[HECnet] Multinet Tunnel Connections to SG1::

[Johnny Billquist]

On 2012-06-07 09:33, Dave McGuire wrote:
On 06/07/2012 03:28 AM, Johnny Billquist wrote:
    SIMH uses libpcap and implements networking for emulated machines by
kicking the interface into promiscuous mode.   That's a bit dangerous
from a traffic perspective; I'm not certain that'd be a good idea for
anything other than the most casual use on a quiet network.

Dangerous in which way?

    Traffic.   My main desktop machine sits on a busy network, and I am
uncomfortable with the notion of it sitting in promiscuous mode.
Remember that I also do all of my work (meaning "make money for food"
work) on this system.

Ok. Good. We're on the same page about where the potential problem lies then.

It will create a larger load on the system, but that's about it. And
todays machines are fast enough that you really need a lot of traffic
before it will become a serious problem from that point of view.
Remember that most of the filtering out of unwanted packets are done in
the kernel anyway, and not in user space. That's what the bpf interface
and filter programs do for you.

    Yes I understand, but while not in promiscuous mode, the filtering is
done by the network interface chip, rather than by one of my cores which
are typically pretty busy doing other things.   I have a very fast
machine on my desk, but that doesn't mean I want to waste the cycles...I
use those cycles for lots of stuff. ;)

Your machine is most likely wasting lots of cycles. You probably don't even want to know... ;-)
Networks are slow, compared to CPUs.
A friend of mine wrote his PhD about the concept that you basically can increase network bandwidth by just compressing data you want to send as much as humanly possible for this simple reason. The CPU will wait on data anyway, so why not use that time to compress the data to send, which means you get more data through on the same network bandwidth.

The main reason in the past for changing the MAC address has been that
you want to control the source MAC address. However, most systems now
allows you to spoof the source MAC when outputting packets on the
ethernet, so that problem is solved.

    Well, and to receive those packets!

Right.

Load is the one remaining reason to even worry, and that is a rather
small worry for most people.

    I agree, but it is a bigger concern for me as above.   There is also a
"purist" aspect...I also eschew things like "winprinters" (if you
remember those!)...my main CPU cores have other work to be doing.