



Internet Engineering Task Force (IETF)                          G. White
Request for Comments: 9956                                     CableLabs
Updates: 8325                                                 T. Fossati
Category: Standards Track                                         Linaro
ISSN: 2070-1721                                                  R. Geib
                                                        Deutsche Telekom
                                                                May 2026


   A Non-Queue-Building Per-Hop Behavior (NQB PHB) for Differentiated
                                Services

Abstract

   This document specifies characteristics of a Non-Queue-Building Per-
   Hop Behavior (NQB PHB).  The NQB PHB provides a shallow-buffered,
   best-effort service as a complement to a Default deep-buffered, best-
   effort service for Internet services.  The purpose of this NQB PHB is
   to provide a separate queue that enables smooth (i.e., non-bursty),
   low-data-rate, application-limited traffic microflows, to avoid the
   delay, delay variation and loss that would ordinarily be caused by
   sharing a queue with bursty, capacity-seeking traffic.  This PHB is
   implemented without prioritization and can be implemented without
   rate policing, making it suitable for environments where the use of
   these features is restricted.  The NQB PHB has been developed
   primarily for use by access network segments, where queuing delay and
   queuing loss caused by Queue-Building (QB) protocols are manifested;
   however, its use is not limited to such segments.  In particular, the
   application of NQB PHB to cable broadband links, Wi-Fi links, and
   mobile network radio/core segments are discussed in this document.
   This document recommends a specific Differentiated Services Code
   Point (DSCP) to identify NQB microflows and updates the guidance in
   RFC 8325 on mapping Differentiated Services (Diffserv or DS) to IEEE
   802.11 for this codepoint.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9956.

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.  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
   2.  Requirements Language
   3.  Context
     3.1.  NQB Behavior
     3.2.  Relationship to the Diffserv Architecture
     3.3.  Relationship to L4S
     3.4.  Applicability
   4.  NQB Sender Requirements
   5.  NQB PHB Requirements
     5.1.  Primary Requirements
     5.2.  Traffic Protection
     5.3.  Limiting Packet Bursts from Links
     5.4.  Treatment of the Explicit Congestion Notification Field
   6.  Operational Considerations
     6.1.  NQB Traffic Identification
     6.2.  Aggregation of the NQB DSCP into Another Diffserv PHB
     6.3.  Aggregation of Other DSCPs into the NQB PHB
     6.4.  Cross-Domain Usage and DSCP Re-marking
       6.4.1.  Interoperability with Non-DS-Capable Domains
     6.5.  The NQB DSCP and Tunnels
     6.6.  Guidance for Lower-Rate Links
   7.  Mapping NQB to Standards of Other SDOs
     7.1.  DOCSIS Access Networks
     7.2.  Mobile Networks
     7.3.  Wi-Fi Networks
       7.3.1.  Interoperability with Existing Wi-Fi Networks
       7.3.2.  The Updates to RFC 8325
   8.  IANA Considerations
   9.  Security Considerations
   10. References
     10.1.  Normative References
     10.2.  Informative References
   Appendix A.  DSCP Re-marking Policies
   Appendix B.  Comparison with Expedited Forwarding
   Appendix C.  Impact on Higher Layer Protocols
   Appendix D.  Alternative Diffserv Codepoints
   Acknowledgements
   Authors' Addresses

1.  Introduction

   This document defines a Diffserv PHB called the "Non-Queue-Building
   Per-Hop Behavior" (or "NQB PHB").  The NQB PHB isolates traffic
   microflows (application-to-application flows, see Section 1.2 of
   [RFC2475]) that have relatively low data rates and that do not,
   themselves, materially contribute to queuing delay and loss.  This
   isolation allows these traffic microflows to avoid the queuing delay
   and losses caused by other traffic.

   NQB microflows such as interactive voice, game sync packets, certain
   machine-to-machine applications, DNS lookups, and some real-time
   Internet of Things (IoT) analytics data are low-data-rate,
   application-limited microflows.  These can be distinguished from
   bursty traffic microflows and high-data-rate traffic microflows
   managed by a classic congestion control algorithm (both of which
   cause queuing delay and loss).  The term "classic congestion control"
   is defined in [RFC9330] to mean congestion control that coexists with
   standard Reno congestion control [RFC5681].  In this document, we
   will use "Queue-Building" (or "QB") to refer to microflows that cause
   queuing delay and loss.  See Section 1.2 of [RFC2475] for definitions
   of other terminology used in this document.

   In accordance with IETF guidance in [RFC2914] and [RFC8085], most
   packets carried by access networks are managed by an end-to-end
   congestion control algorithm.  Many of the commonly deployed
   congestion control algorithms, such as Reno [RFC5681], CUBIC
   [RFC9438], or BBR [BBR-CC], are designed to seek the available
   capacity of the path from sender to receiver (which can frequently be
   the access network link capacity).  In doing so, they generally
   overshoot the available capacity, causing a queue to build up at the
   bottleneck link.  This queue buildup results in variable delay and
   packet loss that can affect all the applications that are sharing the
   bottleneck link.  Moreover, many bottleneck links implement a
   relatively deep buffer (100 ms or more) (see [Gettys2012],
   [Cardwell2017], [WikipediaBufferbloat], and [BBR-CC]) in order to
   enable these congestion control algorithms to use the link
   efficiently, which exacerbates the delay and delay variation
   experienced.

   In contrast to applications that frequently cause queuing delay,
   there are a variety of relatively low data rate applications that do
   not materially contribute to queuing delay and loss; nonetheless,
   they are subjected to it by sharing the same bottleneck link.  Many
   of these applications can be sensitive to delay or delay variation,
   as well as packet loss; thus, they produce a poor Quality of
   Experience (QoE) in such conditions.

   Active Queue Management (AQM) mechanisms intended for single queues
   (such as Proportional Integral Controller Enhanced (PIE) [RFC8033],
   DOCSIS-PIE [RFC8034], PI2 [RFC9332], or CoDel [RFC8289]) can improve
   the QoE for delay-sensitive applications, but there are practical
   limits to the amount of improvement that can be achieved without
   impacting the throughput of capacity-seeking applications.  For
   example, AQMs generally allow a significant amount of queue depth
   variation to accommodate the behaviors of congestion control
   algorithms such as Reno and CUBIC.  If the AQM attempted to control
   the queue depth much more tightly, applications using those
   algorithms would not fully utilize the link.  Alternatively, flow-
   queuing systems, such as fq_codel [RFC8290], can be employed to
   isolate microflows from one another; however, they are not
   appropriate for all bottleneck links due to reasons that include
   complexity.

   The NQB PHB supports differentiating between these two classes of
   traffic in bottleneck links and queuing them separately so that both
   classes can deliver satisfactory QoE for their applications.  In
   particular, the NQB PHB provides a shallow-buffered, best-effort
   service as a complement to a Default (see [RFC2474]) deep-buffered,
   best-effort service.  This PHB is designed for broadband access
   network links (where there is minimal aggregation of traffic),
   especially when buffers are deep (see Section 3.4).  The
   applicability of this PHB to lower-rate links is discussed in
   Section 6.6.

   To be clear, a network implementing the NQB PHB solely provides
   isolation for traffic classified as behaving in conformance with the
   behaviors discussed in Section 3.1.  A node supporting the NQB PHB
   makes no guarantees on delay or data rate for NQB-marked microflows
   (beyond the delay bound provided by the shallow buffer), it is the
   NQB senders' behavior itself that results in low delay and low loss.

   Sections 6 and 7 of this document provide guidance for network
   operators regarding appropriate forwarding of traffic marked with the
   NQB DSCP.  The guidance includes recommendations for the
   configuration of network nodes that support the NQB PHB as well as
   for network nodes that do not support the PHB; this allows NQB
   traffic to be forwarded in a way that aligns with the goals for NQB
   treatment and supports the use of this codepoint by other nodes and
   other networks.

2.  Requirements Language

   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.

3.  Context

3.1.  NQB Behavior

   There are applications that send traffic at relatively low data rates
   and/or in a fairly smooth and consistent manner such that they are
   unlikely to exceed the available capacity of the network path between
   sender and receiver, even at an inter-packet timescale.  Some of
   these applications are transactional in nature; they might only send
   one packet (or a few packets) per RTT.  These applications might
   themselves only cause very small, transient queues to form in network
   buffers; nonetheless, they can be subjected to delay and delay
   variation as a result of sharing a network buffer with applications
   that tend to cause large and/or standing queues to form.  These
   applications typically implement a response to network congestion
   that consists of discontinuing (or significantly reducing)
   transmissions.  These applications can be negatively affected by
   excessive delay and delay variation.  Such applications are ideal
   candidates to be queued separately from the applications that are the
   cause of queue buildup, delay, and loss.

   In contrast, QB microflows include those that probe for link capacity
   and induce delay and loss as a result, for example, microflows that
   use CUBIC, Reno, or other TCP/QUIC congestion control algorithms in a
   capacity-seeking manner.  Other types of QB microflows include those
   that send at a high burst rate even if the long-term average data
   rate is much lower.

3.2.  Relationship to the Diffserv Architecture

   The IETF has defined the Diffserv architecture [RFC2475] with the
   intention that it allows traffic to be marked in a manner that
   conveys the performance requirements of that traffic either
   qualitatively or in a relative sense (e.g., priority).  The
   architecture defines the use of the DSCP field [RFC2474] for this
   purpose, and numerous RFCs have been written that describe
   recommended interpretations of the values (Diffserv Codepoints
   [RFC2474]) of the field, and standardized treatments (traffic
   conditioning and per-hop-behaviors [RFC2475]) that can be implemented
   to satisfy the performance requirements of traffic so marked.

   While this architecture is powerful and flexible enough to be
   configured to meet the performance requirements of a variety of
   applications and traffic categories, or to achieve differentiated
   service offerings, meeting the performance requirements of an
   application across the entire sender-to-receiver path involves all
   the networks in the path agreeing on what those requirements are and
   sharing an interest in meeting them.  In many cases, this is made
   more difficult since the performance "requirements" are not strict
   ones (e.g., applications will degrade in some manner as loss, delay,
   and/or delay-variation increase), so the importance of meeting them
   for any particular application in some cases involves a judgment as
   to the value of avoiding some amount of degradation in quality for
   that application in exchange for an increase in the degradation of
   another application.

   Further, in many cases, the implementation of Diffserv PHBs has
   historically involved prioritization of service classes with respect
   to one another, which sets up the zero-sum game alluded to in the
   previous paragraph and which results in the need to limit access to
   higher priority classes via mechanisms such as access control,
   admission control, traffic conditioning and rate policing, and/or to
   meter and bill for carriage of such traffic.  These mechanisms can be
   difficult or impossible to implement in the Internet.

