Internet DRAFT - draft-ietf-avt-rtp-rfc3984bis
draft-ietf-avt-rtp-rfc3984bis
Obsoletes RFC 3984
Audio/Video Transport WG Y.-K. Wang
Internet Draft Huawei Technologies
Intended status: Standards track R. Even
Expires: April 2011 Self-employed
T. Kristensen
Tandberg
R. Jesup
WorldGate Communications
October 9, 2010
RTP Payload Format for H.264 Video
draft-ietf-avt-rtp-rfc3984bis-12.txt
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document authors. All rights reserved.
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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, excluding the
Scalable Video Coding (SVC) extension and the Multivew Video Coding
extension, for which the RTP payload formats are defined elsewhere.
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.
This memo obsoletes RFC 3984. Changes from RFC 3984 are summarized
in section 15. Issues on backward compatibility to RFC 3984 are
discussed in section 14.
Table of Contents
Table of Contents................................................2
1. Introduction..................................................4
1.1. The H.264 Codec..........................................4
1.2. Parameter Set Concept....................................6
1.3. Network Abstraction Layer Unit Types.....................6
2. Conventions...................................................7
3. Scope.........................................................8
4. Definitions and Abbreviations.................................8
4.1. Definitions..............................................8
4.2. Abbreviations...........................................10
5. RTP Payload Format...........................................11
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5.1. RTP Header Usage........................................11
5.2. Payload Structures......................................13
5.3. NAL Unit Header Usage...................................14
5.4. Packetization Modes.....................................16
5.5. Decoding Order Number (DON).............................18
5.6. Single NAL Unit Packet..................................20
5.7. Aggregation Packets.....................................21
Table 4. Type field for STAPs and MTAPs........................22
5.7.1. Single-Time Aggregation Packet.....................23
5.7.2. Multi-Time Aggregation Packets (MTAPs).............26
5.7.3. Fragmentation Units (FUs)..........................29
6. Packetization Rules..........................................33
6.1. Common Packetization Rules..............................33
6.2. Single NAL Unit Mode....................................34
6.3. Non-Interleaved Mode....................................34
6.4. Interleaved Mode........................................35
7. De-Packetization Process.....................................35
7.1. Single NAL Unit and Non-Interleaved Mode................35
7.2. Interleaved Mode........................................36
7.2.1. Size of the De-interleaving Buffer.................36
7.2.2. De-interleaving Process............................37
7.3. Additional De-Packetization Guidelines..................38
8. Payload Format Parameters....................................39
8.1. Media Type Registration.................................39
8.2. SDP Parameters..........................................58
8.2.1. Mapping of Payload Type Parameters to SDP..........58
8.2.2. Usage with the SDP Offer/Answer Model..............59
8.2.3. Usage in Declarative Session Descriptions..........69
8.3. Examples................................................70
Offer SDP:......................................................76
Answer SDP:.....................................................76
8.4. Parameter Set Considerations............................77
8.5. Decoder Refresh Point Procedure using In-Band Transport of
Parameter Sets (Informative).................................80
8.5.1. IDR Procedure to Respond to a Request for a Decoder
Refresh Point.............................................80
8.5.2. Gradual Recovery Procedure to Respond to a Request for
a Decoder Refresh Point...................................81
9. Security Considerations......................................82
10. Congestion Control..........................................82
11. IANA Consideration..........................................83
12. Informative Appendix: Application Examples..................83
12.1. Video Telephony according to ITU-T Recommendation H.241
Annex A......................................................84
12.2. Video Telephony, No Slice Data Partitioning, No NAL Unit
Aggregation..................................................84
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12.3. Video Telephony, Interleaved Packetization Using NAL Unit
Aggregation..................................................84
12.4. Video Telephony with Data Partitioning.................85
12.5. Video Telephony or Streaming with FUs and Forward Error
Correction...................................................86
12.6. Low Bit-Rate Streaming.................................88
12.7. Robust Packet Scheduling in Video Streaming............89
13. Informative Appendix: Rationale for Decoding Order Number...90
13.1. Introduction...........................................90
13.2. Example of Multi-Picture Slice Interleaving............90
13.3. Example of Robust Packet Scheduling....................92
13.4. Robust Transmission Scheduling of Redundant Coded Slices96
13.5. Remarks on Other Design Possibilities..................96
14. Backward Compatibility to RFC 3984..........................97
15. Changes from RFC 3984.......................................99
16. Acknowledgements...........................................101
17. References.................................................101
17.1. Normative References..................................101
17.2. Informative References................................102
18. Authors' Addresses.........................................104
1. Introduction
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). In this memo the name H.264 is
used for the codec and the standard, but the memo is equally
applicable to the ISO/IEC counterpart of the coding standard.
This memo obsoletes RFC 3984. Changes from RFC 3984 are summarized
in section 15. Issues on backward compatibility to RFC 3984 are
discussed in section 14.
1.1. The H.264 Codec
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
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1.5 Mbit/s, compared to the current operation point of MPEG 2 video
at around 3.5 Mbit/s [10].
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 [10].
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
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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 4629 [11] or MPEG-4 Visual's Header Extension
Code (HEC) [12] 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 [13], [14],
and [15].
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.
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The syntax and semantics of the NAL unit type octet are specified
in [1], but the essential properties of the NAL unit type octet are
summarized below. The NAL unit type octet has the following format:
+---------------+
|0|1|2|3|4|5|6|7|
+-+-+-+-+-+-+-+-+
|F|NRI| Type |
+---------------+
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 RFC 2119 [4].
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).
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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
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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].
NALU-time: The value that the RTP timestamp would have if the
NAL unit would be transported in its own RTP packet.
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 and remove
those packets whose elimination produces the least adverse
effect on the user experience.
static macroblock: A certain amount of macroblocks in the video
stream can be defined as static, as defined in section 8.3.2.8
in [3]. Static macroblocks free up additional processing
cycles for the handling of non-static macroblocks. Based on a
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given amount of video processing resources and a given
resolution, a higher number of static macroblocks enables a
correspondingly higher frame rate.
default sub-profile: The subset of coding tools, which may be
all coding tools of one profile or the common subset of coding
tools of more than one profile, indicated by the profile-level-
id parameter.
default level: The level indicated by the profile-level-id
parameter, which consists of three octets, profile_idc, profile-
iop, and level_idc. The default level is indicated by level_idc
in most cases, and, in some cases, additionally by profile-iop.
4.2. 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
SAR: Sample Aspect Ratio
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
VUI: Video Usability Information
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5. RTP Payload Format
5.1. RTP Header Usage
The format of the RTP header is specified in RFC 3550 [5] 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
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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.
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).
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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.
5.2. Payload Structures
The payload format defines three different basic payload structures.
A receiver can identify the payload structure by the first byte of
the RTP packet 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.
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.
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Table 1 summarizes NAL unit types and the corresponding RTP packet
types when each of these NAL units is directly used as a packet
payload, and where the types are described in this memo.
Table 1. Summary of NAL unit types and the corresponding packet
types
NAL Unit Packet Packet Type Name Section
Type Type
---------------------------------------------------------
0 reserved -
1-23 NAL unit Single NAL unit packet 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 reserved -
5.3. NAL Unit Header Usage
The structure and semantics of the NAL unit header were introduced
in section 1.3. For convenience, the format of the NAL unit header
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
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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 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 [14]. Other mappings MAY also be
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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
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o Interleaved mode
The single NAL unit mode is targeted for conversational systems
that comply with ITU-T Recommendation H.241 [3] (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 media type parameter. The used
packetization mode governs which NAL unit types are allowed in RTP
payloads. Table 3 summarizes the allowed packet payload types for
each packetization 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)
Payload Packet Single NAL Non-Interleaved Interleaved
Type Type Unit Mode Mode Mode
-------------------------------------------------------------
0 reserved 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 reserved ig ig ig
Some NAL unit or payload type values (indicated as reserved in
Table 3) are reserved for future extensions. NAL units of those
types SHOULD NOT be sent by a sender (direct as packet payloads, or
as aggregation units in aggregation packets, or as fragmented units
in FU packets) and MUST be ignored by a receiver. For example, the
payload 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". However, NAL units of NAL
unit types 1-23 can be used in "Interleaved Mode" as aggregation
units in STAP-B, MTAP16 and MTAP24 packets as well as fragmented
units in FU-A and FU-B packets. Similarly, NAL units of NAL unit
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types 1-23 can also be used in the "Non-Interleaved Mode" as
aggregation units in STAP-A packets or fragmented units in FU-A
packets, in addition to being directly used as packet payloads.
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 media type parameter as
follows. When the value of the OPTIONAL sprop-interleaving-depth
media type parameter is equal to 0 (explicitly or per default), the
transmission order of NAL units MUST conform to the NAL unit
decoding order. When the value of the OPTIONAL sprop-interleaving-
depth media type parameter is greater than 0,
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 de-packetizing 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
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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.
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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 de-packetization 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 de-
packetizing single NAL unit packets in RTP sequence number order
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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.
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.
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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
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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
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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.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RTP Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|STAP-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 containing
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 containing
two single-time aggregation units
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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
NAL unit contained in a multi-time aggregation 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.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: NAL 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 NAL units
contained in 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
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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 containing 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.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RTP Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|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 containing two multi-time aggregation units
5.7.3. 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 NAL unit
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
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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.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 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
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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. An 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,
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where sometimes several macroblocks occupy zero bits, this is
undesirable and can add delay. However, the (potential) use of
zero-length NALU fragments should be carefully weighed against
the increased risk of the loss of at least a part 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; however, for
delay-critical systems, they SHOULD be sent in their original
decoding order to minimize the delay. Note that the decoding
order is the order of the NAL units in the bitstream.
o Parameter sets are handled in accordance with the rules and
recommendations given in section 8.4.
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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 media type parameter is equal to 0 or the packetization-mode
is not present. 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 [3] (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 media type parameter is equal to 1. 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
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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 media type parameter is equal to 2. 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
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 as
long as the output for the same input is the same as the process
described below. The same output means that the resulting NAL
units, and their order, are identical. 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
de-packetization 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 sequence 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 de-
packetized in RTP sequence number order. If a de-packetized packet
is a single NAL unit packet, the NAL unit contained in the packet
is passed directly to the decoder. If a de-packetized 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.
For all the FU-A packets containing fragments of a single NAL unit,
the de-packetized fragments are concatenated in their sending order
to recover the NAL unit, which is then passed to the decoder.
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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 NAL units
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 de-interleaving buffer in this section. Receivers
SHOULD also prepare for transmission delay jitter; i.e., either
reserve separate buffers for transmission delay jitter buffering
and de-interleaving buffering or use a receiver buffer for both
transmission delay jitter and de-interleaving. 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 de-interleaving buffer. Subsection
7.2.2 specifies the receiver process on how to organize received
NAL units to the NAL unit decoding order.
7.2.1. Size of the De-interleaving Buffer
In either Offer/Answer or declarative SDP usage, the sprop-deint-
buf-req media type parameter signals the requirement for the de-
interleaving buffer size. It is therefore RECOMMENDED to set the
de-interleaving buffer size, in terms of number of bytes, equal to
or greater than the value of sprop-deint-buf-req media type
parameter.
When the 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 de-interleaving buffer with the
deint-buf-cap media type parameter. See section 8.1 for further
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information on deint-buf-cap and sprop-deint-buf-req media type
parameters and section 8.2.2 for further information on their use
in the SDP Offer/Answer model.
7.2.2. De-interleaving 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 are 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 de-interleaving buffer as follows.
NAL units of aggregation packets are stored in the de-interleaving
buffer individually. The value of DON is calculated and stored for
each NAL unit.
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
media type parameter (see section 8.1) incremented by 1.
Initial buffering lasts until one of the following conditions is
fulfilled:
o There are N or more VCL NAL units in the de-interleaving 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 media
type parameter.
The NAL units to be removed from the de-interleaving buffer are
determined as follows:
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o If the de-interleaving buffer contains at least N VCL NAL units,
NAL units are removed from the de-interleaving 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 de-interleaving 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 NAL units in the
de-interleaving buffer.
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 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 detected,
after taking into account possible retransmission and FEC, 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.
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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 media
subtype registration for the ITU-T H.264 | ISO/IEC 14496-10 codec.
A mapping of the parameters into the Session Description Protocol
(SDP) [6] is also provided for applications that use SDP.
Equivalent parameters could be defined elsewhere for use with
control protocols that do not use SDP.
Some parameters provide a receiver with the properties of the
stream that will be sent. The names of all these parameters start
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.
8.1. Media Type Registration
The media subtype for the ITU-T H.264 | ISO/IEC 14496-10 codec is
allocated from the IETF tree.
Media Type name: video
Media subtype name: H264
Required parameters: none
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OPTIONAL parameters:
profile-level-id:
A base16 [7] (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,
constraint_set3_flag, and reserved_zero_4bits in bit-
significance order, starting from the most significant bit,
and 3) level_idc. Note that reserved_zero_4bits is required
to be equal to 0 in [1], but other values for it may be
specified in the future by ITU-T or ISO/IEC.
The profile-level-id parameter indicates the default sub-
profile, i.e. the subset of coding tools that may have been
used to generate the stream or that the receiver supports,
and the default level of the stream or the receiver supports.
The default sub-profile is indicated collectively by the
profile_idc byte and some fields in the profile-iop byte.
Depending on the values of the fields in the profile-iop byte,
the default sub-profile may be the set of coding tools
supported by one profile, or a common subset of coding tools
of multiple profiles, as specified in subsection 7.4.2.1.1 of
[1]. The default level is indicated by the level_idc byte,
and, when profile_idc is equal to 66, 77 or 88 (the Baseline,
Main, or Extended profile) and level_idc is equal to 11,
additionally by bit 4 (constraint_set3_flag) of the profile-
iop byte. When profile_idc is equal to 66, 77 or 88 (the
Baseline, Main, or Extended profile) and level_idc is equal
to 11, and bit 4 (constraint_set3_flag) of the profile-iop
byte is equal to 1, the default level is level 1b.
