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Network Working Group                                         Y. T'Joens
Request for Comments: 3301                                      B. Sales
Category: Standards Track                                        Alcatel
                                                           P. Crivellari
                                                                Belgacom
                                                               June 2002


                 Layer Two Tunnelling Protocol (L2TP):
                     ATM access network extensions

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 augments the procedures described in RFC 2661 to
   further support ATM SVC (Switched Virtual Circuits) or PVC (Permanent
   Virtual Circuits) based access networks.  L2TP (Layer 2 Tunneling
   Protocol) specifies a protocol for tunnelling PPP packets over packet
   based networks and over IP networks in particular.  L2TP supports
   remote access by ISDN and PSTN networks.  The extensions defined
   within this document allow for asymmetric bi-directional call
   establishment and service selection in the ATM access network.

Table Of Contents

   1. Introduction ..................................................  2
   1.1 Conventions ..................................................  2
   2. Assumptions ...................................................  3
   2.1 Topology .....................................................  3
   2.2 Connection Establishment .....................................  3
   2.3 LCP Negotiation ..............................................  3
   3. ATM access enhanced procedures ................................  3
   3.1 ATM connectivity .............................................  4
   3.2 Tunnel establishment .........................................  4
   3.3 Call establishment ...........................................  5
   3.3.1 Incoming Call Establishment ................................  5
   3.3.2 Outgoing Call Establishment ................................  6



T'Joens, et al.             Standards Track                     [Page 1]

RFC 3301          L2TP: ATM access network extensions          June 2002


   3.4 Framing ......................................................  6
   4. Service model issues ..........................................  7
   4.1 Authentication ...............................................  7
   4.2 Authorization ................................................  7
   5. New and extended AVPs .........................................  7
   5.1 New AVP Summary ..............................................  7
   5.2 New AVP definition ...........................................  8
   5.3 Changed AVP Definition ....................................... 12
   6. IANA considerations ........................................... 16
   7. Security considerations ....................................... 17
   8. Acknowledgements .............................................. 17
   9. References .................................................... 17
   10. Authors Addresses ............................................ 18
   11. Full Copyright Statement ..................................... 19

1. Introduction

   L2TP [RFC2661] defines the procedures for tunneling PPP sessions
   between a so called L2TP Access Concentrator (LAC) and an L2TP
   Network Server (LNS).  The main focus of [RFC2661] is on supporting
   HDLC based ISDN/PSTN access networks.

   This document augments the procedures described in [RFC2661] to
   further support ATM SVC or PVC based access networks.  Support for
   ATM access networks requires extensions to the present L2TP
   procedures so as to cope with :

   (a) the traffic management aspects of ATM connections (e.g.
       asymmetric bandwidth allocation and service category selection
       capabilities),

   (b) the addressing format to be used in switched ATM networks [AESA]
       and

   (c) the limitations imposed on LCP negotiation by transporting PPP
       over AAL5 over the access network segment of the PPP connection
       [RFC2364].

   Within this document, the necessary extensions to [RFC2661] are
   defined to cope with issues (a) and (b), issue (c) which is not
   specific to ATM may be solved as described in [L2TP_link].

1.1 Conventions

   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].




T'Joens, et al.             Standards Track                     [Page 2]

RFC 3301          L2TP: ATM access network extensions          June 2002


2. Assumptions

   In this section we describe some assumptions that have lead to the
   extensions described in this document.

2.1 Topology

   The procedures as defined in [RFC2661] apply mainly to access network
   technology such as PSTN and ISDN, which may be respectively
   asynchronous HDLC and synchronous HDLC based.  The aim of this
   document is to extend L2TP support to allow for user / LAC
   communication based on ATM access network technology.

2.2 Connection Establishment

   Due to the wide variety of existing signalling protocols and ATM
   service categories, and their support or non-support within ATM based
   access networks, this document takes as approach to provide for a
   flexible identification of ATM connection characteristics while
   establishing outgoing and incoming L2TP calls.  The procedures as
   defined within this document allow the allocation of asymmetric
   bandwidth and service category selection in terms of real or non-real
   time requirements on the ATM portion of the access network.

   As such, the detailed signalling protocol specific information
   elements that are necessary for switched VC service, are explicitly
   not negotiated during call establishment over the L2TP tunnel.

   In order to identify the endpoint of the ATM connection within the
   ATM access network, SVCs can be established on the basis of the ATM
   end system addressing format [AESA].  For PVC based services, the PVC
   can either be referred to by using the ATM end system addressing
   procedure (Called/Calling Number), or by making use of a textual name
   (Service Name).  The latter is inspired by the procedures defined
   within [Auto_PVC].

2.3 LCP negotiation

   The procedures described within this document may be combined with
   the procedures described in [L2TP_link] to limit LCP negotiation
   between LNS and user, so as to enforce PPP over AAL5 specific LCP
   negotiation [RFC2364].

3. ATM access enhanced procedures

   In order to illustrate the procedures specified within this document,
   this section will provide an operational description of Virtual
   dial-up access through an ATM based access network (e.g., ADSL).



T'Joens, et al.             Standards Track                     [Page 3]

RFC 3301          L2TP: ATM access network extensions          June 2002


   Note that the emphasis is on the changes proposed within this
   document relative to [RFC2661].

3.1 ATM connectivity

   Prior to initiating the PPP protocol layer, a Virtual Connection (VC)
   MUST be established between the user and the Network Access Server
   (LAC).  This virtual connection MAY either be a preconfigured
   Permanent VC(PVC), where the access network provider, NAS and user
   agree beforehand on the characteristics of the PVC, or MAY be an on-
   demand switched VC(SVC), where the negotiation between user, network
   and NAS takes place by means of an ATM signalling protocol.  Note
   that for establishing PVCs, alternative use may be made of the
   procedures as described in [Auto_PVC].

   In both cases, the user is referred to as the virtual dial-in user.

   Prior to accepting the switched connection from the virtual dial-in
   user, the LAC MAY check with the LNS whether the call should be
   accepted.  In the latter situation, the LAC MAY determine based upon
   parameters available within the call establishment message that this
   concerns a virtual dial in user, or MAY undertake a partial
   authentication of the end system/user, in order to bind the end
   system/user with a specific LNS.

   For PVC based users, the LAC MAY be triggered by the arrival of an
   LCP Configure Request, or PPP Authentication request message from the
   virtual dial-in user to initiate conversation with the LNS.  Note
   that the exact timing of triggering communication between LAC and LNS
   is outside the scope of this document.

3.2 Tunnel establishment

   If no tunnel connection currently exists to the desired LNS, one is
   initiated.  During the tunnel establishment, LNS and LAC indicate
   bearer and framing capabilities to each other, according to normal
   procedures.

   The bearer capability is extended to allow the LAC to indicate its
   support of ATM bearer devices.  Positive receipt of this indication,
   allows both LAC and LNS to use the extensions as defined within this
   document to support ATM based incoming and outgoing calls.

   If no compatibility between LNS and LAC exists according to the
   extensions defined within this document, no tunnel establishment can
   take place.  This would be because the LAC does not support any
   bearer capability which is expected by the LNS (e.g., an ATM based
   LAC, that only signals the "Broadband" Bearer Capability), or vice



T'Joens, et al.             Standards Track                     [Page 4]

RFC 3301          L2TP: ATM access network extensions          June 2002


   versa.  It is however encouraged that LAC or LNS implementations
   would allow for seamless interworking with peer devices which do not
   implement the extensions defined within this document.  This could be
   implemented by allowing a graceful fallback to digital bearer
   capability.

3.3 call establishment

   During incoming and outgoing broadband call establishment, the
   following extensions are defined to existing procedures.

3.3.1 Incoming Call Establishment

   The ATM connection between the virtual Dial-in user and LAC MAY
   either be dynamically or statically established.  When the VC
   connection is dynamically established (Switched VC), the LAC will
   receive a SETUP message over the interface that connects it to the
   ATM network.  This specification does not assume any specific
   interface type (UNI or NNI).  Permanent VC connections MAY either be
   manually configured, or configured by use of the extensions to the
   ILMI procedures as defined by [Auto_PVC].

   For switched VC connections, the LAC MAY select the peer LNS on the
   basis of connection establishment information, or by allowing partial
   PPP authentication of the virtual Dial-in user.  The connection
   establishment information that can be used by the LAC include Called
   Party AESA, Called Party AESA Subaddress, Calling Party AESA or
   Calling Party AESA Subaddress.

