Internet DRAFT - draft-dnoveck-nfsv4-nfsulb
draft-dnoveck-nfsv4-nfsulb
Network File System Version 4 D. Noveck
Internet-Draft July 25, 2017
Intended status: Standards Track
Expires: January 26, 2018
Transport-generic Network File System (NFS) Upper Layer Bindings To RPC-
Over-RDMA
draft-dnoveck-nfsv4-nfsulb-01
Abstract
This document specifies Upper Layer Bindings to allow use of RPC-
over-RDMA by protocols related to the Network File System (NFS).
Such bindings are required when using RPC-over-RDMA, in order to
enable use of Direct Data Placement and for a number of other
reasons. These bindings are structured to be applicable to all known
version of the RPC-over-RDMA transport, including optional
extensions. All versions of NFS are addressed.
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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This Internet-Draft will expire on January 26, 2018.
Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conveying NFS Operations On RPC-Over-RDMA . . . . . . . . . . 3
2.1. Direct Placement of Request Data . . . . . . . . . . . . 4
2.2. Direct Placement of Response Data . . . . . . . . . . . . 5
2.3. Scatter-gather when Using DDP . . . . . . . . . . . . . . 5
2.4. DDP-eligibility Violations . . . . . . . . . . . . . . . 5
2.5. Long Calls and Replies . . . . . . . . . . . . . . . . . 6
3. Preparatory Material for Multiple Bindings . . . . . . . . . 7
3.1. Reply Size Estimation . . . . . . . . . . . . . . . . . . 7
3.2. Retry to Deal with Reply Size Mis-estimation . . . . . . 8
4. Upper Layer Binding for NFS Versions 2 And 3 . . . . . . . . 9
4.1. Auxiliary Protocols . . . . . . . . . . . . . . . . . . . 9
4.1.1. MOUNT, NLM, And NSM Protocols . . . . . . . . . . . . 10
4.1.2. NFSACL Protocol . . . . . . . . . . . . . . . . . . . 10
5. Upper Layer Binding for NFS Version 4 . . . . . . . . . . . . 10
5.1. DDP-Eligibility . . . . . . . . . . . . . . . . . . . . . 11
5.1.1. READ_PLUS Replies . . . . . . . . . . . . . . . . . . 11
5.2. NFS Version 4 Reply Size Estimation . . . . . . . . . . . 12
5.2.1. Reply Size Estimation for Minor Version 0 . . . . . . 12
5.2.2. Reply Size Estimation for Minor Version 1 And Newer . 12
5.3. NFS Version 4 COMPOUND Requests . . . . . . . . . . . . . 13
5.3.1. NFS Version 4 COMPOUND Example . . . . . . . . . . . 13
5.4. NFS Version 4 Callback . . . . . . . . . . . . . . . . . 14
5.4.1. NFS Version 4.0 Callback . . . . . . . . . . . . . . 14
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5.4.2. NFS Version 4.1 Callback . . . . . . . . . . . . . . 14
5.5. Session-Related Considerations . . . . . . . . . . . . . 15
5.6. Connection Keep-Alive . . . . . . . . . . . . . . . . . . 16
6. Extending NFS Upper Layer Bindings . . . . . . . . . . . . . 16
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
8. Security Considerations . . . . . . . . . . . . . . . . . . . 17
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 17
9.1. Normative References . . . . . . . . . . . . . . . . . . 18
9.2. Informative References . . . . . . . . . . . . . . . . . 18
Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . 19
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 20
1. Introduction
An RPC-over-RDMA transport, such as the ones defined in [RFC8166] and
[rpcrdmav2], may employ direct data placement to convey data payloads
associated with RPC transactions. To enable successful
interoperation, RPC client and server implementations must agree as
to which XDR data items in what particular RPC procedures are
eligible for direct data placement (DDP). Specifying those data
items is a major component of a protocol's Upper Layer Binding.
In addition, Upper Layer Bindings are required to include additional
information to assure that adequate resources are allocated to
receive RPC replies, and for a number of other reasons.
This document contains material required of Upper Layer Bindings, as
specified in [RFC8166], for the NFS protocol versions listed below.
In addition, bindings are provided, when necessary, for auxiliary
protocols used together with NFS versions 2 and 3.
o NFS Version 2 [RFC1094]
o NFS Version 3 [RFC1813]
o NFS Version 4.0 [RFC7530]
o NFS Version 4.1 [RFC5661]
o NFS Version 4.2 [RFC7862]
2. Conveying NFS Operations On RPC-Over-RDMA
This document is written to apply to multiple versions of the RPC-
over-RDMA transport, only the first of which is currently specified
by a Sroposed Standard (in [RFC8166]). However, it is expected that
other versions will be created, and this document has been structured
to support future versions by focusing on the functions to be
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provided by the transport and the transport limitations which the
Upper Layer Protocols need to accommodate, allowing the transport
specification and the specifications for associated extensions to
define how those functions will be provided and the details of the
transport limitations.
In the subsections that follow, we will describe the generic function
to be provided or limitation to be accommodated and follow it with
material that describes, in general, how that issue is dealt with in
Version One. For more detail about Version One, [RFC8166] should be
consulted. How these issues are to be dealt with in future versions
is left to the specification documents for those versions and
associated documents defining optional extensions.
For example:
o ULBs within this document define which data items are eligible for
Direct Data Placement while transport versions might differ as how
this is to be effected. See Sections 2.1 and 2.2 for more detail.
In both cases, Section 2.3 discusses issues connected with the use
of discontiguous areas for Direct Data Placement.
o Section 2.4 defines the concept of a DDP-eligibility violation and
requires that such violations be reported while transport versions
might differ as to the manner in which the reporting is to be
done.
o Section 2.5 discusses issues arising from limits on the size of
messages conveyed using RDMA SENDs. Different transport version
may have different size limits, while, in the case of replies the
ULBs are responsible for specifying how limits on reply sizes are
to be determined.
2.1. Direct Placement of Request Data
When DDP-eligible XDR data items appear in a request the requester
needs to take special actions in order to provide for the direct
placement of those items in the responder's memory. The specific
actions to be taken are defined by the transport version being used.
In Version One, the Read list in each RPC-over-RDMA transport header
represents a set of memory regions containing a single item of DDP-
eligible NFS argument data. Large data items, such as the data
payload of an NFS version 3 WRITE procedure, can be referenced by the
Read list. The NFS server pulls such payloads from the client and
places them directly into its own memory.
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2.2. Direct Placement of Response Data
When a request is such that it is possible for DDP-eligible data
items to appear in the corresponding reply, the requester needs to
take special actions in order to provide for the direct placement of
those items in the requester's memory, if such placement is desired.
The specific actions to be taken are defined by the transport version
being used as is the means to indicate that such direct placement is
not to be done
In Version One, the Write list in each RPC-over-RDMA transport header
represents a set of memory regions that can receive DDP-eligible NFS
result data. Large data items, such as the payload of an NFS version
3 READ procedure, can be referenced by the Write list. The NFS
server pushes such payloads to the client, placing them directly into
the client's memory, using target addresses provided by the client
when sending the request.
Each Write chunk corresponds to a specific XDR data item in an NFS
reply. This document describes how NFS client and server
implementations determine the correspondence between Write chunks and
XDR results.
2.3. Scatter-gather when Using DDP
In order to accommodate the storage of multiple data blocks within
individual cache buffers, the RPC-over-RDMA transport allows the
addresses to which a DDP-eligible data item to be discontiguous. How
these addresses are indicated depends on the transport version.
Within Version One, a chunk typically corresponds to exactly one XDR
data item. Each Read chunk is represented as a list of segments at
the same XDR Position. Each Write chunk is represented as an array
of segments. An NFS client thus has the flexibility to advertise a
set of discontiguous memory regions in which to convey a single DDP-
eligible XDR data item.
2.4. DDP-eligibility Violations
When the transport header uses the means defined to directly place on
an XDR item is applied to an XDR item not define in the ULB as DDP-
eligible, a DDP-eligibility violation is recognized. The means by
which such violations are to be reported is defined by the particular
transport version being used.
To report a DDP-eligibility violation within Version One, an NFS
server returns one of:
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o An RPC-over-RDMA message of type RDMA_ERROR, with the rdma_xid
field set to the XID of the matching NFS Call, and the rdma_error
field set to ERR_CHUNK
o An RPC message (via an RDMA_MSG message) with the xid field set to
the XID of the matching NFS Call, the mtype field set to REPLY,
the stat field set to MSG_ACCEPTED, and the accept_stat field set
to GARBAGE_ARGS.
