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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

PPP Control Messages and Code Values

The Code field indicates the type of control frame within the particular control protocol. Some protocols have a unique set of codes used only by that particular protocol; examples include the authentication protocols (PAP and CHAP) and the Bandwidth Allocation Protocol (BAP). Since the NCPs and many of the feature control protocols like CCP and ECP are based on LCP, they use a common set of message codes and types. Table 36 shows these common message codes, as well as a cross-reference table indicating which control protocols use them:

The contents of the Data field depends entirely on the type of control message. In some cases, no extra data needs to be sent at all, in which case the Data field may be omitted. In other control messages it carries information relevant to the message type. For example, a Code-Reject message carries in the Data field a copy of the frame that was rejected.

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