Source: https://tools.ietf.org/id/draft-ietf-rmt-flute-revised-13.html
Timestamp: 2019-07-15 18:44:57
Document Index: 92262339

Matched Legal Cases: ['art-1', 'art-2', 'art-1', 'art 1', 'art-2', 'art 2']

Reliable Multicast Transport (RMT) T. Paila
Internet-Draft R. Walsh
Obsoletes: 3926 (if approved) Nokia
Intended status: Standards Track M. Luby
Expires: July 14, 2012 Qualcomm, Inc.
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. This document obsoletes RFC3926.
This Internet-Draft will expire on July 14, 2012.
7.1. Problem Statement
7.2. Attacks against the data flow
7.2.1. Access to confidential files
7.2.2. File corruption
7.3. Attacks against the session control parameters and associated Building Blocks
7.3.1. Attacks against the Session Description
7.3.2. Attacks against the FDT Instances
7.3.3. Attacks against the ALC/LCT parameters
7.3.4. Attacks against the associated Building Blocks
7.4. Other Security Considerations
7.5. Minimum Security Recommendations
8.1. Registration Request for XML Schema of FDT Instance
8.2. Media-Type Registration Request for application/fdt+xml
8.3. Content Encoding Algorithm Registration Request
8.3.1. Explicit IANA Assignment Guidelines
8.4. Registration of EXT_FDT LCT Header Extension Type
8.5. Registration of EXT_CENC LCT Header Extension Type
11.1. RFC3926 to draft-ietf-rmt-flute-revised-12
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 addressing.
In-band signaling of the transport parameters of the ALC 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 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 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] 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 specification.
The differences between [RFC3926] and this document are listed in Section 11.
This document updates ALC [RFC5775] and Layered Coding Transport (LCT) [RFC5651] in the sense it defines two new header extensions, EXT_FDT and EXT_CENC.
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 [RFC4566] files which enable user applications to access multimedia sessions.
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 Layered Coding Transport (LCT) Building Block [RFC5651] and the FEC Building Block [RFC5052].
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) [RFC1112] and the Source-Specific Multicast (SSM) [PAPER.SSM].
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 these networks.
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 document.
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".
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.
Identifier of the file, expressed as a URI. The identifier MAY provide a location for the file. In the above example: "http://www.example.com/docs/file.txt".
File type, expressed as MIME media type. 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.
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-DSIG [RFC3275] to digitally sign the file. XML-DSIG can also be used to provide tamper prevention e.g. on 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
ALC is a protocol instantiation of Layered Coding Transport building block (LCT) [RFC5651]. 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 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). 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 session.
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 extension header (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) 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. Alternately, a new receiver MAY discover a binding between TSI and FLUTE protocol version via a session discovery protocol that is out of scope in this document.
If the sender is not assigned a permanent IP address accessible to receivers, then packets that can be received by receivers contain a temporary IP address. In this case 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. In this case the TSI is scoped by the temporary 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 a permanent IP address or a temporary IP address, is outside the scope of this document.
The ALC/LCT session is called a file delivery session.
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 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.
Size of the transmission object carrying the file
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 is 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 this occurs). 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, a subset of, 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.
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.
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 is 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 which 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. 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 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 an 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 case of the prime epoch (era 0) [RFC5905]. The handling of time wraparound (to happen in 2036) requires to consider the associated epoch. In any case, both a sender and a receiver can determine to which (136 year) epoch the FDT Instance expiration time value pertains to by choosing the epoch for which the expiration time is closest in time to the current time.
Here is an example. Let us imagine 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 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 understand the FEC Encoding ID in a FDT Instance, 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 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 (rather than in-band, within one or several FDT Instances). 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 with as much or greater reliability 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 higher a delivery probability than the file recovery probability.
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 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 [RFC5775], all ALC/LCT packets are sent using UDP.
FLUTE Payload: Encoding Symbol(s)
~             (for FDT Instance in a FDT packet)                ~
The FDT Instance Header (EXT_FDT) is a new fixed length, ALC PI specific LCT header extension [RFC5651]. The Header Extension Type (HET) for the extension is 192. Its format is defined below:
This document specifies FLUTE version 2. 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 '2'.
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 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. Senders MUST NOT re-use 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 SHOULD be considered as an error case. Receiver behavior in this case 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.
