[ntp:questions] Re: Question about RFC-2030 SNTP Version4

David J Taylor david-taylor at blueyonder.co.uk
Fri Aug 1 17:12:45 UTC 2003


"Christopher J. Holland" <msnews at microsoft.com> wrote in message
news:PSwWa.97$It3.109169 at twister.socal.rr.com...
> What is T4?
> What does scarfed mean?
>
> The roundtrip delay d and local clock offset t are defined as
>
> Originate Timestamp     T1   time request sent by client
> Receive Timestamp       T2   time request received by server
> Transmit Timestamp      T3   time reply sent by server
> Destination Timestamp   T4   time reply received by client

As I understand it, when you ask a server for the time the server fills in
T2 and T3, and you as the client fill in fields T1 and T4.  You need to
keep a copy of T1 yourself, because the server may replace the T1 value by
the time at which it got its last update (I paraphrase).  So you can make
a first-order estimate of the server's sanity looking at the new value of
T1 when the packer returns.  By manipulating your saved T1, T2, T3 and T4,
you can get the round-trip delay and clock offset.

The key thing is that (your copy of) T1 and T4 are completed by the client
using its timeframe, and the server does T2 and T3 using the server's
timeframe.  The copy of T1 in the return packet is overwritten by the
server.

(I'm not an expert so I hope I've got this right!).

Cheers,
David





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