





Network Working Group                                          M. Watson
Request for Comments: 5052                                       M. Luby
Obsoletes: 3452                                              L. Vicisano
Category: Standards Track                               Digital Fountain
                                                             August 2007


             Forward Error Correction (FEC) Building Block

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

   This document describes how to use Forward Error Correction (FEC)
   codes to efficiently provide and/or augment reliability for bulk data
   transfer over IP multicast.  This document defines a framework for
   the definition of the information that needs to be communicated in
   order to use an FEC code for bulk data transfer, in addition to the
   encoded data itself, and for definition of formats and codes for
   communication 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 that 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.  This document obsoletes RFC 3452.











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Table of Contents

   1. Introduction ....................................................3
   2. Definitions and Abbreviations ...................................4
   3. Requirements Notation ...........................................4
   4. Rationale .......................................................5
   5. Applicability Statement .........................................6
   6. Functionality ...................................................6
      6.1. FEC Schemes ................................................8
      6.2. FEC Object Transmission Information .......................10
           6.2.1. Transport of FEC Object Transmission Information ...11
           6.2.2. Opacity of FEC Object Transmission Information .....12
           6.2.3. Mandatory FEC Object Transmission
                  Information Elements ...............................12
           6.2.4. Common FEC Object Transmission Information
                  Elements ...........................................12
           6.2.5. Scheme-Specific FEC Object Transmission
                  Information Element ................................13
      6.3. FEC Payload ID ............................................13
   7. FEC Scheme Specifications ......................................14
   8. CDP Specifications .............................................17
   9. Common Algorithms ..............................................18
      9.1. Block Partitioning Algorithm ..............................18
           9.1.1. First Step .........................................18
           9.1.2. Second step ........................................19
   10. Requirements from Other Building Blocks .......................20
   11. Security Considerations .......................................20
   12. IANA Considerations ...........................................21
      12.1. Explicit IANA Assignment Guidelines ......................21
   13. Changes from RFC 3452 .........................................22
   14. Acknowledgments ...............................................23
   15. References ....................................................23
      15.1. Normative References .....................................23
      15.2. Informative References ...................................23

















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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 [10], specifically Section
   4.2 of that document, and follows the general guidelines provided in
   [5].

   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/schemes, 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 and may support any kind of
   application requiring reliable transport -- for example, object
   delivery and streaming applications.

   This document also provides 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 -- see Section
   4).

   RFC 3452 [3], which is obsoleted by this document, contained a
   previous version, which was published in the "Experimental" category.
   RFC 3452 was published as an Experimental RFC in part due to the lack
   at that time of specified congestion control strategies suitable for
   use with Reliable Multicast protocols.

   This Proposed Standard specification is thus based on RFC 3452 [3]
   updated according to accumulated experience and growing protocol
   maturity since the publication of RFC 3452 [3].  Said experience
   applies both to this specification itself and to congestion control
   strategies related to the use of this specification.

   The differences between RFC 3452 [3] and this document are listed in
   Section 13.








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2.  Definitions and Abbreviations

   Object:  An ordered sequence of octets 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.  That is, 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

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

   An FEC code, in the general sense, is 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', which we
   distinguish from the concept of an 'FEC code' or 'FEC algorithm'.  An
   FEC scheme defines the ancillary information and procedures which,
   combined with an FEC code or algorithm specification, fully define
   how the FEC code can be used with CDPs.  An FEC scheme may be
   associated with a single standardized FEC code (A 'Fully-Specified'
   FEC scheme) or may be applicable to many FEC codes (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 CDP that is compliant to the
   requirements specified in this document can make use of any FEC
   scheme that 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 CDPs or particular FEC schemes.  The
   primary functionality described in this document that is common to
   all such CDPs that use FEC codes is the definition and transport of
   three kinds of information from sender to receiver(s):

      1) encoding symbols themselves,
      2) ancillary 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
      3) ancillary information associated with the whole object being
         transferred.





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   It is important to note that this information is only required by the
   receiver if one or more of the encoding symbols to which it relates
   are received.

   This document does not describe how receivers may request
   transmission of particular encoding symbols for an object.  This is
   because although there are CDPs where requests for transmission are
   of use, there are also CDPs that do not require such requests.

   The companion document [4] 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.

5.  Applicability Statement

   The FEC building block does not provide any support for congestion
   control.  Any complete multicast CDP MUST provide congestion control
   that conforms to [6], in particular, Section 3.2 of that document.
   Thus, congestion control MUST be provided by another building block
   when the FEC building block is used in a CDP.

   A more complete description of the applicability of FEC codes can be
   found in the companion document [4].

6.  Functionality

   This section describes FEC information that is to be sent either 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 CDPs 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.




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   This document specifies both the FEC information that must be carried
   in data packets and the FEC information that must be communicated
   from sender to receiver(s) either out-of-band or in data packets.
   Specification of protocol mechanisms for transporting this
   information, for example, field and packet formats, is out of scope
   of this document.  Instead, this document specifies at a higher level
   the information that must be communicated and provides detailed
   requirements for FEC Scheme and Content Delivery Protocol
   specifications, which are where the detailed field and packet formats
   should be defined.

   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 that 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
       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 -- in
       this case, 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
   correspondence 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



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       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
   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.







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   The FEC Encoding ID for Fully-Specified FEC Schemes and both the FEC
   Encoding ID and FEC Instance ID for Under-Specified FEC Schemes are
   essential for the decoder to decode an object.  Thus, they 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 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-specific 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.

   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 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.







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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
      each mandatory for at least one of the two types of FEC Scheme.
      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.

   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
      resulting octet string is known as the "encoded Scheme-specific
      FEC Object Transmission Information".  Each CDP specifies how the
      encoded Scheme-specific FEC Object Transmission is communicated
      reliably to the receiver(s), i.e., exactly where it shall be
      carried within packets of the CDP.  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.

   Each FEC scheme specifies an encoding format for the Common and
   Scheme-specific FEC Object Transmission Information.  Each CDP must
   specify at least one of the following:

   1.  A means to reliably communicate the Common FEC Object
       Transmission Information elements to the receiver(s) using the
       encoding format defined by the FEC scheme.



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   2.  An alternative, CDP-specific, encoding format for each of the
       Common FEC Object Transmission Information elements.

   The Mandatory and Common FEC Object Transmission Information elements
   are defined in the sections below.

6.2.1.  Transport of FEC Object Transmission Information

   It is the responsibility of the CDP to reliably transport the FEC
   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) 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 the Mandatory FEC Object Transmission Information
   element.

   Common FEC Object Transmission Information elements can be
   transported in two different ways: (a) the FEC Scheme defines an
   encoding format for the Common FEC Object Transmission Information
   elements that it uses, and the CDP transports this encoded data
   block, or (b) the CDP defines an encoding format for each Common FEC
   Object Transmission Information element and transports the
   information in this format.

   An FEC Scheme MUST define an encoding format for the Common FEC
   Object Transmission Information elements that it uses.  The resulting
   octet string is known as the "encoded Common FEC Object Transmission
   Information".  A CDP MAY define individual encoding formats for each
   of the Common FEC Object Transmission Information elements.  The
   choice of which way the Common FEC Object Transmission Information
   elements shall be transported, (a) or (b), is made by the Content
   Delivery Protocol, and a particular method SHOULD be defined in the
   Content Delivery Protocol specification.  Note that a CDP may provide
   support for one or both options.

   In the case that the CDP uses the encoding format specified by the
   FEC scheme, it may simply concatenate the encoded Common FEC Object
   Transmission Information and the encoded Scheme-specific FEC Object
   Transmission Information, or it may carry each in a separate field or
   wrapper within the CDP.  In the former case, the concatenated octet
   string is known as the encoded FEC Object Transmission Information.
   The FEC scheme must define the encoding format for the Common FEC
   Object Transmission Information elements that it uses in such a way
   that the length of each element is either fixed or can be determined
   from the encoded data itself.



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   The encoding format of the Scheme-specific FEC Object Transmission
   Information element is defined by the FEC scheme.  CDPs specify only
   how the resulting octet sequence is communicated.  As with the
   encoding format for the Common FEC Object Transmission Information
   elements, the length of the Scheme-specific FEC Object Transmission
   Information must either be fixed or be possible to determine from the
   encoded data itself.

6.2.2.  Opacity of FEC 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.

   Any 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.

   Any 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 element is:

   FEC Encoding ID:  an integer between 0 and 255 inclusive identifying
      a specific FEC scheme (Fully-Specified or Under-Specified.)

