RFC3355: Layer Two Tunnelling Protocol (L2TP) Over ATM Adaptation Layer 5 (AAL5)

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Network Working Group                                           A. Singh
Request for Comments: 3355                                      Motorola
Category: Standards Track                                      R. Turner
                                                                  R. Tio
                                                                S. Nanji
                                                        Redback Networks
                                                             August 2002

                  Layer Two Tunnelling Protocol (L2TP)
                   Over ATM Adaptation Layer 5 (AAL5)

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.


   The Layer Two Tunneling Protocol (L2TP) provides a standard method
   for transporting the link layer of the Point-to-Point Protocol (PPP)
   between a dial-up server and a Network Access Server, using a network
   connection in lieu of a physical point-to-point connection.  This
   document describes the use of an Asynchronous Transfer Mode (ATM)
   network for the underlying network connection.  ATM User-Network
   Interface (UNI) Signaling Specification Version 4.0 or Version 3.1
   with ATM Adaptation Layer 5 (AAL5) are supported as interfaces to the
   ATM network.


   This specification is intended for implementations of L2TP that use
   ATM to provide the communications link between the L2TP Access
   Concentrator and the L2TP Network Server.

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RFC 3355                     L2TP Over AAL5                  August 2002

1. Introduction

   The Point-to-Point Protocol (PPP) [RFC1661], is frequently used on
   the link between a personal computer with a dial modem and a network
   service provider, or NSP.  The Layer Two Tunneling Protocol (L2TP)
   [RFC2661] enables a dial-up server to provide access to a remote NSP
   by extending the PPP connection through a tunnel in a network to
   which both it and the NSP are directly connected.  A "tunnel" is a
   network layer connection between two nodes, used in the role of a
   data link layer connection between those nodes, possibly as part of a
   different network.  In [RFC2661] the dial-up server is called an L2TP
   Access Concentrator, or LAC.  The remote device that provides access
   to a network is called an L2TP Network Server, or LNS.  L2TP uses a
   packet delivery service to create a tunnel between the LAC and the
   LNS.  "L2TP is designed to be largely insulated from the details of
   the media over which the tunnel is established; L2TP requires only
   that the tunnel media provide packet oriented point-to-point
   connectivity" [RFC2661].  An ATM network with AAL5 offers a suitable
   form of packet oriented connection.  This standard supplements
   [RFC2661] by providing details specific to the use of AAL5 for a
   point-to-point connection between LAC and LNS.

2. Conventions

   Requirements keywords The key words "MUST", "MUST NOT", "REQUIRED",
   and "OPTIONAL" in this document are to be interpreted as described in

   A list of acronyms used in this document is given at the end of the
   document as Appendix A.

3. AAL5 Layer Service Interface

   L2TP treats the underlying ATM AAL5 layer service as a bit-
   synchronous point-to-point link.  In this context, the L2TP link
   corresponds to an ATM AAL5 virtual circuit (VC).  The VC MUST be
   full-duplex, point to point, and it MAY be either dedicated (i.e.,
   permanent, set up by provisioning) or switched (set up on demand.)

   The AAL5 message mode service, in the non-assured mode of operation,
   without the corrupted delivery option MUST be used.

   Interface Format - The L2TP/AAL5 layer boundary presents an octet
   service interface to the AAL5 layer.  There is no provision for sub-
   octets to be supplied or accepted.

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3.1 Maximum Transfer Unit

   Each L2TP PDU MUST be transported within a single AAL5 PDU.
   Therefore the maximum transfer unit (MTU) of the AAL5 connection
   constrains the MTU of the L2TP tunnel that uses the connection and
   the MTU of all PPP connections that use the tunnel.  ([RFC1661]
   refers to this as Maximum Receive Unit, or MRU.  In [SIG31], it is
   the Forward and Backward Maximum CPCS-SDU Size.)

   An implementation MUST support a PPP MRU of at least 1500 bytes.

