 | Reserved |R|A|O| Rsrvd |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Linknum | Reason code (bytes 1-3) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|RsnCode byte 4 | |
+-+-+-+-+-+-+-+-+ -+
| |
+- -+
| |
+- -+
| |
+- Reserved -+
| |
+- -+
| |
+- -+
| |
+- -+
| |
+- -+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 35 DELETE LINK LLC message format
When the client or server detects that a QP or SMC-R link goes down
or needs to come down, it sends this message over one of the other
links in the link group.
When the DELETE Link is sent from the client it only serves as a
notification, and the client expects the server to send a DELETE LINK
Request in response. To avoid races, only the server will initiate
the actual DELETE LINK Request and Response sequence that results
from notification from the client.
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The server can also initiate the DELETE Link without notification
from the client if it detects an error or if orderly link termination
was initiated.
The client may also request termination of the entire link group and
the server may terminate the entire link group using this message.
The contents of this message are:
Type
Type 4 indicates DELETE LINK
Length
All LLC messages are 44 bytes long
R
Reply flag. When set indicates this is an DELETE LINK REPLY
A
All flag. When set indicates that all links in the link group
are to be terminated. This terminates the link group.
O
Orderly flag. Indicates orderly termination. Orderly termination
is generally caused by an operator command rather than an error
on the link. When the client requests orderly termination, the
server may wait to complete other work before terminating.
LinkNum
The link number of the link to be terminated. If the A flag is
set, this field has no meaning and is set to 0.
RsnCode
The termination reason code. Currently defined reason codes are:
Request Reason Codes:
o X'00010000' = lost path
o X'00020000' = operator initiated termination
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o X'00030000' = Program initiated termination (link inactivity)
o X'00040000' = LLC protocol violation
o X'00050000' = Asymmetric link no longer needed
Response Reason Codes:
o X'00100000' = Unknown Link ID (no link)
o Others TBD
A.3.5. CONFIRM RKEY LLC message format
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| type = 6 | length = 44 | Reserved |R|0|Z|C|Rsrvd |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NumTkns | New RMB Rkey for this link (bytes 1-3) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|ThisLink byte 4| |
+-+-+-+-+-+-+-+-+ -+
| New RMB virtual address for this link |
+- +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | |
+-+-+-+-+-+-+-+-+ -+
| |
+- Other link RMB specification or zeros -+
| |
+- +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ -+
| |
+- -+
| Other link RMB specification or zeroes |
+- +-+-+-+-+-+-+-+-+
| | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 36 CONFIRM RKEY LLC message format
The CONFIRM_RKEY flow can be sent at any time from either the client
or the server, to inform the peer that an RMB has been created or
deleted. The creator of a new RMB must inform its peer of the new
RMB's RToken for all SMC-R links in the SMC-R link group. The
deleter of an RMB must inform its peer of the deleted RMB's RToken
for all SMC-R links.
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For RMB creation, the creator sends this message over the SMC link
that the first TCP connection that uses the new RMB is using. This
message contains the new RMB RToken for the SMC link that the message
is sent over, then it lists the sender's SMC links in the link group
paired with the new RToken for the new RMB for that link. This
message can communicate the new RTokens for 3 QPs: the QP for the
link this message is sent over, and 2 others. If there are more than
3 links in the SMC-R link group, CONFIRM_RKEY_CONTINUATION will be
required.
For RMB deletion, the creator sends the same format of message with a
delete flag set, to inform the peer that the RMB's RTokens on all
links in the group are deleted.
In both cases, the peer responds by simply echoing the message with
the response flag set. If the response is a negative response, the
sender must recalculate the RToken set and start a new CONFIRM_RKEY
exchange from the beginning. The timing of this retry is controlled
by the C flag as described below.
The contents of this message are:
Type
Type 6 indicates CONFIRM RKEY
Length
All LLC messages are 44 bytes long
R
Reply flag. When set indicates this is a CONFIRM RKEY REPLY
0
Reserved bit
Z
Negative response flag
C
Configuration Retry bit. If this is a negative response and this
flag is set, the originator should recalculate the Rkey set and
retry this exchange as soon as the current configuration change
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is completed. If this flag is not set on a negative response, the
originator must wait for the next natural stimulus (for example,
a new TCP connection started that requires a new RMB) before
retrying.
NumTkns
The number of other link/RToken pairs, including those provided
in this message, to be communicated. Note that this value does
not include the Rtoken for the link this message was sent on
(i.e., the maximum value is 2). If this value is three or fewer
this is the only message in the exchange. If this value is
greater than three, a CONFIRM RKEY CONTINUATION message will be
required.
Note: in this version of the architecture, 8 is the maximum
number of links supported in a link group.
New RMB Rkey for this link
The new RMB's Rkey as assigned on the link this message is being
sent over.
New RMB virtual address for this link
The new RMB's virtual address as assigned on the link this
messages is being sent over.
