RMT T. Paila
Internet-Draft Nokia
Expires: January 30, 2004 M. Luby
Digital Fountain
R. Lehtonen
TeliaSonera
V. Roca
INRIA Rhone-Alpes
August 2003
FLUTE - File Delivery over Unidirectional Transport
draft-ietf-rmt-flute-02.txt
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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This Internet-Draft will expire on January 30, 2004.
Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
This document defines FLUTE, a protocol for the unidirectional
delivery of files over the Internet, which is particularly suited to
multicast networks. The specification builds on Asynchronous Layered
Coding, the base protocol designed for massively scalable multicast
distribution.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions used in this document . . . . . . . . . . . . . 4
3. File delivery . . . . . . . . . . . . . . . . . . . . . . . 5
3.1 File delivery session . . . . . . . . . . . . . . . . . . . 5
3.2 File Delivery Table . . . . . . . . . . . . . . . . . . . . 6
3.3 Dynamics of FDT Instances within file delivery session . . . 8
3.4 Structure of FDT Instance . . . . . . . . . . . . . . . . . 9
3.4.1 Format of FDT Instance Header . . . . . . . . . . . . . . . 10
3.4.2 Syntax of FDT Instance Payload . . . . . . . . . . . . . . . 10
3.5 Multiplexing of files within a file delivery session . . . . 12
4. Channels, congestion control and timing . . . . . . . . . . 13
5. Delivering FEC Object Transmission Information . . . . . . . 14
5.1 Use of EXT_FTI for delivery of FEC Object Transmission
Information . . . . . . . . . . . . . . . . . . . . . . . . 14
5.1.1 General EXT_FTI format . . . . . . . . . . . . . . . . . . . 14
5.1.2 FEC Encoding ID specific formats for EXT_FTI . . . . . . . . 15
5.2 Use of FDT for delivery of FEC Object Transmission
Information . . . . . . . . . . . . . . . . . . . . . . . . 17
6. Describing file delivery sessions . . . . . . . . . . . . . 19
7. Security considerations . . . . . . . . . . . . . . . . . . 20
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 24
Normative references . . . . . . . . . . . . . . . . . . . . 23
Informative references . . . . . . . . . . . . . . . . . . . 24
A. Receiver operation (informative) . . . . . . . . . . . . . . 26
B. Example of FDT Instance Payload (informative) . . . . . . . 28
Intellectual Property and Copyright Statements . . . . . . . 29
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1. Introduction
This document defines FLUTE, a protocol for unidirectional delivery
of files over the Internet. The specification builds on Asynchronous
Layered Coding (ALC), version 1 [3], the base protocol designed for
massively scalable multicast distribution. ALC defines transport of
arbitrary binary objects. For file delivery applications mere
transport of objects is not enough, however. The end systems need to
know what do the objects actually represent. This document specifies
a technique called FLUTE - a mechanism for signalling and mapping the
properties of files to concepts of ALC in a way that allows receivers
to assign those parameters for received objects. Consequently,
throughout this document the term 'file' relates to an 'object' as
discussed in ALC. Although this specification frequently makes use
of multicast addressing as an example, the techniques are similarly
applicable for use with unicast addressing.
This specification answers the following questions:
* How does an ALC session represent a file delivery session?
* How can the properties of delivered files be signaled in-band
within the file delivery session?
* How to describe the file delivery session, its transport details
and its schedule in a general case?
* What is the internal structure of file delivery sessions wherein
several files can be delivered within a single session?
This specification is structured as follows. Chapter 3 begins by
defining the concept of the file delivery session. Following that it
introduces the File Delivery Table that forms the core part of this
specification. Further, it discusses multiplexing issues of
transport objects within a file delivery session. Chapter 4
describes the use of congestion control and channels with FLUTE.
Chapter 5 defines how the FEC Object Transmission Information is to
be delivered within a file delivery session. Chapter 6 defines the
required parameters for describing file delivery sessions in a
general case. Chapter 7 outlines security considerations regarding
file delivery with FLUTE. Last, there are two informative
appendixes. The first appendix gives an example of File Delivery
Table. The second appendix describes an envisioned receiver
operation for the receiver of the file delivery session.
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2. Conventions used in this document
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 RFC 2119 [2].
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3. File delivery
Asynchronous Layered Coding is a protocol designed for delivery of
arbitrary binary objects. It is especially suitable for massively
scalable, unidirectional, multicast distribution. ALC provides the
basic transport for FLUTE.
