[ntpwg] Documents, slides, etc. from WG meeting

[Martin Burnicki]

Dave,

David L. Mills wrote:
> Guys,
>
> The 1588 provisions for switches and routers is called a boundary clock.
> It requires intricate management of various fixed latencies and queueing
> delays. 1588 NICs known to me use the Intel chipset which has an onboard
> ARM processor very definitely provisioned by firmware. As for modifying
> NTP to simulate the 1588 two-step protocol, a description on how to do
> this is in the architecture briefing on the NTP project page.

The boundary clock is just _one_ possible way to eliminate the latency of 
intermediate nodes between the grandmaster and the final client. 

In the NTP world this is comparable to an NTP server running on a company's 
firewall or router, which gets its time from the outside world and serves it 
to the internal network.

Since each boundary clock includes a control loop, a number of cascaded 
boundary clocks results in cascaded control loops, maybe with unknown 
characteristics, which may let the overall control loop between the final 
client and the grandmaster become instable.

So another approach is the transparent clock, which just adds to every packet 
how much the packet has been delayed when it has traversed an intermediate 
node. So even if there are several intermediate nodes, there's only on big 
control loop which includes the grandmaster and the final client, which can 
possibly easier be made stable than a number of cascaded loops.

For existing NTP installations the similar configuration is when clients 
behind a firewall directly send queries to a server in the outer world, and 
the queries and replies just go through the firewall, though of course a  
firewall or router would not try to compensate any latency it adds to the NTP 
packets.

The advantages and disadvantages of boundary clocks and transparent clocks for 
this purpose have been discussed on several PTP plug fests and meetings.

Martin