[ntp:questions] Windows and Wi-Fi - starts well, frequency steps?

Rod Dorman rodd at panix.com
Wed Jan 4 19:55:06 UTC 2012


In article <LWIMq.52547$Ee3.24264 at newsfe04.iad>,
unruh  <unruh at invalid.ca> wrote:
>On 2012-01-03, Rod Dorman <rodd at panix.com> wrote:
>> In article <jdqcs5$ppn$1 at dont-email.me>,
>> David Woolley  <david at ex.djwhome.demon.invalid> wrote:
>>>Rod Dorman wrote:
>>>> But thats my point, it says nothing about transport layer
>>>> protocols. I'm just trying to understand Dave Hart's statement
>>>
>>>As it says nothing about them, it means that all transport protocols get 
>>>the same resilience, other things being equal (UDP opens the possibility 
>>>of multicast).
>>>
>>>> which appears to claim the UDP over WiFi is guaranteed which I've
>>>> never seen stated before.
>>>
>>>In a network with a WiFi element, the WiFi element is the most likely 
>>>one to lose packets and force retransmissions, and therefore cause NTP 
>>>packets to arrive with large delays.  To a large extent it does 
>>>guarantee delivery compared with what would happen if it didn't retransmit.
>>
>> I take "guaranteed delivery" when mentioning a transport protocol to
>> mean end-to-end, not just that one hop of it will retransmit.
>
>The network IS hop to hop. 

Only if the WAP is the NTP server. If its some other host in your LAN
or if its out in the WAN then theres going to be more that one hop and
any router along the route could decide to drop the UDP packet.

>And we are getting really far away from the original question. The
>answer seems to be that wireless can typically have large, assymetric
>delays, which plays havoc with ntp. Esp if some link which typically has
>a delay, suddenly has a shorter delay (due to typical retransmission,
>and suddenly none on some packet). (the ntp filter algorithm tends to
>throw away packets with longer delays, but grabs and uses packets with
>shorter delays. Thus if there is an occasional longer delay, that does
>not matter, but if there is only an occasional shorter asymmetric delay,
>ntp will use that.)

I've got no issues with that, my only objection was the implied claim
that if WiFi was involved the UDP transport protocol was suddenly
redefined to be guaranteed.

-- 
					-- Rod --
rodd(at)polylogics(dot)com



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