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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 Frame Fragmentation Process

To accomplish this fragmentation process, a three step process is followed:

1. Original PPP Frame Creation: The data or other information to be sent is first formatted as a “whole” PPP frame, but in a modified form, as described below.
   
   
2. Fragmentation: The full-sized PPP frame is chopped into fragments by MP.
   
   
3. Encapsulation: Each fragment is encapsulated in the Information field of a new PPP MP fragment frame, along with control information to allow the fragments to be reassembled by the recipient.

Several of the fields that normally appear in a “whole” PPP frame aren’t needed if that frame is going to then be divided and placed into other PPP Multilink frames, so when fragmentation is to occur, they are omitted when the original PPP frame is constructed for efficiency’s sake. Specifically:

• The Flag fields at the start and end are used only for framing for transmission and aren’t needed in the logical frame being fragmented.
  
  
• The FCS field is not needed, because each fragment has its own FCS field.
  
  
• The special “compression” options that are possible for any PPP frame are used when creating this original frame: Address and Control Field Compression (APCP) and Protocol Field Compression (PFC). This means that there are no Address or Control fields in the frame, and the Protocol field is only one byte in size. Note that this inherently restricts fragments to carrying only certain types of information.

These changes save a full eight bytes on each PPP frame to be fragmented. As a result, the original PPP frame has a very small header, consisting of only a one-byte Protocol field. The Protocol value of each fragment is set to 0x003D to indicate a MP fragment, while the Protocol field of the original frame becomes the first byte of “data” in the first fragment.

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