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Versions: 00 01 02 03                                                   
Internet Engineering Task Force                                P. Savola
Internet Draft                                                 CSC/FUNET
Expiration Date: August 2004
                                                           February 2004

                    IPv6 Multicast Deployment Issues


Status of this Memo

   This document is an Internet-Draft and is subject to all provisions
   of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
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   There are many issues concerning the deployment and implementation,
   and to a lesser degree, specification of IPv6 multicast.  This memo
   describes known problems, trying to raise awareness.  Currently,
   global IPv6 interdomain multicast is impossible except using SSM:
   there is no way to convey information about multicast sources between
   PIM-SM RPs; the situation is analyzed, and a technique called
   Embedded RP is offered as a solution.  The deploability of SSM and
   Embedded RP is also considered.  In addition, an issue regarding
   link-local multicast-blocking Ethernet switches is brought up.
   Finally, the requirement for functionality like MLD snooping is

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Table of Contents

   1.  Introduction  ...............................................   2
   2.  Issues with Multiple PIM Domains and Any Source Multicast  ..   3
     2.1.  Changing the Multicast Usage Model  .....................   3
     2.2.  Implementing MSDP for IPv6 Interdomain Multicast  .......   4
     2.3.  Implementing Another Multicast Routing Protocol  ........   4
     2.4.  Embedding the RP Address in an IPv6 Multicast Address  ..   4
     2.5.  Site-local Group Scoping  ...............................   5
   3.  Issues with SSM and Embedded RP  ............................   5
     3.1.  SSM Deployment  .........................................   5
     3.2.  RP Failover with Embedded RP   ..........................   6
   4.  Neighbor Discovery Using Multicast  .........................   6
   5.  Functionality Like MLD Snooping  ............................   7
   6.  Security Considerations  ....................................   7
   7.  Acknowledgements  ...........................................   8
   8.  References  .................................................   8
     8.1.  Normative References  ...................................   8
     8.2.  Informative References  .................................   8
   Author's Address  ...............................................   9
   Intellectual Property Statement  ................................   9
   Full Copyright Statement  .......................................  10

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, trying to raise awareness.

   Currently, global IPv6 interdomain multicast is impossible except
   using SSM: there is no way to convey information about multicast
   sources between PIM-SM RPs; site-scoped multicast problematic.  A few
   possible solutions, such as Embedded RP [EMBEDRP] are outlined or
   referred to.  These are discussed in section 2.  Deployment issues
   with both SSM and Embedded RP are discussed separately in section 3.

   In addition, an issue regarding link-local multicast -blocking
   Ethernet switches is brought up.  Finally, the requirement for
   functionality like MLD snooping is noted.  These are discussed in
   sections 4 and 5, respectively.

   [MULTIGAP] analyses the more generic set of issues with multicast;
   this memo focuses on critical issues regarding IPv6.

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2. Issues with Multiple PIM Domains and Any Source Multicast

   For both administrative and technical reasons, there must be multiple
   Protocol-Independent Multicast - Sparse Mode (PIM-SM) [PIM-SM]
   domains in the Internet, which means there will be multiple PIM-SM
   Rendezvous Points (RPs) -- and communication mechanisms between these
   RPs will become critical.

   These issues only come up with classical Any Source Multicast;
   Source-Specific Multicast [SSM] does not require RPs and is not
   affected, as the multicast "channel" is identified by the combination
   <source address, group address> and can be communicated out-of-band.

   In IPv4, notification of multicast sources between these PIM RPs is
   done with Multicast Source Discovery Protocol (MSDP) [MSDP].  Many
   consider this a sub-optimal, but unfortunately necessary, solution;
   when it was specified, it was only meant as a "stop-gap" measure.

   Below, some issues and solutions or work-arounds are described.

2.1. Changing the Multicast Usage Model

   As "Any Source Multicast" -model has been found to be complex and a
   bit problematic, there may be an 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.

   In any case, even though SSM would avoid mentioned problems, it is
   far from being generally implemented, much less deployed, yet.

