



SCONE                                                           N. Zhang
Internet-Draft                                                     X. Xu
Intended status: Informational                             China Telecom
Expires: 6 December 2026                                     4 June 2026


 Endpoint Handling of SCONE Throughput Advice Across QUIC Path Changes
                 draft-zhang-scone-migration-advice-00

Abstract

   SCONE throughput advice is scoped to the path and direction in which
   it is received.  When a QUIC connection changes path, the endpoint
   can retain an old advice value while that value is no longer
   applicable to the active path.  This document describes endpoint-
   local handling of that transition.  It focuses on the distinction
   between retained advice and active-path advice, the interval in which
   no fresh path-applicable advice is available, and the observability
   needed to avoid treating historical advice as current guidance.  This
   document defines no new SCONE protocol elements, does not modify QUIC
   path validation, does not define an application API, and does not
   change congestion control behavior.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 6 December 2026.

Copyright Notice

   Copyright (c) 2026 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.



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   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Relationship to Existing SCONE Work . . . . . . . . . . . . .   4
   3.  Conventions and Terminology . . . . . . . . . . . . . . . . .   4
   4.  Scope and Non-Goals . . . . . . . . . . . . . . . . . . . . .   5
   5.  Problem Statement . . . . . . . . . . . . . . . . . . . . . .   6
     5.1.  Protocol Semantics and Endpoint State . . . . . . . . . .   6
     5.2.  Path Change and Advice Applicability  . . . . . . . . . .   7
     5.3.  The Advice Gap  . . . . . . . . . . . . . . . . . . . . .   7
     5.4.  Reusing Old Advice on a New Path  . . . . . . . . . . . .   7
     5.5.  Directional Asymmetry . . . . . . . . . . . . . . . . . .   8
     5.6.  Implementation Ambiguity Examples . . . . . . . . . . . .   8
   6.  Why This Needs Explicit Handling  . . . . . . . . . . . . . .   8
   7.  Endpoint Handling Guidance  . . . . . . . . . . . . . . . . .   9
     7.1.  General Principle . . . . . . . . . . . . . . . . . . . .   9
     7.2.  Advice State Model  . . . . . . . . . . . . . . . . . . .   9
       7.2.1.  State Diagram . . . . . . . . . . . . . . . . . . . .  10
     7.3.  Behavioral Consequences of the Advice Gap . . . . . . . .  12
     7.4.  Behavior Upon Path Change . . . . . . . . . . . . . . . .  12
     7.5.  Behavior During the Advice Gap  . . . . . . . . . . . . .  13
     7.6.  Using Existing Signaling Opportunities on New Paths . . .  13
     7.7.  Arrival of Fresh Advice . . . . . . . . . . . . . . . . .  13
     7.8.  Advice Expiry on the Active Path  . . . . . . . . . . . .  14
     7.9.  Advice-State Transition Summary . . . . . . . . . . . . .  14
     7.10. Worked Example: Cellular to Wi-Fi Migration . . . . . . .  15
     7.11. Illustrative Representation of Advice State . . . . . . .  17
   8.  Manageability Considerations  . . . . . . . . . . . . . . . .  18
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  20
   10. Future Work . . . . . . . . . . . . . . . . . . . . . . . . .  20
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  20
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  20
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  20
     12.2.  Informative References . . . . . . . . . . . . . . . . .  21
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  21










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1.  Introduction

   SCONE provides throughput advice to endpoints for a particular
   network path and direction.  A QUIC [RFC9000] connection, however,
   can change the path that carries application traffic.  This can occur
   during connection migration, interface change, NAT rebinding, or
   similar path-level events.

   The SCONE protocol already defines the signaling mechanism and the
   path-scoped semantics of throughput advice.  In particular, advice
   received on one path is not assumed to apply to another path.  This
   document does not restate that rule as a new protocol requirement.
   Instead, it describes the endpoint-side state consequences of that
   rule.

