Internet-Draft T. Talpey
Internet-Draft J. Pinkerton
Updates: 5040, 7306 (if approved) Microsoft
Intended status: Standards Track
Expires: August 22, 2016 February 19, 2016
RDMA Durable Write Commit
draft-talpey-rdma-commit-00
Abstract
This document specifies extensions to RDMA protocols to provide
capabilities in support of enhanced remotely-directed data
consistency. The extensions include a new operation supporting
remote commitment to durability of remotely-managed buffers, which
can provide enhanced guarantees and improve performance for low-
latency storage applications. In addition to, and in support of
these, extensions to local behaviors are described, which may be used
to guide implementation, and to ease adoption. This document would
extend the IETF Remote Direct Memory Access Protocol (RDMAP),
RFC5040, and RDMA Protocol Extensions, RFC7306.
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 RFC 2119 [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
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on August 22, 2016.
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Copyright Notice
Copyright (c) 2016 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
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Glossary . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Requirements . . . . . . . . . . . . . . . . . . . . . . 7
2.1.1. Non-Requirements . . . . . . . . . . . . . . . . . . 9
2.2. Additional Semantics . . . . . . . . . . . . . . . . . . 10
3. Proposed Extensions . . . . . . . . . . . . . . . . . . . . . 11
3.1. Local Extensions . . . . . . . . . . . . . . . . . . . . 11
3.1.1. Registration Semantics . . . . . . . . . . . . . . . 11
3.1.2. Completion Semantics . . . . . . . . . . . . . . . . 12
3.1.3. Platform Semantics . . . . . . . . . . . . . . . . . 12
3.2. RDMAP Extensions . . . . . . . . . . . . . . . . . . . . 12
3.2.1. RDMA Commit Request Header Format . . . . . . . . . . 15
3.2.2. RDMA Commit Response Header Format . . . . . . . . . 16
3.2.3. Ordering . . . . . . . . . . . . . . . . . . . . . . 16
3.2.4. Atomicity . . . . . . . . . . . . . . . . . . . . . . 17
3.2.5. Discovery of RDMAP Extensions . . . . . . . . . . . . 17
4. Ordering and Completions Table . . . . . . . . . . . . . . . 18
5. Error Processing . . . . . . . . . . . . . . . . . . . . . . 18
5.1. Errors Detected at the Local Peer . . . . . . . . . . . . 18
5.2. Errors Detected at the Remote Peer . . . . . . . . . . . 19
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
7. Security Considerations . . . . . . . . . . . . . . . . . . . 20
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 20
8.1. Normative References . . . . . . . . . . . . . . . . . . 20
8.2. Informative References . . . . . . . . . . . . . . . . . 21
8.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Appendix A. DDP Segment Formats for RDMA Extensions . . . . . . 22
A.1. DDP Segment for RDMA Commit Request . . . . . . . . . . . 22
A.2. DDP Segment for RDMA Commit Response . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 24
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1. Introduction
The RDMA Protocol (RDMAP) [RFC5040] and RDMA Protocol Extensions
(RDMAPEXT) [RFC7306] provide capabilities for secure, zero copy data
communications that preserve memory protection semantics, enabling
more efficient network protocol implementations. The RDMA Protocol
is part of the iWARP family of specifications which also include the
Direct Data Placement Protocol (DDP) [RFC5041], and others as
described in the relevant documents. For additional background on
RDMA Protocol applicability, see "Applicability of Remote Direct
Memory Access Protocol (RDMA) and Direct Data Placement Protocol
(DDP)" RFC5045 [RFC5045].
RDMA protocols are enjoying good success in improving the performance
of remote storage access, and have been well-suited to semantics and
latencies of existing storage solutions. However, new storage
solutions are emerging with much lower latencies, driving new
workloads and new performance requirements. Also, storage
programming paradigms SNIANVM [SNIANVM] are driving new requirements
of the remote storage layers, in addition to driving down latency
tolerances. Overcoming these latencies, and providing the means to
achieve durability without invoking upper layers and remote CPUs for
each such request, are the motivators for the extensions proposed by
this document.
This document specifies the following extensions to the RDMA Protocol
(RDMAP) and its local memory ecosystem:
o RDMA Commit - support for RDMA requests and responses with
enhanced placement semantics.
o Enhanced memory registration semantics in support of durability.
The extensions defined in this document do not require the RDMAP
version to change.
1.1. Glossary
This document is an extension of RFC 5040 and RFC 7306, and key words
are additionally defined in the glossaries of the referenced
documents.
The following additional terms are defined in this document.
Commit: The placement of data into storage referenced by a target
Tagged Buffer in a durable fashion.
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Durability: The property that data is present and remains stable
after recovery from a power failure or other fatal error in an
upper layer or hardware. <https://en.wikipedia.org/wiki/
Durability_(database_systems)>, <https://en.wikipedia.org/wiki/
Disk_buffer#Cache_control_from_the_host>[SCSI],
2. Problem Statement
RDMA is widely deployed in support of storage and shared memory over
increasingly low-latency and high-bandwidth networks. The state of
the art today yields end-to-end network latencies on the order of one
to two microseconds for message transfer, and bandwidths exceeding 40
gigabit/s. These bandwidths are expected to increase over time, with
latencies decreasing as a direct result.
In storage, another trend is emerging - greatly reduced latency of
persistently storing data blocks. While best-of-class Hard Disk
Drives (HDDs) have delivered latencies of several milliseconds for
many years, Solid State Disks (SSDs) have improved this by one to two
orders of magnitude. Technologies such as NVM Express NVMe [1] yield
even higher-performing results by eliminating the traditional storage
interconnect. The latest technologies providing memory-based
persistence, such as Nonvolatile Memory DIMM NVDIMM [2], places
storage-like semantics directly on the memory bus, reducing latency
to less than a microsecond and bandwidth to potentially many tens of
gigabyte/s. [supporting data to be added]
RDMA protocols, in turn, are used for many storage protocols,
including NFS/RDMA RFC5661 [RFC5661] RFC5666 [RFC5666] RFC5667
[RFC5667], SMB Direct MS-SMB2 [SMB3] MS-SMBD [SMBDirect] and iSER
RFC7145 [RFC7145], to name just a few. These protocols allow storage
and computing peers to take full advantage of these highly performant
networks and storage technologies to achieve remarkable throughput,
while minimizing the CPU overhead needed to drive their workloads.
This leaves more computing resources available for the applications,
which in turn can scale to even greater levels. Within the context
of Cloud-based environments, and through scale-out approaches, this
can directly reduce the number of servers that need to be deployed,
making such attributes compelling.
However, limiting factors come into play when deploying ultra-low
latency storage in such environments:
o The latency of the fabric, and of the necessary RDMA message
exchanges to ensure reliable transfer is now higher than that of
the storage itself.
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o The requirement that storage be resilient to failure requires that
multiple copies be committed in multiple locations across the
fabric, adding extra hops which increase the latency and computing
demand placed on implementing the resiliency.
o Processing is required at the receiver in order to ensure that the
storage data has reached a persistent state, and acknowledge the
transfer so that the sender can proceed.
o Typical latency optimizations, such as polling a receive memory
location for a key that determines when the data arrives, can
create both correctness and security issues because the buffer may
not remain stable after the application determines that the IO has
completed. This is of particular concern in security conscious
environments.
The first issue is fundamental, and due to the nature of serial,
shared communication channels, presents challenges that are not
easily bypassed. Therefore, an RDMA solution which reduces the
exchanges which encounter such latencies is highly desirable.
The second issue requires that outbound transfers be made as
efficient as possible, so that replication of data can be done with
minimal overhead and delay (latency). A reliable "push" RDMA
transfer method is highly suited to this.
The third issue requires that the transfer be performed without an
upper-layer exchange required. Within security contraints, RDMA
transfers arbitrated only by lower layers into well-defined and pre-
advertised buffers present an ideal solution.
The fourth issue requires significant CPU activity, consuming power
and valuable resources, and additionally is not guaranteed by the
RDMA protocols, which make no guarantee of the order in which
received data is placed or becomes visible; such guarantees are made
only after signaling a completion to upper layers.
The RDMAP and DDP protocols, together, provide data transfer
semantics with certain consistency guarantees to both the sender and
receiver. Delivery of data transferred by these protocols is said to
have been Placed in destination buffers upon Completion of specific
operations. In general, these guarantees are limited to the
visibility of the transferred data within the hardware domain of the
receiver (data sink). Significantly, the guarantees do not
necessarily extend to the actual storage of the data in memory cells,
nor do they convey any guarantee of durability, that is, that the
data may not be present after a catastrophic failure such as power
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loss. These guarantees may be provided by upper layers, such as the
ones mentioned.
The NFSv4.1 and iSER protocols are, respectively, file and block
oriented, and have been used extensively for providing access to hard
disk and solid state flash drive media. Such devices incur certain
latencies in their operation, from the millisecond-order rotational
and seek delays of rotating disk hardware, or the 100-microsecond-
order erase/write and translation layers of solid state flash. These
file and block protocols have benefited from the increased bandwidth,
lower latency, and markedly lower CPU overhead of RDMA to provide
excellent performance for such media, approximately 30-50
microseconds for 4KB writes in leading implementations.
These protocols employ a "pull" model for write: the client, or
initiator, sends an upper layer write request which contains a
reference to the data to be written. The upper layer protocols
encode this as one or more memory regions. The server, or target,
then prepares the request for local write execution, and "pulls" the
data with an RDMA Read. After processing the write, a response is
returned. There are therefore two or more roundtrips on the RDMA
network in processing the request. This is desirable for several
reasons, as described in the relevant specifications, but it incurs
latency. However, since as mentioned the network latency has been so
much less than the storage processing, this has been a sound
approach.
Today, a new class of Storage Class Memory is emerging, in the form
of Non-Volatile DIMM and NVM Express devices, among others. These
devices are characterized by further reduced latencies, in the 10-
microsecond-order range for NVMe, and sub-microsecond for NVDIMM.
The 30-50 microsecond write latencies of the above file and block
protocols are therefore from one to two orders of magnitude larger
than the storage media! The client/server processing model of
traditional storage protocols are no longer amortizable at an
acceptable level into the overall latency of storage access, due to
their requiring request/response communication, CPU processing by the
both server and client (or target and initiator), and the interrupts
to signal such requests.
Another important property of certain such devices is the requirement
for explicitly requesting that the data written to them be made
durable. Because durability requires that data be committed to
memory cells, it is a relatively expensive operation in time (and
power), and in order to maintain the highest device throughput and
most efficient operation, the "commit" operation is explicit. When
the data is written by an application on the local platform, this
responsibility naturally falls to that application (and the CPU on
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which it runs). However, when data is written by current RDMA
protocols, no such semantic is provided. As a result, upper layer
stacks, and the target CPU, must be invoked to perform it, adding
overhead and latency that is now highly undesirable.
When such devices are deployed as the remote server, or target,
storage, and when such a durability can be requested and guaranteed
remotely, a new transfer model can be considered. Instead of relying
on the server, or target, to perform requested processing and to
reply after the data is durably stored, it becomes desirable for the
client, or initiator, to perform these operations itself. By
altering the transfer models to support a "push mode", that is, by
allowing the requestor to push data with RDMA Write and subsequently
make it durable, a full round trip can be eliminated from the
operation. Additionally, the signaling, and processing overheads at
the remote peer (server or target) can be eliminated. This becomes
an extremely compelling latency advantage.
Together the above discussion argues for a new transfer model
supporting remote durability guarantees, provided by the RDMA
transport, and used directly by upper layers on a data source, to
control durable storage of data on a remote data sink without
requiring its remote interaction. Existing, or new, upper layers can
use such a model in several ways, and evolutionary steps to support
durability guarantees without required protocol changes are explored
in the remainder of this document.
Note that is intended that the requirements and concept of these
extensions can be applied to any similar RDMA protocol, and that a
compatible remote durability model can be applied broadly.
2.1. Requirements
The fundamental new requirement for extending RDMA protocols is to
define the property of _durability_. This new property drives the
operations to extend Placement as defined in existing RDMA protocols.
When Placed, these protocols require only that the data be visible
consistently to both the platform on which the buffer resides, and to
remote peers across the network via RDMA. In modern hardware
designs, this buffer can reside in memory, or also in cache, if that
cache is part of the hardware consistency domain. Many designs use
such caches extensively to improve performance of local access.
Durability, by contrast, requires that the data not only be
consistently visible, it further requires that the buffer contents be
preserved across catastrophic failures. While it is possible for
caches to be durable, they are typically not. Efficient designs, in
fact, lead many implementations to make them volatile. In these
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designs, an explicit flush operation, often followed by an explicit
commit, is required to provide this guarantee.
For the RDMA protocol to remotely provide durability guarantees, the
new requirement is mandatory. Note that this does not imply support
for durability by the RDMA hardware implementation itself; it is
entirely acceptable for the RDMA implementation to request durability
from another subsystem, for example, by requesting that the CPU
perform the flush and commit. But, in an ideal implementation, the
RDMA implementation will be able to act as a master and provide these
services without further work requests. Note, it is possible that
different buffers will require different durability processing, for
example one buffer may reside in persistent memory, while another may
place its durable blocks in a persistent storage device. Many such
memory-addressable designs are entering the market, from NVDIMM to
NVMe and even to SSDs and hard drives.
Therefore, any local memory registration primitive will be enhanced
to specify an optional durability attribute, along with any local
information required to achieve it. These attributes remain local -
like existing local memory registration, the region is fully
described by a { handle, offset, length } descriptor, and such
aspects of the local physical address, memory type, protection
(remote read, remote write, protection key), etc are not instantiated
in the protocol. The RDMA implementation maintains these, and
strictly performs processing based on them, but they are not known to
the peer, and therefore are not a matter for the protocol.
Note, additionally, that by describing durability only through the
presence of an optional durability attribute, it is possible to
describe regions as both durable and non-durable, in order to enable
efficient processing. When commit is remotely requested of a non-
durable region, the result is not required to be that the data is
durable. This can be used by upper layers to enable bulk-type
processing with low overhead, by assigning specific durability
through use of the Steering Tag.
