RFC9339: OSPF Reverse Metric

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Related keywords:  (IGP) (OSPF)

Internet Engineering Task Force (IETF)                K. Talaulikar, Ed.
Request for Comments: 9339                                     P. Psenak
Category: Standards Track                            Cisco Systems, Inc.
ISSN: 2070-1721                                              H. Johnston
                                                               AT&T Labs
                                                           December 2022

                          OSPF Reverse Metric


   This document specifies the extensions to OSPF that enable a router
   to use Link-Local Signaling (LLS) to signal the metric that receiving
   OSPF neighbor(s) should use for a link to the signaling router.  When
   used on the link to the signaling router, the signaling of this
   reverse metric (RM) allows a router to influence the amount of
   traffic flowing towards itself.  In certain use cases, this enables
   routers to maintain symmetric metrics on both sides of a link between

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

Copyright Notice

   Copyright (c) 2022 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   Trust Legal Provisions and are provided without warranty as described
   in the Revised BSD License.

Table of Contents

   1.  Introduction
     1.1.  Requirements Language
   2.  Use Cases
     2.1.  Link Maintenance
     2.2.  Adaptive Metric Signaling
   3.  Solution
   4.  LLS Reverse Metric TLV
   5.  LLS Reverse TE Metric TLV
   6.  Procedures
   7.  Operational Guidelines
   8.  Backward Compatibility
   9.  IANA Considerations
   10. Security Considerations
   11. References
     11.1.  Normative References
     11.2.  Informative References
   Authors' Addresses

1.  Introduction

   A router running the OSPFv2 [RFC2328] or OSPFv3 [RFC5340] routing
   protocols originates a Router-LSA (Link State Advertisement) that
   describes all its links to its neighbors and includes a metric that
   indicates its "cost" to reach the neighbor over that link.  Consider
   two routers, R1 and R2, that are connected via a link.  In the
   direction R1->R2, the metric for this link is configured on R1.  In
   the direction R2->R1, the metric for this link 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 a router is required
   to signal what we call the "reverse metric" (RM) to its neighbor to
   adjust the routing metric in the inbound direction.  When R1 signals
   its RM on its link to R2, 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, all other routers do their
   computation using this value.  This may result in the desired change
   in the traffic distribution that R1 wanted to achieve towards itself
   over the link from R2.

   This document describes extensions to OSPF LLS [RFC5613] to signal
   OSPF RMs.  Section 4 specifies the LLS Reverse Metric TLV and
   Section 5 specifies the LLS Reverse TE Metric TLV.  The related
   procedures are specified in 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 are addressed by the
   OSPF RM.  The usage of the OSPF RM need not be limited to these
   cases; it is intended to be a generic mechanism.

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

              Figure 1: Reference Dual Hub-and-Spoke Topology

   Consider a deployment scenario, such as the one shown in Figure 1,
   where routers R1 through RN are dual-home connected to AGGR1 and
   AGGR2 that are aggregating their traffic towards a core network.

2.1.  Link Maintenance

   Before network maintenance events are performed on individual links,
   operators substantially increase (to maximum value) the OSPF metric
   simultaneously on both routers attached to the same link.  In doing
   so, the routers generate new Router LSAs that are flooded throughout
   the network and cause all routers to shift traffic onto alternate
   paths (where available) with limited disruption (depending on the
   network topology) to in-flight communications by applications or end
   users.  When performed successfully, this allows the operator to
   perform disruptive augmentation, fault diagnosis, or repairs on a
   link in a production network.

