Internet Engineering Task Force                                P. Savola
Internet-Draft                                                 CSC/FUNET
Obsoletes:                                             December 20, 2004
3913,2189,2201,1584,1585 (if
Expires: June 20, 2005

        Overview of the Internet Multicast Routing Architecture

Status of this Memo

   This document is an Internet-Draft and is subject to all provisions
   of section 3 of RFC 3667.  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 become aware will be disclosed, in accordance with
   RFC 3668.

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Copyright Notice

   Copyright (C) The Internet Society (2004).


   The lack of up-to-date documentation on IP multicast routing
   protocols and procedures has caused a great deal of confusion.  To
   clarify the situation, this memo describes the routing protocols and
   techniques currently (as of this writing) in use.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1   Multicast-related Abbreviations  . . . . . . . . . . . . .  4
   2.  Multicast Routing  . . . . . . . . . . . . . . . . . . . . . .  4
     2.1   Setting up Multicast Forwarding State  . . . . . . . . . .  4
       2.1.1   PIM-SM . . . . . . . . . . . . . . . . . . . . . . . .  4
       2.1.2   PIM-DM . . . . . . . . . . . . . . . . . . . . . . . .  4
       2.1.3   Bidirectional PIM  . . . . . . . . . . . . . . . . . .  5
       2.1.4   DVMRP  . . . . . . . . . . . . . . . . . . . . . . . .  5
       2.1.5   Obsolete Protocols . . . . . . . . . . . . . . . . . .  6
       2.1.6   Interactions and Summary . . . . . . . . . . . . . . .  6
     2.2   Distributing Topology Information  . . . . . . . . . . . .  6
       2.2.1   Multi-protocol BGP . . . . . . . . . . . . . . . . . .  7
       2.2.2   OSPF/IS-IS Multi-topology Extensions . . . . . . . . .  7
       2.2.3   Issue: Overlapping Unicast/multicast Topology  . . . .  7
     2.3   Learning (Active) Sources  . . . . . . . . . . . . . . . .  8
       2.3.1   SSM  . . . . . . . . . . . . . . . . . . . . . . . . .  8
       2.3.2   MSDP . . . . . . . . . . . . . . . . . . . . . . . . .  9
       2.3.3   Embedded-RP  . . . . . . . . . . . . . . . . . . . . .  9
     2.4   Configuring and Distributing PIM-SM RP Information . . . .  9
       2.4.1   Manual Configuration with an Anycast Address . . . . .  9
       2.4.2   Embedded-RP  . . . . . . . . . . . . . . . . . . . . . 10
       2.4.3   BSR and Auto-RP  . . . . . . . . . . . . . . . . . . . 10
     2.5   Mechanisms for Enhanced Redundancy . . . . . . . . . . . . 11
       2.5.1   Anycast RP . . . . . . . . . . . . . . . . . . . . . . 11
       2.5.2   Stateless RP Failover  . . . . . . . . . . . . . . . . 11
       2.5.3   Bidirectional PIM  . . . . . . . . . . . . . . . . . . 11
     2.6   Interactions with Hosts  . . . . . . . . . . . . . . . . . 11
       2.6.1   Hosts Sending Multicast  . . . . . . . . . . . . . . . 12
       2.6.2   Hosts Receiving Multicast  . . . . . . . . . . . . . . 12
     2.7   Restricting Multicast Flooding in the Link Layer . . . . . 12
       2.7.1   Router-to-Router Flooding Reduction  . . . . . . . . . 12
       2.7.2   Host/Router Flooding Reduction . . . . . . . . . . . . 13
   3.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 13
   4.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 13
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 13
   6.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
   6.1   Normative References . . . . . . . . . . . . . . . . . . . . 14
   6.2   Informative References . . . . . . . . . . . . . . . . . . . 15
       Author's Address . . . . . . . . . . . . . . . . . . . . . . . 17
   A.  Other Issues . . . . . . . . . . . . . . . . . . . . . . . . . 17
     A.1   Multicast Payload Transport Extensions . . . . . . . . . . 17
       A.1.1   Reliable Multicast . . . . . . . . . . . . . . . . . . 17
       A.1.2   Multicast Group Security . . . . . . . . . . . . . . . 18
       Intellectual Property and Copyright Statements . . . . . . . . 19

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1.  Introduction

   Good, up-to-date documentation of IP multicast is close to
   non-existent.  This issue severely felt with multicast routing
   protocols and techniques.  The consequence is that those who wish to
   learn of IP multicast and how the routing works in the real world do
   not know where to begin.

