[ntp:questions] Is a doubling of I/O bytes expected for NTP over Wi-Fi?

David Taylor david-taylor at blueyonder.co.uk.invalid
Tue Nov 26 10:56:40 UTC 2013

Brief background.  I have five Raspberry Pi NTP servers, all with a 
nominally identical configuration, all with GPS/PPS receivers.  Only one 
of these is doing any significant work other than running NTP.  None of 
the devices are acting as servers for other clients.  Two are LAN 
attached, and three are connected over Wi-Fi.  They all have three LAN 
servers polled at 32s intervals, and four pool servers (WAN) polled at 
1024s intervals.  The I/O shows that input considerably exceeds output.

Looking at the I/O bytes reported by MRTG for the LAN or Wi-Fi over USB 
adapters, I see:

raspi-1  LAN  in: 207 B/s, out: 35 B/s
raspi-2  LAN  in: 202 B/s, out: 36 B/s

raspi-4: Wi-Fi  in: 406 B/s, out 40 B/s
raspi-5: Wi-Fi  in: 426 B/s, out: 53 B/s

It puzzles me that the bytes sent to the RPi when Wi-Fi is attached are 
about double the bytes sent over the LAN.  I would expect most of that 
traffic to be the RPi interrogating the three local stratum-1 NTP servers.

I don't see why there should be double the bandwidth just because Wi-Fi 
is used.  Is this expected?  Could it be that the USB Wi-Fi adapter is 
reporting incorrectly?

I ask here, as most of the traffic will be NTP packets (or it /should/ be!).
Web: http://www.satsignal.eu

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