T1: A Survival Guide

PPP Logical Link Statesand State Machines

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.

Table 9-1. Steps to initialize link in Figure 9-2

Step	Physical	LCP	Authentication	NCP (IP)
0	Down	Down	Down	Down, but datais ready fortransmission
1	Down	Down	Down	IP stack sends Open event to IPCP to initialize link ...