Network Working Group W. Mark Townsley Internet-Draft cisco Systems Category: Standards Track <draft-ietf-l2tpext-pwe3-hdlc-03.txt> March 2004 HDLC Frames over L2TPv3 Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. 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 High Level Data Link Control (HDLC) frames over L2TPv3. Townsley Standards Track [Page 1]
INTERNET DRAFT HDLC Frames over L2TPv3 March 2004 Contents Status of this Memo.......................................... 1 1. Introduction.............................................. 2 1.1 Abbreviations......................................... 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 3.3 Use of Circuit Status AVP for HDLC.................... 5 4. Encapsulation............................................. 6 4.1 Data Packet Sequencing................................ 6 5. Security Considerations................................... 6 6. IANA Considerations....................................... 7 7. Acknowledgments........................................... 7 8. References................................................ 7 8.1 Normative References.................................. 7 8.2 Informative References................................ 7 9. Contacts.................................................. 7 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]. 1. Introduction [L2TPv3] 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 Townsley Standards Track [Page 2]
INTERNET DRAFT HDLC Frames over L2TPv3 March 2004 L2TPv3 nodes) frame encapsulation, and simple interface up and interface down notifications. The reader is expected to be very familiar with the terminology and protocol constructs defined in [L2TPv3]. 1.1 Abbreviations HDLCPW HDLC Pseudo-Wire LCCE L2TP Control Connection Endpoint (See [L2TPv3]) 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 [L2TPv3]. The L2TPv3 SCCRQ Control Message and corresponding SCCRP Control Message MUST include the HDLC PW Type of TBD1 (See IANA Considerations Section), in the Pseudo Wire Capabilities List as defined in 5.4.3 of [L2TPv3]. This identifies the control connection as able to establish L2TP sessions to support HDLC Pseudo-Wires (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 [L2TPv3], though the unstructured Hostname AVP [L2TPv3] MAY be used if both endpoints support an application (as defined by the Application Code AVP [L2TPv3]) to identify LCCEs via this value. 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 ssession 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 [L2TPv3]. 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 interfacing becoming INACTIVE. Townsley Standards Track [Page 3]
INTERNET DRAFT HDLC Frames over L2TPv3 March 2004 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. The Circuit Status AVP (see Section 4) MUST be present in the ICRQ, ICRP messages and MAY be present in the SLI message for HDLCPWs. Following is an example of the L2TP messages exchanged for an HDLCPW which 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 [L2TPv3] 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) to associate the L2TP session with. The Remote End ID AVP defined in [L2TPv3] 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 Townsley Standards Track [Page 4]
INTERNET DRAFT HDLC Frames over L2TPv3 March 2004 exhibit a periodic retry mechanism. 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 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 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 [L2TPv3]. General Result Codes regarding L2TP session establishment are defined in [L2TPv3]. Additional HDLC result codes are defined as follows: TBD2: HDLC Link was deleted permanently (no longer provisioned) TBD3: HDLC Link has been INACTIVE for an extended period of time 3.3 L2TPv3 Session Maintenance HDLC PW over L2TP makes use of the Set Link Info (SLI) control message defined in [L2TPv3] 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 exception to this is 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 [L2TPv3] for session keepalive. This gives all sessions basic dead peer and path detection between PEs. 3.3 Use of Circuit Status AVP for HDLC HDLC reports Circuit Status with the Circuit Status AVP defined in [L2TPv3]. For reference, this AVP is shown below: Townsley Standards Track [Page 5]
INTERNET DRAFT HDLC Frames over L2TPv3 March 2004 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Reserved |A|N| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 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 A (Active) bit indicates whether the HDLC interface is ACTIVE (1) or INACTIVE (0). The N (New) bit SHOULD be set to one (1) if this is the first time this interface has transitioned to ACTIVE, zero (0) otherwise. 4. Encapsulation HDLC PWs use the default encapsulations defined in [L2TPv3] for demultiplexing, sequencing, and flags. The HDLC PW 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, transported over the PW. Since all packets are passed in a largely transparent manner over the HDLC PW, any protocol which has HDLC-like framing may utilize the HDLC PW mode, including PPP, Frame-Relay, X.25, etc. Exceptions include cases where direct access to the HDLC interface is required, or modes which 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. 4.1 Data Packet Sequencing Data Packet Sequencing MAY be enabled for HDLC PWs. The sequencing mechanisms described in [L2TPv3] MUST be used for signaling sequencing support. HDLC PW 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. 5. Security Considerations HDLC over L2TPv3 is subject to the security considerations defined in [L2TPv3]. There are no additional considerations specific to carrying HDLC that are not present carrying other data link types. Townsley Standards Track [Page 6]
INTERNET DRAFT HDLC Frames over L2TPv3 March 2004 6. IANA Considerations The signaling mechanisms defined in this document rely upon the assignment of an HDLC Pseudowire Type. IANA assignment of this value should take place within the PWE3 WG. 7. Acknowledgments Thanks to Sudhir Rustogi and George Wilkie for valuable input. 8. References 8.1 Normative References [L2TPv3] J. Lau, M. Townsley, I. Goyret, "Layer Two Tunneling Protocol (Version 3)", work in progress, draft-ietf-l2tpext-l2tp-base-10.txt, August 2003. [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 9. Contacts W. Mark Townsley cisco Systems 7025 Kit Creek Road PO Box 14987 Research Triangle Park, NC 27709 mark@townsley.net Full Copyright Statement Copyright (C) The Internet Society (2004). 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 Townsley Standards Track [Page 7]
INTERNET DRAFT HDLC Frames over L2TPv3 March 2004 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." Intellectual Property The IETF takes no position regarding the validity or scope of any intellectual property or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; neither does it represent that it has made any effort to identify any such rights. Information on the IETF's procedures with respect to rights in standards-track and standards-related documentation can be found in BCP-11 [RFC2028]. Copies of claims of rights made available for publication and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementors or users of this specification can be obtained from the IETF Secretariat. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights which may cover technology that may be required to practice this standard. Please address the information to the IETF Executive Director. Townsley Standards Track [Page 8]
