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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: BCP                                      France Telecom
Expires: August 3, 2009                                 January 30, 2009


    Application Mechanism for maintaining alive the Network Address
           Translator (NAT) mappings associated to RTP flows.
                  draft-ietf-avt-app-rtp-keepalive-05

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
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   This Internet-Draft will expire on August 3, 2009.

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   Copyright (c) 2009 IETF Trust and the persons identified as the
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Abstract

   This document lists the different mechanisms that enable applications



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   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.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Requirements . . . . . . . . . . . . . . . . . . . . . . . . .  4
   4.  List of Alternatives for Performing RTP Keepalive  . . . . . .  5
     4.1.  Transport Packet of 0-byte . . . . . . . . . . . . . . . .  5
     4.2.  RTP Packet with Comfort Noise Payload  . . . . . . . . . .  5
     4.3.  RTCP Packets Multiplexed with RTP Packets  . . . . . . . .  6
     4.4.  STUN Indication Packet . . . . . . . . . . . . . . . . . .  6
     4.5.  RTP Packet with Incorrect Version Number . . . . . . . . .  6
     4.6.  RTP Packet with Unknown Payload Type . . . . . . . . . . .  7
   5.  Recommended Solution for Keepalive Mechanism . . . . . . . . .  7
   6.  Media Format Exceptions  . . . . . . . . . . . . . . . . . . .  8
     6.1.  Real-time Text Payload Format Keepalive Mechanism  . . . .  8
   7.  Timing and Transport Considerations  . . . . . . . . . . . . .  8
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  9
     9.1.  Registration of the SDP 'rtp-keepalive' Attribute  . . . .  9
   10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  9
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     11.1. Normative references . . . . . . . . . . . . . . . . . . . 10
     11.2. Informative references . . . . . . . . . . . . . . . . . . 10
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11





















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1.  Introduction

   Documents [RFC4787] and [RFC5382] 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.

   When applied to applications using RTP [RFC3550], the RTP media
   stream packets themselves normally fulfill this requirement.  However
   there exist some cases where RTP does not generate the minimum
   required 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 [RFC3264].
      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 [RFC4103], 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 important to note that the above examples also require the
   agents to use symmetric RTP [RFC4961] 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.  Section 5 finally states
   the recommended solution for RTP keepalive.

   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 [DRAFT-ICE].

   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 [RFC3550] recommends a minimum interval of 5
         seconds and that "on hold" procedures of [RFC3264] 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 additional 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
   [RFC2119].


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.








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

   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  Impacts on existing software SHOULD be minimized.

   REQ-7  Remote peer SHOULD NOT be impacted.

   REQ-8  The support for RTP keepalive SHOULD be described in the SDP.

   REQ-9  More than one mechanism MAY exist.


4.  List of Alternatives for Performing RTP Keepalive

   This section lists, in no particular order, some alternatives that
   can be used to perform a keepalive message within RTP media streams.

4.1.  Transport Packet of 0-byte

   The application sends an empty transport packet (e.g.  UDP packet,
   DCCP packet ...).

   Cons:
   o  This alternative is specific to each transport protocol.

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

4.2.  RTP Packet with Comfort Noise Payload

   The application sends an RTP packet with a comfort-noise payload
   [RFC3389].

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





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4.3.  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) [DRAFT-RTP-RTCP].  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.

4.4.  STUN Indication Packet

   The application sends a STUN [RFC5389] Binding Indication packet as
   specified in ICE [DRAFT-ICE].

   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 [DRAFT-ICE].

4.5.  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 [RFC3550], 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] and [RFC3264] 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 not be used for RTP keepalive.

4.6.  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 [RFC3550] states
   that "a receiver MUST ignore packets with payload types that it does
   not understand".

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

   Recommendation:
   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), or specific to a media type
   (Section 4.2).  These mechanisms are thus NOT RECOMMENDED.

   Other mechanisms are dependent on the capabilities of the peer
   (Section 4.3, Section 4.4).  Among these mechanisms, RTCP packets
   multiplexed with RTP packets (Section 4.3) is desirable because it
   reduces the number of ports used.