Table 5 lists all profiles defined in Annex A of [1] and, for
each of the profiles, the possible combinations of
profile_idc and profile-iop that represent the same sub-
profile.
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Table 5. Combinations of profile_idc and profile-iop
representing the same sub-profile corresponding to the
full set of coding tools supported by one profile. In
the following, x may be either 0 or 1, while the profile
names are indicated as follows. CB: Constrained Baseline
profile, B: Baseline profile, M: Main profile, E:
Extended profile, H: High profile, H10: High 10 profile,
H42: High 4:2:2 profile, H44: High 4:4:4 Predictive
profile, H10I: High 10 Intra profile, H42I: High 4:2:2
Intra profile, H44I: High 4:4:4 Intra profile, and C44I:
CAVLC 4:4:4 Intra profile.
Profile profile_idc profile-iop
(hexadecimal) (binary)
CB 42 (B) x1xx0000
same as: 4D (M) 1xxx0000
same as: 58 (E) 11xx0000
same as: 64 (H), 6E (H10), 1xx00000
7A (H42), or F4 (H44)
B 42 (B) x0xx0000
same as: 58 (E) 10xx0000
M 4D (M) 0x0x0000
same as: 64 (H), 6E (H10), 01000000
7A (H42), or F4 (H44)
E 58 00xx0000
H 64 00000000
H10 6E 00000000
H42 7A 00000000
H44 F4 00000000
H10I 64 00010000
H42I 7A 00010000
H44I F4 00010000
C44I 2C 00010000
For example, in the table above, profile_idc equal to 58
(Extended) with profile-iop equal to 11xx0000 indicates the
same sub-profile corresponding to profile_idc equal to 42
(Baseline) with profile-iop equal to x1xx0000. Note that
other combinations of profile_idc and profile-iop (not listed
in Table 5) may represent a sub-profile equivalent to the
common subset of coding tools for more than one profile.
Note also that a decoder conforming to a certain profile may
be able to decode bitstreams conforming to other profiles.
For example, a decoder conforming to the High 4:4:4 profile
at certain level must be able to decode bitstreams conforming
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to the Constrained Baseline, Main, High, High 10 or High
4:2:2 profile at the same or a lower level.
If the profile-level-id parameter is used to indicate
properties of a NAL unit stream, it indicates that, to decode
the stream, the minimum subset of coding tools a decoder has
to support is the default sub-profile, and the lowest level
the decoder has to support is the default level.
If the profile-level-id parameter is used for capability
exchange or session setup procedure, it indicates the subset
of coding tools, which is equal to the default sub-profile,
that the codec supports for both receiving and sending. If
max-recv-level is not present, the default level from
profile-level-id indicates the highest level the codec wishes
to support. If max-recv-level is present it indicates the
highest level the codec supports for receiving. For either
receiving or sending, all levels that are lower than the
highest level supported MUST also be supported.
Informative note: Capability exchange and session setup
procedures should provide means to list the capabilities
for each supported sub-profile separately. For example,
the one-of-N codec selection procedure of the SDP
Offer/Answer model can be used (section 10.2 of [8]).
The one-of-N codec selection procedure may also be used
to provide different combinations of profile_idc and
profile-iop that represent the same sub-profile. When
there are many different combinations of profile_idc and
profile-iop that represent the same sub-profile, using
the one-of-N codec selection procedure may result into a
fairly large SDP message. Therefore, a receiver should
understand the different equivalent combinations of
profile_idc and profile-iop that represent the same sub-
profile, and be ready to accept an offer using any of the
equivalent combinations.
If no profile-level-id is present, the Baseline Profile
without additional constraints at Level 1 MUST be inferred.
max-recv-level:
This parameter MAY be used to indicate the highest level a
receiver supports when the highest level is higher than the
default level (the level indicated by profile-level-id). The
value of max-recv-level is a base16 (hexadecimal)
representation of the two bytes after the syntax element
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profile_idc in the sequence parameter set NAL unit specified
in [1]: profile-iop (as defined above) and level_idc. If
(the level_idc byte of max-recv-level is equal to 11 and bit
4 of the profile-iop byte of max-recv-level is equal to 1) or
(the level_idc byte of max-recv-level is equal to 9 and bit 4
of the profile-iop byte of max-recv-level is equal to 0), the
highest level the receiver supports is level 1b. Otherwise,
the highest level the receiver supports is equal to the
level_idc byte of max-recv-level divided by 10.
max-recv-level MUST NOT be present if the highest level the
receiver supports is not higher than the default level.
max-mbps, max-smbps, 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 highest level conveyed in the
value of the profile-level-id parameter or the max-recv-level
parameter MUST be such that the receiver is fully capable of
supporting. max-mbps, max-smbps, 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 highest level, as specified below.
When more than one parameter from the set (max-mbps, max-
smbps , 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 highest 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
highest level specified in the value of the profile-level-id
parameter or the max-recv-level parameter.
If a receiver can support all the properties of level A, the
highest level specified in the value of the profile-level-id
parameter or the max-recv-level parameter MUST be level A
(i.e. MUST NOT be lower than level A). In other words, a
receiver MUST NOT signal values of max-mbps, max-fs, max-cpb,
max-dpb, and max-br that taken together meet the requirements
of a higher level compared to the highest level specified in
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the value of the profile-level-id parameter or the max-recv-
level parameter.
Informative note: When the OPTIONAL media type parameters
are used to signal the properties of a NAL unit stream,
max-mbps, max-smbps, 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 highest level conveyed in the value
of the profile-level-id parameter or the max-recv-level
parameter. When max-mbps is signaled, the receiver MUST be
able to decode NAL unit streams that conform to the signaled
highest level, with the exception that the MaxMBPS value in
Table A-1 of [1] for the signaled highest 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 given in Table
A-1 of [1] for the highest level. Senders MAY use this
knowledge to send pictures of a given size at a higher
picture rate than is indicated in the signaled highest level.
max-smbps: The value of max-smbps is an integer indicating the
maximum static macroblock processing rate in units of static
macroblocks per second, under the hypothetical assumption
that all macroblocks are static macroblocks. When max-smbps
is signaled the MaxMBPS value in Table A-1 of [1] should be
replaced with the result of the following computation:
o If the parameter max-mbps is signaled, set a variable
MaxMacroblocksPerSecond to the value of max-mbps.
Otherwise, set MaxMacroblocksPerSecond equal to the value
of MaxMBPS in Table A-1 [1] for the signaled highest level
conveyed in the value of the profile-level-id parameter or
the max-recv-level parameter.
o Set a variable P_non-static to the proportion of non-
static macroblocks in picture n.
o Set a variable P_static to the proportion of static
macroblocks in picture n.
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o The value of MaxMBPS in Table A-1 of [1] should be
considered by the encoder to be equal to:
MaxMacroblocksPerSecond * max-smbps / (P_non-static *
max-smbps + P_static * MaxMacroblocksPerSecond)
The encoder should recompute this value for each picture. The
value of max-smbps MUST be greater than or equal to the value
of MaxMBPS given explicitly as the value of the max-mbps
parameter or implicitly in Table A-1 of [1] for the signaled
highest level. Senders MAY use this knowledge to send
pictures of a given size at a higher picture rate than is
indicated in the signaled highest 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 highest level
conveyed in the value of the profile-level-id parameter or
the max-recv-level parameter. When max-fs is signaled, the
receiver MUST be able to decode NAL unit streams that conform
to the signaled highest level, with the exception that the
MaxFS value in Table A-1 of [1] for the signaled highest
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
given in Table A-1 of [1] for the highest level. Senders MAY
use this knowledge to send larger pictures at a
proportionally lower frame rate than is indicated in the
signaled highest 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 highest level conveyed in the
value of the profile-level-id parameter or the max-recv-level
parameter. When max-cpb is signaled, the receiver MUST be
able to decode NAL unit streams that conform to the signaled
highest level, with the exception that the MaxCPB value in
Table A-1 of [1] for the signaled highest 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 given in Table
A-1 of [1] for the highest level. Senders MAY use this
knowledge to construct coded video streams with greater
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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 highest level conveyed in the
value of the profile-level-id parameter or the max-recv-level
parameter. When max-dpb is signaled, the receiver MUST be
able to decode NAL unit streams that conform to the signaled
highest level, with the exception that the MaxDPB value in
Table A-1 of [1] for the signaled highest 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 given in Table A-1 of [1] for the highest
level. Senders MAY use this knowledge to construct coded
video streams with improved compression.
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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. 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 highest level conveyed in
the value of the profile-level-id parameter or the max-recv-
level parameter.
When max-br is signaled, the video codec of the receiver MUST
be able to decode NAL unit streams that conform to the
signaled highest level, with the following exceptions in the
limits specified by the highest level:
o The value of max-br replaces the MaxBR value in Table A-1
of [1] for the highest level.
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 highest 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 given in Table A-1 of [1] for the signaled
highest level.
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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
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.
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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 can be placed in the NAL unit
stream to 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 a comma (',') separated list of base64 [7]
representations of parameter set NAL units as specified in
sections 7.3.2.1 and 7.3.2.2 of [1]. 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 buffer all
sprop-parameter-sets and make them available to the
decoder instance that decodes a certain payload type.
The "sprop-parameter-sets" parameter MUST only contain
parameter sets that are conforming to the profile-level-id,
i.e., the subset of coding tools indicated by any of the
parameter sets MUST be equal to the default sub-profile, and
the level indicated by any of the parameter sets MUST be
equal to the default level.
sprop-level-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 can be placed in the NAL unit
stream to precede any other NAL units in decoding order and
that are associated with one or more levels different than
the default level. The parameter MUST NOT be used to
indicate codec capability in any capability exchange
procedure.
The sprop-level-parameter-sets parameter contains parameter
sets for one or more levels which are different than the
default level. All parameter sets associated with one level
are clustered and prefixed with a three-byte field which has
the same syntax as profile-level-id. This enables the
receiver to install the parameter sets for one level and
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discard the rest. The three-byte field is named PLId, and
all parameter sets associated with one level are named PSL,
which has the same syntax as sprop-parameter-sets. Parameter
sets for each level are represented in the form of PLId:PSL,
i.e., PLId followed by a colon (':') and the base64 [7]
representation of the initial parameter set NAL units for the
level. Each pair of PLId:PSL is also separated by a colon.
Note that a PSL can contain multiple parameter sets for that
level, separated with commas (',').
The subset of coding tools indicated by each PLId field MUST
be equal to the default sub-profile, and the level indicated
by each PLId field MUST be different than the default level.
All sequence parameter sets contained in each PSL MUST have
the three bytes from profile_idc to level_idc, inclusive,
equal to the preceding PLId.
Informative note: This parameter allows for efficient
level downgrade or upgrade in SDP Offer/Answer and out-
of-band transport of parameter sets, simultaneously.
use-level-src-parameter-sets:
This parameter MAY be used to indicate a receiver capability.
The value MAY be equal to either 0 or 1. When the parameter
is not present, the value MUST be inferred to be equal to 0.
The value 0 indicates that the receiver does not understand
the sprop-level-parameter-sets parameter, and does not
understand the "fmtp" source attribute as specified in
section 6.3 of [9], and will ignore sprop-level-parameter-
sets when present, and will ignore sprop-parameter-sets when
conveyed using the "fmtp" source attribute. The value 1
indicates that the receiver understands the sprop-level-
parameter-sets parameter, and understands the "fmtp" source
attribute as specified in section 6.3 of [9], and is capable
of using parameter sets contained in the sprop-level-
parameter-sets or contained in the sprop-parameter-sets that
is conveyed using the "fmtp" source attribute.
Informative note: An RFC 3984 receiver does not
understand sprop-level-parameter-sets, use-level-src-
parameter-sets, or the "fmtp" source attribute as
specified in section 6.3 of [9]. Therefore, during SDP
Offer/Answer, an RFC 3984 receiver as the answerer will
simply ignore sprop-level-parameter-sets, when present in
an offer, and sprop-parameter-sets conveyed using the
"fmtp" source attribute as specified in section 6.3 of
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[9]. Assume that the offered payload type was accepted
at a level lower than the default level. If the offered
payload type included sprop-level-parameter-sets or
included sprop-parameter-sets conveyed using the "fmtp"
source attribute, and the offerer sees that the answerer
has not included use-level-src-parameter-sets equal to 1
in the answer, the offerer knows that in-band transport
of parameter sets is needed.
in-band-parameter-sets:
This parameter MAY be used to indicate a receiver capability.
The value MAY be equal to either 0 or 1. The value 1
indicates that the receiver discards out-of-band parameter
sets in sprop-parameter-sets and sprop-level-parameter-sets,
therefore the sender MUST transmit all parameter sets in-band.
The value 0 indicates that the receiver utilizes out-of-band
parameter sets included in sprop-parameter-sets and/or sprop-
level-parameter-sets. However, in this case, the sender MAY
still choose to send parameter sets in-band. When in-band-
parameter-sets is equal to 1, use-level-src-parameter-sets
MUST NOT be present or MUST be equal to 0. When the
parameter is not present, this receiver capability is not
specified, and therefore the sender MAY send out-of-band
parameter sets only, or it MAY send in-band-parameter-sets
only, or it MAY send both.
level-asymmetry-allowed:
This parameter MAY be used in SDP Offer/Answer to indicate
whether level asymmetry, i.e., sending media encoded at a
different level in the offerer-to-answerer direction than the
level in the answerer-to-offerer direction, is allowed. The
value MAY be equal to either 0 or 1. When the parameter is
not present, the value MUST be inferred to be equal to 0.
The value 1 in both the offer and the answer indicates that
level asymmetry is allowed. The value of 0 in either the
offer or the answer indicates the level asymmetry is not
allowed.
If "level-asymmetry-allowed" is equal to 0 (or not present)
in either the offer or the answer, level asymmetry is not
allowed. In this case, the level to use in the direction
from the offerer to the answerer MUST be the same as the
level to use in the opposite direction.
packetization-mode:
This parameter signals the properties of an RTP payload type
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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 MUST
be used. This mode is in use in standards using ITU-T
Recommendation H.241 [3] (see section 12.1). When the value
of packetization-mode is equal to 1, the non-interleaved mode
MUST be used. When the value of packetization-mode is equal
to 2, the interleaved mode 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 an RTP packet stream.
