Internet Engineering Task Force                                P. Savola
Internet-Draft                                                 CSC/FUNET
Obsoletes:                                                 June 26, 2006
3913,2189,2201,1584,1585 (if
Intended status: Best Current
Expires: December 28, 2006

        Overview of the Internet Multicast Routing Architecture

Status of this Memo

   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 becomes
   aware will be disclosed, in accordance with Section 6 of BCP 79.

   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-

   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

   The list of Internet-Draft Shadow Directories can be accessed at

   This Internet-Draft will expire on December 28, 2006.

Copyright Notice

   Copyright (C) The Internet Society (2006).


   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.

Savola                  Expires December 28, 2006               [Page 1]

Internet-Draft         Multicast Routing Overview              June 2006

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.  Bi-directional PIM . . . . . . . . . . . . . . . . . .  5
       2.1.4.  DVMRP  . . . . . . . . . . . . . . . . . . . . . . . .  5
       2.1.5.  MOSPF  . . . . . . . . . . . . . . . . . . . . . . . .  6
       2.1.6.  BGMP . . . . . . . . . . . . . . . . . . . . . . . . .  6
       2.1.7.  CBT  . . . . . . . . . . . . . . . . . . . . . . . . .  6
       2.1.8.  Interactions and Summary . . . . . . . . . . . . . . .  6
     2.2.  Distributing Topology Information  . . . . . . . . . . . .  7
       2.2.1.  Multi-protocol BGP . . . . . . . . . . . . . . . . . .  7
       2.2.2.  OSPF/IS-IS Multi-topology Extensions . . . . . . . . .  8
       2.2.3.  Issue: Overlapping Unicast/multicast Topology  . . . .  8
     2.3.  Learning (Active) Sources  . . . . . . . . . . . . . . . .  8
       2.3.1.  SSM  . . . . . . . . . . . . . . . . . . . . . . . . .  9
       2.3.2.  MSDP . . . . . . . . . . . . . . . . . . . . . . . . .  9
       2.3.3.  Embedded-RP  . . . . . . . . . . . . . . . . . . . . .  9
     2.4.  Configuring and Distributing PIM-SM RP Information . . . . 10
       2.4.1.  Manual Configuration with an Anycast Address . . . . . 10
       2.4.2.  Embedded-RP  . . . . . . . . . . . . . . . . . . . . . 10
       2.4.3.  BSR and Auto-RP  . . . . . . . . . . . . . . . . . . . 11
     2.5.  Mechanisms for Enhanced Redundancy . . . . . . . . . . . . 11
       2.5.1.  Anycast RP . . . . . . . . . . . . . . . . . . . . . . 11
       2.5.2.  Stateless RP Failover  . . . . . . . . . . . . . . . . 12
       2.5.3.  Bi-directional PIM . . . . . . . . . . . . . . . . . . 12
     2.6.  Interactions with Hosts  . . . . . . . . . . . . . . . . . 12
       2.6.1.  Hosts Sending Multicast  . . . . . . . . . . . . . . . 12
       2.6.2.  Hosts Receiving Multicast  . . . . . . . . . . . . . . 12
     2.7.  Restricting Multicast Flooding in the Link Layer . . . . . 13
       2.7.1.  Router-to-Router Flooding Reduction  . . . . . . . . . 13
       2.7.2.  Host/Router Flooding Reduction . . . . . . . . . . . . 13
   3.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 14
   4.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 14
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 14
   6.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
     6.1.  Normative References . . . . . . . . . . . . . . . . . . . 14
     6.2.  Informative References . . . . . . . . . . . . . . . . . . 16
   Appendix A.  Multicast Payload Transport Extensions  . . . . . . . 18
     A.1.  Reliable Multicast . . . . . . . . . . . . . . . . . . . . 18
     A.2.  Multicast Group Security . . . . . . . . . . . . . . . . . 19
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 19
   Intellectual Property and Copyright Statements . . . . . . . . . . 20

Savola                  Expires December 28, 2006               [Page 2]

Internet-Draft         Multicast Routing Overview              June 2006

1.  Introduction

   Good, up-to-date documentation of IP multicast is close to non-
   existent.  This issue is 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.  Multicast addressing is described in a
   companion document [I-D.ietf-mboned-addrarch].

   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 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).

   Some multicast data transport issues are also introduced in
   Appendix A.

   This memo obsoletes and re-classifies to Historic [RFC2026] Border
   Gateway Multicast Protocol (BGMP), Core Based Trees (CBT), Multicast
   OSPF (MOSPF) RFCs: [RFC3913], [RFC2189], [RFC2201], [RFC1584], and
   [RFC1585].  The purpose of the re-classification is to give the
   readers (both implementors and deployers) an idea what the status of
   a protocol is; 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 protocol.

