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Network Working Group A. Ballardie Request for Comments: 2189 Consultant Category: Experimental September 1997

         Core Based Trees (CBT version 2) Multicast Routing
  1. - Protocol Specification –

Status of this Memo

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


 This document describes the Core Based Tree (CBT version 2) network
 layer multicast routing protocol. CBT builds a shared multicast
 distribution tree per group, and is suited to inter- and intra-domain
 multicast routing.
 CBT may use a separate multicast routing table, or it may use that of
 underlying unicast routing, to establish paths between senders and
 receivers. The CBT architecture is described in [1].
 This document is progressing through the IDMR working group of the
 IETF.  CBT related documents include [1, 5, 6]. For all IDMR-related
 documents, see


1. Changes Since Previous version............................. 2
2. Introduction & Terminology................................. 3
3. CBT Functional Overview.................................... 3
4. CBT Protocol Specificiation Details........................ 6
   4.1 CBT HELLO Protocol..................................... 6
       4.1.1 Sending HELLOs................................... 7
       4.1.2 Receiving HELLOs................................. 7
   4.2 JOIN_REQUEST Processing................................ 8
       4.2.1 Sending JOIN_REQUESTs............................ 8
       4.2.2 Receiving JOIN_REQUESTs.......................... 8
   4.3 JOIN_ACK Processing.................................... 9
       4.3.1 Sending JOIN_ACKs................................ 9
       4.3.2 Receiving JOIN_ACKs.............................. 9

Ballardie Experimental [Page 1] RFC 2189 CBTv2 Protocl Specification September 1997

   4.4 QUIT_NOTIFICATION Processing........................... 10
       4.4.1 Sending QUIT_NOTIFICATIONs....................... 10
       4.4.2 Receiving QUIT_NOTIFICATIONs..................... 10
   4.5 CBT ECHO_REQUEST Processing............................ 11
       4.5.1 Sending ECHO_REQUESTs............................ 11
       4.5.2 Receiving ECHO_REQUESTs.......................... 12
   4.6 ECHO_REPLY Processing.................................. 12
       4.6.1 Sending ECHO_REPLYs.............................. 12
       4.6.2 Receiving ECHO_REPLYs............................ 12
   4.7 FLUSH_TREE Processing.................................. 13
       4.7.1 Sending FLUSH_TREE Messages...................... 13
       4.7.2 Receiving FLUSH_TREE Messages.................... 13
5. Non-Member Sending......................................... 13
6. Timers and Default Values.................................. 13
7. CBT Packet Formats and Message Types....................... 14
   7.1 CBT Common Control Packet Header....................... 14
   7.2 HELLO Packet Format.................................... 15
   7.3 JOIN_REQUEST Packet Format............................. 16
   7.4 JOIN_ACK Packet Format................................. 16
   7.5 QUIT_NOTIFICATION Packet Format........................ 17
   7.6 ECHO_REQUEST Packet Format............................. 18
   7.7 ECHO_REPLY Packet Format............................... 18
   7.8 FLUSH_TREE Packet Format............................... 19
8. Core Router Discovery...................................... 19
   8.1  "Bootstrap" Mechanism Overview........................ 20
   8.2  Bootstrap Message Format.............................. 21
   8.3  Candidate Core Advertisement Message Format........... 21
9. Interoperability Issues.................................... 21
10.  Security Considerations.................................. 21
Acknowledgements.............................................. 22
References.................................................... 22
Author Information............................................ 23

1. Changes from CBT version 1

 This version of the CBT protocol specification differs significantly
 from the previous version. Consequently, this version represents
 version 2 of the CBT protocol.  CBT version 2 is not, and was not,
 intended to be backwards compatible with version 1; we do not expect
 this to cause extensive compatibility problems because we do not
 believe CBT is at all widely deployed at this stage. However, any
 future versions of CBT can be expected to be backwards compatible
 with this version.

Ballardie Experimental [Page 2] RFC 2189 CBTv2 Protocl Specification September 1997

 The most significant changes to version 2 compared to version 1
 o new LAN mechanisms, including the incorporation of an HELLO
 o new simplified packet formats, with the definition of a common CBT
   control packet header.
 o each group shared tree has only one active core router.
   This specification revision is a complete re-write of the previous

2. Introduction & Terminology

 In CBT, a "core router" (or just "core") is a router which acts as a
 "meeting point" between a sender and group receivers. The term
 "rendezvous point (RP)" is used equivalently in some contexts [2]. A
 core router need not be configured to know it is a core router.
 A router that is part of a CBT distribution tree is known as an "on-
 tree" router. An on-tree router maintains active state for the group.
 We refer to a broadcast interface as any interface that supports
 multicast transmission.
 An "upstream" interface (or router) is one which is on the path
 towards the group's core router with respect to this interface (or
 router). A "downstream" interface (or router) is one which is on the
 path away from the group's core router with respect to this interface
 (or router).
 Other terminology is introduced in its context throughout the text.

3. CBT Functional Overview

 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 [3] 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 8). This join

Ballardie Experimental [Page 3] RFC 2189 CBTv2 Protocl Specification September 1997

 message must be explicitly acknowledged (JOIN_ACK) either by the core
 router itself, or by another router that is on the 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 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. A router is not considered "on-
 tree" until it has received a JOIN_ACK for a previously sent
 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.

Ballardie Experimental [Page 4] RFC 2189 CBTv2 Protocl Specification September 1997

 Data from non-member senders must be encapsulated (IP-in-IP) by the
 first-hop router, and is unicast to the group's core router.
 Consequently, no group state is required in the network between the
 first hop router and the group's core. On arriving at the core
 router, the data packet's outer encapsulating header is removed and
 the packet is disemminated over the group shared tree as described
 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 CBT "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
 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
 prompts a response (ECHO_REPLY), which is either unicast or
 multicast, as appropriate.  The ECHO_REPLY message carries a list of
 groups for which the corresponding interface is a child interface.
 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

Ballardie Experimental [Page 5] RFC 2189 CBTv2 Protocl Specification September 1997

 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. The IP source address of CBT
 control messages is the outgoing interface of the sending router. The
 IP destination address of CBT control messages is either the "all-
 cbt- routers" group address, or a unicast address, as appropriate.
 All the necessary addressing information is obtained by on-tree
 routers as part of tree set up.
 If CBT is implemented over a tunnelled topology, when sending a CBT
 control packet over a tunnel interface, the sending router uses as
 the packet's IP source address the local tunnel end point address,
 and the remote tunnel end point address as the packet's IP
 destination address.

4. Protocol Specification Details

 Details of the CBT protocol are presented in the context of a single
 router implementation.

4.1. CBT HELLO Protocol

 The HELLO protocol is used to elect a designated router (DR) on
 broadcast-type links. It is also used to elect a designated border
 router (BR) when interconnecting a CBT domain with other domains (see
 [5]). Alternatively, the designated BR may be elected as a matter of
 local policy.
 A router represents its status as a link's DR by setting the DR-flag
 on that interface; a DR flag is associated with each of a router's
 broadcast interfaces. This flag can only assume one of two values:
 TRUE or FALSE. By default, this flag is FALSE.
 A network manager can preference a router's DR eligibility by
 optionally configuring an HELLO preference, which is included in the
 router's HELLO messages.  Valid configuration values range from 1 to
 254 (decimal), 1 representing the "most eligible" value. In the
 absence of explicit configuration, a router assumes the default HELLO
 preference value of 255. The elected DR uses HELLO preference zero
 (0) in HELLO advertisements, irrespective of any configured
 preference.  The DR continues to use preference zero for as long as
 it is running.

