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Versions: (draft-marjou-behave-app-rtp-keepalive) 00 01 02 03 04 05 06 07 08 09 10 RFC 6263

Network Working Group                                          X. Marjou
Internet-Draft                                                A. Sollaud
Intended status: Best Current                             France Telecom
Practice                                                February 8, 2008
Expires: August 11, 2008


    Application Mechanism for maintaining alive the Network Address
           Translator (NAT) mappings associated to RTP flows.
                  draft-ietf-avt-app-rtp-keepalive-02

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Copyright Notice

   Copyright (C) The IETF Trust (2008).

Abstract

   This document lists the different mechanisms that enable applications
   using Real-time Transport Protocol (RTP) to maintain their RTP
   Network Address Translator (NAT) mappings alive.  It also makes a
   recommendation for a preferred mechanism.  This document is not
   applicable to Interactive Connectivity Establishment (ICE) agents.




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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Requirements . . . . . . . . . . . . . . . . . . . . . . . . .  4
   4.  List of Alternatives for Performing RTP Keepalive  . . . . . .  5
     4.1.  UDP Packet of 0-byte . . . . . . . . . . . . . . . . . . .  5
     4.2.  DCCP Packet of 0-byte  . . . . . . . . . . . . . . . . . .  5
     4.3.  RTP Packet with Comfort Noise Payload  . . . . . . . . . .  5
     4.4.  RTCP Packets Multiplexed with RTP Packets  . . . . . . . .  6
     4.5.  STUN Indication Packet . . . . . . . . . . . . . . . . . .  6
     4.6.  RTP Packet with No-OP Payload  . . . . . . . . . . . . . .  6
     4.7.  RTP Packet with Incorrect Version Number . . . . . . . . .  7
     4.8.  RTP Packet with Unknown Payload Type . . . . . . . . . . .  7
   5.  Recommended solution for keepalive mechanism . . . . . . . . .  8
   6.  Additional considerations  . . . . . . . . . . . . . . . . . .  8
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . .  8
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  9
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  9
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     10.1. Normative references . . . . . . . . . . . . . . . . . . .  9
     10.2. Informative references . . . . . . . . . . . . . . . . . .  9
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10
   Intellectual Property and Copyright Statements . . . . . . . . . . 11



























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1.  Introduction

   Documents [4] and [5] describe NAT behaviors and point out that two
   key aspects of NAT are mappings (a.k.a. bindings) and their
   refreshment.  This introduces a derived requirement for applications
   engaged in a multimedia session involving NAT traversal: they need to
   generate a minimum of flow activity in order to create NAT mappings
   and maintain them alive.

   When applied to applications using RTP [2], the RTP media stream
   packets themselves normally fulfill this requirement.  However there
   exist some cases where RTP do not generate a minimum flow activity.

   The examples are:

   o  In some RTP usages, such as SIP, agents can negotiate a
      unidirectional media stream by using the SDP "recvonly" attribute
      on one agent and "sendonly" on the peer, as defined in RFC 3264
      [7].  RFC 3264 directs implementations not to transmit media on
      the receiving agent.  In case the agent receiving the media is
      located in the private side of a NAT, it will never receive RTP
      packets from the public peer if the NAT mapping has not been
      created.

   o  Similarly, a bidirectional media stream can be "put on hold".
      This is accomplished by using the SDP "sendonly" or "inactive"
      attributes.  Again RFC 3264 directs implementations to cease
      transmission of media in these cases.  However, doing so may cause
      NAT bindings to timeout, and media won't be able to come off hold.

   o  In case of audio media, if silence suppression is in use, long
      periods of silence may cause media transmission to cease
      sufficiently long for NAT bindings to time out.

   o  Some RTP payload formats, such as the payload format for text
      conversation [12], may send packets so infrequently that the
      interval exceeds the NAT binding timeouts.

   To solve these problems, an agent therefore needs to periodically
   send keepalive data within the outgoing RTP session of an RTP media
   stream regardless of whether the media stream is currently inactive,
   sendonly, recvonly or sendrecv, and regardless of the presence or
   value of the bandwidth attribute.

   It is also important to note that the above examples also require the
   agents to use symmetric RTP [13] in addition to RTP keepalive.

   This document first states the requirements that must be supported to



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   perform RTP keepalives (Section 3).  In a second step, the document
   reports the different mechanisms to overcome this problem (Section 4)
   and makes recommendations about their use.

   The scope of the draft is limited to non-ICE agents.  Indeed, ICE
   agents need to follow the RTP keepalive mechanism specified in the
   ICE specification [6].

   The scope of the draft is also limited to RTP flows.  In particular,
   this document does not address keepalive activity related to:

   o  Session signaling flows, such as the Session Initiation Protocol
      (SIP).

   o  RTCP flows.
      *  Recall that [2] recommends a minimum interval of 5 seconds and
         that "on hold" procedures of [7] do not impact RTCP
         transmissions.  Therefore, when in use, there is always some
         RTCP flow activity.

