21.3. The Hypertext Transfer Protocol

Version 1.1 of HTTP is specified in RFC 2616. HTTP is also a requestresponse protocol. A request 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 are ASCII, with each line terminated by a carriage return (\r) and a line feed (\n). The start line consists of a method that indicates what 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. A header 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

The start line in an HTTP response message contains a version string followed by a numeric status code and a status message, terminated by a carriage return and a line feed. The remainder of the HTTP response message has the same format as the request message: headers followed by a blank line and an optional entity body.

The following is a sample HTTP header for a print request for the author's printer:

    POST /phaser860/ipp HTTP/1.1^M
    Content-Length: 21931^M
    Content-Type: application/ipp^M
    Host: phaser860:ipp^M
    ^M

The ^M at the end of the each line is the carriage return that precedes the line feed. The line feed doesn't show up as a printable character. Note that the last line of the header is empty, except for the carriage return and line feed.