This phase is complete when a configuration-acknowledgment frame has been both sent and received. Before any network-layer datagrams (for example, IP) can be exchanged, LCP first must open the connection and negotiate configuration parameters. First, link establishment and configuration negotiation occurs.The PPP LCP provides a method of establishing, configuring, maintaining, and terminating the point-to-point connection. Modified frames, however, always will be clearly distinguishable from standard frames. The LCP can negotiate modifications to the standard PPP frame structure. By prior agreement, consenting PPP implementations can use a 32-bit (4-byte) FCS for improved error detection. Frame Check Sequence (FCS)-Normally 16 bits (2 bytes).
By prior agreement, consenting PPP implementations can use other values for the maximum information field length. The default maximum length of the information field is 1,500 bytes. The end of the information field is found by locating the closing flag sequence and allowing 2 bytes for the FCS field. Data-Zero or more bytes that contain the datagram for the protocol specified in the protocol field.The most up-to-date values of the protocol field are specified in the most recent Assigned Numbers Request for Comments (RFC). Protocol-Two bytes that identify the protocol encapsulated in the information field of the frame.(For more information about LLC types and frame types, refer to "Synchronous Data Link Control and Derivatives,") A connectionless link service similar to that of Logical Link Control (LLC) Type 1 is provided. Control-A single byte that contains the binary sequence 00000011, which calls for transmission of user data in an unsequenced frame.PPP does not assign individual station addresses. Address-A single byte that contains the binary sequence 11111111, the standard broadcast address.The flag field consists of the binary sequence 01111110. Flag-A single byte that indicates the beginning or end of a frame.The following descriptions summarize the PPP frame fields illustrated in Figure 13-1 :
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