|  | 9.1 | The TCP/IP Protocol Suite
| 9.1.3 | TCP/IP protocol stack and the transport layer
The transport layer performs two functions - flow control, provided by sliding windows; and reliability, provided by sequence numbers and acknowledgments. The transport layer also provides two protocols: TCP -- a connection-oriented, reliable protocol; provides flow control by providing sliding windows, and reliability by providing sequence numbers and acknowledgments. TCP re-sends anything that is not received and supplies a virtual circuit between end-user applications. The advantage of TCP is that it provides guaranteed delivery of the segments. UDP -- connectionless and unreliable; although responsible for transmitting messages, no software checking for segment delivery is provided at this layer. The advantage that UDP provides is speed. Since UDP provides no acknowledgments, less traffic is sent across the network, making the transfer faster.