Internet DRAFT - draft-dong-ospf-purge-lsa
draft-dong-ospf-purge-lsa
Network Working Group J. Dong
Internet-Draft X. Zhang
Intended status: Standards Track Huawei Technologies
Expires: April 26, 2012 October 24, 2011
Purge LSA for OSPF flushing
draft-dong-ospf-purge-lsa-00
Abstract
In some scenarios current OSPF flushing mechanism may incur problem
of delaying the deletion of invalid Link State Advertisement (LSA)
and result in desynchronization of link state database. This
document proposes a backward compatible solution to solve this
problem.
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.
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This Internet-Draft will expire on April 26, 2012.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Problem with OSPF Flushing Mechanism . . . . . . . . . . . . . 3
3. Proposed Flushing Mechanism . . . . . . . . . . . . . . . . . . 4
4. Backward Compatibility . . . . . . . . . . . . . . . . . . . . 4
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
6. Security Considerations . . . . . . . . . . . . . . . . . . . . 5
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 5
8. Normative References . . . . . . . . . . . . . . . . . . . . . 5
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 5
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1. Introduction
The LSA flushing mechanism of OSPF is described in [RFC2328]. In
some scenarios such as route oscillation, this flushing mechanism may
incur problem of delaying the deletion of invalid LSA on some
routers, and result in desynchronization of link state database.
This document proposes a backward compatible solution to solve this
problem.
2. Problem with OSPF Flushing Mechanism
As described in section 14.1 of [RFC2328], an LSA can be flushed from
the routing domain by setting its LS age to MaxAge, while leaving its
LS sequence number alone, and then reflooding the LSA. The MaxAge
LSA must be removed immediately from the router's link state database
as soon as both a) it is no longer contained on any neighbor Link
state retransmission lists and b) none of the router's neighbors are
in states Exchange or Loading. And section 12.1.6 of [RFC2328]
specifies that "As soon as this flood has been acknowledged by all
adjacent neighbors, a new instance can be originated with sequence
number of InitialSequenceNumber." Thus after the MaxAge LSA is
removed, a new instance with sequence number equal to
InitialSequenceNumber would be originated.
Under some scenarios such as route oscillation, such procedure may
delay the deletion of invalid LSA in the link state database of some
routers thus cause desynchronization of link state database. One
example is described as below:
a. Router X and router Y formed OSPF adjacency, X advertised an AS-
external-LSA with sequence number set to InitialSequenceNumber to Y.
Then route oscillation happens to this external route.
b. X flushes the AS-external-LSA by setting the LS age to MaxAge and
sends it to Y. After some time X does not receive the acknowledgment
from Y, then X retransmits the flushing LSA to Y.
c. Y receives the first flushing LSA and sends Acknowledgment back
to X. Then Y receives the second flushing LSA and sends back the
second acknowledgment.
d. X receives the first Acknowledgment and remove the flushing LSA
from its link state database. Then X originates this AS-external-LSA
with InitialSequenceNumber and sends it to Y. Soon the LSA is flushed
again due to route oscillation. After doing this, X receives the
second acknowledgment for the previous flushing from Y, then the
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flushing LSA is removed from X's link state database.
e. Y receives this new LSA and install it into its link state
database. Then in a short time Y receives the flushing LSA. If the
time interval between this flushing LSA and previous LSA is shorter
than MinLSArrival, this flushing LSA would be discarded by Y.
As a result, router X does not have this LSA in its link state
database, but router Y will keep this LSA until the LS age reaches
MaxAge. Such desynchronization of link state database may cause
traffic blackholing or forwarding loops.
The root cause of this problem is that after LSA flushing the router
would set the LS sequence number of the new LSA instance back to
InitialSequenceNumber. And in some cases the LSA updates and the
acknowledgments may become out of sync.
3. Proposed Flushing Mechanism
This section defines a flushing mechanism to solve the problem
described in section 2. This flushing mechanism SHOULD be used when
LS sequence number is not to wrap. When it is time for the LS
sequence number is to wrap, procedures in Section 14.1 of [RFC2328]
MUST be used.
When an LSA needs to be flushed, the router SHOULD keep only the
header of the LSA, set its LS age to 0 and increment the LS sequence
number by 1. Such an LSA is called Purge LSA. The router SHOULD
flood this Purge LSA to neighbors for LSA flushing. The Purge LSA
SHOULD be retained in the database until the LS age reaches MaxAge.
When a new instance of the LSA is to be originated, if a
corresponding Purge LSA exists in the database, the router SHOULD
advance the LSA's LS sequence number one past the LS sequence number
of the corresponding Purge LSA. Then the corresponding Purge LSA
SHOULD be removed from the router's link state database.
4. Backward Compatibility
The proposed flushing mechanism is backward compatible with legacy
OSPF implementations. Legacy OSPF routers would treat the received
Purge LSA as a newer instance than its database copy, since the
sequence number is larger. And since the Purge LSA contains only LSA
header, it will not be used in the routing table calculation. When
the LS age of this purge LSA reaches MaxAge, it would be removed from
the router's link state database.
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5. IANA Considerations
This document makes no request of IANA.
6. Security Considerations
This document does not change the security properties of OSPF.
7. Acknowledgements
The authors would like to thank Yongming Fu and Yong Miao for their
helps and suggestions to this document.
8. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.
Authors' Addresses
Jie Dong
Huawei Technologies
Huawei Building, No.156 Beiqing Rd.
Beijing 100095
China
Email: jie.dong@huawei.com
Xudong Zhang
Huawei Technologies
Huawei Building, No.156 Beiqing Rd.
Beijing 100095
China
Email: zhangxudong@huawei.com
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