| Internet-Draft | Flex-Algorithm Reverse Affinity | January 2025 |
| Psenak, et al. | Expires 1 August 2025 | [Page] |
An IGP Flexible Algorithm (Flex-Algorithm) allows IGPs to compute constraint-based paths.¶
This document extends IGP Flex-Algorithm with additional constraints for inclusion or exclusion of links in the path based on Admin Groups associated with the reverse direction of the SPF path computation.¶
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An IGP Flex-Algorithm as specified in [RFC9350] computes a constraint-based path.¶
This document extends IGP Flex-Algorithm with additional constraints for inclusion or exclusion of links in the path based on Admin Groups associated with the reverse direction of the SPF path computation.¶
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.¶
The Flexible Algorithm definition can specify Admin Groups that are used by the operator to include or exclude links during the Flex-Algorithm path computation. These link Admin Groups are checked in the path direction of the SPF computation, e.g., in the direction from the root vertex toward verticies of increasing distance.¶
In some cases, it is beneficial to check the Admin Groups of the link from the reverse direction of the path computation. For example, on a point-to-point link between endpoints A and B and for the path copmputed in a direction from A to B, the input errors can only be detected at node B. An operator may measure the rate of such input errors and set certain Admin Groups on a link locally on node B when the input error rate exceeds a certain threshold. When the Flex-Algorithm calculation processes the link A to B, it may look at the Admin Groups of link's reverse direction, e.g., from B to A. This would allow the operator to exclude such link from the Flex-Algorithm topology.¶
The IS-IS Flexible Algorithm Exclude Reverse Admin Group (FAERAG) Sub-TLV is used to advertise the exclude rule that is used during the Flex-Algorithm path calculation as specified in Section 10.¶
The IS-IS FAERAG Sub-TLV is a Sub-TLV of the IS-IS FAD Sub-TLV. It has the following format:¶
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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Extended Admin Group | +- -+ | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ where:¶
The IS-IS FAERAG Sub-TLV MUST NOT appear more than once in a single IS-IS FAD Sub-TLV. If it appears more than once, the IS-IS FAD Sub-TLV MUST be ignored by the receiver.¶
The IS-IS FAERAG Sub-TLV MUST NOT appear more than once in the set of FAD sub-TLVs for a given Flex-Algorithm from a given IS. If it appears more than once in such set, the IS-IS FAERAG Sub-TLV in the first occurrence in the lowest numbered LSP from a given IS MUST be used and any other occurrences MUST be ignored by the receiver.¶
The IS-IS Flexible Algorithm Include-Any Reverse Admin Group Sub-TLV is used to advertise the include-any rule that is used during the Flex-Algorithm path calculation as specified in Section 10.¶
The format of the IS-IS Flexible Algorithm Include-Any Reverse Admin Group Sub-TLV is identical to the format of the FAERAG Sub-TLV in Section 4.¶
IANA has assigned Type value of 11 for the IS-IS Flexible Algorithm Include-Any Reverse Admin Group Sub-TLV.¶
The IS-IS Flexible Algorithm Include-Any Reverse Admin Group Sub-TLV MUST NOT appear more than once in a single IS-IS FAD Sub-TLV. If it appears more than once, the IS-IS FAD Sub-TLV MUST be ignored by the receiver.¶
The IS-IS Flexible Algorithm Include-Any Reverse Admin Group Sub-TLV MUST NOT appear more than once in the set of FAD sub-TLVs for a given Flex-Algorithm from a given IS. If it appears more than once in such set, the IS-IS Flexible Algorithm Include-Any Reverse Admin Group Sub-TLV in the first occurrence in the lowest numbered LSP from a given IS MUST be used and any other occurrences MUST be ignored by the receiver.¶
The IS-IS Flexible Algorithm Include-All Reverse Admin Group Sub-TLV is used to advertise the include-all rule that is used during the Flex-Algorithm path calculation as specified in Section 10.¶
The format of the IS-IS Flexible Algorithm Include-All Reverse Admin Group Sub-TLV is identical to the format of the FAERAG Sub-TLV in Section 4.¶
IANA has assigned Type value of 12 for the IS-IS Flexible Algorithm Include-All Reverse Admin Group Sub-TLV.¶
The IS-IS Flexible Algorithm Include-All Reverse Admin Group Sub-TLV MUST NOT appear more than once in a single IS-IS FAD Sub-TLV. If it appears more than once, the IS-IS FAD Sub-TLV MUST be ignored by the receiver.¶
The IS-IS Flexible Algorithm Include-All Reverse Admin Group Sub-TLV MUST NOT appear more than once in the set of FAD sub-TLVs for a given Flex-Algorithm from a given IS. If it appears more than once in such set, the IS-IS Flexible Algorithm Include-All Reverse Admin Group Sub-TLV in the first occurrence in the lowest numbered LSP from a given IS MUST be used and any other occurrences MUST be ignored by the receiver.¶
The OSPF Flexible Algorithm Exclude Reverse Admin Group (FAERAG) Sub-TLV is used to advertise the exclude rule that is used during the Flex-Algorithm path calculation as specified in Section 10.¶
The OSPF FAERAG Sub-TLV is a Sub-TLV of the OSPF FAD TLV. It has the following format:¶
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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Extended Admin Group | +- -+ | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ where:¶
The OSPF FAERAG Sub-TLV MUST NOT appear more than once in an OSPF FAD TLV. If it appears more than once, the OSPF FAD TLV MUST be ignored by the receiver.¶
The OSPF Flexible Algorithm Include-Any Reverse Admin Group Sub-TLV is used to advertise the include-any rule that is used during the Flex-Algorithm path calculation as specified in Section 10.¶
The format of the OSPF Flexible Algorithm Include-Any Reverse Admin Group Sub-TLV is identical to the format of the OSPF FAERAG Sub-TLV in Section 7.¶
IANA has assigned Type value of 11 for the OSPF Flexible Algorithm Include-Any Reverse Admin Group Sub-TLV.¶
The OSPF Flexible Algorithm Include-Any Reverse Admin Group Sub-TLV MUST NOT appear more than once in an OSPF FAD TLV. If it appears more than once, the OSPF FAD TLV MUST be ignored by the receiver.¶
The OSPF Flexible Algorithm Include-All Reverse Admin Group Sub-TLV is used to advertise the include-all rule that is used during the Flex-Algorithm path calculation as specified in Section 10.¶
The format of the OSPF Flexible Algorithm Include-All Reverse Admin Group Sub-TLV is identical to the format of the OSPF FAERAG Sub-TLV in Section 7.¶
IANA has assigned Type value of 12 for OSPF Flexible Algorithm Include-All Reverse Admin Group Sub-TLV.¶
The OSPF Flexible Algorithm Include-All Reverse Admin Group Sub-TLV MUST NOT appear more than once in an OSPF FAD TLV. If it appears more than once, the OSPF FAD TLV MUST be ignored by the receiver.¶
Three new rules are added to the existing rules specified in Section 13 of [RFC9350]:¶
Please see rules 8, 9, and 10 in Section 11.3 for these added rules.¶
IANA has assigned the following Sub-Sub-TLVs in the "ISIS Sub-Sub-TLVs for Flexible Algorithm Definition Sub-TLV" registry:¶
IANA has assigned the following Sub-TLVs in the "OSPF TLVs for Flexible Algorithm Definition TLV" registry:¶
This document creates a new registry called "IGP Flex-Algorithm Path Computation Rules Registry" within the "Interior Gateway Protocol (IGP) Parameters" registry group. The registration procedure for the new registry is "Expert Review".¶
The "IGP Flex-Algo Path Computation Rules Registry" specifies the ordered set of rules that MUST be used to prune links from the topology during the Flex-Algorithm path computation.¶
| Number | Description | Reference |
|---|---|---|
| 1 | Check if any exclude Administrative Group rule is part of the Flex-Algorithm Definition. If such exclude rule exists, check if any color that is part of the exclude rule is also set on the link. If such a color is set, the link MUST be pruned from the computation | Section 13 of [RFC9350] |
| 2 | Check if any exclude SRLG rule is part of the Flex-Algorithm Definition. If such exclude rule exists, check if the link is part of any SRLG that is also part of the SRLG exclude rule. If the link is part of such SRLG, the link MUST be pruned from the computation. | Section 13 of [RFC9350] |
| 3 | Check if any include-any Administrative Group rule is part of the Flex-Algorithm Definition. If such include-any rule exists, check if any color that is part of the include-any rule is also set on the link. If no such color is set, the link MUST be pruned from the computation | Section 13 of [RFC9350] |
| 4 | Check if any include-all Administrative Group rule is part of the Flex-Algorithm Definition. If such include-all rule exists, check if all colors that are part of the include-all rule are also set on the link. If all such colors are not set on the link, the link MUST be pruned from the computation. | Section 13 of [RFC9350] |
| 5 | If the Flex-Algorithm Definition uses something other than the IGP metric (Section 5 of [RFC9350]), and such metric is not advertised for the particular link in a topology for which the computation is done, such link MUST be pruned from the computation. A metric of value 0 MUST NOT be assumed in such a case. | Section 13 of [RFC9350] |
| 6 | Check if any exclude FAEMB rule is part of the Flex-Algorithm definition. If such exclude rule exists and the link has Maximum Link Bandwidth advertised, check if the link bandwidth satisfies the FAEMB rule. If the link does not satisfy the FAEMB rule, the link MUST be pruned from the Flex-Algorithm computation | Section 6 of [I-D.ietf-lsr-flex-algo-bw-con] |
| 7 | Check if any exclude FAEMD rule is part of the Flex-Algorithm definition. If such exclude rule exists and the link has Min Unidirectional link delay advertised, check if the link delay satisfies the FAEMD rule. If the link does not satisfy the FAEMD rule, the link MUST be pruned from the Flex-Algorithm computation. | Section 6 of [I-D.ietf-lsr-flex-algo-bw-con] |
| 8 | Check if any exclude reverse Admin Group rule is part of the Flex-Algorithm definition. If such exclude rule exists, check if any Admin Group that is part of the exclude rule is also set on the link in the reverse direction. If such Admin Group is set on the link in the reverse direction, the link MUST be pruned from the computation. | This document, Section 10 |
| 9 | Check if any include-any reverse Admin Group rule is part of the Flex-Algorithm definition. If such include-any rule exists, check if any Admin Group that is part of the include-any rule is also set on the link in the reverse direction. If no such Admin Group is set on the link in the reverse direction, the link MUST be pruned from the computation | This document, Section 10 |
| 10 | Check if any include-all reverse Admin Group rule is part of the Flex-Algorithm definition. If such include-all rule exists, check if all Admin Groups that are part of the include-all rule are also set on the link in the reverse direction. If all such Admin Groups are not set on the link in the reverse direction, the link MUST be pruned from the computation. | This document, Section 10 |
This document inherits security considerations from [RFC9350].¶
TBD¶