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<draft-ietf-idmr-cbt-spec-02.txt>
Inter-Domain Multicast Routing (IDMR) A. J. Ballardie
INTERNET-DRAFT University College London
N. Jain
Bay Networks, Inc.
S. Reeve
Bay Networks, Inc.
June 20th, 1995
Core Based Trees (CBT) Multicast
-- Protocol Specification --
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) multicast 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].
The 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
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will be called CBT "native mode" and obviates the need to insert a
CBT header into data packets before forwarding over CBT interfaces.
Native mode is only relevant to CBT-only domains or ``clouds''.
The CBT architecture is described in an accompanying document:
draft-ietf-idmr-arch-00.txt. Other related documents include [4, 5].
_1. _D_o_c_u_m_e_n_t _L_a_y_o_u_t
We describe the protocol details by means of example using the topol-
ogy shown in figure 1. Examples show how a host joins a group and
leaves a group, and we also show various tree maintenance scenarios.
In this figure member hosts are shown as capital letters, routers are
prefixed with R, and subnets are prefixed with S.
Figure 1 is shown over...
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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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_2. _P_r_o_t_o_c_o_l _S_p_e_c_i_f_i_c_a_t_i_o_n
_2._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. Group initiation could be car-
ried 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 inter-
face (GUI), known as the CBT User Group Management Interface.
NOTE: Work is currently in progress to address the issue of core
placement.
_2._2. _T_r_e_e _J_o_i_n_i_n_g _P_r_o_c_e_s_s
The following steps are involved in a host establishing itself as
part of a CBT multicast tree:
o+ the joining host must inform all routers on its subnet that it
requires a Designated Router (DR) for the group it wishes to
join (it is a requirement that only one router, the DR, forward
to and from upstream to avoid loops).
o+ the establishment of a DR for the group.
o+ once established, the DR must proceed to join the distribution
tree.
The following CBT control messages come into play during the host
joining process:
NOTE: all CBT message types are described in section 8 irrespective
of some of the comments included with certain message types below.
o+ CORE_NOTIFICATION (sent only by a group initiating host to
inform each core for the group that it has been elected as a
core for the group).
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o+ CORE_NOTIFICATION_ACK
o+ DR_SOLICITATION
o+ DR_ADVERTISEMENT_NOTIFICATION (sent only by a local CBT-capable
router when that router is unaware of a DR for the group on the
same subnet, and believes it is candidate for the best next-hop
router off the LAN to the core address as specified in the
DR_SOLICITATION. This message acts as a tie-breaker in the case
where there are two or more such routers on a subnet).
o+ DR_ADVERTISEMENT
o+ TAG_REPORT (sent by a joining host to the DR subsequent to
receiving a DR_ADVERTISEMENT. This message serves to invoke the
DR to become part of the distribution tree, if not already, by
sending a JOIN_REQUEST).
o+ JOIN_REQUEST (sent only by the group's DR iff it is not yet part
of, or in the process of, joining the corresponding CBT tree).
o+ JOIN_ACK
o+ HOST_JOIN_ACK (multicast across the subnet by the local DR as an
indication that the DR is part of the distribution tree. This
message may be sent in immediate response to receiving a
TAG_REPORT, depending on whether the DR is already part of the
CBT tree or not. If not it is sent subsequent to the DR receiv-
ing a JOIN_ACK).
A group-initiating host sends a CORE-NOTIFICATION message to each of
the elected cores for the group. This message is acknowledged
(CORE_NOTIFICATION_ACK) by each core individually. Provided at least
one ACK is received a host will not be prevented from joining the
tree.
The purpose of the CORE_NOTIFICATION is twofold: firstly, to communi-
cate the identities of all of the cores, together with their rank-
ings, to each of them individually; secondly, to invoke the building
of the core backbone or core tree. 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.
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Taking our example topology in figure 1, host A is the group initia-
tor. The elected cores are router R4 (primary core) and R9 (secon-
dary core). Host A first sends a CORE_NOTIFICATION to each of R4 and
R9, and each responds positively with a CORE_NOTIFICATION_ACK.
CORE_NOTIFICATION messages are always unicast.
Subsequent to sending a CORE_NOTIFICATION_ACK, each secondary core
router (in this case there is only one secondary, R9) proceeds to
join the primary core, and thus forms the core tree, or backbone; R9
unicasts a JOIN_REQUEST (subcode CORE_JOIN) to R8, its best next-hop
to the primary core, R4. JOIN_REQUESTs (and corresponding ACKs) are
processed by all intervening CBT-capable routers, and forwarded if
necessary. R8 forwards the JOIN_REQUEST to R4, remembering the incom-
ing and outgoing interfaces of the JOIN_REQUEST.
