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Inter-Domain Multicast Routing (IDMR)                       A. Ballardie
INTERNET-DRAFT                                                Consultant

                                                              March 1997

         Core Based Trees (CBT) Multicast Routing Architecture

Status of this Memo

   This document is an Internet Draft.  Internet Drafts are working doc-
   uments of the Internet Engineering Task Force (IETF), its Areas, and
   its Working Groups. Note that other groups may also distribute work-
   ing documents as Internet Drafts).

   Internet Drafts are draft documents valid for a maximum of six
   months. Internet Drafts may be updated, replaced, or obsoleted by
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   Drafts as reference material or to cite them other than as a "working
   draft" or "work in progress."

   Please check the I-D abstract listing contained in each Internet
   Draft directory to learn the current status of this or any other In-
   ternet Draft.


   CBT is a multicast routing architecture that builds a single delivery
   tree per group which is shared by all of the group's senders and re-
   ceivers.  Most multicast algorithms build one multicast tree per
   sender (subnetwork), the tree being rooted at the sender's subnet-
   work. The primary advantage of the shared tree approach is that it
   typically offers more favourable scaling characteristics than all
   other multicast algorithms.

   The CBT protocol [1] is a network layer multicast routing protocol
   that builds and maintains a shared delivery tree for a multicast
   group.  The sending and receiving of multicast data by hosts on a
   subnetwork conforms to the traditional IP multicast service model

   CBT is progressing through the IDMR working group of the IETF.  The
   CBT protocol is described in an accompanying document [1]. For this,
   and all IDMR-related documents, see http://www.cs.ucl.ac.uk/ietf/idmr

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  1. Background................................................... 3

  2. Introduction................................................. 3

  3. Source Based Tree Algorithms................................. 5

     3.1 Distance-Vector Multicast Algorithm...................... 5

     3.2 Link State Multicast Algorithm........................... 6

     3.3 The Motivation for Shared Trees.......................... 7

  4. CBT - The New Architecture................................... 8

     4.1 Design Requirements...................................... 8

     4.2 Components & Functions................................... 10

         4.2.1 Core Router Discovery ............................. 13

         4.2.2 CBT Control Message Retransmission Strategy ....... 14

         4.2.3 Non-Member Sending................................. 15

  5. Interoperability with Other Multicast Routing Protocols ..... 15

  6. Summary ..................................................... 16

  Acknowledgements ............................................... 16

  References ..................................................... 16

  Author Information.............................................. 18

  Appendix........................................................ 19

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1.  Background

   Shared trees were first described by Wall in his investigation into
   low-delay approaches to broadcast and selective broadcast [3]. Wall
   concluded that delay will not be minimal, as with shortest-path
   trees, but the delay can be kept within bounds that may be accept-
   able.  Back then, the benefits and uses of multicast were not fully
   understood, and it wasn't until much later that the IP multicast
   address space was defined (class D space [4]). Deering's work [2] in
   the late 1980's was pioneering in that he defined the IP multicast
   service model, and invented algorithms which allow hosts to arbitrar-
   ily join and leave a multicast group. All of Deering's multicast
   algorithms build source-rooted delivery trees, with one delivery tree
   per sender subnetwork. These algorithms are documented in [2].

   After several years practical experience with multicast,  we see a
   diversity of multicast applications and correspondingly, a wide vari-
   ety of multicast application requirements.  For example, distributed
   interactive simulation (DIS) applications have strict requirements in
   terms of join latency, group membership dynamics, group sender popu-
   lations, far exceeding the requirements of many other multicast

    The multicast-capable part of the Internet, the MBONE, continues to
   expand rapidly.  The obvious popularity and growth of multicast means
   that the scaling aspects of wide-area multicasting cannot be over-
   looked; some predictions talk of thousands of groups being present at
   any one time in the Internet.

   We evaluate scalability in terms of network state maintenance, band-
   width efficiency, and protocol overhead. Other factors that can
   affect these parameters include sender set size, and wide-area dis-
   tribution of group members.

2.  Introduction

   Multicasting on the local subnetwork does not require either the
   presence of a multicast router or the implementation of a multicast
   routing algorithm; on most shared media (e.g. Ethernet), a host,
   which need not necessarily be a group member, simply sends a multi-
   cast data packet, which is received by any member hosts connected to
   the same medium.

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   For multicasts to extend beyond the scope of the local subnetwork,
   the subnet must have a multicast-capable router attached, which
   itself is attached (possibly "virtually") to another multicast-
   capable router, and so on. The collection of these (virtually) con-
   nected multicast routers forms the Internet's MBONE.

