(as letters) abbr. A connection-oriented transport layer protocol. Short for Transmission Control Protocol. RFC 793. When using TCP to send packets over an IP network, a connection must first be established between the sending and receiving nodes. This connection should be torn down once all data has been received. While the connection is in place, the TCP protocol handles the details of packet fragmentation and reassembly, of retries for lost packets, and of reordering at the receiver. This is why TCP is said to be a reliable protocol compared to its unreliable UDP competitor. However, all this reliability comes at a cost: TCP is 10 times harder to implement (because of all the connection and packet states to be tracked) and generally involves more code and processing at each end even without losses. So if you only need UDP, you can write the protocol stack yourself; if you need TCP, you should buy one instead.