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Internet Engineering Task Force                                 RMT WG
INTERNET-DRAFT                                  M.Luby/Digital Fountain
draft-ietf-rmt-bb-fec-04.txt                            L.Vicisano/Cisco
                                                     J.Gemmell/Microsoft
                                            L.Rizzo/ACIRI and Univ. Pisa
                                                         M.Handley/ACIRI
                                                        J. Crowcroft/UCL
                                                         18 October 2001
                                                     Expires: April 2002


                 RMT BB Forward Error Correction Codes

This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC2026.

Internet-Drafts are working documents of the Internet Engineering Task
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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.


                                Abstract


     This memo describes the abstract packet formats and IANA
     registration procedures for use of Forward Error Correction
     (FEC) codes within the context of reliable IP multicast
     transport. This memo should be read in conjunction with and
     uses the terminology of the companion memo [1], which
     describes the use of Forward Error Correction (FEC) codes
     within the context of reliable IP multicast transport and



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     provides an introduction to some commonly used FEC codes.


1.  FEC Abstract Packet Fields and Out-of-Band Information

This section describes FEC information that is either to be sent out-of-
band or in packets.  The FEC information is associated with transmission
of data about 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.

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 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 a complete protocol instantiation that uses the FEC
building block. FEC information is classified as follows:


 1) FEC Encoding ID

      Identifies the FEC encoder being used and allows receivers to
      select the appropriate FEC decoder.  The value of the FEC Encoding
      ID MUST be the same for all transmission of data related to a
      particular object, but MAY vary across different transmissions of
      data about different objects, even if transmitted to the same set
      of multicast channels and/or using a single upper-layer session.
      The FEC encoding ID is subject to IANA registration.

 2) FEC Encoding Name

      Provides a more specific identification of the FEC encoder being
      used for an Under-Specified FEC scheme.  This value is not used
      for Fully-Specified FEC schemes.  (See Section 1.1 for the
      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
      subject to IANA registration.

 3) FEC payload ID

      Identifies the encoding symbol(s) in the payload of the packet.
      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
      combination of block number and encoding symbol ID).

 4) FEC Object Transmission Information

      This is information regarding the encoding of a specific object
      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
      object length).  This might also include specific parameters of
      the FEC encoder.


The FEC Encoding ID, FEC Encoding Name (for Under-Specified FEC schemes)
and 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.  In any case, the means for communicating this to a
receiver is out of the scope of this document.  The FEC Payload ID MUST
be included in the data packet header fields, as it provides a
description of the data contained in the packet.

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.


1.1.  FEC Encoding ID and FEC Encoding Name


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
format.

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.  The FEC Encoding ID uniquely identifies a Fully-Specified FEC
scheme. Companion documents of this specification may specify Fully-
Specified FEC schemes and associate them with FEC Encoding ID values.
These documents MUST also specify a format for the FEC Payload ID and
specify the information in the FEC Object Transmission Information.

It is possible that a FEC scheme cannot be a Fully-Specified FEC scheme,
because a specification is simply not available or that a party exists
that owns the encoding scheme and is not willing to disclose the
algorithm or specification. We refer to such an FEC encoding schemes as
an Under-Specified FEC scheme.  The following holds for an Under-
Specified FEC scheme:

  o The format of the FEC Payload ID and the specific information in the
    FEC Object Transmission Information MUST be defined for the Under-
    Specified FEC scheme.

  o A value for the FEC Encoding ID MUST be reserved and associated with
    the format of the FEC Payload ID and the specific information in the
    FEC Object Transmission Information.  An already reserved FEC
    Encoding ID value MUST be reused if it is associated with the same
    format of FEC Payload ID and the same information in the FEC Object
    Transmission Information as the ones needed for the new Under-
    Specified FEC scheme.

  o A value for the FEC Encoding Name MUST be reserved.


An Under-specified FEC scheme is fully identified by the tuple (FEC
Encoding ID, FEC Encoding Name). The tuple MUST identify a single scheme
that has at least one implementation. The party that owns this tuple
MUST be able to provide information on how to obtain the Under-Specified
FEC scheme 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.

Different Under-Specified FEC schemes that share the same FEC Encoding
ID -- but have different FEC Encoding Names -- also share the same
format of FEC Payload ID and specify the same information in the FEC
Object Transmission Information.

This specification reserves the range 0-127 for the values of FEC



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Encoding IDs for Fully-Specified FEC schemes and the range 128-255 for
the values of Under-Specified FEC schemes.


1.2.  FEC Payload ID and FEC Object Transmission Information


     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
specify the information in the FEC Object Transmission Information
according to the needs of the encoding scheme. This applies to documents
that reserve values of FEC Encoding IDs for both Fully-Specified and
Under-Specified FEC schemes.

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.
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
protocol instantiations.


2.  Preassigned FEC Encoding IDs

This section specifies the FEC Encoding ID and the associated FEC
Payload ID format and the specific information in the FEC Object
Transmission Information for a number of known Under-Specified FEC
schemes.  Under-specified FEC schemes that use the same FEC Payload ID
format and specific information in the FEC Object Transmission
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
be specified for other Under-Specified FEC schemes in companion
documents.


2.1.  Small Block, Large Block and Expandable FEC Codes

This subsection reserves the FEC Encoding ID value 128 for the Under-
Specified FEC schemes described in [1] that are called Small Block FEC
codes, Large Block FEC codes and Expandable FEC codes.

