[ntpwg] ntpwg Updated NTPv4 Protocol Specification/timestamp location

[Al Morton]

At 11:45 PM 2/18/2008, Danny Mayer wrote:
>Kurt Roeckx wrote:
>...
> > We start with A sending a packet at time 0.0.  A takes the time
> > when the packets starts to be transmitted, so sets T1 to 0.0.
> >
> > The packet will start to arrive at B at 2.0, and stop at
> > 3.0.  B takes the timestamp at the end of the packet
> > and will set T2 to 3.0.
> >
> > It takes B 1 second to process the packet.  It will start to transmit
> > the packet at 4.0.  But since it timestamps the packets after
> > it's completly send, it will set T3 1 second later to 5.0.  B will inform
> > A about T3 with an other packet it will send later.
> >
> > The packet starts to arrive at A at 6.0, which is when A takes
> > the timestamp, getting us T4 = 6.0.  The packet completly arrives at
> > 7.0, but that doesn't matter.
> >
> > This gives:
> > offset = 1/2 * [(T2-T1) + (T3-T4)] = 1/2 * [(3-0)+(5-6)] = 1.
> >
> > While we expected the result to be 0.
> >
>
>Why would you expect that? The offset should always be non-zero as you
>yourself have demonstrated that it's taken time for the packet to travel
>between A and B.
>
>Danny

Kurt,

B set T2 at the end of the packet, so it includes serialization time.
B set T3 at the end of the packet, where it excludes serialization time.
(A sets T1 and T4 at the beginning of the packets).

Consistency among senders and receivers seems to be important here.

movin' on,
Al