A Description of the Point-to-Point Protocol Over Ethernet Discovery Stage (282079)

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To establish a PPPoE connection, the Ethernet address of the remote PPPoE server must be obtained. Additionally, a unique session identifier must also be negotiated. The method to obtain this information is referred to as the PPPoE Discovery Stage.

The PPPoE Discovery Stage is made up of four steps: initiation, offer, request, and session confirmation:

1. The PPPoE Active Discovery Initiation (PADI) packet:
   The PPPoE client sends out a PADI packet to the broadcast address. This packet can also populate the "service-name" field if a service name has been entered on the dial-up networking properties of the PPPoE broadband connectoid. If a service name has not been entered, this field cannot be populated.
2. The PPPoE Active Discovery Offer (PADO) packet:
   The PPPoE server, or Access Concentrator, should respond to the PADI with a PADO if the Access Concentrator is able to service the "service-name" field that had been listed in the PADI packet. If no "service-name" field had been listed, the Access Concentrator should respond with a PADO packet that has the "service-name" field populated with the service names that the Access Concentrator can service. The PADO packet is sent to the unicast address of the PPPoE client.
3. The PPPoE Active Discovery Request (PADR) packet:
   When a PADO packet is received, the PPPoE client responds with a PADR packet. This packet is sent to the unicast address of the Access Concentrator. The client may receive multiple PADO packets, but the client responds to the first valid PADO that the client received. If the initial PADI packet had a blank "service-name" field filed, the client populates the "service-name" field of the PADR packet with the first service name that had been returned in the PADO packet.
4. The PPPoE Active Discovery Session-confirmation (PADS) packet:
   When the PADR is received, the Access Concentrator generates a unique session identification (ID) for the Point-to-Point Protocol (PPP) session and returns this ID to the PPPoE client in the PADS packet. This packet is sent to the unicast address of the client.

When this process has completed, the client is aware of the address of the Access Concentrator and a session ID has been established. At this point, a normal PPP session begins. This session can remain established until a PPPoE Active Discovery Terminate (PADT) packet is sent. The PADT may be sent by either the Access Concentrator or the PPPoE client.