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Internet Engineering Task Force (IETF)                          Z. ZhangRequest for Comments: 8042                                       L. WangUpdates:2328                                     Juniper Networks, Inc.Category: Standards Track                                      A. LindemISSN: 2070-1721                                            Cisco Systems                                                           December 2016OSPF Two-Part MetricAbstract   This document specifies an optional OSPF protocol extension to   represent router metrics in a multi-access network in two parts: the   metric from the router to the network and the metric from the network   to the router.  For such networks, the router-to-router metric for   OSPF route computation is the sum of the two parts.  This document   updatesRFC 2328.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 inSection 2 of RFC 7841.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc8042.Copyright Notice   Copyright (c) 2016 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 78 and the IETF Trust's Legal   Provisions Relating to IETF Documents   (http://trustee.ietf.org/license-info) 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   the Trust Legal Provisions and are provided without warranty as   described in the Simplified BSD License.Zhang, et al.                Standards Track                    [Page 1]

RFC 8042                  OSPF Two-Part Metric             December 2016Table of Contents1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .21.1.  Requirements Language . . . . . . . . . . . . . . . . . .32.  Proposed Enhancement  . . . . . . . . . . . . . . . . . . . .33.  Specifications  . . . . . . . . . . . . . . . . . . . . . . .43.1.  Router Interface Parameters . . . . . . . . . . . . . . .43.2.  Advertising Network-to-Router Metric in OSPFv2  . . . . .4     3.3.  Advertising Network-to-Router Traffic Engineering (TE)           Metric  . . . . . . . . . . . . . . . . . . . . . . . . .53.4.  Advertising Network-to-Router Metric in OSPFv3  . . . . .53.5.  OSPF Stub Router Behavior . . . . . . . . . . . . . . . .53.6.  SPF Calculation . . . . . . . . . . . . . . . . . . . . .53.7.  Backward Compatibility  . . . . . . . . . . . . . . . . .64.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .65.  Security Considerations . . . . . . . . . . . . . . . . . . .66.  References  . . . . . . . . . . . . . . . . . . . . . . . . .76.1.  Normative References  . . . . . . . . . . . . . . . . . .76.2.  Informative References  . . . . . . . . . . . . . . . . .7   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .8   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .8   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .91.  Introduction   With Open Shortest Path First (OSPF) [RFC2328] [RFC5340]), a Network-   LSA (Link State Advertisement) is advertised to list all routers on a   broadcast network.  Additionally, each router on the broadcast   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 point-to-point links   to all routers on the same network with individual metrics.   Broadcast capability is still used to optimize database   synchronization and adjacency maintenance.   This works well for broadcast networks where the metric between   different pairs of routers are really independent, for example,   Virtual Private LAN Service (VPLS) networks.   With certain types of broadcast networks, further optimization can be   made to reduce the size of Router-LSAs and the number of updates.Zhang, et al.                Standards Track                    [Page 2]

RFC 8042                  OSPF Two-Part Metric             December 2016   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, [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.  In addition, the router in question determines   that its metric to reach all others also changed and it 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 and   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",   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this   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   to it from all other terminals 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.   Instead of using the hybrid interface type described in [RFC6845],   the network is 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   metrics can be calculated.   With the proposed enhancement, the size of the Router-LSA will be   significantly reduced.  In addition, when a router's communication   capability changes, only that router needs to update its Router-LSA.Zhang, et al.                Standards Track                    [Page 3]

RFC 8042                  OSPF Two-Part Metric             December 2016   Note that while the example uses the satellite as the relay point at   the radio level (layer 2), the satellite does not participate in   packet forwarding at layer 3.  In fact, the satellite does not need   to run any layer-3 protocol.  Therefore, for generality, the metric   is abstracted as to/from the "network" rather than specifically to/   from the "satellite".3.  Specifications   The following 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 inSection 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 a 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.  Defaults to "Interface output cost" but      is 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 4.  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-ID    |        0      |          MT Metric            |      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   Multiple such sub-TLVs can exist in a single OSPF Extended Link TLV,   one for each topology [RFC4915].  Each sub-TLV will have a unique   Multi-Topology Identifier (MT-ID) and will adhere to the   advertisement rules defined inSection 3.4 of [RFC4915].  The OSPFZhang, et al.                Standards Track                    [Page 4]

