FEC Framework M. Watson
Internet-Draft Netflix, Inc.
Intended status: Standards Track A. Begen
Expires: May 24, 2011 Cisco
November 20, 2010
Forward Error Correction (FEC) Framework
draft-ietf-fecframe-framework-11
Abstract
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 FEC 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 FEC 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.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on May 24, 2011.
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Definitions and Abbreviations . . . . . . . . . . . . . . . . 6
3. Architecture Overview . . . . . . . . . . . . . . . . . . . . 9
4. Procedural Overview . . . . . . . . . . . . . . . . . . . . . 13
4.1. General . . . . . . . . . . . . . . . . . . . . . . . . . 13
4.2. Sender Operation . . . . . . . . . . . . . . . . . . . . . 14
4.3. Receiver Operation . . . . . . . . . . . . . . . . . . . . 17
5. Protocol Specification . . . . . . . . . . . . . . . . . . . . 21
5.1. General . . . . . . . . . . . . . . . . . . . . . . . . . 21
5.2. Structure of the Source Block . . . . . . . . . . . . . . 21
5.3. Packet Format for FEC Source Packets . . . . . . . . . . . 21
5.3.1. Generic Explicit Source FEC Payload ID . . . . . . . . 23
5.4. Packet Format for FEC Repair Packets . . . . . . . . . . . 23
5.4.1. Packet Format for FEC Repair Packets over RTP . . . . 23
5.5. FEC Framework Configuration Information . . . . . . . . . 24
5.6. FEC Scheme Requirements . . . . . . . . . . . . . . . . . 26
6. Feedback . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
7. Transport Protocols . . . . . . . . . . . . . . . . . . . . . 30
8. Congestion Control . . . . . . . . . . . . . . . . . . . . . . 31
8.1. Motivation . . . . . . . . . . . . . . . . . . . . . . . . 31
8.2. Normative Requirements . . . . . . . . . . . . . . . . . . 32
9. Security Considerations . . . . . . . . . . . . . . . . . . . 33
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 34
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 35
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 36
12.1. Normative references . . . . . . . . . . . . . . . . . . . 36
12.2. Informative references . . . . . . . . . . . . . . . . . . 36
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 38
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1. Introduction
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.
Forward Error Correction (FEC) 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 [RFC5052]
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 and, the term "Forward
Error Correction" is more widely used.
This document defines a framework for the definition of CDPs which
provide for FEC protection for arbitrary packet flows over unreliable
transports such as UDP. As such, this document complements the FEC
Building Block of [RFC5052], 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 CDP, but rather defines only those aspects that are
expected to be common to all CDPs based on this framework.
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 CDP specification which makes use of
this framework will address these signaling requirements. However,
this document does specify the information which is required by the
FEC Framework at the sender and receiver, e.g., 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.
FEC schemes designed for use with this framework must fulfill a
number of requirements defined in this document. These requirements
are different from those defined in [RFC5052] for FEC schemes for
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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 in this document.
Since the RTP protocol is (often) used over UDP, this framework can
be applied to RTP flows as well. FEC repair packets may be sent
directly over UDP or RTP. The latter approach has the advantage that
RTP instrumentation, based on RTP Control Protocol (RTCP), can be
used for the repair flow. Additionally, the post-repair RTCP
extended reports [RFC5725] may be used to obtain information about
the loss rate after FEC recovery.
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).
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2. Definitions and Abbreviations
Application Data Unit (ADU): The unit of source data provided as
payload to the transport layer.
ADU Flow: A sequence of ADUs associated with a transport-layer flow
identifier (such as the standard 5-tuple {Source IP address, source
port, destination IP address, destination port, transport protocol}).
AL-FEC: Application-layer Forward Error Correction.
Application Protocol: Control protocol used to establish and control
the source flow being protected, e.g., RTSP.
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 FEC
capabilities.
FEC Code: An algorithm for encoding data such that the encoded data
flow is resilient to data loss. Note that in general FEC codes may
also be used to make a data flow resilient to corruption, but that is
not considered in this document.
FEC Framework: A protocol framework for definition of Content
Delivery Protocols using FEC, such as the framework defined in this
document.
FEC Framework Configuration Information: Information which controls
the operation of the FEC Framework.