   In contrast, the NQB PHB has been designed with the goal that it
   avoids many of these issues; thus, it could conceivably be deployed
   across the Internet.  The intent of the NQB DSCP is that it signals
   verifiable behavior that permits the sender to request differentiated
   treatment.  Also, the NQB traffic is to be given a separate queue
   with forwarding preference equal to Default traffic and given no
   reserved capacity other than any minimum capacity that it shares with
   Default traffic.  As a result, the NQB PHB does not aim to meet
   specific application performance requirements.  Instead, the sole
   goal of the NQB PHB is to isolate NQB traffic from other traffic that
   causes an increase in loss, delay, and/or delay-variation, given that
   the NQB traffic is, itself, only an insignificant contributor to
   those degradations.  The PHB is also designed to reduce the
   incentives for a sender to mis-mark its traffic since neither higher
   capacity nor reserved capacity are being offered.  These attributes
   eliminate many of the trade-offs that underlie the handling of
   differentiated service classes in the Diffserv architecture as it has
   previously been defined.  These attributes also significantly
   simplify access control and admission control functions, reducing
   them to simple verification of behavior.  This aspect is discussed
   further in Sections 4 and 5.2.

   Therefore, the NQB PHB is intended for the situation where the
   performance requirements of applications cannot be assured across the
   whole sender-to-receiver path; as a result, applications cannot
   feasibly place requirements on the network.  Instead, many
   applications have evolved to make the best out of the network
   environment that they find themselves in.  In this context, the NQB
   PHB is intended to provide a better network environment for
   applications that send data at relatively low and non-bursty data
   rates.

   In regard to a comparison between the NQB PHB and other standardized
   PHBs in the Diffserv series, the closest similarity is to the
   Expedited Forwarding (EF) PHB [RFC3246], which also intends to enable
   services that provide low loss, low delay, and low-delay variation.
   Unlike EF, NQB has no requirement for a guaranteed minimum rate, nor
   does it have a requirement to police incoming traffic to such a rate:
   NQB is expected to be given the same forwarding preference as Default
   traffic.  See Appendix B for a more detailed comparison of the NQB
   and EF PHBs.

   In nodes that support multiple Diffserv Service Classes, NQB traffic
   is intended to be handled as a part of the Default treatment.
   Traffic assigned to this class does not receive better forwarding
   treatment (e.g., prioritization) with respect to other classes, AFxx,
   EF, etc.  Of course, traffic marked as NQB could (like other Default
   traffic) receive better forwarding treatment with respect to Lower-
   Effort (LE) [RFC8622] (e.g., the NQB queue could be emptied in a
   priority sequence before the LE queue).

3.3.  Relationship to L4S

   In this document, the NQB DSCP and PHB have been defined to operate
   independently of the Low Latency, Low Loss, and Scalable throughput
   (L4S) architecture [RFC9330].  Nonetheless, traffic marked with the
   NQB DSCP is intended to be compatible with L4S [RFC9330], with the
   result being that NQB traffic and L4S traffic can share the low-
   latency queue in an L4S DualQ node [RFC9332].  A network node's
   compliance with the DualQ Coupled AQM requirements (see Section 2.5
   of [RFC9332]) is considered sufficient to support the NQB PHB
   requirement of fair allocation of capacity between the QB and NQB
   queues (Section 5).  Note that these requirements, in turn, require
   compliance with all the requirements in Section 5 of [RFC9331].

   Applications that comply with both the NQB sender requirements in
   Section 4 and the "Prague L4S requirements" in Section 4 of [RFC9331]
   could mark their packets both with the NQB DSCP and with the ECT(1)
   value.

   In nodes that support both the NQB PHB and L4S, the L4S network
   functions SHOULD treat packets marked with the NQB DSCP and ECT(1) or
   Congestion Experienced (CE) the same as packets marked with the
   Default DSCP and the same Explicit Congestion Notification (ECN)
   value.  Here, "L4S network functions" refers to the L4S Network Node
   functions (see Section 5 of [RFC9331]), and any mechanisms designed
   to protect the L4S queue (such as those discussed in Section 8.2 of
   [RFC9330]).  The processing by an L4S node of an ECT(0) packet that
   is classified to the L queue (e.g., as a result of being marked with
   a NQB DSCP) is specified in Section 5.4.1.1 of [RFC9331] and
   Section 2.5.1.1 of [RFC9332].

   Additionally, Section 5.4 places requirements on treatment of ECN-
   marked packets by a node that supports the NQB PHB.

3.4.  Applicability

   This PHB is primarily applicable for high-speed broadband access
   network links, where there is minimal aggregation of traffic and deep
   buffers are common.

   In many other links, forwarding NQB-marked packets using the Default
   treatment might be sufficient to preserve the loss, delay, and delay-
   variation performance for NQB traffic.  This is generally true in
   links that do not typically experience congestion (for example, many
   backbone and core network links) and in highly aggregated links
   (links designed to carry a large number of simultaneous microflows)
   where individual microflow burstiness is averaged out and, thus, is
   unlikely to cause much actual delay.  Section 6.2 provides
   recommendations for configuration of network nodes in such cases.

4.  NQB Sender Requirements

   Microflows that are eligible to be marked with the NQB DSCP are ones
   that send non-bursty traffic at a low data rate relative to typical
   network path capacities.  Here, the data rate is limited by the
   application itself rather than by network capacity: these microflows
   send at a data rate of no more than about 1% of the "typical" network
   path capacity.  In addition, these microflows are required to be sent
   in a smooth (i.e., paced) manner, where the number of IP bytes sent
   in any time interval "T" is less than or equal to (R * T) + MTU,
   where "R" is the maximum rate described in the preceding sentence,
   expressed in bytes-per-second.  For example, at the time of writing,
   access network data rates are typically on the order of 50 Mbps or
   more in the Internet (see Section 6.6 for a discussion of cases where
   this isn't true): this implies 500 kbps as an upper limit.  Note that
   microflows are unidirectional, while application traffic is often
   bidirectional (i.e., an application instance might consist of one or
   more microflows in each direction).  For a particular application, it
   could be the case that some of its microflows are eligible to be
   marked with the NQB DSCP while others are not.  For example, an
   interactive video streaming application might consist of a high-
   bandwidth video stream (not eligible for NQB marking) in one
   direction and a low-bandwidth control stream (eligible for NQB
   marking) in the other.

   Microflows marked with the NQB DSCP are expected to comply with
   existing guidance for safe deployment on the Internet, including the
   guidance related to response to network congestion, for example the
   requirements in [RFC8085] and Section 2 of [RFC3551] (also see the
   circuit breaker limits in Section 4.3 of [RFC8083] and the
   description of inelastic pseudowires in Section 4 of [RFC7893]).  The
   fact that a microflow's data rate is low relative to typical network
   capacities is no guarantee that sufficient capacity exists in any
   particular network, and it is the responsibility of the application
   to detect and react appropriately if the network capacity is
   insufficient.  To be clear, the description of NQB-marked microflows
   in this document is not to be interpreted as suggesting that
   applications generating such microflows are in any way exempt from
   this responsibility.  One way that an application marking its traffic
   as NQB can handle this is to implement a scalable congestion control
   mechanism as described in [RFC9331].

   The DS field specification requires the definition of a recommended
   DSCP to be associated with each standardized PHB (see Section 5 of
   [RFC2474]).  In accordance with this, applications are RECOMMENDED to
   use the DSCP value 45 (decimal) to mark microflows as NQB.  The
   choice of the DSCP value 45 (decimal) is motivated, in part, by the
   desire to achieve separate queuing in existing Wi-Fi networks (see
   Section 7.3) and by the desire to make implementation of the PHB
   simpler in network equipment that has the ability to classify traffic
   based on ranges of DSCP values (see Section 6.3 for further
   discussion).

   The two primary considerations for whether an application chooses to
   mark its traffic as NQB involve the risks of being subjected to a
   traffic protection algorithm (see Section 5.2) and/or to the
   consequences of overrunning the NQB shallow buffer if (in either
   case) the traffic contributes to the formation of a queue in a node
   that supports the PHB.  In both cases, the result could be that
   excess traffic is discarded or queued separately as Default traffic
   (and, thus, potentially is delivered out of order).  To avoid these
   risks, if a microflow's traffic exceeds the rate equation provided in
   the first paragraph of this section, the application MUST NOT mark
   this traffic with the NQB DSCP.  In such a case, the application
   could instead consider using a scalable congestion control mechanism
   as described in [RFC9331].

   The sender requirements outlined in this section are all related to
   observable attributes of the packet stream, which makes it possible
   for network elements (including nodes implementing the PHB) to
   monitor for inappropriate usage of the DSCP and take action (such as
   discarding or re-marking) on traffic that does not comply.  This
   functionality, when implemented as part of the PHB, is described in
   Section 5.2.

5.  NQB PHB Requirements

   For the NQB PHB to become widely deployed, it is important that
   incentives are aligned correctly, i.e., that there is a benefit to
   the application in marking its packets correctly and a disadvantage
   (or at least no benefit) to an application in intentionally mis-
   marking its traffic.  Thus, a useful property of nodes (i.e., network
   switches and routers) that support separate queues for NQB and QB
   microflows is that each queue tends to be the better choice for the
   traffic it is designed for: the NQB queue for microflows consistent
   with the NQB sender requirements in Section 4 and the QB queue for
   those that are not.  By adhering to these principles, there is little
   incentive for senders to mis-mark their traffic as NQB.

   This principle of incentive alignment ensures a system is robust to
   the behavior of the large majority of individuals and organizations
   who can be expected to act in their own interests (including
   application developers and service providers who act in the interests
   of their users).  Malicious behavior is not necessarily based on
   rational self-interest, so incentive alignment is not a sufficient
   defense, but the large majority of users do not act out of malice.
   Protection against malicious attacks (and accidents) is addressed in
   Section 5.2 and summarized in Section 9.  As mentioned previously,
   the NQB designation and marking is intended to convey verifiable
   traffic behavior, as opposed to simply a desire for differentiated
   treatment.  As a result, any mis-marking can be identified by the
   network.

5.1.  Primary Requirements

   A node supporting the NQB PHB MUST provide a queue for NQB traffic
   separate from the queue used for Default traffic.

   A node supporting the NQB PHB SHOULD NOT rate limit or rate police
   the aggregate of NQB traffic separately from Default traffic.  An
   exception to this recommendation for traffic sent towards a non-DS-
   capable domain is discussed in Section 6.4.1.  Note also that
   Section 5.2 discusses potential uses of per-microflow (rather than
   aggregate) rate policing.

   The NQB queue SHOULD be given equivalent forwarding preference
   compared to the Default queue.  The node SHOULD provide a scheduler
   that allows NQB and Default traffic to share the link in a manner
   that treats the two classes equally, e.g., a Deficit Round-Robin
   (DRR) scheduler with equal weights or two Wireless Multimedia Access
   Categories with the same Enhanced Distributed Channel Access (EDCA)
   parameters.  The use of equal weights for DRR is given as a
   reasonable example and is not intended to preclude other scheduling
   weights (see below for details).  A node that provides rate limits or
   rate guarantees for Default traffic SHOULD ensure that such limits
   and/or guarantees are shared with NQB traffic in a manner that treats
   the two classes equally.  This could be supported using a
   hierarchical scheduler where the rate limits and guarantees are
   configured on a parent class, and the two queues (Default and NQB)
   are arranged as the children of the parent class and given equal
   access to the capacity configured for the parent class (e.g., with
   equal DRR scheduling).  Compliance with these recommendations reduces
   the incentives for QB traffic to be mis-marked as NQB and is most
   important in nodes that are likely bottlenecks, where deviation from
   them could result in a discernible benefit for mis-marked traffic (to
   the detriment of other traffic).  In network nodes that are rarely
   bottlenecks, these recommendations are less critical.

