Internet Draft S. Wenger
Document: draft-ietf-avt-rtp-h264-02.txt M.M. Hannuksela
Expires: December 2003 T. Stockhammer
M. Westerlund
D. Singer
June 2003
Expires December 2003
RTP payload Format for H.264 Video
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas,
and its working groups. Note that other groups may also distribute
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Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
This memo describes an RTP Payload format for the ITU-T
Recommendation H.264 video codec. This codec was designed as a
joint project of the Video Coding Experts Group (VCEG) of ITU-T and
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the Moving Picture Experts Group (MPEG) of ISO/IEC. Recommendation
H.264 was approved by ITU-T on May 2003, and the approved draft
specification is available for public review. ISO/IEC
International Standard 14496-10 will be technically identical to
ITU-T Recommendation H.264.
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Table of Contents
1. Introduction..............................................5
1.1. The H.264 codec.........................................5
1.2. Parameter Set Concept...................................6
1.3. Network Abstraction Layer Unit Types......................7
2. Conventions...............................................9
3. Scope ....................................................9
4. Definitions...............................................9
5. RTP Payload Format........................................ 11
5.1. RTP Header Usage....................................... 11
5.2. Common structure of the RTP payload format................ 13
5.3. Decoding Order Number (DON)............................. 14
5.4. Single NAL Unit Packet ................................. 16
5.5. Aggregation Packets.................................... 17
5.6. Fragmentation Units (FUs)............................... 25
6. Packetization Rules....................................... 28
6.1. Unrestricted Mode (Multiple Picture Model)................ 29
6.2. Restricted Mode (Single Picture Model).................... 30
7. De-Packetization Process (Informative)....................... 30
8. Payload Format Parameters.................................. 33
8.1. MIME Registration...................................... 33
8.2. SDP Parameters ........................................ 37
9. Security Considerations.................................... 38
10. Informative Appendix: Application Examples .................. 39
10.1. Video Telephony, No Slice Data Partitioning, No Packet
Aggregation ............................................... 39
10.2. Video Telephony, Interleaved Packetization Using Packet
Aggregation ............................................... 40
10.3. Video Telephony, with Data Partitioning.................. 41
10.4. Low-Bit-Rate Streaming................................. 41
10.5. Robust Packet Scheduling in Video Streaming.............. 42
11. Informative Appendix: Rationale for Decoding Order Number..... 43
11.1. Introduction......................................... 43
11.2. Example of Multi-Picture Slice Interleaving.............. 43
11.3. Example of Robust Packet Scheduling..................... 45
11.4. Robust Transmission Scheduling of Redundant Coded Slices... 49
11.5. Remarks on Other Design Possibilities.................... 50
12. Open Issues............................................. 51
13. Full Copyright Statement.................................. 51
14. Intellectual Property Notice............................... 52
15. References.............................................. 52
15.1. Normative References.................................. 52
15.2. Informative References................................. 53
Annex A: Changes relative to draft-ietf-avt-rtp-h264-01.txt....... 55
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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 and ISO/IEC
International Standard 14496 Part 10 (also known as MPEG-4 Advanced
Video Coding). Recommendation H.264 was approved by ITU-T on May
2003, and the approved draft specification is available for public
review [1]. 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 near loss-less coding. Most, if not all,
relevant companies in all of these fields (including Video-
Conferencing, Streaming, TV broadcast, and Digital Cinema) have
participated in the standardization, which gives hope that this
wide application range is more than an illusion and may
materialize, probably in a relatively short time frame. The
overall performance of H.264 is as such that bit rate savings of
50% or more, compared to the current state of technology, 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 [6].
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, things such as transform, quantization, motion
search/compensation, and the loop filter. It follows the general
concept of most of today's video codecs, a macroblock-based coder
that utilizes 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).
Macroblocks in slices are ordered in scan order unless a different
macroblock allocation is specified, using the so-called Flexible
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Macroblock Ordering syntax. In-picture prediction is used only
within a slice. More information is provided in [6].
The NAL encoder encapsulates the slice output of the VCL encoder
into Network Abstraction Layer Units (NAL units), which are
suitable for the transmission over packet networks or the 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.
Internally, the NAL uses NAL Units. A NAL unit consists of a one-
byte header and the payload byte string. The header co-serves as
the RTP payload header and 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
common in the form of the Temporal Reference in earlier video
compression standards). Also, there are NAL units that are
affecting many pictures and are, hence, inherently time-less. For
this reason, the handling of the RTP timestamp requires some
special considerations for those 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 RFC2429 [8] or MPEG-4's HEC [9] unnecessary. The
way how this was achieved is to decouple information that is
relevant to more than one slice from the media stream. This higher
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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 such 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
o picture size,
o optional coding modes employed, and
o macroblock to slice group map.
In order to be able to change picture parameters (such as the
picture size), without having the need 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.
This mechanism allows to decouple the transmission of parameter
sets from the packet stream, and transmit 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 get 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 [10],
[11] and [12].
All NAL units consist of a single NAL unit type octet, which also
co-serves as the payload header. The payload of a NAL unit follows
immediately.
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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 |
+---------------+
F: 1 bit
forbidden_zero_bit, when zero, indicates a bit error free NAL
unit. The H.264 specification declares a value of 1 as a syntax
violation. Hence, when set, the decoder is advised that bit
errors or any other syntax violation may be present in the
payload or in the NAL unit type octet. A prudent reaction of
decoders that are incapable of handling erroneous bit streams is
to discard such NAL units.
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 above
00 indicate that the decoding of the NAL unit is required to
maintain the integrity of the reference pictures. Furthermore,
values above 00 indicate the relative transport priority, as
determined by the encoder. Intelligent network elements can use
this information to protect more important NAL units better than
less important NAL units. 11 is the highest transport priority,
followed by 10, then by 01 and, finally, 00 is the lowest.
Type: 5 bits
nal_unit_type. The NAL Unit payload type as defined in table 7-
1 of [1], and later within this memo. Note that the NAL unit
types defined in this memo are marked as unspecified in [1].
For a reference of all currently defined NAL unit types and their
semantics please refer to section 7.4.1 in [1].
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2. Conventions
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 RFC 2119 [2].
This specification uses the notion of setting and clearing a bit
when handling bit fields. Setting a bit is the same as assigning
that bit the value of 1 (On). Clearing a bit is the same as
assigning that bit the value of 0 (Off).
3. Scope
This payload specification can only be used to carry the "naked"
H.264 NAL unit stream over RTP. Likely, the first applications of
this specification will be in the conversational multimedia field,
video telephone or video conference. The draft is not intended for
the use in conjunction with the byte stream format of Annex B of
H.264.
4. Definitions and Abbreviations
4.1. Definitions
This document uses the definitions of [1]. The following terms
defined in [1] are summed up below for convenience:
access unit: A set of NAL units always containing a primary
coded picture. In addition to the primary coded picture, an
access unit may also contain one or more redundant coded
pictures or other NAL units not containing slices or slice data
partitions of a coded picture. The decoding of an access unit
always results in a decoded picture.
coded video sequence: A sequence of access units that consists,
in decoding order, of an IDR access unit followed zero or more
non-IDR access units including all subsequent access units up to
but not including any subsequent IDR access unit.
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instantaneous decoding refresh (IDR) access unit: An access unit
in which the primary coded picture is an IDR picture.
instantaneous decoding refresh (IDR) picture: A coded picture
containing only slices with I or SI slice types that causes a
"reset" in the decoding process. After the decoding of an IDR
picture all following coded pictures in decoding order can be
decoded without inter prediction from any picture decoded prior
to the IDR picture.
primary coded picture: The coded representation of a picture to
be used by the decoding process for a bitstream conforming to
H.264. The primary coded picture contains all macroblocks of
the picture.
redundant coded picture: A coded representation of a picture or
a part of a picture. The content of a redundant coded picture
shall not be used by the decoding process for a bitstream
conforming to H.264. The content of a redundant coded picture
may be used by the decoding process for a bitstream that
contains errors or losses.
VCL NAL unit: A collective term used to refer to coded slice and
coded data partition NAL units.
In addition, the following definitions apply:
decoding order number (DON): A field in the payload structure or
a derived variable indicating NAL unit decoding order. Values
of DON are in the range of 0 to 65535, inclusive. After
reaching the maximum value, the value of DON wraps around to 0.
NAL unit decoding order: A NAL unit order that conforms to the
constraints on NAL unit order given in section 7.4.1.2 in [1].
transmission order: The order of packets in ascending RTP
sequence number order (in modulo arithmetic). Within an
aggregation packet, the NAL unit transmission order is the same
as the order of appearance of NAL units in the packet.