In this specification the above-mentioned arbitrary binary objects
are files. The core of this specification is to define how the
properties of the files are carried in-band together with the
delivered files.
As an example, let us consider a file referred by "www.ex.com/docs/
file.txt". Using the example, the following properties describe the
properties that need to be conveyed by the file delivery protocol.
* Location of the file, expressed as either absolute or relative
URL. In the above example: "www.ex.com/docs/file.txt"
* File name (usually, this can be concluded from the URL). In the
above example: "file.txt"
* File type, expressed as MIME media type (usually, this can also be
concluded from the extension of the file name). In the above
example: "text/plain"
* File size, expressed as bytes. In the above example (imaginary):
"5200"
* Content encoding of the file, within transport. In the above
example, the file could be encoded using ZLIB [10].
* Security properties of the file such as digital signatures,
message digestives, etc.
3.1 File delivery session
ALC is a protocol instantiation of Layered Coding Transport building
block (LCT) [4]. Thus ALC inherits the session concept of LCT. In
this document we will use concept ALC/LCT session to collectively
denote the interchangeable terms ALC session and LCT session.
An ALC/LCT session consists of a set of logically grouped ALC/LCT
channels associated with a single sender sending packets with ALC/LCT
headers for one or more objects. An ALC/LCT channel is defined by
the combination of a sender and an address associated with the
channel by the sender. A receiver joins a channel to start receiving
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the data packets sent to the channel by the sender, and a receiver
leaves a channel to stop receiving data packets from the channel.
One of the fields carried in the ALC/LCT header is the Transport
Session Identifier (TSI). The TSI is scoped by the source IP
address, and the (source IP address, TSI) pair uniquely identifies a
session, i.e., the receiver uses this pair carried in each packet to
uniquely identify from which session the packet was received. In
case multiple objects are carried within a session another field
within the ALC/LCT header, the Transport Object Identifier (TOI),
identifies from which object within the session the data in the
packet was generated. Note that each object is associated with a
unique TOI within the scope of a session.
When FLUTE is used for file delivery over ALC the following rules
apply:
* The ALC/LCT session is called file delivery session.
* The ALC/LCT concept of 'transport object' denotes either a 'file'
or a 'File Delivery Table Instance (section 3.2)'
* The TOI field MUST be used in ALC/LCT packets.
* The TOI value '0' is reserved for delivery of File Delivery Table
* Each file in a file delivery session MUST be associated with a TOI
(>0) in the scope of that session.
3.2 File Delivery Table
The File Delivery Table (FDT) provides a means to describe various
attributes associated with files that are to be delivered within the
file delivery session. Such attributes are for example the
following.
Attributes related to the delivery of file:
- TOI value that represents the file
- FEC Instance ID
- FEC Object Transmission Information
- Aggregate rate of sending packets to all channels
Attributes related to the file itself:
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- Location of file
- Name of file
- MIME media type of file
- Size of file
- Encoding of file
- Message digest of file
Some of these attributes are mandatory, others optional, as defined
in section 3.4.2.
Logically, the FDT is a set of file description entries. Each file
description entry is identified by a unique identifier in the given
session. In FLUTE, this identifier is the TOI of an instance of the
file. Each file description entry consequently contains one or more
descriptors that map the above-mentioned attributes to the identified
file. At minimum the mapping from TOI to URL value MUST be given.
Each file delivery session MUST have an FDT that is local to the
given session. The FDT SHOULD provide mapping for every TOI
appearing within the session. Handling of unmapped TOIs (those that
are not resolved by the FDT) is out of scope of this specification.
Within the file delivery session the FDT is delivered as FDT
Instances. An FDT Instance contains one or more file description
entries of the FDT. Any FDT Instance can be equal to, a subset of, a
superset of, or complement any other FDT Instance. A certain FDT
Instance may be repeated several times during a session, even after
subsequent FDT Instances (with higher FDT Instance ID numbers) have
been transmitted. In minimum the FDT Instance contains a single file
description entry. In maximum the FDT Instance contains the complete
FDT of the file delivery session.
A receiver of the file delivery session keeps an FDT database for
received file description entries. The receiver maintains the
database, for example, upon reception of FDT Instances. Thus, at any
given time the contents of the FDT database represent the receiver's
current view of the FDT of the file delivery session. Since each
receiver behaves independently of other receivers, it SHOULD NOT be
assumed that the contents of the FDT database are the same for all
the receivers of a given file delivery session.
