TCP operates in the transport layer, along with its friend UDP, the User Datagram Protocol. TCP is more complex; it performs error-checking, and it tries very hard to deliver your packets. There is a lot of back-and-forth communication with TCP as it transmits and verifies transmission, and when packets get lost it resends them. UDP is simpler and has less overhead. It sends out datagrams once, and UDP neither knows nor cares if they reach their destination.
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