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4.2. Abbreviations
DON: Decoding Order Number
DONB: Decoding Order Number Base
DOND: Decoding Order Number Difference
FU: Fragmentation Unit
IDR: Instantaneous Decoding Refresh
IEC: International Engineering Consortium
ISO: International Organization for Standardization
ITU-T: International Telecommunication Union,
Telecommunication Standardization Sector
MTAP: Multi-Time Aggregation Packet
MTAP16: MTAP with 16-bit timestamp offset
MTAP24: MTAP with 24-bit timestamp offset
NAL: Network Adaptation Layer
NALU: NAL Unit
STAP: Single-Time Aggregation Packet
STAP-A: STAP type A
STAP-B: STAP type B
TS: Timestamp
VCL: Video Coding Layer
5. RTP Payload Format
5.1. RTP Header Usage
The format of the RTP header is specified in RFC 1889 [3] and
reprinted in Figure 1 for convenience. This payload format uses
the fields of the header in a manner consistent with that
specification.
When encapsulating one NAL unit per RTP packet, the RECOMMENDED RTP
payload format is specified in section 5.2. The RTP payload (and
the settings for some RTP header bits) for aggregation packets and
fragmentation units are specified in sections 5.5 and 5.6,
respectively.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P|X| CC |M| PT | sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| synchronization source (SSRC) identifier |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| contributing source (CSRC) identifiers |
| .... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: RTP header according RFC 1889.
The RTP header information is set as follows:
Version (V): 2 bits
Set to 2 according to RFC 1889.
Padding (P): 1 bit
Used according to RFC 1889.
Extension (X): 1 bit
Used according to RFC 1889 and profile definitions.
CSRC count (CC): 4 bits
Used according to RFC 1889.
Marker bit (M): 1 bit
Set for the very last packet of the access unit indicated by the
RTP timestamp, in line with the normal use of the M bit and to
allow an efficient playout buffer handling. Decoders MAY use
this bit as an early indication of the last packet of an access
unit, but MUST NOT rely on this property.
Informative note: Only one M bit is associated with an
aggregation packet carrying multiple NAL units, and thus if a
gateway has re-packetized an aggregation packet into several
packets, it cannot reliably set the M bit of those packets.
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Payload type (PT): 7 bits
The assignment of an RTP payload type for this new packet format
is outside the scope of this document, and will not be specified
here. The assignment of a payload type needs to be performed
either through the profile used or in a dynamic way.
Sequence number (SN): 16 bit
Increased by one for each sent packet. Set to a random value
during startup as per RFC1889
Timestamp: 32 bits
The RTP timestamp is set to the sampling timestamp of the
content. A 90 kHz clock rate MUST be used. If the content is a
part of a coded frame that was sampled as two fields having
distinct sampling times and that is supposed to be displayed as
fields having distinct display times, the RTP timestamp is set
to the sampling timestamp of the latest sampled field. In
addition, the picture timing SEI message (subclauses D.1.2 and
D.2.2 of [1]) SHOULD be used to convey the timestamps for
display, and the last clock timestamp in decoding order conveyed
in a picture timing SEI message MUST correspond to the RTP
timestamp of the primary coded picture of the same access unit.
If the NAL unit has no own timing properties (e.g. parameter set
and SEI NAL units), the RTP timestamp is set to the RTP
timestamp of the primary coded picture of the access unit to
which the NAL unit is included according to section 7.4.1.2 of
[1]. The setting of the RTP Timestamp for MTAPs is defined in
section 5.5.2.
Synchronization source (SSRC) identifier: 32 bits
Used according to RFC 1889.
Contributing source (CSRC) identifiers: 0 to 15 items, 32 bits each
Used according to RFC 1889.
5.2. Common structure of the RTP payload format
The payload format is defined as a number of different payload
structures depending on need. However which structure that a
received RTP packet contains is evident from the first byte of the
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payload. This byte will always be structured as a NAL unit header.
The NAL unit type field indicates which structure that is present.
The possible structures are:
Single NAL Unit Packet: Contains only a single NAL unit in the
payload. The NAL header type field will be equal to the original
NAL type, i.e., in the range of 1 to 23, inclusive. Specified in
section 5.4.
Aggregation packet: Packet type used to aggregate multiple NAL
units into a single RTP payload. This packet exists in four
versions, the Single-Time Aggregation Packet type A (STAP-A), the
Single-Time Aggregation Packet type B (STAP-B), Multi-Time
Aggregation Packet (MTAP) with 16 bit offset (MTAP16), and Multi-
Time Aggregation Packet (MTAP) with 24 bit offset (MTAP24). The
NAL type numbers assigned for STAP-A, STAP-B, MTAP16, and MTAP24
are 24, 25, 26, and 27 respectively. Specified in section 5.5.
Fragmentation packet: Used to fragment a single NAL unit over
multiple RTP packets. Identified with the NAL type number 27.
Specified in section 5.6.
Table 1 - Summary of NAL types and their payload structures.
Type Packet Type name Section
--------------------------------------------------------
1-23 NAL unit A single NAL unit packet 5.4
24 STAP-A Single-time aggregation packet 5.5.1
25 STAP-B Single-time aggregation packet 5.5.1
26 MTAP16 Multi-time aggregation packet 5.5.2
27 MTAP24 Multi-time aggregation packet 5.5.2
28 FU Fragmentation unit 5.6
5.3. Decoding Order Number (DON)
This memo allows transmission of NAL units out of their decoding
order. Decoding order number (DON) is a field in the payload
structure or a derived variable that indicates the NAL unit
decoding order. Rationale and example use cases for transmission
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out of decoding order and for the use of DON are given in section
11.
The coupling of transmission and decoding order is controlled by
the optional interleaving-depth MIME parameter as follows. When
the value of the optional interleaving-depth MIME parameter is
equal to 0 and transmission of NAL units out of their decoding
order is disallowed by external means, the transmission order of
NAL units MUST conform to the NAL unit decoding order. When the
value of the optional interleaving-depth MIME parameter is greater
than 0 or transmission of NAL units out of their decoding order is
allowed by external means,
o the order of NAL units in an STAP-B, an MTAP16, and an MTAP24 is
NOT REQUIRED to be the NAL unit decoding order, and
o the order of NAL units composed by decapsulating STAP-Bs, MTAPs,
and FUs in two consecutive packets, when neither of these packets
is a single NAL unit packet or an STAP-A, is NOT REQUIRED to be
the NAL unit decoding order.
The RTP payload structures for a single NAL unit packet and an
STAP-A do not include DON. STAP-B and FU structures include DON,
and the structure of MTAPs enables derivation of DON as specified
in section 5.5.2.
Informative note: If a transmitter wants to encapsulate one NAL
unit per packet and transmit packets out of their decoding order,
STAP-B packet type can be used.
The transmission order of NAL units in single NAL unit packets and
STAP-As MUST be the same as their NAL unit decoding order. The NAL
units within an STAP-A MUST appear in the NAL unit decoding order.
The transmission order of a single NAL unit packet or an STAP-A
MUST succeed the transmission order of any NAL unit that precedes
the single NAL unit packet or the STAP-A, respectively, in NAL unit
decoding order. The transmission order of a single NAL unit packet
or an STAP-A MUST precede the transmission order of any NAL unit
that succeeds the single NAL unit packet or the STAP-A,
respectively, in NAL unit decoding order.
Informative note: Due to the restrictions in the transmission order
of single NAL unit packets and STAP-As, the decoding order of
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single NAL unit packets and STAP-As is the same as their
transmission order, and all NAL units preceding a single NAL unit
packet or an STAP-A in transmission order are decoded before the
single NAL unit packet or the STAP-A, respectively.
The DON of the first NAL unit in transmission order MAY be set to
any value. Values of DON are in the range of 0 to 65535,
inclusive. After reaching the maximum value, the value of DON
wraps around to 0.
The decoding order of two NAL units is determined as follows. Let
the value of DON of one NAL unit be D1 and the value of DON of
another NAL unit be D2. If D1 < D2 and D2 - D1 < 32768, or if D1 >
D2 and D1 - D2 >= 32768, then the NAL unit having a value of DON
equal to D1 precedes the NAL unit having a value of DON equal to D2
in NAL unit decoding order. If D1 < D2 and D2 - D1 >= 32768, or if
D1 > D2 and D1 - D2 < 32768, then the NAL unit having a value of
DON equal to D2 precedes the NAL unit having a value of DON equal
to D1 in NAL unit decoding order.
Values of DON related fields (DON, DONB, and DOND, see section 5.5)
MUST be such that the decoding order determined by the values of
DON as specified above conforms to the NAL unit decoding order. If
the order of two consecutive NAL units in the NAL unit stream is
switched and the new order still conforms to the NAL unit decoding
order, the NAL units MAY have the same value of DON. For example,
when arbitrary slice order is allowed by the video coding profile
in use, all the coded slice NAL units of a coded picture are
allowed to have the same value of DON. Consequently, NAL units
having the same value of DON can be decoded in any order, and two
NAL units having a different value of DON should be passed to the
decoder in the order specified above.
An example decapsulation process to recover the NAL unit decoding
order is given in section 7.
