Internet Engineering Task Force (IETF) T. Paila
Request for Comments: 6726 Nokia
Obsoletes: 3926 R. Walsh
Category: Standards Track Nokia/TUT
ISSN: 2070-1721 M. Luby
Qualcomm Technologies, Inc.
November 2012 FLUTE - File Delivery over Unidirectional Transport
This document defines File Delivery over Unidirectional Transport
(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.
This document obsoletes RFC 3926.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
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Table of Contents
1. Introduction ....................................................31.1. Applicability Statement ....................................51.1.1. The Target Application Space ........................51.1.2. The Target Scale ....................................51.1.3. Intended Environments ...............................51.1.4. Weaknesses ..........................................62. Conventions Used in This Document ...............................63. File Delivery ...................................................73.1. File Delivery Session ......................................83.2. File Delivery Table .......................................103.3. Dynamics of FDT Instances within a File Delivery Session ..123.4. Structure of FDT Instance Packets .........................153.4.1. Format of FDT Instance Header ......................163.4.2. Syntax of FDT Instance .............................173.4.3. Content Encoding of FDT Instance ...................213.5. Multiplexing of Files within a File Delivery Session ......224. Channels, Congestion Control, and Timing .......................235. Delivering FEC Object Transmission Information .................246. Describing File Delivery Sessions ..............................26
7. Security Considerations ........................................277.1. Problem Statement .........................................277.2. Attacks against the Data Flow .............................287.2.1. Access to Confidential Files .......................287.2.2. File Corruption ....................................287.3. Attacks against the Session Control Parameters and
Associated Building Blocks ................................307.3.1. Attacks against the Session Description ............307.3.2. Attacks against the FDT Instances ..................317.3.3. Attacks against the ALC/LCT Parameters .............317.3.4. Attacks against the Associated Building Blocks .....327.4. Other Security Considerations .............................327.5. Minimum Security Recommendations ..........................338. IANA Considerations ............................................348.1. Registration of the FDT Instance XML Namespace ............348.2. Registration of the FDT Instance XML Schema ...............348.3. Registration of the application/fdt+xml Media Type ........358.4. Creation of the FLUTE Content Encoding Algorithms
Registry ..................................................368.5. Registration of LCT Header Extension Types ................369. Acknowledgments ................................................3610. Contributors ..................................................3711. Change Log ....................................................3711.1. RFC 3926 to This Document ................................3712. References ....................................................4012.1. Normative References .....................................4012.2. Informative References ...................................41Appendix A. Receiver Operation (Informative) ......................44Appendix B. Example of FDT Instance (Informative) .................451. Introduction
This document defines FLUTE version 2, a protocol for unidirectional
delivery of files over the Internet. This specification is not
backwards compatible with the previous experimental version defined
in [RFC3926] (see Section 11 for details). The specification builds
on Asynchronous Layered Coding (ALC), version 1 [RFC5775], 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
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 signaling of the transport parameters of the ALC session.
- In-band signaling of the properties of delivered files.
- Details associated with the multiplexing of multiple files within
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, which forms the core part of
this specification. Further, it discusses multiplexing issues of
transmission 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. Appendix A describes an
envisioned receiver operation for the receiver of the file delivery
session. Readers who want to see a simple example of FLUTE in
operation should refer to Appendix A right away. Appendix B gives an
example of a File Delivery Table.
This specification contains part of the definitions necessary to
fully specify a Reliable Multicast Transport (RMT) protocol in
accordance with [RFC2357].
This document obsoletes [RFC3926], which contained a previous version
of this specification and was published in the "Experimental"
category. This Proposed Standard specification is thus based on
[RFC3926] and has been updated according to accumulated experience
and growing protocol maturity since the publication of [RFC3926].
Said experience applies both to this specification itself and to
congestion control strategies related to the use of this
The differences between [RFC3926] and this document are listed in
This document updates ALC [RFC5775] and Layered Coding Transport
(LCT) [RFC5651] in the sense that it defines two new header
extensions, EXT_FDT and EXT_CENC.
