Inter-Domain Multicast Routing (IDMR) A. J. Ballardie
INTERNET-DRAFT University College London
April 18th, 1995
Core Based Trees (CBT) Multicast
-- Architectural Overview and Specification --
<draft-ietf-idmr-cbt-spec-01.txt>
Status of this Memo
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Abstract
CBT is a new architecture for local- and wide-area IP multicasting,
being unique in its utilization of just one shared delivery tree, as
opposed to the source-based delivery trees of traditional IP multi-
cast schemes.
The primary advantages of the CBT approach are that it typically
offers more favourable scaling characteristics than do existing mul-
ticast algorithms. The definition of a new network layer multicast
protocol has also meant that it has been possible to integrate an en-
riched functionality into multicast that is not possible under other
IP multicast schemes, for example, the incorporation of security
features. Besides this functionality providing the ability to authen-
ticate tree-joining host's and routers, optional in-built protocol
mechanisms provide a scalable solution to the multicast key distribu-
tion problem [RFC 1704].
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CBT is backwards compatible with traditional IP-style multicast. Host
changes are not required, and a local CBT-capable router is mandatory
if CBT-style multicasts are to be forwarded beyond the local subnet-
work.
_1. _B_a_c_k_g_r_o_u_n_d
Centre based forwarding was first described in the early 1980s by
Wall in his PhD thesis on broadcast and selective broadcast. At this
time, multicast was in its very earliest stages of development, and
researchers were only just beginning to realise the benefits that
could be gained from it, and some of the uses it could be put to. It
was only later that the class-D multicast address space was defined,
and later again that intrinsic multicast support was taken advantage
of for broadcast media, such as Ethernet.
Now that we have several years practical experience with multicast, a
diversity of multicast applications, and an internetwork infrastruc-
ture that wants to support it to an ever-increasing degree, we re-
visit the centre-based forwarding paradigm introduced by Wall, and
mould and adapt it specifically for today's multicast environment.
_2. _I_n_t_r_o_d_u_c_t_i_o_n
Multicast group communication is an increasingly important capability
in many of today's data networks. Most LANs and more recent wide-area
network technologies such as SMDS and ATM specify multicast as part
of their service.
Since the wide-area introduction of multicasting there has been a
large increase in the number and diversity of multicast applications,
examples of which include audio and video conferencing, replicated
database updating and querying, software update distribution, stock
market information services, and more recently, resource discovery.
Multimedia is another fast expanding area for which multicast offers
an invaluable service. It has therefore been necessary of late to
address the topic of scalability with regards to multicast algo-
rithms, since, if they do not scale to an internetwork size that is
expected (given the growth rate of the last several years), they can-
not be of longlasting benefit. This motivates the need for new multi-
casting techniques to be investigated.
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This draft describes a new multicast routing architecture and proto-
col which is applicable to a datagram network. The CBT architecture
has attractive scaling characteristics. We measure scalability in
terms of network state maintenance, bandwidth- and processing costs.
_3. _D_o_c_u_m_e_n_t _L_a_y_o_u_t
The remainder of this document is divided into three parts: Part A
offers a general architectural overview and discussion on the CBT
architecture. This section also includes a description of CBT ``any-
casting'' [see RFC 1546].
Parts B and C comprise the protocol specification. Part B describes
protocol engineering design features, such as CBT group initiation,
the tree joining process, tree maintenance issues, the tree leaving
process, LAN issues, data packet forwarding, and data packet encapsu-
lation and translation (see footnote 1)
Part C illustrates and describes in detail, individual CBT packet
formats and message types.
Part D looks briefly at some other related issues.
9_________________________
9 1 We will refer to the copying (and sometimes altera-
tion) of various fields of the IP header to a CBT
header as translation throughout. This may not be in
total agreement with how the term is used elsewhere.
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Part A
_1. _C_B_T - _T_h_e _N_e_w _A_r_c_h_i_t_e_c_t_u_r_e
_2. _A_r_c_h_i_t_e_c_t_u_r_a_l _O_v_e_r_v_i_e_w
A core-based tree involves having a single node, in our case a router
(with additional routers for robustness), known as the core of the
tree, from which branches emmanate. These branches are made up of
other routers, so-called non-core routers, which form a shortest for-
ward path between a member-host's directly attached router, and the
core. A router at the end of a branch shall be known as a leaf router
on the tree.
The CBT protocol builds a delivery tree reflecting the architecture
just described. This architecture allows for the enhancement of the
scalability of the multicast algorithm with regards to group-specific
state maintained in the network, particularly for the case where
there are many active senders in a particular group. The CBT archi-
tecture offers an improvement in scalability over existing techniques
by a factor of the number of active sources (where a source is a sub-
network aggregate). Hence, a core-based architecture allows us to
significantly improve the overall scaling factor of S * N we have in
the source-based tree architecture, to just N. This is the result of
having just one multicast tree per group as opposed to one tree per
(source, group) pair.
It is also interesting to note that routers between a non-member
sender and the CBT delivery tree need no knowledge of the multicast
tree/group whatsoever in order to forward CBT multicasts, since these
are unicast towards the core. This two-phase routing approach is
unique to the CBT architecture. One such application that can take
advantage of this two-phase routing is resource discovery, whereby a
resource, for example, a replicated database, is distributed in dif-
ferent locations throughout the Internet. The databases in the dif-
ferent locations make up a single multicast group, linked by a CBT
tree. A client need only know the address of (one of) the core(s) for
the group in order to send (unicast) a request to it. Such a request
would not span the tree in this case, but would be answered by the
first tree router encountered, making it quite likely that the
request is answered by the ``nearest'' server. Effectively, this
corresponds to an ``anycast'' service [RFC 1546] (see section X).
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A diagram showing a single-core CBT tree is shown in the figure
below. Only one core is shown to demonstrate the principle.
b b b-----b
\ | |
\ | |
b---b b------b
/ \ / KEY....
