One document matched: draft-ietf-rmt-fcast-07.xml
<?xml version="1.0" encoding="US-ASCII"?>
<!-- $Id: draft-ietf-rmt-fcast-06.xml 79 2012-10-10 07:31:11Z roca $ -->
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<front>
<title abbrev="FCAST: Scalable Object Delivery">FCAST: Scalable Object
Delivery for the ALC and NORM Protocols</title>
<author fullname="Vincent Roca" initials="V." surname="Roca">
<organization>INRIA</organization>
<address>
<postal>
<street>655, av. de l'Europe</street>
<street>Inovallee; Montbonnot</street>
<city>ST ISMIER cedex</city>
<code>38334</code>
<country>France</country>
</postal>
<email>vincent.roca@inria.fr</email>
<uri>http://planete.inrialpes.fr/people/roca/</uri>
</address>
</author>
<author fullname="Brian Adamson" initials="B." surname="Adamson">
<organization>Naval Research Laboratory</organization>
<address>
<postal>
<street/>
<city>Washington, DC</city>
<code>20375</code>
<country>USA</country>
</postal>
<email>adamson@itd.nrl.navy.mil</email>
<uri>http://cs.itd.nrl.navy.mil</uri>
</address>
</author>
<date/>
<area>Transport</area>
<workgroup>RMT</workgroup>
<keyword>ALC, NORM</keyword>
<abstract>
<t>This document introduces the FCAST object (e.g., file) delivery
application on top of the ALC and NORM reliable multicast protocols.
FCAST is a highly scalable application that provides a reliable object
delivery service.</t>
</abstract>
</front>
<middle>
<!--
<t>
<list style="empty">
<t> *** Editor's note:
XXX </t>
</list>
</t>
-->
<section anchor="intro" title="Introduction">
<!-- ==================================== -->
<t>This document introduces the FCAST reliable and scalable object
(e.g., file) delivery application. Two variants of FCAST exist: <list
style="symbols">
<t>FCAST/ALC that relies on the Asynchronous Layer Coding (ALC)
<xref target="RFC5775"/> and the Layered Coding Transport (LCT)
<xref target="RFC5651"/> reliable multicast transport protocol,
and</t>
<t>FCAST/NORM that relies on the NACK-Oriented Reliable Multicast
(NORM) <xref target="RFC5740"/> reliable multicast transport
protocol.</t>
</list> 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.</t>
<t>A design goal behind FCAST is to define a streamlined solution, in
order to enable lightweight implementations of the protocol stack, and
limit the operational processing and storage requirements. A consequence
of this choice is that FCAST cannot be considered as a versatile
application, capable of addressing all the possible use-cases. On the
contrary, FCAST has some intrinsic limitations. From this point of view
it differs from FLUTE <xref target="RFC6726"/> which favors flexibility
at the expense of some additional complexity.</t>
<t>A good example of the design choices meant to favor simplicity is the
way FCAST manages the object meta-data: by default, the meta-data and
the object content are sent together, in a compound object. This
solution has many advantages in terms of simplicity as will be described
later on. However this solution has an intrinsic limitation since it
does not enable a receiver to decide in advance, before beginning the
reception of the compound object, whether the object is of interest or
not, based on the information that may be provided in the meta-data.
Therefore this document discusses additional techniques that may be used
to mitigate this limitation. When use-cases require that each receiver
download the whole set of objects sent in the session (e.g., with
mirroring tools), this limitation is not considered a problem.</t>
<t>FCAST is expected to work in many different environments and is
designed to be flexible. The service provided by FCAST can differ
according to the exact conditions FCAST is used, like the presence or
not of reception feedbacks. For example, environments exist where no
feedback is possible with FCAST/ALC, which guarantees a maximum
scalability but at the same time may reduce transmission reliability.
<xref target="op_considerations"/> discusses such operational
considerations in detail. Finally, <xref
target="compliance_requirements"/> provides the guidance for compliant
implementation of the specification and identifies those features that
are optional.</t>
<section title="Requirements Notation">
<!-- ==================================== -->
<t>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 <xref
target="RFC2119"/>.</t>
</section>
<section title="Definitions, Notations and Abbreviations">
<!-- ================================================= -->
<t>This document uses the following terms: </t>
<texttable align="left" style="none">
<ttcol align="right" width="24%"/>
<ttcol align="center" width="1%"/>
<ttcol/>
<c>FCAST/ALC</c>
<c>:</c>
<c>Denotes the FCAST application running on top of the ALC/LCT
reliable transport protocol</c>
<c>FCAST/NORM</c>
<c>:</c>
<c>Denotes the FCAST application running on top of the NORM reliable
transport protocol</c>
<c>FCAST</c>
<c>:</c>
<c>Denotes either FCAST/ALC or FCAST/NORM</c>
<c>Compound Object</c>
<c>:</c>
<c>An ALC or NORM transport object composed of the Compound Object
Header</c>
<c>Carousel</c>
<c>:</c>
<c>The process of sending Compound Objects implemented by a FCAST
sender</c>
<c>Carousel Instance</c>
<c>:</c>
<c>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.</c>
<c>Carousel Instance Descriptor (CID)</c>
<c>:</c>
<c>Special object that lists the Compound Objects comprising a given
Carousel Instance</c>
<c>Carousel Instance IDentifier (CIID)</c>
<c>:</c>
<c>Numeric value that identifies a Carousel Instance</c>
<c>Carousel Cycle</c>
<c>:</c>
<c>A transmission round within which all the Compound Objects
registered in the Carousel Instance are transmitted a certain number
of times; By default, Compound Objects are transmitted once per
cycle, but higher values are possible, that might differ on a
per-object basis. </c>
<c>Transport Object Identifier (TOI)</c>
<c>:</c>
<c>Numeric identifier associated to a specific object by the
underlying transport protocol. In the case of ALC, this corresponds
to the TOI described in <xref target="RFC5651"/>; In the case of
NORM, this corresponds to the NormTransportId described in <xref
target="RFC5740"/>.</c>
<c>FEC Object Transmission Information (FEC OTI)</c>
<c>:</c>
<c>FEC information associated with an object that is essential for
the FEC decoder to decode a specific object.</c>
</texttable>
</section>
</section>
<section anchor="fcast_specifications" title="FCAST Data Formats">
<t>This section details the various data formats used by FCAST.</t>
<section anchor="compound_obj_format" title="Compound Object Format">
<t>In an FCAST session, Compound Objects are constructed by prepending
the Compound Object Header (which contains the meta-data of the
object) before the original object data content (see <xref
target="meta-data_tx"/>). <xref target="header_with_md_format"/>
illustrates the associated Compound Object header format. All
multi-byte fields are in network (Big Endian) byte order.</t>
<figure anchor="header_with_md_format" title="Compound Object Format.">
<artwork align="center"><![CDATA[
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ^
|Ver| Resvd |G|C| MDFmt | MDEnc | Checksum | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| Compound Object Header Length | h
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-| d
| Object Meta-Data (variable length) | r
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| | Padding (optional) | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ v
| |
. Object Data (optional, variable length) .
. .
. .]]></artwork>
</figure>
<t>The Compound Object Header fields are:</t>
<texttable style="all">
<ttcol align="left" width="15%">Field</ttcol>
<ttcol align="left">Description</ttcol>
<c>Version</c>
<c>2-bit field that MUST be set to 0 in this specification and
indicates the protocol version number.</c>
<c>Reserved</c>
<c>4-bit field that MUST be set to 0 in this specification and is
reserved for future use. Receivers MUST ignore this field.</c>
<c>G</c>
<c>1-bit field that, when set to 1, indicates that the checksum
encompasses the whole Compound Object (Global checksum). When set to
0, this field indicates that the checksum encompasses only the
Compound Object header.</c>
<c>C</c>
<c>1-bit field that, when set to 1, indicates the object is a
Carousel Instance Descriptor (CID). When set to 0, this field
indicates that the transported object is a standard object.</c>
<c>Meta-Data Format (MDFmt)</c>
<c>4-bit field that defines the format of the object meta-data (see
<xref target="iana"/>). An HTTP/1.1 metainformation format <xref
target="RFC2616"/> MUST be supported and is associated to value 0.
