Networking Working Group L. Ginsberg
Internet-Draft P. Wells
Intended status: Standards Track Cisco Systems
Expires: February 18, 2017 B. Decraene
Orange
T. Przygienda
Juniper
H. Gredler
Private Contributer
August 17, 2016
IS-IS Minimum Remaining Lifetime
draft-ietf-isis-remaining-lifetime-04.txt
Abstract
Corruption of the Remainining Lifetime Field in a Link State PDU can
go undetected. In certain scenarios this may cause or exacerbate
flooding storms. It is also a possible denial of service attack
vector. This document defines a backwards compatible solution to
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.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
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."
This Internet-Draft will expire on February 18, 2017.
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Copyright Notice
Copyright (c) 2016 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
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 2
2. Solution . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Deployment Considerations . . . . . . . . . . . . . . . . . . 5
3.1. Inconsistent Values for MaxAge . . . . . . . . . . . . . 5
3.2. Reporting Corrupted Lifetime . . . . . . . . . . . . . . 5
3.3. Impact of Delayed LSP Purging . . . . . . . . . . . . . . 6
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
5. Security Considerations . . . . . . . . . . . . . . . . . . . 7
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7
7. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 7
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
8.1. Normative References . . . . . . . . . . . . . . . . . . 7
8.2. Informational References . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8
1. Problem Statement
[ISO10589] defines the format of a Link State PDU (LSP) which
includes a Remaining Lifetime field. This field is set by the
originator based on local configuration and then decremented by all
systems once the entry is stored in their Link State PDU Database
(LSPDB) consistent with the passing of time. This allows all
Intermediate Systems (ISs) to age out the LSP at approximately the
same time.
Each LSP also has a checksum field to allow receiving systems to
detect errors which may have occurred during transmission. [ISO
10589] mandates that the checksum is calculated by the originator of
the LSP and cannot be modified by other routers. Therefore the
Remaining Lifetime is deliberately excluded from the checksum
calculation. In cases where cryptographic authentication is included
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in an LSP ([RFC5304] or [RFC5310]) the Remaining Lifetime field is
also excluded from the hash calculation. If the Remaining Lifetime
field gets corrupted during flooding this corruption is therefore
undetectable. The consequences of such corruption depend upon how
the Remaining Lifetime is altered.
In cases where the Remaining Lifetime becomes larger than the
originator intended the impact is benign. As the originator is
responsible for refreshing the LSP before it ages out a new version
of the LSP will be generated before the LSP ages out - so no harm is
done.
In cases where the Remaining Lifetime field becomes smaller than the
originator intended the LSP may age out prematurely (i.e. before the
originator refreshes the LSP). This has two negative consequences:
1. The LSP will be purged by an IS when the Remaining Lifetime
expires. This will cause a temporary loss of reachability to
destinations impacted by the content of that LSP.
2. Unnecessary LSP flooding will occur as a result of the premature
purge and subsequent regeneration/flooding of a new version of
the LSP by the originator
If the corrupted Remaining Lifetime is only modestly shorter than the
lifetime assigned by the originator the negative impacts are also
modest. If, however, the corrupted Remaining Lifetime becomes very
small, then the negative impacts can become significant - especially
in cases where the cause of the corruption is persistent so that the
cycle repeats itself frequently.
A backwards compatible solution to this problem is defined in the
following sections.
2. Solution
As discussed in the previous section, the problematic case is when
Remaining Lifetime is corrupted and becomes much smaller than it
should be. The goal of the solution is then to prevent premature
purging.
Under normal circumstances updates to an LSP - including purging if
appropriate - are the responsibility of the originator of the LSP.
There is a maximum time between generations of a given LSP. Once
this time has expired it is the responsibility of the originator to
refresh the LSP (i.e. issue a new version with higher sequence
number) even if the contents of the LSP have not changed. [ISO10589]
defines maximumLSPGenerationInterval to be sufficiently less than the
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maximum lifetime of an LSP so that the new version can be flooded
network wide before the old version ages out on any IS.
