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Sampling of the Group Membership in RTP (RFC2762)

IP.com Disclosure Number: IPCOM000003360D
Original Publication Date: 2000-Feb-01
Included in the Prior Art Database: 2000-Sep-13
Document File: 10 page(s) / 24K

Publishing Venue

Internet Society Requests For Comment (RFCs)

Related People

J. Rosenberg: AUTHOR [+2]

Abstract

In large multicast groups, the size of the group membership table maintained by RTP (Real Time Transport Protocol) participants may become unwieldy, particularly for embedded devices with limited memory and processing power. This document discusses mechanisms for sampling of this group membership table in order to reduce the memory requirements. Several mechanisms are proposed, and the performance of each is considered.

This text was extracted from a ASCII Text document.
This is the abbreviated version, containing approximately 12% of the total text.

Network Working Group J. Rosenberg

Request for Comments: 2762 dynamicsoft

Category: Experimental H. Schulzrinne

Columbia U.

February 2000

Sampling of the Group Membership in RTP

Status of this Memo

This memo defines an Experimental Protocol for the Internet

community. It does not specify an Internet standard of any kind.

Discussion and suggestions for improvement are requested.

Distribution of this memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (2000). All Rights Reserved.

Abstract

In large multicast groups, the size of the group membership table

maintained by RTP (Real Time Transport Protocol) participants may

become unwieldy, particularly for embedded devices with limited

memory and processing power. This document discusses mechanisms for

sampling of this group membership table in order to reduce the memory

requirements. Several mechanisms are proposed, and the performance of

each is considered.

1 Introduction

RTP, the Real Time Transport Protocol [1], mandates that RTCP packets

be transmitted from each participant with a period roughly

proportional to the group size. The group size is obtained by storing

a table, containing an entry for each unique SSRC seen in RTP and

RTCP packets. As members leave or time out, entries are deleted, and

as new members join, entries are added. The table is thus highly

dynamic.

For large multicast sessions, such as an mbone broadcast or IP-based

TV distribution, group sizes can be extremely large, on the order of

hundreds of thousands to millions of participants. In these

environments, RTCP may not always be used, and thus the group

membership table isn't needed. However, it is highly desirable for

RTP to scale well for groups with one member to groups with one

million members, without human intervention to "turn off" RTCP when

it's no longer appropriate. This means that the same tools and

systems can be used for both small conferences and TV broadcasts in a

smooth, scalable fashion.

Previous work [2] has identified three major scalability problems

with RTP. These are:

1. Congestion due to floods of RTCP packets in highly dynamic groups;

2. Large delays between receipt of RTCP packets from a single user;

3. Large size of the group membership table.

The reconsideration algorithm [2] helps to alleviate the first of

these. This document addresses the third, that of large group size

tables.

Storage of an SSRC table with one million members, for example,

requires at least four megabytes. As a result, embedded devices with

small memory capacity may have difficulty under these conditions. To

solve this problem, SSRC sampling has been proposed. SSRC sampling

uses statistical sampling to...