Inter-Domain Multicast Routing (IDMR) A. J. Ballardie
INTERNET-DRAFT University College London
S. Reeve
Bay Networks, Inc.
N. Jain
Bay Networks, Inc.
February 9th, 1996
Core Based Trees (CBT) Multicast
-- Protocol Specification --
<draft-ietf-idmr-cbt-spec-04.txt>
Status of this Memo
This document is an Internet Draft. Internet Drafts are working do-
cuments 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 protocol specification. CBT is a next-generation multicast
protocol that makes use of a shared delivery tree rather than
separate per-sender trees utilized by most other multicast schemes
[1, 2, 3].
This specification includes a description of an optimization whereby
native IP-style multicasts are forwarded over tree branches as well
as subnetworks with group member presence. This mode of operation
will be called CBT "native mode" and obviates the need to encapsulate
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data packets before forwarding over CBT tree interfaces. Native mode
is only relevant to CBT-only domains or ``clouds''. Also included are
some new "data-driven" features.
A special authors' note is included explaining the latest updates to
the CBT specification, together with some nomenclature, and miscel-
laneous items.
This document is progressing through the IDMR working group of the
IETF. The CBT architecture is described in an accompanying document:
ftp://cs.ucl.ac.uk/darpa/IDMR/draft-ietf-idmr-arch-00.txt. Other
related documents include [4, 5]. For all IDMR-related documents, see
http://www.cs.ucl.ac.uk/ietf/idmr.
1. Authors' Note
The purpose of this note is to explain how the CBT protocol has
evolved since the previous version (November 1995).
Since the previous release, CBT has been assigned official IP proto-
col and UDP port numbers (section 8).
The CBT designers have constantly been seeking to streamline the pro-
tocol and seek new mechanisms to simplify the group initiation pro-
cedure. Especially, it has been a high priority to ensure that join
latency be kept to an absolute minimum. The November '95 draft intro-
duced the re-invented subnet designated router (DR) election pro-
cedure, described here in section 2.3.
The concept of proxy-ACKs was introduced in the November '95 draft,
but these have been removed since the extra message overhead does not
warrant the negligible gain they provide.
The CBT loop detection mechanism (comprising rejoin-active and
rejoin-nactive) has been slightly modified, and is now simpler and
more straighforward. The revised mechanism incorporates a new join
ack subcode, and is explained in section 5.3.
Core selection, placement, and management, which have prevented sim-
ple group initiation/joining, apparent in data-driven schemes (like
DVMRP), have been separated out from the protocol itself. Core
management is not a problem unique to CBT, but also PIM-Sparse Mode.
Separate, protocol-independent core management mechanisms are
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currently being proposed/developed [8, 9]. In the absence of core
management/distribution protocol, the task could be manually handled
by network management facilities.
In CBT, the core routers for a particular group are categorised into
PRIMARY CORE, and NON-PRIMARY (secondary) CORES.
The core tree, the part of a tree linking all core routers together,
is built on-demand (section 2.4). That is, the core tree is only
built subsequent to a non-primary core receiving a join-request
(non-primary core routers join the primary core router -- the primary
need do nothing). Join-requests carry an ordered list of core routers
(and the identity of the primary core in its own separate field),
making it possible for the non-primary cores to know where to join.
On-demand core tree building is explained as part of section 2.4.
CBT now supports the aggregation of neighbour keepalives, which pre-
viously were sent on a per group basis. Any two adjacent CBT routers
need only send a single keepalive between each other, rather than one
per group. Additional aggregation strategies are currently being
worked on, and we present some ideas on aggregated rejoins in Appen-
dix A. An updated draft fully specifying CBT aggregation strategy
should appear soon.
The end result of these developments is that the CBT protocol is much
simplified and more efficient.
2. Protocol Specification
2.1. CBT Group Initiation
The requirement of hosts to discover the identity of candidate core
routers (or RPs) differentiates the role of hosts in shared tree mul-
ticast protocols and shortest-path tree multicast protocols; the
latter need only announce their desire to join a group by means of an
IGMP membership report. It is highly desirable that hosts wishing to
join a shared tree need only do the same, leaving local multicast
routers to discover <core, group> mappings, or have local routers
configured with the identity of core(s) in the next level of a
hierarchy, as suggested by Hierarchical PIM [8].
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If the latter approach is eventually adopted by the IETF, then host
operations need not differ due to the type of multicast tree being
joined, and indeed, the type of tree being joined for a particular
group can remain transparent to the host.
If the latter approach is not adopted, then hosts need to inform
their local multicast router of a <core, group> mapping for each
group joined. This requires hosts to discover <core, group> mappings,
which in turn requires the existence of a (global) core advertisement
protocol. Hosts subsequently need a means of advertising <core,
group> mappings to the local multicast router so it can initiate a
join. This requires an extension to IGMP, for example, the presence
of IGMP RP/Core Reports, as suggested in IGMP version 3 [7], or the
protocol itself must provide a means (message) for advertising cores
to the local router. In the absence of H-PIM, some similar mechanism,
or IGMPv3, CBT implementors may wish to extend CBT to include a core
reporting message for group initiators/joiners (for example, whenever
a group is initiated/joined, a configuration file is read which holds
<core, group> mappings).
Alternatively, <core, group> mappings can be downloaded to local mul-
ticast routers by means of network management tools.
2.2. Tree Joining Process -- Overview
A local CBT router is notified, by IGMP, of a host's desire to join a
group. If more than one CBT router is present on the subnetwork, each
will receive the IGMP membership report. However, only one, the
default subnet designated router (DEFAULT DR) will act upon the
receipt of a report by initiating a CBT join. Note, a CBT join is
only initiated if the subnetwork is not yet part of the delivery
tree. Also, we assume that the local CBT default DR discovers <core,
group> mappings by one of the mechanisms described in the previous
section. DR election is described in section 2.3.
