Protocol Overview

Any PPPoE session typically comprises two peers communicating over an Ethernet link. Each peer must learn the Ethernet address of the remote peer and also establish a unique session identifier that identifies the PPPoE session between the peers. Figure 1-2 “When Client and Server Are on the Same Link” depicts how the PPPoE client and the server, on the same link, communicate with each other.

Figure 1-2 When Client and Server Are on the Same Link

You need an optional relay when there is a router in between the PPPoE client and the server. In such cases, the relay receives packets from the clients and transmits them on the other link to the server and from the server to the client. See Figure 1-3 “When Client and Server Are on Different Links” for information on how the client connects to the server via a relay.

Figure 1-3 When Client and Server Are on Different Links

Each PPPoE session includes the following stages:

Discovery stage
PPPoE functions as a client/server model in the discovery stage. Before a host tries to initiate a PPP session, it searches for a suitable PPPoE server (access concentrator) that can provide the services required by the host. The host then learns the server’s Media Access Control (MAC) address and a unique session number called the session ID. During this process, if the host finds more than one access concentrator, it chooses the most appropriate one with which to establish connection. After the host selects the access concentrator, both the host and the chosen access concentrator have the necessary information to establish their PPP connection over the Ethernet.
The host and the server exchange packets as follows:
1. The host broadcasts the initiation packet, PPPoE Active Discovery Initiation ( PADI).
2. One or more access concentrators (servers) send offer packets, PPPoE Active Discover Offer ( PADO), to the host.
3. The host sends the unicast session request packet, PPPoE Active Discovery Request ( PADR), to the access concentrator from which it requests service.
4. The selected access concentrator sends the confirmation packet, PPPoE Active Discovery Session-confirmation (PADS), to the host.
5. Either the host or the access concentrator sends the termination packet, PPPoE Active Discovery Terminate (PADT), to indicate the close of a session.
Session stage
PPPoE functions as a peer-peer protocol in the session stage. After learning the Ethernet address and the session number, both the host and the server start the pppd daemon. The PPPoE session frames encapsulate the PPP frames. All Ethernet packets are unicast.

Protocol Overview

Technical documentation

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