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2.2. Implementing MSDP for IPv6 Interdomain Multicast

   One could argue that currently, the easiest stop-gap solution (to a
   stop-gap solution) would be to specify IPv6 TLV's for MSDP.  This
   would be fairly straightforward, and existing implementations would
   probably be relatively easily modifiable.

   There has been some resistance to this, as MSDP was not supposed to
   last this long in the first place.

2.3. Implementing Another Multicast Routing Protocol

   One possibility might be to specify and/or implement a different
   multicast routing protocol.  In fact, Border Gateway Multicast
   Protocol (BGMP) [BGMP] has been specified for a few years; however,
   it seems quite complex and there have been no implementations.
   Lacking deployment experience and specification analysis, it is
   difficult to say which problems it might solve (and possibly, which
   new ones to introduce).

   In conclusion, looking for a solution in BGMP may not be realistic in
   this time frame.

2.4. Embedding the RP Address in an IPv6 Multicast Address

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

   At the first glance, this appears to be an impossible problem: the
   address of the RP, as well as the multicast address, are both 128
   bits long; in the general case, embedding one in the other is

   However, making some assumptions about multicast addressing, this is
   can be done -- a proposed solution is presented in a different memo
   [EMBEDRP].  Additionally, this requires a PIM-SM implementation of
   the Embedded RP group-to-RP mapping mechanism which takes this
   encoding to the account.

   One should note that MSDP is also used in "Anycast RP" [ANYCASTRP]
   -technique, for sharing the state information between different RP's
   in one PIM domain; unless other proposals, such as [ANYPIMRP], are
   deployed, or MSDP for IPv6 implemented, Anycast-RP technique cannot
   be used.

   However, a "cold failover" variant of anycast-RP (for long-term
   redundancy only) would still be possible. In this mechanism, multiple

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   routers would be configured with the RP address, but only one would
   be active at the time: if the RP goes down, another takes its place.
   The multicast state stored in the RP would be lost, though, unless it
   is copied by some out-of-band mechanism (e.g. placing the backup RP
   absolutely on-the-path and have it snoop all the relevant packets).

2.5. Site-local Group Scoping

   Site-local groups must be their own PIM domains to prevent site-local
   data leaking to other sites.  A more complex possibility would be to
   implement something resembling "BSR border" feature which would
   filter out all site-local components in PIM packets: if this is not
   done very carefully, site-local information will leak to the global
   network.  This is operationally difficult, and PIM working group has
   come to consensus that a scope boundary MUST also be a a site
   boundary for certain critical PIM messages (e.g. C-RP and Bootstrap).

   Especially if site-local multicast is used (and the site also wants
   to engage in global multicast), there will be a huge number of
   domains and communication required between them.  This will increase
   the need for a global multicast solution.

3. Issues with SSM and Embedded RP

   This section briefly describes some challenges with SSM deployment,
   and gaps caused by the introduction of Embedded RP.

3.1. SSM Deployment

   To be deployed, SSM requires changes to routers, MLD-snooping
   Ethernet switches, host systems, APIs, and the multicast usage

   Introducing SSM support in the routers has been straightforward.

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

   Host systems require MLDv2 support.  The situation has become
   slightly better with respect to MLDv2 support for end systems.  The
   multicast source filtering API specification has also been completed;
   its deployment is likely roughly equal (or slightly worse) than
   MLDv2.  The API is required for creating SSM applications.

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   Now the most difficult problem, multicast usage models, remains a
   problem.  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, Any Source Multicast -like
   techniques seem to be required.

3.2. 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, called Anycast-RP, should still be supported.
   MSDP could still be used for that; an alternative is doing the same
   in PIM itself [PIMANYRP].

   However, 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

4. Neighbor Discovery Using Multicast

   Neighbor Discovery [NDISC] uses link-local multicast in Ethernet
   media, not broadcast as does ARP with IPv4.  This may cause some
   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-
   layer multicast packets to other ports at all.  In essence, native
   IPv6 is impossible with this equipment.  Investigation is still going
   on whether these issues can be worked around.

   However, it seems likely 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 [MLDSNOOP].

