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Network Working Group S. Wenger
Request for Comments: 3984 M.M. Hannuksela
Category: Standards Track T. Stockhammer
M. Westerlund
D. Singer
February 2005
RTP Payload Format for H.264 Video
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This memo describes an RTP Payload format for the ITU-T
Recommendation H.264 video codec and the technically identical
ISO/IEC International Standard 14496-10 video codec. The RTP payload
format allows for packetization of one or more Network Abstraction
Layer Units (NALUs), produced by an H.264 video encoder, in each RTP
payload. The payload format has wide applicability, as it supports
applications from simple low bit-rate conversational usage, to
Internet video streaming with interleaved transmission, to high bit-
rate video-on-demand.
Table of Contents
1. Introduction.................................................. 3
1.1. The H.264 Codec......................................... 3
1.2. Parameter Set Concept................................... 4
1.3. Network Abstraction Layer Unit Types.................... 5
2. Conventions................................................... 6
3. Scope......................................................... 6
4. Definitions and Abbreviations................................. 6
4.1. Definitions............................................. 6
5. RTP Payload Format............................................ 8
5.1. RTP Header Usage........................................ 8
5.2. Common Structure of the RTP Payload Format.............. 11
5.3. NAL Unit Octet Usage.................................... 12
Wenger, et al. Standards Track [Page 1]
RFC 3984 RTP Payload Format for H.264 Video February 2005
5.4. Packetization Modes..................................... 14
5.5. Decoding Order Number (DON)............................. 15
5.6. Single NAL Unit Packet.................................. 18
5.7. Aggregation Packets..................................... 18
5.8. Fragmentation Units (FUs)............................... 27
6. Packetization Rules........................................... 31
6.1. Common Packetization Rules.............................. 31
6.2. Single NAL Unit Mode.................................... 32
6.3. Non-Interleaved Mode.................................... 32
6.4. Interleaved Mode........................................ 33
7. De-Packetization Process (Informative)........................ 33
7.1. Single NAL Unit and Non-Interleaved Mode................ 33
7.2. Interleaved Mode........................................ 34
7.3. Additional De-Packetization Guidelines.................. 36
8. Payload Format Parameters..................................... 37
8.1. MIME Registration....................................... 37
8.2. SDP Parameters.......................................... 52
8.3. Examples................................................ 58
8.4. Parameter Set Considerations............................ 60
9. Security Considerations....................................... 62
10. Congestion Control............................................ 63
11. IANA Considerations........................................... 64
12. Informative Appendix: Application Examples.................... 65
12.1. Video Telephony according to ITU-T Recommendation H.241
Annex A................................................. 65
12.2. Video Telephony, No Slice Data Partitioning, No NAL
Unit Aggregation........................................ 65
12.3. Video Telephony, Interleaved Packetization Using NAL
Unit Aggregation........................................ 66
12.4. Video Telephony with Data Partitioning.................. 66
12.5. Video Telephony or Streaming with FUs and Forward
Error Correction........................................ 67
12.6. Low Bit-Rate Streaming.................................. 69
12.7. Robust Packet Scheduling in Video Streaming............. 70
13. Informative Appendix: Rationale for Decoding Order Number..... 71
13.1. Introduction............................................ 71
13.2. Example of Multi-Picture Slice Interleaving............. 71
13.3. Example of Robust Packet Scheduling..................... 73
13.4. Robust Transmission Scheduling of Redundant Coded
Slices.................................................. 77
13.5. Remarks on Other Design Possibilities................... 77
14. Acknowledgements.............................................. 78
15. References.................................................... 78
15.1. Normative References.................................... 78
15.2. Informative References.................................. 79
Authors’ Addresses................................................ 81
Full Copyright Statement.......................................... 83
Wenger, et al. Standards Track [Page 2]
RFC 3984 RTP Payload Format for H.264 Video February 2005
1. Introduction
1.1. The H.264 Codec
This memo specifies an RTP payload specification for the video coding
standard known as ITU-T Recommendation H.264 [1] and ISO/IEC
International Standard 14496 Part 10 [2] (both also known as Advanced
Video Coding, or AVC). Recommendation H.264 was approved by ITU-T on
May 2003, and the approved draft specification is available for
public review [8]. In this memo the H.264 acronym is used for the
codec and the standard, but the memo is equally applicable to the
ISO/IEC counterpart of the coding standard.
The H.264 video codec has a very broad application range that covers
all forms of digital compressed video from, low bit-rate Internet
streaming applications to HDTV broadcast and Digital Cinema
applications with nearly lossless coding. Compared to the current
state of technology, the overall performance of H.264 is such that
bit rate savings of 50% or more are reported. Digital Satellite TV
quality, for example, was reported to be achievable at 1.5 Mbit/s,
compared to the current operation point of MPEG 2 video at around 3.5
Mbit/s [9].
The codec specification [1] itself distinguishes conceptually between
a video coding layer (VCL) and a network abstraction layer (NAL).
The VCL contains the signal processing functionality of the codec;
mechanisms such as transform, quantization, and motion compensated
prediction; and a loop filter. It follows the general concept of
most of today’s video codecs, a macroblock-based coder that uses
inter picture prediction with motion compensation and transform
coding of the residual signal. The VCL encoder outputs slices: a bit
string that contains the macroblock data of an integer number of
macroblocks, and the information of the slice header (containing the
spatial address of the first macroblock in the slice, the initial
quantization parameter, and similar information). Macroblocks in
slices are arranged in scan order unless a different macroblock
allocation is specified, by using the so-called Flexible Macroblock
Ordering syntax. In-picture prediction is used only within a slice.
More information is provided in [9].
The Network Abstraction Layer (NAL) encoder encapsulates the slice
output of the VCL encoder into Network Abstraction Layer Units (NAL
units), which are suitable for transmission over packet networks or
use in packet oriented multiplex environments. Annex B of H.264
defines an encapsulation process to transmit such NAL units over
byte-stream oriented networks. In the scope of this memo, Annex B is
not relevant.
Wenger, et al. Standards Track [Page 3]
RFC 3984 RTP Payload Format for H.264 Video February 2005
Internally, the NAL uses NAL units. A NAL unit consists of a one-
byte header and the payload byte string. The header indicates the
type of the NAL unit, the (potential) presence of bit errors or
syntax violations in the NAL unit payload, and information regarding
the relative importance of the NAL unit for the decoding process.
This RTP payload specification is designed to be unaware of the bit
string in the NAL unit payload.
One of the main properties of H.264 is the complete decoupling of the
transmission time, the decoding time, and the sampling or
presentation time of slices and pictures. The decoding process
specified in H.264 is unaware of time, and the H.264 syntax does not
carry information such as the number of skipped frames (as is common
in the form of the Temporal Reference in earlier video compression
standards). Also, there are NAL units that affect many pictures and
that are, therefore, inherently timeless. For this reason, the
handling of the RTP timestamp requires some special considerations
for NAL units for which the sampling or presentation time is not
defined or, at transmission time, unknown.
1.2. Parameter Set Concept
One very fundamental design concept of H.264 is to generate self-
contained packets, to make mechanisms such as the header duplication
of RFC 2429 [10] or MPEG-4’s Header Extension Code (HEC) [11]
unnecessary. This was achieved by decoupling information relevant to
more than one slice from the media stream. This higher layer meta
information should be sent reliably, asynchronously, and in advance
from the RTP packet stream that contains the slice packets.
(Provisions for sending this information in-band are also available
for applications that do not have an out-of-band transport channel
appropriate for the purpose.) The combination of the higher-level
parameters is called a parameter set. The H.264 specification
includes two types of parameter sets: sequence parameter set and
picture parameter set. An active sequence parameter set remains
unchanged throughout a coded video sequence, and an active picture
parameter set remains unchanged within a coded picture. The sequence
and picture parameter set structures contain information such as
picture size, optional coding modes employed, and macroblock to slice
group map.
To be able to change picture parameters (such as the picture size)
without having to transmit parameter set updates synchronously to the
slice packet stream, the encoder and decoder can maintain a list of
more than one sequence and picture parameter set. Each slice header
contains a codeword that indicates the sequence and picture parameter
set to be used.
Wenger, et al. Standards Track [Page 4]
RFC 3984 RTP Payload Format for H.264 Video February 2005
This mechanism allows the decoupling of the transmission of parameter
sets from the packet stream, and the transmission of them by external
means (e.g., as a side effect of the capability exchange), or through
a (reliable or unreliable) control protocol. It may even be possible
that they are never transmitted but are fixed by an application
design specification.
1.3. Network Abstraction Layer Unit Types
Tutorial information on the NAL design can be found in [12], [13],
and [14].
All NAL units consist of a single NAL unit type octet, which also
co-serves as the payload header of this RTP payload format. The
payload of a NAL unit follows immediately.
The syntax and semantics of the NAL unit type octet are specified in
[1], but the essential properties of the NAL unit type octet are
summarized below. The NAL unit type octet has the following format:
+---------------+
|0|1|2|3|4|5|6|7|
+-+-+-+-+-+-+-+-+
|F|NRI| Type |
+---------------+
The semantics of the components of the NAL unit type octet, as
specified in the H.264 specification, are described briefly below.
F: 1 bit
forbidden_zero_bit. The H.264 specification declares a value of
1 as a syntax violation.
NRI: 2 bits
nal_ref_idc. A value of 00 indicates that the content of the NAL
unit is not used to reconstruct reference pictures for inter
picture prediction. Such NAL units can be discarded without
risking the integrity of the reference pictures. Values greater
than 00 indicate that the decoding of the NAL unit is required to
maintain the integrity of the reference pictures.
Type: 5 bits
nal_unit_type. This component specifies the NAL unit payload type
as defined in table 7-1 of [1], and later within this memo. For a
reference of all currently defined NAL unit types and their
semantics, please refer to section 7.4.1 in [1].
Wenger, et al. Standards Track [Page 5]
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