Advanced Programming in the UNIX® Environment

ptg10805159

790 Communicating with a Network Printer Chapter 21

IPP is specified in a series of documents (Requests For Comments, or RFCs)

available at http://www.ietf.org/rfc.html.Proposed draft standards are

developed by the Printer Working Group, which is associated with the IEEE. These

drafts areavailable athttp://www.pwg.org/ipp.The main documents arelisted in

Figure21.1, although many other documents areavailable to further specify

administrative procedures, job attributes, and the like.

Document Title

RFC 2567 Design Goals for an Internet Printing Protocol

RFC 2568 Rationale for the Structure of the Model and Protocol for the Internet Printing

Protocol

RFC 2911Internet Printing Protocol/1.1: Model and Semantics

RFC 2910 Internet Printing Protocol/1.1: Encoding and Transport

RFC 3196 Internet Printing Protocol/1.1: Implementor’s Guide

Candidate Standard Internet Printing Protocol Version 2.0, Second Edition

5100.12- 2011

Figure 21.1Primary IPP documents

Candidate Standard5100.12- 201 1specifies all features that implementations must

support to conform to different versions of the IPP standard. Thereare many proposed

extensions to the IPP protocol (specific features aredefined in other IPP-related

documents). These features aredivided into groups to create different conformance

levels; each level is a different protocol version. For compatibility,each higher level of

conformance requires that implementations meet most of the requirements defined by

lower versions of the standard. In this chapter, we will use IPP version 1.1 in our simple

example.

IPP is built on top of HTTP,the Hypertext Transfer Protocol (Section 21.3). HTTP,in

turn, is built on top of TCP/IP.Figure21.2 shows the structure of an IPP message.

Ethernet

header

IP

header

TCP

header

HTTP

header

IPP

header

data to be

printed

Figure 21.2 Structure of an IPP message

IPP is a request–response protocol. A client sends a request message to a server,

and the server answers with a response message. The IPP header contains a field that

indicates the requested operation. Operations aredefined to submit print jobs, cancel

print jobs, get job attributes, get printer attributes, pause and restart the printer,place a

job on hold, and release a held job.

Figure21.3 shows the structure of an IPP message header.The first 2 bytes arethe

IPP version number.For protocol version 1.1, each byte has a value of 1. For a protocol

request, the next 2 bytes contain a value identifying the type of operation requested.

For a protocol response, these 2 bytes contain a status code instead.