[Docs] [txt|pdf|xml|html] [Tracker] [WG] [Email] [Diff1] [Diff2] [Nits] [IPR]

Versions: (draft-wang-lsr-ospf-prefix-originator-ext) 00 01 02 03 04 05

LSR Working Group                                                A. Wang
Internet-Draft                                             China Telecom
Intended status: Standards Track                               A. Lindem
Expires: May 28, 2020                                      Cisco Systems
                                                                 J. Dong
                                                     Huawei Technologies
                                                               P. Psenak
                                                           K. Talaulikar
                                                           Cisco Systems
                                                       November 25, 2019


                    OSPF Prefix Originator Extension
                draft-ietf-lsr-ospf-prefix-originator-05

Abstract

   This document defines Open Shortest Path First (OSPF) encodings to
   advertise the router-id of the originator of inter-area prefixes for
   OSPFv2 and OSPFv3 Link-State Advertisements (LSAs).  The source
   originator is needed in several multi-area OSPF use cases.

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 May 28, 2020.

Copyright Notice

   Copyright (c) 2019 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



Wang, et al.              Expires May 28, 2020                  [Page 1]


Internet-Draft           OSPF Prefix Originator            November 2019


   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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions used in this document . . . . . . . . . . . . . .   3
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Inter-Area Prefix Source Advertisement Use Cases  . . . . . .   4
   5.  External Prefix Source Advertisement Use Cases  . . . . . . .   5
   6.  Prefix Source Router-ID sub-TLV . . . . . . . . . . . . . . .   6
   7.  Elements of Procedure . . . . . . . . . . . . . . . . . . . .   7
     7.1.  Inter-Area Prefixes . . . . . . . . . . . . . . . . . . .   7
     7.2.  External Prefixes . . . . . . . . . . . . . . . . . . . .   7
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   10. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . .   8
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     11.1.  Normative References . . . . . . . . . . . . . . . . . .   9
     11.2.  Informative References . . . . . . . . . . . . . . . . .  10
   Appendix A.  Inter-Area Topology Retrieval Process  . . . . . . .  10
   Appendix B.  Special Considerations on Inter-Area Topology
                Retrieval  . . . . . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   [I-D.ietf-ospf-mpls-elc] defines mechanisms to advertise Entropy
   Readable Label Depth (ERLD) for ingress Label Switching Routers (LSR)
   to discover other LSR's capability of performing Entropy Label based
   load-balancing in MPLS networks.  The ingress LSR can use this
   information to construct the appropriate label stack for specific
   traffic requirements, especially in segment routed networks and other
   deployments requiring stacked LSPs.

   However, in inter-area scenarios, the Area Border Router (ABR) does
   not advertise the originating OSPF router-id for inter-area prefixes.
   An OSPF router in one area doesn't know the origin area of inter-area
   prefixes and can't determine the router that originated these
   prefixes or the ERLD capabilities of the destination.  Therefore, it
   is necessary to advertise the originator of these inter-area prefixes
   to ensure the ingress LSR can construct the appropriate label stack.

   More generally, [RFC8476] defines a mechanism to advertise multiple
   types of supported Maximum SID Depths (MSD) at node and/or link



Wang, et al.              Expires May 28, 2020                  [Page 2]


Internet-Draft           OSPF Prefix Originator            November 2019


   granularity.  This information will be referred when the head-end
   router starts to send traffic to destination prefixes.  In inter-area
   scenario, it is also necessary for the sender to learn the
   capabilities of the receivers associated with the inter-area
   prefixes.

   There is another scenario where knowing the originator of inter-area
   prefixes is useful.  For example, Border Gateway Protocol Link-State
   (BGP-LS) [RFC7752] describes mechanisms using the BGP protocol to
   advertise Link-State information.  This information can enable a
   Software Definition Network (SDN) controller to automatically
   determine the underlay network topology.

   However, if the underlay network is partitioned into multiple areas
   and running the OSPF protocol, it is not easy for the SDN controller
   to rebuild the multi-area topology since ABR that connects multiple
   areas will normally hide the detailed topology for these non-backbone
   areas.  If only the internal routers within backbone area run the
   BGP-LS protocol, they just learn and report the summary network
   information from the non-backbone areas.  If the SDN controller can
   learn the originator of the inter-area prefixes, it is possible to
   rebuild the inter-area topology.

