One document matched: draft-ietf-fecframe-framework-10.xml
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<?rfc toc="yes" ?>
<rfc category="std" docName="draft-ietf-fecframe-framework-10"
ipr="pre5378Trust200902">
<front>
<title abbrev="FEC Framework">Forward Error Correction (FEC)
Framework</title>
<author fullname="Mark Watson" initials="M." surname="Watson">
<organization>Netflix, Inc.</organization>
<address>
<postal>
<street>100 Winchester Circle</street>
<city>Los Gatos, CA</city>
<region>CA</region>
<code>95032</code>
<country>U.S.A.</country>
</postal>
<email>watsonm@netflix.com</email>
</address>
</author>
<date day="7" month="September" year="2010" />
<area>Transport</area>
<workgroup>FEC Framework Working Group</workgroup>
<abstract>
<t>This document describes a framework for using forward error
correction (FEC) codes with applications in public and private IP
networks to provide protection against packet loss. The framework
supports applying Forward Error Correction to arbitrary packet flows
over unreliable transport and is primarily intended for real-time, or
streaming, media. This framework can be used to define Content Delivery
Protocols that provide Forward Error Correction for streaming media
delivery or other packet flows. Content Delivery Protocols defined using
this framework can support any FEC Scheme (and associated FEC codes)
which is compliant with various requirements defined in this document.
Thus, Content Delivery Protocols can be defined which are not specific
to a particular FEC Scheme and FEC Schemes can be defined which are not
specific to a particular Content Delivery Protocol.</t>
</abstract>
</front>
<middle>
<section title="Introduction">
<t>Many applications have a requirement to transport a continuous stream
of packetized data from a source (sender) to one or more destinations
(receivers) over networks which do not provide guaranteed packet
delivery. Primary examples are real-time, or streaming, media
applications such as broadcast, multicast or on-demand audio, video or
multimedia.</t>
<t>Forward Error Correction is a well-known technique for improving
reliability of packet transmission over networks which do not provide
guaranteed packet delivery, especially in multicast and broadcast
applications. The FEC Building Block defined in <xref
target="RFC5052"></xref> provides a framework for definition of Content
Delivery Protocols (CDPs) for object delivery (including, primarily,
file delivery) which make use of separately defined FEC Schemes. Any CDP
defined according to the requirements of the FEC Building Block can then
easily be used with any FEC Scheme which is also defined according to
the requirements of the FEC Building Block. (Note that the term "Forward
Erasure Correction" is sometimes used, 'erasures' being a type of error
in which data is lost and this loss can be detected, rather than being
received in corrupted form - the focus of this document is strictly on
erasures, however the term Forward Error Correction is more widely
used).</t>
<t>This document defines a framework for the definition of CDPs which
provide for FEC protection of arbitrary packet flows over unreliable
transports such as UDP. As such, this document complements the FEC
Building Block of <xref target="RFC5052"></xref>, by providing for the
case of arbitrary packet flows over unreliable transport, the same kind
of framework as that document provides for object delivery. This
document does not define a complete Content Delivery Protocol, but
rather defines only those aspects that are expected to be common to all
Content Delivery Protocols based on this framework.</t>
<t>This framework does not define how the flows to be protected are
determined, nor how the details of the protected flows and the FEC
streams which protect them are communicated from sender to receiver. It
is expected that any complete Content Delivery Protocol specification
which makes use of this framework will address these signalling
requirements. However, this document does specify the information which
is required by the FEC Framework at the sender and receiver - for
example details of the flows to be FEC protected, the flow(s) that will
carry the FEC protection data and an opaque container for
FEC-Scheme-specific information.</t>
<t>FEC Schemes designed for use with this framework must fulfil a number
of requirements defined in this document. Note that these requirements
are different from those defined in <xref target="RFC5052"></xref> for
FEC Schemes for object delivery. However there is a great deal of
commonality and FEC Schemes defined for object delivery may be easily
adapted for use with the framework defined here.</t>
<t>Since the RTP protocol layer is used over UDP, this framework can be
applied to RTP flows as well. FEC repair packets may be sent directly
over UDP or over RTP. The latter approach has the advantage that RTP
instrumentation, based on RTCP, can be used for the repair flow.
Additionally, the post-repair RTCP extended report <xref
target="RFC5725"></xref> may be used to obtain information about the
loss rate after FEC recovery.</t>
<t>The use of RTP for repair flows is defined for each FEC Scheme by
defining an RTP Payload Format for that particular FEC Scheme (possibly
in the same document).</t>
</section>
<section title="Definitions/Abbreviations">
<t><list style="hanging">
<t hangText="'ADU Flow'">A sequence of ADUs associated with a
transport layer flow identifier (such as the standard 5-tuple {
Source IP Address, Source Transport Port, Destination IP Address,
Destination Transport Port, Transport Protocol } in the case of
UDP)</t>
<t hangText="'AL-FEC'">Application Layer Forward Error
Correction</t>
<t hangText="'Application Data Unit'">The unit of source data
provided as payload to the transport layer</t>
<t hangText="'Application protocol'">Control protocol used to
establish and control the source data flow being protected - e.g.
