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Versions: 00 01 02 03 04 05 06 07 08 RFC 6968

RMT                                                              V. Roca
Internet-Draft                                                     INRIA
Intended status: Standards Track                              B. Adamson
Expires: June 3, 2013                          Naval Research Laboratory
                                                       November 30, 2012


     FCAST: Scalable Object Delivery for the ALC and NORM Protocols
                        draft-ietf-rmt-fcast-07

Abstract

   This document introduces the FCAST object (e.g., file) delivery
   application on top of the ALC and NORM reliable multicast protocols.
   FCAST is a highly scalable application that provides a reliable
   object delivery service.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on June 3, 2013.

Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.



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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Requirements Notation  . . . . . . . . . . . . . . . . . .  5
     1.2.  Definitions, Notations and Abbreviations . . . . . . . . .  5
   2.  FCAST Data Formats . . . . . . . . . . . . . . . . . . . . . .  6
     2.1.  Compound Object Format . . . . . . . . . . . . . . . . . .  6
     2.2.  Carousel Instance Descriptor Format  . . . . . . . . . . .  8
   3.  FCAST Principles . . . . . . . . . . . . . . . . . . . . . . . 12
     3.1.  FCAST Content Delivery Service . . . . . . . . . . . . . . 12
     3.2.  Meta-Data Transmission . . . . . . . . . . . . . . . . . . 12
     3.3.  Meta-Data Content  . . . . . . . . . . . . . . . . . . . . 13
     3.4.  Carousel Transmission  . . . . . . . . . . . . . . . . . . 14
     3.5.  Carousel Instance Descriptor Special Object  . . . . . . . 14
     3.6.  Compound Object Identification . . . . . . . . . . . . . . 16
     3.7.  FCAST Sender Behavior  . . . . . . . . . . . . . . . . . . 17
     3.8.  FCAST Receiver Behavior  . . . . . . . . . . . . . . . . . 18
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . . 18
     4.1.  Problem Statement  . . . . . . . . . . . . . . . . . . . . 19
     4.2.  Attacks Against the Data Flow  . . . . . . . . . . . . . . 19
       4.2.1.  Access to Confidential Objects . . . . . . . . . . . . 19
       4.2.2.  Object Corruption  . . . . . . . . . . . . . . . . . . 20
     4.3.  Attacks Against the Session Control Parameters and
           Associated Building Blocks . . . . . . . . . . . . . . . . 21
       4.3.1.  Attacks Against the Session Description  . . . . . . . 22
       4.3.2.  Attacks Against the FCAST CID  . . . . . . . . . . . . 22
       4.3.3.  Attacks Against the Object Meta-Data . . . . . . . . . 22
       4.3.4.  Attacks Against the ALC/LCT and NORM Parameters  . . . 23
       4.3.5.  Attacks Against the Associated Building Blocks . . . . 23
     4.4.  Other Security Considerations  . . . . . . . . . . . . . . 24
     4.5.  Minimum Security Recommendations . . . . . . . . . . . . . 24
   5.  Requirements for Compliant Implementations . . . . . . . . . . 24
     5.1.  Requirements Related to the Object Meta-Data . . . . . . . 24
     5.2.  Requirements Related to the Carousel Instance
           Descriptor (CID) Mechanism . . . . . . . . . . . . . . . . 26
   6.  Operational Considerations . . . . . . . . . . . . . . . . . . 26
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 27
     7.1.  Creation of the FCAST Object Meta-Data Format Registry . . 27
     7.2.  Creation of the FCAST Object Meta-Data Encoding
           Registry . . . . . . . . . . . . . . . . . . . . . . . . . 28
     7.3.  Creation of the FCAST Object Meta-Data Types Registry  . . 28
   8.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 29
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 29
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 29
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 30
   Appendix A.  FCAST Examples  . . . . . . . . . . . . . . . . . . . 31
     A.1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . 31
     A.2.  Regular Compound Object Example  . . . . . . . . . . . . . 32



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     A.3.  Carousel Instance Descriptor Example . . . . . . . . . . . 32
   Appendix B.  Additional Meta-Data Transmission Mechanisms  . . . . 33
     B.1.  Supporting Additional Mechanisms . . . . . . . . . . . . . 33
     B.2.  Using NORM_INFO Messages with FCAST/NORM . . . . . . . . . 34
       B.2.1.  Example  . . . . . . . . . . . . . . . . . . . . . . . 34
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 36













































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1.  Introduction

   This document introduces the FCAST reliable and scalable object
   (e.g., file) delivery application.  Two variants of FCAST exist:

   o  FCAST/ALC that relies on the Asynchronous Layer Coding (ALC)
      [RFC5775] and the Layered Coding Transport (LCT) [RFC5651]
      reliable multicast transport protocol, and

   o  FCAST/NORM that relies on the NACK-Oriented Reliable Multicast
      (NORM) [RFC5740] reliable multicast transport protocol.

   Hereafter, the term FCAST denotes either FCAST/ALC or FCAST/NORM.
   FCAST is not a new protocol specification per se.  Instead it is a
   set of data format specifications and instructions on how to use ALC
   and NORM to implement a file-casting service.

   A design goal behind FCAST is to define a streamlined solution, in
   order to enable lightweight implementations of the protocol stack,
   and limit the operational processing and storage requirements.  A
   consequence of this choice is that FCAST cannot be considered as a
   versatile application, capable of addressing all the possible use-
   cases.  On the contrary, FCAST has some intrinsic limitations.  From
   this point of view it differs from FLUTE [RFC6726] which favors
   flexibility at the expense of some additional complexity.

   A good example of the design choices meant to favor simplicity is the
   way FCAST manages the object meta-data: by default, the meta-data and
   the object content are sent together, in a compound object.  This
   solution has many advantages in terms of simplicity as will be
   described later on.  However this solution has an intrinsic
   limitation since it does not enable a receiver to decide in advance,
   before beginning the reception of the compound object, whether the
   object is of interest or not, based on the information that may be
   provided in the meta-data.  Therefore this document discusses
   additional techniques that may be used to mitigate this limitation.
   When use-cases require that each receiver download the whole set of
   objects sent in the session (e.g., with mirroring tools), this
   limitation is not considered a problem.

   FCAST is expected to work in many different environments and is
   designed to be flexible.  The service provided by FCAST can differ
   according to the exact conditions FCAST is used, like the presence or
   not of reception feedbacks.  For example, environments exist where no
   feedback is possible with FCAST/ALC, which guarantees a maximum
   scalability but at the same time may reduce transmission reliability.
   Section 6 discusses such operational considerations in detail.
   Finally, Section 5 provides the guidance for compliant implementation



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   of the specification and identifies those features that are optional.

1.1.  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 [RFC2119].

1.2.  Definitions, Notations and Abbreviations

   This document uses the following terms:

           FCAST/ALC : Denotes the FCAST application running on top of
                       the ALC/LCT reliable transport protocol
          FCAST/NORM : Denotes the FCAST application running on top of
                       the NORM reliable transport protocol
               FCAST : Denotes either FCAST/ALC or FCAST/NORM
     Compound Object : An ALC or NORM transport object composed of the
                       Compound Object Header
            Carousel : The process of sending Compound Objects
                       implemented by a FCAST sender
   Carousel Instance : Fixed set of registered Compound Objects that are
                       sent by the carousel during a certain number of
                       cycles; Whenever Compound Objects need to be
                       added or removed, a new Carousel Instance is
                       defined.
   Carousel Instance : Special object that lists the Compound Objects
    Descriptor (CID)   comprising a given Carousel Instance
   Carousel Instance : Numeric value that identifies a Carousel Instance
   IDentifier (CIID)
      Carousel Cycle : A transmission round within which all the
                       Compound Objects registered in the Carousel
                       Instance are transmitted a certain number of
                       times; By default, Compound Objects are
                       transmitted once per cycle, but higher values are
                       possible, that might differ on a per-object
                       basis.
    Transport Object : Numeric identifier associated to a specific
    Identifier (TOI)   object by the underlying transport protocol.  In
                       the case of ALC, this corresponds to the TOI
                       described in [RFC5651]; In the case of NORM, this
                       corresponds to the NormTransportId described in
                       [RFC5740].
          FEC Object : FEC information associated with an object that is
        Transmission   essential for the FEC decoder to decode a
    Information (FEC   specific object.
                OTI)




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2.  FCAST Data Formats

   This section details the various data formats used by FCAST.