The following CBT control messages come into play subequent to the
host sending an IGMP join (host membership report):
+ JOIN_REQUEST
+ JOIN_ACK
A join-request is generated by a locally-elected DR (see next
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section) in response to receiving an IGMP group membership report
from a directly connected host. The join is sent to the next-hop on
the path to the target core, as specified in the join packet. The
join is processed by each such hop on the path to the core, until
either the join reaches the target core itself, or hits a router that
is already part of the corresponding distribution tree (as identified
by the group address). In both cases, the router concerned terminates
the join, and responds with a join-ack, which traverses the reverse-
path of the corresponding join. This is possible due to the transient
path state created by a join traversing a CBT router. The ack fixes
that state.
2.3. DR Election
Multiple CBT routers may be connected to a multi-access subnetwork.
In such cases it is necessary to elect a (sub)network designated
router (DR) that is responsible for sending IGMP host membership
queries, and generating join-requests in response to receiving IGMP
group membership reports. Such joins are forwarded upstream by the
DR.
The IGMP querier election is as follows (note, here we talk about
"CBT routers", but the described mechanism also applies to the gen-
eral case). At start-up, a CBT router assumes it is the only CBT-
capable router on its subnetwork. It therefore sends two IGMP-HOST-
MEMBERSHIP-QUERYs in short succession (within 5 secs) (for robust-
ness) in order to quickly learn about any group memberships on the
subnet. If other CBT routers are present on the same subnet, they
will receive these IGMP queries, and depending on which router was
already the elected querier, yield querier duty to the new router iff
the new router is lower-addressed. If it is not, then the newly-
started CBT router will yield when it hears a query from the already
established querier.
The CBT DEFAULT DR (D-DR) is always (footnote 1) the subnet's IGMP-
_________________________
1 This document does not address the case where some
routers on a multi-access subnet may be running multi-
cast routing protocols other than CBT. In such cases,
IGMP querier may be a non-CBT router, in which case the
CBT DR election breaks. This will be discussed in a CBT
interoperability document, to appear shortly.
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querier; in CBT these two roles go hand-in-hand. As a result, there
is no protocol overhead whatsoever associated with electing the CBT
D-DR.
2.4. Tree Joining Process -- Details
The receipt of an IGMP group membership report by a CBT D-DR for a
CBT group not previously heard from triggers the tree joining pro-
cess.
Immediately subsequent to receiving an IGMP group membership report
for a CBT group not previously heard from, the D-DR unicasts a JOIN-
REQUEST to the first hop on the (unicast) path to the target core
specified in the CBT join packet.
Each CBT-capable router traversed on the path between the sending DR
and the core processes the join. However, if a join hits a CBT router
that is already on-tree (footnote), the join is not propogated
further, but ACK'd downstream from that point.
JOIN-REQUESTs carry the identity of all cores for the group. Assuming
there are no on-tree routers in between, once the join (subcode
ACTIVE_JOIN) reaches the target core, if the target core is not the
primary core (as indicated in a separate field of the join packet) it
first acknowledges the received join by means of a JOIN-ACK, then
sends a JOIN-REQUEST, subcode REJOIN-ACTIVE, to the primary core
router. Either the primary core, or the first on-tree router encoun-
tered, acknowledges the received rejoin by means of a JOIN-ACK. In
the former case, the primary core responds by sending a join-ack,
subcode PRIMARY-REJOIN-ACK, which traverses the reverse-path of the
join. In the latter case, the join-ack is returned with subcode NOR-
MAL; the receiving router responds to this with a rejoin-Nactive, for
loop detection. Note that loop detection is not necessary subsequent
to receiving a join-ack with subcode PRIMARY-REJOIN-ACK. Loop detec-
tion is described further in section 5.3.
To facilitate detailed protocol description, we use a sample topol-
ogy, illustrated in Figure 1 (shown over). Member hosts are shown as
individual capital letters, routers are prefixed with R, and subnets
_________________________
"on-tree" describes whether a router has a FIB entry
for the corresponding group.
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are prefixed with S.
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A B
| S1 S4 |
------------------- -----------------------------------------------
| | | |
------ ------ ------ ------
| R1 | | R2 | | R5 | | R6 |
------ ------ ------ ------
C | | | | |
| | | | S2 | S8 |
---------- ------------------------------------------ -------------
S3 |
------
| R3 |
| ------ D
| S9 | | S5 |
| | ---------------------------------------------
| |----| | |
---| R7 |-----| ------
| |----| |------------------| R4 |
| S7 | ------ F
| | | S6 |
|-E | ---------------------------------
| |
| ------
|---| |---------------------| R8 |
|R12 -----| ------ G
|---| | | | S10
| S14 ----------------------------
| |
I --| ------
| | R9 |
------
| S12
| ----------------------------
S15 | |
| ------
|----------------------|R10 |
J ---| ------ H
| | |
| ----------------------------
| S13
Figure 1. Example Network Topology
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Taking the example topology in figure 1, host A is the group initia-
tor, and has elected core routers R4 (primary core) and R9 (secondary
core) by some external protocol. We assume the local CBT DR discovers
<core,group> mappings by "some means", possible one of the mechanisms
described in section 2.1.
Router R1 receives an IGMP host membership report, and proceeds to
unicast a JOIN-REQUEST, subcode ACTIVE-JOIN to the next-hop on the
path to R4 (R3), the target core. R3 receives the join, caches the
necessary group information, and forwards it to R4 -- the target of
the join.