5.4. Single NAL Unit Packet
The single NAL unit packet defined here MUST contain one and only
one NAL unit of the types defined in [1]. This means that neither
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an aggregation packet nor a fragmentation unit can be used within a
single NAL unit packet. A NAL unit stream composed by
decapsulating single NAL unit packets in RTP sequence number order
MUST conform to the NAL unit decoding order. The structure of the
single NAL unit packet is shown in Figure 2.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RTP Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Single NAL unit |
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :...OPTIONAL RTP padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2. RTP payload format for single NAL unit packet.
5.5. Aggregation Packets
Aggregation packets are the NALU unit aggregation scheme of this
payload specification. The scheme is introduced to reflect the
dramatically different MTU sizes of two key target networks --
wireline IP networks (with an MTU size that is often limited by the
Ethernet MTU size -- roughly 1500 bytes), and IP or non-IP (e.g.
ITU-T H.324/M) based wireless communication systems with preferred
transmission unit sizes of 254 bytes or less. In order to prevent
media transcoding between the two worlds, and to avoid undesirable
packetization overhead, a NAL unit aggregation scheme is
introduced.
Two types of aggregation packets are defined by this specification:
o Single-time aggregation packet (STAP) aggregates NAL units with
identical NALU-time. Two types of STAPs are defined, one without
DON (STAP-A) and another one including DON (STAP-B).
o Multi-time aggregation packet (MTAP) aggregates NAL units with
potentially differing NALU-time. Two different MTAPs are defined
that differ in the length of the NAL unit timestamp offset.
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The term NALU-time is defined as the value that the RTP timestamp
would have if that NAL unit would be transported in its own RTP
packet.
Each NAL unit to be carried in an aggregation packet is
encapsulated in an aggregation unit. Please see below for the
three different aggregation units and their characteristics.
The structure of the RTP payload format for aggregation packets is
presented in Figure 3.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|F|NRI| type | |
+-+-+-+-+-+-+-+-+ |
| |
| one or more aggregation units |
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :...OPTIONAL RTP padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3. RTP payload format for aggregation packets.
MTAPs and STAPs share the following packetization rules: The RTP
timestamp MUST be set to the earliest of the NALU times of all the
NAL units to be aggregated. The type field of the NAL unit type
octet MUST be set to the appropriate value as indicated in Table 2.
The F bit MUST be cleared if all F bits of the aggregated NAL units
are zero, otherwise it MUST be set.
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Table 2: Type field for STAPs and MTAPs
Type Packet Timestamp offset DON related fields
field length (DON, DONB, DOND)
(in bits) present
--------------------------------------------------------
24 STAP-A 0 no
25 STAP-B 0 yes
26 MTAP16 16 yes
27 MTAP24 24 yes
The marker bit in the RTP header MUST be set to the value the
marker bit of the last NAL unit of the aggregated packet would have
if it were transported in its own RTP packet.
The payload of an aggregation packet consists of one or more
aggregation units. See section 5.5.1 and 5.5.2 for the three
different types of aggregation units. An aggregation packet can
carry as many aggregation units as necessary, however the total
amount of data in an aggregation packet obviously MUST fit into an
IP packet, and the size SHOULD be chosen such that the resulting IP
packet is smaller than the MTU size. An aggregation packet MUST
NOT contain fragmentation units specified in section 5.6.
5.5.1. Single-Time Aggregation Packet
Single-time aggregation packet (STAP) SHOULD be used whenever
aggregating NAL units that share the same NALU-time. The payload
of an STAP-A does not include DON and consists of at least one
single-time aggregation unit as presented in Figure 4. The payload
of an STAP-B consists of a 16-bit unsigned decoding order number
(DON) followed by at least one single-time aggregation unit as
presented in Figure 5.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: |
+-+-+-+-+-+-+-+-+ |
| |
| single-time aggregation units |
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4. Payload format for STAP-A.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: decoding order number (DON) | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
| single-time aggregation units |
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5. Payload format for STAP-B.
A single-time aggregation unit consists of 16-bit unsigned size
information that indicates the size of the following NAL unit in
bytes (excluding these two octets, but including the NAL unit type
octet of the NAL unit), followed by the NAL unit itself including
its NAL unit type byte. A single-time aggregation unit is byte-
aligned within the RTP payload but it may not be aligned on a 32-
bit word boundary. Figure 6 presents the structure of the single-
time aggregation unit.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: NAL unit size | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
| NAL unit |
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6. Structure for single-time aggregation unit.
The NAL units within an STAP-A MUST appear in the NAL unit decoding
order. If the value of the optional interleaving-depth MIME
parameter is equal to 0 or transmission of NAL units out of their
decoding order is disallowed by other means, the NAL units within
an STAP-B MUST appear in the NAL unit decoding order. Otherwise,
the NAL units within an STAP-B are NOT REQUIRED to appear in the
NAL unit decoding order.
Figure 7 presents an example of an RTP packet that contains an
STAP-B. The STAP contains two single-time aggregation units,
labeled as 1 and 2 in the figure.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RTP Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|STAP-B NAL HDR | DON | NALU 1 Size |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NALU 1 Size | NALU 1 HDR | NALU 1 Data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
: |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | NALU 2 Size | NALU 2 HDR |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NALU 2 Data |
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :...OPTIONAL RTP padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7. An example of an RTP packet including an STAP-B and two
single-time aggregation units.
5.5.2. Multi-Time Aggregation Packets (MTAPs)
The NAL unit payload of MTAPs consists of a 16-bit unsigned
decoding order number base (DONB) and one or more multi-time
aggregation units as presented in Figure 8. DONB MUST contain the
smallest value of DON among the NAL units of the MTAP. The choice
between the different MTAP types (MTAP16 and MTAP24) is application
dependent -- the larger the timestamp offset is the higher is the
flexibility of the MTAP, but the higher is also the overhead.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: decoding order number base | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
| multi-time aggregation units |
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 8. NAL unit payload format for MTAPs.
Two different multi-time aggregation units are defined in this
specification. Both of them consist of 16 bits unsigned size
information of the following NAL unit, an 8-bit unsigned decoding
order number delta (DOND), and n bits of timestamp offset (TS
offset) for this NAL unit, whereby n can be 16 or 24. The
structure of the multi-time aggregation units for MTAP16 and MTAP24
are presented in Figure 9 and Figure 10 respectively. Note that
the starting or ending position of an aggregation unit within a
packet is NOT REQUIRED to be on a 32-bit word boundary. DON of the
following NAL unit is equal to DONB + DOND. The derived DON is
REQUIRED to be in the range of 0 to 65535, inclusive. This memo
does not specify how the NAL units within an MTAP are ordered, but,
in most cases, NAL unit decoding order SHOULD be used. The
timestamp offset field MUST be set to a value equal to the value of
the following formula: (the NALU-time of the NAL unit - the RTP
timestamp of the packet).
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: NAL unit size | DOND | TS offset |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TS offset | |
+-+-+-+-+-+-+-+-+ NAL unit |
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 9. Multi-time aggregation unit for MTAP16
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: NALU unit size | DOND | TS offset |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TS offset | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| NAL unit |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 10. Multi-time aggregation unit for MTAP24
For the "earliest" multi-time aggregation unit in an MTAP the
timing offset MUST be zero. Hence, the RTP timestamp of the MTAP
itself is identical to the earliest NALU-time.
Figure 11 presents an example of an RTP packet that contains a
multi-time aggregation packet of type MTAP16 that contains two
multi-time aggregation units, labeled as 1 and 2 in the figure.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RTP Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|MTAP16 NAL HDR | decoding order number base | NALU 1 Size |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NALU 1 Size | NALU 1 DOND | NALU 1 TS offset |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NALU 1 HDR | NALU 1 DATA |
+-+-+-+-+-+-+-+-+ +
: |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | NALU 2 SIZE | NALU 2 DOND |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NALU 2 TS offset | NALU 2 HDR | NALU 2 DATA |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :...OPTIONAL RTP padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 11. An example of an RTP packet including a multi-time
aggregation packet of type MTAP16 and two multi-time aggregation
units.
5.6. Fragmentation Units (FUs)
This payload type allows fragmenting a NAL unit into several RTP
packets. Doing so on the application layer instead of relying on
lower layer fragmentation (e.g. by IP) has the following
advantages:
o The payload format is capable of transporting NAL units bigger
than 64 kbytes over an IPv4 network that may be present in pre-
recorded video, particularly in High Definition formats (there is
a limit of the number of slices per picture, which results in a
limit of NAL units per picture, which may result in big NAL
units)
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o The fragmentation mechanisms allows fragmenting a single picture
and applying generic forward error correction, see e.g. [15], to
fragmentation units with identical timestamp.
Fragmentation is defined only for a single NAL unit, and not for
any aggregation packets. A fragment of a NAL unit consists of an
integer number of consecutive octets of that NAL unit. Each octet
of the NAL unit MUST be part of exactly one fragment of that NAL
unit. Fragments of the same NAL unit MUST be sent in consecutive
order with ascending RTP sequence numbers (with no other RTP
packets within the same RTP packet stream being sent between the
first and last fragment). Similarly, a NAL unit MUST be
reassembled in RTP sequence number order.