2.1.  Compound Object Format

   In an FCAST session, Compound Objects are constructed by prepending
   the Compound Object Header (which contains the meta-data of the
   object) before the original object data content (see Section 3.2).
   Figure 1 illustrates the associated Compound Object header format.
   All multi-byte fields are in network (Big Endian) byte order.

     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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  ^
    |Ver| Resvd |G|C| MDFmt | MDEnc |           Checksum            |  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
    |                 Compound Object Header Length                 |  h
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|  d
    |               Object Meta-Data (variable length)              |  r
    |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
    |                               |      Padding (optional)       |  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  v
    |                                                               |
    .          Object Data (optional, variable length)              .
    .                                                               .
    .                                                               .

                     Figure 1: Compound Object Format.

   The Compound Object Header fields are:

   +------------+------------------------------------------------------+
   | Field      | Description                                          |
   +------------+------------------------------------------------------+
   | Version    | 2-bit field that MUST be set to 0 in this            |
   |            | specification and indicates the protocol version     |
   |            | number.                                              |
   +------------+------------------------------------------------------+
   | Reserved   | 4-bit field that MUST be set to 0 in this            |
   |            | specification and is reserved for future use.        |
   |            | Receivers MUST ignore this field.                    |
   +------------+------------------------------------------------------+
   | G          | 1-bit field that, when set to 1, indicates that the  |
   |            | checksum encompasses the whole Compound Object       |
   |            | (Global checksum).  When set to 0, this field        |
   |            | indicates that the checksum encompasses only the     |
   |            | Compound Object header.                              |



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   | C          | 1-bit field that, when set to 1, indicates the       |
   |            | object is a Carousel Instance Descriptor (CID).      |
   |            | When set to 0, this field indicates that the         |
   |            | transported object is a standard object.             |
   +------------+------------------------------------------------------+
   | Meta-Data  | 4-bit field that defines the format of the object    |
   | Format     | meta-data (see Section 7).  An HTTP/1.1              |
   | (MDFmt)    | metainformation format [RFC2616] MUST be supported   |
   |            | and is associated to value 0.  Other formats (e.g.,  |
   |            | XML) MAY be defined in the future.                   |
   +------------+------------------------------------------------------+
   | Meta-Data  | 4-bit field that defines the optional encoding of    |
   | Encoding   | the Object Meta-Data field (see Section 7).  By      |
   | (MDEnc)    | default, a plain text encoding is used and is        |
   |            | associated to value 0.  GZIP encoding MUST also be   |
   |            | supported and is associated to value 1.  Other       |
   |            | encodings MAY be defined in the future.              |
   +------------+------------------------------------------------------+
   | Checksum   | 16-bit field that contains the checksum computed     |
   |            | over either the whole Compound Object (when G is set |
   |            | to 1), or over the Compound Object header (when G is |
   |            | set to 0), using the Internet checksum algorithm     |
   |            | specified in [RFC1071].  More precisely, the         |
   |            | checksum field is the 16-bit one's complement of the |
   |            | one's complement sum of all 16-bit words to be       |
   |            | considered.  If a segment contains an odd number of  |
   |            | octets to be checksummed, the last octet is padded   |
   |            | on the right with zeros to form a 16-bit word for    |
   |            | checksum purposes (this pad is not transmitted).     |
   |            | While computing the checksum, the checksum field     |
   |            | itself MUST be set to zero.                          |
   +------------+------------------------------------------------------+
   | Compound   | 32-bit field indicating total length (in bytes) of   |
   | Object     | all fields of the Compound Object Header, except the |
   | Header     | optional padding.  A header length field set to      |
   | Length     | value 8 means that there is no meta-data included.   |
   |            | When this size is not multiple to 32-bits words and  |
   |            | when the Compound Object Header is followed by a non |
   |            | null Compound Object Data, padding MUST be added.    |
   |            | It should be noted that the meta-data field maximum  |
   |            | size is equal to (2^32 - 8) bytes.                   |
   +------------+------------------------------------------------------+









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   +------------+------------------------------------------------------+
   | Object     | Variable length field that contains the meta-data    |
   | Meta-Data  | associated to the object.  The format and encoding   |
   |            | of this field are defined respectively by the MDFmt  |
   |            | and MDEnc fields.  With the default HTTP/1.1 format  |
   |            | and plain text encoding, the Meta-Data is            |
   |            | NULL-terminated plain text that follows the "TYPE"   |
   |            | ":" "VALUE" "<CR-LF>" format used in HTTP/1.1 for    |
   |            | metainformation [RFC2616].  The various meta-data    |
   |            | items can appear in any order.  The associated       |
   |            | string, when non empty, MUST be NULL-terminated.     |
   |            | When no meta-data is communicated, this field MUST   |
   |            | be empty and the Compound Object Header Length MUST  |
   |            | be equal to 8.                                       |
   +------------+------------------------------------------------------+
   | Padding    | Optional, variable length field of zero-value bytes  |
   |            | to align the start of the Object Data to 32-bit      |
   |            | boundary.  Padding is only used when the Compound    |
   |            | Object Header Length value, in bytes, is not         |
   |            | multiple of 4 and when the Compound Object Header is |
   |            | followed by non null Compound Object Data.           |
   +------------+------------------------------------------------------+

   The Compound Object Header is then followed by the Object Data, i.e.,
   the original object possibly encoded by FCAST.  Note that the length
   of this content is the transported object length (e.g., as specified
   by the FEC OTI) minus the Compound Object Header Length and optional
   padding if any.

2.2.  Carousel Instance Descriptor Format

   In an FCAST session, a Carousel Instance Descriptor (CID) MAY be sent
   in order to carry the list of Compound Objects that are part of a
   given Carousel Instance (see Section 3.5).  The format of the CID,
   that is sent as a special Compound Object, is given in Figure 2.
   Being a special case of Compound Object, this format is in line with
   the format described in Section 2.1.














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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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  ^
    |Ver| Resvd |G|C| MDFmt | MDEnc |           Checksum            |  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
    |                 Compound Object Header Length                 |  h
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|  d
    |               Object Meta-Data (variable length)              |  r
    |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
    |                               |      Padding (optional)       |  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  v
    .                                                               .  ^
    .                Object List (variable length)                  .  |
    .                                                               .  o
    .                                               +-+-+-+-+-+-+-+-+  b
    .                                               |                  j
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                  v

              Figure 2: Carousel Instance Descriptor Format.

   Because the CID is transmitted as a special Compound Object, the
   following CID-specific meta-data entries are defined:

   o  Fcast-CID-Complete: when set to '1', it indicates that no new
      object in addition to the ones whose TOI are specified in this
      CID, or the ones that have been specified in the previous CID(s),
      will be sent in the future.  Otherwise it MUST be set to 0.  This
      entry is optional.  If absent, a receiver MUST conclude that the
      session is complete.

   o  Fcast-CID-ID: this entry contains the Carousel Instance
      IDentifier, or CIID.  It starts from 0 and is incremented by 1 for
      each new carousel instance.  This entry is optional if the FCAST
      session consists of a single, complete, carousel instance.  In all
      other cases, this entry MUST be defined.  In particular, the CIID
      is used by the TOI equivalence mechanism thanks to which any
      object is uniquely identified, even if the TOI is updated (e.g.,
      after re-enqueuing the object with NORM).  The Fcast-CID-ID value
      can also be useful to detect possible gaps in the Carousel
      Instances, for instance caused by long disconnection periods.
      Finally, it can also be useful to avoid problems when TOI wrapping
      to 0 takes place to differentiate the various incarnations of the
      TOIs if need be.

   The motivation for making the Fcast-CID-Complete and Fcast-CID-ID
   entries optional is to simplify the simple case of a session
   consisting of a single, complete, carousel instance, with an Object
   List given in plain text, without any content encoding.  In that



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   case, the CID does not need to contain any meta-data entry.

   The following standard meta-data entry types are also used
   (Section 3.3):

   o  Content-Length: it specifies the size of the object list, before
      any content encoding (if any).

   o  Content-Encoding: it specifies the optional encoding of the object
      list, performed by FCAST.  For instance:
           Content-Encoding: gzip
      indicates that the Object List field has been encoded with GZIP
      [RFC1952].  If there is no Content-Encoding entry, the receiver
      MUST assume that the Object List field is plain text (default).
      The support of GZIP encoding, in addition to the plain text form,
      is REQUIRED.  The Content-Encoding entry MAY be used to indicate
      other encoding types to support non-standard FCAST implementation
      or future extended specifications.

