A Method for Transmitting PPP Over Ethernet (PPPoE)

A Method for Transmitting PPP Over Ethernet (PPPoE)

Status of this Memo

This memo provides information for the Internet community. It does

not specify an Internet standard of any kind. Distribution of this

memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (1999). All Rights Reserved.

Abstract

The Point-to-Point Protocol (PPP) [1] provides a standard method for

transporting multi-protocol datagrams over point-to-point links.

This document describes how to build PPP sessions and encapsulate PPP

packets over Ethernet.

Applicability

This specification is intended to provide the facilities which are

defined for PPP, such as the Link Control Protocol, Network-layer

Control Protocols, authentication, and more. These capabilities

require a point-to-point relationship between the peers, and are not

designed for the multi-point relationships which are available in

Ethernet and other multi-access environments.

This specification can be used by multiple hosts on a shared,

Ethernet to open PPP sessions to multiple destinations via one or

more bridging modems. It is intended to be used with broadband

remote access technologies that provide a bridged Ethernet topology,

when access providers wish to maintain the session abstraction

associated with PPP.

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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.

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RFC 2516 Transmitting PPP Over Ethernet February 1999

The Discovery stage remains stateless until a PPP session is

established. Once a PPP session is established, both the Host and

the Access Concentrator MUST allocate the resources for a PPP virtual

interface.

4. Payloads

The following packet formats are defined here. The payload contents

will be defined in the Discovery and PPP sections.

An Ethernet frame is as follows:

1

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| DESTINATION_ADDR |

| (6 octets) |

| |

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| SOURCE_ADDR |

| (6 octets) |

| |

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| ETHER_TYPE (2 octets) |

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

~ ~

~ payload ~

~ ~

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| CHECKSUM |

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The DESTINATION_ADDR field contains either a unicast Ethernet

destination address, or the Ethernet broadcast address (0xffffffff).

For Discovery packets, the value is either a unicast or broadcast

address as defined in the Discovery section. For PPP session

traffic, this field MUST contain the peer's unicast address as

determined from the Discovery stage.

The SOURCE_ADDR field MUST contains the Ethernet MAC address of the

source device.

The ETHER_TYPE is set to either 0x8863 (Discovery Stage) or 0x8864

(PPP Session Stage).

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RFC 2516 Transmitting PPP Over Ethernet February 1999

The Ethernet payload for PPPoE is as follows:

1 2 3

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| VER | TYPE | CODE | SESSION_ID |

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| LENGTH | payload ~

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The VER field is four bits and MUST be set to 0x1 for this version of

the PPPoE specification.

The TYPE field is four bits and MUST be set to 0x1 for this version

of the PPPoE specification.

The CODE field is eight bits and is defined below for the Discovery

and PPP Session stages.

The SESSION_ID field is sixteen bits. It is an unsigned value in

network byte order. It's value is defined below for Discovery

packets. The value is fixed for a given PPP session and, in fact,

defines a PPP session along with the Ethernet SOURCE_ADDR and

DESTINATION_ADDR. A value of 0xffff is reserved for future use and

MUST NOT be used

The LENGTH field is sixteen bits. The value, in network byte order,

indicates the length of the PPPoE payload. It does not include the

length of the Ethernet or PPPoE headers.

5. Discovery Stage

There are four steps to the Discovery stage. When it completes, both

peers know the PPPoE SESSION_ID and the peer's Ethernet address,

which together define the PPPoE session uniquely. The steps consist

of the Host broadcasting an Initiation packet, one or more Access

Concentrators sending Offer packets, the Host sending a unicast

Session Request packet and the selected Access Concentrator sending a

Confirmation packet. When the Host receives the Confirmation packet,

it may proceed to the PPP Session Stage. When the Access

Concentrator

...