Network Working Group T. Paila

Network Working Group T. Paila
Request for Comments: 3926 Nokia
Category: Experimental M. Luby
Digital Fountain
R. Lehtonen
TeliaSonera
V. Roca
INRIA Rhone-Alpes
R. Walsh
Nokia
October 2004

FLUTE - File Delivery over Unidirectional Transport

Status of this Memo

This memo defines an Experimental Protocol for the Internet
community. It does not specify an Internet standard of any kind.
Discussion and suggestions for improvement are requested.
Distribution of this memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (2004).

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.

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Applicability Statement . . . . . . . . . . . . . . . . 3
1.1.1. The Target Application Space . . . . . . . . . . 3
1.1.2. The Target Scale . . . . . . . . . . . . . . . . 4
1.1.3. Intended Environments . . . . . . . . . . . . . 4
1.1.4. Weaknesses . . . . . . . . . . . . . . . . . . . 4
2. Conventions used in this Document. . . . . . . . . . . . . . . 5
3. File delivery . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. File delivery session . . . . . . . . . . . . . . . . . 6
3.2. File Delivery Table. . . . . . . . . . . . . . . . . . . 8
3.3. Dynamics of FDT Instances within file delivery session . 9
3.4. Structure of FDT Instance packets. . . . . . . . . . . . 11

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3.4.1. Format of FDT Instance Header . . . . . . . . . 12
3.4.2. Syntax of FDT Instance . . . . . . . . . . . . . 13
3.4.3. Content Encoding of FDT Instance . . . . . . . . 16
3.5. Multiplexing of files within a file delivery session . . 17
4. Channels, congestion control and timing . . . . . . . . . . . 18
5. Delivering FEC Object Transmission Information . . . . . . . . 19
5.1. Use of EXT_FTI for delivery of FEC Object Transmission
Information. . . . . . . . . . . . . . . . . . . . . . . 20
5.1.1. General EXT_FTI format . . . . . . . . . . . . . 20
5.1.2. FEC Encoding ID specific formats for EXT_FTI . . 21
5.2. Use of FDT for delivery of FEC Object Transmission
Information. . . . . . . . . . . . . . . . . . . . . . . 25
6. Describing file delivery sessions. . . . . . . . . . . . . . . 25
7. Security Considerations . . . . . . . . . . . . . . . . . . . 26
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 29
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 29
Normative References . . . . . . . . . . . . . . . . . . . . . 29
Informative References . . . . . . . . . . . . . . . . . . . . 30
A. Receiver operation (informative) . . . . . . . . . . . . . . . 32
B. Example of FDT Instance (informative). . . . . . . . . . . . . 33
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 34
Full Copyright Statement . . . . . . . . . . . . . . . . . . . 35

1. Introduction

This document defines FLUTE version 1, a protocol for unidirectional
delivery of files over the Internet. The specification builds on
Asynchronous Layered Coding (ALC), version 1 [2], 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 the objects actually represent. This
document specifies a technique called FLUTE - a mechanism for
signaling 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 document defines a specific transport application of ALC, adding
the following specifications:

- Definition of a file delivery session built on top of ALC,
including transport details and timing constraints.

- In-band signalling of the transport parameters of the ALC session.

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- In-band signalling of the properties of delivered files.

- Details associated with the multiplexing of multiple files within
a session.

This specification is structured as follows. Section 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. Section 4
describes the use of congestion control and channels with FLUTE.
Section 5 defines how the Forward Error Correction (FEC) Object
Transmission Information is to be delivered within a file delivery
session. Section 6 defines the required parameters for describing
file delivery sessions in a general case. Section 7 outlines
security considerations regarding file delivery with FLUTE. Last,
there are two informative appendices. The first appendix describes
an envisioned receiver operation for the receiver of the file
delivery session. The second appendix gives an example of File
Delivery Table.

Statement of Intent

This memo contains part of the definitions necessary to fully
specify a Reliable Multicast Transport protocol in accordance with
RFC2357. As per RFC2357, the use of any reliable multicast
protocol in the Internet requires an adequate congestion control
scheme.

While waiting for such a scheme to be available, or for an
existing scheme to be proven adequate, the Reliable Multicast
Transport working group (RMT) publishes this Request for Comments
in the "Experimental" category.

It is the intent of RMT to re-submit this specification as an IETF
Proposed Standard as soon as the above condition is met.

1.1. Applicability Statement

1.1.1. The Target Application Space

FLUTE is applicable to the delivery of large and small files to many
hosts, using delivery sessions of several seconds or more. For
instance, FLUTE could be used for the delivery of large software
updates to many hosts simultaneously. It could also be used for
continuous, but segmented, data such as time-lined text for
subtitling - potentially leveraging its layering inheritance from ALC
and LCT to scale the richness of the session to the congestion status

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of the network. It is also suitable for the basic transport of
metadata, for example SDP [12] files which enable user applications
to access multimedia sessions.

1.1.2. The Target Scale

Massive scalability is a primary design goal for FLUTE. IP multicast
is inherently massively scalable, but the best effort service that it
provides does not provide session management functionality,
congestion control or reliability. FLUTE provides all of this using
ALC and IP multicast without sacrificing any of the inherent
scalability of IP multicast.

