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Versions: (draft-keyupate-ospf-ospfv2-hbit) 00 01 02 03 04 05 06 07 08

OSPF                                                            K. Patel
Internet-Draft                                                    Arrcus
Updates: 2328,6987 (if approved)                       P. Pillay-Esnault
Intended status: Standards Track                               Futurewei
Expires: January 9, 2020                                     M. Bhardwaj
                                                            S. Bayraktar
                                                           Cisco Systems
                                                            July 8, 2019


                     Host Router Support for OSPFv2
                     draft-ietf-ospf-ospfv2-hbit-08

Abstract

   The OSPFv2 specifies an SPF algorithm that identifies transit
   vertices based on their adjacencies.  Therefore, OSPFv2 does not have
   a mechanism to prevent traffic transiting a participating node if it
   is a transit vertex in the only existing or shortest path to the
   destination.  The use of metrics to make the node undesirable can
   only help to repel traffic if an alternative better route exists.
   This document defines the Host-bit functionality to prevent other
   OSPFv2 routers from using the router for transit traffic in OSPFv2
   routing domains.  This document updates the Open Shortest Path First
   v2 specification (OSPFv2 rfc2328) by assigning a new bit (Host-bit)
   in the OSPF Router-LSA bit registry.  In addition, if the Host-bit is
   set, the calculation of the shortest-path tree for an area, as
   described in OSPFv2, is modified by including a new check to verify
   that transit vertices have the Host-bit clear.  In addition, this
   document updates OSPF Stub Router Advertisement (rfc6987) to
   advertise for type-2 External and NSSA LSAs with a high cost in order
   to repel traffic effectively.

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 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."




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   This Internet-Draft will expire on January 9, 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
   carefully, as they describe your rights and restrictions with respect
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   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.  Requirements Language . . . . . . . . . . . . . . . . . . . .   3
   3.  Host-bit Support  . . . . . . . . . . . . . . . . . . . . . .   3
   4.  SPF Modifications . . . . . . . . . . . . . . . . . . . . . .   5
   5.  Auto Discovery and Backward Compatibility . . . . . . . . . .   6
   6.  OSPF AS-External-LSAs/NSSA LSAs with Type 2 Metrics . . . . .   7
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   8
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     10.1.  Normative References . . . . . . . . . . . . . . . . . .   9
     10.2.  Informative References . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   The OSPFv2 specifies an SPF algorithm that identifies transit
   vertices based on their adjacencies.  Therefore, OSPFv2 does not have
   a mechanism to prevent traffic transiting a participating node if it
   is a transit vertex in the only existing or shortest path to the
   destination.  The use of metrics to make the node undesirable can
   only help to repel traffic if an alternative better route exists.

   This functionality is particularly useful for a number of use cases:

   1.  To isolate a router to avoid blackhole scenarios when there is a
       reload and possible long reconvergence times.





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   2.  Closet Switches are usually not used for transit traffic but need
       to participate in the topology.

   3.  Overloaded routers could use such a capability to temporarily
       repel traffic until they stabilize.

   4.  BGP Route reflectors known as virtual Route Reflectors (vRRs),
       that are not in the forwarding path but are in central locations
       such as data centers.  Such Route Reflectors typically are used
       for route distribution and are not capable of forwarding transit
       traffic.  However, they need to learn the OSPF topology to
       perform spf computation for optimal routes and reachbility
       resolution for its clients
       [I-D.ietf-idr-bgp-optimal-route-reflection].

   This document defines the Host-bit (H-Bit)functionality to prevent
   other OSPFv2 routers from using the router for transit traffic in
   OSPFv2 routing domains.  This document updates the [RFC2328] by -
   assigning the Host-bit in the OSPFv2 Router Properties Registry - if
   the host-bit is set then the calculation of the shortest-path tree
   for an area, as described in section 16.1 of [RFC2328], is modified
   by including a new check to verify that transit vertices DO NOT have
   the host-bit set.

2.  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.

3.  Host-bit Support

   This document defines a new router-LSA bit known as the Host Bit or
   the H-bit.  An OSPFv2 router advertising a router-LSA with the H-bit
   set indicates to other OSPFv2 routers in the area supporting the
   functionality that it MUST NOT be used as a transit router (see
   section 4).

   If the host-bit is NOT set routers MUST act transit routers as
   described in [RFC2328] ensuring backward compatibility.









