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

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Document Type
This is an older version of an Internet-Draft whose latest revision state is "Active".
Authors Greg White , Thomas Fossati
Last updated 2023-03-15 (Latest revision 2023-03-13)
Replaces draft-white-tsvwg-nqb
RFC stream Internet Engineering Task Force (IETF)
Additional resources Mailing list discussion
Stream WG state WG Document
Revised I-D Needed - Issue raised by WGLC
Associated WG milestone
Dec 2023
Submit "A Non-Queue-Building Per-Hop Behavior (NQB PHB) for Differentiated Services" as a Proposed Standard RFC
Document shepherd Gorry Fairhurst
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Send notices to David Black <>,
Transport Area Working Group                                    G. White
Internet-Draft                                                 CableLabs
Updates: rfc8325 (if approved)                                T. Fossati
Intended status: Standards Track                                     ARM
Expires: 14 September 2023                                 13 March 2023

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


   This document specifies properties and 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,
   low-data-rate, application-limited traffic flows, which would
   ordinarily share a queue with bursty and capacity-seeking traffic, to
   avoid the latency, latency variation and loss caused by such 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 delays
   and queuing loss caused by Queue-Building protocols are manifested,
   but its use is not limited to such segments.  In particular,
   applications to cable broadband links, Wi-Fi links, and mobile
   network radio and core segments are discussed.  This document
   recommends a specific Differentiated Services Code Point (DSCP) to
   identify Non-Queue-Building flows.

   [NOTE (to be removed by RFC-Editor): This docuement references an ISE
   submission draft (I-D.briscoe-docsis-q-protection) that is approved
   for publication as an RFC.  This draft should be held for publication
   until the queue protection RFC can be referenced.]

Status of This Memo

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

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

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   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
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   This Internet-Draft will expire on 14 September 2023.

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   Copyright (c) 2023 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
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   Please review these documents carefully, as they describe your rights
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   4
   3.  Context . . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Non-Queue-Building Behavior . . . . . . . . . . . . . . .   4
     3.2.  Relationship to the Diffserv Architecture . . . . . . . .   5
     3.3.  Relationship to L4S . . . . . . . . . . . . . . . . . . .   6
   4.  DSCP Marking of NQB Traffic . . . . . . . . . . . . . . . . .   7
     4.1.  Non-Queue-Building Sender Requirements  . . . . . . . . .   7
     4.2.  Aggregation of the NQB DSCP with other Diffserv PHBs  . .   8
     4.3.  Aggregation of other DSCPs in the NQB PHB . . . . . . . .   9
     4.4.  End-to-end usage and DSCP Re-marking  . . . . . . . . . .  10
       4.4.1.  Unmanaged Networks  . . . . . . . . . . . . . . . . .  10
     4.5.  The NQB DSCP and Tunnels  . . . . . . . . . . . . . . . .  11
   5.  Non-Queue-Building PHB Requirements . . . . . . . . . . . . .  12
     5.1.  Primary Requirements  . . . . . . . . . . . . . . . . . .  12
     5.2.  Traffic Protection  . . . . . . . . . . . . . . . . . . .  13
     5.3.  Guidance for Very Low-Rate Links  . . . . . . . . . . . .  14
   6.  Impact on Higher Layer Protocols  . . . . . . . . . . . . . .  14
   7.  Configuration and Management  . . . . . . . . . . . . . . . .  15
   8.  Example Use Cases . . . . . . . . . . . . . . . . . . . . . .  15
     8.1.  DOCSIS Access Networks  . . . . . . . . . . . . . . . . .  15
     8.2.  Mobile Networks . . . . . . . . . . . . . . . . . . . . .  15
     8.3.  WiFi Networks . . . . . . . . . . . . . . . . . . . . . .  16
       8.3.1.  Interoperability with Existing WiFi Networks  . . . .  16
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  19

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   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  19
   11. Implementation Status . . . . . . . . . . . . . . . . . . . .  19
   12. Security Considerations . . . . . . . . . . . . . . . . . . .  20
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  21
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  21
     13.2.  Informative References . . . . . . . . . . . . . . . . .  21
   Appendix A.  DSCP Re-marking Policies . . . . . . . . . . . . . .  24
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  25

1.  Introduction

   This document defines a Differentiated Services per-hop behavior
   (PHB) called "Non-Queue-Building Per-Hop Behavior" (NQB PHB), which
   isolates traffic flows that are relatively low data rate and that do
   not themselves materially contribute to queueing delay and loss,
   allowing them to avoid the queuing delays and losses caused by other
   traffic.  Such Non-Queue-Building flows (for example: interactive
   voice, game sync packets, machine-to-machine applications, DNS
   lookups, and real-time IoT analytics data) are application limited
   flows that are distinguished from the high-data-rate traffic flows
   that are typically managed by an end-to-end congestion control

   Most packets carried by broadband access networks are managed by an
   end-to-end congestion control algorithm, such as Reno, Cubic or BBR.
   These congestion control algorithms attempt to seek the available
   capacity of the end-to-end path (which can frequently be the access
   network link capacity), and in doing so generally overshoot the
   available capacity, causing a queue to build-up at the bottleneck
   link.  This queue build-up results in queuing delay (variable
   latency) and possibly 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)
   in order to enable traditional congestion-controlled applications to
   effectively use the link, which exacerbates the latency and latency
   variation experienced.