RTSP.</t>
<t hangText="'Content Delivery Protocol (CDP)'">A complete
application protocol specification which, through the use of the
framework defined in this document, is able to make use of FEC
Schemes to provide Forward Error Correction capabilities</t>
<t hangText="'FEC'">Forward Error Correction.</t>
<t hangText="'FEC Code'">An algorithm for encoding data such that
the encoded data flow is resilient to data loss (Note: in general
FEC Codes may also be used to make a data flow resilient to
corruption, but that is not considered here).</t>
<t hangText="'FEC Framework'">A protocol framework for definition of
Content Delivery Protocols using FEC, such as the framework defined
in this document.</t>
<t hangText="'FEC Framework Configuration Information'">Information
which controls the operation of the FEC Framework.</t>
<t hangText="'FEC Payload ID'">Information which identifies the
contents of a packet with respect to the FEC Scheme.</t>
<t hangText="'FEC Repair Packet'">At a sender (respectively, at a
receiver) a payload submitted to (respectively, received from) the
Transport protocol containing one or more repair symbols along with
a Repair FEC Payload ID and possibly an RTP header.</t>
<t hangText="'FEC Scheme'">A specification which defines the
additional protocol aspects required to use a particular FEC code
with the FEC Framework, or, in the context of RMT, with the RMT FEC
Building Block.</t>
<t hangText="'FEC Source Packet'">At a sender (respectively, at a
receiver) a payload submitted to (respectively, received from) the
Transport protocol containing an ADU along with an optional Source
FEC Payload ID.</t>
<t hangText="'Protection amount'">The relative increase in data sent
due to the use of FEC.</t>
<t hangText="'Repair data flow'">The packet flow or flows carrying
forward error correction data</t>
<t hangText="'Repair FEC Payload ID'">An FEC Payload ID specifically
for use with repair packets.</t>
<t hangText="'Source data flow'">The packet flow or flows to which
FEC protection is to be applied. A source data flow consists of
ADUs.</t>
<t hangText="'Source FEC Payload ID'">An FEC Payload ID specifically
for use with source packets.</t>
<t hangText="'Source protocol'">A protocol used for the source data
flow being protected - e.g. RTP.</t>
<t hangText="'Transport protocol'">The protocol used for transport
of the source and repair data flows - e.g. UDP, DCCP.</t>
</list>The following definitions are aligned with <xref
target="RFC5052"></xref><list style="hanging">
<t hangText="'Code rate'">the ratio between the number of source
symbols and the number of encoding symbols. By definition, the code
rate is such that: 0 < code rate <= 1. A code rate close to 1
indicates that a small number of repair symbols have been produced
during the encoding process.</t>
<t hangText="'Encoding symbol'">unit of data generated by the
encoding process. With systematic codes, source symbols are part of
the encoding symbols.</t>
<t hangText="'Packet Erasure Channel'">a communication path where
packets are either dropped (e.g., by a congested router, or because
the number of transmission errors exceeds the correction
capabilities of the physical layer codes) or received. When a packet
is received, it is assumed that this packet is not corrupted.</t>
<t hangText="'Repair symbol'">encoding symbol that is not a source
symbol.</t>
<t hangText="'Source Block'">group of ADUs which are to be FEC
protected as a single block.</t>
<t hangText="'Source symbol'">unit of data used during the encoding
process.</t>
<t hangText="'Systematic code'">FEC code in which the source symbols
are part of the encoding symbols. The Reed-Solomon codes introduced
in this document are systematic.</t>
</list></t>
<t>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 <xref
target="RFC2119"></xref>.</t>
</section>
<section title="Architecture Overview">
<t>The FEC Framework is described in terms of an additional layer
between the transport layer (e.g. UDP or DCCP) and protocols running
over this transport layer. Examples of such protocols are RTP, RTCP,
etc. As such, the data path interface between the FEC Framework and both
underlying and overlying layers can be thought of as being the same as
the standard interface to the transport layer - i.e. the data exchanged
consists of datagram payloads each associated with a single ADU flow
identified (in the case of UDP) by the standard 5-tuple { Source IP
Address, Source Transport Port, Destination IP Address, Destination
Transport Port, Transport Protocol }. In the case that RTP is used for
the repair flows, the source and repair data may be multiplexed using
RTP onto a single UDP flow and must consequently be demultiplexed at the
receiver. There are various ways in which this multiplexing can be done,
for example as described in <xref target="RFC4588"></xref>.</t>
<t>It is important to understand that the main purpose of the FEC
Framework architecture is to allocate functional responsibilities to
separately documented components in such a way that specific instances
of the components can be combined in different ways to describe
different protocols.</t>
<t>The FEC Framework makes use of an FEC Scheme, in a similar sense to
that defined in <xref target="RFC5052"></xref> and uses the terminology
of that document. The FEC Scheme defines the FEC encoding and decoding
and defines the protocol fields and procedures used to identify packet
payload data in the context of the FEC Scheme. The interface between the
FEC Framework and an FEC Scheme, which is described in this document, is
a logical one, which exists for specification purposes only. At an
encoder, the FEC Framework passes ADUs to the FEC Scheme for FEC
encoding. The FEC Scheme returns repair symbols with their associated
Repair FEC Payload IDs, and in some case Source FEC Payload IDs,
depending on the FEC Scheme. At a decoder, the FEC Framework passes
transport packet payloads (source and repair) to the FEC Scheme and the
FEC Scheme returns additional recovered source packet payloads.</t>
<t>This document defines certain FEC Framework Configuration Information
which MUST be available to both sender and receiver(s). For example,
this information includes the specification of the ADU flows which are
to be FEC protected, specification of the ADU flow(s) which will carry
the FEC protection (repair) data and the relationship(s) between these
source and repair flows (i.e. which source flow(s) are protected by each
repair flow. The FEC Framework Configuration Information also includes
information fields which are specific to the FEC Scheme. This
information is analogous to the FEC Object Transmission Information
defined in <xref target="RFC5052"></xref>.</t>
<t>The FEC Framework does not define how the FEC Framework Configuration
Information for the stream is communicated from sender to receiver. This
must be defined by any Content Delivery Protocol specification as
described in the following sections.</t>
<t>In this architecture we assume that the interface to the transport
layer supports the concepts of data units (referred to here as
Application Data Units) to be transported and identification of ADU
flows on which those data units are transported. Since this is an
interface internal to the architecture, we do not specify this interface
explicitly. We do require that ADU flows which are distinct from the
transport layer point of view (for example, distinct UDP flows as
identified by the UDP source/destination ports/addresses) are also
distinct on the interface between the transport layer and the FEC
Framework.</t>
<t>As noted above, RTP flows are a specific example of ADU flows which
might be protected by the FEC Framework. From the FEC Framework point of
view, RTP source flows are ADU flows like any other, with the RTP header
