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

RMT                                                              V. Roca
Internet-Draft                                                     INRIA
Intended status: Experimental                                 B. Adamson
Expires: September 27, 2013                    Naval Research Laboratory
                                                          March 26, 2013


         FCAST: Object Delivery for the ALC and NORM Protocols
                        draft-ietf-rmt-fcast-08

Abstract

   This document introduces the FCAST reliable object (e.g., file)
   delivery application.  It is designed to operate either on top of the
   underlying Asynchronous Layer Coding (ALC)/Layered Coding Transport
   (LCT) or the NACK-Oriented Reliable Multicast (NORM) reliable
   multicast transport protocols.

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 September 27, 2013.

Copyright Notice

   Copyright (c) 2013 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



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   described in the Simplified BSD License.


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  . . . . . . . . . . .  9
   3.  FCAST Principles . . . . . . . . . . . . . . . . . . . . . . . 12
     3.1.  FCAST Content Delivery Service . . . . . . . . . . . . . . 12
     3.2.  Compound Object and Meta-Data Transmission . . . . . . . . 12
     3.3.  Meta-Data Content  . . . . . . . . . . . . . . . . . . . . 13
     3.4.  Carousel Transmission  . . . . . . . . . . . . . . . . . . 14
     3.5.  Carousel Instance Descriptor Special Object  . . . . . . . 15
     3.6.  Compound Object Identification . . . . . . . . . . . . . . 16
     3.7.  FCAST Sender Behavior  . . . . . . . . . . . . . . . . . . 17
     3.8.  FCAST Receiver Behavior  . . . . . . . . . . . . . . . . . 18
   4.  Requirements for Compliant Implementations . . . . . . . . . . 19
     4.1.  Requirements Related to the Object Meta-Data . . . . . . . 19
     4.2.  Requirements Related to the Carousel Instance
           Descriptor (CID) . . . . . . . . . . . . . . . . . . . . . 21
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 21
     5.1.  Problem Statement  . . . . . . . . . . . . . . . . . . . . 21
     5.2.  Attacks Against the Data Flow  . . . . . . . . . . . . . . 22
       5.2.1.  Attacks Meant to Gain Access to Confidential
               Objects  . . . . . . . . . . . . . . . . . . . . . . . 22
       5.2.2.  Attacks Meant to Corrupt Objects . . . . . . . . . . . 22
     5.3.  Attacks Against the Session Control Parameters and
           Associated Building Blocks . . . . . . . . . . . . . . . . 24
       5.3.1.  Attacks Against the Session Description  . . . . . . . 24
       5.3.2.  Attacks Against the FCAST CID  . . . . . . . . . . . . 24
       5.3.3.  Attacks Against the Object Meta-Data . . . . . . . . . 25
       5.3.4.  Attacks Against the ALC/LCT and NORM Parameters  . . . 25
       5.3.5.  Attacks Against the Associated Building Blocks . . . . 25
     5.4.  Other Security Considerations  . . . . . . . . . . . . . . 26
     5.5.  Minimum Security Recommendations . . . . . . . . . . . . . 26
   6.  Operational Considerations . . . . . . . . . . . . . . . . . . 27
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 28
     7.1.  Creation of the FCAST Object Meta-Data Format Registry . . 28
     7.2.  Creation of the FCAST Object Meta-Data Encoding
           Registry . . . . . . . . . . . . . . . . . . . . . . . . . 29
     7.3.  Creation of the FCAST Object Meta-Data Types Registry  . . 29
   8.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 30
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 31
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 31



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     9.2.  Informative References . . . . . . . . . . . . . . . . . . 32
   Appendix A.  FCAST Examples  . . . . . . . . . . . . . . . . . . . 33
     A.1.  Simple Compound Object Example . . . . . . . . . . . . . . 33
     A.2.  Carousel Instance Descriptor Example . . . . . . . . . . . 34
   Appendix B.  Additional Meta-Data Transmission Mechanisms  . . . . 34
     B.1.  Supporting Additional Mechanisms . . . . . . . . . . . . . 35
     B.2.  Using NORM_INFO Messages with FCAST/NORM . . . . . . . . . 35
       B.2.1.  Example  . . . . . . . . . . . . . . . . . . . . . . . 36
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 38










































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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.