   The aim of this document is to provide a brief overview of multicast
   routing protocols and techniques.

   This memo deals with:

   o  setting up multicast forwarding state (Section 2.1),

   o  distributing multicast topology information (Section 2.2),

   o  learning active sources (Section 2.3),

   o  configuring and distributing the PIM-SM RP information (Section

   o  mechanisms for enhanced redundancy (Section 2.5),

   o  interacting with hosts (Section 2.6), and

   o  restricting the multicast flooding in the link layer (Section

   Multicast data delivery issues are also introduced in Appendix A.1.

   This memo obsoletes and re-classifies to Historic Border Gateway
   Multicast Protocol (BGMP), Core-based Trees (CBT), Multicast OSPF
   (MOSPF) RFCs: [RFC3913], [RFC2189], [RFC2201], [RFC1584], and

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1.1  Multicast-related Abbreviations

   ASM     Any Source Multicast
   BGMP Border Gateway Multicast Protocol (obsolete)
   BSR     Bootstrap Router
   CBT  Core-based Trees (obsolete)
   CGMP    Cisco Group Management Protocol
   DR      Designated Router
   DVMRP   Distance Vector Multicast Routing Protocol
   GARP    Group Address Resolution Protocol
   IGMP    Internet Group Management Protocol
   MLD     Multicast Listener Discovery
   MSNIP   Multicast Source Notification of Interest Protocol
   MOSPF   Multicast OSPF (obsolete)
   MBGP    Multi-protocol BGP (*not* "Multicast BGP")
   MSDP    Multicast Source Discovery Protocol
   PGM     Pragmatic General Multicast
   PIM     Protocol Independent Multicast
   PIM-DM  Protocol Independent Multicast - Dense Mode
   PIM-SM  Protocol Independent Multicast - Sparse Mode
   PIM-SSM Protocol Independent Multicast - (Source-specific) Sparse Mode
   RGMP    (Cisco's) Router Group Management Protocol
   RP      Rendezvous Point
   SSM     Source-specific Multicast

2.  Multicast Routing

2.1  Setting up Multicast Forwarding State

   The most important part of multicast routing is setting up the
   multicast forwarding state.  This section describes the protocols
   commonly used for this purpose.

2.1.1  PIM-SM

   By far, the most common multicast routing protocol is PIM-SM
   [I-D.ietf-pim-sm-v2-new].  The PIM-SM protocol includes both Any
   Source Multicast (ASM) and Source-Specific Multicast (SSM)
   functionality; PIM-SSM is a subset of PIM-SM.  Most current routing
   platforms support PIM-SM.

2.1.2  PIM-DM

   Whereas PIM-SM is designed to avoid unnecessary flooding of multicast
   data, PIM-DM [I-D.ietf-pim-dm-new-v2] operates in a "dense" mode,
   flooding the multicast transmissions throughout the network ("flood
   and prune") unless the leaf parts of the network periodically

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   indicate that they are not interested in that particular traffic.

   PIM-DM may be some fit in small and/or simple networks, where setting
   up an RP would be unnecessary, and possibly in cases where a large
   number of users is expected to be able to wish to receive the
   transmission so that the amount of state the network has to keep is
   minimal.  Therefore PIM-DM has typically only been used in special
   deployments, never currently in, e.g., ISPs' networks.

   PIM-DM never really got popular due to its reliance of data plane and
   potential for loops, and the over-reliance of the complex Assert
   mechanism.  Further, it was a non-starter with high-bandwidth

   Many implementations also support so-called "sparse-dense" mode,
   where Sparse mode is is used by default, but Dense is used for
   configured multicast group ranges (such as Auto-RP in Section 2.4.3)
   only.  Lately, many networks have been transitioned away from
   sparse-dense to only sparse mode.