   An empty Object List is valid and indicates that the current carousel
   instance does not include any objects (Section 3.5).  This can be
   specified by using the following meta-data entry:
           Content-Length: 0
   or simply by leaving the Object List empty.  In both cases, padding
   MUST NOT be used and consequently the transported object length
   (e.g., as specified by the FEC OTI) minus the Compound Object Header
   Length equals zero.

   The non-encoded (i.e., plain text) Object List, when non empty, is a
   NULL-terminated ASCII string.  It can contain two things:

   o  a list of TOI values, and

   o  a list of TOI equivalences;

   A list of TOIs included in the current carousel instance is specified
   as an ASCII string containing comma-delimited individual TOI values
   and/or TOI intervals.  Inidividual TOIs consist of a single integer
   value while TOI intervals are a hyphen-delimited pair of TOI values
   to indicate a inclusive range of TOI values (e.g., "1,2,4-6" would
   indicate the list of TOI values of 1,2,4,5, and 6).  For a TOI
   Interval indicated by ""TOI_a-TOI_b", the 'TOI_a' value MUST be
   strictly inferior to the 'TOI_b' value.  If a TOI wrapping to 0
   occurs in an interval, then two TOI intervals MUST be specified,
   TOI_a-MAX_TOI and 0-TOI_b.

   This string can also contain the TOI equivalences, if any.  The
   format is a comma-separated list of equivalence TOI value pairs with



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   a delimiting equals size '=' to indicate the equivalence assignment
   (e.g., " newTOI "=" 1stTOI "/" 1stCIID ").  Each equivalence
   indicates that the new TOI, for the current Carousel Instance, is
   equivalent to (i.e., refers to the same object as) the provided
   identifier, 1stTOI, for the Carousel Instance of ID 1stCIID.  In the
   case of the NORM protocol where NormTransportId values need to
   monotonically increase for NACK-based protocol operation, this allows
   an object from a prior Carousel Instance to be reincluded in a
   subsequent Carousel Instance with the receiver set informed of the
   equivalence so that unnecessary retransmission requests can be
   avoided.

   The ABNF [RFC5234] specification is the following:
   cid-list   =  *(list-elem *( "," list-elem))
   list-elem    =  toi-elem / toieq-elem
   toi-elem     =  toi-value / toi-interval
   toi-value    =  1*DIGIT
   toi-interval =  toi-value "-" toi-value
                   ; additionally, the first toi-value MUST be
                   ; strictly inferior to the second toi-value
   toieq-elem   =  "(" toi-value "=" toi-value "/" ciid-value ")"
   ciid-value   = 1*DIGIT
   DIGIT        =  %x30-39
                   ; a digit between 0 and 9, inclusive

   For readability purposes and to simplify processing, the TOI values
   in the list MUST be given in increasing order handling wrap of the
   TOI space appropriately.  TOI equivalence elements MUST be grouped
   together at the end of the list in increasing newTOI order.
   Specifying a TOI equivalence for a given newTOI relieves the sender
   from specifying newTOI explicitly in the TOI list.  A receiver MUST
   be able to handle situations where the same TOI appears both in the
   TOI value and TOI equivalence lists.  Finally, a given TOI value or
   TOI equivalence item MUST NOT be included multiple times in either
   list.

   For instance, the following object list specifies that the current
   Carousel Instance is composed of 8 objects, and that TOIs 100 to 104
   are equivalent to the TOIs 10 to 14 of Carousel Instance ID 2 and
   refer to the same objects:

   97,98,99,(100=10/2),(101=11/2),(102=12/2),(103=13/2),(104=14/2)

   or equivalently:

   97-104,(100=10/2),(101=11/2),(102=12/2),(103=13/2),(104=14/2)





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3.  FCAST Principles

   This section details the principles of FCAST.

3.1.  FCAST Content Delivery Service

   The basic goal of FCAST is to transmit objects to a group of
   receivers in a reliable way, where the receiver set may be restricted
   to a single receiver or may include possibly a very large number of
   receivers.  FCAST supports two forms of operation:

   1.  FCAST/ALC, where the FCAST application works on top of the ALC/
       LCT reliable multicast transport protocol, without any feedback
       from the receivers, and

   2.  FCAST/NORM, where the FCAST application works on top of the NORM
       reliable multicast transport protocol, that requires positive/
       negative acknowledgements from the receivers.

   This specification is designed such that both forms of operation
   share as much commonality as possible.  Section 6 discusses some
   operational aspects and the content delivery service that is provided
   by FCAST for a given use-case.

3.2.  Meta-Data Transmission

   FCAST carries meta-data elements by prepending them to the object
   they refer to.  As a result, a Compound Object is created that is
   composed of a header followed by the original object data.  This
   header is itself composed of the meta-data as well as several fields,
   for instance to indicate the boundaries between the various parts of
   this Compound Object (Figure 3).

     <------------------------ Compound Object ----------------------->
     +-------------------------+--------------------------------------+
     |  Compound Object Header |              Object Data             |
     | (can include meta-data) |       (can be encoded by FCAST)      |
     +-------------------------+--------------------------------------+

                  Figure 3: Compound Object composition.

   Attaching the meta-data to the object is an efficient solution, since
   it guaranties that meta-data are received along with the associated
   object, and it allows the transport of the meta-data to benefit from
   any transport-layer erasure protection of the Compound Object (e.g.,
   using FEC encoding and/or NACK-based repair).  However a limit of
   this scheme is that a client does not know the meta-data of an object
   before beginning its reception, and in case of erasures affecting the



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   meta-data, not until the object decoding is completed.  The details
   of course depend upon the transport protocol and the FEC code used.

   Appendix B describes extensions that provide additional means to
   carry meta-data, e.g., to communicate meta-data ahead of time.

3.3.  Meta-Data Content

   A compliant FCAST implementation MUST support at least the following
   meta-data types defined in [RFC2616]:

   o  Content-Location: the URI of the object, which gives the name and
      location of the object;

   o  Content-Type: a string that contains the MIME type of the object;

   o  Content-Length: an unsigned 64-bit integer that contains the size
      of the initial object, before any content encoding (if any) and
      without considering the Compound Object header.  Note that the use
      of certain FEC schemes MAY further limit the maximum value of the
      object;

   o  Content-Encoding: a string that contains the optional encoding of
      the object performed by FCAST.  If there is no Content-Encoding
      entry, the receiver MUST assume that the object is not encoded
      (default).  The support of GZIP encoding, or any other solution,
      remains optional.

   o  Content-MD5: a binary content coded as "base64" that contains the
      MD5 message digest of the object in order to check its integrity.
      This digest is meant to protect from transmission and processing
      errors, not from deliberate attacks by an intelligent attacker
      (see Section 4).  This digest only protects the object, not the
      header, and therefore not the meta-data.  A separate checksum is
      provided to that purpose (Section 2.1);

   The following additional REQUIRED meta-data types are used for
   dealing with very large objects (e.g., that largely exceed the
   working memory of a receiver).  When this happens, the meta-data
   associated to each sub-object MUST include the following entries:

   o  Fcast-Obj-Slice-Nb: an unsigned 32-bit integer that contains the
      total number of slices.  A value greater than 1 indicates that
      this object is the result of a split of the original object;

   o  Fcast-Obj-Slice-Idx: an unsigned 32-bit integer that contains the
      slice index (in the {0 ..  SliceNb - 1} interval);




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   o  Fcast-Obj-Slice-Offset: an unsigned 64-bit integer that contains
      the offset at which this slice starts within the original object;

   Future standards actions can extend the set of meta-data types
   supported by FCAST.

3.4.  Carousel Transmission

   A set of FCAST Compound Objects scheduled for transmission are
   considered a logical "Carousel".  A given "Carousel Instance" is
   comprised of a fixed set of Compound Objects.  Whenever the FCAST
   application needs to add new Compound Objects to or remove old
   Compound Objects from the transmission set, a new Carousel Instance
   is defined since the set of Compound Objects changes.  Because of the
   native object multiplexing capability of both ALC and NORM, sender
   and receiver(s) are both capable to multiplex and demultiplex FCAST
   Compound Objects.

