PPPoE

PPPoE, or Point-to-Point Protocol over Ethernet, is a network protocol that encapsulates PPP frames inside Ethernet frames. It combines PPP’s link-layer features—such as authentication and IP address negotiation—with the Ethernet access method used by many broadband networks. PPPoE is commonly used to provide DSL broadband connections where an ISP assigns a unique PPP session per customer.

How it works: PPPoE operates in two stages. First is the discovery phase, in which a client finds an access concentrator on the local Ethernet by sending a PADI (PPPoE Active Discovery Initiation) frame, receiving a PADO, and then sending a PADR, followed by the access concentrator replying with PADS to establish a session. Once a session is established, PPP negotiation proceeds as in standard PPP: Link Control Protocol (LCP) for setup, optional PAP or CHAP authentication, and IPCP to configure IP addresses. After authentication, IP traffic is carried in PPP frames over the Ethernet connection.

Overhead and MTU: PPPoE adds header overhead, typically reducing the standard Ethernet MTU from 1500 bytes to about 1492 bytes in common deployments.

Deployment and considerations: PPPoE is widely deployed in DSL networks and provides per-user accounting and session management. It introduces additional processing and overhead compared to native IP over Ethernet. In some modern networks, IP over Ethernet without PPPoE (IPoE) or DHCP-based methods are used instead, but PPPoE remains common in many older or legacy configurations.

Standards: PPPoE was defined in RFC 2516 (PPP over Ethernet). See also PPP and PPPoA for related technologies.