MBONE Deployment WG                                            P. Savola
Internet-Draft                                                 CSC/FUNET
Expires: March 3, 2005                                 September 2, 2004

                    IPv6 Multicast Deployment Issues

Status of this Memo

   This document is an Internet-Draft and is subject to all provisions
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Copyright Notice

   Copyright (C) The Internet Society (2004).


   This memo describes known issues with IPv6 multicast, and provides
   historical reference of how some earlier problems have been resolved.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1   Multicast-related Abbreviations  . . . . . . . . . . . . .  3
   2.  Justification for IPv6 Inter-domain ASM  . . . . . . . . . . .  3
     2.1   SSM Deployment Issues  . . . . . . . . . . . . . . . . . .  3
     2.2   Groups of Different Non-global Scope . . . . . . . . . . .  4
   3.  Different Solutions to Inter-domain Multicast  . . . . . . . .  5
     3.1   Changing the Multicast Usage Model . . . . . . . . . . . .  5
     3.2   Implementing MSDP for IPv6 . . . . . . . . . . . . . . . .  6
     3.3   Implementing Another Multicast Routing Protocol  . . . . .  6
     3.4   Embedding the RP Address in an IPv6 Multicast Address  . .  6
   4.  Issues with IPv6 Multicast . . . . . . . . . . . . . . . . . .  7
     4.1   Issues with Embedded RP  . . . . . . . . . . . . . . . . .  7
       4.1.1   RP Failover with Embedded RP . . . . . . . . . . . . .  7
       4.1.2   Embedded RP and Control Mechanisms . . . . . . . . . .  7
     4.2   Neighbor Discovery Using Multicast . . . . . . . . . . . .  8
     4.3   Functionality Like MLD Snooping  . . . . . . . . . . . . .  8
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
   6.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  9
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  9
   7.1   Normative References . . . . . . . . . . . . . . . . . . . .  9
   7.2   Informative References . . . . . . . . . . . . . . . . . . . 10
       Author's Address . . . . . . . . . . . . . . . . . . . . . . . 11
       Intellectual Property and Copyright Statements . . . . . . . . 12

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1.  Introduction

   There are many issues concerning the deployment and implementation,
   and to a lesser degree, specification of IPv6 multicast.  This memo
   describes known problems to raise awareness, and documents how
   previous problems have been resolved.

   Section 2 describes the justifications for providing an inter-domain
   multicast solution using Any Source Multicast (ASM) with IPv6.
   Section 3 in turn describes which options were considered for filling
   those the requirements for the IPv6 inter-domain multicast solutions.
   These sections are provided for historical reference of the
   discussion and consensus in the IETF MBONED working group.

   Section 4 lists issues that have come up with IPv6 multicast but have
   not yet been at least fully resolved, and may require raised

1.1  Multicast-related Abbreviations

   ASM     Any Source Multicast
   BSR     Bootstrap Router
   CGMP    Cisco Group Management Protocol
   DR      Designated Router
   IGMP    Internet Group Management Protocol
   MLD     Multicast Listener Discovery
   MSDP    Multicast Source Discovery Protocol
   PIM     Protocol Independent Multicast
   PIM-SM  Protocol Independent Multicast - Sparse Mode
   RP      Rendezvous Point
   SSM     Source-specific Multicast

2.  Justification for IPv6 Inter-domain ASM

   This section documents the reasons and the discussion which led to
   the agreement why a solution to IPv6 inter-domain ASM was necessary.

   The main reason was that SSM [I-D.ietf-ssm-arch] was not considered
   to solve all the relevant problems (e.g., many-to-many applications,
   source discovery), and that SSM was not sufficiently widely deployed
   and used.

2.1  SSM Deployment Issues

   To be deployed, SSM requires changes to:

   1.  routers

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   2.  IGMP/MLD-snooping Ethernet switches

   3.  hosts

   4.  application programming interfaces (APIs)

   5.  multicast usage models

   Introducing SSM support in the routers has been straightforward as
   PIM-SSM is a subset of PIM-SM [I-D.ietf-pim-sm-v2-new].

   IGMP-snooping Ethernet switches have been a more difficult issue
   [SSMSNOOP]; some which perform IGMPv2 snooping discard IGMPv3 reports
   or queries, or multicast transmissions associated to them.  If MLDv1
   snooping had been implemented (or is implemented in a similar
   manner), this would likely have affected that as well.

