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Versions: 00 01 02 03 04 05 06 07 RFC 4591

Network Working Group                                   W. Mark Townsley
Internet-Draft                                             George Wilkie
Category: Standards Track                                     Skip Booth
Expiration Date: October 2005                                    Jed Lau
                                                          Stewart Bryant
                                                           cisco Systems

                                                              April 2005

                        Frame-Relay over L2TPv3

                   draft-ietf-l2tpext-pwe3-fr-05.txt


Status of this Memo

   By submitting this Internet-Draft, I certify that any applicable
   patent or other IPR claims of which I am aware have been disclosed,
   and any of which I become aware will be disclosed, in accordance with
   RFC 3668.

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Copyright Notice

   Copyright (C) The Internet Society (2004). All Rights Reserved.

Abstract

   The Layer 2 Tunneling Protocol, Version 3, (L2TPv3) defines a
   protocol for tunneling a variety of data link protocols over IP
   networks.  This document describes the specifics of how to tunnel
   Frame-Relay over L2TPv3, including frame encapsulation,
   virtual-circuit creation, deletion, and status change notification.



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   Contents

   Status of this Memo..........................................    1

   1. Introduction..............................................    3
      1.1 Abbreviations.........................................    3

   2. Control Connection Establishment..........................    3

   3. PVC Status Notification and Session Establishment.........    3
      3.1 L2TPv3 Session Establishment..........................    4
      3.2 L2TPv3 Session Teardown...............................    5
      3.3 L2TPv3 Session Maintenance............................    5
      3.4 Use of the Circuit Status AVP for Frame-Relay.........    6
      3.5 Frame-Relay Header Length AVP.........................    6

   4. Encapsulation.............................................    7
      4.1 Data Packet Encapsulation.............................    7
      4.2 Data Packet Sequencing................................    9
      4.3 MTU Considerations....................................    9

   5. Security Considerations...................................    9

   6. IANA Considerations.......................................    9
      6.1 Pseudowire Type.......................................    9
      6.2 Result Code AVP Values................................   10
      6.3 Control Message Attribute Value Pairs (AVPs)..........   10

   7. Acknowledgments...........................................   10

   8. References................................................   10
      8.1 Normative References..................................   10
      8.2 Informative References................................   11

   9. Authors' Addresses........................................   11

Specification of Requirements

   In this document, several words are used to signify the requirements
   of the specification.  These words are often capitalized.  The key
   words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD",
   "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document
   are to be interpreted as described in [RFC2119].








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1. Introduction

   [RFC3931] defines a base protocol for Layer 2 Tunneling over IP
   networks. This document defines the specifics necessary for tunneling
   Frame-Relay over L2TPv3. Such emulated circuits are referred to as
   Frame-Relay Pseudowires (FRPWs).

   Protocol specifics defined in this document for L2TPv3 FRPWs include
   those necessary for frame encapsulation, PVC creation, deletion, and
   status change notification.  Support for Switched Virtual Circuits
   (SVCs) and Switched/soft Permanent Virtual Circuits (SPVCs) are
   outside the scope of this document.

   The reader is expected to be very familiar with the terminology and
   protocol constructs defined in [RFC3931].

1.1 Abbreviations

   FR    Frame-Relay
   FRPW  Frame-Relay Pseudo-Wire
   LCCE  L2TP Control Connection Endpoint (See [RFC3931])
   PVC   Permanent virtual circuit
   PW    Pseudo-Wire
   VC    Virtual circuit

2. Control Connection Establishment

   In order to tunnel a Frame-Relay circuit over IP using L2TPv3, an
   L2TPv3 Control Connection MUST first be established as described in
   [RFC3931]. The L2TPv3 SCCRQ Control Message and corresponding SCCRP
   Control Message MUST include the Frame-Relay DLCI PW Type of 0x0001
   (See IANA Considerations Section), in the Pseudo Wire Capabilities
   List as defined in 5.4.3 of [RFC3931]. This identifies the control
   connection as able to establish L2TP sessions to support Frame-Relay
   Pseudo-Wires (FRPWs).

   An LCCE MUST be able to uniquely identify itself in the SCCRQ and
   SCCRP messages via a globally unique value. By default, this is
   advertised via the structured Router ID AVP [RFC3931], though the
   unstructured Hostname AVP [RFC3931] MAY be used to identify LCCEs as
   well.

3. PVC Status Notification and Session Establishment

   This section specifies how the status of a PVC is reported between
   two LCCEs. This includes what should happen when a PVC is created,
   deleted or when it changes state between ACTIVE and INACTIVE.