   All multicast routing protocols make use of IGMP [5], a protocol that
   operates between hosts and multicast router(s) belonging to the same
   subnetwork. IGMP enables the subnet's multicast router(s) to monitor
   group membership presence on its directly attached links, so that if
   multicast data arrives, it knows over which of its links to send a
   copy of the packet.

   In our description of the MBONE so far, we have assumed that all mul-
   ticast routers on the MBONE are running the same multicast routing
   protocol. In reality, this is not the case; the MBONE is a collection
   of autonomously administered multicast regions, each region defined
   by one or more multicast-capable border routers. Each region indepen-
   dently chooses to run whichever multicast routing protocol best suits
   its needs, and the regions interconnect via the "backbone region",
   which currently runs the Distance Vector Multicast Routing Protocol
   (DVMRP) [6]. Therefore, it follows that a region's border router(s)
   must interoperate with DVMRP.

   Different algorithms use different techniques for establishing a dis-
   tribution tree. If we classify these algorithms into source-based
   tree algorithms and shared tree algorithms, we'll see that the dif-
   ferent classes have considerably different scaling characteristics,
   and the characteristics of the resulting trees differ too, for exam-
   ple, average delay. Let's look at source-based tree algorithms first.

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3.  Source-Based Tree Algorithms

   The strategy we'll use for motivating (CBT) shared tree multicast is
   based, in part, in explaining the characteristics of source-based
   tree multicast, in particular its scalability.

   Most source-based tree multicast algorithms are often referred to as
   "dense-mode" algorithms; they assume that the receiver population
   densely populates the domain of operation, and therefore the accompa-
   nying overhead (in terms of state, bandwidth usage, and/or processing
   costs) is justified.  Whilst this might be the case in a local envi-
   ronment, wide-area group membership tends to be sparsely distributed
   throughout the Internet.  There may be "pockets" of denseness, but if
   one views the global picture, wide-area groups tend to be sparsely

   Source-based multicast trees are either built by a distance-vector
   style algorithm, which may be implemented separately from the unicast
   routing algorithm (as is the case with DVMRP), or the multicast tree
   may be built using the information present in the underlying unicast
   routing table (as is the case with PIM-DM [7]). The other algorithm
   used for building source-based trees is the link-state algorithm (a
   protocol instance being M-OSPF [8]).

3.1.  Distance-Vector Multicast Algorithm

   The distance-vector multicast algorithm builds a multicast delivery
   tree using a variant of the Reverse-Path Forwarding technique [9].
   The technique basically is as follows: when a multicast router
   receives a multicast data packet, if the packet arrives on the inter-
   face used to reach the source of the packet, the packet is forwarded
   over all outgoing interfaces, except leaf subnets with no members
   attached.  A "leaf" subnet is one which no router would use to reach
   the souce of a multicast packet. If the data packet does not arrive
   over the link that would be used to reach the source, the packet is

   This constitutes a "broadcast & prune" approach to multicast tree
   construction; when a data packet reaches a leaf router, if that
   router has no membership registered on any of its directly attached
   subnetworks, the router sends a prune message one hop back towards
   the source. The receiving router then checks its leaf subnets for
   group membership, and checks whether it has received a prune from all
   of its downstream routers (downstream with respect to the source).

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   If so, the router itself can send a prune upstream over the interface
   leading to the source.

   The sender and receiver of a prune message must cache the <source,
   group> pair being reported, for a "lifetime" which is at the granu-
   larity of minutes. Unless a router's prune information is refreshed
   by the receipt of a new prune for <source, group> before its "life-
   time" expires, that information is removed, allowing data to flow
   over the branch again. State that expires in this way is referred to
   as "soft state".

   Interestingly, routers that do not lead to group members are incurred
   the state overhead incurred by prune messages. For wide-area multi-
   casting, which potentially has to support many thousands of active
   groups, each of which may be sparsely distributed, this technique
   clearly does not scale.

3.2.  Link-State Multicast Algorithm

   Routers implementing a link state algorithm periodically collect
   reachability information to their directly attached neighbours, then
   flood this throughout the routing domain in so-called link state
   update packets. Deering extended the link state algorithm for multi-
   casting by having a router additionally detect group membership
   changes on its incident links before flooding this information in
   link state packets.

   Each router then, has a complete, up-to-date image of a domain's
   topology and group membership. On receiving a multicast data packet,
   each router uses its membership and topology information to calculate
   a shortest-path tree rooted at the sender subnetwork. Provided the
   calculating router falls within the computed tree, it forwards the
   data packet over the interfaces defined by its calculation. Hence,
   multicast data packets only ever traverse routers leading to members,
   either directly attached, or further downstream. That is, the deliv-
   ery tree is a true multicast tree right from the start.

