|  | 12.1 | The Transport Layer
| 12.1.2 | Layer 4 protocols
The emphasis of this curriculum is on TCP/IP Ethernet networks. The TCP/IP protocol of the OSI model Layer 4 (transport layer) has two protocols - TCP and UDP. TCP supplies a virtual circuit between end-user applications. These are its characteristics: connection-oriented reliable divides outgoing messages into segments reassembles messages at the destination station re-sends anything not received reassembles messages from incoming segments. UDP transports data unreliably between hosts. Following are the characteristics of UDP: connectionless unreliable transmit messages (called user datagrams) provides no software checking for message delivery (unreliable) does not reassemble incoming messages uses no acknowledgements provides no flow control