The frame delimiters used in PPP protocol are delimiters.

the flag field is the delimiter of PPP frame. If two flag fields appear continuously, it means that this is an empty frame direct discarding process. Address field A is specified as xff, which is meaningless and deleted through negotiation. The control field C is specified as x3, which has no practical significance and can be deleted through negotiation. The length of the protocol (the fourth field in the header) is 2 bytes. When it is x21, the information field of PPP frame is IP datagram. ?

PPP was originally designed to provide an encapsulation protocol for IP traffic transmission between two peer nodes. In the TCP-IP protocol set, it is a data link layer protocol for synchronous modulation connection, which replaces the original non-standard layer 2 protocol, namely SLIP. Besides IP, PPP can also carry other protocols, including Internet Packet Exchange (IPX) of DECnet and Novell.

Related information

A method of encapsulating multi-protocol datagrams. PPP encapsulation provides a multiplexing technology in which different network layer protocols are simultaneously transmitted on the same link. PPP encapsulation is a multi-purpose, point-to-point protocol that can maintain compatibility with most commonly used hardware and overcome the shortcomings of SLIP. The WAN data link encapsulation service it provides is similar to the closed service provided by LAN.

PPP not only provides frame demarcation, but also provides protocol identification and bit-level integrity check services. In order to establish point-to-point link communication, each end of PPP link must first send LCP packets to set up and test the data link. After the link is established and the optional functions required by LCP are selected, PPP must send NCP packets to select and set one or more network layer protocols.