When a NAL unit is fragmented and conveyed within fragmentation
units (FUs), it is referred to as fragmented NAL unit. STAPs and
MTAPs MUST NOT be fragmented.
The RTP timestamp of an RTP packet carrying an FU is set to the
NALU time of the fragmented NAL unit.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| FU indicator | FU header | DON (optional) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
| |
| FU payload |
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :...OPTIONAL RTP padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 12. RTP payload format for fragmentation unit.
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The RTP payload of a fragmentation unit consists of fragmentation
unit indicator of one octet, a fragmentation unit header of one
octet, a 16-bit unsigned decoding order number (DON) that is
conditionally present, and a fragmentation unit payload.
A value equal to 28 in the NAL unit type field of the FU indicator
octet identifies an FU. The FU indicator octet has the following
format:
+---------------+
|0|1|2|3|4|5|6|7|
+-+-+-+-+-+-+-+-+
|F|NRI| 28 |
+---------------+
The use of the F bit is described in section 1.3. The value of the
NRI field MUST be set according to the value of the NRI field in
the fragmented NAL unit.
The FU header has the following format:
+---------------+
|0|1|2|3|4|5|6|7|
+-+-+-+-+-+-+-+-+
|S|E|R| Type |
+---------------+
S: 1 bit
The Start bit, when one, indicates the start of a fragmented NAL
unit. Otherwise, when the following FU payload is not the start
of a fragmented NAL unit payload, the Start bit is set to zero.
E: 1 bit
The End bit, when one, indicates the end of a fragmented NAL
unit, i.e. the last byte of the payload is also the last byte of
the fragmented NAL unit. Otherwise, when the following FU
payload is not the last fragment of a fragmented NAL unit, the
End bit is set to zero.
R: 1 bit
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The Reserved bit MUST be equal to 0 and MUST be ignored by the
receiver.
Type: 5 bits
The NAL Unit payload type as defined in table 7-1 of [1].
DON is present when the S bit in the FU header is equal to 1. The
value of DON is selected as described in section 5.3.
Informative note: The DON field in FUs allows gateways to fragment
NAL units to FUs without organizing the incoming NAL units to the
NAL unit decoding order.
A fragmented NAL unit MUST NOT be transmitted in one FU, i.e.,
Start bit and End bit MUST NOT both be set to one in the same FU
header. A fragmentation unit MUST NOT contain an aggregation
packet. Fragmentation units of a NAL unit MUST be sent in
consecutive packets of the same RTP stream.
The FU payload consists of fragments of the payload of the
fragmented NAL unit such that if the fragmentation unit payloads of
consecutive FUs are sequentially concatenated, the payload of the
fragmented NAL unit is reconstructed. Note that the NAL unit type
octet of the fragmented NAL unit is not included as such in the
fragmentation unit payload, but rather the information of the NAL
unit type octet of the fragmented NAL unit is conveyed in F and NRI
fields of the FU indicator octet of the fragmentation unit and in
the type field of the FU header. A FU payload MAY have any number
of octets and MAY be empty.
If a fragmentation unit is lost, the receiver SHOULD discard all
following fragmentation units in transmission order corresponding
to the same fragmented NAL unit.
6. Packetization Rules
Two cases of packetization rules have to be distinguished by the
possibility to put packets belonging to more than a single picture
into a single aggregated packet (using MTAPs).
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6.1. Unrestricted Mode (Multiple Picture Model)
This mode MAY be supported by some receivers. Usually, the
capability of a receiver to support this mode is implied by the
application, or indicated by external control protocol means. The
use of this mode MUST be signaled with the optional mtap-allowed
MIME or SDP parameter, if MIME or SDP signaling is in use. The
following packetization rules MUST be enforced by the sender:
o All packet types specified in Error! Reference source not found.
MAY be used.
o The transmission order of packets and NAL units is constrained as
specified in section 5.3. Some receivers MAY NOT support a
transmission order that does not conform to the NAL unit decoding
order.
o Coded slice NAL units or coded slice data partition NAL units
belonging to the same coded picture (and hence share the same RTP
timestamp value) MAY be sent in any order permitted by the
applicable profile defined in [1], although, for delay-critical
systems, they SHOULD be sent in their original coding order to
minimize the delay. Note that the coding order is not
necessarily the scan order, but the order the NAL packets become
available to the RTP stack.
o Sequence and picture parameter set NAL units MUST NOT be sent in
an RTP session whose parameter sets were already changed by
control protocol messages during the lifetime of the RTP session.
o Network elements such as gateways MAY convert single NAL unit
packets into one aggregation packet, convert an aggregation
packet into several single NAL unit packets, or mix both
concepts. However, when doing so they SHOULD take into account
at least the following parameters: path MTU size, unequal
protection mechanisms (e.g. through packet-based FEC according to
RFC2398, carried by RFC2198, especially for sequence and picture
parameter set NAL units and coded slice data partition A NAL
units), bearable latency of the system, and buffering
capabilities of the receiver.
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o Network elements such as gateways MUST NOT duplicate any NAL unit
except for sequence or picture parameter set NAL units, because
neither this memo nor the H.264 specification provides means to
identify duplicated NAL units. Sequence and picture parameter
set NAL units MAY be duplicated to make their correct reception
more probable, but any such duplication MUST NOT affect the
contents of any active sequence or picture parameter set.
6.2. Restricted Mode (Single Picture Model)
This mode MUST be supported by all receivers. It is primarily
intended for low-delay applications. Its main difference from the
Unrestricted Mode is to forbid the packetization of data belonging
to more than one picture in a single RTP packet. Hence, MTAPs MUST
NOT be used. The following packetization rules MUST be enforced by
the sender:
o All rules of the Unrestricted Mode above, but MTAPs MUST NOT be
used. This implies that aggregation packets MUST NOT include
coded slices or coded slice data partitions belonging to
different access units.
7. De-Packetization Process (Informative)
The de-packetization process is implementation dependent. Hence,
the following description should be seen as an example of a
suitable implementation. Other schemes may be used as well.
Optimizations relative to the described algorithms are likely
possible.
The general concept behind these de-packetization rules is to
reorder NAL units from transmission order to the NAL unit decoding
order.
The receiver includes a receiver buffer, which is used to reorder
packets from transmission or to the NAL unit decoding order. The
receiver may use the following guidelines when determining the size
of the receiver buffer. The optional interleaving-depth MIME
parameter indicates the size of the receiver buffer as the number
of VCL NAL units. The size of the receiver buffer in bytes may be
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estimated by multiplying the optional initial-buffering-time MIME
parameter with the bandwidth-value of the corresponding media level
SDP description parameter, if available (and taking into account
the necessary conversions between the units of initial-buffering-
time and bandwidth-value). The receiver should take buffering for
transmission delay jitter into account too and either reserve a
separate buffer for transmission delay jitter buffering or combine
the buffer for transmission delay jitter with the receiver buffer.
The receiver stores incoming NAL units in reception order into the
receiver buffer as follows. NAL units contained in a single NAL
unit packet are stored into the receiver buffer as such. NAL units
of aggregation packets are stored into the receiver buffer
individually. For each NAL unit of a single NAL unit packet and an
aggregation packet, the RTP sequence number of the packet that
contained the NAL unit is stored and associated with the stored NAL
unit. All fragments of a fragmented NAL unit are collected and
stored into the receiver buffer. For a fragmented NAL unit, the
RTP sequence number of the RTP packet containing the first fragment
in transmission order is stored and associated with the fragmented
NAL unit. Moreover, for each NAL unit, the packet type (single NAL
unit packet, STAP-A, STAP-B, MTAP16, MTAP24, or fragmentation unit)
that contained the NAL unit is stored and associated with each
stored NAL unit. Furthermore, for NAL units carried in STAP-Bs,
MTAPs, and FUs, decoding order number (DON) is calculated and
stored.
Hereinafter, let N be the value of the optional interleaving-depth
MIME type parameter (see section 8.1). When the RTP session is
initialized, the receiver buffers at least N VCL NAL units or at
least for the duration equal to the value of the optional initial-
buffering-time MIME parameter, if available, whichever results into
shorter initial buffering duration, before passing any packet to
the decoder.
If the receiver buffer contains at least N VCL NAL units, NAL units
are removed from the receiver buffer and passed to the decoder in
the order specified below until the buffer contains N-1 VCL NAL
units. Variable ts is set to the value of system timer that was
initialized to 0 when the first packet of the NAL unit stream was
received. If the receiver buffer contains a NAL unit whose
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reception time tr fulfills the condition that ts - tr > initial-
buffering-time, NAL units are passed to the decoder (and removed
from the receiver buffer) in the order specified below until the
receiver buffer contains no NAL unit whose reception time tr
fulfills the specified condition. Note that transmission delay
jitter should be taken into account in the calculations with
timestamps.
