| Internet-Draft | IGP Flex-Algorithm with Link Loss | October 2023 |
| Xu & Wang | Expires 15 April 2024 | [Page] |
IGP Flexible Algorithms allow IGPs to compute constraint-based paths. Since link loss 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 link loss constraint to prune unsatisfied links in Flexible Algorithms.¶
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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] and [RFC7471], 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].¶
IGP Flexible Algorithms allow IGPs to compute constraint-based paths in [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 minimum 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 draft. 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.¶
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].¶
A new Exclude Link Loss Constraint Sub-TLV is proposed to spcify 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.¶
IS-IS Flex-Algorithm Exclude Maximum Link Loss Sub-TLV (FAEML) 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Max Link Loss |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type: 252¶
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.¶
The FAEML sub-TLV MUST appear at most once in the FAD Sub-TLV. If it appears more than once, the IS-IS FAD Sub-TLV MUST be ignored by the receiver.¶
The maximum link loss advertised in FAEML Sub-TLV MUST be compared with the link loss advertised in Sub-Sub-TLV 36 [RFC8570] of ASLA Sub- TLV [RFC9479]. If L-Flag is set in the ASLA sub-TLV, the maximum link loss advertised in FAEML sub-TLV MUST be compared with the link loss advertised by the sub-TLV 36 of the TLV 22/222/23/223/141 [RFC5305] as defined in [RFC9479] Section 4.2.¶
If the link loss is larger than the maximum link loss advertised in FAEML sub-TLV, the link MUST be excluded from the Flex-Algorithm topology. If a link does not have the link loss advertised but the FAD contains the FAEML sub-TLV, then it MUST NOT be excluded from the Flex-Algorithm topology.¶
OSPF Flex-Algorithm Exclude Maximum Link Loss Sub-TLV (FAEML) is a sub-TLV of the OSPF 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Max Link Loss |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type: 252¶
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.¶
The FAEML sub-TLV MUST appear at most once in the FAD Sub-TLV. If it appears more than once, the IS-IS FAD Sub-TLV MUST be ignored by the receiver.¶
The maximum link loss advertised in FAEML Sub-TLV MUST be compared with the link loss advertised in Sub-Sub-TLV 30 [RFC7471] of ASLA Sub- TLV [RFC9492]. The ASLA Sub-TLV is advertised in Extended Link Opaque LSAs [RFC7684] for OSPFv2 and E-Router-LSAs [RFC8362] for OSPFv3.¶
If the link loss is larger than the maximum link loss advertised in FAEML sub-TLV, the link MUST be excluded from the Flex-Algorithm topology. If a link does not have the link loss advertised but the FAD contains the FAEML sub-TLV, then it MUST NOT be excluded from the Flex-Algorithm topology.¶
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].¶
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 the link does not satisfy the FAEML rule, the link MUST be pruned from the computation.¶
Type: 252(TBA)¶
Description: IS-IS Exclude Maximum Link Loss Sub-TLV¶
Reference: This document Section 2.1¶
Type: 252(TBA)¶
Description: OSPF Exclude Maximum Link Loss Sub-TLV¶
Reference: This document Section 2.2¶