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PROPOSED STANDARD
Errata Exist
Internet Engineering Task Force (IETF) Y.-K. Wang
Request for Comments: 6184 R. Even
Obsoletes: 3984 Huawei Technologies
Category: Standards Track T. Kristensen
ISSN: 2070-1721 Tandberg
R. Jesup
WorldGate Communications
May 2011
RTP Payload Format for H.264 Video
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 Multiview 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 bitrate
conversational usage, to Internet video streaming with interleaved
transmission, to high bitrate video-on-demand.
This memo obsoletes RFC 3984. Changes from RFC 3984 are summarized
in Section 14. Issues on backward compatibility to RFC 3984 are
discussed in Section 15.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6184.
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RFC 6184 RTP Payload Format for H.264 Video May 2011
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction ....................................................4
1.1. The H.264 Codec ............................................4
1.2. Parameter Set Concept ......................................5
1.3. Network Abstraction Layer Unit Types .......................6
2. Conventions .....................................................7
3. Scope ...........................................................7
4. Definitions and Abbreviations ...................................7
4.1. Definitions ................................................7
4.2. Abbreviations ..............................................9
5. RTP Payload Format .............................................10
5.1. RTP Header Usage ..........................................10
5.2. Payload Structures ........................................12
5.3. NAL Unit Header Usage .....................................13
5.4. Packetization Modes .......................................16
5.5. Decoding Order Number (DON) ...............................17
5.6. Single NAL Unit Packet ....................................19
5.7. Aggregation Packets .......................................20
5.7.1. Single-Time Aggregation Packet (STAP) ..............22
5.7.2. Multi-Time Aggregation Packets (MTAPs) .............25
5.8. 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 ..........................................34
7. De-Packetization Process .......................................35
7.1. Single NAL Unit and Non-Interleaved Mode ..................35
7.2. Interleaved Mode ..........................................35
7.2.1. Size of the De-Interleaving Buffer .................36
7.2.2. De-Interleaving Process ............................36
7.3. Additional De-Packetization Guidelines ....................38
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8. Payload Format Parameters ......................................39
8.1. Media Type Registration ...................................39
8.2. SDP Parameters ............................................57
8.2.1. Mapping of Payload Type Parameters to SDP ..........57
8.2.2. Usage with the SDP Offer/Answer Model ..............58
8.2.3. Usage in Declarative Session Descriptions ..........66
8.3. Examples ..................................................68
8.4. Parameter Set Considerations ..............................75
8.5. Decoder Refresh Point Procedure Using In-Band
Transport of Parameter Sets (Informative)..................78
8.5.1. IDR Procedure to Respond to a Request for
a Decoder Refresh Point ............................78
8.5.2. Gradual Recovery Procedure to Respond to
a Request for a Decoder Refresh Point ..............79
9. Security Considerations ........................................79
10. Congestion Control ............................................80
11. IANA Considerations ...........................................81
12. Informative Appendix: Application Examples ....................81
12.1. Video Telephony According to Annex A of ITU-T
Recommendation H.241 .....................................81
12.2. Video Telephony, No Slice Data Partitioning, No
NAL Unit Aggregation .....................................82
12.3. Video Telephony, Interleaved Packetization Using
NAL Unit Aggregation .....................................82
12.4. Video Telephony with Data Partitioning ...................83
12.5. Video Telephony or Streaming with FUs and Forward
Error Correction .........................................83
12.6. Low Bitrate Streaming ....................................86
12.7. Robust Packet Scheduling in Video Streaming ..............86
13. Informative Appendix: Rationale for Decoding Order Number .....87
13.1. Introduction .............................................87
13.2. Example of Multi-Picture Slice Interleaving ..............88
13.3. Example of Robust Packet Scheduling ......................89
13.4. Robust Transmission Scheduling of Redundant Coded
Slices ...................................................93
13.5. Remarks on Other Design Possibilities ....................94
14. Changes from RFC 3984 .........................................94
15. Backward Compatibility to RFC 3984 ............................96
16. Acknowledgements ..............................................98
17. References ....................................................98
17.1. Normative References .....................................98
17.2. Informative References ...................................99
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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 14496-10 [2] (both also known as Advanced
Video Coding (AVC)). In this memo, the name H.264 is used for the
codec and the standard, but this 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 14. Issues on backward compatibility to RFC 3984 are
discussed in Section 15.
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 bitrate 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
bitrate savings of 50% or more are reported. Digital Satellite TV
quality, for example, was reported to be achievable at 1.5 Mbit/s,
compared to the current operation point of MPEG 2 video at around 3.5
Mbit/s [10].
The codec specification [1] itself conceptually distinguishes 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 using the syntax of slice groups. In-picture
prediction is used only within a slice. More information is provided
in [10].
