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Versions: 00 01 02 03 04                                 Standards Track
INTERNET-DRAFT                                            H.K. Jerry Chu
<draft-ietf-ipoib-link-multicast-04.txt>                Sun Microsystems
                                                           Vivek Kashyap
Expires: December, 2003                                       June, 2003

             IP link and multicast over InfiniBand networks

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that other
   groups may also distribute working documents as Internet-Drafts.

   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."

   The list of current Internet-Drafts can be accessed at

   The list of Internet-Draft Shadow Directories can be accessed at

   Copyright (C) The Internet Society (2003).  All Rights Reserved.


   This document specifies a method for setting up IP subnets and
   multicast services over InfiniBand(TM) networks. Discussions in this
   document are applicable to both IPv4 and IPv6, unless explicitly
   specified. A separate document will cover unicast and encapsulation
   of IP datagrams over InfiniBand networks.

Table of Contents
   1.0     Introduction
   2.0     Terminology
   3.0     Basic IPoIB Transport - Unreliable Datagram
   4.0     IB Multicast Architecture
   5.0     IB Links vs. IPoIB Links

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   6.0     Setting up an IPoIB Link
   6.1     Maximum Transmission Unit
   6.2     IPoIB Link Q_Key
   6.3     Other Link Attributes
   7.0     The IPoIB Broadcast Group
   8.0     Mapping for other Multicast Groups
   9.0     Sending and Receiving IP Multicast Packets
   10.0    IP Multicast Routing
   11.0    New Types of Vulnerability in IB Multicast
   12.0    Security Considerations
   13.0    Acknowledgments
   14.0    References
   15.0    Author's Address
   16.0    Full Copyright Statement

1.0 Introduction

   InfiniBand Architecture (IBA) defines four layers of network services
   corresponding to layer one through layer four of the OSI reference
   model.  For the purpose of running IP over an InfiniBand (IB)
   network, the IB link, network, and transport layers collectively
   constitute the data link layer to the IP stack. One can find a
   general overview of IB architecture related to IP networks in

   This document will focus on the necessary steps in order to lay out
   an IP network on top of an IB network. It will describe all the
   elements of an IP over InfiniBand (IPoIB) link, how to configure its
   associated attributes, and how to set up basic broadcast and
   multicast services for it. IPoIB links are the building blocks upon
   which an IP network consisting of many IP subnets connected by
   routers can be built.  Subnetting allows the containment of broadcast
   traffic within a single link. It also provides certain degree of
   isolation for the administration purpose between nodes on different

2.0 Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

3.0 Basic IPoIB Transport - Unreliable Datagram

   InfiniBand defines four types of transport services [IBTA]. They are
   reliable connection, unreliable connection, reliable datagram,
   unreliable datagram. IBA also defines a special raw datagram service

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   for encapsulation purpose. Both unreliable datagram and raw datagram
   define support for multicast. They provide the basic transport
   mechanism that best matches the IP datagram paradigm.

   IB unreliable datagram provides many additional features such as the
   partition key (P_Key) protection, multiple queue pairs (QPs), and
   Q_Key protection. Moreover, it defines a 32-bit invariant CRC
   checksum, which provides a much stronger protection against data
   corruption, compared with the 16-bit CRC that a raw datagram carries.

   For these reasons, IB unreliable datagram is considered to be a much
   better choice as the basic IPoIB transport than the raw datagram, and
   is chosen as the default IPoIB transport mechanism ([IPoIB_ARCH],

4.0 IB Multicast Architecture

   The following discussion gives a short overview of the multicast
   architecture in InfiniBand. For a complete specification, the reader
   is referred to [IBTA].

   IBA defines two layers of multicast services. Its link layer uses
   multicast LIDs (MLIDs) in the Local Route Header (LRH). LIDs are
   allocated by the Subnet Manager (SM) and fall in the range between
   0xC0000 to 0xFFFE (approximately 16k). MLIDs are used by IB switches
   to program their multicast forwarding tables. An IB switch
   implementation may support much fewer MLIDs in its forwarding table

   The IB network layer uses multicast GIDs (MGIDs) in the Global Route
   Header (GRH). MGIDs closely resemble IPv6 multicast addresses [AARCH]
   shown below.

   |   8    |  4 |  4 |                  112 bits                   |
   +------ -+----+----+---------------------------------------------+
   |11111111|flgs|scop|                  group ID                   |

                                 Figure 1

   [IPoIB_ARCH] describes each field in more details.

   Since every IB multicast packet is required to carry a LRH and a GRH,
   both a valid MGID and a valid MLID are needed before an IB multicast
   packet can be constructed.

   An IB multicast group is uniquely identified by a valid MGID. Before
   a MGID can be used within an IB subnet, either as a destination

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   address of a multicast packet, or to represent a multicast group that
   an IB node can join, an IB multicast group corresponding to the MGID
   must be created through the Subnet Administrator (SA). Besides the
   the MGID, the creator of an IB multicast group must supply values of
   path MTU, P_Key, Q_Key, Service Level (SL), FlowLabel, TClass that
   are appropriate for all the potential clients of the multicast group
   to use. In return, SA will allocate a MLID to be used by switches in
   the local IB subnet.

   Unreliable multicast is defined by IBA as an optional functionality
   for channel adaptors (CAs) and switches. In today's IP technology,
   link multicast has become an indispensable function for better
   supporting a modern IP network. For this reason, it is required that
   an IPoIB fabric supports multicast. This includes all the CAs and
   switches that are part of an IP network.

5.0 IB Links vs. IPoIB Links

   A link segment on top of which an IP subnet can be configured is
   defined in [IPV6] as a communication facility or medium over which
   nodes can communicate at the "link" layer.  For most types of
   communication media, the boundary between different data link
   segments closely follows the physical topology of the network. For
   instance, an Ethernet network connected by switches, hubs, or bridges
   usually forms a single link segment and broadcast/multicast domain.
   Different Ethernet segments can be connected by IP routers at the
   network layer to form an IP network.

   InfiniBand defines its own link-layer and subnets consisting of nodes
   connected by IB switches and routers. However, the IPoIB link
   boundary need not follow the IB link boundary. Nodes residing on
   different IB subnets can still communicate directly with one another
   through IB routers at the InfiniBand network layer. This
   communication at the network layer applies to unicast as well as

   The ultimate requirement for two nodes in the same IB fabric to
   communicate at the IB level, besides physical connectivity, is a
   common P_Key.

   Partitioning in IB provides an isolation mechanism among nodes in an
   IB fabric, much like VLANs in the Ethernet network.  Each port of an
   HCA (Host Channel Adaptor) contains a P_Key table holding all the
   valid P_Keys the port is allowed to use. The P_Key table is set up by
   the SM of the local IB subnet. Each QP is programmed with a P_Key
   from the local P_Key table. This P_Key is carried in all the outgoing
   packets from the QP, and is used to compare against the P_Key of all
   incoming packets to the QP. Any packet with an invalid P_Key will be

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   discarded by the QP and a P_Key violation trap will be generated.  IB
   switches may optionally enforce partition checking too.

   Following the above, IB partitions are the natural choice for
   defining IPoIB link boundary. It also provides much needed
   flexibility for a network administrator to group nodes logically into
   different subnets in a large network.

6.0 Setting up an IPoIB Link

   A network administrator defines an IPoIB link by setting up an IB
   partition and assigning it a unique P_Key. Since a full-duplex
   communication is required among IP nodes, full-membership P_Keys,
   that is, those with the high-order bit set to 1 shall be used. An IB
   partition may or may not span multiple IB subnets; and whether it
   does or not is mostly transparent to IPoIB.

   Each node attached to an IB partition MUST have one of its HCAs
   assigned the P_Key to use. Note that the P_key table of an HCA port
   may contain many P_Keys. It is up to the implementation to define the
   method by which the P_Key relevant to a particular IPoIB subnet is
   determined and conveyed to the IPoIB stack. For instance,
   implementations may resort to a manual configuration when choosing
   the P_key or a set of P_Keys for IPoIB, and rely on DHCP [DHCP] to
   assign an IP subnet number to each IPoIB link.

   Once an IB partition is established for IPoIB use, the link MTU and
   Q_Key are two other attributes that must be chosen before an IPoIB
   link can be configured.