/ \/
b X---b-----b X = Core
/ \ b = non-core router
/ \
/ \
b b------b
/ \ |
/ \ |
b b b
Figure 1: Single-Core CBT Tree
_2._1. _A_r_c_h_i_t_e_c_t_u_r_a_l _J_u_s_t_i_f_i_c_a_t_i_o_n
First of all, exactly what is a core-based tree (CBT) architecture?
Core-based, or centre-based forwarding trees, were first described by
Wall in his investigation into low-delay approaches to broadcast and
selective broadcast. Wall concluded that delay will not be minimal,
as with shortest-path trees, but the delay can be kept within bounds
that may be acceptable. Simulations have recently been carried out
to compare the maximum and average delays of centre-based and
shortest-path trees. A summary of these simulations can be found in
In the context of multicast, the extent to which the delay charac-
teristics of a shared tree are less optimal than SPTs, is question-
able. The simulation results state that CBTs incur, on average, a 10%
increase in delay over SPTs. Slight discrepancies in delay may not
be a critical factor for many multicast applications, such as
resource discovery or database updating/querying. Even for real-time
applications such as voice and video conferencing, a core based tree
may indeed be acceptable, especially if the majority of branches of
that tree span high-bandwidth links, such as optical fibre. In
several years' time it is easy to envisage the Internet being host to
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thousands of active multicast groups, and similarly, the bandwidth
capacity on many of the Internet links may well far exceed those of
today.
An important question raised in the SPT vs. CBT debate is: how effec-
tively can load sharing be achieved by the different schemes? It
would seem that SPT schemes cannot achieve load balancing because of
the nature of their forwarding: nodes on a SPT do not have the option
to forward incoming packets over different links (i.e. load balance)
because of the danger of loops forming in the multicast tree topol-
ogy.
With shared tree schemes however, each receiver can choose which of
the small selection of cores it wishes to join. Cores and on-tree
nodes can be configured to accept only a certain number of joins,
forcing a receiver to join via a different path. This flexibility
gives shared tree schemes the ability to achieve load balancing.
In general, spread over all groups, CBT has the ability to randomize
the group set over different trees (spanning different links around
the centre of the network), something that would not seem possible
under SPT schemes.
Finally, the CBT protocol requires each receiver to explicitly join
the delivery tree, resulting in a tree spanning only a group's
receivers. As a result, data flows only over those links that lead to
receivers, and thus there is no requirement for off-tree routers to
maintain prune state, which prevents data flow where it is not
needed.
_2._2. _T_h_e _I_m_p_l_i_c_a_t_i_o_n_s _o_f _S_h_a_r_e_d _T_r_e_e_s
The trade-offs introduced by the CBT architecture focus primarily
between a reduction in the overall state the network must maintain
(given that a group has a significant proportion of active senders),
and the potential increased delay imposed by a shared delivery tree.
We have emphasized CBT's much improved scalability over existing
schemes for the case where there are {\m active} group senders. How-
ever, because of CBT's ``hard-state'' approach to tree building, i.e.
group tree link information does not time out after a period of inac-
tivity, as is the case with most source-based architecutures,
source-based architectures scale best when there are no senders to a
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multicast group. This is because multicast routers in the network
eventually time out all information pertaining to an inactive group.
Source-based trees are said to be built ``on-demand'', and are
``data-driven''.
A consequence of the ``hard-state'' approach is that multicast tree
branches do not automatically adapt to underlying multicast route
changesotnote{If multicast were part of the global internetwork
infrastructure, multicast routes are gleaned exclusively from {\m
unicast} routes.}. This is in contrast to the ``soft-state'', data-
driven approach -- data always follows the path as specified in the
routing table. Provided reachability is not lost, it is advantageous,
from the perspective of uninterrupted packet flow, that a multicast
route is kept constant, but the two disadvantages are: a route may
not be optimal for its entire duration, and, ``hard-state'' requires
the incorporation of {\m control messages} that monitor reachability
between adjacent routers on the multicast tree. This control message
overhead can be quite considerable unless some form of message aggre-
gation is employed.
In terms of the effectiveness of the CBT approach to multicasting,
the increased delay factor imposed by a shared delivery tree may not
always be acceptable, particularly if a portion of the delivery tree
spans low bandwidth links. This is especially relevant for real-time
applications, such as voice conferencing.
Another consequence of one shared delivery tree is that the cores for
a particular group, especially large, widespread groups with numerous
active senders, can potentially become traffic ``hot-spots'' or
``bottlenecks''. This has been referred to as the {\m traffic concen-
tration} effect in
The branches of a CBT tree are made up of a collection of branches,
rooted at the tree node that originated a join-request, and terminat-
ing at the tree node that acknowledged the same join. This has impli-
cations where asymmetric routes are concerned (similar to source-
based schemes based on RPF) -- whilst the same CBT branch is used for
data packet flow in {\m both} directions, the child-to-parent direc-
tion constitutes a valid route reflecting the underlying unicast
route (at least at the time the branch was created). However, in the
parent-to-child direction, the path does not necessarily reflect
underlying unicast routing at any instant, and therefore, in a
policy-oriented environment, this {\m might} have disadvantageous
side-effects.
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Finally, there are questions concerning the {\m cores} of a group
tree: how are they selected, where are they placed, how are they
managed, and how do new group members get to know about them? We have
attempted to implement some very simple heuristics to address some of
these questions in section X, but these may not be appropriate for
large-scale implementation of CBT. Work is currently underway in the
development of a core placement/location protocol.
We conclude in section X that most aspects of core management are
topics of further research.
_3. _C_B_T _a_n_d ``_A_n_y_c_a_s_t_i_n_g''
_3._1. _O_v_e_r_v_i_e_w _o_f ``_A_n_y_c_a_s_t_i_n_g''
Anycasting [RFC 1546] is a proposed best-effort, stateless, datagram
delivery service which is used by hosts primarily to locate particular
services on an internetwork. The goal of anycast is for a client to
transmit one request to a resource ``anycast address'', and for a sin-
gle, preferably nearest, server to receive the request and respond to
it.
