Audio/Video Transport WG Y.-K. Wang
Internet Draft S. Wenger
Intended status: Standards track M.M. Hanuksela
Expires: January 2009 Nokia
T. Stockhammer
Nomor Research
M. Westerlund
Ericsson
D. Singer
Apple
July 14, 2008
RTP Payload Format for H.264 Video
draft-wang-avt-rfc3984bis-01.txt
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Abstract
This memo describes an RTP Payload format for the ITU-T
Recommendation H.264 video codec and the technically identical
ISO/IEC International Standard 14496-10 video codec. The RTP payload
format allows for packetization of one or more Network Abstraction
Layer Units (NALUs), produced by an H.264 video encoder, in each RTP
payload. The payload format has wide applicability, as it supports
applications from simple low bit-rate conversational usage, to
Internet video streaming with interleaved transmission, to high bit-
rate video-on-demand.
This memo intends to obsolete RFC 3984.
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.............................................9
5.1. RTP Header Usage..........................................9
5.2. Common Structure of the RTP Payload Format...............12
5.3. NAL Unit Octet Usage.....................................13
5.4. Packetization Modes......................................15
5.5. Decoding Order Number (DON)..............................16
5.6. Single NAL Unit Packet...................................19
5.7. Aggregation Packets......................................20
5.7.1. Single-Time Aggregation Packet......................22
5.7.2. Multi-Time Aggregation Packets (MTAPs)..............24
5.7.3. Fragmentation Units (FUs)...........................28
6. Packetization Rules...........................................32
6.1. Common Packetization Rules...............................32
6.2. Single NAL Unit Mode.....................................33
6.3. Non-Interleaved Mode.....................................33
6.4. Interleaved Mode.........................................33
7. De-Packetization Process......................................34
7.1. Single NAL Unit and Non-Interleaved Mode.................34
7.2. Interleaved Mode.........................................34
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7.2.1. Size of the Deinterleaving Buffer...................35
7.2.2. Deinterleaving Process..............................35
7.3. Additional De-Packetization Guidelines...................37
8. Payload Format Parameters.....................................38
8.1. MIME Registration........................................38
8.2. SDP Parameters...........................................50
8.2.1. Mapping of MIME Parameters to SDP...................50
8.2.2. Usage with the SDP Offer/Answer Model...............50
8.2.3. Usage in Declarative Session Descriptions...........55
8.3. Examples.................................................56
8.4. Parameter Set Considerations.............................61
9. Security Considerations.......................................63
10. Congestion Control...........................................64
11. IANA Consideration...........................................65
12. Informative Appendix: Application Examples...................65
12.1. Video Telephony according to ITU-T Recommendation H.241
Annex A.......................................................65
12.2. Video Telephony, No Slice Data Partitioning, No NAL Unit
Aggregation...................................................65
12.3. Video Telephony, Interleaved Packetization Using NAL Unit
Aggregation...................................................66
12.4. Video Telephony with Data Partitioning..................66
12.5. Video Telephony or Streaming with FUs and Forward Error
Correction....................................................67
12.6. Low Bit-Rate Streaming..................................69
12.7. Robust Packet Scheduling in Video Streaming.............70
13. Informative Appendix: Rationale for Decoding Order Number....71
13.1. Introduction............................................71
13.2. Example of Multi-Picture Slice Interleaving.............71
13.3. Example of Robust Packet Scheduling.....................73
13.4. Robust Transmission Scheduling of Redundant Coded Slices77
13.5. Remarks on Other Design Possibilities...................77
14. Acknowledgements.............................................78
15. References...................................................78
15.1. Normative References....................................78
15.2. Informative References..................................79
Authors' Addresses...............................................80
Intellectual Property Statement..................................82
Disclaimer of Validity...........................................82
Acknowledgement..................................................83
16. Backward compatibility to RFC 3984...........................83
17. Changes from RFC 3984........................................83
17.1. Technical changes.......................................83
17.2. Editorial changes.......................................86
18. Open issues..................................................97
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1. Introduction
This memo intends to obsolete RFC 3984.
1.1. The H.264 Codec
This memo specifies an RTP payload specification for the video coding
standard known as ITU-T Recommendation H.264 [1] and ISO/IEC
International Standard 14496 Part 10 [2] (both also known as Advanced
Video Coding, or AVC). Recommendation H.264 was approved by ITU-T on
May 2003, and the approved draft specification is available for
public review [8]. In this memo the H.264 acronym is used for the
codec and the standard, but the memo is equally applicable to the
ISO/IEC counterpart of the coding standard.
The H.264 video codec has a very broad application range that covers
all forms of digital compressed video from, low bit-rate Internet
streaming applications to HDTV broadcast and Digital Cinema
applications with nearly lossless coding. Compared to the current
state of technology, the overall performance of H.264 is such that
bit rate savings of 50% or more are reported. Digital Satellite TV
quality, for example, was reported to be achievable at 1.5 Mbit/s,
compared to the current operation point of MPEG 2 video at around 3.5
Mbit/s [9].
The codec specification [1] itself distinguishes conceptually between
a video coding layer (VCL) and a network abstraction layer (NAL).
The VCL contains the signal processing functionality of the codec;
mechanisms such as transform, quantization, and motion compensated
prediction; and a loop filter. It follows the general concept of
most of today's video codecs, a macroblock-based coder that uses
inter picture prediction with motion compensation and transform
coding of the residual signal. The VCL encoder outputs slices: a bit
string that contains the macroblock data of an integer number of
macroblocks, and the information of the slice header (containing the
spatial address of the first macroblock in the slice, the initial
quantization parameter, and similar information). Macroblocks in
slices are arranged in scan order unless a different macroblock
allocation is specified, by using the so-called Flexible Macroblock
Ordering syntax. In-picture prediction is used only within a slice.
More information is provided in [9].
The Network Abstraction Layer (NAL) encoder encapsulates the slice
output of the VCL encoder into Network Abstraction Layer Units (NAL
units), which are suitable for transmission over packet networks or
use in packet oriented multiplex environments. Annex B of H.264
defines an encapsulation process to transmit such NAL units over
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byte-stream oriented networks. In the scope of this memo, Annex B is
not relevant.
Internally, the NAL uses NAL units. A NAL unit consists of a one-
byte header and the payload byte string. The header indicates the
type of the NAL unit, the (potential) presence of bit errors or
syntax violations in the NAL unit payload, and information regarding
the relative importance of the NAL unit for the decoding process.
This RTP payload specification is designed to be unaware of the bit
string in the NAL unit payload.
One of the main properties of H.264 is the complete decoupling of the
transmission time, the decoding time, and the sampling or
presentation time of slices and pictures. The decoding process
specified in H.264 is unaware of time, and the H.264 syntax does not
carry information such as the number of skipped frames (as is common
in the form of the Temporal Reference in earlier video compression
standards). Also, there are NAL units that affect many pictures and
that are, therefore, inherently timeless. For this reason, the
handling of the RTP timestamp requires some special considerations
for NAL units for which the sampling or presentation time is not
defined or, at transmission time, unknown.
1.2. Parameter Set Concept
One very fundamental design concept of H.264 is to generate self-
contained packets, to make mechanisms such as the header duplication
of RFC 2429 [10] or MPEG-4's Header Extension Code (HEC) [11]
unnecessary. This was achieved by decoupling information relevant to
more than one slice from the media stream. This higher layer meta
information should be sent reliably, asynchronously, and in advance
from the RTP packet stream that contains the slice packets.
(Provisions for sending this information in-band are also available
for applications that do not have an out-of-band transport channel
appropriate for the purpose.) The combination of the higher-level
parameters is called a parameter set. The H.264 specification
includes two types of parameter sets: sequence parameter set and
picture parameter set. An active sequence parameter set remains
unchanged throughout a coded video sequence, and an active picture
parameter set remains unchanged within a coded picture. The sequence
and picture parameter set structures contain information such as
picture size, optional coding modes employed, and macroblock to slice
group map.
To be able to change picture parameters (such as the picture size)
without having to transmit parameter set updates synchronously to the
slice packet stream, the encoder and decoder can maintain a list of
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more than one sequence and picture parameter set. Each slice header
contains a codeword that indicates the sequence and picture parameter
set to be used.
This mechanism allows the decoupling of the transmission of parameter
sets from the packet stream, and the transmission of them by external
means (e.g., as a side effect of the capability exchange), or through
a (reliable or unreliable) control protocol. It may even be possible
that they are never transmitted but are fixed by an application
design specification.
1.3. Network Abstraction Layer Unit Types
Tutorial information on the NAL design can be found in [12], [13],
and [14].
All NAL units consist of a single NAL unit type octet, which also co-
serves as the payload header of this RTP payload format. The payload
of a NAL unit follows immediately.
The syntax and semantics of the NAL unit type octet are specified in
[1], but the essential properties of the NAL unit type octet are
summarized below. The NAL unit type octet has the following format:
+---------------+
|0|1|2|3|4|5|6|7|
+-+-+-+-+-+-+-+-+
|F|NRI| Type |
+---------------+
The semantics of the components of the NAL unit type octet, as
specified in the H.264 specification, are described briefly below.
F: 1 bit
forbidden_zero_bit. The H.264 specification declares a value of
1 as a syntax violation.
NRI: 2 bits
nal_ref_idc. A value of 00 indicates that the content of the NAL
unit is not used to reconstruct reference pictures for inter
picture prediction. Such NAL units can be discarded without
risking the integrity of the reference pictures. Values greater
than 00 indicate that the decoding of the NAL unit is required to
maintain the integrity of the reference pictures.
Type: 5 bits
nal_unit_type. This component specifies the NAL unit payload
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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 [3].
This specification uses the notion of setting and clearing a bit when
bit fields are handled. Setting a bit is the same as assigning that
bit the value of 1 (On). Clearing a bit is the same as assigning
that bit the value of 0 (Off).
3. Scope
This payload specification can only be used to carry the "naked"
H.264 NAL unit stream over RTP, and not the bitstream format
discussed in Annex B of H.264. Likely, the first applications of
this specification will be in the conversational multimedia field,
video telephony or video conferencing, but the payload format also
covers other applications, such as Internet streaming and TV over IP.
4. Definitions and Abbreviations
4.1. Definitions
This document uses the definitions of [1]. The following terms,
defined in [1], are summed up for convenience:
access unit: A set of NAL units always containing a primary coded
picture. In addition to the primary coded picture, an access
unit may also contain one or more redundant coded pictures or
other NAL units not containing slices or slice data partitions of
a coded picture. The decoding of an access unit always results
in a decoded picture.
coded video sequence: A sequence of access units that consists,
in decoding order, of an instantaneous decoding refresh (IDR)
access unit followed by zero or more non-IDR access units
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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.
media aware network element (MANE): A network element, such as a
middlebox or application layer gateway that is capable of parsing
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certain aspects of the RTP payload headers or the RTP payload and
reacting to the contents.
Informative note: The concept of a MANE goes beyond normal
routers or gateways in that a MANE has to be aware of the
signaling (e.g., to learn about the payload type mappings of
the media streams), and in that it has to be trusted when
working with SRTP. The advantage of using MANEs is that they
allow packets to be dropped according to the needs of the
media coding. For example, if a MANE has to drop packets due
to congestion on a certain link, it can identify those packets
whose dropping has the smallest negative impact on the user
experience and remove them in order to remove the congestion
and/or keep the delay low.
4.2. Abbreviations
DON: Decoding Order Number
DONB: Decoding Order Number Base
DOND: Decoding Order Number Difference
FEC: Forward Error Correction
FU: Fragmentation Unit
IDR: Instantaneous Decoding Refresh
IEC: International Electrotechnical Commission
ISO: International Organization for Standardization
ITU-T: International Telecommunication Union,
Telecommunication Standardization Sector
MANE: Media Aware Network Element
MTAP: Multi-Time Aggregation Packet
MTAP16: MTAP with 16-bit timestamp offset
MTAP24: MTAP with 24-bit timestamp offset
NAL: Network Abstraction Layer
NALU: NAL Unit
SEI: Supplemental Enhancement Information
STAP: Single-Time Aggregation Packet
STAP-A: STAP type A
STAP-B: STAP type B
TS: Timestamp
VCL: Video Coding Layer
5. RTP Payload Format
5.1. RTP Header Usage
The format of the RTP header is specified in RFC 3550 [4] and
reprinted in Figure 1 for convenience. This payload format uses the
fields of the header in a manner consistent with that specification.
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When one NAL unit is encapsulated per RTP packet, the RECOMMENDED RTP
payload format is specified in section 5.6. The RTP payload (and the
settings for some RTP header bits) for aggregation packets and
fragmentation units are specified in sections 5.7 and 5.8,
respectively.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P|X| CC |M| PT | sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| synchronization source (SSRC) identifier |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| contributing source (CSRC) identifiers |
| .... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1 RTP header according to RFC 3550
The RTP header information to be set according to this RTP payload
format is set as follows:
Marker bit (M): 1 bit
Set for the very last packet of the access unit indicated by the
RTP timestamp, in line with the normal use of the M bit in video
formats, to allow an efficient playout buffer handling. For
aggregation packets (STAP and MTAP), the marker bit in the RTP
header MUST be set to the value that the marker bit of the last
NAL unit of the aggregation packet would have been if it were
transported in its own RTP packet. Decoders MAY use this bit as
an early indication of the last packet of an access unit, but
MUST NOT rely on this property.
Informative note: Only one M bit is associated with an
aggregation packet carrying multiple NAL units. Thus, if a
gateway has re-packetized an aggregation packet into several
packets, it cannot reliably set the M bit of those packets.
Payload type (PT): 7 bits
The assignment of an RTP payload type for this new packet format
is outside the scope of this document and will not be specified
here. The assignment of a payload type has to be performed
either through the profile used or in a dynamic way.
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Sequence number (SN): 16 bits
Set and used in accordance with RFC 3550. For the single NALU
and non-interleaved packetization mode, the sequence number is
used to determine decoding order for the NALU.
Timestamp: 32 bits
The RTP timestamp is set to the sampling timestamp of the
content. A 90 kHz clock rate MUST be used.
If the NAL unit has no timing properties of its own (e.g.,
parameter set and SEI NAL units), the RTP timestamp is set to the
RTP timestamp of the primary coded picture of the access unit in
which the NAL unit is included, according to section 7.4.1.2 of
[1].
The setting of the RTP Timestamp for MTAPs is defined in section
5.7.2.
Receivers SHOULD ignore any picture timing SEI messages included
in access units that have only one display timestamp. Instead,
receivers SHOULD use the RTP timestamp for synchronizing the
display process.
RTP senders SHOULD NOT transmit picture timing SEI messages for
pictures that are not supposed to be displayed as multiple
fields.
If one access unit has more than one display timestamp carried in
a picture timing SEI message, then the information in the SEI
message SHOULD be treated as relative to the RTP timestamp, with
the earliest event occurring at the time given by the RTP
timestamp, and subsequent events later, as given by the
difference in SEI message picture timing values. Let tSEI1,
tSEI2, ..., tSEIn be the display timestamps carried in the SEI
message of an access unit, where tSEI1 is the earliest of all
such timestamps. Let tmadjst() be a function that adjusts the
SEI messages time scale to a 90-kHz time scale. Let TS be the
RTP timestamp. Then, the display time for the event associated
with tSEI1 is TS. The display time for the event with tSEIx,
where x is [2..n] is TS + tmadjst (tSEIx - tSEI1).
Informative note: Displaying coded frames as fields is needed
commonly in an operation known as 3:2 pulldown, in which film
content that consists of coded frames is displayed on a
display using interlaced scanning. The picture timing SEI
message enables carriage of multiple timestamps for the same
coded picture, and therefore the 3:2 pulldown process is
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perfectly controlled. The picture timing SEI message
mechanism is necessary because only one timestamp per coded
frame can be conveyed in the RTP timestamp.
Informative note: Because H.264 allows the decoding order to
be different from the display order, values of RTP timestamps
may not be monotonically non-decreasing as a function of RTP
sequence numbers. Furthermore, the value for interarrival
jitter reported in the RTCP reports may not be a trustworthy
indication of the network performance, as the calculation
rules for interarrival jitter (section 6.4.1 of RFC 3550)
assume that the RTP timestamp of a packet is directly
proportional to its transmission time.
5.2. Common Structure of the RTP Payload Format
The payload format defines three different basic payload structures.
A receiver can identify the payload structure by the first byte of
the RTP payload, which co-serves as the RTP payload header and, in
some cases, as the first byte of the payload. This byte is always
structured as a NAL unit header. The NAL unit type field indicates
which structure is present. The possible structures are as follows:
Single NAL Unit Packet: Contains only a single NAL unit in the
payload. The NAL header type field will be equal to the original NAL
unit type; i.e., in the range of 1 to 23, inclusive. Specified in
section 5.6.
Aggregation packet: Packet type used to aggregate multiple NAL units
into a single RTP payload. This packet exists in four versions, the
Single-Time Aggregation Packet type A (STAP-A), the Single-Time
Aggregation Packet type B (STAP-B), Multi-Time Aggregation Packet
(MTAP) with 16-bit offset (MTAP16), and Multi-Time Aggregation Packet
(MTAP) with 24-bit offset (MTAP24). The NAL unit type numbers
assigned for STAP-A, STAP-B, MTAP16, and MTAP24 are 24, 25, 26, and
27, respectively. Specified in section 5.7.
Fragmentation unit: Used to fragment a single NAL unit over multiple
RTP packets. Exists with two versions, FU-A and FU-B, identified
with the NAL unit type numbers 28 and 29, respectively. Specified in
section 5.8.
Informative note: This specification does not limit the size of
NAL units encapsulated in single NAL unit packets and
fragmentation units. The maximum size of a NAL unit encapsulated
in any aggregation packet is 65535 bytes.
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Table 1 summarizes NAL unit types and the corresponding RTP packet
types when each of these NAL units is directly used 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 undefined -
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 undefined -
5.3. NAL Unit Octet Usage
The structure and semantics of the NAL unit octet were introduced in
section 1.3. For convenience, the format of the NAL unit type octet
is reprinted below:
+---------------+
|0|1|2|3|4|5|6|7|
+-+-+-+-+-+-+-+-+
|F|NRI| Type |
+---------------+
This section specifies the semantics of F and NRI according to this
specification.
F: 1 bit
forbidden_zero_bit. A value of 0 indicates that the NAL unit
type octet and payload should not contain bit errors or other
syntax violations. A value of 1 indicates that the NAL unit type
octet and payload may contain bit errors or other syntax
violations.
MANEs SHOULD set the F bit to indicate detected bit errors in the
NAL unit. The H.264 specification requires that the F bit is
equal to 0. When the F bit is set, the decoder is advised that
bit errors or any other syntax violations may be present in the
payload or in the NAL unit type octet. The simplest decoder
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reaction to a NAL unit in which the F bit is equal to 1 is to
discard such a NAL unit and to conceal the lost data in the
discarded NAL unit.
NRI: 2 bits
nal_ref_idc. The semantics of value 00 and a non-zero value
remain unchanged from the H.264 specification. In other words, a
value of 00 indicates that the content of the NAL unit is not
used to reconstruct reference pictures for inter picture
prediction. Such NAL units can be discarded without risking the
integrity of the reference pictures. Values greater than 00
indicate that the decoding of the NAL unit is required to
maintain the integrity of the reference pictures.
In addition to the specification above, according to this RTP
payload specification, values of NRI indicate the relative
transport priority, as determined by the encoder. MANEs can use
this information to protect more important NAL units better than
they do less important NAL units. The highest transport priority
is 11, followed by 10, and then by 01; finally, 00 is the lowest.
Informative note: Any non-zero value of NRI is handled
identically in H.264 decoders. Therefore, receivers need not
manipulate the value of NRI when passing NAL units to the
decoder.
An H.264 encoder MUST set the value of NRI according to the H.264
specification (subclause 7.4.1) when the value of nal_unit_type
is in the range of 1 to 12, inclusive. In particular, the H.264
specification requires that the value of NRI SHALL be equal to 0
for all NAL units having nal_unit_type equal to 6, 9, 10, 11, or
12.
For NAL units having nal_unit_type equal to 7 or 8 (indicating a
sequence parameter set or a picture parameter set, respectively),
an H.264 encoder SHOULD set the value of NRI to 11 (in binary
format). For coded slice NAL units of a primary coded picture
having nal_unit_type equal to 5 (indicating a coded slice
belonging to an IDR picture), an H.264 encoder SHOULD set the
value of NRI to 11 (in binary format).
For a mapping of the remaining nal_unit_types to NRI values, the
following example MAY be used and has been shown to be efficient
in a certain environment [13]. Other mappings MAY also be
desirable, depending on the application and the H.264/AVC Annex A
profile in use.
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Informative note: Data Partitioning is not available in
certain profiles; e.g., in the Main or Baseline profiles.
Consequently, the NAL unit types 2, 3, and 4 can occur only if
the video bitstream conforms to a profile in which data
partitioning is allowed and not in streams that conform to the
Main or Baseline profiles.
Table 2. Example of NRI values for coded slices and coded slice data
partitions of primary coded reference pictures
NAL Unit Type Content of NAL unit NRI (binary)
----------------------------------------------------------------
1 non-IDR coded slice 10
2 Coded slice data partition A 10
3 Coded slice data partition B 01
4 Coded slice data partition C 01
Informative note: As mentioned before, the NRI value of non-
reference pictures is 00 as mandated by H.264/AVC.
An H.264 encoder SHOULD set the value of NRI for coded slice and
coded slice data partition NAL units of redundant coded reference
pictures equal to 01 (in binary format).
Definitions of the values for NRI for NAL unit types 24 to 29,
inclusive, are given in sections 5.7 and 5.8 of this memo.
No recommendation for the value of NRI is given for NAL units
having nal_unit_type in the range of 13 to 23, inclusive, because
these values are reserved for ITU-T and ISO/IEC. No
recommendation for the value of NRI is given for NAL units having
nal_unit_type equal to 0 or in the range of 30 to 31, inclusive,
as the semantics of these values are not specified in this memo.
5.4. Packetization Modes
This memo specifies three cases of packetization modes:
o Single NAL unit mode
o Non-interleaved mode
o Interleaved mode
The single NAL unit mode is targeted for conversational systems that
comply with ITU-T Recommendation H.241 [15] (see section 12.1). The
non-interleaved mode is targeted for conversational systems that may
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not comply with ITU-T Recommendation H.241. In the non-interleaved
mode, NAL units are transmitted in NAL unit decoding order. The
interleaved mode is targeted for systems that do not require very low
end-to-end latency. The interleaved mode allows transmission of NAL
units out of NAL unit decoding order.
The packetization mode in use MAY be signaled by the value of the
OPTIONAL packetization-mode MIME parameter. 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 undefined ig ig ig
1-23 NAL unit yes yes no
24 STAP-A no yes no
25 STAP-B no no yes
26 MTAP16 no no yes
27 MTAP24 no no yes
28 FU-A no yes yes
29 FU-B no no yes
30-31 undefined ig ig ig
Some UAL unit or payload type values (indicated as undefined in Table
3) are reserved for future extensions. NAL units of those types
SHOULD NOT be sent by a sender (direct as packet payloads, or as
aggregation units in aggregation packets, or as fragmented units in
FU packets) and MUST be ignored by a receiver. For example, the
payload types 1-23, with the associated packet type "NAL unit", are
allowed in "Single NAL Unit Mode" and in "Non-Interleaved Mode", but
disallowed in "Interleaved Mode". However, NAL units of NAL unit
types 1-23 can be used in "Interleaved Mode" as aggregation units in
STAP-B, MTAP16 and MTAP14 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.
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Decoding order number (DON) is a field in the payload structure or a
derived variable that indicates the NAL unit decoding order.
Rationale and examples of use cases for transmission out of decoding
order and for the use of DON are given in section 13.
The coupling of transmission and decoding order is controlled by the
OPTIONAL sprop-interleaving-depth MIME parameter as follows. When
the value of the OPTIONAL sprop-interleaving-depth MIME parameter is
equal to 0 (explicitly or per default), the transmission order of NAL
units MUST conform to the NAL unit decoding order. When the value of
the OPTIONAL sprop-interleaving-depth MIME parameter is greater than
0,
o the order of NAL units in an MTAP16 and an MTAP24 is NOT REQUIRED
to be the NAL unit decoding order, and
o the order of NAL units generated by decapsulating STAP-Bs, MTAPs,
and FUs in two consecutive packets is NOT REQUIRED to be the NAL
unit decoding order.
The RTP payload structures for a single NAL unit packet, an STAP-A,
and an FU-A do not include DON. STAP-B and FU-B structures include
DON, and the structure of MTAPs enables derivation of DON as
specified in section 5.7.2.
Informative note: When an FU-A occurs in interleaved mode, it
always follows an FU-B, which sets its DON.
Informative note: If a transmitter wants to encapsulate a single
NAL unit per packet and transmit packets out of their decoding
order, STAP-B packet type can be used.
In the single NAL unit packetization mode, the transmission order of
NAL units, determined by the RTP sequence number, MUST be the same as
their NAL unit decoding order. In the non-interleaved packetization
mode, the transmission order of NAL units in single NAL unit packets,
STAP-As, and FU-As MUST be the same as their NAL unit decoding order.
The NAL units within an STAP MUST appear in the NAL unit decoding
order. Thus, the decoding order is first provided through the
implicit order within a STAP, and second provided through the RTP
sequence number for the order between STAPs, FUs, and single NAL unit
packets.
Signaling of the value of DON for NAL units carried in STAP-B, MTAP,
and a series of fragmentation units starting with an FU-B is
specified in sections 5.7.1, 5.7.2, and 5.8, respectively. The DON
value of the first NAL unit in transmission order MAY be set to any
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value. Values of DON are in the range of 0 to 65535, inclusive.
After reaching the maximum value, the value of DON wraps around to 0.
The decoding order of two NAL units contained in any STAP-B, MTAP, or
a series of fragmentation units starting with an FU-B is determined
as follows. Let DON(i) be the decoding order number of the NAL unit
having index i in the transmission order. Function don_diff(m,n) is
specified as follows:
If DON(m) == DON(n), don_diff(m,n) = 0
If (DON(m) < DON(n) and DON(n) - DON(m) < 32768),
don_diff(m,n) = DON(n) - DON(m)
If (DON(m) > DON(n) and DON(m) - DON(n) >= 32768),
don_diff(m,n) = 65536 - DON(m) + DON(n)
If (DON(m) < DON(n) and DON(n) - DON(m) >= 32768),
don_diff(m,n) = - (DON(m) + 65536 - DON(n))
If (DON(m) > DON(n) and DON(m) - DON(n) < 32768),
don_diff(m,n) = - (DON(m) - DON(n))
A positive value of don_diff(m,n) indicates that the NAL unit having
transmission order index n follows, in decoding order, the NAL unit
having transmission order index m. When don_diff(m,n) is equal to 0,
then the NAL unit decoding order of the two NAL units can be in
either order. A negative value of don_diff(m,n) indicates that the
NAL unit having transmission order index n precedes, in decoding
order, the NAL unit having transmission order index m.
Values of DON related fields (DON, DONB, and DOND; see section 5.7)
MUST be such that the decoding order determined by the values of DON,
as specified above, conforms to the NAL unit decoding order. If the
order of two NAL units in NAL unit decoding order is switched and the
new order does not conform to the NAL unit decoding order, the NAL
units MUST NOT have the same value of DON. If the order of two
consecutive NAL units in the NAL unit stream is switched and the new
order still conforms to the NAL unit decoding order, the NAL units
MAY have the same value of DON. For example, when arbitrary slice
order is allowed by the video coding profile in use, all the coded
slice NAL units of a coded picture are allowed to have the same value
of DON. Consequently, NAL units having the same value of DON can be
decoded in any order, and two NAL units having a different value of
DON should be passed to the decoder in the order specified above.
When two consecutive NAL units in the NAL unit decoding order have a
different value of DON, the value of DON for the second NAL unit in
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decoding order SHOULD be the value of DON for the first, incremented
by one.
An example of the decapsulation process to recover the NAL unit
decoding order is given in section 7.
Informative note: Receivers should not expect that the absolute
difference of values of DON for two consecutive NAL units in the
NAL unit decoding order will be equal to one, even in error-free
transmission. An increment by one is not required, as at the
time of associating values of DON to NAL units, it may not be
known whether all NAL units are delivered to the receiver. For
example, a gateway may not forward coded slice NAL units of non-
reference pictures or SEI NAL units when there is a shortage of
bit rate in the network to which the packets are forwarded. In
another example, a live broadcast is interrupted by pre-encoded
content, such as commercials, from time to time. The first intra
picture of a pre-encoded clip is transmitted in advance to ensure
that it is readily available in the receiver. When transmitting
the first intra picture, the originator does not exactly know how
many NAL units will be encoded before the first intra picture of
the pre-encoded clip follows in decoding order. Thus, the values
of DON for the NAL units of the first intra picture of the pre-
encoded clip have to be estimated when they are transmitted, and
gaps in values of DON may occur.
5.6. Single NAL Unit Packet
The single NAL unit packet defined here MUST contain only one NAL
unit, of the types defined in [1]. This means that neither an
aggregation packet nor a fragmentation unit can be used within a
single NAL unit packet. A NAL unit stream composed by decapsulating
single NAL unit packets in RTP sequence number order MUST conform to
the NAL unit decoding order. The structure of the single NAL unit
packet is shown in Figure 2.
Informative note: The first byte of a NAL unit co-serves as the
RTP payload header.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|F|NRI| Type | |
+-+-+-+-+-+-+-+-+ |
| |
| Bytes 2..n of a Single NAL unit |
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :...OPTIONAL RTP padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2 RTP payload format for single NAL unit packet
5.7. Aggregation Packets
Aggregation packets are the NAL unit aggregation scheme of this
payload specification. The scheme is introduced to reflect the
dramatically different MTU sizes of two key target networks: wireline
IP networks (with an MTU size that is often limited by the Ethernet
MTU size; roughly 1500 bytes), and IP or non-IP (e.g., ITU-T H.324/M)
based wireless communication systems with preferred transmission unit
sizes of 254 bytes or less. To prevent media transcoding between the
two worlds, and to avoid undesirable packetization overhead, a NAL
unit aggregation scheme is introduced.
Two types of aggregation packets are defined by this specification:
o Single-time aggregation packet (STAP): aggregates NAL units with
identical NALU-time. Two types of STAPs are defined, one without
DON (STAP-A) and another including DON (STAP-B).
o Multi-time aggregation packet (MTAP): aggregates NAL units with
potentially differing NALU-time. Two different MTAPs are defined,
differing in the length of the NAL unit timestamp offset.
Each NAL unit to be carried in an aggregation packet is encapsulated
in an aggregation unit. Please see below for the four different
aggregation units and their characteristics.
The structure of the RTP payload format for aggregation packets is
presented in Figure 3.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|F|NRI| Type | |
+-+-+-+-+-+-+-+-+ |
| |
| one or more aggregation units |
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :...OPTIONAL RTP padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3 RTP payload format for aggregation packets
MTAPs and STAPs share the following packetization rules: The RTP
timestamp MUST be set to the earliest of the NALU-times of all the
NAL units to be aggregated. The type field of the NAL unit type
octet MUST be set to the appropriate value, as indicated in Table 4.
The F bit MUST be cleared if all F bits of the aggregated NAL units
are zero; otherwise, it MUST be set. The value of NRI MUST be the
maximum of all the NAL units carried in the aggregation packet.
Table 4. Type field for STAPs and MTAPs
Type Packet Timestamp offset DON related fields
field length (DON, DONB, DOND)
(in bits) present
--------------------------------------------------------
24 STAP-A 0 no
25 STAP-B 0 yes
26 MTAP16 16 yes
27 MTAP24 24 yes
The marker bit in the RTP header is set to the value that the marker
bit of the last NAL unit of the aggregated packet would have if it
were transported in its own RTP packet.
The payload of an aggregation packet consists of one or more
aggregation units. See sections 5.7.1 and 5.7.2 for the four
different types of aggregation units. An aggregation packet can
carry as many aggregation units as necessary; however, the total
amount of data in an aggregation packet obviously MUST fit into an IP
packet, and the size SHOULD be chosen so that the resulting IP packet
is smaller than the MTU size. An aggregation packet MUST NOT contain
fragmentation units specified in section 5.8. Aggregation packets
MUST NOT be nested; i.e., an aggregation packet MUST NOT contain
another aggregation packet.
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5.7.1. Single-Time Aggregation Packet
Single-time aggregation packet (STAP) SHOULD be used whenever NAL
units are aggregated that all share the same NALU-time. The payload
of an STAP-A does not include DON and consists of at least one
single-time aggregation unit, as presented in Figure 4. The payload
of an STAP-B consists of a 16-bit unsigned decoding order number
(DON) (in network byte order) followed by at least one single-time
aggregation unit, as presented in Figure 5.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: |
+-+-+-+-+-+-+-+-+ |
| |
| single-time aggregation units |
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4 Payload format for STAP-A
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: decoding order number (DON) | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
| single-time aggregation units |
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5 Payload format for STAP-B
The DON field specifies the value of DON for the first NAL unit in an
STAP-B in transmission order. For each successive NAL unit in
appearance order in an STAP-B, the value of DON is equal to (the
value of DON of the previous NAL unit in the STAP-B + 1) % 65536, in
which '%' stands for the modulo operation.