Other link RMB specification
The new RMB's specification on the other links in the link group,
as shown in Figure 38.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link number | RMB's Rkey for the specified link (bytes 1-3) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|New Rkey byte 4| |
+-+-+-+-+-+-+-+-+ -+
| RMB's virtual address for the specified link |
+- +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+
Figure 37 Format of link number/Rkey pairs
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Link number
The link number for a link in the link group
RMB's Rkey for the specified link
The Rkey used to reach the RMB over the link whose number was
specified in the link number field.
RMB's virtual address for the specified link
The virtual address used to reach the RMB over the link whose
number was specified in the link number field.
A.3.6. CONFIRM RKEY CONTINUATION LLC message format
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| type = 8 | length = 44 | Reserved |R|0|Z| Rsrvd |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NumTknsLeft | |
+-+-+-+-+-+-+-+-+ -+
| |
+- Other link RMB specification -+
| |
+- +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | |
+-+-+-+-+-+-+-+-+ -+
| |
+- Other link RMB specification or zeros -+
| |
+- +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ -+
| |
+- -+
| Other link RMB specification or zeroes |
+- +-+-+-+-+-+-+-+-+
| | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The CONFIRM RKEY CONTINUATION LLC message is used to communicate any
additional RMB RTokens that did not fit into the CONFIRM RKEY
message. Each of these messages can hold up to 3 RMB RTokens. The
Numlinks field indicates how many RMB RTokens are to be communicated,
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including the ones in this message. If the value is 3 or less, this
is the last message of the group. If the value is 4 or higher,
additional CONFIRM RKEY CONTINUATION messages will follow, and the
Numlinks value will be a countdown until all are communicated.
Like the CONFIRM RKEY message, the peer responds by echoing the
message back with the reply flag set.
The contents of this message are:
Type
Type 8 indicates CONFIRM RKEY CONTINUATION
Length
All LLC messages are 44 bytes long
R
Reply flag. When set indicates this is a CONFIRM RKEY
CONTINUATION REPLY
0
Reserved bit
Z
Negative response flag
NumTknsLeft
The number of link/RToken pairs, including those provided in this
message, that are remaining to be communicated. If this value is
three or fewer this is the last message in the exchange. If this
value is greater than three, another CONFIRM RKEY CONTINUATION
message will be required. Note that in this version of the
architecture, 8 is the maximum number of links supported in a
link group.
Other link RMB specifications
The new RMB's specification on other links in the link group, as
shown in Figure 38.
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A.3.7. DELETE RKEY LLC message format
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| type = 9 | length = 44 | Reserved |R|0|Z| Rsrvd |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Count | Error Mask | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| First deleted Rkey |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Second deleted Rkey or zeros |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Third deleted Rkey or zeros |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Fourth deleted Rkey or zeros |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Fifth deleted Rkey or zeros |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sixth deleted Rkey or zeros |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Seventh deleted Rkey or zeros |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Eighth deleted Rkey or zeros |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The DELETE_RKEY flow can be sent at any time from either the client
or the server, to inform the peer that one or more RMBs have been
deleted. Because the peer already knows every RMB's Rkey on each
link in the link group, this message only specifies one Rkey for each
RMB being deleted. The Rkey provided for each deleted RMB will be its
Rkey as known on the SMC-R link that this message is sent over.
It is not necessary to provide the entire RToken. The Rkey alone is
sufficient for identifying an existing RMB.
The peer responds by simply echoing the message with the response
flag set. If the peer did not recognize an Rkey, a negative response
flag will be set, however no aggressive recovery action beyond
logging the error will be taken.
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The contents of this message are:
Type
Type 9 indicates DELETE RKEY
Length
All LLC messages are 44 bytes long
R
Reply flag. When set indicates this is a DELETE RKEY REPLY
0
Reserved bit
Z
Negative response flag
Count
Number of RMBs being deleted by this message. Maximum value is 8
Error Mask
If this is a negative response, indicates which RMBs were not
successfully deleted. Each bit corresponds to a listed RMB. So
for example b'01010000' indicates that the second and fourth
Rkeys weren't successfully deleted.
Deleted Rkeys
A list of Count Rkeys. Provided on the request flow and echoed
back on the response flow. Each Rkey is valid on the link this
message is sent over, and represents a deleted RMB. Up to eight
RMBs can be deleted in this message.
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A.3.8. TEST LINK LLC message format
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| type = 7 | length = 44 | Reserved |R| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+- -+
| |
+- User Data -+
| |
+- -+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+- -+
| |
+- -+
| Reserved |
+- -+
| |
+- -+
| |
+- -+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 38 TEST LINK LLC message format
The TEST_LINK request can be sent from either peer to the other on an
existing SMC-R link at any time to test that the SMC-R link is active
and healthy at the software level. A peer which receives a TEST_LINK
LLC message immediately sends back a TEST_LINK reply, echoing back
the user data. Also refer to 4.5.3. TCP Keepalive processing.