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 of the network. It is also suitable for the basic transport
of metadata, for example, Session Description Protocol (SDP)
[RFC4566] files that enable user applications to access multimedia
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 by
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 RMT building blocks used by FLUTE shall also apply to FLUTE.
These are the ALC protocol instantiation [RFC5775], the LCT building
block [RFC5651], and the FEC building block [RFC5052].
FLUTE can be used with both multicast and unicast delivery, but its
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. Because of its
low expectations, FLUTE works with most types of networks, including
LANs, WANs, Intranets, the Internet, asymmetric networks, wireless
networks, and satellite networks.
FLUTE is compatible with both IPv4 and IPv6, as no part of the packet
is IP version specific. FLUTE works with both multicast models:
Any-Source Multicast (ASM) [RFC1112] and Source-Specific Multicast
FLUTE is applicable for both shared networks, such as the Internet,
with a suitable congestion control building block; and provisioned/
controlled networks, such as wireless broadcast radio systems, with a
traffic-shaping building block.
FLUTE congestion control protocols depend on the ability of a
receiver to change multicast subscriptions between multicast groups
supporting different rates and/or layered codings. If the network
does not support this, then the FLUTE congestion control protocols
may not be amenable to such a network.
FLUTE can also be used for point-to-point (unicast) communications.
At a minimum, implementations of ALC MUST support the Wave and
Equation Based Rate Control (WEBRC) [RFC3738] multiple-rate
congestion control scheme [RFC5775]. However, since WEBRC has been
designed for massively scalable multicast flows, it is not clear how
appropriate it is to the particular case of unicast flows. Using a
separate point-to-point congestion control scheme is another
alternative. How to do that is outside the scope of the present
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 [RFC2119].
The terms "object" and "transmission object" are consistent with the
definitions in ALC [RFC5775] and LCT [RFC5651]. The terms "file" and
"source object" are pseudonyms for "object".
3. File Delivery
Asynchronous Layered Coding [RFC5775] 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 [RFC3986]. The
identifier MAY provide a location for the file. In the above
* File name (usually, this can be concluded from the URI). In the
above example: "file.txt".
* File type, expressed as Internet Media Types (often referred to as
"Media Types"). In the above example: "text/plain".
* File size, expressed in octets. In the above example: "5200". If
the file is content encoded, then this is the file size before
* Content encoding of the file, within transport. In the above
example, the file could be encoded using ZLIB [RFC1950]. In this
case, the size of the transmission object carrying the file would
probably differ from the file size. The transmission 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 [RFC5751]
as the content encoding type for files with this authentication
wrapper, and one could use XML Digital Signatures (XML-DSIG)
[RFC3275] to digitally sign the file. XML-DSIG can also be used
to provide tamper prevention, e.g., in the Content-Location field.
Content encoding is applied to file data before FEC protection.
For each unique file, FLUTE encodes the attributes listed above and
other attributes as children of an XML file element. A table of XML
file elements is transmitted as a special file called a 'File
Delivery Table' (FDT), which is further described in the next
subsection and in Section 3.2.
3.1. File Delivery Session
ALC is a protocol instantiation of the Layered Coding Transport (LCT)
building block [RFC5651]. Thus, ALC inherits the session concept of
LCT. In this document, we will use the concept of the 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 ALC/LCT packets 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), an integer carried in a field of size 16,
32, or 48 bits (note that the TSI may be carried by other means, in
which case it is absent from the LCT header [RFC5651]). The (source
IP address, TSI) pair uniquely identifies a session. Note that the
TSI is scoped by the IP address, so the same TSI may be used by
several source IP addresses at once. Thus, the receiver uses the
(source IP address, TSI) pair from each packet to uniquely identify
the session sending each packet. When a session carries multiple
objects, the Transmission Object Identifier (TOI) field within the
ALC/LCT header names the object used to generate each packet. Note
that each object is associated with a unique TOI within the scope of
A FLUTE session consistent with this specification MUST use FLUTE
version 2 as specified in this document. Thus, all sessions
consistent with this specification MUST set the FLUTE version to 2.
