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A Non-Queue-Building Per-Hop Behavior (NQB PHB) for Differentiated Services
draft-ietf-tsvwg-nqb-21

Document Type Active Internet-Draft (tsvwg WG)
Authors Greg White , Thomas Fossati , Ruediger Geib
Last updated 2023-11-07
Replaces draft-white-tsvwg-nqb
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Intended RFC status Proposed Standard
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Submit "A Non-Queue-Building Per-Hop Behavior (NQB PHB) for Differentiated Services" as a Proposed Standard RFC
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Send notices to David Black <david.black@dell.com>, gorry@erg.abdn.ac.uk
draft-ietf-tsvwg-nqb-21
Transport Area Working Group                                    G. White
Internet-Draft                                                 CableLabs
Updates: rfc8325 (if approved)                                T. Fossati
Intended status: Standards Track                                  Linaro
Expires: 10 May 2024                                             R. Geib
                                                        Deutsche Telekom
                                                         7 November 2023

   A Non-Queue-Building Per-Hop Behavior (NQB PHB) for Differentiated
                                Services
                        draft-ietf-tsvwg-nqb-21

Abstract

   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
   (i.e. non-bursty), low-data-rate, application-limited traffic
   microflows, 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 microflows.

   [NOTE (to be removed by RFC-Editor): This document 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.

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Copyright Notice

   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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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   4
   3.  Context . . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Non-Queue-Building Behavior . . . . . . . . . . . . . . .   5
     3.2.  Relationship to the Diffserv Architecture . . . . . . . .   5
     3.3.  Relationship to L4S . . . . . . . . . . . . . . . . . . .   7
   4.  DSCP Marking of NQB Traffic . . . . . . . . . . . . . . . . .   8
     4.1.  Non-Queue-Building Sender Requirements  . . . . . . . . .   8
     4.2.  Aggregation of the NQB DSCP into another Diffserv PHB . .   9
     4.3.  Aggregation of other DSCPs in the NQB PHB . . . . . . . .  10
     4.4.  End-to-end usage and DSCP Re-marking  . . . . . . . . . .  11
       4.4.1.  Interoperability with Non-DS-Capable Domains  . . . .  11
     4.5.  The NQB DSCP and Tunnels  . . . . . . . . . . . . . . . .  12
   5.  Non-Queue-Building PHB Requirements . . . . . . . . . . . . .  13
     5.1.  Primary Requirements  . . . . . . . . . . . . . . . . . .  13
     5.2.  Traffic Protection  . . . . . . . . . . . . . . . . . . .  14
     5.3.  Impact on Higher Layer Protocols  . . . . . . . . . . . .  16
   6.  Configuration and Management  . . . . . . . . . . . . . . . .  17
     6.1.  Guidance for Lower-Rate Links . . . . . . . . . . . . . .  17
   7.  Mapping NQB to standards of other SDOs  . . . . . . . . . . .  17

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     7.1.  DOCSIS Access Networks  . . . . . . . . . . . . . . . . .  18
     7.2.  Mobile Networks . . . . . . . . . . . . . . . . . . . . .  18
     7.3.  Wi-Fi Networks  . . . . . . . . . . . . . . . . . . . . .  19
       7.3.1.  Interoperability with Existing Wi-Fi Networks . . . .  19
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  21
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  22
   10. Implementation Status . . . . . . . . . . . . . . . . . . . .  22
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  23
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  24
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  24
     12.2.  Informative References . . . . . . . . . . . . . . . . .  24
   Appendix A.  DSCP Re-marking Policies . . . . . . . . . . . . . .  27
   Appendix B.  Comparison to Expedited Forwarding . . . . . . . . .  28
   Appendix C.  Alternate Diffserv Code Points . . . . . . . . . . .  30
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  30

1.  Introduction

   This document defines a Differentiated Services per-hop behavior
   (PHB) called "Non-Queue-Building Per-Hop Behavior" (NQB PHB), which
   isolates traffic microflows (see [RFC2475] for the definition of a
   microflow) that are relatively low data rate and that do not
   themselves materially contribute to queuing delay and loss, allowing
   them to avoid the queuing delays and losses caused by other traffic.
   Such Non-Queue-Building microflows (for example: interactive voice,
   game sync packets, machine-to-machine applications, DNS lookups, and
   real-time IoT analytics data) are low-data-rate application-limited
   microflows that are distinguished from bursty traffic microflows and
   high-data-rate traffic microflows managed by a classic congestion
   control algorithm (as defined in [RFC9330]), both of which cause
   queuing delay and loss.

