





Internet Engineering Task Force (IETF)                       L. Ginsberg
Request for Comments: 7987                                      P. Wells
Category: Standards Track                                  Cisco Systems
ISSN: 2070-1721                                              B. Decraene
                                                                  Orange
                                                           T. Przygienda
                                                                 Juniper
                                                              H. Gredler
                                                            RtBrick Inc.
                                                            October 2016


                    IS-IS Minimum Remaining Lifetime

Abstract

   Corruption of the Remaining Lifetime field in a Link State Protocol
   Data Unit (LSP) 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.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7987.
















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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 ...............................................3
      1.1. Requirements Language ......................................4
   2. Solution ........................................................4
   3. Deployment Considerations .......................................5
      3.1. Inconsistent Values for MaxAge .............................5
      3.2. Reporting Corrupted Lifetime ...............................6
      3.3. Impact of Delayed LSP Purging ..............................7
   4. Security Considerations .........................................7
   5. References ......................................................7
      5.1. Normative References .......................................7
      5.2. Informative References .....................................8
   Acknowledgements ...................................................8
   Contributors .......................................................8
   Authors' Addresses .................................................9




















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1.  Problem Statement

   [ISO10589] defines the format of a Link State PDU (LSP) that 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 LSP 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 that may have occurred during transmission.  [ISO10589]
   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 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.





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   A backwards-compatible solution to this problem is defined in the
   following sections.

1.1.  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].

2.  Solution

   As discussed in the previous section, the problematic case is when
   the 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 a
   higher sequence number) even if the contents of the LSP have not
   changed.  [ISO10589] defines maximumLSPGenerationInterval to be
   sufficiently less than the 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 pseudonode 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.



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   Section 7.3.16 of [ISO10589] 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 a zero Remaining
      Lifetime (a purged LSP) is newer than the same LSP with a non-zero
      Remaining Lifetime.

   o  If both the received and local copy of the LSP have a non-zero
      Remaining Lifetime, they are considered the same even if the
      Remaining Lifetimes differ.

   Section 7.3.15.1.e(1) of [ISO10589] defines the actions to take on
   receipt of an LSP generated by another IS that 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.

   This additional action ensures 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 Section 7.3.15.1.b of
   [ISO10589] still apply, i.e., an LSP received with a zero Remaining
   Lifetime is still considered newer than a matching LSP with a non-
   zero Remaining Lifetime.  Therefore, the changes proposed here will
   not result in LSPDB inconsistency among routers in the network.

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 the 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



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   is therefore possible for the value of MaxAge on the IS that
   originates an LSP to be higher or lower than the value of MaxAge on
   the ISs that receive the LSP.

   If the value of MaxAge of the IS that 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 ensure 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 to have an LSP purged prematurely when using the
   extension defined in the previous section.  Implementors of this
   extension may wish to protect against this case by making the value
   to which the 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
   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
      Section 7.3.15.1 of [ISO10589].

   o  The LSP is newer than the copy in the local LSPDB (including the
      case of not being present in the LSPDB).

   o  The 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 a 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 the Remaining
   Lifetime received when the LSP was added to the LSPDB.





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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 -- this implies that the originator is no longer
   reachable.  In such a case, its neighbors will update their own LSPs
   to report the loss of connectivity to the originator.  [ISO10589]
   specifies that LSPs from a node that is unreachable (failure of the
   two-way connectivity check) will not be used.  Note that 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 (e.g., LSP corruption and
   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.

4.  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 that 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.

5.  References

5.1.  Normative References

   [ISO10589] International Organization for Standardization,
              "Information technology -- Telecommunications and
              information exchange between systems -- 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,
              November 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>.

5.2.  Informative References

   [PROB-STATEMENT]
              Decraene, B. and C. Schmitz, "IS-IS LSP lifetime
              corruption - Problem Statement", Work in Progress,
              draft-decraene-isis-lsp-lifetime-problem-statement-02,
              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>.

Acknowledgements

   The problem statement in [PROB-STATEMENT] motivated this work.

Contributors

   The following individual contributed substantially to the content of
   this document and should be considered a co-author:

   Stefano Previdi
   Cisco Systems
   Email: sprevidi@cisco.com

















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Authors' Addresses

   Les Ginsberg
   Cisco Systems
   510 McCarthy Blvd.
   Milpitas, CA  95035
   United States of America

   Email: ginsberg@cisco.com


   Paul Wells
   Cisco Systems
   170 W. Tasman Dr.
   San Jose, CA  95035
   United States of America

   Email: pauwells@cisco.com


   Bruno Decraene
   Orange
   44 avenue de la Republique, CS 50010
   92326 Chatillon Cedex  92794
   France

   Email: bruno.decraene@orange.com


   Tony Przygienda
   Juniper
   1137 Innovation Way
   Sunnyvale, CA  94089
   United States of America

   Email: prz@juniper.net


   Hannes Gredler
   RtBrick Inc.

   Email: hannes@rtbrick.com









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