Internet Engineering Task Force                                P. Savola
Internet-Draft                                                 CSC/FUNET
Obsoletes:                                                August 3, 2007
(if approved)
Intended status: Best Current
Expires: February 4, 2008

        Overview of the Internet Multicast Routing Architecture

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
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Copyright Notice

   Copyright (C) The IETF Trust (2007).


   This document describes multicast routing architectures that are
   currently deployed on the Internet.  This document briefly describes
   those protocols and references their specifications.

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   This memo also obsoletes and reclassifies to Historic several older
   RFCs.  These RFCs describe multicast routing protocols that were
   never widely deployed or have fallen into disuse.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Multicast-related Abbreviations  . . . . . . . . . . . . .  5
   2.  Multicast Routing  . . . . . . . . . . . . . . . . . . . . . .  5
     2.1.  Setting up Multicast Forwarding State  . . . . . . . . . .  6
       2.1.1.  PIM-SM . . . . . . . . . . . . . . . . . . . . . . . .  6
       2.1.2.  PIM-DM . . . . . . . . . . . . . . . . . . . . . . . .  6
       2.1.3.  Bi-directional PIM . . . . . . . . . . . . . . . . . .  7
       2.1.4.  DVMRP  . . . . . . . . . . . . . . . . . . . . . . . .  7
       2.1.5.  MOSPF  . . . . . . . . . . . . . . . . . . . . . . . .  7
       2.1.6.  BGMP . . . . . . . . . . . . . . . . . . . . . . . . .  7
       2.1.7.  CBT  . . . . . . . . . . . . . . . . . . . . . . . . .  8
       2.1.8.  Interactions and Summary . . . . . . . . . . . . . . .  8
     2.2.  Distributing Topology Information  . . . . . . . . . . . .  8
       2.2.1.  Multi-protocol BGP . . . . . . . . . . . . . . . . . .  9
       2.2.2.  OSPF/IS-IS Multi-topology Extensions . . . . . . . . .  9
       2.2.3.  Issue: Overlapping Unicast/multicast Topology  . . . . 10
       2.2.4.  Summary  . . . . . . . . . . . . . . . . . . . . . . . 10
     2.3.  Learning (Active) Sources  . . . . . . . . . . . . . . . . 11
       2.3.1.  SSM  . . . . . . . . . . . . . . . . . . . . . . . . . 11
       2.3.2.  MSDP . . . . . . . . . . . . . . . . . . . . . . . . . 11
       2.3.3.  Embedded-RP  . . . . . . . . . . . . . . . . . . . . . 12
       2.3.4.  Summary  . . . . . . . . . . . . . . . . . . . . . . . 12
     2.4.  Configuring and Distributing PIM RP Information  . . . . . 12
       2.4.1.  Manual RP Configuration  . . . . . . . . . . . . . . . 12
       2.4.2.  Embedded-RP  . . . . . . . . . . . . . . . . . . . . . 13
       2.4.3.  BSR and Auto-RP  . . . . . . . . . . . . . . . . . . . 13
       2.4.4.  Summary  . . . . . . . . . . . . . . . . . . . . . . . 14
     2.5.  Mechanisms for Enhanced Redundancy . . . . . . . . . . . . 14
       2.5.1.  Anycast RP . . . . . . . . . . . . . . . . . . . . . . 14
       2.5.2.  Stateless RP Failover  . . . . . . . . . . . . . . . . 14
       2.5.3.  Bi-directional PIM . . . . . . . . . . . . . . . . . . 15
       2.5.4.  Summary  . . . . . . . . . . . . . . . . . . . . . . . 15
     2.6.  Interactions with Hosts  . . . . . . . . . . . . . . . . . 15
       2.6.1.  Hosts Sending Multicast  . . . . . . . . . . . . . . . 15
       2.6.2.  Hosts Receiving Multicast  . . . . . . . . . . . . . . 16
       2.6.3.  Summary  . . . . . . . . . . . . . . . . . . . . . . . 16
     2.7.  Restricting Multicast Flooding in the Link Layer . . . . . 16
       2.7.1.  Router-to-Router Flooding Reduction  . . . . . . . . . 17
       2.7.2.  Host/Router Flooding Reduction . . . . . . . . . . . . 17
       2.7.3.  Summary  . . . . . . . . . . . . . . . . . . . . . . . 18
   3.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 18