   In contrast to traditional congestion-controlled applications, there
   are a variety of relatively low data rate applications that do not
   materially contribute to queueing delay and loss but are nonetheless
   subjected to it by sharing the same bottleneck link in the access
   network.  Many of these applications can be sensitive to latency or
   latency variation, as well as packet loss, and thus produce a poor
   quality of experience in such conditions.

   Active Queue Management (AQM) mechanisms (such as PIE [RFC8033],
   DOCSIS-PIE [RFC8034], or CoDel [RFC8289]) can improve the quality of
   experience for latency sensitive applications, but there are

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   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 much more tightly, applications using those algorithms
   would not perform well.  Alternatively, flow queueing systems, such
   as fq_codel [RFC8290] can be employed to isolate flows from one
   another, but these are not appropriate for all bottleneck links, due
   to complexity or other reasons.

   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 quality of experience for their
   applications.  In particular, the NQB PHB provides a shallow-
   buffered, best-effort service as a complement to a Default deep-
   buffered best-effort service.

   To be clear, a network implementing the NQB PHB solely provides
   isolation for traffic classified as behaving in conformance with the
   NQB DSCP (and optionally enforces that behavior).  It is the NQB
   senders' behavior itself which results in low latency and low loss.

2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "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.  Non-Queue-Building Behavior

   There are many applications that send traffic at relatively low data
   rates and/or in a fairly smooth and consistent manner such that they
   are highly unlikely to exceed the available capacity of the network
   path between source and sink.  These applications might themselves
   only cause very small, transient queues to form in network buffers,
   but nonetheless they can be subjected to packet 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.  Many of
   these applications are negatively affected by excessive packet delay
   and delay variation.  Such applications are ideal candidates to be
   queued separately from the applications that are the cause of queue
   build-up, latency and loss.

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   In contrast, Queue-Building (QB) flows include those that use TCP or
   QUIC, with Cubic, Reno or other TCP congestion control algorithms
   that probe for the link capacity and induce latency and loss as a
   result.  Other types of QB flows 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 Differentiated Services architecture
   [RFC2475] with the intention that it allows traffic to be marked in a
   manner that conveys the performance requirements of that traffic
   either quantitatively or in a relative sense (i.e. priority).  The
   architecture defines the use of the Diffserv field [RFC2474] for this
   purpose, and numerous RFCs have been written that describe
   recommended interpretations of the values (Diffserv Code Points) of
   the field, and standardized treatments (traffic conditioning and per-
   hop-behaviors) 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, it has not been used for these purposes end-to-end
   across the Internet.

   This difficulty is in part due to the fact that meeting the
   performance requirements of an application in an end-to-end context
   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/latency/jitter 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 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 an end-to-end context.

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   Finally, some jurisdictions impose regulations that limit the ability
   of networks to provide differentiation of services, in large part
   based on the belief that doing so necessarily involves prioritization
   or privileged access to bandwidth, and thus a benefit to one class of
   traffic always comes at the expense of another.

   In contrast, the NQB PHB has been designed with the goal that it
   avoids many of these issues, and thus could conceivably be deployed
   end-to-end 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 priority equal to Default traffic and given no
   reserved bandwidth other than the bandwidth that it shares with
   Default traffic.  As a result, the NQB PHB does not aim to meet
   specific application performance requirements.  Instead, the goal of
   the NQB PHB is to provide statistically better loss, latency, and
   jitter performance for traffic that is itself only an insignificant
   contributor to those degradations.  The PHB is also designed to
   minimize any incentives for a sender to mismark its traffic, since
   neither higher priority nor reserved bandwidth 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 traditionally been defined.  These attributes
   also significantly simplify access control and admission control
   functions, reducing them to simple verification of behavior.

3.3.  Relationship to L4S

   The NQB DSCP and PHB described in this draft have been defined to
   operate independently of the experimental L4S Architecture
   [I-D.ietf-tsvwg-l4s-arch].  Nonetheless, the NQB-marked traffic flows
   are intended to be compatible with [I-D.ietf-tsvwg-l4s-arch], with
   the result being that NQB traffic and L4S traffic can share the low-
   latency queue in an L4S DualQ node
   [I-D.ietf-tsvwg-aqm-dualq-coupled].  Compliance with the DualQ
   Coupled AQM requirements (Section 2.5 of
   [I-D.ietf-tsvwg-aqm-dualq-coupled]) is considered sufficient to
   support the NQB PHB requirement of fair allocation of bandwidth
   between the QB and NQB queues (Section 5).  Note that these
   requirements in turn require compliance with all the requirements in
   Section 5 of [I-D.ietf-tsvwg-ecn-l4s-id].

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   Applications that comply with both the NQB sender requirements in
   Section 4.1 and the L4S "Prague" requirements in Section 4 of
   [I-D.ietf-tsvwg-ecn-l4s-id] could mark their packets both with the
   NQB DSCP and with the ECT(1) value.  Packets marked with both the NQB
   DSCP and the ECT(1) codepoint SHOULD be treated the same as packets
   marked with either codepoint alone by the traffic protection function
   and by any re-marking/traffic policing function designed to protect
   unmanaged networks (as described in Section 4.4.1).