   Host systems require MLDv2 [RFC3810] support.  The situation has
   improved with respect to MLDv2 support for end systems, and
   interoperability has increased after the publication of the RFC due
   to the stabilization of the ICMP types used.

   The multicast source filtering API specification has also been
   completed [RFC3678]; its deployment is likely roughly equal (or
   slightly worse) than MLDv2.  The API is required for creating
   (cross-platform) SSM applications.

   The most difficult issue, multicast usage models, remains a problem
   as of this writing.  SSM is an excellent fit for one-to-many
   distribution topologies, and porting such applications to use SSM
   would likely be rather simple.  However, a significant number of
   current applications are many-to-many (e.g., conferencing
   applications) which cannot be converted to SSM without significant
   effort, including, for example, out-of-band source discovery.  For
   such applications to be usable for IPv6 at least in a short to medium
   term, ASM -like techniques seem to be required.

2.2  Groups of Different Non-global Scope

   Many ASM applications are used with a smaller scope than global; some
   of these have a wider scope than others.  However, groups of smaller
   scope typically need to be in their own PIM-SM domains to prevent
   inappropriate data leakage.  Therefore if a site has groups of
   different scopes, having multiple PIM domain borders becomes a
   requirement unless inter-domain multicast is used instead; further,
   configuring such nesting scopes would likely be an operational
   challenge.  In consequence, if these applications of non-global scope
   need to be used, inter-domain multicast support is practically

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   In consequence, especially if multicast with different non-global
   scopes is used, there will be a need for inter-domain multicast
   solutions.  As many applications are relying on ASM characteristics,
   this further increases a need for an inter-domain ASM solution.

3.  Different Solutions to Inter-domain Multicast

   When ASM is used, the Internet must be divided to multiple PIM-SM for
   both administrative and technical reasons, which means there will be
   multiple PIM-SM RPs which need to communicate the information of
   sources between themselves.

   On the other hand, SSM does not require RPs and also works in the
   inter-domain without such communication.  Section 2 describes the
   justification why Inter-domain ASM was still considered to be
   required.  This section describes different solutions which were
   discussed to providing inter-domain multicast for IPv6.

   For inter-domain multicast, there is consensus to continue using SSM,
   and also use Embedded-RP as appropriate.

   This section provides historical reference of the discussion and

3.1  Changing the Multicast Usage Model

   As ASM model has been found to be complex and a bit problematic, some
   felt that this is a good incentive to move to SSM for good (at least
   for most cases).  Below two paragraphs are adapted from

      The most serious criticism of the SSM architecture is that it does
      not support shared trees which may be useful for supporting
      many-to-many applications.  In the short-term this is not a
      serious concern since the multicast application space is likely to
      be dominated by one-to-many applications.  Some other classes of
      multicast applications that are likely to emerge in the future are
      few-to-few (e.g.  private chat rooms, whiteboards), few-to-many
      (e.g., video conferencing, distance learning) and many-to-many
      (e.g., large chat rooms, multi-user games).  The first two classes
      can be easily handled using a few one-to-many source-based trees.

      The issue of many-to-many multicasting service on top of a SSM
      architecture is an open issue at this point.  However, some feel
      that even many-to-many applications should be handled with
      multiple one- to-many instead of shared trees.

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   In any case, even though SSM would avoid the problems of ASM, it was
   felt that SSM is not sufficiently widely available to completely
   replace ASM (see Section 2.1), and that the IETF should not try to
   force the application writers to change their multicast usage models.

3.2  Implementing MSDP for IPv6

   In IPv4, notification of multicast sources between these PIM-SM RPs
   is done with Multicast Source Discovery Protocol (MSDP) [RFC3618].
   The protocol is widely considered a sub-optimal solution and even
   dangerous to deploy; when it was specified, it was only meant as a
   "stop-gap" measure.

   The easiest stop-gap solution (to a stop-gap solution) would have
   been to specify IPv6 TLV's for MSDP.  This would be fairly
   straightforward, and existing implementations would probably be
   relatively easy to modify.

   There is and has been resistance to this, as MSDP was not supposed to
   last this long in the first place; there is clear consensus that
   there should be no further work on it [I-D.ietf-mboned-msdp-deploy].

