The PPP link layer is a nested set of state machines. The state machines do not reflect protocol layers in the strict sense of passing encapsulated packets between layers, but some of the “layers” depend on lower layers for correct functionality. At the bottom is the HDLC layer running directly on the physical link. Above the HDLC layer is the LCP layer, which negotiates transmission options for the HDLC layer. If authentication is required, it occurs before the NCPs complete negotiation of network layer options. The relationship between the state machines is shown in Figure 9-2. Multiple state machines may be involved at the network-protocol layer; Figure 9-2 shows state machines for IP, IPX, and SNA.

Figure 9-2. PPP layering

Each layer in Figure 9-2 must receive an administrative Open from the higher layer, instruct the lower layer to initialize, and then negotiate parameters for its own layer. At that point, it responds to the Open command with an Up event of its own. To transmit IP data, for example, the link must be initialized for IP. The steps shown in Figure 9-2 are summarized in Table 9-1.

The series of steps needed to set up the link for transmission of IP packets is shown in Figure 9-2 by the numbered sequence of steps in the diagram. When the IP layer has packets to transmit, but is not initialized, an administrative Open command is sent from the IP layer to the IP Control Protocol. IPCP is responsible for negotiating IP parameters. However, data cannot be transmitted until the lower layers have been configured to transmit data. Therefore, IPCP sends an Open command of its own to the authentication state machine. Once again, lower layers are not available, so the authentication layer sends its own Open command to the Link Control Protocol. LCP then proceeds to initialize the physical link layer by sending an Open command down to the appropriate physical layer. Depending on the physical layer, the Open command may be simple or quite complex. Asynchronous modems must establish circuit-switched paths through the telephone network and establish a carrier signal before returning an Up event. Dedicated leased lines normally have an available physical layer at all times, so they return an Up event much more quickly.

When the Up event has been received from the physical layer, LCP negotiates parameters for link encapsulation. Negotiation of encapsulation parameters is an involved process, as detailed in the next section. When negotiation completes after LCP has established a set of parameters used for encapsulation on the link, LCP will send an Up event to the authentication layer. Authentication may or may not be employed on any particular link. If it is not, the authentication layer simply does nothing. Placing the authentication layer before the network-protocol initialization prevents network attacks from outsiders who are not authorized to use the network. If it does not receive network layer parameters, the host is not on the network and cannot transmit data.

When authentication completes, an Up event is sent to the NCP, which is IPCP in this case. IPCP negotiates addressing and sends an Up event to the IP layer. The IP layer can then transmit the packets that were queued in the beginning. Once LCP is initialized, adding network protocols is much less time-consuming. If IPX packets were to be transmitted, only IPXCP would need to negotiate because the other layers would have previously been completed. Figure 9-3 shows the procedure that would be used once the IP initialization was complete. When the Open event goes down the layers, it reaches the authentication layer. However, authentication was already performed when the link initialized for IP, so the authentication layer can respond immediately with an Up notification. The only negotiation that must take place is the IPXCP negotiation to configure the link for use with IPX. The steps shown in Figure 9-3 are summarized in Table 9-2.

Figure 9-3. Adding another network protocol to a configured link

Figure 9-4 shows another view of the link-initialization process, through the PPP link state diagram.

Figure 9-4. PPP link state diagram

Five link states are defined: