Advanced Programming in the UNIX® Environment

ptg10805159

Section 21.2 The Internet Printing Protocol 791

version number

operation ID (request)/status code (response)

request ID

attributes

end-of-attributes tag

(2 bytes)

(2 bytes)

(4 bytes)

(0-nbytes)

(1 byte)

Figure 21.3 Structure of an IPP header

The next 4 bytes contain an integer identifying the request, which allows requests to

be matched up with responses. Optional attributes follow this, terminated by an end-

of-attributes tag. Any data that might be associated with the request follows

immediately after the end-of-attributes tag.

In the header,integers arestored as signed, two’s-complement, binary values in

big-endian byte order (i.e., network byte order). Attributes arestored in groups. Each

group starts with a single byte identifying the group. Within each group, an attribute is

generally represented as a 1-byte tag, followed by a 2-byte name length, followed by the

name of the attribute, followed by a 2-byte value length, and finally by the value itself.

The value can be encoded as a string, a binary integer, or a morecomplex structure,

such as a date/timestamp.

Figure21.4 shows how theattributes-charsetattribute would be encoded

with a value ofutf-8.

attribute tag = 0x47

size of attribute name = 18

name =attributes-charset

size of attribute value = 5

value =utf-8

(1 byte)

(2 bytes)

(18 bytes)

(2 bytes)

(5 bytes)

Figure 21.4Sample IPP attribute encoding

Depending on the operation requested, some attributes arerequired to be provided

in the request message, whereas others areoptional. For example, Figure21.5 shows

the attributes defined for a print-job request.