Q & A on TCP: Connection-oriented or connectionless? Q: Part of the definition, if I understand correctly, of connection oriented service is that a virtual connection is arranged between sender and receiver for the whole of the transfer. How can TCP be connection oriented, if its segments are encapsulated in IP datagrams, which are a connectionless service? A: You are correct that TCP uses a connectionless service (IP) to exchange segments. The key to understanding TCP is to observe that although TCP provides a CO (i.e., virtual circuit) abstraction to applications that use it, the TCP protocol modules on the sending and receiving machines do not require a CO network. In fact, the whole idea of building TCP over IP seems like a paradox because: TCP provides IP provides --------------------------- ---------------------------- connection-oriented service connectionless service stream interface datagram interface flow control no flow control congestion control no congestion control reliability: no guarantees about no loss loss no duplication duplication no out-of-order delivery out-of-order delivery In other words, TCP provides *more* service than IP. To achieve such features TCP uses mechanisms such as acknowledgements, window advertisements, and timers that allow it to detect and recover from loss, duplication, and out-of-order delivery.