   However, the flooding (reliable broadcasting) of group membership
   information is the predominant factor preventing the link state mul-
   ticast algorithm being applicable over the wide-area.  The other lim-
   iting factor is the processing cost of the Dijkstra calculation to
   compute the shortest-path tree for each active source.

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3.3.  The Motivation for Shared Trees

   The algorithms described in the previous sections clearly motivate
   the need for a multicast algorithm(s) that is more scalable. CBT was
   designed primarily to address the topic of scalability; a shared tree
   architecture offers an improvement in scalability over source tree
   architectures by a factor of the number of active sources (where
   source is usually a subnetwork aggregate).  Source trees scale O(S *
   G), since a distinct delivery tree is built per active source. Shared
   trees eliminate the source (S) scaling factor; all sources use the
   same shared tree, and hence a shared tree scales O(G).  The implica-
   tion of this is that applications with many active senders, such as
   distributed interactive simulation applications, and distributed
   video-gaming (where most receivers are also senders), have a signifi-
   cantly lesser impact on underlying multicast routing if shared trees
   are used.

   In the "back of the envelope" table below we compare the amount of
   state required by CBT and DVMRP for different group sizes with dif-
   ferent numbers of active sources:

     |  Number of   |                |                |                 |
     |    groups    |        10      |       100      |        1000     |
     |  Group size  |                |                |                 |
     | (# members)  |        20      |       40       |         60      |
     | No. of srcs  |    |     |     |    |     |     |    |     |      |
     |  per group   |10% | 50% |100% |10% | 50% |100% |10% | 50% | 100% |
     | No. of DVMRP |    |     |     |    |     |     |    |     |      |
     |    router    |    |     |     |    |     |     |    |     |      |
     |   entries    | 20 | 100 | 200 |400 | 2K  | 4K  | 6K | 30K | 60K  |
     | No. of CBT   |                |                |                 |
     |  router      |                |                |                 |
     |  entries     |       10       |       100      |       1000      |

            Figure 1: Comparison of DVMRP and CBT Router State

   Shared trees also incur significant bandwidth and state savings com-
   pared with source trees; firstly, the tree only spans a group's

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   receivers (including links/routers leading to receivers) -- there is
   no cost to routers/links in other parts of the network. Secondly,
   routers between a non-member sender and the delivery tree are not
   incurred any cost pertaining to multicast, and indeed, these routers
   need not even be multicast-capable -- packets from non-member senders
   are encapsulated and unicast to a core on the tree.

   The figure below illustrates a core based tree.

           b      b     b-----b
            \     |     |
             \    |     |
              b---b     b------b
             /     \  /                   KEY....
            /       \/
           b         X---b-----b          X = Core
                    / \                   b = on-tree router
                   /   \
                  /     \
                  b      b------b
                 / \     |
                /   \    |
               b     b   b

                            Figure 2: CBT Tree

4.  CBT - The New Architecture

4.1.  Design Requirements

   The CBT shared tree design was geared towards several design objec-

   +o    scalability - the CBT designers decided not to sacrifice CBT's
        O(G) scaling characteric to optimize delay using SPTs, as does
        PIM.  This was an important design decision, and one, we think,
        was taken with foresight; once multicasting becomes ubiquitous,
        router state maintenance will be a predominant scaling factor.
        It is possible in some circumstances to improve/optimize the

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        delay of shared trees by other means. For example, a broadcast-
        type lecture with a single sender (or limited set of infre-
        quently changing senders) could have its core placed in the
        locality of the sender, allowing the CBT to emulate a shortest-
        path tree (SPT) whilst still maintaining its O(G) scaling char-
        acteristic. More generally, because CBT does not incur source-
        specific state, it is particularly suited to many sender appli-

   +o    routing protocol independence - a facet of some source tree
        algorithms is that they are tied inextricably, one way or
        another, to a particular routing protocol. For example, DVMRP
        relies for its correct operation on some of the features of RIP
        (e.g. poison reverse). Similarly, M-OSPF can only be implemented
        in routers supporting OSPF, the corresponding unicast protocol.

        If multicast routing is to become ubiquitous in the Internet,
        multicast routing protocol operation should remain independent
        of particular unicast protocols to facilitate inter-domain mul-
        ticast routing in a heterogeneous unicast routing environment.

   +o    robustness - source-based tree algorithms are clearly robust; a
        sender simply sends its data, and intervening routers "conspire"
        to get the data where it needs to, creating state along the way.
        This is the so-called "data driven" approach -- there is no set-
        up protocol involved.

        It is not as easy to achieve the same degree of robustness in
        shared tree algorithms; a shared tree's core router maintains
        connectivity between all group members, and is thus a single
        point of failure.  Protocol mechanisms must be present that
        ensure a core failure is detected quickly, and the tree recon-
        nected quickly using a replacement core router.