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   4.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 18
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 19
   6.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 19
     6.1.  Normative References . . . . . . . . . . . . . . . . . . . 19
     6.2.  Informative References . . . . . . . . . . . . . . . . . . 20
   Appendix A.  Multicast Payload Transport Extensions  . . . . . . . 23
     A.1.  Reliable Multicast . . . . . . . . . . . . . . . . . . . . 23
     A.2.  Multicast Group Security . . . . . . . . . . . . . . . . . 23
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 23
   Intellectual Property and Copyright Statements . . . . . . . . . . 24

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1.  Introduction

   This document provides a brief overview of multicast routing
   architectures that are currently deployed on the Internet and how
   those protocols fit together.  It also describes those multicast
   routing protocols that were never widely deployed or have fallen into
   disuse.  A companion document [I-D.ietf-mboned-addrarch] describes
   multicast addressing architectures.

   Specifically, 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 RP information (Section 2.4),

   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 2.7).

   Section 2 starts by describing a simplistic example how these classes
   of mechanisms fit together.  Some multicast data transport issues are
   also introduced in Appendix A.

   This memo obsoletes and re-classifies to Historic [RFC2026] the
   following RFCs:

   o  Border Gateway Multicast Protocol (BGMP) [RFC3913],

   o  Core Based Trees (CBT) [RFC2189] [RFC2201],

   o  Multicast OSPF (MOSPF) [RFC1584] and [RFC1585].

   For the most part, these protocols have fallen into disuse.  There
   may be legacy deployments of some of these protocols, which are not
   affected by this reclassification.  See Section 2.1 for more on each

   Further historical perspective may be found in, for example,
   [RFC1458], [IMRP-ISSUES], and [IM-GAPS].

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1.1.  Multicast-related Abbreviations

   ASM             Any Source Multicast
   BGMP            Border Gateway Multicast Protocol
   BSR             Bootstrap Router
   CBT             Core Based Trees
   CGMP            Cisco Group Management Protocol
   DR              Designated Router
   DVMRP           Distance Vector Multicast Routing Protocol
   GARP            (IEEE 802.1D-2004) Generic Attribute Reg. Protocol
   GMRP            GARP Multicast Registration Protocol
   IGMP            Internet Group Management Protocol
   MBGP            Multi-protocol BGP (*not* "Multicast BGP")
   MLD             Multicast Listener Discovery
   MRP             (IEEE 802.1ak) Multiple Registration Protocol
   MMRP            (IEEE 802.1ak) Multicast Multiple Registration Proto.
   MOSPF           Multicast OSPF
   MSDP            Multicast Source Discovery Protocol
   PGM             Pragmatic General Multicast
   PIM             Protocol Independent Multicast
   PIM-DM          PIM - Dense Mode
   PIM-SM          PIM - Sparse Mode
   PIM-SSM         PIM - Source-Specific Multicast
   RGMP            (Cisco's) Router Group Management Protocol
   RP              Rendezvous Point
   SSM             Source-specific Multicast

2.  Multicast Routing

   In order to give a simplified summary how each of these class of
   mechanisms fits together, consider the following multicast receiver

   Certain protocols and configuration needs to be in place before
   multicast routing can work.  Specifically, when ASM is employed, a
   router will need to know its RP address(es) (Section 2.4,
   Section 2.5).  With IPv4, RPs need to be connected to other RPs using
   MSDP so information about sources connected to other RPs can be
   distributed (Section 2.3).  Further, routers need to know if or how
   multicast topology differs from unicast topology, and routing
   protocol extensions can provide that information (Section 2.2).

