



Internet Engineering Task Force (IETF)                           A. Wang
Request for Comments: 9084                                 China Telecom
Category: Standards Track                                      A. Lindem
ISSN: 2070-1721                                      Cisco Systems, Inc.
                                                                 J. Dong
                                                     Huawei Technologies
                                                               P. Psenak
                                                      K. Talaulikar, Ed.
                                                     Cisco Systems, Inc.
                                                             August 2021


                   OSPF Prefix Originator Extensions

Abstract

   This document defines OSPF extensions to include information
   associated with the node originating a prefix along with the prefix
   advertisement.  These extensions do not change the core OSPF route
   computation functionality but provide useful information for network
   analysis, troubleshooting, and use cases like traffic engineering.

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
   https://www.rfc-editor.org/info/rfc9084.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction
     1.1.  Requirements Language
   2.  Protocol Extensions
     2.1.  Prefix Source OSPF Router-ID Sub-TLV
     2.2.  Prefix Source Router Address Sub-TLV
   3.  Elements of Procedure
   4.  Security Considerations
   5.  Operational Considerations
   6.  IANA Considerations
   7.  References
     7.1.  Normative References
     7.2.  Informative References
   Acknowledgement
   Authors' Addresses

1.  Introduction

   Prefix attributes are advertised in OSPFv2 [RFC2328] using the
   Extended Prefix Opaque Link State Advertisement (LSA) [RFC7684] and
   in OSPFv3 [RFC5340] using the various Extended Prefix LSA types
   [RFC8362].

   The procedures for identification of the originating router for a
   prefix in OSPF vary by the type of the prefix and, currently, it is
   not always possible to produce an accurate result.  For intra-area
   prefixes, the originating router is identified by the Advertising
   Router field of the area-scoped LSA used for those prefix
   advertisements.  However, for inter-area prefixes advertised by an
   Area Border Router (ABR), the Advertising Router field of their area-
   scoped LSAs is set to the ABR itself and the information about the
   router originating the prefix advertisement is lost in the process of
   prefix propagation across areas.  For Autonomous System (AS) external
   prefixes, the originating router may be considered as the Autonomous
   System Border Router (ASBR) and is identified by the Advertising
   Router field of the AS-scoped LSA used.  However, the actual
   originating router for the prefix may be a remote router outside the
   OSPF domain.  Similarly, when an ABR performs translation of Not-So-
   Stubby Area (NSSA) [RFC3101] LSAs to AS-external LSAs, the
   information associated with the NSSA ASBR (or the router outside the
   OSPF domain) is not propagated across the OSPF domain.

   While typically the originator of information in OSPF is identified
   by its OSPF Router ID, it does not necessarily represent a reachable
   address for the router since the OSPF Router ID is a 32-bit number
   that is unique in the OSPF domain.  For OSPFv2, a common practice is
   to use one of the IPv4 addresses of the node (e.g., a loopback
   interface) as the OSPF Router ID.  However, this cannot always be
   assumed and this approach does not apply to IPv6 addresses with
   OSPFv3.  The IPv4/IPv6 Router Address, as respectively defined in
   [RFC3630] and [RFC5329] for OSPFv2 and OSPFv3, provides an address to
   reach the advertising router.

   The primary use case for the extensions proposed in this document is
   to be able to identify the originator of a prefix in the network.  In
   cases where multiple prefixes are advertised by a given router, it is
   also useful to be able to associate all these prefixes with a single
   router even when prefixes are advertised outside of the area in which
   they originated.  It also helps to determine when the same prefix is
   being originated by multiple routers across areas.

   This document proposes extensions to the OSPF protocol for the
   inclusion of information associated with the router originating the
   prefix along with the prefix advertisement.  These extensions do not
   change the core OSPF route computation functionality.  They provide
   useful information for topology analysis and traffic engineering,
   especially on a controller when this information is advertised as an
   attribute of the prefixes via mechanisms such as Border Gateway
   Protocol - Link State (BGP-LS) [RFC7752] [RFC9085].

1.1.  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.