A single-time aggregation unit consists of 16-bit unsigned size
information (in network byte order) that indicates the size of the
following NAL unit in bytes (excluding these two octets, but
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including the NAL unit type octet of the NAL unit), followed by the
NAL unit itself, including its NAL unit type byte. A single-time
aggregation unit is byte aligned within the RTP payload, but it may
not be aligned on a 32-bit word boundary. Figure 6 presents the
structure of the single-time aggregation unit.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: NAL unit size | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
| NAL unit |
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6 Structure for single-time aggregation unit
Figure 7 presents an example of an RTP packet that contains an STAP-
A. The STAP contains two single-time aggregation units, labeled as 1
and 2 in the figure.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RTP Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|STAP-A NAL HDR | NALU 1 Size | NALU 1 HDR |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NALU 1 Data |
: :
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | NALU 2 Size | NALU 2 HDR |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NALU 2 Data |
: :
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :...OPTIONAL RTP padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7 An example of an RTP packet including an STAP-A containing
two single-time aggregation units
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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.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: decoding order number base | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
| multi-time aggregation units |
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 9 NAL unit payload format for MTAPs
Two different multi-time aggregation units are defined in this
specification. Both of them consist of 16 bits unsigned size
information of the following NAL unit (in network byte order), an 8-
bit unsigned decoding order number difference (DOND), and n bits (in
network byte order) of timestamp offset (TS offset) for this NAL
unit, whereby n can be 16 or 24. The choice between the different
MTAP types (MTAP16 and MTAP24) is application dependent: the larger
the timestamp offset is, the higher the flexibility of the MTAP, but
the overhead is also higher.
The structure of the multi-time aggregation units for MTAP16 and
MTAP24 are presented in Figures 10 and 11, respectively. The
starting or ending position of an aggregation unit within a packet is
NOT REQUIRED to be on a 32-bit word boundary. The DON of the NAL
unit contained in a multi-time aggregation unit is equal to (DONB +
DOND) % 65536, in which % denotes the modulo operation. This memo
does not specify how the NAL units within an MTAP are ordered, but,
in most cases, NAL unit decoding order SHOULD be used.
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).
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: NAL unit size | DOND | TS offset |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TS offset | |
+-+-+-+-+-+-+-+-+ NAL unit |
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 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.
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Figure 12 presents an example of an RTP packet that contains a multi-
time aggregation packet of type MTAP16 that contains two multi-time
aggregation units, labeled as 1 and 2 in the figure.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RTP Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|MTAP16 NAL HDR | decoding order number base | NALU 1 Size |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NALU 1 Size | NALU 1 DOND | NALU 1 TS offset |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NALU 1 HDR | NALU 1 DATA |
+-+-+-+-+-+-+-+-+ +
: :
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | NALU 2 SIZE | NALU 2 DOND |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NALU 2 TS offset | NALU 2 HDR | NALU 2 DATA |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
: :
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :...OPTIONAL RTP padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 12 An RTP packet including a multi-time aggregation packet of
type MTAP16 containing two multi-time aggregation units
Figure 13 presents an example of an RTP packet that contains a multi-
time aggregation packet of type MTAP24 that contains two multi-time
aggregation units, labeled as 1 and 2 in the figure.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RTP Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|MTAP24 NAL HDR | decoding order number base | NALU 1 Size |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NALU 1 Size | NALU 1 DOND | NALU 1 TS offs |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|NALU 1 TS offs | NALU 1 HDR | NALU 1 DATA |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
: :
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | NALU 2 SIZE | NALU 2 DOND |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NALU 2 TS offset | NALU 2 HDR |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NALU 2 DATA |
: :
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :...OPTIONAL RTP padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 13 An RTP packet including a multi-time aggregation packet of
type MTAP24 containing two multi-time aggregation units
5.7.3. Fragmentation Units (FUs)
This payload type allows fragmenting a NAL unit into several RTP
packets. Doing so on the application layer instead of relying on
lower layer fragmentation (e.g., by IP) has the following advantages:
o The payload format is capable of transporting NAL units bigger
than 64 kbytes over an IPv4 network that may be present in pre-
recorded video, particularly in High Definition formats (there is
a limit of the number of slices per picture, which results in a
limit of NAL units per picture, which may result in big NAL
units).
o The fragmentation mechanism allows fragmenting a single NAL unit
and applying generic forward error correction as described in
section 12.5.
Fragmentation is defined only for a single NAL unit and not for any
aggregation packets. A fragment of a NAL unit consists of an 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.
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Fragments of the same NAL unit MUST be sent in consecutive order with
ascending RTP sequence numbers (with no other RTP packets within the
same RTP packet stream being sent between the first and last
fragment). Similarly, a NAL unit MUST be reassembled in RTP sequence
number order.
When a NAL unit is fragmented and conveyed within fragmentation units
(FUs), it is referred to as a fragmented NAL unit. STAPs and MTAPs
MUST NOT be fragmented. FUs MUST NOT be nested; i.e., an FU MUST NOT
contain another FU.
The RTP timestamp of an RTP packet carrying an FU is set to the NALU-
time of the fragmented NAL unit.
Figure 14 presents the RTP payload format for FU-As. An FU-A
consists of a fragmentation unit indicator of one octet, a
fragmentation unit header of one octet, and a fragmentation unit
payload.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| FU indicator | FU header | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
| FU payload |
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :...OPTIONAL RTP padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 14 RTP payload format for FU-A
Figure 15 presents the RTP payload format for FU-Bs. An FU-B
consists of a fragmentation unit indicator of one octet, a
fragmentation unit header of one octet, a decoding order number (DON)
(in network byte order), and a fragmentation unit payload. In other
words, the structure of FU-B is the same as the structure of FU-A,
except for the additional DON field.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| FU indicator | FU header | DON |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
| |
| FU payload |
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :...OPTIONAL RTP padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 15 RTP payload format for FU-B
NAL unit type FU-B MUST be used in the interleaved packetization mode
for the first fragmentation unit of a fragmented NAL unit. NAL unit
type FU-B MUST NOT be used in any other case. In other words, in the
interleaved packetization mode, each NALU that is fragmented has an
FU-B as the first fragment, followed by one or more FU-A fragments.
The FU indicator octet has the following format:
+---------------+
|0|1|2|3|4|5|6|7|
+-+-+-+-+-+-+-+-+
|F|NRI| Type |
+---------------+
Values equal to 28 and 29 in the Type field of the FU indicator octet
identify an FU-A and an FU-B, respectively. The use of the F bit is
described in section 5.3. The value of the NRI field MUST be set
according to the value of the NRI field in the fragmented NAL unit.
The FU header has the following format:
+---------------+
|0|1|2|3|4|5|6|7|
+-+-+-+-+-+-+-+-+
|S|E|R| Type |
+---------------+
S: 1 bit
When set to one, the Start bit indicates the start of a
fragmented NAL unit. When the following FU payload is not the
start of a fragmented NAL unit payload, the Start bit is set to
zero.
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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; i.e., the
Start bit and End bit MUST NOT both be set to one in the same FU
header.
The FU payload consists of fragments of the payload of the fragmented
NAL unit so that if the fragmentation unit payloads of consecutive
FUs are sequentially concatenated, the payload of the fragmented NAL
unit can be reconstructed. The NAL unit type octet of the fragmented
NAL unit is not included as such in the fragmentation unit payload,
but rather the information of the NAL unit type octet of the
fragmented NAL unit is conveyed in F and NRI fields of the FU
indicator octet of the fragmentation unit and in the type field of
the FU header. An FU payload MAY have any number of octets and MAY
be empty.
Informative note: Empty FUs are allowed to reduce the latency of
a certain class of senders in nearly lossless environments.
These senders can be characterized in that they packetize NALU
fragments before the NALU is completely generated and, hence,
before the NALU size is known. If zero-length NALU fragments
were not allowed, the sender would have to generate at least one
bit of data of the following fragment before the current fragment
could be sent. Due to the characteristics of H.264, 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
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the increased risk of the loss of at least a part of the NALU
because of the additional packets employed for its transmission.
If a fragmentation unit is lost, the receiver SHOULD discard all
following fragmentation units in transmission order corresponding to
the same fragmented NAL unit.
A receiver in an endpoint or in a MANE MAY aggregate the first n-1
fragments of a NAL unit to an (incomplete) NAL unit, even if fragment
n of that NAL unit is not received. In this case, the
forbidden_zero_bit of the NAL unit MUST be set to one to indicate a
syntax violation.
6. Packetization Rules
The packetization modes are introduced in section 5.2. The
packetization rules common to more than one of the packetization
modes are specified in section 6.1. The packetization rules for the
single NAL unit mode, the non-interleaved mode, and the interleaved
mode are specified in sections 6.2, 6.3, and 6.4, respectively.
6.1. Common Packetization Rules
All senders MUST enforce the following packetization rules regardless
of the packetization mode in use:
o Coded slice NAL units or coded slice data partition NAL units
belonging to the same coded picture (and thus sharing the same RTP
timestamp value) MAY be sent in any order; however, for delay-
critical systems, they SHOULD be sent in their original decoding
order to minimize the delay. Note that the decoding order is the
order of the NAL units in the bitstream.
o Parameter sets are handled in accordance with the rules and
recommendations given in section 8.4.
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 MANEs MAY convert single NAL unit packets into one aggregation
packet, convert an aggregation packet into several single NAL unit
packets, or mix both concepts, in an RTP translator. The RTP
translator SHOULD take into account at least the following
parameters: path MTU size, unequal protection mechanisms (e.g.,
through packet-based FEC according to RFC 2733 [18], especially
for sequence and picture parameter set NAL units and coded slice
data partition A NAL units), bearable latency of the system, and
buffering capabilities of the receiver.
Informative note: An RTP translator is required to handle RTCP
as per RFC 3550.
6.2. Single NAL Unit Mode
This mode is in use when the value of the OPTIONAL packetization-mode
MIME parameter is equal to 0 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 [15] (see section 12.1). Only
single NAL unit packets MAY be used in this mode. STAPs, MTAPs, and
FUs MUST NOT be used. The transmission order of single NAL unit
packets MUST comply with the NAL unit decoding order.
6.3. Non-Interleaved Mode
This mode is in use when the value of the OPTIONAL packetization-mode
MIME parameter is equal to 1. This mode SHOULD be supported. It is
primarily intended for low-delay applications. Only single NAL unit
packets, STAP-As, and FU-As MAY be used in this mode. STAP-Bs,
MTAPs, and FU-Bs MUST NOT be used. The transmission order of NAL
units MUST comply with the NAL unit decoding order.
6.4. Interleaved Mode
This mode is in use when the value of the OPTIONAL packetization-mode
MIME parameter is equal to 2. 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 be used as well as long as the
output for the same input is the same as the process described below.
The output is the same meaning that the number of NAL units and their
order are both the 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 decapsulation
guidelines for intelligent receivers.
All normal RTP mechanisms related to buffer management apply. In
particular, duplicated or outdated RTP packets (as indicated by the
RTP sequences number and the RTP timestamp) are removed. To
determine the exact time for decoding, factors such as a possible
intentional delay to allow for proper inter-stream synchronization
must be factored in.
7.1. Single NAL Unit and Non-Interleaved Mode
The receiver includes a receiver buffer to compensate for
transmission delay jitter. The receiver stores incoming packets in
reception order into the receiver buffer. Packets are decapsulated
in RTP sequence number order. If a decapsulated packet is a single
NAL unit packet, the NAL unit contained in the packet is passed
directly to the decoder. If a decapsulated packet is an STAP-A, the
NAL units contained in the packet are passed to the decoder in the
order in which they are encapsulated in the packet. For all the FU-A
packets containing fragments of a single NAL unit, the decapsulated
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,
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the receiver operation is described under the assumption that there
is no transmission delay jitter. To make a difference from a
practical receiver buffer that is also used for compensation of
transmission delay jitter, the receiver buffer is here after called
the deinterleaving buffer in this section. Receivers SHOULD also
prepare for transmission delay jitter; i.e., either reserve separate
buffers for transmission delay jitter buffering and deinterleaving
buffering or use a receiver buffer for both transmission delay jitter
and deinterleaving. Moreover, receivers SHOULD take transmission
delay jitter into account in the buffering operation; e.g., by
additional initial buffering before starting of decoding and
playback.
This section is organized as follows: subsection 7.2.1 presents how o
calculate the size of the deinterleaving buffer. Subsection 7.2.2
specifies the receiver process how to organize received NAL units to
the NAL unit decoding order.
7.2.1. Size of the Deinterleaving Buffer
When SDP Offer/Answer model or any other capability exchange
procedure is used in session setup, the properties of the received
stream SHOULD be such that the receiver capabilities are not
exceeded. In the SDP Offer/Answer model, the receiver can indicate
its capabilities to allocate a deinterleaving buffer with the deint-
buf-cap MIME parameter. The sender indicates the requirement for the
deinterleaving buffer size with the sprop-deint-buf-req MIME
parameter. It is therefore RECOMMENDED to set the deinterleaving
buffer size, in terms of number of bytes, equal to or greater than
the value of sprop-deint-buf-req MIME parameter. See section 8.1 for
further information on deint-buf-cap and sprop-deint-buf-req MIME
parameters and section 8.2.2 for further information on their use in
SDP Offer/Answer model.
When a declarative session description is used in session setup, the
sprop-deint-buf-req MIME parameter signals the requirement for the
deinterleaving buffer size. It is therefore RECOMMENDED to set the
deinterleaving buffer size, in terms of number of bytes, equal to or
greater than the value of sprop-deint-buf-req MIME parameter.
7.2.2. Deinterleaving Process
There are two buffering states in the receiver: initial buffering and
buffering while playing. Initial buffering occurs when the RTP
session is initialized. After initial buffering, decoding and
playback are started, and the buffering-while-playing mode is used.
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Regardless of the buffering state, the receiver stores incoming NAL
units, in reception order, in the deinterleaving buffer as follows.
NAL units of aggregation packets are stored in the deinterleaving
buffer individually. The value of DON is calculated and stored for
each NAL unit.
The receiver operation is described below with the help of the
following functions and constants:
o Function AbsDON is specified in section 8.1.
o Function don_diff is specified in section 5.5.
o Constant N is the value of the OPTIONAL sprop-interleaving-depth
MIME type parameter (see section 8.1) incremented by 1.
Initial buffering lasts until one of the following conditions is
fulfilled:
o There are N or more VCL NAL units in the deinterleaving buffer.
o If sprop-max-don-diff is present, don_diff(m,n) is greater than
the value of sprop-max-don-diff, in which n corresponds to the NAL
unit having the greatest value of AbsDON among the received NAL
units and m corresponds to the NAL unit having the smallest value
of AbsDON among the received NAL units.
o Initial buffering has lasted for the duration equal to or greater
than the value of the OPTIONAL sprop-init-buf-time MIME parameter.
The NAL units to be removed from the deinterleaving buffer are
determined as follows:
o If the deinterleaving buffer contains at least N VCL NAL units,
NAL units are removed from the deinterleaving buffer and passed to
the decoder in the order specified below until the buffer contains
N-1 VCL NAL units.
o If sprop-max-don-diff is present, all NAL units m for which
don_diff(m,n) is greater than sprop-max-don-diff are removed from
the deinterleaving buffer and passed to the decoder in the order
specified below. Herein, n corresponds to the NAL unit having the
greatest value of AbsDON among the NAL units in the deinterleaving
buffer.
The order in which NAL units are passed to the decoder is specified
as follows:
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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 found, a
gateway may decide not to send the corresponding coded slice data
partitions B and C, as their information is meaningless for H.264
decoders. In this way a MANE can reduce network load by
discarding useless packets without parsing a complex bitstream.
o Intelligent RTP receivers (e.g., in gateways) may identify lost
FUs. If a lost FU is found, a gateway may decide not to send the
following FUs of the same fragmented NAL unit, as their
information is meaningless for H.264 decoders. In this way a MANE
can reduce network load by discarding useless packets without
parsing a complex bitstream.
o Intelligent receivers having to discard packets or NALUs should
first discard all packets/NALUs in which the value of the NRI
field of the NAL unit type octet is equal to 0. This will
minimize the impact on user experience and keep the reference
pictures intact. If more packets have to be discarded, then
packets with a numerically lower NRI value should be discarded
before packets with a numerically higher NRI value. However,
discarding any packets with an NRI bigger than 0 very likely leads
to decoder drift and SHOULD be avoided.
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8. Payload Format Parameters
This section specifies the parameters that MAY be used to select
optional features of the payload format and certain features of the
bitstream. The parameters are specified here as part of the MIME
subtype registration for the ITU-T H.264 | ISO/IEC 14496-10 codec. A
mapping of the parameters into the Session Description Protocol (SDP)
[5] is also provided for applications that use SDP. Equivalent
parameters could be defined elsewhere for use with control protocols
that do not use MIME or SDP.
Some parameters provide a receiver with the properties of the stream
that will be sent. The 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. The media sender selects all "sprop"
parameters rather than the receiver. This uncommon characteristic of
the "sprop" parameters may not be compatible with some signaling
protocol concepts, in which case the use of these parameters SHOULD
be avoided.
8.1. MIME Registration
The MIME subtype for the ITU-T H.264 | ISO/IEC 14496-10 codec is
allocated from the IETF tree.
The receiver MUST ignore any unspecified parameter.
Media Type name: video
Media subtype name: H264
Required parameters: none
OPTIONAL parameters:
profile-level-id:
A base16 [6] (hexadecimal) representation of the following
three bytes in the sequence parameter set NAL unit specified
in [1]: 1) profile_idc, 2) a byte herein referred to as
profile-iop, composed of the values of constraint_set0_flag,
constraint_set1_flag,constraint_set2_flag, and
reserved_zero_5bits in bit-significance order, starting from
the most significant bit, and 3) level_idc. Note that
reserved_zero_5bits is required to be equal to 0 in [1], but
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other values for it may be specified in the future by ITU-T or
ISO/IEC.
