| Rfc | 1584 |
|---|
| Title | Multicast Extensions to OSPF |
|---|
| Author | J. Moy |
|---|
| Date | March 1994 |
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| Format: | TXT, PS,
PDF, HTML |
|---|
| Status: | HISTORIC |
|---|
|
Network Working Group J. Moy
Request for Comments: 1584 Proteon, Inc.
Category: Standards Track March 1994
Multicast Extensions to OSPF
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is
unlimited.
Abstract
This memo documents enhancements to the OSPF protocol enabling the
routing of IP multicast datagrams. In this proposal, an IP multicast
packet is routed based both on the packet's source and its multicast
destination (commonly referred to as source/destination routing). As
it is routed, the multicast packet follows a shortest path to each
multicast destination. During packet forwarding, any commonality of
paths is exploited; when multiple hosts belong to a single multicast
group, a multicast packet will be replicated only when the paths to
the separate hosts diverge.
OSPF, a link-state routing protocol, provides a database describing
the Autonomous System's topology. A new OSPF link state
advertisement is added describing the location of multicast
destinations. A multicast packet's path is then calculated by
building a pruned shortest-path tree rooted at the packet's IP
source. These trees are built on demand, and the results of the
calculation are cached for use by subsequent packets.
The multicast extensions are built on top of OSPF Version 2. The
extensions have been implemented so that a multicast routing
capability can be introduced piecemeal into an OSPF Version 2
routing domain. Some of the OSPF Version 2 routers may run the
multicast extensions, while others may continue to be restricted to
the forwarding of regular IP traffic (unicasts).
Please send comments to mospf@gated.cornell.edu.
1. Introduction
This memo documents enhancements to OSPF Version 2 to support IP
multicast routing. The enhancements have been added in a backward-
compatible fashion; routers running the multicast additions will
interoperate with non-multicast OSPF routers when forwarding regular
(unicast) IP data traffic. The protocol resulting from the addition
of the multicast enhancements to OSPF is herein referred to as the
MOSPF protocol.
IP multicasting is an extension of LAN multicasting to a TCP/IP
internet. Multicasting support for TCP/IP hosts has been specified
in [RFC 1112]. In that document, multicast groups are represented by
IP class D addresses. Individual TCP/IP hosts join (and leave)
multicast groups through the Internet Group Management Protocol
(IGMP, also specified in [RFC 1112]). A host need not be a member of
a multicast group in order to send datagrams to the group. Multicast
datagrams are to be delivered to each member of the multicast group
with the same "best-effort" delivery accorded regular (unicast) IP
data traffic.
MOSPF provides the ability to forward multicast datagrams from one
IP network to another (i.e., through internet routers). MOSPF
forwards a multicast datagram on the basis of both the datagram's
source and destination (this is sometimes called source/destination
routing). The OSPF link state database provides a complete
description of the Autonomous System's topology. By adding a new
type of link state advertisement, the group-membership-LSA, the
location of all multicast group members is pinpointed in the
database. The path of a multicast datagram can then be calculated by
building a shortest-path tree rooted at the datagram's source. All
branches not containing multicast members are pruned from the tree.
These pruned shortest-path trees are initially built when the first
datagram is received (i.e., on demand). The results of the shortest
path calculation are then cached for use by subsequent datagrams
having the same source and destination.
OSPF allows an Autonomous System to be split into areas. However,
when this is done complete knowledge of the Autonomous System's
topology is lost. When forwarding multicasts between areas, only
incomplete shortest-path trees can be built. This may lead to some
inefficiency in routing. An analogous situation exists when the
source of the multicast datagram lies in another Autonomous System.
In both cases (i.e., the source of the datagram belongs to a
different OSPF area, or to a different Autonomous system) the
neighborhood immediately surrounding the source is unknown. In these
cases the source's neighborhood is approximated by OSPF summary link
advertisements or by OSPF AS external link advertisements
respectively.
Routers running MOSPF can be intermixed with non-multicast OSPF
routers. Both types of routers can interoperate when forwarding
regular (unicast) IP data traffic. Obviously, the forwarding extent
of IP multicasts is limited by the number of MOSPF routers present
in the Autonomous System (and their interconnection, if any). An
ability to "tunnel" multicast datagrams through non-multicast
routers is not provided. In MOSPF, just as in the base OSPF
protocol, datagrams (multicast or unicast) are routed "as is" --
they are not further encapsulated or decapsulated as they transit
the Autonomous System.
1.1. Terminology
This memo uses the terminology listed in section 1.2 of [OSPF].
For this reason, terms such as "Network", "Autonomous System"
and "link state advertisement" are assumed to be understood. In
addition, the abbreviation LSA is used for "link state
advertisement". For example, router links advertisements are
referred to as router-LSAs and the new link state advertisement
describing the location of members of a multicast group is
referred to as a group-membership-LSA.
[RFC 1112] discusses the data-link encapsulation of IP multicast
datagrams. In contrast to the normal forwarding of IP unicast
datagrams, on a broadcast network the mapping of an IP multicast
destination to a data-link destination address is not done with
the ARP protocol. Instead, static mappings have been defined
from IP multicast destinations to data-link addresses. These
mappings are dependent on network type; for some networks IP
multicasts are algorithmically mapped to data-link multicast
addresses, for other networks all IP multicast destinations are
mapped onto the data-link broadcast address. This document
loosely describes both of these possible mappings as data-link
multicast.
The following terms are also used throughout this document:
o Non-multicast router. A router running OSPF Version 2, but
not the multicast extensions. These routers do not forward
multicast datagrams, but can interoperate with MOSPF routers
in the forwarding of unicast packets. Routers running the
MOSPF protocol are referred to herein as either multicast-
capable routers or MOSPF routers.
o Non-broadcast networks. A network supporting the attachment
of more than two stations, but not supporting the delivery
of a single physical datagram to multiple destinations
(i.e., not supporting data-link multicast). [OSPF] describes
these networks as non-broadcast, multi-access networks. An
example of a non-broadcast network is an X.25 PDN.
o Transit network. A network having two or more OSPF routers
attached. These networks can forward data traffic that is
neither locally-originated nor locally-destined. In OSPF,
with the exception of point-to-point networks and virtual
links, the neighborhood of each transit network is described
by a network links advertisement (network-LSA).
o Stub network. A network having only a single OSPF router
attached. A network belonging to an OSPF system is either a
transit or a stub network, but never both.
1.2. Acknowledgments
The multicast extensions to OSPF are based on Link-State
Multicast Routing algorithm presented in [Deering]. In addition,
the [Deering] paper contains a section on Hierarchical Multicast
Routing (providing the ideas for MOSPF's inter-area multicasting
scheme) and several Distance Vector (also called Bellman-Ford)
multicast algorithms. One of these Distance Vector multicast
algorithms, Truncated Reverse Path Broadcasting, has been
implemented in the Internet (see [RFC 1075]).
The MOSPF protocol has been developed by the MOSPF Working Group
of the Internet Engineering Task Force. Portions of this work
have been supported by DARPA under NASA contract NAG 2-650.
2. Multicast routing in MOSPF
This section describes MOSPF's basic multicast routing algorithm.
The basic algorithm, run inside a single OSPF area, covers the case
where the source of the multicast datagram is inside the area
itself. Within the area, the path of the datagram forms a tree
rooted at the datagram source.
2.1. Routing characteristics
As a multicast datagram is forwarded along its shortest-path
tree, the datagram is delivered to each member of the
destination multicast group. In MOSPF, the forwarding of the
multicast datagram has the following properties:
o The path taken by a multicast datagram depends both on the
datagram's source and its multicast destination. Called
source/destination routing, this is in contrast to most
unicast datagram forwarding algorithms (like OSPF) that
route based solely on destination.
o The path taken between the datagram's source and any
particular destination group member is the least cost path
available. Cost is expressed in terms of the OSPF link-state
metric. For example, if the OSPF metric represents delay, a
minimum delay path is chosen. OSPF metrics are configurable.
A metric is assigned to each outbound router interface,
representing the cost of sending a packet on that interface.
The cost of a path is the sum of its constituent (outbound)
router interfaces[1].
o MOSPF takes advantage of any commonality of least cost paths
to destination group members. However, when members of the
multicast group are spread out over multiple networks, the
multicast datagram must at times be replicated. This
replication is performed as few times as possible (at the
tree branches), taking maximum advantage of common path
segments.
o For a given multicast datagram, all routers calculate an
identical shortest-path tree. There is a single path between
the datagram's source and any particular destination group
member. This means that, unlike OSPF's treatment of regular
(unicast) IP data traffic, there is no provision for equal-
cost multipath.
o On each packet hop, MOSPF normally forwards IP multicast
datagrams as data-link multicasts. There are two exceptions.
First, on non-broadcast networks, since there are no data-
link multicast/broadcast services the datagram must be
forwarded to specific MOSPF neighbors (see Section 2.3.3).
Second, a MOSPF router can be configured to forward IP
multicasts on specific networks as data-link unicasts, in
order to avoid datagram replication in certain anomalous
situations (see Section 6.4).
While MOSPF optimizes the path to any given group member, it
does not necessarily optimize the use of the internetwork as a
whole. To do so, instead of calculating source-based shortest-
path trees, something similar to a minimal spanning tree
(containing only the group members) would need to be calculated.
This type of minimal spanning tree is called a Steiner tree in
the literature. For a comparison of shortest-path tree routing
to routing using Steiner trees, see [Deering2] and [Bharath-
Kumar].
2.2. Sample path of a multicast datagram
As an example of multicast datagram routing in MOSPF, consider
the sample Autonomous System pictured in Figure 1. This figure
has been taken from the OSPF specification (see [OSPF]). The
larger rectangles represent routers, the smaller rectangles
hosts. Oblongs and circles represent multi-access networks[2].
Lines joining routers are point-to-point serial connections. A
cost has been assigned to each outbound router interface.
All routers in Figure 1 are assumed to be running MOSPF. A
number of hosts have been added to the figure. The hosts
labelled Ma have joined a particular multicast group (call it
Group A) via the IGMP protocol. These hosts are located on
networks N2, N6 and N11. Similarly, using IGMP the hosts
labelled Mb have joined a separate multicast group B; these
hosts are located on networks N1, N2 and N3. Note that hosts can
join multiple multicast groups; it is only for clarity of
presentation that each host has joined at most one multicast
group in this example. Also, hosts H2 through H5 have been
added to the figure to serve as sources for multicast datagrams.
Again, the datagrams' sources have been made separate from the
group members only for clarity of presentation.
To illustrate the forwarding of a multicast datagram, suppose
that Host H2 (attached to Network N4) sends a multicast datagram
to multicast group B. This datagram originates as a data-link
layer multicast on Network N4. Router RT3, being a multicast
router, has "opened up" its interface data-link multicast
filters. It therefore receives the multicast datagram, and
attempts to forward it to the members of multicast group B
(located on networks N1, N2 and N3). This is accomplished by
sending a single copy of the datagram onto Network N3, again as
a data-link multicast[3]. Upon receiving the multicast datagram
from RT3, routers RT1 and RT2 will then multicast the datagram
on their connected stub networks (N1 and N2 respectively). Note
that, since the datagram is sent onto Network N3 as a data-link
multicast, Router RT4 will also receive a copy. However, it will
not forward the datagram, since it does not lie on a shortest
path between the source (Host H2) and any members of multicast
group B.
Note that the path of the multicast datagram depends on the
datagram's source network. If the above multicast datagram was
instead originated by Host H3, the path taken would be
identical, since hosts H2 and H3 lie on the same network
(Network N4). However, if the datagram was originated by Host
H4, its path would be different. In this case, when Router RT3
+
| 3+---+ +--+ +--+ N12 N14
N1|--|RT1|\1 |Mb| |H4| \ N13 /
_| +---+ \ +--+ /+--+ 8\ |8/8
| + \ _|__/ \|/
+--+ +--+ / \ 1+---+8 8+---+6
|Mb| |Mb| * N3 *---|RT4|------|RT5|--------+
+--+ /+--+ \____/ +---+ +---+ |
+ / | |7 |
| 3+---+ / | | |
N2|--|RT2|/1 |1 |6 |
__| +---+ +---+8 6+---+ |
| + |RT3|--------------|RT6| |
+--+ +--+ +---+ +--+ +---+ |
|Ma| |H3|_ |2 _|H2| Ia|7 |
+--+ +--+ \ | / +--+ | |
+---------+ | |
N4 | |
| |
| |
N11 | |
+---------+ | |
| \ | | N12
|3 +--+ | |6 2/
+---+ |Ma| | +---+/
|RT9| +--+ | |RT7|---N15
+---+ | +---+ 9
|1 + | |1
_|__ | Ib|5 __|_ +--+
/ \ 1+----+2 | 3+----+1 / \--|Ma|
* N9 *------|RT11|----|---|RT10|---* N6 * +--+
\____/ +----+ | +----+ \____/
| | |
|1 + |1
+--+ 10+----+ N8 +---+
|H1|-----|RT12| |RT8|
+--+SLIP +----+ +---+ +--+
|2 |4 _|H5|
| | / +--+
+---------+ +--------+
N10 N7
Figure 1: A sample MOSPF configuration
receives the datagram, RT3 will drop the datagram instead of
forwarding it (since RT3 is no longer on the shortest path to
any member of Group B).
Note that the path of the multicast datagram also depends on the
destination multicast group. If Host H2 sends a multicast to
Group A, the path taken is as follows. The datagram again starts
as a multicast on Network N4. Router RT3 receives it, and
creates two copies. One is sent onto Network N3 which is then
forwarded onto Network N2 by RT2. The other copy is sent to
Router RT10 (via RT6), where the datagram is again split,
eventually to be delivered onto networks N6 and N11. Note that,
although multiple copies of the datagram are produced, the
datagram itself is not modified (except for its IP TTL) as it is
forwarded. No encapsulation of the datagram is performed; the
destination of the datagram is always listed as the multicast
group A.
2.3. MOSPF forwarding mechanism
Each MOSPF router in the path of a multicast datagram bases its
forwarding decision on the contents of a data cache. This cache
is called the forwarding cache. There is a separate forwarding
cache entry for each source/destination combination[4]. Each
cache entry indicates, for multicast datagrams having matching
source and destination, which neighboring node (i.e., router or
network) the datagram must be received from (called the upstream
node) and which interfaces the datagram should then be forwarded
out of (called the downstream interfaces).
A forwarding cache entry is actually built from two component
pieces. The first of these components is called the local group
database. This database, built by the IGMP protocol, indicates
the group membership of the router's directly attached networks.
The local group database enables the local delivery of multicast
datagrams. The second component is the datagram's shortest path
tree. This tree, built on demand, is rooted at a multicast
datagram's source. The datagram's shortest path tree enables the
delivery of multicast datagrams to distant (i.e., not directly
attached) group members.
2.3.1. IGMP interface: the local group database
The local group database keeps track of the group membership
of the router's directly attached networks. Each entry in
the local group database is a [group, attached network]
pair, which indicates that the attached network has one or
more IP hosts belonging to the IP multicast destination
group. This information is then used by the router when
deciding which directly attached networks to forward a
received IP multicast datagram onto, in order to complete
delivery of the datagram to (local) group members.
The local group database is built through the operation of
the Internet Group Management Protocol (IGMP; see [RFC
1112]). When a MOSPF router becomes Designated Router on an
attached network (call the network N1), it starts sending
periodic IGMP Host Membership Queries on the network. Hosts
then respond with IGMP Host Membership Reports, one for each
multicast group to which they belong. Upon receiving a Host
Membership Report for a multicast group A, the router
updates its local group database by adding/refreshing the
entry [Group A, N1]. If at a later time Reports for Group A
cease to be heard on the network, the entry is then deleted
from the local group database.
It is important to note that on any particular network, the
sending of IGMP Host Membership Queries and the listening to
IGMP Host Membership Reports is performed solely by the
Designated Router. A MOSPF router ignores Host Membership
Reports received on those networks where the router has not
been elected Designated Router[5]. This means that at most
one router performs these IGMP functions on any particular
network, and ensures that the network appears in the local
group database of at most one router. This prevents
multicast datagrams from being replicated as they are
delivered to local group members. As a result, each router
in the Autonomous System has a different local group
database. This is in contrast to the MOSPF link state
database, and the datagram shortest-path trees (see Section
2.3.2), all of which are identical in each router belonging
to the Autonomous System.
The existence of local group members must be communicated to
the rest of the routers in the Autonomous System. This
ensures that a remotely-originated multicast datagram will
be forwarded to the router for distribution to its local
group members. This communication is accomplished through
the creation of a group-membership-LSA. Like other link
state advertisements, the group-membership-LSA is flooded
throughout the Autonomous System. The router originates a
separate group-membership-LSA for each multicast group
having one or more entries in the router's local group
database. The router's group-membership-LSA (say for Group
A) lists those local transit vertices (i.e., the router
itself and/or any directly connected transit networks) that
should not be pruned from Group A's datagram shortest-path
trees. The router lists itself in its group-membership-LSA
for Group A if either 1) one or more of the router's
attached stub networks contain Group A members or 2) the
router itself is a member of Group A. The router lists a
directly connected transit network in the group-membership-
LSA for Group A if both 1) the router is Designated Router
on the network and 2) the network contains one or more Group
A members.
Consider again the example pictured in Figure 1. If Router
RT3 has been elected Designated Router for Network N3, then
Table 1: lists the local group database for the routers
RT1-RT4.
In this case, each of the routers RT1, RT2 and RT3 will
originate a group-membership-LSA for Group B. In addition,
RT2 will also be originating a group-membership-LSA for
Group A. RT1 and RT2's group-membership-LSAs will list
solely the routers themselves (N1 and N2 are stub networks).
RT3's group-membership-LSA will list the transit Network N3.
Figure 2 displays the Autonomous System's link state
database. A router/transit network is labelled with a
multicast group if (and only if) it has been mentioned in a
group-membership-LSA for the group When building the
shortest-path tree for a particular multicast datagram, this
labelling enables those branches without group members to be
pruned from the tree. The process of building a multicast
datagram's shortest path tree is discussed in Section 2.3.2.
Note that none of the hosts in Figure 1 belonging to
multicast groups A and B show up explicitly in the link
state database (see Figure 2). In fact, looking at the link
state database you cannot even determine which stub networks
Router local group database
_____________________________________
RT1 [Group B, N1]
RT2 [Group A, N2], [Group B, N2]
RT3 [Group B, N3]
RT4 None
Table 1: Sample local group databases
**FROM**
|RT|RT|RT|RT|RT|RT|RT|RT|RT|RT|RT|RT|
|1 |2 |3 |4 |5 |6 |7 |8 |9 |10|11|12|N3|N6|N8|N9|
----- ---------------------------------------------
RT1| | | | | | | | | | | | |0 | | | |
RT2| | | | | | | | | | | | |0 | | | |
RT3| | | | | |6 | | | | | | |0 | | | |
RT4| | | | |8 | | | | | | | |0 | | | |
RT5| | | |8 | |6 |6 | | | | | | | | | |
RT6| | |8 | |7 | | | | |5 | | | | | | |
RT7| | | | |6 | | | | | | | | |0 | | |
* RT8| | | | | | | | | | | | | |0 | | |
* RT9| | | | | | | | | | | | | | | |0 |
T RT10| | | | | |7 | | | | | | | |0 |0 | |
O RT11| | | | | | | | | | | | | | |0 |0 |
* RT12| | | | | | | | | | | | | | | |0 |
* N1|3 | | | | | | | | | | | | | | | |
N2| |3 | | | | | | | | | | | | | | |
N3|1 |1 |1 |1 | | | | | | | | | | | | |
N4| | |2 | | | | | | | | | | | | | |
N6| | | | | | |1 |1 | |1 | | | | | | |
N7| | | | | | | |4 | | | | | | | | |
N8| | | | | | | | | |3 |2 | | | | | |
N9| | | | | | | | |1 | |1 |1 | | | | |
N10| | | | | | | | | | | |2 | | | | |
N11| | | | | | | | |3 | | | | | | | |
N12| | | | |8 | |2 | | | | | | | | | |
N13| | | | |8 | | | | | | | | | | | |
N14| | | | |8 | | | | | | | | | | | |
N15| | | | | | |9 | | | | | | | | | |
H1| | | | | | | | | | | |10| | | | |
Figure 2: The MOSPF database.
Networks and routers are represented by vertices.
An edge of cost X connects Vertex A to Vertex B iff
the intersection of Column A and Row B is marked
with an X. In addition, RT1, RT2 and N3 are labelled
with multicast group A and RT1, N6 and RT9 are
labelled with multicast group B.
contain multicast group members. The link state database
simply indicates those routers/transit networks having
attached group members. This is all that is necessary for
successful forwarding of multicast datagrams.