FEC Payload ID: Information which identifies the contents of a packet
with respect to the FEC scheme.
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.
FEC Scheme: A specification which defines the additional protocol
aspects required to use a particular FEC code with the FEC Framework.
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 Explicit Source FEC
Payload ID.
Protection Amount: The relative increase in data sent due to the use
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of FEC.
Repair Flow: The packet flow carrying FEC data.
Repair FEC Payload ID: An FEC Payload ID specifically for use with
repair packets.
Source Flow: The packet flow to which FEC protection is to be
applied. A source flow consists of ADUs.
Source FEC Payload ID: An FEC Payload ID specifically for use with
source packets.
Source Protocol: A protocol used for the source flow being protected,
e.g., RTP.
Transport Protocol: The protocol used for transport of the source and
repair flows, e.g., UDP and DCCP.
The following definitions are aligned with [RFC5052]:
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.
Encoding Symbol: Unit of data generated by the encoding process.
With systematic codes, source symbols are part of the encoding
symbols.
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.
Repair Symbol: Encoding symbol that is not a source symbol.
Source Block: Group of ADUs which are to be FEC protected as a single
block.
Source Symbol: Unit of data used during the encoding process.
Systematic Code: FEC code in which the source symbols are part of the
encoding symbols.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
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"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
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3. Architecture Overview
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. 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 by the
standard 5-tuple {Source IP address, source port, destination IP
address, destination port, transport protocol}. In the case that RTP
is used for the repair flows, the source and repair data can be
multiplexed using RTP onto a single UDP flow and needs to be
consequently demultiplexed at the receiver. There are various ways
in which this multiplexing can be done, for example as described in
[RFC4588].
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.
The FEC Framework makes use of an FEC scheme, in a similar sense to
that defined in [RFC5052] 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 cases 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.
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(s)). 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 [RFC5052].
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The FEC Framework does not define how the FEC Framework Configuration
Information for the stream is communicated from sender to receiver.
This has to be defined by any CDP specification as described in the
following sections.
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 (ADUs)) 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
addresses/ports) are also distinct on the interface between the
transport layer and the FEC Framework.
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.
Depending on the FEC scheme, RTP can also be used as a transport for
repair packet flows. In this case an FEC scheme has to define an RTP
payload format for the repair data.
The architecture outlined above is illustrated in the Figure 1. In
this architecture, two (optional) 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 one considers 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).
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+--------------------------------------------+
| Application |
+--------------------------------------------+
|
|
|
+ - - - - - - - - - - - - - - - - - - - - - - - -+
| +--------------------------------------------+ |
| Application Layer |
| +--------------------------------------------+ |
| |
| + -- -- -- -- -- -- -- -- -- -- --+ | |
| RTP (Optional) | |
| | | |- Configuration/Coordination
+- -- -- -- -- -- -- -- -- -- -- -+ |
| | | |
| ADU flows |
| | v |
+--------------------------------------------+ +----------------+
| | FEC Framework (This document) |<--->| FEC Scheme |
+--------------------------------------------+ +----------------+
| | | |
Source | Repair |
| | | |
+-- -- -- -- --|-- --+ -- -- -- -- -- + -- --+
| | RTP Layer | | RTP Processing | | |
| (Optional) | +-- -- -- |- -- -+ |
| | +-- -- -- -- -- -- -- |--+ | |
| | RTP (De)multiplexing | |
| +-- -- -- --- -- -- -- -- -- -- -- -- -- -- -+ |
|
| +--------------------------------------------+ |
| Transport Layer (e.g., UDP) |
| +--------------------------------------------+ |
|
| +--------------------------------------------+ |
| IP |
| +--------------------------------------------+ |
| Content Delivery Protocol |
+ - - - - - - - - - - - - - - - - - - - - - - - +
Figure 1: FEC Framework architecture
The content 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
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corresponds to defining an RTP payload format for the specific FEC
scheme.
The use of RTP for repair packets is independent of the protocols
used for source packets: if RTP is used for source packets, 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.
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4. Procedural Overview
4.1. General
The mechanism defined in this document does not place any
restrictions on the ADUs which can be protected together, except that
the ADU is carried over a supported transport protocol (See
Section 7). The data can be from multiple source flows that are
protected jointly. The FEC Framework handles the source flows as a
sequence of source blocks each consisting of a set of ADUs, possibly
from multiple source flows which are to be protected together. For
example, each source block can be constructed from those ADUs related
to a particular segment in time of the flow.
