Internet DRAFT - draft-cel-nfsv4-rpcrdma-reliable-reply
draft-cel-nfsv4-rpcrdma-reliable-reply
Network File System Version 4 C. Lever
Internet-Draft Oracle
Intended status: Experimental May 20, 2019
Expires: November 21, 2019
Improving the Performance and Reliability of RPC Replies on RPC-over-
RDMA Transports
draft-cel-nfsv4-rpcrdma-reliable-reply-05
Abstract
RPC transports such as RPC-over-RDMA version 1 require reply buffers
to be in place before an RPC Call is sent. However, RPC consumers
sometimes have difficulty estimating the expected maximum size of a
particular RPC reply. This introduces the risk that an RPC Reply
message can overrun reply resources provided by the requester,
preventing delivery of the message, through no fault of the
requester. This document describes a mechanism that eliminates the
need for pre-allocation of reply resources for unpredictably large
replies.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on November 21, 2019.
Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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carefully, as they describe your rights and restrictions with respect
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 4
3. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Reply Chunk Overrun . . . . . . . . . . . . . . . . . . . 4
3.2. Reply Size Calculation . . . . . . . . . . . . . . . . . 5
3.3. Requester Registration Costs . . . . . . . . . . . . . . 5
3.4. Denial of Service . . . . . . . . . . . . . . . . . . . . 6
3.5. Estimating Transport Header Size . . . . . . . . . . . . 6
4. Responder-Provided Read Chunks . . . . . . . . . . . . . . . 7
4.1. Specification . . . . . . . . . . . . . . . . . . . . . . 7
4.1.1. Responder Duties . . . . . . . . . . . . . . . . . . 8
4.1.2. Requester Duties . . . . . . . . . . . . . . . . . . 8
4.1.3. Pull Completion Notification . . . . . . . . . . . . 8
4.1.4. Remote Invalidation . . . . . . . . . . . . . . . . . 9
5. Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . 9
5.1. Benefits . . . . . . . . . . . . . . . . . . . . . . . . 9
5.1.1. Less Frequent Use of Explicit RDMA . . . . . . . . . 9
5.1.2. Support for Arbitrarily Large Replies . . . . . . . . 10
5.1.3. Protection of Connection After RPC Cancellation . . . 10
5.1.4. Asynchronous Chunk Invalidation . . . . . . . . . . . 10
5.2. Costs . . . . . . . . . . . . . . . . . . . . . . . . . . 10
5.2.1. Responder Memory Exposure . . . . . . . . . . . . . . 10
5.2.2. Round Trip Penalty . . . . . . . . . . . . . . . . . 10
5.2.3. Credit Accounting Complexity . . . . . . . . . . . . 11
5.3. Selecting a Reply Mechanism . . . . . . . . . . . . . . . 11
5.3.1. Requester . . . . . . . . . . . . . . . . . . . . . . 11
5.3.2. Responder . . . . . . . . . . . . . . . . . . . . . . 12
5.4. Implementation Complexity . . . . . . . . . . . . . . . . 12
5.4.1. RPC Call Path . . . . . . . . . . . . . . . . . . . . 13
5.4.2. RPC Reply Path . . . . . . . . . . . . . . . . . . . 13
5.4.3. Managing RDMA_DONE messages . . . . . . . . . . . . . 13
5.5. Alternatives . . . . . . . . . . . . . . . . . . . . . . 14
6. Interoperation Considerations . . . . . . . . . . . . . . . . 14
7. Security Considerations . . . . . . . . . . . . . . . . . . . 14
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 15
9.1. Normative References . . . . . . . . . . . . . . . . . . 15
9.2. Informative References . . . . . . . . . . . . . . . . . 15
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 16
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 16
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1. Introduction
One way in which RPC-over-RDMA version 1 improves transport
efficiency is by ensuring resources for RPC replies are available in
advance of each RPC transaction [RFC8166]. These resources are
typically provisioned before a requester sends each RPC Call message.
They are provided to the responder to use for transmiting the
associated RPC Reply message back to the requester.
In particular, when the Payload Stream of an RPC Reply message is
expected to be large, the requester allocates and registers a Reply
chunk. The responder transfers the RPC Reply message's Payload
stream directly into the requester memory associated with that chunk,
then indicates that the RPC Reply is ready. The requester
invalidates the memory region.
In most cases, Upper Layer Protocols are capable of accurately
calculating the maximum size of RPC Reply messages. In addition, the
average size of RPC Reply messages is small, making the risk of Reply
chunk overrun exceptionally small.
