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rfc:rfc3301

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.

T'Joens, et al. Standards Track [Page 8] RFC 3301 L2TP: ATM access network extensions June 2002

       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|
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

T'Joens, et al. Standards Track [Page 9] RFC 3301 L2TP: ATM access network extensions June 2002

       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.

T'Joens, et al. Standards Track [Page 10] RFC 3301 L2TP: ATM access network extensions June 2002

       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.

T'Joens, et al. Standards Track [Page 11] RFC 3301 L2TP: ATM access network extensions June 2002

       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'.

T'Joens, et al. Standards Track [Page 12] RFC 3301 L2TP: ATM access network extensions June 2002

 (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...
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

T'Joens, et al. Standards Track [Page 13] RFC 3301 L2TP: ATM access network extensions June 2002

       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].

T'Joens, et al. Standards Track [Page 14] RFC 3301 L2TP: ATM access network extensions June 2002

       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]:

T'Joens, et al. Standards Track [Page 15] RFC 3301 L2TP: ATM access network extensions June 2002

 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) :
  1. Rx Minimum BPS
  1. Rx Maximum BPS
  1. Service Category
  1. Service Name
  1. Calling Sub-Address
  1. VPI/VCI Identifier
 This document further defines a new bit (B) in the bearer
 capabilities and bearer type AVPs (section 5.3).

T'Joens, et al. Standards Track [Page 16] RFC 3301 L2TP: ATM access network extensions June 2002

 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

T'Joens, et al. Standards Track [Page 17] RFC 3301 L2TP: ATM access network extensions June 2002

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

T'Joens, et al. Standards Track [Page 18] 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.

T'Joens, et al. Standards Track [Page 19]

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