The intention is that if the underlying region is marked as non-
volatile, the placement of data into it is also non-volatile (i.e.
any volatile buffering between the network and the underlying storage
has been flushed).
To enable the maximum generality, the commit operation is specified
to act on a list of { handle, offset, length } regions. The
requirement is that each byte of each specified region be made
durable before the response to the commit is generated. However,
depending on the implementation, other bytes in other regions may be
made durable as part of processing any commit. Any data in any
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buffer destined for persistent, durable storage, may become durable
at any time, even if not requested explicitly. For example, a simple
and stateless approach would be for all data be flushed and
committed, system-wide. A possibly more efficient implementation
might track previously written bytes, or blocks with "dirty" bytes,
and commit only those. Either result provides the required
guarantee. The length of the region list, and the maximum amount of
data that can be made durable in a single request, are implementation
dependent and its protocol expression is to be described.
The commit operation is specified to return a status, which may be
zero on success but may take other values to be determined. Several
possibilities present themselves. The commit operation may fail to
make the data durable, perhaps due to a hardware failure, or a change
in device capability (device read-only, device wear, etc). The data,
however, may not have been lost and is still present in the buffer.
Or, the device may support an integrity check, similar to modern
error checking memory or media error detection on hard drive
surfaces, and its status is returned. Or, the request may exceed
device limits in size or even transient attribute such as temporary
media failure. The behavior of the device itself is beyond the scope
of this specification.
Because the commit involves processing on the local platform and the
actual device, it is expected to take a certain time to be performed.
For this reason, the commit operation is required to be defined as a
"queued" operation on the RDMA device, and therefore also the
protocol. The RDMA protocol supports RDMA Read and Atomic in such a
fashion. The iWARP family defines a "queue number" with queue-
specific processing that is naturally suited for this. Queuing
provides a convenient means for supporting ordering among other
operations, and for flow control. Flow control for RDMA Reads and
Atomics share incoming and outgoing crediting depths ("IRD/ORD");
commit will either share these, or define their own separate values.
2.1.1. Non-Requirements
The protocol does not include a "RDMA Write with durability", that
is, a modifier on the existing RDMA Write operation. While it might
seem a logical approach, several issues become apparent:
The existing RDMA Write operation is unacknowledged at the RDMA
layer. Requiring it to provide an indication of remote durability
would require it to have an acknowledgement, which would be an
undesirable extension to the operation.
Such an operation would require flow control and therefore also
buffering on the responding peer. Existing RDMA Write semantics
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are not flow controlled and as tagged transfers are by design
zero-copy i.e. unbuffered. Requiring these would introduce
potential pipeline stalls and increase implementation complexity
in a critical performance path.
The operation at the initiator would stall until the
acknowledgement of completion, significantly changing the semantic
of the existing operation, and complicating software by blocking
the send work queue. As each operation would be self-describing
with respect to durability, individual operations would therefore
block with differing semantics.
Even for the possibly-common case of commiting after every write,
it is highly undesirable to impose new optional semantics on an
existing operation. And, the same result can be achieved by
sending the commit in the same network packet, and since the RDMA
Write is unacknowledged while the commit is always replied-to, no
additional overhead is imposed on the combined exchange.
[Further expand on the undesirable nature of such a change.]
2.2. Additional Semantics
Ordering w.r.t. RDMA Write, receives, RDMA Read, other commits.
Also, ensure ordering ensures similar remote semantics to local
The commit operation is ordered with respect to certain other
operations, and it may be advantageous to combine certain actions
into the same request, or requests with specific ordering to the
commit. Examples to be discussed include:
Additional optional payload to be placed and made durable in an
atomic fashion after the requested commit. A small (64 bit)
payload, sent in the same, or other single, request, and aligned
such that it can be made durable in a single hardware operation,
can be used to satisfy the "log update" scenario (describe this in
more detail).
Immediate data to be optionally provided in a completion to an
upper layer on the remote peer. Such an indication can be used to
signal the upper layer that certain data has been placed in the
peer's buffer, and has been made available durably.
Remote invalidation, as optionally performed by existing RDMA
protocols for other operations.
Upper Layer message, an optional full message to be provided in a
completion after the commit.
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Integrity check for committed data, which could take the form of a
value to be verified before returning, or a value computed and
returned which the initiator can use to verify. Specification of
the checksum or hash algorithm, or its negotiation by an upper
layer, will be necessary if adopted.
3. Proposed Extensions
The extensions in this document fall into two categories:
o Local behavior extensions
o Protocol extensions
These categories are described, and may be implemented, separately.
3.1. Local Extensions
Here discuss memory registration, new memory and protection
attributes, and applicability to both remote and "local" (receives).