   Note that if a given media uses a codec that already integrates a
   keepalive mechanism, no keepalive mechanism is required at the RTP
   level.


2.  Terminology

   In this document, the key words "MUST", "MUST NOT", "REQUIRED",
   "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
   and "OPTIONAL" are to be interpreted as described in RFC 2119 [1].


3.  Requirements

   This section outlines the key requirements that need to be satisfied
   in order to provide RTP media keepalive.

   REQ-1  Some data is sent periodically within the outgoing RTP session
          for the whole duration of the RTP media stream.

   REQ-2  Any type of transport (e.g.  UDP, TCP) MUST be supported.

   REQ-3  Any media type (e.g. audio, video, text) MUST be supported.








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   REQ-4  Any media format (e.g.  G.711, H.263) MUST be supported.

   REQ-5  Session signaling protocols SHOULD not be impacted.

   REQ-6  Session description protocols SHOULD not be impacted.

   REQ-7  Impacts on existing software SHOULD be minimized.

   REQ-8  Remote peer SHOULD not be impacted.

   REQ-9  More than one mechanism MAY exist.


4.  List of Alternatives for Performing RTP Keepalive

   This section lists some alternatives that can be used in order to
   perform a keepalive message within RTP media streams.

4.1.  UDP Packet of 0-byte

   The application sends an empty UDP packet.

   Cons:
   o  This alternative is specific to UDP.

   Recommendation:
   o  This method should not be used for RTP keepalive.

4.2.  DCCP Packet of 0-byte

   The application sends an empty DCCP packet.

   Cons:
   o  This alternative is specific to DCCP.

   Recommendation:
   o  This method should not be used for RTP keepalive.

4.3.  RTP Packet with Comfort Noise Payload

   The application sends an RTP packet with a comfort-noise payload
   [10].

   Cons:
   o  This alternative is limited to audio formats only.
   o  Comfort Noise needs to be supported by the remote peer.





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   o  Comfort Noise needs to be signalled in SDP offer/answer.
   o  The peer is likely to render comfort noise at the other side, so
      the content of the payload (the noise level) needs to be carefully
      chosen.

   Recommendation:
   o  This method may be used when the media allows for it.

4.4.  RTCP Packets Multiplexed with RTP Packets

   The application sends RTCP packets in the RTP media path itself (i.e.
   same tuples for both RTP and RTCP packets) [3].  RTCP packets
   therefore maintain the NAT mappings open.

   Cons:
   o  Multiplexing RTP and RTCP must be supported by the remote peer.
   o  Multiplexing RTP and RTCP must be signalled in SDP offer/answer.
   o  Some RTCP monitoring tools expect that RTCP are not multiplexed.

   Recommendation:
   o  This method must only be used for RTP keepalive when negotiated
      between agents.

4.5.  STUN Indication Packet

   The application sends a STUN [8] Binding Indication packet as
   specified in ICE [6].

   Thanks to the RTP validity check, STUN packets will be ignored by the
   RTP stack.

   Cons:
   o  The sending agent needs support STUN.

   Recommendation:
   o  This method must only be used for sessions between ICE agents, as
      specified in [6].

4.6.  RTP Packet with No-OP Payload

   The application sends an RTP No-OP payload [11] .

   Cons:
   o  This payload type needs to be supported by the remote peer.
   o  This payload type needs to be signalled in the SDP offer/answer.
   o  RFC4566 [9] and RFC3264 [7] mandate not to send media with
      inactive and recvonly attributes.




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   Recommendation:
   o  This method may be used for RTP keepalive when negotiated between
      agents.

   [Authors note: this draft is now expired, do we keep it here?]

4.7.  RTP Packet with Incorrect Version Number

   The application sends an RTP packet with an incorrect version number,
   which value is zero.

   Based on RTP specification [2], the peer should perform a header
   validity check, and therefore ignore these types of packet.

   Cons:
   o  Only four version numbers are possible.  Using one of them for RTP
      keepalive would be wasteful.
   o  RFC4566 [9] and RFC3264 [7] mandate not to send media with
      inactive and recvonly attributes.

   Recommendation:
   o  This method should not be used for RTP keepalive.

4.8.  RTP Packet with Unknown Payload Type

   The application sends an RTP packet of 0 length with a dynamic
   payload type that has not been negotiated by the peers (e.g. not
   negotiated within the SDP offer/answer, and thus not mapped to any
   media format).

   The sequence number is incremented by one for each packet, as it is
   sent within the same RTP session as the actual media.  The timestamp
   contains the same value a media packet would have at this time.  The
   marker bit is not significant for the keepalive packets and is thus
   set to zero.

