OSPF Reverse Metric

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Link State Routing                                         K. Talaulikar
Internet-Draft                                                 P. Psenak
Intended status: Standards Track                     Cisco Systems, Inc.
Expires: March 6, 2019                                       H. Johnston
                                                               AT&T Labs
                                                       September 2, 2018

                          OSPF Reverse Metric


   This document specifies the extensions to OSPF that enables a router
   to signal to its neighbor the metric that the neighbor should use
   towards itself using link-local advertisement between them.  The
   signalling of this reverse metric, to be used on link(s) towards
   itself, allows a router to influence the amount of traffic flowing
   towards itself and in certain use-cases enables routers to maintain
   symmetric metric on both sides of a link between them.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

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
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   This Internet-Draft will expire on March 6, 2019.

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Copyright Notice

   Copyright (c) 2018 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
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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.  Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Symmetrical Metric Based on Reference Bandwidth . . . . .   3
     2.2.  Adaptive Metric Signaling . . . . . . . . . . . . . . . .   4
   3.  Solution  . . . . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  LLS Reverse Metric TLV  . . . . . . . . . . . . . . . . . . .   6
   5.  Procedures  . . . . . . . . . . . . . . . . . . . . . . . . .   6
   6.  Backward Compatibility  . . . . . . . . . . . . . . . . . . .   8
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   9.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .   8
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   8
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     11.1.  Normative References . . . . . . . . . . . . . . . . . .   8
     11.2.  Informative References . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   Routers running the Open Shortest Path First (OSPFv2) [RFC2328] and
   OSPFv3 [RFC5340] routing protocols originate a Router-LSA (Link State
   Advertisement) that describes all its links to its neighbors and
   includes a metric which indicates its "cost" of reaching the neighbor
   over that link.  Consider two routers R1 and R2 that are connected
   via a link.  The metric for this link in direction R1->R2 is
   configured on R1 and in the direction R2->R1 is configured on R2.
   Thus the configuration on R1 influences the traffic that it forwards
   towards R2 but does not influence the traffic that it may receive
   from R2 on that same link.

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   This document describes certain use-cases where it is desirable for
   R1 to be able to signal what we call as the reverse metric (RM) that
   R2 should use on the link towards R1.  Once R1 signals its reverse
   metric on its link to R2, then R2 advertises this value as its metric
   to R1 in its Router-LSA instead of its locally configured value.
   Once this information is part of the topology then all other routers
   do their computation using this value which results in the desired
   change in traffic distribution that R1 wanted to achieve towards
   itself over the link from R2.

   This document proposes an extension to OSPF link-local signaling
   (LLS) [RFC5613] for signalling the OSPF Reverse Metric using the LLS
   Reverse Metric TLV in Section 4 and describes the related procedures
   in section Section 5.

2.  Use Cases

   This section describes certain use-cases that OSPF reverse metric
   helps to address.  The usage of OSPF reverse metric need not be
   limited to these cases and is intended to be a generic mechanism.

2.1.  Symmetrical Metric Based on Reference Bandwidth

   Certain OSPF implementations and deployments deduce the metric of
   links based on their bandwidth using a reference bandwidth.  The OSPF
   MIB [RFC4750] has ospfReferenceBandwidth that is used by entries in
   the ospfIfMetricTable.  This mechanism is leveraged in deployments
   where the link metrics get lowered or increased as bandwidth capacity
   is removed or added e.g. consider layer-2 links bundled as a layer-3
   interface on which OSPF is enabled.  In the situations where these
   layer-2 links are directly connected to the two routers, the link and
   bandwidth availability is detected and updated on both sides.  This
   allows for schemes where the metric is maintained to be symmetric in
   both directions based on the bandwidth.

   Now consider variation of the same deployment where the links between
   routers are not directly connected and instead are provisioned over a
   layer-2 network consisting of switches or other mechanisms for a
   layer-2 emulation.  In such scenarios, as show in Figure 1, the
   router on one side of the link would not detect when the neighboring
   router has lost one of its layer-2 link and has reduced capacity to
   its layer-2 switch.  Note that the number of links and their
   capacities on the router R0 may not be the same as those on R1, R2
   and R3.  The left hand side diagram shows the actual physical
   topology in terms of the layer-2 links while the right hand side
   diagram shows the logical layer-3 link topology between the routers.