included within the ADU.</t>
<t>Depending on the FEC Scheme, RTP may also be used as a transport for
repair packet flows. In this case an FEC Scheme must define an RTP
Payload Format for the repair data.</t>
<t>The architecture outlined above is illustrated in the <xref
target="architecturefigure"></xref>. In this architecture, two RTP
instances are shown, for the source and repair data respectively. This
is because the use of RTP for the source data is separate from and
independent of the use of RTP for the repair data. The appearance of two
RTP instances is more natural when you consider that in many FEC codes,
the repair payload contains repair data calculated across the RTP
headers of the source packets. Thus a repair packet carried over RTP
starts with an RTP header of its own which is followed (after the Repair
Payload ID) by repair data containing bytes which protect the source RTP
headers (as well as repair data for the source RTP payloads).</t>
<figure anchor="architecturefigure" title="FEC Framework Architecture">
<artwork><![CDATA[
+--------------------------------------------+
| Application |
+--------------------------------------------+
|
|
|
+ - - - - - - - - - - - - - - - - - - - - - - - -+
| +--------------------------------------------+ |
| Application Layer |
| +--------------------------------------------+ |
| |
| + -- -- -- -- -- -- -- -- -- -- --+ | |
| RTP (optional) | |
| | | |-Configuration/Coordination
+- -- -- -- -- -- -- -- -- -- -- -+ |
| | | |
| ADU flows |
| | v |
+--------------------------------------------+ +----------------+
| | FEC Framework (this document) |<--->| FEC Scheme |
+--------------------------------------------+ +----------------+
| | | |
Source | Repair |
| | | |
+-- -- -- -- --|-- --+ -- -- -- -- -- + -- --+
| | RTP | | RTP processing | |<--- Optional
| | +-- -- -- |- -- -+ | - dependent on
| | +-- -- -- -- -- -- -- |--+ | | FEC Scheme
| | RTP (de)multiplexing | |
| +-- -- -- --- -- -- -- -- -- -- -- -- -- -- -+ |
|
| +--------------------------------------------+ |
| Transport Layer (e.g. UDP) |
| +--------------------------------------------+ |
|
| +--------------------------------------------+ |
| IP |
| +--------------------------------------------+ |
Content Delivery Protocol
+ - - - - - - - - - - - - - - - - - - - - - - - +]]></artwork>
</figure>
<t>The contents of the transport payload for repair packets is fully
defined by the FEC Scheme. For a specific FEC Scheme, a means MAY be
defined for repair data to be carried over RTP, in which case the repair
packet payload format starts with the RTP header. This corresponds to
defining an RTP Payload Format for the specific FEC Scheme. Guidelines
for writers of RTP Payload Formats are provided in <xref
target="RFC2736"></xref>.</t>
<t>The use of RTP for repair packets is independent of the protocols
used for source packets: if RTP is used for source packets then repair
packets may or may not use RTP and vice versa (although it is unlikely
that there are useful scenarios where non-RTP source flows are protected
by RTP repair flows). FEC Schemes are expected to recover entire
transport payloads for recovered source packets in all cases. For
example if RTP is used for source flows, the FEC Scheme is expected to
recover the entire UDP payload, including the RTP header.</t>
</section>
<section title="Procedural overview">
<section title="General">
<t>The mechanism defined in this document does not place any
restrictions on the Application Data Units which can be protected
together, except that the Application Data Unit is carried over a
supported transport protocol (See <xref
target="TransportProtocols"></xref>). The data may be from multiple
Source Data Flows that are protected jointly. The FEC framework
handles the Source Data Flows as a sequence of 'source blocks' each
consisting of a set of Application Data Units, possibly from multiple
Source Data Flows which are to be protected together. For example,
each source block may be constructed from those Application Data Units
related to a particular segment in time of the flow.</t>
<t>At the sender, the FEC Framework passes the payloads for a given
block to the FEC Scheme for FEC encoding. The FEC Scheme performs the
FEC encoding operation and returns the following information: <list
style="symbols">
<t>optionally, FEC Payload IDs for each of the source payloads
(encoded according to an FEC-Scheme-specific format)</t>
<t>one or more FEC repair packet payloads</t>
<t>FEC Payload IDs for each of the repair packet payloads (encoded
according to an FEC-Scheme-specific format)</t>
</list>The FEC framework then performs two operations: Firstly, it
appends the FEC payload IDs, if provided, to each of the Application
Data Units, and sends the resulting packets, known as 'FEC source
packets', to the receiver and secondly it places the provided 'FEC
repair packet payloads' and corresponding 'FEC Repair Payload IDs'
appropriately to construct 'FEC repair packets' and send them to the
receiver. Note that FEC repair packets MAY be sent to a different
multicast group or groups from the source packets.</t>
<t>This document does not define how the sender determines which
Application Data Units are included in which source blocks or the
sending order and timing of FEC source and FEC repair packets. A
specific Content Delivery Protocol MAY define this mapping or it MAY
be left as implementation dependent at the sender. However, a CDP
specification MUST define how a receiver determines a minimum length
of time that it should wait to receive FEC repair packets for any
given source block. FEC Schemes MAY define limitations on this
mapping, such as maximum size of source blocks, but SHOULD NOT attempt
to define specific mappings. The sequence of operations at the sender
is described in more detail in <xref
target="senderoperation"></xref>.</t>
<t>At the receiver, original Application Data Units are recovered by
the FEC Framework directly from any FEC Source Packets received simply
by removing the Source FEC Payload ID, if present. The receiver also
passes the contents of the received Application Data Units, plus their
FEC Payload IDs to the FEC Scheme for possible decoding.</t>
<t>If any Application Data Units related to a given source block have
been lost, then the FEC Scheme may perform FEC decoding to recover the
missing Application Data Units (assuming sufficient FEC Source and FEC
Repair packets related to that source block have been received).</t>
<t>Note that the receiver may need to buffer received source packets
to allow time for the FEC Repair packets to arrive and FEC decoding to
be performed before some or all of the received or recovered packets
are passed to the application. If such a buffer is not provided, then
the application must be able to deal with the severe re-ordering of
packets that may occur. However, such buffering is Content Delivery
Protocol and/or implementation-specific and is not specified here. The
receiver operation is described in more detail in <xref
target="receiveroperation"></xref></t>
<t>The FEC Source packets MUST contain information which identifies
the source block and the position within the source block (in terms
specific to the FEC Scheme) occupied by the Application Data Unit.
This information is known as the 'Source FEC Payload ID'. The FEC
Scheme is responsible for defining and interpreting this information.
This information MAY be encoded into a specific field within the FEC
Source packet format defined in this specification, called the
Explicit Source FEC Payload ID field. The exact contents and format of
the Explicit Source FEC Payload ID field are defined by the FEC
Scheme. Alternatively, the FEC Scheme MAY define how the Source FEC
Payload ID is derived from other fields within the source packets.