6.1 Maximum Transmission Unit

   IB defines five permissible maximum payload sizes (MTUs). They are
   256, 512, 1024, 2048 and 4096 bytes. [IPV6] requires a link MTU of
   1280 bytes or greater. To be better compatible with Ethernet, the
   dominant network media in both the LAN and WAN environment, the IPoIB
   link MTU should be 1500 bytes or greater. This leaves only 2048 and
   4096 bytes as the two acceptable MTUs for IPoIB. Channel adaptors
   supporting a MTU less than the minimal requirement can still expose
   an acceptable MTU to IP through an adaptation layer that fragments
   larger messages into smaller IB packets, and reassembles them on the
   receiving end. But this must be done in a way that is transparent to
   the IP stack.

   It is up to the network administrator to select a link MTU to use
   when configuring an IPoIB link. The link MTU SHALL not be greater
   than the MTU of any IB devices on the IPoIB link. Here the IB devices
   include IB switches, CAs, or routers.

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   In general, a maximum link MTU should be employed whenever possible
   to attain a better throughput performance. One caveat is that once a
   link MTU is chosen for a given IPoIB link, nodes connected by CAs of
   a smaller MTU won't be able to join the link unless the whole link
   and all the devices attached to it are reconfigured to use the
   smaller MTU.

   It may be desirable in some case to use a smaller link MTU than the
   full size. For example, bridging an IPoIB link with an Ethernet link
   could be made much easier if the IPoIB link MTU is reduced to 1500
   bytes. For IPv4, this may require a manual configuration of a
   different link MTU than the maximum that all the nodes support.  For
   IPv6, one can use the MTU option of the router advertisement [DISC]
   to announce a smaller MTU to all the nodes.

   In case an IPoIB link spans more than one IB subnet, the IPoIB link
   MTU MUST not exceed the path MTU of any path connecting two nodes in
   the same IB partition. It is up to the network administrator to
   determine the appropriate path MTU value that will work for any node
   in the same IPoIB link.

6.2 IPoIB Link Q_Key

   A Q_Key is programmed by the source QP in every IB datagram, and is
   compared against the Q_Key of the destination QP.  A Q_Key violation
   will cause the offending datagram to be dropped, and a Q_Key
   violation counter to be incremented on the receiving port. A trap is
   also generated if the feature is supported on that port.

   A single Q_Key must be selected for all the QPs attached to an IPoIB
   link to use. It is recommended that a controlled Q_Key be used with
   the high order bit set. This is to prevent non-privileged software
   from fabricating and sending out bogus IP datagrams. All QPs
   configured for a given IPoIB link SHALL be assigned the same per-link

6.3 Other Link Attributes

   TClass, FlowLabel, HopLimit, and SL are four other attributes that
   are required if an IPoIB link covers more than a single IB subnet.
   The selection of these values are implementation dependent.
   Implementations must take into account the topology of IB subnets
   comprising the IPoIB link to ensure a successful communication
   between any two nodes in the same IPoIB link.

7.0 The IPoIB Broadcast Group

   Once an IB partition is created with link attributes identified for

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   an IPoIB link, the network administrator must create a special IB
   all-node multicast group (henceforth referred to as the broadcast
   group) with these link attributes for every node on the IPoIB link to
   join.  The creation of an IB multicast group is through the use of
   the "MCMemberRecord" SA attribute as described in the IBA

   The MGID of an IPoIB broadcast group will embed in it the P_Key of
   the IB partition that defines the IPoIB link. A special signature is
   also embedded to identify all the MGIDs for IPoIB use only. For IPv4
   over IB, the signature will be "0x401B". For IPv6 over IB, the
   signature will be "0x601B".

   For an IPv4 subnet, the MGID for this special IB multicast group
   SHALL have the following format:

   |   8    |  4 |  4 |     16 bits    | 16 bits | 48 bits  | 32 bits |
   |11111111|0001|scop|0100000000011011|< P_Key >|00.......0|<all 1's>|

                                 Figure 2

   For an IPv6 subnet, the format of the MGID SHALL look like this:

   |   8    |  4 |  4 |     16 bits    | 16 bits |       80 bits      |
   |11111111|0001|scop|0110000000011011|< P_Key >|000.............0001|

                                 Figure 3

   As for the scop bits, if the IPoIB link is fully contained within a
   single IB subnet, the scop bits SHALL be set to 2 (link-local).
   Otherwise the scope will be set higher.