The FLUTE version is carried within the EXT_FDT Header Extension
(defined in Section 3.4.1) in the ALC/LCT layer. A FLUTE session
consistent with this specification MUST use ALC version 1 as
specified in [RFC5775], and LCT version 1 as specified in [RFC5651].
If multiple FLUTE sessions are sent to a channel, then receivers MUST
determine the FLUTE protocol version, based on version fields and the
(source IP address, TSI) pair carried in the ALC/LCT header of the
packet. Note that when a receiver first begins receiving packets, it
might not know the FLUTE protocol version, as not every LCT packet
carries the EXT_FDT header (containing the FLUTE protocol version).
A new receiver MAY keep an open binding in the LCT protocol layer
between the TSI and the FLUTE protocol version, until the EXT_FDT
header arrives. Alternatively, a new receiver MAY discover a binding
between TSI and FLUTE protocol version via a session discovery
protocol that is out of scope of this document.
If the sender's IP address is not accessible to receivers, then
packets that can be received by receivers contain an intermediate IP
address. In this case, the TSI is scoped by this intermediate 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 an IP address for the sender. In
this case, the TSI is scoped by the intermediate IP address assigned
by the NAT. As another example, the sender may send its original
packets using IPv6, but some portions of the network may not be IPv6
capable. 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 the sender's IP address or an intermediate IP address,
is outside the scope of this document.
When FLUTE is used for file delivery over ALC, the ALC/LCT session is
called a file delivery session, and the ALC/LCT concept of 'object'
denotes either a 'file' or a 'File Delivery Table Instance'
Additionally, the following rules apply:
* 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 [RFC5651] for the LCT definition of the Close
Session flag, and see Section 4.2 of [RFC5775] for an example of
the use of a TOI within an ALC packet.
* The TOI value '0' is reserved for the delivery of File Delivery
Table Instances. Each non-expired File Delivery Table Instance is
uniquely identified by an FDT Instance ID within the EXT_FDT
header defined in Section 3.4.1.
* 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
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 or
Attributes related to the delivery of a 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 transmission 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)
- 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 carried in each file description entry
is how FLUTE names the ALC/LCT data packets used for delivery of the
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,
other than the FDT itself, is not considered a 'file' belonging to
the file delivery session. This object received with an unmapped TOI
(non-zero TOI that is not resolved by the FDT) SHOULD in general be
ignored by a FLUTE receiver. The details of how to do that are out
of scope of this specification.
Note that a client that joins an active file delivery session MAY
receive data packets for a TOI > 0 before receiving any FDT Instance
(see Section 3.3 for recommendations on how to limit the probability
that this situation will occur). Even if the TOI is not mapped to
any file description entry, this is hopefully a transient situation.
When this happens, system performance might be improved by caching
such packets within a reasonable time window and storage size. Such
optimizations are use-case and implementation specific, and further
details are beyond the scope of this document.
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, be a subset
of, be a superset of, overlap with, or complement any other FDT
Instance. A certain FDT Instance may be repeated multiple 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
exhaustive set of file description entries of the files being
delivered in 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 the FDT database is an abstract concept, the structure and the
maintenance 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 a 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 (i.e., an integer > 0) for
FDT Instances is outside the scope of this specification.
* The FDT Instance is identified by the use of a new fixed-length
LCT Header Extension, EXT_FDT (defined later in this section).
Each non-expired FDT Instance is uniquely identified within the
file delivery session by its FDT Instance ID, carried by the
EXT_FDT Header Extension. Any ALC/LCT packet carrying an FDT
Instance MUST include EXT_FDT.
* It is RECOMMENDED that an FDT Instance that contains the file
description entry for a file be sent at least once before sending
the described file within a file delivery session. This
recommendation is intended to minimize the amount of file data
that may be received by receivers in advance of the FDT Instance
containing the entry for a file (such data must either be
speculatively buffered or discarded). Note that this possibility
cannot be completely eliminated, since the first transmission of
FDT data might be lost.
