Internet Engineering Task Force Audio/video Transport Wg Internet-draft Issues with Rtp Sampling

Status of this Memo This document is an Internet-Draft. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as \work in progress". To learn the current status of any Internet-Draft, please check the \1id-abstracts.txt" listing contained in the Internet-Drafts Shadow Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe), 1 Abstract In order to control the ow of RTCP packets in large multicast groups, session participants are required to transmit their packets with a period proportional to the group size. Obtaining a group size estimate generally requires a participant to maintain a list of group members. As this can require signiicant memory, particularly for embedded systems, RTP has been revised to allow for a statistical sampling procedure which allows the memory size to be bounded for all group sizes. However, this sampling algorithm can interact with other aspects of RTP. In particular, we have identiied three problems. First, RTP sampling has an adverse aaect on the performance of BYE reconsideration. Secondly, it can cause inaccurate estimates with dynamic group sizes that decrease rapidly. Thirdly, the current SSRC sampling algorithm grossly overestimates the group size when there are a few senders. We discuss these problems in detail and present simple solutions. 2 Introduction In order to control the ow of RTCP packets in large multicast groups, session participants are required to transmit their packets with a period proportional to the group size. Obtaining a group size estimate generally requires a participant to maintain a list of group members. As this can require signiicant memory, particularly for embedded systems, RTP 1] has been revised to allow for a statistical sampling procedure which allows the memory size to be bounded for all group sizes. This algorithm operates by applying an SSRC mask with m bits equal to one to each packet received. Initially, m starts at 0. If the bitwise AND of the SSRC of the incoming packet, and the mask, equals the bitwise AND of some key and the mask, the packet is accepted and the SSRC counted in the group size estimate. When the memory requirements …