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                |   R0   |
                | Router |
                +--------+                       +--------+
    (a) Physical   ^ ^ ^           (b) Layer-3   |   R0   |
        Topology   | | |              Topology   +--------+
                   v v v                           ^ ^ ^
             +----------------+                    | | |
             | Layer 2 Switch |                    | | |
             |  (Aggregation) |                +---+ | +---+
             +----------------+                |     |     |
              ^^  ^ ^ ^ ^   ^                  v     |     v
              ||  | | | |   |              +------+  |  +------+
         +----+|  | | | |   |              |  R1  |  |  |  R3  |
         | +---+  | | | |   +----+         +------+  |  +------+
         v v      v v v v        v                   v
    +--------+  +--------+  +--------+           +--------+
    |   R1   |  |   R2   |  |   R3   |           |   R2   |
    | Router |  | Router |  | Router |           +--------+
    +-- -----+  +--------+  +--------+

           Figure 1: Routers Interconnected over Layer-2 Network

   In such a scenario, the amount of traffic that can be forwarded in
   bidirectional manner between say R0 and R1 is dictated by the lower
   of the link capacity of R0 and R1 to the layer-2 transport network.
   In this scenario, when one of the link from R1 to the switch goes
   down, it would increase its link metric to R0 from say 20 to 40.
   However, similarly R0 also needs to increase its link metric to R1 as
   well from 20 to 40 as otherwise, the traffic will hit congestion and
   get dropped.

   When R1 has the ability to signal the OSPF reverse metric of 40
   towards itself to R0, then R0 can also update its metric without any
   manual intervention to ensure the correct traffic distribution.
   Consider if some destinations were reachable from R0 via R1
   previously and this automatic metric adjustment now makes some of
   those destinations reachable from R0 via R3.  This allows some
   traffic load on the link R0 to R1 to now flow via R3 to these

2.2.  Adaptive Metric Signaling

   Now consider another deployment scenario where, as show in Figure 2,
   two routers AGGR1 and AGGR2 are connected to a bunch of routers R1
   thru RN that are dual homed to them and aggregating the traffic from
   them towards a core network.  At some point T, AGGR1 loses some of
   its capacity towards the core or is facing some congestion issue

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   towards the core and it needs to reduce the traffic going through it
   and perhaps redirect some of that load via AGGR2 which is not facing
   a similar issue.  Altering its own metric towards Rx routers would
   influence the traffic flowing through it in the direction from core
   to the Rx but not the other way around as desired.

              Core Network
          ^                ^
          |                |
          V                v
     +----------+    +----------+
     |  AGGR1   |    |  AGGR2   |
     +----------+    +----------+
       ^      ^        ^      ^
       |      |        |      |
       |      +-----------+   |
       |               |  |   |
       |      +--------+  |   |
       v      v           v   v
    +-----------+      +-----------+
    |    R1     |      |    RN     |
    |  Router   | ...  |  Router   |
    +-----------+      +-----------+

                Figure 2: Adaptive Metric for Dual Gateways

   In such a scenario, the AGGR1 router could signal an incremental
   value of OSPF reverse metric towards some or all of the Rx routers.
   When the Rx routers apply this signaled reverse metric offset value
   to the original metric on their links towards AGGR1 then the path via
   AGGR2 becomes a better path causing traffic towards the core getting
   diverted away from it.  Note that the reverse metric mechanism allows
   such adaptive metric changes to be applied on the AGGR1 as opposed to
   being provisioning statically on the possibly large number of Rx

3.  Solution

   To address the use-cases described earlier and to allow an OSPF
   router to indicate its reverse metric for a specific point-to-point
   or point-to-multipoint link to its neighbor, this document proposes
   to extend OSPF link-local signaling to advertise the Reverse Metric
   TLV in OSPF Hello packets.  This ensures that the RM signaling is
   scoped ONLY to each specific link individually.  The router continues
   to include the Reverse Metric TLV in its Hello packets on the link as
   long as it needs its neighbor to use that metric value towards
   itself.  Further details of the procedures involve are specified in
   Section 5.

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   The RM signaling specified in this document is not required for
   broadcast or non-broadcast-multi-access (NBMA) links since the same
   objective is achieved there using the OSPF Two-Part Metric mechanism

4.  LLS Reverse Metric TLV

   The Reverse Metric TLV is a new LLS TLV.  It has 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            |
   |              Flags        |O|H|        Reverse Metric         |


      Type: TBD, suggested value 19

      Length: 4 octet

      Flags: Following are defined currently and the rest MUST be set to
      0 and ignored on reception.

      *  H (0x1) : Indicates that neighbor should use value only if
         higher than its current metric value in use

      *  O (0x2) : Indicates that the reverse metric value provided is
         an offset that is to be added to the original metric

      Reverse Metric: The value or offset of reverse metric to be used

5.  Procedures

   When a router needs to signal a RM value that its neigbhor(s) should
   use towards itself, it includes the Reverse Metric TLV in the LLS
   block of its hello messages sent on the link and continues to include
   this TLV for as long as it needs it's neighbor to use this value.
   The mechanisms used to determine the value to be used for the RM is
   specific to the implementation and use-case and is outside the scope
   of this document.  e.g. in the use-case related to symmetric metric
   described in Section 2.1, the RM value may be derived based on the
   router's link's bandwidth with respect to the reference bandwidth.