1.1.3. Intended Environments

All of the environmental requirements and considerations that apply
to the ALC building block [2] and to any additional building blocks
that FLUTE uses also apply to FLUTE.

FLUTE can be used with both multicast and unicast delivery, but it's
primary application is for unidirectional multicast file delivery.
FLUTE requires connectivity between a sender and receivers but does
not require connectivity from receivers to a sender. FLUTE
inherently works with all types of networks, including LANs, WANs,
Intranets, the Internet, asymmetric networks, wireless networks, and
satellite networks.

FLUTE is compatible with both IPv4 or IPv6 as no part of the packet
is IP version specific. FLUTE works with both multicast models:
Any-Source Multicast (ASM) [13] and the Source-Specific Multicast
(SSM) [15].

FLUTE is applicable for both Internet use, with a suitable congestion
control building block, and provisioned/controlled systems, such as
delivery over wireless broadcast radio systems.

1.1.4. Weaknesses

Some networks are not amenable to some congestion control protocols
that could be used with FLUTE. In particular, for a satellite or
wireless network, there may be no mechanism for receivers to
effectively reduce their reception rate since there may be a fixed
transmission rate allocated to the session.

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FLUTE provides reliability using the FEC building block. This will
reduce the error rate as seen by applications. However, FLUTE does
not provide a method for senders to verify the reception success of
receivers, and the specification of such a method is outside the
scope of this document.

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 [1].

The terms "object" and "transport object" are consistent with the
definitions in ALC [2] and LCT [3]. The terms "file" and "source
object" are pseudonyms for "object".

3. File delivery

Asynchronous Layered Coding [2] 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, and thus FLUTE inherits the requirements
of ALC.

This specification is designed for the delivery of 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 5200 byte file referred to by
"http://www.example.com/docs/file.txt". Using the example, the
following properties describe the properties that need to be conveyed
by the file delivery protocol.

* Identifier of the file, expressed as a URI. This identifier may
be globally unique. The identifier may also provide a location
for the file. In the above example: "http://www.example.com/docs/
file.txt".

* File name (usually, this can be concluded from the URI). 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". If an explicit value for the MIME type is
provided separately from the file extension and does not match the
MIME type of the file extension then the explicitly provided value
MUST be used as the MIME type.

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* File size, expressed in bytes. In the above example: "5200". If
the file is content encoded then this is the file size before
content encoding.

* Content encoding of the file, within transport. In the above
example, the file could be encoded using ZLIB [10]. In this case
the size of the transport object carrying the file would probably
differ from the file size. The transport object size is delivered
to receivers as part of the FLUTE protocol.

* Security properties of the file such as digital signatures,
message digests, etc. For example, one could use S/MIME [18] as
the content encoding type for files with this authentication
wrapper, and one could use XML-DSIG [19] to digitally sign an FDT
Instance.

3.1. File delivery session

ALC is a protocol instantiation of Layered Coding Transport building
block (LCT) [3]. Thus ALC inherits the session concept of LCT. In
this document we will use the 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
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, the Transport
Object Identifier (TOI) field within the ALC/LCT header identifies
from which object the data in the packet was generated. Note that
each object is associated with a unique TOI within the scope of a
session.

If the sender is not assigned a permanent IP address accessible to
receivers, but instead, packets that can be received by receivers
containing a temporary IP address for packets sent by the sender,
then the TSI is scoped by this temporary IP address of the sender for
the duration of the session. As an example, the sender may be behind
a Network Address Translation (NAT) device that temporarily assigns

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an IP address for the sender that is accessible to receivers, and in
this case the TSI is scoped by the temporary IP address assigned by
the NAT that will appear in packets received by the receiver. As
another example, the sender may send its original packets using IPv6,
but some portions of the network may not be IPv6 capable and thus
there may be an IPv6 to IPv4 translator that changes the IP address
of the packets to a different IPv4 address. In this case, receivers
in the IPv4 portion of the network will receive packets containing
the IPv4 address, and thus the TSI for them is scoped by the IPv4
address. How the IP address of the sender to be used to scope the
session by receivers is delivered to receivers, whether it is a
permanent IP address or a temporary IP address, is outside the scope
of this document.

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 'object' denotes either a 'file' or a 'File
Delivery Table Instance' (section 3.2)

* The TOI field MUST be included in ALC packets sent within a FLUTE
session, with the exception that ALC packets sent in a FLUTE
session with the Close Session (A) flag set to 1 (signaling the
end of the session) and that contain no payload (carrying no
information for any file or FDT) SHALL NOT carry the TOI. See
Section 5.1 of RFC 3451 [3] for the LCT definition of the Close
Session flag, and see Section 4.2 of RFC 3450 [2] for an example
of its use within an ALC packet.

* The TOI value '0' is reserved for delivery of File Delivery Table
Instances. Each File Delivery Table Instance is uniquely
identified by an FDT Instance ID.

* Each file in a file delivery session MUST be associated with a TOI
(>0) in the scope of that session.

* Information carried in the headers and the payload of a packet is
scoped by the source IP address and the TSI. Information
particular to the object carried in the headers and the payload of
a packet is further scoped by the TOI for file objects, and is
further scoped by both the TOI and the FDT Instance ID for FDT
Instance objects.

