MOSPF: Analysis and Experience
RFC 1585
|
Document |
Type |
|
RFC - Informational
(March 1994; No errata)
|
|
Last updated |
|
2013-03-02
|
|
Stream |
|
Legacy
|
|
Formats |
|
plain text
pdf
htmlized
bibtex
|
Stream |
Legacy state
|
|
(None)
|
|
Consensus Boilerplate |
|
Unknown
|
|
RFC Editor Note |
|
(None)
|
IESG |
IESG state |
|
RFC 1585 (Informational)
|
|
Telechat date |
|
|
|
Responsible AD |
|
(None)
|
|
Send notices to |
|
(None)
|
Network Working Group J. Moy
Request for Comments: 1585 Proteon, Inc.
Category: Informational March 1994
MOSPF: Analysis and Experience
Status of this Memo
This memo provides information for the Internet community. This memo
does not specify an Internet standard of any kind. Distribution of
this memo is unlimited.
Abstract
This memo documents how the MOSPF protocol satisfies the requirements
imposed on Internet routing protocols by "Internet Engineering Task
Force internet routing protocol standardization criteria" ([RFC
1264]).
Please send comments to mospf@gated.cornell.edu.
1. Summary of MOSPF features and algorithms
MOSPF is an enhancement of OSPF V2, enabling the routing of IP
multicast datagrams. OSPF is a link-state (unicast) routing
protocol, providing a database describing the Autonomous System's
topology. IP multicast is an extension of LAN multicasting to a
TCP/IP Internet. IP Multicast permits an IP host to send a single
datagram (called an IP multicast datagram) that will be delivered to
multiple destinations. IP multicast datagrams are identified as
those packets whose destinations are class D IP addresses (i.e.,
addresses whose first byte lies in the range 224-239 inclusive).
Each class D address defines a multicast group.
The extensions required of an IP host to participate in IP
multicasting are specified in "Host extensions for IP multicasting"
([RFC 1112]). That document defines a protocol, the Internet Group
Management Protocol (IGMP), that enables hosts to dynamically join
and leave multicast groups.
MOSPF routers use the IGMP protocol to monitor multicast group
membership on local LANs through the sending of IGMP Host Membership
Queries and the reception of IGMP Host Membership Reports. A MOSPF
router then distributes this group location information throughout
the routing domain by flooding a new type of OSPF link state
advertisement, the group-membership-LSA (type 6). This in turn
enables the MOSPF routers to most efficiently forward a multicast
Moy [Page 1]
RFC 1585 MOSPF: Analysis and Experience March 1994
datagram to its multiple destinations: each router calculates the
path of the multicast datagram as a shortest-path tree whose root is
the datagram source, and whose terminal branches are LANs containing
group members.
A separate tree is built for each [source network, multicast
destination] combination. To ease the computational demand on the
routers, these trees are built "on demand", i.e., the first time a
datagram having a particular combination of source network and
multicast destination is received. The results of these "on demand"
tree calculations are then cached for later use by subsequent
matching datagrams.
MOSPF is meant to be used internal to a single Autonomous System.
When supporting IP multicast over the entire Internet, MOSPF would
have to be used in concert with an inter-AS multicast routing
protocol (something like DVMRP would work).
The MOSPF protocol is based on the work of Steve Deering in
[Deering]. The MOSPF specification is documented in [MOSPF].
1.1. Characteristics of the multicast datagram's path
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.
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. This is possible since the
shortest-path tree is rooted at the datagram source, instead
Moy [Page 2]
RFC 1585 MOSPF: Analysis and Experience March 1994
Show full document text