   After a path change, an endpoint can still retain an advice value
   that was received on the previous path.  That retained value might be
   useful for diagnostics, telemetry, or local policy.  However,
   retaining a value is different from treating that value as current
   advice for the active path.  If an implementation does not preserve
   this distinction, stale advice can be exposed to local adaptation
   logic or operational telemetry as if it still represented the active
   path.

   This document describes the transition between old-path advice and
   fresh advice on the active path.  It uses the term "advice gap" for
   the interval during which previous-path advice is no longer
   applicable to the active path and fresh advice for the active path is
   not yet available.  The advice gap is an endpoint-local condition
   used to describe state handling and observability.  It is not a new
   SCONE protocol state and does not require any new wire-visible
   signal.

   The guidance in this document is intentionally limited.  It does not
   define a fallback sending-rate algorithm, does not modify QUIC loss
   recovery or congestion control, and does not define how an
   application adapts its media rate.  Its purpose is to help
   implementations avoid conflating three different conditions: explicit
   low-rate advice, absence of path-applicable advice, and retained
   historical advice.

   This document focuses on cases in which application traffic
   transitions to a different active path for a single-path QUIC
   connection.  Multipath-specific handling is outside the scope of this
   document.






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2.  Relationship to Existing SCONE Work

   The SCONE protocol [SCONE-PROTOCOL] defines the wire mechanism for
   communicating throughput advice and the semantics of that advice.  It
   also describes opportunities for endpoints to send SCONE packets on a
   path so that network elements can provide advice.  This document does
   not change the SCONE protocol, does not add a new signaling trigger,
   and does not define a new requirement for network elements.

   The SCONE applicability and manageability document [SCONE-APPMAN]
   discusses broader deployment, operational, and monitoring
   considerations.  This document is narrower.  It focuses only on
   endpoint-visible advice-state transitions caused by QUIC path
   changes.

   The specific gap addressed by this document is the implementation
   boundary between the protocol semantics of path-scoped advice and the
   endpoint's local representation of advice state.  An implementation
   might store an old advice value, expose advice to an application
   component, log advice for operations, or apply local policy based on
   advice.  This document describes how those uses can remain distinct
   from current active-path advice after a path change.

   This document is intended as implementation and manageability
   guidance.  If the working group determines that some of this guidance
   belongs in the SCONE protocol or applicability and manageability
   documents, the text here can be used as input to that discussion.

3.  Conventions and Terminology

   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 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   old path:  A path that previously carried application data for the
      QUIC connection and for which throughput advice had been received.

   new path:  A path that has become active, or is becoming active, for
      the same QUIC connection after migration or path re-establishment.

   fresh advice:  Throughput advice that is applicable to the currently
      active path and remains valid according to the semantics defined
      by the SCONE protocol.

   stale advice:  Throughput advice that is retained by an endpoint but




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      is no longer known to be applicable to the currently active path,
      for example because it was received on a previous path.

   advice gap:  An endpoint-local transition condition during which
      advice associated with a previous path is no longer applicable to
      the active path, while fresh advice for the active path is not yet
      available.  An advice gap is not a new SCONE protocol state.

   path re-establishment:  A transition in which endpoint traffic begins
      using a different path for the same QUIC connection, whether due
      to migration, interface change, rebinding, or similar path-level
      events.

   historical advice:  Advice information retained by an endpoint after
      it ceases to be known to apply to the active path.  Historical
      advice can be retained for diagnostics, telemetry, or local
      policy, but it is not current advice for the active path.

   active-path advice state:  The endpoint's internal representation of
      whether fresh SCONE advice is currently available for the active
      path and direction.  This is an implementation concept and is not
      a wire-visible SCONE field.

   path context:  An endpoint-local representation of the network path
      to which advice applicability is associated.  This document does
      not define a wire-visible path identifier.

4.  Scope and Non-Goals

   This document is limited to endpoint handling guidance for SCONE
   throughput advice across QUIC path changes.

