What Is PPP Callback?
When companies need to manage and negotiate secure access to network resources, one method of accomplishing this is to implement a callback control protocol such as point-to-point protocol (PPP) callback service. In PPP callback, two routers initiate and receive calls internally from a verified source, much like dial-up Internet. PPP callback presents companies with a win-win scenario for workers who telecommute from home via dial-up networking (through heightened security), while also accounting for and limiting toll costs associated with the telecommute.
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Purpose
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PPP callback serves two purposes: It controls access between routers by requiring user authentication and verification, and it allows companies to control costs between routers. This permits salespeople on the road to call securely from different physical locations. Also, the PPP callback system enables employers to pick up the toll phone costs of system administrators who work remotely.
Configuration
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For PPP callback to operate, two routers must establish a PPP link and have link control protocol, or LCP, enabled. LCP negotiates user authentication options. One router must be designated the "client," while the other acts as the "server." The system administrator configures the client to initiate PPP callback requests and sets the server to accept callback requests and place calls. The system administrator also selects the compression, identification and network layer protocols that will be used in the callback process.
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Sequence of Events
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PPP callback follows a set sequence of events, with user authentication and verification occurring at both the client and server ends of the router. A remote user, or client, initiates a call to the server and requests access using his credentials. The server verifies this information, and if authentication is successful the server disconnects the session, lets a certain amount of time lapse and then calls the client. However, if authentication is unsuccessful, the server terminates the session and does not return the call. When the client computer answers the remote call, it must supply the authenticating credentials to the server before final access to network resources is granted.
Platform Specificity
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PPP callback configurations vary from platform to platform, with each system issuing its own set of commands to execute callback protocols. Unix uses the ppp.secret file to define how callback processes will run, while Microsoft Windows uses dial-up networking to establish its callback routines. Cisco routers support stand-alone configurations, or configurations that perform within the Microsoft operating system environments through MS Callback.
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References
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