Ballardie Experimental [Page 6] RFC 2189 CBTv2 Protocl Specification September 1997

 HELLO messages are multicast periodically to the all-cbt-routers
 group,, using IP TTL 1. The advertisement period is
 [HELLO_INTERVAL] seconds.
 HELLO messages have a suppressing effect on those routers which would
 advertise a "lesser preference" in their HELLO messages; a router
 resets its [HELLO_INTERVAL] if the received HELLO is "better" than
 its own. Thus, in steady state, the HELLO protocol incurs very little
 traffic overhead.
 The DR election winner is that which advertises the lowest HELLO
 preference, or the lowest-addressed in the event of a tie.
 The situation where two or more routers attached to the same
 broadcast link areadvertising HELLO preference 0 should never arise.
 However, should this situation arise, all but the lowest addressed
 zero advertising router relinquishes its claim as DR immediately by
 unsetting the DR flag on the corresponding interface. The
 relinquishing router(s) subsequently advertise their previously used
 preference value in HELLO advertisements.

4.1.1. Sending HELLOs

 When a router starts up, it multicasts two HELLO messages over each
 of its broadcast interfaces in successsion. The DR flag is initially
 unset (FALSE) on each broadcast interface.  This avoids the situation
 in which each router on a multi-access subnet believes it is the DR,
 thus preventing the multiple forwarding of join-requests should they
 arrive during this start up period.  If no "better" HELLO message is
 received after HOLDTIME seconds, the router assumes the role of DR on
 the corresponding interface.
 A router sends an HELLO message whenever its [HELLO_INTERVAL]
 expires.  Whenever a router sends an HELLO message, it resets its
 hello timer.

4.1.2. Receiving HELLOs

 A router does not respond to an HELLO message if the received HELLO
 is "better" than its own, or equally preferenced but lower addressed.
 A router must respond to an HELLO message if that received is lesser
 preferenced (or equally preferenced but higher addressed) than would
 be sent by this router over the same interface. This response is sent
 on expiry of an interval timer which is set between zero (0) and
 [HOLDTIME] seconds when the lesser preferenced HELLO message is

Ballardie Experimental [Page 7] RFC 2189 CBTv2 Protocl Specification September 1997

4.2. JOIN_REQUEST Processing

 A JOIN_REQUEST is the CBT control message used to register a member
 host's interest in joining the distribution tree for the group.

4.2.1. Sending JOIN_REQUESTs

 A JOIN_REQUEST can only ever be originated by a leaf router, i.e. a
 router with directly attached member hosts. This join message is sent
 hop-by-hop towards the core router for the group (see section 8).
 The originating router caches <group, NULL, upstream interface> state
 for each join it originates. This state is known as "transient join
 state".  The absence of a "downstream interface" (NULL) indicates
 that this router is the join message originator, and is therefore
 responsible for any retransmissions of this message if a response is
 not received within [RTX_INTERVAL].  It is an error if no response is
 received after [JOIN_TIMEOUT] seconds.  If this error condition
 occurs, the joining process may be re-invoked by the receipt of the
 next IGMP host membership report from a locally attached member host.
 Note that if the interface over which a JOIN_REQUEST is to be sent
 supports multicast, the JOIN_REQUEST is multicast to the all-cbt-
 routers group, using IP TTL 1.  If the link does not support
 multicast, the JOIN_REQUEST is unicast to the next hop on the unicast
 path to the group's core.

4.2.2. Receiving JOIN_REQUESTs

 On broadcast links, JOIN_REQUESTs which are multicast may only be
 forwarded by the link's DR. Other routers attached to the link may
 process the join (see below). JOIN_REQUESTs which are multicast over
 a point-to-point link are only processed by the router on the link
 which does not have a local interface corresponding to the join's
 network layer (IP) source address. Unicast JOIN_REQUESTs may only be
 processed by the router which has a local interface corresponding to
 the join's network layer (IP) destination address.
 With regard to forwarding a received JOIN_REQUEST, if the receiving
 router is not on-tree for the group, and is not the group's core
 router, and has not already forwarded a join for the same group, the
 join is forwarded to the next hop on the path towards the core. The
 join is multicast, or unicast, according to whether the outgoing
 interface supports multicast.  The router caches the following
 information with respect to the forwarded join: <group, downstream
 interface, upstream interface>. Subsequent JOIN_REQUESTs received for
 the same group are cached until this router has received a JOIN_ACK
 for the previously sent join, at which time any cached joins can also
 be acknowledged.

Ballardie Experimental [Page 8] RFC 2189 CBTv2 Protocl Specification September 1997

 If this transient join state is not "confirmed" with a join
 acknowledgement (JOIN_ACK) message from upstream, the state is timed
 out after [TRANSIENT_TIMEOUT] seconds.
 If the receiving router is the group's core router, the join is
 "terminated" and acknowledged by means of a JOIN_ACK. Similarly, if
 the router is on-tree and the JOIN_REQUEST arrives over an interface
 that is not the upstream interface for the group, the join is
 If a JOIN_REQUEST for the same group is scheduled to be sent over the
 corresponding interface (i.e. awaiting a timer expiry), the
 JOIN_REQUEST is unscheduled.
 If this router has a cache-deletion-timer [CACHE_DEL_TIMER] running
 on the arrival interface for the group specified in a multicast join,
 the timer is cancelled.

4.3. JOIN_ACK Processing

 A JOIN_ACK is the mechanism by which an interface is added to a
 router's multicast forwarding cache; thus, the interface becomes part
 of the group distribution tree.

4.3.1. Sending JOIN_ACKs

 The JOIN_ACK is sent over the same interface as the corresponding
 JOIN_REQUEST was received. The sending of the acknowledgement causes
 the router to add the interface to its child interface list in its
 forwarding cache for the group, if it is not already.
 A JOIN_ACK is multicast or unicast, according to whether the outgoing
 interface supports multicast transmission or not.

4.3.2. Receiving JOIN_ACKs

 The group and arrival interface must be matched to a <group, ....,
 upstream interface> from the router's cached transient state. If no
 match is found, the JOIN_ACK is discarded.  If a match is found, a
 CBT forwarding cache entry for the group is created, with "upstream
 interface" marked as the group's parent interface.
 If "downstream interface" in the cached transient state is NULL, the
 JOIN_ACK has reached the originator of the corresponding
 JOIN_REQUEST; the JOIN_ACK is not forwarded downstream.  If
 "downstream interface" is non-NULL, a JOIN_ACK for the group is sent

Ballardie Experimental [Page 9] RFC 2189 CBTv2 Protocl Specification September 1997

 over the "downstream interface" (multicast or unicast, accordingly).
 This interface is installed in the child interface list of the
 group's forwarding cache entry.
 Once transient state has been confirmed by transferring it to the
 forwarding cache, the transient state is deleted.


 A CBT tree is "pruned" in the direction downstream-to-upstream
 whenever a CBT router's child interface list for a group becomes


 A QUIT_NOTIFICATION is sent to a router's parent router on the tree
 whenever the router's child interface list becomes NULL. If the link
 over which the quit is to be sent supports multicast transmission, if
 the sending router is the link's DR the quit is unicast, otherwise it
 is multicast.
 A QUIT_NOTIFICATION is not acknowledged; once sent, all information
 pertaining to the group it represents is deleted from the forwarding
 cache immediately.
 To help ensure consistency between a child and parent router given
 the potential for loss of a QUIT_NOTIFICATION, a total of [MAX_RTX]
 QUIT_NOTIFICATIONs are sent, each HOLDTIME seconds after the previous
 The sending of a quit (the first) also invokes the sending of a
 FLUSH_TREE message over each downstream interface for the
 corresponding group.

4.4.2. Receiving QUIT_NOTIFICATIONs

 The group reported in the QUIT_NOTIFICATION must be matched with a
 forwarding cache entry. If no match is found, the QUIT_NOTIFICATION
 is ignored and discarded.  If a match is found, if the arrival
 interface is a valid child interface in the group entry, how the
 router proceeds depends on whether the QUIT_NOTIFICATION was
 multicast or unicast.
 If the QUIT_NOTIFICATION was unicast, the corresponding child
 interface is deleted from the group's forwarding cache entry, and no
 further processing is required.

Ballardie Experimental [Page 10] RFC 2189 CBTv2 Protocl Specification September 1997

 If the QUIT_NOTIFICATION was multicast, and the arrival interface is
 a valid child interface for the specified group, the router sets a
 cache-deletion-timer [CACHE_DEL_TIMER].
 Because this router might be acting as a parent router for multiple
 downstream routers attached to the arrival link, [CACHE_DEL_TIMER]
 interval gives those routers that did not send the  QUIT_NOTIFICA-
 TION, but received it over their parent interface, the opportunity to
 ensure that the parent router does not remove the link from its child
 interface list.  Therefore, on receipt of a multicast
 QUIT_NOTIFICATION over a parent interface, a receiving router
 schedules a JOIN_REQUEST for the group for sending at a random
 interval between 0 (zero) and HOLDTIME seconds.  If a multicast
 JOIN_REQUEST is received over the corresponding interface (parent)
 for the same group before this router sends its own scheduled
 JOIN_REQUEST, it unschedules the multicasting of its own

4.5. ECHO_REQUEST Processing

 The ECHO_REQUEST message allows a child to monitor reachability to
 its parent router for a group (or range of groups if the parent
 router is the parent for multiple groups). Group information is not
 carried in ECHO_REQUEST messages.

4.5.1. Sending ECHO_REQUESTs

 Whenever a router creates a forwarding cache entry due to the receipt
 of a JOIN_ACK, the router begins the periodic sending of ECHO_REQUEST
 messages over its parent interface. The ECHO_REQUEST is multicast to
 the "all-cbt-routers" group over multicast-capable interfaces, unless
 the sending router is the DR on the interface over which the
 ECHO_REQUEST is being sent, in which case it is unicast (as is the
 corresponding ECHO_REPLY).
 ECHO_REQUEST messages are sent at [ECHO_INTERVAL] second intervals.
 Whenever an ECHO_REQUEST is sent, [ECHO_INTERVAL] is reset.
 If no response is forthcoming, any groups present on the parent
 interface will eventually expire [GROUP_EXPIRE_TIME]. This results in
 the sending of a QUIT_NOTIFICATION upstream, and sends a FLUSH_TREE
 message downstream for each group for which the upstream interface
 was the parent interface.

Ballardie Experimental [Page 11] RFC 2189 CBTv2 Protocl Specification September 1997

4.5.2. Receiving ECHO_REQUESTs

 If an ECHO_REQUEST is received over any valid child interface, the
 receiving router schedules an ECHO_REPLY message for sending over the
 same interface; the scheduled interval is between 0 (zero) and
 HOLDTIME seconds. This message is multicast to the "all-cbt-routers"
 group over multicast-capable interfaces, and unicast otherwise.
 If a multicast ECHO_REQUEST message arrives via any valid parent
 interface, the router resets its [ECHO_INTERVAL] timer for that
 upstream interface, thereby suppressing the sending of its own
 ECHO_REQUEST over that upstream interface.

4.6. ECHO_REPLY Processing

 ECHO_REPLY messages allow a child to monitor the reachability of its
 parent, and help ensure the group state information is consistent
 between them.

4.6.1. Sending ECHO_REPLY messages

 An ECHO_REPLY message is sent in response to receiving an
 ECHO_REQUEST message, provided the ECHO_REQUEST is received over any
 one of this router's valid child interfaces. An ECHO_REPLY reports
 all groups for which the link is its child.
 ECHO_REPLY messages are unicast or multicast, as appropriate.

