L2TP PPP LCP extensions: Layer-Two Tunneling Protocol Extensions for PPP Link Control Protocol Negotiation

Network Working Group                                          W. Palter
Request for Comments: 3437                                       zev.net
Category: Standards Track                                    W. Townsley
                                                           Cisco Systems
                                                           December 2002

              Layer-Two Tunneling Protocol Extensions for
                 PPP Link Control Protocol Negotiation

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2002).  All Rights Reserved.

Abstract

   This document defines extensions to the Layer Two Tunneling Protocol
   (L2TP) for enhanced support of link-specific Point to Point Protocol
   (PPP) options.  PPP endpoints typically have direct access to the
   common physical media connecting them and thus have detailed
   knowledge about the media that is in use.  When the L2TP is used, the
   two PPP peers are no longer directly connected over the same physical
   media.  Instead, L2TP inserts a virtual connection over some or all
   of the PPP connection by tunneling PPP frames over a packet switched
   network such as IP.  Under some conditions, an L2TP endpoint may need
   to negotiate PPP Link Control Protocol (LCP) options at a location
   which may not have access to all of the media information necessary
   for proper participation in the LCP negotiation.  This document
   provides a mechanism for communicating desired LCP options between
   L2TP endpoints in advance of PPP LCP negotiation at the far end of an
   L2TP tunnel, as well as a mechanism for communicating the negotiated
   LCP options back to where the native PPP link resides.

Table of Contents

   1. Introduction...............................................  2
     1.1 Specification of Requirements...........................  3
   2. LCP Options From LAC to LNS................................  3
     2.1 LCP Want Options (iccn, occn)...........................  4
     2.2 LCP Allow Options (iccn, occn)..........................  6
     2.3 LCP Options From LNS to LAC.............................  7
   3. Security Considerations....................................  8
   4. IANA Considerations........................................  8
   5. Normative References.......................................  8
   6. Author's Addresses.........................................  9
   7. Full Copyright Statement................................... 10

1. Introduction

   L2TP [RFC2661] provides a very limited amount of guidance to the LNS
   as to what type of interface a tunneled PPP session arrived on at an
   LAC.  Such information is limited to whether the interface was
   "synchronous" or "asynchronous", "digital" or "analog."  These
   indications provide some guidance when negotiating PPP LCP at the
   LNS, but they are not as robust as they could be.

   This document defines a more robust way to inform the LAC of LCP
   negotiated options, and provides guidance to the LNS on the limits
   and values that the LAC requires during LCP negotiation.  Deep
   knowledge of PPP [RFC1661] and L2TP [RFC2661] are expected for the
   remainder of this document.

   L2TP Proxy LCP allows options to be negotiated where the native PPP
   link resides, thus circumventing issues with ACCM, Alternate FCS, and
   other LCP Options that the LNS would not necessarily know how to
   properly negotiate without access to the physical media for the
   native PPP connection, interface type, or configuration.  However,
   use of Proxy LCP introduces other problems as well as there are
   options within LCP PPP negotiation which should be set or adjusted by
   the LNS, such as the PPP Authentication Type and MRU.  Finally, the
   PPP Client may reinitiate LCP negotiation at any time, and unless the
   LAC is sniffing every PPP data packet it forwards, it would not be
   aware that this is even occurring.

   LCP options may be classified into roughly three different categories
   with respect to their affect on L2TP; (1) options which affect
   framing in a way that the LAC may need to know about or handle
   specifically (e.g., ALT-FCS, ACCM, MRU), (2) options that are mostly
   transparent to the LAC (e.g., AUTH-TYPE), and (3) options that the
   LAC may wish to influence because they are dependent on the media
   type (ACFC, PFC).  We are most concerned with options that fall into
   category (1) and (3).

   This document defines new AVPs to allow the LAC and the LNS to
   communicate complete LCP information in order to react accordingly.
   LCP option information is structured in the same way as the Proxy LCP
   AVPs are in [RFC2661].  This essentially involves encapsulation of a
   PPP LCP Configure-Request or Configure-Ack packet within an L2TP AVP.

1.1 Specification of Requirements

   In this document, several words are used to signify the requirements
   of the specification.  These words are often capitalized.  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].

2. LCP Options From LAC to LNS

   The LAC may utilize the following AVPs within an ICCN or OCCN message
   in order to influence the LNS to negotiate LCP in a specific manner.
   If these AVPs are supported by the LNS, they should override any
   suggestions for LCP options implied by the Bearer Type or Framing
   Type AVPs.

