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Alternate Formats: rfc3984.txt | rfc3984.txt.pdf
RFC 3984 - RTP Payload Format for H.264 Video
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RFC3984 - RTP Payload Format for H.264 Video
Network Working Group S. Wenger
Request for Comments: 3984 M.M. Hannuksela
Category: Standards Track T. Stockhammer
M. Westerlund
D. Singer
February 2005
RTP Payload Format for H.264 Video
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This memo describes an RTP Payload format for the ITU-T
Recommendation H.264 video codec and the technically identical
ISO/IEC International Standard 14496-10 video codec. The RTP payload
format allows for packetization of one or more Network Abstraction
Layer Units (NALUs), produced by an H.264 video encoder, in each RTP
payload. The payload format has wide applicability, as it supports
applications from simple low bit-rate conversational usage, to
Internet video streaming with interleaved transmission, to high bit-
rate video-on-demand.
Table of Contents
1. Introduction.................................................. 3
1.1. The H.264 Codec......................................... 3
1.2. Parameter Set Concept................................... 4
1.3. Network Abstraction Layer Unit Types.................... 5
2. Conventions................................................... 6
3. Scope......................................................... 6
4. Definitions and Abbreviations................................. 6
4.1. Definitions............................................. 6
5. RTP Payload Format............................................ 8
5.1. RTP Header Usage........................................ 8
5.2. Common Structure of the RTP Payload Format.............. 11
5.3. NAL Unit Octet Usage.................................... 12
5.4. Packetization Modes..................................... 14
5.5. Decoding Order Number (DON)............................. 15
5.6. Single NAL Unit Packet.................................. 18
5.7. Aggregation Packets..................................... 18
5.8. Fragmentation Units (FUs)............................... 27
6. Packetization Rules........................................... 31
6.1. Common Packetization Rules.............................. 31
6.2. Single NAL Unit Mode.................................... 32
6.3. Non-Interleaved Mode.................................... 32
6.4. Interleaved Mode........................................ 33
7. De-Packetization Process (Informative)........................ 33
7.1. Single NAL Unit and Non-Interleaved Mode................ 33
7.2. Interleaved Mode........................................ 34
7.3. Additional De-Packetization Guidelines.................. 36
8. Payload Format Parameters..................................... 37
8.1. MIME Registration....................................... 37
8.2. SDP Parameters.......................................... 52
8.3. Examples................................................ 58
8.4. Parameter Set Considerations............................ 60
9. Security Considerations....................................... 62
10. Congestion Control............................................ 63
11. IANA Considerations........................................... 64
12. Informative Appendix: Application Examples.................... 65
12.1. Video Telephony according to ITU-T Recommendation H.241
Annex A................................................. 65
12.2. Video Telephony, No Slice Data Partitioning, No NAL
Unit Aggregation........................................ 65
12.3. Video Telephony, Interleaved Packetization Using NAL
Unit Aggregation........................................ 66
12.4. Video Telephony with Data Partitioning.................. 66
12.5. Video Telephony or Streaming with FUs and Forward
Error Correction........................................ 67
12.6. Low Bit-Rate Streaming.................................. 69
12.7. Robust Packet Scheduling in Video Streaming............. 70
13. Informative Appendix: Rationale for Decoding Order Number..... 71
13.1. Introduction............................................ 71
13.2. Example of Multi-Picture Slice Interleaving............. 71
13.3. Example of Robust Packet Scheduling..................... 73
13.4. Robust Transmission Scheduling of Redundant Coded
Slices.................................................. 77
13.5. Remarks on Other Design Possibilities................... 77
14. Acknowledgements.............................................. 78
15. References.................................................... 78
15.1. Normative References.................................... 78
15.2. Informative References.................................. 79
Authors' Addresses................................................ 81
Full Copyright Statement.......................................... 83
1. Introduction
1.1. The H.264 Codec
This memo specifies an RTP payload specification for the video coding
standard known as ITU-T Recommendation H.264 [1] and ISO/IEC
International Standard 14496 Part 10 [2] (both also known as Advanced
Video Coding, or AVC). Recommendation H.264 was approved by ITU-T on
May 2003, and the approved draft specification is available for
public review [8]. In this memo the H.264 acronym is used for the
codec and the standard, but the memo is equally applicable to the
ISO/IEC counterpart of the coding standard.
The H.264 video codec has a very broad application range that covers
all forms of digital compressed video from, low bit-rate Internet
streaming applications to HDTV broadcast and Digital Cinema
applications with nearly lossless coding. Compared to the current
state of technology, the overall performance of H.264 is such that
bit rate savings of 50% or more are reported. Digital Satellite TV
quality, for example, was reported to be achievable at 1.5 Mbit/s,
compared to the current operation point of MPEG 2 video at around 3.5
Mbit/s [9].
The codec specification [1] itself distinguishes conceptually between
a video coding layer (VCL) and a network abstraction layer (NAL).
The VCL contains the signal processing functionality of the codec;
mechanisms such as transform, quantization, and motion compensated
prediction; and a loop filter. It follows the general concept of
most of today's video codecs, a macroblock-based coder that uses
inter picture prediction with motion compensation and transform
coding of the residual signal. The VCL encoder outputs slices: a bit
string that contains the macroblock data of an integer number of
macroblocks, and the information of the slice header (containing the
spatial address of the first macroblock in the slice, the initial
quantization parameter, and similar information). Macroblocks in
slices are arranged in scan order unless a different macroblock
allocation is specified, by using the so-called Flexible Macroblock
Ordering syntax. In-picture prediction is used only within a slice.
More information is provided in [9].
The Network Abstraction Layer (NAL) encoder encapsulates the slice
output of the VCL encoder into Network Abstraction Layer Units (NAL
units), which are suitable for transmission over packet networks or
use in packet oriented multiplex environments. Annex B of H.264
defines an encapsulation process to transmit such NAL units over
byte-stream oriented networks. In the scope of this memo, Annex B is
not relevant.
Internally, the NAL uses NAL units. A NAL unit consists of a one-
byte header and the payload byte string. The header indicates the
type of the NAL unit, the (potential) presence of bit errors or
syntax violations in the NAL unit payload, and information regarding
the relative importance of the NAL unit for the decoding process.
This RTP payload specification is designed to be unaware of the bit
string in the NAL unit payload.
One of the main properties of H.264 is the complete decoupling of the
transmission time, the decoding time, and the sampling or
presentation time of slices and pictures. The decoding process
specified in H.264 is unaware of time, and the H.264 syntax does not
carry information such as the number of skipped frames (as is common
in the form of the Temporal Reference in earlier video compression
standards). Also, there are NAL units that affect many pictures and
that are, therefore, inherently timeless. For this reason, the
handling of the RTP timestamp requires some special considerations
for NAL units for which the sampling or presentation time is not
defined or, at transmission time, unknown.
1.2. Parameter Set Concept
One very fundamental design concept of H.264 is to generate self-
contained packets, to make mechanisms such as the header duplication
of RFC 2429 [10] or MPEG-4's Header Extension Code (HEC) [11]
unnecessary. This was achieved by decoupling information relevant to
more than one slice from the media stream. This higher layer meta
information should be sent reliably, asynchronously, and in advance
from the RTP packet stream that contains the slice packets.
(Provisions for sending this information in-band are also available
for applications that do not have an out-of-band transport channel
appropriate for the purpose.) The combination of the higher-level
parameters is called a parameter set. The H.264 specification
includes two types of parameter sets: sequence parameter set and
picture parameter set. An active sequence parameter set remains
unchanged throughout a coded video sequence, and an active picture
parameter set remains unchanged within a coded picture. The sequence
and picture parameter set structures contain information such as
picture size, optional coding modes employed, and macroblock to slice
group map.
To be able to change picture parameters (such as the picture size)
without having to transmit parameter set updates synchronously to the
slice packet stream, the encoder and decoder can maintain a list of
more than one sequence and picture parameter set. Each slice header
contains a codeword that indicates the sequence and picture parameter
set to be used.
This mechanism allows the decoupling of the transmission of parameter
sets from the packet stream, and the transmission of them by external
means (e.g., as a side effect of the capability exchange), or through
a (reliable or unreliable) control protocol. It may even be possible
that they are never transmitted but are fixed by an application
design specification.
1.3. Network Abstraction Layer Unit Types
Tutorial information on the NAL design can be found in [12], [13],
and [14].
All NAL units consist of a single NAL unit type octet, which also
co-serves as the payload header of this RTP payload format. The
payload of a NAL unit follows immediately.