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3.1 L2TPv3 Session Establishment

   PVC creation (provisioning) results in establishment of an L2TP
   session via the standard three-way handshake described in Section
   3.4.1 of [RFC3931]. An LCCE MAY initiate the session immediately upon
   PVC creation, or wait until the PVC state transitions to ACTIVE
   before attempting to establish a session for the PVC. Waiting until
   the PVC transitions to ACTIVE may be preferred as it delays
   allocation of L2TP resources until absolutely necessary.

   The Pseudowire Type AVP defined in Section 5.4.4 of [RFC3931],
   Attribute Type 68, MUST be present in the ICRQ messages and MUST
   include the Frame-Relay DLCI PW Type of 0x0001 for FRPWs.

   The Circuit Status AVP (see Section 3.4) MUST be present in the ICRQ
   and ICRP messages, and MAY be present in the SLI message for FRPWs.

   The Frame-Relay Header Length AVP (see Section 3.5) MAY be present in
   the ICRQ and ICRP messages.

   Following is an example of the L2TP messages exchanged for an FRPW
   which is initiated after a new PVC is provisioned and becomes ACTIVE.

         LCCE (LAC) A                     LCCE (LAC) B
      ------------------               ------------------
      FR PVC Provisioned
                                       FR PVC Provisioned
      FR PVC ACTIVE

                   ICRQ (status = 0x03) ---->

                                       FR PVC ACTIVE

                   <---- ICRP (status = 0x03)

      L2TP session established,
      OK to send data into tunnel

                       ICCN ----->
                                    L2TP session established,
                                    OK to send data into tunnel

   In the example above, an ICRQ is sent after the PVC is created and
   becomes ACTIVE. The Circuit Status AVP indicates that this PVC is
   ACTIVE and New (0x03). The Remote End ID AVP [RFC3931] MUST be
   present in the ICRQ in order to identify the PVC (together with the
   identity of the LCCE itself as defined in Section 2) to associate the
   L2TP session with. The Remote End ID AVP defined in [RFC3931] is of



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   opaque form and variable length, though one MUST at a minimum support
   use of an unstructured four-octet value that is known to both LCCEs
   (either by direct configuration, or some other means). The exact
   method of how this value is configured, retrieved, discovered, or
   otherwise determined at each LCCE is outside the scope of this
   document.

   As with the ICRQ, the ICRP is sent only after the FR PVC transitions
   to ACTIVE as well. If LCCE B had not been provisioned for the PVC
   identified in the ICRQ, a CDN would have been immediately returned
   indicating that the circuit was not provisioned or available at this
   LCCE.  LCCE A should then exhibit a periodic retry mechanism. The
   period and maximum number of retries MUST be configurable.

   An Implementation MAY send an ICRQ or ICRP before a PVC is ACTIVE, as
   long as the Circuit Status AVP reflects that the PVC is INACTIVE and
   an SLI is sent when the PVC becomes ACTIVE (see Section 3.3).

   The ICCN is the final stage in the session establishment, confirming
   the receipt of the ICRP with acceptable parameters to allow
   bidirectional traffic.

3.2 L2TPv3 Session Teardown

   In the event a PVC is deleted (unprovisioned) at either LCCE, the
   associated L2TP session MUST be torn down via the CDN message defined
   in Section 3.4.3 of [RFC3931].

   General Result Codes regarding L2TP session establishment are defined
   in [RFC3931]. Additional Frame-Relay result codes are defined as
   follows:

       RC-TBD-1: FR PVC was deleted permanently (no longer provisioned)
       RC-TBD-2: FR PVC has been INACTIVE for an extended period of time
       RC-TBD-3: Mismatched FR Header Length

3.3 L2TPv3 Session Maintenance

   FRPW over L2TP makes use of the Set Link Info (SLI) control message
   defined in [RFC3931] to signal Frame-Relay link status notifications
   between LCCEs. This includes ACTIVE or INACTIVE notifications of the
   VC, or any other parameters that may need to be shared between the
   tunnel endpoints or LCCEs in order to provide proper PW emulation.
   The SLI message is a single message that is sent over the L2TP
   control channel signaling the state change. Since the message is
   delivered reliably, there is no additional response or action
   required of the PW subsytem to ensure that the state change
   notification was received by the tunnel peer.



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   The SLI message MUST be sent any time there is a circuit status
   change which may be reported by any values identified in the Circuit
   Status AVP. The only exception to this are the initial ICRQ, ICRP and
   CDN messages which establish and teardown the L2TP session itself
   when the PVC is created or deleted.  The SLI message may be sent from
   either LCCE at any time after the first ICRQ is sent (and perhaps
   before an ICRP is received, requiring the peer to perform a reverse
   Session ID lookup).

