[1]These messages (or their equivalent) are usually displayed on the browser status line at the bottom of the window.
When a user clicks on a highlighted hyperlink within a WWW page, her browser uses the information in the associated URL to obtain and display the desired page.![]()
* Note that we are, for the moment, ignoring the process of "looking up" the remote host -- discovering the network address associated with its domain name.
What this means is that the browser process uses the facilities of the network connecting the two computers to send a "connection request" message to a server program (process) running on the computer whose name was given in the URL.
If the remote server process is prepared to accept the connection, it responds with a "connection accepted" message.
The two process now have an open "communications channel" between them.
The browser then sends a request, in ordinary, plain old ASCII text, to the server process thus:
GET /home.html
The string "GET something
" is one of many commands defined
in the HyperText Transfer Protocol, HTTP[2]. The server responds by returning the
contents of the file /home.html
, also in ordinary plain
(ASCII) text.Finally, the browser process interprets the HTML markup in the returned file, and displays it to the user.
[2] for the pedantic: this request syntax is from HTTP 0.9, the original version of the protocol, which is no longer in common use. See later lectures for the "real" protocol.
This module provides a (so-called) transport service to processes. The transport service module is responsible for breaking an incoming stream of data into chunks (more formally: Transport Protocol Data Units or in Internet terminology, segments) for transfer across the network Each segment is prefixed with a transport header, to indicate to the remote transport service module what data it contains, and to carry other communications-related information.
As each segment is received from the network, an acknowledgement (or ACK)is sent back, so that damaged or lost segments can be resent, ensuring reliable communications. The transport protocol defines the rules by which this is achieved.
Note that the need for reliable interprocess communications is independent of the nature of the application (in our example, the WWW), hence this service can be shared by many different applications which need reliable data transfer across the network.
In the Internet, the reliable transport service module implements the TCP protocol.
The network service module builds a packet containing the entire TPDU as its data part, and prefixes a network header containing, among other information, the network address of the destination computer. It then passes the packet to the actual network for delivery.
In the Internet, the network service module implements the IP protocol, and NPDUs are normally called IP packets or datagrams.![]()
Note that the network service need not guarantee reliable delivery of packets -- why not?
Notes:![]()