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AVT                                                         F. Andreasen
Internet-Draft                                                   D. Oran
Expires: November 18, 2005                                       D. Wing
                                                     Cisco Systems, Inc.
                                                            May 17, 2005


                     A No-Op Payload Format for RTP
                       draft-wing-avt-rtp-noop-03

Status of this Memo

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   This Internet-Draft will expire on November 18, 2005.

Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   This document defines an no-op payload format for the Real-time
   Transport Protocol (RTP), and a mechanism to request transmission of
   an early RTCP report.  This can be used to verify RTP connectivity
   and to keep Network Address Translator (NAT) bindings and Firewall
   pinholes open.





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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  RTP Payload Format for No-Op . . . . . . . . . . . . . . . . .  4
     2.1   Registration . . . . . . . . . . . . . . . . . . . . . . .  4
     2.2   Use of RTP Header Fields . . . . . . . . . . . . . . . . .  4
     2.3   Payload Format . . . . . . . . . . . . . . . . . . . . . .  4
     2.4   Sender Operation . . . . . . . . . . . . . . . . . . . . .  5
     2.5   Mixer, Translator Operation  . . . . . . . . . . . . . . .  5
     2.6   Receiver Operation . . . . . . . . . . . . . . . . . . . .  5
     2.7   Indication of No-OP Capability using SDP . . . . . . . . .  6
   3.  Example SDP Offer/Answer . . . . . . . . . . . . . . . . . . .  6
   4.  MIME Registration  . . . . . . . . . . . . . . . . . . . . . .  7
     4.1   audio/no-op  . . . . . . . . . . . . . . . . . . . . . . .  7
     4.2   video/no-op  . . . . . . . . . . . . . . . . . . . . . . .  8
     4.3   text/no-op . . . . . . . . . . . . . . . . . . . . . . . .  8
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  9
   7.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . .  9
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     8.1   Normative References . . . . . . . . . . . . . . . . . . .  9
     8.2   Informational References . . . . . . . . . . . . . . . . . 10
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 10
       Intellectual Property and Copyright Statements . . . . . . . . 12



























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1.  Introduction

   This memo defines a new RTP payload format called "no-op".  This
   payload behaves like a normal RTP payload, except the RTP packet is
   not used to play out media.  It is also explicitly designed to
   interact constructively with the RTCP feedback profile [6].

   This new payload format is useful for:

   o  media session reception quality assessment, such as at the
      beginning of a session;
   o  keepalives to keep NAT bindings and/or firewall pinholes open when
      RTP media traffic is not otherwise being transmitted.

   In addition it has a number of uses whose utility is speculative but
   for which it is easy pressed into service:

   o  measurement-based admission control by probing available
      bandwidth, and
   o  synthetic load generation for performance testing and other
      minimally-intrusive instrumentation.

   When an endpoint mas a media stream marked as 'recvonly' or
   'inactive' the endpoint is not supposed to send any media (i.e.  RTP
   packets).  However, to keep a NAT binding alive, the endpoint will
   need to send packets over the RTP and RTCP ports.  RTP No-Op is
   ideally suited to this.  In comparison, if one participant in an
   audio multicast conference has a 'recvonly' or 'inactive' media
   stream yet occasionally sends comfort noise packets in order to keep
   its NAT binding open, these comfort noise packets are interpreted as
   audio packets by receivers and mixers which can cause undesirable
   behavior -- such as selection of the primary speaker or the playout
   of comfort noise when no audio should be played.

   Unlike Comfort noise [9], which is specific to voice RTP streams, RTP
   No-Op is applicable to any kind of RTP stream including video, audio,
   realtime text, or any other media type that would benefit from the
   capabilities listed above.  This gives RTP No-Op an advantage as a
   NAT keepalive mechanism.  Certain functions and RTP payload types can
   use RTP No-Op without re-inventing their own payload-specific NAT
   keepalive mechanism -- such as video muting, Clearmode [10], and text
   [11].

   Some audio codecs have their own 'silence' packets.  However, some
   codecs only send such 'silence' packets if the noise floor changes;
   G.729b [12] is an example of such a codec.  RTP No-Op allows the RTP
   stack itself, rather than the codec, to send periodic packets as a
   keepalive mechanism.



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   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 RFC 2119 [1].

2.  RTP Payload Format for No-Op

2.1  Registration

   The RTP payload format is designated as "no-op" and the MIME types
   are "audio/no-op", "video/no-op", and "text/no-op".  The default
   clock rate is 8000 Hz, but other rates MAY be used.  In accordance
   with current practice, this payload format does not have a static
   payload type number, but uses a RTP payload type number established
   dynamically out-of-band, e.g. through SDP [4].

