OSPF                                                            K. Patel
Internet-Draft                                                    Arrcus
Updates: 6987 (if approved)                            P. Pillay-Esnault
Intended status: Standards Track                          PPE Consulting
Expires: March 16, 2020                                      M. Bhardwaj
                                                            S. Bayraktar
                                                           Cisco Systems
                                                      September 13, 2019

                     Host Router Support for OSPFv2


   The Open Shortest Path First Version 2 (OSPFv2) does not have a
   mechanism for a node to repel transit traffic if it is on the
   shortest path.  This document assigns a new bit (Host-bit) in the
   OSPF Router-LSA bit registry and in the OSPF Router Informational
   Capability Bits Registry that enables a host router to advertise that
   it is a non-transit router.  It also describes the changes needed to
   support the Host-bit in the domain.  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

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on March 16, 2020.

Copyright Notice

   Copyright (c) 2019 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  . . . . . . . . . . . . . . . . . . . . . . . .   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 a Shortest Path First (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 help to repel traffic only 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.

   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.

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   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 reachability
       resolution for its clients

   This document describes the Host-bit (H-Bit)functionality that
   prevents other OSPFv2 routers from using the router for transit
   traffic in OSPFv2 routing domains.  This document defines the Host-
   bit in the OSPFv2 Router Properties Registry and 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

2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "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 that it MUST NOT be used as a transit router (see
   section 4) by other OSPFv2 routers in the area supporting the

   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                             |
       |                              ...                              |

                                  Figure 1

                          Host Bit in Router-LSA

                 0 1 2 3 4 5 6 7

                                 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.  In this mode, the other OSPFv2 routers
   in the area MUST NOT use the host router for transit traffic, but use
   the host router only for its local destinations.

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   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
   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
   advertisement of prefixes from other routers in the area in its
   summary LSAs.  Only IPv4 prefixes associated with its local
   interfaces MUST 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 the host router cannot act as an AS Boundary
   Router (ASBR).  Indeed, 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 permanently 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 that 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 an 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 that 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

   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

   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-

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 the event that a rogue or buggy router advertises
   incorrectly its capability there are two possible cases:

   o  The router does not have the capability but sends 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 (MaxLinkMetric)
      of this router 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 and to be avoided.

   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,

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

   [RFC6987]  Retana, A., Nguyen, L., Zinin, A., White, R., and D.
              McPherson, "OSPF Stub Router Advertisement", RFC 6987,
              DOI 10.17487/RFC6987, September 2013,

   [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

              Raszuk, R., Cassar, C., Aman, E., Decraene, B., and K.
              Wang, "BGP Optimal Route Reflection (BGP-ORR)", draft-
              ietf-idr-bgp-optimal-route-reflection-19 (work in
              progress), July 2019.

Authors' Addresses

   Keyur Patel

   Email: keyur@arrcus.com

   Padma Pillay-Esnault
   PPE Consulting

   Email: padma.ietf@gmail.com

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

   Email: manbhard@cisco.com

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

   Email: serpil@cisco.com

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