The NAL encoder encapsulates the slice output of the VCL encoder into
Network Abstraction Layer Units (NALUs), which are suitable for
transmission over packet networks or for use in packet-oriented
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multiplex environments. Annex B of H.264 defines an encapsulation
process to transmit such NALUs over bytestream-oriented networks. In
the scope of this memo, Annex B is not relevant.
Internally, the NAL uses NAL units. A NAL unit consists of a one-
byte header and the payload byte string. The header indicates the
type of the NAL unit, the (potential) presence of bit errors or
syntax violations in the NAL unit payload, and information regarding
the relative importance of the NAL unit for the decoding process.
This RTP payload specification is designed to be unaware of the bit
string in the NAL unit payload.
One of the main properties of H.264 is the complete decoupling of the
transmission time, the decoding time, and the sampling or
presentation time of slices and pictures. The decoding process
specified in H.264 is unaware of time, and the H.264 syntax does not
carry information such as the number of skipped frames (as is common
in the form of the Temporal Reference in earlier video compression
standards). Also, there are NAL units that affect many pictures and
that are, therefore, inherently timeless. For this reason, the
handling of the RTP timestamp requires some special considerations
for NAL units for which the sampling or presentation time is not
defined or, at transmission time, is 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 sets and
picture parameter sets. 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.
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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. A
description of the payload of a NAL unit follows.
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.
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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).
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.
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coded video sequence: A sequence of access units that consists, in
decoding order, of an instantaneous decoding refresh (IDR) access
unit followed by zero or more non-IDR access units including all
subsequent access units up to but not including any subsequent IDR
access unit.
IDR access unit: An access unit in which the primary coded picture
is an IDR picture.
IDR picture: A coded picture containing only slices with I or SI
slice types that causes a "reset" in the decoding process. After
the decoding of an IDR picture, all following coded pictures in
decoding order can be decoded without inter prediction from any
picture decoded prior to the IDR picture.
primary coded picture: The coded representation of a picture to be
used by the decoding process for a bitstream conforming to H.264.
The primary coded picture contains all macroblocks of the picture.
redundant coded picture: A coded representation of a picture or a
part of a picture. The content of a redundant coded picture shall
not be used by the decoding process for a bitstream conforming to
H.264. The content of a redundant coded picture may be used by
the decoding process for a bitstream that contains errors or
losses.
VCL NAL unit: A collective term used to refer to coded slice and
coded data partition NAL units.
In addition, the following definitions apply:
decoding order number (DON): A field in the payload structure or a
derived variable indicating NAL unit decoding order. Values of
DON are in the range of 0 to 65535, inclusive. After reaching the
maximum value, the value of DON wraps around to 0.
NAL unit decoding order: A NAL unit order that conforms to the
constraints on NAL unit order given in Section 7.4.1.2 in [1].
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.
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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 that it has to be trusted when working
with Secure Real-time Transport Protocol (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 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
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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
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 5.7.2 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
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formats, to allow an efficient playout buffer handling. For
aggregation packets (STAP and MTAP), the marker bit in the RTP
header MUST be set to the value that the marker bit of the last
NAL unit of the aggregation packet would have been if it were
transported in its own RTP packet. Decoders MAY use this bit as
an early indication of the last packet of an access unit but MUST
NOT rely on this property.
Informative note: Only one M bit is associated with an
aggregation packet carrying multiple NAL units. Thus, if a
gateway has re-packetized an aggregation packet into several
packets, it cannot reliably set the M bit of those packets.
Payload type (PT): 7 bits
The assignment of an RTP payload type for this new packet format
is outside the scope of this document and will not be specified
here. The assignment of a payload type has to be performed either
through the profile used or in a dynamic way.
Sequence number (SN): 16 bits
Set and used in accordance with RFC 3550. For the single NALU and
non-interleaved packetization mode, the sequence number is used to
determine decoding order for the NALU.
Timestamp: 32 bits
The RTP timestamp is set to the sampling timestamp of the content.
A 90 kHz clock rate MUST be used.
If the NAL unit has no timing properties of its own (e.g.,
parameter set and SEI NAL units), the RTP timestamp is set to the
RTP timestamp of the primary coded picture of the access unit in
which the NAL unit is included, according to Section 7.4.1.2 of
[1].
The setting of the RTP timestamp for MTAPs is defined in Section
5.7.2.
Receivers SHOULD ignore any picture timing SEI messages included
in access units that have only one display timestamp. Instead,
receivers SHOULD use the RTP timestamp for synchronizing the
display process.
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 picture time values carried in the picture timing SEI message.
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Let tSEI1, tSEI2, ..., tSEIn be the display timestamps carried in
the SEI message of an access unit, where tSEI1 is the earliest of
all such timestamps. Let tmadjst() be a function that adjusts the
SEI messages time scale to a 90-kHz time scale. Let TS be the RTP
timestamp. Then, the display time for the event associated with
tSEI1 is TS. The display time for the event with tSEIx, where x
is [2..n], is TS + tmadjst (tSEIx - tSEI1).
