Applications

Elliot Schwartz <elliot@mit.edu>

The Web

Client-Server Architecture

HTTP: Hyper Text Transfer Protocol

HTTP is the protocol that the web browsers use to communicate with the web servers.

Requirements

Format

The browser starts a TCP connection to the server.
The browser selects a method by sending a line of text:

<METHOD> <PARAMETERS> <VERSION>

The browser sends a list of parameters, terminated with a double newline:

<NAME1>: <VALUE1>

<NAME2>: <VALUE2>

<NAME3>: <VALUE3>

The server sends back a one-line status message.
The server sends back a list of parameters describing the content:

<NAME1>: <VALUE1>

<NAME2>: <VALUE2>

<NAME3>: <VALUE3>

The server sends back the content.
The server closes the connection.

Methods

For example:
athena% telnet -u www.mit.edu 80
Trying 18.181.0.31...
Connected to DANDELION-PATCH.MIT.EDU.
Escape character is '^]'.
GET / HTTP/1.0

HTTP/1.1 200 OK
Date: Fri, 20 Feb 1998 22:45:44 GMT
Server: Apache/1.2.5-dev
Last-Modified: Fri, 20 Feb 1998 18:55:16 GMT
ETag: "431a-b83-34edd194"
Content-Length: 2947
Accept-Ranges: bytes
Connection: close
Content-Type: text/html

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN">
<html>
<head>
<title>MIT SIPB Home Page</title>
[...]
</div></body>
</html>

Connection closed by foreign host.

This opens up an unencrypted (-u) TCP connection to port 80 (the http port)
The client sends this to invoke the GET method on the root page, with no parameters.
The server sends back this status message, saying everything is ok. It is followed by a list of parameters.

Date gives the current date on the server.
Server returns the kind of web server that is running.
Last-Modified gives the date the document was last modified on.
Content-Length indicates the length of the content that will follow the parameters.
Content-Type indicates what format the content will be in. Web pages are "text/html", while graphics use other content types.

The content starts here, and is in HTML format.
The connection is closed by the server.

An example of the browser using a parameter is in loading cached pages, where the browser reports how old its version of the page is, and the content is only sent if the page has been modified since then. Here's an example of a successfully cached page:

athena% telnet -u www.mit.edu 80
Trying 18.181.0.31...
Connected to DANDELION-PATCH.MIT.EDU.
Escape character is '^]'.
GET / HTTP/1.0
If-Modified-Since: Fri, 20 Feb 1998 20:00:00 GMT

HTTP/1.1 304 Not Modified
Date: Fri, 20 Feb 1998 23:01:53 GMT
Server: Apache/1.2.5-dev
Connection: close
ETag: "431a-b83-34edd194"

Connection closed by foreign host.

This says only send the page if it's been modified since the date give.
The status says that it hasn't been modified, so it wont return the page.

HEAD

POST

Interaction with TCP and the Internet