Accommodating a Maximum Transit Unit/Maximum Receive Unit (MTU/MRU) Greater Than 1492 in the Point-to-Point Protocol over Ethernet (PPPoE)
RFC 4638

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Network Working Group                                          P. Arberg
Request for Comments: 4638                               D. Kourkouzelis
Category: Informational                                 Redback Networks
                                                              M. Duckett
                                                             T. Anschutz
                                                               BellSouth
                                                              J. Moisand
                                                        Juniper Networks
                                                          September 2006

  Accommodating a Maximum Transit Unit/Maximum Receive Unit (MTU/MRU)
                       Greater Than 1492 in the
             Point-to-Point Protocol 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 (2006).

IESG Note

   As of this writing, no current IEEE standard supports the use of
   "jumbo frames" (MTU greater than 1500).  Although this document
   contains recommended mechanisms to detect problems in the path,
   interoperability and reliability of non-standard extensions cannot be
   assured.  Both implementors and users of the protocol described here
   should exercise caution in its use.

Abstract

   The Point-to-Point Protocol over Ethernet (PPPoE), as described in
   RFC 2516, mandates a maximum negotiated Maximum Receive Unit (MRU) of
   1492.  This document outlines a solution that relaxes this
   restriction and allows a maximum negotiated MRU greater than 1492 to
   minimize fragmentation in next-generation broadband networks.

Arberg, et al.               Informational                      [Page 1]
RFC 4638                 PPPoE MRU/MTU Increase           September 2006

Table of Contents

   1. Introduction ....................................................2
   2. Terminology .....................................................4
   3. Proposed Solution ...............................................4
   4. PPPoE Discovery Stage ...........................................5
   5. LCP Considerations ..............................................5
      5.1. MRU Negotiations ...........................................5
      5.2. MRU Test and Troubleshooting ...............................6
   6. Security Considerations .........................................7
   7. IANA Considerations .............................................7
   8. Acknowledgements ................................................7
   9. Normative References ............................................7
   10. Informative References .........................................8

1.  Introduction

   As broadband network designs are changing from PC-initiated PPPoE [1]
   sessions in a combined Ethernet/Asynchronous Transfer Mode (ATM)
   setup, as shown in Figure 1, to more intelligent PPPoE-capable
   Residential Gateway (RG) and Gigabit Ethernet/ATM broadband network
   designs, as shown in Figures 2 and 3, the need to increase the
   maximum transmit and receive unit in the PPPoE protocol is becoming
   more important in order to reduce fragmentation in the network.

         <------------------ PPPoE session ------------------>

                                         +-----+           +-----+
       +--+              +---+           |     |           |     |
       |PC|--------------|CPE|-----------|DSLAM|-----------| BRAS|
       +--+  <Ethernet>  +---+   <ATM>   |     |   <ATM>   |     |
                                         +-----+           +-----+

        Figure 1.  Initial broadband network designs with PPPoE

   In the network design shown in Figure 1, fragmentation is typically
   not a problem, since the subscriber session is PPPoE end to end from
   the PC to the BRAS.  Therefore, a PPP-negotiated MRU of 1492 octets
   is fully acceptable, as it makes the largest PPPoE frame adhere to
   the standard Ethernet MTU of 1500 octets.

Arberg, et al.               Informational                      [Page 2]
RFC 4638                 PPPoE MRU/MTU Increase           September 2006

         <----- IPoE -----> <--------- PPPoE session --------->

                                         +-----+            +-----+
       +--+              +---+           |     |            |     |
       |PC|--------------| RG|-----------|DSLAM|------------| BRAS|
       +--+  <Ethernet>  +---+   <ATM>   |     |   <GigE>   |     |
                                         +-----+            +-----+

     Figure 2.  Next-generation broadband network designs with PPPoE

   In the network design shown in Figure 2, fragmentation becomes a
   major problem, since the subscriber session is a combination of IPoE
   and PPPoE.  The IPoE typically uses a Maximum Transit Unit (MTU) of