2.1.3  Bidirectional PIM

   Bidirectional-PIM [I-D.ietf-pim-bidir] aims to offer streamlined
   PIM-SM operation, without data-driven events and data-encapsulation,
   inside a PIM-SM domain.  The usage of bidir-PIM may be on the
   increase especially inside sites leveraging multicast.

   As of this writing, it cannot be applied in Inter-domain multicast or
   for Embedded-RP.  XXX: explain why not, someone send text? (one
   paragraph please!).

2.1.4  DVMRP

   Distance Vector Multicast Routing Protocol (DVMRP)
   [RFC1075][I-D.ietf-idmr-dvmrp-v3][I-D.ietf-idmr-dvmrp-v3-as] is the
   first protocol designed for multicasting, and to get around initial
   deployment hurdles, it also included the tunneling capabilities which
   were part of its multicast topology functions.

   Nowadays, DVMRP is used only very rarely in operator networks, having
   been replaced with PIM-SM.  The most typical deployment of DVMRP is
   at a leaf network, to run from a legacy firewall only supporting
   DVMRP to the internal network.  However, GRE tunneling [RFC2784]
   seems to have overtaken DVMRP in this functionality, and there is
   relatively little use for DVMRP except in legacy deployments.

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2.1.5  Obsolete Protocols

   Three protocols which are considered completely obsolete are MOSPF
   [RFC1584] (which was implemented and deployed at a time, but PIM
   later replaced it), BGMP [RFC3913] (which did not get sufficient
   support to get adopted and moved forward), and CBT [RFC2201] (which
   was never significantly deployed, losing to PIM in the standards

2.1.6  Interactions and Summary

   It is worth noting that is it is possible to run different protocols
   with different groups ranges (e.g., treat some groups as dense mode
   in an other-wise PIM-SM network; this typically requires manual
   configuration of the groups) or interact between different protocols
   (e.g., use DVMRP in the leaf network, but PIM-SM upstream).  The
   basics for interactions among different protocols have been outlined
   in [RFC2715].

   The following figure gives a concise summary of the different
   protocols as of this writing.

                      | Interdomain | Intradomain | Status     |
   | PIM-SM           |     Yes     |     Yes     | De facto   |
   | PIM-DM           | Not feasible|     Yes     | Little use |
   | Bidir-PIM        |      No     |     Yes     | Wait & see |
   | DVMRP            | Not anymore |  Yes (stub) | Going out  |
   | CBT, MOSPF, BGMP |      No     |     No      | Obsolete   |

2.2  Distributing Topology Information

   When unicast and multicast topologies are the same ("congruent"),
   i.e., use the same routing tables (routing information base, RIB), it
   has been considered sufficient to just distribute one set of
   reachability information.

   However, when PIM -- which by default built multicast topology based
   on the unicast topology -- gained popularity, it became apparent that
   it would be necessary to be able to distribute also non-congruent
   multicast reachability information in the regular unicast protocols.

   The topology information is needed to perform efficient distribution
   of multicast transmissions and to prevent transmission loops by
   applying it to Reverse Path Forwarding (RPF) check.  XXX: does the

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   use of RPF need to be expanded?

   This subsection introduces these protocols.

2.2.1  Multi-protocol BGP

   Multiprotocol Extensions for BGP-4 [RFC2858] (often referred to as
   "MBGP"; however, it is worth noting that "MBGP" does *not* stand for
   "Multicast BGP") specifies a mechanism how BGP can be used to
   distribute different reachability information for unicast and
   multicast traffic (using SAFI=2 for multicast).  Multiprotocol BGP
   has been widely deployed for years, and is also neded for routing for
   example IPv6.

