IS-IS Traffic Engineering (TE) Metric Extensions
obsoleted by
- rfc-8570 — IS-IS Traffic Engineering (TE) Metric Extensions
Extracted elements (30)
Residual bandwidth (Type 37) differs from unreserved bandwidth (RFC 5305): residual bandwidth subtracts RSVP-TE tunnel reservations from maximum link capacity and provides an aggregated remainder across priorities, making it suitable for Weighted ECMP, whereas unreserved bandwidth is per-priority and suited for call admission control.
The A bit was intentionally omitted from the Delay Variation and bandwidth sub-TLVs to help mitigate oscillations. Only sub-TLVs where anomaly-triggered fast failover is operationally useful include the A bit.
To prevent oscillations, only the high threshold or the low threshold (but not both) may be used to trigger re-advertisement for any given sub-TLV that supports both thresholds.
As per RFC 5305, unrecognized sub-TLVs should be silently ignored, ensuring backward compatibility with IS-IS implementations that do not support these extensions.
IS-IS TE Metric Extensions sub-TLVs are not confined to the TE context and can be used for computing paths in the absence of a TE subsystem, provided Interface and Neighbor Address sub-TLVs are present.
All values except residual bandwidth MUST be calculated as rolling averages, where the averaging period MUST be a configurable period of time.
Announcements MUST be able to be throttled using configurable inter-update throttle timers; the minimum announcement periodicity is 1 announcement per second, with a default value that SHOULD be 120 seconds.
Bandwidth advertisements MUST be encoded as IEEE floating-point values; delay and delay-variation advertisements MUST be encoded as integer values. Delay values MUST be in microseconds, packet loss MUST be a percentage of packets sent, and bandwidth MUST be in bytes per second.
Implementations MUST make it possible to individually enable or disable each sub-TLV based on configuration.
Implementations SHOULD provide configurable accelerated advertisement thresholds per sub-TLV so that immediate advertisement occurs when a measured parameter falls outside configured bounds or when the difference from the last advertised value exceeds a configured threshold.
Interface Address and Neighbor Address sub-TLVs (IPv4 or IPv6) MUST be present when using IS-IS TE Metric Extensions sub-TLVs, even outside the TE context.
Measured link-loss values larger than the 24-bit field maximum (50.331642%) SHOULD be encoded as the maximum value, since high-speed links with over 50% loss are considered unusable.
RESERVED fields in all sub-TLVs MUST be set to 0 when sent and MUST be ignored when received.
The default measurement interval for all sub-TLVs SHOULD be 30 seconds; implementations SHOULD NOT permit the inter-update timer to be lower than the measurement interval.
The delay advertised by the Unidirectional Link Delay and Min/Max sub-TLVs MUST be the delay from the local neighbor to the remote one (i.e., the forward-path latency).
The measurement interval, any filter coefficients, and any advertisement intervals MUST be configurable per sub-TLV.
The measurement of delay SHOULD NOT vary significantly based upon offered traffic load, and queuing delays SHOULD NOT be included in the delay measurement.
When link-performance values change by small amounts below advertisement thresholds, implementations SHOULD suppress sub-TLV re-advertisement and/or lengthen the refresh period to reduce unnecessary flooding.
Seven new IS-IS TE sub-TLVs are defined for distributing network-performance information: Unidirectional Link Delay (33), Min/Max Unidirectional Link Delay (34), Unidirectional Delay Variation (35), Unidirectional Link Loss (36), Unidirectional Residual Bandwidth (37), Unidirectional Available Bandwidth (38), and Unidirectional Utilized Bandwidth (39). These are registered in the 'Sub-TLVs for TLVs 22, 23, 141, 222, and 223' registry.
The Anomalous (A) bit is included in several sub-TLVs (delay, min/max delay, link loss). When set, it indicates that the measured value exceeds a configured maximum threshold; when cleared, it indicates steady-state performance or that the value has fallen below a configured reuse threshold.
IANA registered seven new sub-TLV type values (33–39) in the 'Sub-TLVs for TLVs 22, 23, 141, 222, and 223' IS-IS registry: Unidirectional Link Delay (33), Min/Max Unidirectional Link Delay (34), Unidirectional Delay Variation (35), Unidirectional Link Loss (36), Unidirectional Residual Bandwidth (37), Unidirectional Available Bandwidth (38), and Unidirectional Utilized Bandwidth (39).
The sub-TLVs allow advertisement of sensitive link-state information (bandwidth, delay) that operators may not wish to disclose. Additionally, IS-IS TE sub-TLVs sent over insecure links expose the network to man-in-the-middle attacks that could manipulate path selection (e.g., delaying real-time financial data). IS-IS cryptographic authentication per RFC 5304 is recommended as a mitigation.
Unstable or too-frequent metric advertisements can destabilize large networks. Implementations SHOULD follow the announcement threshold, suppression, and periodicity guidelines in Sections 5-7 to mitigate instability risk.
Min/Max Unidirectional Link Delay sub-TLV (Type 34, Length 8): one-bit A field, 7 reserved bits, 24-bit Min Delay field, 8 reserved bits, and 24-bit Max Delay field, all in microseconds as integer values.
Unidirectional Available Bandwidth sub-TLV (Type 38, Length 4): 8 reserved bits and a 32-bit Available Bandwidth field in IEEE floating-point format, bytes per second. Available bandwidth is residual bandwidth minus measured bandwidth used for non-RSVP-TE forwarding.
Unidirectional Delay Variation sub-TLV (Type 35, Length 4): 8 reserved bits and a 24-bit Delay Variation field in microseconds as an integer. This sub-TLV omits the A bit (intentionally, to mitigate oscillations). A value of 0 indicates the metric has not been measured.
Unidirectional Link Delay sub-TLV (Type 33, Length 4): one-bit A field, 7 reserved bits, and a 24-bit Delay field encoding average link delay in microseconds as an integer. Maximum encodable value is 16,777,215 microseconds (approximately 16.8 seconds).
Unidirectional Link Loss sub-TLV (Type 36, Length 4): one-bit A field, 7 reserved bits, and a 24-bit Link Loss field representing packet loss as a percentage with basic unit 0.000003%; maximum encodable loss is 50.331642%.
Unidirectional Residual Bandwidth sub-TLV (Type 37, Length 4): 8 reserved bits and a 32-bit Residual Bandwidth field in IEEE floating-point format with units of bytes per second. No A bit is included.
Unidirectional Utilized Bandwidth sub-TLV (Type 39, Length 4): 8 reserved bits and a 32-bit Utilized Bandwidth field in IEEE floating-point format, bytes per second, representing actual measured link utilization.