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MAGMA Working Group                           Bill Fenner, AT&T Research
INTERNET-DRAFT                              Haixiang He, Nortel Networks
draft-ietf-magma-igmp-proxy-04.txt      Brian Haberman, Caspian Networks
                                      Hal Sandick, Sheperd Middle School
Expire: March, 2004                                      September, 2003



       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 Internet
   Group Management Protocol (IGMP) or Multicast Listener Discovery
   (MLD) membership information.



1. Introduction

   This document applies spanning tree multicast routing [Deering91] to
   an Internet Group Management Protocol (IGMP) or Multicast Listener



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   Discovery (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. Motivation

   In a simple tree topology, it is not necessary to run a multicast
   routing protocol. It is sufficient to learn and proxy group
   membership information and simply forward multicast packets based
   upon that information. One typical example of such tree topology can
   be found on an edge aggregation box such as Digital Subscriber Line
   Access Multiplexer (DSLAM). In most deployment scenarios, an edge box
   has only one connection to the core network side and has many
   connections to the customer side.

   Using IGMP/MLD-based forwarding to replicate multicast traffic on
   devices such as the edge boxes can greatly simplify the design and
   implementation of those devices. By not supporting more complicated
   multicast routing protocols such as PIM or DVMRP, it reduces not only
   the cost of the devices but also the operational overhead.  Another
   advantage is that it makes the proxy devices independent of the
   multicast routing protocol used by the core network routers. Hence
   proxy devices can be easily deployed in any multicast network.

   Robustness in an edge box is usually achieved by using a hot spare
   connection to the core network. When the first connection fails, the
   edge box fails over to the second connection. IGMP/MLD-based
   forwarding can benefit from such mechanism and use the spare
   connection for its redundant or backup connection to multicast
   routers. When an edge box fails over to the second connection, the
   proxy upstream connection can also be updated to the new connection.



1.2. Applicability Statement

   The IGMP/MLD-based forwarding only works in a simple tree topology.
   The tree must be manually configured by designating upstream and
   downstream interfaces on each proxy device. There are no multicast
   routers within the tree and the root of the tree is expected to be
   connected to a wider multicast infrastructure. This protocol is
   limited to a single administrative domain.

   In more complicated scenarios where the topology is not a tree, a
   more robust failover mechanism is desired,  or more than one
   administrative domains are involved, a multicast routing protocol



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   should be used.


1.3. 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].

   This document is a product of the Multicast & Anycast Group
   Membership (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.


2. Definitions


2.1. Upstream Interface

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


2.2. Downstream Interface

   Each of a proxy device'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 environments, for each multicast group, a group is in IGMP
   version 1 (IGMPv1) [Deering89] mode if an IGMPv1 report is heard. A
   group is in IGMP version 2 (IGMPv2) [Fenner97] mode if an IGMPv2
   report is heard but no IGMPv1 report is heard. A group is in IGMP
   version 3 (IGMPv3) [CDFKT02] mode if an IGMPv3 report is heard but no
   IGMPv1 or IGMPv2 report is heard.

   In IPv6 environments, for each multicast group, a group is in MLD
   version 1 (MLDv1) [DFH99] mode if a MLDv1 report is heard. MLDv1 is
   equivalent to IGMPv2.  A group is in MLD version 2 (MLDv2)
   [VCFDFKH02] 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



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   group membership on an interface.  When a group is in IGMPv3/MLDv2
   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 proxy device into which the
   membership information of each of its downstream interfaces is
   merged. The membership database is a set of membership records of the
   form:

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

   Please refer to IGMPv3/MLDv2 [CDFKT02, VCFDFKH02] specifications for
   the definition of the fields "filter-mode" and "source-list". The
   operational behaviors of the membership database is defined in
   section 4.1.


3.  Abstract protocol definition

   A proxy device 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 proxy device MUST
   NOT perform the router portion of IGMP/MLD on its upstream interface.

   The proxy device 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 proxy device sends IGMP/MLD membership reports on the upstream
   interface when queried, and sends unsolicited reports or leaves when
   the database changes.

   When the proxy device receives a packet destined for a multicast
   group (channel in Source-Specific Multicast (SSM)), 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 (this may include



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   the upstream interface).

   Note that the rule that a proxy device must be the querier in order
   to forward packets restricts the IP addressing scheme used; in
   particular, the IGMP/MLD-based forwarding devices 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.  IGMP/MLD Querier
   election rule defines that the Querier that has the lowest IP address
   wins the election. So in an IGMP/MLD-based forwarding only
   environment, if non proxy device wins the IGMP/MLD Querier election,
   no packets will flow.

   For example, in an IGMP/MLD-based forwarding only environment as
   shown in the figure below:

           LAN 1  --------------------------------------
                  Upstream |              | Upstream
                           A(non-proxy)   B(proxy)
                Downstream |(lowest IP)   | Downstream
           LAN 2  --------------------------------------

   device A has the lowest IP address on LAN 2 but it is not a proxy
   device. According to IGMP/MLD Querier election rule, A will win the
   election on LAN 2 since it has the lowest IP address. Device B will
   not forward traffic to LAN 2 since it is not the querier on LAN 2.

   The election of a single forwarding proxy is necessary to avoid local
   loops and redundant traffic for links which are considered to be
   downstream links by multiple IGMP/MLD-based forwarders. This rule
   "piggy-backs" forwarder election on IGMP/MLD Querier election. The
   use of the IGMP/MLD Querier election process to choose the forwarding
   proxy delivers similar functionality on the local link as the
   Protocol Independent Multicast (PIM) Assert mechanism.  On a link
   with only one IGMP/MLD-based forwarding device, this rule MAY be
   disabled (i.e. the device 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.

   For example, for redundancy purpose, as shown in the figure below:

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

   LAN 2 can have two proxy devices A and B. In such configuration, one
   proxy device must be elected to forward the packets. This document
   requires that the forwarder must be the IGMP/MLD querier. So proxy



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   device A will forward packets to LAN 2 only if A is the querier. In
   the above figure, if A is the only proxy device, A can be configured
   to forward packets even though B is the querier.


   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 proxy device, 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 Protocol Independent Multicast - Sparse Mode (PIM-SM)
   [FHHK02], 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.  Proxy Device Behavior

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


4.1.  Membership Database

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



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   If the proxy device performs IGMPv3/MLDv2 on the 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 proxy device were a host performing the action. If the
   proxy device performs IGMPv1 or IGMPv2/MLDv1 on the 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 proxy device reports using IGMPv1 or
   IGMPv2/MLDv1, only the multicast address field in the membership
   record is used.


4.2.  Forwarding Packets

   A proxy device forwards packets received on its upstream interface to
   each downstream interface based upon the downstream interface's
   subscriptions and whether or not this proxy device is the IGMP/MLD
   Querier on each interface. A proxy device 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 proxy
   device is the IGMP/MLD Querier on each interface. A proxy device 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 proxy device should
   be compliant with the specification about using IGMPv3 for SSM
   [HC01].  Note that the proxy device should be compliant with both the
   IGMP Host Requirement and the IGMP Router Requirement for SSM since
   it performs IGMP Host Portion on the 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 proxy device 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



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   elected Querier.  An attacker on a downstream LAN can cause itself to
   be elected Querier resulting in no packets being forwarded.

   IGMP/MLD-based forwarding does not provide "upstream loop" detection
   mechanism as described in section 3. Hence to avoid the problems
   caused by an "upstream loop", it MUST be administratively assured
   that such loops don't exist when deploying IGMP/MLD Proxying.

   The IGMP/MLD information presented by the proxy to its upstream
   routers is the aggregation of all its downstream group membership
   information.  Any access control applied on the group membership
   protocol at the upstream router can not be performed on a single
   subscriber. That is, the access control will apply equally to all the
   interested hosts reachable via the proxy device.


Acknowledgments

   The authors would like to thank Eric Nordmark, Dave Thaler, Pekka
   Savola, Karen Kimball and others for reviewing the specification and
   providing their comments.



Normative 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.

   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-



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               Specific Multicast", Work in Progress.



Informative References

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

   FHHK02      Fenner, W., Handley, M., Holbrook, H., and Kouvelas, I.,
               "Protocol Independent Multicast - Sparse Mode (PIM-SM):
               Protocol Specification (Revised)", 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
     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


Full Copyright Statement

   Copyright (C) The Internet Society (2002).  All Rights Reserved.




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Acknowledgement:

   Funding for the RFC Editor function is currently provided by the
   Internet Society.























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