   These extensions are in widespread use wherever BGP is used to
   distribute unicast topology information.  Those having multicast
   infrastructure and using BGP should use Multiprotocol BGP to also
   distribute multicast reachability information explicitly even if the
   topologies are congruent (using SAFI=3).  A number of significant
   multicast transit providers even require this, by doing the RPF
   lookups solely based on explicitly advertised multicast address

2.2.2  OSPF/IS-IS Multi-topology Extensions

   Similar to BGP, some IGPs also provide the capability for signalling
   a differing multicast topology, for example IS-IS multi-topology
   extensions [I-D.ietf-isis-wg-multi-topology].  Similar work exists
   for OSPF [I-D.ietf-ospf-mt].

   It is worth noting that interdomain incongruence and intradomain
   incongruence are orthogonal, so one doesn't require the other.
   Specifically, interdomain incongruence is quite common, while
   intradomain incongruence isn't, so you see much more deployments of
   MBGP than MT-ISIS/OSPF.  Commonly deployed networks have managed well
   without protocols handling intradomain incongruence.  However, the
   availability of multi-topology mechanisms may in part replace the
   typically used workarounds such as tunnels.

2.2.3  Issue: Overlapping Unicast/multicast Topology

   An interesting case occurs when some do not distribute multicast
   topology  explicitly while some do.  In particular, this happens when
   some multicast sites in the Internet are using plain BGP while some
   use MBGP.

   Different implementations deal with this using different means.
   Sometimes, multicast RPF mechanisms first look up the multicast

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   routing table, or RIB ("topology database") with longest prefix match
   algorithm, and if they find any entry (including a default route),
   that is used; if no match is found, unicast routing table is looked

   An alternative approach is to use longest prefix match on the union
   of multicast and unicast routing tables; an implementation technique
   here is to copy the whole unicast routing table over to the multicast
   routing table.  The important point to remember here, though, is to
   not override the multicast-only routes; if the longest prefix match
   would find both a (copied) unicast route and a multicast-only route,
   the latter should be treated as preferable.

   One implemented approach is to just look up the information in the
   unicast routing table, and provide the user capabilities to change
   that as appropriate, using for example copying functions discussed

2.3  Learning (Active) Sources

   Typically, multicast routing protocols must either assume that the
   receivers know the IP addresses of the (active) sources for a group a
   priori, possibly using a rendezvous mechanism (SSM), or the sources
   must be discovered by the network protocols automatically (ASM).

   Learning active sources is a relatively straightforward process with
   a single PIM-SM domain and with a single RP, but having a single
   PIM-SM domain is a completely unscalable model for many reasons  for
   the whole Internet.  Therefore it is required to be able to split up
   the multicast routing infrastructures to smaller domains, and there
   must be a way to share information about active sources using some
   mechanism, if ASM model is to be supported.

   This section discusses the options.

2.3.1  SSM

   Source-specific Multicast [I-D.ietf-ssm-arch] (sometimes also
   referred to as "single-source Multicast") does not count on learning
   active sources in the network; it is assumed that the recipients know
   these using out of band mechanisms, and when subscribing to a (S,G)
   channel, indicate toward which source(s) the multicast routing
   protocol should send the Join messages.

   As of this writing, there are attempts to analyze and/or define
   out-of-band source discovery functions which would help SSM in

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2.3.2  MSDP

   Multicast Source Discovery Mechanism [RFC3618] was invented as a
   stop-gap mechanism, when it became apparent that multiple PIM-SM
   domains (and RPs) were needed in the network, and information about
   the active sources needed to be propagated between the PIM-SM domains
   using some other protocol.

   MSDP is also used to share the state about sources between multiple
   RPs in a single domain for, e.g., redundancy purposes [RFC3446].
   There is also work in progress to achieve the same using PIM
   extensions [I-D.ietf-pim-anycast-rp].  See Section 2.5 for more.

   There is no intent to define MSDP for IPv6, but instead use only SSM
   and Embedded-RP instead [I-D.ietf-mboned-ipv6-multicast-issues].

