Network Working Group Vipin Jain Internet-Draft Riverstone Networks Category: Standards Track Editor Expires April 2006 October 2005 Fail Over extensions for L2TP "failover" draft-ietf-l2tpext-failover-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 L2TP is a connection-oriented protocol that has shared state between active endpoints. Some of this shared state is vital for operation but may be rather volatile in nature, such as packet sequence numbers used on the L2TP Control Connection. When failure of one side of a control connection occurs, a new control connection is created and associated with the old connection by exchanging information about the old connection. Such a mechanism is not intended as a replacement for an active fail over with some mirrored connection states, but as an aid just for those parameters that are particularly difficult to have immediately available. Protocol extensions to L2TP defined in this document are intended to facilitate state recovery, providing additional resiliency in an L2TP network and improving a remote system's layer 2 connectivity. Jain, et al. Standards Track [Page 1] INTERNET DRAFT FAILOVER April 2006 Table of Contents Status of this Memo.......................................... 1 1.0 Introduction............................................. 3 2.0 Protocol Operation....................................... 4 2.1 Pre Failover Operation................................ 4 2.2 Failover Recovery Procedure........................... 5 2.2.1 Recovery tunnel establishment.................... 5 2.2.2 Control and/or Data Channel Reset................ 8 2.3 Session State Synchronization......................... 9 3.0 IANA Considerations...................................... 11 4.0 Security Considerations.................................. 12 5.0 Acknowledgements......................................... 12 6.0 Author Information....................................... 12 7.0 References............................................... 13 8.0 Intellectual Property Statement.......................... 13 9.0 Disclaimer of Validity................................... 13 10.0 Copyright Statement..................................... 13 Appendix A................................................... 14 Appendix B................................................... 15 Appendix C................................................... 17 Appendix D................................................... 18 Contributors Following is the list of contributors to this document. Paul Howard Juniper Networks Vipin Jain Riverstone Networks Sam Henderson Cisco Systems Keyur Parikh Harris Communications Terminology Endpoint: L2TP control connection endpoint i.e. either LAC or LNS. Also known as LCCE in [L2TPv3] Active Endpoint: An endpoint that is currently providing service. Backup Endpoint: A redundant endpoint standing by for the active endpoint. Failover: The action of a Backup Endpoint taking over the service of an active endpoint. This could be due to administrative action or failure of the active endpoint. Jain, et al. Standards Track [Page 2] INTERNET DRAFT FAILOVER April 2006 Old Tunnel: A control connection that existed before failure and is subjected to recovery upon failover. Recovery Tunnel: A new control connection established only to recover an old tunnel. Recovered tunnel: After an Old Tunnel is recovered (i.e. control connection and its sessions are restored) using the mechanism described in this document it is referred as Recovered Tunnel. 1.0 Introduction The goal of this draft is to aid the overall resiliency of an L2TP endpoint by introducing extensions to RFC 2661 [L2TPv2] and RFC 3931 [L2TPv3] that will minimize the recovery time of the L2TP layer after a failover, while minimizing the impact on its performance. Therefore it is assumed that the endpoint's overall architecture is also supportive in the resiliency effort. To ensure proper operation of an L2TP endpoint after a failover, the associated information of the control connection and sessions between them must be correct and consistent. This includes both the configured and dynamic information. The configured information is assumed to be correct and consistent after a failover, otherwise the tunnels and sessions would not have been setup in the first place. The dynamic information, which is also referred to as stateful information, changes with the processing of the tunnel's control and data packets. Currently, the only such information that is essential to the tunnel's