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RTT matters

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RTT matters

  1. 1. RTT matters Matsuzaki ‘maz’ Yoshinobu <maz@iij.ad.jp> Internet Initiative Japan Inc. (IIJ/AS2497) 12015/11/10
  2. 2. Round Trip Time (RTT) RTT t0 t1 Internet Initiative Japan Inc. (IIJ/AS2497) 22015/11/10
  3. 3. TCP 3 way handshake and RTT RTT RTT Internet Initiative Japan Inc. (IIJ/AS2497) 32015/11/10
  4. 4. TCP and RTT RTT window size = TCP RX buffer RTT RTT Internet Initiative Japan Inc. (IIJ/AS2497) 42015/11/10
  5. 5. packet loss and RTT RTO RTO is computed based on RTT -- see rfc6298 retransmission timeout Internet Initiative Japan Inc. (IIJ/AS2497) 52015/11/10
  6. 6. RTT • the lower, the better – faster TCP handshake – better TCP throughput – faster TCP recovery from a packet loss • There are various efforts to improve TCP performance even in a high latency situation Internet Initiative Japan Inc. (IIJ/AS2497) 62015/11/10
  7. 7. Wi-Fi is getting popular wirelesswired Internet server 2 server 1 Internet Initiative Japan Inc. (IIJ/AS2497) 72015/11/10
  8. 8. I faced a trouble wirelesswired • RTT was not stable from a wireless client to the server 2. L 64 bytes time=398.122 ms 64 bytes time=316.330 ms 64 bytes time=337.251 ms 64 bytes time=358.155 ms 64 bytes time=299.480 ms 64 bytes time=387.789 ms server 2 server 1 Internet Initiative Japan Inc. (IIJ/AS2497) 82015/11/10
  9. 9. let’s try to isolate the problem wireless 64 bytes time=169.588 ms 64 bytes time=170.666 ms 64 bytes time=169.793 ms 64 bytes time=168.185 ms 64 bytes time=169.783 ms 64 bytes time=169.957 ms server 1 64 bytes time=3.757 ms 64 bytes time=3.789 ms 64 bytes time=3.768 ms 64 bytes time=3.718 ms 64 bytes time=3.644 ms 64 bytes time=3.762 ms 1) the local wifi network looks pretty stable 2) RTT to server 1 looks also stable server 2 Internet Initiative Japan Inc. (IIJ/AS2497) 92015/11/10
  10. 10. hmmm... wirelesswired server 2 server 1 Internet Initiative Japan Inc. (IIJ/AS2497) 10 64 bytes time=296.040 ms 64 bytes time=296.105 ms 64 bytes time=296.442 ms 64 bytes time=296.186 ms 64 bytes time=296.103 ms 64 bytes time=296.070 ms 3) from the wired host in the same network, RTT to the server 2 looks stable 2015/11/10
  11. 11. so... wireless server 1 server 2 wired • this strange behavior happens only for this combination L Internet Initiative Japan Inc. (IIJ/AS2497) 112015/11/10
  12. 12. RTT distribution to server 1 from wired host to server 1 from wifi host to server 1 wired wifi Internet Initiative Japan Inc. (IIJ/AS2497) 122015/11/10
  13. 13. RTT distribution to server 2 from wired host to server 2 from wifi host to server 2 wired wifi Internet Initiative Japan Inc. (IIJ/AS2497) 132015/11/10
  14. 14. the wifi AP was buffering packets wirelesswired Internet server 2 server 1 Internet Initiative Japan Inc. (IIJ/AS2497) 14 • and this caused the unstable RTT L 2015/11/10
  15. 15. My wifi adapter does sleep • to reduce battery usage • before sleeping, the client send a notification to the wifi AP, and the AP keeps packets until the client wake up • so, my PC was asking the buffering! Internet Initiative Japan Inc. (IIJ/AS2497) 152015/11/10
  16. 16. wifi AP sends beacon • beacon interval – time interval between beacon transmissions – usually 100msec, but it’s configurable • TIM (Traffic Indication Map) – to tell any sleeping clients if the AP has any buffered frames present for it • wifi adapter can sleep between beacons, and wake up to check a beacon (TIM can indicate if the adapter need to receive data or not) Internet Initiative Japan Inc. (IIJ/AS2497) 162015/11/10
  17. 17. the scenario • My wifi adapter went to sleep after 200msec of no traffic – that’s why the unstable RTT happens only when I was communicating with server 2 (average RTT is 300msec) • Based on the beacon interval information (which was 100msec in my case), it woke up and received a response – that’s why most RTT distribution is within 100msec Internet Initiative Japan Inc. (IIJ/AS2497) 172015/11/10
  18. 18. sleeping and buffering Internet Initiative Japan Inc. (IIJ/AS2497) 18 200msec no traffic goes to sleep until the next beacon buffered 2015/11/10
  19. 19. Summary • Strange RTT behavior happens if your communication is between: – a host connected to a wifi network and – a far end host (RTT>200msec) • Your wifi adapter goes to sleep – “200msec of no traffic” seems a common trigger • The sleep duration is manageable by setting beacon interval of your wifi AP – 100msec would be reasonable – You might be able to reduce battery usage by setting it as 1000msec, but this could introduce more RTT penalty Internet Initiative Japan Inc. (IIJ/AS2497) 192015/11/10

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