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A Non-Queue-Building Per-Hop Behavior (NQB PHB) for Differentiated Services
draft-ietf-tsvwg-nqb-18

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Authors Greg White , Thomas Fossati
Last updated 2023-07-10 (Latest revision 2023-03-25)
Replaces draft-white-tsvwg-nqb
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Submit "A Non-Queue-Building Per-Hop Behavior (NQB PHB) for Differentiated Services" as a Proposed Standard RFC
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draft-ietf-tsvwg-nqb-18
Transport Area Working Group                                    G. White
Internet-Draft                                                 CableLabs
Updates: rfc8325 (if approved)                                T. Fossati
Intended status: Standards Track                                     ARM
Expires: 11 January 2024                                    10 July 2023

   A Non-Queue-Building Per-Hop Behavior (NQB PHB) for Differentiated
                                Services
                        draft-ietf-tsvwg-nqb-18

Abstract

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

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

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

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   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 11 January 2024.

Copyright Notice

   Copyright (c) 2023 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   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 Revised BSD License text as
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   4
   3.  Context . . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Non-Queue-Building Behavior . . . . . . . . . . . . . . .   4
     3.2.  Relationship to the Diffserv Architecture . . . . . . . .   5
     3.3.  Relationship to L4S . . . . . . . . . . . . . . . . . . .   7
   4.  DSCP Marking of NQB Traffic . . . . . . . . . . . . . . . . .   7
     4.1.  Non-Queue-Building Sender Requirements  . . . . . . . . .   8
     4.2.  Aggregation of the NQB DSCP with other Diffserv PHBs  . .   9
     4.3.  Aggregation of other DSCPs in the NQB PHB . . . . . . . .  10
     4.4.  End-to-end usage and DSCP Re-marking  . . . . . . . . . .  10
       4.4.1.  Interoperability with Non-DS-Capable Domains  . . . .  11
     4.5.  The NQB DSCP and Tunnels  . . . . . . . . . . . . . . . .  12
   5.  Non-Queue-Building PHB Requirements . . . . . . . . . . . . .  13
     5.1.  Primary Requirements  . . . . . . . . . . . . . . . . . .  13
     5.2.  Traffic Protection  . . . . . . . . . . . . . . . . . . .  14
   6.  Impact on Higher Layer Protocols  . . . . . . . . . . . . . .  16
   7.  Configuration and Management  . . . . . . . . . . . . . . . .  16
     7.1.  Guidance for Lower-Rate Links . . . . . . . . . . . . . .  16
   8.  Example Use Cases . . . . . . . . . . . . . . . . . . . . . .  17
     8.1.  DOCSIS Access Networks  . . . . . . . . . . . . . . . . .  17
     8.2.  Mobile Networks . . . . . . . . . . . . . . . . . . . . .  17
     8.3.  WiFi Networks . . . . . . . . . . . . . . . . . . . . . .  18
       8.3.1.  Interoperability with Existing WiFi Networks  . . . .  18
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  21

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   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  21
   11. Implementation Status . . . . . . . . . . . . . . . . . . . .  21
   12. Security Considerations . . . . . . . . . . . . . . . . . . .  22
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  23
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  23
     13.2.  Informative References . . . . . . . . . . . . . . . . .  23
   Appendix A.  DSCP Re-marking Policies . . . . . . . . . . . . . .  26
   Appendix B.  Comparison to Expedited Forwarding . . . . . . . . .  27
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  28

1.  Introduction

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

   In accordance with IETF guidance in [RFC2914] and [RFC8085], most
   packets carried by broadband access networks are managed by an end-
   to-end congestion control algorithm.  Many of the commonly-deployed
   congestion control algorithms, such as Reno, Cubic or BBR, are
   designed to seek the available capacity of the end-to-end path (which
   can frequently be the access network link capacity), and in doing so
   generally overshoot the available capacity, causing a queue to build
   up at the bottleneck link.  This queue build-up results in delay
   (variable latency) and packet loss that can affect all the
   applications that are sharing the bottleneck link.  Moreover, many
   bottleneck links implement a relatively deep buffer (100 ms or more)
   in order to enable these congestion control algorithms to effectively
   use the link, which exacerbates the latency and latency variation
   experienced.