operation is its sequence numbers. For the tunnel control channel, the inconsistencies in its sequence numbers can result in the termination of the entire tunnel. For tunnel sessions, the inconsistency in its sequence numbers, when used, can cause significant data loss thus giving perception of "service loss" to the end user. Thus, an optimal resilient architecture that aims to minimize "service loss" after a failover must make provision for the tunnel's essential stateful information - i.e. its sequence numbers. Currently, there are two options available: the first option is to ensure that the backup endpoint is completely synchronized with the active with respect to the control and data sessions sequence numbers. The other option is to re-establish all the tunnels and its sessions after a failover. The drawback of the first option is that it adds significant performance and complexity impact to the endpoint's architecture, especially as tunnel and session aggregation increases. The drawback of the second option is that it increases the "service loss" time, especially as the architecture scales. Jain, et al. Standards Track [Page 3] INTERNET DRAFT FAILOVER April 2006 To alleviate the above-mentioned drawbacks of the current options, this draft introduces a mechanism to bring the dynamic stateful information of a tunnel to correct and consistent state after a failure. The proposed mechanism, defines the recovery of tunnels and sessions that were in established state prior to the failure. 2.0 Protocol Operation The failover protocol consists of three phases - pre failover, failover recovery, and session state synchronization. Pre failover operation allows an endpoint to specify its failover capabilities and timer values, attributes that are used when failover occurs. Failover recovery is started at the failed endpoint when it initiates a new L2TP control connection (called recovery tunnel), for every old tunnel that needs recovery. The recovery tunnel serves three purposes: 1) It provides a means of authentication and a three-way handshake to ensure both ends agree on the failover for a given tunnel. 2) It identifies the old tunnel that needs recovery. 3) It exchanges the Ns and Nr values to be used in the recovered tunnel on both ends. Upon establishing the recovery tunnel two endpoints reset their control and/or data channel; after which recovery tunnel could be torn down. The sessions that were in established state resume traffic. Data channel recovery is a process of resetting sequence numbers when applicable, hence there is no recovery tunnel established if there is no control channel failure. Session state synchronization process allows two endpoints to agree on the state of various sessions in the tunnel after failover. The inconsistency could arise due to failure on one of the endpoints. To synchronize, two endpoints first silently clear the sessions that were not in established state. At this point they can allow new sessions to establish on the recovered tunnel. Then, they utilize two new messages Failed Session Query (FSQ) and Failed Session Response (FSR) over the recovered tunnel or over the control channel (for data-channel-only failure) to obtain the state of sessions on the peer. 2.1 Pre Failover Operation An endpoint that supports the failover protocol defined in this document MUST include Failover Capability AVP in SCCRQ or SCCRP during control connection establishment. Jain, et al. Standards Track [Page 4] INTERNET DRAFT FAILOVER April 2006 Failover Capability AVP 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |M|H| rsvd | Length | Vendor Id [IETF] | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Attribute Type 76 | Reserved |D|C| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Recovery Time (in milliseconds) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The AVP MAY be hidden (the H-bit set to 0 or 1). The AVP is not mandatory (the M-bit MUST be set to 0). The C bit, when set indicates an endpoint's capability to initiate failover and its ability to respond to a failure on the other endpoint by implementing the protocol described in this document. The D bit, when set indicates that an endpoint is capable of resetting Nr value based on received Ns value(s) from one or more 'out of order but in sequence' packets from the peer. This bit is applicable only for the sessions using sequence numbers on the data channel i.e. data channel failure on the system not exhibiting D bit capability could still recover sessions that