RMT V. Roca
Internet-Draft INRIA
Intended status: Experimental B. Adamson
Expires: January 15, 2009 Naval Research Laboratory
July 14, 2008
FCAST: Scalable Object Delivery for the ALC and NORM Protocols
draft-roca-rmt-newfcast-02
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Abstract
This document introduces the FCAST object (e.g., file) delivery
application on top of the ALC and NORM reliable multicast protocols.
FCAST is a highly scalable application that provides a reliable
object delivery service.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Applicability . . . . . . . . . . . . . . . . . . . . . . 4
2. Requirements notation . . . . . . . . . . . . . . . . . . . . 4
3. Definitions, Notations and Abbreviations . . . . . . . . . . . 5
3.1. Definitions . . . . . . . . . . . . . . . . . . . . . . . 5
3.2. Abbreviations . . . . . . . . . . . . . . . . . . . . . . 5
4. FCAST Principles . . . . . . . . . . . . . . . . . . . . . . . 6
4.1. FCAST Content Delivery Service . . . . . . . . . . . . . . 6
4.2. Meta-Data Transmission . . . . . . . . . . . . . . . . . . 6
4.3. Meta-Data Content . . . . . . . . . . . . . . . . . . . . 7
4.4. Carousel Transmission . . . . . . . . . . . . . . . . . . 8
4.5. Carousel Instance Object . . . . . . . . . . . . . . . . . 9
4.6. FCAST Sender Behavior . . . . . . . . . . . . . . . . . . 10
4.7. FCAST Receiver Behavior . . . . . . . . . . . . . . . . . 11
4.8. FCAST Object Identification . . . . . . . . . . . . . . . 12
4.9. FCAST/ALC Additional Specificities . . . . . . . . . . . . 13
4.10. FCAST/NORM Additional Specificities . . . . . . . . . . . 13
5. FCAST Specifications . . . . . . . . . . . . . . . . . . . . . 14
5.1. Compound Object Header Format . . . . . . . . . . . . . . 14
5.2. Carousel Instance Object (CIO) Format . . . . . . . . . . 16
6. Security Considerations . . . . . . . . . . . . . . . . . . . 18
6.1. Problem Statement . . . . . . . . . . . . . . . . . . . . 18
6.2. Attacks Against the Data Flow . . . . . . . . . . . . . . 19
6.2.1. Access to Confidential Objects . . . . . . . . . . . . 19
6.2.2. Object Corruption . . . . . . . . . . . . . . . . . . 19
6.3. Attacks Against the Session Control Parameters and
Associated Building Blocks . . . . . . . . . . . . . . . . 20
6.3.1. Attacks Against the Session Description . . . . . . . 21
6.3.2. Attacks Against the FCAST CIO . . . . . . . . . . . . 21
6.3.3. Attacks Against the Object Meta-Data . . . . . . . . . 22
6.3.4. Attacks Against the ALC/LCT Parameters . . . . . . . . 22
6.3.5. Attacks Against the Associated Building Blocks . . . . 22
6.4. Other Security Considerations . . . . . . . . . . . . . . 23
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 23
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 24
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 24
9.1. Normative References . . . . . . . . . . . . . . . . . . . 24
9.2. Informative References . . . . . . . . . . . . . . . . . . 24
Appendix A. FCAST in practice . . . . . . . . . . . . . . . . . . 25
Appendix B. FCAST Examples . . . . . . . . . . . . . . . . . . . 26
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 28
Intellectual Property and Copyright Statements . . . . . . . . . . 29
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1. Introduction
This document introduces the FCAST reliable and scalable object
(e.g., file) delivery application. Two versions of FCAST exist:
o FCAST/ALC that relies on the Asynchronous Layer Coding (ALC)
[RMT-PI-ALC] and the Layered Coding Transport (LCT) [RMT-BB-LCT]
reliable multicast transport protocol, and
o FCAST/NORM that relies on the NACK-Oriented Reliable Multicast
(NORM) [RMT-PI-NORM] reliable multicast transport protocol.
Hereafter, the term FCAST denotes either FCAST/ALC or FCAST/NORM.
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, see
Appendix A), then the service can be considered as fully reliable.
On the opposite, if FCAST operates in PUSH mode, then the service is
usually only partially reliable, and a receiver that is disconnected
during a sufficient time will perhaps not have the possibility to
download the object.
Depending on the target use case, the FCAST scalability is more or
less important. 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 opposite, the FCAST/NORM scalability is
typically limited by NORM scalability itself. Similarly, if FCAST is
used along with a session control application that collects reception
information from the receivers, then this session control application
limits 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 is out of the
scope of the present document.
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 opposite, FCAST has some intrinsic limitations. From
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this point of view it differs from FLUTE [RMT-FLUTE] which favors
flexibility at the expense of some additional complexity.
A good example of the design choices that are meant to favor
simplicity, is the way FCAST manages the meta-data of an object: with
FCAST, the meta-data are simply prepended to the object. This
solution has many advantages in terms of simplicity as will be
described later on. But it also has an intrinsic limitation since it
does not enable a receiver to decide in advance, before beginning
reception of the object, whether the object is of interest or not.
