Internet Engineering Task Force G. Liebl
Internet Draft LNT, Munich Univ. of
Technology
Document: draft-ietf-avt-uxp-06.txt
October 2003 M. Wagner, J. Pandel,
W. Weng
Expires: April 2004 Siemens AG, Munich
An RTP Payload Format for Erasure-Resilient Transmission of
Progressive Multimedia Streams
Status of this Memo
This document is an Internet-Draft and is in full conformance
with all provisions of Section 10 of RFC2026.
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Abstract
This document specifies an efficient way to ensure erasure-
resilient transmission of progressively encoded multimedia
sources via RTP using Reed-Solomon (RS) codes together with
interleaving. The level of erasure protection can be explicitly
adapted to the importance of the respective parts in the source
stream, thus allowing a graceful degradation of application
quality with increasing packet loss rate on the network. Hence,
this type of unequal erasure protection (UXP) schemes is intended
to cope with the rapidly varying channel conditions on wireless
access links to the Internet backbone. Furthermore, protection of
non-progressive multimedia streams is ensured, since equal
erasure protection (EXP) represents a subset of generic UXP. By
applying interleaving and RS codes a payload format is defined,
which can be easily integrated into the existing framework for
RTP.
Table of Contents
1. Introduction.................................................2
2. Conventions used in this Document............................3
3. Reed-Solomon Codes...........................................6
4. Progressive Source Coding....................................7
5. General Structure of UXP Schemes.............................8
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6. RTP payload structure.......................................13
7. Indication of UXP in SDP....................................20
8. Security Considerations.....................................21
9. Application Statement.......................................22
10. Intellectual Property Considerations.......................23
11. References.................................................23
12. Acknowledgments............................................24
13. Author's Addresses.........................................24
1. Introduction
Due to the increasing popularity of high-quality multimedia
applications over the Internet and the high level of public
acceptance of existing mobile communication systems, there is a
strong demand for a future combination of these two techniques:
One possible scenario consists of an integrated communication
environment, where users can set up multimedia connections
anytime and anywhere via radio access links to the Internet.
For this reason, several packet-oriented transmission modes like
EGPRS (Enhanced General Packet Radio Service) or UMTS (Universal
Mobile Telecommunications System) can be used, which are mostly
based on the same principle: Long message blocks, i.e. IP
packets, that enter the wireless part of the network are split up
into segments of desired length, which can be multiplexed onto
link layer packets of fixed size. The latter are then transmitted
sequentially over the wireless link, reassembled, and passed on
to the next network element.
However, compared to the rather benign channel characteristics on
today's fixed networks, wireless links suffer from severe fading,
noise, and interference conditions in general, thus resulting in
a comparably high residual bit error rate after detection and
decoding. By use of efficient CRC-mechanisms, these bit errors
are usually detected with very high probability, and every
corrupted segment, i.e. which contains at least one erroneous
bit, is discarded to prevent error propagation through the
network. But if only one single segment is missing at the
reassemble stage, the upper layer IP packet cannot be
reconstructed anymore. The result is a significant increase in
packet loss rate at IP level.
Since most multimedia applications can only recover from a very
limited number of lost IP packets, it is vitally necessary to
keep packet loss at IP level within a certain acceptable range
depending on the individual quality-of-service requirements.
However, due to the delay constraints typically imposed by most
audio or video codecs, the use of ARQ-schemes is often prohibited
both at link level and at transport level. In addition,
retransmission strategies cannot be applied to any broadcast or
multicast scenarios. Thus, forward erasure correction strategies
have to be considered, which provide a simple means to
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reconstruct the content of lost packets at the receiver from the
redundancy that has been spread out over a certain number of
consecutive packets.
There already exist some previous studies and proposals regarding
erasure-resilient packet transmission [1,8]. Since most of them
are based on the assumption that all parts in a message block are
equally important to the receiver, i.e. the respective
application cannot operate on partly complete blocks, they were
optimized with respect to assigning equal erasure protection over
the whole message block. However, recent developments both in
audio and video coding have introduced the notion of
progressively encoded media streams, for which unequal erasure
protection strategies seem to be more promising, as it will be
explained in more detail below. Although the scheme defined in
[1] is in principle capable of supporting some kind of unequal
erasure protection, possible implementations seem to be quite
complex with respect to the gain in performance. Finally, in [1]
it is assumed that consecutive RTP packets can have variable
length, which would cause significant segmentation overhead at
the link layer of almost all wireless systems.
This document defines a payload format for RTP, such that
different elements in a progressively encoded multimedia stream
can be protected against packet erasures according to their
respective quality-of-service requirement. The general principle,
including the use of Reed-Solomon codes together with an
appropriate interleaving scheme for adding redundancy, follows
the ideas already presented in [3], but allows for finer
granularity in the structure of the progressive media stream. The
proposed scheme is generic in the way that it (1) is independent
of the type of media stream, be it audio or video, and (2) can be
adapted to varying transmission quality very quickly by use of
inband-signaling.
2. Conventions used in this Document
The following terms are used throughout this document:
1.) Segment: denotes a link layer transport unit.
2.) Segmentation/Reassembly Process: If the size of the
transport units at the link layer is smaller than that at
the upper layers, message blocks have to be split up into
several parts, i.e. segments, which are then transmitted
subsequently over the link. If nothing is lost, the original
message block can be restored at the receiving entity
(reassembly).
3.) Codec: denotes a functional pair consisting of a source
encoding unit at the sender and a corresponding source
decoding unit at the receiver; usually standardized for
different media applications like audio or video.
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4.) Media stream: A bitstream. which results at the output of an
encoder for a specific media type, e.g. H.263, MPEG-4
Visual.
5.) Progressive media stream: A media stream which can be
divided into successive elements. The distinct elements are
of different importance to the decoding process and are
commonly ordered from highest to least importance, where the
latter elements depend on the previous.
6.) Progressive source coding: results in a progressive media
stream.
7.) Reed-Solomon (RS) code: belongs to the class of linear
nonbinary block codes, and is uniquely specified by the
block length n, the number of parity symbols t, and the
symbol alphabet.
8.) n: is a variable, which denotes both the block length of a
RS codeword, and the number of columns in a TB (see 19).
9.) k: is a variable, which denotes the number of information
symbols in an RS codeword.
10.) t: is a variable, which denotes the number of parity symbols
in an RS codeword.
11.) Erasure: When a packet is lost during transmission, an
erasure is said to have happened. Since the position of the
erased packet in a sequence is usually known, a
corresponding erasure marker can be set at the receiving
entity.
12.) Base layer: comprises the first and most important elements
of the progressive media stream, without which all
subsequent information is useless.
13.) Enhancement layer: comprises one or more sets of the less
important subsequent elements of the progressive media
stream. A specific enhancement layer can be decoded, if and
only if the base layer and all previous enhancement layer
data (of higher importance) are available.
14.) Info stream: denotes the bitstream which has to be
protected by the UXP scheme. It usually consists of the
media stream (progressively source encoded or not), which is
arranged according to a desired syntax (e.g. to achieve an
appropriate framing, see Sect. 6.3 ). In any case, it is
assumed that every info stream is already octet-aligned
according to the standard procedures defined in the context
of the used syntax specifications.
15.) Info octet: Denotes one element of the info stream.
16.) Transmission block (TB): denotes a memory array of L rows
and n columns. Each row of a TB represents a RS codeword,
whereas each column, together with the respective UXP header
(see 36) in front, forms the payload of a single RTP packet.
Each TB consists of at least two distinct transmission sub
blocks (TSB, see20): The first L_s rows belong to the
signaling TSB, whereas the last L_d=(L-L_s) rows belong to
one or more data TSB.
17.) Transmission sub block (TSB): denotes a memory array of
0<l<L rows and n columns, which is a horizontal slice of a
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TB. Depending on whether the info octet positions are filled
with descriptors (see31) or media data, the TSB is of type
signaling or data, respectively.
18.) L: is a variable, which denotes both the number of rows in a
TB and the payload length (without UXP header, see 36) of an
RTP packet in octets.
19.) Unequal erasure protection (UXP): denotes a specific
strategy which varies the level of erasure protection across
a TB according to a given redundancy profile.
20.) Equal erasure protection (EXP): is a subset of UXP, for
which the level of erasure protection is kept constant
across a TB.
21.) Redundancy profile: describes the size of the different
erasure protection classes in a TB, i.e. the number of rows
(codewords) per class.
22.) Erasure protection class: contains a set of rows (codewords)
of the TB with same erasure correction capability.
23.) i: is a variable, which denotes the number of parity
symbols for each row in erasure protection class i.
24.) EPC_i: is a variable, which denotes the set of rows
contained in erasure protection class i.
25.) R_i: is a variable, which denotes the total number of rows
contained in erasure protection class i, i.e. the
cardinality of EPC_i.
26.) T: is a variable, which denotes the number of parity
symbols for each row in the highest erasure protection class
(with respect to application data) in a TB.
27.) EPV: denotes the erasure protection vector of length (T+1)
used to describe a certain redundancy profile.
28.) DP: descriptor used for in-band signaling of the erasure
protection vector.
29.) SI: stuffing indicator, which contains the number of media
stuffing symbols at the end of a data TSB (see 34).
30.) Descriptor Stuffing: insertion of otherwise unused
descriptor values (i.e. 0x00) at the end of the signaling
TSB. Descriptor stuffing is performed, if the final sequence
of descriptors and stuffing indicators for a valid
redundancy profile is shorter than the space initially
reserved for it in the signaling TSB.
31.) Media Stuffing: insertion of additional symbols at the end
of a data TSB. Media stuffing is performed, if the info
stream (see 17) is shorter than the space reserved for it in
the data TSB for a desired redundancy profile. Since the
number of stuffing symbols is signaled in the respective SI,
any octet value may be used (e.g. 0x00).
32.) Interleaver: performs the spreading of a codeword, i.e. a
row in the TB, over n successive packets, such that the
probability of an erasure burst in a codeword is kept small.
33.) UXP header: is the additional header information contained
in each RTP packet after UXP has been applied. It is always
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present at the start of the payload section of an RTP
packet.
34.) X: denotes a currently not used extension field of 1 bit in
the UXP header.
35.) P: is a variable which denotes the number of parity symbols
per row used to protect the inband signaling of the
redundancy profile.
36.) ceil(.): denotes the ceiling function, i.e. rounding up to
the next integer.
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. Reed-Solomon Codes
Reed-Solomon (RS) codes are a special class of linear nonbinary
block codes, which are known to offer maximum erasure correction
capability with minimum amount of redundancy.
An arbitrary t-erasure-correcting (n,k) RS code defined over
Galois field GF(q) has the following parameters [2]:
- Block length: n=q-1
- No. of information symbols in a codeword: k
- No. of parity-check symbols in a codeword: n-k=t
- Minimum distance: d=t+1
In what follows, only systematic RS codes over GF(2^8) shall be
considered, i.e. the symbols of interest can be directly related
to a tuple of eight bits, which is commonly called an octet in
packet transmission. The principle structure of a codeword is
shown in Fig. 1.
By shortening the initial (n=255,n-t) RS code, any desired
(n',n'-t) RS code for a given erasure correction capability t may
be obtained.
block of n octets
<----------------->
+-+-+-+-+-+-+-+-+-+
|&|&|&|&|&|&|&|*|*|
+-+-+-+-+-+-+-+-+-+
<------------><--->
k=n-t t
(&:info) (*:parity)
Fig. 1: Structure of a systematic RS codeword
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4. Progressive Source Coding
The output of an encoder for a specific media type, e.g. H.263 or
MPEG-4 Visual is said to be a media stream. If the media stream
consists of several distinct elements, which are of different
importance with respect to the quality of the decoding process at
the receiver, then the media stream is progressive. The
progressive media stream is often organized in separate layers.
Hence, there exists at least one layer, often called base layer,
without which decoding fails at all, whereas all the other
layers, often called enhancement layers, just help to continually
improve the quality. Consequently, the different layers are
usually contained in the (source-)encoded media stream in
decreasing order of importance, i.e. the base layer data is
followed by the various enhancement layers.
An example can be found in the fine granular scalability modes
which have been proposed to various standardization bodies like
MPEG, where the resolution of the scaling process in the
progressive source encoder is as low as one symbol in the
enhancement layer [4]. Another example is given by data
partitioning which can be applied to the ITU/MPEG H.26L standard
[5], MPEG-4, and H.263++. Also, the existence of I,P, and B
frames in streams which comply with standards like MPEG-2 can be
interpreted as progressive.
From the above definition, it is quite obvious that the most
important base layer data must be protected as strongly as
possible against packet loss during transmission. However, the
protection of the enhancement layers can be continually lowered,
since a loss at these stages has only minor consequences for the
decoding process. Thus, by using a suitable unequal erasure
protection strategy across a progressive media stream, the
overhead due to redundancy is reduced. Furthermore, if channel
conditions get worse during transmission, only more and more
enhancement layers are lost, i.e. a graceful degradation in
application quality at the receiver is achieved [6].
Nevertheless, it should be mentioned that the specific structure
of the media stream strongly depends on the actual media codec in
use and does not always provide suitable mechanisms for transport
over data networks, like framing (see also Sect. 6.3 ). In order
to keep the description of the unequal erasure protection
strategy in Sect. 5 as general as possible, the final bitstream
which has to be protected by the proposed UXP scheme will be
called "info stream" in the following. Furthermore, it is assumed
that every info stream is already octet-aligned according to the
standard procedures defined in the context of the used syntax
specifications.
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5. General Structure of UXP Schemes
In this section, the principle features of the proposed UXP
scheme are described with a special focus on the protection and
reconstruction procedure which is applied to the info stream. In
addition, the behavior of the sender and receiver is specified as
far as it concerns the reconstruction of the info stream.
However, the complete UXP payload structure, including the
additional UXP header, is described in Sect. 6.
The reason for using the term "info stream" as well as the
details of the construction are described in Sect. 6.3 . For now,
we assume that we have an info stream which has to be protected.
Fig. 1 already illustrated the structure of a systematic RS
codeword, which shall be represented by a single row with n
successive symbols that contain the information and the parity
octets. This structure shall now be extended by forming a
transmission block (TB) consisting of L codewords of length n
octets each, which amounts to a total of L rows and n columns
[7]: Each column, together with the respective UXP header in
front, shall represent the payload of an RTP packet, i.e. the
whole data of a TB is transmitted via a sequence of n RTP packets
all carrying a payload of length (L+2) octets (UXP header
included).
Each TB usually consists of two or more horizontal sub blocks,
the so-called transmission sub blocks (TSB), as can be seen in
Fig. 2: The first L_s rows always belong to the signaling TSB,
which is used to convey the actual redundancy profile in the data
part to the receiver (see 6.4.). The following L_d=(L-L_s) rows
belong to one or more data TSBs, which contain the interleaved
and RS encoded info stream, as will be described below.
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Transmission Block (TB)
/\ +-+-+-+-+-+-+-+-+-+ /\
| | signaling TSB | | L_s octets
| +-+-+-+-+-+-+-+-+-+ \/
| | | /\ /\
| + data TSB #1 + | L_d(1) octets |
| | | | |
| +-+-+-+-+-+-+-+-+-+ \/ |
L octets | | | /\ |
payload | + data TSB #2 + | L_d(2) octets |
per packet | + | | | L_d oct.
| +-+-+-+-+-+-+-+-+-+ \/ |
| | . | . |
| + . + . |
| | . | . |
| +-+-+-+-+-+-+-+-+-+ /\ |
| | data TSB #z | | L_d(z) octets |
\/ +-+-+-+-+-+-+-+-+-+ \/ \/
<----------------->
n packets
Fig. 2: General structure of a TB
Since the UXP procedure is mainly applied to the data TSBs, it
will be described next, whereas the content and syntax of the
signaling TSB will be defined in section 6.4.
For means of simplification, only one single data TSB will be
assumed throughout the following explanation of the encoding and
decoding procedure. However, an extension to more than one data
TSB per TB is straightforward, and will be shown in section 6.5.
As depicted in Fig. 3, the rows of a transmission sub block shall
be assembled into T+1 different classes EPC_i, where i=0...T,
such that each class contains exactly R_i=|EPC_i| consecutive
rows of the matrix, where the R_i have to satisfy the following
relationship:
R_0+R_1+...+R_T=L_d
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Data Transmission Sub Block (data TSB)
T
<------->
/\ +-+-+-+-+-+-+-+-+-+ /\
| |&|&|&|&|&|*|*|*|*| |
| +-+-+-+-+-+-+-+-+-+ | R_T=3
| |&|&|&|&|&|*|*|*|*| |
| +-+-+-+-+-+-+-+-+-+ |
L_d octets | |&|&|&|&|&|*|*|*|*| \/
per packet | +-+-+-+-+-+-+-+-+-+ /\
| |%|%|%|%|%|%|*|*|*| | R_(T-1)=1
| +-+-+-+-+-+-+-+-+-+ \/
| |$|$|$|$|$|$|$|*|*| .
| +-+-+-+-+-+-+-+-+-+ .
| |!|!|!|!|!|!|!|!|*| .
| +-+-+-+-+-+-+-+-+-+ /\
| |#|#|#|#|#|#|#|#|#| | R_0=1
\/ +-+-+-+-+-+-+-+-+-+ \/
<----------------->
n packets
&,%,$,!,# : info octets belonging to a certain info stream in
decreasing order of importance
* : parity octets gained from Reed-Solomon coding
Fig. 3: General structure for coding with unequal erasure
protection
Furthermore, all rows in a particular class EPC_i shall contain
exactly the same number of parity octets, which is equal to the
index i of the class. For each row in a certain class EPC_i, the
same (n,n-i) RS code shall be applied.
As can be observed from Fig. 3, class EPC_T contains the largest
number of parity octets per row, i.e. offers the highest erasure
protection capability in the block. Consequently, the most
important element in the info stream must be assigned to class
EPC_T, where the value of T should be chosen according to the
desired outage threshold of the application given a certain
packet erasure rate on the link.
All other classes EPC_(T-1)...EPC_0 shall be sequentially filled
with the remaining elements of the info stream in decreasing
order of importance, where the optimal choice for the size of
each class (0 or more rows), i.e. the structure of the redundancy
profile, should depend on the quality-of-service requirements for
the various (progressively-encoded) layers.
