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Versions: 00 01 02 03 RFC 4638

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
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   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.


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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.




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      <----- 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.

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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.

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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.









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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.



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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












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Acknowledgment

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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.



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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.



























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