2.  Protocol Extensions

   This document defines the Prefix Source OSPF Router-ID and the Prefix
   Source Router Address Sub-TLVs.  They are used, respectively, to
   include the Router ID of, and a reachable address of, the router that
   originates the prefix as a prefix attribute.

2.1.  Prefix Source OSPF Router-ID Sub-TLV

   For OSPFv2, the Prefix Source OSPF Router-ID Sub-TLV is an optional
   sub-TLV of the OSPFv2 Extended Prefix TLV [RFC7684].  For OSPFv3, the
   Prefix Source OSPF Router-ID Sub-TLV is an optional sub-TLV of the
   Intra-Area-Prefix TLV, Inter-Area-Prefix TLV, and External-Prefix TLV
   [RFC8362] when originating either an IPv4 [RFC5838] or an IPv6 prefix
   advertisement.

   The Prefix Source OSPF Router-ID Sub-TLV has the following format:

     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           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                        OSPF Router ID                         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

           Figure 1: Prefix Source OSPF Router-ID Sub-TLV Format

   Where:
      Type:  4 for OSPFv2 and 27 for OSPFv3

      Length:  4

      OSPF Router ID:  the OSPF Router ID of the OSPF router that
         originated the prefix advertisement in the OSPF domain

   The parent TLV of a prefix advertisement MAY include more than one
   Prefix Source OSPF Router-ID Sub-TLV, one corresponding to each of
   the Equal-Cost Multipath (ECMP) nodes that originated the advertised
   prefix.

   For intra-area prefix advertisements, the Prefix Source OSPF Router-
   ID Sub-TLV MUST be considered invalid and ignored if the OSPF Router
   ID field is not the same as the Advertising Router field in the
   containing LSA.  Similar validation cannot be reliably performed for
   inter-area and external prefix advertisements.

   A received Prefix Source OSPF Router-ID Sub-TLV with the OSPF Router
   ID field set to 0 MUST be considered invalid and ignored.
   Additionally, reception of such sub-TLVs SHOULD be logged as an error
   (subject to rate limiting).

2.2.  Prefix Source Router Address Sub-TLV

   For OSPFv2, the Prefix Source Router Address Sub-TLV is an optional
   sub-TLV of the OSPFv2 Extended Prefix TLV [RFC7684].  For OSPFv3, the
   Prefix Source Router Address Sub-TLV is an optional sub-TLV of the
   Intra-Area-Prefix TLV, Inter-Area-Prefix TLV, and External-Prefix TLV
   [RFC8362] when originating either an IPv4 [RFC5838] or an IPv6 prefix
   advertisement.

   The Prefix Source Router Address Sub-TLV has the following format:

     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           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |              Router Address (4 or 16 octets)                  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

           Figure 2: Prefix Source Router Address Sub-TLV Format

   Where:
      Type:  5 for OSPFv2 and 28 for OSPFv3

      Length:  4 or 16

      Router Address:  A reachable IPv4 or IPv6 router address for the
         router that originated the IPv4 or IPv6 prefix advertisement,
         respectively.  Such an address would be semantically equivalent
         to what may be advertised in the OSPFv2 Router Address TLV
         [RFC3630] or in the OSPFv3 Router IPv6 Address TLV [RFC5329].

   The parent TLV of a prefix advertisement MAY include more than one
   Prefix Source Router Address Sub-TLV, one corresponding to each of
   the Equal-Cost Multipath (ECMP) nodes that originated the advertised
   prefix.

   A received Prefix Source Router Address Sub-TLV that has an invalid
   length (i.e., not consistent with the prefix's address family) MUST
   be considered invalid and ignored.  Additionally, reception of such
   sub-TLVs SHOULD be logged as an error (subject to rate limiting).

3.  Elements of Procedure

   This section describes the procedure for the advertisement of the
   Prefix Source OSPF Router-ID and Prefix Source Router Address Sub-
   TLVs along with the prefix advertisement.

   The OSPF Router ID of the Prefix Source OSPF Router-ID is set to the
   OSPF Router ID of the node originating the prefix in the OSPF domain.