   For a given Carousel Instance, one or more transmission cycles are
   possible.  During each cycle, all of the Compound Objects comprising
   the Carousel are sent.  By default, each object is transmitted once
   per cycle.  However, in order to allow different levels of priority,
   some objects MAY be transmitted more often that others during a
   cycle, and/or benefit from higher FEC protection than others.  For
   example, this can be the case for the CID objects (Section 3.5) where
   extra protection can benefit overall carousel integrity.  For some
   FCAST usage (e.g., a unidirectional "push" mode), a Carousel Instance
   may be sent in a single transmission cycle.  In other cases it may be
   conveyed in a large number of transmission cycles (e.g., in "on-
   demand" mode, where objects are made available for download during a
   long period of time).

3.5.  Carousel Instance Descriptor Special Object

   The FCAST sender can transmit an OPTIONAL Carousel Instance
   Descriptor (CID).  The CID carries the list of the Compound Objects
   that are part of a given Carousel Instance, by specifying their
   respective Transmission Object Identifiers (TOI).  However the CID
   does not describe the objects themselves (i.e., there is no meta-
   data).  Additionally, the CID MAY include an "FCAST-CID-Complete"
   meta-data entry set to '1' to indicate that no further modification
   to the enclosed list will be done in the future.  Finally, the CID
   MAY include a Carousel Instance ID (CIID) that identifies the
   Carousel Instance it pertains to.  These aspects are discussed in
   Section 2.2.

   There is no reserved TOI value for the CID Compound Object itself,
   since this special object is regarded by ALC/LCT or NORM as a



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   standard object.  On the contrary, the nature of this object (CID) is
   indicated by means of a specific Compound Object header field (the
   "C" flag from Figure 1) so that it can be recognized and processed by
   the FCAST application as needed.  A direct consequence is the
   following: since a receiver does not know in advance which TOI will
   be used for the following CID (in case of a dynamic session), it MUST
   NOT filter out packets that are not in the current CID's TOI list.
   Said differently, the goal of CID is not to setup ALC or NORM packet
   filters (this mechanism would not be secure in any case).

   The use of a CID remains OPTIONAL.  If it is not used, then the
   clients progressively learn what files are part of the carousel
   instance by receiving ALC or NORM packets with new TOIs.  However
   using a CID has several benefits:

   o  When an "FCAST-CID-Complete" meta-data entry set to '1' is
      included, the receivers know when they can leave the session,
      i.e., when they have received all the objects that are part of the
      last carousel instance of this delivery session;

   o  In case of a session with a dynamic set of objects, the sender can
      reliably inform the receivers that some objects have been removed
      from the carousel with the CID.  This solution is more robust than
      the "Close Object flag (B)" of ALC/LCT since a client with an
      intermittent connectivity might loose all the packets containing
      this B flag.  And while NORM provides a robust object cancellation
      mechanism in the form of its NORM_CMD(SQUELCH) message in response
      to receiver NACK repair requests, the use of the CID provides an
      additional means for receivers to learn of objects for which it is
      futile to request repair;

   o  The TOI equivalence (Section 3.6) can be signaled with the CID.
      This is often preferable to the alternative solution where the
      equivalence is identified by examining the object meta-data,
      especially in case of erasures.

   During idle periods, when the carousel instance does not contain any
   object, a CID with an empty TOI list MAY be transmitted.  In that
   case, a new carousel instance ID MUST be used to differentiate this
   (empty) carousel instance from the other ones.  This mechanism can be
   useful to inform the receivers that:

   o  all the previously sent objects have been removed from the
      carousel.  It therefore improves the FCAST robustness even during
      "idle" period;

   o  the session is still active even if there is currently no content
      being sent.  Said differently, it can be used as a heartbeat



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      mechanism.  If an "FCAST-CID-Complete" meta-data entry is not
      included (or if set to '0'), it informs the receivers the carousel
      instance may be modified and that new objects could be sent in the
      future;

3.6.  Compound Object Identification

   The FCAST Compound Objects are directly associated with the object-
   based transport service that the ALC and NORM protocols provide.  In
   each of these protocols, the messages containing transport object
   content are labeled with a numeric transport object identifier (i.e.,
   the ALC TOI and the NORM NormTransportId).  For the purposes of this
   document, this identifier in either case (ALC or NORM) is referred to
   as the TOI.

   There are several differences between ALC and NORM:

   o  the ALC use of TOI is rather flexible, since several TOI field
      sizes are possible (from 16 to 112 bits), since this size can be
      changed at any time, on a per-packet basis, and since the TOI
      management is totally free as long as each object is associated to
      a unique TOI (if no wraparound happened);

   o  the NORM use of TOI is more directive, since the TOI field is 16
      bit long and since TOIs MUST be managed sequentially;

   In both NORM and ALC, it is possible that the transport
   identification space may eventually wrap for long-lived sessions
   (especially with NORM where this phenomenon is expected to happen
   more frequently).  This can possibly introduce some ambiguity in
   FCAST object identification if a sender retains some older objects in
   newer Carousel Instances with updated object sets.  To avoid
   ambiguity the active TOIs (i.e., the TOIs corresponding to objects
   being transmitted) can only occupy half of the TOI sequence space.
   If an old object, whose TOI has fallen outside the current window,
   needs to be transmitted again, a new TOI must be used for it.  In
   case of NORM, this constraint limits to 32768 the maximum number of
   objects that can be part of any carousel instance.  In order to allow
   receivers to properly combine the transport packets with a newly-
   assigned TOI to those of associated to the previously-assigned TOI, a
   mechanism is required to equate the objects with the new and the old
   TOIs.

   The preferred mechanism consists in signaling, within the CID, that
   the newly assigned TOI, for the current Carousel Instance, is
   equivalent to the TOI used within a previous Carousel Instance.  By
   convention, the reference tuple for any object is the {TOI; CI ID}
   tuple used for its first transmission within a Carousel Instance.



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   This tuple MUST be used whenever a TOI equivalence is provided.

   An alternative solution, when meta-data can be processed rapidly
   (e.g., by using NORM-INFO messages), consists for the receiver in
   identifying that both objects are the same, after examining the meta-
   data.  The receiver can then take appropriate measures.

3.7.  FCAST Sender Behavior

   This section provides an informative description of expected FCAST
   sender behavior.  The following operations can take place at a
   sender:

   1.  The user (or another application) selects a set of objects (e.g.,
       files) to deliver and submits them, along with their meta-data,
       to the FCAST application;

   2.  For each object, FCAST creates the Compound Object and registers
       this latter in the carousel instance;

   3.  The user then informs FCAST that all the objects of the set have
       been submitted.  If the user knows that no new object will be
       submitted in the future (i.e., if the session's content is now
       complete), the user informs FCAST.  Finally, the user specifies
       how many transmission cycles are desired (this number may be
       infinite);

   4.  At this point, the FCAST application knows the full list of
       Compound Objects that are part of the Carousel Instance and can
       create a CID if desired, possibly with the complete flag set;

   5.  The FCAST application can now define a transmission schedule of
       these Compound Objects, including the optional CID.  This
       schedule defines in which order the packets of the various
       Compound Objects should be sent.  This document does not specify
       any scheme.  This is left to the developer within the provisions
       of the underlying ALC or NORM protocol used and the knowledge of
       the target use-case.

   6.  The FCAST application then starts the carousel transmission, for
       the number of cycles specified.  Transmissions take place until:

       *  the desired number of transmission cycles has been reached, or

       *  the user wants to prematurely stop the transmissions, or

       *  the user wants to add one or several new objects to the
          carousel, or on the contrary wants to remove old objects from



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          the carousel.  In that case a new carousel instance must be
          created.

   7.  If the session is not finished, then continue at Step 1 above;

3.8.  FCAST Receiver Behavior

   This section provides an informative description of expected FCAST
   receiver behavior.  The following operations can take place at a
   receiver:

   1.  The receiver joins the session and collects incoming packets;

   2.  If the header portion of a Compound Object is entirely received
       (which may happen before receiving the entire object with some
       ALC/NORM configurations), or if the meta-data is sent by means of
       another mechanism prior to the object, the receiver processes the
       meta-data and chooses to continue to receive the object content
       or not;

   3.  When a Compound Object has been entirely received, the receiver
       processes the header, retrieves the object meta-data, perhaps
       decodes the meta-data, and processes the object accordingly;

   4.  When a CID is received, which is indicated by the 'C' flag set in
       the Compound Object header, the receiver decodes the CID, and
       retrieves the list of objects that are part of the current
       carousel instance.  This list can be used to remove objects sent
       in a previous carousel instance that might not have been totally
       decoded and that are no longer part of the current carousel
       instance;

   5.  When a CID is received, the receiver also retrieves the list of
       TOI equivalences, if any, and takes appropriate measures, for
       instance by informing the transport layer;

   6.  When a receiver receives a CID with an "FCAST-CID-Complete" meta-
       data entry set to '1', and has successfully received all the
       objects of the current carousel instance, it can safely exit from
       the current FCAST session;

   7.  Otherwise continue at Step 2 above.


4.  Security Considerations






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4.1.  Problem Statement

   A content delivery system is potentially subject to attacks.  Attacks
   may target:

   o  the network (to compromise the routing infrastructure, e.g., by
      creating congestion),

   o  the Content Delivery Protocol (CDP) (e.g., to compromise the
      normal behavior of FCAST), or

   o  the content itself (e.g., to corrupt the objects being
      transmitted).

   These attacks can be launched either:

   o  against the data flow itself (e.g., by sending forged packets),

   o  against the session control parameters (e.g., by corrupting the
      session description, the CID, the object meta-data, or the ALC/LCT
      control parameters), that are sent either in-band or out-of-band,
      or

   o  against some associated building blocks (e.g., the congestion
      control component).

   In the following sections we provide more details on these possible
   attacks and sketch some possible counter-measures.  We finally
   provide recommendations in Section 4.5.

4.2.  Attacks Against the Data Flow

   Let us consider attacks against the data flow first.  At least, the
   following types of attacks exist:

   o  attacks that are meant to give access to a confidential object
      (e.g., in case of a non-free content) and

   o  attacks that try to corrupt the object being transmitted (e.g., to
      inject malicious code within an object, or to prevent a receiver
      from using an object, which is a kind of Denial of Service (DoS)).

4.2.1.  Access to Confidential Objects

   Access control to the object being transmitted is typically provided
   by means of encryption.  This encryption can be done over the whole
   object (e.g., by the content provider, before submitting the object
   to FCAST), or be done on a packet per packet basis (e.g., when IPsec/



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   ESP is used [RFC4303], see Section 4.5).  If confidentiality is a
   concern, it is RECOMMENDED that one of these solutions be used.

4.2.2.  Object Corruption

   Protection against corruptions (e.g., if an attacker sends forged
   packets) is achieved by means of a content integrity verification/
   sender authentication scheme.  This service can be provided at the
   object level, but in that case a receiver has no way to identify
   which symbol(s) is(are) corrupted if the object is detected as
   corrupted.  This service can also be provided at the packet level.
   In this case, after removing all corrupted packets, the file may be
   in some cases recovered.  Several techniques can provide this content
   integrity/sender authentication service:

   o  at the object level, the object can be digitally signed, for
      instance by using RSASSA-PKCS1-v1_5 [RFC3447].  This signature
      enables a receiver to check the object integrity, once this latter
      has been fully decoded.  Even if digital signatures are
      computationally expensive, this calculation occurs only once per
      object, which is usually acceptable;

   o  at the packet level, each packet can be digitally signed
      [RFC6584].  A major limitation is the high computational and
      transmission overheads that this solution requires.  To avoid this
      problem, the signature may span a set of packets (instead of a
      single one) in order to amortize the signature calculation.  But
      if a single packets is missing, the integrity of the whole set
      cannot be checked;

   o  at the packet level, a Group Message Authentication Code (MAC)
      [RFC2104][RFC6584] scheme can be used, for instance by using HMAC-
      SHA-256 with a secret key shared by all the group members, senders
      and receivers.  This technique creates a cryptographically secured
      digest of a packet that is sent along with the packet.  The Group
      MAC scheme does not create prohibitive processing load nor
      transmission overhead, but it has a major limitation: it only
      provides a group authentication/integrity service since all group
      members share the same secret group key, which means that each
      member can send a forged packet.  It is therefore restricted to
      situations where group members are fully trusted (or in
      association with another technique as a pre-check);

   o  at the packet level, Timed Efficient Stream Loss-Tolerant
      Authentication (TESLA) [RFC4082][RFC5776] is an attractive
      solution that is robust to losses, provides a true authentication/
      integrity service, and does not create any prohibitive processing
      load or transmission overhead.  Yet checking a packet requires a



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      small delay (a second or more) after its reception;

   o  at the packet level, IPsec/ESP [RFC4303] can be used to check the
      integrity and authenticate the sender of all the packets being
      exchanged in a session (see Section 4.5).

   Techniques relying on public key cryptography (digital signatures and
   TESLA during the bootstrap process, when used) require that public
   keys be securely associated to the entities.  This can be achieved by
   a Public Key Infrastructure (PKI), or by a PGP Web of Trust, or by
   pre-distributing securely the public keys of each group member.

   Techniques relying on symmetric key cryptography (Group MAC) require
   that a secret key be shared by all group members.  This can be
   achieved by means of a group key management protocol, or simply by
   pre-distributing securely the secret key (but this manual solution
   has many limitations).

   It is up to the developer and deployer, who know the security
   requirements and features of the target application area, to define
   which solution is the most appropriate.  In any case, whenever there
   is any concern of the threat of file corruption, it is RECOMMENDED
   that at least one of these techniques be used.

4.3.  Attacks Against the Session Control Parameters and Associated
      Building Blocks

   Let us now consider attacks against the session control parameters
   and the associated building blocks.  The attacker has at least the
   following opportunities to launch an attack:

   o  the attack can target the session description,

   o  the attack can target the FCAST CID,

   o  the attack can target the meta-data of an object,

   o  the attack can target the ALC/LCT parameters, carried within the
      LCT header or

   o  the attack can target the FCAST associated building blocks, for
      instance the multiple rate congestion control protocol.

   The consequences of these attacks are potentially serious, since they
   can compromise the behavior of content delivery system or even
   compromise the network itself.





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4.3.1.  Attacks Against the Session Description

   An FCAST receiver may potentially obtain an incorrect Session
   Description for the session.  The consequence of this is that
   legitimate receivers with the wrong Session Description are unable to
   correctly receive the session content, or that receivers
   inadvertently try to receive at a much higher rate than they are
   capable of, thereby possibly disrupting other traffic in the network.

   To avoid these problems, it is RECOMMENDED that measures be taken to
   prevent receivers from accepting incorrect Session Descriptions.  One
   such measure is the sender authentication to ensure that receivers
   only accept legitimate Session Descriptions from authorized senders.
   How these measures are achieved is outside the scope of this document
   since this session description is usually carried out-of-band.

4.3.2.  Attacks Against the FCAST CID

   Corrupting the FCAST CID is one way to create a Denial of Service
   attack.  For example, the attacker can insert an "FCAST-CID-Complete"
   meta-data entry to make the receivers believe that no further
   modification will be done.

   It is therefore RECOMMENDED that measures be taken to guarantee the
   integrity and to check the sender's identity of the CID.  To that
   purpose, one of the counter-measures mentioned above (Section 4.2.2)
   SHOULD be used.  These measures will either be applied on a packet
   level, or globally over the whole CID object.  When there is no
   packet level integrity verification scheme, it is RECOMMENDED to
   digitally sign the CID.

4.3.3.  Attacks Against the Object Meta-Data

   Corrupting the object meta-data is another way to create a Denial of
   Service attack.  For example, the attacker changes the MD5 sum
   associated to a file.  This possibly leads a receiver to reject the
   files received, no matter whether the files have been correctly
   received or not.  When the meta-data are appended to the object,
   corrupting the meta-data means that the Compound Object will be
   corrupted.

   It is therefore RECOMMENDED that measures be taken to guarantee the
   integrity and to check the sender's identity of the Compound Object.
   To that purpose, one of the counter-measures mentioned above
   (Section 4.2.2) SHOULD be used.  These measures will either be
   applied on a packet level, or globally over the whole Compound
   Object.  When there is no packet level integrity verification scheme,
   it is RECOMMENDED to digitally sign the Compound Object.



