linux-kernel - Re: [PATCH] IPv6: Allow the MTU of ipip6 tunnel to be set below 1280

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Message-ID: <CABfLueHOR1HNsRC_-+Phc=9LMPTiOVuWoEjguG64L=9hiZLeVg@mail.gmail.com>
Date:	Sun, 29 Sep 2013 17:33:37 +0100
From:	Oussama Ghorbel <ou.ghorbel@...il.com>
To:	Oussama Ghorbel <oghorbell@...il.com>,
	"David S. Miller" <davem@...emloft.net>,
	Alexey Kuznetsov <kuznet@....inr.ac.ru>,
	James Morris <jmorris@...ei.org>,
	Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
	Patrick McHardy <kaber@...sh.net>, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Oussama Ghorbel <ou.ghorbel@...il.com>
Subject: Re: [PATCH] IPv6: Allow the MTU of ipip6 tunnel to be set below 1280

On Sun, Sep 29, 2013 at 4:45 PM, Hannes Frederic Sowa
<hannes@...essinduktion.org> wrote:
> On Sun, Sep 29, 2013 at 10:40:11AM +0100, Oussama Ghorbel wrote:
>> On Fri, Sep 27, 2013 at 6:03 PM, Hannes Frederic Sowa
>> <hannes@...essinduktion.org> wrote:
>> > Ok, let's go with one function per protocol type. Seems easier.
>> >
>> > It seems to get more hairy, because it depends on the tunnel driver if the
>> > prepended ip header is accounted in hard_header_len. :/
>> >
>> > I don't know if it works out cleanly. Otherwise I would be ok if the checks
>> > just get repeated in ip6_tunnel and leave the rest as-is.
>> >
>> Yes, It will be the clean way to do it.
>
> Fine. :)
>
>> >
>> > Linux currently cannot create "jumbograms" (only the receiving side
>> > is supported).
>> >
>> I understand, but what are the benefit from this limit or the harm
>> from not specifying it?
>> Please check this comment from eth.c
>>
>> /**
>>  * eth_change_mtu - set new MTU size
>>  * @dev: network device
>>  * @new_mtu: new Maximum Transfer Unit
>>  *
>>  * Allow changing MTU size. Needs to be overridden for devices
>>  * supporting jumbo frames.
>>  */
>> int eth_change_mtu(struct net_device *dev, int new_mtu)
>
> Hmm, I cannot judge without the full patch. Will it be applicable
> to all net_devices or just ethernet ones? The name could be a bit
> misleading. Remindes me a lot of dev_set_mtu based on the signature, btw.

Normally to all net_devices, otherwise it could get complicated to
check for every dev separately ...
But, never mind, the comment below solve the issue

>
>> So wouldn't be a good idea to let our function open for jumbo frames...?
>
> Hm, we can document the fact that the function would needed to be updated in
> that case. But we should not allow to set a mtu which would require jumbograms
> currently.

OK, sounds a good. I will check the mtu against the limit
IPV6_MAXPLEN, and document the jumbo restriction ...

>
> Greetings,
>
>   Hannes
>

Regards,
Oussama
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