   +o    simplicity - the CBT protocol is relatively simple compared to
        most other multicast routing protocols. This simplicity can lead
        to enhanced performance compared to other protocols.

   +o    interoperability - from a multicast perspective, the Internet is
        a collection of heterogeneous multicast regions. The protocol
        interconnecting these multicast regions is currently DVMRP [6];
        any regions not running DVMRP connect to the DVMRP "backbone" as
        stub regions. Thus, the current Internet multicast infrastruc-
        ture resembles a tree structure. CBT has well-defined interoper-
        ability mechanisms with DVMRP [15].

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        Over the longer term, the imposing of a tree structure on the
        multicast infrastructure is unacceptable for various reasons
        (e.g. administrative burden). Ideally, we want to be able to
        randomly interconnect multicast regions and use a shared tree
        protocol, such as CBT, to dynamically build inter-domain shared
        trees spanning only those domains with interested receivers (or
        those leading to interested receivers).  It is still not clear
        how we can achieve this efficiently and effectively in the pres-
        ence of heterogeneous multicast regions/domains, each with their
        differing multicast forwarding rules. Besides this, there is the
        issue of core router discovery in the inter-domain environment.
        These, and other outstanding issues regarding inter-domain mul-
        ticast routing, are discussed in [10].

        Clearly therefore, significant aspects of the inter-domain mul-
        ticast routing (IDMR) architecture remain areas of ongoing
        research.  When an IDMR architecture can be fully defined, new
        CBT interoperability mechanisms will be specified as deemed nec-
        essary to accommodate the IDMR architecture.

4.2.  CBT Components & Functions

   The CBT protocol is designed to build and maintain a shared multicast
   distribution tree that spans only those networks and links leading to
   interested receivers.

   To achieve this, a host first expresses its interest in joining a
   group by multicasting an IGMP host membership report [5] across its
   attached link. On receiving this report, a local CBT aware router
   invokes the tree joining process (unless it has already) by generat-
   ing a JOIN_REQUEST message, which is sent to the next hop on the path
   towards the group's core router (how the local router discovers which
   core to join is discussed in section 4.2.1). This join message must
   be explicitly acknowledged (JOIN_ACK) either by the core router
   itself, or by another router that is on the unicast path between the
   sending router and the core, which itself has already successfully
   joined the tree.

   The join message sets up transient join state in the routers it tra-
   verses, and this state consists of <group, incoming interface, outgo-
   ing interface>. "Incoming interface" and "outgoing interface" may be
   "previous hop" and "next hop", respectively, if the corresponding
   links do not support multicast transmission. "Previous hop" is taken
   from the incoming control packet's IP source address, and "next hop"

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   is gleaned from the routing table - the next hop to the specified
   core address. This transient  state eventually times out unless it is
   "confirmed" with a join acknowledgement (JOIN_ACK) from upstream. The
   JOIN_ACK traverses the reverse path of the corresponding join mes-
   sage, which is possible due to the presence of the transient join
   state. Once the acknowledgement reaches the router that originated
   the join message, the new receiver can receive traffic sent to the

   Loops cannot be created in a CBT tree because a) there is only one
   active core per group, and b) tree building/maintenance scenarios
   which may lead to the creation of tree loops are avoided.  For exam-
   ple, if a router's upstream neighbour becomes unreachable, the router
   immediately "flushes" all of its downstream branches, allowing them
   to individually rejoin if necessary.  Transient unicast loops do not
   pose a threat because a new join message that loops back on itself
   will never get acknowledged, and thus eventually times out.

   The state created in routers by the sending or receiving of a
   JOIN_ACK is bi-directional - data can flow either way along a tree
   "branch", and the state is group specific - it consists of the group
   address and a list of local interfaces over which join messages for
   the group have previously been acknowledged. There is no concept of
   "incoming" or "outgoing" interfaces, though it is necessary to be
   able to distinguish the upstream interface from any downstream inter-
   faces. In CBT, these interfaces are known as the "parent" and "child"
   interfaces, respectively. We recommend the parent be distinguished as
   such by a single bit in each multicast forwarding cache entry.

   With regards to the information contained in the multicast forwarding
   cache, on link types not supporting native multicast transmission an
   on-tree router must store the address of a parent and any children.
   On links supporting multicast however, parent and any child informa-
   tion is represented with local interface addresses (or similar iden-
   tifying information, such as an interface "index") over which the
   parent or child is reachable.

