Internet DRAFT - draft-cel-nfsv4-rpcrdma-version-two
draft-cel-nfsv4-rpcrdma-version-two
Network File System Version 4 C. Lever, Ed.
Internet-Draft Oracle
Intended status: Standards Track D. Noveck
Expires: November 7, 2019 NetApp
May 6, 2019
RPC-over-RDMA Version 2 Protocol
draft-cel-nfsv4-rpcrdma-version-two-09
Abstract
This document specifies a new version of the transport protocol that
conveys Remote Procedure Call (RPC) messages on physical transports
capable of Remote Direct Memory Access (RDMA). The new version of
this protocol is extensible.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 4
3. RPC-over-RDMA Version 2 Headers and Chunks . . . . . . . . . 5
3.1. rpcrdma_common: Common Transport Header Prefix . . . . . 5
3.2. rpcrdma2_hdr_prefix: Version 2 Transport Header Prefix . 6
3.3. rpcrdma2_chunk_lists: Describe External Data Payload . . 7
4. Transport Properties . . . . . . . . . . . . . . . . . . . . 8
4.1. Transport Properties Model . . . . . . . . . . . . . . . 8
4.2. Current Transport Properties . . . . . . . . . . . . . . 10
4.2.1. Receive Buffer Size . . . . . . . . . . . . . . . . . 11
4.2.2. Reverse Request Support . . . . . . . . . . . . . . . 12
5. RPC-over-RDMA Version 2 Transport Messages . . . . . . . . . 13
5.1. Overall Transport Message Structure . . . . . . . . . . . 13
5.2. Transport Header Types . . . . . . . . . . . . . . . . . 13
5.3. Header Types Defined in RPC-over-RDMA version 2 . . . . . 14
5.3.1. RDMA2_MSG: Convey RPC Message Inline . . . . . . . . 15
5.3.2. RDMA2_NOMSG: Convey External RPC Message . . . . . . 15
5.3.3. RDMA2_ERROR: Report Transport Error . . . . . . . . . 15
5.3.4. RDMA2_CONNPROP: Advertise Transport Properties . . . 18
6. XDR Protocol Definition . . . . . . . . . . . . . . . . . . . 19
6.1. Code Component License . . . . . . . . . . . . . . . . . 20
6.2. Extraction and Use of XDR Definitions . . . . . . . . . . 22
6.3. XDR Definition for RPC-over-RDMA Version 2 Core
Structures . . . . . . . . . . . . . . . . . . . . . . . 24
6.4. XDR Definition for RPC-over-RDMA Version 2 Base Header
Types . . . . . . . . . . . . . . . . . . . . . . . . . . 26
6.5. Use of the XDR Description Files . . . . . . . . . . . . 27
7. Protocol Version Negotiation . . . . . . . . . . . . . . . . 29
7.1. Server Does Support RPC-over-RDMA Version 2 . . . . . . . 29
7.2. Server Does Not Support RPC-over-RDMA Version 2 . . . . . 29
7.3. Client Does Not Support RPC-over-RDMA Version 2 . . . . . 30
8. Differences from the RPC-over-RDMA Version 1 Protocol . . . . 30
8.1. Transport Properties . . . . . . . . . . . . . . . . . . 30
8.2. Credit Management Changes . . . . . . . . . . . . . . . . 30
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8.3. Inline Threshold Changes . . . . . . . . . . . . . . . . 32
8.4. Support for Remote Invalidation . . . . . . . . . . . . . 33
8.4.1. Reverse Direction Remote Invalidation . . . . . . . . 33
8.5. Error Reporting Changes . . . . . . . . . . . . . . . . . 34
9. Extending the Version 2 Protocol . . . . . . . . . . . . . . 34
9.1. Adding New Header Types to RPC-over-RDMA Version 2 . . . 35
9.2. Adding New Transport properties to the Protocol . . . . . 36
9.3. Adding New Error Codes to the Protocol . . . . . . . . . 37
9.4. Adding New Header Flags to the Protocol . . . . . . . . . 38
10. Relationship to other RPC-over-RDMA Versions . . . . . . . . 38
10.1. Relationship to RPC-over-RDMA Version 1 . . . . . . . . 38
10.2. Extensibility Beyond RPC-over-RDMA Version 2 . . . . . . 40
11. Security Considerations . . . . . . . . . . . . . . . . . . . 40
11.1. Security Considerations (Transport Properties) . . . . . 40
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 41
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 41
13.1. Normative References . . . . . . . . . . . . . . . . . . 41
13.2. Informative References . . . . . . . . . . . . . . . . . 41
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 42
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 42
1. Introduction
Remote Direct Memory Access (RDMA) [RFC5040] [RFC5041] [IBARCH] is a
technique for moving data efficiently between end nodes. By
directing data into destination buffers as it is sent on a network
and placing it using direct memory access implemented by hardware,
the complementary benefits of faster transfers and reduced host
overhead are obtained.
RPC-over-RDMA version 1 enables ONC RPC [RFC5531] messages to be
conveyed on RDMA transports. That protocol is specified in
[RFC8166]. RPC-over-RDMA version 1 is deployed and in use, although
there are known shortcomings to this protocol:
o The protocol's default size of Receive buffers forces the use of
RDMA Read and Write transfers for small payloads, and limits the
size of reverse direction messages.
o It is difficult to make optimizations or protocol fixes that
require changes to on-the-wire behavior.
To address these issues in a way that is compatible with existing
RPC-over-RDMA version 1 deployments, a new version of the RPC-over-
RDMA transport protocol is presented in this document.
This new version of RPC-over-RDMA is extensible, enabling OPTIONAL
extensions to be added without impacting existing implementations.
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To enable protocol extension, the XDR definition for RPC-over-RDMA
version 2 is organized differently than the definition version 1.
These changes, which are discussed in Section 10.1, do not affect the
on-the-wire format.
In addition, RPC-over-RDMA version 2 contains a set of incremental
changes that relieve certain performance constraints and enable
recovery from certain abnormal corner cases. These changes include:
o The exchange of transport properties as described in Section 8.1.
o A more flexible credit account mechanism, detailed in Section TBD.
o Larger default inline thresholds as described in Section 8.3.
o Support for remote invalidation as explained in Section 8.4.
o Support for reverse direction operation, as described in
[RFC8167], is now REQUIRED. Details are in Section 3.2.
o An expansion of error reporting capabilities, described in
Section 5.3.3. A summary of the reasons for this expansion
appears in Section 8.5. This expansion supports the addition of
new error codes as described in Section 9.3.
Because of the way in which RPC-over-RDMA version 2 builds upon the
facilities present in RPC-over-RDMA version 1, a knowledge of the
basic structure of RPC-over-RDMA version 1, as described in
[RFC8166], is assumed in this document.
As in that document, the terms "RPC Payload Stream" and "Transport
Header Stream" (defined in Section 3.2 of that document) are used to
distinguish between an RPC message as defined by [RFC5531] and the
header whose job it is to describe the RPC message and its associated
memory resources. In that regard, the reader is assumed to
understand how RDMA is used to transfer chunks between client and
server, the use of Position-Zero Read chunks and Reply chunks to
convey Long RPC messages, and the role of DDP-eligibility in
constraining how data payloads are to be conveyed.
2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
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3. RPC-over-RDMA Version 2 Headers and Chunks
Most RPC-over-RDMA version 2 data structures are derived from
corresponding structures in RPC-over-RDMA version 1. As is typical
for new versions of an existing protocol, the XDR data structures
have new names and there are a few small changes in content. In some
cases, there have been structural re-organizations to enabled
protocol extensibility.
3.1. rpcrdma_common: Common Transport Header Prefix
The rpcrdma_common prefix describes the first part of each RDMA-over-
RPC transport header for version 2 and subsequent versions.
<CODE BEGINS>
struct rpcrdma_common {
uint32 rdma_xid;
uint32 rdma_vers;
uint32 rdma_credit;
uint32 rdma_htype;
};
<CODE ENDS>
RPC-over-RDMA version 2's use of these first four words matches that
of version 1 as required by [RFC8166]. However, there are important
structural differences in the way that these words are described by
the respective XDR descriptions:
o The header type is represented as a uint32 rather than as an enum
that would need to be modified to reflect additions to the set of
header types made by later extensions.
o The header type field is part of an XDR structure devoted to
representing the transport header prefix, rather than being part
of a discriminated union, that includes the body of each transport
header type.
o There is now a prefix structure (see Section 3.2) of which the
rpcrdma_common structure is the initial segment. This is a newly
defined XDR object within the protocol description, in contrast
with RPC-over-RDMA version 1, which limits the common portion of
all header types to the four words in rpcrdma_common.
