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Versions: 00

   Internet Draft                                  Editors:    D. Meyer
                                                                 Sprint
   Document:                                                B. Nickless
   draft-ietf-mboned-iesg-gap-analysis-00.txt          Argonne National
                                                             Laboratory
   Expires:                                                   July 2002
   January 2003


                      Internet Multicast Gap Analysis
                       from the MBONED Working Group
                               for the IESG


1. Status of this Memo

   This document is an Internet-Draft and is in full conformance
   with 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
   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

   The list of Internet-Draft Shadow Directories can be accessed at
        http://www.ietf.org/shadow.html.


2. Abstract

   An overview of IP multicast as deployed in the Internet today, from
   the perspective of the MBONED working group.  Existing
   infrastructure is examined critically.  Suggestions for possible
   improvement of the overall architecture are presented for the IESG.


3. Table of Contents

   1. Status of this Memo.............................................1
   2. Abstract........................................................1
   4. Overview and Background.........................................2
   5. Conventions used in this document...............................2
   6. RFC 1112........................................................3
   7. Source-Specific Multicast.......................................3
   8. Host Extensions for IP Multicast................................3

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   9. Mapping of Multicast Group Addresses to Ethernet MAC Addresses..4
   10. Local Subnet Receiver Interest Protocol (IGMP).................4
   11. Collision-Sense Media Access Sender Model......................4
   12. Multicast Gateways.............................................5
   13. Dense Mode Internet Multicast Routing..........................5
   14. Reachability Protocol Independent Multicast Routing............6
   15. Sparse Mode Internet Multicast Routing.........................7
   16. Mixed Dense/Sparse Mode Internet Multicast Routing.............7
   17. Bursty Sources vs. Sparse Mode Forwarding State Maintenance....8
   18. Co-mingled Source Knowledge and Packet Forwarding..............8
   19. Co-mingled IP and Ethernet Routing.............................9
   20. Inter-Domain IP Multicast Exchange Points......................9
   21. IP Multicast Architectural Gaps...............................11
   22. Recommendations from MBONED to IESG...........................11
   23. Acknowledgements..............................................13
   24. Security Considerations.......................................13
   25. References....................................................14
   26. EditorsÆ Addresses............................................14


4. Overview and Background

   At the IETF-54 meeting, the MBONED working group recommended that
   the MSDP working group publish their current work as an
   Informational RFC and shut down.  Some participants in the MBONED
   and MSDP working groups believed that the recurring discussions
   about the operation of MSDP were proxy arguments about the IP
   Multicast service model, and how that model can be supported in the
   Internet.  Participants came to rough consensus that the best place
   for these overall service model and deployment questions is the
   MBONED working group.

   A two phase approach was adopted.  The short-term objective is to
   document existing MSDP implementations and deployments.  A longer-
   term objective for the MBONED working group is to perform a ôgap
   analysisö of the existing IP multicast service model, protocols, and
   deployment.

   This document represents that ôgap analysis,ö and is intended as
   advice to IESG.  The MBONED participants hope the IESG will consider
   this advice in the context of IESG guidance for further IP multicast
   protocol development and deployment work.

5. Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and "OPTIONAL" in
   this document are to be interpreted as described in RFC-2119 [1].


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6. RFC 1112

   The seminal specification for IP Multicast is RFC 1112.  It
   describes five elements required for IP Multicast on a local subnet:
   extensions to host software, an IPv4 address range reserved for
   group addresses, a method for mapping multicast group addresses to
   Ethernet Media Access Control (MAC) addresses, a protocol to
   discover receivers interested in packets addressed to a group
   (Internet Group Management Protocol), and a collision-sense multiple
   access (CSMA) method for multicast packet senders.

   This model was inspired by Ethernet.  The IEEE 802.3 Ethernet
   specification includes a bit in the MAC address format that
   indicates the frame may be intended for multiple receivers.
   Ethernet interfaces can be programmed to receive these multicast MAC
   addresses and forward them to the host for processing.  On a
   traditional 10Base5 Ethernet, any station can put a frame on the
   wire, with the only requirement being to sense collisions and
   retransmit if necessary.

   RFC 1112 also suggests that gateways may exist for moving IP
   multicast datagrams to other subnets with interested receivers.

   The RFC 1112 service model has since become known as Any Source
   Multicast (ASM).  When a receiver registers interest in a group, it
   will be delivered datagrams from any source that transmits datagrams
   addressed to that group.