   If the originating node is advertising an OSPFv2 Router Address TLV
   [RFC3630] or an OSPFv3 Router IPv6 Address TLV [RFC5329], then the
   same address MUST be used in the Router Address field of the Prefix
   Source Router Address Sub-TLV.  When the originating node is not
   advertising such an address, implementations can select a unique and
   reachable local address (for example, advertised with the N-Flag set
   [RFC7684] or N-bit set [RFC8362]) on the originating node to
   advertise in the Router Address field.

   When an ABR generates inter-area prefix advertisements into its non-
   backbone areas corresponding to an inter-area prefix advertisement
   from the backbone area, the only way to determine the originating
   node information is based on the Prefix Source OSPF Router-ID and
   Prefix Source Router Address Sub-TLVs present in the inter-area
   prefix advertisement originated into the backbone area by an ABR from
   another non-backbone area.  The ABR performs its prefix calculation
   to determine the set of nodes that contribute to ECMP paths for the
   prefix.  It MUST only use the prefix originator information from this
   set of nodes.  The ABR MUST NOT include the Prefix Source OSPF
   Router-ID or the Prefix Source Router Address Sub-TLVs when it is
   unable to determine the information for the originating nodes
   contributing ECMP paths.

   Implementations may support the propagation of the originating node
   information along with a redistributed prefix into the OSPF domain
   from another routing domain.  The details of such mechanisms are
   outside the scope of this document.  Such implementations may also
   provide control on whether the Router Address in the Prefix Source
   Router Address Sub-TLV is set as the ASBR node address or as the
   address of the actual node outside the OSPF domain that owns the
   prefix.

   When translating NSSA prefix advertisements [RFC3101] to AS external
   prefix advertisements, the NSSA ABR follows the same procedures as an
   ABR generating inter-area prefix advertisements for the propagation
   of the originating node information.

4.  Security Considerations

   Since this document extends the OSPFv2 Extended Prefix LSA, the
   security considerations for [RFC7684] are applicable.  Similarly,
   since this document extends the OSPFv3 E-Intra-Area-Prefix-LSA, E-
   Inter-Area-Prefix-LSA, E-AS-External-LSA, and E-NSSA-LSA, the
   security considerations for [RFC8362] are applicable.  The new sub-
   TLVs introduced in this document are optional and do not affect the
   OSPF route computation and therefore do not affect the security
   aspects of OSPF protocol operations.

   A rogue node that can inject prefix advertisements may use the
   extensions introduced in this document to advertise bogus prefix
   source information.

5.  Operational Considerations

   Consideration should be given to the operational impact of the
   increase in the size of the OSPF Link-State Database as a result of
   the protocol extensions in this document.  Based on deployment design
   and requirements, a subset of prefixes may be identified for which
   originating node information is required to be included in prefix
   advertisements.

   The propagation of prefix source node information for prefix
   advertisements advertised across an OSPF area or domain boundaries
   will expose information outside of an area or domain where it would
   normally be hidden or abstracted by the base OSPF protocol.  Based on
   deployment design and requirements, the propagation of node
   information across area or domain boundaries may be limited to a
   subset of prefixes in the ABRs or ASBRs, respectively.

   The identification of the node that is originating a specific prefix
   in the network may aid in the debugging of issues related to prefix
   reachability within an OSPF network.

6.  IANA Considerations

   Per this document, IANA has allocated the following codepoints from
   the "OSPFv2 Extended Prefix TLV Sub-TLVs" registry under the "Open
   Shortest Path First v2 (OSPFv2) Parameters" registry.

           +=======+==============================+===========+
           | Value | Description                  | Reference |
           +=======+==============================+===========+
           | 4     | Prefix Source OSPF Router-ID | RFC 9084  |
           +-------+------------------------------+-----------+
           | 5     | Prefix Source Router Address | RFC 9084  |
           +-------+------------------------------+-----------+

              Table 1: Codepoints in OSPFv2 Extended Prefix
                               TLV Sub-TLVs

   Per this document, IANA has allocated the following codepoints from
   the "OSPFv3 Extended-LSA Sub-TLVs" registry under the "Open Shortest
   Path First v3 (OSPFv3) Parameters" registry.

