Sampling of the Group Membership in RTP (RFC2762)
Original Publication Date: 2000-Feb-01
Included in the Prior Art Database: 2019-Feb-10
Internet Society Requests For Comment (RFCs)
J. Rosenberg: AUTHOR [+1]
This document discusses mechanisms for sampling of this group membership table in order to reduce the memory requirements. This memo defines an Experimental Protocol for the Internet community.
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 (C) The Internet Society (2000). All Rights Reserved.
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.
RTP, the Real Time Transport Protocol , 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
Rosenberg & Schulzrinne Experimental [Page 1]
RFC 2762 RTP Sampling February 2000
systems can be used for both small conferences and TV broadcasts in a smooth, scalable fashion.
Previous work  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  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 obtain a stochastic estimate of the group membership. There are many issues that arise when this is done. This document reviews these issues and discusses the mechanisms which can be applied by implementors. In particular, it foc...