STORM A. Kanevsky, Ed.
Internet-Draft Dell Inc.
Updates: 5043, 5044 (if approved) C. Bestler, Ed.
Intended status: Standards Track Nexenta Systems
Expires: April 24, 2012 R. Sharp
Intel
S. Wise
Open Grid Computing
October 22, 2011
Enhanced RDMA Connection Establishment
draft-ietf-storm-mpa-peer-connect-08
Abstract
This document updates RFC 5043 and RFC 5044 by extending Marker
Protocol Data Unit (PDU) Aligned Framing (MPA) negotiation for Remote
Direct Memory Access (RDMA) connection establishment. The first
enhancement extends RFC 5044, enabling peer-to-peer connection
establishment over MPA/ Transmission Control Protocol (TCP). The
second enhancement extends both RFC 5043 and RFC 5044, by providing
an option for standardized exchange of RDMA-layer connection
configuration.
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 April 24, 2012.
Copyright Notice
Copyright (c) 2011 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
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Provisions Relating to IETF Documents
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Summary of changes affecting RFC 5044 . . . . . . . . . . 4
1.2. Summary of changes affecting RFC 5043 . . . . . . . . . . 4
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 4
3. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Motivations . . . . . . . . . . . . . . . . . . . . . . . . . 6
4.1. Standardization of RDMA Read Parameter Configuration . . . 7
4.2. Enabling MPA Mode . . . . . . . . . . . . . . . . . . . . 8
4.3. Lack of Explicit RTR in MPA Request/Reply Exchange . . . . 9
4.4. Limitations on ULP Workaround . . . . . . . . . . . . . . 10
4.4.1. Transport Neutral APIs . . . . . . . . . . . . . . . . 11
4.4.2. Work/Completion Queue Accounting . . . . . . . . . . . 11
4.4.3. Host-based Implementation of MPA Fencing . . . . . . . 12
5. Enhanced MPA Connection Establishment . . . . . . . . . . . . 12
6. Enhanced MPA Request/Reply Frames . . . . . . . . . . . . . . 13
7. Enhanced SCTP Session Control Chunks . . . . . . . . . . . . . 14
8. MPA Error Reporting . . . . . . . . . . . . . . . . . . . . . 16
9. Enhanced RDMA Connection Establishment Data . . . . . . . . . 16
9.1. IRD and ORD Negotiation . . . . . . . . . . . . . . . . . 17
9.2. Peer-to-Peer Connection Negotiation . . . . . . . . . . . 19
9.3. Enhanced Connection Negotiation Flow . . . . . . . . . . . 20
10. Interoperability . . . . . . . . . . . . . . . . . . . . . . . 20
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21
12. Security Considerations . . . . . . . . . . . . . . . . . . . 22
13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 22
14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 22
14.1. Normative References . . . . . . . . . . . . . . . . . . . 22
14.2. Informative References . . . . . . . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 23
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1. Introduction
When used over Transmission Control Protocol (TCP), the current
Remote Direct Data Placement (RDDP) [RFC5041] suite of protocols
relies on MPA [RFC5044] protocol for both connection establishment
and for markers for TCP layering.
A typical model for establishing an RDMA connection has the following
steps:
o The passive side (responder) Upper Layer Protocol (ULP) listens
for connection requests.
o The active side (initiator) ULP submits a connection request using
an RDMA endpoint, the desired destination and the parameters to be
used for the connection. Those parameters include both RDMA layer
characteristics, such as the number of simultaneous RDMA Read
Requests to be allowed and application specific data.
o The passive side ULP receives a connection request, which includes
the identity of the active side and the requested connection
characteristics. The passive side ULP uses this information to
decide whether to accept the connection, and if it is to be
accepted, how to create and/or configure the local RDMA endpoint.
o If accepting, responder submits its acceptance of the connection
request, which in turn, generates the accept message to initiator.
This responder accept operation includes the RDMA endpoint to be
used and the connection characteristics (both the RDMA
configuration and any application specific private data to be
transferred to initiator).
o The active side receives confirmation that the connection has been
accepted, what the configured connection characteristics are, and
any application supplied private data.
Currently, MPA only supports a client-server model for connection
establishment, forcing peer-to-peer applications to interact as
though they had a client/server relationship. In addition
negotiation of some of Remote Direct Memory Access Protocol (RDMAP)
[RFC5040] specific parameters are left to ULP negotiation. Providing
an optional ULP-independent format for exchanging these parameters
would be of benefit to transport neutral Remote Direct Memory Access
(RDMA) applications.
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1.1. Summary of changes affecting RFC 5044
This draft enhances [RFC5044] MPA connection setup protocol. First,
it adds exchange and negotiation of the parameters necessary to
support RDMA Read Requests. Second, it adds a Ready to Receive (RTR)
message from the initiator to the responder as the last message of
connection establishment and adds negotiation of an RTR message type
into MPA request/reply frames.
1.2. Summary of changes affecting RFC 5043
This draft enhances [RFC5043] by adding new Enhanced Session Control
Chunks that extends the currently defined Chunks with the addition of
Inbound RDMA Read Queue Depth (IRD) and Outbound RDMA Read Queue
Depth (ORD) negotiation.
2. 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].
3. Definitions
Active Side: See Initiator.