   [RFC7794] introduces the Intermediate System to Intermediate System
   (IS-IS) "IPv4/IPv6 Source Router IDs" Type-Length-Value (TLV) to
   advertise the source of prefixes leaked from a different IS-IS level.
   This TLV can be used in the above scenarios.  Such solution can also
   be applied in networks that run the OSPF protocol, but existing OSPF
   LSAs TLVs must be extended to include the router originating the
   prefix.

   This draft provides such solution for the OSPFv2 [RFC2328] and OSPFv3
   [RFC5340] protocols.

2.  Conventions used in this document

   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.

3.  Terminology

   The following terms are used in this document:

   o  ABR: Area Border Router




Wang, et al.              Expires May 28, 2020                  [Page 3]


Internet-Draft           OSPF Prefix Originator            November 2019


   o  ASBR: Autonomous System Border Router

   o  ERLD: Entropy Readable Label Depth

   o  EL: Entropy Label

   o  IS-IS: Intermediate System to Intermediate System

   o  LSA: Link-State Advertisement

   o  MSD: Maximum SID Depths

   o  NLRI: Network Layer Reachability Information

   o  OSPF: Open Shortest Path First

   o  SID: Segment IDentifier

   o  SDN: Software Definition Network

4.  Inter-Area Prefix Source Advertisement Use Cases

   Figure 1 illustrates a topology where OSPF is running in multiple
   areas.  R0-R4 are routers in the backbone area, S1-S4 are internal
   routers in area 1, and T1-T4 are internal routers in area 2.  R1 and
   R3 are ABRs between area 0 and area 1.  R2 and R4 are ABRs between
   area 0 and area 2.  N1 is the network between router S1 and S2 and N2
   is the network between router T1 and T2.  Ls1 is the loopback address
   of Node S1 and Lt1 is the loopback address of Node T1.






















Wang, et al.              Expires May 28, 2020                  [Page 4]


Internet-Draft           OSPF Prefix Originator            November 2019


                            +-----------------+
                            |IP SDN Controller|
                            +--------+--------+
                                     |
                                     | BGP-LS
                                     |
        +---------------------+------+--------+-----+--------------+
        | +--+        +--+   ++-+   ++-+    +-++   + -+        +--+|
        | |S1+--------+S2+---+R1+---|R0+----+R2+---+T1+--------+T2||
        | +-++   N1   +-++   ++-+   +--+    +-++   ++++   N2   +-++|
        |   |           |     |               |     ||           | |
        |   |           |     |               |     ||           | |
        | +-++        +-++   ++-+           +-++   ++++        +-++|
        | |S4+--------+S3+---+R3+-----------+R4+---+T3+--------+T4||
        | +--+        +--+   ++-+           +-++   ++-+        +--+|
        |                     |               |                    |
        |                     |               |                    |
        |         Area 1      |     Area 0    |      Area 2        |
        +---------------------+---------------+--------------------+

              Figure 1: OSPF Inter-Area Prefix Originator Scenario

   If S1 wants to send traffic to prefix Lt1 that is connected to T1 in
   another area, it should know the ERLD and MSD values associated with
   the node T1, and then construct the right label stack at the ingress
   node for traffic destined to prefix Lt1.

   In another scenario, If R0 has some method to learn the originator of
   network N1 and reports such information to IP SDN controller, then it
   is possible for the controller to reconstruct the topology in the
   non-backbone areas.  The topology reconstruction process and its
   limitations are described in the Appendix A and Appendix B.

5.  External Prefix Source Advertisement Use Cases

   Figure 2 illustrates a topology where OSPF is running in different
   domain that is connected by an Autonomous System Border Router
   (ASBR).  A, B, and C are routers in the Domain 1; C, D, and E are
   routers in Domain 2.  Router C is the ASBR between the two domains.

   When router E receives an external prefix, it will redistribute it as
   an AS-External LSA within domain 2.  When C receives such LSA, the
   originator information for such external prefix will be lost when it
   encodes the prefix information with the current LSA format field.  In
   some situations, it will be helpful if C can advertise such
   originator information.





