Transport Area Working Group                                    G. White
Internet-Draft                                                 CableLabs
Intended status: Standards Track                              T. Fossati
Expires: 13 January 2022                                             ARM
                                                            12 July 2021

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


   This document specifies properties and characteristics of a Non-
   Queue-Building Per-Hop Behavior (NQB PHB).  The purpose of this NQB
   PHB is to provide a separate queue that enables smooth, low-data-
   rate, application-limited traffic flows, which would ordinarily share
   a queue with bursty and capacity-seeking traffic, to avoid the
   latency, latency variation and loss caused by such traffic.  This PHB
   is implemented without prioritization and without rate policing,
   making it suitable for environments where the use of either these
   features may be restricted.  The NQB PHB has been developed primarily
   for use by access network segments, where queuing delays and queuing
   loss caused by Queue-Building protocols are manifested, but its use
   is not limited to such segments.  In particular, applications to
   cable broadband links, Wi-Fi links, and mobile network radio and core
   segments are discussed.  This document recommends a specific
   Differentiated Services Code Point (DSCP) to identify Non-Queue-
   Building flows.

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on 13 January 2022.

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Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
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   extracted from this document must include Simplified BSD License text
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   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   4
   3.  Non-Queue-Building Behavior . . . . . . . . . . . . . . . . .   4
   4.  The NQB PHB and its Relationship to the Diffserv
           Architecture  . . . . . . . . . . . . . . . . . . . . . .   5
   5.  DSCP Marking of NQB Traffic . . . . . . . . . . . . . . . . .   6
     5.1.  End-to-end usage and DSCP Re-marking  . . . . . . . . . .   7
     5.2.  Aggregation of the NQB PHB with other Diffserv PHBs . . .   8
   6.  Non-Queue-Building PHB Requirements . . . . . . . . . . . . .   8
   7.  Impact on Higher Layer Protocols  . . . . . . . . . . . . . .   9
   8.  The NQB PHB and Tunnels . . . . . . . . . . . . . . . . . . .  10
   9.  Relationship to L4S . . . . . . . . . . . . . . . . . . . . .  10
   10. Configuration and Management  . . . . . . . . . . . . . . . .  11
   11. Example Use Cases . . . . . . . . . . . . . . . . . . . . . .  11
     11.1.  DOCSIS Access Networks . . . . . . . . . . . . . . . . .  11
     11.2.  Mobile Networks  . . . . . . . . . . . . . . . . . . . .  11
     11.3.  WiFi Networks  . . . . . . . . . . . . . . . . . . . . .  12
       11.3.1.  Interoperability with Existing WiFi Networks . . . .  12
   12. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  13
   13. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
   14. Security Considerations . . . . . . . . . . . . . . . . . . .  14
   15. References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     15.1.  Normative References . . . . . . . . . . . . . . . . . .  14
     15.2.  Informative References . . . . . . . . . . . . . . . . .  15
   Appendix A.  DSCP Remarking Pathologies . . . . . . . . . . . . .  17
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  18

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1.  Introduction

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

   The vast majority of packets that are carried by broadband access
   networks are managed by an end-to-end congestion control algorithm,
   such as Reno, Cubic or BBR.  These congestion control algorithms
   attempt to seek the available capacity of the end-to-end path (which
   can frequently be the access network link capacity), and in doing so
   generally overshoot the available capacity, causing a queue to build-
   up at the bottleneck link.  This queue build up results in queuing
   delay (variable latency) and possibly packet loss that affects all of
   the applications that are sharing the bottleneck link.

   In contrast to traditional congestion-controlled applications, there
   are a variety of relatively low data rate applications that do not
   materially contribute to queueing delay and loss, but are nonetheless
   subjected to it by sharing the same bottleneck link in the access
   network.  Many of these applications may 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,
   particularly when only a few of such flows are present.

   The NQB PHB supports differentiating between these two classes of
   traffic in bottleneck links and queuing them separately in order that
   both classes can deliver satisfactory quality of experience for their

   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.