R4, being the target of the join, sends a JOIN_ACK back out of the
receiving interface to the previous-hop sender of the join, R3. A
JOIN-ACK, like a JOIN-REQUEST, is processed hop-by-hop by each router
on the reverse-path of the corresponding join. The receipt of a
join-ack establishes the receiving router on the corresponding CBT
tree, i.e. the router becomes part of a branch on the delivery tree.
Finally, R3 sends a join-ack to R1. A new CBT branch has been
created, attaching subnet S1 to the CBT delivery tree for the
corresponding group (footnote 2).
For the period between any CBT-capable router forwarding (or ori-
ginating) a JOIN_REQUEST and receiving a JOIN_ACK the corresponding
router is not permitted to acknowledge any subsequent joins received
for the same group; rather, the router caches such joins till such
time as it has itself received a JOIN_ACK for the original join. Only
then can it acknowledge any cached joins. A router is said to be in a
pending-join state if it is awaiting a JOIN_ACK itself.
Note that the presence of underlying transient asymmetric routes is
irrelevant to the tree-building process; CBT tree branches are sym-
metric by the nature in which they are built. Joins set up transient
state (incoming and outgoing interface state) in all routers along a
path to a particular core. The corresponding join-ack traverses the
reverse-path of the join as dictated by the transient state, and not
the path that underlying routing would dictate. Whilst permanent
asymmetric routes could pose a problem for CBT, transient
_________________________
2 At this point, it is proposed that IGMP (v3) group
multicasts a notification across the subnet indicating
to member hosts that the delivery tree has been joined
successfully. Such a message would greatly benefit mul-
ticast protocols requiring explicit joins [5, 10].
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asymmetricity is detected by the CBT protocol.
2.5. Default DRs and Group DRs
The DR election mechanism does not guarantee that the DR will be the
router that actually forwards a join off a multi-access network; the
first hop on the path to a particular core might be via another
router on the same (sub)network, which actually forwards off-subnet.
The CBT router that becomes the interface between the subnet and the
rest of the CBT tree, i.e. the CBT router at which a join-ack arrives
on the subnet, becomes the CBT GROUP DR. This group-specific DR (G-
DR) is a token (implicit) identity. In the normal case where there is
no subnet extra hop, the receipt of a JOIN-ACK means that the D-DR
becomes the G-DR for the specified group.
Although very much the same, let's see another example using our
example topology of figure 1 of a host joining a CBT tree for the
case where more than one CBT router exists on the host subnetwork.
B's subnet, S4, has 3 CBT routers attached. Assume also that R6 has
been elected IGMP-querier and CBT D-DR.
R6 (S4's D-DR) receives an IGMP group membership report. By some
means, R6 discovers the <core, group> mapping for the group specified
in the report; R4 is the target core for the group. R6 generates a
join-request for target core R4, subcode ACTIVE_JOIN. R6's routing
table says the next-hop on the path to R4 is R2, which is on the same
subnet as R6. This is irrelevant to R6, which unicasts it to R2. R2
unicasts it to R3, which happens to be already on-tree for the speci-
fied group (from R1's join). R3 therefore can acknowledge the arrived
join and unicast it back to R2. R2 realises it is not the origin of
the corresponding join-request, but sees that the origin (R6) is on
the same subnet as itself, and that over which the join-ack should be
forwarded to the origin, R6. R2 unicasts the join-ack on its final
hop. R2 has thus become the group's G-DR, with R6 remaining the D-DR
for all groups.
If an IGMP membership report is received by a D-DR with a join for
the same group already pending, or if the D-DR is already on-tree for
the group, it takes no action.
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2.6. Tree Teardown
There are two scenarios whereby a tree branch may be torn down:
+ During a re-configuration. If a router's best next-hop to the
specified core is one of its existing children, then before
sending the join it must tear down that particular downstream
branch. It does so by sending a FLUSH_TREE message which is pro-
cessed hop-by-hop down the branch. All routers receiving this
message must process it and forward it to all their children.
Routers that have received a flush message will re-establish
themselves on the delivery tree if they have directly connected
subnets with group presence.
+ If a CBT router has no children it periodically checks all its
directly connected subnets for group member presence. If no
member presence is ascertained on any of its subnets it sends a
QUIT_REQUEST upstream to remove itself from the tree.
The following example, using the example topology of figure 1, shows
how a tree branch is gracefully torn down using a QUIT_REQUEST.
Assume group member B leaves group G on subnet S4. B issues an IGMP
HOST-MEMBERSHIP-LEAVE (relevant only to IGMPv2 and later versions)
message which is multicast to the "all-routers" group (224.0.0.2).
R6, the subnet's D-DR and IGMP-querier, responds with a group-
specific-QUERY. No hosts respond within the required response inter-
val, so D-DR assumes group G traffic is no longer wanted on subnet
S4.
Since R6 has no CBT children, and no other directly attached subnets
with group G presence, it immediately follows on by sending a
QUIT_REQUEST to R2, its parent on the tree for group G. R2 responds
with a QUIT-ACK, unicast to R6; R2 removes the corresponding child
information. R2 in turn sends a QUIT upstream to R3 (since it has no
other children or subnet(s) with group presence).
NOTE: immediately subsequent to sending a QUIT-REQUEST, the sender
removes the corresponding parent information, i.e. it does not
wait for the receipt of a QUIT-ACK.
R3 responds to the QUIT by unicasting a QUIT-ACK to R2. R3 subse-
quently checks whether it in turn can send a quit by checking group G
presence on its directly attached subnets, and any group G children.
It has the latter (R1 is its child on the group G tree), and so R3
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cannot itself send a quit. However, the branch R3-R2-R6 has been
removed from the tree.
