Transport Area Working Group                                    G. White
Internet-Draft                                                 CableLabs
Intended status: Standards Track                              T. Fossati
Expires: August 26, 2021                                             ARM
                                                       February 22, 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 low latency and, when
   possible, low loss for application-limited traffic flows that would
   ordinarily share a queue with capacity-seeking 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 and mobile network radio and core segments are
   discussed.  This document defines a standard Differentiated Services
   Code Point (DSCP) to identify Non-Queue-Building flows.

Status of This Memo

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   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 August 26, 2021.

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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
   ( in effect on the date of
   publication of this document.  Please review these documents
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   3
   3.  Overview: Non-Queue-Building Flows  . . . . . . . . . . . . .   3
   4.  The NQB PHB and its Relationship to the DiffServ Architecture   4
   5.  DSCP Marking of NQB Traffic . . . . . . . . . . . . . . . . .   5
     5.1.  End-to-end usage and DSCP Re-marking  . . . . . . . . . .   6
     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  . . . . . . . . . . . . . .  10
   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
   12. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  13
   13. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
   14. Security Considerations . . . . . . . . . . . . . . . . . . .  14
   15. Informative References  . . . . . . . . . . . . . . . . . . .  14
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  16

1.  Introduction

   This document defines a Differentiated Services (DS) per-hop behavior
   (PHB) called "Non-Queue-Building Per-Hop Behavior" (NQB PHB), which
   is intended to enable networks to provide low latency and low loss
   for traffic flows that are relatively low data rate and that do not
   themselves materially contribute to queueing delay and loss.  Such
   Non-Queue-Building flows (for example: interactive voice and video,
   gaming, machine to machine applications) are application limited

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   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, in fact, 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

   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

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.  Overview: Non-Queue-Building Flows

   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 cause queues

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   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.  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 capacity-seeking 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 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
   - in many cases these applications only send a few packets per RTT.
   In contrast, Queue-building (QB) flows include traffic which uses TCP
   or QUIC, with Cubic, Reno or other TCP congestion controllers.

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
   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 hard ones (e.g. applications will
   degrade in some manner as loss/latency/jitter increase), so the
   importance of meeting them for any particular application 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.

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   Further, in many cases the implementation of DiffServ PHBs involves
   prioritization of service classes with respect to one another, which
   results in the need to limit access to higher priority classes via
   mechanisms such as access control, admission control, traffic
   conditioning and rate policing, and/or to meter and bill for carriage
   of such traffic.  These mechanisms can be difficult or impossible to
   implement in an end-to-end context.

   Finally, some jurisdictions impose regulations that limit the ability
   of networks to provide differentiation of services, in large part
   based on the understanding that doing so ordinarily 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 equal priority
   as 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, nor does it aim to provide a differentiated service
   class as defined in [RFC4594].  Instead the goal of the NQB PHB is to
   provide statistically better loss, latency, and jitter performance
   for traffic that is not itself the cause of those degradations.
   These attributes eliminate the inherent value judgments 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 42 so that their packets can be queued
   separately from QB flows.  The choice of the value 42 is motivated in
   part by the desire to achieve separate queuing in existing WiFi
   networks (see Section 11.3).

   There are many application flows that fall very neatly into one or
   the other of these two categories: NQB and QB, but there are also
   application flows that may be in a gray area in between (e.g. they
   are NQB on higher-speed links, but QB on lower-speed links).  If
   there is uncertainty as to whether an application's traffic aligns
   with the description of NQB behavior in the preceding section, the
   application SHOULD NOT mark its traffic with the NQB DSCP.  In such a

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   case, the application SHOULD instead implement a congestion control
   mechanism, for example as described in [RFC8085] or

   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 no benefit to an application in intentionally
   mismarking its traffic.  Thus, a useful property of nodes that
   support separate queues for NQB and QB flows would be that for NQB
   flows, the NQB queue provides better performance than the QB queue;
   and for QB flows, the QB queue provides better performance than the
   NQB queue.  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.  Shallow-buffered backbone and core network
   switches, 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 traffic.

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

   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.  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 42) to DSCP 2 prior to interconnection, unless agreed otherwise
   between the interconnecting partners.  The fact that this DSCP is
   intended for end-to-end usage does not preclude networks from mapping
   the NQB DSCP to a value other than 42 or 2 for internal usage, as

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   long as the appropriate NQB DSCP is restored when forwarding to
   another network.  Additionally, it is not precluded for
   interconnecting networks to negotiate (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 (as discussed
   in Section 11.3) networks SHOULD remap NQB traffic (e.g.  DSCP 2) to
   DSCP 42 prior to a customer access link.

   Reports on existing deployments of DSCP manipulation [Custura][Barik]
   categorize the remarking 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 value of 42, there were no observations of DSCP
   manipulation reported in which traffic was marked 42 by any of these
   policies.  Thus it appears that these remarking policies would be
   unlikely to result in QB traffic being marked as NQB (42).  In terms
   of the fate of NQB-marked traffic that is subjected to one of these
   policies, the result would be that NQB marked traffic would be
   indistinguishable from some subset (possibly all) of other traffic.
   In the policies where all traffic is 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 entirely.
   In the policies where the top three bits are overwritten, NQB would
   receive the same marking as AF41, AF31, AF21, AF11 (as well as the
   currently unassigned DSCPs 50 and 58), with all of these code points
   getting mapped to DSCP=2, AF11 or AF21 (depending on the overwrite
   value used).  Since the recommended usage of the standardized code
   points in that list include high throughput data for store and
   forward applications (and it is impossible to predict what future use
   would be assigned to the currently unassigned values) it would seem
   inadvisable for a node to attempt to treat all such traffic as if it
   were NQB marked.  For the policy in which the low three bits are set
   to 0b000, the NQB value would be mapped to CS5 and would be
   indistinguishable from CS5, VA, EF (and the unassigned DSCPs 41, 43,
   45).  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).  Furthermore, as this remarking policy results
   in an overt enforcement of the IP Precedence compatibility
   configuration discussed in [RFC4594] Section 1.5.4, and to the extent
   that this compatibility is maintained in the future, any future
   recommended usages of the currently unassigned DSCPs in that list
   would be likely to similarly be somewhat compatible with NQB

