FEC Framework Working Group M. Watson
Internet-Draft Digital Fountain
Intended status: Standards Track November 16, 2007
Expires: May 19, 2008
Forward Error Correction (FEC) Framework
draft-ietf-fecframe-framework-01
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Abstract
This document describes for 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.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Definitions/Abbreviations . . . . . . . . . . . . . . . . . . 6
3. Requirements notation . . . . . . . . . . . . . . . . . . . . 8
4. Architecture Overview . . . . . . . . . . . . . . . . . . . . 9
5. Procedural overview . . . . . . . . . . . . . . . . . . . . . 12
5.1. General . . . . . . . . . . . . . . . . . . . . . . . . . 12
5.2. Sender Operation . . . . . . . . . . . . . . . . . . . . . 13
5.3. Receiver Operation . . . . . . . . . . . . . . . . . . . . 15
6. Protocol Specification . . . . . . . . . . . . . . . . . . . . 18
6.1. General . . . . . . . . . . . . . . . . . . . . . . . . . 18
6.2. Structure of the source block . . . . . . . . . . . . . . 18
6.3. Packet format for FEC Source packets . . . . . . . . . . . 18
6.4. Packet Format for FEC Repair packets . . . . . . . . . . . 20
6.5. FEC Framework Configuration Information . . . . . . . . . 20
6.6. FEC Scheme requirements . . . . . . . . . . . . . . . . . 22
7. Transport Protocols . . . . . . . . . . . . . . . . . . . . . 25
8. Congestion Control . . . . . . . . . . . . . . . . . . . . . . 26
8.1. Normative requirements . . . . . . . . . . . . . . . . . . 27
9. Security Considerations . . . . . . . . . . . . . . . . . . . 29
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 30
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 31
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 32
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 33
Intellectual Property and Copyright Statements . . . . . . . . . . 34
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1. Introduction
Many applications have a requirement to transport a continuous stream
of packetised 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 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 [3] 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).
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 [3], 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.
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.
FEC Schemes designed for use with this framework must fulfil a number
of requirements defined in this document. Note that these
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requirements are different from those defined in [3] 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.
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2. Definitions/Abbreviations
'FEC' Forward Error Correction.
'AL-FEC' Application Layer Forward Error Correction
'FEC Framework' A protocol framework for definition of Content
Delivery Protocols using FEC, such as the framework defined in
this document.
'Source data flow' The packet flow or flows to which FEC protection
is to be applied.
'Repair data flow' The packet flow or flows carrying forward error
correction data
'Source protocol' A protocol used for the source data flow being
protected - e.g. RTP.
'Transport protocol' The protocol used for transport of the source
data flow being protected - e.g. UDP, DCCP.
'Transport payload' Data used as the payload for the transport layer
(e.g. UDP or DCCP packet payload)
'Application protocol' Control protocols used to establish and
control the source data flow being protected - e.g. RTSP.
'FEC Code' An algorithm for encoding data such that the encoded data
flow is resiliant to data loss or corruption.
'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.
'Source Block' the group of source data packets which are to be FEC
protected as a single block
'Protection amount' The relative increase in data sent due to the
use of FEC.
FEC Framework Configuration Information: Information which controls
the operation of the FEC Framework.
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FEC Payload ID: Information which identifies the contents of a
packet with respect to the FEC Scheme.
Source FEC Payload ID: An FEC Payload ID specifically for use with
source packets.
Repair FEC Payload ID: An FEC Payload ID specifically for use with
repair packets.
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.
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3. Requirements notation
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 [1].
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4. Architecture Overview
The FEC Framework is described in terms of an additional protocol
layer between the transport layer (e.g. UDP or DCCP) and Application
and Transport 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 transport flow identified by
the standard 5-tuple { Source IP Address, Source Transport Port,
Destination IP Address, Destination Transport Port, Transport
Protocol }.