   When a host wants to receive a transmission, it first needs to find
   out the multicast group address (and with SSM, source address) using
   unspecified means.  Then it will signal its interest to its first-hop
   router using IGMP or MLD (Section 2.6).  The router initiates setting
   up hop-by-hop multicast forwarding state (Section 2.1) to the source

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   (in SSM) or first through the RP (in ASM).  Routers use an RP to find
   out all the sources for a group (Section 2.3).  When multicast
   transmission arrives at the receiver's LAN, it is flooded to every
   port unless flooding reduction such as IGMP snooping is employed
   (Section 2.7).

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
   [RFC4601].  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.  It
   builds a unidirectional, group-specific distribution tree consisting
   of the interested receivers of a group.

2.1.2.  PIM-DM

   Whereas PIM-SM has been designed to avoid unnecessary flooding of
   multicast data, PIM-DM [RFC3973] assumed that almost every subnet at
   a site had at least one receiver for a group.  PIM-DM floods
   multicast transmissions throughout the network ("flood and prune")
   unless the leaf parts of the network periodically indicate that they
   are not interested in that particular group.

   PIM-DM may be an acceptable fit in small and/or simple networks,
   where setting up an RP would be unnecessary, and possibly in cases
   where a large percentage of users is expected to want to receive the
   transmission so that the amount of state the network has to keep is

   PIM-DM was used as a first step in transitioning away from DVMRP.  It
   also became apparent that most networks would not have receivers for
   most groups, and to avoid the bandwidth and state overhead, the
   flooding paradigm was gradually abandoned.  Transitioning from PIM-DM
   to PIM-SM was easy as PIM-SM was designed to use compatible packet
   formats and dense-mode operation could also be satisfied by a sparse
   protocol.  PIM-DM is no longer in widespread use.

   Many implementations also support so-called "sparse-dense"
   configuration, where Sparse mode 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 transitioned away

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   from sparse-dense to only sparse mode.

2.1.3.  Bi-directional PIM

   Bi-directional PIM [I-D.ietf-pim-bidir] is a multicast forwarding
   protocol that establishes a common shared-path for all sources with a
   single root.  It can be used as an alternative to PIM-SM inside a
   single domain.  It doesn't have data-driven events or data-
   encapsulation.  As it doesn't keep source-specific state, it may be
   an appealing approach especially in sites with a large number of

   As of this writing, there is no inter-domain solution to configure a
   group range to use bi-directional PIM.

2.1.4.  DVMRP

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

   Currently, 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

2.1.5.  MOSPF

   MOSPF [RFC1584] was implemented by several vendors and has seen some
   deployment in intra-domain networks.  However, since it is based on
   intra-domain OSPF it does not scale to the inter-domain case,
   operators have found it is easier to deploy a single protocol for use
   in both intra-domain and inter-domain networks and so it is no longer
   being actively deployed.

2.1.6.  BGMP

   BGMP [RFC3913] did not get sufficient support within the service
   provider community to get adopted and moved forward in the IETF
   standards process.  There were no reported production implementations
   and no production deployments.

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2.1.7.  CBT

   CBT [RFC2201][RFC2189] was an academic project that provided the
   basis for PIM sparse mode shared trees.  Once the shared tree
   functionality was incorporated into PIM implementations, there was no
   longer a need for a production CBT implementation.  Therefore, CBT
   never saw production deployment.

