[ntp:questions] Clarification on the org, rec, xmt header fields

Hal Murray hal-usenet at ip-64-139-1-69.sjc.megapath.net
Sun May 17 03:46:17 UTC 2009


>> If you assume a client-server model, from the client's view
>> there are 4 time stamps:
>>   when the request leaves the client
>>   when the request arrives at the server
>>   when the response leaves the server
>>   when the response arrives at the client

> This model makes complete sense.  What I'm curious about is that I
> would imagine that mode = 3, the only timestamp that is struck is the
> 'request leaves' (org) but what I see is request arrives (rec) and the
> response leaves (xmt) in the client packet.  What else is odd is that
> the (org) is 10 milliseconds after the (rec) and the (xmt) is 8
> minutes in the future! 

One thing I wasn't clear on...  The code that adds the time stamps doesn't 
know if it's client or server.  The transmit side puts its timestamp into the 
3rd slot.  The Receive side puts its time stamp into the 4th slot.  If you 
are using tcpdump/Etherreal, you won't see that because it doesn't happen 
until the packet gets to the user (ntpd).

For the simple client/server case, the server copies the data from slots 3 
and 4 to slots 1 and 2, then sends the packet back.


-- 
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