   In the DRR example above, equal scheduling weights is only an
   example.  Ideally, the DRR weight would be chosen to match the
   highest fraction of capacity that NQB-compliant flows are likely to
   use on a particular network segment.  Given that NQB-compliant flows
   are not capacity seeking (in contrast to QB flows, which can be), and
   since DRR allows unused capacity in one class to be used by traffic
   in the other, providing a higher-than-needed NQB scheduler weight
   could be considered less problematic than the reverse.  That said,
   providing a higher-than-needed NQB scheduler weight does increase the
   likelihood that a non-compliant microflow mis-marked as NQB is able
   to use more than its fair share of network capacity.  NQB microflows
   are expected to each consume no more than 1% of the link capacity,
   and in low stat-mux environments (such as at the edge of the network)
   would be unlikely in aggregate to consume 50% of the link capacity.
   Thus, 50% seems a reasonable upper bound on the weight for the NQB
   PHB in these environments.

   By default, a node supporting the NQB PHB SHOULD classify packets
   marked with the DSCP value 45 (decimal) into the queue for NQB
   traffic.  In accordance with the requirement in Section 3 of
   [RFC2474], a node supporting the NQB PHB MUST support the ability to
   configure the DSCP that is used to classify packets into the queue
   for NQB traffic.  A node supporting the NQB PHB MAY support the
   ability to configure multiple DSCPs that are used to classify packets
   into the queue for NQB traffic.

   Support for the NQB PHB is advantageous at bottleneck nodes.  Many
   bottleneck nodes have a relatively deep buffer for Default traffic
   (e.g., roughly equal to the base RTT of the expected connections,
   which could be tens or hundreds of milliseconds).  Providing a
   similarly deep buffer for the NQB queue would be at cross purposes to
   providing very low queueing delay and would erode the incentives for
   QB traffic to be marked correctly at such a bottleneck node.  The NQB
   queue MUST have a buffer size that is significantly smaller than the
   buffer provided for Default traffic.  It is RECOMMENDED to configure
   an NQB buffer size less than or equal to 10 ms at the shared NQB/
   Default egress rate.

   In order to enable network operators to monitor the usage of the NQB
   PHB and, in particular, to monitor for potential mis-marking of QB
   traffic, a node supporting the NQB PHB MUST provide statistics that
   can be used by the network operator to detect whether abuse is
   occurring (e.g., packet and drop counters).  Support for such
   counters ensures that operators who configure the NQB PHB have the
   ability to track the amount of packet drop that is occurring due to
   traffic overrunning the shallow buffer and then take action if they
   believe the PHB is causing more issues than it is solving in their
   environment.  Those actions could include disabling the PHB,
   identifying and dealing with the sources of malicious traffic
   directly, enabling traffic protection (Section 5.2) if it is
   available, or pursuing a feature request with the equipment
   manufacturer to add a traffic protection function if it isn't
   currently available.

   To prevent propagation of degradation of service for NQB traffic
   caused by potential mis-marking of QB traffic, network equipment that
   supports this PHB and handles traffic for multiple users SHOULD
   support provisioning of capacity and related forwarding resources on
   a per-user basis and SHOULD support enforcement of the resulting per-
   user limits on the aggregate of NQB and QB traffic for each user.  In
   this context, the term "user" should be read as meaning an entity
   that the equipment is designed to serve more-or-less independently,
   such as a subscriber in the case of access network equipment, a
   station (STA) in the case of a Wi-Fi Access Point (AP) that
   implements per-STA queuing and airtime fairness, or an end user in
   the case of a networking co-op that serves a set of end users.  This
   functionality is commonly available in the class of network equipment
   for which this PHB is primarily applicable (see Section 3.4).
   Provisioning methodology as well as decisions on whether and how to
   enforce the resulting limits may vary by network operator.

   While not fully described in this document, it may be possible for
   network equipment to implement a separate QB/NQB pair of queues for
   additional service classes beyond the Default PHB / NQB PHB pair.

   In some cases, existing network equipment has been deployed that
   cannot readily be upgraded or configured to support the PHB
   requirements.  However, this equipment might be capable of loosely
   supporting an NQB service; see Section 7.3.1 for details and an
   example where this is particularly important.  A similar approach
   might prove to be useful in other network environments.

5.2.  Traffic Protection

   It is possible that, due to an implementation error or
   misconfiguration, a QB microflow could end up being mis-marked as NQB
   or vice versa.  It is also possible that a malicious actor could
   introduce a QB microflow marked as NQB with the intention of causing
   disruptions.  In the case of a low-data-rate microflow that isn't
   marked as NQB and therefore ends up in the QB queue, it would only
   impact its own Quality of Service (QoS); therefore, it seems to be of
   lesser concern.  However, a QB microflow that is mis-marked as NQB is
   able to contribute to NQB queue formation at a network node that
   would cause queuing delay and/or loss for all the other microflows
   that are sharing the NQB queue.

   To prevent this situation from harming the performance of the
   microflows that comply with the requirements in Section 4, network
   elements that support the NQB PHB SHOULD support a "traffic
   protection" function that can identify microflows or packets that are
   inconsistent with the sender requirements in Section 4 and either
   reclassify those microflows/packets to the QB queue or discard the
   offending traffic.  In the case of a traffic protection algorithm
   that reclassifies offending traffic, the implementation MAY provide
   an option to additionally re-mark such traffic to Default (or
   possibly to another local use codepoint) so that the result of the
   traffic protection decision can be used by further hops.  This sort
   of re-marking could provide a limited layer of protection in
   situations where downstream network nodes support separate queuing
   for NQB-marked packets but lack support for traffic protection.

   If traffic protection is not supported or is not effective in
   preventing queue formation and growth in the NQB queue, then QB
   traffic that is mis-marked as NQB is able to form a queue that
   overflows the shallow buffer provided for NQB traffic; this is
   expected to result in redirecting the excess packets to the QB queue
   or discarding them.  Both actions degrade service for not only the
   mis-marked QB traffic, but also for any correctly marked NQB traffic.
   This will likely cause a significant degradation of service for NQB
   traffic.  Even if mis-marked QB traffic does not cause buffer
   overflow, the queue that forms results in QB traffic obtaining the
   reduced loss and delay benefits of the NQB service while causing
   queuing delay for all the other microflows that are sharing the
   queue.  These increased abilities of QB traffic to damage the NQB
   service in the absence of a traffic protection function need to be
   considered.  This is the motivation for the "SHOULD" requirement to
   support traffic protection (in the previous paragraph).  An NQB PHB
   implementation that does not support traffic protection risks being
   limited to deployment situations where traffic protection is
   potentially not necessary.  One example of such a situation could be
   a controlled environment (e.g., enterprise LAN) where a network
   administrator is expected to manage the usage of DSCPs.

   As it is defined here, traffic protection differs from Traffic
   Conditioning implemented in other Diffserv contexts.  Traffic
   Conditioning is commonly performed at the edge of a DS domain (either
   ingress or egress, depending on Traffic Conditioning Agreements
   (TCAs) in place).  In contrast, traffic protection is intended to be
   implemented in the nodes that implement the PHB.  By placing the
   traffic protection at the PHB node, an implementation can monitor the
   actual NQB queue and take action only if a queue begins to form.
   Implementation of traffic protection at PHB nodes that are most
   likely to be a bottleneck is particularly important because these are
   the nodes that would be expected to show the most queue buildup in
   the presence of QB traffic mis-marked as NQB.

   This specification does not mandate a particular algorithm for
   traffic protection.  This is intentional since this will probably be
   an area where implementers innovate.  The specifics of traffic
   protection could need to be different in different network equipment
   and in different network contexts.  Instead, this specification
   provides guidelines and some examples of traffic protection
   algorithms that could be employed.

   The traffic protection function SHOULD NOT base its decisions upon
   application-layer constructs (such as the port number used by the
   application or the source/destination IP address).  Instead, it ought
   to base its decisions on the actual behavior of each microflow (i.e.,
   the pattern of packet arrivals).

   A conventional implementation of such a traffic protection algorithm
   is a per-microflow rate policer, designed to identify microflows that
   exceed the bound provided in Section 4, where the value R is set to
   1% of the egress link capacity available for NQB traffic.  An
   alternative is to use a traffic protection algorithm that bases its
   decisions on the detection of actual queuing (i.e., by monitoring the
   queuing delay experienced by packets in the NQB queue) in correlation
   with the arrival of packets for each microflow.  While a per-
   microflow rate policer is conceptually simpler (and is based directly
   on the NQB sender requirements), it could often end up being more
   strict than is necessary (for example, by policing a flow that
   exceeds the rate equation even when the link is underutilized).  One
   example traffic protection algorithm based on the detection of actual
   queuing can be found in [RFC9957].  This algorithm maintains per-
   microflow state for a certain number of simultaneous QB microflows
   (e.g., 32), and shared state for any additional microflows above that
   number.

   In the case of a traffic protection algorithm that reclassifies
   offending traffic, different levels of hysteresis could be
   considered.  For example, the reclassify decision could be made on a
   packet-by-packet basis, which could result in significant out-of-
   order delivery for offending microflows as some portion of the
   microflow's packets remain in the NQB queue and some are reclassified
   to the Default queue.  Alternatively, a traffic protection function
   could employ a certain level of hysteresis to prevent borderline
   microflows from being reclassified capriciously, thus causing less
   potential for out-of-order delivery.  As a third option, the decision
   could be made to take action on all the future packets of the
   microflow, though sufficient logic would be needed to ensure that a
   future microflow (e.g., with the same 5-tuple) isn't misidentified as
   the current offending microflow.

   In the case of a traffic protection algorithm that discards offending
   traffic, similar levels of hysteresis could be considered.  In this
   case, it is RECOMMENDED that the decision thresholds be set higher
   than in the case of designs that reclassify since the degradation of
   communications caused by the packet being discarded is likely to be
   greater than the degradation caused by out-of-order delivery.

   The traffic protection function described here might require that the
   network element maintain microflow state.  The traffic protection
   function MUST be designed such that the node implementing the NQB PHB
   does not fail (e.g., crash) in the case that the microflow state is
   exhausted.  This might be accomplished simply by controlling/limiting
   the resources dedicated to tracking misbehaving flows.

   Some networks might prefer to implement a Traffic Conditioning
   approach that polices the application of the NQB DSCP at the ingress
   edge so that per-hop traffic protection is not needed.  This could be
   accomplished via the use of a per-microflow rate policer that polices
   microflows at 1% of the minimum link capacity of the network.  This
   approach would generally be expected to be inferior to per-hop
   traffic protection because:

   *  on one hand, it would be difficult for edge nodes to guarantee
      that there would never be more than 100 NQB flows that would share
      a single internal bottleneck, and

   *  on the other hand, there could be internal links that have much
      greater capacity than the minimum.

