[ntp:questions] NTPD concurrent clients limit

David L. Mills mills at udel.edu
Thu Jul 31 13:31:22 UTC 2008


See the Rate Management and the Kiss-o'-Dearh page in the current NTP 
documentation on the web.


Unruh wrote:

> "Richard B. Gilbert" <rgilbert88 at comcast.net> writes:
>>j. wrote:
>>>Hi all,
>>>I'm testing an embedded linux device, which implement an NTP server,
>>>based on the ntpd demon.
>>>It looks like ntpd accepts only a limited number of requests from a
>>>test clientIi've set up.
>>>Do you know if there's such limit or what's the logic behind it?
>>>Maybe ntpd rejects bursts of requests coming from the same IP?
>>>Thanks in advance,
>>>Gianandrea Gobbo.
>>If you poll the server continuously at intervals of less than 64 
>>seconds, most modern NTP servers will send you a "Kiss of Death" packet.
>>Polling this frequently is considered abusive!  It's also unnecessary, 
>>NTP is designed to work with poll intervals between 64 seconds and 1024 
>>seconds and will adjust its poll interval within that range as needed.
> His question can be rephrased, what does ntpd do after it has sent the Kiss of Death?
> does it drop all subsequent packets? -- That sounds like a huge cost on the
> ntp server-- ie imagine a popular server with 10,000 machines it has sent
> the KoD to. It then has to scan that whole list for each packet to see if
> it is in there-- something which takes time and destroys the ability of ntp
> to deliver its time base rapidly.
> Note that how ntpd handles this situation depends on which version of ntpd
> you are running. 
>>There are two exceptions to the above.  You may specify the "iburst" 
>>keyword for a server and NTPD will send an INITIAL burst of eight 
>>request packets at intervals of two seconds.  This is designed for fast 
>>startup.  After the initial burst, polling continues at intervals 
>>between 64 and 1024 seconds.
> So how does the server know whether this burst is an iburst or is a rogue
> client to which it should send a KoD?
>>If you are using a dialup telephone connection for short periods three 
>>or four times a day, you may specify the "burst" keyword which sends 
>>eight requests two seconds apart at EACH poll interval.  "Burst" is to 
>>be used ONLY for brief periods with LONG intervals between them!
>>It is customary to request permission from the owner of the server 
>>before using "burst".

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