Thus, if there is no out-of-band mechanism to enable receivers to
obtain the meta-data (or a subset) in advance, then all the objects
sent in the FCAST session should be of interest to all receivers. If
this is not the case, a receiver will probably waste time and
resources to receive and decode objects that will turn out to be
useless to him.
1.1. Applicability
FCAST is compatible with any congestion control protocol designed for
ALC/LCT or NORM. However, depending on the use-case, the data flow
generated by the FCAST application might not be constant, but instead
be bursty in nature. Similarly, depending on the use-case, an FCAST
session might be very short. Whether and how this will impact the
congestion control protocol is out of the scope of the present
document.
FCAST is compatible with any security mechanism designed for ALC/LCT
or NORM. The use of a security scheme is strongly RECOMMENDED (see
Section 6).
FCAST is compatible with any FEC scheme designed for ALC/LCT or NORM.
Whether FEC is used or not, and the kind of FEC scheme used, is to
some extent transparent to FCAST.
FCAST is compatible with both IPv4 and IPv6. Nothing in the FCAST
specification has any implication on the source or destination IP
address.
2. 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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3. Definitions, Notations and Abbreviations
3.1. Definitions
This document uses the following definitions:
FCAST/ALC denotes the FCAST application running on top of the ALC/
LCT reliable transport protocol;
FCAST/NORM denotes the FCAST application running on top of the
NORM reliable transport protocol;
FCAST denotes either FCAST/ALC or FCAST/NORM;
Compound Object denotes an ALC or NORM transport object composed
of the Compound Object Header Section 5.1, including any meta-data
and the content of the original application object (e.g., a file);
Carousel denotes the compound object transmission system of an
FCAST sender;
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 Object (CIO) denotes a specific object that
lists the compound objects that comprise a given carousel
instance;
Carousel Cycle denotes a transmission round within which all the
compound objects registered in the Carousel Instance are
transmitted a certain number of times. By default, compound
objects are transmitted once per cycle, but higher values are
possible, that might differ on a per-object basis;
The Transmission Object Identifier (TOI) refers the numeric
identifier associated to a specific object by the underlying
transport layer. In the case of ALC, this corresponds to the TOI
described in that specification while for the NORM specification
this corresponds to the NormObjectId described there.
3.2. Abbreviations
This document uses the following abbreviations:
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+------------------+-------------------------------------+
| Abbreviation | Definition |
+------------------+-------------------------------------+
| CIO | Carousel Instance Object |
| FEC OTI | FEC Object Transmission Information |
| TOI | Transmission Object Identifier |
+------------------+-------------------------------------+
4. FCAST Principles
4.1. FCAST Content Delivery Service
The basic goal of FCAST is to transmit objects to a group of
receivers in a reliable way. The receiver set MAY be restricted to a
single receiver or MAY include possibly a very large number of
receivers. FCAST is specified to support two forms of operation.
1. FCAST/ALC: where the FCAST application is meant to run on top of
the ALC/LCT reliable multicast transport protocol, and
2. FCAST/NORM: where the FCAST application is meant to run on top of
the NORM reliable multicast transport protocol.
This specification is designed such that both forms of operation
share as much commonality as possible.
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. The transmission might be fully
reliable, or only partially reliable depending upon the way ALC or
NORM is used (e.g., whether FEC encoding and/or NACK-based repair
requests are used or not), the way the FCAST carousel is used (e.g.,
whether the objects are made available for a long time span or not),
and 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).
FCAST is designed to be as self-sufficient as possible, in particular
in the way object meta-data is attached to object data content.
However, for some uses, meta-data MAY also be communicated by an out-
of-band mechanism that is out of the scope of the present document.
4.2. Meta-Data Transmission
FCAST usually carries meta-data elements by prepending them to the
object it refers to. As a result, a compound object is created that
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is composed of a header followed by the original object content.
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.
Attaching the meta-data to the object is an efficient solution, since
it guaranties that meta-data be received along with the associated
object, and it allows the transport of the meta-data to benefit from
any transport-layer FEC erasure protection of the compound object.
However a limit of this scheme, as such, is that a client does not
know the meta-data of an object before it begins receiving the object
and perhaps not until decoding the object completely depending upon
the transport protocol used and its particular FEC code type and
parameters.
However, this solution can be associated to another in-band (e.g.,
via NORM INFO messages, Section 4.10) or out-of-band signaling
mechanism (Appendix A) in order to carry the whole meta-data (or a
subset of it) possibly ahead of time.
4.3. Meta-Data Content
The meta-data associated to an object can be composed of, but are not
limited to:
o Content-Location: the URI of the object, which gives the name and
location of the object;
o Content-Type: the MIME type of the object;
o Content-Length: the size of the initial object, before any content
encoding (if any). Note that this content length does not include
the meta-data nor the header of the compound object;
o Content-Encoding: the optional encoding of the object performed by
FCAST;
o Content-MD5: the MD5 message digest of the object in order to
check its integrity. Note that this digest is meant to protect
from transmission and processing errors, not from deliberate
attacks by an intelligent attacker. Note also that this digest
only protects the object, not the header, and therefore not the
meta-data;
o a digital signature for this object;
This list is not limited and new meta-data information can be added.
