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)

PPP Bandwidth Allocation Protocol (BAP) and Bandwidth Allocation Control Protocol (BACP) 1 PPP General Frame Format

The PPP protocol suite includes a number of different protocols used to send both data and control information in different ways. Each of these packages information into messages called frames, each of which follows a particular frame format. PPP starts with a general frame format that encompasses all frames sent on the link, and then includes more specific formats for different purposes. Understanding these formats not only makes diagnosing PPP issues easier, it also helps make more clear how the key PPP protocols function.

In this section I illustrate the most common frame formats used for sending both data and control information over PPP. I begin with an explanation of the overall format used for all PPP frames. I also describe the general format used for the various control protocols and the option format that most of them use. (One of the nice things about PPP is that so many of the protocols use control frames with a common format.)

I then specifically list the frames used for the Link Control Protocol (LCP) and the authentication protocols (PAP and CHAP). I also describe the special format used by the PPP Multilink Protocol (MP) to transport fragments of data over bundled links.

PPP Bandwidth Allocation Protocol (BAP) and Bandwidth Allocation Control Protocol (BACP) 1 PPP General Frame Format