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Versions: 00 01 02 03                                                   
Internet Engineering Task Force                     R. Perlman
INTERNET DRAFT                                      Sun Microsystems
February 1999                                       C-Y Lee
                                                    Nortel Networks
                                                    A. Ballardie
                                                    Research Consultant
                                                    J. Crowcroft
                                                    Z. Wang
                                                    Lucent Technologies
                                                    T. Maufer
                                                    3Com Corporation
                                                    C. Diot
                                                    J. Thoo
                                                    Nortel Networks
                                                    M. Green
                                                    @Home Networks

    Simple Multicast: A Design for Simple, Low-Overhead Multicast^M


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 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."

     To view the list Internet-Draft Shadow Directories, see

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   This paper describes a design for multicast that is simple to
   understand and low enough overhead for routers that a single scheme
   can work both within and between domains. It also eliminates the need
   for coordinated multicast address allocation across the Internet. It
   is not very different from the tree-based schemes CBT, PIM-SM, and
   BGMP. Essentially all of the mechanisms to support this have already
   been implemented in the other designs. The contribution of this
   protocol is in what is NOT required to be implemented.

   The main idea for simplifying multicast is to consider the identity
   of a group to be the 8-byte combination of a "core node" C, and the
   multicast address M. The identity of the group is carried in join
   messages and data messages. M no longer has to be unique across the
   Internet. It only has to be unique per C. The other idea, which is
   independent of the first, it to build a bi-directional tree (as is
   done in CBT and BGMP) instead of building per-source trees from each
   sender.  This reduces the state necessary in routers to support

Changes from revision 1
   - use a Simple Multicast (SM) header instead of a new IP option

   - modified branch creation and deletion to avoid loops

   - added tree splicing mechanism

   - added multicast scoping

   - allow both IGMP and host SM Join

   - added sender only joins

   - third party independence

   - layer 2 filtering

   - host API and kernel changes

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1.0 Introduction

   IP Multicast has been around for over a decade, and several multicast
   protocols have been developed over the years. However, the solutions
   are either difficult to understand or expensive to deploy or both. In
   particular, we believe that multicast address allocation protocols
   are too complex and BGMP in combination with MASC will not scale

   In this paper, we present a design we call Simple Multicast that
   reduces the complexity and overhead of multicast. It is not really
   "yet another multicast protocol". Instead, it is more like a subset
   of other protocols, with one variation; to have the identifier of a
   group consist of both C (the core) and M (the multicast address).
   This eliminates the need to have unique multicast addresses and
   coordinate multicast addresses across the Internet.

1.1 Previous Work

   DVMRP is the first multicast routing protocol proposed. It uses a
   simple mechanism of flooding and pruning.

   The scalability issues with DVMRP led to the development of CBT. In
   CBT, a multicast group is formed by choosing a distinguished node,
   the "core", and having all members join by sending special join
   messages towards the core. The routers along the path keep state
   about which ports are in the group. If a router along the path of the
   join already has state about that group the join does not proceed
   further. Instead the router just "grafts" the new limb onto the tree.
   The result is a tree of shortest paths from the core, with only the
   routers along the path knowing anything about that group.

   In PIM-SM, each node could independently decide whether the volume of
   traffic from a particular source is worth switching from a shared
   tree to a per-source tree.  Thus, there are two possible trees for
   traffic from a particular source for group M; the shared tree and the
   source tree. To prevent loops, the shared tree had to be
   unidirectional, i.e., to send to the shared tree, the data has to be
   encapsulated and unicast to the core.

   The other issue that makes current protocols complex is the necessity
   for routers to be able to figure out the location of the core based
   solely on the multicast address M.  In PIM-SM, this resulted in a
   protocol whereby "core-capable" routers are being continuously
   advertised. All routers keep track of the current set of live core-
   capable routers, and there is a hashing function to map a multicast
   address to one of the set of core-capable routers. This advertisement
   protocol is confined to within a domain because it was recognized

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   that this mechanism would not scale to the entire Internet.

   For inter-domain multicast, a set of new protocols has been proposed.
   The MASC protocol deals with hierarchical block allocation of Class D
   address space.  Essentially, it creates a prefix structure in
   multicast address space in a way similar to unicast address space.
   Because of the limited multicast address space, the allocation has to
   be dynamic.  MASC contains mechanisms for collision detection and
   de-allocation. Once a block of multicast addresses is allocated, and
   no collision is detected for a period of time, the address block is
   then given to MAAS servers for actual assignment to multicast groups.
   The address block has to be propagated through BGP+ so that routers
   throughout the Internet can know the mapping of multicast addresses
   to cores, even in other domains. BGMP then uses this information to
   know the direction in which a join to multicast address M should be

1.2 Overview of Simple Multicast

   The Simple Multicast proposal tries to reduce or eliminate some of
   the complexity and overhead of multicast by taking a slightly
   different approach.  The basic idea in Simple Multicast is that a
   multicast group is created by generating:

   - a distinguished node C known as the "core"

   - a multicast address M

   The multicast group is then identified by the pair (C,M) rather than
   just M as in conventional IP multicast. Note that the address M does
   not have to be unique across the Internet now. Instead, only the pair
   (C,M) has to be unique. That means that every node C in the Internet
   can assign the full 28 bits worth of multicast addresses.

   In Simple Multicast, multicast address allocation and core placement
   (i.e., choosing a multicast address M and a core C for a multicast
   group) are taken out of the basic multicast protocol. End systems may
   find out about the multicast address M and the core C for a group
   through one of several possible mechanisms including email
   announcement, web advertising, SDR, DNS lookup etc.  Both SM-aware
   endnodes and SM-aware routers must recognize the combination of (C,M)
   as the identity of the group.

   Once the end systems have M and C, they then join the group by
   sending a special join message towards the core C, creating state in
   the routers along the path until the join packet hits the core or a
   router that is already on the tree for this multicast group. This
   creates a branch in the bi-directional distribution tree for the

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   group. The current IGMP mechanism for joining groups is fine,
   provided that both C and M appear in the IGMP reply. Until IGMP is
   modified to support this, the join message itself can be sent from
   the end system. If both C and M appear in the join message, then the
   first hop router can initiate the join.

   To enable incremental deployment of Simple Multicast, we provide a
   mechanism for the join message traverses non-SM aware routers. (See
   Joining a Group).

