Network Working Group W. Mark Townsley Internet-Draft George Wilkie Category: Standards Track Skip Booth Jed Lau March 2004 Stewart Bryant cisco Systems Frame-Relay 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 Frame-Relay over L2TPv3, including frame encapsulation, virtual- circuit creation, deletion, and line status change notification. Townsley, et al. Standards Track [Page 1] INTERNET DRAFT Frame-Relay over L2TPv3 March 2004 Contents Status of this Memo.......................................... 1 1. Introduction.............................................. 2 1.1 Abbreviations......................................... 3 2. Control Connection Establishment.......................... 3 3. PVC 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 the Circuit Status AVP for Frame-Relay......... 6 4. Encapsulation............................................. 6 4.1 Data Packet Encapsulation............................. 6 4.2 Data Packet Sequencing................................ 7 5. Security Considerations................................... 8 6. IANA Considerations....................................... 8 7. Acknowledgments........................................... 8 8. References................................................ 8 8.1 Normative References.................................. 8 8.2 Informative References................................ 8 9. Contacts.................................................. 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 [L2TPv3] 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 Townsley, et al. Standards Track [Page 2] INTERNET DRAFT Frame-Relay over L2TPv3 March 2004 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 [L2TPv3]. 1.1 Abbreviations FR Frame-Relay FRPW Frame-Relay Pseudo-Wire LCCE L2TP Control Connection Endpoint (See [L2TPv3]) 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 [L2TPv3]. The L2TPv3 SCCRQ Control Message and corresponding SCCRP Control Message MUST include the Frame-Relay 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 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 [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. 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. 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 [L2TPv3]. 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 Townsley, et al. Standards Track [Page 3] INTERNET DRAFT Frame-Relay over L2TPv3 March 2004 the PVC transitions to ACTIVE may be preferred as it delays allocation of L2TP resources until absolutely necessary. The Circuit Status AVP (see Section 4) MUST be present in the ICRQ and ICRP messages, and MAY be present in the SLI message for FRPWs. 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 [L2TPv3] 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 [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 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 Townsley, et al. Standards Track [Page 4] INTERNET DRAFT Frame-Relay over L2TPv3 March 2004 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 [L2TPv3]. General Result Codes regarding L2TP session establishment are defined in [L2TPv3]. Additional Frame-Relay result codes are defined as follows: TBD2: FR PVC was deleted permanently (no longer provisioned) TBD3: FR PVC has been INACTIVE for an extended period of time 3.3 L2TPv3 Session Maintenance FRPW over L2TP makes use of the Set Link Info (SLI) control message defined in [L2TPv3] 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. 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 is 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 factility defined in Section 4.4 of [L2TPv3] for session keepalive. This gives all sessions basic dead peer and path detection between LCCEs. Townsley, et al. Standards Track [Page 5] INTERNET DRAFT Frame-Relay over L2TPv3 March 2004 3.4 Use of the Circuit Status AVP for Frame-Relay Frame-relay circuit status is reported via the Circuit Status AVP defined in [L2TPv3]. 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 |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 FR PVC is ACTIVE (1) or INACTIVE (0). The N (New) bit indicates whether the circuit status indication is for a new FR PVC (1) or an existing FR PVC (0). 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 [L2TPv3]. If sequencing or other features require presence of an L2-Specific Sublayer, the Default format defined in section 4.6 of [L2TPv3] 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 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| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Townsley, et al. Standards Track [Page 6] INTERNET DRAFT Frame-Relay over L2TPv3 March 2004 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. 4.2 Data Packet Sequencing Data Packet Sequencing MAY be enabled for FRPWs. The sequencing mechanisms described in [L2TPv3] 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. Townsley, et al. Standards Track [Page 7] INTERNET DRAFT Frame-Relay over L2TPv3 March 2004 5. Security Considerations Frame Relay over L2TPv3 is subject to the security considerations defined in [L2TPv3]. There are no additional considerations specific to carrying Frame Relay that are not present carrying other data link types. 6. IANA Considerations The following value needs assignment by IANA (number space should be created as part of publication of [L2TPv3]): TBD1 - Frame Relay Pseudowire Type (see Pseudo Wire Capabilities List as defined in 5.4.3 of [L2TPv3]). Two new L2TP Result Codes appear in section 3.2 which need assignment by IANA as described in section 9.1 of [BCP0068]. TBD2 - PVC was deleted permanently (no longer provisioned) TBD3 - PVC has been INACTIVE for an extended period of time 7. Acknowledgments The first Frame Relay over L2TP document was published as "Frame Relay Service Type for L2TP," draft-vasavada-l2tpext-fr-svctype- 00.txt 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. 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", Townsley, et al. Standards Track [Page 8] INTERNET DRAFT Frame-Relay over L2TPv3 March 2004 RFC3438, BCP0068, December 2002 [Q922] ITU-T Recommendation Q.922, ISDN Data Link Layer Specification for Frame Mode Bearer Services, ITU, Geneva, 1992. 9. Contacts 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 Uxbridge UB11 1BL United Kingdom stbryant@cisco.com 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 Townsley, et al. Standards Track [Page 9] INTERNET DRAFT Frame-Relay over L2TPv3 March 2004 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. 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