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rfc:rfc2201

Network Working Group A. Ballardie Request for Comments: 2201 Consultant Category: Experimental September 1997

       Core Based Trees (CBT) Multicast Routing Architecture

Status of this Memo

 This memo defines an Experimental Protocol for the Internet
 community.  This memo does not specify an Internet standard of any
 kind.  Discussion and suggestions for improvement are requested.
 Distribution of this memo is unlimited.

Abstract

 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
 receivers.  Most multicast algorithms build one multicast tree per
 sender (subnetwork), the tree being rooted at the sender's
 subnetwork.  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
 [2].
 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

TABLE OF CONTENTS

 1. Background...................................................  2
 2. Introduction.................................................  2
 3. Source Based Tree Algorithms.................................  3
    3.1 Distance-Vector Multicast Algorithm......................  4
    3.2 Link State Multicast Algorithm...........................  5
    3.3 The Motivation for Shared Trees..........................  5
 4. CBT - The New Architecture...................................  7
    4.1 Design Requirements......................................  7
    4.2 Components & Functions...................................  8
        4.2.1 CBT Control Message Retransmission Strategy........ 10
        4.2.2 Non-Member Sending................................. 11
 5. Interoperability with Other Multicast Routing Protocols ..... 11

Ballardie Experimental [Page 1] RFC 2201 CBT Multicast Routing Architecture September 1997

 6. Core Router Discovery........................................ 11
    6.1 Bootstrap Mechanism Overview............................. 12
 7. Summary ..................................................... 13
 8. Security Considerations...................................... 13
 Acknowledgements ............................................... 14
 References ..................................................... 14
 Author Information.............................................. 15

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
 acceptable.  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
 arbitrarily 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
 variety of multicast application requirements.  For example,
 distributed interactive simulation (DIS) applications have strict
 requirements in terms of join latency, group membership dynamics,
 group sender populations, far exceeding the requirements of many
 other multicast applications.
 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
 overlooked; 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,
 bandwidth efficiency, and protocol overhead. Other factors that can
 affect these parameters include sender set size, and wide-area
 distribution 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,

Ballardie Experimental [Page 2] RFC 2201 CBT Multicast Routing Architecture September 1997

 which need not necessarily be a group member, simply sends a
 multicast data packet, which is received by any member hosts
 connected to the same medium.
 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)
 connected 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
 multicast 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
 independently 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
 distribution tree. If we classify these algorithms into source-based
 tree algorithms and shared tree algorithms, we'll see that the
 different classes have considerably different scaling
 characteristics, and the characteristics of the resulting trees
 differ too, for example, average delay. Let's look at source-based
 tree algorithms first.

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
 accompanying overhead (in terms of state, bandwidth usage, and/or
 processing costs) is justified.  Whilst this might be the case in a
 local environment, wide-area group membership tends to be sparsely

Ballardie Experimental [Page 3] RFC 2201 CBT Multicast Routing Architecture September 1997

 distributed throughout the Internet.  There may be "pockets" of
 denseness, but if one views the global picture, wide-area groups tend
 to be sparsely distributed.
 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
 interface 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 discarded.
 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).
 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
 granularity of minutes. Unless a router's prune information is
 refreshed by the receipt of a new prune for <source, group> before
 its "lifetime" 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
 multicasting, which potentially has to support many thousands of
 active groups, each of which may be sparsely distributed, this
 technique clearly does not scale.

Ballardie Experimental [Page 4] RFC 2201 CBT Multicast Routing Architecture September 1997

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
 multicasting 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
 delivery 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
 multicast algorithm being applicable over the wide-area.  The other
 limiting factor is the processing cost of the Dijkstra calculation to
 compute the shortest-path tree for each active source.

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
 implication 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 significantly lesser impact on underlying multicast routing if
 shared trees are used.

Ballardie Experimental [Page 5] RFC 2201 CBT Multicast Routing Architecture September 1997

 In the "back of the envelope" table below we compare the amount of
 state required by CBT and DVMRP for different group sizes with
 different 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
 compared with source trees; firstly, the tree only spans a group's
 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.

