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TCP Performance Optimization in Wide Area Networks

Learn how to configure routers, switches, and PCs for high TCP performance over local and wide area networks. Understand TCP algorithms, tuning parameters, and Gigabit Ethernet adapter performance for optimal network throughput.

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TCP Performance Optimization in Wide Area Networks

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  1. HENP Working Group High TCP performance over wide area networksArlington, VAMay 8, 2002 Sylvain Ravot <sylvain.ravot@cern.ch> CalTech

  2. HENP WG Goal #3 Share information and provide advice on the configuration of routers, switches, PCs and network interfaces, and network testing and problem resolution, to achieve high performance over local and wide area networks in production.

  3. Overview • TCP • TCP congestion avoidance algorithm • TCP parameters tuning • Gigabit Ethernet Adapter performance

  4. TCP Algorithms Connection opening : cwnd = 1 segment Congestion Avoidance Slow Start cwnd = SSTHRESH Exponential increase for cwnd until cwnd = SSTHRESH Additiveincrease for cwnd Retransmission timeout SSTHRESH:=cwnd/2 cwnd:= 1 segment 3 duplicate ack received 3 duplicate ack received Retransmission timeout SSTHRESH:=cwnd/2 Fast Recovery Expected ack received cwnd:=cwnd/2 Exponentialincrease beyond cwnd Retransmission timeout SSTHRESH:=cwnd/2

  5. TCP Congestion Avoidance behavior (I) • Assumption • The time spent in slow start is neglected • The time to recover a loss is neglected • No buffering (Max. congestion window size = Bandwidth Delay Product) • Constant RTT W W/2 W/2 W (RTT) • The congestion window is opened at the constant rate of one segment per RTT, so each cycle is W/2. • The throughput is the area under the curve.

  6. Example • Assumption • Bandwidth = 600 Mbps • RTT = 170 ms (CERN – CalTech) • BDB = 12.75 Mbytes • Cycle = 12.3 minutes • Time to transfer 10 Gbyte? 12.3 Min W Initial SSTRESH Initial SSTRESH W/2 W/2 W (RTT) 3.8 minutes to transfer 10 GBytes if cwnd = 6.45 Mbytes at the beginning of the congestion avoidance state.(Throughput = 350 Mbps) 2.4 minutes to transfer 10 Gbyte if cwnd = 12.05 Mbyte at the beginning of the congestion avoidance state(Throughput = 550 Mbps)

  7. Area #1 Cwnd<BDP => Throughput < Bandwidth RTT constant Throughput = Cwnd / RTT Area #2 Cwnd > BDP => Througput = Bandwith RTT increase (proportional to cwnd) TCP Congestion Avoidance behavior (II) • We take into account the buffering space. (cwnd) W Buffering capacity BDP W/2 Area #1 Area #2 (RTT) W/2 W

  8. Limit the maximum congestion avoidance window size In the application In the OS Smaller backoff TCP Multi-streams After a loss : Cwnd := Cwnd × back_off 0.5 < Back_off < 1 Tuning • Keep the congestion window size in the yellow area : • Limit the maximum congestion widow size to avoid loss • Smaller backoff (Cwnd) (Cwnd) W W BDP BDP (Time) (Time) • Limiting the maximum congestion avoidancewidow size and setting a large initial ssthresh, we reached 125 Mbps throughput between CERN and Caltech and 143 Mbps throughput between CERN and Chicago through the 155 Mbps of the transatlantic link.

  9. Tuning TCP parameters Buffer space that the kernel allocates for each socket • Kernel 2.2 • echo 262144 > /proc/sys/net/core/rmem_max echo 262144 > /proc/sys/net/core/wmem_max • Kernel 2.4 • echo "4096 87380 4194304" > /proc/sys/net/ipv4/tcp_rmemecho "4096 65536 4194304" > /proc/sys/net/ipv4/tcp_wmem • The 3 values are respectively min, default, and max. Socket buffer settings: • Setsockopt() of SO_RCVBUF and SO_SNDBUF • Has to be set after calling socket() but before bind() • Kernel 2.2 : default value is 32KB • Kernel 2.4 : default value can be set in /proc/sys/net/ipv4 (see above) Initial SSTRHESH • Set the initial ssthresh to a value larger than the bandwidth delay product • No parameter to set this value in Linux 2.2 and 2.4 => Modified linux kernel Slow Start Exponential increase for cwnd until cwnd = SSTHRESH Congestion Avoidance Additive increase for cwnd Cwnd = SSTHRESH Connection opening : cwnd = 1 segment