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 expiration time of the FDT Instance. In addition, the "FDT-Instance" element MAY contain the "Complete" attribute (boolean), which, when TRUE, signals that this "FDT Instance" includes the set of "File" entries that exhausts both the set of files delivered so far and also 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 identical file parameters 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").
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 [RFC2616] which identifies the object to be delivered, for example a URI with the "http" or "file" URI scheme. 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 newer instance (e.g. version) of the same "File".
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 as 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 length.
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 [RFC2616].
Where the MD5 message digest is described, the attribute "Content-MD5" MUST be used for the purpose as defined in [RFC2616].
The following specifies the XML Schema [XML-Schema-Part-1][XML-Schema-Part-2] for FDT Instance:
<xs:schema xmlns="urn:ietf:params:xml:ns:fdt"
targetNamespace="urn:ietf:params:xml:ns:fdt"
<xs:element name="FDT-Instance" type="FDT-InstanceType"/>
<xs:complexType name="FDT-InstanceType">
<xs:element name="File" type="FileType" maxOccurs="unbounded"/>
<xs:attribute name="FEC-OTI-Scheme-Specific-Info"
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 are in the format of MIME fields and are either defined in the HTTP/1.1 specification [RFC2616] or another well-known specification.
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 LCT header extension [RFC5651]. 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:
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.
This field MUST be set to all '0'. This field MUST be ignored on reception.
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.
ALC/LCT has a concept of channels and congestion control. There are four scenarios in which FLUTE is envisioned to be applied.
Use of a single channel and a single-rate congestion control protocol.
Use of multiple channels and a multiple-rate congestion control protocol. In this case the FDT Instances MAY be delivered on more than one channel.
Use of 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.
Use of 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.
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 transmission object must complete 'prior' to the second one, it is RECOMMENDED that only a single channel is used for the file delivery session.
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 okay 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 okay 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 Compact No-code FEC Encoding ID 0 [RFC5445] 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.
FLUTE inherits the use of FEC building block [RFC5052] 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. There are two methods to achieve this: the use of ALC specific LCT extension header EXT_FTI [RFC5775] and the use of FDT. The latter method is specified in this section. The use of EXT_FTI requires repetition of the FEC Object Transmission Information to ensure reception (though not necessarily in every packet) and thus may entail higher overhead than the use of the FDT, but may also provide more timely delivery of the FEC Object Transmission Information.
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). The FEC Object Transmission Information that MUST be delivered to receivers is defined by the FEC Scheme. This section 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 prioritize the sources in the following way (in the order of decreasing priority).
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.
"Transfer-Length" carries the Transfer-Length Object Transmission Information element defined in [RFC5052].
"FEC-OTI-FEC-Encoding-ID" carries the "FEC Encoding ID" Object Transmission Information element defined in [RFC5052], as carried in the Codepoint field of the ALC/LCT header.
"FEC-OTI-FEC-Instance-ID" carries the "FEC Instance ID" Object Transmission Information element defined in [RFC5052] for Under-specified FEC Schemes.
"FEC-OTI-Maximum-Source-Block-Length" carries the "Maximum Source Block Length" Object Transmission Information element defined in [RFC5052], if required by the FEC Scheme.
"FEC-OTI-Encoding-Symbol-Length" carries the "Encoding Symbol Length" Object Transmission Information element defined in [RFC5052], if required by the FEC Scheme.
"FEC-OTI-Max-Number-of-Encoding-Symbols" carries the "Maximum Number of Encoding Symbols" Object Transmission Information element defined in [RFC5052], if required by the FEC Scheme.
"FEC-OTI-Scheme-specific-information" carries the "encoded scheme-specific FEC Object Transmission Information" as defined in [RFC5052], if required by the FEC Scheme.
In FLUTE, the FEC Encoding ID (8 bits) for a given TOI MUST be carried in the Codepoint field of the ALC/LCT header. When the FEC Object Transmission Information for this TOI is delivered through the FDT, then the associated "FEC-OTI-FEC-Encoding-ID" attribute and the Codepoint field of all packets for this TOI MUST be the same.
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 [RFC5775], the transport parameters of an ALC/LCT session that the receiver needs to know are:
Some information that tells receiver, in the first place, that the session contains files that are of interest;
Definition and configuration of congestion control mechanism for the session ;
Security parameters relevant for the session.
