[ntp:hackers] Use MSG_CONFIRM flag to keep OS ARP table up to date?

todd glassey tglassey at earthlink.net
Sun May 6 22:41:59 UTC 2012


On 5/6/2012 1:37 PM, Kurt Roeckx wrote:
> On Wed, May 02, 2012 at 12:10:25PM +0200, Heiko Gerstung wrote:
>> Hi!
>>
>> We are currently running stress tests with more than 100k simulated NTP clients against one single NTP server and found out
>> that Linux is sending ARP queries for every single NTP client every 30s, putting significant load onto the NTP server
>> (running NTP 4.2.6p3) we use as the NTP reference for the simulated clients.
>
> And those 100k NTP clients are all in the local subnet?  I only
> have my gateway in the ARP table, and that's really all I need.
>
>> This seems to be caused by Linux not automatically updating the ARP cache when NTP sends its NTP responses to the clients,
>> resulting in all ARP table entries to become stale after a period of 30s (this can be changed but would require users to
>> mess with their kernel / network configuration). This triggers a new ARP request for each stale table entry
>
> Yes, that's normal and expected behaviour.  And if you want to
> avoid ARP and other broadcast storms, I suggest you use smaller
> subnets.
>

Yes but in the interest of building a more versatile tool, perhaps if 
there was some way of setting up connection dynamics which could be 
passed to the OS, the listener and below it for some things, this could 
be a valuable addition.

We are moving towards a tool which needs to be able to synchronize 
totally virtualized inferences and in that provide a set of proofs for 
those operations.

That's really just the way it is and whether we like it or not, this 
issue - i.e. controlling the transport context is going to become an 
issue for us I think, especially as we move into cloud type systems as 
well.

Just my two cents.

Todd

>
> Kurt
>
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Todd S. Glassey
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