draft-ietf-rmt-bb-fec-05.txt   draft-ietf-rmt-bb-fec-06.txt 
Internet Engineering Task Force RMT WG Internet Engineering Task Force RMT WG
INTERNET-DRAFT M.Luby/Digital Fountain INTERNET-DRAFT M.Luby/Digital Fountain
draft-ietf-rmt-bb-fec-05.txt L.Vicisano/Cisco draft-ietf-rmt-bb-fec-06.txt L.Vicisano/Cisco
J.Gemmell/Microsoft J.Gemmell/Microsoft
L.Rizzo/ACIRI and Univ. Pisa L.Rizzo/ACIRI and Univ. Pisa
M.Handley/ACIRI M.Handley/ACIRI
J. Crowcroft/UCL J. Crowcroft/UCL
21 November 2001 8 February 2002
Expires: May 2002 Expires: August 2002
RMT BB Forward Error Correction Codes Forward Error Correction building block
This document is an Internet-Draft and is in full conformance with all Status of this Document
provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering Task This document is an Internet-Draft and is in full conformance with all
Force (IETF), its areas, and its working groups. Note that other groups provisions of Section 10 of RFC2026 [1]. Internet-Drafts are working
may also distribute working documents as Internet-Drafts. documents of the Internet Engineering Task Force (IETF), its areas, and
its working groups. Note that other groups may also distribute working
documents as Internet-Drafts.
Internet-Drafts are valid for a maximum of six months and may be Internet-Drafts are valid for a maximum of six months and may be
updated, replaced, or obsoleted by other documents at any time. It is updated, replaced, or obsoleted by other documents at any time. It is
inappropriate to use Internet-Drafts as reference material or to cite inappropriate to use Internet-Drafts as reference material or to cite
them other than as a "work in progress". them other than as a "work in progress".
The list of current Internet-Drafts can be accessed at The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt http://www.ietf.org/ietf/1id-abstracts.txt
To view the list Internet-Draft Shadow Directories, see To view the list Internet-Draft Shadow Directories, see
http://www.ietf.org/shadow.html. http://www.ietf.org/shadow.html.
This document is a product of the IETF RMT WG. Comments should be This document is a product of the IETF RMT WG. Comments should be
addressed to the authors, or the WG's mailing list at rmt@isi.edu. addressed to the authors, or the WG's mailing list at rmt@lbl.gov.
Abstract Abstract
This memo describes the abstract packet formats and IANA This document describes the abstract packet formats and IANA
registration procedures for use of Forward Error Correction registration procedures for use of Forward Error Correction
(FEC) codes within the context of reliable IP multicast (FEC) codes within the context of reliable IP multicast
transport. This memo should be read in conjunction with and transport. This document should be read in conjunction with
uses the terminology of the companion memo [1], which and uses the terminology of the companion document [6], which
describes the use of Forward Error Correction (FEC) codes describes the use of Forward Error Correction (FEC) codes
within the context of reliable IP multicast transport and within the context of reliable IP multicast transport and
provides an introduction to some commonly used FEC codes.
^L Table of Contents
1. Introduction. . . . . . . . . . . . . . . . . . . . . . 4
2. Rationale . . . . . . . . . . . . . . . . . . . . . . . 4
3. Functionality . . . . . . . . . . . . . . . . . . . . . 5
3.1. FEC Encoding ID and FEC Encoding Name. . . . . . . . 6
3.2. FEC Payload ID and FEC Object Transmission
Information . . . . . . . . . . . . . . . . . . . . . . . 7
4. Applicability Statement . . . . . . . . . . . . . . . . 8
5. Packet Header Fields. . . . . . . . . . . . . . . . . . 9
5.1. Small Block, Large Block and Expandable FEC
Codes . . . . . . . . . . . . . . . . . . . . . . . . . . 9
5.2. Small Block Systematic FEC Codes . . . . . . . . . . 10
6. Requirements from other building blocks . . . . . . . . 12
7. Security Considerations . . . . . . . . . . . . . . . . 12
8. IANA Considerations . . . . . . . . . . . . . . . . . . 13
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . 14
10. References . . . . . . . . . . . . . . . . . . . . . . 14
11. Authors' Addresses . . . . . . . . . . . . . . . . . . 15
12. Full Copyright Statement . . . . . . . . . . . . . . . 17
1. Introduction
This document describes how to use Forward Error Correction (FEC) codes
to provide support for reliable delivery of content using IP multicast.
This document should be read in conjunction with and uses the
terminology of the companion document [6], which describes the use of
FEC codes within the context of reliable IP multicast transport and
provides an introduction to some commonly used FEC codes. provides an introduction to some commonly used FEC codes.
1. FEC Abstract Packet Fields and Out-of-Band Information This document describes a building block as defined in RFC3048 [11].
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 RFC2119 [2].
2. Rationale
FEC codes are a valuable basic component of any transport protocol that
is to provide reliable delivery of content. Using FEC codes is valuable
in the context of IP multicast and reliable delivery because FEC
encoding symbols can be useful to all receivers for reconstructing
content 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.
The goal of the FEC building block is to describe functionality directly
related to FEC codes that is common to all reliable content delivery IP
multicast protocols, and to leave out any additional functionality that
is specific to particular protocols. The primary functionality
described in this document that is common to all such protocols that use
FEC codes are FEC encoding symbols for an object that is included in
packets that flow from a sender to receivers. 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 requests.
The companion document [6] 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.
3. Functionality
This section describes FEC information that is either to be sent out-of- This section describes FEC information that is either to be sent out-of-
band or in packets. The FEC information is associated with transmission band or in packets. The FEC information is associated with transmission
of data about a particular object. There are three classes of packets of data about a particular object. There are three classes of packets
that may contain FEC information: data packets, session-control packets that may contain FEC information: data packets, session-control packets
and feedback packets. They generally contain different kinds of FEC and feedback packets. They generally contain different kinds of FEC
information. Note that some protocols may not use session-control or information. Note that some protocols may not use session-control or
feedback packets. feedback packets.
Data packets may sometimes serve as session-control packets as well; Data packets may sometimes serve as session-control packets as well;
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ID MUST be the same for all transmission of data related to a ID MUST be the same for all transmission of data related to a
particular object, but MAY vary across different transmissions of particular object, but MAY vary across different transmissions of
data about different objects, even if transmitted to the same set data about different objects, even if transmitted to the same set
of multicast channels and/or using a single upper-layer session. of multicast channels and/or using a single upper-layer session.
The FEC encoding ID is subject to IANA registration. The FEC encoding ID is subject to IANA registration.
2) FEC Encoding Name 2) FEC Encoding Name
Provides a more specific identification of the FEC encoder being Provides a more specific identification of the FEC encoder being
used for an Under-Specified FEC scheme. This value is not used used for an Under-Specified FEC scheme. This value is not used
for Fully-Specified FEC schemes. (See Section 1.1 for the for Fully-Specified FEC schemes. (See Section 3.1 for the
definition of Under-Specified and Fully-Specified FEC schemes.) definition of Under-Specified and Fully-Specified FEC schemes.)
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The FEC encoding name is scoped by the FEC encoding ID, and is The FEC encoding name is scoped by the FEC encoding ID, and is
subject to IANA registration. subject to IANA registration.
3) FEC payload ID 3) FEC payload ID
Identifies the encoding symbol(s) in the payload of the packet. Identifies the encoding symbol(s) in the payload of the packet.
The fields in the FEC Payload ID depend on the encoder being used The fields in the FEC Payload ID depend on the encoder being used
(e.g. in Block and Expandable FEC codes this may be the (e.g. in Block and Expandable FEC codes this may be the
combination of block number and encoding symbol ID). combination of block number and encoding symbol ID).
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This is information regarding the encoding of a specific object This is information regarding the encoding of a specific object
needed by the FEC decoder (e.g. for Block and Expandable FEC codes needed by the FEC decoder (e.g. for Block and Expandable FEC codes
this may be the combination of the source block lengths and the this may be the combination of the source block lengths and the
object length). This might also include specific parameters of object length). This might also include specific parameters of
the FEC encoder. the FEC encoder.
