Re: [PATCH net-next] ipv6: RTAX_FEATURE_ALLFRAG causes inefficient TCP segment sizing

["Maciej Żenczykowski" <maze@google.com>]

>> (*) Would it be legal for a tunnel endpoint to support ipv6 packets up
>> to 1280 bytes in size
>> but still send back a 'packet to big please use 1K mtu' message?
>
>
> I don't think this is a valid thing to do - either the tunnel server would
> forward the packet through the tunnel (fragmenting the underlaying IPv4 if
> necessary), and *not* send back a ICMPv6 PTB error at the same time, OR: it
> would need to drop the packet and *do* send back the ICMPv6 PTB. Not both
> at the same time.
>
> However, if it is going to drop the large packets and reply with the PTB, it
> will cause a blackhole with current Linux, because if it get the PTB with
> MTU=1000, it will include the frag header, but *not* reduce the actual
> packet
> size. And since this is pure IPv6 routing, the tunnel server will not be
> able
> to fragment the IPv6 packets, since IPv6 routers don't do that (the presense
> of the Fragmentation header does not change this fact).
>
> So the tunnel routers pretty much *must* support an IPv6 MTU of 1280, either
> by ensuring the outer IPv4 MTU is 1300 or larger, or by performing IPv4
> fragmentation and reassembly "under the hood".

I think you could still accept 1280 mtu packets, but for packets >= 1281 mtu
you could return packet to big, please use mtu=1000.

>
> I don't think it is worth while (or even appropriate) for the Linux IPv6
> stack
> to try to optimize for this.

Agreed.  Let's ignore this.