Advanced Programming in the UNIX® Environment

ptg10805159

792 Communicating with a Network Printer Chapter 21

Attribute Status Description

attributes-charset required the character set used by attributes of typetextorname

attributes-natural-languagerequired the natural language used by attributes of typetextor

name

printer-uri required the printer’s Universal Resource Identifier

requesting-user-name optional name of user submitting job (used for authentication, if

enabled)

job-name optional name of job used to distinguish between multiple jobs

ipp-attribute-fidelity optional if true, tells printer to reject job if all attributes can’t be

met; otherwise, printer does its best to print the job

document-name optional the name of the document (suitable for printing in a

banner,for example)

document-format optional the format of the document (e.g., plaintext, PostScript)

document-natural-language optional the natural language of the document

compression optional the algorithm used to compress the document data

job-k-octets optional size of the document in 1,024-octet units

job-impressions optional number of impressions (images imposed on a page)

submitted in this job

job-media-sheets optional number of sheets printed by this job

Figure 21.5Attributes of print-job request

The IPP header contains a mixture of text and binary data. Attribute names are

stored as text, but sizes arestored as binary integers. This complicates the process of

building and parsing the header,since we need to worry about such things as network

byte order and our host processor ’s ability to address an integer on an arbitrary byte

boundary.Abetter alternative would have been to design the header to contain text

only.This simplifies processing at the cost of slightly larger protocol messages.

21.3 The Hyper text Transfer Protocol

Version 1.1 of HTTP is specified in RFC 2616. HTTP is also a request–response protocol.

Arequest message contains a start line, followed by header lines, a blank line, and an

optional entity body.The entity body contains the IPP header and data in this case.

HTTP headers areASCII, with each line terminated by a carriage return (\r)and a

line feed (\n). The start line consists of amethodthat indicates which operation the

client is requesting, a Uniform Resource Locator(URL)that describes the server and

protocol, and a string indicating the HTTP version. The only method used by IPP is

POST,which is used to send data to a server.

The header lines specify attributes, such as the format and length of the entity body.

Aheader line consists of an attribute name followed by a colon, optional white space,

and the attribute value, and is terminated by a carriage return and a line feed. For

example, to specify that the entity body contains an IPP message, we include the header

line

Content-Type: application/ipp