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:
o Optionally, FEC Payload IDs for each of the source payloads
(encoded according to an FEC-Scheme-Specific format).
o One or more FEC repair packet payloads.
o FEC Payload IDs for each of the repair packet payloads (encoded
according to an FEC-Scheme-Specific format).
The FEC Framework then performs two operations. First, it appends
the Source FEC Payload IDs, if provided, to each of the ADUs, and
sends the resulting packets, known as FEC source packets, to the
receiver, and second it places the provided FEC repair packet
payloads and corresponding Repair FEC Payload IDs appropriately to
construct FEC repair packets and send them to the receiver.
This document does not define how the sender determines which ADUs
are included in which source blocks or the sending order and timing
of FEC source and repair packets. A specific CDP 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 needs to 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 Section 4.2.
At the receiver, original ADUs 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 ADUs, plus their FEC Payload IDs to the FEC
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scheme for possible decoding.
If any ADUs related to a given source block have been lost, then the
FEC scheme can perform FEC decoding to recover the missing ADUs
(assuming sufficient FEC source and repair packets related to that
source block have been received).
Note that the receiver might 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 has to be able to deal with the severe
re-ordering of packets that can occur. However, such buffering is
CDP and/or implementation-specific and is not specified here. The
receiver operation is described in more detail in Section 4.3.
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 ADU. 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 schemes. 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.
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 schemes.
The FEC scheme MAY use different FEC Payload ID field formats for
source and repair packets.
4.2. Sender Operation
It is assumed that the sender has constructed or received original
data packets for the session. These could be carrying any type of
data. The following operations, illustrated in Figure 2, for the
case of UDP repair flows and Figure 3 for the case of RTP repair
flows, describe a possible way to generate compliant source and
repair flows:
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1. ADUs are provided by the application.
2. A source block is constructed as specified in Section 5.2.
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.
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.
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.
6. The FEC Framework constructs FEC source packets according to
Section 5.3 and FEC repair packets according to Section 5.4 using
the FEC Payload IDs and repair packet payloads provided by the
FEC scheme.
7. The FEC source and 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).
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+----------------------+
| Application |
+----------------------+
|
|(1) ADUs
|
v
+----------------------+ +------------------+
| FEC Framework | | |
| |-------------------------->| FEC Scheme |
|(2) Construct source |(3) Source Block | |
| blocks | | (4) FEC Encoding |
|(6) Construct FEC |<--------------------------| |
| source and repair | | |
| packets |(5) Explicit Source FEC | |
+----------------------+ Payload IDs +------------------+
| Repair FEC Payload IDs
| Repair symbols
|
|(7) FEC source and repair packets
v
+----------------------+
| Transport Layer |
| (e.g., UDP) |
+----------------------+
Figure 2: Sender operation
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+----------------------+
| Application |
+----------------------+
|
|(1) ADUs
|
v
+----------------------+ +------------------+
| FEC Framework | | |
| |-------------------------->| FEC Scheme |
|(2) Construct source |(3) Source Block | |
| blocks | | (4) FEC Encoding |
|(6) Construct FEC |<--------------------------| |
| source packets and| | |
| repair payloads |(5) Explicit Source FEC | |
+----------------------+ Payload IDs +------------------+
| | Repair FEC Payload IDs
| | Repair symbols
| |
|(7) Source |(7') Repair payloads
| packets |
| |
| + -- -- -- -- -+
| | RTP |
| +-- -- -- -- --+
v v
+----------------------+
| Transport Layer |
| (e.g., UDP) |
+----------------------+
Figure 3: Sender operation with RTP repair flows
4.3. Receiver Operation
The following describes a possible receiver algorithm, illustrated in
Figure 4 and Figure 5 for the case of RTP repair flows, when
receiving an FEC source or repair packet:
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 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.
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
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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).
2. The FEC Framework extracts the Explicit Source FEC Payload ID
field (if present) from the source packets and the Repair FEC
Payload ID from the repair packets.
3. The Explicit Source FEC Payload IDs (if present), Repair FEC
Payload IDs, FEC source and repair payloads are passed to the FEC
scheme.