These changes are part of a larger structural change in the XDR
description of RPC-over-RDMA version 2 that enables a cleaner
treatment of protocol extension. The XDR appearing in Section 6
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reflects these changes, which are discussed in further detail in
Section 10.1.
3.2. rpcrdma2_hdr_prefix: Version 2 Transport Header Prefix
The following prefix structure appears at the start of any RPC-over-
RDMA version 2 transport header.
<CODE BEGINS>
const RPCRDMA2_F_RESPONSE 0x00000001;
struct rpcrdma2_hdr_prefix
struct rpcrdma_common rdma_start;
uint32 rdma_flags;
};
<CODE ENDS>
The rdma_flags is new to RPC-over-RDMA version 2. Currently, the
only flag defined within this word is the RPCRDMA2_F_RESPONSE flag.
The other bits are reserved for future use as described in
Section 9.4. The sender MUST set these to zero.
The RPCRDMA2_F_RESPONSE flag qualifies the values contained in the
transport header's rdma_start.rdma_xid and rdma_start.rdma_credits
fields. The RPCRDMA2_F_RESPONSE flag enables a receiver to reliably
avoid performing an XID lookup on incoming reverse direction Call
messages, and apply the value of the rdma_start.rdma_credits field
correctly, based on the direction of the message being conveyed.
In general, when a message carries an XID that was generated by the
message's receiver (that is, the receiver is acting as a requester),
the message's sender sets the RPCRDMA2_F_RESPONSE flag. Otherwise
that flag is clear. For example:
o When the rdma_start.rdma_htype field has the value RDMA2_MSG or
RDMA2_NOMSG, the value of the RPCRDMA2_F_RESPONSE flag MUST be the
same as the value of the associated RPC message's msg_type field.
o When the header type is anything else and a whole or partial RPC
message payload is present, the value of the RPCRDMA2_F_RESPONSE
flag MUST be the same as the value of the associated RPC message's
msg_type field.
o When no RPC message payload is present, a Requester MUST set the
value of RPCRDMA2_F_RESPONSE to reflect how the receiver is to
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interpret the rdma_start.rdma_credits and rdma_start.rdma_xid
fields.
o When the rdma_start.rdma_htype field has the value RDMA2_ERROR,
the RPCRDMA2_F_RESPONSE flag MUST be set.
3.3. rpcrdma2_chunk_lists: Describe External Data Payload
The rpcrdma2_chunk_lists structure specifies how an RPC message is
conveyed using explicit RDMA operations.
<CODE BEGINS>
struct rpcrdma2_chunk_lists {
uint32 rdma_inv_handle;
struct rpcrdma2_read_list *rdma_reads;
struct rpcrdma2_write_list *rdma_writes;
struct rpcrdma2_write_chunk *rdma_reply;
};
<CODE ENDS>
For the most part this structure parallels its RPC-over-RDMA version
1 equivalent. That is, rdma_reads, rdma_writes, rdma_reply provide,
respectively, descriptions of the chunks used to read a Long request
or directly placed data from the requester, to write directly placed
response data into the requester's memory, and to write a long reply
into the requester's memory.
An important addition relative to the corresponding RPC-over-RDMA
version 1 rdma_header structures is the rdma_inv_handle field. This
field supports remote invalidation of requester memory registrations
via the RDMA Send With Invalidate operation.
To request Remote Invalidation, a requester sets the value of the
rdma_inv_handle field in an RPC Call's transport header to a non-zero
value that matches one of the rdma_handle fields in that header. If
none of the rdma_handle values in the header conveying the Call may
be invalidated by the responder, the requester sets the RPC Call's
rdma_inv_handle field to the value zero.
If the responder chooses not to use remote invalidation for this
particular RPC Reply, or the RPC Call's rdma_inv_handle field
contains the value zero, the responder uses RDMA Send to transmit the
matching RPC reply.
If a requester has provided a non-zero value in the RPC Call's
rdma_inv_handle field and the responder chooses to use Remote
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Invalidation for the matching RPC Reply, the responder uses RDMA Send
With Invalidate to transmit that RPC reply, and uses the value in the
corresponding Call's rdma_inv_handle field to construct the Send With
Invalidate Work Request.
4. Transport Properties
RPC-over-RDMA version 2 provides a mechanism for connection endpoints
to communicate information about implementation properties, enabling
compatible endpoints to optimize data transfer. Initially only a
small set of transport properties are defined and a single operation
is provided to exchange transport properties (see Section 5.3.4).
Both the set of transport properties and the operations used to
communicate may be extended. Within RPC-over-RDMA version 2, all
such extensions are OPTIONAL. For information about existing
transport properties, see Sections 4.1 through 4.2. For discussion
of extensions to the set of transport properties, see Section 9.2.
4.1. Transport Properties Model
A basic set of receiver and sender properties is specified in this
document. An extensible approach is used, allowing new properties to
be defined in future Standards Track documents.
Such properties are specified using:
o A code point identifying the particular transport property being
specified.
o A nominally opaque array which contains within it the XDR encoding
of the specific property indicated by the associated code point.
The following XDR types are used by operations that deal with
transport properties:
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<CODE BEGINS>
typedef rpcrdma2_propid uint32;
struct rpcrdma2_propval {
rpcrdma2_propid rdma_which;
opaque rdma_data<>;
};
typedef rpcrdma2_propval rpcrdma2_propset<>;
typedef uint32 rpcrdma2_propsubset<>;
<CODE ENDS>
An rpcrdma2_propid specifies a particular transport property. In
order to facilitate XDR extension of the set of properties by
concatenating XDR definition files, specific properties are defined
as const values rather than as elements in an enum.
An rpcrdma2_propval specifies a value of a particular transport
property with the particular property identified by rdma_which, while
the associated value of that property is contained within rdma_data.
An rdma_data field which is of zero length is interpreted as
indicating the default value or the property indicated by rdma_which.
While rdma_data is defined as opaque within the XDR, the contents are
interpreted (except when of length zero) using the XDR typedef
associated with the property specified by rdma_which. As a result,
when rpcrdma2_propval does not conform to that typedef, the receiver
is REQUIRED to return the error RDMA2_ERR_BAD_XDR using the header
type RDMA2_ERROR as described in Section 5.3.3. For example, the
receiver of a message containing a valid rpcrdma2_propval returns
this error if the length of rdma_data is such that it extends beyond
the bounds of the message being transferred.
In cases in which the rpcrdma2_propid specified by rdma_which is
understood by the receiver, the receiver also MUST report the error
RDMA2_ERR_BAD_XDR if either of the following occur:
o The nominally opaque data within rdma_data is not valid when
interpreted using the property-associated typedef.
o The length of rdma_data is insufficient to contain the data
represented by the property-associated typedef.
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Note that no error is to be reported if rdma_which is unknown to the
receiver. In that case, that rpcrdma2_propval is not processed and
processing continues using the next rpcrdma2_propval, if any.
A rpcrdma2_propset specifies a set of transport properties. No
particular ordering of the rpcrdma2_propval items within it is
imposed.
A rpcrdma2_propsubset identifies a subset of the properties in a
previously specified rpcrdma2_propset. Each bit in the mask denotes
a particular element in a previously specified rpcrdma2_propset. If
a particular rpcrdma2_propval is at position N in the array, then bit
number N mod 32 in word N div 32 specifies whether that particular
rpcrdma2_propval is included in the defined subset. Words beyond the
last one specified are treated as containing zero.
4.2. Current Transport Properties
Although the set of transport properties may be extended, a basic set
of transport properties is defined in Table 1.
In that table, the columns contain the following information:
o The column labeled "Property" identifies the transport property
described by the current row.
o The column labeled "Code" specifies the rpcrdma2_propid value used
to identify this property.
o The column labeled "XDR type" gives the XDR type of the data used
to communicate the value of this property. This data type
overlays the data portion of the nominally opaque field rdma_data
in a rpcrdma2_propval.
o The column labeled "Default" gives the default value for the
property which is to be assumed by those who do not receive, or
are unable to interpret, information about the actual value of the
property.
o The column labeled "Sec" indicates the section within this
document that explains the semantics and use of this transport
property.
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+---------+-----+------------------+----------------------+---------+
| Propert | Cod | XDR type | Default | Sec |
| y | e | | | |
+---------+-----+------------------+----------------------+---------+
| Receive | 1 | uint32 | 4096 | Section |
| Buffer | | | | 4.2.1 |
| Size | | | | |
| Reverse | 2 | enum rpcrdma2_rv | RDMA2_RVREQSUP_INLIN | Section |
| Request | | reqsup | E | 4.2.2 |
| Support | | | | |
+---------+-----+------------------+----------------------+---------+
Table 1
4.2.1. Receive Buffer Size
The Receive Buffer Size specifies the minimum size, in octets, of
pre-posted receive buffers. It is the responsibility of the endpoint
sending this value to ensure that its pre-posted receive buffers are
at least the size specified, allowing the endpoint receiving this
value to send messages that are of this size.