   So, Traffic Conditioning at the edge could simultaneously be too
   lenient and too strict.

5.3.  Limiting Packet Bursts from Links

   Some link technologies introduce burstiness by briefly storing
   packets prior to forwarding them.  A common cause of this burstiness
   is link discontinuity (i.e., where the link is not continuously
   available for transmission by the device), for example, time-
   division-duplex links or time-division-multiple-access (TDMA) links.
   Some link technologies that fall into this category are Passive
   Optical Networks (PONs), Wi-Fi, LTE/5G, and Data-Over-Cable Service
   Interface Specification (DOCSIS).

   As well as NQB senders needing to limit packet bursts (see
   Section 4), traffic designated for the NQB PHB would benefit from
   configuring these link technologies to limit the burstiness
   introduced.  This is for three reasons:

   1.  Burstiness, whether caused by the sender or by a link on the
       path, could cause queuing delay at downstream bottlenecks and,
       thus, degrade QoE.

   2.  Burstiness in links typically means that packets have been
       delayed by a variable amount.  That is, for packets that are
       being aggregated awaiting a transmission opportunity, some
       packets would generally have arrived just after the last
       transmission opportunity and would have to wait the longest while
       others would generally arrive just in time for the next
       transmission opportunity and would wait the least.  This
       manifests as delay variation that can also degrade QoE for
       applications that desire NQB treatment.

   3.  A downstream bottleneck that implements the NQB PHB could have
       implemented a traffic protection mechanism (Section 5.2) that
       responds to queuing delay by re-marking/reclassifying/dropping
       packets.  Bursty arrivals caused by an upstream link could
       introduce queuing delay in the NQB queue and these would be more
       likely to be subjected to traffic protection effects.

   This document does not set any quantified requirements for links to
   limit bursts; this is primarily because link technologies are outside
   the remit of Diffserv specifications.  However, it would not seem
   necessary to limit bursts lower than roughly 10% of the minimum base
   RTT expected in the typical deployment scenario (e.g., 250 µs burst
   duration for links within the public Internet).  This observation
   aligns with a similar one in Section 5.5 of [RFC9331].

5.4.  Treatment of the Explicit Congestion Notification Field

   NQB network functions MUST treat packets marked with the NQB DSCP
   uniformly, regardless of the value of the ECN field.  Here, the term
   "NQB network functions" refers to the traffic protection function
   (defined in Section 5.2) and any re-marking/traffic policing function
   designed to protect unmanaged networks (as described in
   Section 6.4.1).

6.  Operational Considerations

   This section describes considerations for network operators regarding
   the identification, marking, and handling of NQB traffic.  It
   outlines aggregation behaviors and special considerations in
   unmanaged and inter-network scenarios.  Additionally, Section 6.2
   contains configuration recommendations for nodes that do not support
   the NQB PHB.  Section 6.4.1 contains configuration recommendations
   for networks that interconnect with non-DS-capable domains.

6.1.  NQB Traffic Identification

   As required in Section 5, nodes supporting the NQB PHB provide for
   the configuration of classifiers that can be used to differentiate
   between QB and NQB traffic of equivalent importance.  The assigned
   NQB DSCP value (45 decimal) is recommended for use as the default
   classifier to distinguish NQB traffic from traffic classified as
   Default (DSCP 0) (see Sections 4 and 5.1).

6.2.  Aggregation of the NQB DSCP into Another Diffserv PHB

   It is RECOMMENDED that networks and nodes that do not support the NQB
   PHB be configured to treat traffic marked with the NQB DSCP the same
   as traffic with the Default DSCP.  This includes networks (such as
   MPLS) and nodes that aggregate service classes as discussed in
   [RFC5127] and [RFC8100]; in this case, this recommendation would
   result in traffic marked with the NQB DSCP being aggregated into the
   Elastic Treatment Aggregate (for networks as described in [RFC5127])
   or the Default / Elastic Treatment Aggregate (for networks as
   described in [RFC8100]).

   Networks and nodes that do not support the NQB PHB ought to only
   classify packets with the NQB DSCP value into the appropriate
   treatment aggregate, or encapsulate such packets for purposes of
   aggregation, and SHOULD NOT re-mark them with a different DSCP.  This
   preservation of the NQB DSCP value enables hops further along the
   path to provide the NQB PHB successfully.  This aligns with
   recommendations in [RFC5127].

   In nodes that do not typically experience congestion (for example,
   many backbone and core network switches), forwarding packets with the
   NQB DSCP using the Default treatment might be sufficient to preserve
   the loss, delay, and delay-variation performance for NQB traffic.

   In nodes that do experience congestion, forwarding packets with the
   NQB DSCP using the Default treatment could result in degradation of
   the loss, delay, and delay-variation performance but nonetheless
   preserves the incentives described in Section 5.

   Aggregating traffic marked with the NQB DSCP into a PHB designed for
   real-time, delay-sensitive traffic (e.g., the Real-Time Treatment
   Aggregate [RFC5127] or the Bulk Real-Time Treatment Aggregate
   [RFC8100]), might better preserve the loss, delay, and delay-
   variation performance in the presence of congestion; however, it
   would need to be done with consideration of the risk of creating an
   incentive for non-compliant traffic to be mis-marked as NQB.

6.3.  Aggregation of Other DSCPs into the NQB PHB

   The Diffserv model provides flexibility for operators to control the
   way they choose to aggregate traffic marked with a specific DSCP.
   Operators of nodes that support the NQB PHB could choose to aggregate
   other service classes into the NQB queue.  This is particularly
   useful in cases where specialized PHBs for these other service
   classes had not been provided at a potential bottleneck, perhaps
   because it was too complex to manage traffic contracts and
   conditioning.  Candidate service classes for this aggregation would
   include those that carry low-data-rate inelastic traffic that has low
   to very-low tolerance for loss, delay, and/or delay variation.
   Operators would need to use their own judgment based on the actual
   traffic characteristics in their networks in deciding whether or not
   to aggregate other service classes / DSCPs with NQB.  For networks
   that use the service class definitions from [RFC4594], this could
   include Telephony (EF/VA), Signaling (CS5), and possibly Real-Time
   Interactive (CS4) (depending on data rate).  In the preceding
   sentence, "VA" and "CSx" refer to VOICE-ADMIT ([RFC5865]) and Class
   Selector ([RFC2474]), respectively.  In some networks, equipment
   limitations may necessitate aggregating a range of DSCPs (e.g.,
   traffic marked with DSCPs 40-47 (decimal), i.e., those whose three
   most significant bits are 0b101).  As noted in Section 4, the choice
   of the DSCP value 45 (decimal) is motivated in part by the desire to
   make this aggregation simpler in network equipment that can classify
   packets via comparing the DSCP value to a range of configured values.

   A node providing only a NQB queue and a Default queue may obtain an
   NQB performance similar to that of EF, for example, as described by
   Appendix A.3.1 of [RFC2598].  Some caveats and differences are
   discussed in Appendix B.

6.4.  Cross-Domain Usage and DSCP Re-marking

   In contrast to some existing standard PHBs, which are typically only
   used within a DS domain (e.g., an Autonomous System (AS) or an
   enterprise network), this PHB is expected to be used across the
   Internet, wherever suitable operator agreements apply.  Under the
   model described in [RFC2474], this requires that the corresponding
   DSCP is recognized and mapped across network boundaries accordingly.

   If NQB support is extended across a DS domain boundary, the
   interconnected networks agreeing to support NQB SHOULD use the DSCP
   value 45 (decimal) for NQB at network interconnection, unless a
   different DSCP is explicitly documented in the TCA (see [RFC2475])
   for that interconnection.  If a Diffserv-Intercon Interconnection
   Class (see Section 4 of [RFC8100]) is operational between
   interconnected domains, the receiving domain may prefer a different
   DSCP for NQB traffic that allows for a DSCP range-based
   classification for the Default / Elastic Treatment Aggregate.
   Similar to the handling of DSCPs for other PHBs (and as discussed in
   [RFC2475]), networks can re-mark NQB traffic to a DSCP value other
   than 45 (decimal) for internal usage.  To ensure reliable NQB PHB
   treatment on the entire path, the appropriate NQB DSCP would need to
   be restored when forwarding to another network.

6.4.1.  Interoperability with Non-DS-Capable Domains

   As discussed in Section 4 of [RFC2475], there may be cases where a
   network operator that supports Diffserv is delivering traffic to
   another network domain (e.g., a network outside of their
   administrative control) where there is an understanding that the
   downstream domain does not support Diffserv or there is no knowledge
   of the traffic management capabilities of the downstream domain, and
   no agreement in place.  In such cases, Section 4 of [RFC2475]
   suggests that the upstream domain opportunistically re-mark traffic
   with a Class Selector Codepoint or DSCP 0 (Default) under the
   assumption that traffic so marked would be handled in a predictable
   way by the downstream domain.

   In the case of a network that supports the NQB PHB (and carries
   traffic marked with the recommended DSCP value 45 (decimal)), the
   same concerns apply.  In particular, since the recommended NQB DSCP
   value 45 (decimal) could be given high priority in some non-DS-
   compliant network equipment (e.g., legacy Wi-Fi APs as described in
   Section 7.3.1), it is RECOMMENDED that the operator of the upstream
   domain implement one of the following safeguards before delivering
   traffic into a non-DS-capable domain:

   1.  One option for such a safeguard is to re-mark NQB traffic to DSCP
       0 (Default) (or another Class Selector DSCP) before delivering
       traffic into a non-DS-capable domain, in accordance with the
       suggestion in Section 4 of [RFC2475].  Network equipment designed
       for such environments SHOULD, by default, re-mark NQB traffic to
       DSCP 0 (Default) and SHOULD support the ability to change and
       disable this re-marking.  Re-marking NQB traffic to DSCP 0
       (Default) could be considered the "safest" approach since the
       upstream domain can thereby ensure that NQB traffic is not given
       inappropriate treatment in the non-DS-capable domain.  That said,
       it comes with the downside that the re-marking ruins any
       possibility of NQB isolation in any further downstream domain
       (not just the immediate neighbor).

   2.  As an alternative to re-marking all NQB traffic, such an operator
       could deploy a traffic protection (see Section 5.2) or a shaping/
       policing function on traffic marked with the NQB DSCP that
       minimizes the potential for negative impacts on Default traffic,
       should the downstream domain treat traffic with the NQB DSCP as
       high priority.

       In the case that a traffic protection function is used, it MUST
       either re-mark offending traffic to DSCP 0 (or another Class
       Selector DSCP) or discard it.  Note that a traffic protection
       function, as defined in this document, might only provide
       protection from issues occurring in subsequent network hops if
       the device implementing the traffic protection function is the
       bottleneck link on the path, so it might not be a solution for
       all situations.

