Jerry, I have modified the timing diagrams to account for the two possible sources of start-time (a user or the requesterNSA). See attachment. Actually, thinking about this some more I don't think we need to make a distinction about the origin of the start-time timestamp... based on my reasoning below. NTP, if correctly configured, is a protocol for synchronising PCs to a global reference standard (rather than synchronising the clocks between each other). http://en.wikipedia.org/wiki/Network_Time_Protocol So I think we can get around the problem of an unknown originator of the time if we assume (this must be an explicitly stated assumption) that the BOTH the 'user', the 'requesterNSA' and the 'providerNSA' clocks have NTP enabled. If this is true, we don't need to care about the origin of the start-time timestamp and set the value of X (skew) to be 2xNTP accuracy (approx 20ms). Does this make sense to you? Guy From: Guy Roberts Sent: 01 October 2010 14:22 To: 'Jerry Sobieski' Subject: clock skews Jerry, Thinking about the clock skew problem some more - in my diagram where we have put 'skew' this needs to be the skew between the clock of the point where the timestamp for the requested start-time was added and the time of the provider NSA. We can *NOT* assume (as I have done) that the requesterNSA added the start-time. More often than not this is added by hand by a human or forwarded from anther machine.... Guy