7. Source-Specific Multicast

   Just as early Ethernet controllers were programmable to only receive
   frames with certain MAC addresses, RFC 1112 and IGMP Version 2 only
   allowed IP Multicast receivers to elect to receive datagrams
   addressed to specific group addresses.  Receivers could not select
   the sources participating in a group from which they would receive.

   Later Ethernet controllers allowed more sophisticated filtering,
   including the capability of choosing from which senders the host
   wished to accept frames.  Similarly, Source-Specific Multicast (SSM)
   is an extension to the basic IP multicast model that allows
   receivers to select the source addresses from which to receive
   datagrams.  IGMP Version 3 implements this service model extension
   for IPv4, and the Multicast Listener Discovery (MLD) protocol
   Version 2 implements this service model extension for IPv6.


8. Host Extensions for IP Multicast

   Ultimately, user applications originate and accept IP multicast
   datagrams.  RFC 1112 describes the extensions to various host
   software modules to support applications sending and receiving

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   datagrams.  It also describes the operations a user application can
   perform to transmit and receive multicast datagrams.


9. Mapping of Multicast Group Addresses to Ethernet MAC Addresses

   When preparing an IP datagram for transmission on an Ethernet, it is
   necessary for the host to specify the destination MAC address.  (The
   source MAC address is typically constant, based on the IEEE-
   controlled address hard-wired into the Ethernet controller.)  Before
   sending unicast datagrams, the Address Resolution Protocol (ARP) is
   generally used by a host to learn the destination MAC address for a
   given destination IPv4 address.  RFC 2461 describes the equivalent
   Neighbor Discovery protocol procedure used for IPv6.

   IPv4 multicast datagrams are not addressed to specific host
   addresses; instead, they are addressed to group addresses in the
   224.0.0.0/4 range.  Likewise, IPv6 multicast datagrams are addressed
   to group addresses in the FF00::/8 range.

   RFC 1112 specifies a static 32:1 mapping from IPv4 multicast group
   addresses to Ethernet MAC addresses.  One reason to define the 32:1
   mapping was financial; to reserve enough Ethernet MAC addresses from
   the IEEE for a 1:1 mapping would have cost USD$16,000 in 1988.  The
   32:1 mapping reduced that cost to USD$1,000.

   RFC 2464 (section 7) specifies a static 2^96:1 (that is,
   79,228,162,514,264,337,593,543,950,336:1) mapping from IPv6
   multicast group addresses to Ethernet MAC addresses.  Note that it
   is impossible to determine the RFC 2375 scope directly from the
   Ethernet 802.3 MAC address, as the RFC 2464 mapping does not include
   the scope octet.


10. Local Subnet Receiver Interest Protocol (IGMP)

   RFCs 1112 and 2236 define the Internet Group Management Protocol
   (IGMP) Version 2.  IGMPv2 notifies the network of the interest of a
   host for datagrams addressed to a given group address.  Again, this
   is analogous to a host notifying an Ethernet controller to accept
   frames with a given MAC address.

   The IPv6 equivalent of IGMP is the Multicast Listener Discovery
   Protocol (MLD), originally defined in RFC 2710.


11. Collision-Sense Media Access Sender Model

   Any host connected to a 10Base5 Ethernet can choose to transmit a
   frame at any time, subject only to the operation of the collision-
   sense media access (CSMA) protocol.


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   Given the static mapping of IPv4 group addresses to Ethernet MAC
   addresses, RFC 1112 specifies that an IPv4 datagram, addressed to a
   multicast group, can be transmitted by a host at any time.

   Similarly, RFC 2464 implies that an IPv6 datagram addressed to a
   multicast group can be transmitted by a host at any time.


12. Multicast Gateways

   RFC 1112 suggested that gateways might exist to pass multicast
   traffic between networks.  Through the operation of the local subnet
   receiver interest protocol IGMP, these gateways can learn of the
   interest of receivers in multicast group datagrams.

   As the technology for internetwork routing was unknown at the time
   of publication, RFC 1112 does not specify how that routing is to
   take place.


13. Dense Mode Internet Multicast Routing

   Following the Ethernet broadcast model, the first scheme for routing
   IP multicast datagrams between Ethernets was for a multi-homed
   gateway to flood all multicast datagrams on each Ethernet to all
   other Ethernets.  In other words, gateways become the IP multicast
   equivalent of Ethernet repeaters.

   Ethernet bridges generally run the Spanning Tree Protocol (IEEE
   Specification 802.1d) to eliminate forwarding loops.  Forwarding
   loops can also happen when there is more than one multi-homed
   gateway in an internetwork.  The Distance Vector Multicast Routing
   Protocol (DVMRP) [RFC 1075] operates to eliminate forwarding loops.
   DVMRP, operating on a gateway, keeps track of the Reverse Path
   Forwarding (RPF) interface from which a multicast datagram with a
   given source address should arrive.  If such multicast datagrams
   arrive on the appropriate RPF interfaces, they are replicated and
   flooded to all other interfaces by the gateway.

   The effect of this procedure is to replicate all IP multicast
   datagrams transmitted by any host to all subnets.  This limits the
   available bandwidth for all multicast traffic in the internetwork to
   that bandwidth available on the slowest link.

   One optimization to this procedure is for gateways to use a receiver
   interest protocol such as IGMP to limit traffic flooded out an
   interface.  Only if there are receivers interested in a group, on a
   network attached to a given interface, will the gateway flood
   datagrams addressed to that group out that interface.  Of course,
   gateways are generally assumed by fellow gateways to be interested
   in all groups, so this optimization does not apply to networks with
   more than one gateway.


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   A second optimization further limits flooding.  Consider a situation
   where a gateway has no interested receivers on all attached networks
   for a given group, yet receives an IP multicast datagram addressed
   to that group.  DVMRP provides for gateways to send Prune messages
   out the appropriate RPF interface to notify fellow gateways that
   they have no interested receivers.

   A third optimization provides for this limiting process to continue
   recursively.   Once a gateway receives Prune messages from all other
   gateways on a network, and has no interested hosts, it stops
   forwarding messages out the attached interface.  If all interfaces
   have such stoppages for a given group, it can generate its own Prune
   towards out the appropriate RPF interface to notify upstream
   gateways to stop sending datagrams addressed to a given group.

   Through repeated application of this procedure, the distribution of
   multicast datagrams is limited only to the networks that have
   attached interested receivers, and to intermediate networks between
   sources and interested receivers.  Multicast datagrams are
   distributed down a tree rooted at the source.  Vertexes of the tree
   are the gateways, and the edges of the tree are the networks
   connecting gateways.

   This general strategy is known as ôdense modeö multicast routing.

   As the number of multicast sources and receivers increase, the core
   of the multicast-enabled internetwork becomes more and more heavily
   loaded.  Fewer opportunities for pruning occur.

   As dense mode routing was experimentally deployed, a meta-stable
   failure mode was discovered.  A gateway (or its attached network)
   can be overwhelmed with multicast traffic.  Even though the gateway
   may have no interested receivers, it can fail to generate the
   required number of Prune messages.  Unfortunately this failure mode
   can spread, because upstream gateways (closer to the sources) assume
   that packet replication and transmission is required, adding to
   their own load.  Eventually the whole multicast internet collapses
   under the weight of un-Pruned traffic.


14. Reachability Protocol Independent Multicast Routing

   In addition to controlling whether forwarding occurs (based on
   receiver interest), DVMRP maintains the topology of the forwarding
   trees from source to receivers.  As its name implies, a distance-
   vector procedure similar to RIP is used.

   Experience has shown that a distance-vector reachability protocol
   does not scale for large internets.  Link-state protocols such as
   IS-IS and OSPF are generally used within administrative domains, and
   the Border Gateway Protocol is generally used between administrative
   domains.  Convergence speed, policy flexibility, and other
   considerations motivate this diversity of reachability protocol use.

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   The Protocol Independent Multicast (PIM) multicast routing protocol
   takes its name from the fact that it can take its reachability
   information from any underlying reachability protocol.  PIM
   concentrates on maintaining and controlling the multicast forwarding
   tree along the topology provided by whatever underlying reachability
   protocol(s) is/are used.


15. Sparse Mode Internet Multicast Routing

   One way to eliminate the dense mode meta-stable failure mode is by
   adjusting the inter-gateway forwarding procedure to require
   downstream gateways to explicitly request datagrams for a given
   group based on receiver interest.

   In this regime, the forwarding tree is built from the interested
   receivers towards the source.  Datagrams from the source are
   distributed back down the tree to the interested receivers.

   Through IGMPv3 (or any similar SSM-style receiver interest discovery
   protocol) the receivers provide both pieces of information necessary
   for the internetwork to create and maintain the source-rooted
   forwarding tree: the IP addresses of the source and multicast group.