2.3.2. A datagram's shortest-path tree
While the local group database facilitates the local
delivery of multicast datagrams, the datagram's shortest-
path tree describes the intermediate hops taken by a
multicast datagram as it travels from its source to the
individual multicast group members. As mentioned above, the
datagram's shortest-path tree is a pruned shortest-path tree
rooted at the datagram's source. Two datagrams having
differing [source net, multicast destination] pairs may
have, and in fact probably will have, different pruned
shortest-path trees.
A datagram's shortest path tree is built "on demand"[6],
i.e., when the first multicast datagram is received having a
particular [source net, multicast destination] combination.
To build the datagram's shortest-path tree, the following
calculations are performed. First, the datagram's source IP
network is located in the link state database. Then using
the router-LSAs and network-LSAs in the link state database,
a shortest-path tree is built having the source network as
root. To complete the process, the branches that do not
contain routers/transit networks that have been labelled
with the particular multicast destination (via a group-
membership-LSA) are pruned from the tree.
As an example of the building of a datagram's shortest path
tree, again consider the Autonomous System in Figure 1. The
Autonomous System's link state database is pictured in
Figure 2. When a router initially receives a multicast
datagram sent by Host H2 to the multicast group A, the
following steps are taken: Host H2 is first determined to be
on Network N4. Then the shortest path tree rooted at net N4
is calculated[7], pruning those branches that do not contain
routers/transit networks that have been labelled with the
multicast group A. This results in the pruned shortest-path
tree pictured in Figure 3. Note that at this point all the
leaves of the tree are routers/transit networks labelled
with multicast group A (routers RT2 and RT9 and transit
Network N6).
In order to forward the multicast datagram, each router must
find its own position in the datagram's shortest path tree.
o RT3 (N4, origin)
/ \
1/ \8
/ \
N3 (Mb) o o RT6
/ \
0/ \7
/ \
RT2 (Ma,Mb) o o RT10
/ \
3/ \1
/ \
N8 o o N6 (Ma)
/
0/
/
RT11 o
/
1/
/
N9 o
/
0/
/
RT9 (Ma) o
Figure 3: Sample datagram's shortest-path tree,
source N4, destination Group A
The router's (call it Router RTX) position in the datagram's
pruned shortest-path tree consists of 1) RTX's parent in the
tree (this will be the forwarding cache entry's upstream
node) and 2) the list of RTX's interfaces that lead to
downstream routers/transit networks that have been labelled
with the datagram's destination (these will be added to the
forwarding cache entry as downstream interfaces). Note that
after calculating the datagram's shortest path tree, a
router may find that it is itself not on the tree. This
would be indicated by a forwarding cache entry having no
upstream node or an empty list of downstream interfaces.
As an example of a router describing its position on the
datagram's shortest-path tree, consider Router RT10 in
Figure 3. Router RT10's upstream node for the datagram is
Router RT6, and there are two downstream interfaces: one
connecting to Network N6 and the other connecting to Network
N8.
2.3.3. Support for Non-broadcast networks
When forwarding multicast datagrams over non-broadcast
networks, the datagram cannot be sent as a link-level
multicast (since neither link-level multicast nor broadcast
are supported on these networks), but must instead be
forwarded separately to specific neighbors. To facilitate
this, forwarding cache entries can also contain downstream
neighbors as well as downstream interfaces.
The IGMP protocol is not defined over non-broadcast
networks. For this reason, there cannot be group members
directly attached to non-broadcast networks, nor do non-
broadcast networks ever appear in local group database
entries.
As an example, suppose that Network N3 in Figure 1 is an
X.25 PDN. Consider Router RT3's forwarding cache entry for
datagrams having source Network N4 and multicast destination
Group B. In place of having the interface to Network N3
appear as the downstream interface in the matching
forwarding cache entry, the neighboring routers RT1 and RT2
would instead appear as separate downstream neighbors. In
addition, in this case there could not be a Group B member
directly attached to Network N3.
2.3.4. Details concerning forwarding cache entries
Each of the downstream interface/neighbors in the cache
entry is labelled with a TTL value. This value indicates the
number of hops a datagram forwarded out of the interface (or
forwarded to the neighbor) would have to travel before
encountering a router/transit network requesting the
multicast destination. The reason that a hop count is
associated with each downstream interface/neighbor is so
that IP multicast's expanding ring search procedure can be
more efficiently implemented. By expanding ring search is
meant the following. Hosts can restrict the frowarding
extent of the IP multicast datagrams that they send by
appropriate setting of the TTL value in the datagram's IP
header. Then, for example, to search for the nearest server
the host can send multicasts first with TTL set to 1, then
2, etc. By attaching a hop count to each downstream
interface/neighbor in the forwarding cache, datagrams will
not be forwarded unless they will ultimately reach a
multicast destination before their TTL expires[8]. This
avoids wasting network bandwidth during an expanding ring
search.
As an example consider Router RT10's forwarding cache in
Figure 3. Router RT10's cache entry has two downstream
interfaces. The first, connecting to Network N6, is labelled
as having a group member one hop away (Network N6). The
second, which connects to Network N8, is labelled as having
a group member two hops away (Router RT9).
Both the datagram shortest path tree and the local group
database may contribute downstream interfaces to the
forwarding cache entries. As an example, if a router has a
local group database entry of [Group G, NX], then a
forwarding cache entry for Group G, regardless of
destination, will list the router interface to Network NX as
a downstream interface. Such a downstream interface will
always be labelled with a TTL of 1.
As an example of forwarding cache entries, again consider
the Autonomous System pictured in Figure 1. Suppose Host H2
sends a multicast datagram to multicast group A. In that
case, some routers will not even attempt to build a
forwarding cache entry (e.g, router RT5) because they will
never receive the multicast datagram in the first place.
Other routers will receive the multicast datagram (since
they are forwarded as link-level multicasts), but after
building the pruned shortest path tree will notice that they
themselves are not a part of the tree (routers RT1, RT4,
RT7, RT8 and RT12). These latter routers will install an
empty cache entry, indicating that they do not participate
in the forwarding of the multicast datagram. A sample of the
forwarding cache entries built by the other routers in the
Autonomous System is pictured in Table 2.
A MOSPF router must clear its entire forwarding cache when
the Autonomous System's topology changes, because all the
datagram shortest-path trees must be rebuilt. Likewise, when
the location of a multicast group's membership changes
(reflected by a change in group-membership-LSAs), all cache
entries associated with the particular multicast destination
group must be cleared. Other than these two cases,
forwarding cache entries need not ever be deleted or
otherwise modified; in particular, the forwarding cache
entries do not have to be aged. However, forwarding cache
entries can be freely deleted after some period of
inactivity (i.e., garbage collected), if router memory
Router Upstream Downstream interfaces
node (interface:hops)
___________________________________________
RT10 Router RT6 (N6:1), (N8:2)
RT11 Net N8 (N9:1)
RT3 Net N4 (N3:1), (RT6:3)
RT6 Router RT3 (RT10:2)
RT2 Net N3 (N2:1)
Table 2: Sample forwarding cache entries,
for source N4 and destination Group A.
resources are in short supply.
3. Inter-area multicasting
Up to this point this memo has discussed multicast forwarding when
the entire Autonomous System is a single OSPF area. The logic for
when the multicast datagram's source and its destination group
members belong to the same OSPF area is the same. This section
explains the behavior of the MOSPF protocol when the datagram's
source and (at least some of) its destination group members belong
to different OSPF areas. This situation is called inter-area
multicast.
Inter-area multicast brings up the following issues, which are
resolved in succeeding sections:
o Are the group-membership-LSAs specific to a single area? And if
they are, how is group membership information conveyed from one
area to the next?
o How are the datagram shortest-path trees built in the inter-area
case, since complete information concerning the topology of the
datagram source's neighborhood is not available to routers in
other areas?
o In an area border router, multiple datagram shortest-path trees
are built, one for each attached area. How are these separate
datagram shortest-path trees combined into a single forwarding
cache entry?
It should be noted in the following that the basic protocol
mechanisms in the inter-area case are the same as for the intra-area
case. Forwarding of multicasts is still defined by the contents of
the forwarding cache. The forwarding cache is still built from the
same two components: the local group database and the datagram
shortest-path trees. And while the calculation of the datagram
shortest-path trees is different in the inter-area case (see Section
3.2), the local group database is built exactly the same as in the
intra-area case (i.e., MOSPF's interface with IGMP remains unchanged
in the presence of areas). Finally, the forwarding algorithm
described in Section 11 is the same for both the intra-area and
inter-area cases.
The following discussion uses the area configuration pictured in
Figure 4 as an example. This figure, taken from the OSPF
specification, shows an Autonomous System split into three areas
(Area 1, Area 2 and Area 3). A single backbone area has been
configured (everything outside of the shading). Since the backbone
area must be contiguous, a single virtual link has been configured
between the area border routers RT10 and RT11. Additionally, an area
address range has been configured in Router RT11 so that Networks
N9-N11 and Host H1 will be reported as a single route outside of
Area 3 (via summary-link-LSAs).
3.1. Extent of group-membership-LSAs
Group-membership-LSAs are specific to a single OSPF area. This
means that, just as with OSPF router-LSAs, network-LSAs and
summary-link-LSAs, a group-membership-LSA is flooded throughout
a single area only[9]. A router attached to multiple areas
(i.e., an area border router) may end up originating several
group-membership-LSAs concerning a single multicast destination,
one for each attached area. However, as we will see below, the
contents of these group-membership-LSAs will vary depending on
their associated areas.
Just as in OSPF, each MOSPF area has its own link state
database. The MOSPF database is simply the OSPF link state
database enhanced by the group-membership-LSAs. Consider again
the area configuration pictured in Figure 4. The result of
adding the group-membership-LSAs to the area databases yields
the databases pictured in Figures 6 and 7. Figure 6 shows Area
1's MOSPF database. Figure 7 shows the backbone's MOSPF
database. Superscripts indicate which transit vertices have been
advertised as requesting particular multicast destinations. A
superscript of "w" indicates that the router is advertising
itself as a wild-card multicast receiver (see below). The dashed
lines are OSPF summary-link-LSAs or AS external-link-LSAs. Note
in Figure 7 that Router RT11 has condensed its routes to
Networks N9-N11 and Host H1 into a single summary-link-LSA.
..................................
. + .
. | 3+---+ +--+ +--+ . N12 N14
. N1|--|RT1|\1 |Mb| |H4| . \ N13 /
. _| +---+ \ +--+ /+--+ . 8\ |8/8
. | + \ _|__/ . \|/
. +--+ +--+ / \ 1+---+8. 8+---+6
. |Mb| |Mb| * N3 *---|RT4|------|RT5|--------+
. +--+ /+--+ \____/ +---+ . +---+ |
. + / | . |7 |
. | 3+---+ / | . | |
. N2|--|RT2|/1 |1 . |6 |
. __| +---+ +---+8 . 6+---+ |
. | + |RT3|--------------|RT6| |
. +--+ +--+ +---+ +--+. +---+ |
. |Ma| |H3|_ |2 _|H2|. Ia|7 |
. +--+ +--+ \ | / +--+. | |
. +---------+ . | |
.Area 1 N4 . | |
.................................. | |
................................ | |
. N11 . | |
. +---------+ . | |
. | \ . | | N12
. |3 +--+ . | |6 2/
. +---+ |Ma| . | +---+/
. |RT9| +--+ . | |RT7|---N15
. +---+ ....... | +---+ 9
. |1 .. + ...|..........|1........
. _|__ .. | Ib|5 __|_ +--+.
. / \ 1+----+2.. | 3+----+1 / \--|Ma|.
. * N9 *------|RT11|----|---|RT10|---* N6 * +--+.
. \____/ +----+ .. | +----+ \____/ .
. | !*******|*****! | .
. |1 Virtual + Link |1 .
. +--+ 10+----+ ..N8 +---+ .
. |H1|-----|RT12| .. |RT8| .
. +--+SLIP +----+ .. +---+ +--+.
. |2 .. |4 _|H5|.
. | .. | / +--+.
. +---------+ .. +--------+ .
. N10 Area 3..Area 2 N7 .
.............................................................
Figure 4: A sample MOSPF area configuration
Suppose an OSPF router has a local group database entry for
[Group Y, Network X]. The router then originates a group-
membership-LSA for Group Y into the area containing Network X.
For example, in the area configuration pictured in Figure 4,
Router RT1 originates a group-membership-LSA for Group B. This
group-membership-LSA is flooded throughout Area 1, and no
further. Likewise, assuming that Router RT3 has been elected
Designated Router for Network N3, RT3 originates a group-
membership-LSA into Area 1 listing the transit Network N3 as
having group members. Note that in the link state database for
Area 1 (Figure 6) both Router RT1 and Network N3 have
accordingly been labelled with Group B.
In OSPF, the area border routers forward routing information and
data traffic between areas. In MOSPF. a subset of the area
border routers, called the inter-area multicast forwarders,
forward group membership information and multicast datagrams
between areas. Whether a given OSPF area border router is also a
MOSPF inter-area multicast forwarder is configuration dependent
(see Section B.1). In Figure 4 we assume that all area border
routers are also inter-area multicast forwarders.
In order to convey group membership information between areas,
inter-area multicast forwarders "summarize" their attached
areas' group membership to the backbone. This is very similar
functionality to the summary-link-LSAs that are generated in the
base OSPF protocol. An inter-area multicast forwarder
calculates which groups have members in its attached non-
backbone areas. Then, for each of these groups, the inter-area
multicast forwarder injects a group-membership-LSA into the
backbone area. For example, in Figure 4 there are two groups
having members in Area 1: Group A and Group B. For that reason,
both of Area 1's inter-area multicast forwarders (Routers RT3
and RT4) inject group-membership-LSAs for these two groups into
the backbone. As a result both of these routers are labelled
membership +------------------+ datagrams
+ > > > >>| Backbone |< < < < +
^ +------------------+ ^
^ / | \ ^
^ / | \ ^
+----^-----+/ +----------+ \+----^-----+
| Area 1 | | Area 2 | | Area 3 |
+----------+ +----------+ +----------+
Figure 5: Inter-area routing architecture
with Groups A and B in the backbone link state database (see
Figure 7).
However, unlike the summarization of unicast destinations in the
base OSPF protocol, the summarization of group membership in
MOSPF is asymmetric. While a non-backbone area's group
membership is summarized to the backbone, this information is
not then readvertised into other non-backbone areas. Nor is the
backbone's group membership summarized for the non-backbone
areas. Going back to the example in Figure 4, while the presence
of Area 3's group (Group A) is advertised to the backbone, this
information is not then redistributed to Area 1. In other words,
routers internal to Area 1 have no idea of Area 3's group
membership.
At this point, if no extra functionality was added to MOSPF,
multicast traffic originating in Area 1 destined for Multicast
Group A would never be forwarded to those Group A members in
Area 3. To accomplish this, the notion of wild-card multicast
receivers is introduced. A wild-card multicast receiver is a
router to which all multicast traffic, regardless of multicast
destination, should be forwarded. A router's wild-card multicast
reception status is per-area. In non-backbone areas, all inter-
area multicast forwarders[10] are wild-card multicast receivers.
This ensures that all multicast traffic originating in a non-
backbone area will be forwarded to its inter-area multicast
forwarders, and hence to the backbone area. Since the backbone
has complete knowledge of all areas' group membership, the
datagram can then be forwarded to all group members. Note that
in the backbone itself there is no need for wild-card multicast
receivers[11]. As an example, note that Routers RT3 and RT4 are
wild-card multicast receivers in Area 1 (see Figure 6), while
there are none in the backbone (see Figure 7).
This yields the inter-area routing architecture pictured in
Figure 5. All group membership is advertised by the non-
backbone areas into the backbone. Likewise, all IP multicast
traffic arising in the non-backbone areas is forwarded to the
backbone. Since at this point group membership information meets
the multicast datagram traffic, delivery of the multicast
datagrams becomes possible.
3.2. Building inter-area datagram shortest-path trees
When building datagram shortest-path trees in the presence of
areas, it is often the case that the source of the datagram and
(at least some of) the destination group members are in separate
areas. Since detailed topological information concerning one
**FROM**
|RT|RT|RT|RT|RT|RT|
|1 |2 |3 |4 |5 |7 |N3|
----- -------------------
RT1| | | | | | |0 |
RT2| | | | | | |0 |
RT3| | | | | | |0 |
* RT4| | | | | | |0 |
* RT5| | |14|8 | | | |
T RT7| | |20|14| | | |
O N1|3 | | | | | | |
* N2| |3 | | | | | |
* N3|1 |1 |1 |1 | | | |
N4| | |2 | | | | |
Ia,Ib| | |15|22| | | |
N6| | |16|15| | | |
N7| | |20|19| | | |
N8| | |18|18| | | |
N9-N11,H1| | |19|16| | | |
N12| | | | |8 |2 | |
N13| | | | |8 | | |
N14| | | | |8 | | |
N15| | | | | |9 | |
Figure 6: Area 1's MOSPF database.
Networks and routers are represented by vertices.
An edge of cost X connects Vertex A to Vertex B iff
the intersection of Column A and Row B is marked
with an X. In addition, RT1, RT2 and N3 are labelled
with multicast group A, RT1 is labelled with multicast
group B, and both RT3 and RT4 are labelled as
wild-card multicast receivers.
**FROM**
|RT|RT|RT|RT|RT|RT|RT
|3 |4 |5 |6 |7 |10|11|
------------------------
RT3| | | |6 | | | |
RT4| | |8 | | | | |
RT5| |8 | |6 |6 | | |
RT6|8 | |7 | | |5 | |
RT7| | |6 | | | | |
* RT10| | | |7 | | |2 |
* RT11| | | | | |3 | |
T N1|4 |4 | | | | | |
O N2|4 |4 | | | | | |
* N3|1 |1 | | | | | |
* N4|2 |3 | | | | | |
Ia| | | | | |5 | |
Ib| | | |7 | | | |
N6| | | | |1 |1 |3 |
N7| | | | |5 |5 |7 |
N8| | | | |4 |3 |2 |
N9-N11,H1| | | | | | |1 |
N12| | |8 | |2 | | |
N13| | |8 | | | | |
N14| | |8 | | | | |
N15| | | | |9 | | |
Figure 7: The backbone's MOSPF database.
Networks and routers are represented by vertices.
An edge of cost X connects Vertex A to Vertex B iff
the intersection of Column A and Row B is marked
with an X. In addition, RT3 and RT4 are labelled
with both multicast groups A and B, and RT7, RT10,
and RT11 are labelled with multicast group A.
OSPF area is not distributed to other OSPF areas (the flooding
of router-LSAs, network-LSAs and group-membership-LSAs is
restricted to a single OSPF area only), the building of complete
datagram shortest-path trees is often impossible in the inter-
area case. To compensate, approximations are made through the
use of wild-card multicast receivers and OSPF summary-link-LSAs.
When it first receives a datagram for a particular [source net,
destination group] pair, a router calculates a separate datagram
shortest-path tree for each of the router's attached areas. Each
datagram shortest-path tree is built solely from LSAs belonging
to the particular area's link state database. Suppose that a
router is calculating a datagram shortest-path tree for Area A.
It is useful then to separate out two cases.
The first case, Case 1: The source of the datagram belongs to
Area A has already been described in Section 2.3.2. However, in
the presence of OSPF areas, during tree pruning care must be
taken so that the branches leading to other areas remain, since
it is unknown whether there are group members in these (remote)
areas. For this reason, only those branches having no group
members nor wild-card multicast receivers are pruned when
producing the datagram shortest-path tree.
As an example, suppose in Figure 4 that Host H2 sends a
multicast datagram to destination Group A. Then the datagram's
shortest-path tree for Area 1, built identically by all routers
in Area 1 that receive the datagram, is shown in Figure 8. Note
that both inter-area multicast forwarders (RT3 and RT4) are on
the datagram's shortest-path tree, ensuring the delivery of the
datagram to the backbone and from there to Areas 2 and 3.
o Case 2: The source of the datagram belongs to an area other
than Area A. In this case, when building the datagram
shortest-path tree for Area A, the immediate neighborhood of
the datagram's source is unknown. However, there are
summary-link-LSAs in the Area A link state database
indicating the cost of the paths between each of Area A's
inter-area multicast forwarders and the datagram source.
These summary links are used to approximate the neighborhood
of the datagram's source; the tree begins with links
directly connecting the source to each of the inter-area
multicast forwarders. These links point in the reverse
o RT3 (W, origin=N4)
|
1|
|
N3 (Mb) o
/ \
0/ \0
/ \
RT2 (Ma,Mb) o o RT4 (W)
Figure 8: Datagram's shortest-path tree,
Area 1, source N4, destination Group A
direction (towards instead of away from the datagram source)
from the links considered in Case 1 above. All additional
links added to the tree also point in the reverse direction.
The final datagram shortest-path tree is then produced by,
as before, pruning all branches having no group-members nor
wild-card multicast receivers.
