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

["Fischer, Anna" <anna.fischer@xxxxxx>]

> > 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?

Also, note that bulk data transfers between the Linux guest and any other 
*Linux* machine does work just fine as well. The problem is only when talking 
to Windows Vista and Windows 7 (I have not yet tried Windows XP) machines. So 
if this was a checksum problem, then communication with Linux machines should 
fail, too, as Linux also discards packets with invalid checksums.

Anna

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