Reliable Multicast M. Watson
Internet-Draft M. Luby
Expires: October 31, 2005 Digital Fountain
L. Vicisano
Cisco Systems, Inc.
April 29, 2005
Forward Error Correction (FEC) Building Block
draft-ietf-rmt-fec-bb-revised-00
Status of this Memo
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Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This document describes how to use Forward Error Correction (FEC)
codes to efficiently provide and/or augment reliability for data
transport. This document defines a framework for the definition of
the information that needs to be communicated in order to use an FEC
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code for delivering content, in addition to the encoded data itself,
and for definition of formats and codes for communcation of that
information. Both information communicated with the encoded data
itself and information that needs to be communicated 'out-of-band'
are considered. The procedures for specifying new FEC codes,
defining the information communication requirements associated with
those codes and registering them with the Internet Assigned Numbers
Authority (IANA) are also described. The requirements on Content
Delivery Protocols which wish to use FEC codes defined within this
framework are also defined. The companion document titled, "The Use
of Forward Error Correction (FEC) in Reliable Multicast" describes
some applications of FEC codes for delivering content.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Definitions/Abbreviations . . . . . . . . . . . . . . . . . . 5
3. Requirements notation . . . . . . . . . . . . . . . . . . . . 6
4. Rationale . . . . . . . . . . . . . . . . . . . . . . . . . . 7
5. Applicability Statement . . . . . . . . . . . . . . . . . . . 9
6. Functionality . . . . . . . . . . . . . . . . . . . . . . . . 10
6.1 FEC Schemes . . . . . . . . . . . . . . . . . . . . . . . 11
6.2 FEC Object Transmission Information . . . . . . . . . . . 13
6.2.1 Transport of Object Transmission Information . . . . . 14
6.2.2 Opacity of Object Transmission Information . . . . . . 15
6.2.3 Mandatory FEC Object Transmission Information
elements . . . . . . . . . . . . . . . . . . . . . . . 15
6.2.4 Common FEC Object Transmission Information elements . 15
6.2.5 Scheme-specific FEC Object Transmission
Information element . . . . . . . . . . . . . . . . . 16
6.3 FEC Payload ID . . . . . . . . . . . . . . . . . . . . . . 16
7. FEC scheme specifications . . . . . . . . . . . . . . . . . . 18
8. CDP specifications . . . . . . . . . . . . . . . . . . . . . . 21
9. Common algorithms . . . . . . . . . . . . . . . . . . . . . . 22
9.1 Block partitioning algorithm . . . . . . . . . . . . . . . 22
9.1.1 First Step . . . . . . . . . . . . . . . . . . . . . . 22
9.1.2 Second step . . . . . . . . . . . . . . . . . . . . . 23
10. Requirements from other building blocks . . . . . . . . . . 25
11. Security Considerations . . . . . . . . . . . . . . . . . . 26
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . 27
12.1 Explicit IANA Assignment Guidelines . . . . . . . . . . . 27
13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 29
14. References . . . . . . . . . . . . . . . . . . . . . . . . . 29
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 30
Intellectual Property and Copyright Statements . . . . . . . . 31
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1. Introduction
This document describes how to use Forward Error Correction (FEC)
codes to provide support for reliable delivery of content within the
context of a Content Delivery Protocol (CDP). This document
describes a building block as defined in [11]. This document is a
product of the IETF RMT WG and follows the general guidelines
provided in [9].
The purpose of this building block is to define a framework for
forward error correction such that:
1. CDPs can be designed to operate with a range of different FEC
codes/scheme, without needing to know details of the specific FEC
code/scheme that may be used
2. FEC schemes can be designed to operate with a range of different
CDPs, without needing to know details of the specific CDPs
Note that a 'CDP' in the context of this document may consist of
several distinct protocol mechanisms.
Editor's note (to be removed): This document proposes an update to
RFC3452. The primary goals of this update are:
1. Move the FEC Building Block from Experimental to Proposed
Standard
2. Clarify some confusing language in the current RFC 3452, which
is not incorrect but has been misinterpreted on some occasions
(based on comments on the mailing list and during the RMT
meetings).
3. Provide detailed guidelines on how to write an RFC for an FEC
scheme corresponding to a new FEC Encoding ID (for both Fully-
Specified and Under-Specified FEC Schemes)
4. Ensure that the FEC building block is applicable to multiple
protocols, including FLUTE and NORM. The FEC Building Block
will not preclude the application of FEC Schemes to streaming
protocols, for FEC schemes that support it.
5. Ensure that protocols using this building block can be defined
in a way whch is 'FEC Scheme independent' so that new FEC
schemes can be defined without the need to amend the CDPs.
6. Ensure that new FEC schemes can be defined in a way which is
'CDP independent' so that new CDPs can be defined without the
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need to amend the FEC schemes.
