TVWeb-URI-Requirements-19981110

TV URI Schemes
Requirements

10th November 1998

Editors:: Warner ten Kate<tenkate@natlab.research.philips.com>.
Copyright:: Copyright ) 1998 W3C (MIT, INRIA, Keio), All Rights Reserved.

Abstract

This document lists the requirements posed to URI schemes for use in TV-based environments. The document summarizes the outcome of discussions on this subject by the W3C TV-Web Interest Group. See the references for the starters of that discussion.

TV Broadcast

Definition: In this document TV Broadcast is used as the generic term to refer to currently existing TV systems, their transport protocols, and their typical operation of content provision and distribution. TV Broadcast includes systems like DVB, ATSC, DSS, NTSC, and PAL. The TV Broadcast 'network layer' is typically non-IP based.

Requirements on TV Broadcast URI schemes

The URI scheme should comply with RFC 2396.
It must be possible for the resource referenced by a URI to be a channel/service (i.e. a concatenation of programs), an entire program/event, or just a single component of a program/event. Fragments within a component are outside the current scope of requirements.
Given a URI, it must be possible for a receiver to actually locate the resource, or conclude that it is not reachable.
Given a URI, it must be possible for a receiver to determine the time period(s) within which the resource can be retrieved from the (also resolved) location.
A URI should be invariant with respect to the normal range of transport stream transformations, both in referencing the time and location of that resource in that transport stream.
The URI scheme should support the spectrum of transport protocols applied and standardized in TV Broadcast systems. This includes both audio/video and data broadcast protocols.
A URI must be meaningful when interpreted from any of the following locations relative to the resource being referenced:
- From the same TV Broadcast network
- From another TV Broadcast network
- From an arbitrary location in the Internet
[Note: this means that the system can detect it concerns a URI pointing to a TV Broadcast network.]
A URI should be resolvable under any of the following network access conditions:
- TV Broadcast, same or another network
- Internet
- In Home/local storage
- Other (future) networks
The actual resource's retrieved content data may differ in terms of content quality, performance, and edit version.
[Note the difference with the previous requirement, particularly the use of 'must' and 'should'.]
The URI scheme must be compatible with solutions already adopted in standardisation bodies such as ATSC, DVB, and DAVIC.
The URI scheme should interoperate with the Internet access schemes, in particular http.
[Note: the scheme, not per se the access protocol it calls; it means that hierarchies in the URI scheme should map as much as possible. This should assist seamless transitions when the client decides to use another access protocol (and network).]
Ideally, the URI should support referencing various instantiations of the same content (quality/compression ratio, versions/edits).
The URI scheme should support relative referencing such that a TV-program with all its associated resources can be referenced against a common base, which is the TV Broadcast URI of that aggregate.

Exceptions in TV Broadcast URIs

TV Broadcast differs from the conventional Internet in several ways. The TV Broadcast URI scheme is affected by that in the following aspects:

The host is not necessarily a server identifyable through an IP-address. For instance, the 'host' is a transport stream.
The resource access and retrieval scheme is not necessarily IP-stack based.
The resource's availability implicitly depends on, or at least relates to, a transmission schedule.

References