Tcp Tutorial-4

There is yet another way of detecting data corruption-: Handshaking.

This feature ensures demands that both the source and destination must transmit and receive acknowledgement messages, that confirm transfer of uncorrupted data. Such acknowledgement messages are known as ACK messages.

Let's take an example of a typical scenario of data transfer between two systems. Source Sends MSG1 to Destination. It will not send MSG2 to Destination unless and until it gets the MSG ACK and destination will not send more requests for data or the next request message (MSG2) unless it gets the ACK from Source confirming that the MSG1 ACK was received by it. If the source does not get a ACK message from the destination, then something which is called a timed-out occurs and the source will re send the data to destination.

So this means that if A sends a data packet to B and B checksums the data packet and finds the data corrupted, then it can simply delete for a time out to take place. Once the time out takes place, A will re send the data packet to B. But this kind of system of deleting corrupt data is not used as it is inefficient and time consuming.

Instead of deleting the corrupt data and waiting for a time out to take place, the destination (B) sends a not acknowledged or NACK message to source(A). When A gets the NACK message, instead of waiting for a time out to take place, it straightaway resends the data packet.

An ACK message of 1000 would mean that all data up to 1000 octets has been received till now.