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 Fundamentals and Operation

PPP Overview, History and Benefits 1 2 PPP Link Setup and Phases

PPP General Operation

The fact that the PPP suite includes literally dozens of protocols often makes it seem like it must be a really complex technology. In fact, the general operation of PPP is really quite straight-forward. The existence of all those PPP protocols allows PPP to be flexible and extensible, supporting many higher layer datagram types and various features.

The bottom line, however, is that PPP operation involves just three basic steps. Beginning in a state where there is no PPP link between the devices, these are the operations that occur in PPP (also illustrated in Figure 24):

1. Link Setup and Configuration: Before the two devices can exchange information, they must make contact and set up a link between them. During link setup, all the parameters needed to manage the operation of the link are agreed upon by the two devices. The LCP begins this process, and invokes the help of support protocols as they are needed, for options like authentication. After the link is set up in general terms, the appropriate NCP is called for whatever layer three technology is being carried on the link to complete link setup.
   
   
2. Link Operation: The devices on the link use it to send datagrams. Each device transmits by taking layer three datagrams, encapsulating them and sending them down to layer one to be transmitted. Each device receives by taking PPP frames sent up from its own physical layer, stripping off the PPP header and passing the datagram up to layer three. Where appropriate, optional feature protocols are used here, such as CCP for compression.
   
   
3. Link Termination: When either device decides it no longer wants to communicate, it terminates the link. The link can of course be re-established if desired.
   

   Figure 24: Overview of PPP Operation

   In simplest terms, PPP consists of only three basic steps: link setup, link operation and link termination.

   
   

Link setup is by far the most complicated of these general steps, as it involves several substeps used to negotiate link parameters and options. The next topic describes the steps in link setup, and discusses the phases that a link passes through as it is set up, used, and eventually terminated.

PPP Overview, History and Benefits 1 2 PPP Link Setup and Phases