For instance, when dealing with very large objects (e.g., that
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largely exceed the working memory of a receiver), it can be
interesting to split this object into several sub-objects. When a
file is split into several objects by FCAST, the meta-data includes:
o Fcast_Obj_Slice_Nb: the total number of slices. A value strictly
greater than 1 indicates that this object is the result of a split
of the original object;
o Fcast_Obj_Slice_Idx: the slice index (in the [0 .. SliceNb[
interval);
o Fcast_Obj_Slice_Offset: the offset at which this slice starts
within the original object;
When meta-data elements are communicated out-of-band, in advance of
data transmission, the following pieces of information may also be
useful:
o TOI: the Transmission Object Identifier (TOI) of the object, in
order to enable a receiver to easily associate the meta-data to
the object for which he receives packets;
o FEC Object Transmission Information (FEC OTI). In this case the
FCAST sender does not need to use the optional EXT_FTI mechanism
of ALC or NORM protocols.
4.4. Carousel Transmission
A set of FCAST compound objects scheduled for transmission are
considered a logical "Carousel". A single "Carousel Instance" is
comprised of a fixed set of compound objects. Whenever the FCAST
application needs to add new objects to or remove old objects from
the transmission set, a new Carousel Instance is defined since the
set of compound objects changes.
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. This
can be the case for instance of the CIO objects (Section 4.5). For
some FCAST usage (e.g., a unidirectional "push" mode), a Carousel
Instance may have only a single transmission cycle. In other cases
there may be a large number of transmission cycles (e.g., such as an
"on-demand" mode where objects are made available for download during
a possibly very long period of time).
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4.5. Carousel Instance Object
The FCAST sender MAY transmit an OPTIONAL Carousel Instance Object
(CIO). The CIO carries a list of the compound objects that are part
of a given Carousel Instance. The objects are listed using their
respective Transmission Object Identifiers (TOI). There is no
reserved TOI value for the CIO, since this object is regarded by ALC/
LCT or NORM as a standard object. The nature of this object is
indicated by means of a specific meta-data field so that it can be
recognized and processed by the FCAST application as needed.
The CIO includes a Carousel Instance ID (CID) that identifies the
Carousel Instance. The CIO includes a "Complete" flag that is used
to indicate that no other modification to the enclosed list will be
done in the future. However the CIO does not describe the objects
themselves (i.e., there is no meta-data). Any objects that are not
incuded in the CIO list MUST NOT be considered as part of the current
Carousel Instance, even if they were part of any previous Carousel
Instances.
Note use of a CIO is NOT mandatory. If it is not used, then the
clients will progressively learn what files are part of the carousel
instance by receiving ALC or NORM packets with new TOIs. However use
of the CIO has several benefits:
o Receivers know when they can leave the session, i.e., when they
have received all the objects that are part of the delivery
session, thanks to the "Complete" flag;
o In case of a session with a dynamic set of objects, the sender can
reliably inform the receivers that some objects have been removed
from the carousel with the CIO. 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 CIO provides an
additional means for receivers to learn of objects for which it is
futile to request repair
The decision of whether a CIO should be used, as well as how often
and when it should be sent, is left to the sender and depends on many
parameters, including the target use case and the session dynamics.
In case of an FCAST session in a strictly unidirectional, proactive
transmission mode (i.e., "push" mode), the CIO SHOULD be sent before
the objects (and repeated periodically during the Carousel Instance
transmission to enable late receivers to catch up, if this is
desired). In case of a highly dynamic FCAST session, a CIO will
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probably be sent at the beginning of each new carousel instance, and
then periodically. The period of CIO repetition depends on the
desired maximum latency that could be experienced by late receivers
who joined the FCAST session in the middle of a carousel instance
transmission cycle, and therefore missed the initial CIO
transmission. These operational aspects are out of the scope of the
present document.
4.6. FCAST Sender Behavior
The following operations take place at a sender:
1. The user (or another application) selects a set of objects (e.g.,
files) to deliver and submits them to the FCAST application. The
user also specifies how many times each object should be sent in
this carousel instance. Said differently, if objects have
similar lengths, assigning them a different number of
transmissions leads to define different transmission priorities
to each of them;
2. For each object, FCAST creates the compound object and registers
this latter in the carousel instance.
3. The user then informs FCAST when all the objects of the set have
been submitted. If no new object will be submitted later to
FCAST (i.e., if the session's content is now complete), the user
SHOULD also provide FCAST this information;
4. At this point, the FCAST application knows the full list of
compound objects that are part of the carousel instance and can
define a transmission schedule of these objects. This
specification does not mandate any transmission schedule scheme.
This is left to the developer within the provisions of the
underlying ALC or NORM protocol used.
5. The FCAST application will create a CIO as needed. If no new
object will be submitted, then the sender includes the "Complete"
keyword in any CIO created to inform the receivers that no object
in addition to the ones specified in this carousel instance will
be sent. While this specification RECOMMENDS that the sender
SHOULD send the CIO prior to the transmission of the associated
objects, it does not mandate if or how the CIO transmission
should be repeated during the associated carousel instance. This
is left to the developer;
6. The FCAST application then starts the carousel transmission, for
the number of cycles specified (which might be infinite), taking
into account the possible transmission specificities of each
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object. The 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 opposite wants to remove old objects from
the carousel. In that case a new carousel instance must be
created.