   The multicast tree formed is bi-directional, meaning that traffic can
   be injected from any point. The core is just another node in the
   tree.  The data packet contains both C and M, and routers look up the
   group based on the combination (C,M).

   Data packets would need to carry both C and M. There has been a few
   suggestions on how this may be done:  1) Define a new IP option and
   specify both C and M in it.  2) Define a new protocol and specify the
   new protocol in the 'protocol' field of the IPv4 header. Encapsulate
   the payload inside this new protocol.  This new protocol header will
   contain both C and M.  3) Map (C,M) to a unique class-D address on
   the data-link. The destination address of the data packet would be
   re-written to a unique class-D address before being forwarded on that

   Although option processing in general is more expensive, in this case
   the option processing is merely, forwarding packets by looking at an
   extra IP address in the option field. In contrast, other IP options
   such as LSR, SSR and Router Alert are more involved.  Hence, from a
   purely technical point of view, the first and second approach can be
   implemented in hardware and there is no significant difference
   between these two approaches. However, due to current hardware
   implementation convention, option processing is more likely done in
   software. As a result, we have opted to use the SM header instead.

   The third approach does not require data packets or join messages to
   carry the core address. SM nodes obtain the unique class-D address
   which maps to a group (C,M) from a special node(s) on the data-link.
   This approach is appealing because it allows SM applications to join
   a group by joining a class-D address just like conventional IP
   multicast. On the other hand, it also introduces concerns not unlike
   label switching, e.g. vulnerability to loops, ensuring the uniqueness
   of addresses at all times, ensuring all nodes on the LAN use the same
   address for a group at all times and address recycling, among others.
   In this approach, if a unique address on the data-link is not
   available for use, data cannot be forwarded. In contrast, if a packet
   cannot be label switched, it can be routed.  We are investigating the
   feasibility of this approach.

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   The SM header will carry both C and M. The reason for carrying both C
   and M in the option instead of carrying at least one of them in the
   destination address is to allow SM aware routers to co-exist with
   non-SM aware routers. The destination address in the IP packet is set
   to a reserved multicast address, the ALL-SM-NODES, when sending to
   networks with SM aware routers.  This ensures that non-SM routers
   will not forward SM multicast data packets. When the packet must hop
   over non-SM routers, the IP destination address is set to the next
   SM-aware router in the path.

   A nice feature of Simple Multicast is that, since both C and M are in
   the SM header, the destination address in the IP packet can be
   replaced with the tunnel endpoint address, and packets can be
   'tunneled' with very little work. Instead of having to add and delete
   IP headers (if the packet is encapsulated IPIP), the only work is to
   write the tunnel endpoint address into the destination address of the
   IP header..

1.3 Why Simple Multicast

   We now discuss some of the advantages of Simple Multicast.

   - One protocol is all that is needed.  Currently, we need to deal
   with two sets of multicast protocols in order to support multicast in
   the Internet: DVMRP, PIM-DM, PIM-SM and CBT etc for intra-domain
   multicast and MASC, MAAS and BGMP for inter-domain support. The
   beauty of the Simple Multicast proposal is only one multicast
   protocol is needed for both intra-domain and inter-domain.  This is
   possible because Simple Multicast is designed to be scalable.

   - Scalability.  Simple Multicast is scalable to the global Internet.
   This scalability is achieved by using a trivial multicast address
   allocation scheme, decoupling core selection and discovery from the
   multicast protocol and using bi-directional trees.  If core discovery
   is decoupled from multicast routing protocols such as PIM-SM or CBT,
   these protocols would not have to use the bootstrap mechanism to
   discover and select cores, a mechanism generally considered to be not

   - Trivial multicast address allocation. IP Multicast address
   allocation is still an unresolved problem. Dynamically allocating
   addresses such that addresses are allocated in aggregatable blocks,
   while ensuring low probability of address collision (non-uniqueness)
   is non-trivial. In Simple Multicast, since (C,M) is the identifier
   for a multicast group, address assignment becomes totally trivial,
   since addresses only have to be unique per core. Each core can have
   the full 28 bit space (over 200 million address) so we have virtually
   unlimited multicast addresses. Each core can allocate these addresses

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   independently without Internet-wide coordination.

   - Cost effective and efficient delivery trees.  It takes less state
   in routers to support a group with n senders with a single shared
   tree than with n per-sender trees. A bi-directional shared tree is as
   cost effective for delivery of traffic from source S,even if S is not
   the core, as a per-source tree rooted at S. The bi-directional shared
   tree is much more efficient for delivery of traffic from non-core
   source S than a unidirectional tree where the data from S must be
   tunneled to the core before being multicast.

   Bi-directional trees are more robust. In a unidirectional tree, the
   core is needed for relaying packets from all senders. If the core is
   down, the tree is gone. For a bi-directional tree, the core does not
   hold any particular significance. The core is just another node in
   the tree. If the core is down, the tree is merely partitioned and may
   still be used for traffic delivery if the application chooses to do

   - Incremental deployment.  Simple Multicast routers may be deployed
   along side unicast routers and other multicast routers. Traffic is
   effectively tunneled (although the actual mechanism used is more
   efficient than tunnels) through routers which do not support Simple
   Multicast. Therefore a network manager may incrementally add Simple
   Multicast routers as multicast users spread in the network.

2.0 The Design

   In this section, we describe the design of Simple Multicast and its
   basic operations in detail.

2.1 Creating a Multicast Group

   To create a group, one needs to select a core address and a multicast

   Typically most applications consist of a single high-volume source.
   For those applications, the core should be the source. For others,
   any node close to any member of the group would be a logical choice
   for core. Because the tree-building strategy (like BGMP) uses a
   single exit point from a domain or any region separated from the rest
   of the Internet through expensive links, the traffic pattern
   resembles individual trees within domains hooked together with
   inter-domain paths. In other words, if S is in your domain, then you
   will receive traffic from S through a path internal to your domain
   even if the core of the group is outside the domain. Therefore, even
   if most of the members of the group are in Europe, and one member of
   the group is in Australia, and the Australian is chosen as the core,

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   the tree will still be a very good tree. Traffic between the
   Europeans would be multicast through the tree confined within Europe,
   even though the core was in Australia.