It specifies the maximum number of VCL NAL units that precede
any VCL NAL unit in the RTP packet 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 de-
interleaving buffer for the RTP packet 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 de-
interleaving buffer that is specified in section 7.2. It is
guaranteed that receivers can perform the de-interleaving of
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interleaved NAL units into NAL unit decoding order, when the
de-interleaving 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 de-interleaving 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 de-interleaving
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 de-interleaving 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 an RTP
packet 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 wait before starting decoding to recover the
NAL unit decoding order from the transmission order. The
parameter is the maximum value of (decoding time of the NAL
unit - transmission time of a NAL unit), assuming reliable
and instantaneous transmission, the same timeline for
transmission and decoding, and that decoding starts when the
first packet arrives.
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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 an RTP
packet 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,
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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.
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
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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.
sar-understood:
This parameter MAY be used to indicate a receiver capability
and not anything else. The parameter indicates the maximum
value of aspect_ratio_idc (specified in [1]) smaller than 255
that the receiver understands. Table E-1 of [1] specifies
aspect_ratio_idc equal to 0 as "unspecified", 1 to 16,
inclusive, as specific Sample Aspect Ratios (SARs), 17 to 254,
inclusive, as "reserved", and 255 as the Extended SAR, for
which SAR width and SAR height are explicitly signaled.
Therefore, a receiver with a decoder according to [1]
understands aspect_ratio_idc in the range of 1 to 16,
inclusive and aspect_ratio_idc equal to 255, in the sense
that the receiver knows what exactly the SAR is. For such a
receiver, the value of sar-understood is 16. If in the
future Table E-1 of [1] is extended, e.g., such that the SAR
for aspect_ratio_idc equal to 17 is specified, then for a
receiver with a decoder that understands the extension, the
value of sar-understood is 17. For a receiver with a decoder
according to the 2003 version of [1], the value of sar-
understood is 13, as the minimum reserved aspect_ratio_idc
therein is 14.
When sar-understood is not present, the value MUST be
inferred to be equal to 13.
sar-supported:
This parameter MAY be used to indicate a receiver capability
and not anything else. The value of this parameter is an
integer in the range of 1 to sar-understood, inclusive, equal
to 255. The value of sar-supported equal to N smaller than
255 indicates that the receiver supports all the SARs
corresponding to H.264 aspect_ratio_idc values (see Table E-1
of [1]) in the range from 1 to N, inclusive, without
geometric distortion. The value of sar-supported equal to
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255 indicates that the receiver supports all sample aspect
ratios which are expressible using two 16-bit integer values
as the numerator and denominator, i.e., those that are
expressible using the H.264 aspect_ratio_idc value of 255
(Extended_SAR, see Table E-1 of [1]), without geometric
distortion.
H.264 compliant encoders SHOULD NOT send an aspect_ratio_idc
equal to 0, or an aspect_ratio_idc larger than sar-understood
and smaller than 255. H.264 compliant encoders SHOULD send
an aspect_ratio_idc that the receiver is able to display
without geometrical distortion. However, H.264 compliant
encoders MAY choose to send pictures using any SAR.
Note that the actual sample aspect ratio or extended sample
aspect ratio, when present, of the stream is conveyed in the
Video Usability Information (VUI) part of the sequence
parameter set.
Encoding considerations:
This type is only defined for transfer via RTP (RFC 3550).
Security considerations:
See section 9 of RFC xxxx.
Public specification:
Please refer to RFC xxxx 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:
Ye-Kui Wang, yekuiwang@huawei.com
Intended usage: COMMON
Author:
Ye-Kui Wang, yekuiwang@huawei.com
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Change controller:
IETF Audio/Video Transport working group delegated from the
IESG.
8.2. SDP Parameters
The receiver MUST ignore any parameter unspecified in this memo.
8.2.1. Mapping of Payload Type Parameters to SDP
The media type video/H264 string is mapped to fields in the Session
Description Protocol (SDP) [6] 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 media subtype).
o The clock rate in the "a=rtpmap" line MUST be 90000.
o The OPTIONAL parameters "profile-level-id", "max-recv-level",
"max-mbps", "max-smbps", "max-fs", "max-cpb", "max-dpb", "max-
br", "redundant-pic-cap", "use-level-src-parameter-sets", "in-
band-parameter-sets", "level-asymmetry-allowed", "packetization-
mode", "sprop-interleaving-depth", "sprop-deint-buf-req",
"deint-buf-cap", "sprop-init-buf-time", "sprop-max-don-diff",
"max-rcmd-nalu-size", "sar-understood", and "sar-supported",
when present, MUST be included in the "a=fmtp" line of SDP.
These parameters are expressed as a media type string, in the
form of a semicolon separated list of parameter=value pairs.
o The OPTIONAL parameters "sprop-parameter-sets" and "sprop-level-
parameter-sets", when present, MUST be included in the "a=fmtp"
line of SDP or conveyed using the "fmtp" source attribute as
specified in section 6.3 of [9]. For a particular media format
(i.e., RTP payload type), a "sprop-parameter-sets" or "sprop-
level-parameter-sets" MUST NOT be both included in the "a=fmtp"
line of SDP and conveyed using the "fmtp" source attribute.
When included in the "a=fmtp" line of SDP, these parameters are
expressed as a media type string, in the form of a semicolon
separated list of parameter=value pairs. When conveyed using
the "fmtp" source attribute, these parameters are only
associated with the given source and payload type as parts of
the "fmtp" source attribute.
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Informative note: Conveyance of "sprop-parameter-sets" and
"sprop-level-parameter-sets" using the "fmtp" source
attribute allows for out-of-band transport of parameter sets
in topologies like Topo-Video-switch-MCU [29].
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;
packetization-mode=1;
sprop-parameter-sets=<parameter sets data>
8.2.2. Usage with the SDP Offer/Answer Model
When H.264 is offered over RTP using SDP in an Offer/Answer model
[8] 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" and "packetization-mode". These
media format configuration parameters (except for the level part
of "profile-level-id") 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. Note that the
level part of "profile-level-id" includes level_idc, and, for
indication of level 1b when profile_idc is equal to 66, 77 or 88,
bit 4 (constraint_set3_flag) of profile-iop. The level part of
"profile-level-id" is changeable.
Informative note: The requirement for symmetric use does not
apply for the level part of "profile-level-id", and does not
apply for the other stream properties and capability
parameters.
Informative note: In H.264 [1], all the levels except for
level 1b are equal to the value of level_idc divided by 10.
Level 1b is a level higher than level 1.0 but lower than
level 1.1, and is signaled in an ad-hoc manner, due to that
the level was specified after level 1.0 and level 1.1. For
the Baseline, Main and Extended profiles (with profile_idc
equal to 66, 77 and 88, respectively), level 1b is indicated
by level_idc equal to 11 (i.e. same as level 1.1) and
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constraint_set3_flag equal to 1. For other profiles, level
1b is indicated by level_idc equal to 9 (but note that level
1b for these profiles are still higher than level 1, which
has level_idc equal to 10, and lower than level 1.1). In SDP
Offer/Answer, an answer to an offer may indicate a level
equal to or lower than the level indicated in the offer. Due
to the ad-hoc indication of level 1b, offerers and answerers
must check the value of bit 4 (constraint_set3_flag) of the
middle octet of the parameter "profile-level-id", when
profile_idc is equal to 66, 77 or 88 and level_idc is equal
to 11.
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 [8]. An answer MUST NOT
contain a payload type number used in the offer unless the
configuration is exactly the same as in the offer.
Informative note: When an offerer receives an answer, it has
to compare payload types not declared in the offer based on
the media type (i.e., video/H264) and the above media
configuration parameters with any payload types it has
already declared. This will enable it to determine whether
the configuration in question is new or if it is equivalent
to configuration already offered, since a different payload
type number may be used in the answer.
o The parameter "max-recv-level", when present, declares the
highest level supported for receiving. In case "max-recv-level"
is not present, the highest level supported for receiving is
equal to the default level indicated by the level part of
"profile-level-id". "max-recv-level", when present, MUST be
higher than the default level.
o The parameter "level-asymmetry-allowed" indicates whether level
asymmetry is allowed.
If "level-asymmetry-allowed" is equal to 0 (or not present) in
either the offer or the answer, level asymmetry is not allowed.
In this case, the level to use in the direction from the offerer
to the answerer MUST be the same as the level to use in the
opposite direction, and the common level to use is equal to the
lower value of the default level in the offer and the default
level in the answer.
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Otherwise ("level-asymmetry-allowed" equals to 1 in both the
offer and the answer), level asymmetry is allowed. In this case,
the level to use in the offerer-to-answerer direction MUST be
equal to the highest level the answerer supports for receiving,
and the level to use in the answerer-to-offerer direction MUST
be equal to the highest level the offerer supports for receiving.
When level asymmetry is not allowed, level upgrade is not
allowed, i.e. the default level in the answer MUST be equal to
or lower than the default level in the offer.
o The parameters "sprop-deint-buf-req", "sprop-interleaving-depth",
"sprop-max-don-diff", and "sprop-init-buf-time" describe the
properties of the RTP packet stream that the offerer or answerer
is sending for the media format configuration. This differs
from the normal usage of the Offer/Answer parameters: normally
such parameters declare the properties of the stream that the
offerer or the answerer is able to receive. When dealing with
H.264, the offerer assumes that the answerer will be able to
receive media encoded using the configuration being offered.
Informative note: The above parameters apply for any stream
sent by the declaring entity with the same configuration;
i.e., they are dependent on their source. Rather than being
bound to the payload type, the values may have to be applied
to another payload type when being sent, as they apply for
the configuration.
o The capability parameters "max-mbps", "max-smbps", "max-fs",
"max-cpb", "max-dpb", "max-br", ,"redundant-pic-cap", "max-rcmd-
nalu-size", "sar-understood", "sar-supported" MAY be used to
declare further capabilities of the offerer or answerer for
receiving. These parameters MUST NOT be present when the
direction attribute is sendonly, and the parameters describe the
limitations of what the offerer or answerer accepts for
receiving streams.
o An offerer has to include the size of the de-interleaving buffer,
"sprop-deint-buf-req", in the offer for an interleaved H.264
stream. To enable the offerer and answerer to inform each other
about their capabilities for de-interleaving buffering in
receiving streams, both parties are RECOMMENDED to include
"deint-buf-cap". For interleaved streams, it is also
RECOMMENDED to consider offering multiple payload types with
different buffering requirements when the capabilities of the
receiver are unknown.
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o The "sprop-parameter-sets" or "sprop-level-parameter-sets"
parameter, when present (included in the "a=fmtp" line of SDP or
conveyed using the "fmtp" source attribute as specified in
section 6.3 of [9]), is used for out-of-band transport of
parameter sets. However, when out-of-band transport of
parameter sets is used, parameter sets MAY still be additionally
transported in-band.
The answerer MAY use either out-of-band or in-band transport of
parameter sets for the stream it is sending, regardless of
whether out-of-band parameter sets transport has been used in
the offerer-to-answerer direction. Parameter sets included in
an answer are independent of those parameter sets included in
the offer, as they are used for decoding two different video
streams, one from the answerer to the offerer, and the other in
the opposite direction.
The following rules apply to transport of parameter sets in the
offerer-to-answerer direction.
o An offer MAY include either or both of "sprop-parameter-
sets" and "sprop-level-parameter-sets". If neither "sprop-
parameter-sets" nor "sprop-level-parameter-sets" is present
in the offer, then only in-band transport of parameter sets
is used.
o If the answer includes "in-band-parameter-sets" equal to 1,
then the offerer MUST transmit parameter sets in-band.
Otherwise, the following applies.
o If the level to use in the offerer-to-answerer
direction is equal to the default level in the offer,
the following applies.
When there is a "sprop-parameter-sets" included
in the "a=fmtp" line in the offer, the answerer
MUST be prepared to use the parameter sets
included in the "sprop-parameter-sets" for
decoding the incoming NAL unit stream.
When there is a "sprop-parameter-sets" conveyed
using the "fmtp" source attribute in the offer,
the following applies. If the answer includes
"use-level-src-parameter-sets" equal to 1 or the
"fmtp" source attribute, the answerer MUST be
prepared to use the parameter sets included in
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the "sprop-parameter-sets" for decoding the
incoming NAL unit stream; Otherwise, the offerer
MUST transmit parameter sets in-band.
When "sprop-parameter-sets" is not present in the
offer, the offerer MUST transmit parameter sets
in-band.
The answerer MUST ignore "sprop-level-parameter-
sets", when present (either included in the
"a=fmtp" line or conveyed using the "fmtp" source
attribute) in the offer.
o Otherwise (the level to use in the offerer-to-answerer
direction is not equal to the default level in the
offer), the following applies.
The answerer MUST ignore "sprop-parameter-sets",
when present (either included in the "a=fmtp"
line or conveyed using the "fmtp" source
attribute) in the offer.
When neither "use-level-src-parameter-sets" equal
to 1 nor the "fmtp" source attribute is present
in the answer, the answerer MUST ignore "sprop-
level-parameter-sets", when present in the offer,
and the offerer MUST transmit parameter sets in-
band.
When either "use-level-src-parameter-sets" equal
to 1 or the "fmtp" source attribute is present in
the answer, the answerer MUST be prepared to use
the parameter sets that are included in "sprop-
level-parameter-sets" for the accepted level (i.e.
the default level in the answer), when present in
the offer, for decoding the incoming NAL unit
stream, and ignore all other parameter sets
included in "sprop-level-parameter-sets".
When no parameter sets for the level to use in
the offerer-to-answerer direction are present in
"sprop-level-parameter-sets" in the offer, the
offerer MUST transmit parameter sets in-band.
