Internet Engineering Task Force               Bill Fenner, AT&T Research
INTERNET-DRAFT                              Haixiang He, Nortel Networks
draft-ietf-magma-igmp-proxy-00.txt       Brian Haberman, Nortel Networks
                                            Hal Sandick, Nortel Networks
Expire: May, 2002                                         November, 2001

           IGMP-based Multicast Forwarding ("IGMP Proxying")

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC 2026.

   Internet Drafts are working documents of the Internet Engineering
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   In certain topologies, it is not necessary to run a multicast routing
   protocol.  It is sufficient to learn and proxy group membership
   information and simply forward based upon that information.  This
   draft describes a mechanism for forwarding based solely upon IGMP
   membership information.

   This document is a product of the IDMR working group within the
   Internet Engineering Task Force.  Comments are solicited and should
   be addressed to the working group's mailing list at
   and/or the authors.

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1. Introduction

   This document applies spanning tree multicast routing[Deering91] to
   an IGMP-only environment.  The topology is limited to a tree, since
   we specify no protocol to build a spanning tree over a more complex
   topology.  The root of the tree is assumed to be connected to a wider
   multicast infrastructure.

1.1. Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [Bradner97].

2. Definitions

2.1. Upstream Interface

   A router's interface in the direction of the root of the tree.  Also
   called the "Host interface".

2.2. Downstream Interface

   Each of a router's interfaces that is not in the direction of the
   root of the tree.  Also called the "Router interfaces".

2.3. Group Mode

   For each multicast group, a group is in IGMPv1 mode if an IGMPv1
   report is heard. A group is in IGMPv2 mode if an IGMPv2 report is
   heard but no IGMPv1 report is heard. A group is in IGMPv3 mode if an
   IGMPv3 is heard but no IGMPv1 or IGMPv2 report is heard.

2.4.  Subscription

   When a group is in IGMPv1 or IGMPv2 mode, the subscription is a group
   membership on an interface.  When a group is in IGMPv3 mode, the
   subscription is an IGMPv3 state entry (i.e. a (multicast address,
   group timer, filter-mode, source-element list) tuple) on an

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2.5.  Membership Database

   The database maintained at each router into which the membership
   information of each of its downstream interfaces is merged.

3.  Abstract protocol definition

   A router performing IGMP-based forwarding has a single upstream
   interface and one or more downstream interfaces.  These designations
   are explicitly configured; there is no protocol to determine what
   type each interface is.  It performs the router portion of the IGMP
   [Deering89, Fenner97, CDFKT01] protocol on its downstream interfaces,
   and the host portion of IGMP on its upstream interface.  The router
   MUST NOT perform the router portion of IGMP on its upstream

   The router maintains a database consisting of the merger of all
   subscriptions on any downstream interface. Refer to section 4 for the
   details about the construction and maintenance of the membership

   The router sends IGMP membership reports on the upstream interface
   when queried, and sends unsolicited reports or leaves when the
   database changes.

   When the router receives a packet destined for a multicast group, it
   uses a list consisting of the upstream interface and any downstream
   interface which has a subscription pertaining to this packet and on
   which it is the IGMP Querier.  This list may be built dynamically or
   cached. It removes the interface on which this packet arrived from
   the list and forwards the packet to the remaining interfaces.

   Note that the rule that a router must be the querier in order to
   forward packets restricts the IP addressing scheme used; in
   particular, the IGMP-based forwarding routers must be given the
   lowest IP addresses of any potential IGMP Querier on the link, in
   order to win the IGMP Querier election.  If another device wins the
   IGMP Querier election, no packets will flow.

   Forwarder election is necessary for links which are considered to be
   downstream links by multiple IGMP-based forwarders. This rule
   "piggy-backs" forwarder election on IGMP Querier election.  On a link
   with only one IGMP-based forwarding router, this rule MAY be disabled
   (i.e. the router MAY be configured to forward packets to an interface
   on which it is not the querier).  However, the default configuration
   MUST include the querier rule.

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   Note that this does not protect against an "upstream loop". For
   example, as shown in the figure below:

           LAN 1  --------------------------------------
                  Upstream |              | Downstream
                           A              B
                Downstream |              | Upstream
           LAN 2  --------------------------------------

   B will unconditionally forward packets from LAN 1 to LAN 2, and A
   will unconditionally forward packets from LAN 2 to LAN 1.  This will
   cause an upstream loop. A multicast routing protocol which employs a
   tree building algorithm is requried to resolve loops like this.

3.1. Topology Restrictions

   This specification describes a protocol that works only in a simple
   tree topology.  The tree must be manually configured by designating
   upstream and downstream interfaces on each router, and the root of
   the tree is expected to be connected to a wider multicast

3.2. Supporting Senders

   In order for senders to send from inside the proxy tree, all traffic
   is forwarded towards the root.  The multicast router(s) connected to
   the wider multicast infrastructure should be configured to treat all
   systems inside the proxy tree as though they were directly connected
   -- e.g., for PIM-SM, these routers should Register-encapsulate
   traffic from new sources within the proxy tree just as they would
   directly-connected sources.

   This information is likely to be manually configured; IGMP-based
   multicast forwarding provides no way for the routers upstream of the
   proxy tree to know what networks are connected to the proxy tree.  If
   the proxy topology is congruent with some routing topology, this
   information MAY be learned from the routing protocol running on the
   topology; e.g. a router may be configured to treat multicast packets
   from all prefixes learned from routing protocol X via interface Y as
   though they were from a directly connected system.