The syntax and semantics of the NAL unit type octet are specified in
[1], but the essential properties of the NAL unit type octet are
summarized below. The NAL unit type octet has the following format:
+---------------+
|0|1|2|3|4|5|6|7|
+-+-+-+-+-+-+-+-+
|F|NRI| Type |
+---------------+
The semantics of the components of the NAL unit type octet, as
specified in the H.264 specification, are described briefly below.
F: 1 bit
forbidden_zero_bit. The H.264 specification declares a value of
1 as a syntax violation.
NRI: 2 bits
nal_ref_idc. A value of 00 indicates that the content of the NAL
unit is not used to reconstruct reference pictures for inter
picture prediction. Such NAL units can be discarded without
risking the integrity of the reference pictures. Values greater
than 00 indicate that the decoding of the NAL unit is required to
maintain the integrity of the reference pictures.
Type: 5 bits
nal_unit_type. This component specifies the NAL unit payload type
as defined in table 7-1 of [1], and later within this memo. For a
reference of all currently defined NAL unit types and their
semantics, please refer to section 7.4.1 in [1].
This memo introduces new NAL unit types, which are presented in
section 5.2. The NAL unit types defined in this memo are marked as
unspecified in [1]. Moreover, this specification extends the
semantics of F and NRI as described in section 5.3.
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 BCP 14, RFC 2119 [3].
This specification uses the notion of setting and clearing a bit when
bit fields are handled. 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, and not the bitstream format
discussed in Annex B of H.264. Likely, the first applications of
this specification will be in the conversational multimedia field,
video telephony or video conferencing, but the payload format also
covers other applications, such as Internet streaming and TV over IP.
4. Definitions and Abbreviations
4.1. Definitions
This document uses the definitions of [1]. The following terms,
defined in [1], are summed up 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 instantaneous decoding refresh (IDR) access
unit followed by zero or more non-IDR access units including all
subsequent access units up to but not including any subsequent IDR
access unit.
IDR access unit: An access unit in which the primary coded picture
is an IDR picture.
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.
media aware network element (MANE): A network element, such as a
middlebox or application layer gateway that is capable of parsing
certain aspects of the RTP payload headers or the RTP payload and
reacting to the contents.
Informative note: The concept of a MANE goes beyond normal
routers or gateways in that a MANE has to be aware of the
signaling (e.g., to learn about the payload type mappings of
the media streams), and in that it has to be trusted when
working with SRTP. The advantage of using MANEs is that they
allow packets to be dropped according to the needs of the media
coding. For example, if a MANE has to drop packets due to
congestion on a certain link, it can identify those packets
whose dropping has the smallest negative impact on the user
experience and remove them in order to remove the congestion
and/or keep the delay low.
Abbreviations
DON: Decoding Order Number
DONB: Decoding Order Number Base
DOND: Decoding Order Number Difference
FEC: Forward Error Correction
FU: Fragmentation Unit
IDR: Instantaneous Decoding Refresh
IEC: International Electrotechnical Commission
ISO: International Organization for Standardization
ITU-T: International Telecommunication Union,
Telecommunication Standardization Sector
MANE: Media Aware Network Element
MTAP: Multi-Time Aggregation Packet
MTAP16: MTAP with 16-bit timestamp offset
MTAP24: MTAP with 24-bit timestamp offset
NAL: Network Abstraction Layer
NALU: NAL Unit
SEI: Supplemental Enhancement Information
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 3550 [4] and
reprinted in Figure 1 for convenience. This payload format uses the
fields of the header in a manner consistent with that specification.
When one NAL unit is encapsulated per RTP packet, the RECOMMENDED RTP
payload format is specified in section 5.6. The RTP payload (and the
settings for some RTP header bits) for aggregation packets and
fragmentation units are specified in sections 5.7 and 5.8,
respectively.
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 to RFC 3550
The RTP header information to be set according to this RTP payload
format is set as follows:
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 in video
formats, to allow an efficient playout buffer handling. For
aggregation packets (STAP and MTAP), the marker bit in the RTP
header MUST be set to the value that the marker bit of the last
NAL unit of the aggregation packet would have been if it were
transported in its own RTP packet. 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. Thus, if a
gateway has re-packetized an aggregation packet into several
packets, it cannot reliably set the M bit of those packets.
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 has to be performed either
through the profile used or in a dynamic way.
Sequence number (SN): 16 bits
Set and used in accordance with RFC 3550. For the single NALU and
non-interleaved packetization mode, the sequence number is used to
determine decoding order for the NALU.
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 NAL unit has no timing properties of its own (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 in
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
5.7.2.
Receivers SHOULD ignore any picture timing SEI messages included
in access units that have only one display timestamp. Instead,
receivers SHOULD use the RTP timestamp for synchronizing the
display process.
RTP senders SHOULD NOT transmit picture timing SEI messages for
pictures that are not supposed to be displayed as multiple fields.
If one access unit has more than one display timestamp carried in
a picture timing SEI message, then the information in the SEI
message SHOULD be treated as relative to the RTP timestamp, with
the earliest event occurring at the time given by the RTP
timestamp, and subsequent events later, as given by the difference
in SEI message picture timing values. Let tSEI1, tSEI2, ...,
tSEIn be the display timestamps carried in the SEI message of an
access unit, where tSEI1 is the earliest of all such timestamps.
Let tmadjst() be a function that adjusts the SEI messages time
scale to a 90-kHz time scale. Let TS be the RTP timestamp. Then,
the display time for the event associated with tSEI1 is TS. The
display time for the event with tSEIx, where x is [2..n] is TS +
tmadjst (tSEIx - tSEI1).
Informative note: Displaying coded frames as fields is needed
commonly in an operation known as 3:2 pulldown, in which film
content that consists of coded frames is displayed on a display
using interlaced scanning. The picture timing SEI message
enables carriage of multiple timestamps for the same coded
picture, and therefore the 3:2 pulldown process is perfectly
controlled. The picture timing SEI message mechanism is
necessary because only one timestamp per coded frame can be
conveyed in the RTP timestamp.
Informative note: Because H.264 allows the decoding order to be
different from the display order, values of RTP timestamps may
not be monotonically non-decreasing as a function of RTP
sequence numbers. Furthermore, the value for interarrival
jitter reported in the RTCP reports may not be a trustworthy
indication of the network performance, as the calculation rules
for interarrival jitter (section 6.4.1 of RFC 3550) assume that
the RTP timestamp of a packet is directly proportional to its
transmission time.
5.2. Common Structure of the RTP Payload Format
The payload format defines three different basic payload structures.
A receiver can identify the payload structure by the first byte of
the RTP payload, which co-serves as the RTP payload header and, in
some cases, as the first byte of the payload. This byte is always
structured as a NAL unit header. The NAL unit type field indicates
which structure is present. The possible structures are as follows:
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
unit type; i.e., in the range of 1 to 23, inclusive. Specified in
section 5.6.
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 unit type numbers
assigned for STAP-A, STAP-B, MTAP16, and MTAP24 are 24, 25, 26, and
27, respectively. Specified in section 5.7.
Fragmentation unit: Used to fragment a single NAL unit over multiple
RTP packets. Exists with two versions, FU-A and FU-B, identified
with the NAL unit type numbers 28 and 29, respectively. Specified in
section 5.8.
Table 1. Summary of NAL unit types and their payload structures
Type Packet Type name Section
---------------------------------------------------------
0 undefined -
1-23 NAL unit Single NAL unit packet per H.264 5.6
24 STAP-A Single-time aggregation packet 5.7.1
25 STAP-B Single-time aggregation packet 5.7.1
26 MTAP16 Multi-time aggregation packet 5.7.2
27 MTAP24 Multi-time aggregation packet 5.7.2
28 FU-A Fragmentation unit 5.8
29 FU-B Fragmentation unit 5.8
30-31 undefined -
Informative note: This specification does not limit the size of
NAL units encapsulated in single NAL unit packets and
fragmentation units. The maximum size of a NAL unit encapsulated
in any aggregation packet is 65535 bytes.
5.3. NAL Unit Octet Usage
The structure and semantics of the NAL unit octet were introduced in
section 1.3. For convenience, the format of the NAL unit type octet
is reprinted below:
+---------------+
|0|1|2|3|4|5|6|7|
+-+-+-+-+-+-+-+-+
|F|NRI| Type |
+---------------+
This section specifies the semantics of F and NRI according to this
specification.
F: 1 bit
forbidden_zero_bit. A value of 0 indicates that the NAL unit type
octet and payload should not contain bit errors or other syntax
violations. A value of 1 indicates that the NAL unit type octet
and payload may contain bit errors or other syntax violations.
