RFC 2516 (rfc2516) - Page 2 of 17
A Method for Transmitting PPP Over Ethernet (PPPoE)
Alternative Format: Original Text Document
RFC 2516 Transmitting PPP Over Ethernet February 1999
This document describes the PPP Over Ethernet encapsulation that is
being deployed by RedBack Networks, RouterWare, UUNET and others.
1. Introduction
Modern access technologies are faced with several conflicting goals.
It is desirable to connect multiple hosts at a remote site through
the same customer premise access device. It is also a goal to
provide access control and billing functionality in a manner similar
to dial-up services using PPP. In many access technologies, the most
cost effective method to attach multiple hosts to the customer
premise access device, is via Ethernet. In addition, it is desirable
to keep the cost of this device as low as possible while requiring
little or no configuration.
PPP over Ethernet (PPPoE) provides the ability to connect a network
of hosts over a simple bridging access device to a remote Access
Concentrator. With this model, each host utilizes it's own PPP stack
and the user is presented with a familiar user interface. Access
control, billing and type of service can be done on a per-user,
rather than a per-site, basis.
To provide a point-to-point connection over Ethernet, each PPP
session must learn the Ethernet address of the remote peer, as well
as establish a unique session identifier. PPPoE includes a discovery
protocol that provides this.
2. Conventions
The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
document, are to be interpreted as described in [2].
3. Protocol Overview
PPPoE has two distinct stages. There is a Discovery stage and a PPP
Session stage. When a Host wishes to initiate a PPPoE session, it
must first perform Discovery to identify the Ethernet MAC address of
the peer and establish a PPPoE SESSION_ID. While PPP defines a
peer-to-peer relationship, Discovery is inherently a client-server
relationship. In the Discovery process, a Host (the client)
discovers an Access Concentrator (the server). Based on the network
topology, there may be more than one Access Concentrator that the
Host can communicate with. The Discovery stage allows the Host to
discover all Access Concentrators and then select one. When
Discovery completes successfully, both the Host and the selected
Access Concentrator have the information they will use to build their
point-to-point connection over Ethernet.
Mamakos, et. al. Informational