   In deployments such as a hub-and-spoke topology (as shown in
   Figure 1), it is quite common to have routers with several hundred
   interfaces and individual interfaces that move anywhere from several
   hundred gigabits to terabits per second of traffic.  The challenge in
   such conditions is that the operator must accurately identify the
   same point-to-point (P2P) link on two separate devices to increase
   (and afterward decrease) the OSPF metric appropriately and to do so
   in a coordinated manner.  When considering maintenance for PE-CE
   links when many Customer Edge (CE) routers connect to a Provider Edge
   (PE) router, an additional challenge related to coordinating access
   to the CE routers may arise when the CEs are not managed by the

   The OSPF RM mechanism helps address these challenges.  The operator
   can set the link on one of the routers (generally the hub, like AGGR1
   or a PE) to a "maintenance mode".  This causes the router to
   advertise the maximum metric for that link and to signal its neighbor
   on the same link to advertise maximum metric via the reverse metric
   signaling mechanism.  Once the link maintenance is completed and the
   "maintenance mode" is turned off, the router returns to using its
   provisioned metric for the link and stops the signaling of RM on that
   link, resulting in its neighbor also reverting to its provisioned
   metric for that link.

2.2.  Adaptive Metric Signaling

   In Figure 1, consider that at some point in time (T), AGGR1 loses
   some of its capacity towards the core.  This may result in a
   congestion issue towards the core on AGGR1 that it needs to mitigate
   by redirecting some of its traffic load to transit via AGGR2, which
   is not experiencing a similar issue.  Altering its link metric
   towards the R1-RN routers would influence the traffic from the core
   towards R1-RN, but not the other way around as desired.

   In such a scenario, the AGGR1 router could signal an incremental OSPF
   RM to some or all the R1-RN routers.  When the R1-RN routers add this
   signaled RM offset to the provisioned metric on their links towards
   AGGR1, the path via AGGR2 becomes a better path.  This causes traffic
   towards the core to be diverted away from AGGR1.  Note that the RM
   mechanism allows such adaptive metric changes to be applied on the
   AGGR1 as opposed to being provisioned on a possibly large number of
   R1-RN routers.

   The RM mechanism may be similarly applied between spine and leaf
   nodes in a Clos network [CLOS] topology deployment.

3.  Solution

   To address the use cases described earlier and to allow an OSPF
   router to indicate its RM for a specific link to its neighbor(s),
   this document proposes to extend OSPF link-local signaling to signal
   the Reverse Metric TLV in OSPF Hello packets.  This ensures that the
   RM signaling is scoped only to a specific link.  The router continues
   to include the Reverse Metric TLV in its Hello packets on the link
   for as long as it needs its neighbor to use that metric value towards
   itself.  Further details of the procedures involved are specified in
   Section 6.

   The RM mechanism specified in this document applies only to P2P,
   Point-to-Multipoint (P2MP), and hybrid-broadcast-P2MP ([RFC6845])
   links.  It is not applicable for broadcast or Non-Broadcast Multi-
   Access (NBMA) links since the same objective is achieved there using
   the OSPF Two-Part Metric mechanism [RFC8042] for OSPFv2.  The OSPFv3
   solution for broadcast or NBMA links is outside the scope of this

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         |

                        Figure 2: Reverse Metric TLV


   Type:  19

   Length:  4 octets

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

   Flags:  1 octet.  The following flags are defined currently and the
      rest MUST be set to 0 on transmission and ignored on reception:

      H (0x1):  Indicates that the neighbor should use the value only if
         it is higher than its provisioned metric value for the link.

      O (0x2):  Indicates that the RM value provided is an offset that
         is to be added to the provisioned metric.

   Reverse Metric:  Unsigned integer of 2 octets that carries the value
      or offset of RM to replace or be added to the provisioned link

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                         |

                      Figure 3: Reverse TE Metric TLV


   Type:  20

   Length:  4 octets

   Flags:  1 octet.  The following flags are defined currently and the
      rest MUST be set to 0 on transmission and ignored on reception:

      H (0x1):  Indicates that the neighbor should use the value only if
         it is higher than its provisioned TE metric value for the link.

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

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

   Reverse TE Metric:  Unsigned integer of 4 octets that carries the
      value or offset of reverse traffic engineering metric to replace
      or to be added to the provisioned TE metric of the link.

6.  Procedures

   When a router needs to signal an RM value that its neighbor(s) should
   use for a link towards the router, it includes the Reverse Metric TLV
   in the LLS block of its Hello packets sent on that link and continues
   to include this TLV for as long as the router needs 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.  For example, the RM value may
   be derived based on the router's link bandwidth with respect to a
   reference bandwidth.

