A TCP Tuning Daemon SC2002 November 19, 2002 Tom Dunigan Matt Mathis Brian Tierney

UT-BATTELLE U.S. Department of Energy Oak Ridge National Laboratory Roadmap Motivation Net100 project –Web100 –network probes & sensors –protocol analysis A TCP tuning daemon Tuning experiments rg … and now a word from our sponsors DOE-funded project (Office of Science) $1M/yr, 3 yrs beginning 9/01 LBL, ORNL, PSC, NCAR Net100 project objectives: (network-aware operating systems) measure, understand, and improve end-to-end network/application performance tune network protocols and applications (grid and bulk transfer) first year emphasis: TCP bulk transfer over high delay/bandwidth nets

UT-BATTELLE U.S. Department of Energy Oak Ridge National Laboratory Motivation Poor network application performance –High bandwidth paths, but app’s slow – Is it application? OS? network? … Yes –Often need a network “wizard” Changing : bandwidths –9.6 Kbs… 1.5 Mbs..45 …100…1000…? Gbs Unchanging: TCP –speed of light (RTT) –MTU (still 1500 bytes) –TCP congestion avoidance TCP is lossy by design ! –2x overshoot at startup, sawtooth –recovery after a loss can be very slow on today’s high delay/bandwidth links –Recovery proportional to MSS/RTT 2 Linear recovery at 0.5 Mb/s! Instantaneous bandwidth Average bandwidth Early startup losses ORNL to NERSC ftp 8 Mbs GigE/OC12 80ms RTT

UT-BATTELLE U.S. Department of Energy Oak Ridge National Laboratory TCP tuning set optimal (?) buffer size –need buffer = bandwidth*RTT ORNL/NERSC (80 ms, OC12) need 6 MB avoid losses –modified slow-start –reduce bursts –anticipate loss (ECN,Vegas?) –reorder threshold speed recovery –bigger MTU or “virtual MSS” –modified AIMD (0.5,1) –delayed ACKs and initial window avoid congestion collapse be fair (?) … intranets, QoS ns simulation: 500 mbs link, 80 ms RTT Packet loss early in slow start. Standard TCP with del ACK takes 10 minutes to recover!

UT-BATTELLE U.S. Department of Energy Oak Ridge National Laboratory Net100 components for tuning TCP protocol analysis –simulation/emulation –kernel tuning extensions Web100 Linux kernel (NSF) –instrumented TCP stack (IETF MIB draft) –100+ variables per flow (/proc/web100) –socket open/close event notification –API and tools for tracing and tuning, e.g., bw tester: firebird.ccs.ornl.gov:7123 Path characterization –Network Tuning and Analysis Framework (NTAF) –both active and passive measurement iperf, pipechar –schedule probes and distribute/archive results –data base of measurements –NTAF/Net100 hosts at PSC, NCAR,LBL,ORNL, NERSC,CERN,UT,SLAC TCP tuning daemon

UT-BATTELLE U.S. Department of Energy Oak Ridge National Laboratory TCP Tuning Daemon Work-around Daemon (WAD) –tune unknowing sender/receiver at startup and/or during flow –Web100 kernel extensions pre-set windowscale to allow dynamic tuning uses netlink to alert daemon of socket open/close (or poll) besides existing Web100 buffer tuning, new tuning options using WAD_* variables knobs to disable Linux 2.4 caching, burst mgt., and sendstall –config file with static tuning data mode specifies dynamic tuning (Floyd AIMD, NTAF buffer size, concurrent streams) –daemon periodically polls NTAF for fresh tuning data –written in C (also python version) WAD config file [bob] src_addr: src_port: 0 dst_addr: dst_port: 0 mode: 1 sndbuf: rcvbuf: wadai: 6 wadmd: 0.3 maxssth: 100 divide: 1 reorder: 9 sendstall: 0 delack: 0 floyd: 1

UT-BATTELLE U.S. Department of Energy Oak Ridge National Laboratory Experimental results Evaluating the tuning daemon in the wild –emphasis: bulk transfers over high delay/bandwidth nets (Internet2, ESnet) –tests over: 10GigE,OC48, OC12, OC3, ATM/VBR, GigE,FDDI,100/10T,cable, ISDN,wireless (802.11b),dialup –tests over NistNET 100T testbed Various TCP tuning options –buffer tuning –AIMD mods (including Floyd, both in-kernel and in WAD) –slow-start mods –parallel vs single Results are anecdotal –more systematic testing is on-going –Your mileage may vary …. Network professionals on a closed course. Do not attempt this at home.

