|  | 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 acknowledgments
- provides no flow control