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

Mikael Abrahamsson <> Sat, 25 August 2018 06:32 UTC

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Date: Sat, 25 Aug 2018 08:32:41 +0200 (CEST)
From: Mikael Abrahamsson <>
To: Toerless Eckert <>
cc: Tom Herbert <>, int-area <>,
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Subject: Re: [Int-area] WG Adoption Call: IP Fragmentation Considered Fragile
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On Sat, 25 Aug 2018, Toerless Eckert wrote:

> On Fri, Aug 03, 2018 at 09:48:25AM +0200, Mikael Abrahamsson wrote:
>> I've kept saying "Networks must support ip fragmentation properly.
> Why ? Wheren't you also saying that you've got (like probably many
> else on this thread) all the experience that only TCP MSS gets you
> working connectivity in many case (like hotels) ?

Correct. The reason for this is that whatever we design must be resilient 
to failure. Networks should work properly, but applications should 
handle it when they don't. Degraded performance is ok.

> IMHO, we (network layer) should accept defeat on network layer
> fragmentation and agree that we should make it easier for the
> transport layer to resolve the problem.

I want to keep the fragmentation requirement for the network.

> Aka: I would lvoe to see a new ICMPv4/ICMPv6 reply and/or PTB reply option
> indicating "Fragmented Packets Not Permitted". Any network device which
> for whatever reason does not like Fragemnts would simply drop
> fragmented packets and send this as a reply. Allows then the
> transport layer to automatically use packetization  (such as TCP MSS)
> to get packets through.

I am not opposed to this option being created, but you still need PLPMTUD. 
This option might trigger faster PLPMTUD, but it doesn't make the problem 
go away. If the application still keeps sending packets that needs to be 
fragmented, what should the stack do, just send an error to the 
application? Yes, this will mean we will fail faster, but apart from that?

Mikael Abrahamsson    email: