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Versions: 00 01 02 03                                                   
Internet Engineering Task Force                                P. Savola
Internet Draft                                                 CSC/FUNET
Expiration Date: April 2003
                                                            October 2002


                    IPv6 Multicast Deployment Issues

               draft-savola-v6ops-multicast-issues-00.txt

Status of this Memo

   This document is an Internet-Draft.  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-Drafts.

   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
   http://www.ietf.org/ietf/1id-abstracts.txt.

   To view the list Internet-Draft Shadow Directories, see
   http://www.ietf.org/shadow.html.

Abstract

   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 completely impossible except
   using SSM: there is no way to convey information about multicast
   sources between PIM RPs. Site-scoped multicast is also problematic
   when used alongside to global multicast because of that.  A few
   possible solutions are outlined or referred to.  In addition, an
   issue regarding link-local multicast-blocking Ethernet switches is
   brought up.  Finally, a feature request for MLD snooping switches is
   noted.










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Table of Contents

   1.  Introduction  ...............................................   2
   2.  Issues with Multiple PIM Domains and Any Source Multicast  ..   2
     2.1.  Changing the Multicast Usage Model  .....................   3
     2.2.  Implementing MSDP for IPv6  .............................   3
     2.3.  Implementing Another Multicast Routing Protocol  ........   4
     2.4.  Embedding the Address of the RP in Multicast Address  ...   4
     2.5.  Site-local Group Scoping  ...............................   4
   3.  Neighbor Discovery Using Multicast  .........................   5
   4.  MLD Snooping Ethernet Switches  .............................   5
   5.  Security Considerations  ....................................   6
   6.  Acknowledgements  ...........................................   6
   7.  References  .................................................   6
     7.1.  Normative References  ...................................   6
     7.2.  Informative References  .................................   6
   Author's Address  ...............................................   7




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 completely impossible
   except using SSM: there is no way to convey information about
   multicast sources between PIM RPs. Site-scoped multicast is also
   problematic when used alongside to global multicast because of that.
   A few possible solutions are outlined or referred to.  These are
   discussed in section 2.

   In addition, an issue regarding link-local multicast-blocking
   Ethernet switches is brought up.  Finally, a feature request for MLD
   snooping switches is noted.  These are discussed in sections 3 and 4,
   respectively.

2. Issues with Multiple PIM Domains and Any Source Multicast

   For both administrative and technical reasons, there must be multiple
   Protocol-Independent Multicast (PIM) [PIM] domains in the Internet,
   which means there will be multiple PIM 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



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   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/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).

   SSM is ideal for "broadcast" -type applications, but can be argued to
   be a bit lacking in e.g. "videoconference" -type applications (as it
   may be desirable to have data flow directly between each
   participant).  However, in the latter case, as with telephone
   teleconferences, it is often very desirable for one party to act as a
   chair or a "hub". In this case, other participants could send their
   data using unicast to the hub and hub could retransmit all data using
   SSM, though.

   One could argue that the added latency of circulating the data
   through the hub may be unacceptable; however, at least in the bigger
   conferences (where usually using multicast really matters) this does
   not seem like a big problem because there must be some control of the
   order of speech anyway.

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

   Nonetheless, few seem to realize that SSM is currently the only way
   to get global interdomain multicast in IPv6.

2.2. Implementing MSDP for IPv6

   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, though.  Whether this is a "good"
   or "bad" decision is a matter of opinion.




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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 Address of the RP in 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
   impossible.

   However, making some assumptions about multicast addressing, this is
   can be done -- a proposed solution is presented in a different memo
   [V6RPADDR].  Some minor changes in existing PIM specifications would
   have to be done to take advantage of this, though (but non-modified
   implementations would be no worse than today).

   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; without further specification, anycast-RP
   technique could not be used with "embedded RP address" mechanism.

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



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   domains and communication required between them.  This will increase
   the need for a global multicast solution.

3. Neighbor Discovery Using Multicast

   Neighbor Discovery [NDISC] uses link-local multicast in the most
   common link-layer media, not broadcast as does ARP with IPv4.  This
   may cause some operational problems with some equipment.

   The author has seen two brands of managed Ethernet 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
   (and whether there are workarounds).

4. MLD Snooping Ethernet Switches

   Especially if multicast traffic is relatively heavy (e.g. video
   streaming), it becomes particularly important to have Multicast
   Listener Discovery (MLD) snooping implemented in some 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.  Clarifications
   and analysis on what MLD snooping switches can reasonably expect
   would be very important.












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5. 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
   specification.

   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.

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.

7. References

7.1. Normative References

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

7.2. Informative References

   [ANYCASTRP] Dorian, K. et al, q(Anycast RP mechanism using PIM and
               MSDP", work-in-progress, draft-ietf-mboned-anycast-
               rp-08.txt, May 2001.

   [BGMP]      Thaler, D., "Border Gateway Multicast Protocol (BGMP)",
               work-in-progress, draft-ietf-bgmp-spec-03.txt. June 2002.

   [MLDSNOOP]  Christensen, M., Solensky, F., "IGMP and MLD snooping
               switches", work-in-progress, draft-ietf-magma-
               snoop-02.txt, June 2002.

   [MSDP]      Farinacci, D. et al, "Multicast Source Discovery Protocol
               (MSDP)", work-in-progress, draft-ietf-msdp-spec-13.txt
               (expired), 2002.

   [PIM]       Fenner, B. et al, "Protocol Independent Multicast -
               Sparse Mode (PIM-SM): Protocol Specification (Revised)",
               work-in-progress, draft-ietf-pim-sm-v2-new-05.txt,
               March 2002.




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   [SSM]       Holbrook, H. et al, "Source-Specific Multicast for IP",
               work-in-progress, draft-ietf-ssm-arch-00.txt,
               November 2001.

   [V6RPADDR]  Savola, P., "Embedding the Address of RP in IPv6
               Multicast Address", work-in-progress,
               draft-savola-mboned-mcast-rpaddr-00.txt, October 2002.

Author's Address

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





































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