3. Data Packet Forwarding Rules
When a router receives (non-locally originated) data packets for for-
warding over directly attached member subnets, it only does so over
the set of outgoing member subnets (interfaces) for which that router
is DR, irrespective of whether group membership is registered on
other local interfaces. In addition, in native mode, packets are for-
warded over any remaining interfaces specified by the FIB entry for
the group that are not in the above set (excluding the incoming
interface). In CBT mode, encapsulated data packets are forwarded over
the full set of interfaces specified by the FIB entry, except the
incoming interface.
A router only forwards data packets originated by directly attached
hosts iff the router is the DR on the interface over which those
packets were received.
4. Data Packet Forwarding -- Encapsulation Details
In "native mode" all data packets are forwarded over CBT tree inter-
faces as native IP multicasts, i.e. there are no encapsulations
required. This assumes that CBT is the multicast routing protocol in
operation within the domain (or "cloud") in question, and that all
routers within the domain of operation are CBT-capable, i.e. there
are no "tunnels".
In a multi-protocol environment, whose infrastructure may include
non-multicast-capable routers, it is necessary to tunnel data packets
between CBT-capable routers. This is called "CBT mode". Data packets
are de-capsulated by CBT routers (such that they become native mode
data packets) before being forwarded over subnets with member hosts.
When multicasting (native mode) to member hosts, the TTL value of the
original IP header is set to one. CBT mode encapsulation is as fol-
lows:
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++++++++++++++++++++++++++++++++++++++++++++++++++++++++
| encaps IP hdr | CBT hdr | original IP hdr | data ....|
++++++++++++++++++++++++++++++++++++++++++++++++++++++++
Figure 2. Encapsulation for CBT mode
The TTL value of the CBT header is set by the encapsulating CBT
router directly attached to the origin of a data packet. This value
is decremented each time it is processed by a CBT router. An encap-
sulated data packet is discarded when the CBT header TTL value
reaches zero.
The purpose of the (outer) encapsulating IP header is to "tunnel"
data packets between CBT-capable routers (or "islands"). The outer IP
header's TTL value is set to the "length" of the corresponding tun-
nel, or MAX_TTL (255)if this is not known, or subject to change.
For native mode IP multicasts, i.e. those without any extra encapsu-
lation, the TTL value of the IP header is decremented each time the
packet is received by a multicast router.
It is worth pointing out here the distinction between subnetworks and
tree branches, although they can be one and the same. For example, a
multi-access subnetwork containing routers and end-systems could
potentially be both a CBT tree branch and a subnetwork with group
member presence. A tree branch which is not simultaneously a subnet-
work is either a "tunnel" or a point-to-point link.
In CBT mode there are three forwarding methods used by CBT routers:
+ IP multicasting. This method is used to send a data packet
across a directly-connected subnetwork with group member pres-
ence. System host changes are not required for CBT. Similarly,
end-systems originating multicast data do so in traditional IP-
style.
+ CBT unicasting. This method is used for sending data packets
encapsulated (as illustrated above) across a tunnel or point-
to-point link. En/de-capsulation takes place in CBT routers.
+ CBT multicasting. Routers on multi-access links use this method
to send data packets encapsulated (as illustrated above) but the
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outer encapsulating IP header contains a multicast address. This
method is used when a parent or multiple children are reachable
over a single physical interface, as could be the case on a
multi-access Ethernet. The IP module of end-systems subscribed
to the same group will discard these multicasts since the CBT
payload type (protocol id) of the outer IP header is not recog-
nizable by hosts.
CBT routers create Forwarding Information Base (FIB) entries whenever
they send or receive a JOIN_ACK. The FIB describes the parent-child
relationships on a per-group basis. A FIB entry dictates over which
tree interfaces, and how (unicast or multicast) a data packet is to
be sent. Additionally, a data packet is IP multicast over any
directly-connected subnetworks with group member presence. Such
interfaces are kept in a separate table relating to IGMP. A FIB entry
is shown below:
32-bits 4 4 4 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 3. CBT FIB entry
Note that a CBT FIB is required for both CBT-mode and native-mode
multicasting.
The field lengths shown above assume a maximum of 16 directly con-
nected neighbouring routers.
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When a data packet arrives at a CBT router, the following rules
apply:
+ if the packet is an IP-style multicast, it is checked to see if
it originated locally (i.e. if the arrival interface subnetmask
bitwise ANDed with the packet's source IP address equals the
arrival interface's subnet number, then the packet was sourced
locally). If the packet is not of local origin, it is discarded.
+ the packet is IP multicast to all directly connected subnets
with group member presence. The packet is sent with an IP TTL
value of 1 in this case.
+ the packet is encapsulated for CBT forwarding (see figure 2) and
unicast to parent and children. However, if more than one child
is reachable over the same interface the packet will be CBT mul-
ticast. Therefore, it is possible that an IP-style multicast and
a CBT multicast will be forwarded over a particular subnetwork.
NOTE: the TTL value of encapsulated data packets is manipulated as
described at the beginning of this section.
Using our example topology in figure 1, let's assume member G ori-
ginates an IP multicast packet. R8 is the DR for subnet S10. R8 CBT
unicasts the packet to each of its children, R9 and R12. These chil-
dren are not reachable over the same interface. R8, being the DR for
subnets S14 and S10 also IP multicasts the packet to S14 (S10
received the IP style packet already from the originator). R9, the DR
for S12, need not IP multicast onto S12 since there are no members
present there. R9 CBT unicasts the packet to R10, which is the DR for
S13 and S15. It IP multicasts to both S13 and S15.