This document defines the way that the Explicit Source FEC Payload ID
field is appended to source packets to form FEC Source packets.</t>
<t>The FEC Repair packets MUST contain information which identifies
the source block and the relationship between the contained repair
payloads and the original source block. This is known as the 'Repair
FEC Payload ID'. This information MUST be encoded into a specific
field, the Repair FEC Payload ID field, the contents and format of
which are defined by the FEC Scheme.</t>
<t>The FEC Scheme MAY use different FEC Payload ID field formats for
FEC Source packets and FEC Repair packets.</t>
</section>
<section anchor="senderoperation" title="Sender Operation">
<t>It is assumed that the sender has constructed or received original
data packets for the session. These may be RTP, RTCP, MIKEY or indeed
any other type of packet. The following operations, illustrated in
<xref target="senderfigure"></xref>, for the case of UDP repair flows
and <xref target="senderfigurertp"></xref> for the case of RTP repair
flows, describe a possible way to generate compliant FEC Source packet
and FEC repair packet streams: <list>
<t>1. Application Data Units are provided by the application.</t>
<t>2. A source block is constructed as specified in <xref
target="sourceblock"></xref>.</t>
<t>3. The source block is passed to the FEC Scheme for FEC
encoding. The Source FEC Payload ID information of each Source
packet is determined by the FEC Scheme. If required by the FEC
Scheme the Source FEC Payload ID is encoded into the Explicit
Source FEC Payload ID field.</t>
<t>4. The FEC Scheme performs FEC Encoding, generating repair
packet payloads from a source block and a Repair FEC Payload ID
field for each repair payload.</t>
<t>5. The Explicit Source FEC Payload IDs (if used), Repair FEC
Payload IDs and repair packet payloads are provided back from the
FEC Scheme to the FEC Framework.</t>
<t>6. The FEC Framework constructs FEC Source packets according to
<xref target="sourcepackets"></xref> and FEC Repair packets
according to <xref target="repairpackets"></xref> using the FEC
Payload IDs and repair packet payloads provided by the FEC
Scheme.</t>
<t>7. The FEC Source and FEC Repair packets are sent using normal
transport layer procedures. The port(s) and multicast group(s) to
be used for FEC Repair packets are defined in the FEC Framework
Configuration Information. The FEC Source packets are sent using
the same ADU flow identification information as would have been
used for the original source packets if the FEC Framework were not
present (for example, in the UDP case, the UDP source and
destination addresses and ports on the IP datagram carrying the
Source Packet will be the same whether or not the FEC Framework is
applied).</t>
</list></t>
<figure anchor="senderfigure" title="Sender operation">
<artwork><![CDATA[
+----------------------+
| Application |
+----------------------+
|
| (1) Application Data Units
|
v
+----------------------+ +------------------+
| FEC Framework | | |
| |-------------------------->| FEC Scheme |
|(2) Construct source | (3) Source Block | |
| blocks | | (4) FEC Encoding |
|(6) Construct FEC src |<--------------------------| |
| packets and FEC | | |
| repair packets |(5) Ex src FEC Payload Ids,| |
+----------------------+ Repair FEC Payload Ids,+------------------+
| Repair symbols
|
| (7) FEC Source packets and FEC repair packets
v
+----------------------+
| Transport Layer |
| (e.g. UDP ) |
+----------------------+
]]></artwork>
</figure>
<figure anchor="senderfigurertp"
title="Sender operation with RTP repair flows">
<artwork><![CDATA[
+----------------------+
| Application |
+----------------------+
|
| (1) Application Data Units
v
+----------------------+ +------------------+
| FEC Framework | | |
| |-------------------------->| FEC Scheme |
|(2) Construct source | (3) Source Block | |
| blocks | | (4) FEC Encoding |
|(6) Construct FEC src |<--------------------------| |
| packets and FEC | | |
| repair payloads |(5) Ex src FEC Payload Ids,| |
+----------------------+ Repair FEC Payload Ids,+------------------+
| | Repair symbols
|(7) Source |
| |(7') Repair RTP payloads
| + -- -- -- -- -+
| | RTP |
| +-- -- -- -- --+
v v
+----------------------+
| Transport Layer |
| (e.g. UDP ) |
+----------------------+
]]></artwork>
</figure>
</section>
<section anchor="receiveroperation" title="Receiver Operation">
<t>The following describes a possible receiver algorithm, illustrated
in <xref target="receiverfigure"></xref> and <xref
target="receiverfigurertp"></xref> for the case of RTP repair flows,
when receiving an FEC source or repair packet: <list>
<t>1. FEC Source Packets and FEC Repair packets are received and
passed to the FEC Framework. The type of packet (Source or Repair)
and the Source Data Flow to which it belongs (in the case of
source packets) is indicated by the ADU flow information which
identifies the flow at the transport layer (for example source and
destination ports and addresses in the case of UDP).</t>
<t>1a. In the special case that RTP is used for repair packets and
source and repair packets are multiplexed onto the same UDP flow,
then RTP demultiplexing is required to demultiplex source and
repair flows. However, RTP processing is applied only to the
repair packets at this stage: source packets continue to be
handled as UDP payloads (i.e. including their RTP headers).</t>
<t>2. The FEC Framework extracts the Explicit Source FEC Payload
ID field (if present) from FEC Source Packets and the Repair FEC
Payload ID from FEC Repair Packets.</t>
<t>3. The Explicit Source FEC Payload IDs (if present), Repair FEC
Payload IDs, FEC Source payloads and FEC Repair payloads are
passed to the FEC Scheme.</t>
<t>4. The FEC Scheme uses the received FEC Payload IDs (and
derived FEC Source Payload IDs in the case that the Explicit
Source FEC Payload ID field is not used) to group source and
repair packets into source blocks. If at least one source packet
is missing from a source block, and at least one repair packet has
been received for the same source block then FEC decoding may be
performed in order to recover missing source payloads. The FEC
Scheme determines whether source packets have been lost and
whether enough data for decoding of any or all of the missing
source payloads in the source block has been received.</t>
<t>5. The FEC Scheme returns the Application Data Units to the FEC
Framework in the form of source blocks containing received and
decoded Application Data Units and indications of any Application
Data Units which were missing and could not be decoded.</t>
<t>6. The FEC Framework passes the received and recovered
Application Data Units to the application.</t>
</list>Note that the description above defines functionality
responsibilities but does not imply a specific set of timing
relationships. For example, ADUs may be provided to the application as
soon as they are received or recovered (and hence potentially
out-of-order) or they may be buffered and delivered to the application
in-order.</t>
<figure anchor="receiverfigure" title="Receiver Operation">
<preamble></preamble>
<artwork><![CDATA[
+----------------------+
| Application |
+----------------------+
^
| (6) Application Data Units
|
+----------------------+ +------------------+
| FEC Framework | | |
| |<---------------------------| FEC Scheme |
|(2)Extract FEC Payload| (5) Application Data Units | |
| IDs and pass IDs & | | (4) FEC Decoding |
| Payloads to FEC |--------------------------->| |
| Scheme | (3) Ex src FEC Payload IDs,| |
+----------------------+ FEC Repair Payload IDs,+------------------+
^ FEC Source Payloads,
| FEC Repair Payloads
|
| (1) FEC Source packets and FEC repair packets
|
+----------------------+
| Transport Layer |
| (e.g. UDP ) |
+----------------------+ ]]></artwork>
<postamble></postamble>
</figure>
<figure anchor="receiverfigurertp" title="Receiver Operation">
<preamble></preamble>
<artwork><![CDATA[
+----------------------+
| Application |
+----------------------+
^
| (6) Application Data Units
|
+----------------------+ +------------------+
| FEC Framework | | |
| |<---------------------------| FEC Scheme |
|(2)Extract FEC Payload| (5) Application Data Units | |
| IDs and pass IDs & | | (4) FEC Decoding |
| Payloads to FEC |--------------------------->| |
| Scheme | (3) Ex src FEC Payload IDs,| |
+----------------------+ FEC Repair Payload IDs,+------------------+
^ ^ FEC Source Payloads,
| | FEC Repair Payloads
|Source pkts |
| |(1a) FEC repair payloads
+-- |- -- -- -- -- -- -+
|RTP| | RTP processing |
| | +-- -- -- --|-- -+
| +-- -- -- -- -- |--+ |
| | RTP demux | |
+-- -- -- -- -- -- -- -+
| (1) FEC Source packets and FEC repair packets
+----------------------+
| Transport Layer |
| (e.g. UDP ) |
+----------------------+ ]]></artwork>
<postamble></postamble>
</figure>
<t>Note that the above procedure may result in a situation in which
not all ADUs are recovered.</t>
<t>Source packets which are correctly received and those which are
reconstructed MAY be delivered to the application out of order and in
a different order from the order of arrival at the receiver.