The motivation for anycasting is that it simplifies the task of finding
the appropriate server in a network, and obviates the need to configure
applications with particular server address(es), for example, as in DNS
resolvers.
Questions that, as yet, remain unanswered regarding anycasting, include:
how best can anycasting be achieved, and should anycast addresses be a
special class of IP address?
As for how best to achieve anycast, there are two possible approaches:
use existing IP multicast, or, answering our second question, define a
special class of IP anycast address within the IP address space, and
have servers additionally bind an anycast address on which they listen
for client requests.
Using existing IP multicast has problems associated with it. Firstly,
using expanding ring search to locate a network resource is inefficient
for two reasons: it requires potentially many re-transmissions of the
request from the client, each iteration requiring a larger TTL (see
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footnote 11) value. This continues until a response is received.
The other problem with using IP multicast is that, for any multicast
transmission, potentially more than one response may be received. To
summarize, using existing IP multicast for anycast is inefficient in its
use of network resources, and does not necessarily achieve the desired
goal of anycast, namely that only one server respond to a client
request. Also, anycasting should not require managing the IP TTL value
of client request packets -- the goal of anycast is to send a single
packet, which follows a single path, in order to locate a single,
preferably nearest, server.
Defining a special class of ``anycast'' addresses has several problems
associated with it. For example, routing must be adapted to support yet
another class of IP address, and routing tables would be required to
support anycast routes. Furthermore, segmenting the IP address space
yet further not only involves significant administrative burden, but
also assumes that existing applications will recognise particular
addresses as being anycast [RFC 1546].
_3._2. _T_h_e _C_B_T ``_A_n_y_c_a_s_t'' _S_o_l_u_t_i_o_n
It so happens that the CBT multicast architecture provides an effective
solution to the anycasting problem, without requiring the definition of
special anycast addresses.
The CBT architecture was explained in section 2. CBT is especially
attractive for resource discovery applications, where it is assumed that
different network resources for distinct CBT groups. The reason CBT is
particularly suited to resource discovery, as described, is because it
typically involves many senders, whereby a sender is not a group member.
As we have already explained, CBT multicast, unlike other IP multicast
schemes, involves maintaining group-specific state in the network that
is independent of the number of active sources. Moreover, this state is
constrained to the tree links that span only a group's receivers.
In CBT multicast, non-member senders actually utilize unicast to route
_________________________
9 11 This is a field of the IP header which is decre-
mented each time the corresponding packet traverses a
router. If the TTL field reaches zero, a router will
discard the packet.
9
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multicast data to the CBT delivery tree. This is known as CBT's 2-phase
routing. These packets are unicast addressed to a single core router (of
which there may be several), and will first encounter the delivery tree
either at the addressed core, or at an on-tree (non-core) router that is
on the unicast path between the sender and the addressed core.
For typical multicast applications, the receiving on-tree router disem-
minates the received packet(s) to adjacent outgoing on-tree neighbours,
and neighbours proceed similarly on receipt of a packet. This is how
multicast data packets span a CBT tree.
For anycast (and resource discovery applications) however, the first
on-tree node encountered does not disemminate the packet further, but
responds to the received request.
Thus, we believe that CBT offers an effective solution to ``anycasting''
and resource discovery in general. However, some questions remain: what
level of fault tolerance does the CBT solution offer, by what means does
a sender establish the unicast address of a CBT core router, and
finally, is there a guarantee that a client request will hit the CBT
tree, i.e. reach a server, at the nearest point to the sender?
The question of fault tolerance is indirectly related to the question of
establishing a core address. A CBT tree should never comprise only one
core router for reasons of robustness. We envisage there should be at
least two cores for local groups, and possibly up to five for wide-area
groups. By whatever means a client establishes the identity of a core,
it will always simultaneously establish the identities of all cores for
a particular tree.
So, how could core addresses be found out about? One obvious solution
would be to advertise core addresse, together with their associated net-
work resource, in an application such as, or very much like, ``sd''.
With regards to our final question, the choice of core will determine if
a packet reaches a nearest server. Since users can not be expected to
know about network topology, it is assumed that the choice of core will
be fairly random. Hence, our scheme makes no guarantees that a client
request will reach the nearest server.
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Part B
_1. _P_r_o_t_o_c_o_l _O_v_e_r_v_i_e_w
_1._1. _C_B_T _G_r_o_u_p _I_n_i_t_i_a_t_i_o_n
Like any of the other multicast schemes, one user, the group initia-
tor, initiates a CBT multicast group. The procedures involved in ini-
tiating and joining a CBT group involves a little more user interac-
tion than current IP multicast schemes, for example, it is necessary
to supply information such as desired group scope, as well as select
the primary core from a selection of pre-configured core routers.
Explicit core rankings help prevent loops when the core tree is ini-
tially set up. It also assists in the tree maintenance process should
the tree become partitioned.
Group initiation could be carried out by a network management centre,
or by some other external means, rather than have a user act as group
initiator. However, in the author's implementation, this flexibility
has been afforded the user, and a CBT group is invoked by means of a
graphical user interface (GUI), known as the CBT User Group Manage-
ment Interface.
NOTE: Work is currently in progress to address the issue of core
placement.
_1._2. _T_r_e_e _J_o_i_n_i_n_g _P_r_o_c_e_s_s
Once the cores have been enumerated by a group's initiator, and the
application, port number etc. have been selected, the group-
initiating host sends a special CORE-NOTIFICATION message to each of
them, which is acknowledged. The purpose of this message is twofold:
firstly, to communicate the identities of all of the cores, together
with their rankings, to each of them individually; secondly, to
invoke the building of the core backbone. These two procedures follow
on one to the other in the order just described. New receivers
attempting to join whilst the building of the core backbone is still
in progress have their explicit JOIN-REQUEST messages stored by
whichever CBT-capable router, involved in the core joining process,
is encountered first. Routers on the core backbone will usually
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include not only the cores themselves, but intervening CBT-capable
routers on the unicast path between them. Once this set up is com-
plete, any pending joins for the same group can be acknowledged.