   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.  For instance the
   delivery service provided by FCAST might be fully reliable, or only
   partially reliable depending upon the exact way FCAST is used.
   Indeed, if FCAST/ALC is used for a finite duration over purely
   unidirectional networks (where no feedback is possible), a fully
   reliable service may not be possible in practice.  This is different
   with NORM that can collect reception and loss feedbacks from
   receivers.  This is discussed in Section 6.

   The delivery service provided by FCAST might also differ in terms of
   scalability with respect to the number of receivers.  The FCAST/ALC
   service is naturally massively scalable since neither FCAST nor ALC
   limit the number of receivers (there is no feedback message at all).
   On the opposite FCAST/NORM scalability is typically limited by NORM
   itself as NORM relies on feedback messages from the receivers.
   However NORM is designed in such a way to offer a reasonably scalable
   service (e.g. through the use of proactive Forward Erasure
   Corrections (FEC) codes), and so does the service provided by FCAST/
   NORM.  This aspect is also discussed in Section 6.

   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.




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   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.

   Finally, Section 4 provides guidance for compliant implementation 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 definitions:

   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:  denotes an ALC or NORM transport object composed of
          the FCAST Header and the Object Data (some Compound Objects
          may not include any Object Data);

   FCAST Header:  denotes the header prepended to the Object Data, that
          together form the Compound Object.  This FCAST Header usually
          contains the Object meta-data, among other things;

   Object Data:  denotes the original object (e.g., a file) that forms
          the payload of the Compound Object;






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   Carousel:  denotes the building block that enables an FCAST sender to
          transmit Compound Objects in a cyclic manner;

   Carousel Instance:  denotes a 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 Descriptor (CID):  denotes a special object that
          lists the Compound Objects that comprise a given Carousel
          Instance;

   Carousel Instance IDentifier (CIID):  numeric value that identifies a
          Carousel Instance.

   Carousel Cycle:  denotes 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 Identifier (TOI):  denotes the numeric identifier
          associated to a specific 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 Transmission Information (FEC OTI):  FEC information
          associated with an object and that is essential for the FEC
          decoder to decode a specific object.


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 FCAST Header (which usually contains the meta-data of the object)
   to the Object Data (see Section 3.2).  Figure 1 illustrates the
   associated format.  All multi-byte fields MUST be in network (Big
   Endian) byte order.








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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            |  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
    |                      FCAST Header Length                      |  h
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|  d
    |               Object Meta-Data (variable length)              |  r
    |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
    |                               |      Padding (optional)       |  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  v
    |                                                               |
    .          Object Data (optional, variable length)              .
    .                                                               .
    .                                                               .

                     Figure 1: Compound Object Format.

   The FCAST Header fields are:

   +------------+------------------------------------------------------+
   |      Field | Description                                          |
   +------------+------------------------------------------------------+
   |    Version | 3-bit field that MUST be set to 0 in this            |
   |            | specification and indicates the FCAST protocol       |
   |            | version number.                                      |
   |   Reserved | 3-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     |
   |            | FCAST header.                                        |
   |          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 field (see Section 7).  An HTTP/1.1 meta   |
   |    (MDFmt) | information format [RFC2616] MUST be supported and   |
   |            | is associated to value 0.  Other formats (e.g., XML) |
   |            | may be defined in the future.                        |








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   |  Meta-Data | 4-bit field that defines the optional encoding of    |
   |   Encoding | the Object Meta-Data field (see Section 7).  Two     |
   |    (MDEnc) | values are currently defined.  A value of 0          |
   |            | indicates that the field contains UTF-8 [RFC2279]    |
   |            | encoded text.  A value of 1 indicates that the field |
   |            | contains GZIP[RFC1952] compressed UTF-8 encoded      |
   |            | text.                                                |
   |   Checksum | 16-bit field that contains the checksum computed     |
   |            | over either the whole Compound Object (when G is set |
   |            | to 1), or over the FCAST 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.                                 |
   |      FCAST | 32-bit field indicating total length (in bytes) of   |
   |     Header | all fields of the FCAST Header, except the optional  |
   |     Length | padding.  A header length field set to value 8 means |
   |            | that there is no meta-data included.  When this size |
   |            | is not multiple to 32-bits words and when the FCAST  |
   |            | Header is followed by a non null Object Data,        |
   |            | padding MUST be added.  It should be noted that the  |
   |            | meta-data field maximum size is equal to (2^32 - 8)  |
   |            | bytes.                                               |
   |     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 format and       |
   |            | encoding, the Object Meta-Data field, if not empty,  |
   |            | MUST contain a UTF-8 encoded text that follows the   |
   |            | "TYPE" ":" "VALUE" "<CR-LF>" format used in HTTP/1.1 |
   |            | for meta information [RFC2616].  The various         |
   |            | meta-data items can appear in any order.  The        |
   |            | receiver MUST NOT assume this string is              |
   |            | NULL-terminated.  When no meta-data is communicated, |
   |            | this field MUST be empty and the FCAST Header Length |
   |            | MUST be equal to 8.                                  |









