A broad class of security threats can be classified as denial-of-service (DoS) attacks. As the name suggests, a DoS attack renders a network, host, or other piece of infrastructure unusable by legitimate users.
There are many forms of DoS attacks, including 'Packet Flooding':
* Bandwidth Flooding.
The attacker sends a deluge of packets to the targeted host - so many packets that the target's access link (router) becomes clogged, preventing legitimate packets from reaching the server.
* Connection Flooding.
The attacker establishes a large number of half-open or fully open TCP connections at the target host. The host can become so bogged down with these half-bogus connections that it stops accepting legitimate connections.
See also: Everyday tea for information problems, or DDOS attack (for everyone), TCP Connections & UDP Connectionless Transport.