   A router receiving a Hello packet from its neighbor that contains the
   Reverse Metric TLV on a link MUST use the RM value to derive the
   metric for the link to the advertising router in its Router-LSA when
   the RM feature is enabled (refer to Section 7 for details on
   enablement of RM).  When the O flag is set, the metric value to be
   advertised is derived by adding the value in the TLV to the
   provisioned metric for the link.  The metric value 0xffff (maximum
   interface cost) is advertised when the sum exceeds the maximum
   interface cost.  When the O flag is clear, the metric value to be
   advertised is copied directly from the value in the TLV.  When the H
   flag is set and the O flag is clear, the metric value to be
   advertised is copied directly from the value in the TLV only when the
   RM value signaled is higher than the provisioned metric for the link.
   The H and O flags are mutually exclusive; the H flag is ignored when
   the O flag is set.

   A router stops including the Reverse Metric TLV in its Hello packets
   when it needs its neighbors to go back to using their own provisioned
   metric values.  When this happens, a router that has modified its
   metric in response to receiving a Reverse Metric TLV from its
   neighbor MUST revert to using its provisioned metric value.

   In certain scenarios, two or more routers may start the RM signaling
   on the same link.  This could create collision scenarios.  The
   following guidelines are RECOMMENDED for adoption to ensure that
   there is no instability in the network due to churn in their metric
   caused by the signaling of RM:

   *  The RM value that is signaled by a router to its neighbor should
      not be derived from the RM being signaled by any of its neighbors
      on any of its links.

   *  The RM value that is signaled by a router to its neighbor should
      not be derived from the RM being signaled by any of its neighbors
      on any of its links.  RM signaling from other routers can affect
      the router's metric advertised in its Router-LSA.  When deriving
      the RM values that a router signals to its neighbors, it should
      use its provisioned local metric values not influenced by any RM

   Based on these guidelines, a router would not start, stop, or change
   its RM signaling based on the RM signaling initiated by some other
   routers.  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 cases when symmetrical metrics are desired (e.g., when
   metrics are derived based on link bandwidth), the RM signaling can be
   enabled on routers on either end of a link.  In other use cases (as
   described in Section 2.1), RM signaling may need to be enabled 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 packet (one for each of the topologies for which it
   desires to signal the RM).  A router MUST NOT include more than one
   instance of this TLV per MTID.  If more than a single instance of
   this TLV per MTID is present, the receiving router MUST only use the
   value from the first instance and ignore the others.

   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.  Using similar procedures to
   those described above, the Reverse TE Metric TLV MAY be used to
   signal the reverse TE metric for router links.  The neighbor MUST use
   the reverse TE metric value to derive the TE metric advertised in the
   TE Metric sub-TLV of the Link TLV in its TE Opaque LSA [RFC3630] when
   the reverse metric feature is enabled (refer Section 7 for details on
   enablement of RM).  The rules for doing so are analogous to those
   given above for the Router-LSA.

7.  Operational Guidelines

   The signaled RM does not alter the OSPF metric parameters stored in a
   receiving router's persistent provisioning database.

   Routers that receive an RM advertisement SHOULD log an event to
   notify system administration.  This will assist in rapidly
   identifying the node in the network that is advertising an OSPF
   metric or TE metric different from what is configured locally on the

   When the link TE metric is raised to the maximum value, either due to
   the RM mechanism or by explicit user configuration, this SHOULD
   immediately trigger the CSPF (Constrained Shortest Path First)
   recalculation to move the TE traffic away from that link.

   An implementation MUST NOT signal RM to neighbors by default and MUST
   provide a configuration option to enable the signaling of RM on
   specific links.  An implementation MUST NOT accept the RM from its
   neighbors by default.  An implementation MAY provide configuration to
   accept the RM globally on the device, or per area, but an
   implementation MUST support configuration to enable/disable
   acceptance of the RM from neighbors on specific links.  This is to
   safeguard against inadvertent use of RM.

