Skip to main content

Accommodating a Maximum Transit Unit/Maximum Receive Unit (MTU/MRU) Greater Than 1492 in the Point-to-Point Protocol over Ethernet (PPPoE)
draft-arberg-pppoe-mtu-gt1492-03

The information below is for an old version of the document that is already published as an RFC.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 4638.
Authors Mike Duckett , Jerome Moisand , Tom Anschutz , Diamantis Kourkouzelis , Peter Arberg
Last updated 2015-10-14 (Latest revision 2006-03-21)
RFC stream Internet Engineering Task Force (IETF)
Intended RFC status Informational
Formats
Additional resources Mailing list discussion
Stream WG state (None)
Document shepherd (None)
IESG IESG state Became RFC 4638 (Informational)
Action Holders
(None)
Consensus boilerplate Unknown
Telechat date (None)
Responsible AD Mark Townsley
Send notices to james.d.carlson@sun.com
draft-arberg-pppoe-mtu-gt1492-03
PPP Extensions Working Group
Internet Draft                                              Peter Arberg
                                                  Diamantis Kourkouzelis
Intended status: Informational                          Redback Networks
Expiration date: September 2006 
                                                            Mike Duckett
                                                            Tom Anschutz
                                                               BellSouth

                                                          Jerome Moisand
                                                        Juniper Networks
                                                              March 2006

          Accommodating an MTU/MRU greater than 1492 in PPPoE
                <draft-arberg-pppoe-mtu-gt1492-03.txt>

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
   aware will be disclosed, in accordance with Section 6 of BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-
   Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt.

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on September 9, 2006.

Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   Point-to-Point Protocol Over Ethernet (PPPoE), as described in RFC
   2516 [1], mandates a maximum negotiated MRU of 1492. This document 
   outlines a solution to relax that restriction and allow a maximum 
   negotiated MRU greater than 1492 to minimize fragmentation in next 
   generation broadband networks. 

Arberg                   Expires September 2006                 [Page 1]
Internet Draft             PPPoE MRU/MTU Increase             March 2006

1. Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC2119 [3].

      ATM          - Asynchronous Transfer Mode .
      PPP          - Point-to-Point Protocol.
      PPPoA        - PPP over AAL5.
      PPPoE        - PPP over Ethernet.
      MTU          - Maximum Transmit Unit
      MRU          - Maximum Receive Unit
      PC           - Personal Computer.
      CPE          - Customer Premises Equipment.
      RG           - Residential Gateway.
      BRAS         - Broadband Remote Access Server.
      DSLAM        – Digital Subscriber Line Access Multiplexer
      PPPoE client - PC, RG or CPE which initiates a PPPoE session
      PPPoE server - BRAS terminating PPPoE sessions initiated by client

2. Introduction

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

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

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

    Fig. 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, so 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                   Expires September 2006                 [Page 2]
Internet Draft             PPPoE MRU/MTU Increase             March 2006

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

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

    Fig. 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 use a MTU of 1500 octets.
    However, when the Residential Gateway and the BRAS are the PPPoE 
    session endpoints, and therefore negotiate a MTU/MRU of 1492 octets 
    resulting in a large number of fragmented packets in the network.

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

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

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

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

    Fig. 3: Broadband network designs with PPPoA to PPPoE conversion.

    In the network design shown in figure 3, which is studied by the 
    DSL-Forum in the context of the migration to Ethernet for broadband 
    aggregation networks, fragmentation is not the only problem when 
    MRU differences exist in PPPoA and PPPoE sessions.

    The subscriber session is a PPP session running over a combination 
    of PPPoA and PPPoE. The PPP/PPPoA host typically negotiates a 
    1500 octets MRU. Widely deployed PPP/PPPoA hosts in CPE equipment
    do not support an 1492 octets MRU, which creates an issue in turn 
    for the BRAS (PPPoE server) if strict compliance to RFC2516 [1] is 
    mandated. For PPP/PPPoA hosts capable of negotiating a 1492 octets 
    MRU size, then we are back to a fragmentation issue.

Arberg                   Expires September 2006                 [Page 3]
Internet Draft             PPPoE MRU/MTU Increase             March 2006

3. Proposed solution

   The procedure described in this document do not strictly conform 
   to IEEE standards for Ethernet packet size, but rely on a widely 
   deployed behavior of supporting jumbo frames on Ethernet segments.

   Since next generation broadband networks are built around Ethernet 
   systems supporting baby-giants and jumbo frames with payload sizes 
   larger than the normal Ethernet MTU of 1500 octets, a BRAS acting
   as a PPPoE server MUST support PPPoE MRU negotiations larger than 
   1492 octets in order to limit the amount of fragmented packets in 
   network designs shown in section 1.

   By default, the Maximum-Receive-Unit (MRU) option MUST follow the
   rules set forward in RFC1661 [2], but MUST NOT be negotiated to a
   larger size than 1492 to guarantee compatibility with Ethernet
   network segments limited to 1500 octets frames. In such a case, 
   the PPPoE header being 6 octets and the PPP Protocol ID being 
   2 octets, the PPP MRU MUST NOT be greater than 1492.