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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. The following lists are examples of such
attributes, and are not intended to be mutually exclusive nor
exhaustive.

Attributes related to the delivery of file:

- TOI value that represents the file

- FEC Object Transmission Information (including the FEC Encoding ID
and, if relevant, the FEC Instance ID)

- Size of the transport object carrying the file

- Aggregate rate of sending packets to all channels

Attributes related to the file itself:

- Name, Identification and Location of file (specified by the URI)

- MIME media type of file

- Size of file

- Encoding of file

- Message digest of file

Some of these attributes MUST be included in the file description
entry for a file, others are optional, as defined in section 3.4.2.

Logically, the FDT is a set of file description entries for files to
be delivered in the session. Each file description entry MUST
include the TOI for the file that it describes and the URI
identifying the file. The TOI is included in each ALC/LCT data
packet during the delivery of the file, and thus the TOI carried in
the file description entry is how the receiver determines which
ALC/LCT data packets contain information about which file. Each file
description entry may also contain one or more descriptors that map
the above-mentioned attributes to the file.

Each file delivery session MUST have an FDT that is local to the
given session. The FDT MUST provide a file description entry mapped
to a TOI for each file appearing within the session. An object that
is delivered within the ALC session, but not described in the FDT, is

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not considered a 'file' belonging to the file delivery session.
Handling of these unmapped TOIs (TOIs 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. Each FDT Instance contains at least a single file
description entry and at most 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
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:

* For every file delivered within a file delivery session there MUST
be a file description entry included in at least one FDT Instance
sent within the session. A file description entry contains at a
minimum the mapping between the TOI and the URI.

* An FDT Instance MAY appear in any part of the file delivery
session and packets for an FDT Instance MAY be interleaved with
packets for other files or other FDT Instances within a session.

* 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.

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* FDT Instance is identified by the use of a new fixed length LCT
Header Extension EXT_FDT (defined later in this section). 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 > 0 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 include it, or complement the
description. However, subsequent FDT Instances MUST NOT change
the parameters already described for a specific TOI.

* An FDT Instance is valid until its expiration time. The
expiration time is expressed within the FDT Instance payload as a
32 bit data field. The value of the data field represents the 32
most significant bits of a 64 bit Network Time Protocol (NTP) [5]
time value. These 32 bits provide an unsigned integer
representing the time in seconds relative to 0 hours 1 January
1900. Handling of wraparound of the 32 bit time is outside the
scope of NTP and FLUTE.

* The receiver SHOULD NOT use a received FDT Instance to interpret
packets received beyond the expiration time of the FDT Instance.

* A sender MUST use an expiry time in the future upon creation of an
FDT Instance relative to its Sender Current Time (SCT).

* 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. (Note that since FEC Encoding ID 0 is the default for
FLUTE, this implies that Source Block Number and Encoding Symbol
ID lengths both default to 16 bits each.)

Generally, a receiver needs to receive an FDT Instance describing a
file before it is able to recover the file itself. In this sense FDT
Instances are of higher priority than files. Thus, it is RECOMMENDED
that FDT Instances describing a file be sent with at least as much
reliability within a session (more often or with more FEC protection)
as the files they describe. In particular, if FDT Instances are
longer than one packet payload in length it is RECOMMENDED that an
FEC code that provides protection against loss be used for delivering
FDT Instances. How often the description of a file is sent in an FDT

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Instance or how much FEC protection is provided for each FDT Instance
(if the FDT Instance is longer than one packet payload) is dependent
on the particular application and outside the scope of this document.

3.4. Structure of FDT Instance packets

FDT Instances are carried in ALC packets with TOI = 0 and with an
additional REQUIRED LCT Header extension called the FDT Instance
Header. The FDT Instance Header (EXT_FDT) 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 Encoding Symbols for the FDT Instance which contains one
or more file description entries. A FDT Instance MAY span several
ALC packets - the number of ALC packets is a function of the file
attributes associated with the FDT Instance. The FDT Instance Header
is carried in each ALC packet carrying the FDT Instance. The FDT
Instance Header is identical for all ALC/LCT packets for a particular
FDT Instance.

The overall format of ALC/LCT packets carrying an FDT Instance is
depicted in the Figure 1 below. All integer fields are carried in
"big-endian" or "network order" format, that is, most significant
byte (octet) first. As defined in [2], all ALC/LCT packets are sent
using UDP.

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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| UDP header |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Default LCT header (with TOI = 0) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LCT header extensions (EXT_FDT, EXT_FTI, etc.) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| FEC Payload ID |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Encoding Symbol(s) for FDT Instance |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 1 - Overall FDT Packet

3.4.1. Format of FDT Instance Header

FDT Instance Header (EXT_FDT) is a new fixed length, ALC PI specific
LCT header extension [3]. 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 | V | FDT Instance ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Version of FLUTE (V), 4 bits:

This document specifies FLUTE version 1. Hence in any ALC packet
that carries FDT Instance and that belongs to the file delivery
session as specified in this specification MUST set this field to
'1'.