Consumer: The ULPs or applications that lie above MPA and Direct
Data Placement (DDP). The Consumer is responsible for making TCP
or SCTP connections, starting MPA and DDP connections, and
generally controlling operations. See [RFC5044] and [RFC5043].
CRC: Cyclic Redundancy Check
Completion Queue (CQ): A consumer accessible queue where the RDMA
device reports completions of Work Requests. A Consumer is able
to reap completions from a CQ without requiring per transaction
support from the kernel or other privileged entity. See [RDMAC].
Completion Queue Entry (CQE): Transport and device specific
representation of a Work Completion. A Completion Queue holds
CQEs. See [RDMAC].
FULPDU: Framed Upper Layer Protocol PDU. See FPDU of [RFC5044].
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Inbound RDMA Read Request Queue (IRRQ): A queue that is associated
with an RDMA Connection that tracks active incoming simultaneous
RDMA Read Request Messages. See [RDMAC].
Inbound RDMA Read Queue Depth (IRD): The maximum number of incoming
simultaneous RDMA Read Request Messages an RDMA connection can
handle. See [RDMAC].
Initiator: The endpoint of a connection that sends the MPA Request
Frame. Initiator is the active side of the connection
establishment. See [RFC5044].
IRD: See Inbound RDMA Read Queue Depth.
MPA Fencing: MPA responder Connection Establishment logic that
ensures that no ULP messages will be transferred until the
initiator first message has been received.
MPA Request Frame: Data sent from the MPA initiator to the MPA
responder during the Startup Phase. See [RFC5044].
MPA Reply Frame: Data sent from the MPA responder to the MPA
initiator during the Startup Phase. See [RFC5044].
ORD: See Outbound RDMA Read Queue Depth.
Outbound RDMA Read Queue Depth (ORD): The maximum number of
simultaneous RDMA Read Requests that can be issued for the RDMA
connection. This should be less than or equal to the peer's IRD.
See [RDMAC].
Passive Side: See Responder.
Private Data: A block of data exchanged between MPA endpoints during
initial connection setup. See [RFC5044].
Queue Pair (QP): The traditional name for a local Endpoint in a
[VIA] derived local interface. A Queue Pair is the set of Work
Queues associated exclusively with a single Endpoint. The Send
Queue (SQ), Receive Queue (RQ) and Inbound RDMA Read Queue (IRQ)
are considered to be part of the Queue Pair. The potentially
shared Completion Queue (CQ) and Shared Receive Queue (SRQ) are
not. See [RDMAC].
Remote Peer: The MPA protocol implementation on the opposite end of
the connection. Used to refer to the remote entity when
describing protocol exchanges or other interactions between two
Nodes. See [RFC5044].
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Responder: The connection endpoint that responds to an incoming MPA
connection request (the MPA Request Frame). Responder is the
passive side of the connection establishment. See [RFC5044].
Ready to Receive (RTR): RTR is the last connection establishment
message sent from the initiator to the responder indicating that
the initiator is ready to receive messages and that connection
establishment is completed. See [IBTA].
Startup Phase: The initial exchanges of an MPA connection that
serves to more fully identify MPA endpoints to each other and pass
connection specific setup information to each other. See
[RFC5044].
Shared Receive Queue(SRQ): A shared pool of Receive Work Requests
posted by the Consumer that can be allocated by multiple RDMA
endpoints (Queue Pair). See [RDMAC].
Tagged (DDP) Message: - A DDP Message that targets a Tagged Buffer
that is explicitly Advertised to the Remote Peer through exchange
of an STag (memory handle), offset in the memory region identified
by STag, and length [RFC5040].
Untagged (DDP) Message: - A DDP Message that targets an Untagged
Buffer associated with a queue specified by Queue Number (QN).
[RFC5040].
Work Queue: An element of a [VIA] derived local interface that
allows user-space applications to submit Work Requests directly to
network hardware. Specific Work Queues include the Send Queue
(SQ) for transmit requests, Receive Queue (RQ) for receive
requests specific to a single Endpoint and Shared Receive Queues
(SRQs) for receive requests that can be allocated by one or more
Endpoints. See [RDMAC].
Work Queue Element (WQE): Transport and device specific
representation of a Work Request. See [RDMAC].
Work Request: An elementary object used by Consumers to enqueue a
requested operation (WQEs) onto a Work Queue. See [RDMAC].
4. Motivations
The goal of this draft is twofold. One is to extend support from the
current client-server model for RDMA connection setup to a peer-to-
peer model. The second is to add negotiation of RDMA Read queue size
for both sides of an RDMA connection.
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4.1. Standardization of RDMA Read Parameter Configuration
Most RDMA applications are developed using a transport neutral
Application Programming Interface (API) to access RDMA services based
on a "queue pair" paradigm as originally defined by the Virtual
Interface Architecture [VIA], refined by the Direct Access
Programming Library [DAPL] and most commonly deployed with the
OpenFabrics API [OFA].
These transport neutral APIs seek to provide a common set of RDMA
services whether the underlying transport is, for example, RDDP over
MPA, RDDP over SCTP or InfiniBand.