MANEs SHOULD set the F bit to indicate detected bit errors in the
NAL unit. The H.264 specification requires that the F bit is
equal to 0. When the F bit is set, the decoder is advised that
bit errors or any other syntax violations may be present in the
payload or in the NAL unit type octet. The simplest decoder
reaction to a NAL unit in which the F bit is equal to 1 is to
discard such a NAL unit and to conceal the lost data in the
discarded NAL unit.
NRI: 2 bits
nal_ref_idc. The semantics of value 00 and a non-zero value
remain unchanged from the H.264 specification. In other words, a
value of 00 indicates that the content of the NAL unit is not used
to reconstruct reference pictures for inter picture prediction.
Such NAL units can be discarded without risking the integrity of
the reference pictures. Values greater than 00 indicate that the
decoding of the NAL unit is required to maintain the integrity of
the reference pictures.
In addition to the specification above, according to this RTP
payload specification, values of NRI greater than 00 indicate the
relative transport priority, as determined by the encoder. MANEs
can use this information to protect more important NAL units
better than they do less important NAL units. The highest
transport priority is 11, followed by 10, and then by 01; finally,
00 is the lowest.
Informative note: Any non-zero value of NRI is handled
identically in H.264 decoders. Therefore, receivers need not
manipulate the value of NRI when passing NAL units to the
decoder.
An H.264 encoder MUST set the value of NRI according to the H.264
specification (subclause 7.4.1) when the value of nal_unit_type is
in the range of 1 to 12, inclusive. In particular, the H.264
specification requires that the value of NRI SHALL be equal to 0
for all NAL units having nal_unit_type equal to 6, 9, 10, 11, or
12.
For NAL units having nal_unit_type equal to 7 or 8 (indicating a
sequence parameter set or a picture parameter set, respectively),
an H.264 encoder SHOULD set the value of NRI to 11 (in binary
format). For coded slice NAL units of a primary coded picture
having nal_unit_type equal to 5 (indicating a coded slice
belonging to an IDR picture), an H.264 encoder SHOULD set the
value of NRI to 11 (in binary format).
For a mapping of the remaining nal_unit_types to NRI values, the
following example MAY be used and has been shown to be efficient
in a certain environment [13]. Other mappings MAY also be
desirable, depending on the application and the H.264/AVC Annex A
profile in use.
Informative note: Data Partitioning is not available in certain
profiles; e.g., in the Main or Baseline profiles.
Consequently, the nal unit types 2, 3, and 4 can occur only if
the video bitstream conforms to a profile in which data
partitioning is allowed and not in streams that conform to the
Main or Baseline profiles.
Table 2. Example of NRI values for coded slices and coded slice
data partitions of primary coded reference pictures
NAL Unit Type Content of NAL unit NRI (binary)
----------------------------------------------------------------
1 non-IDR coded slice 10
2 Coded slice data partition A 10
3 Coded slice data partition B 01
4 Coded slice data partition C 01
Informative note: As mentioned before, the NRI value of non-
reference pictures is 00 as mandated by H.264/AVC.
An H.264 encoder SHOULD set the value of NRI for coded slice and
coded slice data partition NAL units of redundant coded reference
pictures equal to 01 (in binary format).
Definitions of the values for NRI for NAL unit types 24 to 29,
inclusive, are given in sections 5.7 and 5.8 of this memo.
No recommendation for the value of NRI is given for NAL units
having nal_unit_type in the range of 13 to 23, inclusive, because
these values are reserved for ITU-T and ISO/IEC. No
recommendation for the value of NRI is given for NAL units having
nal_unit_type equal to 0 or in the range of 30 to 31, inclusive,
as the semantics of these values are not specified in this memo.
5.4. Packetization Modes
This memo specifies three cases of packetization modes:
o Single NAL unit mode
o Non-interleaved mode
o Interleaved mode
The single NAL unit mode is targeted for conversational systems that
comply with ITU-T Recommendation H.241 [15] (see section 12.1). The
non-interleaved mode is targeted for conversational systems that may
not comply with ITU-T Recommendation H.241. In the non-interleaved
mode, NAL units are transmitted in NAL unit decoding order. The
interleaved mode is targeted for systems that do not require very low
end-to-end latency. The interleaved mode allows transmission of NAL
units out of NAL unit decoding order.
The packetization mode in use MAY be signaled by the value of the
OPTIONAL packetization-mode MIME parameter or by external means. The
used packetization mode governs which NAL unit types are allowed in
RTP payloads. Table 3 summarizes the allowed NAL unit types for each
packetization mode. Some NAL unit type values (indicated as
undefined in Table 3) are reserved for future extensions. NAL units
of those types SHOULD NOT be sent by a sender and MUST be ignored by
a receiver. For example, the Types 1-23, with the associated packet
type "NAL unit", are allowed in "Single NAL Unit Mode" and in "Non-
Interleaved Mode", but disallowed in "Interleaved Mode".
Packetization modes are explained in more detail in section 6.
Table 3. Summary of allowed NAL unit types for each packetization
mode (yes = allowed, no = disallowed, ig = ignore)
Type Packet Single NAL Non-Interleaved Interleaved
Unit Mode Mode Mode
-------------------------------------------------------------
0 undefined ig ig ig
1-23 NAL unit yes yes no
24 STAP-A no yes no
25 STAP-B no no yes
26 MTAP16 no no yes
27 MTAP24 no no yes
28 FU-A no yes yes
29 FU-B no no yes
30-31 undefined ig ig ig
5.5. Decoding Order Number (DON)
In the interleaved packetization mode, the transmission order of NAL
units is allowed to differ from the decoding order of the NAL units.
Decoding order number (DON) is a field in the payload structure or a
derived variable that indicates the NAL unit decoding order.
Rationale and examples of use cases for transmission out of decoding
order and for the use of DON are given in section 13.
The coupling of transmission and decoding order is controlled by the
OPTIONAL sprop-interleaving-depth MIME parameter as follows. When
the value of the OPTIONAL sprop-interleaving-depth MIME parameter is
equal to 0 (explicitly or per default) or 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 sprop-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 MTAP16 and an MTAP24 is NOT REQUIRED
to be the NAL unit decoding order, and
o the order of NAL units generated by decapsulating STAP-Bs, MTAPs,
and FUs in two consecutive packets is NOT REQUIRED to be the NAL
unit decoding order.
The RTP payload structures for a single NAL unit packet, an STAP-A,
and an FU-A do not include DON. STAP-B and FU-B structures include
DON, and the structure of MTAPs enables derivation of DON as
specified in section 5.7.2.
Informative note: When an FU-A occurs in interleaved mode, it
always follows an FU-B, which sets its DON.
Informative note: If a transmitter wants to encapsulate a single
NAL unit per packet and transmit packets out of their decoding
order, STAP-B packet type can be used.
In the single NAL unit packetization mode, the transmission order of
NAL units, determined by the RTP sequence number, MUST be the same as
their NAL unit decoding order. In the non-interleaved packetization
mode, the transmission order of NAL units in single NAL unit packets,
STAP-As, and FU-As MUST be the same as their NAL unit decoding order.
The NAL units within an STAP MUST appear in the NAL unit decoding
order. Thus, the decoding order is first provided through the
implicit order within a STAP, and second provided through the RTP
sequence number for the order between STAPs, FUs, and single NAL unit
packets.
Signaling of the value of DON for NAL units carried in STAP-B, MTAP,
and a series of fragmentation units starting with an FU-B is
specified in sections 5.7.1, 5.7.2, and 5.8, respectively. The DON
value 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 contained in any STAP-B, MTAP, or
a series of fragmentation units starting with an FU-B is determined
as follows. Let DON(i) be the decoding order number of the NAL unit
having index i in the transmission order. Function don_diff(m,n) is
specified as follows:
If DON(m) == DON(n), don_diff(m,n) = 0
If (DON(m) < DON(n) and DON(n) - DON(m) < 32768),
don_diff(m,n) = DON(n) - DON(m)
If (DON(m) > DON(n) and DON(m) - DON(n) >= 32768),
don_diff(m,n) = 65536 - DON(m) + DON(n)
If (DON(m) < DON(n) and DON(n) - DON(m) >= 32768),
don_diff(m,n) = - (DON(m) + 65536 - DON(n))
If (DON(m) > DON(n) and DON(m) - DON(n) < 32768),
don_diff(m,n) = - (DON(m) - DON(n))
A positive value of don_diff(m,n) indicates that the NAL unit having
transmission order index n follows, in decoding order, the NAL unit
having transmission order index m. When don_diff(m,n) is equal to 0,
then the NAL unit decoding order of the two NAL units can be in
either order. A negative value of don_diff(m,n) indicates that the
NAL unit having transmission order index n precedes, in decoding
order, the NAL unit having transmission order index m.