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   |    Padding | Optional, variable length field of zero-value bytes  |
   |            | to align the start of the Object Data to a 32-bit    |
   |            | boundary.  Padding is only used when the FCAST       |
   |            | Header Length value, in bytes, is not multiple of 4  |
   |            | and when the FCAST Header is followed by non null    |
   |            | Object Data.                                         |
   +------------+------------------------------------------------------+

   The FCAST Header is then followed by the Object Data, i.e., either an
   original object (possibly encoded by FCAST) or a CID.  Note that the
   length of the Object Data content is the ALC or NORM transported
   object length (e.g., as specified by the FEC OTI) minus the FCAST
   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.

     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            |  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
    |                      FCAST 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 and MUST be
   supported:





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   o  Fcast-CID-Complete: this is an optional entry that, when set to
      "Fcast-CID-Complete: 1", indicates no new object (if we ignore CID
      Compound Objects) in addition to the ones whose TOI are listed in
      this CID or the ones that have been listed in the previous CID(s),
      will be sent in the future.  Conversely, if it is set to "Fcast-
      CID-Complete: 0", or if this entry is absent, it indicates that
      the session is not complete..  An FCAST sender MUST NOT use any
      other value for this entry.

   o  Fcast-CID-ID: this entry contains the Carousel Instance
      IDentifier, or CIID.  It starts from 0 upon FCAST session creation
      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 (no need for the FCAST sender to
      specify it and for the FCAST receiver to process it).  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 following standard meta-data entry types are also used
   (Section 3.3):

   o  Content-Length: it specifies the size in bytes 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.

   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 ALC or NORM transported object
   length (e.g., as specified by the FEC OTI) minus the FCAST Header
   Length equals zero.

   The Object List, when non empty and with MDEnc=0, is a UTF-8 encoded
   text that is not necessarily NULL-terminated.  It can contain two
   things:





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   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.  Individual 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
   a delimiting equals sign '=' 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 relisted 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



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   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)


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.  Compound Object and 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 an FCAST Header followed by the Object Data (Figure 3).
   This header is itself composed of the object meta-data (if any) as
   well as several fields (e.g., to indicate format, encoding or



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   boundaries Section 2.1).

     <------------------------ Compound Object ----------------------->
     +-------------------------+--------------------------------------+
     |       FCAST 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
   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

   The following meta-data types are 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
      in bytes of the initial object, before any content encoding (if
      any) and without considering the FCAST 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.  For instance:
           Content-Encoding: gzip
      indicates that the object has been encoded with GZIP [RFC1952].
      If there is no Content-Encoding entry, the receiver MUST assume
      that FCAST did not modify the original encoding of the object
      (default).

   The following additional meta-data type is defined to check object
   integrity:



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   o  Fcast-Obj-Digest-SHA256: a string that contains the "base64"
      [RFC4648] encoding of the SHA-256 message digest of the object
      [RFC3174][RFC6234], before any content encoding is applied (if
      any) and without considering the FCAST Header.  This digest is
      meant to protect from transmission and processing errors, not from
      deliberate attacks by an intelligent attacker (see Section 5).
      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);

   o  Fcast-Obj-Digest-SHA1: it is similar to Fcast-Obj-Digest-SHA256
      with the only exception that SHA-256 is replaced by SHA-1.  An
      FCAST sender MAY include both an Fcast-Obj-Digest-SHA1 and an
      Fcast-Obj-Digest-SHA256 message digest in the meta-data, in order
      to let a receiver select the most appropriate algorithm (e.g.,
      depending on local processing power);

   The following additional 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);

   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



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   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: 1"
   meta-data entry 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
   standard object.  On the contrary, the nature of this object (CID) is
   indicated by means of a specific FCAST 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 "Fcast-CID-
      Complete: 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;




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   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) is signaled within the CID.