All the CBT-capable routers traversed by a JOIN-ACKnowlegement change
their status to CBT-non-core routers for the group identified by
group-id. It is the JOIN-ACK that actually creates a tree branch.
The JOIN-ACK carries the complete core list for the group, which is
stored by each of the routers it traverses. Between sending a JOIN-
REQUEST and receiving a JOIN-ACK, a router is in a state of pending
membership. A router that is in the join pending state can not send
join acknowledgements in response to other join requests received for
the same group, but rather caches them for acknowledgement subsequent
to its own join being acknowledged.
Non-member senders, and new group receivers, are expected to know the
address of at least one of the corresponding group's cores in order
to send to/join a group. The current specification does not state how
this information is gleaned, but it might be obtainable from a direc-
tory such as ``sd'' (the multicast session directory) (see footnote
2) or from the Domain Name System (DNS). (see footnote 3)
In accordance with existing IP multicast schemes, if the scope of
multicasts is to extend beyond the local area, at least one CBT-
capable router must be present on the local subnetwork for hosts on
that subnetwork to utilize CBT multicast delivery. Only one local
router, the designated router, is allowed to send to/receive from
uptree (i.e. the branch leading to/from the core) for a particular
group. We therefore make a clear distinction between a group member-
ship interrogator -- the router responsible for sending IGMP host-
membership queries onto the local subnet, and the designated router.
However, they may or may not be one and the same. LAN specifics are
discussed in sections 1.6, 1.7 and 1.8.
Once the designated router (DR) has been established, i.e. the router
_________________________
9 2 By Van Jacobson et al., LBL.
9 3 We considered disseminating core identities by in-
cluding them in link-state routing updates. However,
this does not provide scalability since it involves
global group information distribution. Further, it in-
volves a dependency on link-state routing
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that is on the shortest-path to the corresponding core, the new
receiver (host) sends a special CBT report to it, requesting that it
join the corresponding delivery tree if it has not already. If the DR
has already joined the corresponding tree, then the DR multicasts to
the group a notification to that effect back across the subnet.
Information included in this notification include whether the DR was
successful in joining the corresponding tree, and actual core affili-
ation.
NOTE: the actual core affiliation of a tree router may differ from
the core specified in the join request, if that join is terminated
by an on-tree router whose affiliation is to a different core.
If the local DR has not joined the tree, then it proceeds to send a
JOIN-REQUEST and awaits an acknowledgement, at which time the notifi-
cation, as described above, is multicast across the subnetwork.
_1._3. _T_r_e_e _L_e_a_v_i_n_g _P_r_o_c_e_s_s
A QUIT-REQUEST is a request by a CBT router to leave a group. A
QUIT-REQUEST may be sent by a router to detach itself from a tree if
and only if it has no members for that group on any directly attached
subnets, AND it has received a QUIT-REQUEST on each of its child
interfaces for that group (if it has any). The QUIT-REQUEST can only
be sent to the parent router. The parent immediately acknowledges
the QUIT-REQUEST with a QUIT-ACK and removes that child interface
from the tree. Any CBT router that sends a QUIT-ACK in response to
receiving a QUIT-REQUEST should itself send a QUIT-REQUEST upstream
if the criteria described above are satisfied.
Failure to receive a QUIT-ACK despite several re-transmissions gives
the sending router the right to remove the relevant parent interface
information, and by doing so, removes itself from the CBT tree for
that group.
_1._4. _T_r_e_e _M_a_i_n_t_e_n_a_n_c_e _I_s_s_u_e_s
Robustness features/mechanisms have been built into the CBT protocol
as has been deemed appropriate to ensure timely tree re-configuration
in the event of a node or core failure. These mechanisms are imple-
mented in the form of request-response messages. Their frequency is
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configurable, with the trade-off being between protocol overhead and
timeliness in detecting a node failure, and recovering from that
failure.
_1._4._1. _N_o_d_e _F_a_i_l_u_r_e
The CBT protocol treats core- and non-core failure in the same way,
using the same mechanisms to re-establish tree connectivity.
Each child node on a CBT tree monitors the status of its
parent/parent link at fixed intervals by means of a ``keepalive''
mechanism operating between them. The ``keepalive'' mechanism is
implemented by means of two CBT control messages: CBT-ECHO-REQUEST
and CBT-ECHO-REPLY.
For any non-core router, if its parent router, or path to the parent,
fails, that non-core router is initially responsible for re-attaching
itself, and therefore all routers subordinate to it on the same
branch, to the tree (Note: re-joining is not necessary just because
unicast calculates a new next-hop to the core).
Subsequent to sending a QUIT-REQUEST on the parent link, a non-core
router initially attempts to re-join the tree by sending a RE-JOIN-
REQUEST (see section 1.4.4) on an alternate path (the alternate path
is derived from unicast routing) to an arbitrary alternate core
selected from the core list. The corresponding core is tested for
reachability before the re-join is sent, by means of the control mes-
sage: CBT-CORE-PING. Failure to receive a response from the selected
core will result in another being selected, and the process continues
to repeat itself until a reachable core is found.
The significance of sending a RE-JOIN-REQUEST (as opposed to a JOIN-
REQUEST) is because of the presence of subordinate routers, i.e.
there exists a downstream branch connected to the re-joining router.
Care must be taken in this case to avoid loops forming on the tree.
If the joining router did not have downstream routers connected to
it, it would not be necessary to take precautions to avoid loops
since they could not occur (this is explained in more detail in sec-
tion 1.4.3).
NOTE: It was an engineering design decision not to flush the com-
plete (downstream) branch when some (upstream) router detects a
failure. Whilst each router would join via its shortest-path to
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the corresponding core, it would result in an overall longer re-
connectivity latency.