3.1.1. Registration Semantics
New platform-specific attributes to RDMA registration, allows them to
be processed at the server *only* without client knowledge, or
protocol exposure. No client knowledge - ensures future interop
New local PM memory registration example:
Register(region[], PMType, mode) -> Handle
PMType includes type of PM i.e. plain RAM, or "commit
required", or PCIe-resident, or any other local platform-
specific processing
Mode includes disposition of data Read and/or write e.g.
Cacheable after operation (needed by CPU on data sink)
Handle is processed in receiving NIC during RDMA operation to
specified region, under control of original Mode.
Also consider whether potential "integrity check" behavior can be
made per-region. If so, piggybacking it on the registration enables
selecting the integrity hash, and making its processing optional and
straightforward.
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Any other per-region durability processing to be explored.
3.1.2. Completion Semantics
Transparency is possible when upper layer provides Completions (e.g.
messages or immediate data)
Commit to durability can be piggybacked by data sink upon signaling.
Upper layer may not need to explicitly Commit in this case, which of
course are dependent on upper layer and workload.
Can apply this concept to RDMA Write with Immediate Or ...ordinary
receives. Strong possibilities exist - explore here.
Ordering of operations is critical: Such RDMA Writes cannot be
allowed to "pass" durability. Therefore, protocol implications may
also exist.
Discuss optional behaviors explored in prior section, and whether/how
they generate completions.
3.1.3. Platform Semantics
Writethrough behavior on durable regions and reasons for same.
Consider requiring/recommending a local writethrough behavior on any
durable region, to support a nonblocking hurry-up to avoid future
stalls on a subsequent cache flush, prior to a commit. Also, it
would enhance durability.
PCI extension to support Commit Allow NIC to provide durability
directly and efficiently To Memory, CPU, PCI Root, PM device, PCIe
device, ... Avoids CPU interaction Supports strong data consistency
model Performs equivalent of: CLFLUSHOPT (region list) PCOMMIT Or if
NIC is on memory bus or within CPU complex... Other possibilities
exist
3.2. RDMAP Extensions
This document defines a new RDMA operation, "RDMA Commit". The wire-
related aspects of the proposed protocol are discussed in this
section.
This section and the ones following present one possible approach
toward defining the wire protocol defined by the above discussion.
The definitions are included for initial discussion and do not
comprise a complete specification. Certain additional protocol
features of any potential new extension, such as any associated
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Immediate Data, Solicited Events, Remote Invalidation, ULP Message
inclusion, etc, are left to a later version.
For reference, Figure 1 depicts the format of the DDP Control and
RDMAP Control Fields, in the style and convention of RFC 5040 and
RFC7306:
The DDP Control Field consists of the T (Tagged), L (Last), Resrv,
and DV (DDP protocol Version) fields RFC 5041. The RDMAP Control
Field consists of the RV (RDMA Version), Rsv, and Opcode fields RFC
5040.
This specification adds values for the RDMA Opcode field to those
specified in RFC 5040. Table 1 defines the new values of the RDMA
Opcode field that are used for the RDMA Messages defined in this
specification.
As shown in Table 1, STag (Steering Tag) and Tagged Offset are valid
only for certain RDMA Messages defined in this specification.
Table 1 also shows the appropriate Queue Number for each Opcode.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|T|L| Resrv | DV| RV|R| Opcode |
| | | | | |s| |
| | | | | |v| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Invalidate STag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: DDP Control and RDMAP Control Fields
All RDMA Messages defined in this specification MUST carry the
following values:
o The RDMA Version (RV) field: 01b.
o Opcode field: Set to one of the values in Table 1.
o Invalidate STag: Set to zero, or optionally to non-zero by the
sender, processed by the receiver.
Note: N/A in the table below means Not Applicable
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-------+-----------+-------+------+-------+-----------+--------------
RDMA | Message | Tagged| STag | Queue | Invalidate| Message
Opcode | Type | Flag | and | Number| STag | Length
| | | TO | | | Communicated
| | | | | | between DDP
| | | | | | and RDMAP
-------+-----------+-------+------+-------+-----------+--------------
-------+-----------+-------------------------------------------------
01100b | RDMA | 0 | N/A | 1 | opt | Yes
| Commit | | | | |
| Request | | | | |
-------+-----------+-------------------------------------------------
01101b | RDMA | 0 | N/A | 3 | N/A | Yes
| Commit | | | | |
| Response | | | | |
-------+-----------+-------------------------------------------------
Table 1: Additional RDMA Usage of DDP Fields
This extension adds RDMAP use of Queue Number 1 for Untagged Buffers
for issuing RDMA Commit Requests, and use of Queue Number 3 for
Untagged Buffers for tracking RDMA Commit Responses.
All other DDP and RDMAP Control Fields are set as described in RFC
5040 and RFC 7306.
Table 2 defines which RDMA Headers are used on each new RDMA Message
and which new RDMA Messages are allowed to carry ULP payload.
