IGP Flexible Algorithms Reverse Affinity Constraint

IGP protocols historically computed the best paths over the network solely based on the IGP metric assigned to the links. An IGP Flex-Algorithm as specified in allows IGPs to compute constraint-based paths. Several mechanisms to include or exclude the link during the Flex-Algorithm path calculation have been defined already: - link inclusion or exclusion based on the presence of a specific Admin Group(s) - - link exclusion based on the presence of a specific Shared Risk Link Group (SRLG) - - link exclusion based on minimum bandwidth - - link exclusion based on maximum delay - 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 term OSPF in this document is used to cover both OSPFv2 and OSPFv3 protocols.

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 when, and only when, they appear in all capitals, as shown here.

The Flexible Algorithm definition (FAD) can specify Extended Administrative Groups that are used by the operator to include or exclude links during the Flex-Algorithm path computation . These link Extended Administrative 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 certain scenarios, it is beneficial to evaluate the Extended Administrative Groups associated with the reverse direction of a link, rather than solely those in the direction of path computation. Consider a point-to-point link represented as a pair of directed edges between two nodes, A and B. When computing a path from A to B, issues such as input errors on the link, detectable only at the receiving node B, may be operationally significant. An operator might monitor metrics like CRC errors or other input-related faults at node B and apply thresholds over a defined observation period. If such a threshold is exceeded, node B may locally assign specific Extended Administrative Groups to the link in the direction from B to A. To accommodate this operational intent, the Flex-Algorithm can be extended to inspect the Extended Administrative Groups of the reverse-direction edge (from B to A) when evaluating the forward-direction edge (from A to B) during path computation. This enables the exclusion of links from the computed topology based on conditions detected at the far end of the link, improving network reliability and policy control.

Threshold based setting of the link Extended Administrative Groups must be done with care to avoid excessive flooding and constant Flex-Algorithm path re-calculation. One possible approach is to use two different thresholds when setting and un-setting the Extended Administrative Group on the link. For example, when setting the Extended Administrative Group on the link based on the percentage of certain incoming errors, higher threshold is used for setting and lower threshold is used for un-setting the Extended Administrative Group on the link. Many implementations provide a throttling mechanism that limits the rate at which the LSP in case of ISIS, or LSA in case of OSPFv2 and OSPFv3, is updated at the originator. Such mechanism is typically not specific to any particular link attribute, but rather considers any change in the LSP or LSA. Usage of such throttling mechanism can also be used to avoid frequent changes in the setting of the Extended Administrative Group on a link to affect the stability of the receivers.

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 . The IS-IS FAERAG Sub-TLV is a Sub-TLV of the IS-IS FAD Sub-TLV. It has the following format:

Type (1 octet): 10 Length (1 octet): Variable, dependent on the size of the Extended Admin Group. The length MUST be a multiple of 4 octets. If the length is not a multiple of 4 octets, the entire Sub-TLV MUST be ignored by the receiver. Extended Administrative Group: Extended Administrative Group as defined in . 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 . 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 . 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 . 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 . 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 . The OSPF FAERAG Sub-TLV is a Sub-TLV of the OSPF FAD TLV. It has the following format:

Type (2 octets): 10 Length (2 octets): Variable, dependent on the size of the Extended Admin Group. The length MUST be a multiple of 4 octets. If the length is not a multiple of 4 octets, the entire Sub-TLV MUST be ignored by the receiver. Extended Administrative Group: Extended Administrative Group as defined in . 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 . 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 . 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 . 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 . 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.

The following procedures augment the rules defined in Section 13 of by introducing additional constraints based on Administrative Groups (AGs) associated with the reverse direction of a link. 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. 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. 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. Please see rules 8, 9, and 10 in for these added rules.

This document makes the following registration in the "IS-IS Sub-Sub-TLVs for Flexible Algorithm Definition Sub-TLV" registry under "IS-IS TLV Codepoints" registry group: Type: 10 Description: Flexible Algorithm Exclude Reverse Admin Group MP-TLV Applicability: N Reference: This document (). Type: 11 Description: Flexible Algorithm Include-Any Reverse Admin Group MP-TLV Applicability: N Reference: This document (). Type: 12 Description: Flexible Algorithm Include-All Reverse Admin Group MP-TLV Applicability: N Reference: This document ().

This document makes the following registration in the "OSPF Flexible Algorithm Definition TLV Sub-TLVs" registry under "Open Shortest Path First (OSPF) Parameters" registry group: Type: 10 Description: Flexible Algorithm Exclude Reverse Admin Group Reference: This document (). Type: 11 Description: Flexible Algorithm Include-Any Reverse Admin Group Reference: This document (). Type: 12 Description: Flexible Algorithm Include-All Reverse Admin Group Reference: This document ().

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" specifies the ordered set of rules that is used to prune links from the topology during the Flex-Algorithm path computation. There is no upper bound on the number of rules that the registry supports. IGP Flex-algo Path Calculation Rules

Sequence 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
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
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
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
5	If the Flex-Algorithm Definition uses something other than the IGP metric (Section 5 of ), 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
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
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
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,
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,
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,

This section provides the guidance for designated expert on evaluating new registrations in the "IGP Flex-Algorithm Path Computation Rules" registry:

When a new constraint is defined, the rule associated with that constraint MAY be inserted at any position. Backwards compatibility is guaranteed because nodes which don't support the new constraint will not participate in an algorithm where the FAD specifies a constraint they don't support.
The relative ordering of existing rules MUST NOT be altered. Doing so has the potential to create backwards compatibility issues.
Deletion of the rules MUST NOT be done. Given that the rules are only used conditionally based on the information carried in the winning FAD, deletion of the rule is not necessary.
Merging or repeating of the rules MUST NOT be done.

This document inherits security considerations from .

TBD