[Docs] [txt|pdf] [draft-ietf-rmt-fcast] [Diff1] [Diff2]

EXPERIMENTAL

Internet Engineering Task Force (IETF)                           V. Roca
Request for Comments: 6968                                         INRIA
Category: Experimental                                        B. Adamson
ISSN: 2070-1721                                Naval Research Laboratory
                                                               July 2013


  FCAST: Object Delivery for the Asynchronous Layered Coding (ALC) and
           NACK-Oriented Reliable Multicast (NORM) Protocols

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 Layered Coding (ALC) / Layered Coding
   Transport (LCT) reliable multicast transport protocol or the NACK-
   Oriented Reliable Multicast (NORM) transport protocol.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for examination, experimental implementation, and
   evaluation.

   This document defines an Experimental Protocol for the Internet
   community.  This document is a product of the Internet Engineering
   Task Force (IETF).  It represents the consensus of the IETF
   community.  It has received public review and has been approved for
   publication by the Internet Engineering Steering Group (IESG).  Not
   all documents approved by the IESG are a candidate for any level of
   Internet Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6968.
















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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
   described in the Simplified BSD License.

Table of Contents

   1. Introduction ....................................................3
      1.1. Requirements Notation ......................................4
      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 Metadata Transmission .................13
      3.3. Metadata Content ..........................................13
      3.4. Carousel Transmission .....................................15
      3.5. Carousel Instance Descriptor Special Object ...............15
      3.6. Compound Object Identification ............................17
      3.7. FCAST Sender Behavior .....................................18
      3.8. FCAST Receiver Behavior ...................................19
   4. Requirements for Compliant Implementations .....................20
      4.1. Requirements Related to the Object Metadata ...............20
      4.2. Requirements Related to the Carousel Instance Descriptor ..21
   5. Security Considerations ........................................22
      5.1. Problem Statement .........................................22
      5.2. Attacks against the Data Flow .............................22
           5.2.1. Attacks Meant to Gain Access to
                  Confidential Objects ...............................23
           5.2.2. Attacks Meant to Corrupt Objects ...................23
      5.3. Attacks against the Session Control Parameters and
           Associated Building Blocks ................................24
           5.3.1. Attacks against the Session Description ............25
           5.3.2. Attacks against the FCAST CID ......................25
           5.3.3. Attacks against the Object Metadata ................25
           5.3.4. Attacks against the ALC/LCT and NORM Parameters ....26
           5.3.5. Attacks against the Associated Building Blocks .....26



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      5.4. Other Security Considerations .............................27
      5.5. Minimum Security Recommendations ..........................27
   6. Operational Considerations .....................................28
   7. IANA Considerations ............................................29
      7.1. Creation of the FCAST Object Metadata Format Registry .....29
      7.2. Creation of the FCAST Object Metadata Encoding Registry ...30
      7.3. Creation of the FCAST Object Metadata Types Registry ......30
   8. Acknowledgments ................................................32
   9. References .....................................................32
      9.1. Normative References ......................................32
      9.2. Informative References ....................................33
   Appendix A. FCAST Examples ........................................35
     A.1. Simple Compound Object Example .............................35
     A.2. Carousel Instance Descriptor Example .......................36
   Appendix B. Additional Metadata Transmission Mechanisms ...........37
     B.1. Supporting Additional Mechanisms ...........................37
     B.2. Using NORM_INFO Messages with FCAST/NORM ...................38
       B.2.1. Example ................................................38

1.  Introduction

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

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

   o  FCAST/NORM, which relies on the NACK-Oriented Reliable Multicast
      (NORM) [RFC5740] 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 under which 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, which can collect reception and loss feedback
   from receivers.  This is discussed in Section 6.





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   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
   limits the number of receivers (there is no feedback message at all).
   Conversely, the scalability of FCAST/NORM 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 Error
   Correction (FEC) codes [RFC6363]), 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
   to limit the operational processing and storage requirements.  A
   consequence of this choice is that FCAST cannot be considered 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 the File Delivery over
   Unidirectional Transport (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 metadata: by default, the metadata 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 metadata.  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].








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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 transport protocol.

   FCAST/NORM:  denotes the FCAST application running on top of the NORM
      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, which
      together form the Compound Object.  This FCAST Header usually
      contains the object metadata, among other things.