   An implementation SHOULD use a larger MTU than the minimum value
   specified above.  It is RECOMMENDED that an implementation support an
   IP packet of at least 9180 bytes in the PPP PDU.

3.2 Quality of Service

   In order to provide a desired Quality of Service (QoS), and possibly
   different qualities of service to different client connections, an
   implementation MAY use more than one AAL5 connection between LAC and

   QoS mechanisms, such as Differentiated UBR [DUBR], that could involve
   inverse multiplexing a tunnel across multiple VCs are for further
   study.  QoS mechanisms applicable to a single tunnel corresponding to
   a single VC, are independent of the ATM transport and out of scope of
   this document.

3.3 ATM Connection Parameters

   The L2TP layer does not impose any restrictions regarding
   transmission rate or the underlying ATM layer traffic descriptor

   Specific traffic parameters MAY be set for a PVC connection by
   agreement between the communicating parties.  The caller MAY request
   specific traffic parameters at the time an SVC connection is set up.

   Autoconfiguration of end-systems for PVCs can be facilitated by the
   use of the optional ILMI 4.0 extensions documented in [ILMIA].  This
   provides comparable information to the IEs used for control plane
   connection establishment.

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RFC 3355                     L2TP Over AAL5                  August 2002

4. Multi-Protocol Encapsulation

   This specification uses the principles, terminology, and frame
   structure described in "Multiprotocol Encapsulation over ATM
   Adaptation Layer 5" [RFC2684].  The purpose of this specification is
   not to reiterate what is already standardized in [RFC2684], but to
   specify how the mechanisms described in [RFC2684] are to be used to
   map L2TP onto an AAL5-based ATM network.

   As specified in [RFC2684], L2TP PDUs shall be carried in the payload
   field of Common Part Convergence Sublayer (CPCS) PDUs of AAL5, and
   the Service Specific Convergence Sublayer (SSCS) of AAL5 shall be

   Section 1 of [RFC2684] defines two mechanisms for identifying the
   protocol encapsulated in the AAL5 PDU's payload field:

      1. Virtual circuit (VC) based multiplexing.
      2. Logical Link Control (LLC) encapsulation.

   In the first mechanism, the payload's protocol type is implicitly
   agreed to by the end points for each virtual circuit using
   provisioning or control plane procedures.  This mechanism will be
   referred to as "VC-multiplexed L2TP".

   In the second mechanism, the payload's protocol type is explicitly
   identified in each AAL5 PDU by an IEEE 802.2 LLC header.  This
   mechanism will be referred to as "LLC encapsulated L2TP".

   An L2TP implementation:

      1. MUST support LLC encapsulated L2TP on PVCs.
      2. MAY support LLC encapsulated L2TP on SVCs.
      3. MAY support VC-multiplexed L2TP on PVCs or SVCs.

   When a PVC is used, the endpoints must be configured to use one of
   the two encapsulation methods.

   If an implementation supports SVCs, it MUST use the [Q.2931] Annex C
   procedure to negotiate connection setup, encoding the Broadband Lower
   Layer Interface (B-LLI) information element (IE) to signal either
   VC-multiplexed L2TP or LLC encapsulated L2TP.  The details of this
   control plane procedure are described in section 7.

   If an implementation is connecting through a Frame Relay/ATM FRF.8
   [FRF8] service inter-working unit, then it MUST use LLC encapsulated

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5. LLC Encapsulated L2TP over AAL5

   When LLC encapsulation is used, the payload field of the AAL5 CPCS
   PDU SHALL be encoded as shown in Figure 1.  The pertinent fields in
   that diagram are:

      1. IEEE 802.2 LLC header:  Source and destination SAP of 0xAA
         followed by a frame type of Un-numbered Information (value
         0x03).  This LLC header indicates that an IEEE 802.1a SNAP
         header follows [RFC2684].

      2. IEEE 802.1a SNAP Header:  The three octet Organizationally
         Unique Identifier (OUI) value of 0x00-00-5E identifies IANA
         (Internet Assigned Numbers Authority.)  The two octets Protocol
         Identifier (PID) identifies L2TP as the encapsulated protocol.
         The PID value is 0x0007.