Going upstream from R8, R8 CBT unicasts to R4. It is DR for all
directly connected subnets and therefore IP multicasts the data
packet onto S5, S6 and S7, all of which have member presence. R4 uni-
casts the packet to all outgoing children, R3 and R7 (NOTE: R4 does
not have a parent since it is the primary core router for the group).
R7 IP multicasts onto S9. R3 CBT unicasts to R1 and R2, its children.
Finally, R1 IP multicasts onto S1 and S3, and R2 IP multicasts onto
S4.
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4.1. Non-Member Sending
For a multicast data packet to span beyond the scope of the originat-
ing subnetwork at least one CBT-capable router must be present on
that subnetwork. The default DR (D-DR) for the group on the subnet-
work must encapsulate the (native) IP-style packet and unicast it to
a core for the group. In native mode this encapsualation constitutes
IP-in-IP. In CBT mode, the encapsulation required is shown in figure
2. In both cases, CBT routers are required to know <core, group> map-
pings. The alternatives for discovering these are discussed in sec-
tion 2.1. Beyond this, this topic is beyond the scope of this docu-
ment.
5. Eliminating the Topology-Discovery Protocol in the Presence of Tun-
nels
Traditionally, multicast protocols operating within a virtual topol-
ogy, i.e. an overlay of the physical topology, have required the
assistance of a multicast topology discovery protocol, such as that
present in DVMRP. However, it is possible to have a multicast proto-
col operate within a virtual topology without the need for a multi-
cast topology discovery protocol. One way to achieve this is by hav-
ing a router configure all its tunnels to its virtual neighbours in
advance. A tunnel is identified by a local interface address and a
remote interface address. Routing is replaced by "ranking" each such
tunnel interface associated with a particular core address; if the
highest-ranked route is unavailable (tunnel end-points are required
to run an Hello-like protocol between themselves) then the next-
highest ranked available route is selected, and so on. The exact
specification of the Hello protocol is outside the scope of this
document.
CBT trees are built using the same join/join-ack mechanisms as
before, only now some branches of a delivery tree run in native mode,
whilst others (tunnels) run in CBT mode. Underlying unicast routing
dictates which interface a packet should be forwarded over. Each
interface is configured as either native mode or CBT mode, so a
packet can be encapsulated (decapsulated) accordingly.
As an example, router R's configuration would be as follows:
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intf type mode remote addr
-----------------------------------
#1 phys native -
#2 tunnel cbt 128.16.8.117
#3 phys native -
#4 tunnel cbt 128.16.6.8
#5 tunnel cbt 128.96.41.1
core backup-intfs
--------------------
A #5, #2
B #3, #5
C #2, #4
The CBT FIB needs to be slightly modified to accommodate an extra
field, "backup-intfs" (backup interfaces). The entry in this field
specifies a backup interface whenever a tunnel interface specified in
the FIB is down. Additional backups (should the first-listed backup
be down) are specified for each core in the core backup table. For
example, if interface (tunnel) #2 were down, and the target core of a
CBT control packet were core A, the core backup table suggests using
interface #5 as a replacement. If interface #5 happened to be down
also, then the same table recommends interface #2 as a backup for
core A.
6. Tree Maintenance
Once a tree branch has been created, i.e. a CBT router has received a
JOIN_ACK for a JOIN_REQUEST previously sent (forwarded), a child
router is required to monitor 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. Adja-
cent CBT routers only need to send one keepalive per link, regardless
of how many groups are present on that link. This aggregation stra-
tegy is expected to conserve considerable bandwidth on "busy" links,
such as those nearer the "centre" of the network.
The keepalive protocol is simple, as follows: a child unicasts a
CBT-ECHO-REQUEST to its parent, which unicasts a CBT-ECHO-REPLY in
response.
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For any CBT router, if its parent router, or path to the parent,
fails, the child is initially responsible for re-attaching itself,
and therefore all routers subordinate to it on the same branch, to
the tree.
6.1. Router Failure
An on-tree router can detect a failure from the following two cases:
+ if the child responsible for sending keepalives across a partic-
ular link stops receiving CBT_ECHO_REPLY messages. In this case
the child realises that its parent has become unreachable and
must therefore try and re-connect to the tree for all groups
represented on the parent/child link. Until an aggregation stra-
tegy is fully worked out, a (re)join must be sent for each group
individually. (We present some ideas on rejoin aggregation in
Appendix A).
The rejoining router (that which is immediately subordinate to
the failure) sends a JOIN_REQUEST (subcode ACTIVE_JOIN if it has
no children attached, and subcode ACTIVE_REJOIN if at least one
child is attached) to the best next-hop router on the path to
the elected core. If no JOIN-ACK is received after three
retransmissions, each transmission being at PEND-JOIN-INTERVAL
(10 secs), an alternate core is elected from the core list, and
the process repeated. If all cores have been tried unsuccess-
fully, the D-DR has no option but to give up.
+ if a parent stops receiving CBT_ECHO_REQUESTs from a child. In
this case the parent simply removes the child interface from FIB
entries that are represented by that parent/child link.
6.2. Router Re-Starts
There are two cases to consider here:
+ Core re-start. All JOIN-REQUESTs (all types) carry the identi-
ties (i.e. addresses) of each of the cores for a group. If a
router is a core for a group, but has only recently re-started,
it will not be aware that it is a core for any group(s). In such
circumstances, a core only becomes aware that it is such by
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receiving a JOIN-REQUEST. Subsequent to a core learning its
status in this way, if it is not the primary core it ack-
nowledges the received join, then sends a JOIN_REQUEST (subcode
ACTIVE_REJOIN) to the primary core. If the re-started router is
the primary core, it need take no action, i.e. in all cir-
cumstances, the primary core simply waits to be joined by other
routers.
