[ofa-general] Re: [PATCHv2] IB/ipoib: S/G and HW checksum support

[James Lentini]

On Tue, 4 Sep 2007, Michael S. Tsirkin wrote:

> > Quoting James Lentini <jlentini at netapp.com>:
> > Subject: Re: [PATCHv2] IB/ipoib: S/G and HW checksum support
> > 
> > 
> > 
> > On Tue, 4 Sep 2007, Michael S. Tsirkin wrote:
> > 
> > > > Quoting James Lentini <jlentini at netapp.com>:
> > > > Subject: Re: [PATCHv2] IB/ipoib: S/G and HW checksum support
> > > > 
> > > > 
> > > > 
> > > > On Tue, 4 Sep 2007, Jason Gunthorpe wrote:
> > > > 
> > > > > On Tue, Sep 04, 2007 at 12:11:33PM +0300, Michael S. Tsirkin wrote:
> > > > > 
> > > > > > I know some people find this approach controversial,
> > > > > > but from my perspective, this is not worse than e.g.
> > > > > > SDP which does not have SW checksums pretty much by design.
> > > > > 
> > > > > This would be alot better in my mind of the option was negotiated as
> > > > > part of the CM setup process. Otherwise this becomes a network wide
> > > > > all or nothing kind of feature..
> > > > > 
> > > > > What if the RXing Linux IB side is acting as a forwarder to ethernet?
> > > > > It will forward corrupt packets if this option is set, right?
> > > > 
> > > > So this break all gateway devices?
> > > 
> > > It won't. The gateway will calculate the checksums.
> > > 
> > > > How would packets be routed with this change?
> > > 
> > > As usual.
> > 
> > A Linux system setup as a router with an IPoIB interface and an 
> > Ethernet interface will work if this feature is turned on?
> 
> I am yet to test this setup, but yes, it should.

I has this scenario in mind: 

A ------- B ------- C
   IPoIB      Eth

A and C are Linux hosts, B is a Linux host setup as a router.
 
If the link between A and B has this checksum change turned on, then 
then TCP connections between A anc C will fail with TCP checksum 
errors. 

Technically an IPoIB network with these changes can route IP packets 
to other networks, but with the missing transport layer checksums 
the contents are unintelligible.