16. Mixed Dense/Sparse Mode Internet Multicast Routing

   A straightforward sparse mode forwarding protocol alone cannot
   support the RFC 1112 Any Source Multicast service model.  Although
   the receivers supply the group address for which theyÆre interested
   in receiving datagrams, the internetwork is responsible for
   identifying active sources so the source-rooted forwarding trees can
   be created.

   The hybrid approach taken in RFC 2362 (PIM Sparse Mode Version 2)
   and MSDP is to flood the initial datagrams from any sender,
   typically under strict rate controls.  When a gateway receives one
   of these flooded datagrams from a given sender, whose group address
   matches that of an attached interested receiver, the gateway grafts
   itself to the source-rooted forwarding tree for that sender.

   So long as the source continues to transmit packets, the forwarding
   tree associated with the source is preserved.  After a period of
   quiescence the forwarding tree is torn down.

   The result is a dual-plane routing architecture.  A dense-mode,
   rate-limited, flooding plane distributes datagrams from newly active
   sources.  A sparse-mode, source-rooted tree based forwarding plane
   distributes and replicates datagrams from established sources.



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17. Bursty Sources vs. Sparse Mode Forwarding State Maintenance

   Prior to the development of the client-server-based World Wide Web,
   a session announcement protocol (SAP) was developed to allow
   interested parties to discover and participate in multilateral
   multimedia conference.  Periodically a multicast datagram would be
   generated and sourced, providing the multicast group addresses,
   media formats, and other such information needed for interested
   parties to join the conference.

   As in any real-world application protocol, two factors required an
   engineering trade-off: the bandwidth consumed by the announcements
   vs. the frequency of announcements.  Recall that early internetwork
   multicast routing used a dense mode approach; datagrams were flooded
   everywhere unless they were explicitly known to be unwanted.   Given
   the propensity of the entire internetwork multicast infrastructure
   to collapse under load, great emphasis was placed on limiting the
   total bandwidth consumed by the announcements.  Thus, the SAP
   announcement frequency was often measured in tens of minutes.

   As sparse-mode multicast routing became more widely deployed, this
   tens-of-minutes frequency of SAP announcements became a problem.
   Each time an SAP announcement was sourced, the sparse-mode source-
   based distribution tree would be created towards interested
   receivers.  But due to the low frequency of each independent
   announcement, the distribution tree would have been deemed quiescent
   and would be torn down.

   The resulting ôbursty sourceö traffic would often follow only the
   dense-mode, rate-limited flooding routing plane.  The sparse-mode,
   higher-performance forwarding plane would assume the source has gone
   quiescent long before the next ôburstö.

18. Co-mingled Source Knowledge and Packet Forwarding

   ThereÆs a chicken-and-egg problem at the heart of internetwork
   multicast routing.  On the one hand, experience has shown that a
   source-based distribution tree is the most efficient way to forward
   datagrams from a source to all interested listeners.  On the other
   hand, such a source-based distribution tree cannot be created until
   the source is known, and RFC 1112 decrees that sources be able to
   transmit at any time without warning.

   In other words, RFC 1112 defines an active source as a source that
   has placed a datagram on the wire.  But by the time the datagram has
   been placed on the wire, itÆs too late to create a source-based
   distribution tree to all interested receivers.

   There have been several approaches taken to resolve this problem.

   The first was to not use source-based distribution trees at all.
   Unfortunately this approach resulted in an internetwork that would
   collapse under load.

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   The second approach has been to create dual routing planes: a dense-
   mode plane to forward the initial datagrams from a source, and as a
   side effect create a source-based distribution tree in the sparse-
   mode plane.  Unfortunately these dual routing planes lead to a great
   deal of complexity.

   The third approach has been SSM: put the burden of spreading the
   knowledge of active sources on the application rather than the
   network.  This approach has two major drawbacks: first, it requires
   the replacement or upgrade of edge IEEE 802.x devices to support
   IGMPv3 snooping, along with a wholesale upgrade to host operating
   systems.  Second, it requires applications to develop their own
   rendezvous mechanisms.

19. Co-mingled IP and Ethernet Routing

   For primarily historical reasons, the IETF has pushed vendors of
   nominally IEEE 802.x compliant equipment to also become IPv4-aware
   enough to understand the IGMP Version 2 protocol.

   IEEE has responded with the GARP/GMRP protocol suite, which are
   intended to allow 802.x hosts to control MAC-layer multicast
   replication and filtering.  However, the IETF has continued work on
   IGMP and MLD while ignoring media-specific protocols like GARP/GMRP.