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

   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, which
      might differ on a per-object basis, are possible.








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   Transport Object Identifier (TOI):  denotes the numeric identifier
      associated with 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 metadata 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.

    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 Metadata (variable length)               |  r
   |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
   |                               |      Padding (optional)       |  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  v
   |                                                               |
   .          Object Data (optional, variable length)              .
   .                                                               .
   .                                                               .

                     Figure 1: Compound Object Format












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   The FCAST Header fields are:

   +------------+------------------------------------------------------+
   |      Field | Description                                          |
   +------------+------------------------------------------------------+
   |    Version | 3-bit field that MUST be set to 0 in this            |
   |            | specification and that 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 that the  |
   |            | object is a CID.  When set to 0, this field          |
   |            | indicates that the transported object is a standard  |
   |            | object.                                              |
   |            |                                                      |
   |   Metadata | 4-bit field that defines the format of the Object    |
   |     Format | Metadata field (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.                   |
   |            |                                                      |
   |   Metadata | 4-bit field that defines the optional encoding of    |
   |   Encoding | the Object Metadata field (see Section 7).  Two      |
   |    (MDEnc) | values are currently defined.  A value of 0          |
   |            | indicates that the field contains UTF-8 encoded      |
   |            | [RFC3629] text.  A value of 1 indicates that the     |
   |            | field contains GZIP [RFC1952] compressed UTF-8       |
   |            | encoded text.                                        |
   |            |                                                      |













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   |   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.  An FCAST Header Length field set to value  |
   |            | 8 means that there is no metadata included.  When    |
   |            | this size is not a multiple of 32-bit words and when |
   |            | the FCAST Header is followed by non-null Object      |
   |            | Data, padding MUST be added.  It should be noted     |
   |            | that the Object Metadata field maximum size is equal |
   |            | to (2^32 - 8) bytes.                                 |
   |            |                                                      |
   |     Object | Variable-length field that contains the metadata     |
   |   Metadata | associated to the object.  The format and encoding   |
   |            | of this field are defined by the MDFmt and MDEnc     |
   |            | fields, respectively.  With the default format and   |
   |            | encoding, the Object Metadata field, if not empty,   |
   |            | MUST contain UTF-8 encoded text that follows the     |
   |            | "TYPE" ":" "VALUE" "<CR-LF>" format used in HTTP/1.1 |
   |            | for metainformation [RFC2616].  The various          |
   |            | metadata items can appear in any order.  The         |
   |            | receiver MUST NOT assume that this string is NULL-   |
   |            | terminated.  When no metadata is communicated, this  |
   |            | field MUST be empty and the FCAST 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 a 32-bit    |
   |            | boundary.  Padding is only used when the FCAST       |
   |            | Header Length value, in bytes, is not a multiple of  |
   |            | 4 and when the FCAST Header is followed by non-null  |
   |            | Object Data.                                         |
   +------------+------------------------------------------------------+






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   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 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 Metadata (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 metadata entries are defined and MUST be
   supported:

   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 TOIs 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.




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   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 for detecting possible gaps in the Carousel
      Instances, for instance, gaps caused by long disconnection
      periods.  Finally, it can also be useful for avoiding problems
      when TOI wrapping to 0 takes place to differentiate the various
      incarnations of the TOIs if need be.

   The following standard metadata entry types are also used
   (Section 3.3):

   o  Content-Length: specifies the size in bytes of the Object List,
      before any content encoding (if any).

   o  Content-Encoding: 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 metadata 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 UTF-8-encoded
   text that is not necessarily NULL-terminated.  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.  Individual TOIs consist of a single integer
   value, while TOI intervals are a hyphen-delimited pair of TOI values



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   to indicate an 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] is as follows:

   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.






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   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 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
       transport protocol, which requires positive/negative
       acknowledgments 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.

















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3.2.  Compound Object and Metadata Transmission

   FCAST carries metadata 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 metadata (if any) as well as
   several fields (e.g., to indicate format, encoding, or boundaries
   (Section 2.1)).

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

                   Figure 3: Compound Object Composition

   Attaching the metadata to the object is an efficient solution, since
   it guarantees that metadata are received along with the associated
   object, and it allows the transport of the metadata 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 metadata of an object
   before beginning its reception, and in the case of erasures affecting
   the metadata, 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 metadata, e.g., to communicate metadata ahead of time.

