The PPP protocol requires a point-to-point relationship between the two communicating parties, which is not suitable for broadcast Ethernet and other multi-point access networks, so the PPPOE protocol (Point-to-Point Protocol Over Ethernet) was created. It not only provides a broadband access method for users who use bridged Ethernet access, but also provides convenient access control and billing. Each access user establishes a unique PPP session, so the MAC address of the remote access centralized device must be known before the session is established. The PPPOE protocol can be obtained through the discovery protocol.
As defined in RFC2516, PPPoE has two obvious stages, the discovery stage (PPPoE Discovery stage) and the PPP session stage (PPPoE Session Stage).
When the host wants to start a PPPOE session, it must first perform a discovery process to identify the other party's MAC address, and then establish a unique PPPOE session ID. PPPOE uses a discovery protocol to solve this problem, which is based on a client/server model. Due to the broadcast characteristics of Ethernet, the host (client) can discover all access concentrators (servers) in this process, and select one of them, and establish a point-to-point connection between the two based on the information obtained. When a PPP session is established, the entire discovery phase of PPPOE is completed.
There are four steps in the discovery phase. When it is completed, both PPPoE parties know the PPPoE SESSION_ID and the other party's Ethernet address, and together they uniquely define the PPPoE session:
- The host broadcasts an initial data packet PADI (PPPOE Active Discovery Initiation) {PADI: The host sends a broadcast to find the PPPoE server, the source is the mac address of the local interface, and the destination is FFFF.FFFF.FFFF.FFFF}
- The access concentrator sends effective discovery to provide data packet response request PADO (PPPOE Active Discovery Offer) {PADO: The destination mac address of the response to the client is the mac address of the client}
- The user host sends the session request packet PADR (PPPOE Active Discovery Request) in unicast to the access concentrator {Unicast request, request to establish a link}
- The selected access concentrator sends a confirmation packet PADS (PPPOE Active Discovery Session-confirmation) {confirmation, if the user name and password are all correct, they are assigned to the ip address and start to connect to the network} Then PPP goes through standard LCP and NCP (IPCP) process, the
following figure shows how PPPoE works:
The following is a packet capture analysis of the process: