[ntp:hackers] Minor twitches and flakes

David L. Mills mills at udel.edu
Sun Apr 6 21:25:52 UTC 2008


Guys,

The ntpq billboards have been changed in very minor ways to agree with 
the names used in the NTPv4 specification. Only the receive timestamp is 
reported, as the other timestamps are either clobbered (to avoid a 
replay vulnerability) or misleading after the on-wire checks.

There is a new restriction bit called flake. When lit, a fraction (10 
percent) of arriving NTP packets are simply dropped. The idea is to make 
sure the on-wire and Autokey protocols operate correctly in case of 
moderate to high packet losss. The on-wire protocol works just fine, 
even in symmetric modes with Autokey, when the packet loss is as high as 
50 percent.

However, packet loss is more critical in broadcast mode with Autokey. If 
an ordinary packet (ASSOC message) is lost, no problem; however, if an 
autokey values packet (AUTO message) is lost, the autokey sequence is 
broken. When this happens the client eventually times out and restarts 
the protocol. With a packet loss of 10 percent, one AUTO message in ten 
can be dropped. With the current default key list regeneration interval, 
this happens about once or twice a day. I don't think this is 
significant, as broadcast mode would ordinarily not be used over 
moderate to high loss networks.

Why flake? Once upon a time in the early Internet, Bob Braden operated a 
"flakeway" that randomly discarded packets from one net to another. That 
was very useful in exactly the kind of tests reported here.

Dave


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