3.3.  Metadata Content

   The following metadata 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.






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   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 metadata types are defined to check object
   integrity:

   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 metadata.  A separate checksum is provided for
      that purpose (Section 2.1).

   o  Fcast-Obj-Digest-SHA1: similar to Fcast-Obj-Digest-SHA256, except
      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 metadata, in order to let a receiver select
      the most appropriate algorithm (e.g., depending on local
      processing power).

   The following additional metadata types are used for dealing with
   very large objects (e.g., objects that largely exceed the working
   memory of a receiver).  When this happens, the metadata 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 IANA assignments to extend the set of metadata types supported
   by FCAST are to be made through Expert Review [RFC5226].




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3.4.  Carousel Transmission

   A set of FCAST Compound Objects scheduled for transmission is
   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, a
   sender and receiver(s) are both capable of multiplexing and
   demultiplexing 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 than 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 CID.  The CID carries the
   list of the Compound Objects that are part of a given Carousel
   Instance by specifying their respective Transport Object Identifiers
   (TOIs).  However, the CID does not describe the objects themselves
   (i.e., there is no metadata).  Additionally, the CID MAY include an
   "Fcast-CID-Complete: 1" metadata 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 that since a
   receiver does not know in advance which TOI will be used for the
   following CID (in the case of a dynamic session), it MUST NOT filter




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   out packets that are not in the current CID's TOI list.  Said
   differently, the goal of the CID is not to set up 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" metadata 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.

   o  In the 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 "B" flag of ALC/LCT, since a client
      with intermittent connectivity might lose 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.  This therefore improves the robustness of FCAST even
      during "idle" periods.

   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" metadata entry is included
      (or if set to "Fcast-CID-Complete: 0"), it informs the receivers
      that the Carousel Instance may be modified and that new objects
      could be sent in the future.







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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  ALC's use of the 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
      management of the TOI is totally free as long as each object is
      associated to a unique TOI (if no wraparound occurred).

   o  NORM's use of the TOI serves a more "directive" purpose, since the
      TOI field is 16 bits 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 the
   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 of 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.





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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 metadata, to
       the FCAST application.

   2.  For each object, FCAST creates the Compound Object and registers
       it 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 "Fcast-CID-Complete: 1" if
       no new objects will be sent in the future.

   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.





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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 metadata is sent by means of
       another mechanism prior to the object, the receiver processes the
       metadata and chooses whether or not to continue to receive the
       object content.

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

   4.  When a CID is received, as 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.  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"
       metadata 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.















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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 Metadata

   Metadata transmission mechanisms:

   +------------------+------------------------------------------------+
   | Feature          | Status                                         |
   +------------------+------------------------------------------------+
   | metadata         | An FCAST sender MUST send metadata with the    |
   | transmission     | in-band mechanism provided by FCAST, i.e.,     |
   | using FCAST's    | within the FCAST Header.  All the FCAST        |
   | in-band          | receivers MUST be able to process metadata     |
   | mechanism        | sent with this FCAST in-band mechanism.        |
   |                  |                                                |
   | metadata         | In addition to the FCAST in-band transmission  |
   | transmission     | of metadata, an FCAST sender MAY send a subset |
   | using other      | or all of the metadata using another           |
   | mechanisms       | 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 metadata sent in this way.  If that is     |
   |                  | not the case, the FCAST receiver will receive  |
   |                  | and process metadata sent in-band anyway.      |
   |                  | See Appendix B.                                |
   +------------------+------------------------------------------------+

   Metadata format and encoding:

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





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   Metadata 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.                |
   +-------------------------------+-----------------------------------+

4.2.  Requirements Related to the Carousel Instance Descriptor

   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 Metadata items (Section 2.2):

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











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

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

   o  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 countermeasures.  Recommendations are
   made in Section 5.5.

5.2.  Attacks against the Data Flow

   The following types of attacks against the data flow exist:

   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; this would be a denial-of-service (DoS)
      attack).









