PPI is a standard for link layer packet encapsulation, proposed as generic extensible container to store both captured in-band data and out-of-band data. Originally it was developed to provide 802.11n radio information, but can be used for other purposes as well.
Sample capture: https://wiki.wireshark.org/SampleCaptures?action=AttachFile&do=get&target=Http.cap
This page hosts a formal specification of packet_ppi using Kaitai Struct. This specification can be automatically translated into a variety of programming languages to get a parsing library.