Internet Draft

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

July 1997

Core Based Trees (CBT version 2) Multicast Routing

-- Protocol Specification --

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
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draft" or "work in progress."

Please check the I-D abstract listing contained in each Internet
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Internet Draft.

Abstract

This document describes the Core Based Tree (CBT version 2) network
layer multicast routing protocol. CBT builds a shared multicast dis-
tribution 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 http://www.cs.ucl.ac.uk/ietf/idmr.

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TABLE OF CONTENTS

1. Changes Since Previous version............................. 4

2. Introduction & Terminology................................. 4

3. CBT Functional Overview.................................... 5

4. CBT Protocol Specificiation Details........................ 8

4.1 CBT HELLO Protocol..................................... 8

4.1.1 Sending HELLOs................................... 9

4.1.2 Receiving HELLOs................................. 9

4.2 JOIN_REQUEST Processing................................ 9

4.2.1 Sending JOIN_REQUESTs............................ 10

4.2.2 Receiving JOIN_REQUESTs.......................... 10

4.3 JOIN_ACK Processing.................................... 11

4.3.1 Sending JOIN_ACKs................................ 11

4.3.2 Receiving JOIN_ACKs.............................. 11

4.4 QUIT_NOTIFICATION Processing........................... 12

4.4.1 Sending QUIT_NOTIFICATIONs....................... 12

4.4.2 Receiving QUIT_NOTIFICATIONs..................... 12

4.5 CBT ECHO_REQUEST Processing............................ 13

4.5.1 Sending ECHO_REQUESTs............................ 13

4.5.2 Receiving ECHO_REQUESTs.......................... 14

4.6 ECHO_REPLY Processing.................................. 14

4.6.1 Sending ECHO_REPLYs.............................. 14

4.6.2 Receiving ECHO_REPLYs............................ 14

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4.7 FLUSH_TREE Processing.................................. 15

4.7.1 Sending FLUSH_TREE Messages...................... 15

4.7.2 Receiving FLUSH_TREE Messages.................... 15

5. Non-Member Sending......................................... 16

6. Timers and Default Values.................................. 16

7. CBT Packet Formats and Message Types....................... 17

7.1 CBT Common Control Packet Header....................... 17

7.2 HELLO Packet Format.................................... 18

7.3 JOIN_REQUEST Packet Format............................. 19

7.4 JOIN_ACK Packet Format................................. 19

7.5 QUIT_NOTIFICATION Packet Format........................ 20

7.6 ECHO_REQUEST Packet Format............................. 21

7.7 ECHO_REPLY Packet Format............................... 21

7.8 FLUSH_TREE Packet Format............................... 22

8. Core Router Discovery...................................... 22

8.1 "Bootstrap" Mechanism Overview........................ 23

8.2 Bootstrap Message Format.............................. 24

8.3 Candidate Core Advertisement Message Format........... 24

9. Interoperability Issues.................................... 25

Acknowledgements.............................................. 26

References.................................................... 26

Author Information............................................ 28

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1. Changes from CBT version 1

This version of the CBT protocol specification differs significantly
from the previous version. Consequently, this version represents ver-
sion 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.

The most significant changes to version 2 compared to version 1
include:

+o new LAN mechanisms, including the incorporation of an HELLO proto-
col.

+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
revision.

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 "ren-
dezvous 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 mul-
ticast 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).

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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 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 8). This join 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 tra-
verses, and this state consists of . "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 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
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 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.

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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. A router is not considered "on-tree" until
it has received a JOIN_ACK for a previously sent JOIN_REQUEST.

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.

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. Conse-
quently, 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 below.

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
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 224.0.0.15); this has a
suppressing effect on any other router for which the link is its

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parent link. If a parent link does not support multicast transmis-
sion, keepalives are unicast.

The receipt of a keepalive message over a valid child interface
prompts a response (ECHO_REPLY), which is either unicast or multi-
cast, 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 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
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. The IP source address of CBT control
messages is the outgoing interface of the sending router. The IP des-
tination 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 destina-
tion address.

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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 option-
ally 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 prefer-
ence. The DR continues to use preference zero for as long as it is
running.

HELLO messages are multicast periodically to the all-cbt-routers
group, 224.0.0.15, 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 broad-
cast link are advertising HELLO preference 0 should never arise. How-
ever, should this situation arise, all but the lowest addressed zero-

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advertising router relinquishes its claim as DR immediately by unset-
ting 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
received.

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.

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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 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 multi-
cast, 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 infor-
mation with respect to the forwarded join: . 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.

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If this transient join state is not "confirmed" with a join acknowl-
edgement (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 "ter-
minated" 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 acknowl-
edged.

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 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.

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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 "down-
stream interface" is non-NULL, a JOIN_ACK for the group is sent 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.

4.4. QUIT_NOTIFICATION Processing

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

4.4.1. Sending QUIT_NOTIFICATIONs

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
one.

The sending of a quit (the first) also invokes the sending of a
FLUSH_TREE message over each downstream interface for the correspond-
ing 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

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interface is a valid child interface in the group entry, how the
router proceeds depends on whether the QUIT_NOTIFICATION was multi-
cast or unicast.

If the QUIT_NOTIFICATION was unicast, the corresponding child inter-
face is deleted from the group's forwarding cache entry, and no fur-
ther processing is required.

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_NOTIFICA-
TION 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 JOIN_REQUEST.

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.

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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.

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 HOLD-
TIME 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

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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
tree(s).

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 partic-
ular group. The flush message is multicast to the "all-cbt-routers"
group when sent over multicast-capable interfaces, and unicast other-
wise.

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
(INADDR_ANY).

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.

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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 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.

+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).

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+o [ECHO_INTERVAL]: interval between sending ECHO_REQUEST to parent
routers. Default: 60 seconds.

+o [EXPECTED_REPLY_TIME]: consider parent unreachable. Default: 70
seconds.

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.

Figure 1. CBT Common Control Packet Header

This CBT specification is version 2.

CBT packet types are:

+o type 0: HELLO

+o type 1: JOIN_REQUEST

+o type 2: JOIN_ACK

+o type 3: QUIT_NOTIFICATION

+o type 4: ECHO_REQUEST

+o type 5: ECHO_REPLY

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+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

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 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.

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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
INADDR_ANY.

+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
JOIN_REQUEST.

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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 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

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.

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7.6. ECHO_REQUEST Packet Format

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

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+o oringinating parent router: address of the router originating this
ECHO_REPLY.

+o group address: a list of multicast group addresses for which 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 "boot-
strap" 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.

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The other option is to manually configure leaf routers with 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
available.

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 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

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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 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
downstream.

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

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9. Interoperability Issues

Interoperability between CBT and DVMRP is specified in [5].

Interoperability with other multicast protocols will be fully speci-
fied as the need arises.

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Acknowledgements

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 sug-
gested 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 specifica-
tion.

Others that have contributed to the progress of CBT include Ken Carl-
berg, 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] Core Based Trees (CBT) Multicast Routing Architecture;
A. Ballardie; ftp://ds.internic.net/internet-drafts/draft-ietf-idmr-
cbt-arch-**.txt. Working draft, April 1997.

[2] Protocol Independent Multicast (PIM) Sparse Mode/Dense Mode; D.
Estrin et al; ftp://netweb.usc.edu/pim Working drafts, 1996.

[3] Internet Group Management Protocol, version 2 (IGMPv2); W. Fenner;
ftp://ds.internic.net/internet-drafts/draft-ietf-idmr-igmp-v2-**.txt.
Working draft, 1996.

[4] Assigned Numbers; J. Reynolds and J. Postel; RFC 1700, October
1994.

[5] CBT Border Router Specification for Interconnecting a CBT Stub
Region to a DVMRP Backbone; A. Ballardie; ftp://ds.internic.net/inter-
net-drafts/draft-ietf-idmr-cbt-dm-interop-**.txt. Working draft,
March 1997.

[6] Scalable Multicast Key Distribution; A. Ballardie; RFC 1949, July

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1996.

[7] A Dynamic Bootstrap Mechanism for Rendezvous-based Multicast Rout-
ing; D. Estrin et al.; Technical Report; ftp://catarina.usc.edu/pim

[8] 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

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Author Information:

Tony Ballardie,
Research Consultant,

e-mail: ABallardie@acm.org

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