PPPoE/v6 Administrator's Guide

Introduction
Protocol Overview
Chapter 114
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.