INTERNET DRAFT Stephen N. Zilles
<draft-ietf-ipp-rat-02.txt> Adobe Systems Inc.
November 21, 1997 Expires: May 21, 1998
Rationale for the Structure of the Model and Protocol
for the Internet Printing Protocol
STATUS OF THIS MEMO
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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:
Requirements for an Internet Printing Protocol
Internet Printing Protocol/1.0: Model and Semantics
Internet Printing Protocol/1.0: Protocol Specification
Rationale for the Structure of the Model and Protocol for
the Internet Printing Protocol
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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 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 architecture for IPP defines (in the Model document
[IPP-MOD]) 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 [IPP-PRO]) 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
[IPP-PRO]) 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.
Another part of the IPP architecture is the Directory Schema
(described in the model document).. The role of the
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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 [IPP-MOD] 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
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
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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 bynumeric Job
ID), when the Printer object successfully processes a create
request and creates a new Job, the Printer object SHALL
generate 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 no matter the 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
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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 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
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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
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. 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 mechanism that are currently deployed or being
deployed. HTTP will run over TLS/SSL. The digest
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.
7. HTTP provides an extensibility model that a new
protocol would have to develop independently. In
particular, the headers, content-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.
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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 where that is relevant. 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. Because there were no strong
arguments for or against using a new method, the POST
method was used.
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 TLS/SSL [TLS-PRO] secure communication mechanism.
To provide access to both HTTP security and TLS security,
IPP Printers provide two URLs for accessing the Printer
object: one URL for which the scheme is HTTP for normal HTTP
1.1 access and one URL for which the scheme is HTTPS for
access to the Printer using TLS/SSL protected communication.
Two URLs are necessary because all allowed modes of TLS
require some level of security beyond HTTP security so a TLS
capable URL cannot be used to negotiate down to HTTP
security.
7. COPYRIGHT
This document and translations of it may be copied and
furnished to others, and derivative works that comment on or
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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.
8. REFERENCES
[RFC1759]
Smith, R., Wright, F., Hastings, T., Zilles, S., and
Gyllenskog, J., "Printer MIB", RFC 1759, March 1995.
[RFC1905]
J. Case, et al. "Protocol Operations for Version 2 of
the Simple Network Management Protocol (SNMPv2)", RFC 1905,
January 1996.
[RFC1906]
J. Case, et al. "Transport Mappings for Version 2 of
the Simple Network Management Protocol (SNMPv2)", RFC 1906,
January 1996.
[IPP-MOD]
Isaacson, S, deBry, R, Hastings, T, Herriot, R, Powell,
P, "Internet Printing Protocol/1.0: Model and Semantics"
draft-ipp-mod-07.txt, November, 1997.
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[IPP-PRO]
Herriot, R., Butler, S., Moore, P., Tuner, R., " Internet
Printing Protocol/1.0: Protocol Specifications", draft-ipp-pro-
03.txt, November, 1997.
[IPP-REQ]
Wright, D., "Requirements for an Internet Printing Protocol",
draft-ipp-req-01.txt, November, 1997.
[TLS-PRO]
Dierks, T., Allen, C., "The TLS Protocol Version 1.0",
<draft-ietf-tls-protocol-05.txt>, November, 1997
9. 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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