IGP Flexible Algorithm Utilizing Link Bit Error Rate Metric

Link Bit Error Rate (hereafter "link BER metric") refers to the percentage of transmitted bits that are received in error over a network link. It is a key metric for network performance evaluation. A high BER metric can impact service quality and forwarding efficiency. To maintain optimal forwarding paths, it is desirable to avoid links with excessive BER metric values during IGP path computation. The IGP Flexible Algorithms enable IGPs to compute constraint-based paths . Current path computation methods focus on determining the minimum path cost from source to destination. Flex-Algorithm already supports path computation based on IGP cost, minimum link delay, and traffic-engineering metrics. defines additional generic metric types and constraints. defines BER metric advertisement extensions for IS-IS and OSPF. This document specifies a BER-metric-based exclusion constraint for Flex-Algorithm 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 when, and only when, they appear in all capitals, as shown here.

BER: Bit Error Rate. EMA: Exponential Moving Average. FA-EMB: Flex-Algorithm Exclude Maximum Link BER. FAD: Flexible Algorithm Definition. OSPF: Open Shortest Path First. TLV: Type-Length-Value.

This document defines a new Flexible Algorithm Definition (FAD) that exclude links exceeding a configured maximum BER threshold. Existing metric-type (for example, IGP, TE, and delay) remains unchanged. defines extensions for advertising Packets with Bit Errors (PER) metric for links. Utilizing the link PER metric for Flex-Algorithm path computation will be covered in the future revision of this document.

A new sub-sub-TLV, the "Exclude Maximum Link BER Sub-TLV", is defined as part of the FAD TLV. To ensure loop-free forwarding, all routers participating in a Flex-Algorithm MUST agree on the FAD definition. Selected nodes within the IGP domain MUST advertise FADs by including them in routing updates, as specified in Sections 5, 6, and 7 of . The "Exclude Maximum Link BER Sub-TLV" defines the maximum allowable BER metric value for links in the Flex-Algorithm topology. When this sub-sub-TLV is present, links whose advertised BER metric exceeds the configured maximum BER metric value are excluded from path computation.

The IS-IS FA-EMB Sub-TLV is defined as a sub-sub-TLV of the IS-IS FAD Sub-TLV.

IS-IS FA-EMB Sub-TLV Type: TBA1 (to be assigned by IANA) Length: 3 octets Max Link BER: 24-bit unsigned integer representing the maximum allowable BER percentage. Encoded with a resolution of 0.000003% per unit, providing a maximum expressible value of 50.331642% (0xFFFFFF * 0.000003). Values exceeding this limit MUST be advertised as 0xFFFF FF. The FA-EMB sub-sub-TLV MUST appear at most once in the IS-IS FAD Sub-TLV. If it appears more than once, the IS-IS FAD Sub-TLV MUST be ignored by the receiving node. The maximum link BER metric advertised in the FA-EMB sub-sub-TLV MUST be compared with the BER metric advertised for the link as defined in . If the advertised link BER metric exceeds the maximum BER metric in FA-EMB, the link MUST be excluded from the Flex-Algorithm topology. If a link does not advertise the BER metric but the FAD contains the FA-EMB sub-sub-TLV, the link MUST NOT be excluded from the Flex-Algorithm topology.

The OSPF FA-EMB Sub-TLV is defined as a sub-sub-TLV of the OSPF FAD Sub-TLV.

OSPF FA-EMB Sub-TLV Type: TBA2 (to be assigned by IANA) Length: 3 octets Max Link BER: 24-bit unsigned integer representing the maximum allowable BER percentage. Encoded with a resolution of 0.000003% per unit, providing a maximum expressible value of 50.331642% (0xFFFFFF * 0.000003). Values exceeding this limit MUST be advertised as 0xFFFF FF. The FA-EMB sub-sub-TLV MUST appear at most once in the OSPF FAD Sub-TLV. If it appears more than once, the OSPF FAD Sub-TLV MUST be ignored by the receiving node. The maximum link BER metric advertised in the FA-EMB sub-sub-TLV MUST be compared with the BER metric advertised for the link as defined in . If the advertised link BER metric exceeds the maximum BER metric in FA-EMB, the link MUST be excluded from the Flex-Algorithm topology. If a link does not advertise the BER metric but the FAD contains the FA-EMB sub-sub-TLV, the link MUST NOT be excluded from the Flex-Algorithm topology.

The following rule is added to the topology pruning rules in Section 13 of :

Check if any exclude FA-EMB rule is part of the Flex-Algorithm definition. If such exclude rule exists and the link has BER metric advertised, check whether the link satisfies the FA-EMB rule. If not, the link MUST be pruned from the computation.

In some deployments, the BER metric may oscillate around the configured threshold, causing repeated topology updates and recomputations. The following mechanisms can improve stability:

Delayed collection: BER metric values advertised into IGP can be computed over a sufficiently long observation interval.
Averaging and normalization: BER metric values SHOULD be derived from averaged data (for example EMA) and MAY be normalized to avoid advertising insignificant changes.
Flapping suppression: Implementations MAY apply dampening or hold-down timers before advertising BER metric changes that repeatedly cross the exclusion threshold.

The security considerations in , , , , , and apply.

This document requests allocation of two new code points.

IANA maintains the registry for IS-IS sub-sub-TLVs in the Flexible Algorithm Definition Sub-TLV. IANA is requested to allocate the following IS-IS sub-sub-TLV type: IS-IS Sub-Sub-TLV Types

Value	Description	Reference
TBA1	IS-IS Exclude Maximum Link BER Sub-TLV	This document

IANA maintains the registry for OSPF sub-sub-TLVs in the Flexible Algorithm Definition Sub-TLV. IANA is requested to allocate the following OSPF sub-sub-TLV type: OSPF Sub-Sub-TLV Types

Value	Description	Reference
TBA2	OSPF Exclude Maximum Link BER Sub-TLV	This document