The following rules apply to transport of parameter sets in the
answerer-to-offerer direction.
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o An answer MAY include either "sprop-parameter-sets" or
"sprop-level-parameter-sets", but MUST NOT include both of
the two. If neither "sprop-parameter-sets" nor "sprop-
level-parameter-sets" is present in the answer, then only
in-band transport of parameter sets is used.
o If the offer includes "in-band-parameter-sets" equal to 1,
the answerer MUST NOT include "sprop-parameter-sets" or
"sprop-level-parameter-sets" in the answer and MUST
transmit parameter sets in-band. Otherwise, the following
applies.
o If the level to use in the answerer-to-offerer
direction is equal to the default level in the answer,
the following applies.
When there is a "sprop-parameter-sets" included
in the "a=fmtp" line in the answer, the offerer
MUST be prepared to use the parameter sets
included in the "sprop-parameter-sets" for
decoding the incoming NAL unit stream.
When there is a "sprop-parameter-sets" conveyed
using the "fmtp" source attribute in the answer,
the following applies. If the offer includes
"use-level-src-parameter-sets" equal to 1 or the
"fmtp" source attribute, the offerer MUST be
prepared to use the parameter sets included in
the "sprop-parameter-sets" for decoding the
incoming NAL unit stream; Otherwise, the
answerer MUST transmit parameter sets in-band.
When "sprop-parameter-sets" is not present in the
answer, the answerer MUST transmit parameter sets
in-band.
The offerer MUST ignore "sprop-level-parameter-
sets", when present (either included in the
"a=fmtp" line or conveyed using the "fmtp" source
attribute) in the answer.
o Otherwise (the level to use in the answerer-to-offerer
direction is not equal to the default level in the
answer), the following applies.
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The offerer MUST ignore "sprop-parameter-sets",
when present (either included in the "a=fmtp"
line of SDP or conveyed using the "fmtp" source
attribute) in the answer.
When neither "use-level-src-parameter-sets" equal
to 1 nor the "fmtp" source attribute is present
in the offer, the offerer MUST ignore "sprop-
level-parameter-sets", when present, and the
answerer MUST transmit parameter sets in-band.
When either "use-level-src-parameter-sets" equal
to 1 or the "fmtp" source attribute is present in
the offer, the offerer MUST be prepared to use
the parameter sets that are included in "sprop-
level-parameter-sets" for the level to use in the
answerer-to-offerer direction, when present in
the answer, for decoding the incoming NAL unit
stream, and ignore all other parameter sets
included in "sprop-level-parameter-sets" in the
answer.
When no parameter sets for the level to use in
the answerer-to-offerer direction are present in
"sprop-level-parameter-sets" in the answer, the
answerer MUST transmit parameter sets in-band.
When "sprop-parameter-sets" or "sprop-level-parameter-sets" is
conveyed using the "fmtp" source attribute as specified in
section 6.3 of [9], the receiver of the parameters MUST store
the parameter sets included in the "sprop-parameter-sets" or
"sprop-level-parameter-sets" for the accepted level and
associate them to the source given as a part of the "fmtp"
source attribute. Parameter sets associated with one source
MUST only be used to decode NAL units conveyed in RTP packets
from the same source. When this mechanism is in use, SSRC
collision detection and resolution MUST be performed as
specified in [9].
Informative note: Conveyance of "sprop-parameter-sets" and
"sprop-level-parameter-sets" using the "fmtp" source
attribute may be used in topologies like Topo-Video-switch-
MCU [29] to enable out-of-band transport of parameter sets.
For streams being delivered over multicast, the following rules
apply:
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o The media format configuration is identified by "profile-level-
id", including the level part, and "packetization-mode". These
media format configuration parameters (including the level part
of "profile-level-id") MUST be used symmetrically; i.e., the
answerer MUST either maintain all configuration parameters or
remove the media format (payload type) completely. Note that
this implies that the level part of "profile-level-id" for
Offer/Answer in multicast is not changeable.
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 [8]. An answer MUST NOT
contain a payload type number used in the offer unless the
configuration is the same as in the offer.
o Parameter sets received MUST be associated with the originating
source, and MUST be only used in decoding the incoming NAL unit
stream from the same source.
o The rules for other parameters are the same as above for unicast
as long as the above rules are obeyed.
Table 6 lists the interpretation of all the media type parameters
that MUST be used for the different direction attributes.
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Table 6. Interpretation of parameters for different direction
attributes.
sendonly --+
recvonly --+ |
sendrecv --+ | |
| | |
profile-level-id C C P
max-recv-level R R -
packetization-mode C C P
sprop-deint-buf-req P - P
sprop-interleaving-depth P - P
sprop-max-don-diff P - P
sprop-init-buf-time P - P
max-mbps R R -
max-smbps R R -
max-fs R R -
max-cpb R R -
max-dpb R R -
max-br R R -
redundant-pic-cap R R -
deint-buf-cap R R -
max-rcmd-nalu-size R R -
sar-understood R R -
sar-supported R R -
in-band-parameter-sets R R -
use-level-src-parameter-sets R R -
level-asymmetry-allowed O - -
sprop-parameter-sets S - S
sprop-level-parameter-sets S - S
Legend:
C: configuration for sending and receiving streams
O: offer/answer mode
P: properties of the stream to be sent
R: receiver capabilities
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S: out-of-band parameter sets
-: not usable, when present SHOULD be ignored
Parameters used for declaring receiver capabilities are in general
downgradable; i.e., they express the upper limit for a sender's
possible behavior. Thus a sender MAY select to set its encoder
using only lower/less or equal values of these parameters.
Parameters declaring a configuration point are not changeable, with
the exception of the level part of the "profile-level-id" parameter
for unicast usage. These express values a receiver expects to be
used and must be used verbatim on the sender side.
When a sender's capabilities are declared, and non-downgradable
parameters are used in this declaration, then these parameters
express a configuration that is acceptable for the sender to
receive streams. In order to achieve high interoperability levels,
it is often advisable to offer multiple alternative configurations;
e.g., for the packetization mode. It is impossible to offer
multiple configurations in a single payload type. Thus, when
multiple configuration offers are made, each offer requires its own
RTP payload type associated with the offer.
A receiver SHOULD understand all media type parameters, even if it
only supports a subset of the payload format's functionality. This
ensures that a receiver is capable of understanding when an offer
to receive media can be downgraded to what is supported by the
receiver of the offer.
An answerer MAY extend the offer with additional media format
configurations. However, to enable their usage, in most cases a
second offer is required from the offerer to provide the stream
property parameters that the media sender will use. This also has
the effect that the offerer has to be able to receive this media
format configuration, not only to send it.
If an offerer wishes to have non-symmetric capabilities between
sending and receiving, the offerer can allow asymmetric levels via
"level-asymmetry-allowed" equal to 1. Alternatively, the offerer
could offer different RTP sessions; i.e., different media lines
declared as "recvonly" and "sendonly", respectively. This may have
further implications on the system, and may require additional
external semantics to associate the two media lines.
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8.2.3. Usage in Declarative Session Descriptions
When H.264 over RTP is offered with SDP in a declarative style, as
in RTSP [27] or SAP [28], the following considerations are
necessary.
o All parameters capable of indicating both stream properties and
receiver capabilities are used to indicate only stream
properties. For example, in this case, the parameter "profile-
level-id" declares only the values used by the stream, not the
capabilities for receiving streams. This results in that the
following interpretation of the parameters MUST be used:
Declaring actual configuration or stream properties:
- profile-level-id
- packetization-mode
- sprop-interleaving-depth
- sprop-deint-buf-req
- sprop-max-don-diff
- sprop-init-buf-time
Out-of-band transporting of parameter sets:
- sprop-parameter-sets
- sprop-level-parameter-sets
Not usable(when present, they SHOULD be ignored):
- max-mbps
- max-smbps
- max-fs
- max-cpb
- max-dpb
- max-br
- max-recv-level
- redundant-pic-cap
- max-rcmd-nalu-size
- deint-buf-cap
- sar-understood
- sar-supported
- in-band-parameter-sets
- level-asymmetry-allowed
- use-level-src-parameter-sets
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o A receiver of the SDP is required to support all parameters and
values of the parameters provided; otherwise, the receiver MUST
reject (RTSP) or not participate in (SAP) the session. It falls
on the creator of the session to use values that are expected to
be supported by the receiving application.
8.3. Examples
An SDP Offer/Answer exchange wherein both parties are expected to
both send and receive could look like the following. Only the
media codec specific parts of the SDP are shown. Some lines are
wrapped due to text constraints.
Offerer -> Answerer SDP message:
m=video 49170 RTP/AVP 100 99 98
a=rtpmap:98 H264/90000
a=fmtp:98 profile-level-id=42A01E; packetization-mode=0;
sprop-parameter-sets=<parameter sets data#0>
a=rtpmap:99 H264/90000
a=fmtp:99 profile-level-id=42A01E; packetization-mode=1;
sprop-parameter-sets=<parameter sets data#1>
a=rtpmap:100 H264/90000
a=fmtp:100 profile-level-id=42A01E; packetization-mode=2;
sprop-parameter-sets=<parameter sets data#2>;
sprop-interleaving-depth=45; sprop-deint-buf-req=64000;
sprop-init-buf-time=102478; deint-buf-cap=128000
The above offer presents the same codec configuration in three
different packetization formats. PT 98 represents single NALU mode,
PT 99 represents non-interleaved mode, and PT 100 indicates the
interleaved mode. In the interleaved mode case, the interleaving
parameters that the offerer would use if the answer indicates
support for PT 100 are also included. In all three cases the
parameter "sprop-parameter-sets" conveys the initial parameter sets
that are required by the answerer when receiving a stream from the
offerer when this configuration is accepted. Note that the value
for "sprop-parameter-sets" could be different for each payload type.
Answerer -> Offerer SDP message:
m=video 49170 RTP/AVP 100 99 97
a=rtpmap:97 H264/90000
a=fmtp:97 profile-level-id=42A01E; packetization-mode=0;
sprop-parameter-sets=<parameter sets data#3>
a=rtpmap:99 H264/90000
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a=fmtp:99 profile-level-id=42A01E; packetization-mode=1;
sprop-parameter-sets=<parameter sets data#4>;
max-rcmd-nalu-size=3980
a=rtpmap:100 H264/90000
a=fmtp:100 profile-level-id=42A01E; packetization-mode=2;
sprop-parameter-sets=<parameter sets data#5>;
sprop-interleaving-depth=60;
sprop-deint-buf-req=86000; sprop-init-buf-time=156320;
deint-buf-cap=128000; max-rcmd-nalu-size=3980
As the Offer/Answer negotiation covers both sending and receiving
streams, an offer indicates the exact parameters for what the
offerer is willing to receive, whereas the answer indicates the
same for what the answerer accepts to receive. In this case the
offerer declared that it is willing to receive payload type 98.
The answerer accepts this by declaring an equivalent payload type
97; i.e., it has identical values for the two parameters "profile-
level-id" and "packetization-mode" (since "packetization-mode" is
equal to 0, "sprop-deint-buf-req" is not present). As the offered
payload type 98 is accepted, the answerer needs to store parameter
sets included in sprop-parameter-sets=<parameter sets data#0> in
case the offer finally decides to use this configuration. In the
answer, the answerer includes the parameter sets in sprop-
parameter-sets=<parameter sets data#3> that the answerer would use
in the stream sent from the answerer if this configuration is
finally used.
The answerer also accepts the reception of the two configurations
that payload types 99 and 100 represent. Again, the answerer needs
to store parameter sets included in sprop-parameter-sets=<parameter
sets data#1> and sprop-parameter-sets=<parameter sets data#2> in
case the offer finally decides to use either of these two
configurations. The answerer provides the initial parameter sets
for the answerer-to-offerer direction, i.e. the parameter sets in
sprop-parameter-sets=<parameter sets data#4> and sprop-parameter-
sets=<parameter sets data#5>, for payload types 99 and 100,
respectively, that it will use to send the payload types. The
answerer also provides the offerer with its memory limit for de-
interleaving operations by providing a "deint-buf-cap" parameter.
This is only useful if the offerer decides on making a second offer,
where it can take the new value into account. The "max-rcmd-nalu-
size" indicates that the answerer can efficiently process NALUs up
to the size of 3980 bytes. However, there is no guarantee that the
network supports this size.
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In the following example, the offer is accepted without level
downgrading (i.e. the default level, 3.0, is accepted), and both
"sprop-parameter-sets" and "sprop-level-parameter-sets" are present
in the offer. The answerer must ignore sprop-level-parameter-
sets=<parameter sets data#1> and store parameter sets in sprop-
parameter-sets=<parameter sets data#0> for decoding the incoming
NAL unit stream. The offerer must store the parameter sets in
sprop-parameter-sets=<parameter sets data#2> in the answer for
decoding the incoming NAL unit stream. Note that in this example,
parameter sets in sprop-parameter-sets=<parameter sets data#2> must
be associated with level 3.0.
Offer SDP:
m=video 49170 RTP/AVP 98
a=rtpmap:98 H264/90000
a=fmtp:98 profile-level-id=42A01E; //Baseline profile, Level 3.0
packetization-mode=1;
sprop-parameter-sets=<parameter sets data#0>;
sprop-level-parameter-sets=<parameter sets data#1>
Answer SDP:
m=video 49170 RTP/AVP 98
a=rtpmap:98 H264/90000
a=fmtp:98 profile-level-id=42A01E; //Baseline profile, Level 3.0
packetization-mode=1;
sprop-parameter-sets=<parameter sets data#2>
In the following example, the offer (Baseline profile, level 1.1)
is accepted with level downgrading (the accepted level is 1b), and
both "sprop-parameter-sets" and "sprop-level-parameter-sets" are
present in the offer. The answerer must ignore sprop-parameter-
sets=<parameter sets data#0> and all parameter sets not for the
accepted level (level 1b) in sprop-level-parameter-sets=<parameter
sets data#1>, and must store parameter sets for the accepted level
(level 1b) in sprop-level-parameter-sets=<parameter sets data#1>
for decoding the incoming NAL unit stream. The offerer must store
the parameter sets in sprop-parameter-sets=<parameter sets data#2>
in the answer for decoding the incoming NAL unit stream. Note that
in this example, parameter sets in sprop-parameter-sets=<parameter
sets data#2> must be associated with level 1b.
