A YANG Data Model for PFC Queue Telemetry

Introduction A pause-frame count is not the duration for which a queue contained traffic while its applicable PFC gate was closed. Pause requests may overlap, refresh a timer, end early, or occur when no traffic awaits service. Management systems comparing queue measurements require the measurement interval, queue mapping, and treatment of unavailable observations as well as the duration value. Existing facilities are substantial: exposes queue pause/deadlock state and counters, describes implemented pause-duration monitoring, covers Ethernet PFC management, and and provide QoS management. includes a pause fraction in a link-quality proposal. This document does not introduce pause measurement. Its candidate increment is a common per-queue backlog/gate intersection with explicit interval continuity and uncertainty. The module is an independently reviewable representation of that contract. Integration into existing queue operational models should be considered before allocating another namespace. The underlying PFC behavior remains an IEEE function; coordination with relevant IEEE management work, including , is needed. A standalone IETF RFC is not assumed to be the only outcome.

Requirements Language The key words MUST, MUST NOT, REQUIRED, SHOULD, SHOULD 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.

Measurement Definition Consider one instantiated egress queue over a half-open monotonic interval [t0,t1), with T=t1-t0. Let Q(t) be true when at least one packet in that queue awaits transmission. A packet already handed to the transmission engine is not queued backlog. The service candidate is the packet selected by the implementation's ordinary ordering within that queue before received-PFC gating; it need not have won arbitration against other queues. Let P(t) be true when received PFC closes the gate applicable to that candidate. The candidate's applicable priority and the actual gate must be identifiable. If several priorities share a queue, a union or sum of all their pause durations is not a substitute for the candidate's gate. If the candidate or gate cannot be determined, the record has unknown quality. Q = integral over [t0,t1) of Q(t) dt I = integral over [t0,t1) of (Q(t) AND P(t)) dt 0 <= I <= Q <= T The module reports Q as backlog-ns, I as gated-backlog-ns, and T as duration-ns. Other constraints, such as shaping, arbitration, or link availability, may be active at the same time. They are not subtracted from I. Thus I measures an observed conjunction; it is not the counterfactual extra delay caused solely by PFC, nor a packet-level blocked-time sum. A continuously backlogged queue with a closed PFC gate contributes to I even if a shaper also blocks it. PFC timer refresh, zero-quantum resume, and watchdog overrides MUST be reflected in the actual gate state. Summing requested pause quanta is not permitted. The observation MUST NOT alter the IEEE gate state, scheduling order, or watchdog behavior. A change of candidate selection method or queue-to-priority mapping terminates the current epoch before a new mapping is observed.

Queue and Interval Identity A record is identified by interface, queue-id, epoch-id, and interval-id. queue-id is stable only within its interface and epoch. epoch-id is a freshly generated UUID whenever the queue is recreated, mapping or measurement method changes, the measurement subsystem restarts, or interval identity/clock state is lost. It MUST NOT be reused for the same interface and queue. The mapped-priority set is constant during an epoch and includes one through eight PFC priorities. start-offset-ns is a monotonic offset from a local origin fixed for that queue epoch. The origin has no cross-device meaning. The first interval can start at a nonzero offset. duration-ns is positive and at most one day. The end offset MUST be no greater than 2^53-1 nanoseconds; the server starts a new epoch before this limit is reached. This bounded representation permits exact comparisons in YANG XPath. Epoch renewal at this limit is ordinary behavior, not a network fault. interval-id starts at one and increments for each completed interval, including unknown-quality intervals. IDs MUST NOT wrap. Within an epoch, intervals MUST NOT overlap or move backward. Missing acquisition time can be represented by a gap in offsets; no duration is invented for it. A mapping change closes a positive-duration interval under the old epoch and starts the next under a fresh epoch. If no positive-duration record can be formed, the new epoch begins without a zero-length record. Optional wall-clock-start permits correlation with external events. It does not replace the monotonic offset and duration; no clock synchronization accuracy is implied. Wall-clock correction alone need not change an epoch. Loss of monotonic-clock validity requires a fresh epoch and missing coverage, not a fabricated interval length.