Then continue at Step 1 above.
_*** Editor's note: Question: should a sender use a CIO with an
empty list of objects when he has reached the desired number of
cycles? Do we say "SHOULD" or "MUST"? Possible wording (to
discuss): When the desired number of transmission cycles has been
reached, after a small duration during which the user did not
submit any new object and did not tell FCAST to add some more
transmission cycles, the sender SHOULD create and send a CIO with
an empty list of objects. However, it should be noted that doing
so is sub-optimal if some of the objects are to be sent once again
latter on, since the receiver will destroy those objects that have
not been totally decoded upon receiving this CIO._
4.7. FCAST Receiver Behavior
The following operations take place at an FCAST receiver:
1. The receiver joins the session and collects symbols;
2. As the header portion of compound objects are received (which may
be received before the entire object is received with some ALC/
NORM transport configurations), the receiver processes the meta-
data and may choose to continue to receive the object content or
not;
3. When a compound object has been entirely received, the receiver
processes the header, retrieves the object meta-data, perhaps
decodes the meta-data, and processes the object accordingly;
4. When a CIO is received, which is indicated by the 'I' flag set in
the compound object header, the receiver decodes the CIO, and
retrieves the list of objects that are part of the current
carousel instance. This list is used to remove objects sent in a
previous carousel instance that might not have been totally
decoded. This list is also used to set up a new filter, since
all the received content for objects from the given sender that
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are not part of this list SHOULD be immediately discarded;
5.
_*** Editor's note: there is an exception: the TOI for the
following CIO. This TOI is perhaps not yet known, but it MUST
NOT be filtered. This is where having a floating TOI for CIOs
makes things a bit more complex. How to address this problem
is TBD._
6. When a receiver has received a CIO with the "Complete" flag set,
and has successfully received all the objects of the current
carousel instance, it can safely exit from the current FCAST
session;
4.8. FCAST Object Identification
FCAST objects are directly associated with the object-based transport
service that the ALC and NORM protocols provide. In each of these
protocols, messages containing transport object content are labeled
with a numeric transport object identifier (i.e., the ALC TOI and the
NORM _NormTransportId_). For purposes of this document, this
identifier in either case (ALC or NORM) is referred to as the TOI.
The FCAST Compound Object Header meta-data can include an attribute
that identifies the given object's TOI. Additionally, the CIO lists
objects for the applicable Carousel Instance by using the TOI.
In both NORM and ALC, it is possible that the transport
identification space may eventually wrap for very long-lived
sessions. 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 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 allow receivers to
properly combine new transport symbols for any olders objects with
newly-assigned TOIs to achieve reliable transfer, a mechanism is
required to equate the object(s) with new TOI with the older object
TOI. _This mechanism is TBD._
_*** Editor's note: Perhaps a way to disambiguate possible
wrapping of TOI is by concatenation of the Carousel Instance Id
and TOI? And also provide a mechanism to equate an object with a
new TOI in a new Carousel Instance with an older TOI in an older
Carousel Instance if it represents the same content. This way the
transport object id could "move forward" as needed and receivers
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could possibly combine symbols from the new transmission with the
older content. Vincent had a scheme that partially addressed this
notion in an email._
4.9. FCAST/ALC Additional Specificities
There are no additional details or options for FCAST/ALC operation.
4.10. FCAST/NORM Additional Specificities
The NORM Protocol provides a few additional capabilities that can be
used to specifically support FCAST operation:
1. The NORM_INFO message for conveying "out-of-band" content with
respect to a given transport object MAY be used to provide the
FCAST compound object header and meta-data to the receiver group.
NORM's NACK-based repair request signaling allows for an object's
NORM_INFO content to be requested separately and more quickly
than the object's "in-band" data content that is typically
encoded using FEC. However, the limitation here is that the
Compound Object Header and its meta-data MUST fit within the byte
size limit defined by the NORM sender's configured "segment size"
(typically a little less than the network MTU).
2. The NORM_CMD(SQUELCH) messages used by the NORM protocol sender
to inform receivers of objects that have been canceled when
receivers make repair requests for such invalid objects.
3. 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.
Receivers automatically learn of the availability of NORM_INFO for a
given object from a flag in the NORM_DATA message header. When
NORM_INFO is used for FCAST/NORM operation, the NORM_INFO content
MUST contain the FCAST Compound Object Header and meta-data for that
object. In this case, the data content portion of the NORM transport
object is the original application object. When NORM_INFO is not
used for a given sender object (i.e., the corresponding NORM_DATA
header flag is not set), the NORM transport object data content sent
MUST contain the FCAST Compound Object Header unless this information
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is signaled by another means (out of scope of this document) prior to
the carousel transmission.
It should also 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.
5. FCAST Specifications
This section details the technical aspects of FCAST.