3.3  Implementing Another Multicast Routing Protocol

   One possibility might have been to specify and/or implement a
   different multicast routing protocol.

   In fact, Border Gateway Multicast Protocol (BGMP)
   [I-D.ietf-bgmp-spec] has been specified; however, it is widely held
   to be quite complex and there have been no implementations, nor will
   to make any.  Lacking deployment experience and specification
   analysis, it is difficult to say which problems it might solve (and
   possibly, which new ones to introduce).  One probable reason why BGMP
   failed to attract continuing interest was it's dependance on
   similarly heavy-weight multicast address allocation/assignment

   As of this writing, no other inter-domain protocols have been
   specified, and BGMP is not considered a realistic option.

3.4  Embedding the RP Address in an IPv6 Multicast Address

   One way to work around these problems was to allocate and assign
   multicast addresses in such a fashion that the address of the RP
   could be automatically calculated from the IPv6 multicast address.

   Making some assumptions about how the RPs would configure Interface
   Identifiers, this is can achieved as described in

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   [I-D.ietf-mboned-embeddedrp]; PIM-SM implementations need to
   implement the Embedded RP group-to-RP mapping mechanism which
   processes this encoding.

   To completely replace the need for MSDP for IPv6, a different way to
   implement "Anycast RP" [RFC3446] -technique, for sharing the state
   information between different RP's in one PIM-SM domain, is also
   needed.  One such approach is described in [I-D.ietf-pim-anycast-rp].

4.  Issues with IPv6 Multicast

   This section describes issues that have come up with IPv6 multicast
   but have not yet been at least fully resolved.

4.1  Issues with Embedded RP

4.1.1  RP Failover with Embedded RP

   Embedded RP provides a means for ASM multicast without inter-domain
   MSDP.  However, to continue providing failover mechanisms for RPs, a
   form of state sharing, Anycast-RP, should still be supported.
   Instead of MSDP, this can be achieved using a PIM-SM extension

   One should note that as Embedded RP does not require MSDP peerings
   between the RPs, it's possible to deploy more RPs in a PIM domain.
   For example, the scalability and redundancy could be achieved by
   co-locating RP functionality in the DRs: each major source, which
   "owns" a group, could have its own DR act as the RP.  This has about
   the same redundancy characteristics as using SSM -- so there may not
   be an actually very urgent need for Anycast-RP if operational methods
   to include fate-sharing of the groups is followed.

   In any case, "cold failover" redundancy without state sharing is
   still an option.  This does not offer any load-balancing of RPs or
   shared trees, but provides only long-term redundancy.  In this
   mechanism, multiple routers would be configured with the RP address
   (with appropriate unicast metrics), but only one of them would be
   active at any time: if the main RP goes down, another takes its
   place.  However, the multicast state stored in the RP would be lost,
   unless it is synchronized by some out-of-band mechanism.

4.1.2  Embedded RP and Control Mechanisms

   With ASM and MSDP deployment, the ISPs can better control who is
   using their RPs.

   With Embedded RP, anyone could use a third-party RP to host their

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   groups unless some mechanisms, for example access-lists, are in place
   to control the use of the RP [I-D.ietf-mboned-embeddedrp].

   Such abuse is of questionable benefit, though, as anyone with a /64
   could form an RP of its own.

   Whether this is a sufficiently serious problem worth designing a
   (potentially complex) solution for is still under debate, as of this

4.2  Neighbor Discovery Using Multicast

   Neighbor Discovery [RFC2461] uses link-local multicast in Ethernet
   media, not broadcast as ARP does with IPv4.  This has been seen to
   cause operational problems with some equipment.

   The author has seen one brand of managed Ethernet switches, and heard
   reports of a few unmanaged switches, that do not forward IPv6
   link-local multicast packets to other ports at all.  In essence,
   native IPv6 is impossible with this equipment.  These problems have
   likely been fixed in later revisions of the equipment, but this does
   not fix the equipment on the field, and it is likely that similar
   problems will surface again.

   It seems likely that this may be a problem with some switches that
   build multicast forwarding state based on Layer 3 information (and do
   not support IPv6); state using Layer 2 information would work just
   fine [I-D.ietf-magma-snoop].  Therefore the snooping swich developers
   should be aware of the tradeoff of using Layer 2 vs Layer 3
   information on multicast data forwarding, especially if IPv6 snooping
   is not supported.

