IGP Unreachable Prefix Announcement
draft-ppsenak-lsr-igp-ureach-prefix-announce-01

Networking Working Group                                  P. Psenak, Ed.
Internet-Draft                                               C. Filsfils
Intended status: Informational                              S. Litkowski
Expires: 30 March 2023                                     Cisco Systems
                                                                D. Voyer
                                                             Bell Canada
                                                              A. Dhamija
                                                                 Rakuten
                                                       26 September 2022

                  IGP Unreachable Prefix Announcement
            draft-ppsenak-lsr-igp-ureach-prefix-announce-01

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
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   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 30 March 2023.

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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
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   extracted from this document must include Revised BSD License text as
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Supporting UPA in IS-IS . . . . . . . . . . . . . . . . . . .   3
     2.1.  Advertisement of UPA in IS-IS . . . . . . . . . . . . . .   3
     2.2.  Propagation of UPA in IS-IS . . . . . . . . . . . . . . .   4
   3.  Supporting UPA in OSPF  . . . . . . . . . . . . . . . . . . .   4
     3.1.  Advertisement of UPA in OSPF  . . . . . . . . . . . . . .   5
     3.2.  Propagation of UPA in OSPF  . . . . . . . . . . . . . . .   5
   4.  Deployment Considerations for UPA . . . . . . . . . . . . . .   6
   5.  Processing of the UPA . . . . . . . . . . . . . . . . . . . .   6
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   7
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

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/

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   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
   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
   Unreachable Prefix Announcement (UPA).

2.  Supporting UPA 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 UPA in IS-IS

   Existing nodes in a network which receive UPA 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 UPA is only required on routers
   which have a use case for this information.  Area Border Routers
   (ABRs), which would be responsible for propagating UPA 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 UPA.

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

   UPA in ISIS is supported for all IS-IS Sub-TLVs Advertising Prefix
   Reachability, e.g., SRv6 Locator [I-D.ietf-lsr-isis-srv6-extensions],
   Extended IP reachability [RFC5305], MT IP Reach [RFC5120], IPv6 IP
   Reach [RFC5308], MT IPv6 IP Reach [RFC5120], IPv4 Algorithm Prefix
   Reachability TLV [I-D.ietf-lsr-ip-flexalgo], and IPv6 Algorithm
   Prefix Reachability TLV [I-D.ietf-lsr-ip-flexalgo]

2.2.  Propagation of UPA in IS-IS

   ISIS allows propagation of IP prefixes in both directions between
   level 1 and level 2.  For reachable prefixes this is only done if the
   prefix is reachable in source level - e.g., the prefix needs to be
   reachable in level 1 to be propagated to level 2 and vice verse.
   Such requirement of reachability MUST not be applied for UPAs, as
   they are propagating unreachability.

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

3.  Supporting UPA 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:

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   "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:

   "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."

   UPA in OSPFv2 is supported for OSPFv2 Summary-LSA [RFC2328], AS-
   external-LSAs [RFC2328], NSSA AS-external LSA.[RFC3101], and OSPFv2
   Extended Prefix TLV [I-D.ietf-lsr-ip-flexalgo].

   UPA in OSPFv3 is supported for Inter-Area-Prefix-LSA [RFC2328], AS-
   External-LSA [RFC2328], NSSA-LSA [RFC2328], E-Inter-Area-Prefix-LSA
   [RFC8362], E-AS-External-LSA [RFC8362], E-Type-7-LSA [RFC8362], and
   SRv6 Locator LSA [I-D.ietf-lsr-ospfv3-srv6-extensions].

3.1.  Advertisement of UPA 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 UPA.

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

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

3.2.  Propagation of UPA in OSPF

   Advertising prefix reachability between OSPF areas assumes prefix
   reachability in a source area.  Such requirement of reachability MUST
   not be applied for UPAs, as they are propagating unreachability.