           +=======+==============================+===========+
           | Value | Description                  | Reference |
           +=======+==============================+===========+
           | 27    | Prefix Source OSPF Router-ID | RFC 9084  |
           +-------+------------------------------+-----------+
           | 28    | Prefix Source Router Address | RFC 9084  |
           +-------+------------------------------+-----------+

           Table 2: Codepoints in OSPFv3 Extended-LSA Sub-TLVs

7.  References

7.1.  Normative References

   [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>.

   [RFC3630]  Katz, D., Kompella, K., and D. Yeung, "Traffic Engineering
              (TE) Extensions to OSPF Version 2", RFC 3630,
              DOI 10.17487/RFC3630, September 2003,
              <https://www.rfc-editor.org/info/rfc3630>.

   [RFC5329]  Ishiguro, K., Manral, V., Davey, A., and A. Lindem, Ed.,
              "Traffic Engineering Extensions to OSPF Version 3",
              RFC 5329, DOI 10.17487/RFC5329, September 2008,
              <https://www.rfc-editor.org/info/rfc5329>.

   [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>.

   [RFC7684]  Psenak, P., Gredler, H., Shakir, R., Henderickx, W.,
              Tantsura, J., and A. Lindem, "OSPFv2 Prefix/Link Attribute
              Advertisement", RFC 7684, DOI 10.17487/RFC7684, November
              2015, <https://www.rfc-editor.org/info/rfc7684>.

   [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>.

7.2.  Informative References

   [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>.

   [RFC5838]  Lindem, A., Ed., Mirtorabi, S., Roy, A., Barnes, M., and
              R. Aggarwal, "Support of Address Families in OSPFv3",
              RFC 5838, DOI 10.17487/RFC5838, April 2010,
              <https://www.rfc-editor.org/info/rfc5838>.

   [RFC7752]  Gredler, H., Ed., Medved, J., Previdi, S., Farrel, A., and
              S. Ray, "North-Bound Distribution of Link-State and
              Traffic Engineering (TE) Information Using BGP", RFC 7752,
              DOI 10.17487/RFC7752, March 2016,
              <https://www.rfc-editor.org/info/rfc7752>.

   [RFC9085]  Previdi, S., Talaulikar, K., Ed., Filsfils, C., Gredler,
              H., and M. Chen, "Border Gateway Protocol - Link State
              (BGP-LS) Extensions for Segment Routing", RFC 9085,
              DOI 10.17487/RFC9085, August 2021,
              <https://www.rfc-editor.org/info/rfc9085>.

Acknowledgement

   Many thanks to Les Ginsberg for his suggestions on this document.
   Also, thanks to Jeff Tantsura, Rob Shakir, Gunter Van de Velde,
   Goethals Dirk, Smita Selot, Shaofu Peng, John E. Drake, and Baalajee
   S. for their review and valuable comments.  The authors would also
   like to thank Alvaro Retana for his detailed review and suggestions
   for the improvement of this document.

Authors' Addresses

   Aijun Wang
   China Telecom
   Beiqijia Town
   Changping District
   Beijing
   102209
   China

   Email: wangaj3@chinatelecom.cn


   Acee Lindem
   Cisco Systems, Inc.
   301 Midenhall Way
   Cary, NC 27513
   United States of America

   Email: acee@cisco.com


   Jie Dong
   Huawei Technologies
   Huawei Campus, No. 156 Beiqing Rd.
   Beijing
   100095
   China

   Email: jie.dong@huawei.com


   Peter Psenak
   Cisco Systems, Inc.
   Eurovea Centre, Central 3
   Pribinova Street 10
   81109 Bratislava
   Slovakia

   Email: ppsenak@cisco.com


   Ketan Talaulikar (editor)
   Cisco Systems, Inc.
   India

   Email: ketant@cisco.com