   For Permanent VC connections, the LAC MAY be triggered by (a) the
   establishment of the PVC, (b) by an LCP configure request, (c) by
   partially authenticating the virtual Dial-in user, or (d) by means
   outside the scope of this specification.

   Within the ICRQ, the LAC MUST indicate a broadband bearer in the
   Bearer Type AVP (B bit set to 1), MAY include the Service Category
   AVP, and MAY include the Service Name AVP.  If the LNS would not
   support the B Bearer bit, it will return an error on the ICRQ
   message.  In such a case, the implementation MAY decide to fall back
   to digital bearer capability, and SHOULD refrain from using the
   extensions defined within this document.  Further, the ICRQ message
   MAY contain the VPI/VCI identifier AVP.  This identifier can further
   be used at the LNS for management purposes next to or alternative to
   the Physical Channel ID AVP.

   Within the ICCN, both Tx Connect Speed AVP and Rx Connect Speed
   SHOULD be used if an asymetric connection has been established.




T'Joens, et al.             Standards Track                     [Page 5]

RFC 3301          L2TP: ATM access network extensions          June 2002


3.3.2 Outgoing Call Establishment

   Within an OCRQ, the LNS MUST indicate to the LAC minimum and maximum
   speeds for receive and transmit traffic (from the LAC perspective).
   This is to allow for the bi-directional asymmetric nature of ATM
   traffic contracts.  Note that in order to support UBR connections
   between LAC and user, the Minimum BPS MUST be set to zero.

   Further during OCRQ, the LNS MAY include the required Service
   Category AVP, i.e., indicating real time (rt) or non-real time (nrt)
   transport services.  The combination of minimum and maximum receive
   and transmit speed, and the indication of the required service
   category allows the LAC to establish an ATM connection according to
   its own capabilities, and the ATM access network capabilities,
   however within the service requirement for the PPP layer.

   Real time connectivity can be provided by either CBR or rt-VBR ATM
   service categories, non-real time connectivity can be provided by
   UBR, nrt-VBR, ABR or GFR ATM service categories.

   Further the LNS MUST indicate to the LAC in OCRQ message the called
   number according to the format described in this document (NSAP
   format).  When the called number carries an all zero payload, the LAC
   SHOULD look at the Service Name AVP to bind the tunnel call to an ATM
   VC connection.

   Next to the normal AVPs, the OCRP message MAY contain the VPI/VCI
   identifier AVP.  This identifier can further be used at the LNS for
   management purposes next to or alternative to the Physical Channel ID
   AVP.

3.4 Framing

   Within this document the PPP PDU refers to the concatenation of PPP
   protocol ID, PPP Information and PPP padding fields.

   In the direction of user to LNS, the PPP PDU will be carried on top
   of an AAL5 connection between user and LAC.  The LAC MUST strip off
   the AAL5 specific fields based on the encapsulation mechanism in use
   on the ATM connection, i.e. VC multiplexed or LLC encapsulated
   [RFC2364], and MUST encapsulate the PPP PDU with address and control
   field, as per HDLC procedures, on the L2TP tunnel.

   In the direction of LNS to user, the PPP PDU will be carried on top
   of an AAL5 connection between LAC and user.  The LAC MUST strip the
   PPP PDU from the address and control field on the L2TP tunnel, and





T'Joens, et al.             Standards Track                     [Page 6]

RFC 3301          L2TP: ATM access network extensions          June 2002


   insert the AAL5 specific fields based on the encapsulation mechanism
   in use on the ATM connection, i.e. VC multiplexed or LLC
   encapsulated.

4. Service model issues

4.1 Authentication

   In case of ATM switched VC establishment, calling party number
   information may be used for first level authentication much in the
   same way as for PSTN or ISDN access.  In case of permanent VC
   establishment, authentication may not be an issue from the LAC side,
   because of the permanent character of the VC.  Bilateral agreement
   between LAC and LNS providers may eliminate the authentication phase
   in the latter case.