Informative note: Displaying coded frames as fields is needed
commonly in an operation known as 3:2 pulldown, in which film
content that consists of coded frames is displayed on a display
using interlaced scanning. The picture timing SEI message
enables carriage of multiple timestamps for the same coded
picture, and therefore the 3:2 pulldown process is perfectly
controlled. The picture timing SEI message mechanism is
necessary because only one timestamp per coded frame can be
conveyed in the RTP timestamp.
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 is 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.
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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.
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.
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MANEs SHOULD set the F bit to indicate detected bit errors in
the NAL unit. The H.264 specification requires that the F bit
be equal to 0. When the F bit is set, the decoder is advised
that bit errors or any other syntax violations may be present
in the payload or in the NAL unit type octet. The simplest
decoder reaction to a NAL unit in which the F bit is equal to 1
is to discard such a NAL unit and to conceal the lost data in
the discarded NAL unit.
NRI: 2 bits
nal_ref_idc. The semantics of value 00 and a non-zero value
remain unchanged from the H.264 specification. In other words,
a value of 00 indicates that the content of the NAL unit is not
used to reconstruct reference pictures for inter picture
prediction. Such NAL units can be discarded without risking
the integrity of the reference pictures. Values greater than
00 indicate that the decoding of the NAL unit is required to
maintain the integrity of the reference pictures.
In addition to the specification above, according to this RTP
payload specification, values of NRI 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).
Wang, et al. Standards Track [Page 14]
RFC 6184 RTP Payload Format for H.264 Video May 2011
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 desirable, depending on the application and the H.264
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.
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.
Wang, et al. Standards Track [Page 15]
RFC 6184 RTP Payload Format for H.264 Video May 2011
5.4. Packetization Modes
This memo specifies three cases of packetization modes:
o Single NAL unit mode
o Non-interleaved mode
o Interleaved mode
The single NAL unit mode is targeted for conversational systems that
comply with ITU-T Recommendation H.241 [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, 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
Wang, et al. Standards Track [Page 16]
RFC 6184 RTP Payload Format for H.264 Video May 2011
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 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
Wang, et al. Standards Track [Page 17]
RFC 6184 RTP Payload Format for H.264 Video May 2011
mode, the transmission order of NAL units in single NAL unit packets,
STAP-As, and FU-As MUST be the same as their NAL unit decoding order.
The NAL units within an STAP MUST appear in the NAL unit decoding
order. Thus, the decoding order is first provided through the
implicit order within an STAP and then provided through the RTP
sequence number for the order between STAPs, FUs, and single NAL unit
packets.
The 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,
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.
Wang, et al. Standards Track [Page 18]
RFC 6184 RTP Payload Format for H.264 Video May 2011
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 bitrate 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 MUST conform to
the NAL unit decoding order. The structure of the single NAL unit
packet is shown in Figure 2.
Wang, et al. Standards Track [Page 19]
RFC 6184 RTP Payload Format for H.264 Video May 2011
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-based or non-IP-based (e.g.,
ITU-T H.324/M) 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-times. 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-times. 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.
Wang, et al. Standards Track [Page 20]
RFC 6184 RTP Payload Format for H.264 Video May 2011
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:
o The RTP timestamp MUST be set to the earliest of the NALU-times of
all the NAL units to be aggregated.
o The type field of the NAL unit type octet MUST be set to the
appropriate value, as indicated in Table 4.
o The F bit MUST be cleared if all F bits of the aggregated NAL
units are zero; otherwise, it MUST be set.
o 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.
Wang, et al. Standards Track [Page 21]
RFC 6184 RTP Payload Format for H.264 Video May 2011
The payload of an aggregation packet consists of one or more
aggregation units. See Sections 5.7.1 and 5.7.2 for the four
different types of aggregation units. An aggregation packet can
carry as many aggregation units as necessary; however, the total
amount of data in an aggregation packet obviously MUST fit into an IP
packet, and the size SHOULD be chosen so that the resulting IP packet
is smaller than the MTU size. An aggregation packet MUST NOT contain
fragmentation units, as specified in Section 5.8. Aggregation
packets MUST NOT be nested; that is, an aggregation packet MUST NOT
contain another aggregation packet.
5.7.1. Single-Time Aggregation Packet (STAP)
A 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
Wang, et al. Standards Track [Page 22]
RFC 6184 RTP Payload Format for H.264 Video May 2011
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: decoding order number (DON) | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
| single-time aggregation units |
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5. Payload format for STAP-B
The DON field specifies the value of DON for the first NAL unit in an
STAP-B in transmission order. For each successive NAL unit in
appearance order in an STAP-B, the value of DON is equal to (the
value of DON of the previous NAL unit in the STAP-B + 1) % 65536, in
which '%' stands for the modulo operation.