The FEC Encoding ID, FEC Encoding Name (for Under-Specified FEC schemes) The FEC Encoding ID, FEC Encoding Name (for Under-Specified FEC schemes)
and the FEC Object Transmission Information can be sent to a receiver and the FEC Object Transmission Information can be sent to a receiver
within the data packet headers, within session control packets, or by within the data packet headers, within session control packets, or by
some other means. In any case, the means for communicating this to a some other means. In any case, the means for communicating this to a
receiver is out of the scope of this document. The FEC Payload ID MUST receiver is outside the scope of this document. The FEC Payload ID MUST
be included in the data packet header fields, as it provides a be included in the data packet header fields, as it provides a
description of the data contained in the packet. description of the data contained in the packet.
Within the context of FEC repair schemes, feedback packets are 3.1. FEC Encoding ID and FEC Encoding Name
(optionally) used to request FEC retransmission. The FEC-related
information present in feedback packets usually contains an FEC Block ID
that defines the block that is being repaired, and the number of Repair
Symbols requested. Although this is the most common case, variants are
possible in which the receivers provide more specific information about
the Repair Symbols requested (e.g. an index range or a list of symbols
accepted). It is also possible to include multiple of these requests in
a single feedback packet.
This document does not provide any detail about feedback schemes used in
combination with FEC nor the format of FEC information in feedback
packets. If feedback packets are used in a complete protocol
instantiation, these details must be provided in the protocol
instantiation specification.
1.1. FEC Encoding ID and FEC Encoding Name
The FEC Encoding ID is a numeric index that identifies a specific FEC The FEC Encoding ID is a numeric index that identifies a specific FEC
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scheme OR a class of encoding schemes that share the same FEC Payload ID scheme OR a class of encoding schemes that share the same FEC Payload ID
format. format.
An FEC scheme is a Fully-Specified FEC scheme if the encoding scheme is 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 formally and fully specified, in a way that independent implementors can
implement both encoder and decoder from a specification that is an IETF implement both encoder and decoder from a specification that is an IETF
RFC. The FEC Encoding ID uniquely identifies a Fully-Specified FEC RFC. The FEC Encoding ID uniquely identifies a Fully-Specified FEC
scheme. Companion documents of this specification may specify Fully- scheme. Companion documents of this specification may specify Fully-
Specified FEC schemes and associate them with FEC Encoding ID values. Specified FEC schemes and associate them with FEC Encoding ID values.
These documents MUST also specify a format for the FEC Payload ID and These documents MUST also specify a format for the FEC Payload ID and
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FEC scheme identified by the tuple, e.g. a pointer to a publicly FEC scheme identified by the tuple, e.g. a pointer to a publicly
available reference-implementation or the name and contacts of a company available reference-implementation or the name and contacts of a company
that sells it, either separately or embedded in another product. that sells it, either separately or embedded in another product.
Different Under-Specified FEC schemes that share the same FEC Encoding Different Under-Specified FEC schemes that share the same FEC Encoding
ID -- but have different FEC Encoding Names -- also share the same ID -- but have different FEC Encoding Names -- also share the same
format of FEC Payload ID and specify the same information in the FEC format of FEC Payload ID and specify the same information in the FEC
Object Transmission Information. Object Transmission Information.
This specification reserves the range 0-127 for the values of FEC This specification reserves the range 0-127 for the values of FEC
^L
Encoding IDs for Fully-Specified FEC schemes and the range 128-255 for Encoding IDs for Fully-Specified FEC schemes and the range 128-255 for
the values of Under-Specified FEC schemes. the values of Under-Specified FEC schemes.
1.2. FEC Payload ID and FEC Object Transmission Information 3.2. FEC Payload ID and FEC Object Transmission Information
A document that specifies an FEC scheme and reserves a value of FEC A document that specifies an FEC scheme and reserves a value of FEC
Encoding ID MUST define a packet format for the FEC Payload ID and Encoding ID MUST define a packet format for the FEC Payload ID and
specify the information in the FEC Object Transmission Information specify the information in the FEC Object Transmission Information
according to the needs of the encoding scheme. This applies to documents according to the needs of the encoding scheme. This applies to documents
that reserve values of FEC Encoding IDs for both Fully-Specified and that reserve values of FEC Encoding IDs for both Fully-Specified and
Under-Specified FEC schemes. Under-Specified FEC schemes.
The packet format definition for the FEC Payload ID MUST specify the The packet format definition for the FEC Payload ID MUST specify the
meaning and layout of the fields down to the level of specific bits. meaning and layout of the fields down to the level of specific bits.
The FEC Payload ID MUST have a length that is a multiple of a 4-byte The FEC Payload ID MUST have a length that is a multiple of a 4-byte
word. This requirement facilitates the alignment of packet fields in word. This requirement facilitates the alignment of packet fields in
protocol instantiations. protocol instantiations. This building block describes the abstract
2. Preassigned FEC Encoding IDs packet formats for carrying FEC encoding symbols that are sent from a
sender to a receiver.
4. Applicability Statement
The FEC building block applies to creating and sending encoding symbols
for objects that are to be reliably transported using IP multicast or
unicast. The FEC building block does not provide higher level session
support. Thus, for example, many objects may be transmitted within the
same session, in which case a higher level building block may carry a
unique Transport Object ID (TOI) for each object in the session to allow
the receiver to demultiplex packets within the session based on the TOI
within each packet. As another example, a receiver may subscribe to
more than one session at a time. In this case a higher level building
block may carry a unique Transport Session ID (TSI) for each session to
allow the receiver to demultiplex packets based on the TSI within each
packet.
Other building blocks may supply direct support for carrying out-of-band
information directly relevant to the FEC building block to receivers.
For example, the length of the object is part of the FEC Object
Transmission Information that may in some cases be communicated out-of-
band to receivers, and one mechanism for providing this to receivers is
within the context of another building block that provides this
information.
Some protocols may use FEC codes as a mechanism for repairing the loss
of packets. Within the context of FEC repair schemes, feedback packets
are (optionally) used to request FEC retransmission. The FEC-related
information present in feedback packets usually contains an FEC Block ID
that defines the block that is being repaired, and the number of Repair
Symbols requested. Although this is the most common case, variants are
possible in which the receivers provide more specific information about
the Repair Symbols requested (e.g. an index range or a list of symbols
accepted). It is also possible to include multiple of these requests in
a single feedback packet. This document does not provide any detail
about feedback schemes used in combination with FEC nor the format of
FEC information in feedback packets. If feedback packets are used in a
complete protocol instantiation, these details must be provided in the
protocol instantiation specification.
The FEC building block does not provide any support for congestion
control. Any complete protocol MUST provide congestion control that
conforms to RFC2357 [7], 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 [6].
5. Packet Header Fields
This section specifies the FEC Encoding ID and the associated FEC This section specifies the FEC Encoding ID and the associated FEC
Payload ID format and the specific information in the FEC Object Payload ID format and the specific information in the FEC Object
Transmission Information for a number of known Under-Specified FEC Transmission Information for a number of known Under-Specified FEC
schemes. Under-specified FEC schemes that use the same FEC Payload ID schemes. Under-specified FEC schemes that use the same FEC Payload ID
format and specific information in the FEC Object Transmission format and specific information in the FEC Object Transmission
Information as for one of the FEC Encoding IDs specified in this section Information as for one of the FEC Encoding IDs specified in this section
MUST use the corresponding FEC Encoding ID. Other FEC Encoding IDs may MUST use the corresponding FEC Encoding ID. Other FEC Encoding IDs may
be specified for other Under-Specified FEC schemes in companion be specified for other Under-Specified FEC schemes in companion
documents. documents.
2.1. Small Block, Large Block and Expandable FEC Codes 5.1. Small Block, Large Block and Expandable FEC Codes
This subsection reserves the FEC Encoding ID value 128 for the Under- This subsection reserves the FEC Encoding ID value 128 for the Under-
Specified FEC schemes described in [1] that are called Small Block FEC Specified FEC schemes described in [6] that are called Small Block FEC
codes, Large Block FEC codes and Expandable FEC codes. codes, Large Block FEC codes and Expandable FEC codes.
The FEC Payload ID is composed of a Source Block Number and an Encoding The FEC Payload ID is composed of a Source Block Number and an Encoding
Symbol ID structured as follows: Symbol ID structured as follows:
^L
0 1 2 3 0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Block Number | | Source Block Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Encoding Symbol ID | | Encoding Symbol ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Source Block Number idenfities from which source block of the object The Source Block Number identifies from which source block of the object
the encoding symbol(s) in the payload are generated. These blocks are the encoding symbol(s) in the payload are generated. These blocks are
numbered consecutively from 0 to N-1, where N is the number of source numbered consecutively from 0 to N-1, where N is the number of source
blocks in the object. blocks in the object.