The contents of this message are:
Type
Type 7 indicates TEST LINK
Length
All LLC messages are 44 bytes long
R
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Reply flag. When set indicates this is a TEST LINK REPLY
User Data
The receiver of this message echoes the sender's data back in a
TEST_LINK response LLC message
A.4. Connection Data Control (CDC) message format
The RMBE control data is communicated using Connection Data Control
(CDC) messages, which use RDMA message passing using inline data,
similar to LLC messages. Also similar to LLC messages, this data
block is 44 bytes long to ensure that it can it into private data
areas of receive WQEs, without requiring the receiver to post receive
buffers.
Unlike LLC messages, this data is integral to the data path so its
processing must be prioritized and optimized similarly to other data
path processing. While LLC messages may be processed on a slower
path than data, these messages cannot be.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
0 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = x'FE' | Length = 44 | Sequence number |
4 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SMC-R alert token |
8 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Producer cursor wrap seqno |
12 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Producer Cursor |
16 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Consumer cursor wrap seqno |
20 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Consumer Cursor |
24 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|B|P|U|R|F|Rsrvd|D|C|A| Reserved |
28 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
32 +- -+
| |
36 +- Reserved -+
| |
40 +- -+
| |
44 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 39 Connection Data Control (CDC) Message Format
Type = x'FE'
This type number has the two high order bits turned on to enable
processing to quickly distinguish it from an LLC message
Length = 44
The length of inline data that does not require posting of a
receive buffer.
Sequence number
A 2 byte unsigned integer that represents a wrapping sequence
number. The initial value is one and this value can wrap to 0.
Incremented with every control message send, except for the
failover data validation message, and used to guard against
processing an old control message out of sequence, and also used
in failover data validation. In normal usage, if this number is
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less than the last received value, discard this message. If
greater, processes this message. Old control messages can be
lost with no ill effect, but cannot be processed after newer
ones.
If this is a failover validation CDC message (F flag set), then
the receiver must verify that it has received and fully processed
the RDMA write that was described by the CDC message with the
sequence number in this message. If not, the TCP connection must
be reset, to guard against data loss. Details of this processing
are in section 4.6.1.
SMC-R alert token
The endpoint-assigned alert token that identifies which TCP
connection on the link group this control message refers to.
Producer cursor wrap seqno
A 2 byte unsigned integer that represents wrapping counter
incremented by the producer whenever the data written into this
RMBE receiver buffer causes a wrap (i.e. the producer cursor
wraps). This is used by the receiver to determine when new data
is available even though the cursors appear unchanged such as
when a full window size write is completed (Producer cursor of
this RMBE sent by peer = Local Consumer Cursor) or in scenarios
where the Producer Cursor sent for this RMBE < Local Consumer
Cursor).
Producer cursor
Unsigned, 4 byte integer that is a wrapping offset into the RMBE
data area. Points to the next byte of data to be written by the
sender. Can advance up to the receiver's Consumer Cursor as known
by the sender. When the urgent data present indicator is on then
points one byte beyond the last byte of urgent data. When
computing this cursor, the presence of the eyecatcher in the RMBE
data area must be accounted for. The first writeable data
location in the RMBE is at offset 4, so this cursor begins at 4
and wraps to 4.
Consumer cursor wrap seqno
2 byte unsigned integer that mirrors the value of the Producer
cursor wrap sequence number when the last read from this RMBE
occurred. Used as an indicator on how far along the consumer is
in reading data (i.e. processed last wrap point or not). The
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producer side can use this indicator to detect whether more data
can be written to the partner in full window write scenarios
(where the Producer Cursor = Consumer Cursor as known on the
remote RMBE). In this scenario if the consumer sequence number
equals the local producer sequence number the producer knows that
more data can be written.
Consumer Cursor
Unsigned 4 byte integer that is a wrapping offset into the
sender's RMBE data area. Points to the offset of the next byte
of data to be consumed by the peer in its own RMBE. When
computing this cursor, the presence of the eyecatcher in the RMBE
data area must be accounted for. The first writeable data
location in the RMBE is at offset 4, so this cursor begins at 4
and wraps to 4. The sender cannot write beyond this cursor into
the peer's RMBE without causing data loss.
B-bit
Writer blocked indicator: Sender is blocked for writing, requires
explicit notification when receive buffer space is available.
P-bit
Urgent data pending: Sender has urgent data pending for this
connection
U-bit
Urgent data present: Indicates that urgent is data present in the
RMBE data area, and the producer cursor points to one byte beyond
the last byte of urgent data.
R-bit
Request for consumer cursor update: Indicates that a consumer
cursor update is requested bypassing any window size optimization
algorithms.