* Within a file delivery session, any TOI > 0 MAY be described more
than once. For example, a previous FDT Instance 0 describes a TOI
of value '3'. Now, subsequent FDT Instances can either keep TOI
'3' unmodified in the table, not include it, or augment 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
UTF-8 decimal representation of a 32-bit unsigned integer. The
value of this integer represents the 32 most significant bits of a
64-bit Network Time Protocol (NTP) [RFC5905] time value. These
32 bits provide an unsigned integer representing the time in
seconds relative to 0 hours 1 January 1900 in the case of the
prime epoch (era 0) [RFC5905]. The handling of time wraparound
(to happen in 2036) requires that the associated epoch be
considered. In any case, both a sender and a receiver easily
determine to which (136-year) epoch the FDT Instance expiration
time value pertains by choosing the epoch for which the expiration
time is closest in time to the current time.
Here is an example. Let us imagine that a new FLUTE session is
started on February 7th, 2036, 0h, i.e., at NTP time
4,294,944,000, a few hours before the end of epoch 0. In order to
define an FDT Instance valid for the next 48 hours, The FLUTE
sender sets an expiry time of 149,504. This FDT Instance will
expire exactly on February 9th, 2036, 0h. A client that receives
this FDT Instance on the 7th, 0h, just after it has been sent,
immediately understands that this value corresponds to epoch 1. A
client that joins the session on February 8th, 0h, i.e., at NTP
time 63,104, epoch 1, immediately understands that the 149,504 NTP
timestamp corresponds to epoch 1.
* The space of FDT Instance IDs is limited by the associated field
size (i.e., 20 bits) in the EXT_FDT Header Extension
(Section 3.4.1). Therefore, senders should take care to always
have a large enough supply of available FDT Instance IDs when
specifying FDT expiration times.
* The receiver MUST NOT use a received FDT Instance to interpret
packets received beyond the expiration time of the FDT Instance.
* A sender MUST use an expiration 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 the Compact No-Code FEC Encoding ID 0
[RFC5445] 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.)
* If the receiver does not support the FEC Scheme indicated by the
FEC Encoding ID, the receiver MUST NOT decode the associated FDT.
* It is RECOMMENDED that the mechanisms used for file attribute
delivery SHOULD achieve a delivery probability that is higher than
the file recovery probability and the file attributes SHOULD be
delivered at this higher priority before the delivery of the
associated files begins.
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. Additionally, a
FLUTE sender SHOULD assume that receivers will not receive all
packets pertaining to FDT Instances. The way FDT Instances are
transmitted has a large impact on satisfying the recommendation
above. When there is a single file transmitted in the session, one
way to satisfy the recommendation above is to repeatedly transmit on
a regular enough basis FDT Instances describing the file while the
file is being transmitted. If an FDT Instance is longer than one
packet payload in length, it is RECOMMENDED that an FEC code that
provides protection against loss be used for delivering this FDT
Instance. When there are multiple files in a session concurrently
being transmitted to receivers, the way the FDT Instances are
structured and transmitted also has a large impact. As an example, a
way to satisfy the recommendation above is to transmit an FDT
Instance that describes all files currently being transmitted, and to
transmit this FDT Instance reliably, using the same techniques as
explained for the case when there is a single file transmitted in a
session. If instead the concurrently transmitted files are described
in separate FDT Instances, another way to satisfy this recommendation
is to transmit all the relevant FDT Instances reliably, using the
same techniques as explained for the case when there is a single file
transmitted in a session.
In any case, how often the description of a file is sent in an FDT
Instance, how often an FDT Instance is sent, and how much FEC
protection is provided for an FDT Instance (if longer than one packet
payload) are dependent on the particular application and are outside
the scope of this document.
Sometimes the various attributes associated with files that are to be
delivered within the file delivery session are sent out-of-band. The
details of how this is done are out of the scope of this document.
