[time-nuts] IEEE 1588 PTP support on raspberry pi 4 compute module

[Hal Murray]

> If you have fully symmetric delays, then there is no need for 1588. NTP will
> do just fine. Unfortunately as even home networking hardware loads up,  ( =
> lots of traffic in only one direction) this may not be the case. When things
> *do* go asymmetric, then you *do* need 1588 compatible switches. 

Even if the network is lightly loaded, there can be asymmetry in CPU 
processing delays.

In traditional NTP, the transmit time stamp is grabbed as late as possible 
before sending a packet..  That cancels out if both CPUs are the same speed.  
Adding authentication (crypto type computations) increases delay from grab to 
send.  That will increase the offset if the CPUs are not matched.

There is work in progress at the IETF working group on an "interleaved" mode 
that would send the time stamp from one packet in the next packet.  That adds 
a delay in getting the data which can't be good but I don't know how bad it 
is.  I've been thinking that a system could measure that delay and add it to 
the time stamp.  No code yet.


-- 
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