       In the case that a traffic policing function or a rate-shaping
       function is applied to the aggregate of NQB traffic destined to
       such a downstream domain, the policer/shaper rate SHOULD be set
       to either 5% of the interconnection data rate or 5% of the
       typical rate for such interconnections, whichever is greater,
       with excess traffic being re-marked and classified for Default
       forwarding (or dropped, as a last resort).  A traffic policing
       function SHOULD allow approximately 100 ms of burst tolerance
       (e.g., a token bucket depth equal to 100 ms multiplied by the
       policer rate).  A traffic-shaping function SHOULD allow
       approximately 10 ms of burst tolerance and no more than 50 ms of
       buffering.  The burst tolerance values recommended here are
       intended to reduce the degradation that could be introduced to
       delay- and loss-sensitive traffic marked NQB without
       significantly degrading Default traffic and that could be
       adjusted based on local network policy.  Increasing the burst
       tolerance would further reduce the potential for degradation
       (increased loss or increased delay) of traffic marked NQB but
       would come at the cost of an increased risk of degradation
       (increased loss or increased delay) of Default traffic.

       The recommendation to limit NQB traffic to 5% is based on an
       assumption that internal links in the downstream domain could
       have data rates as low as one tenth of the interconnect rate; in
       which case, if the entire aggregate of NQB traffic traversed a
       single instance of such a link, the aggregate would consume no
       more than 50% of that link's capacity.  The limit for NQB traffic
       SHOULD be adjusted based on any knowledge of the local network
       environment that is available.

6.5.  The NQB DSCP and Tunnels

   [RFC2983] discusses tunnel models that support Diffserv.  It
   describes a "uniform model" in which the inner DSCP is copied to the
   outer header at encapsulation and the outer DSCP is copied to the
   inner header at decapsulation.  It also describes a "pipe model" in
   which the outer DSCP is not copied to the inner header at
   decapsulation.  Both models can be used in conjunction with the NQB
   PHB.  In the case of the pipe model, any DSCP manipulation (re-
   marking) of the outer header by intermediate nodes would be discarded
   at tunnel egress.  In some cases, this could improve the possibility
   of achieving NQB treatment in subsequent nodes; in other cases, it
   could degrade that possibility (e.g., if the re-marking was designed
   specifically to preserve NQB treatment in downstream domains).

   As is discussed in [RFC2983], tunnel protocols that are sensitive to
   reordering (such as IPsec [RFC4301] or Layer 2 Tunneling Protocol
   (L2TP) [RFC2661]) can result in undesirable interactions if multiple
   DSCP PHBs are signaled for traffic within a tunnel instance.  This is
   true for tunnels containing a mix of QB and NQB traffic.
   Additionally, since networks supporting the NQB PHB could implement a
   traffic protection mechanism (see Section 5.2) and/or responses to
   NQB buffer overrun that result in out-of-order delivery for traffic
   marked with the NQB DSCP, even tunnels solely containing NQB traffic
   could have issues if they are sensitive to reordering and the outer
   header retains the NQB DSCP.  As a result, the use of a reordering-
   sensitive tunnel protocol to carry NQB traffic, or a mix of QB and
   NQB traffic, necessitates that the outer tunnel header be re-marked
   with a non-NQB DSCP (e.g., Default); in this case, the "pipe" model
   is preferable because it preserves the marking differentiation at
   tunnel decapsulation.

6.6.  Guidance for Lower-Rate Links

   The NQB sender requirements in Section 4 place responsibility in the
   hands of the application developer to determine the likelihood that
   the application's sending behavior could result in a queue forming
   along the path.  These requirements rely on application developers
   having a reasonable sense for the network context in which their
   application is to be deployed.  Even so, there will undoubtedly be
   networks that contain links having a data rate that is below the
   lower end of what is considered "typical"; some of these links could
   even be below the instantaneous sending rate of some NQB-marked
   applications.

   To limit the consequences of this scenario, operators of networks
   with lower rate links SHOULD consider utilizing a traffic protection
   function on those links that is more tolerant of burstiness (i.e., a
   temporary queue).  This will have the effect of allowing a larger set
   of NQB-marked microflows to remain in the NQB queue; however, it will
   come at the expense of a greater potential for delay variation.  In
   implementations that support [RFC9957], the burst tolerance can be
   configured via the CRITICALqLSCORE_us input parameter.

   Alternatively, operators of networks with lower rate links MAY choose
   to disable NQB support (and thus aggregate traffic marked with the
   NQB DSCP with Default traffic) on these lower rate links.  For links
   that have a data rate that is less than 10% of "typical" path rates,
   it is RECOMMENDED that the NQB PHB be disabled and that traffic
   marked with the NQB DSCP is therefore carried using the Default PHB
   (without being re-marked to the Default DSCP (0)).

7.  Mapping NQB to Standards of Other SDOs

   This section provides recommendations for the support of the NQB PHB
   in certain use cases.  This section is not exhaustive.

7.1.  DOCSIS Access Networks

   Residential cable broadband Internet services are commonly configured
   with a single bottleneck link (the access network link) upon which
   the service definition is applied.  The service definition, typically
   an upstream/downstream data rate tuple, is implemented as a
   configured pair of rate shapers that are applied to the user's
   traffic.  In such networks, the QoS that each application receives,
   and as a result, the QoE that it generates for the user is influenced
   by the characteristics of the access network link.

   To support the NQB PHB, cable broadband services would need to be
   configured to provide a separate queue for traffic marked with the
   NQB DSCP.  The NQB queue would need to be configured to share the
   service's rate-shaped capacity with the queue for QB traffic.
   Further discussion about support of the NQB PHB in DOCSIS networks
   can be found in [LOW_LATENCY_DOCSIS].

7.2.  Mobile Networks

   Historically, 3GPP mobile networks have utilized "bearers" to
   encapsulate each user's user plane traffic through the radio access
   and core networks.  Bearers are also associated with a QoS that
   determines how packets are prioritized at queues and radio
   schedulers.  In LTE networks, these are Evolved Packet System (EPS)
   bearers, part of the EPS, which comprises the core and access network
   architecture.  Typically, an LTE operator provides a dedicated EPS
   bearer for IP Multimedia Subsystem (IMS) Voice over LTE (VoLTE)
   traffic to meet regulatory obligations for call completion rates and
   a best-effort default EPS bearer for Internet traffic.  This default
   EPS bearer is typically non-Guaranteed Bit Rate (non-GBR) and
   provides no guarantees; its buffering characteristics generally are
   not compatible with low-latency traffic.  In 5G networks, similar
   functionality is provided using QoS flows within a PDU Session in the
   core network, which are mapped to Data Radio Bearers (DRBs) on the
   radio network.  5G systems offer more flexibility in QoS handling,
   allowing traffic to be treated according to type (e.g., loss-
   tolerant, low-latency); hence, they support more suitable treatment
   of Internet real-time microflows.

   To support the NQB PHB, an LTE network could be configured to provide
   the User Equipment (UE, the subscriber's device) with a dedicated
   low-latency, non-GBR EPS bearer, in addition to the default EPS
   bearer.  For example, this dedicated EPS bearer could use QCI 7 (QoS
   Class Identifier 7), which is typically used for low-latency, non-GBR
   services.  Alternatively, in a 5G system, a Data Radio Bearer (DRB)
   with 5QI 7 (5G QoS Identifier 7, also used for low-latency traffic),
   could be provisioned (see Table 5.7.4-1: Standardized 5QI to QoS
   characteristics mapping in [SA-5G]).

   A packet carrying the NQB DSCP could then be routed through this
   dedicated low-latency path, while packets without the NQB marking
   would be routed through the default bearer.

7.3.  Wi-Fi Networks

   Wi-Fi networking equipment compliant with 802.11e/n/ac/ax
   [IEEE802-11] generally supports either four or eight transmit queues
   and four sets of associated EDCA parameters (corresponding to the
   four Wi-Fi Multimedia (WMM) Access Categories) that are used to
   enable differentiated medium access characteristics.  The four WMM
   Access Categories, referred to as Voice Access Category (AC_VO),
   Video Access Category (AC_VI), Best Effort Access Category (AC_BE),
   and Background Access Category (AC_BK), provide four levels of
   prioritized access to the wireless medium.  As discussed in
   [RFC8325], it has been a common practice for Wi-Fi implementations to
   use a default DSCP to User Priority (UP) mapping that utilizes the
   most significant three bits of the DSCP field to select "User
   Priority", which is then mapped to the four WMM Access Categories.
   [RFC8325] also provides an alternative mapping that more closely
   aligns with the DSCP recommendations provided by the IETF.  In the
   case of some managed Wi-Fi equipment, this mapping can be controlled
   by the network operator, e.g., via TR-369 [TR-369].

   In addition to the requirements provided in other sections of this
   document, to support the NQB PHB, Wi-Fi equipment (including
   equipment compliant with [RFC8325]) SHOULD map the DSCP value 45
   (decimal) into a separate queue in the same Access Category as the
   queue that carries Default traffic (i.e., the Best Effort Access
   Category).  It is RECOMMENDED that Wi-Fi equipment provide a separate
   queue in UP 0 and map the DSCP value 45 (decimal) to that queue.  If
   a separate queue in UP 0 cannot be provided (due to hardware
   limitations, etc.), a Wi-Fi device MAY map the DSCP value 45
   (decimal) to UP 3.

7.3.1.  Interoperability with Existing Wi-Fi Networks

   While some existing Wi-Fi equipment might be capable (in some cases
   via firmware update) of supporting the NQB PHB requirements, many
   currently deployed devices cannot be configured in this way.  As a
   result, the remainder of this section discusses interoperability with
   these existing Wi-Fi networks, as opposed to PHB compliance.

   Since this equipment is widely deployed, and the Wi-Fi link can
   become a bottleneck link, the performance of traffic marked with the
   NQB DSCP across such links could have a significant impact on the
   viability and adoption of the NQB DSCP and PHB.  Depending on the
   DSCP used to mark NQB traffic, existing Wi-Fi equipment that uses the
   default mapping of DSCPs to Access Categories (see Section 2.3 of
   [RFC8325]) and the default EDCA parameters will support either (but
   not both) of the following characteristics:

   *  the NQB PHB requirement for separate queuing of NQB traffic from
      Default traffic (Section 5.1)

   *  the recommendation to treat NQB traffic with forwarding preference
      equal to that used for Default traffic (Section 5.1)

   The DSCP value 45 (decimal) is recommended for NQB (see Section 4).
   This maps NQB to UP 5 using the default mapping, which is in the
   Video Access Category.  While this choice of DSCP enables these Wi-Fi
   systems to support the NQB PHB requirement for separate queuing,
   existing Wi-Fi devices generally utilize EDCA parameters that result
   in statistical prioritization of the Video Access Category above the
   Best Effort Access Category.  In addition, this equipment does not
   support the remaining NQB PHB recommendations in Section 5.  The
   rationale for the choice of DSCP value 45 (decimal) as well as its
   ramifications and remedies for its limitations are discussed further
   below.

   The choice of separated queuing rather than equal forwarding
   preference in existing Wi-Fi networks was motivated by the following:

   *  Separate queuing is necessary in order to provide a benefit for
      traffic marked with the NQB DSCP.

   *  The arrangement of queues in Wi-Fi equipment is typically fixed,
      whereas the relative priority of the Access Category queues is
      configurable.  Most Wi-Fi equipment has hardware support (albeit
      generally not exposed for user control), which could be used to
      adjust the EDCA parameters in order to meet the equal forwarding
      preference recommendation.  This is discussed further below.