If the profile-level-id parameter is used to indicate
properties of a NAL unit stream, it indicates the profile and
level that a decoder has to support in order to comply with
[1] when it decodes the stream. The profile-iop byte
indicates whether the NAL unit stream also obeys all
constraints of the indicated profiles as follows. If bit 7
(the most significant bit), bit 6, or bit 5 of profile-iop is
equal to 1, all constraints of the Baseline profile, the Main
profile, or the Extended profile, respectively, are obeyed in
the NAL unit stream. If the profile-level-id parameter is
used for capability exchange or session setup procedure, it
indicates the profile that the codec supports and the highest
level supported for the signaled profile. The profile-iop
byte indicates whether the codec has additional limitations
whereby only the common subset of the algorithmic features and
limitations of the profiles signaled with the profile-iop byte
and of the profile indicated by profile_idc is supported by
the codec. For example, if a codec supports only the common
subset of the coding tools of the Baseline profile and the
Main profile at level 2.1 and below, the profile-level-id
becomes 42E015, in which 42 stands for the Baseline profile,
E0 indicates that only the common subset for all profiles is
supported, and 15 indicates level 2.1.
Informative note: Capability exchange and session setup
procedures should provide means to list the capabilities
for each supported codec profile separately. For example,
the one-of-N codec selection procedure of the SDP
Offer/Answer model can be used (section 10.2 of [7]).
If no profile-level-id is present, the Baseline Profile
without additional constraints at Level 1 MUST be implied.
max-mbps, max-fs, max-cpb, max-dpb, and max-br:
These parameters MAY be used to signal the capabilities of a
receiver implementation. These parameters MUST NOT be used for
any other purpose. The profile-level-id parameter MUST be
present in the same receiver capability description that
contains any of these parameters. The level conveyed in the
value of the profile-level-id parameter MUST be such that the
receiver is fully capable of supporting. max-mbps, max-fs,
max-cpb, max-dpb, and max-br MAY be used to indicate
capabilities of the receiver that extend the required
capabilities of the signaled level, as specified below.
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When more than one parameter from the set (max-mbps, max-fs,
max-cpb, max-dpb, max-br) is present, the receiver MUST
support all signaled capabilities simultaneously. For
example, if both max-mbps and max-br are present, the signaled
level with the extension of both the frame rate and bit rate
is supported. That is, the receiver is able to decode NAL
unit streams in which the macroblock processing rate is up to
max-mbps (inclusive), the bit rate is up to max-br
(inclusive), the coded picture buffer size is derived as
specified in the semantics of the max-br parameter below, and
other properties comply with the level specified in the value
of the profile-level-id parameter.
If a receiver can support all the properties of level A, the
level specified in the value of the profile-level-id MUST be
level A (i.e. MUST NOT be lower than level A). In other
words, a sender or receiver MUST NOT signal values of max-
mbps, max-fs, max-cpb, max-dpb, and max-br that meet the
requirements of a higher level compared to the level specified
in the value of the profile-level-id parameter.
Informative note: When the OPTIONAL MIME type parameters
are used to signal the properties of a NAL unit stream,
max-mbps, max-fs, max-cpb, max-dpb, and max-br are not
present, and the value of profile-level-id must always be
such that the NAL unit stream complies fully with the
specified profile and level.
max-mbps: The value of max-mbps is an integer indicating the
maximum macroblock processing rate in units of macroblocks per
second. The max-mbps parameter signals that the receiver is
capable of decoding video at a higher rate than is required by
the signaled level conveyed in the value of the profile-level-
id parameter. When max-mbps is signaled, the receiver MUST be
able to decode NAL unit streams that conform to the signaled
level, with the exception that the MaxMBPS value in Table A-1
of [1] for the signaled level is replaced with the value of
max-mbps. The value of max-mbps MUST be greater than or equal
to the value of MaxMBPS for the level given in Table A-1 of
[1]. Senders MAY use this knowledge to send pictures of a
given size at a higher picture rate than is indicated in the
signaled level.
max-fs: The value of max-fs is an integer indicating the maximum
frame size in units of macroblocks. The max-fs parameter
signals that the receiver is capable of decoding larger
picture sizes than are required by the signaled level conveyed
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in the value of the profile-level-id parameter. When max-fs
is signaled, the receiver MUST be able to decode NAL unit
streams that conform to the signaled level, with the exception
that the MaxFS value in Table A-1 of [1] for the signaled
level is replaced with the value of max-fs. The value of max-
fs MUST be greater than or equal to the value of MaxFS for the
level given in Table A-1 of [1]. Senders MAY use this
knowledge to send larger pictures at a proportionally lower
frame rate than is indicated in the signaled level.
max-cpb: The value of max-cpb is an integer indicating the
maximum coded picture buffer size in units of 1000 bits for
the VCL HRD parameters (see A.3.1 item i of [1]) and in units
of 1200 bits for the NAL HRD parameters (see A.3.1 item j of
[1]). The max-cpb parameter signals that the receiver has
more memory than the minimum amount of coded picture buffer
memory required by the signaled level conveyed in the value of
the profile-level-id parameter. When max-cpb is signaled, the
receiver MUST be able to decode NAL unit streams that conform
to the signaled level, with the exception that the MaxCPB
value in Table A-1 of [1] for the signaled level is replaced
with the value of max-cpb. The value of max-cpb MUST be
greater than or equal to the value of MaxCPB for the level
given in Table A-1 of [1]. Senders MAY use this knowledge to
construct coded video streams with greater variation of bit
rate than can be achieved with the MaxCPB value in Table A-1
of [1].
Informative note: The coded picture buffer is used in the
hypothetical reference decoder (Annex C) of H.264. The use
of the hypothetical reference decoder is recommended in
H.264 encoders to verify that the produced bitstream
conforms to the standard and to control the output bitrate.
Thus, the coded picture buffer is conceptually independent
of any other potential buffers in the receiver, including
de-interleaving and de-jitter buffers. The coded picture
buffer need not be implemented in decoders as specified in
Annex C of H.264, but rather standard-compliant decoders
can have any buffering arrangements provided that they can
decode standard-compliant bitstreams. Thus, in practice,
the input buffer for video decoder can be integrated with
de-interleaving and de-jitter buffers of the receiver.
max-dpb: The value of max-dpb is an integer indicating the
maximum decoded picture buffer size in units of 1024 bytes.
The max-dpb parameter signals that the receiver has more
memory than the minimum amount of decoded picture buffer
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memory required by the signaled level conveyed in the value of
the profile-level-id parameter. When max-dpb is signaled, the
receiver MUST be able to decode NAL unit streams that conform
to the signaled level, with the exception that the MaxDPB
value in Table A-1 of [1] for the signaled level is replaced
with the value of max-dpb. Consequently, a receiver that
signals max-dpb MUST be capable of storing the following
number of decoded frames, complementary field pairs, and non-
paired fields in its decoded picture buffer:
Min(1024 * max-dpb / ( PicWidthInMbs * FrameHeightInMbs *
256 * ChromaFormatFactor ), 16)
PicWidthInMbs, FrameHeightInMbs, and ChromaFormatFactor are
defined in [1].
The value of max-dpb MUST be greater than or equal to the
value of MaxDPB for the level given in Table A-1 of [1].
Senders MAY use this knowledge to construct coded video
streams with improved compression.
Informative note: This parameter was added primarily to
complement a similar codepoint in the ITU-T Recommendation
H.245, so as to facilitate signaling gateway designs. The
decoded picture buffer stores reconstructed samples. There
is no relationship between the size of the decoded picture
buffer and the buffers used in RTP, especially de-
interleaving and de-jitter buffers.
max-br: The value of max-br is an integer indicating the maximum
video bit rate in units of 1000 bits per second for the VCL
HRD parameters (see A.3.1 item i of [1]) and in units of 1200
bits per second for the NAL HRD parameters (see A.3.1 item j
of [1]).
The max-br parameter signals that the video decoder of the
receiver is capable of decoding video at a higher bit rate
than is required by the signaled level conveyed in the value
of the profile-level-id parameter.
When max-br is signaled, the video codec of the receiver MUST
be able to decode NAL unit streams that conform to the
signaled level, conveyed in the profile-level-id parameter,
with the following exceptions in the limits specified by the
level:
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o The value of max-br replaces the MaxBR value of the signaled
level (in Table A-1 of [1]).
o When the max-cpb parameter is not present, the result of the
following formula replaces the value of MaxCPB in Table A-1
of [1]: (MaxCPB of the signaled level) * max-br / (MaxBR of
the signaled level).
For example, if a receiver signals capability for Level 1.2
with max-br equal to 1550, this indicates a maximum video
bitrate of 1550 kbits/sec for VCL HRD parameters, a maximum
video bitrate of 1860 kbits/sec for NAL HRD parameters, and a
CPB size of 4036458 bits (1550000 / 384000 * 1000 * 1000).
The value of max-br MUST be greater than or equal to the value
MaxBR for the signaled level given in Table A-1 of [1].
Senders MAY use this knowledge to send higher bitrate video as
allowed in the level definition of Annex A of H.264, to
achieve improved video quality.
Informative note: This parameter was added primarily to
complement a similar codepoint in the ITU-T Recommendation
H.245, so as to facilitate signaling gateway designs. No
assumption can be made from the value of this parameter
that the network is capable of handling such bit rates at
any given time. In particular, no conclusion can be drawn
that the signaled bit rate is possible under congestion
control constraints.
redundant-pic-cap:
This parameter signals the capabilities of a receiver
implementation. When equal to 0, the parameter indicates that
the receiver makes no attempt to use redundant coded pictures
to correct incorrectly decoded primary coded pictures. When
equal to 0, the receiver is not capable of using redundant
slices; therefore, a sender SHOULD avoid sending redundant
slices to save bandwidth. When equal to 1, the receiver is
capable of decoding any such redundant slice that covers a
corrupted area in a primary decoded picture (at least partly),
and therefore a sender MAY send redundant slices. When the
parameter is not present, then a value of 0 MUST be used for
redundant-pic-cap. When present, the value of redundant-pic-
cap MUST be either 0 or 1.
When the profile-level-id parameter is present in the same
capability signaling as the redundant-pic-cap parameter, and
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the profile indicated in profile-level-id is such that it
disallows the use of redundant coded pictures (e.g., Main
Profile), the value of redundant-pic-cap MUST be equal to 0.
When a receiver indicates redundant-pic-cap equal to 0, the
received stream SHOULD NOT contain redundant coded pictures.
Informative note: Even if redundant-pic-cap is equal to 0,
the decoder is able to ignore redundant codec pictures
provided that the decoder supports such a profile
(Baseline, Extended) in which redundant coded pictures are
allowed.
Informative note: Even if redundant-pic-cap is equal to 1,
the receiver may also choose other error concealment
strategies to replace or complement decoding of redundant
slices.
sprop-parameter-sets:
This parameter MAY be used to convey any sequence and picture
parameter set NAL units (herein referred to as the initial
parameter set NAL units) that MUST precede any other NAL units
in decoding order. The parameter MUST NOT be used to indicate
codec capability in any capability exchange procedure. The
value of the parameter is the base64 [6] representation of the
initial parameter set NAL units as specified in sections
7.3.2.1 and 7.3.2.2 of [1]. The parameter sets are conveyed
in decoding order, and no framing of the parameter set NAL
units takes place. A comma is used to separate any pair of
parameter sets in the list. Note that the number of bytes in
a parameter set NAL unit is typically less than 10, but a
picture parameter set NAL unit can contain several hundreds of
bytes.
Informative note: When several payload types are offered in
the SDP Offer/Answer model, each with its own sprop-
parameter-sets parameter, then the receiver cannot assume
that those parameter sets do not use conflicting storage
locations (i.e., identical values of parameter set
identifiers). Therefore, a receiver should double-buffer
all sprop-parameter-sets and make them available to the
decoder instance that decodes a certain payload type.
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
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declared, multiple configuration points (RTP payload types)
must be used.
When the value of packetization-mode is equal to 0 or
packetization-mode is not present, the single NAL mode, as
defined in section 6.2 of RFC 3984, MUST be used. This mode
is in use in standards using ITU-T Recommendation H.241 [15]
(see section 12.1). When the value of packetization-mode is
equal to 1, the non-interleaved mode, as defined in section
6.3 of RFC 3984, MUST be used. When the value of
packetization-mode is equal to 2, the interleaved mode, as
defined in section 6.4 of RFC 3984, MUST be used. The value
of packetization-mode MUST be an integer in the range of 0 to
2, inclusive.
sprop-interleaving-depth:
This parameter MUST NOT be present when packetization-mode is
not present or the value of packetization-mode is equal to 0
or 1. This parameter MUST be present when the value of
packetization-mode is equal to 2.
This parameter signals the properties of an RTP packet stream.
It specifies the maximum number of VCL NAL units that precede
any VCL NAL unit in the RTP packet stream in transmission
order and follow the VCL NAL unit in decoding order.
Consequently, it is guaranteed that receivers can reconstruct
NAL unit decoding order when the buffer size for NAL unit
decoding order recovery is at least the value of sprop-
interleaving-depth + 1 in terms of VCL NAL units.
The value of sprop-interleaving-depth MUST be an integer in
the range of 0 to 32767, inclusive.
sprop-deint-buf-req:
This parameter MUST NOT be present when packetization-mode is
not present or the value of packetization-mode is equal to 0
or 1. It MUST be present when the value of packetization-mode
is equal to 2.
sprop-deint-buf-req signals the required size of the
deinterleaving 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
deinterleaving buffer that is specified in section 7.2 of RFC
3984. It is guaranteed that receivers can perform the
deinterleaving of interleaved NAL units into NAL unit decoding
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order, when the deinterleaving buffer size is at least the
value of sprop-deint-buf-req in terms of bytes.
The value of sprop-deint-buf-req MUST be an integer in the
range of 0 to 4294967295, inclusive.
Informative note: sprop-deint-buf-req indicates the
required size of the deinterleaving buffer only. When
network jitter can occur, an appropriately sized jitter
buffer has to be provisioned for as well.
deint-buf-cap:
This parameter signals the capabilities of a receiver
implementation and indicates the amount of deinterleaving
buffer space in units of bytes that the receiver has available
for reconstructing the NAL unit decoding order. A receiver is
able to handle any stream for which the value of the sprop-
deint-buf-req parameter is smaller than or equal to this
parameter.
If the parameter is not present, then a value of 0 MUST be
used for deint-buf-cap. The value of deint-buf-cap MUST be an
integer in the range of 0 to 4294967295, inclusive.
Informative note: deint-buf-cap indicates the maximum
possible size of the deinterleaving buffer of the receiver
only. When network jitter can occur, an appropriately
sized jitter buffer has to be provisioned for as well.
sprop-init-buf-time:
This parameter MAY be used to signal the properties of an RTP
packet stream. The parameter MUST NOT be present, if the
value of packetization-mode is equal to 0 or 1.
The parameter signals the initial buffering time that a
receiver MUST wait before starting decoding to recover the NAL
unit decoding order from the transmission order. The
parameter is the maximum value of (decoding time of the NAL
unit - transmission time of a NAL unit), assuming reliable and
instantaneous transmission, the same timeline for transmission
and decoding, and that decoding starts when the first packet
arrives.
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
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decoding time and the transmission order is from left to
right:
0 2 1 3 5 4 6 8 7 ...
Assuming a steady transmission rate of NAL units, the
transmission times are:
0 1 2 3 4 5 6 7 8 ...
Subtracting the decoding time from the transmission time
column-wise results in the following series:
0 -1 1 0 -1 1 0 -1 1 ...
Thus, in terms of intervals of NAL unit transmission times,
the value of sprop-init-buf-time in this example is 1. The
parameter is coded as a non-negative base10 integer
representation in clock ticks of a 90-kHz clock. If the
parameter is not present, then no initial buffering time value
is defined. Otherwise the value of sprop-init-buf-time MUST
be an integer in the range of 0 to 4294967295, inclusive.
In addition to the signaled sprop-init-buf-time, receivers
SHOULD take into account the transmission delay jitter
buffering, including buffering for the delay jitter caused by
mixers, translators, gateways, proxies, traffic-shapers, and
other network elements.
sprop-max-don-diff:
This parameter MAY be used to signal the properties of an RTP
packet stream. It MUST NOT be used to signal transmitter or
receiver or codec capabilities. The parameter MUST NOT be
present if the value of packetization-mode is equal to 0 or 1.
sprop-max-don-diff is an integer in the range of 0 to 32767,
inclusive. If sprop-max-don-diff is not present, the value of
the parameter is unspecified. sprop-max-don-diff is
calculated as follows:
sprop-max-don-diff = max{AbsDON(i) - AbsDON(j)},
for any i and any j>i,
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
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first NAL unit in transmission order (whose index is 0),
AbsDON(0) = DON(0). For other NAL units, AbsDON is calculated
as follows:
If DON(m) == DON(n), AbsDON(n) = AbsDON(m)
If (DON(m) < DON(n) and DON(n) - DON(m) < 32768),
AbsDON(n) = AbsDON(m) + DON(n) - DON(m)
If (DON(m) > DON(n) and DON(m) - DON(n) >= 32768),
AbsDON(n) = AbsDON(m) + 65536 - DON(m) + DON(n)
If (DON(m) < DON(n) and DON(n) - DON(m) >= 32768),
AbsDON(n) = AbsDON(m) - (DON(m) + 65536 - DON(n))
If (DON(m) > DON(n) and DON(m) - DON(n) < 32768),
AbsDON(n) = AbsDON(m) - (DON(m) - DON(n))
where DON(i) is the decoding order number of the NAL unit
having index i in the transmission order. The decoding order
number is specified in section 5.5 of RFC 3984.