FDT Instance ID, 20 bits:

For each file delivery session the numbering of FDT Instances starts
from '0' and is incremented by one for each subsequent FDT Instance.
After reaching the maximum value (2^20-1), the numbering starts again
from '0'. When wraparound from 2^20-1 to 0 occurs, 0 is considered
higher than 2^20-1. A new FDT Instance reusing a previous FDT
Instance ID number, due to wraparound, may not implicitly expire the
previous FDT Instance with the same ID. It would be reasonable for

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FLUTE Senders to only construct and deliver FDT Instances with
wraparound IDs after the previous FDT Instance using the same ID has
expired. However, mandatory receiver behavior for handling FDT
Instance ID wraparound and other special situations (for example,
missing FDT Instance IDs resulting in larger increments than one) is
outside the scope of this specification and left to individual
implementations of FLUTE.

3.4.2. Syntax of FDT Instance

The FDT Instance contains file description entries that provide the
mapping functionality described in 3.2 above.

The FDT Instance is an XML structure that has a single root element
"FDT-Instance". The "FDT-Instance" element MUST contain "Expires"
attribute, which tells the expiry time of the FDT Instance. In
addition, the "FDT-Instance" element MAY contain the "Complete"
attribute (boolean), which, when TRUE, signals that no new data will
be provided in future FDT Instances within this session (i.e., that
either FDT Instances with higher ID numbers will not be used or if
they are used, will only provide identical file parameters to those
already given in this and previous FDT Instances). For example, this
may be used to provide a complete list of files in an entire FLUTE
session (a "complete FDT").

The "FDT-Instance" element MAY contain attributes that give common
parameters for all files of an FDT Instance. These attributes MAY
also be provided for individual files in the "File" element. Where
the same attribute appears in both the "FDT-Instance" and the "File"
elements, the value of the attribute provided in the "File" element
takes precedence.

For each file to be declared in the given FDT Instance there is a
single file description entry in the FDT Instance. Each entry is
represented by element "File" which is a child element of the FDT
Instance 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. The value of the XML attribute name corresponds to MIME
field name and the XML attribute value corresponds to the value of
the MIME field body. 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 URI as defined in [6].

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In addition to mandatory attributes, the "FDT-Instance" element and
the "File" element MAY contain other attributes of which the
following are specifically pointed out.

* Where the MIME type is described, the attribute "Content-Type"
MUST be used for the purpose as defined in [6].

* Where the length is described, the attribute "Content-Length" MUST
be used for the purpose as defined in [6]. The transfer length is
defined to be the length of the object transported in bytes. It
is often important to convey the transfer length to receivers,
because the source block structure needs to be known for the FEC
decoder to be applied to recover source blocks of the file, and
the transfer length is often needed to properly determine the
source block structure of the file. There generally will be a
difference between the length of the original file and the
transfer length if content encoding is applied to the file before
transport, and thus the "Content-Encoding" attribute is used. If
the file is not content encoded before transport (and thus the
"Content-Encoding" attribute is not used) then the transfer length
is the length of the original file, and in this case the
"Content-Length" is also the transfer length. However, if the
file is content encoded before transport (and thus the "Content-
Encoding" attribute is used), e.g., if compression is applied
before transport to reduce the number of bytes that need to be
transferred, then the transfer length is generally different than
the length of the original file, and in this case the attribute
"Transfer-Length" MAY be used to carry the transfer length.

* Where the content encoding scheme is described, the attribute
"Content-Encoding" MUST be used for the purpose as defined in [6].

* Where the MD5 message digest is described, the 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:

<?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-Instance">
<xs:complexType>
<xs:sequence>

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<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="Transfer-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"
type="xs:base64Binary"
use="optional"/>
<xs:attribute name="FEC-OTI-FEC-Encoding-ID"
type="xs:unsignedLong"
use="optional"/>
<xs:attribute name="FEC-OTI-FEC-Instance-ID"
type="xs:unsignedLong"
use="optional"/>
<xs:attribute name="FEC-OTI-Maximum-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-Encoding-Symbols"
type="xs:unsignedLong"
use="optional"/>
<xs:anyAttribute processContents="skip"/>
</xs:complexType>
</xs:element>
</xs:sequence>

<xs:attribute name="Expires"
type="xs:string"
use="required"/>
<xs:attribute name="Complete"
type="xs:boolean"

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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="FEC-OTI-FEC-Encoding-ID"
type="xs:unsignedLong"
use="optional"/>
<xs:attribute name="FEC-OTI-FEC-Instance-ID"
type="xs:unsignedLong"
use="optional"/>
<xs:attribute name="FEC-OTI-Maximum-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-Encoding-Symbols"
type="xs:unsignedLong"
use="optional"/>
<xs:anyAttribute processContents="skip"/>
</xs:complexType>
</xs:element>
</xs:schema>

Any valid FDT Instance must use the above XML Schema. 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, for attributes applying to a single file, those MUST be
placed within the attributes of the element "File". For attributes
applying to all files described by the current FDT Instance, those
MUST be placed within the element "FDT-Instance". It is RECOMMENDED
that the new descriptors applied in the FDT are in the format of MIME
fields and are either defined in the HTTP/1.1 specification [6] or
another well-known specification.

