Congestion Control Algorithms: Open Questions Benno Overeinder NLnet Labs

NLnet Labs What This Talk is Not About Details of TCP congestion avoidance and control algorithms Research on improvements of TCP congestion avoidance algorithms Measurements of TCP congestion avoidance algorithm performance None of this, but –highlight current open question and future research

NLnet Labs Congestion Control Over the Years Global congestion collapse (1986) TCP Tahoe (1988) and TCP Reno (1990) TCP New Reno (1998) … TCP over long fat networks (2002–2003) TCP and bufferbloat (2011)

NLnet Labs Common Congestion Control Algorithms FreeBSD/Solaris –TCP New Reno –Reno: “classic” congestion avoidance –improves retransmission during the fast-recovery phase Linux –TCP CUBIC –BIC: optimized congestion control algorithm for LFN –CUBIC: less aggressive and more systematic derivative Windows –Compound TCP –achieve good performance for LFNs, while not harm fairness

NLnet Labs Fairness One mechanism at a time Mixed mechanisms

NLnet Labs SHAPE OF CONGESTION WINDOW INCREASE FUNCTION

NLnet Labs Shape of Congestion Window Increase Function

NLnet Labs Convex vs. Concave-Convex: H-TCP vs. CUBIC

NLnet Labs Distribution of cwnd for Convex and Concave Increase Distribution of cwnd at back-off for convex and concave updates versus loss probability. Key: + convex, o concave-convex

NLnet Labs BUFFERING AND BUFFERBLOAT

NLnet Labs Impact of Buffering Throughput and cwnd for Reno, buffer size 1 x BPD, 10 MBps link, 100 ms RTT. With buffering, flow throughput and cwnd are fundamentally different quantities, and only weakly related.

NLnet Labs Bufferbloat Trend to provide large buffers to network equipment –rule of thumb buffer to accommodate 250 ms traffic –e.g., 1 Gb/s interface requires 32 MB buffer Flow of packets slows down traveling from fast to slow network –buffer absorbs, temporary delay packets –packets queued in network only drops if buffer is full TCP congestion algorithm –relies on packet drops to determine available bandwidth –keeps speeding up and slowing down the transmission rate to find equilibrium –packet drops must occur in a timely manner –…

NLnet Labs Active Queue Management (AQM) Random Early Detection (RED) –random and early notification of congestion –variants FRED, SRED, with notion of flows –no synchronisation à la drop-tail CHOKe –penalize misbehaving flows –similar to SRED, but less complex CoDel –improve overall performance of RED –easier to manage, does not require manual configuration

NLnet Labs Congestion Control, Latency, and AQM

NLnet Labs CONCLUDING

NLnet Labs Remy Computer-Generated Congestion Control Specify –prior knowledge and assumptions of network –objective to achieve (e.g., throughput and delay) Outperforms existing (w/ ns2 simulations) –TCP New Reno, TCP Cubic, Compound (at end-points) –in many cases outperforms Cubic/FQ-CoDeL (requires network changes) Results for dumbbell topology, n=12, 15 Mbps.

NLnet Labs Summary Congestion control problem has changed –from: there is congestion, what do we do? –via: networks are empty, what do we do? –to: how do we get all this stuff deployed and let it interoperate? After 20+ years still interesting and important problem One size does not fit all? –FreeBSD modCC dynamic load/unload CC algorithm –For discussion: Internet at large might agree on model to prepare a “one-size-fits-all” Remy? IETF/IRTF –IETF: AQM, CONEX, RMCAT –IRTF: ICCRG