[ISO10589] defines two cases where a system other than the originator
of an LSP is allowed to purge an LSP:
1. The LSP ages out. This should only occur if the originating IS
is no longer reachable and therefore is unable to update the LSP
2. There is a Designated Intermediate System (DIS) change on a LAN.
The pseudo-node LSPs generated by the previous DIS are no longer
required and may be purged by the new DIS.
In both of these cases purging is not necessary for correct operation
of the protocol. It is provided as an optimization to remove stale
entries from the LSPDB.
In cases where the Remaining Lifetime in a received LSP has been
corrupted and is smaller than the Remaining Lifetime at the
originating node when the Remaining Lifetime expires on the receiving
node it can appear as if the originating IS has failed to regenerate
the LSP before it ages out (case #1 above). To prevent this from
having a negative impact a modest change to the storage of "new" LSPs
in the LSPDB is specified.
[ISO10589] Section 7.3.16 defines the rules to determine whether a
received LSP is older, the same, or newer than the copy of the same
LSP in the receiver's LSPDB. The key elements are:
o Higher sequence numbers are newer
o If sequence numbers are the same, an LSP with zero Remaining
Lifetime (a purged LSP) is newer than the same LSP w non-zero
Remaining Lifetime
o If both the received and local copy of the LSP have non-zero
Remaining Lifetime they are considered the same even if the
Remaining Lifetimes differ
[ISO10589] Section 7.3.15.1.e(1) defines the actions to take on
receipt of an LSP generated by another IS which is newer than the
local copy and has a non-zero Remaining Lifetime. An additional
action is defined by this document:
vi. If the Remaining Lifetime of the new LSP is less than MaxAge it
is set to MaxAge
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This additional action insures that no matter what value of Remaining
Lifetime is received a system other than the originator of an LSP
will never purge the LSP until the LSP has existed in the database
for at least MaxAge.
It is important to note that no change is proposed for handling the
receipt of purged LSPs. The rules specified in [ISO10589]
Section 7.3.15.1b still apply i.e., an LSP received with zero
Remaining Lifetime is still considered newer than a matching LSP with
non-zero Remaining Lifetime. Therefore the changes proposed here
will not result in LSPDB inconsistency among routers in the newtork.
3. Deployment Considerations
This section discusses some possible deployment issues for this
protocol extension.
3.1. Inconsistent Values for MaxAge
[ISO10589] defines MaxAge (the maximum value for Remaining Lifetime
in an LSP) as an architectural constant of 20 minutes (1200 seconds).
However, in practice, implementations have supported allowing this
value to be configurable. The common intent of a configurable value
is to support longer lifetimes than the default - thus reducing the
periodic regeneration of LSPs in the absence of topology changes.
See a discussion of this point in [RFC3719]. It is therefore
possible for the value of MaxAge on the IS which originates an LSP to
be higher or lower than the value of MaxAge on the ISs which receive
the LSP.
If the value of MaxAge of the IS which originated the LSP is smaller
than the value of MaxAge of the receiver of an LSP, then setting the
Remaining Lifetime of the received LSP to the local value of MaxAge
will insure that it is not purged prematurely. However, if the value
of MaxAge on the receiver is less than that of the originator then it
is still possible when using the extension defined in the previous
section to have an LSP purged prematurely. Implementors of this
extension may wish to protect against this case by making the value
to which Remaining Lifetime is set under the conditions described in
the previous section configurable. If that is done the configured
value MUST be greater than or equal to the locally configured value
of MaxAge.
3.2. Reporting Corrupted Lifetime
Reporting reception of an LSP with a possible corrupt Remaining
Lifetime field can be useful in identifying a problem in the network.
In order to minimize the reports of false positives the following
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algorithm SHOULD be used in determining whether the Remaining
Lifetime in the received LSP is possibly corrupt:
o The LSP has passed all acceptance tests as specified in [ISO10589]
Section 7.3.15.1
o The LSP is newer than the copy in the local LSPDB (including the
case of not being present in the LSPDB)
o Remaining Lifetime in the received LSP is less than
ZeroAgeLifetime
o The adjacency to the neighbor from which the LSP is received has
been up for a minimum of ZeroAgeLifetime
In such a case an IS SHOULD generate a CorruptRemainingLifetime
event.
