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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
STATUS OF THIS MEMO
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
documents at any time. It is inappropriate to use Internet-
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
the ''1id-abstracts.txt'' listing contained in the Internet-
Drafts Shadow Directories on ftp.is.co.za (Africa), nic.nordu.net
(Europe), munnari.oz.au (Pacific Rim), ds.internic.net (US East
Coast), or ftp.isi.edu (US West Coast).
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 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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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 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
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 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.
3. RATIONALE FOR THE MODEL
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
encodings.
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
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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.
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 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
much.
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.
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 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. RATIONALE FOR SECURITY
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
Protocol:"
[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
Email: szilles@adobe.com
+
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