7. Ensure on-the-wire backwards compatibility with existing
implementations of protocols that are compliant with the
current Experimental RFCs, e.g., existing implementations of
FLUTE and NORM.
It is anticipated that updates to other RMT RFCs to Proposed
Standard status will be carried out independently and in parallel
with this work. Whilst on-the-wire backwards compatibility is a
mandatory requirement, some reorganisation of material amongst the
RFCs is implied by the approach proposed in this document.
Specifically:
8. FEC Scheme definitions from RFC3452 should be moved to a
separate document. The scheme definitions will be revised to
follow the format specified in this document.
9. FEC scheme specific information in FLUTE (RFC3926) should be
moved into the FEC Scheme specification document referred to
above, for example the formats of the FEC Object Transmission
Information when carried in EXT_FTI.
10. Provision for transport of FEC scheme specific Object
Transmission Information needs to be included in FLUTE and
NORM (in a backwards-compatible way).
It is also proposed that the algorithm for partitioning objects
into source blocks currently included in the FLUTE specification
should be moved into this document as a common algorithm to be
used by multiple FEC Schemes.
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2. Definitions/Abbreviations
Object: An ordered sequence of bytes to be transferred by the
transport protocol. For example, a file or stream.
Symbol: A unit of data processed by the Forward Error Correction
code. A symbol is always considered as a unit i.e. it is either
completely received or completely lost.
Source symbol: A symbol containing information from the original
object.
Repair symbol: A symbol containing information generated by the FEC
code which can be used to recover lost source symbols.
Encoding symbol: A source symbol or a repair symbol.
Encoder: The FEC scheme specific functions required to transform a
object into FEC encoded data. i.e. the functions that produce
repair symbols using source symbols.
Decoder: The FEC scheme specific functions required to transform
received FEC encoded data into a copy of the original object
Receiver: A system supporting the receiving functions of a CDP and
FEC scheme according to this specification
Sender: A system supporting the sending functions of a CDP and FEC
scheme according to this specification
Source Block: A part of the object formed from a subset of the
object's source symbols
CDP: Content Delivery Protocol
FEC: Forward Error Correction
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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. Rationale
FEC codes are a valuable basic component of any CDP that is to
provide reliable delivery of an object. Using FEC codes is effective
in the context of IP multicast and reliable delivery because FEC
encoding symbols can be useful to all receivers for reconstructing an
object even when the receivers have received different encoding
symbols. Furthermore, FEC codes can ameliorate or even eliminate the
need for feedback from receivers to senders to request retransmission
of lost packets.
Central to this document is the concept of an 'FEC Scheme'. An FEC
scheme defines ancilliary information and procedures which, combined
with an FEC encoding algorithm, fully define how the FEC code can be
used with CDPs. An FEC scheme may be associated with a single
standardised FEC algorithm (A 'Fully specified' FEC scheme) or may be
applicable to many FEC algorithms (An 'under-specified' FEC scheme).
This document describes a framework for the definition of FEC
schemes. Definition of actual FEC schemes is outside the scope of
this document. This document also defines requirements for reliable
CDPs that make use of FEC schemes. Any such protocol which is
compliant to the requirements specified in this document can make use
of any FEC scheme which is defined within the framework described
here. Note that FEC schemes may place restrictions on the types of
CDP they are intended to be used with. For example, some FEC schemes
may be specific to particular types of application, such as file
delivery or streaming.
The goal of the FEC building block is to describe functionality
directly related to FEC codes that is common to all reliable CDPs and
to all FEC schemes, and to leave out any additional functionality
that is specific to particular protocols or particular FEC schemes.
The primary functionality described in this document that is common
to all such protocols that use FEC codes is the definition and
transport of three kinds of information from sender to receiver(s):
encoding symbols themselves, anciliary information associated with
encoding symbols (or groups of such symbols, such as the group of
symbols in a single packet, or the group of symbols related to a
single source block) and anciliary information associated with the
whole object being transfered. It is important to note that this
ancilliary information is only required by the receiver if one or
more of the encoding symbols to which it relates are received.
This document for example does not describe how receivers may request
transmission of particular encoding symbols for an object. This is
because although there are protocols where requests for transmission
are of use, there are also protocols that do not require such
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requests.
The companion document [3] should be consulted for a full explanation
of the benefits of using FEC codes for reliable content delivery
using IP multicast. FEC codes are also useful in the context of
unicast, and thus the scope and applicability of this document is not
limited to IP multicast.
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5. Applicability Statement
The FEC building block does not provide any support for congestion
control. Any complete multicast protocol MUST provide congestion
control that conforms to [5], and thus this MUST be provided by
another building block when the FEC building block is used in a
protocol.