The order that NAL units are passed to the decoder is specified as
follows:
o Let PDON be a variable that is initialized to 0 at the beginning
of the an RTP session.
o If the oldest RTP sequence number in the buffer corresponds to a
single NAL unit packet or an STAP-A, the NAL unit in the single
NAL unit packet or the STAP-A, respectively, is the next NAL unit
in the NAL unit decoding order. The value of PDON is set to 0.
o If the oldest RTP sequence number in the buffer corresponds to An
STAP-B, an MTAP, or an FU, the NAL unit decoding order is
recovered among the NAL units conveyed in STAP-Bs, MTAPs, and FUs
in RTP sequence number order until the next single NAL unit
packet or the next STAP-A (exclusive). This set of NAL units is
hereinafter referred to as the candidate NAL units. If no NAL
units conveyed in single NAL unit packets or STAP-As reside in
the receiver buffer, all NAL units belong to candidate NAL units.
o For each NAL unit among the candidate NAL units, a DON distance
is calculated as follows. If the value of DON of the NAL unit is
larger than the value of PDON, the DON distance is equal to DON -
PDON. Otherwise, the DON distance is equal to 65535 - PDON + DON
+ 1. NAL units are delivered to the decoder in ascending order
of DON distance. When a desired amount of NAL units have been
passed to the decoder, the value of PDON is set to the value of
DON for the last NAL unit passed to the decoder.
o If several NAL units share the same DON distance, they can be
passed to the decoder in any order.
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o If the video decoder in use does not support Arbitrary Slice
Order, the decoding order of coded slices and coded slice data
partitions A is ordered in ascending order of the
first_mb.in.slice syntax element in the slice header. Moreover,
coded slice data partition B and C immediately follow the
corresponding coded slice data partition A in decoding order.
The following additional de-packetization rules may be used to
implement an operational H.264 de-packetizer:
o Intelligent RTP receivers (e.g. in gateways) may identify lost
coded slice data partitions A (DPAs). If a lost DPA is found, a
gateway may decide not to send the corresponding coded slice data
partitions B and C, as their information is meaningless for H.264
decoders. In this way a network element can reduce network load
by discarding useless packets, without parsing a complex bit
stream.
o Intelligent receivers may discard all packets in which the value
of the NRI field of the NAL unit type octet is equal to 0.
However, they process those packets if possible, because the user
experience may suffer if the packets are discarded.
8. Payload Format Parameters
This section specifies the parameters that MAY be used to select
optional features of the payload format. The parameters are
specified here as part of the MIME subtype registration for the
ITU-T H.264 | ISO/IEC 14496-10 codec. A mapping of the parameters
into the Session Description Protocol (SDP) [4] is also provided
for those applications that use SDP. Equivalent parameters could
be defined elsewhere for use with control protocols that do not use
MIME or SDP.
8.1. MIME Registration
The MIME subtype for the ITU-T H.264 | ISO/IEC 14496-10 codec is
allocated from the IETF tree.
Any unspecified parameter MUST be ignored by the receiver.
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Media Type name: video
Media subtype name: H264
Required parameters: none
Optional parameters:
profile-level-id: A profile-level element used in specifying the
value of this parameter is generated by
forming a string of hexadecimal
representations of the following three bytes
in the sequence parameter set NAL unit
specified in [1]: 1) profile_idc, 2) a byte
herein referred to as profile-iop, composed of
the values of constrained_set0_flag,
constrained_set1_flag, constrained_set2_flag,
and reserved_zero_5bits in bit-significance
order starting from the most significant bit,
and 3) level_idc. Note that
reserved_zero_5bits is required to be equal to
0 in [1], but other values for it may be
specified in the future by ITU-T or ISO/IEC.
The value of profile-level-id is a sequence of
profile-level elements. If this parameter is
used for indicating properties of a NAL unit
stream, it indicates the profiles that are in
use in the stream and the highest level that
is in use for each signaled profile. The
profile-iop byte for each signaled profile
indicates whether the NAL unit stream also
obeys all constraints of the indicated
profiles as follows. If bit 7 (the most
significant bit), bit 6, or bit 5 of profile-
iop is equal to 1, all constraints of the
Baseline profile, the Main profile, or the
Extended profile, respectively, are obeyed in
the NAL unit stream. If the profile-level-id
parameter is used for capability exchange or
session setup procedure, it indicates the
profiles that the codec supports and the
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highest level that is supported for each
signaled profile. The profile-iop byte for
each signaled profile indicates whether the
codec has such additional limitations that
only the common subset of the algorithmic
features and limitations of the profiles
signaled with the profile-iop byte and the
profile indicated by profile_idc is supported
by the codec. For example, if a codec
supports the Baseline Profile at level 3 and
below and the Main Profile at level 2.1 and
below without any additional limitations, the
profile-level-id becomes 42A01E4D4015. If a
codec supports only the common subset of the
coding tools of the Baseline profile and the
Main profile at level 2.1 and below, the
profile-level-id becomes 42E015. If no
profile-level-id is present, the Baseline
Profile without additional constraints at
Level 1 MUST be implied.
parameter-sets: This parameter MAY be used to convey such
sequence and picture parameter set NAL units,
herein referred to as the initial parameter
set NAL units, that MUST precede any other NAL
units in decoding order. The parameter MUST
NOT be used to indicate codec capability in
any capability exchange procedure. The value
of the parameter is the hexadecimal
representation of the initial parameter set
NAL units as specified in sections 7.3.2.1 and
7.3.2.2 of [1]. The parameter sets are
conveyed in decoding order and no framing of
the parameter set NAL units takes place. A
comma is used to separate any pair of
parameter sets in the list. Note that the
number of bytes in a parameter set NAL unit is
typically less than 10 bytes, but a picture
parameter set NAL unit can contain several
hundreds of bytes.
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interleaving-depth: This parameter MAY be used to signal the
properties of a NAL unit stream or the
capabilities of a transmitter or receiver
implementation. The parameter specifies the
maximum amount of VCL NAL units that precede
any VCL NAL unit in the NAL unit stream in
transmission order and follow the VCL NAL unit
in decoding order. If the parameter is not
present, then a value of 0 MUST be used for
interleaving-depth. The value of
interleaving-depth MUST be an integer in the
range from 0 to 32767, inclusive.
initial-buffering-time: This parameter MAY be used to signal the
properties of a NAL unit stream or the
capabilities of a transmitter or receiver
implementation. The parameter is a decimal
representation of the maximum time in clock
ticks of a 90-kHz clock between the
transmission time of any NAL unit and the
decoding time of the NAL unit assuming
reliable and instantaneous transmission and
the same timeline for transmission and
decoding. If the parameter is not present,
then a value of 0 MUST be used for initial-
buffering-time. The value of initial-
buffering-time MUST be an integer in the range
from 0 to 4 294 967 295, inclusive. Receivers
SHOULD take transmission delay jitter
buffering, including buffering for the delay
jitter caused by mixers, translators,
gateways, proxies, traffic-shapers and other
network elements, into account in addition to
the signaled initial-buffering-time.
mtap-allowed: Permissible values are 0 and 1. If 0 or not
present, MTAPs MUST NOT be present in the NAL
unit stream. If 1, MTAPs MAY be present in the
NAL unit stream.
Encoding considerations:
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This type is only defined for transfer via RTP
(RFC 1889).
Security considerations:
See section 9 of RFC XXXX. [Ed.Note: to be
replaced with the RFC number of this
specification]
Public specification:
Please refer to RFC XXXX [Ed.Note: to be
replaced with the RFC number of this
specification] and its section 15.
Additional information:
None
File extensions: none
Macintosh file type code: none
Object identifier or OID: none
Person & email address to contact for further information:
stewe@cs.tu-berlin.de
Intended usage: COMMON.
Author/Change controller:
stewe@cs.tu-berlin.de
IETF Audio/Video transport working group
8.2. SDP Parameters
The MIME media type video/H264 string is mapped to fields in the
Session Description Protocol (SDP) [4] as follows:
o The media name in the "m=" line of SDP MUST be video.
o The encoding name in the "a=rtpmap" line of SDP MUST be H264 (the
MIME subtype).
o The clock rate in the "a=rtpmap" line MUST be 90000.
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o The "a=fmtp" line of SDP MUST contain the optional parameters
"profile-level-id", "parameter-sets", "interleaving-depth",
"initial-buffering-time", and "mtap-allowed", if any, to indicate
the coder capability and configuration, respectively. These
parameters are expressed as a MIME media type string, in the form
of as a semicolon separated list of parameter=value pairs.
An example of media representation in SDP is as follows (Baseline
Profile, Level 3.0, more than one slice group, arbitrary slice
ordering, and redundant slices are in use):
m=video 49170/2 RTP/AVP 98
a=rtpmap:98 H264/90000
a=fmtp:98 profile-level-id=42A01E
9. Security Considerations
RTP packets using the payload format defined in this specification
are subject to the security considerations discussed in the RTP
specification [1], and any appropriate RTP profile (for example
[2]).
This implies that confidentiality of the media streams is achieved
by encryption. Because the data compression used with this payload
format is applied end-to-end, encryption may be performed after
compression so there is no conflict between the two operations.
