PPP Over Ethernet (PPPoE) Extensions for Credit Flow and Link Metrics
RFC 4938

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Network Working Group                                           B. Berry
Request for Comments: 4938                                    H. Holgate
Category: Informational                               Cisco Systems,Inc.
                                                               June 2007

                 PPP Over Ethernet (PPPoE) Extensions
                   for Credit Flow and Link Metrics

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 IETF Trust (2007).

IESG Note

   The PPP Extensions Working Group (PPPEXT) has reservations about the
   desirability of the feature described in this document.  In
   particular, it solves a general problem at an inappropriate layer and
   it may have unpredictable interactions with higher and lower level
   protocols.  The techniques described in this document are intended
   for use with a particular deployment technique that uses a PPP
   termination separated from a radio termination by an Ethernet, and
   that has radio-side flow control for a slower PPP-only link to remote
   nodes.  Implementors are better advised to avoid split termination
   with inter-media protocol translation, and use standard Internet
   Protocol routing instead.

Abstract

   This document extends the Point-to-Point over Ethernet (PPPoE)
   Protocol with a credit-based flow control mechanism and Link Quality
   Metric report.  This optional extension should improve the
   performance of PPPoE over media with variable bandwidth and limited
   buffering, such as mobile radio links.

Berry & Holgate              Informational                      [Page 1]
RFC 4938           PPPoE with Credit Flow and Metrics          June 2007

Table of Contents

   1. Introduction ....................................................2
   2. Payload .........................................................3
   3. Overview of Protocol Extensions .................................3
   4. Discovery Stage .................................................3
      4.1. PPPoE Active Discovery Request (PADR) ......................4
      4.2. PPPoE Active Discovery Session-confirmation (PADS) .........4
      4.3. PPPoE Active Discovery Session-Grant (PADG) ................5
      4.4. PPPoE Active Discovery Session-Credit Response (PADC) ......5
      4.5. PPPoE Active Discovery Quality (PADQ) ......................6
   5. PPP Session Stage ...............................................7
   6. Credit Flow Considerations ......................................7
   7. PADG and PADC Retransmission ....................................8
   8. Other Considerations ............................................9
   9. IANA Considerations .............................................9
   10. Security Considerations ........................................9
   Appendix A: Tag Values.............................................10
   Appendix B: Example Message Formats................................11
   Acknowledgements...................................................15
   Normative References...............................................15

1.  Introduction

   PPP over Ethernet (PPPoE) [2] is a protocol for establishing and
   encapsulating sessions between hosts and traffic aggregators (Access
   Concentrators) for PPP [1] transport over real or emulated Ethernet.
   PPPoE works well when both session endpoints have similar bandwidth,
   forwarding, and buffering capabilities that do not vary over time.
   However, it is insufficient for applications with variable bandwidth
   and limited buffering (for example, mobile radio links).  This
   document addresses this problem by suggesting an extension to PPPoE
   to support credit-based session flow control and session-based link
   metric exchanges.

   The diagram below illustrates the problem that this extension is
   intended to solve, for the case of a radio link.  Here PPPoE sessions
   are used between access concentrators (routers) and radio
   transmission systems that are shown as radio neighbors.  Each radio
   transmission system establishes point-to-point Radio Link Protocol
   (RLP) sessions with its neighbors and establishes a corresponding
   PPPoE session for each neighbor with the transmission system's
   associated access concentrator (router).  The radio logically
   associates the PPPoE session with the corresponding RLP session.

Berry & Holgate              Informational                      [Page 2]
RFC 4938           PPPoE with Credit Flow and Metrics          June 2007

   +--------+       +-------+       +-------+       +--------+
   | Access |       | Host  |       | Host  |       | Access |
   | Conc.  |=======| Radio |~~~~~~~| Radio |=======| Conc.  |
   +--------+       +-------+       +-------+       +--------+
            |       |       |       |       |       |