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   A router receiving a hello packet from its neighbor that contains the
   Reverse Metric TLV on its link SHOULD use the RM value to derive the
   metric for the link in its Router-LSA to the advertising router.
   When the O flag is set, the value in the TLV needs to be added to the
   existing original metric provisioned on the link to derive the new
   metric value to be used.  When the O flag is clear, the value in the
   TLV should be directly used as the metric to be used.  When H flag is
   set and O flag is clear, this is done only when the RM value signaled
   is higher than the provisioned metric value being used already.  This
   mechanism applies only for point-to-point, point-to-multipoint and
   hybrid broadcast point-to-multipoint ( [RFC6845]) links.  For
   broadcast and NBMA links the OSPF Two-Part Metric mechanism [RFC8042]
   should be used in similar use-cases.

   Implementations SHOULD provide a configuration option to enable the
   signaling of RM from a router to its neighbors and MAY provide a
   configuration option to disable the acceptance of the RM from its

   A router stops including the Reverse Metric TLV in its hello messages
   when it needs its neighbors to go back to using their own provisioned
   metric values.  When that happens, a router which had modified its
   metric in response to receiving a Reverse Metric TLV from its
   neighbor should revert back to using its original provisioned metric

   In certain scenarios, it is possible that two or more routers start
   the RM signaling on the same link.  This could create collision
   scenarios.  The following rules MUST be adopted by routers to ensure
   that there is no instability in the network due to churn in their
   metric due to signaling of RM:

   o  The RM value that is signaled by a router to its neighbor MUST NOT
      be derived from the reverse metric being signaled by any of its
      neighbor on any of its links.

   o  The RM value that is signaled by a router MUST NOT be derived from
      its own metric which has been modified on account of a RM signaled
      from any of its neighbors on any of its links.  RM signaling from
      other routers can affect the router's own metric advertised in its
      Router-LSA.  When deriving the RM values that a router signals to
      its neighbors, it should use its "original" local metric values
      not influenced by any RM signaling.

   Based on these rules, a router MUST never start or stop or change its
   RM metric signaling based on the RM metric signaling initiated by
   some other router.  Based on the local configuration policy, each
   router would end up accepting the RM value signaled by its neighbor

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   and there would be no churn of metrics on the link or the network on
   account of RM signaling.

   In certain use-case as described in Section 2.1 when symmetrical
   metrics are desired, the RM signaling can be enabled on routers on
   either ends of a link.  In other use-cases as described in
   Section 2.2 RM signaling may need to be enabled on only router at one
   end of a link.

6.  Backward Compatibility

   The signaling specified in this document happens at a link-local
   level between routers on that link.  A router which does not support
   this specification would ignore the Reverse Metric LLS TLV and take
   no actions to updates its metric in the other LSAs.  As a result, the
   behavior would be the same as before this specification.  Therefore,
   there are no backward compatibility related issues or considerations
   that need to be taken care of when implementing this specification.

7.  IANA Considerations

   This specification updates Link Local Signalling TLV Identifiers

   Following values are requested for allocation:

   o TBD (Suggested value 19) - Reverse Metric TLV

8.  Security Considerations

   Implementations must assure that malformed LLS TLV and Sub-TLV
   permutations do not result in errors which cause hard OSPF failures.

9.  Contributors

   Thanks to Jay Karthik for his contributions on the use-cases related
   to symmetric metric and the review of the solution.

10.  Acknowledgements

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,

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   [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,

   [RFC5613]  Zinin, A., Roy, A., Nguyen, L., Friedman, B., and D.
              Yeung, "OSPF Link-Local Signaling", RFC 5613,
              DOI 10.17487/RFC5613, August 2009,

11.2.  Informative References

   [RFC4750]  Joyal, D., Ed., Galecki, P., Ed., Giacalone, S., Ed.,
              Coltun, R., and F. Baker, "OSPF Version 2 Management
              Information Base", RFC 4750, DOI 10.17487/RFC4750,
              December 2006, <https://www.rfc-editor.org/info/rfc4750>.

   [RFC6845]  Sheth, N., Wang, L., and J. Zhang, "OSPF Hybrid Broadcast
              and Point-to-Multipoint Interface Type", RFC 6845,
              DOI 10.17487/RFC6845, January 2013,

   [RFC8042]  Zhang, Z., Wang, L., and A. Lindem, "OSPF Two-Part
              Metric", RFC 8042, DOI 10.17487/RFC8042, December 2016,

Authors' Addresses

   Ketan Talaulikar
   Cisco Systems, Inc.
   S.No. 154/6, Phase I, Hinjawadi

   Email: ketant@cisco.com

   Peter Psenak
   Cisco Systems, Inc.
   Apollo Business Center
   Mlynske nivy 43
   Bratislava  821 09

   Email: ppsenak@cisco.com

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   Hugh Johnston
   AT&T Labs

   Email: hugh_johnston@labs.att.com

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