Network Working Group E. Berger
Internet-Draft S. Nandakumar
Intended status: Standards Track M. Zanaty
Expires: September 14, 2017 Cisco Systems
March 13, 2017
Frame Marking RTP Header Extension
draft-ietf-avtext-framemarking-04
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 decryption 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
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on September 14, 2017.
Copyright Notice
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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. Extension for Non-Scalable Streams . . . . . . . . . . . 4
3.2. Extension for Scalable Streams . . . . . . . . . . . . . 5
3.2.1. Layer ID Mappings for Scalable Streams . . . . . . . 6
3.2.1.1. H265 LID Mapping . . . . . . . . . . . . . . . . 6
3.2.1.2. VP9 LID Mapping . . . . . . . . . . . . . . . . . 7
3.2.1.3. VP8 LID Mapping . . . . . . . . . . . . . . . . . 7
3.2.1.4. H264-SVC LID Mapping . . . . . . . . . . . . . . 7
3.2.1.5. H264 (AVC) LID Mapping . . . . . . . . . . . . . 7
3.3. Signaling Information . . . . . . . . . . . . . . . . . . 8
3.4. Usage Considerations . . . . . . . . . . . . . . . . . . 8
3.4.1. Relation to Layer Refresh Request (LRR) . . . . . . . 8
4. Security Considerations . . . . . . . . . . . . . . . . . . . 8
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
7.1. Normative References . . . . . . . . . . . . . . . . . . 9
7.2. Informative References . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
Many widely deployed RTP [RFC3550] topologies [RFC7667] 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 where the switch has no access to media
decryption keys. 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 [RFC6464], and select the corresponding video stream
for transmission to participants; see Figure 1.
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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 [RFC7667].
+---+ +------------+ +---+
| 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 payload
format-agnostic way which is not specific to each different video
codec. Most modern video codecs share common concepts around
frame types and other critical information to make this codec-
agnostic handling possible.
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.
By providing meta-information about the RTP streams outside the
encrypted media payload, an RTP switch can do codec-agnostic
selective forwarding without decrypting the payload. This document
specifies the necessary meta-information in an RTP header extension.
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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
This specification 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.
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 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.
This extension is only specified for Source (not Redunadancy) RTP
Streams [RFC7656] that carry video payloads. It is not specified for
audio payloads, nor is it specified for Redundancy RTP Streams. The
(separate) specifications for Redudancy RTP Streams often include
provisions for recovering any header extensions that were part of the
original source packet. Such provisions SHALL be followed to recover
the Frame Marking RTP header extension of the original source packet.
3.1. Extension for Non-Scalable Streams
The following RTP header extension is RECOMMENDED for non-scalable
streams. It MAY also be used for scalable streams if the sender has
limited or no information about stream scalability. The ID is
assigned per [RFC5285], and the length is encoded as L=0 which
indicates 1 octet of data.
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ID=? | L=0 |S|E|I|D|0 0 0 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The following information are extracted from the media payload and
sent in the Frame Marking RTP header extension.
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o S: Start of Frame (1 bit) - MUST be 1 in the first packet in a
frame; otherwise MUST be 0.
o E: End of Frame (1 bit) - MUST be 1 in the last packet in a frame;
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 IDR/CRA/BLA/RAP [RFC7798];
otherwise MUST be 0.
o D: Discardable Frame (1 bit) - MUST be 1 for frames that can be
discarded, and still provide a decodable media stream; otherwise
MUST be 0.
o The remaining (4 bits) - MUST be 0 for non-scalable streams.
3.2. Extension for Scalable Streams
The following RTP header extension is RECOMMENDED for scalable
streams. It MAY also be used for non-scalable streams, in which case
TID, LID and TL0PICIDX MUST be 0. The ID is assigned per [RFC5285],
and the length is encoded as L=2 which indicates 3 octets of data.
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=? | L=2 |S|E|I|D|B| TID | LID | TL0PICIDX |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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 IDR/CRA/BLA/RAP [RFC7798];
otherwise MUST be 0.
o D: Discardable Frame (1 bit) - MUST be 1 for frames that can be
discarded, 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. If no scalability is
used, this 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. If
no scalability is used, this MUST be 0.