4.  DSCP Marking of NQB Traffic

4.1.  Non-Queue-Building Sender Requirements

   Flows that are eligible to be marked with the NQP DSCP are typically
   UDP flows that send 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
   applications send at most a few packets per RTT or a data rate of no
   more than about 1 percent of the "typical" network path capacity.  In
   today's network, where access network data rates are typically on the
   order of 100 Mbps, this implies 1 Mbps as an upper limit.  In
   addition, these applications send their traffic in a smooth (i.e.
   paced) manner, where the number of bytes sent in any time interval
   "T" is less than or equal to R * T + 1500 bytes, where "R" is the
   maximum rate described above.

   When such a flow is generated by a sender that does not implement a
   traditional congestion control mechanism, it nonetheless is expected
   to comply with existing guidance for safe deployment on the Internet,
   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]).
   To be clear, the description of NQB-marked flows in this document is
   not to be interpreted as suggesting that such flows are in any way
   exempt from this responsibility.

   Applications that align with the description of behavior in the
   preceding paragraphs in this section SHOULD identify themselves to
   the network using a Diffserv Code Point (DSCP) of 45 (decimal) so
   that their packets can be queued separately from QB flows.  The
   choice of the value 45 is motivated in part by the desire to achieve
   separate queuing in existing WiFi networks (see Section 8.3) and by
   the desire to make implementation of the PHB simpler in network gear
   that has the ability to classify traffic based on ranges of DSCP
   value (see Section 4.2 for further discussion).

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   In networks where another (e.g., a local-use) codepoint is designated
   for NQB traffic, or where specialized PHBs are available that can
   meet specific application requirements (e.g., a guaranteed-latency
   path for voice traffic), it could be preferred to use another DSCP.
   In end systems where the choice of using DSCP 45 is not available to
   the application, the CS5 DSCP (40 decimal) could be used as a
   fallback.  See Section 4.2 for rationale as to why this choice could
   be fruitful.

   If the application's traffic exceeds the rate equation provided in
   the first paragraph of this section, the application SHOULD NOT mark
   its traffic with the NQB DSCP.  In such a case, the application could
   instead consider implementing a low latency congestion control
   mechanism as described in [I-D.ietf-tsvwg-ecn-l4s-id].  At the time
   of writing, it is believed that 1 Mbps is a reasonable upper bound on
   instantaneous traffic rate for an NQB-marked application, but this
   value is of course subject to the context in which the application is
   expected to be deployed.

   An application that marks its traffic as NQB runs the risk of being
   subjected to a traffic protection algorithm (see Section 5.2) if it
   contributes to the formation of a queue in a node that supports the
   PHB.  This could result in the excess traffic being discarded or
   queued separately as default traffic (and thus potentially delivered
   out of order).  As a result, applications that aren't clearly beneath
   the threshold described above would need to weigh the risk of
   additional loss or out-of-order delivery against the expected latency
   benefits of NQB treatment in determining whether to mark their
   packets as NQB.

   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 re-mark traffic that
   does not comply.  This functionality, when implemented as part of the
   PHB is described in Section 5.2.

4.2.  Aggregation of the NQB DSCP with other Diffserv PHBs

   It is RECOMMENDED that networks and nodes that do not support the NQB
   PHB be configured to treat NQB-marked traffic the same as traffic
   marked "Default".  It is additionally RECOMMENDED that such networks
   and nodes simply classify the NQB DSCP into the same treatment
   aggregate as Default traffic, or encapsulate the NQB-marked packet,
   rather than re-marking NQB traffic as Default.  This preservation of
   the NQB marking enables hops further along the path to provide the
   NQB PHB successfully.

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   In backbone and core network switches (particularly if shallow-
   buffered), as well as in nodes that do not typically experience
   congestion, treating NQB-marked traffic the same as Default might be
   sufficient to preserve loss/latency/jitter performance for NQB
   traffic.  In other nodes, treating NQB-marked traffic as Default
   could result in degradation of loss/latency/jitter performance but
   nonetheless preserves the incentives described in Section 5.  An
   alternative, in controlled environments where there is no risk of
   mismarking of traffic, would be to aggregate NQB-marked traffic with
   real-time, latency sensitive traffic.  Similarly, networks and nodes
   that aggregate service classes as discussed in [RFC5127] and
   [RFC8100] might not be able to provide a PDB/PHB that meets the
   requirements of this document.  In these cases it is RECOMMENDED that
   NQB-marked traffic be aggregated into the Elastic Treatment Aggregate
   (for [RFC5127] networks) or the Default / Elastic Treatment Aggregate
   (for [RFC8100] networks), although in some cases a network operator
   might instead choose to aggregate NQB traffic into the (Bulk) Real-
   Time Treatment Aggregate.  Either approach comes with trade-offs:
   when the aggregated traffic encounters a bottleneck, aggregating with
   Default/Elastic traffic could result in a degradation of
   loss/latency/jitter performance for NQB traffic, while aggregating
   with Real-Time (assuming such traffic is provided a prioritized PHB)
   risks creating an incentive for mismarking of non-compliant traffic
   as NQB (except in controlled environments).  In either case, the NQB
   DSCP SHOULD be preserved (possibly via encapsulation) in order to
   limit the negative impact that such networks would have on end-to-end
   performance for NQB traffic.  This aligns with recommendations in

