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Network Working Group                                          L. NguyenRequest for Comments: 4812                                        A. RoyCategory: Informational                                    Cisco Systems                                                                A. Zinin                                                          Alcatel-Lucent                                                              March 2007OSPF Restart SignalingStatus of This Memo   This memo provides information for the Internet community.  It does   not specify an Internet standard of any kind.  Distribution of this   memo is unlimited.Copyright Notice   Copyright (C) The IETF Trust (2007).Abstract   OSPF is a link-state intra-domain routing protocol used in IP   networks.  Routers find new and detect unreachable neighbors via the   Hello subprotocol.  Hello OSPF packets are also used to ensure two-   way connectivity within time.  When a router restarts its OSPF   software, it may not know its neighbors.  If such a router sends a   Hello packet on an interface, its neighbors are going to reset the   adjacency, which may not be desirable in certain conditions.   This memo describes a vendor-specific mechanism that allows OSPF   routers to inform their neighbors about the restart process.  Note   that this mechanism requires support from neighboring routers.  The   mechanism described in this document was proposed before Graceful   OSPF Restart, as described inRFC 3623, came into existence.  It is   implemented/supported by at least one major vendor and is currently   deployed in the field.  The purpose of this document is to capture   the details of this mechanism for public use.  This mechanism is not   an IETF standard.Nguyen, et al.                Experimental                      [Page 1]

RFC 4812                 OSPF Restart Signaling               March 2007Table of Contents1. Introduction ....................................................22. Proposed Solution ...............................................22.1. Sending Hello Packets with the RS-bit Set ..................32.2. Receiving Hello Packets with the RS-Bit Set ................32.3. Ensuring Topology Stability ................................43. Backward Compatibility ..........................................44. Security Considerations .........................................45. IANA Considerations .............................................46. References ......................................................56.1. Normative References .......................................56.2. Informative References .....................................5Appendix A.  Acknowledgements ......................................61.  Introduction   While performing a graceful restart of OSPF software [RFC3623],   routers need to prevent their neighbors from resetting their   adjacencies.  However, after a reload, routers may not be aware of   the neighbors they had adjacencies with in their previous   incarnations.  If such a router sends a Hello packet on an interface   and this packet does not list some neighbors, those neighbors will   reset the adjacency with the restarting router.   This document describes a technique that allows restarting routers to   inform their neighbors that they may not know about some neighbors   yet and the absence of some router IDs in the Hello packets should be   ignored.2.  Proposed Solution   With this Restart Signaling Solution, a new bit, called RS (restart   signal), is introduced into the Extended Options (EO) TLV in the   Link-Local Signaling (LLS) block (see [RFC4813]).  The value of this   bit is 0x00000002; see Figure 1 below.   +---+---+---+---+---+---+---+- -+---+---+---+---+---+---+---+---+   | * | * | * | * | * | * | * |...| * | * | * | * | * | * | RS| LR|   +---+---+---+---+---+---+---+- -+---+---+---+---+---+---+---+---+                  Figure 1.  Bits in Extended Options TLV   For a definition of the LR-bit, see [RFC4811].Nguyen, et al.                Experimental                      [Page 2]

