Network Working Group
Request for Comments: 3926
Category: Experimental

T. Paila
Nokia
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

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.

   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
   
             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.

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 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.

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.

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 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.

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 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.

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

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.

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         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

   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].

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>
       <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"
   
                    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 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.

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:

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 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.

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).

FEC Object Transmission Information that is available in EXT_FTI.

FEC Object Transmission Information that is available in the FDT.

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:

as defined in [2].

Header Extension Length (HEL), 8 bits:

The length of the whole Header Extension field, expressed in multiples of 32-bit words. This length includes the FEC Encoding ID specific format part.

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 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.

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.

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 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.

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;

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 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 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 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.
   
   [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.
   
   [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.

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.

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.

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.

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.

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.

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.

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.

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 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.

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.

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>

Authors' Addresses

   Toni Paila
   Nokia
   Itamerenkatu 11-13
   Helsinki  FIN-00180
   Finland
   
   EMail: toni.paila@nokia.com
   
   Michael Luby
   Digital Fountain
   39141 Civic Center Dr.
   Suite 300
   Fremont, CA  94538
   USA

EMail:

          luby@digitalfountain.com
   
   Rami Lehtonen
   TeliaSonera
   Hatanpaan valtatie 18
   Tampere  FIN-33100
   Finland
   
   EMail: rami.lehtonen@teliasonera.com
   
   Vincent Roca
   INRIA Rhone-Alpes
   655, av. de l'Europe
   Montbonnot
   St Ismier cedex  38334
   France
   
   EMail: vincent.roca@inrialpes.fr
   
   Rod Walsh
   Nokia
   Visiokatu 1
   Tampere  FIN-33720
   Finland
   
   EMail: rod.walsh@nokia.com

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