4.2 Authorization

   Because of the flexibility of establishing ATM connections with
   varying parameters, some authorization may be required prior to
   accepting the establishment of a switched ATM connection from the
   user with certain ATM traffic parameters.  This authorization may be
   performed against the ATM specific authentication information (e.g.
   calling line id), or may be performed after partial authentication of
   the user at the PPP level.  Non authorized access requests result in
   connection release.

5. New and extended AVPs

5.1 New AVP Summary

   The following table lists the extra AVPs that are defined within this
   document.  The "attr" column indicates the integer value assigned to
   this attribute.  Note that the attribute value is relative compared
   to the vendor ID.  The "M" column indicates the setting of the
   "Mandatory" bit of the AVP header for each attribute.  The "LEN"
   column indicates the size of the AVP including the AVP header.  A "+"
   in this column indicates that the length varies depending upon the
   length of the actual contents of the value field.

   The usage list for each entry indicates the message types that
   utilize each AVP.  An abbreviation shown in mixed or upper case
   letters indicates that the corresponding AVP MUST be present in this
   message type.  All lower case indicates that the AVP MAY optionally
   appear in this message type.  Some AVPs MAY be present only when a
   corresponding optional AVP or specific setting within the AVP is
   present, these AVPs are shown in lower case as well.




T'Joens, et al.             Standards Track                     [Page 7]

RFC 3301          L2TP: ATM access network extensions          June 2002


   Attr    M       Len     Attribute Name (usage)

   40      0       10      Rx Minimum BPS (ocrq)
      32-bit integer indicating the lowest acceptable line speed for the
      call in the receive direction.  Rx indicates the user to LAC
      direction.
   41      0       10      Rx Maximum BPS (ocrq)
      32-bit integer indicating the highest acceptable line speed for
      call in the receive direction.  Rx indicates the user to LAC
      direction.
   42      0       8       Service Category (ocrq, icrq)
      The Service Category indicates the service expected for the call,
      e.g., real time or non-real time.
   43      0       6+      Service Name (ocrq, icrq)
      The Service Name indicates the service name linked to a
      preestablished PVC.
   44      0       26      Calling Sub-Address(icrq)
      20 octet binary encoded NSAP subaddress indicating the Calling
      Party Sub-Address.
   45      0       10      VPI/VCI identifier (icrq, ocrp)
      10 octet binary encoded identification of VPI/VCI values used for
      incoming calls.

5.2 New AVP definition

   The following lists the new AVPs defined within this document, and
   describes the expected behaviour when this AVP would be present
   within a message.

   Rx Minimum BPS (OCRQ)

         The Rx Minimum BPS, Attribute Type 40, encodes the lowest
         acceptable line speed for this call in the receive direction,
         for these cases where asymmetric transmission is required.

         The Attribute Value field for this AVP has the following
         format:

   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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Rx Minimum BPS                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

         The Rx Minimum BPS is a 32 bit value indicating the speed in
         bits per second.





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         This AVP MAY be included within the OCRQ, and SHOULD only be
         included when the LAC indicated broadband bearer support in the
         bearer capabilities AVP during tunnel establishment.

         This AVP may be hidden (the H-bit may be set to 0 or 1).  The
         M- bit for this AVP must be set to 0.  The Length (before
         hiding) of this AVP is 10.

   Rx Maximum BPS

         The Rx Maximum BPS, Attribute Type 41, encodes the highest
         acceptable line speed for this call in the receive direction,
         for these cases where asymmetric transmission is required.

         The Attribute Value field for this AVP has the following
         format:

   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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Rx Maximum BPS                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

         The Rx Maximum BPS is a 32 bit value indicating the speed in
         bits per second.

         This AVP MAY be included within the OCRQ, and SHOULD only be
         included when the LAC indicated broadband bearer support in the
         bearer capabilities AVP during tunnel establishment.

         This AVP may be hidden (the H-bit may be set to 0 or 1).  The
         M- bit for this AVP must be set to 0.  The Length (before
         hiding) of this AVP is 10.

   Service Category

         The Service Category AVP, Attribute type 42, indicates optional
         extra information on the Quality of Service expected for the
         call establishment on the broadband bearer medium.

         The Attribute Value field for this AVP has the following
         format:

   0                   1
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Resvd for future QoS ind.   |S|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



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         The Attribute Value field is a 16-bit mask, with one bit
         defined.  The S bit indicates either non real time (S bit set
         to 0) or real time (S bit set to 1) service requirement.  The
         other bit fields are reserved for future use.

         The Service Category AVP MAY be present in OCRQ and ICRQ
         messages, and SHOULD only be included when the LAC indicated
         broadband bearer support in the bearer capabilities AVP during
         tunnel establishment.

         This AVP may be hidden (the H-bit may be set to 0 or 1).  The
         M- bit for this AVP must be set to 0.  The Length (before
         hiding) of this AVP is 8.

   Service Name

         The Service Name AVP, Attribute Type 43, provides the peer with
         an textual name for referring to an ATM VC connection.

         The Attribute Value field for this AVP has the following
         format:

   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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Service Name (arbitrary number of octets) ....
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

         The Service Name is of arbitrary length, but must be at least 1
         octet.  The Service Name is UTF-8 encoded. [10646]

         The Service Name should be unique at least to the LNS/LAC
         combination.

         The Service Name AVP MAY only be provided when the Called
         Number field is encoded as all zeros in OCRQ.  The Service Name
         AVP MAY be present in OCRQ and ICRQ messages, and SHOULD only
         be included when the LAC indicated broadband bearer support in
         the bearer capabilities AVP during tunnel establishment.

         This AVP may be hidden (the H-bit may be set to 0 or 1).  The
         M- bit for this AVP must be set to 0.  The length of this
         attribute is arbitrary, however at least 7.

   Calling Sub-Address (ICRQ)

         The Calling Sub-Address AVP, Attribute Type 44, encodes
         additional Calling Party subaddress information.



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         The Attribute Value field for this AVP has the following
         format:

   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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                              NSAP                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         NSAP (cont'd)                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         NSAP (cont'd)                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         NSAP (cont'd)                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         NSAP (cont'd)                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

         The Calling Sub-Address AVP MUST be encoded as a 20 octet
         binary encoded NSAP address when the B bit is set in the Bearer
         Type AVP.  The NSAP binary encoded address provides a broader
         range of address encapsulation methods than an ASCII field.
         The structure of the NSAP address (e.g., E.164, ICD, DCC) is
         defined in [AESA].

         The Calling Sub-Address number AVP MAY be present in ICRQ, and
         SHOULD only be available if the Calling Party number is also
         within the message.

         This AVP may be hidden (the H-bit may be 0 or 1).  The M-bit
         for this AVP MUST be set to 0.  The Length (before hiding) of
         this AVP is 26.

   VPI/VCI identifier(icrq, ocrp)

         The VPI/VCI identifier, Attribute Type 45, encodes the VPI/VCI
         value used at the ATM interface at the LAC.

         The Attribute Value field for this AVP has the following
         format:

   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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |resvd  |           VPI         |              VCI              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

         The VPI/VCI identifier is a 32 bit value encoding the VPI(12
         bits) and VCI (16 bits) value.



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         This AVP MAY be included within the ICRQ and OCRP, and SHOULD
         only be included when the LAC indicated broadband bearer
         support in the bearer capabilities AVP during tunnel
         establishment.

         This AVP may be hidden (the H-bit may be set to 0 or 1).  The
         M- bit for this AVP must be set to 0.  The Length (before
         hiding) of this AVP is 10.

5.3 Changed AVP Definition

   The following AVPs see their contents changed relative to [RFC2661]
   in order to support the procedures described in this document.

   Bearer Capabilities

   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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Resvd for future bearer capability definitions         |B|A|D|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

         The bearer Capabilities AVP within a SCCRQ or SCCRP indicates
         the bearer capabilities that the sender of this message can
         provide for outgoing calls.  This document extends the existing
         AVP with the B bit.  If bit B is set, broadband access is
         supported (ATM).

         Attempts to establish an outgoing call with the bearer type set
         to B, while the bearer capability did not indicate this
         capability will result in the call being rejected with Result
         Code 5 'Call failed due to lack of appropriate facilities being
         available (permanent condition)'.

         In these cases where the LAC only supports the B bit, and the
         LNS would not recognize the B bit, no outgoing calls are
         possible.  Note that when the LAC only has ATM based devices,
         it may still opt for seamless fall back to digital bearer
         types.

