LSR Working Group L. Gong
Internet-Draft W. Cheng
Updates: 6987, 8770 (if approved) China Mobile
Intended status: Standards Track C. Lin
Expires: 12 June 2025 New H3C Technologies
A. Lindem
LabN Consulting LLC
R. Chen
ZTE Corporation
9 December 2024
Advertising Unreachable Links in OSPF
draft-ietf-lsr-ospf-ls-link-infinity-02
Abstract
In certain scenarios, it is necessary to advertise unreachable links
in OSPF, which should be explicitly excluded from the related SPF
calculation. This document specifies using LSLinkInfinity(0xffff) to
advertise an OSPF link as unreachable.
Stub Router Advertisement (RFC 6987) defines MaxLinkMetric (0xffff)
to indicate a router-LSA link should not be used for transit traffic.
This document updates RFC 6987 and RFC 8770. When an OSPFv2 router
supports the Unreachable Link support capability defined in this
document, the OSPFv2 stub router MaxLinkMetric(0xffff) MUST be
updated to MaxReachableLinkMetric(0xfffe).
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
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Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on 12 June 2025.
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Copyright Notice
Copyright (c) 2024 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 (https://trustee.ietf.org/
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Please review these documents carefully, as they describe your rights
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provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Case 1: Traffic Engineering . . . . . . . . . . . . . . . 3
2.2. Case 2: Flexible Algorithm . . . . . . . . . . . . . . . 3
3. Solution based on LSLinkInfinity . . . . . . . . . . . . . . 4
4. Backward Compatibility . . . . . . . . . . . . . . . . . . . 5
4.1. LSLinkInfinity Backward Compatibility . . . . . . . . . . 5
4.2. Stub Router Advertisement Backward Compatibility . . . . 6
5. Management Considerations . . . . . . . . . . . . . . . . . . 6
6. Security Considerations . . . . . . . . . . . . . . . . . . . 7
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
8. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 7
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
9.1. Normative References . . . . . . . . . . . . . . . . . . 7
9.2. Informative References . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction
In specific scenarios, there is a requirement to advertise
unreachable links in OSPF, which MUST NOT be considered during the
standard SPF computation. For example, a link may be available for
Traffic Engineering (TE) purposes but not suitable for hop-by-hop
routing. Another example is an OSPF link with dedicated resources
for a network slice included in a Flexible Algorithm (Flex-
Algorithm) but excluded from the default topology.
This document proposes a mechanism to advertise infinity links in
OSPF.
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1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"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
2.1. Case 1: Traffic Engineering
A network topology is shown in Figure 1. There is a link available
for Traffic Engineering between Node A and E. If this link is used
for SPF calculations, best-effort traffic will be routed on the link.
TE Link
---------
/ \
/ \
A------C------E
| | |
| | |
| | |
B------D------F
Figure 1: Network Topology
2.2. Case 2: Flexible Algorithm
A network topology is shown in Figure 2. The links between nodes A,
B, C, and D are to be used exclusively for a flex-algorithm used for
a specific network slice. These links have an Extended
Administrative Group (EAG) [RFC7308] attribute specifying the "red"
color.
******
A------C------E
|* |* |
|* |* | ******: "red" link
|* |* |
B------D------F
******
Figure 2: Network Topology
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Flex-Algorithm 128 is enabled on Nodes A, B, C, and D, with an EAG
rule including "red" and the Metric-Type is designated to be a type
other than the IGP metric. Flex-Algorithm allows OSPF to compute the
paths along the constrained topology. The topology used by Flex-
Algorithm 128 is shown in Figure 3.