The FEC Framework makes use of an FEC Scheme, in a similar sense to
that defined in [3] and uses the terminology of that document. The
FEC Scheme provides FEC encoding and decoding and describes the
protocol fields and or 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 groups of transport packet payloads
to the FEC Scheme for FEC Encoding. The FEC Scheme returns FEC
repair packet payloads, encoded FEC Payload ID information for each
of the repair packets and, in some cases, encoded FEC Payload ID
information for each of the source packets. 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 transport flows
which are to be FEC protected, specification of the transport 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 analagous to the FEC
Object Transmission Information defined in [3].
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.
In this architecture we assume that the interface to the transport
layer supports the concepts of payloads to be transported and
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identification transport flows on which those payloads are
transported. Since this is an interface internal to the
architecture, we do not specify this interface explicitly, except to
say that transport 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.
The architecture outlined above is illustrated in the Figure 1.
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+--------------------------------------------+
| |
| Application |
| |
+--------------------------------------------+
^
|
v
+ - - - - - - -- - - - - - - - - - - - - - - - - +
| |
+--------------------------------------------+
| | | |
| Application Layer |
| | | |
+--------------------------------------------+
| +---------------------------------+ | |
| | |
| | Application/Transport Protocol | | |
| (e.g. RTP) | |-Configuration/Coordination
| +---------------------------------+ | |
^ |
| | Transport flows | |
v v
| +--------------------------------------------+ | +----------------+
| | | |
| | FEC Framework (this document) |------| FEC Scheme |
| | | |
| +--------------------------------------------+ | +----------------+
^
| | Transport flows |
v
| +--------------------------------------------+ |
| |
| | Transport Layer (e.g. UDP) | |
| |
+--------------------------------------------+
| +--------------------------------------------+ |
| |
| | IP | |
| |
| +--------------------------------------------+ |
Content Delivery Protocol
+ - - - - - - - - - - - - - - - - - - - - - - - +
Figure 1: FEC Framework Architecture
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5. Procedural overview
5.1. General
The mechanism defined in this document does not place any
restrictions on the source transport payloads which can be protected
together, except that the source transport payload is carried over a
supported transport protocol (See Section 7). The data may be from
multiple transport flows that are protected jointly. The FEC
framework handles the packet flows as a sequence of 'source blocks'
each consisting of a set of source transport payloads, possibly from
multiple flows which are to be protected together. For example, each
source block may be constructed from those source transport payloads
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, encoded FEC Payload IDs for each of the source
payloads
o one or more FEC repair packet payloads
o encoded FEC Payload IDs for each of the repair packet payloads
The FEC framework then performs two operations: Firstly, it appends
the FEC payload IDs, if provided, to each of the source transport
payloads, 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.
This document does not define how the sender determines which source
transport payloads 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 the length of
time it should wait to receive FEC repair packets for any given
source block. The sequence of operations at the sender is described
in more detail in Section 5.2.
At the receiver, original source transport payloads are recovered by
the FEC Framework directly from any FEC Source packets received
simply by removing the Source FEC Payload ID, if present. The
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receiver also passes the contents of the received FEC Source
transport payloads, plus their FEC Payload IDs to the FEC Scheme for
possible decoding.
If any FEC source transport payloads related to a given source block
have been lost, then the FEC Scheme may perform FEC decoding to
recover the missing source transport payloads (assuming sufficient
FEC Source and FEC Repair packets related to that source block have
been received).
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 will be required. 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
Section 5.3
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 packet. 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.
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.
The FEC Scheme MAY use different FEC Payload ID field formats for FEC
Source packets and FEC Repair packets.
5.2. Sender Operation
It is assumed that the sender has constructed or received original
data packets for the session. These may be RTP, RTCP, MIKEY or
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indeed any other type of packet. The following operations,
illustrated in Figure 2 describe a possible way to generate compliant
FEC Source packet and FEC repair packet streams:
1. Source transport payloads are provided by the application.
2. A source block is constructed as specified in Section 6.2.
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 6.3 and FEC Repair packets according to Section 6.4
using the FEC Payload IDs and repair packet payloads provided by
the FEC Scheme.
