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

Mikael Abrahamsson <swmike@swm.pp.se> Sat, 28 July 2018 06:13 UTC

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Date: Sat, 28 Jul 2018 08:13:52 +0200 (CEST)
From: Mikael Abrahamsson <swmike@swm.pp.se>
To: Tom Herbert <tom@herbertland.com>
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Subject: Re: [Int-area] WG Adoption Call: IP Fragmentation Considered Fragile
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On Fri, 27 Jul 2018, Tom Herbert wrote:

> I don't think that can be called a general approach. Setting MTU to 1400 
> bytes only avoids fragmentation if all the tunnel headers being inserted 
> are less than 100 bytes. If you start using more tunnel options or the 
> tunnel ingress inserts extension headers then that 100 byte budget may 
> be exceeded may be exceeded. So a requirement to avoid fragmentation in 
> this manner would have to be that the MTU needs to be low enough to 
> account for all possible encapsulation overhead that may be applied to a 
> packet-- the emerging use of in-situ OAM, segment routing, and even just 
> switching from IPv4 to IPv6 for the underlay puts downward pressure on 
> MTUs. Lowering the MTU increase ratio of overhead to user data thus 
> making communications less efficient.

A PLPMTUD mechanism for the tunnel as a complement to regular PMTUD would 
be enough. OpenVPN doesn't have this, if it did, I would use it.

> Fragmentation for tunneling is a special case since tunnels are often 
> used within a controlled network and precisley two fragments are always 
> generated. I know of at least one very large data center that relies on 
> fragmentation for tunneling. It seems to work fine in such an 
> environment and is preferable to lowering the MTU for everyone (even 
> non-tunnels) or turning on jumbo frames (complex to do at large scale).

I run my tunnel over the Internet, using multiple ISPs. I prefer to avoid 
using IP fragments.

-- 
Mikael Abrahamsson    email: swmike@swm.pp.se