   The RECOMMENDED solution is thus the "RTCP packets multiplexed with
   RTP packets" (Section 4.3).  However, when this mechanism cannot be
   negotiated, it is RECOMMENDED to use the fallback "RTP Packet with
   Unknown Payload Type" mechanism (Section 4.6) as it will always work.

   When using SDP, an agent supporting the fallback solution MUST
   indicate its support by adding an a=rtp-keepalive SDP attribute.
   This attribute is declarative only and can not be negotiated.  It
   indicates that the fallback solution will be used if the recommended



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   solution can not be used.

   When using the SDP offer-answer [RFC3264], the agent SHOULD offer
   both the "a=rtcp-mux" and "a=rtp-keepalive" attributes.  If "a=rtcp-
   mux"attribute is present in the answer, the agent uses RTCP packets
   being multiplexed on the RTP port as a keepalive.  Otherwise, the
   agent uses RTP packets with an invalid payload type as a keepalive.


6.  Media Format Exceptions

   When a given media format does not allow the keepalive solution
   recommended in Section 5, an alternative mechanism SHOULD be defined
   in the payload format specification for this media format.

   Real-time text payload format [RFC4103] is an example of such a media
   format.

6.1.  Real-time Text Payload Format Keepalive Mechanism

   Real-time text payload format [RFC4103] does not allow to use
   different payloads within a same RTP session, so the fallback
   mechanism does not work.

   For real-time text, the RECOMMENDED solution is 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
   [RFC4103].


7.  Timing and Transport Considerations

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

   When using the "RTCP packets multiplexed with RTP packets" solution
   for keepalive, Tr MUST comply with the RTCP timing rules of
   [RFC3550].  The fallback "RTP Packet with Unknown Payload Type"
   solution uses RTP, and thus does not have these RTCP constraints.

   Keepalive 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



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   regular RTP packets.


8.  Security Considerations

   The RTP keepalive packets are sent on the same path as regular RTP
   media packets and may be perceived as an attack by the peer.  The
   peer needs to react as described by [RFC3550] by ignoring them.  Not
   only does it not hurt to drop them, but not dropping them would be
   harmful.


9.  IANA Considerations

9.1.  Registration of the SDP 'rtp-keepalive' Attribute

   This section instructs the IANA to register the following SDP att-
   field under the Session Description Protocol (SDP) Parameters
   registry:

   Contact name: xavier.marjou@orange-ftgroup.com

   Attribute name: rtp-keepalive

   Long-form attribute name: RTP keepalive

   Type of attribute Media level

   Subject to charset: No

   Purpose of attribute: The 'rtp-keepalive' attribute declares that the
   agent supports the fallback RTP keepalive mechanism (Section 4.6).

   Allowed attribute values: None


10.  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, Hadriel Kaplan, Randell Jesup,
   Remi Denis-Courmont, and Steve Casner for their useful inputs and
   comments.


11.  References





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11.1.  Normative references

   [DRAFT-RTP-RTCP]
              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.

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

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

   [RFC4961]  Wing, D., "Symmetric RTP / RTP Control Protocol (RTCP)",
              BCP 131, RFC 4961, July 2007.

11.2.  Informative references

   [DRAFT-ICE]
              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.

   [RFC3264]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
              with Session Description Protocol (SDP)", RFC 3264,
              June 2002.

   [RFC3389]  Zopf, R., "Real-time Transport Protocol (RTP) Payload for
              Comfort Noise (CN)", RFC 3389, September 2002.

   [RFC4103]  Hellstrom, G. and P. Jones, "RTP Payload for Text
              Conversation", RFC 4103, June 2005.

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

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

   [RFC5382]  Guha, S., Biswas, K., Ford, B., Sivakumar, S., and P.
              Srisuresh, "NAT Behavioral Requirements for TCP", BCP 142,
              RFC 5382, October 2008.

   [RFC5389]  Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,



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              "Session Traversal Utilities for NAT (STUN)", RFC 5389,
              October 2008.


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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