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

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   Active Queue Management (AQM) mechanisms (such as PIE [RFC8033],
   DOCSIS-PIE [RFC8034], or CoDel [RFC8289]) can improve the quality of
   experience for latency sensitive applications, but there are
   practical limits to the amount of improvement that can be achieved
   without impacting the throughput of capacity-seeking applications.
   For example, AQMs generally allow a significant amount of queue depth
   variation to accommodate the behaviors of congestion control
   algorithms such as Reno and Cubic.  If the AQM attempted to control
   the queue much more tightly, applications using those algorithms
   would not perform well.  Alternatively, flow queueing systems, such
   as fq_codel [RFC8290] can be employed to isolate flows from one
   another, but these are not appropriate for all bottleneck links, due
   to complexity or other reasons.

   The NQB PHB supports differentiating between these two classes of
   traffic in bottleneck links and queuing them separately so that both
   classes can deliver satisfactory quality of experience for their
   applications.  In particular, the NQB PHB provides a shallow-
   buffered, best-effort service as a complement to a Default deep-
   buffered best-effort service.  This PHB is primarily applicable for
   high-speed broadband access network links, where there is minimal
   aggregation of traffic, and deep buffers are common.  The
   applicability of this PHB to lower-speed links is discussed in
   Section 5.

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

2.  Requirements Language

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

3.  Context

3.1.  Non-Queue-Building Behavior

   There are many applications that send traffic at relatively low data
   rates and/or in a fairly smooth and consistent manner such that they
   are highly unlikely to exceed the available capacity of the network
   path between source and sink.  Some of these applications are
   transactional in nature, and might only send one packet (or a few
   packets) per RTT.  These applications might themselves only cause

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   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) flows include those that use TCP or
   QUIC, with Cubic, Reno or other TCP congestion control algorithms
   that probe for the link capacity and induce latency and loss as a
   result.  Other types of QB flows include those that send at a high
   burst rate even if the long-term average data rate is much lower.

3.2.  Relationship to the Diffserv Architecture

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

   While this architecture is powerful and flexible enough to be
   configured to meet the performance requirements of a variety of
   applications and traffic categories, or to achieve differentiated
   service offerings, it has not been used for these purposes end-to-end
   across the Internet.

   This is in part due to the fact that meeting the performance
   requirements of an application in an end-to-end context involves all
   the networks in the path agreeing on what those requirements are and
   sharing an interest in meeting them.  In many cases this is made more
   difficult since the performance "requirements" are not strict ones
   (e.g., applications will degrade in some manner as loss/latency/
   jitter increase), so the importance of meeting them for any
   particular application in some cases involves a judgment as to the
   value of avoiding some amount of degradation in quality for that
   application in exchange for an increase in the degradation of another
   application.

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

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

   In contrast, the NQB PHB has been designed with the goal that it
   avoids many of these issues, and thus could conceivably be deployed
   end-to-end across the Internet.  The intent of the NQB DSCP is that
   it signals verifiable behavior that permits the sender to request
   differentiated treatment.  Also, the NQB traffic is to be given a
   separate queue with priority equal to Default traffic and given no
   reserved bandwidth other than the bandwidth that it shares with
   Default traffic.  As a result, the NQB PHB does not aim to meet
   specific application performance requirements.  Instead, the goal of
   the NQB PHB is to provide statistically better loss, latency, and
   jitter performance for traffic that is itself only an insignificant
   contributor to those degradations.  The PHB is also designed to
   minimize any incentives for a sender to mismark its traffic, since
   neither higher priority nor reserved bandwidth are being offered.
   These attributes eliminate many of the trade-offs that underlie the
   handling of differentiated service classes in the Diffserv
   architecture as it has traditionally been defined.  These attributes
   also significantly simplify access control and admission control
   functions, reducing them to simple verification of behavior.

   The NQB PHB is therefore intended for the prevalent situation where
   the performance requirements of applications cannot be assured end-
   to-end, and as a result, applications cannot feasibly place
   requirements on the network.  Instead, many applications have evolved
   to make the best out of the network environment that they find
   themselves in.  In this context, the NQB PHB provides a better
   network environment for many applications that send data at
   relatively low data rates.

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   In regards to comparison between the NQB PHB and other standardized
   PHBs in the Diffserv series, the closest similarity is to the
   Expedited Forwarding (EF) PHB [RFC3246], which also intends to enable
   low loss, low delay, and low jitter services.  The main distinctions
   between NQB and EF are discussed in Appendix B.

