[ntp:questions] Client doesn't drop failed source

Rob nomail at example.com
Thu Jan 21 09:06:02 UTC 2010


David Woolley <david at ex.djwhome.demon.invalid> wrote:
> E-Mail Sent to this address will be added to the BlackLists wrote:
>
>> 
>> I can perhaps see idling the connection to keep it open when the
>>  poll rate is at ~ 1 minute, however what about when the poll
>>  rate decreases to ~ 17 minutes? (or less often if so configured)
>
> There are no network cost in keeping a TCP connection "idling", as there 
> is no traffic in that state.  (The exception is if you enable 
> keepalives, and you wait hours between real traffic, but even then the 
> traffic is very very small.)

It would be very unwise to use TCP for something like NTP.
Information sent by an application via a TCP socket will be re-sent by
the OS when no acknowledgement is received.  When the original network
packet had been lost, the receiver will get a retransmitted copy which
contains the original timestamp but which arrives much later in time.
(the re-try timer in TCP is usually in the order of seconds)

It is better to send a time message and lose it (with UDP), than to
receive it later because it has been re-transmitted.  With UDP the
re-transmit is done at the application layer, and the application can
put fresh time information in the re-transmission.




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