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Versions: 00 01 02 03 rfc4755                            Standards Track
INTERNET DRAFT                                               V. Kashyap
<draft-ietf-ipoib-connected-mode-03.txt>                            IBM
Expiration Date: September 2006                              March 2006

                    IP over InfiniBand: Connected Mode

Status of this memo

         By submitting this Internet-Draft, each author represents that
         any applicable patent or other IPR claims of which he or she is
         aware have been or will be disclosed, and any of which he or she
         becomes aware will be disclosed, in accordance with Section 6 of
         BCP 79.

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Copyright Notice

         Copyright (C) The Internet Society (2006).  All Rights Reserved.


         This document specifies transmission of IPv4/IPv6 packets and
         address resolution over the connected modes of InfiniBand.

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Table of Contents

         1.0       Introduction
         2.0       IPoIB-connected mode
         2.1       Multicasting
         2.2       Outline of Address Resolution
         2.3       Outline of Connection Setup
         3.0       Address Resolution
         3.1       Link-layer Address
         3.2       IB Connection Setup
         3.3       Simultaneous IB Connections
         3.4       IPoIB-CM IB Connection Teardown
         3.5       Service-ID
         4.0       Frame Format
         5.0       Maximum Transmission Unit
         5.1       Per-Connection MTU
         6.0       Private-Data Format
         7.0       IPoIB-CM Considerations
         7.1       A Cautionary Note on IPoIB-RC
         7.2       IPoIB-CM Per-Destination MTU
         8.0       Security Considerations
         9.0       IANA Considerations
         10.0      Acknowledgements
         11.0      References
         12.0      Author's Address

1.0 Introduction

         The InfiniBand specification [IB_ARCH] can be found at
         www.infinibandta.org.  The document [IPoIB_ARCH] provides a
         short overview of InfiniBand architecture along with
         consideration for specifying IP over InfiniBand networks.

         The InfiniBand architecture (IBA) defines multiple modes of
         transports. Of these the unreliable datagram (UD) transport
         method best matches the needs of IP. IP over InfiniBand (IPoIB)
         over UD is described in [IPoIB_UD]. This document describes IP
         transmission over the connected modes of IBA.

         IBA defines two connected modes:

              1. Reliable Connected (RC)
              2. Unreliable Connected (UC)

         As is evident from the nomenclature, the two modes differ mainly
         in providing reliability of data delivery across the connection.
         This document applies equally to both the connected modes.
         IPoIB over these two modes is referred to as IPoIB-CM (connected

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         mode) in this document.  For clarity, IPoIB over the unreliable
         datagram mode as described in [IPoIB_UD], is referred to as

         IBA requires that all Host Channel Adapters (HCAs) support the
         reliable and unreliable connected modes [IB_ARCH]. It is
         optional for Target Channel Adapters (TCAs) to support the
         connected modes.

         The connected modes offer link MTUs of up to 2^31 octets in
         length.  Thus, the use of connected modes can offer significant
         benefits by supporting reasonably large MTUs. The datagram modes
         of InfiniBand Architecture (IBA) are limited to 4096 octets.

         Reliability is also enhanced if the underlying feature of
         "automatic path migration" supported by the connected modes is

         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].

2.0 IPoIB-connected mode

         IPoIB over connected mode is an OPTIONAL extension to IPoIB-UD.
         Every IPoIB implementation MUST support [IPoIB-UD] and MAY
         support the extensions described in this document.

         Therefore, IP encapsulation, default MTU, link layer address
         format and the IPv6 stateless autoconfiguration mechanism apply
         to IPoIB-CM exactly as described in [IPoIB_UD].

2.1 Multicasting

         The connected modes of IBA define a non-broadcast, multiple
         access network. The connected modes of IBA do not support
         multicasting though every node can communicate with every other
         node if desired.

         This requires that multicasting be emulated in some form by the
         network.  However, in the case of an InfiniBand network, instead
         of an emulation, an unreliable datagram (UD)  queue pair (QP)
         can be used to support multicasting while the connected mode  QP
         is used for unicast traffic.  Since every IPoIB implementation
         is required to support the UD mode, every implementation
         supporting IPoIB-CM will be able to utilize the pre-existing
         IPoIB-UD QP for all broadcast/multicast communications.

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         Multicast mapping, transmission and reception of multicast
         packets and multicast routing MUST use the UD QP associated with
         the IPoIB interface.