   *  Traffic that is compliant with the NQB sender requirements in
      Section 4 is expected to cause minimal degradation to traffic in
      lower priority Access Categories.  In any case, it would be
      unlikely to cause more degradation to lower priority Access
      Categories than the existing recommended Video Access Category
      traffic types: Broadcast Video, Multimedia Streaming, Multimedia
      Conferencing from [RFC8325], and AudioVideo, ExcellentEffort from
      [QOS_TRAFFIC_TYPE].

   *  Several existing client applications that are compatible with the
      NQB sender requirements already select the Video Access Category;
      thus, they would not see a degradation in performance by
      transitioning to the NQB DSCP, regardless of whether the network
      supported the PHB.

   *  Application instances on Wi-Fi client devices are already free to
      choose any Access Category that they wish, regardless of their
      sending behavior, without any policing of usage.  So, the choice
      of using DSCP value 45 (decimal) for NQB creates no new avenues
      for non-NQB-compliant client applications to exploit the
      prioritization function in Wi-Fi.

   *  For application traffic that originates outside of the Wi-Fi
      network, and, thus, is transmitted by the Access Point, the choice
      of DSCP value 45 (decimal) does create a potential for abuse by
      non-compliant applications.  However, opportunities exist in the
      network components upstream of the Wi-Fi Access Point to police
      the usage of the NQB DSCP and potentially re-mark traffic that is
      considered non-compliant, as is recommended in Section 6.4.1.
      Furthermore, it is reasonable to expect that ISPs currently manage
      the DSCPs on traffic destined to their customers' networks and
      will continue to do so whether or not they support NQB.  This
      includes the practice in residential broadband networks of re-
      marking the DSCP field to zero on all traffic.  Any change to
      these practices done to enable the NQB DSCP to pass through could
      be done alongside the implementation of the recommendations in
      Section 6.4.1.

   The choice of Video Access Category rather than the Voice Access
   Category was motivated by the desire to minimize the potential for
   degradation of Best Effort Access Category traffic.  The choice of
   Video Access Category rather than the Background Access Category was
   motivated by the much greater potential of degradation to NQB traffic
   that would be caused by the vast majority of traffic in most Wi-Fi
   networks, which utilizes the Best Effort Access Category.

   As stated above, the use of DSCP value 45 (decimal) for NQB is not
   expected to create incentives for abuse by non-compliant applications
   in the Wi-Fi uplink direction.  The fact that the NQB DSCP brings
   with it the potential for degradation of non-compliant applications
   (traffic protection and/or a shallow queue resulting in reordering
   and/or packet loss at some hop along the path) plus the existence of
   multiple other DSCP values that don't carry the risk of degradation
   and that could be readily used to obtain prioritization (AC_VI or
   even AC_VO) leads to the conclusion that NQB non-compliant
   applications that are seeking prioritization in the Wi-Fi uplink
   would be better off selecting one of those other DSCPs.  This
   conclusion is not expected to be disturbed by network support for NQB
   increasing the likelihood of DSCP value 45 (decimal) traffic
   traversing network boundaries without change to the DSCP, as that
   likelihood of increased network boundary traversal is balanced by a
   likelihood of NQB traffic encountering the traffic-limiting aspects
   of NQB support, traffic protection, and shallow buffers, which limit
   the potential for abuse.

   In the case of traffic originating outside of the Wi-Fi network, the
   prioritization of traffic marked with the NQB DSCP via the Video
   Access Category (if left unchanged) could potentially erode the
   principle of alignment of incentives discussed in Section 5.  In
   order to preserve the incentives principle for NQB, Wi-Fi systems MAY
   be configured such that the EDCA parameters for the Video Access
   Category match those of the Best Effort Access Category, which will
   mean AC_VI is at the same priority level as AC_BE.  These changes
   might not be possible on all Access Points; in any case, the
   requirements and recommendations in Section 6.4.1 would apply in this
   situation.

   Systems that utilize [RFC8325] but cannot provide a separate AC_BE
   queue for NQB traffic SHOULD map the DSCP value 45 (decimal) (or the
   locally determined alternative) to UP 5 in the Video Access Category
   as well (see Section 7.3.2).

7.3.2.  The Updates to RFC 8325

   Section 4.2.9 of [RFC8325] describes the recommendation for the
   handling of Standard service class traffic that carries the Default
   DSCP.  This update to [RFC8325] changes the title of Section 4.2.9 of
   [RFC8325] from "Standard" to "Standard and Non-Queue-Building".  This
   update additionally adds a paragraph at the end of Section 4.2.9 of
   [RFC8325] as follows:

   |  RFC 9956 defines a shallow-buffered, best-effort service for
   |  traffic described as Non-Queue-Building and recommends the
   |  following treatment in Wi-Fi equipment.  It is RECOMMENDED that
   |  Wi-Fi equipment map Non-Queue-Building traffic into a separate
   |  queue in the same Access Category as the queue that carries
   |  Default traffic (i.e., the Best Effort Access Category).  It is
   |  RECOMMENDED that Wi-Fi equipment provide a separate queue in UP 0
   |  and map Non-Queue-Building traffic to that queue.  If a separate
   |  queue in UP 0 cannot be provided (due to hardware limitations,
   |  etc.), a Wi-Fi device MAY map Non-Queue-Building traffic to UP 3.
   |  If neither of these options provides a separate queue from Default
   |  traffic, it is RECOMMENDED that Wi-Fi equipment map Non-Queue-
   |  Building traffic to UP 5 (which corresponds to the default mapping
   |  described in Section 2.3).  RFC 9956 provides additional
   |  recommendations and requirements for support of the NQB PHB that
   |  aren't available in the QoS model described in Section 6 but
   |  nonetheless could be supported in implementations.

   In another update to [RFC8325] captured below, a new row for "Non-
   Queue-Building" traffic is inserted between the existing "Low-Latency
   Data" and "OAM" rows in Figure 1 of [RFC8325] as follows:

  +===============+======+==========+==================================+
  | IETF Diffserv | PHB  |Reference |         IEEE 802.11              |
  | Service Class |      |   RFC    |User Priority|  Access Category   |
  |===============+======+==========+=============+====================|
  |     ...       | ...  |   ...    |    ...      |        ...         |
  +---------------+------+----------+-------------+--------------------+
  |    Low-       | AF21 |          |             |                    |
  |    Latency    | AF22 | RFC 2597 |     3       | AC_BE (Best Effort)|
  |    Data       | AF23 |          |             |                    |
  +---------------+------+----------+-------------+--------------------+
  |     Non-      |      |          |    0, 3     | AC_BE (Best Effort)|
  |    Queue-     | NQB  | RFC 9956 |            OR                    |
  |   Building    |      |          |     5       |    AC_VI (Video)   |
  |               |      |          |      See Section 4.2.9           |
  +---------------+------+----------+-------------+--------------------+
  |     OAM       | CS2  | RFC 2474 |     0       | AC_BE (Best Effort)|
  +---------------+------+----------+-------------+--------------------+

   A third update adds the following sentence to the end of the first
   paragraph in Section 5.3 of [RFC8325]:

   |  An exception to this is the NQB DSCP value 45 (decimal), which
   |  encodes for best-effort service.

8.  IANA Considerations

   IANA has assigned the Differentiated Services Field Codepoint (DSCP)
   45 from the "Differentiated Services Field Codepoints (DSCP)"
   registry (https://www.iana.org/assignments/dscp-registry/) ("DSCP
   Pool 3 Codepoints", Codepoint Space xxxx01, Standards Action
   ([RFC8126]) for NQB behavior.

   IANA has updated this registry as follows:

   Name:  NQB

   Value (Binary):  101101

   Value (Decimal):  45

   Reference:  RFC 9956

9.  Security Considerations

   The security considerations for the NQB PHB relate to the potential
   to impact the capacity available or delay experienced by other flows
   that share a bottleneck on the path with traffic that is marked with
   the recommended NQB DSCP.

   Full support for the NQB PHB in bottleneck links limits the
   incentives for a QB application to mis-mark its packets as NQB,
   particularly for implementations that support traffic protection.  If
   a QB microflow were to mis-mark its packets as NQB, it would be
   unlikely to receive a benefit by doing so, and it would usually
   experience a degradation, in contrast to mis-marking its packets for
   a higher-priority PHB, e.g., the EF PHB [RFC3246].  The nature of the
   degradation would depend on the specifics of the PHB implementation,
   including response to NQB buffer overflow (and on the presence or
   absence of a traffic protection function) but could include excessive
   packet loss, excessive delay variation, and/or excessive out-of-order
   delivery.  If an NQB microflow were to fail to mark its packets as
   NQB, it could suffer the delay and loss typical of sharing a queue
   with capacity-seeking traffic.

   To preserve low-delay performance for NQB traffic, networks that
   support the NQB PHB will need to ensure that mechanisms are in place
   to prevent malicious traffic marked with the NQB DSCP from causing
   excessive queue delay.  Section 5.2 recommends the implementation of
   a traffic protection mechanism to achieve this goal.  The
   recommendations on traffic protection mechanisms in this document
   presume that some type of "flow" state be maintained in order to
   differentiate between microflows that are causing queuing delay and
   those that aren't.  Since this flow state is likely finite, this
   opens up the possibility of flow-state exhaustion attacks.  While
   this document requires that traffic protection mechanisms be designed
   with this possibility in mind, the outcomes of flow-state exhaustion
   would depend on the implementation.

   If traffic protection is not implemented or is not able to prevent
   queue formation in the NQB shallow buffer, the limited size of that
   buffer will cause a growing queue to overrun that buffer, resulting
   in negative effects (e.g., reforwarding as Default, discarding) that
   potentially impact multiple NQB-marked microflows, independent of
   whether each affected microflow contributed to queue formation.  As
   discussed elsewhere in this document, those negative effects serve to
   discourage misuse and abuse of NQB by QB traffic, but the negative
   side effects on NQB traffic that is using NQB (and the associated
   shallow buffer) as intended motivates limiting the effects of shallow
   buffer overrun via per-user provisioning limits that prevent queue
   overrun effects from affecting other users (see Section 5.1).

   Notwithstanding the above, the choice of DSCP for NQB does allow
   existing Wi-Fi networks to readily (and by default) support some of
   the PHB requirements, but without a traffic protection function, and
   (when left in the default state) by giving NQB traffic higher
   priority than QB traffic.  This is not considered to be a compliant
   implementation of the PHB.  These existing Wi-Fi networks currently
   provide priority to half of the DSCP space, whether or not the DSCP
   value 45 (decimal) is assigned to the NQB DSCP.  While the DSCP value
   45 (decimal) could also be abused to gain priority on such links, the
   potential presence of traffic protection functions in other hops
   along the path (which likely act on the DSCP value 45 (decimal)
   alone) would make it less attractive for such abuse than any of the
   other 31 DSCP values that are given priority.

   This document discusses the potential use of the NQB DSCP and NQB PHB
   in network technologies that are standardized in other SDOs.  Any
   security considerations that relate to deployment and operation of
   NQB solely in specific network technologies are not discussed here.

