FLUTE File Delivery Table

Description

The FLUTE File Delivery Table (FDT) is a core component of the FLUTE protocol, standardized by the IETF and adopted within 3GPP for Multimedia Broadcast Multicast Service (MBMS) and evolved MBMS (eMBMS). It is an XML-based structure that acts as a manifest or catalog for files within a FLUTE session. The FDT itself is transmitted as a special file within the session, identified by a specific Transport Object Identifier (TOI). It contains a series of FDT Instance elements, each providing a snapshot of the files available at a given point in time. Each file entry within an FDT Instance includes mandatory attributes such as the TOI, Content-Location (a URI), Content-Length, and Content-Type, along with optional attributes like transfer length, expiration time, and content encoding parameters (e.g., for FEC).

The FDT's primary operational role is to provide receivers with the necessary metadata to correctly identify, request (in the case of repair flows), reassemble, and process the delivered file objects. In a unidirectional broadcast scenario, files are sent repeatedly in carousels. The FDT is also carouseled, with new instances published to add, modify, or remove file entries. A receiver tunes into the session, acquires the current FDT, and uses it to map incoming data packets (identified by their TOI and source block numbers) to the correct file and to understand when the complete file has been received. For reliable delivery, the FDT includes information for Forward Error Correction (FEC) decoding, specifying the FEC encoding ID and instance ID used for each file.

Architecturally, the FDT sits within the application layer of the FLUTE protocol stack. The BM-SC (Broadcast Multicast Service Centre) in the 3GPP network generates and manages the FDT for a given MBMS session. The FDT's structure and semantics are defined to be extensible, allowing for the inclusion of session-specific and application-specific metadata in separate namespaces. This enables advanced services like dynamic playlist updates for streaming or conditional access information. Its standardized format ensures interoperability between different network equipment and user devices, forming the essential glue that links the transport-layer delivery of data packets to the application-layer consumption of complete files.

Purpose & Motivation

The FDT was created to solve the fundamental problem of delivering discrete, identifiable files over unidirectional, best-effort IP multicast or broadcast channels. Prior to FLUTE and the FDT, broadcast systems often used proprietary encapsulation methods, making large-scale, interoperable file delivery challenging. The FDT provides a standardized, flexible, and efficient mechanism to announce and describe file content within a session. This allows receivers, which may join a session at any time, to immediately understand what files are being delivered and how to process them, without relying on out-of-band signaling or pre-configured knowledge.

Its development was motivated by the rise of MBMS in 3GPP, which aimed to enable efficient point-to-multipoint services like mobile TV, software updates, and group communications. These services required a reliable file delivery mechanism that could scale to millions of devices. The FDT addresses the limitations of simple data carousels by providing rich metadata, supporting advanced features like FEC for reliability, content compression, and dynamic updates. It abstracts the complexity of the underlying transport, allowing application developers to focus on content creation while the FLUTE protocol, guided by the FDT, handles the logistics of delivery, reassembly, and verification.