Alternatively, buffering and packet re-ordering MAY be applied to
re-order received and reconstructed source packets into the order they
were placed into the source block, if that is necessary according to
the application.</t>
</section>
</section>
<section title="Protocol Specification">
<section title="General">
<t>This section specifies the protocol elements for the FEC Framework.
Three components of the protocol are defined in this document and are
described in the following sections: <list>
<t>1. Construction of a source block from Application Data Units.
The FEC code will be applied to this source block to produce the
repair payloads.</t>
<t>2. A format for packets containing source data.</t>
<t>3. A format for packets containing repair data.</t>
</list>The operation of the FEC Framework is governed by certain FEC
Framework Configuration Information. This configuration information is
also defined in this section. A complete protocol specification that
uses this framework MUST specify the means to determine and
communicate this information between sender and receiver.</t>
</section>
<section anchor="sourceblock" title="Structure of the source block">
<t>The FEC Framework and FEC Scheme exchange Application Data Units in
the form of source blocks. A source block is generated by the FEC
Framework from an ordered sequence of Application Data Units. The
allocation of Application Data Units to blocks is dependent on the
application. Note that some Application Data Units may not be included
in any block. Each Source Block provided to the FEC scheme consists of
an ordered sequence of Application Data Units where the following
information is provided for each ADU: <list style="symbols">
<t>A description of the Source Data Flow with which the
Application Data Unit is associated (See 6.5)</t>
<t>The Application Data Unit itself</t>
<t>The length of the Application Data Unit</t>
</list></t>
<t></t>
</section>
<section anchor="sourcepackets"
title="Packet format for FEC Source packets">
<t>The packet format for FEC Source packets MUST be used to transport
the payload of an original source packet. As depicted in <xref
target="sourcepacketfigure"></xref>, it consists of the original
packet, optionally followed by the Explicit Source FEC Payload ID
field. The FEC Scheme determines whether the Explicit Source FEC
Payload ID field is required. This determination is specific to each
ADU flow.</t>
<figure anchor="sourcepacketfigure"
title="Structure of the FEC packet format for FEC Source packets">
<artwork><![CDATA[
+------------------------------------+
| IP header |
+------------------------------------+
| Transport header |
+------------------------------------+
| Application Data Unit |
+------------------------------------+
| Explicit Source FEC Payload ID |
+------------------------------------+
]]></artwork>
</figure>
<t>The FEC Source packets MUST be sent using the same ADU flow as
would have been used for the original source packets if the FEC
Framework were not present. The transport payload of the FEC Source
packet MUST consist of the Application Data Unit followed by the
Explicit Source FEC Payload ID field, if required.</t>
<t>The Explicit Source FEC Payload ID field contains information
required to associate the source packet with a source block and for
the operation of the FEC algorithm and is defined by the FEC Scheme.
