RFC 3984
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 packet5.7. 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 units5.7.2. 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 units5.8. 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.
(page 31 continued on part 2)