4.6.2. Receiving ECHO_REPLY messages

 An ECHO_REPLY message must be received via a valid parent interface.
 For each group reported in an ECHO_REPLY, the downstream router
 attempts to match the group with one in its forwarding cache for
 which the arrival interface is the group's parent interface. For each
 successful match, the entry is "refreshed". If however, after
 [GROUP_EXPIRE_TIME] seconds a group has not been "refreshed", a
 QUIT_NOTIFICATION is sent upstream, and a FLUSH_TREE message is sent
 downstream, for the group.
 If this router has directly attached members for any of the flushed
 groups, the receipt of an IGMP host membership report for any of
 those groups will prompt this router to rejoin the corresponding

Ballardie Experimental [Page 12] RFC 2189 CBTv2 Protocl Specification September 1997

4.7. FLUSH_TREE Processing

 The FLUSH_TREE (flush) message is the mechanism by which a router
 invokes the tearing down of all its downstream branches for a
 particular group. The flush message is multicast to the "all-cbt-
 routers" group when sent over multicast-capable interfaces, and
 unicast otherwise.

4.7.1. Sending FLUSH_TREE messages

 A FLUSH_TREE message is sent over each downstream (child) interface
 when a router has lost reachability with its parent router for the
 group (detected via ECHO_REQUEST and ECHO_REPLY messages). All group
 state is removed from an interface over which a flush message is
 sent.  A flush can specify a single group, or all groups

4.7.2. Receiving FLUSH_TREE messages

 A FLUSH_TREE message must be received over the parent interface for
 the specified group, otherwise the message is discarded.
 The flush message must be forwarded over each child interface for the
 specified group.
 Once the flush message has been forwarded, all state for the group is
 removed from the router's forwarding cache.

5. Non-Member Sending

 Data can be sent to a CBT tree by a sender not attached to the group
 tree.  The sending host originates native multicast data, which is
 promiscuously received by a local router, which must be CBT capable.
 It is assumed the local CBT router knows about the relevant <core,
 group> mapping, and thus can encapsulate (IP-in-IP) the data packet
 and unicast it to the corresponding core router. On arriving at the
 core router, the data packet is decapsulated and disemminated over
 the group tree in the manner already described.

6. Timers and Default Values

 This section provides a summary of the timers described above,
 together with their recommended default values. Other values may be
 configured; if so, the values used should be consistent across all
 CBT routers attached to the same network.
 o    [HELLO_INTERVAL]: the interval between sending an HELLO message.
      Default: 60 seconds.

Ballardie Experimental [Page 13] RFC 2189 CBTv2 Protocl Specification September 1997

 o    [HELLO_PREFERENCE]: Default: 255.
 o    [HOLDTIME]: generic response interval. Default: 3 seconds.
 o    [MAX_RTX]: default maximum number of retransmissions. Default 3.
 o    [RTX_INTERVAL]: message retransmission time. Default: 5 seconds.
 o    [JOIN_TIMEOUT]: raise exception due to tree join failure.
      Default: 3.5 times [RTX_INTERVAL].
 o    [TRANSIENT_TIMEOUT]: delete (unconfirmed) transient state.
      Default: (1.5*RTX_INTERVAL) seconds.
 o    [CACHE_DEL_TIMER]: remove child interface from forwarding cache.
      Default: (1.5*HOLDTIME) seconds.
 o    [GROUP_EXPIRE_TIME]: time to send a QUIT_NOTIFICATION to our
      non-responding parent.  Default: (1.5*ECHO_INTERVAL).
 o    [ECHO_INTERVAL]: interval between sending ECHO_REQUEST to parent
      routers.  Default: 60 seconds.
 o    [EXPECTED_REPLY_TIME]: consider parent unreachable. Default: 70

7. CBT Packet Formats and Message Types

 CBT control packets are encapsulated in IP. CBT has been assigned IP
 protocol number 7 by IANA [4].