5.1. Compound Object Header Format
In an FCAST session, its compound objects are constructed by
prepending the Compound Object Header including any meta-data content
as shown in Figure 1 before the original object data content.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|rsvd |I|MDE|MDF| Header Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Object Meta-Data Content (optional, variable length) |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | Padding (optional) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. Object Data Content (optional, variable length) .
. .
. .
Figure 1: Compound Object Header with Meta-Data
The Compound Object Header fields are:
+------------+------------------------------------------------------+
| Field | Description |
+------------+------------------------------------------------------+
| I | 1-bit field that, when set to 1, indicates the |
| | object is a Carousel Instance Object (CIO). When |
| | set to 0, this field indicates that the transported |
| | object is a standard object. |
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| Meta-Data | 2-bit field that defines the optional encoding of |
| Encoding | the Object Meta-Data Content field (see Section 7). |
| (MDEnc) | A plain text encoding is the default encoding and is |
| | associated value 0. A gzip encoding MAY be |
| | supported and is associated to value 1. Other |
| | encodings MAY be defined in the future. |
| Meta-Data | 2-bit field that defines the format of the object |
| Format | meta-data (see Section 7). An HTTP/1.1 |
| (MDFmt) | metainformation format [RFC2068] MUST be supported |
| | and is associated to value 0. Other formats (e.g., |
| | XML) MAY be defined in the future. |
| Header | 24-bit field indicating total length (in bytes) of |
| Length | all fields of the Compound Object Header, except the |
| | optional padding. A header length field set to |
| | value 4 means that there is no meta-data included. |
| | When this size is not multiple to 32 bits words, |
| | padding is added. It should be noted that the |
| | meta-data field maximum size is equal to 2^24 - 4 |
| | bytes. |
| Object | Optional, variable length field that contains the |
| Meta-Data | meta-data associated to the object, either in plain |
| | text or encoded, as specified by the MDEnc field. |
| | The Meta-Data is NULL-terminated plain text of the |
| | "TYPE" ":" "VALUE" "<CR-LF>" format used in HTTP/1.1 |
| | for metainformation [RFC2068]. 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. |
| Padding | Optional, variable length field of zero-value bytes |
| | to align start of object data content to 32-bit |
| | boundary. Padding is only used when the header |
| | length value, in bytes, is not multiple of 4. |
| Object | Data content of original object represented by this |
| Data | Compound Object. Note that the length of this |
| Content | content is the transported object size minus the |
| | Compound Object Header Length |
+------------+------------------------------------------------------+
_*** Editor's note: Should we add a checksum to protect the header
itself? Since meta-data do not use an XML encoding, there is no
way to digitally sign it to check its integrity. A checksum could
offer some integrity guaranty (not security of course). _
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5.2. Carousel Instance Object (CIO) Format
The format of the CIO, which is a particular compound object, is
given in Figure 2. Because the CIO is transmitted as a special
compound object, the following CIO-specific meta-data entry is
defined:
o Fcast_CIO_complete: when set to 1, it indicates that no new
objects in addition to the ones whose TOI are specified in this
CIO, or the ones that have been specified in the previous CIO(s),
will be generated;
o Fcast_CIO_ID: this value identifies the carousel instance. It
starts from 0 and is incremented by 1 for each new carousel
instance. This entry is not mandatory since the TOI numbering of
the compound objects carrying a CIO can be used to identify the
latest CIO instance. However, this value can be useful to detect
possible gaps in the carousel instances, for instance caused by
long disconnection periods. It can also be usefull to avoid
problems when TOI wrapping to 0 takes place.
Additionaly, the following standard meta-data entries are often used:
o Content-Encoding: the optional encoding of the CIO object, by
FCAST. For instance:
Content-Encoding: gzip
indicates that the Object List field has been encoded with gzip
[RFC1952]. When set to 0, this flag indicates the the Object List
field is plain text. The support of gzip encoding is MANDATORY,
both for an FCAST sender and for an FCAST receiver
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ^
|rsvd |1|MDE|MDF| Header Length | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ h
| | d
| Object Meta-Data Content (optional, variable length) | r
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| | Padding (optional) | v
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ^
. . |
. Object List (variable length) . O
. . b
. +-+-+-+-+-+-+-+-+ j
. | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ v
Figure 2: Carousel Instance Object Format
The CIO fields are:
+------------+------------------------------------------------------+
| Field | Description |
+------------+------------------------------------------------------+
| Object | List of TOIs included in the current carousel |
| List | instance, in an exhaustive way. This list, whose |
| | format is defined below, can be either in plain text |
| | (if Z is not set) or gzip'ed (if Z is set). An |
| | empty list (0 length field) indicates that the |
| | current carousel instance does not include any |
| | object. |
+------------+------------------------------------------------------+
The non-encoded (i.e., plain text) Object List is a NULL-terminated,
ASCII string containing the list of TOIs included in the current
carousel instance, specified either as the individual TOIs of each
object, or as TOI spans, or combinations of these. The format of the
ASCII string is a comma-separated list of individual "TOI" values or
"TOI_a-TOI_b" elements. This latter case means that all values
between TOI_a and TOI_b, inclusive, are part of the list. We further
require that TOI_a be strictly inferior to TOI_b. The ABNF
specification is the following:
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cio-list = *(list-elem *( "," list-elem))
list-elem = toi-value / toi-range
toi-value = 1*DIGIT
toi-range = toi-value "-" toi-value
; additionally, the first toi-value MUST be
; strictly inferior to the second toi-value
DIGIT = %x30-39
; a digit between O and 9, inclusive
It is RECOMMENDED, for processing reasons, that all the TOI values in
the list be given in increasing order. However a receiver MUST be
able to handle non-monotonically increasing values. It is
RECOMMENDED, for processing reasons, that a given TOI value NOT be
included mutiple times in the list.
