[ntp:questions] Re: an example of a negative RootDelay, please...

David L. Mills mills at udel.edu
Fri Aug 8 16:53:53 UTC 2003


The root delay format is described on p. 51 of RFC-1305 as I'm sure you 
have seen. As in almost everything Internet, this and other NTP packet 
variables are in twos-complement representation and big-endian network 
byte order. That completely describes both positive and negative values 
except for scaling. The decimal point for the root delay and root 
dispersion is between bits 15 and 16. Once you understand how 
twos-complement arithmetic works, the answer to your question is obvious.

Negative values for roundtrip delay and by implication root delay are in 
principle possible, but very rare. See Section 8.3 of RFC-1305 for an 
explanation of errors due to frequency skew between server and client. 
When the skew is large and the propagation times very small, the delay 
calculation can indeed result in negative values.


Morpheus wrote:
> Hi,
> would someone give me an example of a negative RootDelay field in bit format
> and second format? i'm writing an algorithm to understand the NTP Packet
> and i only saw positive RootDelay so i cannot test that algorithm with
> negative RootDelay
> Thanks for you help

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