| To be able to transport PPP protocol units over Ethernet, they are inserted as payload in Ethernet frames. For this purpose, two new ethertype values were defined, which show the receiver that the Ethernet frame contains PPP payload. The two different types serve to distinguish between two phases within PPoE: the discovery stage, and the session stage. A typical discovery stage consists of four steps, which appear as follows (in Figure 9-2): The host sends a PADI (PPPoE Active Discovery Initiation) packet to the Ethernet broadcast address to find out which access concentrators are available in the Ethernet. One (or several) of these access concentrators replies by sending a PADO (PPPoE Active Discovery Offer) packet, informing the host about the Ethernet address where an access concentrator is available, which may specify additional services. The host selects one from the available access concentrators and requests that this concentrator establish a connection by sending a PADR (PPPoE Active Discovery Request) packet. The access concentrator replies by sending a PADS (PPPoE Active Discovery Session Confirmation) packet. Figure 9-2. Typical sequence for PPPoE Active Discovery. 
Subsequently, the discovery stage is left behind and the session stage begins, where PPP payload is packed transparently in Ethernet frames having ethertype value 0x8864 (in contrast to packets in the discovery stage, which have ethertype value 0x8863). Figure 9-3 shows what a PPPoE packet looks like in the session stage. The underlying Ethernet already forms frames, so PPPoE does not require character stuffing, in contrast to the asynchronous case described in Chapter 8. Figure 9-3. Protocol data unit of the PPPoE session stage. 
The PPP protocol should generally initiate a PPPoE connection to be torn down, but there is also a PADT (PPPoE Active Discovery Terminate) packet, which can be used to terminate a PPPoE connection. Once a PADT packet has been sent or received, not even a normal PPP connection termination packet may be sent. | 