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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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |            LS age             |     Options   |       1       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                        Link State ID                          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                     Advertising Router                        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                     LS sequence number                        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |         LS checksum           |             length            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |H|0|0|N|W|V|E|B|        0      |            # links            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                          Link ID                              |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         Link Data                             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |     Type      |     # TOS     |            metric             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                              ...                              |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |      TOS      |        0      |          TOS  metric          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                          Link ID                              |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         Link Data                             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                              ...                              |


                          Host Bit in router-LSA

                 0 1 2 3 4 5 6 7
                 +-+-+-+-+-+-+-+-+
                 |H|0|0|N|W|V|E|B|
                 +-+-+-+-+-+-+-+-+

                                 Host Bit

   Bit H is the high-order bit of the OSPF as shown above.  When set, an
   OSPFv2 router is a Host (non-transit) router and is incapable of
   forwarding transit traffic.

   An OSPFv2 router originating a router-LSA with the H-bit set MUST
   advertise all its router links with a link cost of MaxLinkMetric
   [RFC6987].  This is to increase the applicability of the H-bit to



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   partial deployments where it is the responsibility of the operator to
   ensure that OSPFv2 routers not supporting the H-bit do not install
   routes causing routing loops.

   When the H-bit is set, an Area Border Router (ABR) MUST advertise the
   same H-bit setting in its self-originated router-LSAs for all
   attached areas.  The consistency of the setting will prevent inter-
   area traffic transiting through the router by suppressing the
   suppressing advertisement of prefixes from other routers in the area
   in its summary LSAs.  ONLY IPv4 prefixes associated with its local
   interfaces MAY be advertised in summary LSAs to provide reachability
   to end hosts attached behind a router with the H-bit set.

   When the H-bit is set cannot act as an AS Boundary Router (ASBR), as
   ASBR are transit routers to prefixes that are typically imported
   through redistribution of prefixes of other routing protocols.
   Therefore, non-local IPv4 prefixes, e.g., those exported from other
   routing protocols, MUST NOT be advertised in AS-external-LSAs for
   routers acting permanly as a host.  However, in use cases such as an
   overloaded router or a router being gracefully isolated, these
   routers are only temporarily acting as host routers and therefore
   should continue to advertise their External LSAs but ensure they do
   not attract traffic.  In addition to the procedure described above,
   temporary host routers advertising type 2-metric External LSAs MUST
   set the metrics to LSInfinity to repel traffic.(see Section 6 of this
   document).

4.  SPF Modifications

   The SPF calculation described in section 16.1 [RFC2328] will be
   modified to ensure that the routers originating router-LSAs with the
   H-bit set will not be used for transit traffic.  Step 2 is modified
   as follows:


















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                2) Call the vertex just added to the
                   tree vertex V.  Examine the LSA
                   associated with vertex V.  This is
                   a lookup in the Area A's link state
                   database based on the Vertex ID. If
                   this is a router-LSA, and the H-bit
                   of the router-LSA is set, and
                   vertex V is not the root, then the
                   router should not be used for transit
                   and step (3) should be executed
                   immediately. If this is a router-LSA,
                   and bit V of the router-LSA (see
                   Section A.4.2) is set, set Area A's
                   TransitCapability to TRUE. In any case,
                   each link described by the LSA gives
                   the cost to an adjacent vertex.  For
                   each described link, (say it joins
                   vertex V to vertex W):


5.  Auto Discovery and Backward Compatibility

   To avoid the possibility of any routing loops due to partial
   deployment, this document defines a OSPF Router Information (RI) LSA
   [RFC7770] with and area flooding scope and a new bit assigned in the
   OSPF Router Informational Capability Bits Registry.  Bit:

                  Bit       Capabilities

                  7         Host Router Support capability

   Auto Discovery via announcement of the Host Support Functional
   Capability ensures that the H-bit functionality and its associated
   SPF changes MUST only take effect if all the routers in a given OSPF
   area support this functionality.

   In normal operations, there is no guarantee that the RI LSA will
   reach all routers in an area in a timely manner which may result in
   rooting loops in partial deployments.  For example, in a new router
   joins an area which previous had only H-bit capable routers with
   H-bit set then it may take some time for the RI to propagate to all
   routers.

   The following recommendations will mitigate transient routing loops:

   o  Implementations are RECOMMENDED to provide a configuration
      parameter to manually override enforcement of the H-bit
      functionality in partial deployments where the topology guarantees



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      that OSPFv2 routers not supporting the H-bit do not compute routes
      resulting in routing loops.

   o  All routers, with the H-bit set, MUST advertise all of the
      router's non-local links with a metric equal to MaxLinkMetric in
      its LSAs in order to avoid OSPFv2 (unless last resort) routers not
      supporting the H-bit from attempting to use it for transit
      traffic.

   o  All routers supporting H-Bit MUST check all the RI LSAs of nodes
      in the area before actively running the modified SPF to account
      for the H-bit in order to verify that all routers are in routing
      capability.  If any router does not have the H-Bit support then
      all routers in the areas MUST run the normal SPF.

   o  Any router not supporting the H-bit capability is detected (by
      examination of RI- LSA or RTR LSA in the area database) then all
      routers in the area MUST revert back to normal operations.

