Internet Core Protocols: The Definitive Guide

Application design issues

Some applications open a connection and keep it open for long periods of time, while others open and close connections rapidly, using many circuits for a single operation.

For example, if you instruct your web browser to open a document from an HTTP 1.0 Web server, the HTTP client issues an active open to the destination HTTP 1.0 server, which then sends the document to the client and close the TCP connection. If there are any graphic objects on that document, the HTTP client has to open multiple unique connections for each of those objects. Thus, opening a single web page could easily result in twenty or more circuits being established and destroyed, depending on the number of objects embedded in the requested web page.

Since this model generates a lot of traffic (and uses a lot of network resources on the server), this process was changed with HTTP 1.1, which now allows a single circuit to be used for multiple operations. With HTTP 1.1, a client may request a page and then reuse the existing circuit to download objects embedded within that page. This model results in significantly fewer virtual circuits being used, although it also makes the download process synchronous rather than asynchronous.

Most applications use a single circuit for everything, keeping that circuit open even when there may not be any noticeable activity. TELNET is one example of this, where the TELNET client will issue an active open during the initial connection, and then use that ...