2.2  Use of RTP Header Fields

   Timestamp: The RTP timestamp reflects the measurement point for the
      current packet.  The receiver calculates jitter for RTCP receiver
      reports based on all packets with a given timestamp.  Note: The
      jitter value should primarily be used as a means for comparing the
      reception quality between two users or two time-periods, not as an
      absolute measure.
   Marker bit: The RTP marker bit has no special significance for this
      payload type.

2.3  Payload Format

   The payload format is shown below.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |R|                         reserved                            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      padding (OPTIONAL)                       |
      |                             ....                              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The payload contains at least 4 bytes.  The first 32 bits are defined
   as follows:

   bit 0:      "R", "Request Early RTCP", is used to request invocation
               of RTCP feedback by timely transmission of an RTCP report
               (see Section 2.6).






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   bits 1-31:  Reserved; contents are ignored.

   Additional padding bytes MAY be appended up to the ptime value in SDP
   (see Section 2.7).  These bytes are ignored.  Padding may be useful
   to generate RTP packets that are the same size as a normal media
   payload.

2.4  Sender Operation

   As discussed in the introduction, endpoints MUST occasionally send a
   packet to their RTP and RTCP peer to keep NAT and firewall bindings
   active, even if the media stream is marked 'recvonly' or 'inactive'.
   If no other RTP packet has been sent for approximately 30 seconds, an
   RTP NoOp packet SHOULD be sent.  It is permissible to send a NoOp
   packet even for media streams marked 'recvonly' or 'inactive'.

2.5  Mixer, Translator Operation

   An RTP mixer or unicast-to-unicast RTP translator SHOULD forward RTP
   No-Op payload packets normally.  A unicast-to-multicast RTP
   translator SHOULD replicate RTP No-Op payload packets normally.

   A multicast-to-unicast RTP translator SHOULD NOT replicate an RTP
   No-Op packet with the Request Early RTCP bit set unless:

   1.  all receivers are known to be operating under the bandwidth
       limitations rules of [6], and
   2.  the restriction of applicability to "small groups" in [6] is
       observed
   Otherwise the sender may be flooded with RTCP reports.

2.6  Receiver Operation

   Upon receipt of an RTP packet with the No-Op payload format and the
   'Request Early RTCP Report' bit set to 0, the receiver performs
   normal RTP receive operations on it -- incrementing the RTP receive
   counter, calculating jitter, and so on.  The receiver then discards
   the packet -- it is not used to play out media.

   Upon receipt of an RTP packet with the No-Op payload format and the
   'Request Early RTCP Report' bit set to 1, the receiver adjusts
   counters as described above and then also performs the following
   steps (with reference to the definitions and procedures of the AVPF
   profile [6]):

   1.  ascertains whether the associated RTP session is operating under
       the AVPF RTP profile (or one derived from it via combination with
       another RTP profile).  If not the receiver takes no further



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       action on this packet - specifically, if the RTP/AVPF profile (or
       one derived from it) is not used the receiver MUST NOT send an
       early RTCP report.  If so, it continues as follows.
   2.  generates a feedback "Event" which in turn may trigger the
       generation of a "FB message".
   3.  sends the FB message as an "early RTCP packet" assuming the
       bandwidth constraints for feedback messages are satisfied.
   4.  Otherwise, takes no further action

2.7  Indication of No-OP Capability using SDP

   Senders and receivers may indicate support for the No-Op payload
   format, for example, by using the Session Description Protocol SDP
   [4].  If the payload format is being used for connectivity
   verification (e.g. in conjunction with [5]) senders and receivers
   MUST advertise the AVPF profile (or a profile used in combination
   with it).

   The default packetization interval for this payload type is 20ms
   (ptime:20) but alternate values can be advertised in SDP using the
   ptime attribute value [4].

3.  Example SDP Offer/Answer

   Offer:
         v=0
         o=alice 2890844526 2890844526 IN IP4 host.atlanta.example.com
         s=-
         c=IN IP4 host.atlanta.example.com
         t=0 0
         m=audio 49170 RTP/AVPF 0 33
         a=rtpmap:0 PCMU/8000
         a=rtpmap:33 no-op/8000
         m=video 41372 RTP/AVPF 31 36
         a=rtpmap:31 H261/90000
         a=rtpmap:36 no-op/90000















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   Answer:
         v=0
         o=bob 2808844564 2808844564 IN IP4 host.biloxi.example.com
         s=-
         c=IN IP4 host.biloxi.example.com
         t=0 0
         m=audio 59174 RTP/AVPF 0 33
         a=rtpmap:0 PCMU/8000
         a=rtpmap:33 no-op/8000
         m=video 59170 RTP/AVPF 32 36
         a=rtpmap:31 H261/90000
         a=rtpmap:36 no-op/90000

4.  MIME Registration

   This section registers MIME types for audio/no-op, video/no-op, and
   text/no-op.