Measurement Method and Error Bounds event-based quality means that relevant state transitions were integrated. It does not mean infinite timestamp precision. sampled quality means that predicate samples were used, with sample-period-ns reporting the maximum spacing, including jitter, during that interval. The period MUST be positive and no greater than T. A shortened interval that lacks valid samples is unknown. Both known-quality modes MUST include I, Q, and absolute-error-ns, denoted E. For each reported duration separately, the implementation MUST have a defensible worst-case bound |reported-true|<=E, including quantization, capture uncertainty and sampling error. E MUST lie in [0,T], and the reported point values MUST satisfy I<=Q<=T. A sample period alone does not bound arbitrary aliasing. When no tighter bound is justified, E=T is permitted; unknown is preferable when the measurement itself is unavailable. unknown quality MUST include unknown-reason and MUST omit both measured durations, their error bound, and sample period. Unsupported measurement, unresolved multi-priority candidates, or lost measurement events MUST NOT appear as zero durations. An empty queue with a valid observation is different: Q=I=0 with known quality. This version uses one quality classification for the paired measurement rather than partial validity of individual leaves.

Publication, Retrieval, and Aggregation A server MUST publish a completed queue record atomically. Reads and subscriptions observe either the previous complete record or the next complete record. Repeated reads of the same identity MUST contain identical measurement fields. The module holds only the latest completed interval; it does not promise a history buffer. The server SHOULD document its measurement cadence and the implementation limits on event collection and sampling. NETCONF, RESTCONF, and YANG-Push can retrieve these records. Subscription update cadence is distinct from acquisition duration; a subscription may skip completed intervals. Filtered reads may omit required context at the client's request; clients MUST obtain a complete row before validating or aggregating it. Atomicity is per queue record, not across all queues or devices. A consumer MUST deduplicate by the complete identity, detect report-ID gaps and offset gaps, and reject same-identity conflicting content or overlapping/backward intervals. It MUST NOT sum a repeated record. It MUST NOT fill unobserved intervals with zeros. An epoch change creates a measurement boundary, even if the queue name and priority set appear unchanged. A disappearing queue or subscription disconnect does not establish an interval end. For contiguous records with one epoch and mapping, compute aggregate I/T as sum(I)/sum(T), and aggregate Q/T similarly. Do not average ratios without duration weights. Known and unknown intervals MUST NOT be combined into a complete-coverage result. A collector can present a partial-coverage result if it explicitly reports the missing time. Across epochs, it may aggregate separately qualified groups, but MUST NOT claim continuity. For one record, clip the interval for I to [max(0,I-E), min(T,I+E)] and similarly for Q. Ratios to T retain these bounds divided by T. For a sum, add absolute errors and clip to the aggregate duration. I/Q is undefined when Q=0. If Q-E<=0, a nontrivial bound on I/Q is generally unavailable; [0,1] is the conservative range. No probability distribution or confidence level is implied by an absolute error bound.

Use and Limits in Congestion Diagnosis A collector can distinguish a busy paused queue from pause activity observed only while a queue was empty. It can also identify intervals for which the paired measurement is unavailable. These are operational observations, not a proof of the remote bottleneck or the identity of traffic causing it. The model MUST NOT be interpreted as an instruction to suppress ECN, clear CE, change Not-ECT handling, or retransmit packets. and remain applicable. PFC and other service restrictions can overlap; a high I/Q ratio cannot establish that the local queue contributes no congestion. Cross-correlation with RoCEv2 observations also cannot prove packet loss or validate an ECN control loop. Controllers that choose to consume this telemetry need separate stability and performance evaluation under . The measurement does not standardize a reward function, pause-aware marking exception, or control-loop algorithm.