Since FDT database is an abstract concept, the structure and the
maintaining of the FDT database are left to individual
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implementations and are thus out of scope of this specification.
3.3 Dynamics of FDT Instances within file delivery session
The following rules define the dynamics of the FDT Instances within a
file delivery session:
* Within a file delivery session, the complete FDT MUST be sent at
least once. The complete FDT is defined as an FDT that has file
description entry for every file sent within the file delivery
session. In minimum, each file description entry contains the
mapping to TOI and the URL.
* An FDT Instance MAY appear in any part of the file delivery
session and even multiplexed with other files or other FDT
Instances.
* The TOI value of '0' MUST be reserved for delivery of FDT
Instances. The use of other TOI values for FDT Instances is
outside the scope of this specification.
* FDT Instance is identified by the use of a new fixed length LCT
Header Extension EXT_FDT (defined later in this chapter). Each
FDT Instance is uniquely identified within the file delivery
session by its FDT Instance ID. Any ALC/LCT packet carrying FDT
Instance (indicated by TOI = 0) MUST include EXT_FDT.
* It is RECOMMENDED that FDT Instance that contains the file
description entry for a file is sent prior to the sending of the
described file within a file delivery session.
* Within a file delivery session, any TOI MAY be described more than
once. An example: previous FDT Instance 0 describes TOI of value
'3'. Now, subsequent FDT Instances can either keep TOI '3'
unmodified on the table, not to include it, complement the
description or modify the description. In the last case the
receiver interpretation of such a situation is specific to
implementation and therefore is left out of scope of this
specification.
* An FDT Instance is valid until its expiration time. The expiry
time is expressed within the FDT Instance payload as a 32 bit
Network Time Protocol (NTP) time value in seconds.
* The receiver behaviour upon expiration of the FDT Instance is out
of scope of this specification.
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* A sender MUST use an expiry time in the future upon creation of an
FDT Instance.
* Any FEC Encoding ID MAY be used for the sending of FDT Instances.
The default is to use FEC Encoding ID 0 for the sending of FDT
Instances.
3.4 Structure of FDT Instance
The FDT Instance consists of two parts: FDT Instance Header and FDT
Instance Payload. The FDT Instance Header is a new fixed length LCT
Header extension (EXT_FDT). It contains the FDT Instance ID that
uniquely identifies FDT instances within a file delivery session.
The FDT Instance Header is placed in the same way as any other LCT
extension header. There MAY be other LCT extension headers in use.
The LCT extension headers are followed by the FEC Payload ID, and
finally the FDT Instance Payload which contains one or more file
description entries. The FDT Instance Payload MAY span over several
ALC packets - the number of ALC packets is indicated by the FEC
Object Transmission Information associated to this FDT Instance. The
FDT Instance Header is carried in each ALC packet carrying FDT
Instance. The FDT Instance Header is identical for all the ALC/LCT
packets carrying parts of a particular FDT Instance.
The overall format of ALC/LCT packets carrying FDT Instance is
depicted in the Figure 1 below.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| UDP header |
| |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| Default LCT header |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LCT header extensions (EXT_FDT, EXT_FTI, etc.) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| FEC Payload ID |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Encoding Symbol(s) of FDT Instance Payload |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1 - Overall FDT Packet
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3.4.1 Format of FDT Instance Header
FDT Instance Header (EXT_FDT) is a new fixed length, ALC PI specific
LCT header extension [4]. The Header Extension Type (HET) for the
extension is 192. Its format is defined below:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| HET = 192 | FDT Instance ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
FDT Instance ID, 24 bits:
For each file delivery session the numbering of FDT Instances starts
from '0' and is incremented by exactly one for each subsequent FDT
Instance. After reaching the maximum value (2^24-1), the numbering
starts again from '0'. When wraparound from 2^24-1 to 0 occurs, 0 is
considered higher than 2^24-1. Receiver handling of wraparound and
other special situations (for example, missing FDT Instance IDs
resulting in longer increments than one) is left out of this
specification to individual implementations of FLUTE.
3.4.2 Syntax of FDT Instance Payload
The FDT Instance Payload contains file description entries that
provide the mapping functionality described in 3.2 above.
The FDT Instance Payload is an XML structure that has a single root
element "FDT-Payload". The "FDT-Payload" element MUST contain
"Expires" attribute, which tells the expiry time of the FDT Instance
Payload. In addition, the "FDT-Payload" element MAY contain
"Complete" attribute (boolean), which MAY be used to signal that the
given FDT Instance is the last FDT Instance to be expected on this
file delivery session. For each file to be declared in the given FDT
Instance there is a single file description entry in the FDT Instance
Payload. Each entry is represented by element "File" which is a
child element of the FDT Payload structure.
The attributes of "File" element in the XML structure represent the
attributes given to the file that is delivered in the file delivery
session. Each "File" element MUST contain at least two attributes
"TOI" and "Content-Location". "TOI" MUST be assigned a valid TOI
value as described in section 3.3 above. "Content-Location" MUST be
assigned a valid URL as defined in [6].
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In addition to mandatory attributes, the "File" entity MAY contain
other attributes of which the following are specifically pointed out.
* If the MIME type of the file is described, attribute
"Content-Type" MUST be used for the purpose as defined in [6].
* If the length of the file is described, attribute "Content-Length"
MUST be used for the purpose as defined in [6]. Note that
Content-Length describes the transferred object length, not the
actual file size after decoding. If the actual size of the file
length without any content encoding is described, attribute
"Entity-Length" (value in bytes) MUST be used.
* If the encoding scheme of the file is described, attribute
"Content-Encoding" MUST be used for the purpose as defined in [6].
* If the MD5 message digest of the file is described, attribute
"Content-MD5" MUST be used for the purpose as defined in [6].
* The FEC Object Transmission Information attributes as described in
section 5.2.
The following specifies the XML Schema [8][9] for FDT Instance
Payload:
<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs=http://www.w3.org/2001/XMLSchema xmlns:fl="http://www.example.com/flute" elementFormDefault:xs="qualified" targetNamespace:xs="http://www.example.com/flute">
<xs:element name="FDT-Payload">
<xs:complexType>
<xs:attribute name="Expires" type="xs:string" use="required"/>
<xs:attribute name="Complete" type="xs:boolean" use="optional"/>
<xs:sequence>
<xs:element name="File" maxOccurs="unbounded">
<xs:complexType>
<xs:attribute name="Content-Location"
type="xs:anyURI" use="required"/>
<xs:attribute name="TOI"
type="xs:positiveInteger" use="required"/>
<xs:attribute name="Content-Length"
type="xs:unsignedLong" use="optional"/>
<xs:attribute name="Entity-Length"
type="xs:unsignedLong" use="optional"/>
<xs:attribute name="Content-Type"
type="xs:string" use="optional"/>
<xs:attribute name="Content-Encoding"
type="xs:string" use="optional"/>
<xs:attribute name="Content-MD5"
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type="xs:base64Binary" use="optional"/>
<xs:attribute name="FEC-OTI-FEC-Instance-ID"
type="xs:unsignedLong" use="optional"/>
<xs:attribute name="FEC-OTI-Source-Block-Length"
type="xs:unsignedLong" use="optional"/>
<xs:attribute name="FEC-OTI-Encoding-Symbol-Length"
type="xs:unsignedLong" use="optional"/>
<xs:attribute name="FEC-OTI-Max-Number-of-Data-Symbols-per-Block"
type="xs:unsignedLong" use="optional"/>
<xs:attribute name="FEC-OTI-Max-Number-of-Encoding-Symbols"
type="xs:unsignedLong" use="optional"/>
<xs:anyAttribute processContents="skip" />
</xs:complexType>
</xs:element>
</xs:sequence>
</xs:complexType>
</xs:element>
</xs:schema>
Any XML document that conforms with the above XML Schema is a valid
FDT. This way FDT provides extensibility to support private
attributes within the file description entries. Those could be, for
example, the attributes related to the delivery of the file (timing,
packet transmission rate, etc.).
In case the basic FDT XML Schema is extended in terms of new
descriptors, those MUST be placed within the attributes of the
element "File". It is RECOMMENDED that the new descriptors applied
in the FDT are in the format of MIME fields and are either defined in
HTTP/1.1 specification [6] or otherwise well-known specification.
3.5 Multiplexing of files within a file delivery session
The delivered files appear as objects (identified with TOIs) within
the file delivery session. All the objects, including the FDT
Instances, MAY be multiplexed in any order and in parallel with each
other.
Especially multiple FDT Instances MAY be delivered during the session
in a particular TOI. In this case, it is RECOMMENDED that the
sending of a previous FDT Instance SHOULD end before the sending of
the next FDT Instance starts. However, due to unexpected network
conditions the FDT Instances MAY be multiplexed packetwise. In that
case, the FDT Instances are uniquely identified by their FDT Instance
ID carried in the EXT_FDT headers.