   In accordance with IETF guidance in [RFC2914] and [RFC8085], most
   packets carried by broadband access networks are managed by an end-
   to-end congestion control algorithm.  Many of the commonly-deployed
   congestion control algorithms, such as Reno, Cubic or BBR, are
   designed 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 delay
   (variable latency) 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)
   in order to enable these congestion control algorithms to effectively
   use the link, which exacerbates the latency and latency variation
   experienced.

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   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 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
   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 queuing systems, such as
   fq_codel [RFC8290] can be employed to isolate microflows 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.  This PHB is primarily applicable for
   high-speed broadband access network links, where there is minimal
   aggregation of traffic, and deep buffers are common.  The
   applicability of this PHB to lower-speed links is discussed in
   Section 5.

   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",
   "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

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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.  Some of these applications are
   transactional in nature, and 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, 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.  These applications
   typically implement a response to network congestion that consists of
   discontinuing (or significantly reducing) transmissions.  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.

   In contrast, Queue-Building (QB) microflows 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 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 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 qualitatively 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 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

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   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.

   Finally, some jurisdictions impose regulations that limit the ability
   of networks to provide differentiation of services, in large part
   this seems to be 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.  This
   aspect is discussed further in Section 4.1 and Section 5.2.

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   The NQB PHB is therefore intended for the prevalent situation where
   the performance requirements of applications cannot be assured end-
   to-end, and 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 provides a better
   network environment for many applications that send data at
   relatively low data rates.

   In regards to 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
   low loss, low delay, and low jitter services.  The main distinctions
   between NQB and EF are discussed in Appendix B.

   In nodes that support multiple DiffServ Service Classes, NQB traffic
   is to be treated as a part of the Default treatment.  Capacity
   assigned to this class is not prioritized with respect to other
   classes, AFxx, EF, etc.  Of course, traffic marked as NQB could (like
   other Default traffic) be prioritized with respect to Lower-Effort
   (LE) [RFC8622] (i.e. the NQB queue would be emptied in a priority
   sequence before the LE queue).

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 [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].  Compliance with the DualQ Coupled AQM requirements
   (Section 2.5 of [RFC9332]) 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
   [RFC9331].

   Applications that comply with both the NQB sender requirements in
   Section 4.1 and the L4S "Prague" requirements in Section 4 of
   [RFC9331] could mark their packets both with the NQB DSCP and with
   the ECT(1) value.  NQB network functions SHOULD treat packets marked
   with the NQB DSCP uniformly, regardless of the value of the ECN
   field.  Here, NQB network functions means 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 4.4.1).  L4S network functions SHOULD treat packets marked
   with the NQB DSCP and ECT(1) or CE the same as packets marked with
   the Default DSCP and the same ECN value.  Here, L4S network functions

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   means the L4S Network Node functions (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].

4.  DSCP Marking of NQB Traffic

4.1.  Non-Queue-Building Sender Requirements

   Microflows that are eligible to be marked with the NQB DSCP are
   typically UDP microflows 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 microflows send at 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 50 Mbps or
   more (and see Section 6.1 for a discussion of cases where this isn't
   true), this implies 500 kbps as an upper limit.  In addition, these
   microflows are required to be sent 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.

   Microflows marked with the NQB DSCP are expected to comply with
   existing guidance for safe deployment on the Internet, including the
   guidance around 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 low latency congestion
   control mechanism as described in [RFC9331].

   Microflows that align with the description of behavior in the
   preceding paragraphs in this section SHOULD be identified to the
   network using a Diffserv Code Point (DSCP) of 45 (decimal) so that
   their packets can be queued separately from QB microflows.  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

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   Section 7.3) 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 values (see Section 4.3 for further
   discussion).

   The consideration as to whether an application chooses to mark its
   traffic as NQB involves 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, if a microflow's traffic exceeds the rate equation provided
   in the first paragraph of this section, the application SHOULD NOT
   mark this traffic with the NQB DSCP.  In such a case, the application
   could instead consider implementing a low latency congestion control
   mechanism as described in [RFC9331].  At the time of writing, it is
   believed that 500 kbps is a reasonable upper bound on instantaneous
   traffic rate for a microflow marked with the NQB DSCP on the
   Internet.  This value is of course subject to the context in which
   the application is expected to be deployed.

   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.

4.2.  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 and nodes
   that aggregate service classes as discussed in [RFC5127] and
   [RFC8100], in which case this recommendation would result in traffic
   marked with the NQB DSCP being aggregated into the Elastic Treatment
   Aggregate (for [RFC5127] networks) or the Default / Elastic Treatment
   Aggregate (for [RFC8100] networks).

   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
   loss/latency/jitter performance for NQB traffic.

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

   Aggregating traffic marked with the NQB DSCP into a PHB designed for
   real-time, latency sensitive traffic (e.g. the (Bulk) Real-Time
   Treatment Aggregate), might better preserve loss/latency/jitter
   performance in the presence of congestion, but would need to be done
   with consideration of the risk of creating an incentive for non-
   compliant traffic to be mis-marked as NQB.

   Networks and nodes that do not support the NQB PHB should 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].

4.3.  Aggregation of other DSCPs in the NQB PHB

   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 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.  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 [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 (decimal), i.e., those whose
   three MSBs are 0b101).  As noted in Section 4.1, 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, as described by [RFC2598].
   Some caveats and differences are discussed in Appendix B.

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

   In contrast to some existing standard PHBs, many of which are
   typically only used 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 DSCP
   value 45 (decimal) 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
   (decimal) for internal usage.  To ensure reliable end-to-end NQB PHB
   treatment, the appropriate NQB DSCP would need to be restored when
   forwarding to another network.

4.4.1.  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 NQB DSCP value) the same concerns
   apply.  In particular, since the recommended NQB DSCP value could be
   given high priority in some non-DS-compliant network gear (e.g.,
   legacy Wi-Fi APs as described in Section 7.3.1), it is RECOMMENDED
   that the operator of the upstream domain re-mark NQB traffic to DSCP
   0 (Default) before delivering traffic into a non-DS-capable domain.
   Network equipment designed for such environments, SHOULD by default
   re-mark NQB traffic to DSCP 0, and SHOULD support the ability to
   disable this re-marking.

   As an alternative to re-marking, 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

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   negative impacts on Default traffic, should the downstream domain
   treat traffic with the NQB DSCP as high priority.  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 re-marked and classified for Default
   forwarding or dropped.  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 latency and loss sensitive traffic marked NQB without
   significantly degrading 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.  This SHOULD be adjusted based on any
   knowledge of the local network environment that is available.

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 traffic marked with the NQB DSCP 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

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   reordering-sensitive tunnel protocol.  Additionally, since networks
   supporting the NQB PHB could implement a traffic protection mechanism
   (see Section 5.2) that results in out-of-order delivery to microflows
   that are marked with the NQB DSCP, it is RECOMMENDED that reordering-
   sensitive tunnel protocols not be used with NQB-marked traffic.

5.  Non-Queue-Building PHB Requirements

   For the NQB PHB to succeed, 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 microflows is that for
   microflows 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 microflows 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 11).  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 microflows, but instead aims to provide an upper-
   bound to queuing delay for as many such marked microflows 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 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 is discussed in Section 4.4.1.  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 Default.  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 scheduler with equal
   weights.  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

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   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 helps to ensure that there are
   no incentives for QB traffic to be mismarked as NQB.

   A node supporting the NQB PHB SHOULD by default classify packets
   marked with the NQB DSCP 45 (decimal) into the queue for Non-Queue-
   Building traffic.  A node supporting the NQB PHB MUST support the
   ability to configure the DSCP that is used to classify packets into
   the queue for Non-Queue-Building traffic.  A node supporting the NQB
   PHB SHOULD support the ability to configure multiple DSCPs that are
   used to classify packets into the queue for Non-Queue-Building
   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 ms).  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 SHOULD
   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.