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

3.3.  Relationship to L4S

   The NQB DSCP and PHB described in this draft have been defined to
   operate independently of the experimental L4S Architecture [RFC9330].
   Nonetheless, traffic marked with the NQB DSCP is intended to be
   compatible with [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) the same as packets marked with the
   Default DSCP and ECT(1).  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]).

4.  DSCP Marking of NQB Traffic

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4.1.  Non-Queue-Building Sender Requirements

   Flows that are eligible to be marked with the NQP DSCP are typically
   UDP flows that send traffic at a low data rate relative to typical
   network path capacities.  Here the data rate is limited by the
   application itself rather than by network capacity - these
   applications send at 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 7.1 for a discussion of cases where this isn't
   true), this implies 500 kbps as an upper limit.  In addition, these
   applications are required to send their traffic in a smooth (i.e.
   paced) manner, where the number of bytes sent in any time interval
   "T" is less than or equal to R * T + 1500 bytes, where "R" is the
   maximum rate described above.

   Flows 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 an
   application'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 flows in this document is not
   to be interpreted as suggesting that such flows are in any way exempt
   from this responsibility.  One way that an application marking its
   flows as NQB can handle this is to implement a low latency congestion
   control mechanism as described in [RFC9331].

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

   In networks where another (e.g., a local-use) DSCP is designated for
   NQB traffic, or where specialized PHBs are available that can meet
   specific application requirements (e.g., a guaranteed-latency path
   for voice traffic), it could be preferred to use another DSCP.  In
   end systems where the choice of using DSCP 45 (decimal) is not

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   available to the application, the CS5 DSCP (40 decimal) could be used
   as a fallback.  See Section 4.2 for rationale as to why this choice
   could be fruitful.

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

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

   The sender requirements outlined in this section are all related to
   observable attributes of the packet stream, which makes it possible
   for network elements (including nodes implementing the PHB) to
   monitor for inappropriate usage of the DSCP, and re-mark traffic that
   does not comply.  This functionality, when implemented as part of the
   PHB is described in Section 5.2.

4.2.  Aggregation of the NQB DSCP with other Diffserv PHBs

   It is RECOMMENDED that networks and nodes that do not support the NQB
   PHB be configured to treat traffic marked with the NQB DSCP the same
   as traffic with the "Default" DSCP.  This includes networks and nodes
   that aggregate service classes as discussed in [RFC5127] and
   [RFC8100], in which case this recommendation would result in traffic
   marked with the NQB DSCP being aggregated into the Elastic Treatment
   Aggregate (for [RFC5127] networks) or the Default / Elastic Treatment
   Aggregate (for [RFC8100] networks).

   In nodes that do not typically experience congestion (for example,
   many backbone and core network switches), forwarding packets with the
   NQB DSCP using the Default treatment might be sufficient to preserve
   loss/latency/jitter performance for NQB traffic.

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

   An alternative would be to aggregate traffic marked with the NQB DSCP
   with real-time, latency sensitive traffic (e.g. the (Bulk) Real-Time
   Treatment Aggregate), although this risks creating an incentive for
   mismarking of non-compliant traffic as NQB and so is NOT RECOMMENDED
   in general.

   Networks and nodes that do not support the NQB PHB should simply
   classify packets with the NQB DSCP value into the appropriate
   treatment aggregate, or encapsulate such packets for purposes of
   aggregation, and SHOULD NOT re-mark them with a different DSCP.  This
   preservation of the NQB DSCP value enables hops further along the
   path to provide the NQB PHB successfully.  This aligns with
   recommendations in [RFC5127].

4.3.  Aggregation of other DSCPs in the NQB PHB

   Operators of nodes that support the NQB PHB may choose to aggregate
   other service classes into the NQB queue.  This is particularly
   useful in cases where specialized PHBs for these other service
   classes are not provided.  Candidate service classes for this
   aggregation would include those that carry low-data-rate inelastic
   traffic that has low to very-low tolerance for loss, latency and/or
   jitter.  An operator would need to use their own judgement based on
   the actual traffic characteristics in their network in deciding
   whether or not to aggregate other service classes / DSCPs with NQB.
   For networks that use the [RFC4594] service class definitions, this
   could include Telephony (EF/VA), Signaling (CS5), and possibly Real-
   Time Interactive (CS4) (depending on data rate).  In some networks,
   equipment limitations may necessitate aggregating a range of DSCPs
   (e.g. traffic marked with DSCPs 40-47 (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.