<CODE BEGINS>
const uint32 RDMA2_PROPID_RBSIZ = 1;
typedef uint32 rpcrdma2_prop_rbsiz;
<CODE ENDS>
The sender may use his knowledge of the receiver's buffer size to
determine when the message to be sent will fit in the preposted
receive buffers that the receiver has set up. In particular,
o Requesters may use the value to determine when it is necessary to
provide a Position-Zero Read chunk when sending a request.
o Requesters may use the value to determine when it is necessary to
provide a Reply chunk when sending a request, based on the maximum
possible size of the reply.
o Responders may use the value to determine when it is necessary,
given the actual size of the reply, to actually use a Reply chunk
provided by the requester.
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4.2.2. Reverse Request Support
The value of this property is used to indicate a client
implementation's readiness to accept and process messages that are
part of reverse direction RPC requests.
<CODE BEGINS>
enum rpcrdma2_rvreqsup {
RDMA2_RVREQSUP_NONE = 0,
RDMA2_RVREQSUP_INLINE = 1,
RDMA2_RVREQSUP_GENL = 2
};
const uint32 RDMA2_PROPID_BRS = 2;
typedef rpcrdma2_rvreqsup rpcrdma2_prop_brs;
<CODE ENDS>
Multiple levels of support are distinguished:
o The value RDMA2_RVREQSUP_NONE indicates that receipt of reverse
direction requests and replies is not supported.
o The value RDMA2_RVREQSUP_INLINE indicates that receipt of reverse
direction requests or replies is only supported using inline
messages and that use of explicit RDMA operations or other form of
Direct Data Placement for reverse direction requests or responses
is not supported.
o The value RDMA2_RVREQSUP_GENL that receipt of reverse direction
requests or replies is supported in the same ways that forward
direction requests or replies typically are.
When information about this property is not provided, the support
level of servers can be inferred from the reverse direction requests
that they issue, assuming that issuing a request implicitly indicates
support for receiving the corresponding reply. On this basis,
support for receiving inline replies can be assumed when requests
without Read chunks, Write chunks, or Reply chunks are issued, while
requests with any of these elements allow the client to assume that
general support for reverse direction replies is present on the
server.
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5. RPC-over-RDMA Version 2 Transport Messages
5.1. Overall Transport Message Structure
Each transport message consists of multiple sections:
o A transport header prefix, as defined in Section 3.2. Among other
things, this structure indicates the header type.
o The transport header proper, as defined by one of the sub-sections
below. See Section 5.2 for the mapping between header types and
the corresponding header structure.
o Potentially, an RPC message being conveyed as an addendum to the
header.
This organization differs from that presented in the definition of
RPC-over-RDMA version 1 [RFC8166], which presented the first and
second of the items above as a single XDR item. The new organization
is more in keeping with RPC-over-RDMA version 2's extensibility model
in that new header types can be defined without modifying the
existing set of header types.
5.2. Transport Header Types
The new header types within RPC-over-RDMA version 2 are set forth in
Table 2. In that table, the columns contain the following
information:
o The column labeled "Operation" specifies the particular operation.
o The column labeled "Code" specifies the value of header type for
this operation.
o The column labeled "XDR type" gives the XDR type of the data
structure used to describe the information in this new message
type. This data immediately follows the universal portion on the
transport header present in every RPC-over-RDMA transport header.
o The column labeled "Msg" indicates whether this operation is
followed (or not) by an RPC message payload.
o The column labeled "Sec" indicates the section (within this
document) that explains the semantics and use of this operation.
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+----------------------+------+-------------------+-----+-----------+
| Operation | Code | XDR type | Msg | Sec |
+----------------------+------+-------------------+-----+-----------+
| Convey Appended RPC | 0 | rpcrdma2_msg | Yes | Section |
| Message | | | | 5.3.1 |
| Convey External RPC | 1 | rpcrdma2_nomsg | No | Section |
| Message | | | | 5.3.2 |
| Report Transport | 4 | rpcrdma2_err | No | Section |
| Error | | | | 5.3.3 |
| Specify Properties | 5 | rpcrdma2_connprop | No | Section |
| at Connection | | | | 5.3.4 |
+----------------------+------+-------------------+-----+-----------+
Table 2
Suppport for the operations in Table 2 is REQUIRED. Support for
additional operations will be OPTIONAL. RPC-over-RDMA version 2
implementations that receive an OPTIONAL operation that is not
supported MUST respond with an RDMA2_ERROR message with an error code
of RDMA2_ERR_INVAL_HTYPE.
5.3. Header Types Defined in RPC-over-RDMA version 2
The header types defined and used in RPC-over-RDMA version 1 are all
carried over into RPC-over-RDMA version 2, although there may be
limited changes in the definition of existing header types.
In comparison with the header types of RPC-over-RDMA version 1, the
changes can be summarized as follows:
o To simplify interoperability with RPC-over-RDMA version 1, only
the RDMA2_ERROR header (defined in Section 5.3.3) has an XDR
definition that differs from that in RPC-over-RDMA version 1, and
its modifications are all compatible extensions.
o RDMA2_MSG and RDMA2_NOMSG (defined in Sections Section 5.3.1 and
Section 5.3.2) have XDR definitions that match the corresponding
RPC-over-RDMA version 1 header types. However, because of the
changes to the header prefix, the version 1 and version 2 header
types differ in on-the-wire format.
o RDMA2_CONNPROP (defined in Section 5.3.4) is a completely new
header type devoted to enabling connection peers to exchange
information about their transport properties.
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5.3.1. RDMA2_MSG: Convey RPC Message Inline
RDMA2_MSG is used to convey an RPC message that immediately follows
the Transport Header in the Send buffer. This is either an RPC
request that has no Position-Zero Read chunk or an RPC reply that is
not sent using a Reply chunk.
<CODE BEGINS>
const rpcrdma2_proc RDMA2_MSG = 0;
struct rpcrdma2_msg {
struct rpcrdma2_chunk_lists rdma_chunks;
/* The rpc message starts here and continues
* through the end of the transmission. */
uint32 rdma_rpc_first_word;
};
<CODE ENDS>
5.3.2. RDMA2_NOMSG: Convey External RPC Message
RDMA2_NOMSG is used to convey an entire RPC message using explicit
RDMA operations. Usually this is because the RPC message does not
fit within the size limits that result from the receiver's inline
threshold. The message may be a Long request, which is read from a
memory area specified by a Position-Zero Read chunk; or a Long reply,
which is written into a memory area specified by a Reply chunk.
<CODE BEGINS>
const rpcrdma2_proc RDMA2_NOMSG = 1;
struct rpcrdma2_nomsg {
struct rpcrdma2_chunk_lists rdma_chunks;
};
<CODE ENDS>
5.3.3. RDMA2_ERROR: Report Transport Error
RDMA2_ERROR provides a way of reporting the occurrence of transport
errors on a previous transmission. This header type MUST NOT be
transmitted by a requester. [ cel: how is the XID field set when
sending an error report from a requester, or when the error occurred
on a non-RPC message? ]
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<CODE BEGINS>
const rpcrdma2_proc RDMA2_ERROR = 4;
struct rpcrdma2_err_vers {
uint32 rdma_vers_low;
uint32 rdma_vers_high;
};
struct rpcrdma2_err_write {
uint32 rdma_chunk_index;
uint32 rdma_length_needed;
};
union rpcrdma2_error switch (rpcrdma2_errcode rdma_err) {
case RDMA2_ERR_VERS:
rpcrdma2_err_vers rdma_vrange;
case RDMA2_ERR_READ_CHUNKS:
uint32 rdma_max_chunks;
case RDMA2_ERR_WRITE_CHUNKS:
uint32 rdma_max_chunks;
case RDMA2_ERR_SEGMENTS:
uint32 rdma_max_segments;
case RDMA2_ERR_WRITE_RESOURCE:
rpcrdma2_err_write rdma_writeres;
case RDMA2_ERR_REPLY_RESOURCE:
uint32 rdma_length_needed;
default:
void;
};
<CODE ENDS>
Error reporting is addressed in RPC-over-RDMA version 2 in a fashion
similar to RPC-over-RDMA version 1. Several new error codes, and
error messages never flow from requester to responder. RPC-over-RDMA
version 1 error reporting is described in Section 5 of [RFC8166].