Other formats (e.g., XML) MAY be defined in the future.</c>
<c>Meta-Data Encoding (MDEnc)</c>
<c>4-bit field that defines the optional encoding of the Object
Meta-Data field (see <xref target="iana"/>). By default, a plain
text encoding is used and is associated to value 0. GZIP encoding
MUST also be supported and is associated to value 1. Other encodings
MAY be defined in the future.</c>
<c>Checksum</c>
<c>16-bit field that contains the checksum computed over either the
whole Compound Object (when G is set to 1), or over the Compound
Object header (when G is set to 0), using the Internet checksum
algorithm specified in <xref target="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.</c>
<c>Compound Object Header Length</c>
<c>32-bit field indicating total length (in bytes) of all fields of
the Compound Object Header, except the optional padding. A header
length field set to value 8 means that there is no meta-data
included. When this size is not multiple to 32-bits words and when
the Compound Object Header is followed by a non null Compound Object
Data, padding MUST be added. It should be noted that the meta-data
field maximum size is equal to (2^32 - 8) bytes.</c>
<c>Object Meta-Data</c>
<c>Variable length field that contains the meta-data associated to
the object. The format and encoding of this field are defined
respectively by the MDFmt and MDEnc fields. With the default
HTTP/1.1 format and plain text encoding, the Meta-Data is
NULL-terminated plain text that follows the "TYPE" ":" "VALUE"
"<CR-LF>" format used in HTTP/1.1 for metainformation <xref
target="RFC2616"/>. The various meta-data items can appear in any
order. The associated string, when non empty, MUST be
NULL-terminated. When no meta-data is communicated, this field MUST
be empty and the Compound Object Header Length MUST be equal to
8.</c>
<c>Padding</c>
<c>Optional, variable length field of zero-value bytes to align the
start of the Object Data to 32-bit boundary. Padding is only used
when the Compound Object Header Length value, in bytes, is not
multiple of 4 and when the Compound Object Header is followed by non
null Compound Object Data.</c>
</texttable>
<t>The Compound Object Header is then followed by the Object Data,
i.e., the original object possibly encoded by FCAST. Note that the
length of this content is the transported object length (e.g., as
specified by the FEC OTI) minus the Compound Object Header Length and
optional padding if any.</t>
</section>
<section anchor="carousel_instance_descr"
title="Carousel Instance Descriptor Format">
<!-- ==================================== -->
<t>In an FCAST session, a Carousel Instance Descriptor (CID) MAY be
sent in order to carry the list of Compound Objects that are part of a
given Carousel Instance (see <xref target="cio"/>). The format of the
CID, that is sent as a special Compound Object, is given in <xref
target="cid_format"/>. Being a special case of Compound Object, this
format is in line with the format described in <xref
target="compound_obj_format"/>.</t>
<figure anchor="cid_format"
title="Carousel Instance Descriptor Format.">
<artwork align="center"><![CDATA[
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ^
|Ver| Resvd |G|C| MDFmt | MDEnc | Checksum | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| Compound Object Header Length | h
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-| d
| Object Meta-Data (variable length) | r
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| | Padding (optional) | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ v
. . ^
. Object List (variable length) . |
. . o
. +-+-+-+-+-+-+-+-+ b
. | j
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ v ]]></artwork>
</figure>
<t>Because the CID is transmitted as a special Compound Object, the
following CID-specific meta-data entries are defined: <list
style="symbols">
<t>Fcast-CID-Complete: when set to '1', it indicates that no new
object in addition to the ones whose TOI are specified in this
CID, or the ones that have been specified in the previous CID(s),
will be sent in the future. Otherwise it MUST be set to 0. This
entry is optional. If absent, a receiver MUST conclude that the
session is complete.</t>
<t>Fcast-CID-ID: this entry contains the Carousel Instance
IDentifier, or CIID. It starts from 0 and is incremented by 1 for
each new carousel instance. This entry is optional if the FCAST
session consists of a single, complete, carousel instance. In all
other cases, this entry MUST be defined. In particular, the CIID
is used by the TOI equivalence mechanism thanks to which any
object is uniquely identified, even if the TOI is updated (e.g.,
after re-enqueuing the object with NORM). The Fcast-CID-ID value
can also be useful to detect possible gaps in the Carousel
Instances, for instance caused by long disconnection periods.
Finally, it can also be useful to avoid problems when TOI wrapping
to 0 takes place to differentiate the various incarnations of the
TOIs if need be.</t>
</list>The motivation for making the Fcast-CID-Complete and
Fcast-CID-ID entries optional is to simplify the simple case of a
session consisting of a single, complete, carousel instance, with an
Object List given in plain text, without any content encoding. In that
case, the CID does not need to contain any meta-data entry.</t>
<t>The following standard meta-data entry types are also used (<xref
target="meta-data_content"/>): <list style="symbols">
<t>Content-Length: it specifies the size of the object list,
before any content encoding (if any).</t>
<t>Content-Encoding: it specifies the optional encoding of the
object list, performed by FCAST. For instance: <figure>
<artwork align="left"><![CDATA[ Content-Encoding: gzip ]]></artwork>
</figure> indicates that the Object List field has been encoded
with GZIP <xref target="RFC1952"/>. If there is no
Content-Encoding entry, the receiver MUST assume that the Object
List field is plain text (default). The support of GZIP encoding,
in addition to the plain text form, is REQUIRED. The
Content-Encoding entry MAY be used to indicate other encoding
types to support non-standard FCAST implementation or future
extended specifications.</t>
</list></t>
<t>An empty Object List is valid and indicates that the current
carousel instance does not include any objects (<xref target="cio"/>).
This can be specified by using the following meta-data entry: <figure>
<artwork align="left"><![CDATA[ Content-Length: 0 ]]></artwork>
</figure> or simply by leaving the Object List empty. In both cases,
padding MUST NOT be used and consequently the transported object
length (e.g., as specified by the FEC OTI) minus the Compound Object
Header Length equals zero.</t>
<t>The non-encoded (i.e., plain text) Object List, when non empty, is
a NULL-terminated ASCII string. It can contain two things: <list
style="symbols">
<t>a list of TOI values, and</t>
<t>a list of TOI equivalences;</t>
</list>A list of TOIs included in the current carousel instance is
specified as an ASCII string containing comma-delimited individual TOI
values and/or TOI intervals. Inidividual TOIs consist of a single
integer value while TOI intervals are a hyphen-delimited pair of TOI
values to indicate a inclusive range of TOI values (e.g., "1,2,4-6"
would indicate the list of TOI values of 1,2,4,5, and 6). For a TOI
Interval indicated by ""TOI_a-TOI_b", the 'TOI_a' value MUST be
strictly inferior to the 'TOI_b' value. If a TOI wrapping to 0 occurs
in an interval, then two TOI intervals MUST be specified,
TOI_a-MAX_TOI and 0-TOI_b.</t>
<t>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 size '=' to indicate the equivalence assignment
(e.g., " newTOI "=" 1stTOI "/" 1stCIID "). Each equivalence indicates
that the new TOI, for the current Carousel Instance, is equivalent to
(i.e., refers to the same object as) the provided identifier, 1stTOI,
for the Carousel Instance of ID 1stCIID. In the case of the NORM
protocol where NormTransportId values need to monotonically increase
for NACK-based protocol operation, this allows an object from a prior
Carousel Instance to be reincluded in a subsequent Carousel Instance
with the receiver set informed of the equivalence so that unnecessary
retransmission requests can be avoided.</t>
<t>The ABNF <xref target="RFC5234"/> specification is the
following:</t>
<figure>
<artwork align="left"><![CDATA[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]]></artwork>
</figure>
<t>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.</t>
<t>For instance, the following object list specifies that the current
Carousel Instance is composed of 8 objects, and that TOIs 100 to 104
are equivalent to the TOIs 10 to 14 of Carousel Instance ID 2 and
refer to the same objects:</t>
<figure>
<artwork align="left"><![CDATA[
97,98,99,(100=10/2),(101=11/2),(102=12/2),(103=13/2),(104=14/2)
]]></artwork>
</figure>
<t>or equivalently:</t>
<figure>
<artwork align="left"><![CDATA[
97-104,(100=10/2),(101=11/2),(102=12/2),(103=13/2),(104=14/2)
]]></artwork>
</figure>
</section>
</section>
<!-- ======================================================================= -->
<section anchor="fcast_principles" title="FCAST Principles">
<!-- ==================================== -->
<t>This section details the principles of FCAST.</t>
<section anchor="fcast_service" title="FCAST Content Delivery Service">
<!-- ==================================== -->
<t>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: <list
style="numbers">
<t>FCAST/ALC, where the FCAST application works on top of the
ALC/LCT reliable multicast transport protocol, without any
feedback from the receivers, and</t>
<t>FCAST/NORM, where the FCAST application works on top of the
NORM reliable multicast transport protocol, that requires
positive/negative acknowledgements from the receivers.</t>
</list></t>
<t>This specification is designed such that both forms of operation
share as much commonality as possible. <xref
target="op_considerations"/> discusses some operational aspects and
the content delivery service that is provided by FCAST for a given
use-case.</t>
</section>
<section anchor="meta-data_tx" title="Meta-Data Transmission">
<!-- ==================================== -->
<t>FCAST carries meta-data elements by prepending them to the object
they refer to. As a result, a Compound Object is created that is
composed of a header followed by the original object data. This header
is itself composed of the meta-data as well as several fields, for
instance to indicate the boundaries between the various parts of this
Compound Object (<xref target="fig_compound_obj"/>).</t>
<figure anchor="fig_compound_obj" title="Compound Object composition.">
<artwork align="center"><![CDATA[
<------------------------ Compound Object ----------------------->
+-------------------------+--------------------------------------+
| Compound Object Header | Object Data |
| (can include meta-data) | (can be encoded by FCAST) |
+-------------------------+--------------------------------------+
]]></artwork>
</figure>
<t>Attaching the meta-data to the object is an efficient solution,
since it guaranties that meta-data are received along with the
associated object, and it allows the transport of the meta-data to
benefit from any transport-layer erasure protection of the Compound
Object (e.g., using FEC encoding and/or NACK-based repair). However a
limit of this scheme is that a client does not know the meta-data of
an object before beginning its reception, and in case of erasures
affecting the meta-data, not until the object decoding is completed.