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 transport 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 eventual IP FEC Source
Packet will be the same whether or not the FEC Framework is
applied).
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+-------------------------------------+
| |
| Application |
| |
+-------------------------------------+
|
| (1) Source transport payloads
|
v
+-------------------------------------+ +---------------------+
| FEC Framework | | |
| |------------------------------->| FEC Scheme |
| (2) Construct source blocks | (3) Source Block | |
| | | (4) FEC Encoding |
| (6) Construct FEC source packets |<-------------------------------| |
| and FEC repair packets | (5) Ex. Source FEC Payload Ids,| |
+-------------------------------------+ Repair FEC Payload Ids, +---------------------+
| Repair payloads
|
| (7) FEC Source packets and FEC repair packets
v
+-------------------------------------+
| |
| Transport Layer (e.g. UDP) |
| |
+-------------------------------------+
Figure 2: Sender operation
5.3. Receiver Operation
The following describes a possible receiver algorithm, illustrated in
Figure 3, 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 transport flow to which it belongs (in the case of
source packets ) is indicated by the transport flow information
which identifies the flow at the transport layer (for example
source and destination ports and addresses in the case of UDP).
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.
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.
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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.
5. The FEC Scheme returns the source transport payload to the FEC
Framework in the form of source blocks containing received and
decoded source packets and indications of any source packets which
were missing and could not be decoded.
6. The FEC Framework passes the received and recovered source
packet payloads to the application.
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+-------------------------------------+
| |
| Application |
| |
+-------------------------------------+
^
| (6) Source transport payloads
|
|
+-------------------------------------+ +---------------------+
| FEC Framework | | |
| |<-------------------------------| FEC Scheme |
| (2) Extract FEC Payload IDs and | (5) Source Transport Payloads | |
| pass Payloads and Payload IDs | | (4) FEC Decoding |
| to FEC Scheme |------------------------------->| |
| | (3) Ex. FEC Source 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) |
| |
+--------------------------------------------+
Figure 3: Receiver Operation
Note that the above procedure may result in a situation in which not
all original source packets are recovered.
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 required 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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6. Protocol Specification
6.1. 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 source payloads. 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 Configuation 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.
6.2. Structure of the source block
The FEC Framework and FEC Scheme exchange source transport payload in
the form of source blocks. A source block is generated by the FEC
Framework from an ordered sequence of source transport payloads. The
allocation of transport payloads to blocks is dependent on the
application. Note that some source transport payloads may not be
included in any block. For each source transport payload included in
a source block, the following information is provided to the FEC
Scheme:
o A description of the source transport flow with which the
transport payload is associated (See 6.5)
o The source transport payload itself
o The length of the source transport payload
6.3. 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 4,
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 transport flow.
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+------------------------------------+
| IP header |
+------------------------------------+
| Transport header |
+------------------------------------+
| Original transport Payload |
+------------------------------------+
| Explicit Source FEC Payload ID |
+------------------------------------+
Figure 4: Structure of the FEC packet format for FEC Source packets
The FEC Source packets MUST be sent using the same transport flow as
would have been used for the original source packets if the FEC
Framework were not present. The Original transport Payload field
MUST be identical to the source transport payload. The transport
payload of the FEC Source packet MUST consist of the Original
Transport Payload 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. 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 the a sequence number of some kind 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.
Since the addition of the Explicit Source FEC Payload ID increases
the packet length, then 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
source transport payloads that will be delivered using the FEC
Framework.
Note: The Explicit Source FEC Payload ID is placed at the end of the
packet so that in the case that Robust Header Compression [2] 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.
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6.4. Packet Format for FEC Repair packets
The packet format for FEC Repair packets is shown in Figure 5. 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 5: 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.
6.5. FEC Framework Configuration Information
The FEC Framework Configuration Information is information that the
FEC Framework needs in order to apply FEC protection to the transport
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.