2.1.8.  Interactions and Summary

   It is worth noting that it is possible to run different protocols
   with different multicast group ranges.  For example, treat some
   groups as dense or bi-dir in an otherwise PIM-SM network; this
   typically requires manual configuration of the groups or a mechanism
   like BSR (Section 2.4.3).  It is also possible to interact between
   different protocols, for example 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 deployment status
   of different protocols as of this writing.

                | Interdomain | Intradomain | Status         |
   | PIM-SM     |     Yes     |     Yes     | Active         |
   | PIM-DM     | Not anymore | Not anymore | Little use     |
   | Bi-dir PIM |      No     |     Yes     | Some uptake    |
   | DVMRP      | Not anymore |  Stub only  | Going out      |
   | MOSPF      |      No     | Not anymore | Inactive       |
   | CBT        |      No     |     No      | Never deployed |
   | BGMP       |      No     |     No      | Never deployed |

   From this table, it is clear that PIM-Sparse Mode is the only
   multicast routing protocol that is deployed inter-domain and,
   therefore, is most frequently used within multicast domains as well.

2.2.  Distributing Topology Information

   PIM has become the de-facto multicast forwarding protocol, but as its
   name implies, it is independent of the underlying unicast routing
   protocol.  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 just to distribute one
   set of reachability information to be used in conjunction with a
   protocol that sets up multicast forwarding state (e.g., PIM-SM).

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   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.
   This was previously not an issue, because DVMRP built its own
   reachability information.

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

   This subsection introduces these protocols.

2.2.1.  Multi-protocol BGP

   Multiprotocol Extensions for BGP-4 [RFC4760] (often referred to as
   "MBGP"; however, it is worth noting that "MBGP" does *not* stand for
   "Multicast BGP") specifies a mechanism by which BGP can be used to
   distribute different reachability information for unicast (SAFI=1)
   and multicast traffic (SAFI=2).  Multiprotocol BGP has been widely
   deployed for years, and is also needed to route IPv6.  Note that
   SAFI=3 was originally specified for "both unicast and multicast" but
   has since then been deprecated.

   These extensions are in widespread use wherever BGP is used to
   distribute unicast topology information.  Multicast-enabled networks
   that use BGP should use Multiprotocol BGP to distribute multicast
   reachability information explicitly even if the topologies are
   congruent to make an explicit statement about multicast reachability.
   A number of significant multicast transit providers even require
   this, by doing the RPF lookups solely based on explicitly advertised
   multicast address family.

2.2.2.  OSPF/IS-IS Multi-topology Extensions

   Similar to BGP, some IGPs also provide the capability for signalling
   a differing topologies, for example IS-IS multi-topology extensions
   [I-D.ietf-isis-wg-multi-topology].  These can be used for a multicast
   topology that differs from unicast.  Similar but not so widely
   implemented work exists for OSPF [RFC4915].

   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 deployment 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

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   typically used workarounds such as tunnels.

2.2.3.  Issue: Overlapping Unicast/multicast Topology

   An interesting case occurs when some routers do not distribute
   multicast topology information explicitly while others 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 in different ways.
   Sometimes, multicast RPF mechanisms first look up the multicast
   routing table, or M-RIB ("topology database") with a longest prefix
   match algorithm, and if they find any entry (including a default
   route), that is used; if no match is found, the unicast routing table
   is used instead.

   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.

   Another 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 above.

2.2.4.  Summary

   A congruent topology can be deployed using unicast routing protocols
   that provide no support for a separate multicast topology.  In intra-
   domain that approach is often adequate.  However, it is recommended
   that if interdomain routing uses BGP, multicast-enabled sites should
   use MP-BGP SAFI=2 for multicast and SAFI=1 for unicast even if the
   topology was congruent to explicitly signal "yes, we use multicast".

   The following table summarizes the approaches that can be used to
   distribute multicast topology information.

                          | Interdomain  | Intradomain  |
   +--------------------- +--------------+--------------+
   | MP-BGP SAFI=2        | Recommended  |     Yes      |
   | MP-BGP SAFI=3        | Doesn't work | Doesn't work |
   | IS-IS multi-topology |     No       |     Yes      |
   | OSPF multi-topology  |     No       | Few implem.  |

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2.3.  Learning (Active) Sources

   To build a multicast distribution tree, the routing protocol needs to
   find out where the sources for the group are.  In case of SSM, the
   user specifies the source IP address or it is otherwise learned out
   of band.  In ASM, RPs are used to find out the sources (which in turn
   requires a mechanism to find the RPs as discussed in Section 2.4).

