[Openib-windows] IPoIB GUID to MAC conversion

[Dror Goldenberg]

> -----Original Message-----
> From: Fab Tillier [mailto:ftillier at silverstorm.com] 
> Sent: Thursday, May 26, 2005 10:43 PM

> > > > 4) Use the 48 bits to contain the path information. A non
> > > IB routable
> > > >      approach will use the bits as follows: 16 LID, 4 
> SL, 24 QPN,
> > > >      2 static rate (might be not enough to hold all static
> > > rate options)
> > > >      2 reserved (local/global, unicast/mcast) - total 48 bits.
> > >
> > > This is interesting, as it would allow creating static 
> ARP entries.  
> > > Does it break the ability of IPoIB routers to route 
> traffic from IB 
> > > to Ethernet?
> >
> > No. This approach is IP routable (IPoIB to IPoEther). It is not IB 
> > routable because we won't be able to add a GID when sending packets.
> 
> Actually, does it prevent IB global routing?  These generated 
> MAC addresses are only used to interface with the OS.  On the 
> wire, the MAC is always the 20-byte IPoIB MAC.
> 

It is not IB routable because you lose the routing (GID) information.

Imagine 2 nodes behind an IB router talking to you. And let's assume that
they use same QP number (because the drivers there are identical and they
just came out of reset). They will end up having the same MAC address in
your code too: same QPN, same LID/SL (the IB router LID/SL). You just
lost the fact that they are behind an IB  router and that they need to use
a GRH to be able to talk.
When the OS hands you a packet for transmission, you can't really tell
to whom to send it to... you need to be able to guess the GID.
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