do not use sequence numbers. Section 2.2.2 contain more details on data channel reset. The Failover Capability AVP MUST set at least one of the two capability bits i.e. set C bit and/or D bit. Recovery Time, applicable only when C bit is set, is the time in milliseconds an endpoint asks its peer to wait before assuming the recovery process has failed. This timer starts with when an endpoint's control channel timeout ([L2TPv2] section 5.8, [L2TPv3] section 4.2) is started, and is not terminated (before expiry) until an endpoint successfully authenticate its peer during recovery. A value of zero doesn't mean that no failover will occur, it means no additional time is requested from the peer. 2.2 Failover Recovery Procedure Failover recovery procedure consists of two steps: 1) Recovery tunnel establishment 2) Control and/or data channel reset 2.2.1 Recovery tunnel establishment For control channel failure, failed endpoint establishes a new Jain, et al. Standards Track [Page 5] INTERNET DRAFT FAILOVER April 2006 control connection called recovery tunnel for every old tunnel it wishes to recover. The purpose of the recovery tunnel is solely to recover the corresponding old tunnel. An endpoint SHOULD not send any control message on this tunnel, other than those required to manage the life of the recovery tunnel. Recovery tunnel MUST also not indicate it is failover capable i.e. MUST not include Failover Capability AVP in SCCRQ or SCCRP messages. Recovery tunnel MUST use the same L2TP version and establishment procedures that were used for the control connection being recovered. It MUST follow the procedures described in [L2TPv2] or [L2TPv3] to establish the recovery tunnel. To identify the old control connection, SCCRQ message for recovery tunnel MUST include Tunnel Recovery AVP. Tunnel Recovery AVP for L2TPv3 tunnels: 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |M|H| rsvd | Length | Vendor Id [IETF] | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Attribute Type 77 | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Recover Tunnel Id | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Recover Remote Tunnel Id | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Tunnel Recovery AVP for L2TPv2 tunnels: 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |M|H| rsvd | Length | Vendor Id [IETF] | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Attribute Type 77 | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Reserved | Recover Tunnel Id | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Reserved | Recover Remote Tunnel Id | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ This AVP MUST not be hidden (the H-bit is set to 0). The AVP is mandatory (the M-bit is set to 1). Recover Tunnel Id encodes the local tunnel id that it wants recovered. Similarly, Recover Remote Tunnel Id encodes the remote Jain, et al. Standards Track [Page 6] INTERNET DRAFT FAILOVER April 2006 tunnel id corresponding to the old tunnel. Upon getting an SCCRQ with Tunnel Recovery AVP, the peer endpoint validates Recover Tunnel Id and Recover Remote Tunnel Id and responds with an SCCRP. It MUST terminate the recovery tunnel if: - Recover Tunnel Id or Remote Recover Tunnel Id is unknown. - Failed or non failed endpoint did not indicate it was failover capable. - The L2TP version of recovery tunnel is different from the version used in the old tunnel. If non failed endpoint accepts the SCCRQ, it MAY include Suggested Control Sequence AVP in the SCCRP. Suggested Control Sequence AVP 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |M|H| rsvd | Length | Vendor Id [IETF] | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Attribute Type 78 | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Suggested Ns | Suggested Nr | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ This AVP MAY be hidden (the H-bit set to 0 or 1). The AVP is not mandatory (the M-bit is set to 0). This is an optional AVP, suggesting Ns and Nr values to be used by the failed endpoint. If this AVP is present in an SCCRP message, the failed endpoint MUST set the Ns and Nr values of the recovered tunnel to the respective suggested values. When this AVP is not sent in SCCRP or not present in an incoming SCCRP, the Ns and Nr values for the recovered tunnel are set to zero. It is recommended that the non failed endpoint suggest the Ns and Nr values to help avoid the interference in recovered tunnel's control channel with old control packets. In case of L2TPv3, Recovery tunnel MUST use the