R4 receives the JOIN_REQUEST (subcode CORE_JOIN), realises it is the
target of the join, and therefore sends a JOIN_ACK back out of the
receiving interface to the previous-hop sender of the join. R8
receives the JOIN_ACK and forwards it to R9 over the interface the
join was received from R9. On receipt of the JOIN_ACK, R9 need take
no further action. Core tree set up is complete.
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, at
which time it can acknowledge any cached joins. A router is said to
be in a pending-join state if it is awaiting a JOIN_ACK itself.
Returning to host A which has just received both
CORE_NOTIFICATION_ACKs, it must now establish which local CBT router
is DR for the group. Since A is the group initiator it is highly
unlikely that a DR for the group will already exist. If A was joining
an existing group a DR may already be present.
Host A sends a DR_SOLICITATION (IP TTL 1) to the "all-CBT-routers"
address (224.0.0.7). The solicitation contains one of core addresses
as elected by the host, to which it wishes a join to be sent. Any
routers on the same subnet receiving the solicitation establish
whether they are the best next-hop to the specified core or not. If a
router does consider itself a candidate and has no record for a DR
for the group, it multicasts a DR_ADV_NOTIFICATION to the "all-CBT-
routers" group (224.0.0.7). This message acts as a tie-breaker in the
case where there is more than one CBT router on the subnet which
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thinks it is the best next-hop to the core. The lowest-addressed
source of a DR_ADV_NOTIFICATION wins the election and subsequently
advertises itself as DR by means of a DR_ADVERTISEMENT, multicast to
the "all-systems group (224.0.0.1). As R1 is the only router on A's
subnet, it responds with a DR_ADV_NOTIFICATION followed by a
DR_ADVERTISEMENT.
The time between sending a DR_ADV_NOTIFICATION and a DR_ADVERTISEMENT
should be configurable and ideally less than one second so as to keep
join latency to a minimum.
The DR election for subnet S4 is more complex. When host B sends a
DR_SOLICITATION routers R2, R5 and R6 receive it. Assuming R2 and R5
both believe they are the best next-hop to R4 (the specified core)
both send a DR_ADV_NOTIFICATION. R2 (the lower addressed) wins the
tie-breaker and subsequently multicasts a DR_ADVERTISEMENT to S4. All
subnets with joining hosts proceed similarly.
A DR candidate is a router whose outgoing interface, as specified in
its routing table entry for the destination, is different than the
interface over which the DR_SOLICITATION arrived.
On receiving a DR_ADVERTISEMENT host A sends a TAG_REPORT to the DR,
R1. R1 responds by unicasting a JOIN_REQUEST (subcode ACTIVE_JOIN) to
R3 -- the best next-hop to R4, the desired target of the join. R3
forwards (unicast) the received join to R4, remembering incoming and
outgoing interfaces. R4, now already established on tree for the
group responds to the JOIN_REQUEST with a JOIN_ACK, and sends it to
R3, which in turn sends it to R1. The branch R1-R3-R4 is now complete
and part of the distribution tree.
On receipt of the JOIN_ACK, R1 multicasts to the "all-systems"
address (224.0.0.1) a HOST_JOIN_ACK which is a notification to the
joining end-system that the DR has been successful in joining the
tree. The multicast application running on host A can now send data.
Host B proceeds to join the group in a similar fashion, but there are
some subtle differences. Host B is not the group initiator and it
need not send CORE_NOTIFICATIONs. Host B's first step is to elect a
DR, as described above. On receipt of a DR_ADVERTISEMENT from router
R2 in this case, B unicasts a TAG_REPORT to R2. The core specified in
the TAG_REPORT is R4. In response the the TAG_REPORT, R2 unicasts a
JOIN_REQUEST (subcode ACTIVE_JOIN) to R3, the best next-hop to R4. R3
however, has just joined the tree and so can acknowledge the received
join, i.e. it need not travel all the way to R4. R3 unicasts a
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JOIN_ACK to R2, which results in R2 multicasting a HOST_JOIN_ACK
across subnet S4.
_3. _D_a_t_a _P_a_c_k_e_t _F_o_r_w_a_r_d_i_n_g (_C_B_T _m_o_d_e)
"CBT mode" as opposed to "native mode" describes the
forwarding/sending of data packets over CBT tree interfaces contain-
ing a CBT header encapsulation. For efficiency, this encapsulation is
as follows:
++++++++++++++++++++++++++++++++++++++++++++++++++++++++
| encaps IP hdr | CBT hdr | original IP hdr | data ....|
++++++++++++++++++++++++++++++++++++++++++++++++++++++++
Figure 2. Encapsulation for CBT mode
By using the encapsulations above there is virtually no necessity to
modify a packet's original IP header, and decapsulation is relatively
efficient.