A single-time aggregation unit consists of 16-bit unsigned size
information (in network byte order) that indicates the size of the
following NAL unit in bytes (excluding these two octets, but
including the NAL unit type octet of the NAL unit), followed by the
NAL unit itself, including its NAL unit type byte. A single-time
aggregation unit is byte aligned within the RTP payload, but it may
not be aligned on a 32-bit word boundary. Figure 6 presents the
structure of the single-time aggregation unit.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: NAL unit size | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
| NAL unit |
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6. Structure for single-time aggregation unit
Wang, et al. Standards Track [Page 23]
RFC 6184 RTP Payload Format for H.264 Video May 2011
Figure 7 presents an example of an RTP packet that contains an STAP-
A. The STAP contains two single-time aggregation units, labeled as 1
and 2 in the figure.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RTP Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|STAP-A NAL HDR | NALU 1 Size | NALU 1 HDR |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NALU 1 Data |
: :
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | NALU 2 Size | NALU 2 HDR |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NALU 2 Data |
: :
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :...OPTIONAL RTP padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7. An example of an RTP packet including an STAP-A
containing two single-time aggregation units
Wang, et al. Standards Track [Page 24]
RFC 6184 RTP Payload Format for H.264 Video May 2011
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
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.
Wang, et al. Standards Track [Page 25]
RFC 6184 RTP Payload Format for H.264 Video May 2011
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 of 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.
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).
Wang, et al. Standards Track [Page 26]
RFC 6184 RTP Payload Format for H.264 Video May 2011
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 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.
Wang, et al. Standards Track [Page 27]
RFC 6184 RTP Payload Format for H.264 Video May 2011
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
Wang, et al. Standards Track [Page 28]
RFC 6184 RTP Payload Format for H.264 Video May 2011
Figure 13 presents an example of an RTP packet that contains a multi-
time aggregation packet of type MTAP24 that contains two multi-time
aggregation units, labeled as 1 and 2 in the figure.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RTP Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|MTAP24 NAL HDR | decoding order number base | NALU 1 Size |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NALU 1 Size | NALU 1 DOND | NALU 1 TS offs |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|NALU 1 TS offs | NALU 1 HDR | NALU 1 DATA |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
: :
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | NALU 2 SIZE | NALU 2 DOND |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NALU 2 TS offset | NALU 2 HDR |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NALU 2 DATA |
: :
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :...OPTIONAL RTP padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 13. An RTP packet including a multi-time aggregation
packet of type MTAP24 containing two multi-time
aggregation units
5.8. Fragmentation Units (FUs)
This payload type allows fragmenting a NAL unit into several RTP
packets. Doing so on the application layer instead of relying on
lower-layer fragmentation (e.g., by IP) has the following advantages:
o The payload format is capable of transporting NAL units bigger
than 64 kbytes over an IPv4 network that may be present in pre-
recorded video, particularly in High-Definition formats (there is
a limit of the number of slices per picture, which results in a
limit of NAL units per picture, which may result in big NAL
units).
o The fragmentation mechanism allows fragmenting a single NAL unit
and applying generic forward error correction as described in
Section 12.5.
Wang, et al. Standards Track [Page 29]
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Fragmentation is defined only for a single NAL unit and not for any
aggregation packets. A fragment of a NAL unit consists of an integer
number of consecutive octets of that NAL unit. Each octet of the NAL
unit MUST be part of exactly one fragment of that NAL unit.
Fragments of the same NAL unit MUST be sent in consecutive order with
ascending RTP sequence numbers (with no other RTP packets within the
same RTP packet stream being sent between the first and last
fragment). Similarly, a NAL unit MUST be reassembled in RTP sequence
number order.
When a NAL unit is fragmented and conveyed within fragmentation units
(FUs), it is referred to as a fragmented NAL unit. STAPs and MTAPs
MUST NOT be fragmented. FUs MUST NOT be nested; that is, 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
start of a fragmented NAL unit payload, the Start bit is set
to zero.
Wang, et al. Standards Track [Page 31]
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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; that is, 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 the 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, 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.
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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.
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).
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Senders using the non-interleaved mode and the interleaved mode MUST
enforce the following packetization rule:
o In an RTP translator, 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. 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 5109 [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 RTP
Control Protocol (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 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.
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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 also be used 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.
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
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is no transmission delay jitter. To differentiate the receiver
buffer from a practical receiver buffer that is also used for
compensation of transmission delay jitter, the receiver buffer is
hereafter 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 Session Description Protocol
(SDP) usage, the sprop-deint-buf-req media type parameter signals the
requirement for the de-interleaving buffer size. Therefore, it is
RECOMMENDED to set the de-interleaving buffer size, in terms of
number of bytes, equal to or greater than the value of the 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
information on the 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.
Wang, et al. Standards Track [Page 36]
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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:
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.
Wang, et al. Standards Track [Page 37]
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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.
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
Wang, et al. Standards Track [Page 38]
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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 has
been allocated from the IETF tree.
