MAC addresses, hostnames, and DHCP

[Ben Scott]

On 12/2/05, Bill McGonigle <bill at bfccomputing.com> wrote:
> >   However, there are some caveats.  If any traffic occurs (payload, or
> > SSH control traffic like rekeying, keep-alive, etc.), TCP (or higher
> > level protocols) may time out before the IP layer finishes coming back
> > up.  This is especially likely if you're using SSH to tunnel X11.
>
> There seems to be a two-minute window where you can get away with this.
>  TCP sees it as packet loss.  I forget why 120 seconds is a magic
> number - something either in the spec/defaults for TCP packet retries.

  2 minutes is the default TCP... MSL (maximum segment life) or RTT
(re-transmission timeout).  I can't remember which, but I rember that
RTT = 2 * MSL.  IIRC, the 2 minute figure was somewhat arbitrary, but
based on some accurate real-world experience, and was noted somewhere
else as meaning "FTP to Mars" would not be possible, but noted
somewhere else as changeable, so FTP to Mars would be possible, or
something.  My memory is fuzzy.  :)

  Of course, that only comes into play if TCP is trying to transmit
data -- hence the "idle" part.  If data isn't flowing, then you can
unplug the cable for hours and nothing will notice.  Other protocols
often have their own timeouts as well (such as the SSH keep-alives,
which become "make-deads" in that case).

-- Ben