A FLUSH-TREE control message is however sent if the best next-hop of
the re-join is a child on the same tree.
_1._4._2. _C_o_r_e _F_a_i_l_u_r_e
Once the core tree has been established as the initial step of group
initiation, core router failure thereafter is handled no differently
than non-core router failure, with a core attempting to re-connect
itself to the corresponding tree by means of either a join or re-
join.
When a core router re-starts subsequent to failure, it will have no
knowledge of the tree for which it is supposed to be currently a
core. The only means by which it can find out, and therefore re-
establish itself on the corresponding tree is if some other on-tree
router sends it a CBT-CORE-PING message. This message, by default,
always contains the identities of all the cores for a group, together
with the group-id.
On receipt of a CBT-CORE-PING, a recently re-started core will re-
join the tree by means of a JOIN-REQUEST.
_1._4._3. _U_n_i_c_a_s_t _T_r_a_n_s_i_e_n_t _L_o_o_p_s
Routers rely on underlying unicast routing to carry JOIN-REQUESTs
towards the core of a core-based tree. However, subsequent to a
topology change, transient routing loops, so called because of their
short-lived nature, can form in routing tables whilst the routing
algorithm is in the process of converging or stabilizing.
There are two cases to consider with respect to CBT and unicast tran-
sient loops, namely:
o+ a join is sent over a transient loop, but no part of the
corresponding CBT tree forms part of that loop. In this case,
the join will never get acknowledged and will therefore timeout.
Subsequent re-tries will succeed after the transient loop has
disappeared.
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o+ a join is sent over a transient loop, and the loop consists
either partly or entirely of routers on the corresponding CBT
tree. If the loop consists only partly of routers on the tree
and the join originated at a router that is not attempting to
re-join the tree, then the JOIN-REQUEST will be acknowledged. No
further action is necessary since a loop-free path exists from
the originating router to the tree.
If the loop consists entirely of routers on the tree, then the
router originating the join is attempting to re-join the tree.
In this case also, the join could be acknowledged which would
result in a loop forming on the tree, so we have designed a
loop-detection mechanism which is described below.
_1._4._4. _L_o_o_p _D_e_t_e_c_t_i_o_n
The CBT protocol incorporates an explicit loop-detection mechanism.
Loop detection is only necessary when a router, with at least one
child, is attempting to re-connect itself to the corresponding tree.
We distinguish between three types of JOIN-REQUEST: active; active
re-join; and non-active re-join (see Part C, section 1.3).
An active JOIN-REQUEST for group A is one which originates from a
router which has no chilren belonging to group A.
An active re-join for group A is one which originates from a router
that has children belonging to group A.
A non-active re-join is one that originally started out as an active
re-join, but has reached an on-tree router for the corresponding
group. At this point, the router changes the join status to non-
active re-join and forwards it on its parent branch, as does each CBT
router that receives it. Should the router that originated the active
re-join subsequently receive the non-active re-join, a loop is obvi-
ously present in the tree. The router must therefore immediately send
a QUIT-REQUEST to its parent router, and attempt to re-join again. In
this way the re-join acts as a loop-detection packet.
Another scenario that requires consideration is when there is a break
in the path (tunnel) between a child and its parent. Although the
parent is active, the child believes that the parent is down -- the
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child cannot distinguish between the parent being down and the path
to it being down. If the path failure is short-lived, whilst the
child will have chosen a new route to the core, the parent will be
unaware of this, and will continue forwarding over its child inter-
faces, the potential risk being apparent.
We guard against this using a child assert mechanism, which is impli-
cit, i.e. no control message overhead is incurred for this mechanism.
If no CBT-ECHO-REQUEST is heard, after a certain interval the
corresponding child interface is removed by the parent.
As an additional precaution against packet looping, multicast data
packets that are in the process of spanning a CBT's delivery tree
branches (remember, we distinguish between actual tree branches and
attached subnetworks, although there are cases when they are one and
the same) carry an on-tree indicator in the CBT header of the packet.
Provided a data packet arrives via a valid tree interface, all
routers are obliged to check that the on-tree indicator is set
accordingly. A data packet arriving at the tree for the first time
from a non-member sender will have the on-tree indicator bits set by
the receiving router. These bits should never subsquently be modified
by any router. Should a packet be erroneously forwarded by an on-
tree router over an off-tree interface, should that packet somehow
work its way back on tree, it can be immediately recognised and dis-
carded.
_1._5. _C_o_r_e _P_l_a_c_e_m_e_n_t
As it stands, the current implementation of CBT uses trivial heuris-
tics for core placement.
Careful placement of core(s) no doubt assists in optimizing the
routes between any sender and group members on the tree. Depending
on particular group dynamics, such as sender/receiver population, and
traffic patterns, it may well be counter-productive to place a
core(s) near or at the centre of a group. In any event, there exists
no polynomial time algorithm that can find the centre of a dynamic
multicast spanning tree.
One suggestion might be that cores be statically configured
throughout the Internet - there need only be some relatively small
number of cores per backbone network (see footnote 4),
_________________________
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and the addresses of these cores would be ``well-known''.
Work is currently in progress to develop a core location/placement
mechanism.
_1._6. _L_A_N _D_e_s_i_g_n_a_t_e_d _R_o_u_t_e_r
As we have said, there must only ever exist one DR for any particular
group that is responsible for uptree forwarding/reception of data
packets.
A group's DR is elected by means of an explicit mechanism. Whenever a
host initiates/joins a group, part of the process is for it to send a
CBT-DR-SOLICITATION message, addressed to the CBT ``all-routers''
address, which is a request for the best next-hop router to a speci-
fied core.
If the group is being initiated, a DR will almost certainly not be
present on the local subnet for the group, whereas if a group is
being joined, the DR may or may not be present, depending on whether
there exist other group members on the LAN (subnet).