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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] (see
   also 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
   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.

   o  At the packet level, a Group-keyed 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 itself.  The Group-keyed MAC scheme does not
      create prohibitive processing loads or 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; this means that each member can send a
      forged packet.  It is therefore restricted to situations where




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      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 to 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; this
      delay 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
   via a Public Key Infrastructure (PKI), a Pretty Good Privacy (PGP)
   Web of Trust, or by securely preplacing the public keys of each group
   member.

   Techniques relying on symmetric key cryptography (Group-keyed 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.

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 can target, among
   other things, the following:

   o  the session description,

   o  the FCAST CID,

   o  the metadata of an object,



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   o  the ALC/LCT parameters, carried within the LCT header, or

   o  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 is that legitimate
   receivers with the wrong session description will be unable to
   correctly receive the session content or 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 DoS attack.  For
   example, the attacker can insert an "Fcast-CID-Complete: 1" metadata
   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 countermeasures mentioned above (Section 5.2.2)
   SHOULD be used.  These measures will either be applied at the 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 Metadata

   Modifying the object metadata 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 metadata processing.  For instance, a receiver SHOULD
   NOT try to follow links (e.g., the URI contained in the



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   Content-Location metadata).  As another example, malformed HTTP
   content can be used as an attack vector, and a receiver should take
   great care with such content.

   It is therefore RECOMMENDED that measures be taken to guarantee the
   integrity and to check the identity of the sender of the Compound
   Object.  To that purpose, one of the countermeasures mentioned above
   (Section 5.2.2) SHOULD be used.  These measures will either be
   applied at the 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 "A" flag
   set to 1 can lead the receiver to prematurely close the session.
   Similarly, sending forged ALC packets with the Close Object "B" flag
   set to 1 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 in each ALC or NORM
   packet received.  To that purpose, one of the countermeasures
   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 and 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 authenticated as legitimate receivers
   before they can join the session.  If authenticating a receiver does
   not prevent that receiver from launching an attack, the network
   operator will still be able to easily identify the receiver that
   launched the attack and take countermeasures.  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



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   reaction.  It is therefore RECOMMENDED [RFC5775] that a receiver
   using TESLA quickly reduce its subscription level when the receiver
   believes that 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 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 might be preferable, or a Group-
   keyed MAC scheme could be used in parallel with 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 apply to FCAST.  Finally, the security discussion of
   [RMT-SEC] also applies here.

5.5.  Minimum Security Recommendations

   We now introduce a security configuration that is mandatory to
   implement but not necessarily mandatory to use, 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 with
   [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
   that an automated key management scheme be used, especially in the
   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.






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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 might
   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, are 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 deployed (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
   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 scalability is at a maximum, since
   neither FCAST, ALC, UDP, nor IP generates any feedback message.  On
   the contrary, the scalability of FCAST/NORM is typically limited by
   the scalability of NORM 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,



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   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 CID (Section 3.5).  The decision of whether the CID
   mechanism should be used or not is left to the sender.  When it is
   used, the question of how often and when a CID should be sent is also
   left to the sender.  These considerations 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

   Per this specification, IANA has created three new registries.

7.1.  Creation of the FCAST Object Metadata Format Registry

   IANA has created a new registry, "FCAST Object Metadata 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 defines the format of the object
   metadata (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].

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









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7.2.  Creation of the FCAST Object Metadata Encoding Registry

   IANA has created a new registry, "FCAST Object Metadata 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
   Metadata 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   |    [RFC3629]    | This specification |
   |         |       text       |                 |                    |
   |         |                  |                 |                    |
   |    1    |  GZIP'ed UTF-8   |    [RFC1952],   | This specification |
   |         |   encoded text   |    [RFC3629]    |                    |
   +---------+------------------+-----------------+--------------------+

