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rfc:rfc4349

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.

Pignataro & Townsley Standards Track [Page 1] RFC 4349 HDLC Frames over L2TPv3 February 2006

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

Pignataro & Townsley Standards Track [Page 2] RFC 4349 HDLC Frames over L2TPv3 February 2006

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.

Pignataro & Townsley Standards Track [Page 3] RFC 4349 HDLC Frames over L2TPv3 February 2006

 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.

Pignataro & Townsley Standards Track [Page 4] RFC 4349 HDLC Frames over L2TPv3 February 2006

 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.

Pignataro & Townsley Standards Track [Page 5] RFC 4349 HDLC Frames over L2TPv3 February 2006

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.

Pignataro & Townsley Standards Track [Page 6] RFC 4349 HDLC Frames over L2TPv3 February 2006

    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].

Pignataro & Townsley Standards Track [Page 7] RFC 4349 HDLC Frames over L2TPv3 February 2006

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.

Pignataro & Townsley Standards Track [Page 8] RFC 4349 HDLC Frames over L2TPv3 February 2006

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.

Pignataro & Townsley Standards Track [Page 9] RFC 4349 HDLC Frames over L2TPv3 February 2006

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

Pignataro & Townsley Standards Track [Page 10] RFC 4349 HDLC Frames over L2TPv3 February 2006

Full Copyright Statement

 Copyright (C) The Internet Society (2006).
 This document is subject to the rights, licenses and restrictions
 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
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 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
 ENGINEERING TASK FORCE DISCLAIM 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.

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Pignataro & Townsley Standards Track [Page 11]

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