Network Working Group C. Lin
Internet Draft M. Chen
Intended status: Standards Track New H3C Technologies
Expires: September 3, 2024 W. Cheng
L. Gong
China Mobile
March 3, 2024
Advertisement of Dedicated Metric for Flexible Algorithm in IGP
draft-lin-lsr-flex-algo-metric-04
Abstract
This document proposes a method to advertise dedicated metric for
Flex-Algorithm in IGP.
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Table of Contents
1. Introduction...................................................2
1.1. Requirements Language.....................................2
2. Problem Statement..............................................3
3. Dedicated Metric for Flexible Algorithm........................4
4. Security Considerations........................................5
5. IANA Considerations............................................6
6. References.....................................................6
6.1. Normative References......................................6
6.2. Informative References....................................6
Authors' Addresses................................................7
1. Introduction
Flexible Algorithm (Flex-Algorithm) allows IGP to compute
constraint-based paths. [RFC9350] specifies the usage of Flex-
Algorithm in Segment Routing (SR) data planes - SR MPLS and SRv6.
[RFC9350] extends the Flex-Algorithm for native IPv4 and IPv6 data
planes.
However, links shared among multiple algorithms cannot be configured
by different metrics, which may limit the flexibility of Flex-
Algorithm.
This document proposes a method to advertise dedicated metric for
Flex-Algorithm in IGP.
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.
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2. Problem Statement
Flex-Algorithm allows IGP to compute the best paths over a network
based on user-defined constraints and metrics.
We use Figure 1 to illustrate the problem. In this scenario, all
the IGP link metrics are 1.
Flex-Algorithm 128 are enabled on Node A, B, C and D. The topology
used by Flex-Algorithm 128 is shown in Figure 2.
Another Flex-Algorithm 129 is also enabled on Node A, B, C and D.
The metric-type of Flex-Algorithm 129 is the same with Flex-
Algorithm 128. So the topology used by Flex-Algorithm 129 is the
same as Figure 2.
A------C------E
| | |
| | |
| | |
B------D------F
Figure 1
A------C
| |
| |
| |
B------D
Figure 2
Assume that the traffics are from A to D. Since the metrics of A->B-
>D and A->C->D are equal, the traffics are forwarded along both of
the two paths by ECMP load sharing.
The network operator expects to use link A->B->D as the primary path
and link A->C->D as the backup path in Flex-Algorithm 128.
Meanwhile, in Flex-Algorithm 129, the link A->C->D is expected to be
the primary path and the link A->B->D is expected to be the backup
path. So the traffics steered into Flex-Algorithm 128 and the
traffics steered into Flex-Algorithm 129 can be carried by different
paths separately. If a failure occurs on one path, the other path
can still be used as protection.
However, it cannot be satisfied because Flex-Algorithm 128 and 129
use the same metrics in path computation.
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If the metric-type of an algorithm is IGP Metric, the metrics
advertised for links are used in path computation. For example, the
default metric field of TLV-22 in IS-IS (The extended IS
reachability TLV [RFC5305]). So Flex-Algorithm 128 and Flex-
Algorithm 129 share the same link metrics with the default algorithm
of normal SPF calculation.
If the metric-type of an algorithm is a kind of link attribute, such
as Min Unidirectional Link Delay or Traffic Engineering Default
Metric, the link attributes advertised in Application-Specific Link
Attribute (ASLA) [RFC8919] [RFC8920] are used in path computation.
So Flex-Algorithm 128 and Flex-Algorithm 129 also share the link
attributes.
3. Dedicated Metric for Flexible Algorithm
This document defines a new optional metric sub-TLV called the Flex-
Algorithm-associated Generic Metric sub-TLV to advertise dedicated
metric for Flex-Algorithm.
The format of Flex-Algorithm-associated Generic Metric sub-TLV in
IS-IS 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 | Metric-Type | Algorithm |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
o Type: TBD.
o Length: 6.
o Metric-Type: A value from the IGP metric-type registry.
o Algorithm: Associated algorithm from 1 to 255.
o Metric: Metric value from 1 to 16,777,215.
The format of Flex-Algorithm-associated Generic Metric sub-TLV in
OSPF is as the following:
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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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Metric-Type | Algorithm | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
o Type: TBD.
o Length: 8.
o Metric-Type: A value from the IGP metric-type registry.
o Algorithm: Associated algorithm from 1 to 255.
o Metric: Metric value from 1 to 16,777,215.
The Flex-Algorithm-associated Generic Metric sub-TLV is carried in
the Application-Specific Link Attribute (ASLA) defined in [RFC8919]
(IS-IS) and [RFC8920] (OSPF) which is advertised for Flex-Algorithm
application with 'X' Application Identifier Bit. If the Metric-Type
and Algorithm fields is consistent with the FAD of a Flex-Algorithm,
that Flex-Algorithm should use the metric in the new defined sub-TLV
during path calculation.
For example, in IS-IS, if the metric-type of a Flex-Algorithm is 0
(IGP-Metric) and the Flex-Algorithm-associated Generic Metric sub-
TLV of the same metric-type is advertised in ASLA carried by TLV-22
(The extended IS reachability TLV [RFC5305]), the metric in the new
defined sub-TLV, other than the default metric field in TLV-22,
should be used by the associated Flex-Algorithm.
For another example, in OSPFv3, if the metric-type of a Flex-
Algorithm is 2 (Traffic Engineering Default Metric) and the Flex-
Algorithm-associated Generic Metric sub-TLV of the same metric-type
is advertised in ASLA carried by OSPFv3 Router-Link TLV [RFC8362],
the metric in the new defined sub-TLV should be used by the
associated Flex-Algorithm, ignoring the TE Metric sub-tlv.
4. Security Considerations
TBD
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5. IANA Considerations
Flex-Algorithm-associated Generic Metric sub-TLV (TBD)
6. References
6.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, May 2017
[RFC9350] Psenak, P., Ed., Hegde, S., Filsfils, C., Talaulikar, K.,
and A. Gulko, "IGP Flexible Algorithm", RFC 9350, DOI
10.17487/RFC9350, February 2023, <https://www.rfc-
editor.org/info/rfc9350>.
[RFC8919] Ginsberg, L., Psenak, P., Previdi, S., Henderickx, W., and
J. Drake, "IS-IS Application-Specific Link Attributes",
RFC 8919, DOI 10.17487/RFC8919, October 2020,
<https://www.rfc-editor.org/info/rfc8919>.
[RFC8920] Psenak, P., Ed., Ginsberg, L., Henderickx, W., Tantsura,
J., and J. Drake, "OSPF Application-Specific Link
Attributes", RFC 8920, DOI 10.17487/RFC8920, October 2020,
<https://www.rfc-editor.org/info/rfc8920>.
6.2. Informative References
[RFC9350] Psenak, P., Ed., Hegde, S., Filsfils, C., Talaulikar, K.,
and A. Gulko, "IGP Flexible Algorithm", RFC 9350, DOI
10.17487/RFC9350, February 2023, <https://www.rfc-
editor.org/info/rfc9350>.
[RFC5305] Li, T. and H. Smit, "IS-IS Extensions for Traffic
Engineering", RFC 5305, DOI 10.17487/RFC5305, October
2008, <https://www.rfc-editor.org/info/rfc5305>.
[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>.
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Authors' Addresses
Changwang Lin
New H3C Technologies
Email: linchangwang.04414@h3c.com
Mengxiao Chen
New H3C Technologies
Email: chen.mengxiao@h3c.com
Liyan Gong
China Mobile
Email: gongliyan@chinamobile.com
Weiqiang Cheng
China Mobile
Email: chengweiqiang@chinamobile.com
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