The details of course depend upon the transport protocol and the FEC
code used.</t>
<t><xref target="annex_add_meta_data_tx_mechanisms"/> describes
extensions that provide additional means to carry meta-data, e.g., to
communicate meta-data ahead of time.</t>
</section>
<section anchor="meta-data_content" title="Meta-Data Content">
<!-- ==================================== -->
<t>A compliant FCAST implementation MUST support at least the
following meta-data types defined in <xref target="RFC2616"/>: <list
style="symbols">
<t>Content-Location: the URI of the object, which gives the name
and location of the object;</t>
<t>Content-Type: a string that contains the MIME type of the
object;</t>
<t>Content-Length: an unsigned 64-bit integer that contains the
size of the initial object, before any content encoding (if any)
and without considering the Compound Object header. Note that the
use of certain FEC schemes MAY further limit the maximum value of
the object;</t>
<t>Content-Encoding: a string that contains the optional encoding
of the object performed by FCAST. If there is no Content-Encoding
entry, the receiver MUST assume that the object is not encoded
(default). The support of GZIP encoding, or any other solution,
remains optional.</t>
<t>Content-MD5: a binary content coded as "base64" that contains
the MD5 message digest of the object in order to check its
integrity. This digest is meant to protect from transmission and
processing errors, not from deliberate attacks by an intelligent
attacker (see <xref target="security"/>). This digest only
protects the object, not the header, and therefore not the
meta-data. A separate checksum is provided to that purpose (<xref
target="compound_obj_format"/>);</t>
<!-- <t>a digital signature for this object;</t> -->
</list></t>
<t>The following additional REQUIRED meta-data types are used for
dealing with very large objects (e.g., that largely exceed the working
memory of a receiver). When this happens, the meta-data associated to
each sub-object MUST include the following entries: <list
style="symbols">
<t>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;</t>
<t>Fcast-Obj-Slice-Idx: an unsigned 32-bit integer that contains
the slice index (in the {0 .. SliceNb - 1} interval);</t>
<t>Fcast-Obj-Slice-Offset: an unsigned 64-bit integer that
contains the offset at which this slice starts within the original
object;</t>
</list>Future standards actions can extend the set of meta-data
types supported by FCAST.</t>
</section>
<section anchor="carousel" title="Carousel Transmission">
<t>A set of FCAST Compound Objects scheduled for transmission are
considered a logical "Carousel". A given "Carousel Instance" is
comprised of a fixed set of Compound Objects. Whenever the FCAST
application needs to add new Compound Objects to or remove old
Compound Objects from the transmission set, a new Carousel Instance is
defined since the set of Compound Objects changes. Because of the
native object multiplexing capability of both ALC and NORM, sender and
receiver(s) are both capable to multiplex and demultiplex FCAST
Compound Objects.</t>
<t>For a given Carousel Instance, one or more transmission cycles are
possible. During each cycle, all of the Compound Objects comprising
the Carousel are sent. By default, each object is transmitted once per
cycle. However, in order to allow different levels of priority, some
objects MAY be transmitted more often that others during a cycle,
and/or benefit from higher FEC protection than others. For example,
this can be the case for the CID objects (<xref target="cio"/>) 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).</t>
</section>
<section anchor="cio"
title="Carousel Instance Descriptor Special Object">
<t>The FCAST sender can transmit an OPTIONAL Carousel Instance
Descriptor (CID). The CID carries the list of the Compound Objects
that are part of a given Carousel Instance, by specifying their
respective Transmission Object Identifiers (TOI). However the CID does
not describe the objects themselves (i.e., there is no meta-data).
