[ntp:questions] Very rapid polling

Uwe Klein uwe_klein_habertwedt at t-online.de
Sun Feb 15 19:49:35 UTC 2009


Richard B. Gilbert wrote:
> Uwe Klein wrote:
> 
>> gary.limanapier at elisys.co.uk wrote:
>>
>>> On Feb 15, 9:15 am, Uwe Klein <uwe_klein_habertw... at t-online.de>
>>> wrote:
>>>
>>>> gary.limanap... at elisys.co.uk wrote:
>>>>
>>>>> Just joining this discussion as we have noticed NTP request from our
>>>>> Authorised Server every 300ms on our LAN. They occur only between the
>>>>> Time Server and the WiFi Tablet during a socket connection while there
>>>>> is a low signal strength. The Tablet sends its own NTP request with a
>>>>> Poll Interval of 4 (16 seconds). The server returns its response
>>>>> increasing the Poll Interval to 10 (1024 secs). The server then keeps
>>>>> sending this same packet every 300-500ms.
>>>>
>>>>
>>>> Does the ntp process submit these packets repeatedly or
>>>> are you seeing some (uncalled for) "collision/resend" action due to
>>>> issues with the wlan connection?
>>>>
>>>> uwe
>>>
>>>
>>>
>>> Would it matter if the packet colided? NTP is using UDP and I can't
>>> see from the packet analyser that it is expects a response back. When
>>> it does work with a known good machine there is no acknowledgement of
>>> the packet being sent. Do you think that there another level of
>>> operation in the network that I am not seeing with the packet analyser?
>>
>>
>> you usually get retransmission on collision from the hardware ( or the 
>> driver ).
>> ( ethernet i am certain about, no firm knowledge about wlan though )
>> This would be independent of the protocol used.
>>
>> this might help ( look for collision .. RTS/CTS packet ):
>>     http://wlan.nat.sdu.dk/802_11standard.htm
>>
>>  From my side this is an absolutely blind guess. But it could
>> well be a point to look at before trying to find an app bug
>> that may not be there.
>>
>> uwe
> 
> 
> Remember that NTP uses UDP.  UDP does not guarantee delivery, does not 
> resend packets, etc.  It sends a response to a request and doesn't know 
> or care if the response was received or not!

Watch my lips ( or this wrongfull usage of that idiom) :

The hardware does automatic resend on collision using an
exponential+random backoff algorithm.

In Ethernet hardware this is buried in the chipset.

Wlan does similar stuff but buried in the driver OS side and/or Firmware side.
( And the newfangled stuff tends to have more and nefarious implementation issues )

It has nothing to do  with UDP, TCP or whatever type of protocol you think you
are using ( wrong layer ).

An eternity ( in internet time ) ago I used to design ethernet interface cards.
You could have very subtle errors in the transceiver or cabling where some party
on the cable saw a collision while others did not.

uwe




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