RFC 4812                 OSPF Restart Signaling               March 20072.1.  Sending Hello Packets with the RS-bit Set   OSPF routers should set the RS-bit in the EO-TLV attached to a Hello   packet when it is not known that all neighbors are listed in this   packet, but the restarting router wants them to preserve their   adjacencies.  The RS-bit must not be set in Hello packets longer than   RouterDeadInterval seconds.2.2.  Receiving Hello Packets with the RS-Bit Set   When an OSPF router receives a Hello packet containing the LLS block   with the EO-TLV that has the RS-bit set, the router should skip the   two-way connectivity check with the announcing neighbor (i.e., the   router should not generate a 1-WayReceived event for the neighbor if   it does not find its own router ID in the list of neighbors as   described inSection 10.5 of [RFC2328]), provided that the neighbor   Finite State Machine (FSM) for this neighbor is in the Full state.   The router should also send a unicast Hello back to the sender in   reply to a Hello packet with RS-bit set.  This is to speed up   learning of previously known neighbors.  When sending such a reply   packet, care must be taken to ensure that the RS-bit is clear in it.   Two additional fields are introduced in the neighbor data structure:   RestartState flag and ResyncTimeout timer.  RestartState flag   indicates that a Hello packet with the RS-bit set has been received   and the local router expects its neighbor to go through the Link   State Database (LSDB) resynchronization procedure using [RFC4811].   ResyncTimeout is a single-shot timer limiting the delay between the   first seen Hello packet with the RS-bit set and initialization of the   LSDB resynchronization procedure.  The length of ResyncTimeout timer   is RouterDeadInterval seconds.   When a Hello packet with the RS-bit set is received and RestartState   flag is not set for the neighbor, the router sets RestartState flag   and starts ResyncTimeout timer.  If ResyncTimeout expires,   RestartState flag is cleared and a 1-WayReceived event is generated   for the neighbor.  If, while ResyncTimeout timer is running, the   neighbor starts LSDB resynchronization procedure using [RFC4811],   ResyncTimeout timer is canceled.  The router also clears RestartState   flag on completion of the LSDB resynchronization process.   Two or more routers on the same segment cannot have Hello packets   with the RS-bit set at the same time, as can be the case when two or   more routers restart at about the same time.  In such a scenario, the   routers should clear the RestartState flag, cancel the ResyncTimeout   timer, and generate a 1-WayReceived event.Nguyen, et al.                Experimental                      [Page 3]

RFC 4812                 OSPF Restart Signaling               March 20072.3.  Ensuring Topology Stability   Under certain circumstances, it might be desirable to stop announcing   the restarting router as fully adjacent if this may lead to possible   routing loops.  In order to provide this functionality, a   configurable option is provided on the neighboring routers that   instructs the OSPF process to follow the logics described below.   When an OSPF router schedules a routing table calculation due to a   change in the contents of its LSDB, it should also reset all   adjacencies with restarting routers (those with RestartState set to   TRUE) by clearing the RestartState neighbor flags, canceling   ResyncTimeout timers (if running), and generating the 1-WayReceived   events for the neighbor FSMs.3.  Backward Compatibility   The described technique requires cooperation from neighboring   routers.  However, if neighbors do not support this technique, they   will just reset the adjacency.4.  Security Considerations   The described technique does not introduce any new security issues   into the OSPF protocol.5.  IANA Considerations   Please refer to the "IANA Considerations" section of [RFC4813] for   more information on the Extended Options bit definitions.Nguyen, et al.                Experimental                      [Page 4]

RFC 4812                 OSPF Restart Signaling               March 20076.  References6.1.  Normative References   [RFC2328]  Moy, J., "OSPF Version 2", STD 54,RFC 2328, April 1998.   [RFC3623]  Moy, J., Pillay-Esnault, P., and A. Lindem, "Graceful OSPF              Restart",RFC 3623, November 2003.6.2.  Informative References   [RFC4813]  Friedman, B., Nguyen, L., Roy, A., Yeung, D., and A.              Zinin, "OSPF Link-Local Signaling",RFC 4813, March 2007.   [RFC4811]  Nguyen, L., Roy, A., and A. Zinin, "OSPF Out-of-Band Link              State Database (LSDB) Resynchronization",RFC 4811, March              2007.Nguyen, et al.                Experimental                      [Page 5]

RFC 4812                 OSPF Restart Signaling               March 2007Appendix A.  Acknowledgments   The authors would like to thank John Moy, Russ White, Don Slice, and   Alvaro Retana for their valuable comments.Authors' Addresses   Liem Nguyen   Cisco Systems   225 West Tasman Drive   San Jose, CA  95134   USA   EMail: lhnguyen@cisco.com   Abhay Roy   Cisco Systems   225 West Tasman Drive   San Jose, CA  95134   USA   EMail: akr@cisco.com   Alex Zinin   Alcatel-Lucent   Mountain View, CA   USA   EMail: alex.zinin@alcatel-lucent.comNguyen, et al.                Experimental                      [Page 6]

RFC 4812                 OSPF Restart Signaling               March 2007Full Copyright Statement   Copyright (C) The IETF Trust (2007).   This document is subject to the rights, licenses and restrictions   contained inBCP 78, and except as set forth therein, the authors   retain all their rights.   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, THE IETF TRUST 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.Intellectual Property   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 inBCP 78 andBCP 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 athttp://www.ietf.org/ipr.   The 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.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Nguyen, et al.                Experimental                      [Page 7]