   The broadcast group for IPv4 will serve to provide a broadcast
   service for protocols like ARP to use.

   When a node is first brought up on an IPoIB link identified by a
   P_Key, it must look for the right broadcast group to join. This is
   done by querying the SA MCMemberRecord database for a multicast group
   with a MGID matching the one constructed from the link P_Key and the
   IPoIB signature. The node SHOULD always look for a MGID of a link-
   local scope first before attempting one with a greater scope.

   Once the right MGID and broadcast group are identified, the local

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   node SHOULD use the MTU associated with the broadcast group.  In case
   the MTU of the broadcast group is greater than what the local HCA can
   support, the node can not join the IPoIB link and operate as an IP
   node. Otherwise the local node must join the broadcast group as a
   "full member" and use the rest of link attributes associated with the
   group for all future communication to the link.

   In addition to the special all-node multicast group for broadcast
   purpose, an all-router multicast group may be created at link
   configuration time if an IP router will be attached to the link. This
   is to facilitate IP multicast operations described later. An IB
   multicast group for the all-router MGID must cover every IB subnet
   that the IPoIB link encompasses.  The format of the all-router MGID
   will be covered in the next section.

8.0 Mapping for other Multicast Groups

   The general IP multicast [IPMULT] support over IB is similar to the
   case of the special broadcast group discussed above. An algorithmic
   mapping is used so that given an IP multicast address, individual
   host can compute the corresponding IB multicast address (MGID) all by
   itself without having to consult an external entity. This also
   removes the need for an externally maintained IP to IB multicast
   mapping table.

   The IPoIB multicast mapping is depicted in Figure 4. The same mapping
   function is used for both IPv4 and IPv6 except the IPoIB signature

   |   8    |  4 |  4 |     16 bits     | 16 bits |      80 bits       |
   +------ -+----+----+-----------------+---------+--------------------+
   |11111111|0001|scop|<IPoIB signature>|< P_Key >|      group ID      |

                                 Figure 4

   Since a MGID allocated for transporting IP multicast datagrams is
   considered only a transient link-layer multicast address, all IB
   MGIDs allocated for IPoIB purpose SHOULD have T = 1. The scope bits
   SHALL be the same as that of the all-node MGID for the same IPoIB

   An IP multicast address is used together with a given IPoIB link
   P_Key to form the MGID of the IB multicast group. For IPv6 the lower
   80-bit of the group ID is used directly in the lower 80-bit of the
   MGID. For IPv4, the group ID is only 28-bit long and the rest of the
   80 bits are filled with 0.

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   The rest of the bits are the same as those of the broadcast MGID.
   For example, on an IPoIB link that is fully contained within a single
   IB subnet with a P_Key of 0x8006, the MGIDs for the all-router
   multicast group with group ID 2 [AARCH, IGMP2] are:




   for IPv4 in a compressed format, and




   for IPv6 in a compressed format.

   A special case exists for the IPv4 limited broadcast address
   "" [HOSTS]. The address SHALL be mapped to the
   broadcast MGID for IPv4 networks as described in section 7 above.
   Also the IPv6 all-node multicast address "FF0X::1" [AARCH] maps
   naturally to the the special broadcast MGID for IPv6 networks.

9.0 Sending and Receiving IP Multicast Packets

   Multicast in InfiniBand differs in a number of ways from multicast in
   Ethernet. This adds some complexity to an IPoIB implementation when
   supporting IP multicast over IB.

   A) An IB multicast group must be explicitly created through the SA
   before it can be used.