   All sessions established by a given control connection utilize the
   L2TP Hello facility defined in Section 4.4 of [RFC3931] for session
   keepalive. This gives all sessions basic dead peer and path detection
   between LCCEs.

3.4 Use of the Circuit Status AVP for Frame-Relay

   Frame-relay circuit status is reported via the Circuit Status AVP
   defined in [RFC3931], Attribute Type 71. For reference, this AVP is
   shown below:

    0                   1
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Reserved        |N|A|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Value is a 16 bit mask with the two least significant bits
   defined and the remaining bits reserved for future use. Reserved bits
   MUST be set to 0 when sending, and ignored upon receipt.

   The N (New) bit indicates whether the Circuit Status indication is
   for a new FR PVC (1) or an existing FR PVC (0).

   The A (Active) bit indicates whether the FR PVC is ACTIVE (1) or
   INACTIVE (0).

3.5 Frame-Relay Header Length AVP

   The "Frame-Relay Header Length AVP", Attribute type AVP-TBD-1,
   indicates the number of bytes in the Frame Relay header. The two peer
   LCCEs MUST agree on the length of the Frame Relay header.

   This AVP is exchanged during session negotiation (in ICRQ, ICRP).  If
   the other LCCE supports a different Frame Relay header length, the
   associated L2TP session MUST be torn down via CDN message with result
   code RC-TBD-3 (see Section 3.2).

   If the Frame-Relay Header Length AVP is not signaled, it MUST be



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   assumed that the peer uses a 2-byte Frame Relay header.

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

   Frame-Relay Header Length (ICRQ, ICRP)

       0                   1
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |   Frame Relay Header Length   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Frame Relay Header Length Type is a 2-octet unsigned integer with
   the following values defined in this document:

      2 - Two-octet Frame Relay Header
      4 - Four-octet Frame Relay Header

   This AVP MAY be hidden (the H bit MAY be 0 or 1). The M bit for this
   AVP MAY be set to 0, but MAY vary (see Section 5.2 of [RFC3931]).
   The length (before hiding) of this AVP is 8.


4. Encapsulation

4.1 Data Packet Encapsulation

   The FR PDU is transported in its entirety, excluding the opening and
   closing HDLC flags and the FCS. Bit stuffing is undone. The L2TPv3
   Session Header is that as defined in [RFC3931]. If sequencing or
   other features require presence of an L2-Specific Sublayer, the
   Default format defined in Section 4.6 of [RFC3931] MUST be used.

   The FR header is defined in [Q922], however the notation used differs
   from that used in IETF specifications. For reference the FR header in
   IETF notation is:

    0                   1
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | hi dlci   |C|0|lo dlci|F|B|D|1|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Two-octet FR Header







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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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | hi dlci   |C|0| dlci  |F|B|D|0|   dlci      |0| dlci_lo   |0|1|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Four-octet FR  Header

   C/R (bit 6)
   FR frame C/R (command/response) bit [Q922].

   F - FECN (bit 12):
   FR FECN (Forward Explicit Congestion Notification) bit [Q922].

   B - BECN (bit 13):
   FR BECN (Backward Explicit Congestion Notification) bit [Q922].

   D - DE (bit 14)
   FR DE bit indicates the discard eligibility [Q922].

   Usage of the C/R, FECN, BECN and DE bits is as specified in [Q922].

   The C/R bit is conveyed transparently. Its value MUST NOT be changed
   by the LCCE.

   The FECN bit MAY be set by the LCCE to notify the receiving end-user
   that the frames it recieves have encountered congestion. The end-user
   may use this indication for destination controlled transmit rate
   adjustment. The bit must never be cleared by the LCCE. If the LCCE
   does not support FECN it shall pass the bit unchanged.

   The BECN bit MAY be set by the LCCE to notify the receiving end-user
   that frames it transmits may encounter congestion. The end-user may
   use this indication to adjust its transmit rate. The bit must never
   be cleared by the LCCE. If the LCCE does not support BECN it shall
   pass the bit unchanged.

   The DE bit MAY be set by a policing function on the LCCE to indicate
   that this frame SHOULD be discarded in preference to other frames in
   a congestion situation. The bit must never be cleared by the LCCE. If
   the LCCE does not support DE it shall pass the bit unchanged.

   The encapsulation of Frame Relay frames with Two-octet FR Header is
   REQUIRED. The encapsulation of Frame Relay frames with Four-octet FR
   Header is OPTIONAL.






