RFC 2102 (rfc2102) - Page 3 of 23

Multicast Support for Nimrod : Requirements and Solution Approaches

RFC 2102                Nimrod Multicast Support           February 1997


2  Multicast vs Unicast

   We begin by looking at the similarities and differences between
   unicast routing and multicast routing.  Both unicast and multicast
   routing require two phases - route generation and packet forwarding.
   In the case of unicast routing, Nimrod specifies modes of packet
   forwarding; route generation itself is not specified but left to the
   particular routing agent.  For multicasting, Nimrod leaves both route
   generation and packet forwarding mechanisms unspecified.  To explain
   why, we first point out three aspects that make multicasting quite
   different from unicasting :

o Groups and group dynamism.  In multicasting, the destinations are part
  of a group, whose membership is dynamic.  This brings up the following
  issues :

  -  An association between the multicast group and the EIDs and
     locators of the members comprising that group.  This is especially
     relevant in the case of sender initiated multicasting and policy
     support.

  -  A mechanism to accommodate new group members in the delivery in
     response to addition of members, and a mechanism to "prune" the
     delivery in response to departures.

o State creation.  Most solutions to multicasting can essentially be
  viewed as creating state in routers for multicast packet forwarding.
  Based on who creates the state, multicasting solutions differ.  In
  multicasting, we have several options for this - e.g., the sender, the
  receivers or the intermediate routers.

o Route generation.  Even more so than in unicast routing, one can choose
  from a rich spectrum of heuristics with different tradeoffs between a
  number of parameters (such as cost and delay, algorithmic time
  complexity and optimality etc.).  For instance, some heuristics produce
  a low-cost tree with high end-to-end delay and some produce trees that
  give the shortest path to each destination but with a higher cost.
  Heuristics for multicasting are a significant research area today, and
  we expect advances to result in sophisticated heuristics in the near
  future.

   Noting that there are various possible combinations of route
   generation, group dynamism handling and state creation for a solution
   and that each solution conceivably has applications for which it is
   the most suitable, we do not specify one particular approach to
   multicasting in Nimrod.  Every implementation of Nimrod is free to
   use its own multicasting technique, as long as it meets the goals and
   requirements of Nimrod.  However, for interoperability, it is



Ramanathan                   Informational