A more complete description of the applicability of FEC codes can be
found in the companion document [3].
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6. Functionality
This section describes FEC information that is either to be sent in
packets also containing FEC encoding symbols or 'out-of-band'. The
FEC information is associated with transmission of encoding symbols
related to a particular object. There are three classes of packets
that may contain FEC information: data packets, session-control
packets and feedback packets. They generally contain different kinds
of FEC information. Note that some protocols may not use session-
control or feedback packets.
Data packets may sometimes serve as session-control packets as well;
both data and session-control packets generally travel downstream
from the sender towards receivers and are sent to a multicast channel
or to a specific receiver using unicast. Session-control packets may
additionally travel upstream from receivers to senders.
As a general rule, feedback packets travel upstream from receivers to
the sender. Sometimes, however, they might be sent to a multicast
channel or to another receiver or to some intermediate node or
neighboring router that provides recovery services.
This document specifies the FEC information that must be carried in
data packets and the other FEC information that must be communicated
from sender to receiver(s) either out-of-band or in data packets.
This document does not specify out-of-band methods nor does it
specify the way out-of-band FEC information is associated with FEC
information carried in data packets. These methods must be specified
in CDP specifications that use this FEC building block.
FEC information is classified as follows:
1. FEC information associated with an object
This is information that is essential for the FEC decoder to
decode a specific object. An example of this information is the
identity of the FEC scheme that is being used to encode the
object, in the form of the FEC Encoding ID. The FEC Encoding ID
is described further below. This information may also include
FEC scheme-specific parameters for the FEC decoder.
2. FEC information associated with specific encoding symbols for an
object
This is information which is associated with one or more encoding
symbols and is thus needed by the decoder whenever one or more of
those encoding symbols have been received. Depending on the FEC
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scheme, information may be associated with individual symbols
and/or with groups of symbols. One common such grouping is the
group of symbols included within a single packet. Many FEC
schemes also segment the object being encoded into multiple
'source blocks', each of which is processed independently for FEC
purposes. Information about each source block is another type of
information associated with a group of encoding symbols, this
time the group of symbols which are related to a given source
block.
Two 'containers' are provided for communicating the FEC information
described above, but there is not necessarily a one-to-one
correspondance between the class of FEC information and the mechanism
used. The two mechanisms are:
a. FEC Object Transmission Information
CDPs must provide a reliable mechanism for communicating certain
FEC information from sender to receiver(s). This information is
known as 'FEC Object Transmission Information' and its contents
depend on the particular FEC scheme. It includes all information
of the first class above and may include information of the
second class. The FEC Object Transmission Information can be
sent to a receiver within the data packet headers, within session
control packets, or by some other means.
b. FEC Payload ID
CDPs must provide a mechanism for communicating information which
identifies (for FEC purposes) the encoding symbols carried by a
packet. This information is known as the FEC Payload ID and its
contents depend on the FEC scheme. It includes only information
of the second class above. A data packet that carries encoding
symbols MUST include an FEC Payload ID.
6.1 FEC Schemes
Two types of FEC scheme are defined by this document: 'Fully-
specified' FEC schemes and 'Under-specified' FEC schemes. An FEC
scheme is a Fully-Specified FEC scheme if the encoding scheme is
formally and fully specified, in a way that independent implementors
can implement both encoder and decoder from a specification that is
an IETF RFC.
It is possible that an FEC scheme may not be a Fully-Specified FEC
scheme, because either a specification is simply not available or a
party exists that owns the encoding scheme and is not willing to
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disclose the algorithm or specification. We refer to such an FEC
encoding scheme as an Under-Specified FEC scheme.
FEC schemes are identified by an FEC Encoding ID, which is an integer
identifier assigned by IANA. The FEC Encoding ID allows receivers to
select the appropriate FEC decoder. The value of the FEC Encoding ID
MUST be the same for all transmission of encoding symbols related to
a particular object, but MAY vary across different transmissions of
encoding symbols about different objects, even if transmitted to the
same set of multicast channels and/or using a single upper-layer
session.
The FEC Instance ID is an integer value that identifies a specific
instance of an Under-Specified FEC scheme. This value is not used
for Fully-Specified FEC schemes. The FEC Instance ID is scoped by
the FEC Encoding ID, and FEC Instance ID values are subject to IANA
registration.
The FEC Encoding ID and FEC Instance ID are essential for the decoder
to decode an object and thus are part of the FEC Object Transmission
Information.
The following requirements apply to all FEC schemes, whether Fully-
Specified or Under-Specified:
o The type, semantics and an encoding format for the FEC Payload ID
and the FEC Object Transmission Information MUST be defined.
o A value for the FEC Encoding ID MUST be reserved and associated
with the types, semantics and encoding format of the FEC Payload
ID and the the FEC Object Transmission Information.