If a DR is present for the specified group, it responds to the soli-
citation with a CBT-DR-ADVERTISEMENT, which is addressed to the
group.
If no DR is present, each CBT router inspects its unicast routing
table to establish whether it is the next best-hop to the specified
core.
A router which considers itself the best next-hop does not respond
immediately with an advertisement, but rather sends a CBT-DR-ADV-
NOTIFICATION to the CBT ``all-routers'' address. This is a precau-
tionary measure to prevent more than one router advertising itself as
_________________________
4 The storage and switching overhead incurred by
these core routers increases linearly with the number
of groups traversing them. A threshold value could be
introduced indicating the maximum number of groups per-
mitted to traverse a core router. Once exceeded, addi-
tional core routers would need to be assigned to the
backbone.
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the DR for the group (it is conceivable that more than one router
might think itself as the best next-hop to the core). If this
scenario does indeed occur, the advertisement notification acts as a
tie-breaker, the router with the lowest address winning the election.
The lowest addressed router subsequently advertises itself as DR for
the group.
_1._7. _N_o_n-_M_e_m_b_e_r _S_e_n_d_i_n_g
For non-member senders wishing to send multicasts beyond the scope of
the local subnetwork, the presence of a local CBT-capable router is
mandatory. The sending of multicast packets from a non-member host to
a particular group is two-phase: the first phase involves a host uni-
casting the packet from the originating host to one of the group's
cores (the destination field of the IP header carries the unicast
address of the core). The second phase is the disemmination of the
the packet by the receiving router to neighbouring (adjacent) routers
on the corresponding tree. Similarly, when an on-tree neighbour
receives the packet, it distributes it in the same fashion.
Before the multicast leaves the originating subnetwork, it is neces-
sary for the local CBT DR to append a CBT header to the packet
(behind the IP header), and change the IP destination address field
from a multicast address to the unicast address of a core for the
group. How does the CBT DR know that this multicast address is asso-
ciated with a CBT group? The answer is that there must be some form
of mapping mechanism, which has information about which group address
correspond to CBT multicast groups. This mechanism maps an IP multi-
cast address to a unicast core address.
Packets sent from a non-member sender will first encounter the
corresponding delivery tree either at the addressed core, or hit an
on-tree router that is on the shortest-path between the sender and
the core. What happens when a CBT packet hits the corresponding
delivery tree is dealt with under ``Data Packet Forwarding'' in sec-
tion 1.8 below.
NOTE: No host changes are required for CBT. CBT hosts are simply
required to run the CBT application-level software that provides the
CBT user group management interface.
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_1._8. _D_a_t_a _P_a_c_k_e_t _F_o_r_w_a_r_d_i_n_g
In this section we describe how multicast data packets span a CBT
tree.
It is important to note that CBT uses the Internet Group Management
Protocol (IGMP) in much the same way as traditional IP schemes,
namely to establish group presence on directly-connected subnets,
and to exchange CBT routing information. A new IGMP message type
has been created for exchanging CBT routing messages.
We must again bring to the reader's attention the distinction between
tree branches and subnets, although there are cases where they are
one and the same.
It has been an important engineering design goal for CBT to be back-
wards compatible with IP-style multicasts. Until the interface with
other multicast protocols is clearly defined, CBT routing information
is not exchanged with that of any other schemes.
IP-style multicast data packets arriving at a CBT router are checked
to see if they originated locally. If not, they are discarded. Other-
wise, the local CBT DR for the group first sends a copy of the IP-
style packet over any directly-connected subnetworks with group
member presence (provided the TTL allows), then appends a CBT header
to the packet for forwarding over outgoing tree interfaces.
CBT-style packets arriving at a CBT router are forwarded over tree
interfaces for the group, and sent IP-style over any directly-
connected subnetworks with group member presence. The conversion from
a CBT-style packet to an IP-style packet requires the copying of
various fields of the CBT header to the IP header.
The child(ren) or parent of a CBT router may be reachable over a
multi-access LAN. This is the case where a subnetwork and a tree
branch are one and the same. In this case, the forwarding of the
CBT-style packets is achieved with multicast as opposed to unicast.
End-systems subscribed to the same group may receive these packets,
but they will not be processed, since end-systems will not recognise
the upper-layer protocol identifier, i.e. CBT.
NOTE: it was an engineering design decision to multicast data pack-
ets with a CBT header on multi-access links -- the case of unicast-
ing separately from parent to n children is clearly more costly.
Multicasting also reduces traffic -- when a parent receives a
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packet, it does not need to re-send the packet to any of its other
children that may be present on the multi-access link, since they
will have received a copy from the child's multicast.
Data arriving at a CBT router is always multicast first IP-style onto
any directly-connected subnets with group member presence, and only
subsequently unicast (multicast on multi-access links) to
parent/children with a CBT header.
A CBT router will not forward IP-style multicsat data packets unless
that router has a forwarding information base (FIB) entry for the
specified group, The exception to this is if a multicast originates
on a local subnetwork. In this case, the local CBT DR for the group
needs to insert a CBT header in the packet (behind the IP hdr) and
unicast it to one of the cores for the group.