The common model for establishing an RDMA connection has the
following steps:
o The passive side ULP listens for connection requests.
o The active side ULP submits a connection request using an RDMA
endpoint ("queue pair"), the desired destination and the
parameters to be used for the connection. Those parameters
include both RDMA layer characteristics, such as the RDMA Read
credits to be allowed and application specific data (typically
referred to as "private data").
o The passive side ULP receives a connection request, which includes
the identity of the active side and the requested connection
characteristics. The passive side ULP uses this information to
decide whether to accept the connection, and if it is to be
accepted, how to create and/or configure the RDMA endpoint.
o If accepting, the passive side ULP submits its acceptance of the
connection request. This local accept operation includes the RDMA
endpoint to be used and the connection characteristics (both the
RDMA configuration and any application specific private data to be
returned).
o The active side receives confirmation that the connection has been
accepted, what the configured connection characteristics are, and
any application supplied private data.
As currently defined, DDP connection establishment requires the ULP
to encode the RDMA configuration in the application specific private
data. This results in undesirable duplication of logic to cover both
InfiniBand and RDDP, and to specify the extraction of the RDMA
characteristics from the ULP for each specific Upper Layer Protocol.
Both RDDP and InfiniBand support an initial private data exchange,
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therefore a standard definition of the RDMA characteristics within
the private data section would enable common connection establishment
APIs to format the RDMA characteristics based on the same API
information used when establishing either protocol to form the
connection. The application would then only have to indicate that it
was using this standard format to enable common connection
establishment procedures to apply common code to properly parse these
fields and configure the RDMA endpoints accordingly. Exchange of
parameters necessary to perform RDMA Read operations is a common
usage of the initial private data exchange.
One of the RDMA operations that is defined in [RDMAC] is an RDMA
Read. RDMA Read operations are performed using an untagged message
sent from a Queue Pair (QP) on the local endpoint to a QP on the
remote endpoint targeting the Inbound RDMA Read Request Queue (QN=1
or Inbound RDMA Read Request Queue (IRRQ)) associated with the
connection. RDMA Read responses transfer data associated with each
RDMA Read Request from the remote endpoint to the local endpoint
using tagged messages. An inbound RDMA Read Request remains on the
IRRQ from the time that it is received until the time that the last
tagged message associated with the RDMA request is acknowledged. The
IRRQ is associated with a QP but is not a Work Queue. Instead the
IRRQ is a standalone queue that is used to manage RDMA read requests
associated with a QP. See [RDMAC] section 6 for more information
regarding QPs and IRRQ. One of the characteristics that must be
configured for a QP is the size of the IRRQ. This parameter is
called the Inbound RDMA Read Queue Depth (IRD). Another
characteristic of a QP that must be configured a local limit on the
number of simultaneous outbound RDMA Read Requests based on the size
of the remote endpoint QP's IRRQ. This parameter is call the
Outbound RDMA Read Queue Depth (ORD). ORD is used to limit the
number of simultaneous RDMA read requests such that the local
endpoint does not overrun the remote endpoint's IRRQ depth or IRD.
Note that outbound RDMA Reads are submitted to a QP's Send Queue at
the local peer, not to a separate outbound RDMA read request queue on
the local peer. The local endpoint uses ORD to strictly limit
simultaneous read requests so that IRRQ overruns do not occur at the
remote endpoint.
Determination of the values of the ORD and IRD are left to the ULP by
the current RDDP suite of protocols and also by [RDMAC]. Since this
negotiation of ORD and IRD is typical, it is desirable to provide a
common mechanism described in this draft.
4.2. Enabling MPA Mode
MPA defines encoding of DDP Segments in Framed Upper Layer Protocol
PDUs (FULPDUs). Generation of FULPDUs requires the ability to
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periodically insert MPA Markers and to generate the MPA CRC-32c for
each frame. Reception may require parsing/removing the markers after
using them to identify MPA Frame boundaries, and validation of the
MPA-CRC32c.
A major design objective for MPA was to ensure that the resulting TCP
stream would be a fully compliant TCP stream for any and all TCP-
aware middle-boxes. The challenge is that while only some TCP
payload streams are a valid stream of MPA FULPDUs, any sequence of
bytes is a valid TCP payload stream. The determination that a given
stream is in a specific MPA mode cannot be made at the MPA or TCP
layer. Therefore enabling of MPA mode is handled by the ULP.
The MPA protocol can be viewed as having two parts.
o a specification of generation and reception of MPA FULPDUs. This
is unchanged by enhanced RDMA connection establishment.
o a pre-MPA exchange of messages to enable a specific MPA mode for
the TCP connection. Enhanced RDMA connection establishment
extends this protocol with two new features.
In typical implementations, generation and reception of MPA FULPDUs
is handled by hardware. The exchange of the MPA Request and Reply
frames is then handled by host software. As will be explained, this
implementation split impedes applications which are not compatible
with the client-server assumptions in the current MPA Request/Reply
exchange.
4.3. Lack of Explicit RTR in MPA Request/Reply Exchange
The exchange of MPA Request and Reply messages to place a TCP
connection in MPA mode is specified in [RFC5044]. This protocol
provides many benefits to the design of MPA FULPDU hardware:
o The ULP is responsible for specifying the exact MPA Mode (Markers
enabled or disabled, CRC-32c enabled or suppressed) and the point
in the TCP streams (inbound and outbound) where MPA frames will
begin.
o Before the first MPA frame is transmitted, all pre-MPA mode TCP
payload will have been acknowledged by the peer. Therefore it is
never necessary to generate a retransmission that mixes pre-MPA
and MPA payload.
o Before MPA reception is enabled, all incoming pre-MPA mode TCP
payload will have been acknowledged. Therefore the host will
never receive a TCP segment that mixes pre-MPA and MPA payload.