F-bit
Failover validation indicator: sent by a peer to guard against
data loss during failover when the TCP connection is being moved
to another SMC-R link in the link group. When this bit is set
the only other fields in the CDC message that are significant are
the type, length, SMC-R alert token and the sequence number. The
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receiver must validate that it has fully processed the RDMA write
described by the previous CDC message bearing the same sequence
number as this validation message. If it has, no further action
is required. If it has not, the TCP connection must be reset.
This processing is described in detail in section 4.6.1.
D-bit
Sending done indicator: Sent by a peer when it is done writing
new data into the receiver's RMBE data area.
C-bit
Peer Closed Connection indicator: Sent by a peer when it is
completely done with this connection and will no longer be making
any updates to the receiver's RMBE, and will also not be sending
any more control messages.
A-bit
Abnormal Close indicator: Sent by a peer when the connection is
abnormally terminated (for example, the TCP connection was
Reset). When sent it indicates that the peer is completely done
with this connection and will no longer be making any updates to
this RMBE or sending any more control messages. It also indicates
that the RMBE owner must flush any remaining data on this
connection and surface an error return code to any outstanding
socket APIs on this connection (same processing as receiving an
RST segment on a TCP connection).
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Appendix B. Socket API considerations
A key design goal for SMC-R is to require no application changes for
exploitation. It is confined to socket applications using stream
(i.e. TCP protocol) sockets over IPv4 or IPv6. By virtue of the fact
that the switch to the SMC-R protocol occurs after a TCP connection
is established no changes are required in socket address family or in
the IP addresses and ports that the socket application are using.
Existing socket APIs that allow the application to retrieve local and
remote socket address structures for an established TCP connection
(for example, getsockname() and getpeername()) will continue to
function as they have before. Existing DNS setup and APIs for
resolving hostnames to IP addresses and vice versa also continue to
function without any changes. In general all of the usual socket APIs
that are used for TCP communicates (send APIs, recv APIs, etc.) will
continue to function as they do today even if SMC-R is used as the
underlying protocol.
Each SMC-R enabled implementation does however need to pay special
attention to any socket APIs that have a reliance on the underlying
TCP and IP protocols and ensure that their behavior in an SMC-R
environment is reasonable and minimizes impact to the application.
While the basic socket API set is fairly similar across different
Operating Systems, when it comes to advanced socket API options there
is more variability. Each implementation needs to perform a detailed
analysis of its API options and SMC-R impact and implications. As
part of that step a discussion or review with other implementations
supporting SMC-R would be useful to ensure a consistent
implementation.
setsockopt()/ getsockopt() considerations
These APIs allow socket applications to manipulate socket, transport
(TCP/UDP) and IP level options associated with a given socket.
Typically, a platform restricts the number of IP options available to
stream (TCP) socket applications given their connection oriented
nature. The general guideline here is to continue processing these
APIs in a manner that allows for application compatibility. Some
options will be relevant to the SMC-R protocol and will require
special processing under the covers. For example, the ability to
manipulate TCP send and receive buffer sizes is still valid for SMC-
R. However, other options may have no meaning for SMC-R. For
example, if an application enabled the TCP_NODELAY option to disable
Nagle's algorithm it should have no real effect in SMC-R
communications as there is no notion of Nagle's algorithm with this
new protocol. But the implementation must accept the TCP_NODELAY
option as it does today and save it so that it can be later extracted
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via getsockopt() processing. Note that any TCP or IP level options
will still have an effect on any TCP/IP packets flowing for an SMC-R
connection (i.e. as part of TCP/IP connection establishment and
TCP/IP connection termination packet flows).
Under the covers manipulation of the TCP options will also include
the SMC layer setting and reading the SMC-R experimental option
before and after completion of the 3 way TCP handshake.
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Appendix C. Rendezvous Error scenarios
Error scenarios in setting up and managing SMC-R links are discussed
in this section.
C.1. SMC Decline during CLC negotiation
A peer to the SMC-R CLC negotiation can send SMC Decline in lieu of
any expected CLC message to decline SMC and force the TCP connection
back to IP fabric. There can be several reasons for an SMC Decline
during the CLC negotiation including: RNIC went down, SMC-R forbidden
by local policy, subnet (IPv4) or prefix (IPv6) doesn't match, lack
of resources to perform SMC-R. In all cases when an SMC Decline is
sent in lieu of an expected CLC message, no confirmation is required
and the TCP connection immediately falls back to using the IP fabric.
To prevent ambiguity between CLC messages and application data, an
SMC Decline cannot "chase" another CLC message. SMC Decline can only
be sent in lieu of an expected CLC message. For example, if the
client sends SMC Proposal then its RNIC goes down, it must wait for
the SMC Accept for the server and then it can reply to that with an
SMC Decline.
This "no chase" rule means that if this TCP connection is not a first
contact between RoCE peers, a server cannot send SMC Decline after
sending SMC Accept - it can only either break the TCP connection.
Similarly, once the client sends SMC Confirm on a TCP connection that
isn't first contact, it is committed to SMC-R for this TCP connection
and cannot fall back to IP.