   This document does not:

   *  define any new SCONE packet format, frame, capsule, transport
      parameter, or other protocol element;

   *  define any application-facing or browser-facing API;

   *  modify QUIC loss recovery or congestion control;

   *  define a multipath scheduling algorithm; or

   *  require a network element to retain, transfer, or reinterpret
      advice across paths.






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   This document also does not require an endpoint to compute a specific
   fallback sending rate during the advice gap.  Such policies remain an
   implementation matter, subject to existing transport behavior,
   congestion control, application adaptation logic, and deployment
   needs.

   This document does not define a normative endpoint state machine.
   The states and transitions described here are implementation-facing
   distinctions that help prevent stale or historical advice from being
   consumed as current active-path advice.

   The manageability considerations in this document are limited to
   events created by path changes and advice transitions at endpoints.
   They complement, rather than replace, more general SCONE deployment
   and logging guidance in [SCONE-APPMAN].

   This document describes a minimum set of endpoint-side distinctions
   that are useful across path changes: whether advice is fresh, stale,
   expired, absent, or retained only as historical advice; whether the
   endpoint is in an advice gap for a given direction; and when that
   advice gap begins and ends.

5.  Problem Statement

5.1.  Protocol Semantics and Endpoint State

   The protocol-level rule is that throughput advice is scoped to the
   path and direction for which it is received.  The implementation
   question is how an endpoint represents that rule after a path change.
   This distinction matters because a stored advice value can have
   several different uses inside an endpoint.

   For example, the value can be retained for diagnostics, reported
   through telemetry, made visible to a local adaptation component, or
   used by local policy.  These uses are not equivalent.  A value that
   is safe to retain as historical information is not necessarily safe
   to expose as current active-path advice.

   The problem addressed by this document is therefore not how to
   transfer advice from one path to another.  It is how to avoid
   confusing retained old-path advice with fresh advice for the active
   path.









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5.2.  Path Change and Advice Applicability

   SCONE throughput advice is meaningful only in the context in which it
   was generated.  When a QUIC endpoint migrates a connection from one
   path to another, advice associated with the old path cannot be
   assumed to remain applicable to the new path.  The bottleneck, access
   network, and traffic treatment on the new path can differ from those
   of the old path.

   This creates a transition condition distinct from formal expiry.
   Advice can cease to be applicable to the active path when the path
   changes, while protocol-defined expiry remains a separate condition.
   These conditions need not coincide.

5.3.  The Advice Gap

   In practice, a new path can start carrying application data before
   fresh SCONE advice has arrived for that path.  This creates an advice
   gap: advice associated with the previous path is no longer applicable
   to the active path, while fresh advice for the active path is not yet
   available.  This condition is especially visible when:

   *  migration occurs across access technologies;

   *  the new path was not previously active;

   *  signaling cadence is coarse relative to path-change timing; or

   *  the endpoint begins using the new path for application data as
      soon as QUIC path validation permits.

5.4.  Reusing Old Advice on a New Path

   Reusing old advice on a new path can be too conservative.  If the new
   path offers a higher achievable rate than the old path, carrying
   forward prior advice can unnecessarily suppress endpoint sending
   behavior.

   Reusing old advice can also be too aggressive.  If the new path is
   narrower or is subject to different policy treatment, carrying
   forward prior advice can cause the endpoint to act on guidance that
   no longer reflects current path conditions.









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5.5.  Directional Asymmetry

   SCONE advice is directional.  Following a connection migration, fresh
   advice for one direction can become available before fresh advice for
   the opposite direction.  As a result, an endpoint can simultaneously
   hold fresh advice for one direction and stale or absent advice for
   the other.

   This can require implementations to track advice transitions on a
   per-direction basis where applicable.

5.6.  Implementation Ambiguity Examples

   The transition described above can lead to materially different
   endpoint behavior if implementations do not preserve a clear
   distinction between stored advice values and active-path
   applicability.