Offer SDP:
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m=video 49170 RTP/AVP 98
a=rtpmap:98 H264/90000
a=fmtp:98 profile-level-id=42A00B; //Baseline profile, Level 1.1
packetization-mode=1;
sprop-parameter-sets=<parameter sets data#0>;
sprop-level-parameter-sets=<parameter sets data#1>
Answer SDP:
m=video 49170 RTP/AVP 98
a=rtpmap:98 H264/90000
a=fmtp:98 profile-level-id=42B00B; //Baseline profile, Level 1b
packetization-mode=1;
sprop-parameter-sets=<parameter sets data#2>;
use-level-src-parameter-sets=1
In the following example, the offer (Baseline profile, level 1.1)
is accepted with level downgrading (the accepted level is 1b), and
both "sprop-parameter-sets" and "sprop-level-parameter-sets" are
present in the offer. However, the answerer is a legacy RFC 3984
implementation and does not understand "sprop-level-parameter-sets",
hence it does not include "use-level-src-parameter-sets" (which the
answerer does not understand, either) in the answer. Therefore,
the answerer must ignore both sprop-parameter-sets=<parameter sets
data#0> and sprop-level-parameter-sets=<parameter sets data#1>, and
the offerer must transport parameter sets in-band.
Offer SDP:
m=video 49170 RTP/AVP 98
a=rtpmap:98 H264/90000
a=fmtp:98 profile-level-id=42A00B; //Baseline profile, Level 1.1
packetization-mode=1;
sprop-parameter-sets=<parameter sets data#0>;
sprop-level-parameter-sets=<parameter sets data#1>
Answer SDP:
m=video 49170 RTP/AVP 98
a=rtpmap:98 H264/90000
a=fmtp:98 profile-level-id=42B00B; //Baseline profile, Level 1b
packetization-mode=1
In the following example, the offer is accepted without level
downgrading, and "sprop-parameter-sets" is present in the offer.
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Parameter sets in sprop-parameter-sets=<parameter sets data#0> must
be stored and used used by the encoder of the offerer and the
decoder of the answerer, and parameter sets in sprop-parameter-
sets=<parameter sets data#1>must be used by the encoder of the
answerer and the decoder of the offerer. Note that sprop-
parameter-sets=<parameter sets data#0> is basically independent of
sprop-parameter-sets=<parameter sets data#1>.
Offer SDP:
m=video 49170 RTP/AVP 98
a=rtpmap:98 H264/90000
a=fmtp:98 profile-level-id=42A01E; //Baseline profile, Level 3.0
packetization-mode=1;
sprop-parameter-sets=<parameter sets data#0>
Answer SDP:
m=video 49170 RTP/AVP 98
a=rtpmap:98 H264/90000
a=fmtp:98 profile-level-id=42A01E; //Baseline profile, Level 3.0
packetization-mode=1;
sprop-parameter-sets=<parameter sets data#1>
In the following example, the offer is accepted without level
downgrading, and neither "sprop-parameter-sets" nor "sprop-level-
parameter-sets" is present in the offer, meaning that there is no
out-of-band transmission of parameter sets, which then have to be
transported in-band.
Offer SDP:
m=video 49170 RTP/AVP 98
a=rtpmap:98 H264/90000
a=fmtp:98 profile-level-id=42A01E; //Baseline profile, Level 3.0
packetization-mode=1
Answer SDP:
m=video 49170 RTP/AVP 98
a=rtpmap:98 H264/90000
a=fmtp:98 profile-level-id=42A01E; //Baseline profile, Level 3.0
packetization-mode=1
In the following example, the offer is accepted with level
downgrading and "sprop-parameter-sets" is present in the offer. As
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sprop-parameter-sets=<parameter sets data#0> contains level_idc
indicating Level 3.0, therefore cannot be used as the answerer
wants Level 2.0 and must be ignored by the answerer, and in-band
parameter sets must be used.
Offer SDP:
m=video 49170 RTP/AVP 98
a=rtpmap:98 H264/90000
a=fmtp:98 profile-level-id=42A01E; //Baseline profile, Level 3.0
packetization-mode=1;
sprop-parameter-sets=<parameter sets data#0>
Answer SDP:
m=video 49170 RTP/AVP 98
a=rtpmap:98 H264/90000
a=fmtp:98 profile-level-id=42A014; //Baseline profile, Level 2.0
packetization-mode=1
In the following example, the offer is also accepted with level
downgrading, and neither "sprop-parameter-sets" nor "sprop-level-
parameter-sets" is present in the offer, meaning that there is no
out-of-band transmission of parameter sets, which then have to be
transported in-band.
Offer SDP:
m=video 49170 RTP/AVP 98
a=rtpmap:98 H264/90000
a=fmtp:98 profile-level-id=42A01E; //Baseline profile, Level 3.0
packetization-mode=1
Answer SDP:
m=video 49170 RTP/AVP 98
a=rtpmap:98 H264/90000
a=fmtp:98 profile-level-id=42A014; //Baseline profile, Level 2.0
packetization-mode=1
In the following example, the offer is accepted with level
upgrading, and neither "sprop-parameter-sets" nor "sprop-level-
parameter-sets" is present in the offer or the answer, meaning that
there is no out-of-band transmission of parameter sets, which then
have to be transported in-band. The level to use in the offerer-
to-answerer direction is Level 3.0, and the level to use in the
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answerer-to-offerer direction is Level 2.0. The answerer is
allowed to send at any level up to and including level 2.0, and the
offerer is allowed to send at any level up to and including level
3.0.
Offer SDP:
m=video 49170 RTP/AVP 98
a=rtpmap:98 H264/90000
a=fmtp:98 profile-level-id=42A014; //Baseline profile, Level 2.0
packetization-mode=1; level-asymmetry-allowed=1
Answer SDP:
m=video 49170 RTP/AVP 98
a=rtpmap:98 H264/90000
a=fmtp:98 profile-level-id=42A01E; //Baseline profile, Level 3.0
packetization-mode=1; level-asymmetry-allowed=1
In the following example, the offerer is a Multipoint Control Unit
(MCU) in a Topo-Video-switch-MCU like topology [29], offering
parameter sets received (using out-of-band transport) from three
other participants B, C, and D, and receiving parameter sets from
the participant A, which is the answerer. The participants are
identified by their values of CNAME, which are mapped to different
SSRC values. The same codec configuration is used by all the four
participants. The participant A stores and associates the
parameter sets included in <parameter sets data#B>, <parameter sets
data#C>, and <parameter sets data#D> to participants B, C, and D,
respectively, and uses <parameter sets data#B> for decoding NAL
units carried in RTP packets originated from participant B only,
uses <parameter sets data#C> for decoding NAL units carried in RTP
packets originated from participant C only, and uses <parameter
sets data#D> for decoding NAL units carried in RTP packets
originated from participant D only.
Offer SDP:
m=video 49170 RTP/AVP 98
a=ssrc:SSRC-B cname:CNAME-B
a=ssrc:SSRC-C cname:CNAME-C
a=ssrc:SSRC-D cname:CNAME-D
a=ssrc:SSRC-B fmtp:98
sprop-parameter-sets=<parameter sets data#B>
a=ssrc:SSRC-C fmtp:98
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sprop-parameter-sets=<parameter sets data#C>
a=ssrc:SSRC-D fmtp:98
sprop-parameter-sets=<parameter sets data#D>
a=rtpmap:98 H264/90000
a=fmtp:98 profile-level-id=42A01E; //Baseline profile, Level 3.0
packetization-mode=1
Answer SDP:
m=video 49170 RTP/AVP 98
a=ssrc:SSRC-A cname:CNAME-A
a=ssrc:SSRC-A fmtp:98
sprop-parameter-sets=<parameter sets data#A>
a=rtpmap:98 H264/90000
a=fmtp:98 profile-level-id=42A01E; //Baseline profile, Level 3.0
packetization-mode=1
8.4. Parameter Set Considerations
The H.264 parameter sets are a fundamental part of the video codec
and vital to its operation; see section 1.2. Due to their
characteristics and their importance for the decoding process, lost
or erroneously transmitted parameter sets can hardly be concealed
locally at the receiver. A reference to a corrupt parameter set
has normally fatal results to the decoding process. Corruption
could occur, for example, due to the erroneous transmission or loss
of a parameter set NAL unit, but also due to the untimely
transmission of a parameter set update. A parameter set update
refers to a change of at least one parameter in a picture parameter
set or sequence parameter set for which the picture parameter set
or sequence parameter set identifier remains unchanged. Therefore,
the following recommendations are provided as a guideline for the
implementer of the RTP sender.
Parameter set NALUs can be transported using three different
principles:
A. Using a session control protocol (out-of-band) prior to the
actual RTP session.
B. Using a session control protocol (out-of-band) during an ongoing
RTP session.
C. Within the RTP packet stream in the payload (in-band) during an
ongoing RTP session.
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It is recommended to implement principles A and B within a session
control protocol. SIP and SDP can be used as described in the SDP
Offer/Answer model and in the previous sections of this memo.
Section 8.2.2 includes a detailed discussion on transport of
parameter sets in-band or out-of-band in SDP Offer/Answer using
media type parameters "sprop-parameter-sets", "sprop-level-
parameter-sets", "use-level-src-parameter-sets" and "in-band-
parameter-sets". This section contains guidelines on how
principles A and B should be implemented within session control
protocols. It is independent of the particular protocol used.
Principle C is supported by the RTP payload format defined in this
specification. There are topologies like Topo-Video-switch-MCU [29]
for which the use of principle C may be desirable.
If in-band signaling of parameter sets is used, the picture and
sequence parameter set NALUs SHOULD be transmitted in the RTP
payload using a reliable method of delivering of RTP (see below),
as a loss of a parameter set of either type will likely prevent
decoding of a considerable portion of the corresponding RTP packet
stream.
If in-band signaling of parameter sets is used, the sender SHOULD
take the error characteristics into account and use mechanisms to
provide a high probability for delivering the parameter sets
correctly. Mechanisms that increase the probability for a correct
reception include packet repetition, FEC, and retransmission. The
use of an unreliable, out-of-band control protocol has similar
disadvantages as the in-band signaling (possible loss) and, in
addition, may also lead to difficulties in the synchronization (see
below). Therefore, it is NOT RECOMMENDED.
Parameter sets MAY be added or updated during the lifetime of a
session using principles B and C. It is required that parameter
sets are present at the decoder prior to the NAL units that refer
to them. Updating or adding of parameter sets can result in
further problems, and therefore the following recommendations
should be considered.
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- When parameter sets are added or updated, care SHOULD be taken
to ensure that any parameter set is delivered prior to its usage.
When new parameter sets are added, previously unused parameter
set identifiers are used. It is common that no synchronization
is present between out-of-band signaling and in-band traffic.
If out-of-band signaling is used, it is RECOMMENDED that a
sender does not start sending NALUs requiring the added or
updated parameter sets prior to acknowledgement of delivery from
the signaling protocol.
- When parameter sets are updated, the following synchronization
issue should be taken into account. When overwriting a
parameter set at the receiver, the sender has to ensure that the
parameter set in question is not needed by any NALU present in
the network or receiver buffers. Otherwise, decoding with a
wrong parameter set may occur. To lessen this problem, it is
RECOMMENDED either to overwrite only those parameter sets that
have not been used for a sufficiently long time (to ensure that
all related NALUs have been consumed), or to add a new parameter
set instead (which may have negative consequences for the
efficiency of the video coding).
Informative note: In some topologies like Topo-Video-switch-
MCU [29] the origin of the whole set of parameter sets may
come from multiple sources that may use non-unique parameter
sets identifiers. In this case an offer may overwrite an
existing parameter set if no other mechanism that enables
uniqueness of the parameter sets in the out-of-band channel
exists.
- In a multiparty session, one participant MUST associate
parameter sets coming from different sources with the source
identification whenever possible, e.g. by conveying out-of-band
transported parameter sets, as different sources typically use
independent parameter set identifier value spaces.
- Adding or modifying parameter sets by using both principles B
and C in the same RTP session may lead to inconsistencies of the
parameter sets because of the lack of synchronization between
the control and the RTP channel. Therefore, principles B and C
MUST NOT both be used in the same session unless sufficient
synchronization can be provided.
In some scenarios (e.g., when only the subset of this payload
format specification corresponding to H.241 is used) or topologies,
it is not possible to employ out-of-band parameter set transmission.
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In this case, parameter sets have to be transmitted in-band. Here,
the synchronization with the non-parameter-set-data in the
bitstream is implicit, but the possibility of a loss has to be
taken into account. The loss probability should be reduced using
the mechanisms discussed above. In case a loss of a parameter set
is detected, recovery may be achieved by using a Decoder Refresh
Point procedure, for example, using RTCP feedback Full Intra
Request (FIR) [30]. Two example Decoder Refresh Point procedures
are provided in the informative Section 8.5.
- When parameter sets are initially provided using principle A and
then later added or updated in-band (principle C), there is a
risk associated with updating the parameter sets delivered out-
of-band. If receivers miss some in-band updates (for example,
because of a loss or a late tune-in), those receivers attempt to
decode the bitstream using out-dated parameters. It is
therefore RECOMMENDED that parameter set IDs be partitioned
between the out-of-band and in-band parameter sets.
8.5. Decoder Refresh Point Procedure using In-Band Transport of
Parameter Sets (Informative)
When a sender with a video encoder according to [1] receives a
request for a decoder refresh point, the encoder shall enter the
fast update mode by using one of the procedures specified
in Section 8.5.1 or 8.5.2 below. The procedure in 8.5.1 is the
preferred response in a lossless transmission environment. Both
procedures satisfy the requirement to enter the fast update mode
for H.264 video encoding.