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4.3.4.  Attacks Against the ALC/LCT and NORM Parameters

   By corrupting the ALC/LCT header (or header extensions) one can
   execute attacks on the underlying ALC/LCT implementation.  For
   example, sending forged ALC packets with the Close Session flag (A)
   set to one can lead the receiver to prematurely close the session.
   Similarly, sending forged ALC packets with the Close Object flag (B)
   set to one can lead the receiver to prematurely give up the reception
   of an object.  The same comments can be made for NORM.

   It is therefore RECOMMENDED that measures be taken to guarantee the
   integrity and to check the sender's identity of each ALC or NORM
   packet received.  To that purpose, one of the counter-measures
   mentioned above (Section 4.2.2) SHOULD be used.

4.3.5.  Attacks Against the Associated Building Blocks

   Let us first focus on the congestion control building block that may
   be used in an ALC or NORM session.  A receiver with an incorrect or
   corrupted implementation of the multiple rate congestion control
   building block may affect the health of the network in the path
   between the sender and the receiver.  That may also affect the
   reception rates of other receivers who joined the session.

   When congestion control is applied with FCAST, it is therefore
   RECOMMENDED that receivers be required to identify themselves as
   legitimate before they receive the Session Description needed to join
   the session.  If authenticating a receiver does not prevent this
   latter to launch an attack, it will enable the network operator to
   identify him and to take counter-measures.  This authentication can
   be made either toward the network operator or the session sender (or
   a representative of the sender) in case of NORM.  The details of how
   it is done are outside the scope of this document.

   When congestion control is applied with FCAST, it is also RECOMMENDED
   that a packet level authentication scheme be used, as explained in
   Section 4.2.2.  Some of them, like TESLA, only provide a delayed
   authentication service, whereas congestion control requires a rapid
   reaction.  It is therefore RECOMMENDED [RFC5775] that a receiver
   using TESLA quickly reduces its subscription level when the receiver
   believes that a congestion did occur, even if the packet has not yet
   been authenticated.  Therefore TESLA will not prevent DoS attacks
   where an attacker makes the receiver believe that a congestion
   occurred.  This is an issue for the receiver, but this will not
   compromise the network.  Other authentication methods that do not
   feature this delayed authentication could be preferred, or a group
   MAC scheme could be used in parallel to TESLA to prevent attacks
   launched from outside of the group.



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4.4.  Other Security Considerations

   Lastly, we note that the security considerations that apply to, and
   are described in, ALC [RFC5775], LCT [RFC5651], NORM [RFC5740] and
   FEC [RFC5052] also apply to FCAST as FCAST builds on those
   specifications.  In addition, any security considerations that apply
   to any congestion control building block used in conjunction with
   FCAST also applies to FCAST.  Finally, the security discussion of
   [I-D.ietf-rmt-sec-discussion] also applies here.

4.5.  Minimum Security Recommendations

   We now introduce a mandatory to implement but not necessarily to use
   security configuration, in the sense of [RFC3365].  Since FCAST/ALC
   relies on ALC/LCT, it inherits the "baseline secure ALC operation" of
   [RFC5775].  Similarly, since FCAST/NORM relies on NORM, it inherits
   the "baseline secure NORM operation" of [RFC5740].  More precisely,
   in both cases security is achieved by means of IPsec/ESP in transport
   mode.  [RFC4303] explains that ESP can be used to potentially provide
   confidentiality, data origin authentication, content integrity, anti-
   replay and (limited) traffic flow confidentiality.  [RFC5775]
   specifies that the data origin authentication, content integrity and
   anti-replay services SHALL be used, and that the confidentiality
   service is RECOMMENDED.  If a short lived session MAY rely on manual
   keying, it is also RECOMMENDED that an automated key management
   scheme be used, especially in case of long lived sessions.

   Therefore, the RECOMMENDED solution for FCAST provides per-packet
   security, with data origin authentication, integrity verification and
   anti-replay.  This is sufficient to prevent most of the in-band
   attacks listed above.  If confidentiality is required, a per-packet
   encryption SHOULD also be used.


5.  Requirements for Compliant Implementations

   This section lists the features that any compliant FCAST/ALC or
   FCAST/NORM implementation MUST support, and those that remain
   OPTIONAL, e.g., in order to enable some optimizations for a given
   use-case, at a receiver.

5.1.  Requirements Related to the Object Meta-Data

   Meta-data transmission mechanisms:







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   +-----------------------+-------------------------------------------+
   | Feature               | Status                                    |
   +-----------------------+-------------------------------------------+
   | meta-data             | An FCAST sender MUST send meta-data with  |
   | transmission using    | the in-band mechanism provided by FCAST,  |
   | FCAST's in-band       | i.e., within the Compound Object header.  |
   | mechanism             | All the FCAST receivers MUST be able to   |
   |                       | process meta-data sent with this FCAST    |
   |                       | in-band mechanism.                        |
   | meta-data             | In addition to the FCAST in-band          |
   | transmission using    | transmission of meta-data, an FCAST       |
   | other mechanisms      | sender MAY send a subset or all of the    |
   |                       | meta-data using another mechanism.        |
   |                       | Supporting this mechanism in a compliant  |
   |                       | FCAST receiver is OPTIONAL, and its use   |
   |                       | is OPTIONAL too.  An FCAST receiver MAY   |
   |                       | support this mechanism and take advantage |
   |                       | of the meta-data sent in this way.  If it |
   |                       | is not the case, the FCAST receiver will  |
   |                       | anyway receive and process meta-data sent |
   |                       | in-bound.  See Annex Appendix B.          |
   +-----------------------+-------------------------------------------+

   Meta-data format and encoding:

   +-----------------------+-------------------------------------------+
   | Feature               | Status                                    |
   +-----------------------+-------------------------------------------+
   | Meta-Data Format      | All FCAST implementations MUST support an |
   | (MDFmt field)         | HTTP/1.1 metainformation format           |
   |                       | [RFC2616].  Other formats (e.g., XML) MAY |
   |                       | be defined in the future.                 |
   | Meta-Data Encoding    | All FCAST implementations MUST support    |
   | (MDEnc field)         | both a plain text and a GZIP encoding     |
   |                       | [RFC1952] of the Object Meta-Data field.  |
   |                       | Other encodings MAY be defined in the     |
   |                       | future.                                   |
   +-----------------------+-------------------------------------------+

   Meta-data items (Section 3.3):











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              +------------------------+-------------------+
              | Feature                | Status            |
              +------------------------+-------------------+
              | Content-Location       | MUST be supported |
              | Content-Type           | MUST be supported |
              | Content-Length         | MUST be supported |
              | Content-Encoding       | MUST be supported |
              | Content-MD5            | MUST be supported |
              | Fcast-Obj-Slice-Nb     | MUST be supported |
              | Fcast-Obj-Slice-Idx    | MUST be supported |
              | Fcast-Obj-Slice-Offset | MUST be supported |
              +------------------------+-------------------+

5.2.  Requirements Related to the Carousel Instance Descriptor (CID)
      Mechanism

   Any compliant FCAST implementation MUST support the CID mechanism, in
   order to list the Compound Objects that are part of a given Carousel
   Instance.  However its use is OPTIONAL.


6.  Operational Considerations

   FCAST is compatible with any congestion control protocol designed for
   ALC/LCT or NORM.  However, depending on the use-case, the data flow
   generated by the FCAST application might not be constant, but instead
   be bursty in nature.  Similarly, depending on the use-case, an FCAST
   session might be very short.  Whether and how this will impact the
   congestion control protocol is out of the scope of the present
   document.

   FCAST is compatible with any security mechanism designed for ALC/LCT
   or NORM.  The use of a security scheme is strongly RECOMMENDED (see
   Section 4).

   FCAST is compatible with any FEC scheme designed for ALC/LCT or NORM.
   Whether FEC is used or not, and the kind of FEC scheme used, is to
   some extent transparent to FCAST.

   FCAST is compatible with both IPv4 and IPv6.  Nothing in the FCAST
   specification has any implication on the source or destination IP
   address type.

   The delivery service provided by FCAST might be fully reliable, or
   only partially reliable depending upon:

   o  the way ALC or NORM is used (e.g., whether FEC encoding and/or
      NACK-based repair requests are used or not),



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   o  the way the FCAST carousel is used (e.g., whether the objects are
      made available for a long time span or not), and

   o  the way in which FCAST itself is employed (e.g., whether there is
      a session control application that might automatically extend an
      existing FCAST session until all receivers have received the
      transmitted content).