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4.2 Data Packet Sequencing

   Data Packet Sequencing MAY be enabled for FRPWs. The sequencing
   mechanisms described in [RFC3931] MUST be used for signaling
   sequencing support. FRPW over L2TP MUST request the presence of the
   L2TPv3 Default L2-Specific Sublayer when sequencing is enabled, and
   MAY request its presence at all times.

   If the FRPW is known to be carrying data which does not require
   packet order to be strictly maintained (such as IP), then packet
   sequencing for the FRPW SHOULD NOT be enabled.

4.3 MTU Considerations

   With L2TPv3 as the tunneling protocol, the packet resulted from the
   encapsulation is N bytes longer than Frame-Relay frame without the
   opening and closing HDLC flags or FCS. The value of N depends on the
   following fields:

      L2TP Session Header:
         Flags, Ver, Res - 4 octets (L2TPv3 over UDP only)
         Session ID      - 4 octets
         Cookie Size     - 0, 4 or 8 octets
      L2-Specific Sublayer - 0 or 4 octets (i.e., using sequencing)

   Hence the range for N in octets is:

      N = 4-16,  L2TPv3 data messages are over IP;
      N = 16-28, L2TPv3 data messages are over UDP;
      (N does not include the IP header).

   The MTU and fragmentation implications resulting from this are
   discussed in Section 4.1.4 of [RFC3931].

5. Security Considerations

   Frame Relay over L2TPv3 is subject to the security considerations
   defined in [RFC3931]. There are no additional considerations specific
   to carrying Frame Relay that are not present carrying other data link
   types.

6. IANA Considerations

6.1 Pseudowire Type

   The following value for the Frame Relay DLCI PW Type (see Pseudo Wire
   Capabilities List as defined in 5.4.3 of [RFC3931] and L2TPv3
   Pseudowire Types in 10.6 of [RFC3931]) needs allocation by the IANA



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   (number space already created as part of publication of [RFC3931]):

      L2TPv3 Pseudowire Types
      -----------------------

      0x0001 - Frame Relay DLCI Pseudowire Type

6.2 Result Code AVP Values

   Three new L2TP Result Codes appear in section 3.2 which need
   assignment by IANA as described in section 2.3 of [BCP0068].

      Result Code AVP (Attribute Type 1) Values
      -----------------------------------------

      RC-TBD-1 - PVC was deleted permanently (no longer provisioned)
      RC-TBD-2 - PVC has been INACTIVE for an extended period of time
      RC-TBD-3 - Mismatched FR Header Length

6.3 Control Message Attribute Value Pairs (AVPs)

   An additional AVP Attribute is specified in section 3.5. It is
   required to be defined by IANA as described in Section 2.2 of
   [BCP0068].

      Control Message Attribute Value Pairs
      -------------------------------------

      AVP-TBD-1 - Frame-Relay Header Length

7. Acknowledgments

   The first Frame Relay over L2TP document was published as
   draft-vasavada-l2tpext-fr-svctype-00.txt, "Frame Relay Service Type
   for L2TP", in Feburary of 2001 by Nishit Vasavada, Jim Boyle, Chris
   Garner, Serge Maskalik, and Vijay Gill. This document is
   substantially different, but the basic concept of carrying Frame
   Relay over L2TP is the same.

   Thanks to Lloyd Wood for a razor-sharp review.

   Carlos Pignataro helped with review and editing of the document.

8. References

8.1 Normative References

      [RFC3931]  J. Lau, M. Townsley, I. Goyret, "Layer Two Tunneling



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                 Protocol - Version 3 (L2TPv3)", RFC 3931, March 2005.

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

8.2 Informative References

      [BCP0068] Townsley, W., Layer Two Tunneling Protocol (L2TP)
                Internet Assigned Numbers Authority (IANA)
                Considerations Update", RFC3438, BCP0068, December 2002

      [Q922]    ITU-T Recommendation Q.922, ISDN Data Link Layer
                Specification for Frame Mode Bearer Services, ITU,
                Geneva, 1992.



9. Authors' Addresses

   W. Mark Townsley
   cisco Systems
   7025 Kit Creek Road
   PO Box 14987
   Research Triangle Park, NC 27709
   mark@townsley.net

   George Wilkie
   cisco Systems
   96 Commercial Street
   Edinburgh, EH6 6LX
   United Kingdom
   gwilkie@cisco.com

   Jed Lau
   cisco Systems
   170 W. Tasman Drive
   San Jose, CA  95134
   jedlau@cisco.com

   Skip Booth
   cisco Systems
   7025 Kit Creek Road
   PO Box 14987
   Research Triangle Park, NC 27709
   ebooth@cisco.com

   Stewart Bryant
   cisco Systems



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   Uxbridge UB11 1BL
   United Kingdom
   stbryant@cisco.com


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