The specification for an Under-Specified FEC Scheme MAY allocate a
sub-field within the Scheme-specific FEC Object Transmission
Information element which is for instance-specific information. Each
specific instance of the Under-Specified FEC Scheme may then use this
field in an instance-specific way. The FEC scheme should define the
scheme-specific FEC Object Transmission Information element in such a
way that receivers that do not support the received FEC Instance ID
can still parse and interpret the scheme-speific FEC Object
Transmission Information element with the exception of the instance-
specific field.
An already defined Under-Specified FEC Scheme (i.e. FEC Encoding ID
value) MUST be reused if the associated FEC Payload ID and FEC Object
Transmission Information have the required fields and encoding
formats for a new Under-Specified FEC scheme instance.
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An instance of an Under-Specified FEC scheme is fully identified by
the tuple (FEC Encoding ID, FEC Instance ID). The tuple MUST
identify a single scheme instance that has at least one
implementation. The party that owns this tuple MUST be able to
provide information on how to obtain the the Under-Specified FEC
scheme instance identified by the tuple, e.g., a pointer to a
publicly available reference-implementation or the name and contacts
of a company that sells it, either separately or embedded in another
product.
This specification reserves the range 0-127 for the values of FEC
Encoding IDs for Fully-Specified FEC schemes and the range 128-255
for the values of Under-Specified FEC schemes.
6.2 FEC Object Transmission Information
The FEC Object Transmission Information contains information which is
essential to the decoder in order to decode the encoded object. It
may also contain information which is required to decode certain
groups of encoding symbols, for example individual Source Blocks
within the object. This information is communicated reliably by the
CDP to the receiver(s) as described in Section 8.
The FEC Object Transmission Information may consist of several
elements and each element may be one of three types, as follows:
Mandatory: These elements are defined in this specification and are
mandatory for all FEC Schemes. Each FEC scheme specifies how the
values of the Mandatory FEC Object Transmission Information
elements are determined and each CDP specifies how this
information is encoded and reliably communicated to the
receiver(s). The Mandatory FEC Object Transmission Information
includes the identification of the FEC Scheme, which is needed by
the receiver to determine whether it supports the FEC Scheme.
Common: These elements are defined in this specification and are
optional to be used by an FEC scheme. Each FEC scheme specifies
which of the Common FEC Object Transmission Information elements
it uses and how the values of these elements are determined. Each
FEC scheme also specifies an encoding format for the information.
Each CDP must specify at least one of the following:
1. A means to reliably communicated the Common FEC Object
Transmission Information elements to the receiver(s) using the
encoding format defined by the FEC scheme.
2. An alternative, CDP specific, encoding format for each of the
Common FEC Object Transmission Information elements.
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Scheme-specific: An FEC scheme may specify a single Scheme-specific
FEC Object Transmission Information element. The FEC scheme
specifies the type, semantics and encoding format of the Scheme-
specific FEC Object Transmission Information element. The
encoding format may assume that the receiver can determine the
length of the Scheme-specific FEC Object Transmission Information
element from information communicated outside that element. Each
CDP specifies how the Scheme-specific FEC Object Transmission
Element is communicated reliably to the receiver(s) i.e. exactly
where it shall be carried within packets of the CDP. CDPs also
specify how the length of the Scheme-specific FEC Object
Transmission Information can be determined by the receiver. Note
that although from the point of view of this specification and of
CDPs there is only a single Scheme-specific FEC Object
Transmission Information element, the FEC scheme may specify this
element to contain multiple distinct pieces of information.
The Mandatory and Common FEC Object Transmission Information elements
are defined in the sections below.
6.2.1 Transport of Object Transmission Information
It is the responsibility of the CDP to reliably transport the Object
Transmission Information to the receiver(s).
It is important to note that the encoding format of the Mandatory FEC
Object Transmission Information elements (The FEC Encoding ID and FEC
Instance ID) is defined by the CDP. This is so that the receiver can
identify the FEC Scheme to be used for interpreting the remaining FEC
Object Transmission Information elements. All CDPs must define
encoding formats for all the Mandatory FEC Object Transmission
Information elements.
Common FEC Object Transmission Information elements can be
transported in two different ways: (a) the FEC Scheme defines an
encoding format for each Common Object Transmission Information
element and the CDP transports it, or (b) the CDP defines an encoding
format and transports the information in this format.
An FEC Scheme MUST define encoding formats for the Common FEC Object
Transmission Information elements. A CDP MAY define encoding formats
for the Common FEC Object Transmission Information elements. The CDP
determines which way the Common FEC Object Transmission Information
elements shall be transported, (a) or (b). Note that a CDP may
provide support for one or both options.
In the case that the CDP uses the encoding formats specified by the
FEC scheme then it may simply concatenate the encoding formats
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defined by the FEC scheme or it may carry each element in a seperate
field or wrapper within the CDP. The FEC scheme must define the
encoding formats of the Common FEC Object Transmission Information
elements in such a way that the length of each element is either
fixed or can be determined from the encoded data itself.
The encoding format of the Scheme-specific FEC Object Transmission
Information element is defined by the FEC scheme. CDPs specify only
how the resulting byte sequence is communicated. As with encoding
formats for the Common FEC Oject Transmission Information elements
the length of the Scheme-specific FEC Object Transmission Information
must either be fixed or it must be possible to determine the length
from the encoded data itself.
6.2.2 Opacity of Object Transmission Information
The Scheme-specific FEC Object Transmission Information element is
opaque to the CDP in the sense that inspecting the contents of this
element can only be done if FEC scheme-specific logic is included in
the CDP.
The encoding formats defined by the FEC scheme for the Common FEC
Object Transmission Information elements are also opaque to the CDP
in the same sense.
The encoding formats defined by the CDP for the Common FEC Object
Transmission Information elements are not opaque in this sense,
although it must be considered that different FEC Schemes may use
different combinations of the Common FEC Object Transmission
Information elements.
6.2.3 Mandatory FEC Object Transmission Information elements
The Mandatory FEC Object Transmission Information elements are:
FEC Encoding ID: an integer between 0 and 255 inclusive identifying a
specific FEC scheme (Fully-Specified or Under-Specified.)
FEC Instance ID (if the FEC Encoding ID identifies an Under-Specified
FEC scheme): an integer between 0 and 65535 inclusive identifying an
instance of an Under-specifed FEC scheme
6.2.4 Common FEC Object Transmission Information elements
The Common FEC Object Transmission Information elements are described
below. Note that this specification does not provide complete
definitions of these fields. Instead only aspects of the abstract
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type are defined. The precise type and semantics are defined for
each FEC scheme in the FEC scheme specification.
Transfer-Length: a non-negative integer indicating the length of the
object in bytes
Encoding-Symbol-Length: a non-negative integer indicating the length
of each encoding symbol in bytes
Maximum-Source-Block-Length: a non-negative integer indicating the
maximum number of source symbols in a source block
Max-Number-of-Encoding-Symbols: a non-negative integer indicating the
maximum number of encoding symbols (i.e. source plus repair
symbols in the case of a systematic code)
FEC Schemes define the precise type of these elements and in
particular may restrict the value range of each element. FEC Schemes
also define an encoding format for each of the above elements that
they require. CDPs may also provide an encoding format for each
element in which case this encoding format MUST be capable of
representing values up to (2^^48)-1 in the case of the Transfer-
Length and up to (2^^32)-1 for the other elements. CDPs may
additionally or alternatively provide a mechanism to transport these
elements encoded according to the encoding format defined by the FEC
scheme. For example, FLUTE [9] specifies an XML-based encoding
format for these elements, but can also transport FEC scheme-specific
encoding formats within the EXT-FTI header extension.
6.2.5 Scheme-specific FEC Object Transmission Information element
The Scheme-specific FEC Object Transmission Information element may
be used by an FEC Scheme to communicate information which is
essential to the decoder which cannot adequately be represented
within the Mandatory or Common FEC Object Transmission Information
elements.
From the point of view of a CDP, the Scheme-specific FEC Object
Transmission Information element is an opaque, variable length,
bitstring. The FEC Scheme defines the structure of this bitstring,
which may contain multiple distinct elements.
6.3 FEC Payload ID
The FEC Payload ID contains information which indicates to the FEC
decoder the relationships between the encoding symbols carried by a
particular packet and the FEC encoding transformation. For example
if the packet carries source symbols then the FEC Payload ID
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indicates which source symbols of the object are carried by the
packet. If the packet carries repair symbols, then the FEC Payload
ID indicates how those repair symbols were constructed from the
object.
The FEC Payload ID may also contain information about larger groups
of encoding symbols of which those contained in the packet are part.
For example, the FEC Payload ID may contain information about the
source block the symbols are related to.
The FEC Payload ID for a given packet is essential to the decoder if
and only if the packet itself is received. Thus it must be possible
to obtain the FEC Payload ID from the recieved packet. Usually, the
FEC Payload ID is simply carried explicitly as a separate field
within each packet. Some FEC schemes may specify means for deriving
the relationship between the carried encoding symbols and the object
implicitly from other information within the packet, such as protocol
headers already present. Such FEC schemes could obviously only be
used with CDPs which provided the appropriate information from which
the FEC Payload ID could be derived.
The encoding format of the FEC Payload ID is defined by the FEC
Scheme. CDPs specify how the FEC Payload ID is carried within data
packets i.e. the position of the FEC Payload ID within the CDP packet
format and the how it is associated with encoding symbols.
