PPP is the standard protocol to bring TCP/IP on a serial line (RS232/Modem,...)

LCP and IPCP are Link and network negociation protocol. They are used during PPP connection.

IP and ICMP are network layer protocol (IP for addressing management and ICMP for small message on IP (for ping ))

UDP is a protocol connectionless-oriented (easy to use for small command to send to an embedded system)

TCP is the same than UPD but connection-oriented and reliable transfert.

I want to implement PAP (password Auth. protocol to manage login/password during PPP connection) and SMTP to give possibility to send Email...