[time-nuts] SLIP vs Ethernet for NTP

Tijd Dingen tijddingen at yahoo.com
Sun Oct 23 12:57:03 EDT 2011

> >The danger of Ethernet is that it has high capacity and a interconnect 
> >friendly interface. Thus, you might feel inclined to toss data over it 
> >carlessly causing packet delays and you can hook it into a switch and 
> >get delays and packet losses there.

> I usually cure that by using fiber-based ethernet, which leaves a lot
> less to plug it into.  Also firewall rules preventing anything but
> port 123 packets helps a lot.

Purely within the original timing related context:

If you hit the firewall, the damage is already done. Point being that if you have to traverse the ruleset (be that on a linux box with iptables or a dedicated piece of network hardware), then this is going to cause variations in delay. If you want to get the best out of ethernet for timing applications, I'd say a point-to-point link, and on the source side only generate the timing related ethernet frames.

Besides, the firewall has no added value, since you /obviously/ will use a dedicated ethernet link for this. "What?" you say, "I don't use a dedicated link for that!". Which is of course fine, but then you might as well not bother nitpicking about all the other details, since other (non-timing related) traffic will cause too much variation. And yes, you can do priority based routing, and no that's no good since it also leads to variable routing delays (on the device doing the priority routing).

For normal networking who cares (I don't), but if you attempt to get the best out of your ethernet link for timing stuff, I'd say a dedicated link is in order.


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