Additionally, the CID MAY include an "FCAST-CID-Complete" meta-data
entry set to '1' to indicate that no further modification to the
enclosed list will be done in the future. Finally, the CID MAY include
a Carousel Instance ID (CIID) that identifies the Carousel Instance it
pertains to. These aspects are discussed in <xref
target="carousel_instance_descr"/>.</t>
<t>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 Compound Object header field (the "C" flag from
<xref target="header_with_md_format"/>) so that it can be recognized
and processed by the FCAST application as needed. A direct consequence
is the following: since a receiver does not know in advance which TOI
will be used for the following CID (in case of a dynamic session), it
MUST NOT filter out packets that are not in the current CID's TOI
list. Said differently, the goal of CID is not to setup ALC or NORM
packet filters (this mechanism would not be secure in any case).</t>
<t>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: <list style="symbols">
<t>When an "FCAST-CID-Complete" meta-data entry set to '1' is
included, the receivers know when they can leave the session,
i.e., when they have received all the objects that are part of the
last carousel instance of this delivery session;</t>
<t>In case of a session with a dynamic set of objects, the sender
can reliably inform the receivers that some objects have been
removed from the carousel with the CID. This solution is more
robust than the "Close Object flag (B)" of ALC/LCT since a client
with an intermittent connectivity might loose all the packets
containing this B flag. And while NORM provides a robust object
cancellation mechanism in the form of its NORM_CMD(SQUELCH)
message in response to receiver NACK repair requests, the use of
the CID provides an additional means for receivers to learn of
objects for which it is futile to request repair;</t>
<t>The TOI equivalence (<xref target="object_id"/>) can be
signaled with the CID. This is often preferable to the alternative
solution where the equivalence is identified by examining the
object meta-data, especially in case of erasures.</t>
</list></t>
<t>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: <list style="symbols">
<t>all the previously sent objects have been removed from the
carousel. It therefore improves the FCAST robustness even during
"idle" period;</t>
<t>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 an "FCAST-CID-Complete" meta-data entry is
not included (or if set to '0'), it informs the receivers the
carousel instance may be modified and that new objects could be
sent in the future;</t>
</list></t>
</section>
<section anchor="object_id" title="Compound Object Identification">
<!-- ==================================== -->
<t>The FCAST Compound Objects are directly associated with the
object-based transport service that the ALC and NORM protocols
provide. In each of these protocols, the messages containing transport
object content are labeled with a numeric transport object identifier
(i.e., the ALC TOI and the NORM NormTransportId). For the purposes of
this document, this identifier in either case (ALC or NORM) is
referred to as the TOI.</t>
<t>There are several differences between ALC and NORM: <list
style="symbols">
<t>the ALC use of TOI is rather flexible, since several TOI field
sizes are possible (from 16 to 112 bits), since this size can be
changed at any time, on a per-packet basis, and since the TOI
management is totally free as long as each object is associated to
a unique TOI (if no wraparound happened);</t>
<t>the NORM use of TOI is more directive, since the TOI field is
16 bit long and since TOIs MUST be managed sequentially;</t>
</list></t>
<t>In both NORM and ALC, it is possible that the transport
identification space may eventually wrap for long-lived sessions
(especially with NORM where this phenomenon is expected to happen more
frequently). This can possibly introduce some ambiguity in FCAST
object identification if a sender retains some older objects in newer
Carousel Instances with updated object sets. <!-- Thus, when an updated
object set, for a new Carousel Instance, would transport identifiers
that exceed one-half of the TOI sequence space (or otherwise exceed
the sender repair window capability in the case of NORM), it MAY be
necessary to re-enqueue old objects within the Carousel with new TOI
to stay within transport identifier limits. --> To avoid ambiguity the
active TOIs (i.e., the TOIs corresponding to objects being
transmitted) can only occupy half of the TOI sequence space. If an old
object, whose TOI has fallen outside the current window, needs to be
transmitted again, a new TOI must be used for it. In case of NORM,
this constraint limits to 32768 the maximum number of objects that can
be part of any carousel instance. In order to allow receivers to
properly combine the transport packets with a newly-assigned TOI to
those of associated to the previously-assigned TOI, a mechanism is
required to equate the objects with the new and the old TOIs.</t>
<t>The preferred mechanism consists in signaling, within the CID, that
the newly assigned TOI, for the current Carousel Instance, is
equivalent to the TOI used within a previous Carousel Instance. By
convention, the reference tuple for any object is the {TOI; CI ID}
tuple used for its first transmission within a Carousel Instance. This
tuple MUST be used whenever a TOI equivalence is provided.</t>
<t>An alternative solution, when meta-data can be processed rapidly
(e.g., by using NORM-INFO messages), consists for the receiver in
identifying that both objects are the same, after examining the
meta-data. The receiver can then take appropriate measures.</t>
</section>
<!--
<section anchor="alc_specifics"
title="FCAST/ALC Additional Specificities">
<t>There are no additional detail or option for FCAST/ALC
operation.</t>
</section>
<section anchor="norm_specifics"
title="FCAST/NORM Additional Specificities">
<t>The NORM Protocol provides a few additional capabilities that can
be used to specifically support FCAST operation:</t>
<t><list style="numbers">
<t>The NORM_INFO message can convey "out-of-band" content with
respect to a given transport object. With FCAST, it MAY be used to
provide to the receivers a new, associated, Compound Object which
contains the main Compound Object meta-data, or a subset of it.
In that case the NORM_INFO Compound Object MUST NOT contain any
Object Data field (i.e., it is only
composed of the header), it MUST feature a non global checksum, and
it MUST NOT include any padding field. The main Compound
Object MUST in any case contain the whole meta-data (e.g., because
a receiver MAY not support the NORM_INFO facility). Additionally, the
meta-data entries contained in the NORM_INFO MUST be identical to
the same entries in the main Compound Object. Finally, note that
the availability of NORM_INFO for a given object is signaled
through the use of a dedicated flag in the NORM_DATA message
header. Along with NORM's NACK-based repair request signaling, it
allows a receiver to quickly (and independently) request an
object's NORM_INFO content. However, a limitation here is that the
NORM_INFO Compound Object header MUST fit within the byte size
limit defined by the NORM sender's configured "segment size"
(typically a little less than the network MTU);</t>
<t>The NORM_CMD(SQUELCH) messages are used by the NORM protocol
sender to inform receivers of objects that have been canceled when
receivers make repair requests for such invalid objects. Along
with the CID mechanism, a receiver has two efficient and reliable
ways to discover old objects that have been removed from the
carousel instance;</t>
<t>NORM also supports an optional positive acknowledgment
mechanism that can be used for small-scale multicast receiver
group sizes. Also, it may be possible in some cases for the sender
to infer, after some period without receiving NACKs at the end of
its transmission that the receiver set has fully received the
transmitted content. In particular, if the sender completes its
end-of-transmission series of NORM_CMD(FLUSH) messages without
receiving repair requests from the group, it may have some
assurance that the receiver set has received the content prior to
that point. These mechanisms are likely to help FCAST in achieving
fully reliable transmissions;</t>
</list></t>
<t>It should be noted that the NORM_INFO message header may carry
the EXT_FTI extension. The reliable delivery of the NORM_INFO content
allows the individual objects' FEC Transmission Information to be
provided to the receivers without burdening every packet (i.e.
NORM_DATA messages) with this additional, but important, content.
Examples are provided in <xref target="fcast_exple"></xref>.</t>
</section>
-->
<section anchor="sender_behavior" title="FCAST Sender Behavior">
<!-- ==================================== -->
<t>This section provides an informative description of expected FCAST
sender behavior. The following operations can take place at a sender:
<list style="numbers">
<t>The user (or another application) selects a set of objects
(e.g., files) to deliver and submits them, along with their
meta-data, to the FCAST application;</t>
<t>For each object, FCAST creates the Compound Object and
registers this latter in the carousel instance;</t>
<t>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);</t>
<t>At this point, the FCAST application knows the full list of
Compound Objects that are part of the Carousel Instance and can
create a CID if desired, possibly with the complete flag set;</t>
<t>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.</t>
<t>The FCAST application then starts the carousel transmission,
for the number of cycles specified. Transmissions take place
until: <list style="symbols">
<t>the desired number of transmission cycles has been reached,
or</t>
<t>the user wants to prematurely stop the transmissions,
or</t>
<t>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.</t>
</list></t>
<t>If the session is not finished, then continue at Step 1
above;</t>
</list></t>
</section>
<section anchor="receiver_behavior" title="FCAST Receiver Behavior">
<!-- ==================================== -->
<t>This section provides an informative description of expected FCAST
receiver behavior. The following operations can take place at a
receiver: <list style="numbers">
<t>The receiver joins the session and collects incoming
packets;</t>
<t>If the header portion of a Compound Object is entirely received
(which may happen before receiving the entire object with some
ALC/NORM configurations), or if the meta-data is sent by means of
another mechanism prior to the object, the receiver processes the
meta-data and chooses to continue to receive the object content or
not;</t>
<t>When a Compound Object has been entirely received, the receiver
processes the header, retrieves the object meta-data, perhaps
decodes the meta-data, and processes the object accordingly;</t>
<t>When a CID is received, which is indicated by the 'C' flag set
in the Compound Object header, the receiver decodes the CID, and
retrieves the list of objects that are part of the current
carousel instance. This list can be used to remove objects sent in
a previous carousel instance that might not have been totally
decoded and that are no longer part of the current carousel
instance;</t>
<t>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;</t>
<t>When a receiver receives a CID with an "FCAST-CID-Complete"
meta-data entry set to '1', and has successfully received all the
objects of the current carousel instance, it can safely exit from
the current FCAST session;</t>
<t>Otherwise continue at Step 2 above.</t>
</list></t>
</section>
</section>
<section anchor="security" title="Security Considerations">
<!-- ==================================== -->
<!-- XXX: this section is taken from FLUTE. It is therefore very ALC centric... -->
<section anchor="pb_statement" title="Problem Statement">
<!-- ==================================== -->
<t>A content delivery system is potentially subject to attacks.