2.3.3  Embedded-RP

   Embedded-RP [RFC3956] is an IPv6-only technique to map the address of
   the RP to the multicast group address.  Using this method, it is
   possible to avoid the use of MSDP while still allowing multiple
   multicast domains (in the traditional sense).  The model works by
   defining a single RP for a particular group for all of the Internet,
   so there is no need to share state about that with any other RPs
   (except, possibly, for redundancy purposes with Anycast-RP using

2.4  Configuring and Distributing PIM-SM RP Information

   For PIM-SM, there exist configuration mechanisms which are used to
   configure the RP addresses and which groups are to use those RPs in
   the routers.  This section outlines the approaches.

2.4.1  Manual Configuration with an Anycast Address

   It is often the easiest to just manually configure the RP information
   on the routers when PIM-SM is used.

   Originally, static RP mapping was considered suboptimal since it
   required explicit configuration chages every time the RP address
   changed.  However, with the advent of anycast RP addressing, the RP
   address is unlikely to ever change.  Therefore, the administrative
   burden is generally limited to initial configuration.  Since there is
   usually a fair amount of multicast configuration required on all
   routers anyway (eg, PIM on all interfaces), adding the RP address
   statically isn't really an issue.  Further, static anycast RP mapping
   provides the benefits of RP load balancing and redundancy (see
   Section 2.5) without the complexity found in dynamic mechanisms like

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   Auto-RP and Bootstrap Router (BSR).

   With such design, an anycast RP uses a "portable" address, which is
   configured on a loopback interfaces of the routers currently acting
   as RPs, as described in [RFC3446].

   Using this technique, each router might only need to be configured
   with one, portable RP address.

2.4.2  Embedded-RP

   Embedded-RP provides the information about the RP's address in the
   group addresses which are delegated to those who use the RP, so
   unless no other ASM than Embedded-RP is used, one only needs to
   configure the RP routers themselves.

   While Embedded-RP in many cases is sufficient for IPv6, other methods
   of RP configuration are needed if one needs to provide ASM service
   for other than Embedded-RP group addresses.  In particular, service
   discovery type of applications may need hard-coded addresses that are
   not dependent on local RP addresses.

   As the RP's address is exposed to the users and applications, it is
   very important to ensure it does not change often, e.g., by using
   manual configuration of an anycast address.

2.4.3  BSR and Auto-RP

   BSR [I-D.ietf-pim-sm-bsr] is a mechanism for configuring the RP
   address for groups.  It may no longer be in as wide use with IPv4 it
   was ealier, and for IPv6, Embedded-RP will in many cases be

   Cisco's Auto-RP is an older, proprietary method for distributing
   group to RP mappings, similar to BSR.  Auto-RP has little use today.

   Both Auto-RP and BSR require some form of control at the routers to
   ensure that only valid routers are able to advertise themselves as
   RPs.  Further, flooding of BSR and Auto-RP messages must be prevented
   at PIM borders.  Additionally, routers require monitoring that they
   are actually using the RP(s) the administrators think they should be
   using, for example if a router (maybe in customer's control) is
   advertising itself inappropriately.  All in all, while BSR and
   Auto-RP provide easy configuration, they also provide very
   significant configuration and management complexity.

   It is worth noting that both Auto-RP and BSR were deployed before the
   use of a manually configured anycast-RP address became relatively

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   commonplace, and there is actually relatively little use for them

2.5  Mechanisms for Enhanced Redundancy

   A couple of mechanisms, already described in this document, have been
   used as a means to enhance redundancy, resilience against failures,
   and to recover from failures quickly.  This section summarizes these
   techniques explicitly.

2.5.1  Anycast RP

   As mentioned in Section 2.3.2, MSDP is also used to share the state
   about sources between multiple RPs in a single domain for, e.g.,
   redundancy purposes [RFC3446].  The purpose of MSDP in this context
   is to share the same state information on multiple RPs for the same
   groups to enhance the robustness of the service.

   There is also work in progress to achieve the same using PIM
   extensions [I-D.ietf-pim-anycast-rp].  This is a required method to
   be able to use Anycast RP with IPv6.

