PPPoE principle Overview

We have to understand the principles of a protocol well. Here we will explain the principles of PPPoE. First, let's take a look at the definition of this PPPoE protocol. Let's take a look at it from the basics.

PPPoE principle-Definition

With PPPOE, multiple hosts on a shared Ethernet can have multiple PPP sessions with the Remote Access Concentrator through one or more simple Bridge access devices. With this model, each host uses its own PPP protocol stack and provides users with a familiar user interface. Access control, billing, and service types can be handled by each user rather than by each site. PPPOE consists of two phases: discovery and PPP session. The discovery phase is a stateless Client/Server mode. The objective is to obtain the Ethernet MAC address of the PPPOE end and create a unique PPPOESESSION_ID. After the discovery stage ends, it enters the standard PPP session stage.

PPPoE principle-basic Protocol framework

For PPPOE protocol, refer to RFC2516. PPPOE supports the adaptation of PPP frames on the Ethernet and provides PPP connections on the Ethernet. Figure 1 and figure 2 respectively refer to the PPPOE protocol stack on Ethernet and the PPPOE protocol stack on AAL5.

PPPOE connection

Figure 3 shows the typical PPPOE connection mode

PPPoE principle-PPPOE communication process

PPPOE has two different phases: discovery phase and PPP session phase. When a host wants to start a PPPOE session, it must first identify the Ethernet MAC address of the Peer end and establish a PPPOESESSION_ID. In the discovery phase, the host can discover Multiple Access Concentrator based on the network topology. The discovery phase allows the host to discover all Access Concentrator, and then select one. When the discovery phase is completed successfully, both the host and the selected Access Concentrator have their information for establishing PPP connections over Ethernet. The discovery phase remains stateless until the PPP session is established. Once a PPP session is established, both the host and the Access Concentrator must allocate resources to the PPP Virtual Interface. Figure 4 shows the PPPOE communication process.

Discovery phase

There are four steps in the discovery phase. When this phase is complete, both ends of the communication will know the PPPOESESSION_ID and the peer ethernet address, which together uniquely define the PPPOE session. These steps include: the host broadcasts an initiator group PADI), one or more Access Concentrator sends messages to the group PADO), and the host sends unicast Session requests to the group PADR ), the selected Access Concentrator sends a validation group PADS ). When the host receives a confirmation group, it can start the PPP session phase. When the Access Concentrator sends a confirmation group, it can start the PPP session phase.

When the host does not receive PADO within the specified time, it should resend its PADI group and double the wait time. This process will be repeated as expected. If the host is waiting to receive PADS, use a similar timeout mechanism with the host resending PADR. After a specified number of retries, the host should resend the PADI group.

PPPOE also has a PADT group, which can be sent at any time after the session is established to terminate the PPPOE session. It can be sent by the host or Access Concentrator. When a PADT is received, this session is no longer allowed to send PPP services. After sending or receiving PADT, you do not have to send it even if the normal PPP terminates the group. The PPP peer end should use the PPP protocol itself to terminate the PPPOE session. However, when the PPP cannot be used, the PADT can be used.

PPP session phase

Once the PPPOE session starts, PPP data can be sent in any other form of PPP encapsulation. All Ethernet frames are unicast. The SESSION_ID of the pppoe session must not be changed and must be the value assigned by the discovery phase.

PPPoE principle-pppoe lcp configuration options

PPPoverEthernetRFC2516) we recommend that you negotiate the magic number option. We recommend that you do not negotiate the Protocol domain compression option PFC. In implementation, you must not request any of the following options for negotiation, and reject the following options for negotiation: FieldCheckSequence (FCS) Alternatives, Address-and-Control-Field-Compression (ACFC ), asynchronous-Control-Character-Map (ACCM) MRU must not be greater than 1492. It is recommended that the Access Concentrator occasionally send an Echo_Request message to the host to determine the session status. Otherwise, if the host terminates the session without sending the Terminate_Request message, the Access Concentrator cannot determine whether the session has been terminated. When the LCP ends, the host and Access Concentrator must stop using this PPPOE session. If the host wants to start another PPP session, it must return to the PPPOE discovery phase.