PPP Network Controland the IP Control Protocol
After LCP negotiation has completed, the LCP state engine sends an Up event to the NCP state engine. The Up event from LCP triggers network layer negotiations. PPP is commonly used on links primarily devoted to IP, in part because its implementation is required on all IPv4 routers with serial interfaces. RFC 1332 defines the IP Control Protocol (IPCP), which is the Network Control Protocol for IP and has the PPP protocol number 0x8021. Figure 9-24 illustrates IPCP.
Tip
Other Network Control Protocols have been specified for a wide variety of network layer protocols. A complete list is found in Appendix F.
Figure 9-24. IPCP frame format
IPCP shares several common features with LCP, as can be seen in Figure 9-24. Frame formats are identical, but the code list is a subset of LCP’s. IPCP uses only codes 1 (Configure-Request) through 7 (Code-Reject). IPCP will trade configuration requests and responses until a common set of options is established. Configuration options are, of course, completely different; they are listed in Table 9-8 and described in the following sections. Terminate-Request and Terminate-Ack are used to tear down the IP layer only. After terminating IPCP, it is possible to re-open IP with another IPCP configuration round. Other network protocols are completely unaffected.
Table 9-8. IPCP configuration options
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