A PPP link is established using LCP negotiation and NCP negotiation between two communicating devices:
LCP negotiation: The two communicating devices negotiate the link layer attributes such as the MRU and authentication mode.
NCP negotiation: The two communicating devices negotiate the format and type of packets to be transmitted.
The following figure shows the PPP link establishment process.
Figure 1 PPP link establishment process
The PPP link establishment process is as follows:
PPP starts and ends with the Dead phase. Two communicating devices stay in the Dead phase for a very short period and then enter the Establishment phase after detecting that the physical connection status is Up.
In the Establishment phase, the two devices perform LCP negotiation to negotiate the following items: the working mode such as single-link PPP (SP) or MP, MRU, authentication mode, magic number, and asynchronous character mapping. If LCP negotiation is successful, LCP enters the Opened state, indicating that the lower-layer link has been established.
If authentication is configured, the two devices enter the Authentication phase and perform CHAP or PAP authentication. If no authentication is configured, the two devices enter the Network phase.
In the Authentication phase, the two devices enter the Termination phase to tear down the link if authentication fails. At this time, LCP enters the Closed state. If authentication is successful, the two devices enter the Network phase. LCP remains Opened, whereas the NCP status changes from Initial to Starting.
In the Network phase, the two devices perform NCP negotiation, which includes the Internet Protocol Control Protocol (IPCP) negotiation and Multiprotocol Label Switching Control Protocol (MPLSCP) negotiation. During IPCP negotiation, the two devices negotiate IP addresses for their connected interfaces. During NCP negotiation, the two devices select a network layer protocol. After the negotiation succeeds (that is, the NCP negotiation status is Opened), network layer protocol packets can be sent over the PPP link.
After NCP negotiation succeeds, packets can be sent over the PPP link. If the PPP connection is interrupted during PPP operation, the two devices enter the Termination phase, the physical link is disconnected, PPP authentication fails, or the negotiation timeout period expires.
In the Termination phase, the two devices enter the Dead phase after all resources are released.
While configuring, maintaining and terminating a point-to-point link, PPP goes through the following phases.
Link Dead Phase
The Link Dead phase is also known as the physical-layer not ready phase. A PPP link begins and ends with this phase.
When two communicating devices detect that the physical link between them is activated (for example, carrier signals are detected on the physical link), PPP enters the Link Establishment phase.
In the Link Establishment phase, the two devices negotiate link layer attributes. The LCP status changes according to different events. In the Link Dead phase, LCP is in the Initial or Starting state. Once a physical link becomes available, the LCP status changes.
If a link is terminated, PPP also enters the Link Dead phase.
PPP stays in the Link Dead phase for a short period of time.
Link Establishment Phase
In the Link Establishment phase, two communicating devices exchange Configure packets to negotiate link layer attributes. After the two devices finish exchanging Configure packets, PPP enters either the Authentication phase or the Network-Layer Protocol phase, depending on whether authentication is required.
In the Link Establishment phase, the LCP status changes twice.
In the Link Dead phase, LCP is in the Initial or Starting state. When detecting that a link is available, the physical layer sends an Up event to the link layer. After receiving the Up event, the link layer changes the LCP status to Request-Sent. Then a Configure-Request packet is sent to establish a link.
The receipt of a Configure-Ack packet causes the LCP status to change to Opened. After LCP enters the Opened state, the device that receives a Configure-Ack packet enters the next phase.
The process occurs in the same manner on the other device. Any non-LCP packets received in this phase are discarded.
Authentication Phase
The Authentication phase is optional. By default, PPP does not perform authentication during PPP link establishment. If authentication is required, the authentication protocol must be specified in the Link Establishment phase.
PPP authentication is performed on:
Hosts and switchs that connect to a PPP network server through switched circuits or dial-up lines
Dedicated links
PPP provides two authentication modes: PAP authentication and CHAP authentication.
The authentication mode is negotiated by the two communicating devices in the Link Establishment phase. Link quality monitoring may occur concurrently but cannot delay authentication indefinitely.
Only LCP packets, authentication packets, and link quality monitoring packets are allowed in the Authentication phase. All other packets received in this phase are discarded. The receipt of a Configure-Request packet will cause a return to the Link Establishment phase from the Authentication phase.
In the Network-Layer Protocol phase, each network layer protocol, such as IP, IPX, or AppleTalk must be separately configured by using the appropriate NCP. For example, IPCP is used to configure the IP protocol. Each NCP may be in Opened or Closed state at any time. After an NCP enters the Opened state, PPP will transmit network-layer protocol packets.
The receipt of a Configure-Request packet will cause a return to the Link Establishment phase from the Network-Layer Protocol phase.
In the Link Termination phase, a link is terminated. PPP can terminate a link at any time. This may occur because of the loss of carrier, an authentication failure, a link quality monitoring failure, or the administrative termination of the link.
In the Link Establishment phase, LCP Terminate packets may be exchanged to close a link. When the link is closing, the link layer instructs the network layer protocols to take appropriate actions and instructs the physical layer to disconnect to terminate the link.
