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A Non-Queue-Building Per-Hop Behavior (NQB PHB) for Differentiated Services
draft-ietf-tsvwg-nqb-23

Document Type Active Internet-Draft (tsvwg WG)
Authors Greg White , Thomas Fossati , Ruediger Geib
Last updated 2024-06-11 (Latest revision 2024-05-07)
Replaces draft-white-tsvwg-nqb
RFC stream Internet Engineering Task Force (IETF)
Intended RFC status Proposed Standard
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Stream WG state Waiting for WG Chair Go-Ahead
Revised I-D Needed - Issue raised by WGLC, Doc Shepherd Follow-up Underway
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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 <david.black@dell.com>, gorry@erg.abdn.ac.uk
draft-ietf-tsvwg-nqb-23
Transport Area Working Group                                    G. White
Internet-Draft                                                 CableLabs
Updates: 8325 (if approved)                                   T. Fossati
Intended status: Standards Track                                  Linaro
Expires: 8 November 2024                                         R. Geib
                                                        Deutsche Telekom
                                                              7 May 2024

   A Non-Queue-Building Per-Hop Behavior (NQB PHB) for Differentiated
                                Services
                        draft-ietf-tsvwg-nqb-23

Abstract

   This document specifies characteristics of a Non-Queue-Building Per-
   Hop Behavior (NQB PHB).  The NQB PHB provides a shallow-buffered,
   best-effort service as a complement to a Default deep-buffered best-
   effort service for Internet services.  The purpose of this NQB PHB is
   to provide a separate queue that enables smooth (i.e. non-bursty),
   low-data-rate, application-limited traffic microflows, 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, and updates the RFC8325
   guidance on mapping Diffserv to IEEE 802.11 for this codepoint.

   [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) 2024 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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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 . . . . . . . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  Non-Queue-Building Behavior . . . . . . . . . . . . . . .   5
     3.2.  Relationship to the Diffserv Architecture . . . . . . . .   5
     3.3.  Relationship to L4S . . . . . . . . . . . . . . . . . . .   8
   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 . .  10
     4.3.  Aggregation of other DSCPs into the NQB PHB . . . . . . .  11
     4.4.  Cross-domain usage and DSCP Re-marking  . . . . . . . . .  11
       4.4.1.  Interoperability with Non-DS-Capable Domains  . . . .  12
     4.5.  The NQB DSCP and Tunnels  . . . . . . . . . . . . . . . .  13
   5.  Non-Queue-Building PHB Requirements . . . . . . . . . . . . .  14
     5.1.  Primary Requirements  . . . . . . . . . . . . . . . . . .  14
     5.2.  Traffic Protection  . . . . . . . . . . . . . . . . . . .  16
     5.3.  Limiting Packet Bursts from Links . . . . . . . . . . . .  19
   6.  Configuration and Management  . . . . . . . . . . . . . . . .  19
     6.1.  Guidance for Lower-Rate Links . . . . . . . . . . . . . .  20
   7.  Mapping NQB to standards of other SDOs  . . . . . . . . . . .  20

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     7.1.  DOCSIS Access Networks  . . . . . . . . . . . . . . . . .  21
     7.2.  Mobile Networks . . . . . . . . . . . . . . . . . . . . .  21
     7.3.  Wi-Fi Networks  . . . . . . . . . . . . . . . . . . . . .  22
       7.3.1.  Interoperability with Existing Wi-Fi Networks . . . .  22
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  24
   9.  Implementation Status . . . . . . . . . . . . . . . . . . . .  25
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  25
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  26
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  26
     11.2.  Informative References . . . . . . . . . . . . . . . . .  27
   Appendix A.  DSCP Re-marking Policies . . . . . . . . . . . . . .  30
   Appendix B.  Comparison with Expedited Forwarding . . . . . . . .  31
   Appendix C.  Impact on Higher Layer Protocols . . . . . . . . . .  33
   Appendix D.  Alternative Diffserv Code Points . . . . . . . . . .  33
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  34
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  34

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 (application-to-application flows, see
   [RFC2475]) 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 (defined in [RFC9330] to mean one that coexists
   with standard Reno congestion control [RFC5681]), 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 path from sender to
   receiver (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 use
   the link efficiently, 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 intended for single queues
   (such as PIE [RFC8033], DOCSIS-PIE [RFC8034], PI2 [RFC9332], 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 fully utilize the link.
   Alternatively, flow queuing systems, such as fq_codel [RFC8290] can
   be employed to isolate microflows from one another, but not all
   operators think they are 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).  A node supporting
   the NQB PHB makes no guarantees on latency or data rate for NQB-
   marked microflows, 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.