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o LID: Layer ID (8 bits) - Identifies the spatial and quality layer
encoded. If no scalability is used, this MUST be 0 or omitted.
When omitted, TL0PICIDX MUST also be omitted.
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. If no scalability
is used, this MUST be 0 or omitted. When omitted, LID MUST also
be omitted.
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
and/or quality layers with higher resolutions and/or bitrates.
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 for 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.
3.2.1. Layer ID Mappings for Scalable Streams
3.2.1.1. H265 LID Mapping
The following shows the H265 [RFC7798] LayerID (6 bits) and TID (3
bits) from the NAL unit header mapped to the generic LID and TID
fields.
The I bit MUST be 1 when the NAL unit type is 16-23 (inclusive),
otherwise it MUST be 0.
The S and E bits MUST match the corresponding bits in PACI:PHES:TSCI
payload structures.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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3.2.1.2. VP9 LID Mapping
The following shows VP9 Layer encoding information (4 bits for
spatial and quality, 3 bits for temporal layer) mapped to the generic
LID and TID fields.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
3.2.1.3. VP8 LID Mapping
The following shows the header extension for VP8 that contains only
temporal 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
3.2.1.4. H264-SVC LID Mapping
The following shows H264-SVC [RFC6190] Layer encoding information (3
bits for spatial/dependency layer, 4 bits for quality layer and 3
bits for temporal layer) mapped to the generic LID and TID fields.
The S, E, I and D bits MUST match the corresponding bits in PACSI
payload structures.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
3.2.1.5. H264 (AVC) LID Mapping
The following shows the header extension for H264 (AVC) that contains
only temporal 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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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. Usage Considerations
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.
3.4.1. Relation to Layer Refresh Request (LRR)
Receivers can use the Layer Refresh Request (LRR)
[I-D.ietf-avtext-lrr] RTCP feedback message to upgrade to a higher
layer in scalable encodings. The TID/LID values and formats used in
LRR messages correspond to the same values and formats specified in
Section 3.2.
4. Security Considerations
In the Secure Real-Time Transport Protocol (SRTP) [RFC3711], RTP
header extensions are authenticated but usually not encrypted. When
header extensions are used some of the payload type information are
exposed and 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.
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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: mzanaty@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>.
7.2. Informative References
[RFC7656] Lennox, J., Gross, K., Nandakumar, S., Salgueiro, G., and
B. Burman, Ed., "A Taxonomy of Semantics and Mechanisms
for Real-Time Transport Protocol (RTP) Sources", RFC 7656,
DOI 10.17487/RFC7656, November 2015,
<http://www.rfc-editor.org/info/rfc7656>.
[RFC7667] Westerlund, M. and S. Wenger, "RTP Topologies", RFC 7667,
DOI 10.17487/RFC7667, November 2015,
<http://www.rfc-editor.org/info/rfc7667>.
[RFC6464] Lennox, J., Ed., Ivov, E., and E. Marocco, "A Real-time
Transport Protocol (RTP) Header Extension for Client-to-
Mixer Audio Level Indication", RFC 6464,
DOI 10.17487/RFC6464, December 2011,
<http://www.rfc-editor.org/info/rfc6464>.
[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>.
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[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>.
[RFC6190] Wenger, S., Wang, Y., Schierl, T., and A. Eleftheriadis,
"RTP Payload Format for Scalable Video Coding", RFC 6190,
DOI 10.17487/RFC6190, May 2011,
<http://www.rfc-editor.org/info/rfc6190>.
[RFC7798] Wang, Y., Sanchez, Y., Schierl, T., Wenger, S., and M.
Hannuksela, "RTP Payload Format for High Efficiency Video
Coding (HEVC)", RFC 7798, DOI 10.17487/RFC7798, March
2016, <http://www.rfc-editor.org/info/rfc7798>.
[I-D.ietf-avtext-lrr]
Lennox, J., Hong, D., Uberti, J., Holmer, S., and M.
Flodman, "The Layer Refresh Request (LRR) RTCP Feedback
Message", draft-ietf-avtext-lrr-03 (work in progress),
July 2016.
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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