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

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

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   If NQB support is extended across a DiffServ domain boundary, the
   interconnected networks agreeing to support NQB SHOULD use the DSCP
   value 45 (decimal) for NQB at network interconnection, unless a
   different DSCP is explicitly documented in the TCA (Traffic
   Conditioning Agreement, see [RFC2475]) for that interconnection.
   Similar to the handling of DSCPs for other PHBs (and as discussed in
   [RFC2475]), networks can re-mark NQB traffic to a DSCP other than 45
   (decimal) for internal usage.  To ensure reliable end-to-end NQB PHB
   treatment, the appropriate NQB DSCP should 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 WiFi APs as described in Section 8.3.1), it is RECOMMENDED
   that the operator of the upstream domain re-mark NQB traffic to DSCP
   0 (Default) before delivering traffic into a non-DS-capable domain.

   Network equipment that is intended to deliver traffic into networks
   that are expected to be non-DS-compliant (e.g., an access network
   gateway for a residential ISP) SHOULD by default ensure that NQB
   traffic is re-marked with a DSCP that is unlikely to result in
   prioritized treatment in the downstream domain, such as DSCP 0
   (Default).  It is RECOMMENDED that this equipment supports the
   ability to configure the re-marking, so that (when it is appropriate)
   traffic can be delivered as NQB-marked.

   As an alternative to re-marking, 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.  It should be noted
   that a traffic protection function as defined in this document might

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   only provide protection from issues occuring in subsequent network
   hops if the device implementing the traffic protection function is
   the bottleneck link on the path, so it might not be a solution for
   all situations.  In the case that a traffic policing function or a
   rate shaping function is applied to the aggregate of NQB traffic
   destined to such a downstream domain, the policer/shaper rate SHOULD
   be set to either 5% of the interconnection data rate, or 5% of the
   typical rate for such interconnections, whichever is greater, with
   excess traffic being either re-marked and classified for Default
   forwarding or dropped.  A traffic policing function SHOULD allow
   approximately 100 ms of burst tolerance (e.g. a token bucket depth
   equal to 100 ms multiplied by the policer rate).  A traffic shaping
   function SHOULD allow approximately 10 ms of burst tolerance, and no
   more than 50 ms of buffering.  The burst tolerance values recommended
   here are intended to reduce the degradation that could be introduced
   to latency and loss sensitive traffic marked NQB without
   significantly degrading Default traffic.

   The recommendation to limit NQB traffic to 5% is based on an
   assumption that internal links in the downstream domain could have
   data rates as low as one tenth of the interconnect rate, in which
   case if the entire aggregate of NQB traffic traversed a single
   instance of such a link, the aggregate would consume no more than 50%
   of that link's capacity.  This SHOULD be adjusted based on any
   knowledge of the local network environment that is available.

4.5.  The NQB DSCP and Tunnels

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

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

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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 flows is that for flows
   consistent with the NQB sender requirements in Section 4.1, the NQB
   queue would likely be a better choice than the QB queue; and for
   flows inconsistent with those requirements, the QB queue would likely
   be a better choice than the NQB queue (this is discussed further in
   this section and Section 12).  By adhering to these principles, there
   is no incentive for senders to mismark their traffic as NQB.  As
   mentioned previously, the NQB designation and marking is intended to
   convey verifiable traffic behavior, as opposed to simply a desire for
   differentiated treatment.  As a result, any mismarking can be
   identified by the network.

5.1.  Primary Requirements

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

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

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

   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 fair manner, e.g., a
   deficit round-robin scheduler with equal weights.  Compliance with
   these recommendations helps to ensure that there are no incentives
   for QB traffic to be mismarked as NQB.

   A node supporting the NQB PHB SHOULD 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 classify packets into the queue for Non-Queue-Building traffic.

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   The NQB queue SHOULD have a buffer size that is significantly smaller
   than the buffer provided for Default traffic.  It is expected that
   most Default traffic is engineered to work well when the network
   provides a relatively deep buffer (e.g., on the order of tens or
   hundreds of ms) in nodes where support for the NQB PHB is
   advantageous (i.e., bottleneck nodes).  Providing a similarly deep
   buffer for the NQB queue would be at cross purposes to providing very
   low queueing delay and would erode the incentives for QB traffic to
   be marked correctly.  An NQB buffer size less than or equal to 10 ms
   is RECOMMENDED.

