RTP Payload Format for the 1998 Version of ITU-T Rec. H.263 Video (H.263+) [RFC2429]
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (1998). All Rights Reserved.
1. Introduction
This document specifies an RTP payload header format applicable to
the transmission of video streams generated based on the 1998 version
of ITU-T Recommendation H.263 [4]. Because the 1998 version of H.263
is a superset of the 1996 syntax, this format can also be used with
the 1996 version of H.263 [3], and is recommended for this use by new
implementations. This format does not replace RFC 2190, which
continues to be used by existing implementations, and may be required
for backward compatibility in new implementations. Implementations
using the new features of the 1998 version of H.263 shall use the
format described in this document.
The 1998 version of ITU-T Recommendation H.263 added numerous coding
options to improve codec performance over the 1996 version. The 1998
version is referred to as H.263+ in this document. Among the new
options, the ones with the biggest impact on the RTP payload
specification and the error resilience of the video content are the
slice structured mode, the independent segment decoding mode, the
reference picture selection mode, and the scalability mode. This
section summarizes the impact of these new coding options on
packetization. Refer to [4] for more information on coding options.
The slice structured mode was added to H.263+ for three purposes: to
provide enhanced error resilience capability, to make the bitstream
more amenable to use with an underlying packet transport such as RTP,
and to minimize video delay. The slice structured mode supports
fragmentation at macroblock boundaries.
With the independent segment decoding (ISD) option, a video picture
frame is broken into segments and encoded in such a way that each
segment is independently decodable. Utilizing ISD in a lossy network
environment helps to prevent the propagation of errors from one
segment of the picture to others.
The reference picture selection mode allows the use of an older
reference picture rather than the one immediately preceding the
current picture. Usually, the last transmitted frame is implicitly
used as the reference picture for inter-frame prediction. If the
reference picture selection mode is used, the data stream carries
information on what reference frame should be used, indicated by the
temporal reference as an ID for that reference frame. The reference
picture selection mode can be used with or without a back channel,
which provides information to the encoder about the internal status
of the decoder. However, no special provision is made herein for
carrying back channel information.
H.263+ also includes bitstream scalability as an optional coding
mode. Three kinds of scalability are defined: temporal, signal-to-
noise ratio (SNR), and spatial scalability. Temporal scalability is
achieved via the disposable nature of bi-directionally predicted
frames, or B-frames. (A low-delay form of temporal scalability known
as P-picture temporal scalability can also be achieved by using the
reference picture selection mode described in the previous
paragraph.) SNR scalability permits refinement of encoded video
frames, thereby improving the quality (or SNR). Spatial scalability
is similar to SNR scalability except the refinement layer is twice
the size of the base layer in the horizontal dimension, vertical
dimension, or both.
2. Usage of RTP
When transmitting H.263+ video streams over the Internet, the output
of the encoder can be packetized directly. All the bits resulting
from the bitstream including the fixed length codes and variable
length codes will be included in the packet, with the only exception
being that when the payload of a packet begins with a Picture, GOB,
Slice, EOS, or EOSBS start code, the first two (all-zero) bytes of
the start code are removed and replaced by setting an indicator bit
in the payload header.
For H.263+ bitstreams coded with temporal, spatial, or SNR
scalability, each layer may be transported to a different network
address. More specifically, each layer may use a unique IP address
and port number combination. The temporal relations between layers
shall be expressed using the RTP timestamp so that they can be
synchronized at the receiving ends in multicast or unicast
applications.
The H.263+ video stream will be carried as payload data within RTP
packets. A new H.263+ payload header is defined in section 4. This
section defines the usage of the RTP fixed header and H.263+ video
packet structure.
2.1 RTP Header Usage
Each RTP packet starts with a fixed RTP header. The following fields
of the RTP fixed header are used for H.263+ video streams:
Marker bit (M bit): The Marker bit of the RTP header is set to 1 when
the current packet carries the end of current frame, and is 0
otherwise.
Payload Type (PT): The Payload Type shall specify the H.263+ video
payload format.