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4. Channels, congestion control and timing
ALC/LCT has a concept of channels and congestion control. There are
four scenarios FLUTE is envisioned to be applied.
(a) Use a single channel and a single-rate congestion control
protocol.
(b) Use multiple channels and a multiple-rate congestion control
protocol. In this case the FDT Instances MAY be delivered on more
than one channel.
(c) Use a single channel without congestion control supplied by ALC,
but only when in a controlled network environment where flow/
congestion control is being provided by other means.
(d) Use multiple channels without congestion control supplied by ALC,
but only when in a controlled network environment where flow/
congestion control is being provided by other means. In this case
the FDT Instances MAY be delivered on more than one channel.
When using just one channel for a file delivery session, like in (a)
and (c), the notion of 'prior' and 'after' are intuitively defined
for the delivery of objects with respect to their delivery times.
However, if multiple channels are used, like in (b) and (d), it is
not straightforward to state that an object was delivered 'prior' to
the other. An object may begin to be delivered on one or more of
those channels before the delivery of a second object begins.
However, the use of multiple channels/layers may complete the
delivery of the second object before the first. This is not a
problem when objects are delivered sequentially using a single
channel. Thus, if the application of FLUTE has a mandatory or
critical requirement that the first object must complete 'prior' to
the second one, it is RECOMMENDED that only a single channel is used
for the file delivery session.
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5. Delivering FEC Object Transmission Information
FLUTE inherits the use of FEC building block [5] from ALC. When
using FLUTE for file delivery over ALC the FEC Object Transmission
Information MUST be delivered in-band within the file delivery
session. In this chapter, two methods are specified for FLUTE for
this purpose: the use of ALC specific LCT extension header EXT_FTI
[3], and, the use of FDT.
The receiver of file delivery session MUST support delivery of FEC
Object Transmission Information using the EXT_FTI for the FDT
Instances carried using TOI value 0. For the TOI values other than 0
either method MAY be applied: the use of EXT_FTI and the use of FDT.
The FEC Object Transmission Information regarding a given TOI may be
available from several sources. In this case, it is RECOMMENDED that
the receiver of the file delivery session prioritizes the sources in
the following way (in the order of decreasing priority).
1. FEC Object Transmission Information that is available in EXT_FTI.
2. FEC Object Transmission Information that is available in the FDT.
3. FEC Object Transmission Information that is available out of
band.
5.1 Use of EXT_FTI for delivery of FEC Object Transmission Information
As specified in [3], the EXT_FTI header extension is intended to
carry in band the FEC Object Transmission Information for an object.
It is left up to individual implementations to decide how frequently
and in which ALC packets the EXT_FTI header extension occurs.
The ALC specification does not define the format or the processing of
the EXT_FTI header extension. The following sections specify EXT_FTI
when used in FLUTE.
In FLUTE, the FEC Encoding ID (8 bits) is carried in the Codepoint
field of the ALC/LCT header.
5.1.1 General EXT_FTI format
The general EXT_FTI format specifies the structure and those
attributes of FEC Object Transmission Information that are applicable
to any FEC Encoding ID.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| HET = 64 | HEL | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| Object Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| FEC Instance ID | FEC Enc. ID Specific Format |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Header Extension Type (HET), 8 bits:
64 as defined in [3]
Header Extension Length (HEL), 8 bits:
The length of the whole Header Extension field, expressed in
multiples of 32-bit words. This length includes the FEC Encoding ID
specific format part.
Object Length, 48 bits:
As specified in [3]. The length of the object in bytes.
FEC Instance ID, optional, 16 bits:
This field is used for FEC Instance ID. It is only present if the
value of FEC Encoding ID is in the range of 128-255. When the value
of FEC Encoding ID is in the range of 0-127, this field is set to 0.
FEC Encoding ID Specific Format:
Different FEC encoding schemes will need different sets of encoding
parameters. Thus, the structure and length of this field depends on
FEC Encoding ID. The next sections specify structure of this field
for FEC Encoding ID numbers 0, 128, 129 and 130.
5.1.2 FEC Encoding ID specific formats for EXT_FTI
5.1.2.1 FEC Encoding IDs 0, 128, and 130
FEC Encoding ID 0 is 'Compact No-Code FEC' (Fully-Specified) [7].
