A cumulative negative acknowledgment (CNAK) approach for scalable reliable multicast

Error recovery by retransmission for large scale multicast faces two intrinsic scalability challenges: acknowledgment implosion and exposure to retransmission. This paper proposes a new scheme to reduce the cost of error recovery by using the idea of cumulative negative acknowledgment (CNAK). In the scheme, we assume a window control of size W. A receiver does not send back a NAK immediately upon the discovery of a packet loss. Instead, the receiver accumulates the losses and returns a CNAK to the source at about half of the window interval. Retransmission is carried out on multiple multicast channels which are mapped to window size W. The A /sub i/ multicast channel is mapped to the sequence number i where i = 1,2,...,W. Our scheme has these advantages: one CNAK replaces W/2 number of conventional NAK, thus, greatly reducing the NAK overhead. Moreover, the use of multiple multicast channels for packet retransmission completely eliminates the exposure hazard. We apply the CNAK scheme to the DR local recovery model and show that the proposed C4 protocol is the most efficient protocol among all. The protocol is highly scalable and free of unwanted repair packets.