UT-BATTELLE U.S. Department of Energy Oak Ridge National Laboratory WAD tuning results Classic buffer tuning ORNL to PSC, OC12, 80ms RTT network-challenged app. gets 10 Mbs same app., WAD/NTAF tuned buffer gets 143 Mbs Virtual MSS tune TCP’s additive increase (WAD_AI) add k segments per RTT during recovery k =6 like GigE jumbo frame, but: interrupt rate not reduced doesn’t do k segments for initial window

UT-BATTELLE U.S. Department of Energy Oak Ridge National Laboratory Tuning around Linux (2.4) TCP Tunable ssthresh caching Tunable “sendstall” (TXQUELEN) 600 mbs Amsterdam-Chicago GigE via 10GigE, 100 ms RTT sendstalls UDP event Floyd AIMD Standard AIMD Floyd AIMD : as cwnd grows increase AI and decrease MD, do the reverse when cwnd shrinks Added to Net100 kernel and to WAD (WAD tunable)

UT-BATTELLE U.S. Department of Energy Oak Ridge National Laboratory WAD tuning Modified slow-start and AI ORNL to NERSC, OC12, 80 ms RTT often losses in slow-start WAD tuned Floyd slow-start and fixed AI (6) WAD-tuned AIMD and slow-start ORNL to CERN, OC12, 150ms RTT parallel streams AIMD (1/(2k),k) WAD-tuned single stream (0.125,4)

UT-BATTELLE U.S. Department of Energy Oak Ridge National Laboratory GridFTP tuning Can tuned single stream compete with parallel streams? Mostly not with “equivalence” tuning, but sometimes…. Parallel streams have slow-start advantage. WAD can divide buffer among concurrent flows—fairer/faster? Tests inconclusive so far…. Testing on real Internet is problematic. Is there a “congestion metric”? Per unit of time? Flow Mbs congestion re-xmits untuned tuned parallel untuned tuned parallel Data/plots from Web100 tracer Buffers: 64K I/O, 4MB TCP (untuned 64K TCP: 8 mbs, 200s)

UT-BATTELLE U.S. Department of Energy Oak Ridge National Laboratory Future TCP tuning Reorder threshold seeing more out of order packets WAD tune a bigger reorder threshold for path 40x improvement! Linux 2.4 does a good job already adjusts and caches reorder threshold “undo” congestion avoidance Delayed ACKs WAD could turn off delayed ACKs -- 2x improvement in recovery rate and slow-start Linux 2.4 already turns off delayed ACKs for initial slow- start ns simulation: 500 mbs link, 80 ms RTT Packet loss early in slow-start. Standard TCP with del ACK takes 10 minutes to recover! NOTE aggressive static AIMD (Floyd pre-tune) LBL to ORNL (using our TCP-over-UDP) : dup3 case had 289 retransmits, but all were unneeded!

UT-BATTELLE U.S. Department of Energy Oak Ridge National Laboratory Futures Net100 –analyze effectiveness/fairness of current tuning options simulation emulation on the net (systematic tests) –NTAF probes -- characterizing a path to tune a flow router data (passive) monitoring applications with Web100 –additional tuning algorithms Vegas,ECN non-TCP identify non-congestive loss? –parallel/multipath selection/tuning –WAD-to-WAD tuning –jumbo frames experiments… the quest for bigger and bigger MTUs –more user -friendly Web100 extensions –refine user interface and API –port to other OS’s

UT-BATTELLE U.S. Department of Energy Oak Ridge National Laboratory Summary Novel approaches –non-invasive dynamic tuning of legacy applications –using TCP to tune TCP (Web100) –tuning on a per flow/destination Effective evaluation framework –protocol analysis and tuning + net/app/OS debugging –out-of-kernel tuning Beneficial interactions –TCP protocols (Floyd, Wu Feng (DRS), Web100, parallel/non-TCP) –Path characterization research (SciDAC, CAIDA, Pinger) –Scientific application and Data grids (SciDAC, CERN) Performance improvements