Parse L2TP in PCAP++

Question

I need to parse difficult traffic with Layer 2 Tunneling Protocol:
Eth->IP->UDP->L2TP->PTPP->IP->TCP->...
.. and get inner and outer IP addresses.
I use loop over layer->getNextLayer()) and it stops after first UDP.
Is there any BestPractise to cope with L2TP?

Thanks in advance