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

Joe Touch <touch@strayalpha.com> Wed, 11 September 2019 04:45 UTC

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From: Joe Touch <touch@strayalpha.com>
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Date: Tue, 10 Sep 2019 21:45:04 -0700
Cc: Fernando Gont <fgont@si6networks.com>, Bob Hinden <bob.hinden@gmail.com>, "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>
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To: "Templin (US), Fred L" <Fred.L.Templin@boeing.com>
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Subject: Re: [Int-area] Discussion about Section 6.1 in draft-ietf-intarea-frag-fragile
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+1

You have no  way of knowing how many tunnels are being traversed.

There is no packet size that *guarantees* you have avoided fragmentation somewhere along an Internet path.

Joe

> On Sep 10, 2019, at 6:29 AM, Templin (US), Fred L <Fred.L.Templin@boeing.com> wrote:
> 
> 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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