However, on rare occasions an Upper Layer Protocol might not be able
to derive a reply size upper bound. An example of this is the NFS
version 4.1 GETATTR operation [RFC5661] [RFC8267] where a reply can
contain an unpredictable number of data content and hole descriptors.
Further, since the average size of actual RPC Replies is small,
requesters frequently allocate and register a Reply chunk for a reply
that, once it has been constructed by the responder, is small enough
to be sent inline. In this case, a responder is free to either
populate the Reply chunk or send the RPC Reply without the use of the
Reply chunk. The requester's cost of preparing the Reply chunk has
been wasted, and the extra registration and invalidation adds
unwanted latency to the operation.
A better method of handling RPC replies could ensure that RPC Replies
can be received even when the maximum possible size of some replies
cannot be calculated in advance. This method could also ensure that
no extra memory registration/invalidation operations are necessary to
make this guarantee.
This document resurrects the responder-provided Read chunk mechanism
that was briefly outlined in [RFC5666] to achieve these goals. The
discussion in this document assumes the reader is familiar with
[RFC8166].
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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.
3. Problem Statement
RPC-over-RDMA version 1 uses an RDMA Send request to transmit
transport headers and small RPC messages.
Each peer on an RPC-over-RDMA transport connection provisions Receive
buffers in which to capture incoming RDMA Send messages. There is a
limited number of these buffers, necessitating accounting in the
transport protocol to prevent a peer from emitting more Send
operations than the receiver is prepared for.
Because the selection of Receive Work Request to handle an incoming
Send is outside the control of the host O/S, the smallest buffer in
this pool determines the largest size message that can be received.
The size of the largest message that can be received via RDMA Send is
known as the receiver's "inline threshold" [RFC8166].
When marshaling an RPC transaction, a requester allocates and
registers a Reply chunk whenever the maximum possible size of the
corresponding RPC-over-RDMA reply is larger than the requester's
receive inline threshold. The Reply chunk is presented to the
responder as part of the RPC Call. The responder may place the
associated RPC Reply message in the memory region linked with this
Reply chunk.
3.1. Reply Chunk Overrun
If a responder overruns a Reply chunk during an RDMA Write, a memory
protection error occurs. This typically results in connection loss.
Any RPC transactions running on that connection must be
retransmitted. The failing RPC transaction will never get a reply,
and retransmitting it may result in additional connection loss
events.
A smart responder compares the size of an RPC Reply with the size of
the target Reply chunk before initiating the placement of data in
that chunk. A generic RDMA_ERROR message reports the problem and the
requester can terminate the RPC transaction.
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In either case, the RPC is executed by the responder, but the
requester does not receive the results or acknowledgement of its
completion.
3.2. Reply Size Calculation
To determine when a Reply chunk is needed, requesters calculate the
maximum possible size of the RPC Reply message expected for each
transaction. Upper Layer Bindings, such as [RFC8267] provide
guidance on how to calculate Reply sizes and in what cases the Upper
Layer Protocol might have difficulty giving an exact upper bound.
Unfortunately, there are rare cases where an upper bound cannot be
computed. For instance, there is no way to know how large an NFS
Access Control List (ACL) is until it is retrieved from an NFS server
[RFC5661]. There is no protocol-specified limit on the size of NFS
ACLs. When retrieving an NFS ACL, there is always a risk, albeit a
small one, that the NFS client has not provided a large enough Reply
chunk, and that therefore the NFS server will not be able to return
that ACL to the client (unless somehow a larger Reply chunk can be
provided).
3.3. Requester Registration Costs
For an Upper Layer Protocol such as NFS version 4.2 [RFC7862], NFS
COMPOUND Call and Reply messages can be large on occasion. For
instance, an NFSv4.2 COMPOUND can contain a LOOKUP operation together
with a GETATTR operation. The size of a LOOKUP result is relatively
small. However, the GETATTR in that COMPOUND may request attributes,
such as ACLs or security labels, that can grow arbitrarily large and
whose size is not known in advance.
Thus a requester can be responsible for provisioning quite a large
reply buffer for each LOOKUP COMPOUND, which is a frequent request.
If the maximum possible reply message can be large, the requester is
required to provide a Reply chunk. Most of the time, however, the
actual size of a LOOKUP COMPOUND reply is small enough to be sent
using one RDMA Send.
In other words, an NFS version 4 client provides a Reply chunk quite
frequently during RPC transactions, but NFS version 4 servers almost
never need to use it because the actual size of replies is typically
less than the inline threshold. The overhead of registering and
invalidating this chunk is significant. Moreover it is unnecessary
whenever the size of an actual RPC reply is small.
Before an RPC transaction is terminated, a requester is responsible
for fencing the Reply chunk from the responder [RFC8166]. That makes
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RPC completion synchronous with Reply chunk invalidation. Therefore
the latency of Reply chunk invalidation adds to the total execution
time of the RPC transaction.
3.4. Denial of Service
When an RPC transaction is canceled or aborted (for instance, because
an application process exited prematurely), a requester must
invalidate or set aside Write and Reply chunks associated with that
transaction [RFC8166].
This is because that RPC transaction is still running on the
responder. The responder remains obligated to return the result of
that transaction via RDMA Write, if there are Write or Reply chunks.
If memory registered on behalf of that transaction is re-used, the
requester must protect that memory from server RDMA Writes associated
with previous transactions by fencing it from the responder. The
responder triggers a memory protection error when it writes into
those memory regions, and the connection is lost.
A malfunctioning application or a malicious user on the requester can
create a situation where RPCs are continuously initiated and then
aborted, resulting in responder replies that repeatedly terminate the
underlying RPC-over-RDMA connection.
A rogue responder can purposely overrun a Reply chunk to kill a
connection. Repeated connection loss can result in a Denial of
Service.
3.5. Estimating Transport Header Size
To determine whether a Reply chunk is needed, a requester computes
the size of the Reply's Transport Header and the maximum possible
size of the RPC Reply message, and sums the two. If the sum is
smaller than the requester's receive inline threshold, a Reply chunk
is not required.
The size of a Transport Header depends on how many Write chunks the
requester provides, whether a Reply chunk is needed, and how many
segments are contained in provided Write and Reply chunks.
When the total size of the Reply message is already near the inline
threshold, therefore, a requester has to know whether a Reply chunk
is needed (and how many segments it contains) before it can determine
if a Reply chunk is needed.
A requester can resort to limiting Transport Header size to a fixed
value that ensures this computation does not become a recursion.
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However, as in earlier sections, this can mean that some RPC
transactions where a Reply chunk is not strictly necessary must incur
the cost of preparing a Reply chunk.
4. Responder-Provided Read Chunks
A potential mechanism for resolving these issues is suggested in
Section 3.4 of [RFC5666]:
In the absence of a server-provided read chunk list in the reply,
if the encoded reply overflows the posted receive buffer, the RPC
will fail with an RDMA transport error.
When sending a large RPC Call message, requesters already employ Read
chunks. There is no advance indication or limit on the size of any
RPC Call message. To achieve the same flexibility for RPC Replies,
Read chunks can be used in the reverse direction (e.g., responder
exposes memory, requester initiates RDMA Read).
Rather than a requester providing a Reply chunk for conveying an as-
yet-unconstructed large reply, a responder can expose a Read chunk
containing the actual Payload stream of the RPC Reply message. A
responder would employ a Read chunk to return a reply any time
requester-provided reply resources are not adequate.
The requester does not have to calculate a reply size maximum or
register and invalidate a Reply chunk in these cases. Without a
requester-provided Reply chunk, the responder sends each reply
inline, except when the actual size of an RPC Reply message is larger
than the receiver's inline threshold.
This results in no wasted activity on the requester and arbitrarily
large RPC Replies can be received reliably.
Current RPC-over-RDMA version 1 implementations do not support
responder-provided Read chunks, although RPC-over-RDMA version 1 did
have this support in the past [RFC5666]. Adapting this deprecated
mechanism for new RPC-over-RDMA transports is straightforward.
4.1. Specification
A responder MAY choose to send an RPC Reply using a Position Zero
Read chunk comprised of one or more RDMA segments. Position Zero
Read chunks are defined in Section 3.5.3 of [RFC8166].
Similar to its use in an RPC Call, a Position Zero Read chunk in an
RPC Reply contains an RPC Reply's Payload stream. Position Zero Read
chunks are always sent using an RPC-over-RDMA RDMA_NOMSG message.
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In other words, a responder-provided Read chunk can replace the use
of a Reply chunk in Long Replies. And, as with Reply chunks, a
responder must still make use of Write chunks provided by the
requester.
4.1.1. Responder Duties
A responder MUST send a Position Zero Read chunk when the actual size
of the RPC Reply's Payload stream exceeds all requester-provided
reply resources; that is, when the inline threshold and any provided
Reply chunk are both too small to accommodate the Payload stream of
the reply.
