Java Network Programming, Second Edition

HTTP

HTTP, the Hypertext Transfer Protocol, is the standard protocol for communication between web browsers and web servers. HTTP specifies how a client and server establish a connection, how the client requests data from the server, how the server responds to that request, and finally how the connection is closed. HTTP connections use the TCP/IP protocol for data transfer.

HTTP 1.0 is the currently accepted version of the protocol. It uses MIME to encode data. The basic protocol defines a sequence of four steps for each request from a client to the server:

1. Making the connection. The client establishes a TCP connection to the server, on port 80 by default; other ports may be specified in the URL.

2. Making a request. The client sends a message to the server requesting the page at a specified URL. The format of this request is typically something like:

   GET /index.html HTTP 1.0

   GET is a keyword. /index.html is a relative URL to a file on the server. The file is assumed to be on the machine that receives the request, so there is no need to prefix it with http://www.thismachine.com/. HTTP 1.0 is the version of the protocol that the client understands. The request is terminated with two carriage return/linefeed pairs (\r\n\r\n in Java parlance) regardless of how lines are terminated on the client or server platform.

   Although the GET line is all that is required, a client request can include other information as well. This takes the following form:

   Keyword: Value

   The most common such keyword ...