Rationale for the Structure of the Model and Protocol

for the Internet Printing Protocol

Status of this Memo

This memo defines an Experimental Protocol for the Internet community. It does not specify an Internet standard of any kind. Discussion and suggestions for improvement are requested. Distribution of this memo is unlimited.

Copyright Notice

Copyright © The Internet Society (1999). All Rights Reserved.

IESG Note

This document defines an Experimental protocol for the Internet community. The IESG expects that a revised version of this protocol will be published as Proposed Standard protocol. The Proposed Standard, when published, is expected to change from the protocol defined in this memo. In particular, it is expected that the standards-track version of the protocol will incorporate strong authentication and privacy features, and that an "ipp:" URL type will be defined which supports those security measures. Other changes to the protocol are also possible. Implementors are warned that future versions of this protocol may not interoperate with the version of IPP defined in this document, or if they do interoperate, that some protocol features may not be available.

The IESG encourages experimentation with this protocol, especially in combination with Transport Layer Security (TLS) [RFC2246], to help determine how TLS may effectively be used as a security layer for IPP.

ABSTRACT

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.

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 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 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.

2. THE PRINTER

Because the (abstract) server/Printer encompasses a wide range of implementations, it is necessary to make some assumptions about a minimal implementation. The most likely minimal implementation is one that is embedded in an output device running a specialized real time operating system and with limited processing, memory and storage capabilities. This printer will be connected to the Internet and will have at least a TCP/IP capability with (likely) SNMP [RFC1905, RFC1906] support for the Internet connection. In addition, it is likely the the Printer will be an HTML/HTTP server to allow direct user access to information about the printer.

3. RATIONALE FOR THE MODEL

The Model [RFC2566] is defined independently of any encoding of the Model data both to support the likely uses of IPP and to be robust with respect to the possibility of alternate encoding.

It is expected that a client or server/Printer would represent the Model data in some data structure within the applications/servers that support IPP. Therefore, the Model was designed to make that representation straightforward. Typically a parser or formatter would be used to convert from or to the encoded data format. Once in an internal form suitable to a product, the data can be manipulated by the product. For example, the data sent with a Print Job can be used to control the processing of that Print Job.

The semantics of IPP are attached to the (abstract) Model. Therefore, the application/server is not dependent on the encoding of the Model data, and it is possible to consider alternative mechanisms and formats by which the data could be transmitted from a client to a server; for example, a server could have a direct, client-less GUI interface that might be used to accept some kinds of Print Jobs. This independence would also allow a different encoding and/or transmission mechanism to be used if the ones adopted here were shown to be overly limiting in the future. Such a change could be migrated into new products as an alternate protocol stack/parser for the Model data.

Having an abstract Model also allows the Model data to be aligned with the (abstract) model used in the Printer [RFC1759], Job and Host Resources MIBs. This provides consistency in interpretation of the data obtained independently of how the data is accessed, whether via IPP or via SNMP [RFC1905, RFC1906] and the Printer/Job MIBs.

There is one aspect of the Model that deserves some extra explanation. There are two ways for identifying a Job object: (a) with a Job URI and (b) using a combination of the Printer URI and a Job ID (a 32 bit positive integer). Allowing Job objects to have URIs allows for flexibility and scalability. For example a job could be moved from a printer with a large backlog to one with a smaller load and the job identification, the Job object URI, need not change. However, many existing printing systems have local models or interface constraints that force Job objects to be identified using only a 32-bit positive integer rather than a URI. This numeric Job ID is only unique within the context of the Printer object to which the create request was originally submitted. In order to allow both types of client access to Jobs (either by Job URI or by numeric Job ID), when the Printer object successfully processes a create request and creates a new Job, the Printer object generates both a Job URI and a Job ID for the new Job object. This requirement allows all clients to access Printer objects and Job objects independent of any local constraints imposed on the client implementation.

4. RATIONALE FOR THE PROTOCOL

There are two parts to the Protocol: (1) the encoding of the Model data and (2) the mechanism for transmitting the model data between client and server.

4.1 The Encoding

To make it simpler to develop embedded printers, a very simple binary encoding has been chosen. This encoding is adequate to represent the kinds of data that occur within the Model. It has a simple structure consisting of sequences of attributes. Each attribute has a name, prefixed by a name length, and a value. The names are strings constrained to characters from a subset of ASCII. The values are either scalars or a sequence of scalars. Each scalar value has a length specification and a value tag which indicates the type of the value. The value type has two parts: a major class part, such as integer or string, and a minor class part which distinguishes the usage of the major class, such as dateTime string. Tagging of the values with type information allows for introducing new value types at some future time.

A fully encoded request/response has a version number, an operation (for a request) or a status and optionally a status message (for a response), associated parameters and attributes which are encoded Model data and, optionally (for a request), print data following the Model data.

4.2 The Transmission Mechanism

The chosen mechanism for transmitting the encoded Model data is HTTP 1.1 Post (and associated response). No modifications to HTTP 1.1 are proposed or required. The sole role of the Transmission Mechanism is to provide a transfer of encoded Model data from/to the client to/from the server. This could be done using any data delivery mechanism. The key reasons why HTTP 1.1 Post is used are given below. The most important of these is the first. With perhaps this exception, these reasons could be satisfied by other mechanisms. There is no claim that this list uniquely determines a choice of mechanism.

1. HTTP 1.0 is already widely deployed and, based on the recent evidence, HTTP 1.1 is being widely deployed as the manufacturers release new products. The performance benefits of HTTP 1.1 have been shown and manufactures are reacting positively.