If a responder does not support responder-provided Read chunks in
this case, it MUST return an appropriate permanent transport error to
terminate the requester's RPC transaction.
4.1.2. Requester Duties
Upon receipt of an RDMA_NOMSG message containing a Position Zero Read
chunk, the requester pulls the RPC Reply's Payload stream from the
responder.
After RDMA Read operations have completed (successfully or in error),
the requester MUST inform the responder that it may invalidate the
Read chunk containing the RPC Reply message. This is referred to as
"pull completion notification".
4.1.3. Pull Completion Notification
Pull completion notification is accomplished in one of two ways:
o The requester can send an RDMA_DONE message with the rdma_xid
field set to the same value as the rdma_xid field in the
RDMA_NOMSG request. Or,
o The requester can piggyback the pull completion notification in
the transport header of a subsequent RPC Call, if the transport
protocol has such a facility.
When an RPC transaction is aborted on a requester, the requester
normally forgets its XID. If a requester receives a reply bearing a
Position Zero Read chunk and does not recognize the XID, the
requester MUST notify the responder of pull completion.
Whenever a responder receives a pull completion notification for an
XID for which there is no Read chunk waiting to be invalidated, the
responder MUST silently drop the notification.
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If a requester receives an RPC Reply via a responder-provided Read
chunk, but does not support such chunks, it MUST inform the responder
of pull completion and terminate the RPC transaction.
A malicious or broken requester might neglect to send pull completion
notifications for one or more RPC transactions that included
responder-provided Read chunks. To prevent exhaustion of responder
resources, a responder can choose to invalidate its Read chunks after
waiting for a short period. If the requester attempts additional
RDMA Read operations against that Read chunk, a remote access error
occurs and the connection is lost.
4.1.4. Remote Invalidation
Remote Invalidation can reduce or eliminate the need for the
responder to explicitly invalidate memory containing an RPC Reply
message.
Remote Invalidation might be done by transmitting an RDMA_DONE
message using RDMA Send With Invalidate. If instead pull completion
notification is piggybacked on a subsequent RPC Call, a facility for
Remote Invalidation would have to be built into RPC Call processing.
If Remote Invalidate support is not indicated by one or both peers,
messages carrying pull completion notification MUST be transmitted
using RDMA Send. If Remote Invalidation support is indicated by both
peers, messages carrying pull completion messages SHOULD be
transmitted using RDMA Send With Invalidate.
The rule for choosing the value of the Send With Invalidate Work
Request's inv_handle field depends on the version of the transport
protocol that is use. If the responder has provided an R_key that
may be invalidated, the requester MUST present only that R_key when
using RDMA Send With Invalidate.
5. Analysis
5.1. Benefits
5.1.1. Less Frequent Use of Explicit RDMA
The vast majority of RPC Replies can be conveyed via RDMA_MSG. No
extra Reply chunk registration and invalidation cost is incurred when
a large RPC Reply message is possible but the actual reply size is
small. This reduces or even eliminates the use of explicit RDMA for
frequent small-to-moderate-size replies, improving the average
latency of individual RPCs and allowing RNIC and platform resources
to scale better.
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5.1.2. Support for Arbitrarily Large Replies
The responder-provided Read chunk approach accommodates arbitrarily
large replies. Requesters no longer need to calculate the maximum
size of RPC Reply messages, even if a Reply chunk is provided.
5.1.3. Protection of Connection After RPC Cancellation
When an RPC is canceled on the requester (say, because the requesting
application has been terminated), and no Reply chunk is provided, the
requester is no longer responsible for invalidating that RPC's Reply
chunk. When the responder sends the reply, it provides a Position
Zero Read chunk and does not use RDMA Write to transmit the RPC Reply
message. The transport connection is preserved because no memory
protection violation can occur.
5.1.4. Asynchronous Chunk Invalidation
Registration of a responder-provided Read chunk must be completed
before sending the RDMA_NOMSG message conveying the chunk
information. However, pull completion notification and subsequent
responder-side memory invalidation can be performed after the RPC
transaction has completed on the requester. Because those are
asynchronous to RPC completion, the additional latency is not
attributed to the execution time of the RPC transaction.
5.2. Costs
5.2.1. Responder Memory Exposure
Responder memory is registered and exposed to requesters when
replying. When a responder has properly allocated a Protection
Domain for each connection and uses appropriate R_key rotation
techniques (see Section 7), the exposure is minimal. However,
because current RPC-over-RDMA responder implementations do not expose
memory to requesters, they typically share one Protection Domain
among all connections.
5.2.2. Round Trip Penalty
Using a Read chunk for large replies introduces a round-trip penalty.
A requester can provide a Reply chunk to avoid this penalty.
However:
o The Read chunk round-trip penalty would be paid much less often
than the Reply chunk registration cost is paid today, since
responder-provided Read chunks are used only when necessary
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o Read chunk frequency is reduced even further as the inline
threshold is increased past the average size of the Upper Layer
Protocol's RPC Replies
o Invalidation of a Reply chunk is synchronous with RPC completion,
and may take as long as a round trip to the responder
o Read chunks are typically used for large payloads, where it is
likely that data transmission time greatly exceeds the round-trip
time
There are a few particular situations where the frequency of large
replies is high. For example, the use of the krb5i or krb5p GSS
services with RPC-over-RDMA require that Payload reduction is not
used. Thus, RPC-over-RDMA peers use only pure RDMA Sends or Long
messages when these services are in use. The actual size of a
READDIR reply is often unpredictable but is frequently large. In
these two cases, using a Reply chunk could be the more efficient
default choice.
5.2.3. Credit Accounting Complexity
Credit accounting is made more complex by the use of RDMA_DONE
messages after RDMA Read operations have completed. Sending an
RDMA_DONE message consumes one credit, temporarily reducing RPC
concurrency on the connection. There is no response to RDMA_DONE, so
it is not clear to the sender when that credit becomes available
again. One way to resolve this is to add a new message type to the
protocol, RDMA_ACK, which could be used any time there is a uni-
directional transport message to maintain the proper balance of
credit grants and responses.
Alternately, if the transport protocol supports piggybacking pull
completion notification on RPC Call messages, the requester can
piggyback in most cases to simplify credit accounting. An explicit
RDMA_DONE would be necessary only during light workloads, or the ULP
could post an RPC NULL containing a piggybacked pull completion
notification in these cases.
5.3. Selecting a Reply Mechanism
This section illustrates some possible implementation choices.
5.3.1. Requester
As an RPC Call is constructed, a requester might choose a reply
mechanism based on its estimation of the range of possible sizes of
the reply.
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Responder-provided Read chunk
The requester knows the minimum size of the reply is smaller than
the inline threshold, but the maximum size of the reply is larger
than the inline threshold; or the requester cannot calculate the
maximum size of the reply. The client does not provide a Reply
chunk, and relies on a responder-provider Read chunk to handle
large replies.
Reply chunk
The requester knows the minimum and maximum size of the reply is
larger than the inline threshold. The requester provides a Reply
chunk.
Send-only
The requester knows the maximum size of the reply is smaller than
the inline threshold. The requester does not provide a Reply
chunk, and relies on a responder-provider Read chunk to handle
large replies.
A requester whose design requires Reply chunk invalidation after an
RPC transaction is canceled might choose to never use Reply chunks,
in favor of minimizing opportunities for connection loss.
5.3.2. Responder
After a responder has constructed an RPC Reply, it might choose which
reply mechanism to employ based on the actual size of the Payload
stream of the RPC Reply message.
Responder-provided Read chunk
The Payload stream is larger than the inline threshold and either
no Reply chunk was provided or the provided Reply chunk is too
small. The responder uses a responder-provided Read chunk.
Reply chunk
If a usable Reply chunk is available, the responder uses the Reply
chunk.
Send-only
If no Reply chunk is available and the Payload stream fits within
the inline threshold, the responder uses only Send or Send With
Invalidate to transmit the reply.
5.4. Implementation Complexity
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5.4.1. RPC Call Path
Implementation of responder-provided Read chunks introduces little or
no additional complexity to the end-to-end RPC Call path. Unless a
requester implementer chooses to implement support for both Reply
chunks and responder-provided Read chunks, there could be a net loss
of code and run-time complexity in the RPC Call hot path.
The responder's RPC Call path needs to recognize RDMA_DONE messages
and initiate invalidation of Read chunks. Because invalidation can
be asynchronous, it is possible to perform Read chunk invalidation in
a separate worker thread.
5.4.2. RPC Reply Path
On the RPC Reply path side, logic to initiate registration of Read
chunks and wait for completion is added to the responder. This path
is not part of the hot path because it is used only infrequently.
The requester's reply handling hot path must recognize when Read
chunks are present in an RDMA_NOMSG message, and shunt execution to
code that can initiate an RDMA Read and wait for completion. Once
complete, the requester posts an RDMA_DONE message.