Informative note: Receivers may use sprop-max-don-diff to
trigger which NAL units in the receiver buffer can be
passed to the decoder.
max-rcmd-nalu-size:
This parameter MAY be used to signal the capabilities of a
receiver. The parameter MUST NOT be used for any other
purposes. The value of the parameter indicates the largest
NALU size in bytes that the receiver can handle efficiently.
The parameter value is a recommendation, not a strict upper
boundary. The sender MAY create larger NALUs but must be
aware that the handling of these may come at a higher cost
than NALUs conforming to the limitation.
The value of max-rcmd-nalu-size MUST be an integer in the
range of 0 to 4294967295, inclusive. If this parameter is not
specified, no known limitation to the NALU size exists.
Senders still have to consider the MTU size available between
the sender and the receiver and SHOULD run MTU discovery for
this purpose.
This parameter is motivated by, for example, an IP to H.223
video telephony gateway, where NALUs smaller than the H.223
transport data unit will be more efficient. A gateway may
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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.
spatial-resolution:
This parameter MAY be used to indicate the maximum spatial
resolution of a NAL unit stream or the preferred spatial
resolution of a receiver. The value is a base16 [6]
(hexadecimal) representation of the width and height of the
spatial resolution, in pixels, separated by a comma.
Encoding considerations:
This type is only defined for transfer via RTP (RFC 3550).
Security considerations:
See section 9 of RFC xxxx.
Public specification:
Please refer to RFC xxxx and its section 15.
Additional information:
None
File extensions: none
Macintosh file type code: none
Object identifier or OID: none
Person & email address to contact for further information:
Ye-Kui Wang, ye-kui.wang@nokia.com
Intended usage: COMMON
Author:
Ye-Kui Wang, ye-kui.wang@nokia.com
Change controller:
IETF Audio/Video Transport working group delegated from the
IESG.
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8.2. SDP Parameters
8.2.1. Mapping of MIME Parameters to SDP
The MIME media type video/H264 string is mapped to fields in the
Session Description Protocol (SDP) [5] as follows:
o The media name in the "m=" line of SDP MUST be video.
o The encoding name in the "a=rtpmap" line of SDP MUST be H264 (the
MIME subtype).
o The clock rate in the "a=rtpmap" line MUST be 90000.
o The OPTIONAL parameters "profile-level-id", "max-mbps", "max-fs",
"max-cpb", "max-dpb", "max-br", "redundant-pic-cap", "sprop-
parameter-sets", "packetization-mode", "sprop-interleaving-depth",
"deint-buf-cap", "sprop-deint-buf-req", "sprop-init-buf-time",
"sprop-max-don-diff", "max-rcmd-nalu-size", and "spatial-
resolution", when present, MUST be included in the "a=fmtp" line
of SDP. These parameters are expressed as a MIME media type
string, in the form of a semicolon separated list of
parameter=value pairs.
An example of media representation in SDP is as follows (Baseline
Profile, Level 3.0, some of the constraints of the Main profile may
not be obeyed):
m=video 49170 RTP/AVP 98
a=rtpmap:98 H264/90000
a=fmtp:98 profile-level-id=42A01E;
packetization-mode=1;
spatial-resolution=704,576;
sprop-parameter-sets=<base64 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 [7]
for negotiation for unicast usage, the following limitations and
rules apply:
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o The parameters identifying a media format configuration for H.264
are "profile-level-id", "packetization-mode", and, if required by
"packetization-mode", "sprop-deint-buf-req". These media format
configuration parameters (except for the level part of "profile-
level-id") MUST be used symmetrically; i.e., the answerer MUST
either maintain all configuration parameters or remove the media
format (payload type) completely, if one or more of the parameter
values are not supported. For the level part of "profile-level-
id", the answerer MUST maintain the same or a lower level or
remove the media format (payload type) completely.
Informative note: The requirement for symmetric use applies
only for the above media format configuration parameters and
not for the other stream properties and capability parameters,
including the level part of "profile-level-id".
Informative note: In H.264, all the levels except for level 1b
are equal to the value of level_idc divided by 10. Level 1b
is a level higher than level 1.0 but lower than level 1.1, and
is signaled in an ad-hoc manner, due to that the level was
specified after level 1.0 and level 1.1. For the Baseline,
Main and Extended profiles (with profile_idc equal to 66, 77
and 88, respectively), level 1b is indicated by level_idc
equal to 11 (i.e. same as level 1.1) and 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).
To simplify handling and matching of these configurations, the
same RTP payload type number used in the offer SHOULD also be
used in the answer, as specified in [7]. An answer MUST NOT
contain a payload type number used in the offer unless the
configuration ("profile-level-id", "packetization-mode", and, if
present, "sprop-deint-buf-req") is the same as in the offer or
the configuration only differs from that in the offer with a
lower level indicated by "profile-level-id".
Informative note: An offerer, when receiving the answer, has
to compare payload types not declared in the offer based on
media type (i.e., video/h264) and the above media format
configuration parameters with any payload types it has already
declared, in order to determine whether the configuration in
question is new or equivalent to a configuration already
offered.
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o The parameters "packetization-mode", and if present, "sprop-deint-
buf-req", "sprop-parameter-sets", "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 this media format configuration. This differs from
the normal usage of the Offer/Answer parameters: normally such
parameters declare the properties of the stream that the offerer
or the answerer is able to receive. When dealing with H.264, the
offerer assumes that the answerer will be able to receive media
encoded using the configuration being offered.
Informative note: The above parameters apply for any stream
sent by the declaring entity with the same configuration;
i.e., they are dependent on their source. Rather than being
bound to the payload type, the values may have to be applied
to another payload type when being sent, as they apply for the
configuration.
o The capability parameters ("max-mbps", "max-fs", "max-cpb", "max-
dpb", "max-br", ,"redundant-pic-cap", "max-rcmd-nalu-size") MAY be
used to declare further capabilities. These parameters can only
be present when the direction attribute is sendrecv or recvonly,
and the parameters describe the limitations of what the receiver
accepts.
o As specified above, an offerer has to include the size of the
deinterleaving buffer in the offer for an interleaved H.264
stream. To enable the offerer and answerer to inform each other
about their capabilities for deinterleaving buffering, both
parties are RECOMMENDED to include "deint-buf-cap". This
information MAY be used when the value for "sprop-deint-buf-req"
is selected in a second round of offer and answer. 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.
For streams being delivered over multicast, the following rules
apply:
o The media format configuration is identified by the same
parameters as above for unicast (i.e. "profile-level-id",
"packetization-mode", and, if required by "packetization-mode",
"sprop-deint-buf-req"). These media format configuration
parameters (including the level part of "profile-level-id") MUST
be used symmetrically; i.e., the answerer MUST either maintain all
configuration parameters or remove the media format (payload type)
completely.
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o The stream properties parameters ("sprop-parameter-sets", "sprop-
interleaving-depth", "sprop-max-don-diff", and "sprop-init-buf-
time") MUST NOT be changed by the answerer. Thus, a payload type
can either be accepted unaltered or removed.
o The receiver capability parameters "max-mbps", "max-fs", "max-
cpb", "max-dpb", "max-br", and "max-rcmd-nalu-size" MUST be
supported by the answerer for all streams declared as sendrecv or
recvonly; otherwise, one of the following actions MUST be
performed: the media format is removed, or the session rejected.
o The receiver capability parameter redundant-pic-cap SHOULD be
supported by the answerer for all streams declared as sendrecv or
recvonly as follows: The answerer SHOULD NOT include redundant
coded pictures in the transmitted stream if the offerer indicated
redundant-pic-cap equal to 0. Otherwise (when redundant_pic_cap
is equal to 1), it is beyond the scope of this memo to recommend
how the answerer should use redundant coded pictures.
Below are the complete lists of how the different parameters shall be
interpreted in the different combinations of offer or answer and
direction attribute.
o In offers and answers for which "a=sendrecv" or no direction
attribute is used, or in offers and answers for which "a=recvonly"
is used, the following interpretation of the parameters MUST be
used.
Declaring actual configuration or properties for receiving:
- profile-level-id
- packetization-mode
Declaring actual properties of the stream to be sent (applicable
only when "a=sendrecv" or no direction attribute is used):
- sprop-deint-buf-req
- sprop-interleaving-depth
- sprop-parameter-sets
- sprop-max-don-diff
- sprop-init-buf-time
- spatial-resolution
Informative note: When "a=sendrecv" or no direction attribute
is used, "spatial-resolution" indicates both the maximum
spatial resolution of the stream sent by the declaring entity
and the preferred spatial resolution for receiving a stream.
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Declaring receiver capabilities:
- max-mbps
- max-fs
- max-cpb
- max-dpb
- max-br
- redundant-pic-cap
- deint-buf-cap
- max-rcmd-nalu-size
Declaring receiver preference:
- spatial-resolution
o In an offer or answer for which the direction attribute
"a=sendonly" is included for the media stream, the following
interpretation of the parameters MUST be used:
Declaring actual configuration and properties of stream proposed
to be sent:
- profile-level-id
- packetization-mode
- sprop-deint-buf-req
- sprop-max-don-diff
- sprop-init-buf-time
- sprop-parameter-sets
- sprop-interleaving-depth
- spatial-resolution
Furthermore, the following considerations are necessary:
o Parameters used for declaring receiver capabilities are in general
downgradable; i.e., they express the upper limit for a sender's
possible behavior. Thus a sender MAY select to set its encoder
using only lower/less or equal values of these parameters.
"sprop-parameter-sets" MUST NOT be used in a sender's declaration
of its capabilities, as the limits of the values that are carried
inside the parameter sets are implicit with the profile and level
used.
o Parameters declaring a configuration point are not downgradable,
with the exception of the level part of the "profile-level-id"
parameter. This expresses values a receiver expects to be used
and must be used verbatim on the sender side.
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o When a sender's capabilities are declared, and non-downgradable
parameters are used in this declaration, then these parameters
express a configuration that is acceptable. 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.
o A receiver SHOULD understand all MIME 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.
o 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
properties 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.
o If an offerer wishes to have non-symmetric capabilities between
sending and receiving, the offerer has to offer different RTP
sessions; i.e., different media lines declared as "recvonly" and
"sendonly", respectively. This may have further implications on
the system.
8.2.3. Usage in Declarative Session Descriptions
When H.264 over RTP is offered with SDP in a declarative style, as in
RTSP [27] or SAP [28], the following considerations are necessary.
o All parameters capable of indicating the properties of both an RTP
packet stream and a receiver are used to indicate the properties
of an RTP packet stream. For example, in this case, the parameter
"profile-level-id" declares the values used by the stream, instead
of the capabilities of the sender. This results in that the
following interpretation of the parameters MUST be used:
Declaring actual configuration or stream properties:
- profile-level-id
- sprop-parameter-sets
- packetization-mode
- sprop-interleaving-depth
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- sprop-deint-buf-req
- sprop-max-don-diff
- sprop-init-buf-time
Not usable:
- max-mbps
- max-fs
- max-cpb
- max-dpb
- max-br
- redundant-pic-cap
- max-rcmd-nalu-size
- deint-buf-cap
o A receiver of the SDP is required to support all parameters and
values of the parameters provided; otherwise, the receiver MUST
reject (RTSP) or not participate in (SAP) the session. It falls
on the creator of the session to use values that are expected to
be supported by the receiving application.
8.3. Examples
An SDP Offer/Answer exchange wherein both parties are expected to
both send and receive could look like the following. Only the media
codec specific parts of the SDP are shown. Some lines are wrapped
due to text constraints.
Offerer -> Answerer SDP message:
m=video 49170 RTP/AVP 100 99 98
a=rtpmap:98 H264/90000
a=fmtp:98 profile-level-id=42A01E; packetization-mode=0;
sprop-parameter-sets=<base64 data#0>
a=rtpmap:99 H264/90000
a=fmtp:99 profile-level-id=42A01E; packetization-mode=1;
sprop-parameter-sets=<base64 data#1>
a=rtpmap:100 H264/90000
a=fmtp:100 profile-level-id=42A01E; packetization-mode=2;
sprop-parameter-sets=<base64 data#2>;
sprop-interleaving-depth=45; sprop-deint-buf-req=64000;
sprop-init-buf-time=102478; deint-buf-cap=128000
The above offer presents the same codec configuration in three
different packetization formats. PT 98 represents single NALU mode,
PT 99 represents non-interleaved mode, and PT 100 indicates the
interleaved mode. In the interleaved mode case, the interleaving
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parameters that the offerer would use if the answer indicates support
for PT 100 are also included. In all three cases the parameter
"sprop-parameter-sets" conveys the initial parameter sets that are
required by the answerer when receiving a stream from the offerer
when this configuration (profile-level-id and packetization mode) is
accepted. Note that the value for "sprop-parameter-sets" could be
different for each payload type.
Answerer -> Offerer SDP message:
m=video 49170 RTP/AVP 100 99 97
a=rtpmap:97 H264/90000
a=fmtp:97 profile-level-id=42A01E; packetization-mode=0;
sprop-parameter-sets=<base64 data#3>
a=rtpmap:99 H264/90000
a=fmtp:99 profile-level-id=42A01E; packetization-mode=1;
sprop-parameter-sets=<base64 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=<base64 data#5>;
sprop-interleaving-depth=60;
sprop-deint-buf-req=86000; sprop-init-buf-time=156320;
deint-buf-cap=128000; max-rcmd-nalu-size=3980
As the Offer/Answer negotiation covers both sending and receiving
streams, an offer indicates the exact parameters for what the offerer
is willing to receive, whereas the answer indicates the same for what
the answerer accepts to receive. In this case the offerer declared
that it is willing to receive payload type 98. The answerer accepts
this by declaring an equivalent payload type 97; i.e., it has
identical values for the two parameters "profile-level-id" and
"packetization-mode" (since "packetization-mode" is equal to 0,
"sprop-deint-buf-req" is not present). As the offered payload type
98 is accepted, the answerer needs to store parameter sets included
in sprop-parameter-sets=<base64 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=<base64 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=<base64
data#1> and sprop-parameter-sets=<base64 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-
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offerer direction, i.e. the parameter sets in sprop-parameter-
sets=<base64 data#4> and sprop-parameter-sets=<base64 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 deinterleaving operations by providing a "deint-buf-
cap" parameter. This is only useful if the offerer decides on making
a second offer, where it can take the new value into account. The
"max-rcmd-nalu-size" indicates that the answerer can efficiently
process NALUs up to the size of 3980 bytes. However, there is no
guarantee that the network supports this size.
Below are some more examples.
In the following example, the original offer is accepted without
downgrading the level part of "profile-level-id", and "sprop-
parameter-sets" is present (i.e. there is out-of-band transmission of
parameter sets). This case needs only one round of offer/answer.
Note that sprop-parameter-sets=<base-64 data#0> is basically
independent of sprop-parameter-sets=<base-64 data#1>. The former is
to be used by the encoder of the offerer and the decoder of the
answerer. The latter is to be used by the encoder of the answerer
and the decoder of the offerer. Note that requiring them (including
both sequence parameter sets and picture parameter sets) to be
identical or the former to be a subset of the latter would seriously
limit independent encoding optimizations by the independent encoders.
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
spatial-resolution=704,576;
packetization-mode=1;
sprop-parameter-sets=<base-64 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
spatial-resolution=704,576;
packetization-mode=1;
sprop-parameter-sets=<base-64 data#1>
In the following example, the original offer is also accepted without
downgrading the level part of "profile-level-id", but "sprop-
parameter-sets" is not present, meaning that there is no out-of-band
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transmission of parameter sets, which then have to be transmitted in-
band. This case also needs only one round of offer/answer.
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;
spatial-resolution=704,576
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;
spatial-resolution=704,576
In the following example, the original offer is accepted with
downgrading the level part of "profile-level-id", and "sprop-
parameter-sets" is present (i.e. there is out-of-band transmission of
parameter sets). This case needs at least two rounds of offer/answer
unless the offer includes multiple payload types for different
levels, which makes the SDP message larger. The reason for the need
of more than one offer/answer round is because the "sprop-parameter-
sets" in the original offer is not applicable to any level lower than
the one indicated by the level part of "profile-level-id".
Note that sprop-parameter-sets=<base-64 data#0> contains level_idc
indicating Level 3.0, therefore cannot be used as the answerer wants
Level 2.0 and does not need to be stored by the answerer. The sprop-
parameter-sets=<base-64 data#2> is to be used by the encoder of the
offerer and the decoder of the answerer. The sprop-parameter-
sets=<base-64 data#1> is to be used by the encoder of the answerer
and the decoder of the offerer.