3.4.3. Content Encoding of FDT Instance

The FDT Instance itself MAY be content encoded, for example
compressed. This specification defines FDT Instance Content Encoding
Header (EXT_CENC). EXT_CENC is a new fixed length, ALC PI specific
LCT header extension [3]. The Header Extension Type (HET) for the

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extension is 193. If the FDT Instance is content encoded, the
EXT_CENC MUST be used to signal the content encoding type. In that
case, EXT_CENC header extension MUST be used in all ALC packets
carrying the same FDT Instance ID. Consequently, when EXT_CENC
header is used, it MUST be used together with a proper FDT Instance
Header (EXT_FDT). Within a file delivery session, FDT Instances that
are not content encoded and FDT Instances that are content encoded
MAY both appear. If content encoding is not used for a given FDT
Instance, the EXT_CENC MUST NOT be used in any packet carrying the
FDT Instance. The format of EXT_CENC 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 = 193 | CENC | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Content Encoding Algorithm (CENC), 8 bits:

This field signals the content encoding algorithm used in the FDT
Instance payload. The definition of this field is outside the scope
of this specification. Applicable content encoding algorithms
include, for example, ZLIB [10], DEFLATE [16] and GZIP [17].

Reserved, 16 bits:

This field MUST be set to all '0'.

3.5. Multiplexing of files within a file delivery session

The delivered files are carried as transport objects (identified with
TOIs) in the file delivery session. All these objects, including the
FDT Instances, MAY be multiplexed in any order and in parallel with
each other within a session, i.e., packets for one file MAY be
interleaved with packets for other files or other FDT Instances
within a session.

Multiple FDT Instances MAY be delivered in a single session using TOI
= 0. 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, packets for
the FDT Instances MAY be interleaved. A receiver can determine which
FDT Instance a packet contains information about since 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, as 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, as 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 transport object must complete 'prior' to the second one, it is
RECOMMENDED that only a single channel is used for the file delivery
session.

Furthermore, if multiple channels are used then a receiver joined to
the session at a low reception rate will only be joined to the lower
layers of the session. Thus, since the reception of FDT Instances is
of higher priority than the reception of files (because the reception
of files depends on the reception of an FDT Instance describing it),
the following is RECOMMENDED:

1. The layers to which packets for FDT Instances are sent SHOULD NOT
be biased towards those layers to which lower rate receivers are
not joined. For example, it is ok to put all the packets for an
FDT Instance into the lowest layer (if this layer carries enough

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packets to deliver the FDT to higher rate receivers in a
reasonable amount of time), but it is not ok to put all the
packets for an FDT Instance into the higher layers that only high
rate receivers will receive.

2. If FDT Instances are generally longer than one Encoding Symbol in
length and some packets for FDT Instances are sent to layers that
lower rate receivers do not receive, an FEC Encoding other than
FEC Encoding ID 0 SHOULD be used to deliver FDT Instances. This
is because in this case, even when there is no packet loss in the
network, a lower rate receiver will not receive all packets sent
for an FDT Instance.

5. Delivering FEC Object Transmission Information

FLUTE inherits the use of FEC building block [4] 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 section, two methods are specified for FLUTE for
this purpose: the use of ALC specific LCT extension header EXT_FTI
[2] 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
the receiver MUST support both methods: the use of EXT_FTI and the
use of FDT.

The FEC Object Transmission Information that needs to be delivered to
receivers MUST be exactly the same whether it is delivered using
EXT_FTI or using FDT (or both). Section 5.1 describes the required
FEC Object Transmission Information that MUST be delivered to
receivers for various FEC Encoding IDs. In addition, it describes
the delivery using EXT_FTI. Section 5.2 describes the delivery using
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.

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5.1. Use of EXT_FTI for delivery of FEC Object Transmission Information

As specified in [2], the EXT_FTI header extension is intended to
carry the FEC Object Transmission Information for an object in-band.
It is left up to individual implementations to decide how frequently
and in which ALC packets the EXT_FTI header extension is included.
In environments with higher packet loss rate, the EXT_FTI might need
to be included more frequently in ALC packets than in environments
with low error probability. The EXT_FTI MUST be included in at least
one sent ALC packet for each FDT Instance.

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.

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 | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| Transfer Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| FEC Instance ID | FEC Enc. ID Specific Format |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Header Extension Type (HET), 8 bits:

64 as defined in [2].

Header Extension Length (HEL), 8 bits:

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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.

Transfer Length, 48 bits:

The length of the transport object that carries the file in bytes.
(This is the same as the file length if the file is not content
encoded.)

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'
(Under-Specified) [4]. 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Maximum Source Block Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Encoding Symbol Length, 16 bits:

Length of Encoding Symbol in bytes.

All Encoding Symbols of a transport object MUST be equal to this
length, with the optional exception of the last source symbol of the
last source block (so that redundant padding is not mandatory in this

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last symbol). This last source symbol MUST be logically padded out
with zeroes when another Encoding Symbol is computed based on this
source symbol to ensure the same interpretation of this Encoding
Symbol value by the sender and receiver. However, this padding does
not actually need to be sent with the data of the last source symbol.

Maximum Source Block Length, 32 bits:

The maximum number of source symbols per source block.

