Inter-Domain Multicast Routing (IDMR) A. Ballardie
INTERNET-DRAFT Consultant
March 1997
Core Based Trees (CBT) 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
months. Internet Drafts may be updated, replaced, or obsoleted by
other documents at any time. It is not appropriate to use Internet
Drafts as reference material or to cite them other than as a "working
draft" or "work in progress."
Please check the I-D abstract listing contained in each Internet
Draft directory to learn the current status of this or any other
Internet Draft.
Abstract
This document describes the Core Based Tree (CBT) network layer mul-
ticast routing protocol. CBT builds a shared multicast distribution
tree per group, and is suited to inter- and intra-domain multicast
routing.
CBT is protocol independent in that it makes use of unicast routing
to establish paths between senders and receivers. The CBT architec-
ture 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 Revision............................ 3
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................................ 10
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.............................. 12
4.4 QUIT_NOTIFICATION Processing........................... 12
4.4.1 Sending QUIT_NOTIFICATIONs....................... 12
4.4.2 Receiving QUIT_NOTIFICATIONs..................... 13
4.5 CBT ECHO_REQUEST Processing............................ 14
4.5.1 Sending ECHO_REQUESTs............................ 14
4.5.2 Receiving ECHO_REQUESTs.......................... 14
4.6 ECHO_REPLY Processing.................................. 15
4.6.1 Sending ECHO_REPLYs.............................. 15
4.6.2 Receiving ECHO_REPLYs............................ 15
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4.7 FLUSH_TREE Processing.................................. 16
4.7.1 Sending FLUSH_TREE Messages...................... 16
4.7.2 Receiving FLUSH_TREE Messages.................... 16
5. Timers and Default Values.................................. 16
6. CBT Packet Formats and Message Types....................... 17
6.1 CBT Common Control Packet Header....................... 18
6.2 HELLO Packet Format.................................... 19
6.3 JOIN_REQUEST Packet Format............................. 19
6.4 JOIN_ACK Packet Format................................. 20
6.5 QUIT_NOTIFICATION Packet Format........................ 21
6.6 ECHO_REQUEST Packet Format............................. 21
6.7 ECHO_REPLY Packet Format............................... 22
6.8 FLUSH_TREE Packet Format............................... 23
7. Core Router Discovery...................................... 23
7.1 Bootstrap Message Format.............................. 25
7.2 Candidate Core Advertisement Message Format........... 25
8. Interoperability Issues.................................... 25
Acknowledgements.............................................. 26
References.................................................... 26
Author Information............................................ 27
1. Changes since Previous Revision (05)
This revision of the CBT protocol specification differs significantly
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from the previously released revision (05). Consequently, this revi-
sion 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. How-
ever, any future versions of CBT can be expected to be backwards com-
patible 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 pro-
tocol.
+o new simplified packet formats, with the definition of a common
CBT control packet header.
+o a generic intra-domain core discovery ("bootstrap") mechanism,
to be specified separately, and published soon.
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 configured
to act as a "meeting point" between a sender and group receivers. The
term "rendezvous point (RP)" is used equivalently in some contexts
[2]. Each core router is 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 router. A "down-
stream" interface (or router) is one which is on the path away from
the group's core router with respect to this router.
Other terminology is introduced in its context throughout the text.
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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 7). This join message must be
explicitly acknowledged (JOIN_ACK) either by the core router itself,
or by another router that is on the unicast path between the sending
router and the core, which itself has already successfully joined the
tree.
The join message sets up transient join state in the routers it tra-
verses, and this state consists of <group, incoming interface, outgo-
ing interface>. "Incoming interface" and "outgoing interface" may be
"previous hop" and "next hop", respectively, if the corresponding
links do not support multicast transmission. "Previous hop" is taken
from the incoming control packet's IP source address, and "next hop"
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.
