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Li 6 China Mobile 7 October 31, 2016 9 LSA Flushing Problem Mitigation in OSPF Networks 10 draft-dong-ospf-flush-mitigation-00 12 Abstract 14 In OSPF protocol, LSAs with the LS age at MaxAge are not used in 15 routing table calculation and MUST be flushed in the network. In 16 some cases, the flushing of OSPF MaxAge LSAs may cause flooding storm 17 of OSPF packets and severely impact network stability and the 18 services provided by the network. This document specifies a backward 19 compatible mechanism to mitigate the impact of MaxAge LSA flushing in 20 OSPF networks. 22 Requirements Language 24 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 25 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 26 document are to be interpreted as described in RFC 2119 [RFC2119]. 28 Status of This Memo 30 This Internet-Draft is submitted in full conformance with the 31 provisions of BCP 78 and BCP 79. 33 Internet-Drafts are working documents of the Internet Engineering 34 Task Force (IETF). Note that other groups may also distribute 35 working documents as Internet-Drafts. The list of current Internet- 36 Drafts is at http://datatracker.ietf.org/drafts/current/. 38 Internet-Drafts are draft documents valid for a maximum of six months 39 and may be updated, replaced, or obsoleted by other documents at any 40 time. It is inappropriate to use Internet-Drafts as reference 41 material or to cite them other than as "work in progress." 43 This Internet-Draft will expire on May 4, 2017. 45 Copyright Notice 47 Copyright (c) 2016 IETF Trust and the persons identified as the 48 document authors. All rights reserved. 50 This document is subject to BCP 78 and the IETF Trust's Legal 51 Provisions Relating to IETF Documents 52 (http://trustee.ietf.org/license-info) in effect on the date of 53 publication of this document. Please review these documents 54 carefully, as they describe your rights and restrictions with respect 55 to this document. Code Components extracted from this document must 56 include Simplified BSD License text as described in Section 4.e of 57 the Trust Legal Provisions and are provided without warranty as 58 described in the Simplified BSD License. 60 Table of Contents 62 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 63 2. Proposed Solution . . . . . . . . . . . . . . . . . . . . . . 3 64 3. Deployment Considerations . . . . . . . . . . . . . . . . . . 4 65 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4 66 5. Security Considerations . . . . . . . . . . . . . . . . . . . 5 67 6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 5 68 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 5 69 7.1. Normative References . . . . . . . . . . . . . . . . . . 5 70 7.2. Informative References . . . . . . . . . . . . . . . . . 5 71 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 5 73 1. Introduction 75 In OSPF protocol [RFC2328], Link State Updates (LSAs) are exchanged 76 in Link State Update (LSU) packets to achieve link-state database 77 (LSDB) synchronization and consistent route calculation. LSAs with 78 the LS age at MaxAge are not used in routing table calculation and 79 MUST be flushed in the network. In some cases, the flushing of 80 MaxAge LSAs can cause flooding storm of OSPF packets and severely 81 impact network stability and the services provided by the network. 82 [I-D.dong-ospf-maxage-flush-problem-statement] analyzes the problem 83 of MaxAge LSA flushing, and gives the requirements on potential 84 solutions. 86 This document proposes a backward compatible mechanism to mitigate 87 the impacts of MaxAge LSA flushing in OSPF networks. 89 2. Proposed Solution 91 In normal cases, the flushing of router-LSA indicates that the 92 originator of the LSA is no longer reachable in the network and is 93 unable to refresh the LSA. The flushing of other types of LSAs 94 indicate the routing information carried in the LSAs is no longer 95 applicable. Since usually the removal of a node is a significant 96 change to the network and can also be informed by the update of LSAs 97 of its adjacent routers, the flushing of router-LSA MUST be processed 98 carefully to avoid unnecessary routing churns caused by improper LSA 99 flushing. 