It is worth pointing out at this point the distinction between sub-
networks 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 subnetwork is a "tunnel" or a point-to-point link.
In CBT forwarding mode there are three forwarding methods used by CBT
routers:
o+ IP multicasting. This method is used to send a data packet
across a directly-connected subnetwork with group member pres-
ence. Thus, system host changes are not required for CBT. Simi-
larly, end-systems originating multicast data do so in tradi-
tional IP-style.
o+ CBT unicasting. This method is used for sending data packets
encapsulated (as illustrated above) across a tunnel or point-
to-point link.
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o+ CBT multicasting. This method sends data packets encapsulated
(as illustrated above) but the outer encapsulating IP header
contains a multicast address. This method is used when a parent
or multiple children are reachable over a single physical inter-
face, 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 will not be recog-
nized.
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 4 | 4
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 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
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The field lengths shown above assume a maximum of 16 directly con-
nected neighbouring routers.
When a data packet arrives at a CBT router, the following rules
apply:
o+ if the packet is an IP-style multicast, it is checked to see if
it originated locally (i.e. if the arrival interface subnetmask
ANDed with the packet's source IP address equals the arrival
interface's subnet number, the packet was sourced locally). If
it does not the packet is discarded.
o+ 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.
o+ 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.
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 (R4 is DR
for all its attached subnets). R8 CBT unicasts the packet to each of
its children, R9 and R12. These children are not reachable over the
same interface. R8, being the DR for subnets S14 and S10 also IP mul-
ticasts 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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_3._1. _N_o_n-_M_e_m_b_e_r _S_e_n_d_i_n_g
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 DR for the group on the subnetwork must encap-
sulate the IP-style packet and unicast it to a core for the group.
This requires CBT routers to have access to a mapping mechanism
between group addresses and core routers. This mechanism is
currently beyond the scope of this document.
_4. _D_a_t_a _P_a_c_k_e_t _F_o_r_w_a_r_d_i_n_g (_n_a_t_i_v_e _m_o_d_e)
In CBT "native mode" only one forwarding method is used, namely all
data packets are forwarded over CBT tree interfaces as native IP mul-
ticasts, 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. It also assumes that all routers within the
domain of operation are CBT-capable, i.e. there are no "tunnels". If
this latter constraint cannot be satisfied it is necessary to encap-
sulate IP-over-IP before forwarding to a child or parent reachable
via non-CBT-capable router(s).
Besides the structural characteristics of "native mode" data packets,
described above, the data packet forwarding rules are identical to
those described in section 3.
_4._1. _N_o_n-_M_e_m_b_e_r _S_e_n_d_i_n_g (_n_a_t_i_v_e _m_o_d_e)
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 DR for the group on the subnetwork must encap-
sulate (IP-over-IP) the IP-style packet and unicast it to a core for
the group. This requires CBT routers to have access to a mapping
mechanism between group addresses and core routers. This mechanism
is currently beyond the scope of this document.
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_5. _T_r_e_e _M_a_i_n_t_e_n_a_n_c_e
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.
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.
_5._1. _R_o_u_t_e_r _F_a_i_l_u_r_e
A non-core router can detect a failure from the following two cases:
o+ if a child 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. It does so by
arbitrarily choosing an alternate core from its list of cores
for this group. It establishes a chosen core's reachability by
unicasting a CBT_CORE_PING message to it, to which the core
responds with a CBT_PING_REPLY. On receipt of the latter, the
re-joining router sends a JOIN_REQUEST (subcode ACTIVE_REJOIN)
to the best next-hop router on the path to the core. A router
will continue arbitrarily choosing an alternate core until a
CBT_PING_REPLY is received.
o+ if a parent stops receiving CBT_ECHO_REQUESTs from a child. In
this case the parent simply removes the child interface from its
FIB entry for the particular group.
_5._2. _R_o_u_t_e_r _R_e-_S_t_a_r_t_s
There are two cases to consider here:
o+ Core re-start. In this case, the core router relies on receiving
a CBT_CORE_PING message, which contains the list of cores for
the specified group. Obviously, one of the core addresses will
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be its own. If a core realises its core status for a group in
this way, if it is not the primary it sends a JOIN_REQUEST (sub-
code ACTIVE_JOIN) to the primary core. If the router in ques-
tion is the primary it need not send a join, but rather awaits
joins and considers itself part of the tree again.
o+ 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.