As an example, suppose again that Host H2 in Figure 4 sends
a multicast datagram to destination Group A. The datagram's
shortest-path tree for the backbone is shown in Figure 9.
The neighborhood around the source (Network N4) has been
approximated by the summary links advertised by routers RT3
and RT4. Note that all links in Figure 9's datagram
shortest-path tree have arrows pointing in the reverse
direction, towards Network N4 instead of away from it.
The reverse costs used for the entire tree in Case 2 are forced
because summary-link-LSAs only specify the cost towards the
datagram source. In the presence of asymmetric link costs, this
may lead to less efficient routes when forwarding multicasts
o N4
/ \
2/ \3
/ \
RT3 (Ma,Mb) o o RT4 (Ma,Mb)
/ \
6/ \8
/ \
RT6 o o RT5
| |
5| |6
| |
RT10 (Ma) o o RT7 (Ma)
|
2|
|
RT11 (Ma) o
Figure 9: Datagram shortest-path tree: Backbone,
source N4, destination Group A. Note that
reverse costs (i.e., toward origin) are
used throughout.
between areas.
Those routers attached to multiple areas must calculate multiple
trees and then merge them into a single forwarding cache entry.
As shown in Section 2.3.2, when connected to a single area the
router's position on the datagram shortest-path tree determines
(in large part) its forwarding cache entry. When attached to
multiple areas, and hence calculating multiple datagram
shortest-path trees, each tree contributes to the forwarding
cache entry's list of downstream interfaces/neighbors. However,
only one of the areas' datagram shortest-path trees will
determine the forwarding cache entry's upstream node. When one
of the attached areas contains the datagram source, that area
will determine the upstream node. Otherwise, the tiebreaking
rules of Section 12.2.7 are invoked.
Consider again the example of Host H2 in Figure 4 sending a
multicast datagram to destination Group A. Router RT3 will
calculate two datagram shortest-path trees, one for Area 1 and
one for the backbone. Since the source of the datagram (Host
H2) belongs to Area 1, the Area 1 datagram shortest-path tree
determines RT3's upstream node: Network N4. Router RT3
calculates two downstream interfaces for the datagram: the
interface to Network N3 (which comes from Area 1's datagram
shortest-path tree) and the serial line to Router RT6 (which
comes from the backbone's datagram shortest-path tree). As for
Router RT10, it calculates two trees, determining its upstream
node from the backbone tree and its two downstream interfaces
from the Area 2 tree. Finally, Router RT11 calculates three
trees, determining its upstream node from the Area 2 tree and
its downstream interface from the Area 3 tree.
4. Inter-AS multicasting
This section explains how MOSPF deals with the forwarding of
multicast datagrams between Autonomous Systems. Certain AS boundary
routers in a MOSPF system will be configured as inter-AS multicast
forwarders. It is assumed that these routers will also be running an
inter-AS multicast routing protocol. This specification does not
dictate the operation of such an inter-AS multicast routing
protocol. However, the following interactions between MOSPF and the
inter-AS routing protocol are assumed:
(1) MOSPF guarantees that the inter-AS multicast forwarders will
receive all multicast datagrams; but it is up to each router so
designated to determine whether the datagram should be forwarded
to other Autonomous Systems. This determination will probably be
made via the inter-AS routing protocol.
(2) MOSPF assumes that the inter-AS routing protocol is forwarding
multicast datagrams in an RPF (reverse path forwarding; see
[Deering] for an explanation of this terminology) fashion. In
other words, it is assumed that a multicast datagram whose
source (call it X) lies outside the MOSPF domain will enter the
MOSPF domain at those points that are advertising (into OSPF)
the best routes back to X. MOSPF calculates the path of the
datagram through the MOSPF domain based on this assumption.
MOSPF designates an inter-AS multicast forwarder as a wild-card
multicast receiver in all of its attached areas. As in the inter-
area case, this ensures that the routers remain on all pruned
shortest-path trees and thereby receive all multicast datagrams,
regardless of destination.
As an example, suppose that in Figure 1 both RT5 and RT7 were
configured as inter-AS multicast forwarders. Then the link state
database would look like the one pictured in Figure 2, with the
addition of a) wild-card status for RT5 and RT7 (they would appear
with superscripts of "w") and b) the external links originated by
RT5 and RT7 being labelled as multicast-capable[12].
As another example, consider the area configuration in Figure 4.
Again suppose RT5 and RT7 are configured as inter-AS multicast
forwarders. Then in Area 1's link state database (Figure 6), the
external links originated by RT5 and RT7 would again be labelled as
multicast-capable. However, note that in Area 1's database RT5 and
RT7 are not labelled as wild-card multicast receivers. This is
unnecessary; since Area 1's inter-area multicast forwarders (RT3 and
RT4) are wild-cards, all multicast datagrams will be forwarded to
the backbone. And in the backbone's link state database (Figure 7),
RT5 and RT7 will be labelled as wild-cards.
4.1. Building inter-AS datagram shortest-path trees.
When multicast datagrams are to be forwarded between Autonomous
Systems, the datagram shortest-path tree is built as follows.
Remember that the router builds a separate tree for each area to
which it is attached; these trees are then merged into a single
forwarding cache entry. Suppose that the router is building the
tree for Area A. We break up the tree building into three cases.
This first two cases have already been described earlier in this
memo: Case 1 (the source of the datagram belongs to Area A)
having been described in Section 2.3.2 and Case 2 (the source of
the datagram belongs to another OSPF area) having been described
in Section 3.2. The only modification to these cases is that
inter-AS multicast forwarders, as well as group members and
inter-area multicast forwarders, must remain on the pruned
trees. The new case is as follows:
o Case 3: The source of the datagram belongs to another
Autonomous System. The immediate neighborhood of the source
is then unknown. In this case the multicast-capable AS
external links are used to approximate the neighborhood of
the source; the tree begins with links directly attaching
the source to one or more inter-AS multicast forwarders. The
approximating AS external links point in the reverse
direction (i.e., towards the source), just as with the
approximating summary links in Case 2. Also, as in Case 2,
all links included in the tree must point in the reverse
direction. The final datagram shortest-path tree is then
produced (as always) by pruning those branches having no
group members nor wild-card multicast receivers.
As an example, suppose that a host on Network N12 (see
Figure 4) originates a multicast datagram for Destination
Group B. Assume that all external costs pictured are OSPF
external type 1 metrics. Then any routers in Area 1
receiving the datagram would build the datagram shortest-
path tree pictured in Figure 10. Note that all links in the
tree point in the reverse direction, towards the source. The
tree indicates that the routers expect the datagram to enter
the Autonomous System at Router RT7, and then to enter the
area at Router RT4.
Note that in those cases where the "best" inter-AS multicast
forwarder is not directly attached to the area, the
neighborhood of the source is actually approximated by the
concatenation of a summary link and a multicast-capable AS
external link. This is in fact the case in Figure 10.
In Case 3 (datagram source in another AS) the requirement that
all tree links point in the reverse direction (towards the
source) accommodates the fact that summary links and AS external
links already point in the reverse direction. This also leads to
the requirement that the inter-AS multicast routing protocol
operate in a reverse path forwarding fashion (see condition 2 of
Section 4). Note that Reverse path forwarding can lead to sub-
optimal routing when costs are configured asymmetrically. And it
can even lead to non-delivery of multicast datagrams in the case
of asymmetric reachability.
Inter-AS multicast forwarders may end up calculating a
forwarding cache entry's upstream node as being external to the
AS. As an example, Router RT7 in Figure 10 will end up
calculating an external router (via its external link to Network
o N12
|
2|
|
o RT7
|
14|
|
o RT4 (W)
|
0|
|
o N3 (Mb)
/|\
/ | \
1/ | 1\
/ 1| \
/ | \
RT1 (Mb) o | o RT3 (W)
o
RT2 (Ma,Mb)
Figure 10: Datagram shortest-path tree: Area 1,
source N12, destination Group B. Note that
reverse costs (i.e., toward origin) are
used throughout.
N12) as the upstream node for the datagram. This means that RT7
must receive the datagram from a router in another AS before
injecting the datagram into the MOSPF system.
4.2. Stub area behavior
AS external links are not imported into stub areas. Suppose that
the source of a particular datagram lies outside of the
Autonomous System, and that the datagram is forwarded into a
stub area. In the stub area's datagram shortest-path tree the
neighborhood of the datagram's source cannot be approximated by
AS external links. Instead the neighborhood of the source is
approximated by the default summary links (see Section 3.6 of
[OSPF]) that are originated by the stub area's intra-area
multicast forwarders.
Except for this small change to the construction of a stub
area's datagram shortest-path trees, all other MOSPF algorithms
(e.g., merging with other areas' datagram shortest-path trees to
form the forwarding cache) function the same for stub areas as
they do for non-stub areas.
4.3. Inter-AS multicasting in a core Autonomous System
It may be the case that the MOSPF routing domain connects
together many different Autonomous Systems, thereby serving as a
"core Autonomous System" (e.g, the old NSFNet backbone). In this
case, it could very well be that the majority of the MOSPF
routers are also inter-AS multicast forwarders. Having each
inter-AS multicast forwarder then declare itself a wild-card
multicast receiver could very well waste considerable network
bandwidth. However, as an alternative to declaring themselves
wild-card multicast receivers, the inter-AS multicast routers
could instead explicitly advertise all groups that they were
interested in forwarding (to other "client" Autonomous Systems)
in group-membership-LSAs. These advertised groups would have to
be learned through an inter-AS multicast routing protocol (or
possibly even statically configured).
This in essence allows the clients of the core Autonomous System
to advertise their group membership into the core. However,
since any client MOSPF domains will still have their inter-AS
multicast forwarders configured as wild-card multicast
receivers, this advertisement will be asymmetric: the core will
not advertise its or others' group membership to the clients.
The achieves the same inter-AS multicast routing architecture
that MOSPF uses for inter-area multicast routing (see Figure 5).
5. Modelling internal group membership
A MOSPF router may itself contain multicast applications. A typical
example of this is a UNIX workstation that doubles as a multicast
router. This section concerns two alternative ways of representing
the group membership of the MOSPF router's internal applications.
Both representations have advantages. For maximum flexibility, the
MOSPF forwarding algorithm (see Section 11) has been specified so
that either representation can be used in a MOSPF router (and in
fact, both representations can be used at once, depending on the
application).
The first representation is based on the paradigm presented in RFC
1112. In this case, an application joins a multicast group on one or
more specific physical interfaces. The application then receives a
multicast datagram if and only if it is received on one of the
specified interfaces. If a datagram is received on multiple
specified interfaces, the application receives multiple copies.
Figure 11 shows this algorithm as it is implemented in (modified)
BSD UNIX kernels. The figure shows the processing of a multicast
datagram, starting with its reception on a particular interface.
First copies of the datagram are given to those applications that
have joined on the receiving interface. Then the forwarding decision
(pictured as a box containing a question mark) is made, and the
packet is (possibly) forwarded out certain interfaces. If these
interfaces are not capable of receiving their own multicasts, a copy
of the datagram must be internally looped back to appropriately
joined applications.
The advantages to the RFC 1112 representation are as follows:
o It is the standard for the way an IP host joins multicast
groups. It is simplest to use the same membership model for
hosts and routers; most would consider an IP router to be a
special case of an IP host anyway.
o It is the way group membership has been implemented in BSD UNIX.
Existing multicast applications are written to join multicast
groups on specific interfaces.
o The possibility of receiving multiple datagram copies may
improve fault tolerance. If the datagram is dropped due to an
+-------+
|receive|
+-------+
|
|---> To application
|
+-------------------+
|forwarding decision|
+-------------------+
|
/ \
/---\----> To application
/ \------> To application
/ \
/ \
+--------+ +--------+
|transmit| |transmit|
+--------+ +--------+
Figure 11: RFC 1112 representation of internal
group membership
error on the path to some interface, another interface may still
receive a copy.
o The ability to specify a particular receiving interface may
improve the accuracy of IP multicast's expanding ring search
mechanism (see Section 2.3.4).
o Membership in the non-routable multicast groups (224.0.0.1 -
224.0.0.255) must be on a per-interface basis. An OSPF router
always belongs to 224.0.0.5 (AllSPFRouters) on its OSPF
interfaces, and may belong to 224.0.0.6 (AllDRouters) on one or
more of its OSPF interfaces.
The second representation is MOSPF-specific. In this case, an
application joins a multicast group on an interface-independent
basis. In other words, group membership is associated with the
router as a whole, not separately on each interface. The application
then receives a copy of a multicast datagram if and only if the
datagram would actually be forwarded by the MOSPF router. Figure 12
shows how this algorithm would be implemented. The datagram is
received on a particular interface. If the datagram is validated for
forwarding (i.e., the receiving interface connects to the matching
forwarding cache entry's upstream node), a copy of the datagram is
also given to appropriately joined applications. Note that this
model of group membership is not as general as the RFC 1112 model,
in that it can only be implemented in MOSPF routers and not in
arbitrary IP hosts. However, it has the following advantages:
o The application does not need to have knowledge of the router
interfaces. It does not need to know what kind or how many
interfaces there are; this will be taken care of by the MOSPF
protocol itself.
o As long as any interface is operational, the application will
continue to receive multicast datagrams. This happens
automatically, without the application modifying its group
membership.
o The application receives only one copy of the datagram. Using
the RFC1112 representation, whenever an application joins on
more than one interface (which must be done if the application
does not want to rely on a single interface), multiple datagram
copies will be received during normal operation.
6. Additional capabilities
This section describes the MOSPF configuration options that allow
routers of differing capabilities to be mixed together in the same
+-------+
|receive|
+-------+
|
|
|
+-------------------+
|forwarding decision|---> to application
+-------------------+
|
/ \
/ \
/ \
/ \
/ \
+--------+ +--------+
|transmit| |transmit|
+--------+ +--------+
Figure 12: MOSPF-specific representation of internal
group membership
routing domain. Note that these options handle special circumstances
that may not be encountered in normal operation. Default values for
the configuration settings are specified in Appendix B.
6.1. Mixing with non-multicast routers
MOSPF routers can be mixed freely with routers that are running
only the base OSPF algorithm (called non-multicast routers in
the following). This allows MOSPF to be deployed in a piecemeal
fashion, thereby speeding deployment and allowing
experimentation with multicast routing on a limited scale.
When a MOSPF router builds a datagram shortest-path tree, it
omits all non-multicast routers. For example, in Figure 1, if
Router RT6 was not a multicast router, the datagram shortest-
path tree in Figure 3 would be built with a more circuitous
branch through Router RT5, instead of through Router RT6. In
addition, non-multicast routers do not participate in the
flooding of the new group-membership-LSAs. This adheres to the
general principle that a router should not have to handle those
link state advertisements whose format (or contents) the router
does not understand.
Mixing MOSPF routers with non-multicast routers creates a number
of potential problems. Certain mixings of MOSPF and non-
multicast routers can cause multicast datagrams to take
suboptimal paths, or in other cases can lead to the non-delivery
of multicast datagrams. In addition, mixing MOSPF routers and
non-multicast routers can cause the paths of multicast datagrams
to diverge radically from the path of unicast datagrams. Such
divergences can make routing problems harder to debug.
In particular, the following specific difficulties may arise
when mixing MOSPF routers with non-multicast routers:
o Even though there is unicast connectivity to a destination,
there may not be multicast connectivity. For example, if
Router RT10 in Figure 1 becomes a non-multicast router, the
group member connected to Network N11 will no longer be able
to receive multicasts sourced by Host H2. But the two hosts
will be able to exchange unicasts (e.g., ICMP pings).
o When the Designated Router for a multi-access network is a
non-multicast router, the network will not be used for
forwarding multicast datagrams. For example, if in Figure 1
Router RT4 is Designated Router for Network N3, and RT4 is
non-multicast, Network N3 will not be used to forward IP
multicasts. This would mean that multicast datagrams
originated by Hosts H2 and H3 would not be forwarded beyond
their local network (N4), even though it seems that the
needed multicast connectivity exists.
o When forwarding multicast datagrams between areas, mixing of
MOSPF routers and non-multicast routers in the source area
may cause unexpected loss of multicast connectivity. This is
because in the inter-area routing of multicast datagrams the
neighborhood of the datagram's source is approximated by
OSPF summary links, and OSPF summary-link-LSAs do not carry
indications/guarantees of the summarized path's multicast
routing capability.
6.2. TOS-based multicast
MOSPF allows a separate datagram shortest-path tree to be built
for each IP Type of Service. This means that the path of a
multicast datagram can vary depending on the datagram's TOS
classification, as well as its source and destination.
For each router interface, OSPF allows a separate metric to be
configured for each IP TOS. When building the shortest path tree
for TOS X, the cost of a path is the sum of the component
interfaces' TOS X metrics. Note that OSPF requires that a TOS 0
metric be specified for each interface. However, as a form of
data compression, metrics need only be specified for non-zero
TOS if they are different than the TOS 0 metric.
Additionally, OSPF routers can be configured to ignore TOS when
forwarding packets. Such routers, called TOS-incapable, build
only the TOS 0 portion of the routing table. TOS-incapable
routers can be mixed freely with TOS-capable routers when
forwarding unicast packets. The way this is handled for unicast
packets is that the unicast is forwarded along the TOS 0 route
whenever the TOS X route does not exist. However, MOSPF must
treat this situation somewhat differently, since each router
must build the exact same tree rooted at the datagram's source.
Like OSPF, MOSPF allows TOS-based routing to be optional. TOS-
capable and TOS-incapable multicast routers can be mixed freely
in the routing domain. TOS-incapable routers will only ever
build TOS 0 datagram shortest-path trees. TOS-capable routers
will first build TOS 0 datagram shortest-path trees. If these
trees contain only TOS-capable routers, datagram shortest-path
trees are then built separately for non-zero TOS values.
Otherwise, the TOS 0 datagram shortest-path tree is used to
forward all traffic, regardless of its TOS designation. Using
this logic, all routers in essence continue to utilize identical
datagram shortest-path trees. See Section 12.2.8 for more
details.
6.3. Assigning multiple IP networks to a physical network
Assigning multiple IP networks/subnets to a single physical
network causes some confusion in MOSPF. This is because the
underlying OSPF protocol treats these IP networks/subnets as
entirely separate entities, originating separate network-LSAs
for each and forming separate adjacencies for each, while IGMP
recognizes only the single underlying physical network. Adding
to the problem is the fact that when a multicast datagram is
received from such a multiply-addressed physical wire, there is
no good way to choose the datagram's upstream node (which must
be done in order to make the forwarding decision; see Section 11
for details). As a result, unless this situation is dealt with
through configuration, unwanted replication of multicast
datagrams may occur when they are forwarded over multiply-
addressed wires.
As a remedy, MOSPF allows multicast forwarding to be disabled on
certain IP networks/subnets. When multicast forwarding is
disabled on the wire's "extra" subnets (i.e., all but one), the
extra subnets will not appear in datagram shortest-path trees,
nor will they appear in local group database or forwarding cache
entries. As a result, the possibility of unwanted datagram
replication is eliminated. The actual disabling of multicast
forwarding on a subnet is done through setting the
IPMulticastForwarding parameter to disabled on all router
interfaces connecting to the subnet (see Section B.2).
6.4. Networks on Autonomous System boundaries
Another complication can arise on IP networks/subnets that lie
on the boundary of a MOSPF Autonomous System. Similar to the
unicast situation where these networks may be running multiple
IGPs (Interior Gateway Protocols), these networks may also be
running multiple multicast routing protocols. It may then become
impossible for a MOSPF router to determine whether a multicast
datagram is being forwarded along the datagram shortest-path
tree, or whether it has been inadvertently received from the
other Autonomous System. Guessing wrong can lead to either
unwanted replication or non-delivery of the multicast datagram.
In addition, in order to prevent receiving duplicate multicast
datagrams, group members on these boundary networks will
probably want to declare their membership to one Autonomous
System and not another.
For example, consider the two Autonomous Systems pictured in
Figure 13. Network X is on the boundary of both ASes. One
possible multicast datagram path is shown; the datagram
originates in a third Autonomous System, and is then delivered
to both AS #1 and AS #2 separately. The paths through the two
Autonomous Systems may end up having certain boundary networks
as common segments. In Figure 13, Network X is common to both
paths. In this case, if both Autonomous Systems were running
(separate copies of) MOSPF, the same datagram would appear twice
on Network X as a data-link multicast. This would cause
duplicate datagrams to be received by any group members on
Network X or downstream from Network X.
MOSPF has two mechanisms to eliminate this replication of
multicast datagrams. First, a system administrator can configure
certain networks to forward multicast datagrams as data-link
unicasts instead of data-link multicasts. This is done by
setting the IPMulticastForwarding parameter to data-link unicast
on those router interfaces attaching to the network (see Section
B.2). As an example, in Figure 13 the routers in AS #2 could be
configured so that Router C would send the multicast datagram
out onto Network X as a data-link unicast addressed directly to
Router D. Router D would accept this data-link unicast, but
<-Datagram path->*
* *
* *
* .....*.........
