Rationale for the Structure of the Model and Protocol for the Internet Printing Protocol

This document is one of a set of documents, which together describe all aspects of a new Internet Printing Protocol (IPP). IPP is an application level protocol that can be used for distributed printing using Internet tools and technologies. This document describes IPP from a high level view, defines a roadmap for the various documents that form the suite of IPP specifications, and gives background and rationale for the IETF working group’s major decisions. Zilles Experimental [Page 1] RFC 2568 Rationale for IPP April 1999 The full set of IPP documents includes: Design Goals for an Internet Printing Protocol [RFC2567] Rationale for the Structure and Model and Protocol for the Internet Printing Protocol (this document) Internet Printing Protocol/1.0: Model and Semantics [RFC2566] Internet Printing Protocol/1.0: Encoding and Transport [RFC2565] Internet Printing Protocol/1.0: Implementer’s Guide [ipp-iig] Mapping between LPD and IPP Protocols [RFC2569] The "Design Goals for an Internet Printing Protocol" document takes a broad look at distributed printing functionality, and it enumerates real-life scenarios that help to clarify the features that need to be included in a printing protocol for the Internet. It identifies requirements for three types of users: end users, operators, and administrators. The Design Goals document calls out a subset of end user requirements that are satisfied in IPP/1.0. Operator and administrator requirements are out of scope for version 1.0. The "Internet Printing Protocol/1.0: Model and Semantics" document describes a simplified model consisting of abstract objects, their attributes, and their operations that is independent of encoding and transport. The model consists of a Printer and a Job object. The Job optionally supports multiple documents. This document also addresses security, internationalization, and directory issues. The "Internet Printing Protocol/1.0: Encoding and Transport" document is a formal mapping of the abstract operations and attributes defined in the model document onto HTTP/1.1. It defines the encoding rules for a new Internet media type called "application/ipp". The "Internet Printing Protocol/1.0: Implementer’s Guide" document gives insight and advice to implementers of IPP clients and IPP objects. It is intended to help them understand IPP/1.0 and some of the considerations that may assist them in the design of their client and/or IPP object implementations. For example, a typical order of processing requests is given, including error checking. Motivation for some of the specification decisions is also included. The "Mapping between LPD and IPP Protocols" document gives some advice to implementers of gateways between IPP and LPD (Line Printer Daemon) implementations. 1. ARCHITECTURAL OVERVIEW The Internet Printing Protocol (IPP) is an application level protocol that can be used for distributed printing on the Internet. This protocol defines interactions between a client and a server. The Zilles Experimental [Page 2] RFC 2568 Rationale for IPP April 1999 protocol allows a client to inquire about capabilities of a printer, to submit print jobs and to inquire about and cancel print jobs. The server for these requests is the Printer; the Printer is an abstraction of a generic document output device and/or a print service provider. Thus, the Printer could be a real printing device, such as a computer printer or fax output device, or it could be a service that interfaced with output devices. The protocol is heavily influenced by the printing model introduced in the Document Printing Application (DPA) [ISO10175] standard. Although DPA specifies both end user and administrative features, IPP version 1.0 (IPP/1.0) focuses only on end user functionality. The architecture for IPP defines (in the Model and Semantics document [RFC2566]) an abstract Model for the data which is used to control the printing process and to provide information about the process and the capabilities of the Printer. This abstract Model is hierarchical in nature and reflects the structure of the Printer and the Jobs that may be being processed by the Printer. The Internet provides a channel between the client and the server/Printer. Use of this channel requires flattening and sequencing the hierarchical Model data. Therefore, the IPP also defines (in the Encoding and Transport document [RFC2565]) an encoding of the data in the model for transfer between the client and server. This transfer of data may be either a request or the response to a request. Finally, the IPP defines (in the Encoding and Transport document [RFC2565]) a protocol for transferring the encoded request and response data between the client and the server/Printer. An example of a typical interaction would be a request from the client to create a print job. The client would assemble the Model data to be associated with that job, such as the name of the job, the media to use, the number of pages to place on each media instance, etc. This data would then be encoded according to the Protocol and would be transmitted according to the Protocol. The server/Printer would receive the encoded Model data, decode it into a form understood by the server/Printer and, based on that data, do one of two things: (1) accept the job or (2) reject the job. In either case, the server must construct a response in terms of the Model data, encode that response according to the Protocol and transmit that encoded Model data as the response to the request using the Protocol. Another part of the IPP architecture is the Directory Schema described in the model document. The role of a Directory Schema is to provide a standard set of attributes which might be used to query a Zilles Experimental [Page 3] RFC 2568 Rationale for IPP April 1999 directory service for the URI of a Printer that is likely to meet the needs of the client. The IPP architecture also addresses security issues such as control of access to server/Printers and secure transmissions of requests, response and the data to be printed.