IGP Flexible Algorithm with Link Loss

Abstract

IGP Flexible Algorithms allow IGPs to compute constraint-based paths. Since link packet loss rate plays an important role in network evaluation, links with high packet loss rate should be bypassed during forwarding. This draft proposes a path computation method based on a maximum link loss constraint to prune unsatisfied links in Flexible Algorithms.¶

1. Introduction

Link packet loss rate (link loss) is a measure of the percentage of data packets that are lost during transmission over a network. It is an important performance metric that directly impacts the quality of service, network congestion, security, and overall network efficiency. Ensuring a low packet loss rate is essential for maintaining efficient and secure network operations. Consequently, It is necessary to avoid passing through links with a high packet loss rate during forwarding.¶

The link loss is advertised by the Unidirectional Link Loss Sub-TLV defined in [RFC8570] by IS-IS and [RFC7471] by OSPF, which describes the loss (as a packet percentage) between two directly connected IS-IS neighbors. This Sub-TLV is carried in the Application-Specific Link Attributes Sub-TLV advertised by IS-IS [RFC9479] or OSPF [RFC9492]. The link packet loss rate can be measured by methods such as TWAMP [RFC5357] and STAMP [RFC8762], which is beyond the scope of this document. The link-loss measurement should be consistent in the IGP routing domain.¶

IGP Flexible Algorithms allow IGPs to compute constraint-based paths [RFC9350]. Current path computation methods are based on calculating the minimum cost of the path from the source to the destination. Flex-Algorithm has already supported path computation with the IGP cost, the minimum link delay and the traffic-engineering metric. [I-D.ietf-lsr-flex-algo-bw-con] defines a family of generic metrics (e.g. bandwidth based metric type) and bandwidth related constraints to support path computation based on bandwidth. However, current calculation types and metric types cannot support path computation based on link loss, since the cost of the path should be defined as the maximum/minimum value among all passing links.¶

To overcome the above issue, there are two solutions. First, new operators like maximum value operator can be defined, which works as a step function. When the link loss exceeds a threshold, the cost of the link is set to the maximum. Second, new Flexible Algorithm Definition (FAD) constraints can be defined to exclude links that do not meet the link loss requirements during path calculation. The second method is specifically demonstrated in this document. The general ideas are as below.¶

• 1. The link loss is used as a link constraint for path computation. That is, the link whose loss rate is greater than the specified value is excluded.¶

• 2. Metric-type remains unchanged: igp, te, and delay.¶

With a new FAD constraint Sub-TLV advertised by IGP, links with low packet loss rate will be selected for path computation. The new Exclude Maximum Link Loss Sub-TLVs are defined in Section 2. The Flex-Algorithm calculation method based on link loss is presented in Section 3. Link packet loss rate is obtained from the existing Unidirectional Link Loss Sub-TLV defined in RFC9479 and RFC9492.¶

2. Exclude Maximum Link Loss Sub-TLV

A new Exclude Maximum Link Loss Sub-TLV is defined as a sub-TLV of the FAD TLV. To guarantee loop free forwarding, all routers that participate in a Flex-Algorithm MUST agree on the FAD. Selected nodes within the IGP domain MUST advertise FADs as described in Sections 5, 6, and 7 of [RFC9350].¶

The Exclude Maximum Link Loss Sub-TLV is proposed to specify the upper limit of the link loss. When this Sub-TLV is carried in a FAD TLV, all links with packet loss rate larger than the defined maximum link loss value will be excluded from the Flex-Algorithm topology.¶

    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     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                Max Link Loss                  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Type: 252(TBA)

      Length: 3 octets

      Max Link Loss:  This 24-bit field carries link packet loss as a
    percentage of the total traffic sent over a configurable interval.
    The basic unit is 0.000003%, where (2^24 - 2) is 50.331642%. This
    value is the highest packet-loss percentage that can be expressed.
    Therefore, measured values that are larger than the field maximum
    SHOULD be encoded as the maximum value.

    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           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                Max Link Loss                  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Type: 252(TBA)

      Length: 3 octets

      Max Link Loss:  This 24-bit field carries link packet loss as a
    percentage of the total traffic sent over a configurable interval.
    The basic unit is 0.000003%, where (2^24 - 2) is 50.331642%. This
    value is the highest packet-loss percentage that can be expressed.
    Therefore, measured values that are larger than the field maximum
    SHOULD be encoded as the maximum value.

3. Calculation of Flexible Algorithm Paths

A new rule is added to the rules used to prune links from the topology during the Flex-Algorithm computation in Section 13 of [RFC9350].¶

• 1. Check if any exclude FAEML rule is part of the Flex-Algorithm definition. If such exclude rule exists and the link has link loss advertised, check if the link satisfies the FAEML rule. If not, the link MUST be pruned from the computation.¶

4. Operational Considerations

In some scenarios, the link status can be frequently changed between available and unavailable since the link packet loss rate may fluctuate around the threshold value. Consequently, flex-algo calculation may be triggered frequently. There are a few mechanisms to solve this problem.¶

• 1. Delayed collection: IGP advertised loss could be calculated on an interval which is long enough, such as 10 minutes.¶

• 2. Averaging and normalization: IGP advertised loss should be some form of average (e.g. exponential weighted average) from the collected loss values and the advertised loss can be normalized to avoid advertising non-significant changes in loss.¶

• 3. Flapping suppression: When the IGP advertised loss is detected on a frequent change, a timer can be set to delay the update process.¶

6. References