2.5.2  Stateless RP Failover

   It is also possible to use some mechanisms for smaller amount of
   redundancy as Anycast RP, without sharing state between the RPs.  A
   traditional mechanism has been to use Auto-RP or BSR (see Section
   2.4.3) to select another RP when the active one failed.  However, the
   same functionality could be achieved using a shared-unicast RP
   address ("anycast RP without state sharing") without the complexity
   of a dynamic mechanism.  Further, Anycast RP offers a significantly
   more extensive failure mitigation strategy, so today there is
   actually very little need to use stateless failover mechanisms,
   especially dynamic ones, for redundancy purposes.

2.5.3  Bidirectional PIM

   Bidirectional PIM (see Section 2.1.3) was designed with faster
   multicast convergence in mind, and if fast failover is required, it
   may seem like an attractive solution.

   XXX: someone want to send a bit more text about the convergence
   differences wrt PIM-SM?

2.6  Interactions with Hosts

   Previous sections have dealt with the components required by routers
   to be able to do multicast routing.  Obviously, the the real users of

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   multicast are the hosts: either sending or receiving multicast.

   This section describes the required interactions with hosts.

2.6.1  Hosts Sending Multicast

   Hosts don't need to do any signalling prior to sending multicast to a
   group; they just send to the link-layer multicast address, and the
   designated router will listen to the multicast packets and start
   forwarding them as appropriate.

   There is a proposal, Multicast Source Notification of Interest
   Protocol (MSNIP) [I-D.ietf-magma-msnip] which could be used for both
   the sending hosts to register as having interest in sending if there
   would be receivers, and for the receivers (or routers) to signal
   their interest in receiving the multicast transmission.  XXX: to be
   removed if MSNIP is not finalized before this goes forward.

2.6.2  Hosts Receiving Multicast

   Hosts signal their interest in receiving a multicast group or channel
   by the use of IGMP [RFC3376] and MLD [RFC3810].  IGMPv2 and MLDv1 are
   also commonplace, but most new deployments support the latest

2.7  Restricting Multicast Flooding in the Link Layer

   Multicast transmission in the link layer, for example Ethernet,
   typically includes some form of flooding the packets through a LAN.
   This causes unnecessary bandwidth usage and discarding unwanted
   frames on those nodes which did not want to receive the multicast

   Therefore a number of techniques have been developed, to be used in
   Ethernet switches between routers, or between routers and hosts, to
   limit the flooding.

   These options are discussed in this section.

2.7.1  Router-to-Router Flooding Reduction

   Only a proprietary solution, Cisco's RGMP [RFC3488] is known to have
   been developed to reduce the amount of router-to-router flooding on a
   LAN.  This is typically only considered a problem in some
   Ethernet-based Internet Exchange points.

   There have been similar proposals to snoop PIM messages
   [I-D.tsenevir-pim-sm-snoop][I-D.serbest-l2vpn-vpls-mcast] to achieve

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   the same effect.

2.7.2  Host/Router Flooding Reduction

   There are a number of techniques to help reduce flooding both from a
   router to hosts, and from a host to the routers (and other hosts).

   Cisco's proprietary CGMP [CGMP] provides a solution where the routers
   notify the switches, but also allows the switches to snoop IGMP
   packets to enable faster notification of hosts no longer wishing to
   receive a group.  IPv6 is not supported.

   IEEE specifications mention Group Address Resolution Protocol (GARP)
   [GARP] as a link-layer method to perform the same functionality.  Its
   implementation status is unknown.

   IGMP snooping [I-D.ietf-magma-snoop] appears to be the most widely
   implemented technique, often used alongside with IGMP proxying
   [I-D.ietf-magma-igmp-proxy].  IGMP snooping requires that the
   switches implement a significant amount of IP-level packet
   inspection; this appears to be something that is difficult to get
   right, and often the upgrades are also a challenge.  To allow the
   snooping switches to identify at which ports the routers reside (and
   therefore where to flood the packets) instead of requiring manual
   configuration, Multicast Router Discovery protocol is being specified

3.  Acknowledgements

   Tutoring a couple multicast-related papers, the latest by Kaarle
   Ritvanen [RITVANEN] convinced the author that the up-to-date
   multicast routing and address assignment/allocation documentation is

   Leonard Giuliano, James Lingard, Jean-Jacques Pansiot, Dave Meyer,
   and Stig Venaas provided good comments, helping in improving this

4.  IANA Considerations

   This memo includes no request to IANA.