.........*..... | . * AS #2
AS #1 * . |*****+---+
+---+*****|*----|RTC|
|RTA|----*|* . +---+
+---+ . *|* .
. *|* .
. *|* . +---+
+---+ . *|*----|RTD|
|RTB|----*|*****+---+
+---+*****| .....*..........
.........*.... | *
* | *
* Network X *
*
Figure 13: Networks on AS boundaries
would reject any data-link multicast forwarded by Router A. This
would eliminate replication of multicast datagrams downstream
from Network X. In addition, if the IPMulticastForwarding
parameter is set to data-link unicast on Network X, group
membership will not be monitored on the network. This will
prevent group members attached directly to Network X from
receiving multiple datagram copies, since group membership on
the boundary network will be monitored from only one AS (AS #1
in our example).
It should be noted that forwarding IP multicasts as data-link
unicasts has some disadvantages when three or more MOSPF routers
are attached to the network. First of all, it is more work for a
router to send multiple unicasts than a single multicast.
Second, the multiple unicasts consume more network bandwidth
than a single multicast. And last, it increases the delay for
some group members since multiple unicasts also take longer to
send than a single multicast.
6.5. Recommended system configuration
In order to make MOSPF's selection of routes more predictable,
it is recommended that all routers in any particular OSPF area
have the same multicast and TOS capabilities.Keeping areas
homogeneous ensures that IP multicast packets will follow
relatively the same path as IP unicasts. In contrast, while
heterogeneous areas will function, and will probably be
necessary at least during the initial introduction of multicast
routing, such areas may produce seemingly sub-optimal and
unexpected routes. For example, see Section 6.1 above for a
detailed description of the possible pitfalls when mixing
multicast and non-multicast routers.
As for the other options presented above, to achieve the most
predictable results it is recommended that a router interface's
IPMulticastForwarding parameter be set to a value other than
data-link multicast only when either a) multiple IP networks
have been assigned to a single physical wire or b) multiple
multicast routing protocols are running on the attached network.
7. Basic implementation requirements
An implementation of MOSPF requires the following pieces of system
support. Note that this support is in addition to that required for
the base OSPF implementation as outlined in Section 4.4 of [OSPF].
o Promiscuous multicast reception. In a multicast router, it is
necessary to receive all IP multicasts at the data-link level.
On those interfaces where IP multicast datagrams are
encapsulated by a wide range of data-link multicast destination
addresses (e.g, ethernet and FDDI), this is most easily
accomplished by disabling any hardware filtering of multicast
destinations (i.e., by "opening up" the interface's multicast
filter).
o Data-link multicast/broadcast detection. To avoid unwanted
replication of multicast datagrams in certain exceptional
conditions, it is necessary for the multicast router to
determine whether a datagram was received as a data-link
multicast/broadcast or as a data-link unicast, for later use by
the MOSPF forwarding mechanism. See Section 6.4 for more
details.
o An implementation of IGMP. MOSPF uses the Internet Group
Management Protocol (IGMP, documented in [RFC 1112]) to monitor
multicast group membership. See Section 9 for details.
8. Protocol data structures
The MOSPF protocol is described herein in terms of its operation on
various protocol data structures. These data structures are included
for explanatory uses only, and are not intended to constrain a MOSPF
implementation. Besides the data structures listed below, this
specification will also reference the various data structures (e.g.,
OSPF interfaces and neighbors) defined in [OSPF].
In a MOSPF router, the following items are added to the list of
global OSPF data structures described in Section 5 of [OSPF]:
o Local group database. This database describes the group
membership on all attached networks for which the router is
either Designated Router or Backup Designated Router. This in
turn determines the group-membership-LSAs that the router will
originate, and the local delivery of multicast datagrams (see
Sections 2.3.1 and 10).
o Forwarding cache. Each entry in the forwarding cache describes
the path of a multicast datagram having a particular [source
net, multicast destination, TOS] combination. These cache
entries are calculated when building the datagram shortest-path
trees. See Sections 2.3.4 and 11 for more details.
o Multicast routing capability. Indicates whether the router is
running the multicast extensions defined in this memo. A router
running the multicast extensions must still run the base OSPF
algorithm as set forth in [OSPF]. Such a router will continue to
interoperate with non-multicast-capable OSPF routers when
forwarding IP unicast traffic.
o Inter-area multicast forwarder. Indicates whether the router
will forward IP multicasts from one OSPF area to another. Such a
router declares itself a wild-card multicast receiver in its
non-backbone area router-LSAs (see Section 14.6), and also
summarizes its attached areas' group membership to the backbone
in group-membership-LSAs. When building inter-area datagram
shortest-path trees, it is these routers that appear immediately
adjacent to the datagram source at the root of the tree (see
Section 3.2). Not all multicast-capable area border routers need
be configured as inter-area multicast forwarders. However,
whenever both ends of a virtual link are multicast-capable, they
must both be configured as inter-area multicast forwarders (see
Section 14.11).
o Inter-AS multicast forwarder. Indicates whether the router will
forward IP multicasts between Autonomous Systems. Such a router
declares itself a wild-card multicast receiver in its router-
LSAs (see Section 14.6). These routers are also assumed to be
running some kind of inter-AS multicast protocol. They mark all
external routes that they import into the OSPF domain as to
whether they provide multicast connectivity (see Section 14.9).
When building inter-AS multicast datagram trees, it is these
routers that appear immediately adjacent to the datagram source
at the root of the tree.
8.1. Additions to the OSPF area structure
The OSPF area data structure is described in Section 6 of
[OSPF]. In a MOSPF router, the following item is added to the
OSPF area structure:
o List of group-membership-LSAs. These link state
advertisements describe the location of the area's multicast
group members. Group-membership-LSAs are flooded throughout
a single area only. Area border routers also summarize their
attached areas' membership by originating group-membership-
LSAs into the backbone area. For more information, see
Sections 3.1 and 10.
8.2. Additions to the OSPF interface structure
The OSPF interface structure is described in Section 9 of
[OSPF]. In a MOSPF router, the following items are added to the
OSPF interface structure. Note that the IPMulticastForwarding
parameter is really a description of the attached network. As
such, it should be configured identically on all routers
attached to a common network; otherwise incorrect routing of
multicast datagrams may result[13].
o IPMulticastForwarding. This configurable parameter indicates
whether IP multicasts should be forwarded over the attached
network, and if so, how the forwarding should be done. The
parameter can assume one of three possible values: disabled,
data-link multicast and data-link unicast. When set to
disabled, IP multicast datagrams will not be forwarded out
the interface. When set to data-link multicast, IP multicast
datagrams will be forwarded as data-link multicasts. When
set to data-link unicast, IP multicast datagrams will be
forwarded as data-link unicasts. The default value for this
parameter is data-link multicast. The other two settings are
for use in the special circumstances described in Sections
6.3 and 6.4. When set to disabled or to data-link unicast,
IGMP group membership is not monitored on the attached
network.
o IGMPPollingInterval. When the router is actively monitoring
group membership on the attached network, it periodically
sends IGMP Host Membership Queries. IGMPPollingInterval is a
configurable parameter indicating the number of seconds
between IGMP Host Membership Queries. The router actively
monitors group membership on the attached network when both
a) the interface's IPMulticastForwarding parameter is set to
data-link multicast and b) the router has been elected
Designated Router on the attached network. See Section 9 for
details.
o IGMPTimeout. This configurable parameter indicates the
length of time (in seconds) that a local group database
entry associated with this interface will persist without
another matching IGMP Host Membership Report being received.
See Section 9 for details.
o IGMP polling timer. The firing of this interval timer causes
an IGMP Host Membership Query to be sent out the interface.
The length of this timer is the configurable parameter
IGMPPollingInterval. See Section 9 for details.
8.3. Additions to the OSPF neighbor structure
The OSPF neighbor structure is defined in Section 10 of [OSPF].
In a MOSPF router, the following items are added to the OSPF
neighbor structure:
o Neighbor Options. This field was already defined in the OSPF
specification. However, in MOSPF there is a new option which
indicates the neighbor's multicast capability. This new
option is learned in the Database Exchange process through
reception of the neighbor's Database Description packets,
and determines whether group-membership-LSAs are flooded to
the neighbor. See the items concerning flooding in Section
14 for a more detailed explanation.
8.4. The local group database
The local group database has already been introduced in Section
2.3.1. The current section attempts a more precise definition.
The local group database tracks the group membership of the
router's directly attached networks. Database entries are
created and maintained by the IGMP protocol. Database entries
can cause group-membership-LSAs to be originated, which in turn
enable the pruning of datagram shortest-path trees. The local
group database also dictates the router's responsibility for the
delivery of multicast datagrams to directly attached group
members.
Each entry in the local group database has three components: the
multicast group, the attached network and the entry's age. A
database entry is indexed by the first two components: multicast
group and attached network. A database lookup function is
assumed to exist, so that given a [multicast group, attached
network] pair, the matching database entry (if any) can be
discovered. A database entry for [Group A, Network N1] exists if
and only if there are Group A members currently located on
Network N1.
The three components of a local group database entry are defined
as follows:
o MulticastGroup. The multicast group whose members are being
tracked by this entry. Each multicast group is represented
as a class D IP address. For the semantics of multicast
group membership, see [RFC 1112].
o AttachedNetwork. Each database entry is concerned with the
group members belonging to a single attached network. To get
a complete picture of the local group membership (when for
example building a group-membership-LSA), it may be
necessary to consult multiple database entries, one for each
attached network. Note that a router is only required to
maintain entries for those attached networks on which the
router has been elected Designated Router or Backup
Designated Router (see Section 9).
o Age. Indicates the number of seconds since an IGMP Host
Membership Report for multicast Group A has been seen on
Network N1. If the age field hits Network N1's configured
IGMPTimeout value, the local group database entry is removed
(i.e., the entry has "aged out"). See Sections 9.2 and 9.3
for more information.
8.5. The forwarding cache
The forwarding cache has already been defined in Section 2.3.
The current section attempts a more precise definition. Each
entry in the forwarding cache indicates how a multicast datagram
having a particular [source network, destination multicast
group, IP TOS] will be forwarded. A forwarding cache entry is
built on demand from the local group database and the datagram's
shortest-path tree. For more details, consult Sections 2.3.4 and
12.
Each entry in the forwarding cache has six components: the
multicast datagram's source network, the destination multicast
group, the IP TOS, the upstream node, the list of downstream
interfaces and (possibly) a list of downstream neighbors. A
forwarding cache entry is indexed by source network, destination
multicast group and IP TOS. A lookup function is assumed to
exist, so that given a multicast datagram with a particular [IP
source, destination multicast group, IP TOS], a matching cache
entry (if any) can be found.
The six components of a forwarding cache entry are defined as
follows:
o Source network. The datagram's source network is described
by a network/subnet/supernet number and its corresponding
mask. The source network for a datagram is discovered via a
routing table/database lookup of the datagram's IP source
address, as described in Section 11.2.
o Destination multicast group. The destination group to which
matching datagrams are being forwarded. For the semantics of
multicast group membership, see [RFC 1112].
o IP TOS. The IP Type of Service specified by matching
datagrams. Note that this means that the path of the
multicast datagram depends on its TOS classification.
o Upstream node. The attached network/neighboring router from
which the datagram must be received. If received from a
different attached network/neighboring router, the matching
datagram is dropped instead of forwarded. This prevents
unwanted replication of multicast datagrams. It is possible
that the upstream node is unspecified (i.e., set to NULL).
In this case, matching datagrams will always be dropped, no
matter where they are received from. It is also possible
that the upstream node is specified as the placeholder
EXTERNAL. This means that the datagram must be received on a
non-MOSPF interface in order to be forwarded.
o List of downstream interfaces. These are the router
interfaces that the matching datagram should be forwarded
out of (assuming that the datagram was received from
upstream node). Each interface is also listed with a TTL
value. The TTL value is the minimum number of hops necessary
to reach the closest (in terms of router hops) group member.
This allows the router to drop datagrams that have no chance
of reaching a destination group member.
o List of downstream neighbors. When the datagram is to be
forwarded out a non-broadcast multi-access network, or if
the interface's IPMulticastForwarding parameter is set to
data-link unicast, the datagram must be forwarded separately
to each downstream neighbor (see Sections 2.3.3 and 6.4). As
done for downstream interfaces, each downstream neighbor is
specified together with the smallest TTL that will actually
reach a group member.
9. Interaction with the IGMP protocol
MOSPF uses the IGMP protocol (see [RFC 1112]) to monitor multicast
group membership. In short, the Designated Router on a network
periodically sends IGMP Host Membership Queries (see Section 9.1),
which in turn elicit IGMP Host Membership Reports from the network's
multicast group members. These Host Membership Reports are then
recorded in the Designated Router's and Backup Designated Router's
local group databases (see Section 9.2).
9.1. Sending IGMP Host Membership Queries
Only the network's Designated Router sends Host Membership
Queries. This minimizes the amount of group membership
information on the network, both in terms of queries and
responses.
When a MOSPF router becomes Designated Router on a network, it
checks to see that the network's IPMulticastForwarding parameter
is set to data-link multicast (see Section B.2). If so, it
starts the interface's IGMP polling timer. Then, whenever the
timer fires (every IGMPPollingInterval seconds), the MOSPF
router sends a Host Membership Query out the interface. The
destination of the query is the IP address 224.0.0.1. For the
format of the query, see [RFC 1112]. If/when the MOSPF router
ceases to be the network's Designated Router, the IGMP polling
timer is disabled and no more Hosts Membership Queries are sent.
Unusual behavior can result when multiple IP networks are
assigned to a single physical network. MOSPF treats each such IP
network separately, electing (possibly) a different Designated
Router for each network. However, IGMP operates on a physical
network basis only: when a Host Membership Query is sent, all
group members on the physical network respond, regardless of
their IP addresses. So unless the IPMulticastForwarding
parameter is set to a value other than data-link multicast on
all but one of the physical network's IP networks, excess
multicast membership reporting will result.
9.2. Receiving IGMP Host Membership Reports
Received Host Membership Reports are processed by both the
network's Designated Router and Backup Designated Router. It is
the Designated Router's responsibility to distribute the
network's group membership information throughout the routing
domain, by originating group-membership-LSAs (see Section 10).
The Backup Designated Router processes Reports so that it too
has a complete picture of the network's group membership,
enabling a quick cutover upon Designated Router failure.
An IGMP Host Membership Report concerns membership in a single
IP multicast group (call it Group A). The Report is sent to the
Group A address so that other group members may see the Report
and avoid sending duplicates (see [RFC 1112] for details). When
an IGMP Host Membership Report, sent on Network N[14], is
received by a MOSPF router, the following steps are executed:
(1) If the router is neither the Designated Router nor the
Backup Designated Router on the network, the Report is
discarded and processing stops.
(2) If the Report concerns a multicast group in the range
224.0.0.1 - 224.0.0.255, the Report is discarded and
processing stops. This range of multicast groups are for
local use (single hop) only, and datagrams sent to these
destinations are never forwarded by multicast routers.
(3) Locate the entry for [Group A, Network N] in the local group
database. If no such entry exists, create one. In any case,
set the age of the entry to 0. Note that even if multiple
hosts attached to Network N report membership in the same
group, only a single local group database entry will be
formed. See Section 8.4 for more details concerning the
local group database.
(4) If the router is the network's Designated Router, and a
local group database entry was created in the previous step,
it may be necessary to originate a new group-membership-LSA.
See Section 10 for details.
9.3. Aging local group database entries
Every local database entry has an age field. Suppose that there
is a database entry for [Group A, Network N1]. The age field
then indicates the length of time (in seconds) since the last
Host Membership Report for Group A was received on Network N1.
If the age of the entry reaches Network N1's configured
IGMPTimeout value (see Section B.2), the entry is considered
invalid and is removed from the database.
Note that when a router, after having been either Network N1's
Designated Router or Backup Designated Router, but now being
neither, will (after IGMPTimeout seconds) automatically age out
all of its local group database entries associated with Network
N1. For this reason, it is not necessary to purge local group
database entries on OSPF interface state changes.
9.4. Receiving IGMP Host Membership Queries
If a MOSPF router has internal multicast applications, and if
the applications have bound themselves to certain interfaces
(using the RFC 1112 representation described in Section 5), then
the MOSPF router responds to received Host Membership Queries by
issuing Host Membership Reports. Identical to the operation of
any IP host supporting multicast applications, the exact
procedure for issuing these Host Membership Reports is specified
in [RFC 1112]. Note that in this case, if the router has been
elected Designated Router on a network, it must receive its own
Host Membership Reports and Host Membership Queries.
If instead all of its applications have joined groups in an
interface-independent fashion (using the MOSPF-specific
representation described in Section 5), the MOSPF router does
not respond to Host Membership Queries. Instead, the MOSPF
router communicates this membership information by originating
appropriate group-membership-LSAs (see Section 10.1).
10. Group-membership-LSAs
Group-membership-LSAs provide the means of distributing membership
information throughout the MOSPF routing domain. Group-membership-
LSAs are specific to a single OSPF area (see Section 3.1). Each
group-membership-LSA concerns a single multicast group. Essentially,
the group-membership-LSA lists those networks which are directly
connected to the LSA's originator and which contain one or more
group members. For more details on how the group-membership-LSA
augments the OSPF link state database, see Section 2.3.1.
The creation of group-membership-LSAs is discussed in Section 10.1.
The format of the group-membership-LSA is described in Section A.3.
A router will originate a group membership-LSA for multicast group A
when one or more of the following conditions hold:
(1) The router is Designated Router on a network (call it Network
X), the interface to Network X has its IPMulticastForwarding
parameter set to data-link multicast (see Section B.2), and
Network X contains one or more members of Group A.
(2) The router is an inter-area multicast forwarder (see Section
B.1), and one or more of the router's attached non-backbone
areas contain Group A members. In this case, the router will
originate a group-membership-LSA for Group A into the backbone.
This is the way group membership is conveyed between areas (see
Section 3.1).
(3) The router itself has applications that are requesting
membership in Group A, in an interface-independent fashion (see
Section 5).
As for all other types of OSPF link state advertisements (e.g,
router-LSAs, network-LSAs, etc.), group-membership-LSAs are aged as
they are held in a router's link state database. To prevent valid
advertisements from "aging out", a router must refresh its self-
originated group-membership-LSAs every LSRefreshTime interval, by
incrementing their LS sequence numbers and reissuing them. In
addition, when an event occurs that would alter one of the router's
self-originated group-membership-LSAs, a new instance of the LSA is
issued with an updated (i.e., incremented by 1) LS sequence number.
Note however that a router is not allowed to originate two new
instances of the same advertisement within MinLSInterval seconds.
For that reason, occasionally advertisement originations will need
to be deferred. Also, an event may occur that makes it inappropriate
for the router to continue to originate a particular LSA. In that
case, the router flushes the advertisement from the routing domain
by "premature aging". For more information concerning the
maintenance of LSAs, see Sections 12, 12.4, 14 and 14.1 of [OSPF].
When one of the following events occurs, it may be necessary for a
router to (re)issue one or more group-membership-LSAs:
(1) One of the router's interfaces changes state. For example, the
router may have become Designated Router on a particular
network, causing the router to start advertising the network's
group membership to the rest of the MOSPF system in group-
membership-LSAs.
(2) The router receives an IGMP Host Membership Report, causing a
new local group database entry to be formed (see Section 9.2).
(3) One of the router's local group database entries "ages out",
because it is no longer being refreshed by received IGMP Host
Membership Reports (see Section 9.3).
(4) The router is an inter-area multicast forwarder, and the group
membership of one of the router's attached non-backbone areas
changes. This is detected by the reception of a new, or the
flushing of an old, group-membership-LSA into/from the non-
backbone area's link state database.
(5) The group membership of one of the router's internal
applications changes.
10.1. Constructing group-membership-LSAs
This section details how to build a group-membership-LSA. The
format of a group-membership-LSA is described in Section A.3.
Each group-membership-LSA concerns a single multicast group. The
body of the advertisement is a list of the local transit nodes
(the router itself and directly attached transit networks) that
contain group members. Section 10 listed the conditions
requiring the (re)origination of a group-membership-LSA. Note
that if the router is an area border router, it may be necessary
to originate a separate group-membership-LSA for each attached
area.
