}
6.3.1.3 PPPoE Discovery Stage
In the PPPoE discovery stage, a PPPoE client will find a proper server, and then build
the connection. When a client initiates a PPPoE session, it should perform discovery
to indentify the PPPoE server
’s Ethernet MAC address, and establish a PPPoE
session ID.
PPPoE Client
PPPoE Server
PADI
PADO
PADR
PADS
Figure 6-10 PPPoE Discovery Stage Flows
The discovery stage includes the following four steps:
1)
PADI (PPPoE Active Discovery Initiation):
At the beginning, a PPPoE client
broadcasts a PADI packet to find all the servers that can be connected possibly.
Until it receives PADO packets from one or more servers. The PADI packet must
contain a service name which indicates the service requested by the client.
2)
PADO (PPPoE Active Discovery Offer):
When a PPPoE server receives a PADI
packet in its service range, it will send a PADO response packet. The PADO
packet must contain the server
’s name, and a service name identical to the one in
the PADI, and any number of other service names which indicate other services
that the PPPoE server can offer. If a PPPoE server receives a PADI packet
beyond its service range, it cannot respond with a PADO packet.
3)
PADR (PPPoE Active Discovery Request):
The client may receive more than
one PADO packet as the PADI was broadcast. The client chooses one server
according to the server
’s name or the services offered. Then the host sends a
PADR packet to the selected server. The PADR packet must contain a service
name which indicates the service requested by the client.
4)
PADS (PPPoE Active Discovery Session- confirmation):
When a PPPoE
server receives a PADR packet; it prepares to begin a PPP session. It generates
a unique PPPoE session ID, and respond to the client with a PADS packet. The
PADS packet must contain a service name which indicates the service provided
to the client.
When the discovery stage completes successfully, both the server and client know the
PPPoE session ID and the peer's Ethernet MAC address, which together define the
PPPoE session uniquely.