C.2. SMC Decline during LLC negotiation
For a TCP connection that represents first contact between RoCE
pairs, it is possible for SMC to fail back to IP during the LLC
negotiation. This is possible until the first contact SMC link is
confirmed. For example, see Figure 40. After a first contact SMC
link is confirmed, fallback to IP is no longer possible. The rule
that this translates to is: a first contact peer can send SMC Decline
at any time during LLC negotiation until it has successfully sent its
CONFIRM LINK (request or response) flow. After that point, it cannot
fall back to IP.
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Host X -- Server Host Y -- Client
+-------------------+ +-------------------+
| PeerID = PS1 | | PeerID = PC1 |
| +------+ +------+ |
| QP 8 |RNIC 1| SMC-R link 1 |RNIC 2| QP 64 |
| RKey X | |MAC MA|<-------------------->|MAC MB| | |
| | |GID GA| attempted setup |GID GB| | RKey Y2|
| \/ +------+ +------+ \/ |
|+--------+ | | +--------+ |
|| RMB | | | | RMB | |
|+--------+ | | +--------+ |
| /\ +------+ +------+ /\ |
| | |RNIC 3| |RNIC 4| | Rkey W2|
| | |MAC MC| |MAC MD| | |
| QP 9 |GID GC| |GID GD| QP65 |
| +------+ +------+ |
+-------------------+ +-------------------+
SYN / SYN-ACK / ACT TCP 3-way handshake with TCP option
<--------------------------------------------------------->
SMC Proposal / SMC Accept / SMC Confirm exchange
<-------------------------------------------------------->
CONFIRM LINK(request, link 1)
.........................................................>
CONFIRM LINK(response, link 1)
X...................................
:
: ROCE write faliure
:.................................>
SMC Decline(PC1, reason code)
<--------------------------------------------------------
Connection data flows over IP fabric
<------------------------------------------------------->
Legend:
------------ TCP/IP and CLC flows
............ RoCE (LLC) flows
Figure 40 SMC Decline during LLC negotiation
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C.3. The SMC Decline window
Because SMC-R does not support fall-back to IP for a TCP connection
that is already using RDMA, there are specific rules on when SMC
Decline, which signals a fall-back to IP because of an error or
problem with the RoCE fabric, can be sent during TCP connection
setup. There is a point of no return after which a connection cannot
fall back to IP, and RoCE errors that occur after this point require
the connection to be broken with a RST flow in the IP fabric.
For first contact, that point of no return is after the Add Link LLC
message has been successfully sent for the second SMC-R link.
Specifically, the server cannot fall back to IP after receiving
either a positive write completion indication for the Add Link
request, or after receiving the Add Link response from the client,
whichever comes first. The client cannot fall back to IP after
either sending a negative Add Link response, receiving a positive
write complete on a positive Add Link response, or receiving a
Confirm Link for the second SMC-R link from the server, whichever
comes first.
For subsequent contact, that point of no return is after the last
send of the CLC negotiation completes. This, in combination with the
rule that error "chasers" are not allowed during CLC negotiation,
means that the server cannot send SMC Decline after sending an SMC
Accept, and the client cannot send an SMC Decline after sending an
SMC Confirm.
C.4. Out of synch conditions during SMC-R negotiation
The SMC Accept CLC message contains a "first contact" flag that
indicates to the client whether or not the server believes it is
setting up a new link group, or using an existing link group. This
flag is used to detect an out of synch condition between the client
and the server. The scenario detected is as follows: There is a
single existing SMC-R link between the peers. After the client sends
the SMC Proposal CLC message, the existing SMC-R link between the
client and the server fails. The client cannot chase the SMC
Proposal CLC message with an SMC Decline CLC message in this case
because the client does not yet know that the server would have
wanted to choose the SMC-R link that just crashed. The QP that
failed recovers before the server returns its SMC Accept CLC message.
This means that there is a QP but no SMC link. Since the server had
not yet learned of the SMC link failure when it sent the SMC Accept
CLC message, it attempts to re-use the SMC link that just failed.
This means the server would not set the "first contact" flag,
indicating to the client that the server thinks it is reusing an SMC-
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R link. However the client does not have an SMC-R link that matches
the server's specification. Because the "first contact" flag is off,
the client realizes it is out of synch with the server and sends SMC
Decline to cause the connection to fall back to IP.
C.5. Timeouts during CLC negotiation
Because the SMC-R negotiation flows as TCP data, there are built-in
timeouts and retransmits at the TCP layer for individual messages.
Implementations also must to protect the overall TCP/CLC handshake
with a timer or timers to prevent connections from hanging
indefinitely due to SMC-R processing. This can be done with
individual timers for individual CLC messages or an overall timer for
the entire exchange, which may include the TCP handshake and the CLC
handshake under one timer or separate timers. This decision is
implementation dependent.
