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) andNACK-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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RFC 6968 FCAST Object Delivery July 20135.4. Other Security Considerations .............................275.5. Minimum Security Recommendations ..........................276. Operational Considerations .....................................287. IANA Considerations ............................................297.1. Creation of the FCAST Object Metadata Format Registry .....297.2. Creation of the FCAST Object Metadata Encoding Registry ...307.3. Creation of the FCAST Object Metadata Types Registry ......308. Acknowledgments ................................................329. References .....................................................329.1. Normative References ......................................329.2. Informative References ....................................33Appendix A. FCAST Examples ........................................35A.1. Simple Compound Object Example .............................35A.2. Carousel Instance Descriptor Example .......................36Appendix B. Additional Metadata Transmission Mechanisms ...........37B.1. Supporting Additional Mechanisms ...........................37B.2. Using NORM_INFO Messages with FCAST/NORM ...................38B.2.1. Example ................................................381. 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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RFC 6968 FCAST Object Delivery July 2013
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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RFC 6968 FCAST Object Delivery July 20131.2. 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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RFC 6968 FCAST Object Delivery July 2013
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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RFC 6968 FCAST Object Delivery July 2013
| 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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RFC 6968 FCAST Object Delivery July 2013
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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RFC 6968 FCAST Object Delivery July 2013
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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RFC 6968 FCAST Object Delivery July 2013
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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RFC 6968 FCAST Object Delivery July 2013
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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RFC 6968 FCAST Object Delivery July 20133.2. 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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RFC 6968 FCAST Object Delivery July 2013
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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RFC 6968 FCAST Object Delivery July 20133.4. 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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RFC 6968 FCAST Object Delivery July 2013
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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RFC 6968 FCAST Object Delivery July 20133.6. 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.
Roca & Adamson Experimental [Page 17]

RFC 6968 FCAST Object Delivery July 20133.7. 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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RFC 6968 FCAST Object Delivery July 20133.8. 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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RFC 6968 FCAST Object Delivery July 20134. 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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RFC 6968 FCAST Object Delivery July 20135. Security Considerations5.1. 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).
Roca & Adamson Experimental [Page 22]

RFC 6968 FCAST Object Delivery July 20135.2.1. 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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RFC 6968 FCAST Object Delivery July 2013
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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RFC 6968 FCAST Object Delivery July 2013
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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RFC 6968 FCAST Object Delivery July 2013
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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RFC 6968 FCAST Object Delivery July 2013
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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RFC 6968 FCAST Object Delivery July 20136. 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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RFC 6968 FCAST Object Delivery July 2013
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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RFC 6968 FCAST Object Delivery July 2013Appendix B. Additional Metadata Transmission MechanismsB.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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RFC 6968 FCAST Object Delivery July 2013B.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).
Roca & Adamson Experimental [Page 38]