The Encoding Symbol ID identifies which specific encoding symbol(s) The Encoding Symbol ID identifies which specific encoding symbol(s)
generated from the source block are carried in the packet payload. The generated from the source block are carried in the packet payload. The
exact details of the correspondence between Encoding Symbol IDs and the exact details of the correspondence between Encoding Symbol IDs and the
encoding symbol(s) in the packet payload are dependent on the particular encoding symbol(s) in the packet payload are dependent on the particular
encoding algorithm used as identified by the Fec Encoding ID and by the encoding algorithm used as identified by the Fec Encoding ID and by the
FEC Encoding Name, and these details may be proprietary. FEC Encoding Name, and these details may be proprietary.
The FEC Object Transmission Information has the following specific The FEC Object Transmission Information has the following specific
information: information:
o The FEC Encoding ID 128.
o The FEC Encoding Name associated with the FEC Encoding ID 128 to be
used.
o The total length of the object in bytes. o The total length of the object in bytes.
o The number of source blocks that the object is partitioned into, and o The number of source blocks that the object is partitioned into, and
the length of each source block in bytes. the length of each source block in bytes.
How this out of band information is communicated is outside the scope of How this out-of-band information is communicated is outside the scope of
this document. As an example the source block lengths may be derived by this document. As an example the source block lengths may be derived by
a fixed algorithm from the object length. As another example, it may be a fixed algorithm from the object length. As another example, it may be
that all source blocks are the same length and this is what is passed that all source blocks are the same length and this is what is passed
out of band to the receiver. As a third example, it could be that the out-of-band to the receiver. As a third example, it could be that the
full sized source block length is provided and this is the length used full sized source block length is provided and this is the length used
for all but the last source block, which is calculated based on the full for all but the last source block, which is calculated based on the full
source block length and the object length. source block length and the object length.
2.2. Small Block Systematic FEC Codes 5.2. Small Block Systematic FEC Codes
This subsection reserves the FEC Encoding ID value 129 for the Under- This subsection reserves the FEC Encoding ID value 129 for the Under-
Specified FEC schemes described in [1] that are called Small Block Specified FEC schemes described in [6] that are called Small Block
Systematic FEC codes. For Small Block Systematic FEC codes, each source Systematic FEC codes. For Small Block Systematic FEC codes, each source
block is of length at most 65536 source symbols. block is of length at most 65536 source symbols.
^L
Although these codes can generally be accommodated by the FEC Encoding Although these codes can generally be accommodated by the FEC Encoding
ID described in Section 2.1, a specific FEC Encoding ID is defined for ID described in Section 5.1, a specific FEC Encoding ID is defined for
Small Block Systematic FEC codes to allow more flexibility and to retain Small Block Systematic FEC codes to allow more flexibility and to retain
header compactness. The small source block length and small expansion header compactness. The small source block length and small expansion
factor that often characterize systematic codes may require that the factor that often characterize systematic codes may require the data
data source changes frequently the source block length. To allow the source to frequently change the source block length. To allow the
dynamic variation of the source block length and to communicate it to dynamic variation of the source block length and to communicate it to
the receivers with low overhead, the block length is included in the FEC the receivers with low overhead, the block length is included in the FEC
Payload ID. Payload ID.
The FEC Payload ID is composed of the Source Block Number, Source Block The FEC Payload ID is composed of the Source Block Number, Source Block
Length and the Encoding Symbol ID: Length and the Encoding Symbol ID:
0 1 2 3 0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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encoding symbol is either an original source symbol or a redundant encoding symbol is either an original source symbol or a redundant
symbol generated by the encoder. The exact details of the symbol generated by the encoder. The exact details of the
correspondence between Encoding Symbol IDs and the encoding symbol(s) in correspondence between Encoding Symbol IDs and the encoding symbol(s) in
the packet payload are dependent on the particular encoding algorithm the packet payload are dependent on the particular encoding algorithm
used as identified by the Fec Encoding ID and by the FEC Encoding Name, used as identified by the Fec Encoding ID and by the FEC Encoding Name,
and these details may be proprietary. and these details may be proprietary.
The FEC Object Transmission Information has the following specific The FEC Object Transmission Information has the following specific
information: information:
o The FEC Encoding ID 129.
o The FEC Encoding Name associated with the FEC Encoding ID 129 to be
used.
o The total length of the object in bytes. o The total length of the object in bytes.
o The maximum number of encoding symbols that can be generated for any o The maximum number of encoding symbols that can be generated for any
source block. This field is provided for example to allow receivers source block. This field is provided for example to allow receivers
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to preallocate buffer space that is suitable for decoding to recover to preallocate buffer space that is suitable for decoding to recover
any source block. any source block.
o For each source block, the length in bytes of encoding symbols for o For each source block, the length in bytes of encoding symbols for
the source block. the source block.
How this out of band information is communicated is outside the scope of How this out-of-band information is communicated is outside the scope of
this document. As an example the length in bytes of encoding symbols this document. As an example the length in bytes of encoding symbols
for each source block may be the same for all source blocks. As another for each source block may be the same for all source blocks. As another
example, the encoding symbol length may be the same for all source example, the encoding symbol length may be the same for all source
blocks of a given object and this length is communicated for each blocks of a given object and this length is communicated for each
object. As a third example, it may be that there is a threshold value object. As a third example, it may be that there is a threshold value
I, and for all source blocks consisting of less than I source symbols, I, and for all source blocks consisting of less than I source symbols,
the encoding symbol length is one fixed number of bytes, but for all the encoding symbol length is one fixed number of bytes, but for all
source blocks consisting of I or more source symbols, the encoding source blocks consisting of I or more source symbols, the encoding
symbol length is a different fixed number of bytes. symbol length is a different fixed number of bytes.
Note that each encoding symbol, i.e., each source symbol and redundant Note that each encoding symbol, i.e., each source symbol and redundant
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object. As a third example, it may be that there is a threshold value object. As a third example, it may be that there is a threshold value
I, and for all source blocks consisting of less than I source symbols, I, and for all source blocks consisting of less than I source symbols,
the encoding symbol length is one fixed number of bytes, but for all the encoding symbol length is one fixed number of bytes, but for all
source blocks consisting of I or more source symbols, the encoding source blocks consisting of I or more source symbols, the encoding
symbol length is a different fixed number of bytes. symbol length is a different fixed number of bytes.
Note that each encoding symbol, i.e., each source symbol and redundant Note that each encoding symbol, i.e., each source symbol and redundant
symbol, must be the same length for a given source block, and this symbol, must be the same length for a given source block, and this
implies that each source block length is a multiple of its encoding implies that each source block length is a multiple of its encoding
symbol length. If the original source block length is not a multiple of symbol length. If the original source block length is not a multiple of
the encodin symbol length, it is up to the sending application to the encoding symbol length, it is up to the sending application to
appropriately pad the original source block to form the source block to appropriately pad the original source block to form the source block to
be encoded, and to communicate this padding to the receiving be encoded, and to communicate this padding to the receiving
application. The form of this padding, if used, and how it is application. The form of this padding, if used, and how it is
communicated to the receiving application, is out of the scope of this communicated to the receiving application, is outside the scope of this
document, and must be handled at the application level. document, and must be handled at the application level.
3. IANA Considerations 6. 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 RFC2357 [7], 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.
7. 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 especially 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 especially a concern for the FEC BB because the
use of even one corrupted packet containing encoding data may result in
the decoding of content that is completely corrupted and unusable. It
is thus RECOMMENDED that the decoded content be checked for integrity
before delivering the content to an application. For example, an MD5
hash [10] of the content may be appended before transmission, and the
MD5 hash is computed and checked after the content 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 [9] 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. Thus, it
is RECOMMENDED that the receiver authenticate the session description,
for example by using either the ESP (with enabled authentication
service) [5] or AH [4] protocols of IPSEC [3] to ensure the authenticity
of the session description.