FEC schemes for systematic FEC codes may specify two FEC Payload ID
formats, one for packets carrying only source symbols and another for
packets carrying at least one repair symbol. CDPs must include an
indication of which of the two FEC Payload ID formats is included in
each packet if they wish to support such FEC Schemes.
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7. FEC scheme specifications
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 12.
2. The type, semantics and encoding format of one or two FEC Payload
IDs. Where two FEC Payload ID formats are specified, then the
FEC scheme MUST be a systematic FEC code and one FEC Payload ID
format MUST be designated for use with packets carrying only
source symbols and the other FEC Payload ID format MUST be
designated for use with packets carrying at least one repair
symbol.
3. The type and semantics of the FEC Object Transmission
Information. The FEC Scheme MAY define additional restrictions
on the type (including value range) of the Common FEC Object
Transmission Information elements.
4. An encoding format for the Common FEC Object Transmission
Information elements used by the FEC Scheme.
Fully-specified FEC schemes MUST further specify:
1. A full specification of the FEC code.
This specification MUST precisely define the valid FEC Object
Transmission Information values, the valid FEC Payload ID values
and the valid packet payload sizes for any given object (where
packet payload refers to the space - not necessarily contiguous -
within a packet dedicated to carrying encoding symbol bytes).
Furthermore, given an object, a valid Object Transmission
Information value, a valid 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 packet
with the given 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 an object, a valid FEC Object
Transmission Information for the object, a valid FEC Payload ID
and packet payload length as input produces the exact value of
the encoding symbol bytes as output.
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2. A description of practical encoding and decoding algorithms.
This description need not be to the same level of detail as for
(1) 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 a Scheme-specific FEC
Object Transmission Information element.
Note that if an FEC sheme does not define a Scheme-specific FEC
Object Transmission Information then such an element MUST NOT be
introduced in future versions of the FEC Scheme. This requirement is
included to ensure backwards-compatibility of CDPs designed to
support only FEC Schemes which do not use the Scheme-specific FEC
Object Transmission Information element.
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 schemes SHOULD make use of the Common FEC Object Transmission
Information elements in preference to including infomation in a
Scheme-specific FEC Object Transmission Information element.
FEC scheme specifications SHOULD use the terminology defined in this
document and SHOULD follow the following format:
1. Introduction <define whether the scheme is Fully-Specified or
Under-Specified>
<describe the use-cases addressed by this FEC scheme>
2. Formats and Codes
2.1 FEC Payload ID(s) <define the type and format of one or two
FEC Payload IDs>
2.2 FEC Object Transmission Information
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2.2.1 Mandatory <define the value of the FEC Encoding ID for
this FEC scheme>
<in the case of Under-Specified FEC schemes, state the
requirement for the FEC Instance ID to be part of the FEC
Object Transmission Information>
2.2.2 Common <describe which Common FEC Object Transmission
Information elements are used by this FEC scheme, define
their value ranges and define an encoding format for them>
2.2.3 Scheme-Specific <define the Scheme-specific FEC Object
Transmission Information, including an encoding format, if
required>
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 Object Transmission
Information.>
4. FEC code specification (for Fully-Specified FEC schemes
only) <provide a complete specification of the FEC Code>
Specifications MAY include additional sections, for example,
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.
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8. CDP specifications
A specification for a CDP that uses this building block MUST include
the following things:
1. Definitions of encoding formats for the Mandatory FEC Object
Transmission Information elements.
2. A means to reliably communicate the Mandatory FEC Object
Transmission Information elements from sender to receiver(s)
using the encoding format defined in (1).
3. A means to reliably communicate the Scheme-specific FEC Object
Transmission Information element from sender to receiver(s) using
the encoding format of the Scheme-specific FEC Object
Transmission Information element defined by the FEC scheme.
4. Means to reliably communicate the Common FEC Object Transmission
Information element from sender to receiver(s) using either or
both of (a) encoding formats defined by the FEC Scheme or (b)
encoding formats defined by the CDP
5. A means to communicate the FEC Payload ID in association with a
data packet. Note that the encoding format of the FEC Payload ID
is defined by the FEC Scheme.
If option (b) of (4) above is used, then the CDP MUST specify an
encoding format for the Common FEC Object Transmission Information
elements.
CDPs MAY additionally specify the following things:
1. A means to indicate whether the FEC Payload ID within a packet is
encoded according to the format for packets including only source
symbols or according to the format for packets including at least
one repair symbol.
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9. Common algorithms
This section describes certain algorithms which are expected to be
commonly required by FEC schemes or by CDPs. FEC Schemes and CDPs
SHOULD use these algorithms in preference to scheme or protocol
specific algorithms where appropriate.