-------+-----------+-------------------+-------------------------
RDMA | Message | RDMA Header Used | ULP Message allowed in
Message| Type | | the RDMA Message
OpCode | | |
| | |
-------+-----------+-------------------+-------------------------
-------+-----------+-------------------+-------------------------
01100b | RDMA | None | TBD
| Commit | |
| Request | |
-------+-----------+-------------------+-------------------------
01101b | RDMA | None | No
| Commit | |
| Response | |
-------+-----------+---------------------------------------------
Table 2: RDMA Message Definitions
Further discussion.
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3.2.1. RDMA Commit Request Header Format
The RDMA Commit Request Message makes use of the DDP Untagged Buffer
Model. RDMA Commit Request messages MUST use the same Queue Number
as RDMA Read Requests and RDMA Extensions Atomic Operation Requests
(QN=1). Reusing the same queue number for RMDA Commit Requests
allows the operations to reuse the same infrastructure (e.g.
Outbound and Inbound RDMA Read Queue Depth (ORD/IRD) flow control) as
that defined for RDMA Read Requests.
The RDMA Commit Request Message carries an RDMA Commit header that
describes the ULP Buffer address in the Responder's memory. The RDMA
Write Request header immediately follows the DDP header. The RDMAP
layer passes an RDMAP Control Field to the DDP layer. Figure 2
depicts the RDMA Commit Request Header that is used for all RDMA
Commit Request Messages:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Request Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data Sink STag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data Sink Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data Sink Tagged Offset |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: RDMA Commit Request Header
Request Identifier: 32 bits. The Request Identifier specifies a
number that is used to identify the RDMA Commit Request Message.
The value used in this field is selected by the RNIC that sends
the message, and it is reflected back to the Local Peer in the
RDMA Commit Response message. N.B. Is this field really useful
to the RNIC, or does ordering suffice???
Data Sink STag: 32 bits The Data Sink STag identifies the Remote
Peer's Tagged Buffer targeted by the RDMA Commit Request. The
Data Sink STag is associated with the RDMAP Stream through a
mechanism that is outside the scope of the RDMAP specification.
Data Sink Tagged Offset: 64 bits The Data Sink Tagged Offset
specifies the starting offset, in octets, from the base of the
Remote Peer's Tagged Buffer targeted by the RDMA Commit Request.
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... Additional region identifiers to be committed in processing the
RDMA Commit Request, and/or upper layer message to be passed to
upper layer after commit completion (TBD).
3.2.2. RDMA Commit Response Header Format
The RDMA Commit Response Message makes use of the DDP Untagged Buffer
Model. RDMA Commit Response messages MUST use the same Queue Number
as RDMA Extensions Atomic Operation Responses (QN=3). The RDMAP
layer passes the following payload to the DDP layer on Queue Number
3.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Original Request Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Status |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: RDMA Commit Response Header
Original Request Identifier: 32 bits. The Original Request
Identifier is set to the value specified in the Request Identifier
field that was originally provided in the corresponding RDMA
Commit Request Message. N.B. ditto previous question.
Status: 32 bits. Zero if the RDMA Commit was successfully processed,
or any other value if not.
3.2.3. Ordering
Ordering and completion rules for RDMA Commit Request are similar to
those for an Atomic operation as described in section 5 of RFC 7306.
The queue number field of the RDMA Commit Request for the DDP layer
MUST be 1, and the RDMA Commit Response for the DDP layer MUST be 3.
There are no ordering requirements for the placement of the data to
be committed, nor are there any requirements for the order in which
the data is made durable. Data received by prior operations (e.g.
RDMA Write) MAY be submitted for placement at any time, and
durability MAY occur before the commit is requested. Data committed
after placement MAY become durable at any time, in the course of
operation of the persistency management of the storage device, or by
other actions resulting in durability. Any data specified by the
commit operation, in any case, MUST be made durable before successful
return of the commit.
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3.2.4. Atomicity
There are no atomicity guarantees provided on the Responder's node by
the RDMA Commit Operation with respect to any other operations.
While the Completion of the RDMA Commit Operation ensures that the
requested data was placed and committed to the target Tagged Buffer,
other operations might have also placed or fetched overlapping data.
The upper layer is responsible for arbitrating any shared access.
(To discuss) The commit operation provides an optional block of data
which is committed to a specified region after the successful
completion of the requested commit. This specified region MAY be
constrained in size and alignment by the implementation, and the
implementation MUST fail the operation and send a terminate message
if the subsequent commit cannot be performed atomically. The
implementation MUST NOT perform the subsequent commit if an error
occurred on the requested commit, and SHOULD return a non-zero status
indicating the error.
(Sidebar) It would be useful to make a statement about other RDMA
Commit to the target buffer and RDMA Read from the target buffer on
the same connection. Use of QN 1 for these operations provides
ordering guarantees which imply that they will "work" (see #7 below).