Timestamp: The RTP Timestamp encodes the sampling instance of the
first video frame data contained in the RTP data packet. The RTP
timestamp shall be the same on successive packets if a video frame
occupies more than one packet. In a multilayer scenario, all
pictures corresponding to the same temporal reference should use the
same timestamp. If temporal scalability is used (if B-frames are
present), the timestamp may not be monotonically increasing in the
RTP stream. If B-frames are transmitted on a separate layer and
address, they must be synchronized properly with the reference
frames. Refer to the 1998 ITU-T Recommendation H.263 [4] for
information on required transmission order to a decoder. For an
H.263+ video stream, the RTP timestamp is based on a 90 kHz clock,
the same as that of the RTP payload for H.261 stream [5]. Since both
the H.263+ data and the RTP header contain time information, it is
required that those timing information run synchronously. That is,
both the RTP timestamp and the temporal reference (TR in the picture
header of H.263) should carry the same relative timing information.
Any H.263+ picture clock frequency can be expressed as
1800000/(cd*cf) source pictures per second, in which cd is an integer
from 1 to 127 and cf is either 1000 or 1001. Using the 90 kHz clock
of the RTP timestamp, the time increment between each coded H.263+
picture should therefore be a integer multiple of (cd*cf)/20. This
will always be an integer for any "reasonable" picture clock
frequency (for example, it is 3003 for 29.97 Hz NTSC, 3600 for 25 Hz
PAL, 3750 for 24 Hz film, and 1500, 1250 and 1200 for the computer
display update rates of 60, 72 and 75 Hz, respectively). For RTP
packetization of hypothetical H.263+ bitstreams using "unreasonable"
custom picture clock frequencies, mathematical rounding could become
necessary for generating the RTP timestamps.
2.2 Video Packet Structure
A section of an H.263+ compressed bitstream is carried as a payload
within each RTP packet. For each RTP packet, the RTP header is
followed by an H.263+ payload header, which is followed by a number
of bytes of a standard H.263+ compressed bitstream. The size of the
H.263+ payload header is variable depending on the payload involved
as detailed in the section 4. The layout of the RTP H.263+ video
packet is shown as:
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 ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| H.263+ Payload Header ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| H.263+ Compressed Data Stream ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Any H.263+ start codes can be byte aligned by an encoder by using the
stuffing mechanisms of H.263+. As specified in H.263+, picture,
slice, and EOSBS starts codes shall always be byte aligned, and GOB
and EOS start codes may be byte aligned. For packetization purposes,
GOB start codes should be byte aligned; however, since this is not
required in H.263+, there may be some cases where GOB start codes are
not aligned, such as when transmitting existing content, or when
using H.263 encoders that do not support GOB start code alignment.
In this case, follow-on packets (see section 5.2) should be used for
packetization.
All H.263+ start codes (Picture, GOB, Slice, EOS, and EOSBS) begin
with 16 zero-valued bits. If a start code is byte aligned and it
occurs at the beginning of a packet, these two bytes shall be removed
from the H.263+ compressed data stream in the packetization process
and shall instead be represented by setting a bit (the P bit) in the
payload header.
3. Design Considerations
The goals of this payload format are to specify an efficient way of
encapsulating an H.263+ standard compliant bitstream and to enhance
the resiliency towards packet losses. Due to the large number of
different possible coding schemes in H.263+, a copy of the picture
header with configuration information is inserted into the payload
header when appropriate. The use of that copy of the picture header
along with the payload data can allow decoding of a received packet
even in such cases in which another packet containing the original
picture header becomes lost.
There are a few assumptions and constraints associated with this
H.263+ payload header design. The purpose of this section is to
point out various design issues and also to discuss several coding
options provided by H.263+ that may impact the performance of
network-based H.263+ video.
o The optional slice structured mode described in Annex K of H.263+
[4] enables more flexibility for packetization. Similar to a
picture segment that begins with a GOB header, the motion vector
predictors in a slice are restricted to reside within its
boundaries. However, slices provide much greater freedom in the
selection of the size and shape of the area which is represented as
a distinct decodable region. In particular, slices can have a size
which is dynamically selected to allow the data for each slice to
fit into a chosen packet size. Slices can also be chosen to have a
rectangular shape which is conducive for minimizing the impact of
errors and packet losses on motion compensated prediction. For
these reasons, the use of the slice structured mode is strongly
recommended for any applications used in environments where
significant packet loss occurs.