   An optional PPPoE tag "PPP-Max-Payload" allows a PPPoE client to 
   override this default behavior by providing a maximum size for the 
   PPP payload it can support in both the sending and receiving 
   directions. When such a tag is received by the PPPoE server, the 
   server MAY allow the negotiation of a larger MRU than 1492 and the 
   use of a larger MTU than 1492 subject to limitations of its local 
   configuration and according to the rules set forward in RFC1661 [2], 
   and within the limits of the maximum payload size being indicated by
   the PPPoE client.

4. PPPoE Discovery Stage

   If a PPPoE client wants to use a higher MTU/MRU than 1492 octets, 
   then it MUST include an optional PPP-Max-Payload Tag in the PADI 
   and PADR packets.
   If the PPPoE server can support a higher MTU/MRU than 1492 octets, it
   MUST respond with an echo of the clients tag in the PADO and PADS 
   packets when the PPP-Max-Payload tag is received from the client.

   Tag-name:   PPP-Max-Payload
   Tag-value:  0x0120
   Tag-length: 2 octets
   Tag-value:  binary encoded value (max PPP payload in octets)

   Tag-description:
   This TAG indicates that the client and server are capable of 
   supporting a given maximum PPP payload greater than 1492 octets for 
   both the sending and receiving directions. 
   Note that this value represents the PPP payload, so it is directly
   comparable with the value used in the PPP MRU negotiation.

Arberg                   Expires September 2006                 [Page 4]
Internet Draft             PPPoE MRU/MTU Increase             March 2006

5. LCP Considerations

5.1 MRU Negotiations

   Since Ethernet (without jumbo frames) has a maximum payload size of
   1500 octets, the PPPoE header is 6 octets and the PPP Protocol ID is 
   2 octets, the Maximum-Receive-Unit (MRU) option MUST NOT be 
   negotiated to a larger size than 1492, unless both the PPPoE client 
   and server have indicated the ability to support a larger MRU in the 
   PPPoE Discovery Stage.

   The initial MRU negotiation for the PPP/PPPoE server MUST follow a 
   flow as shown below:

   If PPPoE {
      PPP_MRU_Max = 1492
      If (PPP-Max-Payload-Tag) AND (PPP-Max-Payload-Tag > 1492)
        Then PPP_MRU_Max = min (PPP-Max-Payload-Tag, Interface MTU-8)
   }
   "Normal" PPP_MRU_Negotiation (PPP_MRU_Max)

   If the PPP-Max-Payload tag is present and greater than 1492, it MUST
   be considered along with the server's interface MTU settings when 
   selecting the maximum value for the normal RFC1661 [2] MRU 
   negotiation which decides the actual MRU to use.

   If the PPP-Max-Payload tag isn’t present, or is present but below 
   1492, then the existing MRU constraint of 1492 octets MUST stay 
   applicable, hence preserving backward compatibility.

   This in summary indicates the following behavior:
   1. when a "PPP-Max-Payload" tag is received, 
     a. the value in this tag will indicate the maximum allowed 
        MRU to accept and suggest in a MRU negotiation, 
     b. if MRU is not negotiated then RFC1661 [2] will set the default 
        MRU at 1500. This will say that the "PPP-Max-Payload" tag can 
        have a greater value than 1500, but in this case RFC1661 [2] 
        sets the default MRU to 1500, and only if MRU is negotiated 
        higher (up to maximum payload) will the "PPP-Max-Payload" tag 
        value be used.

   2. when a "maximum-payload" tag is not received by either end,
      then RFC2516 [1] sets the rule.

Arberg                   Expires September 2006                 [Page 5]
Internet Draft             PPPoE MRU/MTU Increase             March 2006

5.2 MRU test and troubleshooting

   If the MRU is negotiated to a larger value than 1492 octets, the 
   sending side SHOULD have the option to send one or more MRU-sized 
   Echo-Request packets once the session is opened. This allows it to 
   test that the receiving side and any intermediate equipment can 
   handle such packet size.
   If no Echo-Replies are received, the sending side MAY choose to 
   repeat the test with 1492 octets Echo-Request packets. If these 
   packets receive replies, the sending side MUST not send packets 
   bigger than 1492 octets for this session.

  This capability SHOULD be enabled by default. It SHOULD be 
  configurable and MAY be disabled on networks where there is some 
  prior knowledge indicating that the test is not necessary.

6. Security Considerations

   This document does not introduce new security issues. The security
   considerations pertaining to the original PPPoE protocol [1] remain
   relevant.

7. IANA Considerations

   No IANA action is required.

8. Acknowledgments

   The authors would like to thank Prakash Jayaraman, Amit Cohen, 
   Jim Ellis, David Thorne, John Reid, Oliver Thorp, Wojciech Dec, 
   Jim Wilks, Mark Townsley, Bart Salaets, Tom Mistretta, Paul Howard,
   Dave Bernard and Darren Nobel for their contributions and comments 
   to this document.