   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 for the whole Internet is a completely unscalable model
   for many reasons.  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 the ASM model is to be supported.

   This section discusses the options.

2.3.1.  SSM

   Source-specific Multicast [RFC4607] (sometimes also referred to as
   "single-source Multicast") does not count on learning active sources
   in the network.  Recipients need to know the source IP addresses
   using an out of band mechanism which are used to subscribe to the
   (source, group) channel.  The multicast routing uses the source
   address to set up the state and no further source discovery is

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

2.3.2.  MSDP

   Multicast Source Discovery Protocol [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].  The
   same can be achieved using PIM extensions [RFC4610].  See Section 2.5
   for more information.

   There is no intent to define MSDP for IPv6, but instead use only SSM

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   and Embedded-RP [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 address for a particular
   group for all of the Internet, so there is no need to share state
   about that with any other RPs.  If necessary, RP redundancy can still
   be achieved with Anycast-RP using PIM.

2.3.4.  Summary

   The following table summarizes the source discovery approaches and
   their status.

                          | IPv4 | IPv6 | Status                       |
   | Bi-dir single domain | Yes  | Yes  | OK but for intra-domain only |
   | PIM-SM single domain | Yes  | Yes  | OK                           |
   | PIM-SM with MSDP     | Yes  | No   | De-facto v4 inter-domain ASM |
   | PIM-SM w/ Embedded-RP| No   | Yes  | Best inter-domain ASM option |
   | SSM                  | Yes  | Yes  | No major uptake yet          |

2.4.  Configuring and Distributing PIM RP Information

   PIM-SM and Bi-dir PIM configuration mechanisms exist 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 RP Configuration

   It is often easiest just to 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 changes 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 (e.g., PIM on all interfaces), adding the RP address
   statically isn't really an issue.  Further, static anycast RP mapping

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   provides the benefits of RP load sharing and redundancy (see
   Section 2.5) without the complexity found in dynamic mechanisms like
   Auto-RP and Bootstrap Router (BSR).

   With such design, an anycast RP uses an address that is configured on
   a loopback interfaces of the routers currently acting as RPs, and
   state is distributed using PIM [RFC4610] or MSDP [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, the network
   administrator only needs to configure the RP routers.

   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 as
   it was earlier, 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.

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   It is worth noting that both Auto-RP and BSR were deployed before the
   use of a manually configured anycast-RP address became relatively
   commonplace, and there is actually relatively little need for them
   today unless there is a need to configure different properties (e.g.,
   sparse, dense, bi-dir) in a dynamic fashion.

2.4.4.  Summary

   The following table summarizes the RP discovery mechanisms and their
   status.  With the exception of Embedded-RP, each mechanism operates
   within a PIM domain.

                        | IPv4 | IPv6 | Deployment            |
   | Static RP          | Yes  | Yes  | Especially in ISPs    |
   | Auto-RP            | Yes  | No   | Legacy deployment     |
   | BSR                | Yes  | Yes  | Some, anycast simpler |
   | Embedded-RP        | No   | Yes  | Growing               |

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.

   Recent PIM extensions [RFC4610] also provide this functionality.  In
   contrast to MSDP, this approach works for both IPv4 and IPv6.