9.1 Block partitioning algorithm
This algorithm computes a partitioning of a object into source blocks
so that all source blocks are as close to being equal length as
possible. A first number of source blocks are of the same larger
length, and the remaining second number of source blocks of the same
smaller length.
This algorithm is described in two steps, the second of which may be
useful in itself as an independent algorithm in some cases. In the
first step, the number of source symbols (T) and the number of source
blocks (N) are derived from the Object transfer length (L), Maximum
Source Block Length (B) and Symbol Length (E).
In the second step, the partitioning of the object is derived from
the number of source symbols (T) and the number of source blocks (N).
The partitioning is defined in terms of a first number of source
blocks (I), a second number of source blocks (N-I), the length of
each of the first source blocks (A_large) and the length of each of
the second source blocks (A_small).
This algorithm is identical to the one specified in Section 5.1.2.3
of FLUTE [9]
The following notation is used in the description below:
ceil[x] denotes x rounded up to the nearest integer.
floor[x] denotes x rounded down to the nearest integer.
9.1.1 First Step
Input:
B -- Maximum Source Block Length, i.e., the maximum number of source
symbols per source block
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L -- Transfer Length in bytes
E -- Encoding Symbol Length in bytes
Output:
T -- the number of source symbols in the object.
N -- The number of source blocks into which the object shall be
partitioned.
Algorithm:
1. The number of source symbols in the transport object is computed
as T = ceil[L/E].
2. The transport object shall be partitioned into N = ceil[T/B]
source blocks.
9.1.2 Second step
Input:
T -- the number of source symbols in the object.
N -- The number of source blocks into which the object is
partitioned.
Output:
I -- the number of larger source blocks.
A_large -- the length of each of the larger source blocks in symbols.
A_small -- the length of each of the smaller source blocks in
symbols.
Algorithm:
1. A_large = ceil[T/N]
2. A_small = floor[T/N]
3. I = T - A_small * N
Each of the first I source blocks then consists of A_large source
symbols, each source symbol is E bytes in length. Each of the
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remaining N-I source blocks consist of A_small source symbols, each
source symbol is E bytes in length except that the last source symbol
of the last source block is L-(((L-1)/E) rounded down to the nearest
integer)*E bytes in length.
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10. Requirements from other building blocks
The FEC building block does not provide any support for congestion
control. Any complete protocol MUST provide congestion control that
conforms to [5], and thus this MUST be provided by another building
block when the FEC building block is used in a protocol.
There are no other specific requirements from other building blocks
for the use of this FEC building block. However, any protocol that
uses the FEC building block will inevitably use other building blocks
for example to provide support for sending higher level session
information within data packets containing FEC encoding symbols.
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11. Security Considerations
Data delivery can be subject to denial-of-service attacks by
attackers which send corrupted packets that are accepted as
legitimate by receivers. This is particularly a concern for
multicast delivery because a corrupted packet may be injected into
the session close to the root of the multicast tree, in which case
the corrupted packet will arrive to many receivers. This is
particularly a concern for the FEC building block because the use of
even one corrupted packet containing encoding data may result in the
decoding of an object that is completely corrupted and unusable. It
is thus RECOMMENDED that the decoded objects be checked for integrity
before delivering objects to an application. For example, an MD5
hash [6] of an object may be appended before transmission, and the
MD5 hash is computed and checked after the object is decoded but
before it is delivered to an application. Moreover, in order to
obtain strong cryptographic integrity protection a digital signature
verifiable by the receiver SHOULD be computed on top of such a hash
value. It is also RECOMMENDED that a packet authentication protocol
such as TESLA [10] be used to detect and discard corrupted packets
upon arrival. Furthermore, it is RECOMMENDED that Reverse Path
Forwarding checks be enabled in all network routers and switches
along the path from the sender to receivers to limit the possibility
of a bad agent successfully injecting a corrupted packet into the
multicast tree data path.
Another security concern is that some FEC information may be obtained
by receivers out-of-band in a session description, and if the session
description is forged or corrupted then the receivers will not use
the correct protocol for decoding content from received packets. To
avoid these problems, it is RECOMMENDED that measures be taken to
prevent receivers from accepting incorrect session descriptions,
e.g., by using source authentication to ensure that receivers only
accept legitimate session descriptions from authorized senders.
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12. IANA Considerations
Values of FEC Encoding IDs and FEC Instance IDs are subject to IANA
registration. FEC Encoding IDs and FEC Instance IDs are
hierarchical: FEC Encoding IDs scope independent ranges of FEC
Instance IDs. Only FEC Encoding IDs that correspond to Under-
Specified FEC schemes scope a corresponding set of FEC Instance IDs.
The FEC Encoding ID and FEC Instance IDs are non-negative integers.