5.  Security Considerations

   This memo only describes different approaches to multicast routing,
   and this has no security considerations; the security analysis of the
   mentioned protocols is out of scope of this memo.

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6.  References

6.1  Normative References

              Pusateri, T., "Distance Vector Multicast Routing
              Protocol", draft-ietf-idmr-dvmrp-v3-11 (work in progress),
              December 2003.

              Pusateri, T., "Distance Vector Multicast Routing Protocol
              Applicability Statement", draft-ietf-idmr-dvmrp-v3-as-01
              (work in progress), May 2004.

              Przygienda, T., Shen, N. and N. Sheth, "M-ISIS: Multi
              Topology (MT)Routing in IS-IS",
              draft-ietf-isis-wg-multi-topology-07 (work in progress),
              June 2004.

              Psenak, P., "MT-OSPF: Multi Topology (MT) Routing in
              OSPF", draft-ietf-ospf-mt-00 (work in progress), October

              Handley, M., Kouvelas, I., Speakman, T. and L. Vicisano,
              "Bi-directional Protocol Independent Multicast
              (BIDIR-PIM)", draft-ietf-pim-bidir-07 (work in progress),
              July 2004.

              Adams, A., Nicholas, J. and W. Siadak, "Protocol
              Independent Multicast - Dense Mode (PIM-DM): Protocol
              Specification  (Revised)", draft-ietf-pim-dm-new-v2-05
              (work in progress), June 2004.

              Fenner, B., Handley, M., Holbrook, H. and I. Kouvelas,
              "Protocol Independent Multicast - Sparse Mode PIM-SM):
              Protocol Specification  (Revised)",
              draft-ietf-pim-sm-v2-new-11 (work in progress), October

              Holbrook, H. and B. Cain, "Source-Specific Multicast for
              IP", draft-ietf-ssm-arch-06 (work in progress), September

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   [RFC2858]  Bates, T., Rekhter, Y., Chandra, R. and D. Katz,
              "Multiprotocol Extensions for BGP-4", RFC 2858, June 2000.

   [RFC3376]  Cain, B., Deering, S., Kouvelas, I., Fenner, B. and A.
              Thyagarajan, "Internet Group Management Protocol, Version
              3", RFC 3376, October 2002.

   [RFC3618]  Fenner, B. and D. Meyer, "Multicast Source Discovery
              Protocol (MSDP)", RFC 3618, October 2003.

   [RFC3810]  Vida, R. and L. Costa, "Multicast Listener Discovery
              Version 2 (MLDv2) for IPv6", RFC 3810, June 2004.

   [RFC3956]  Savola, P. and B. Haberman, "Embedding the Rendezvous
              Point (RP) Address in an IPv6 Multicast Address", RFC
              3956, November 2004.

6.2  Informative References

   [CGMP]     "Cisco Group Management Protocol",

   [GARP]     Tobagi, F., Molinero-Fernandez, P. and M. Karam, "Study of
              IEEE 802.1p GARP/GMRP Timer Values", 1997.

              Fenner, B., He, H., Haberman, B. and H. Sandick,
              "IGMP/MLD-based Multicast Forwarding ('IGMP/MLD
              Proxying')", draft-ietf-magma-igmp-proxy-06 (work in
              progress), April 2004.

              Haberman, B. and J. Martin, "Multicast Router Discovery",
              draft-ietf-magma-mrdisc-03 (work in progress), October

              Fenner, B., "Multicast Source Notification of Interest
              Protocol (MSNIP)", draft-ietf-magma-msnip-05 (work in
              progress), March 2004.

              Christensen, M., Kimball, K. and F. Solensky,
              "Considerations for IGMP and MLD Snooping Switches",
              draft-ietf-magma-snoop-11 (work in progress), May 2004.

