Date: Fri, 18 Dec 1998 17:11:36 -0600 (CST) Message-Id: <199812182311.RAA23755@isis.visi.com> From: "Craig A. Finseth" <email@example.com> To: firstname.lastname@example.org Cc: email@example.com Subject: Re: submission This message is to follow up your request from several days ago to provide an analysis of how the proposed scheme meets the URI, URL, and URN requirements. It's a first cut document and undoubtedly can be improved. Craig ------------------------------------------------------------ URI Scheme Viewed as URI, URL, URN This document briefly analyzes the URI scheme proposed in  and  in terms of the separate views as a URI, a URL, and a URN. AS A URI (Uniform Resource Identifier) To be a URI, a scheme must meet the requirements as spelled out in RFC 2396. They are: Uniformity: The proposed scheme identifies a wide variety of resources in a highly uniform manner. This manner is one that isolates the identification of the resources from the mechanism used to access them. It can be readily extended to handle new or additional resource types in a smooth fashion. It is also uniform syntactically, in that it conforms to the syntax specified in RFC 2396. Resource: The content identified by the proposed scheme fully meets the requirements of being a resource as specified in RFC 2396. The characteristic that all of the identified resources have in common is also the unifying force behind the creation of the scheme: all resources identified by this scheme are supplied over one or more TV broadcasts. Identifier: Again, the proposed scheme is an identifier by the definition supplied in RFC 2396. AS A URL (Uniform Resource Locator) The proposed scheme also meets the requirements (as specified in RFC 2396) for being a URL, albeit in a trivial way. The scheme assumes that each transport carries the information required to map each URL to the associated data, much as a file system maps a name to the actual storage holding the bytes of the file. The URI (viewed as a URL) is mapped by a transport-specific mechanism to the correct bytes. Admittedly, the proposed scheme only meets the URL requirements in a trivial way. However, the key point is that no other URL scheme is required in order to fully use the proposed scheme. AS A URN (Uniform Resource Name) Finally, the proposed scheme meets the requirements (as specified in RFC 2396) for being a URN. For proper operation, URIs must remain globally unique: once a URI is assigned to a purpose (e.g,. to designate a particular background GIF or to designate a channel), it can be used for no other purpose. The authority that assigns the URIs is in full control of what properties are to be preserved in this assignment. Further, the assigning authority is assumed to retain the memory of what URIs have been used and for what purposes, thus satisfying the requirement that URNs remain persistant.  Finseth, C,. Thomas, G., "Specifications for a TV Broadcast URI Scheme", Internet-Draft ???????????-00.txt  Finseth, C,. Thomas, G., "An Example Instantiation of the TV Broadcast URI Scheme for ATSC", Internet-Draft ???????????-00.txt ...from RFC 2396... 1.1 Overview of URI URI are characterized by the following definitions: Uniform Uniformity provides several benefits: it allows different types of resource identifiers to be used in the same context, even when the mechanisms used to access those resources may differ; it allows uniform semantic interpretation of common syntactic conventions across different types of resource identifiers; it allows introduction of new types of resource identifiers without interfering with the way that existing identifiers are used; and, it allows the identifiers to be reused in many different contexts, thus permitting new applications or protocols to leverage a pre-existing, large, and widely-used set of resource identifiers. Resource A resource can be anything that has identity. Familiar examples include an electronic document, an image, a service (e.g., "today's weather report for Los Angeles"), and a collection of other resources. Not all resources are network "retrievable"; e.g., human beings, corporations, and bound books in a library can also be considered resources. The resource is the conceptual mapping to an entity or set of entities, not necessarily the entity which corresponds to that mapping at any particular instance in time. Thus, a resource can remain constant even when its content---the entities to which it currently corresponds---changes over time, provided that the conceptual mapping is not changed in the process. Identifier An identifier is an object that can act as a reference to something that has identity. In the case of URI, the object is a sequence of characters with a restricted syntax. Having identified a resource, a system may perform a variety of operations on the resource, as might be characterized by such words as `access', `update', `replace', or `find attributes'. 1.2. URI, URL, and URN A URI can be further classified as a locator, a name, or both. The term "Uniform Resource Locator" (URL) refers to the subset of URI that identify resources via a representation of their primary access mechanism (e.g., their network "location"), rather than identifying the resource by name or by some other attribute(s) of that resource. The term "Uniform Resource Name" (URN) refers to the subset of URI that are required to remain globally unique and persistent even when the resource ceases to exist or becomes unavailable. The URI scheme (Section 3.1) defines the namespace of the URI, and thus may further restrict the syntax and semantics of identifiers using that scheme. This specification defines those elements of the URI syntax that are either required of all URI schemes or are common to many URI schemes. It thus defines the syntax and semantics that are needed to implement a scheme-independent parsing mechanism for URI references, such that the scheme-dependent handling of a URI can be postponed until the scheme-dependent semantics are needed. We use the term URL below when describing syntax or semantics that only apply to locators. Although many URL schemes are named after protocols, this does not imply that the only way to access the URL's resource is via the named protocol. Gateways, proxies, caches, and name resolution services might be used to access some resources, independent of the protocol of their origin, and the resolution of some URL may require the use of more than one protocol (e.g., both DNS and HTTP are typically used to access an "http" URL's resource when it can't be found in a local cache). A URN differs from a URL in that it's primary purpose is persistent labeling of a resource with an identifier. That identifier is drawn from one of a set of defined namespaces, each of which has its own set name structure and assignment procedures. The "urn" scheme has been reserved to establish the requirements for a standardized URN namespace, as defined in "URN Syntax" [RFC2141] and its related specifications. Most of the examples in this specification demonstrate URL, since they allow the most varied use of the syntax and often have a hierarchical namespace. A parser of the URI syntax is capable of parsing both URL and URN references as a generic URI; once the scheme is determined, the scheme-specific parsing can be performed on the generic URI components. In other words, the URI syntax is a superset of the syntax of all URI schemes.