This EXT_FTI specification requires that an algorithm is known to
both sender and receivers for determining the size of all source
blocks of the transport object that carries the file identified by
the TOI (or within the FDT Instance identified by the TOI and the FDT
Instance ID). The algorithm SHOULD be the same for all files using
the same FEC Encoding ID within a session.

Section 5.1.2.3 describes an algorithm that is RECOMMENDED for this
use.

For the FEC Encoding IDs 0, 128 and 130, this algorithm is the only
well known way the receiver can determine the length of each source
block. Thus, the algorithm does two things: (a) it tells the
receiver the length of each particular source block as it is
receiving packets for that source block - this is essential to all of
these FEC schemes; and, (b) it provides the source block structure
immediately to the receiver so that the receiver can determine where
to save recovered source blocks at the beginning of the reception of
data packets for the file - this is an optimization which is
essential for some implementations.

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.

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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Maximum Source Block Length | Max. Num. of Encoding Symbols |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Encoding Symbol Length, 16 bits:

Length of Encoding Symbol in bytes.

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All Encoding Symbols of a transport object MUST be equal to this
length, with the optional exception of the last source symbol of the
last source block (so that redundant padding is not mandatory in this
last symbol). This last source symbol MUST be logically padded out
with zeroes when another Encoding Symbol is computed based on this
source symbol to ensure the same interpretation of this Encoding
Symbol value by the sender and receiver. However, this padding need
not be actually sent with the data of the last source symbol.

Maximum Source Block Length, 16 bits:

The maximum number of source symbols per source block.

Maximum Number of Encoding Symbols, 16 bits:

Maximum number of Encoding Symbols that can be generated for a source
block.

This EXT_FTI specification requires that an algorithm is known to
both sender and receivers for determining the size of all source
blocks of the transport object that carries the file identified by
the TOI (or within the FDT Instance identified by the TOI and the FDT
Instance ID). The algorithm SHOULD be the same for all files using
the same FEC Encoding ID within a session.

Section 5.1.2.3 describes an algorithm that is RECOMMENDED for this
use. For FEC Encoding ID 129 the FEC Payload ID in each data packet
already contains the source block length for the source block
corresponding to the Encoding Symbol carried in the data packet.
Thus, the algorithm for computing source blocks for FEC Encoding ID
129 could be to just use the source block lengths carried in data
packets within the FEC Payload ID. However, the algorithm described
in Section 5.1.2.3 is useful for the receiver to compute the source
block structure at the beginning of the reception of data packets for
the file. If the algorithm described in Section 5.1.2.3 is used then
it MUST be the case that the source block lengths that appear in data
packets agree with the source block lengths calculated by the
algorithm.

5.1.2.3. Algorithm for Computing Source Block Structure

This algorithm computes a source block structure so that all source
blocks are as close to being equal length as possible. A first
number of source blocks are of the same larger length, and the
remaining second number of source blocks are sent of the same smaller
length. The total number of source blocks (N), the first number of

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source blocks (I), the second number of source blocks (N-I), the
larger length (A_large) and the smaller length (A_small) are
calculated thus,

Input:
B -- Maximum Source Block Length, i.e., the maximum number of
source symbols per source block
L -- Transfer Length in bytes
E -- Encoding Symbol Length in bytes

Output:
N -- The number of source blocks into which the transport
object is partitioned.

The number and lengths of source symbols in each of the N
source blocks.

Algorithm:
(a) The number of source symbols in the transport object is
computed as T = L/E rounded up to the nearest integer.
(b) The transport object is partitioned into N source blocks,
where N = T/B rounded up to the nearest integer
(c) The average length of a source block, A = T/N
(this may be non-integer)
(d) A_large = A rounded up to the nearest integer
(it will always be the case that the value of A_large is at
most B)
(e) A_small = A rounded down to the nearest integer
(if A is an integer A_small = A_large,
and otherwise A_small = A_large - 1)
(f) The fractional part of A, A_fraction = A - A_small
(g) I = A_fraction * N
(I is an integer between 0 and N-1)
(h) Each of the first I source blocks consists of A_large source
symbols, each source symbol is E bytes in length. Each of the
remaining N-I source blocks consist of A_small source symbols,
each source symbol is E bytes in length except that the last
source symbol of the last source block is L-(((L-1)/E) rounded
down to the nearest integer)*E bytes in length.

Note, this algorithm does not imply implementation by floating point
arithmetic and integer arithmetic may be used to avoid potential
floating point rounding errors.

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5.2. Use of FDT for delivery of FEC Object Transmission Information

The FDT delivers FEC Object Transmission Information for each file
using an appropriate attribute within the "FDT-Instance" or the
"File" element of the FDT structure. For future FEC Encoding IDs, if
the attributes listed below do not fulfill the needs of describing
the FEC Object Transmission Information then additional new
attributes MAY be used.

* "Transfer-Length" is semantically equivalent with the field
"Transfer Length" of EXT_FTI.

* "FEC-OTI-FEC-Encoding-ID" is semantically equivalent with the
field "FEC Encoding ID" as carried in the Codepoint field of the
ALC/LCT header.

* "FEC-OTI-FEC-Instance-ID" is semantically equivalent with the
field "FEC Instance ID" of EXT_FTI.