A potential denial-of-service threat exists for data encodings
using compression techniques that have non-uniform receiver-end
computational load. The attacker can inject pathological datagrams
into the stream which are complex to decode and cause the receiver
to be overloaded. H.264 is particularly vulnerable to such attacks
because it is extremely simple to generate datagrams containing NAL
units that affect the decoding process of many future NAL units.
As with any IP-based protocol, in some circumstances a receiver may
be overloaded simply by the receipt of too many packets, either
desired or undesired. Network-layer authentication may be used to
discard packets from undesired sources, but the processing cost of
the authentication itself may be too high. In a multicast
environment, pruning of specific sources may be implemented in
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future versions of IGMP [5] and in multicast routing protocols to
allow a receiver to select which sources are allowed to reach it.
10. Informative Appendix: Application Examples
This payload specification is very flexible in its use, to cover
the extremely wide application space that is anticipated for the
H.264. However, such a great flexibility also makes it difficult
for an implementer to decide on a reasonable packetization scheme.
Some information how to apply this specification to real-world
scenarios is likely to appear in the form of academic publications
and a test model software and description in the near future.
However, some preliminary usage scenarios are described here as
well.
10.1. Video Telephony according to ITU-T Recommendation H.241
Annex A
H.323-based video telephony systems that use H.264 as an optional
video compression scheme are required to support H.241 Annex A as a
packetization scheme. The packetization mechanism defined in this
Annex is technically identical with a small subset of this
specification.
When operating according to H.241 Annex A, parameter sets NAL units
are sent in-band. Only Single NAL unit packets are used. A
typical packet stream generated by such a system consists of all
sequence and picture parameter sets used for the future video
sequence, possibly sent in more than one copy to raise the
likeliness of their arrival at the receiver, followed by the
packets carrying the NAL units of the IDR picture, and followed by
packets carrying the subsequent pictures. Many such systems are
not sending IDR pictures regularly, but only when required by user
interaction or by control protocol means, e.g. when switching
between video channels in an Multipoint Control Unit.
10.2. Video Telephony, No Slice Data Partitioning, No NAL unit
Aggregation
The RTP part of this scheme is implemented and tested (though not
the control-protocol part, see below).
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In most real-world video telephony applications, the picture
parameters such as picture size or optional modes never change
during the lifetime of a connection. Hence, all necessary
parameter sets (usually only one) are sent as a side effect of the
capability exchange/announcement process e.g. according to the SDP
syntax specified in section 8.2 of this document. Since all
necessary parameter set information is established before the RTP
session starts, there is no need for sending any parameter set NAL
units. Slice data partitioning is not used either. Hence, the RTP
packet stream consists basically of NAL units that carry single
coded slices.
The size of those single-slice NAL units is chosen by the encoder
such that they offer the best performance. Often, this is done by
adapting the coded slice size to the MTU size of the IP network.
For small picture sizes this may result in a one-picture-per-one-
packet strategy. The loss of packets and the resulting drift-
related artifacts are cleaned up by Intra refresh algorithms.
10.3. Video Telephony, Interleaved Packetization Using NAL unit
Aggregation
This scheme allows better error concealment and is widely used in
H.263 based designed using RFC2429 packetization. It is also
implemented and good results were reported [10].
The VCL encoder codes the source picture such that all macroblocks
(MBs) of one MB line are assigned to one slice. All slices with
even MB row addresses are combined into one STAP, and all slices
with odd MB row addresses into another STAP. Those STAPs are
transmitted as RTP packets. The establishment of the parameter
sets is performed as discussed above.
Note that the use of STAPs is essential here, because the high
number of individual slices (18 for a CIF picture) would lead to
unacceptably high IP/UDP/RTP header overhead (unless the source
coding tool FMO is used, which is not assumed in this scenario).
Furthermore, some wireless video transmission systems, such as
H.324M and the IP-based video telephony specified in 3GPP, are
likely to use relatively small transport packet size. For example,
a typical MTU size of H.223 AL3 SDU is around 100 bytes [13].
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Coding individual slices according to this packetization scheme
provides a further advantage in communication between wired and
wireless networks, as individual slices are likely to be smaller
than the preferred maximum packet size of wireless systems.
Consequently, a gateway can convert the STAPs used in a wired
network to several RTP packets with only one NAL unit that are
preferred in a wireless network and vice versa.
10.4. Video Telephony, with Data Partitioning
This scheme is implemented and was shown to offer good performance
especially at higher packet loss rates [10].
Data Partitioning is known to be useful only when some form of
unequal error protection is available. Normally, in single-session
RTP environments, even error characteristics are assumed --
statistically, the packet loss probability of all packets of the
session is the same. However, there are means to reduce the packet
loss probability of individual packets in an RTP session. RFC 2198
[14], for example, allows carrying a redundant copy of a essential
packet in the next RTP packet. Packet-based Forward Error
Correction [15] carried in RFC2198 is also an appropriate means to
protect high priority information.
In all cases, the incurred overhead is substantial, but in the same
order of magnitude as the number of bits that have otherwise be
spent for intra information. However, this mechanism is not adding
any delay to the system.
Again, the complete parameter set establishment is performed
through control protocol means.
10.5. Low-Bit-Rate Streaming
This scheme has been implemented with H.263 and gave good results
[16]. There is no technical reason why similarly good results
could not be achievable with H.264.
In today's Internet streaming, some of the offered bit-rates are
relatively low in order to allow terminals with dial-up modems to
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access the content. In wired IP networks, relatively large
packets, say 500 - 1500 bytes, are preferred to smaller and more
frequently occurring packets in order to reduce network congestion.
Moreover, use of large packets decreases the amount of RTP/UDP/IP
header overhead. For low-bit-rate video, the use of large packets
means that sometimes up to few pictures should be encapsulated in
one packet.
However, loss of a packet including many coded pictures would have
drastic consequences in visual quality, as there is practically no
other way to conceal a loss of an entire picture than to repeat the
previous one. One way to construct relatively large packets and
maintain possibilities for successful loss concealment is to
construct MTAPs that contain slices from several pictures in an
interleaved manner. An MTAP should not contain spatially adjacent
slices from the same picture or spatially overlapping slices from
any picture. If a packet is lost, it is likely that a lost slice
is surrounded by spatially adjacent slices of the same picture and
spatially corresponding slices of the temporally previous and
succeeding pictures. Consequently, concealment of the lost slice
is likely to succeed relatively well.
10.6. Robust Packet Scheduling in Video Streaming
This scheme has been implemented with MPEG-4 Part 2 and simulated
in a wireless streaming environment [17]. There is no technical
reason why similar or better results could not be achievable with
H.264.
Streaming clients typically have a receiver buffer that is capable
of storing a relatively large amount of data. Initially, when a
streaming session is established, a client does not start playing
the stream back immediately, but rather it typically buffers the
incoming data for a few seconds. This buffering helps to maintain
continuous playback, because, in case of occasional increased
transmission delays or network throughput drops, the client can
decode and play buffered data. Otherwise, without initial
buffering, the client has to freeze the display, stop decoding, and
wait for incoming data. The buffering is also necessary for either
automatic or selective retransmission in any protocol level. If
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any part of a picture is lost, a retransmission mechanism may be
used to resend the lost data. If the retransmitted data is
received before its scheduled decoding or playback time, the loss
is perfectly recovered. Coded pictures can be ranked according to
their importance in the subjective quality of the decoded sequence.
For example, non-reference pictures, such as conventional B
pictures, are subjectively least important, because their absence
does not affect decoding of any other pictures. In addition to
non-reference pictures, the ITU-T H.264 | ISO/IEC 14496-10 standard
includes a temporal scalability method called sub-sequences [18].
Subjective ranking can also be made on coded slice data partition
or slice group basis. Coded slices and coded slice data partitions
that are subjectively the most important can be sent earlier than
their decoding order indicates, whereas coded slices and coded
slice data partitions that are subjectively the least important can
be sent later than their natural coding order indicates.
Consequently, any retransmitted parts of the most important slices
and coded slice data partitions are more likely to be received
before their scheduled decoding or playback time compared to the
least important slices and slice data partitions.
11. Informative Appendix: Rationale for Decoding Order Number
11.1. Introduction
The Decoding Order Number (DON) concept was introduced mainly to
enable efficient multi-picture slice interleaving (see section
10.5) and robust packet scheduling (see section 10.6). In both of
these applications NAL units are transmitted out of decoding order.
DON indicates the decoding order of NAL units and should be used in
the receiver the recover the decoding order. Example use cases for
efficient multi-picture slice interleaving and for robust packet
scheduling are given in sections 11.2 and 11.3 respectively.
Section 11.4 describes the benefits of the DON concept in error
resiliency achieved by redundant coded pictures. Section 11.5
summarizes considered alternatives to DON and justifies why DON was
chosen to this RTP payload specification.