2.5. Long Calls and Replies
Because of the use of pre-posted receive buffers whose size is fixed,
all RPC-over-RDMA transport versions have limits on the size of
messages which can be conveyed without use of explicit RDMA
operations, although different transport versions may have different
limits. In particular, when the transport version allows messages to
be continued across multiple RDMA SENDs, the limit can be
substantially greater than the receive buffer size. Also note that
the size of the messages allowed may be reduced because of space
taken up by the transport header fields.
Each transport version is responsible for defining the message size
limits and the means by which the transfer of messages that exceed
these limits is to be provided for. These means may be different in
the cases of long calls and replies.
When using Version One, if an NFS request is too large to be conveyed
within the NFS server's responder inline threshold, even after any
DDP-eligible data items have been removed, an NFS client must send
the request in the form of a Long Call. The entire NFS request is
sent in a special Read chunk called a Position Zero Read chunk.
Also when using Version One, if an NFS client determines that the
maximum size of an NFS reply could be too large to be conveyed within
its own inline threshold, it provides a Reply chunk in the RPC-over-
RDMA transport header conveying the NFS request. The server places
the entire NFS reply in the Reply chunk.
There exist cases in which an NFS client needs to provide both a
Position Zero Read chunk and a Reply chunk for the same RPC. One
common source of such situations is when the RPC authentication
flavor being used requires that DDP-eligible data items never be
removed from RPC messages.
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3. Preparatory Material for Multiple Bindings
Although each of the NFS versions and each of the auxiliary protocols
discussed in Section 4.1 has its own ULB, there is important
preparatory material in the subsections below that applies to
multiple ULPs. In particular:
o The material in Section 3.1 applies to all of the ULPs discussed
in this document.
o The material in Section 3.2 applies to NFSv2, NFSv3, NFSv4.0, the
MOUNT protocol, and the NFSACL protocol.
3.1. Reply Size Estimation
During the construction of each RPC Call message, a client is
responsible for allocating appropriate resources for receiving the
matching Reply message. The resources required depends on the
maximum reply size expected, whether DDP-eligible can removed from
the reply and the transport version being used. The ULB is
responsible for defining how the maximum reply size is to be
determined while the specifiction of the transport version being used
is responsible for defining how this maximum affects the resources to
be allocated. Because the responder may not be able to send the
required response when these resources have not been allocated,
reliable reply size estimation is necessary to allow successful
interoperation.
In many cases the Upper Layer Protocol's XDR definition provides
enough information to enable the client to make a reliable prediction
of the maximum size of the expected Reply message. However, If there
are variable-size data items in the result, the maximum size of the
RPC Reply message can be reliably estimated in many cases:
o The client requests only a specific portion of an object (for
example, using the "count" and "offset" fields in an NFS READ).
o The client has already cached the size of the whole object it is
about to request (e.g., via a previous NFS GETATTR request).
o The client specifies a reply size limit for the particular reply,
as it does by setting the count field of READDIR request.
It is sometimes not possible to determine the maximum Reply message
size based solely on the above criteria. Client implementers can
choose to provide the largest possible Reply buffer in those cases,
based on, for instance, the largest possible NFS READ or WRITE
payload (which is negotiated at mount time).
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There exist cases in which a client cannot be sure any a priori
determination is fully reliable. Handling of such cases is discussed
in Section 3.2.
3.2. Retry to Deal with Reply Size Mis-estimation
For some of the protocols discussed in this document, it is possible
for a compliant responder to send a valid reply whose length exceeds
the client's a priori estimate. In such cases, the client needs to
expect an error indication that indicates the existence of the
oversize reply. When this happens, the client can either terminate
that RPC transaction, or retry it with a larger reply size estimate.
In the case of the NFSv4.0, the use of NFS COMPOUND operations raises
the possibility of non-idempotent requests that combine a non-
idempotent operation with an operation whose maximum reply size
cannot be determined with certainty. This makes retrying the
operation problematic. It should be noted that many operations
normally considered non-idempotent (e.g. WRITE, SETATTR) are
actually idempotent. Truly non-idempotent operations are quite
unusual in COMPOUNDs that include operations with uncertain reply
sizes.
Depending on the transport version used, the client's choices may be
restricted as follows:
o The client may be required to treat the error as permanent, with
retry not allowed.
o The client may be allowed to reissue the request with a larger
reply estimate, unless it is a non-idempotent request. In this
case, non-idempotent requests may not be retried and will result
in errors being reported to the issuer in this case.
o The client may be allowed to reissue the request with a larger
reply estimate, in essentially all cases. In this case, the
client has sufficient information to avoid re-executing a non-
idempotent request and may, if it chooses, retry all requests with
a larger reply size.
In the case of Version One, the absence of a itinct error code to
signal a reply chunk of inadequate size meanss that retry in this
situation is not available.
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4. Upper Layer Binding for NFS Versions 2 And 3
This Upper Layer Binding specification applies to NFS Version 2
[RFC1094] and NFS Version 3 [RFC1813]. For brevity, in this section
a "legacy NFS client" refers to an NFS client using NFS version 2 or
NFS version 3 to communicate with an NFS server. Likewise, a "legacy
NFS server" is an NFS server communicating with clients using NFS
version 2 or NFS version 3.
The following XDR data items in NFS versions 2 and 3 are DDP-
eligible:
o The opaque file data argument in the NFS WRITE procedure
o The pathname argument in the NFS SYMLINK procedure
o The opaque file data result in the NFS READ procedure
o The pathname result in the NFS READLINK procedure
All other argument or result data items in NFS versions 2 and 3 are
not DDP-eligible.
A legacy NFS client determines the maximum reply size for each
operation using the basic criteria outlined in Section 3.1. Such
clients deal with reply sizes beyond the maximum as escribed in
Section 2.5.
4.1. Auxiliary Protocols
NFS versions 2 and 3 are typically deployed with several other
protocols, sometimes referred to as "NFS auxiliary protocols." These
are separate RPC programs that define procedures which are not part
of the NFS version 2 or version 3 RPC programs. These include:
o The MOUNT and NLM protocols, introduced in an appendix of
[RFC1813]
o The NSM protocol, described in Chapter 11 of [NSM]
o The NFSACL protocol, which does not have a public definition
(NFSACL here is treated as a de facto standard as there are
several interoperating implementations).
RPC-over-RDMA treats these programs as distinct Upper Layer Protocols
[RFC8166]. To enable the use of these ULPs on an RPC-over-RDMA
transport, an Upper Layer Binding specification is provided here for
each.
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4.1.1. MOUNT, NLM, And NSM Protocols
Typically MOUNT, NLM, and NSM are conveyed via TCP, even in
deployments where NFS operations on RPC-over-RDMA. When a legacy
server supports these programs on RPC-over-RDMA, it advertises the
port address via the usual rpcbind service [RFC1833].
No operation in these protocols conveys a significant data payload,
and the size of RPC messages in these protocols is uniformly small.
Therefore, no XDR data items in these protocols are DDP-eligible.
The largest variable-length XDR data item is an xdr_netobj. In most
implementations this data item is not larger than 1024 bytes, making
this size a reasonable basis for reply size estimation. However,
since this limit is not specified as part of the protocol, the
techniques described in Section 3.1 should be used to deal with
situations where these sizes are exceeded.
4.1.2. NFSACL Protocol
Legacy clients and servers that support the NFSACL RPC program
typically convey NFSACL procedures on the same connection as the NFS
RPC program. This obviates the need for separate rpcbind queries to
discover server support for this RPC program.
ACLs are typically small, but even large ACLs must be encoded and
decoded to some degree. Thus, no data item in this Upper Layer
Protocol is DDP-eligible.
For procedures whose replies do not include an ACL object, the size
of a reply is determined directly from the NFSACL program's XDR
definition.
There is no protocol-wide size limit for NFS version 3 ACLs, and
there is no mechanism in either the NFSACL or NFS programs for a
legacy client to ascertain the largest ACL a legacy server can store.
Legacy client implementations should choose a maximum size for ACLs
based on their own internal limits. A recommended lower bound for
this maximum is 32,768 bytes, though a larger Reply chunk (up to the
negotiated rsize setting) can be provided. Since no limit is
specified as part of the protocol, the techniques described in
Section 3.1 should be used to deal with situations where these
recommended bounds are exceeded.
5. Upper Layer Binding for NFS Version 4
This Upper Layer Binding specification applies to all protocols
defined in NFS Version 4.0 [RFC7530], NFS Version 4.1 [RFC5661], and
NFS Version 4.2 [RFC7862].
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5.1. DDP-Eligibility
Only the following XDR data items in the COMPOUND procedure of all
NFS version 4 minor versions are DDP-eligible:
o The opaque data field in the WRITE4args structure
o The linkdata field of the NF4LNK arm in the createtype4 union
o The opaque data field in the READ4resok structure
o The linkdata field in the READLINK4resok structure
o In minor version 2 and newer, the rpc_data field of the
read_plus_content union (further restrictions on the use of this
data item follow below).
5.1.1. READ_PLUS Replies
The NFS version 4.2 READ_PLUS operation returns a complex data type
[RFC7862]. The rpr_contents field in the result of this operation is
an array of read_plus_content unions, one arm of which contains an
opaque byte stream (d_data).
The size of d_data is limited to the value of the rpa_count field,
but the protocol does not bound the number of elements which can be
returned in the rpr_contents array. In order to make the size of
READ_PLUS replies predictable by NFS version 4.2 clients, the
following restrictions are placed on the use of the READ_PLUS
operation on RPC-over-RDMA transports:
o An NFS version 4.2 client MUST NOT provide more than one Write
chunk for any READ_PLUS operation. When providing a Write chunk
for a READ_PLUS operation, an NFS version 4.2 client MUST provide
a Write chunk that is either empty (which forces all result data
items for this operation to be returned inline) or large enough to
receive rpa_count bytes in a single element of the rpr_contents
array.
o If the Write chunk provided for a READ_PLUS operation by an NFS
version 4.2 client is not empty, an NFS version 4.2 server MUST
use that chunk for the first element of the rpr_contents array
that has an rpc_data arm.
o An NFS version 4.2 server MUST NOT return more than two elements
in the rpr_contents array of any READ_PLUS operation. It returns
as much of the requested byte range as it can fit within these two
elements. If the NFS version 4.2 server has not asserted rpr_eof
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in the reply, the NFS version 4.2 client SHOULD send additional
READ_PLUS requests for any remaining bytes.
5.2. NFS Version 4 Reply Size Estimation
An NFS version 4 client provides a Reply chunk when the maximum
possible reply size is larger than the client's responder inline
threshold.
There are certain NFS version 4 data items whose size cannot be
estimated by clients reliably, however, because there is no protocol-
specified size limit on these structures. These include:
o The attrlist4 field
o Fields containing ACLs such as fattr4_acl, fattr4_dacl,
fattr4_sacl
o Fields in the fs_locations4 and fs_locations_info4 data structures
o Opaque fields which pertain to pNFS layout metadata, such as
loc_body, loh_body, da_addr_body, lou_body, lrf_body,
fattr_layout_types and fs_layout_types,
5.2.1. Reply Size Estimation for Minor Version 0
The items enumerated above in Section 5.2 make it difficult to
predict the maximum size of GETATTR replies that interrogate
variable-length attributes. As discussed in Section 3.1, client
implementations can rely on their own internal architectural limits
to bound the reply size. However, since such limits are not
guaranteed to be reliable, use of the techniques discussed in
Section 3.2 may sometimes be necessary.
It is best to avoid issuing single COMPOUNDs that contain both non-
idempotent operations and operations where the maximum reply size
cannot be reliably predicted.
5.2.2. Reply Size Estimation for Minor Version 1 And Newer
In NFS version 4.1 and newer minor versions, the csa_fore_chan_attrs
argument of the CREATE_SESSION operation contains a
ca_maxresponsesize field. The value in this field can be taken as
the absolute maximum size of replies generated by a replying NFS
version 4 server.
This value can be used in cases where it is not possible to estimate
a reply size upper bound precisely. In practice, objects such as
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ACLs, named attributes, layout bodies, and security labels are much
smaller than this maximum.
5.3. NFS Version 4 COMPOUND Requests
The NFS version 4 COMPOUND procedure allows the transmission of more
than one DDP-eligible data item per Call and Reply message. An NFS
version 4 client provides XDR Position values in each Read chunk to
disambiguate which chunk is associated with which argument data item.
However, NFS version 4 server and client implementations must agree
in advance on how to pair Write chunks with returned result data
items.
The mechanism specified in Section 4.3.2 of [RFC8166]) is applied
here, with additional restrictions that appear below. In the
following list, an "NFS Read" operation refers to any NFS Version 4
operation which has a DDP-eligible result data item (i.e., either a
READ, READ_PLUS, or READLINK operation).
o If an NFS version 4 client wishes all DDP-eligible items in an NFS
reply to be conveyed inline, it leaves the Write list empty.
o The first chunk in the Write list MUST be used by the first READ
operation in an NFS version 4 COMPOUND procedure. The next Write
chunk is used by the next READ operation, and so on.
o If an NFS version 4 client has provided a matching non-empty Write
chunk, then the corresponding READ operation MUST return its DDP-
eligible data item using that chunk.
o If an NFS version 4 client has provided an empty matching Write
chunk, then the corresponding READ operation MUST return all of
its result data items inline.
o If an READ operation returns a union arm which does not contain a
DDP-eligible result, and the NFS version 4 client has provided a
matching non-empty Write chunk, an NFS version 4 server MUST
return an empty Write chunk in that Write list position.
o If there are more READ operations than Write chunks, then
remaining NFS Read operations in an NFS version 4 COMPOUND that
have no matching Write chunk MUST return their results inline.
5.3.1. NFS Version 4 COMPOUND Example
The following example shows a Write list with three Write chunks, A,
B, and C. The NFS version 4 server consumes the provided Write
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chunks by writing the results of the designated operations in the
compound request (READ and READLINK) back to each chunk.
Write list:
A --> B --> C
NFS version 4 COMPOUND request:
PUTFH LOOKUP READ PUTFH LOOKUP READLINK PUTFH LOOKUP READ
| | |
v v v
A B C
If the NFS version 4 client does not want to have the READLINK result
returned via RDMA, it provides an empty Write chunk for buffer B to
indicate that the READLINK result must be returned inline.
5.4. NFS Version 4 Callback
The NFS version 4 protocols support server-initiated callbacks to
notify clients of events such as recalled delegations.
5.4.1. NFS Version 4.0 Callback
NFS version 4.0 implementations typically employ a separate TCP
connection to handle callback operations, even when the forward
channel uses a RPC-over-RDMA transport.
No operation in the NFS version 4.0 callback RPC program conveys a
significant data payload. Therefore, no XDR data items in this RPC
program is DDP-eligible.
A CB_RECALL reply is small and fixed in size. The CB_GETATTR reply
contains a variable-length fattr4 data item. See Section 5.2.1 for a
discussion of reply size prediction for this data item.
An NFS version 4.0 client advertises netids and ad hoc port addresses
for contacting its NFS version 4.0 callback service using the
SETCLIENTID operation.
5.4.2. NFS Version 4.1 Callback
In NFS version 4.1 and newer minor versions, callback operations may
appear on the same connection as is used for NFS version 4 forward
channel client requests. NFS version 4 clients and servers MUST use
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the mechanism described in [RFC8167] when backchannel operations are
conveyed on RPC-over-RDMA transports.
The csa_back_chan_attrs argument of the CREATE_SESSION operation
contains a ca_maxresponsesize field. The value in this field can be
taken as the absolute maximum size of backchannel replies generated
by a replying NFS version 4 client.
There are no DDP-eligible data items in callback procedures defined
in NFS version 4.1 or NFS version 4.2. However, some callback
operations, such as messages that convey device ID information, can
be large, in which case a Long Call or Reply might be required.
When an NFS version 4.1 client reports a backchannel
ca_maxrequestsize that is larger than the connection's inline
thresholds, the NFS version 4 client can support Long Calls.
Otherwise an NFS version 4 server MUST use Short messages to convey
backchannel operations.
5.5. Session-Related Considerations
Typically, the presence of an NFS session [RFC5661] has no effect on
the operation of RPC-over-RDMA. None of the operations introduced to
support NFS sessions contain DDP-eligible data items. There is no
need to match the number of session slots with the number of
available RPC-over-RDMA credits.