8.5.1. IDR Procedure to Respond to a Request for a Decoder Refresh
Point
This section gives one possible way to respond to a request for a
decoder refresh point.
The encoder shall, in the order presented here:
1) Immediately prepare to send an IDR picture.
2) Send a sequence parameter set to be used by the IDR picture to
be sent. The encoder may optionally also send other sequence
parameter sets.
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3) Send a picture parameter set to be used by the IDR picture to be
sent. The encoder may optionally also send other picture
parameter sets.
4) Send the IDR picture.
5) From this point forward in time, send any other sequence or
picture parameter sets that have not yet been sent in this
procedure, prior to their reference by any NAL unit, regardless
of whether such parameter sets were previously sent prior to
receiving the request for a decoder refresh point. As needed,
such parameter sets may be sent in a batch, one at a time, or in
any combination of these two methods. Parameter sets may be re-
sent at any time for redundancy. Caution should be taken when
parameter set updates are present, as described above in Section
8.4.
8.5.2. Gradual Recovery Procedure to Respond to a Request for a
Decoder Refresh Point
This section gives another possible way to respond to a request for
a decoder refresh point.
The encoder shall, in the order presented here:
1) Send a recovery point SEI message (see Sections D.1.7 and D.2.7
of [1]).
2) Repeat any sequence and picture parameter sets that were sent
before the recovery point SEI message, prior to their reference
by a NAL unit.
The encoder shall ensure that the decoder has access to all
reference pictures for inter prediction of pictures at or after the
recovery point, which is indicated by the recovery point SEI
message, in output order, assuming that the transmission from now
on is error-free.
The value of the recovery_frame_cnt syntax element in the recovery
point SEI message should be small enough to ensure a fast recovery.
As needed, such parameter sets may be re-sent in a batch, one at a
time, or in any combination of these two methods. Parameter sets
may be re-sent at any time for redundancy. Caution should be taken
when parameter set updates are present, as described above in
Section 8.4.
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9. Security Considerations
RTP packets using the payload format defined in this specification
are subject to the security considerations discussed in the RTP
specification [5], and in any appropriate RTP profile (for example,
[16]). This implies that confidentiality of the media streams is
achieved by encryption; for example, through the application of
SRTP [26]. Because the data compression used with this payload
format is applied end-to-end, any encryption needs to be performed
after compression. A potential denial-of-service threat exists for
data encodings using compression techniques that have non-uniform
receiver-end computational load. The attacker can inject
pathological datagrams into the stream that are complex to decode
and that cause the receiver to be overloaded. H.264 is
particularly vulnerable to such attacks, as it is extremely simple
to generate datagrams containing NAL units that affect the decoding
process of many future NAL units. Therefore, the usage of data
origin authentication and data integrity protection of at least the
RTP packet is RECOMMENDED; for example, with SRTP [26].
Note that the appropriate mechanism to ensure confidentiality and
integrity of RTP packets and their payloads is very dependent on
the application and on the transport and signaling protocols
employed. Thus, although SRTP is given as an example above, other
possible choices exist.
Decoders MUST exercise caution with respect to the handling of user
data SEI messages, particularly if they contain active elements,
and MUST restrict their domain of applicability to the presentation
containing the stream.
End-to-End security with either authentication, integrity or
confidentiality protection will prevent a MANE from performing
media-aware operations other than discarding complete packets. And
in the case of confidentiality protection it will even be prevented
from performing discarding of packets in a media aware way. To
allow any MANE to perform its operations, it will be required to be
a trusted entity which is included in the security context
establishment.
10. Congestion Control
Congestion control for RTP SHALL be used in accordance with RFC
3550 [5], and with any applicable RTP profile; e.g., RFC 3551 [16].
An additional requirement if best-effort service is being used is:
users of this payload format MUST monitor packet loss to ensure
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that the packet loss rate is within acceptable parameters. Packet
loss is considered acceptable if a TCP flow across the same network
path, and experiencing the same network conditions, would achieve
an average throughput, measured on a reasonable timescale, that is
not less than the RTP flow is achieving. This condition can be
satisfied by implementing congestion control mechanisms to adapt
the transmission rate (or the number of layers subscribed for a
layered multicast session), or by arranging for a receiver to leave
the session if the loss rate is unacceptably high.
The bit rate adaptation necessary for obeying the congestion
control principle is easily achievable when real-time encoding is
used. However, when pre-encoded content is being transmitted,
bandwidth adaptation requires the availability of more than one
coded representation of the same content, at different bit rates,
or the existence of non-reference pictures or sub-sequences [22] in
the bitstream. The switching between the different representations
can normally be performed in the same RTP session; e.g., by
employing a concept known as SI/SP slices of the Extended Profile,
or by switching streams at IDR picture boundaries. Only when non-
downgradable parameters (such as the profile part of the
profile/level ID) are required to be changed does it become
necessary to terminate and re-start the media stream. This may be
accomplished by using a different RTP payload type.
MANEs MAY follow the suggestions outlined in section 7.3 and remove
certain unusable packets from the packet stream when that stream
was damaged due to previous packet losses. This can help reduce
the network load in certain special cases.
11. IANA Consideration
The H264 media subtype name specified by RFC 3984 should be updated
as defined in section 8.1 of this memo.
12. Informative Appendix: Application Examples
This payload specification is very flexible in its use, in order to
cover the extremely wide application space anticipated for H.264.
However, this great flexibility also makes it difficult for an
implementer to decide on a reasonable packetization scheme. Some
information on how to apply this specification to real-world
scenarios is likely to appear in the form of academic publications
and a test model software and description in the near future.
However, some preliminary usage scenarios are described here as
well.
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12.1. Video Telephony according to ITU-T Recommendation H.241 Annex A
H.323-based video telephony systems that use H.264 as an optional
video compression scheme are required to support H.241 Annex A [3]
as a packetization scheme. The packetization mechanism defined in
this Annex is technically identical with a small subset of this
specification.
When a system operates according to H.241 Annex A, parameter set
NAL units are sent in-band. Only Single NAL unit packets are used.
Many such systems are not sending IDR pictures regularly, but only
when required by user interaction or by control protocol means;
e.g., when switching between video channels in a Multipoint Control
Unit or for error recovery requested by feedback.
12.2. Video Telephony, No Slice Data Partitioning, No NAL Unit
Aggregation
The RTP part of this scheme is implemented and tested (though not
the control-protocol part; see below).
In most real-world video telephony applications, picture parameters
such as picture size or optional modes never change during the
lifetime of a connection. Therefore, all necessary parameter sets
(usually only one) are sent as a side effect of the capability
exchange/announcement process, e.g., according to the SDP syntax
specified in section 8.2 of this document. As all necessary
parameter set information is established before the RTP session
starts, there is no need for sending any parameter set NAL units.
Slice data partitioning is not used, either. Thus, the RTP packet
stream basically consists of NAL units that carry single coded
slices.
The encoder chooses the size of coded slice NAL units so that they
offer the best performance. Often, this is done by adapting the
coded slice size to the MTU size of the IP network. For small
picture sizes, this may result in a one-picture-per-one-packet
strategy. Intra refresh algorithms clean up the loss of packets
and the resulting drift-related artifacts.
12.3. Video Telephony, Interleaved Packetization Using NAL Unit
Aggregation
This scheme allows better error concealment and is used in H.263
based designs using RFC 4629 packetization [11]. It has been
implemented, and good results were reported [13].
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The VCL encoder codes the source picture so that all macroblocks
(MBs) of one MB line are assigned to one slice. All slices with
even MB row addresses are combined into one STAP, and all slices
with odd MB row addresses into another. Those STAPs are
transmitted as RTP packets. The establishment of the parameter
sets is performed as discussed above.
Note that the use of STAPs is essential here, as the high number of
individual slices (18 for a CIF picture) would lead to unacceptably
high IP/UDP/RTP header overhead (unless the source coding tool FMO
is used, which is not assumed in this scenario). Furthermore, some
wireless video transmission systems, such as H.324M and the IP-
based video telephony specified in 3GPP, are likely to use
relatively small transport packet size. For example, a typical MTU
size of H.223 AL3 SDU is around 100 bytes [17]. Coding individual
slices according to this packetization scheme provides further
advantage in communication between wired and wireless networks, as
individual slices are likely to be smaller than the preferred
maximum packet size of wireless systems. Consequently, a gateway
can convert the STAPs used in a wired network into several RTP
packets with only one NAL unit, which are preferred in a wireless
network, and vice versa.
12.4. Video Telephony with Data Partitioning
This scheme has been implemented and has been shown to offer good
performance, especially at higher packet loss rates [13].
Data Partitioning is known to be useful only when some form of
unequal error protection is available. Normally, in single-session
RTP environments, even error characteristics are assumed; i.e., the
packet loss probability of all packets of the session is the same
statistically. However, there are means to reduce the packet loss
probability of individual packets in an RTP session. A FEC packet
according to RFC 2733 [18], for example, specifies which media
packets are associated with the FEC packet.
In all cases, the incurred overhead is substantial but is in the
same order of magnitude as the number of bits that have otherwise
been spent for intra information. However, this mechanism does not
add any delay to the system.
Again, the complete parameter set establishment is performed
through control protocol means.
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12.5. Video Telephony or Streaming with FUs and Forward Error
Correction
This scheme has been implemented and has been shown to provide good
performance, especially at higher packet loss rates [19].
The most efficient means to combat packet losses for scenarios
where retransmissions are not applicable is forward error
correction (FEC). Although application layer, end-to-end use of
FEC is often less efficient than an FEC-based protection of
individual links (especially when links of different
characteristics are in the transmission path), application layer,
end-to-end FEC is unavoidable in some scenarios. RFC 5109 [18]
provides means to use generic, application layer, end-to-end FEC in
packet-loss environments. A binary forward error correcting code
is generated by applying the XOR operation to the bits at the same
bit position in different packets. The binary code can be
specified by the parameters (n,k) in which k is the number of
information packets used in the connection and n is the total
number of packets generated for k information packets; i.e., n-k
parity packets are generated for k information packets.
When a code is used with parameters (n,k) within the RFC 5109
framework, the following properties are well known:
a) If applied over one RTP packet, RFC 5109 provides only packet
repetition.
b) RFC 5109 is most bit rate efficient if XOR-connected packets
have equal length.
c) At the same packet loss probability p and for a fixed k, the
greater the value of n is, the smaller the residual error
probability becomes. For example, for a packet loss probability
of 10%, k=1, and n=2, the residual error probability is about 1%,
whereas for n=3, the residual error probability is about 0.1%.
d) At the same packet loss probability p and for a fixed code rate
k/n, the greater the value of n is, the smaller the residual
error probability becomes. For example, at a packet loss
probability of p=10%, k=1 and n=2, the residual error rate is
about 1%, whereas for an extended Golay code with k=12 and n=24,
the residual error rate is about 0.01%.
For applying RFC 5109 in combination with H.264 baseline coded
video without using FUs, several options might be considered:
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1) The video encoder produces NAL units for which each video frame
is coded in a single slice. Applying FEC, one could use a
simple code; e.g., (n=2, k=1). That is, each NAL unit would
basically just be repeated. The disadvantage is obviously the
bad code performance according to d), above, and the low
flexibility, as only (n, k=1) codes can be used.
2) The video encoder produces NAL units for which each video frame
is encoded in one or more consecutive slices. Applying FEC, one
could use a better code, e.g., (n=24, k=12), over a sequence of
NAL units. Depending on the number of RTP packets per frame, a
loss may introduce a significant delay, which is reduced when
more RTP packets are used per frame. Packets of completely
different length might also be connected, which decreases bit
rate efficiency according to b), above. However, with some care
and for slices of 1kb or larger, similar length (100-200 bytes
difference) may be produced, which will not lower the bit
efficiency catastrophically.
3) The video encoder produces NAL units, for which a certain frame
contains k slices of possibly almost equal length. Then,
applying FEC, a better code, e.g., (n=24, k=12), can be used
over the sequence of NAL units for each frame. The delay
compared to that of 2), above, may be reduced, but several
disadvantages are obvious. First, the coding efficiency of the
encoded video is lowered significantly, as slice-structured
coding reduces intra-frame prediction and additional slice
overhead is necessary. Second, pre-encoded content or, when
operating over a gateway, the video is usually not appropriately
coded with k slices such that FEC can be applied. Finally, the
encoding of video producing k slices of equal length is not
straightforward and might require more than one encoding pass.
Many of the mentioned disadvantages can be avoided by applying FUs
in combination with FEC. Each NAL unit can be split into any
number of FUs of basically equal length; therefore, FEC with a
reasonable k and n can be applied, even if the encoder made no
effort to produce slices of equal length. For example, a coded
slice NAL unit containing an entire frame can be split to k FUs,
and a parity check code (n=k+1, k) can be applied. However, this
has the disadvantage that unless all created fragments can be
recovered, the whole slice will be lost. Thus a larger section is
lost than would be if the frame had been split into several slices.
The presented technique makes it possible to achieve good
transmission error tolerance, even if no additional source coding
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layer redundancy (such as periodic intra frames) is present.
Consequently, the same coded video sequence can be used to achieve
the maximum compression efficiency and quality over error-free
transmission and for transmission over error-prone networks.
Furthermore, the technique allows the application of FEC to pre-
encoded sequences without adding delay. In this case, pre-encoded
sequences that are not encoded for error-prone networks can still
be transmitted almost reliably without adding extensive delays. In
addition, FUs of equal length result in a bit rate efficient use of
RFC 5109.
If the error probability depends on the length of the transmitted
packet (e.g., in case of mobile transmission [15]), the benefits of
applying FUs with FEC are even more obvious. Basically, the
flexibility of the size of FUs allows appropriate FEC to be applied
for each NAL unit and unequal error protection of NAL units.
When FUs and FEC are used, the incurred overhead is substantial but
is in the same order of magnitude as the number of bits that have
to be spent for intra-coded macroblocks if no FEC is applied. In
[19], it was shown that the overall performance of the FEC-based
approach enhanced quality when using the same error rate and same
overall bit rate, including the overhead.
