Callback Option of PPP

[Michael Zheng <[e-mail]>]

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     I wonder if anyone can clarify a couple of things for me about the 
     callback option of the LCP.
     
     1. The use of conf-nak for callback:
     
        If an implementation doesn't like the "operation" in a callback 
     confreq, presumably it confnak's it with the desired operation and 
     leaves the "message" portion blank.  Can anybody confirm this?
     
        Also, are there any implementations out there that use confnak to 
     suggest a callback request from the peer?
     
     2. The use of conf-rej for callback:
     
        Some of the operations (e.g., 0 and 2) are designed to work with 
     the authentication information, which is available only after the 
     authentication phase finishes.  Therefore, the validity of the 
     callback request is not determined till the authentication is done.  
     As a result, those confreq should always be ack'd.  And later on, 
     depending on whether the callback is authorized or not, the callback 
     may or may not happen.  
     
        Some other operations (e.g, 1, 3 and 4) contains complete 
     information to determine the validity of the request.  If those 
     requests are invalid (e.g., operation 1 contains unknown syntax to the 
     called unit), should a confrej be sent right away?  Notice that this 
     behavior is different than the one above.
     
     3. Could someone please provide a little more information about 
     operations 2 and 4?  How is operation 4 intended to be used?
     
     Thank you for your help.
     
     -mz