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Versions: 00 01 02 03 04 05 06 RFC 4605

Internet Engineering Task Force               Bill Fenner, AT&T Research
INTERNET-DRAFT                              Haixiang He, Nortel Networks
draft-ietf-magma-igmp-proxy-01.txt      Brian Haberman, Caspian Networks
                                      Hal Sandick, Sheperd Middle School
Expire: May, 2003                                         November, 2002



       IGMP/MLD-based Multicast Forwarding ("IGMP/MLD 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
   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
   http://www.ietf.org/ietf/1id-abstracts.txt.

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.



Abstract

   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/MLD
   membership information.

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






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1. Introduction

   This document applies spanning tree multicast routing[Deering91] to
   an IGMP/MLD-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",
   "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and "OPTIONAL" in this
   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

   In IPv4 environment, 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 report is heard but no IGMPv1 or IGMPv2
   report is heard.

   In IPv6 environment, for each multicast group, a group is in MLDv1
   mode if a MLDv1 report is heard. MLDv1 is equivalent to IGMPv2.  A
   group is in MLDv2 mode if an MLDv2 report is heard but no MLDv1
   report is heard. MLDv2 is equivalent to IGMPv3.


2.4.  Subscription

   When a group is in IGMPv1 or IGMPv2/MLDv1 mode, the subscription is a
   group membership on an interface.  When a group is in IGMPv3/MLDv2



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   mode, the subscription is a an IGMPv3/MLDv2 state entry (i.e. a
   (multicast address, group timer, filter-mode, source-element list)
   tuple) on an interface.




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/MLD-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, CDFKT02] or MLD [DFH99, VCFDFKH02] protocol on
   its downstream interfaces, and the host portion of IGMP/MLD on its
   upstream interface.  The router MUST NOT perform the router portion
   of IGMP/MLD on its upstream interface.

   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
   database.

   The router sends IGMP/MLD 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/MLD 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/MLD-based forwarding routers must be given the
   lowest IP addresses of any potential IGMP/MLD Querier on the link, in
   order to win the IGMP/MLD Querier election.  If another device wins
   the IGMP/MLD Querier election, no packets will flow.

   Forwarder election is necessary for links which are considered to be
   downstream links by multiple IGMP/MLD-based forwarders. This rule



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   "piggy-backs" forwarder election on IGMP/MLD Querier election.  On a
   link with only one IGMP/MLD-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.

   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 required 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
   infrastructure.


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/MLD-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



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   though they were from a directly connected system.


4.  Router Behavior

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


4.1.  Membership Database

   The router performs the router portion of the IGMP/MLD protocol on
   each downstream interface.  For each interface, the version of
   IGMP/MLD 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/MLDv1 subscriptions, these
   subscriptions are converted to IGMPv3/MLDv2 subscriptions. The
   IGMPv3/MLDv2 and the converted subscriptions are first preprocessed
   to remove the timers in the subscriptions, and if the filter mode is
   EXCLUDE, to remove every source whose source timer > 0. Then the
   preprocessed subscriptions are merged using the merging rules for
   multiple memberships on a single interface specified in the
   IGMPv3/MLDv2 specification[CDFKT02,VCFDFKH02] 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/MLDv1 subscription that is (G). I2 has an IGMPv3/MLDv2
   subscription that has membership information (G, INCLUDE, (S1, S2)).
   The I1's subscription is converted to an IGMPv3/MLDv2 subscription
   that has membership information (G, EXCLUDE, NULL). Then the
   subscriptions are preprocessed and merged and final membership record
   is (G, EXCLUDE, NULL).

   The router performs the host portion of the IGMP/MLD protocol on
   upstream interface. If there is an IGMPv1 or IGMPv2/MLDv1 querier on
   upstream network, then the router will perform IGMPv1 or IGMPv2/MLDv1
   on upstream interface accordingly. Otherwise, it will perform
   IGMPv3/MLDv2.



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   If the router performs IGMPv3/MLDv2 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/MLDv1 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/MLDv1, only the multicast address field in the
   membership record is used.


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/MLD 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/MLD 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
   addresses.


5. Security Considerations

   Since only the Querier forwards packets, the IGMP/MLD 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



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   get elected Querier resulting in no packets being forwarded.



References

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

   CDFKT02     Cain, B., S. Deering, B. Fenner, I. Kouvelas and A.
               Thyagarajan, "Internet Group Management Protocol,
               Version 3", RFC 3376, October 2002.

   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.

   DFH99       Deering, S., Fenner, W., and Haberman, B., "Multicast
               Listener Discovery (MLD) for IPv6", RFC 2710,
               October 1999.

   VCFDFKH02   Vida, R., Costa, L., Fdida, S., Deering, S., Fenner, B.,
               Kouvelas, I., and Haberman, B., "Multicast Listener
               Discovery Version 2 (MLDv2) for IPv6", Work in Progress.

   HC01        Holbrook, H., and Cain, B., "Using IGMPv3 For Source-
               Specific Multicast", Work in Progress.

Author's  Address:

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

     Haixiang He
     Nortel Networks
     600 Technology Park Drive
     Billerica, MA 01821
     Phone: +1 978 288-7482



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     Email: haixiang@nortelnetworks.com

     Brian Haberman
     Caspian Networks
     One Park Drive, Suite 400
     Research Triangle Park, NC  27709
     Phone: +1-919-949-4828
     EMail: bkhabs@nc.rr.com

     Hal Sandick
     Sheperd Middle School
     2401 Dakota St.
     Durham, NC 27707
     Email: sandick@nc.rr.com






































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