A PPP frame consists of six fields. The following descriptions summarize the PPP frame fields illustrated in the figure:
- Flag - A single byte that indicates the beginning or end of a frame. The flag field consists of the binary sequence 01111110. In successive PPP frames, only a single Flag character is used.
- Address - A single byte that contains the binary sequence 11111111, the standard broadcast address. PPP does not assign individual station addresses.
- Control - A single byte that contains the binary sequence 00000011, which calls for transmission of user data in an unsequenced frame. This provides a connectionless link service that does require the establishment of data links or links stations. On a point-to-point link, the destination node does not need to be addressed. Therefore, for PPP, the Address field is set to 0xFF, the broadcast address. If both PPP peers agree to perform address and control field compression during the LCP negotiation, the Address field is not included.
- Protocol - Two bytes that identify the protocol encapsulated in the information field of the frame. The 2-byte Protocol field identifies the protocol of the PPP payload. If both PPP peers agree to perform protocol field compression during LCP negotiation, the Protocol field is one byte for the protocol identification in the range 0x00-00 to 0x00-FF. The most up-to-date values of the protocol field are specified in the most recent Assigned Numbers Request For Comments (RFC).
- Data - Zero or more bytes that contain the datagram for the protocol specified in the protocol field. The end of the information field is found by locating the closing flag sequence and allowing 2 bytes for the FCS field. The default maximum length of the information field is 1,500 bytes. By prior agreement, consenting PPP implementations can use other values for the maximum information field length.
- Frame Check Sequence (FCS) - Normally 16 bits (2 bytes). By prior agreement, consenting PPP implementations can use a 32-bit (4-byte) FCS for improved error detection. If the receiver’s calculation of the FCS does not match the FCS in the PPP frame, the PPP frame is silently discarded.
LCPs can negotiate modifications to the standard PPP frame structure. Modified frames, however, are always distinguishable from standard frames.