6.2.4.  Common FEC Object Transmission Information Elements

   The Common FEC Object Transmission Information elements are described
   below.  Note that with the exception of the FEC Instance ID, this
   specification does not provide complete definitions of these fields.
   Instead, only aspects of the abstract type are defined.  The precise
   type and semantics are defined for each FEC scheme in the FEC scheme
   specification.

   FEC Instance ID:  an integer between 0 and 65535 inclusive
      identifying an instance of an Under-Specified FEC scheme

   Transfer-Length:  a non-negative integer indicating the length of the
      object in octets





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   Encoding-Symbol-Length:  a non-negative integer indicating the length
      of each encoding symbol in octets

   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)

   The FEC Instance ID MUST be used by all Under-Specified FEC schemes
   and MUST NOT be used by Fully-Specified FEC Schemes.

   FEC Schemes define the precise type of those of the above elements
   that they use and in particular may restrict the value range of each
   element.  FEC Schemes also define an encoding format for the subset
   of the above elements that they use.  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^^16)-1 in the case of
   the FEC Instance ID, (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 the encoded Common FEC
   Object Transmission information defined by the FEC scheme.  For
   example, FLUTE [8] specifies an XML-based encoding format for these
   elements, but can also transport FEC scheme-specific encoding formats
   within the EXT-FTI LCT 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 that is essential
   to the decoder and that 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, octet
   string.  The FEC Scheme defines the structure of this octet string,
   which may contain multiple distinct elements.

6.3.  FEC Payload ID

   The FEC Payload ID contains information that 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
   indicates which source symbols of the object are carried by the
   packet.  If the packet carries repair symbols, then the FEC Payload




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   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 received packet.  Usually, the
   FEC Payload ID is simply carried explicitly as a separate field
   within each packet.  In this case, the size of the FEC Payload ID
   field SHOULD be a small fraction of the packet size.  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, including its size, 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 (that is, those codes in which
   the original source data is included within the encoded data) 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.

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



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       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 octets).

       Furthermore, given an object, valid values for each of the FEC
       Object Transmission Information elements used by the FEC Scheme,
       a valid FEC Payload ID value, and a valid packet payload size,
       the specification MUST uniquely define the values of the encoding
       symbol octets to be included in the packet payload of a 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, valid values for each
       of the FEC Object Transmission Information elements used by the
       FEC Scheme for the object, a valid FEC Payload ID, and packet
       payload length as input produces the exact value of the encoding
       symbol octets as output.

   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.




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   Note that if an FEC scheme does not define a Scheme-specific FEC
   Object Transmission Information element, 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 that 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 octets
   that 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 octets themselves.  For example,
   the initial octets may include some kind of length indication.

   FEC schemes SHOULD make use of the Common FEC Object Transmission
   Information elements in preference to including information 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

          2.2.1 Mandatory  <define the value of the FEC Encoding ID for
              this FEC scheme>

          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 that are specific to this FEC
      scheme, in particular derivation and interpretation of the fields
      in the FEC Payload ID and FEC Object Transmission Information.>



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   4. FEC code specification (for Fully-Specified FEC schemes only)
      <provide a complete specification of the FEC Code>

   Specifications MAY include additional sections such as those
   containing 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 a larger specification.

8.  CDP Specifications

   A specification for a CDP that uses this building block MUST include
   the following things:

   1.  Definitions of an encoding format for the Mandatory FEC Object
       Transmission Information element.

   2.  A means to reliably communicate the Mandatory FEC Object
       Transmission Information element from sender to receiver(s) using
       the encoding format defined in (1).

   3.  Means to reliably communicate the Common FEC Object Transmission
       Information element from sender to receiver(s) using either or
       both of (a) the encoding format defined by the FEC Scheme or (b)
       encoding formats defined by the CDP

   4.  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.

   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 (3) 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 that 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 an 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 are 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).

   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

   L  -- Transfer Length in octets

   E  -- Encoding Symbol Length in octets







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   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 octets in length.  Each of the
   remaining N-I source blocks consist of A_small source symbols; each
   source symbol is E octets 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 octets 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 CDP MUST provide congestion control that
   conforms to [6], and thus this MUST be provided by another building
   block when the FEC building block is used in a CDP.

   There are no other specific requirements from other building blocks
   for the use of this FEC building block.  However, any CDP that uses
   the FEC building block may use other building blocks, for example, to
   provide support for sending higher level session information within
   data packets containing FEC encoding symbols.

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 at 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 source authentication and integrity checking
   are applied to decoded objects before delivering objects to an
   application.  For example, a SHA-1 hash [7] of an object may be
   appended before transmission, and the SHA-1 hash is computed and
   checked after the object is decoded, but before it is delivered to an
   application.  Source authentication SHOULD be provided, for example,
   by including a digital signature verifiable by the receiver and
   computed on top of the hash value.  It is also RECOMMENDED that a
   packet authentication protocol such as Timed Efficient Stream Loss-
   Tolerant Authentication (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.  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.  They are in the registry named "Reliable Multicast
   Transport (RMT) FEC Encoding IDs and FEC Instance IDs" located at
   time of publication at:
               http://www.iana.org/assignments/rmt-fec-parameters

   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 [2].  Section 7 defines explicit requirements
   that documents defining new FEC Encoding IDs should meet.

   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 [2].  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".




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   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].

   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.

13.  Changes from RFC 3452

   This section lists the changes between the Experimental version of
   this specification, [3], and this version:

   o  The requirements for definition of a new FEC Scheme and the
      requirements for specification of new Content Delivery Protocols
      that use FEC Schemes are made more explicit to permit independent
      definition of FEC Schemes and Content Delivery Protocols.

   o  The definitions of basic FEC Schemes have been removed with the
      intention of publishing these separately.

   o  The FEC Object Transmission Information (OTI) is more explicitly
      defined, and in particular, three classes of FEC OTI (Mandatory,
      Common, and Scheme-specific) are introduced to permit reusable
      definition of explicit fields in Content Delivery Protocols to
      carry these elements.

   o  FEC Schemes are required to specify a complete encoding for the
      FEC Object Transmission, which can be carried transparently by
      Content Delivery protocols (instead of defining explicit
      elements).

   o  The possibility for FEC Schemes to define two FEC Payload ID
      formats for use with source and repair packets, respectively, in
      the case of systematic FEC codes is introduced.





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   o  The file blocking algorithm from FLUTE is included here as a
      common algorithm that is recommended to be reused by FEC Schemes
      when appropriate.

14.  Acknowledgments

   This document is largely based on RFC 3452 [3], and thus thanks are
   due to the additional authors of that document: J. Gemmell, L. Rizzo,
   M.  Handley, and J. Crowcroft.

15.  References

15.1.  Normative References

   [1]   Bradner, S., "Key words for use in RFCs to Indicate Requirement
         Levels", BCP 14, RFC 2119, March 1997.

   [2]   Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
         Considerations Section in RFCs", BCP 26, RFC 2434, October
         1998.

15.2.  Informative References

   [3]   Luby, M., Vicisano, L., Gemmell, J., Rizzo, L., Handley, M.,
         and J. Crowcroft, "Forward Error Correction (FEC) Building
         Block", RFC 3452, December 2002.

   [4]   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.

   [5]   Kermode, R. and L. Vicisano, "Author Guidelines for Reliable
         Multicast Transport (RMT) Building Blocks and Protocol
         Instantiation documents", RFC 3269, April 2002.

   [6]   Mankin, A., Romanov, A., Bradner, S., and V. Paxson, "IETF
         Criteria for Evaluating Reliable Multicast Transport and
         Application Protocols", RFC 2357, June 1998.

   [7]   Federal Information Processing Standards Publication (FIPS PUB)
         180-1, Secure Hash Standard, 17 April 1995.


   [8]   Paila, T., Luby, M., Lehtonen, R., Roca, V., and R. Walsh,
         "FLUTE - File Delivery over Unidirectional Transport", RFC
         3926, October 2004.





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   [9]   Perrig, A., Song, D., Canetti, R., Tygar, J., and B. Briscoe,
         "Timed Efficient Stream Loss-Tolerant Authentication (TESLA):
         Multicast Source Authentication Transform Introduction", RFC
         4082, June 2005.

   [10]  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
   Digital Fountain
   39141 Civic Center Drive
   Suite 300
   Fremont, CA  94538
   U.S.A.

   EMail: lorenzo@digitalfountain.com












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Full Copyright Statement

   Copyright (C) The IETF Trust (2007).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
   THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
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Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.







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