In this document, the range of values for FEC Encoding IDs is 0 to
255. Values from 0 to 127 are reserved for Fully-Specified FEC
schemes and Values from 128 to 255 are reserved for Under-Specified
FEC schemes, as described in more detail in Section 6.1.
12.1 Explicit IANA Assignment Guidelines
This document defines a name-space for FEC Encoding IDs named:
ietf:rmt:fec:encoding
The values that can be assigned within the "ietf:rmt:fec:encoding"
name-space are numeric indexes in the range [0, 255], boundaries
included. Assignment requests are granted on a "IETF Consensus"
basis as defined in [8].
This document also defines a name-space for FEC Instance IDs named:
ietf:rmt:fec:encoding:instance
The "ietf:rmt:fec:encoding:instance" name-space is a sub-name-space
associated with the "ietf:rmt:fec:encoding" name-space. Each value
of "ietf:rmt:fec:encoding" assigned in the range [128, 255] has a
separate "ietf:rmt:fec:encoding:instance" sub-name-space that it
scopes. Values of "ietf:rmt:fec:encoding" in the range [0, 127] do
not scope a "ietf:rmt:fec:encoding:instance" sub-name-space.
The values that can be assigned within each "ietf:rmt:fec:encoding:
instance" sub-name-space are non-negative integers less than 65536.
Assignment requests are granted on a "First Come First Served" basis
as defined in [8]. The same value of "ietf:rmt:fec:encoding:
instance" can be assigned within multiple distinct sub-name-spaces,
i.e., the same value of "ietf:rmt:fec:encoding:instance" can be used
for multiple values of "ietf:rmt:fec:encoding".
Requestors of "ietf:rmt:fec:encoding:instance" assignments MUST
provide the following information:
o The value of "ietf:rmt:fec:encoding" that scopes the "ietf:rmt:
fec:encoding:instance" sub-name-space. This must be in the range
[128, 255].
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o Point of contact information
o A pointer to publicly accessible documentation describing the
Under-Specified FEC scheme, associated with the value of "ietf:
rmt:fec:encoding:instance" assigned, and a way to obtain it (e.g.,
a pointer to a publicly available reference-implementation or the
name and contacts of a company that sells it, either separately or
embedded in a product).
It is the responsibility of the requestor to keep all the above
information up to date.
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13. Acknowledgments
This document is largely based on RFC3452 [2] and thus thanks are due
to the additional authors of that document: J. Gemmell, L. Rizzo, M.
Handley, J. Crowcroft.
14. References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Luby, M., Vicisano, L., Gemmell, J., Rizzo, L., Handley, M.,
and J. Crowcroft, "Forward Error Correction (FEC) Building
Block", RFC 3452, December 2002.
[3] Luby, M., Vicisano, L., Gemmell, J., Rizzo, L., Handley, M.,
and J. Crowcroft, "The Use of Forward Error Correction (FEC) in
Reliable Multicast", RFC 3453, December 2002.
[4] Kermode, R. and L. Vicisano, "Author Guidelines for Reliable
Multicast Transport (RMT) Building Blocks and Protocol
Instantiation documents", RFC 3269, April 2002.
[5] Mankin, A., Romanov, A., Bradner, S., and V. Paxson, "IETF
Criteria for Evaluating Reliable Multicast Transport and
Application Protocols", RFC 2357, June 1998.
[6] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
April 1992.
[7] Masinter, L., "Hyper Text Coffee Pot Control Protocol
(HTCPCP/1.0)", RFC 2324, April 1998.
[8] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 26, RFC 2434,
October 1998.
[9] Paila, T., Luby, M., Lehtonen, R., Roca, V., and R. Walsh,
"FLUTE - File Delivery over Unidirectional Transport",
RFC 3926, October 2004.
[10] Perrig, A., Cametti, R., Song, D., and J. Tygar, "Efficient and
Secure Source Authentication for Multicast", February 2001.
Network and Distributed System Security Symposium
NDDS 2001
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pp. 35-46
[11] Whetten, B., Vicisano, L., Kermode, R., Handley, M., Floyd, S.,
and M. Luby, "Reliable Multicast Transport Building Blocks for
One-to-Many Bulk-Data Transfer", RFC 3048, January 2001.
Authors' Addresses
Mark Watson
Digital Fountain
39141 Civic Center Drive
Suite 300
Fremont, CA 94538
U.S.A.
Email: mark@digitalfountain.com
Michael Luby
Digital Fountain
39141 Civic Center Drive
Suite 300
Fremont, CA 94538
U.S.A.
Email: luby@digitalfountain.com
Lorenzo Vicisano
Cisco Systems, Inc.
170 West Tasman Dr.
San Jose, CA 95134
U.S.A.
Email: lorenzo@cisco.com
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