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
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address and a list of local interfaces over which join messages for
the group have previously been acknowledged. There is no concept of
"incoming" or "outgoing" interfaces, though it is necessary to be
able to distinguish the upstream interface from any downstream inter-
faces. In CBT, these interfaces are known as the "parent" and "child"
interfaces, respectively. We recommend the parent be distinguished as
such by a single bit in each multicast forwarding cache entry.
With regards to the information contained in the multicast forwarding
cache, on link types not supporting native multicast transmission an
on-tree router must store the address of a parent and any children.
On links supporting multicast however, parent and any child informa-
tion is represented with local interface addresses (or similar iden-
tifying information, such as an interface "index") over which the
parent or child is reachable.
When a multicast data packet arrives at a router, the router uses the
group address as an index into the multicast forwarding cache. A copy
of the incoming multicast data packet is forwarded over each inter-
face (or to each address) listed in the entry except the incoming
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
non-NULL. A child interface is one over which a member host is
directly attached, or one over which a downstream on-tree router is
attached. This "tree maintenance" is achieved by each downstream
router periodically sending a 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 par-
ent link. If a parent link does not support multicast transmission,
keepalives are unicast.
The receipt of a keepalive message over a valid child interface imme-
diately prompts a response (ECHO_REPLY), which is either unicast or
multicast, as appropriate.
The ECHO_REQUEST does not contain any group information; the
ECHO_REPLY does, but only periodically. To maintain consistent infor-
mation between parent and child,
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the parent periodically reports, in an ECHO_REPLY, all groups for
which it has state, over each of its child interfaces for those
groups. This group-carrying echo reply is not prompted explicitly by
the receipt of an echo request message. A child is notified of the
time to expect the next echo reply message containing group informa-
tion in an echo reply prompted by a child's echo request. The fre-
quency of parent group reporting is at the granularity of minutes.
It cannot be assumed all of the routers on a multi-access link have a
uniform view of unicast routing; this is particularly the case when a
multi-access link spans two or more unicast routing domains. This
could lead to multiple upstream tree branches being formed (an error
condition) unless steps are taken to ensure all routers on the link
agree which is the upstream router for a particular group. CBT
routers attached to a multi-access link participate in an explicit
election mechanism that elects a single router, the designated router
(DR), as the link's upstream router for all groups. Since the DR
might not be the link's best next-hop for a particular core router,
this may result in join messages being re-directed back across a
multi-access link. If this happens, the re-directed join message is
unicast across the link by the DR to the best next-hop, thereby pre-
venting a looping scenario. This re-direction only ever applies to
join messages. Whilst this is suboptimal for join messages, which
are generated infrequently, multicast data never traverses a link
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 the IP address of a router reachable over
one of the sending router's interfaces, depending on whether the
sender's outgoing link supports multicast transmission. All the nec-
essary addressing information is obtained 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]).
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.
HELLO messages are multicast periodically to the all-cbt-routers
group, 224.0.0.15, using IP TTL 1. The advertisement period is
[HELLO_TIMER] seconds. [HELLO_TIMER] comprises a configured
[HELLO_INTERVAL], to which is added [RND_RSP] seconds - a random
response interval. This random response additive is required to
avoid the potential problem of synchronisation between HELLO adver-
tisements (or other control messages) from different routers. The
HELLO protocol's convergence time is set at [HELLO_CONV] seconds -
the time after which no further HELLOs are expected in any one round
of the protocol.
Each HELLO advertising router includes the upper bound of its
[RND_RSP] timer in its HELLO advertisements. This is necessary so
that all routers attached to the link can agree on a common HELLO
convergence time [HELLO_CONV]; in any one round of the HELLO proto-
col, a router assumes the minimum of the upper bound of its config-
ured [RND_RSP] and that of any received advertisement's. The minimum
upper bound is then used as this router's [RND_RSP] upper bound in
the next round of the protocol. [HELLO_CONV] is set to this minimum
upper bound + 2 seconds (the 2 seconds being a response "safety mar-
gin") for the next round of the protocol.
