INTERNET-DRAFT Stephen N. Zilles
Adobe Systems Inc.
July 14, 1997
Rationale for the Structure of the Model and Protocol
for the Internet Printing Protocol (IPP)
<draft-ietf-ipp-rat-00.txt>
Expires Jan 14, 1998
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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 rational for the decisions made in
structuring the architecture.
The full set of IPP documents include:
Internet Printing Protocol/1.0: Requirements
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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Rationale for the Structure of the Model and Protocol July 14, 1997
TABLE OF CONTENTS
1. ARCHITECTURAL OVERVIEW..............................................3
2. THE PRINTER.........................................................4
3. RATIONAL FOR THE MODEL..............................................4
4. RATIONAL FOR THE PROTOCOL...........................................5
4.1 The Encoding .....................................................5
4.2 Smission Mechanism ...............................................5
5. RATIONAL FOR THE DIRECTORY SCHEMA...................................6
6. RATIONAL FOR SECURITY...............................................6
7. AUTHOR'S ADDRESSES..................................................7
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Rationale for the Structure of the Model and Protocol July 14, 1997
1 Rationale for the Structure of the Model and Protocol
2 for the Internet Printing Protocol (IPP)
3 1. Architectural Overview
4 The Internet Printing Protocol (IPP) is an application level
5 protocol that can be used for distributed printing on the
6 Internet. This protocol defines interactions between a
7 client and a server. The client is provided the capability
8 to inquire about capabilities of a printer, to submit print
9 jobs and to inquire about and cancel print jobs. The server
10 for these requests is the Printer; the Printer is an
11 abstraction a generic document output device and/or a print
12 service provider. Thus, the Printer could be a real printing
13 device, such as a computer printer or fax output device, or
14 it could be a service that interfaced with output devices.
15 The architecture for IPP defines (in the Model document) an
16 abstract Model for the data which is used to control the
17 printing process and to provide information about the
18 process and the capabilities of the Printer. This abstract
19 Model is hierarchical in nature and reflects the structure
20 of the Printer and the Jobs that may be being processed by
21 the Printer. The Internet provides a channel between the
22 client and the server/Printer. Use of this channel requires
23 flattening and sequencing the hierarchical Model data.
24 Therefore, the IPP also defines (in the Protocol document)
25 an encoding of the data in the model for transfer between
26 the client and server. This transfer of data may be either a
27 request or the response to a request. Finally, the IPP
28 defines (in the Protocol document) a protocol for
29 transfering the encoded request and response data between
30 the client and the server/Printer.
31 An example of a typical interaction would by a request from
32 the client to create a print job. The client would assemble
33 the Model data to be associated with that job, such as the
34 name of the job, the media to use, the number of pages to
35 place on each media instance, etc. This data would then be
36 encoded according to the Protocol and would be transmitted
37 according to the Protocol. The server/Printer would receive
38 the encoded Model data, decode it into a form understood by
39 the server/Printer and, based on that data, do one of two
40 things: (1) accept the job or (2) reject the job. In either
41 case, the server must construct a response in terms of the
42 Model data, encode that response according to the Protocol
43 and transmit that encoded Model data as the response to the
44 request using the Protocol.
45 Another part of the IPP architecture is the Directory
46 Schema. The role of the Directory Schema is to provide a
47 standard set of attributes which might be used to query a
48 directory service for the URI of a Printer that is likely to
49 meet the needs of the client.
50 The IPP architecture also addresses security issues such as
51 control of access to server/Printers and secure
52 transmissions of requests, response and the data to be
53 printed.
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54 2. The Printer
55 Because the (abstract) server/Printer encompasses a wide
56 range of implementations, it is necessary to make some
57 assumptions about a minimal implementation. The most likely
58 minimal implementation is one that is embedded in an output
59 device running a specialized real time operating system and
60 with limited processing, memory and storage capabilities.
61 This Printer will be connected to the Internet and will have
62 at least a TCP/IP capability with (likely) SNMP support for
63 the Internet connection. In addition, it is likely the the
64 Printer will be an HTML/HTTP server to allow direct user
65 access to information about the printer.
66 3. Rationale For The Model
67 The Model is defined independently of any encoding of the
68 Model data both to support the likely uses of IPP and to be
69 robust with respect to the possibility of alternate
70 encodings.
71 It is expected that a client or server/Printer would
72 represent the Model data in some data structure within the
73 applications/servers that support IPP. Therefore, the Model
74 was designed to make that representation straightforward.
75 Typically, a parser or formatter would be used to convert
76 from or to the encoded data format. Once in an internal form
77 suitable to a product, the data can be manipulated by the
78 product. For example, the data sent with a Print Job can be
79 used to control the processing of that Print Job.
80 The semantics of IPP are attached to the (abstract) Model.
81 Therefore, the application/server is not dependent on the
82 encoding of the Model data, and it is possible to consider
83 alternative mechanisms and formats by which the data could
84 be transmitted from a client to a server; for example, a
85 server could have a direct, client-less GUI interface that
86 might be used to accept some kinds of Print Jobs. This
87 independence would also allow a different encoding and/or
88 transmission mechanism to be used if the ones adopted here
89 were shown to be overly limiting in the future. Such a
90 change could be migrated into new products as an alternate
91 protocol stack/parser for the Model data.