Attacks may target: <list style="symbols">
<t>the network (to compromise the routing infrastructure, e.g., by
creating congestion),</t>
<t>the Content Delivery Protocol (CDP) (e.g., to compromise the
normal behavior of FCAST), or</t>
<t>the content itself (e.g., to corrupt the objects being
transmitted).</t>
</list></t>
<t>These attacks can be launched either: <list style="symbols">
<t>against the data flow itself (e.g., by sending forged
packets),</t>
<t>against the session control parameters (e.g., by corrupting the
session description, the CID, the object meta-data, or the ALC/LCT
control parameters), that are sent either in-band or out-of-band,
or</t>
<t>against some associated building blocks (e.g., the congestion
control component).</t>
</list></t>
<t>In the following sections we provide more details on these possible
attacks and sketch some possible counter-measures. We finally provide
recommendations in <xref target="min-sec-recommendations"/>.</t>
</section>
<section anchor="attacks_against_data"
title="Attacks Against the Data Flow">
<!-- ==================================== -->
<t>Let us consider attacks against the data flow first. At least, the
following types of attacks exist: <list style="symbols">
<t>attacks that are meant to give access to a confidential object
(e.g., in case of a non-free content) and</t>
<t>attacks that try to corrupt the object being transmitted (e.g.,
to inject malicious code within an object, or to prevent a
receiver from using an object, which is a kind of Denial of
Service (DoS)).</t>
</list></t>
<section anchor="access_to_confidential_obj"
title="Access to Confidential Objects">
<!-- ==================================== -->
<t>Access control to the object being transmitted is typically
provided by means of encryption. This encryption can be done over
the whole object (e.g., by the content provider, before submitting
the object to FCAST), or be done on a packet per packet basis (e.g.,
when IPsec/ESP is used <xref target="RFC4303"/>, see <xref
target="min-sec-recommendations"/>). If confidentiality is a
concern, it is RECOMMENDED that one of these solutions be used.</t>
</section>
<section anchor="obj_corruption" title="Object Corruption">
<!-- ==================================== -->
<t>Protection against corruptions (e.g., if an attacker sends forged
packets) is achieved by means of a content integrity
verification/sender authentication scheme. This service can be
provided at the object level, but in that case a receiver has no way
to identify which symbol(s) is(are) corrupted if the object is
detected as corrupted. This service can also be provided at the
packet level. In this case, after removing all corrupted packets,
the file may be in some cases recovered. Several techniques can
provide this content integrity/sender authentication service: <list
style="symbols">
<t>at the object level, the object can be digitally signed, for
instance by using RSASSA-PKCS1-v1_5 <xref target="RFC3447"/>.
This signature enables a receiver to check the object integrity,
once this latter has been fully decoded. Even if digital
signatures are computationally expensive, this calculation
occurs only once per object, which is usually acceptable;</t>
<t>at the packet level, each packet can be digitally signed
<xref target="RFC6584"/>. A major limitation is the high
computational and transmission overheads that this solution
requires. To avoid this problem, the signature may span a set of
packets (instead of a single one) in order to amortize the
signature calculation. But if a single packets is missing, the
integrity of the whole set cannot be checked;</t>
<t>at the packet level, a Group Message Authentication Code
(MAC) <xref target="RFC2104"/><xref target="RFC6584"/> scheme
can be used, for instance by using HMAC-SHA-256 with a secret
key shared by all the group members, senders and receivers. This
technique creates a cryptographically secured digest of a packet
that is sent along with the packet. The Group MAC scheme does
not create prohibitive processing load nor transmission
overhead, but it has a major limitation: it only provides a
group authentication/integrity service since all group members
share the same secret group key, which means that each member
can send a forged packet. It is therefore restricted to
situations where group members are fully trusted (or in
association with another technique as a pre-check);</t>
<t>at the packet level, Timed Efficient Stream Loss-Tolerant
Authentication (TESLA) <xref target="RFC4082"/><xref
target="RFC5776"/> is an attractive solution that is robust to
losses, provides a true authentication/integrity service, and
does not create any prohibitive processing load or transmission
overhead. Yet checking a packet requires a small delay (a second
or more) after its reception;</t>
<t>at the packet level, IPsec/ESP <xref target="RFC4303"/> can
be used to check the integrity and authenticate the sender of
all the packets being exchanged in a session (see <xref
target="min-sec-recommendations"/>).</t>
</list></t>
<t>Techniques relying on public key cryptography (digital signatures
and TESLA during the bootstrap process, when used) require that
public keys be securely associated to the entities. This can be
achieved by a Public Key Infrastructure (PKI), or by a PGP Web of
Trust, or by pre-distributing securely the public keys of each group
member.</t>
<t>Techniques relying on symmetric key cryptography (Group MAC)
require that a secret key be shared by all group members. This can
be achieved by means of a group key management protocol, or simply
by pre-distributing securely the secret key (but this manual
solution has many limitations).</t>
<t>It is up to the developer and deployer, who know the security
requirements and features of the target application area, to define
which solution is the most appropriate. In any case, whenever there
is any concern of the threat of file corruption, it is RECOMMENDED
that at least one of these techniques be used.</t>
</section>
</section>
<section anchor="attack_against_ctrl_param"
title="Attacks Against the Session Control Parameters and Associated Building Blocks">
<!-- ==================================== -->
<t>Let us now consider attacks against the session control parameters
and the associated building blocks. The attacker has at least the
following opportunities to launch an attack: <list style="symbols">
<t>the attack can target the session description,</t>
<t>the attack can target the FCAST CID,</t>
<t>the attack can target the meta-data of an object,</t>
<t>the attack can target the ALC/LCT parameters, carried within
the LCT header or</t>
<t>the attack can target the FCAST associated building blocks, for
instance the multiple rate congestion control protocol.</t>
</list></t>
<t>The consequences of these attacks are potentially serious, since
they can compromise the behavior of content delivery system or even
compromise the network itself.</t>
<section anchor="attack_against_sdp"
title="Attacks Against the Session Description">
<!-- ==================================== -->
<t>An FCAST receiver may potentially obtain an incorrect Session
Description for the session. The consequence of this is that
legitimate receivers with the wrong Session Description are unable
to correctly receive the session content, or that receivers
inadvertently try to receive at a much higher rate than they are
capable of, thereby possibly disrupting other traffic in the
network.</t>
<t>To avoid these problems, it is RECOMMENDED that measures be taken
to prevent receivers from accepting incorrect Session Descriptions.
One such measure is the sender authentication to ensure that
receivers only accept legitimate Session Descriptions from
authorized senders. How these measures are achieved is outside the
scope of this document since this session description is usually
carried out-of-band.</t>
</section>
<section anchor="attack_against_cio"
title="Attacks Against the FCAST CID">
<!-- ==================================== -->
<t>Corrupting the FCAST CID is one way to create a Denial of Service
attack. For example, the attacker can insert an "FCAST-CID-Complete"
meta-data entry to make the receivers believe that no further
modification will be done.</t>
<t>It is therefore RECOMMENDED that measures be taken to guarantee
the integrity and to check the sender's identity of the CID. To that
purpose, one of the counter-measures mentioned above (<xref
target="obj_corruption"/>) SHOULD be used. These measures will
either be applied on a packet level, or globally over the whole CID
object. When there is no packet level integrity verification scheme,
it is RECOMMENDED to digitally sign the CID.</t>
</section>
<section anchor="attack_against_meta_data"
title="Attacks Against the Object Meta-Data">
<!-- ==================================== -->
<t>Corrupting the object meta-data is another way to create a Denial
of Service attack. For example, the attacker changes the MD5 sum
associated to a file. This possibly leads a receiver to reject the
files received, no matter whether the files have been correctly
received or not. When the meta-data are appended to the object,
corrupting the meta-data means that the Compound Object will be
corrupted.</t>
<t>It is therefore RECOMMENDED that measures be taken to guarantee
the integrity and to check the sender's identity of the Compound
Object. To that purpose, one of the counter-measures mentioned above
(<xref target="obj_corruption"/>) SHOULD be used. These measures
will either be applied on a packet level, or globally over the whole
Compound Object. When there is no packet level integrity
verification scheme, it is RECOMMENDED to digitally sign the
Compound Object.</t>
<!-- XXX: how can we digitally sign a compound object? -->
</section>
<section anchor="attack_against_alc"
title="Attacks Against the ALC/LCT and NORM Parameters">
<!-- ==================================== -->
<t>By corrupting the ALC/LCT header (or header extensions) one can
execute attacks on the underlying ALC/LCT implementation. For
example, sending forged ALC packets with the Close Session flag (A)
set to one can lead the receiver to prematurely close the session.