   For the use case in Section 2.1, it is RECOMMENDED that the network
   operator limit the period of enablement of the reverse metric
   mechanism to be only the duration of a network maintenance window.

   [RFC9129] specifies the base OSPF YANG data model.  The required
   configuration and operational elements for this feature are expected
   to be introduced as an augmentation to this base OSPF YANG data

8.  Backward Compatibility

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

9.  IANA Considerations

   IANA has registered code points from the "Link Local Signalling TLV
   Identifiers (LLS Types)" registry in the "Open Shortest Path First
   (OSPF) Link Local Signalling (LLS) - Type/Length/Value Identifiers
   (TLV)" registry group for the TLVs introduced in this document as

   *  19 - Reverse Metric TLV

   *  20 - Reverse TE Metric TLV

10.  Security Considerations

   The security considerations for "OSPF Link-Local Signaling" [RFC5613]
   also apply to the extension described in this document.  The purpose
   of using 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 traffic being misrouted.  If
   authentication is being used in the OSPFv2 routing domain
   [RFC5709][RFC7474], then the Cryptographic Authentication TLV
   [RFC5613] MUST also be used to protect the contents of the LLS block.

   A router that is misbehaving or misconfigured may end up signaling
   varying values of RMs or toggle the state of RM.  This can result in
   a neighbor router having to frequently update its Router LSA, causing
   network churn and instability despite existing OSPF protocol
   mechanisms (e.g., MinLSInterval, and [RFC8405]).  It is RECOMMENDED
   that implementations support the detection of frequent changes in RM
   signaling and ignore the RM (i.e., revert to using their provisioned
   metric value) during such conditions.

   The reception of malformed LLS TLVs or sub-TLVs SHOULD be logged, but
   such logging MUST be rate-limited to prevent Denial of Service (DoS)

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,

   [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, <https://www.rfc-editor.org/info/rfc8174>.

11.2.  Informative References

   [CLOS]     Clos, C., "A study of non-blocking switching networks",
              The Bell System Technical Journal, Vol. 32, Issue 2,
              DOI 10.1002/j.1538-7305.1953.tb01433.x, March 1953,

   [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, <https://www.rfc-editor.org/info/rfc5709>.

   [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,

   [RFC8405]  Decraene, B., Litkowski, S., Gredler, H., Lindem, A.,
              Francois, P., and C. Bowers, "Shortest Path First (SPF)
              Back-Off Delay Algorithm for Link-State IGPs", RFC 8405,
              DOI 10.17487/RFC8405, June 2018,

   [RFC8500]  Shen, N., Amante, S., and M. Abrahamsson, "IS-IS Routing
              with Reverse Metric", RFC 8500, DOI 10.17487/RFC8500,
              February 2019, <https://www.rfc-editor.org/info/rfc8500>.

   [RFC9129]  Yeung, D., Qu, Y., Zhang, Z., Chen, I., and A. Lindem,
              "YANG Data Model for the OSPF Protocol", RFC 9129,
              DOI 10.17487/RFC9129, October 2022,


   The authors would like to thank:

   *  Jay Karthik for his contributions to the use cases and the review
      of the solution.

   *  Les Ginsberg, Aijun Wang, Gyan Mishra, Matthew Bocci, Thomas
      Fossati, and Steve Hanna for their review and feedback.

   *  Acee Lindem for a detailed shepherd's review and comments.

   *  John Scudder for his detailed AD review and suggestions for

   The document leverages the concept of RM for IS-IS, its related use
   cases, and applicability aspects from [RFC8500].

Authors' Addresses

   Ketan Talaulikar (editor)
   Cisco Systems, Inc.
   Email: ketant.ietf@gmail.com

   Peter Psenak
   Cisco Systems, Inc.
   Apollo Business Center
   Mlynske nivy 43
   821 09 Bratislava
   Email: ppsenak@cisco.com

   Hugh Johnston
   AT&T Labs
   United States of America
   Email: hugh.johnston@att.com