RFC 2102 (rfc2102) - Page 3 of 23
Multicast Support for Nimrod : Requirements and Solution Approaches
Alternative Format: Original Text Document
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