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Versions: 00

Network Working Group                                          E. Berger
Internet-Draft                                             S. Nandakumar
Intended status: Standards Track                           Cisco Systems
Expires: April 24, 2014                                 October 21, 2013


                     Frame marking for RTP packets
                  draft-avtext-berger-framemarking-00

Abstract

   This document describes a mechanisms to provide frame markings to
   allow RTP switches to perform stream operations on encrypted payload.
   The mechanisms support extensions to allow for codec specific
   information.

Status of this Memo

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   This Internet-Draft will expire on April 24, 2014.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Solution  . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
     2.1.  RTP header extension  . . . . . . . . . . . . . . . . . . . 4
     2.2.  Signaling information . . . . . . . . . . . . . . . . . . . 5
     2.3.  Considerations on use . . . . . . . . . . . . . . . . . . . 5
   3.  Security Considerations . . . . . . . . . . . . . . . . . . . . 6
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
   5.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 6
     5.1.  Normative References  . . . . . . . . . . . . . . . . . . . 6
     5.2.  Informative References  . . . . . . . . . . . . . . . . . . 6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 7






































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1.  Introduction

   It is common practice in modern voice and video conferencing systems
   to implement a centralized component that acts as a RTP switch.  It
   receives voice and video streams from each participant, which may be
   encrypted using SRTP [RFC3711].  The goal is to provide a set of
   streams back to the participants which enable them to render the
   right media content.  In a simple video configuration, for example,
   the goal will be that each participant sees and hears just the active
   speaker.  In that case, the goal of the switch is to receive the
   voice and video streams from each participant, determine the active
   speaker based on energy in the voice packets, and select the
   corresponding video stream for transmission to participants, see
   Figure 1

   In this document, an "RTP switch" is used as a common short term for
   the terms switching RTP mixer", "source projecting middlebox", and
   "video switching MCU" as discussed in
   [I-D.ietf-avtcore-rtp-topologies-update].

            +---+      +------------+      +---+
            | A |<---->|            |<---->| B |
            +---+      |            |      +---+
                       |   RTP      |
            +---+      |  Switch    |      +---+
            | C |<---->|            |<---->| D |
            +---+      +------------+      +---+


                           Figure 1: RTP switch

   In order to properly support switching of video streams, the RTP
   switch typically needs information in order to do a proper job:
   o  Because of inter-frame dependencies, it should ideally switch
      video streams at a point where the first frame from the new
      speaker can be decoded by recipients without prior frames, e.g
      switch on an intra-frame.
   o  In many cases, the switch may need to drop frames in order to
      realize congestion control techniques, and needs to know which
      frames can be dropped with minimal impact to video quality
   o  Furthermore, it is highly desirable to do this in a way which is
      not specific to the video codec.  Nearly all modern video codecs
      share common concepts around I, P, B frames.
   o  It is also desirable to be able to do this for SRTP without
      requiring the video switch to decrypt the packets.  SRTP will
      encrypt the RTP payload format contents and consequently this data
      is not usable for the switching function.




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   By providing meta-information about the RTP streams outside the
   encrypted media payload a RTP switch can do selective forwarding
   without decrypting the payload.  This document provides a solution to
   this problem.


2.  Solution

   The solution uses RTP header extensions as defined in [RFC5285].  A
   subset of meta-information from the video stream is provided as an
   header extension to allow a RTP switch to do generic video switching
   handling of video streams encoded with different video codecs.

   The following information are extracted from the media payload.
   o  Discardable - The flag must be true for packets that can be
      dropped, and still provide a decodable media stream.
   o  Switching point (1 bit) - The flag must be true for RTP packets in
      a frame that can be used as a switcing point.  A switching point
      is the first packet where a new receiver can start decoding a
      video stream without prior frames, e.g an IDR frame from
      [RFC6184].
   o  Temporal ID (3 bits) - The base temporal quality starts with 0,
      and increases with 1 for each temporal layer.
   o  frame Type (3 bits) - Abstract frame type; P-frame=0, IDR=1,
      GDR=2.  The abstracted frame types are:
      *  P-frame - a frame depending on a previous frame
      *  IDR - a frame without references to other frames
      *  GDR - a Gradual Decoder Refresh (GDR) packet includes both
         p-frame and frame information to allow a receiver to build of a
         IDR frame over a short period.