   These AVPs may coexist with the Proxy LCP and Proxy Authentication
   AVPs (Proxy AVPs) defined in the base L2TP specification.  If Proxy
   AVPs are received, the LNS may choose to accept these parameters, or
   renegotiate LCP with the options suggested by the AVPs defined in
   this document.  If the LAC wishes to force negotiation of LCP by the
   LNS, it should simply omit all Proxy AVPs during call initialization.

   By default, the AVPs defined in this document are not mandatory (M-
   bit is set to zero).  However, if an implementation needs to strongly
   enforce adherence to the options defined within the AVPs, it MAY set
   the M-bit to 1, thus forcing the peer to discontinue the session if
   it does not support this AVP.  This is NOT recommended unless it is
   known that the result of operating without these extensions is
   completely unacceptable.

   If the AVPs in sections 2.1 and 2.2 are sent to the LNS, the LAC MUST
   be prepared to accept the AVPs as defined in section 2.3.

2.1 LCP Want Options (iccn, occn)

   The LCP Allow Options AVP, Attribute Type 49, contains a list of
   options that the LAC wants to be negotiated by the LNS.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |M|H| rsvd  |      Length       |           Vendor ID           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         Attribute Type        |      LCP Configure-Req ...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                   ... LCP Configure-Req (continued) ...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Vendor ID is the IETF Vendor ID of 0.

   This AVP MAY be hidden (the H bit MAY be 0 or 1).

   The M bit for this AVP may be set to 0 or 1.  If the sender of this
   AVP does not wish to establish a connection to a peer which does not
   understand this L2TP extension, it SHOULD set the M bit to 1,
   otherwise it MUST be set to 0.

   The Length (before hiding) of this AVP is 6 plus the length of the
   LCP Configure Request.

   The AVP SHOULD be present in the following messages: ICCN, OCCN

   The LCP Configure-Req Value for this AVP is identical to the
   information field of a PPP LCP Configure-Req Packet (much like a
   Proxy LCP AVP in [RFC2661]).  It is sent from the LAC to the LNS, and
   is intended to guide PPP LCP negotiations at an LNS.  In some cases,
   each individual PPP LCP option carried in this AVP maps to a desired
   value (e.g., MRU) and in some cases it maps to a specific option that
   is desired to be enabled (e.g., ACFC).  The LNS should use these
   suggestions when building its initial Configure-Request.

   The following chart defines some of the more common LCP options that
   may be included in this AVP with guidance on how to handle them at
   the LAC and LNS.  This table is provided for some of the more common
   or problematic LCP options.  It is not intended to be an exhaustive
   representation of all LCP options available.

   LCP Want Option     LAC Action              LNS Action

     MRU               LAC provides a          LNS SHOULD begin LCP
                                               negotiation maximum
                                               value with this value.
                                               However, it MAY reduce
                                               MRU if necessary.

     ACCM              LAC Provides a mask     LNS SHOULD begin LCP
                                               negotiation with this
                                               value.  LNS may add
                                               bit(s) while
                                               negotiating.

     PFC               LAC provides PFC        LNS SHOULD begin LCP
                       on the link type        negotiation if it is
                                               desired with
                       the link type           this value.
                       (e.g. AHDLC)

     ACFC              LAC provides ACCOMP     LNS SHOULD begin LCP
                       if it is desired on     negotiation with this
                       the link type           value.
                       (e.g. AHDLC)

     FCS-ALT           LAC indicates required  LNS SHOULD begin
                       values for the link     negotiation with this
                       type                    value.  Note that this
                                               value is of no
                                               consequence to the LNS
                                               as FCS is stripped at
                                               the LAC, however some
                                               PPP media types require
                                               this option.

2.2 LCP Allow Options (iccn, occn)

   The LCP Allow Options AVP, Attribute Type 50 contains a list of
   options that the LAC will allow to be negotiated by the LNS.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |M|H| rsvd  |      Length       |           Vendor ID           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         Attribute Type        |      LCP Configure-Ack ...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                   ... LCP Configure-Ack (continued) ...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Vendor ID is the IETF Vendor ID of 0.

   This AVP MAY be hidden (the H bit MAY be 0 or 1).

   The M bit for this AVP may be set to 0 or 1.  If the sender of this
   AVP does not wish to establish a connection to a peer which does not
   understand this L2TP extension, it SHOULD set the M bit to 1,
   otherwise it MUST be set to 0.

   The Length (before hiding) of this AVP is 6 plus the length of the
   LCP Configure Request.