6. Security Considerations
6.1. Problem Statement
A content delivery system is potentially subject to attacks. Attacks
may target:
o the network (to compromise the routing infrastructure, e.g., by
creating congestion),
o the Content Delivery Protocol (CDP) (e.g., to compromise the
normal behavior of FCAST) or
o the content itself (e.g., to corrupt the objects being
transmitted).
These attacks can be launched either:
o against the data flow itself (e.g., by sending forged packets),
o against the session control parameters (e.g., by corrupting the
session description, the CIO, the object meta-data, or the ALC/LCT
control parameters), that are sent either in-band or out-of-band,
or
o against some associated building blocks (e.g., the congestion
control component).
In the following sections we provide more details on these possible
attacks and sketch some possible counter-measures.
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6.2. Attacks Against the Data Flow
Let us consider attacks against the data flow first. At least, the
following types of attacks exist:
o attacks that are meant to give access to a confidential object
(e.g., in case of a non-free content) and
o attacks that try to corrupt the object being transmitted (e.g., to
inject malicious code within an object, or to prevent a receiver
from using an object, which is a kind of Denial of Service (DoS)).
6.2.1. Access to Confidential Objects
Access control to the object being transmitted is typically provided
by means of encryption. This encryption can be done over the whole
object (e.g., by the content provider, before submitting the object
to FCAST), or be done on a packet per packet basis (e.g., when IPSec/
ESP is used [RFC4303]). If confidentiality is a concern, it is
RECOMMENDED that one of these solutions be used.
6.2.2. Object Corruption
Protection against corruptions (e.g., in case of forged packets) is
achieved by means of a content integrity verification/sender
authentication scheme. This service can be provided at the object
level, but in that case a receiver has no way to identify which
symbol(s) is(are) corrupted if the object is detected as corrupted.
This service can also be provided at the packet level. In this case,
after removing all corrupted packets, the file may be in some cases
recovered. Several techniques can provide this content integrity/
sender authentication service:
o at the object level, the object can be digitally signed (with
public key cryptography), for instance by using RSASSA-PKCS1-v1_5
[RFC3447]. This signature enables a receiver to check the object
integrity, once this latter has been fully decoded. Even if
digital signatures are computationally expensive, this calculation
occurs only once per object, which is usually acceptable;
o at the packet level, each packet can be digitally signed. A major
limitation is the high computational and transmission overheads
that this solution requires (unless perhaps if Elliptic Curve
Cryptography (ECC) is used). 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;
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o at the packet level, a Group Message Authentication Code (MAC)
[RFC2104] scheme can be used, for instance by using HMAC-SHA-1
with a secret key shared by all the group members, senders and
receivers. This technique creates a cryptographically secured
digest of a packet that is sent along with the packet. The Group
MAC scheme does not create prohibitive processing load nor
transmission overhead, but it has a major limitation: it only
provides a group authentication/integrity service since all group
members share the same secret group key, which means that each
member can send a forged packet. It is therefore restricted to
situations where group members are fully trusted (or in
association with another technique as a pre-check);
o at the packet level, Timed Efficient Stream Loss-Tolerant
Authentication (TESLA) [RFC4082] 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;
o at the packet level, IPSec/AH [RFC4302] (possibly associated to
IPSec/ ESP) can be used to protect all the packets being exchanged
in a session.
Techniques relying on public key cryptography (digital signatures and
TESLA during the bootstrap process, when used) require that public
keys be securely associated to the entities. This can be achieved by
a Public Key Infrastructure (PKI), or by a PGP Web of Trust, or by
pre-distributing securely the public keys of each group member.
Techniques relying on symmetric key cryptography (Group MAC) require
that a secret key be shared by all group members. This can be
achieved by means of a group key management protocol, or simply by
pre-distributing securely the secret key (but this manual solution
has many limitations).
It is up to the developer and deployer, who know the security
requirements and features of the target application area, to define
which solution is the most appropriate. In any case, whenever there
is any concern of the threat of file corruption, it is RECOMMENDED
that at least one of these techniques be used.
6.3. Attacks Against the Session Control Parameters and Associated
Building Blocks
Let us now consider attacks against the session control parameters
and the associated building blocks. The attacker has at least the
following opportunities to launch an attack:
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o the attack can target the session description,
o the attack can target the FCAST CIO,
o the attack can target the meta-data of an object,
o the attack can target the ALC/LCT parameters, carried within the
LCT header or
o the attack can target the FCAST associated building blocks.
The latter one is particularly true with the multiple rate congestion
control protocol which may be required.