o In non-rectangular slice structured mode, only complete slices
should be included in a packet. In other words, slices should not
be fragmented across packet boundaries. The only reasonable need
for a slice to be fragmented across packet boundaries is when the
encoder which generated the H.263+ data stream could not be
influenced by an awareness of the packetization process (such as
when sending H.263+ data through a network other than the one to
which the encoder is attached, as in network gateway
implementations). Optimally, each packet will contain only one
slice.
o The independent segment decoding (ISD) described in Annex R of [4]
prevents any data dependency across slice or GOB boundaries in the
reference picture. It can be utilized to further improve
resiliency in high loss conditions.
o If ISD is used in conjunction with the slice structure, the
rectangular slice submode shall be enabled and the dimensions and
quantity of the slices present in a frame shall remain the same
between each two intra-coded frames (I-frames), as required in
H.263+. The individual ISD segments may also be entirely intra
coded from time to time to realize quick error recovery without
adding the latency time associated with sending complete INTRA-
pictures.
o When the slice structure is not applied, the insertion of a
(preferably byte-aligned) GOB header can be used to provide resync
boundaries in the bitstream, as the presence of a GOB header
eliminates the dependency of motion vector prediction across GOB
boundaries. These resync boundaries provide natural locations for
packet payload boundaries.
o H.263+ allows picture headers to be sent in an abbreviated form in
order to prevent repetition of overhead information that does not
change from picture to picture. For resiliency, sending a complete
picture header for every frame is often advisable. This means that
(especially in cases with high packet loss probability in which
picture header contents are not expected to be highly predictable),
the sender may find it advisable to always set the subfield UFEP in
PLUSPTYPE to '001' in the H.263+ video bitstream. (See [4] for the
definition of the UFEP and PLUSPTYPE fields).
o In a multi-layer scenario, each layer may be transmitted to a
different network address. The configuration of each layer such as
the enhancement layer number (ELNUM), reference layer number
(RLNUM), and scalability type should be determined at the start of
the session and should not change during the course of the session.
o All start codes can be byte aligned, and picture, slice, and EOSBS
start codes are always byte aligned. The boundaries of these
syntactical elements provide ideal locations for placing packet
boundaries.
o We assume that a maximum Picture Header size of 504 bits is
sufficient. The syntax of H.263+ does not explicitly prohibit
larger picture header sizes, but the use of such extremely large
picture headers is not expected.
4. H.263+ Payload Header
For H.263+ video streams, each RTP packet carries only one H.263+
video packet. The H.263+ payload header is always present for each
H.263+ video packet. The payload header is of variable length. A 16
bit field of the basic payload header may be followed by an 8 bit
field for Video Redundancy Coding (VRC) information, and/or by a
variable length extra picture header as indicated by PLEN. These
optional fields appear in the order given above when present.
If an extra picture header is included in the payload header, the
length of the picture header in number of bytes is specified by PLEN.
The minimum length of the payload header is 16 bits, corresponding to
PLEN equal to 0 and no VRC information present.
The remainder of this section defines the various components of the
RTP payload header. Section five defines the various packet types
that are used to carry different types of H.263+ coded data, and
section six summarizes how to distinguish between the various packet
types.
4.1 General H.263+ payload header
The H.263+ payload header is structured as follows:
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RR |P|V| PLEN |PEBIT|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
RR: 5 bits
Reserved bits. Shall be zero.
P: 1 bit
Indicates the picture start or a picture segment (GOB/Slice) start
or a video sequence end (EOS or EOSBS). Two bytes of zero bits
then have to be prefixed to the payload of such a packet to compose
a complete picture/GOB/slice/EOS/EOSBS start code. This bit allows
the omission of the two first bytes of the start codes, thus
improving the compression ratio.
V: 1 bit
Indicates the presence of an 8 bit field containing information for
Video Redundancy Coding (VRC), which follows immediately after the
initial 16 bits of the payload header if present. For syntax and
semantics of that 8 bit VRC field see section 4.2.
PLEN: 6 bits
Length in bytes of the extra picture header. If no extra picture
header is attached, PLEN is 0. If PLEN>0, the extra picture header
is attached immediately following the rest of the payload header.
