[Wylug-discuss] Using vi over SSH over VPN over 3G connection.

[John Hodrien]

On Thu, 10 Mar 2011, Paul Brook wrote:

> My guess is this is an MTU issue.  Typical symptoms are that initially tings
> work fine, but TCP connections block once large blocks of data are
> transferred.  Ethernet usually has an MTU of 1500 bytes. The average terminal
> displays about 2000 characters, so vim "instantly" printing a screenful of
> data on startup is probably sufficient to trigger this.
>
> In theory the TCP connection should automatically accommodate network segments
> (e.g. VPN tunnels) with a lower MTU[1]. However it's common for firewalls to
> interfere with this process by blocking ICMP messages.
>
> Try manually reducing the MTU on the 3G and/or VPN interfaces. If this
> confirms the above diagnosis then you have a few options: Beat the
> router/firewall with a clue stick until PMTU discovery works, have linux hack
> round the problem for you by enabling TCP MSS clamping, or live with the
> manual MTU hacks.

That was exactly my suspicion too, hence why I asked about scp.  You often see
a typical ssh connection work fine, but when you scp a big file you're
guaranteed to use big packets that cause it to stall.  I've seen this with
Talk Talk who run with a lower than normal MTU.

So despite not saying that with my initialy post, my thinking tallies exactly
with this.

jh