Fix serialization for QinQ

[teodoro-vargas]

Current implementation of libtins uses Dot1Q frame to represent both Dot1Q and Dot1AD (QinQ). The serialization of a packet with double tagging writes the PIDs in the wrong order: 0x8100 (Dot1Q) followed by 0x88a8 (Dot1AD), when the order should be 0x88a8 (Dot1AD) followed by 0x8100 (Dot1Q).

[mfontanini]

Wouldn't it be simpler to simply fix Dot1Q instead of introducing a new class to fix this? e.g. flip the check and instead of doing what it's currently doing with the inner_pdu, do it with the parent_pdu.

[teodoro-vargas]

@mfontanini That would require to assume that the parent frame is an EthernetII, since the method parent_pdu returns a PDU and a cast would be necessary. We can try that option, but we think that that casting can be a little dangerous.

[mfontanini]

You don't have to cast the parent PDU, you can just call PDU::pdu_type and see if it's the ethernet one.

[teodoro-vargas]

Ok, that didn't work for me. However, keeping the change in the write_serialization method in EthernetII and removing the Dot1AD frame did.
Still, having to get the Dot1Q frame and then check if there's another Dot1Q frame inside, instead of just looking for the Dot1AD frame, would be a little annoyance for some people.

[teodoro-vargas]

@mfontanini I believe that the approach you mentioned didn't work because of the pdu_type value during the serialization process (which is a static value of 0x8100, in Dot1Q). In the latest commit of this PR, It's checked directly in the EthernetII frame if the following two frames are Dot1Q, in which case, a QinQ PID is written (0x88a8).
Please, let us know if the change is still incorrect.

[teodoro-vargas]

@mfontanini friendly ping. We applied the fixes you requested.
Thanks.

[mfontanini]

Looks good! Do you mind adding a test? Specifically something that constructs a simple packet like EthernetII() / Dot1Q() / Dot1Q() / something using some tag for each dot1q frame, the serializing it, parsing it back and making sure everything works?

[teodoro-vargas]

Updated PR is in #316