However, there are some rare error conditions which require special
handling when an NFS session is operating on an RPC-over-RDMA
transport. For example, a requester might receive, in response to an
RPC request, an RDMA_ERROR message with an rdma_err value of
ERR_CHUNK, or an RDMA_MSG containing an RPC_GARBAGEARGS reply.
Within RPC-over-RDMA Version One, this class of error can be
generated for two different reasons:
o There was an XDR error detected parsing the RPC-over-RDMA headers.
o There was an error sending the response, because, for example, a
necessary reply chunk was not provided or the one provided is of
insufficient length.
These two situations, which arise due to incorrect implementations or
underestimation of reply size, have different implications with
regard to Exactly-Once Semantics. An XDR error in decoding the
request precludes the execution of the request on the responder, but
failure to send a reply indicates that some or all of the operations
were executed.
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In both instances, the client SHOULD NOT retry the operation without
addressing reply resource inadequacy. Such a retry can result in the
same sort of error seen previously. Instead, it is best to consider
the operation as completed unsuccessfully and report an error to the
consumer who requested the RPC.
In addition, within the error response, the requester does not have
the result of the execution of the SEQUENCE operation, which
identifies the session, slot, and sequence id for the request which
has failed. The xid associated with the request, obtained from the
rdma_xid field of the RDMA_ERROR or RDMA_MSG message, must be used to
determine the session and slot for the request which failed, and the
slot must be properly retired. If this is not done, the slot could
be rendered permanently unavailable.
5.6. Connection Keep-Alive
NFS version 4 client implementations often rely on a transport-layer
keep-alive mechanism to detect when an NFS version 4 server has
become unresponsive. When an NFS server is no longer responsive,
client-side keep-alive terminates the connection, which in turn
triggers reconnection and RPC retransmission.
Some RDMA transports (such as Reliable Connections on InfiniBand)
have no keep-alive mechanism. Without a disconnect or new RPC
traffic, such connections can remain alive long after an NFS server
has become unresponsive. Once an NFS client has consumed all
available RPC-over-RDMA credits on that transport connection, it will
forever await a reply before sending another RPC request.
NFS version 4 clients SHOULD reserve one RPC-over-RDMA credit to use
for periodic server or connection health assessment. This credit can
be used to drive an RPC request on an otherwise idle connection,
triggering either a quick affirmative server response or immediate
connection termination.
6. Extending NFS Upper Layer Bindings
RPC programs such as NFS are required to have an Upper Layer Binding
specification to interoperate on RPC-over-RDMA transports [RFC8166].
Via standards action, the Upper Layer Binding specified in this
document can be extended to cover versions of the NFS version 4
protocol specified after NFS version 4 minor version 2, or separately
published extensions to an existing NFS version 4 minor version, as
described in [RFC8178].
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7. IANA Considerations
NFS use of direct data placement introduces a need for an additional
NFS port number assignment for networks that share traditional UDP
and TCP port spaces with RDMA services. The iWARP [RFC5041]
[RFC5040] protocol is such an example (InfiniBand is not).
NFS servers for versions 2 and 3 [RFC1094] [RFC1813] traditionally
listen for clients on UDP and TCP port 2049, and additionally, they
register these with the portmapper and/or rpcbind [RFC1833] service.
However, [RFC7530] requires NFS version 4 servers to listen on TCP
port 2049, and they are not required to register.
An NFS version 2 or version 3 server supporting RPC-over-RDMA on such
a network and registering itself with the RPC portmapper MAY choose
an arbitrary port, or MAY use the alternative well-known port number
for its RPC-over-RDMA service. The chosen port MAY be registered
with the RPC portmapper under the netid assigned by the requirement
in [RFC8166].
An NFS version 4 server supporting RPC-over-RDMA on such a network
MUST use the alternative well-known port number for its RPC-over-RDMA
service. Clients SHOULD connect to this well-known port without
consulting the RPC portmapper (as for NFS version 4 on TCP
transports).
The port number assigned to an NFS service over an RPC-over-RDMA
transport is available from the IANA port registry [RFC3232].
8. Security Considerations
RPC-over-RDMA supports all RPC security models, including RPCSEC_GSS
security and transport-level security [RFC2203]. The choice of RDMA
Read and RDMA Write to convey RPC argument and results does not
affect this, since it changes only the method of data transfer.
