Multicast Support for Nimrod : Requirements and Solution Approaches
RFC 2102

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Network Working Group                                     R. Ramanathan
Request for Comments: 2102                 BBN Systems and Technologies
Category: Informational                                   February 1997

  Multicast Support for Nimrod :  Requirements and Solution Approaches

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

   Nimrod does not specify a particular solution for multicasting.
   Rather, Nimrod may use any of a number of emerging multicast
   techniques.  We identify the requirements that Nimrod has of a
   solution for multicast support.  We compare existing approaches for
   multicasting within an internetwork and discuss their advantages and
   disadvantages.  Finally, as an example, we outline the mechanisms to
   support multicast in Nimrod using the scheme currently being
   developed within the IETF - namely, the Protocol Indpendent Multicast
   (PIM) protocol.

Table of Contents

   1  Introduction.................................................  2
   2  Multicast vs Unicast.........................................  3
   3  Goals and Requirements.......................................  4
   4  Approaches...................................................  6
   5  A Multicasting Scheme based on PIM........................... 10
      5.1 Overview ................................................ 10
      5.2 Joining and Leaving a Tree .............................. 12
          5.2.1 An Example ........................................ 15
      5.3 Establishing a Shared Tree .............................. 16
      5.4 Switching to a Source-Rooted Shortest Path Tree.......... 18
      5.5 Miscellaneous Issues..................................... 20
   6  Security Considerations...................................... 21
   7  Summary...................................................... 21
   8  References................................................... 22
   9  Acknowledgements............................................. 23
   10 Author's Address............................................. 23

Ramanathan                   Informational                      [Page 1]
RFC 2102                Nimrod Multicast Support           February 1997

1  Introduction

   The nature of emerging applications such as videoconferencing, remote
   classroom, etc.  makes the support for multicasting essential for any
   future routing architecture.  Multicasting is performed by using a
   multicast delivery tree whose leaves are the multicast destinations.

   Nimrod does not propose a solution for the multicasting problem.
   There are two chief reasons for this.  First, multicasting is a non-
   trivial problem whose requirements are still not well understood.
   Second, a number of groups (for instance the IDMR working group of
   the IETF) are studying the problem by itself and it is not our
   intention to duplicate those efforts.

   This attitude towards multicasting is consistent with Nimrod's
   general philosophy of flexibility, adaptability and incremental
   change.

   While a multicasting solution per se is not part of the "core" Nimrod
   architecture, Nimrod does require that the solution have certain
   characteristics.  It is the purpose of this document to discuss some
   of these requirements and evaluate approaches towards meeting them.

   This document is organized as follows.  In section 2 we discuss why
   multicasting is treated a little differently than unicast despite the
   fact that the former is essentially a generalization of the latter.
   Following that, in section 4 we discuss current approaches toward
   multicasting .  In section 5, we give an example of how Nimrod
   multicasting may be done using PIM [DEF+94a].  For readers who do not
   have the time to go through the entire document, a summary is given
   at the end.

   This document uses many terms and concepts from the Nimrod
   Architecture document [CCS96] and some terms and concepts (in section
   5) from the Nimrod Functionality document [RS96].  Much of the
   discussion assumes that you have read at least the Nimrod
   Architecture document [CCS96].

Ramanathan                   Informational                      [Page 2]
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