   The receiver set can be restricted to a single receiver or possibly
   possibly a very large number of receivers.  While the choice of the
   underlying transport protocol (i.e., ALC or NORM) and its parameters
   may limit the practical receiver group size, nothing in FCAST itself
   limits it.  For instance, if FCAST/ALC is used on top of purely
   unidirectional transport channels, with no feedback information at
   all, which is the default mode of operation, then the scalability is
   maximum since neither FCAST, nor ALC, UDP or IP generates any
   feedback message.  On the contrary, the FCAST/NORM scalability is
   typically limited by NORM scalability itself.  For example, NORM can
   be configured to operate using proactive FEC without feedback similar
   to ALC, with receivers configured to provide NACK and optionally ACK
   feedback, or a hybrid combination of these.  Similarly, if FCAST is
   used along with a session control application that collects reception
   information from the receivers, then this session control application
   may limit the scalability of the global object delivery system.  This
   situation can of course be mitigated by using a hierarchy of feedback
   message aggregators or servers.  The details of this are out of the
   scope of the present document.

   The content of a carousel instance MAY be described by means of an
   OPTIONAL Carousel Instance Descriptor (CID) (Section 3.5).  The
   decisions of whether a CID should be used or not, how often and when
   a CID should be sent, are left to the sender and depend on many
   parameters, including the target use case and the session dynamics.
   For instance it may be appropriate to send a CID at the beginning of
   each new carousel instance, and then periodically.  These operational
   aspects are out of the scope of the present document.


7.  IANA Considerations

   This specification requires IANA to create two new registries.

7.1.  Creation of the FCAST Object Meta-Data Format Registry

   This document requires IANA to create a new registry, "FCAST Object
   Meta-Data Format" (MDFmt), with a reference to this document.  The
   registry entries consist of a numeric value from 0 to 15, inclusive
   (i.e., they are 4-bit positive integers) that define the format of



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   the object meta-data (see Section 2.1).

   The initial value for this registry registry is defined below.
   Future assignments are to be made through Expert Review with
   Specification Required [RFC5226].

   +-----------+---------------------+-----------------+---------------+
   | Value     |     Format Name     |      Format     | Reference     |
   |           |                     |    Reference    |               |
   +-----------+---------------------+-----------------+---------------+
   | 0         |       HTTP/1.1      |    [RFC2616],   | This          |
   | (default) |   metainformation   |   Section 7.1   | specification |
   |           |        format       |                 |               |
   +-----------+---------------------+-----------------+---------------+

7.2.  Creation of the FCAST Object Meta-Data Encoding Registry

   This document requires IANA to create a new registry, "FCAST Object
   Meta-Data Encoding" (MDEnc), with a reference to this document.  The
   registry entries consist of a numeric value from 0 to 15, inclusive
   (i.e., they are 4-bit positive integers) that define the optional
   encoding of the Object Meta-Data field (see Section 2.1).

   The initial values for this registry registry are defined below.
   Future assignments are to be made through Expert Review [RFC5226].

   +-------------+-------------+-------------------+-------------------+
   |    Value    |   Encoding  |      Encoding     |     Reference     |
   |             |     Name    |     Reference     |                   |
   +-------------+-------------+-------------------+-------------------+
   | 0 (default) |  Plain Text |        This       |        This       |
   |             |             |   specification   |   specification   |
   |      1      |     GZIP    |     [RFC1952]     |        This       |
   |             |             |                   |   specification   |
   +-------------+-------------+-------------------+-------------------+

7.3.  Creation of the FCAST Object Meta-Data Types Registry

   This document requires IANA to create a new registry, "FCAST Object
   Meta-Data Types" (MDType), with a reference to this document.  The
   registry entries consist of additional text meta-data type
   identifiers and descriptions for meta-data item types that are
   specific to FCAST operation and not previously defined in [RFC1952].
   The initial values are those described in Section 3.3 of this
   specification.  This table summarizes those initial registry entries.
   Future assignments are to be made through Expert Review [RFC5226].





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   +------------------------+--------------------------+---------------+
   | Meta-Data Type         | Description              |   Reference   |
   +------------------------+--------------------------+---------------+
   | Fcast-Obj-Slice-Nb     | Unsigned 32-bit integer  |      This     |
   |                        | that contains the total  | specification |
   |                        | number of slices.  A     |               |
   |                        | value greater than 1     |               |
   |                        | indicates that this      |               |
   |                        | object is the result of  |               |
   |                        | a split of the original  |               |
   |                        | object                   |               |
   | Fcast-Obj-Slice-Idx    | Unsigned 32-bit integer  |      This     |
   |                        | that contains the slice  | specification |
   |                        | index (in the {0 ..      |               |
   |                        | SliceNb - 1} interval)   |               |
   | Fcast-Obj-Slice-Offset | Unsigned 64-bit integer  |      This     |
   |                        | that contains the byte   | specification |
   |                        | offset at which this     |               |
   |                        | slice starts within the  |               |
   |                        | original object          |               |
   +------------------------+--------------------------+---------------+


8.  Acknowledgments

   The authors are grateful to the authors of [ALC-00] for specifying
   the first version of FCAST/ALC.  The authors are also grateful to
   David Harrington, Gorry Fairhurst and Lorenzo Vicisano for their
   valuable comments.


9.  References

9.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

   [RFC5651]  Luby, M., Watson, M., and L. Vicisano, "Layered Coding
              Transport (LCT) Building Block", RFC 5651, October 2009.

   [RFC5740]  Adamson, B., Bormann, C., Handley, M., and J. Macker,
              "NACK-Oriented Reliable Multicast (NORM) Transport
              Protocol", RFC 5740, November 2009.



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   [RFC5775]  Luby, M., Watson, M., and L. Vicisano, "Asynchronous
              Layered Coding (ALC) Protocol Instantiation", RFC 5775,
              April 2010.

   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.

9.2.  Informative References

   [ALC-00]   Luby, M., Gemmell, G., Vicisano, L., Crowcroft, J., and B.
              Lueckenhoff, "Asynchronous Layered Coding: a Scalable
              Reliable Multicast Protocol", March 2000.

   [RFC6726]  Paila, T., Walsh, R., Luby, M., Roca, V., and R. Lehtonen,
              "FLUTE - File Delivery over Unidirectional Transport",
              RFC 6726, November 2012.

   [I-D.ietf-rmt-sec-discussion]
              Adamson, B., Roca, V., and H. Asaeda, "Security and
              Reliable Multicast Transport Protocols: Discussions and
              Guidelines", draft-ietf-rmt-sec-discussion-06 (work in
              progress), March 2011.

   [RFC3365]  Schiller, J., "Strong Security Requirements for Internet
              Engineering Task Force Standard Protocols", BCP 61,
              RFC 3365, August 2002.

   [RFC1071]  Braden, R., Borman, D., Partridge, C., and W. Plummer,
              "Computing the Internet checksum", RFC 1071,
              September 1988.

   [RFC1952]  Deutsch, P., Gailly, J-L., Adler, M., Deutsch, L., and G.
              Randers-Pehrson, "GZIP file format specification version
              4.3", RFC 1952, May 1996.

   [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104,
              February 1997.

   [RFC3447]  Jonsson, J. and B. Kaliski, "Public-Key Cryptography
              Standards (PKCS) #1: RSA Cryptography Specifications
              Version 2.1", RFC 3447, February 2003.

   [RFC4082]  Perrig, A., Song, D., Canetti, R., Tygar, J., and B.



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              Briscoe, "Timed Efficient Stream Loss-Tolerant
              Authentication (TESLA): Multicast Source Authentication
              Transform Introduction", RFC 4082, June 2005.

   [RFC4303]  Kent, S., "IP Encapsulating Security Payload (ESP)",
              RFC 4303, December 2005.

   [RFC5052]  Watson, M., Luby, M., and L. Vicisano, "Forward Error
              Correction (FEC) Building Block", RFC 5052, August 2007.

   [RFC5510]  Lacan, J., Roca, V., Peltotalo, J., and S. Peltotalo,
              "Reed-Solomon Forward Error Correction (FEC) Schemes",
              RFC 5510, April 2009.

   [RFC5776]  Roca, V., Francillon, A., and S. Faurite, "Use of Timed
              Efficient Stream Loss-Tolerant Authentication (TESLA) in
              the Asynchronous Layered Coding (ALC) and NACK-Oriented
              Reliable Multicast (NORM) Protocols", RFC 5776,
              April 2010.