   As the multicast addresses only need to be unique per core, each core
   has over 200 million multicast addresses for allocation. Once the
   core is chosen, some very simple mechanisms can be used to generate
   the multicast address for the chosen core, for example, querying the
   core for an address or random  generation as it is done in SDR (the
   collision rate will be significantly lower). Some permanent mapping
   of "well-known" addresses for popular groups is also feasible.

2.2 Joining a Group

   To join a group, one first has to find the core address C and
   multicast address M. It is appropriate to have a variety of
   mechanisms. A web page advertising a "singles chat group" might
   advertise its (C,M) on its web page. Or a provider of some other sort
   of service, like stock quotes, might advertise on a web page.
   Ideally, clicking on the web page would cause M and C to be
   downloaded to the client machine, which would then join the group.
   Another mechanism, for instance when arranging a private conference,
   might be to be told about M and C via the telephone, or via email.
   Yet another mechanism is to have the group (together with a name or a
   description) advertised in a directory such as SDR.

   If IGMP is extended to support SM, the host sends a membership report
   for group (C,M). The SM DR is responsible for forwarding the join off
   the LAN. This  message is sent towards the core, creating state in
   the routers along the path, so that each router knows which ports are
   in the group (C,M).

   If there are no SM routers on the LAN, a host may send an SM Join
   itself. The destination IP address of the join message is set to the
   core IP address. If a non-SM router on the LAN receives the join
   message, it will forward it to the core. Data will be tunneled to
   this endnode by an upstream SM router.  As there could be potentially
   multiple tunnels to the LAN, host SM Join should only be used when
   there is no local SM support as may be the case during initial
   deployment or when there are very few local members to justify a
   network upgrade.  If the next hop towards the core on the LAN is an
   SM router, and if it is not an SM DR itself, it will redirect the
   join to the SM DR. In this case, if data is tunneled from upstream,
   it will be tunneled to the SM router that forwards the join off the
   LAN, instead of the endnode. [Note: This approach provides a
   migration path whereby as more SM routers are deployed on the LAN,
   less tunnels are used. It also allows the co-existence of IGMP (with
   or without SM support) and host SM Join during the migration

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   If a router receives a join formulticast address (C,M), and it
   already has state for (C,M), then it merely adds that port to its set
   of ports for (C,M) and does not forward the join further.  The result
   is a tree of shortest paths from the core to each member.  Each
   router on the tree has a database of (C,M, {ports}) that tells it,
   for group (C,M), the ports that data should be forwarded to.

   The join message is sent with the Router Alert option. Since the join
   message has C as the destination address, if an intermediate router
   is not SM aware, it will just forward the join towards the core. When
   the join message reaches an SM-aware router R2, it looks at the IP
   source address of the join message, say R1. If R1 is a neighbor, R2
   adds the port from which the join was received to its list of ports
   for (C,M). If R1 is not a neighbor, R2 will add a join-ack to R1. If
   R2 is not a neighbor, R1 adds the 'tunnel port' to R2 as its 'parent
   port' for (C,M). If R2 is a neighbor, R1 just adds the port as its
   parent port for (C,M), since the packet will not need to be tunneled
   to get to R2.

   A non-member sender may join the group as a sender-only (cf uni-
   directional join in CBT). The sender will be on-tree and thus will be
   sending keep-alives and receiving heartbeat messages, and hence will
   be aware about core liveliness. Data will not be forwarded to a
   sender-only branch.

2.3 Transmitting to multicast group (C,M)

   A sender who is a member of the group, sends an IP packet with C and
   M in the SM header. The destination IP address is set to ALL-SM-
   NODES. This ensures non-SM aware nodes will ignore the packet. Only
   SM aware routers will forward the packet.

   A router that receives an SM packet looks up (C,M) in its forwarding
   table. If it knows about (C,M), it checks if the port it received the
   packet on is in its database. If not, it drops the packet. If so, it
   forwards the packet onto all the other ports listed in its database
   for (C,M). If the outgoing port is a tunnel port, the destination
   address of the IP header is replaced by the tunnel endpoint, and will
   therefore travel across routers that are not SM-aware. At the other
   end of the tunnel, the SM-aware router will replace the destination
   address with ALL-SM-NODES, or with another tunnel endpoint's address,
   depending on whether the

   packet is being forwarded on a "real port" or a "tunnel port.

   If you are not a member of the group but want to transmit to the

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   group, you place C into the IP destination address, and put C and M
   in the SM header. The packet might travel all the way to the core,
   but if it instead hits an SM-aware router R with state about (C,M)
   before it gets to the core, R will inject the packet into the tree.
   A sender-only member may transmit like a member, but will not be
   receiving any packets for this group.

2.4 Inter-domain Multicast

   Simple Multicast works both for intra-domain and inter-domain
   multicast. Because the join message of Simple Multicast carries the
   core IP address, and unicast routing already knows how to reach any
   IP address, the join message will be delivered based on the unicast
   forwarding table.

   2.4.1 Incongruent unicast and multicast topologies

   Where the unicast and multicast topologies are incongruent, BGP-4+
   [MBGP] allows a network provider to specify the path it would accept
   multicast traffic independent of the path unicast traffic would
   traverse. In the figure below, AS1 may have a peering agreement with
   AS2 to forward its unicast traffic, but a peering agreement with AS3
   to forward multicast traffic. A join from AS1 towards any cores in
   AS4 would be sent via AS3. A finer granularity of policy may specify
   certain network or core ranges that AS3 would carry traffic for.

         *     *
        *       *
      AS1       AS4
        *       *
         *     *

   The join message to C should be routed towards the exit router
   specified by BGP4+, for delivery of multicast traffic outside of the

   2.4.2 "3rd Party" Independence

   For the case in which SM is used both within and between domains,
   joins from different parts of the domain might only converge (merge)
   outside the domain. It is not desirable for a domain to depend on
   another, "3rd party", domain for the distribution of internally
   sourced traffic to other internal receivers. It is therefore
   necessary to ensure that joins from different internal receivers
   merge at a common point inside the domain.

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   BGP-4 operates on border routers (BRs) of transit domains, and
   ensures that all BRs know which of them acts as egress for a
   particular unicast prefix. Some transit domains (the elected egress
   router) inject external route information internally, and therefore,
   internal routers know in which direction to forward packets destined
   to a particular unicast prefix. In other cases, and in stub domains,
   external route information is not injected inside the domain.
   Nevertheless, the BRs of these domains know for which unicast
   prefix(es) each of them is acting as egress. Thus, domain BR routing
   knowledge ensures that joins originated inside a domain converge at a
   common point inside the domain.

