Network Working Group Carlos Pignataro Internet-Draft W. Mark Townsley Category: Standards Track cisco Systems Expiration Date: December 2005 June 2005 HDLC Frames over L2TPv3 draft-ietf-l2tpext-pwe3-hdlc-06.txt Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. 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/1id-abstracts.html The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html Copyright Notice Copyright (C) The Internet Society (2005). 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. Pignataro, et al. Standards Track [Page 1] INTERNET DRAFT HDLC Frames over L2TPv3 June 2005 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 3.4 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. Security Considerations................................... 8 6. IANA Considerations....................................... 8 6.1 Pseudowire Type....................................... 8 6.2 Result Code AVP Values................................ 8 7. Acknowledgments........................................... 8 8. References................................................ 9 8.1 Normative References.................................. 9 8.2 Informative References................................ 9 9. Authors' Addresses........................................ 9 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 [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 Pignataro, et al. Standards Track [Page 2] INTERNET DRAFT HDLC Frames over L2TPv3 June 2005 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 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 Pseudo-Wire LAC L2TP Access Concentrator (See [RFC3931]) LCCE L2TP Control Connection Endpoint (See [RFC3931]) PW Pseudo-Wire 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 PW Type of 0x0006 (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 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 [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 Pignataro, et al. Standards Track [Page 3] INTERNET DRAFT HDLC Frames over L2TPv3 June 2005 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. 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 HDLC PW Type of 0x0006 for HDLCPWs. The Circuit Status AVP (see Section 3.4) 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 [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) to associate the L2TP session with. The Remote End ID AVP defined Pignataro, et al. Standards Track [Page 4] INTERNET DRAFT HDLC Frames over L2TPv3 June 2005 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. 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: RC-TBD-1 - HDLC Link was deleted permanently (no longer provisioned) RC-TBD-2 - 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 [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 exception to this are the initial ICRQ, ICRP and CDN messages which establish and Pignataro, et al. Standards Track [Page 5] INTERNET DRAFT HDLC Frames over L2TPv3 June 2005 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, 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 HDLC PWs use the default encapsulations defined in [RFC3931] 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 ("port to port" Frame-Relay transport), X.25 (LAPB), etc. In such cases, the negotiations and signaling of the specific protocols transported over the HDLC PW take Pignataro, et al. Standards Track [Page 6] INTERNET DRAFT HDLC Frames over L2TPv3 June 2005 place between the Remote Systems. A non-exhaustive list of examples and considerations of this transparent nature include: o When the HDLC PW 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 HDLC PW 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 HDLC PW 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 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. 4.2 Data Packet Sequencing Data Packet Sequencing MAY be enabled for HDLC PWs. The sequencing mechanisms described in Section 4.6.1 of [RFC3931] MUST be used for signaling sequencing support. HDLC PW 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 resulted from the encapsulation is N bytes longer than 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: Pignataro, et al. Standards Track [Page 7] INTERNET DRAFT HDLC Frames over L2TPv3 June 2005 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 HDLC over L2TPv3 is subject to the security considerations defined in [RFC3931]. There are no additional considerations specific to carrying HDLC that are not present carrying other data link types. 6. IANA Considerations 6.1 Pseudowire Type The signaling mechanisms defined in this document rely upon the allocation of an HDLC Pseudowire Type (see Pseudo Wire Capabilities List as defined in 5.4.3 of [RFC3931] and L2TPv3 Pseudowire Types in 10.6 of [RFC3931]) by the IANA (number space already created as part of publication of [RFC3931]): L2TPv3 Pseudowire Types ----------------------- 0x0006 - HDLC Pseudowire Type 6.2 Result Code AVP Values Two new L2TP Result Codes for CDN 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 - HDLC Link was deleted permanently (no longer provisioned) RC-TBD-2 - HDLC Link has been INACTIVE for an extended period of time 7. Acknowledgments 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, et al. Standards Track [Page 8] INTERNET DRAFT HDLC Frames over L2TPv3 June 2005 8. References 8.1 Normative References [RFC3931] J. Lau, M. Townsley, 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. 8.2 Informative References [BCP0068] Townsley, W., Layer Two Tunneling Protocol (L2TP) Internet Assigned Numbers Authority (IANA) Considerations Update", RFC3438, BCP0068, December 2002 9. Authors' Addresses Carlos Pignataro cisco Systems 7025 Kit Creek Road PO Box 14987 Research Triangle Park, NC 27709 cpignata@cisco.com W. Mark Townsley cisco Systems 7025 Kit Creek Road PO Box 14987 Research Triangle Park, NC 27709 mark@townsley.net Intellectual Property Statement The IETF takes no position regarding the validity or scope of any Intellectual Property Rights 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; nor does it represent that it has made any independent effort to identify any such rights. 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Disclaimer of Validity This document and the information contained herein are provided on an "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. Copyright Statement Copyright (C) The Internet Society (2005). 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. Acknowledgment Funding for the RFC Editor function is currently provided by the Internet Society. Pignataro, et al. Standards Track [Page 10]