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The limitation of the current MPA Request/Reply exchange is that it
does not define a Ready to Receive (RTR) message that the active side
would send, so that the passive side can know that the last non-MPA
payload (the MPA Reply) had been received.
Instead, the role of an RTR message is piggy-backed on the first MPA
FULPDU sent by the active side. This is actually a valuable
optimization for all applications that fit the classic client/server
model. The client only initiates the connection when it has a
request to send to the server, and the server has nothing to send
until it has received and processed the client request.
Even applications where the server sends some configuration data
immediately can easily send the same information as application
private data in the MPA Reply. So the currently defined exchange
works for almost all applications.
Many peer-to-peer applications, especially those involving cluster
calculations (frequently using Message Passing Interface (MPI)
[UsingMPI], or [RDS]), have no natural client or server roles
([PPMPI], [OpenMP]). Typically one member of the cluster is
arbitrarily selected to initiate the connection when the distributed
task is launched, while the other accepts it. At startup time,
however, there is no way to predict which node will have the first
message to actually send. Establishing the connections immediately,
however, is valuable because it reduces latency once results are
ready to transmit and it validates connectivity throughout the
cluster.
The lack of an explicit RTR message in the MPA Request/Reply exchange
forces all applications to have a first message from the connection
initiator, whether this matches the application communication model
or not.
4.4. Limitations on ULP Workaround
The requirement that the RDMA connection initiator sends the first
message does not appear to be onerous on first examination. The
natural question is why the application layer would not simply
generate a dummy message when there was no other message to submit.
There are three factors that make this workaround unsuitable for many
peer-to-peer applications.
o Transport Neutral APIs.
o Work/Completion Queue Accounting.
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o Host-based implementation of MPA Fencing.
4.4.1. Transport Neutral APIs
Many of these applications access RDMA services using a transport
neutral API such as [DAPL] or [OFA]. Only RDDP over TCP [RFC5044]
has a first message requirement. Other RDMA transports, including
RDDP over SCTP (see [RFC5043]) and InfiniBand (see [IBTA]), do not.
Application or middleware communications can be expressed as
transport neutral RDMA operations, allowing lower software layers to
translate to transport and device specifics. Having a distinct extra
message that is required only for one transport undermines the
application's goal of being transport neutral.
4.4.2. Work/Completion Queue Accounting
RDMA local APIs conventionally use work queues to submit requests
(work queue elements or WQEs) and to asynchronously receive
completions (in completion queues or CQs).
Each work request can generate a completion queue entry (CQE).
Completions for successful transmit work requests are frequently
suppressed, but the completion queue capacity must account for the
possibility that each will complete in error. A completion queue can
receive completions from multiple work queues.
Completion Queues are defined so as to allow hardware RDMA
implementations to generate CQEs directly to a user-space mapped
buffer. This enables a user-space RDMA consumer to reap completions
without requiring kernel intervention.
A hardware RDMA implementation cannot reasonably wait for an
available slot in the completion queue. The queue must be sized such
that an overflow will not occur. When an overflow does occur it is
considered catastrophic and will typically require tearing down all
RDMA connections using that CQ.
This style of interface is very efficient, but places a burden on the
application to properly size each Completion Queue to match the Work
Queues that feed it.
While the format of both WQEs and CQEs is transport and device
dependent, a transport neutral API can deal with WQEs and CQEs as
abstract transport and device neutral objects. Therefore the number
of WQEs and CQEs required for an application can be transport and
device neutral.
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The capacity of the work queues and completion queues can be
calculated in an abstract transport/device neutral fashion. If a
dummy operation approach was used, it would require lower layers to
know the usage model, and would disrupt the calculations by inserting
a dummy "operation" Work Request and filtering out the matching
completion. The lower layer does not know the usage model on which
the queue sizes are built, nor does it know how frequently an
insertion will be required.
4.4.3. Host-based Implementation of MPA Fencing
Many hardware implementations of RDDP using MPA/TCP do not handle the
MPA Request/Reply exchange in hardware, rather they are handled by
the host processor in software. With such designs it is common for
the MPA Fencing to be implemented in the user-space device-specific
library (commonly referred to as a 'User Verbs' library or module).
When the generation and reception of MPA FULPDUs is already dedicated
to hardware, a Work Completion can only be generated by an untagged
message since arrival of a message for tagged buffer does not
necessarily generate a completion and is done without any interaction
with ULP [RFC5040].
5. Enhanced MPA Connection Establishment
Below we provide an overview of Enhanced Connection Setup. The goal
is to allow standard negotiation of ORD/IRD setting on both sides of
the RDMA connection and/or to negotiate the initial data transfer
operation by the initiator when the existing 'client sends first'
rule does not match application requirements.
The RDMA connection initiator sends an MPA Request, as specified in
[RFC5044]; the new format defined here allows for:
o Standardized negotiation of ORD and IRD.
o Negotiation of an RTR message.
