I. Overview
To establish communication through a point-to-point link, each end of the PPP link must first send lcppackets to set and test the data link. The peer can be authenticated only after the link is established. Then, PPP must send ncppackets to select and set one or more network layer protocols. Once each selected network-layer protocol is set, mongorams from each network-layer protocol can be sent on the connection. The link remains the communication settings unchanged until the external LCP and NCP close the link, or when some external events occur (timer in the expired state or network administrator interference ).
Ii. phase division Diagram
During point-to-point link setup, maintenance, and termination, the PPP link goes through several clear phases, as shown in the diagram. This figure does not show all status transitions.
3. Link death (physical connection does not exist)
The link must start and end at this stage. When an external event (such as a carrier listener or set by the network administrator) indicates that the physical layer is ready, PPP enters the link establishment phase. In this phase, the LCP automatic machine will be in the initial state, and the conversion to the link establishment phase will give the LCP automatic machine an up event signal.
Execution record:
Typically, the link will automatically return to this phase after it is disconnected from the modem. In the hardware-implemented link, this phase is quite short-only enough to detect the existence of the device.
Iv. link establishment stage
LCP is used to exchange configuration packages and establish connections. Once a configuration package (configure-ackpacket) is sent and received, the switching is completed and the LCP is enabled. All configuration options are assumed to use the default value unless it is changed by the configuration exchange. Note that only the configuration options that do not depend on the special network layer protocol are equal to the LCP configuration. In the network layer protocol phase, some network layer protocols are configured by some network control protocols (NCP. Any non-lcppackets received at this stage must be silentlydiscarded (quietly discarded ). The lcpconfigure-Request (LCP Configuration Requirements) enables the link to return from the network layer protocol or authentication phase to the link establishment phase.
V. Authentication phase
On some links, one end of the link may need peer to authenticate it before the network layer protocol packets is allowed to exchange. By default, authentication does not need to be enforced. If an execution requires peer to authenticate based on a specific authentication protocol, it must use that authentication protocol at the link establishment stage. Authentication should be performed immediately after the link is established as much as possible. However, the link quality check can occur at the same time. During one execution, it is prohibited to postpone authentication uncertain due to the switch link quality check packets. Before the authentication is completed, it is prohibited to move from the authentication phase to the network layer protocol phase. If the authentication fails, the authenticator should jump to the end of the link.
In this phase, only the Link Control Protocol, authentication protocol, and packets of The Link Quality Monitoring Protocol are allowed. Other packages received in this phase must be quietly discarded.
Execution record:
During a single execution, authentication failure is caused only by timeout or no response. The authentication should allow some type of re-transmission. The end of the link can only be reached after several failed authentication attempts. During execution, if either party rejects the authentication from the other party, the other party is responsible for starting the link termination phase.
Vi. Network Layer Protocol phase
Once PPP completes the previous phase, each network layer protocol (such as IP, IPX, or appletalk) must be set separately by the appropriate Network Control Protocol (NCP. Each NCP can be opened and closed at any time.
Execution record:
Because an execution may initially require a great amount of time for link quality detection, when waiting for the Peer to set NCP, the execution should avoid using a fixed timeouts. When an NCP is in the opened State, PPP carries the corresponding network layer protocol packets. When the corresponding NCP is not in the opened State, any received network-layer protocol packets will be quietly discarded.
Execution record:
when the LCP is in the opened State, any protocol packets that is not supported by the execution must be returned in protocol-reject. Only supported protocols are quietly discarded. In this phase, link traffic is composed of any possible combination of LCP, NCP, and network layer protocol packets.
7. Link termination phase
PPP can terminate the link at any time. There are many causes for Link termination: Carrier loss, authentication failure, link quality failure, expiration of idle cycle timer, or link shutdown by the Administrator. LCP terminates the link by switching terminate (terminate) packets. When the link is being closed, PPP notifies the network layer protocol so that they can take the correct action. After the switch terminate (terminate) packets, the execution should notify the physical layer of disconnection to force link termination, especially when authentication fails. The sender of the terminate-Request (termination-Request) should be disconnected after receiving the terminate-ack (termination-Allow) or after the counter restarts. The party that receives the terminate-request should wait for the Peer to cut off. After the terminate-request is sent, it must go through at least one restarttime (restart time) to allow disconnection. PPP should move forward to the link death stage. Any non-lcppackets received at this stage must be quietly discarded.
execution record:
LCP is enough to close the link, and each NCP does not need to send a terminatepackets. On the contrary, disabling an NCP is not enough to cause the termination of the PPP link, even if the NCP is currently the only NCP in the opened State.