Values of DON related fields (DON, DONB, and DOND; see section 5.7)
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 NAL units in NAL unit decoding order is switched and the
new order does not conform to the NAL unit decoding order, the NAL
units MUST NOT have the same value of DON. 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.
When two consecutive NAL units in the NAL unit decoding order have a
different value of DON, the value of DON for the second NAL unit in
decoding order SHOULD be the value of DON for the first, incremented
by one.
An example of the decapsulation process to recover the NAL unit
decoding order is given in section 7.
Informative note: Receivers should not expect that the absolute
difference of values of DON for two consecutive NAL units in the
NAL unit decoding order will be equal to one, even in error-free
transmission. An increment by one is not required, as at the time
of associating values of DON to NAL units, it may not be known
whether all NAL units are delivered to the receiver. For example,
a gateway may not forward coded slice NAL units of non-reference
pictures or SEI NAL units when there is a shortage of bit rate in
the network to which the packets are forwarded. In another
example, a live broadcast is interrupted by pre-encoded content,
such as commercials, from time to time. The first intra picture
of a pre-encoded clip is transmitted in advance to ensure that it
is readily available in the receiver. When transmitting the first
intra picture, the originator does not exactly know how many NAL
units will be encoded before the first intra picture of the pre-
encoded clip follows in decoding order. Thus, the values of DON
for the NAL units of the first intra picture of the pre-encoded
clip have to be estimated when they are transmitted, and gaps in
values of DON may occur.
5.6. Single NAL Unit Packet
The single NAL unit packet defined here MUST contain only one NAL
unit, of the types defined in [1]. This means that neither 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.
Informative note: The first byte of a NAL unit co-serves as the
RTP payload header.
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 | |
+-+-+-+-+-+-+-+-+ |
| |
| Bytes 2..n of a Single NAL unit |
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :...OPTIONAL RTP padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2. RTP payload format for single NAL unit packet
5.7. Aggregation Packets
Aggregation packets are the NAL 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. 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 including DON (STAP-B).
o Multi-time aggregation packet (MTAP): aggregates NAL units with
potentially differing NALU-time. Two different MTAPs are defined,
differing in the length of the NAL unit timestamp offset.
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 four 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 4.
The F bit MUST be cleared if all F bits of the aggregated NAL units
are zero; otherwise, it MUST be set. The value of NRI MUST be the
maximum of all the NAL units carried in the aggregation packet.
Table 4. 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 is set to the value that 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 sections 5.7.1 and 5.7.2 for the four
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 so that the resulting IP packet
is smaller than the MTU size. An aggregation packet MUST NOT contain
fragmentation units specified in section 5.8. Aggregation packets
MUST NOT be nested; i.e., an aggregation packet MUST NOT contain
another aggregation packet.
5.7.1. Single-Time Aggregation Packet
Single-time aggregation packet (STAP) SHOULD be used whenever NAL
units are aggregated that all 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) (in network byte order) followed by at least one single-time
aggregation unit, as presented in Figure 5.
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
The DON field specifies the value of DON for the first NAL unit in an
STAP-B in transmission order. For each successive NAL unit in
appearance order in an STAP-B, the value of DON is equal to (the
value of DON of the previous NAL unit in the STAP-B + 1) % 65536, in
which '%' stands for the modulo operation.
A single-time aggregation unit consists of 16-bit unsigned size
information (in network byte order) 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.
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
Figure 7 presents an example of an RTP packet that contains an STAP-
A. The STAP contains two single-time aggregation units, labeled as 1
and 2 in the figure.
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-A NAL HDR | 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-A and two
single-time aggregation units
Figure 8 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.
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 8. An example of an RTP packet including an STAP-B and two
single-time aggregation units
5.7.2. Multi-Time Aggregation Packets (MTAPs)
The NAL unit payload of MTAPs consists of a 16-bit unsigned decoding
order number base (DONB) (in network byte order) and one or more
multi-time aggregation units, as presented in Figure 9. DONB MUST
contain the value of DON for the first NAL unit in the NAL unit
decoding order among the NAL units of the MTAP.
Informative note: The first NAL unit in the NAL unit decoding
order is not necessarily the first NAL unit in the order in which
the NAL units are encapsulated in an MTAP.
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 9. 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 (in network byte order), an 8-
bit unsigned decoding order number difference (DOND), and n bits (in
network byte order) of timestamp offset (TS offset) for this NAL
unit, whereby n can be 16 or 24. The choice between the different
MTAP types (MTAP16 and MTAP24) is application dependent: the larger
the timestamp offset is, the higher the flexibility of the MTAP, but
the overhead is also higher.
The structure of the multi-time aggregation units for MTAP16 and
MTAP24 are presented in Figures 10 and 11, respectively. The
starting or ending position of an aggregation unit within a packet is
NOT REQUIRED to be on a 32-bit word boundary. The DON of the
following NAL unit is equal to (DONB + DOND) % 65536, in which %
denotes the modulo operation. 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: If the NALU-time is larger than or equal to
the RTP timestamp of the packet, then the timestamp offset equals
(the NALU-time of the NAL unit - the RTP timestamp of the packet).
If the NALU-time is smaller than the RTP timestamp of the packet,
then the timestamp offset is equal to the NALU-time + (2^32 - the RTP
timestamp of the packet).
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: NAL unit size | DOND | TS offset |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TS offset | |
+-+-+-+-+-+-+-+-+ NAL unit |
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 10. 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 11. Multi-time aggregation unit for MTAP24
For the "earliest" multi-time aggregation unit in an MTAP the
timestamp offset MUST be zero. Hence, the RTP timestamp of the MTAP
itself is identical to the earliest NALU-time.
Informative note: The "earliest" multi-time aggregation unit is
the one that would have the smallest extended RTP timestamp among
all the aggregation units of an MTAP if the aggregation units were
encapsulated in single NAL unit packets. An extended timestamp is
a timestamp that has more than 32 bits and is capable of counting
the wraparound of the timestamp field, thus enabling one to
determine the smallest value if the timestamp wraps. Such an
"earliest" aggregation unit may not be the first one in the order
in which the aggregation units are encapsulated in an MTAP. The
"earliest" NAL unit need not be the same as the first NAL unit in
the NAL unit decoding order either.
Figure 12 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.
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 12. An RTP packet including a multi-time aggregation
packet of type MTAP16 and two multi-time aggregation
units
Figure 13 presents an example of an RTP packet that contains a
multi-time aggregation packet of type MTAP24 that contains two
multi-time aggregation units, labeled as 1 and 2 in the figure.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|MTAP24 NAL HDR | decoding order number base | NALU 1 Size |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NALU 1 Size | NALU 1 DOND | NALU 1 TS offs |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|NALU 1 TS offs | 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 13. An RTP packet including a multi-time aggregation
packet of type MTAP24 and two multi-time aggregation
units
5.8. 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).
o The fragmentation mechanism allows fragmenting a single picture
and applying generic forward error correction as described in
section 12.5.
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 a fragmented NAL unit. STAPs and MTAPs
MUST NOT be fragmented. FUs MUST NOT be nested; i.e., an FU MUST NOT
contain another FU.
The RTP timestamp of an RTP packet carrying an FU is set to the NALU
time of the fragmented NAL unit.
Figure 14 presents the RTP payload format for FU-As. An FU-A
consists of a fragmentation unit indicator of one octet, a
fragmentation unit header of one octet, and a fragmentation unit
payload.
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 | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
| FU payload |
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :...OPTIONAL RTP padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 14. RTP payload format for FU-A
Figure 15 presents the RTP payload format for FU-Bs. An FU-B
consists of a fragmentation unit indicator of one octet, a
fragmentation unit header of one octet, a decoding order number (DON)
(in network byte order), and a fragmentation unit payload. In other
words, the structure of FU-B is the same as the structure of FU-A,
except for the additional DON field.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| FU indicator | FU header | DON |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
| |
| FU payload |
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :...OPTIONAL RTP padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 15. RTP payload format for FU-B
NAL unit type FU-B MUST be used in the interleaved packetization mode
for the first fragmentation unit of a fragmented NAL unit. NAL unit
type FU-B MUST NOT be used in any other case. In other words, in the
interleaved packetization mode, each NALU that is fragmented has an
FU-B as the first fragment, followed by one or more FU-A fragments.