FEC Encoding ID 128 is 'Small Block, Large Block and Expandable FEC'
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(Under-Specified) [5]. FEC Encoding ID 130 is 'Compact FEC'
(Under-Specified) [7]. For these FEC Encoding IDs, the FEC Encoding
ID specific format of EXT_FTI is defined as follows.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
General EXT_FTI format | Encoding Symbol Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Block Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Encoding Symbol Length, 16 bits:
Length of encoding symbol in bytes.
Source Block Length, 32 bits
The number of source symbols in a full-sized source block. In this
EXT_FTI specification, it is assumed that:
1. All source blocks MUST have a predictable size. The blocking
scheme is defined by an algorithm shared by the source and the
receivers, and the information contained in the EXT_FTI. The
blocking scheme is outside the scope of this document. Yet an
example is to have all blocks the same size 'Source Block Length',
except perhaps the last two blocks whose size is shorter when the
total 'Object Length' is not a multiple of 'Source Block Length'
times 'Encoding Symbol Length'. The advantage of this blocking
scheme is that no block is shorter than half the 'Source Block
Length'.
2. All source symbols of all source blocks are of the same size,
except perhaps some of them. This size MUST be predictable and
only depends on the blocking algorithm and the information
contained in the EXT_FTI. For instance, the last symbol of a
block can be shorter when the block size in bytes is not a
multiple of the symbol size.
3. The size of a full-sized source symbol is equal to the size of an
encoding symbol.
5.1.2.2 FEC Encoding ID 129
Small Block Systematic FEC (Under-Specified). The FEC Encoding ID
specific format of EXT_FTI is defined as follows.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
General EXT_FTI format | Max. Data Symbols per Block |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Max. Num. of Encoding Symbols | Encoding Symbol Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Maximum Number of Data Symbols per Block, 16 bits:
Indicates the current maximum number of user data segments per FEC
coding block to be used by the sender during the session.
Maximum Number of Encoding Symbols, 16 bits:
Maximum number of encoding symbols that can be generated for a source
block.
Encoding Symbol Length, 16 bits:
Length of encoding symbol in bytes.
5.2 Use of FDT for delivery of FEC Object Transmission Information
The receiver of file delivery session MAY support delivery of FEC
Object Transmission Information using FDT. In that case the
following attributes within the "File" element of the FDT structure
MUST be used when applicable. If the listed attributes do not
fulfill the needs of describing the FEC Object Transmission
Information, additional new attributes MAY be used.
* "Content-Length" is semantically equivalent with the field "Object
Length" of EXT_FTI.
* "FEC-OTI-FEC-Instance-ID" is semantically equivalent with the
field "FEC Instance ID" of EXT_FTI.
* "FEC-OTI-Source-Block-Length" is semantically equivalent with the
field "Source Block Length" of EXT_FTI for FEC Encoding IDs 0, 128
and 130.
* "FEC-OTI-Encoding-Symbol-Length" is semantically equivalent with
the field "Encoding Symbol Length" of EXT_FTI for FEC Encoding IDs
0, 128, 129 and 130.
* "FEC-OTI-Max-Number-of-Data-Symbols-per-Block" is semantically
equivalent with the field "Maximum Number of Data Symbols per
Block" of EXT_FTI for FEC Encoding ID 129.
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* "FEC-OTI-Max-Number-of-Encoding-Symbols" is semantically
equivalent with the field "Maximum Number of Encoding Symbols" of
EXT_FTI for FEC Encoding ID 129.
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6. Describing file delivery sessions
To start receiving a file delivery session, the receiver needs to
know transport parameters associated with the session. Interpreting
these parameters and starting the reception therefore represents the
entry point from which on the receiver operation falls into the scope
of this specification. According to [3], the transport parameters of
an ALC/LCT session that the receiver needs to know are:
* The sender IP address;
* The number of channels in the session;
* The destination IP address and port number for each channel in the
session;
* The Transport Session Identifier (TSI) of the session;
* An indication of whether or not the session carries packets for
more than one object;
Optionally, the following parameters MAY be associated with the
session (Note, the list is not exhaustive):
* The start time and end time of the session;
* FEC Encoding ID and FEC Instance ID when the default FEC Encoding
ID 0 is not used for the delivery of FDT;
* Compression format if optional compression of FDT Instance Payload
is used;
* The FEC Object Transmission Information when this information is
neither available in the EXT_FTI nor FDT as described in section
5.