2.5.2.  Stateless RP Failover

   While Anycast RP shares state between RPs so that RP failure causes
   only small disturbance, stateless approaches are also possible with a
   more limited resiliency.  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,

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   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.  Bi-directional PIM

   Because bi-directional PIM (see Section 2.1.3) does not switch to
   shortest path tree (SPT), the final multicast tree is may be
   established faster.  On the other hand, PIM-SM or SSM may converge
   more quickly especially in scenarios (e.g., unicast routing change)
   where bi-directional needs to re-do the Designated Forwarder

2.5.4.  Summary

   The following table summarizes the techniques for enhanced

                        | IPv4 | IPv6 | Deployment            |
   | Anycast RP w/ MSDP | Yes  | No   | De-facto approach     |
   | Anycast RP w/ PIM  | Yes  | Yes  | Newer approach        |
   | Stateless RP fail. | Yes  | Yes  | Causes disturbance    |
   | Bi-dir PIM         | Yes  | Yes  | Deployed at some sites|

2.6.  Interactions with Hosts

   Previous sections have dealt with the components required by routers
   to be able to do multicast routing.  Obviously, the real users of
   multicast are the hosts: either sending or receiving multicast.  This
   section describes the required interactions with hosts.

2.6.1.  Hosts Sending Multicast

   After choosing a multicast group through a variety of means, hosts
   just send the packets to the link-layer multicast address, and the
   designated router will receive all the multicast packets and start
   forwarding them as appropriate.

   In intra-domain or Embedded-RP scenarios, ASM senders may move to a
   new IP address without significant impact on the delivery of their
   transmission.  SSM senders cannot change the IP address unless
   receivers join the new channel or the sender uses an IP mobility
   technique that is transparent to the receivers.

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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
   still commonplace, but are also often used in new deployments.  Some
   vendors also support SSM mapping techniques for receivers which use
   an older IGMP/MLD version where the router maps the join request to
   an SSM channel based on various, usually complex means of

2.6.3.  Summary

   The following table summarizes the techniques host interaction.

                        | IPv4  | IPv6 | Notes                |
   | Host sending       | Yes   | Yes  | No support needed    |
   | Host receiving ASM | IGMP  | MLD  | Any IGMP/MLD version |
   | Host receiving SSM | IGMPv3| MLDv2| Also SSM-mapping     |

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.

   Some mechanisms operate with IP addresses, others with MAC addresses.
   If filtering is done based on MAC addresses, hosts may receive
   unnecessary multicast traffic (filtered out in the hosts' IP layer)
   if more than one IP multicast group addresses maps into the same MAC
   address, or if IGMPv3/MLDv2 source filters are used.  Filtering based
   on IP destination addresses, or destination and sources addresses,
   will help avoid these but requires parsing of the Ethernet frame

   These options are discussed in this section.

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2.7.1.  Router-to-Router Flooding Reduction

   A proprietary solution, Cisco's RGMP [RFC3488] has been developed to
   reduce the amount of flooding between routers in a switched networks.
   This is typically only considered a problem in some Ethernet-based
   Internet Exchange points or VPNs.

   There have been proposals to observe and possibly react ("snoop") PIM
   messages [I-D.ietf-l2vpn-vpls-pim-snooping].

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.  Fast leave behaviour support for IGMPv3 hasn't been
   implemented.  Due to IGMP report suppression in IGMPv1 and IGMPv2,
   multicast is still flooded to ports which were once members of a
   group as long as there is at least one receiver on the link.
   Flooding restrictions are done based on multicast MAC addresses.
   IPv6 is not supported.

   IEEE 802.1D-2004 specification describes Generic Attribute
   Registration Protocol (GARP), and GARP Multicast Registration
   Protocol (GMRP) [GMRP] is a link-layer multicast group application of
   GARP that notifies switches about MAC multicast group memberships.
   If GMRP is used in conjunction with IP multicast, then the GMRP
   registration function would become associated with an IGMP "join."
   However, this GMRP-IGMP association is beyond the scope of GMRP.
   GMRP requires support at the host stack and it has not been widely
   implemented.  Further, IEEE 802.1 considers GARP and GMRP obsolete
   being replaced by Multiple Registration Protocol (MRP) and Multicast
   Multiple Registration Protocol (MMRP) that are being specified in
   IEEE 802.1ak [802.1ak].  MMRP is expected to be mainly used between
   bridges.  Some further information about GARP/GMRP is also available
   in Appendix B of [RFC3488].

