HTTP, an Application Protocol

Now let's get back to our example. Web browsers and servers speak an application protocol that runs on top of TCP/IP, using it simply as a way to pass strings of bytes back and forth. This protocol is called HTTP (Hyper-Text Transfer Protocol) and we've already seen one command in it — the GET shown above.

When the GET command goes to www.google.com webserver with service number 80, it will be dispatched to a server daemon listening on port 80. Most Internet services are implemented by server daemons that do nothing but wait on ports, watching for and executing incoming commands.

If the design of the Internet has one overall rule, it's that all the parts should be as simple and human-accessible as possible. HTTP, and its relatives (like the Simple Mail Transfer Protocol, SMTP, that is used to move electronic mail between hosts) tend to use simple printable-text commands that end with a carriage-return/line feed.

This is marginally inefficient; in some circumstances you could get more speed by using a tightly-coded binary protocol. But experience has shown that the benefits of having commands be easy for human beings to describe and understand outweigh any marginal gain in efficiency that you might get at the cost of making things tricky and opaque.

Therefore, what the server daemon ships back to you via TCP/IP is also text. The beginning of the response will look something like this (a few headers have been suppressed):

HTTP/1.1 200 OK
Date: Sat, 10 Oct 1998 18:43:35 GMT
Server: Apache/1.2.6 Red Hat
Last-Modified: Thu, 27 Aug 1998 17:55:15 GMT
Content-Length: 2982
Content-Type: text/html

These headers will be followed by a blank line and the text of the web page (after which the connection is dropped). Your browser just displays that page. The headers tell it how (in particular, the Content-Type header tells it the returned data is really HTML).