HDLC over L2TPv3: High-Level Data Link Control (HDLC) Frames over Layer 2 Tunneling Protocol, Version 3 (L2TPv3)

Network Working Group                                       C. Pignataro
Request for Comments: 4349                                   M. Townsley
Category: Standards Track                                  Cisco Systems
                                                           February 2006


              High-Level Data Link Control (HDLC) Frames
          over Layer 2 Tunneling Protocol, Version 3 (L2TPv3)

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 (2006).

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
   High-Level Data Link Control (HDLC) frames over L2TPv3.

Table of Contents

   1. Introduction ....................................................2
      1.1. Abbreviations ..............................................2
      1.2. Specification of Requirements ..............................3
   2. Control Connection Establishment ................................3
   3. HDLC Link Status Notification and Session Establishment .........3
      3.1. L2TPv3 Session Establishment ...............................3
      3.2. L2TPv3 Session Teardown ....................................5
      3.3. L2TPv3 Session Maintenance .................................5
      3.4. Use of Circuit Status AVP for HDLC .........................6
   4. Encapsulation ...................................................6
      4.1. Data Packet Encapsulation ..................................6
      4.2. Data Packet Sequencing .....................................7
      4.3. MTU Considerations .........................................7
   5. Applicability Statement .........................................8
   6. Security Considerations .........................................9
   7. IANA Considerations .............................................9
      7.1. Pseudowire Type ............................................9
      7.2. Result Code AVP Values .....................................9
   8. Acknowledgements ................................................9
   9. References .....................................................10
      9.1. Normative References ......................................10
      9.2. Informative References ....................................10

1.  Introduction

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

   Protocol specifics defined in this document for L2TPv3 HDLCPWs
   include those necessary for simple point-to-point (e.g., between two
   L2TPv3 nodes) frame encapsulation, and for simple interface up and
   interface down notifications.

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

1.1 Abbreviations

   HDLC    High-Level Data Link Control
   HDLCPW  HDLC Pseudowire
   LAC     L2TP Access Concentrator (see [RFC3931])
   LCCE    L2TP Control Connection Endpoint (see [RFC3931])
   PW      Pseudowire

1.2.  Specification of Requirements

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

2.  Control Connection Establishment

   In order to tunnel an HDLC link 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 HDLC Pseudowire Type of 0x0006 (see
   Section 7, "IANA Considerations"), in the Pseudowire Capabilities
   List as defined in 5.4.3 of [RFC3931].  This identifies the control
   connection as able to establish L2TP sessions to support HDLC
   Pseudowires (HDLCPWs).

   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.  HDLC Link Status Notification and Session Establishment

   This section specifies how the status of an HDLC interface is
   reported between two LCCEs, and the associated L2TP session creation
   and deletion that occurs.

3.1.  L2TPv3 Session Establishment

   Associating an HDLC serial interface with a PW and its transition to
   "Ready" or "Up" results in the establishment of an L2TP session via
   the standard three-way handshake described in Section 3.4.1 of
   [RFC3931].  For purposes of this discussion, the action of locally
   associating an interface running HDLC with a PW by local
   configuration or otherwise is referred to as "provisioning" the HDLC
   interface.  The transition of the interface to "ready" or "up" will
   be referred to as the interface becoming ACTIVE.  The transition of
   the interface to "not-ready" or "down" will be referred to as the
   interface becoming INACTIVE.

   An LCCE MAY initiate the session immediately upon association with an
   HDLC interface or wait until the interface becomes ACTIVE before
   attempting to establish an L2TP session.  Waiting until the interface
   transitions to ACTIVE may be preferred, as it delays allocation of
   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 Pseudowire Type of 0x0006 for HDLCPWs.

   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 HDLCPWs.

   Following is an example of the L2TP messages exchanged for an HDLCPW
   that is initiated after an HDLC interface is provisioned and becomes
   ACTIVE.

