PPP

PPP negotiation consists of three phases: LCP to set up link options (the most important of which is authentication), the optional authentication phase, and IPCP to configure Internet protocol.

The PPP control protocols LCP and IPCP work the same way. Each end sends a configuration-request message that contains some proposed options for the connection. If all the options in the configuration-request message are acceptable to the other end, the other end sends a configuration-acknowledge message; otherwise, the other end sends either a configuration-nak message that contains an acceptable set of values or a configuration-reject for any options that are completely unacceptable. A configuration protocol is complete when both ends have sent a config-ack.

The details of the authentication protocols PAP and CHAP differ, but in general the MOB-IP-SIM sends a challenge message to the mobile and examines the mobile's response. The MOB-IP-SIM then sends a message indicating whether the response was acceptable. The only difference is the contents of the challenge and response packets.

If no frames at all appear in the PPP log (see the Section called tcpdump (show packet contents) in Chapter 5), verify that the E5515C is set to use an external PDSN. If a long string of authentication challenge frames with no replies appears,the mobile probably failed to reject the proposed authentication protocol during LCP. As a workaround, you can disable PPP authentication (see Propose PPP authentication), but the mobile is behaving incorectly and needs correcting.