8. IANA Considerations
Values of FEC Encoding IDs and FEC Encoding Names are subject to IANA Values of FEC Encoding IDs and FEC Encoding Names are subject to IANA
registration. FEC Encoding IDs and FEC Encoding Names are hierarchical: registration. FEC Encoding IDs and FEC Encoding Names are hierarchical:
FEC Encoding IDs scope ranges of FEC Encoding Names. Only FEC Encoding FEC Encoding IDs scope ranges of FEC Encoding Names. Only FEC Encoding
IDs that correspond to Under-Specified FEC schemes scope a corresponding IDs that correspond to Under-Specified FEC schemes scope a corresponding
set of FEC Encoding Names. set of FEC Encoding Names.
The FEC Encoding ID is a numeric non-negative index. In this document, The FEC Encoding ID is a numeric non-negative index. In this document,
the range of values for FEC Encoding IDs is 0 and 255. Values from 0 to the range of values for FEC Encoding IDs is 0 and 255. Values from 0 to
127 are reserved for Fully-Specified FEC schemes, as described in more 127 are reserved for Fully-Specified FEC schemes and Values from 128 to
detail in Section 1.1. The assignment of a FEC Encoding ID in this range 255 are reserved for Under-Specified FEC schemes, as described in more
can only be granted if the requestor can provide such a specification detail in Section 3.1. This specification already assigns the values 128
published as an IETF RFC, as described in more detail in Section 1.1. and 129, as described in Section 5. The assignment of additional FEC
Values from 128 to 255 are reserved for Under-Specified FEC schemes, as Encoding IDs is on a Specification Required basis as defined in RFC2434
described in more detail in Section 1.1. This specification already [8], and in particular a specification published as an IETF RFC is
assigns the values 128 and 129, as described in Section 2. required providing the information described in more detail in Section
3.1.
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Values of FEC Encoding IDs can only be assigned if the required format
for the FEC Payload ID and the specific information in the FEC Object
Transmission Information are specified in an IETF RFC.
Each FEC Encoding ID assigned to an Under-Specified FEC scheme scopes an Each FEC Encoding ID assigned to an Under-Specified FEC scheme scopes an
independent range of FEC Encoding Names (i.e. the same value of FEC independent range of FEC Encoding Names (i.e. the same value of FEC
Encoding Name can be reused for different FEC Encoding IDs). An FEC Encoding Name can be reused for different FEC Encoding IDs). An FEC
Encoding Name is a numeric non-negative index. Encoding Name is a numeric non-negative index.
Under the scope of a FEC Encoding ID, FEC Encoding Names are assigned on Under the scope of a FEC Encoding ID, FEC Encoding Names are assigned on
a First Come First Served base to requestors that are able to provide a First Come First Served basis as defined in RFC2434 [8] to requestors
point of contact information and a pointer to publicly accessible that are able to provide point of contact information and a pointer to
documentation describing the Under-Specified FEC scheme and ways to publicly accessible documentation describing the Under-Specified FEC
obtain it (e.g. a pointer to a publicly available reference- scheme and ways to obtain it (e.g. a pointer to a publicly available
implementation or the name and contacts of a company that sells it, reference-implementation or the name and contacts of a company that
either separately or embedded in another product). The requestor is sells it, either separately or embedded in another product). The
responsible for keeping this information up to date. requestor is responsible for keeping this information up to date.
4. Acknowledgments 9. Acknowledgments
Brian Adamson contributed to this document by shaping Section 2.2 and Brian Adamson contributed to this document by shaping Section 5.2 and
providing general feedback. We also wish to thank Vincent Roca for his providing general feedback. We also wish to thank Vincent Roca, Justin
extensive comments and Justin Chapweske for his comments. Chapweske and Roger Kermode for their extensive comments.
5. References 10. References
[1] Luby, M., Vicisano, Gemmell, J., L., Rizzo, L., Handley, M., [1] Bradner, S., "The Internet Standards Process -- Revision 3",
RFC2026, October 1996.
[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", RFC2119, March 1997.
[3] Kent, S., Atkinson, R., "Security Architecture for the Internet
Protocol", RFC2401, November 1998.
[4] Kent, S., Atkinson, R., "IP Authentication Header", RFC2406,
November 1998.
[5] Kent, S., Atkinson, R., "IP Encapsulating Security Payload (ESP)",
RFC2406, November 1998.