   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 case problems like these

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   (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 in the same
   segment one wishes to communicate with.

5. 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
   streaming), it becomes particularly important to have some feature
   like Multicast Listener Discovery (MLD) snooping implemented in the
   equipment, most importantly Ethernet switches [MLDSNOOP].

   In addition, there have been some misunderstandings wrt. which
   multicast addresses (in particular, link-locals) MLD reports
   (utilized in the snooping) should be generated for.  If all
   implementations do not generate enough MLD reports, the introduction
   of MLD snooping could cause them being blocked out.  To clarify, a
   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 [MULTIGAP]: the whole idea of
   L2-only devices having be able to peek into L3 datagrams seems like a
   severe layering violation -- and often the devices aren't upgradeable
   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].

6. Security Considerations

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

   One fairly obvious issue raised in this memo is that if there is no
   adminsitrative PIM domain border between site-local multicast
   domains, the site-local traffic could very easily flow into other
   sites and the Internet.

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7. 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; HÂ…vard Eidnes made
   a case for a protocol like CGMP. Leonard Giuliano pointed out a more
   complete analysis of SSM with different kind of applications.

8. References

8.1. Normative References

   [ANYCASTRP] Kim, D. et al, "Anycast RP mechanism using PIM and
               MSDP", RFC 3446, January 2003.

   [EMBEDRP]   Savola, P., Haberman, B., "Embedding the RP Address
               in an IPv6 Multicast Address", work-in-progress,
               draft-ietf-mboned-embeddedrp-01.txt, Feb 2004.

   [MSDP]      Fenner, B., Meyer, D., "Multicast Source Discovery
               Protocol", RFC 3618, Oct 2003.

   [NDISC]     Narten, T., Nordmark, E., Simpson W., "Neighbor Discovery
               for IP Version 6 (IPv6)", RFC2461, December 1998.

   [PIM-SM]    Fenner, B. et al, "Protocol Independent Multicast -
               Sparse Mode (PIM-SM): Protocol Specification (Revised)",
               work-in-progress, draft-ietf-pim-sm-v2-new-08.txt,
               October 2003.

   [SSM]       Holbrook, H. et al, "Source-Specific Multicast for IP",
               work-in-progress, draft-ietf-ssm-arch-04.txt,
               Oct 2003.

8.2. Informative References

   [ANYPIMRP]  Farinacci, D., Cai, Y., "Anycast-RP using PIM",
               work-in-progress, draft-ietf-pim-anycast-rp-00.txt,
               November 2003.

   [BGMP]      Thaler, D., "Border Gateway Multicast Protocol (BGMP)",
               work-in-progress, draft-ietf-bgmp-spec-06.txt,
               January 2004.

   [BSR]       Fenner, B., et al., "Bootstrap Router (BSR) Mechanism for
               PIM Sparse Mode", work-in-progress, draft-ietf-pim-sm-

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               bsr-03.txt, February 2003.

   [CGMP]      Cisco, "Cisco Group Management Protocol", e.g.

   [GARP]      Tobagi, F., et al, "Study of IEEE 802.1p GARP/GMRP Timer
               Values", (for introduction to GARP/GMRP, see section 2),
               Sep 1997.

   [MLDSNOOP]  Christensen, M., Solensky, F., "IGMP and MLD snooping
               switches", work-in-progress, draft-ietf-magma-
               snoop-10.txt, October 2003.

   [MULTIGAP]  Meyer, D., Nickless, B., "Internet Multicast Gap
               Analysis [...]", work-in-progress,
               draft-ietf-mboned-iesg-gap-analysis-00.txt, July 2002.

   [PIMSO]     Bhattacharyya, S. et al, "Deployment of PIM-SO at Sprint
               (PIM-SO)", work-in-progress,
               draft-bhattach-diot-pimso-00.txt (expired), March 2000.

   [SSMSNOOP]  Thaler, D., "Operational Problems with IGMP snooping
               switches", presentation in MAGMA WG at IETF56,
               March 2003.

Author's Address

   Pekka Savola
   Espoo, Finland
   EMail: psavola@funet.fi

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