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 can 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.
5. The FEC scheme returns the ADUs to the FEC Framework in the form
of source blocks containing received and decoded ADUs and
indications of any ADUs which were missing and could not be
decoded.
6. The FEC Framework passes the received and recovered ADUs to the
application.
The description above defines functionality responsibilities but does
not imply a specific set of timing relationships. 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.
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+----------------------+
| Application |
+----------------------+
^
|
|(6) ADUs
|
+----------------------+ +------------------+
| FEC Framework | | |
| |<---------------------------| FEC Scheme |
|(2)Extract FEC Payload|(5) ADUs | |
| IDs and pass IDs & | | (4) FEC Decoding |
| payloads to FEC |--------------------------->| |
| scheme |(3) Explicit Source FEC | |
+----------------------+ Payload IDs +------------------+
^ Repair FEC Payload IDs
| Source payloads
| Repair payloads
|
|(1) FEC source and repair packets
|
+----------------------+
| Transport Layer |
| (e.g., UDP) |
+----------------------+
Figure 4: Receiver operation
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+----------------------+
| Application |
+----------------------+
^
|
|(6) ADUs
|
+----------------------+ +------------------+
| FEC Framework | | |
| |<---------------------------| FEC Scheme |
|(2)Extract FEC Payload|(5) ADUs | |
| IDs and pass IDs & | | (4) FEC Decoding |
| payloads to FEC |--------------------------->| |
| scheme |(3) Explicit Source FEC | |
+----------------------+ Payload IDs +------------------+
^ ^ Repair FEC Payload IDs
| | Source payloads
| | Repair payloads
| |
|Source |Repair payloads
|packets |
| |
+-- |- -- -- -- -- -- -+
|RTP| | RTP Processing |
| | +-- -- -- --|-- -+
| +-- -- -- -- -- |--+ |
| | RTP Demux | |
+-- -- -- -- -- -- -- -+
^
|(1) FEC source and repair packets
|
+----------------------+
| Transport Layer |
| (e.g., UDP) |
+----------------------+
Figure 5: Receiver operation with RTP repair flows
Note that the above procedure might result in a situation in which
not all ADUs are recovered.
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5. Protocol Specification
5.1. General
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:
1. Construction of a source block from ADUs. The FEC code will be
applied to this source block to produce the repair payloads.
2. A format for packets containing source data.
3. A format for packets containing repair data.
The operation of the FEC Framework is governed by certain FEC
Framework Configuration Information, which is 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.
5.2. Structure of the Source Block
The FEC Framework and FEC scheme exchange ADUs in the form of source
blocks. A source block is generated by the FEC Framework from an
ordered sequence of ADUs. The allocation of ADUs to blocks is
dependent on the application. Note that some ADUs may not be
included in any block. Each source block provided to the FEC scheme
consists of an ordered sequence of ADUs where the following
information is provided for each ADU:
o A description of the source flow with which the ADU is associated
with.
o The ADU itself.
o The length of the ADU.
5.3. Packet Format for FEC Source Packets
The packet format for FEC source packets MUST be used to transport
the payload of an original source packet. As depicted in Figure 6,
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.
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+------------------------------------+
| IP Header |
+------------------------------------+
| Transport Header |
+------------------------------------+
| Application Data Unit |
+------------------------------------+
| Explicit Source FEC Payload ID |
+------------------------------------+
Figure 6: Structure of the FEC packet format for FEC source packets
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 ADU followed by the Explicit Source FEC Payload
ID field, if required.
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. 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.
In applications where avoidance of IP packet fragmentation is a goal,
CDPs SHOULD consider the Explicit Source FEC Payload ID size when
determining the size of ADUs that will be delivered using the FEC
Framework. This is because the addition of the Explicit Source FEC
Payload ID increases the packet length.
The Explicit Source FEC Payload ID is placed at the end of the packet
so that in the case that Robust Header Compression (ROHC) [RFC3095]
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 are used with this
framework need to consider that FEC schemes can add this Explicit
Source FEC Payload ID and thereby increase the packet size.
In many applications, support for FEC is added to a pre-existing
protocol and in this case use of the Explicit Source FEC Payload ID
can break backwards compatibility, since source packets are modified.
