1.
RTT matters
Matsuzaki ‘maz’ Yoshinobu
<maz@iij.ad.jp>
Internet Initiative Japan Inc. (IIJ/AS2497) 12015/11/10

2.
Round Trip Time (RTT)
RTT
t0
t1
Internet Initiative Japan Inc. (IIJ/AS2497) 22015/11/10

3.
TCP 3 way handshake and RTT
RTT
RTT
Internet Initiative Japan Inc. (IIJ/AS2497) 32015/11/10

4.
TCP and RTT
RTT
window size
= TCP RX buffer
RTT
RTT
Internet Initiative Japan Inc. (IIJ/AS2497) 42015/11/10

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.
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.
Wi-Fi is getting popular
wirelesswired
Internet
server 2
server 1
Internet Initiative Japan Inc. (IIJ/AS2497) 72015/11/10

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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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