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

From: "H.K. Jerry Chu" <[email-masked]>
Date: Fri, 2 Sep 2005 13:44:33 -0700 (PDT)

>I think that there is an even more fundamental issue than ICMPs being
>filtered. ICMPs for "Fragmentation Needed and DF Set" are generated by
>routers. So, for local subnet traffic, RFC 1191 (PMTU Discovery) won't work
>to my understanding. PMTU discovery is a layer 3 protocol, not layer 2.
>For the specific case of ipoib-cm with local subnet traffic. A local "huge"
>frame will be handed over to the layer 2 IB interface for transmission. If
>it doesn't manage to send that large MTU because there isn't such a large
>MTU between the local node and its peer, then the frame is silenty discarded
>by the TRANSMITTER. One way to go now is to generate a local ICMP message
>saying that this was a too large frame. But I don't think it's the right way
>to go.
>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.

Thanks for clarifying this for me. Per-neighbor MTU is a much better term
than my "per-destination" MTU, which caused some confusion.

I haven't followed the PMTUD WG but its new proposal might be wothwhile
looking too. (The drawback could be it takes time to learn the optimal
PMTU...)

Jerry

>-Dror

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