RFC 8042                  OSPF Two-Part Metric             December 2016   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 Traffic Engineering (TE) Metric   A Traffic Engineering Network-to-Router Metric Sub-TLV is defined,   similar to the Traffic Engineering Metric Sub-TLV defined inSection 2.5.5 of [RFC3630].  The only difference is the TLV type,   which is 35.  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 such a Link TLV.3.4.  Advertising Network-to-Router Metric in OSPFv3   Network-to-Router metric advertisement in OSPFv3 Extended Router-LSA   [OSPFV3-EXTENDED-LSA] will be described in a separate document.3.5.  OSPF Stub Router Behavior   When an OSPF router with interfaces to multi-access networks using   two-part metrics is advertising itself as a stub router [RFC6987],   only the Router-to-Network metric in the stub router's OSPF Router-   LSA links for those networks is set to the MaxLinkMetric.  This is   fully backward compatible and will result in the same behavior as   described in [RFC6987].3.6.  SPF Calculation   The first stage of the shortest-path tree calculation is described inSection 16.1 of [RFC2328].  With a 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   follows:   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.   o  OSPFv3 [RFC5340] Shortest Path First (SPF) changes will be      described in a separate document.Zhang, et al.                Standards Track                    [Page 5]

RFC 8042                  OSPF Two-Part Metric             December 20163.7.  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:             Bit       Capabilities             6         Two-Part Metric support   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   IANA has made the following assignments per this document:   o  Two-Part Metric support (6) was added to the "OSPF Router      Informational Capability Bits" registry.   o  Network-to-Router Metric Sub-TLV (4) has been added to the "OSPFv2      Extended Link TLV Sub-TLVs" registry.   o  Network-to-Router TE Metric Sub-TLV (35) has been added to the      "Types for sub-TLVs of TE Link TLV (Value 2)" registry.5.  Security Considerations   This document does not introduce new security risks.  Existing   security considerations in OSPFv2 and OSPFv3 apply.Zhang, et al.                Standards Track                    [Page 6]

RFC 8042                  OSPF Two-Part Metric             December 20166.  References6.1.  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,              <http://www.rfc-editor.org/info/rfc2119>.   [RFC2328]  Moy, J., "OSPF Version 2", STD 54,RFC 2328,              DOI 10.17487/RFC2328, April 1998,              <http://www.rfc-editor.org/info/rfc2328>.   [RFC3630]  Katz, D., Kompella, K., and D. Yeung, "Traffic Engineering              (TE) Extensions to OSPF Version 2",RFC 3630,              DOI 10.17487/RFC3630, September 2003,              <http://www.rfc-editor.org/info/rfc3630>.   [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,              <http://www.rfc-editor.org/info/rfc4915>.   [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,              <http://www.rfc-editor.org/info/rfc5329>.   [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, <http://www.rfc-editor.org/info/rfc7684>.   [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, <http://www.rfc-editor.org/info/rfc7770>.6.2.  Informative References   [OSPFV3-EXTENDED-LSA]              Lindem, A., Mirtorabi, S., and A. Roy, "OSPFv3 LSA              Extendibility", Work in Progress,draft-ietf-ospf-ospfv3-lsa-extend-13.txt, October 2016.   [RFC5340]  Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF              for IPv6",RFC 5340, DOI 10.17487/RFC5340, July 2008,              <http://www.rfc-editor.org/info/rfc5340>.Zhang, et al.                Standards Track                    [Page 7]

RFC 8042                  OSPF Two-Part Metric             December 2016   [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,              <http://www.rfc-editor.org/info/rfc6845>.   [RFC6987]  Retana, A., Nguyen, L., Zinin, A., White, R., and D.              McPherson, "OSPF Stub Router Advertisement",RFC 6987,              DOI 10.17487/RFC6987, September 2013,              <http://www.rfc-editor.org/info/rfc6987>.Acknowledgements   The authors would like to thank Abhay Roy, Hannes Gredler, Peter   Psenak, and Eric Wu for their comments and suggestions.Contributors   David Dubois   General Dynamics C4S   400 John Quincy Adams Road   Taunton, MA 02780   United States of America   Email: dave.dubois@gd-ms.com   Vibhor Julka   Individual Contributor   Email: vjulka1@yahoo.com   Tom McMillan   L3 Communications, Linkabit   9890 Towne Centre Drive   San Diego, CA 92121   United States of America   Email: tom.mcmillan@l-3com.comZhang, et al.                Standards Track                    [Page 8]

RFC 8042                  OSPF Two-Part Metric             December 2016Authors' Addresses   Zhaohui Zhang   Juniper Networks, Inc.   10 Technology Park Drive   Westford, MA 01886   United States of America   Email: zzhang@juniper.net   Lili Wang   Juniper Networks, Inc.   10 Technology Park Drive   Westford, MA 01886   United States of America   Email: liliw@juniper.net   Acee Lindem   Cisco Systems   301 Midenhall Way   Cary, NC 27513   United States of America   Email: acee@cisco.comZhang, et al.                Standards Track                    [Page 9]