A CBT FIB entry is shown below:
32-bits 8 8 4 8 | 8
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| group-id | parent addr | parent vif | No. of | |
| | index | index |children | children |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|chld addr |chld vif |
| index | index |
|+-+-+-+-+-+-+-+-+-+-+
|chld addr |chld vif |
| index | index |
|+-+-+-+-+-+-+-+-+-+-+
|chld addr |chld vif |
| index | index |
|+-+-+-+-+-+-+-+-+-+-+
| |
| etc. |
|+-+-+-+-+-+-+-+-+-+-+
Figure 2. CBT FIB entry
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The CBT DR for the specified group fills in the CBT and IP headers as
follows (the CBT header is shown over):
o+ the multicast group address (group-id) is inserted into the
group-id field of the CBT header.
o+ the unicast address of a core router for the corresponding group
is placed in the core address field of the CBT hdr.
o+ the IP address of the originating host is inserted into the ori-
gin field of the CBT header.
o+ the proto field of the CBT header is set to identify the upper-
layer (transport) protocol.
o+ the ttl field of the CBT header is either decremented (if CBT-
style packet was received) or it is set to the value reflected
in the packet's IP hdr (if the pkt originated locally).
o+ the on-tree field of the CBT header is set (provided this CBT
router is on-tree for the specified group). It is left unset
otherwise.
o+ the source address field of the IP header is set to the unicast
address of the originating host (the IP src addr changes as the
CBT-style packet is passed router-to-router on a CBT tree).
o+ the destination field of the IP header is set to the unicast
address of the on-tree neighbour (set to group address if more
than one neighbour is reachable over the same interface).
o+ the protocol field of the IP header is set to the CBT protocol
value.
o+ the TTL value of the IP header is set to MAX_TTL.
The packet is now ready for sending. Once this packet arrives at a
CBT router, the packet is ``reverse-engineered'' (using the informa-
tion carried in the CBT hdr) to produce an IP-style multicast for
sending on directly-connected subnets with group presence.
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Part C
_1. _C_B_T _P_a_c_k_e_t _F_o_r_m_a_t_s _a_n_d _M_e_s_s_a_g_e _T_y_p_e_s
CBT packets travel in IP datagrams. We distinguish between two types
of CBT packet: CBT data packets, and CBT control packets.
CBT data packets carry a CBT header when these packets are traversing
CBT tree branches. The CBT header is positioned immediately behind
the IP header.
CBT control packets carry a CBT control header. All CBT control mes-
sages are implemented over UDP. This makes sense for several reasons:
firstly, all the information required to build a CBT delivery tree is
kept in user space. Secondly, implementation is made considerably
easier.
CBT control messages fall into two categories: primary maintenance
messages, which are concerned with tree-building, re-configuration,
and teardown, and auxiliary maintenance messsages, which are mainly
concerned with general tree maintenance.
_1._1. _C_B_T _H_e_a_d_e_r _F_o_r_m_a_t
See over....
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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 |unused | type | hdr length | protocol |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| checksum | IP TTL | on-tree|unused|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| group identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| core address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| packet origin |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| flow identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| security fields |
| (T.B.D) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3. CBT Header
Each of the fields is described below:
o+ Vers: Version number -- this release specifies version 1.
o+ type: indicates whether the payload is data or control infor-
mation.
o+ hdr length: length of the header, for purpose of checksum
calculation.
o+ protocol: upper-layer protocol number.
o+ checksum: the 16-bit one's complement of the one's complement
of the CBT header, calculated across all fields.
o+ IP TTL: TTL value gleaned from the IP header where the packet
originated. It is decremented each time it traverses a CBT
router.
o+ on-tree: indicates whether the packet is on- or off-tree.
Once this field is set (i.e. on-tree), it is non-changing.
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o+ group identifier: multicast group address.
o+ core address: the unicast address of a core for the group. A
core address is always inserted into the CBT header by an
originating host, since at any instant, it does not know if
the local DR for the group is on-tree. If it is not, the
local DR must unicast the packet to the specified core.
o+ packet origin: source address of the originating end-system.
o+ flow-identifier: value uniquely identifying a previously set
up data stream.
o+ security fields: these fields (T.B.D.) will ensure the
authenticity and integrity of the received packet.
_1._2. _C_o_n_t_r_o_l _P_a_c_k_e_t _H_e_a_d_e_r _F_o_r_m_a_t
The individual fields are described below. It should be noted that the
contents of the fields beyond ``group identifier'' are empty in some
control messages:
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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 |unused | type | code | unused |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| hdr length | checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| group identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| packet origin |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| core address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Core #1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Core #2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Core #3 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Core #4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Core #5 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Resource Reservation fields |
| (T.B.D) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| security fields |
| (T.B.D) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4. CBT Control Packet Header
o+ Vers: Version number -- this release specifies version 1.
o+ type: indicates control message type (see sections 1.3, 1.4).
o+ code: indicates sub-code of control message type.
o+ header length: length of the header, for purpose of checksum
calculation.
o+ checksum: the 16-bit one's complement of the one's complement
of the CBT control header, calculated across all fields.
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o+ group identifier: multicast group address.
o+ packet origin: source address of the originating end-system.
o+ core address: desired/actual core affiliation of control mes-
sage.
o+ Core #Z: Maximum of 5 core addresses may be specified for any
one group. An implementation is not expected to utilize more
than, say, 3.
NOTE: It was an engineering design decision to have a fixed max-
imum number of core addresses, to avoid a variable-sized packet.
o+ Resource Reservation fields: these fields (T.B.D.) are used
to reserve resources as part of the CBT tree set up pro-
cedure.
o+ Security fields: these fields (T.B.D.) ensure the authenti-
city and integrity of the received packet.
_1._3. _P_r_i_m_a_r_y _M_a_i_n_t_e_n_a_n_c_e _M_e_s_s_a_g_e _T_y_p_e_s
There are six types of CBT primary maintenance message, namely:
o+ JOIN-REQUEST: invoked by an end-system, generated and sent
(unicast) by a CBT router to the specified core address. Its
purpose is to establish the sending CBT router as part of the
corresponding delivery tree.
o+ JOIN-ACK: an acknowledgement to the above. The full list of
core addresses is carried in a JOIN-ACK, together with the
actual core affiliation (the join may have been terminated by
an on-tree router on its journey to the specified core, and
the terminating router may or may not be affiliated to the
core specified in the original join). A JOIN-ACK traverses
the same path as the corresponding JOIN-REQUEST, and it is
the receipt of a JOIN-ACK that actually creates a tree
branch.