Note the length reflects the omission of the first two bytes of the
picture start code (PSC). See section 5.1.
PEBIT: 3 bits
Indicates the number of bits that shall be ignored in the last byte
of the picture header. If PLEN is not zero, the ignored bits shall
be the least significant bits of the byte. If PLEN is zero, then
PEBIT shall also be zero.
4.2 Video Redundancy Coding Header Extension
Video Redundancy Coding (VRC) is an optional mechanism intended to
improve error resilience over packet networks. Implementing VRC in
H.263+ will require the Reference Picture Selection option described
in Annex N of [4]. By having multiple "threads" of independently
inter-frame predicted pictures, damage of individual frame will cause
distortions only within its own thread but leave the other threads
unaffected. From time to time, all threads converge to a so-called
sync frame (an INTRA picture or a non-INTRA picture which is
redundantly represented within multiple threads); from this sync
frame, the independent threads are started again. For more
information on codec support for VRC see [7].
P-picture temporal scalability is another use of the reference
picture selection mode and can be considered a special case of VRC in
which only one copy of each sync frame may be sent. It offers a
thread-based method of temporal scalability without the increased
delay caused by the use of B pictures. In this use, sync frames sent
in the first thread of pictures are also used for the prediction of a
second thread of pictures which fall temporally between the sync
frames to increase the resulting frame rate. In this use, the
pictures in the second thread can be discarded in order to obtain a
reduction of bit rate or decoding complexity without harming the
ability to decode later pictures. A third or more threads can also
be added as well, but each thread is predicted only from the sync
frames (which are sent at least in thread 0) or from frames within
the same thread.
While a VRC data stream is - like all H.263+ data - totally self-
contained, it may be useful for the transport hierarchy
implementation to have knowledge about the current damage status of
each thread. On the Internet, this status can easily be determined
by observing the marker bit, the sequence number of the RTP header,
and the thread-id and a circling "packet per thread" number. The
latter two numbers are coded in the VRC header extension.
The format of the VRC header extension is as follows:
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| TID | Trun |S|
+-+-+-+-+-+-+-+-+
TID: 3 bits
Thread ID. Up to 7 threads are allowed. Each frame of H.263+ VRC
data will use as reference information only sync frames or frames
within the same thread. By convention, thread 0 is expected to be
the "canonical" thread, which is the thread from which the sync
frame should ideally be used. In the case of corruption or loss of
the thread 0 representation, a representation of the sync frame
with a higher thread number can be used by the decoder. Lower
thread numbers are expected to contain equal or better
representations of the sync frames than higher thread numbers in
the absence of data corruption or loss. See [7] for a detailed
discussion of VRC.
Trun: 4 bits
Monotonically increasing (modulo 16) 4 bit number counting the
packet number within each thread.
S: 1 bit
A bit that indicates that the packet content is for a sync frame.
An encoder using VRC may send several representations of the same
"sync" picture, in order to ensure that regardless of which thread
of pictures is corrupted by errors or packet losses, the reception
of at least one representation of a particular picture is ensured
(within at least one thread). The sync picture can then be used
for the prediction of any thread. If packet losses have not
occurred, then the sync frame contents of thread 0 can be used and
those of other threads can be discarded (and similarly for other
threads). Thread 0 is considered the "canonical" thread, the use
of which is preferable to all others. The contents of packets
having lower thread numbers shall be considered as having a higher
processing and delivery priority than those with higher thread
numbers. Thus packets having lower thread numbers for a given sync
frame shall be delivered first to the decoder under loss-free and
low-time-jitter conditions, which will result in the discarding of
the sync contents of the higher-numbered threads as specified in
Annex N of [4].
5. Packetization schemes
5.1 Picture Segment Packets and Sequence Ending Packets (P=1)
A picture segment packet is defined as a packet that starts at the
location of a Picture, GOB, or slice start code in the H.263+ data
stream. This corresponds to the definition of the start of a video
picture segment as defined in H.263+. For such packets, P=1 always.
An extra picture header can sometimes be attached in the payload
header of such packets. Whenever an extra picture header is attached
as signified by PLEN>0, only the last six bits of its picture start
code, '100000', are included in the payload header. A complete
H.263+ picture header with byte aligned picture start code can be
conveniently assembled on the receiving end by prepending the sixteen
leading '0' bits.