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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 even at an inter-packet timescale.  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 across the
   Internet.

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   This is in part due to the fact that meeting the performance
   requirements of an application across the entire sender-to-receiver
   path involves all the networks in the path agreeing on what those
   requirements are and sharing an interest in meeting them.  In many
   cases this is made more difficult since the performance
   "requirements" are not strict ones (e.g., applications will degrade
   in some manner as loss/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 the Internet.

   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.

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   In contrast, the NQB PHB has been designed with the goal that it
   avoids many of these issues, and thus could conceivably be deployed
   across the Internet.  The intent of the NQB DSCP is that it signals
   verifiable behavior that permits the sender to request differentiated
   treatment.  Also, the NQB traffic is to be given a separate queue
   with forwarding preference equal to Default traffic and given no
   reserved bandwidth other than any minimum bandwidth that it shares
   with Default traffic.  As a result, the NQB PHB does not aim to meet
   specific application performance requirements.  Instead, the sole
   goal of the NQB PHB is to isolate NQB traffic from other traffic that
   degrades loss, latency, and jitter performance, given that the NQB
   traffic 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.

   The NQB PHB is therefore intended for the prevalent situation where
   the performance requirements of applications cannot be assured across
   the whole sender-to-receiver path, 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 and smooth 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.  Unlike EF, NQB has no
   requirement for a guaranteed minimum rate, nor to police incoming
   traffic to such a rate, and NQB is expected to be treated with the
   same priority as Default (see Appendix B for details).

   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).

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3.3.  Relationship to L4S

   The NQB DSCP and PHB described in this document 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
   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

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   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 are marked with the NQB DSCP SHOULD align with the
   description of behavior in the preceding paragraphs in this section.
   Applications are RECOMMENDED to use the Diffserv Code Point (DSCP) 45
   (decimal) to mark microflows as NQB.  The choice of the DSCP value 45
   (decimal) is motivated in part by the desire to achieve separate
   queuing in existing Wi-Fi networks (see Section 7.3) and by the
   desire to make implementation of the PHB simpler in network 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

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   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).

   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].

   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.

   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.

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4.3.  Aggregation of other DSCPs into 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 had not been provided at a potential bottleneck, perhaps
   because it was too complex to manage traffic contracts and
   conditioning.  Candidate service classes for this aggregation would
   include those that carry low-data-rate inelastic traffic that has low
   to very-low tolerance for loss, 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, for example as described by
   Appendix A.3.1 of [RFC2598].  Some caveats and differences are
   discussed in Appendix B.

   [NOTE (to be removed by RFC-Editor): this section references the
   obsoleted RFC2598 instead of its replacement RFC3246, because the
   former contains the description of EF performance.]

4.4.  Cross-domain 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 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

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   (decimal) for internal usage.  To ensure reliable NQB PHB treatment
   on the entire path, the appropriate NQB DSCP would need to be
   restored when forwarding to another network.

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 implement certain safeguards
   before delivering traffic into a non-DS-capable domain.

