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The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Network Interface Layer (OSI Data Link Layer) Protocols            9  TCP/IP Serial Line Internet Protocol (SLIP) and Point-to-Point Protocol (PPP)                 9  Point-to-Point Protocol (PPP)                      9  PPP Protocol Frame Formats

PPP General Frame Format 1 2 3 4 PPP Link Control Protocol (LCP) Frame Formats

Summary of PPP Control Message Formatting

Well now… My intention in this topic was to show you the general format used for the different control protocols since they are so similar and I don't have the time or space to describe each protocol's frames individually. I concede that this may have ended up being more confusing than enlightening, since it may be difficult to see how PPP moves from the general to the specific in its formatting. If it will help, here's a quick summary:

1. The PPP general frame format is used for all frames, including all control frames. Its Information field contains the payload, which carries the entire control message within it for control frames.
   
   
2. The control frame is structured using the general format I gave at the start of this topic. The Code value indicates the type of control frame for each control protocol. The Data field is variable in length, and contains data for that control frame, which in some cases may include one or more configuration options.
   
   
3. For configuration control frames like Configure-Request and Configure-Ack, the Data field contains an encapsulated set of options using the general structure in the second table in this topic. Each option has its own Data subfield that contains data specific to that option.

To help make this more clear, the next two topics provide more specific examples of frame formats for LCP and the authentication protocols.

PPP General Frame Format 1 2 3 4 PPP Link Control Protocol (LCP) Frame Formats