





Internet Engineering Task Force (IETF)                       U. Chunduri
Request for Comments: 7602                                         W. Lu
Category: Standards Track                                        A. Tian
ISSN: 2070-1721                                            Ericsson Inc.
                                                                 N. Shen
                                                     Cisco Systems, Inc.
                                                               July 2015


                   IS-IS Extended Sequence Number TLV

Abstract

   This document defines the Extended Sequence Number TLV to protect
   Intermediate System to Intermediate System (IS-IS) PDUs from replay
   attacks.

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 5741.

   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/rfc7602.

Copyright Notice

   Copyright (c) 2015 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.






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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
     1.2.  Acronyms  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Replay Attacks and Impact on IS-IS Networks . . . . . . . . .   4
     2.1.  IIHs  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.2.  LSPs  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.3.  SNPs  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Extended Sequence Number TLV  . . . . . . . . . . . . . . . .   4
     3.1.  Sequence Number Wrap  . . . . . . . . . . . . . . . . . .   5
   4.  Mechanism and Packet Encoding . . . . . . . . . . . . . . . .   5
     4.1.  IIHs  . . . . . . . . . . . . . . . . . . . . . . . . . .   6
     4.2.  SNPs  . . . . . . . . . . . . . . . . . . . . . . . . . .   6
   5.  Backward Compatibility and Deployment . . . . . . . . . . . .   6
     5.1.  IIHs and SNPs . . . . . . . . . . . . . . . . . . . . . .   7
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Appendix A.  ESSN Encoding Mechanisms . . . . . . . . . . . . . .  10
     A.1.  Using Timestamps  . . . . . . . . . . . . . . . . . . . .  10
     A.2.  Using Non-volatile Storage  . . . . . . . . . . . . . . .  10
   Appendix B.  Operational/Implementation Considerations  . . . . .  11
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  11
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   Intermediate System to Intermediate System (IS-IS) [ISO10589] has
   been adopted widely in various Layer 2 / Layer 3 routing and
   switching deployments of data centers and for critical business
   operations.  Its flexibility and scalability make it well suited for
   the rapid development of new data center infrastructures.  Also,
   while technologies such as Software-Defined Networking (SDN) may
   improve network management and enable new applications, their use has
   an effect on the security requirements of the routing infrastructure.

   A replayed IS-IS PDU can potentially cause many problems in IS-IS
   networks, including bouncing adjacencies, blackholing, and even some
   form of Denial-of-Service (DoS) attacks as explained in Section 2.
   This problem is also discussed in the Security Considerations
   section, in the context of cryptographic authentication work as
   described in [RFC5304] and [RFC5310].





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   Currently, there is no mechanism to protect IS-IS Hello (IIH) PDUs
   and Sequence Number PDUs (SNPs) from replay attacks.  However, Link
   State PDUs (LSPs) have a sequence number in the LSP header as defined
   in [ISO10589], with which they can effectively mitigate intra-session
   replay attacks.  But, LSPs are still susceptible to inter-session
   replay attacks.

   This document defines the Extended Sequence Number (ESN) TLV to
   protect IS-IS PDUs from replay attacks.

   The new ESN TLV defined here thwarts these threats and can be
   deployed with the authentication mechanisms specified in [RFC5304]
   and [RFC5310] for a more secure network.

   Replay attacks can be effectively mitigated by deploying a group key
   management protocol (being developed as defined in [GROUP-IKEv2] and
   [MRKMP]) with a frequent key change policy.  Currently, there is no
   such mechanism defined for IS-IS.  Even if such a mechanism is
   defined, usage of this TLV can be helpful to avoid replays before the
   keys are changed.

   Also, it is believed that, even when such a key management system is
   deployed, there always will be some systems based on manual keying
   that coexist with systems based on key management protocols.  The ESN
   TLV defined in this document is helpful for such deployments.

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].

1.2.  Acronyms

   CSNP    -  Complete Sequence Number PDU

   ESN     -  Extended Sequence Number

   IIH     -  IS-IS Hello

   IS      -  Intermediate System

   LSP     -  IS-IS Link State PDU

   PDU     -  Protocol Data Unit






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   PSNP    -  Partial Sequence Number PDU

   SNP     -  Sequence Number PDU

2.  Replay Attacks and Impact on IS-IS Networks

   Replaying a captured protocol packet to cause damage is a common
   threat for any protocol.  Securing the packet with cryptographic
   authentication information alone cannot mitigate this threat
   completely.  This section explains the replay attacks and their
   applicability to each IS-IS PDU.

2.1.  IIHs

   When an adjacency is brought up, an IS sends an IIH packet with an
   empty neighbor list (TLV 6); it can be sent with or without
   authentication information.  Packets can be replayed later on the
   broadcast network, and this may cause all ISs to bounce the
   adjacency, thus churning the network.  Note that mitigating replay is
   only possible when authentication information is present.

2.2.  LSPs

   Normal operation of the IS-IS update process as specified in
   [ISO10589] provides timely recovery from all LSP replay attacks.
   Therefore, the use of the extensions defined in this document is
   prohibited in LSPs.  Further discussion of the vulnerability of LSPs
   to replay attacks can be found in [ISIS-ANALYSIS].

2.3.  SNPs

   A replayed CSNP can result in the sending of unnecessary PSNPs on a
   given link.  A replayed CSNP or PSNP can result in unnecessary LSP
   flooding on the link.

3.  Extended Sequence Number TLV

   The Extended Sequence Number (ESN) TLV is composed of 1 octet for the
   Type, 1 octet that specifies the number of bytes in the Value field,
   and a 12-byte Value field.  This TLV is defined only for IIH and SNP
   PDUs.

   Code - 11.

   Length - total length of the value field, which is 12 bytes.






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   Value - 64-bit Extended Session Sequence Number (ESSN), which is
      followed by a 32-bit, monotonically increasing, per-packet
      sequence number.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+
   |    Type       |
   +-+-+-+-+-+-+-+-+
   |    Length     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Extended Session Sequence Number (High-Order 32 Bits)      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Extended Session Sequence Number (Low-Order 32 Bits)       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |             Packet Sequence Number (32 Bits)                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

           Figure 1: Extended Sequence Number (ESN) TLV

   The ESN TLV defined here is optional.  Though this is an optional
   TLV, it can be mandatory on a link when 'verify' mode is enabled as
   specified in Section 5.1.  The ESN TLV MAY be present only in IIH
   PDUs and SNPs.  A PDU with multiple ESN TLVs is invalid and MUST be
   discarded on receipt.

   The 64-bit ESSN MUST be nonzero and MUST contain a number that is
   increased whenever it is changed due any situation, as specified in
   Section 3.1.  Encoding the 64-bit unsigned integer ESSN value is a
   local matter, and implementations MAY use one of the alternatives
   provided in Appendix A.  Effectively, for each PDU that contains the
   ESN TLV, the 96-bit unsigned integer value consisting of the 64-bit
   ESSN and 32-bit Packet Sequence Number (PSN) -- where the ESSN is the
   higher-order 64 bits -- MUST be greater than the most recently
   received value in a PDU of the same type originated by the same IS.

3.1.  Sequence Number Wrap

   If the 32-bit Packet Sequence Number in the ESN TLV wraps or the
   router performs a cold restart, the 64-bit ESSN value MUST be set
   higher than the previous value.  IS-IS implementations MAY use the
   guidelines provided in Appendix A for accomplishing this.

4.  Mechanism and Packet Encoding

   The encoding of the ESN TLV in each applicable IS-IS PDU is detailed
   below.  Please refer to Section 5 for appropriate operations on how
   to interoperate with legacy node(s) that do not support the



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   extensions defined in this document.  If the received PDU with the
   ESN TLV is accepted, then the stored value for the corresponding
   originator and PDU type MUST be updated with the latest value
   received.  Please note that level information is included in the PDU
   type.

4.1.  IIHs

   ESN TLV information is maintained for each type of IIH PDU being sent
   on a given circuit.  The procedures for encoding, verification, and
   sequence number wrapping are explained in Section 3.

4.2.  SNPs

   Separate CSNP/PSNP ESN TLV information is maintained per PDU type,
   per originator, and per link.  The procedures for encoding,
   verification, and sequence number wrapping are explained in Section
   3.

5.  Backward Compatibility and Deployment

   The implementation and deployment of the ESN TLV can be done to
   support backward compatibility and gradual deployment in the network
   without requiring a flag day.  This feature can also be deployed for
   the links in a certain area of the network where the maximum security
   mechanism is needed, or it can be deployed for the entire network.

