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RE: [Xen-users] Slow TCP performance between Windows Vista and XenPV-on-HVM guest

> Subject: RE: [Xen-users] Slow TCP performance between Windows Vista and
> XenPV-on-HVM guest
> >
> > I am running a Xen HVM guest with netfront PV drivers. This is running
> SLES10
> > SP3 inside the guest. The Dom0 is also SLES10 SP3.
> >
> > Now I am trying to communicate from that HVM guest to a Windows Visa
> or also
> > Windows 7 machine and I am getting really poor TCP performance. When
> tracing
> > on the network traffic, I can see that no packets are dropped or
> missing or
> > anything, but what happens is that the Vista machine does not seem to
> send an
> > ACK back to the Xen guest, and then the Xen guest is doing a
> retransmit after
> > a while.
> Are you capturing packets on the windows machine or on the Dom0?

Dom0. Note that the Windows machine doesn't even run Xen or anything, it is 
just some random machine on the network. Only the Linux guest runs on Xen.

> If you are using tcpdump on dom0, make sure you use '-s0' so that you
> capture the entire packup, and possibly '-v' as well. Without capturing
> the entire packet, tcpdump can't tell you if the checksum is correct or
> not. Even if the checksum is incorrect on Dom0 it doesn't necessarily
> tell you that there is a problem though. A bad checksum on received
> packets on the windows machine would definitely suggest a problem
> though.

I capture with Ethereal. I definitely catch all packet. If this was a checksum 
problem, then communication wouldn't work at all. However, SSH and other 
(slower) connections work just fine. The problem is only on bulk data transfer 
using TCP. If the Linux guest was sending a packet with an invalid checksum, 
then the Windows guest would *never* send out the ACK. However, it is actually 
sending out the ACK, but only after the retransmit, to ACK the *retransmitted* 
packet. If this was a checksum problem, then the retransmitted packet would 
also have an invalid checksum and so it would basically never be ACKed. 

I have read about Vista's TCP "auto-tuning" feature, and I wonder if something 
like this might be the problem here that the Xen guest cannot cope with?


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