11.2. Example of Multi-Picture Slice Interleaving
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An example of multi-picture slice interleaving follows. A subset
of a coded video sequence is depicted below in output order. R
denotes a reference picture, N denotes a non-reference picture, and
the number indicates a relative output time.
... R1 N2 R3 N4 R5 ...
The decoding order of these picture is from left to right as
follows:
... R1 R3 N2 R5 N4 ...
The NAL units of pictures R1, R3, N2, R5, and N4 are marked with a
DON equal to 1, 2, 3, 4, and 5, respectively.
Each reference picture consists of three slice groups that are
scattered as follows (a number denotes the slice group number for
each macroblock in a QCIF frame):
0 1 2 0 1 2 0 1 2 0 1
2 0 1 2 0 1 2 0 1 2 0
1 2 0 1 2 0 1 2 0 1 2
0 1 2 0 1 2 0 1 2 0 1
2 0 1 2 0 1 2 0 1 2 0
1 2 0 1 2 0 1 2 0 1 2
0 1 2 0 1 2 0 1 2 0 1
2 0 1 2 0 1 2 0 1 2 0
1 2 0 1 2 0 1 2 0 1 2
For the sake of simplicity, we assume that all the macroblocks of a
slice group are included in one slice. Three MTAPs are constructed
from three consecutive reference pictures so that each MTAP
contains three aggregation units, each of which contains all the
macroblocks from one slice group. The first MTAP contains slice
group 0 of picture R1, slice group 1 of picture R2, and slice group
2 of picture R3. The second MTAP contains slice group 1 of picture
R1, slice group 2 of picture R2, and slice group 0 of picture R3.
The third MTAP contains slice group 2 of picture R1, slice group 0
of picture R2, and slice group 1 of picture R3. Each non-reference
picture is encapsulated into an STAP-B.
Consequently, the transmission order of NAL units is the following:
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R1, slice group 0, DON 1
R3, slice group 1, DON 2
R5, slice group 2, DON 4
R1, slice group 1, DON 1
R3, slice group 2, DON 2
R5, slice group 0, DON 4
R1, slice group 2, DON 1
R3, slice group 1, DON 2
R5, slice group 0, DON 4
N2, DON 3
N4, DON 5
The receiver is able to organize the NAL units back in decoding
order based on the value of DON associated with each NAL unit.
If one the MTAPs is lost, the spatially adjacent and temporally co-
located macroblocks are received and can be used to conceal the
loss efficiently. If one of the STAPs is lost, the effect of the
loss does not propagate temporally.
11.3. Example of Robust Packet Scheduling
An example of robust packet scheduling follows. The communication
system used in the example consists of the following components in
the order that the video is processed from source to sink:
o camera and capturing
o pre-encoding buffer
o encoder
o encoded picture buffer
o transmitter
o transmission channel
o receiver
o receiver buffer
o decoder
o decoded picture buffer
o display
The video communication system used in the example operates as
follows. Note that processing of the video stream happens
gradually and at the same time in all components of the system.
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The source video sequence is shot and captured to a pre-encoding
buffer. The pre-encoding buffer can be used to order pictures from
sampling order to encoding order or to analyze multiple
uncompressed frames for bitrate rate control purposes, for
example. In some cases the pre-encoding buffer may not exist, but
rather the sampled pictures are encoded right away. The encoder
encodes pictures from the pre-encoding buffer and stores the
output, i.e., coded pictures, to the encoded picture buffer. The
transmitter encapsulates the coded pictures from the encoded
picture buffer to transmission packets and sends them to a receiver
through a transmission channel. The receiver stores the received
packets to the receiver buffer. The receiver buffering process
typically includes buffering for transmission delay jitter. The
receiver buffer can also be used to recover correct decoding order
of coded data. The decoder reads coded data from the receiver
buffer and produces decoded pictures as output into the decoded
picture buffer. The decoded picture buffer is used to recover the
output (or display) order of pictures. Finally, pictures are
displayed.
In the following example figures, I denotes an IDR picture, R
denotes a reference picture, N denotes a non-reference picture, and
the number after I, R, or N indicates a relative sampling time
proportional to the previous IDR picture in decoding order. Values
below the sequence of pictures indicate scaled system clock
timestamps. The system clock is initialized arbitrarily in this
example, and time runs from left to right. Each I, R, and N
picture is mapped into the same timeline compared to the previous
processing step, if any, assuming that encoding, transmission, and
decoding take no time. Thus, events happening at the same time are
located in the same column throughout all example figures.
A subset of a sequence of coded pictures is depicted below in
sampling order.
... N58 N59 I00 N01 N02 R03 N04 N05 R06 ... N58 N59 I00 N01 ...
... --|---|---|---|---|---|---|---|---|- ... -|---|---|---|- ...
... 58 59 60 61 62 63 64 65 66 ... 128 129 130 131 ...
The sampled pictures are buffered in the pre-encoding buffer to
arrange them in encoding order. In this example, we assume that
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the non-reference pictures are predicted from both the previous and
the next reference picture in output order. Thus, the pre-encoding
buffer has to contain at least two pictures and the buffering
causes a delay of two picture intervals. The output of the pre-
encoding buffering process, and the encoding (and decoding) order
of the pictures is as follows:
... N58 N59 I00 R03 N01 N02 R06 N04 N05 ...
... -|---|---|---|---|---|---|---|---|- ...
... 60 61 62 63 64 65 66 67 68 ...
The encoder or the transmitter can set the value of DON for each
picture to a value of DON for the previous picture in decoding
order plus one.
For the sake of simplicity, let us assume that:
o the frame rate of the sequence is constant,
o each picture consists on only one slice,
o each slice is encapsulated in a single NAL unit packet,
o pictures are transmitted in decoding order, and
o pictures are transmitted at constant intervals (that is equal to
1 / frame rate).
Thus, pictures are received in decoding order:
... N58 N59 I00 R03 N01 N02 R06 N04 N05 ...
... -|---|---|---|---|---|---|---|---|- ...
... 60 61 62 63 64 65 66 67 68 ...
The optional interleaving-depth MIME type parameter is set to 0,
because no buffering is needed to recover the correct decoding
order from transmission (or reception order).
The decoder has to buffer for one picture interval initially in its
decoded picture buffer to organize pictures from decoding order to
output order as depicted below:
... N58 N59 I00 N01 N02 R03 N04 N05 R06 ...
... -|---|---|---|---|---|---|---|---|- ...
... 61 62 63 64 65 66 67 68 69 ...
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The amount of required initial buffering in the decoded picture
buffer can be signaled in the buffering period SEI message or with
the num_reorder_frames syntax element of H.264 video usability
information. num_reorder_frames indicates the maximum number of
frames, complementary field pairs, or non-paired fields that
precede any frame, complementary field pair, or non-paired field in
the sequence in decoding order and follow it in output order. For
the sake of simplicity, we assume that num_reorder_frames is used
to indicate the initial buffer in the decoded picture buffer. In
this example, num_reorder_frames is equal to 1.
It can be observed that if the IDR picture I00 is lost during
transmission and a retransmission request is issued when the value
of the system clock is 62, there is one picture interval of time
(until the system clock reaches timestamp 63) to receive the
retransmitted IDR picture I00.
Let us then assume that IDR pictures are transmitted two frame
intervals earlier than their decoding position, i.e., the pictures
are transmitted as follows:
... I00 N58 N59 R03 N01 N02 R06 N04 N05 ...
... --|---|---|---|---|---|---|---|---|- ...
... 62 63 64 65 66 67 68 69 70 ...
The optional interleaving-depth MIME type parameter is set equal to
1 according to its definition. (The value of interleaving-depth in
this example can be derived as follows: Picture I00 is the only
picture preceding picture N58 or N59 in transmission order and
following it in decoding order. Except for pictures I00, N58, and
N59, the transmission order is the same as the decoding order of
pictures. Since a coded picture is encapsulated into exactly one
NAL unit, the value of interleaving-depth is equal to the maximum
number of pictures preceding any picture in transmission order and
following the picture in decoding order.)
The receiver buffering process contains one picture at a time
according to the value of the interleaving-depth parameter and
orders pictures from the reception order to the correct decoding
order based on the value of DON associated with each picture. The
output of the receiver buffering process is the following:
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... N58 N59 I00 R03 N01 N02 R06 N04 N05 ...
... -|---|---|---|---|---|---|---|---|- ...
... 63 64 65 66 67 68 69 70 71 ...
Again, an initial buffering delay of one picture interval is needed
to organize pictures from decoding order to output order as
depicted below:
... N58 N59 I00 N01 N02 R03 N04 N05 ...
... -|---|---|---|---|---|---|---|- ...
... 64 65 66 67 68 69 70 71 ...
It can be observed that the maximum delay that IDR pictures can
undergo during transmission, including possible application,
transport, or link layer retransmission, is equal to three picture
intervals. Thus, the loss resiliency of IDR pictures is improved
in systems supporting retransmission compared to the case in which
pictures were transmitted in their decoding order.