* "FEC-OTI-Maximum-Source-Block-Length" is semantically equivalent
with the field "Maximum Source Block Length" of EXT_FTI for FEC
Encoding IDs 0, 128 and 130, and semantically equivalent with the
field "Maximum Source Block Length" of EXT_FTI for FEC Encoding ID
129.

* "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-Encoding-Symbols" is semantically
equivalent with the field "Maximum Number of Encoding Symbols" of
EXT_FTI for FEC Encoding ID 129.

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 thereafter the receiver
operation falls into the scope of this specification. According
to [2], the transport parameters of an ALC/LCT session that the
receiver needs to know are:

* The source IP address;

* The number of channels in the session;

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* The destination IP address and port number for each channel in the
session;

* The Transport Session Identifier (TSI) of the session;

* An indication that the session is a FLUTE session. The need to
demultiplex objects upon reception is implicit in any use of
FLUTE, and this fulfills the ALC requirement of an indication of
whether or not a session carries packets for more than one object
(all FLUTE sessions carry 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;

* Content Encoding format if optional content encoding of FDT
Instance is used, e.g., compression;

* Some information that tells receiver, in the first place, that the
session contains files that are of interest.

It is envisioned that these parameters would be described according
to some session description syntax (such as SDP [12] or XML based)
and held in a file which would be acquired by the receiver before the
FLUTE session begins by means of some transport protocol (such as
Session Announcement Protocol [11], email, HTTP [6], SIP [22], manual
pre-configuration, etc.) However, the way in which the receiver
discovers the above-mentioned parameters is out of scope of this
document, as it is for LCT and ALC. In particular, this
specification does not mandate or exclude any mechanism.

7. Security Considerations

The security considerations that apply to, and are described in, ALC
[2], LCT [3] and FEC [4] also apply to FLUTE. In addition, any
security considerations that apply to any congestion control building
block used in conjunction with FLUTE also apply to FLUTE.

Because of the use of FEC, FLUTE is especially vulnerable to denial-
of-service attacks by attackers that try to send forged packets to
the session which would prevent successful reconstruction or cause
inaccurate reconstruction of large portions of the FDT or file by
receivers. Like ALC, FLUTE is particularly affected by such an

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attack because many receivers may receive the same forged packet. A
malicious attacker may spoof file packets and cause incorrect
recovery of a file.

Even more damaging, a malicious forger may spoof FDT Instance
packets, for example sending packets with erroneous FDT-Instance
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.
It is thus STRONGLY RECOMMENDED that the FLUTE delivery service at
the receiver does not have write access to the system files or
directories, or any other critical areas. As described for MIME
[20][21], special consideration should be paid to the security
implications of any MIME types that can cause the remote execution of
any actions in the recipient's environment. Note, RFC 1521 [21]
describes important security issues for this environment, even though
its protocol is obsoleted by RFC 2048 [20].

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.

At the application level, it is RECOMMENDED that an integrity check
on the entire received object be done once the object is
reconstructed to ensure it is the same as the sent object, especially
for objects that are FDT Instances. Moreover, in order to obtain
strong cryptographic integrity protection a digital signature
verifiable by the receiver SHOULD be used to provide this application
level integrity check. However, if even one corrupted or forged
packet is used to reconstruct the object, it is likely that the
received object will be reconstructed incorrectly. This will
appropriately cause the integrity check to fail and, in this case,
the inaccurately reconstructed object SHOULD be discarded. Thus, the
acceptance of a single forged packet can be an effective denial of
service attack for distributing objects, but an object integrity
check at least prevents inadvertent use of inaccurately reconstructed
objects. The specification of an application level integrity check
of the received object is outside the scope of this document.

At the packet level, it is RECOMMENDED that a packet level
authentication be used to ensure that each received packet is an
authentic and uncorrupted packet containing FEC data for the object
arriving from the specified sender. Packet level authentication has
the advantage that corrupt or forged packets can be discarded

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individually and the received authenticated packets can be used to
accurately reconstruct the object. Thus, the effect of a denial of
service attack that injects forged packets is proportional only to
the number of forged packets, and not to the object size. Although
there is currently no IETF standard that specifies how to do
multicast packet level authentication, TESLA [14] is a known
multicast packet authentication scheme that would work.

In addition to providing protection against reconstruction of
inaccurate objects, packet level authentication can also provide some
protection against denial of service attacks on the multiple rate
congestion control. Attackers can try to inject forged packets with
incorrect congestion control information into the multicast stream,
thereby potentially adversely affecting network elements and
receivers downstream of the attack, and much less significantly the
rest of the network and other receivers. Thus, it is also
RECOMMENDED that packet level authentication be used to protect
against such attacks. TESLA [14] can also be used to some extent to
limit the damage caused by such attacks. However, with TESLA a
receiver can only determine if a packet is authentic several seconds
after it is received, and thus an attack against the congestion
control protocol can be effective for several seconds before the
receiver can react to slow down the session reception rate.

Reverse Path Forwarding checks SHOULD be enabled in all network
routers and switches along the path from the sender to receivers to
limit the possibility of a bad agent injecting forged packets into
the multicast tree data path.

A receiver with an incorrect or corrupted implementation of the
multiple rate congestion control building block may affect health of
the network in the path between the sender and the receiver, and may
also affect the reception rates of other receivers joined to the
session. 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.