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

   As an alternative to re-marking all NQB traffic, such an operator
   could deploy a traffic protection (see Section 5.2) or a shaping/
   policing function on traffic marked with the NQB DSCP that minimizes
   the potential for negative impacts on Default traffic, should the
   downstream domain treat traffic with the NQB DSCP as high priority.
   In the case that a traffic protection function is used, it SHOULD
   either re-mark offending traffic to DSCP 0 or discard it.  It should
   be noted that a traffic protection function as defined in this
   document might only provide protection from issues occurring in

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   subsequent network hops if the device implementing the traffic
   protection function is the bottleneck link on the path, so it might
   not be a solution for all situations.  In the case that a traffic
   policing function or a rate shaping function is applied to the
   aggregate of NQB traffic destined to such a downstream domain, the
   policer/shaper rate SHOULD be set to either 5% of the interconnection
   data rate, or 5% of the typical rate for such interconnections,
   whichever is greater, with excess traffic being re-marked and
   classified for Default forwarding (or dropped, as a last resort).  A
   traffic policing function SHOULD allow approximately 100 ms of burst
   tolerance (e.g. a token bucket depth equal to 100 ms multiplied by
   the policer rate).  A traffic shaping function SHOULD allow
   approximately 10 ms of burst tolerance, and no more than 50 ms of
   buffering.  The burst tolerance values recommended here are intended
   to reduce the degradation that could be introduced to 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.  In some cases, this could improve the possibility
   of achieving NQB treatment in subsequent nodes, but in other cases it
   could degrade that possibility (e.g. if the re-marking was designed
   specifically to preserve NQB treatment in downstream domains).

   As is discussed in [RFC2983], tunnel protocols that are sensitive to
   reordering (such as IPSec [RFC4301] or L2TP [RFC2661]) 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 reordering-

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   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.  By
   adhering to these principles, there is no incentive for senders to
   mismark their traffic as NQB.

   This principle of incentive alignment ensures a system is robust to
   the behavior of the large majority of individuals and organizations
   who can be expected to act in their own interests (including
   application developers and service providers who act in the interests
   of their users).  Malicious behavior is not necessarily based on
   rational self-interest, so incentive alignment is not a sufficient
   defense, but the large majority of users do not act out of malice.
   Protection against malicious attacks (and accidents) is addressed in
   Section 5.2 and summarized in Section 10.  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 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 for traffic sent towards a non-DS-
   capable domain is discussed in Section 4.4.1.  Note also that
   Section 5.2 discusses potential uses of per-microflow (rather than
   aggregate) rate policing.

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   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 (DRR) scheduler with
   equal weights, or two Wireless Multimedia Access Categories with the
   same channel access (EDCA) parameters.  The use of equal weights for
   DRR is given as a reasonable example, and is not intended to preclude
   other scheduling weights (see below for details).  A node that
   provides rate limits or rate guarantees for Default traffic SHOULD
   ensure that such limits and/or guarantees are shared with NQB traffic
   in a manner that treats the two classes equally.  This could be
   supported using a hierarchical scheduler where the rate limits and
   guarantees are configured on a parent class, and the two queues
   (Default and NQB) are arranged as the children of the parent class
   and given equal access to the capacity configured for the parent
   class (e.g. with equal DRR scheduling).  Compliance with these
   recommendations helps to ensure that there are no incentives for QB
   traffic to be mismarked as NQB.

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

   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,

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   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
   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 comply with the requirements in Section 4.1, network
   elements that support the NQB PHB SHOULD support a "traffic
   protection" function that can identify microflows or packets that are
   inconsistent with the sender requirements in Section 4.1, and either
   reclassify those microflows/packets to the QB queue or discard the
   offending traffic.  In the case of a traffic protection algorithm
   that reclassifies offending traffic, the implementation MAY
   additionally re-mark such traffic to Default (or possibly to another
   local use code point) so that the result of the traffic protection
   decision can be used by further hops.  This sort of re-marking could
   provide a limited layer of protection in situations where downstream
   network nodes support separate queuing for NQB marked packets but
   lack support for traffic protection.

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

   This specification does not mandate a particular algorithm for
   traffic protection.  This is intentional, since this will probably be
   an area where implementers innovate, and 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.

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

   A conventional implementation of such a traffic protection algorithm
   is a per-microflow rate policer, designed to identify microflows that
   exceed the bound provided in Section 4.1, 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.  While a per-
   microflow rate policer is conceptually simpler (and is based directly
   on the NQB sender requirements), it could often end up being more
   strict than is necessary (for example by policing a flow that exceeds
   the rate equation even when the link is underutilized).  One example
   traffic protection algorithm based on the detection of actual queuing
   can be found in [I-D.briscoe-docsis-q-protection].  This algorithm
   maintains per-microflow state for a certain number of simultaneous
   "queue-building" microflows (e.g. 32), and shared state for any
   additional microflows above that number.