+ Non-core re-start. In this case, the router can only join the
tree again if a downstream router sends a JOIN_REQUEST through
it, or it is elected DR for one of its directly attached sub-
nets, and subsequently receives an IGMP membership report.
6.3. Route Loops
Routing loops are only a concern when a router with at least one
child is attempting to re-join a CBT tree. In this case the re-
joining router sends a JOIN_REQUEST (subcode ACTIVE REJOIN) to the
best next-hop on the path to an elected core. This join is forwarded
as normal until it reaches either the specified core, another core,
or a non-core router that is already part of the tree. If the rejoin
reaches the primary core, loop detection is not necessary. The pri-
mary core acks an active-rejoin by means of a JOIN-ACK, subcode
PRIMARY-REJOIN-ACK. This ack must be processed by each router on the
reverse-path of the active-rejoin. If an active-rejoin is terminated
by any router on the tree other than the primary core, loop detection
must take place, as we now describe.
If, in response to an active-rejoin, a JOIN-ACK is returned, subcode
NORMAL (as opposed to an ack with subcode PRIMARY-REJOIN-ACK), the
router receiving the ack subsequently generates a JOIN-REQUEST, sub-
code NACTIVE-REJOIN (non-active rejoin). This packet serves only to
detect loops; it does not create any transient state in the routers
it traverses, other than the originating router. Any on-tree router
receiving a non-active rejoin is required to forward it over its
parent interface for the specified group. In this way, it will either
reach the primary core, which returns, directly to the sender, a join
ack with subcode PRIMARY-NACTIVE-ACK (so the sender knows no loop is
present), or the sender receives the non-active rejoin it sent, via
one of its child interfaces, in which case the rejoin obviously
formed a loop.
If a loop is present, the non-active join originator immediately
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sends a QUIT_REQUEST to its newly-established parent and the loop is
broken.
Using figure 4 (over) to demonstrate this, if R3 is attempting to
re-join the tree (R1 is the core in figure 4) and R3 believes its
best next-hop to R1 is R6, and R6 believes R5 is its best next-hop to
R1, which sees R4 as its best next-hop to R1 -- a loop is formed. R3
begins by sending a JOIN_REQUEST (subcode ACTIVE_REJOIN, since R4 is
its child) to R6. R6 forwards the join to R5. R5 is on-tree for the
group, so responds to the active-rejoin with a JOIN-ACK, subcode NOR-
MAL (the ack traverses R6 on its way to R3). R3 now generates a
JOIN-REQUEST, subcode NACTIVE-REJOIN, and forwards this to its
parent, R6. R6 forwards the non-active rejoin to R5, its parent. R5
does similarly, as does R4. Now, the non-active rejoin has reached
R3, which originated it, so R3 concludes a loop is present on the
parent interface for the specified group. It immediately sends a
QUIT_REQUEST to R6, which in turn sends a quit if it has not received
an ACK from R5 already AND has itself a child or subnets with member
presence. If so it does not send a quit -- the loop has been broken
by R3 sending the first quit.
QUIT_REQUESTs are typically acknowledged by means of a QUIT_ACK. A
child removes its parent information immediately subsequent to send-
ing its first QUIT-REQUEST. The ack here serves to notify the (old)
child that it (the parent) has in fact removed its child information.
However, there might be cases where, due to failure, the parent can-
not respond. The child sends a QUIT-REQUEST a maximum of three
times, at PEND-QUIT-INTERVAL (10 sec) intervals.
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------
| R1 |
------
|
---------------------------
|
------
| R2 |
------
|
---------------------------
| |
------ |
| R3 |--------------------------|
------ |
| |
--------------------------- |
| | ------
------ | | |
| R4 | |-------| R6 |
------ | |----|
| |
--------------------------- |
| |
------ |
| R5 |--------------------------|
------ |
|
Figure 4: Example Loop Topology
In another scenario the rejoin travels over a loop-free path, and the
first on-tree router encountered is the primary core, R1. In figure
4, R3 sends a join, subcode REJOIN_ACTIVE to R2, the next-hop on the
path to core R1. R2 forwards the re-join to R1, the primary core,
which returns a JOIN-ACK, subcode PRIMARY-REJOIN-ACK, over the
reverse-path of the rejoin-active. Whenever a router receives a
PRIMARY-REJOIN-ACK no loop detection is necessary.
If we assume R2 is on tree for the corresponding group, R3 sends a
join, subcode REJOIN_ACTIVE to R2, which replies with a join ack,
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subcode NORMAL. R3 must then generate a loop detection packet (join
request, subcode REJOIN-NACTIVE) which is forwarded to its parent,
R2, which does similarly. On receipt of the rejoin-Nactive, the pri-
mary core unicasts a join ack back directly to R3, with subcode
PRIMARY-NACTIVE-ACK. This confirms to R3 that its rejoin does not
form a loop.
7. Data Packet Loops
The CBT protocol builds a loop-free distribution tree. If all routers
that comprise a particular tree function correctly, data packets
should never traverse a tree branch more than once.
CBT routers will only forward native-style data packets if they are
received over a valid on-tree interface. A native-style data packet
that is not received over such an interface is discarded.
Encapsulated CBT data packets from a non-member sender can arrive via
an "off-tree" interface (this is how CBT-mode sends data across tun-
nels, and how data from non-member senders in native-mode or CBT-mode
reaches a tree). The encapsulating CBT data packet header includes
an "on-tree" field, which contains the value 0x00 until the data
packet reaches an on-tree router. At this point, the router must con-
vert this value to 0xff to indicate the data packet is now on-tree.
This value remains unchanged, and from here on the packet should
traverse only on-tree interfaces. If an encapsulated packet happens
to "wander" off-tree and back on again, the latter on-tree router
will receive the CBT encapsulated packet via an off-tree interface.