The following defines the contents of a group-membership-LSA, as
originated by Router X into Area A. It is assumed that the
group-membership-LSA is to report membership in multicast group
G:
o The advertisement fields that are not type-specific (LS age,
LS sequence number, LS checksum and length) are set
according to Section 12.1 of [OSPF].
o The Options field of a group-membership-LSA is not processed
on receipt. However, for consistency, the Option field in
these advertisements should have its MC-bit set, T-bit
clear, and the E-bit should match the configuration of Area
A (i.e., set if and only if Area A is not a stub area). The
rest of the Options field is set to 0.
o The Link State ID is set to the group whose membership is
being reported (Group G).
o The Advertising Router is set to the OSPF Router ID of the
router originating the advertisement (Router X).
o The body of the advertisement is a list of local transit
vertices that should be labelled with Group G membership
(see Section 2.3.1). This list may include the advertising
router itself, and any of the transit networks that are
directly attached to said router. The following steps
determine which of these transit vertices are actually
included in the group-membership-LSA. Note that any
particular vertex should be listed at most once, even though
the following may indicate multiple reasons for a particular
vertex to be listed. Also note that if no transit vertices
are listed by the advertisement, the advertisement should
not be (re)originated; if an instance of the advertisement
already exists, it should then be flushed from the link
state database using the premature aging procedure specified
in Section 14.1 of [OSPF].
a. Consider those entries in the local group database that
describe Group G membership (see Section 8.4). Consider
each such entry in turn. Each entry references one of
Router X's attached networks (call it Network N). If
either Network N does not belong to Area A, or if Router
X is not Network N's Designated Router[15], Network N
should not be added to the group-membership-LSA, and the
next local group database entry should be examined.
Otherwise, if N is a stub network (e.g., Router X is the
only OSPF router attached to N), Router X adds itself to
the advertisement by adding a vertex with Vertex type
set to 1 (router) and Vertex ID set to Router X's OSPF
Router ID. Otherwise, N is a transit network. In this
case, Network N should be added to the advertisement by
adding a vertex with Vertex type set to 2 (network) and
Vertex ID set to the IP address of Network N's
Designated Router (i.e., Router X's IP interface address
on Network N).
b. If Router X itself has applications requesting Group G
membership on an interface-independent basis (see
Section 5), it should add itself to the advertisement by
adding a vertex with Vertex type set to 1 (router) and
Vertex ID set to Router X's OSPF Router ID.
c. If Router X is an inter-area multicast forwarder (see
Section 3.1), Area A is the backbone area (Area ID
0.0.0.0), and at least one of Router X's attached non-
backbone areas has Group G members (indicated by the
presence of one or more advertisements in the areas'
link state databases having Link State ID set to Group G
and LS age set to a value other than MaxAge[16]), then
Router X should add itself to the advertisement by
adding a vertex with Vertex type set to 1 (router) and
Vertex ID set to Router X's OSPF Router ID.
Consider as an example the network configuration in Figure 4.
Suppose that Router RT2 has been elected Designated Router for
Network N3. Router RT2 would then originate (into Area 1) the
following group-membership-LSA for Group B:
; RT2's group-membership-LSA for Group B
LS age = 0 ;always true on origination
Options = (E-bit|MC-bit)
LS type = 6 ;group-membership-LSA
Link State ID = Group B
Advertising Router = RT2's Router ID
Vertex type = 1 ;RT2 itself (for stub N2)
Vertex ID = RT2's Router ID
Vertex type = 2 ;Network N3 (since RT2 is DR)
Vertex ID = RT2's IP interface address on N3
10.2. Flooding group-membership-LSAs
When MOSPF routers and non-multicast OSPF routers are mixed
together in a routing domain, the group-membership-LSAs are not
flooded to the non-multicast routers[17]. As a general design
principle, optional OSPF advertisements are only flooded to
those routers that understand them.
A MOSPF router learns of its neighbor's multicast-capability at
the beginning of the "Database Exchange Process" (see Section
10.6 of [OSPF], receiving Database Description packets from a
neighbor in state Exstart). A neighbor is multicast-capable if
and only if it sets the MC-bit in the Options field of its
Database Description packets. Then, in the next step of the
Database Exchange process, group-membership-LSAs are included in
the Database summary list sent to the neighbor (see Sections 7.2
and 10.3 of [OSPF]) if and only if the neighbor is multicast-
capable.
When flooding group-membership-LSAs to adjacent neighbors, a
MOSPF router looks at the neighbor's multicast-capability.
Group-membership-LSAs are only flooded to multicast-capable
neighbors. To be more precise, in Section 13.3 of [OSPF],
group-membership-LSAs are only placed on the Link state
retransmission lists of multicast-capable neighbors[18]. Note
however that when sending Link State Update packets as
multicasts, a non-multicast neighbor may (inadvertently) receive
group-membership-LSAs. The non-multicast router will then simply
discard the LSA (see Section 13 of [OSPF], receiving LSAs having
unknown LS types).
11. Detailed description of multicast datagram forwarding
This section describes in detail the way MOSPF forwards a multicast
datagram. The forwarding process has already been informally
presented in Section 2.2. However, there are several obscure
configuration options (e.g., the IPMulticastForwarding interface
parameter) that have been presented elsewhere in this document,
which may influence the forwarding process. This section gathers
together all the influencing factors into a single algorithm.
It is assumed in the following that the datagram under consideration
has actually be received on one of the router's interfaces. Locally
generated datagrams (i.e., originated by one of the router's
internal applications) are handled instead by the algorithm in
Section 11.3.
Assume that the datagram's IP destination is Group G. The forwarding
process then consists of the following steps:
(1) Upon reception of the datagram, the MOSPF router notes the
following parameters. These parameters are examined in later
steps, to determine whether the datagram should be forwarded.
a. The receiving MOSPF interface associated with the datagram.
Based on the receiving physical interface, the receiving
MOSPF interface is selected by the algorithm in Section
11.1.
b. Whether the datagram was received as a link-level
multicast/broadcast or as a link-level unicast. This
information is used later in Step 7 to help determine
whether the datagram should be forwarded.
(2) A copy of the datagram should be passed to each internal
application that has joined Group G on the receiving MOSPF
interface (see Section 5).
(3) If the datagram's IP source address matches the receiving MOSPF
interface's IP address, the datagram should not be forwarded
further, and should instead be discarded, completing the
forwarding process. This keeps the router's own locally
originated datagrams from being mistakenly replicated, in those
cases where the receiving MOSPF interface receives its own
multicast transmissions.
(4) If Group G falls into the range 224.0.0.1 through 224.0.0.255
inclusive, the datagram should not be forwarded further. This
range of addresses has been dedicated for use on a local network
segment only.
(5) Associate a source network (SourceNet) with the multicast
datagram, as described in Section 11.2. If SourceNet cannot be
determined (i.e., there is no available unicast route back to
the datagram source), the datagram should not be forwarded
further.
(6) Look up the forwarding cache entry (see Section 8.5) matching
the datagram's [SourceNet, Group G, TOS] combination. If the
cache entry does not yet exist, one is built by the calculation
in Section 12. In order for the datagram to be forwarded, the
contents of the forwarding cache entry must be further verified
against the received datagram's characteristics as follows:
a. If the forwarding cache entry's upstream node is unspecified
(i.e., NULL), then the datagram should not be forwarded
further.
b. Otherwise, suppose that the forwarding cache entry's
upstream node is set to EXTERNAL. In this case, the datagram
is forwarded further if and only if the receiving MOSPF
interface is set to NULL (i.e., if and only if the datagram
was received on a non-MOSPF interface).
c. Otherwise, if the datagram's receiving MOSPF interface does
not attach to the forwarding cache entry's upstream node,
the datagram should not be forwarded further.
(7) If the receiving MOSPF interface's IPMulticastForwarding
parameter is set to data-link unicast, the datagram should be
forwarded further only if it was received as a data-link
unicast.
(8) At this point the datagram is eligible for further forwarding.
Before forwarding, the router checks to see whether it has any
internal applications that have joined Group G on an interface-
independent basis. If so, a copy of the datagram should be
passed to each such requesting application process.
(9) Examine each of the downstream interfaces listed in the
forwarding cache entry. If the TTL in the datagram is greater
than or equal to the TTL specified for the downstream interface,
a copy of the datagram should be forwarded out the downstream
interface. Before forwarding the datagram copy, the copy's TTL
should be decremented by 1. On most interfaces, the datagram is
forwarded as a data-link multicast/broadcast. The exact data-
link encapsulation is dependent on the attached network's type:
o On ethernet and IEEE 802.3 networks, the datagram is
forwarded as a data-link multicast. The destination data-
link multicast address is selected as an algorithmic
translation of the IP multicast destination. See [RFC 1112]
for details.
o On FDDI networks, the datagram is forwarded as a data-link
multicast. The destination data-link multicast address is
selected as an algorithmic translation of the IP multicast
destination. See [RFC 1390] for details.
o On SMDS networks, the datagram is forwarded using the same
SMDS address that is used by IP broadcast datagrams. See
[RFC 1209] for details.
o On networks that support broadcast, but not multicast (e.g.,
the Experimental Ethernet), the datagram is forwarded as a
data-link broadcast. See [RFC 1112] for details.
o On point-to-point networks, the datagram is forwarded in the
same way that unicast datagrams are forwarded. See [RFC
1112] for details.
(10)
Examine each of the downstream neighbors listed in the
forwarding cache entry. If the TTL in the datagram is greater
than or equal to the TTL specified for the downstream neighbor,
a copy of the datagram should be forwarded to the downstream
neighbor (as a data-link unicast). Before forwarding the
datagram copy, the copy's TTL should be decremented by 1.
ICMP error messages are never generated in response to received IP
multicasts. In particular, ICMP destination unreachables and ICMP
TTL expired messages are not generated by the above procedure if the
router refuses to forward a multicast datagram.
11.1. Associating a MOSPF interface with a received datagram
A MOSPF interface must be associated with a received multicast
datagram before it is forwarded (see Step 1a of Section 11), and
with received IGMP Host Membership Reports before they are
processed (see Section 9.2).
When there is only a single IP network assigned to the physical
interface that received the datagram, the choice of receiving
MOSPF interface is clear. When there are multiple logical IP
networks attached to the receiving physical interface, the
receiving MOSPF interface is selected as follows. Examine all of
the MOSPF interfaces associated with the receiving physical
interface. Discard those interfaces whose IPMulticastForwarding
parameter has been set to disabled. The receiving MOSPF
interface is then the remaining interface having the highest IP
interface address (or NULL if there are no remaining
interfaces)[19].
11.2. Locating the source network
MOSPF forwarding cache entries are indexed by the datagram's
source IP network/subnet/supernet. For this reason, whenever an
IP multicast datagram is received, the IP network belonging to
the datagram's IP source address must be found. This is
accomplished by the following algorithm:
Look up the OSPF TOS 0 routing table entry[20] corresponding to
the datagram's IP source address, as described in Section 11.1
of [OSPF]. If this routing table entry describes an OSPF
intra-area or inter-area route, the source network is set to be
the network defined by the routing table entry's Destination ID
and Address Mask (see Section 11 of [OSPF]). Otherwise (i.e.,
the routing table entry specifies an external route, or there is
no matching routing table entry), the list of matching AS
external-link-LSAs is examined. A matching AS external-link-LSA
is one that describes a network which contains the datagram's IP
source address. The list of matching AS external-link-LSAs is
pruned in the following steps to determine the source network:
(1) Those AS external-link-LSAs with MC-bit clear (see Section
A.1), or with LS age set to MaxAge, or which have been
originated by unreachable AS boundary routers are discarded.
(2) AS external-link-LSAs specifying Type 1 external metrics are
always preferred over those specifying Type 2 external
metrics.
(3) If there are still multiple AS external-link-LSAs remaining,
those specifying the best matching (i.e., most specific)
network are selected. The source network is then set to the
network/subnet/supernet (possibly even the default route)
described by the best matching AS external-link-LSAs. Note
that AS external-link-LSAs specifying a cost of LSInfinity
are eligible for this best match, as long as their MC-bit is
set.[21]
It is possible that two different MOSPF routers may calculate
the same multicast datagram's source network differently. For
example, consider the network configuration shown in Figure 4.
When calculating the source network for a datagram whose source
is Network N10 and destination is Group Ma, Router RT11 would
calculate the source network as Network N10 itself, while Router
RT10 would calculate the source network as the aggregate of
Networks N9-N11 and Host H1 (advertised in a single summary-
link-LSA by Router RT11). However, despite the possibility of
routers selecting different source networks, all routers will
still agree on the datagram's shortest-path tree.
External sources are treated differently in the above
calculation since it is likely that the Internet will have
separate multicast and unicast topologies for some time to come.
When the multicast and unicast topologies do merge, the MC-bit
will be set on all AS external-link-LSAs and the above use of
the LSInfinity metric (to indicate a route that is to be used
for multicast traffic, but not unicast traffic), will no longer
be necessary. At that time, the determination of source network
for external sources will revert to the same simple routing
table lookup that is used for internal sources.
As an example of the logic for external sources, suppose a
multicast datagram is received having the IP source address
10.1.1.1. Suppose also that the three AS external-link-LSAs
shown in Table 3 are in the router's OSPF database. The OSPF
routing table lookup would yield the network 10.1.1.0 with a
mask of 255.255.255.0, however the above calculation would
choose a source network of 10.1.0.0 with a mask of 255.255.0.0,
despite the fact that its matching LSA has a cost of LSInfinity.
11.3. Forwarding locally originated multicasts
This section describes how a MOSPF router forwards a multicast
datagram that has been originated by one of the router's own
internal applications. The process begins with one of the
router's internal applications formatting and addressing the
datagram. Forwarding the locally originated multicast then
consists of the following steps:
(1) Find the router interface whose IP address matches the
datagram's source address. Multicast the datagram out that
interface, according to the Host extensions for IP
multicasting specified in [RFC 1112].
(2) If the router interface found in the previous step has been
configured for MOSPF, and if its IPMulticastForwarding
parameter is not equal to disabled, then set the receiving
MOSPF interface to that interface. Otherwise, set the
receiving MOSPF interface to NULL.
(3) Execute the MOSPF forwarding process described in Section
11, beginning with its Step 4.
Network Mask Cost MC-bit
______________________________________________________
10.1.1.0 255.255.255.0 Type 1: 10 clear
10.1.0.0 255.255.0.0 Type 2: LSInfinity set
10.0.0.0 255.0.0.0 Type 2: 1 set
Table 3: Sample AS external-link-LSAs
The above algorithm amounts to the router always multicasting
the datagram out the source interface, and the executing the
basic forwarding algorithm (in Section 11) as if the datagram
had actually been received on the source interface. In those
cases where the router receives its own multicast transmissions,
unwanted replication is prevented by Step 3 of Section 11. In
fact, this specification has purposely presented the forwarding
algorithm (both for received and for locally originated
datagrams) so that the correct forwarding actions are taken
independent of whether the router receives its own multicast
transmissions.
12. Construction of forwarding cache entries
This section details the building of a MOSPF forwarding cache entry.
A high level discussion of this construction has already been
presented in Sections 2.3, 2.3.1, 2.3.2, 3.2, and 4.1. Forwarding
cache entries are built on demand, when a multicast datagram is
received and no matching forwarding cache entry is found (see Step 6
of Section 11). The parameters passed to the forwarding cache entry
build process are: the datagram's source network (see Section 11.2)
and its destination group address. These two parameters are called
SourceNet and Group G in the following algorithm. The main steps in
the build process are the following:
(1) Allocate the forwarding cache entry. Initialize its Source
network to SourceNet, its Destination multicast group to Group G
and its IP TOS field to match the multicast datagram's TOS.
Initialize its upstream node and list of downstream interfaces
to NULL.
(2) For each Area A to which the calculating router is attached:
a. Calculate Area A's datagram shortest-path tree. This
calculation is described in Section 12.2 below. In many ways
it is similar to the calculation of OSPF's intra-area
routes, described in Section 16.1 of [OSPF]. The main
differences between the multicast datagram shortest-path
tree calculation and OSPF's intra-area unicast calculation
are listed in Section 12.2.9 below. As a product of each
area's datagram shortest-path tree, the forwarding cache
entry's list of outgoing interfaces is (possibly) updated.
Area A's datagram shortest-path tree is dependent on the
datagram's IP TOS. Section 12.2 describes the TOS 0 datagram
shortest-path tree. The modifications necessary for non-zero
TOS values are detailed in Section 12.2.8.
b. Possibly set the forwarding cache entry's upstream node.
Only one of the calculating router's attached areas will
determine the forwarding cache entry's upstream node. This
area is called the datagram's RootArea. The RootArea is
initially set to NULL. After completing Area A's datagram
shortest-path tree, the calculation in Section 12.2.7 will
determine whether Area A is the datagram's RootArea.
(3) Update the forwarding cache entry's list of outgoing interfaces,
according to the contents of the local group database. This
ensures multicast delivery to group members residing on the
calculating router's directly attached networks. This process is
described in Section 12.3.
These main steps are described in more detail below. The detailed
description begins with an explanation of the major data structure
used by the datagram shortest-path tree calculation: The Vertex data
structure.
12.1. The Vertex data structure
A datagram shortest-path tree is built by the Dijkstra or SPF
algorithm. The algorithm is stated herein using graph-oriented
language: vertices and links. Vertices are the area's routers
and transit networks, and links are the router interfaces and
point-to-point lines that connect them. Each vertex has the
following state information attached to it. Basically, this
information indicates the current best path from the SourceNet
to the vertex, and the position of the vertex relative to the
calculating router. Note that a separate datagram shortest-path
tree is built for each area, and that the vertices described
below are also specific to a single area (called Area A).
o Vertex type. Set to 1 for routers, 2 for transit networks.
Note that this coding matches the coding for vertices listed
in the group-membership-LSA (see Section A.3).
o Vertex ID. A 32-bit identifier for the vertex. For routers,
set to the router's OSPF Router ID. For transit networks,
set the IP address of the network's Designated Router. Note
that this coding matches the coding for vertices listed in
the group-membership-LSA (see Section A.3).
o LSA. The link state advertisement describing the vertex'
immediate neighborhood. Can be discovered by performing a
database lookup in Area A's link state database (see Section
12.2 of [OSPF]), with LS type set to Vertex type and Link
State ID set to Vertex ID.
o Parent. In the current best path from SourceNet to the
vertex, the router/transit network immediately preceding the
vertex. Note that the parent can change as better and better
paths are found, up until the vertex is installed on the
shortest-path tree.
o IncomingLinkType. This parameter is set to the type of link
that led to Vertex's inclusion on the shortest-path tree.
Listed in order of decreasing preference[22], the possible
types are: ILVirtual (virtual links), ILDirect (vertex is
directly attached to SourceNet), ILNormal (either router-
to-router or router-to-network links), ILSummary (OSPF
summary links), ILExternal (OSPF AS external links), or
ILNone (the vertex is not on the shortest-path tree).
o AssociatedInterface/Neighbor. If the current best path from
SourceNet to the vertex goes through the calculating router,
this parameter indicates the calculating router's interface
(or neighbor) which leads to the vertex.
o Cost. The cost, in terms of the OSPF link state metric, of
the current best path from SourceNet to the vertex. Note
that if the cost of the path is a combination of both
external type 2 and internal OSPF metrics, that the vertex'
cost parameter reflects both cost components. Remember that
the type 2 cost component is always more significant than
the type 1 component.
o TTL. If the current best path from SourceNet to vertex goes
through the calculating router, TTL is set to the number of
routers between the calculating router and the vertex. This
includes the calculating router, but does not include the
vertex itself.
12.2. The SPF calculation
This section details the construction of datagram shortest-path
trees. Such a tree describes the path of a multicast datagram
as it traverses an OSPF area. For a given datagram, each router
in an OSPF area builds an identical tree. A router connected to
multiple areas builds a separate datagram shortest-path tree for
each area.
The datagram shortest-path tree is built by the Dijkstra or SPF
algorithm, which is the same algorithm used to discover OSPF's
intra-area unicast routes (see Section 16.1 of [OSPF]). The
algorithm is stated herein and in [OSPF] using graph-oriented
language: vertices and links. Vertices are the area's routers
and transit networks, and links are the router interfaces and
point-to-point lines that connect them. Basically, the algorithm
manipulates two lists of vertices: the candidate list and the
forming shortest-path tree. The candidate list consists of those
vertices to which paths have been discovered, but for which the
optimality of the discovered paths is yet unknown. At each cycle
of the algorithm, the vertex closest to the tree's root, yet
still remaining on the candidate list, is moved from the
candidate list to the shortest-path tree. Then the neighbors of
the just processed vertex are examined for possible addition
to/modification of the candidate list. The algorithm terminates
when the candidate list is empty.
The datagram shortest-path tree for Area A is constructed in the
following steps. The datagram's SourceNet and its destination
group G are inputs to the calculation (see Step 6 of Section
11). The datagram shortest-path tree also depends on the IP Type
of service specified in the datagrams' IP Header. However, a
discussion of TOS is deferred until Section 12.2.8; all
calculations and costs in the current section concern TOS 0
only. Call the router performing the calculation Router RTX. At
each step (and in the subordinate Sections 12.2.1 through
12.2.8) LSAs from Area A's link state database are examined. In
all cases, any LSA having LS age equal to MaxAge is ignored. The
main body of the calculation is in Steps 4 and 5, which are
repeated until the candidate list becomes empty:
(1) Initialize the algorithm's data structures. Clear the
shortest-path tree. Initialize the state of each vertex in
Area A (i.e., the area's routers and transit networks) to:
Parent set to NULL, IncomingLinkType set to ILNone and
AssociatedInterface/Neighbor set to NULL.
