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IETF RFC 8042
Last modified on Wednesday, December 28th, 2016
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Internet Engineering Task Force (IETF) Z. Zhang
Request for Comments: 8042 L. Wang
Updates: 2328 Juniper Networks, Inc.
Category: Standards Track A. Lindem
ISSN: 2070-1721 Cisco Systems
December 2016
OSPF Two-Part Metric
 Abstract
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
updates RFC 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 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
http://www.rfc-editor.org/info/RFC 8042.
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
(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 2016
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Proposed Enhancement . . . . . . . . . . . . . . . . . . . . 3
3. Specifications . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Router Interface Parameters . . . . . . . . . . . . . . . 4
3.2. Advertising Network-to-Router Metric in OSPFv2 . . . . . 4
3.3. Advertising Network-to-Router Traffic Engineering (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 . . . . . . . . . . . . . . . . . . . . . . . . . 7
6.1. Normative References . . . . . . . . . . . . . . . . . . 7
6.2. Informative References . . . . . . . . . . . . . . . . . 7
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 8
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
With Open Shortest Path First (OSPF) [RFC 2328] [RFC 5340]), 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. [RFC 6845] 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, [RFC 6845] 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 [RFC 2119].
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 [RFC 6845],
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 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 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 [RFC 7684], 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 [RFC 4915]. Each sub-TLV will have a unique
Multi-Topology Identifier (MT-ID) and will adhere to the
advertisement rules defined in Section 3.4 of [RFC 4915]. The OSPF
Zhang, 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 in
Section 2.5.5 of [RFC 3630]. 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) [RFC 5329], 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 [RFC 6987],
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 [RFC 6987].
3.6. SPF Calculation
The first stage of the shortest-path tree calculation is described in
Section 16.1 of [RFC 2328]. 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 [RFC 5340] 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 2016
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
[RFC 7770] 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 2016
6. References
6.1. Normative References
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC 2119, March 1997,
<http://www.rfc-editor.org/info/RFC 2119>.
[RFC 2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328,
DOI 10.17487/RFC 2328, April 1998,
<http://www.rfc-editor.org/info/RFC 2328>.
[RFC 3630] Katz, D., Kompella, K., and D. Yeung, "Traffic Engineering
(TE) Extensions to OSPF Version 2", RFC 3630,
DOI 10.17487/RFC 3630, September 2003,
<http://www.rfc-editor.org/info/RFC 3630>.
[RFC 4915] Psenak, P., Mirtorabi, S., Roy, A., Nguyen, L., and P.
Pillay-Esnault, "Multi-Topology (MT) Routing in OSPF",
RFC 4915, DOI 10.17487/RFC 4915, June 2007,
<http://www.rfc-editor.org/info/RFC 4915>.
[RFC 5329] Ishiguro, K., Manral, V., Davey, A., and A. Lindem, Ed.,
"Traffic Engineering Extensions to OSPF Version 3",
RFC 5329, DOI 10.17487/RFC 5329, September 2008,
<http://www.rfc-editor.org/info/RFC 5329>.
[RFC 7684] Psenak, P., Gredler, H., Shakir, R., Henderickx, W.,
Tantsura, J., and A. Lindem, "OSPFv2 Prefix/Link Attribute
Advertisement", RFC 7684, DOI 10.17487/RFC 7684, November
2015, <http://www.rfc-editor.org/info/RFC 7684>.
[RFC 7770] 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/RFC 7770,
February 2016, <http://www.rfc-editor.org/info/RFC 7770>.
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.
[RFC 5340] Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
for IPv6", RFC 5340, DOI 10.17487/RFC 5340, July 2008,
<http://www.rfc-editor.org/info/RFC 5340>.
Zhang, et al. Standards Track PAGE 7
RFC 8042 OSPF Two-Part Metric December 2016
[RFC 6845] Sheth, N., Wang, L., and J. Zhang, "OSPF Hybrid Broadcast
and Point-to-Multipoint Interface Type", RFC 6845,
DOI 10.17487/RFC 6845, January 2013,
<http://www.rfc-editor.org/info/RFC 6845>.
[RFC 6987] Retana, A., Nguyen, L., Zinin, A., White, R., and D.
McPherson, "OSPF Stub Router Advertisement", RFC 6987,
DOI 10.17487/RFC 6987, September 2013,
<http://www.rfc-editor.org/info/RFC 6987>.
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.com
Zhang, et al. Standards Track PAGE 8
RFC 8042 OSPF Two-Part Metric December 2016
Authors' 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.com
Zhang, et al. Standards Track PAGE 9
RFC TOTAL SIZE: 18496 bytes
PUBLICATION DATE: Wednesday, December 28th, 2016
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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