   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
      mechanism.  If no "Fcast-CID-Complete" meta-data entry is included
      (or if set to "Fcast-CID-Complete: 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 protocol, the packets containing transport object content are
   labeled with a numeric transport object identifier: the TOI with ALC,
   and the NormTransportId with NORM.  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);




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   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 eventually wraps 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 associated to the previously-
   assigned TOI, a mechanism is required to equate the objects with the
   new and the old TOIs.  This 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; CIID} tuple used for its first transmission within a Carousel
   Instance.  This tuple MUST be used whenever a TOI equivalence is
   provided.  Section 2.2 details how to describe these TOI
   equivalences.

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);




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   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
          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 FCAST Header, the receiver decodes the CID, and retrieves the
       list of objects that are part of the current carousel instance.



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       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 'Fcast-CID-Complete: 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.  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.

4.1.  Requirements Related to the Object Meta-Data

   Meta-data transmission mechanisms:

   +-----------------------+-------------------------------------------+
   | 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 FCAST Header.  All the   |
   | mechanism             | FCAST receivers MUST be able to process   |
   |                       | meta-data sent with this FCAST in-band    |
   |                       | mechanism.                                |















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   | 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-band.  See Appendix B.                 |
   +-----------------------+-------------------------------------------+

   Meta-data format and encoding:

   +-----------------------+-------------------------------------------+
   | Feature               | Status                                    |
   +-----------------------+-------------------------------------------+
   | Meta-Data Format      | All FCAST implementations MUST support an |
   | (MDFmt field)         | HTTP/1.1 meta information format          |
   |                       | [RFC2616].                                |
   | Meta-Data Encoding    | All FCAST implementations MUST support    |
   | (MDEnc field)         | both UTF-8 encoded text and a GZIP        |
   |                       | compressed [RFC1952] of UTF-8 encoded     |
   |                       | text, for the Object Meta-Data field.     |
   +-----------------------+-------------------------------------------+

   Meta-data items (Section 3.3):

   +-------------------------+-----------------------------------------+
   | Feature                 | Status                                  |
   +-------------------------+-----------------------------------------+
   | Content-Location        | MUST be supported                       |
   | Content-Type            | MUST be supported                       |
   | Content-Length          | MUST be supported                       |
   | Content-Encoding        | MUST be supported.  All FCAST           |
   |                         | implementations MUST support GZIP       |
   |                         | encoding [RFC1952]                      |
   | Fcast-Obj-Digest-SHA1   | MUST be supported                       |
   | Fcast-Obj-Digest-SHA256 | MUST be supported                       |
   | Fcast-Obj-Slice-Nb      | MUST be supported                       |
   | Fcast-Obj-Slice-Idx     | MUST be supported                       |
   | Fcast-Obj-Slice-Offset  | MUST be supported                       |
   +-------------------------+-----------------------------------------+






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4.2.  Requirements Related to the Carousel Instance Descriptor (CID)

   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.

   CID-specific Meta-data items (Section 2.2):

               +-----------------------+-------------------+
               | Feature               | Status            |
               +-----------------------+-------------------+
               | Fcast-CID-Complete    | MUST be supported |
               | Fcast-CID-ID          | MUST be supported |
               +-----------------------+-------------------+


5.  Security Considerations

5.1.  Problem Statement

   A content delivery system may be subject to attacks that target:

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

   o  the Content Delivery Protocol (CDP), to compromise the delivery
      mechanism (i.e., FCAST in this case), or

   o  the content itself, to corrupt the objects being transmitted.

   These attacks can be launched against all or any subset of the
   following:

   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).

   More details on these possible attacks are provided in the following
   sections along with possible counter-measures.  Recommendations are
   made in Section 5.5.





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5.2.  Attacks Against the Data Flow

   The following types of attacks exist against the data flow:

   o  attacks that are meant to gain unauthorized access to a
      confidential object (e.g., obtaining non-free content without
      purchasing it) 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)).

