Networking Working Group                                  P. Psenak, Ed.
Internet-Draft                                               L. Ginsberg
Intended status: Informational                             Cisco Systems
Expires: 8 September 2022                                       D. Voyer
                                                             Bell Canada
                                                            7 March 2022


                IGP Prefix Reachability Loss Anouncement
                draft-ppsenak-lsr-igp-pfx-reach-loss-00

Abstract

   In the presence of summarization, there is a need to signal loss of
   reachability to an individual prefix covered by the summary in order
   to enable fast convergence away from paths to the node which owns the
   prefix which is no longer reachable.  This document describes how to
   use existing protocol mechanisms in IS-IS and OSPF to advertise such
   prefix reachability loss.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119][RFC8174] when, and only when, they appear in all
   capitals, as shown here.

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 https://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 8 September 2022.







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Copyright Notice

   Copyright (c) 2022 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 (https://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 Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Supporting PRLA in IS-IS  . . . . . . . . . . . . . . . . . .   3
     2.1.  Advertisement of PRLA in IS-IS  . . . . . . . . . . . . .   3
     2.2.  Propagation of PRLA in IS-IS  . . . . . . . . . . . . . .   4
   3.  Supporting PRLA in OSPF . . . . . . . . . . . . . . . . . . .   4
     3.1.  Advertisement of PRLA in OSPF . . . . . . . . . . . . . .   5
     3.2.  Propagation of PRLA in OSPF . . . . . . . . . . . . . . .   5
   4.  Deployment Considerations for PRLA  . . . . . . . . . . . . .   5
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   6
   8.  Normative References  . . . . . . . . . . . . . . . . . . . .   6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   Link-state IGP protocols like IS-IS and OSPF are primarily used to
   distribute routing information between routers belonging to a single
   Autonomous System (AS) and to calculate the reachability for IPv4 or
   IPv6 prefixes advertised by the individual nodes inside the AS.  Each
   node advertises the state of its local adjacencies, connected
   prefixes, capabilities, etc.  The collection of these states from all
   the routers inside the area form a link-state database (LSDB) that
   describes the topology of the area and holds additional state
   information about the prefixes, router capabilities, etc.

   The growth of networks running a link-state routing protocol results
   in the addition of more state which leads to scalability and
   convergence challenges.  The organization of networks into levels/
   areas and IGP domains helps limit the scope of link-state information
   within certain boundaries.  However, the state related to prefix
   reachability often requires propagation across a multi-area/ level



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   and/or multi-domain IGP network.  Techniques such as summarization
   have been used traditionally to address the scale challenges
   associated with advertising prefix state outside of the local area/
   domain.  However, this results in suppression of the individual
   prefix state that is useful for triggering fast-convergence
   mechanisms outside of the IGPs - e.g., BGP PIC Edge [I-D.ietf-rtgwg-
   bgp-pic].

   This document describes how the use of existing protocol mechanisms
   can support the necessary functionality without the need for any
   protocol extensions.  The functionality being described is called
   Prefix Reachability Loss Announcement (PRLA).

2.  Supporting PRLA in IS-IS

   [RFC5305] defines the encoding for advertising IPv4 prefixes using 4
   octets of metric information.  Section 4 specifies:

   "If a prefix is advertised with a metric larger then MAX_PATH_METRIC
   (0xFE000000, see paragraph 3.0), this prefix MUST NOT be considered
   during the normal SPF computation.  This allows advertisement of a
   prefix for purposes other than building the normal IP routing table.
   "

   Similarly, [RFC5308] defines the encoding for advertising IPv6
   prefixes using 4 octets of metric information.  Section 2 states:

   "...if a prefix is advertised with a metric larger than
   MAX_V6_PATH_METRIC (0xFE000000), this prefix MUST NOT be considered
   during the normal Shortest Path First (SPF) computation.  This will
   allow advertisement of a prefix for purposes other than building the
   normal IPv6 routing table."

   This functionality can be used to advertise a prefix (IPv4 or IPv6)
   in a manner which indicates that reachability has been lost - and to
   do so without requiring all nodes in the network to be upgraded to
   support the functionality.

2.1.  Advertisement of PRLA in IS-IS

   Existing nodes in a network which receive PRLA advertisements will
   ignore them.  This allows flooding of such advertisements to occur
   without the need to upgrade all nodes in a network.








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   Recognition of the advertisement as PRLA is only required on routers
   which have a use case for this information.  Area Border Routers
   (ABRs), which would be responsible for propagating PRLA
   advertisements into other areas would need to recognize such
   advertisements.

   As per the definitions referenced in the preceding section, any
   prefix advertisement with a metric value greater than 0xFE000000 can
   be used for purposes other than normal routing calculations.  Such an
   advertisement can be interpreted by the receiver as a PRLA.

   Optionally, an implementation may use local configuration to limit
   the set of metric values which will be interpreted as PRLA.  The only
   restriction is that such values MUST be greater than 0xFE000000.

2.2.  Propagation of PRLA in IS-IS

   ISIS L1/L2 routers may wish to advertise received PRLAs into other
   areas (upwards and/or downwards).  When propagating PRLAs the
   original metric value MUST be preserved.  The cost to reach the
   originator of the received PRLA MUST NOT be considered when
   readvertising the PRLA.

3.  Supporting PRLA in OSPF

   [RFC2328] Appendix B defines the following architectural constant for
   OSPF:

   "LSInfinity The metric value indicating that the destination
   described by an LSA is unreachable.  Used in summary-LSAs and AS-
   external-LSAs as an alternative to premature aging (see
   Section 14.1).  It is defined to be the 24-bit binary value of all
   ones: 0xffffff."