4.3.  Aggregation of other DSCPs in the NQB PHB

   Operators of nodes that support the NQB PHB may choose to aggregate
   other service classes into the NQB queue.  This is particularly
   useful in cases where specialized PHBs for these other service
   classes are not provided.  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, latency and/or
   jitter.  An operator would need to use their own judgement based on
   the actual traffic characteristics in their network in deciding
   whether or not to aggregate other service classes / DSCPs with NQB.
   For networks that use the [RFC4594] service class definitions, this
   could include Telephony (EF/VA), Signaling (CS5), and possibly Real-
   Time Interactive (CS4) (depending on data rate).  In some networks,
   equipment limitations may necessitate aggregating a range of DSCPs
   (e.g. traffic marked with DSCPs 40-47, i.e., those whose three MSBs
   are 0b101).  As noted in Section 4.1, the choice of the value 45 is
   motivated in part by the desire to make this aggregation simpler in
   network equipment that can classify packets via comparing the DSCP

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   value to a range of configured values.

4.4.  End-to-end usage and DSCP Re-marking

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

   If NQB support is extended across a DiffServ domain boundary, the
   interconnected networks agreeing to support NQB SHOULD use the value
   45 for NQB at network interconnection, unless a different DSCP is
   explicitly documented in the TCA (Traffic Conditioning Agreement, see
   [RFC2475]) for that interconnection.  Similar to the handling of
   DSCPs for other PHBs (and as discussed in [RFC2475]), networks can
   re-mark NQB traffic to a DSCP other than 45 for internal usage.  To
   ensure reliable end-to-end NQB PHB treatment, the appropriate NQB
   DSCP should be restored when forwarding to another network.

   In order to enable interoperability with WiFi equipment as described
   in Section 8.3.1, networks SHOULD ensure NQB traffic is marked DSCP
   45 prior to a customer access link, subject to the safeguards
   described below and in that section.

4.4.1.  Unmanaged Networks

   As discussed in Section 4 of [RFC2475], there may be cases where a
   network operator is delivering traffic into a network outside of
   their control, where there is no knowledge of the traffic management
   capabilities of the downstream domain, and no agreement in place
   (e.g., a residential ISP delivering traffic to a customer's home
   network that may contain a legacy WiFi AP).  In such cases, the
   network equipment in the downstream domain might not support the NQB
   PHB and might instead prioritize traffic marked with the NQB DSCP.
   In these cases, certain precautions can prevent issues that would
   otherwise be caused by excessive NQB traffic and/or traffic mismarked
   as NQB.

   Network equipment that is intended to deliver traffic into such
   unmanaged networks (e.g., an access network gateway for a residential
   ISP) SHOULD by default ensure that NQB traffic is re-marked with a
   DSCP that is unlikely to result in prioritized treatment in the
   downstream domain, such as DSCP 0 (Default).  Such equipment SHOULD
   support the ability to configure the re-marking, so that (when it is
   appropriate) traffic can be delivered as NQB-marked.  As an
   alternative to re-marking, the operator could deploy a traffic

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   protection (see Section 5.2) or a shaping/policing function on NQB-
   marked traffic that minimizes the potential for negative impacts on
   Default traffic.  It should be noted that a traffic protection
   function as defined in this document might only provide protection
   from issues occuring 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
   either dropped or re-marked and classified for Default forwarding.  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 approximately 50 ms of

   The recommendation to limit NQB traffic to 5% in these situations is
   based on an expectation of support for at least 5 simultaneous NQB
   streams, and SHOULD be adjusted according to local network policy.

4.5.  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, potentially improving the possibility of achieving
   NQB treatment in subsequent nodes.

   As is discussed in [RFC2983], tunnel protocols that are sensitive to
   reordering can result in undesirable interactions if multiple DSCP
   PHBs are signaled for traffic within a tunnel instance.  This is true
   for NQB-marked traffic as well.  If a tunnel contains a mix of QB and
   NQB traffic, and this is reflected in the outer DSCP in a network
   that supports the NQB PHB, it would be necessary to avoid a
   reordering-sensitive tunnel protocol.

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5.  Non-Queue-Building PHB Requirements

   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 mismarking its traffic.  Thus, a useful
   property of nodes (i.e. network switches and routers) that support
   separate queues for NQB and QB flows is that for flows consistent
   with the NQB sender requirements in Section 4.1, the NQB queue would
   likely be a better choice than the QB queue; and for flows
   inconsistent with those requirements, the QB queue would likely be a
   better choice than the NQB queue (this is discussed further in this
   section and Section 12).  By adhering to these principles, there is
   no incentive for senders to mismark their traffic as NQB.  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 mismarking can be
   identified by the network.

5.1.  Primary Requirements

   A node supporting the NQB PHB makes no guarantees on latency or data
   rate for NQB-marked flows, but instead aims to provide an upper-bound
   to queuing delay for as many such marked flows as it can and shed
   load when needed.

   A node supporting the NQB PHB MUST provide a queue for Non-Queue-
   Building traffic separate from any queue used for Queue-Building

   A node supporting the NQB PHB SHOULD NOT rate limit or rate police
   the aggregate of NQB traffic separately from Queue-Building traffic
   of equivalent importance.  An exception to this recommendation is
   discussed in Section 4.4.1.

   The NQB queue SHOULD be given equivalent forwarding preference
   compared to Queue-Building traffic of equivalent importance.  The
   node SHOULD provide a scheduler that allows QB and NQB traffic of
   equivalent importance to share the link in a fair manner, e.g., a
   deficit round-robin scheduler with equal weights.  Compliance with
   these recommendations helps to ensure that there are no incentives
   for QB traffic to be mismarked as NQB.

   A node supporting the NQB PHB SHOULD treat traffic marked as Default
   (DSCP=0) as QB traffic having equivalent importance to the NQB-marked
   traffic.  A node supporting the NQB PHB MUST support the ability to
   configure the DSCPs that are used to identify QB and NQB traffic of
   equivalent importance.

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   The NQB queue SHOULD have a buffer size that is significantly smaller
   than the buffer provided for QB traffic.  It is expected that most QB
   traffic is engineered to work well when the network provides a
   relatively deep buffer (e.g., on the order of tens or hundreds of ms)
   in nodes where support for the NQB PHB is advantageous (i.e.,
   bottleneck nodes).  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.
   An NQB buffer size less than or equal to 10 ms is RECOMMENDED.

   In some cases, existing network gear has been deployed that cannot
   readily be upgraded or configured to support the PHB requirements.
   This equipment might however be capable of loosely supporting an NQB
   service – see Section 8.3.1 for details and an example where this is
   particularly important.  A similar approach might prove necessary in
   other network environments.

5.2.  Traffic Protection

   It is possible that due to an implementation error or
   misconfiguration, a QB flow would end up getting mismarked as NQB, or
   vice versa.  In the case of a low data rate flow that isn't marked as
   NQB and therefore ends up in the QB queue, it would only impact its
   own quality of service, and so it seems to be of lesser concern.
   However, a QB flow that is mismarked as NQB would cause queuing
   delays and/or loss for all the other flows that are sharing the NQB

   To prevent this situation from harming the performance of the flows
   that are in compliance with the requirements in Section 4.1, network
   elements that support the NQB PHB SHOULD support a "traffic
   protection" function that can identify flows that are inconsistent
   with the sender requirements in Section 4.1, and either re-mark those
   flows/packets as Default and reclassify them to the QB queue or
   discard the offending traffic.  Such a function SHOULD be implemented
   in an objective and verifiable manner, basing its decisions upon the
   behavior of the flow rather than on application-layer constructs.
   While it is possible to utilize a per-flow rate policer to perform
   this function, it is RECOMMENDED that traffic protection algorithms
   base their decisions on the detection of actual queuing, as opposed
   to simply packet arrival rate or data rate.  It could be advantageous
   for a traffic protection function to employ hysteresis to prevent
   borderline flows from being reclassified capriciously.

   The traffic protection function described here requires that the
   network element maintain some sort of flow state.  The traffic
   protection function MUST be designed to fail gracefully in the case
   that the flow state is exhausted.

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   One example traffic protection algorithm can be found in
   [I-D.briscoe-docsis-q-protection].  This algorithm maintains per-flow
   state for up to 32 simultaneous "queue-building" flows, and shared
   state for any additional flows in excess of that number.

   There are some situations where such a function is potentially not
   necessary.  For example, a network element designed for use in
   controlled environments (e.g., enterprise LAN).  Additionally, some
   networks might prefer to police the application of the NQB DSCP at
   the ingress edge, so that per-hop traffic protection is not needed.

5.3.  Guidance for Very Low-Rate Links

   The NQB sender requirements in Section 4.1 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", and some of these links
   could even be below the instantaneous sending rate of some NQB-marked

   To limit the consequences of this scenario, operators of such
   networks SHOULD utilize a traffic protection function that is more
   tolerant of burstiness (i.e., a temporary queue).  Alternatively,
   operators of such networks MAY choose to disable NQB support (and
   thus aggregate NQB-marked traffic with Default traffic) on these low-
   speed links.  For links that are less than ten percent of "typical"
   path rates, it is RECOMMENDED that NQB support be disabled and for
   NQB-marked traffic to thus be carried using the Default PHB.

6.  Impact on Higher Layer Protocols

   Network elements that support the NQB PHB and that support traffic
   protection as discussed in the previous section introduce the
   possibility that flows classified into the NQB queue could experience
   out of order delivery or packet loss if their behavior is not
   consistent with NQB.  This is particularly true if the traffic
   protection algorithm makes decisions on a packet-by-packet basis.  In
   this scenario, a flow 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 queueing latency 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

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   that an increased amount of out of order delivery or packet loss will
   be experienced.  This characteristic provides one disincentive for
   mismarking of traffic.