7.1. CBT Common Control Packet Header

 All CBT control messages have a common fixed length header.
     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  |  addr len     |         checksum              |
                Figure 1. CBT Common Control Packet Header
 This CBT specification is version 2.

Ballardie Experimental [Page 14] RFC 2189 CBTv2 Protocl Specification September 1997

 CBT packet types are:
 o    type 0: HELLO
 o    type 1: JOIN_REQUEST
 o    type 2: JOIN_ACK
 o    type 4: ECHO_REQUEST
 o    type 5: ECHO_REPLY
 o    type 6: FLUSH_TREE
 o    type 7: Bootstrap Message (optional)
 o    type 8: Candidate Core Advertisement (optional)
 o    Addr Length: address length in bytes of unicast or multicast
      addresses carried in the control packet.
 o    Checksum: the 16-bit one's complement of the one's complement
      sum of the entire CBT control packet.

7.2. HELLO Packet Format

  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
 |                    CBT Control Packet Header                  |
 |  Preference   |  option type  |  option len   |  option value |
                   Figure 2. HELLO Packet Format
 HELLO Packet Field Definitions:
 o    preference: sender's HELLO preference.
 o    option type: the type of option present in the "option value"
      field.  One option type is currently defined: option type 0
      (zero) = BR_HELLO; option value 0 (zero); option length 0
      (zero). This option type is used with HELLO messages sent by a

Ballardie Experimental [Page 15] RFC 2189 CBTv2 Protocl Specification September 1997

      border router (BR) as part of designated BR election (see [5]).
 o    option len: length of the "option value" field in bytes.
 o    option value: variable length field carrying the option value.

7.3. JOIN_REQUEST Packet Format

     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
    |                    CBT Control Packet Header                  |
    |                          group address                        |
    |                          target router                        |
    |                        originating router                     |
    |  option type  |  option len   |        option value           |
                   Figure 3. JOIN_REQUEST Packet Format
    JOIN_REQUEST Field Definitions
 o    group address: multicast group address of the group being joined.
      For a "wildcard" join (see [5]), this field contains the value of
 o    target router: target (core) router for the group.
 o    originating router: router that originated this JOIN_REQUEST.
 o    option type, option len, option value: see HELLO packet format,
      section 7.2.

7.4. JOIN_ACK Packet Format

    JOIN_ACK Field Definitions
 o    group address: multicast group address of the group being joined.
 o    target router: router (DR) that originated the corresponding

Ballardie Experimental [Page 16] RFC 2189 CBTv2 Protocl Specification September 1997

     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
    |                    CBT Control Packet Header                  |
    |                          group address                        |
    |                           target router                       |
    |  option type  |  option len   |         option value          |
                     Figure 4. JOIN_ACK Packet Format
 o    option type, option len, option value: see HELLO packet format,
      section 7.2.

7.5. QUIT_NOTIFICATION Packet Format

     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
    |                    CBT Control Packet Header                  |
    |                          group address                        |
    |                    originating child router                   |
                Figure 5. QUIT_NOTIFICATION Packet Format
    QUIT_NOTIFICATION Field Definitions
 o    group address: multicast group address of the group being joined.
 o    originating child router: address of the router that
      originates the QUIT_NOTIFICATION.

Ballardie Experimental [Page 17] RFC 2189 CBTv2 Protocl Specification September 1997

7.6. ECHO_REQUEST Packet Format

     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
    |                    CBT Control Packet Header                  |
    |                    originating child router                   |
                   Figure 6. ECHO_REQUEST Packet Format
    ECHO_REQUEST Field Definitions
 o    originating child router: address of the router that
      originates the ECHO_REQUEST.

7.7. ECHO_REPLY Packet Format

     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
    |                    CBT Control Packet Header                  |
    |                    originating parent router                  |
    |                       group address #1                        |
    |                       group address #2                        |
    |                           ......                              |
    |                       group address #n                        |
                    Figure 7. ECHO_REPLY Packet Format
    ECHO_REPLY Field Definitions
 o    oringinating parent router: address of the router originating
      this ECHO_REPLY.
 o    group address: a list of multicast group addresses for which

Ballardie Experimental [Page 18] RFC 2189 CBTv2 Protocl Specification September 1997

      this router considers itself a parent router w.r.t. the link
      over which this message is sent.