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2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  Non-Queue-Building Behavior

   There are many applications that send traffic at relatively low data
   rates and/or in a fairly smooth and consistent manner such that they
   are highly unlikely to exceed the available capacity of the network
   path between source and sink.  These applications do not on their own
   cause queues to form in network buffers, but nonetheless can be
   subjected to packet delay and delay variation as a result of sharing
   a network buffer with applications that do cause queues to form.
   Many of these applications are negatively affected by excessive
   packet delay and delay variation.  Such applications are ideal
   candidates to be queued separately from the applications that are the
   cause of queue buildup, latency and loss.

   These Non-queue-building (NQB) flows are typically UDP flows that
   don't seek the maximum capacity of the link (examples: online games,
   voice chat, DNS lookups, real-time IoT analytics data).  Here the
   data rate is limited by the application itself rather than by network
   capacity - these applications send, at most, the equivalent of a few
   well-spaced packets per RTT, even if the packets are not actually
   RTT-clocked.  In today's network this corresponds to an instantaneous
   data rate (packet size divided by packet inter-arrival time) of no
   more than about 1 Mbps (e.g. no more than one 1250 B packet every 10
   ms), but there is no precise bound since it depends on the conditions
   in which the application is operating.

   Note that, while such flows ordinarily don't implement a traditional
   congestion control mechanism, they nonetheless are expected to comply
   with existing guidance for safe deployment on the Internet, for
   example the requirements in [RFC8085] and Section 2 of [RFC3551]
   (also see the circuit breaker limits in Section 4.3 of [RFC8083] and
   the description of inelastic pseudowires in Section 4 of [RFC7893]).
   To be clear, the description of NQB flows in this document should not
   be interpreted as suggesting that such flows are in any way exempt
   from this responsibility.

   In contrast, Queue-building (QB) flows include traffic which uses TCP
   or QUIC, with Cubic, Reno or other TCP congestion control algorithms
   that probe for the link capacity and induce latency and loss as a
   result.  Other types of QB flows include those that frequently send

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   at a high burst rate (e.g. several consecutive packets sent well in
   excess of 1 Mbps) even if the long-term average data rate is much

4.  The NQB PHB and its Relationship to the Diffserv Architecture

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

   While this architecture is powerful, and can be configured to meet
   the performance requirements of a variety of applications and traffic
   categories, or to achieve differentiated service offerings, it has
   proven problematic to enable its use for these purposes end-to-end
   across the Internet.

   This difficulty is in part due to the fact that meeting (in an end-
   to-end context) the performance requirements of an application
   involves all of 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 due to the fact that the
   performance "requirements" are not strict ones (e.g. applications
   will degrade in some manner as loss/latency/jitter increase), so the
   importance of meeting them for any particular application in some
   cases involves a judgment as to the value of avoiding some amount of
   degradation in quality for that application in exchange for an
   increase in the degradation of another application.

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

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

   In contrast, the NQB PHB has been designed with the goal that it
   avoids many of these issues, and thus could conceivably be deployed
   end-to-end across the Internet.  The intent of the NQB DSCP is that
   it signals verifiable behavior as opposed to wants and needs.  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.  These attributes eliminate many of the tradeoffs that
   underlie the handling of differentiated service classes in the
   Diffserv architecture as it has traditionally been defined.  They
   also significantly simplify access control and admission control
   functions, reducing them to simple verification of behavior.

5.  DSCP Marking of NQB Traffic

   Applications that align with the description of NQB behavior in the
   preceding section SHOULD identify themselves to the network using a
   Diffserv Code Point (DSCP) of 45 (decimal) so that their packets can
   be queued separately from QB flows.  If the application's traffic
   exceeds more than a few packets per RTT, or exceeds approximately 1
   Mbps on an instantaneous (inter-packet) basis, the application SHOULD
   NOT mark its traffic with the NQB DSCP.  In such a case, the
   application SHOULD instead implement a congestion control mechanism,
   for example as described in Section 3.1 of [RFC8085] or

   The choice of the value 45 is motivated in part by the desire to
   achieve separate queuing in existing WiFi networks (see
   Section 11.3).