   This principle can be applied recursively across a multiple levels of
   routing hierarchy.

2.5 Failure Recovery

   The situations to detect are:

   - branch unused

   - loop

   - path to core broken or changed

   - core dead or unreachable

   Any of the tree building schemes (CBT, PIM-SM, BGMP) need to solve
   these problems, and there is no need to do anything radically new.
   The only extra mechanism we've introduced is for loop detection.
   Since packets can quickly proliferate in a multicast loop, it is
   desirable to detect a loop as soon as it is formed forms.  Since SM
   uses an SM header, we can make use of a flag that will enable us to
   detect a loop on a data packet.

   The other mechanisms we specify are similar to those already in place
   for PIM, CBT, and BGMP.

2.5.1 Unused Branch

   A branch must be kept alive with a "keep-alive" message. If R
   receives at least one keep-alive message from a child in tree (C,M),
   R sends a keep-alive to its parent port for (C,M). If no keep-alive
   is received for some amount of time (at least a few keep-alive
   intervals) from some child port for (C,M), that port is removed from
   the list of ports. If there are no more child ports, then R stops
   sending keep-alives, or as an optimization "unjoins" from its parent.

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2.5.2 Loop

   It would be easy to detect a loop if we could assume that any data
   packet for which TTL became zero implied there was a loop.
   Unfortunately, some applications do an "expanding ring search" or a
   traceroute in which packets are launched with very small TTLs. It
   would be wrong to conclude there was a loop when the TTL on those
   packets expired.

   We use a flag in the SM header to indicate a packet that would
   indicate a loop if its TTL reached 0. An application launching a
   packet with a low TTL would not set that flag. SM routers do not need
   to look at the flag except on packets for which TTL expires.

   Loops can also be detected on keep-alive and heartbeat messages
   (which are sent outwards from the core...see next section). The
   keep-alive message indicates "hops from furthest leaf". A router
   collects keep-alives from its child ports and transmits a keep-alive
   that is one hop more than the maximum "hops" it receives in any keep
   alive from a child.

   The heartbeat is like a keep-alive, but from the parent. Likewise it
   carries a "distance from the core". In either case (heartbeat or
   keep-alive) if the distance gets too great a loop is suspected and
   the port is removed from the tree and the child rejoins to the core.

2.5.3 Path to core broken or changed

   A parent transmits a "heartbeat" message to its children at regular
   intervals. The heartbeat indicates whether the core is known to be
   alive. A parent continues sending heartbeat messages even if it stops
   receiving "core-alive" heartbeats from its parent. In this way a
   subtree will continue functioning even if the core is dead.  And if
   the core is not dead, the parent can simply rejoin without causing
   disruption to the nodes below it in the tree, where feasible.

   If unicast routing indicates the path to the core has changed, R
   rejoins to the core, again, without disrupting the subtree below it,
   where feasible.

   To avoid loops from forming, the parent would rejoin the core using a
   special join to splice the sub-trees. This splice message must be
   forwarded all the way to the core, creating state where there is no
   existing state. The core will acknowledge the splice message.

   If the splice message hits a downstream router, it will be forwarded
   until it reaches the router that originated this splice message. At

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   this point, the router would realize that it cannot splice the sub-
   trees without causing loops. Depending on application requirement
   which is conveyed to routers from core via heartbeat messages, the
   router could either flush the sub-tree and let leaf routers or hosts
   rejoin, or if the application desire, allow the sub-trees to continue
   functioning separately, but attempts to splice the sub-trees again
   when unicast route to the core changes. The latter makes more sense
   when there is a network partition, and the core is not reachable.

   Since the heartbeat message is generated at regular intervals even if
   a heartbeat is not received from the parent, a very long tree does
   not suffer from delay variance that might cause nodes very far from
   the core to incorrectly assume the tree was broken.

2.5.4 Core dead or unreachable

   When the core transmits a heartbeat message it sets the "core alive"
   flag. If a router has received a heartbeat message from its parent
   with the "core alive" flag set recently enough (3 heartbeat
   intervals), then it sets the "core alive" flag in its heartbeat
   messages to its children.

   If it stops receiving heartbeats with "core alive", it prunes itself
   from the old parent and rejoin (by sending a splice message) the

   The only purpose of knowing whether the core is alive or not is for
   applications to decide, if there are multiple trees for a group,
   which tree they should transmit on. (see next section)

2.5.5 Multiple Trees for Reliability

   The core should be selected to be a node that is reliable. However,
   if a group will be long-lived and there is the worry that the core
   might die, a simple mechanism is to create multiple trees (C1, M1)
   and (C2, M2) for this group. All members join both groups. They can
   transmit on either group. If "core alive" heartbeat is only received
   on group (C1, M1) that is the group that should be transmitted to.

   For applications for which instantaneous switchover is more important
   than overhead, senders should transmit on both trees.

2.6 Access Control

   We accomplish access control by allowing the core for the group to be
   configured with the set of allowed senders. The core can put the
   access rules into the heartbeat message. The heartbeat message

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   contains a list of address prefixes of authorized senders and
   unauthorized senders. If the rules do not fit into the heartbeat, or
   the core for privacy reasons does not want to advertise in advance
   all the allowed senders, it can specify that no senders other than It
   is allowed. In that case, all senders must tunnel packets to the core
   and the core will forward them. Once a sender gets permission to
   send, and is known to have data to send, the core can add that
   sender's address to the heartbeat message.

   For example, if there is some sort of authentication that must be
   done in order to get permission, the core initially disallows all
   senders, but then when S1 gets permission, it gets added to the list
   in the heartbeat message.

   Since the heartbeat message gives the access rules, all SM routers
   will refuse to forward a packet from a sender disallowed by the
   access rules.

   Border/Access routers may also have an additional Access Control List
   locally.  For instance, it may have a list of sender
   prefixes/addresses allowed to transmit multicast data.  All multicast
   traffic with source address matching these prefixes/ addresses will
   not be filtered. The Include/Exclude Senders List from the core will
   prevent these senders from sending to a group that they are not
   permitted to.