   The implementation SHOULD allow the configuration of ESN TLV features
   on each IS-IS link level.  The implementation SHOULD also allow
   operators to control the configuration of the 'send' and/or 'verify'
   feature of IS-IS PDUs for the links and for the node.  In this
   document, the 'send' mode is to include the ESN TLV in its own IS-IS
   PDUs, and the 'verify' mode is to process the ESN TLV in the
   receiving IS-IS PDUs from neighbors.

   When an adversary is actively attacking, it is possible to have
   inconsistent data views in the network, if there is a considerable
   delay in enabling the 'verify' mode where nodes were configured to
   the 'send' mode, e.g., from the first to the last node or all nodes
   of a particular LAN segment.  This happens primarily because replay
   PDUs can potentially be accepted by the nodes where the 'verify' mode
   is still not provisioned at the time of the attack.  To minimize such
   a window, it is recommended that provisioning of 'verify' SHOULD be
   done in a timely fashion by the network operators.







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5.1.  IIHs and SNPs

   On the link level, the ESN TLV involves the IIH PDUs and SNPs (both
   CSNP and PSNP).  The 'send' and 'verify' modes described above can be
   set independently on each link and, in the case of a broadcast
   network, independently on each level.

   To introduce ESN support without disrupting operations, ISs on a
   given interface are first configured to operate in 'send' mode.  Once
   all routers operating on an interface are operating in 'send' mode,
   'verify' mode can be enabled on each IS.  Once 'verify' mode is set
   for an interface, all the IIH PDUs and SNPs being sent on that
   interface MUST contain the ESN TLV.  Any such PDU received without an
   ESN TLV MUST be discarded when 'verify' mode is enabled.  Similarly,
   to safely disable ESN support on a link, 'verify' mode is disabled on
   all ISs on the link.  Once 'verify' mode is disabled on all routers
   operating on an interface, 'send' mode can be disabled on each IS.
   Please refer to Section 5 for considerations on enabling or disabling
   'verify' mode on all ISs on a link.

6.  IANA Considerations

   A new TLV codepoint, as defined in this document, has been assigned
   by IANA from the "IS-IS TLV Codepoints" registry.  It is referred to
   as the Extended Sequence Number TLV and has the following attributes:

      Value  Name                   IIH  LSP  SNP  Purge
      -----  ---------------------  ---  ---  ---  -----
      11     ESN TLV                 y    n    y    n

7.  Security Considerations

   This document describes a mechanism to mitigate the replay attack
   threat as discussed in the Security Considerations sections of
   [RFC5304] and [RFC5310].  If an adversary interferes either by not
   forwarding packets or by delaying messages as described in Section
   3.3 of [RFC6862], the mechanism specified in this document cannot
   mitigate those threats.  Also, some of the threats described in
   Section 2.3 of [ISIS-ANALYSIS] are not addressable with the ESN TLV
   as specified in this document.  This document does not introduce any
   new security concerns to IS-IS or any other specifications
   referenced.









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8.  References

8.1.  Normative References

   [ISO10589] International Organization for Standardization,
              "Intermediate system to intermediate system intra-domain-
              routing routine 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>.

   [RFC5905]  Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
              "Network Time Protocol Version 4: Protocol and Algorithms
              Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010,
              <http://www.rfc-editor.org/info/rfc5905>.

8.2.  Informative References

   [MRKMP]    Hartman, S., Zhang, D., and G. Lebovitz, "Multicast Router
              Key Management Protocol (MaRK)", Work in Progress,
              draft-hartman-karp-mrkmp-05, September 2012.

   [ISIS-ANALYSIS]
              Chunduri, U., Tian, A., and W. Lu, "KARP IS-IS security
              analysis", Work in Progress, draft-ietf-karp-isis-
              analysis-07, July 2015.

   [GROUP-IKEv2] Rowles, S., Yeung, A., Ed., Tran, P., and Y. Nir,
              "Group Key Management using IKEv2", Work in Progress,
              draft-yeung-g-ikev2-08, October 2014.

   [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>.







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   [RFC6862]  Lebovitz, G., Bhatia, M., and B. Weis, "Keying and
              Authentication for Routing Protocols (KARP) Overview,
              Threats, and Requirements", RFC 6862,
              DOI 10.17487/RFC6862, March 2013,
              <http://www.rfc-editor.org/info/rfc6862>.