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o+ JOIN-NACK: a negative acknowledgement, indicating that the
tree join process has not been successful.
o+ QUIT-REQUEST: a request, sent from a child to a parent, to be
removed as a child to that parent.
o+ QUIT-ACK: acknowledgement to the above. If the parent, or the
path to it is down, no acknowledgement will be received
within the timeout period. This results in the child
nevertheless removing its parent information.
o+ FLUSH-TREE: a message sent from parent to all children, which
traverses a complete branch. This message results in all tree
interface information being removed from each router on the
branch, possibly because of a re-configuration scenario.
The JOIN-REQUEST has three valid sub-codes, namely JOIN-ACTIVE, RE-
JOIN-ACTIVE, and RE-JOIN-NACTIVE.
A JOIN-ACTIVE is sent from a CBT router that has no children for the
specified group.
A RE-JOIN-ACTIVE is sent from a CBT router that has at least one
child for the specified group.
A RE-JOIN-NACTIVE originally started out as an active re-join, but
has reached an on-tree router for the corresponding group. At this
point, the router changes the join status to non-active re-join and
forwards it on its parent branch, as does each CBT router that
receives it. Should the router that originated the active re-join
subsequently receive the non-active re-join, it must immediately send
a QUIT-REQUEST to its parent router. It then attempts to re-join
again. In this way the re-join acts as a loop-detection packet.
_1._4. _A_u_x_i_l_l_i_a_r_y _M_a_i_n_t_e_n_a_n_c_e _M_e_s_s_a_g_e _T_y_p_e_s
There are eleven CBT auxilliary maintenance message types:
o+ CBT-DR-SOLICITATION: a request sent from a host to the CBT
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``all-routers'' multicast address, for the address of the
best next-hop CBT router on the LAN to the core as specified
in the solicitation.
o+ CBT-DR-ADVERTISEMENT: a reply to the above. Advertisements
are addressed to the ``all-systems'' multicast group.
o+ CBT-CORE-NOTIFICATION: unicast from a group initiating host
to each core selected for the group, this message notifies
each core of the identities of each of the other core(s) for
the group, together with their core ranking. The receipt of
this message invokes the building of the core tree by all
cores other than the highest-ranked (primary core).
o+ CBT-CORE-NOTIFICATION-REPLY: a notification of acceptance to
becoming a core for a group, to the corresponding end-system.
o+ CBT-ECHO-REQUEST: once a tree branch is established, this
messsage acts as a ``keepalive'', and is unicast from child
to parent.
o+ CBT-ECHO-REPLY: positive reply to the above.
o+ CBT-CORE-PING: unicast from a CBT router to a core when a
tree router's parent has failed. The purpose of this message
is to establish core reachability before sending a JOIN-
REQUEST to it.
o+ CBT-PING-REPLY: positive reply to the above.
o+ CBT-TAG-REPORT: unicast from an end-system to the designated
router for the corresponding group, subsequent to the end-
system receiving a designated router advertisement (as well
as a core notification reply if group-initiating host). This
message invokes the sending of a JOIN-REQUEST if the receiv-
ing router is not already part of the corresponding tree.
o+ CBT-CORE-CHANGE: group-specific multicast by a CBT router
that originated a JOIN-REQUEST on behalf of some end-system
on the same LAN (subnet). The purpose of this message is to
notify end-systems on the LAN belonging to the specified
group of such things as: success in joining the delivery
tree; actual core affiliation.
o+ CBT-DR-ADV-NOTIFICATION: multicast to the CBT ``all-routers''
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address, this message is sent subsequent to receiving a CBT-
DR-SOLICITATION, but prior to any CBT-DR-ADVERTISEMENT being
sent. It acts as a tie-breaking mechanism should more than
one router on the subnet think itself the best next-hop to
the addressed core. It also promts an already established DR
to announce itself as such if it has not already done so in
response to a CBT-DR-SOLICITATION.
Part D
_1. _I_n_t_e_r_o_p_e_r_a_b_i_l_i_t_y _I_s_s_u_e_s
One of the design goals of CBT is for it to fully interwork with
other IP multicast schemes. We have already described how CBT-style
packets are transformed into IP-style multicasts, and vice-versa.
In order for CBT to fully interwork with other schemes, it is neces-
sary to define the interface(s) between a ``CBT cloud'' and the cloud
of another scheme. The CBT authors are currently working out the
details of the ``CBT-other'' interface, and therefore we omit further
discussion of this topic at the present time.
_2. _A _R_o_u_t_e_r _O_p_t_i_m_i_z_a_t_i_o_n
In a CBT-only environment it is possible to optimize the performance
of CBT with respect to data packet forwarding in CBT-capable routers.
In such an environment the presence of a CBT header is not necessary,
and its absence is likely to improve switching times by around 50 per
cent. However, the downside is that the functionality the CBT header
provides, such as CBT security, is lost.
_3. _C_B_T _S_e_c_u_r_i_t_y _A_r_c_h_i_t_e_c_t_u_r_e
see current I-D: draft-ballardie-mkd-00.{ps,txt}
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INTERNET-DRAFT Core Based Trees (CBT) Multicast April 1995
_4. _A_c_k_n_o_w_l_e_d_g_e_m_e_n_t_s
Special thanks goes to Paul Francis, NTT Japan, for the original
brainstorming sessions that brought about this work.
Steve Ostrowitz (Bay Networks Inc.) for his suggestions and comments
on making a CBT router implemention as optimal as possible.
I would also like to thank the participants of the IETF IDMR working
group meetings for their general constructive comments and sugges-
tions since the inception of CBT.
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INTERNET-DRAFT Core Based Trees (CBT) Multicast April 1995
Author's Address:
Tony Ballardie,
Department of Computer Science,
University College London,
Gower Street,
London, WC1E 6BT,
ENGLAND, U.K.
Tel: ++44 (0)71 387 7050 x. 3462
e-mail: A.Ballardie@cs.ucl.ac.uk
NOTE: For a version of this draft containing all diagrams and refer-
ences, you are recommended to retrieve the .ps version.
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