[Simh] Cluster communications errors

[Paul Koning]

> On Jul 18, 2018, at 7:18 PM, Johnny Billquist <bqt at softjar.se> wrote:
> 
>> ...
> 
> It's probably worth pointing out that the reason I implemented that was not because of hardware problems, but because of software problems. DECnet can degenerate pretty badly when packets are lost. And if you shove packets fast enough at the interface, the interface will (obviously) eventually run out of buffers, at which point packets will be dropped.
> This is especially noticeable in DECnet/RSX at least. I think I know how to improve that software, but I have not had enough time to actually try fixing it. And it is especially noticeable when doing file transfers over DECnet.

All ARQ protocols suffer dramatically with packet loss.  The other day I was reading a recent paper about high speed long distance TCP.  It showed a graph of throughput vs. packet loss rate.  I forgot the exact numbers, but it was something like 0.01% packet loss rate causes a 90% throughput drop.  Compare that with the old (1970s) ARPAnet rule of thumb that 1% packet loss means 90% loss of throughput.  Those both make sense; the old one was for "high speed" links running at 56 kbps, rather than the multi-Gbps of current links.

The other thing with nontrivial packet loss is that any protocol with congestion control algorithms triggered by packet loss (such as recent versions of DECnet), the flow control machinery will severely throttle the link under such conditions.

So yes, anything you can do in the infrastructure to keep the packet loss well under 1% is going to be very helpful indeed.

	paul