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5.3.1. Generic Explicit Source FEC Payload ID
In order to apply FEC protection using multiple FEC schemes to a
single source flow, all schemes have to 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.
The Generic Explicit Source FEC Payload ID has a length of two octets
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 two octets (65535)
to 0). Sequence numbers SHOULD NOT be reused until all values in the
sequence number space have been used.
5.4. Packet Format for FEC Repair Packets
The packet format for FEC repair packets is shown in Figure 7. The
transport payload consists of a Repair FEC Payload ID field followed
by repair data generated in the FEC encoding process.
+------------------------------------+
| IP Header |
+------------------------------------+
| Transport Header |
+------------------------------------+
| Repair FEC Payload ID |
+------------------------------------+
| Repair Symbols |
+------------------------------------+
Figure 7: Packet format for repair packets
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.
5.4.1. Packet Format for FEC Repair Packets over RTP
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
Figure 8.
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+------------------------------------+
| IP header |
+------------------------------------+
| Transport Header (UDP) |
+------------------------------------+
| RTP Header |
+------------------------------------+
| Repair FEC Payload ID |
+------------------------------------+
| Repair Symbols |
+------------------------------------+
Figure 8: Packet format for repair packets
5.5. FEC Framework Configuration Information
The FEC Framework Configuration Information is information that the
FEC Framework needs in order to apply FEC protection to the ADU
flows. A complete CDP specification that uses the framework
specified here MUST include details of how this information is
derived and communicated between sender and receiver.
The FEC Framework Configuration Information includes identification
of the set of source flows. For example, in the case of UDP, each
source flow is uniquely identified by a tuple {Source IP address,
source UDP port, destination IP address, destination UDP port}. In
some applications some of these fields can contain wildcards, so that
the flow is identified by a subset of the fields. In particular, in
many applications the limited tuple {Destination IP address,
destination UDP port} is sufficient.
A single instance of the FEC Framework provides FEC protection for
packets of the specified set of source 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:
1. Identification of the repair flows.
2. For each source flow protected by the repair flow(s):
A. Definition of the source flow.
B. An integer identifier for this flow definition (i.e., tuple).
This identifier MUST be unique amongst all source flows that
are protected by the same FEC repair flow. Integer
identifiers can be allocated starting from zero and
increasing by one for each flow. However, any random (but
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still unique) allocation is also possible. A source flow
identifier need not be carried in source packets since source
packets are directly associated with a flow by virtue of
their packet headers.
3. The FEC Encoding ID, identifying the FEC scheme.
4. The length of the Explicit Source FEC Payload ID (in octets).
5. Zero or more FEC-Scheme-Specific Information (FSSI) elements,
each consisting of a name and a value where the valid element
names and value ranges are defined by the FEC scheme.
Multiple instances of the FEC Framework, with separate and
independent FEC Framework Configuration Information, can be present
at a sender or receiver. A single instance of the FEC Framework
protects packets of the source flows identified in (2) above, i.e.,
all packets sent on those flows MUST be FEC source packets as defined
in Section 5.3. A single source flow can be protected by multiple
instances of the FEC Framework.
The integer flow identifier identified in (2b) above 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 can use it to identify the source
flow with which a recovered packet is associated. The integer flow
identifier can therefore take the place of the complete flow
description (e.g., UDP 4-tuple).
Whether and how this flow identifier is used is defined by the FEC
scheme. Since repair packets can provide protection for multiple
source flows, repair packets would either not carry the identifier at
all or can carry multiple identifiers. However, in any case, the
flow identifier associated with a particular source packet can be
recovered from the repair packets as part of an FEC decoding
operation.
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 Section 5.4 and MUST relate to the same
FEC Framework instance.
In the case that RTP is used for repair packets, the identification
of the repair packet flow can also include the RTP payload type to be
used for repair packets.
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FSSI includes the information that is specific to the FEC scheme used
by the CDP. FSSI is used to communicate the information that cannot
be adequately represented otherwise and is essential for proper FEC
encoding and decoding operations. The motivation behind separating
the FSSI required only by the sender (which is carried in Sender-Side
FEC-Scheme-Specific Information (SS-FSSI) container) from the rest of
the FSSI is to provide the receiver or the third party entities a
means of controlling the FEC operations at the sender. Any FSSI
other than the one solely required by the sender MUST be communicated
via the FSSI container.