4.  Router Behavior

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   This section describes an IGMP-based multicast forwarding router's
   actions in more detail.

4.1.  Membership Database

   The router performs the router portion of the IGMP protocol on each
   downstream interface.  For each interface, the version of IGMP used
   is explicitly configured and default to the highest version supported
   by the system.

   The output of this protocol is a set of subscriptions; this set is
   maintained separately on each downstream interface.  In addition, the
   subscriptions on each downstream interface are merged into the
   membership database.

   The membership database is a set of membership records of the form:

           (multicast-address, filter-mode, source-list)

   Each record is the result of the merge of all subscriptions for that
   record's multicast-address on downstream interfaces. If some
   subscriptions are IGMPv1 or IGMPv2 subscriptions, these subscriptions
   are converted to IGMPv3 subscriptions. The IGMPv3 subscriptions and
   the converted subscriptions are merged using the merging rules for
   multiple memberships on a single interface specified in the IGMPv3
   specification[CDFKT01] to create the membership record. For example,
   there are two downstream interfaces I1 and I2 that have subscriptions
   for multicast address G. I1 has an IGMPv2 subscription that is (G).
   I2 has an IGMPv3 subscription that is (G, INCLUDE, (S1, S2)). The
   I1's subscription is converted to (G, EXCLUDE, NULL). Then the
   subscriptions are merged and final membership record is (G, EXCLUDE,

   The router performs the host portion of the IGMP protocol on upstream
   interface. If there is an IGMPv1 or IGMPv2 querier on upstream
   network, then the router will perform IGMPv1 or IGMPv2 on upstream
   interface accordingly. Otherwise, it will perform IGMPv3.

   If the router performs IGMPv3 on upstream interface, then when the
   composition of the membership database changes, the change in the
   database is reported on the upstream interface as though this router
   were a host performing the action. If the router performs IGMPv1 or
   IGMPv2 on upstream interface, then when the membership records are
   created or deleted, the changes are reported on the upstream
   interface.  All other changes are ignored. When the router reports
   using IGMPv1 or IGMPv2, only the multicast address field in the
   membership record is used.

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4.2.  Forwarding Packets

   A router forwards packets received on its upstream interface to each
   downstream interface based upon the downstream interface's
   subscriptions and whether or not this router is the IGMP Querier on
   each interface.  A router forwards packets received on any downstream
   interface to the upstream interface, and to each downstream interface
   other than the incoming interface based upon the downstream
   interfaces' subscriptions and whether or not this router is the IGMP
   Querier on each interface.  A router MAY use a forwarding cache in
   order not to make this decision for each packet, but MUST update the
   cache using these rules any time any of the information used to build
   it changes.

4.3. SSM Considerations

   To support Source-Specific Multicast (SSM), the router should be
   compliant with the specification about using IGMPv3 for SSM [HC01].
   Note that the router should be compliant with both the IGMP Host
   Requirement and the IGMP Router Requirement for SSM since it performs
   IGMP Host Portion on upstream interface and IGMP Router Portion on
   each downstream interface.

   An interface can be configured to perform IGMPv1 or IGMPv2. In this
   scenario, the SSM semantic will not be maintained for that interface.
   However, a router that supports this document should ignore those
   IGMPv1 or IGMPv2 subscriptions sent to SSM addresses. And more
   importantly, the packets with source-specific addresses SHOULD not be
   forwarded to interfaces with IGMPv2 or IGMPv1 subscriptions for these

5. Security Considerations

   Since only the Querier forwards packets, the IGMP Querier election
   process may lead to black holes if a non-forwarder is elected
   Querier.  An attacker on a downstream LAN can cause itself to get
   elected Querier resulting in no packets being forwarded.


   Bradner97   Bradner, S., "Key words for use in RFCs to Indicate
               Requirement Levels", RFC 2119/BCP 14, Harvard
               University, March 1997.

   CDFKT01     Cain, B., S. Deering, B. Fenner, I. Kouvelas and A.

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Internet Draft     draft-ietf-magma-igmp-proxy-00.txt          May, 2002

               Thyagarajan, "Internet Group Management Protocol,
               Version 3", Work in progress.  (draft-ietf-idmr-igmp-

   Deering91   Deering, S., "Multicast Routing in a Datagram
               Internetwork", Ph.D. Thesis, Stanford University,
               December 1991.

   Fenner97    Fenner, W., "Internet Group Management Protocol,
               Version 2", RFC 2236, Xerox PARC, November 1997.

   Deering89   Deering, S., "Host Extensions for IP Multicasting",
               RFC 1112, August 1989.

   HC01        Holbrook, H., and Cain, B., "Using IGMPv3 For Source-
               Specific Multicast", draft-holbrook-idmr-igmpv3-
               ssm-01.txt, March 2001.

Author's  Address:

     William C. Fenner
     AT&T Labs - Research
     75 Willow Rd
     Menlo Park, CA 94025
     Phone: +1 650 330 7893

     Haixiang He
     Nortel Networks
     600 Technology Park Drive
     Billerica, MA 01821
     Phone: 978-288-7482

     Brian Haberman
     Nortel Networks
     300 Perimter Park
     Morrisvile, NC 27560

     Hal Sandick
     Nortel Networks
     300 Perimter Park
     Morrisvile, NC 27560

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