The consequences of these attacks are potentially serious, since they
can compromise the behavior of content delivery system or even
compromise the network itself.
6.3.1. Attacks Against the Session Description
An FCAST receiver may potentially obtain an incorrect Session
Description for the session. The consequence of this is that
legitimate receivers with the wrong Session Description are unable to
correctly receive the session content, or that receivers
inadvertently try to receive at a much higher rate than they are
capable of, thereby possibly disrupting other traffic in the network.
To avoid these problems, it is RECOMMENDED that measures be taken to
prevent receivers from accepting incorrect Session Descriptions. One
such measure is the sender authentication to ensure that receivers
only accept legitimate Session Descriptions from authorized senders.
How these measures are archived is outside the scope of this document
since this session description is usually carried out-of-band.
6.3.2. Attacks Against the FCAST CIO
Corrupting the FCAST CIO is one way to create a Denial of Service
attack. For example, the attacker removes legitimate object TOIs
from the list.
It is therefore RECOMMENDED that measures be taken to guarantee the
integrity and to check the sender's identity of the CIO. To that
purpose, one of the counter-measures mentioned above (Section 6.2.2)
SHOULD be used. These measures will either be applied on a packet
level, or globally over the whole CIO object. When there is no
packet level integrity verification scheme, it is RECOMMENDED to
digitally sign the CIO.
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6.3.3. Attacks Against the Object Meta-Data
Corrupting the object meta-data is another way to create a Denial of
Service attack. For example, the attacker changes the MD5 sum
associated to a file. This possibly leads a receiver to reject the
files received, no matter whether the files have been correctly
received or not. When the meta-data are appended to the object,
corrupting the meta-data means that the compound object will be
corrupted.
It is therefore RECOMMENDED that measures be taken to guarantee the
integrity and to check the sender's identity of the compound object.
To that purpose, one of the counter-measures mentioned above
(Section 6.2.2) SHOULD be used. These measures will either be
applied on a packet level, or globally over the whole compound
object. When there is no packet level integrity verification scheme,
it is RECOMMENDED to digitally sign the compound object.
6.3.4. Attacks Against the ALC/LCT Parameters
By corrupting the ALC/LCT header (or header extensions) one can
execute attacks on the underlying ALC/LCT implementation. For
example, sending forged ALC packets with the Close Session flag (A)
set one can lead the receiver to prematurely close the session.
Similarly, sending forged ALC packets with the Close Object flag (B)
set one can lead the receiver to prematurely give up the reception of
an object.
It is therefore RECOMMENDED that measures be taken to guarantee the
integrity and to check the sender's identity of each ALC packet
received. To that purpose, one of the counter-measures mentioned
above (Section 6.2.2) SHOULD be used.
6.3.5. Attacks Against the Associated Building Blocks
Let us first focus on the congestion control building block that may
be used in the ALC session. A receiver with an incorrect or
corrupted implementation of the multiple rate congestion control
building block may affect the health of the network in the path
between the sender and the receiver. That may also affect the
reception rates of other receivers who joined the session.
When congestion control building block 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. How receivers identify themselves as
legitimate is outside the scope of this document. If authenticating
a receiver does not prevent this latter to launch an attack, it will
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enable the network operator to identify him and to take counter-
measures.
When congestion control building block is applied with FCAST/ALC, it
is also RECOMMENDED that a packet level authentication scheme be
used, as explained in Section 6.2.2. Some of them, like TESLA, only
provide a delayed authentication service, whereas congestion control
requires a rapid reaction. It is therefore RECOMMENDED [2] 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 since no congestion actually
occurred. 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 reduce the probability of this
attack.
6.4. Other Security Considerations
Lastly, we note that the security considerations that apply to, and
are described in, ALC [2], LCT [3] and FEC [4] 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.
7. IANA Considerations
This document requires a IANA registration for the following
attributes:
Object meta-data format (MDFmt): All implementations MUST support
format 0 (default).
+----------------------------------------+-------------+
| format name | Value |
+----------------------------------------+-------------+
| as per HTTP/1.1 metainformation format | 0 (default) |
+----------------------------------------+-------------+
Object Meta-Data Encoding (MDENC): All implementations MUST support
value 0 (default).
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+------------+-------------+
| Name | Value |
+------------+-------------+
| plain text | 0 (default) |
| gzip | 1 |
+------------+-------------+
8. Acknowledgments
The authors would like to thank Toni Paila and Rod Walsh for their
challenging comments throughout the design of FCAST. The authors are
grateful to the authors of [ALC_00] for specifying the first version
of FCAST/ALC.
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RMT-PI-ALC]
Luby, M., Watson, M., and L. Vicisano, "Asynchronous
Layered Coding (ALC) Protocol Instantiation", Work
in Progress, November 2007.
[RMT-BB-LCT]
Luby, M., Watson, M., and L. Vicisano, "Layered Coding
Transport (LCT) Building Block", Work in Progress,
February 2007.
[RMT-PI-NORM]
Adamson, B., Bormann, C., Handley, M., and J. Macker,
"Negative-acknowledgment (NACK)-Oriented Reliable
Multicast (NORM) Protocol", Work in Progress, May 2008.
