RE: Why is MTU an issue? (RE: Please read - proposed W G termination)

> From: Mark Townsley [mailto:[email protected]]
> Sent: Friday, September 02, 2005 11:12 PM

> Dror Goldenberg wrote:
> 
> >
> >
> > > From: Mark Townsley [mailto:[email protected]]
> > > Sent: Friday, September 02, 2005 10:43 PM
> >
> > > Dror Goldenberg wrote:
> > >
> > > >
> > > >
> > > > > From: Mark Townsley [mailto:[email protected]]
> > > > > Sent: Friday, September 02, 2005 4:21 PM
> > > > >
> > > > > Harald Tveit Alvestrand wrote:
> > > > >
> > >
> > > > I think that the right approach is to have a per-neighbor
> > > MTU in the
> > > > L2 interface. And furthermore, MTU can be different between
> > > multicast
> > > > and unicast. We're actually trying to see whether a per
> > > neighbor MTU
> > > > can be added to Linux.
> > > >
> > > On the face of it, "per-neighbor" MTU sounds like a
> > > management nightmare
> > > for anything but a handful of hosts.
> > >
> >
> > You just need to configure for each one of your hosts an MTU for 
> > ipoib-cm, or use the default one. When a connection is 
> made, ipoib-cm 
> > chooses MTU=min(local MTU, remote MTU) and you're done.
> >
> > Maybe you can elaborate more on why you think it's a nightmare ?
> >
> I probably (well, definitely) do not know enough about IB to 
> be making 
> such grand statements. But it seemed to me that if each host 
> may have a 
> different MTU, then all hosts which can talk to one another within a 
> given subnet would have to know the MTU of every other host. So, a 
> classic N^2 problem. Is IB natively point 2 point? Perhaps that would 
> ease my fears.
> 
> - Mark
> 
IB is a fabric based protocol. For each ipoib-cm there is a QP which is
point
to point. When you create this QP and connect it with the peer, you
negotiate
the MTU. Once you're done, you have a QP connected point to point with the
ipoib-cm
stack of your peer. Other peers are connected through other QPs, for which
the 
MTU decision is independent.

The MTU problem is  a n^2 problem but it is solved distributively... Each
host decides which MTU to use for each one of the connections to the peers
it has. In an N hosts fabric, each host may have (N-1) connections to peers.
The MTU selection is trivial, it's the minimum between my local MTU and my
peer MTU, negotiated at the time the connection is established.

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