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

At 01:26 PM 9/2/2005, Dror Goldenberg wrote:


> > From: Mark Townsley [<mailto:[email protected]>mailto:[email protected]]
> > Sent: Friday, September 02, 2005 11:12 PM
>
> > Dror Goldenberg wrote:
> >
> > >
> > >
> > > > From: Mark Townsley 
> [<mailto:[email protected]>mailto:[email protected]]
> > > > Sent: Friday, September 02, 2005 10:43 PM
> > >
> > > > Dror Goldenberg wrote:
> > > >
> > > > >
> > > > >
> > > > > > From: Mark Townsley 
> [<mailto:[email protected]>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.

It should also be noted that one would not be limited to only one 
connection pair since from a technology perspective, there is no reason why 
multiples cannot be used for QoS purposes / bandwidth management 
purposes.  The same applies to the UD method though few people have 
implemented it as such.

Mike

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