5.2.1.  Attacks Meant to Gain Access to Confidential Objects

   Modern cryptographic mechanisms can provide access control to
   transmitted objects.  One way to do this is by encrypting the entire
   object prior to transmission, knowing that authenticated receivers
   have the cryptographic mechanisms to decrypt the content.  Another
   way is to encrypt individual packets using IPsec/ESP [RFC4303]
   (Section 5.5).  These two techniques can also provide confidentiality
   to the objects being transferred.

   If access control and/or confidentiality services are desired, one of
   these mechanisms is RECOMMENDED and SHOULD be deployed.

5.2.2.  Attacks Meant to Corrupt Objects

   Protection against attacks on the data integrity of the object may be
   achieved by a mechanism agreed upon between the sender and receiver,
   that features sender authentication and a method to verify that the
   object integrity has remained intact during transmission.  This
   service can be provided at the object level, but in that case a
   receiver has no way to identify what symbols are corrupted if the
   object is detected as corrupted.  This service can also be provided
   at the packet level.  In some cases, after removing all corrupted
   packets, the object may be recovered.  Several techniques can provide
   the data integrity and sender authentication services:

   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.  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;




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   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 preliminary check to
      quickly detect attacks initiated by non-group members and discard
      their packets;

   o  at the packet level, Timed Efficient Stream Loss-Tolerant
      Authentication (TESLA) [RFC4082][RFC5776] is an attractive
      solution that is robust to losses, provides an authentication and
      integrity verification service, and does not create any
      prohibitive processing load or transmission overhead.  Yet, a
      delay is incurred in checking a TESLA authenticated packet which
      may be more than what is desired in some use-cases;

   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 5.5).

   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
   securely preplacing 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
   securely preplacing 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 a threat of object corruption, it is RECOMMENDED that at least one
   of these techniques be used.  Section 5.5 defines minimum security
   recommendations that can be used to provide such services.






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5.3.  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 the content delivery system or even
   compromise the network itself.

5.3.1.  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 will be
   unable to correctly receive the session content, or that receivers
   will 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 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.

5.3.2.  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:
   1" 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



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   purpose, one of the counter-measures mentioned above (Section 5.2.2)
   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.

5.3.3.  Attacks Against the Object Meta-Data

   Modifying the object meta-data is another way to launch an attack.
   For example, the attacker may change the message digest associated to
   an object, leading a receiver to reject an object, even if it has
   been correctly received.  More generally, a receiver SHOULD be very
   careful during meta-data processing.  For instance a receiver SHOULD
   NOT try to follow links (e.g., the URI contained in th Content-
   Location meta-data).  As another example, malformed HTTP contents can
   be used as an attack vector and a receiver should take great care.

   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 5.2.2) 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.

5.3.4.  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 5.2.2) SHOULD be used.

5.3.5.  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.



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   When congestion control is applied with FCAST, it is therefore
   RECOMMENDED that receivers be authenticated as legitimate receivers
   before they can join the session.  If authenticating a receiver does
   not prevent this latter to launch an attack, it will enable the
   network operator to easily identify him and to take counter-measures.
   The details of how this 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 5.2.2.  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.

5.4.  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.

5.5.  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].  Therefore, IPsec/
   ESP in transport mode MUST be implemented, but not necessarily used,
   in accordance to [RFC5775] and [RFC5740].  [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



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   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.


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 5).

   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),

   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 a
   very large number of receivers.  While the choice of the underlying



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   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 servers or feedback
   message aggregation.  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 three 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
   the object meta-data (see Section 2.1).

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








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

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 defines the encoding of
   the Object Meta-Data field (see Section 2.1).

