Re: L2TPv3 offset

On 19/02/2019 09:17, t.martitz@avm.de wrote:

Hello,

I saw that you removed the offset option from l2tp sessions in Linux
4.16 (commit 900631ee6a2651dc4fbaecb8ef9fa5f1e3378853 l2tp: remove
configurable payload offset). Since we need something like that I'm
reaching out to you.

Adding netdev.

Our use case is pseudo-wire over l2tp, so we encapsulate ethernet
within l2tpv3. This has lots of benifits for us, but one remarkable
drawback is that the receiver, who's removing the encapsulation, will
see a misaligned inner IP header.

So we planned to use the offset to correct that. But I saw that the
offset was removed not too long ago. Now I see that there is still
"l2specific_len" / L2TP_ATTR_L2SPEC_LEN that we can use for pretty
much the same purpose but I get the feeling that this should only be 0
or 4 while we would need 2 or 6. I get this feeling because "ip l2tp"
of the iproute2 package does not allow setting this at all and
hardcodes 0 or 4 based on the type.

In L2TPv3, any payload offset would need to be implemented by defining a
new L2SpecificSublayer type since the L2TPv3 header has no offset field.
There are two standard L2TPv3 sublayers supported - None and Default.
These are fixed size (0 and 4 bytes respectively), hence the kernel now
ignores L2TP_ATTR_L2SPEC_LEN since the length can be derived from
L2TP_ATTR_L2SPEC_TYPE. For reference, the set of defined types are
listed at
https://www.iana.org/assignments/l2tp-parameters/l2tp-parameters.xhtml#l2tp-parameters-37.
No L2SpecificSublayer type was ever defined for L2TPv3 to allow a
configurable payload offset in ethernet pseudowires.

So my question is what should we do know? Being based on removed
functionality is kind of bad, but we must fix the misaligned inner ip
header.
We control the sender and receiver so we could apply all kinds of
hacks, but we would rather find a solution compliant with the Internet
community.

If you really need to insert padding in transmitted L2TPv3 packets
between the L2TPv3 header and its payload, one option is to define a new
L2SpecificHeaderType and patch the kernel to accept it.

If you control both the sender and receiver, is using FOU an option?

James