NOTE: this does not, however, extend to RDMA Write, which is not
sequenced nor does it employ a DDP QN.
3.2.5. Discovery of RDMAP Extensions
As for RFC 7306, explicit negotiation by the RDMAP peers of the
extensions covered by this document is not required. Instead, it is
RECOMMENDED that RDMA applications and/or ULPs negotiate any use of
these extensions at the application or ULP level. The definition of
such application-specific mechanisms is outside the scope of this
specification. For backward compatibility, existing applications
and/or ULPs should not assume that these extensions are supported.
In the absence of application-specific negotiation of the features
defined within this specification, the new operations can be
attempted, and reported errors can be used to determine a remote
peer's capabilities. In the case of RDMA Commit, an operation to a
previously Advertised buffer with remote write permission can be used
to determine the peer's support. A Remote Operation Error or
Unexpected OpCode error will be reported by the remote peer if the
Operation is not supported by the remote peer.
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4. Ordering and Completions Table
Table 3 summarizes the ordering relationships for the RDMA Commit
operation from the standpoint of the Requester. Note that in the
table, Send Operation includes Send, Send with Invalidate, Send with
Solicited Event, and Send with Solicited Event and Invalidate. Also
note that Immediate Operation includes Immediate Data and Immediate
Data with Solicited Event.
As for the prior section, the text below presents one possible
approach, and is included in skeletal form to be filled-in when
appropriate.
Note: N/A in the table below means Not Applicable
----------+------------+-------------+-------------+-----------------
First | Second | Placement | Placement | Ordering
Operation | Operation | Guarantee at| Guarantee at| Guarantee at
| | Remote Peer | Local Peer | Remote Peer
----------+------------+-------------+-------------+-----------------
RDMA | TODO | No Placement| N/A | Completed in
Commit | | Guarantee | | Order
| | between Foo | |
| | and Bar | |
----------+------------+-------------+-------------+-----------------
TODO | RDMA | No Placement| N/A | TODO
| Commit | Guarantee | |
| | between Foo | |
| | and Bar | |
----------+------------+-------------+-------------+-----------------
TODO | TODO | Etc | Etc | Etc
----------+------------+-------------+-------------+-----------------
----------+------------+-------------+-------------+-----------------
Table 3: Ordering of Operations
5. Error Processing
In addition to error processing described in section 7 of RFC 5040
and section 8 of RFC 7306, the following rules apply for the new RDMA
Messages defined in this specification.
5.1. Errors Detected at the Local Peer
The Local Peer MUST send a Terminate Message for each of the
following cases:
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1. For errors detected while creating a RDMA Commit Request or other
reasons not directly associated with an incoming Message, the
Terminate Message and Error code are sent instead of the Message.
In this case, the Error Type and Error Code fields are included
in the Terminate Message, but the Terminated DDP Header and
Terminated RDMA Header fields are set to zero.
2. For errors detected on an incoming RDMA Commit Request or RDMA
Commit Response, the Terminate Message is sent at the earliest
possible opportunity, preferably in the next outgoing RDMA
Message. In this case, the Error Type, Error Code, and
Terminated DDP Header fields are included in the Terminate
Message, but the Terminated RDMA Header field is set to zero.
3. For errors detected in the processing of the RDMA Commit itself,
that is, the act of making the data durable, no Terminate Message
is generated. Because the data is not lost, the connection MUST
NOT terminate and the peer MUST inform the requester of the
status, and allow the requester to perform further action, for
instance, recovery.
5.2. Errors Detected at the Remote Peer
On incoming RDMA Commit Requests, the following MUST be validated:
o The DDP layer MUST validate all DDP Segment fields.
The following additional validation MUST be performed:
o If the RDMA Commit cannot be satisfied, due to transient or
permanent errors detected in the processing by the Responder, a
status MUST be returned to the Requestor. Valid status values are
to be specified.
6. IANA Considerations
This document requests that IANA assign the following new operation
codes in the "RDMAP Message Operation Codes" registry defined in
section 3.4 of [RFC6580].
0xC RDMA Commit Request, this specification
0xD RDMA Commit Response, this specification
Additionally, the name of the listed entry in "RDMAP DDP Untagged
Queue Numbers" as defined in section 10.2 of [RFC7306] is requested
to be updated as follows:
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0x00000003 Queue 3 Modify name to "Atomic Response and RDMA Commit
Response operations" and add reference to this specification
Note to RFC Editor: this section may be edited and updated prior to
publication as an RFC.
7. Security Considerations
This document specifies extensions to the RDMA Protocol specification
in RFC 5040 and RDMA Protocol Extensions in RFC 7306, and as such the
Security Considerations discussed in Section 8 of RFC 5040 and
Section 9 of RFC 7306 apply. In particular, RDMA Commit Operations
use ULP Buffer addresses for the Remote Peer Buffer addressing used
in RFC 5040 as required by the security model described in [RDMAP
Security [RFC5042]].