The FU indicator octet has the following format:
+---------------+
|0|1|2|3|4|5|6|7|
+-+-+-+-+-+-+-+-+
|F|NRI| Type |
+---------------+
Values equal to 28 and 29 in the Type field of the FU indicator octet
identify an FU-A and an FU-B, respectively. The use of the F bit is
described in section 5.3. 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
When set to one, the Start bit indicates the start of a fragmented
NAL unit. 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
When set to one, the End bit 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. 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
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].
The value of DON in FU-Bs is selected as described in section 5.5.
Informative note: The DON field in FU-Bs allows gateways to
fragment NAL units to FU-Bs 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., the
Start bit and End bit MUST NOT both be set to one in the same FU
header.
The FU payload consists of fragments of the payload of the fragmented
NAL unit so that if the fragmentation unit payloads of consecutive
FUs are sequentially concatenated, the payload of the fragmented NAL
unit can be reconstructed. 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.
Informative note: Empty FUs are allowed to reduce the latency of a
certain class of senders in nearly lossless environments. These
senders can be characterized in that they packetize NALU fragments
before the NALU is completely generated and, hence, before the
NALU size is known. If zero-length NALU fragments were not
allowed, the sender would have to generate at least one bit of
data of the following fragment before the current fragment could
be sent. Due to the characteristics of H.264, where sometimes
several macroblocks occupy zero bits, this is undesirable and can
add delay. However, the (potential) use of zero-length NALUs
should be carefully weighed against the increased risk of the loss
of the NALU because of the additional packets employed for its
transmission.
If a fragmentation unit is lost, the receiver SHOULD discard all
following fragmentation units in transmission order corresponding to
the same fragmented NAL unit.
A receiver in an endpoint or in a MANE MAY aggregate the first n-1
fragments of a NAL unit to an (incomplete) NAL unit, even if fragment
n of that NAL unit is not received. In this case, the
forbidden_zero_bit of the NAL unit MUST be set to one to indicate a
syntax violation.
6. Packetization Rules
The packetization modes are introduced in section 5.2. The
packetization rules common to more than one of the packetization
modes are specified in section 6.1. The packetization rules for the
single NAL unit mode, the non-interleaved mode, and the interleaved
mode are specified in sections 6.2, 6.3, and 6.4, respectively.
6.1. Common Packetization Rules
All senders MUST enforce the following packetization rules regardless
of the packetization mode in use:
o Coded slice NAL units or coded slice data partition NAL units
belonging to the same coded picture (and thus sharing the same RTP
timestamp value) MAY be sent in any order permitted by the
applicable profile defined in [1]; however, 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 Parameter sets are handled in accordance with the rules and
recommendations given in section 8.4.
o MANEs MUST NOT duplicate any NAL unit except for sequence or
picture parameter set NAL units, as 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. Duplication SHOULD be
performed on the application layer and not by duplicating RTP
packets (with identical sequence numbers).
Senders using the non-interleaved mode and the interleaved mode MUST
enforce the following packetization rule:
o MANEs MAY convert single NAL unit packets into one aggregation
packet, convert an aggregation packet into several single NAL unit
packets, or mix both concepts, in an RTP translator. The RTP
translator SHOULD take into account at least the following
parameters: path MTU size, unequal protection mechanisms (e.g.,
through packet-based FEC according to RFC 2733 [18], 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.
Informative note: An RTP translator is required to handle RTCP as
per RFC 3550.
6.2. Single NAL Unit Mode
This mode is in use when the value of the OPTIONAL packetization-mode
MIME parameter is equal to 0, the packetization-mode is not present,
or no other packetization mode is signaled by external means. All
receivers MUST support this mode. It is primarily intended for low-
delay applications that are compatible with systems using ITU-T
Recommendation H.241 [15] (see section 12.1). Only single NAL unit
packets MAY be used in this mode. STAPs, MTAPs, and FUs MUST NOT be
used. The transmission order of single NAL unit packets MUST comply
with the NAL unit decoding order.
6.3. Non-Interleaved Mode
This mode is in use when the value of the OPTIONAL packetization-mode
MIME parameter is equal to 1 or the mode is turned on by external
means. This mode SHOULD be supported. It is primarily intended for
low-delay applications. Only single NAL unit packets, STAP-As, and
FU-As MAY be used in this mode. STAP-Bs, MTAPs, and FU-Bs MUST NOT
be used. The transmission order of NAL units MUST comply with the
NAL unit decoding order.
6.4. Interleaved Mode
This mode is in use when the value of the OPTIONAL packetization-mode
MIME parameter is equal to 2 or the mode is turned on by external
means. Some receivers MAY support this mode. STAP-Bs, MTAPs, FU-As,
and FU-Bs MAY be used. STAP-As and single NAL unit packets MUST NOT
be used. The transmission order of packets and NAL units is
constrained as specified in section 5.5.
7. De-Packetization Process (Informative)
The de-packetization process is implementation dependent. Therefore,
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. Section
7.1 presents the de-packetization process for the single NAL unit and
non-interleaved packetization modes, whereas section 7.2 describes
the process for the interleaved mode. Section 7.3 includes
additional decapsulation guidelines for intelligent receivers.
All normal RTP mechanisms related to buffer management apply. In
particular, duplicated or outdated RTP packets (as indicated by the
RTP sequences number and the RTP timestamp) are removed. To
determine the exact time for decoding, factors such as a possible
intentional delay to allow for proper inter-stream synchronization
must be factored in.
7.1. Single NAL Unit and Non-Interleaved Mode
The receiver includes a receiver buffer to compensate for
transmission delay jitter. The receiver stores incoming packets in
reception order into the receiver buffer. Packets are decapsulated
in RTP sequence number order. If a decapsulated packet is a single
NAL unit packet, the NAL unit contained in the packet is passed
directly to the decoder. If a decapsulated packet is an STAP-A, the
NAL units contained in the packet are passed to the decoder in the
order in which they are encapsulated in the packet. If a
decapsulated packet is an FU-A, all the fragments of the fragmented
NAL unit are concatenated and passed to the decoder.
Informative note: If the decoder supports Arbitrary Slice Order,
coded slices of a picture can be passed to the decoder in any
order regardless of their reception and transmission order.
7.2. Interleaved Mode
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 compensate
for transmission delay jitter and to reorder packets from
transmission order to the NAL unit decoding order. In this section,
the receiver operation is described under the assumption that there
is no transmission delay jitter. To make a difference from a
practical receiver buffer that is also used for compensation of
transmission delay jitter, the receiver buffer is here after called
the deinterleaving buffer in this section. Receivers SHOULD also
prepare for transmission delay jitter; i.e., either reserve separate
buffers for transmission delay jitter buffering and deinterleaving
buffering or use a receiver buffer for both transmission delay jitter
and deinterleaving. Moreover, receivers SHOULD take transmission
delay jitter into account in the buffering operation; e.g., by
additional initial buffering before starting of decoding and
playback.
This section is organized as follows: subsection 7.2.1 presents how
to calculate the size of the deinterleaving buffer. Subsection 7.2.2
specifies the receiver process how to organize received NAL units to
the NAL unit decoding order.
7.2.1. Size of the Deinterleaving Buffer
When SDP Offer/Answer model or any other capability exchange
procedure is used in session setup, the properties of the received
stream SHOULD be such that the receiver capabilities are not
exceeded. In the SDP Offer/Answer model, the receiver can indicate
its capabilities to allocate a deinterleaving buffer with the deint-
buf-cap MIME parameter. The sender indicates the requirement for the
deinterleaving buffer size with the sprop-deint-buf-req MIME
parameter. It is therefore RECOMMENDED to set the deinterleaving
buffer size, in terms of number of bytes, equal to or greater than
the value of sprop-deint-buf-req MIME parameter. See section 8.1 for
further information on deint-buf-cap and sprop-deint-buf-req MIME
parameters and section 8.2.2 for further information on their use in
SDP Offer/Answer model.
When a declarative session description is used in session setup, the
sprop-deint-buf-req MIME parameter signals the requirement for the
deinterleaving buffer size. It is therefore RECOMMENDED to set the
deinterleaving buffer size, in terms of number of bytes, equal to or
greater than the value of sprop-deint-buf-req MIME parameter.
7.2.2. Deinterleaving Process
There are two buffering states in the receiver: initial buffering and
buffering while playing. Initial buffering occurs when the RTP
session is initialized. After initial buffering, decoding and
playback is started, and the buffering-while-playing mode is used.
Regardless of the buffering state, the receiver stores incoming NAL
units, in reception order, in the deinterleaving buffer as follows.
NAL units of aggregation packets are stored in the deinterleaving
buffer individually. The value of DON is calculated and stored for
all NAL units.