However, it is still RECOMMENDED that any out-of-band transmission be
managed in such a way that a receiver will be able to recover the
attributes associated with a file at least as reliably as the
receiver is able to receive enough packets containing encoding
symbols to recover the file. For example, the probability of a
randomly chosen receiver being able to recover a given file can often
be estimated based on a statistical model of reception conditions,
the amount of data transmitted, and the properties of any Forward
Error Correction in use. The recommendation above suggests that
mechanisms used for file attribute delivery should achieve a higher
delivery probability than the file recovery probability. The sender
MAY also continue sending the various file attributes in-band, in
addition to the out-of-band transmission.
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. Placement of the FDT Instance Header is the same as that of
any other LCT Header Extension. There MAY be other LCT Header
Extensions in use.
The FDT Instance is encoded for transmission, like any other object,
using an FEC Scheme (which MAY be the Compact No-Code FEC Scheme).
The LCT Header Extensions are followed by the FEC Payload ID, and
finally the Encoding Symbols for the FDT Instance, which contains one
or more file description entries. An 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
The overall format of ALC/LCT packets carrying an FDT Instance is
depicted in Figure 1 below. All integer fields are carried in
"big-endian" or "network order" format (i.e., most significant byte
(octet) first). As defined in [RFC5775], all ALC/LCT packets are
sent using UDP.
| UDP header |
| Default LCT header (with TOI = 0) |
| LCT Header Extensions (EXT_FDT, EXT_FTI, etc.) |
| FEC Payload ID |
FLUTE Payload: Encoding Symbol(s)
~ (for FDT Instance in an FDT packet) ~
Figure 1: Overall FDT Packet
3.4.1. Format of FDT Instance Header
The FDT Instance Header (EXT_FDT) is a new fixed-length, ALC
Protocol-Instantiation-specific LCT Header Extension [RFC5651]. The
Header Extension Type (HET) for the extension is 192. Its format is
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 |
Figure 2: EXT_FDT Format
Version of FLUTE (V), 4 bits:
This document specifies FLUTE version 2. Hence, in any ALC packet
that carries an FDT Instance and that belongs to the file delivery
session as specified in this specification MUST set this field
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 from
the smallest FDT Instance ID value assigned to an expired FDT
Instance. When wraparound from a greater FDT Instance ID value to a
smaller FDT Instance ID value occurs, the smaller FDT Instance ID
value is considered logically higher than the greater FDT Instance ID
value. Then, the subsequent FDT Instances are assigned the next
available smallest FDT Instance ID value, in order to always keep the
FDT Instance ID values logically increasing.
Senders MUST NOT reuse an FDT Instance ID value that is already in
use for a non-expired FDT Instance. Sender behavior when all the FDT
Instance IDs are used by non-expired FEC Instances is outside the
scope of this specification and left to individual implementations of
FLUTE. Receipt of an FDT Instance that reuses an FDT Instance ID
value that is currently used by a non-expired FDT Instance MUST be
considered an error case. Receiver behavior in this case (e.g.,
leave the session or ignore the new FDT Instance) is outside the
scope of this specification and left to individual implementations of
FLUTE. Receivers MUST be ready to handle FDT Instance ID wraparound
and situations where missing FDT Instance IDs result in increments
larger than one.
3.4.2. Syntax of FDT Instance
The FDT Instance contains file description entries that provide the
mapping functionality described in Section 3.2 above.
The FDT Instance is an Extensible Markup Language (XML) structure
that has a single root element "FDT-Instance". The "FDT-Instance"
element MUST contain the "Expires" attribute, which provides the
expiration time of the FDT Instance. In addition, the "FDT-Instance"
element MAY contain the "Complete" attribute, a boolean that can be
either set to '1' or 'true' for TRUE, or '0' or 'false' for FALSE.
When TRUE, the "Complete" attribute signals that this "FDT Instance"
includes the set of "File" entries that exhausts both the set of
files delivered so far and the set of files to be delivered in the
session. This implies 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 file parameters identical to those already given in this
and previous FDT Instances). The "Complete" attribute is therefore
used to provide a complete list of files in an entire FLUTE session
(a "complete FDT"). Note that when all the FDT Instances received so
far have no "Complete" attribute, the receiver MUST consider that the
session is not complete and that new data MAY be provided in future
FDT Instances. This is equivalent to receiving FDT Instances having
the "Complete" attribute set to FALSE.
