Re: [Int-area] WG Adoption Call: IP Fragmentation Considered Fragile

Fernando Gont <fernando@gont.com.ar> Fri, 27 July 2018 21:01 UTC

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To: Ole Troan <otroan@employees.org>, Brian E Carpenter <brian.e.carpenter@gmail.com>
Cc: Tom Herbert <tom@herbertland.com>, "internet-area@ietf.org" <int-area@ietf.org>, "intarea-chairs@ietf.org" <intarea-chairs@ietf.org>
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From: Fernando Gont <fernando@gont.com.ar>
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Date: Fri, 27 Jul 2018 22:51:40 +0200
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Subject: Re: [Int-area] WG Adoption Call: IP Fragmentation Considered Fragile
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On 07/27/2018 10:28 PM, Ole Troan wrote:
> 
> 
>> On 27 Jul 2018, at 22:12, Brian E Carpenter <brian.e.carpenter@gmail.com> wrote:
>>
>> Fragmentation, (PL)PMTUD, extension headers, and innovative
>> L4 protocols are very possibly not viable on the open Internet.
>> At least, we can't assume that they will work on arbitrary paths.
>> Sad but apparently true.
> 
> Hasn’t this been discussed ad infinitum in the ossification work?
> If you want to generalize, nothing is guaranteed to work across an arbitrary path in the Internet. 
> 
> So what? This is part of a tussle and it would be making a self fulfilling prophecy for us to take all policy based filtering or other brokenness into consideration when designing protocols. 

I see your point. However, how do you engineer something that "works" if
you ignore all brokenness etc.?

We do normally engineer protocols taking this things into account. e.g.

* PLPMTUD is a response to ICMP filtering
* ECN had a backup mechanism that would switch to non-ECN for cases
where e.g. firewalls were complaining about previously-unspecified bits
* Quic is most likely implemented over UDP to be able to survive NATs
and firewalls


Yes, and one hand is not nice to have to account for all types of
brokenness and filtering. OTOH, it would make any sense to enigneer a
protocol that only works on paper.


-- 
Fernando Gont
e-mail: fernando@gont.com.ar || fgont@si6networks.com
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