The receiver operation is described below with the help of the
following functions and constants:
o Function AbsDON is specified in section 8.1.
o Function don_diff is specified in section 5.5.
o Constant N is the value of the OPTIONAL sprop-interleaving-depth
MIME type parameter (see section 8.1) incremented by 1.
Initial buffering lasts until one of the following conditions is
fulfilled:
o There are N VCL NAL units in the deinterleaving buffer.
o If sprop-max-don-diff is present, don_diff(m,n) is greater than
the value of sprop-max-don-diff, in which n corresponds to the NAL
unit having the greatest value of AbsDON among the received NAL
units and m corresponds to the NAL unit having the smallest value
of AbsDON among the received NAL units.
o Initial buffering has lasted for the duration equal to or greater
than the value of the OPTIONAL sprop-init-buf-time MIME parameter.
The NAL units to be removed from the deinterleaving buffer are
determined as follows:
o If the deinterleaving buffer contains at least N VCL NAL units,
NAL units are removed from the deinterleaving buffer and passed to
the decoder in the order specified below until the buffer contains
N-1 VCL NAL units.
o If sprop-max-don-diff is present, all NAL units m for which
don_diff(m,n) is greater than sprop-max-don-diff are removed from
the deinterleaving buffer and passed to the decoder in the order
specified below. Herein, n corresponds to the NAL unit having the
greatest value of AbsDON among the received NAL units.
The order in which 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 For each NAL unit associated with a value of DON, 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.
o NAL units are delivered to the decoder in ascending order of DON
distance. If several NAL units share the same value of DON
distance, they can be passed to the decoder in any order.
o When a desired number 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.
7.3. Additional De-Packetization Guidelines
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 MANE can reduce network load by
discarding useless packets without parsing a complex bitstream.
o Intelligent RTP receivers (e.g., in gateways) may identify lost
FUs. If a lost FU is found, a gateway may decide not to send the
following FUs of the same fragmented NAL unit, as their
information is meaningless for H.264 decoders. In this way a MANE
can reduce network load by discarding useless packets without
parsing a complex bitstream.
o Intelligent receivers having to discard packets or NALUs should
first discard all packets/NALUs in which the value of the NRI
field of the NAL unit type octet is equal to 0. This will
minimize the impact on user experience and keep the reference
pictures intact. If more packets have to be discarded, then
packets with a numerically lower NRI value should be discarded
before packets with a numerically higher NRI value. However,
discarding any packets with an NRI bigger than 0 very likely leads
to decoder drift and SHOULD be avoided.
8. Payload Format Parameters
This section specifies the parameters that MAY be used to select
optional features of the payload format and certain features of the
bitstream. 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)
[5] is also provided for applications that use SDP. Equivalent
parameters could be defined elsewhere for use with control protocols
that do not use MIME or SDP.
Some parameters provide a receiver with the properties of the stream
that will be sent. The name of all these parameters starts with
"sprop" for stream properties. Some of these "sprop" parameters are
limited by other payload or codec configuration parameters. For
example, the sprop-parameter-sets parameter is constrained by the
profile-level-id parameter. The media sender selects all "sprop"
parameters rather than the receiver. This uncommon characteristic of
the "sprop" parameters may not be compatible with some signaling
protocol concepts, in which case the use of these parameters SHOULD
be avoided.
8.1. MIME Registration
The MIME subtype for the ITU-T H.264 | ISO/IEC 14496-10 codec is
allocated from the IETF tree.
The receiver MUST ignore any unspecified parameter.
Media Type name: video
Media subtype name: H264
Required parameters: none
OPTIONAL parameters:
profile-level-id:
A base16 [6] (hexadecimal) representation 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
constraint_set0_flag, constraint_set1_flag,
constraint_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.
If the profile-level-id parameter is used to
indicate properties of a NAL unit stream, it
indicates the profile and level that a decoder
has to support in order to comply with [1] when
it decodes the stream. The profile-iop byte
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 profile that the codec
supports and the highest level
supported for the signaled profile. The
profile-iop byte indicates whether the codec
has additional limitations whereby only the
common subset of the algorithmic features and
limitations of the profiles signaled with the
profile-iop byte and of the profile indicated
by profile_idc is supported by the codec. For
example, 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, in
which 42 stands for the Baseline profile, E0
indicates that only the common subset for all
profiles is supported, and 15 indicates level
2.1.
Informative note: Capability exchange and
session setup procedures should provide
means to list the capabilities for each
supported codec profile separately. For
example, the one-of-N codec selection
procedure of the SDP Offer/Answer model can
be used (section 10.2 of [7]).
If no profile-level-id is present, the Baseline
Profile without additional constraints at Level
1 MUST be implied.
max-mbps, max-fs, max-cpb, max-dpb, and max-br:
These parameters MAY be used to signal the
capabilities of a receiver implementation.
These parameters MUST NOT be used for any other
purpose. The profile-level-id parameter MUST
be present in the same receiver capability
description that contains any of these
parameters. The level conveyed in the value of
the profile-level-id parameter MUST be such
that the receiver is fully capable of
supporting. max-mbps, max-fs, max-cpb, max-
dpb, and max-br MAY be used to indicate
capabilities of the receiver that extend the
required capabilities of the signaled level, as
specified below.
When more than one parameter from the set (max-
mbps, max-fs, max-cpb, max-dpb, max-br) is
present, the receiver MUST support all signaled
capabilities simultaneously. For example, if
both max-mbps and max-br are present, the
signaled level with the extension of both the
frame rate and bit rate is supported. That is,
the receiver is able to decode NAL unit
streams in which the macroblock processing rate
is up to max-mbps (inclusive), the bit rate is
up to max-br (inclusive), the coded picture
buffer size is derived as specified in the
semantics of the max-br parameter below, and
other properties comply with the level
specified in the value of the profile-level-id
parameter.
A receiver MUST NOT signal values of max-
mbps, max-fs, max-cpb, max-dpb, and max-br that
meet the requirements of a higher level,
referred to as level A herein, compared to the
level specified in the value of the profile-
level-id parameter, if the receiver can support
all the properties of level A.
Informative note: When the OPTIONAL MIME
type parameters are used to signal the
properties of a NAL unit stream, max-mbps,
max-fs, max-cpb, max-dpb, and max-br are
not present, and the value of profile-
level-id must always be such that the NAL
unit stream complies fully with the
specified profile and level.
max-mbps: The value of max-mbps is an integer indicating
the maximum macroblock processing rate in units
of macroblocks per second. The max-mbps
parameter signals that the receiver is capable
of decoding video at a higher rate than is
required by the signaled level conveyed in the
value of the profile-level-id parameter. When
max-mbps is signaled, the receiver MUST be able
to decode NAL unit streams that conform to the
signaled level, with the exception that the
MaxMBPS value in Table A-1 of [1] for the
signaled level is replaced with the value of
max-mbps. The value of max-mbps MUST be
greater than or equal to the value of MaxMBPS
for the level given in Table A-1 of [1].
Senders MAY use this knowledge to send pictures
of a given size at a higher picture rate than
is indicated in the signaled level.
max-fs: The value of max-fs is an integer indicating
the maximum frame size in units of macroblocks.
The max-fs parameter signals that the receiver
is capable of decoding larger picture sizes
than are required by the signaled level conveyed
in the value of the profile-level-id parameter.
When max-fs is signaled, the receiver MUST be
able to decode NAL unit streams that conform to
the signaled level, with the exception that the
MaxFS value in Table A-1 of [1] for the
signaled level is replaced with the value of
max-fs. The value of max-fs MUST be greater
than or equal to the value of MaxFS for the
level given in Table A-1 of [1]. Senders MAY
use this knowledge to send larger pictures at a
proportionally lower frame rate than is
indicated in the signaled level.
max-cpb The value of max-cpb is an integer indicating
the maximum coded picture buffer size in units
of 1000 bits for the VCL HRD parameters (see
A.3.1 item i of [1]) and in units of 1200 bits
for the NAL HRD parameters (see A.3.1 item j of
[1]). The max-cpb parameter signals that the
receiver has more memory than the minimum
amount of coded picture buffer memory required
by the signaled level conveyed in the value of
the profile-level-id parameter. When max-cpb
is signaled, the receiver MUST be able to
decode NAL unit streams that conform to the
signaled level, with the exception that the
MaxCPB value in Table A-1 of [1] for the
signaled level is replaced with the value of
max-cpb. The value of max-cpb MUST be greater
than or equal to the value of MaxCPB for the
level given in Table A-1 of [1]. Senders MAY
use this knowledge to construct coded video
streams with greater variation of bit rate
than can be achieved with the
MaxCPB value in Table A-1 of [1].