92 Having an abstract Model also allow the Model data to be
93 aligned with the (abstract) model used in the Printer, Job
94 and Host Resources MIBs. This provides consistency in
95 interpretation of the data obtained independently of how the
96 data is accessed, whether via IPP or via SNMP and the
97 Printer/Job MIBs.
98 4. Rationale For The Protocol
99 There are two parts to the Protocol: (1) the encoding of the
100 Model data and (2) the mechanism for transmitting the model
101 data between client and server.
102 4.1 The Encoding
103 The TranTo make it simpler to develop embedded printers, a
104 very simple binary encoding has been chosen. This encoding
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105 is adequate to represent the kinds of data that occur within
106 the Model. It has a simple structure consisting of name
107 value pairs in which the names are length specified strings
108 constrained to characters from a subset of ASCII and the
109 values are either scalars or a sequence of scalars. Each
110 scalar value has a length specification.
111 A fully encoded request/response has a version number, an
112 operation (for a request) or a status (for a response),
113 associated parameters which are encoded Model data and,
114 optionally, print data following the Model data.
115 [ISSUE: what should be said about Parameters and Attributes]
116 4.2 Smission Mechanism
117 The chosen mechanism for transmitting the encoded Model data
118 is HTTP 1.1 Post (and associated response). No modifications
119 to HTTP 1.1 are proposed or required.
120 The sole role of the Transmission Mechanism is to provide a
121 transfer of encoded Model data from/to the client to/from
122 the server. This could be done using any data delivery
123 mechanism. The key reasons why HTTP 1.1 Post is used are:
124 1. HTTP 1.0 is already widely deployed and, based on the
125 recent evidence, HTTP 1.1 will be widely deployed as the
126 manufactures release new products. The performance
127 benefits of HTTP 1.1 have been shown and manufactures are
128 reacting postively.
129 2. Wide deployment has meant that many of the problems of
130 making a protocol work in a wide range of environments
131 from local net to intranet to internet have been solved
132 and will stay solved with HTTP 1.1 deployment.
133 3. HTTP 1.1 solves most of the problems that might have
134 required a new protocol to be developed. HTTP 1.1 allows
135 persistent connections that make a multi-message protocol
136 be more efficient; for example it is practical to have a
137 separate CreatJob and SendJob messages. Chunking allows
138 the transmission of large print files without having to
139 prescan the file to determine the file length. The accept
140 headers allow the client's protocol and localization
141 desires to be transmitted with the IPP operations and
142 data. The HTTP redirect response allows a client to be
143 informed when a Job he is interested in is moved to
144 another server/Printer for any reason.
145 4. Most network Printers will be implementing HTTP servers
146 for reasons other than IPP. These network attached
147 Printers want to provide information on how to use the
148 printer, its current state, etc. in HTML. This requires
149 having an HTTP server which would be available to do IPP
150 functions as well.
151 5. Most of the complexity of HTTP 1.1 is concerned with the
152 implementation of proxies and not the implementation of
153 clients and/or servers. Work is proceding in the HTTP
154 Working Group to help identify what must be done by a
155 server. As the Protocol document shows, that is not very
156 much.
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157 6. An HTTP based solution fits will with the Internet
158 security mechanism that are currently deployed or being
159 deployed. HTTP will run over SSL/TLS. The digest
160 authentication mechanism of HTTP 1.1 provides an adequate
161 level of access control (at least for intranet usage).
162 These solutions are deployed and in practical use; a new
163 solution would require extensive use to have the same
164 degree of confidence in its security.
165 5. Rationale For The Directory Schema
166 Successful use of IPP depends on the client finding a
167 suitable IPP enabled Printer to which to send a IPP
168 requests, such as print a job. This task is simplified if
169 there is a Directory Service which can be queried for a
170 suitable Printer. The purpose of the Directory Schema is to
171 have a standard description of Printer attributes that can
172 be associated the the URI for the printer. These attributes
173 are a subset of the Model attributes and can be encoded in
174 the appropriate query syntax for the Directory Service being
175 used by the client.
176 6. Rationale For Security
177 Security is an areas of active work on the Internet.
178 Complete solutions to a wide range of security concerns are
179 not yet available. Therefore, in the design of IPP, the
180 focus has been on identifying a set of security
181 protocols/features that are implemented (or currently
182 implementable) and solve real problems with distributed
183 printing. The two areas that seem appropriate to support
184 are: (1) authorization to use a Printer and (2) secure
185 interaction with a printer. The chosen mechanisms are the
186 digest authentication mechanism of HTTP 1.1 and the SSL/TLS
187 secure communication mechanism, which also includes
188 authorization.
189 7. Author's Addresses
190 Stephen Zilles
191 Adobe Systems Incorporated
192 345 Park Avenue
193 MailStop W14
194 San Jose, CA 95110-2704
195
196 Phone: +1 408 536-4766
197 Fax: +1 408 537-4042
198 Email: szilles@adobe.com
199
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