If the TCP and/or CLC handshakes time out, the TCP connection must be
terminated as it would be in a legacy IP environment when connection
setup doesn't complete in a timely manner. Because the CLC flows are
TCP messages, if they cannot be sent and received in a timely
fashion, the TCP connection is not healthy and would not work if
fallback to IP were attempted.
C.6. Protocol errors during CLC negotiation
Protocol errors occur during CLC negotiation when a message is
received that is not expected. For example, a peer that is expecting
a CLC message but instead receives application data has experienced a
protocol error, and also indicates a likely software error as the two
sides are out of synch. When application data is expected, this data
is not parsed to ensure it's not a CLC message.
When a peer is expecting a CLC negotiation message, any parsing error
except a bad enumerated value in that message must be treated as
application data. The CLC negotiation messages are designed with
beginning and ending eyecatchers to help verify that they are
actually the expected message. If other parsing errors in an
expected CLC message occur, such as incorrect length fields or
incorrectly formatted fields, the message must be treated as
application data.
All protocol errors with the exception of bad enumerated values must
result in termination of the TCP connection. No fallback to IP is
allowed in the case of a protocol error because if the protocols are
out of synch, mismatched, or corrupted, then data and security
integrity cannot be ensured.
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The exception to this rule is enumerated values, for example the QP
MTU values on SMC Accept and SMC Confirm. If a reserved value is
received, the proper error response is to send SMC Decline and fall
back to IP. The reason for this is that use of a reserved enumerated
value indicates that the other partner likely has additional support
that the receiving partner does not have. This indicated mismatch of
SMC-R capabilities is not an integrity problem, but indicates that
SMC-R cannot be used for this connection
C.7. Timeouts during LLC negotiation
Whenever a peer sends an LLC message to which a reply is expected, it
sets a timer after the send posts to wait for the reply. An expected
response may be a reply flavor of the LLC message (for example
CONFIRM LINK REPLY) or a new LLC message (for example an ADD LINK
CONTINUATION expected from the server by the client if there are more
Rkeys to communicate).
On LLC flows that are part of a first contact setup of a link group,
the value of the timer is implementation dependent but should be long
enough to allow the other peer have a write complete timeout and 2-3
retransmits of an SMC Decline on the TCP fabric. For LLC flows
that are maintaining the link group and not part of first contact
setup of a link group, the timers may be shorter. Upon receipt of an
expected reply the timer is cancelled. If a timer pops without a
reply having been received, the sender must initiate a recovery
action
During first contact processing, failure of an LLC verification timer
is a should-not-occur which indicates a problem with one of the
endpoints. The reason for this is that if there is a "routine"
failure in the RoCE fabric that causes an LLC verification send to
fail, the sender will get a write completion failure and will then
send SMC Decline to the partner. The only time an LLC verification
timer will expire on a first contact is when the sender thinks the
send succeeded but it actually didn't. Because of the reliable
connected nature of QP connections on the RoCE fabric, this is
indicates a problem with one of the peers, not with the RoCE fabric.
After the reliable connected QP for the first SMC-R link in a link
group is set up on initial contact, the client sets a timer to wait
for a RoCE verification message from the server that the QP is
actually connected and usable. If the server experiences a failure
sending its QP confirmation message, it will send SMC Decline, which
should arrive at the client before the client's verification timer
expires. If the client's timer expires without receiving either an
SMC Decline or a RoCE message confirmation from the server, there is
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a problem either with the server or with the TCP fabric. In either
case the client must break the TCP connection and clean up the SMC-R
link.
There are two scenarios in which the client's response to the QP
verification message fails to reach the server. The main difference
is whether or not the client has successfully completed the send of
the CONFIRM LINK response.
In the normal case of a problem with the RoCE path, the client will
learn of the failure by getting a write completion failure, before
the server's timer expires. In this case, the client sends an SMC
Decline CLC message to the server and the TCP connection falls back
to IP.
If the client's send of the Confirmation message receives a positive
return code but for some reason still does not reach the server, or
the client's SMC Decline CLC message fails to reach the server after
the client fails to send its RoCE confirmation message, then the
server's timer will time out and the server must break the TCP
connection by sending RST. This is expected to be a very rare case,
because if the client cannot send its CONFIRM LINK RSP LLC message,
the client should get a negative return code and initiate fallback to
IP. A client receiving a positive return code on a send that fails
to reach the server should be extremely rare.
C.7.1. Recovery actions for LLC timeouts and failures
The following table describes recovery actions for LLC timeouts. A
write completion failure or other indication of failure to send on
the send of the LLC command is treated the same as a timeout.
LLC Message: CONFIRM LINK from server (first contact, first link in
the link group)
Timer waits for: CONFIRM LINK reply from client
Recovery action: Break the TCP connection by sending RST and
clean up the link. The server should have received an SMC
Decline from the client by now if the client had an LLC send
failure.
LLC Message: CONFIRM LINK from server (first contact, second link in
the link group)
Timer waits for: CONFIRM LINK reply from client
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Recovery action: The second link was not successfully set up.
