A Protocol Analysis of a UNIX Network
This article provides a detailed analysis of the protocols in use on your UNIX network.
Some protocols are easy to identify while others need more investigation to understand what they are doing and what information they are exchanging. This IBM developerWorks "Deep-protocol analysis of UNIX networks" article provides details on how to take a closer look at your network and find out what is going on.
"With both TCP/IP and UDP/IP communications, the key elements are the IP addresses used to identify the hosts and the port number. The port number is used to provide additional communication channels so that you can support multiple connections between two hosts. There are some standards in the port definitions. For example, port 25 is for email (SMTP) traffic, and most websites operate on port 80 (HTTP). These conventions are used to allow programs to communicate with each other over a known channel in the same way as you would choose a phone or fax number.
"While these conventions exist, there is no limit or restriction on what ports you use. In fact, in many cases, subversive network applications and some security methods will deliberately use non-standard ports. For example, some will hide content by misusing a standard port with a different protocol, like using HTTP over port 25. Other examples include using a different port from the standard so that it is not obvious which port is being used for the traffic (like using port 99 for HTTP), or by encapsulating specific protocol traffic within another protocol. This last method is actually the one used by network tunneling and virtual private networks (VPNs).