   IGMP snooping [RFC4541] appears to be the most widely implemented
   technique.  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.

   Snooping switches also need to identify the ports where routers
   reside and therefore where to flood the packets.  This can be

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   accomplished using Multicast Router Discovery protocol [RFC4286],
   looking at certain IGMP queries [RFC4541], looking at PIM Hello and
   possibly other messages, or by manual configuration.  An issue with
   PIM snooping at LANs is that PIM messages can't be turned off or
   encrypted, leading to security issues [I-D.ietf-pim-lasthop-threats].

   IGMP proxying [RFC4605] is sometimes used either as a replacement of
   a multicast routing protocol on a small router, or to aggregate IGMP/
   MLD reports when used with IGMP snooping.

2.7.3.  Summary

   The following table summarizes the techniques for multicast flooding
   reduction inside a single link for router-to-router and last-hop

                           | R-to-R | LAN | Notes                     |
   | Cisco's RGMP          |  Yes   | No  | Replaced by PIM snooping  |
   | PIM snooping          |  Yes   | No  | Security issues in LANs   |
   | IGMP/MLD snooping     |  No    | Yes | Common, IGMPv3 or MLD bad |
   | Multicast Router Disc |  No    | Yes | Few if any implem. yet    |
   | IEEE GMRP and MMRP    |  No    | No  | No host/router deployment |
   | Cisco's CGMP          |  No    | Yes | Replaced by other snooping|

3.  Acknowledgements

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

   Leonard Giuliano, James Lingard, Jean-Jacques Pansiot, Dave Meyer,
   Stig Venaas, Tom Pusateri, Marshall Eubanks, Dino Farinacci, Bharat
   Joshi, Albert Manfredi, Jean-Jacques Pansiot, Spencer Dawkins, Sharon
   Chisholm, John Zwiebel, Dan Romascanu, Thomas Morin, and Ron Bonica
   provided good comments, helping in improving this document.

4.  IANA Considerations

   This memo includes no request to IANA.

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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.

   However, there has been analysis of the security of multicast routing
   infrastructures [RFC4609], IGMP/MLD [I-D.daley-magma-smld-prob], and
   PIM last-hop issues [I-D.ietf-pim-lasthop-threats].

6.  References

6.1.  Normative References

              Handley, M., "Bi-directional Protocol Independent
              Multicast (BIDIR-PIM)", draft-ietf-pim-bidir-09 (work in
              progress), February 2007.

   [RFC2026]  Bradner, S., "The Internet Standards Process -- Revision
              3", BCP 9, RFC 2026, October 1996.

   [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.

   [RFC4601]  Fenner, B., Handley, M., Holbrook, H., and I. Kouvelas,
              "Protocol Independent Multicast - Sparse Mode (PIM-SM):
              Protocol Specification (Revised)", RFC 4601, August 2006.

   [RFC4607]  Holbrook, H. and B. Cain, "Source-Specific Multicast for
              IP", RFC 4607, August 2006.

   [RFC4760]  Bates, T., Chandra, R., Katz, D., and Y. Rekhter,
              "Multiprotocol Extensions for BGP-4", RFC 4760,
              January 2007.

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   [RFC4915]  Psenak, P., Mirtorabi, S., Roy, A., Nguyen, L., and P.
              Pillay-Esnault, "Multi-Topology (MT) Routing in OSPF",
              RFC 4915, June 2007.

6.2.  Informative References

   [802.1ak]  "IEEE 802.1ak - Multiple Registration Protocol",

   [CGMP]     "Cisco Group Management Protocol",

   [GMRP]     "GARP Multicast Registration Protocol",

              Daley, G. and G. Kurup, "Trust Models and Security in
              Multicast Listener Discovery",
              draft-daley-magma-smld-prob-00 (work in progress),
              July 2004.