Send DELETE LINK to the client. Connection data cannot flow in
the first link in the link group, until the reply to this DELETE
LINK is received, to prevent the peers from being out of synch on
the state of the link group.
LLC Message: CONFIRM LINK from server (not first contact)
Timer Waits for: CONFIRM LINK reply from client
Recovery action: Clean up the new link and set a timer to retry.
Send DELETE LINK to the client, in case the client has a longer
timer interval, so the client can stop waiting
LLC Message: CONFIRM LINK REPLY from client (first contact)
Timer waits for: ADD LINK from server
Recovery action: Clean up the SMC-R link and break the TCP
connection by sending RST over the IP fabric. There is a problem
with the server. If the server had a send failure, it should
have have sent SMC Decline by now.
LLC Message: ADD LINK from server (first contact)
Timer waits for: ADD LINK reply from client
Recovery action: Break the TCP connection with RST and clean up
RoCE resources. The connection is past the point where the
server can fall back to IP, and if the client had a send problem
it should have sent SMC Decline by now.
LLC Message: ADD LINK from server (not first contact)
Timer waits for: ADD LINK reply from client
Recovery action: Clean up resources (QP, RMB keys, etc) for the
new link and treat the link that the ADD LINK was sent over as if
it had failed. If there is another link available to resend the
ADD LINK and the link group still needs another link, retry the
ADD LINK over another link in the link group.
LLC Message: ADD LINK REPLY from client (and there are more Rkeys to
be communicated)
Timer waits for: ADD LINK CONTINUATION from server
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Recovery action: Treat the same as ADD LINK timer failure
LLC Message: ADD LINK REPLY or ADD LINK CONTINUATION reply from
client (and there are no more Rkeys to be communicated, for the
second link in a first contact scenario)
Timer waits for: CONFIRM LINK from the server on the new link
Recovery action: The new link has failed to set up. Send DELETE
LINK to the server. Do not consider the socket opened to the
client application until receiving confirmation from the server
in the form of a DELETE LINK request for this link and sending
the reply (to prevent the partners from being out of synch on the
state of the link group).
Set a timer to send another ADD LINK to the server if there is
still an unused RNIC on the client side.
LLC Message: ADD LINK REPLY or ADD LINK CONTINUATION reply from the
client (and there are no more Rkeys to be communicated)
Timer waits for: CONFIRM LINK from the server, over the new link
Recovery action: Send a DELETE LINK to the server for the new
link, then clean up any resource allocated for the new link and
set a timer to send ADD LINK to the server if there is still an
unused RNIC on the client side. The new link has failed to set
up, but the link that the ADD LINK exchange occurred over is
unaffected.
LLC Message: ADD LINK CONTINUATION from server
Timer waits for: ADD LINK CONTINUATION REPLY from client
Recovery action: Treat the same as ADD LINK timer failure
LLC Message: ADD LINK CONTINUATION reply from client (first contact,
and RMB count fields indicate that the server owes more ADD LINK
CONTINUATION messages)
Timer waits for: ADD LINK CONTINUATION from the server
Recovery action: Clean up the SMC link and break the TCP
connection by sending RST. There is a problem with the server.
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If the server had a send failure, it should have have sent SMC
Decline by now.
LLC Message: ADD LINK CONTINUATION reply from client (not first
contact and RMB count fields indicate that the server owes more ADD
LINK CONTINUATION messages)
Timer waits for: ADD LINK CONTINUATION from server
Recovery action: Treat as is if client detected link failure on
the link the ADD LINK exchange is using. Send DELETE LINK to
the server over another active link if one exists, otherwise
clean up the link group.
LLC Message: DELETE LINK from client
Timer waits for: DELETE LINK request from server
Recovery action: If the scope of the request is to delete a
single link, the surviving link, over which the client sent the
DELETE LINK is no longer usable either. If this is the last link
in the link group, end TCP connections over the link group by
sending RST packets. If there are other surviving links in the
link group, resend over a surviving link. Also send a DELETE
LINK over a surviving link for the link that the client attempted
to send the initial DELETE LINK message over. If the scope of
the request is to delete the entire link group, try resending on
other links in the link group until success is achieved. If all
sends fail, tear down the link group and any TCP connections that
exist on it.
LLC Message: DELETE LINK from server (scope: entire link group)
Timer waits for: Confirmation from the adapter that the message
was delivered.
Recovery action: Tear down the link group and any TCP connections
that exist over it.
LLC Message: DELETE LINK from server (scope: single link)
Timer waits for: DELETE LINK reply from the client
Recovery action: The link over which the client sent the DELETE
LINK is no longer usable either. If this is the last link in the
link group, end TCP connections over the link group by sending
RST packets. If there are other surviving links in the link
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group, resend over a surviving link. Also send a DELETE LINK
over a surviving link for the link that the server attempted to
send the initial DELETE LINK message over. If the scope of the
request is to delete the entire link group, try resending on
other links in the link group until success is achieved. If all
sends fail, tear down the link group and any TCP connections that
exist on it.