[6] Luby, M., Vicisano, Gemmell, J., L., Rizzo, L., Handley, M.,
Crowcroft, J., "The use of Forward Error Correction in Reliable Crowcroft, J., "The use of Forward Error Correction in Reliable
Multicast", Internet draft draft-ietf-rmt-info-fec-01.txt, October 2001. Multicast", Internet draft draft-ietf-rmt-info-fec-02.txt, January 2002.
6. Authors' Addresses [7] Mankin, A., Romanow, A., Bradner, S., Paxson V., "IETF Criteria for
Evaluating Reliable Multicast Transport and Application Protocols",
RFC2357, June 1998.
[8] Narten, T., Alvestrand, H., "Guidelines for Writing an IANA
Considerations Section in RFCs", RFC2434, October 1998.
[9] Perrig, A., Canetti, R., Song, D., Tygar, J.D., "Efficient and
Secure Source Authentication for Multicast", Network and Distributed
System Security Symposium, NDSS 2001, pp. 35-46, February 2001.
[10] Rivest, R., "The MD5 Message-Digest Algorithm", RFC1321, April
1992.
[11] Whetten, B., Vicisano, L., Kermode, R., Handley, M., Floyd, S.,
Luby, M., "Reliable Multicast Transport Building Blocks for One-to-Many
Bulk-Data Transfer", RFC3048, January 2001.
11. Authors' Addresses
Michael Luby Michael Luby
luby@digitalfountain.com luby@digitalfountain.com
Digital Fountain, Inc. Digital Fountain, Inc.
39141 Civic Center Drive 39141 Civic Center Drive
Suite 300 Suite 300
Fremont, CA 94538 Fremont, CA 94538
Lorenzo Vicisano Lorenzo Vicisano
lorenzo@cisco.com lorenzo@cisco.com
cisco Systems, Inc. cisco Systems, Inc.
170 West Tasman Dr., 170 West Tasman Dr.,
San Jose, CA, USA, 95134 San Jose, CA, USA, 95134
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Jim Gemmell Jim Gemmell
jgemmell@microsoft.com jgemmell@microsoft.com
Microsoft Research Microsoft Research
301 Howard St., #830 301 Howard St., #830
San Francisco, CA, USA, 94105 San Francisco, CA, USA, 94105
Luigi Rizzo Luigi Rizzo
luigi@iet.unipi.it luigi@iet.unipi.it
ACIRI, 1947 Center St., Berkeley CA 94704 ACIRI, 1947 Center St., Berkeley CA 94704
and and
skipping to change at page 10, line 25 skipping to change at page 16, line 4
and and
Dip. di Ing. dell'Informazione Dip. di Ing. dell'Informazione
Universita` di Pisa Universita` di Pisa
via Diotisalvi 2, 56126 Pisa, Italy via Diotisalvi 2, 56126 Pisa, Italy
Mark Handley Mark Handley
mjh@aciri.org mjh@aciri.org
ACIRI ACIRI
1947 Center St. 1947 Center St.
Berkeley CA, USA, 94704 Berkeley CA, USA, 94704
Jon Crowcroft Jon Crowcroft
J.Crowcroft@cs.ucl.ac.uk J.Crowcroft@cs.ucl.ac.uk
Department of Computer Science Department of Computer Science
University College London University College London
Gower Street, Gower Street,
London WC1E 6BT, UK London WC1E 6BT, UK
^L 12. Full Copyright Statement
7. Full Copyright Statement
Copyright (C) The Internet Society (2001). All Rights Reserved. Copyright (C) The Internet Society (2001). All Rights Reserved.
This document and translations of it may be copied and furnished to This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it or others, and derivative works that comment on or otherwise explain it or
assist in its implementation may be prepared, copied, published and assist in its implementation may be prepared, copied, published and
distributed, in whole or in part, without restriction of any kind, distributed, in whole or in part, without restriction of any kind,
provided that the above copyright notice and this paragraph are included provided that the above copyright notice and this paragraph are included
on all such copies and derivative works. However, this document itself on all such copies and derivative works. However, this document itself
may not be modified in any way, such as by removing the copyright notice may not be modified in any way, such as by removing the copyright notice
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The limited permissions granted above are perpetual and will not be The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns. revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an "AS This document and the information contained herein is provided on an "AS
IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK
FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT
LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT
INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR
FITNESS FOR A PARTICULAR PURPOSE." FITNESS FOR A PARTICULAR PURPOSE."
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