Retransmission (data networks)

Positive Acknowledgement with Retransmission ( PAR, German: positive acknowledgment with retransmission ) is within the Transmission Control Protocol ( TCP) used method to avoid the loss of data packets in a data transfer.

Perhaps the simplest and most basic technique when reliable data exchange is to send a data packet and start a timer with a value previously selected as favorable as possible. If the packet to the receiver, so it sends an acknowledgment ( acknowledgment / ACK) back to the sender. Only after the transmitter has received the confirmation, and therefore knows that the data packet has safely arrived at the receiver, it sends the next packet. But he does not receive an acknowledgment before the timer expires, it sends the packet again ( retransmission ).

Problems arise in that course packets and thus also the acknowledgments can not only be lost, but can be greatly delayed or even duplicated. This problem is generally solved by the fact that the packets are numbered and this number includes the acknowledgment.

Overall, this method is not very efficient, since the sender must wait idly for a long time at a high latency. If one considers, for example, a satellite link with a latency of one second, the transmitter must wait two seconds ( there and back ) until it can send the next packet. Even with a packet size of 100 kB, only a data throughput rate of 50 kB / sec is achieved even though the satellite actually could transfer several MB / sec. The network used is thus not optimally utilized. To circumvent this, several packets are sent by the TCP, so that the first packet is already acknowledged, while the last is still transmitted. The recipient shall for this purpose a reception window for several packages available.