[RMT-FLUTE]
Paila, T., Walsh, R., Luby, M., Lehtonen, R., and V. Roca,
"FLUTE - File Delivery over Unidirectional Transport",
Work in Progress, October 2007.
9.2. Informative References
[ALC_00] Luby, M., Gemmell, G., Vicisano, L., Crowcroft, J., and B.
Lueckenhoff, "Asynchronous Layered Coding: a Scalable
Reliable Multicast Protocol",
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draft-ietf-rmt-pi-alc-00.txt, March 2000.
[RFC1952] Deutsch, P., Gailly, J-L., Adler, M., Deutsch, L., and G.
Randers-Pehrson, "GZIP file format specification version
4.3", RFC 1952, May 1996.
[RFC2068] Fielding, R., Gettys, J., Mogul, J., Nielsen, H., and T.
Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1",
RFC 2068, January 1997.
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104,
February 1997.
[RFC3447] Jonsson, J. and B. Kaliski, "Public-Key Cryptography
Standards (PKCS) #1: RSA Cryptography Specifications
Version 2.1", RFC 3447, February 2003.
[RFC4082] Perrig, A., Song, D., Canetti, R., Tygar, J., and B.
Briscoe, "Timed Efficient Stream Loss-Tolerant
Authentication (TESLA): Multicast Source Authentication
Transform Introduction", RFC 4082, June 2005.
[RFC4302] Kent, S., "IP Authentication Header", RFC 4302,
December 2005.
[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)",
RFC 4303, December 2005.
Appendix A. FCAST in practice
This section discusses how FCAST/ALC and FCAST/NORM can be used in
practise.
Out-of-band transmission of the object meta-data: In some use-
cases, the meta-data (or a subset of them) will be communicated to
the receivers by means of an out-of-band mechanism. In some use-
cases, this out-of-band mechanism can itself be a dedicated FCAST
session. It is also possible that the TOI of each object be known
in advance (e.g., the TOI can be reserved). When this is the
case, sending this TOI along with the meta-data makes it possible
for a receiver to know in advance the meta-data associated to each
object, which enables the end-user (or the terminal when a set of
preferences or selection criteria have been filled) to filter the
incoming packets and discard those associated to a non-desired
object.
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SDP: The FCAST session parameters can be communicated in numerous
ways. One of them consists in using the Session Description
Protocol (SDP) [REF].
RoHC: In some situations, for instance in low-bandwidth wireless
environments, it can be desirable to compress the various protocol
headers (in our case IP, UDP, and possibly ALC/LCT) in a robust
way. The Robust Header Compression (RoHC) family of compression
schemes [REF] can be used to that purpose.
Object aggregation:
Session-level protocol: It is often desirable to use FCAST as a
robust transport solution under the control of a session level
protocol. This session level protocol can for instance have a
certain knowledge of the set of receivers and perform receiver
management operations. Examples of such operations include but
are not limited to accepting new receivers in the group or
performing cleaning operations after the departure of a receiver,
managing security aspects like group keying or performing AAA.
This session level protocol can also provide a higher level
reliability framework, in order to make sure that each active
receiver has received correctly a given object, and in case this
is not the case, it can launch a recovery mechanism that might
sometimes imply a direct point-to-point retransmission of missing
symbols, or when the number of receivers concerned is higher than
a certain threshold, a new multicast recovery session.
Appendix B. FCAST Examples
Figure 3 shows a compound object:
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0 |0| 0 | 0 | 37 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. meta-data ASCII null terminated string (33 bytes) .
. .
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. Object data .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: Compound Object Example
where the meta-data ASCII string, in HTTP/1.1 meta-information format
contains:
Content-Location: example.txt <CR-LF>
This string is 33 bytes long, including the NULL-termination
character. There is no gzip encoding of the meta-data (Z=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.
Figure 4 shows a compound object without any meta-data:
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| 0 | 0 | 4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. Object data .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: Compound Object Example with no Meta-Data.
The fact there is no meta-data is indicated by the value 3 of the
Header Length field.
Figure 5 shows an example CIO object, in the case of a static FCAST
session, i.e., a session where the set of objects is set once and for
all.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0 |1| 0 | 0 | 4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. Object List string .
. .
. +-+-+-+-+-+-+-+-+
. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: Example of CIO, in case of a static session.
The object list contains the following string:
1,2,3,100-104,200-203,299
There are therefore a total of 3+5+4+1 = 13 objects in the carousel
instance, and therefore in the FCAST session. There is no meta-data
associated to this CIO. The session being static the sender did not
feel the necessity to carry a Carousel Instance ID meta-data.
Authors' Addresses
Vincent Roca
INRIA
655, av. de l'Europe
Inovallee; Montbonnot
ST ISMIER cedex 38334
France
Email: vincent.roca@inria.fr
URI: http://planete.inrialpes.fr/~roca/
Brian Adamson
Naval Research Laboratory
Washington, DC 20375
USA
Email: adamson@itd.nrl.navy.mil
URI: http://cs.itd.nrl.navy.mil
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Full Copyright Statement
Copyright (C) The IETF Trust (2008).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
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