openssh-based file transfers (e.g. rsync, scp, ...) are running 40 (!!) times faster via IPv4 than IPv6

[Philipp Marek]

> first post to list, hopefully on-topic. Haven't found anything on the
> net, tried to ask at first in OpenSuse forums a
> while ago
> (https://forums.opensuse.org/showthread.php/533588-rsnapshot-rsync-massive-performance-decrease)
> and today
> opened a bug in OpenSuse's Bugzilla
> (https://bugzilla.opensuse.org/show_bug.cgi?id=1115075)
> 
> As written in bug report, as general network issues could be excluded,
> any hints where to start, how to narrow down if
> this is a specific openssh issue or something else?

Your IPv6 _route_ has 3 times the latency:

   $ time /usr/sbin/traceroute vserver.domain.tld
   ...
    7  domain.tld (IPv4)  26.002 ms !X  24.859 ms !X  25.230 ms !X

   $ time /usr/sbin/traceroute6 vserver.domain.tld
   ...
    8  domain.tld (IPv6)  80.430 ms !X  79.301 ms !X  80.444 ms !X

The big time waster seems to be this link:
    6  nug-d-i40-v6.telia.net (2001:2000:3018:8d::1)  32.046 ms  23.850 
ms  24.097 ms
    7  contabo-ic-305268-ffm-b11.c.telia.net (2001:2000:3080:953::2)  
74.829 ms  77.704 ms  78.066 ms

I'll now speculate that you might as well have some MTU mismatches,
meaning that packages have to get split up along the way, further
slowing down your connection.

Do you have native IPv6 all the way, or is there an 6-in-4 tunnel
inbetween (from the fritzbox on)?

Please try
    # ping -M do -s <size> <ip>
for sizes between 1300 and 1500 and find the largest (even) packet size
that still works, and then do the same with ping6 as well.

Also note whether some packetsize works, but just larger ones vanish
along the way.