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rfc:rfc2568

Network Working Group S. Zilles Request for Comments: 2568 Adobe Systems Inc. Category: Experimental April 1999

       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 (C) 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.

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.

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.

Zilles Experimental [Page 4] RFC 2568 Rationale for IPP April 1999

 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.

Zilles Experimental [Page 5] RFC 2568 Rationale for IPP April 1999

 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.

Zilles Experimental [Page 6] RFC 2568 Rationale for IPP April 1999

    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.

Zilles Experimental [Page 7] RFC 2568 Rationale for IPP April 1999

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.

Zilles Experimental [Page 8] RFC 2568 Rationale for IPP April 1999

 [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

Zilles Experimental [Page 9] RFC 2568 Rationale for IPP April 1999

9. Full Copyright Statement

 Copyright (C) The Internet Society (1999).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 English.
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Zilles Experimental [Page 10]

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