A network manager can preference a router's DR eligibility by option-
ally configuring a HELLO preference. 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
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zero (0) in HELLO advertisements, irrespective of any configured
preference. The DR continues to use preference zero for as long as
it is running.
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-
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.
A router sends a HELLO message whenever its [HELLO_TIMER] expires.
Whenever a router sends a HELLO message, it resets its [HELLO_TIMER].
4.1.2. Receiving HELLOs
On receipt of any HELLO message, a router adjusts its [RND_RSP] upper
bound to the minimum of this router's configured [RND_RSP] upper
bound and that received in the received HELLO. The router also
adjusts its [HELLO_CONV] as described above.
A router need not respond to a HELLO message if the received HELLO is
"better" than its own. Thus, in steady state, the HELLO protocol
incurs very little traffic overhead.
If the received HELLO message is "better" (lower preferenced, or
equally preferenced but lower addressed) than it would send itself,
it immediately unsets its DR flag on the arriving interface if the DR
flag is set on that interface. It also resets its [HELLO_TIMER].
If the received HELLO message is not "better" than this router would
send itself, it sets its [RND_RSP] random response timer; on expiry,
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the router responds with its own HELLO message . If no "better" HELLO
message is received within the current [HELLO_CONV], the router sets
the DR flag on the corresponding interface.
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 7).
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 [JOIN_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.
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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, 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 follow-
ing information with respect to the forwarded join: <group, down-
stream interface, upstream interface>.
If this transient join state is not "confirmed" with a join acknowl-
edgement (JOIN_ACK) message from upstream, the state is timed out
after 1.5 times [JOIN_RTX_INTERVAL].
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 [RND_RSP] pertaining to a JOIN_REQUEST is active (i.e. running),
if a JOIN_REQUEST is received for the same group over that group's
parent interface, cancel [RND_RSP] for the impending JOIN_REQUEST.
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.
If a multicast JOIN_REQUEST is received and the QUIT_TIME bit (see
section 4.4.1) is set on the arrival interface for the specified
group, unset the QUIT_TIME bit.
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. If the router
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does not yet have active state for this group, this router must be
the core router for the group; the core creates a forwarding cache
entry and includes the interface in its child interface list, and
sends the JOIN_ACK downstream.
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 "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.
A QUIT_NOTIFICATION is not acknowledged; once sent, all information
pertaining to the group it represents is deleted from the forwarding
cache after a short interval.
To ensure consistency between a child and parent router given the
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potential for loss of a QUIT_NOTIFICATION, there is a QUIT_TIME bit
associated with the parent of each group entry; whenever a
QUIT_NOTIFICATION is sent for a group, the QUIT_TIME bit for that
group entry is set for a maximum of [QUIT_TIME] seconds before the
entry is deleted and the QUIT_TIME bit unset. By default, this bit is
unset.
When the QUIT_TIME bit is set, if the router detects multicast traf-
fic for the group arriving over a to-be-deleted parent interface (one
over which a quit has recently been sent), the router sends another
QUIT_NOTIFICATION over that interface. This is multicast, or unicast,
as appropriate for the outgoing link. It continues to do so at
[QUIT_RATE] second intervals so long as data continues to arrive, and
provided [QUIT_TIME] has not yet expired.
If, after sending a QUIT_NOTIFICATION a multicast JOIN_REQUEST for
the specified group arrives over the interface the quit was sent, the
QUIT_TIME bit is immediately unset if it is set (any traffic arriving
over the interface will be for/from another child router attached to
the same link).
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 inter-
face 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 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_NOTIFICATION, 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.
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Therefore, on receipt of a multicast QUIT_NOTIFICATION over a parent
interface, a receiving router starts a random response interval timer
which is set to [RND_RSP] seconds.
If a multicast JOIN_REQUEST is received over the same interface (par-
ent) for the same group before this router's [RND_RSP] timer expires,
it suppresses the multicasting of its own similar JOIN_REQUEST.
If a multicast JOIN_REQUEST is not received via the router's parent
link before [RND_RSP] expires, a JOIN_REQUEST is multicast over the
link for the previously quit group, with IP TTL 1.
