MOSPF: Analysis and Experience
RFC 1585

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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