TCP

Transmission Control Protocol

In the Internet protocol suite, TCP is the intermediate Layer between the Internet Protocol (IP) below it, and an application above it. Applications often need reliable pipe-like connections to each other, whereas the Internet Protocol does not provide such streams, but rather only best effort delivery (i.e., unreliable packets). TCP does the task of the transport layer in the simplified OSI model of computer networks. The other main transport-level Internet protocol is UDP.

http://en.wikipedia.org/wiki/Transmission_Control_Protocol

http://www.faqs.org/rfcs/rfc793.html