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Real Time Control Protocol (RTCP) attribute in Session Description Protocol (SDP) (RFC3605)

IP.com Disclosure Number: IPCOM000019910D
Original Publication Date: 2003-Oct-01
Included in the Prior Art Database: 2003-Oct-10
Document File: 9 page(s) / 17K

Publishing Venue

Internet Society Requests For Comment (RFCs)

Related People

C. Huitema: AUTHOR

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.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 19% of the total text.

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, [RFC3261]) 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 [RFC2766]. 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 [RFC2327] to

describe the IP addresses and TCP or UDP ports used by the various

media. Audio and video are typically sent using RTP [RFC3550], 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 [Page 1]

RFC 3605 RTCP attribute in SDP October 2003

states that "other ports used by the media application (such as the

RTCP port) should be derived algorithmically from the base media

port." RTCP port numbers were necessarily derived from the base

media port in older versions of RTP (such as [RFC1889]), but now that

this restriction has been lifted, there is a need to specify RTCP

ports explicitly in SDP. Note, however, that implementations of RTP

adhering to the earlier [RFC1889] specification may not be able to

make use of the SDP attributes specified in this document.

When the NAT device performs port mapping, there is no guarantee that

the mappings of two separate ports reflects the sequencing and the

parity of the original port numbers; in fact, when the NAT manages a

pool of IP addresses, it is even possible that the RTP and the RTCP

ports may be mapped to different addresses. In order to successfully

establish connections despite th...