Wang, et al.              Expires May 28, 2020                  [Page 5]


Internet-Draft           OSPF Prefix Originator            November 2019


   +-------------------------------------------------------------------+
   |                              |                    External Prefix |
   | +---+       +---+          +---+         +---+         +-|-+      |
   | | A +-------+ B +----------| C +---------+ D +---------+ E |------|
   | +---+       +---+          +---+         +---+         +---+      |
   |        Domain 1              |                 Domain 2           |
   +-------------------------------------------------------------------+

          Figure 2: OSPF External Prefix Originator Scenario

   From the above scenarios, we can conclude that it is useful to define
   the OSPF prefix originator sub TLV .

6.  Prefix Source Router-ID sub-TLV

   [RFC7684] and [RFC8362] respectively define TLV-based LSAs for OSPFv2
   and OSPFv3.  These documents facilitate addition of new attributes
   for prefixes and provide the basis for a sub-TLV to advertise the
   "Prefix Source Router ID".  For OSPFv2, this sub-TLV is a sub-TLV of
   OSPFv2 Extended Prefix TLV which SHOULD be included in the "OSPFv2
   Extended Prefix Opaque LSA" [RFC7684] for inter-area prefixes.  For
   OSPFv3, this sub-TLV is a sub-TLV of "Inter-Area-Prefix TLV", which
   SHOULD be included in the "E-Inter-Area-Prefix-LSA" [RFC8362].

   The "Prefix Source 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           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                Prefix Source Router-ID                        |
    +---------------------------------------------------------------+
           Figure 3: Prefix Source Router-ID sub-TLV Format

   o  Source Router-ID Sub-TLV Type: 4 (IANA TEMPORARY allocation)
      [RFC7684] or 27 (IANA TEMPORARY allocation) [RFC8362]

   o  Length: 4

   o  Value: Router-ID of OSPFv2/OSPFv3 router that is the source of the
      prefix.

   This sub-TLV provides the same functionality as the IS-IS "IPv4/IPv6
   Source Router" TLV defined in [RFC7794].






Wang, et al.              Expires May 28, 2020                  [Page 6]


Internet-Draft           OSPF Prefix Originator            November 2019


7.  Elements of Procedure

   The following sections describe the procedure to include the newly
   defined "Source Router-ID Sub-TLV" in the related LSA for inter-area
   prefixes and external prefixes respectively.

7.1.  Inter-Area Prefixes

   When an ABR, for example R2 in Figure 1, receives a Router-LSA
   advertisement in area 2, it SHOULD originate the corresponding
   "OSPFv2 Extended Prefix Opaque LSA" for OSPFv2 or "E-Inter-Area-
   Prefix-LSA" for OSPFv3 that includes the Source Router-ID sub-TLV for
   the network prefixes.  For example, to identify the source router
   prefix Lt1 and other inter-area prefixes in Figure 1.

   When a router in another area, e.g., S1, receives such LSA, it then
   can ascertain that prefix Lt1 is associated with node T1 and obtain
   the ERLD or MSD value from T1's Router-Information LSA [RFC7770] and
   construct the right label stack at the ingress node S1 for traffic
   destined to prefix Lt1.

   When a router in another area, e.g., R0, receives such LSA, it learns
   the Prefix Source Router-id and includes it in the prefix information
   advertised to an SDN controller as described in
   [I-D.ietf-idr-bgp-ls-segment-routing-ext].  The SDN controller can
   then use such information to build the inter-area topology according
   to the process described in the Appendix A.  The topology retrieval
   process may not suitable for some environments as stated in
   Appendix B.

7.2.  External Prefixes

   When an ASBR, for example C in Figure 2, receives an AS-External LSA
   for an external prefix in domain 2, it SHOULD extract the originator
   information from the "Advertising Router" field from the LSA header.
   When the prefix is advertised into domain 1 as an AS-External LSA,
   router C may also advertise the Source Router-ID using a AS-scoped
   OSPFv2 Extended Prefix Opaque LSA or as a Sub-TLV in the OSPFv3 AS-
   External LSA.