LLC Message: CONFIRM RKEY from the client
Timer waits for: CONFIRM RKEY REPLY from the server
Recovery action: Perform normal client procedures for detection
of failed link. The link over which the message was sent has
failed.
LLC Message: CONFIRM RKEY from the server
Timer waits for : CONFIRM RKEY REPLY from the client
Recovery action: Perform normal server procedures for detection
of failed link. The link over which the message was sent has
failed.
LLC Message: TEST LINK from the client
Timer waits for: TEST LINK REPLY from the server
Recovery action: Perform normal client procedures for detection
of failed link. The link over which the message was sent has
failed.
LLC Message: TEST LINK from the server
Timer waits for : TEST LINK REPLY from the client
Recovery action: Perform normal server procedures for detection
of failed link. The link over which the message was sent has
failed.
The following table describes recovery actions for invalid LLC
messages. These could be misformatted or contain out of synch data.
LLC Message received: CONFIRM LINK from server
What could be bad: Incorrect link information
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Recovery action: Protocol error. The link must be brought down
by sending a DELETE LINK for the link over another link in the
link group if one exists. If this is first contact, fall back to
IP by sending SMC Decline to server.
LLC Message received: ADD LINK
What could be bad: Undefined enumerated MTU value
Recovery action: Send negative ADD LINK reply with reason code
x'2'
LLC Message received: ADD LINK reply from client
What could be bad: Client side link information that would result
in a parallel link being set up
Recovery action: Parallel links are not permitted. Delete the
link by sending DELETE LINK to the client over another link in
the link group.
LLC Message received: Any link group command from the server except
DELETE LINK for the entire link group
What could be bad: Client has sent DELETE LINK for the link that
the message was received on
Recovery action: Ignore the LLC message. Worst case the server
will time out. Best case the DELETE LINK crosses with the
command from the server and the server realizes it failed.
LLC Message received: ADD LINK CONTINUATION from the server or ADD
LINK CONTINUATION REPLY from the client
What could be bad: Number of RMBs provided doesn't match count
given on initial ADD LINK or ADD LINK reply message
Recovery action: Protocol error. Treat as if detected link outage
LLC Message received: DELETE LINK from client
What could be bad: Link indicated doesn't exist
Recovery action: If the link is in the process of being cleaned
up, assume timing window and ignore message. Otherwise, send
DELETE LINK REPLY with reason code 1.
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LLC Message received: DELETE LINK from server
What could be bad: Link indicated doesn't exist
Recovery action: Send DELETE LINK REPLY with reason code 1.
LLC Message received: CONFIRM RKEY form either client or server
What could be bad: No Rkey provided for one or more of the links
in the link group
Recovery action: Treat as if detected failure of the link(s) for
which no RKEY was provided
LLC message received: DELETE RKEY
Specified RKey doesn't exist
Send negative DELETE RKEY response.
LLC message received: TEST LINK reply
What could be bad: User data doesn't match what was sent in the
TEST LINK request
Recovery action: Treat as if detected that the link has gone
down. This is a protocol error
LLC message received: Unknown LLC type with high order bits of opcode
equal b'10'
What could be bad: This is an optional LLC message which the
receiver does not support
Recovery action: Ignore (silently discard) the message
LLC message received: any unambiguously incorrect or out of synch LLC
message
What it indicates: Link is out of sync
Recovery action: Treat as if detected that the link has gone
down. Note that an unsupported or unknown LLC opcode whose two
high order bits are b'10' is not an error, and must be silently
discarded. Any other unknown or unsupported LLC opcode is an
error.
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C.8. Failure to add second SMC-R link to a link group
When there is any failure in setting up the second SMC-R link in an
SMC-R link group, including confirmation timer expiration, the SMC-R
link group is allowed to continue, without available failover.
However this situation is extremely undesirable and the server must
endeavor to correct it as soon as it can.
The server peer in the SMC-R link group must set a timer to drive it
to retry setup of a failed additional SMC-R link. The server will
immediately retry the SMC-R link setup when the first of the
following events occurs:
o The retry timer expires
o A new RNIC becomes available to the server, on the same LAN as the
SMC-R link group
o An "Add Link" LLC request message is received from the client,
which indicates availability of a new RNIC on the client side.
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Authors' Addresses
Mike Fox
IBM
3039 Cornwallis Rd.
Research Triangle Park, NC 27709
Email: mjfox@us.ibm.com
Constantinos (Gus) Kassimis
IBM
3039 Cornwallis Rd.
Research Triangle Park, NC 27709
Email: kassimis@us.ibm.com
Jerry Stevens
IBM
3039 Cornwallis Rd.
Research Triangle Park, NC 27709
Email: sjerry@us.ibm.com
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