   Video codec specific information can be provided as an extension.

2.1.  RTP header extension

   The values of frame information can be carried as RTP header
   extensions encoded using the one-byte header as described in
   [RFC5285].  Only the one-byte header version is listed with examples
   in the document.

   0                   1
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  ID=2 |  L=0  |D|S|TID  |Type |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   The frame marking can be extended with codec specific information



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   using a longer length value in the one-byte header.  The codec
   specific information included in the header extension MUST match the
   SDP negotiated payload format for the RTP stream.

     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 2
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  ID=2 |  L=2 |D|S|TID  |Type | video codec specific information |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   In the following example the H265 LayerID is included as video codec
   specific information.  The length field is 1 to add another 1 byte of
   data, the H265 LayerId is a 6-bit field and a 2-bit PADding at the
   end.

     0                   1                   2
    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  ID=2 |  L=1  |D|S|TID  |Type | H265-LayerId|PAD|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

2.2.  Signaling information

   The URI for declaring this header extension in an extmap attribute is
   "urn:ietf:params:rtp-hdrext:framemarkinginfo".  It does not contain
   any extension attributes.

   An example attribute line in SDP:

      a=extmap:3 urn:ietf:params:rtp-hdrext:framemarkinginfo

2.3.  Considerations on use

   The header extension values MUST represent what is already in the RTP
   payload.

   When a RTP switch needs to discard a received video frame due to
   congestion control considerations, it is RECOMMENDED that it
   preferably drop frames marked with the "discardable" bit.

   When a RTP switch want to forward a new video stream to a receiver,
   its RECOMMENDED to forward the new video stream from the first
   switching point and forward.  A RTP switch can request a media source
   to generate a switching point for H264 by sending Full Intra Request
   (RTCP FIR) as defined in [RFC5104].






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3.  Security Considerations

   In the Secure Real-Time Transport Protocol (SRTP) [RFC3711], RTP
   header extensions are authenticated but not encrypted.  When header
   extensions are used some of the payload type information are exposed
   and is visible to middle boxes.  The encrypted media data is not
   exposed, so this is not seen as a high risk exposure.


4.  IANA Considerations

   This document defines a new extension URI to the RTP Compact
   HeaderExtensions sub-registry of the Real-Time Transport Protocol
   (RTP) Parameters registry, according to the following data:

   Extension URI: urn:ietf:params:rtp-hdrext:framemarkinginfo
   Description: Frame marking information for video streams
   Contact: espeberg@cisco.com
   Reference: RFC XXXX

   Note to RFC Editor: please replace RFC XXXX with the number of this
   RFC.


5.  References

5.1.  Normative References

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

5.2.  Informative References

   [I-D.ietf-avtcore-rtp-topologies-update]
              Westerlund, M. and S. Wenger, "RTP Topologies",
              draft-ietf-avtcore-rtp-topologies-update (work in
              progress), April 2013.

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

   [RFC3711]  Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
              Norrman, "The Secure Real-time Transport Protocol (SRTP)",
              RFC 3711, March 2004.

   [RFC5104]  Wenger, S., Chandra, U., Westerlund, M., and B. Burman,



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              "Codec Control Messages in the RTP Audio-Visual Profile
              with Feedback (AVPF)", RFC 5104, February 2008.

   [RFC5285]  Singer, D. and H. Desineni, "A General Mechanism for RTP
              Header Extensions", RFC 5285, July 2008.

   [RFC6184]  Wang, Y., Even, R., Kristensen, T., and R. Jesup, "RTP
              Payload Format for H.264 Video", RFC 6184, May 2011.


Authors' Addresses

   Espen Berger
   Cisco Systems

   Phone: +47 98228179
   Email: espeberg@cisco.com


   Suhas Nandakumar
   Cisco Systems
   170 West Tasman Drive
   San Jose, CA  95134
   US

   Email: snandaku@cisco.com

























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