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   treatment.  Here there may be an opportunity for a node to provide
   the NQB PHB or the CS5 PHB and retain some of the benefits of NQB
   marking.  As a result, nodes supporting the NQB PHB MAY additionally
   classify CS5 marked traffic into the NQB queue.

   Regarding the DSCP value of 2, some of these remappings are more
   problematic.  In particular, in policies where the top three bits are
   overwritten with 0b000, the DSCP values AF41, AF31, AF21, and AF11
   (as well as the currently unassigned DSCPs 50 and 58) would all get
   mapped to the NQB value.

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 with standard,
   elastic, best-effort traffic, although in some cases a network
   operator may instead choose to aggregate NQB traffic with Real-Time
   traffic.  Either approach comes with trade-offs: aggregating with
   best-effort could result in a degradation of loss/latency/jitter
   performance, while aggregating with Real-Time may create an incentive
   for mismarking of non-compliant traffic.  In either case, [RFC5127]
   requires that such aggregations preserve the notion of each end-to-
   end service class that is aggregated, and recommends preservation of
   the DSCP as a way of accomplishing this.  Compliance with this
   recommendation would serve to limit the negative impact that such
   networks would have on end-to-end performance for NQB traffic.

   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, Signaling,
   Real-Time Interactive 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 the queue used for queue-building

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   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 equal priority 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.

   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 having equivalent importance.

   The NQB queue SHOULD have a buffer size that is significantly smaller
   than the buffer provided for QB traffic.  It is expected that most QB
   traffic is optimized to make use of 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 reclassify those flows/packets
   to the QB queue.  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.  Similarly, flow queueing
   systems obviate the need for an explicit 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.

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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.  This is particularly true if the traffic
   protection algorithm makes decisions on a packet-by-packet basis.  In
   this scenario, a flow that is (mis)marked as NQB and that causes a
   queue to form in this bottleneck link could see some of its packets
   forwarded by the NQB queue, and some of them redirected to the QB
   queue.  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 out of order delivery will be

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 in order to avoid these
   undesirable interactions.

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 dual-queue node
   [I-D.ietf-tsvwg-aqm-dualq-coupled].  Compliance with the DualQ
   coupled AQM requirements 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 shaping 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 be routed through the default EPS bearer.

11.3.  WiFi Networks

   WiFi networking equipment compliant with 802.11e 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.

   While some WiFi equipment may be capable (in some cases via firmware
   update) of supporting the NQB PHB requirements by providing a
   separate queue for NQB marked traffic that shares an Access Category
   with default traffic, many currently deployed devices cannot be
   configured in this way.

   Implementations typically utilize the IP DSCP field to select a
   transmit queue, but should be considered as Non-Differentiated
   Services-Compliant Nodes as described in Section 4 of [RFC2475]
   because in widely deployed WiFi networks, this transmit queue
   selection is a local implementation characteristic that is not part
   of a consistently operated DiffServ domain or region.  As a result
   this document discusses interoperability with these existing WiFi
   networks, in addition to PHB compliance.

   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.  In
   order to increase the likelihood that NQB traffic is provided a
   separate queue from QB traffic in existing WiFi equipment, the 42
   code point is preferred for NQB.  This would map NQB to UP_5 which is
   in the "Video" Access Category.  Similarly, systems that utilize
   [RFC8325], SHOULD map the NQB code point to UP_5 in the "Video"
   Access Category.

   While the DSCP to User Priority mapping can enable WiFi systems to
   support the NQB PHB requirement for segregated queuing, many
   currently deployed WiFi systems may not be capable of supporting the
   remaining NQB PHB requirements in Section 6.  This is 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.

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   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
   equal prioritization, and could erode the principle of alignment of
   incentives.  In order to preserve the incentives principle, 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 starvation of traffic marked
   Default, for example by limiting the rate of such traffic to a set
   fraction of the customer's service rate.

   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-

12.  Acknowledgements

   Thanks to 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

13.  IANA Considerations

   This document requests that IANA assign the Differentiated Services
   Field Codepoints (DSCP) 2 ('0b000010', 0x02) and 42 ('0b101010',
   0x2A) from the "Differentiated Services Field Codepoints (DSCP)"
   registry ( ("DSCP

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   Pool 1 Codepoints", Codepoint Space xxxxx0, Standards Action) to
   denote Non-Queue-Building behavior.

14.  Security Considerations

   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 could experience excessive packet loss (in the
   case that traffic protection is not supported by a node) or it could
   receive no benefit (in the case that traffic protection is
   supported).  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

   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.  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", draft-briscoe-docsis-q-protection-00 (work
              in progress), July 2019.

              Schepper, K., Briscoe, B., and G. White, "DualQ Coupled
              AQMs for Low Latency, Low Loss and Scalable Throughput
              (L4S)", draft-ietf-tsvwg-aqm-dualq-coupled-13 (work in
              progress), November 2020.

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

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

   [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,

   [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,

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

   [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, <>.

   [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,

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   [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, <>.

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

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

   [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,

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

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

Authors' Addresses

   Greg White


   Thomas Fossati


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