However, this router will recognise that the "on-tree" field of the
encapsulating CBT header is set to 0xff, and so immediately discards
the packet.
8. CBT Packet Formats and Message Types
CBT packets travel in IP datagrams. We distinguish between two types
of CBT packet: CBT data packets, and CBT control packets. CBT con-
trol packets carry a CBT control header. All CBT control messages are
implemented over UDP. CBT mode data (figure 2) requires a CBT data
packet header.
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8.1. CBT Header Format (for CBT Mode data)
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 | on-tree|unused|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| checksum | IP TTL | unused |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| group identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| reserved | reserved | Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| .....VALUE.... |
| (for flow-id and/or security options) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5. CBT Header
Each of the fields is described below:
+ Vers: Version number -- this release specifies version 1.
+ type: indicates CBT payload is data. The only value defined
for this field is 255 (0xff).
+ hdr length: length of the header, for purpose of checksum
calculation.
+ on-tree: indicates whether the packet is on-tree (0xff) or
off-tree (0x00). Once this field is set (i.e. on-tree), it
is non-changing. This field can only be set by a router that
has a FIB entry for the corresponding group, i.e. a router
that has received a join-ack for a join-request previously
sent/forwarded.
+ checksum: the 16-bit one's complement of the one's complement
of the CBT header, calculated across all fields.
+ IP TTL: TTL value gleaned from the IP header where the packet
originated. It is decremented each time it traverses a CBT
router.
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+ group identifier: multicast group address.
+ The TLV fields at the end of the header are for a flow-
identifier, and/or security options, if and when implemented.
A "type" value of zero implies a "length" of zero, implying
there is no "value" field.
8.2. Control Packet Header Format
The individual fields are described below. It should be noted that only
certain fields beyond ``group identifier'' are processed for the dif-
ferent control messages.
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 | # cores |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| hdr length | checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| group identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| packet origin |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| primary core address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| target core address (core #1) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Core #2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Core #3 |
| .... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| reserved | reserved | Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| .....VALUE.... |
| (for flow-id and/or security options) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6. CBT Control Packet Header
+ Vers: Version number -- this release specifies version 1.
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+ type: indicates control message type (see sections 7.3,
7.3.1).
+ code: indicates subcode of control message type.
+ # cores: number of core addresses carried by this control
packet (does not include "primary core address" field).
+ header length: length of the header, for purpose of checksum
calculation.
+ checksum: the 16-bit one's complement of the one's complement
of the CBT control header, calculated across all fields.
+ group identifier: multicast group address.
+ packet origin: address of the CBT router that originated the
control packet.
+ primary core address: the address of the primary core for the
group.
+ target core address: desired core affiliation of control mes-
sage.
+ Core #Z: Z refers to some arbitrary IP address representing a
core.
+ The TLV fields at the end of the header are for a flow-
identifier, and/or security options, if implemented. A "type"
value of zero implies a "length" of zero, implying there is
no "value" field.
8.3. CBT Control Message Types
There are eight types of CBT message. All are encoded in the CBT con-
trol header, shown in figure 6.
+ JOIN-REQUEST (type 1): generated by a router and unicast to
the specified core address. It is processed hop-by-hop on its
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way to the specified core. Its purpose is to establish the
sending CBT router, and all intermediate CBT routers, as part
of the corresponding delivery tree. Note that all cores are
carried in join-requests.
+ JOIN-ACK (type 2): 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 ter-
minated 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,
with each CBT router on the path processing the ack. It is
the receipt of a JOIN-ACK that actually creates a tree
branch.
+ JOIN-NACK (type 3): a negative acknowledgement, indicating
that the tree join process has not been successful.
+ QUIT-REQUEST (type 4): a request, sent from a child to a
parent, to be removed as a child to that parent.
+ QUIT-ACK (type 5): 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.
+ FLUSH-TREE (type 6): a message sent from parent to all chil-
dren, 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.
+ CBT-ECHO-REQUEST (type 7): once a tree branch is established,
this messsage acts as a ``keepalive'', and is unicast from
child to parent (one per link, NOT one per group).
+ CBT-ECHO-REPLY (type 8): positive reply to the above.
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8.3.1. CBT Control Message Subcodes
The JOIN-REQUEST has three valid subcodes:
+ ACTIVE-JOIN (code 0) - sent from a CBT router that has no
children for the specified group.
+ REJOIN-ACTIVE (code 1) - sent from a CBT router that has at
least one child for the specified group.
+ REJOIN-NACTIVE (code 2) - generated by a router subsequent to
receiving a join ack, subcode NORMAL, in response to a
active-rejoin.
A JOIN-ACK has three valid subcodes:
+ NORMAL (code 0) - sent by a core router, or on-tree non-core
router acknowledging joins with subcodes ACTIVE-JOIN and
REJOIN-ACTIVE.
+ PRIMARY-REJOIN-ACK (code 1) - sent by a primary core to ack-
nowledge the receipt of a join-request received with subcode
REJOIN-ACTIVE. This message traverses the reverse-path of the
corresponding re-join, and is processed by each router on
that path.
+ PRIMARY-NACTIVE-ACK (code 2) - sent by a primary core to ack-
nowledge the receipt of a join-request received with subcode
REJOIN-NACTIVE. This ack is unicast directly to the router
that generated the rejoin-Nactive, i.e. the ack it is not
processed hop-by-hop.
9. CBT Protocol and Port Numbers
CBT mode (data) encapsulation (figure 2) requires an IP protocol
number assignment for CBT. An official protocol number has recently
been approved by the IANA; CBT has IP protocol number 7.
