Positive Acknowledgment with Retransmission

Positive Acknowledgment with Retransmission (PAR, German : positive confirmation with retransmission ) is a procedure used within the Transmission Control Protocol (TCP) to avoid the loss of data packets in a data transmission .

Probably the simplest and most basic technique for reliable data exchange is to send a data packet and start a timer with a value that has been selected as favorably as possible beforehand. If the packet arrives at the receiver, the latter sends an acknowledgment ( ACK ) back to the sender. Only when the sender has received this confirmation and thus knows that the data packet has safely arrived at the recipient does it send the next packet. However, if it does not receive a confirmation of receipt before the timer expires, it sends the packet again ( retransmission ).

Problems arise from the fact that packets and thus of course also the confirmations can not only be lost, but can also be very much delayed or even duplicated. This problem is generally solved by numbering the packages and also including this number on the confirmation.

Overall, however, this method is not very efficient because the sender has to wait a long time inactive if there is a high latency period . If one considers z. B. a satellite connection with a latency of one second, the sender has to wait two seconds (there and back) before it can send the next packet. Even with a packet size of 100 kB, a data throughput of 50 kB / sec is achieved, although the satellite could actually transmit several MB / sec. The network used is therefore not being used optimally. In order to avoid this, several packets are sent with TCP so that the first packet is already acknowledged while the last is still being transmitted. For this purpose, the receiver provides a reception window for several packets.

See also

• High-level data link control (HDLC)