2.2 Outline of Address Resolution

         Every IPoIB-CM interface MUST have two sets of QPs associated
         with it:

                 1) One unreliable datagram QP
                 2) One or more connected mode QPs

         [IPoIB_UD] describes the address resolution method to determine
         the link address of the peer. This response is received on the
         UD QP associated with the IPoIB interface.

2.3 Outline of Connection setup

         Once the link address of the remote node is known, an IB
         connection must be setup between the nodes before any IP
         communication may occur.

         To make a connection, the sender must know the service-ID to use
         in the request to make a connection [IB_ARCH]. It must also
         supply the "connection mode" queue pair to the remote node. The
         peer replies with its queue pair. Each IB connection is peer to
         peer and uses one connected mode QP at each end.

         Though the address resolution occurs at an individual IP address
         level, the connection between the nodes is at the IB layer.
         Therefore, every individual address resolution does not imply a
         new connection between the peers.

3.0 Address Resolution

         Address resolution queries are sent out on the "broadcast-GID"
         over the UD QP associated with the IPoIB interface. A unicast
         reply is received on the UD QP.

3.1 Link-layer Address

         IPoIB encapsulation [IPoIB_UD] describes the link-layer address
         as follows:

             <1 octet reserved>:QP: GID

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         This document extends the link-layer address as follows:



                 This is a single octet field. The bits indicate the
                 connected modes supported by the interface.

                 Bit 0 specifies the support for the "reliable connected"
                 (RC) mode.  Bit 1 indicates the support for the
                 "unreliable connected" (UC) mode.  All other bits in the
                 octet are reserved and MUST be set to 0 on transmits and
                 ignored on receives.  The format of the flags is:

                     |RC|UC| 0| 0| 0| 0| 0| 0|

                 Both the RC and UC MAY be set at the same time if the
                 interface supports both the modes. Since the IPoIB-UD
                 mode is always supported there are no flags to indicate
                 IPoIB-UD support.

                 If IPoIB-CM is not supported i.e. if the implementation
                 only supports IPoIB-UD, then the implementation MUST
                 ignore the <Flags> on reception. It MUST set the <Flags>
                 octet to all zeroes on transmission as specified in


                 The queue-pair number (QPN) on which the unicast address
                 resolution reply will be received [IPoIB_UD]. An IPoIB
                 interface has only one UD QP associated with it whether
                 it supports this extension or not.

                 The QPN also serves another purpose. It is used to form
                 the Service-ID that is used to setup the IB connection.

         On receiving the multicast/broadcast address resolution request,
         the receiver replies with its own link-address, including the
         associated UD QPN and the appropriate flags.

         The receiver's reply is unicast back to the sender after the
         receiver has, as in the case of IPoIB-UD, resolved the GID to
         the LID, and determined other required parameters [IPoIB_UD].

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         Once the address resolution is completed the underlying IB
         connection on the supported connection modes can be set up. An
         implementation is NOT REQUIRED to setup a connection merely
         because the peer indicates the capability. The decision to make
         such a connection is left to the implementation.

3.2 IB Connection Setup

         Once the address resolution is complete the IB connection can be
         setup by either of the peers. To setup a connection IB
         Management Datagrams (MADs) are directed to the peer's
         communication manager (CM). The connection request always
         contains a Service-ID for the peer to associate the request with
         the appropriate service. If the request is accepted, the peer
         returns the relevant connected mode QPN in the response MAD. The
         format of the CM connection messages and the IB connection setup
         process is described in [IB_ARCH]. The overall handshake is of
         the form:

             REQ ---->
                  <---- REP [or REJ(reject)]
             RTA ---->
             [or REJ(reject)]

         The CM messages include, among other parameters, the Service-ID,
         Local connection-mode QPN, and the payload size to use over the

               The IB connection is setup using the Service-ID as defined
               in section 3.5 below. The node MUST keep a record of IB
               connections it is participating in. The node MAY attempt
               another connection to the remote peer using the same
               Service-ID as used for an existing IB connection.
               Similarly, the receiver of such a connection MAY drop the
               request with a suitable error indication in the CM
               response. The decision to accept or initiate multiple
               connections from or to an IPoIB interface is left to the

         The node that initiated the connection is aware of the target
         node's IP address as described above. The node receiving the IB
         connection request, however cannot determine the initiating
         node's link address.  To enable this determination, every CM
         message exchanged in setting up the IB connection, MUST include
         the sender's IPoIB-UD QPN in the "private data" [IB_ARCH] field.
         The IPoIB-UD QPN MUST be included in all "REJ" [IB_ARCH]
         messages too.

