Link State Routing                                         K. Talaulikar
Internet-Draft                                                 P. Psenak
Intended status: Standards Track                     Cisco Systems, Inc.
Expires: July 3, October 29, 2021                                    H. Johnston
                                                               AT&T Labs
                                                       December 30, 2020
                                                          April 27, 2021

                          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
   signaling of this reverse metric, to be used on link(s) the links 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.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   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.  LLS Reverse TE Metric TLV . . . . . . . . . . . . . . . . . .   6
   6.  Procedures  . . . . . . . . . . . . . . . . . . . . . . . . .   7
   7.  Backward Compatibility  . . . . . . . . . . . . . . . . . . .   9
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   10. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  10
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  10
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  10
     12.2.  Informative References . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

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 that 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.

   This document describes certain use-cases where it is desirable for a router to be able is required
   to signal what we call as the "reverse metric" (RM) to its neighbor to
   adjust the routing metric on in the inbound direction.  When 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 the 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 signaling the OSPF reverse metric using the LLS
   Reverse Metric TLV in Section 4, the reverse Traffic Engineering (TE)
   metric [RFC3630] using the LLS Reverse TE Metric TLV in Section 5 and
   describes the related procedures in section Section 6.

1.1.  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.

2.  Use Cases

   This section describes certain use-cases that OSPF reverse metric
   helps to address.  The usage of the 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 are 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 a 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 shown 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 links 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 left-hand side diagram shows the actual
   physical topology in terms of the layer-2 links while the right hand right-hand
   side diagram shows the logical layer-3 link topology between the

                |   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 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 can signal the OSPF reverse metric of 40 towards itself to
   R0, then R0 can also update updates 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 of the traffic load on
   the link R0 to R1 to now flow via R3 to these destinations.

2.2.  Adaptive Metric Signaling

   Now consider another deployment scenario where, as show shown in Figure 2,
   two routers AGGR1 and AGGR2 are connected to a bunch of routers R1
   thru RN that are dual homed 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
   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 the
   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 6.

   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            |
   |     MTID      | Flags     |O|H|        Reverse Metric         |


      Type: TBD, suggested value 19

      Length: 4 octet

      MTID : the multi-topology identifier of the link ([RFC4915])

      Flags: 1 octet, 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: 2 octets, the value or offset of reverse metric to
      be used

5.  LLS Reverse TE Metric TLV

   The Reverse TE Metric TLV is a new LLS TLV.  It has the following

    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|                 RESERVED                      |
   |                     Reverse TE Metric                         |


      Type: TBD, suggested value 20

      Length: 4 octet

      Flags: 1 octet, 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 TE metric value in use

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

      RESERVED: 24-bit field.  SHOULD be set to 0 on transmission and
      MUST be ignored on receipt.

      Reverse TE Metric: 4 octets, the value or offset of reverse
      traffic engineering metric to be used

6.  Procedures

   When a router needs to signal a an RM value that its neigbhor(s) neighbor(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 its 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.

   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 the H
   flag is set and the 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 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 this happens, a router which that 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 may 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 an 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
   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 on the router at
   one end of a link.

   When using multi-topology routing with OSPF [RFC4915] a router MAY
   include multiple instances of the Reverse Metric TLV in the LLS block
   of its hello message - one for each of the topology for which it
   desires to signal the reserve metric for.

   In certain scenarios, the OSPF router may also require the
   modification of the TE metric being advertised by its neighbor router
   towards itself in the inbound direction.  The Reverse TE Metric TLV,
   using similar procedures as described above, MAY be used to signal
   the reverse TE metric by a router.  The neighbor SHOULD use the
   reverse TE metric value to derive the TE metric to be used in the TE
   Metric sub-TLV of the Link TLV in its TE Opaque LSA [RFC3630].

7.  Backward Compatibility

   The signaling specified in this document happens at a link-local
   level between routers on that link.  A router which that does not support
   this specification would ignore the Reverse Metric and Reverse TE
   Metric LLS TLVs 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.

8.  IANA Considerations

   This specification updates Link Local Signalling TLV Identifiers

   Following values are requested for have been assigned via early allocation:

   o TBD (Suggested value 19) 19 - Reverse Metric TLV

   o TBD (Suggested value 20) 20 - Reverse TE Metric TLV

9.  Security Considerations

   The security considerations for "OSPF Link-Local Signaling" [RFC5613]
   also apply to the extension described in this document.  The usage of
   the reverse metric TLVs is to alter the metrics used by routers on
   the link and influence the flow and routing of traffic over the
   network.  Hence, modification of the Reverse Metric and Reverse TE
   Metric TLVs may result in misrouting of traffic.  If authentication
   is being used in the OSPF routing domain [RFC5709][RFC7474], then the
   Cryptographic Authentication TLV [RFC5613] SHOULD also be used to
   protect the contents of the LLS block.

   Receiving a malformed LLS Reverse Metric or Reverse TE Metric TLVs
   MUST NOT result in a hard router or OSPF process failure.  The
   reception of malformed LLS TLVs or sub-TLVs SHOULD be logged, but
   such logging MUST be rate- limited rate-limited to prevent denial-of-service (DoS)

10.  Contributors

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

11.  Acknowledgements

12.  References

12.1.  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,

   [RFC3630]  Katz, D., Kompella, K., and D. Yeung, "Traffic Engineering
              (TE) Extensions to OSPF Version 2", RFC 3630,
              DOI 10.17487/RFC3630, September 2003,

   [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,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

12.2.  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, <>.

   [RFC4915]  Psenak, P., Mirtorabi, S., Roy, A., Nguyen, L., and P.
              Pillay-Esnault, "Multi-Topology (MT) Routing in OSPF",
              RFC 4915, DOI 10.17487/RFC4915, June 2007,

   [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, <>.

   [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,

   [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,

   [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.


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

   Hugh Johnston
   AT&T Labs