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
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
The attributes of the "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
the MIME field name, and the XML attribute value corresponds to the
value of the MIME field body [RFC2045]. 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.
"Content-Location" [RFC2616] MUST be assigned a syntactically valid
URI, as defined in [RFC3986], which identifies the file to be
delivered. For example, it can be a URI with the "http" or "file"
URI scheme. Only one "Content-Location" attribute is allowed for
each file. The "Content-Location" field MUST be considered a string
that identifies a file (i.e., two different strings are two different
identifiers). Any use of the "Content-Location" field for anything
else other than to identify the object is out of scope of this
specification. The semantics for any two "File" elements declaring
the same "Content-Location" but differing "TOI" is that the element
appearing in the FDT Instance with the greater FDT Instance ID is
considered to declare a newer instance (e.g., version) of the same
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:
* The attribute "Content-Type" SHOULD be included and, when present,
MUST be used for the purpose defined in [RFC2616].
* Where the length is described, the attribute "Content-Length" MUST
be used for the purpose defined in [RFC2616]. The transfer length
is defined to be the length of the object transported in octets.
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 octets 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
* Whenever content encoding is applied, the attribute
"Content-Encoding" MUST be included. Whenever the attribute
"Content-Encoding" is included, it MUST be used as described in
* Where the MD5 message digest is described, the attribute
"Content-MD5" MUST be used for the purpose defined in [RFC2616].
Note that the goal is to provide a decoded object integrity
service in cases where transmission and/or FLUTE/ALC processing
errors may occur (the probability of collision is in that case
negligible). It MUST NOT be regarded as a security mechanism (see
Section 7 for information regarding security measures).
* The FEC Object Transmission Information attributes are described
in Section 5.
The following specifies the XML Schema [XML-Schema-Part-1]
[XML-Schema-Part-2] for the FDT Instance:
<?xml version="1.0" encoding="UTF-8"?>
<xs:element name="FDT-Instance" type="FDT-InstanceType"/>
<xs:element name="File" type="FileType" maxOccurs="unbounded"/>
<xs:any namespace="##other" processContents="skip"
Figure 3: XML Schema for the FDT Instance
Any valid FDT Instance MUST use the above XML Schema. This way, FDT
provides extensibility to support private elements and 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.). Unsupported private elements and
attributes SHOULD be silently ignored by a FLUTE receiver.
In case the basic FDT XML Schema is extended in terms of new
descriptors (attributes or elements), for descriptors applying to a
single file, those MUST be placed within the element "File". For
descriptors 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 attributes applied in the FDT be in the
format of message header fields and be either defined in the HTTP/1.1
specification [RFC2616] or another well-known specification, or in an
IANA registry [IANAheaderfields]. However, this specification
doesn't prohibit the use of other formats to allow private attributes
to be used when interoperability is not a concern.
3.4.3. Content Encoding of FDT Instance
The FDT Instance itself MAY be content encoded (e.g., compressed).
This specification defines the FDT Instance Content Encoding Header
(EXT_CENC). EXT_CENC is a new fixed-length LCT Header Extension
[RFC5651]. The Header Extension Type (HET) for the extension is 193.
If the FDT Instance is content encoded, EXT_CENC MUST be used to
signal the content encoding type. In that case, the EXT_CENC Header
Extension MUST be used in all ALC packets carrying the same FDT
Instance ID. Consequently, when the 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, 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 |
Figure 4: EXT_CENC Format
Content Encoding Algorithm (CENC), 8 bits:
This field signals the content encoding algorithm used in the FDT
Instance payload. This subsection reserves the Content Encoding
Algorithm values 0, 1, 2, and 3 for null, ZLIB [RFC1950], DEFLATE
[RFC1951], and GZIP [RFC1952], respectively.
Reserved, 16 bits:
This field MUST be set to all '0's. This field MUST be ignored on
3.5. Multiplexing of Files within a File Delivery Session
The delivered files are carried as transmission 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 might 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.