Security issues are the primary topic of this RFC.
The interaction of the authentication protocols within PPP are highly implementation dependent. This is indicated by the use of SHOULD throughout the document.
For example, upon failure of authentication, some implementations do not terminate the link. Instead, the implementation limits the kind of traffic in the Network-Layer Protocols to a filtered subset, which in turn allows the user opportunity to update secrets or send mail to the network administrator indicating a problem.
There is no provision for re-tries of failed authentication. However, the LCP state machine can renegotiate the authentication protocol at any time, thus allowing a new attempt. It is recommended that any counters used for authentication failure not be reset until after successful authentication, or subsequent termination of the failed link.
There is no requirement that authentication be full duplex or that the same protocol be used in both directions. It is perfectly acceptable for different protocols to be used in each direction. This will, of course, depend on the specific protocols negotiated.
In practice, within or associated with each PPP server, there is a database which associates "user" names with authentication information ("secrets"). It is not anticipated that a particular named user would be authenticated by multiple methods. This would make the user vulnerable to attacks which negotiate the least secure method from among a set (such as PAP rather than CHAP). Instead, for each named user there should be an indication of exactly one method used to authenticate that user name. If a user needs to make use of different authentication method under different circumstances, then distinct user names SHOULD be employed, each of which identifies exactly one authentication method.
Passwords and other secrets should be stored at the respective ends such that access to them is as limited as possible. Ideally, the secrets should only be accessible to the process requiring access in order to perform the authentication.
The secrets should be distributed with a mechanism that limits the number of entities that handle (and thus gain knowledge of) the secret. Ideally, no unauthorized person should ever gain knowledge of the secrets. It is possible to achieve this with SNMP Security Protocols , but such a mechanism is outside the scope of this specification.
Other distribution methods are currently undergoing research and experimentation. The SNMP Security document also has an excellent overview of threats to network protocols.