The variable-length SS-FSSI and FSSI containers transmit the
information in textual representation and contain zero or more
distinct elements, whose descriptions are provided by the fully-
specified FEC schemes.
For the CDPs that choose the Session Description Protocol (SDP)
[RFC4566] as their session description protocol, the ABNF [RFC5234]
syntax for the SS-FSSI and FSSI containers is provided in Section 4.5
of [I-D.ietf-fecframe-sdp-elements].
5.6. FEC Scheme Requirements
In order to be used with this framework, an FEC scheme MUST be
capable of processing data arranged into blocks of ADUs (source
blocks).
A specification for a new FEC scheme MUST include the following:
1. The FEC Encoding ID value that uniquely identifies the FEC
scheme. This value MUST be registered with IANA as described in
Section 10.
2. The type, semantics and encoding format of the Repair FEC Payload
ID.
3. The name, type, semantics and text value encoding rules for zero
or more FEC-Scheme-Specific Information elements.
4. A full specification of the FEC code.
This specification MUST precisely define the valid FEC-Scheme-
Specific 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
symbols).
Furthermore, given a source block as defined in Section 5.2,
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valid values of the FEC-Scheme-Specific Information, a valid
Repair FEC Payload ID value and a valid packet payload size, the
specification MUST uniquely define the values of the encoding
symbols to be included in the repair packet payload of a packet
with the given Repair FEC Payload ID value.
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 Information, a valid Repair FEC Payload
ID value and a valid packet payload size as input produces the
exact value of the encoding symbols as output.
5. A description of practical encoding and decoding algorithms.
This description need not be to the same level of detail as for
the encoding above, however it has to be sufficient to
demonstrate that encoding and decoding of the code is both
possible and practical.
FEC scheme specifications MAY additionally define the following:
1. Type, semantics and encoding format of an Explicit Source FEC
Payload ID.
Whenever an FEC scheme specification defines an 'encoding format' for
an element, this has to 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 can include some kind of length indication.
FEC scheme specifications SHOULD use the terminology defined in this
document and SHOULD follow the following format:
1. Introduction <Describe the use-cases addressed by this FEC
scheme>
2. Formats and Codes
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>
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2.2 Repair FEC Payload ID <Define the type and format of the
Repair FEC Payload ID>
2.3 FEC Framework Configuration Information <Define the names,
types and text value encoding formats of the FEC-Scheme-
Specific Information elements>
3. Procedures <Describe any procedures which are specific to this
FEC scheme, in particular derivation and interpretation of the
fields in the FEC Payload IDs and FEC-Scheme-Specific Information>
4. FEC Code Specification <Provide a complete specification of the
FEC Code>
Specifications can include additional sections including examples.
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 can include portions of another by
reference). Where an RTP Payload Format is defined for repair data
for a specific FEC scheme, the RTP Payload Format and the FEC scheme
can be specified within the same document.
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6. Feedback
Many applications require some kind of feedback on transport
performance. E.g., how much data arrived at the receiver, at what
rate and when? When FEC is added to such applications, feedback
mechanisms can also need to be enhanced to report on the performance
of the FEC. E.g., how much lost data was recovered by the FEC?
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). Thus, feedback information
that is averaged over longer periods of time will likely not provide
sufficient information for engineering purposes. More detailed
feedback over shorter time scales might be preferred. For example,
for applications using RTP transport, see [RFC5725].
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.
New applications which require such feedback SHOULD use RTP/RTCP
[RFC3550].
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7. Transport Protocols
This framework is intended to be used to define CDPs that operate
over transport protocols providing an unreliable datagram service,
including in particular the User Datagram Protocol (UDP) and the
Datagram Congestion Control Protocol (DCCP).
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8. Congestion Control
This section starts with some informative background on the
motivation of the normative requirements for congestion control,
which are spelled out in Section 8.2.
8.1. Motivation
o The enforcement of congestion control principles has gained a lot
of momentum in the IETF over the recent years. While the need for
congestion control 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.
o The authors of this document 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, e.g., 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 can be so high that
the required end-to-end reliability will be difficult to achieve.