   Normally the peer will ignore this packet, as RTP [2] states that "a
   receiver MUST ignore packets with payload types that it does not
   understand".

   Cons:
   o  RFC4566 [9] and RFC3264 [7] mandate not to send media with
      inactive and recvonly attributes, however this is mitigated as no
      real media is sent with this mechanism.

   Recommendation:





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   o  This method should be used for RTP keepalive.


5.  Recommended solution for keepalive mechanism

   Some mechanisms do not meet the requirements as they are either
   specific to the transport (Section 4.1, Section 4.2), specific to a
   media type (Section 4.3).  These mechanisms are thus NOT RECOMMENDED.

   Other mechanisms are dependent on the capabilities of the peer
   (Section 4.4, Section 4.5, Section 4.6).  Among these mechanisms,
   RTCP packets multiplexed with RTP packets (Section 4.4) is desirable
   because it reduces the number of ports used.

   The RECOMMENDED solution is thus the "RTCP packets multiplexed with
   RTP packets" (Section 4.4).  However, when this mechanism cannot be
   negotiated, it is RECOMMENDED to use the fallback "RTP Packet with
   Unknown Payload Type" mechanism of Section 4.8 as it will always
   work.

   The exception to the above recommendation is for real-time text as
   [12] does not allow to use different payloads within a same RTP
   session.  For real-time text, the RECOMMENDED solution is thus the
   "RTCP packets multiplexed with RTP packets".  When this mechanism
   cannot be negotiated, it is RECOMMENDED to use an empty T140block
   containing no data in the same manner as for the idle procedure
   defined in [12].


6.  Additional considerations

   An application supporting this specification must transmit keepalive
   packets every Tr seconds during the whole duration of the media
   session.  Tr SHOULD be configurable, and otherwise MUST default to 15
   seconds.

   Keepalives packets within a particular RTP session MUST use the tuple
   (source IP address, source TCP/UDP ports, target IP address, target
   TCP/UDP Port) of the regular RTP packets.

   The agent SHOULD only send RTP keepalive when it does not send
   regular RTP packets.


7.  Security Considerations

   The keepalive packets are sent on the same path as regular RTP media
   packets.  In addition, they do not convey any valuable information.



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   So the mechanism described here does not imply new security issues.


8.  IANA Considerations

   This document has no actions for IANA.


9.  Acknowledgements

   Jonathan Rosenberg provided the major inputs for this draft via the
   ICE specification.  In addition, thanks to Alfred E. Heggestad, Colin
   Perkins, Dan Wing, Gunnar Hellstrom, and Randell Jesup for their
   useful inputs and comments.


10.  References

10.1.  Normative references

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

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

   [3]   Perkins, C. and M. Magnus, "Multiplexing RTP Data and Control
         Packets on a Single Port", draft-ietf-avt-rtp-and-rtcp-mux-07
         (work in progress), August 2007.

10.2.  Informative references

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

   [5]   Guha, S., Biswas, K., Ford, B., Francis, P., Sivarkumar, S.,
         and P. Srisuresh, "NAT Behavioral Requirements for TCP",
         draft-ietf-behave-tcp-07 (work in progress), April 2007.

   [6]   Rosenberg, J., "Interactive Connectivity Establishment (ICE): A
         Methodology for Network Address Translator (NAT) Traversal for
         Offer/Answer Protocols", draft-ietf-mmusic-ice-19 (work in
         progress), October 2007.

   [7]   Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
         Session Description Protocol (SDP)", RFC 3264, June 2002.



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   [8]   Rosenberg, J., Huitema, C., Mahy, R., and D. Wing, "Simple
         Traversal Underneath Network Address Translators (NAT) (STUN)",
         draft-ietf-behave-rfc3489bis-13 (work in progress),
         November 2007.

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

   [10]  Zopf, R., "Real-time Transport Protocol (RTP) Payload for
         Comfort Noise (CN)", RFC 3389, September 2002.

   [11]  Andreason, F., Oran, D., and D. Wing, "A No-OP Payload Format
         for RTP", draft-ietf-avt-rtp-no-op-04 (work in progress),
         May 2007.

   [12]  Hellstrom, G. and P. Jones, "RTP Payload for Text
         Conversation", RFC 4103, June 2005.

   [13]  Wing, D., "Symmetric RTP / RTP Control Protocol (RTCP)",
         BCP 131, RFC 4961, July 2007.


Authors' Addresses

   Xavier Marjou
   France Telecom
   2, avenue Pierre Marzin
   Lannion  22307
   France

   Email: xavier.marjou@orange-ftgroup.com


   Aurelien Sollaud
   France Telecom
   2, avenue Pierre Marzin
   Lannion  22307
   France

   Email: aurelien.sollaud@orange-ftgroup.com











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