Network Working Group E. Berger
Internet-Draft S. Nandakumar
Intended status: Standards Track M. Zanaty
Expires: January 19, 2017 Cisco Systems
July 18, 2016
Frame Marking RTP Header Extension
draft-ietf-avtext-framemarking-02
Abstract
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.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on January 19, 2017.
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to this document. Code Components extracted from this document must
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Key Words for Normative Requirements . . . . . . . . . . . . 4
3. Frame Marking RTP Header Extension . . . . . . . . . . . . . 4
3.1. Mandatory Extension . . . . . . . . . . . . . . . . . . . 4
3.2. Layer ID Mappings . . . . . . . . . . . . . . . . . . . . 5
3.2.1. H265 LID Mapping . . . . . . . . . . . . . . . . . . 5
3.2.2. VP9 LID Mapping . . . . . . . . . . . . . . . . . . . 5
3.2.3. VP8 LID Mapping . . . . . . . . . . . . . . . . . . . 6
3.2.4. H264-SVC LID Mapping . . . . . . . . . . . . . . . . 6
3.2.5. H264 (AVC) LID Mapping . . . . . . . . . . . . . . . 6
3.3. Signaling information . . . . . . . . . . . . . . . . . . 6
3.4. Considerations on use . . . . . . . . . . . . . . . . . . 6
4. Security Considerations . . . . . . . . . . . . . . . . . . . 7
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
7.1. Normative References . . . . . . . . . . . . . . . . . . 7
7.2. Informative References . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction
Many widely deployed RTP [RFC3550] 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",
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"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.
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 frame types.
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 without decryption, which
may not even be possible in the case of end-to-end encryption of
private media.
A comprehensive discussion of SFU considerations around codec
agnostic selective forwarding of RTP media is described in
[I-D.aboba-avtcore-sfu-rtp]
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.
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2. Key Words for Normative Requirements
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. Frame Marking RTP Header Extension
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 an RTP switch to do generic selective
forwarding of video streams encoded with potentially different video
codecs.
3.1. Mandatory Extension
The following information are extracted from the media payload and
sent in the Frame Marking RTP header extension.
o S: Start of Frame (1 bit) - MUST be 1 in the first packet in a
frame within a layer; otherwise MUST be 0.
o E: End of Frame (1 bit) - MUST be 1 in the last packet in a frame
within a layer; otherwise MUST be 0.
o 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.
o 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.
o B: Base Layer Sync (1 bit) - MUST be 1 if this frame only depends
on the base layer; otherwise MUST be 0.
o TID: Temporal ID (3 bits) - The base temporal layer starts with 0,
and increases with 1 for each higher temporal layer/sub-layer.
o LID: Layer ID (8 bits) - Identifies the spatial and quality layer
encoded.
o TL0PICIDX: Temporal Layer 0 Picture Index (8 bits) - Running index
of base temporal layer 0 frames when TID is 0. When TID is not 0,
this indicates a dependency on the given index.
The layer information contained in TID and LID convey useful aspects
of the layer structure that can be utilized in selective forwarding.
Without further information about the layer structure, these
identifiers can only be used for relative priority of layers. They
convey a layer hierarchy with TID=0 and LID=0 identifying the base
layer. Higher values of TID identify higher temporal layers with
higher frame rates. Higher values of LID identify higher spatial or
quality layers with higher resolutions and bitrates.
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With further information, for example, possible future RTCP SDES
items that convey full layer structure information, it may be
possible to map these TIDs and LIDs to specific frame rates,
resolutions and bitrates. Such additional layer information may be
useful to forwarding decisions in the RTP switch, but is beyond the
scope of this memo. The relative layer information is still useful
for many selective forwarding decisions even without such additional
layer information.
The Frame Marking RTP header extension is encoded using the one-byte
header or two-byte header as described in [RFC5285]. The one-byte
header format is shown below and used for examples in this memo. The
two-byte header format is used when other two-byte header extensions
are present in the same RTP packet, since mixing one-byte and two-
byte extensions is not possible in the same RTP packet.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ID=2 | L=2 |S|E|I|D|B| TID | LID | TL0PICIDX |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
3.2. Layer ID Mappings
3.2.1. H265 LID Mapping
The following shows H265-LayerID (6 bits) mapped to the generic LID
field.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ID=2 | L=2 |S|E|I|D|B| TID |0|0| LayerID | TL0PICIDX |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
3.2.2. VP9 LID Mapping
The following shows VP9 Layer encoding information (4 bits for
spatial and quality) mapped to the generic LID field.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ID=2 | L=2 |S|E|I|D|B| TID |0|0|0|0| RS| RQ| TL0PICIDX |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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3.2.3. VP8 LID Mapping
The following shows the header extension for VP8 that contains no
layer information.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ID=2 | L=2 |S|E|I|D|B| TID |0|0|0|0|0|0|0|0| TL0PICIDX |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
3.2.4. H264-SVC LID Mapping
The following shows H264-SVC Layer encoding information (3 bits for
spatial and 4 bits quality) mapped to the generic LID field.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ID=2 | L=2 |S|E|I|D|B| TID |0| DID | QID | TL0PICIDX |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
3.2.5. H264 (AVC) LID Mapping
The following shows the header extension for H264 (AVC) that contains
no layer information.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ID=2 | L=2 |S|E|I|D|B| TID |0|0|0|0|0|0|0|0| TL0PICIDX |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
3.3. Signaling information
The URI for declaring this header extension in an extmap attribute is
"urn:ietf:params:rtp-hdrext:framemarking". It does not contain any
extension attributes.
An example attribute line in SDP:
a=extmap:3 urn:ietf:params:rtp-hdrext:framemarking
3.4. Considerations on use
The header extension values MUST represent what is already in the RTP
payload.
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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.
4. 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.
5. Acknowledgements
Many thanks to Bernard Aboba, Jonathan Lennox, and Stephan Wenger for
their inputs.
6. 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.
7. References
7.1. 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,
<http://www.rfc-editor.org/info/rfc2119>.
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7.2. Informative References
[I-D.ietf-avtcore-rtp-topologies-update]
Westerlund, M. and S. Wenger, "RTP Topologies", draft-
ietf-avtcore-rtp-topologies-update-10 (work in progress),
July 2015.
[I-D.aboba-avtcore-sfu-rtp]
Aboba, B., "Codec-Independent Selective Forwarding",
draft-aboba-avtcore-sfu-rtp-00 (work in progress), July
2015.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550,
July 2003, <http://www.rfc-editor.org/info/rfc3550>.
[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, DOI 10.17487/RFC3711, March 2004,
<http://www.rfc-editor.org/info/rfc3711>.
[RFC5104] Wenger, S., Chandra, U., Westerlund, M., and B. Burman,
"Codec Control Messages in the RTP Audio-Visual Profile
with Feedback (AVPF)", RFC 5104, DOI 10.17487/RFC5104,
February 2008, <http://www.rfc-editor.org/info/rfc5104>.
[RFC5285] Singer, D. and H. Desineni, "A General Mechanism for RTP
Header Extensions", RFC 5285, DOI 10.17487/RFC5285, July
2008, <http://www.rfc-editor.org/info/rfc5285>.
[RFC6184] Wang, Y., Even, R., Kristensen, T., and R. Jesup, "RTP
Payload Format for H.264 Video", RFC 6184,
DOI 10.17487/RFC6184, May 2011,
<http://www.rfc-editor.org/info/rfc6184>.
Authors' Addresses
Espen Berger
Cisco Systems
Phone: +47 98228179
Email: espeberg@cisco.com
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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
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