Similarly, sending forged ALC packets with the Close Object flag (B)
set to one can lead the receiver to prematurely give up the
reception of an object. The same comments can be made for NORM.</t>
<t>It is therefore RECOMMENDED that measures be taken to guarantee
the integrity and to check the sender's identity of each ALC or NORM
packet received. To that purpose, one of the counter-measures
mentioned above (<xref target="obj_corruption"/>) SHOULD be
used.</t>
</section>
<section anchor="attack_against_bb"
title="Attacks Against the Associated Building Blocks">
<!-- ==================================== -->
<t>Let us first focus on the congestion control building block that
may be used in an ALC or NORM session. A receiver with an incorrect
or corrupted implementation of the multiple rate congestion control
building block may affect the health of the network in the path
between the sender and the receiver. That may also affect the
reception rates of other receivers who joined the session.</t>
<t>When congestion control is applied with FCAST, it is therefore
RECOMMENDED that receivers be required to identify themselves as
legitimate before they receive the Session Description needed to
join the session. If authenticating a receiver does not prevent this
latter to launch an attack, it will enable the network operator to
identify him and to take counter-measures. This authentication can
be made either toward the network operator or the session sender (or
a representative of the sender) in case of NORM. The details of how
it is done are outside the scope of this document.</t>
<t>When congestion control is applied with FCAST, it is also
RECOMMENDED that a packet level authentication scheme be used, as
explained in <xref target="obj_corruption"/>. Some of them, like
TESLA, only provide a delayed authentication service, whereas
congestion control requires a rapid reaction. It is therefore
RECOMMENDED <xref target="RFC5775"/> that a receiver using TESLA
quickly reduces its subscription level when the receiver believes
that a congestion did occur, even if the packet has not yet been
authenticated. Therefore TESLA will not prevent DoS attacks where an
attacker makes the receiver believe that a congestion occurred. This
is an issue for the receiver, but this will not compromise the
network. Other authentication methods that do not feature this
delayed authentication could be preferred, or a group MAC scheme
could be used in parallel to TESLA to prevent attacks launched from
outside of the group.</t>
</section>
</section>
<section title="Other Security Considerations">
<!-- ==================================== -->
<t>Lastly, we note that the security considerations that apply to, and
are described in, ALC <xref target="RFC5775"/>, LCT <xref
target="RFC5651"/>, NORM <xref target="RFC5740"/> and FEC <xref
target="RFC5052"/> also apply to FCAST as FCAST builds on those
specifications. In addition, any security considerations that apply to
any congestion control building block used in conjunction with FCAST
also applies to FCAST. Finally, the security discussion of <xref
target="I-D.ietf-rmt-sec-discussion"/> also applies here.</t>
</section>
<section anchor="min-sec-recommendations"
title="Minimum Security Recommendations">
<!-- ==================================== -->
<t>We now introduce a mandatory to implement but not necessarily to
use security configuration, in the sense of <xref target="RFC3365"/>.
Since FCAST/ALC relies on ALC/LCT, it inherits the "baseline secure
ALC operation" of <xref target="RFC5775"/>. Similarly, since
FCAST/NORM relies on NORM, it inherits the "baseline secure NORM
operation" of <xref target="RFC5740"/>. More precisely, in both cases
security is achieved by means of IPsec/ESP in transport mode. <xref
target="RFC4303"/> explains that ESP can be used to potentially
provide confidentiality, data origin authentication, content
integrity, anti-replay and (limited) traffic flow confidentiality.
<xref target="RFC5775"/> specifies that the data origin
authentication, content integrity and anti-replay services SHALL be
used, and that the confidentiality service is RECOMMENDED. If a short
lived session MAY rely on manual keying, it is also RECOMMENDED that
an automated key management scheme be used, especially in case of long
lived sessions.</t>
<t>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.</t>
</section>
</section>
<!-- ======================================================================= -->
<section anchor="compliance_requirements"
title="Requirements for Compliant Implementations">
<!-- ==================================== -->
<t>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.</t>
<section title="Requirements Related to the Object Meta-Data">
<!-- ==================================== -->
<t>Meta-data transmission mechanisms:</t>
<texttable>
<preamble/>
<ttcol align="left" width="33%">Feature</ttcol>
<ttcol align="left">Status</ttcol>
<c>meta-data transmission using FCAST's in-band mechanism</c>
<c>An FCAST sender MUST send meta-data with the in-band mechanism
provided by FCAST, i.e., within the Compound Object header. All the
FCAST receivers MUST be able to process meta-data sent with this
FCAST in-band mechanism.</c>
<c>meta-data transmission using other mechanisms</c>
<c>In addition to the FCAST in-band transmission of meta-data, an
FCAST sender MAY send a subset or all of the meta-data using another
mechanism. Supporting this mechanism in a compliant FCAST receiver
is OPTIONAL, and its use is OPTIONAL too. An FCAST receiver MAY
support this mechanism and take advantage of the meta-data sent in
this way. If it is not the case, the FCAST receiver will anyway
receive and process meta-data sent in-bound. See Annex <xref
target="annex_add_meta_data_tx_mechanisms"/>.</c>
</texttable>
<t>Meta-data format and encoding:</t>
<texttable>
<preamble/>
<ttcol align="left" width="33%">Feature</ttcol>
<ttcol align="left">Status</ttcol>
<c>Meta-Data Format (MDFmt field)</c>
<c>All FCAST implementations MUST support an HTTP/1.1
metainformation format <xref target="RFC2616"/>. Other formats
(e.g., XML) MAY be defined in the future.</c>
<c>Meta-Data Encoding (MDEnc field)</c>
<c>All FCAST implementations MUST support both a plain text and a
GZIP encoding <xref target="RFC1952"/> of the Object Meta-Data
field. Other encodings MAY be defined in the future.</c>
</texttable>
<t>Meta-data items (<xref target="meta-data_content"/>):</t>
<texttable>
<preamble/>
<ttcol align="left" width="33%">Feature</ttcol>
<ttcol align="left">Status</ttcol>
<c>Content-Location</c>
<c>MUST be supported</c>
<c>Content-Type</c>
<c>MUST be supported</c>
<c>Content-Length</c>
<c>MUST be supported</c>
<c>Content-Encoding</c>
<c>MUST be supported</c>
<c>Content-MD5</c>
<c>MUST be supported</c>
<c>Fcast-Obj-Slice-Nb</c>
<c>MUST be supported</c>
<c>Fcast-Obj-Slice-Idx</c>
<c>MUST be supported</c>
<c>Fcast-Obj-Slice-Offset</c>
<c>MUST be supported</c>
</texttable>
</section>
<section title="Requirements Related to the Carousel Instance Descriptor (CID) Mechanism">
<!-- ==================================== -->
<t>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.</t>
</section>
</section>
<!-- ======================================================================= -->
<section anchor="op_considerations" title="Operational Considerations">
<!-- ==================================== -->
<t>FCAST is compatible with any congestion control protocol designed for
ALC/LCT or NORM. However, depending on the use-case, the data flow
generated by the FCAST application might not be constant, but instead be
bursty in nature. Similarly, depending on the use-case, an FCAST session
might be very short. Whether and how this will impact the congestion
control protocol is out of the scope of the present document.</t>
<t>FCAST is compatible with any security mechanism designed for ALC/LCT
or NORM. The use of a security scheme is strongly RECOMMENDED (see <xref
target="security"/>).</t>
<t>FCAST is compatible with any FEC scheme designed for ALC/LCT or NORM.
Whether FEC is used or not, and the kind of FEC scheme used, is to some
extent transparent to FCAST.</t>
<t>FCAST is compatible with both IPv4 and IPv6. Nothing in the FCAST
specification has any implication on the source or destination IP
address type.</t>
<!--
<t>Depending on the target use case, the delivery service provided by
FCAST is more or less reliable. For instance, with FCAST/ALC used in
ON-DEMAND mode over a time period that largely exceeds the typical
download time, the service can be considered as fully reliable.
