What mechanisms provide tcp its reliability?

Answer 1

ACK packets, and sequencing.

Every piece of data sent over a TCP stream is acknowledged by the recipient when it is received, this tells the sender that the packet arrived.

Every packet sent has a unique, sequentially increasing number associated with it.

If a recipient receives packets out of order, it knows how to shuffle them back into place. If a sender doesn't receive an acknowledgement, it knows to resend.

Answer 2

Windowing and acknowledgments.