7.8. FLUSH_TREE Packet Format

     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
    |                    CBT Control Packet Header                  |
    |                         group address                         |
    |                           ......                              |
    |                       group address #n                        |
                    Figure 8. FLUSH_TREE Packet Format
    FLUSH_TREE Field Definitions
 o    group address(es): multicast group address(es) of the group(s)
      being "flushed".

8. Core Router Discovery

 There are two available options for CBTv2 core discovery; the
 "bootstrap" mechanism (as currently specified with the PIM sparse
 mode protocol [2]) 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, to be applicable, all CBT
 routers within a domain must implement the bootstrap mechanism.
 The other option is to manually configure leaf routers with <core,
 group> mappings (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

Ballardie Experimental [Page 19] RFC 2189 CBTv2 Protocl Specification September 1997

8.1. "Bootstrap" Mechanism Overview

 It is unlikely that the bootstrap mechanism will be appended to a
 well-known network layer protocol, such as IGMP [3], 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
 (details are provided in [7]). 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
 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.
 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

Ballardie Experimental [Page 20] RFC 2189 CBTv2 Protocl Specification September 1997

8.2. Bootstrap Message Format

   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
  |             CBT common control packet header                  |
  |      For full Bootstrap Message specification, see [7]        |
                 Figure 9. Bootstrap Message Format

8.3. Candidate Core Advertisement Message Format

   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
  |              CBT common control packet header                 |
  |   For full Candidate Core Adv. Message specification, see [7] |
       Figure 10. Candidate Core Advertisement Message Format

9. Interoperability Issues

 Interoperability between CBT and DVMRP is specified in [5].
 Interoperability with other multicast protocols will be fully
 specified as the need arises.

10. 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.
 [9] offers a synopsis of multicast security threats and proposes some
 possible counter measures.

Ballardie Experimental [Page 21] RFC 2189 CBTv2 Protocl Specification September 1997

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


 Special thanks goes to Paul Francis, NTT Japan, for the original
 brainstorming sessions that brought about this work.
 The emergence of CBTv2 owes much to Clay Shields and his work on
 Ordered CBT (OCBT) [8]. Clay identified and proved several failure
 modes of CBT as it was specified with multiple cores, and also
 suggested using an unreliable quit mechanism, which appears in this
 specification as the QUIT_NOTIFICATION. Clay has also provided more
 general constructive comments on the CBT architecture and
 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.


 [1] Core Based Trees (CBT) Multicast Routing Architecture; A.
 Ballardie; RFC 2201, September 1997.
 [2] Protocol Independent Multicast (PIM) Sparse Mode/Dense Mode; D.
 Estrin et al;   Working drafts, 1996.
 [3] Internet Group Management Protocol, version 2 (IGMPv2); W.
 v2-**.txt.  Working draft, 1996.
 [4] Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, RFC 1700,
 October 1994.
 [5] CBT Border Router Specification for Interconnecting a CBT Stub
 Region to a DVMRP Backbone; A. Ballardie;
 interop-**.txt.  Working draft,  March 1997.
 [6] Ballardie, A., "Scalable Multicast Key Distribution", RFC 1949,
 July 1996.

Ballardie Experimental [Page 22] RFC 2189 CBTv2 Protocl Specification September 1997

 [7] A Dynamic Bootstrap Mechanism for Rendezvous-based Multicast
 Routing; D. Estrin et al.; Technical Report;
 [8] The Ordered Core Based Tree Protocol; C. Shields and J.J. Garcia-
 Luna-Aceves; In Proceedings of IEEE Infocom'97, Kobe, Japan, April
 [9]  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

Ballardie Experimental [Page 23]

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