8.  Security Considerations

   Since this document extends the "OSPFv2 Extended Prefix LSA" and
   "OSPFv3 E-Inter-Area-Prefix LSA", the security considerations for
   [RFC7684] and [RFC8362] are applicable.

   Modification of the "Prefix Source Sub-TLV" could be used for a
   Denial-of-Service attack and could inhibit the use cases described in



Wang, et al.              Expires May 28, 2020                  [Page 7]


Internet-Draft           OSPF Prefix Originator            November 2019


   Section 4.  If the OSPF domain is vulnerable to such attacks, OSPF
   authentication should be used as defined for OSPFv2 in [RFC5709] and
   [RFC7474] and for OSPFv3 in [RFC7166].

   Additionally, advertisement of the prefix source for inter-area
   prefixes facilitates reconstruction of the OSPF topology for other
   areas.  Network operators may consider their topologies to be
   sensitive confidential data.  For OSPFv3, IPsec can be used to
   provide confidentiality [RFC4552].  Since there is no standard
   defined for native OSPFv2 IPsec, some form of secure tunnel is
   required to provide confidentiality.

9.  IANA Considerations

   This specification defines the Prefix Source Router-ID sub-TLV as
   described in Section 6.  This value should be added to the both
   existing "OSPFv2 Extended Prefix TLV Sub-TLVs" and "OSPFv3 Extended-
   LSA Sub-TLVs" registries.

   The following sub-TLV is added to the "OSPFv2 Extended Prefix TLV
   Sub-TLVs" registry.  The allocation policy is IETF Review as defined
   in [RFC7684]

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Code Point  |   Description         |           Status       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       4        | Prefix Source Sub-TLV |   Allocation from IANA |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   Figure 4:  Code Point in "OSPFv2 Extended Prefix TLV Sub-TLVs"

   The following sub-TLV is added to the "OSPFv3 Extended-LSA Sub-TLVs"
   registry.  The allocation policy is IETF Review as defined in
   [RFC8362]

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Code Point  |   Description         |           Status       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       27       | Prefix Source Sub-TLV |   Allocation from IANA |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   Figure 5:  Code Point in "OSPFv3 Extended-LSA Sub-TLVs"

10.  Acknowledgement

   Many thanks to Les Ginsberg for his suggestions on this draft.  Also
   thanks to Jeff Tantsura, Rob Shakir, Gunter Van De Velde, Goethals
   Dirk, Smita Selot, Shaofu Peng, and John E Drake for their valuable
   comments.




Wang, et al.              Expires May 28, 2020                  [Page 8]


Internet-Draft           OSPF Prefix Originator            November 2019


11.  References

11.1.  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>.

   [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>.

   [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>.

   [RFC5709]  Bhatia, M., Manral, V., Fanto, M., White, R., Barnes, M.,
              Li, T., and R. Atkinson, "OSPFv2 HMAC-SHA Cryptographic
              Authentication", RFC 5709, DOI 10.17487/RFC5709, October
              2009, <https://www.rfc-editor.org/info/rfc5709>.

   [RFC7166]  Bhatia, M., Manral, V., and A. Lindem, "Supporting
              Authentication Trailer for OSPFv3", RFC 7166,
              DOI 10.17487/RFC7166, March 2014,
              <https://www.rfc-editor.org/info/rfc7166>.

   [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>.

   [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>.

   [RFC7770]  Lindem, A., Ed., Shen, N., Vasseur, JP., Aggarwal, R., and
              S. Shaffer, "Extensions to OSPF for Advertising Optional
              Router Capabilities", RFC 7770, DOI 10.17487/RFC7770,
              February 2016, <https://www.rfc-editor.org/info/rfc7770>.






Wang, et al.              Expires May 28, 2020                  [Page 9]


Internet-Draft           OSPF Prefix Originator            November 2019


   [RFC7794]  Ginsberg, L., Ed., Decraene, B., Previdi, S., Xu, X., and
              U. Chunduri, "IS-IS Prefix Attributes for Extended IPv4
              and IPv6 Reachability", RFC 7794, DOI 10.17487/RFC7794,
              March 2016, <https://www.rfc-editor.org/info/rfc7794>.

   [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>.

