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OSPF Two-part Metric

The information below is for an old version of the document.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 8042.
Authors Zhaohui (Jeffrey) Zhang , Lili Wang , Acee Lindem
Last updated 2016-10-13 (Latest revision 2016-08-29)
Replaces draft-zzhang-ospf-two-part-metric
RFC stream Internet Engineering Task Force (IETF)
Additional resources Mailing list discussion
Stream WG state Submitted to IESG for Publication
Revised I-D Needed - Issue raised by AD
Document shepherd Yingzhen Qu
Shepherd write-up Show Last changed 2016-04-29
IESG IESG state Became RFC 8042 (Proposed Standard)
Consensus boilerplate Yes
Telechat date (None)
Responsible AD Alia Atlas
Send notices to "Yingzhen Qu" <>
IANA IANA review state IANA OK - Actions Needed
Network Working Group                                           Z. Zhang
Internet-Draft                                                   L. Wang
Updates: 2328, 5340 (if approved)                 Juniper Networks, Inc.
Intended status: Standards Track                               A. Lindem
Expires: March 2, 2017                                     Cisco Systems
                                                         August 29, 2016

                          OSPF Two-part Metric


   This document specifies an optional extension to the OSPF protocol,
   to represent the metric on a multi-access network as two parts: the
   metric from a router to the network, and the metric from the network
   to the router.  The router to router metric would be the sum of the
   two.  This document updates RFC 2328.

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
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

   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."

   This Internet-Draft will expire on March 2, 2017.

Copyright Notice

   Copyright (c) 2016 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
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of

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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  Proposed Enhancement  . . . . . . . . . . . . . . . . . . . .   3
   3.  Speficications  . . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Router Interface Parameters . . . . . . . . . . . . . . .   4
     3.2.  Advertising Network-to-Router Metric in OSPFv2  . . . . .   4
     3.3.  Advertising Network-to-Router TE Metric . . . . . . . . .   5
     3.4.  Advertising Network-to-Router Metric in OSPFv3  . . . . .   5
     3.5.  OSPF Stub Router Behavior . . . . . . . . . . . . . . . .   5
     3.6.  SPF Calculation . . . . . . . . . . . . . . . . . . . . .   5
     3.7.  Backward Compatibility  . . . . . . . . . . . . . . . . .   6
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .   8
   Appendix B.  Contributors' Addreses . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   With Open Shortest Path First (OSPF, [RFC2328], [RFC5340]) protocol,
   for a broadcast network, a Network-LSA is advertised to list all
   routers on the network, and each router on the network includes a
   link in its Router-LSA to describe its connection to the network.
   The link in the Router-LSA includes a metric but the listed routers
   in the Network LSA do not include a metric.  This is based on the
   assumption that from a particular router, all others on the same
   network can be reached with the same metric.

   With some broadcast networks, different routers can be reached with
   different metrics.  [RFC6845] extends the OSPF protocol with a hybrid
   interface type for that kind of broadcast network, where no Network
   LSA is advertised and Router-LSAs simply include p2p links to all
   routers on the same network with individual metrics.  Broadcast
   capability is still utilized to optimize database synchronization and
   adjacency maintenance.

   That works well for broadcast networks where the metric between
   different pair of routers are really independent.  For example, VPLS

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   With certain types of broadcast networks, further optimization can be
   made to reduce the size of the Router-LSAs and number of updates.

   Consider a satellite radio network with fixed and mobile ground
   terminals.  All communication goes through the satellite.  When the
   mobile terminals move about, their communication capability may
   change.  When OSPF runs over the radio network (routers being or in
   tandem with the terminals), [RFC6845] hybrid interface can be used,
   but with the following drawbacks.

   Consider that one terminal/router moves into an area where its
   communication capability degrades significantly.  Through the radio
   control protocol, all other routers determine that the metric to this
   particular router changed and they all need to update their Router-
   LSAs accordingly.  The router in question also determines that its
   metric to reach all others also changed and it also needs to update
   its Router-LSA.  Consider that there could be many terminals and many
   of them can be moving fast and frequently, the number/frequency of
   updates of those large Router-LSAs could inhibit network scaling.

1.1.  Requirements Language

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

2.  Proposed Enhancement

   Notice that in the above scenario, when one terminal's communication
   capability changes, its metric to all other terminals and the metric
   from all other terminals to it will all change in a similar fashion.
   Given this, the above problem can be easily addressed by breaking the
   metric into two parts: the metric to the satellite and the metric
   from the satellite.  The metric from terminal R1 to R2 would be the
   sum of the metric from R1 to the satellite and the metric from the
   satellite to R2.

   Now instead of using the [RFC6845] hybrid interface type, the network
   is just treated as a regular broadcast network.  A router on the
   network no longer lists individual metrics to each neighbor in its
   Router-LSA.  Instead, each router advertises the metric from the
   network to itself in addition to the normal metric for the network.
   With the normal Router-to-Network and additional Network-to-Router
   metrics advertised for each router, individual router-to-router
   metric can be calculated.

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   With the proposed enhancement, the size of Router-LSA will be
   significantly reduced.  In addition, when a router's communication
   capability changes, only that router needs to update its Router-LSA.

   Note that while the example uses the satellite as the relay point at
   the radio level (layer-2), at layer-3, the satellite does not
   participate in packet forwarding.  In fact, the satellite does not
   need to be running any layer-3 protocol.  Therefore for generality,
   the metric is abstracted as to/from the "network" rather that
   specifically to/from the "satellite".

3.  Speficications

   The following protocol specifications are added to or modified from
   the base OSPF protocol.  If an area contains one or more two-part
   metric networks, then all routers in the area MUST support the
   extensions specified herein.  This is ensured by procedures described
   in Section 3.7.

3.1.  Router Interface Parameters

   The "Router interface parameters" have the following additions:

   o  Two-part metric: TRUE if the interface connects to a multi-access
      network that uses two-part metric.  All routers connected to the
      same network SHOULD have the same configuration for their
      corresponding interfaces.

   o  Interface input cost: Link state metric from the two-part-metric
      network to this router.  Defaulted to "Interface output cost" but
      not valid for normal networks using a single metric.  May be
      configured or dynamically adjusted to a value different from the
      "Interface output cost".

3.2.  Advertising Network-to-Router Metric in OSPFv2

   For OSPFv2, the Network-to-Router metric is encoded in an OSPF
   Extended Link TLV Sub-TLV [RFC7684], defined in this document as the
   Network-to-Router Metric Sub-TLV.  The type of the Sub-TLV is TBD2.
   The length of the Sub-TLV is 4 (for the value part only).  The value
   part of the Sub-TLV is defined as follows:

       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
      |      MT       |        0      |          MT   metric          |

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   Multiple such Sub-TLVs can exist in a single OSPF Extended Link TLV,
   one for each topology [RFC4915].  The OSPF Extended Link TLV
   identifies the transit link to the network, and is part of an OSPFv2
   Extended-Link Opaque LSA.  The Sub-TLV MUST ONLY appear in Extended-
   Link TLVs for Link Type 2 (link to transit network), and MUST be
   ignored if received for other link types.

3.3.  Advertising Network-to-Router TE Metric

   A Traffic Engineering Network-to-Router Metric Sub-TLV is defined,
   similar to the Traffic Engineering Metric Sub-TLV defined in
   Section 2.5.5 of [RFC3630].  The only difference is the TLV type,
   which is TBD3.  The Sub-TLV MUST only appear in type 2 Link TLVs
   (Multi-access) of Traffic Engineer LSAs (OSPF2) or Intra-Area-TE-LSAs
   (OSPFv3) [RFC5329], and MUST appear at most once in one such Link

3.4.  Advertising Network-to-Router Metric in OSPFv3

   Network-to-Router metric advertisement in OSPFv3 Extended-Router-LSA
   [I-D.ietf-ospf-ospfv3-lsa-extend] will be described in a separate

3.5.  OSPF Stub Router Behavior

   When an OSPF router with interfaces including two-part metric is
   advertising itself as a stub router [RFC6987], only the Router-to-
   Network metric in the stub router's OSPF Router-LSA links is set to
   the MaxLinkMetric.  This is fully backward compatible and will result
   in the same behavior as [RFC6987].