The following set of rules contains a compact description of all
the operations that must be performed for each transmission
block:
1.) The total number of columns n of the TB shall be chosen
according to the actual delay constraints of the application.
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2.) Next, the expected number of rows reserved for the signaling
TSB has to selected, which limits the data TSB to L_d=(L-L_s)
rows.
3.) The maximum erasure correction capability T in the data TSB
should be chosen according to the desired outage threshold of the
application given the actual packet erasure rate on the link.
4.) The redundancy profile for the rest of the data TSB should
depend on the size and number of the various layers in the info
stream, as well as the desired probability of successful decoding
for each of them (quality-of-service requirement).
5.) Any suitable optimization algorithm may be used for deriving
an adequate redundancy profile. However, the result has to
satisfy the following constraints:
a) All available info octet positions in the data TSB have to be
completely filled. If the info stream is too short for a desired
profile, media stuffing may be applied to the empty info octet
positions at the end of the data TSB by appending a sufficient
number of octets (with arbitrary value, e.g. 0x00). The actual
number of stuffing symbols per data TSB is then signaled via the
respective stuffing indicator (see Sect. 6.4.). However, before
resorting to any stuffing, it should be checked whether it is
possible to strengthen the protection of certain rows instead,
thus improving the overall robustness of the decoding process.
b) The info stream SHOULD be fully contained within the data TSB
(unless cutting it off at a specific point is explicitly allowed
by the properties of the info stream).
c) The number of required descriptors and stuffing indicators
(see section 6.4.) to signal the profile SHALL NOT exceed the
space initially reserved for them in the signaling TSB.
Constraints a) and b) should be already incorporated in the
optimization algorithm. However, if constraint c) is not met, the
data TSB has to be reduced by one row in favor of the signaling
TSB to accommodate more space for the descriptors and stuffing
indicators, i.e. steps 2-5 have to be repeated until a valid
redundancy profile has been obtained.
6.) For each nonempty class EPC_i, i=T...0, in the data TSB, the
following steps have to be performed:
a) All rows of this specific class SHALL be filled from left to
right and top to bottom with data octets of the info stream.
b) For each row in the class, the required i parity-check octets
are computed from the same set of codewords of an (n,n-i) RS
code, and filled in the empty positions at the end of each row.
Thus, every row in the class constitutes a valid codeword of the
chosen RS code.
7.) After having filled the whole data TSB with information and
parity octets, the redundancy profile is mapped to the signaling
TSB as described in section 6.4.
8.) Each column of the resulting TB is now read out octet-wise
from top to bottom and, together with the respective UXP header
(see section 6.2.) in front, is mapped onto the payload section
of one and only one RTP packet.
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9.) The n resulting RTP packets SHALL be transmitted
consecutively to the remote host, starting with the leftmost one.
10.) At the corresponding protocol entity at the remote host, the
payload (without the UXP header) of all successfully received RTP
packets belonging to the same sending TB SHALL be filled into a
similar receiving TB column-wise from top to bottom and left to
right.
11.) For every erased packet of a received TB, the respective
column in the TB shall be filled with a suitable erasure marker.
12.) Before any other operations can be performed, the redundancy
profile has to be restored from the signaling TSB according to
the procedure defined in Sect. 6.4.. If the attempt fails because
of too many lost packets, the whole TB SHALL be discarded and the
receiving entity should wait for the next incoming TB.
13.) If the attempt to recover the redundancy profile has been
successful, a decoding operation shall be performed for each row
of the data TSB by applying any suitable algorithm for erasure
decoding.
14.) For all rows of the data TSB for which the decoding
operation has been successful, the reconstructed data octets are
read out from left to right and top to bottom, and appended to
the reconstructed version of the info stream.
One can easily realize that the above rules describe an
interleaver, i.e. at the sender a single codeword of a TB is
spread out over n successive packets. Thus, each codeword of a
transmitted TB experiences the same number of erasures at exactly
the same positions.
Two important conclusions can be drawn from this:
a) Since the same RS code is applied to all rows contained in a
specific class, either all of them can be correctly decoded or
none. Hence, there exist no partly decodable classes at the
receiver.
b) If decoding is successful for a certain class EPC_i, all the
classes EPC_(i+1)...EPC_T can also be decoded, since they are
protected by at least one more parity octet per row. Together
with rule 6, it is therefore always ensured, that in case a
decodable enhancement layer exists, all other layers it depends
on can also be reconstructed!
Given the maximum erasure protection value T, the redundancy
profile for a data TSB of size (L_d x n) shall be denoted by a
so-called erasure protection vector EPV of length (T+1), where
EPV:=(R_0,R_1,...,R_(T-1),R_T)
From the above definition, it is easy to realize that the trivial
cases of no erasure protection and EXP are a subset of UXP:
a) no erasure protection at all: all application data is mapped
onto
class EPC_0, i.e. EPV=(L_d,0,0,...,0).
b) EXP: all application data is mapped onto class EPC_T, i.e.
EPV=(0,0,...,0,R_T=L_d).
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Hence, the UXP payload format also can be used with info streams
which are non progressive.
6. RTP payload structure
This section is organized as follows. First, the specific
settings in the RTP header are shown. Next, the RTP payload
header for UXP (the so-called UXP header) is specified. After
that, the structure of the bitstream which is protected by UXP,
the so-called info stream, is discussed. Finally, the in-band
signaling of the erasure protection vector is introduced.
For every packet, the UXP payload is formed by reading out a
column of the TB and prefixing it with the UXP header. Thus, an
UXP-compliant RTP packet looks as follows:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
|RTP Header| UXP Header| one column of the TB |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
6.1 Specific Settings in the RTP Header
The timestamp of each RTP packet is set to the sampling time of
the first octet of the progressive media stream in the
corresponding TB. If several data TSBs are included in one TB,
the sampling time of data TSB #1 is relevant. This results in the
TS value being the same for all RTP packets belonging to a
specific TB.
The payload type is of dynamic type, and obtained through out-of-
band signaling similar to [1]. End systems, which cannot
recognize a payload type, must discard it.
The marker bit is set to 1 in the last packet of a TB; otherwise,
its value is 0.