Control Message authentication (i.e. exchange the nonce values) as described in [L2TPv3] section 4.3, if the old tunnel was configured to do Control Message authentication. An L2TP Version 3 recovered tunnel MUST reset their nonce values (local and remote) to the nonce values exchanged in the recovery tunnel. To authenticate an endpoint during recovery, an endpoint MUST follow the procedure described in either [L2TPv2] section 5.1.1 or Jain, et al. Standards Track [Page 7] INTERNET DRAFT FAILOVER April 2006 [L2TPv3] section 4.3. It SHOULD use the same secret that was used to authenticate the old tunnel. Not being able to authenticate could be a reason to terminate the recovery tunnel. If, for any reason, the failed endpoint could not establish the recovery tunnel then it MUST silently clear the old tunnel and sessions within, assuming the recovery process has failed. Any control packet received on the recovered tunnel, before control channel reset, MUST be silently discarded. An endpoint MUST use Tie Breaker AVP (section 4.4.3 [L2TPv2]) or Control Connection Tie Breaker AVP (section 5.4.3 [L2TPv3]) in the setup of the recovery tunnel to ensure that only a single recovery tunnel (when both endpoints failover) is established for each tunnel to be recovered. The scope of tie breaker AVP's action, when used in a recovery tunnel, is restricted to the recovery tunnel(s) for a single tunnel to be recovered as opposed to the non-recovery usage where the scope is the LAC-LNS pair. Thus an implementation MUST apply the tiebreaker only to those tunnels that are a) recovery tunnels, and b) associated with the same tunnel to be recovered. It must not impact the operation of non- recovery tunnels nor or of recovery tunnels associated with different tunnels to be recovered. The tunnel that wins the tie is used to decide the suggested Ns, Nr values on the recovered tunnel. Therefore, the endpoint that looses the tie, should reset the Ns and Nr values as if it were a non failed endpoint (section 2.2.2). Appendix C illustrates double failover scenario. 2.2.2 Control and/or Data Channel Reset Control channel reset procedure SHOULD flush the transmit and receive windows, and reset the control channel sequence numbers (i.e. Ns and Nr values) on recovered tunnel. The control channel on failed endpoint is reset upon getting a valid SCCRP, whereas control channel on non failed endpoint is reset upon getting a valid SCCCN. If failed endpoint does not receive Suggested sequence number AVP in SCCRP then it MUST reset Ns and Nr values to zero. Similarly, if non failed endpoint opts not to send suggested sequence number AVP then it MUST reset Ns and Nr values to zero. Either endpoint can tear down the recovery tunnel after control channel reset. For control channel failure an endpoint MUST prevent establishment of new sessions until it has cleared (or marked for clearance) the sessions that were not in established state i.e. until after Step 1, section 2.3 is complete. Data channel is reset only for the sessions using sequence Jain, et al. Standards Track [Page 8] INTERNET DRAFT FAILOVER April 2006 numbers. For L2TPv3 data channel, terms Nr and Ns are used to mean 'expected sequence number' and 'sequence number' respectively. Data channel reset requires the failed endpoint to set the Ns value to zero, where as non failed endpoint continues to use the Ns values it was using previously. To reset Nr values during failover, if an endpoint receives 'n' out of order but in sequence packets then it MUST set the Nr value based on the Ns value of the incoming packets, as suggested in Appendix C [L2TPv3]. The value of 'n' SHOULD be configurable. For sessions requiring data channel reset, if one of the endpoints doesn't exhibit the capability (indicated in 'D' bit in Failover Capability AVP) to reset the Nr value, then data channel using sequence numbers can't be recovered. Such sessions SHOULD be torn down by the failed endpoint by sending a CDN. For data-channel- only failure, two endpoints MAY use FSQ/FSR messages on the control channel synchronize the state of sessions as described in section 2.3 below. 