In all cases below, the responder copies the values of the
rdma_start.rdma_xid and rdma_start.rdma_vers fields from the incoming
transport header that generated the error to transport header of the
error response. The responder sets the rdma_start.rdma_htype field
of the transport header prefix to RDMA2_ERROR, and the
rdma_start.rdma_credit field is set to the credit grant value for
this connection. The receiver of this header type MUST ignore the
value of the rdma_start.rdma_credits field.
RDMA2_ERR_VERS
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This is the equivalent of ERR_VERS in RPC-over-RDMA version 1.
The error code value, semantics, and utilization are the same.
RDMA2_ERR_INVAL_HTYPE
If a responder recognizes the value in the rdma_start.rdma_vers
field, but it does not recognize the value in the
rdma_start.rdma_htype field or does not support that header type,
it MUST set the rdma_err field to RDMA2_ERR_INVAL_HTYPE.
RDMA2_ERR_BAD_XDR
If a responder recognizes the values in the rdma_start.rdma_vers
and rdma_start.rdma_proc fields, but the incoming RPC-over-RDMA
transport header cannot be parsed, it MUST set the rdma_err field
to RDMA2_ERR_BAD_XDR. This includes cases in which a nominally
opaque property value field cannot be parsed using the XDR typedef
associated with the transport property definition. The error code
value of RDMA2_ERR_BAD_XDR is the same as the error code value of
ERR_CHUNK in RPC-over-RDMA version 1. The responder MUST NOT
process the request in any way except to send an error message.
RDMA2_ERR_READ_CHUNKS
If a requester presents more DDP-eligible arguments than the
responder is prepared to Read, the responder MUST set the rdma_err
field to RDMA2_ERR_READ_CHUNKS, and set the rdma_max_chunks field
to the maximum number of Read chunks the responder can receive and
process.
If the responder implementation cannot handle any Read chunks for
a request, it MUST set the rdma_max_chunks to zero in this
response. The requester SHOULD resend the request using a
Position-Zero Read chunk. If this was a request using a Position-
Zero Read chunk, the requester MUST terminate the transaction with
an error.
RDMA2_ERR_WRITE_CHUNKS
If a requester has constructed an RPC Call message with more DDP-
eligible results than the server is prepared to Write, the
responder MUST set the rdma_err field to RDMA2_ERR_WRITE_CHUNKS,
and set the rdma_max_chunks field to the maximum number of Write
chunks the responder can process and return.
If the responder implementation cannot handle any Write chunks for
a request, it MUST return a response of RDMA2_ERR_REPLY_RESOURCE
(below). The requester SHOULD resend the request with no Write
chunks and a Reply chunk of appropriate size.
RDMA2_ERR_SEGMENTS
If a requester has constructed an RPC Call message with a chunk
that contains more segments than the responder supports, the
responder MUST set the rdma_err field to RDMA2_ERR_SEGMENTS, and
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set the rdma_max_segments field to the maximum number of segments
the responder can process.
RDMA2_ERR_WRITE_RESOURCE
If a requester has provided a Write chunk that is not large enough
to fully convey a DDP-eligible result, the responder MUST set the
rdma_err field to RDMA2_ERR_WRITE_RESOURCE.
The responder MUST set the rdma_chunk_index field to point to the
first Write chunk in the transport header that is too short, or to
zero to indicate that it was not possible to determine which chunk
is too small. Indexing starts at one (1), which represents the
first Write chunk. The responder MUST set the rdma_length_needed
to the number of bytes needed in that chunk in order to convey the
result data item.
Upon receipt of this error code, a responder MAY choose to
terminate the operation (for instance, if the responder set the
index and length fields to zero), or it MAY send the request again
using the same XID and more reply resources.
RDMA2_ERR_REPLY_RESOURCE
If an RPC Reply's Payload stream does not fit inline and the
requester has not provided a large enough Reply chunk to convey
the stream, the responder MUST set the rdma_err field to
RDMA2_ERR_REPLY_RESOURCE. The responder MUST set the
rdma_length_needed to the number of Reply chunk bytes needed to
convey the reply.
Upon receipt of this error code, a responder MAY choose to
terminate the operation (for instance, if the responder set the
index and length fields to zero), or it MAY send the request again
using the same XID and larger reply resources.
RDMA2_ERR_SYSTEM
If some problem occurs on a responder that does not fit into the
above categories, the responder MAY report it to the sender by
setting the rdma_err field to RDMA2_ERR_SYSTEM.
This is a permanent error: a requester that receives this error
MUST terminate the RPC transaction associated with the XID value
in the rdma_start.rdma_xid field.
5.3.4. RDMA2_CONNPROP: Advertise Transport Properties
The RDMA2_CONNPROP message type allows an RPC-over-RDMA endpoint,
whether client or server, to indicate to its partner relevant
transport properties that the partner might need to be aware of.
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The message definition for this operation is as follows:
<CODE BEGINS>
struct rpcrdma2_connprop {
rpcrdma2_propset rdma_props;
};
<CODE ENDS>
All relevant transport properties that the sender is aware of should
be included in rdma_props. Since support of each of the properties
is OPTIONAL, the sender cannot assume that the receiver will
necessarily take note of these properties. The sender should be
prepared for cases in which the receiver continues to assume that the
default value for a particular property is still in effect.
Generally, a participant will send a RDMA2_CONNPROP message as the
first message after a connection is established. Given that fact,
the sender should make sure that the message can be received by peers
who use the default Receive Buffer Size. The connection's initial
receive buffer size is typically 1KB, but it depends on the initial
connection state of the RPC-over-RDMA version in use.
Properties not included in rdma_props are to be treated by the peer
endpoint as having the default value and are not allowed to change
subsequently. The peer should not request changes in such
properties.
Those receiving an RDMA2_CONNPROP may encounter properties that they
do not support or are unaware of. In such cases, these properties
are simply ignored without any error response being generated.
6. XDR Protocol Definition
This section contains a description of the core features of the RPC-
over-RDMA version 2 protocol expressed in the XDR language [RFC4506].
Because of the need to provide for protocol extensibility without
modifying an existing XDR definition, this description has some
important structural differences from the corresponding XDR
description for RPC-over-RDMA version 1, which appears in [RFC8166].
This description is divided into three parts:
o A code component license which appears in Section 6.1.
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o An XDR description of the structures that are generally available
for use by transport header types including both those defined in
this document and those that may be defined as extensions. This
includes definitions of the chunk-related structures derived from
RPC-over-RDMA version 1, the transport property model introduced
in this document, and a definition of the transport header
prefixes that precede the various transport header types. This
appears in Section 6.3.
o An XDR description of the transport header types defined in this
document, including those derived from RPC-over-RDMA version 1 and
those introduced in RPC-over-RDMA version 2. This appears in
Section 6.4.
This description is provided in a way that makes it simple to extract
into ready-to-compile form. To enable the combination of this
description with the descriptions of subsequent extensions to RPC-
over-RDMA version 2, the extracted description can be combined with
similar descriptions published later, or those descriptions can be
compiled separately. Refer to Section 6.2 for details.
6.1. Code Component License
Code components extracted from this document must include the
following license text. When the extracted XDR code is combined with
other complementary XDR code which itself has an identical license,
only a single copy of the license text need be preserved.
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<CODE BEGINS>
/// /*
/// * Copyright (c) 2010-2018 IETF Trust and the persons
/// * identified as authors of the code. All rights reserved.
/// *
/// * The authors of the code are:
/// * B. Callaghan, T. Talpey, C. Lever, and D. Noveck.
/// *
/// * Redistribution and use in source and binary forms, with
/// * or without modification, are permitted provided that the
/// * following conditions are met:
/// *
/// * - Redistributions of source code must retain the above
/// * copyright notice, this list of conditions and the
/// * following disclaimer.
/// *
/// * - Redistributions in binary form must reproduce the above
/// * copyright notice, this list of conditions and the
/// * following disclaimer in the documentation and/or other
/// * materials provided with the distribution.
/// *
/// * - Neither the name of Internet Society, IETF or IETF
/// * Trust, nor the names of specific contributors, may be
/// * used to endorse or promote products derived from this
/// * software without specific prior written permission.
/// *
/// * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS
/// * AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED
/// * WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
/// * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
/// * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO
/// * EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
/// * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
/// * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
/// * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
/// * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
/// * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
/// * LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
/// * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING
/// * IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
/// * ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
/// */
///
<CODE ENDS>
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6.2. Extraction and Use of XDR Definitions
The reader can apply the following sed script to this document to
produce a machine-readable XDR description of the RPC-over-RDMA
version 2 protocol without any OPTIONAL extensions.
<CODE BEGINS>
sed -n -e 's:^ */// ::p' -e 's:^ *///$::p'
<CODE ENDS>
That is, if this document is in a file called "spec.txt" then the
reader can do the following to extract an XDR description file and
store it in the file rpcrdma-v2.x.
<CODE BEGINS>
sed -n -e 's:^ */// ::p' -e 's:^ *///$::p' \
< spec.txt > rpcrdma-v2.x
<CODE ENDS>
Although this file is a usable description of the base protocol, when
extensions are to supported, it may be desirable to divide into
multiple files. The following script can be used for that purpose:
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<CODE BEGINS>
#!/usr/local/bin/perl
open(IN,"rpcrdma-v2.x");
open(OUT,">temp.x");
while(<IN>)
{
if (m/FILE ENDS: (.*)$/)
{
close(OUT);
rename("temp.x", $1);
open(OUT,">temp.x");
}
else
{
print OUT $_;
}
}
close(IN);
close(OUT);
<CODE ENDS>
Running the above script will result in two files:
o The file common.x, containing the license plus the common XDR
definitions which need to be made available to both the base
operations and any subsequent extensions.
o The file baseops.x containing the XDR definitions for the base
operations, defined in this document.
Optional extensions to RPC-over-RDMA version 2, published as
Standards Track documents, will have similar means of providing XDR
that describes those extensions. Once XDR for all desired extensions
is also extracted, it can be appended to the XDR description file
extracted from this document to produce a consolidated XDR
description file reflecting all extensions selected for an RPC-over-
RDMA implementation.
Alternatively, the XDR descriptions can be compiled separately. In
this case the combination of common.x and baseops.x serves to define
the base transport, while using as XDR descriptions for extensions,
the XDR from the document defining that extension, together with the
file common.x, obtained from this document.
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6.3. XDR Definition for RPC-over-RDMA Version 2 Core Structures
<CODE BEGINS>
/// /*******************************************************************
/// * Transport Header Prefixes
/// ******************************************************************/
///
/// struct rpcrdma_common {
/// uint32 rdma_xid;
/// uint32 rdma_vers;
/// uint32 rdma_credit;
/// uint32 rdma_htype;
/// };
///
/// const RPCRDMA2_F_RESPONSE 0x00000001;
///
/// struct rpcrdma2_hdr_prefix
/// struct rpcrdma_common rdma_start;
/// uint32 rdma_flags;
/// };
///
/// /*******************************************************************
/// * Chunks and Chunk Lists
/// ******************************************************************/
///
/// struct rpcrdma2_segment {
/// uint32 rdma_handle;
/// uint32 rdma_length;
/// uint64 rdma_offset;
/// };
///
/// struct rpcrdma2_read_segment {
/// uint32 rdma_position;
/// struct rpcrdma2_segment rdma_target;
/// };
///
/// struct rpcrdma2_read_list {
/// struct rpcrdma2_read_segment rdma_entry;
/// struct rpcrdma2_read_list *rdma_next;
/// };
///
/// struct rpcrdma2_write_chunk {
/// struct rpcrdma2_segment rdma_target<>;
/// };
///
/// struct rpcrdma2_write_list {
/// struct rpcrdma2_write_chunk rdma_entry;
/// struct rpcrdma2_write_list *rdma_next;
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/// };
///
/// struct rpcrdma2_chunk_lists {
/// uint32 rdma_inv_handle;
/// struct rpcrdma2_read_list *rdma_reads;
/// struct rpcrdma2_write_list *rdma_writes;
/// struct rpcrdma2_write_chunk *rdma_reply;
/// };
///
/// /*******************************************************************
/// * Transport Properties
/// ******************************************************************/
///
/// /*
/// * Types for transport properties model
/// */
/// typedef rpcrdma2_propid uint32;
///
/// struct rpcrdma2_propval {
/// rpcrdma2_propid rdma_which;
/// opaque rdma_data<>;
/// };
///
/// typedef rpcrdma2_propval rpcrdma2_propset<>;
/// typedef uint32 rpcrdma2_propsubset<>;
///
/// /*
/// * Transport propid values for basic properties
/// */
/// const uint32 RDMA2_PROPID_RBSIZ = 1;
/// const uint32 RDMA2_PROPID_BRS = 2;
///
/// /*
/// * Types specific to particular properties
/// */
/// typedef uint32 rpcrdma2_prop_rbsiz;
/// typedef rpcrdma2_rvreqsup rpcrdma2_prop_brs;
///
/// enum rpcrdma2_rvreqsup {
/// RDMA2_RVREQSUP_NONE = 0,
/// RDMA2_RVREQSUP_INLINE = 1,
/// RDMA2_RVREQSUP_GENL = 2
/// };
///
/// /* FILE ENDS: common.x; */
<CODE ENDS>
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6.4. XDR Definition for RPC-over-RDMA Version 2 Base Header Types
<CODE BEGINS>
/// /*******************************************************************
/// * Descriptions of RPC-over-RDMA Header Types
/// ******************************************************************/
///
/// /*
/// * Header Type Codes.
/// */
/// const rpcrdma2_proc RDMA2_MSG = 0;
/// const rpcrdma2_proc RDMA2_NOMSG = 1;
/// const rpcrdma2_proc RDMA2_ERROR = 4;
/// const rpcrdma2_proc RDMA2_CONNPROP = 5;
///
/// /*
/// * Header Types to Convey RPC Messages.
/// */
/// struct rpcrdma2_msg {
/// struct rpcrdma2_chunk_lists rdma_chunks;
///
/// /* The rpc message starts here and continues
/// * through the end of the transmission. */
/// uint32 rdma_rpc_first_word;
/// };
///
/// struct rpcrdma2_nomsg {
/// struct rpcrdma2_chunk_lists rdma_chunks;
/// };
///
/// /*
/// * Header Type to Report Errors.
/// */
/// const uint32 RDMA2_ERR_VERS = 1;
/// const uint32 RDMA2_ERR_BAD_XDR = 2;
/// const uint32 RDMA2_ERR_INVAL_HTYPE = 3;
/// const uint32 RDMA2_ERR_READ_CHUNKS = 4;
/// const uint32 RDMA2_ERR_WRITE_CHUNKS = 5;
/// const uint32 RDMA2_ERR_SEGMENTS = 6;
/// const uint32 RDMA2_ERR_WRITE_RESOURCE = 7;
/// const uint32 RDMA2_ERR_REPLY_RESOURCE = 8;
/// const uint32 RDMA2_ERR_SYSTEM = 9;
///
/// struct rpcrdma2_err_vers {
/// uint32 rdma_vers_low;
/// uint32 rdma_vers_high;
/// };
///
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/// struct rpcrdma2_err_write {
/// uint32 rdma_chunk_index;
/// uint32 rdma_length_needed;
/// };
///
/// union rpcrdma2_error switch (rpcrdma2_errcode rdma_err) {
/// case RDMA2_ERR_VERS:
/// rpcrdma2_err_vers rdma_vrange;
/// case RDMA2_ERR_READ_CHUNKS:
/// uint32 rdma_max_chunks;
/// case RDMA2_ERR_WRITE_CHUNKS:
/// uint32 rdma_max_chunks;
/// case RDMA2_ERR_SEGMENTS:
/// uint32 rdma_max_segments;
/// case RDMA2_ERR_WRITE_RESOURCE:
/// rpcrdma2_err_write rdma_writeres;
/// case RDMA2_ERR_REPLY_RESOURCE:
/// uint32 rdma_length_needed;
/// default:
/// void;
/// };
///
/// /*
/// * Header Type to Exchange Transport Properties.
/// */
/// struct rpcrdma2_connprop {
/// rpcrdma2_propset rdma_props;
/// };
///
/// /* FILE ENDS: baseops.x; */
<CODE ENDS>
6.5. Use of the XDR Description Files
The three files common.x and baseops.x, when combined with the XDR
descriptions for extension defined later, produce a human-readable
and compilable description of the RPC-over-RDMA version 2 protocol
with the included extensions.
Although this XDR description can be useful in generating code to
encode and decode the transport and payload streams, there are
elements of the structure of RPC-over-RDMA version 2 which are not
expressible within the XDR language as currently defined. This
requires implementations that use the output of the XDR processor to
provide additional code to bridge the gaps.
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o The values of transport properties are represented within XDR as
opaque values. However, the actual structures of each of the
properties are represented by XDR typedefs, with the selection of
the appropriate typedef described by text in this document. The
determination of the appropriate typedef is not specified by XDR,
which does not possess the facilities necessary for that
determination to be specified in an extensible way.
This is similar to the way in which NFSv4 attributes are handled
[RFC7530] [RFC5661]. As in that case, implementations that need
to encode and decode these nominally opaque entities need to use
the protocol description to determine the actual XDR
representation that underlays the items described as opaque.
o The transport stream is not represented as a single XDR object.
Instead, the header prefix is described by one XDR object while
the rest of the header is described as another XDR object with the
mapping between the header type in the header prefix and the XDR
object representing the header type represented by tables
contained in this document, with additional mappings being
specifiable by a later extension document.