The RDMA connection responder processes the MPA Request and generates
an MPA Reply, as specified in [RFC5044]; the new format completes the
negotiation.
The local interface needs to provide a way for a ULP to request the
use of explicit RTR messages per-application or per-connection basis
when an explicit RTR message will be required. Piggy-backing the RTR
on a Client's first message is a valuable optimization for most
connections.
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The RDMA connection initiator MUST NOT allow any later FULPDUs to be
transmitted before the RTR message. One method to achieve that is to
delay notifying the ULP that the RDMA connection has been established
until after any required RTR Message has been transmitted.
All MPA exchanges are performed via TCP prior to RDMA establishment,
and are therefore signaled via TCP and not via RDMA completion.
6. Enhanced MPA Request/Reply Frames
Enhanced RDMA connection establishment uses an alternate format for
MPA Requests and Replies, as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
0 | |
+ Key (16 bytes containing "MPA ID Req Frame") +
4 | (4D 50 41 20 49 44 20 52 65 71 20 46 72 61 6D 65) |
+ Or (16 bytes containing "MPA ID Rep Frame") +
8 | (4D 50 41 20 49 44 20 52 65 70 20 46 72 61 6D 65) |
+ +
12 | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
16 |M|C|R|S| Res | Rev | PD_Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ ~
~ Private Data ~
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Key: Unchanged from [RFC5044].
M: Unchanged from [RFC5044].
C: Unchanged from [RFC5044].
R: Unchanged from [RFC5044].
S: One if the Private Data begins with the enhanced RDMA connection
establishment data. Zero otherwise.
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Res: One bit smaller than in [RFC5044], otherwise unchanged. In
[RFC5044] 'Res' field, in which the newly defined 'S' bit resides,
is reserved for future use. [RFC5044] specifies that 'RES' MUST
be set to zero when sending, and MUST NOT be checked on reception,
making use of S bit backwards compatible with the original MPA
frame format. When the S bit is set to zero, no additional
private data is used for enhanced RDMA connection establishment,
and therefore the resulting MPA request and reply frames are
identical to the unenhanced protocol.
Rev: This field contains the revision of MPA. To use any enhanced
connection establishment feature this MUST be set to two or
higher, If no enhanced connection establishment features are
desired it MAY be set to one. A host accepting MPA connections
MUST continue to accept MPA Requests with version one even if it
supports version two.
PD_Length: Unchanged from [RFC5044]. This is the total length of
the Private Data field, including the enhanced RDMA connection
establishment data if present.
Private Data: Unchanged from [RFC5044]. However, if the 'S' flag is
set, Private Data begins with enhanced RDMA connection
establishment data.
7. Enhanced SCTP Session Control Chunks
Enhanced RDMA Connection Establishment uses the first 32 bits of the
Private data field for IRD and ORD negotiation in the "DDP Stream
Session Initiate" and "DDP Stream Session Accept" SCTP Session
Control Chunks.
The type of the SCTP Session Control Chunk is defined by a Function
Code (see [RFC4960]). [RFC5043] already defines codes for 'DDP
Stream Session Initiate' and 'DDP Stream Session Accept', which are
equivalent to a MPA Request Frame and an accepting MPA Reply Frame.
Enhanced RDMA connection establishment requires three additional
Function codes listed below:
Enhanced DDP Stream Session Initiate: 0x005
Enhanced DDP Stream Session Accept: 0x006
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Enhanced DDP Stream Session Reject: 0x007
The Enhanced Reject function code MUST be used to indicate rejection
of enhanced DDP stream session for a configuration that would have
been accepted for unenhanced DDP Stream Session negotiation.
The Enhanced DDP stream session establishment follows the same rules
as the standard DDP stream session establishment as defined in
[RFC5043]. ULP-supplied Private Data MUST be included for Enhanced
DDP Stream Session Initiate, Enhanced DDP Stream Session Accept, and
Enhanced DDP Stream Session Reject messages.
Private Data length MUST NOT exceed 512 bytes in any message,
including enhanced RDMA connection establishment data.
Private Data MUST NOT be included in the DDP Stream Session TERM
message.
Received Extended DDP Stream Session Control messages SHOULD be
reported to the ULP. If reported, any supplied Private Data MUST be
available for the ULP to examine. For example, a received Extended
DDP Stream Session Control message is not reported to ULP if none of
the requested RTR message types are supported by receiver. In this
case, Provider MAY generate reject reply message indicating which RTR
message types it supports.
The enhanced DDP stream management MUST use the DDP stream session
termination function code to terminate a stream established using
enhanced DDP stream session function codes.
[RFC5043] already supports either side sending the first DDP Message
since the Payload Protocol Identifier (PPID) already distinguishes
between Session Establishment and DDP Segments. The enhanced RDMA
Connection Establishment provides to the ULP a transport independent
way to support peer-to-peer model.
The following additional Legal Sequences of DDP Stream Session
messages are defined:
o Enhanced Active/Passive Session Accepted: as with section 6.2 of
[RFC5043], but with the extended opcodes as defined in this
document.
o Enhanced Active/Passive Session Rejected: as with section 6.3 of
[RFC5043], but with the extended opcodes as defined in this
document.
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o Enhanced Active/Passive Session Non-ULP Rejected: as with section
6.4 of [RFC5043], but with the extended opcodes as defined in this
document.
