Curbing Timeouts for TCP-Incast in Data Centers via A Cross-Layer Faster Recovery Mechanism
Ahmed M. Abdelmoniem, Brahim Bensaou · 2018
We first study, at a microscopic level, the effects of various types of packet losses on TCP performance in a small data center. Then based on the findings we propose a simple recovery mechanism to combat the drawbacks of the long retransmission timeout. We emphasize through our empirical study that packet losses that occur at the tail of short-lived flows$a$nd/or bursty losses that span a large fraction of the congestion window are frequent in data center networks; and, in most cases, especially for short-lived flows, they result in a loss recovery that incurs waiting for a long retransmission timeout (RTO). The negative effect of frequent RTOs on the FCT is dramatic, yet recovery via RTO is merely a symptom of the pathological design of TCP's minimum RTO mechanism (set by default to the Internet scale). We propose the so-called Timely Retransmitted ACKs (T-RACKs), a very simple recovery mechanism for data centers, implemented as a shim layer between the virtual machines layer and the end-host NIC, to bridge the gap between TCP's huge RTO and the actual round trip times experienced in the data center. Compared to alternative solutions such as DCTCP, our T-RACKS has the virtue of not requiring any modification to TCP, which makes it readily deployable in virtualized multi-tenant public data centers. Experimental results show considerable improvements in the FCT distribution.