         This specification assumes a non-compliant LNS to categorize a
         Bearer Capabilities AVP where the B bit is set as unrecognized
         AVP, upon which the tunnel establishment will fail.  This is to
         be indicated by a Result Code '2-General error - Error Code
         indicates the problem', Error Code '3- Reserved field was non-
         zero'.





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   (Tx) Minimum BPS

         The (Tx) Minimum BPS AVP encodes the lowest acceptable line
         speed for this call in the transmit direction.  The (Tx)
         Minimum BPS AVP MAY be used in OCRQ.  If the Rx Minimum BPS
         AVP, as defined within this document, is not available in the
         message, then symmetric transmission is implied, with both
         minimum receive and transmit bit-rates equal to Minimum BPS.

   (Tx) Maximum BPS

         (Tx) Maximum BPS AVP encodes the highest acceptable line speed
         for this call in the transmit direction.  The (Tx) Maximum BPS
         AVP MAY be used in OCRQ.  If the Rx Maximum BPS AVP, as defined
         within this document, is not available in the message, then
         symmetric transmission is implied, with both maximum receive
         and transmit bitrates equal to Maximum BPS.

   Bearer Type

   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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       Resvd for future bearer types definitions         |B|A|D|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

         The bearer type AVP encodes the bearer type for the requested
         call.  This document extends the bearer types with an
         indication of ATM bearer support (B-bit).  If bit B is set,
         broadband access is requested (ATM).  If bit A is set, analogue
         access is requested.  If bit D is set, Digital access is
         requested.

         Note that in the OCRQ all 3 bits (B,A,D) may be set indicating
         that the call may be of either type.  The B bit SHOULD only be
         set if the Broadband capability was indicated during tunnel
         establishment.

   Q.931 Cause Code

   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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Cause Code           |   Cause Msg   | Advisory Msg...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+






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         The Cause code is not changed from [RFC2661], except for the
         fact that it can also carry Cause Codes specific to ATM
         signalling messages, these cause codes can be found in ATM

         Forum UNI 4.0 [UNI] and the references thereof.  The Cause code
         should be interpreted relative to the Bearer Type in use for
         the specific call.

   Called Number

         The Called Number AVP, Attribute Type 21, encodes the AESA
         number to be called for an OCRQ, and the Called number at the
         LAC for an ICRQ.

         The Attribute Value field for this AVP has a changed encoding
         from [RFC2661]:

   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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                              NSAP                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         NSAP (cont'd)                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         NSAP (cont'd)                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         NSAP (cont'd)                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         NSAP (cont'd)                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

         The Called Number AVP MUST be encoded as a 20 octet binary
         encoded NSAP address when the B bit is set in the Bearer Type
         AVP.  The NSAP binary encoded address provides a broader range
         of address encapsulation methods than an ASCII field.  The
         structure of the NSAP address (e.g., E.164, ICD, DCC) is
         defined in [AESA].

         The Called number AVP MUST be present in OCRQ, and MAY be
         present in ICRQ.

         If the Called Number AVP in an OCRQ carries an all zero NSAP
         address, the Service Name AVP SHOULD provide further
         information to bind the L2TP call to a specific VC connection.
         See also [Auto_PVC].






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         This AVP may be hidden (the H-bit may be 0 or 1).  The M-bit
         for this AVP MUST be set to 0.  The Length (before hiding) of
         this AVP is 26.

   Calling Number

         The Calling Number AVP, Attribute Type 22, encodes the Calling
         Party AESA as received from the Virtual Dial-in User.

         The Attribute Value field for this AVP has a changed encoding
         from [RFC2661]:

   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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                              NSAP                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         NSAP (cont'd)                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         NSAP (cont'd)                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         NSAP (cont'd)                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         NSAP (cont'd)                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

         The Calling Number AVP MUST be encoded as a 20 octet binary
         encoded NSAP address when the B bit is set in the Bearer Type
         AVP.  The NSAP binary encoded address provides a broader range
         of address encapsulation methods than an ASCII field.  The
         structure of the NSAP address (e.g., E.164, ICD, DCC) is
         defined in [AESA].

         The Calling number AVP MAY be present in ICRQ.

         This AVP may be hidden (the H-bit may be 0 or 1).  The M-bit
         for this AVP MUST be set to 0.  The Length (before hiding) of
         this AVP is 26.