         LCCE (LAC) A                     LCCE (LAC) B
      ------------------               ------------------
      HDLC Interface Provisioned
                                       HDLC Interface Provisioned
      HDLC Interface ACTIVE

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

                                       HDLC Interface 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 interface is
   provisioned and becomes ACTIVE.  The Circuit Status AVP indicates
   that this link is ACTIVE and New (0x03).  The Remote End ID AVP
   [RFC3931] MUST be present in the ICRQ in order to identify the HDLC
   link (together with the identity of the LCCE itself as defined in
   Section 2) with which to associate the L2TP session.  The Remote End
   ID AVP defined in [RFC3931] is of 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 associated HDLC
   interface transitions to ACTIVE as well.  If LCCE B had not been
   provisioned for the interface identified in the ICRQ, a CDN would
   have been immediately returned indicating that the associated link
   was not provisioned or available at this LCCE.  LCCE A SHOULD then
   exhibit a periodic retry mechanism.  If so, the period and maximum
   number of retries MUST be configurable.

   An Implementation MAY send an ICRQ or ICRP before an HDLC interface
   is ACTIVE, as long as the Circuit Status AVP reflects that the link
   is INACTIVE and an SLI is sent when the HDLC interface 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 link is removed (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 HDLC result codes are defined as follows:

      20 - HDLC Link was deleted permanently (no longer provisioned)
      21 - HDLC Link has been INACTIVE for an extended period of time

3.3.  L2TPv3 Session Maintenance

   HDLCPWs over L2TP make use of the Set Link Info (SLI) control message
   defined in [RFC3931] to signal HDLC link status notifications between
   PEs.  The SLI message is a single message that is sent over the L2TP
   control channel, signaling the interface state change.

   The SLI message MUST be sent any time there is a status change of any
   values identified in the Circuit Status AVP.  The only exceptions to
   this are the initial ICRQ, ICRP, and CDN messages, which establish
   and teardown the L2TP session itself.  The SLI message may be sent
   from either PE 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 PEs.

3.4.  Use of Circuit Status AVP for HDLC

   HDLC reports Circuit Status with 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 SHOULD be set to one (1) if the Circuit Status
   indication is for a new HDLC circuit; to zero (0) otherwise.

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

4.  Encapsulation

4.1.  Data Packet Encapsulation

   HDLCPWs use the default encapsulations defined in [RFC3931] for
   demultiplexing, sequencing, and flags.  The HDLCPW Type over L2TP is
   intended to operate in an "interface to interface" or "port to port"
   fashion, passing all HDLC data and control PDUs over the PW.  The
   HDLC PDU is stripped of flags and trailing FCS, bit/byte unstuffing
   is performed, and the remaining data, including the address, control,
   and protocol fields, is transported over the PW.

   Since all packets are passed in a largely transparent manner over the
   HDLCPW, any protocol that has HDLC-like framing may utilize the
   HDLCPW mode, including PPP, Frame-Relay ("port to port" Frame-Relay
   transport), X.25 (LAPB), etc.  In such cases, the negotiations and
   signaling of the specific protocols transported over the HDLCPW take
   place between the Remote Systems.  A non-exhaustive list of examples
   and considerations of this transparent nature include:

      o When the HDLCPW transports Point-to-Point Protocol (PPP)
        traffic, PPP negotiations (Link Control Protocol, optional
        authentication, and Network Control Protocols) are performed
        between Remote Systems, and LCCEs do not participate in these
        negotiations.

      o When the HDLCPW transports Frame-Relay traffic, PVC status
        management procedures (Local Management Interface) take place
        between Remote Systems, and LCCEs do not participate in LMI.
        Additionally, individual Frame-Relay virtual-circuits are not
        visible to the LCCEs, and the FECN, BECN, and DE bits are
        transported transparently.

      o When the HDLCPW transports X.25 (LAPB) traffic, LCCEs do not
        function as either LAPB DCE or DTE devices.

   On the other hand, exceptions include cases where direct access to
   the HDLC interface is required, or modes that operate on the flags,
   FCS, or bit/byte unstuffing that is performed before sending the HDLC
   PDU over the PW.  An example of this is PPP ACCM negotiation.

4.2.  Data Packet Sequencing

   Data Packet Sequencing MAY be enabled for HDLCPWs.  The sequencing
   mechanisms described in Section 4.6.1 of [RFC3931] MUST be used for
   signaling sequencing support.  HDLCPWs over L2TP MUST request the
   presence of the L2TPv3 Default L2-Specific Sublayer defined in
   Section 4.6 of [RFC3931] when sequencing is enabled, and MAY request
   its presence at all times.

