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

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