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SubjectRe: [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.

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