Internet DRAFT - draft-thatcher-avtcore-rtp-sframe
draft-thatcher-avtcore-rtp-sframe
Audio/Video Transport Core Maintenance P. Thatcher
Internet-Draft Microsoft
Intended status: Standards Track 10 July 2023
Expires: 11 January 2024
RTP Payload Format for SFrame
draft-thatcher-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 11 January 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 twice,
once before SFrame encryption using media-format-specific RTP
packetization into one large RTP packet, and once after SFrame
encryption using SFrame-specific RTP packetization into many smaller
RTP packets. SFrame encryption is applied to the payload of output
the media-format-specific RTP packetization, and SFrame-specific RTP
packetization is applied to the output of the SFrame encryption.
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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 big RTP packet. The VP8 RTP payload of the big RTP packet
is then encrypted using SFrame, resulting in an SFrame-encrypted RTP
payload of VP8. SFrame-specific packetization is then applied to
SFrame-encrypted RTP payload of VP8, resulting in many smaller RTP
packets.
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 is
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 big
codec-specific RTP payload.
3. The big codec-specific RTP payload is combined with the RTP
headers of the RTP packet with fragment index 0, resulting in a
big media-format-specific RTP packet. The "media PT" from the
SFrame RTP payload header is used as the payload type of the
result.
4. The big media-format-specific RTP packet is passed in a codec-
specific RTP depacetizer, 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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