Discovery

PPPoE includes a Discovery protocol that allows each PPP session to learn the Ethernet address of the remote peer, as well as establish a unique session identifier. When a host wants to initiate a PPPoE session, it must first perform Discovery to identify the Ethernet MAC address of the peer and establish a PPPoE session ID.

Although PPP defines a peer-to-peer relationship, Discovery is inherently a client-server relationship. In the Discovery process, a host acting as a client discovers a remote access concentrator (AC), which acts as the server.

Based on the network topology, there may be more than one remote AC with whom the host can communicate. The Discovery stage allows the host to discover all remote ACs and then select the one to which it wants to connect.

In summary, the Discovery stage consists of the following four steps:

1. The host (PPPoE client) broadcasts a PPPoE Active Discovery Initiation (PADI) packet to all remote ACs in the network.
2. One or more remote ACs respond to the PADI packet by sending a PPPoE Active Discovery Offer (PADO) packet, indicating that they can serve the client request. The PADO packet includes the name of the AC from which it was sent.
3. The host sends a unicast PPPoE Active Discovery Request (PADR) packet to the AC to which it wants to connect.
4. The selected AC sends a PPPoE Active Discovery Session (PADS) packet to confirm the session.

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