The FEC Framework Configuration Information includes identification
of a set of source packet flows. For example, in the case of UDP,
each packet 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 be
wildcarded, 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.
A single instance of the FEC Framework provides FEC protection for
all packets of a specified set of source packet flows, by means of
one or more packet flows consisting of repair packets. The FEC
Framework Configuation Information includes, for each instance of the
FEC Framework:
1. Identification of the packet flow(s) carrying FEC Repair
packets, known as the FEC repair flow(s).
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2. For each source packet flow protected by the FEC repair
flow(s):
a. Defintion of the packet flow carrying source packets (for
example, by means of a tuple as describe above for UDP).
b. An integer identifier for this flow definition (i.e.
tuple). This identifier MUST be unique amongst all source
packet flows which are protected by the same FEC repair flow.
3. The FEC Encoding ID, identifying the FEC Scheme
4. The length of the Explicit Source FEC Payload Id, in bytes
5. An opaque container for FEC-Scheme-specific information
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 all packets of all the source packet flows identified in (2)
above i.e. all packets sent on those flows MUST be FEC Source packets
as defined in Section 6.3. A single source packet flow may be
protected by multiple instances of the FEC Framework.
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).
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, this flow identifier would likely not be
carried directly in repair packets. However, 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.
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 6.4 and MUST relate to the same FEC
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Framework instance.
6.6. FEC Scheme requirements
In order to be used with this framework, an FEC Scheme MUST be
capable of processing data arranged into blocks of source transport
packet payloads (source blocks).
A specification for a new FEC scheme MUST include the following
things:
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 type, semantics and encoding format of the FEC Scheme-
specific FEC Framework Configuration Information.
4. A full specification of the FEC code.
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 - not necessarily
contiguous - within a packet dedicated to carrying encoding
symbol bytes).
Furthermore, given a source block as defined in Section 6.2,
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.
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.
5. A description of practical encoding and decoding algorithms.
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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.
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 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.
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>
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 type and
format of the FEC Scheme-specific FEC Framework configuration
information>
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.>
4. FEC code specification <provide a complete specification of the
FEC Code>
Specifications MAY include additional sections, for example,
examples.
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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).
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7. Transport Protocols
The following transport protocols are supported:
o User Datagram Protocol (UDP)
o Datagram Congestion Control Protocol (DCCP)
Editor's note: This section will contain transport-specific
considerations, if any.
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8. Congestion Control
This section starts with a informative section on the motivation of
the normative requirements for congestion control, which are spelled
out in Section 8.1.
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 heterogenous 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 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.
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 heterogenous 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.
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.
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.
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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.
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.
Taking above considerations into account, the normative text of
this section implements 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).
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.
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.
8.1. Normative requirements
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
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flow bandwidth 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 transport payload 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 source transport
payloads being passed up to the application protocol. Such incorrect
packets 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, for example using
IPsec. Receivers MUST also verify the integrity of source transport
payloads before including the source transport payload into the
source block for FEC purposes.
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 decode 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
permissionions to view.
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10. IANA Considerations
tbd
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11. Acknowledgments
This document is based in large part on [4] 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 [5] and thus thanks are also
due to the participants in 3GPP TSG SA working group 4.
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12. References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] 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.
[3] Watson, M., Luby, M., and L. Vicisano, "Forward Error Correction
(FEC) Building Block", RFC 5052, August 2007.
[4] Watson, M., "Forward Error Correction (FEC) Streaming
Framework", draft-watson-tsvwg-fec-sf-00 (work in progress),
July 2005.
[5] 3GPP, "Multimedia Broadcast/Multicast Service (MBMS); Protocols
and codecs", 3GPP TS 26.346, April 2005.
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Author's Address
Mark Watson
Digital Fountain
39141 Civic Center Drive
Suite 300
Fremont, CA 94538
U.S.A.
Email: mark@digitalfountain.com
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