Similarly, when FCAST is used along with a session control application
that collects reception information and takes appropriate corrective
measures (e.g., a direct point-to-point retransmission of missing
packets, or a new multicast recovery session), then the service can be
considered as fully reliable. On the contrary, if FCAST operates in PUSH
mode, then the service is usually only partially reliable, and a
receiver that is disconnected for a sufficient time will perhaps not
have the possibility to download the object(s).</t>
-->
<t>The delivery service provided by FCAST might be fully reliable, or
only partially reliable depending upon: <list style="symbols">
<t>the way ALC or NORM is used (e.g., whether FEC encoding and/or
NACK-based repair requests are used or not),</t>
<t>the way the FCAST carousel is used (e.g., whether the objects are
made available for a long time span or not), and</t>
<t>the way in which FCAST itself is employed (e.g., whether there is
a session control application that might automatically extend an
existing FCAST session until all receivers have received the
transmitted content).</t>
</list></t>
<t>The receiver set can be restricted to a single receiver or possibly
possibly a very large number of receivers. While the choice of the
underlying transport protocol (i.e., ALC or NORM) and its parameters may
limit the practical receiver group size, nothing in FCAST itself limits
it. For instance, if FCAST/ALC is used on top of purely unidirectional
transport channels, with no feedback information at all, which is the
default mode of operation, then the scalability is maximum since neither
FCAST, nor ALC, UDP or IP generates any feedback message. On the
contrary, the FCAST/NORM scalability is typically limited by NORM
scalability itself. For example, NORM can be configured to operate using
proactive FEC without feedback similar to ALC, with receivers configured
to provide NACK and optionally ACK feedback, or a hybrid combination of
these. Similarly, if FCAST is used along with a session control
application that collects reception information from the receivers, then
this session control application may limit the scalability of the global
object delivery system. This situation can of course be mitigated by
using a hierarchy of feedback message aggregators or servers. The
details of this are out of the scope of the present document.</t>
<t>The content of a carousel instance MAY be described by means of an
OPTIONAL Carousel Instance Descriptor (CID) (<xref target="cio"/>). The
decisions of whether a CID should be used or not, how often and when a
CID should be sent, are left to the sender and depend on many
parameters, including the target use case and the session dynamics. For
instance it may be appropriate to send a CID at the beginning of each
new carousel instance, and then periodically. These operational aspects
are out of the scope of the present document.</t>
</section>
<section anchor="iana" title="IANA Considerations">
<!-- ==================================== -->
<t>This specification requires IANA to create two new registries.</t>
<!--<section title="Namespace declaration for Object Meta-Data Format">-->
<section title="Creation of the FCAST Object Meta-Data Format Registry">
<!-- ==================================== -->
<t>This document requires IANA to create a new registry, "FCAST Object
Meta-Data Format" (MDFmt), with a reference to this document. The
registry entries consist of a numeric value from 0 to 15, inclusive
(i.e., they are 4-bit positive integers) that define the format of the
object meta-data (see <xref target="compound_obj_format"/>).</t>
<t>The initial value for this registry registry is defined below.
Future assignments are to be made through Expert Review with
Specification Required <xref target="RFC5226"/>.</t>
<texttable>
<preamble/>
<ttcol>Value</ttcol>
<ttcol align="center">Format Name</ttcol>
<ttcol align="center">Format Reference</ttcol>
<ttcol>Reference</ttcol>
<c>0 (default)</c>
<c>HTTP/1.1 metainformation format</c>
<c><xref target="RFC2616"/>, Section 7.1</c>
<c>This specification</c>
</texttable>
<!-- <t> All implementations MUST support format 0 (default). </t> -->
</section>
<section title="Creation of the FCAST Object Meta-Data Encoding Registry">
<!-- ==================================== -->
<t>This document requires IANA to create a new registry, "FCAST Object
Meta-Data Encoding" (MDEnc), with a reference to this document. The
registry entries consist of a numeric value from 0 to 15, inclusive
(i.e., they are 4-bit positive integers) that define the optional
encoding of the Object Meta-Data field (see <xref
target="compound_obj_format"/>).</t>
<t>The initial values for this registry registry are defined below.
Future assignments are to be made through Expert Review <xref
target="RFC5226"/>.</t>
<texttable>
<preamble/>
<ttcol align="center">Value</ttcol>
<ttcol align="center">Encoding Name</ttcol>
<ttcol align="center">Encoding Reference</ttcol>
<ttcol align="center">Reference</ttcol>
<c>0 (default)</c>
<c>Plain Text</c>
<c>This specification</c>
<c>This specification</c>
<c>1</c>
<c>GZIP</c>
<c><xref target="RFC1952"/></c>
<c>This specification</c>
</texttable>
<!-- <t> All implementations MUST support both value 0 (plain-text, default) and value 1 (GZIP).</t> -->
</section>
<section title="Creation of the FCAST Object Meta-Data Types Registry">
<t>This document requires IANA to create a new registry, "FCAST Object
Meta-Data Types" (MDType), with a reference to this document. The
registry entries consist of additional text meta-data type identifiers
and descriptions for meta-data item types that are specific to FCAST
operation and not previously defined in <xref target="RFC1952"/>. The
initial values are those described in <xref
target="meta-data_content"/> of this specification. This table
summarizes those initial registry entries. Future assignments are to
be made through Expert Review <xref target="RFC5226"/>.</t>
<texttable align="center">
<preamble/>
<ttcol align="left">Meta-Data Type</ttcol>
<ttcol align="left">Description</ttcol>
<ttcol align="center">Reference</ttcol>
<c>Fcast-Obj-Slice-Nb</c>
<c>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</c>
<c>This specification</c>
<c>Fcast-Obj-Slice-Idx</c>
<c>Unsigned 32-bit integer that contains the slice index (in the {0
.. SliceNb - 1} interval)</c>
<c>This specification</c>
<c>Fcast-Obj-Slice-Offset</c>
<c>Unsigned 64-bit integer that contains the byte offset at which
this slice starts within the original object</c>
<c>This specification</c>
</texttable>
</section>
</section>
<section title="Acknowledgments">
<t>The authors are grateful to the authors of <xref target="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.</t>
</section>
</middle>
<back>
<references title="Normative References">
<?rfc include='reference.RFC.2119'?>
<?rfc include='reference.RFC.5226'?>
<?rfc include='reference.RFC.5651'?>
<?rfc include='reference.RFC.5740'?>
<?rfc include='reference.RFC.5775'?>
<?rfc include='reference.RFC.2616'?>
<?rfc include='reference.RFC.5234'?>
</references>
<references title="Informative References">
<reference anchor="ALC-00">
<front>
<title>Asynchronous Layered Coding: a Scalable Reliable Multicast
Protocol</title>
<author initials="M." surname="Luby">
<organization/>
</author>
<author initials="G." surname="Gemmell">
<organization/>
</author>
<author initials="L." surname="Vicisano">
<organization/>
</author>
<author initials="J." surname="Crowcroft">
<organization/>
</author>
<author initials="B." surname="Lueckenhoff">
<organization/>
</author>
<date month="March" year="2000"/>
</front>
</reference>
<?rfc include="reference.RFC.6726"?>
<?rfc include="reference.I-D.ietf-rmt-sec-discussion"?>
<?rfc include='reference.RFC.3365'?>
<?rfc include='reference.RFC.1071'?>
<?rfc include='reference.RFC.1952'?>
<?rfc include='reference.RFC.2104'?>
<?rfc include='reference.RFC.3447'?>
<?rfc include='reference.RFC.4082'?>
<?rfc include='reference.RFC.4303'?>
<?rfc include='reference.RFC.5052'?>
<?rfc include='reference.RFC.5510'?>
<?rfc include='reference.RFC.5776'?>
<?rfc include='reference.RFC.6584'?>
</references>
<section anchor="fcast_exple" title="FCAST Examples">
<section title="Introduction">
<t>This appendix provides informative examples of FCAST compound
object header and carousel instance descriptor formats.</t>
</section>
<section title="Regular Compound Object Example">
<figure anchor="expl_object" title="Compound Object Example.">
<artwork align="center"><![CDATA[ 0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=0|Resvd=0|1|0|MDFmt=0|MDEnc=0| Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Compound Object Header Length=41 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
. .
. meta-data ASCII null terminated string (33 bytes) .
. .