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

4.  Deployment Considerations for UPA

   The intent of UPA 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.  UPA advertisements should
   therefore be withdrawn after a modest amount of time, that would
   provides sufficient time for UPA to be flooded network-wide and acted
   upon by receiving nodes, but limits the presence of UPA in the
   network to a short time period.  The time the UPA is kept in the
   network SHOULD also reflect the intended use-case for which the UPA
   was advertised.

   As UPA 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, UPAs are advertised in LSPs
   dedicated to this type of advertisement.  This will minimize the
   number of LSPs which need to be updated when UPAs 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.

   It is also recommended that implementations limit the number of UPA
   advertisements which can be originated at a given time.

5.  Processing of the UPA

   Processing of the received UPAs is optional and SHOULD be controlled
   by the configuration at the receiver.  The receiver itself, based on
   its configuration, decides what the UPA will be used for and what
   applications, if any, will be notified when UPA is received.

6.  IANA Considerations

   This document makes no requests to IANA.

7.  Security Considerations

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

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   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.

8.  Acknowledgements

   The authors would like to thank Kamran Raza and Michael MacKenzie for
   their contribution to the overall solution proposed in this document.

9.  References

9.1.  Normative References

   [ISO10589] ISO, "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>.

   [RFC3101]  Murphy, P., "The OSPF Not-So-Stubby Area (NSSA) Option",
              RFC 3101, DOI 10.17487/RFC3101, January 2003,
              <https://www.rfc-editor.org/info/rfc3101>.

   [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>.

   [RFC5120]  Przygienda, T., Shen, N., and N. Sheth, "M-ISIS: Multi
              Topology (MT) Routing in Intermediate System to
              Intermediate Systems (IS-ISs)", RFC 5120,
              DOI 10.17487/RFC5120, February 2008,
              <https://www.rfc-editor.org/info/rfc5120>.

   [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>.

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   [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>.

   [RFC8362]  Lindem, A., Roy, A., Goethals, D., Reddy Vallem, V., and
              F. Baker, "OSPFv3 Link State Advertisement (LSA)
              Extensibility", RFC 8362, DOI 10.17487/RFC8362, April
              2018, <https://www.rfc-editor.org/info/rfc8362>.

9.2.  Informative References

   [I-D.ietf-lsr-ip-flexalgo]
              Britto, W., Hegde, S., Kaneriya, P., Shetty, R., Bonica,
              R., and P. Psenak, "IGP Flexible Algorithms (Flex-
              Algorithm) In IP Networks", Work in Progress, Internet-
              Draft, draft-ietf-lsr-ip-flexalgo-06, 16 May 2022,
              <https://www.ietf.org/archive/id/draft-ietf-lsr-ip-
              flexalgo-06.txt>.

   [I-D.ietf-lsr-isis-srv6-extensions]
              Psenak, P., Filsfils, C., Bashandy, A., Decraene, B., and
              Z. Hu, "IS-IS Extensions to Support Segment Routing over
              IPv6 Dataplane", Work in Progress, Internet-Draft, draft-
              ietf-lsr-isis-srv6-extensions-18, 20 October 2021,
              <https://www.ietf.org/archive/id/draft-ietf-lsr-isis-srv6-
              extensions-18.txt>.

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   [I-D.ietf-lsr-ospfv3-srv6-extensions]
              Li, Z., Hu, Z., Talaulikar, K., and P. Psenak, "OSPFv3
              Extensions for SRv6", Work in Progress, Internet-Draft,
              draft-ietf-lsr-ospfv3-srv6-extensions-08, 14 September
              2022, <https://www.ietf.org/archive/id/draft-ietf-lsr-
              ospfv3-srv6-extensions-08.txt>.

Authors' Addresses

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

   Clarence Filsfils
   Cisco Systems
   Brussels
   Belgium
   Email: cfilsfil@cisco.com

   Stephane Litkowski
   Cisco Systems
   La Rigourdiere
   Cesson Sevigne
   France
   Email: slitkows@cisco.com

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

   Amit Dhamija
   Rakuten
   Email: amit.dhamija@rakuten.com

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