Media Type name: video
Media subtype name: H264
Required parameters: none
OPTIONAL parameters:
profile-level-id:
A base16 [7] (hexadecimal) representation of the following
three bytes in the sequence parameter set NAL unit is 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, constraint_set4_flag,
constraint_set5_flag, and reserved_zero_2bits in bit-
significance order, starting from the most-significant bit, and
3) level_idc. Note that reserved_zero_2bits 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.
Wang, et al. Standards Track [Page 39]
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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 Section 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), 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.
Wang, et al. Standards Track [Page 40]
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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
B 42 (B) x0xx0000
same as: 58 (E) 10xx0000
M 4D (M) 0x0x0000
E 58 00xx0000
H 64 00000000
H10 6E 00000000
H42 7A 00000000
H44 F4 00000000
H10I 6E 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.
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.
Wang, et al. Standards Track [Page 41]
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If the profile-level-id parameter is used for capability
exchange or session setup, 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 in 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
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 if 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.
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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 bitrate 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 bitrate 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 the 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
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.
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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.
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
Wang, et al. Standards Track [Page 44]
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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 and in units of 1200 bits for the NAL HRD
parameters. Note that this parameter does not use units of
cpbBrVclFactor and cpbBrNALFactor (see Table A-1 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 (after taking cpbBrVclFactor and cpbBrNALFactor into
consideration when needed). The value of max-cpb (after taking
cpbBrVclFactor and cpbBrNALFactor into consideration when
needed) 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 variation of bitrate 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 a video decoder can be integrated with
de-interleaving and de-jitter buffers of the receiver.
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max-dpb: The value of max-dpb is an integer indicating the maximum
decoded picture buffer size in units of 8/3 macroblocks. 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 MaxDpbMbs value in Table A-1 of [1]
for the signaled highest level is replaced with the value of
max-dpb * 3 / 8. 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(max-dpb * 3 / 8 / ( PicWidthInMbs * FrameHeightInMbs),
16)
Wherein PicWidthInMbs and FrameHeightInMbs are defined in [1].
The value of max-dpb MUST be greater than or equal to the value
of MaxDpbMbs * 3 / 8, wherein the value of MaxDpbMbs is given
in Table A-1 of [1] for the highest level. Senders MAY use
this knowledge to construct coded video streams with improved
compression.
Informative note: This parameter was added primarily to
complement a similar codepoint in the ITU-T Recommendation
H.245, so as to facilitate signaling gateway designs. The
decoded picture buffer stores reconstructed samples. 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.
Informative note: In RFC 3984, which this document
obsoletes, the unit of this parameter was 1024 bytes. The
unit has been changed to 8/3 macroblocks in this document.
The reason for this change was due to the changes from the
2003 version of the H.264 specification referenced by RFC
3984 to the 2010 version of the H.264 specification
referenced by this document, particularly the changes to
Table A-1 in the H.264 specification due to addition of
color formats and bit depths not supported earlier. The
changed semantics of this parameter keeps backward
compatibility to RFC 3984 and supports all profiles defined
in the 2010 version of the H.264 specification.
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max-br: The value of max-br is an integer indicating the maximum
video bitrate in units of 1000 bits per second for the VCL HRD
parameters and in units of 1200 bits per second for the NAL HRD
parameters. Note that this parameter does not use units of
cpbBrVclFactor and cpbBrNALFactor (see Table A-1 of [1]).
The max-br parameter signals that the video decoder of the
receiver is capable of decoding video at a higher bitrate 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 (after taking cpbBrVclFactor and
cpbBrNALFactor into consideration when needed) 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 Main profile
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 (after taking cpbBrVclFactor and
cpbBrNALFactor into consideration when needed) MUST be greater
than or equal to the value MaxBR given in Table A-1 of [1] for
the signaled highest level.
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. The
assumption that the network is capable of handling such
bitrates at any given time cannot be made from the value of
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this parameter. In particular, no conclusion can be drawn
that the signaled bitrate 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, 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 a profile (Baseline,
Extended) in which redundant coded pictures are allowed.
Informative note: Even if redundant-pic-cap is equal to 1,
the receiver may also choose other error concealment
strategies to replace or complement decoding of redundant
slices.
sprop-parameter-sets:
This parameter MAY be used to convey any sequence and picture
parameter set NAL units (herein referred to as the initial
parameter set NAL units) that 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
Wang, et al. Standards Track [Page 48]
RFC 6184 RTP Payload Format for H.264 Video May 2011
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 hundred bytes.
Informative note: When several payload types are offered in
the SDP Offer/Answer model, each with its own sprop-
parameter-sets parameter, 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 that are different than the default
level. All parameter sets associated with one level are
clustered and prefixed with a three-byte field that has the
same syntax as profile-level-id. This enables the receiver to
install the parameter sets for one level and 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:PSLs
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
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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, does not understand the
"fmtp" source attribute as specified in Section 6.3 of [9],
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,
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 [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 if 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
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RFC 6184 RTP Payload Format for H.264 Video May 2011
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, 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 that 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 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.