The format of the Source FEC Payload ID field is defined by the FEC
Scheme. Note that in the case that the FEC Scheme or CDP defines a
means to derive the Source FEC Payload ID from other information in
the packet (for example a sequence number used by the application
protocol), then the Source FEC Payload ID field is not included in the
packet. In this case the original source packet and FEC Source Packet
are identical.</t>
<t>In applications where avoidance of IP packet fragmentation is a
goal, Content Delivery Protocols SHOULD consider the Explicit Source
FEC Payload ID size when determining the size of Application Data
Units that will be delivered using the FEC Framework. This is because
the addition of the Explicit Source FEC Payload ID increases the
packet length.</t>
<t>Note: The Explicit Source FEC Payload ID is placed at the end of
the packet so that in the case that Robust Header Compression <xref
target="RFC3095"></xref> or other header compression mechanisms are
used and in the case that a ROHC profile is defined for the protocol
carried within the transport payload (for example RTP), then ROHC will
still be applied for the FEC Source packets. Applications that may be
used with this Framework should consider that FEC Schemes may add this
Explicit Source FEC Payload ID and thereby increase the packet
size.</t>
<t>In many applications, support for Forward Error Correction is added
to a pre-existing protocol and in this case use of the Explicit Source
FEC Payload ID may break backwards compatibility, since source packets
are modified.</t>
<section title="Generic Explicit Source FEC Payload Id">
<t>In order to apply FEC protection using multiple FEC Schemes to a
single source flow all schemes must use the same Explicit Source FEC
Payload Id format. In order to enable this, it is RECOMMENDED that
FEC Schemes support the Generic Explicit Source FEC Payload Id
format described below.</t>
<t>The Generic Explicit Source FEC Payload Id has length of 2 bytes
and consists of an unsigned packet sequence number in network byte
order. The allocation of sequence numbers to packets is independent
of any FEC Scheme and of the Source Block construction, except that
the use of this sequence number places a constraint on source block
construction. Source packets within a given source block MUST have
consecutive sequence numbers (where consecutive includes wrap-around
from the maximum value which can be represented in 2 bytes - 65535 -
to 0). Sequence numbers SHOULD NOT be reused until all values in the
sequence number space have been used.</t>
</section>
</section>
<section anchor="repairpackets"
title="Packet Format for FEC Repair packets">
<t>The packet format for FEC Repair packets is shown in <xref
target="repairpacketfigure"></xref>. The transport payload consists of
a Repair FEC Payload ID field followed by repair data generated in the
FEC encoding process. <figure anchor="repairpacketfigure"
title="Packet format for repair packets">
<artwork><![CDATA[
+------------------------------------+
| IP header |
+------------------------------------+
| Transport header |
+------------------------------------+
| Repair FEC Payload ID |
+------------------------------------+
| Repair Symbols |
+------------------------------------+
]]></artwork>
</figure></t>
<t>The Repair FEC Payload ID field contains information required for
the operation of the FEC algorithm at the receiver. This information
is defined by the FEC Scheme. The format of the Repair FEC Payload ID
field is defined by the FEC Scheme.</t>
<section title="Packet Format for FEC Repair packets over RTP">
<t>For FEC Schemes which specify the use of RTP for repair packets,
the packet format for repair packets includes an RTP header as shown
in <xref target="repairpacketfigureRTP"></xref>.</t>
<t><figure anchor="repairpacketfigureRTP"
title="Packet format for repair packets">
<artwork><![CDATA[
+------------------------------------+
| IP header |
+------------------------------------+
| Transport header (UDP) |
+------------------------------------+
| RTP Header |
+------------------------------------+
| Repair FEC Payload ID |
+------------------------------------+
| Repair Symbols |
+------------------------------------+
]]></artwork>
</figure></t>
</section>
</section>
<section anchor="config" title="FEC Framework Configuration Information">
<t>The FEC Framework Configuration Information is information that the
FEC Framework needs in order to apply FEC protection to the ADU flows.
A complete Content Delivery Protocol specification that uses the
framework specified here MUST include details of how this information
is derived and communicated between sender and receiver.</t>
<t>The FEC Framework Configuration Information includes identification
of the set of Source Data Flows. For example, in the case of UDP, each
Source Data Flow is uniquely identified by a tuple { Source IP
Address, Destination IP Address, Source UDP port, Destination UDP port
}. Note that in some applications some of these fields may contain
wildcards, so that the flow is identified by a subset of the fields
and in particular in many applications the limited tuple { Destination
IP Address, Destination UDP port } is sufficient.</t>
<t>A single instance of the FEC Framework provides FEC protection for
packets of the specified set of Source Data Flows, by means of one or
more packet flows consisting of repair packets. The FEC Framework
Configuration Information includes, for each instance of the FEC
Framework: <list>
<t>1. Identification of the packet flow(s) carrying FEC Repair
packets, known as the FEC repair flow(s).</t>
<t>2. For each Source Data Flow protected by the FEC repair
flow(s): <list>
<t>a. Definition of the Source Data Flow carrying source
packets (for example, by means of a tuple as described above
for UDP).</t>
<t>b. An integer identifier for this Source Data Flow. This
identifier MUST be unique amongst all Source Data Flows which
are protected by the same FEC repair flow.</t>
</list></t>
<t>3. The FEC Encoding ID, identifying the FEC Scheme</t>
<t>4. The length of the Explicit Source FEC Payload Id, in
bytes</t>
<t>5. Zero or more FEC-Scheme-specific information elements, each
consisting of a name and a value where the valid element names and
value ranges are defined by the FEC Scheme</t>
</list></t>
<t>Multiple instances of the FEC Framework, with separate and
independent FEC Framework Configuration Information, may be present at
a sender or receiver. A single instance of the FEC Framework protects
packets of the Source Data Flows identified in (2) above i.e. all
packets sent on those flows MUST be FEC Source packets as defined in
<xref target="sourcepackets"></xref>. A single Source Data Flow may be
protected by multiple instances of the FEC Framework.</t>
<t>The integer flow identifier identified in 2(b) is a "shorthand" to
identify source flows between the FEC Framework and the FEC Scheme.
The reason for defining this as an integer, and including it in the
FEC Framework Configuration Information is so that the FEC Scheme at
the sender and receiver may use it to identify the source flow with
which a recovered packet is associated. The integer flow identifier
may therefore take the place of the complete flow description (e.g.
UDP 4-tuple).</t>
<t>Whether and how this flow identifier is used is defined by the FEC
Scheme. Since source packets are directly associated with a flow by
virtue of their packet headers, this identifier need not be carried in
source packets. Since repair packets may provide protection for
multiple source flows, repair packets would either not carry the
identifier at all or may carry multiple identifiers. However, in any
case, the flow identifier associated with a particular source packet
may be recovered from the repair packets as part of an FEC decoding
operation. Integer flow identifiers SHOULD be allocated starting from
zero and increasing by one for each flow.</t>
<t>A single FEC repair flow provides repair packets for a single
instance of the FEC Framework. Other packets MUST NOT be sent within
this flow i.e. all packets in the FEC repair flow MUST be FEC repair
packets as defined in <xref target="repairpackets"></xref> and MUST