   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 8.3.1 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 flow could end up being mismarked as NQB, or
   vice versa.  It is also possible that a malicious actor could
   introduce a QB flow marked as NQB with the intention of causing
   disruptions.  In the case of a low data rate flow that isn't marked
   as NQB and therefore ends up in the QB queue, it would only impact
   its own quality of service, and so it seems to be of lesser concern.
   However, a QB flow that is mismarked as NQB would cause queuing
   delays and/or loss for all the other flows that are sharing the NQB
   queue.

   To prevent this situation from harming the performance of the flows
   that are in compliance with the requirements in Section 4.1, network
   elements that support the NQB PHB SHOULD support a "traffic
   protection" function that can identify flows that are inconsistent
   with the sender requirements in Section 4.1, and either re-mark those
   flows/packets as Default and reclassify them to the QB queue or
   discard the offending traffic.  Such a function SHOULD base its
   decisions upon the behavior of each flow rather than on application-
   layer constructs (such as the port number used by the application or
   the source/destination IP address).

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   This specification does not mandate a particular algorithm for
   traffic protection.  This is intentional, since the specifics of
   traffic protection could need to be different in different network
   equipment and in different network contexts.  Instead this
   specification provides guidelines and some examples of traffic
   protection algorithms which could be employed.

   One potential implementation of such a traffic protection algorithm
   is a per-flow rate policer, designed to identify flows 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 flow.  One example traffic
   protection algorithm along these lines can be found in
   [I-D.briscoe-docsis-q-protection].  This algorithm maintains per-flow
   state for up to 32 simultaneous "queue-building" flows, and shared
   state for any additional flows in excess of that number.

   In the case of a traffic protection algorithm that re-marks and
   reclassifies offending traffic, different levels of hysteresis could
   be considered.  For example, the re-mark/reclassify decision could be
   made on a packet-by-packet basis, which could result in significant
   out-of-order delivery for offending flows as some portion of the
   flow's packets remain in the NQB queue and some are re-marked and
   reclassified to the Default queue.  Alternatively, a traffic
   protection function could employ a certain level of hysteresis to
   prevent borderline flows 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 flow, though sufficient logic would be needed to ensure that a
   future flow (e.g. with the same 5-tuple) isn't misidentified as the
   current offending flow.

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

   The traffic protection function described here requires that the
   network element maintain some sort of flow 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 flow
   state is exhausted.

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

6.  Impact on Higher Layer Protocols

   Network elements that support the NQB PHB and that support traffic
   protection as discussed in the previous section introduce the
   possibility that flows classified into the NQB queue could experience
   out of order delivery or packet loss if their behavior is not
   consistent with 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
   flow that is (mis)marked as NQB and that causes a queue to form in
   this bottleneck link could see some of its packets forwarded by the
   NQB queue, and some of them either discarded or redirected to the QB
   queue.  In the case of redirection, depending on the queueing latency
   and scheduling within the network element, this could result in
   packets being delivered out of order.  As a result, the use of the
   NQB DSCP by a higher layer protocol carries some risk 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.

7.  Configuration and Management

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

7.1.  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

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   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 flows 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.

8.  Example Use Cases

8.1.  DOCSIS Access Networks

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

   To support the NQB PHB, cable broadband services MUST be configured
   to provide a separate queue for 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.

8.2.  Mobile Networks

   Historically, 3GPP mobile networks have utilised "bearers" to
   encapsulate each user's user plane traffic through the radio and core
   networks.  A "dedicated bearer" can be allocated a Quality of Service
   (QoS) to apply any prioritisation to its flows at queues and radio
   schedulers.  Typically, an LTE operator provides a dedicated bearer
   for IMS VoLTE (Voice over LTE) traffic, which is prioritised in order
   to meet regulatory obligations for call completion rates; and a "best
   effort" default bearer, for Internet traffic.  The "best effort"

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   bearer provides no guarantees, and hence its buffering
   characteristics are not compatible with low-latency traffic.  The 5G
   radio and core systems offer more flexibility over bearer allocation,
   meaning bearers can be allocated per traffic type (e.g., loss-
   tolerant, low-latency etc.) and hence support more suitable treatment
   of Internet real-time flows.