This situation is similar to that in which RPC message headers
contain program and procedure numbers, so that the XDR for those
request and replies can be used to encode and decode the
associated messages without requiring that all be present in a
single XDR specification. As in that case, implementations need
to use the header specification to select the appropriate XDR-
generated code to be used in message processing.
o The relationship between the transport stream and the payload
stream is not specified in the XDR itself, although comments
within the XDR text make clear where transported messages,
described by their own XDR, need to appear. Such data by its
nature is opaque to the transport, although its form differs XDR
opaque arrays.
Potential extensions allowing continuation of RPC messages across
transport message boundaries will require that message assembly
facilities, not specifiable within XDR, also be part of transport
implementations.
To summarize, the role of XDR in this specification is more limited
than for protocols which are themselves XDR programs, where the
totality of the protocol is expressible within the XDR paradigm
established for that purpose. This more limited role reflects the
fact that XDR lacks facilities to represent the embedding of
transported material within the transport framework. In addition,
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the need to cleanly accommodate extensions has meant that those using
rpcgen in their applications need to take a more active role in
providing the facilities that cannot be expressed within XDR.
7. Protocol Version Negotiation
When an RPC-over-RDMA version 2 client establishes a connection to a
server, its first order of business is to determine the server's
highest supported protocol version.
As with RPC-over-RDMA version 1, upon connection establishment a
client MUST NOT send more than a single RPC-over-RDMA message at a
time until it receives a valid non-error RPC-over-RDMA message from
the server that grants client credits.
The second word of each transport header is used to convey the
transport protocol version. In the interest of simplicity, we refer
to that word as rdma_vers even though in the RPC-over-RDMA version 2
XDR definition it is described as rdma_start.rdma_vers.
First, the client sends a single valid RPC-over-RDMA message with the
value two (2) in the rdma_vers field. Because the server might
support only RPC-over-RDMA version 1, this initial message can be no
larger than the version 1 default inline threshold of 1024 bytes.
7.1. Server Does Support RPC-over-RDMA Version 2
If the server does support RPC-over-RDMA version 2, it sends RPC-
over-RDMA messages back to the client with the value two (2) in the
rdma_vers field. Both peers may use the default inline threshold
value for RPC-over-RDMA version 2 connections (4096 bytes).
7.2. Server Does Not Support RPC-over-RDMA Version 2
If the server does not support RPC-over-RDMA version 2, it MUST send
an RPC-over-RDMA message to the client with the same XID, with
RDMA2_ERROR in the rdma_start.rdma_htype field, and with the error
code RDMA2_ERR_VERS. This message also reports a range of protocol
versions that the server supports. To continue operation, the client
selects a protocol version in the range of server-supported versions
for subsequent messages on this connection.
If the connection is lost immediately after an RDMA2_ERROR /
RDMA2_ERR_VERS message is received, a client can avoid a possible
version negotiation loop when re-establishing another connection by
assuming that particular server does not support RPC-over-RDMA
version 2. A client can assume the same situation (no server support
for RPC-over-RDMA version 2) if the initial negotiation message is
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lost or dropped. Once the negotiation exchange is complete, both
peers may use the default inline threshold value for the transport
protocol version that has been selected.
7.3. Client Does Not Support RPC-over-RDMA Version 2
If the server supports the RPC-over-RDMA protocol version used in
Call messages from a client, it MUST send Replies with the same RPC-
over-RDMA protocol version that the client uses to send its Calls.
The client MUST NOT change the version during the duration of the
connection.
8. Differences from the RPC-over-RDMA Version 1 Protocol
This section describes the substantive changes made in RPC-over-RDMA
version 2, as opposed to the structural changes to enable
extensibility, which are discussed in Section 10.1.
8.1. Transport Properties
RPC-over-RDMA version 2 provides a mechanism for exchanging the
transport's operational properties. This mechanism allows connection
endpoints to communicate the properties of their implementation at
connection setup. The mechanism could be expanded to enable an
endpoint to request changes in properties of the other endpoint and
to notify peer endpoints of changes to properties that occur during
operation. Transport properties are described in Section 4.
8.2. Credit Management Changes
RPC-over-RDMA transports employ credit-based flow control to ensure
that a requester does not emit more RDMA Sends than the responder is
prepared to receive. Section 3.3.1 of [RFC8166] explains the purpose
and operation of RPC-over-RDMA version 1 credit management in detail.
In the RPC-over-RDMA version 1 design, each RDMA Send from a
requester contains an RPC Call with a credit request, and each RDMA
Send from a responder contains an RPC Reply with a credit grant. The
credit grant implies that enough Receives have been posted on the
responder to handle the credit grant minus the number of pending RPC
transactions (the number of remaining Receive buffers might be zero).
In other words, each RPC Reply acts as an implicit ACK for a previous
RPC Call from the requester, indicating that the responder has posted
a Receive to replace the Receive consumed by the requester's RDMA
Send. Without an RPC Reply message, the requester has no way to know
that the responder is properly prepared for subsequent RPC Calls.
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Aside from being a bit of a layering violation, there are basic (but
rare) cases where this arrangement is inadequate:
o When a requester retransmits an RPC Call on the same connection as
an earlier RPC Call for the same transaction.
o When a requester transmits an RPC operation that requires no
reply.
o When more than one RPC-over-RDMA message is needed to complete the
transaction (e.g., RDMA_DONE).
Typically, the connection must be replaced in these cases. This
resets the credit accounting mechanism but has an undesirable impact
on other ongoing RPC transactions on that connection.
Because credit management accompanies each RPC message, there is a
strict one-to-one ratio between RDMA Send and RPC message. There are
interesting use cases that might be enabled if this relationship were
more flexible:
o RPC-over-RDMA operations which do not carry an RPC message; e.g.,
control plane operations.
o A single RDMA Send that conveys more than one RPC message for the
purpose of interrupt mitigation.
o An RPC message that is conveyed via several sequential RDMA Sends
to reduce the use of explicit RDMA operations for moderate-sized
RPC messages.
o An RPC transaction that needs multiple exchanges or an odd number
of RPC-over-RDMA operations to complete.
Bi-directional RPC operation also introduces an ambiguity. If the
RPC-over-RDMA message does not carry an RPC message, then it is not
possible to determine whether the sender is a requester or a
responder, and thus whether the rdma_credit field contains a credit
request or a credit grant.
A more sophisticated credit accounting mechanism is provided in RPC-
over-RDMA version 2 in an attempt to address some of these
shortcomings. This new mechanism is detailed in Section TBD.
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8.3. Inline Threshold Changes
The term "inline threshold" is defined in Section 3.3.2 of [RFC8166].
An "inline threshold" value is the largest message size (in octets)
that can be conveyed on an RDMA connection using only RDMA Send and
Receive. Each connection has two inline threshold values: one for
messages flowing from client-to-server (referred to as the "client-
to-server inline threshold") and one for messages flowing from
server-to-client (referred to as the "server-to-client inline
threshold"). Note that [RFC8166] uses somewhat different
terminology. This is because it was written with only forward-
direction RPC transactions in mind.
A connection's inline thresholds determine when RDMA Read or Write
operations are required because the RPC message to be sent cannot be
conveyed via a single RDMA Send and Receive pair. When an RPC
message does not contain DDP-eligible data items, a requester
prepares a Long Call or Reply to convey the whole RPC message using
RDMA Read or Write operations.
RDMA Read and Write operations require that each data payload resides
in a region of memory that is registered with the RNIC. When an RPC
is complete, that region is invalidated, fencing it from the
responder. Memory registration and invalidation typically have a
latency cost that is insignificant compared to data handling costs.
When a data payload is small, however, the cost of registering and
invalidating the memory where the payload resides becomes a
relatively significant part of total RPC latency. Therefore the most
efficient operation of RPC-over-RDMA occurs when explicit RDMA Read
and Write operations are used for large payloads, and are avoided for
small payloads.
When RPC-over-RDMA version 1 was conceived, the typical size of RPC
messages that did not involve a significant data payload was under
500 bytes. A 1024-byte inline threshold adequately minimized the
frequency of inefficient Long Calls and Replies.
With NFS version 4.1 [RFC5661], the increased size of NFS COMPOUND
operations resulted in RPC messages that are on average larger and
more complex than previous versions of NFS. With 1024-byte inline
thresholds, RDMA Read or Write operations are needed for frequent
operations that do not bear a data payload, such as GETATTR and
LOOKUP, reducing the efficiency of the transport.
To reduce the need to use Long Calls and Replies, RPC-over-RDMA
version 2 increases the default size of inline thresholds. This also
increases the maximum size of reverse-direction RPC messages.