101 The proposed solution aims to distinguish persistent LSA flushing 102 from normal LSA flushing, so that the impact of persistent flushing 103 can be alleviated without slowing down normal route convergence. 104 Specifically, the flushing of router-LSA and the subsequent flushing 105 of LSAs belonging to the same originator are further examined. 106 During the examination time, the old instance of the LSAs and the 107 MaxAge LSAs are kept in LSDB and the route recalculation is 108 postponed. 110 Two types of timers are used in this solution: 112 o T1: the examination time of the suspicious persistent LSA flushing 113 of a particular router. When a MaxAge router-LSA of a particular 114 router is received, timer T1 fires and the originator of the 115 router-LSA is marked as in Restrain state. The value of timer T1 116 is configurable, and the RECOMMENED value is 1800 seconds. 118 o T2: the examination time of a received MaxAge LSA, the originator 119 of which is currently in Restrain state. When a Maxage LSA is 120 received and the orginator of the LSA is in Restrain state, timer 121 T2 fires and the old instance of the LSA is still in use, which 122 means the Maxage LSA does not trigger route recalculation. The 123 value of timer T2 is configurable, and the RECOMMENDED value is 10 124 seconds. 126 The detailed procedures are described as follows : 128 a. When a MaxAge router-LSA is received, 130 o If the originator of the LSA is not in Restrain state, mark the 131 originator of the LSA as in Restrain state, timer T1 is started 132 for that router, and timer T2 is started for the router-LSA. The 133 MaxAge LSA is flushed further in the newtork, while the old 134 instance of the LSA is still in use in route calculation until T2 135 expires. 137 o If the originator of the LSA is already in Restrain state, then T1 138 is restarted for that router. If timer T2 does not exist for this 139 LSA, timer T2 is started for the LSA. 141 b. When a MaxAge LSA with LSA-type other than router LSA is 142 received, 144 o If the originator of the LSA is in Restrain state, timer T2 is 145 started for the LSA. The MaxAge LSA is flushed further in the 146 newtork, while the old instance of the LSA is in use in route 147 calculation. 149 o If the originator of the LSA is not in Restrain state, the 150 processing is according to [RFC2328]. 152 c. When a newer LSA instance originated by a router in Restrain 153 state is received, 155 o If timer T2 for this LSA exists, the newer instance replaces the 156 old LSA instance in link-state database and triggers route 157 recalculation, timer T2 for this LSA is stopped. 159 o If timer T2 for this LSA does not exist, the processing is 160 according to [RFC2328]. 162 d. When timer T2 for a particular LSA expires, the MaxAge LSA 163 triggers route recalculation and is removed from link-state database. 165 e. When timer T1 for a particular router expires, the router is 166 marked as in normal state. 168 3. Deployment Considerations 170 While it is RECOMMENDED that the proposed mechanism deployed on all 171 the routers in the same OSPF network, this mechanism can also be 172 deployed into the network incrementally. 174 4. IANA Considerations 176 This document makes no request of IANA. 178 Note to RFC Editor: this section may be removed on publication as an 179 RFC. 181 5. Security Considerations 183 TBD 185 6. Acknowledgements 187 TBD 189 7. References 191 7.1. Normative References 193 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 194 Requirement Levels", BCP 14, RFC 2119, 195 DOI 10.17487/RFC2119, March 1997, 196 . 198 [RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328, 199 DOI 10.17487/RFC2328, April 1998, 200 . 202 7.2. Informative References 204 [I-D.dong-ospf-maxage-flush-problem-statement] 205 Dong, J., Zhang, X., and Z. Li, "OSPF Corrupted MaxAge LSA 206 Flushing Problem Statement", draft-dong-ospf-maxage-flush- 207 problem-statement-00 (work in progress), March 2016. 209 Authors' Addresses 211 Jie Dong 212 Huawei Technologies 213 Huawei Campus, No.156 Beiqing Rd. 214 Beijing 100095 215 China 217 Email: jie.dong@huawei.com 219 Xudong Zhang 220 Huawei Technologies 221 Huawei Campus, No.156 Beiqing Rd. 222 Beijing 100095 223 China 225 Email: zhangxudong@huawei.com 226 Zhenqiang Li 227 China Mobile 228 No.32 Xuanwumenxi Ave., Xicheng District 229 Beijing 100032 230 China 232 Email: lizhenqiang@chinamobile.com