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, and
unicast to the parent router otherwise.
ECHO_REQUEST messages are sent at [ECHO_INTERVAL] second intervals.
Whenever an ECHO_REQUEST is sent, [ECHO_INTERVAL] is reset.
If, for any echo-request sent to a parent, the expected response
(ECHO_REPLY) is not forthcoming within [ECHO_RTX_INTERVAL], the echo
request message is retransmitted. If no response is forthcoming
within [ECHO_TIMEOUT] seconds, the router sends a FLUSH_TREE message
over each of its child interfaces for the group, then removes all
forwarding cache state for the group.
4.5.2. Receiving ECHO_REQUESTs
If a ECHO_REQUEST is received over any valid child interface, the
receiving router responds with an ECHO_REPLY message over the same
interface. This message is multicast to the "all-cbt-routers" group
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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 ensure the group state information is consistent between
them.
4.6.1. Sending ECHO_REPLY messages
An ECHO_REPLY message is sent in direct response to receiving an
ECHO_REQUEST message, provided the ECHO_REQUEST is received over any
one of this router's valid child interfaces. Additionally, an
ECHO_REPLY is sent periodically by a parent router over each of its
child links, reporting 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.
When received, the child router resets its [ECHO_INTERVAL] timer for
this upstream interface. The child router also caches the reported
"group report interval" (seconds) - the time at which the next group
carrying ECHO_REPLY will be sent by the parent router. Like
[ECHO_INTERVAL], this is cached per upstream interface. If the group
carrying ECHO_REPLY does not arrive shortly after "group report
interval" has expired, a QUIT_NOTIFICATION is sent for each group for
which the non-reporting router is the parent.
If this echo reply carries a list of groups, the child router must
match all those of its forwarding cache entries for which the arrival
interface is the upstream interface. If the parent router does not
consider itself the parent router for group(s) which the child thinks
is its parent, the child sends a FLUSH_TREE message downstream for
each such group. If this router has directly attached members for any
of the flushed groups, the receipt of an IGMP host membership report
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for any of those groups will prompt this router to rejoin the corre-
sponding tree(s).
If the upstream router considers itself the parent for more groups
than does the receiving router, this router sends a QUIT_NOTIFICATION
for each of those groups for which the QUIT_TIME bit is set in the
forwarding cache. Otherwise, the router takes no action.
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.
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. Timers and Default Values
This section provides a summary of the timers described above,
together with their default values.
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+o [HELLO_INTERVAL]: a base value making up the bulk of the inter-
val between sending a HELLO message. Default: 60 seconds.
+o [RND_RSP]: router's random response interval. Default: 2 sec-
onds.
+o [HELLO_TIMER]: (variable) interval between sending HELLO mes-
sages. [HELLO_TIMER] = [HELLO_INTERVAL + RND_RSP]
+o [HELLO_CONV]: convergence time of one round of the HELLO proto-
col. [HELLO_CONV] = [min(RND_RSP) + 2 seconds].
+o [JOIN_RTX_INTERVAL]: retransmission time for JOIN_REQUESTs.
Default: 5 seconds.
+o [JOIN_TIMEOUT]: time to raise exception due to tree join fail-
ure. Default: 3.5 times [JOIN_RTX_INTERVAL].
+o [CACHE_DEL_TIMER]: time to remove child interface from forward-
ing cache. Default: 2 seconds.
+o [QUIT_TIME]: time to remove parent interface from forwarding
cache entry. Unset QUIT_TIME bit. Default: 60 seconds.
+o [QUIT_RATE]: period for sending QUIT_NOTIFICATION if traffic
persists. Default: 15 seconds.
+o [ECHO_INTERVAL]: interval between sending ECHO_REQUEST to parent
routers. Default: 60 seconds.
+o [ECHO_RTX_INTERVAL]: retransmission time for ECHO_REQUESTs.
Default 2 seconds.