   [RFC7474]  Bhatia, M., Hartman, S., Zhang, D., and A. Lindem, Ed.,
              "Security Extension for OSPFv2 When Using Manual Key
              Management", RFC 7474, DOI 10.17487/RFC7474, April 2015,
              <http://www.rfc-editor.org/info/rfc7474>.









































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Appendix A.  ESSN Encoding Mechanisms

   IS-IS nodes implementing this specification SHOULD use available
   mechanisms to preserve the 64-bit Extended Session Sequence Number's
   strictly increasing property, whenever it is changed for the deployed
   life of the IS-IS node (including cold restarts).

   This appendix provides guidelines for maintaining the strictly
   increasing property of the 64-bit ESSN in the ESN TLV, and
   implementations can resort to any similar method as long as this
   property is maintained.

A.1.  Using Timestamps

   One mechanism for accomplishing this is by encoding the 64-bit ESSN
   as the system time represented by a 64-bit unsigned integer value.
   This MAY be similar to the system timestamp encoding for the NTP long
   format as defined in Appendix A.4 of [RFC5905].  The new current time
   MAY be used when the IS-IS node loses its sequence number state
   including when the Packet Sequence Number wraps.

   Implementations MUST make sure while encoding the 64-bit ESN value
   with the current system time that it does not default to any previous
   value or some default node time of the system, especially after cold
   restarts or any other similar events.  In general, system time must
   be preserved across cold restarts in order for this mechanism to be
   feasible.  One example of such implementation is to use a battery
   backed real-time clock (RTC).

A.2.  Using Non-volatile Storage

   One other mechanism for accomplishing this is similar to the one
   specified in [RFC7474] -- use the 64-bit ESSN as a wrap/boot count
   stored in non-volatile storage.  This value is incremented anytime
   the IS-IS node loses its sequence number state, including when the
   Packet Sequence Number wraps.

   There is a drawback to this approach, which is described as follows
   in Section 8 of [RFC7474].  It requires the IS-IS implementation to
   be able to save its boot count in non-volatile storage.  If the non-
   volatile storage is ever repaired or router hardware is upgraded such
   that the contents are lost, keys MUST be changed to prevent replay
   attacks.








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Appendix B.  Operational/Implementation Considerations

   Since the ESN is maintained per PDU type, per originator, and per
   link, this scheme can be useful for monitoring the health of the
   IS-IS adjacency.  A Packet Sequence Number skip that occurs upon
   receiving an IIH can be recorded by the neighbors and can be used
   later to correlate adjacency state changes over the interface.  For
   instance, in multi-access media, completely different issues on the
   network may be indicated when all neighbors record skips from the
   same IIH sender versus when only one neighbor records skips.  For
   operational issues, effective usage of the TLV defined in this
   document MAY also need more system information before making concrete
   conclusions; defining all that information is beyond the scope of
   this document.

Acknowledgements

   As some sort of sequence number mechanism to thwart protocol replays
   is a old concept, the authors of this document do not make any claims
   on the originality of the overall protection idea described.  The
   authors are thankful for the review and the valuable feedback
   provided by Acee Lindem and Joel Halpern.  Thanks to Alia Atlas,
   Chris Hopps, Nevil Brownlee, and Adam W. Montville for their reviews
   and suggestions during IESG directorate review.  The authors also
   thank Christer Holmberg, Ben Campbell, Barry Leiba, Stephen Farrell,
   and Alvaro Retana for their reviews of this document.

Contributors

   The authors would like to thank Les Ginsberg for his significant
   contribution in detailed reviews and suggestions.




















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Authors' Addresses

   Uma Chunduri
   Ericsson Inc.
   300 Holger Way,
   San Jose, California  95134
   United States

   Phone: 408 750-5678
   Email: uma.chunduri@ericsson.com


   Wenhu Lu
   Ericsson Inc.
   300 Holger Way,
   San Jose, California  95134
   United States

   Email: wenhu.lu@ericsson.com


   Albert Tian
   Ericsson Inc.
   300 Holger Way,
   San Jose, California  95134
   United States

   Phone: 408 750-5210
   Email: albert.tian@ericsson.com


   Naiming Shen
   Cisco Systems, Inc.
   225 West Tasman Drive,
   San Jose, California  95134
   United States

   Email: naiming@cisco.com













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