3.6.  SPF Calculation

   The first stage of the shortest-path tree calculation is described in
   section 16.1 of [RFC2328].  With two-part metric, when a vertex V
   corresponding to a Network-LSA has just been added to the Shortest
   Path Tree (SPT) and an adjacent vertex W (joined by a link in V's
   corresponding Network-LSA) is being added to the candidate list, the
   cost from V to W (W's network-to-router cost) is determined as

   o  For OSPFv2, if vertex W has a corresponding Extended-Link Opaque
      LSA with an Extended Link TLV for the link from W to V, and the
      Extended Link TLV has a Network-to-Router Metric Sub-TLV for the
      corresponding topology, then the cost from V to W is the metric in
      the Sub-TLV.  Otherwise, the cost is 0.

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   o  OSPFv3 [RFC5340] SPF changes will be described in a separate

3.7.  Backward Compatibility

   Due to the change of procedures in the SPF calculation, all routers
   in an area that includes one or more two-part metric networks must
   support the changes specified in this document.  To ensure that, if
   an area is provisioned to support two-part metric networks, all
   routers supporting this capability must advertise a Router
   Information (RI) LSA with a Router Functional Capabilities TLV
   [RFC7770]  that includes the following Router Functional Capability

             Bit       Capabilities

             TBD1      OSPF Two-part Metric (TPM)

   Upon detecting the presence of a reachable Router-LSA without a
   companion RI LSA that has the bit set, all routers MUST recalculate
   routes without considering any network-to-router costs.

4.  IANA Considerations

   This document requests the following IANA assignments:

   o  A new bit (TBD1) in Registry for OSPF Router Informational
      Capability Bits, to indicate the capability of supporting two-part

   o  A new Sub-TLV type (TBD2) in OSPF Extended Link TLV Sub-TLV
      registry, for the Network-to-Router Metric Sub-TLV.

   o  A new Sub-TLV type (TBD3) in Types for sub-TLVs of TE Link TLV
      (Value 2) registry, for the Network-to-Router TE Metric Sub-TLV.

5.  Security Considerations

   This document does not introduce new security risks.  Existing
   security considerations in OSPFv2 and OSPFv3 apply.

6.  References

6.1.  Normative References

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   [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,

   [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,

   [RFC5329]  Ishiguro, K., Manral, V., Davey, A., and A. Lindem, Ed.,
              "Traffic Engineering Extensions to OSPF Version 3",
              RFC 5329, DOI 10.17487/RFC5329, September 2008,

   [RFC7684]  Psenak, P., Gredler, H., Shakir, R., Henderickx, W.,
              Tantsura, J., and A. Lindem, "OSPFv2 Prefix/Link Attribute
              Advertisement", RFC 7684, DOI 10.17487/RFC7684, November
              2015, <>.

   [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, <>.

6.2.  Informative References

              Lindem, A., Mirtorabi, S., Roy, A., and F. Baker, "OSPFv3
              LSA Extendibility", draft-ietf-ospf-ospfv3-lsa-extend-10
              (work in progress), May 2016.

   [RFC5340]  Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
              for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008,

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   [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,

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

Appendix A.  Acknowledgements

   The authors would like to thank Abhay Roy, Hannes Gredler, Peter
   Psenak and Eric Wu for their comments and suggestions.

   The RFC text was produced using Marshall Rose's xml2rfc tool.

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Appendix B.  Contributors' Addreses

   David Dubois
   General Dynamics C4S
   400 John Quincy Adams Road
   Taunton, MA 02780


   Vibhor Julka


   Tom McMillan
   L3 Communications, Linkabit
   9890 Towne Centre Drive
   San Diego, CA 92121


Authors' Addresses

   Zhaohui Zhang
   Juniper Networks, Inc.
   10 Technology Park Drive
   Westford, MA 01886


   Lili Wang
   Juniper Networks, Inc.
   10 Technology Park Drive
   Westford, MA 01886


   Acee Lindem
   Cisco Systems
   301 Midenhall Way
   Cary, NC 27513


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