Savola                  Expires December 28, 2006               [Page 3]

Internet-Draft         Multicast Routing Overview              June 2006

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            Group Address Resolution Protocol
   IGMP            Internet Group Management Protocol
   MBGP            Multi-protocol BGP (*not* "Multicast BGP")
   MLD             Multicast Listener Discovery
   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

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 [RFC3973] operates in a "dense" mode, flooding the
   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 traffic.

Savola                  Expires December 28, 2006               [Page 4]

Internet-Draft         Multicast Routing Overview              June 2006

   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
   minimal.  PIM-DM has been used to transition to PIM-SM but it is no
   longer in widespread use.

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

   Many implementations also support so-called "sparse-dense" mode,
   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 been transitioned away from sparse-
   dense to only sparse mode.

2.1.3.  Bi-directional PIM

   Bi-directional 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.  As it doesn't keep source-specific state, it
   may be a lucrative approach especially in sites with a large number
   of sources.

   As of this writing, in IPv6 or inter-domain multicast there is no
   standards based mechanism for alerting routers that a group range is
   to be used for 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

Savola                  Expires December 28, 2006               [Page 5]

Internet-Draft         Multicast Routing Overview              June 2006

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.

2.1.7.  CBT

   CBT [RFC2201] 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 implemention.  Therefore, CBT never saw production

2.1.8.  Interactions and Summary

   It is worth noting that it is possible to run different protocols
   with different multicast group ranges (e.g., treat some groups as
   dense mode in an otherwise PIM-SM network; this typically requires
   manual configuration of the groups) or interaction 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 deployment status
   of different protocols as of this writing.

                | Interdomain | Intradomain | Status         |
   | PIM-SM     |     Yes     |     Yes     | Active         |
   | PIM-DM     | Not feasible|     Yes     | Little use     |
   | Bi-dir PIM |      No     |     Yes     | Wait & see     |
   | DVMRP      | Not anymore |  Stub only  | Going out      |
   | MOSF       |      No     | Not anymore | Inactive       |
   | CBT        |      No     |     No      | Never deployed |
   | BGMP       |      No     |     No      | Never deployed |

Savola                  Expires December 28, 2006               [Page 6]

Internet-Draft         Multicast Routing Overview              June 2006

   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

   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).

   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 [RFC2858] (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 and
   multicast traffic (using SAFI=2 for multicast).  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 been deprecated [I-D.ietf-idr-rfc2858bis].

   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.

Savola                  Expires December 28, 2006               [Page 7]

Internet-Draft         Multicast Routing Overview              June 2006

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 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
   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.

2.3.  Learning (Active) Sources

   Typically, multicast routing protocols must either assume that the
   receivers know the IP addresses of the (active) sources for a group
   in advance, possibly using an out-of-band mechanism (SSM), or the

Savola                  Expires December 28, 2006               [Page 8]

Internet-Draft         Multicast Routing Overview              June 2006

   sources must be discovered by the network protocols automatically

   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 [I-D.ietf-ssm-arch] (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 [I-D.ietf-pim-anycast-rp].
   See Section 2.5 for more information.

   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

Savola                  Expires December 28, 2006               [Page 9]

Internet-Draft         Multicast Routing Overview              June 2006

   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.

2.4.  Configuring and Distributing PIM-SM RP Information

   For PIM-SM, 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 Configuration with an Anycast Address

   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 (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 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, 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, 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

Savola                  Expires December 28, 2006              [Page 10]

Internet-Draft         Multicast Routing Overview              June 2006

   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 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
   commonplace, and there is actually relatively little need 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.

   Recent PIM extensions [I-D.ietf-pim-anycast-rp] also provide this
   functionality.  In contrast to MSDP, this approach works for both

Savola                  Expires December 28, 2006              [Page 11]

Internet-Draft         Multicast Routing Overview              June 2006

   IPv4 and 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.  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 built faster
   and converges faster after failures.  On the other hand, PIM-SM or
   SSM may converge more quickly especially in scenarios where bi-
   directional needs to re-do the Designated Forwarder election.

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.

   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.

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

Savola                  Expires December 28, 2006              [Page 12]

Internet-Draft         Multicast Routing Overview              June 2006

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

   A proprietary solution, Cisco's RGMP [RFC3488] has 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 proposals to observe and possibly react ("snoop") PIM
   messages [I-D.tsenevir-pim-sm-snoop][I-D.serbest-l2vpn-vpls-mcast] to
   achieve 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.  The
   implementation status is unknown.