7.  Configuration and Management

   As required above, 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 default for
   such classifiers is recommended to be the assigned NQB DSCP (to
   identify NQB traffic) and the Default (0) DSCP (to identify QB

8.  Example Use Cases

8.1.  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 quality of service that each
   application receives, and as a result, the quality of experience that
   it generates for the user is influenced by the characteristics of the
   access network link.

   To support the NQB PHB, cable broadband services MUST be configured
   to provide a separate queue for NQB-marked traffic.  The NQB queue
   MUST be configured to share the service's rate shaped bandwidth with
   the queue for QB traffic.

8.2.  Mobile Networks

   Historically, 3GPP mobile networks have utilised "bearers" to
   encapsulate each user's user plane traffic through the radio and core
   networks.  A "dedicated bearer" can be allocated a Quality of Service
   (QoS) to apply any prioritisation to its flows at queues and radio
   schedulers.  Typically, an LTE operator provides a dedicated bearer
   for IMS VoLTE (Voice over LTE) traffic, which is prioritised in order
   to meet regulatory obligations for call completion rates; and a "best
   effort" default bearer, for Internet traffic.  The "best effort"
   bearer provides no guarantees, and hence its buffering
   characteristics are not compatible with low-latency traffic.  The 5G
   radio and core systems offer more flexibility over bearer allocation,
   meaning bearers can be allocated per traffic type (e.g., loss-
   tolerant, low-latency etc.) and hence support more suitable treatment
   of Internet real-time flows.

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   To support the NQB PHB, the mobile network SHOULD be configured to
   give User Equipment a dedicated, low-latency, non-GBR, EPS bearer,
   e.g., one with QCI 7, in addition to the default EPS bearer; or a
   Data Radio Bearer with 5QI 7 in a 5G system (see Table 5.7.4-1:
   Standardized 5QI to QoS characteristics mapping in [SA-5G]).

   A packet carrying the NQB DSCP SHOULD be routed through the dedicated
   low-latency EPS bearer.  A packet that has no associated NQB marking
   SHOULD NOT be routed through the dedicated low-latency EPS bearer.

8.3.  WiFi Networks

   WiFi networking equipment compliant with 802.11e/n/ac/ax [IEEE802-11]
   generally supports either four or eight transmit queues and four sets
   of associated Enhanced Multimedia Distributed Control Access (EDCA)
   parameters (corresponding to the four WiFi Multimedia (WMM) Access
   Categories) that are used to enable differentiated media access
   characteristics.  As discussed in [RFC8325], it has been a common
   practice for WiFi implementations to use a default DSCP to User
   Priority mapping that utilizes the most significant three bits of the
   Diffserv 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 WiFi gear, this
   mapping can be controlled by the network operator, e.g., via TR-369

   In addition to the requirements provided in other sections of this
   document, to support the NQB PHB, WiFi equipment (including equipment
   compliant with [RFC8325]) SHOULD map the NQB codepoint 45 into a
   separate queue in the same Access Category as the queue that carries
   Default traffic (i.e. the Best Effort Access Category).

8.3.1.  Interoperability with Existing WiFi Networks

   While some existing WiFi 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 WiFi networks, as opposed to PHB compliance.

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   Since this equipment is widely deployed, and the WiFi link is
   commonly a bottleneck link, the performance of NQB-marked traffic
   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 WiFi equipment that uses the default
   mapping of DSCPs to Access Categories and the default EDCA parameters
   will support either the NQB PHB requirement for separate queuing of
   NQB traffic, or the recommendation to treat NQB traffic with priority
   equal to Default traffic, but not both.

   The DSCP value 45 is recommended for NQB.  This maps NQB to UP_5
   using the default mapping, which is in the "Video" Access Category.
   While this choice of DSCP enables these WiFi systems to support the
   NQB PHB requirement for separate queuing, existing WiFi 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 45 as well as its ramifications, and remedies for
   its limitations are discussed further below.

   The choice of separated queuing rather than equal priority in
   existing WiFi networks was motivated by the following:

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

   *  WiFi gear typically has hardware support (albeit generally not
      exposed for user control) for adjusting the EDCA parameters in
      order to meet the equal priority recommendation.  This is
      discussed further below.

   *  Traffic that is compliant with the NQB sender requirements
      Section 4.1 is 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].

   *  Application instances on WiFi 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 45 for NQB creates no new avenues for non-NQB-compliant client
      applications to exploit the prioritization function in WiFi.

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

   *  For application traffic that originates outside of the WiFi
      network, and thus is transmitted by the Access Point,
      opportunities exist in the network components upstream of the WiFi
      Access Point to police the usage of the NQB codepoint and
      potentially re-mark traffic that is considered non-compliant, as
      is recommended in Section 4.4.1.  A residential ISP that re-marks
      the Diffserv field to zero, bleaches all DSCPs and hence would not
      be impacted by the introduction of traffic marked as NQB.
      Furthermore, any change to this practice ought to be done
      alongside the implementation of those recommendations in the
      current document.

   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 WiFi
   networks, which utilizes the Best Effort Access Category.

