PPP over Ethernet (PPPOE) BOF
8/25/98, 0900 - 1000
Chair(s): Louis Mamakos <louie@uu.net>, UUNET
David Carrel <carrel@RedBack.net>, RedBack Networks
Reported by: Jim Solomon
I. "Service Requirements and Motivation" -- Louis Mamakos
Louis presented the service and operational requirements for high-speed, consumer-
oriented, Internet access. The proposed protocol does not attempt to solve every
problem in every setting; however, it solves the 90%-case very simply and elegantly and
is extensible for adaptation to future needs.
Service Provider Requirements:
- unobtrusive to existing network configuration; i.e., the presence of a high-speed
modem should not require renumbering the installed systems.
- provide a session abstraction to the user; i.e., provide a familiar interface to the user
that leverages the existing dial experience.
- provide multiple sessions to the same PC; allow many PCs to set up sessions to
possibly many Broadband Access Servers (BAS) through possibly many modems;
and allow many PCs to share a single modem while allowing them to have sessions
to different Network Service Providers, perhaps simultaneously.
- provide operation over a single ATM or Frame Relay virtual circuit; specifically, do
not require the telco to provision an additional PVC for every new host or Network
Service Provider.
- work with the existing, installed base of CPE devices (modems), which all support
bridged RFC 1483 or bridged RFC 1490 encapsulation.
- require no configuration of the CPE device; such devices should be cheap and easy
to use.
II. "Protocol Overview" -- Dave Carrel
Internet Draft: draft-carrel-info-pppoe-00.txt
The PPP over Ethernet (PPPOE) protocol has two distinct stages:
1. Discovery -- the process by which the PC and the BAS discover each other's MAC
address and by which the BAS assigns a Session ID for the "Session" phase of PPPOE.
2. Session -- the data transfer phase in which PPP frames are encapsulated within
Ethernet payloads.
Discovery consists of the exchange of four packets: initiation, offer, session, confirmation.
Once discovery completes, the protocol enters the session phase and uses the Session ID
assigned during discovery.
III. "Discussion" -- Many
The most frequent comment regarded a desire to run L2TP from the PC to the BAS,
rather than the PPPOE protocol. It was noted that LT2P over UDP over IP requires
configuration of the CPE device (it needs, minimally, an IP address) and unecessarily
burns additional IP addresses.
The suggestion was then made to run L2TP directly over Ethernet. It was noted that
L2TP's "control channel" and other functionality were unnecessarily complex for the
target environment and, by the time this functionality was stripped out, one was left with
something that looks a lot like PPPOE. It was also noted that L2TP over Ethernet does
not solve the objection of PPPOE's MTU being something less than 1500 bytes (an issue
only for BCP and protocols other than IPv4 and IPv6). Others noted that requiring an
L2TP tunnel for every PPP session to the Internet does not scale well.