   In the case of a traffic protection algorithm that reclassifies
   offending traffic, different levels of hysteresis could be
   considered.  For example, the reclassify decision could be made on a

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

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

   The traffic protection function described here might require that the
   network element maintain microflow state.  The traffic protection
   function MUST be designed such that the node implementing the NQB PHB
   does not fail (e.g. crash) in the case that the microflow state is
   exhausted.

   There are some situations where traffic protection is potentially not
   necessary.  One example could be 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.  Another
   example could be highly aggregated links (links designed to carry a
   large number of simultaneous microflows), where individual microflow
   burstiness is averaged out and thus is unlikely to cause much actual
   delay.

   Some networks might prefer to implement a more traditional Traffic
   Conditioning approach, and police the application of the NQB DSCP at
   the ingress edge so that per-hop traffic protection is not needed.
   This could be accomplished via the use of a per-microflow rate
   policer that polices microflows at 1 percent of the minimum link
   capacity of the network.  This approach would generally be expected
   to be inferior to per-hop traffic protection, because on one hand it
   would be difficult for edge nodes to guarantee that there would never
   be more than 100 NQB flows that would share a single internal
   bottleneck, and on the other hand there could be internal links that
   have much greater capacity than the minimum.  So, Traffic
   Conditioning at the edge could simultaneously be too lenient and too
   strict.

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5.3.  Limiting Packet Bursts from Links

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

   As well as NQB senders needing to limit packet bursts (see
   Section 4.1), traffic designated for the NQB PHB would benefit from
   configuring these link technologies to limit the burstiness
   introduced.  This is for three reasons.  The first reason is that
   burstiness, whether caused by the sender or by a link on the path,
   could cause queuing delays at downstream bottlenecks and thus degrade
   Quality of Experience.  The second reason is that burstiness in links
   typically means that packets have been delayed by a variable amount,
   i.e. for packets that are being aggregated awaiting a transmission
   opportunity, some packets would generally have arrived just after the
   last transmission opportunity, and thus have to wait the longest,
   while others would generally arrive just in time for the next
   transmission opportunity, and thus would wait the least.  This
   manifests as latency variation (jitter) which can also degrade
   Quality of Experience for applications that desire NQB treatment.
   The third reason is that a downstream bottleneck that implements the
   NQB PHB could have implemented a traffic protection mechanism
   (Section 5.2) that responds to queuing delays by re-
   marking/reclassifying/dropping packets, and bursty arrivals caused by
   an upstream link could introduce queuing delays in the NQB queue and
   thus be more likely to be subjected to traffic protection effects.

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

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
   classifier to distinguish NQB traffic from traffic classified as
   Default (DSCP 0) is recommended to be the assigned NQB DSCP (45
   decimal).

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   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.
   However, the NQB DSCP SHOULD NOT be re-marked to the Default DSCP
   (0).

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.  Further discussion about
   support of the NQB PHB in DOCSIS networks can be found in
   [LOW_LATENCY_DOCSIS].

7.2.  Mobile Networks

   Historically, 3GPP mobile networks have utilized "bearers" to
   encapsulate each user's user plane traffic through the radio 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 can
   become a bottleneck link, the performance of traffic marked with the
   NQB DSCP across such links could have a significant impact on the
   viability and adoption of the NQB DSCP and PHB.  Depending on the
   DSCP used to mark NQB traffic, existing Wi-Fi equipment that uses the
   default mapping of DSCPs to Access Categories and the default EDCA
   parameters will support either the NQB PHB requirement for separate
   queuing of NQB traffic from Default, or the recommendation to treat
   NQB traffic with forwarding preference 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 forwarding
   preference in existing Wi-Fi networks was motivated by the following:

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

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

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

   *  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.

   *  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.

   *  For application traffic that originates outside of the Wi-Fi
      network, and thus is transmitted by the Access Point, the choice
      of DSCP 45 does create a potential for abuse by non-compliant
      applications.  But, opportunities exist in the network components

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      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.  Furthermore, it is
      a common practice for residential ISPs to re-mark the Diffserv
      field to zero on all traffic destined to their customers'
      networks, and any change to this practice done to enable the NQB
      DSCP to pass through could be done alongside the implementation of
      the recommendations in Section 4.4.1.

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

   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.  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

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   *  Value (Binary): 101101

   *  Value (Decimal): 45

   *  Reference: this document

9.  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.

10.  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

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   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, whether or not 45 is
   assigned to the NQB DSCP.  While the NQB DSCP value could also be
   abused to gain priority on such links, the potential presence of
   traffic protection functions in other hops along the path (which
   likely act on the NQB DSCP value alone) would make it less attractive
   for such abuse than any of the other 31 DSCP values that are given
   priority.

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

   NQB uses the 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.
   Nonetheless, an on-path attacker can also alter other mutable fields
   in the IP header (e.g. the TTL), which can wreak much more havoc than
   just altering QoS treatment.

11.  References

11.1.  Normative References

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

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

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

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

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

11.2.  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.

   [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-07, 23
              November 2023, <https://datatracker.ietf.org/doc/html/
              draft-briscoe-docsis-q-protection-07>.

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

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

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   [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>.

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

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

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

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

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

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

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

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

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

   [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>.

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

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

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

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

   [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.  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, it would be

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   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 [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 classify CS5-marked traffic into the NQB queue (as
   discussed in Section 4.3).

Appendix B.  Comparison with 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

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   guidelines in Section 1.5.1 of [RFC4594].  [RFC3246] goes on to
   formalize the definition of EF by requiring that an EF node be
   characterizable in terms of the fidelity with which it is able to
   provide a guaranteed rate.

   While the NQB PHB is not required to be configured with a guaranteed
   minimum rate, [RFC2474] and [RFC4594] recommend assigning some
   minimum resources for the Default PHB, in particular some dedicated
   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 could
   effectively receive a rate guarantee of (e.g.) 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 at the domain
   border, this is only one option for NQB.  NQB primarily recommends
   traffic protection located at each potential bottleneck, where actual
   queuing can be detected and where excess traffic can be reclassified
   into the Default PHB rather than dropping it.  Local traffic
   protection is more feasible for NQB, given the focus is on access
   networks, where one node is typically designed to be the known
   bottleneck where traffic control functions all reside.  In contrast,
   EF is presumed to follow the Diffserv architecture [RFC2475] for core
   networks, where traffic conditioning is delegated to border nodes, in
   order to simplify high capacity interior nodes.  Further, NQB
   recommends a microflow-based mechanism to limit the performance
   impact of excess traffic to those microflows causing potential
   congestion of the NQB queue, whereas EF ignores microflow properties.
   Note that under congestion, low loss for NQB conformant flows is only
   ensured if such a mechanism is operational.  Note also that this
   mechanism for NQB operates at the available forwarding rate for the
   PHB (which could vary based on other traffic load) as opposed to a
   configured guaranteed rate, as in EF.

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

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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.  Impact on Higher Layer Protocols

   The NQB PHB itself has no impact on higher layer protocols, because
   it only isolates NQB traffic from non-NQB.  However, traffic
   protection of the PHB can have unintended side-effects on higher
   layer protocols.  Traffic protection introduces the possibility that
   microflows classified into the NQB queue could experience out-of-
   order delivery or packet loss if their behavior is not consistent
   with the NQB sender requirements.  Out-of-order delivery could be
   particularly likely if the traffic protection algorithm makes
   decisions on a packet-by-packet basis.  In this scenario, a microflow
   that is (mis)marked as NQB and that causes a queue to form in this
   bottleneck link could see some of its packets forwarded by the NQB
   queue, and some of them either discarded or redirected to the QB
   queue.  In the case of redirection, depending on the queuing 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.

Appendix D.  Alternative Diffserv Code Points

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

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   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.

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.

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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