8. MPA Error Reporting
The RDMA connection establishment protocol is layered upon [RFC5040]
and [RFC5041]. Any enhanced RDMA connection establishment error
generates an MPA termination message to a peer. [RFC5040] defines a
triplet of protocol layers, error types and error codes for error
specification. MPA negotiation for RDMA connection establishment
uses the following layer and error type for MPA error reporting:
Layer: 0x2 - LLP
Error Type: 0x0 - MPA
While [RFC5044] defines four error codes, [RFC5043] does not define
any. Enhanced RDMA connection establishment extends [RFC5044] error
codes by adding three new error codes. Thus, enhanced RDMA
connection establishment is backward compatible with both [RFC5043]
and [RFC5044].
The following error codes are defined for enhanced RDMA connection
establishment negotiation:
Error Code Description
--------------------------------------------------------
0x05 Local catastrophic
0x06 Insufficient IRD resources
0x07 No matching RTR option
9. Enhanced RDMA Connection Establishment Data
Enhanced RDMA Connection Establishment places the following 32 bits
at the beginning of the Private data field of the MPA Request and
Reply Frames or the "DDP Stream Session Initiate" and "DDP Stream
Session Accept" SCTP Session Control Chunks. ULP specified private
data follows this field. The maximum amount of ULP specified private
data is therefore reduced by 4 bytes. Note that this field MUST be
sent in network byte order, with IRD and ORD encoded as 14 bit
unsigned integers.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
0 |A|B| IRD |C|D| ORD |
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4 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
IRD: Inbound RDMA Read Queue Depth.
ORD: Outbound RDMA Read Queue Depth.
A: Control Flag for connection model.
B: Control Flag for zero length FULPDU (Send) RTR message.
C: Control Flag for zero length RDMA Write RTR message.
D: Control Flag for zero length RDMA Read RTR message.
9.1. IRD and ORD Negotiation
IRD and ORD are used for negotiation of Inbound RDMA Read Request
Queue depths for both endpoints of the RDMA connection. IRD is used
to configure the depth of the Inbound RDMA Read Request Queue (IRRQ)
on each endpoint. ORD is used to limit the number of simultaneous
outbound RDMA Read Requests allowed at at given point in time in
order to avoid IRRQ overruns at the remote endpoint. In order to
describe the negotiation of both local endpoint and remote endpoint
ORD and IRD values, four terms are defined:
Initiator IRD: IRD value sent in the MPA request or "DDP Stream
Session Initiate" SCTP Session Control Chunk. This is the value
of the initiator's IRD at the time of the MPA Request generation.
The responder sets its local ORD value to this value or less.
Initiator IRD is the maximum number of simultaneous inbound RDMA
Read Requests which the initiator can support for the requested
connection.
Initiator ORD: ORD value in the MPA request or "DDP Stream Session
Initiate" SCTP Session Control Chunk. This is the initial value
of the initiator's ORD at the time of the MPA Request generation
and also a request to the responder to support a responder IRD of
at least this value. Initiator ORD is the maximum number of
simultaneous outbound RDMA Read operations that the initiator
desires the responder to support for the requested connection.
Responder IRD: IRD value returned in the MPA reply or "DDP Stream
Session Accept" SCTP Session Control Chunk. This is the actual
value that the responder set for its local IRD. This value is
greater than or equal to initiator ORD for successful
negotiations. Responder IRD is the maximum number of simultaneous
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inbound RDMA Read Requests that the responder actually can support
for the requested connection.
Responder ORD: ORD value returned in the MPA reply or "DDP Stream
Session Accept" SCTP Session Control Chunk. This is the actual
value that the responder used for ORD and is less than or equal to
initiator IRD for successful negotiations. Responder ORD is the
maximum number of simultaneous outbound RDMA Read operations that
the responder will allow for the requested connection.
The relationships between these parameters after a successful
negotiation is complete are the following:
initiator ORD <= responder IRD
responder ORD <= initiator IRD
The responder and initiator MUST pass the peer's provided IRD and ORD
values to the ULP, in addition to using the values as calculated by
the preceding rules.
Responder ORD SHOULD be set to a value less than or equal to
initiator IRD. If initiator ORD is insufficient to support the
selected connection model, responder IRD MAY be increased, for
example if initiator ORD is 0 (RDMA Reads will not be used by the
ULP) and the responder supports a zero length RDMA Read RTR message,
then responder IRD can be set to 1. The responder MUST set its ORD
at most to initiator IRD. The responder MAY reject the connection
request if initiator IRD is not sufficient for the ULP required ORD
and specify the required ORD in the MPA Reject frame responder ORD.
Thus, the TERM message MUST contain Layer 2, Error Type 0, Error Code
6.
Upon receiving the MPA Accept frame from the responder, the initiator
MUST set its IRD at least to responder ORD and its ORD at most to
responder IRD. If the initiator does not have sufficient resources
for the required IRD, it MUST send a TERM message to the responder
indicating insufficient resources, and terminate the connection due
to insufficient resources. Thus, the TERM message MUST contain Layer
2, Error Type 0, Error Code 6.