YANG Tree Diagram The tree is generated from the module using the notation in . module: ietf-pfc-queue-telemetry augment /if:interfaces/if:interface: +--ro pfc-queue-telemetry +--ro queue* [queue-id] +--ro queue-id string +--ro epoch-id yang:uuid +--ro interval-id uint64 +--ro start-offset-ns uint64 +--ro duration-ns uint64 +--ro wall-clock-start? yang:date-and-time +--ro mapped-priority* uint8 +--ro quality enumeration +--ro unknown-reason enumeration +--ro gated-backlog-ns interval-value +--ro backlog-ns interval-value +--ro absolute-error-ns interval-value +--ro sample-period-ns uint64

YANG Module The module uses YANG 1.1 , NMDA , the interface model , and common types . Review follows . RFC XXXX in the module is an RFC Editor placeholder. module ietf-pfc-queue-telemetry { yang-version 1.1; namespace "urn:ietf:params:xml:ns:yang:ietf-pfc-queue-telemetry"; prefix pqt; import ietf-yang-types { prefix yang; reference "RFC 9911: Common YANG Data Types"; } import ietf-interfaces { prefix if; reference "RFC 8343: A YANG Data Model for Interface Management"; } organization "Individual Submission"; contact "Author: Jin Yang <mailto:yangjinwl@chinamobile.com> Author: Weiqiang Cheng <mailto:chengweiqiang@chinamobile.com> Author: Tingting Han <mailto:hantingting@chinamobile.com> Author: Junjie Wang <mailto:wangjj@centec.com> Author: Siyuan Zhang <mailto:zhangsy@centec.com>"; description "Completed intervals of queue backlog and received PFC gating. These observations do not identify the sole cause of delay. Copyright (c) 2026 IETF Trust and the persons identified as authors of the code. All rights reserved. Redistribution and use in source and binary forms, with or without modification, is permitted pursuant to, and subject to the license terms contained in, the Revised BSD License set forth in Section 4.c of the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info). This version of this YANG module is part of RFC XXXX (https://www.rfc-editor.org/info/rfcXXXX); see the RFC itself for full legal notices."; revision 2026-10-30 { description "Initial revision."; reference "RFC XXXX: A YANG Data Model for PFC Queue Telemetry"; } typedef interval-value { type uint64 { range "0..86400000000000"; } units "nanoseconds"; description "Duration or absolute error within a one-day interval."; } augment "/if:interfaces/if:interface" { description "Completed PFC queue observations on this interface."; container pfc-queue-telemetry { config false; description "Latest completed interval for each observed queue."; list queue { must 'start-offset-ns <= 9007199254740991 - duration-ns'; must 'not(gated-backlog-ns) or ' + 'gated-backlog-ns <= backlog-ns'; must 'not(backlog-ns) or backlog-ns <= duration-ns'; must 'not(absolute-error-ns) or absolute-error-ns <= duration-ns'; must 'not(sample-period-ns) or sample-period-ns <= duration-ns'; key "queue-id"; description "One coherent completed record; never a partly updated row. Queue identity is scoped to this interface and epoch."; leaf queue-id { type string { length "1..128"; } description "Stable local queue identifier."; } leaf epoch-id { type yang:uuid; mandatory true; description "Fresh identity on restart, queue reuse, mapping or measurement-method change."; } leaf interval-id { type uint64 { range "1..max"; } mandatory true; description "Starts at one; advances for every completed interval without reuse."; } leaf start-offset-ns { type uint64 { range "0..9007199254740991"; } units "nanoseconds"; mandatory true; description "Monotonic start relative to this queue epoch; not a wall clock."; } leaf duration-ns { type uint64 { range "1..86400000000000"; } units "nanoseconds"; mandatory true; description "Monotonic duration T of the half-open interval."; } leaf wall-clock-start { type yang:date-and-time; description "Optional correlation timestamp; clock synchronization accuracy is external."; } leaf-list mapped-priority { type uint8 { range "0..7"; } min-elements 1; max-elements 8; description "PFC priorities mapped to this queue in the epoch."; } leaf quality { type enumeration { enum event-based { description "State-transition integration with bounded error."; } enum sampled { description "State samples with a worst-case error bound."; } enum unknown { description "Joint measurement unavailable; durations omitted."; } } mandatory true; description "Measurement method and validity for this interval."; } leaf unknown-reason { when "../quality = 'unknown'"; type enumeration { enum unavailable { description "Measurement not supported."; } enum candidate-unresolved { description "Cannot identify the queue service candidate."; } enum capture-gap { description "Measurement events lost."; } enum other { description "Other measurement failure."; } } mandatory true; description "Reason that both duration values are unavailable."; } leaf gated-backlog-ns { when "../quality != 'unknown'"; type interval-value; mandatory true; description "Time I when a nonempty queue candidate has a closed received-PFC gate; concurrent shaping or scheduling delays are not excluded."; } leaf backlog-ns { when "../quality != 'unknown'"; type interval-value; mandatory true; description "Time Q when at least one packet awaits service."; } leaf absolute-error-ns { when "../quality != 'unknown'"; type interval-value; mandatory true; description "Worst-case absolute error applicable separately to I and Q; includes quantization and sampling uncertainty."; } leaf sample-period-ns { when "../quality = 'sampled'"; type uint64 { range "1..86400000000000"; } units "nanoseconds"; mandatory true; description "Maximum spacing between samples, including jitter."; } } } } }