   Sub-Address

         The Sub-Address AVP, Attribute Type 23, encodes additional
         Called Party subaddress information.

         The Attribute Value field for this AVP has a changed encoding
         from [RFC2661]:





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   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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                              NSAP                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         NSAP (cont'd)                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         NSAP (cont'd)                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         NSAP (cont'd)                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         NSAP (cont'd)                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

         The Sub-Address AVP MUST be encoded as a 20 octet binary
         encoded NSAP address when the B bit is set in the Bearer Type
         AVP.  The NSAP binary encoded address provides a broader range
         of address encapsulation methods than an ASCII field.  The
         structure of the NSAP address (e.g., E.164, ICD, DCC) is
         defined in [AESA].

         The Sub-Address number AVP MAY be present in ICRQ and OCRQ, and
         SHOULD only be available if the Called Party number is also
         within the message.

         This AVP may be hidden (the H-bit may be 0 or 1).  The M-bit
         for this AVP MUST be set to 0.  The Length (before hiding) of
         this AVP is 26.

6. IANA Considerations

   This document requires IANA to allocate 6 new type values for the
   following AVPs (see section 5.2) :

   - Rx Minimum BPS

   - Rx Maximum BPS

   - Service Category

   - Service Name

   - Calling Sub-Address

   - VPI/VCI Identifier

   This document further defines a new bit (B) in the bearer
   capabilities and bearer type AVPs (section 5.3).



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   This document defines a flag field in the Service Category AVP, only
   one bit in this flag has been assigned within this document (S).
   Further assignments fall under the rule of "Specification Required",
   i.e. Values and their meaning must be documented in an RFC or other
   permanent and readily available reference, in sufficient detail so
   that interoperability between independent implementations is
   possible.

7. Security Considerations

   No extra security risk outside these specified by [RFC2661] are
   foreseen.

8. Acknowledgements

   The authors would like to thank Laurent Hermans for his work on
   earlier versions of this document, Juha Heinanen (Telia) and David
   Allen (Nortel Networks) for their constructive discussion on the
   document during the Minneapolis IETF meeting, Mark Townsley (cisco)
   for his hint on the use of the VPI/VCI identifier AVP.

9. References

   [RFC2661]   Townsley, W., Valencia, A., Rubens, A., Singh Pall, G.,
               Zorn, G. and B. Palter, "Layer Two Tunnelling Protocol
               (L2TP)", RFC 2661, August 1999.

   [RFC2119]   Bradner, S., "Key words for use in RFCs to Indicate
               Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2364]   Gross, G., Kaycee, M., Lin, A., Malis, A. and J.
               Stephens, "PPP over AAL5", RFC 2364, July 1998.

   [UNI]       User-Network Interface (UNI) Specification, Version 4.0,
               ATM Forum, July, 1996

   [AESA]      ATM Forum Addressing : Reference Guide, version 1.0, ATM
               Forum, Final Ballot, January 1999

   [L2TP_link] Townsley, M. and W. Palter, "L2TP Link Extensions", Work
               in Progress.

   [Auto_PVC]  ATM Forum, "Auto-configuration of PVCs", af-nm-0122.000,
               March 1999

   [10646]     ISO/IEC, Information Technology - Universal Multiple-
               Octet Coded Character Set (UCS) - Part 1: Architecture
               and Basic Multilingual Plane, May 1993, with amendments



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10. Authors Addresses

   Yves T'joens
   Alcatel Network Strategy Group
   Francis Wellesplein 1, 2018 Antwerp, Belgium
   Phone : +32 3 240 7890
   EMail : yves.tjoens@alcatel.be

   Paolo Crivellari
   Belgacom
   bd du Roi Albert II 27
   B-1030 Bruxelles
   Phone: +32 2 202 9698
   EMail: paolo.crivellari@belgacom.be

   Bernard Sales
   Alcatel Network Strategy Group
   Francis Wellesplein 1, 2018 Antwerp, Belgium
   Phone : +32 3 240 9574
   EMail : bernard.sales@alcatel.be































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RFC 3301          L2TP: ATM access network extensions          June 2002


11. Full Copyright Statement

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

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS 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.

Acknowledgement

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



















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