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

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

8.3.  WiFi Networks

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

   In addition to the requirements provided in other sections of this
   document, to support the NQB PHB, WiFi equipment (including equipment
   compliant with [RFC8325]) SHOULD map the NQB 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).

8.3.1.  Interoperability with Existing WiFi Networks

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

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

   The DSCP value 45 (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 WiFi systems to
   support the NQB PHB requirement for separate queuing, existing WiFi
   devices generally utilize EDCA parameters that result in statistical
   prioritization of the "Video" Access Category above the "Best Effort"
   Access Category.  In addition this equipment does not support the
   remaining NQB PHB recommendations in Section 5.  The rationale for
   the choice of DSCP 45 (decimal) as well as its ramifications, and
   remedies for its limitations are discussed further below.

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

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

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

   *  Traffic that is compliant with the NQB sender requirements
      Section 4.1 is unlikely to cause more degradation to lower
      priority Access Categories than the existing recommended Video
      Access Category traffic types: Broadcast Video, Multimedia
      Streaming, Multimedia Conferencing from [RFC8325], and AudioVideo,
      ExcellentEffort from [QOS_TRAFFIC_TYPE].

   *  Application instances on WiFi client devices are already free to
      choose any Access Category that they wish, regardless of their
      sending behavior, without any policing of usage.  So, the choice
      of using DSCP 45 (decimal) for NQB creates no new avenues for non-
      NQB-compliant client applications to exploit the prioritization
      function in WiFi.

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

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

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

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

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

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9.  Acknowledgements

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

10.  IANA Considerations

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

   IANA should update this registry as follows:

   *  Name: NQB

   *  Value (Binary): 101101

   *  Value (Decimal): 45

   *  Reference: this document

11.  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

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

12.  Security Considerations

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

   In order to preserve low latency performance for NQB traffic,
   networks that support the NQB PHB will need to ensure that mechanisms
   are in place to prevent malicious traffic marked with the NQB DSCP
   from causing excessive queue delays.  Section 5.2 recommends the
   implementation of a traffic protection mechanism to achieve this goal
   but recognizes that other options might be more desirable in certain
   situations.  The recommendations on traffic protection mechanisms in
   this document presume that some type of "flow" state be maintained in
   order to differentiate between flows that are causing queuing delay
   and those that aren't.  Since this flow state is likely finite, this
   opens up the possibility of flow-state exhaustion attacks.  While
   this document requires that traffic protection mechanisms be designed
   with this possibility in mind, the outcomes of flow-state exhaustion
   would depend on the implementation.

   Notwithstanding the above, the choice of DSCP for NQB does allow
   existing WiFi networks to readily (and by default) support some of
   the PHB requirements, but without a traffic protection function, and
   (when left in the default state) by giving NQB traffic higher
   priority than QB traffic.  This is not considered to be a compliant
   implementation of the PHB.  These existing WiFi networks currently
   provide priority to half of the DSCP space, including the NQB DSCP.
   While the NQB DSCP value could be abused in order 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 seem to make it less attractive for such abuse than any
   of the other 31 DSCP values that are provided high priority.

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   NQB uses the Diffserv code point (DSCP).  The design of Diffserv does
   not include integrity protection for the DSCP, and thus it is
   possible for the DSCP to be changed by an on-path attacker.  The NQB
   PHB and associated DSCP don't change this.  While re-marking DSCPs is
   permitted for various reasons (some are discussed in this document,
   others can be found in [RFC2474] and [RFC2475]), if done maliciously,
   this might negatively affect the QoS of the tampered flow.

13.  References

13.1.  Normative References

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

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

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

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

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

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

13.2.  Informative References

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

   [Custura]  Custura, A., Venne, A., and G. Fairhurst, "Exploring DSCP
              modification pathologies in mobile edge networks", TMA ,
              2017.

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   [I-D.briscoe-docsis-q-protection]
              Briscoe, B. and G. White, "The DOCSIS(r) Queue Protection
              Algorithm to Preserve Low Latency", Work in Progress,
              Internet-Draft, draft-briscoe-docsis-q-protection-06, 13
              May 2022, <https://datatracker.ietf.org/doc/html/draft-
              briscoe-docsis-q-protection-06>.

   [I-D.ietf-tsvwg-dscp-considerations]
              Custura, A., Fairhurst, G., and R. Secchi, "Considerations
              for Assigning a new Recommended DiffServ Codepoint
              (DSCP)", Work in Progress, Internet-Draft, draft-ietf-
              tsvwg-dscp-considerations-13, 3 March 2023,
              <https://datatracker.ietf.org/doc/html/draft-ietf-tsvwg-
              dscp-considerations-13>.