All other fields in the RTP header are set to those values
proposed for regular multimedia transmission using the RTP-format
of the media stream which is protected by UXP, e.g for MPEG-4
Visual as specified in RFC 3016.
6.2. Structure of the UXP Header
The UXP header shall consist of 2 octets, and is shown in Fig. 4:
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0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|X| block PT | block length n|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fig. 4: Proposed UXP header
The fields in the UXP header are defined as follows:
- X (bit 0): extension bit, reserved for future enhancements,
currently not in use -> default value: 0
- block PT (bits 1-7): regular RTP payload type to indicate the
media type contained in the info stream
- block length n (bits 8-15): indicates total number of RTP
- packets
resulting from one TB (which equals
the number of columns of the TB)
The syntax of the info stream which is protected by UXP is
specified by the RTP payload type field contained in the UXP
header. The details of the info stream are described in Sec. 6.3
For example, payload type H.263 means that the info stream
conforms to the specifications of the RTP profile for H.263 and
does not represent the "raw" H.263 media stream produced by an
H.263 encoder.
However, UXP can also be applied to the "raw" media stream (in
case it is already octet-aligned), if this can be signaled to the
receiver via other means, e.g. by use of H.245 or SDP.
Based on the RTP sequence number, the marker bit, and the
repetition of the block length n in each UXP header, the
receiving entity is able to recognize both TB boundaries and the
actual position of packets (both received and lost ones) in the
TB.
6.3 Framing and Timing Mechanism in UXP: The Info Stream
As described in Sect. 5, UXP creates its own packetization scheme
by interleaving. The regular framing and timing structure of RTP
is therefore destroyed. This section describes which kind of
problems arise with interleaving and how they can be solved. This
finally leads to the specification of the info stream.
The timestamp of an RTP packet usually describes the sampling
time of the first octet included in the RTP data packet. This is
in principle also true for UXP RTP packets. According to the time
stamp definition in Sect. 6.1 every packet contains the
timestamp of the sampling time of the first octet in the
corresponding TB. Therefore, all packets which belong to one TB
contain the same timestamp. This can lead to problems since due
to the theoretical size limit of a TB (the limit for the number
of columns is 256, and the limit for the number of rows is the
maximum packet size), it can contain data from different sampling
time instances, e.g. several video frames. Then the timing
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information of the later frames has to be determined from the
media stream itself and not from the RTP timestamp.
A second problem arising with interleaving is that the framing
mechanism of RTP is not supported. Consider a media encoder,
which does not create a fully decodable bitstream, e.g. H.26L
with the video coding layer (VCL) and network adaptation layer
(NAL) concept [9]. In this concept the VCL creates slices which
are prepared for transmission over several networks at the NAL.
Consequently, in case of RTP transmission, header information
which allows to decode the slices is included only in the RTP
packets. Thus, to fill an UXP TB with the "raw" media stream from
the VCL can lead, even without packet losses, to a non-decodable
stream.
The framing problem can be solved in two ways:
One solution could be to use the RTP payload specification of a
given media stream to create a bitstream with an appropriate
framing, resulting in the so-called info stream. For example, to
create an H.263 info stream, the following steps are necessary:
1.) Generate an H.263-compliant media stream, i.e. take a slice
or a video frame directly from the H.263 encoder.
2.) Apply the H.263 payload specification (e.g. RFC 2429) to
create the RTP payload for only one packet.
3.) Insert the latter row by row into one data TSB.
It is possible to apply the procedure mentioned above several
times for different data TSBs (see Sect. 6.5.). Due to the in-
band signaling, it is possible to determine the beginning and end
of every TSB without parsing the whole TB. This allows a fast
decomposition of the TB into the different TSBs.
Another solution of the framing problem would be to rely on the
framing mechanism of the media stream. This is, for example,
possible for media streams which contain start codes.
The timing problem can be solved in two ways.
One solution is to comply with the RTP payload specification of
the media stream. If the specification allows to put into one
packet octets which belong to different sampling times, this
should also be allowed for a TB.
The second solution for the timing problem is to rely on the
timing information contained in the media stream itself, if
available.
Therefore, there are two different modes for framing:
1.) RTP payload framing (if an RTP payload specification exists
for the media stream),
2.) pure media stream framing (if framing is contained in the
media stream),
and two different modes for timing:
1.) timing rules of the RTP payload specification for the media
stream,
2.) timing information within the media stream.
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All combinations of timing and framing modes are possible, but
framing mode 1 and timing mode 1 represent the default mode of
operation for UXP. The use of other timing and framing modes has
to be signaled by non RTP means.
The info stream is thus defined by the media stream together with
framing and timing rules.
In the following, some examples will be given:
1.) The info stream for MPEG-4 Visual according to RFC 3016 is
the pure MPEG-4 compliant media stream, since RFC 3016
specifies (in case of video) to take the MPEG-4 compliant
video stream as payload.
2.) The info stream for H.263+ can be created according to RFC
2429 as follows:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
|H.263+ payload| H.263+ compliant stream (possibly changed with|
|header | respect to RFC 2429) containing a slice/frame |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
This info stream is inserted into one single data TSB.
If necessary, for example, if the slices are too short to achieve
a reasonable TB size, several info streams can be inserted in one
TB by concatenating several data TSBs to a single TB (see Sect.
6.5.).
6.4. In-band Signaling of the Structure of the Redundancy Profile
To enable a dynamic adaptation to varying link conditions, the
actual redundancy profile used in the data TSB as well as the
beginning and end of a TSB must be signaled to the receiving
entity. Since out-of-band signaling either results in excessive
additional control traffic, or prevents quick changes of the
profile between successive TBs, an in-band signaling procedure is
desired.
Since without knowledge of the correct redundancy profile, the
decoding process cannot be applied to any of the erasure
protection classes, the redundancy profile has to be protected at
least as strongly as the most important element in the info
stream. Therefore, an additional class EPC_P is used in the
signaling TSB, where the number of parity symbols is by default
set to the following value:
P=ceil(n/2.0)
Hence, up to 50% of the RTP packets can be lost, before the
redundancy profile cannot be recovered anymore. This seems to be
a reasonable value for the lowest point of operation over a lossy
link. Alternatively, P may be explicitly signaled during session
setup by means of SDP or H.245 protocol.