              Savola, P., "IPv6 Multicast Deployment Issues",

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              draft-ietf-mboned-ipv6-multicast-issues-01 (work in
              progress), September 2004.

              Farinacci, D., "Anycast-RP using PIM",
              draft-ietf-pim-anycast-rp-02 (work in progress), June

              Fenner, B., "Bootstrap Router (BSR) Mechanism for PIM",
              draft-ietf-pim-sm-bsr-04 (work in progress), July 2004.

              Serbest, Y., "Supporting IP Multicast over VPLS",
              draft-serbest-l2vpn-vpls-mcast-01 (work in progress),
              November 2004.

              Senevirathne, T. and S. Vallepali, "Protocol Independent
              Multicast-Sparse Mode (PIM-SM) Snooping",
              draft-tsenevir-pim-sm-snoop-00 (work in progress), April

   [RFC1075]  Waitzman, D., Partridge, C. and S. Deering, "Distance
              Vector Multicast Routing Protocol", RFC 1075, November

   [RFC1584]  Moy, J., "Multicast Extensions to OSPF", RFC 1584, March

   [RFC1585]  Moy, J., "MOSPF: Analysis and Experience", RFC 1585, March

   [RFC2189]  Ballardie, T., "Core Based Trees (CBT version 2) Multicast
              Routing -- Protocol Specification --", RFC 2189, September

   [RFC2201]  Ballardie, T., "Core Based Trees (CBT) Multicast Routing
              Architecture", RFC 2201, September 1997.

   [RFC2715]  Thaler, D., "Interoperability Rules for Multicast Routing
              Protocols", RFC 2715, October 1999.

   [RFC2784]  Farinacci, D., Li, T., Hanks, S., Meyer, D. and P. Traina,
              "Generic Routing Encapsulation (GRE)", RFC 2784, March

   [RFC3208]  Speakman, T., Crowcroft, J., Gemmell, J., Farinacci, D.,

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              Lin, S., Leshchiner, D., Luby, M., Montgomery, T., Rizzo,
              L., Tweedly, A., Bhaskar, N., Edmonstone, R.,
              Sumanasekera, R. and L. Vicisano, "PGM Reliable Transport
              Protocol Specification", RFC 3208, December 2001.

   [RFC3446]  Kim, D., Meyer, D., Kilmer, H. and D. Farinacci, "Anycast
              Rendevous Point (RP) mechanism using Protocol Independent
              Multicast (PIM) and Multicast Source Discovery Protocol
              (MSDP)", RFC 3446, January 2003.

   [RFC3488]  Wu, I. and T. Eckert, "Cisco Systems Router-port Group
              Management Protocol (RGMP)", RFC 3488, February 2003.

   [RFC3740]  Hardjono, T. and B. Weis, "The Multicast Group Security
              Architecture", RFC 3740, March 2004.

   [RFC3913]  Thaler, D., "Border Gateway Multicast Protocol (BGMP):
              Protocol Specification", RFC 3913, September 2004.

              Ritvanen, K., "Multicast Routing and Addressing", HUT
              Report, Seminar on Internetworking, May 2004,

Author's Address

   Pekka Savola
   CSC - Scientific Computing Ltd.


Appendix A.  Other Issues

A.1  Multicast Payload Transport Extensions

   A couple of mechanisms have been, and are being specified, to improve
   the characteristics of the data that can be transported over

   These go beyond the scope of multicast routing, but as reliable
   multicast has some relevance, these are briefly mentioned.

A.1.1  Reliable Multicast

   Reliable Multicast Working Group has been working on experimental

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   specifications so that applications requiring reliable delivery
   characteristics, instead of simple unreliable UDP, could use
   multicast as a distribution mechanism.

   One such mechanism is Pragmatic Generic Multicast (PGM) [RFC3208].
   This does not require support from the routers, bur PGM-aware routers
   may act as helpers delivering missing data.

A.1.2  Multicast Group Security

   Multicast Security Working Group has been working on methods how the
   integrity, confidentiality, and authentication of data sent to
   multicast groups can be ensured using cryptographic techniques

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