rfc9843v2.txt   rfc9843.txt 
Internet Engineering Task Force (IETF) S. Hegde Internet Engineering Task Force (IETF) S. Hegde
Request for Comments: 9843 W. Britto Request for Comments: 9843 W. Britto
Category: Standards Track R. Shetty Updates: 9350 R. Shetty
ISSN: 2070-1721 Juniper Networks Inc. Category: Standards Track Juniper Networks Inc.
B. Decraene ISSN: 2070-1721 B. Decraene
Orange Orange
P. Psenak P. Psenak
Cisco Systems Cisco Systems
T. Li T. Li
Juniper Networks Inc. Juniper Networks Inc.
August 2025 August 2025
IGP Flexible Algorithms: Bandwidth, Delay, Metrics, and Constraints IGP Flexible Algorithms: Bandwidth, Delay, Metrics, and Constraints
Abstract Abstract
Many networks configure the IGP link metric relative to the link Many networks configure the IGP link metric relative to the link
capacity, and high bandwidth traffic gets routed per the link capacity, and high bandwidth traffic gets routed per the link
capacity. Flexible Algorithms provide mechanisms to create capacity. Flexible Algorithms provide mechanisms to create
constraint-based paths in an IGP. This specification documents a constraint-based paths in an IGP. This specification documents a
generic metric-type and a set of bandwidth-related constraints to be generic metric-type and a set of bandwidth-related constraints to be
used in Flexible Algorithms. used in Flexible Algorithms.
This document updates RFC 9350.
Status of This Memo Status of This Memo
This is an Internet Standards Track document. This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has (IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841. Internet Standards is available in Section 2 of RFC 7841.
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In some circumstances, path computation constraints, such as In some circumstances, path computation constraints, such as
administrative groups, can be used to ensure that traffic avoids administrative groups, can be used to ensure that traffic avoids
particular portions of the network. These strict constraints are particular portions of the network. These strict constraints are
appropriate when there is an absolute requirement to avoid parts of appropriate when there is an absolute requirement to avoid parts of
the topology, even in failure conditions. However, if the the topology, even in failure conditions. However, if the
requirement is less strict, then using a high metric in a portion of requirement is less strict, then using a high metric in a portion of
the topology may be more appropriate. the topology may be more appropriate.
This document defines a family of generic metrics that can advertise This document defines a family of generic metrics that can advertise
various types of administratively assigned metrics. This document various types of administratively assigned metrics. This document
proposes standard metric-types that have specific semantics and introduces standard metric-types that have specific semantics and
require standardization. This document also specifies user-defined require standardization. This document also specifies user-defined
metric-types where specifics are not defined so that administrators metric-types where specifics are not defined so that administrators
are free to assign semantics as they see fit. are free to assign semantics as they see fit.
Section 3 defines additional FAD [RFC9350] constraints that allow the Section 3 defines additional FAD [RFC9350] constraints that allow the
network administrator to preclude the use of low bandwidth links or network administrator to preclude the use of low bandwidth links or
high delay links. In Section 4, this document specifies a new high delay links. In Section 4, this document specifies a new
bandwidth-based metric-type to be used with Flex-Algorithm and other bandwidth-based metric-type to be used with Flex-Algorithm and other
applications. applications.
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implementation vary the advertised administrative groups based on implementation vary the advertised administrative groups based on
bandwidth, but this seems unnecessarily complex and expressing this bandwidth, but this seems unnecessarily complex and expressing this
requirement as a direct constraint on the topology seems simpler. requirement as a direct constraint on the topology seems simpler.
This is also advantageous if the minimum required bandwidth changes, This is also advantageous if the minimum required bandwidth changes,
as this constraint would provide a single centralized, coordinated as this constraint would provide a single centralized, coordinated
point of control. point of control.