When PLEN>0, the end bit position corresponding to the last byte of
the picture header data is indicated by PEBIT. The actual bitstream
data shall begin on an 8-bit byte boundary following the payload
header.
A sequence ending packet is defined as a packet that starts at the
location of an EOS or EOSBS code in the H.263+ data stream. This
delineates the end of a sequence of H.263+ video data (more H.263+
video data may still follow later, however, as specified in ITU-T
Recommendation H.263). For such packets, P=1 and PLEN=0 always.
The optional header extension for VRC may or may not be present as
indicated by the V bit flag.
5.1.1 Packets that begin with a Picture Start Code
Any packet that contains the whole or the start of a coded picture
shall start at the location of the picture start code (PSC), and
should normally be encapsulated with no extra copy of the picture
header. In other words, normally PLEN=0 in such a case. However, if
the coded picture contains an incomplete picture header (UFEP =
"000"), then a representation of the complete (UFEP = "001") picture
header may be attached during packetization in order to provide
greater error resilience. Thus, for packets that start at the
location of a picture start code, PLEN shall be zero unless both of
the following conditions apply:
1) The picture header in the H.263+ bitstream payload is incomplete
(PLUSPTYPE present and UFEP="000"), and
2) The additional picture header which is attached is not incomplete
(UFEP="001").
A packet which begins at the location of a Picture, GOB, slice, EOS,
or EOSBS start code shall omit the first two (all zero) bytes from
the H.263+ bitstream, and signify their presence by setting P=1 in
the payload header.
Here is an example of encapsulating the first packet in a frame
(without an attached redundant complete picture header):
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RR |1|V|0|0|0|0|0|0|0|0|0| bitstream data without the |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| first two 0 bytes of the PSC ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
5.1.2 Packets that begin with GBSC or SSC
For a packet that begins at the location of a GOB or slice start
code, PLEN may be zero or may be nonzero, depending on whether a
redundant picture header is attached to the packet. In environments
with very low packet loss rates, or when picture header contents are
very seldom likely to change (except as can be detected from the GFID
syntax of H.263+), a redundant copy of the picture header is not
required. However, in less ideal circumstances a redundant picture
header should be attached for enhanced error resilience, and its
presence is indicated by PLEN>0.
Assuming a PLEN of 9 and P=1, below is an example of a packet that
begins with a byte aligned GBSC or a SSC:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RR |1|V|0 0 1 0 0 1|PEBIT|1 0 0 0 0 0| picture header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| starting with TR, PTYPE ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... | bitstream |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| data starting with GBSC/SSC without its first two 0 bytes ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Notice that only the last six bits of the picture start code,
'100000', are included in the payload header. A complete H.263+
picture header with byte aligned picture start code can be
conveniently assembled if needed on the receiving end by prepending
the sixteen leading '0' bits.
5.1.3 Packets that Begin with an EOS or EOSBS Code
For a packet that begins with an EOS or EOSBS code, PLEN shall be
zero, and no Picture, GOB, or Slice start codes shall be included
within the same packet. As with other packets beginning with start
codes, the two all-zero bytes that begin the EOS or EOSBS code at the
beginning of the packet shall be omitted, and their presence shall be
indicated by setting the P bit to 1 in the payload header.
System designers should be aware that some decoders may interpret the
loss of a packet containing only EOS or EOSBS information as the loss
of essential video data and may thus respond by not displaying some
subsequent video information. Since EOS and EOSBS codes do not
actually affect the decoding of video pictures, they are somewhat
unnecessary to send at all. Because of the danger of
misinterpretation of the loss of such a packet (which can be detected
by the sequence number), encoders are generally to be discouraged
from sending EOS and EOSBS.
Below is an example of a packet containing an EOS code:
0 1 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RR |1|V|0|0|0|0|0|0|0|0|0|1|1|1|1|1|1|0|0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
5.2 Encapsulating Follow-On Packet (P=0)
A Follow-on packet contains a number of bytes of coded H.263+ data
which does not start at a synchronization point. That is, a Follow-
On packet does not start with a Picture, GOB, Slice, EOS, or EOSBS
header, and it may or may not start at a macroblock boundary. Since
Follow-on packets do not start at synchronization points, the data at
the beginning of a follow-on packet is not independently decodable.
