No Appls Yet? Be a Good CHAP and Ask PAP
Earlier in this chapter, you read briefly that the servers ask
the user for his username and password. The protocols that exchange the username
and password are built in to most of the application layer protocols. For
instance, back in Chapter 8,
"Shipping Goods over a (Network) Roadway," you read about POP3 and the messages
that a POP3 server uses to ask the POP3 client for the username and password.
ISPs want to authenticate the user, but it's better if they can
authenticate before the user even tries to use an application. ISPs can't count
on every one of their customers to have a web browser (some might just use
e-mail), or they might have customers who just use a web browser but not e-mail.
All the ISP wants to do is authenticate whether the user is allowed to connect,
and more important, whether the user has paid the monthly service fee. As a
result, the ISP has two issues regarding user authentication:
To solve the problem, ISPs use part of Point-to-Point Protocol
(PPP), which defines two options for the exchange of username and password
information.
The names appear in the title to this sectionPAP and CHAP.
Password Authentication Protocol
(PAP) and Challenge Handshake Authentication Protocol
(CHAP) are both parts of PPP, and you know from the past several
chapters that PPP is a data link protocol that's often used in WAN connections,
particularly when using modems or DSL to connect to the Internet. Both PAP and
CHAP provide a protocol for the exchange of the username and password. Figure 17-4 shows the basic process PAP uses
for username and password exchange.
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