The FEC Payload ID is composed of a Source Block Number and an Encoding
Symbol ID structured as follows:









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 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                    Source Block Number                        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                     Encoding Symbol ID                        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


The Source Block Number idenfities from which source block of the object
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
blocks in the object.

The Encoding Symbol ID identifies which specific encoding symbol(s)
generated from the source block are carried in the packet payload.  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 algorithm used as identified by the Fec Encoding ID and by the
FEC Encoding Name, and these details may be proprietary.

The FEC Object Transmission Information has the following specific
information:


  o The total length of the object in bytes.

  o The number of source blocks that the object is partitioned into, and
    the length of each source block in bytes.



2.2.  Small Block Systematic FEC Codes

This subsection reserves the FEC Encoding ID value 129 for the Under-
Specified FEC schemes described in [1] that are called Small Block
Systematic FEC codes.  For Small Block Systematic FEC codes, each source
block is of length at most 65536 bytes.

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
Small Block Systematic FEC codes to allow more flexibility and to retain
header compactness. The small source block length and small exapansion
factor that often characterize systematic codes may require that the
data source changes frequently the source block length. To allow the
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
Payload ID.



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The FEC Payload ID is composed of the Source Block Number, Source Block
Length and the Encoding Symbol ID:


 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                    Source Block Number                        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|      Source Block Length      |       Encoding Symbol ID      |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


The Source Block Number idenfities from which source block of the object
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
blocks in the object.

The Source Block Length is the length in units of source symbols of the
source block identified by the Source Block Number.

The Encoding Symbol ID identifies which specific encoding symbol(s)
generated from the source block are carried in the packet payload.  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 algorithm used as identified by the Fec Encoding ID and by the
FEC Encoding Name, and these details may be proprietary.

The FEC Object Transmission Information has the following specific
information:


  o The total length of the object in bytes.

  o The maximum length in bytes of the encoding symbols that can be
    generated for any source block.  This field is provided to allow
    receivers to preallocate buffer space that is suitable for decoding
    to recover any source block.

  o The length in bytes of a source symbol.



3.  IANA Considerations

Values of FEC Encoding IDs and FEC Encoding Names are subject to IANA
registration. FEC Encoding IDs and FEC Encoding Names are hierarchical:
FEC Encoding IDs scope ranges of FEC Encoding Names.  Only FEC Encoding



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IDs that correspond to Under-Specified FEC schemes scope a corresponding
set of FEC Encoding Names.

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
127 are reserved for Fully-Specified FEC schemes, as described in more
detail in Section 1.1. The assignment of a FEC Encoding ID in this range
can only be granted if the requestor can provide such a specification
published as an IETF RFC, as described in more detail in Section 1.1.
Values from 128 to 255 are reserved for Under-Specified FEC schemes, as
described in more detail in Section 1.1. This specification already
assigns the values 128 and 129, as described in Section 2.

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
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 is a numeric non-negative index.

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
point of contact information and a pointer to publicly accessible
documentation describing the Under-Specified FEC scheme and ways 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 another product). The requestor is
responsible for keeping this information up to date.


4.  Acknowledgments

Brian Adamson contributed to this document by shaping Section 2.2 and
providing general feedback. We also wish to thank Vincent Roca and
Justin Chapweske for their comments.


5.  References


[1] Luby, M., Vicisano, Gemmell, J., L., Rizzo, L., Handley, M.,
Crowcroft, J., "The use of Forward Error Correction in Reliable
Multicast", Internet draft draft-ietf-rmt-info-fec-01.txt, October 2001.






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6.  Authors' Addresses

   Michael Luby
   luby@digitalfountain.com
   Digital Fountain, Inc.
   39141 Civic Center Drive
   Suite 300
   Fremont, CA  94538

   Lorenzo Vicisano
   lorenzo@cisco.com
   cisco Systems, Inc.
   170 West Tasman Dr.,
   San Jose, CA, USA, 95134

   Jim Gemmell
   jgemmell@microsoft.com
   Microsoft Research
   301 Howard St., #830
   San Francisco, CA, USA, 94105

   Luigi Rizzo
   luigi@iet.unipi.it
   ACIRI, 1947 Center St., Berkeley CA 94704
   and
   Dip. di Ing. dell'Informazione
   Universita` di Pisa
   via Diotisalvi 2, 56126 Pisa, Italy

   Mark Handley
   mjh@aciri.org
   ACIRI
   1947 Center St.
   Berkeley CA, USA, 94704

   Jon Crowcroft
   J.Crowcroft@cs.ucl.ac.uk
   Department of Computer Science
   University College London
   Gower Street,
   London WC1E 6BT, UK










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7.  Full Copyright Statement

Copyright (C) The Internet Society (2001).  All Rights Reserved.

This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it or
assist in its implementation may be prepared, copied, published and
distributed, in whole or in part, without restriction of any kind,
provided that the above copyright notice and this paragraph are included
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
or references to the Internet Society or other Internet organizations,
except as needed for the purpose of developing Internet standards in
which case the procedures for copyrights defined in the Internet
languages other than English.

The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.

This document and the information contained herein is provided on an "AS
IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK
FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT
LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT
INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR
FITNESS FOR A PARTICULAR PURPOSE."


























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