4.3.  MTU Considerations

   With L2TPv3 as the tunneling protocol, the packet resulting from the
   encapsulation is N bytes longer than the HDLC frame without the 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.  Applicability Statement

   HDLC Pseudowires support a "port to port" or "interface to interface"
   deployment model operating in a point-to-point fashion.  In addition
   to the transport of HDLC frames, a natural application of HDLCPWs
   allows for the transport of any protocol using an HDLC-like framing.

   The HDLCPW emulation over a packet-switched network (PSN) has the
   following characteristics in relationship to the native service:

      o HDLC data and control fields are transported transparently (see
        Section 4.1).  The specific negotiations and signaling of the
        protocol being transported are performed between Remote Systems
        transparently, and the LCCE does not participate in them.

      o The trailing FCS (Frame Check Sequence) containing a CRC (Cyclic
        Redundancy Check) is stripped at the ingress LCCE and not
        transported over HDLCPWs.  It is therefore regenerated at the
        egress LCCE (see Section 4.1).  This means that the FCS may not
        accurately reflect errors on the end-to-end HDLC link.  Errors
        or corruption introduced in the HDLCPW payload during
        encapsulation or transit across the packet-switched network may
        not be detected.  This lack of integrity-check transparency may
        not be of concern if it is known that the inner payloads or
        upper protocols transported perform their own error and
        integrity checking.  To allow for payload integrity-checking
        transparency on HDLCPWs using L2TP over IP or L2TP over UDP/IP,
        the L2TPv3 session can utilize IPSec as specified in Section
        4.1.3 of [RFC3931].

      o HDLC link status notification is provided using the Circuit
        Status AVP in the SLI message (see Section 3.4).

      o The length of the resulting L2TPv3 packet is longer than the
        encapsulated HDLC frame without flags and FCS (see Section 4.3),
        with resulting MTU and fragmentation implications discussed in
        Section 4.1.4 of [RFC3931].

      o The packet-switched network may reorder, duplicate, or silently
        drop packets.  Sequencing may be enabled in the HDLCPW for some
        or all packets to detect lost, duplicate, or out-of-order
        packets on a per-session basis (see Section 4.2).

      o The faithfulness of an HDLCPW may be increased by leveraging
        Quality of Service features of the LCCEs and the underlying PSN.

6.  Security Considerations

   HDLC over L2TPv3 is subject to the security considerations defined in
   [RFC3931].  Beyond the considerations when carrying other data link
   types, there are no additional considerations specific to carrying
   HDLC.

7.  IANA Considerations

7.1.  Pseudowire Type

   The signaling mechanisms defined in this document rely upon the
   allocation of an HDLC Pseudowire Type (see Pseudowire Capabilities
   List as defined in 5.4.3 of [RFC3931] and L2TPv3 Pseudowire Types in
   10.6 of [RFC3931]) by the IANA (number space created as part of
   publication of [RFC3931]).  The HDLC Pseudowire Type is defined in
   Section 2 of this specification:

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

      0x0006 - HDLC Pseudowire Type

7.2.  Result Code AVP Values

   This number space is managed by IANA as described in section 2.3 of
   [BCP0068].  Two new L2TP Result Codes for the CDN message appear in
   Section 3.2. The following is a summary:

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

      20 - HDLC Link was deleted permanently (no longer provisioned)
      21 - HDLC Link has been INACTIVE for an extended period of time

8.  Acknowledgements

   Thanks to Sudhir Rustogi and George Wilkie for valuable input.  Maria
   Alice Dos Santos provided helpful review and comment.  Many thanks to
   Mark Lewis for providing review and clarifying comments during IETF
   Last Call.

9.  References

9.1.  Normative References

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

9.2.  Informative References

   [BCP0068]  Townsley, W., "Layer Two Tunneling Protocol (L2TP)
              Internet Assigned Numbers Authority (IANA) Considerations
              Update", BCP 68, RFC 3438, December 2002.

Authors' Addresses

   Carlos Pignataro
   Cisco Systems
   7025 Kit Creek Road
   PO Box 14987
   Research Triangle Park, NC 27709

   EMail: cpignata@cisco.com


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

   EMail: mark@townsley.net

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