The initiator MUST pass the responder provided IRD and ORD to the ULP
for both MPA Accept and Reject messages. The initiator ULP can
decide its course of action. For example, the initiator ULP may
terminate the established connection and renegotiate responder ORD.
An all ones value (0x3FFF) indicates that automatic negotiation of
the IRD or ORD is not desired, and that the ULP will be responsible
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for it. The responder MUST respond to an initiator ORD value of
0x3FFF by leaving its local endpoint IRD value unchanged, and setting
IRD to 0x3FFF in its reply message. The initiator MUST leave its
local endpoint ORD value unchanged upon receiving a responder IRD
value of 0x3FFF. The responder MUST respond to an initiator IRD
value of 0x3FFF by leaving its local endpoint ORD value unchanged,
and setting ORD to 0x3FFF in its reply message. The initiator MUST
leave its local endpoint IRD value unchanged upon receiving a
responder ORD value of 0x3FFF.
9.2. Peer-to-Peer Connection Negotiation
Control Flag A value 1 indicates that a peer-to-peer connection model
is being performed, and value 0 indicates a client-server model.
Control Flag B value 1 indicates that a zero length FULPDU (Send) RTR
message is requested for the initiator and supported by the
responder, respectively, 0 otherwise. Control Flag C value 1
indicates that a zero length RDMA Write RTR message is requested for
the initiator and supported by the responder, respectively, 0
otherwise. Control Flag D value 1 indicates that a zero length RDMA
Read RTR message is requested for the initiator and supported by the
responder, respectively, 0 otherwise. The initiator MUST set Control
Flag A to 1 for peer-to-peer model. The initiator MUST set each
Control Flag B, C and D to 1 for each of the options it supports, if
Control Flag A is set to 1.
The responder MUST support at least one RTR message option if it
supports Enhanced RDMA connection establishment. If Control Flag A
is 1 in the MPA request message then the responder MUST set Control
Flag A to 1 in the MPA reply message. For each initiator supported
RTR message option the responder SHOULD set the corresponding Control
Flag if the responder can support that option in an MPA reply. The
responder is not required to specify all RTR message options it
supports. The responder MUST set at least one RTR message option if
it supports more than one initiator specified RTR message option.
The responder MAY include additional RTR message options it supports,
even if not requested by any initiator specified RTR message options.
If the responder does not support any of the initiator specified RTR
message options then the responder MUST set at least one RTR message
type option it supports.
Upon receiving the MPA accept frame with Control Flag A set to 1, the
initiator MUST generate one of the negotiated RTR messages. If the
initiator is not able to generate any of the responder supported RTR
messages, then it MUST send a TERM message to the responder
indicating failure to negotiate a mutually compatible connection
model or RTR option, and terminate the connection. Thus, the TERM
message MUST contain Layer 2, Error Type 0, Error Code 7. The ULP
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can negotiate a ULP level RTR message when a Provider level RTR
message cannot be negotiated.
The initiator MUST set Control Flag A to 0 for client-server model.
The responder MUST set Control Flag A to 0 if Control Flag A is 0 in
request. If Control Flag A is set to 0 then Control Flags B, C and D
MUST also be set to 0. On reception if Control Flag A is set to 0
then Control Flags B, C, and D MUST be ignored.
9.3. Enhanced Connection Negotiation Flow
The RTR message type and ORD/IRD negotiation follows the following
order:
initiator (MPA Request) --> Set Control Flag A to 1 to indicate
peer-to-peer connection model and initiator IRD, ORD setting on
local Endpoint of the connection. Set Control Flags B, C, and D
to 1 for each initiator-supported option of RTR message.
responder (MPA Reply) <-- Match the initiator Control Flag A value
and set ORD/IRD to the responder local endpoint values based upon
the initiator initial ORD/IRD values and the number of
simultaneous RDMA Read Requests required by the ULP. Sets Control
Flags B, C, and D to 1 for responder-supported options of RTR
message options for peer-to-peer connection model and sets the
responder IRD/ORD actual values.
initiator (First RDMA Message) --> After the initiator modifies its
ORD/IRD to match the responder's values as stated above, the
initiator sends the first message of negotiated RTR message
option. If no matching RTR message option exists then the
initiator sends a TERM message.
The initiator or responder MUST generate the TERM message that
contains Layer 2, Error Type 0, Error Code 5 when it encounters any
error locally for which the special Error Code is not defined in
section Section 8 before resetting the connection.
10. Interoperability
The initiator MUST NOT use the enhanced RDMA connection establishment
formats or function codes when no enhanced functionality is desired.
The responder MUST continue to accept unenhanced connection requests.
There are three initiator/responder cases that involve enhanced MPA:
both the initiator and responder, only the responder, and only the
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initiator. The enhanced MPA frame is defined by field 'S' set to 1.
Enhanced MPA initiator and responder: If the responder receives an
enhanced MPA message, it MUST respond with an enhanced MPA
message.
Enhanced MPA responder only: If the responder receives an unenhanced
MPA message ('S' is set to 0), it MUST respond with an unenhanced
MPA message.
Enhanced MPA initiator only: If the responder receives an enhanced
MPA message and it does not support enhanced RDMA connection
establishment, it MUST close the TCP connection and exit MPA.
From a standard RDMA connection establishment point of view
enhanced MPA frame is improperly formatted as stated in [RFC5044].