   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 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 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 mismarked 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

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   impact its own quality of service, and so it seems to be of lesser
   concern.  However, a QB microflow that is mismarked as NQB would
   cause queuing delays 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 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 microflows that are
   inconsistent with the sender requirements in Section 4.1, and either
   re-mark those microflows/packets as Default and reclassify them to
   the QB queue or discard the offending traffic.  Such a function
   SHOULD base its decisions upon the behavior of each microflow rather
   than on application-layer constructs (such as the port number used by
   the application or the source/destination IP address).

   This specification does not mandate a particular algorithm for
   traffic protection.  This is intentional, since 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 which could be employed.

   One potential implementation of such a traffic protection algorithm
   is a per-microflow rate policer, designed to identify microflows that
   exceed the bound provided in Section 4.1, where the value R is set to
   1 percent 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.  One example traffic
   protection algorithm along these lines can be found in
   [I-D.briscoe-docsis-q-protection].  This algorithm maintains per-
   microflow state for up to 32 simultaneous "queue-building"
   microflows, and shared state for any additional microflows in excess
   of that number.

   In the case of a traffic protection algorithm that re-marks and
   reclassifies offending traffic, different levels of hysteresis could
   be considered.  For example, the re-mark/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 re-marked
   and 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

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   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 use re-mark/reclassify, since the
   degradation of communications caused by packet discard are likely to
   be greater than the degradation caused by out-of-order delivery.

   The traffic protection function described here requires 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.

   There are some situations where traffic protection is potentially not
   necessary.  For example, a network element designed for use in
   controlled environments (e.g., enterprise LAN) where a network
   administrator is expected to manage the usage of DSCPs.
   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.  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 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 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
   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.

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6.  Configuration and Management

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

   Additionally, Section 4.2 contains configuration recommendations for
   nodes that do not support the NQB PHB, and Section 4.4.1 contains
   configuration recommentations for networks that interconnect with
   non-DS-capable domains.

6.1.  Guidance for Lower-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
   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, but will come at
   the expense of a greater potential for latency variation.  In
   implementations that support [I-D.briscoe-docsis-q-protection], 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 ten percent of "typical" path
   rates, it is RECOMMENDED that the NQB PHB be disabled and for traffic
   marked with the NQB DSCP to thus be carried using the Default PHB.

7.  Mapping NQB to standards of other SDOs

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

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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 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 traffic marked with the NQB DSCP.
   The NQB queue MUST be configured to share the service's rate shaped
   bandwidth with the queue for QB traffic.

7.2.  Mobile Networks

   Historically, 3GPP mobile networks have utilized "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 microflows at queues and
   radio schedulers.  Typically, an LTE operator provides a dedicated
   bearer for IMS VoLTE (Voice over LTE) traffic, which is prioritized
   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 microflows.

   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.

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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 Enhanced Multimedia Distributed Control
   Access (EDCA) parameters (corresponding to the four Wi-Fi Multimedia
   (WMM) Access Categories) that are used to enable differentiated media
   access characteristics.  As discussed in [RFC8325], it has been a
   common practice for Wi-Fi 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
   Wi-Fi gear, 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 NQB DSCP 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 NQB DSCP 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 NQB DSCP 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 is
   commonly 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 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.

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   The DSCP value 45 (decimal) 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 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 45 (decimal) 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 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.

   *  Wi-Fi 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 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 45 (decimal) for NQB creates no new avenues for non-
      NQB-compliant client applications to exploit the prioritization
      function in Wi-Fi.

   *  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 Wi-Fi
      network, and thus is transmitted by the Access Point,
      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 4.4.1.  A residential ISP that re-marks

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      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 Wi-Fi
   networks, which utilizes the Best Effort Access Category.

   If left unchanged, the prioritization of traffic marked with the NQB
   DSCP via the Video Access Category (particularly in the case of
   traffic originating outside of the Wi-Fi 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,
   Wi-Fi 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 Wi-Fi 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 Wi-Fi 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
   DSCP 45 (decimal) (or the locally determined alternative) to UP_5 in
   the "Video" Access Category.

8.  Acknowledgements

   Thanks to Gorry Fairhurst, Diego Lopez, Stuart Cheshire, Brian
   Carpenter, Bob Briscoe, Greg Skinner, Toke Hoeiland-Joergensen, Luca
   Muscariello, David Black, Sebastian Moeller, 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.

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9.  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
   (https://www.iana.org/assignments/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

10.  Implementation Status

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

   The NQB PHB is implemented in equipment compliant with the current
   DOCSIS 3.1 specification, published by CableLabs at: CableLabs
   Specifications Search (https://www.cablelabs.com/specifications/searc
   h?query=&category=DOCSIS&subcat=DOCSIS%203.1&doctype=Specifications&c
   ontent=false&archives=false&currentPage=1).

   CableLabs maintains a list of production cable modem devices that are
   Certified as being compliant to the DOCSIS Specifications, this list
   is available at https://www.cablelabs.com/wp-content/uploads/2013/10/
   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
   to the specification, but which have been tested in non-public
   Interoperability Events.  These implementations are all proprietary,
   not available as open source.

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11.  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 microflow 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 microflow was to fail to mark its packets as
   NQB, it could suffer the latency and loss typical of sharing a queue
   with capacity seeking traffic.

   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 traffic marked with the NQB DSCP 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 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.

   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, including the NQB DSCP.
   While the NQB DSCP value could 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 NQB DSCP value
   alone) would make it less attractive for such abuse than any of the
   other 31 DSCP values that are provided priority.

   This draft discusses the potential use of the NQB DSCP and NQB PHB in
   network technologies that are standardized in other SDOs.  The
   details of any security considerations that relate to deployment and
   operation of NQB in these network technologies are not discussed
   here.

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   NQB uses the Diffserv field.  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 microflow.

12.  References

12.1.  Normative References

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

   [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>.

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

   [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>.

12.2.  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 ,
              2017.

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   [I-D.briscoe-docsis-q-protection]
              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, <https://datatracker.ietf.org/doc/html/draft-
              briscoe-docsis-q-protection-06>.

   [IEEE802-11]
              IEEE-SA, "IEEE 802.11-2020", IEEE 802, December 2020,
              <https://standards.ieee.org/standard/802_11-2020.html>.

   [QOS_TRAFFIC_TYPE]
              Microsoft, Corporation, "QOS_TRAFFIC_TYPE enumeration",
              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>.

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

   [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>.

   [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>.

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   [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>.

   [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>.

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   [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>.

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

   [TR-369]   Broadband Forum, "The User Services Platform", January
              2022, <https://usp.technology/specification/index.html>.

Appendix A.  DSCP Re-marking Policies

   Some network operators typically bleach (zero out) the Diffserv field
   on ingress into their network [RFC9435][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 (decimal), there were no observations of
   DSCP manipulation reported in which traffic was marked 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 (45).  In terms of the fate of traffic marked with the NQB DSCP
   that is subjected to one of these policies, the result would be that
   traffic marked with the NQB DSCP 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.

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   In the policies where the top three bits are overwritten (see
   Section 4.2 of [RFC9435]), the NQB DSCP (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 DSCPs 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 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 DSCP (i.e. 5, 13, or 21) to the NQB PHB, or re-mark such
   traffic as 45 (decimal), they risk treating as NQB other traffic,
   which 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 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 4.3) classify CS5-marked
   traffic into NQB queue.

Appendix B.  Comparison to Expedited Forwarding

   The Expedited Forwarding 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.

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   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
   bandwidth.  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
   effectively receives a rate guarantee of 50% of the rate guaranteed
   to the combined NQB/Default PHBs, and so 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, thereby ensuring that low latency and low latency variation are
   provided.  NQB is intended as a best effort service, and 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, this is only
   one option for NQB.  NQB alternatively recommends that the
   implementation re-mark and forward excess traffic using the Default
   PHB, rather than dropping it.  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.

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   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 packet delay and 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 priority as Default (and sometimes aggregated with
   Default).

Appendix C.  Alternate Diffserv Code Points

   In networks where another (e.g., a local-use) DSCP 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 (decimal) is not
   available to the application, the CS5 DSCP (40 decimal) could be used
   as a fallback.  See Section 4.3 for rationale as to why this choice
   could be fruitful.

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

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