* Some information that tells receiver, in the first place, that the
session contains files that are of interest
How the receiver acquires the above-mentioned parameters is out of
scope of this document. The specification, in particular, does not
mandate or exclude any mechanism. The description can be conveyed to
the receiver via techniques such as Session Announcement Protocol
[11], email, accessing URL, manual configuration, etc. Similarly the
format of this session description is out of the scope of this
document.
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7. Security considerations
There is a risk of forged file delivery sessions. A malicious
attacker may spoof file delivery (ALC/LCT) packets in order to
initiate an attack. The attacker may have several objectives he or
she wishes to achieve, like Denial of Service (DoS). The following
are the most obvious risks, however not exhaustive.
The attacker can focus on the FDT information, sending forged packets
with erroneous FDT-Payload fields. Many attacks can follow this
approach. For instance a malicious attacker may alter the
Content-Location field of TOI 'n', to make it point to a system file
or a user configuration file. Then, TOI 'n' can carry a Trojan horse
or some other type of virus. Another example is generating a bad
Content-MD5 sum, leading receivers to reject the associated file that
will be declared corrupted. The Content-Encoding can also be
modified, which also prevents the receivers to correctly handle the
associated file.
These examples show that the FDT information is critical to the FLUTE
delivery service. It is therefore highly RECOMMENDED that the FDT
information be protected by the appropriate security measures. For
instance TESLA [13] can be used for authenticating the FDT source,
along with the other packets exchanged during the ALC/LCT session.
In some cases a group authentication service can be sufficient. In
that case, simple and efficient cryptographic transforms can then be
used [12]. The FDT content may also be digitally signed, which
provides both source authentication and packet integrity. This is
feasible if the FDT packet rate is kept sufficiently low (generating/
verifying digital signatures are computationally demanding tasks).
In that case, the number of signature verifications at a receiver
should be rate limited in order to prevent DoS attacks consisting in
sending a high number of forged FDT packets. Finally it is
RECOMMENDED that the FLUTE delivery service does not have write
access to the system files or directories, or any other critical
areas.
An attacker can also eavesdrop on the FLUTE session. Packets
containing the FDT information are critical from that point of view
since they contain information on the session content. When this is
an issue it is RECOMMENDED that the FDT packets be encrypted (as well
as the data packets) using a confidentiality service. The MSEC IETF
Working Group defines security transforms, Group Key Management and
Group Security Associations building blocks that can be used to that
purpose.
A difficulty is the unidirectional feature of FLUTE. Many protocols
providing application-level security are based on bidirectional
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communications. The application of these security protocols in case
of strictly unidirectional links is not considered in the present
document.
In addition to the attacks on the FDT information, FLUTE is subject
to attacks on the ALC/LCT session itself. Therefore, the security
considerations of [3] and [4] also apply to FLUTE.
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8. Acknowledgements
The following persons have contributed to this specification: Rod
Walsh, Juha-Pekka Luoma, Esa Jalonen, Sami Peltotalo, Jani Peltotalo
and Brian Adamson. The authors would like to thank all the
contributors for their valuable work in reviewing and providing
feedback regarding this specification.
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Normative references
[1] Bradner, S., "The Internet Standards Process -- Revision 3", RFC
2026, BCP 9, October 1996.
[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", RFC 2119, BCP 14, March 1997.
[3] Luby, M., Gemmel, J., Vicisano, L., Rizzo, L. and J. Crowcroft,
"Asynchronous Layered Coding (ALC) Protocol Instantiation", RFC
3450, December 2002.
[4] Luby, M., Gemmel, J., Vicisano, L., Rizzo, L. and J. Crowcroft,
"Layered Coding Transport (LCT) Building Block", RFC 3451,
December 2002.
[5] Luby, M., Gemmel, J., Vicisano, L., Rizzo, L., Crowcroft, J. and
M. Handley, "Forward Error Correction (FEC) Building Block", RFC
3452, December 2002.
[6] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L.,
Leach, P. and T. Berners-Lee, "Hypertext Transfer Protocol --
HTTP/1.1", RFC 2616, June 1999.
[7] Luby, M. and L. Vicisano, "Compact Forward Error Correction
(FEC) Schemes", draft-ietf-rmt-bb-fec-supp-compact-01 (work in
progress), May 2003.
[8] Thompson, H., Beech, D., Maloney, M. and N. Mendelsohn, "XML
Schema Part 1: Structures", W3C Recommendation, May 2001.
[9] Biron, P. and A. Malhotra, "XML Schema Part 2: Datatypes", W3C
Recommendation, May 2001.