   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) using
   Multicast Router Discovery protocol [RFC4286], looking at certain
   IGMP queries [RFC4541], or by manual configuration.  IGMP proxying
   [I-D.ietf-magma-igmp-proxy] is sometimes used either as a replacement

Savola                  Expires December 28, 2006              [Page 13]

Internet-Draft         Multicast Routing Overview              June 2006

   of a multicast routing protocol on a small router, or to aggregate
   IGMP/MLD reports when used with IGMP 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, and Spencer Dawkins
   provided good comments, helping in improving this document.

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.

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

6.  References

6.1.  Normative References

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

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


Savola                  Expires December 28, 2006              [Page 14]

Internet-Draft         Multicast Routing Overview              June 2006

              Savola, P., "Overview of the Internet Multicast Addressing
              Architecture", draft-ietf-mboned-addrarch-04 (work in
              progress), March 2006.

              Psenak, P., "Multi-Topology (MT) Routing in OSPF",
              draft-ietf-ospf-mt-06 (work in progress), February 2006.

              Handley, M., "Bi-directional Protocol Independent
              Multicast (BIDIR-PIM)", draft-ietf-pim-bidir-08 (work in
              progress), October 2005.

              Fenner, B., "Protocol Independent Multicast - Sparse Mode
              (PIM-SM): Protocol  Specification (Revised)",
              draft-ietf-pim-sm-v2-new-12 (work in progress),
              March 2006.

              Holbrook, H. and B. Cain, "Source-Specific Multicast for
              IP", draft-ietf-ssm-arch-07 (work in progress),
              October 2005.

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

   [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.

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

Savola                  Expires December 28, 2006              [Page 15]

Internet-Draft         Multicast Routing Overview              June 2006

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.

              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
              Applicability Statement", draft-ietf-idmr-dvmrp-v3-as-01
              (work in progress), May 2004.

              Bates, T., "Multiprotocol Extensions for BGP-4",
              draft-ietf-idr-rfc2858bis-10 (work in progress),
              March 2006.

              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.

              Savola, P., "IPv6 Multicast Deployment Issues",
              draft-ietf-mboned-ipv6-multicast-issues-02 (work in
              progress), February 2005.

              Savola, P., Lehtonen, R., and D. Meyer, "PIM-SM Multicast
              Routing Security Issues and Enhancements",
              draft-ietf-mboned-mroutesec-04 (work in progress),
              October 2004.

              Farinacci, D. and Y. Cai, "Anycast-RP using PIM",
              draft-ietf-pim-anycast-rp-07 (work in progress),
              February 2006.

              Bhaskar, N., "Bootstrap Router (BSR) Mechanism for PIM",

Savola                  Expires December 28, 2006              [Page 16]

Internet-Draft         Multicast Routing Overview              June 2006

              draft-ietf-pim-sm-bsr-09 (work in progress), June 2006.

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

              Lingard, J. and P. Savola, "Last-hop Threats to Protocol
              Independent Multicast (PIM)",
              draft-savola-pim-lasthop-threats-02 (work in progress),
              June 2006.

              Serbest, Y., "Supporting IP Multicast over VPLS",
              draft-serbest-l2vpn-vpls-mcast-03 (work in progress),
              July 2005.

              Senevirathne, T. and S. Vallepali, "Protocol Independent
              Multicast-Sparse Mode (PIM-SM) Snooping",
              draft-tsenevir-pim-sm-snoop-00 (work in progress),
              April 2002.

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

   [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.

   [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.

Savola                  Expires December 28, 2006              [Page 17]

Internet-Draft         Multicast Routing Overview              June 2006

   [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.

   [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.

              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

Savola                  Expires December 28, 2006              [Page 18]

Internet-Draft         Multicast Routing Overview              June 2006

   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.


Savola                  Expires December 28, 2006              [Page 19]

Internet-Draft         Multicast Routing Overview              June 2006

Full Copyright Statement

   Copyright (C) The Internet Society (2006).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on an

Intellectual Property

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; nor does it represent that it has
   made any independent effort to identify any such rights.  Information
   on the procedures with respect to rights in RFC documents can be
   found in BCP 78 and BCP 79.

   Copies of IPR disclosures made to the IETF Secretariat and any
   assurances of licenses to be made available, or the result of an
   attempt made to obtain a general license or permission for the use of
   such proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository at

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard.  Please address the information to the IETF at


   Funding for the RFC Editor function is provided by the IETF
   Administrative Support Activity (IASA).

Savola                  Expires December 28, 2006              [Page 20]