Bug#726676: Debian bug #726676: linux-image-3.10-3-amd64: TCP packet loss when using proxy ARP IP addresses
Hi Andris, I've run across a bug that looks very like the one you
reported, but my analysis is rather different.
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=729567
As far as I can tell the problem I'm seeing is:
The LAN NIC on the machine running the OpenVPN server has the
"generic-receive-offload" (GRO) option set, so it combines TCP segments
coming from the LAN and destined for one of the OpenVPN clients into one
big segment.
However this segment is bigger than the MTU on the OpenVPN tunnel, so
when it gets routed out to the tunnel it gets split up into smaller
segments.
But the kernel seems to forget to calculate the TCP checksum for these
segments, so when they are received by the OpenVPN client they are
discarded, and have to be retransmitted one by one.
Do you have GRO set on the LAN interface of your OpenVPN server?
Does turning it off make your system work better?
--
John Hughes, Atlantic Technologies.
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