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OSPF Extension for Prefix Originator

The information below is for an old version of the document.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 9084.
Authors Aijun Wang , Acee Lindem , Jie Dong , Ketan Talaulikar , Peter Psenak
Last updated 2019-05-09 (Latest revision 2019-02-28)
Replaces draft-wang-lsr-ospf-prefix-originator-ext
RFC stream Internet Engineering Task Force (IETF)
OPSDIR Last Call Review Incomplete, due 2021-03-29
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LSR Working Group                                                A. Wang
Internet-Draft                                             China Telecom
Intended status: Standards Track                               A. Lindem
Expires: September 2, 2019                                 Cisco Systems
                                                                 J. Dong
                                                     Huawei Technologies
                                                           K. Talaulikar
                                                               P. Psenak
                                                           Cisco Systems
                                                           March 1, 2019

                  OSPF Extension for Prefix Originator


   This document describes OSPFv2 and OSPFv3 encodings to advertise the
   router-id of the originator of inter-area prefixes for OSPFv2 and
   OSPFv3 LSAs, which are needed in several use cases 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
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   Drafts is at

   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 September 2, 2019.

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
   ( in effect on the date of
   publication of this document.  Please review these documents

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   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.  Scenario Description  . . . . . . . . . . . . . . . . . . . .   3
   4.  Prefix Source Router-ID sub-TLV . . . . . . . . . . . . . . .   4
   5.  Extended LSA Elements of Procedure  . . . . . . . . . . . . .   5
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   8.  Acknowledgement . . . . . . . . . . . . . . . . . . . . . . .   6
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Appendix A.  Inter-Area Topology Retrieval Process  . . . . . . .   7
   Appendix B.  Special Considerations on Inter-Area Topology
                Retrieval  . . . . . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   [I-D.ietf-ospf-mpls-elc] defines mechanisms to signal Entropy Label
   Capability (ELC) and Entropy Readable Label Depth (ERLD) for ingress
   LSR to discover each LSR's capability of reading the maximum label
   stack depth and performing EL-based load-balancing in MPLS networks.
   The ingress LSR can use this information to push the appropriate
   label stack for specific FEC trafffic, especially in segment routing
   environments and other stacked LSPs scenarios.

   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 where the prefixes really
   came from and can't determine the router that originated inter-area
   prefixes and then can't judge the ELC and ERLD capabilities of the
   destination.  It is necessary to transfer the originator information
   of these inter-area prefixes to ensure the ingress LSR constructs the
   right Label stack.

   More generally, draft [I-D.ietf-ospf-segment-routing-msd] defines a
   mechanism to advertise multiple types of supported Maximum SID Depths
   (MSD) at node and/or link 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

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   for the sender to learn the capabilities of the receivers associated
   with the inter-area prefixes.

   There is also another scenario where knowing the originator of inter-
   area prefixes is useful.  For example, BGP-LS [RFC7752] describes
   mechanisms using the BGP protocol to advertise Link-State
   information.  This can enable an SDN controller to collect the
   underlay network topology automatically.

   But if the underlay network is divided into multiple areas and
   running the OSPF protocol, it is not easy for the SDN controller to
   rebuild the multi-area topology, because normally an Area Border
   Router (ABR) that connects multiple areas will hide the detailed
   topology information for these non-backbone areas, and the router in
   backbone area that runs the BGP-LS protocol can only 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 for them to rebuild the inter-area topology

   [RFC7794] introduces the IS-IS "IPv4/IPv6 Source Router IDs" TLV to
   advertise the source of the prefixes redistributed 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 the related Link state Advertisements (LSAs) must be extended.

   This draft provides such solution for the OSPFv2 and OSPFv3

2.  Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119] .

3.  Scenario Description

   Fig.1 illustrates the topology scenario when OSPF is running in
   multi-area.  R0-R4 are routers in backbone area, S1-S4,T1-T4 are
   internal routers in area 1 and area 2 respectively.  R1 and R3 are
   area border routers between area 0 and area 1.  R2 and R4 are area
   border routers 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.