   For the deployment of IPv6, it would be important to find out how
   this can be fixed (e.g., how exactly this breaks specifications) and
   how one can identify which equipment could cause problems like these
   (and whether there are workarounds).

   One workaround might be to implement a toggle in the nodes that would
   use link-layer broadcast instead of multicast as a fallback solution.
   However, this would have to be used in all the systems on the same
   link, otherwise local communication is impaired.

4.3  Functionality Like MLD Snooping

   On Ethernet, multicast frames are forwarded to every port, even
   without subscribers (or IPv6 support).

   Especially if multicast traffic is relatively heavy (e.g., video

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   streaming), it becomes particularly important to have some feature
   like Multicast Listener Discovery (MLD) snooping implemented, to
   reduce the amount of flooding [I-D.ietf-magma-snoop].

   In addition, some vendors have not realized which multicast addresses
   (in particular, link-local addresses) MLD reports -- utilized in the
   snooping -- should be generated for.  The introduction of MLD
   snooping could cause hosts which do not send MLD reports
   appropriately to be blocked out.  As specified in [RFC2461], an MLD
   report must be generated for every group except all-nodes (ff02::1 --
   which is forwarded to all ports); this also includes all the other
   link-local groups.

   Looking at the actual problem from a higher view, it is not clear
   that MLD snooping is the right long-term solution.  It makes the
   switches complex, requires the processing of datagrams above the
   link-layer, and should be discouraged
   [I-D.ietf-mboned-iesg-gap-analysis]: the whole idea of L2-only
   devices having to peek into L3 datagrams seems like a severe layering
   violation -- and often the devices aren't upgradeable (if there are
   bugs or missing features, which could be fixed later) in any way.
   Better mechanisms could be having routers tell switches which
   multicasts to forward where (e.g., [CGMP]) or by using some other
   mechanisms [GARP].

5.  Security Considerations

   Only deployment and implementation issues are considered, and these
   do not have any particular security considerations; security
   considerations for each technology are covered in the respective

6.  Acknowledgements

   Early discussions with Stig Venaas, Jerome Durand, Tim Chown et al.
   led to the writing of this draft.  Brian Haberman offered extensive
   comments along the way.  "Itojun" Hagino brought up the need for MLD
   snooping in a presentation.  Bill Nickless pointed out issues in the
   gap analysis and provided a pointer to GARP/GMRP; Havard Eidnes made
   a case for a protocol like CGMP.  Leonard Giuliano pointed out a more
   complete analysis of SSM with different kind of applications.

7.  References

7.1  Normative References

              Thaler, D., "Border Gateway Multicast Protocol (BGMP):

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              Protocol Specification", draft-ietf-bgmp-spec-06 (work in
              progress), January 2004.

              Savola, P. and B. Haberman, "Embedding the Rendezvous
              Point (RP) Address in an IPv6 Multicast Address",
              draft-ietf-mboned-embeddedrp-07 (work in progress), July

              McBride, M., "Multicast Source Discovery Protocol (MSDP)
              Deployment Scenarios", draft-ietf-mboned-msdp-deploy-06
              (work in progress), March 2004.

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

              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-10 (work in progress), July 2004.

              Holbrook, H. and B. Cain, "Source-Specific Multicast for
              IP", draft-ietf-ssm-arch-05 (work in progress), July 2004.

   [RFC2461]  Narten, T., Nordmark, E. and W. Simpson, "Neighbor
              Discovery for IP Version 6 (IPv6)", RFC 2461, December

   [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.

   [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.

7.2  Informative References

   [CGMP]     "Cisco Group Management Protocol",

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

              Bhattacharyya, S., Diot, C., Giuliano, L. and R. Rockell,
              "Deployment of PIM-SO at Sprint (PIM-SO)", March 2000.

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

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

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

   [RFC3678]  Thaler, D., Fenner, B. and B. Quinn, "Socket Interface
              Extensions for Multicast Source Filters", RFC 3678,
              January 2004.

              "Operational Problems with IGMP snooping switches", March
              2003, <http://www.ietf.org/proceedings/03mar/148.htm>.

Author's Address

   Pekka Savola

   EMail: psavola@funet.fi

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