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The PPP protocol provides a standard mode to transmit packets of multiple network-layer protocols on the point-to-point link.
Support point-to-point connections. The PPP protocol is different from other protocols at the data link layer such as X.25, frame relay; it contains the Challenge Handshake Authentication Protocol (CHAP) and Password Authentication Protocol (PAP) and guarantees the network security.
The physical layer of the PPP protocol supports asynchronous mode that contains eight bits without parity check and a bit-oriented synchronization connection. For example, the frame relay must be a synchronized circuit.
The PPP protocol contains network control protocols that are suitable for different networks, such as IPCP and IPXCP. With the PPP protocol or SLIP protocol, two ends can negotiate whether to compress the packet header.
Three Main Components
A method for encapsulating multi-protocol datagrams
Link Control Protocol (LCP)
Network Control Protocol
The Protocol field is two octets, and its value identifies the protocol type in the Information field of the packet.
The structure of this field is consistent with the ISO 3309 an extension mechanism for address fields.
Three main components of the PPP protocol are encapsulation format, LCP, and NCP.
The PPP protocol is a protocol at the data link layer. The frame of the encapsulation format is similar to that of High-Level Data Link Control (HDLC).
The PPP protocol distinguishes the data type of the payload in the Data field.
The LCP protocol implements the configuration and test of data links.
The NCP protocol implements the configuration of parameters that are necessary for network communication between point-to-point devices.
The magic number is negotiated during the configuration phase of the link. The packets with the magic number can check whether a loop happens in the PPP link.
PAP is a twice-handshake protocol. Its security is not high because it transmits a username and clear text password over the network.
CHAP is a triple-handshake protocol. It does not transmit the password but transmits only the hostnames of the authenticator and authenticated. Therefore, it is safer than the PAP.
The maximum receive unit (MRU) of the PPP protocol is 1500 by default. Two communication ends can negotiate the value of MRU.
The state transition diagram of the PPP protocol consists of the following phases: Link Dead, Link Establishment, Authentication, According to the packet function, the LCP protocol contains Link.
Configuration packets Link Termination packets and Link.
Link Configuration packets are used to negotiate some optional
Configuration Options.
Link Termination packets are used to terminate a PPP link.
Link Maintenance packets are used to test and debug a PPP link.
The NCP protocol manages the negotiation of Configuration Options at the network layer, including static and dynamic negotiation.
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