TCP is a connection-oriented protocol that guarantees reliable data delivery. TCP communication is like a telephone call. A host originating a TCP call first will check to see if the destination host is alive and responding prior to sen ding data. This is achieved by a three-way handshake, whereby the two hosts exchange information like the initial sequence number of the packets that will be used. Within the TCP header are fields that can be used to identify whether a packet is initiating a conversation or is part of an existing conversation. Applications that use TCP as the Transport Layer protocol include FTP, HTTP, Simple Mail Transfer Protocol (SMTP) and telnet.

UDP communication is more like using the postal service. A host originating a UDP packet will send the packet without checking to see if the intended host is ready to receive it. There are no sequence numbers in the UDP header, and no packet recovery procedure built in to the protocol. Additionally, no fields in the UDP header can be examined to determine if the packet is initiating communications or is part of an existing conversation. Applications that use UDP as the transport layer protocol include TFTP and Domain Name System (DNS).