Examples and Conformance Cases For T=1000 ns, actual gate closure over [100,900) and continuous backlog produce I=800 and Q=1000. A timer extension at 400 ns must not double-count [400,600). If the queue is empty on [100,300), I=600 and Q=800. Concurrent shaping on [500,700) does not change those values under this definition. A multi-priority queue with no observable service-candidate priority reports unknown and candidate-unresolved. A valid queue with Q=0 reports I=0, even if every mapped priority is paused. If event loss makes the joint duration unreliable, the record reports capture-gap instead of zero. Contiguous intervals (T,I)=(100,50) and (900,90) yield I/T=140/1000=0.14, not the unweighted ratio 0.30. With E=10 and E=20, the aggregate error is at most 30/1000. Re-reading the first interval contributes nothing new. Negative cases include missing durations for event-based quality, any duration with unknown quality, I>Q, Q>T, E>T, a missing sampled period, conflicting same-identity reports, and overlapping offsets. Both schema constraints and procedural checks are required; schema validation cannot verify that physical gates were measured truthfully. The following operational-state example uses the JSON encoding in . uint64 values are strings. The interface and queue identity are illustrative; no deployment result is implied. { "ietf-interfaces:interfaces": { "interface": [ { "name": "Ethernet1/1", "type": "iana-if-type:ethernetCsmacd", "oper-status": "up", "ietf-pfc-queue-telemetry:pfc-queue-telemetry": { "queue": [ { "queue-id": "q3", "epoch-id": "31c38fe2-f97e-4a88-a0a7-bae9ed5d4131", "interval-id": "2", "start-offset-ns": "100000000", "duration-ns": "100000000", "mapped-priority": [ 3 ], "quality": "event-based", "gated-backlog-ns": "18500000", "backlog-ns": "64000000", "absolute-error-ns": "256" } ] }, "admin-status": "up", "if-index": 1, "statistics": { "discontinuity-time": "2026-10-30T00:00:00Z" } } ] } }

Security Considerations The module is intended for NETCONF or RESTCONF using their secure transports. NETCONF over SSH is specified in ; RESTCONF uses HTTPS. Access control such as NACM MUST restrict which users can read queue measurements and establish subscriptions. This model contains no writable nodes, RPCs, actions, or notifications. Readable interface/queue identities, priorities, timing, backlog and gating expose traffic activity and shared-resource conditions. Operators SHOULD limit access by tenant and operational role, limit retained precision where appropriate, and bound polling/subscription load. Opaque queue IDs do not alone anonymize a workload. An authenticated device can supply false but internally consistent telemetry. Epoch, interval, and arithmetic checks detect contradictions and discontinuities, not authenticity of hardware events. Consumers MUST preserve uncertainty and missing coverage rather than convert unavailable data into apparent absence of congestion.

IANA Considerations IANA is requested to register the following namespace in the IETF XML Registry , and the following module in the YANG Module Names registry . These are requested registrations, not assigned values. Requested Registration

Field	Value
URI	urn:ietf:params:xml:ns:yang:ietf-pfc-queue-telemetry
Registrant Contact	IESG
XML	N/A; XML namespace
Name	ietf-pfc-queue-telemetry
Prefix	pqt
Maintained by IANA	N
Reference	RFC XXXX