   When a multicast data packet arrives at a router, the router uses the
   group address as an index into the multicast forwarding cache. A copy
   of the incoming multicast data packet is forwarded over each inter-
   face (or to each address) listed in the entry except the incoming

   Each router that comprises a CBT multicast tree, except the core
   router, is responsible for maintaining its upstream link, provided it

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   has interested downstream receivers, i.e. the child interface list is
   non-NULL. A child interface is one over which a member host is
   directly attached, or one over which a downstream on-tree router is
   attached.  This "tree maintenance" is achieved by each downstream
   router periodically sending a "keepalive" message (ECHO_REQUEST) to
   its upstream neighbour, i.e. its parent router on the tree. One
   keepalive message is sent to represent entries with the same parent,
   thereby improving scalability on links which are shared by many
   groups.  On multicast capable links, a keepalive is multicast to the
   "all-cbt-routers" group (IANA assigned as; this has a
   suppressing effect on any other router for which the link is its par-
   ent link.  If a parent link does not support multicast transmission,
   keepalives are unicast.

   The receipt of a keepalive message over a valid child interface imme-
   diately prompts a response (ECHO_REPLY), which is either unicast or
   multicast, as appropriate.

   The ECHO_REQUEST does not contain any group information; the
   ECHO_REPLY does, but only periodically. To maintain consistent infor-
   mation between parent and child,
    the parent periodically reports, in a ECHO_REPLY, all groups for
   which it has state, over each of its child interfaces for those
   groups. This group-carrying echo reply is not prompted explicitly by
   the receipt of an echo request message.  A child is notified of the
   time to expect the next echo reply message containing group informa-
   tion in an echo reply prompted by a child's echo request. The fre-
   quency of  parent group reporting is at the granularity of minutes.

   It cannot be assumed all of the routers on a multi-access link have a
   uniform view of unicast routing; this is particularly the case when a
   multi-access link spans two or more unicast routing domains. This
   could lead to multiple upstream tree branches being formed (an error
   condition) unless steps are taken to ensure all routers on the link
   agree which is the upstream router for a particular group. CBT
   routers attached to a multi-access link participate in an explicit
   election mechanism that elects a single router, the designated router
   (DR), as the link's upstream router for all groups. Since the DR
   might not be the link's best next-hop for a particular core router,
   this may result in join messages being re-directed back across a
   multi-access link. If this happens, the re-directed join message is
   unicast across the link by the DR to the best next-hop, thereby pre-
   venting a looping scenario. This re-direction only ever applies to
   join messages.  Whilst this is suboptimal for join messages, which
   are generated infrequently, multicast data never traverses a link

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   more than once (either natively, or encapsulated).

   In all but the exception case described above, all CBT control mes-
   sages are multicast over multicast supporting links to the "all-cbt-
   routers" group, with IP TTL 1. When a CBT control message is sent
   over a non-multicast supporting link, it is explicitly addressed to
   the appropriate next hop.

4.2.1.  Core Router Discovery

   Core router discovery is by far the most controversial and difficult
   aspect of shared tree multicast architectures, particularly in the
   context of inter-domain multicast routing (IDMR).  There have been
   many proposals over the past three years or so, including advertising
   core addresses in a multicast session directory like "sdr" [11], man-
   ual placement, and the HPIM [12] approach of strictly dividing up the
   multicast address space into many "hierarchical scopes" and using
   explicit advertising of core routers between scope levels.

   For intra-domain core discovery, CBT has decided to adopt the "boot-
   strap" mechanism currently specified with the PIM sparse mode proto-
   col [7]. This bootstrap mechanism is scalable, robust, and does not
   rely on underlying multicast routing support to deliver core router
   information; this information is distributed via traditional unicast
   hop-by-hop forwarding.

   It is expected that the bootstrap mechanism will be specified inde-
   pendently as a "generic" RP/Core discovery mechanism in its own sepa-
   rate document. It is unlikely at this stage that the bootstrap mecha-
   nism will be appended to a well-known network layer protocol, such as
   IGMP [5] or ICMP [13], though this would facilitate its ubiquitous
   (intra-domain) deployment. Therefore, each multicast routing protocol
   requiring the bootstrap mechanism must implement it as part of the
   multicast routing protocol itself.

   A summary of the operation of the bootstrap mechanism follows. It is
   assumed that all routers within the domain implement the "bootstrap"
   protocol, or at least forward bootstrap protocol messages.

   A subset of the domain's routers are configured to be CBT candidate
   core routers. Each candidate core router periodically (default every
   60 secs) advertises itself to the domain's Bootstrap Router (BSR),
   using  "Core Advertisement" messages.  The BSR is itself elected
   dynamically from all (or participating) routers in the domain.  The

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   domain's elected BSR collects "Core Advertisement" messages from can-
   didate core routers and periodically advertises a candidate core set
   (CC-set) to each other router in the domain, using traditional hop-
   by-hop unicast forwarding. The BSR uses "Bootstrap Messages" to
   advertise the CC-set. Together, "Core Advertisements" and "Bootstrap
   Messages" comprise the "bootstrap" protocol.