(2) Initialize the candidate list. One or more vertices are
initially placed on the candidate list, depending on the
location of SourceNet with respect to Area A and Router RTX.
This breaks down into the following cases (which are named
for later reference):
o Case SourceIntraArea: SourceNet belongs to Area A. In
this case, the candidate list is initialized as in
Section 12.2.1.
o Case SourceInterArea1: SourceNet belongs to an OSPF area
that is not directly attached to Router RTX. In this
case, the candidate list is initialized as in Section
12.2.2.
o Case SourceInterArea2: SourceNet does not belong to Area
A, but it still belongs to an OSPF area that is directly
attached to Router RTX. In this case, the candidate
list is initialized as in Section 12.2.3.
o Case SourceExternal: SourceNet is external to the OSPF
routing domain, and Area A is not an OSPF stub area. In
this case, the candidate list is initialized as in
Section 12.2.4.
o Case SourceStubExternal: SourceNet is external to the
OSPF routing domain, and Area A is an OSPF stub area. In
this case, the candidate list is initialized as in
Section 12.2.5.
Two different routers in Area A may select different
initialization cases above. For example, consider the
network configuration shown in Figure 4. When calculating
the Area 3 datagram shortest-path tree for a datagram whose
source is Network N7 (e.g., from Host H5) and destination is
Group Ma, Router RT11 would initialize the candidate list
using Case SourceInterArea2 while Router RT9 would use Case
SourceInterArea1. Likewise, if Area 3 were configured as an
OSPF stub area and the datagram source was the external
Network N12, Router RT11 would use Case SourceStubExternal
while Router RT9 would use Case SourceInterArea1! However,
despite the possibility of routers selecting different
cases, all routers in an area will still initialize the
candidate list (and in fact, run the rest of the SPF
calculation) identically.
(3) If the candidate list is empty, the algorithm terminates.
(4) Move the closest candidate vertex to the shortest-path tree.
Select the vertex on the candidate list that is closest to
SourceNet (i.e., has the smallest Cost value). If there are
multiple possibilities, select transit networks over
routers. If there are still multiple possibilities
remaining, select the vertex having the highest Vertex ID.
Call the chosen vertex Vertex V. Remove Vertex V from the
candidate list, and install it on the shortest-path tree.
Next, determine whether Vertex V has been labelled with the
Destination multicast Group G. If so, it may cause the
forwarding cache entry's list of outgoing
interfaces/neighbors to be updated. See Section 12.2.6 for
details.
(5) Examine Vertex V's neighbors for possible inclusion in the
candidate list. Consider Vertex V's LSA. Each link in the
LSA describes a connection to a neighboring router/network.
If the link connects to a stub network, examine the next
link in the LSA. Otherwise, the link (Link L) connects to a
neighboring transit node. Call this node Vertex W. Perform
the following steps on Vertex W:
a. If W is already on the shortest-path tree, or if W's LSA
does not contain a link back to vertex V, or if W's LSA
has LS age of MaxAge, or if W is not multicast-capable
(indicated by the MC-bit in the LSA's Options field),
examine the next link in V's LSA.
b. Otherwise determine the cost to associate with the link
from V to W. If SourceNet belongs to Area A (Case
SourceIntraArea in Step 2), use the cost listed for Link
L in V's LSA. Otherwise, use the link's reverse cost:
Examine W's LSA, and find the cost listed for the link
connecting back to V. Actually, when V and W are both
routers, there may be multiple links between them. In
this case, use the smallest cost listed in W's LSA for
any of the links connecting back to V and having the
same Type (as specified in the Router-LSA; must be
either: point-to-point connection or virtual link) as
Link L[23].
c. Calculate the cost from SourceNet to W, when using Link
L. It is the sum of the cost of SourceNet to V (i.e.,
V's Cost parameter) plus the link cost calculated in
Step 5b. Let this sum be Cost C. If W is not yet on the
candidate list, install W on the candidate list,
modifying its parameters as specified below (Step 5d).
Otherwise, W is on the candidate list already. In this
case, if:
o C is less than W's current Cost, update W's
parameters on the candidate list as specified below
(Step 5d).
o C is equal to W's current Cost, then the following
tiebreakers are invoked. The type of Link L is
compared to W's current IncomingLinkType, and
whichever link has the preferred type is chosen (the
preference order of link types is listed in Section
12.1's definition of IncomingLinkType). If the link
types are the same, then a link whose Parent is a
transit network is preferred over one whose Parent
is a router. If the links are still equivalent, the
link whose Parent has the higher Vertex ID is
chosen. Whenever Link L is chosen, W's parameters
are modified as below (Step 5d). Whenever the
previously discovered link is chosen, the next link
in V's LSA is examined instead.
o C is greater than W's current Cost, examine the next
link in V's LSA.
d. At this point, a better candidate path has been found to
Vertex W, using Link L. Modify Vertex W's parameters
accordingly. W's Parent is set to Vertex V. W's
IncomingLinkType is set to ILVirtual if Link L is a
virtual link, otherwise IncomingLinkType is set to
ILNormal. W's Cost parameter is set to C. W's TTL and
AssociatedInterface/Neighbor parameters are set
according to one of the following cases:
o Vertex V is the calculating router itself. In this
case, W's TTL parameter is set to 1. If Link L is a
virtual link, W's AssociatedInterface/Neighbor is
set to NULL. Otherwise, W's
AssociatedInterface/Neighbor is set to the non-
virtual interface connecting the calculating router
to W which has the smallest cost value. Note that,
in the reverse cost (inter-area and inter-AS
multicast) cases, this may not be the interface
corresponding to Link L. However, since W is only
concerned with the node it is receiving the datagram
from (the upstream node; see Section 11), and not
with the particular interface the datagram is
received on, the calculating router is free to pick
the sending interface when there are multiple
connecting links.
o Vertex V is upstream of the calculating router
(i.e., V's AssociatedInterface/Neighbor is equal to
NULL). In this case, Vertex W's TTL parameter is set
to 0, and its AssociatedInterface/Neighbor is set to
NULL.
o V is a transit network, and is directly downstream
from the calculating router (i.e., V's
AssociatedInterface/Neighbor is non-NULL and V's TTL
is set to 1). W is then one of the calculating
router's neighbors. In this case, W's TTL parameter
is also set to 1. If network V has been configured
for data-link unicasting (see Section B.2) or if V
is a non-broadcast network, W's
AssociatedInterface/Neighbor is set to W itself (a
neighbor of the calculating router). Otherwise, W's
AssociatedInterface/Neighbor is set to the
calculating router's interface to Network V.
o Vertex V is downstream from the calculating router
(i.e., V's AssociatedInterface/Neighbor is non-
NULL), and either a) V is a router or b) V's TTL
parameter is greater than 1. In these cases, W's
AssociatedInterface/Neighbor parameter is copied
directly from V. If V is a router, W's TTL
parameter is set to V's TTL parameter incremented by
one. If V is a transit network, W's TTL parameter is
set directly to V's TTL parameter.
(6) If the candidate list is non-empty, go to Step 4. Otherwise,
the algorithm terminates.
After the datagram shortest-path tree for Area A is complete,
the calculating router (RTX) must decide whether Area A, out of
all of RTX's attached areas, determines the forwarding cache
entry's upstream node. This determination is described in
Section 12.2.7.
Examples of the above SPF calculation, with particular emphasis
on the tiebreaking rules, are given in Appendix C.
12.2.1. Candidate list Initialization: Case SourceIntraArea
In this case, SourceNet belongs to Area A. The candidate
list is then initialized as follows. Start with the LSA
listed as Link State Origin in the matching OSPF routing
table entry. If this LSA is not multicast-capable (i.e, its
Options field has the MC-bit clear) the candidate list
should be set to NULL. Otherwise, the vertex identified by
the LSA is installed on the candidate list, setting its
vertex parameters as follows: IncomingLinkType set to
ILDirect, Cost set to 0, Parent to NULL and
AssociatedInterface/Neighbor to NULL.
As a consequence of this initialization, note that if
SourceNet is a stub network, then the datagram shortest-path
tree will not actually be rooted at the datagram source, but
will instead be rooted at the MOSPF router that attaches the
stub network to the rest of the MOSPF system. For example,
consider the network configuration shown in Figure 4. When
calculating the Area 2 datagram shortest-path tree for a
datagram whose source is Network N7 (e.g., from Host H5) and
destination is Group Ma, Router RT11 (and all other routers
attached to Area 2) will begin with the candidate list set
to Router RT8. As another example, the datagram shortest-
path tree pictured in Figure 3 is really rooted at Router
RT3 instead of Network N4.
12.2.2. Candidate list Initialization: Case SourceInterArea1
In this case, SourceNet belongs to an OSPF area that is not
directly attached to the calculating router (RTX). The
candidate list is then initialized as follows. Examine the
Area A summary-link-LSAs advertising SourceNet. For each
such summary-link-LSA: if both a) the MC-bit is set in the
LSA's Options field and b) the advertised cost is not equal
to LSInfinity, then the vertex representing the LSA's
advertising area border router is added to the candidate
list. An added vertex' state is initialized as:
IncomingLinkType set to ILSummary, Cost to whatever is
advertised in the LSA, Parent to NULL and
AssociatedInterface/Neighbor to NULL.
For example, consider the network configuration shown in
Figure 4. When calculating the Area 1 datagram shortest-
path tree for a datagram whose source is Network N7 (e.g.,
from Host H5) and destination is Group Ma, Router RT2 would
initialize the candidate list to contain the two area border
routers RT3 (with a cost of 20) and RT4 (with a cost of 19).
See Figure 6 for more details.
12.2.3. Candidate list Initialization: Case SourceInterArea2
In this case, SourceNet belongs to an OSPF area other than
Area A, but one that is still directly attached to the
calculating router (RTX). The candidate list is then
initialized in the following two steps:
(1) Find the Area A summary-link-LSA that best matches
SourceNet, excluding those summary-link-LSAs specifying
cost LSInfinity or having unreachable Advertising
Routers[24]. A matching summary-link-LSA is one that
advertises a range of addresses containing SourceNet;
the best matching is as usual the most specific match.
Let SourceRange be the network described by the best
matching summary-link-LSA.
(2) Similar to the logic in the SourceInterArea1 case,
examine all the Area A summary-link-LSAs which advertise
SourceRange. For each such summary-link-LSA: if both a)
the MC-bit is set in the LSA's Options field, b) the
advertised cost is not equal to LSInfinity and c) the
Advertising Router is reachable, then the vertex
representing the LSA's Advertising Router is added to
the candidate list. An added vertex' state is
initialized as: IncomingLinkType set to ILSummary, Cost
to whatever is advertised in the LSA, Parent to NULL and
AssociatedInterface/Neighbor to NULL.
The reason why SourceRange is used, instead of simply using
SourceNet (as was done in case SourceInterArea1), is that
routing information may have been collapsed at area
boundaries. In order for Area A's area border routers and
its internal routers to construct the same Area A datagram
shortest-path tree, they must both start at SourceRange -
Area A's internal routers know nothing about SourceNet. Note
that SourceRange is not discovered simply by looking at the
calculating router's configured set of area address ranges,
in order to avoid dependence on the configured area address
ranges being synchronized across all area border routers.
For example, consider the network configuration shown in
Figure 4. When calculating the Area 2 datagram shortest-
path tree for a datagram whose source is Network N11 and
destination is Group Ma, Router RT11 would calculate
SourceRange to be the collection: Networks N9-N11 and Host
H1. It would then initialize the candidate list to contain
itself (RT11) only, with an associated Cost of 1 (since RT11
is advertising Networks N9-N11 and Host H1 in a summary-
link-LSA with a cost of 1).
12.2.4. Candidate list Initialization: Case SourceExternal
In this case, SourceNet is external to the OSPF routing
domain, and Area A is not an OSPF stub area. The candidate
list is then initialized as follows. Note that an attempt
may be made to add a Vertex W to the candidate list when W
already belongs to the candidate list. When this happens,
W's vertex parameters are updated if the Cost parameter it
would be added with is better[25] (closer to SourceNet) than
its previous value. When the costs are the same, W's
parameters are still modified if the IncomingLinkType it
would be added with is better (see IncomingLinkType's
definition in Section 12.1) than its previous value.
For each AS external-link-LSA advertising SourceNet, the
following steps are performed:
o If the AS external-link-LSA's MC-bit is clear or if its
advertising router is not reachable, then the AS
external-link-LSA is not used. AS external-link-LSAs
having their MC-bit set and advertising a cost of
LSInfinity can be used; these LSAs describe paths that
can be used for multicast, but not unicast, data traffic
(see Section 11.2).
o If the AS external-link-LSA's Forwarding address field
is 0.0.0.0, the following vertices are added to the
candidate list. If the Advertising AS boundary router
(call it ASBR) belongs to Area A, the vertex
representing the AS boundary router is added to the
candidate list using parameters: IncomingLinkType set to
ILExternal, Cost to whatever is advertised in the LSA,
Parent to NULL and AssociatedInterface/Neighbor to NULL.
Then, regardless of whether ASBR belongs to Area A, all
Area A area border routers that are advertising
reachable multicast-capable (MC-bit set) type 4
summary-link-LSAs for ASBR are added to the candidate
list. Each such area border router is added with the
parameters: IncomingLinkType set to ILSummary, Cost to
the sum of whatever is advertised in the type 4
summary-link-LSA plus the value in the original AS
external-link-LSA, Parent to NULL and
AssociatedInterface/Neighbor to NULL.
o If the AS external-link-LSA's Forwarding address field
is non-zero, the Forwarding address is looked up in the
OSPF routing table. Then processing breaks into one of
the following cases:
o The Forwarding address is not usable. In this case,
nothing is added to the candidate list. The
Forwarding address is not usable if either it has no
matching routing table entry, or if the matching
routing table entry is neither of type intra-area
nor of type inter-area.
o The Forwarding address belongs to Area A[26]: the
Forwarding address' matching routing table entry has
Path-type of intra-area and its Associated area is
Area A. In this case, the vertex represented by the
matching routing table entry's Link State Origin
field is added to the candidate list (assuming that
the vertex is multicast-capable). The vertex is
added with the parameters: IncomingLinkType set to
ILExternal, Cost to whatever was advertised in the
original AS external-link-LSA, Parent to NULL and
AssociatedInterface/Neighbor to NULL.
o The Forwarding address belongs to an area that is
not attached to Router RTX[27]: the Forwarding
address' matching routing table entry has Path-type
of inter-area. Call the network represented by the
matching routing table entry ForwardNet. For each
reachable multicast-capable summary-link-LSA (in
Area A) advertising ForwardNet, add the LSA's
advertising area border router to the candidate list
using parameters: IncomingLinkType set to ILSummary,
Cost to the sum of whatever is advertised in the
summary-link-LSA plus the value in the original AS
external-link-LSA, Parent to NULL and
AssociatedInterface/Neighbor to NULL.
o The Forwarding address belongs to another one of
Router RTX's attached areas[28]: the Forwarding
address' matching routing table entry has Path-type
of intra-area and its associated Area is other than
Area A. Call the network represented by the
matching routing table entry ForwardNet. First find
the Area A summary-link-LSA that best matches
ForwardNet, excluding those summary-link-LSAs
specifying cost LSInfinity or having unreachable
Advertising Routers. Let ForwardRange be the network
described by the best matching summary-link-LSA.
Then, for each reachable multicast-capable summary-
link-LSA (in Area A) advertising ForwardRange, add
the LSA's advertising area border router to the
candidate list using parameters: IncomingLinkType
set to ILSummary, Cost to the sum of whatever is
advertised in the summary-link-LSA plus the value in
the original AS external-link-LSA, Parent to NULL
and AssociatedInterface/Neighbor to NULL.
The above calculation can be restated as follows. Each of
Area A's inter-area multicast forwarders and inter-AS
multicast forwarders are examined. Those that have
multicast-capable paths to SourceNet (represented as either
a multicast-capable AS external link or the concatenation of
a Type 4 summary link and a multicast-capable AS external
link) are added to the candidate list as router vertices.
(It is possible that, when considering a router that is both
an inter-area multicast forwarder and an inter-AS multicast
forwarder, two equal cost paths exist to SourceNet, one an
AS external link and the other a concatenation of a Type 4
summary link and an AS external link. In this case, the
concatenation of the Type 4 summary link and the AS external
link is preferred). The added vertex' state is set as
follows: IncomingLinkType set to ILSummary if the path is
represented as a concatenation of a Type 4 summary link and
an AS external link, IncomingLinkType set to ILExternal
otherwise, Cost set to the cost of the shortest path from
vertex to SourceNet, Parent set to NULL and
AssociatedInterface/Neighbor set to NULL.
For example, consider the network configuration shown in
Figure 4. When calculating the Area 2 datagram shortest-
path tree for a datagram whose source is Network N14 and
destination is Group Ma, the candidate list would be
initialized to the two routers RT7 at a cost of 14 and RT10
at a cost of 19. This assumes that the external costs
pictured in Figure 4 are external type 1s.
12.2.5. Candidate list Initialization: Case
SourceStubExternal
In this case, SourceNet is external to the OSPF routing
domain, and Area A is an OSPF stub area. The candidate list
is then initialized similarly to case SourceInterArea1. The
Area A summary-link-LSAs advertising DefaultDestination are
examined. For each such summary-link-LSA having both its
MC-bit set and its advertised cost not equal to LSInfinity,
the vertex representing the LSA's advertising area border
router is added to the candidate list. An added vertex'
state is initialized as: IncomingLinkType set to ILSummary,
Cost to whatever is advertised in the LSA, Parent to NULL
and AssociatedInterface/Neighbor to NULL.
The most likely outcome of the above is that all of stub
Area A's inter-area multicast forwarders will be installed
on the candidate list, with appropriate costs.
12.2.6. Processing labelled vertices
When encountered during the SPF calculation, vertices
labelled with the destination multicast group (Group G) may
cause the forwarding cache entry's list of downstream
interfaces/neighbors to be modified. A Vertex V in Area A
is labelled with Group G if and only if at least one of the
following holds:
(1) V is a router, and its router-LSA indicates that it is a
wild-card multicast receiver (i.e., bit W in its
router-LSA is set). This may be true when V is an
inter-area or inter-AS multicast forwarder.
(2) V is listed in the body of a group membership-LSA. In
particular, find the originator of Vertex V's LSA; call
it Router Y. Then find the group-membership-LSA in Area
A's link state database which has Link State ID = Group
G and Advertising Router = Router Y (see Section A.3).
If this group-membership-LSA exists, and if Vertex V is
listed in the body of the LSA (see Sections 10 and A.3),
then Vertex V is labelled with Group G.
When Vertex V is added to the shortest-path tree in Step 4
of Section 12.2, and if Vertex V is both downstream from the
calculating router (i.e., Vertex V's
AssociatedInterface/Neighbor is non-NULL) and labelled with
Group G, then Vertex V's AssociatedInterface/Neighbor is
added to the forwarding cache entry's list of downstream
interfaces/neighbors. In addition, Vertex V's TTL value is
attached to the added downstream interface/neighbor. If the
particular interface/neighbor had already been added to the
list of downstream interfaces/neighbors, the list is simply
modified by setting the downstream interface/neighbor's TTL
value to the minimum of its existing TTL value and Vertex
V's TTL value.
12.2.7. Merging datagram shortest-path trees
After the datagram shortest-path tree for Area A is
complete, the calculating router (RTX) must decide whether
Area A, out of all of its attached areas, determines the
forwarding cache entry's upstream node. This is done by
examining RTX's position on the Area A datagram shortest-
path tree, which is in turn described by RTX's Area A Vertex
data structure. If RTX's Vertex parameter IncomingLinkType
is either ILNone (RTX is not on the tree), ILVirtual or
ILSummary, then some area other than Area A will determine
the upstream node. Otherwise, Area A might possibly
determine the upstream node (i.e., may be selected the
RootArea), depending on the following tiebreakers[29]:
o If RootArea has not been set, then set RootArea to Area
A. Otherwise, compare the present RootArea to Area A in
the following:
o Choose the area that is "nearest to the source". Nearest
to the source depends on each area's candidate list
initialization case, as it occurs in Step 2 of Section
12.2. The initialization cases, listed in order of
decreasing preference (or nearest to farthest) are:
SourceIntraArea, SourceInterArea1, SourceExternal and
SourceStubExternal. Areas whose candidate list
initialization falls into case SourceInterArea2 are
never used as the RootArea. As an example, consider the
network configuration shown in Figure 4. When
calculating the datagram shortest-path tree for a
datagram whose source is Network N7 (e.g., from Host H5)
and destination is Group Ma, Router RT11 would set its
RootArea to Area 2 (Case SourceIntraArea) instead of
Area 3 (Case SourceInterArea2) or the backbone Area 0
(Case SourceInterArea).
o If there are still two equally good areas, and one of
them is the backbone, set RootArea to the backbone (Area
0).
o If there are still two equally good areas, set RootArea
to the area whose datagram shortest-path tree provides
the shortest path from SourceNet to RTX. This is a
comparison of RTX's Vertex parameter Cost in the two
areas.
o If there are still two equally good areas, set RootArea
to one with the highest OSPF Area ID.
