Wednesday, January 7, 2009

How cumulative acknowledgement can compensate for a lost ACK?

Cumulative ACK can compensate for a lost ACK with the subsequent ACKs of higher number. For example, A is a sender and B is a receiver. A sends packets to B from sequence number 101 to 201. After receiving all bytes from A, B sends a cumulative ACK of 202. Here, ACK Number 202 means, B received all the bytes up to 201. Let’s assume another case that there is no cumulative acknowledgement and all the packets are received by the receiver but one ACK is lost. In that case even if the packet is successfully received by the receiver, sender retransmits the packets, which it didn’t receive the ACK. In this case if there is cumulative acknowledgement, even one ACK is lost, if sender receives higher ACK number, sender determines that packets are received. The ACK with higher value compensate the previously lost ACK.

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