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

Joe Touch <touch@strayalpha.com> Mon, 30 July 2018 13:36 UTC

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From: Joe Touch <touch@strayalpha.com>
In-Reply-To: <alpine.DEB.2.20.1807300726500.14354@uplift.swm.pp.se>
Date: Mon, 30 Jul 2018 06:36:16 -0700
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To: Mikael Abrahamsson <swmike@swm.pp.se>
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Subject: Re: [Int-area] WG Adoption Call: IP Fragmentation Considered Fragile
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> On Jul 29, 2018, at 10:29 PM, Mikael Abrahamsson <swmike@swm.pp.se> wrote:
> 
> On Sun, 29 Jul 2018, Joe Touch wrote:
> 
>> You’re engaging in a game of escalation - whatever layer you add fragmentation will end up being a layer that a vendor puts a device that does DPI that fails.
> 
> Yes, but I can filter those UDP packets by looking in the UDP header, that's all the DPI I need in that box. It doesn't need to understand the upper-protocol level fragmentation, because I do not require it to understand that protocol at all. I just need for it to understand that it's UDP and look at the UDP port number.

Right. You need just UDP ports right now for YOUR DPI.

Others need to look at the payload (the D in DPI).

> 
> The biggest mistake of TCP and UDP combined with IP level fragmentation is that the port information isn't available in every packet.

The biggest mistake of protocol X with X-1 level fragmentation is that the entire headers of X aren’t available in every X-1 packet.

Replace X with your favorite protocol and you’ll see how and why this can’t continue to work. The packets would eventually burst with all the headers.

Joe