relate to the same FEC Framework instance.</t>
<t>In the case that RTP is used for repair packets, the identification
of the repair packet flow MAY also include the RTP Payload Type to be
used for repair packets.</t>
<t>FEC Scheme-specific information elements MAY be encoded into a text
string for transport within Content Delivery Protocols. See Section
4.5 of <xref target="I-D.ietf-fecframe-sdp-elements"></xref> for the
ABNF <xref target="RFC5234"></xref> syntax.</t>
</section>
<section anchor="fecscheme" title="FEC Scheme requirements">
<t>In order to be used with this framework, an FEC Scheme MUST be
capable of processing data arranged into blocks of Application Data
Units (source blocks).</t>
<t>A specification for a new FEC scheme MUST include the following
things: <list style="numbers">
<t>The FEC Encoding ID value that uniquely identifies the FEC
scheme. This value MUST be registered with IANA as described in
<xref target="iana"></xref>.</t>
<t>The type, semantics and encoding format of the Repair FEC
Payload ID.</t>
<t>The name, type, semantics and text value encoding rules for
zero or more FEC Scheme-specific FEC Framework Configuration
Information elements. Names must conform to the <spanx
style="verb">name</spanx> production and values encodings to the
<spanx style="verb">value</spanx> production defined in <xref
target="config"></xref></t>
<t>A full specification of the FEC code. <vspace
blankLines="1" />This specification MUST precisely define the
valid FEC-Scheme-Specific FEC Framework Configuration Information
values, the valid FEC Payload ID values and the valid packet
payload sizes (where packet payload refers to the space within a
packet dedicated to carrying encoding symbol bytes). <vspace
blankLines="1" />Furthermore, given a source block as defined in
<xref target="sourceblock"></xref>, valid values of the
FEC-Scheme-Specific FEC Framework Configuration Information, a
valid Repair FEC Payload ID value and a valid packet payload size,
the specification MUST uniquely define the values of the encoding
symbol bytes to be included in the repair packet payload of a
packet with the given Repair FEC Payload ID value.<vspace
blankLines="1" />A common and simple way to specify the FEC code
to the required level of detail is to provide a precise
specification of an encoding algorithm which, given a source
block, valid values of the FEC-Scheme-Specific FEC Framework
Configuration Information, a valid Repair FEC Payload ID value and
a valid packet payload size as input produces the exact value of
the encoding symbol bytes as output.</t>
<t>A description of practical encoding and decoding
algorithms.<vspace blankLines="1" />This description need not be
to the same level of detail as for the encoding above, however it
must be sufficient to demonstrate that encoding and decoding of
the code is both possible and practical.</t>
</list></t>
<t>FEC scheme specifications MAY additionally define the following:
<list style="numbers">
<t>Type, semantics and encoding format of an Explicit Source FEC
Payload ID.</t>
</list></t>
<t>Whenever an FEC scheme specification defines an 'encoding format'
for an element, this must be defined in terms of a sequence of bytes
which can be embedded within a protocol. The length of the encoding
format MUST either be fixed or it must be possible to derive the
length from examining the encoded bytes themselves. For example, the
initial bytes may include some kind of length indication.</t>
<t>FEC scheme specifications SHOULD use the terminology defined in
this document and SHOULD follow the following format: <list
style="hanging">
<t hangText="1. Introduction"><describe the use-cases addressed
by this FEC scheme><vspace blankLines="1" /></t>
<t hangText="2. Formats and Codes"><list style="hanging">
<t hangText="2.1 Source FEC Payload ID(s)"><Either, define
the type and format of the Explicit Source FEC Payload ID, or
define how Source FEC Payload ID information is derived from
source packets><vspace blankLines="1" /></t>
<t hangText="2.2 Repair FEC Payload Id"><Define the type
and format of the Repair FEC Payload ID></t>
<t
hangText="2.3 FEC Framework Configuration Information"><Define
the names, types and text value encoding formats of the FEC
Scheme-specific FEC Framework configuration information
elements></t>
</list></t>
<t hangText="3. Procedures"><describe any procedures which are
specific to this FEC scheme, in particular derivation and
interpretation of the fields in the FEC Payload ID and FEC
Scheme-specific FEC Framework configuration information.></t>
<t hangText="4. FEC code specification"><provide a complete
specification of the FEC Code></t>
</list></t>
<t>Specifications MAY include additional sections, for example,
examples.</t>
<t>Each FEC scheme MUST be specified independently of all other FEC
schemes; for example, in a separate specification or a completely
independent section of larger specification (except, of course, a
specification of one FEC Scheme may include portions of another by
reference).</t>
<t>Where an RTP Payload Format is defined for repair data for a
specific FEC Scheme, the RTP Payload Format and the FEC Scheme MAY be
specified within the same document.</t>
</section>
</section>
<section title="Feedback">
<t>Many applications require some kind of feedback on transport
performance: how much data arrived at the receiver, at what rate, when
etc. When FEC is added to such applications, feedback mechanisms may
also need to be enhanced to report on the performance of the FEC (for
example how much lost data was recovered by the FEC).</t>
<t>When used to provide instrumentation for engineering purposes, it is
important to remember that FEC is generally applied to relatively small
blocks of data (in the sense that each block is transmitted over a
relatively small period of time) and so feedback information averaged
over longer periods of time than the FEC block transmission time will
likely not provide sufficient information for engineering purposes. For
example see <xref target="RFC5725"></xref>.</t>
<t>Applications which used feedback for congestion control purposes MUST
calculate such feedback on the basis of packets received before FEC
recovery is applied. If this requirement conflicts with other uses of
the feedback information then the application MUST be enhanced to
support both information calculated pre- and post- FEC recovery. This is
to ensure that congestion control mechanisms operate correctly based on
congestion indications received from the network, rather than on
post-FEC recovery information which would give an inaccurate picture of
congestion conditions.</t>
<t>New applications which require such feedback SHOULD use RTP/RTCP
<xref target="RFC3550"></xref>.</t>
</section>
<section anchor="TransportProtocols" title="Transport Protocols">
<t>This framework is intended to be used to define Content Delivery
Protocols which operate over transport protocols which provide an
unreliable datagram service, including in particular the User Datagram
Protocol (UDP) and the Datagram Congestion Control Protocol (DCCP).</t>
</section>
<section title="Congestion Control">
<t>This section starts with a informative section on the motivation of
the normative requirements for congestion control, which are spelled out
in <xref target="normativecongestion"></xref>. <list>
<t>Informative Note: The enforcement of Congestion Control (CC)
principles has gained a lot of momentum in the IETF over the recent
years. While the need of CC over the open Internet is unquestioned,
and the goal of TCP friendliness is generally agreed for most (but
not all) applications, the subject of congestion detection and
measurement in heterogeneous networks can hardly be considered as
solved. Most congestion control algorithms detect and measure
congestion by taking (primarily or exclusively) the packet loss rate
into account. This appears to be inappropriate in environments where
a large percentage of the packet losses are the result of link-layer
errors and independent of the network load. Note that such
environments exist in the "open Internet", as well as in "closed" IP
based networks. An example for the former would be the use of