   When a router receives an IGMP host membership report from one of its
   directly attached hosts, the local router uses a hash function on the
   reported group address, the result of which is used as an index into
   the CC-set. This is how local routers discover which core to use for
   a particular group.

   Note the hash function is specifically tailored such that a small
   number of consecutive groups always hash to the same core. Further-
   more, bootstrap messages can carry a "group mask", potentially limit-
   ing a CC-set to a particular range of groups. This can help reduce
   traffic concentration at the core.

   If a BSR detects a particular core as being unreachable (it has not
   announced its availability within some period), it deletes the rele-
   vant core from the CC-set sent in its next bootstrap message. This is
   how a local router discovers a group's core is unreachable; the
   router must re-hash for each affected group and join the new core
   after removing the old state. The removal of the "old" state follows
   the sending of a QUIT_NOTIFICATION upstream, and a FLUSH_TREE message

4.2.2.  CBT Control Message Retransmission Strategy

   Certain CBT control messages illicit a response of some sort. Lack of
   response may be due to an upstream router crashing, or the loss of
   the original message, or its response. To detect these events, CBT
   retransmits those control messages for which it expects a response,
   if that response is not forthcoming within the retransmission-
   interval, which varies depending on the type of message involved.
   There is an upper bound (typically 3) on the number of retransmis-
   sions of the original message before an exception condition is

   For example, the exception procedure for lack of response to a
   JOIN_REQUEST is to rehash the corresponding group to another core
   router from the advertised CC-set, then restart the joining process.

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   The exception procedure for lack of response to an ECHO_REQUEST is to
   send a QUIT_NOTIFICATION upstream and a FLUSH_TREE message downstream
   for the group. If this is router has group members attached, it
   establishes if the group's core is still active (it appears in the
   most recent CC-set advertisement from the BSR), and if so restarts
   the joining process to that core. If the contents of the CC-set indi-
   cate the core is unreachable, the router must rehash the group, then
   restart the joining process towards the newly elected core.

4.2.3.  Non-Member Sending

   If a non-member sender's local router is already on-tree for the
   group being sent to, the subnet's upstream router simply forwards the
   data packet over all outgoing interfaces corresponding to that
   group's forwarding cache entry. This is in contrast to PIM-SM [7]
   which must encapsulate data from a non-member sender, irrespective of
   whether the local router has joined the tree. This is due to PIM's
   uni-directional state.

   If the sender's subnet is not attached to the group tree, the local
   DR must encapsulate the data packet and unicast it to the group's
   core router, where it is decapsulated and disseminated over all tree
   interfaces, as specified by the core's forwarding cache entry for the
   group. The data packet encapsulation method is IP-in-IP [14].

   Routers in between a non-member sender and the group's core need not
   know anything about the multicast group, and indeed may even be mul-
   ticast-unaware. This makes CBT particulary attractive for applica-
   tions with non-member senders.

5.  Interoperability with Other Multicast Routing Protocols

   See "interoperability" in section 4.1.

   The interoperability mechanisms for interfacing CBT with DVMRP are
   defined in [15].

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6.  Summary

   This document presents an architecture for intra- and inter-domain
   multicast routing, though some aspects of inter-domain multicast
   routing remain to be solved (e.g. inter-domain core router discov-
   ery). We motivated this architecture by describing how an inter-
   domain multicast routing algorithm must scale to large numbers of
   groups present in the internetwork, and discussed why most other
   existing algorithms are less suited to inter-domain multicast rout-
   ing.  We followed by describing the features and components of the
   architecture, illustrating its simplicity and scalability. Finally,
   the Appendix summarizes simulation results comparing CBT with PIM.

7.  Acknowledgements

   Special thanks goes to Paul Francis, NTT Japan, for the original
   brainstorming sessions that brought about this work.

   Thanks also to Bibb Cain et al. (Harris Corporation) for allowing the
   publication of their simulation results in the Appendix, and the
   duplication of figures 3 and 4.

   The participants of the IETF IDMR working group have provided useful
   feedback since the inception of CBT.


  [1] Core Based Trees (CBT) Multicast Routing: Protocol Specification;
  A.  Ballardie; ftp://ds.internic.net/internet-drafts/draft-ietf-idmr-
  cbt-spec-**.txt.  Working draft, March 1997.

  [2] Multicast Routing in a Datagram Internetwork; S. Deering, PhD The-
  sis, 1991; ftp://gregorio.stanford.edu/vmtp/sd-thesis.ps.

  [3] Mechanisms for Broadcast and Selective Broadcast; D. Wall; PhD
  thesis, Stanford University, June 1980. Technical Report #90.