Consequently, since all other classes must have equal or less
erasure protection capability, the maximum allowable value for
class EPC_T in the data TSB is now limited to T<=P.
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The signaling of the erasure protection vector is accomplished by
means of descriptors. In the following we describe an efficient
encoding scheme for the descriptors.
For each class EPC_i with R_i>0, there is a descriptor DP_i
providing information about the size of class EPC_i (i.e. the
value of R_i) and establishing a relationship between the erasure
protection of class EPC_i and that of the class EPC_(i+j), where
j>0 and j is the smallest value for which R_(i+j)>0 is true. A
descriptor DP_i is mapped onto one octet, which is sub-divided
into two half-octets (i.e. the higher and the lower four bits).
The first half-octet is of type unsigned and contains the 4-bit
representation of the decimal value R_i. The second half-octet is
of type signed and contains the difference in erasure protection
between class EPC_i and class EPC_(i+j), i.e. the signed 4-bit
representation of the decimal value (-j) (where the MSB denotes
the sign, and the lower three bits the absolute value). Note that
the erasure protection P of class EPC_p is fixed, whereas the
size R_P may vary.
Thus, the data to be filled into class EPC_P shall consist of a
sequence of descriptors separated by stuffing indicators (see
below), where the number of descriptors is primarily given by the
number of protection classes EPC_i, 0<=i<=T, in the data TSB with
R_i>0.
Without a-priori knowledge, the initial value for the size of the
signaling TSB, R_P, should be set to one (row). When the number
of necessary descriptors and stuffing indicators exceeds the (n-
P) information positions, one or more additional rows have to be
reserved. This is usually done by increasing the value for L_s to
R_P>1, i.e. the data TSB is reduced to (L-R_P) rows. Hence, in
order to indicate the actual size of the signaling TSB, an
additional descriptor is inserted at the very beginning, which
takes on the value 0xq0, where q denotes the (octal) four bit
representation of the decimal value R_P.
Furthermore, the end of each data TSB is signaled by the
otherwise unused descriptor value 0x00, followed by exactly one
stuffing indicator (SI). The latter is mapped onto an octet,
which is of type unsigned and contains the 8-bit representation
of the decimal value of the number of media stuffing symbols used
at the end of the respective data TSB.
The (extended) sequence of descriptors and stuffing indicators is
then mapped to the octet positions in the R_P rows of the
signaling TSB from left to right and top to bottom. Each row is
then encoded with the same (n,n-P) RS code.
If the number of descriptors and stuffing indicators is less than
the available octet positions, however, empty positions in class
EPC_P may be filled up with the otherwise unused descriptor 0x00.
At the receiving entity, the sequence of descriptors shall be
recovered by performing erasure decoding on the first row of the
TB (which definitely belongs to the signaling TSB) using the same
algorithm as later for the data TSB. If successful, the very
first descriptor now indicates the number of rows of the
signaling TSB, and the next (R_P-1) rows are decoded to
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reconstruct the redundancy profile for the data TSB(s), together
with the number of media stuffing symbols denoted by the
respective SI(s).
The complete structure of the TB is now depicted in Fig. 5.
Transmission Block (TB)
P
<--------->
/\ +-+-+-+-+-+-+-+-+-+ /\
| |?|?|?|?|*|*|*|*|*| | R_P=1
| +-+-+-+-+-+-+-+-+-+ \/
| |&|&|&|&|&|*|*|*|*| /\
| +-+-+-+-+-+-+-+-+-+ | R_T=3
| |&|&|&|&|&|*|*|*|*| |
| +-+-+-+-+-+-+-+-+-+ |
L octets | |&|&|&|&|&|*|*|*|*| \/
payload | +-+-+-+-+-+-+-+-+-+ /\
per packet | |%|%|%|%|%|%|*|*|*| | R_(T-1)=1
| +-+-+-+-+-+-+-+-+-+ \/
| |$|$|$|$|$|$|$|*|*| .
| +-+-+-+-+-+-+-+-+-+ .
| |!|!|!|!|!|!|!|!|*| .
| +-+-+-+-+-+-+-+-+-+ /\
| |#|#|#|#|#|#|#|#|#| | R_0=1
\/ +-+-+-+-+-+-+-+-+-+ \/
<----------------->
n packets
? : descriptors and stuffing indicators for in-band
signaling of the redundancy profile
&,%,$,!,# : info octets belonging to a certain element of the
info stream in decreasing order of importance
* : parity octets gained from Reed-Solomon coding
Fig. 5: General structure for UXP with in-band signaling of the
redundancy profile
The following simple example is meant to illustrate the idea
behind using descriptors: Let an erasure protection vector of
length T+1=7 be given as follows:
EPV=(R_0,R_1,...,R_5,R_6)=(7,0,2,2,0,3,10)
Hence, the length L of the TB (including one row for the
signaling TSB) is equal to 7+2+2+3+10+1=25 (rows/octets). If the
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width is assumed to be equal to 20 (columns/packets), then the
erasure protection of the descriptors is P=10.
The corresponding sequence of descriptors can be written as
DP=(DP_6,DP_5,DP_3,DP_2,DP_0)=(0xAC,0x39,0x2A,0x29,0x7A),
where the values of the descriptors are given in hexadecimal
notation. Next, the descriptor indicating the length of the
signaling TSB has to be inserted, the end of the data TSB has to
be marked by 0x00, and the SI has to be appended. If the number
of media stuffing symbols is assumed to be 3, the 10 info octets
in the signaling TSB take on the following values (descriptor
stuffing included):
(0x10,0xAC,0x39,0x2A,0x29,0x7A,0x00,0x03,0x00,0x00)
6.5. Optional Concatenation of Transmission Sub Blocks
The following procedure may be applied if a single info stream
would be too short to achieve an efficient mapping to a
transmission block with respect to the fixed payload length L and
the desired number of packets n. For example, intra-coded video
frames (I-frames) are usually much larger than the following
predicted ones (P-frames). In this case, a certain number z of
successive small info streams should be each mapped to a
transmission sub block with length L_d(y) and width n, such that
L_d(1)+L_d(2)+...+L_d(z)=L_d.
The resulting transmission sub blocks can then be easily
concatenated to form a TB of size L x n having one common
signaling TSB (see Fig. 2): Since the second half-octet of the
descriptors is of type signed (cf. Sect. 6.4.), we are able to
signal both decreasing and increasing erasure protection
profiles.
Again, we will give a simple example to illustrate this idea: Let
the erasure protection vectors for two concatenated data TSBs be
given as follows:
EPV1=(R1_0,R1_1,...,R1_5,R1_6)=(0,0,2,2,0,3,10),
EPV2=(R2_0,R2_1,...,R2_5,R2_6)=(0,0,2,2,0,3,10).
Hence, two single identical data TSBs will be concatenated to
form a TB of length L=2*(2+2+3+10)+2=36 (rows/octets). If the
width is again assumed to be equal to 20 (columns/packets), then
the erasure protection of the descriptors is P=10. We reserve a
total of two rows for the signaling TSB. The corresponding
sequence of descriptors can now be written as
DP=(0xAC,0x39,0x2A,0x29,0xA4,0x39,0x2A,0x29), where the values of
the descriptors are given in hexadecimal notation. The values of
the first four descriptors are taken from the descriptor of EPV1
as described in Sect. 6.4. (without the SI). The last four
descriptors are taken from the descriptor of EPV2 (without SI)
with one exception. The fifth descriptor of DP (i.e. 0xA4) is
created as follows: The first half-octed is created according to
Sect. 6.4. However, the second half-octed describes no longer the
difference between R_P and R2_6. It rather describes the
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difference between R1_2 and R2_6, i.e. R1_2-R2_6, which can be a
positive or negative number. If the number of media stuffing
symbols is assumed to be 3 for each data TSB, the 20 info octet
positions in the signaling TSB are filled with the following
values (descriptor stuffing included):
(0x20,0xAC,0x39,0x2A,0x29,0x00,0x03,0xA4,0x39,0x2A,0x29,0x00,0x03
,
0x00,0x00,0x00,0x00,0x00,0x00,0x00)
Therefore from the example above, the following general rule MUST
be used to create the resulting descriptors for concatenated data
TSB #u and data TSB #v, where v=u+1:
Let EPVu=(Au_0,Au_1,...) and EPVv=(Av_0, Av_1,...) be the
corresponding erasure protection vectors and DPu and DPv the
corresponding descriptors created according to Sect. 6.4. (with
stuffing). Let w be the smallest index for which Au_w >0. Let x
be the largest index for which Av_x >0. The resulting descriptor
can be created by concatenation of DPu and DPv where the first
descriptor of DPv should be changed as follows:
The second half byte is defined by Au_w-Av_x.
7. Indication of UXP in SDP
From the discussion in Sect. 6.3 , we know that UXP encapsulates
and protects the info stream. The info stream consists usually of
a regular RTP-Payload format, e.g. RFC 3016.
There is no static payload type assignment for UXP, so dynamic
payload type numbers MUST be used. The binding to the number is
indicated by an rtpmap attribute. The name used in this binding
is
"UXP". The payload type number of UXP is indicated in the "m"
line of the
media as well as the payload type of the info-stream.
A sample indication of UXP in SDP is as follows:
m = video 8000 RTP/AVP 98 99
a = rtpmap:98 UXP/90000
a = rtpmap:99 MP4V-ES/90000
Here, PT 98 indicates that the payload consists of UXP with the
corresponding info stream "MP4V-ES". Alternatively, PT 99 can be
used which indicates "MP4V-ES" without UXP.
Since UXP is generic, several payload types can be protected. The
lines
m = video 8000 RTP/AVP 98 99 100
a = rtpmap:98 UXP/90000
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a = rtpmap:99 MP4V-ES/90000
a = rtpmap:100 H263-1998/90000
mean that UXP can be used with either "MP4V-ES" or "H263-1998" as
info stream (indicated by PT 98 in the RTP-Header and either
block PT=99 or block PT=100 in the UXP-Header). Alternatively,
PT=99 or PT=100 in the RTP-Header means the use of "MP4V-ES" or
"H263-1998" without UXP.
As described in Sect. 6.4., the parameter P has the default value
P=ceil(n/2.0), if not otherwise stated. The parameter P MAY be
specified explicitly by means of SDP:
a = fmtp:98 UXP-prof: fvalue
where fvalue is a floating point number in the interval (0 <
fvalue <1) and specifies P by P=ceil(n*fvalue). For example, if
we set fvalue=0.5,
a = fmtp:98 UXP-prof: 0.5
we get the default value for P, since P=ceil(n/2.0).
The ABNF for fvalue according to RFC 2234 is
fvalue = "0" "." 1*2DIGIT
8. Security Considerations
The payload of the RTP-packets consists of an interleaved media
and parity stream. Therefore, it is reasonable to encrypt the
resulting stream with one key rather than using different keys
for media and parity data. It should also be noted that
encryption of the media data without encryption of the parity
data could enable known-plaintext attacks.
The overall proportion between parity octets and info octets
should be chosen carefully if the packet loss is due to network
congestion. If the proportion of parity octets per TB is
increased in this case, it could lead to increasing network
congestion. Therefore, the proportion between parity octets and
info octets per TB MUST NOT be increased as packet loss increases
due to network congestion.
The overall ratio between parity and info octets MUST NOT be
higher than 1:1, i.e. the absolute bitrate spent for redundancy
must not be larger than the bitrate required for transmission of
multimedia data itself.
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9. Application Statement
There are currently two different schemes proposed for unequal
error protection in the IETF-AVT: Unequal Level Protection (ULP)
and Unequal Erasure Protection (UXP).
Although both methods seem to address the same problem, the
proposed solutions differ in many respects. This section tries to
describe possible application scenarios and to show the strengths
and weaknesses of both approaches.
The main difference between both approaches is that while ULP
preserves the structure of the packets which have to be protected
and provides the redundancy in extra packets, UXP interleaves the
info stream which has to be protected, inserts the redundancy
information, and thus creates a totally new packet structure.
Another difference concerns multicast compatibility: It cannot be
assumed that all future terminals will be able to apply UXP/ULP.
