IPIP6 Linux Kernel Implementation

[Pietro Paolini]

Hello everyone,

I am writing you regarding the implementation of the IPIP6 tunnel
contained in the Linux Kernel which I am trying to configure in order
to obtain a DS-Lite (IPIP6) compliant tunnel on a router running a
linux OS kernel version 2.6.33.

This my network topology :

lan --> tun1 --> eth0

where the MTU of all links is 1500.

My problem comes up when the packet size of the incoming IPv4 datagram
on my LAN interface is close to the MTU (1496 byte) : then the packet
is "lost" in the file ip6_tunnel.c module at this point :

linux-2.6.33.5/net/ipv6/ip6_tunnel.c
function ip6_tnl_xmit2()
{
....
mtu = dst_mtu(dst) - sizeof (*ipv6h);
if (skb->len > mtu) { --> 1496 > 1460 <--
*pmtu = mtu;
err = -EMSGSIZE;
goto tx_err_dst_release;
}
....
}

As I understand, I could be wrong of course, IPv6 fragmentation is not
handled in that module and packets bigger then the mtu - sizeof(IPv6)
are discarded; I am wondering if I can find a workaround in order to
solve this problem as in some way use the IPv6 stack which I assume
handles fragmentation - I could be again wrong - or implement it in the
ip6_tunnel.c module.

I am not an experienced kernel developer then is important for me
understand if I am in the right direction and Linux kernel really does
not provide this feature in a IPIP6 tunnel or if I am wrong; I need to
have that for the implementation of a DS-Lite tunnel which as defined
in the RFC6333 Section 5.3 requires IPv6 packet fragmentation.

Thanks a lot,
Pietro.
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[Austin S Hemmelgarn]

On 2013-11-07 09:54, Pietro Paolini wrote:
> Hello everyone,
>
> I am writing you regarding the implementation of the IPIP6 tunnel
> contained in the Linux Kernel which I am trying to configure in order
> to obtain a DS-Lite (IPIP6) compliant tunnel on a router running a
> linux OS kernel version 2.6.33.
>
> This my network topology :
>
> lan --> tun1 --> eth0
>
> where the MTU of all links is 1500.

This is probably the cause of the problem, when tunneling without
fragmentation you need to reduce the MTU of the link encapsulated within
the tunnel so that the size of the IP header plus the tunneled packet is
less than or equal to the MTU of the encapsulating link. For an IPIP6
tunnel over a link with an MTU of 1500, the absolute maximum MTU of the
tunnel itself should be 1440. If you use iproute2 or a similar program
to set the tunnel MTU to 1440 things should work.