   [RFC6584]  Roca, V., "Simple Authentication Schemes for the
              Asynchronous Layered Coding (ALC) and NACK-Oriented
              Reliable Multicast (NORM) Protocols", RFC 6584,
              April 2012.


Appendix A.  FCAST Examples

A.1.  Introduction

   This appendix provides informative examples of FCAST compound object
   header and carousel instance descriptor formats.



















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A.2.  Regular Compound Object Example
      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |V=0|Resvd=0|1|0|MDFmt=0|MDEnc=0|           Checksum            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |              Compound Object Header Length=41                 |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
     .                                                               .
     .       meta-data ASCII null terminated string (33 bytes)       .
     .                                                               .
     +               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |               |                    Padding                    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     .                                                               .
     .                         Object data                           .
     .                                                               .
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 4: Compound Object Example.

   Figure 4 shows a regular Compound Object where the meta-data ASCII
   string, in HTTP/1.1 meta-information format (MDFmt=0) contains:

   Content-Location: example.txt <CR-LF>

   This string is 33 bytes long, including the NULL-termination
   character.  There is no GZIP encoding of the meta-data (MDEnc=0) and
   there is no Content-Length information either since this length can
   easily be calculated by the receiver as the FEC OTI transfer length
   minus the header length.  Finally, the checksum encompasses the whole
   Compound Object (G=1).

A.3.  Carousel Instance Descriptor Example

   Figure 5 shows an example CID object, in the case of a static FCAST
   session, i.e., a session where the set of objects is set once and for
   all.  There is no meta-data in this example since Fcast-CID-Complete
   and Fcast-CID-ID are both implicit.












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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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |V=0|Resvd=0|1|1|MDFmt=0|MDEnc=0|           Checksum            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |              Compound Object Header Length=8                  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
     .                                                               .
     .                Object List string                             .
     .                                                               .
     .                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     .                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

          Figure 5: Example of CID, in case of a static session.

   The object list contains the following 26 byte long string, including
   the NULL-termination character:
   1,2,3,100-104,200-203,299

   There are therefore a total of 3+5+4+1 = 13 objects in the carousel
   instance, and therefore in the FCAST session.  There is no meta-data
   associated to this CID.  The session being static and composed of a
   single Carousel Instance, the sender did not feel the necessity to
   carry a Carousel Instance ID meta-data.


Appendix B.  Additional Meta-Data Transmission Mechanisms

B.1.  Supporting Additional Mechanisms

   In certain use-cases, FCAST can take advantage of another in-band
   (e.g., via NORM_INFO messages Appendix B.2) or out-of-band signaling
   mechanism.  This section provides an overview of how other signaling
   mechanism could be employed and a normative specification for how
   FCAST information is embedded when NORM_INFO messages are used for
   FCAST compound message headers.  Such additional signaling schemes
   can be used to carry the whole meta-data, or a subset of it, ahead of
   time, before the associated compound object.  Therefore a receiver
   could be able to decide in advance, before beginning the reception of
   the compound object, whether the object is of interest or not, based
   on the information retrieved by this way, which mitigates FCAST
   limitations.  While out-of-band techniques are out of the scope of
   this document, we explain below how this can be achieved in case of
   FCAST/NORM.

   Supporting additional mechanisms is OPTIONAL in FCAST
   implementations.  In any case, an FCAST sender MUST continue to send



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   all the required meta-data in the compound object, even if the whole
   meta-data, or a subset of it, is sent by another mechanism
   (Section 5).  Additionally, when meta-data is sent several times,
   there MUST NOT be any contradiction in the information provided by
   the different mechanisms.  In case a mismatch is detected, the meta-
   data contained in the Compound Object MUST be used as the definitive
   source.

   When meta-data elements are communicated out-of-band, in advance of
   data transmission, the following piece of information can be useful:

   o  TOI: a positive integer that contains the Transmission Object
      Identifier (TOI) of the object, in order to enable a receiver to
      easily associate the meta-data to the object.  The valid range for
      TOI values is discussed in Section 3.6;

B.2.  Using NORM_INFO Messages with FCAST/NORM

   The NORM_INFO message of NORM can convey "out-of-band" content with
   respect to a given transport object.  With FCAST, this message MAY be
   used as an additional mechanism to transmit meta-data.  In that case,
   the NORM_INFO message carries a new Compound Object that contains all
   the meta-data of the original object, or a subset of it.  The
   NORM_INFO Compound Object MUST NOT contain any Object Data field
   (i.e., it is only composed of the header), it MUST feature a non
   global checksum, and it MUST NOT include any padding field.  Finally,
   note that the availability of NORM_INFO for a given object is
   signaled through the use of a dedicated flag in the NORM_DATA message
   header.  Along with NORM's NACK-based repair request signaling, it
   allows a receiver to quickly (and independently) request an object's
   NORM_INFO content.  However, a limitation here is that the NORM_INFO
   Compound Object header MUST fit within the byte size limit defined by
   the NORM sender's configured "segment size" (typically a little less
   than the network MTU);

B.2.1.  Example

   In case of FCAST/NORM, the FCAST Compound Object meta-data (or a
   subset of it) can be carried as part of a NORM_INFO message, as a new
   Compound Object that does not contain any Compound Object Data.  In
   the following informative example we assume that the whole meta-data
   is carried in such a message for a certain Compound Object.  Figure 6
   shows an example NORM_INFO message that contains the FCAST Compound
   Object Header and meta-data as its payload.  In this example, the
   first 16 bytes are the NORM_INFO base header, the next 12 bytes are a
   NORM EXT_FTI header extension containing the FEC Object Transport
   Information for the associated object, and the remaining bytes are
   the FCAST Compound Object Header and meta-data.  Note that "padding"



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   MUST NOT be used and that the FCAST checksum only encompasses the
   Compound Object Header (G=0).
    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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ --
    |version| type=1|  hdr_len = 7  |          sequence             |  ^
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
    |                           source_id                           |  n
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  o
    |          instance_id          |     grtt      |backoff| gsize |  r
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  m
    |     flags     |  fec_id = 5   |     object_transport_id       |  v
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ --
    |   HET = 64    |    HEL = 3    |                               |  ^
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +  f
    |                     Transfer Length = 41                      |  t
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  i
    |   Encoding Symbol Length (E)  | MaxBlkLen (B) |     max_n     |  v
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ --
    | 0 |   0   |0|0|   0   |   0   |           Checksum            |  ^
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
    |                               41                              |  f
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|  c
    .                                                               .  a
    .       meta-data ASCII null terminated string (33 bytes)       .  s
    .                                                               .  t
    +               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
    |               |                                                  v
    +-+-+-+-+-+-+-+-+                                                 --

     Figure 6: NORM_INFO containing an EXT_FTI header extension and an
                       FCAST Compound Object Header

   The NORM_INFO message shown in Figure 6 contains the EXT_FTI header
   extension to carry the FEC OTI.  In this example, the FEC OTI format
   is that of the Reed-Solomon FEC coding scheme for fec_id = 5 as
   described in [RFC5510].  Other alternatives for providing the FEC OTI
   would have been to either include it directly in the meta-data of the
   FCAST Compound Header, or to include an EXT_FTI header extension to
   all NORM_DATA packets (or a subset of them).  Note that the NORM
   "Transfer_Length" is the total length of the associated FCAST
   Compound Object, i.e., 41 bytes.

   The FCAST Compound Object in this example does contain the same meta-
   data and is formatted as in the example of Figure 4.  With the
   combination of the FEC_OTI and the FCAST meta-data, the NORM protocol
   and FCAST application have all of the information needed to reliably
   receive and process the associated object.  Indeed, the NORM protocol



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   provides rapid (NORM_INFO has precedence over the associated object
   content), reliable delivery of the NORM_INFO message and its payload,
   the FCAST Compound Object.


Authors' Addresses

   Vincent Roca
   INRIA
   655, av. de l'Europe
   Inovallee; Montbonnot
   ST ISMIER cedex  38334
   France

   Email: vincent.roca@inria.fr
   URI:   http://planete.inrialpes.fr/people/roca/


   Brian Adamson
   Naval Research Laboratory
   Washington, DC  20375
   USA

   Email: adamson@itd.nrl.navy.mil
   URI:   http://cs.itd.nrl.navy.mil


























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