   It is worthwhile to note again that the NQB designation and marking
   is intended to convey verifiable traffic behavior, not needs or
   wants.  Also, it is important that incentives are aligned correctly,
   i.e. that there is a benefit to the application in marking its
   packets correctly, and a disadvantage (or at least no benefit) to an
   application in intentionally mismarking its traffic.  Thus, a useful
   property of nodes (i.e. network switches and routers) that support
   separate queues for NQB and QB flows would be that for NQB flows, the

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   NQB queue provides better performance than the QB queue; and for QB
   flows, the QB queue provides better performance than the NQB queue
   (this is discussed further in Section 6 and Section 14).  By adhering
   to these principles, there is no incentive for senders to mismark
   their traffic as NQB, and further, any mismarking can be identified
   by the network.

   Along these lines, nodes that do not support the NQB PHB SHOULD treat
   NQB marked traffic the same as traffic marked "Default", and SHOULD
   preserve the NQB marking.  In backbone and core network switches
   (particularly if shallow-buffered), and nodes that do not typically
   experience congestion, treating NQB marked traffic the same as
   "Default" may be sufficient to preserve latency performance for NQB

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

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

   Absent an explicit agreement to the contrary, networks that support
   the NQB PHB SHOULD preserve a DSCP marking distinction between NQB
   traffic and Default traffic when forwarding via an interconnect from
   or to another network.  To facilitate the default treatment of NQB
   traffic in backbones and core networks, networks SHOULD remap NQB
   traffic (DSCP 45) to DSCP 5 prior to interconnection, unless agreed
   otherwise between the interconnecting partners.  The fact that this
   PHB is intended for end-to-end usage does not preclude networks from
   mapping the NQB DSCP to a value other than 45 or 5 for internal
   usage, as long as the appropriate NQB DSCP is restored when
   forwarding to another network.  Additionally, interconnecting
   networks are not precluded from negotiating (via an SLA or some other
   agreement) a different DSCP to use to signal NQB across the

   In order to enable interoperability with WiFi equipment, networks
   SHOULD remap NQB traffic (e.g.  DSCP 5) to DSCP 45 prior to a
   customer access link, subject to the safeguards described in
   Section 11.3.

   Thus, this document recommends two DSCPs to designate NQB, the value
   45 for use by hosts and in WiFi networks, and the value 5 for use
   across network interconnections.

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5.2.  Aggregation of the NQB PHB with other Diffserv PHBs

   Networks and nodes that aggregate service classes as discussed in
   [RFC5127] and [RFC8100] may not be able to provide a PDB/PHB that
   meets the requirements of this document.  In these cases it is
   recommended that NQB-marked traffic be aggregated into the Elastic
   Treatment Aggregate (for [RFC5127] networks) or the Default / Elastic
   Treatment Aggregate (for [RFC8100] networks), although in some cases
   a network operator may instead choose to aggregate NQB traffic into
   the (Bulk) Real-Time Treatment Aggregate.  Either approach comes with
   trade-offs: aggregating with Default/Elastic traffic could result in
   a degradation of loss/latency/jitter performance for NQB traffic,
   while aggregating with Real-Time risks creating an incentive for
   mismarking of non-compliant traffic as NQB.  In either case, the NQB
   DSCP SHOULD be preserved in order to limit the negative impact that
   such networks would have on end-to-end performance for NQB traffic.
   This aligns with recommendations in [RFC5127].

   Nodes that support the NQB PHB may choose to aggregate other service
   classes into the NQB queue.  Candidate service classes for this
   aggregation would include those that carry inelastic traffic that has
   low to very-low tolerance for loss, latency and/or jitter as
   discussed in [RFC4594].  These could include Telephony (EF/VA),
   Signaling (CS5), Real-Time Interactive (CS4) and Broadcast Video

6.  Non-Queue-Building PHB Requirements

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

   A node supporting the NQB PHB MUST provide a queue for non-queue-
   building traffic separate from any queue used for queue-building

   NQB traffic, in aggregate, SHOULD NOT be rate limited or rate policed
   separately from queue-building traffic of equivalent importance.

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

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

   The NQB queue SHOULD have a buffer size that is significantly smaller
   than the buffer provided for QB traffic (e.g. single-digit
   milliseconds).  It is expected that most QB traffic is engineered to
   work well when the network provides a relatively deep buffer (e.g. on
   the order of tens or hundreds of ms) in nodes where support for the
   NQB PHB is advantageous (i.e. bottleneck nodes).  Providing a
   similarly deep buffer for the NQB queue would be at cross purposes to
   providing very low queueing delay, and would erode the incentives for
   QB traffic to be marked correctly.

