Internet DRAFT - draft-gong-lsr-ospf-unreachable-link
draft-gong-lsr-ospf-unreachable-link
Network Working Group L. Gong
Internet Draft W. Cheng
Intended status: Standards Track China Mobile
Expires: August 21, 2024 C. Lin
New H3C Technologies
A. Lindem
LabN Consulting LLC
R. Chen
ZTE Corporation
February 23, 2024
Advertising Unreachable Links in OSPF
draft-gong-lsr-ospf-unreachable-link-05
Abstract
This document proposes the method to advertise links as unreachable
in OSPF. In some scenarios, there are requirements to advertise
unreachable links in OSPF for purposes other than building the
normal Shortest Path Tree.
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Table of Contents
1. Introduction...................................................2
1.1. Requirements Language.....................................3
2. Use Case.......................................................3
2.1. Case 1: Traffic Engineering...............................3
2.2. Case 2: Flexible Algorithm................................3
3. Solution based on MaxLinkMetric................................4
4. Backward Compatibility.........................................5
4.1. Stub Router Advertisement Backward Compatibility..........6
5. Management Considerations......................................6
6. Security Considerations........................................6
7. IANA Considerations............................................6
8. References.....................................................7
8.1. Normative References......................................7
8.2. Informative References....................................7
Contributors......................................................8
Authors' Addresses................................................8
1. Introduction
In some scenarios, there are requirements to advertise unreachable
links in OSPF for purposes other than building the normal Shortest
Path Tree. One example is a link that is available for Traffic
Engineering (TE), but not for hop-by-hop routing. Another example is
that specific links with dedicated resources for network slicing are
included in Flexible Algorithm (Flex-Algorithm), but should be
excluded in the default topology.
This document proposes the method to advertise unreachable 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 Case
2.1. Case 1: Traffic Engineering
A network topology is shown in Figure 1. There is a link only
available for Traffic Engineering between Node A and E. If that link
is reachable in the SPF computation, undesired flows of best-effort
traffic service may utilize 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. Nodes A, B, C, and D have
an extra link between each other. 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
Flex-Algorithm 128 is enabled on Nodes A, B, C, and D, with an EAG
rule of including "red". Flex-Algorithm allows an IGP to compute the
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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 to transmit particular flows, such as for
a network slice. The "red" links used by Flex-Algorithm 128 are sub-
interfaces with dedicated queues for bandwidth guarantee. So, it is
expected that only the particular flows are transmitted on these
links using Flex-Algorithm 128. However, these links are also
contained in the default topology used by normal SPF calculation,
and unexpected flows of best-effort service may be steered onto
these links. Therefore, it is a problem that the dedicated links for
Flex-Algorithm are still reachable in normal SPF calculation.
If all the "red" links are advertised as unreachable, the default
topology used in normal SPF calculation will be as Figure 4. This
allows only the network slice traffic will be steered into the "red"
links by Flex-Algorithm 128.
A------C------E
| | |
| | |
| | |
B------D------F
Figure 4: SPF Topology after Excluding Unreachable Links
3. Solution based on MaxLinkMetric
This document specifies that if a link is advertised with the
MaxLinkMetric (0xffff), it MUST NOT be considered during the normal
SPF computation.
In OSPF protocol, there are some inconsistencies when a link is
advertised with the MaxLinkMetric (0xffff). [RFC1247] specified
that, if the cost of the link is 0xffff, the link should not be used
for data traffic. However, this was changed in [RFC1583] and
subsequent OSPF versions to not treat links with the cost 0xffff as
unreachable.
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However, such inconsistency may lead to routing loops. For example,
in the network shown as Figure 5, link D-F is advertised with
MaxLinkMetric (65535/0xffff). Router A supports MaxLinkMetric, but
router B does not. Router A sees link D-F as reachable, and the
shortest path to F is A->B->D->F. Router B sees 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
causes routing loops.
40000 40000 Traffic: A->F
A------C------E A sees link D-F as reachable
| | A's shortest path: A->B->D->F
5| |5 B sees link D-F as unreachable
| | B's shortest path: B->A->C->E->F
B------D------F
5 65535
Figure 5: Inconsistency of MaxLinkMetric Causing Loops
To improve backward compatibility, this document defines that all
routers supporting MaxLinkMetric must advertise a Router Information
(RI) LSA with a Router Informational Capabilities TLV [RFC7770]
including the following Router Informational Capability Bit:
Bit Capabilities
TBD MaxLinkMetric support
Upon detecting the presence of a reachable Router-LSA without a
companion RI LSA that has the bit set, all routers in the area MUST
recalculate routes without considering MaxLinkMetric.
MaxLinkMetric is applicable for the following TLVs/LSAs:
o The Router-LSA [RFC2328]
o The OSPFv2 Extended Link TLV of OSPFv2 Extended Link Opaque LSA
[RFC7684]
o The Router-Link TLV of OSPFv3 E-Router-LSA [RFC8362]
4. Backward Compatibility
To avoid topology inconsistency and achieve backward compatibility,
routers MUST advertise the corresponding capability as described in
Section 3.
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Upon detecting the absence of that capability from any router in the
same area, all routers MUST recalculate routes without considering
MaxLinkMetric.
4.1. Stub Router Advertisement Backward Compatibility
Stub Router Advertisement [RFC6987] also uses MaxLinkMetric (0xffff)
to indicate a router-LSA link should not be used for transit
traffic.
When an OSPFv2 router supports [RFC6987] and the MaxLinkMetric
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 MaxLinkMetric - 1 (0xfffe). Since MaxLinkMetric will not be used
to indicate a link is unreachable unless all OSPFv2 routers support
this specification as specified in section 3, all routers will also
support the H-bit and the usage of MaxLinkMetric - 1 to indicate a
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 MaxLinkMetric capability SHOULD be configurable.
In some networks, the operator may still want links with maximum
metric to be treated as reachable. For example, the 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 MaxLinkMetric
capability and still treat links with maximum metric as reachable.
It is also RECOMMENDED that implementations supporting this document
and auto-costing limit the maximum cost to MaxLinkMetric - 1
(0xfffe).
6. Security Considerations
The document does not introduce any new security issues into the
OSPF protocol.
7. IANA Considerations
This document defines a new bit in the registry "OSPF Router
Informational Capability Bits":
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Bit Number Capability Name Reference
-------------------------------------------------------
TBA MaxLinkMetric support This document
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[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, <http://www.rfc-editor.org/info/rfc7770>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, May 2017.
[RFC8362] Lindem, A., Roy, A., Goethals, D., Reddy Vallem, V., and
F. Baker, "OSPFv3 Link State Advertisement (LSA)
Extensibility", DOI 10.17487/RFC8362, RFC 8362, April
2018, <https://www.rfc-editor.org/info/rfc8362>.
8.2. Informative References
[RFC1247] Moy, J., "OSPF Version 2", RFC 1247, July 1991.
[RFC1583] Moy, J., "OSPF Version 2", RFC 1583, March 1994.
[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, <http://www.rfc-
editor.org/info/rfc6987>.
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[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 7308, DOI
10.17487/RFC8770, April 2020, <https://www.rfc-
editor.org/info/rfc8770>.
Contributors
Mengxiao Chen
New H3C Technologies
China
Email: chen.mengxiao@h3c.com
Yanrong Liang
Ruijie Networks Co., Ltd.
China
Email: liangyanrong@ruijie.com.cn
Authors' Addresses
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
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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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