4.1  audio/no-op

   MIME media type name: audio

   MIME subtype name: no-op

   Required parameters: none

   Optional parameters: none

   Encoding considerations: This type is only defined for transfer via
   RTP [2] and Secure RTP [3].

   Security considerations: See Section 5, "Security Considerations", in
   this document.

   Interoperability considerations: none

   Published specification: This document.

   Applications which use this media: The "no-op" application subtype is
   used to maintain network state or verify network connectivity, when a
   more traditional RTP payload type cannot be used.

   Additional information:

   1.  Magic number(s): N/A
   2.  File extension(s): N/A





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   3.  Macintosh file type code: N/A

4.2  video/no-op

   MIME media type name: video

   MIME subtype name: no-op

   Required parameters: none

   Optional parameters: none

   Encoding considerations: This type is only defined for transfer via
   RTP [2] and Secure RTP [3].

   Security considerations: See Section 5, "Security Considerations", in
   this document.

   Interoperability considerations: none

   Published specification: This document.

   Applications which use this media: The "no-op" application subtype is
   used to maintain network state or verify network connectivity, when a
   more traditional RTP payload type cannot be used.

   Additional information:

   1.  Magic number(s): N/A
   2.  File extension(s): N/A
   3.  Macintosh file type code: N/A

4.3  text/no-op

   MIME media type name: text

   MIME subtype name: no-op

   Required parameters: none

   Optional parameters: none

   Encoding considerations: This type is only defined for transfer via
   RTP [2] and Secure RTP [3].

   Security considerations: See Section 5, "Security Considerations", in
   this document.




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   Interoperability considerations: none

   Published specification: This document.

   Applications which use this media: The "no-op" application subtype is
   used to maintain network state or verify network connectivity, when a
   more traditional RTP payload type cannot be used.

   Additional information:

   1.  Magic number(s): N/A
   2.  File extension(s): N/A
   3.  Macintosh file type code: N/A

5.  Security Considerations

   Without security of the RTP stream (via SRTP [3], IPsec [8], or other
   means), it is possible for an attacker to spoof RTP packets,
   including this new packet type.  As this new RTP payload type
   includes a method to request early transmission of RTCP, this could
   be used to cause endpoints to flood the network with RTCP reports.
   Thus, the RTCP transmissions MUST NOT exceed the bandwidth
   recommendations described in section 6.3 of RFC3550 [2].

6.  IANA Considerations

   IANA is requested to make MIME type registrations as specified above
   in Section 4

7.  Acknowledgments

   Thanks to Henning Schulzrinne for suggesting using RTCP as a feedback
   mechanism.

8.  References

8.1  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]  Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
        Norrman, "The Secure Real-time Transport Protocol (SRTP)",
        RFC 3711, March 2004.



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   [4]  Handley, M. and V. Jacobson, "SDP: Session Description
        Protocol", RFC 2327, April 1998.

   [5]  Andreasen, F., "Connectivity Preconditions for Session
        Description Protocol Media Streams",
        draft-andreasen-mmusic-connectivityprecondition-02 (work in
        progress), February 2005.

   [6]  Ott, J. and S. Wenger, "Extended RTP Profile for RTCP-based
        Feedback(RTP/AVPF)", draft-ietf-avt-rtcp-feedback-11 (work in
        progress), August 2004.

   [7]  Rosenberg, J., "Interactive Connectivity Establishment (ICE): A
        Methodology for Network  Address Translator (NAT) Traversal for
        Multimedia Session Establishment Protocols",
        draft-ietf-mmusic-ice-04 (work in progress), February 2005.

8.2  Informational References

   [8]   Kent, S. and R. Atkinson, "Security Architecture for the
         Internet Protocol", RFC 2401, November 1998.

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

   [10]  Kreuter, R., "RTP Payload Format for a 64 kbit/s Transparent
         Call", RFC 4040, April 2005.

   [11]  Hellstrom, G., "RTP Payload for Text Conversation", RFC 2793,
         May 2000.

   [12]  International Telecommunications Union, "G.729 Annex B",
         November 1999,
         <http://www.itu.int/ITU-T/publications/recs.html>.


Authors' Addresses

   Flemming Andreasen
   Cisco Systems, Inc.
   499 Thornall Street, 8th Floor
   Edison, NJ  08837
   USA

   Email: fandreas@cisco.com






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   David Oran
   Cisco Systems, Inc.
   7 Ladyslipper Lane
   Acton, MA  01720
   USA

   Email: oran@cisco.com


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

   Email: dwing@cisco.com



































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Acknowledgment

   Funding for the RFC Editor function is currently provided by the
   Internet Society.















































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