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                            |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        |

              Fig.1 OSPF Inter-Area Prefix Originator Scenario

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

   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 retrieval the topology in non-
   backbone area.  The topology retrieval process and its usage
   limitation are described in the Appendix A and Appendix B.

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

4.  Prefix Source Router-ID sub-TLV

   [RFC7684] and [RFC8362] define the TLV extensions for OSPFv2 and
   OSPFv3 respectively.  These documents facilitate addition of new
   attributes for prefixes and links.  Based on these formats, we can
   define new sub-TLV to advertise the "Prefix Source Router ID", as
   that defined in [RFC7794].

   The "Prefix Source Router-ID" sub-TLV has the following format:

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    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                        |

   o  Source Router-ID Sub-TLV Type: TBD1[RFC7684] or TBD2 [RFC8362]

   o  Length: 4

   o  Value: Router-ID of OSPFv2/OSPFv3 source router

   This sub-TLV can be included in the "OSPFv2 Extended Prefix Opaque
   LSA" [RFC7684] or the "E-Inter-Area-Prefix-LSA" [RFC8362].

5.  Extended LSA Elements of Procedure

   When an ABR, for example R2 in Fig.1, receives the Router-LSA
   announcement 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, e.g., for prefix Lt1, N2. etc., which identifies the source
   router that advertised the prefix.

   When S1 in another area receives such LSA, it then can learn that
   prefix Lt1 is associated with node T1, check the ELC, ERLD, or MSD
   value according to its necessity, and construct the right label stack
   at the ingress node S1 for the traffic destined to Lt1.

   When 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.

6.  Security Considerations


7.  IANA Considerations


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8.  Acknowledgement

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

9.  References

9.1.  Normative References

              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-11
              (work in progress), October 2018.

              Xu, X., Kini, S., Sivabalan, S., Filsfils, C., and S.
              Litkowski, "Signaling Entropy Label Capability and Entropy
              Readable Label-stack Depth Using OSPF", draft-ietf-ospf-
              mpls-elc-07 (work in progress), September 2018.

              Tantsura, J., Chunduri, U., Aldrin, S., and P. Psenak,
              "Signaling MSD (Maximum SID Depth) using OSPF", draft-
              ietf-ospf-segment-routing-msd-25 (work in progress),
              October 2018.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC2328]  Moy, J., "OSPF Version 2", STD 54, RFC 2328,
              DOI 10.17487/RFC2328, April 1998,

   [RFC5340]  Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
              for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008,

   [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, <>.

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   [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,

   [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, <>.

   [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, <>.

9.2.  Informative References

              Wang, A. and H. Chen, "BGP-LS Extension for Inter-AS
              Topology Retrieval", draft-wang-idr-bgpls-inter-as-
              topology-ext-02 (work in progress), August 2018.

Appendix A.  Inter-Area Topology Retrieval Process

   When an IP SDN Controller receives this information, it should
   compare the prefix NLRI that included in the BGP-LS packet.  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 different source routers in non-backbone area.  Belows is
   one example that based on the Fig.1:

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

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

   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 running
       BGP-LS protocol with IP SDN Controller.  The controller then
       knows the prefixes of N1 is from S1.

   d.  Router S2 will do the similar process, and the controller will
       also learn that prefixes N1 is also from S2.

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

   Iterating the above process continuously, an 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 link between 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 and is not unique.

   The Appendix A heuristic to rebuild the topology can normally be used
   if all links are numbered and the anycast prefixes correspond to
   loopbacks and have a host prefix length, i.e., 32 for IPv4 prefixes
   and 128 for IPv6 prefixes.

Authors' Addresses

   Aijun Wang
   China Telecom
   Beiqijia Town, Changping District
   Beijing  102209


   Acee Lindem
   Cisco Systems
   301 Midenhall Way
   Cary, NC  27513


   Jie Dong
   Huawei Technologies


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


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


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