   If left unchanged, the prioritization of NQB-marked traffic via the
   Video Access Category (particularly in the case of traffic
   originating outside of the WiFi network as mentioned above) could
   erode the principle of alignment of incentives discussed in
   Section 5.  In order to preserve the incentives principle for NQB,
   WiFi systems SHOULD be configured such that the EDCA parameters for
   the Video Access Category match those of the Best Effort Access
   Category.  These changes can be deployed in managed WiFi systems or
   those deployed by an ISP and are intended for situations when the
   vast majority of traffic that would use AC_VI is NQB.  In other
   situations (e.g., consumer-grade WiFi gear deployed by an ISP's
   customer) this configuration might not be possible, and the
   requirements and recommendations in Section 4.4.1 would apply.

   Similarly, systems that utilize [RFC8325] but that are unable to
   fully support the PHB requirements, SHOULD map the recommended NQB
   codepoint 45 (or the locally determined alternative) to UP_5 in the
   "Video" Access Category.

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9.  Acknowledgements

   Thanks to Diego Lopez, Stuart Cheshire, Brian Carpenter, Bob Briscoe,
   Greg Skinner, Toke Hoeiland-Joergensen, Luca Muscariello, David
   Black, Sebastian Moeller, Ruediger Geib, Jerome Henry, Steven Blake,
   Jonathan Morton, Roland Bless, Kevin Smith, Martin Dolly, and Kyle
   Rose for their review comments.  Thanks also to Gorry Fairhurst, Ana
   Custura, and Ruediger Geib for their input on selection of
   appropriate DSCPs.

10.  IANA Considerations

   This document requests that IANA assign the Differentiated Services
   Field Codepoint (DSCP) 45 ('0b101101', 0x2D) from the "Differentiated
   Services Field Codepoints (DSCP)" registry
   ( ("DSCP Pool 3
   Codepoints", Codepoint Space xxxx01, Standards Action) as the
   RECOMMENDED codepoint for Non-Queue-Building behavior.

   IANA should update this registry as follows:

   *  Name: NQB

   *  Value (Binary): 101101

   *  Value (Decimal): 45

   *  Reference: this document

11.  Implementation Status

   Note to RFC Editor: This section should be removed prior to

   The NQB PHB is implemented in equipment compliant with the current
   DOCSIS 3.1 specification, published by CableLabs at: CableLabs
   Specifications Search (

   CableLabs maintains a list of production cable modem devices that are
   Certified as being compliant to the DOCSIS Specifications, this list
   is available at
   cert_qual.xlsx.  DOCSIS 3.1 modems certified in CW 134 or greater
   implement the NQB PHB.  This includes products from Arcadyan
   Technology Corporation, Arris, AVM, Castlenet, Commscope, Hitron,
   Motorola, Netgear, Sagemcom and Vantiva.  There are additional
   production implementations that have not been Certified as compliant

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   to the specification, but which have been tested in non-public
   Interoperability Events.  These implementations are all proprietary,
   not available as open source.

12.  Security Considerations

   When the NQB PHB is fully supported in bottleneck links, there is no
   incentive for a Queue-Building application to mismark its packets as
   NQB (or vice versa).  If a Queue-Building flow were to mismark its
   packets as NQB, it would be unlikely to receive a benefit by doing
   so, and it would usually experience a degradation.  The nature of the
   degradation would depend on the specifics of the PHB implementation
   (and on the presence or absence of a traffic protection function),
   but could include excessive packet loss, excessive latency variation
   and/or excessive out-of-order delivery.  If a Non-Queue-Building flow
   were to fail to mark its packets as NQB, it could suffer the latency
   and loss typical of sharing a queue with capacity seeking traffic.

   In order to preserve low latency performance for NQB traffic,
   networks that support the NQB PHB will need to ensure that mechanisms
   are in place to prevent malicious NQB-marked traffic from causing
   excessive queue delays.  Section 5.2 recommends the implementation of
   a traffic protection mechanism to achieve this goal but recognizes
   that other options might be more desirable in certain situations.
   The recommendations on traffic protection mechanisms in this document
   presume that some type of "flow" state be maintained in order to
   differentiate between flows 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.

   Notwithstanding the above, the choice of DSCP for NQB does allow
   existing WiFi 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 WiFi networks currently
   provide priority to half of the DSCP space, including the NQB DSCP.
   While the NQB marking could be abused in order to gain priority on
   such links, the potential presence of traffic protection functions
   along the path (which apply to the NQB marking alone) would seem to
   make it less attractive for such abuse than any of the other 31 DSCP
   values that are provided high priority.

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   NQB uses the Diffserv code point (DSCP).  The design of Diffserv does
   not include integrity protection for the DSCP, and 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 flow.

13.  References

13.1.  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,

   [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,

   [RFC2983]  Black, D., "Differentiated Services and Tunnels",
              RFC 2983, DOI 10.17487/RFC2983, October 2000,

   [RFC8085]  Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage
              Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085,
              March 2017, <>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

   [RFC8325]  Szigeti, T., Henry, J., and F. Baker, "Mapping Diffserv to
              IEEE 802.11", RFC 8325, DOI 10.17487/RFC8325, February
              2018, <>.

13.2.  Informative References

   [Barik]    Barik, R., Welzl, M., Elmokashfi, A., Dreibholz, T., and
              S. Gjessing, "Can WebRTC QoS Work? A DSCP Measurement
              Study", ITC 30, September 2018.