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;
spatial-resolution=704,576;
sprop-parameter-sets=<base-64 data#0>
Answer SDP:
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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;
spatial-resolution=352,288;
sprop-parameter-sets=<base-64 data#1>
Offer SDP:
m=video 49170 RTP/AVP 98
a=rtpmap:98 H264/90000
a=fmtp:97 profile-level-id=42A014; //Baseline profile, Level 2.0
packetization-mode=1;
spatial-resolution=352,288;
sprop-parameter-sets=<base-64 data#2>
Answer SDP: //This SDP is identical to previous 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;
spatial-resolution=352,288;
sprop-parameter-sets=<base-64 data#1>
In the following example, the original offer is also accepted with
downgrading the level part of "profile-level-id", but "sprop-
parameter-sets" is not present, meaning that there is no out-of-band
transmission of parameter sets, which then have to be transmitted in-
band. This case also needs only one round of offer/answer.
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;
spatial-resolution=704,576
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;
spatial-resolution=352,288;
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8.4. Parameter Set Considerations
The H.264 parameter sets are a fundamental part of the video codec
and vital to its operation; see section 1.2. Due to their
characteristics and their importance for the decoding process, lost
or erroneously transmitted parameter sets can hardly be concealed
locally at the receiver. A reference to a corrupt parameter set has
normally fatal results to the decoding process. Corruption could
occur, for example, due to the erroneous transmission or loss of a
parameter set data structure, but also due to the untimely
transmission of a parameter set update. Therefore, the following
recommendations are provided as a guideline for the implementer of
the RTP sender.
Parameter set NALUs can be transported using three different
principles:
A. Using a session control protocol (out-of-band) prior to the actual
RTP session.
B. Using a session control protocol (out-of-band) during an ongoing
RTP session.
C. Within the RTP packet stream in the payload (in-band) during an
ongoing RTP session.
It is necessary 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. This
section contains guidelines on how principles A and B must 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.
The picture and sequence parameter set NALUs SHOULD NOT be
transmitted in the RTP payload unless reliable transport is provided
for RTP, as a loss of a parameter set of either type will likely
prevent decoding of a considerable portion of the corresponding RTP
packet stream. Thus, the transmission of parameter sets using a
reliable session control protocol (i.e., usage of principle A or B
above) is RECOMMENDED.
In the rest of the section it is assumed that out-of-band signaling
provides reliable transport of parameter set NALUs and that in-band
transport does not. 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
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sets correctly. Mechanisms that increase the probability for a
correct reception include packet repetition, FEC, and retransmission.
The use of an unreliable, out-of-band control protocol has similar
disadvantages as the in-band signaling (possible loss) and, in
addition, may also lead to difficulties in the synchronization (see
below). Therefore, it is NOT RECOMMENDED.
Parameter sets MAY be added or updated during the lifetime of a
session using principles B and C. It is required that parameter sets
are present at the decoder prior to the NAL units that refer to them.
Updating or adding of parameter sets can result in further problems,
and therefore the following recommendations should be considered.
- When parameter sets are added or updated, principle C is
vulnerable to transmission errors as described above, and
therefore principle B is RECOMMENDED.
- When parameter sets are added or updated, care SHOULD be taken to
ensure that any parameter set is delivered prior to its usage. It
is common that no synchronization is present between out-of-band
signaling and in-band traffic. If out-of-band signaling is used,
it is RECOMMENDED that a sender does not start sending NALUs
requiring the 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).
- When new parameter sets are added, previously unused parameter set
identifiers are used. This avoids the problem identified in the
previous paragraph. However, in a multiparty session, unless a
synchronized control protocol is used, there is a risk that
multiple entities try to add different parameter sets for the same
identifier, which has to be avoided.
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- 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), 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.
The loss probability should be reduced using the mechanisms discussed
above.
- When parameter sets are initially provided using principle A and
then later added or updated in-band (principle C), there is a risk
associated with updating the parameter sets delivered out-of-band.
If receivers miss some in-band updates (for example, because of a
loss or a late tune-in), those receivers attempt to decode the
bitstream using out-dated parameters. It is therefore RECOMMENDED
that parameter set IDs be partitioned between the out-of-band and
in-band parameter sets.
9. Security Considerations
RTP packets using the payload format defined in this specification
are subject to the security considerations discussed in the RTP
specification [4], and in any appropriate RTP profile (for example,
[16]). This implies that confidentiality of the media streams is
achieved by encryption; for example, through the application of SRTP
[26]. Because the data compression used with this payload format is
applied end-to-end, any encryption needs to be performed after
compression. A potential denial-of-service threat exists for data
encodings using compression techniques that have non-uniform
receiver-end computational load. The attacker can inject
pathological datagrams into the stream that are complex to decode and
that cause the receiver to be overloaded. H.264 is particularly
vulnerable to such attacks, as it is extremely simple to generate
datagrams containing NAL units that affect the decoding process of
many future NAL units. Therefore, the usage of data origin
authentication and data integrity protection of at least the RTP
packet is RECOMMENDED; for example, with SRTP [26].
Note that the appropriate mechanism to ensure confidentiality and
integrity of RTP packets and their payloads is very dependent on the
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application and on the transport and signaling protocols employed.
Thus, although SRTP is given as an example above, other possible
choices exist.
Decoders MUST exercise caution with respect to the handling of user
data SEI messages, particularly if they contain active elements, and
MUST restrict their domain of applicability to the presentation
containing the stream.
End-to-End security with either authentication, integrity or
confidentiality protection will prevent a MANE from performing media-
aware operations other than discarding complete packets. And in the
case of confidentiality protection it will even be prevented from
performing discarding of packets in a media aware way. To allow any
MANE to perform its operations, it will be required to be a trusted
entity which is included in the security context establishment.
10. Congestion Control
Congestion control for RTP SHALL be used in accordance with RFC 3550
[4], and with any applicable RTP profile; e.g., RFC 3551 [16]. An
additional requirement if best-effort service is being used is: users
of this payload format MUST monitor packet loss to ensure that the
packet loss rate is within acceptable parameters. Packet loss is
considered acceptable if a TCP flow across the same network path, and
experiencing the same network conditions, would achieve an average
throughput, measured on a reasonable timescale, that is not less than
the RTP flow is achieving. This condition can be satisfied by
implementing congestion control mechanisms to adapt the transmission
rate (or the number of layers subscribed for a layered multicast
session), or by arranging for a receiver to leave the session if the
loss rate is unacceptably high.
The bit rate adaptation necessary for obeying the congestion control
principle is easily achievable when real-time encoding is used.
However, when pre-encoded content is being transmitted, bandwidth
adaptation requires the availability of more than one coded
representation of the same content, at different bit rates, or the
existence of non-reference pictures or sub-sequences [22] in the
bitstream. The switching between the different representations can
normally be performed in the same RTP session; e.g., by employing a
concept known as SI/SP slices of the Extended Profile, or by
switching streams at IDR picture boundaries. Only when non-
downgradable parameters (such as the profile part of the
profile/level ID) are required to be changed does it become necessary
to terminate and re-start the media stream. This may be accomplished
by using a different RTP payload type.
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MANEs MAY follow the suggestions outlined in section 7.3 and remove
certain unusable packets from the packet stream when that stream was
damaged due to previous packet losses. This can help reduce the
network load in certain special cases.
11. IANA Consideration
IANA has registered one new MIME type; see section 8.1.
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 ITU-T Recommendation H.241 Annex A
H.323-based video telephony systems that use H.264 as an optional
video compression scheme are required to support H.241 Annex A [15]
as a packetization scheme. The packetization mechanism defined in
this Annex is technically identical with a small subset of this
specification.
When a system operates according to H.241 Annex A, parameter set NAL
units are sent in-band. Only Single NAL unit packets are used. Many
such systems are not sending IDR pictures regularly, but only when
required by user interaction or by control protocol means; e.g., when
switching between video channels in a Multipoint Control Unit or for
error recovery requested by feedback.
12.2. Video Telephony, No Slice Data Partitioning, No NAL Unit
Aggregation
The RTP part of this scheme is implemented and tested (though not the
control-protocol part; see below).
In most real-world video telephony applications, picture parameters
such as picture size or optional modes never change during the
lifetime of a connection. Therefore, all necessary parameter sets
(usually only one) are sent as a side effect of the capability
exchange/announcement process, e.g., according to the SDP syntax
specified in section 8.2 of this document. As all necessary
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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 2429 packetization [10]. It has been
implemented, and good results were reported [12].
The VCL encoder codes the source picture so that all macroblocks
(MBs) of one MB line are assigned to one slice. All slices with even
MB row addresses are combined into one STAP, and all slices with odd
MB row addresses into another. Those STAPs are transmitted as RTP
packets. The establishment of the parameter sets is performed as
discussed above.
Note that the use of STAPs is essential here, as the high number of
individual slices (18 for a CIF picture) would lead to unacceptably
high IP/UDP/RTP header overhead (unless the source coding tool FMO is
used, which is not assumed in this scenario). Furthermore, some
wireless video transmission systems, such as H.324M and the IP-based
video telephony specified in 3GPP, are likely to use relatively small
transport packet size. For example, a typical MTU size of H.223 AL3
SDU is around 100 bytes [17]. Coding individual slices according to
this packetization scheme provides further advantage in communication
between wired and wireless networks, as individual slices are likely
to be smaller than the preferred maximum packet size of wireless
systems. Consequently, a gateway can convert the STAPs used in a
wired network into several RTP packets with only one NAL unit, which
are preferred in a wireless network, and vice versa.
12.4. Video Telephony with Data Partitioning
This scheme has been implemented and has been shown to offer good
performance, especially at higher packet loss rates [12].
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Data Partitioning is known to be useful only when some form of
unequal error protection is available. Normally, in single-session
RTP environments, even error characteristics are assumed; i.e., the
packet loss probability of all packets of the session is the same
statistically. However, there are means to reduce the packet loss
probability of individual packets in an RTP session. A FEC packet
according to RFC 2733 [18], for example, specifies which media
packets are associated with the FEC packet.
In all cases, the incurred overhead is substantial but is in the same
order of magnitude as the number of bits that have otherwise been
spent for intra information. However, this mechanism does not add
any delay to the system.
Again, the complete parameter set establishment is performed through
control protocol means.
12.5. Video Telephony or Streaming with FUs and Forward Error Correction
This scheme has been implemented and has been shown to provide good
performance, especially at higher packet loss rates [19].
The most efficient means to combat packet losses for scenarios where
retransmissions are not applicable is forward error correction (FEC).
Although application layer, end-to-end use of FEC is often less
efficient than an FEC-based protection of individual links
(especially when links of different characteristics are in the
transmission path), application layer, end-to-end FEC is unavoidable
in some scenarios. RFC 2733 [18] provides means to use generic,
application layer, end-to-end FEC in packet-loss environments. A
binary forward error correcting code is generated by applying the XOR
operation to the bits at the same bit position in different packets.
The binary code can be specified by the parameters (n,k) in which k
is the number of information packets used in the connection and n is
the total number of packets generated for k information packets;
i.e., n-k parity packets are generated for k information packets.
When a code is used with parameters (n,k) within the RFC 2733
framework, the following properties are well known:
a) If applied over one RTP packet, RFC 2733 provides only packet
repetition.
b) RFC 2733 is most bit rate efficient if XOR-connected packets have
equal length.
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c) At the same packet loss probability p and for a fixed k, the
greater the value of n is, the smaller the residual error
probability becomes. For example, for a packet loss probability
of 10%, k=1, and n=2, the residual error probability is about 1%,
whereas for n=3, the residual error probability is about 0.1%.
d) At the same packet loss probability p and for a fixed code rate
k/n, the greater the value of n is, the smaller the residual error
probability becomes. For example, at a packet loss probability of
p=10%, k=1 and n=2, the residual error rate is about 1%, whereas
for an extended Golay code with k=12 and n=24, the residual error
rate is about 0.01%.
For applying RFC 2733 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
length might also be connected, which decreases bit rate
efficiency according to b), above. However, with some care and
for slices of 1kb or larger, similar length (100-200 bytes
difference) may be produced, which will not lower the bit
efficiency catastrophically.
3) The video encoder produces NAL units, for which a certain frame
contains k slices of possibly almost equal length. Then, applying
FEC, a better code, e.g., (n=24, k=12), can be used over the
sequence of NAL units for each frame. The delay compared to that
of 2), above, may be reduced, but several disadvantages are
obvious. First, the coding efficiency of the encoded video is
lowered significantly, as slice-structured coding reduces intra-
frame prediction and additional slice overhead is necessary.
Second, pre-encoded content or, when operating over a gateway, the
video is usually not appropriately coded with k slices such that
FEC can be applied. Finally, the encoding of video producing k
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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 bit rate efficient use of
RFC 2733.
If the error probability depends on the length of the transmitted
packet (e.g., in case of mobile transmission [14]), the benefits of
applying FUs with FEC are even more obvious. Basically, the
flexibility of the size of FUs allows appropriate FEC to be applied
for each NAL unit and unequal error protection of NAL units.
When FUs and FEC are used, the incurred overhead is substantial but
is in the same order of magnitude as the number of bits that have to
be spent for intra-coded macroblocks if no FEC is applied. In [19],
it was shown that the overall performance of the FEC-based approach
enhanced quality when using the same error rate and same overall bit
rate, including the overhead.
12.6. Low Bit-Rate Streaming
This scheme has been implemented with H.263 and non-standard RTP
packetization and has given good results [20]. There is no technical
reason why similarly good results could not be achievable with H.264.
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In today's Internet streaming, some of the offered bit rates are
relatively low in order to allow terminals with dial-up modems to
access the content. In wired IP networks, relatively large packets,
say 500 - 1500 bytes, are preferred to smaller and more frequently
occurring packets in order to reduce network congestion. Moreover,
use of large packets decreases the amount of RTP/UDP/IP header
overhead. For low bit-rate video, the use of large packets means
that sometimes up to few pictures should be encapsulated in one
packet.
However, loss of a packet including many coded pictures would have
drastic consequences for visual quality, as there is practically no
other way to conceal a loss of an entire picture than to repeat the
previous one. One way to construct relatively large packets and
maintain possibilities for successful loss concealment is to
construct MTAPs that contain interleaved slices from several
pictures. An MTAP should not contain spatially adjacent slices from
the same picture or spatially overlapping slices from any picture.
If a packet is lost, it is likely that a lost slice is surrounded by
spatially adjacent slices of the same picture and spatially
corresponding slices of the temporally previous and succeeding
pictures. Consequently, concealment of the lost slice is likely to
be relatively successful.
12.7. Robust Packet Scheduling in Video Streaming
Robust packet scheduling has been implemented with MPEG-4 Part 2 and
simulated in a wireless streaming environment [21]. There is no
technical reason why similar or better results could not be
achievable with H.264.
Streaming clients typically have a receiver buffer that is capable of
storing a relatively large amount of data. Initially, when a
streaming session is established, a client does not start playing the
stream back immediately. Rather, it typically buffers the incoming
data for a few seconds. This buffering helps maintain continuous
playback, as, in case of occasional increased transmission delays or
network throughput drops, the client can decode and play buffered
data. Otherwise, without initial buffering, the client has to freeze
the display, stop decoding, and wait for incoming data. The
buffering is also necessary for either automatic or selective
retransmission in any protocol level. If any part of a picture is
lost, a retransmission mechanism may be used to resend the lost data.
If the retransmitted data is received before its scheduled decoding
or playback time, the loss is recovered perfectly. Coded pictures
can be ranked according to their importance in the subjective quality
of the decoded sequence. For example, non-reference pictures, such
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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 to this RTP payload specification.
13.2. Example of Multi-Picture Slice Interleaving
An example of multi-picture slice interleaving follows. A subset of
a coded video sequence is depicted below in output order. R denotes
a reference picture, N denotes a non-reference picture, and the
number indicates a relative output time.
... R1 N2 R3 N4 R5 ...
The decoding order of these pictures from left to right is as
follows:
... R1 R3 N2 R5 N4 ...
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The NAL units of pictures R1, R3, N2, R5, and N4 are marked with a
DON equal to 1, 2, 3, 4, and 5, respectively.
Each reference picture consists of three slice groups that are
scattered as follows (a number denotes the slice group number for
each macroblock in a QCIF frame):
0 1 2 0 1 2 0 1 2 0 1
2 0 1 2 0 1 2 0 1 2 0
1 2 0 1 2 0 1 2 0 1 2
0 1 2 0 1 2 0 1 2 0 1
2 0 1 2 0 1 2 0 1 2 0
1 2 0 1 2 0 1 2 0 1 2
0 1 2 0 1 2 0 1 2 0 1
2 0 1 2 0 1 2 0 1 2 0
1 2 0 1 2 0 1 2 0 1 2
For the sake of simplicity, we assume that all the macroblocks of a
slice group are included in one slice. Three MTAPs are constructed
from three consecutive reference pictures so that each MTAP contains
three aggregation units, each of which contains all the macroblocks
from one slice group. The first MTAP contains slice group 0 of
picture R1, slice group 1 of picture R3, and slice group 2 of picture
R5. The second MTAP contains slice group 1 of picture R1, slice
group 2 of picture R3, and slice group 0 of picture R5. The third
MTAP contains slice group 2 of picture R1, slice group 0 of picture
R3, and slice group 1 of picture R5. Each non-reference picture is
encapsulated into an STAP-B.