Ballardie Experimental [Page 6] RFC 2201 CBT Multicast Routing Architecture September 1997

 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
 objectives:
 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
      delay of shared trees by other means. For example, a broadcast-
      type lecture with a single sender (or limited set of
      infrequently 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
      characteristic. More generally, because CBT does not incur
      source-specific state, it is particularly suited to many sender
      applications.
 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.

Ballardie Experimental [Page 7] RFC 2201 CBT Multicast Routing Architecture September 1997

      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
      reconnected 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.  CBT has well-defined interoperability mechanisms
      with DVMRP [15].

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
 generating 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 6). 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
 traverses, and this state consists of <group, incoming interface,
 outgoing 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" 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 message, which is possible due to the presence of
 the transient join state.  Once the acknowledgement reaches the

Ballardie Experimental [Page 8] RFC 2201 CBT Multicast Routing Architecture September 1997

 router that originated the join message, the new receiver can receive
 traffic sent to the group.
 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
 example, 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
 interfaces. In CBT, these interfaces are known as the "parent" and
 "child" interfaces, respectively.
 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
 information is represented with local interface addresses (or similar
 identifying 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
 interface (or to each address) listed in the entry except the
 incoming interface.
 Each router that comprises a CBT multicast tree, except the core
 router, is responsible for maintaining its upstream link, provided it
 has interested downstream receivers, i.e. the child interface list is
 not 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 224.0.0.15); this has a

Ballardie Experimental [Page 9] RFC 2201 CBT Multicast Routing Architecture September 1997

 suppressing effect on any other router for which the link is its
 parent link.  If a parent link does not support multicast
 transmission, keepalives are unicast.
 The receipt of a keepalive message over a valid child interface
 immediately 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
 information 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 information in an echo reply
 prompted by a child's echo request. The frequency 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
 preventing 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
 more than once (either natively, or encapsulated).
 In all but the exception case described above, all CBT control
 messages 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. 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

Ballardie Experimental [Page 10] RFC 2201 CBT Multicast Routing Architecture September 1997

 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
 retransmissions of the original message before an exception condition
 is raised.
 For example, 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 restarts the joining process to the group's core.

4.2.2. 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 [18]
 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
 multicast-unaware. This makes CBT particulary attractive for
 applications 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].

6. 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],
 manual placement, and the HPIM [12] approach of strictly dividing up

Ballardie Experimental [Page 11] RFC 2201 CBT Multicast Routing Architecture September 1997

 the multicast address space into many "hierarchical scopes" and using
 explicit advertising of core routers between scope levels.
 There are currently two options for CBTv2 [1] core discovery; the
 "bootstrap" mechamism, and manual placement. The bootstrap mechanisms
 (as currently specified with the PIM sparse mode protocol [18]) is
 applicable only to intra-domain core discovery, and allows for a
 "plug & play" type operation with minimal configuration. The
 disadvantage of the bootstrap mechanism is that it is much more
 difficult to affect the shape, and thus optimality, of the resulting
 distribution tree. Also, it must be implemented by all CBT routers
 within a domain.
 Manual configuration of leaf routers with <core, group> mappings is
 the other option (note: leaf routers only); this imposes a degree of
 administrative burden - the mapping for a particular group must be
 coordinated across all leaf routers to ensure consistency. Hence,
 this method does not scale particularly well. However, it is likely
 that "better" trees will result from this method, and it is also the
 only available option for inter-domain core discovery currently
 available.

6.1. Bootstrap Mechanism Overview

 It is unlikely at this stage that the bootstrap mechanism 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
 domain's elected BSR collects "Core Advertisement" messages from
 candidate core routers and periodically advertises a candidate core
 set (CC-set) to each other router in the domain, using traditional
 hopby-hop unicast forwarding. The BSR uses "Bootstrap Messages" to
 advertise the CC-set. Together, "Core Advertisements" and "Bootstrap
 Messages" comprise the "bootstrap" protocol.