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Informative references
[10] Deutsch, P. and J-L. Gailly, "ZLIB Compressed Data Format
Specification version 3.3", RFC 1950, May 1996.
[11] Handley, M., Perkins, C. and E. Whelan, "Session Announcement
Protocol", RFC 2974, October 2000.
[12] Hardjono, T. and B. Weis, "The Multicast Security
Architecture", draft-ietf-msec-arch-01 (work in progress), May
2003.
[13] Perrig, A., Canetti, R., Song, D., Tygar, D. and B. Briscoe,
"TESLA: Multicast Source Authentication Transform
Introduction", draft-ietf-msec-tesla-intro-01 (work in
progress), October 2002.
Authors' Addresses
Toni Paila
Nokia
Itamerenkatu 11-13
Helsinki FIN-00180
Finland
EMail: toni.paila@nokia.com
Michael Luby
Digital Fountain
39141 Civic Center Dr.
Suite 300
Fremont, CA 94538
USA
EMail: luby@digitalfountain.com
Rami Lehtonen
TeliaSonera
Hatanpaan valtatie 18
Tampere FIN-33100
Finland
EMail: rami.lehtonen@teliasonera.com
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Vincent Roca
INRIA Rhone-Alpes
655, av. de l'Europe
Montbonnot
St Ismier cedex 38334
France
EMail: vincent.roca@inrialpes.fr
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Appendix A. Receiver operation (informative)
This chapter gives an example how the receiver of the file delivery
session may operate. Instead of a detailed state-by-state
specification the following should be interpreted as a rough sequence
of an envisioned file delivery receiver.
1. The receiver obtains the description of the file delivery session
identified by the pair: (source IP address, Transport Session
Identifier). The receiver also obtains the destination IP
addresses and respective ports associated with the file delivery
session.
2. The receiver joins the channels in order to receive packets
associated with the file delivery session. The receiver may
schedule this join operation utilizing the timing information
contained in a possible description of the file delivery session.
3. The receiver receives ALC/LCT packets associated with the file
delivery session. The receiver checks that the packets match the
declared Transport Session Identifier. If not, packets are
silently discarded.
4. While receiving, the receiver demultiplexes packets based on
their TOI and stores the relevant packet information in an
appropriate area for recovery of the corresponding file. Multiple
files can be reconstructed concurrently.
5. Receiver recovers an object. An object can be recovered when an
appropriate set of packets containing encoding symbols for the
object have been received and the object can be recovered. An
appropriate set of packets is dependent on the properties of the
FEC Encoding ID and FEC Instance ID, and on other information
contained in the FEC Object Transmission Information.
6. If the recovered object was an FDT instance with FDT Instance ID
'N', the receiver parses the payload of the instance 'N' of FDT
and updates its FDT database accordingly. The receiver identifies
FDT instances within a file delivery session by the EXT_FDT header
extension. Any object that is delivered using EXT_FDT header
extension is an FDT instance, uniquely identified by the FDT
Instance ID. Note that TOI '0' is exclusively reserved for FDT
delivery.
7. If the object recovered is not an FDT Instance but a file, the
receiver looks up its FDT database to get the properties described
in the database, and assigns file with the given properties. The
receiver also checks that received content length matches with the
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description in the database. Optionally, if MD5 checksum has been
used, the receiver checks that calculated MD5 matches with the
description in the FDT database.
8. The actions the receiver takes with imperfectly received files
(missing data, mismatching digestive, etc.) is outside the scope
of this specification. When a file is recovered before the
associated file description entry is available, a possible
behavior is to wait until an FDT Instance is received that
includes the missing properties.
9. If the file delivery session end time has not been reached go
back to 3. Otherwise end.
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Appendix B. Example of FDT Instance Payload (informative)
<?xml version="1.0" encoding="UTF-8"?>
<FDT-Payload xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:fl="http://www.example.com/flute" xsi:schemaLocation="http://www.example.com/flute fdt-6a.xsd"
Expires="2890842807">
<File
Content-Location="www.example.com/menu/tracklist.html"
TOI="1"
Content-Type="text/html"/>
<File
Content-Location="www.example.com/tracks/track1.mp3"
TOI="2"
Content-Length="6100"
Content-Type="audio/mp3"
Content-Encoding="gzip"
Content-MD5="Eth76GlkJU45sghK"
Some-Private-Extension-Tag="abc123"/>
</FDT-Payload>
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