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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 that 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
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.
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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 commencement of decoding when the first
packet arrives.
An example of specifying the value of sprop-init-buf-time
follows. A NAL unit stream is sent in the following
interleaved order, in which the value corresponds to the
decoding time and the transmission order is from left to right:
0 2 1 3 5 4 6 8 7 ...
Assuming a steady transmission rate of NAL units, the
transmission times are:
0 1 2 3 4 5 6 7 8 ...
Subtracting the decoding time from the transmission time
column-wise results in the following series:
0 -1 1 0 -1 1 0 -1 1 ...
Thus, in terms of intervals of NAL unit transmission times, the
value of sprop-init-buf-time in this example is 1. The
parameter is coded as a non-negative base10 integer
representation in clock ticks of a 90-kHz clock. If the
parameter is not present, then no initial buffering time value
is defined. Otherwise, the value of sprop-init-buf-time MUST
be an integer in the range of 0 to 4294967295, inclusive.
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RFC 6184 RTP Payload Format for H.264 Video May 2011
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,
receiver, or codec capabilities. The parameter MUST NOT be
present if the value of packetization-mode is equal to 0 or 1.
sprop-max-don-diff is an integer in the range of 0 to 32767,
inclusive. If sprop-max-don-diff is not present, the value of
the parameter is unspecified. sprop-max-don-diff is calculated
as follows:
sprop-max-don-diff = max{AbsDON(i) - AbsDON(j)},
for any i and any j>i,
where i and j indicate the index of the NAL unit in the
transmission order and AbsDON denotes a decoding order number
of the NAL unit that does not wrap around to 0 after 65535. In
other words, AbsDON is calculated as follows: let m and n be
consecutive NAL units in transmission order. For the very
first NAL unit in transmission order (whose index is 0),
AbsDON(0) = DON(0). For other NAL units, AbsDON is calculated
as follows:
If DON(m) == DON(n), AbsDON(n) = AbsDON(m)
If (DON(m) < DON(n) and DON(n) - DON(m) < 32768),
AbsDON(n) = AbsDON(m) + DON(n) - DON(m)
If (DON(m) > DON(n) and DON(m) - DON(n) >= 32768),
AbsDON(n) = AbsDON(m) + 65536 - DON(m) + DON(n)
If (DON(m) < DON(n) and DON(n) - DON(m) >= 32768),
AbsDON(n) = AbsDON(m) - (DON(m) + 65536 - DON(n))
If (DON(m) > DON(n) and DON(m) - DON(n) < 32768),
AbsDON(n) = AbsDON(m) - (DON(m) - DON(n))
where DON(i) is the decoding order number of the NAL unit
having index i in the transmission order. The decoding order
number is specified in Section 5.5.
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Informative note: Receivers may use sprop-max-don-diff to
trigger which NAL units in the receiver buffer can be passed
to the decoder.
max-rcmd-nalu-size:
This parameter MAY be used to signal the capabilities of a
receiver. The parameter MUST NOT be used for any other
purposes. The value of the parameter indicates the largest
NALU size in bytes that the receiver can handle efficiently.
The parameter value is a recommendation, not a strict upper
boundary. The sender MAY create larger NALUs but must be aware
that the handling of these may come at a higher cost than NALUs
conforming to the limitation.
The value of max-rcmd-nalu-size MUST be an integer in the range
of 0 to 4294967295, inclusive. If this parameter is not
specified, no known limitation to the NALU size exists.
Senders still have to consider the MTU size available between
the sender and the receiver and SHOULD run MTU discovery for
this purpose.
This parameter is motivated by, for example, an IP to H.223
video telephony gateway, where NALUs smaller than the H.223
transport data unit will be more efficient. A gateway may
terminate IP; thus, MTU discovery will normally not work beyond
the gateway.
Informative note: Setting this parameter to a lower than
necessary value may have a negative impact.
sar-understood:
This parameter MAY be used to indicate a receiver capability
and nothing 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 exactly what the SAR is. For such a
receiver, the value of sar-understood is 16. In the future, if
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
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RFC 6184 RTP Payload Format for H.264 Video May 2011
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 nothing 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 255 indicates that the receiver
supports all sample aspect ratios that 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 6184.
Public specification:
Please refer to RFC 6184 and its Section 17.
Additional information:
None
File extensions: none
Wang, et al. Standards Track [Page 56]
RFC 6184 RTP Payload Format for H.264 Video May 2011
Macintosh file type code: none
Object identifier or OID: none
Person & email address to contact for further information:
Ye-Kui Wang, yekui.wang@huawei.com
Intended usage: COMMON
Author:
Ye-Kui Wang, yekui.wang@huawei.com
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
Wang, et al. Standards Track [Page 57]
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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.
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; that is, 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, because the level was
Wang, et al. Standards Track [Page 58]
RFC 6184 RTP Payload Format for H.264 Video May 2011
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 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 the 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
the 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 When present, the parameter max-recv-level 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.
When present, max-recv-level 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.
Wang, et al. Standards Track [Page 59]
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Otherwise, level-asymmetry-allowed equals 1 in both the offer and
the answer, and 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 a 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, and 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 when 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.
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
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[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 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.
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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, and 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 is 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 is 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 the transport of parameter sets in
the answerer-to-offerer direction.
o An answer MAY include either sprop-parameter-sets or sprop-
level-parameter-sets but MUST NOT include both. 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.
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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, and the following applies.
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 is 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 is 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-
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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
with 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:
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; that is, 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 the 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 only be used in decoding the incoming NAL unit
stream from the same source.
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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.
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
S: out-of-band parameter sets
-: not usable (when present, SHOULD be ignored)
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Parameters used for declaring receiver capabilities are in general
downgradable; that is, 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.
When a sender's capabilities are declared and non-downgradable
parameters are used in this declaration, 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 being 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.
8.2.3. Usage in Declarative Session Descriptions
When H.264 over RTP is offered with SDP in a declarative style, as in
Real Time Streaming Protocol (RTSP) [27] or Session Announcement
Protocol (SAP) [28], the following considerations are necessary.
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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. The result of this is 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
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.
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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. Payload type 98 represents single
NALU mode, payload type 99 represents non-interleaved mode, and
payload type 100 indicates the interleaved mode. In the interleaved
mode case, the interleaving parameters that the offerer would use if
the answer indicates support for payload type 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.
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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
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 is willing 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; that is, it
has identical values for the two parameters profile-level-id and
packetization-mode (since packetization-mode is equal to 0 and 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
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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.
In the following example, the offer is accepted without level
downgrading (i.e., the default level, 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 Level 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.
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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;
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 Level 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.
Parameter sets in sprop-parameter-sets=<parameter sets data#0> must
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be stored and 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
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In the following example, the offer is accepted with level
downgrading and sprop-parameter-sets is present in the offer. As
sprop-parameter-sets=<parameter sets data#0> contains level_idc
indicating Level 3.0, it 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 answerer-to-
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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 topology like Topo-Video-switch-MCU [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 canonical name (CNAME), which are
mapped to different SSRC values. The same codec configuration is
used by all 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 originating from
participant B only, uses <parameter sets data#C> for decoding NAL
units carried in RTP packets originating from participant C only, and
uses <parameter sets data#D> for decoding NAL units carried in RTP
packets originating from participant D only.
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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
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
normally has 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.
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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.
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
be present at the decoder prior to the NAL units that refer to them.
Update or addition of parameter sets can result in further problems;
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 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
set 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. 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.
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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 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 outdated 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 Sections
8.5.1 or 8.5.2. The procedure in Section 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.
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.
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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.
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
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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. In the case
of confidentiality protection, it will even be prevented from
discarding packets in a media-aware way. To be allowed to perform
its operations, a MANE is required to be a trusted entity that 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]. If
best-effort service is being used, an additional requirement is that
users of this payload format MUST monitor packet loss to ensure 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.
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The bitrate 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 bitrates, 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
restart 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 Considerations
The H264 media subtype name specified by RFC 3984 has been 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.
12.1. Video Telephony According to Annex A of ITU-T Recommendation
H.241
H.323-based video telephony systems that use H.264 as an optional
video compression scheme are required to support Annex A of H.241 [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 Annex A of H.241, 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
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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].
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 are combined 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 Common Intermediate Format (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,
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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; that is,
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 5109 [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.
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 a 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
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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; that is,
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 bitrate 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, 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, 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:
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
lengths might also be connected, which decreases bitrate
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efficiency according to b), above. However, with some care and
for slices of 1 kb 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
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 bitrate 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.
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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
bitrate, including the overhead.
12.6. Low Bitrate 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 bitrates 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 bitrate video, the use of large packets means that
sometimes up to few pictures should be encapsulated in one packet.
However, the loss of a packet including many coded pictures would
have drastic consequences for visual quality, as there is practically
no way to conceal the loss of an entire picture other than repeating
the previous one. One way to construct relatively large packets and
maintain possibilities for successful loss concealment is to
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
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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 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 for this RTP payload specification.
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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 Quarter Common Intermediate Format (QCIF)
frame):
0 1 2 0 1 2 0 1 2 0 1
2 0 1 2 0 1 2 0 1 2 0
1 2 0 1 2 0 1 2 0 1 2
0 1 2 0 1 2 0 1 2 0 1
2 0 1 2 0 1 2 0 1 2 0
1 2 0 1 2 0 1 2 0 1 2
0 1 2 0 1 2 0 1 2 0 1
2 0 1 2 0 1 2 0 1 2 0
1 2 0 1 2 0 1 2 0 1 2
For the sake of simplicity, we assume that all the macroblocks of a
slice group are included in one slice. Three MTAPs are constructed
from three consecutive reference pictures so that each MTAP contains
three aggregation units, each of which contains all the macroblocks
from one slice group. The first MTAP contains slice group 0 of
picture R1, slice group 1 of picture 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.
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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.
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 this 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
bitrate control purposes, for example. In some cases, the pre-
encoding buffer may not exist; instead, the sampled pictures are
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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 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:
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... N58 N59 I00 R03 N01 N02 R06 N04 N05 ...
... -|---|---|---|---|---|---|---|---|- ...
... 60 61 62 63 64 65 66 67 68 ...
Figure 17. Reordered 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)).
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.
Initially, the decoder has to buffer for one picture interval 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
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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; that is, the pictures
are transmitted as follows:
... 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
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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 are transmitted
in their decoding order.
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 packet in transmission order.
In low bitrate video communication, it is relatively common for an
entire coded picture to 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.
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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.
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. 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 Sections 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 Section 7.2.2, changed the sentence
There are N VCL NAL units in the de-interleaving buffer.
to
There are N or more VCL NAL units in the de-interleaving buffer.
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3) In Section 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.
5) In Section 8.1, removed the specification of parameter-add.
Other descriptions of parameter-add (in Sections 8.2 and 8.4)
were also removed.
6) In Section 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 Section 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 Section 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 Section 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 Section 8.2.2, removed that the capability parameters may be
used to declare encoding capabilities.
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11) In Section 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 Section 8.2.2, clarified the rules of using the media type
parameters with SDP Offer/Answer for multicast.
13) In Section 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 Section 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 Section 8.5 (informative) providing example methods for
decoder refresh to handle parameter set losses.
16) Added media type parameters max-recv-level and level-asymmetry-
allowed and adjusted associated text and examples for level
upgrade and asymmetry.
15. 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 14.
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. When such
compatibility issues occur, it is easy to debug and find the reason
for the incompatibility using the following analyses.
Items 1, 2, 3, 7, 9, 10, 12, and 13 are bug-fix types 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 declarative-usage-based applications (only
for the parameter sprop-level-parameter-sets as the other five
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parameters are not usable in declarative usage). For example,
declarative-usage-based applications using RTSP and SAP have a
backward compatibility issue 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.
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;
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.
Item 7 (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 to 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.
Wang, et al. Standards Track [Page 97]
RFC 6184 RTP Payload Format for H.264 Video May 2011
Item 14 does not incur any backward compatibility issues, as out-of-
band transport of parameter sets is still allowed.
Item 15 does not incur any backward compatibility issues, as the
added Section 8.5 is informative.
Item 16 does not create any backward compatibility issues as the
handling of the 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.
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 input during the development
of this memo.
17. References
17.1. Normative References
[1] ITU-T Recommendation H.264, "Advanced video coding for generic
audiovisual services", March 2010.
[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.
Wang, et al. Standards Track [Page 98]
RFC 6184 RTP Payload Format for H.264 Video May 2011
[8] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
Session Description Protocol (SDP)", RFC 3264, June 2002.
[9] Lennox, J., Ott, J., and T. Schierl, "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.),
"Introduction to the special issue on the H.264/AVC video
coding standard", IEEE Transactions on Circuits and Systems for
Video Technology, Vol. 13, No. 7, 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 International Standard 14496-2:2004.
[13] Wenger, S., "H.264/AVC 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, April 2002.
[15] Stockhammer, T., Hannuksela, M.M., and S. Wenger, "H.26L/JVT
Coding Network Abstraction Layer and IP-Based Transport", IEEE
International Conference on Image Processing (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., Ed., "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.
Wang, et al. Standards Track [Page 99]
RFC 6184 RTP Payload Format for H.264 Video May 2011
[21] Kang, S.H. and A. Zakhor, "Packet scheduling algorithm for
wireless video streaming", 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, January 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., Westerlund, M., and B. Burman, "Codec
Control Messages in the RTP Audio-Visual Profile with Feedback
(AVPF)", RFC 5104, February 2008.
Wang, et al. Standards Track [Page 100]
RFC 6184 RTP Payload Format for H.264 Video May 2011
Authors' Addresses
Ye-Kui Wang
Huawei Technologies
400 Crossing Blvd, 2nd Floor
Bridgewater, NJ 08807
USA
Phone: +1-908-541-3518
EMail: yekui.wang@huawei.com
Roni Even
Huawei Technologies
14 David Hamelech
Tel Aviv 64953
Israel
Phone: +972-545481099
EMail: even.roni@huawei.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
3800 Horizon Blvd, Suite #103
Trevose, PA 19053-4947
USA
Phone: +1-215-354-5166
EMail: rjesup@wgate.com, randell_ietf@jesup.org
Wang, et al. Standards Track [Page 101]
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