On Wednesday, November 3, 2021 at 9:34:28 AM UTC-7, Dan Cross wrote:
(snip)
> Sadly, that's not the issue. Even if it were, I would expect to
> see _some_ TCP traffic exchanged; at the minimum, an RST segment
> when the connection was terminated by one side or the other. That
> it doesn't happen when I login to the same TOPS-20 host from a
> machine running Linux on the same network would further suggest
> that this isn't the case.
I only sort-of know how this works, as most often it does.
https://en.wikipedia.org/wiki/Keepalive#TCP_keepalive
Well, one not so unusual one partly indicated by other replies,
is from NAT routers, that have to keep NAT connections open.
But even without that, there is a system for keeping TCP connections
open without any traffic, such as an idle telnet, rsh, or ssh connection.
It would, then, be a lack of keep-alive packets that close the connection.
I do remember, many years ago now, having a problem keeping
connections open, where I had a background program on the
remote end that would periodically send a null character.
I suspect that was related to NAT, as it was close to the beginning
of the need to use NAT.
You might figure out which end closes the connection,
(and goes away) first, to figure out which end to debug.
I haven't thought of the actual keep-alive intervals, though.
I do know that the remote end goes away fast enough after
my Macbook sleeps, that I don't worry about chasing them down.