5.4.3. Managing RDMA_DONE messages
In order for a responder to match incoming RDMA_DONE messages to
reply buffers waiting to be invalidated, it might keep references to
these buffers in a data structure searchable by XID. This is similar
to managing a set of pending backchannel replies.
When an RDMA_DONE message arrives, the responder matches the XID in
the message to a waiting reply buffer, invalidates that buffer, and
removes the XID from the data structure.
This data structure can also be used for housekeeping tasks such as:
o Invalidating waiting buffers after a timeout, in case the
requester never sends RDMA_DONE
o Ignoring retransmitted or garbage RDMA_DONE requests
o Explicitly invalidating waiting Read chunks after a connection
loss, if necessary
o Invalidating waiting buffers on device removal
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5.5. Alternatives
Increasing the inline threshold reduces the likelihood of needing a
Reply chunk, but does not eliminate the risks associated with
unpredictably large replies.
Message Continuation is more efficient than an explicit RDMA
operation, and does not require the exposure of requester or
responder memory
However, Message Continuation still limits the maximum size of a
conveyed message. As with a larger inline threshold, without
responder-provided Read chunks, reply size estimation is still
required to determine when a Reply chunk is required, and therefore
there is still risk associated with unpredictably large replies.
Message Continuation introduces complexity in the management of RPC-
over-RDMA credit grants because the relationship between RPC
transactions and credits is no longer one-to-one. Credit management
logic is an integral part of the RPC Call and Reply hot path on the
requester.
6. Interoperation Considerations
When a requester supports responder-provided Read chunks, it is
likely to neglect providing Reply chunks in some cases. A responder
that does not support responder-provided Read chunks can convey a
transport-level error when it has generated an RPC Reply that is
larger than the available reply resources.
The situation is more problematic if a responder supports responder-
provided Read chunks and sends them to a requester that is not able
to recognize and unmarshal them. The RPC transaction would never
complete, and the requester would never send a pull completion
notification.
Thus responder-provided Read chunks MUST be used only when both peers
support them: Either the base protocol version always has support
enabled, or the base protocol provides an extension mechanism that
indicates when support is available.
7. Security Considerations
The less frequent use of RDMA Write reduces opportunities for memory
overrun on the requester, and reduces the risk of connection loss
after an application is terminated prematurely. This reduces
exposure to accidental or malicious Denial of Service attacks.
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Responder-provided Read chunks are exposed for read-only access.
Remote actors cannot alter the contents of exposed read-only memory,
though a man-in-the-middle can read or alter RDMA payloads while they
are in transit. The use of RPCSEC GSS or a transport-layer
confidentiality service completely blocks payload access by
unintended recipients.
Recommendations about adequate R_key rotation and the appropriate use
of Protection Domains can be found in Section 8.1 of [RFC8166].
These recommendations apply when responders expose memory to convey
the Payload stream of an RPC Reply message.
Otherwise, this mechanism does not alter the attack surface of a
transport protocol that employs it.
8. IANA Considerations
This document has no IANA actions.
9. References
9.1. 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>.
[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>.
9.2. Informative References
[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>.
[RFC5666] Talpey, T. and B. Callaghan, "Remote Direct Memory Access
Transport for Remote Procedure Call", RFC 5666,
DOI 10.17487/RFC5666, January 2010,
<https://www.rfc-editor.org/info/rfc5666>.
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[RFC7862] Haynes, T., "Network File System (NFS) Version 4 Minor
Version 2 Protocol", RFC 7862, DOI 10.17487/RFC7862,
November 2016, <https://www.rfc-editor.org/info/rfc7862>.
[RFC8267] Lever, C., "Network File System (NFS) Upper-Layer Binding
to RPC-over-RDMA Version 1", RFC 8267,
DOI 10.17487/RFC8267, October 2017,
<https://www.rfc-editor.org/info/rfc8267>.
Acknowledgments
Many thanks go to Karen Dietke, Chunli Zhang, Dai Ngo, and Tom
Talpey. The author also wishes to thank Bill Baker and Greg Marsden
for their support of this work.
Special thanks go to Transport Area Director Magnus Westerlund, NFSV4
Working Group Chairs Spencer Shepler and Brian Pawlowski, and NFSV4
Working Group Secretary Thomas Haynes for their support.
Author's Address
Charles Lever
Oracle Corporation
United States of America
Email: chuck.lever@oracle.com
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