This document describes a Frame Marking RTP header extension used to convey information about video frames that is critical for error recovery and packet forwarding in RTP middleboxes or network nodes. It is most useful when media is encrypted, and essential when the middlebox or node has no access to the media encryption keys. It is also useful for codec-agnostic processing of encrypted or unencrypted media, while it also supports extensions for codec-specific information.

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1. Introduction

Many widely deployed RTP topologies used in modern voice and video conferencing systems include a centralized component that acts as an RTP switch. It receives voice and video streams from each participant, which may be encrypted using SRTP [RFC3711], or extensions that provide participants with private media via end-to-end encryption that excludes the switch. 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, possibly using the client-to-mixer audio level RTP header extension, 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", "source forwarding unit/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 some critical information about video frames in order to start and stop forwarding streams.

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.
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.
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 frame types.
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 without decryption, which may not even be possible in the case of end-to-end encryption of private media.

By providing meta-information about the RTP streams outside the encrypted media payload an 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 RTP header extension to allow a RTP switch to do generic video switching handling of video streams encoded with different video codecs.

2.1. Mandatory Extension

The following information are extracted from the media payload:

S: Start of Frame (1 bit) - MUST be 1 in the first packet in a frame within a layer; otherwise MUST be 0.
E: End of Frame (1 bit) - MUST be 1 in the last packet in a frame within a layer; otherwise MUST be 0.
I: Independent Frame (1 bit) - MUST be 1 for frames that can be decoded independent of prior frames, e.g. intra-frame, VPx keyframe, H.264 IDR [RFC6184], H.265 CRA/BLA; otherwise MUST be 0.
D: Discardable Frame (1 bit) - MUST be 1 for frames that can be dropped, and still provide a decodable media stream; otherwise MUST be 0.
B: Base Layer Sync (1 bit) - MUST be 1 if this frame only depends on the base layer; otherwise MUST be 0.
TID: Temporal ID (3 bits) - The base temporal quality starts with 0, and increases with 1 for each temporal layer/sub-layer.
LID: Layer ID (8 bits) - Identifies the spatial and quality layer encoded.

NOTE:Given the opaque nature of the LID, consider having the layer structure information as RTCP SDES item (either in the RTCP SDES message or as the RTP SDES Header extension) to map the LIDs to specific resolutions and bitrates thus enabling the RTP Switch to make informed decisions

The values of frame information can be carried as RTP header extensions encoded using the one-byte header as described in [RFC5285] as shown below.

 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|  ID=2 |  L=1  |S|E|I|D|B| TID |   LID         |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

2.2. Examples

The following example shows H265-LayerID (6 bits) mapped to the generic LID field.

 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|  ID=2 |  L=1  |S|E|I|D|B| TID |0|0|H265-LayerId|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The following example shows VP9 Layer encoding information (4 bits for spatial and quality) mapped to the generic LID field.

 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|  ID=2 |  L=1  |S|E|I|D|B| TID |0|0|0|0| RS| RQ|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

2.3. 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

2.4. 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 wants to forward a new video stream to a receiver, it is RECOMMENDED to select the new video stream from the first switching point (I bit set) and forward the same. A RTP switch can request a media source to generate a switching point for H.264 by sending Full Intra Request (RTCP FIR) as defined in [RFC5104], for example.

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. Acknowledgements

Many thanks to Bernard Aboba, Jonathan Lennox for their inputs.

5. 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.

6. References

6.1. Normative References

[KEYWORDS]

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

6.2. Informative References

[I-D.ietf-avtcore-rtp-topologies-update]	Westerlund, M. and S. Wenger, "RTP Topologies", Internet-Draft draft-ietf-avtcore-rtp-topologies-update, 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, "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

Mo Zanaty Cisco Systems 170 West Tasman Drive San Jose, CA 95134 US EMail: mzanaty@cisco.com

Abstract

Status of This Memo

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