   For example, one implementation might retain previously received
   advice and continue to surface it to a local adaptation or policy
   module after migration, effectively treating cached old-path advice
   as if it still constrained the active path.  Another implementation
   might clear all advice-related state immediately upon path change.  A
   third implementation might retain the value internally but correctly
   mark the active path as having no current advice.

   These behaviors are not equivalent.  They can lead to different
   sending decisions, different policy outcomes, and different telemetry
   interpretations even when the same SCONE advice was originally
   received.

   A similar ambiguity can arise when advice is directional.  Following
   migration, fresh advice for one direction might become available
   before the other.  An implementation that models advice state only
   once per connection can conflate those cases, while an implementation
   that tracks advice state per direction can preserve the distinction.

6.  Why This Needs Explicit Handling

   The rule that old-path advice is not fresh advice for a new path is
   simple.  However, the consequences of that rule are not always
   captured by a single stored variable.  An endpoint might store the
   last received advice value, an expiry time, the path on which it was
   received, an application-facing estimate, and telemetry fields.  If
   those fields are not updated consistently after a path change,
   different components can observe different interpretations of the
   same advice value.




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   This creates several implementation risks.  First, absence of current
   path-applicable advice can be confused with explicit low-rate advice.
   Second, retained historical advice can be exposed to local policy or
   application adaptation logic as if it were current guidance.  Third,
   telemetry can report that advice is present even when no fresh advice
   is available for the active path.  Fourth, per-direction differences
   can be lost if advice state is tracked only once per connection.

   Explicit endpoint handling is therefore useful even when no new
   protocol mechanism is required.  It provides a common way to describe
   the transition from fresh advice to no fresh active-path advice, and
   from no fresh active-path advice back to fresh advice when a new
   signal is received.  It also gives implementations and operators
   observable events that can be used to diagnose whether stale advice
   was accidentally reused across a path change.

7.  Endpoint Handling Guidance

7.1.  General Principle

   An endpoint that changes to a new QUIC path SHOULD NOT treat advice
   associated with the old path as fresh advice for the new path, unless
   fresh advice has been received that is applicable to the new path.

   This does not imply that the endpoint needs to delete old advice.  An
   implementation can retain old advice as historical information for
   diagnostics, telemetry, or local policy.  The important requirement
   is that retained historical advice remains distinguishable from
   current active-path advice.

   In particular, the endpoint needs to preserve the distinction between
   three conditions: current active-path advice is available; no current
   active-path advice is available; and an old advice value is retained
   but no longer applicable to the active path.

7.2.  Advice State Model

   For endpoint handling across a path change, implementations need to
   distinguish the value of previously received advice from its
   applicability to the active path.  This distinction can be
   represented in different ways.  This document does not require a
   specific data structure or state machine.

   For the relevant direction, an implementation can usefully
   distinguish at least the following conditions:

   *  fresh advice: advice is available, has not expired, and is known
      to apply to the active path;



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   *  stale advice: advice information is retained, but is not known to
      apply to the active path;

   *  expired advice: advice that was previously fresh for the active
      path is no longer fresh due to expiry;

   *  absence of advice: no fresh advice is available for the active
      path; and

   *  advice gap: a path-change transition during which previous-path
      advice is no longer applicable to the active path and fresh advice
      for the active path is not yet available.

   The advice gap is not a replacement for the active-path advice state.
   It is a transition condition that explains why the endpoint currently
   has no fresh advice for the affected active path and direction.

   These distinctions are useful because the same stored advice value
   can be safe to retain for diagnostics but unsafe to consume as
   current active-path guidance.

