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The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Network Interface Layer (OSI Data Link Layer) Protocols            9  TCP/IP Serial Line Internet Protocol (SLIP) and Point-to-Point Protocol (PPP)                 9  Point-to-Point Protocol (PPP)                      9  PPP Core Protocols: Link Control, Network Control and Authentication

PPP Network Control Protocols (IPCP, IPXCP, NBFCP and others) 1 2 3 PPP Feature Protocols

Password Authentication Protocol (PAP).

PAP is a very straight-forward authentication scheme, consisting of only two basic steps, as shown in Figure 29:

1. Authentication Request: The initiating device sends an Authenticate-Request message that contains a name and a password.
   
   
2. Authentication Reply: The responding device looks at the name and password and decides whether to accept the initiating device and continue in setting up the link. If so, it sends back an Authenticate-Ack. Otherwise, it sends an Authenticate-Nak.
   

   Figure 29: PPP Password Authentication Protocol (PAP) Authentication

   PAP works using a simple exchange of a request containing name and password information, and a reply indicating whether or not authentication was successful.

   
   

Simple. Now, remember what Einstein said about simplicity? PAP is another example of something that is just too simple for its own good. Chief amongst its flaws is that it transmits the user name and password in clear text across the link. This is a big “no-no” in security protocols, as it means any eavesdropper can get the password and use it in the future. PAP also provides no protection against various security attacks. For example, an unauthorized user could simply try different passwords indefinitely and hope he or she eventually found one that worked. PAP also puts control of the authentication squarely on the shoulders of the initiating device (usually a client machine) which is not considered desirable.

PPP Network Control Protocols (IPCP, IPXCP, NBFCP and others) 1 2 3 PPP Feature Protocols