To satisfy this requirement, this document defines an Exclude Minimum To satisfy this requirement, this document defines an Exclude Minimum
Bandwidth constraint. When this constraint is advertised in a FAD, a Bandwidth constraint. When this constraint is advertised in a FAD, a
link will be pruned from the Flex-Algorithm topology if the link's link will be pruned from the Flex-Algorithm topology if the link's
advertised Maximum Link Bandwidth is below the advertised Minimum advertised maximum link bandwidth value is below the FAD advertised
Bandwidth value. minimum bandwidth value.
Similarly, this document defines an Exclude Maximum Link Delay Similarly, this document defines an Exclude Maximum Link Delay
constraint. Applications, such as High-Frequency Trading are constraint. Applications, such as High-Frequency Trading are
sensitive to link delays and may perform poorly in networks prone to sensitive to link delays and may perform poorly in networks prone to
delay variability, such as those with transparent Layer 2 link delay variability, such as those with transparent Layer 2 link
recovery mechanisms or satellite links. Mechanisms already exist to recovery mechanisms or satellite links. Mechanisms already exist to
measure the link delay dynamically and advertise it in the IGP. measure the link delay dynamically and advertise it in the IGP.
Networks that employ dynamic link-delay measurement, may want to Networks that employ dynamic link-delay measurement, may want to
exclude links that have a delay over a given threshold. exclude links that have a delay over a given threshold.
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Min Bandwidth (4 octets): Min Bandwidth (4 octets):
A 32-bit field specifying the link bandwidth encoded in IEEE A 32-bit field specifying the link bandwidth encoded in IEEE
floating point format (32 bits) [IEEE754-2019]. The units are floating point format (32 bits) [IEEE754-2019]. The units are
bytes per second. bytes per second.
The FAEMB sub-TLV MUST appear once at most in the FAD sub-TLV. If it The FAEMB sub-TLV MUST appear once at most in the FAD sub-TLV. If it
appears more than once, the IS-IS FAD sub-TLV MUST be ignored by the appears more than once, the IS-IS FAD sub-TLV MUST be ignored by the
receiver. receiver.
The Minimum bandwidth advertised in the FAEMB sub-TLV MUST be The minimum bandwidth value advertised in the FAEMB sub-TLV MUST be
compared with Maximum Link Bandwidth advertised in sub-sub-TLV 9 of compared with maximum link bandwidth value advertised in sub-sub-TLV
the ASLA sub-TLV [RFC9479]. If the L-flag is set in the ASLA sub- 9 of the ASLA sub-TLV [RFC9479]. If the L-flag is set in the ASLA
TLV, the Minimum bandwidth advertised in the FAEMB sub-TLV MUST be sub-TLV, the minimum bandwidth value advertised in the FAEMB sub-TLV
compared with the Maximum Link Bandwidth as advertised in the sub-TLV MUST be compared with the maximum link bandwidth value as advertised
9 of the TLVs 22/222/23/223/141 [RFC5305], as defined in Section 4.2 in the sub-TLV 9 of the TLVs 22/222/23/223/141 [RFC5305], as defined
of [RFC9479]. in Section 4.2 of [RFC9479].
If the Maximum Link Bandwidth is lower than the Minimum Link If the maximum link bandwidth value is lower than the minimum link
Bandwidth advertised in the FAEMB sub-TLV, the link MUST be excluded bandwidth value advertised in the FAEMB sub-TLV, the link MUST be
from the Flex-Algorithm topology. If a link does not have the excluded from the Flex-Algorithm topology. If a link does not have
Maximum Link Bandwidth advertised but the FAD contains this sub-TLV, the Maximum Link Bandwidth advertised but the FAD contains the FAEMB
then that link MUST NOT be excluded from the topology based on the sub-TLV, then that link MUST NOT be excluded from the topology based
Minimum Bandwidth constraint. on the Minimum Bandwidth constraint.
3.1.2. IS-IS Exclude Maximum Delay Sub-TLV 3.1.2. IS-IS Exclude Maximum Delay Sub-TLV
IS-IS Flex-Algorithm Exclude Maximum Delay (FAEMD) sub-TLV is a sub- IS-IS Flex-Algorithm Exclude Maximum Delay (FAEMD) sub-TLV is a sub-
TLV of the IS-IS FAD sub-TLV. It has the following format: TLV of the IS-IS FAD sub-TLV. It has the following format:
0 1 2 3 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 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 | | Type | Length |
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An 8-bit field indicating the total length, in octets, of the An 8-bit field indicating the total length, in octets, of the
subsequent fields. For this sub-TLV, the Length is set to 3. subsequent fields. For this sub-TLV, the Length is set to 3.
Max Link Delay (3 octets): Max Link Delay (3 octets):
A 24-bit field specifying the Maximum link delay in microseconds. A 24-bit field specifying the Maximum link delay in microseconds.
The FAEMD sub-TLV MUST appear only once in the FAD sub-TLV. If it The FAEMD sub-TLV MUST appear only once in the FAD sub-TLV. If it
appears more than once, the IS-IS FAD sub-TLV MUST be ignored by the appears more than once, the IS-IS FAD sub-TLV MUST be ignored by the
receiver. receiver.
The Maximum link delay advertised in the FAEMD sub-TLV MUST be The maximum link delay value advertised in the FAEMD sub-TLV MUST be
compared with Min Unidirectional Link Delay advertised in sub-sub-TLV compared with Min Unidirectional Link Delay advertised in sub-sub-TLV
34 of the ASLA sub-TLV [RFC9479]. If the L-flag is set in the ASLA 34 of the ASLA sub-TLV [RFC9479]. If the L-flag is set in the ASLA
sub-TLV, the Maximum link delay advertised in the FAEMD sub-TLV MUST sub-TLV, the maximum link delay value advertised in the FAEMD sub-TLV
be compared with Min Unidirectional Link Delay as advertised by the MUST be compared with Min Unidirectional Link Delay as advertised by
sub-TLV 34 of the TLVs 22/222/23/223/141 [RFC8570], as defined in the sub-TLV 34 of the TLVs 22/222/23/223/141 [RFC8570], as defined in
Section 4.2 of [RFC9479]. Section 4.2 of [RFC9479].
If the Min Unidirectional Link Delay value is higher than the Maximum If the Min Unidirectional Link Delay value is higher than the Maximum
link delay advertised in the FAEMD sub-TLV, the link MUST be excluded Link Delay advertised in the FAEMD sub-TLV, the link MUST be excluded
from the Flex-Algorithm topology. If a link does not have the Min from the Flex-Algorithm topology. If a link does not have the Min
Unidirectional Link Delay advertised but the FAD contains this sub- Unidirectional Link Delay advertised but the FAD contains the FAEMD
TLV, then that link MUST NOT be excluded from the topology based on sub-TLV, then that link MUST NOT be excluded from the topology based
the Maximum Delay constraint. on the Maximum Delay constraint.
3.2. OSPF FAD Constraint Sub-TLVs 3.2. OSPF FAD Constraint Sub-TLVs
3.2.1. OSPF Exclude Minimum Bandwidth Sub-TLV 3.2.1. OSPF Exclude Minimum Bandwidth Sub-TLV
OSPF Flex-Algorithm Exclude Minimum Bandwidth (FAEMB) sub-TLV is a OSPF Flex-Algorithm Exclude Minimum Bandwidth (FAEMB) sub-TLV is a
sub-TLV of the OSPF FAD TLV. It has the following format: sub-TLV of the OSPF FAD TLV. It has the following format:
0 1 2 3 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 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
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floating point format (32 bits)[IEEE754-2019]. The units are floating point format (32 bits)[IEEE754-2019]. The units are
bytes per second. bytes per second.
The FAEMB sub-TLV MUST only appear once in the FAD sub-TLV. If it The FAEMB sub-TLV MUST only appear once in the FAD sub-TLV. If it
appears more than once, the OSPF FAD TLV MUST be ignored by the appears more than once, the OSPF FAD TLV MUST be ignored by the
receiver. receiver.