_5._3. _R_o_u_t_e _L_o_o_p_s
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 the core. This join is forwarded as nor-
mal until it reaches either the core or a non-core router that is
already part of the tree. If the join reaches the specified core, the
join terminates there and is ACKd as normal. If however, the join is
terminated by non-core router, the ACTIVE_REJOIN is converted to a
NON_ACTIVE_REJOIN and forwarded upstream. A JOIN_ACK is also sent
downstream to acknowledge the received join. The NON_ACTIVE_REJOIN
is a loop detection packet. All routers receiving this must forward
it over their parent interface. If the originator of the correspond-
ing ACTIVE_REJOIN should receive the NON_ACTIVE_REJOIN it immediately
sends a QUIT_REQUEST to its recently established parent and the loop
is broken.
o+ 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 changes the join
subcode to NON_ACTIVE_REJOIN, and forwards this to its parent,
R4. R4 forwards the NON_ACTIVE_REJOIN to R3, its parent. R3
originated the corresponding ACTIVE_REJOIN, and so it immedi-
ately 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 need not send a
quit -- the loop has been broken by R3 sending the first quit.
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QUIT_REQUESTs are typically acknowledged by means of a QUIT_ACK, but
there might be cases where, due to failure, the parent cannot
respond. In this case the child nevertheless removes the parent
information after some small number of re-tries.
------
| R1 |
------
|
---------------------------
|
------
| R2 |
------
|
---------------------------
| |
------ |
| R3 |--------------------------|
------ |
| |
--------------------------- |
| | ------
------ | | |
| R4 | |-------| R6 |
------ | |----|
| |
--------------------------- |
| |
------ |
| R5 |--------------------------|
------ |
|
Figure 4: Example Loop Topology
_6. _D_a_t_a _P_a_c_k_e_t _L_o_o_p_s
NOTE: this is only applicable when CBT header encapsulation is in
use.
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When a data packet hits its first on-tree router, that router is
responsible for setting the on-tree bits in the CBT header. This
indicates to all subsequent routers on the tree that the packet is in
the process of spanning the tree for the group. However, it might be
that a misbehaving router forwards an on-tree packet over a non-tree
interface, and such a packet might work its way back onto the tree,
potentially forming a data packet loop. Therefore, the on-tree bits
in the CBT header serve to identify such packets -- should a router
receive a data packet with its on-tree bits set over a non-tree
interface the packet is immediately discarded.
_7. _T_r_e_e _T_e_a_r_d_o_w_n
There are two scenarios whereby a tree branch may be torn down:
o+ During a re-configuration, if a router's best next-hop to the
specified core is one of its existing children then before send-
ing the re-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. Subsequent to sending a FLUSH_TREE,
the router can send the re-join to its child.
o+ 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.
With regards to the latter scenario, lets see using the example
topology of figure 1 how a tree branch is torn down.
Assume member E leaves the group (if IGMPv2 is in use an explicit
IGMP_LEAVE message will be sent by E). If R7 registers no further
group presence (by means of IGMP) then R7 sends a QUIT_REQUEST to R4.
R4 responds with a QUIT_ACK to R7. R4 has children AND subnets with
group presence, and so does not itself attempt to quit the tree. The
branch R4-R7 has been torn down.
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_8. _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 enscapsulation (for "CBT mode") is shown
below:
++++++++++++++++++++++++++++++++++++++++++++++++++++++++
| encaps IP hdr | CBT hdr | original IP hdr | data ....|
++++++++++++++++++++++++++++++++++++++++++++++++++++++++
Figure 5. Encapsulation for CBT mode
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.
_8._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 6. 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.
_8._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 7. 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.
_8._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. It
is processed hop-by-hop on its 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.
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
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the receipt of a JOIN-ACK that actually creates a tree
branch.
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.
_8._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:
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o+ CBT-DR-SOLICITATION: a request sent from a host to the CBT
``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-ACK: 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-HOST_JOIN_ACK: 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.
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o+ CBT-DR-ADV-NOTIFICATION: multicast to the CBT ``all-routers''
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.
_9. _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.
_1_0. _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-ietf-idmr-mkd-02.txt
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Acknowledgements
Special thanks goes to Paul Francis, NTT Japan, for the original
brainstorming sessions that brought about this work.
Thanks also to team at Bay Networks for their comments and sugges-
tions, in particular Steve Ostrowski for his suggestion of using
"native mode" as a router optimization, Eric Crawley, Scott Reeve,
and Nitin Jain.
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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Author's Address:
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
Nitin Jain,
Bay Networks, Inc.
3 Federal Street,
Billerica, MA 01821,
USA.
Tel: ++1 508 670 8888
e-mail: njain@BayNetworks.com
Scott Reeve,
Bay Networks, Inc.
3 Federal Street,
Billerica, MA 01821,
USA.
Tel: ++1 508 670 8888
e-mail: sreeve@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.
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[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 (draft-ietf-
idmr-mkd-02.txt). Working 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.
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