7.2.1.  State Diagram





























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   +-----------------------+
   | No Advice             |
   | (Initial / Reset)     |
   +-----------+-----------+
               |
               | Fresh advice arrives
               | on active path
               |
               v
   +-----------------------+
   | Fresh Advice          |<---------+
   | (Active Path)         |          |
   +-----------+-----------+          |
               |                      |
               | Path changes         | Fresh advice arrives
               v                      | on new path
   +-----------------------+          |
   | Advice Gap            |----------+
   | (New Path, No         |
   | Fresh Advice)         |
   +-----------+-----------+
               |
               | No fresh advice arrives;
               | retained advice expires
               |
               v
   +-----------------------+
   | No Fresh Active-Path  |
   | Advice                |
   | (Active Path)         |
   +-----------------------+

                                  Figure 1

   Figure 1 illustrates a simplified view of active-path advice-state
   transitions on a per-direction basis.  It is not intended to
   represent all retained internal state.  In particular, historical
   advice may be retained separately from the active-path advice state
   shown here.

   The transitions shown are: initial receipt of fresh advice; path
   change leading to an advice gap; receipt of fresh advice on the new
   path ending the advice gap; and loss of fresh active-path advice
   without immediate replacement.  An implementation may preserve finer
   distinctions internally, including separate expired and absent
   conditions, so long as it preserves at least the behavioral
   distinctions required by this document.




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7.3.  Behavioral Consequences of the Advice Gap

   The advice gap is not merely a bookkeeping condition.  Once an
   endpoint determines that previously received advice is no longer
   applicable to the active path, SCONE-related state needs to be
   interpreted differently by the endpoint and by any local policy logic
   that consumes that state.

   In particular, for the affected direction, the endpoint SHOULD ensure
   that absence of current path-applicable advice is distinguishable
   from explicit low-rate advice.  It should also ensure that any
   retained historical advice is not surfaced to local policy or upper-
   layer components as if it were current advice for the active path.

   Entering an advice gap therefore changes not only the label attached
   to stored advice, but also the conditions under which SCONE-related
   state can be consumed as current path guidance.

7.4.  Behavior Upon Path Change

   When an endpoint determines that a different path is becoming active
   for a QUIC connection, the following handling applies for each
   affected direction:

   1.  The endpoint disassociates advice that was received for the old
       path from the active-path advice state of the new path.

   2.  The endpoint stops representing old-path advice as fresh advice
       for the new path, even if it retains the advice value or related
       metadata locally.

   3.  Unless fresh advice is already available for the new path, the
       endpoint marks the new path as being in an advice gap for the
       affected direction.

   4.  During that interval, the endpoint treats the new path as having
       no current SCONE advice for the affected direction.

   5.  The endpoint can retain prior advice as historical advice for
       diagnostics, telemetry, or local implementation policy, provided
       that such retained state remains distinct from active-path advice
       state.

   This guidance does not require immediate deletion of all previously
   received advice information.  It requires a change in applicability:
   advice that was fresh for the old path is no longer presumed fresh
   for the active path after the path change.




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7.5.  Behavior During the Advice Gap

   The following guidance applies while the endpoint remains in the
   advice-gap condition for a direction.

   During the advice gap, the endpoint has no current path-applicable
   SCONE advice for the affected direction.  This condition is different
   from receiving explicit low-rate advice.  It is also different from
   having never received advice, because the endpoint can retain
   previous-path advice as historical information.

   Accordingly, an endpoint SHOULD NOT apply old-path advice as active
   advice for the new path.  It SHOULD NOT combine stale or historical
   advice with fresh active-path advice in a way that represents them as
   belonging to the same path context.  It SHOULD NOT expose stale or
   historical advice to local policy or upper-layer components as if it
   were fresh advice for the active path.

   An endpoint can retain historical advice for diagnostics, telemetry,
   or local heuristics.  Any such retained information remains
   historical and needs to stay distinguishable from fresh advice for
   the active path.

7.6.  Using Existing Signaling Opportunities on New Paths

   The SCONE protocol allows endpoints to send SCONE packets on a path,
   subject to the rules defined by that protocol.  When an endpoint
   supports SCONE and performs QUIC path validation, existing SCONE
   signaling opportunities on the new path can give network elements an
   earlier opportunity to provide fresh advice for that path.