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8.4. Support for Remote Invalidation
An STag that is registered using the FRWR mechanism in a privileged
execution context or is registered via a Memory Window in an
unprivileged context may be invalidated remotely [RFC5040]. These
mechanisms are available when a requester's RNIC supports
MEM_MGT_EXTENSIONS.
For the purposes of this discussion, there are two classes of STags.
Dynamically-registered STags are used in a single RPC, then
invalidated. Persistently-registered STags live longer than one RPC.
They may persist for the life of an RPC-over-RDMA connection, or
longer.
An RPC-over-RDMA requester may provide more than one STag in one
transport header. It may provide a combination of dynamically- and
persistently-registered STags in one RPC message, or any combination
of these in a series of RPCs on the same connection. Only
dynamically-registered STags using Memory Windows or FRWR (i.e.,
registered via MEM_MGT_EXTENSIONS) may be invalidated remotely.
There is no transport-level mechanism by which a responder can
determine how a requester-provided STag was registered, nor whether
it is eligible to be invalidated remotely. A requester that mixes
persistently- and dynamically-registered STags in one RPC, or mixes
them across RPCs on the same connection, must therefore indicate
which handles may be invalidated via a mechanism provided in the
Upper Layer Protocol. RPC-over-RDMA version 2 provides such a
mechanism.
The RDMA Send With Invalidate operation is used to invalidate an STag
on a remote system. It is available only when a responder's RNIC
supports MEM_MGT_EXTENSIONS, and must be utilized only when a
requester's RNIC supports MEM_MGT_EXTENSIONS (can receive and
recognize an IETH).
8.4.1. Reverse Direction Remote Invalidation
Existing RPC-over-RDMA transport protocol specifications [RFC8166]
[RFC8167] do not forbid direct data placement in the reverse
direction, even though there is currently no Upper Layer Protocol
that makes data items in reverse direction operations elegible for
direct data placement.
When chunks are present in a reverse direction RPC request, Remote
Invalidation allows the responder to trigger invalidation of a
requester's STags as part of sending a reply, the same way as is done
in the forward direction.
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However, in the reverse direction, the server acts as the requester,
and the client is the responder. The server's RNIC, therefore, must
support receiving an IETH, and the server must have registered the
STags with an appropriate registration mechanism.
8.5. Error Reporting Changes
RPC-over-RDMA version 2 expands the repertoire of errors that may be
reported by connection endpoints. This change, which is structured
to enable extensibility, allows a peer to report overruns of specific
resources and to avoid requester retries when an error is permanent.
9. Extending the Version 2 Protocol
RPC-over-RDMA version 2 is designed to be extensible in a way that
enables the addition of OPTIONAL features that may subsequently be
converted to REQUIRED status in a future protocol version. The
protocol may be extended by Standards Track documents in a way
analogous to that provided for Network File System Version 4 as
described in [RFC8178].
This form of extensibility enables limited extensions to the base
RPC-over-RDMA version 2 protocol presented in this document so that
new optional capabilities can be introduced without a protocol
version change, while maintaining robust interoperability with
existing RPC-over-RDMA version 2 implementations. The design allows
extensions to be defined, including the definition of new protocol
elements, without requiring modification or recompilation of the
existing XDR.
A Standards Track document introduces each set of such protocol
elements. Together these elements are considered an OPTIONAL
feature. Each implementation is either aware of all the protocol
elements introduced by that feature or is aware of none of them.
Documents describing extensions to RPC-over-RDMA version 2 should
contain:
o An explanation of the purpose and use of each new protocol element
added.
o An XDR description including all of the new protocol elements, and
a script to extract it.
o A description of interactions with existing extensions.
This includes possible requirements of other OPTIONAL features to
be present for new protocol elements to work, or that a particular
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level of support for an OPTIONAL facility is required for the new
extension to work.
Implementers combine the XDR descriptions of the new features they
intend to use with the XDR description of the base protocol in this
document. This may be necessary to create a valid XDR input file
because extensions are free to use XDR types defined in the base
protocol, and later extensions may use types defined by earlier
extensions.
The XDR description for the RPC-over-RDMA version 2 base protocol
combined with that for any selected extensions should provide an
adequate human-readable description of the extended protocol.
The base protocol specified in this document may be extended within
RPC-over-RDMA version 2 in two ways:
o New OPTIONAL transport header types may be introduced by later
Standards Track documents. Such transport header types will be
documented as described in Section 9.1.
o New OPTIONAL transport properties may be defined in later
Standards Track documents. Such transport properties will be
documented as described in Section 9.2.
The following sorts of ancillary protocol elements may be added to
the protocol to support the addition of new transport properties and
header types.
o New error codes may be created as described in Section 9.3.
o New flags to use within the rdma_flags field may be created as
described in Section 9.4.
New capabilities can be proposed and developed independently of each
other, and implementers can choose among them. This makes it
straightforward to create and document experimental features and then
bring them through the standards process.
9.1. Adding New Header Types to RPC-over-RDMA Version 2
New transport header types are to defined in a manner similar to the
way existing ones are described in Sections Section 5.3.1 through
Section 5.3.4 Specifically what is needed is:
o A description of the function and use of the new header type.
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o A complete XDR description of the new header type including a
description of the use of all fields within the header.
o A description of how errors are reported, including the definition
of a mechanism for reporting errors when the error is outside the
available choices already available in the base protocol or in
other existing extensions.
o An indication of whether a Payload stream must be present, and a
description of its contents and how such payload streams are used
to construct RPC messages for processing.
In addition, there needs to be additional documentation that is made
necessary due to the Optional status of new transport header types.
o Information about constraints on support for the new header types
should be provided. For example, if support for one header type
is implied or foreclosed by another one, this needs to be
documented.
o A preferred method by which a sender should determine whether the
peer supports a particular header type needs to be provided.
While it is always possible for a send a test invocation of a
particular header type to see if support is available, when more
efficient means are available (e.g. the value of a transport
property, this should be noted.
9.2. Adding New Transport properties to the Protocol
The set of transport properties is designed to be extensible. As a
result, once new properties are defined in standards track documents,
the operations defined in this document may reference these new
transport properties, as well as the ones described in this document.
A standards track document defining a new transport property should
include the following information paralleling that provided in this
document for the transport properties defined herein.
o The rpcrdma2_propid value used to identify this property.
o The XDR typedef specifying the form in which the property value is
communicated.
o A description of the transport property that is communicated by
the sender of RDMA2_CONNPROP.
o An explanation of how this knowledge could be used by the peer
receiving this information.
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The definition of transport property structures is such as to make it
easy to assign unique values. There is no requirement that a
continuous set of values be used and implementations should not rely
on all such values being small integers. A unique value should be
selected when the defining document is first published as an internet
draft. When the document becomes a standards track document, the
working group should ensure that:
o rpcrdma2_propid values specified in the document do not conflict
with those currently assigned or in use by other pending working
group documents defining transport properties.
o rpcrdma2_propid values specified in the document do not conflict
with the range reserved for experimental use, as defined in
Section 8.2.
Documents defining new properties fall into a number of categories.
o Those defining new properties and explaining (only) how they
affect use of existing message types.
o Those defining new OPTIONAL message types and new properties
applicable to the operation of those new message types.
o Those defining new OPTIONAL message types and new properties
applicable both to new and existing message types.
When additional transport properties are proposed, the review of the
associated standards track document should deal with possible
security issues raised by those new transport properties.
9.3. Adding New Error Codes to the Protocol
New error codes to be returned when using new header types may be
introduced in the same Standards Track document that defines the new
header type. [ cel: what about adding a new error code that is
returned for an existing header type? ]
For error codes that do not require that additional error information
be returned with them, the existing RDMA_ERR2 header can be used to
report the new error. The new error code is set as the value of
rdma_err with the result that the default switch arm of the
rpcrdma2_error (i.e. void) is selected.
For error codes that do require the return of additional error-
related information together with the error, a new header type should
be defined for the purpose of returning the error together with
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needed additional information. It should be documented just like any
other new header type.
When a new header type is sent, the sender needs to be prepared to
accept header types necessary to report associated errors.
9.4. Adding New Header Flags to the Protocol
There are currently thirty-one flags available for later assignment.
One possible use for such flags would be in a later protocol version,
should that version retain the same general header structure as
version 2.
In addition, it is possible to assign unused flags within extensions
made to version 2, as long as the following practices are adhered to:
o Flags should not be added to the flag word in the prefix structure
if those flags only apply to a single header type. New flags
should only be defined for conditions applying to multiple header
types.
o The document defining the new flag should indicate for which
header types the flag value is meaningful and for which header
types it is an error to set the flag or to leave it unset.
o The sender needs to be provided with a means to determine whether
the receiver is prepared to receive transport headers with the new
flag set. This is most likely to take the form of a transport
property together with the definition of suitable defaults to use
when that property is not supported. Another possibility is to
REQUIRE that receivers supporting a particular header type also
support a set of additional flags.
10. Relationship to other RPC-over-RDMA Versions
10.1. Relationship to RPC-over-RDMA Version 1
In addition to the substantive protocol changes discussed in
Section 8, there are a number of structural XDR changes whose goal is
to enable within-version protocol extensibility.
The RPC-over-RDMA version 1 transport header is defined as a single
XDR object, with an RPC message proper potentially following it. In
RPC-over-RDMA version 2, as described in Section 5.1 there are
separate XDR definitions of the transport header prefix (see
Section 3.2 which specifies the transport header type to be used, and
the specific transport header, defined within one of the subsections
of Section 5). This is similar to the way that an RPC message
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consists of an RPC header (defined in [RFC5531]) and an RPC request
or reply, defined by the Upper Layer protocol being conveyed.
As a new version of the RPC-over-RDMA transport protocol, RPC-over-
RDMA version 2 exists within the versioning rules defined in
[RFC8166]. In particular, it maintains the first four words of the
protocol header as sent and received, as specified in Section 4.2 of
[RFC8166], even though, as explained in Section 3.1 of this document,
the XDR definition of those words is structured differently.
Although each of the first four words retains its semantic function,
there are important differences of field interpretation, besides the
fact that the words have different names and different roles with the
XDR constrict of they are parts.
o The first word of the header, previously the rdma_xid field,
retains the format and function that in had in RPC-over-RDMA
version 1. Within RPC-over-RDMA version 2, this word is the
rdma_xid field of the structure rdma_start. However, to
accommodate the use of request-response pairing of non-RPC
messages and the potential use of message continuation, it cannot
be assumed that it will always have the same value it would have
had in RPC-over-RDMA version 1. As a result, the contents of this
field should not be used without consideration of the associated
protocol version identification.
o The second word of the header, previously the rdma_vers field,
retains the format and function that it had in RPC-over-RDMA
version 1. Within RPC-over-RDMA version 2, this word is the
rdma_vers field of the structure rdma_start. To clearly
distinguish version 1 and version 2 messages, senders MUST fill in
the correct version (fixed after version negotiation) and
receivers MUST check that the content of the rdma_vers is correct
before using referencing any other header field.
o The third word of the header, previously the rdma_credit field,
retains the format and general purpose that it had in RPC-over-
RDMA version 1. Within RPC-over-RDMA version 2, this word is the
rdma_credit field of the structure rdma_start. The RPC-over-RDMA
version 2 protocol provides additional mechanisms that determine
whether the value contained in this field is a credit request or
grant. Also, the way in which credits are accounted for may be
different in RPC-over-RDMA version 2.
o The fourth word of the header, previously the union discriminator
field rdma_proc, retains its format and general function even
though the set of valid values has changed. The value of this
field is now considered an unsigned 32-bit integer rather than an
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enum. Within RPC-over-RDMA version 2, this word is the rdma_htype
field of the structure rdma_start.
Beyond conforming to the restrictions specified in [RFC8166], RPC-
over-RDMA version 2 tightly limits the scope of the changes made in
order to ensure interoperability. It makes no major structural
changes to the protocol, and all existing transport header types used
in version 1 (as defined in [RFC8166]) are retained in version 2.
Chunks are expressed using the same on-the-wire format and are used
in the same way in both versions.
10.2. Extensibility Beyond RPC-over-RDMA Version 2
Subsequent RPC-over-RDMA versions are free to change the protocol in
any way they choose as long as they maintain the first four header
words as currently specified by [RFC8166].
Such changes might involve deletion or major re-organization of
existing transport headers. However, the need for interoperability
between adjacent versions will often limit the scope of changes that
can be made in a single version.
In some cases it may prove desirable to transition to a new version
by using the extension features described for use with RPC-over-RDMA
version 2, by continuing the same basic extension model but allowing
header types and properties that were OPTIONAL in one version to
become REQUIRED in the subsequent version.
11. Security Considerations
The security considerations for RPC-over-RDMA version 2 are the same
as those for RPC-over-RDMA version 1.
11.1. Security Considerations (Transport Properties)
Like other fields that appear in each RPC-over-RDMA header, property
information is sent in the clear on the fabric with no integrity
protection, making it vulnerable to man-in-the-middle attacks.
For example, if a man-in-the-middle were to change the value of the
Receive buffer size or the Requester Remote Invalidation boolean, it
could reduce connection performance or trigger loss of connection.
Repeated connection loss can impact performance or even prevent a new
connection from being established. Recourse is to deploy on a
private network or use link-layer encryption.
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12. IANA Considerations
This document does not require actions by IANA.
13. References
13.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC4506] Eisler, M., Ed., "XDR: External Data Representation
Standard", STD 67, RFC 4506, DOI 10.17487/RFC4506, May
2006, <https://www.rfc-editor.org/info/rfc4506>.
[RFC5531] Thurlow, R., "RPC: Remote Procedure Call Protocol
Specification Version 2", RFC 5531, DOI 10.17487/RFC5531,
May 2009, <https://www.rfc-editor.org/info/rfc5531>.
[RFC8166] Lever, C., Ed., Simpson, W., and T. Talpey, "Remote Direct
Memory Access Transport for Remote Procedure Call Version
1", RFC 8166, DOI 10.17487/RFC8166, June 2017,
<https://www.rfc-editor.org/info/rfc8166>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
13.2. Informative References
[IBARCH] InfiniBand Trade Association, "InfiniBand Architecture
Specification Volume 1", Release 1.3, March 2015,
<http://www.infinibandta.org/content/
pages.php?pg=technology_download>.
[RFC5040] Recio, R., Metzler, B., Culley, P., Hilland, J., and D.
Garcia, "A Remote Direct Memory Access Protocol
Specification", RFC 5040, DOI 10.17487/RFC5040, October
2007, <https://www.rfc-editor.org/info/rfc5040>.
[RFC5041] Shah, H., Pinkerton, J., Recio, R., and P. Culley, "Direct
Data Placement over Reliable Transports", RFC 5041,
DOI 10.17487/RFC5041, October 2007,
<https://www.rfc-editor.org/info/rfc5041>.
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[RFC5661] Shepler, S., Ed., Eisler, M., Ed., and D. Noveck, Ed.,
"Network File System (NFS) Version 4 Minor Version 1
Protocol", RFC 5661, DOI 10.17487/RFC5661, January 2010,
<https://www.rfc-editor.org/info/rfc5661>.
[RFC5662] Shepler, S., Ed., Eisler, M., Ed., and D. Noveck, Ed.,
"Network File System (NFS) Version 4 Minor Version 1
External Data Representation Standard (XDR) Description",
RFC 5662, DOI 10.17487/RFC5662, January 2010,
<https://www.rfc-editor.org/info/rfc5662>.
[RFC7530] Haynes, T., Ed. and D. Noveck, Ed., "Network File System
(NFS) Version 4 Protocol", RFC 7530, DOI 10.17487/RFC7530,
March 2015, <https://www.rfc-editor.org/info/rfc7530>.
[RFC8167] Lever, C., "Bidirectional Remote Procedure Call on RPC-
over-RDMA Transports", RFC 8167, DOI 10.17487/RFC8167,
June 2017, <https://www.rfc-editor.org/info/rfc8167>.
[RFC8178] Noveck, D., "Rules for NFSv4 Extensions and Minor
Versions", RFC 8178, DOI 10.17487/RFC8178, July 2017,
<https://www.rfc-editor.org/info/rfc8178>.
Acknowledgments
The authors gratefully acknowledge the work of Brent Callaghan and
Tom Talpey on the original RPC-over-RDMA version 1 specification (RFC
5666). The authors also wish to thank Bill Baker, Greg Marsden, and
Matt Benjamin for their support of this work.
The XDR extraction conventions were first described by the authors of
the NFS version 4.1 XDR specification [RFC5662]. Herbert van den
Bergh suggested the replacement sed script used in this document.
Special thanks go to Transport Area Director Spencer Dawkins, NFSV4
Working Group Chairs Spencer Shepler and Brian Pawlowski, and NFSV4
Working Group Secretary Thomas Haynes for their support.
Authors' Addresses
Charles Lever (editor)
Oracle Corporation
1015 Granger Avenue
Ann Arbor, MI 48104
United States of America
Phone: +1 248 816 6463
Email: chuck.lever@oracle.com
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David Noveck
NetApp
1601 Trapelo Road
Waltham, MA 02451
United States of America
Phone: +1 781 572 8038
Email: davenoveck@gmail.com
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