FLUTE version number.
It is envisioned that these parameters would be described according to some session description syntax (such as SDP [RFC4566] 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 (SAP) [RFC2974], email, HTTP [RFC2616], SIP [RFC3261], 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.
A content delivery system is potentially subject to attacks. Attacks may target: Section 7.5.
the Content Delivery Protocol (CDP) (e.g., to compromise the normal behavior of FLUTE), or
the content itself (e.g., to corrupt the files being transmitted).
against the session control parameters (e.g., by corrupting the session description, the FDT Instances, or the ALC/LCT control parameters) that are sent either in-band or out-of-band, or
In the following sections we provide more details on these possible attacks and sketch some possible counter-measures. We provide recommendations in
attacks that are meant to give access to a confidential file (e.g., in case of a non-free content) and
attacks that try to corrupt the file being transmitted (e.g., to inject malicious code within a file, or to prevent a receiver from using a file, which is a kind of Denial of Service, DoS).
Access control to the file being transmitted is typically provided by means of encryption. This encryption can be done over the whole file i.e. before applying FEC protection (e.g., by the content provider, before submitting the file to FLUTE), or be done on a packet per packet basis (e.g., when IPsec/ESP is used [RFC4303], see Section 7.5). If confidentiality is a concern, it is RECOMMENDED that one of these solutions be used.
Protection against corruptions (e.g., if an attacker sends forged packets) is achieved by means of a content integrity verification/sender authentication scheme. This service can be provided at the file level i.e. before applying content encoding and forward error correction encoding. In that case a receiver has no way to identify which symbol(s) is(are) corrupted if the file is detected as corrupted. This service can also be provided at the packet level i.e. after applying content encoding and forward error correction encoding, on a packet by packet basis. In this case, after removing all corrupted packets, the file may be in some cases recovered from the remaining correct packets.
Integrity protection applied at the file level has the advantage of lower overhead since only relatively few bits are added to provide the integrity protection compared to the file size. However it has the disadvantage that it cannot distinguish between correct packets and corrupt packets and therefore correct packets, which may form the majority of packets received, may be unusable. Integrity protection applied at the packet level has the advantage that it can distinguish between correct and corrupt packets at the cost of additional per packet overhead.
Several techniques can provide this source authentication/content integrity service:
at the file level, the file MAY be digitally signed, for instance by using RSASSA-PKCS1-v1_5 [RFC3447]. This signature enables a receiver to check the file integrity, once this latter has been fully decoded. Even if digital signatures are computationally expensive, this calculation occurs only once per file, which is usually acceptable;
at the packet level, each packet can be digitally signed [RMT-SIMPLE-AUTH]. A major limitation is the high computational and transmission overheads that this solution requires. To avoid this problem, the signature may span a set of symbols (instead of a single one) in order to amortize the signature calculation, but if a single symbol is missing, the integrity of the whole set cannot be checked;
at the packet level, a Group Message Authentication Code (MAC) [RFC2104][RMT-SIMPLE-AUTH] scheme can be used, for instance by using HMAC-SHA-256 with a secret key shared by all the group members, senders and receivers. This technique creates a cryptographically secured digest of a packet that is sent along with the packet. The Group MAC scheme does not create prohibitive processing load nor transmission overhead, but it has a major limitation: it only provides a group authentication/integrity service since all group members share the same secret group key, which means that each member can send a forged packet. It is therefore restricted to situations where group members are fully trusted (or in association with another technique as a pre-check);
at the packet level, TESLA [RFC4082][RFC5776] is an attractive solution that is robust to losses, provides a true authentication/integrity service, and does not create any prohibitive processing load or transmission overhead. Yet checking a packet requires a small delay (a second or more) after its reception;
at the packet level, IPsec/ESP [RFC4303] can be used to check the integrity and authenticate the sender of all the packets being exchanged in a session (see Section 7.5).
It is up to the developer and deployer, who know the security requirements and features of the target application area, to define which solution is the most appropriate. Nonetheless, in case there is any concern of the threat of file corruption, it is RECOMMENDED that at least one of these techniques be used.
the attack can target the FDT Instances,
the attack can target the FLUTE associated building blocks, for instance the multiple rate congestion control protocol.
The consequences of these attacks are potentially serious, since they might compromise the behavior of content delivery system itself.
A FLUTE receiver may potentially obtain an incorrect Session Description for the session. The consequence of this is that legitimate receivers with the wrong Session Description are unable to correctly receive the session content, or that receivers inadvertently try to receive at a much higher rate than they are capable of, thereby possibly disrupting other traffic in the network.
To avoid these problems, it is RECOMMENDED that measures be taken to prevent receivers from accepting incorrect Session Descriptions. One such measure is source authentication to ensure that receivers only accept legitimate Session Descriptions from authorized senders. How these measures are achieved is outside the scope of this document since this session description is usually carried out-of-band.
Corrupting the FDT Instances is one way to create a Denial of Service attack. For example, the attacker changes the MD5 sum associated to a file. This possibly leads a receiver to reject the files received, no matter whether the files have been correctly received or not.
Corrupting the FDT Instances is also a way to make the reception process more costly than it should be. This can be achieved by changing the FEC Object Transmission Information when the FEC Object Transmission Information is included in the FDT Instance. For example, an attacker may corrupt the FDT Instance in such a way that Reed-Solomon over GF(2^^16) be used instead of GF(2^^8) with FEC Encoding ID 2. This may significantly increase the processing load while compromising FEC decoding.
It is therefore RECOMMENDED that measures be taken to guarantee the integrity and to check the sender's identity of the FDT Instances. To that purpose, one of the counter-measures mentioned above (Section 7.2.2) SHOULD be used. These measures will either be applied on a packet level, or globally over the whole FDT Instance object. Additionally, XML digital signatures [RFC3275] are a way to protect the FDT Instance by digitally signing it. When there is no packet level integrity verification scheme, it is RECOMMENDED to rely on XML digital signatures of the FDT Instances.
By corrupting the ALC/LCT header (or header extensions) one can execute attacks on underlying ALC/LCT implementation. For example, sending forged ALC packets with the Close Session flag (A) set to one can lead the receiver to prematurely close the session. Similarly, sending forged ALC packets with the Close Object flag (B) set to one can lead the receiver to prematurely give up the reception of an object.
It is therefore RECOMMENDED that measures be taken to guarantee the integrity and to check the sender's identity of the ALC packets received. To that purpose, one of the counter-measures mentioned above (Section 7.2.2) SHOULD be used.
Let us first focus on the congestion control building block, that may be used in the ALC session. A receiver with an incorrect or corrupted implementation of the multiple rate congestion control building block may affect the health of the network in the path between the sender and the receiver. That may also affect the reception rates of other receivers who joined the session.
When congestion control building block is applied with FLUTE, it is therefore RECOMMENDED that receivers be required to identify themselves as legitimate before they receive the Session Description needed to join the session. How receivers identify themselves as legitimate is outside the scope of this document. If authenticating a receiver does not prevent this latter to launch an attack, it will enable the network operator to identify him and to take counter-measures.
When congestion control building block is applied with FLUTE, it is also RECOMMENDED that a packet level authentication scheme be used, as explained in Section 7.2.2. Some of them, like TESLA, only provide a delayed authentication service, whereas congestion control requires a rapid reaction. It is therefore RECOMMENDED [RFC5775] that a receiver using TESLA quickly reduces its subscription level when the receiver believes that a congestion did occur, even if the packet has not yet been authenticated. Therefore TESLA will not prevent DoS attacks where an attacker makes the receiver believe that a congestion occurred. This is an issue for the receiver, but this will not compromise the network. Other authentication methods that do not feature this delayed authentication could be preferred, or a group MAC scheme could be used in parallel to TESLA to prevent attacks launched from outside of the group.
Lastly, we note that the security considerations that apply to, and are described in, ALC [RFC5775], LCT [RFC5651] and FEC [RFC5052] also apply to FLUTE as FLUTE builds on those specifications. In addition, any security considerations that apply to any congestion control building block used in conjunction with FLUTE also apply to FLUTE.
