Example: Hypertext Page Transfer

The encapsulation essay presented an example of transferring a hypertext page over a serial link. Let's take another look at the example, from the standpoint of layered, standard protocols.

• A Web browser requests this URL: http://www.FreeSoft.org/Connected/index.html

  A URL (Universal Resource Locator) is a name that identifies a hypertext page. This URL identifies the home page of Connected: An Internet Encyclopedia. I'll explain URLs in more detail later, but for now let's just say that there are three main parts to it. http identifies that the HyperText Transfer Protocol (HTTP) is to be used to obtain the page. www.FreeSoft.org is the name of the Internet host that should be contacted to obtain the Web page. Finally, /Connected/index.html identifies the page itself.

• The DNS protocol converts www.FreeSoft.org into the 32-bit IP address 205.177.42.129

  The Domain Name System (DNS) doesn't fit neatly into our layered protocol model, but it is a very important protocol. The lower levels of the protocol stack all use 32-bit numeric addresses. Therefore, one of the first steps is to translate the textual host name into a numeric IP address, written as four decimal numbers, separated by periods.

• The HTTP protocol constructs a GET /Connected/index.html message, that will be sent to host 205.177.42.129 to request the Web page.

  The HTTP protocol also specifies that TCP will be used to send the message, and that TCP port 80 is used for HTTP operations.

  In the DoD model, this is a Process Layer operation.

• The TCP protocol opens a connection to 205.177.42.129, port 80, and transmits the HTTP GET /Connected/index.html message.

  The TCP protocol specifies that IP will be used for message transport.

  In the DoD model, this is a Host-to-Host Layer operation.

• The IP protocol transmits the TCP packets to 205.177.42.129

  The IP protocol also selects a communication link to perform the first step of the transfer, in this case a modem.

  In the DoD model, this is an Internet Layer operation.

• The PPP protocol encodes the IP/TCP/HTTP packets and transmits them across the modem line.

  In the DoD model, this is a Network Access Layer operation.

Click here if you need to review the 4-layer DoD Model.