The protocol field is two octets its value identifies the datagram encapsulated in the Information field of the packet. In the case of MP the PID also identifies the presence of a 4-octet MP header (or 2-octet, if negotiated).
A PID of Ox003d identifies the packet as MP data with an MP header.
The LCP packets and protocol states of the MLPPP session follow those defined by PPP in RFC 1661, The Point-to-Point Protocol (PPP). The options used during the LCP state for creating an MLPPP NCP session are described below.