We now introduce a mandatory to implement but not necessarily to use security configuration, in the sense of [RFC3365]. Since FLUTE relies on ALC/LCT, it inherits the "baseline secure ALC operation" of [RFC5775]. More precisely, security is achieved by means of IPsec/ESP in transport mode. [RFC4303] explains that ESP can be used to potentially provide confidentiality, data origin authentication, content integrity, anti-replay and (limited) traffic flow confidentiality. [RFC5775] specifies that the data origin authentication, content integrity and anti-replay services SHALL be supported, and that the confidentiality service is RECOMMENDED. If a short lived session MAY rely on manual keying, it is also RECOMMENDED that an automated key management scheme be used, especially in case of long lived sessions.
Therefore, the RECOMMENDED solution for FLUTE provides per-packet security, with data origin authentication, integrity verification and anti-replay. This is sufficient to prevent most of the in-band attacks listed above. If confidentiality is required, a per-packet encryption SHOULD also be used.
This specification contains five separate items for IANA Considerations:
Registration Request for XML Schema of FDT Instance.
Media-Type Registration Request for application/fdt+xml.
Content Encoding Algorithm Registration Request.
Registration of the EXT_FDT LCT Header Extension Type
Registration of the EXT_CENC LCT Header Extension Type
Document [RFC3688] defines an IANA maintained registry of XML documents used within IETF protocols. The following is the registration request for the FDT XML schema.
Registrant Contact: Toni Paila (toni.paila (at) nokia.com)
XML: The XML Schema specified in Section 3.4.2
This section provides the registration request, as per [RFC4288], [RFC4289] and [RFC3023], to be submitted to IANA following IESG approval.
Subtype name: fdt+xml
Encoding considerations: The fdt+xml type consists of UTF-8 ASCII characters [RFC3629] and must be well-formed XML.
Additional content and transfer encodings may be used with fdt+xml files, with the appropriate encoding for any specific file being entirely dependent upon the deployed application.
Restrictions on usage: Only for usage with FDT Instances which are valid according to the XML schema of section 3.4.2.
Security considerations: fdt+xml data is passive, and does not generally represent a unique or new security threat. However, there is some risk in sharing any kind of data, in that unintentional information may be exposed, and that risk applies to fdt+xml data as well.
Published specification: The present document including section 3.4.2. The specified FDT Instance functions as an actual media format of use to the general Internet community and thus media type registration under the Standards Tree is appropriate to maximize interoperability.
Applications which use this media type: Not restricted to any particular application
File extension(s): An FDT Instance may use the extension ".fdt"
Person and email address to contact for further information: Toni Paila (toni.paila (at) nokia.com)
Values of Content Encoding Algorithms are subject to IANA registration. The value of Content Encoding Algorithm is a numeric non-negative index. In this document, the range of values for Content Encoding Algorithms is 0 to 255. This specification already assigns the values 0, 1, 2 and 3 as described in section 3.4.3.
This document defines a name-space called "Content Encoding Algorithms".
IANA has established and manages the new registry for the "FLUTE Content Encoding Algorithm" name-space. The values that can be assigned within this name-space are numeric indexes in the range [0, 255], boundaries included. Assignment requests are granted on a "Specification Required" basis as defined in [RFC5226]. Note that the values 0, 1, 2 and 3 of this registry are already assigned by this document as described in section 3.4.3.
This document registers value 192 for the EXT_FDT LCT Header Extension defined in Section 3.4.1.
This document registers value 193 for the EXT_CENC LCT Header Extension defined in Section 3.4.3.
The following persons have contributed to this specification: Brian Adamson, Mark Handley, Esa Jalonen, Roger Kermode, Juha-Pekka Luoma, Topi Pohjolainen, Lorenzo Vicisano, and Mark Watson. The authors would like to thank all the contributors for their valuable work in reviewing and providing feedback regarding this specification, and in particular .
Email: jani.peltotalo (at) tut.fi
Email: sami.peltotalo (at) tut.fi
EMail: magnus.westerlund (at) ericsson.com
EMail: thorsten.lohmar (at) ericsson.com
Incremented FLUTE protocol version from 1 to 2, due to concerns about backwards compatibility. For instance, the LCT header changed between RFC 3451 and [RFC5651]. In RFC 3451, the T and R fields of the LCT header respectively indicate the presence of Sender Current Time and Expected Residual Time. In [RFC5651], these fields MUST be set to zero and MUST be ignored by receivers (instead the EXT_TIME extension headers can convey this information if needed). Thus, [RFC5651] is not backwards compatible with RFC 3451, even though both have the same LCT version 1. FLUTE version 1 as specified in [RFC3926] MUST use RFC 3451. FLUTE version 2 as specified in this document MUST use [RFC5651]. Therefore an implementation that relies on [RFC3926] and RFC 3451 will not be backwards compatible with FLUTE as specified in this document.