2.3 Session State Synchronization If control channel failover happens while a session is being established or being torn down, it is possible for an endpoint to consider a session in established state, when its peer considers the same session non existent. Two such situations occur when an endpoint fails after sending: - A CDN message that never made it to the peer. - An ICCN message that never made it to the peer. On other hand, a data channel failure could result into sessions not being in recoverable state. Following mechanism MUST be used to identify and clear the sessions that exists on an endpoint but not on its peer: Step1: For control channel failure, after the recovery tunnel is established, the sessions that were not in established state MUST be silently cleared (i.e. without sending a CDN message) by each endpoint. Step2: Both endpoints MAY identify the sessions that might have been in inconsistent states, perhaps based on data channel inactivity. FSQ and FSR messages have been introduced to synchronize session state at any given point during the life of a session between two endpoints. These messages are used when one endpoint determines or suspects in an implementation specific manner that a session state between it and its peer is in inconsistent state. Jain, et al. Standards Track [Page 9] INTERNET DRAFT FAILOVER April 2006 Step3: An endpoint sends Failover Session Query (FSQ) message, message type 21, to query the state of stale sessions on its peer. An FSQ message MUST include at least one Failover Session State (FSS) AVPs. An endpoint MAY send another FSQ message before getting response for its previous FSQs. Failover Session State AVP is described as follows: Failover Session State AVP for L2TPv3 sessions (FSQ, FSR): 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |M|H| rsvd | Length | Vendor Id [IETF] | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Attribute Type 79 | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Session Id | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Remote Session Id | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Failover Session State AVP for L2TPv2 sessions (FSQ, FSR): 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |M|H| rsvd | Length | Vendor Id [IETF] | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Attribute Type 79 | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Reserved | Session Id | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Reserved | Remote Session Id | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ This AVP MAY be hidden (the H-bit set to 0 or 1). The AVP is mandatory (the M-bit is set to 1). Session Id identifies the local session id sender had assigned, for which it would like to query the state on its peer. Remote Session Id is the remote session id for the same session. Before all sessions are synchronized using FSQ/FSR mechanism, if an endpoint receives an ICRQ for a session it believe is already Jain, et al. Standards Track [Page 10] INTERNET DRAFT FAILOVER April 2006 in established state, it MUST respond to such ICRQ with a CDN, setting Assigned/Local Session ID AVP ([L2TPv2] section 4.4.4, [L2TPv3] section 5.4.4) to its local session id, and clear the session that it considered established. An endpoint could assign least recently used session ids to avoid this situation. When an endpoint receives an FSQ message, it MUST ensure that for each FSS AVP in FSQ message it includes an FSS AVP in Failover Session Response (FSR) message, message type 22. There is no one- to-one correspondence between FSQ message and FSR message. Therefore an endpoint could respond to multiple FSQs using one FSR message, or it could respond one FSQ with multiple FSRs. For each FSS AVP received in FSQ, an endpoint MUST validate the Remote Session Id and determine if it is paired with the Session Id specified in the message. If FSS AVP is not valid (i.e. session is non-existing or it is paired with different remote session id), then the Session Id field in FSS AVP in the response MUST be set to zero. When session is discovered to be pairing with mismatching session id, the local session MUST not be cleared, but rather marked stale, to be queried later using another FSQ message. An example dialogue in Appendix D elaborates the endpoints behavior on mismatching session ids. Also, when responding to FSQ with an FSR message, Remote Session Id in FSS AVP is always set to the received value of Session ID in FSS AVP in FSQ message. When an endpoint receives an FSR message, it MUST use the Remote Session Id field to identify the local session and silently (without sending a CDN) clear the session if Session Id in the AVP was zero. Otherwise it can consider the session to be in established state and recovered. FSQ and FSR messages MUST include 'Message Type AVP' and 'FSS AVP'. They MAY include 'Random Vector AVP' and for L2TPv3 'Message digest AVP'. Other AVPs MUST NOT be sent and SHOULD be ignored on receipt. FSS AVP MUST NOT be used in any message other than FSQ and FSR messages. 