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

   To prevent this situation from harming the performance of the real
   NQB flows, network elements that support differentiating NQB traffic
   SHOULD support a "traffic protection" function that can identify QB
   flows that are mismarked as NQB, and either reclassify those flows/
   packets to the QB queue or discard the offending traffic.  Such a
   function SHOULD be implemented in an objective and verifiable manner,
   basing its decisions upon the behavior of the flow rather than on
   application-layer constructs.  One example algorithm can be found in
   [I-D.briscoe-docsis-q-protection].  There are some situations where
   such function may not be necessary.  For example, a network element
   designed for use in controlled environments (e.g. enterprise LAN) may
   not require a traffic protection function.  Additionally, some
   networks may prefer to police the application of the NQB DSCP at the
   ingress edge, so that in-network traffic protection is not needed.

7.  Impact on Higher Layer Protocols

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

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   queue to form in this bottleneck link could see some of its packets
   forwarded by the NQB queue, and some of them either discarded or
   redirected to the QB queue.  In the case of redirection, depending on
   the queueing latency and scheduling within the network element, this
   could result in packets being delivered out of order.  As a result,
   the use of the NQB DSCP by a higher layer protocol carries some risk
   that an increased amount of out of order delivery or packet loss will
   be experienced.  This characteristic provides one disincentive for
   mis-marking of traffic.

8.  The NQB PHB and Tunnels

   [RFC2983] discusses tunnel models that support Diffserv.  It
   describes a "uniform model" in which the inner DSCP is copied to the
   outer header at encapsulation, and the outer DSCP is copied to the
   inner header at decapsulation.  It also describes a "pipe model" in
   which the outer DSCP is not copied to the inner header at
   decapsulation.  Both models can be used in conjunction with the NQB
   PHB.  In the case of the pipe model, any DSCP manipulation (re-
   marking) of the outer header by intermediate nodes would be discarded
   at tunnel egress, potentially improving the possibility of achieving
   NQB treatment in subsequent nodes.

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

9.  Relationship to L4S

   Traffic flows marked with the NQB DSCP as described in this draft are
   intended to be compatible with [I-D.ietf-tsvwg-l4s-arch], with the
   result being that NQB traffic and L4S traffic can share the low-
   latency queue in an L4S DualQ node
   [I-D.ietf-tsvwg-aqm-dualq-coupled].  Compliance with the DualQ
   Coupled AQM requirements (Section 2.5 of
   [I-D.ietf-tsvwg-aqm-dualq-coupled]) is considered sufficient to
   enable fair allocation of bandwidth between the QB and NQB queues.

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10.  Configuration and Management

   As required above, nodes supporting the NQB PHB provide for the
   configuration of classifiers that can be used to differentiate
   between QB and NQB traffic of equivalent importance.  The default for
   such classifiers is recommended to be the assigned NQB DSCP (to
   identify NQB traffic) and the Default (0) DSCP (to identify QB

11.  Example Use Cases

11.1.  DOCSIS Access Networks

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

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

11.2.  Mobile Networks

   Historically, mobile networks have been configured to bundle all
   flows to and from the Internet into a single "default" EPS bearer
   whose buffering characteristics are not compatible with low-latency
   traffic.  The established behaviour is rooted partly in the desire to
   prioritise operators' voice services over competing over-the-top
   services and partly in the fact that the addition of bearers was
   prohibitive due to expense.  Of late, said consideration seems to
   have lost momentum (e.g., with the rise in Multi-RAB (Radio Access
   Bearer) devices) and the incentives might now be aligned towards
   allowing a more suitable treatment of Internet real-time flows.

   To support the NQB PHB, the mobile network SHOULD be configured to
   give UEs 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]).

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   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.