   This document does not define a new signaling trigger and does not
   alter QUIC path validation.  It only notes that using already defined
   signaling opportunities can reduce the duration of operating without
   fresh path-applicable advice after a path change.

7.7.  Arrival of Fresh Advice

   When fresh advice becomes available for the active path, the endpoint
   SHOULD:

   1.  update its active-path advice state for the relevant direction;

   2.  mark the advice gap as ended for that direction; and

   3.  thereafter apply the fresh advice according to the normal rules
       of the SCONE protocol.




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   An endpoint SHOULD NOT combine, average, or otherwise merge fresh
   advice for the active path with stale or historical advice retained
   from the old path as if they referred to the same path context.

   Fresh advice can arrive at different times for different directions.
   Implementations SHOULD therefore manage advice-gap entry, advice-gap
   exit, and fresh-advice state on a per-direction basis where
   applicable.

7.8.  Advice Expiry on the Active Path

   If advice for the currently active path expires before replacement
   advice is received, the endpoint SHOULD stop treating that advice as
   fresh advice for the active path.

   After expiry, the endpoint no longer has fresh path-applicable advice
   for that direction on the active path.  In that respect, expiry and
   path change can lead to a similar operational condition: the endpoint
   is again operating without fresh active-path advice.

   However, expiry on the active path and path change to a different
   active path are distinct events and SHOULD remain distinguishable for
   observability and local policy.

7.9.  Advice-State Transition Summary

   This subsection restates, in summary form, the minimum endpoint-
   visible state transitions and corresponding handling outcomes
   described in the preceding subsections.






















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    +=================+=============+================================+
    | Event           | Resulting   | Endpoint Handling Outcome      |
    |                 | Condition   |                                |
    +=================+=============+================================+
    | Path change, no | Advice gap  | Disassociate old-path advice   |
    | fresh advice on |             | from active-path advice state; |
    | new path        |             | treat new path as having no    |
    |                 |             | current SCONE advice           |
    +-----------------+-------------+--------------------------------+
    | Path change,    | Fresh       | Switch active-path advice      |
    | fresh advice    | advice on   | state to the new path and      |
    | already         | new path    | direction                      |
    | available       |             |                                |
    +-----------------+-------------+--------------------------------+
    | Fresh advice    | Advice gap  | Update active-path advice      |
    | arrives during  | ends        | state; mark gap as ended for   |
    | advice gap      |             | that direction                 |
    +-----------------+-------------+--------------------------------+
    | Advice expires  | No fresh    | Stop treating expired advice   |
    | on active path  | active-path | as fresh advice for the active |
    |                 | advice      | path                           |
    +-----------------+-------------+--------------------------------+

      Table 1: Endpoint Advice-State Transitions Across Path Changes

7.10.  Worked Example: Cellular to Wi-Fi Migration

   Unlike the abstract state summary above, this subsection illustrates
   one concrete single-path migration timeline and shows how the
   distinctions in this document apply in practice.

   This example is intended only to illustrate advice-state transitions.
   It does not specify how an endpoint computes transport behavior or
   application adaptation during the advice gap.

















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   T0: A client establishes a QUIC connection over a cellular
       interface (Path A).

   T1: A network element on Path A sends SCONE throughput advice:
       downstream = 20 Mbps. The endpoint's active-path advice
       state for the downstream direction becomes:
         { path=A, direction=downstream, status=fresh, value=20Mbps }

   T2: The client migrates the QUIC connection to a Wi-Fi
       interface (Path B). Following the path-change handling
       described in this document, the endpoint:
       - disassociates the Path A advice from active-path
         advice state;
       - marks the downstream direction as being in an
         advice gap; and
       - may retain the Path A advice as historical advice.

       Active-path advice state becomes:
         { path=B, direction=downstream, status=absent }

       The endpoint is now in the advice gap for the downstream
       direction.
       In this example, that advice-gap condition corresponds
       to the downstream active-path advice state being absent
       until fresh advice arrives on Path B.