If the above has set the RootArea to be Area A, the
forwarding cache entry's upstream node must be set
accordingly. This setting depends on the IncomingLinkType in
RTX's Area A Vertex structure. If IncomingLinkType is equal
to ILDirect, the upstream node is set to the appropriate
directly-connected stub network. If equal to ILNormal, the
upstream node is set to the Parent field in RTX's Area A
Vertex structure. If equal to ILExternal, the upstream node
is set to the placeholder EXTERNAL.
12.2.8. TOS considerations
The previous sections 12.2 through 12.2.7 described the
construction of a TOS 0 (default TOS) datagram shortest-path
tree. However, in a TOS-capable router, a separate tree may
be built for each TOS. If a TOS-capable router receives a
multicast datagram that specifies a non-zero TOS X, it first
builds the TOS 0 datagram shortest-path tree. Then, if all
the routers on the pruned tree are TOS-capable, a separate
TOS X datagram shortest-path tree is calculated[30].
Otherwise, the TOS 0 tree is used for all datagrams,
regardless of their specified TOS.
To determine whether there are any TOS-incapable routers on
the pruned TOS 0 tree, the following additions are made to
Section 12.2's tree calculation:
o A new piece of state information is added to each
vertex: TOS-capable path. This indicates whether the
present path from SourceNet to vertex, as represented on
the datagram shortest-path tree, contains only TOS-
capable routers.
o The TOS-capable path parameter is calculated when the
vertex is first added to the candidate list and
recalculated when/if the vertex' position on the
candidate list is modified (see Section 12.2's Step 2
and Step 5d). The parameter is set to TRUE if both the
vertex itself is TOS-capable and the vertex' parent has
its TOS-capable path parameter set to TRUE; otherwise,
TOS-capable path is set to FALSE.
o All routers on the TOS 0 datagram shortest-path tree are
TOS-capable if and only if, whenever a vertex labelled
with Group G is added to the shortest-path tree (Section
12.2.6), the value of the vertex' TOS-capable path
parameter is TRUE.
The source of the multicast datagram is always located using
a TOS 0 routing table lookup, regardless of the datagram's
TOS classification (see Section 11.2). If the calculating
router is not capable of TOS-based routing, it calculates
only TOS 0 datagram shortest-path trees, and uses them to
route datagrams independent of TOS value. Otherwise, when
calculating the TOS X datagram shortest-path tree, the
algorithm in Section 12.2 is used, with the modifications
listed below.
o When calculating RangeNet and ForwardRange in Sections
12.2.3 and 12.2.4 respectively, only summary-link-LSAs
having TOS 0 cost of LSInfinity are excluded (no change
from the TOS 0 case). However, when adding vertices to
the candidate list in Sections 12.2.2 through 12.2.5,
the TOS X cost of the summary links and/or AS external
links (and not the TOS 0 cost) are reflected in the
added vertices' Cost parameter.
o In Step 5 of Section 12.2, the TOS X cost of Link L (in
the appropriate direction) is used, not the TOS 0 cost.
o Non-TOS-routers are not added to the candidate list, and
are thus excluded from the trees.
12.2.9. Comparison to the unicast SPF calculation
There are many similarities between the construction of a
multicast datagram's shortest-path trees in Section 12.2 and
OSPF's intra-area route calculation for unicast traffic
(Section 16.1 of [OSPF]). Both have been described in terms
of Dijkstra's algorithm. However, there are some
differences. The major differences are listed below:
o In the multicast case, the datagram SPF calculation is
rooted at the datagram's source. In the unicast case,
each router is the root of its own unicast intra-area
SPF calculation.
o In the multicast case, the datagram shortest-path tree
is a true tree; i.e., between any two nodes on the tree
there is one path. However, due to the provision for
equal-cost multipath in [OSPF], the unicast SPF
calculation may add additional links to the shortest-
path tree.
o In order to avoid unwanted replication of multicast
datagrams, MOSPF ensures that, for any given datagram,
each router builds the exact same datagram shortest-path
tree. This forces two differences from the unicast SPF
calculation. First, it eliminates the possibility of
equal-cost multipath. Secondly, when the MOSPF system
contains multiple alternate paths, the algorithm must
ensure that each MOSPF router deterministically chooses
the same alternative. For this reason, tie-breaking
mechanisms have been specified in Steps 2, 4 and 5b of
Section 12.2.
o The calculation of datagram shortest path trees takes
into account only those links that connect transit nodes
(i.e, router to router or router to transit network
links). The unicast SPF calculation in Section 16.1 of
[OSPF] must additionally examine links to stub networks,
although this is done after all the transit links are
examined.
o While both the multicast and unicast trees select
shortest paths on the basis of the OSPF metric, the
datagram shortest-path trees also keep track of the TTL
values between the root (datagram source) and all
destinations (group members). This enables more
efficient implementation of IP multicast's "expanding
ring search" (see Section 2.3.4).
o In the multicast case, the algorithm is sometimes forced
to use the link state cost for the reverse direction
(i.e, the cost towards, instead of away from, the
source). This is because the costs of OSPF summary-
link-LSAs and AS external-link-LSAs, which sometime form
the base of the multicast datagram shortest-path trees,
are specified in the reverse direction (from the
multicast perspective).
o There are potentially many more datagram shortest-path
trees that need to be calculated (one for each source
net, destination group and TOS combination), than the
limited number of unicast SPF trees (one per each TOS).
This is the main reason that the datagram shortest-path
trees are calculated on demand; it is hoped that this
will spread the cost of the SPF calculations over
time[31].
o The way that the two algorithms handle TOS is different.
In the multicast case, if a TOS-incapable node is
encountered during the calculation of the TOS 0 datagram
shortest-path tree, the TOS 0 datagram shortest-path
tree is used instead of trying to build the TOS X tree
(see Section 12.2.8). In the unicast case, the TOS X
tree is always used, only falling back on the TOS 0
paths when a TOS X path does not exist.
12.3. Adding local database entries to the forwarding cache
After the datagram shortest-path trees have been built for each
attached area, the forwarding cache has an upstream node and a
list of downstream interfaces. In order to ensure the delivery
of the multicast datagram to group members on directly attached
networks, the local group database (Section 8.4) must then be
scanned for possible addition to the list of downstream
interfaces. All local group database entries having Group G as
MulticastGroup are examined. Suppose [Group G, Network N] is
one such entry. If the calculating router (RTX) is Network N's
Designated Router, then RTX's Network N interface is added to
the list of outgoing interfaces, with a TTL of 1. If the Network
N interface was already present in the list of outgoing
interfaces, its TTL is simply set to 1.
For example, consider the network configuration shown in Figure
4 when calculating the forwarding cache entry for a datagram
whose source is Network N4 (e.g., from Host H2) and destination
is Group Mb. After calculating the datagram shortest-path tree
for Area 1, Router RT2 would have set it upstream node to
Network N3 and its list of downstream interfaces to NULL. But
then looking at its local group database, it would add its
Network N2 interface with a TTL of 1 to its list of downstream
interfaces.
13. Maintaining the forwarding cache
A MOSPF router may, for resource reasons, limit the size of its
forwarding cache. At any time cache entries can be purged to make
room for newer entries, since the purged entries can always be
rebuilt when necessary. This memo does not specify an algorithm to
select which entries to purge. However, care should be taken to
ensure that any particular entry is not continually rebuilt and then
purged again (i.e., thrashing should be avoided).
The building of the forwarding cache has been previously described
in Section 12. There are events that force one or more forwarding
cache entries to be deleted; these events are described below. Note
that deleted cache entries will be rebuilt on an as-needed basis.
o When the internal topology of the MOSPF system changes, all
forwarding cache entries must be deleted. This is because
internal topology changes may invalidate the previously
calculated datagram shortest-path trees. Since the multicast
routing calculation depends on the result of the unicast routing
calculations, the forwarding cache should be cleared after the
unicast routing table is rebuilt. Internal topology changes are
indicated when both a) a new instance of either a router-LSA or
a network-LSA is received and b) the contents of the new
advertisement (other than the LS age, LS sequence number and LS
checksum fields) are different from the previous instance. This
covers routers and links going up or down, routers that change
from being multicast-incapable to being multicast-capable, etc.
o When a Type 3 summary-link-LSA (network summary) changes, those
forwarding cache entries specifying datagram sources belonging
to the range of addresses described by the updated summary-
link-LSA must be deleted. See Sections 12.2.3 and 12.2.5.
o Suppose that the content of an AS external-link-LSA changes. If
the AS external-link-LSA describes an external network N, then
all forwarding cache entries specifying an external source
network that is contained in N or that contains N (i.e.,
external sources that are a subset or a superset of N) must be
deleted.
o When membership in a multicast group changes, all forwarding
cache entries for the particular group must be deleted. Group
membership changes are indicated when either a) the content of a
group-membership-LSA changes or b) an entry in the local group
database (see Section 8.4) changes.
o When the cost to an AS boundary router or to a forwarding
address specified by one or more AS external-link-LSAs changes,
all forwarding cache entries specifying an external network as
datagram source must be deleted. In this case, potentially all
inter-AS datagram shortest-path trees have been invalidated. The
forwarding cache entries should be deleted after the new best
cost to the AS boundary router/forwarding address has been
calculated.
14. Other additions to the OSPF specification
MOSPF requires some modifications to the base OSPF protocol. All
these modifications are backward-compatible. A router running MOSPF
will still interoperate with an OSPF router when forwarding unicast
traffic. Most of the modifications have been described earlier in
this document. This section collects together those changes which
have yet to be mentioned, organizing them by the affected Section of
[OSPF].
14.1. The Designated Router
This functionality is described in Section 7.3 of [OSPF]. In
OSPF, a network's Designated Router has two specialized roles.
First, it originates the network's network-LSA. Second, it
controls the flooding on the network, in that all of the routers
on the network synchronize with the Designated Router (and the
Backup Designated Router) only. For these reasons[32], when one
or more of the network's routers are running MOSPF, the
Designated Router should be running MOSPF also. This can be
ensured by assigning all non-multicast routers the Router
Priority of 0.
In MOSPF, the Designated Router also has the additional
responsibility of monitoring the network's multicast group
membership. This is done by periodically sending Host Membership
Queries, and receiving Host Membership Reports in response (see
Section 9). This is yet another reason why the Designated Router
must be multicast-capable.
14.2. Sending Hello packets
This functionality is described in Section 9.5 of [OSPF]. A
MOSPF router sets the MC-bit in the Options field of its Hello
packets. This indicates that the router is multicast-capable; it
does not necessarily indicate the state of the sending
interface's IPMulticastForwarding parameter (see Section B.2).
Setting the MC-bit in Hellos is done strictly for informational
purposes. Neighbors receiving the router's Hello packets do not
act on the state of the MC-bit. A neighbor's multicast-
capability is learned instead during the Database Exchange
Process (see Section 14.4).
14.3. The Neighbor state machine
This functionality is described in Section 10.3 of [OSPF]. When
a neighbor enters state Exchange, the neighbor Database summary
list is initialized (see the OSPF neighbor FSM entry for State:
ExStart and Event: NegotiationDone). This list describes of the
portion of the router's link state database that needs to be
synchronized with the neighbor. Group-membership-LSAs are
included in the neighbor Database summary list if and only if
the neighbor is multicast-capable. The neighbor's multicast
capability is learned by examining the neighbor's Database
Description packets (see Section 14.4).
14.4. Receiving Database Description packets
This functionality is described in Section 10.6 of [OSPF]. A
neighbor's multicast-capability is learned through received
Database Description packets. When the Database Description
packet is received that transitions the neighbor from ExStart to
Exchange, the state of the MC-bit in the packet's Options field
is examined. The neighbor is multicast-capable if and only if
the MC-bit is set.
The neighbor's multicast capability controls whether group-
membership-LSAs are summarized to the neighbor during the
Database Exchange process (see Section 14.3), and whether
group-membership-LSAs are flooded to the neighbor during the
flooding process (see Section 10.2).
14.5. Sending Database Description packets
This functionality is described in Section 10.8 of [OSPF]. A
MOSPF router sets the MC-bit in the Options field of its
Database Description packets. This indicates to its adjacent
neighbors that the router is multicast-capable; it does not
necessarily indicate the state of the sending interface's
IPMulticastForwarding parameter (see Section B.2).
When a router goes from being multicast-capable to multicast-
incapable, or vice-versa, it must indicate this fact to its
adjacent neighbors by restarting the Database Description
process (i.e., rolling back the state of all adjacent neighbors
to Exstart).
14.6. Originating Router-LSAs
This functionality is described in Section 12.4.1 of [OSPF]. A
MOSPF router sets the MC-bit in the Options field of its
router-LSA. This allows the router to be included in datagram
shortest-path trees (see Step 5a of Section 12.2).
In addition, MOSPF has introduced a new flag in the router-LSA's
rtype field: the W-bit. When the W-bit is set, the router is
included on all datagram shortest-path trees, regardless of
multicast group (see Section 12.2.6). Such a router is called a
wild-card multicast receiver. The router sets the W-bit when it
wishes to receive all multicast datagrams, regardless of
destination. This will sometimes be true of inter-area multicast
forwarders (see Section 3.1), and inter-AS multicast forwarders
(see Section 4).
A router must originate a new instance of its router-LSA
whenever an event occurs that would invalidate the LSA's current
contents. In particular, if the router's multicast capability or
its ability to function as either an inter-area or inter-AS
multicast forwarder changes, its router-LSA must be
reoriginated.
14.7. Originating Network-LSAs
This functionality is described in Section 12.4.2 of [OSPF]. In
OSPF, a transit network's network-LSA is originated by the
network's Designated Router. The Designated Router sets the MC-
bit in the Options field of the network-LSA if and only if both
a) the Designated Router is multicast-capable (i.e., running
MOSPF) and b) the Designated Router's interface's
IPMulticastForwarding parameter has been set to a value other
than disabled (see Section B.2). When the network-LSA has the
MC-bit set, the network can be included in datagram shortest-
path trees (see Section 12.2.6).
It is intended that all routers attached to a common network
agree on the network's IPMulticastForwarding capability.
However, this agreement is not enforced. When there are
disagreements, incorrect routing of multicast datagrams can
result.
14.8. Originating Summary-link-LSAs
This functionality is described in Section 12.4.3 of [OSPF].
Inter-area multicast forwarders always set the MC-bit in the
Options field of their summary-link-LSAs, regardless of whether
the path described by the summary-link-LSA is actually
multicast-capable. Indeed, it is possible that there is no
multicast-capable path to the described destination. All other
area border routers (ones that are not inter-area multicast
forwarders) clear the MC-bit in the Options field of their
summary-link-LSAs.
If its MC-bit is clear, the summary-link-LSA will not be used
when initializing the candidate list in Sections 12.2.2, 12.2.3
and 12.2.5.
14.9. Originating AS external-link-LSAs
This functionality is described in Section 12.4.4 of [OSPF].
Unlike in summary-link-LSAs, an inter-AS multicast forwarder
should clear the MC-bit in the Options field of one of its AS
external-link-LSAs if it is known that there is no multicast-
capable path from the described destination to the router
itself. This knowledge may possibly be obtained, for example,
from an inter-AS multicast routing algorithm (see Section 4).
If the inter-AS multicast forwarder is unsure of whether a
multicast-capable path exists between the described destination
and the router itself, the MC-bit should be set in the AS
external-link-LSA. All other AS boundary routers (ones that are
not inter-AS multicast forwarders) clear the MC-bit in the
Options field of their AS external-link-LSAs.
If its MC-bit is clear, the AS external-link-LSA will not be
used when initializing the candidate list in Section 12.2.4.
When multicast connectivity to an external destination exists,
but no unicast connectivity, an AS external-link-LSA can be
originated having its MC-bit set and specifying a cost of
LSInfinity. Such an AS external-link-LSA will still be used by
the multicast routing calculation (see Section 12.2.4). As a
result, when a MOSPF router wishes to stop advertising an AS
external destination, it must use the premature aging procedure
specified in Section 14.1 of [OSPF], rather than simply setting
the AS external-link-LSA's cost to LSInfinity.
14.10. Next step in the flooding procedure
This functionality is described in Section 13.3 of [OSPF].
Group-membership-LSAs are specific to a OSPF single area, and
are flooded to multicast-capable routers only. When flooding a
group-membership-LSA, Section 13.3 of the OSPF specification is
modified as follows: 1) The list of interfaces examined during
flooding (called the eligible interfaces in Section 13.3 of
[OSPF]) is the set of all interfaces attaching to Area A (the
area that the group-membership-LSA is received from), just as
for router-LSAs, network-LSAs and summary-link-LSAs. 2) When
examining each interface, a group-membership-LSA is added to a
neighbor's link state retransmission list if and only if both a)
Step 1d of [OSPF]'s Section 13.3 is reached for the neighbor and
b) the neighbor is multicast-capable. The neighbor's multicast
capability is discovered during the Database Exchange process
(see Section 14.4).
Note that, since on broadcast networks Link State Update packets
are sent initially as multicasts, non-multicast routers may
receive group-membership-LSAs. However, non-multicast routers
will simply drop the group-membership-LSAs, for reasons of
unrecognized LS type (see Step 2 of [OSPF]'s Section 13). Link
State acknowledgments for group-membership-LSAs are not expected
from non-multicast routers, and group-membership-LSAs will never
be retransmitted to non-multicast routers, since the LSAs are
not added to these routers' link state retransmission lists (see
above paragraph).
For more information on flooding group-membership-LSAs, see
Section 10.2.
14.11. Virtual links
This functionality is described in Section 15 of [OSPF]. When a
MOSPF router (i.e., multicast-capable router) is both an area
border router and an endpoint of a virtual link whose other
endpoint is also multicast capable, the router must then also be
an inter-area multicast forwarder. This is necessary to ensure
that multicast datagrams will flow through the virtual link's
transit area, from one endpoint to the other. When the
backbone's datagram shortest-path tree is constructed in Section
12.1, it is assumed that virtual links are capable of forwarding
multicast datagrams whenever both endpoints are multicast-
capable.
15. References
[Bharath-Kumar] Bharath-Kumar, K. and J. Jaffe, "Routing to Multiple
Destinations in Computer Networks", IEEE
Transactions on Communications, COM-31[3], March
1983.
[Deering] Deering, S., "Multicast Routing in Internetworks and
Extended LANs", SIGCOMM Summer 1988 Proceedings,
August 1988.
[Deering2] Deering, S., "Multicast Routing in a Datagram
Internetwork", Stanford Technical Report, STAN-CS-
92-1415, Department of Computer Science, Stanford
University, December 1991.
[OSPF] Moy, J., "OSPF Version 2", RFC 1583, Proteon, Inc.,
March 1994.
[RFC 1075] Waitzman, D., Partridge, C., and S. Deering,
"Distance Vector Multicast Routing Protocol", RFC
1075, BBN STC, Stanford University, November 1988.
[RFC 1112] Deering, S., "Host Extensions for IP Multicasting",
STD 5, RFC 1112, Stanford University, May 1988.
[RFC 1209] Piscitello, D., and J. Lawrence, "Transmission of IP
Datagrams over the SMDS Service", RFC 1209, Bell
Communications Research, March 1991.
[RFC 1340] Reynolds, J. and J. Postel, "Assigned Numbers", STD
2, RFC 1340, USC/Information Sciences Institute,
July 1992.
[RFC 1390] Katz, D., "Transmission of IP and ARP over FDDI
Networks", STD 36, RFC 1390, cisco Systems, Inc.,
January 1993.
Footnotes
[1]Actually, OSPF allows a separate link cost to be configured for
each TOS. MOSPF then potentially calculates separate paths for each
TOS. For details, see Section 6.2.
[2]We also assume in this section that the pictured multi-access
networks provide data-link multicast/broadcast services.
[3]Note that if N3 were a non-broadcast network, Router RT3 would
send separate copies of the datagram to routers RT1 and RT2. Since
the IGMP protocol is not defined on non-broadcast networks, there
could in this case be no Group B member attached to Network N3.
However the multicast datagram would still be delivered to the Group
B members attached to networks N1 and N2.
