Subject: Re: IPV6_USE_MIN_MTU and IPV6_DONT_FRAG questions

From: Karl Auer <[email-masked]>
Date: Wed, 07 Nov 2012 14:33:10 +1100

On Wed, 2012-11-07 at 12:30 +1100, Mark Andrews wrote:
> Should setting IPV6_USE_MIN_MTU to one (1) result in fragmented TCP packets?
> Should setting IPV6_DONTFRAG to one (1) work on TCP sockets?
> [...]
> Both options were set on the socket with the resulting packet trace.

Speaking from a position of ignorance here, but my understanding is that
if you use the first, the second is ignored, which is what seems to be
happening in that trace.

I also believe that the second does not apply to TCP.

I'd expect that if you use the first then PMTUD should be disabled and
the stack will fragment larger packets normally and send atomic
fragments if the whole thing (plus fragment header) fits in a single
fragment. If the implementation doesn't believe in atomic fragments then
it will fragment only packets too large to fit in 1280 bytes and will
send smaller ones without a fragment header.

That said, I reiterate my ignorance.

Regards, K.

-- 
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From: Mark Andrews <[email-masked]>
Date: Wed, 07 Nov 2012 16:55:34 +1100

> send smaller ones without a fragment header.

I'd expect the TCP layer to look at the IP layer MTU and adjust its
segment size to match.

-- 
Mark Andrews, ISC
1 Seymour St., Dundas Valley, NSW 2117, Australia
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