Specifically, the requirements of [RFC8166] ensure that this choice
does not introduce new vulnerabilities.
Because this document defines only the binding of the NFS protocols
atop [RFC8166], all relevant security considerations are therefore to
be described at that layer.
9. References
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9.1. Normative References
[RFC1833] Srinivasan, R., "Binding Protocols for ONC RPC Version 2",
RFC 1833, DOI 10.17487/RFC1833, August 1995,
<http://www.rfc-editor.org/info/rfc1833>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC2203] Eisler, M., Chiu, A., and L. Ling, "RPCSEC_GSS Protocol
Specification", RFC 2203, DOI 10.17487/RFC2203, September
1997, <http://www.rfc-editor.org/info/rfc2203>.
[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,
<http://www.rfc-editor.org/info/rfc5661>.
[RFC7530] Haynes, T., Ed. and D. Noveck, Ed., "Network File System
(NFS) Version 4 Protocol", RFC 7530, DOI 10.17487/RFC7530,
March 2015, <http://www.rfc-editor.org/info/rfc7530>.
[RFC7862] Haynes, T., "Network File System (NFS) Version 4 Minor
Version 2 Protocol", RFC 7862, DOI 10.17487/RFC7862,
November 2016, <http://www.rfc-editor.org/info/rfc7862>.
[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,
<http://www.rfc-editor.org/info/rfc8166>.
[RFC8167] Lever, C., "Bidirectional Remote Procedure Call on RPC-
over-RDMA Transports", RFC 8167, DOI 10.17487/RFC8167,
June 2017, <http://www.rfc-editor.org/info/rfc8167>.
[RFC8178] Noveck, D., "Rules for NFSv4 Extensions and Minor
Versions", RFC 8178, DOI 10.17487/RFC8178, July 2017,
<http://www.rfc-editor.org/info/rfc8178>.
9.2. Informative References
[I-D.ietf-nfsv4-rfc5667bis]
Lever, C., "Remote Direct Memory Access Transport for
Remote Procedure Call, Version One", draft-ietf-
nfsv4-rfc5667bis-11 (work in progress), May 2017.
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[NSM] The Open Group, "Protocols for Interworking: XNFS, Version
3W", February 1998.
[RFC1094] Nowicki, B., "NFS: Network File System Protocol
specification", RFC 1094, DOI 10.17487/RFC1094, March
1989, <http://www.rfc-editor.org/info/rfc1094>.
[RFC1813] Callaghan, B., Pawlowski, B., and P. Staubach, "NFS
Version 3 Protocol Specification", RFC 1813,
DOI 10.17487/RFC1813, June 1995,
<http://www.rfc-editor.org/info/rfc1813>.
[RFC3232] Reynolds, J., Ed., "Assigned Numbers: RFC 1700 is Replaced
by an On-line Database", RFC 3232, DOI 10.17487/RFC3232,
January 2002, <http://www.rfc-editor.org/info/rfc3232>.
[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, <http://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,
<http://www.rfc-editor.org/info/rfc5041>.
[RFC5667] Talpey, T. and B. Callaghan, "Network File System (NFS)
Direct Data Placement", RFC 5667, DOI 10.17487/RFC5667,
January 2010, <http://www.rfc-editor.org/info/rfc5667>.
[rpcrdmav2]
Lever, C., Ed. and D. Noveck, "RPC-over-RDMA Version Two",
July 2017, <http://www.ietf.org/id/
draft-cel-nfsv4-rpcrdma-version-two-05.txt>.
Work in progress.
Appendix A. Acknowledgments
The author gratefully acknowledges the work of Brent Callaghan and
Tom Talpey on the original NFS Direct Data Placement specification
[RFC5667].
A large part of the material in this doccument is taken from
[I-D.ietf-nfsv4-rfc5667bis] written by Chuck Lever. The author
wishes to acknowlege the debt he owes to Chuck for his work in
providing an updated Upper Layer Binding for the NFS-related
protocols.
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The author also wishes to thank Bill Baker and Greg Marsden for their
support of the work to revive RPC-over-RDMA.
Author's Address
David Noveck
1601 Trapelo Road
waltham, MA 02451
Unied States of America
Phone: +1 781 572 8038
Email: davenoveck@gmail.com
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