netdev - Re: ip6-in-ip{4,6} ipsec tunnel issues with 1280 MTU

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Message-ID: <b1df0d3a-701d-7b90-2d15-9ebdb017e811@gmail.com>
Date:   Sat, 28 Apr 2018 19:05:53 -0600
From:   David Ahern <dsahern@...il.com>
To:     Ashwanth Goli <ashwanth@...eaurora.org>
Cc:     Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org,
        maloney@...gle.com, edumazet@...gle.com,
        netdev-owner@...r.kernel.org
Subject: Re: ip6-in-ip{4,6} ipsec tunnel issues with 1280 MTU

On 4/27/18 9:44 AM, Ashwanth Goli wrote:
> On 2018-04-27 20:18, David Ahern wrote:
>> On 4/27/18 5:02 AM, Ashwanth Goli wrote:
>>> On 2018-04-26 17:21, Paolo Abeni wrote:
>>>> Hi,
>>>>
>>>> [fixed CC list]
>>>>
>>>> On Wed, 2018-04-25 at 21:43 +0530, Ashwanth Goli wrote:
>>>>> Hi Pablo,
>>>>
>>>> Actually I'm Paolo, but yours is a recurring mistake ;)
>>>>
>>>>> I am noticing an issue similar to the one reported by Alexis Perez
>>>>> [Regression for ip6-in-ip4 IPsec tunnel in 4.14.16]
>>>>>
>>>>> In my IPsec setup outer MTU is set to 1280, ip6_setup_cork sees an MTU
>>>>> less than IPV6_MIN_MTU because of the tunnel headers. -EINVAL is being
>>>>> returned as a result of the MTU check that got added with below patch.
>>
>> If you know you are running ipsec over the link why are you setting the
>> outer MTU to 1280? RFC 2460 suggests the fragmentation of packets for
>> links with MTU < 1280 should be done below the IPv6 layer:
>>
>> 5. Packet Size Issues
>>
>>    IPv6 requires that every link in the internet have an MTU of 1280
>>    octets or greater.  On any link that cannot convey a 1280-octet
>>    packet in one piece, link-specific fragmentation and reassembly must
>>    be provided at a layer below IPv6.
>>
>>    Links that have a configurable MTU (for example, PPP links [RFC-
>>    1661]) must be configured to have an MTU of at least 1280 octets; it
>>    is recommended that they be configured with an MTU of 1500 octets or
>>    greater, to accommodate possible encapsulations (i.e., tunneling)
>>    without incurring IPv6-layer fragmentation.
> 
> But is this not breaking point (b) from section 7.1 of RFC2473 since the
> inner packet can be smaller than 1280.
> 
> https://tools.ietf.org/html/rfc2473#section-7.1

I don't think so.

Given how Linux works with ipsec (or my understanding of it), your
proposed change seems ok to me.