A******C
* *
* *
* *
B******D
Figure 3: Topology of Flex-Algorithm 128
Flex-Algorithm 128 is used for routing particular flows, such as
those for a network slice. The "red" links used by Flex-Algorithm
128 are sub-interfaces with dedicated queues for guaranteed
bandwidth. Sub-interfaces in other network slices and default
topology are omitted from the example figure for clarity. So, it is
expected that only the particular flows are routed on these links
using Flex-Algorithm 128. However, these links are also contained in
the default topology computed by the normal SPF calculation, and
these links may also be used for best-effort traffic. Therefore, it
is a problem that the dedicated links for Flex-Algorithm are still
reachable in base SPF calculation.
If the IGP metrics for all the "red" links are advertised as
unreachable, the base topology will be as shown in Figure 4,
excluding all the "red" links. This allows only the network slice
traffic to be routed on the "red" links by Flex-Algorithm 128.
A------C------E
| | |
| | |
| | |
B------D------F
Figure 4: Base SPF Topology Excluding Unreachable Links
3. Solution based on LSLinkInfinity
This document specifies that if the IGP metric of a link is
advertised as LSLinkInfinity (0xffff), it MUST NOT be considered
during the related SPF computation. This applies to both the Flex-
Algorithm SPF and the base SPF as long as LSLinkInfinity is specified
for the IGP metric.
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4. Backward Compatibility
4.1. LSLinkInfinity Backward Compatibility
Prior to this specification, OSPF treated links advertised as
LSLinkInfinity as reachable [RFC2328]. Hence, partial deployment of
this specification may result in routing loops due to inconsistent
interpretation of LSLinkInfinity. For example in the network shown
in Figure 5, link D-F is advertised with
LSLinkInfinity(65535/0xffff). Router A supports LSLinkInfinity as
unreachable, but router B does not. Router A considers link D-F as
reachable, and the shortest path to F is A->B->D->F. Router B
considers link D-F as unreachable, and the shortest path to F is
B->A->C->E->F. As a result, A forwards the packets to B, but B
returns them to A, which results in a routing loop.
40000 40000 Traffic: A->F
A------C------E A considers link D-F as reachable
| | A's shortest path: A->B->D->F
5| |5 B considers link D-F as unreachable
| | B's shortest path: B->A->C->E->F
B------D------F
5 65535
Figure 5: Inconsistent LSLinkInfinity Interpretation Causing Loops
To provide backward compatibility, this document defines that routers
supporting LSLinkInfinity for unreachable links MUST advertise a
Router Information (RI) LSA advertisement of a Router Informational
Capabilities TLV [RFC7770] including the following Router
Informational Capability Bit:
+=====+==========================+
| Bit | Capabilities |
+=====+==========================+
| TBA | Unreachable Link support |
+-----+--------------------------+
Table 1
OSPF Routers MUST NOT treat links with an advertised metric of
LSLinkInfinity as unreachable unless all routers in the OSPF area
have advertised this capability. If all OSPF Routers in the area
have advertised this capability, then links with an advertised metric
of LSLinkInfinity MUST be treated as unreachable. Upon detection of
a change in the number of routers in the area not supporting the
Unreachable Link support capability changes to 0 or from 0 to greater
than 0, all OSPF routers in the area MUST recalculate their routes.
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An IGP metic with LSLinkInfinity indicating a link is unreachable is
applicable to the following TLVs/LSAs:
* The Router-LSA [RFC2328] and [RFC5340]
* The OSPFv2 Extended Link TLV of OSPFv2 Extended Link Opaque LSA
[RFC7684]
* The Router-Link TLV of OSPFv3 E-Router-LSA [RFC8362]
4.2. Stub Router Advertisement Backward Compatibility
Stub Router Advertisement [RFC6987] defines MaxLinkMetric (0xffff) to
indicate a router-LSA link should not be used for transit traffic.
This document updates [RFC6987] and [RFC8770]. When an OSPFv2 router
supports the Unreachable Link support capability defined in this
document, the OSPFv2 stub router MaxLinkMetric(0xffff) MUST be
updated to MaxReachableLinkMetric(0xfffe).
When an OSPFv2 router supports [RFC6987] and the Unreachable Link
support capability defined in this document, it MUST also support
[RFC8770]. When announcing itself as a stub router, it MUST set the
H-bit in the router-LSA and advertise all its non-stub links with a
link cost of MaxReachableLinkMetric (0xfffe). Since MaxLinkMetric
will not be used to indicate a link is unreachable unless all OSPFv2
routers in the area support this specification as specified in
section 3, all routers in the area will also support the H-bit and
the usage of MaxReachableLinkMetric to indicate an OSPF stub router
link should not be used for transit traffic.
An OSPFv3 router can simply use the R-bit [RFC5340] for stub router
advertisement.
5. Management Considerations
Support of the Unreachable Link support capability SHOULD be
configurable.
In some networks, the operator may still want links with maximum
metric(0xffff) to be treated as reachable. For example, when auto-
costing of links is used and there is a mix of low-speed and high-
speed links. In such cases, the updated routers can disable the
Unreachable Link support capability and still treat links with
maximum metric as reachable.
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It is also RECOMMENDED that implementations supporting this document
and auto-costing limit the maximum computed cost to
MaxReachableLinkMetric (0xfffe).
6. Security Considerations
The document does not introduce any new security issues for the OSPF
protocol. The security considerations for [RFC2328],[RFC5340],
[RFC6987], and [RFC7770] are applicable to protocol extension.
7. IANA Considerations
This document defines a new bit in the registry "OSPF Router
Functional Capability Bits":
+============+==========================+===============+
| Bit Number | Capability Name | Reference |
+============+==========================+===============+
| 0(TBD) | Unreachable Link support | This document |
+------------+--------------------------+---------------+
Table 2
8. Contributors
The following individuals have contributed to this document:
Mengxiao Chen
New H3C Technologies
China
Email: chen.mengxiao@h3c.com
Yanrong Liang
Ruijie Networks Co., Ltd.
China
Email: liangyanrong@ruijie.com.cn
9. References
9.1. 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,
<https://www.rfc-editor.org/info/rfc2119>.
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[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328,
DOI 10.17487/RFC2328, April 1998,
<https://www.rfc-editor.org/info/rfc2328>.
[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, <https://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, <https://www.rfc-editor.org/info/rfc7770>.
[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>.
[RFC8362] Lindem, A., Roy, A., Goethals, D., Reddy Vallem, V., and
F. Baker, "OSPFv3 Link State Advertisement (LSA)
Extensibility", RFC 8362, DOI 10.17487/RFC8362, April
2018, <https://www.rfc-editor.org/info/rfc8362>.
9.2. Informative References
[RFC5340] Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008,
<https://www.rfc-editor.org/info/rfc5340>.
[RFC6987] Retana, A., Nguyen, L., Zinin, A., White, R., and D.
McPherson, "OSPF Stub Router Advertisement", RFC 6987,
DOI 10.17487/RFC6987, September 2013,
<https://www.rfc-editor.org/info/rfc6987>.
[RFC7308] Osborne, E., "Extended Administrative Groups in MPLS
Traffic Engineering (MPLS-TE)", RFC 7308,
DOI 10.17487/RFC7308, July 2014,
<https://www.rfc-editor.org/info/rfc7308>.
[RFC8770] Patel, K., Pillay-Esnault, P., Bhardwaj, M., and S.
Bayraktar, "Host Router Support for OSPFv2", RFC 8770,
DOI 10.17487/RFC8770, April 2020,
<https://www.rfc-editor.org/info/rfc8770>.
Authors' Addresses
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Liyan Gong
China Mobile
China
Email: gongliyan@chinamobile.com
Weiqiang Cheng
China Mobile
China
Email: chengweiqiang@chinamobile.com
Changwang Lin
New H3C Technologies
China
Email: linchangwang.04414@h3c.com
Acee Lindem
LabN Consulting LLC
United States of America
Email: acee.ietf@gmail.com
Ran Chen
ZTE Corporation
China
Email: chen.ran@zte.com.cn
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