Jesse Gross <je...@kernel.org> writes: > On Wed, Feb 10, 2016 at 3:21 PM, Tom Herbert <t...@herbertland.com> wrote: >> On Wed, Feb 10, 2016 at 12:59 PM, Jesse Gross <je...@kernel.org> wrote: >>> On Wed, Feb 10, 2016 at 12:41 PM, David Wragg <david@weave.works> wrote: >>>> Tom Herbert <t...@herbertland.com> writes: >>>>> The correct thing to do is determine the maximum amount of >>>>> encapsulation overhead that can ever be set in a packet and use for >>>>> setting the MTU. For instance, when RCO is enable in GUE, the size of >>>>> the option is included in tunnel->encap_hlen even though it will not >>>>> be used in all packets (via ip_tunnel_change_mtu). If there is no way >>>>> to determine a maximum overhead a priori from configuration, then >>>>> maximum overhead could be assumed to be maximum possible encapsulation >>>>> header size which for Geneve is 132 bytes IIRC. >>>> >>>> Ok, I'll come up with a patch to address this. >>> >>> I don't think that this really applies in this situation. The concerns >>> here relate to what the MTU is actually set to but this patch affects >>> the range of MTUs allowed to be set by the user. I don't see a reason >>> to disallow the user from setting a precise value if they know what it >>> should be. >>> >> Right, but if the user sets a bad value and packets are silently >> dropped on the floor then that seems like a bad result that could have >> easily been prevented. > > Sure, we might as well prevent the extreme edge cases that can never > be valid. In the case of Geneve though, this would be the minimum > header size, not the maximum, since it's possible that the user > actually knows how big the options are that they want to use. > > But as I said, the practical impact is low because IP_MAX_MTU is so > much larger than the MTU of real devices. There will always be many > values that the MTU can be set to that result in dropped packets. This > is true of all tunnel types.
I agree with Jesse, and if there was a debate about lifting the MTU limit on all tunnel types to IP_MAX_MTU, I'd be for that. But for the other tunnel types, I followed the precedent of the code that was already there, and geneve might as well be consistent. Patch sent to the lists. David
