TCP ACK generation [RFC 1122, RFC 2581 Event TCP Receiver action in-order segment arrival delayed ACK. Wait up to 500ms no gaps, for next segment. If no next segment, everything else already ACKed send ACK in-order segment arrival immediately send single no gaps, cumulative ACK one delayed ACK pending out-of-order segment arrival send duplicate ACK, indicating seg. ligher-than-expect seq of next expected byte gap detected arrival of segment that immediate AcK if segment starts partially or completely fills gap at lower end of gap 3: Transport Layer 3b-63: Transport Layer 3b-6 TCP ACK generation [RFC 1122, RFC 2581] Event in-order segment arrival, no gaps, everything else already ACKed in-order segment arrival, no gaps, one delayed ACK pending out-of-order segment arrival higher-than-expect seq. # gap detected arrival of segment that partially or completely fills gap TCP Receiver action delayed ACK. Wait up to 500ms for next segment. If no next segment, send ACK immediately send single cumulative ACK send duplicate ACK, indicating seq. # of next expected byte immediate ACK if segment starts at lower end of gap