   [Custura]  Custura, A., Venne, A., and G. Fairhurst, "Exploring DSCP
              modification pathologies in mobile edge networks", TMA ,

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              Briscoe, B. and G. White, "The DOCSIS(r) Queue Protection
              Algorithm to Preserve Low Latency", Work in Progress,
              Internet-Draft, draft-briscoe-docsis-q-protection-06, 13
              May 2022, <

              De Schepper, K., Briscoe, B., and G. White, "Dual-Queue
              Coupled Active Queue Management (AQM) for Low Latency, Low
              Loss, and Scalable Throughput (L4S)", Work in Progress,
              Internet-Draft, draft-ietf-tsvwg-aqm-dualq-coupled-25, 29
              August 2022, <

              Custura, A., Fairhurst, G., and R. Secchi, "Considerations
              for Assigning a new Recommended DiffServ Codepoint
              (DSCP)", Work in Progress, Internet-Draft, draft-ietf-
              tsvwg-dscp-considerations-13, 3 March 2023,

              De Schepper, K. and B. Briscoe, "The Explicit Congestion
              Notification (ECN) Protocol for Low Latency, Low Loss, and
              Scalable Throughput (L4S)", Work in Progress, Internet-
              Draft, draft-ietf-tsvwg-ecn-l4s-id-29, 29 August 2022,

              Briscoe, B., De Schepper, K., Bagnulo, M., and G. White,
              "Low Latency, Low Loss, and Scalable Throughput (L4S)
              Internet Service: Architecture", Work in Progress,
              Internet-Draft, draft-ietf-tsvwg-l4s-arch-20, 29 August
              2022, <

              IEEE-SA, "IEEE 802.11-2020", IEEE 802, December 2020,

              Microsoft, Corporation, "QOS_TRAFFIC_TYPE enumeration",
              2022, <

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   [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,

   [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,

   [RFC4594]  Babiarz, J., Chan, K., and F. Baker, "Configuration
              Guidelines for DiffServ Service Classes", RFC 4594,
              DOI 10.17487/RFC4594, August 2006,

   [RFC5127]  Chan, K., Babiarz, J., and F. Baker, "Aggregation of
              Diffserv Service Classes", RFC 5127, DOI 10.17487/RFC5127,
              February 2008, <>.

   [RFC7893]  Stein, Y., Black, D., and B. Briscoe, "Pseudowire
              Congestion Considerations", RFC 7893,
              DOI 10.17487/RFC7893, June 2016,

   [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,

   [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, <>.

   [RFC8083]  Perkins, C. and V. Singh, "Multimedia Congestion Control:
              Circuit Breakers for Unicast RTP Sessions", RFC 8083,
              DOI 10.17487/RFC8083, March 2017,

   [RFC8100]  Geib, R., Ed. and D. Black, "Diffserv-Interconnection
              Classes and Practice", RFC 8100, DOI 10.17487/RFC8100,
              March 2017, <>.

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   [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,

   [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,

   [SA-5G]    3GPP, "System Architecture for 5G", TS 23.501, 2019.

   [TR-369]   Broadband Forum, "The User Services Platform", January
              2022, <>.

Appendix A.  DSCP Re-marking Policies

   Some network operators typically bleach (zero out) the Diffserv field
   on ingress into their network
   [I-D.ietf-tsvwg-dscp-considerations][Custura][Barik], and in some
   cases apply their own DSCP for internal usage.  Bleaching the NQB
   DSCP is not expected to cause harm to Default traffic, but it will
   severely limit the ability to provide NQB treatment end-to-end.
   Reports on existing deployments of DSCP manipulation [Custura][Barik]
   categorize the re-marking behaviors into the following six 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, there were no observations of DSCP
   manipulation reported in which traffic was marked 45 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 (45).  In terms
   of the fate of NQB-marked traffic that is subjected to one of these
   policies, the result would be that NQB-marked traffic 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, 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 [I-D.ietf-tsvwg-dscp-considerations]), the NQB value
   (45) would receive the same marking as would the currently unassigned
   Pool 3 DSCPs 5,13,21,29,37,53,61, with all of these codepoints
   getting re-marked to DSCP = 5, 13 or 21 (depending on the overwrite
   value used).  Since none of the DSCPs in the preceding lists are

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   currently assigned by IANA, and they all are reserved for Standards
   Action, it is believed that they are not widely used currently, but
   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 codepoint (i.e. 5, 13, or 21) to
   the NQB PHB, or re-mark such traffic as 45, they risk treating as NQB
   other traffic, which was not originally marked as NQB.  In addition,
   as described in Section 6 of [I-D.ietf-tsvwg-dscp-considerations]
   future assignments of these 0bxxx101 codepoints 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 NQB
   (45) value would be re-marked to CS5 and would be indistinguishable
   from CS5, VA, EF (and the unassigned DSCPs 41, 42, 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, very low data rate or relatively low and consistent data
   rate).  Similarly, any future recommended usage for DSCPs 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 (as discussed in Section 4.2) classify CS5-marked
   traffic into NQB queue.

Authors' Addresses

   Greg White

   Thomas Fossati

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