If the "push mode" transfer model discussed in section 2 is
implemented by upper layers, new security considerations will be
potentially introduced in those protocols, particularly on the
server, or target, if the new memory regions are not carefully
protected. Therefore, for them to take full advantage of the
extension defined in this document, additional security design is
required in the implementation of those upper layers. The facilities
of RFC5042 [RFC5042] can provide the basis for any such design.
In addition to protection, in "push mode" the server or target will
expose memory resources to the peer for potentially extended periods,
and will allow the peer to perform remote durability requests which
will necessarily consume shared resources, e.g. memory bandwidth,
power, and memory itself. It is recommended that the upper layers
provide a means to gracefully adjust such resources, for example
using upper layer callbacks, without resorting to revoking RDMA
permissions, which would summarily close connections. With the
initiator applications relying on the protocol extension itself for
managing their required durability, the lack of such an approach
would lead to frequent recovery in low-resource situations,
potentially opening a new threat to such applications.
8. References
8.1. 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,
<http://www.rfc-editor.org/info/rfc2119>.
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[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>.
[RFC5042] Pinkerton, J. and E. Deleganes, "Direct Data Placement
Protocol (DDP) / Remote Direct Memory Access Protocol
(RDMAP) Security", RFC 5042, DOI 10.17487/RFC5042, October
2007, <http://www.rfc-editor.org/info/rfc5042>.
[RFC6580] Ko, M. and D. Black, "IANA Registries for the Remote
Direct Data Placement (RDDP) Protocols", RFC 6580,
DOI 10.17487/RFC6580, April 2012,
<http://www.rfc-editor.org/info/rfc6580>.
[RFC7306] Shah, H., Marti, F., Noureddine, W., Eiriksson, A., and R.
Sharp, "Remote Direct Memory Access (RDMA) Protocol
Extensions", RFC 7306, DOI 10.17487/RFC7306, June 2014,
<http://www.rfc-editor.org/info/rfc7306>.
8.2. Informative References
[RFC5045] Bestler, C., Ed. and L. Coene, "Applicability of Remote
Direct Memory Access Protocol (RDMA) and Direct Data
Placement (DDP)", RFC 5045, DOI 10.17487/RFC5045, October
2007, <http://www.rfc-editor.org/info/rfc5045>.
[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>.
[RFC5666] Talpey, T. and B. Callaghan, "Remote Direct Memory Access
Transport for Remote Procedure Call", RFC 5666,
DOI 10.17487/RFC5666, January 2010,
<http://www.rfc-editor.org/info/rfc5666>.
[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>.
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[RFC7145] Ko, M. and A. Nezhinsky, "Internet Small Computer System
Interface (iSCSI) Extensions for the Remote Direct Memory
Access (RDMA) Specification", RFC 7145,
DOI 10.17487/RFC7145, April 2014,
<http://www.rfc-editor.org/info/rfc7145>.
[SCSI] American National Standards Institute, "SCSI Primary
Commands - 3 (SPC-3) (INCITS 408-2005)", May 2005.
[SMB3] Microsoft Corporation, "Server Message Block (SMB)
Protocol Versions 2 and 3 (MS-SMB2)", October 2015.
[SMBDirect]
Microsoft Corporation, "SMB2 Remote Direct Memory Access
(RDMA) Transport Protocol (MS-SMBD)", October 2015.
[SNIANVM] Storage Networking Industry Association NVM TWG, "SNIA NVM
Programming Model v1.0", 2014.
8.3. URIs
[1] http://www.nvmexpress.org
[2] http://www.jedec.org
Appendix A. DDP Segment Formats for RDMA Extensions
This appendix is for information only and is NOT part of the
standard. It simply depicts the DDP Segment format for each of the
RDMA Messages defined in this specification.
A.1. DDP Segment for RDMA Commit Request
Figure 3 depicts an RDMA Commit Request, DDP Segment:
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DDP Control | RDMA Control |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved (Not Used) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DDP (RDMA Commit Request) Queue Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DDP (RDMA Commit Request) Message Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Request Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data Sink STag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data Sink Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data Sink Tagged Offset |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3
A.2. DDP Segment for RDMA Commit Response
Figure 4 depicts an RDMA Commit Response, DDP Segment:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DDP Control | RDMA Control |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved (Not Used) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DDP (RDMA Commit Response) Queue Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DDP (RDMA Commit Response) Message Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Original Request Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Status |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4
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Authors' Addresses
Tom Talpey
Microsoft
One Microsoft Way
Redmond, WA 98052
US
Email: ttalpey@microsoft.com
Jim Pinkerton
Microsoft
One Microsoft Way
Redmond, WA 98052
US
Email: jpink@microsoft.com
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