  10. Gigabit Ethernet NICs performances • NIC tested • 3com: 3C996-T • Syskonnect: SK-9843 SK-NET GE SX • Intel: PRO/1000 T and PRO/1000 XF • 32 and 64 bit PCI Motherboards • Measurements • Back to back linux PCs • Latest drivers available • TCP throughput • Two different tests: Iperf and gensink. Gensink is a tool written at CERN for benchmarking TCP network performance • Performance measurement with Iperf: • We ran 10 consecutive TCP transfers of 20 seconds each. Using the time command, we measured the CPU utilization. • [root@pcgiga-2]#time iperf -c pcgiga-gbe – t 20 • We report the throughput min/avg/max of the 10 transfers. • Performance measurement with gensink: • We ran transfers of 10 Gbyte. Gensink allow us to measure the throughput and the CPU utilization over the last 10 Mbyte transmitted.

  11. Syskonnect - SX, PCI 32 bit 33 MHZ • Setup: • GbE adapter: SK-9843 SK-NET GE SX; Driver included in the kernel • CPU: PIV (1500 Mhz) PCI:32 bit 33MHz • Motherboard: Intel D850GB • RedHat 7.2 Kernel 2.4.17 • Iperf test: • Gensink test: Throughput min / avg / max = 256 / 448 / 451 Mbps CPU utilization average= 0.097 sec/Mbyte

  12. Intel - SX , PCI 32 bit 33 MHZ • Setup: • GbE adapter: Intel PRO/1000 XF; Driver e1000; Version 4.1.7 • CPU: PIV (1500 Mhz) PCI:32 bit 33MHz • Motherboard: Intel D850GB • RedHat 7.2 Kernel 2.4.17 • Iperf test: • Gensink test: Throughput min / avg / max = 380 / 609 / 631 Mbps CPU utilization average= 0.040 sec/Mbyte

  13. 3Com - Cu, PCI 64 bit 66 MHZ • Setup: • GbE adapter: 3C996-T; Driver bcm5700; Version 2.0.18 • CPU: 2 x AMD Athlon MP PCI:64 bit 66MHz • Motherboard: Dual AMD Athlon MP Motherboard • RedHat 7.2 Kernel 2.4.7 • Iperf test • Gensink test: Throughput min / avg / max = 232 / 889 / 945 Mbps CPU utilization average= 0.0066 sec/Mbyte

  14. Intel - Cu, PCI 64 bit 66 MHZ • Setup • GbE adapter: Intel PRO/1000 T; Driver e1000; Version 4.1.7 • CPU: 2 x AMD Athlon MP PCI:64 bit 66MHz • Motherboard: Dual AMD Athlon MP Motherboard • RedHat 7.2 Kernel 2.4.7 • Iperf test : • Gensink test: Throughput min / avg / max = 429 / 905 / 943 Mbps CPU utilization average= 0.0065 sec/Mbyte

  15. Intel - SX, PCI 64 bit 66 MHZ • Setup • GbE adapter: Intel PRO/1000 XF; Driver e1000; Version 4.1.7 • CPU: 2 x AMD Athlon MP PCI:64 bit 66MHz • Motherboard: Dual AMD Athlon MP Motherboard • RedHat 7.2 Kernel 2.4.7 • Iperf test : • Gensink test: Throughput min / avg / max = 222 / 799 / 940 Mbps CPU utilization average= 0.0062 sec/Mbyte

  16. Syskonnect - SX, PCI 64 bit 66 MHZ • Setup • GbE adapter: SK-9843 SK-NET GE SX; Driver included in the kernel • CPU: 2 x AMD Athlon MP PCI:64 bit 66MHz • Motherboard: Dual AMD Athlon MP Motherboard • RedHat 7.2 Kernel 2.4.7 • Iperf test • Gensink test: Throughput min / avg / max = 146 / 936 / 947 Mbps CPU utilization average= 0.0083 sec/Mbyte

  17. Summary • 32 PCI bus • Intel NICs achieved the highest throughput (600 Mbps) with the smallest CPU utilization. Syskonnect NICs achieved only 450 Mbps with a higher CPU utilization. • 32 Vs 64 PCI bus • 64 PCI bus is needed to get high throughput: • We multiplied by 2 the throughput by moving Syskonnect NICs from 32 to 64 PCI buses. • We increased the throughput by 300 Mbps by moving Intel NICs from 32 to 64 PCI buses. • 64 PCI bus • Syskonnect NICs achieved the highest throughput (930 Mbps) with the highest CPU utilization. • Intel NICs performances are unstable. • 3Com NICs are a good compromise between stability, performance, CPU utilization and cost. Unfortunately, we couldn’t test the 3Com NIC with fiber connector. • Cu Vs Fiber connector • We could not measure important differences. • Strange behavior of Intel NICs. The throughout achieve by Intel NICs is unstable.

  18. Questions ?

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