For such packets, P=0 always. If the preceding packet of a Follow-on
packet got lost, the receiver may discard that Follow-on packet as
well as all other following Follow-on packets. Better behavior, of
course, would be for the receiver to scan the interior of the packet
payload content to determine whether any start codes are found in the
interior of the packet which can be used as resync points. The use
of an attached copy of a picture header for a follow-on packet is
useful only if the interior of the packet or some subsequent follow-
on packet contains a resync code such as a GOB or slice start code.
PLEN>0 is allowed, since it may allow resync in the interior of the
packet. The decoder may also be resynchronized at the next segment
or picture packet.
Here is an example of a follow-on packet (with PLEN=0):
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RR |0|V|0|0|0|0|0|0|0|0|0| bitstream data ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
6. Use of this payload specification
There is no syntactical difference between a picture segment packet and
a Follow-on packet, other than the indication P=1 for picture segment or
sequence ending packets and P=0 for Follow-on packets. See the
following for a summary of the entire packet types and ways to
distinguish between them.
It is possible to distinguish between the different packet types by
checking the P bit and the first 6 bits of the payload along with the
header information. The following table shows the packet type for
permutations of this information (see also the picture/GOB/Slice header
descriptions in H.263+ for details):
--------------+--------------+----------------------+-------------------
First 6 bits | P-Bit | PLEN | Packet | Remarks
of Payload |(payload hdr.)| |
--------------+--------------+----------------------+-------------------
100000 | 1 | 0 | Picture | Typical Picture
100000 | 1 | > 0 | Picture | Note UFEP
1xxxxx | 1 | 0 | GOB/Slice/EOS/EOSBS | See possible GNs
1xxxxx | 1 | > 0 | GOB/Slice | See possible GNs
Xxxxxx | 0 | 0 | Follow-on |
Xxxxxx | 0 | > 0 | Follow-on | Interior Resync
--------------+--------------+----------------------+-------------------
The details regarding the possible values of the five bit Group
Number (GN) field which follows the initial "1" bit when the P-bit is
"1" for a GOB, Slice, EOS, or EOSBS packet are found in section 5.2.3
of [4].
As defined in this specification, every start of a coded frame (as
indicated by the presence of a PSC) has to be encapsulated as a
picture segment packet. If the whole coded picture fits into one
packet of reasonable size (which is dependent on the connection
characteristics), this is the only type of packet that may need to be
used. Due to the high compression ratio achieved by H.263+ it is
often possible to use this mechanism, especially for small spatial
picture formats such as QCIF and typical Internet packet sizes around
1500 bytes.
If the complete coded frame does not fit into a single packet, two
different ways for the packetization may be chosen. In case of very
low or zero packet loss probability, one or more Follow-on packets
may be used for coding the rest of the picture. Doing so leads to
minimal coding and packetization overhead as well as to an optimal
use of the maximal packet size, but does not provide any added error
resilience.
The alternative is to break the picture into reasonably small
partitions - called Segments - (by using the Slice or GOB mechanism),
that do offer synchronization points. By doing so and using the
Picture Segment payload with PLEN>0, decoding of the transmitted
packets is possible even in such cases in which the Picture packet
containing the picture header was lost (provided any necessary
reference picture is available). Picture Segment packets can also be
used in conjunction with Follow-on packets for large segment sizes.
7. Security Considerations
RTP packets using the payload format defined in this specification
are subject to the security considerations discussed in the RTP
specification [1], and any appropriate RTP profile (for example [2]).
This implies that confidentiality of the media streams is achieved by
encryption. Because the data compression used with this payload
format is applied end-to-end, encryption may be performed after
compression so there is no conflict between the two operations.
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 which are complex to decode and cause the receiver to
be overloaded. However, this encoding does not exhibit any
significant non-uniformity.
As with any IP-based protocol, in some circumstances a receiver may
be overloaded simply by the receipt of too many packets, either
desired or undesired. Network-layer authentication may be used to
discard packets from undesired sources, but the processing cost of
the authentication itself may be too high. In a multicast