IP/UDP/RTP based streaming from an Internet-connected streaming
server to a device attached to the Internet using cellular
technology.</t>
<t>The authors of this draft are primarily interested in
applications where the application reliability requirements and
end-to-end reliability of the network differ, such that it warrants
higher layer protection of the packet stream - for example due to
the presence of unreliable links in the end-to-end path - and where
real-time, scalability or other constraints prohibit the use of
higher layer (transport or application) feedback. A typical example
for such applications is multicast and broadcast streaming or
multimedia transmission over heterogeneous networks. In other cases,
application reliability requirements may be so high that the
required end-to-end reliability is difficult to achieve even over
wired networks. Furthermore the end-to-end network reliability may
not be known in advance.</t>
<t>This FEC framework is not proposed, nor intended, as a QoS
enhancement tool to combat losses resulting from highly congested
networks. It should not be used for such purposes.</t>
<t>In order to prevent such mis-use, one approach would be to leave
standardisation to bodies most concerned with the problem described
above. However, the IETF defines base standards used by several
bodies, including DVB, 3GPP, 3GPP2, all of which appear to share the
environment and the problem described.</t>
<t>Another approach would be to write a clear applicability
statement - for example restricting use of the framework to networks
with wireless links. However, there may be applications where the
use of FEC may be justified to combat congestion-induced packet
losses - particularly in lightly loaded networks, where congestion
is the result of relatively rare random peaks in instantaneous
traffic load - thereby intentionally violating congestion control
principles. One possible example for such an application could be a
no-matter-what, brute-force FEC protection of traffic generated as
an emergency signal.</t>
<t>We propose a third approach, which is to require at a minimum
that the use of this framework with any given application, in any
given environment, does not cause congestion issues which the
application alone would not itself cause i.e. the use of this
framework must not make things worse.</t>
<t>Taking above considerations into account, <xref
target="normativecongestion"> </xref> specifies a small set of
constraints for the FEC, which are mandatory for all senders
compliant with this FEC framework. Further restrictions may be
imposed for certain Content Delivery Protocols. In this it follows
the spirit of the congestion control section of RTP and its
Audio-Visual Profile (RFC3550/STD64 and RFC3551/STD65).</t>
<t>One of the constraints effectively limits the bandwidth for the
FEC protected packet stream to be no more than roughly twice as high
as the original, non-FEC protected packet stream. This disallows the
(static or dynamic) use of excessively strong FEC to combat high
packet loss rates, which may otherwise be chosen by naively
implemented dynamic FEC-strength selection mechanisms. We
acknowledge that there may be a few exotic applications, e.g. IP
traffic from space-based senders, or senders in certain hardened
military devices, which would warrant a higher FEC strength.
However, in this specification we give preference to the overall
stability and network friendliness of the average application, and
for those a factor of 2 appears to be appropriate.</t>
<t>A second constraint requires that the FEC protected packet stream
be in compliance with the congestion control in use for the
application and network in question.</t>
</list></t>
<section anchor="normativecongestion" title="Normative requirements">
<t>The bandwidth of FEC Repair packet flows MUST NOT exceed the
bandwidth of the source packet flows being protected. In addition,
whenever the source packet flow bandwidth is adapted due to the
operation of congestion control mechanisms, the FEC repair packet flow
bandwidth MUST be similarly adapted.</t>
</section>
</section>
<section title="Security Considerations">
<t>The application of FEC protection to a stream does not provide any
kind of security protection.</t>
<t>If security services are required for the stream, then they MUST
either be applied to the original source data before FEC protection is
applied, or to both the source and repair data, after FEC protection has
been applied.</t>
<t>If integrity protection is applied to source packets before FEC
protection is applied, and no further integrity protection is applied to
repair packets, then a denial of service attack is possible if an
attacker is in a position to inject fake repair transport payloads. If
received by a receiver, such fake repair transport payloads could cause
incorrect FEC decoding resulting in incorrect Application Data Units
being passed up to the application protocol. A similar attack may be
possible if an attacker is in a position to inject fake FEC Framework
Configuration Information or fake FEC Payload IDs. Such incorrect
decoded Application Data Units would then be detected by the source
integrity protection and discarded, resulting in partial or complete
denial of service. Therefore, in such environments, integrity protection
MUST also be applied to the FEC repair transport payloads, FEC Framework
Configuration Information and FEC Payload IDs, for example using IPsec
to integrity protect all packets. Receivers MUST also verify the
integrity of source symbols before including the source symbols into the
source block for FEC purposes.</t>
<t>It is possible that multiple streams with different confidentiality
requirements (for example, the streams may be visible to different sets
of users) can be FEC protected by a single repair stream. This scenario
is not recommended, since resources will be used to distribute and FEC
decode encrypted data which cannot then be decrypted by at least some
receivers. However, in this scenario, confidentiality protection MUST be
applied before FEC encoding of the streams, otherwise repair transport
payload may be used by a receiver to decode unencrypted versions of
source streams which they do not have permissions to view.</t>
</section>
<section anchor="iana" title="IANA Considerations">
<t>FEC Schemes for use with this framework may be identified in
protocols using FEC Encoding IDs. Values of FEC Encoding IDs are subject
to IANA registration. They are in the registry named "FEC Framework
(FECFRAME) FEC Encoding IDs" located at time of publication at
<tbd>.</t>
<t>The values that can be assigned within the FEC Framework (FECFRAME)
FEC Encoding ID registry are numeric indexes in the range [0, 255],
boundaries included. Assignment requests are granted on a "IETF
Consensus" basis as defined in<xref target="RFC5226"> </xref> . <xref
target="fecscheme"></xref> defines explicit requirements that documents
defining new FEC Encoding IDs should meet.</t>
</section>
<section title="Acknowledgments">
<t>This document is based in part on <xref
target="I-D.watson-tsvwg-fec-sf"></xref> and so thanks are due to the
additional authors of that document, Mike Luby, Magnus Westerlund and
Stephan Wenger. That document was in turn based on the FEC streaming
protocol defined by 3GPP in <xref target="MBMSTS"></xref> and thus
thanks are also due to the participants in 3GPP TSG SA working group 4.
Further thanks are due to the members of the FECFRAME working group for
their comments and review.</t>
</section>
</middle>
<back>
<references title="Normative references">
&rfc2119;
&rfc3095;
&rfc5052;
&rfc3550;
&rfc5226;
&rfc5234;
</references>
<references title="Informative references">
&fecsf;
&rfc5725;
&rfc4588;
&rfc2736;
&fecsdp;
<reference anchor="MBMSTS">
<front>
<title>Multimedia Broadcast/Multicast Service (MBMS); Protocols and
codecs</title>
<author>
<organization>3GPP</organization>
</author>
<date day="01" month="April" year="2005" />
</front>
<seriesInfo name="3GPP TS" value="26.346" />
<format target="http://www.3gpp.org/ftp/Specs/html-info/26346.htm"
type="HTML" />
</reference>
</references>
</back>
</rfc>
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