RFC 3605 (rfc3605) - Page 1 of 8
Real Time Control Protocol (RTCP) attribute in Session Description Protocol (SDP)
Alternative Format: Original Text Document
Network Working Group C. Huitema
Request for Comments: 3605 Microsoft
Category: Standards Track October 2003
Real Time Control Protocol (RTCP) attribute in
Session Description Protocol (SDP)
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
The Session Description Protocol (SDP) is used to describe the
parameters of media streams used in multimedia sessions. When a
session requires multiple ports, SDP assumes that these ports have
consecutive numbers. However, when the session crosses a network
address translation device that also uses port mapping, the ordering
of ports can be destroyed by the translation. To handle this, we
propose an extension attribute to SDP.
1. Introduction
The session invitation protocol (SIP, [RFC 3261]) is often used to
establish multi-media sessions on the Internet. There are often
cases today in which one or both ends of the connection are hidden
behind a network address translation device [RFC 2766]. In this case,
the SDP text must document the IP addresses and UDP ports as they
appear on the "public Internet" side of the NAT. In this memo, we
will suppose that the host located behind a NAT has a way to obtain
these numbers. A possible way to learn these numbers is briefly
outlined in section 3, however, just learning the numbers is not
enough.
The SIP messages use the encoding defined in SDP [RFC 2327] to
describe the IP addresses and TCP or UDP ports used by the various
media. Audio and video are typically sent using RTP [RFC 3550], which
requires two UDP ports, one for the media and one for the control
protocol (RTCP). SDP carries only one port number per media, and
Huitema Standards Track
