Network Working Group M. Handley
Request for Comments: 2887 S. Floyd
Category: Informational ACIRI
Digital Fountain, Inc.
The Reliable Multicast Design Space for Bulk Data Transfer
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright (C) The Internet Society (2000). All Rights Reserved.
The design space for reliable multicast is rich, with many possible
solutions having been devised. However, application requirements
serve to constrain this design space to a relatively small solution
space. This document provides an overview of the design space and
the ways in which application constraints affect possible solutions.
The term "general purpose reliable multicast protocol" is something
of an oxymoron. Different applications have different requirements
of a reliable multicast protocol, and these requirements constrain
the design space in ways that two applications with differing
requirements often cannot share a single solution. There are however
many successful reliable multicast protocol designs that serve more
special purpose requirements well.
In this document we attempt to review the design space for reliable
multicast protocols intended for bulk data transfer. The term bulk
data transfer should be taken as having broad meaning - the main
limitations are that the data stream is continuous and long lived -
Handley, et al. Informational [Page 1]RFC 2887 Multicast Design Space for Bulk Data Transfer August 2000
constraints necessary for the forms of congestion control we
currently understand. The purpose of this review is to gather
together an overview of the field and to make explicit the
constraints imposed by particular mechanisms. The aim is to provide
guidance to the standardization process for protocols and protocol
building blocks. In doing this, we cluster potential solutions into
a number of loose categories - real protocols may be composed of
mechanisms from more than one of these clusters.
The main constraint on solutions is imposed by the need to scale to
large receiver sets. For small receiver sets the design space is
much less restricted.
2. Application Constraints
Application requirements for reliable multicast (RM) are as broad and
varied as the applications themselves. However, there are a set of
requirements that significantly affect the design of an RM protocol.
A brief list includes:
o Does the application need to know that everyone received the data?
o Does the application need to constrain differences between
o Does the application need to scale to large numbers of receivers?
o Does the application need to be totally reliable?
o Does the application need ordered data?
o Does the application need to provide low-delay delivery?
o Does the application need to provide time-bounded delivery?
o Does the application need many interacting senders?
o Is the application data flow intermittent?
o Does the application need to work in the public Internet?
o Does the application need to work without a return path (e.g.
o Does the application need to provide secure delivery?
Handley, et al. Informational [Page 2]RFC 2887 Multicast Design Space for Bulk Data Transfer August 2000
In the context of standardizing bulk data transfer protocols, we can
rule out applications with multiple interacting senders and
intermittent data flows. It is not that these applications are
unimportant, but that we do not yet have effective congestion control
for such applications.
2.1. Did everyone receive the data?
In many applications a logically defined unit or units of data is to