Therefore, backward compatibility could be an issue in some
cases. Since ULP does not change the original packet structure,
but only adds some extra packets, it is possible for terminals
which do not
support ULP to discard the extra packets. In case of UXP,
however, two separate streams with and without erasure protection
have to be sent, which increases the overall data rate.
Next, both approaches offer different mechanisms to adjust packet
sizes, if necessary: UXP allows to adjust the packet sizes
arbitrarily. This is an advantage in case the loss probability is
dependent on the packet length, which happens, for example, if
the end-to-end connection contains wireless links. In this case
proper adjustment of the packet size is one essential network
adaptation technique. In addition, if a preencoded stream is sent
over the network, the packet size can be adjusted independently
of slice structures.
Since ULP does not change the existing packetization scheme, this
flexibility does not exist.
The ability of UXP to adjust the packet size arbitrarily can be
especially exploited in a streaming scenario, if a delay of
several hundred milliseconds is acceptable. It is then possible
to fill several video frames into a single TB of desired size,
e.g. a group of pictures consisting of I-frame, P-frames and B-
frames. The redundancy scheme can thus be selected in such a way
as to guarantee the following property: In case of packet loss,
the P-frames are only recoverable if the I-frame on which the
decoding of P-frames depends is recoverable. The same is true for
B-frames, which can only be decoded if the respective P-frames
are recoverable. This prevents situations in which, for example,
the B-frames have been received correctly, but the P-frames have
been lost, i.e. assures a gradual decrease in application quality
also on the frame level. Of course, a similar encoding is
possible with ULP. But in this case one might have to send
several frames within one packet which leads to large packet
sizes.
Furthermore, decoding delay is also a crucial issue in
communications. Again, both approaches have different delay
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properties: UXP introduces a decoding delay because a reasonable
amount of correctly received packets are necessary to start
decoding of a TB. The delay in general depends on the dimensions
of the interleaver. This should be considered for any system
design which includes UXP.
With ULP, every correctly received media packet can be decoded
right away. However, a significant delay is introduced, if
packets are corrupted, because in this case one has to wait for
several redundancy packets. Thus, the delay is in general
dependent on the actual ULP-FEC-packet scheme and cannot be
considered in advance during the system design phase.
Finally, we want to point out that UXP uses RS codes which are
known
to be the most efficient type of block codes in terms of erasure
correction capability.
10. Intellectual Property Considerations
Siemens AG has filed patent applications that might possibly have
technical relations to this contribution.
On IPR related issues, Siemens AG refers to the Siemens Statement
on Patent Licensing, see http://www.ietf.org/ietf/IPR/SIEMENS-
General.
11. References
Normative References
[1] J. Rosenberg and H. Schulzrinne, "An RTP Payload Format for
Generic Forward Error Correction", Request for Comments 2733,
Internet Engineering Task Force, Dec. 1999.
[2] Shu Lin and Daniel J. Costello, Error Control Coding:
Fundamentals and Applications, Prentice-Hall, Inc., Englewood
Cliffs, N.J., 1983.
Informative References
[3] A. Albanese, J. Bloemer, J. Edmonds, M. Luby, and M. Sudan,
"Priority encoding transmission", IEEE Trans. Inform. Theory,
vol. 42, no. 6, pp. 1737-1744, Nov. 1996.
[4] W. Li: "Streaming video profile in MPEG-4", IEEE Trans. on
Circuits and Systems for Video Technology, Vol. 11, no. 3, 301-
317, March 2001.
[5] G. Blaettermann, G. Heising, and D. Marpe: "A Quality
Scalable Mode for H.26L", ITU-T SG16, Q.15, Q15-J24, Osaka, May
2000.
[6] F. Burkert, T. Stockhammer, and J. Pandel, "Progressive A/V
coding for lossy packet networks - a principle approach", Tech.
Rep., ITU-T SG16, Q.15, Q15-I36, Red Bank, N.J., Oct. 1999.
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[7] Guenther Liebl, "Modeling, theoretical analysis, and coding
for wireless packet erasure channels", Diploma Thesis, Inst. for
Communications Engineering, Munich University of Technology,
1999.
[8] U. Horn, K. Stuhlmuller, M. Link, and B. Girod, "Robust
Internet video transmission based on scalable coding and unequal
error protection", Image Com., vol. 15, no. 1-2, pp. 77-94, Sep.
1999.
[9] S. Wenger, "H.26L over IP: The IP-Network Adaptation Layer",
Packet Video 2002, Pittsburgh, Pennsylvania, USA, April 24-
26,2002.
12. Acknowledgments
Many thanks to Philippe Gentric, Stephen Casner, and Hermann
Hellwagner for helpful comments and improvements. The authors
would like to thank Thomas Stockhammer who came up with the
original idea of UXP. Also, the help of Gero Baese, Frank
Burkert, and Minh Ha Nguyen for the development of UXP is well
acknowledged.
13. Author's Addresses
Guenther Liebl
Institute for Communications Engineering (LNT)
Munich University of Technology
D-80290 Munich
Germany
Email: {liebl}@lnt.e-technik.tu-muenchen.de
Marcel Wagner, Juergen Pandel, Wenrong Weng
Siemens AG - Corporate Technology CT IC 2
D-81730 Munich
Germany
Email:
{marcel.wagner,juergen.pandel,wenrong.weng}@mchp.siemens.de
Full Copyright Statement
"Copyright (C) The Internet Society (date). All Rights Reserved.
This document and translations of it may be copied and furnished
to others, and derivative works that comment on or otherwise
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explain it or assist in its implementation may be prepared,
copied, published and distributed, in whole or in part, without
restriction of any kind, provided that the above copyright notice
and this paragraph are included on all such copies and derivative
works. However, this document itself may not be modified in any
way, such as by removing the copyright notice or references to
the Internet Society or other Internet organizations, except as
needed for the purpose of developing Internet standards in which
case the procedures for copyrights defined in the Internet
Standards process must be followed, or as required to translate
it into languages other than English.
The limited permissions granted above are perpetual and will not
be revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on
an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES; EXPRESS OR
IMPLIED; INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE
OF INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR
PURPOSE.
Liebl,Wagner,Pandel,Weng [Page 25]