12.6. Low Bit-Rate Streaming
This scheme has been implemented with H.263 and non-standard RTP
packetization and has given good results [20]. There is no
technical reason why similarly good results could not be achievable
with H.264.
In today's Internet streaming, some of the offered bit rates are
relatively low in order to allow terminals with dial-up modems to
access the content. In wired IP networks, relatively large packets,
say 500 - 1500 bytes, are preferred to smaller and more frequently
occurring packets in order to reduce network congestion. Moreover,
use of large packets decreases the amount of RTP/UDP/IP header
overhead. For low bit-rate video, the use of large packets means
that sometimes up to few pictures should be encapsulated in one
packet.
However, loss of a packet including many coded pictures would have
drastic consequences for visual quality, as there is practically no
other way to conceal a loss of an entire picture than to repeat the
previous one. One way to construct relatively large packets and
maintain possibilities for successful loss concealment is to
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construct MTAPs that contain interleaved slices from several
pictures. An MTAP should not contain spatially adjacent slices
from the same picture or spatially overlapping slices from any
picture. If a packet is lost, it is likely that a lost slice is
surrounded by spatially adjacent slices of the same picture and
spatially corresponding slices of the temporally previous and
succeeding pictures. Consequently, concealment of the lost slice
is likely to be relatively successful.
12.7. Robust Packet Scheduling in Video Streaming
Robust packet scheduling has been implemented with MPEG-4 Part 2
and simulated in a wireless streaming environment [21]. There is
no technical reason why similar or better results could not be
achievable with H.264.
Streaming clients typically have a receiver buffer that is capable
of storing a relatively large amount of data. Initially, when a
streaming session is established, a client does not start playing
the stream back immediately. Rather, it typically buffers the
incoming data for a few seconds. This buffering helps maintain
continuous playback, as, in case of occasional increased
transmission delays or network throughput drops, the client can
decode and play buffered data. Otherwise, without initial
buffering, the client has to freeze the display, stop decoding, and
wait for incoming data. The buffering is also necessary for either
automatic or selective retransmission in any protocol level. If
any part of a picture is lost, a retransmission mechanism may be
used to resend the lost data. If the retransmitted data is
received before its scheduled decoding or playback time, the loss
is recovered perfectly. Coded pictures can be ranked according to
their importance in the subjective quality of the decoded sequence.
For example, non-reference pictures, such as conventional B
pictures, are subjectively least important, as their absence does
not affect decoding of any other pictures. In addition to non-
reference pictures, the ITU-T H.264 | ISO/IEC 14496-10 standard
includes a temporal scalability method called sub-sequences [22].
Subjective ranking can also be made on coded slice data partition
or slice group basis. Coded slices and coded slice data partitions
that are subjectively the most important can be sent earlier than
their decoding order indicates, whereas coded slices and coded
slice data partitions that are subjectively the least important can
be sent later than their natural coding order indicates.
Consequently, any retransmitted parts of the most important slices
and coded slice data partitions are more likely to be received
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before their scheduled decoding or playback time compared to the
least important slices and slice data partitions.
13. Informative Appendix: Rationale for Decoding Order Number
13.1. Introduction
The Decoding Order Number (DON) concept was introduced mainly to
enable efficient multi-picture slice interleaving (see section 12.6)
and robust packet scheduling (see section 12.7). In both of these
applications, NAL units are transmitted out of decoding order. DON
indicates the decoding order of NAL units and should be used in the
receiver to recover the decoding order. Example use cases for
efficient multi-picture slice interleaving and for robust packet
scheduling are given in sections 13.2 and 13.3, respectively.
Section 13.4 describes the benefits of the DON concept in error
resiliency achieved by redundant coded pictures. Section 13.5
summarizes considered alternatives to DON and justifies why DON was
chosen to this RTP payload specification.
13.2. Example of Multi-Picture Slice Interleaving
An example of multi-picture slice interleaving follows. A subset
of a coded video sequence is depicted below in output order. R
denotes a reference picture, N denotes a non-reference picture, and
the number indicates a relative output time.
... R1 N2 R3 N4 R5 ...
The decoding order of these pictures from left to right is as
follows:
... R1 R3 N2 R5 N4 ...
The NAL units of pictures R1, R3, N2, R5, and N4 are marked with a
DON equal to 1, 2, 3, 4, and 5, respectively.
Each reference picture consists of three slice groups that are
scattered as follows (a number denotes the slice group number for
each macroblock in a QCIF frame):
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0 1 2 0 1 2 0 1 2 0 1
2 0 1 2 0 1 2 0 1 2 0
1 2 0 1 2 0 1 2 0 1 2
0 1 2 0 1 2 0 1 2 0 1
2 0 1 2 0 1 2 0 1 2 0
1 2 0 1 2 0 1 2 0 1 2
0 1 2 0 1 2 0 1 2 0 1
2 0 1 2 0 1 2 0 1 2 0
1 2 0 1 2 0 1 2 0 1 2
For the sake of simplicity, we assume that all the macroblocks of a
slice group are included in one slice. Three MTAPs are constructed
from three consecutive reference pictures so that each MTAP
contains three aggregation units, each of which contains all the
macroblocks from one slice group. The first MTAP contains slice
group 0 of picture R1, slice group 1 of picture R3, and slice group
2 of picture R5. The second MTAP contains slice group 1 of picture
R1, slice group 2 of picture R3, and slice group 0 of picture R5.
The third MTAP contains slice group 2 of picture R1, slice group 0
of picture R3, and slice group 1 of picture R5. Each non-reference
picture is encapsulated into an STAP-B.
Consequently, the transmission order of NAL units is the following:
R1, slice group 0, DON 1, carried in MTAP,RTP SN: N
R3, slice group 1, DON 2, carried in MTAP,RTP SN: N
R5, slice group 2, DON 4, carried in MTAP,RTP SN: N
R1, slice group 1, DON 1, carried in MTAP,RTP SN: N+1
R3, slice group 2, DON 2, carried in MTAP,RTP SN: N+1
R5, slice group 0, DON 4, carried in MTAP,RTP SN: N+1
R1, slice group 2, DON 1, carried in MTAP,RTP SN: N+2
R3, slice group 1, DON 2, carried in MTAP,RTP SN: N+2
R5, slice group 0, DON 4, carried in MTAP,RTP SN: N+2
N2, DON 3, carried in STAP-B, RTP SN: N+3
N4, DON 5, carried in STAP-B, RTP SN: N+4
The receiver is able to organize the NAL units back in decoding
order based on the value of DON associated with each NAL unit.
If one of the MTAPs is lost, the spatially adjacent and temporally
co-located macroblocks are received and can be used to conceal the
loss efficiently. If one of the STAPs is lost, the effect of the
loss does not propagate temporally.
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13.3. Example of Robust Packet Scheduling
An example of robust packet scheduling follows. The communication
system used in the example consists of the following components in
the order that the video is processed from source to sink:
o camera and capturing
o pre-encoding buffer
o encoder
o encoded picture buffer
o transmitter
o transmission channel
o receiver
o receiver buffer
o decoder
o decoded picture buffer
o display
The video communication system used in the example operates as
follows. Note that processing of the video stream happens
gradually and at the same time in all components of the system.
The source video sequence is shot and captured to a pre-encoding
buffer. The pre-encoding buffer can be used to order pictures from
sampling order to encoding order or to analyze multiple
uncompressed frames for bit rate control purposes, for example. In
some cases, the pre-encoding buffer may not exist; instead, the
sampled pictures are encoded right away. The encoder encodes
pictures from the pre-encoding buffer and stores the output; i.e.,
coded pictures, to the encoded picture buffer. The transmitter
encapsulates the coded pictures from the encoded picture buffer to
transmission packets and sends them to a receiver through a
transmission channel. The receiver stores the received packets to
the receiver buffer. The receiver buffering process typically
includes buffering for transmission delay jitter. The receiver
buffer can also be used to recover correct decoding order of coded
data. The decoder reads coded data from the receiver buffer and
produces decoded pictures as output into the decoded picture buffer.
The decoded picture buffer is used to recover the output (or
display) order of pictures. Finally, pictures are displayed.
In the following example figures, I denotes an IDR picture, R
denotes a reference picture, N denotes a non-reference picture, and
the number after I, R, or N indicates the sampling time relative to
the previous IDR picture in decoding order. Values below the
sequence of pictures indicate scaled system clock timestamps. The
system clock is initialized arbitrarily in this example, and time
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runs from left to right. Each I, R, and N picture is mapped into
the same timeline compared to the previous processing step, if any,
assuming that encoding, transmission, and decoding take no time.
Thus, events happening at the same time are located in the same
column throughout all example figures.
A subset of a sequence of coded pictures is depicted below in
sampling order.
... N58 N59 I00 N01 N02 R03 N04 N05 R06 ... N58 N59 I00 N01 ...
... --|---|---|---|---|---|---|---|---|- ... -|---|---|---|- ...
... 58 59 60 61 62 63 64 65 66 ... 128 129 130 131 ...
Figure 16 Sequence of pictures in sampling order
The sampled pictures are buffered in the pre-encoding buffer to
arrange them in encoding order. In this example, we assume that
the non-reference pictures are predicted from both the previous and
the next reference picture in output order, except for the non-
reference pictures immediately preceding an IDR picture, which are
predicted only from the previous reference picture in output order.
Thus, the pre-encoding buffer has to contain at least two pictures,
and the buffering causes a delay of two picture intervals. The
output of the pre-encoding buffering process and the encoding (and
decoding) order of the pictures are as follows:
... N58 N59 I00 R03 N01 N02 R06 N04 N05 ...
... -|---|---|---|---|---|---|---|---|- ...
... 60 61 62 63 64 65 66 67 68 ...
Figure 17 Re-ordered pictures in the pre-encoding buffer
The encoder or the transmitter can set the value of DON for each
picture to a value of DON for the previous picture in decoding
order plus one.
For the sake of simplicity, let us assume that:
o the frame rate of the sequence is constant,
o each picture consists of only one slice,
o each slice is encapsulated in a single NAL unit packet,
o there is no transmission delay, and
o pictures are transmitted at constant intervals (that is, 1 /
(frame rate)).
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When pictures are transmitted in decoding order, they are received
as follows:
... N58 N59 I00 R03 N01 N02 R06 N04 N05 ...
... -|---|---|---|---|---|---|---|---|- ...
... 60 61 62 63 64 65 66 67 68 ...
Figure 18 Received pictures in decoding order
The OPTIONAL sprop-interleaving-depth media type parameter is set
to 0, as the transmission (or reception) order is identical to the
decoding order.
The decoder has to buffer for one picture interval initially in its
decoded picture buffer to organize pictures from decoding order to
output order as depicted below:
... N58 N59 I00 N01 N02 R03 N04 N05 R06 ...
... -|---|---|---|---|---|---|---|---|- ...
... 61 62 63 64 65 66 67 68 69 ...
Figure 19 Output order
The amount of required initial buffering in the decoded picture
buffer can be signaled in the buffering period SEI message or with
the num_reorder_frames syntax element of H.264 video usability
information. num_reorder_frames indicates the maximum number of
frames, complementary field pairs, or non-paired fields that
precede any frame, complementary field pair, or non-paired field in
the sequence in decoding order and that follow it in output order.
For the sake of simplicity, we assume that num_reorder_frames is
used to indicate the initial buffer in the decoded picture buffer.
In this example, num_reorder_frames is equal to 1.
It can be observed that if the IDR picture I00 is lost during
transmission and a retransmission request is issued when the value
of the system clock is 62, there is one picture interval of time
(until the system clock reaches timestamp 63) to receive the
retransmitted IDR picture I00.
Let us then assume that IDR pictures are transmitted two frame
intervals earlier than their decoding position; i.e., the pictures
are transmitted as follows:
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... I00 N58 N59 R03 N01 N02 R06 N04 N05 ...
... --|---|---|---|---|---|---|---|---|- ...
... 62 63 64 65 66 67 68 69 70 ...
Figure 20 Interleaving: Early IDR pictures in sending order
The OPTIONAL sprop-interleaving-depth media type parameter is set
equal to 1 according to its definition. (The value of sprop-
interleaving-depth in this example can be derived as follows:
Picture I00 is the only picture preceding picture N58 or N59 in
transmission order and following it in decoding order. Except for
pictures I00, N58, and N59, the transmission order is the same as
the decoding order of pictures. As a coded picture is encapsulated
into exactly one NAL unit, the value of sprop-interleaving-depth is
equal to the maximum number of pictures preceding any picture in
transmission order and following the picture in decoding order.)
The receiver buffering process contains two pictures at a time
according to the value of the sprop-interleaving-depth parameter
and orders pictures from the reception order to the correct
decoding order based on the value of DON associated with each
picture. The output of the receiver buffering process is as
follows:
... N58 N59 I00 R03 N01 N02 R06 N04 N05 ...
... -|---|---|---|---|---|---|---|---|- ...
... 63 64 65 66 67 68 69 70 71 ...
Figure 21 Interleaving: Receiver buffer
Again, an initial buffering delay of one picture interval is needed
to organize pictures from decoding order to output order, as
depicted below:
... N58 N59 I00 N01 N02 R03 N04 N05 ...
... -|---|---|---|---|---|---|---|- ...
... 64 65 66 67 68 69 70 71 ...
Figure 22 Interleaving: Receiver buffer after reordering
Note that the maximum delay that IDR pictures can undergo during
transmission, including possible application, transport, or link
layer retransmission, is equal to three picture intervals. Thus,
the loss resiliency of IDR pictures is improved in systems
supporting retransmission compared to the case in which pictures
were transmitted in their decoding order.
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13.4. Robust Transmission Scheduling of Redundant Coded Slices
A redundant coded picture is a coded representation of a picture or
a part of a picture that is not used in the decoding process if the
corresponding primary coded picture is correctly decoded. There
should be no noticeable difference between any area of the decoded
primary picture and a corresponding area that would result from
application of the H.264 decoding process for any redundant picture
in the same access unit. A redundant coded slice is a coded slice
that is a part of a redundant coded picture.