The Maximum Link Bandwidth as advertised in the Extended Link TLV in The Maximum Link Bandwidth as advertised in the Extended Link TLV in
the Extended Link Opaque LSA in OSPFv2 [RFC7684] or as a sub-TLV of the Extended Link Opaque LSA in OSPFv2 [RFC7684] or as a sub-TLV of
the Router-Link TLV of the E-Router-LSA Router-Link TLV in OSPFv3 the Router-Link TLV of the E-Router-LSA Router-Link TLV in OSPFv3
[RFC8362] MUST be compared against the Minimum bandwidth advertised [RFC8362] MUST be compared against the Minimum Bandwidth advertised
in the FAEMB sub-TLV. If the link bandwidth is lower than the in the FAEMB sub-TLV. If the link bandwidth value is lower than the
Minimum bandwidth advertised in the FAEMB sub-TLV, the link MUST be Minimum Bandwidth advertised in the FAEMB sub-TLV, the link MUST be
excluded from the Flex-Algorithm topology. excluded from the Flex-Algorithm topology.
If a link does not have the Maximum Link Bandwidth advertised but the If a link does not have the Maximum Link Bandwidth advertised but the
FAD contains this sub-TLV, then that link MUST be included in the FAD contains the FAEMB sub-TLV, then that link MUST be included in
topology and proceed to apply further pruning rules for the link. the topology and proceed to apply further pruning rules for the link.
3.2.2. OSPF Exclude Maximum Delay Sub-TLV 3.2.2. OSPF Exclude Maximum Delay Sub-TLV
The OSPF Flex-Algorithm Exclude Maximum Delay (FAEMD) sub-TLV is a The OSPF Flex-Algorithm Exclude Maximum Delay (FAEMD) sub-TLV is a
sub-TLV of the OSPF FAD TLV. It has the following format. sub-TLV of the OSPF FAD TLV. It has the following format.
0 1 2 3 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 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 | | Type | Length |
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Max Link Delay (3 octets): Max Link Delay (3 octets):
A 24-bit field specifying the Maximum link delay in microseconds. A 24-bit field specifying the Maximum link delay in microseconds.
The FAEMD sub-TLV MUST only appear once in the OSPF FAD TLV. If it The FAEMD sub-TLV MUST only appear once in the OSPF FAD TLV. If it
appears more than once, the OSPF FAD TLV MUST be ignored by the appears more than once, the OSPF FAD TLV MUST be ignored by the
receiver. receiver.
The Min Delay value advertised via the Min/Max Unidirectional Link The Min Delay value advertised via the Min/Max Unidirectional Link
Delay of the ASLA sub-TLV [RFC9492] MUST be compared against the Delay of the ASLA sub-TLV [RFC9492] MUST be compared against the
Maximum delay advertised in the FAEMD sub-TLV. If the Min Maximum Delay advertised in the FAEMD sub-TLV. If the Min
Unidirectional Link Delay is higher than the Maximum delay advertised Unidirectional Link Delay is higher than the Maximum Delay advertised
in the FAEMD sub-TLV, the link MUST be excluded from the Flex- in the FAEMD sub-TLV, the link MUST be excluded from the Flex-
Algorithm topology. If the Min/Max Unidirectional Link Delay is not Algorithm topology. If the Min/Max Unidirectional Link Delay is not
advertised for a link but the FAD contains this sub-TLV, then that advertised for a link but the FAD contains this sub-TLV, then that
link MUST NOT be excluded from the topology based on the Maximum link MUST NOT be excluded from the topology based on the Maximum
Delay constraint. Delay constraint.
4. Bandwidth Metric Advertisement 4. Bandwidth Metric Advertisement
Historically, IGP implementations have made default metric Historically, IGP implementations have made default metric
assignments based on link bandwidth. This has proven to be useful assignments based on link bandwidth. This has proven to be useful
 End of changes. 13 change blocks. 
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