3.0 IANA Considerations This document defines following values assigned by IANA - Two new Message Type (Attribute Type 0) Values: Failover Session Query : 21 Failover Session Response : 22 Jain, et al. Standards Track [Page 11] INTERNET DRAFT FAILOVER April 2006 - Four new control message Attribute Value Pairs: Failover Capability : 76 Tunnel Recovery : 77 Suggested Control Sequence : 78 Failover Session State : 79 4.0 Security Considerations The failover mechanism described here leaves a room (1 in 2^16 for L2TPv2 and 1 in 2^32 for L2TPv3) for an intruder to discover the old tunnel id, which could be misused to fake the failover to result into a shutdown of an existing tunnel. To avoid this, control channel authentication described in section 2.2.1 is should be used. L2TPv3 control connections could also use 'Digest AVP' to make it secure. Protecting L2TP with IPSec would also help secure the control connections for failover situations. 5.0 Acknowledgements Leo Huber provided suggestions to help define the failover concept. Mark Townsley reviewed the document and provided valuable suggestions. 6.0 Author Information Vipin Jain Riverstone Networks 5200 Great America Parkway Santa Clara, CA 95054 Email: vipinietf@yahoo.com Paul W. Howard Juniper Networks 10 Technology Park Drive Westford, MA 01886 Email: phoward@juniper.net Sam Henderson Cisco Systems 7025 Kit Creek Rd. PO Box 14987 Research Triangle Park, NC 27709 Email: samh@cisco.com Keyur Parikh Harris Broadcast Communication 4393 Digitalway Mason, OH 45040 Jain, et al. Standards Track [Page 12] INTERNET DRAFT FAILOVER April 2006 Email: kparikh@harris.com 7.0 References [L2TPv2] Townsley, et. al., "Layer Two Tunneling Protocol 'L2TP'", RFC2661 [L2TPv3] Lau, Townsley, Goyret, "Layer Two Tunneling Protocol - version 3 'L2TPv3'", RFC3931 8.0 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. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat 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 implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf- ipr@ietf.org. 9.0 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. 10.0 Copyright Statement Copyright (C) The Internet Society (2005). Jain, et al. Standards Track [Page 13] INTERNET DRAFT FAILOVER April 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. Appendix A This section describes some design considerations that came up during discussions when developing the proposal: A.1 Backward compatibility and extensibility - The mechanism should be backward compatible; i.e. it should not redefine existing behavior of [L2TPv2] and [L2TPv3] compliant systems. - The protocol should allow a peer to detect failover capabilities in advance, for it to fall back to other failover mechanisms should peer does not support proposed failover protocol. - The protocol should allow future extensions to fail-over mechanism at ease. A.2 Less failover recovery time The mechanism should have least possible time to recover from failover (target of 3-5 seconds for 30k tunnels). Specifically it should take following into consideration: - Faster recovery: by utilizing less number of messages exchanged to recover from failover - CPU intensiveness: less cpu intensive a proposal is, better are the chances of faster recovery - Parallel establishment of various tunnels: by keeping different tunnel reestablishments independent of one another. A.3 Less Payload data loss The mechanism should have least possible impact on data flows for sessions with sequencing enabled. A.4 Minimum interference with pre-failure control traffic The mechanism should define a way of clearly distinguishing the messages that were sent before failover from that which are sent Jain, et al. Standards Track [Page 14] INTERNET DRAFT FAILOVER April 2006 after. Specifically, it should define a mechanism that avoid confusion between sequence numbers that were used before and after if the same Tunnel Id is used. A.5 Simplicity Simpler the protocol is, better are the changes of being adopted by everybody. Following would help achieve this: - Use of existing AVPs, messages and packet formats. - Avoid introducing special considerations and mechanisms a new implementation would have to deal with. - Simpler post fail-over synchronization mechanism. A.6 Security The mechanism should provide a mechanism to authenticate peers when resynchronization is happening after a failover. A.7 Scalability It is very important for a proposed protocol to work well for a scalable deployment. This includes dealing with all design considerations discussed above for scalable deployments, having thousands of tunnels or sessions or mix of the two. A target of 30,000 tunnels carrying 150,000 to 200,000 sessions from 300 peers was considered during the design. Appendix B Description below outlines the failover protocol operation for an example tunnel. The failover protocol does not preclude an endpoint from recovering multiple tunnels in parallel. It also allows an endpoint to send multiple FSQs, each including multiple FSS AVPs, to recover quickly. Pre Failover Exchange (section 2.1): Endpoint Peer (assigned tid = x, failover capable) SCCRQ --------------------------------------> validate SCCRQ Jain, et al. Standards Track [Page 15] INTERNET DRAFT FAILOVER April 2006 (assigned tid = y, failover capable) validate <-------------------------------------- send SCCRP SCCRP, etc. .... .... < This Node fails > Failed endpoint establishes recovery tunnel (section 2.2.1). Initiate recovery tunnel establishment for the old tunnel 'x': Failed Endpoint Peer (assigned tid = z, Recovery AVP) SCCRQ -----------------------------------> Detects failover (recover tid = x, recover remote tid = y) validate SCCRQ (Suggested Control Sequence AVP, Suggested Ns/Nr = 3/100) validate <----------------------------------- send SCCRP SCCRP (recover tid = y, recover remote tid = x) reset Ns = 3, Nr = 100 on the recovered tunnel SCCCN -----------------------------------> validate and reset Ns = 100, Nr = 3 on the recovered tunnel Terminate the recovery tunnel tid = 'z' StopCCN --------------------------------------> Cleanup 'w' Session states are synchronized both endpoints may send FSQs and cleanup stale sessions (section 2.3) (FSS AVP for sessions s1, s2, s3..) send FSQ -------------------------------------> compute the state of sessions in FSQ (FSS AVP for sessions s1, s2, s3...) deletes <-------------------------------------- send FSR stale sessions, if any Jain, et al. Standards Track [Page 16] INTERNET DRAFT FAILOVER April 2006 (FSS AVP for sessions s7, s8, s9...) compute <-------------------------------------- send FSQ the sate of sessions in FSQ (FSS AVP for sessions s7, s8, s9...) send FSR --------------------------------------> delete stale sessions, if any Appendix C This section shows an example dialogue to illustrate double failure recovery. The notable difference, as described in section 2.2.1, in the procedure from single failover scenario is the use of tie breaker by one of the failed endpoints to use the recovery tunnel established by its peer (also a failed endpoint) as recovery tunnel. Failed endpoint Failed endpoint (assume old tid = A) (assume old tid = B) Recovery AVP = (A, B) SCCRQ -----------------------+ (with tie (recovery tunnel 'C') | breaker | AVP) | Recovery AVP = (B, A) | +- valid <--------------------------- Send SCCRQ | SCCRQ (recovery tunnel 'D') | (with tie breaker AVP) | This endpoint | | loses tie; | | Discards tunnel 'C' +--> Valid SCCRQ | This endpoint wins tie; | Discards SCCRQ | | (may include SCS AVP) +->Send SCCRP -------------------------> Validate SCCRP Reset 'B'; Set Ns, Nr values --+ | | | Validate SCCN <---------------------- Send SCCN -------+ Reset 'A'; Set Ns, Nr values Jain, et al. Standards Track [Page 17] INTERNET DRAFT FAILOVER April 2006 FSQs and FSRs for the old tunnel (A, B) are exchanged on the recovered tunnel by both endpoints. Appendix D Session id mismatch could not be a result of failure on one of the endpoints. However, failover session recovery procedure could exacerbate the situation, resulting into a permanent mismatch in session ids between two endpoints. Dialogue below outlines the behavior described in section 2.3 to handle such situations gracefully. Failed endpoint Non failed endpoint (assume a mismatch) (assume a mismatch) Sid = A, Remote Sid = B Sid = B, Remote Sid = C Sid = C, Remote Sid = D FSS AVP (A, B) send FSQ -------------------------> No (B, A) pair exist; rather (B, C) exist. If it clears B then peer doesn't know if C is stale on other end. Instead if it marks B stale and queries the session state via FSQ, C would be cleared on the other end. FSS AVP (0, A) Clears A <-------------------------- send FSR ... some time later ... FSS AVP (B, C) No (B,C) <-------------------------- send FSQ Mark C Stale FSS AVP (B, 0) Send FSR --------------------------> Clears B Jain, et al. Standards Track [Page 18]