+o [ECHO_TIMEOUT]: time to consider parent unreachable. Default:
3.5 times [ECHO_RTX_INTERVAL].
6. CBT Packet Formats and Message Types
CBT control packets are encapsulated in IP. CBT has been assigned IP
protocol number 7 by IANA [4].
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6.1. 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.
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
+o type 6: FLUSH_TREE
+o type 7: Bootstrap Message
+o type 8: Candidate Core Advertisement
+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.
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6.2. 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| rnd response | Preference | reserved | option type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| option len | option value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2. HELLO Packet Format
HELLO Packet Field Definitions:
+o rnd response: random response interval in seconds.
+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.
6.3. 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 originating router: router that originated this 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| originating router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| target router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| option type | option len | option value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3. JOIN_REQUEST Packet Format
+o target router: target (core) router for the group.
+o option type: allows the specification of a variety of
JOIN_REQUEST options. One option is currently defined: option
type 0 (zero) = BR_JOIN; option length 0 (zero); option value 0
(zero). This option is used by a CBT domain border router to
join an internal core for all groups that map to that core. The
state instantiated by a JOIN_REQUEST with this option set is
represents (*, core). For further details, see [5].
6.4. JOIN_ACK 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| option type | option len | option value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4. JOIN_ACK Packet Format
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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.
6.5. 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.
6.6. ECHO_REQUEST Packet Format
ECHO_REQUEST Field Definitions
+o originating child router: address of the router that originates
the ECHO_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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| originating child router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6. ECHO_REQUEST Packet Format
6.7. 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 report interval | num groups |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 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 report interval: number of seconds until the sending
router will send its next ECHO_REPLY containing a list of group
addresses.
+o num groups: the number of groups being reported by this
ECHO_REPLY.
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+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.
6.8. 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 8. FLUSH_TREE Packet Format
FLUSH_TREE Field Definitions
+o group address: multicast group address of the group being
"flushed".
7. Core Router Discovery
For intra-domain core discovery, CBT has decided to adopt the "boot-
strap" mechanism currently specified with the PIM sparse mode proto-
col [2]. This bootstrap mechanism is scalable, robust, and does not
rely on underlying multicast routing support to deliver core router
information; this information is distributed via traditional unicast
hop-by-hop forwarding.
It is expected that the bootstrap mechanism will be specified inde-
pendently as a "generic" RP/Core discovery mechanism in its own sepa-
rate document. It is unlikely at this stage that the bootstrap mecha-
nism will be appended to a well-known network layer protocol, such as
IGMP [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 rout-
ing protocol itself.
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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 limit-
ing a CC-set to a particular range of groups. This can help reduce
traffic concentration at the core.
If a BSR detects a particular core as being unreachable (it has not
announced its availability within some period), it deletes the rele-
vant core from the CC-set sent in its next bootstrap message. This is
how a local router discovers a group's core is unreachable; the
router must re-hash for each affected group and join the new core
after removing the old state. The removal of the "old" state follows
the sending of a QUIT_NOTIFICATION upstream, and a FLUSH_TREE message
downstream.
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7.1. 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
7.2. 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
8. 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.
Others that have contributed to the progress of CBT include Ken Carl-
berg, Eric Crawley, Nitin Jain, Steven Ostrowsksi, Radia Perlman,
Scott Reeve, Clay Shields, Sue Thompson, Paul White.
The participants of the IETF IDMR working group have provided useful
feedback since the inception of CBT.
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, 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/internet-drafts/draft-ietf-idmr-cbt-
dvmrp-**.txt. Working draft, March 1997.
[6] Scalable Multicast Key Distribution; A. Ballardie; RFC 1949, July
1996.
[7] A Dynamic Bootstrap Mechanism for Rendezvous-based Multicast Rout-
ing; D. Estrin et al.; Technical Report; ftp://catarina.usc.edu/pim
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Author Information:
Tony Ballardie,
Research Consultant,
e-mail: ABallardie@acm.org
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