              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., "M-ISIS: Multi Topology (MT) Routing in
              IS-IS", draft-ietf-isis-wg-multi-topology-11 (work in
              progress), October 2005.

              Hemige, V., "PIM Snooping over VPLS",
              draft-ietf-l2vpn-vpls-pim-snooping-01 (work in progress),
              March 2007.

              Savola, P., "Overview of the Internet Multicast Addressing
              Architecture", draft-ietf-mboned-addrarch-05 (work in
              progress), October 2006.

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

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              draft-ietf-mboned-ipv6-multicast-issues-02 (work in
              progress), February 2005.

              Savola, P. and J. Lingard, "Last-hop Threats to Protocol
              Independent Multicast (PIM)",
              draft-ietf-pim-lasthop-threats-01 (work in progress),
              June 2007.

              Bhaskar, N., "Bootstrap Router (BSR) Mechanism for PIM",
              draft-ietf-pim-sm-bsr-11 (work in progress), July 2007.

              Lehtonen, R., "Requirements for discovery of dynamic SSM
              sources", draft-lehtonen-mboned-dynssm-req-00 (work in
              progress), February 2005.

   [IM-GAPS]  Meyer, D. and B. Nickless, "Internet Multicast Gap
              Analysis from the MBONED Working Group for the IESG
              [Expired]", draft-ietf-mboned-iesg-gap-analysis-00 (work
              in progress), July 2002.

              Meyer, D., "Some Issues for an Inter-domain Multicast
              Routing Protocol [Expired]",
              draft-ietf-mboned-imrp-some-issues-01 (work in progress),
              September 1997.

   [RFC1075]  Waitzman, D., Partridge, C., and S. Deering, "Distance
              Vector Multicast Routing Protocol", RFC 1075,
              November 1988.

   [RFC1458]  Braudes, B. and S. Zabele, "Requirements for Multicast
              Protocols", RFC 1458, May 1993.

   [RFC1584]  Moy, J., "Multicast Extensions to OSPF", RFC 1584,
              March 1994.

   [RFC1585]  Moy, J., "MOSPF: Analysis and Experience", RFC 1585,
              March 1994.

   [RFC2189]  Ballardie, T., "Core Based Trees (CBT version 2) Multicast
              Routing -- Protocol Specification --", RFC 2189,
              September 1997.

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

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   [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 2000.

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

   [RFC3973]  Adams, A., Nicholas, J., and W. Siadak, "Protocol
              Independent Multicast - Dense Mode (PIM-DM): Protocol
              Specification (Revised)", RFC 3973, January 2005.

   [RFC4286]  Haberman, B. and J. Martin, "Multicast Router Discovery",
              RFC 4286, December 2005.

   [RFC4541]  Christensen, M., Kimball, K., and F. Solensky,
              "Considerations for Internet Group Management Protocol
              (IGMP) and Multicast Listener Discovery (MLD) Snooping
              Switches", RFC 4541, May 2006.

   [RFC4605]  Fenner, B., He, H., Haberman, B., and H. Sandick,
              "Internet Group Management Protocol (IGMP) / Multicast
              Listener Discovery (MLD)-Based Multicast Forwarding
              ("IGMP/MLD Proxying")", RFC 4605, August 2006.

   [RFC4609]  Savola, P., Lehtonen, R., and D. Meyer, "Protocol
              Independent Multicast - Sparse Mode (PIM-SM) Multicast
              Routing Security Issues and Enhancements", RFC 4609,
              October 2006.

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   [RFC4610]  Farinacci, D. and Y. Cai, "Anycast-RP Using Protocol
              Independent Multicast (PIM)", RFC 4610, August 2006.

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

Appendix A.  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.  Reliable Multicast

   Reliable Multicast Working Group has been working on experimental
   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 in router assistance role in the initial delivery and
   potential retransmission of missing data.

A.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

Author's Address

   Pekka Savola
   CSC - Scientific Computing Ltd.


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Full Copyright Statement

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