   The AVP MAY be present in the following messages: ICCN, OCCN

   The LCP Configure-Ack Value for this AVP is identical to the
   information field of a PPP LCP Configure-Req Packet (much like a
   Proxy LCP AVP in [RFC2661]).  It is sent from the LAC to the LNS, and
   is intended to guide PPP LCP negotiations at an LNS.  In some cases,
   each individual PPP LCP option carried in this AVP maps to a maximum
   value (e.g., MRU), while in others it maps to an option that is
   permitted by the LAC (e.g., ACFC).  If the option is not included
   here, it can be assumed by the LNS that the LAC does not understand
   how to perform that particular option at the link layer (and would
   thus Configure-Reject that option).  Information in this AVP should
   be utilized when building PPP Configure-Ack, Configure-Reject and
   Configure-Nak messages.

   The following chart defines some of the more common LCP options that
   may be included in this AVP with guidance on how to handle them at
   the LAC and LNS.  This table is provided for illustration purposes
   for some of the more common or problematic LCP options.  It is not
   intended to be an exhaustive representation of all LCP options
   available.

   LCP Allow Option    LAC Action              LNS Action

     MRU               LAC provides a          LNS may accept reduction
                       maximum value           MRU as requested.

     ACCM              LAC Provides a mask     LNS may accept bit(s)
                                               defined here.  Note that
                                               if ACCM is missing it is
                                               assumed that it is not
                                               applicable to the link
                                               type.

     PFC               LAC provides PFC        LNS may accept PFC.
                       if it is allowed on
                       the link type
                       (e.g. AHDLC)

     ACFC              LAC provides ACFC       LNS may accept ACFC.
                       if it is allowed on
                       the link type
                       (e.g. AHDLC)

     FCS-ALT           LAC indicates valid     Negotiation this option
                       values for the link     is of no consequence to
                       type                    the LNS as the FCS is
                                               stripped at the LAC.
                                               However, the LNS SHOULD
                                               only accept FCS-ALT types
                                               listed here (more than
   one
                                               value may be present).

2.3 LCP Options From LNS to LAC

   In order to communicate negotiated LCP parameters from the LNS to the
   LAC, the format of two existing messages in [RFC2661] are used.
   These are:

      Last Sent LCP Confreq (IETF L2TP Attribute 27)
      Last Received LCP Confreq (IETF L2TP Attribute 28)

   These AVPs are sent from the LAC to the LNS to support Proxy LCP
   negotiation.  In order to report negotiated LCP parameters from the
   LNS to the LAC, two messages of precisely the same format are
   defined:

      LNS Last Sent LCP Confreq (IETF L2TP Attribute 51)
      LNS Last Received LCP Confreq (IETF L2TP Attribute 52)
   When LCP negotiation is completed by the LNS, a Set-Link-Info control
   message MUST be sent with these AVPs contained within.  These AVPs
   MUST contain the last sent and last received (with respect to the
   LNS) LCP packets.

   Rather than simply using the old Attribute values in the SLI Message,
   new AVP Attribute types are defined for these messages due to the
   fact that some existing L2TP implementations might check for what
   could seem like misplacement of known AVP types and generate a false
   error condition.

3. Security Considerations

   There are no known additional significant threats incurred by the
   mechanisms described in this document.

   This document defines additional L2TP AVPs that identify link
   characteristics and interface information of a tunneled PPP link.  If
   these values were snooped, a rogue individual may have access to more
   information about a given network or topology.  Given that these same
   values may be negotiated over the tunneled link in PPP LCP packets
   anyway, this is no more information than is potentially transmitted
   today, it is just in a different form.

4. IANA Considerations

   This document requires four new L2TP "AVP Attribute" numbers to be
   assigned by IANA.

      49, Section 2.1, LCP Want Options
      50, Section 2.2, LCP Allow Options
      51, Section 2.3, LNS Last Sent LCP Confreq
      52, Section 2.3, LNS Last Received LCP Confreq

5. Normative References

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

   [RFC1661] Simpson, W., "The Point-to-Point Protocol (PPP)", STD 51,
              RFC 1661, July 1994.

   [RFC 2661] Townsley, W., Valencia, A., Rubens, A., Pall, G., Zorn, G.
              and B. Palter, "Layer Two Tunneling Layer Two Tunneling
              Protocol (L2TP)", RFC 2661, August 1999.

6. Author's Addresses

   W. Mark Townsley
   Cisco Systems
   7025 Kit Creek Road
   PO Box 14987
   Research Triangle Park, NC 27709

   EMail: mark@townsley.net

   Bill Palter
   EMail: palter.ietf@zev.net

7.  Full Copyright Statement

   Copyright (C) The Internet Society (2002).  All Rights Reserved.

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Acknowledgement

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