Network Working Group L. Gong
Internet Draft W. Cheng
Intended status: Standards Track China Mobile
Expires: April 20, 2024 C. Lin
M. Chen
New H3C Technologies
R. Chen
ZTE Corporation
Y. Liang
Ruijie Networks Co., Ltd.
October 20, 2023
Advertising Unreachable Links in OSPF
draft-gong-lsr-ospf-unreachable-link-02
Abstract
This document proposes the method to advertise unreachable links in
OSPF.
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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 A: Maximum Link Metric................................4
4. Solution B: Unreachable Link Flag..............................6
5. Backward Compatibility.........................................7
6. Security Considerations........................................7
7. IANA Considerations............................................7
8. References.....................................................7
8.1. Normative References......................................7
8.2. Informative References....................................8
Authors' Addresses................................................9
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 SPF computation, unexpected flows of best-effort
service may be steered into it, which is not desired.
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. Node A, B, C and D have an
extra link between each other. These links have EAG attribute of
"red" color.
******
A------C------E
|* |* |
|* |* | ******: "red" link
|* |* |
B------D------F
******
Figure 2: Network Topology
Flex-Algorithm 128 are enabled on Node A, B, C and D, with EAG rule
of including "red". Flex-Algorithm allows IGP to compute the paths
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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 are used to transmit particular flows, such as
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 into
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. So that
only the network slice traffics 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 A: Maximum Link Metric
In OSPF protocol, there are some inconsistencies when a link is
advertised with the maximum link metric (0xffff). [RFC1247]
specifies that, if the cost of the link is 0xffff, the link should
not be used for data traffic. However, if a router performs an
intra-area Dijkstra calculation as specified in [RFC1583] and
higher, it do not treat links with maximum link metric as
unreachable.
[RFC6987] defines the MaxLinkMetric (0xffff) which indicates a
router-LSA link to be unreachable, in order to support stub router
advertisement.
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About the backward compatibility, [RFC6987] states that "Note that
this inconsistency will not lead to routing loops, because if there
are some alternate paths in the network, both types of routers will
agree on using them rather than the path through the stub router. If
the path through the stub router is the only one, the routers of the
first type will not use the stub router for transit (which is the
desired behavior), while the routers of the second type will still
use this path".
However, if the MaxLinkMetric is used for general purposes, 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 Functional Capabilities TLV [RFC7770]
including the following Router Functional 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 MUST recalculate
routes without considering MaxLinkMetric.
In addition, this document extends MaxLinkMetric to be applicable
for the following TLVs/LSAs as well:
o The OSPFv2 Extended Link TLV of OSPFv2 Extended Link Opaque LSA
[RFC7684]
o The Router-Link TLV of OSPFv3 E-Router-LSA [RFC8362]
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4. Solution B: Unreachable Link Flag
A new OSPF Link Flags sub-TLV is defined in OSPF. The format is as
the following:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flags ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
o Type: TBD.
o Length: Variable, dependent on the size of the Flags field. MUST
be a multiple of 4 octets.
o Flags: Following flags are currently defined.
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|U| ~
+-+-+-+-+-+-+-+-+
o U-Flag: Unreachable Link Flag. The associated link MUST be
treated as unreachable during SPF calculation.
The OSPF Link Flags sub-TLV is advertised in the TLVs/sub-TLVs
below:
o OSPFv2 Extended Link TLV of OSPFv2 Extended Link Opaque LSA
[RFC7684]
o Router-Link TLV of OSPFv3 E-Router-LSA [RFC8362]
Due to the change of procedures in the SPF calculation, all routers
in an area must support the changes specified in this section. To
ensure that, if an area is provisioned to support Unreachable Link
Flag, 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:
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Bit Capabilities
TBD Unreachable Link Flag 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 Unreachable Link Flag.
5. Backward Compatibility
Whether using solution A or solution B, all nodes in the same area
must support that feature. To avoid topology inconsistence and
achieve backward compatibility, routers MUST advertise the
corresponding capability as described in Section 3 and Section 4.
Upon detecting the absence of that capability from any router in the
same area, all routers MUST recalculate routes without considering
any unreachable link advertisement.
6. Security Considerations
TBD
7. IANA Considerations
TBD
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.
[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.
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[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.
[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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Authors' Addresses
Liyan Gong
China Mobile
Email: gongliyan@chinamobile.com
Weiqiang Cheng
China Mobile
Email: chengweiqiang@chinamobile.com
Changwang Lin
New H3C Technologies
Email: linchangwang.04414@h3c.com
Mengxiao Chen
New H3C Technologies
Email: chen.mengxiao@h3c.com
Ran Chen
ZTE Corporation
Email: chen.ran@zte.com.cn
Yanrong Liang
Ruijie Networks Co., Ltd.
Email: liangyanrong@ruijie.com.cn
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