   NQB uses the DS field.  The design of Diffserv does not include
   integrity protection for the DSCP; thus, it is possible for the DSCP
   to be changed by an on-path attacker.  The NQB PHB and associated
   DSCP don't change this.  While re-marking DSCPs is permitted for
   various reasons (some are discussed in this document, others can be
   found in [RFC2474] and [RFC2475]), if done maliciously, this might
   negatively affect the QoS of the tampered microflow.  Nonetheless, an
   on-path attacker can also alter other mutable fields in the IP header
   (e.g., the TTL), which can wreak much more havoc than just altering
   QoS treatment.

10.  References

10.1.  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>.

   [RFC2474]  Nichols, K., Blake, S., Baker, F., and D. Black,
              "Definition of the Differentiated Services Field (DS
              Field) in the IPv4 and IPv6 Headers", RFC 2474,
              DOI 10.17487/RFC2474, December 1998,
              <https://www.rfc-editor.org/info/rfc2474>.

   [RFC8085]  Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage
              Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085,
              March 2017, <https://www.rfc-editor.org/info/rfc8085>.

   [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>.

   [RFC8325]  Szigeti, T., Henry, J., and F. Baker, "Mapping Diffserv to
              IEEE 802.11", RFC 8325, DOI 10.17487/RFC8325, February
              2018, <https://www.rfc-editor.org/info/rfc8325>.

10.2.  Informative References

   [Barik]    Barik, R., Welzl, M., Elmokashfi, A., Dreibholz, T., and
              S. Gjessing, "Can WebRTC QoS Work? A DSCP Measurement
              Study", 30th International Teletraffic Congress (ITC 30),
              DOI 10.1109/ITC30.2018.00034, September 2018,
              <https://gitlab2.informatik.uni-wuerzburg.de/itc-
              conference/itc-publications-public/-/raw/master/itc30/
              Barik18ITC30.pdf?inline=true>.

   [BBR-CC]   Cardwell, N., Ed., Swett, I., Ed., and J. Beshay, Ed.,
              "BBR Congestion Control", Work in Progress, Internet-
              Draft, draft-ietf-ccwg-bbr-05, 2 March 2026,
              <https://datatracker.ietf.org/doc/html/draft-ietf-ccwg-
              bbr-04>.

   [Cardwell2017]
              Cardwell, N., Cheng, Y., Gunn, C. S., Yeganeh, S. H., and
              V. Jacobson, "BBR: Congestion-Based Congestion Control",
              Communications of the ACM, vol. 60, no. 2, pp. 58-66,
              DOI 10.1145/3009824, February 2017,
              <https://doi.org/10.1145/3009824>.

   [Custura]  Custura, A., Venne, A., and G. Fairhurst, "Exploring DSCP
              modification pathologies in mobile edge networks", Network
              Traffic Measurement and Analysis Conference (TMA), 2017,
              <https://tma.ifip.org/wp-content/uploads/sites/7/2017/06/
              mnm2017_paper13.pdf>.

   [Gettys2012]
              Gettys, J. and K. Nichols, "Bufferbloat: Dark Buffers in
              the Internet", Communications of the ACM, vol. 55, no. 1,
              pp. 57-65, DOI 10.1145/2063176.2063196, January 2012,
              <https://doi.org/10.1145/2063176.2063196>.

   [IEEE802-11]
              IEEE, "IEEE Standard for Information Technology --
              Telecommunications and Information Exchange between
              Systems - Local and Metropolitan Area Networks -- Specific
              Requirements - Part 11: Wireless LAN Medium Access Control
              (MAC) and Physical Layer (PHY) Specifications", IEEE
              Std 802.11-2024, DOI 10.1109/IEEESTD.2025.10979691, April
              2025, <https://ieeexplore.ieee.org/document/10979691>.

   [LOW_LATENCY_DOCSIS]
              CableLabs, "Low Latency DOCSIS: Technology Overview",
              February 2019, <https://cablela.bs/low-latency-docsis-
              technology-overview-february-2019>.

   [QOS_TRAFFIC_TYPE]
              Microsoft, Corporation, "QOS_TRAFFIC_TYPE enumeration
              (qos2.h)", January 2022, <https://learn.microsoft.com/en-
              us/windows/win32/api/qos2/ne-qos2-qos_traffic_type>.

   [RFC2475]  Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
              and W. Weiss, "An Architecture for Differentiated
              Services", RFC 2475, DOI 10.17487/RFC2475, December 1998,
              <https://www.rfc-editor.org/info/rfc2475>.

   [RFC2598]  Jacobson, V., Nichols, K., and K. Poduri, "An Expedited
              Forwarding PHB", RFC 2598, DOI 10.17487/RFC2598, June
              1999, <https://www.rfc-editor.org/info/rfc2598>.

   [RFC2661]  Townsley, W., Valencia, A., Rubens, A., Pall, G., Zorn,
              G., and B. Palter, "Layer Two Tunneling Protocol "L2TP"",
              RFC 2661, DOI 10.17487/RFC2661, August 1999,
              <https://www.rfc-editor.org/info/rfc2661>.

   [RFC2914]  Floyd, S., "Congestion Control Principles", BCP 41,
              RFC 2914, DOI 10.17487/RFC2914, September 2000,
              <https://www.rfc-editor.org/info/rfc2914>.

   [RFC2983]  Black, D., "Differentiated Services and Tunnels",
              RFC 2983, DOI 10.17487/RFC2983, October 2000,
              <https://www.rfc-editor.org/info/rfc2983>.

   [RFC3246]  Davie, B., Charny, A., Bennet, J.C.R., Benson, K., Le
              Boudec, J.Y., Courtney, W., Davari, S., Firoiu, V., and D.
              Stiliadis, "An Expedited Forwarding PHB (Per-Hop
              Behavior)", RFC 3246, DOI 10.17487/RFC3246, March 2002,
              <https://www.rfc-editor.org/info/rfc3246>.

   [RFC3551]  Schulzrinne, H. and S. Casner, "RTP Profile for Audio and
              Video Conferences with Minimal Control", STD 65, RFC 3551,
              DOI 10.17487/RFC3551, July 2003,
              <https://www.rfc-editor.org/info/rfc3551>.

   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, DOI 10.17487/RFC4301,
              December 2005, <https://www.rfc-editor.org/info/rfc4301>.

   [RFC4594]  Babiarz, J., Chan, K., and F. Baker, "Configuration
              Guidelines for DiffServ Service Classes", RFC 4594,
              DOI 10.17487/RFC4594, August 2006,
              <https://www.rfc-editor.org/info/rfc4594>.

   [RFC5127]  Chan, K., Babiarz, J., and F. Baker, "Aggregation of
              Diffserv Service Classes", RFC 5127, DOI 10.17487/RFC5127,
              February 2008, <https://www.rfc-editor.org/info/rfc5127>.

   [RFC5681]  Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
              Control", RFC 5681, DOI 10.17487/RFC5681, September 2009,
              <https://www.rfc-editor.org/info/rfc5681>.

   [RFC5865]  Baker, F., Polk, J., and M. Dolly, "A Differentiated
              Services Code Point (DSCP) for Capacity-Admitted Traffic",
              RFC 5865, DOI 10.17487/RFC5865, May 2010,
              <https://www.rfc-editor.org/info/rfc5865>.

   [RFC7893]  Stein, Y., Black, D., and B. Briscoe, "Pseudowire
              Congestion Considerations", RFC 7893,
              DOI 10.17487/RFC7893, June 2016,
              <https://www.rfc-editor.org/info/rfc7893>.

   [RFC8033]  Pan, R., Natarajan, P., Baker, F., and G. White,
              "Proportional Integral Controller Enhanced (PIE): A
              Lightweight Control Scheme to Address the Bufferbloat
              Problem", RFC 8033, DOI 10.17487/RFC8033, February 2017,
              <https://www.rfc-editor.org/info/rfc8033>.

   [RFC8034]  White, G. and R. Pan, "Active Queue Management (AQM) Based
              on Proportional Integral Controller Enhanced (PIE) for
              Data-Over-Cable Service Interface Specifications (DOCSIS)
              Cable Modems", RFC 8034, DOI 10.17487/RFC8034, February
              2017, <https://www.rfc-editor.org/info/rfc8034>.

   [RFC8083]  Perkins, C. and V. Singh, "Multimedia Congestion Control:
              Circuit Breakers for Unicast RTP Sessions", RFC 8083,
              DOI 10.17487/RFC8083, March 2017,
              <https://www.rfc-editor.org/info/rfc8083>.

   [RFC8100]  Geib, R., Ed. and D. Black, "Diffserv-Interconnection
              Classes and Practice", RFC 8100, DOI 10.17487/RFC8100,
              March 2017, <https://www.rfc-editor.org/info/rfc8100>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

   [RFC8289]  Nichols, K., Jacobson, V., McGregor, A., Ed., and J.
              Iyengar, Ed., "Controlled Delay Active Queue Management",
              RFC 8289, DOI 10.17487/RFC8289, January 2018,
              <https://www.rfc-editor.org/info/rfc8289>.

   [RFC8290]  Hoeiland-Joergensen, T., McKenney, P., Taht, D., Gettys,
              J., and E. Dumazet, "The Flow Queue CoDel Packet Scheduler
              and Active Queue Management Algorithm", RFC 8290,
              DOI 10.17487/RFC8290, January 2018,
              <https://www.rfc-editor.org/info/rfc8290>.

   [RFC8622]  Bless, R., "A Lower-Effort Per-Hop Behavior (LE PHB) for
              Differentiated Services", RFC 8622, DOI 10.17487/RFC8622,
              June 2019, <https://www.rfc-editor.org/info/rfc8622>.

   [RFC9330]  Briscoe, B., Ed., De Schepper, K., Bagnulo, M., and G.
              White, "Low Latency, Low Loss, and Scalable Throughput
              (L4S) Internet Service: Architecture", RFC 9330,
              DOI 10.17487/RFC9330, January 2023,
              <https://www.rfc-editor.org/info/rfc9330>.

   [RFC9331]  De Schepper, K. and B. Briscoe, Ed., "The Explicit
              Congestion Notification (ECN) Protocol for Low Latency,
              Low Loss, and Scalable Throughput (L4S)", RFC 9331,
              DOI 10.17487/RFC9331, January 2023,
              <https://www.rfc-editor.org/info/rfc9331>.

   [RFC9332]  De Schepper, K., Briscoe, B., Ed., and G. White, "Dual-
              Queue Coupled Active Queue Management (AQM) for Low
              Latency, Low Loss, and Scalable Throughput (L4S)",
              RFC 9332, DOI 10.17487/RFC9332, January 2023,
              <https://www.rfc-editor.org/info/rfc9332>.

   [RFC9435]  Custura, A., Fairhurst, G., and R. Secchi, "Considerations
              for Assigning a New Recommended Differentiated Services
              Code Point (DSCP)", RFC 9435, DOI 10.17487/RFC9435, July
              2023, <https://www.rfc-editor.org/info/rfc9435>.

