RTP Payload Format for SFrame
draft-ietf-avtcore-rtp-sframe-00

Audio/Video Transport Core Maintenance                       P. Thatcher
Internet-Draft                                                 Microsoft
Intended status: Standards Track                         7 November 2023
Expires: 10 May 2024

                     RTP Payload Format for SFrame
                    draft-ietf-avtcore-rtp-sframe-00

Abstract

   This document describes the RTP payload format of SFrame.

Status of This Memo

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   This Internet-Draft will expire on 10 May 2024.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology and Notation  . . . . . . . . . . . . . . . . . .   2
   3.  RTP Packetization of a media frame encrypted by SFrame  . . .   2
   4.  RTP depacketization of SFrame . . . . . . . . . . . . . . . .   3
   5.  SFrame payload type negotiation . . . . . . . . . . . . . . .   4
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   4
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   5
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   5

1.  Introduction

   SFrame [I-D.draft-ietf-sframe-enc-01] describes an end-to-end
   encryption and authentication mechanism for media frames in a
   multiparty conference call, in which central media servers (SFUs) can
   access the media metadata needed to make forwarding decisions without
   having access to the actual media.

   This document describes how to packetize a media frame encrypted
   using SFrame into RTP packets.

2.  Terminology and Notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  RTP Packetization of a media frame encrypted by SFrame

   In order to packetize SFrame into RTP, packetization is done in 2
   stages.  In the first stage, before SFrame encryption, media is
   packetized into RTP packets in a way specific to the media format.
   In the second stage, each RTP packet from the first stage is
   packetized into RTP packets in a way specific to SFrame.  SFrame
   encryption is applied to the payload of each RTP packet between the
   first and second stages.

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   For example, if a media frame to be encrypted by SFrame is encoded
   using VP8, the media frame is first packetized according to [RFC7741]
   into one RTP packets with VP8-specific payloads.  Each of those VP8
   RTP payloads are then encrypted using SFrame, resulting in an SFrame-
   encrypted RTP payload of VP8.  SFrame-specific packetization is then
   applied to the SFrame-encrypted RTP payload of VP8, resulting in RTP
   packets with SFrame-specific RTP payloads.

   SFrame-specific packetization is done by first breaking up the output
   of SFrame encryption into fragments, and then prepending some
   fragment metadata necessary for depacketization.  Finally, fragments
   are combined with values from the RTP header of the output of the
   media-format-specific packetization.

   The SFrame-specific RTP payloads (fragments with prepended metadata)
   have the following format:

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |L| media PT    |  media frame ID               |
   | fragment index                |  fragment ... |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The media PT must be the payload type of the output of the media-
   format-specific packetization.  The frame index of the first fragment
   of each media frame MUST be 0.  The frame index of each subsequent
   fragment MUST be one more than the previous fragment.  The L bit MUST
   be 0 for all fragments except for the last one of the media frame.
   The media frame ID must be unique enough that a depacketizer may be
   able to differentiate the fragments of one media frame from another.
   The SSRC, timestamp, marker bit, CSRCs, and header extensions of the
   SFrame RTP packets MUST be the same as those of the output of the
   media-format-specific packetization.  The payload type of the SFrame
   RTP packets must be a payload type that indicates the payload format
   defined in this document, and it must have a negotiated RTP clock
   rate that is the same as the media-format-specific RTP packet.

4.  RTP depacketization of SFrame

   Depacketization is done by doing the packetization process in
   reverse:

   1.  The fragments of a given media frame ID are grouped together in
       order of fragment index and concatenated together, resulting in a
       media frame encrypted by SFrame.

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   2.  The media frame is decrypted using SFrame, resulting in a media-
       format-specific RTP payload.

   3.  The media-format-specific RTP payload is combined with the RTP
       headers of the RTP packet with fragment index 0, resulting in a
       media-format-specific RTP packet.  The "media PT" from the SFrame
       RTP payload header is used as the payload type of the media-
       format-specific RTP packet.

   4.  The media-format-specific RTP packet is passed into a media-
       format-specific RTP depacketizer, resulting in a media frame.

5.  SFrame payload type negotiation

   Because the payload type of an RTP packet that results from SFrame-
   specific packetization must match the clock rate of the payload type
   of the RTP packet that results from media-format-specific
   packetization, it may be necessary to negotiate more than one SFrame
   payload type.  For example, if one were to use SDP to negotiate
   payload types, the following payload types could be negotiated with
   different clock rates:

   m=audio 50000 RTP/SAVPF 96
   a=rtpmap:96 sframe/48000
   m=video 50002 RTP/SAVPF 97
   a=rtpmap:97 sframe/90000

6.  Security Considerations

   This document is subject to the security considerations of SFrame.

7.  IANA Considerations

   None

8.  References

8.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/rfc/rfc2119>.

   [RFC7741]  Westin, P., Lundin, H., Glover, M., Uberti, J., and F.
              Galligan, "RTP Payload Format for VP8 Video", RFC 7741,
              DOI 10.17487/RFC7741, March 2016,
              <https://www.rfc-editor.org/rfc/rfc7741>.

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   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.

8.2.  Informative References

   [I-D.draft-ietf-sframe-enc-01]
              Omara, E., Uberti, J., Murillo, S. G., Barnes, R., and Y.
              Fablet, "Secure Frame (SFrame)", Work in Progress,
              Internet-Draft, draft-ietf-sframe-enc-01, 13 March 2023,
              <https://datatracker.ietf.org/doc/html/draft-ietf-sframe-
              enc-01>.

Author's Address

   Peter Thatcher
   Microsoft
   Email: pthatcher@microsoft.com

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