Linux Firewalls, Third Edition

Transport Mechanisms

Internet Protocol defines a Network layer protocol of the OSI model. There are also other Network layer protocols, but I will be concentrating solely on IP because it is by far the most popular Network layer protocol in use today. Above the Network layer on the OSI model is the Transport layer. As you might expect, the Transport layer has its own set of protocols. Two of the Transport layer protocols are of interest: UDP and TCP. This section examines each of these protocols.

UDP

UDP, or User Datagram Protocol, is a connectionless protocol used for such services as DNS queries, SNMP, and RADIUS. Being connectionless, UDP is akin to a “fire and forget” type of protocol. The client sends a UDP packet, sometimes referred to ...