[openib-general] How about ib_send_page() ?

[Fab Tillier]

> From: Vivek Kashyap [mailto:kashyapv at us.ibm.com]
> Sent: Tuesday, May 24, 2005 9:57 AM
> 
> On Tue, 24 May 2005, Roland Dreier wrote:
> 
> >     Vivek> I should say it depends. One can utilise a setup that sets
> >     Vivek> the MTU to 2044 or whatever is the UD MTU on the subnet for
> >     Vivek> all modes. The connection will advertise the maximum
> >     Vivek> receive MTU as this value.
> >
> > OK, but that's throwing away the main advantage of connected mode
> > IPoIB, right?
> 
> I was addressing the case of a minimal implementation. For sucn an an
> initial implementation one can go as above. However, as I noted
> in the previous mail, if a large MTU is to be supported then one has to
> split the CM and UD MTUs since the UD MTU is limited to 2 or 4K.
> It is the limitation of the media that we get large MTU with connected
> mode
> whereas we get multicast (absolutely necessary for for address resolution)
> only with UD unless we want to do multicast emulation.

Ok, so this question is from a noob, but here goes anyway.  Why can't IPoIB
advertise a larger MTU than the UD MTU, and then just fragment large IP
packets up if they need to go over the IB UD transport?  Is there any reason
this couldn't work?  If it does, it allows IPoIB to expose a single MTU to
the OS, and take care of the rest under the covers.

Just a thought.

- Fab