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Versions: 00 01 02 03 RFC 4961

BEHAVE                                                           D. Wing
Internet-Draft                                             Cisco Systems
Intended status:  Best Current                         February 26, 2007
Practice
Expires:  August 30, 2007


        Common Local Transmit and Receive Ports (Symmetric RTP)
                 draft-wing-behave-symmetric-rtprtcp-02

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   This Internet-Draft will expire on August 30, 2007.

Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

   This document describes common local transmit and receive ports,
   commonly called "symmetric RTP" and "symmetric RTCP", and explains
   the advantages of using common local transmit and receive ports.







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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Conventions Used in this Document . . . . . . . . . . . . . . . 3
   3.  Definition of Symmetric RTP and Symmetric RTCP  . . . . . . . . 4
   4.  Recommended Usage . . . . . . . . . . . . . . . . . . . . . . . 4
   5.  Security Considerations . . . . . . . . . . . . . . . . . . . . 5
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 5
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 5
     8.1.  Normative References  . . . . . . . . . . . . . . . . . . . 5
     8.2.  Informational References  . . . . . . . . . . . . . . . . . 5
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . . 6
   Intellectual Property and Copyright Statements  . . . . . . . . . . 7





































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1.  Introduction

   TCP [RFC0793], which is inheriently bidirectional, uses common local
   transmit and receive ports.  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 inheriently bidirectional and UDP does not
   require common local transmit and receive ports.  Rather, some UDP
   applications use common local transmit and receive ports (DNS
   [RFC1035]), some applications use different local transmit and
   receive ports with explicit signaling (TFTP [RFC1350]), and other
   applications don't specify the behavior for local transmit and
   receive ports (RTP [RFC3550]).

   Because RTP and RTCP are not inheriently bi-directional protocols,
   and UDP isn't a bi-directional protocol, the usefulness of common
   local transmit and receive ports has been generally ignored for RTP
   and RTCP.  Many firewalls, NATs [RFC3022], and RTP implementations
   expect symmetric RTP, and do not work in the presense of non-
   symmetric 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., SDP [RFC4566] carried in SIP
   [RFC3261], SAP [RFC2974], or MGCP [RFC3435]).

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

   The benefits of common local transmit and receive ports 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].






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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 bi-
   directional 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 a common local
   transmit and receive port for RTP.

   A device which 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 bi-directional media steam.

   A device supports symmetric RTCP if it selects, communicates, and
   uses IP addresses and port numbers such that, when receiving RTCP
   packets for a media stream on UDP port "B" and IP address "b", it
   also transmits RTCP packets for that stream from the same source UDP
   port "B" and IP address "b".  That is, it uses a common local
   transmit and receive port for RTCP.

   A device which doesn't support symmetric RTCP would transmit RTCP
   from a different port, or from a different IP address, than the port
   and IP address used to receive RTCP.


4.  Recommended Usage

   There are two specific instances where symmetric RTP and symmetric
   RTCP are required.

   The first instance is NATs that lack integrated Application Layer
   Gateway (ALG) functionality.  Such NATs require that endpoints use
   UDP port symmetry to establish bi-directional traffic.  This
   requirement exists for all types of NATs described in section 4 of
   [RFC4787].  ALGs are defined in section 4.4 of [RFC3022].

   The second instance is Session Border Controllers (SBCs) and other
   forms of RTP and RTCP relays (e.g., [I-D.ietf-behave-turn]).  Media
   relays are necessary to establish bi-directional UDP communication
   across a NAT that is 'Address-Dependent' or 'Address and Port-
   Dependent' [RFC4787].  However, even with a media relay, UDP port
   symmetry is still required to traverse such a NAT.

   There are other instances where symmetric RTP and symmetric RTCP are
   helpful, but not required.  For example, if a firewall can expect
   symmetric RTP and symmetric RTCP then the firewall's dynamic per-call
   port filter list can be more restrictive compared to non-symmetric
   RTP and non-symmetric RTCP.  Symmetric RTP and symmetric RTCP can



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   also ease debugging and troubleshooting.

   Other UDP-based protocols can also benefit from common local transmit
   and receive ports.

   There are no known cases where symmetric RTP or symmetric RTCP are
   harmful.

   For these reasons it is RECOMMENDED that symmetric RTP and symmetric
   RTCP always be used for bi-directional RTP media streams.


5.  Security Considerations

   There is no additional security exposure if a host uses symmetric RTP
   or symmetric RTCP.


6.  IANA Considerations

   This document doesn't require any IANA registrations.


7.  Acknowledgments

   The author thanks Francois Audet, Sunil Bhargo, Francois Le Faucheur,
   Cullen Jennings, Benny Rodrig, Robert Sparks, and Joe Stone for their
   assistance with this document.


8.  References

8.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

8.2.  Informational References

   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, July 2003.

   [RFC4787]  Audet, F. and C. Jennings, "Network Address Translation
              (NAT) Behavioral Requirements for Unicast UDP", BCP 127,
              RFC 4787, January 2007.

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,



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              RFC 793, September 1981.

   [RFC3022]  Srisuresh, P. and K. Egevang, "Traditional IP Network
              Address Translator (Traditional NAT)", RFC 3022,
              January 2001.

   [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
              Description Protocol", RFC 4566, July 2006.

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, November 1987.

   [RFC1350]  Sollins, K., "The TFTP Protocol (Revision 2)", STD 33,
              RFC 1350, July 1992.

   [I-D.ietf-behave-turn]
              Rosenberg, J., "Obtaining Relay Addresses from Simple
              Traversal Underneath NAT (STUN)",
              draft-ietf-behave-turn-02 (work in progress),
              October 2006.

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              June 2002.

   [RFC2974]  Handley, M., Perkins, C., and E. Whelan, "Session
              Announcement Protocol", RFC 2974, October 2000.

   [RFC3435]  Andreasen, F. and B. Foster, "Media Gateway Control
              Protocol (MGCP) Version 1.0", RFC 3435, January 2003.


Author's Address

   Dan Wing
   Cisco Systems
   170 West Tasman Drive
   San Jose, CA  95134
   USA

   Email:  dwing@cisco.com









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Full Copyright Statement

   Copyright (C) The IETF Trust (2007).

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Acknowledgment

   Funding for the RFC Editor function is provided by the IETF
   Administrative Support Activity (IASA).





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