11.3.  WiFi Networks

   WiFi networking equipment compliant with 802.11e/n/ac/ax [IEEE802-11]
   generally supports either four or eight transmit queues and four sets
   of associated Enhanced Multimedia Distributed Control Access (EDCA)
   parameters (corresponding to the four WiFi Multimedia (WMM) Access
   Categories) that are used to enable differentiated media access
   characteristics.  As discussed in [RFC8325], most existing WiFi
   implementations 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 addition to the requirements provided in other sections of this
   document, to support the NQB PHB, WiFi equipment SHOULD map the NQB
   codepoint 45 into a separate queue that shares an Access Category
   with default traffic (i.e. the Best Effort Access Category).

11.3.1.  Interoperability with Existing WiFi Networks

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

   In order to increase the likelihood that NQB traffic is provided a
   separate queue from QB traffic in existing WiFi equipment that uses
   the default mapping, the 45 code point is recommended for NQB.  This
   maps NQB to UP_5 which is in the "Video" Access Category.  While this
   DSCP to User Priority mapping enables these WiFi systems to support
   the NQB PHB requirement for segregated queuing, it does not support
   the remaining NQB PHB requirements in Section 6.  The ramifications
   of, and remedies for this are discussed further below.

   Existing WiFi devices are unlikely to support a traffic protection
   algorithm, so traffic mismarked as NQB is not likely to be detected
   and remedied by such devices.

   Furthermore, in their default configuration, existing WiFi devices
   utilize EDCA parameters that result in statistical prioritization of
   the "Video" Access Category above the "Best Effort" Access Category.
   If left unchanged, this would violate the NQB PHB requirement for

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   equal prioritization, and could erode the principle of alignment of
   incentives.  In order to preserve the incentives principle for NQB,
   WiFi systems SHOULD configure the EDCA parameters for the Video
   Access Category to match those of the Best Effort Access Category.

   In cases where a network operator is delivering traffic into an
   unmanaged WiFi network outside of their control (e.g. a residential
   ISP delivering traffic to a customer's home network), the network
   operator should presume that the existing WiFi equipment does not
   support the safeguards that are provided by the NQB PHB requirements,
   and thus should take precautions to prevent issues.  When the data
   rate of the access network segment is less than the expected data
   rate of the WiFi network, this is unlikely to be an issue.  However,
   if the access network rate exceeds the expected rate of the WiFi
   network, the operator SHOULD deploy a policing function on NQB marked
   traffic that minimizes the potential for negative impacts on traffic
   marked Default, for example by limiting the rate of such traffic to a
   set fraction of the customer's service rate, with excess traffic
   either dropped or re-marked as Default.

   As an additional safeguard, and to prevent the inadvertent
   introduction of problematic traffic into unmanaged WiFi networks,
   network equipment that is intended to deliver traffic into unmanaged
   WiFi networks (e.g. an access network gateway for a residential ISP)
   MUST by default ensure that NQB traffic is marked with a DSCP that
   selects the "Best Effort" Access Category.  Such equipment MUST
   support the ability to configure the remapping, so that (when
   appropriate safeguards are in place) traffic can be delivered as NQB-

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

12.  Acknowledgements

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

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13.  IANA Considerations

   This document requests that IANA assign the Differentiated Services
   Field Codepoints (DSCP) 5 ('0b000101', 0x05) and 45 ('0b101101',
   0x2D) from the "Differentiated Services Field Codepoints (DSCP)"
   registry ( ("DSCP
   Pool 3 Codepoints", Codepoint Space xxxx01, Standards Action) as the
   RECOMMENDED codepoints for Non-Queue-Building behavior.

14.  Security Considerations

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

   In order to preserve low latency performance for NQB traffic,
   networks that support the NQB PHB will need to ensure that mechanisms
   are in place to prevent malicious NQB-marked traffic from causing
   excessive queue delays.  This document recommends the implementation
   of a traffic protection mechanism to achieve this goal, but
   recognizes that other options may be more desirable in certain

   Notwithstanding the above, the choice of DSCP for NQB does allow
   existing WiFi networks to readily (and by default) support some of
   the PHB requirements, but without a traffic protection function, and
   (when left in the default state) by giving NQB traffic higher
   priority than QB traffic.  This does open up the NQB marking to
   potential abuse on these WiFi links, but since these existing WiFi
   networks already give one quarter of the DSCP space this same
   treatment, and further they give another quarter of the DSCP space
   even higher priority, the NQB DSCP does not seem to be of any greater
   risk for abuse than these others.