Thus, both the initiator and responder report TCP connection
termination to an application locally. In this case the initiator
MAY attempt to establish an RDMA connection using the unenhanced
MPA protocol as defined in [RFC5044] if this protocol is
compatible with the application, and let ULP deal with ORD and
IRD, and peer-to-peer negotiations.
A note for a potential future enhancements for connection
establishment negotiation: It is possible to further extend
formatting of private data of the MPA Request and Reply frames and to
use other bits from "Res" field to indicate that private data
formatting.
11. IANA Considerations
IANA is requested to add the following entries to the "SCTP Function
Codes for DDP Session Control" registry created by Section 3.4 of
[IANA_RDDP_REGISTRY]:
0x0005, Enhanced DDP Stream Session Initiate, [RFCXXXX]
0x0006, Enhanced DDP Stream Session Accept, [RFCXXXX]
0x0007, Enhanced DDP Stream Session Reject, [RFCXXXX]
IANA is requested to add the following entries to the "MPA Errors"
registry created by Section 3.3 of [IANA_RDDP_REGISTRY]
0x2/0x0/0x05, - MPA Error / Local catastrophic error, [RFCXXXX]
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0x2/0x0/0x06 - MPA Error / Insufficient IRD resources, [RFCXXXX]
0x2/0x0/0x07 - MPA Error / No matching RTR option, [RFCXXXX]
RFC Editor: Please replace XXXX in the six instances of [RFCXXXX]
above with the RFC number of this document and remove this note.
12. Security Considerations
The security considerations from RFC 5044 and RFC 5043 apply and the
changes in this document do not introduce new security
considerations. However it is recommended that implementations do
sanity checking for the input parameters, including ORD, IRD, and
RTR.
13. Acknowledgements
The authors wish to thank Sean Hefty, Dave Minturn, Tom Talpey, David
Black and David Harrington for their valuable contributions and
reviews of this document.
14. References
14.1. Normative References
[IANA_RDDP_REGISTRY]
"IANA Registries for the RDDP (Remote Direct Data
Placement) Protocols, Work in Progress", October, 2011, <h
ttp://www.ietf.org/internet-drafts/
draft-ietf-storm-rddp-registries-00.txt>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4960] Stewart, R., "Stream Control Transmission Protocol",
RFC 4960, September 2007.
[RFC5040] Recio, R., Metzler, B., Culley, P., Hilland, J., and D.
Garcia, "A Remote Direct Memory Access Protocol
Specification", RFC 5040, October 2007.
[RFC5041] Shah, H., Pinkerton, J., Recio, R., and P. Culley, "Direct
Data Placement over Reliable Transports", RFC 5041,
October 2007.
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[RFC5043] Bestler, C. and R. Stewart, "Stream Control Transmission
Protocol (SCTP) Direct Data Placement (DDP) Adaptation",
RFC 5043, October 2007.
[RFC5044] Culley, P., Elzur, U., Recio, R., Bailey, S., and J.
Carrier, "Marker PDU Aligned Framing for TCP
Specification", RFC 5044, October 2007.
14.2. Informative References
[DAPL] "Direct Access Programming Library",
<http://www.datcollaborative.org>.
[IBTA] "InfiniBand Architecture Specification Release 1.2.1", <ht
tp://www.infinibandta.org/content/
pages.php?pg=technology_overview>.
[OFA] "OFA verbs & APIs", <http://www.openfabrics.org/>.
[OpenMP] McGraw-Hill, "Parallel Programming in C with MPI and
OpenMP", 2003.
[PPMPI] Morgan Kaufmann Publishers Inc., "Parallel Programming
with MPI", 2008.
[RDMAC] "RDMA Protocol Verbs Specification (Version 1.0)", <http:/
/www.rdmaconsortium.org/home/
draft-hilland-iwarp-verbs-v1.0-RDMAC.pdf>.
[RDS] Open Fabrics Association, "Reliable Datagram Socket",
2008, <http://www.openfabrics.org/archives/
spring2008sonoma/Tuesday/sonoma_2008_0408%20Oracle.ppt>.
[UsingMPI]
MIT Press, "Using MPI-2: Advanced Features of the Message
Passing Interface", 1999.
[VIA] Compaq, Intel, Microsoft, "Virtual Interface Architecture
Specification", 1997, <http://pllab.cs.nthu.edu.tw/cs5403/
Readings/EJB/san_10.pdf>.
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Authors' Addresses
Arkady Kanevsky (editor)
Dell Inc.
One Dell Way, MS PS2-47
Round Rock, TX 78682
USA
Phone: +1-512-728-0000
Email: arkady.kanevsky@gmail.com
Caitlin Bestler (editor)
Nexenta Systems
555 E El Camino Real #104
Sunnyvale, CA 94087
USA
Phone: +1-949-528-3085
Email: Caitlin.Bestler@nexenta.com
Robert Sharp
Intel
LAD High Performance Message Passing, Mailstop: AN1-WTR1
1501 South Mopac, Suite 400
Austin, TX 78746
USA
Phone: +1-512-493-3242
Email: robert.o.sharp@intel.com
Steve Wise
Open Grid Computing
4030 Braker Lane STE 130
Austin, TX 78759
USA
Phone: +1-512-343-9196 x101
Email: swise@opengridcomputing.com
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