Ballardie Experimental [Page 12] RFC 2201 CBT Multicast Routing Architecture September 1997

 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.
 Furthermore, bootstrap messages can carry a "group mask", potentially
 limiting 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
 relevant 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
 downstream.

7. Summary

 This document presents an architecture for intra- and inter-domain
 multicast routing.  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
 routing.  We followed by describing the features and components of
 the architecture, illustrating its simplicity and scalability.

8. Security Considerations

 Security considerations are not addressed in this memo.
 Whilst multicast security is a topic of ongoing research, multicast
 applications (users) nevertheless have the ability to take advantage
 of security services such as encryption or/and authentication
 provided such services are supported by the applications.
 RFCs 1949 and 2093/2094 discuss different ways of distributing
 multicast key material, which can result in the provision of network
 layer access control to a multicast distribution tree.
 [19] offers a synopsis of multicast security threats and proposes
 some possible counter measures.

Ballardie Experimental [Page 13] RFC 2201 CBT Multicast Routing Architecture September 1997

 Beyond these, little published work exists on the topic of multicast
 security.

Acknowledgements

 Special thanks goes to Paul Francis, NTT Japan, for the original
 brainstorming sessions that brought about this work.
 Clay Shields' work on OCBT [17] identified various failure scenarios
 with a multi-core architecture, resulting in the specification of a
 single core architecture.
 Others that have contributed to the progress of CBT include Ken
 Carlberg, Eric Crawley, Jon Crowcroft, Mark Handley, Ahmed Helmy,
 Nitin Jain, Alan O'Neill, Steven Ostrowsksi, Radia Perlman, Scott
 Reeve, Benny Rodrig, Martin Tatham, Dave Thaler, Sue Thompson, Paul
 White, and other participants of the IETF IDMR working group.
 Thanks also to 3Com Corporation and British Telecom Plc for funding
 this work.

References

 [1] Ballardie, A., "Core Based Trees (CBT version 2) Multicast
 Routing: Protocol Specification", RFC 2189, September 1997.
 [2] Multicast Routing in a Datagram Internetwork; S. Deering, PhD
 Thesis, 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] Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, RFC 1700,
 October 1994.
 [5] Internet Group Management Protocol, version 2 (IGMPv2); W.
 Fenner; Work In Progress.
 [6] Distance Vector Multicast Routing Protocol (DVMRP); T. Pusateri;
 Work In Progress.
 [7] Protocol Independent Multicast (PIM) Dense Mode Specification; D.
 Estrin et al; ftp://netweb.usc.edu/pim, Work In Progress.
 [8] Moy, J., "Multicast Extensions to OSPF", 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.

Ballardie Experimental [Page 14] RFC 2201 CBT Multicast Routing Architecture September 1997

 [10] Some Issues for an Inter-Domain Multicast Routing Protocol; D.
 Meyer;  Work In Progress.
 [11] SDP: Session Description Protocol; M. Handley and V. Jacobson;
 Work In Progress.
 [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] Postel, J., "Internet Control Message Protocol (ICMP)", STD 5,
 RFC 792, September 1981.
 [14] Perkins, C., "IP Encapsulation within IP", RFC 2003, October
 1996.
 [15] CBT - Dense Mode Multicast Interoperability; A. Ballardie; Work
 In Progress.
 [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.
 [17] The Ordered Core Based Tree Protocol; C. Shields and J.J.
 Garcia- Luna-Aceves; In Proceedings of IEEE Infocom'97, Kobe, Japan,
 April 1997; http://www.cse.ucsc.edu/research/ccrg/publications/info-
 comm97ocbt.ps.gz
 [18] Estrin, D., et. al., "Protocol Independent Multicast-Sparse Mode
 (PIM-SM): Protocol Specification", RFC 2117, June 1997.
 [19] Multicast-Specific Security Threats and Counter-Measures; A.
 Ballardie and J. Crowcroft; In Proceedings "Symposium on Network and
 Distributed System Security", February 1995, pp.2-16.

Author Information

 Tony Ballardie,
 Research Consultant
 EMail: ABallardie@acm.org

Ballardie Experimental [Page 15]

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