   The NQB signal is not integrity protected and could be flipped by an
   on-path attacker.  This might negatively affect the QoS of the
   tampered flow.

15.  References

15.1.  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,

   [RFC2474]  Nichols, K., Blake, S., Baker, F., and D. Black,
              "Definition of the Differentiated Services Field (DS
              Field) in the IPv4 and IPv6 Headers", RFC 2474,
              DOI 10.17487/RFC2474, December 1998,

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

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

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

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

15.2.  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 ,

              Briscoe, B. and G. White, "Queue Protection to Preserve
              Low Latency", Work in Progress, Internet-Draft, draft-
              briscoe-docsis-q-protection-00, 8 July 2019,

              Schepper, K., Briscoe, B., and G. White, "DualQ Coupled
              AQMs for Low Latency, Low Loss and Scalable Throughput
              (L4S)", Work in Progress, Internet-Draft, draft-ietf-

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              tsvwg-aqm-dualq-coupled-13, 15 November 2020,

              Schepper, K. and B. Briscoe, "Identifying Modified
              Explicit Congestion Notification (ECN) Semantics for
              Ultra-Low Queuing Delay (L4S)", Work in Progress,
              Internet-Draft, draft-ietf-tsvwg-ecn-l4s-id-12, 15
              November 2020, <

              Briscoe, B., Schepper, K., Bagnulo, M., and G. White, "Low
              Latency, Low Loss, Scalable Throughput (L4S) Internet
              Service: Architecture", Work in Progress, Internet-Draft,
              draft-ietf-tsvwg-l4s-arch-08, 15 November 2020,

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

   [RFC2475]  Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
              and W. Weiss, "An Architecture for Differentiated
              Services", RFC 2475, DOI 10.17487/RFC2475, December 1998,

   [RFC3551]  Schulzrinne, H. and S. Casner, "RTP Profile for Audio and
              Video Conferences with Minimal Control", RFC 3551, July
              2003, <>.

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

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

   [RFC7893]  Stein, Y(J)., Black, D., and B. Briscoe, "Pseudowire
              Congestion Considerations", RFC 7893, June 2016,

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   [RFC8033]  Pan, R., Natarajan, P., Baker, F., and G. White,
              "Proportional Integral Controller Enhanced (PIE): A
              Lightweight Control Scheme to Address the Bufferbloat
              Problem", RFC 8033, DOI 10.17487/RFC8033, February 2017,

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

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

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

   [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,

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

Appendix A.  DSCP Remarking Pathologies

   Some network operators typically bleach (zero out) the Diffserv field
   on ingress into their network [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 remark all traffic to a
   particular (non-zero) DSCP value.

   Regarding the DSCP values of 5 & 45, there were no observations of
   DSCP manipulation reported in which traffic was marked 5 or 45 by any
   of these policies.  Thus it appears that these re-marking policies
   would be unlikely to result in QB traffic being marked as NQB (45).
   In terms of the fate of NQB-marked traffic that is subjected to one
   of these policies, the result would be that NQB marked traffic would
   be indistinguishable from some subset (possibly all) of other

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   traffic.  In the policies where all traffic is remarked 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

   In the policies where the top three bits are overwritten, both NQB
   values (5 & 45) would receive the same marking as would the currently
   unassigned Pool 3 DSCPs 13,21,29,37,53,61, with all of these code
   points getting mapped 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 set aside for Standards
   Action, it is believed that they are not widely used currently, but
   this may vary based on local-usage.

   For the policy in which the low three bits are set to 0b000, the NQB
   (45) value would be mapped 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 may 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 5.2) classify CS5-marked
   traffic into NQB queue.  For this same re-marking policy, the NQB (5)
   value would be mapped to CS0/default and would be indistinguishable
   from CS0, LE (and the unassigned DSCPs 2,3,4,6,7).  In this case, NQB
   traffic is likely to be given default treatment in all subsequent
   nodes, which would eliminate the ability to provide NQB treatment in
   those nodes, but would be relatively harmless otherwise.

Authors' Addresses

   Greg White


   Thomas Fossati


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