Combining unacknowledged and queued packets to form ARQ retransmission packet such that queued packet can be recovered from other packets and not sent

摘要

In an automatic repeat request (ARQ) retransmission scheme packets may arrive out of order, triggering an ACKnowledgement or NACK (negative acknowledgement) that results in a retransmission packet being sent (Fig. 14a). The invention XORs the unacknowledged packet P1 with unsent packet P5 from the transmission queue, to form an anticipation packet AP. Which is then transmitted (Fig. 14b). If P1 arrives out of sequence it is XORed with AP to recover P5 (Fig. 14c), this is acknowledged and the original P5 is not transmitted (Fig. 14d). Thereby avoiding duplicated redundant transmission. If P1 is not received it is recovered by XORing AP and P5 (not shown). The invention is useful where TCP is tunnelled through UDP channels (eg. Layer 2 Tunnelling Protocol (L2TP)). The system may also use retransmission packets formed using multiple unacknowledged packets.