Informative note: The coded picture buffer
is used in the hypothetical reference
decoder (Annex C) of H.264. The use of the
hypothetical reference decoder is
recommended in H.264 encoders to verify
that the produced bitstream conforms to the
standard and to control the output bitrate.
Thus, the coded picture buffer is
conceptually independent of any other
potential buffers in the receiver,
including de-interleaving and de-jitter
buffers. The coded picture buffer need not
be implemented in decoders as specified in
Annex C of H.264, but rather standard-
compliant decoders can have any buffering
arrangements provided that they can decode
standard-compliant bitstreams. Thus, in
practice, the input buffer for video
decoder can be integrated with de-
interleaving and de-jitter buffers of the
receiver.
max-dpb: The value of max-dpb is an integer indicating
the maximum decoded picture buffer size in
units of 1024 bytes. The max-dpb parameter
signals that the receiver has more memory than
the minimum amount of decoded picture buffer
memory required by the signaled level conveyed
in the value of the profile-level-id parameter.
When max-dpb is signaled, the receiver MUST be
able to decode NAL unit streams that conform to
the signaled level, with the exception that the
MaxDPB value in Table A-1 of [1] for the
signaled level is replaced with the value of
max-dpb. Consequently, a receiver that signals
max-dpb MUST be capable of storing the
following number of decoded frames,
complementary field pairs, and non-paired
fields in its decoded picture buffer:
Min(1024 * max-dpb / ( PicWidthInMbs *
FrameHeightInMbs * 256 * ChromaFormatFactor ),
16)
PicWidthInMbs, FrameHeightInMbs, and
ChromaFormatFactor are defined in [1].
The value of max-dpb MUST be greater than or
equal to the value of MaxDPB for the level
given in Table A-1 of [1]. Senders MAY use
this knowledge to construct coded video streams
with improved compression.
Informative note: This parameter was added
primarily to complement a similar codepoint
in the ITU-T Recommendation H.245, so as to
facilitate signaling gateway designs. The
decoded picture buffer stores reconstructed
samples and is a property of the video
decoder only. There is no relationship
between the size of the decoded picture
buffer and the buffers used in RTP,
especially de-interleaving and de-jitter
buffers.
max-br: The value of max-br is an integer indicating
the maximum video bit rate in units of 1000
bits per second for the VCL HRD parameters (see
A.3.1 item i of [1]) and in units of 1200 bits
per second for the NAL HRD parameters (see
A.3.1 item j of [1]).
The max-br parameter signals that the video
decoder of the receiver is capable of decoding
video at a higher bit rate than is required by
the signaled level conveyed in the value of the
profile-level-id parameter. The value of max-
br MUST be greater than or equal to the value
of MaxBR for the level given in Table A-1 of
[1].
When max-br is signaled, the video codec of the
receiver MUST be able to decode NAL unit
streams that conform to the signaled level,
conveyed in the profile-level-id parameter,
with the following exceptions in the limits
specified by the level:
o The value of max-br replaces the MaxBR value
of the signaled level (in Table A-1 of [1]).
o When the max-cpb parameter is not present,
the result of the following formula replaces
the value of MaxCPB in Table A-1 of [1]:
(MaxCPB of the signaled level) * max-br /
(MaxBR of the signaled level).
For example, if a receiver signals capability
for Level 1.2 with max-br equal to 1550, this
indicates a maximum video bitrate of 1550
kbits/sec for VCL HRD parameters, a maximum
video bitrate of 1860 kbits/sec for NAL HRD
parameters, and a CPB size of 4036458 bits
(1550000 / 384000 * 1000 * 1000).
The value of max-br MUST be greater than or
equal to the value MaxBR for the signaled level
given in Table A-1 of [1].
Senders MAY use this knowledge to send higher
bitrate video as allowed in the level
definition of Annex A of H.264, to achieve
improved video quality.
Informative note: This parameter was added
primarily to complement a similar codepoint
in the ITU-T Recommendation H.245, so as to
facilitate signaling gateway designs. No
assumption can be made from the value of
this parameter that the network is capable
of handling such bit rates at any given
time. In particular, no conclusion can be
drawn that the signaled bit rate is
possible under congestion control
constraints.
redundant-pic-cap:
This parameter signals the capabilities of a
receiver implementation. When equal to 0, the
parameter indicates that the receiver makes no
attempt to use redundant coded pictures to
correct incorrectly decoded primary coded
pictures. When equal to 0, the receiver is not
capable of using redundant slices; therefore, a
sender SHOULD avoid sending redundant slices to
save bandwidth. When equal to 1, the receiver
is capable of decoding any such redundant slice
that covers a corrupted area in a primary
decoded picture (at least partly), and therefore
a sender MAY send redundant slices. When the
parameter is not present, then a value of 0
MUST be used for redundant-pic-cap. When
present, the value of redundant-pic-cap MUST be
either 0 or 1.
When the profile-level-id parameter is present
in the same capability signaling as the
redundant-pic-cap parameter, and the profile
indicated in profile-level-id is such that it
disallows the use of redundant coded pictures
(e.g., Main Profile), the value of redundant-
pic-cap MUST be equal to 0. When a receiver
indicates redundant-pic-cap equal to 0, the
received stream SHOULD NOT contain redundant
coded pictures.
Informative note: Even if redundant-pic-cap
is equal to 0, the decoder is able to
ignore redundant codec pictures provided
that the decoder supports such a profile
(Baseline, Extended) in which redundant
coded pictures are allowed.
Informative note: Even if redundant-pic-cap
is equal to 1, the receiver may also choose
other error concealment strategies to
replace or complement decoding of redundant
slices.
sprop-parameter-sets:
This parameter MAY be used to convey
any 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
base64 [6] 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, but a picture
parameter set NAL unit can contain several
hundreds of bytes.
Informative note: When several payload
types are offered in the SDP Offer/Answer
model, each with its own sprop-parameter-
sets parameter, then the receiver cannot
assume that those parameter sets do not use
conflicting storage locations (i.e.,
identical values of parameter set
identifiers). Therefore, a receiver should
double-buffer all sprop-parameter-sets and
make them available to the decoder instance
that decodes a certain payload type.
parameter-add: This parameter MAY be used to signal whether
the receiver of this parameter is allowed to
add parameter sets in its signaling response
using the sprop-parameter-sets MIME parameter.
The value of this parameter is either 0 or 1.
0 is equal to false; i.e., it is not allowed to
add parameter sets. 1 is equal to true; i.e.,
it is allowed to add parameter sets. If the
parameter is not present, its value MUST be 1.
packetization-mode:
This parameter signals the properties of an
RTP payload type or the capabilities of a
receiver implementation. Only a single
configuration point can be indicated; thus,
when capabilities to support more than one
packetization-mode are declared, multiple
configuration points (RTP payload types) must
be used.
When the value of packetization-mode is equal
to 0 or packetization-mode is not present, the
single NAL mode, as defined in section 6.2 of
RFC 3984, MUST be used. This mode is in use in
standards using ITU-T Recommendation H.241 [15]
(see section 12.1). When the value of
packetization-mode is equal to 1, the non-
interleaved mode, as defined in section 6.3 of
RFC 3984, MUST be used. When the value of
packetization-mode is equal to 2, the
interleaved mode, as defined in section 6.4 of
RFC 3984, MUST be used. The value of
packetization mode MUST be an integer in the
range of 0 to 2, inclusive.
sprop-interleaving-depth:
This parameter MUST NOT be present
when packetization-mode is not present or the
value of packetization-mode is equal to 0 or 1.
This parameter MUST be present when the value
of packetization-mode is equal to 2.
This parameter signals the properties of a NAL
unit stream. It specifies the maximum number
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.
Consequently, it is guaranteed that receivers
can reconstruct NAL unit decoding order when
the buffer size for NAL unit decoding order
recovery is at least the value of sprop-
interleaving-depth + 1 in terms of VCL NAL
units.
The value of sprop-interleaving-depth MUST be
an integer in the range of 0 to 32767,
inclusive.
sprop-deint-buf-req:
This parameter MUST NOT be present when
packetization-mode is not present or the value
of packetization-mode is equal to 0 or 1. It
MUST be present when the value of
packetization-mode is equal to 2.
sprop-deint-buf-req signals the required size
of the deinterleaving buffer for the NAL unit
stream. The value of the parameter MUST be
greater than or equal to the maximum buffer
occupancy (in units of bytes) required in such
a deinterleaving buffer that is specified in
section 7.2 of RFC 3984. It is guaranteed that
receivers can perform the deinterleaving of
interleaved NAL units into NAL unit decoding
order, when the deinterleaving buffer size is
at least the value of sprop-deint-buf-req in
terms of bytes.
