Re: [Int-area] Discussion about Section 6.1 in draft-ietf-intarea-frag-fragile

Fernando Gont <> Mon, 09 September 2019 23:52 UTC

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To: Joe Touch <>, Bob Hinden <>
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From: Fernando Gont <>
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Date: Mon, 9 Sep 2019 23:46:30 +0300
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Subject: Re: [Int-area] Discussion about Section 6.1 in draft-ietf-intarea-frag-fragile
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Hi, Joe,

Just one nit:

On 7/9/19 20:35, Joe Touch wrote:
> FWIW, in general:
> With all the concern not detecting when frag fails, I’d like to point out that it’s equally impossible to detect when it works, e.g., when it happens on tunnels that start more than one hop away or more than one layer of intermediate headers.
> E.g, PLPMTUD turns of frag *on the connected interface*. There’s no way to disable source fragmentation that happens later in the network (as it would at tunnel ingresses) or deeper in the stack (when what you think is your interface is locally tunneled over a layer you don’t even know about).
> So *all* systems that try to backoff and use smaller MTUs are actually *already* testing whether fragmentation already works in those cases. Even if your app sends a 1-byte packet you have no idea that some set of layers inflates the headers (e.g., with signatures or key exchanges) beyond the MTU somewhere.

This would seem to be incorrect. IP has a minimum MTU of 68 bytes, and
IPv6 has a minimum MTU of 1280. Hence if you send packets smaller than
or equal to the minimum MTU, the packets should go through.

Fernando Gont
SI6 Networks
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