   [RFC8476]  Tantsura, J., Chunduri, U., Aldrin, S., and P. Psenak,
              "Signaling Maximum SID Depth (MSD) Using OSPF", RFC 8476,
              DOI 10.17487/RFC8476, December 2018,
              <https://www.rfc-editor.org/info/rfc8476>.

11.2.  Informative References

   [I-D.ietf-idr-bgp-ls-segment-routing-ext]
              Previdi, S., Talaulikar, K., Filsfils, C., Gredler, H.,
              and M. Chen, "BGP Link-State extensions for Segment
              Routing", draft-ietf-idr-bgp-ls-segment-routing-ext-16
              (work in progress), June 2019.

   [I-D.ietf-ospf-mpls-elc]
              Xu, X., Kini, S., Psenak, P., Filsfils, C., Litkowski, S.,
              and M. Bocci, "Signaling Entropy Label Capability and
              Entropy Readable Label-stack Depth Using OSPF", draft-
              ietf-ospf-mpls-elc-12 (work in progress), October 2019.

   [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>.

Appendix A.  Inter-Area Topology Retrieval Process

   When an IP SDN Controller receives BGP-LS [RFC7752] information, it
   should compare the prefix Network Layer Reachability Information
   (NLRI) that is included in the BGP-LS NLRI.  When it encounters the
   same prefix but with different source router ID, it should extract
   the corresponding area-ID, rebuild the link between these two source
   routers in the non-backbone area.  Below is one example that based on
   the Figure 1:



Wang, et al.              Expires May 28, 2020                 [Page 10]


Internet-Draft           OSPF Prefix Originator            November 2019


   Assuming we want to rebuild the connection between router S1 and
   router S2 located in area 1:

   a.  Normally, router S1 will advertise prefix N1 within its router-
       LSA.

   b.  When this router-LSA reaches the ABR router R1, it will convert
       it into summary-LSA, add the Prefix Source Router-ID sub-TLV,
       which is router id of S1 in this example.

   c.  R1 then floods this extension summary-LSA to R0, which is using
       the BGP-LS protocol with IP SDN Controller.  The controller then
       knows the prefix for N1 is from S1.

   d.  Router S2 will perform a similar process, and the controller will
       also learn that prefix N1 is also from S2.

   e.  Then it can reconstruct the link between S1 and S2, using the
       prefix N1.  The topology within Area 1 can then be reconstructed
       accordingly.

   Iterating the above process continuously, the IP SDN controller can
   retrieve a detailed topology that spans multiple areas.

Appendix B.  Special Considerations on Inter-Area Topology Retrieval

   The above topology retrieval process can be applied in the case where
   each point-to-point or multi-access link connecting routers is
   assigned a unique prefix.  However, there are some situations where
   this heuristic cannot be applied.  Specifically, the cases where the
   link is unnumbered or the prefix corresponding to the link is an
   anycast prefix.

   The Appendix A heuristic to rebuild the topology can normally be used
   if all links are numbered.  For anycast prefixes, if it corresponds
   to the loopback interface and has a host prefix length, i.e., 32 for
   IPv4 prefixes and 128 for IPv6 prefixes, Appendix A can also applied
   since these anycast prefixes are not required to reconstruct the
   topology.

Authors' Addresses










Wang, et al.              Expires May 28, 2020                 [Page 11]


Internet-Draft           OSPF Prefix Originator            November 2019


   Aijun Wang
   China Telecom
   Beiqijia Town, Changping District
   Beijing  102209
   China

   Email: wangaj3@chinatelecom.cn


   Acee Lindem
   Cisco Systems
   301 Midenhall Way
   Cary, NC  27513
   USA

   Email: acee@cisco.com


   Jie Dong
   Huawei Technologies
   Beijing
   China

   Email: jie.dong@huawei.com


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

   Email: ppsenak@cisco.com


   Ketan Talaulikar
   Cisco Systems
   S.No. 154/6, Phase I, Hinjawadi
   Pune  411 057
   India

   Email: ketant@cisco.com









Wang, et al.              Expires May 28, 2020                 [Page 12]


Html markup produced by rfcmarkup 1.129d, available from https://tools.ietf.org/tools/rfcmarkup/