Another vulnerability of FLUTE is the potential of receivers
obtaining an incorrect Session Description for the session. The
consequences of this could be 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 disrupting traffic in
portions of the network. To avoid these problems, it is RECOMMENDED
that measures be taken to prevent receivers from accepting incorrect
Session Descriptions, e.g., by using source authentication to ensure

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that receivers only accept legitimate Session Descriptions from
authorized senders. How this is done is outside the scope of this
document.

8. IANA Considerations

No information in this specification is directly subject to IANA
registration. However, building blocks components used by ALC may
introduce additional IANA considerations. In particular, the FEC
building block used by FLUTE does require IANA registration of the
FEC codec used.

9. Acknowledgements

The following persons have contributed to this specification: Brian
Adamson, Mark Handley, Esa Jalonen, Roger Kermode, Juha-Pekka Luoma,
Jani Peltotalo, Sami Peltotalo, Topi Pohjolainen, and Lorenzo
Vicisano. The authors would like to thank all the contributors for
their valuable work in reviewing and providing feedback regarding
this specification.

Normative References

[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.

[2] Luby, M., Gemmell, J., Vicisano, L., Rizzo, L., and J.
Crowcroft, "Asynchronous Layered Coding (ALC) Protocol
Instantiation", RFC 3450, December 2002.

[3] Luby, M., Gemmell, J., Vicisano, L., Rizzo, L., Handley, M.,
and J. Crowcroft, "Layered Coding Transport (LCT) Building
Block", RFC 3451, December 2002.

[4] Luby, M., Vicisano, L., Gemmell, J., Rizzo, L., Handley, M.,
and J. Crowcroft, "Forward Error Correction (FEC) Building
Block", RFC 3452, December 2002.

[5] Mills, D., "Network Time Protocol (Version 3) Specification,
Implementation", RFC 1305, March 1992.

[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", RFC 3695, February 2004.

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RFC 3926 FLUTE October 2004

[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.

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] Handley, M. and V. Jacobson, "SDP: Session Description
Protocol", RFC 2327, April 1998.

[13] Deering, S., "Host extensions for IP multicasting", STD 5, RFC
1112, August 1989.

[14] Perrig, A., Canetti, R., Song, D., and J. Tygar, "Efficient and
Secure Source Authentication for Multicast, Network and
Distributed System Security Symposium, NDSS 2001, pp. 35-46.",
February 2001.

[15] Holbrook, H., "A Channel Model for Multicast, Ph.D.
Dissertation, Stanford University, Department of Computer
Science, Stanford, California", August 2001.

[16] Deutsch, P., "DEFLATE Compressed Data Format Specification
version 1.3", RFC 1951, May 1996.

[17] Deutsch, P., "GZIP file format specification version 4.3", RFC
1952, May 1996.

[18] Ramsdell, B., "Secure/Multipurpose Internet Mail Extensions
(S/MIME) Version 3.1 Message Specification", RFC 3851, July
2004.

[19] Eastlake, D., Reagle, J., and D. Solo, "(Extensible Markup
Language) XML-Signature Syntax and Processing", RFC 3275, March
2002.

[20] Freed, N., Klensin, J., and J. Postel, "Multipurpose Internet
Mail Extensions (MIME) Part Four: Registration Procedures", RFC
2048, November 1996.

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[21] Moore, K., "MIME (Multipurpose Internet Mail Extensions) Part
Three: Message Header Extensions for Non-ASCII Text", RFC 1521,
November 1996.

[22] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP:
session initiation protocol", RFC 3261, June 2002.

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Appendix A. Receiver operation (informative)

This section 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
transport object have been received. 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.

Appendix B. Example of FDT Instance (informative)

<?xml version="1.0" encoding="UTF-8"?>
<FDT-Instance xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:fl="http://www.example.com/flute"
xsi:schemaLocation="http://www.example.com/flute-fdt.xsd"
Expires="2890842807">
<File
Content-Location="http://www.example.com/menu/tracklist.html"
TOI="1"
Content-Type="text/html"/>
<File
Content-Location="http://www.example.com/tracks/track1.mp3"
TOI="2"
Content-Length="6100"
Content-Type="audio/mp3"
Content-Encoding="gzip"
Content-MD5="+VP5IrWploFkZWc11iLDdA=="
Some-Private-Extension-Tag="abc123"/>
</FDT-Instance>

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Authors' Addresses

Toni Paila
Nokia
Itamerenkatu 11-13
Helsinki FIN-00180
Finland

EMail: [email protected]

Michael Luby
Digital Fountain
39141 Civic Center Dr.
Suite 300
Fremont, CA 94538
USA

EMail: [email protected]

Rami Lehtonen
TeliaSonera
Hatanpaan valtatie 18
Tampere FIN-33100
Finland

EMail: [email protected]

Vincent Roca
INRIA Rhone-Alpes
655, av. de l'Europe
Montbonnot
St Ismier cedex 38334
France

EMail: [email protected]

Rod Walsh
Nokia
Visiokatu 1
Tampere FIN-33720
Finland

EMail: [email protected]

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Full Copyright Statement

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Acknowledgement

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Internet Society.

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