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Network Working Group                                          S. ZillesRequest for Comments: 2568                            Adobe Systems Inc.Category: Experimental                                        April 1999Rationale for the Structure of the Model and Protocolfor the Internet Printing ProtocolStatus 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.  TheZilles                        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 aZilles                        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 19996. 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.comZilles                        Experimental                      [Page 9]

RFC 2568                   Rationale for IPP                  April 19999.  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]