[Xen-users] TCP segmentation issue

[Daniel Gehberger <daniel.gehberger@xxxxxxxxxxxx>]

Hello,

I am facing a segmentation issue in xen and I hope that I can get some 
advices here.
There are two physical computers with ubuntu 14.04 installed. One of 
them is running xen 4.4 also with ubuntu 14.04 guests. I did not change 
any network offloading, based on the parameters I would expect packets 
up to 64k on the interfaces (only LRO is disabled).
When I was testing with iperf (TCP), I can see these large packets on 
all the way during the communication when I am performing it between 2 
guests, from a guest to the other computer, even from the other computer 
to the dom0.
The only case which shows strange behaviour is when I am running the 
iperf server on a guest and the client on the other physical computer. 
In this case in can see the large packets on the physical interfaces but 
on the vif interface I can see that most of the packets are ~MTU sized 
(1448). Actually it seems that if the packet is larger than ~25k on the 
physical side then it is segmented into MTU sized packets (by the 
netback?). With iperf it is actually almost always true without changing 
the buffer size of the client.
I checked that the same happens with linux bridge and open vSwitch. I 
also updated the kernel of dom0 and the guest up to 3.15 without any luck.
Thank You in advance!
Daniel

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