   Arguably, this has marginalized IP multicast deployment, especially
   IPv6 multicast deployment.  Only the very high-end IEEE 802.x
   devices have the sophistication to interpret IPv4/IGMP and IPv6/MLD
   datagrams.


20. Inter-Domain IP Multicast Exchange Points

   Autonomous Systems often wish to exchange traffic.  Exchange points
   have been developed to meet this demand.  One popular type of
   exchange point is realized in an 802.x Ethernet switch.  Each
   participating Autonomous System is provided an 802.x Ethernet port
   and an IP address on the exchange point network, to which the
   Autonomous System connects a router.  Bilateral BGP sessions are
   then established between Autonomous Systems across the 802.x network
   fabric.

   When an Autonomous System router wishes to deliver a unicast
   datagram to another Autonomous System router participating at such
   an exchange point, it follows this procedure:

   - The datagramÆs IP Destination Address is compared to the
     Forwarding Information Base (FIB).  The FIB returns a so-called
     ônext-hopö IP address.  This next-hop address is generally
     assigned to another Autonomous SystemÆs router at the exchange
     point.


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   - Through the operation of the Address Resolution Protocol (ARP),
     the 802.x Ethernet MAC address associated with the next-hop IP
     address is determined.

   - An Ethernet frame is assembled with the destination MAC address
     set to the MAC address determined through ARP, and is transmitted
     to the Exchange Point 802.x Ethernet switch.

   - The exchange point 802.x Ethernet switch examines the destination
     MAC address of the Ethernet frame.  Based on that address, the
     exchange point switch delivers the Ethernet frame to the
     destination Autonomous SystemÆs router.

   Consider an analogous procedure for multicast routing.  The object
   is to graft the Autonomous SystemÆs router onto a source-rooted
   distribution tree across the exchange point.  Here is one procedure
   that a downstream router can follow:

   - The downstream router compares the source address to its Multicast
     Reachability Information Base (M-RIB).  The M-RIB returns the IP
     address of an ôupstreamö router across the exchange point.

   - Through the operation of the PIM Sparse Mode Protocol, the
     downstream router registers interest in that source and group
     addresses to the upstream router across the exchange point.

   - Upon receipt of a matching datagram for the downstream router, the
     upstream router assembles an Ethernet frame and transmits it to
     the exchange point 802.x Ethernet switch.  As per RFC 1112 or RFC
     2464, the destination MAC address of the frame is statically
     derived from the destination group address of the datagram.

   - The exchange point 802.x Ethernet switch examines the destination
     MAC address.  As this MAC address is a multicast address, the
     802.x Ethernet switch replicates this frame and sends it to all
     output ports.

   This presents three problems:

   First, the multicast traffic is needlessly replicated to all
   participants in the exchange point.  In the unicast case above, the
   802.x Ethernet exchange point switch could use the Ethernet
   destination MAC address to uniquely identify which port should
   receive a given frame.  The static mapping of destination multicast
   group address to Ethernet MAC addresses makes that determination
   impossible.

   Second, the needlessly replicated multicast traffic can trigger the
   PIM Assert process, as per RFC 2362 Section 2.9.  The PIM Assert
   process has been observed to override the policy decisions of
   downstream routers in exchange points.


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   Third, it is impossible for multicast traffic to pass through an
   exchange point more than once.  Any given exchange point participant
   may not have a peering agreement with all other participants,
   requiring an intermediate hop through a transit Autonomous System
   participant.  Due to the operation of ARP this is not a problem for
   unicast traffic, but due to the static mapping of multicast groups
   to (e.g.) Ethernet MAC addresses, this cannot work.

   In summary, it is impossible to support IP multicast at an exchange
   point, when that exchange point is based on IEEE 802.3 Ethernet.


21. IP Multicast Architectural Gaps

   In general, the IETF focus is on Internet protocols.  IGMP snooping
   places the requirement of IPv4-awareness on IEEE-standardized 802.x
   Ethernet switches.  The current drafts for IPv6 MLD seek to extend
   that requirement to include IPv6.  The IESG would rightfully refuse
   to allow IETF working groups to impose such requirements on devices
   standardized by organizations outside the IETF (such as IEEE), but
   somehow has excepted the IP multicast work from this discipline.

   The Internet is more complex than a simple CSMA-style Ethernet
   segment, where sources can transmit at any time.  Experience clearly
   indicates that internetwork multicast datagram forwarding is most
   efficiently done by source-rooted distribution trees.  Experience
   compels revisitation of the assumption that sources should be able
   to transmit at any time, yet receive the same level of service as
   that provided by a fully instantiated source-rooted distribution
   tree.

   Registration of soon-to-be-active sources (along the lines of the
   unicast Address Resolution Protocol [ARP]) should be seriously
   considered.

   Part of the registration of soon-to-be-active sources could include
   allocation of link-local media-specific multicast addresses, rather
   than relying solely on the static mappings defined in RFC 1112 and
   RFC 2464.

   The static mapping of IP multicast group addresses to media-specific
   multicast addresses (in particular, Ethernet) cannot operate
   properly at exchange points.


22. Recommendations from MBONED to IESG

   The IESG should direct the MAGMA working group to develop a
   successor to IGMP/MLD.

     - The successor should perform the receiver interest discovery
       functions of existing versions of IGMP/MLD, but in addition
       should support the registration of active sources.

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     - At least three modes of operation should be supported.  As in
       IGMPv2/MLDv1, sources and receivers should be able to transmit
       to and receive from group addresses without respect to the
       identities of sources.  In a second mode, analogous to
       IGMPv3/MLDv2, receivers should be able to select the sources
       from which they want to receive traffic for a particular group.
       A third mode should permit receivers to select the source
       address, group address, and upstream gateway from which to
       receive traffic.

     - The successor should be media-agnostic.  Media-specific
       multicast addresses should be treated as opaque handles.
       Examples of media-specific multicast addresses might include
       802.x MAC addresses, ATM Forum NSAP point-to-multipoint
       addresses, etc.

     - Adaptation layers for this successor protocol should be
       developed to use media-specific mechanisms for multicast
       transport and replication.  For example, the IEEE 802.1p
       GARP/GMRP protocol should be used on Ethernet.  ATM Forum UNI
       point-to-multipoint signaling should be used on ATM networks
       (c.f. RFC 2022).

     - This successor protocol would provide for the dynamic assignment
       of media-specific addresses.  As necessary, the media address
       assignment mechanism might control the creation and maintenance
       of media-specific intra-subnet distribution mechanisms, such as
       ATM point-to-multipoint switched virtual circuits.  When in
       operation on an IEEE 802.3 Ethernet, this mechanism would
       supercede RFC 1112 Section 6.4 and RFC 2464 Section 7.

     - The first application for this successor protocol would be at
       public internetwork exchange points.  The third mode of
       operation (allowing receivers to select the source address,
       group address, and upstream gateway) would allow participants at
       exchange points to select their upstream neighbor towards a
       source based on explicit policy, rather than the vagaries of the
       PIM Assert mechanisms.

     - This successor protocol may not be applicable for IP datagrams
       with TTL=1, so as to preserve semantics for link-local
       rendezvous (e.g. OSPF).  Likewise, it may not be applicable for
       IPv6 RFC 2375 scopes 0, 1, and 2.

   The IESG should encourage the development of a protocol to spread
   the knowledge of active sources to interested gateways.  Given a
   successor to IGMP that supports the registration of active sources,
   this spreading of knowledge can happen independently of actual
   multicast datagram forwarding.

   The IESG should discourage any further work on IGMP or MLD snooping,
   as implemented by otherwise nominally IEEE 802 compliant equipment.

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   Instead, the IESG should encourage the use of GARP/GMRP on IEEE 802
   networks.

   The IESG should guide protocols that use IP multicast to maintain a
   minimum frequency of datagram transmission, so as to preserve
   source-based forwarding trees.

23. Acknowledgements

   This work was supported by the Mathematical, Information, and
   Computational Sciences Division subprogram of the Office of Advanced
   Scientific Computing Research, U.S. Department of Energy, under
   Contract W-31-109-Eng-38.


24. Security Considerations

   Security considerations are not yet discussed in this draft memo.


   Meyer,
   Nickless (Editors)     Informational - Expires January 2002      13 

                   Internet Multicast Gap Analysis          July 2002


25. References

   Most of the references are called out in-line.  This section will be
   completed more fully before final publication.


   1  RFC 2119 Bradner, S., "Key words for use in RFCs to Indicate
      Requirement Levels", BCP 14, RFC 2119, March 1997




26. EditorsÆ Addresses

   David Meyer                     Email: dmm@sprint.net

   Bill Nickless
   Argonne National Laboratory
   9700 South Cass Avenue #221     Phone:  +1 630 252 7390
   Argonne, IL 60439               Email:  nickless@mcs.anl.gov

   Meyer,
   Nickless (Editors)     Informational - Expires January 2002      14


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