   [RFC9438]  Xu, L., Ha, S., Rhee, I., Goel, V., and L. Eggert, Ed.,
              "CUBIC for Fast and Long-Distance Networks", RFC 9438,
              DOI 10.17487/RFC9438, August 2023,
              <https://www.rfc-editor.org/info/rfc9438>.

   [RFC9957]  Briscoe, B., Ed. and G. White, "The DOCSIS Queue
              Protection Algorithm to Preserve Low Latency", RFC 9957,
              DOI 10.17487/RFC9957, May 2026,
              <https://www.rfc-editor.org/info/rfc9957>.

   [SA-5G]    3GPP, "System Architecture for the 5G System (5GS)",
              Version 20.1.0, Release 20, 3GPP TS 23.501, March 2026,
              <https://portal.3gpp.org/desktopmodules/Specifications/
              SpecificationDetails.aspx?specificationId=3144>.

   [TR-369]   Broadband Forum, "The User Services Platform", Issue: 1
              Amendment 4 Corrigendum 2, July 2025,
              <https://usp.technology/specification/index.html>.

   [WikipediaBufferbloat]
              Wikipedia, "Bufferbloat", May 2025,
              <https://en.wikipedia.org/w/
              index.php?title=Bufferbloat&oldid=1292250296>.

Appendix A.  DSCP Re-marking Policies

   Some network operators typically bleach (zero out) the DSCP field on
   ingress into their network (see [RFC9435], [Custura], and [Barik])
   and, in some cases, apply their own DSCP for internal use.  Bleaching
   the DSCP value 45 (decimal) is not expected to cause harm to Default
   traffic, but it will severely limit the ability to provide NQB
   treatment.  Reports on existing deployments of DSCP manipulation (see
   [Custura] and [Barik]) categorize the re-marking behaviors into the
   following policies: bleach all traffic (set DSCP to zero); set the
   top three bits (the former Precedence bits) on all traffic to 0b000,
   0b001, or 0b010; set the low three bits on all traffic to 0b000; or
   re-mark all traffic to a particular (non-zero) DSCP value.

   Regarding the DSCP value 45 (decimal), there were no observations of
   DSCP manipulation reported in which traffic was marked with the DSCP
   value 45 (decimal) by any of these policies.  Thus, it appears that
   these re-marking policies would be unlikely to result in QB traffic
   being marked as NQB.  In terms of the fate of traffic marked with the
   DSCP value 45 (decimal) that is subjected to one of these policies,
   it would be indistinguishable from some subset (possibly all) of
   other traffic.  In the policies where all traffic is re-marked using
   the same (zero or non-zero) DSCP value, the ability for a subsequent
   network hop to differentiate NQB traffic via DSCP would clearly be
   lost entirely.

   In the policies where the top three bits are overwritten (see
   Section 4.2 of [RFC9435]), the DSCP value 45 (decimal) would receive
   the same marking as would the currently unassigned Pool 3 DSCP values
   (5, 13, 21, 29, 37, 53, and 61), with all of these DSCP values
   getting re-marked to DSCP value = 5, 13, or 21 (depending on the
   overwrite value used).  Since none of the DSCP values in the
   preceding lists are currently assigned by IANA, and they all are
   reserved for Standards Action, it is believed that they are not
   widely used currently; however, this could vary based on local-usage
   and could change in the future.  If networks in which this sort of
   re-marking occurs or networks downstream classify the resulting DSCP
   (i.e., 5, 13, or 21) to the NQB PHB or re-mark such traffic with DSCP
   value 45 (decimal), they risk giving NQB treatment to other traffic
   that was not originally marked as NQB.  In addition, as described in
   Section 6 of [RFC9435] future assignments of these 0bxxx101 DSCPs
   would need to be made with consideration of the potential that they
   all are treated as NQB in some networks.

   For the policy in which the low three bits are set to 0b000, the DSCP
   value 45 (decimal) would be re-marked to CS5 and would be
   indistinguishable from CS5, VA, and EF (and the currently unassigned
   DSCP values 41, 42, and 43).  Traffic marked using the existing
   standardized DSCPs in this list are likely to share the same general
   properties as NQB traffic (non-capacity-seeking and very low data
   rate, relatively low data rate, and consistent data rate).
   Similarly, any future recommended usage for DSCP values 41, 42, 43
   would likely be somewhat compatible with NQB treatment, assuming that
   IP Precedence compatibility (see Section 1.5.4 of [RFC4594]) is
   maintained in the future.  Here there might be an opportunity for a
   node to provide the NQB PHB or the CS5 PHB to CS5-marked traffic and
   retain some of the benefits of NQB marking.  This could be another
   motivation to classify CS5-marked traffic into the NQB queue (as
   discussed in Section 6.3).

Appendix B.  Comparison with Expedited Forwarding

   The EF definition [RFC3246] provides the following text to describe
   the EF PHB forwarding behavior:

   |  This specification defines a PHB in which EF packets are
   |  guaranteed to receive service at or above a configured rate

   and

   |  the rate at which EF traffic is served at a given output interface
   |  should be at least the configured rate R, over a suitably defined
   |  interval, independent of the offered load of non-EF traffic to
   |  that interface.

   Notably, this description is true of any class of traffic that is
   configured with a guaranteed minimum rate, including the Default PHB
   if configured per the guidelines in Section 1.5.1 of [RFC4594].
   [RFC3246] goes on to formalize the definition of EF by requiring that
   an EF node be characterizable in terms of the fidelity with which it
   is able to provide a guaranteed rate.

   While the NQB PHB is not required to be configured with a guaranteed
   minimum rate, [RFC2474] and [RFC4594] recommend assigning some
   minimum resources for the Default PHB, in particular, some dedicated
   capacity.  If such a guaranteed minimum rate is configured for the
   Default PHB, it is recommended (Section 5) that NQB traffic share and
   be given equal access to that rate.  In such cases, the NQB PHB could
   effectively receive a rate guarantee of (for example) 50% of the rate
   guaranteed to the combined NQB/Default PHBs; therefore, it
   technically complies with the PHB forwarding behavior defined for EF.

   However, EF is intended to be a managed service and requires that
   traffic be policed such that the arriving rate of traffic into the EF
   PHB doesn't exceed the guaranteed forwarding rate configured for the
   PHB.  This ensures that low delay and low-delay variation are
   provided.  NQB is intended as a best effort service; hence, the
   aggregate of traffic arriving to the NQB PHB queue could exceed the
   forwarding rate available to the PHB.  Section 5.2 discusses the
   recommended mechanism for handling excess traffic in NQB.  While EF
   relies on rate policing and dropping of excess traffic at the domain
   border, this is only one option for NQB.  NQB primarily recommends
   traffic protection located at each potential bottleneck, where actual
   queuing can be detected and where excess traffic can be reclassified
   into the Default PHB rather than dropping it.  Local traffic
   protection is more feasible for NQB, given the focus is on access
   networks, where one node is typically designed to be the known
   bottleneck where traffic control functions all reside.  In contrast,
   EF is presumed to follow the Diffserv architecture [RFC2475] for core
   networks, where traffic conditioning is delegated to border nodes in
   order to simplify high-capacity interior nodes.  Further, NQB
   recommends a microflow-based mechanism to limit the performance
   impact of excess traffic to those microflows causing potential
   congestion of the NQB queue, whereas EF ignores microflow properties.
   Note that, under congestion, low loss for NQB-conformant flows is
   only ensured if such a mechanism is operational.  Note also that this
   mechanism for NQB operates at the available forwarding rate for the
   PHB (which could vary based on other traffic load) as opposed to a
   configured guaranteed rate, as in EF.

   The lack of a requirement of a guaranteed minimum rate, and the lack
   of a requirement to police incoming traffic to such a rate, makes the
   NQB PHB suitable for implementation in networks where link capacity
   is not or cannot be guaranteed.

   There are additional distinctions between EF and NQB arising from the
   intended usage as described in [RFC4594] and the actual usage in
   practice in the Internet.  In Section 1.5.3 of [RFC4594], EF is
   described as generally being used to carry voice or data that
   requires "wire-like" behavior through the network.  The NQB PHB
   similarly is useful to carry application traffic requiring wire-like
   performance, characterized by low delay and low-delay variation, but
   places a pre-condition that each microflow be relatively low data
   rate and sent in a smooth (non-bursty) manner.  In actual practice,
   EF traffic is oftentimes prioritized over Default traffic.  This
   contrasts with NQB traffic, which is to be treated with the same
   forwarding precedence as Default (and sometimes aggregated with
   Default).

Appendix C.  Impact on Higher Layer Protocols

   The NQB PHB itself has no impact on higher layer protocols because it
   only isolates NQB traffic from QB traffic.  However, traffic
   protection of the PHB can have unintended side effects on higher
   layer protocols.  Traffic protection introduces the possibility that
   microflows classified into the NQB queue could experience out-of-
   order delivery or packet loss if their behavior is not consistent
   with the NQB sender requirements.  Out-of-order delivery could be
   particularly likely if the traffic protection algorithm makes
   decisions on a packet-by-packet basis.  In this scenario, a microflow
   that is (mis-)marked as NQB and that causes a queue to form in this
   bottleneck link could see some of its packets forwarded by the NQB
   queue and some of them either discarded or redirected to the QB
   queue.  In the case of redirection, depending on the queuing delay
   and scheduling within the network element, this could result in
   packets being delivered out of order.  As a result, the use of the
   NQB DSCP by a higher layer protocol carries some risk that an
   increased amount of out-of-order delivery or packet loss will be
   experienced.  This characteristic provides one disincentive for
   incorrectly setting the NQB DSCP on traffic that doesn't comply with
   the NQB sender requirements.

Appendix D.  Alternative Diffserv Codepoints

   In networks where the DSCP value 45 (decimal) is already in use for
   another (e.g., a local-use) purpose or where specialized PHBs are
   available that can meet specific application requirements (e.g., a
   guaranteed-delay path for voice traffic), use of another DSCP value
   could be preferred.

   In end systems where the choice of using DSCP value 45 (decimal) is
   not available to the application, the CS5 DSCP (40 decimal) could be
   used as a fallback.  See Section 6.3 for rationale as to why this
   choice could be fruitful.

Acknowledgements

   Thanks to Gorry Fairhurst, Diego Lopez, Stuart Cheshire, Brian
   Carpenter, Bob Briscoe, Greg Skinner, Toke Hoeiland-Joergensen, Luca
   Muscariello, David Black, Jerome Henry, Steven Blake, Jonathan
   Morton, Roland Bless, Kevin Smith, Martin Dolly, and Kyle Rose for
   their review comments.  Thanks also to Gorry Fairhurst and Ana
   Custura for their input on selection of appropriate DSCPs.  Thanks to
   the following for IESG reviews and comments: Mohamed Boucadair, Ketan
   Talaulikar, Mike Bishop, Roman Danyliw, and Éric Vyncke.

Authors' Addresses

   Greg White
   CableLabs
   Email: g.white@cablelabs.com


   Thomas Fossati
   Linaro
   Email: thomas.fossati@linaro.org


   Rüdiger Geib
   Deutsche Telekom
   Email: Ruediger.Geib@telekom.de