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

   +-------+---------------------+--------------------+----------------+
   | Value |    Encoding Name    | Encoding Reference |    Reference   |
   +-------+---------------------+--------------------+----------------+
   |   0   |  UTF-8 encoded text |      [RFC2279]     |      This      |
   |       |                     |                    |  specification |
   |   1   |    GZIP'ed UTF-8    | [RFC1952][RFC2279] |      This      |
   |       |     encoded text    |                    |  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-Digest-SHA1   | a string that contains  |      This     |
   |                         | the "base64" encoding   | specification |
   |                         | of the SHA-1 message    |               |
   |                         | digest of the object    |               |
   |                         | before any content      |               |
   |                         | encoding is applied (if |               |
   |                         | any) and without        |               |
   |                         | considering the FCAST   |               |
   |                         | Header                  |               |
   | Fcast-Obj-Digest-SHA256 | a string that contains  |      This     |
   |                         | the "base64" encoding   | specification |
   |                         | of the SHA-256 message  |               |
   |                         | digest of the object    |               |
   |                         | before any content      |               |
   |                         | encoding is applied (if |               |
   |                         | any) and without        |               |
   |                         | considering the FCAST   |               |
   |                         | Header                  |               |
   | 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.  The authors are also grateful to Jari Arkko,
   Ralph Droms, Wesley Eddy, Roni Even, Stephen Farrell, Russ Housley,



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   Chris Lonvick, Pete Resnick, Joseph Yee, and Martin Stiemerling.


9.  References

9.1.  Normative References

   [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.

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

   [RFC2279]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", RFC 2279, January 1998.

   [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.

   [RFC3174]  Eastlake, D. and P. Jones, "US Secure Hash Algorithm 1
              (SHA1)", RFC 3174, September 2001.

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, October 2006.

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

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

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



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   [RFC6234]  Eastlake, D. and T. Hansen, "US Secure Hash Algorithms
              (SHA and SHA-based HMAC and HKDF)", RFC 6234, May 2011.

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.

   [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.

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

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

   [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.
              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.




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

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


Appendix A.  FCAST Examples

   This appendix provides informative examples of FCAST Compound Objects
   and Carousel Instance Descriptor formats.

A.1.  Simple 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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |Ver=0|  0  |1|0|MDFmt=0|MDEnc=0|           Checksum            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       Header Length=41                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
     .                                                               .
     . "Content-Location: example_1.txt<CR-LF>" meta-data (33 bytes) .
     .                                                               .
     +               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |               |                    Padding                    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     .                                                               .
     .                         Object data                           .
     .                                                               .
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                 Figure 4: Simple Compound Object Example.

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

   Content-Location: example_1.txt<CR-LF>

   This UTF-8 encoded text (since MDEnc=0) is 33 bytes long (there is no
   final '\0' character).  Therefore 3 padding bytes are added.  There
   is no Content-Length meta-data entry for the object transported
   (without FCAST additional encoding) in the Object Data field, 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).



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A.2.  Carousel Instance Descriptor 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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |Ver=0|  0  |1|1|MDFmt=0|MDEnc=0|           Checksum            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       Header Length=31                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
     .                                                               .
     .   "Fcast-CID-Complete: 1<CR-LF>" meta-data string (23 bytes)  .
     .                                                               .
     +                                               +-+-+-+-+-+-+-+-+
     |                                               |    Padding    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
     .                                                               .
     .                Object List string                             .
     .                                                               .
     .               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     .               |
     +-+-+-+-+-+-+-+-+

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

   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.  The meta-data UTF-8 encoded text only contains the following
   entry since Fcast-CID-ID is implicit:

   Fcast-CID-Complete: 1<CR-LF>

   This UTF-8 encoded text (since MDEnc=0) is 23 bytes long (there no
   final '\0' character).  Therefore 1 padding byte is added.

   The object list contains the following 25 byte long string, (there is
   no final '\0' 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





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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
   carrying FCAST 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
   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 4).  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



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   allows a receiver to quickly (and independently) request an object's
   NORM_INFO content.  However, a limitation here is that the FCAST
   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 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 Object Data.  In the following informative
   example we assume that the whole meta-data is carried in such a
   message.  Figure 6 shows an example NORM_INFO message that contains
   the FCAST Header, including meta-data.  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 Header, including meta-data.  Note that "padding" MUST NOT
   be used and that the FCAST checksum only encompasses the Compound
   Object Header (G=0).































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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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ --
    |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 UTF-8 encoded text (32 bytes)            .  s
    .                                                               .  t
    +               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
    |               |                                                  v
    +-+-+-+-+-+-+-+-+                                                 --

     Figure 6: NORM_INFO containing an EXT_FTI header extension and an
                               FCAST 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 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 Compound
   Object, i.e., 41 bytes.

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



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   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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