BlogA Brief Introduction of PPPoE Workflow

A Brief Introduction of PPPoE Workflow

PPPoE (Point to Point Protocol Over Ethernet) is a network tunnel protocol that encapsulates Point to Point Protocol (PPP) in the Ethernet framework.

Because PPP protocol is integrated in the PPPoE protocol, the authentication, encryption, compression and other functions that traditional Ethernet cannot provide are realized. It can also be used in the protocol system where Ethernet protocols such as Cable Modem and Digital Subscriber Line (DSL) provide access services to users.

Compared with traditional access methods, PPPoE has a higher cost performance. It is widely used in a series of applications, including cell networking. The current popular broadband access method ADSL uses the PPPoE protocol.

With the increasing popularity of low-cost broadband technology, DSL is also used more and more frequently. It uses telephone facilities and only invests a very small transformation cost. It uses the unused part of the traditional telephone communication cable to provide high-speed data transmission service, making the communication infrastructure more fully used and sharing voice and data, which is the core foundation of DSL network.

Most computers that access the Internet through ADSL are connected to the Internet through Ethernet cards. The normal TCP/IP mode is also used, and no new protocol is attached. On the other hand, the modem dial-up network uses the PPP protocol, namely Point to Point Protocol, which has the functions of user authentication and IP address notification. PPP over Ethernet (PPPoE) protocol is a technology for relaying PPP frame information in Ethernet, especially for ADSL and other methods.

DSL networks have many forms, but they can be roughly classified into two types: ADSL (Asymmetric Digital Subscriber Line) and SDSL (Symmetric Digital Subscriber Line).

ADSL: It provides asymmetric upload and download speeds. It is one of the most widely deployed broadband technologies in the DSL market. It provides 1.5~8Mbit/s download speeds and 16kbit/s~1Mbit/s upload speeds. It allows voice transmission and data transmission over the telephone line. When users use ADSL network, they can make phone calls as well as surf the Internet.

SDSL: It provides symmetrical upload and download speeds, ranging from 128kbit/s to 2.32mbit/s, and the common speed is 768kbit/s.

PPPoE: It runs PPP over Ethernet. The traditional PPP protocol is a WAN protocol designed for point-to-point links. It is convenient for authentication, monitoring and timing, because there are only two points on the line. One is the PPP access end of the user, and the other is the service end of the ISP receiving the PPP.

Working process of PPPoE:

Discovery phase:

The user host sends the PADI (PPPoE Active Discovery Initiative) package in the broadcast mode, and is ready to obtain all connectable access devices (to obtain their MAC addresses);

After receiving the PADI packet, the access device returns the PADO (PPPoE Active Discovery Offer) as a response;

The user host selects an appropriate access device from multiple PADO packets received according to its name type name or service name, and then sends the PADR (PPPoE Active Discovery Request) packet. In addition, if a user host does not receive the PADO within the specified time after sending the PADI, it will resend the PADI;

After receiving the PADR packet, the access device returns the PAS (PPPoE Active Discovery Session Confirmation) packet, which contains a unique session ID, and both parties enter the PPP session phase.

PPP session phase:

The user host and the access concentrator conduct the PPP session according to the PPP session connection parameters negotiated in the discovery phase.
Once the PPPoE session starts, the PPP data can be sent in any other PPP encapsulation form. All Ethernet frames are unicast.

The SESSION-ID of the PPPoE session must not be changed, and must be the value allocated in the discovery phase.

PPPoE also has a PADT packet, which can be sent at any time after the session is established to terminate the PPPoE session, that is, to release the session. It can be sent by the host or access concentrator. When the other party receives a PADT packet, it is no longer allowed to use this session to send PPP services. The SESSION-ID field value is the session identification number of the PPP session to be terminated.

After sending or receiving the PADT, it is not necessary to send the packet even if the normal PPP terminates the packet. The PPP peer should use the PPP protocol itself to terminate the PPPoE session, but when the PPP cannot be used, PADT can be used.

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