2.7 Dynamically forming more trees

   In some cases dynamically formed auxiliary trees make sense,
   especially in the inter-domain, where policy might prohibit packets
   from A to D to transit domain B. With a core in domain B, or just due
   to the shared tree that happened to get formed, packets from senders
   in A to receivers in D might traverse domain B. One simple method of
   solving the problem is to have A unicast to the core, and have the
   core send the multicast. B is still acting as a transit domain
   between A and D, but it doesn't know it.

   Another solution takes inspiration from the PIM-SM concept of using
   the shared tree to find out about per-source trees. The way it works
   is that the sender in domain A, say X, sends a message to the core C
   telling it that it would like to create a "spin-off" group, (X,M').
   Then the core C, in the heartbeat messages for group (C,M) advertises
   the spin-off trees that members of (C,M) should also join. The spin-
   off tree would, like the original tree, be kept robust through keep-

   Although this does allow creation of multiple trees to support a
   single group, this is less expensive than the PIM-SM scheme because

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   it does not always create a tree for every sender. It only does it
   when necessary, and does not need a totally separate tree for each
   sender. It only needs one per domain in which there are sources (and
   only when the shared tree doesn't work because of transit policy

2.8 Multicast Scoping

   A multicast group address can be scoped such that packets matching
   the group address are not forwarded outside the defined region.  Two
   commonly used scopes are the link-local scope and the global scope
   and they do not require configuration.  Routers merely do not forward
   the statically assigned link-local scope address (

   The third type of scoping requires network administrators to
   configure the perimeter (boundary routers) of the scoped region. This
   is called administratively scoped or local scope. At present, this is
   achieved by configuring multicast border routers (M-BRs) on a scope
   boundary with a boundary scope address range - so-called
   Administratively Scoped address range. Multicast traffic flows which
   are to be confined within a range must use a class-D address which is
   within the range. M-BRs are an impermeable boundary to any multicast
   packet with a class-D destination address that falls within any of
   its configured Administratively Scoped address ranges.

   It is perfectly feasible for SM to use exactly the same mechanism for
   achieving multicast scoping. However, multicast scoping as it is
   currently defined requires a significant amount of configuration, as
   well as co-ordination of the address space for defining scope
   boundary ranges.  Any mis-configurations can lead to multicast
   packets "leaking" across boundaries they should not.

   Multicast scope boundary configurations must conform to certain
   rules, such as the rule that boundaries must be completely contained
   within one another (the term "nesting", or "convex", are often used).
   The MZAP protocol [MZAP] is implemented on M-BRs to detect
   inconsistent administratively scoped boundary configurations. As such
   it is essentially a network management tool, it does not correct

   In SM, the group address (C,M) is scoped according to the unicast
   core address C. The advantage of this compared to Administratively
   Scoped IP  Multicast [RFC2365] is there is no requirement for these
   scoped addresses to be dynamically assigned (via AAP or MAAS) or
   announced in the scoped regions (MZAP).

2.8.1  Multicast Scoping using unicast boundaries and scope mask

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   SM has the unique ability to take advantage of the unicast routing
   system boundaries (e.g.  subnet, area, AS, AS-Confederation etc.) and
   use these as "natural" boundaries for multicast traffic, obviating
   the need for the configuration of explicit multicast boundaries.
   Furthermore, one group identifier (C, M) can be used with multiple
   scopes. It works as follows: assume a (C, M) group identifier is to
   be used for scopes A and B, with A nested inside B. A and B are
   natural unicast routing boundaries, e.g. area, and AS. A unicast
   routing system boundary is implicitly identified by a router
   aggregating routing information before propagating it over outgoing
   interfaces; this is achieved by shortening a prefix mask. For
   example, routing information inside boundary A has an associated mask
   of 24 bits. The boundary router between A and B reduces this is to 16
   bits before propagating inside B.

   Now, if a SM data packet carried a "scope mask(len)" in the SM
   header, the data packet would not pass beyond any unicast routing
   system boundary that itself propagates a shorter mask in unicast
   route updates it sends. The general rule is: a SM data packet
   carrying a "scope mask(len)" is only forwarded over those interfaces
   that aggregate unicast routing information using a mask which is
   equal length or longer than that specified in the SM data packet

                           (c) /16 | (d) /12
                           (a) /8  | (b) /20

   The figure above illustrates a router with 4 interfaces, a, b, c, d,
   each which is aggregating routes with the respective prefix. If a SM
   data packet arrives on interface (b) carrying a "scope mask(len)" of
   12, it is forwarded only over interface (c) and (d).

2.8.2  Multicast Scoping using private network boundaries

   A multicast session can be scoped within a private network if the
   core address belongs to the private address space and is not
   translated to any global address. In this case the boundary routers
   can be the filtering or NAT devices at the edge of the network. Since
   NAT devices can scope the addresses, the SM data packet itself does
   not have to carry the scope mask in the SM header.

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   Note that for administrative scoping purposes, the function in the
   NAT device which is of interest here is the filtering and address
   space separation function, not the address translation function.  An
   public node will not be able to join n private core if the private
   core address is not mapped to any global address. As a result, no
   data packets for this scoped core will be forwarded out of the NAT

   If the boundary routers are NAT devices, there is no requirement for
   the NAT devices to be SM-enabled (i.e. it knows how to translate SM
   specific packets) for the purpose of scoping SM groups. If the NAT is
   not SM-enabled, the join message will be filtered according to the
   core (IP destination) address and hence forwarding states for (C,G)
   will only be created in the defined scope. If the NAT device is SM-
   enabled, data packets can be filtered based on the core address C or
   the source address. In the case of SM dense mode, C=
   If the NAT device is not SM-enabled, since the IP destination
   address=, the packets will be filtered. Hence SM
   dense-mode traffic is scoped by default, i.e. no dense-mode data
   packets will be forwarded across any boundary. If the NAT device is
   SM-enabled, a dense-mode data packet is scoped according to its IP
   source address.  Source address is scoped in the same manner as core

   If two scoped regions intersect topologically, then the address space
   in the overlapped region cannot be used by the outer scope, as stated
   in RFC2365. This applies here as well, i.e. a scoped group address
   cannot have its core address in the address space of the overlapped
   region, to avoid the problem of the same (C,M) belonging to different
   scopes at the intersecting boundary. This implies a core address C,
   scoped within scope X, where scope X is inside scope Y, should be
   unique within scopes X and Y; and no core within scope Y should have
   that same address C. Further, any other addresses scoped within X
   should not be visible to scope Y; all addresses scoped within Y is
   visible to scope X. This address separation is already maintained by
   NAT devices.

2.8.3 Multicast Scoping in IPv6

   In IPv6, if a core address is a site-local scope address, then the
   corresponding (C,*) will be site-local scope as well,

2.9 Additional Features

   We are investigating the following additional features, which are not
   available in other multicast protocols:

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   - the ability to select dense-mode. Currently there are routers that
   implement dense mode and routers that implement sparse mode, and
   typically a domain will implement either sparse or dense mode. There
   is no way to choose, per application, which type of tree is more

   There are cases in which dense mode makes more sense for an
   application.  For example, dense mode is more appropriate if the
   number of receivers is so dense that there is very little
   optimization gained by creating a tree. Dense mode is also
   appropriate when the volume of data is sufficiently low that
   optimizing its delivery is not worth the overhead of creating and
   maintaining a tree.

   With SM we use the convention of core=FF:FF:FF:FF to indicate the
   packet should be sent via dense-mode. For such packets no tree is
   formed and routers merely forward the packet using reverse path
   forwarding.  As in DVMRP, states (S,M), where S is the source IP
   address, are created for dense mode groups.

   Routers find out whether their neighbors support SM, and other
   characteristics of their neighbors, through Hello messages. A dense
   mode SM-packet should only be sent to SM-aware neighbors. As with
   DVMRP, tunnels can be configured between SM-aware nodes to enable a
   wider range for delivery of dense-mode SM packets.

   - the ability to join a set of groups. The join message contains (C,
   M, mask). That facilitates having content parameterized by M. For
   instance, if the set of groups (C,*) is for stock information,
   certain bits in M can encode industry, country, etc. To receive
   information about all stocks, join (C,*). To receive some subset,
   join a more specific (M, mask) for core C.

2.10 SM Issues

2.10.1 Host API and Kernel Changes

   The SM architecture require changes to the host Application
   Programming Interface (API) and kernel. Host may join a group using
   either SM Join - where hosts send joins similarly to an SM router or
   IGMP extended to carry the core address as well as a class-D address.
   As noted before, host SM Join should only be used where appropriate
   e.g. when there is no local SM support.

   Taking the BSD Sockets API as an example, joining a group is achieved
   using a system call; the data structure passed with the system call
   as an argument only supports the specification of a class-D address

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   and interface (IP) address. For SM this data structure needs
   modifying to include a core address element, which can be
   concatenated with the class-D address to form SM's 8 byte group
   identifier. The kernel SM software, or IGMP software, can then make
   use of this information to generate a SM join message, or IGMP
   Report, respectively.

   Similarly, when data is sent to a group, the data structure passed to
   the send system call must include a core address. The kernel SM
   software can then place this core address in the SM header.  When an
   SM packet (identified by the IP protocol field) is received, the
   kernel SM software is invoked and the SM header is decapsulated
   before being send to the upper layer. Extending IGMP

   While not necessary, we propose using TLV in IGMP Membership Report
   messages. It is anticipated that IGMP will be extended for various
   purposes in future. The use of TLV will facilitate that.

   In addition to the class-D address, a field called the extended
   address field, for lack of a better term, is defined to carry the
   additional address require in IGMPv3, Express, SM and Distributed
   Core Multicast (DCM). The IGMP Membership Report message is encoded
   as follow:
    Type     Value
    Classic: S,G (if IGMPv3 with source specific joins)
    Express: S,E
    Simple:  C,M
    DCM:     (S),G where S is a list of channels Hence the extended
   address field carries:  i) the source address for classical IP
   multicast (IGMPv3 with source specific joins) ii) the source address
   for Express iii) the core address for SM iv) the pointer to a list of
   channels for DCM.

   Extending IGMP is perfectly feasible - it has been done before in
   upgrading from IGMPv1 to IGMPv2, and changes will be required for
   IGMPv3 if it gains wider acceptance. The kernel modifications
   required to support SM are mainly to handle the additional address
   field.  The host API change itself require only the addition of two
   parameters.  We do not, therefore, consider host changes as barriers
   to SM deployment.

2.10.2 Layer 2 Filtering

   In conventional IP multicast, each class D could be mapped to a
   distinct MAC address if 28 bits were available at the MAC layer for
   mapping. However, since only 23 bits of the MAC address is used for

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   mapping, 32 IP multicast address could potentially be mapped to one
   MAC layer address.  Hence higher layer filtering of multicast packets
   is required.

   If the low-order 4 bytes of the SM group identifier - the class-D
   address, is similarly mapped, there is the potential for each of a
   subnet's hosts to join different SM groups, with their group-ids
   differing only in the core address portion of the group-id. In this
   worst-case scenario the transmission of packets to one group will be
   received by hosts belonging to all other SM groups on the subnet; a
   group's packets only become distinguishable at the hosts' network
   layers. In a more realistic case we might reasonably expect only a
   small percentage of a subnet's hosts to receive packets

   One possible way to reduce the amount of filtering at the network
   layer, would be to statically map the core address to a multicast
   layer 2 address if we assume groups associated with a core are likely
   to be related. This would still potentially incur higher layer
   filtering of undesired groups, but only those hosts subscribed to
   group(s) associated with a particular core would be affected.

   The problem of mapping a larger-than-usual network identifier to a
   layer 2 address is not unique to SM - the problem manifests itself in
   IPv6 and EXPRESS.

   One possible way of guaranteeing layer-2 multicast destination
   address uniqueness would have special node(s) map unique layer 2
   address to the group-id. Before a node could send, receive or forward
   data, it has to obtain the layer 2 address. IGMP can be extended for
   this purpose.

   Another possible solution is to have hardware filter based on a group
   address at a specific offset and of a specific length. The NIC would
   be snooping the IP header, but software should be able to program it
   to filter addresses at the desired offset.

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3.0 Packet formats

   This section describes all the packet formats. Simple Multicast could
   be implemented as very small modifications to PIM, CBT, or BGMP.

   The packet types are:

   - data packet

   - join-request

   - join-ack

   - keep-alive (sent by child to parent)

   - heartbeat (sent by parent to child)

   - flush-tree (sent by parent to child after a loop is detected, to
   clear out state from looped tree as quickly as possible and cause
   subtree to be reformed)

   For all control packets (JOIN-REQUEST, JOIN-ACK, KEEP-ALIVE,
   HEARTBEAT, FLUSH- TREE), the "Protocol" field in the IPv4 header is
   set to SM (a new protocol field).