CBT control packets travel inside UDP datagrams, as the following
diagram illustrates:
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++++++++++++++++++++++++++++++++++++++++++++
| IP header | UDP header | CBT control pkt |
++++++++++++++++++++++++++++++++++++++++++++
Figure 7. Encapsulation for CBT control messages
CBT therefore requires a UDP port assignment for control messages.
An official UDP port number has recently been approved by the IANA;
CBT control messages are received on UDP port 7777.
10. Default Timer Values
There are several CBT control messages which are transmitted at fixed
intervals. These values, retransmission times, and timeout values,
are given below. Note these are recommended default values only, and
are configurable with each implementation (all times are in seconds):
+ CBT-ECHO-INTERVAL 30 (time between sending successive CBT-ECHO-
REQUESTs to parent).
+ PEND-JOIN-INTERVAL 10 (retransmission time for join-request if
no ack rec'd)
+ PEND-JOIN-TIMEOUT 30 (time to try joining a different core, or
give up)
+ EXPIRE-PENDING-JOIN 90 (remove transient state for join that has
not been ack'd)
+ PEND_QUIT_INTERVAL 10 (retransmission time for quit-request if
no ack rec'd)
+ CBT-ECHO-TIMEOUT 90 (time to consider parent unreachable)
+ CHILD-ASSERT-INTERVAL 90 (increment child timeout if no ECHO
rec'd from a child)
+ CHILD-ASSERT-EXPIRE-TIME 180 (time to consider child gone)
+ IFF-SCAN-INTERVAL 300 (scan all interfaces for group presence.
If none, send QUIT)
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11. Interoperability Issues
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 interoperability, and we expect an interoperability docu-
ment to be available shortly.
12. CBT Security Architecture
see current I-D: ftp://cs.ucl.ac.uk/darpa/IDMR/draft-ietf-idmr-mkd-
01.{ps,txt}
Acknowledgements
Special thanks goes to Paul Francis, NTT Japan, for the original
brainstorming sessions that brought about this work.
Thanks too to Sue Thompson (Bellcore). Her detailed reviews led to
the identification of some subtle protocol flaws, and she suggested
several simplifications.
Thanks also to the networking team at Bay Networks for their comments
and suggestions, in particular Steve Ostrowski for his suggestion of
using "native mode" as a router optimization, and Eric Crawley.
Thanks also to Ken Carlberg (SAIC) for reviewing the text, and gen-
erally providing constructive comments throughout.
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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APPENDIX A
A single rejoin could be sent for all the groups the keepalive
represents. This constitutes an aggregated rejoin strategy; a single
rejoin message can serve to rejoin multiple groups to their respec-
tive trees, provided those groups share a common core (that which is
being rejoined). Therefore, it may be that several rejoins need to be
sent to re-connect all groups traversing the router after a failure.
Similarly, the corresponding join-ack would represent an aggregate.
NOTE: it remains to be worked out how the new parent establishes from
the aggregated rejoin all those groups which the rejoin represents
(so the new parent can create/modify the necessary FIB entries). A
"group aggregate" field may be necessary in the control packet.
Alternatively, when the ack is received in response to the rejoin,
each group represented by the rejoin sends a group-specific echo
until an ack is received for each.
Authors' Addresses:
Tony Ballardie,
Department of Computer Science,
University College London,
Gower Street,
London, WC1E 6BT,
ENGLAND, U.K.
Tel: ++44 (0)71 419 3462
e-mail: A.Ballardie@cs.ucl.ac.uk
Scott Reeve,
Bay Networks, Inc.
3, Federal Street,
Billerica, MA 01821,
USA.
Tel: ++1 508 670 8888
e-mail: sreeve@BayNetworks.com
Nitin Jain,
Bay Networks, Inc.
3, Federal Street,
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Billerica, MA 01821,
USA.
Tel: ++1 508 670 8888
e-mail: njain@BayNetworks.com
References
[1] DVMRP. Described in "Multicast Routing in a Datagram Internet-
work", S. Deering, PhD Thesis, 1990. Available via anonymous ftp from:
gregorio.stanford.edu:vmtp/sd-thesis.ps.
[2] J. Moy. Multicast Routing Extensions to OSPF. Communications of
the ACM, 37(8): 61-66, August 1994.
[3] D. Farinacci, S. Deering, D. Estrin, and V. Jacobson. Protocol
Independent Multicast (PIM) Dense-Mode Specification (draft-ietf-
idmr-pim-spec-01.ps). Working draft, 1994.
[4] A. J. Ballardie. Scalable Multicast Key Distribution
(ftp://cs.ucl.ac.uk/darpa/IDMR/draft-ietf-idmr-mkd-01.{ps,txt}). Work-
ing draft, 1995.
[5] A. J. Ballardie. "A New Approach to Multicast Communication in a
Datagram Internetwork", PhD Thesis, 1995. Available via anonymous ftp
from: cs.ucl.ac.uk:darpa/IDMR/ballardie-thesis.ps.Z.
[6] W. Fenner. Internet Group Management Protocol, version 2 (IGMPv2),
(draft-idmr-igmp-v2-01.txt).
[7] B. Cain, S. Deering, A. Thyagarajan. Internet Group Management
Protocol Version 3 (IGMPv3) (draft-cain-igmp-00.txt).
[8] M. Handley, J. Crowcroft, I. Wakeman. Hierarchical Rendezvous
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(http://www.cs.ucl.ac.uk/staff/M.Handley/hpim.ps) and
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[9] D. Estrin et al. USC/ISI, Work in progress.
(http://netweb.usc.edu/pim/).
[10] D. Estrin et al. PIM Sparse Mode Specification. (draft-ietf-
idmr-pim-sparse-spec-00.txt).
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