      3. The L2TP PDU:

                  Figure 1 - LLC Encapsulated L2TP PDU

                  +-------------------------+ --------
                  |  Destination SAP (0xAA) |     ^
                  +-------------------------+     |
                  |  Source SAP      (0xAA) |  LLC header
                  +-------------------------+     |
                  |  Frame Type = UI (0x03) |     V
                  +-------------------------+ --------
                  |  OUI        (0x00-00-5E)|     |
                  +-+-+-+-+-+-+-+-+-+-+-+-+-|  SNAP Header
                  |  PID        (0x00-07)   |     |
                  +-------------------------+ --------
                  |                         |     |
                  |                         |  L2TP PDU
                  |                         |     |
                  +-------------------------+ --------

   Note: The format of the overall AAL5 CPCS PDU is shown in the next

   The end points MAY be bi-laterally provisioned to send other LLC-
   encapsulated protocols besides L2TP across the same virtual

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RFC 3355                     L2TP Over AAL5                  August 2002

6. Virtual Circuit Multiplexed L2TP over AAL5

   VC-multiplexed L2TP over AAL5 is an alternative technique to LLC
   encapsulated L2TP over AAL5.  In this case, the L2TP PDU is the AAL5
   payload without an LLC header.  This is sometimes called "Null

                     Figure 2 - AAL5 CPCS-PDU Format

                  +-------------------------------+ -------
                  |             .                 |    ^
                  |             .                 |    |
                  |        CPCS-PDU payload       | L2TP PDU
                  |     up to 2^16 - 1 octets)    |    |
                  |             .                 |    V
                  +-------------------------------+ -------
                  |      PAD ( 0 - 47 octets)     |
                  +-------------------------------+ -------
                  |       CPCS-UU (1 octet )      |    ^
                  +-------------------------------+    |
                  |         CPI (1 octet )        |    |
                  +-------------------------------+CPCS-PDU Trailer
                  |        Length (2 octets)      |    |
                  +-------------------------------|    |
                  |         CRC (4 octets)        |    V
                  +-------------------------------+ -------

   The Common Part Convergence Sub-layer (CPCS) PDU payload field
   contains user information up to 2^16 - 1 octets.

   The PAD field pads the CPCS-PDU to fit exactly into the ATM cells
   such that the last 48 octet cell payload created by the SAR sublayer
   will have the CPCS-PDU Trailer right justified in the cell.

   The CPCS-UU (User-to-User indication) field is used to transparently
   transfer CPCS user to user information.  The field has no function
   under the multi-protocol ATM encapsulation and MAY be set to any

   The CPI (Common Part Indicator) field aligns the CPCS-PDU trailer to
   64 bits.  Possible additional functions are for further study in
   ITU-T.  When only the 64 bit alignment function is used, this field
   SHALL be coded as 0x00.

   The Length field indicates the length, in octets, of the payload
   field.  The maximum value for the Length field is 65535 octets.  A
   Length field coded as 0x00 MAY be used for the abort function.

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   The CRC field is computed over the entire CPCS-PDU except the CRC
   field itself.

   The CPCS-PDU payload SHALL consist of an L2TP PDU as defined in

7. Out-of-Band Control Plane Signaling

7.1 Connection Setup

   An SVC connection can originate at either the LAC or the LNS.  An
   implementation that supports the use of SVCs MUST be able to both
   originate and respond to SVC setup requests.  Except for the B-LLI IE
   specified below, all other IEs required for ATM User-Network
   Interface (UNI) Signaling Specification Version 4.0 [SIG40] should be
   encoded as per [RFC2331].

   When originating an SVC AAL5 connection, the caller MUST request in
   the SETUP message either VC-multiplexed L2TP, LLC encapsulated L2TP,
   or both VC-multiplexed and LLC-encapsulated L2TP.  The B-LLI IE SHALL
   be used to specify the requested encapsulation method.  When a caller
   is offering both encapsulations, the two B-LLI IEs SHALL be encoded
   within a Broadband Repeat Indicator information element in the order
   of the sender's preference.

   An implementation MUST be able to accept an incoming call that offers
   LLC encapsulated L2TP in the caller's request.  The called peer's
   implementation MUST reject a call setup request that only offers an
   encapsulation that it does not support.  Implementations originating
   a call offering both protocol encapsulation techniques MUST be able
   to accept the use of either encapsulation techniques.

   When originating an LLC encapsulated call that is to carry an L2TP
   payload, the [Q.2931] B-LLI IE user information layer 2 protocol
   field SHALL be encoded to select LAN Logical Link Control
   (ISO/IEC8802-2) in octet 6.  See [RFC2331] Appendix A for an example.

   When originating a VC-multiplexed call that is to carry an L2TP
   payload, the [Q.2931] B-LLI IE user information layer 2 protocol
   field SHALL be encoded to select no layer 2 protocol in octet 6 and
   layer 3 protocol field SHALL be encoded to select ISO/IEC TR 9577
   [ISO9577] in octet 7.  Furthermore, as per DSL Forum TR-037
   [DSLF037], the extension octets specify VC-multiplexed L2TP by using
   the SNAP IPI, followed by an OUI owned by IANA, followed by the PID
   assigned by IANA for L2TP.  Thus, the User Information layer 3
   protocol field is encoded:  0B 80 00 00 5E 00 07.  The AAL5 frame's

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   payload field will always contain an L2TP PDU.  The SNAP IPI is
   employed only to use the IANA L2TP protocol value to specify the VC-
   multiplexed PDU.

   If the caller offers both encapsulation methods and the called peer
   accepts the call, the called peer SHALL specify the encapsulation
   method by including exactly one B-LLI IE in the Connect message.

   If an SVC tunnel is reset in accordance with section 4.1 of
   [RFC2661], both ends MUST clear the SVC.  Any user sessions on the
   tunnel will be terminated by the reset.  Either end MAY attempt to
   re-establish the tunnel upon receipt of a new request from a client.

7.2 Connection Setup Failure

   When a connection setup fails, the L2TP entity that attempted the
   connection setup MAY consider the called entity unreachable until
   notified that the unreachable entity is available.  The conditions
   under which an entity determines that another is unreachable and how
   it determines that the other is available again are implementation

7.3 Connection Teardown

   When there are no active sessions on an SVC tunnel, either end MAY
   optionally clear the connection.

8. Connection Failure

   Upon notification that an AAL5 SVC connection has been cleared, an
   Implementation SHALL tear down the tunnel and return the control
   connection to the idle state.

9. Security Considerations

   The Layer Two Tunneling Protocol base specification [RFC2661]
   discusses basic security issues regarding L2TP tunneling.  It is
   possible that the L2TP over AAL5 tunnel security may be compromised
   by the attack of ATM transport network itself.  The ATM Forum has
   published a security framework [AFSEC1] and a security specification
   [AFSEC2] that define procedures to guard against common threats to an
   ATM transport network.  Applications that require protection against
   threats to an ATM switching network are encouraged to use
   authentication headers, or encrypted payloads, and/or the ATM-layer
   security services described in [AFSEC2].

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10. Acknowledgments

   This document draws heavily on material from: "PPP Over AAL5" (RFC
   2364) by George Gross, Manu Kaycee, Arthur Lin, Andrew Malis, and
   John Stephens and an earlier document of L2TP over AAL5 by Nagraj
   Arunkumar, Manu Kaycee, Tim Kwok, and Arthur Lin.

   Special thanks to Mike Davison, Arthur Lin, John Stevens for making
   significant contributions to the initial version of this document.

   Special thanks to David Allan of Nortel for his invaluable review of
   this document.

   The security section of this document is based upon RFC 3337, "Class
   Extensions for PPP over Asynchronous Transfer Mode Adaptation Layer 2
   (AAL2)", by Bruce Thompson, Bruce Buffam and Thima Koren.

11. References

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

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

   [SIG31]   The ATM Forum, "ATM User-Network Interface Specification
             V3.1", af-uni-0010.002, 1994.

   [ITU93]   International Telecommunication Union, "B-ISDN ATM
             Adaptation Layer (AAL) Specification", ITU-T Recommendation
             I.363, March 1993.

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

   [RFC2684] Grossman, D. and J. Heinanen, "Multiprotocol Encapsulation
             over ATM Adaptation Layer 5", RFC 2684, September 1999.

   [Q.2931]  International Telecommunication Union, "Broadband
             Integrated Service Digital Network (B-ISDN) Digital
             Subscriber Signaling System No.2 (DSS2) User Network
             Interface Layer 3 Specification for Basic Call/Connection
             Control", ITU-T Recommendation Q.2931, Feb. 1995.

   [FRF8]    The Frame Relay Forum, "Frame Relay/ATM PVC Service
             Interworking Implementation Agreement", FRF.8, April 1995.

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   [ISO9577] ISO/IEC DTR 9577.2, "Information technology -
             Telecommunications and Information exchange between systems
             - Protocol Identification in the network layer", 1995-08-

   [RFC2331] Maher, M., "ATM Signaling Support for IP over ATM - UNI
             Signaling 4.0 Update", RFC 2331, April 1998.

   [DSLF037] DSL Forum Technical Report TR-037, "Auto-Configuration for
             the Connection Between the DSL Broadband Network
             Termination (B-NT) and the Network using ATM", March 2001.

   [SIG40]   ATM Forum, "ATM User-Network Interface (UNI) Signaling
             Specification Version 4.0", af-sig-0061.000, finalized July
             1996; available at ftp://ftp.atmforum.com/pub.

   [DUBR]    ATM Forum, "Addendum to TM 4.1: Differentiated UBR", af-
             tm-0149.000, finalized July, 2000; available at

   [ILMIA]   ATM Forum, "Addendum to the ILMI Auto-configuration
             extension", af-nm-00165.000, April 2001.

   [AFSEC1]  The ATM Forum, "ATM Security Framework Version 1.0", af-
             sec-0096.000, February 1998

   [AFSEC2]  The ATM Forum, "ATM Security Specification v1.1", af-sec-
             0100.002, March 2001

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RFC 3355                     L2TP Over AAL5                  August 2002

Appendix A.  Acronyms

   AAL5    ATM Adaptation Layer Type 5

   ATM     Asynchronous Transfer Mode

   B-LLI   Broadband Low Layer Information

   CPCS    Common Part Convergence Sublayer

   IANA    Internet Assigned Numbers Authority

   IE      Information Element

   L2TP    Layer Two Tunneling Protocol

   LAC     L2TP Access Concentrator

   LLC     Logical Link Control

   LNS     L2TP Network Server

   MTU     Maximum Transfer Unit

   MRU     Maximum Receive Unit

   NSP     Network Service Provider

   OUI     Organizationally Unique Identifier

   PDU     Protocol Data Unit

   PID     Protocol Identifier

   PPP     Point-to-Point Protocol

   PVC     Permanent Virtual Circuit

   SAP     Service Access Point

   SNAP    Subnetwork Address Protocol

   SVC     Switched Virtual Circuit

   VC      Virtual Circuit

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Authors' Addresses

   Rollins Turner
   Paradyne Corporation
   8545 126th Avenue North
   Largo, FL 33773

   EMail: rturner@eng.paradyne.com

   Rene Tio
   Redback Networks, Inc.
   300 Holger Way
   San Jose, CA 95134

   EMail: tor@redback.com

   Ajoy Singh
   1421 West Shure Dr,
   Arlington Heights, IL 60004

   EMail: asingh1@motorola.com

   Suhail Nanji
   Redback Networks, Inc.
   300 Holger Way
   Sunnyvale, CA 95134

   EMail: suhail@redback.com

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