Note that it is not possible to guarantee that all cases of corrupt
Remaining Lifetime will be detected using the above algorithm. It is
also not possible to guarantee that all CorruptRemainingLifetime
events reported using the above algorithm are valid. As a diagnostic
aid an implementation MAY wish to retain the value of Remaining
Lifetime received when the LSP was added to the LSPDB.
3.3. Impact of Delayed LSP Purging
The extensions defined in this document may result in retaining an
LSP longer than its original lifetime. In order for this to occur
the scheduled refresh of the LSP by the originator of the LSP must
fail to occur - which implies the originator is no longer reachable.
In such a case its neighbors will update their own LSPs reporting the
loss of connectivity to the originator. [ISO10589] specifies that
LSPs from a node which is unreachable (failure of the two-way-
connectivity check) will not be used. Note this behavior applies to
ALL information in the set of LSPs from such a node.
Retention of stale LSPs therefore has no negative side effects other
than requiring additional memory for the LSPDB. In networks where a
combination of pathological behaviors (LSP corruption, frequent
resetting of nodes in the network) is seen this could lead to a large
number of stale LSPs being retained - but such networks are already
compromised.
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4. IANA Considerations
None.
5. Security Considerations
The ability to introduce corrupt LSPs is not altered by the rules
defined in this document. Use of authentication as defined in
[RFC5304] and [RFC5310] prevents such LSPs from being intentionally
introduced. A "man-in-the-middle" attack which modifies an existing
LSP by changing the Remaining Lifetime to a small value can cause
premature purges even in the presence of cryptographic
authentication. The mechanisms defined in this document prevent such
an attack from being effective.
6. Acknowledgements
The problem statement in [LIFE-PROB] motivated this work.
7. Contributors
The following people gave a substantial conrtibution to the content
of this document and should be considered as co-authors:
Stefano Previdi
Cisco Systems
Email: sprevidi@cisco.com
8. References
8.1. Normative References
[ISO10589]
International Organization for Standardization,
"Intermediate system to Intermediate system intra-domain
routeing information exchange protocol for use in
conjunction with the protocol for providing the
connectionless-mode Network Service (ISO 8473)", ISO/
IEC 10589:2002, Second Edition, Nov 2002.
[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>.
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[RFC5304] Li, T. and R. Atkinson, "IS-IS Cryptographic
Authentication", RFC 5304, DOI 10.17487/RFC5304, October
2008, <http://www.rfc-editor.org/info/rfc5304>.
[RFC5310] Bhatia, M., Manral, V., Li, T., Atkinson, R., White, R.,
and M. Fanto, "IS-IS Generic Cryptographic
Authentication", RFC 5310, DOI 10.17487/RFC5310, February
2009, <http://www.rfc-editor.org/info/rfc5310>.
8.2. Informational References
[LIFE-PROB]
"IS-IS LSP lifetime corruption - Problem Statement, draft-
decraene-isis-lsp-lifetime-problem-statement-02(work in
progress)", July 2016.
[RFC3719] Parker, J., Ed., "Recommendations for Interoperable
Networks using Intermediate System to Intermediate System
(IS-IS)", RFC 3719, DOI 10.17487/RFC3719, February 2004,
<http://www.rfc-editor.org/info/rfc3719>.
Authors' Addresses
Les Ginsberg
Cisco Systems
510 McCarthy Blvd.
Milpitas, CA 95035
USA
Email: ginsberg@cisco.com
Paul Wells
Cisco Systems
170 W Tasman Dr
San Jose, Ca 95035
USA
Email: pauwells@cisco.com
Bruno Decraene
Orange
38 rue du General Leclerc
Issy Moulineaux cedex 9 92794
France
Email: bruno.decraene@orange.com
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Tony Przygienda
Juniper
1137 Innovation Way
Sunnyvale, Ca 94089
USA
Email: prz@juniper.net
Hannes Gredler
Private Contributer
Email: hannes@gredler.at
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