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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 Authentication Protocol (PAP, CHAP) Frame Formats 1 2 3 4 TCP/IP Network Interface / Internet "Layer Connection" Protocols

PPP Multilink Protocol Fragment Frame Format

The Information field of each fragment uses a substructure that contains a four-field MP header along with one fragment of the original PPP frame, as shown in Table 41.

As you can see, the MP frame format comes in two versions: the long format uses a 4-byte header, while the short format requires only 2 bytes. The default MP header format uses a 24-bit Sequence Number and has 6 reserved bits, as shown in Figure 41. It is possible when MP is set up, for devices to negotiate the Multilink Short Sequence Number Header Format configuration option. If this is done successfully, shorter 12-bit Sequence Numbers are used instead. Four of the reserved bits are also truncated, to save 2 bytes on each frame, as illustrated in Figure 42. (Considering that 12 bits still allows for over 4,000 fragments per PPP frame, this is usually more than enough!)

The Fragment Data field contains the actual fragment to be sent. Since the original PPP header (including the Protocol field) is at the start of the original PPP frame, this will appear at the start of the first fragment. The remaining fragments will have just portions of the Information field of the original PPP frame. The last fragment will end with the last bytes of the original PPP frame.

The receiving device will collect all the fragments for each PPP frame, extract the fragment data and MP headers from each. It will use the Sequence Numbers to reassemble the fragments and then process the resulting PPP frame.

PPP Authentication Protocol (PAP, CHAP) Frame Formats 1 2 3 4 TCP/IP Network Interface / Internet "Layer Connection" Protocols