Acknowledgement or Retransmission

One way of ensuring reliable communications is for the sending host to keep a unique 32-bit sequence number for every packet it sends, the initial number is generated randomly. When this packet is received, the receiving host sends back an acknowledgement.

32 bits ensures uniqueness, at 10 Mb/s it would normally take more than an hour for sequence numbers to wrap around to where they started.

If an acknowledgement is not received within a fixed time, the packet is retransmitted.