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. Object data .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+]]></artwork>
</figure>
<t><xref target="expl_object"/> shows a regular Compound Object where
the meta-data ASCII string, in HTTP/1.1 meta-information format
(MDFmt=0) contains:</t>
<figure>
<artwork align="left"><![CDATA[
Content-Location: example.txt <CR-LF>
]]></artwork>
</figure>
<t>This string is 33 bytes long, including the NULL-termination
character. There is no GZIP encoding of the meta-data (MDEnc=0) and
there is no Content-Length information either since this length can
easily be calculated by the receiver as the FEC OTI transfer length
minus the header length. Finally, the checksum encompasses the whole
Compound Object (G=1).</t>
<!-- removed because only on rare circunstances a compound object will
not contain any meta data -->
<!--
<figure anchor="expl_no_meta-data"
title="Compound Object Example with no Meta-Data.">
<artwork align="center"><![CDATA[ 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0 | 0 |1|0| 0 | 0 | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 8 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. Object data .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+]]></artwork>
</figure>
<t><xref target="expl_no_meta-data"></xref> shows a Compound Object
without any meta-data. The fact there is no meta-data is indicated by
the value 8 of the Compound Object Header Length field. No
padding is required.</t>
-->
</section>
<section title="Carousel Instance Descriptor Example">
<!-- ==================================== -->
<t><xref target="expl_cid"/> shows an example CID object, in the case
of a static FCAST session, i.e., a session where the set of objects is
set once and for all. There is no meta-data in this example since
Fcast-CID-Complete and Fcast-CID-ID are both implicit.</t>
<figure align="center" anchor="expl_cid"
title="Example of CID, in case of a static session.">
<artwork align="center"><![CDATA[ 0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=0|Resvd=0|1|1|MDFmt=0|MDEnc=0| Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Compound Object Header Length=8 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
. .
. Object List string .
. .
. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+]]></artwork>
</figure>
<t>The object list contains the following 26 byte long string,
including the NULL-termination character:</t>
<figure>
<artwork align="left"><![CDATA[1,2,3,100-104,200-203,299]]></artwork>
</figure>
<t>There are therefore a total of 3+5+4+1 = 13 objects in the carousel
instance, and therefore in the FCAST session. There is no meta-data
associated to this CID. The session being static and composed of a
single Carousel Instance, the sender did not feel the necessity to
carry a Carousel Instance ID meta-data.</t>
</section>
</section>
<section anchor="annex_add_meta_data_tx_mechanisms"
title="Additional Meta-Data Transmission Mechanisms">
<!-- ==================================== -->
<section title="Supporting Additional Mechanisms">
<!-- ==================================== -->
<t>In certain use-cases, FCAST can take advantage of another in-band
(e.g., via NORM_INFO messages <xref target="norm_specifics"/>) or
out-of-band signaling mechanism. This section provides an overview of
how other signaling mechanism could be employed and a normative
specification for how FCAST information is embedded when NORM_INFO
messages are used for FCAST compound message headers. Such additional
signaling schemes can be used to carry the whole meta-data, or a
subset of it, ahead of time, before the associated compound object.
Therefore a receiver could be able to decide in advance, before
beginning the reception of the compound object, whether the object is
of interest or not, based on the information retrieved by this way,
which mitigates FCAST limitations. While out-of-band techniques are
out of the scope of this document, we explain below how this can be
achieved in case of FCAST/NORM.</t>
<t>Supporting additional mechanisms is OPTIONAL in FCAST
implementations. In any case, an FCAST sender MUST continue to send
all the required meta-data in the compound object, even if the whole
meta-data, or a subset of it, is sent by another mechanism (<xref
target="compliance_requirements"/>). Additionally, when meta-data is
sent several times, there MUST NOT be any contradiction in the
information provided by the different mechanisms. In case a mismatch
is detected, the meta-data contained in the Compound Object MUST be
used as the definitive source.</t>
<t>When meta-data elements are communicated out-of-band, in advance of
data transmission, the following piece of information can be useful:
<list style="symbols">
<t>TOI: a positive integer that contains the Transmission Object
Identifier (TOI) of the object, in order to enable a receiver to
easily associate the meta-data to the object. The valid range for
TOI values is discussed in <xref target="object_id"/>;</t>
</list></t>
</section>
<section anchor="norm_specifics"
title="Using NORM_INFO Messages with FCAST/NORM">
<!-- ==================================== -->
<t>The NORM_INFO message of NORM can convey "out-of-band" content with
respect to a given transport object. With FCAST, this message MAY be
used as an additional mechanism to transmit meta-data. In that case,
the NORM_INFO message carries a new Compound Object that contains all
the meta-data of the original object, or a subset of it. The NORM_INFO
Compound Object MUST NOT contain any Object Data field (i.e., it is
only composed of the header), it MUST feature a non global checksum,
and it MUST NOT include any padding field. <!--
The main Compound
Object MUST in any case contain the whole meta-data (e.g., because
a receiver MAY not support the NORM_INFO facility). Additionally, the
meta-data entries contained in the NORM_INFO MUST be identical to
the same entries in the main Compound Object.
--> Finally, note that the availability of NORM_INFO for a given
object is signaled through the use of a dedicated flag in the
NORM_DATA message header. Along with NORM's NACK-based repair request
signaling, it allows a receiver to quickly (and independently) request
an object's NORM_INFO content. However, a limitation here is that the
NORM_INFO Compound Object header MUST fit within the byte size limit
defined by the NORM sender's configured "segment size" (typically a
little less than the network MTU);</t>
<section title="Example">
<!-- ==================================== -->
<t>In case of FCAST/NORM, the FCAST Compound Object meta-data (or a
subset of it) can be carried as part of a NORM_INFO message, as a
new Compound Object that does not contain any Compound Object Data.
In the following informative example we assume that the whole
meta-data is carried in such a message for a certain Compound
Object. <xref target="expl_norm_info"/> shows an example NORM_INFO
message that contains the FCAST Compound Object Header and meta-data
as its payload. In this example, the first 16 bytes are the
NORM_INFO base header, the next 12 bytes are a NORM EXT_FTI header
extension containing the FEC Object Transport Information for the
associated object, and the remaining bytes are the FCAST Compound
Object Header and meta-data. Note that "padding" MUST NOT be used
and that the FCAST checksum only encompasses the Compound Object
Header (G=0).</t>
<figure align="center" anchor="expl_norm_info"
title="NORM_INFO containing an EXT_FTI header extension and an FCAST Compound Object Header">
<artwork align="center"><![CDATA[0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ --
|version| type=1| hdr_len = 7 | sequence | ^
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| source_id | n
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ o
| instance_id | grtt |backoff| gsize | r
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ m
| flags | fec_id = 5 | object_transport_id | v
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ --
| HET = 64 | HEL = 3 | | ^
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + f
| Transfer Length = 41 | t
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ i
| Encoding Symbol Length (E) | MaxBlkLen (B) | max_n | v
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ --
| 0 | 0 |0|0| 0 | 0 | Checksum | ^
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| 41 | f
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-| c
. . a
. meta-data ASCII null terminated string (33 bytes) . s
. . t
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| | v
+-+-+-+-+-+-+-+-+ --]]></artwork>
</figure>
<t>The NORM_INFO message shown in <xref target="expl_norm_info"/>
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 <xref target="RFC5510"/>.
Other alternatives for providing the FEC OTI would have been to
either include it directly in the meta-data of the FCAST Compound
Header, or to include an EXT_FTI header extension to all NORM_DATA
packets (or a subset of them). Note that the NORM "Transfer_Length"
is the total length of the associated FCAST Compound Object, i.e.,
41 bytes.</t>
<t>The FCAST Compound Object in this example does contain the same
meta-data and is formatted as in the example of <xref
target="expl_object"/>. With the combination of the FEC_OTI and the
FCAST meta-data, the NORM protocol and FCAST application have all of
the information needed to reliably receive and process the
associated object. Indeed, the NORM protocol provides rapid
(NORM_INFO has precedence over the associated object content),
reliable delivery of the NORM_INFO message and its payload, the
FCAST Compound Object.</t>
</section>
</section>
</section>
</back>
</rfc>
| PAFTECH AB 2003-2026 | 2026-04-23 05:33:46 |