Network Working Group J. Dong
Internet-Draft X. Zhang
Intended status: Standards Track Huawei Technologies
Expires: May 4, 2017 Z. Li
China Mobile
October 31, 2016
LSA Flushing Problem Mitigation in OSPF Networks
draft-dong-ospf-flush-mitigation-00
Abstract
In OSPF protocol, LSAs with the LS age at MaxAge are not used in
routing table calculation and MUST be flushed in the network. In
some cases, the flushing of OSPF MaxAge LSAs may cause flooding storm
of OSPF packets and severely impact network stability and the
services provided by the network. This document specifies a backward
compatible mechanism to mitigate the impact of MaxAge LSA flushing in
OSPF networks.
Requirements Language
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 RFC 2119 [RFC2119].
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on May 4, 2017.
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Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Proposed Solution . . . . . . . . . . . . . . . . . . . . . . 3
3. Deployment Considerations . . . . . . . . . . . . . . . . . . 4
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4
5. Security Considerations . . . . . . . . . . . . . . . . . . . 5
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 5
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
7.1. Normative References . . . . . . . . . . . . . . . . . . 5
7.2. Informative References . . . . . . . . . . . . . . . . . 5
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 5
1. Introduction
In OSPF protocol [RFC2328], Link State Updates (LSAs) are exchanged
in Link State Update (LSU) packets to achieve link-state database
(LSDB) synchronization and consistent route calculation. LSAs with
the LS age at MaxAge are not used in routing table calculation and
MUST be flushed in the network. In some cases, the flushing of
MaxAge LSAs can cause flooding storm of OSPF packets and severely
impact network stability and the services provided by the network.
[I-D.dong-ospf-maxage-flush-problem-statement] analyzes the problem
of MaxAge LSA flushing, and gives the requirements on potential
solutions.
This document proposes a backward compatible mechanism to mitigate
the impacts of MaxAge LSA flushing in OSPF networks.
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2. Proposed Solution
In normal cases, the flushing of router-LSA indicates that the
originator of the LSA is no longer reachable in the network and is
unable to refresh the LSA. The flushing of other types of LSAs
indicate the routing information carried in the LSAs is no longer
applicable. Since usually the removal of a node is a significant
change to the network and can also be informed by the update of LSAs
of its adjacent routers, the flushing of router-LSA MUST be processed
carefully to avoid unnecessary routing churns caused by improper LSA
flushing.
The proposed solution aims to distinguish persistent LSA flushing
from normal LSA flushing, so that the impact of persistent flushing
can be alleviated without slowing down normal route convergence.
Specifically, the flushing of router-LSA and the subsequent flushing
of LSAs belonging to the same originator are further examined.
During the examination time, the old instance of the LSAs and the
MaxAge LSAs are kept in LSDB and the route recalculation is
postponed.
Two types of timers are used in this solution:
o T1: the examination time of the suspicious persistent LSA flushing
of a particular router. When a MaxAge router-LSA of a particular
router is received, timer T1 fires and the originator of the
router-LSA is marked as in Restrain state. The value of timer T1
is configurable, and the RECOMMENED value is 1800 seconds.
o T2: the examination time of a received MaxAge LSA, the originator
of which is currently in Restrain state. When a Maxage LSA is
received and the orginator of the LSA is in Restrain state, timer
T2 fires and the old instance of the LSA is still in use, which
means the Maxage LSA does not trigger route recalculation. The
value of timer T2 is configurable, and the RECOMMENDED value is 10
seconds.
The detailed procedures are described as follows :
a. When a MaxAge router-LSA is received,
o If the originator of the LSA is not in Restrain state, mark the
originator of the LSA as in Restrain state, timer T1 is started
for that router, and timer T2 is started for the router-LSA. The
MaxAge LSA is flushed further in the newtork, while the old
instance of the LSA is still in use in route calculation until T2
expires.
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o If the originator of the LSA is already in Restrain state, then T1
is restarted for that router. If timer T2 does not exist for this
LSA, timer T2 is started for the LSA.
b. When a MaxAge LSA with LSA-type other than router LSA is
received,
o If the originator of the LSA is in Restrain state, timer T2 is
started for the LSA. The MaxAge LSA is flushed further in the
newtork, while the old instance of the LSA is in use in route
calculation.
o If the originator of the LSA is not in Restrain state, the
processing is according to [RFC2328].
c. When a newer LSA instance originated by a router in Restrain
state is received,
o If timer T2 for this LSA exists, the newer instance replaces the
old LSA instance in link-state database and triggers route
recalculation, timer T2 for this LSA is stopped.
o If timer T2 for this LSA does not exist, the processing is
according to [RFC2328].
d. When timer T2 for a particular LSA expires, the MaxAge LSA
triggers route recalculation and is removed from link-state database.
e. When timer T1 for a particular router expires, the router is
marked as in normal state.
3. Deployment Considerations
While it is RECOMMENDED that the proposed mechanism deployed on all
the routers in the same OSPF network, this mechanism can also be
deployed into the network incrementally.
4. IANA Considerations
This document makes no request of IANA.
Note to RFC Editor: this section may be removed on publication as an
RFC.
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5. Security Considerations
TBD
6. Acknowledgements
TBD
7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328,
DOI 10.17487/RFC2328, April 1998,
<http://www.rfc-editor.org/info/rfc2328>.
7.2. Informative References
[I-D.dong-ospf-maxage-flush-problem-statement]
Dong, J., Zhang, X., and Z. Li, "OSPF Corrupted MaxAge LSA
Flushing Problem Statement", draft-dong-ospf-maxage-flush-
problem-statement-00 (work in progress), March 2016.
Authors' Addresses
Jie Dong
Huawei Technologies
Huawei Campus, No.156 Beiqing Rd.
Beijing 100095
China
Email: jie.dong@huawei.com
Xudong Zhang
Huawei Technologies
Huawei Campus, No.156 Beiqing Rd.
Beijing 100095
China
Email: zhangxudong@huawei.com
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Zhenqiang Li
China Mobile
No.32 Xuanwumenxi Ave., Xicheng District
Beijing 100032
China
Email: lizhenqiang@chinamobile.com
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