7.3.  Creation of the FCAST Object Metadata Types Registry

   IANA has created a new registry, "FCAST Object Metadata Types"
   (MDType), with a reference to this document.  The registry entries
   consist of additional text metadata type identifiers and descriptions
   for metadata item types that are specific to FCAST operation and not
   previously defined in [RFC2616].  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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   +-------------------------+-----------------------+-----------------+
   | Metadata Type           | Description           |    Reference    |
   +-------------------------+-----------------------+-----------------+
   | Fcast-Obj-Digest-SHA1   | A string that         |       This      |
   |                         | contains the "base64" |  specification  |
   |                         | encoding 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         |       This      |
   |                         | contains the "base64" |  specification  |
   |                         | encoding 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       |       This      |
   |                         | integer that contains |  specification  |
   |                         | the total 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       |       This      |
   |                         | integer that contains |  specification  |
   |                         | the slice index (in   |                 |
   |                         | the {0 .. SliceNb -   |                 |
   |                         | 1} interval)          |                 |
   |                         |                       |                 |
   | Fcast-Obj-Slice-Offset  | Unsigned 64-bit       |       This      |
   |                         | integer that contains |  specification  |
   |                         | the byte offset at    |                 |
   |                         | which this slice      |                 |
   |                         | starts within the     |                 |
   |                         | original object       |                 |
   +-------------------------+-----------------------+-----------------+




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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,
   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., "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.

   [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.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of
              ISO 10646", STD 63, RFC 3629, November 2003.

   [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.






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   [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.

   [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, J., Vicisano, L., Rizzo, L., Crowcroft,
              J., and B. Lueckenhoff, "Asynchronous Layered Coding: A
              scalable reliable multicast protocol", Work in Progress,
              March 2000.

   [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.







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   [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.

   [RFC6363]  Watson, M., Begen, A., and V. Roca, "Forward Error
              Correction (FEC) Framework", RFC 6363, October 2011.

   [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.

   [RMT-SEC]  Adamson, B., Roca, V., and H. Asaeda, "Security and
              Reliable Multicast Transport Protocols: Discussions and
              Guidelines", Work in Progress, May 2013.






























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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            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     FCAST Header Length=41                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
   .                                                               .
   . "Content-Location: example_1.txt<CR-LF>" metadata (33 bytes)  .
   .                                                               .
   +               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               |                    Padding                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   .                                                               .
   .                         Object Data                           .
   .                                                               .
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                 Figure 4: Simple Compound Object Example

   Figure 4 shows a simple Compound Object where the metadata string, in
   HTTP/1.1 metainformation 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 metadata 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            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                   FCAST Header Length=31                      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
      .                                                               .
      .   "Fcast-CID-Complete: 1<CR-LF>" metadata string (23 bytes)   .
      .                                                               .
      +                                               +-+-+-+-+-+-+-+-+
      |                                               |    Padding    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
      .                                                               .
      .                Object List string                             .
      .                                                               .
      .               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      .               |
      +-+-+-+-+-+-+-+-+

               Figure 5: CID Object Example: 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 metadata 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 is 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 metadata
   associated to this CID.  As the session is static and composed of a
   single Carousel Instance, the sender did not feel the necessity to
   carry a Carousel Instance ID metadata.







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Appendix B.  Additional Metadata 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 mechanisms 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 metadata, or a subset of it, ahead of
   time, before the associated Compound Object.  Therefore, based on the
   information retrieved in this way, a receiver could decide in advance
   (i.e., before beginning the reception of the compound object) whether
   the object is of interest or not; this would mitigate the limitations
   of FCAST.  While out-of-band techniques are out of the scope of this
   document, we explain below how this can be achieved in the 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 metadata in the Compound Object, even if the whole
   metadata, or a subset of it, is sent by another mechanism
   (Section 4).  Additionally, when metadata is sent several times,
   there MUST NOT be any contradiction in the information provided by
   the different mechanisms.  If a mismatch is detected, the metadata
   contained in the Compound Object MUST be used as the definitive
   source.

   When metadata 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 Transport Object
      Identifier (TOI) of the object, in order to enable a receiver to
      easily associate the metadata to the object.  The valid range for
      TOI values is discussed in Section 3.6.















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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 metadata.  In that case,
   the NORM_INFO message carries a new Compound Object that contains all
   the metadata 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 a 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 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 the case of FCAST/NORM, the object metadata (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 metadata is carried in
   such a message.  Figure 6 shows an example NORM_INFO message that
   contains the FCAST Header, including metadata.  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 metadata.  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
   .            metadata UTF-8 encoded text (32 bytes)             .  s
   .                                                               .  t
   +               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
   |               |                                                  v
   +-+-+-+-+-+-+-+-+                                                 --

    Figure 6: NORM_INFO Message 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 metadata 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.











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   The Compound Object in this example does contain the same metadata
   and is formatted as in the example of Figure 4.  With the combination
   of the FEC_OTI and the FCAST metadata, 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,
   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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