Networking Working Group                                     L. Ginsberg
Internet-Draft                                                  P. Wells
Intended status: Standards Track                           Cisco Systems
Expires: February 9, 2017                                    B. Decraene
                                                           T. Przygienda
                                                              H. Gredler
                                                     Private Contributer
                                                         August 08, 2016

                    IS-IS Minimum Remaining Lifetime


   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",
   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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   This Internet-Draft will expire on February 9, 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
   ( in effect on the date of
   publication of this document.  Please review these documents
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   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 . . . . . . . . . . . . . . . . . . . . .   6
   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.  As the
   Remaining Lifetime field changes as it is flooded and as the checksum
   field MUST NOT be altered by receiving ISs the Remaining Lifetime is
   deliberately excluded from the checksum calculation.  In cases where
   cryptographic authentication is included in an LSP ([RFC5304] or

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   [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

   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

   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]
   specifies that maximumLSPGenerationInterval MUST be sufficiently less

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   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.

   There are 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 RemainingLifetime expires on the receiving
   node it can appear as if the originating IS has failed to regenerate
   the LSP (case #1 above) when in fact the LSP still has significant
   lifetime remaining.  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
      RemainingLifetime (a purged LSP) is newer than the same LSP w non-
      zero RemainingLifetime

   o  If both the received and local copy of the LSP have non-zero
      RemainingLifetime they are considered the same even if the
      RemainingLifetimes differ

   [ISO10589] Section 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 RemainingLifetime.  An additional
   action is added:

   vi.  If the RemainingLifetime 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 still apply i.e., an LSP received with zero
   RemainingLifetime is still considered newer than a matching LSP with
   non-zero RemainingLifetime.  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
   RemainingLifetime 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 RemainingLifetime 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
   RemainingLifetime field can be useful in identifying a problem in the
   network.  In order to minimize the reports of false positives the

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   following algorithm SHOULD be used in determining whether the
   RemainingLifetime in the received LSP is possibly corrupt:

   o  The LSP has passed all acceptance tests as specified in [ISO10589]

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

   o  RemainingLifetime 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

   Note that it is not possible to guarantee that all cases of corrupt
   RemainingLifetime 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
   RemainingLifetime 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.  LSPs from a node which is
   unreachable (failure of the two-way-connectivity check) MUST 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

4.  IANA Considerations


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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


8.  References

8.1.  Normative References

              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,

   [RFC5304]  Li, T. and R. Atkinson, "IS-IS Cryptographic
              Authentication", RFC 5304, DOI 10.17487/RFC5304, October
              2008, <>.

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   [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, <>.

8.2.  Informational References

              "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,

Authors' Addresses

   Les Ginsberg
   Cisco Systems
   510 McCarthy Blvd.
   Milpitas, CA  95035


   Paul Wells
   Cisco Systems
   170 W Tasman Dr
   San Jose, Ca  95035


   Bruno Decraene
   38 rue du General Leclerc
   Issy Moulineaux cedex 9  92794


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   Tony Przygienda
   1137 Innovation Way
   Sunnyvale, Ca  94089


   Hannes Gredler
   Private Contributer


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