   [RFC5340] Appendix B states:

   "Architectural constants for the OSPF protocol are defined in
   Appendix B of OSPFV2."

   indicating that these same constants are applicable to OSPFv3.

   [RFC2328] section 14.1. also describes the usage of LSInfinity as a
   way to indicate loss of prefix reachability:








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   "Premature aging can also be used when, for example, one of the
   router's previously advertised external routes is no longer
   reachable.  In this circumstance, the router can flush its AS-
   external-LSA from the routing domain via premature aging.  This
   procedure is preferable to the alternative, which is to originate a
   new LSA for the destination specifying a metric of LSInfinity."

3.1.  Advertisement of PRLA in OSPF

   Using the existing mechanism already defined in the standards, as
   described in previous section, an advertisement of the inter-area or
   external prefix inside OSPF or OSPFv3 LSA that has the age set to
   value lower than MaxAge and metic set to LSInfinity can be
   interpreted by the receiver as a PRLA.

   Existing nodes in a network which receive PRLA advertisements will
   propagate it following existing standard procedures defined by OSPF.

   OSPF Area Border Routers (ABRs), which would be responsible for
   propagating PRLA advertisements into other areas would need to
   recognize such advertisements.

3.2.  Propagation of PRLA in OSPF

   OSPF ABRs may wish to advertise received PRLAs into other connected
   areas.  When doing so, the original LSInfinity metric value in PRLA
   MUST be preserved.  The cost to reach the originator of the received
   PRLA MUST NOT be considered when readvertising the PRLA to connected
   areas.

4.  Deployment Considerations for PRLA

   The economy provided by the use of summary advertisements diminishes
   in the presence of PRLA.  It is therefore recommended that
   implementations limit the number of PRLA advertisements which can be
   originated at a given time.  This implies that PRLA can be used to
   signal the loss of reachablity to a modest number of nodes - but it
   is not a good tool to signal the loss of many nodes simultaneously.

   The intent of PRLA is to provide an event driven signal of the
   transition of a destination from reachable to unreachable.  It is not
   intended to advertise a persistent state.  PRLA advertisements should
   therefore be withdrawn after a modest amount of time, that would
   provides sufficient time for PRLA to be flooded network-wide and
   acted upon by receiving nodes, but limits the presence of PRLA in the
   network to a short time period.  The time the PRLA is kept in the
   network SHOULD also reflect the intended use-case for which the PRLA
   was advertised.



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   As PRLA advertisements in ISIS are advertised in existing Link State
   PDUs (LSPs) and the unit of flooding in IS-IS is an LSP, it is
   recommended that, when possible, PRLAs are advertised in LSPs
   dedicated to this type of advertisement.  This will minimize the
   number of LSPs which need to be updated when PRLAs are advertised and
   withdrawn.

   In OSPF and OSPFv3, each inter-area and external prefix is advertised
   in it's own LSA, so the above optimisation does not apply to OSPF.

5.  IANA Considerations

   This document makes no requests to IANA.

6.  Security Considerations

   The use of PRLAs introduces the possibility that an attacker could
   inject a false, but apparently valid, PRLA.  However, the risk of
   this occurring is no greater than the risk today of an attacker
   injecting any other type of false advertisement .

   The risks can be reduced by the use of existing security extensions
   as described in [RFC5304] and [RFC5310] for IS-IS, in [RFC2328][ and
   [RFC7474] for OSPFv2, and in [RFC5340] and [RFC4552] for OSPFv3.

7.  Acknowledgements

   TBD

8.  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)", 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,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC2328]  Moy, J., "OSPF Version 2", STD 54, RFC 2328,
              DOI 10.17487/RFC2328, April 1998,
              <https://www.rfc-editor.org/info/rfc2328>.





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   [RFC4552]  Gupta, M. and N. Melam, "Authentication/Confidentiality
              for OSPFv3", RFC 4552, DOI 10.17487/RFC4552, June 2006,
              <https://www.rfc-editor.org/info/rfc4552>.

   [RFC5304]  Li, T. and R. Atkinson, "IS-IS Cryptographic
              Authentication", RFC 5304, DOI 10.17487/RFC5304, October
              2008, <https://www.rfc-editor.org/info/rfc5304>.

   [RFC5305]  Li, T. and H. Smit, "IS-IS Extensions for Traffic
              Engineering", RFC 5305, DOI 10.17487/RFC5305, October
              2008, <https://www.rfc-editor.org/info/rfc5305>.

   [RFC5308]  Hopps, C., "Routing IPv6 with IS-IS", RFC 5308,
              DOI 10.17487/RFC5308, October 2008,
              <https://www.rfc-editor.org/info/rfc5308>.

   [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, <https://www.rfc-editor.org/info/rfc5310>.

   [RFC5340]  Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
              for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008,
              <https://www.rfc-editor.org/info/rfc5340>.

   [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,
              <https://www.rfc-editor.org/info/rfc7474>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

Authors' Addresses

   Peter Psenak (editor)
   Cisco Systems
   Pribinova Street 10
   Bratislava 81109
   Slovakia
   Email: ppsenak@cisco.com









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   Les Ginsberg
   Cisco Systems
   821 Alder Drive
   Milpitas, CA 95035
   United States of America
   Email: ginsberg@cisco.com


   Daniel Voyer
   Bell Canada
   Email: daniel.voyer@bell.ca








































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