9. Normative References

   [1] Mamakos L., Lidl K., Evarts J., Carrel D., Simone D., Wheeler R.,
       "A Method for Transmitting PPP Over Ethernet (PPPoE)", 
       RFC 2516, February 1999

   [2] W. Simpson "The Point-to-Point Protocol (PPP)", RFC 1661, 
       July 1994

   [3] Bradner, S., "Key words for use in RFCs to Indicate Requirement 
       Levels", BCP 14, RFC 2119, March 1997.

Arberg                   Expires September 2006                 [Page 6]
Internet Draft             PPPoE MRU/MTU Increase             March 2006

Authors' Addresses

   Peter Arberg 
   Redback Networks, Inc.
   300 Holger Way
   San Jose, CA 95134
 
   Email: parberg@redback.com
 
 
   Diamantis Kourkouzelis 
   Redback Networks, Inc.
   300 Holger Way
   San Jose, CA 95134
 
   Email: diamondk@redback.com

   Mike Duckett
   BellSouth Telecommunications, Inc. 
   575 Morosgo Drive
   Atlanta, GA 30324

   Email: mike.duckett@bellsouth.com

   Tom Anschutz
   BellSouth Science and Technology
   725 W. Peachtree St.
   Atlanta, GA 30308

   Email: tom.anschutz@bellsouth.com

   Jerome Moisand 
   Juniper Networks, Inc.
   10 Technology Park Drive
   Westford, MA 01886

   Email: jmoisand@juniper.net

Arberg                   Expires September 2006                 [Page 7]
Internet Draft             PPPoE MRU/MTU Increase             March 2006

Intellectual Property Statement

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; nor does it represent that it has
   made any independent effort to identify any such rights.  Information
   on the procedures with respect to rights in RFC documents can be
   found in BCP 78 and BCP 79.

   Copies of IPR disclosures made to the IETF Secretariat and any
   assurances of licenses to be made available, or the result of an
   attempt made to obtain a general license or permission for the use of
   such proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository at
   http://www.ietf.org/ipr.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard.  Please address the information to the IETF at
   ietf-ipr@ietf.org.

Disclaimer of Validity

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
   INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Copyright Statement

   Copyright (C) The Internet Society (2006).  This document is subject
   to the rights, licenses and restrictions contained in BCP 78, and
   except as set forth therein, the authors retain all their rights.

Acknowledgment

   Funding for the RFC Editor function is currently provided by the
   Internet Society.

Arberg                   Expires September 2006                 [Page 8]
Internet Draft             PPPoE MRU/MTU Increase             March 2006

Changes from internet-draft version 2.

Section "Status of this Memo": changed to include the IPR statement

Added a "Copyright Notice"

Renamed section 2 from "Motivation" to "Introduction"

Section 2:
Changed:   The IPoE typically negotiate a MTU of 1500 bytes.
To:        The IPoE typically use a MTU of 1500 octets.

Section 5.1:
Changed: The pseudo code example.
      If PPPoE {
        If (PPP-Max-Payload-Tag) Not Present
          Then PPP_MRU_Max = 1492
          Else PPP_MRU_Max = min (PPP-Max-Payload-Tag, Interface MTU-8)
      }
      "Normal" PPP_MRU_Negotiation (PPP_MRU_Max)

To:   If PPPoE {
        PPP_MRU_Max = 1492
        If (PPP-Max-Payload-Tag) AND (PPP-Max-Payload-Tag > 1492)
          Then PPP_MRU_Max = min (PPP-Max-Payload-Tag, Interface MTU-8)
      }
      "Normal" PPP_MRU_Negotiation (PPP_MRU_Max)

Changed:      
   If the PPP-Max-Payload tag is present, it MUST be considered as the
   maximum value for the "normal" MRU negotiation which is the master 
   and decision maker of what the actual MRU will be negotiated to, 
   never higher than the PPP-Max-Payload tag, but it can be negotiated
   to a lower value depending on the server's interface settings and 
   the peer's negotiated MRU value.

To: 
   If the PPP-Max-Payload tag is present and greater than 1492, it MUST
   be considered along with the server's interface MTU settings when 
   selecting the maximum value for the normal RFC1661 [2] MRU 
   negotiation which decides the actual MRU to use.

Changed:
   If the PPP-Max-Payload tag isn’t present, then the existing MRU 
   constraint of 1492 bytes would stay applicable, hence preserving 
   backward compatibility.

Arberg                   Expires September 2006                 [Page 9]
Internet Draft             PPPoE MRU/MTU Increase             March 2006

To:    
   If the PPP-Max-Payload tag isn’t present, or is present but below 
   1492, then the existing MRU constraint of 1492 octets MUST stay 
   applicable, hence preserving backward compatibility.

Section 5.2:
Changed:   This capability SHOULD be disabled by default, and SHOULD 
           only be available for debug, test purpose.

To:        This capability SHOULD be enabled by default. It SHOULD be 
           configurable and MAY be disabled on networks where there is 
           some prior knowledge indicating that the test is not 
           necessary.

Added section "7. IANA Considerations"
Added section "Intellectual Property Statement"
Added section "Disclaimer of Validity"
Added section "Copyright Statement"
Added section "Acknowledgment"

Changed the word bytes to octets in the document.
Editorial Changes to remove the "nits" found in v2.

Arberg                   Expires September 2006                [Page 10]