Consequently, the transmission order of NAL units is the following:
R1, slice group 0, DON 1, carried in MTAP,RTP SN: N
R3, slice group 1, DON 2, carried in MTAP,RTP SN: N
R5, slice group 2, DON 4, carried in MTAP,RTP SN: N
R1, slice group 1, DON 1, carried in MTAP,RTP SN: N+1
R3, slice group 2, DON 2, carried in MTAP,RTP SN: N+1
R5, slice group 0, DON 4, carried in MTAP,RTP SN: N+1
R1, slice group 2, DON 1, carried in MTAP,RTP SN: N+2
R3, slice group 1, DON 2, carried in MTAP,RTP SN: N+2
R5, slice group 0, DON 4, carried in MTAP,RTP SN: N+2
N2, DON 3, carried in STAP-B, RTP SN: N+3
N4, DON 5, carried in STAP-B, RTP SN: N+4
The receiver is able to organize the NAL units back in decoding order
based on the value of DON associated with each NAL unit.
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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 the example operates as
follows. Note that processing of the video stream happens gradually
and at the same time in all components of the system. The source
video sequence is shot and captured to a pre-encoding buffer. The
pre-encoding buffer can be used to order pictures from sampling order
to encoding order or to analyze multiple uncompressed frames for bit
rate control purposes, for example. In some cases, the pre-encoding
buffer may not exist; instead, the sampled pictures are encoded right
away. The encoder encodes pictures from the pre-encoding buffer and
stores the output; i.e., coded pictures, to the encoded picture
buffer. The transmitter encapsulates the coded pictures from the
encoded picture buffer to transmission packets and sends them to a
receiver through a transmission channel. The receiver stores the
received packets to the receiver buffer. The receiver buffering
process typically includes buffering for transmission delay jitter.
The receiver buffer can also be used to recover correct decoding
order of coded data. The decoder reads coded data from the receiver
buffer and produces decoded pictures as output into the decoded
picture buffer. The decoded picture buffer is used to recover the
output (or display) order of pictures. Finally, pictures are
displayed.
In the following example figures, I denotes an IDR picture, R denotes
a reference picture, N denotes a non-reference picture, and the
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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:
... N58 N59 I00 R03 N01 N02 R06 N04 N05 ...
... -|---|---|---|---|---|---|---|---|- ...
... 60 61 62 63 64 65 66 67 68 ...
Figure 17 Re-ordered pictures in the pre-encoding buffer
The encoder or the transmitter can set the value of DON for each
picture to a value of DON for the previous picture in decoding order
plus one.
For the sake of simplicity, let us assume that:
o the frame rate of the sequence is constant,
o each picture consists of only one slice,
o each slice is encapsulated in a single NAL unit packet,
o there is no transmission delay, and
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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 MIME type parameter is set to
0, as the transmission (or reception) order is identical to the
decoding order.
The decoder has to buffer for one picture interval initially in its
decoded picture buffer to organize pictures from decoding order to
output order as depicted below:
... N58 N59 I00 N01 N02 R03 N04 N05 R06 ...
... -|---|---|---|---|---|---|---|---|- ...
... 61 62 63 64 65 66 67 68 69 ...
Figure 19 Output order
The amount of required initial buffering in the decoded picture
buffer can be signaled in the buffering period SEI message or with
the num_reorder_frames syntax element of H.264 video usability
information. num_reorder_frames indicates the maximum number of
frames, complementary field pairs, or non-paired fields that precede
any frame, complementary field pair, or non-paired field in the
sequence in decoding order and that follow it in output order. For
the sake of simplicity, we assume that num_reorder_frames is used to
indicate the initial buffer in the decoded picture buffer. In this
example, num_reorder_frames is equal to 1.
It can be observed that if the IDR picture I00 is lost during
transmission and a retransmission request is issued when the value of
the system clock is 62, there is one picture interval of time (until
the system clock reaches timestamp 63) to receive the retransmitted
IDR picture I00.
Let us then assume that IDR pictures are transmitted two frame
intervals earlier than their decoding position; i.e., the pictures
are transmitted as follows:
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... I00 N58 N59 R03 N01 N02 R06 N04 N05 ...
... --|---|---|---|---|---|---|---|---|- ...
... 62 63 64 65 66 67 68 69 70 ...
Figure 20 Interleaving: Early IDR pictures in sending order
The OPTIONAL sprop-interleaving-depth MIME type parameter is set
equal to 1 according to its definition. (The value of sprop-
interleaving-depth in this example can be derived as follows: Picture
I00 is the only picture preceding picture N58 or N59 in transmission
order and following it in decoding order. Except for pictures I00,
N58, and N59, the transmission order is the same as the decoding
order of pictures. As a coded picture is encapsulated into exactly
one NAL unit, the value of sprop-interleaving-depth is equal to the
maximum number of pictures preceding any picture in transmission
order and following the picture in decoding order.)
The receiver buffering process contains two pictures at a time
according to the value of the sprop-interleaving-depth parameter and
orders pictures from the reception order to the correct decoding
order based on the value of DON associated with each picture. The
output of the receiver buffering process is as follows:
... N58 N59 I00 R03 N01 N02 R06 N04 N05 ...
... -|---|---|---|---|---|---|---|---|- ...
... 63 64 65 66 67 68 69 70 71 ...
Figure 21 Interleaving: Receiver buffer
Again, an initial buffering delay of one picture interval is needed
to organize pictures from decoding order to output order, as depicted
below:
... N58 N59 I00 N01 N02 R03 N04 N05 ...
... -|---|---|---|---|---|---|---|- ...
... 64 65 66 67 68 69 70 71 ...
Figure 22 Interleaving: Receiver buffer after reordering
Note that the maximum delay that IDR pictures can undergo during
transmission, including possible application, transport, or link
layer retransmission, is equal to three picture intervals. Thus, the
loss resiliency of IDR pictures is improved in systems supporting
retransmission compared to the case in which pictures were
transmitted in their decoding order.
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13.4. Robust Transmission Scheduling of Redundant Coded Slices
A redundant coded picture is a coded representation of a picture or a
part of a picture that is not used in the decoding process if the
corresponding primary coded picture is correctly decoded. There
should be no noticeable difference between any area of the decoded
primary picture and a corresponding area that would result from
application of the H.264 decoding process for any redundant picture
in the same access unit. A redundant coded slice is a coded slice
that is a part of a redundant coded picture.
Redundant coded pictures can be used to provide unequal error
protection in error-prone video transmission. If a primary coded
representation of a picture is decoded incorrectly, a corresponding
redundant coded picture can be decoded. Examples of applications and
coding techniques using the redundant codec picture feature include
the video redundancy coding [23] and the protection of "key pictures"
in multicast streaming [24].
One property of many error-prone video communications systems is that
transmission errors are often bursty. Therefore, they may affect
more than one consecutive transmission packets in transmission order.
In low bit-rate video communication, it is relatively common that an
entire coded picture can be encapsulated into one transmission
packet. Consequently, a primary coded picture and the corresponding
redundant coded pictures may be transmitted in consecutive packets in
transmission order. To make the transmission scheme more tolerant of
bursty transmission errors, it is beneficial to transmit the primary
coded picture and redundant coded picture separated by more than a
single packet. The DON concept enables this.
13.5. Remarks on Other Design Possibilities
The slice header syntax structure of the H.264 coding standard
contains the frame_num syntax element that can indicate the decoding
order of coded frames. However, the usage of the frame_num syntax
element is not feasible or desirable to recover the decoding order,
due to the following reasons:
o The receiver is required to parse at least one slice header per
coded picture (before passing the coded data to the decoder).
o Coded slices from multiple coded video sequences cannot be
interleaved, as the frame number syntax element is reset to 0 in
each IDR picture.
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o The coded fields of a complementary field pair share the same
value of the frame_num syntax element. Thus, the decoding order
of the coded fields of a complementary field pair cannot be
recovered based on the frame_num syntax element or any other
syntax element of the H.264 coding syntax.
The RTP payload format for transport of MPEG-4 elementary streams
[25] enables interleaving of access units and transmission of
multiple access units in the same RTP packet. An access unit is
specified in the H.264 coding standard to comprise all NAL units
associated with a primary coded picture according to subclause
7.4.1.2 of [1]. Consequently, slices of different pictures cannot be
interleaved, and the multi-picture slice interleaving technique (see
section 12.6) for improved error resilience cannot be used.
14. Acknowledgements
The authors thank Roni Even, Dave Lindbergh, Philippe Gentric,
Gonzalo Camarillo, Gary Sullivan, Joerg Ott, and Colin Perkins for
careful review.
This document was prepared using 2-Word-v2.0.template.dot.
15. References
15.1. Normative References
[1] ITU-T Recommendation H.264, "Advanced video coding for generic
audiovisual services", May 2003.
[2] ISO/IEC International Standard 14496-10:2003.
[3] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[4] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,
"RTP: A Transport Protocol for Real-Time Applications", STD 64,
RFC 3550, July 2003.
[5] Handley, M. and V. Jacobson, "SDP: Session Description
Protocol", RFC 2327, April 1998.
[6] Josefsson, S., "The Base16, Base32, and Base64 Data Encodings",
RFC 3548, July 2003.
[7] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
Session Description Protocol (SDP)", RFC 3264, June 2002.
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15.2. Informative References
[8] "Draft ITU-T Recommendation and Final Draft International
Standard of Joint Video Specification (ITU-T Rec. H.264 |
ISO/IEC 14496-10 AVC)", available from http://ftp3.itu.int/av-
arch/jvt-site/2003_03_Pattaya/JVT-G050r1.zip, May 2003.
[9] Luthra, A., Sullivan, G.J., and T. Wiegand (eds.), Special
Issue on H.264/AVC. IEEE Transactions on Circuits and Systems
on Video Technology, July 2003.
[10] Bormann, C., Cline, L., Deisher, G., Gardos, T., Maciocco, C.,
Newell, D., Ott, J., Sullivan, G., Wenger, S., and C. Zhu, "RTP
Payload Format for the 1998 Version of ITU-T Rec. H.263 Video
(H.263+)", RFC 2429, October 1998.
[11] ISO/IEC IS 14496-2.
[12] Wenger, S., "H.26L over IP", IEEE Transaction on Circuits and
Systems for Video technology, Vol. 13, No. 7, July 2003.
[13] Wenger, S., "H.26L over IP: The IP Network Adaptation Layer",
Proceedings Packet Video Workshop 02, April 2002.
[14] Stockhammer, T., Hannuksela, M.M., and S. Wenger, "H.26L/JVT
Coding Network Abstraction Layer and IP-based Transport" in
Proc. ICIP 2002, Rochester, NY, September 2002.
[15] ITU-T Recommendation H.241, "Extended video procedures and
control signals for H.300 series terminals", 2004.
[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] Rosenberg, J. and H. Schulzrinne, "An RTP Payload Format for
Generic Forward Error Correction", RFC 2733, December 1999.
[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.
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[20] Varsa, V. and M. Karczewicz, "Slice interleaving in compressed
video packetization", Packet Video Workshop 2000.
[21] Kang, S.H. and A. Zakhor, "Packet scheduling algorithm for
wireless video streaming," International Packet Video Workshop
2002.
[22] Hannuksela, M.M., "Enhanced concept of GOP", JVT-B042,
available http://ftp3.itu.int/av-arch/video-site/0201_Gen/JVT-
B042.doc, anuary 2002.
[23] Wenger, S., "Video Redundancy Coding in H.263+", 1997
International Workshop on Audio-Visual Services over Packet
Networks, September 1997.
[24] Wang, Y.-K., Hannuksela, M.M., and M. Gabbouj, "Error Resilient
Video Coding Using Unequally Protected Key Pictures", in Proc.
International Workshop VLBV03, September 2003.
[25] van der Meer, J., Mackie, D., Swaminathan, V., Singer, D., and
P. Gentric, "RTP Payload Format for Transport of MPEG-4
Elementary Streams", RFC 3640, November 2003.
[26] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)", RFC
3711, March 2004.
[27] Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time Streaming
Protocol (RTSP)", RFC 2326, April 1998.
[28] Handley, M., Perkins, C., and E. Whelan, "Session Announcement
Protocol", RFC 2974, October 2000.
Authors' Addresses
Ye-Kui Wang
Nokia Research Center
P.O. Box 100
33721 Tampere
Finland
Phone: +358-50-466-7004
EMail: ye-kui.wang@nokia.com
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Stephan Wenger
Nokia
955 Page Mill Road
Palo Alto, CA 94304
USA
Phone: +1-650-862-7368
EMail: stewe@stewe.org
Miska M. Hannuksela
Nokia Corporation
P.O. Box 100
33721 Tampere
Finland
Phone: +358-40-521-2845
EMail: miska.hannuksela@nokia.com
Thomas Stockhammer
Nomor Research
D-83346 Bergen
Germany
Phone: +49-8662-419407
EMail: stockhammer@nomor.de
Magnus Westerlund
Multimedia Technologies
Ericsson Research EAB/TVA/A
Ericsson AB
Torshamsgatan 23
SE-164 80 Stockholm
Sweden
Phone: +46-8-7190000
EMail: magnus.westerlund@ericsson.com
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David Singer
QuickTime Engineering
Apple
1 Infinite Loop MS 302-3MT
Cupertino
CA 95014
USA
Phone +1 408 974-3162
EMail: singer@apple.com
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WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
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Copyright Statement
Copyright (C) The IETF Trust (2008).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
16. Backward compatibility to RFC 3984
The current document is a revision of RFC 3984 and intends to
obsolete it. This section addresses the backward compatibility
issues.
TBD.
17. Changes from RFC 3984
17.1. Technical changes
The technical changes (including bug fixes) from RFC 3984 are:
1) In subsections 5.4, 5.5, 6.2, 6,3 and 6.4, removed that the
packetization mode in use may be signaled by external means.
2) In subsection 7.2.2, changed the sentence
There are N VCL NAL units in the deinterleaving buffer.
to
There are N or more VCL NAL units in the deinterleaving buffer.
3) In subsection 7.2.2, changed the sentence
Herein, n corresponds to the NAL unit having the greatest value
of AbsDON among the received NAL units.
to
Herein, n corresponds to the NAL unit having the greatest value
of AbsDON among the NAL units in the deinterleaving buffer.
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4) In subsection 8.1, the semantics of sprop-init-buf-time, paragraph
2, changed the sentence
The parameter is the maximum value of (transmission time of a NAL
unit - decoding time of the NAL unit), assuming reliable and
instantaneous transmission, the same timeline for transmission
and decoding, and that decoding starts when the first packet
arrives.
to
The parameter is the maximum value of (decoding time of the NAL
unit - transmission time of a NAL unit), assuming reliable and
instantaneous transmission, the same timeline for transmission
and decoding, and that decoding starts when the first packet
arrives.
5) In subsection 8.1, removed the specification of parameter-add.
Other descriptions of parameter-add (in subsections 8.2 and 8.4)
are also removed.
6) In subsection 8.1, added the specification text of spatial-
resolution, and the use of this parameter is added to subsections
8.2 and 8.3.
7) In subsection 8.2.2, changed bullet item 1, such that in the SDP
offer/answer model, the use of the level part of "profile-level-
id" does not need to be symmetric, i.e. the value of the level
part in the answer does not have not be the same as in the offer.
In addition, an informative note was added to clarify the
specification of level 1b in H.264.
8) In subsection 8.2.2, changed bullet item 3 from
The capability parameters ("max-mbps", "max-fs", "max-cpb", "max-
dpb", "max-br", ,"redundant-pic-cap", "max-rcmd-nalu-size") MAY
be used to declare further capabilities. Their interpretation
depends on the direction attribute. When the direction attribute
is sendonly, then the parameters describe the limits of the RTP
packets and the NAL unit stream that the sender is capable of
producing. When the direction attribute is sendrecv or recvonly,
then the parameters describe the limitations of what the receiver
accepts.
to
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The capability parameters ("max-mbps", "max-fs", "max-cpb", "max-
dpb", "max-br", ,"redundant-pic-cap", "max-rcmd-nalu-size") MAY
be used to declare further capabilities. These parameters can
only be present when the direction attribute is sendrecv or
recvonly, and the parameters describe the limitations of what the
receiver accepts.
Such that the description matches the semantics of these
parameters defined earlier.
9) In subsection 8.2.2. the following paragraph
For streams being delivered over multicast, the following rules
apply in addition:
was changed to the following (with an item added after the
paragraph):
For streams being delivered over multicast, the following rules
apply:
o The media format configuration is identified by the same
parameters as above for unicast (i.e. "profile-level-id",
"packetization-mode", and, if required by "packetization-
mode", "sprop-deint-buf-req"). These media format
configuration parameters (including the level part of
"profile-level-id") MUST be used symmetrically; i.e., the
answerer MUST either maintain all configuration parameters or
remove the media format (payload type) completely.
Because some items described above for unicast do not apply for
multicast.
