Symmetric RTP / RTP Control Protocol (RTCP)
RFC 4961
Document | Type |
RFC - Best Current Practice
(July 2007; No errata)
Also known as BCP 131
Was draft-wing-behave-symmetric-rtprtcp (individual in gen area)
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Author | Dan Wing | ||
Last updated | 2015-10-14 | ||
Stream | IETF | ||
Formats | plain text html pdf htmlized bibtex | ||
Reviews | |||
Stream | WG state | (None) | |
Document shepherd | No shepherd assigned | ||
IESG | IESG state | RFC 4961 (Best Current Practice) | |
Consensus Boilerplate | Unknown | ||
Telechat date | |||
Responsible AD | Magnus Westerlund | ||
Send notices to | (None) |
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 RTCPShow full document text