3.1 SM-'tunnels'

   Upstream (towards the core) or downstream SM routers may not be
   immediate neighbors, if there are non-SM routers on the path between
   them.  In a traditional tunnel between R1 and R2, R1 must add an
   extra IP header, and R2 must delete the header. SM gets the same
   functionality without adding and deleting headers. Instead all that
   is needed is to overwrite the destination address in the IP header to
   the address of the "tunnel" endpoint. The reason this can be done is
   that the information necessary for SM-routers to route the packet
   (namely C and M) are contained in the SM header.

   JOIN-REQUESTs and JOIN-ACKs allow tunnel-endpoints to learn of each
   other.  The state for a "tunnel" consists of the IP address of the
   endpoint, and the number of actual IP hops in the tunnel. The purpose
   of keeping the count of the tunnel's hops is because SM counts the
   length of the tree, so that senders can know what to set as the TTL
   in data packets.

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3.2 Data Packet Header

   IP Header

   0               1               2               3^M
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1^M
   |Version|  IHL  |Type of Service|          Total Length         |^M
   |         Identification        |Flags|      Fragment Offset    |^M
   |  Time to Live |   Protocol =  |         Header Checksum       |^M
   |               |   PROTO_SM    |                               |
   |                       Source Address                          |^M
   |                    Destination Address                        |^M

   SM Header

   |                         Core Address                          |^M
   |                     Multicast Address                         |^M
   |L|                     Reserved Flag bits                      |^M
   This SM header includes C, M, loop detect flag, where C=FF:FF:FF:FF ^M
   indicates packet should be delivered dense-mode.^M

   The 'L' bit in Flag, if set, indicates the TTL for this packet should
   never reach 0 (See Loops).^M
   The IP Destination address is ALL-SM-NODES except in the following
   - when a non-member sender transmits the packet, the destination is set
   to the core address. The purpose of this is to enable the packet^M
   to be unicasted until it hits a node that is SM-aware, at which point
   the packet is multicast along the tree from the point at which it
   the tree.
   Note that if the non-member sender has joined the group as a 'sender-only'
   (c.f. uni-directional join in CBT), then the destination address in
   the data packet is either ALL-SM-NODES or the tunnel endpoint
   (as described below).

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   - when the packet is transmitted on a tunnel port, in which case the^M
   destination address is set to the IP address of the tunnel endpoint.^M

   Note that at Layer 2, the MAC address is mapped to the Multicast Address
   M of the group (C,M), not to ALL-SM-NODES.^M

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   The following control packet header fields are as defined in CBT:
   addr_len, checksum, Payload Length and # of options.

   0               1               2               3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   |  vers |type=1 |  addr len     |         checksum              |
   |Payload Length |  # of options |           reserved            |
   |                    Join Originating Router                    |
   |                       core address C                          |
   |                       Multicast address M                     |
   |                       Multicast address mask m                |
   |  option type  |  option len   |        option value...        |

   The destination IP address in the IP header is the Core Address.  The
   JOIN-REQUEST is sent with the Router Alert Option.

   The Multicast address and corresponding mask (M,m) may appear
   multiple times. The total length of these fields is specified in the
   "addr_len" field of the common control header.

   The JOIN-REQUEST may contain the following option:

   - Originating TTL. This field is set to the TTL in the IP header of
   this JOIN- REQUEST packet. The receiving SM router ignores this
   option unless the control packet is from a SM router who is not an
   immediate neighbor. The value in this field is used to calculate the
      number of hops in a 'tunnel' = Originating TTL - TTL in the IP
   header for this packet. The value derived is placed in "# of hops in
   tunnel from you to me" in the JOIN-ACK message.

   0               1               2               3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   |     1         |       2       |        Originating TTL        |

   - Sender-Only
   The join would only be successful if the sender is on the Include

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   Senders List or NOT in the Exclude Senders List.
   The sender is attached to the tree as per uni-directional Join in CBT.

   0               1               2               3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   |     2         |       2       |       Reserved                |

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   0               1               2               3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   |  vers |type=2 |  addr len     |         checksum              |
   |Payload Length |  # of options |    # of hops in 'tunnel'      |
   |               |               |       from you to me          |
   |                    Join Originating Router                    |
   |                       core address C                          |
   |                       Multicast address M                     |
   |                       Multicast address mask m                |
   |  option type  |  option len   |        option value...        |

   The destination IP address in the IP header is the downstream IP
   source address of the JOIN-REQUEST. The JOIN_ACK is sent with the
   Router Alert Option.

   The Multicast address and corresponding mask (M,m) may appear
   multiple times. The total length of these fields is specified in the
   "addr_len" field.

   The field "# of hops in tunnel from you to me" is ignored unless the
   control packet is from a SM router who is not an immediate neighbor.
   The value in this field is saved as state for this tunnel port.

   The options from the JOIN-REQUEST are copied into the JOIN-ACK, with
   the exception of the "Originating TTL" option. The Originating TTL is
   set to the TTL in the IP header of this JOIN-ACK packet.

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   0               1               2               3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   |  vers | type=3|  addr len     |         checksum              |
   |Payload Length |  # of options |        reserved               |
   |                KEEP-ALIVE Originating Router                  |
   |                       core address C                          |
   |                       Multicast address M                     |
   |                       Multicast address mask m                |
   |  option type  |  option len   |        option value...        |

   The keep-alive message is sent from a child to a parent (towards
   core), and is sent only if a keep-alive has been received recently
   from a child. The destination IP address in the IP header is ALL-SM-
   NODES or the tunnel endpoint address.

   A single keep-alive can serve as many groups as fit into the list in
   the packet.

   (M,m) may appear multiple times. The total length of these fields is
   specified in the "addr_len" field.

   The KEEP-ALIVE may contain the following options:

   0               1               2               3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   |     1         |       10      |I|     reserved flag bits      |
   |                Include/Exclude Sender Prefix                  |
   |                Include/Exclude Sender Mask                    |

   - Include/Exclude Senders List that upstream routers should filter.
   This option may appear multiple times. The 'I' bit is set if this is
   an include sender list, and is zero if this is an exclude sender

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   0               1               2               3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   |     2         |       10      |        hop count              |
   |     Prune Time                |   # of hops in 'tunnel'       |
   |                               |       from you to me          |

   - KEEP-ALIVE Option. This option should appear the same number of
   times  as the address set (C,M,mask). It corresponds and is
   applicable to the address set (C,M,mask).