6.  OSPF AS-External-LSAs/NSSA LSAs with Type 2 Metrics

   When calculating the path to an OSPF AS-External-LSA or NSSA-LSA with
   a Type-2 metric, the advertised Type-2 metric is taken as more
   significant than the OSPF intra-area or inter-area path.  Hence,
   advertising the links with MaxLinkMetric as specified in [RFC6987]
   does not discourage transit traffic when calculating AS external or
   NSSA routes with Type-2 metrics.

   Consequently, OSPF routers implementing [RFC6987] and required to be
   the last resort transit then they MUST advertise a Type-2 metric of
   LSInfinity-1 for any self-originated type 2 AS-External-LSAs or NSSA-
   LSAs.  However, in situations, the router needs to repel traffic and
   acts as a host router then, in addition of the host bit procedure
   described in this document they MUST advertise a Type-2 metric of
   LSInfinity for any self-originated type 2 AS-External-LSAs or NSSA-
   LSAs.

7.  IANA Considerations

   This document requests the IANA to assign the 0x80 value to the Host-
   Bit (H-bit)in the OSPFv2 Router Properties Registry

       Value          Description                   Reference

       0x80           Host (H-bit)                 This Document






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   This document requests the IANA to assign the Bit Number value of 7
   to the Host Router Support Capability in the OSPF Router
   Informational Capability Bits Registry.  [RFC7770]

       Bit Number    Capability Name               Reference

       7          OSPF Host Router             This Document


8.  Security Considerations

   This document introduces the H-bit which is a capability that
   restricts the use of a router for transit except for its local
   destinations.  This is a subset of the operations of a normal router
   and therefore should not introduce new security considerations beyond
   those already known in OSPF.  The feature however does introduce the
   flooding of a capability information that allows discovery and
   verification that all routers in an area are capable before turning
   on the feature.  In case. a rogue or buggy router advertise
   incorrectly its capability there are two possible cases:

   o  The router does not have the capability but send H-Bit set in its
      LSAs: In this case, there is a possibility of a routing loop.
      However this is mitigated by the fact that this router should be
      avoided anyway.  Moreover, the link metrics cost of this router
      should be MaxLinkMetric and will mitigate this situation.  In any
      case a router advertising the H-bit capability without its links
      cost equal to MaxLinkMetric may be an indicator that this is a
      rogue router.

   o  The router has the capability but sends the H-Bit clear in its
      LSAs: In this case, the router merely prevents support of other
      H-bit routers in the area and all the routers to run the modified
      SPF.  The impact is also mitigated as other H-Bit routers in the
      area also advertise MaxLinkMetric cost so they will still be
      avoided unless they are the last resort path.

9.  Acknowledgements

   The authors would like to acknowledge Hasmit Grover for discovery of
   the limitation in [RFC6987], Acee Lindem, Abhay Roy, David Ward,
   Burjiz Pithawala and Michael Barnes for their comments.

10.  References







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10.1.  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>.

   [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>.

   [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>.

10.2.  Informative References

   [I-D.ietf-idr-bgp-optimal-route-reflection]
              Raszuk, R., Cassar, C., Aman, E., Decraene, B., and K.
              Wang, "BGP Optimal Route Reflection (BGP-ORR)", draft-
              ietf-idr-bgp-optimal-route-reflection-18 (work in
              progress), April 2019.

   [RFC6987]  Retana, A., Nguyen, L., Zinin, A., White, R., and D.
              McPherson, "OSPF Stub Router Advertisement", RFC 6987,
              DOI 10.17487/RFC6987, September 2013,
              <https://www.rfc-editor.org/info/rfc6987>.

Authors' Addresses

   Keyur Patel
   Arrcus

   Email: keyur@arrcus.com


   Padma Pillay-Esnault
   Futurewei
   2330 Central Expressway
   Santa Clara,  CA 95050
   USA

   Email: padma.ietf@gmail.com



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   Manish Bhardwaj
   Cisco Systems
   170 W. Tasman Drive
   San Jose,  CA 95134
   USA

   Email: manbhard@cisco.com


   Serpil Bayraktar
   Cisco Systems
   170 W. Tasman Drive
   San Jose,  CA 95134
   USA

   Email: serpil@cisco.com



































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