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3.3 Simultaneous IB Connections

         To ensure that two IB connections are not setup between the
         peers due to REQ crossing, the following rules MUST be followed:

             The receiver forms the remote node's link-layer address
             using the UD QPN received in the "private data" field of the
             "REQ" message and the GID of the sender included in the
             "REQ" message. The link-layer address is used to determine
             if there is already an outstanding connection request "REQ"
             sent by the local interface to the given received link-layer
             address. If such an outstanding request is determined, then
             the two link-layer addresses (local and remote) are
             numerically compared. If the local link-layer address is
             numerically smaller, then the connection is accepted,
             otherwise rejected. The error code in "REJ" MAD is set to
             "Consumer Reject" [IB_ARCH].

                 The link-layer addresses formed for comparison zero out
                 the connection mode flags specified in section 3.1. The
                 comparison is performed from the most significant octet
                 to the least significant octet of the link-layer

             The above holds even if the receiver supports multiple IB
             connections from the same peer. This is to ensure that only
             one more connection is setup when the "REQ" messages cross.

3.4 IPoIB-CM IB Connection Teardown

         IB connections created through IPoIB-CM are considered part of
         an IPoIB interface. As such, they SHOULD be torn down when the
         IPoIB interfaces they are associated with is torn down.

         Furthermore, the IB connection between two peers MAY be torn
         down by either peer whenever the address resolution entry
         expires. An implementation is free to implement alternative
         policies for tearing down of IB connections between peers.

3.5 Service-ID

         The InfiniBand specification defines a block of service IDs for
         IETF use. The InfiniBand specification has left the definition

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         and management of this block to the IETF [IB_ARCH]. The 64-bit
         block is:

   |00000001|<-------------------IETF use------------------------------>|

         The Service-IDs used by IPoIB will be in the format:

   |00000001|  Type  |         Reserved        |        QPN             |

         The "Type" field MUST be set to 0.

         The "Reserved" field MUST be set to zeroes.

         The QPN MUST be the UD QP exchanged during address resolution.

4.0 Frame Format

         All IP datagrams transported over InfiniBand are prefixed by a
         4-octet encapsulation header as described in [IPoIB_UD].

     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
     |                               |                               |
     |         Type                  |       Reserved                |
     |                               |                               |

         The type field SHALL indicate the encapsulated protocol as per
         the following table.

                         | Type     |    Protocol |
                         | 0x800    |    IPv4     |
                         | 0x86DD   |    IPv6     |

         These values are taken from the "ETHER TYPE" numbers assigned by
         Internet Assigned Numbers Authority (IANA). Other network
         protocols, identified by different values of "ETHER TYPE", may
         use the encapsulation format defined herein, but such use is
         outside of the scope of this document.

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5.0 Maximum Transmission Unit

         The IB connection setup might be used for both IPv4 and IPv6 or
         it could be used for only one of them while a different
         connection is used for the other. The link MTU MUST be able to
         support the minimum MTU required by the protocols.

         The default MTU of the IPoIB-CM interface is 2044 octets i.e.
         2048 octet IPoIB-link MTU minus the 4 octet encapsulation

         However, connected modes of InfiniBand allow message sizes up to
         2^31 octets. Therefore, IPoIB-CM can use a much larger MTU for
         unicast communication between any two endpoints. The maximum
         and/or optimal payload that can be received or sent over an
         InfiniBand connection is dependent on the implementation, HCA
         and the resources configured.

         An implementation MAY utilise the following mechanism to
         exchange the optimal message size across the IB connection.

5.1 Per-Connection MTU

         Every IB connection setup message includes a "private data"
         field [IB_ARCH]. The "private data" field in the connection
         setup message (CM REQ) MUST include the "Receive MTU". This
         indicates the maximum packet size the requester can accept. The
         requester MUST be able to accept smaller MTU sizes as well.