  [4] Assigned Numbers; J. Reynolds and J. Postel; RFC 1700, October

  [5] Internet Group Management Protocol, version 2 (IGMPv2); W. Fenner;

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  Working draft, 1996.

  [6] Distance Vector Multicast Routing Protocol (DVMRP); T. Pusateri;
  Working draft, 1997.

  [7] Protocol Independent Multicast (PIM) Sparse Mode/Dense Mode Speci-
  fications; D. Estrin et al; ftp://netweb.usc.edu/pim   Working drafts,

  [8] Multicast Extensions to OSPF; J. Moy; RFC 1584; March 1994.

  [9] Reverse path forwarding of  broadcast packets; Y.K. Dalal and R.M.
  Metcalfe; Communications of the ACM, 21(12):1040--1048, 1978.

  [10] Some Issues for an Inter-Domain Multicast Routing Protocol; D.
  Meyer; ftp://ds.internic.net/internet-drafts/draft-ietf-mboned-imrp-
  some-issues-**.txt.  Working draft, 1997.

  [11] SDP: Session Description Protocol; M. Handley and V. Jacobson;
  Working draft, 1996.

  [12] Hierarchical Protocol Independent Multicast; M. Handley, J.
  Crowcroft, I. Wakeman.  Available from:
  http://www.cs.ucl.ac.uk/staff/M.Handley/hpim.ps  and
  ftp://cs.ucl.ac.uk/darpa/IDMR/hpim.ps   Work done 1995.

  [13] Internet Control Message Protocol (ICMP); J. Postel; RFC 792;
  September 1981.

  [14] IP Encapsulation within IP; C. Perkins; RFC 2003; October 1996.

  [15] CBT - Dense Mode Multicast Interoperability; A. Ballardie;
  dvmrp-**.txt.  Working draft, March 1997.

  [16] Performance and Resource Cost Comparisons of Multicast Routing
  Algorithms for Distributed Interactive Simulation Applications; T.
  Billhartz, J. Bibb Cain, E.  Farrey-Goudreau, and D. Feig. Available
  from: http://www.epm.ornl.gov/~sgb/pubs.html; July 1995.

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Author Information

   Tony Ballardie,
   Research Consultant

   e-mail: ABallardie@acm.org

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   APPENDIX Part 1: Comparisons & Simulation Results

   Part 1 of this appendix summarises the results of in-depth simula-
   tions carried out by Harris Corp., USA, investigating the performance
   and resource cost comparisons of multicast algorithms for distributed
   interactive simulation (DIS) applications [16].  More precisely, the
   report summarises the work on the Real-time Information Transfer &
   Networking (RITN) program, comparing the cost and performance of PIM
   and CBT in a DIS environment. As we said earlier, DIS applications
   have wide-ranging requirements. We feel it is important to take into
   account a wide range of requirements so that future applications can
   be accommodated with ease; most other multicast architectures are
   tailored to the requirements of applications in the current Internet,
   particularly audio and video applications.  Figure 3 shows a compari-
   son of application requirements.

   We also present results into the study of whether source-based trees
   or shared trees are the best choice for multicasting in the RITN pro-
   gram.  In the study of shortest-path trees (SPTs) vs. shared trees,
   PIM Dense-Mode and PIM-SM with SPTs were used as SPTs, with CBT and
   PIM-SM used as shared trees. This section assumes the reader is
   familiar with the different modes of PIM [7].

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     |              |                    Paradigm                        |
     |              |----------------------------------------------------|
     | Requirement  |             |  audio/video    |    audio/video     |
     |              |    DIS      | lecture dist'n  |     conference     |
     |   senders    |    many     |  small number   |   small number     |
     |  receivers   |also senders |  many recvrs    |  also senders      |
     | no. of grps  |             |                 |                    |
     | per appl'n   |    many     | one or few      |  one or few        |
     | Data tx      |  real time  |  real time      |  real time         |
     | e2e delay    |    small    |  non-critical   |   moderate         |
     |  set up      |  real time  | non real time   |   non real time    |
     | join/leave   | rapid for   | rapid for       | can be rapid for   |
     | dynamics     | participants| receivers       |  participants      |
     |              | must be     | must be scalable|                    |
     | scalability  | scalable to | to large n/ws   |   need scalability |
     |              | large n/ws &| and large nos   |   large n/ws       |
     |              | large grps, | of recvrs per   |                    |
     |              | with large  | group           |                    |
     |              | nos senders |                 |                    |
     |              | & recvrs per|                 |                    |
     |              | group       |                 |                    |
     |              | based upon  |                 |                    |
     | multicast    | the DIS     |  rooted on src  | incl participants  |
     |   tree       | virtual     |  and includes   | and can slowly move|
     |              | space, with |  current recvrs | over phys topology |
     |              | rapid mvmt  |                 |                    |
     |              | over phys   |                 |                    |
     |              | topology    |                 |                    |

              Figure 3: Comparison of Multicast Requirements

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   The following criteria were considered in the simulations:

   +o    end-to-end delay

   +o    group join time

   +o    scalability (all participants are both senders & receivers)

   +o    bandwidth utilization

   +o    overhead traffic

   +o    protocol complexity

   A brief summary of the the results of the evaluation follow. For a
   detailed description of the simulations and test environments, refer
   to [16].

   +o    End-to-end delay. It was shown that PIM-DM and PIM-SM with SPTs
        deliver packets between 13% and 31% faster than CBT. PIM-SM has
        about the same delay characteristics as CBT. Processing delay
        was not taken into account here, and this stands in CBT's
        favour, since PIM routers are likely to have much larger routing
        tables, and thus, much greater search times.

   +o    Join time. There was very little difference between any of the

   +o    Bandwidth efficiency. It was shown that PIM-SM with shared
        trees, and PIM-SM with SPTs both required only about 4% more
        bandwidth in total, to deliver data to hosts. PIM-DM however, is
        very bandwidth inefficient, but this improves greatly as the
        network becomes densely populated with group receivers.

   +o    Overhead comparisons (for tree creation, maintenance, and tear-
        down).  CBT exhibited the lowest overhead percentage, even less
        than PIM-SM with shared trees. PIM-DM was shown to have more
        than double the overhead of PIM-SM with SPTs due to the periodic
        broadcasting & pruning.

        The Harris simulations [16] measured the average number of rout-
        ing table entries required at each router for CBT, PIM-DM, PIM-
        SM with SPTs, and PIM-SM with shared trees. The following param-
        eters were used in the simulations:

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         +  N = number of active multicast groups in the network.

         +  n = average number of senders to a group.

         +  b = fraction of groups moving to source tree in PIM-SM.

         +  c = average percentage of routers on a shared tree for a
         group (or source tree for a particular sender).

         +  s = average percentage of routers on a source-based tree for
         a group (but not on shared tree).

         +  d = average number of hops from a host to the RP.

         +  r = total number of routers in the network.

         The following results were calculated, given b = 1, c = 0.5,
         s = 0.25, and d/r = 0.05. The formulae for the calculation are
         given Part 2 of this Appendix.

         |              |               Group size parameters                |
         |              |----------------------------------------------------|
         |   Protocol   |   N = 1000  | N = 1000  | N = 20,000  | N = 20,000 |
         |              |    n = 10   |  n = 200  |   n = 10    |   n = 200  |
         |              |             |           |             |            |
         |     CBT      |     500     |    500    |   10,000    |   10,000   |
         |              |             |           |             |            |
         |  PIM-Dense   |   10,000    |  200,000  |   200,000   |  4,000,000 |
         |  PIM-Sparse  |             |           |             |            |
         |   with SPT   |    8000     |  150,500  |   160,000   |  3,010,000 |
         | PIM-Sparse,  |             |           |             |            |
         | shared tree  |    1000     |   1,500   |   20,000    |   210,000  |

                Figure 4: Comparison of Router State Requirements

    +o    Complexity comparisons. Protocol complexity, protocol traffic
         overhead, and routing table size were considered here. CBT was
         found to be considerably simpler than all other schemes, on all

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Simulation Conclusions

   In comparing CBT with the other shared tree architecture, PIM, CBT
   was found to be more favourable in terms of scalability, complexity,
   and overhead. Other characteristics were found to be similar.

   When comparing SPTs with shared trees, we find that the end-to-end
   delays of shared trees are usually acceptable, and can be improved by
   strategic core placement.  Routing table size is another important
   factor in support of shared trees, as figures 1 and 4 clearly illus-

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   Appendix: Part 2

   The following formulae were used by Harris Corp. in calculating the
   values in figure 4. The meaning of the formulae arguments precedes
   figure 4.

   +o    average no. of entries in each PIM-DM router is approximated by:

        T(PIM-DM) ~ N * n

   +o    average no. of entries a router maintains is the likelihood, c,
        that the router will be on a shared tree, times the total num-
        ber, N, of shared trees:

        T(CBT) = N * c

   +o    average no. of entries a router maintains due to each src based
        tree is the average no. of hops, d, from a host to the RP,
        divided by the number of routers, r, in the network:

        T(PIM-SM, shared tree) = N * c + N * n * d/r

   +o    average no. of routing table entries for PIM-SM with some groups
        setting up source-based trees:

        T(PIM, SPT) = N * [B * n + 1] * c + b * n * s

   For more detailed information on these formulae, refer to [16].

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