   The fields in this option are:  - Number of hops to furthest leaf for
   (C,M,mask), hop count. The hop count is incremented at every SM hop.
   In addition, when the KEEP-ALIVE is received from a tunnel port, hop
   count = hop count + number of hops in 'tunnel'.

   - Prune Time for (C,M,mask), time after which, if no KEEP-ALIVE is
   received for group (C1, M, mask), the parent should prune off this

   - 'Originating TTL'. This is as described in JOIN-REQUEST.

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   0               1               2               3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   |  vers | type=4|  addr len     |         checksum              |
   |Payload Length |  # of options |      reserved                 |
   |                 HEARTBEAT Originating Router                  |
   |                       core address C                          |
   |                       Multicast address M                     |
   |                       Multicast address mask m                |
   |  option type  |  option len   |        option value...        |

   The heartbeat is sent by a parent to a child. It is sent periodically
   regardless of whether heartbeat is received from its parent.  The
   destination IP address is set to ALL-SM-NODES or the tunnel endpoint

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   The HEARTBEAT may contain the following additional options:  -
   Include/Exclude Senders List. This is the list of allowed/prohibited
   senders to the group. The format of this option is the same the
   KEEP-ALIVE Include/Exclude Senders List, although it serves as a
   different purpose here.

   - spin-off groups (Ci,Mi). One or more spin-off groups (Ci,Mi) may be

   0               1               2               3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   |     1         |  #Groupsx8    |       reserved flag bits      |
   |                       Core Address  Ci                        |
   |                    Multicast Address Mi                       |

   - HEARTBEAT Option. This option should appear the same number of
   times as the address set (C,M,mask). It corresponds and is applicable
   to the address set (C,M,mask).

   The fields in this option are:
   0               1               2               3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   |     2         |       6       |        core distance          |
   |     Time To Shutdown          |   # of hops in 'tunnel'       |
   |                               |      from you to me           |
   |A|                    reserved                                 |

   - distance from core. Number of hops to core (C,M,mask), core
   distance. The core distance is incremented at every SM hop. In
   addition, when the KEEP-ALIVE is received from a tunnel port, core
   distance = core distance + number of hops in 'tunnel' - Time left
   before group should be closed down. (all 'ones' indicates group
   should not be torn down) - The 'A' bit if set indicates the core is
   alive or reachable

   - 'Originating TTL'. This is as described in JOIN-ACK.

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   0               1               2               3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   |  vers | type=5|  addr len     |         checksum              |
   |Payload Length |  # of options |           reserved            |
   |                 HEARTBEAT Originating Router                  |
   |                       core address C                          |
   |                       Multicast address M                     |
   |                       Multicast address mask m                |
   |  option type  |  option len   |        option value...        |

   The destination IP address is set to ALL-SM-NODES or the tunnel
   endpoint address.

   The Multicast address and corresponding mask (M,m) may appear
   multiple times. The total length of these fields is specified in the
   "addr_len" field of the common control header.

   No options are currently defined.

4 Acknowledgments

   Many people have contributed ideas to this proposal, including Harald
   Alvastrand, Joel Halpern and Fred Baker. The fact that SM is based on
   previous work in IP Multicast implies that the authors are grateful
   to everyone who has contributed to the development of IP Multicast.
   We would like to thank all members of IDMR, in particular Dino
   Farinacci, Mark Handley, Brad Cain, Dave Thaler Russ White and Ken
   Carlberg whose helpful comments have improved this proposal. Others
   that have provided helpful technical information include Matthew
   Yuen, Patrick Lee.


      DNS Based RP Placement scheme
      Dino Farinacci's presentation in the MBONED WG, 40th IETF Meeting

      Static Multicast, Internet-Draft, March 1998

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      M. Ohta, J. Crowcroft

      IDMR Mailing List discussion

      CBT, Core Based Tree Multicast Routing,
      Internet-Draft, March 1998
      Ballardie, Cain, Zhang

      PIM-SM, Protocol independent multicast-sparse mode Specification,
      RFC-2117, June 1997
      Estrin, Farinacci, Helmy, Thaler, Deering, Handley,
      Jacobson, Liu, Sharma, and Wei.

      BGMP, Border Gateway Multicast Protocol Specification,
      Internet-Draft, March 1998
      Thaler, Estrin, Meyers

      MASC, Multicast Address Set Claim Protocol,
      Internet-Draft, November 1997
      Estrin, Handley, Kumar, Thaler

      IGMP, Internet Group Management Protocol, Version 3,
      Internet-Draft, November 1998
      Cain, Deering, Thyagarajan

      "A Border Gateway Protocol 4 (BGP-4)", Y. Rekhter & T. Li,
      RFC1771, March 1995

      "Multiprotocol Extensions for BGP-4", RFC 2283, February 1998.
      Bates, T., Chandra, R., Katz, D., and Y. Rekhter,

      "The IP Network Address Translator (NAT)" RFC 1631, May 1994.
      RFC1631 Egevang, K., Francis, P.,

      "Administratively Scoped IP Multicast",
      RFC 2365, July 1998.  Meyer, D.,

      Distributed Core Multicast, L. Blazevic, J-Y. Boudec

      OGMP ftp://cs.ucl.ac.uk/darpa/ogmp.ps.gz

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Authors' Addresses

Radia Perlman
Sun Microsystems Laboratories
2 Elizabeth Drive
Chelmsford, MA 01824

Cheng-Yin Lee
Nortel Networks
PO Box 3511, Station C
Ottawa, ON K1Y 4H7, Canada

Tony Ballardie
Research Consultant

Jon Crowcroft
Department of Computer Science
University College London
Gower Street
London, WC1E 6BT, UK

Zheng Wang
Bell Labs Lucent Technologies
101 Crawfords Corner Road
Holmdel NJ 07733

Thomas Maufer
3Com Corporation
5400 Bayfront Plaza
Santa Clara, CA  95052

Christophe Diot
Sprint ATL
1 Adrian Court
Burlingame CA 94010

Joseph Thoo
Nortel Networks
PO Box 3511, Station C
Ottawa, ON K1Y 4H7, Canada

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Mark Green
@Home Networks

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