Updated dependencies to other RFCs to revised versions, e.g., changed ALC reference from RFC 3450 to [RFC5775], changed LCT reference from RFC 3451 to [RFC5651], etc.
Two additional items are added in the IANA considerations section, specifically the registration of two values in the LCT Header Extension Types registry (192 for EXT_FDT and 193 for EXT_CENC).
Added clarification for the use of FLUTE for unicast communications in Section 1.1.4.
Clarified how to reliably deliver the FDT in Section 3.3 and the possibility of using an out-of-band delivery of FDT information.
Clarified how to address FDT Instance expiration time wraparound with the notion of "epoch" of NTPv4 in Section 3.3.
Clarified what should be considered as erroneous situations in Section 3.4.1 (definition of FDT Instance ID). In particular a receiver MUST be ready to handle FDT Instance ID wraparounds and missing FDT Instances.
Updated the security section to define IPsec/ESP as a mandatory to implement security solution in Section 7.5.
Removed the 'Statement of Intent' from the Section 1. The statement of intent was meant to clarify the "Experimental" status of [RFC3926]. It does not apply to this draft that is intended for "Standard Track" submission.
Added clarification on XML-DSIG in the end of Section 3.
Revised the use of word "complete" in the Section 3.2.
Clarified Figure 1 WRT "Encoding Symbol(s) for FDT Instance".
Clarified the FDT Instance ID wrap-around in the end of Section 3.4.1.
Clarification for "Complete FDT" in the Section 3.4.2.
Added semantics for the case two TOIs refer to same Content-Location. Now it is in line how 3GPP and DVB interpret the case.
In the Section 3.4.2 XML Schema of FDT instance is modified to various advices. For example, extension by element was missing but is now supported. Also namespace definition is changed to URN format.
Clarified FDT-schema extensibility in the end of Section 3.4.2.
The CENC value allocation is added in the end of Section 3.4.3.
Section 5 is modified so that EXT_FTI and the FEC issues are replaced by a reference to LCT specification. We count on revised LCT specification to specify the EXT_FTI.
Added a clarifying paragraph on the use of Codepoint in the very end of Section 5.
Reworked Section 8 - IANA Considerations. Now it contains three IANA registration requests:
Registration Request for XML Schema of FDT Instance (urn:ietf:params:xml:schema:fdt)
Media-Type Registration Request for application/fdt+xml
Content Encoding Algorithm Registration Request (ietf:rmt:cenc)
Added Section 10 - Contributors.
Revised list of both Normative as well as Informative references.
Added a clarification that receiver should ignore reserved bits of Header Extension type 193 upon reception.
Minor changes to remove forward references (use before definition) or refer to forward reference sections.
Elaborate on just what kind of networks cannot support FLUTE congestion control (1.1.4)
In Section 3.2 revise "several" (meaning 3-n vs. "couple" = 2) to "multiple" (meaning 2-n)
Move Section 3.3 requirement to send FDT more reliably than files, to a bulleted RECOMMENDED requirement, making check-off easier for testers.
Sharpen Section 3.3 definition that future FDT file instances can "augment" (meaning enhance) rather than "complement" (sometimes meaning negate, which is not allowed) the file parameters.
Elaborate in Section 3.3 and Section 4 that FEC Encoding ID = 0 is Compact No-code FEC, so that the reader doesn't have to search other RFCs to understand these protocol constants used by FLUTE.
Require in Section 3.3 that FLUTE receivers SHALL NOT attempt to decode FDTs if they do not understand the FEC Encoding ID
Remove restriction of Section 3.3 in bullet #4 that TOI=0 for the FDT, to be consistent with Appendix, bullet 6, and elsewhere. An FDT is signaled by an FDT Instance ID, NOT only by TOI = 0.
Standardize on the term "expiration time" and avoid using the redundant but possibly confusing term "expiry time".
To interwork with experimental flute, stipulate in Section 3.1 that only 1 instantiation of all 3 protocols FLUTE, ALC, and LCT, can be associated with a session (source IP-Address, TSI) and mention in Section 6 that you may (optionally) derive the FLUTE version from the file delivery session description.
Use a software writing tool to lower reading grade level and simplify Section 3.1.
[XML-Schema-Part-1] Thompson, H., Beech, D., Maloney, M. and N. Mendelsohn, "XML Schema Part 1: Structures", W3C Recommendation, May 2001.
[XML-Schema-Part-2] Biron, P. and A. Malhotra, "XML Schema Part 2: Datatypes", W3C Recommendation, May 2001.
[RFC3023] Murata, M, St.Laurent, S and D Kohn, "XML Media Types", RFC 3023, January 2001.
[RFC3629] Yergeau, F, "UTF-8, a transformation format of ISO 10646", RFC 3629, November 2003.
[RFC5226] Narten, T and H Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", RFC 5226, May 2008.
[RFC1950] Deutsch, P. and J-L. Gailly, "ZLIB Compressed Data Format Specification version 3.3", RFC 1950, May 1996.
[RFC1952] Deutsch, P., "GZIP file format specification version 4.3", RFC 1952, May 1996.
[RFC4303] Kent, S, "Encapsulating Security Payload (ESP)", RFC 4303, December 2005.
[RFC3926] Paila, T., Luby, M., Lehtonen, R., Roca, V. and R. Walsh, "FLUTE - File Delivery over Unidirectional Transport", RFC 3926, October 2004.
[RFC4566] Handley, M., Jacobson, V. and C. Perkins, "Session Description Protocol", RFC 4566, July 2006.
[RFC1112] Deering, S., "Host Extensions for IP Multicasting", RFC 1112, STD 5, August 1989.
[PAPER.SSM] Holbrook, H.W., "A Channel Model for Multicast, Ph.D. Dissertation, Stanford University, Department of Computer Science, Stanford, California", August 2001.
[RFC4289] Freed, N. and J. Klensin, "Multipurpose Internet Mail Extensions (MIME) Part Four: Registration Procedures", RFC 4289, December 2005.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A. R., Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP: session initiation protocol", RFC 3261, June 2002.
[RFC3688] Mealling, M, "The IETF XML Registry", RFC 3688, January 2004.
[RFC3447] Jonsson, J and B Kaliski, "Public-Key Cryptography Standards (PKCS) #1: RSA Cryptography Specifications Version 2.1", RFC 3447, February 2003.
[RFC2104] Krawczyk, H, Bellare, M and R Canetti, "HMAC: Keyed-Hashing for Message Authentication", RFC 2104, February 1997.
[RFC4082] Perrig, A, Canetti, R, Tygar, J D and B Briscoe, "Timed Efficient Stream Loss-Tolerant Authentication (TESLA): Multicast Source Authentication Transform Introduction", RFC 4082, June 2005.
[RMT-SIMPLE-AUTH] Roca, V., "Simple Authentication Schemes for the ALC and NORM Protocols", draft-ietf-rmt-simple-auth-for-alc-norm-06.txt (work in progress), December 2011.
Receiver recovers an object. An object can be recovered when an appropriate set of packets containing Encoding Symbols for the transmission 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.
Objects with TOI = 0 are reserved for FDT Instances. All FDT Instances are signaled by including an EXT_FDT header extension in the LCT header. The EXT_FDT header contains an FDT Instance ID (i.e. an FDT version number.) If the object has an FDT Instance ID 'N', the receiver parses the payload of the instance 'N' of FDT and updates its FDT database accordingly.
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 the file the given properties. The receiver also checks that the received content length matches with the description in the database. Optionally, if MD5 checksum has been used, the receiver checks that the calculated MD5 matches the description in the FDT database.
Toni Paila Paila Nokia Itamerenkatu 11-13 Helsinki, 00180 Finland EMail: toni.paila@nokia.com
Rod Walsh Walsh Nokia Visiokatu 1 Tampere, FIN-33720 Finland EMail: rod.walsh@nokia.com
Michael Luby Luby Qualcomm, Inc. 3165 Kifer Rd. Santa Clara, CA 95051 US EMail: luby@qualcomm.com
Rami Lehtonen Lehtonen TeliaSonera Hatanpaan valtatie 18 Tampere, FIN-33100 Finland EMail: rami.lehtonen@teliasonera.com