   T3: The client sends QUIC PATH_CHALLENGE on Path B. The
       endpoint uses existing SCONE signaling opportunities to
       provide an early opportunity for network elements on
       Path B to supply fresh advice.

   T4: A network element on Path B responds with fresh advice:
       downstream = 80 Mbps. Upon receiving that fresh advice,
       the endpoint:
       - updates active-path advice state to:
         { path=B, direction=downstream, status=fresh, value=80Mbps }
       - marks the advice gap as ended for the downstream
         direction.

   Duration of the advice gap: T4 - T2.

                                  Figure 2

   This example illustrates both risks identified earlier: carrying
   forward old-path advice can be either unnecessarily conservative or
   incorrectly aggressive, depending on the new path.





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7.11.  Illustrative Representation of Advice State

   This section provides a purely illustrative example of how the
   advice-state distinctions described in this document might be
   represented internally.  It does not define a required data
   structure, API, or implementation architecture.

   One possible internal representation for per-direction active-path
   advice state is shown below in pseudocode.  The path_context field is
   endpoint-local and does not define a wire-visible path identifier.

           AdviceState {
             path_context:    LocalPathContext
             direction:       enum { upstream, downstream }
             status:          enum { fresh, stale, expired, absent }
             value:           optional<ThroughputAdvice>
             received_time:   optional<Timestamp>
             expiry_time:     optional<Timestamp>
           }

   For example, after a path change (7.4), an implementation might
   update the active-path advice representation by:

   1.  copy the previous active-path advice state to historical advice,
       if retention is useful;

   2.  set active_advice[direction].path_context to the new path
       context;

   3.  set active_advice[direction].status to absent unless fresh advice
       is already available for the new path; and

   4.  record advice-gap entry for the affected direction if no fresh
       advice is available.

   Upon receipt of fresh advice on the new path, an implementation might
   update that representation by:

   1.  set active_advice[direction].status to fresh;

   2.  set active_advice[direction].value to the received throughput
       value;

   3.  set active_advice[direction].received_time to the current time;

   4.  set active_advice[direction].expiry_time according to the SCONE
       protocol semantics; and




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   5.  record advice-gap exit for the affected direction, if the
       endpoint was in an advice gap.

   These illustrations are provided to aid implementers.  Other internal
   representations are equally valid so long as they preserve the
   distinction between current active-path advice and retained
   historical advice.

8.  Manageability Considerations

   The manageability considerations in this document focus on endpoint-
   visible events created by path changes and advice-state transitions.
   They complement, rather than replace, more general SCONE deployment
   and logging guidance in [SCONE-APPMAN].

   Endpoint observability is useful because stale-advice reuse can
   otherwise be difficult to diagnose.  A flow can appear to have advice
   available because an old value remains stored, even though no fresh
   advice is available for the active path.  Similarly, an application
   component might observe a rate value without being able to determine
   whether it is current active-path advice or retained historical
   information.

   Implementations that support SCONE across QUIC path changes can
   provide observability for the following questions:

   *  whether a path change occurred;

   *  whether the endpoint had fresh advice before the path change;

   *  whether the endpoint entered an advice gap for a given direction;

   *  whether old-path advice was retained only as historical advice;

   *  whether historical advice was kept separate from active-path
      advice state;

   *  when fresh advice became available on the new path; and

   *  how long the advice gap lasted.

   The following table suggests observable events that implementations
   can record for manageability purposes.  It is a non-normative example
   of observability support rather than a required event taxonomy.