Redundant coded pictures can be used to provide unequal error
protection in error-prone video transmission. If a primary coded
representation of a picture is decoded incorrectly, a corresponding
redundant coded picture can be decoded. Examples of applications
and coding techniques using the redundant codec picture feature
include the video redundancy coding [23] and the protection of "key
pictures" in multicast streaming [24].
One property of many error-prone video communications systems is
that transmission errors are often bursty. Therefore, they may
affect more than one consecutive transmission packets in
transmission order. In low bit-rate video communication, it is
relatively common that an entire coded picture can be encapsulated
into one transmission packet. Consequently, a primary coded
picture and the corresponding redundant coded pictures may be
transmitted in consecutive packets in transmission order. To make
the transmission scheme more tolerant of bursty transmission errors,
it is beneficial to transmit the primary coded picture and
redundant coded picture separated by more than a single packet.
The DON concept enables this.
13.5. Remarks on Other Design Possibilities
The slice header syntax structure of the H.264 coding standard
contains the frame_num syntax element that can indicate the
decoding order of coded frames. However, the usage of the
frame_num syntax element is not feasible or desirable to recover
the decoding order, due to the following reasons:
o The receiver is required to parse at least one slice header per
coded picture (before passing the coded data to the decoder).
o Coded slices from multiple coded video sequences cannot be
interleaved, as the frame number syntax element is reset to 0 in
each IDR picture.
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o The coded fields of a complementary field pair share the same
value of the frame_num syntax element. Thus, the decoding order
of the coded fields of a complementary field pair cannot be
recovered based on the frame_num syntax element or any other
syntax element of the H.264 coding syntax.
The RTP payload format for transport of MPEG-4 elementary streams
[25] enables interleaving of access units and transmission of
multiple access units in the same RTP packet. An access unit is
specified in the H.264 coding standard to comprise all NAL units
associated with a primary coded picture according to subclause
7.4.1.2 of [1]. Consequently, slices of different pictures cannot
be interleaved, and the multi-picture slice interleaving technique
(see section 12.6) for improved error resilience cannot be used.
14. Backward Compatibility to RFC 3984
The current document is a revision of RFC 3984 and obsoletes it.
The technical changes relative to RFC 3984 are listed in section 15.
This section addresses the backward compatibility issues.
It should be noted that for the majority of cases, there will be no
compatibility issues for legacy implementations per RFC 3984 and
new implementations per this document to interwork. Compatibility
issues may only occur when both of the following conditions are
true: 1) legacy implementations and new implementations are
interworking; and 2) parameter sets are transported out of band.
Even when such compatibility issues occur, it is easy to debug and
find out the reason according to the following analyses.
Items 1), 2), 3), 7), 9), 10), 12) and 13) are bug-fix type of
changes, and do not incur any backward compatibility issues.
Item 4), addition of six new media type parameters, does not incur
any backward compatibility issues for SDP Offer/Answer based
applications, as legacy RFC 3984 receivers ignore these parameters,
and it is fine for legacy RFC 3984 senders not to use these
parameters as they are optional. However, there is a backward
compatibility issue for SDP declarative usage based applications
(only for the parameter sprop-level-parameter-sets as the other
five parameters are not usable in declarative usage), e.g. those
using RTSP and SAP, because the SDP receiver per RFC 3984 cannot
accept a session for which the SDP includes an unrecognized
parameter. Therefore, the RTSP or SAP server may have to prepare
two sets of streams, one for legacy RFC 3984 receivers and one for
receivers according to this memo.
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Items 5), 6), and 11) are related to out-of-band transport of
parameter sets. There are following backward compatibility issues.
1) When a legacy sender per RFC 3984 includes parameter sets for a
level different than the default level indicated by profile-
level-id to sprop-parameter-sets, the parameter value of sprop-
parameter-sets is invalid to the receiver per this memo and
therefore the session may be rejected.
2) In SDP Offer/Answer between a legacy offerer per RFC 3984 and an
answerer per this memo, when the answerer includes in the answer
parameter sets that are not a superset of the parameter sets
included in the offer, the parameter value of sprop-parameter-
sets is invalid to the offerer and the session may not be
initiated properly (related to change item 11).
3) When one endpoint A per this memo includes in-band-parameter-
sets equal to 1, the other side B per RFC 3984 does not
understand that it must transmit parameter sets in-band and B
may still exclude parameter sets in the in-band stream it is
sending. Consequently endpoint A cannot decode the stream it
receives.
Item7), allowance of conveying sprop-parameter-sets and sprop-
level-parameter-sets using the "fmtp" source attribute as specified
in section 6.3 of [9], is similar as item 4). It does not incur
any backward compatibility issues for SDP Offer/Answer based
applications, as legacy RFC 3984 receivers ignore the "fmtp" source
attribute, and it is fine for legacy RFC 3984 senders not to use
the "fmtp" source attribute as it is optional. However, there is a
backward compatibility issue for SDP declarative usage based
applications, e.g. those using RTSP and SAP, because the SDP
receiver per RFC 3984 cannot accept a session for which the SDP
includes an unrecognized parameter (i.e., the "fmtp" source
attribute). Therefore, the RTSP or SAP server may have to prepare
two sets of streams, one for legacy RFC 3984 receivers and one for
receivers according to this memo.
Item 14) removed that use of out-of-band transport of parameter
sets is recommended. As out-of-band transport of parameter sets is
still allowed, this change does not incur any backward
compatibility issues.
Item 15) does not incur any backward compatibility issues as the
added subsection 8.5 is informative.
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Item 16) does not create any backward compatibility issues as the
handling of default level is the same if either end is RFC 3984
compliant, and furthermore, RFC 3984 compliant ends would simply
ignore the new media type parameters, if present.
15. Changes from RFC 3984
Following is the list of technical changes (including bug fixes)
from RFC 3984. Besides this list of technical changes, numerous
editorial changes have been made, but not documented in this
section. Note that section 8.2.2 is where much of the important
changes in this memo occurs and deserves particular attention.
1) In subsections 5.4, 5.5, 6.2, 6,3 and 6.4, removed that the
packetization mode in use may be signaled by external means.
2) In subsection 7.2.2, changed the sentence
There are N VCL NAL units in the deinterleaving buffer.
to
There are N or more VCL NAL units in the de-interleaving buffer.
3) In subsection 8.1, the semantics of sprop-init-buf-time,
paragraph 2, changed the sentence
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.
to
The parameter is the maximum value of (decoding time of the NAL
unit - transmission time of a NAL unit), assuming reliable and
instantaneous transmission, the same timeline for transmission
and decoding, and that decoding starts when the first packet
arrives.
4) Added media type parameters max-smbps, sprop-level-parameter-
sets, use-level-src-parameter-sets, in-band-parameter-sets, sar-
understood and sar-supported.
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5) In subsection 8.1, removed the specification of parameter-add.
Other descriptions of parameter-add (in subsections 8.2 and 8.4)
are also removed.
6) In subsection 8.1, added a constraint to sprop-parameter-sets
such that it can only contain parameter sets for the same
profile and level as indicated by profile-level-id.
7) In subsection 8.2.1, added that sprop-parameter-sets and sprop-
level-parameter-sets may be either included in the "a=fmtp" line
of SDP or conveyed using the "fmtp" source attribute as
specified in section 6.3 of [9].
8) In subsection 8.2.2, removed sprop-deint-buf-req from being part
of the media format configuration in usage with the SDP
Offer/Answer model.
9) In subsection 8.2.2, made it clear that level is downgradable in
the SDP Offer/Answer model, i.e. the use of the level part of
"profile-level-id" does not need to be symmetric (the level
included in the answer can be lower than or equal to the level
included in the offer).
10)In subsection 8.2.2, removed that the capability parameters may
be used to declare encoding capabilities.
11)In subsection 8.2.2, added rules on how to use sprop-parameter-
sets and sprop-level-parameter-sets for out-of-band transport of
parameter sets, with or without level downgrading.
12)In subsection 8.2.2, clarified the rules of using the media type
parameters with SDP Offer/Answer for multicast.
13)In subsection 8.2.2, completed and corrected the list of how
different media type parameters shall be interpreted in the
different combinations of offer or answer and direction
attribute.
14)In subsection 8.4, changed the text such that both out-of-band
and in-band transport of parameter sets are allowed and neither
is recommended or required.
15)Added subsection 8.5 (informative) providing example methods for
decoder refresh to handle parameter set losses.
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16)Added media type parameters max-recv-level, and level-asymmetry-
allowed, and adjusted associated text and examples for level
upgrade and asymmetry.
16. Acknowledgements
Stephan Wenger, Miska Hannuksela, Thomas Stockhammer, Magnus
Westerlund, and David Singer are thanked as the authors of RFC 3984.
Dave Lindbergh, Philippe Gentric, Gonzalo Camarillo, Gary Sullivan,
Joerg Ott, and Colin Perkins are thanked for careful review during
the development of RFC 3984. Stephen Botzko, Magnus Westerlund,
Alex Eleftheriadis, Thomas Schierl, Tom Taylor, Ali Begen, Aaron
Wells, Stuart Taylor, Robert Sparks, Dan Romascanu, and Niclas
Comstedt are thanked for their valuable comments and inputs during
the development of this memo.
This document was prepared using 2-Word-v2.0.template.dot.
17. References
17.1. Normative References
[1] ITU-T Recommendation H.264, "Advanced video coding for
generic audiovisual services", November 2007.
[2] ISO/IEC International Standard 14496-10:2008.
[3] ITU-T Recommendation H.241, "Extended video procedures and
control signals for H.300 series terminals", May 2006.
[4] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[5] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,
"RTP: A Transport Protocol for Real-Time Applications", STD
64, RFC 3550, July 2003.
[6] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[7] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, October 2006.
[8] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
Session Description Protocol (SDP)", RFC 3264, June 2002.
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[9] Lennox, J., Ott, J., and Schierl, T., "Source-Specific Media
Attributes in the Session Description Protocol (SDP)", RFC
5576, June 2009.
17.2. Informative References
[10] Luthra, A., Sullivan, G.J., and T. Wiegand (eds.), Special
Issue on H.264/AVC. IEEE Transactions on Circuits and Systems
on Video Technology, July 2003.
[11] Ott, J., Bormann, C., Sullivan, G., Wenger, S., and R. Even
(Ed.), "RTP Payload Format for ITU-T Rec. H.263 Video", RFC
4629, January 2007.
[12] ISO/IEC IS 14496-2.
[13] Wenger, S., "H.26L over IP", IEEE Transaction on Circuits and
Systems for Video technology, Vol. 13, No. 7, July 2003.
[14] Wenger, S., "H.26L over IP: The IP Network Adaptation Layer",
Proceedings Packet Video Workshop 02, April 2002.
[15] Stockhammer, T., Hannuksela, M.M., and S. Wenger, "H.26L/JVT
Coding Network Abstraction Layer and IP-based Transport" in
Proc. ICIP 2002, Rochester, NY, September 2002.
[16] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and
Video Conferences with Minimal Control", STD 65, RFC 3551,
July 2003.
[17] ITU-T Recommendation H.223, "Multiplexing protocol for low
bit rate multimedia communication", July 2001.
[18] Li, A., "RTP Payload Format for Generic Forward Error
Correction", RFC 5109, December 2007.
[19] Stockhammer, T., Wiegand, T., Oelbaum, T., and F. Obermeier,
"Video Coding and Transport Layer Techniques for H.264/AVC-
Based Transmission over Packet-Lossy Networks", IEEE
International Conference on Image Processing (ICIP 2003),
Barcelona, Spain, September 2003.
[20] Varsa, V. and M. Karczewicz, "Slice interleaving in
compressed video packetization", Packet Video Workshop 2000.
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[21] Kang, S.H. and A. Zakhor, "Packet scheduling algorithm for
wireless video streaming," International Packet Video
Workshop 2002.
[22] Hannuksela, M.M., "Enhanced concept of GOP", JVT-B042,
available http://ftp3.itu.int/av-arch/video-
site/0201_Gen/JVT-B042.doc, anuary 2002.
[23] Wenger, S., "Video Redundancy Coding in H.263+", 1997
International Workshop on Audio-Visual Services over Packet
Networks, September 1997.
[24] Wang, Y.-K., Hannuksela, M.M., and M. Gabbouj, "Error
Resilient Video Coding Using Unequally Protected Key
Pictures", in Proc. International Workshop VLBV03, September
2003.
[25] van der Meer, J., Mackie, D., Swaminathan, V., Singer, D.,
and P. Gentric, "RTP Payload Format for Transport of MPEG-4
Elementary Streams", RFC 3640, November 2003.
[26] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, March 2004.
[27] Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time
Streaming Protocol (RTSP)", RFC 2326, April 1998.
[28] Handley, M., Perkins, C., and E. Whelan, "Session
Announcement Protocol", RFC 2974, October 2000.
[29] Westerlund, M. and S. Wenger, "RTP Topologies", RFC 5117,
January 2008.
[30] Wenger, S., Chandra, U., and M. Westerlund, "Codec Control
Messages in the RTP Audio-Visual Profile with Feedback
(AVPF)", RFC 5104, February 2008.
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18. Authors' Addresses
Ye-Kui Wang
Huawei Technologies
400 Somerset Corp Blvd, Suite 602
Bridgewater, NJ 08807
USA
Phone: +1-908-541-3518
EMail: yekuiwang@huawei.com
Roni Even
14 David Hamelech
Tel Aviv 64953
Israel
Phone: +972-545481099
Email: ron.even.tlv@gmail.com
Tom Kristensen
TANDBERG
Philip Pedersens vei 22
N-1366 Lysaker
Norway
Phone: +47 67125125
Email: tom.kristensen@tandberg.com, tomkri@ifi.uio.no
Randell Jesup
WorldGate Communications
3190 Tremont Ave
Trevose, PA 19053
USA
Phone: +1-215-354-5166
Email: rjesup@wgate.com
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