The value of sprop-deint-buf-req MUST be an
integer in the range of 0 to 4294967295,
inclusive.
Informative note: sprop-deint-buf-req
indicates the required size of the
deinterleaving buffer only. When network
jitter can occur, an appropriately sized
jitter buffer has to be provisioned for
as well.
deint-buf-cap: This parameter signals the capabilities of a
receiver implementation and indicates the
amount of deinterleaving buffer space in units
of bytes that the receiver has available for
reconstructing the NAL unit decoding order. A
receiver is able to handle any stream for which
the value of the sprop-deint-buf-req parameter
is smaller than or equal to this parameter.
If the parameter is not present, then a value
of 0 MUST be used for deint-buf-cap. The value
of deint-buf-cap MUST be an integer in the
range of 0 to 4294967295, inclusive.
Informative note: deint-buf-cap indicates
the maximum possible size of the
deinterleaving buffer of the receiver only.
When network jitter can occur, an
appropriately sized jitter buffer has to
be provisioned for as well.
sprop-init-buf-time:
This parameter MAY be used to signal the
properties of a NAL unit stream. The parameter
MUST NOT be present, if the value of
packetization-mode is equal to 0 or 1.
The parameter signals the initial buffering
time that a receiver MUST buffer before
starting decoding to recover the NAL unit
decoding order from the transmission order.
The parameter is the maximum value of
(transmission time of a NAL unit - decoding
time of the NAL unit), assuming reliable and
instantaneous transmission, the same
timeline for transmission and decoding, and
that decoding starts when the first packet
arrives.
An example of specifying the value of sprop-
init-buf-time follows. A NAL unit stream is
sent in the following interleaved order, in
which the value corresponds to the decoding
time and the transmission order is from left to
right:
0 2 1 3 5 4 6 8 7 ...
Assuming a steady transmission rate of NAL
units, the transmission times are:
0 1 2 3 4 5 6 7 8 ...
Subtracting the decoding time from the
transmission time column-wise results in the
following series:
0 -1 1 0 -1 1 0 -1 1 ...
Thus, in terms of intervals of NAL unit
transmission times, the value of
sprop-init-buf-time in this
example is 1.
The parameter is coded as a non-negative base10
integer representation in clock ticks of a 90-
kHz clock. If the parameter is not present,
then no initial buffering time value is
defined. Otherwise the value of sprop-init-
buf-time MUST be an integer in the range of 0
to 4294967295, inclusive.
In addition to the signaled sprop-init-buf-
time, receivers SHOULD take into account the
transmission delay jitter buffering, including
buffering for the delay jitter caused by
mixers, translators, gateways, proxies,
traffic-shapers, and other network elements.
sprop-max-don-diff:
This parameter MAY be used to signal the
properties of a NAL unit stream. It MUST NOT
be used to signal transmitter or receiver or
codec capabilities. The parameter MUST NOT be
present if the value of packetization-mode is
equal to 0 or 1. sprop-max-don-diff is an
integer in the range of 0 to 32767, inclusive.
If sprop-max-don-diff is not present, the value
of the parameter is unspecified. sprop-max-
don-diff is calculated as follows:
sprop-max-don-diff = max{AbsDON(i) -
AbsDON(j)},
for any i and any j>i,
where i and j indicate the index of the NAL
unit in the transmission order and AbsDON
denotes a decoding order number of the NAL
unit that does not wrap around to 0 after
65535. In other words, AbsDON is calculated as
follows: Let m and n be consecutive NAL units
in transmission order. For the very first NAL
unit in transmission order (whose index is 0),
AbsDON(0) = DON(0). For other NAL units,
AbsDON is calculated as follows:
If DON(m) == DON(n), AbsDON(n) = AbsDON(m)
If (DON(m) < DON(n) and DON(n) - DON(m) <
32768),
AbsDON(n) = AbsDON(m) + DON(n) - DON(m)
If (DON(m) > DON(n) and DON(m) - DON(n) >=
32768),
AbsDON(n) = AbsDON(m) + 65536 - DON(m) + DON(n)
If (DON(m) < DON(n) and DON(n) - DON(m) >=
32768),
AbsDON(n) = AbsDON(m) - (DON(m) + 65536 -
DON(n))
If (DON(m) > DON(n) and DON(m) - DON(n) <
32768),
AbsDON(n) = AbsDON(m) - (DON(m) - DON(n))
where DON(i) is the decoding order number of
the NAL unit having index i in the transmission
order. The decoding order number is specified
in section 5.5 of RFC 3984.
Informative note: Receivers may use sprop-
max-don-diff to trigger which NAL units in
the receiver buffer can be passed to the
decoder.
max-rcmd-nalu-size:
This parameter MAY be used to signal the
capabilities of a receiver. The parameter MUST
NOT be used for any other purposes. The value
of the parameter indicates the largest NALU
size in bytes that the receiver can handle
efficiently. The parameter value is a
recommendation, not a strict upper boundary.
The sender MAY create larger NALUs but must be
aware that the handling of these may come at a
higher cost than NALUs conforming to the
limitation.
The value of max-rcmd-nalu-size MUST be an
integer in the range of 0 to 4294967295,
inclusive. If this parameter is not specified,
no known limitation to the NALU size exists.
Senders still have to consider the MTU size
available between the sender and the receiver
and SHOULD run MTU discovery for this purpose.
This parameter is motivated by, for example, an
IP to H.223 video telephony gateway, where
NALUs smaller than the H.223 transport data
unit will be more efficient. A gateway may
terminate IP; thus, MTU discovery will normally
not work beyond the gateway.
Informative note: Setting this parameter to
a lower than necessary value may have a
negative impact.
Encoding considerations:
This type is only defined for transfer via RTP
(RFC 3550).
A file format of H.264/AVC video is defined in
[29]. This definition is utilized by other
file formats, such as the 3GPP multimedia file
format (MIME type video/3gpp) [30] or the MP4
file format (MIME type video/mp4).
Security considerations:
See section 9 of RFC 3984.
Public specification:
Please refer to RFC 3984 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@stewe.org
Intended usage: COMMON
Author:
stewe@stewe.org
Change controller:
IETF Audio/Video Transport working group
delegated from the IESG.
8.2. SDP Parameters
8.2.1. Mapping of MIME Parameters to SDP
The MIME media type video/H264 string is mapped to fields in the
Session Description Protocol (SDP) [5] 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.
o The OPTIONAL parameters "profile-level-id", "max-mbps", "max-fs",
"max-cpb", "max-dpb", "max-br", "redundant-pic-cap", "sprop-
parameter-sets", "parameter-add", "packetization-mode", "sprop-
interleaving-depth", "deint-buf-cap", "sprop-deint-buf-req",
"sprop-init-buf-time", "sprop-max-don-diff", and "max-rcmd-nalu-
size", when present, MUST be included in the "a=fmtp" line of SDP.
These parameters are expressed as a MIME media type string, in the
form of a semicolon separated list of parameter=value pairs.
An example of media representation in SDP is as follows (Baseline
Profile, Level 3.0, some of the constraints of the Main profile may
not be obeyed):
m=video 49170 RTP/AVP 98
a=rtpmap:98 H264/90000
a=fmtp:98 profile-level-id=42A01E;
sprop-parameter-sets=Z0IACpZTBYmI,aMljiA==
8.2.2. Usage with the SDP Offer/Answer Model
When H.264 is offered over RTP using SDP in an Offer/Answer model [7]
for negotiation for unicast usage, the following limitations and
rules apply:
o The parameters identifying a media format configuration for H.264
are "profile-level-id", "packetization-mode", and, if required by
"packetization-mode", "sprop-deint-buf-req". These three
parameters MUST be used symmetrically; i.e., the answerer MUST
either maintain all configuration parameters or remove the media
format (payload type) completely, if one or more of the parameter
values are not supported.
Informative note: The requirement for symmetric use applies
only for the above three parameters and not for the other
stream properties and capability parameters.
To simplify handling and matching of these configurations, the
same RTP payload type number used in the offer SHOULD also be used
in the answer, as specified in [7]. An answer MUST NOT contain a
payload type number used in the offer unless the configuration
("profile-level-id", "packetization-mode", and, if present,
"sprop-deint-buf-req") is the same as in the offer.
Informative note: An offerer, when receiving the answer, has to
compare payload types not declared in the offer based on media
type (i.e., video/h264) and the above three parameters with any
payload types it has already declared, in order to determine
whether the configuration in qu