4.1 LCP Negotiation of Callback Option for CBCP

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The following describes the message exchange that happens between the caller and the answerer when the caller is requesting the use of CBCP for negotiating callback settings and the answerer accepting the same. For better understanding, the frame bytes, including the PPP encapsulation header, are specified below.

The caller specifies an LCP Configure-Request message as specified in [RFC1661] with the LCP Callback Option. The following describes a sample Configure-Request LCP message that is sent by the caller. The LCP option of relevance here is the Callback option. The others are mentioned here only as an example from a packet capture.

 C0 21 01 00 00 17 02 06 FF FF FF FF 05 06 11 33 
 51 5B 07 02 08 02 0D 03 06 
  
 PPP Protocol Field: C0 21 (LCP)
 Code: 01 (Configure-Request)
 Identifier: 00 
 Length: 00 17 
 Option: Async-Control-Character-Map
     Option Type: 02 (Async-Control-Character-Map)
     Length: 06
     AsyncControlCharacterMap: FF FF FF FF
 Option: Magic-Number
     Option Type: 05 (Magic-Number)
     Length: 06
     Magic Number: 11 33 51 5B
 Option: Protocol-Field-Compression
     Option Type: 07 (Protocol-Field-Compression)
     Length: 02
 Option: Address-and-Control-Field-Compression
     Option Type: 08 (Address-and-Control-Field-Compression)
     Length: 02
 Option: Callback
     Option Type: 0D (Callback)
     Length: 03
     Operation: 06
 

The following specifies a sample response from the answerer for the above-mentioned Configure-Request from the caller.

 C0 21 02 00 00 17 02 06 FF FF FF FF 05 06 11 33 
 51 5B 07 02 08 02 0D 03 06 
  
 PPP Protocol Field: C0 21 (LCP)
 Code: 02 (Configure-Ack)
 Identifier: 00 
 Length: 00 17 
 Option: Async-Control-Character-Map
     Option Type: 02 (Async-Control-Character-Map)
     Length: 06
     AsyncControlCharacterMap: FF FF FF FF
 Option: Magic-Number
     Option Type: 05 (Magic-Number)
     Length: 06
     Magic Number: 11 33 51 5B
 Option: Protocol-Field-Compression
     Option Type: 07 (Protocol-Field-Compression)
     Length: 02
 Option: Address-and-Control-Field-Compression
     Option Type: 08 (Address-and-Control-Field-Compression)
     Length: 02
 Option: Callback
     Option Type: 0D (Callback)
     Length: 03
     Operation: 06