10)In subsection 8.2.2, the bullet item starting with "In an offer or
answer for which the direction attribute "a=sendonly" is included
for the media stream, the following interpretation of the
parameters MUST be used:", removed the following, because, the
direction attribute is sendonly, the sender will not receive
streams.
Declaring the capabilities of the sender when it receives a
stream:
- max-mbps
- max-fs
- max-cpb
- max-dpb
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- max-br
- redundant-pic-cap
- deint-buf-cap
- max-rcmd-nalu-size
17.2. Editorial changes
The editorial changes from RFC 3984 are:
1) In subsection 4.1 (Definitions), added the definition of "NALU-
time" (moved from subsection 5.7 with slight modifications), and
changed the use of "NALU time" to "NALU-time" at two places in
subsection 5.7.
2) Added the subsection number 4.2 for Abbreviations.
3) In subsection 5.2, added the following paragraph right before
Table 1:
Table 1 summarizes NAL unit types and the corresponding RTP
packet types when each of these NAL units is directly used a
packet payload, and where the types are described in this memo.
4) In subsection 5.2, changed Table 1 from
Table 1. Summary of NAL unit types and their payload structures
Type Packet Type name Section
---------------------------------------------------------
0 undefined -
1-23 NAL unit Single NAL unit packet per H.264 5.6
24 STAP-A Single-time aggregation packet 5.7.1
25 STAP-B Single-time aggregation packet 5.7.1
26 MTAP16 Multi-time aggregation packet 5.7.2
27 MTAP24 Multi-time aggregation packet 5.7.2
28 FU-A Fragmentation unit 5.8
29 FU-B Fragmentation unit 5.8
30-31 undefined -
to
Table 1. Summary of NAL unit types and the corresponding packet
types
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NAL Unit Packet Packet Type Name Section
Type Type
---------------------------------------------------------
0 undefined -
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 undefined -
5) In subsection 5.3, removed "greater than 00" from the following
sentence:
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.
6) In subsection 5.3, the second informative note, corrected "nal
unit" to "NAL unit".
7) In the end of subsection 5.4, changed the text starting from the
second sentence of the last text paragraph and Table 3 from
The used packetization mode governs which NAL unit types are
allowed in RTP payloads. Table 3 summarizes the allowed NAL unit
types for each packetization mode. Some NAL unit type values
(indicated as undefined in Table 3) are reserved for future
extensions. NAL units of those types SHOULD NOT be sent by a
sender and MUST be ignored by a receiver. For example, the Types
1-23, with the associated packet type "NAL unit", are allowed in
"Single NAL Unit Mode" and in "Non-Interleaved Mode", but
disallowed in "Interleaved Mode". Packetization modes are
explained in more detail in section 6.
Table 3. Summary of allowed NAL unit types for each packetization
mode (yes = allowed, no = disallowed, ig = ignore)
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Type Packet Single NAL Non-Interleaved Interleaved
Unit Mode Mode Mode
-------------------------------------------------------------
0 undefined ig ig ig
1-23 NAL unit yes yes no
24 STAP-A no yes no
25 STAP-B no no yes
26 MTAP16 no no yes
27 MTAP24 no no yes
28 FU-A no yes yes
29 FU-B no no yes
30-31 undefined ig ig ig
to
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 undefined ig ig ig
1-23 NAL unit yes yes no
24 STAP-A no yes no
25 STAP-B no no yes
26 MTAP16 no no yes
27 MTAP24 no no yes
28 FU-A no yes yes
29 FU-B no no yes
30-31 undefined ig ig ig
Some NAL unit or payload type values (indicated as undefined in
Table 3) are reserved for future extensions. NAL units of those
types SHOULD NOT be sent by a sender (direct as packet payloads,
or as aggregation units in aggregation packets, or as fragmented
units in FU packets) and MUST be ignored by a receiver. For
example, the payload types 1-23, with the associated packet type
"NAL unit", are allowed in "Single NAL Unit Mode" and in "Non-
Interleaved Mode", but disallowed in "Interleaved Mode".
However, NAL units of NAL unit types 1-23 can be used in
"Interleaved Mode" as aggregation units in STAP-B, MTAP16 and
MTAP14 packets as well as fragmented units in FU-A and FU-B
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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.
8) In subsections 5.6 and 5.7, changed "type" to "Type" in Figures 2
and 3.
9) Corrected the titles of Figures 7,8,12, and 13, wherein "and" was
replaced by "containing".
10)In subsection 5.7.2, corrected the sentence
The DON of the following NAL unit is equal to (DONB + DOND) %
65536, in which % denotes the modulo operation.
to
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.
11)In Figure 11, corrected "NALU unit size" to "NAL unit size".
12)In subsection 5.7.2, the informative note under Figure 11,
corrected the first sentence from
The "earliest" multi-time aggregation unit is the one that would
have the smallest extended RTP timestamp among all the
aggregation units of an MTAP if the aggregation units were
encapsulated in single NAL unit packets.
to
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.
13)In subsection 5.7.3, corrected the following sentence by replacing
"picture" with "NAL unit".
The fragmentation mechanism allows fragmenting a single picture
and applying generic forward error correction as described in
section 12.5.
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14)In subsection 5.7.3, changed the following sentence by replacing
'A' with "An".
A FU payload MAY have any number of octets and MAY be empty.
15)In subsection 5.7.3, the last informative note, changed the last
sentence from
However, the (potential) use of zero-length NALUs should be
carefully weighed against the increased risk of the loss of the
NALU because of the additional packets employed for its
transmission.
to
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.
16)In subsection 6.1, corrected the sentence
Coded slice NAL units or coded slice data partition NAL units
belonging to the same coded picture (and thus sharing the same
RTP timestamp value) MAY be sent in any order permitted by the
applicable profile defined in [1]; however, for delay-critical
systems, they SHOULD be sent in their original coding order to
minimize the delay. Note that the coding order is not
necessarily the scan order, but the order the NAL packets become
available to the RTP stack.
to
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.
17)Removed "(Informative)" from the tile of section 7, and changed
the first sentences in the beginning of section 7 from
The de-packetization process is implementation dependent.
Therefore, the following description should be seen as an example
of a suitable implementation. Other schemes may be used as well.
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Optimizations relative to the described algorithms are likely
possible.
to
The de-packetization process is implementation dependent.
Therefore, the following description should be seen as an example
of a suitable implementation. Other schemes may be used as well
as long as the output for the same input is the same as the
process described below. The output is the same meaning that the
number of NAL units and their order are both the identical.
Optimizations relative to the described algorithms are likely
possible.
18)In subsection 7.1, paragraph 1, corrected the last sentence from
If a decapsulated packet is an FU-A, all the fragments of the
fragmented NAL unit are concatenated and passed to the decoder.
to
For all the FU-A packets containing fragments of a single NAL
unit, the decapsulated fragments are concatenated in their
sending order to recover the NAL unit, which is then passed to
the decoder.
19)In subsection 7.2, paragraph 2, corrected the first sentence
(copied below) by replacing "packets" with "NAL units".
The receiver includes a receiver buffer, which is used to
compensate for transmission delay jitter and to reorder packets
from transmission order to the NAL unit decoding order.
20)In subsection 7.2.2, paragraph 1, changed the following sentence
by replacing "is" with "are".
After initial buffering, decoding and playback is started, and
the buffering-while-playing mode is used.
21)In subsection 7.2.2, paragraph 1, changed the sentence
The value of DON is calculated and stored for all NAL units.
to
The value of DON is calculated and stored for each NAL unit.
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22)In subsection 7.2.2, removed "an" from the following sentence.
Let PDON be a variable that is initialized to 0 at the beginning
of the an RTP session.
23)In section 8, paragraph 2, changed the sentence
The name of all these parameters starts with "sprop" for stream
properties.
to
The names of all these parameters start with "sprop" for stream
properties.
24)In subsection 8.1, the semantics of max-mbps, max-fs, max-cpb,
max-dpb, and max-br, changed the last paragraph above the
informative note from
A receiver MUST NOT signal values of max-mbps, max-fs, max-
cpb, max-dpb, and max-br that meet the requirements of a
higher level, referred to as level A herein, compared to the
level specified in the value of the profile-level-id
parameter, if the receiver can support all the properties of
level A.
to
If a receiver can support all the properties of level A, the
level specified in the value of the profile-level-id MUST be
level A (i.e. MUST NOT be lower than level A). In other
words, a sender or receiver MUST NOT signal values of max-
mbps, max-fs, max-cpb, max-dpb, and max-br that meet the
requirements of a higher level compared to the level specified
in the value of the profile-level-id parameter.
25)In subsection 8.1, the semantics of max-dpb, the informative note,
removed "and is a property of the video decoder only" from the
following sentence.
The decoded picture buffer stores reconstructed samples and is a
property of the video decoder only.
26)In subsection 8.1, the semantics of max-br, paragraph 2, removed
the following sentence that is repeated later in the exact form.
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The value of max-br MUST be greater than or equal to the value of
MaxBR for the level given in Table A-1 of [1].
27)In subsection 8.1, the semantics of packetization-mode, changed
the last sentence from
The value of packetization mode MUST be an integer in the range
of 0 to 2, inclusive.
to
The value of packetization-mode MUST be an integer in the range
of 0 to 2, inclusive.
28)In subsection 8.1, the semantics of sprop-init-buf-time, paragraph
2, changed the following sentence by replacing "MUST buffer" by
"MUST wait".
The parameter signals the initial buffering time that a receiver
MUST buffer before starting decoding to recover the NAL unit
decoding order from the transmission order.
29)In subsection 8.1, changed "NAL unit stream" to "RTP packet
stream" at several places in the semantics of sprop-interleaving-
depth, sprop-deint-buf-req, sprop-init-buf-time and sprop-max-don-
diff.
30)In subsection 8.1, removed the paragraph talking about file
formats after the description of "Encoding considerations". The
references 29 and 30 (to file format specifications) were also
removed.
31)In subsection 8.2.1, the example SDP message. Added the required
parameter "packetization-mode", which was missing. The use of
spatial-resolution is also added. The third change is the change
of a hypothetical value of "sprop-parameter-sets" to "<base64
data>", as readers may be confused by the hypothetical value. The
same change regarding hypothetical values of "sprop-parameter-
sets" was made to the SDP examples in subsection 8.3.
32)In subsection 8.2.2, bullet item 2, changed the beginning sentence
from
The parameters "sprop-parameter-sets", "sprop-deint-buf-req",
"sprop-interleaving-depth", "sprop-max-don-diff", and "sprop-
init-buf-time" describe the properties of the NAL unit stream
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that the offerer or answerer is sending for this media format
configuration.
to
The parameters "packetization-mode", and if present, "sprop-
deint-buf-req", "sprop-parameter-sets", "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 this media format configuration.
33)In subsection 8.2.2, bullet item 2, the informative note, the last
sentence, corrected the typo "then" to "than".
34)In subsection 8.2.2, changed the following bullet item for
multicast
o The stream properties parameters ("sprop-parameter-sets",
"sprop-deint-buf-req", "sprop-interleaving-depth", "sprop-
max-don-diff", and "sprop-init-buf-time") MUST NOT be changed
by the answerer. Thus, a payload type can either be accepted
unaltered or removed.
to
o The stream properties parameters ("sprop-parameter-sets",
"sprop-interleaving-depth", "sprop-max-don-diff", and "sprop-
init-buf-time") MUST NOT be changed by the answerer. Thus, a
payload type can either be accepted unaltered or removed.
35)In subsection 8.2.3, changed two uses of "NAL unit stream" to "RTP
packet stream", and changed the sentence "Declaring actual
configuration or properties:" to "Declaring actual configuration
or stream properties:".
36)In subsection 8.3, the first sentence, changed "A SIP Offer/Answer
exchange" to "An SDP Offer/Answer exchange".
37)In subsection 8.3, the first example, changed "Offerer -> Answer
SDP message:" to "Offerer -> Answerer SDP message:".
38)In subsection 8.3, in the paragraph describing the offer SDP in
the first example, changed
PT 98 represents single NALU mode, PT 99 non-interleaved mode; PT
100 indicates the interleaved mode.
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to
PT 98 represents single NALU mode, PT 99 represents non-
interleaved mode, and PT 100 indicates the interleaved mode.
And changed "that are required for the answerer" to "that are
required by the answerer".
And changed
Note that the value for "sprop-parameter-sets", although
identical in the example above, could be different for each
payload type.
to
Note that the value for "sprop-parameter-sets" could be different
for each payload type.
39)In subsection 8.3, changed the two paragraphs describing the
answer SDP in the first example from
As the Offer/Answer negotiation covers both sending and receiving
streams, an offer indicates the exact parameters for what the
offerer is willing to receive, whereas the answer indicates the
same for what the answerer accepts to receive. In this case the
offerer declared that it is willing to receive payload type 98.
The answerer accepts this by declaring a equivalent payload type
97; i.e., it has identical values for the three parameters
"profile-level-id", packetization-mode, and "sprop-deint-buf-
req". This has the following implications for both the offerer
and the answerer concerning the parameters that declare
properties. The offerer initially declared a certain value of
the "sprop-parameter-sets" in the payload definition for PT=98.
However, as the answerer accepted this as PT=97, the values of
"sprop-parameter-sets" in PT=98 must now be used instead when the
offerer sends PT=97. Similarly, when the answerer sends PT=98 to
the offerer, it has to use the properties parameters it declared
in PT=97.
The answerer also accepts the reception of the two configurations
that payload types 99 and 100 represent. It provides the initial
parameter sets for the answerer-to-offerer direction, and for
buffering related parameters that it will use to send the payload
types. It also provides the offerer with its memory limit for
deinterleaving operations by providing a "deint-buf-cap"
parameter. This is only useful if the offerer decides on making
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a second offer, where it can take the new value into account.
The "max-rcmd-nalu-size" indicates that the answerer can
efficiently process NALUs up to the size of 3980 bytes. However,
there is no guarantee that the network supports this size.
to
As the Offer/Answer negotiation covers both sending and receiving
streams, an offer indicates the exact parameters for what the
offerer is willing to receive, whereas the answer indicates the
same for what the answerer accepts to receive. In this case the
offerer declared that it is willing to receive payload type 98.
The answerer accepts this by declaring an equivalent payload type
97; i.e., it has identical values for the two parameters
"profile-level-id" and "packetization-mode" (since
"packetization-mode" is equal to 0, "sprop-deint-buf-req" is not
present). As the offered payload type 98 is accepted, the
answerer needs to store parameter sets included in sprop-
parameter-sets=<base64 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=<base64 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=<base64 data#1> and sprop-parameter-sets=<base64 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=<base64 data#4> and sprop-parameter-
sets=<base64 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 deinterleaving
operations by providing a "deint-buf-cap" parameter. This is
only useful if the offerer decides on making a second offer,
where it can take the new value into account. The "max-rcmd-
nalu-size" indicates that the answerer can efficiently process
NALUs up to the size of 3980 bytes. However, there is no
guarantee that the network supports this size.
40)In the end of subsection 8.3, added four more SDP offer/answer
examples describing level downgrade and using or not using out-of-
band transmission of parameter sets.
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41)In subsection 8.4, changed two uses of "RTP stream" to "RTP packet
stream".
42)In subsection 8.4, changed the sentence
It is RECOMMENDED that parameter set IDs be partitioned between
the out-of-band and in-band parameter sets.
to
It is therefore RECOMMENDED that parameter set IDs be partitioned
between the out-of-band and in-band parameter sets.
43)In subsection 13.3, changed
pictures are transmitted at constant intervals (that is, 1 /
frame rate).
to
pictures are transmitted at constant intervals (that is, 1 /
(frame rate)).
18. Open issues
The issues remaining open are:
1) Rules of SDP usage for spatial-resolution need to be further
specified, e.g., what if a resolution requested by a receiver is
not supported by the sender even the resolution is within the
agreed level limit?
2) In subsection 8.2, the following sentence, what are the "non-
downgradable parameters"? It was defined earlier that a
configuration consists of three parameters, "profile-level-id",
"packetization-mode", and, if required by "packetization-mode",
"sprop-deint-buf-req". Isn't it confusing to say that other
parameters "express a configuration"? What does "acceptable" mean,
acceptable to the sender for receiving a stream using the
configuration?
When a sender's capabilities are declared, and non-downgradable
parameters are used in this declaration, then these parameters
express a configuration that is acceptable.
3) A question related to the first SDP usage example in subsection
8.2.3: If the answer accepts more than one alternative in the
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offer, and the offer/answer stops, is it so that the offerer can
now freely choose one of the alternatives accepted by the
answerer?
4) Subsection 8.2.3, in the following explanatory text for the first
SDP usage example, what does the second sentence mean? If the
offer stops offer/answer, should the offer take into account the
value of the parameter when sending a stream? If yes, then the
sentence should be removed or reworded.
The answerer also provides the offerer with its memory limit for
deinterleaving operations by providing a "deint-buf-cap"
parameter. This is only useful if the offerer decides on making
a second offer, where it can take the new value into account. T
5) Integration of draft-ietf-avt-rtp-h264-params-01.txt remains to be
done.
6) To complete the section on backward compatibility to RFC 3984.
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