   This implies that in order to send a packet destined for an IP
   multicast address, the IPoIB implementation must check with the SA on
   the outbound link first for a "MCMemberRecord" that matches the MGID.
   If one does exist, the MLID associated with the multicast group is
   used as the DLID for the packet. Otherwise, it implies no member
   exists on the local link.  If the scope of the IP multicast group is
   beyond link-local, the packet must be sent to the on-link routers
   through the use of the all-router multicast group or the broadcast
   group. This is to allow local routers to forward the packet to
   multicast listeners on remote networks.  The all-router multicast
   group is preferred over the broadcast group for better efficiency. If
   the all-router multicast group does not exist, the sender can assume

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   that there are no routers on the local link; hence the packet can be
   safely dropped.

   B) A multicast sender must join the target multicast group as a
   "SendOnlyNonMember" before outgoing multicast messages from it can be
   successfully routed. The "SendOnlyNonMember" join is different from
   the regular "FullMember" join in two aspects. First, both types of
   joins enable multicast packets to be routed FROM the local port, but
   only the "FullMember" join causes multicast packets to be routed TO
   the port.  Second, the sender port of a "SendOnlyNonMember" join will
   not be counted as a member of the multicast group for purposes of
   group creation and deletion.

   The following code snippet demonstrates the steps in a typical
   implementation when processing an egress multicast packet.

   if the egress port is already a "SendOnlyNonMember", or a
           => send the packet

   else if the target multicast group exists
           => do "SendOnlyNonMember" join
           => send the packet

   else if scope > link-local AND the all-router multicast group exists
           => send the packet to all routers
           => drop the packet

   Implementations should cache the information about the existence of
   an IB multicast group, its MLID and other attributes. This is to
   avoid expensive SA calls on every outgoing multicast packet. Senders
   MUST subscribe to the multicast group create and delete traps in
   order to monitor the status of specific IB multicast groups. E.g.,
   multicast packets directed to the all-router multicast group due to a
   lack of listener on the local subnet must be forwarded to the right
   multicast group if the group is created later.  This happens when a
   listener shows up on the local subnet.

   A node joining an IP multicast group must first construct a MGID
   according to the rule described in section 8 above. Once the correct
   MGID is calculated, the node must call the SA of the outbound link to
   attempt a "FullMember" join of the IB multicast group corresponding
   to the MGID.  If the IB multicast group doesn't already exist, one
   must be created first with the IPoIB link MTU. For the rest of
   attributes, the same values from the all-node multicast/broadcast
   group SHOULD be used.

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   The join request will cause the local port to be added to the
   multicast group. It also enables the SM to program IB switches and
   routers with the new multicast information to ensure the correct
   forwarding of multicast packets for the group.

   When a node leaves an IP multicast group, it SHOULD make a
   "FullMember" leave request to the SA. This gives SM an opportunity to
   update relevant forwarding information, to delete an IB multicast
   group if the local port is the last FullMember to leave, and free up
   the MLID allocated for it. The specific algorithm is implementation-
   dependent, and is out of the scope of this document.

   Note that for an IPoIB link that spans more than one IB subnet
   connected by IB routers, an adequate multicast forwarding support at
   the IB level is required for multicast packets to reach listeners on
   a remote IB subnet. The specific mechanism for this will be covered
   in [IBTA], and is beyond the scope of IPoIB.

10.0 IP Multicast Routing

   IP multicast routing requires multicast routers to receive a copy of
   every link multicast packet on a locally connected link [IPMULT,
   IP6MLD].  For Ethernet this is usually achieved by turning on the
   promiscuous multicast mode on a locally connected Ethernet interface.

   IBA does not provide any hardware support for promiscuous multicast
   mode.  Fortunately a promiscuous multicast mode can be emulated in
   the software running on a router through the following steps.

   A) Obtain a list of all active IB multicast groups from the local SA.

   B) Make a "NonMember" join request to the SA for every group that has
   a signature in its MGID matching the one for either IPv4 or IPv6.

   C) Subscribe to the IB multicast group creation events using a
   wildcarded MGID so that the router can "NonMember" join all IB
   multicast groups created subsequently for IPv4 or IPv6.

   The "NonMember" join has the same effect as a "FullMember" join
   except that the former will not be counted as a member of the
   multicast group for purposes of group creation or deletion. That is,
   when the last "FullMember" leaves a multicast group, the group can be
   safely deleted by the SA without concerning any "NonMember" routers.