[4]Actually, in MOSPF there is a separate forwarding cache entry for
each combination of source, destination and TOS. For a discussion of
TOS-based multicast routing, see Section 6.2.
[5]The discussion in this section omits mention of the Backup
Designated Router's role in the IGMP protocol. While the Backup
Designated Router does not send IGMP Host Membership Queries, it
does listen to IGMP Host Membership Reports, building "shadow" local
group database entries in the process. These entries do not lead to
group-membership-LSAs, nor do they influence delivery of multicast
datagrams, but are merely maintained to ease the transition from
Backup Designated Router to Designated Router, should the Designated
Router fail. See Sections 2.3.4, 9 and 10 for details.
[6]One might imagine building all possible datagram shortest-path
trees up front. However, this might be expensive, both in router CPU
time and in router memory. It is hoped that building the datagram
shortest-path trees on demand and caching the results will ease
demands on router resources by spreading out the calculations over a
longer period of time.
[7]It is possible that, due to the existence of alternate paths,
several different shortest-path trees are available. MOSPF depends
on all routers constructing the exact same shortest path tree. For
that reason, tie-breaking schemes have been implemented during tree
construction to ensure that identical trees result. See Section 12
for more details.
[8]Note that the expanding ring search yields the nearest server in
terms of hop count, but not necessarily in terms of the OSPF metric.
[9]This means that in MOSPF, just as in OSPF, the only kind of link
state advertisement that can be flooded between areas is the AS
external-link-LSA.
[10]A router indicates that it is a wild-card multicast receiver by
setting the appropriate flag in its router-LSA. See Section 14.6 for
details.
[11]This is not quite true. As we shall see, any inter-AS multicast
forwarders belonging to the backbone are designated as wild-card
multicast receivers. See Section 4.
[12]It is possible that through the operation of an inter-AS
multicast routing protocol, Router RT7 knows that it does not have
multicast connectivity to Network N15 (even though it has unicast
connectivity). In this case, RT7 would not advertise the external
link to N15 as being multicast capable.
[13]Synchronization of the IPMulticastForwarding interface parameter
is not enforced by the MOSPF protocol, since it is not included in
the contents of a MOSPF router's Hello packets.
[14]Actually, when multiple IP networks have been assigned to the
same physical network, the first thing that needs to be done is to
associate an IP network with the received Host Membership Report.
This is done in the same way that a receiving interface is
associated with a received multicast datagram; see Section 11.1.
[15]For this reason when a transit network has both MOSPF routers
and non-multicast OSPF routers attached, care should be taken to
ensure that a MOSPF router is elected Designated Router. This can be
accomplished through proper setting of the routers' configured
Router Priority.
[16]Note that just because these advertisements exist in the link
state database, it does not mean that the Group G members are
reachable. Reachability does not enter into the building of the
transit vertex list, in order to simplify the calculation. This is a
trade-off. As a result, some multicast datagrams may be forwarded
further than necessary, when the described Group G members actually
are unreachable.
[17]Since the Designated Router controls flooding on the network,
this is another reason to ensure that a MOSPF router is elected as
Designated Router.
[18]In other words, group-membership-LSAs will never be
retransmitted to non-multicast routers.
[19]This last step will not be necessary if the configuration
guidelines presented in Section 6.5 are followed.
[20]The TOS 0 routing table entry is examined regardless of the TOS
specified by the multicast datagram.
[21]It is assumed that a MOSPF router that wants to stop advertising
a route to an external destination will use the premature aging
procedure specified in Section 14.1 of [OSPF], rather than setting
the AS external-link-LSA's cost to LSInfinity.
[22]This preference ordering is used in Step 5c of Section 12.2.
[23]No attempt is made to match the links' two halves. See Step 5d.
[24]However, a summary-link-LSA is eligible for matching even if the
MC-bit in its Options field is clear.
[25]Costs may have both a Type 2 and a Type 1 component; the Type 2
component is always most significant.
[26]This case mirrors the SourceIntraArea candidate list
initialization in Section 12.2.1.
[27]This case mirrors the SourceInterArea1 candidate list
initialization in Section 12.2.2.
[28]This case mirrors the SourceInterArea2 candidate list
initialization in Section 12.2.3.
[29]Note that selecting the upstream node in this manner enforces
the inter-area routing architecture outlined in Section 3.1. Namely,
the multicast datagram is forwarded from the source area, over the
backbone and then into the non-backbone areas. This is similar to
the "hub and spoke" architecture for unicast forwarding described in
Section 3.2 of [OSPF].
[30]This procedure seems backwards. One would expect that the TOS X
datagram tree would be built first. However, the SPF calculation
must ensure that all routers participating in the forwarding of that
datagram, both TOS-capable and non-TOS-capable, build the same tree.
Since it is known that the non-TOS-capable routers will use the TOS
0 tree, the only safe way to use the TOS X tree is when you are
guaranteed that the non-TOS-capable routers will decline to forward
the datagram. This guarantee is clearly met when there are only
TOS-capable routers on the TOS 0 datagram tree.
[31]Indeed, there will also be those cases where the router, not
being on a particular datagram shortest-path tree, will never have
to calculate the particular tree, since the router will not receive
the datagram in the first place.
[32]Group-membership-LSAs are not processed by non-multicast routers
(see Section 10.2). Also, if the Designated Router was not running
the multicast extensions, multicast datagrams would not be forwarded
over the network because its network-LSA would have its MC-bit clear
(see Step 5a in Section 12.2).
A. Data Formats
This section documents the format of MOSPF protocol packets and link
state advertisements (LSAs). All changes and additions made to the
OSPF Version 2 data formats have been made in a backward-compatible
manner. In other words, multicast routers running MOSPF can
interoperate with (non-multicast) OSPF Version 2 routers when
forwarding regular (unicast) IP data traffic.
The MOSPF packet formats are the same as for OSPF Version 2
(described in Appendix A of [OSPF]). One additional option has been
added to the Options field that appears in OSPF Hello packets,
Database Description packets and all link state advertisements. This
new option indicates a router's/network's multicast capability, and
is documented in Section A.1. The presence of this new option is
ignored by all non-multicast routers.
To support MOSPF, one of OSPF's link state advertisements has been
modified, and a new link state advertisement has been added. The
format of the router-LSA has been modified (see Section A.2) to
include a new flag indicating whether the router is a wild-card
multicast receiver. A new link state advertisement, called the
group-membership-LSA, has been added to pinpoint multicast group
members in the link state database. This new advertisement is
neither flooded nor processed by non-multicast routers. The group-
membership-LSA is documented in Section A.3.
A.1 The Options field
The OSPF Options field is present in OSPF Hello packets, Database
Description packets and all link state advertisements. The Options
field enables OSPF routers to support (or not support) optional
capabilities, and to communicate their capability level to other
OSPF routers. Through this mechanism routers of differing
capabilities can be mixed within an OSPF routing domain.
When used in Hello packets, the Options field allows a router to
reject a neighbor because of a capability mismatch. Alternatively,
when capabilities are exchanged in Database Description packets a
router can choose not to forward certain LSA types to a neighbor
because of its reduced functionality. Lastly, listing capabilities
in LSAs allows routers to route traffic around reduced functionality
routers, by excluding them from parts of the routing table
calculation.
Three capabilities are currently defined. For each capability, the
effect of the capability's appearance (or lack of appearance) in
Hello packets, Database Description packets and link state
advertisements is specified below. For example, the
ExternalRoutingCapability (below called the E-bit) has meaning only
in OSPF Hello packets.
+---+---+---+---+---+---+---+---+
| * | * | * | * | * |MC | E | T |
+---+---+---+---+---+---+---+-+-+
The OSPF Options field
o T-bit. This describes the router's TOS capability. If the T-bit
is reset, then the router supports only a single TOS (TOS 0).
Such a router is also said to be incapable of TOS-routing. The
absence of the T-bit in a router links advertisement causes the
router to be skipped when building a non-zero TOS shortest-path
tree. In other words, routers incapable of TOS routing will be
avoided as much as possible when forwarding data traffic
requesting a non-zero TOS. The absence of the T-bit in a summary
link advertisement or an AS external link advertisement
indicates that the advertisement is describing a TOS 0 route
only (and not routes for non-zero TOS).
o E-bit. AS external link advertisements are not flooded
into/through OSPF stub areas. The E-bit ensures that all members
of a stub area agree on that area's configuration. The E-bit is
meaningful only in OSPF Hello packets. When the E-bit is reset
in the Hello packet sent out a particular interface, it means
that the router will neither send nor receive AS external link
state advertisements on that interface (in other words, the
interface connects to a stub area). Two routers will not become
neighbors unless they agree on the state of the E-bit.
o MC-bit. The MC-bit describes the multicast capability of the
various pieces of the OSPF routing domain. When calculating the
path of multicast datagrams, only those link state
advertisements having their MC-bit set are used. In addition, a
router uses the MC-bit in its Database Description packets to
tell adjacent neighbors whether the router will participate in
the flooding of the new group-membership-LSAs.
A.2 Router-LSA
An OSPF router originates a router-LSA into each of its attached
areas. The router-LSA describes the state and cost of the router's
interfaces to the area. The contents of the router-LSA are described
in detail in Section A.4.2 of [OSPF]. There are flags in the
router-LSA that indicate whether the router is either a) an area
border router or b) an AS boundary router or c) the endpoint of a
virtual link. One more flag has been added to the router-LSA for
MOSPF; it is called bit W below. This flag indicates whether the
router wishes to receive all multicast datagrams regardless of
destination (i.e., is a wild-card multicast receiver).
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS age | Options | 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link State ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertising Router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS checksum | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| rtype | 0 | # links |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| Link ID | P
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ E
| Link Data | R
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | # TOS | TOS 0 metric | #
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ L
# | TOS | 0 | metric | I
T +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ N
O | ... | K
S +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ S
| | TOS | 0 | metric | |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| ... |
The router LSA
+---+---+---+---+---+---+---+---+
| * | * | * | * | W | V | E | B |
+---+---+---+---+---+---+---+-+-+
The rtype field
The following defines the flags found in the rtype field. Each flag
classifies the router by function:
o bit B. When set, the router is an area border router (B is for
border). These routers forward unicast data traffic between OSPF
areas.
o bit E. When set, the router is an AS boundary router (E is for
external). These routers forward unicast data traffic between
Autonomous Systems.
o bit V. When set, the router is an endpoint of an active virtual
link (V is for virtual) which uses the described area as its
Transit area.
o bit W. When set, the router is a wild-card multicast receiver.
These routers receive all multicast datagrams, regardless of
destination. Inter-area multicast forwarders and inter-AS
multicast forwarders are sometimes wild-card multicast receivers
(see Sections 3 and 4).
A.3 Group-membership-LSA
Group-membership-LSAs are the Type 6 link state advertisements.
Group-membership-LSAs are specific to a particular OSPF area. They
are never flooded beyond their area of origination. A router's
group-membership-LSA for Area A indicates its directly attached
networks which belong to Area A and contain members of a particular
multicast group. A router originates a group-membership-LSA for
multicast group D when the following conditions are met for at least
one directly attached network: 1) the router has been elected
Designated Router for the network and 2) at least one host on the
network has joined Group D via the IGMP protocol.
A router may also originate a group-membership-LSA for Group D if
the router itself has internal applications belonging to Group D. In
addition, area border routers originate group-membership-LSAs into
the backbone area when there are group members in the router's
attached non-backbone areas. See Section 10 for more information
concerning the origination of group-membership-LSAs.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS age | Options | 6 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link State ID = Destination Group |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertising Router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS checksum | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Vertex type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Vertex ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
The group-membership-LSA
The group-membership-LSA consists of the standard 20-byte link state
header (see Section A.4.1 of [OSPF]) followed by a list of transit
vertices to label with the multicast destination. The
advertisement's Link State ID is set to the destination multicast
group address. There is no metric associated with the advertisement.
Each transit vertex is specified by its Vertex type and Vertex ID
(see Section 12.1 for an explanation of this terminology):
o Vertex type. Set equal to 1 for a router, and 2 for a transit
network. Note that the only router that may be included in the
list is the Advertising Router itself.
o Vertex ID. For router vertices, this field indicates the
router's OSPF Router ID. For transit network vertices, this
field indicates the IP address of the network's Designated
Router. Note that the link state advertisement associated with
the transit vertex is the LSA whose LS type = Vertex type, Link
State ID = Vertex ID and Advertising Router = the group-
membership-LSA's Advertising Router.
B. Configurable Constants
This section documents the configurable parameters used by OSPF's
multicast routing extensions. These parameters are in addition to
the configurable constants used by the base OSPF protocol
(documented in Appendix C of [OSPF]). An implementation of MOSPF
must provide the ability to set these parameters, either through
network management or some other means.
B.1 Global parameters
The following parameters apply to the router as a whole.
o Multicast capability. An indication of whether the router is
running MOSPF. If the router is running MOSPF, it will
perform the algorithms as set forth in this specification.
Otherwise, the router is still able to run the basic OSPF
algorithm (as set forth in [OSPF]), and will be able to
interoperate with multicast capable routers (see Section
6.1) when forwarding regular (unicast) IP data traffic.
o Inter-area multicast forwarder. This parameter indicates
whether the router will forward multicast datagrams between
OSPF areas. Such a router summarizes group membership
information to the backbone, and acts as a wild-card
multicast receiver in all its attached non-backbone areas
(see Section 3.1). Not all multicast-capable area border
routers need be configured as inter-area multicast
forwarders. However, whenever both ends of a virtual link
are multicast-capable, they must both be configured as
inter-area multicast forwarders (see Section 14.11). By
default, all multicast-capable area border routers are
configured as inter-area multicast forwarders.
o Inter-AS multicast forwarder. This parameter indicates
whether the router forwards multicast datagrams between
Autonomous Systems. Such a router acts as a wild-card
multicast receiver in all attached areas (see Section 4). It
is also assumed that an inter-AS multicast forwarder runs
some kind of inter-AS multicast routing algorithm.
B.2 Router interface parameters
The following parameters can be configured separately for each
of the router's OSPF interfaces. Remember that an OSPF interface
is the connection between the router and one of its attached IP
networks. Note that the IPMulticastForwarding parameter is
really a description of the attached network. As such, it should
be configured identically on all routers attached to a common
network; otherwise incorrect routing of multicast datagrams may
result.
o IPMulticastForwarding. This configurable parameter indicates
whether IP multicasts should be forwarded over the attached
network, and if so, how the forwarding should be done. The
parameter can assume one of three possible values: disabled,
data-link multicast and data-link unicast. When set to
disabled, IP multicast datagrams will not be forwarded out
the interface. When set to data-link multicast, IP multicast
datagrams will be forwarded as data-link multicasts. When
set to data-link unicast, IP multicast datagrams will be
forwarded as data-link unicasts. The default value for this
parameter is data-link multicast. The other two settings are
for use in the special circumstances described in Sections
6.3 and 6.4. When set to disabled or to data-link unicast,
IGMP group membership is not monitored on the attached
network.
o IGMPPollingInterval. The number of seconds between IGMP Host
Membership Queries sent out this interface. A multicast-
capable router sends IGMP Host Membership Queries only when
it has been elected Designated Router for the attached
network. See [RFC 1112] for a discussion of this parameter's
value.
o IGMP timeout. If no IGMP Host Membership Reports have been
heard on an attached network for a particular multicast
group A after this period of time, the entry [Group A,
attached network] is deleted from the router's local group
database. See Section 9 for more information.
C. Sample datagram shortest-path trees
In MOSPF, all routers must calculate exactly the same datagram
shortest-path trees. In order to ensure this in internetworks having
redundant links, a number of tie-breakers were defined in the MOSPF
routing table calculation (see Steps 4 and 5c of Section 12.2, and
Sections 12.2.4 and 12.2.7). This section illustrates the use of
these tie-breakers on a sample topology.
Three different examples are given. All examples use the same
physical topology and the same set of OSPF interface costs (see the
left side of Figure 14). The source of the datagram is always Host
H1 on the network at the top of the figure (192.9.1.0), and the
destination group members are the two hosts labelled with Group Ma
at the bottom of the figure. The first case shows an example of
intra-area multicast, while the remaining two cases show the
influence of OSPF areas on the path of a multicast datagram.
C.1 An intra-area tree
The datagram shortest-path tree resulting from the intra-area case
is shown on the right of Figure 14. The root of the tree is the
source network (192.9.1.0), and the leaves are the two routers (RT4
and RT3) directly attached to the stub networks containing Group Ma
members.
There are equal-cost paths available to both group members. For the
group member on the left, the path could go either through network
10.1.0.0 or through network 10.2.0.0. By the tie-breaking rules, the
path through 10.2.0.0 is chosen since it has the larger IP network
number (see Step 5c of Section 12.2).
For the group member on the right, the path could go either over
Network 10.2.0.0 or over the serial line connecting routers RT2 and
RT3. The path over Network 10.2.0.0 is chosen after executing two
tie-breaking rules. First, Network 10.2.0.0 is placed on the
shortest-path tree before Router RT3 since networks are always
chosen over routers (see Step 4 of Section 12.2). Then, given a
+--+
|H1|
+--+
Net 192.9.1.0 |
+------------------+
| |
+----------+ |1 |1
| Network | 8+---+ +---+ o 192.9.1.0
| 10.1.0.0 |------|RT1| |RT2| |
+----------+ +---+ +---+ 0|
| |8 8| |
8| +----------+ |8 o RT1
+---+10 | Network | 10+---+ |
|RT4|-------| 10.2.0.0 |----|RT3| 8|
+---+ +----------+ +---+ |
|3 |3 o 10.2.0.0
| | / \
+---------+ +-------+ 0/ \0
| | / \
+--+ +--+ o o
|Ma| |Ma| RT4 RT3
+--+ +--+
Figure 14: An intra-area tree
choice of either Network 10.2.0.0 or Router RT2 for RT3's parent on
the tree, Net 10.2.0.0 is again preferred since it is a network (see
Step 5c of Section 12.2)
C.2 The effect of areas
In Figure 15 below, the previous diagram has been modified by the
inclusion of OSPF areas. The datagram source is now part of the OSPF
backbone (Area 0), while the rest of the topology is in Area 1. In
this case, since the datagram source and the group members belong to
different areas, reverse costs are used when building the tree (see
Step 5b of Section 12.2). This actually eliminates the equal cost
paths from the diagram, and leads to the Area 1 datagram shortest-
path tree on the right of Figure 15.
+--+
|H1|
+--+
Net 192.9.1.0 |
+------------------+
..................... | |
. +----------+ . |1 |1 192.9.1.0
. | Network | 8+---+ +---+ o
. | 10.1.0.0 |------|RT1|........|RT2|... / \
. +----------+ +---+ +---+ . 1/ \1
. | |8 8| . / \
. 8| +----------+ |8 . o RT1 o RT2
. +---+10 | Network | 10+---+ . | \
. |RT4|-------| 10.2.0.0 |----|RT3| . 0| \8
. +---+ +----------+ +---+ . | \
. |3 |3 . o 10.1.0.0 o
. | | . | RT3
. +---------+ +-------+. 8|
. | | . |
. +--+ +--+ . o
. |Ma| |Ma| . RT4
. +--+ Area 1 +--+ .
.........................................
Figure 15: The effect of areas
C.3 The effect of virtual links
In Figure 16 below, Network 10.1.0.0 has been configured as a
separate area (Area 1), while everything else belongs to the OSPF
backbone (Area 0). In addition, a virtual link has been configured
through Area 1, enhancing the backbone connectivity. In this case,
both the source and the group members belong to the same area, so
forward costs are used. However, since virtual links are preferred
over regular links (see Step 5c of Section 12.2), the backbone
datagram shortest-path tree uses Network 10.1.0.0 instead of
10.2.0.0 on the path to the left group member. This leads to the
tree on the right of Figure 16.
+--+
|H1|
+--+
Net 192.9.1.0 |
................ +------------------+
. +----------+ . /1 |
. | Network |8. / |1
. | 10.1.0.0 |-+---+ +---+ o 192.9.1.0
. +----------+*|RT1| |RT2| |
. 8|*******+---+ +---+ 0|
.Area1 |*VL . \8 8| |
.....+---+...... +----------+ |8 o RT1
|RT4|10 | Network | 10+---+ / \
+---+-------| 10.2.0.0 |----|RT3| /8 \8
| +----------+ +---+ / \
|3 |3 o 10.1 o 10.2.0.0
| | | |
+---------+ +-------+ |0 |0
| | | |
+--+ +--+ o o
|Ma| |Ma| RT4 RT3
+--+ +--+
Figure 16: The effect of virtual links
Security Considerations
Security issues are not discussed in this memo.
Author's Address
John Moy
Proteon, Inc.
9 Technology Drive
Westborough, MA 01581
Phone: (508) 898-2800
Email: jmoy@proteon.com