INTERNET DRAFT                                  Stephen N. Zilles
          <draft-ietf-ipp-rat-01.txt>                    Adobe Systems Inc.

          July 30, 1997                               Expires: Jan 30, 1998

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


          This document is an Internet-Draft.  Internet-Drafts are working
          documents of the Internet Engineering Task Force (IETF), its
          areas, and its working groups.  Note that other groups may also
          distribute working documents as Internet-Drafts.

          Internet-Drafts are draft documents valid for a maximum of six
          months and may be updated, replaced, or obsoleted by other
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          Drafts as reference material or to cite them other than as ''work
          in progress.''

          To learn the current status of any Internet-Draft, please check
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          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 on the Internet. There are multiple parts
          to IPP, but the primary architectural components are the Model,
          the Protocol and an interface to Directory Services. This
          document provides a high level overview of the architecture
          and provides the rationale for the decisions made in
          structuring the architecture.

          The full set of IPP documents include:

          Internet Printing Protocol/1.0: Requirements for an Internet
                Printing Protocol
          Internet Printing Protocol/1.0: Model and Semantics
          Internet Printing Protocol/1.0: Security
          Internet Printing Protocol/1.0: Protocol Specification
          Internet Printing Protocol/1.0: Directory Schema

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          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 client is provided the capability 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 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 architecture for IPP defines (in the Model document) 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

          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 Protocol document) 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 Protocol document) a protocol
          for transfering 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.

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          Another part of the IPP architecture is the Directory Schema.
          The role of the 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 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.


          The Model 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

          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

          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

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          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, 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 and the
          Printer/Job MIBs.


          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 name value
          pairs in which the names are length specified strings
          constrained to characters from a subset of ASCII and the values
          are either scalars or a sequence of scalars. Each scalar value
          has a length specification and is represented either as an 4
          byte integer or a string.

          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

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          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 will be widely deployed as the
                manufacturers release new products. The performance
                benefits of HTTP 1.1 have been shown and manufactures
                are reacting postively.

                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 a separate CreatJob and SendJob
                messages. Chunking allows the transmission of large
                print files without having to prescan 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, 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 proxies and not the implementation of
                clients and/or servers. Work is proceding in the HTTP
                Working Group to help identify what must be done by a
                server. As the Protocol document shows, that is not very

             5. An HTTP based solution fits well with the Internet
                security mechanism that are currently deployed or being
                deployed. HTTP will run over SSL/TLS. The digest

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                authentication mechanism of HTTP 1.1 provides an
                adequate level of access control (at least for intranet
                usage). 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.


          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 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.


          Security is an areas 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 the SSL/TLS secure communication mechanism, which also
          includes authorization.

          7. REFERENCES

          [1]  Wright, F. D., "Requirements for an Internet Printing

          [2]  Isaacson, S, deBry, R, Hasting, T, Herriot, R, Powell, P,
               "Internet Printing Protocol/1.0: Model and Semantics"

          [3]  Internet Printing Protocol/1.0: Security

          [4]  Herriot, R, Butler, S, Moore, P, Turner, R, "Internet
               Printing Protocol/1.0: Protocol Specification"

          [5]  Carter, K, Isaacson, S, "Internet Printing Protocol/1.0:
               Directory Schema"

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           8. AUTHORS 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


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