Furthermore, the end-to-end network reliability is not necessarily
known in advance.
o This FEC Framework is not defined, nor intended, as a QoS
enhancement tool to combat losses resulting from highly congested
networks. It should not be used for such purposes.
o In order to prevent such mis-use, one approach is to leave
standardization 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.
o Another approach is to write a clear applicability statement. For
example, one could restrict the use of this framework to networks
with certain loss characteristics (e.g., wireless links).
However, there can be applications where the use of FEC is
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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 a traffic
generated as an emergency signal.
o A third approach 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.
o Taking above considerations into account, Section 8.2 specifies a
small set of constraints for the FEC, which are mandatory for all
senders compliant with this FEC Framework. Further restrictions
can be imposed by certain CDPs.
8.2. Normative Requirements
o The bandwidth of FEC repair data MUST NOT exceed the bandwidth of
the original source data being protected (without the possible
addition of an Explicit Source FEC Payload ID). This disallows
the (static or dynamic) use of excessively strong FEC to combat
high packet loss rates, which can otherwise be chosen by naively
implemented dynamic FEC-strength selection mechanisms. We
acknowledge that there are a few exotic applications, e.g., IP
traffic from space-based senders, or senders in certain hardened
military devices, which could warrant a higher FEC strength.
However, in this specification we give preference to the overall
stability and network friendliness of average applications.
o Whenever the source data rate is adapted due to the operation of
congestion control mechanisms, the FEC repair data rate MUST be
similarly adapted.
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9. Security Considerations
The application of FEC protection to a stream does not provide any
kind of security protection.
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.
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 ADUs being passed
up to the application protocol. A similar attack is possible if an
attacker is in a position to inject fake FEC Framework Configuration
Information or fake FEC Payload IDs. Such incorrect decoded ADUs
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.
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 access.
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10. IANA Considerations
FEC schemes for use with this framework are identified in protocols
using FEC Encoding IDs. Values of FEC Encoding IDs are subject to
IANA registration. For this purposes, this document creates a new
registry called FEC Framework (FECFRAME) FEC Encoding IDs.
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 an IETF
Consensus basis as defined in [RFC5226]. Section 5.6 defines
explicit requirements that documents defining new FEC Encoding IDs
should meet.
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11. Acknowledgments
This document is based in part on [I-D.watson-tsvwg-fec-sf] 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 [MBMSTS], 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.
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12. References
12.1. Normative references
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3095] Bormann, C., Burmeister, C., Degermark, M., Fukushima, H.,
Hannu, H., Jonsson, L-E., Hakenberg, R., Koren, T., Le,
K., Liu, Z., Martensson, A., Miyazaki, A., Svanbro, K.,
Wiebke, T., Yoshimura, T., and H. Zheng, "RObust Header
Compression (ROHC): Framework and four profiles: RTP, UDP,
ESP, and uncompressed", RFC 3095, July 2001.
[RFC5052] Watson, M., Luby, M., and L. Vicisano, "Forward Error
Correction (FEC) Building Block", RFC 5052, August 2007.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
12.2. Informative references
[I-D.watson-tsvwg-fec-sf]
Watson, M., "Forward Error Correction (FEC) Streaming
Framework", draft-watson-tsvwg-fec-sf-00 (work in
progress), July 2005.
[RFC5725] Begen, A., Hsu, D., and M. Lague, "Post-Repair Loss RLE
Report Block Type for RTP Control Protocol (RTCP) Extended
Reports (XRs)", RFC 5725, February 2010.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[RFC4588] Rey, J., Leon, D., Miyazaki, A., Varsa, V., and R.
Hakenberg, "RTP Retransmission Payload Format", RFC 4588,
July 2006.
[I-D.ietf-fecframe-sdp-elements]
Begen, A., "Session Description Protocol Elements for FEC
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Framework", draft-ietf-fecframe-sdp-elements-11 (work in
progress), October 2010.
[MBMSTS] 3GPP, "Multimedia Broadcast/Multicast Service (MBMS);
Protocols and codecs", 3GPP TS 26.346, April 2005.
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Authors' Addresses
Mark Watson
Netflix, Inc.
100 Winchester Circle
Los Gatos, CA 95032
USA
Email: watsonm@netflix.com
Ali Begen
Cisco
181 Bay Street
Toronto, ON M5J 2T3
Canada
Email: abegen@cisco.com
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