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

"Templin (US), Fred L" <Fred.L.Templin@boeing.com> Tue, 10 September 2019 13:29 UTC

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From: "Templin (US), Fred L" <Fred.L.Templin@boeing.com>
To: Fernando Gont <fgont@si6networks.com>, Joe Touch <touch@strayalpha.com>, Bob Hinden <bob.hinden@gmail.com>
CC: "draft-ietf-intarea-frag-fragile@ietf.org" <draft-ietf-intarea-frag-fragile@ietf.org>, "int-area@ietf.org" <int-area@ietf.org>, IESG <iesg@ietf.org>, Suresh Krishnan <suresh@kaloom.com>
Thread-Topic: [Int-area] Discussion about Section 6.1 in draft-ietf-intarea-frag-fragile
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Date: Tue, 10 Sep 2019 13:29:23 +0000
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Subject: Re: [Int-area] Discussion about Section 6.1 in draft-ietf-intarea-frag-fragile
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Fernando,

> -----Original Message-----
> From: Int-area [mailto:int-area-bounces@ietf.org] On Behalf Of Fernando Gont
> Sent: Monday, September 09, 2019 1:47 PM
> To: Joe Touch <touch@strayalpha.com>om>; Bob Hinden <bob.hinden@gmail.com>
> Cc: draft-ietf-intarea-frag-fragile@ietf.org; int-area@ietf.org; IESG <iesg@ietf.org>rg>; Suresh Krishnan <suresh@kaloom.com>
> Subject: Re: [Int-area] Discussion about Section 6.1 in draft-ietf-intarea-frag-fragile
> 
> 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.

Even if the original source uses the IPv6 minimum MTU of 1280, a tunnel somewhere
further down the path could add encapsulations that would cause the (encapsulated)
packet to exceed 1280 bytes. The tunnel therefore has to apply fragmentation.

Fred

> --
> Fernando Gont
> SI6 Networks
> e-mail: fgont@si6networks.com
> PGP Fingerprint: 6666 31C6 D484 63B2 8FB1 E3C4 AE25 0D55 1D4E 7492
> 
> 
> 
> 
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