Non-renegable Selective Acknowledgments (NR-SACKs) for MPTCP

We introduce Non-Renegable Selective Acknowledgments (NR-SACKs) to MPTCP, and investigate their impact in situations where an MPTCP receiver never discards received out-of-order data from the MPTCP receive buffer (i.e., never renegs). NR-SACKs not only allow an MPTCP receiver to report the reception of out-of-order data, but also allow an MPTCP sender to free reported out-of-order data in the MPTCP send buffer sooner than the advance of the MPTCP level cumulative acknowledgement (DATA ACK). We implemented NR-SACKs in the Linux kernel. Experiments show that (i) the MPTCP data transfers with NR-SACKs never perform worse than those without NR-SACKs, and (ii) NR-SACKs improve throughput in MPTCP when the total congestion window (cwnd) of all sub flows is greater than the MPTCP send buffer size (i.e., the send buffer size is the bottleneck).