11.0 New Types of Vulnerability in IB Multicast

   Many IB multicast functions are subject to failures due to a number
   of possible resource constraints. These include the creation of IB

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   multicast groups, the join calls ("SendOnlyNonMember", "FullMember",
   and "NonMember"), and the attaching of a QP to a multicast group.

   In general, the occurrence of these failure conditions is highly
   implementation dependent, and is believed to be rare. Usually a
   failed multicast operation at the IB level can be propagated back to
   the IP level, causing the original operation to fail, and the
   initiator of the operation to be notified. But some IB multicast
   functions are not tied to any foreground operation, making their
   failures hard to detect. E.g., if an IP multicast router attempts to
   "NonMember" join a newly created multicast group in the local subnet,
   but the join call fails, packet forwarding for that particular
   multicast group will likely to fail silently, that is, without the
   attention of local multicast senders. This type of problems can add
   more vulnerability to the already unreliable IP multicast operations.

   Implementations should log error messages upon any failure from an IB
   multicast operation. Network administrators should be aware of this
   vulnerability, and preserve enough multicast resources at the points
   where IP multicast will be used heavily. E.g., HCAs with ample
   multicast resources should be used at any IP multicast router.

12.0 Security Considerations

   All the operations for creating and configuring an IPoIB link
   described in this document, including assigning P_Keys to CAs,
   creating IB multicast groups in SA, creating and attaching QPs to IB
   multicast groups,... etc, are privileged operations, and MUST be
   protected by the underlying operating system. This is to prevent
   malicious, non-privileged software from hijacking important resources
   and configurations.  For example, A bogus IPoIB broadcast group may
   prevent a proper one from being created when the network
   administrator tries to set up a link.

   Controlled Q_Keys SHOULD be used in IPoIB links. This is to prevent
   non-privileged software from fabricating IP datagrams to send, as
   mentioned in section 6.2.

13.0 Acknowledgments

   The authors would like to thank Bruce Beukema, David Brean, Dan
   Cassiday, Aditya Dube, Yaron Haviv, Michael Krause, Thomas Narten,
   Erik Nordmark, Greg Pfister, Renato Recio, Kanoj Sarcar, Satya
   Sharma, and David L. Stevens for their suggestions and many
   clarifications on the IBA specification.

14.0 References

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   [AARCH]   Hinden, R. and S. Deering "IP Version 6 Addressing
             Architecture", RFC 2373, July 1998.

   [DHCP]    R. Droms "Dynamic Host Configuration Protocol", RFC 2131,
             March 1997.

   [DISC]    Narten, T., Nordmark, E. and W. Simpson, "Neighbor
             Discovery for IP Version 6 (IPv6)", RFC 2461, December

   [HOSTS]   Braden R., "Requirements for Internet Hosts --
             Communication Layers", RFC 1122, October 1989

   [IBTA]    InfiniBand Architecture Specification, Release 1.0.a by
             InfiniBand Trade Association at www.infinibandta.org

   [IGMP2]   Fenner W., "Internet Group Management Protocol, Version 2",
             RFC 2236, November 1997.

   [IPMULT]  Deering S., "Host Extensions for IP Multicasting", RFC
             1112, August 1989.

   [IPoIB_ARCH]  draft-ietf-ipoib-architecture-01.txt

   [IPoIB_ENCAP] draft-ietf-ipoib-ip-over-infiniband-01.txt

   [IPV6]    Deering, S. and R. Hinden, "Internet Protocol, Version 6
             (IPv6) Specification", RFC 2460, December 1998.

   [IP6MLD]  Deering S., Fenner W., Haberman B., "Multicast Listener
             Discovery (MLD) for IPv6", RFC 2710, October 1999.

   [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997.

15.0 Author's Address

   H.K. Jerry Chu
   17 Network Circle, UMPK17-201
   Menlo Park, CA 94025

   Phone: +1 650 786-5146
   EMail: jerry.chu@sun.com

   Vivek Kashyap

Chu & Kashyap                                                  [Page 13]

draft-ietf-ipoib-link-multicast-04.txt                         June 2003

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16.0 Full Copyright Statement

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Chu & Kashyap                                                  [Page 14]