On 19/02/2019 09:17, t.mart...@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
