Symmetric RTP / RTP Control Protocol (RTCP)
RFC 4961

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Network Working Group                                            D. Wing
Request for Comments:  4961                                Cisco Systems
BCP:  131                                                      July 2007
Category:  Best Current Practice

              Symmetric RTP / RTP Control Protocol (RTCP)

Status of This Memo

   This document specifies an Internet Best Current Practices for the
   Internet Community, and requests discussion and suggestions for
   improvements.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

   This document recommends using one UDP port pair for both
   communication directions of bidirectional RTP and RTP Control
   Protocol (RTCP) sessions, commonly called "symmetric RTP" and
   "symmetric RTCP".

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 2
   2.  Conventions Used in this Document . . . . . . . . . . . . . . . 2
   3.  Definition of Symmetric RTP and Symmetric RTCP  . . . . . . . . 3
   4.  Recommended Usage . . . . . . . . . . . . . . . . . . . . . . . 3
   5.  Security Considerations . . . . . . . . . . . . . . . . . . . . 4
   6.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 4
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 4
     7.1.  Normative References  . . . . . . . . . . . . . . . . . . . 4
     7.2.  Informative References  . . . . . . . . . . . . . . . . . . 4

Wing                     Best Current Practice                  [Page 1]
RFC 4961                 Symmetric RTP and RTCP                July 2007

1.  Introduction

   TCP [RFC0793], which is inherently bidirectional, transmits and
   receives data using the same local port.  That is, when a TCP
   connection is established from host A with source TCP port "a" to a
   remote host, the remote host sends packets back to host A's source
   TCP port "a".

   However, UDP is not inherently bidirectional and UDP does not require
   using the same port for sending and receiving bidirectional traffic.
   Rather, some UDP applications use a single UDP port to transmit and
   receive (e.g., DNS [RFC1035]), some applications use different UDP
   ports to transmit and receive with explicit signaling (e.g., Trivial
   File Transfer Protocol (TFTP) [RFC1350]), and other applications
   don't specify the choice of transmit and receive ports (RTP
   [RFC3550]).

   Because RTP and RTCP are not inherently bidirectional protocols, and
   UDP is not a bidirectional protocol, the usefulness of using the same
   UDP port for transmitting and receiving has been generally ignored
   for RTP and RTCP.  Many firewalls, Network Address Translators (NATs)
   [RFC3022], and RTP implementations expect symmetric RTP, and do not
   work in the presence of asymmetric RTP.  However, this term has never
   been defined.  This document defines "symmetric RTP" and "symmetric
   RTCP".

   The UDP port number to receive media, and the UDP port to transmit
   media are both selected by the device that receives that media and
   transmits that media.  For unicast flows, the receive port is
   communicated to the remote peer (e.g., Session Description Protocol
   (SDP) [RFC4566] carried in SIP [RFC3261], Session Announcement
   Protocol (SAP) [RFC2974], or Megaco/H.248 [RFC3525]).

   There is no correspondence between the local RTP (or RTCP) port and
   the remote RTP (or RTCP) port.  That is, device "A" might choose its
   local transmit and receive port to be 1234.  Its peer, device "B", is
   not constrained to also use port 1234 for its port.  In fact, such a
   constraint is impossible to meet because device "B" might already be
   using that port for another application.

   The benefits of using one UDP port pair is described below in
   Section 4.

2.  Conventions Used in this Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

Wing                     Best Current Practice                  [Page 2]
RFC 4961                 Symmetric RTP and RTCP                July 2007

3.  Definition of Symmetric RTP and Symmetric RTCP

   A device supports symmetric RTP if it selects, communicates, and uses
   IP addresses and port numbers such that, when receiving a
   bidirectional RTP media stream on UDP port "A" and IP address "a", it
   also transmits RTP media for that stream from the same source UDP
   port "A" and IP address "a".  That is, it uses the same UDP port to
   transmit and receive one RTP stream.

   A device that doesn't support symmetric RTP would transmit RTP from a
   different port, or from a different IP address, than the port and IP
   address used to receive RTP for that bidirectional media steam.

   A device supports symmetric RTCP if it selects, communicates, and
   uses IP addresses and port numbers such that, when receiving RTCP