11.4. Robust Transmission Scheduling of Redundant Coded Slices
A redundant coded picture is a coded representation of a picture or
a part of a picture that is not used in the decoding process if the
corresponding primary coded picture is correctly decoded. There
should be no noticeable difference between any area of the decoded
primary picture and a corresponding area that would result from
application of the H.264 decoding process for any redundant picture
in the same access unit. A redundant coded slice is a coded slice
that is a part of a redundant coded picture.
Redundant coded pictures can be used to provide unequal error
protection in error-prone video transmission. If a primary coded
representation of a picture is decoded incorrectly, a corresponding
redundant coded picture can be decoded. Examples of applications
and coding techniques utilizing the redundant codec picture feature
include the video redundancy coding [19] and protection of "key
pictures" in multicast streaming [20].
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One property of many error-prone video communications systems is
that transmission errors are often bursty and therefore they may
affect more than one consecutive transmission packets in
transmission order. In low bitrate video communication it is
relatively common that an entire coded picture can be encapsulated
into one transmission packet. Consequently, a primary coded
picture and the corresponding redundant coded pictures may be
transmitted in consecutive packets in transmission order. In order
to make the transmission scheme more tolerant of bursty
transmission errors, it is beneficial to transmit a primary coded
picture apart from the corresponding redundant coded pictures. The
DON concept enables this.
11.5. Remarks on Other Design Possibilities
The slice header syntax structure of the H.264 coding standard
contains the frame_num syntax element that can indicates the
decoding order of coded frames. However, the usage of the
frame_num syntax element is not feasible or desirable to recover
the decoding order due to the following reasons:
o The receiver is required to parse at least one slice header per
coded picture (before passing the coded data to the decoder).
o Coded slices from multiple coded video sequences cannot be
interleaved, because the frame number syntax element is reset to
0 in each IDR picture.
o The coded fields of a complementary field pair share the same
value of the frame_num syntax element. Thus, the decoding order
of the coded fields of a complementary field pair cannot be
recovered based on the frame_num syntax element or any other
syntax element of the H.264 coding syntax.
The RTP payload format for transport of MPEG-4 elementary streams
[21] enables interleaving of access units and transmission of
multiple access units in the same RTP packet. An access unit is
specified in the H.264 coding standard to consist of all NAL units
that are associated with a primary coded picture according to
subclause 7.4.1.2 of [1]. Consequently, slices of different
pictures cannot be interleaved and the multi-picture slice
interleaving technique (see section 10.5) for improved error
resilience cannot be used.
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12. Open Issues
Since the length of the value of the optional parameter-sets MIME
parameter may be remarkable, it is desirable to use base64 encoding
instead of the current base16 (hexadecimal) encoding. We will
carry out the change to base64 encoding in the next release of this
memo.
Security section needs review
13. Full Copyright Statement
Copyright (C) The Internet Society (2002). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain
it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph
are included on all such copies and derivative works.
However, this document itself may not be modified in any way, such
as by removing the copyright notice or references to the Internet
Society or other Internet organizations, except as needed for the
purpose of developing Internet standards in which case the
procedures for copyrights defined in the Internet Standards process
must be followed, or as required to translate it into languages
other than English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on
an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
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14. Intellectual Property Notice
The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; neither does it represent that it
has made any effort to identify any such rights. Information on
the IETF's procedures with respect to rights in standards-track and
standards-related documentation can be found in BCP-11. Copies of
claims of rights made available for publication and any assurances
of licenses to be made available, or the result of an attempt made
to obtain a general license or permission for the use of such
proprietary rights by implementors or users of this specification
can be obtained from the IETF Secretariat.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights which may cover technology that may be required to practice
this standard. Please address the information to the IETF
Executive Director.
The IETF has been notified of intellectual property rights claimed
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rights at http://www.ietf.org/ipr.
15. References
15.1. Normative References
[1] "Draft ITU-T Recommendation and Final Draft International
Standard of Joint Video Specification (ITU-T Rec. H.264 |
ISO/IEC 14496-10 AVC)", available from ftp://ftp.imtc-
files.org/jvt-experts/2003_03_Pattaya/JVT-G50r1.zip, May
2003.
[2] S. Bradner, "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
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[3] H. Schulzrinne, S. Casner, R. Frederick, and V. Jacobson,
"RTP: A Transport Protocol for Real-Time Applications", RFC
1889, January 1996.
[4] M. Handley and V. Jacobson, "SDP: Session Description
Protocol", RFC 2327, April 1998.
[5] ITU-T Recommendation T.35 (need to put in Rec name and date)
15.2. Informative References
[6] A. Luthra, G. Sullivan, and T. Wiegand (eds.), Special Issue
on H.264/AVC. IEEE Transactions on Circuits and Systems on
Video Technology, July 2003.
[7] P. Borgwardt, "Handling Interlaced Video in H.26L", VCEG-
N57r2, available from ftp://standard.pictel.com/video-
site/0109_San/VCEG-N57r2.doc, September 2001.
[8] C. Borman et. Al., "RTP Payload Format for the 1998 Version
of ITU-T Rec. H.263 Video (H.263+)", RFC 2429, October 1998.
[9] ISO/IEC IS 14496-2.
[10] S. Wenger, "H.26L over IP", IEEE Transaction on Circuits and
Systems for Video technology, July 2003.
[11] S. Wenger, "H.26L over IP: The IP Network Adaptation Layer",
Proceedings Packet Video Workshop 02, April 2002
[12] T. Stockhammer, M.M. Hannuksela, and S. Wenger, "H.26L/JVT
Coding Network Abstraction Layer and IP-based Transport" in
Proc. ICIP 2002, Rochester, NY, September 2002.
[13] ITU-T Recommendation H.223 (1999).
[14] C. Perkins et. al., "RTP Payload for Redundant Audio Data",
RFC 2198, September 1997.
[15] J. Rosenberg, H. Schulzrinne, "An RTP Payload Format for
Generic Forward Error Correction", RFC 2733, December 1999.
[16] V Varsa, M. Karczewicz, "Slice interleaving in compressed
video packetization", Packet Video Workshop 2000.
[17] S.H. Kang and A. Zakhor, "Packet scheduling algorithm for
wireless video streaming," International Packet Video
Workshop 2002, available http://www.pv2002.org.
[18] M.M. Hannuksela, "Enhanced concept of GOP", JVT-B042,
available ftp://standard.pictel.com/video-site/0201_Gen/JVT-
B042.doc, January 2002.
[19] S. Wenger, "Video Redundancy Coding in H.263+", 1997
International Workshop on Audio-Visual Services over Packet
Networks, September 1997.
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[20] Y.-K. Wang, M.M. Hannuksela, and M. Gabbouj, "Error Resilient
Video Coding Using Unequally Protected Key Pictures", in
Proc. International Workshop VLBV03, September 2003.
[21] J. van der Meer, D. Mackie, V. Swaminathan, D. Singer, and P.
Gentric, "RTP Payload Format for Transport of MPEG-4
Elementary Streams", draft-ietf-avt-mpeg4-simple-07.txt,
February 2003.
Author's Addresses
Stephan Wenger Phone: +49-172-300-0813
TU Berlin / Teles AG Email: stewe@cs.tu-berlin.de
Franklinstr. 28-29
D-10587 Berlin
Germany
Miska M. Hannuksela Phone: +358-7180-73151
Nokia Corporation Email: miska.hannuksela@nokia.com
P.O. Box 100
33721 Tampere
Finland
Thomas Stockhammer Phone: +49-89-28923474
Institute for Communications Eng. Email: stockhammer@ei.tum.de
Munich University of Technology
D-80290 Munich
Germany
Magnus Westerlund Phone: +46-8-4048287
Multimedia Technologies Email:
Ericsson Research EAB/TVA/A magnus.westerlund@ericsson.com
Ericsson AB
Torshamsgatan 23
S-164 80 Stockholm
Sweden
David Singer Phone +1 408 974-3162
QuickTime Engineering Email: singer@apple.com
Apple
1 Infinite Loop MS 302-3MT
Cupertino
CA 95014
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USA
Annex A: Changes relative to draft-ietf-avt-rtp-h264-01.txt
[This section will be removed in a future version of this draft.]
This memo contains the following technical changes relative to the
previous I-D:
o The memo is aligned with the latest version of the H.264
specification. In particular, the concept of access unit defined
in the H.264 specification has been incorporated into the memo.
o The use of RTP timestamps for interlaced video has been
clarified.
o Two forms of single-time aggregation packets are available: one
with decoding order number (DON) and another without DON.
o The assignment rules for the values of DON have been simplified.
NAL units having the same value of DON may be decoded at any
order. NAL units having a different value of DON must be passed
to the decoder in ascending order of DON value.
o Receiver buffer size considerations have been added.
o The depacketization process has been updated to suit lossy
transmission better. The optional initial-buffering-time MIME
parameter forms a part of the solution.
o Example use cases of DON and rationale behind the DON concept
have been added.
o The application examples have been clarified and a new example
has been added
o The security section has been reworked
o numerous editorial fixes
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