linux-kernel - Re: [PATCH net v2] L2TP:Adjust intf MTU,factor underlay L3,overlay L2

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Date:   Tue, 4 Oct 2016 08:53:14 +0100
From:   James Chapman <jchapman@...alix.com>
To:     "R. Parameswaran" <parameswaran.r7@...il.com>
Cc:     kleptog@...na.org, netdev@...r.kernel.org, davem@...hat.com,
        linux-kernel@...r.kernel.org, nprachan@...cade.com,
        rshearma@...cade.com, dfawcus@...cade.com,
        stephen@...workplumber.org, acme@...hat.com, lboccass@...cade.com,
        bhong@...cade.com
Subject: Re: [PATCH net v2] L2TP:Adjust intf MTU,factor underlay L3,overlay L2

On 04/10/16 04:12, R. Parameswaran wrote:
>
> Hi James, 
>
> Please see inline, thanks for the reply:
>
> On Sat, 1 Oct 2016, James Chapman wrote:
>
>> On 30/09/16 03:39, R. Parameswaran wrote:
>>>>> +	/* Adjust MTU, factor overhead - underlay L3 hdr, overlay L2 hdr*/
>>>>> +	if (tunnel->sock->sk_family == AF_INET)
>>>>> +		overhead += (ETH_HLEN + sizeof(struct iphdr));
>>>>> +	else if (tunnel->sock->sk_family == AF_INET6)
>>>>> +		overhead += (ETH_HLEN + sizeof(struct ipv6hdr));
>>>> What about options in the IP header? If certain options are set on the
>>>> socket, the IP header may be larger.
>>>>
>>> Thanks for the reply - It looks like IP options can only be 
>>> enabled through setsockopt on an application's socket (if there's any 
>>> other way to turn on IP options, please let me know - didn't see any 
>>> sysctl setting for transmit). This scenario would come 
>>> into picture when an application opens a raw IP or UDP socket such that it 
>>> routes into the L2TP logical interface.
>> No. An L2TP daemon (userspace) will open a socket for each tunnel that
>> it creates. Control and data packets use the same socket, which is the
>> socket used by this code. It may set any options on its sockets. L2TP
>> tunnel sockets can be created either by an L2TP daemon (managed tunnels)
>> or by ip l2tp commands (unmanaged tunnels).
>>
> One Q I have is whether it would be sufficient to solve this for the
> common case (i.e no IP options) and have an expectation that the 
> administrator will explicitly provision the mtu using the 'ip link ... 
> mtu'  command when dealing with infrequent occurences like IP options? 
>
> But looking at the code, it looks to be possible to pick up whether 
> options are enabled and how long the options are, from the ip_options struct 
> embedded in the tunnel socket. If you want me to, I can repost the patch
> with this change (will need a few days) - please let me know if this is 
> what you had in mind.
>
>
Yes, that's what I had in mind. But my preference would be that this
would be a new function in the ip core, for use by any encap protocol,
where appropriate.