Wide deployment has meant that many of the problems of making a protocol work in a wide range of environments from local net to Intranet to Internet have been solved and will stay solved with HTTP 1.1 deployment.

2. HTTP 1.1 solves most of the problems that might have required a new protocol to be developed. HTTP 1.1 allows persistent connections that make a multi-message protocol be more efficient; for example it is practical to have separate Create-Job and Send- Document messages. Chunking allows the transmission of large print files without having to pre-scan the file to determine the file length. The accept headers allow the client's protocol and localization desires to be transmitted with the IPP operations and data. If the Model were to provide for the redirection of Job requests, such as Cancel-Job, when a Job is moved, the HTTP redirect response allows a client to be informed when a Job he is interested in is moved to another server/Printer for any reason.

3. Most network Printers will be implementing HTTP servers for reasons other than IPP. These network attached Printers want to provide information on how to use the printer, its current state, HELP information, etc. in HTML. This requires having an HTTP server which would be available to do IPP functions as well.

4. Most of the complexity of HTTP 1.1 is concerned with the implementation of HTTP proxies and not the implementation of HTTP clients and/or servers. Work is proceeding in the HTTP Working Group to help identify what must be done by a server. As the Encoding and Transport document shows, that is not very much.

5. HTTP implementations provide support for handling URLs that would have to be provided if a new protocol were defined.

6. An HTTP based solution fits well with the Internet security mechanisms that are currently deployed or being deployed. HTTP will run over SSL3. The digest access authentication mechanism of HTTP 1.1 provides an adequate level of access control. These solutions are deployed and in practical use; a new solution would require extensive use to have the same degree of confidence in its security. Note: SSL3 is not on the IETF standards track.

7. HTTP provides an extensibility model that a new protocol would have to develop independently. In particular, the headers, intent-types (via Internet Media Types) and error codes have wide acceptance and a useful set of definitions and methods for extension.

8. Although not strictly a reason why IPP should use HTTP as the transmission protocol, it is extremely helpful that there are many prototyping tools that work with HTTP and that CGI scripts can be used to test and debug parts of the protocol.

9. Finally, the POST method was chosen to carry the print data because its usage for data transmission has been established, it works and the results are available via CGI scripts or servlets. Creating a new method would have better identified the intended use of the POSTed data, but a new method would be more difficult to deploy. Assigning a new default port for IPP provided the necessary identification with minimal impact to installed infrastructure, so was chosen instead.

5. RATIONALE FOR THE DIRECTORY SCHEMA

Successful use of IPP depends on the client finding a suitable IPP enabled Printer to which to send a IPP requests, such as print a job. This task is simplified if there is a Directory Service which can be queried for a suitable Printer. The purpose of the Directory Schema is to have a standard description of Printer attributes that can be associated the URI for the printer. These attributes are a subset of the Model attributes and can be encoded in the appropriate query syntax for the Directory Service being used by the client.

6. SECURITY CONSIDERATIONS - RATIONALE FOR SECURITY

Security is an area of active work on the Internet. Complete solutions to a wide range of security concerns are not yet available. Therefore, in the design of IPP, the focus has been on identifying a set of security protocols/features that are implemented (or currently implementable) and solve real problems with distributed printing. The two areas that seem appropriate to support are: (1) authorization to use a Printer and (2) secure interaction with a printer. The chosen mechanisms are the digest authentication mechanism of HTTP 1.1 and SSL3 [SSL] secure communication mechanism.

7. REFERENCES

   [ipp-iig]  Hastings, T. and C. Manros, "Internet Printing
              Protocol/1.0:Implementer's Guide", Work in Progress.
   
   [RFC2569]  Herriot, R., Hastings, T., Jacobs, N. and J. Martin,
              "Mapping between LPD and IPP Protocols", RFC 2569, April
              1999.
   
   [RFC2566]  deBry, R., Isaacson, S., Hastings, T., Herriot, R. and P.
              Powell, "Internet Printing Protocol/1.0: Model and
              Semantics", RFC 2566, April 1999.
   
   [RFC2565]  Herriot, R., Butler, S., Moore, P. and R. Tuner, "Internet
              Printing Protocol/1.0: Encoding and Transport", RFC 2565,
              April 1999.
   
   [RFC2567]  Wright, D., "Design Goals for an Internet Printing
              Protocol", RFC 2567, April 1999.
   
   [ISO10175] ISO/IEC 10175 "Document Printing Application (DPA)", June
              1996.
   
   [RFC1759]  Smith, R., Wright, F., Hastings, T., Zilles, S. and J.
              Gyllenskog, "Printer MIB", RFC 1759, March 1995.
   
   [RFC1905]  Case, J., McCloghrie, K., Rose, M. and S. Waldbusser,
              "Protocol Operations for Version 2 of the Simple Network
              Management Protocol (SNMPv2)", RFC 1905, January 1996.
   
   [RFC1906]  Case, J., McCloghrie, K., Rose, M. and S. Waldbusser,
              "Transport Mappings for  Version 2 of the Simple Network
              Management Protocol (SNMPv2)", RFC 1906, January 1996.
   
   [SSL]      Netscape, The SSL Protocol, Version 3, (Text version
              3.02), November 1996.

8. AUTHOR'S ADDRESS

Stephen Zilles
Adobe Systems Incorporated
345 Park Avenue
MailStop W14
San Jose, CA 95110-2704

   Phone: +1 408 536-4766
   Fax:   +1 408 537-4042
   EMail: szilles@adobe.com

9. Full Copyright Statement

Copyright © The Internet Society (1999). All Rights Reserved.

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