RTP Payload Format for SFrame

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.¶

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.¶

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."¶

This Internet-Draft will expire on 11 January 2024.¶

Copyright Notice

Copyright (c) 2023 IETF Trust and the persons identified as the document authors. All rights reserved.¶

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License.¶

Table of Contents

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.¶

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.¶
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¶

Author's Address