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      +=======================+================+===================+
      | Event Name            | Trigger        | Suggested Log     |
      |                       |                | Fields            |
      +=======================+================+===================+
      | advice_path_change    | Active path    | old_path_context, |
      |                       | changes        | new_path_context, |
      |                       |                | timestamp,        |
      |                       |                | had_fresh_advice  |
      +-----------------------+----------------+-------------------+
      | advice_gap_start      | Endpoint       | path_context,     |
      |                       | enters advice  | direction,        |
      |                       | gap            | timestamp         |
      +-----------------------+----------------+-------------------+
      | advice_gap_end        | Fresh advice   | path_context,     |
      |                       | received on    | direction,        |
      |                       | new path       | timestamp,        |
      |                       |                | gap_duration      |
      +-----------------------+----------------+-------------------+
      | advice_expired        | Advice expires | path_context,     |
      |                       | on active path | direction,        |
      |                       | without        | timestamp,        |
      |                       | replacement    | expired_value     |
      +-----------------------+----------------+-------------------+
      | advice_stale_retained | Old advice     | old_path_context, |
      |                       | kept as        | direction,        |
      |                       | historical     | retained_value    |
      +-----------------------+----------------+-------------------+

                   Table 2: Suggested Observable Events

   These event names are illustrative.  Implementations are not required
   to use these specific names.

   Where per-direction advice handling is implemented, these
   observations SHOULD be available per direction.

   Such observability is useful for verifying that an implementation
   does not silently reuse stale advice across path changes, and that
   advice-gap handling remains distinct from handling explicit current-
   path rate limitation.  It can also assist in distinguishing normal
   advice transitions, deployment issues, endpoint policy choices, and
   cases where the new path does not involve SCONE-capable network
   elements.








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9.  Security Considerations

   This document introduces no new protocol mechanism and therefore does
   not create a new signaling attack surface beyond that already
   considered by SCONE and QUIC.

   Incorrect treatment of stale advice as fresh advice can nevertheless
   create security-relevant or policy-relevant misbehavior.  If stale
   high-rate advice is treated as current advice on a narrower or
   differently managed new path, the endpoint can behave inconsistently
   with the active path's policy or capacity.  If stale low-rate advice
   is treated as current advice on a wider new path, the endpoint can
   unnecessarily degrade application behavior.  These are not new
   attacks created by this document, but examples of why active-path
   applicability needs to remain explicit.

   Implementations ought to distinguish clearly between fresh advice,
   stale advice, expired advice, absence of advice, and historical
   advice retained for diagnostics or local policy.  Exposing
   operational state that conflates these conditions can lead to
   incorrect operational decisions or misleading telemetry.

   Logging and telemetry related to path changes can also introduce
   privacy considerations if correlated too precisely across interfaces,
   networks, or access changes.  Implementations ought to apply ordinary
   care when exporting such data.

10.  Future Work

   This document focuses on single-path QUIC connection migration.
   Related topics that may merit future work include multipath cases,
   interaction with application-level adaptation logic, and handling
   when a connection later returns to a previously used path.  These
   topics are intentionally left outside the present scope.

11.  IANA Considerations

   This document has no IANA actions.

12.  References

12.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.




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   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC9000]  Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
              Multiplexed and Secure Transport", RFC 9000,
              DOI 10.17487/RFC9000, May 2021,
              <https://www.rfc-editor.org/info/rfc9000>.

   [SCONE-PROTOCOL]
              Thomson, M., Huitema, C., Oku, K., Joras, M., and L. M.
              Ihlar, "Standard Communication with Network Elements
              (SCONE) Protocol", Work in Progress, Internet-Draft,
              draft-ietf-scone-protocol-04, 14 December 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-scone-
              protocol-04>.

12.2.  Informative References

   [SCONE-APPMAN]
              Mishra, S., Sarker, Z., Tomar, A., and K. Abbas,
              "Applicability & Manageability Considerations for SCONE",
              Work in Progress, Internet-Draft, draft-ietf-scone-
              applicability-manageability-01, 7 February 2026,
              <https://datatracker.ietf.org/doc/html/draft-ietf-scone-
              applicability-manageability-01>.

Authors' Addresses

   N. Zhang
   China Telecom
   Email: zhangn1130@outlook.com


   X. Xu
   China Telecom
   Email: xuxqietf@foxmail.com














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