         It is up to the implementation to utilize this mechanism for
         setting the per IB connection MTU. To calculate the resultant
         IPoIB MTU over the connection the smaller of the two IB "Receive
         MTU" values is used by both the peers.  The IPoIB interface must
         also account for the 4-octet encapsulation header and so the
         IPoIB MTU over the connection will be further reduced by that

6.0 Private-Data Format

         The "private data" field in every CM message for connection
         establishment must include the following values:

                 1. UD QPN of the sender
                 2. Receive MTU supported by the sender

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         The format of "private data" field MUST be:

                 0        7       15       23       31
                 |Reserved|         UD QPN           |
                 |            Receive MTU            |

         The Reserved value MUST be set to zero on transmit and ignored
         on receive.

7.0 IPoIB-CM Considerations

         Every IPoIB interface supports IPoIB-UD. It may additionally
         support one or both of IPoIB-CM modes. Therefore, there can be
         multiple methods of communicating between any two peers. This
         implies that an interface MAY transmit/receive a packet over any
         of the RC, UC or UD modes depending on the modes supported
         between it and the peer. It further follows that every IPoIB
         implementation compliant with this document MUST accept all IP
         unicast transmissions over any of the IPoIB modes it supports.
         Multicast and broadcast packets by their nature will always be
         transmitted and received over the IPoIB-UD QP. Additionally, all
         address resolution responses (ARP or Neighbor Discovery) MUST
         always be encapsulated in a UD mode packet.

7.1 A Cautionary Note on IPoIB-RC

         The RC mode of InfiniBand guarantees in-order delivery of
         packets. Every message transmitted over the RC connection is
         broken into physical MTU sized packets by the RC connection. If
         any packet is lost, it is retransmitted until the complete
         message is exchanged. Therefore, there is a possibility of an
         upper transport layer experiencing a timeout, while the RC layer
         is still in the process of transferring the complete message.
         TCP will view the timeout as an indicator of congestion and
         enter slow-start thereby affecting throughput drastically
         [RFC2581]. Other upper layer protocols might insert
         retransmissions into the fabric adding to the already existing

         The applicability of Infiniband reliability is on a fabric with
         short latencies (not wide area).  Therefore, the RC timer values
         should be short compared with the starting minimum time values
         used by the upper end-to-end transports.  In addition, because
         the RC mode does not have measurement based reliable

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         transmission, its use over fabrics with long latency or very
         dynamic latency may be a concern for congestion- aware traffic
         traversing those fabrics.

7.2 IPoIB-CM Per-Destination MTU

         As described above, interfaces on the same subnet may support
         different link MTUs based on the negotiated value or due to the
         link type (UD or connected mode). Therefore, an implementation
         might choose to define a large IP MTU which is reduced based on
         the MTU to the destination. The relevant MTU may be stored in a
         suitable per-destination object, such as a route cache or a
         neighbour cache. The per-destination MTU is known to the IPoIB-
         CM interface as described in section 5.0.

         Implementations might choose not to support differing MTU values
         and always support an MTU equal to the IPoIB-UD MTU determined
         from the broadcast GID.

8.0 Security Considerations

         A node may be returned a false set of flags by an impostor. This
         may cause unnecessary attempts and some delay/disruption in
         IPoIB communication. The same is the case if wrong/spurious QPN
         values are provided during address resolution

9.0 IANA Considerations

         Future uses of the reserved bits and octets in the link-layer
         address (section 3.1), Service-ID (section 3.5), and "Private-
         Data Format" (section 6.0), MUST be published as RFCs.  This
         document requires that the reserved bits be set to zero on

10.0 Acknowledgements

         The author thanks the IPoIB WG for the various comments and
         suggestions.  A special thanks to Bernie King-Smith and Dror
         Goldenberg for the detailed review and suggestions.

11.0 References


         [IB_ARCH]      InfiniBand Architecture Specification, version 1.2

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         [IPoIB_ARCH]   draft-ietf-ipoib-architecture-04.txt, V. Kashyap

         [IPoIB_UD]     draft-ietf-ipoib-ip-over-infiniband-9.txt,
                        H.K. Jerry Chu, V. Kashyap

         [RFC2119]     RFC 2119, "Key words for use in RFCs to Indicate
                       Requirement Levels", S. Bradner


         [RFC2581] RFC 2581, "TCP Congestion control", M. Allman, V. Paxson,
                   W. Stevens

12.0 Author's Address

         Vivek Kashyap

         15350, SW Koll Parkway
         OR 97006

         Phone: +1 503 578 3422
         Email: vivk@us.ibm.com

Full Copyright Statement

         Copyright (C) The Internet Society (2006).

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