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Abstract-In both TCP and SCTP, selectively acked (SACKed) out-of-order data is implicitly renegable; that is, the receiver can later discard SACKed data. The possibility of reneging forces the transport sender to maintain copies of SACKed data in the send buffer until they are cumulatively acked. In this paper, we investigate the situation where all out-oforder data is non-renegable, such as when the data has been delivered to the application, or when the receiver simply never reneges. Using simulations, we show that SACKs result in inevitable send buffer wastage, which increases as frequency of loss events and loss recovery durations increase. We introduce a fundamentally new ack mechanism, Non-Renegable Selective Acknowledgments (NR-SACKs), for SCTP. Using NR-SACKs, an SCTP receiver can explicitly identify some or all out-of-order data as being non-renegable, allowing the sender to free up send buffer sooner than if the data were only SACKed. We compare and show that NR-SACKs enable efficient utilization of a transport sender's memory. We further investigate the effects of using NR-SACKs in Concurrent Multipath Transfer (CMT). CMT is an experimental SCTP extension that exploits multihoming for simultaneous data transfer over multiple paths [4]. Using simulations, we show that NR-SACKs not only reduce transport sender's memory requirements, but also improve throughput in CMT.