Re: [win-pv-devel] Hi, a question about the checksum offload and WinDivert

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: win-pv-devel [mailto:win-pv-devel-bounces@xxxxxxxxxxxxxxxxxxxx] On
> Behalf Of Haohao Lee
> Sent: 06 November 2018 16:01
> To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>
> Cc: win-pv-devel@xxxxxxxxxxxxxxxxxxxx
> Subject: Re: [win-pv-devel] Hi, a question about the checksum offload and
> WinDivert
> 
> On Tue, Nov 6, 2018 at 7:06 PM Paul Durrant <Paul.Durrant@xxxxxxxxxx>
> wrote:
> >
> > De-htmling...
> >
> > My responses indented:
> >
> > ---
> > From: win-pv-devel [mailto:win-pv-devel-bounces@xxxxxxxxxxxxxxxxxxxx] On
> Behalf Of Haohao Lee
> > Sent: 06 November 2018 08:34
> > To: win-pv-devel@xxxxxxxxxxxxxxxxxxxx
> > Subject: [win-pv-devel] Hi, a question about the checksum offload and
> WinDivert
> >
> > Hi Xen folks,
> >
> > Background:
> > I am a Windows application developer. We developed an application which
> modifies packets and rejects them back into the network stack to do some
> network proxy transparently.
> >
> > We achieved this by using WinDivert (https://reqrypt.org/windivert.html)
> that is a tool/driver allows user-mode applications to capture/modify/drop
> network packets sent to/from the Windows network stack.
> >
> > Our app worked well on physical Windows machines but on Xen virtual
> machines we encountered a problem.
> >
> > Problem:
> > - Everything works well before we start our app.
> > - Network traffic is blocked after the app is started, even a single SYN
> packet couldn't be sent out.
> >
> > Workaround:
> > If we disable the checksum offload in Xen Net Driver, everything starts
> to work again.
> >
> > > Hi Hao,
> > >
> > > Which checksum offload? Just TCP or IPv4 too?
> > > > The driver property does not mention IP or TCP, just checksum
> offload (But our problem only occurs for TCP)
> > > > Here I have a screenshot https://user-
> images.githubusercontent.com/238419/47770931-46b6ad80-dd1c-11e8-8010-
> 42d05baf018b.png

Ok, that tells me you are not using Xen Project PV drivers. Where did you get 
your PV drivers from?

  Paul

> >
> > Testing Environment:
> > Xen Virtual Machine: Windows 7 Sp1 x64 with latest updates
> > Xen Net Driver: Driver version 3.0.144.590 xennet.sys version 2.3.0.144
> >
> > I have a couple of questions:
> > 1. Is this a problem of WinDivert driver or Xen Net driver from your
> perspective?
> >
> > > Since the stable (i.e. 8.x) drivers pass all logo tests (which have
> detailed checks of the semantics of checksum offload, LSO, etc.) the I'd
> say the problem lies in the application. One thing to try is disabling LRO
> though.. this is disabled for logo testing since the version of NDIS we
> use doesn't actually support it. (Moving to a newer NDIS is on the TODO
> list).
> > > > New findings, If I disable checksum offload, the average throughput
> goes up from 600Mbit/s to 1Gbit/s as well. Therefore now I suspect there
> may be something wrong with this option ON.
> >
> > 2. If this belongs to Xen Net driver, does the latest driver fix this?
> > 3. I found many articles on the Internet which teach people to disable
> checksum offload (and other kinds of offload) for Xen virtual machines,
> e.g. some tutorials from AWS. Why is this option ON by default if it
> shouldn't be, or is there any introduction about the context why it is ON
> by default? I know what TCP/IP checksum is, but in virtual machine
> context, I have no idea if it is necessary or not.
> >
> > Any comment or suggestion is appreciated.
> >
> > > The answer is "it's complicated" :-) The default set of offloads is
> the set we use in Citrix branded versions of the drivers for XenServer
> and, whilst there used to be many issues with such offloads in the past
> (pre Xenserver 7.x), we have not had *any* reports from the field to
> suggest there are any current issues with checksum or large packet
> offloads in the 8.x drivers.
> > > > By the way, it seems I cannot install 8.x Xen Net Driver directly.
> The OS keeps telling me my old driver is the best one.
> > >
> > > Cheers,
> > >
> > > Paul
> >
> > Thanks
> >
> > Hao
> 
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