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

   [QOS_TRAFFIC_TYPE]
              Microsoft, Corporation, "QOS_TRAFFIC_TYPE enumeration",
              2022, <https://learn.microsoft.com/en-
              us/windows/win32/api/qos2/ne-qos2-qos_traffic_type>.

   [RFC2475]  Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
              and W. Weiss, "An Architecture for Differentiated
              Services", RFC 2475, DOI 10.17487/RFC2475, December 1998,
              <https://www.rfc-editor.org/info/rfc2475>.

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

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

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

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

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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>.

   [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>.

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

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

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

   [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
   [I-D.ietf-tsvwg-dscp-considerations][Custura][Barik], and in some
   cases apply their own DSCP for internal usage.  Bleaching the NQB
   DSCP is not expected to cause harm to Default traffic, but it will
   severely limit the ability to provide NQB treatment end-to-end.
   Reports on existing deployments of DSCP manipulation [Custura][Barik]
   categorize the re-marking behaviors into the following six policies:
   bleach all traffic (set DSCP to zero), set the top three bits (the
   former Precedence bits) on all traffic to 0b000, 0b001, or 0b010, set
   the low three bits on all traffic to 0b000, or re-mark all traffic to
   a particular (non-zero) DSCP value.

   Regarding the DSCP value 45 (decimal), there were no observations of
   DSCP manipulation reported in which traffic was marked 45 (decimal)
   by any of these policies.  Thus it appears that these re-marking
   policies would be unlikely to result in QB traffic being marked as
   NQB (45).  In terms of the fate of traffic marked with the NQB DSCP
   that is subjected to one of these policies, the result would be that
   traffic marked with the NQB DSCP would be indistinguishable from some
   subset (possibly all) of other traffic.  In the policies where all
   traffic is re-marked using the same (zero or non-zero) DSCP, the
   ability for a subsequent network hop to differentiate NQB traffic via
   DSCP would clearly be lost entirely.

   In the policies where the top three bits are overwritten (see
   Section 4.2 of [I-D.ietf-tsvwg-dscp-considerations]), 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-

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   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 [I-D.ietf-tsvwg-dscp-considerations] future
   assignments of these 0bxxx101 DSCPs would need to be made with
   consideration of the potential that they all are treated as NQB in
   some networks.

   For the policy in which the low three bits are set to 0b000, the NQB
   (45) value would be re-marked to CS5 and would be indistinguishable
   from CS5, VA, EF (and the unassigned DSCPs 41, 42, 43).  Traffic
   marked using the existing standardized DSCPs in this list are likely
   to share the same general properties as NQB traffic (non capacity-
   seeking, very low data rate or relatively low and consistent data
   rate).  Similarly, any future recommended usage for DSCPs 41, 42, 43
   would likely be somewhat compatible with NQB treatment, assuming that
   IP Precedence compatibility (see Section 1.5.4 of [RFC4594]) is
   maintained in the future.  Here there might be an opportunity for a
   node to provide the NQB PHB or the CS5 PHB to CS5-marked traffic and
   retain some of the benefits of NQB marking.  This could be another
   motivation to (as discussed in Section 4.2) classify CS5-marked
   traffic into NQB queue.

Appendix B.  Comparison to Expedited Forwarding

   The Expedited Forwarding definition [RFC3246] provides the following
   intuitive description of the EF PHB: "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."  This differs from the
   definition of the NQB PHB, in which NQB traffic is treated as being
   the same forwarding preference (and hence the same likelihood of
   being deferred if other traffic is being served) as Default traffic.
   As a result there is no rate R that can be configured for the NQB
   PHB, and moreover the NQB PHB does not guarantee any serving rate for
   NQB-marked traffic, let alone one that is independent of the offered
   load of non-NQB-marked traffic.  This difference additionally makes
   the NQB PHB suitable for implementation in networks where link
   capacity is not guaranteed.

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   The Formal Definition of the EF PHB Section 2.2 of [RFC3246] requires
   support for the configured rate R as well as a guarantee on departure
   times of EF-marked packets within a characterizable bound (E_a), as
   described by a pair of equations (eq_1 and eq_2).  Since the NQB PHB
   cannot be configured with a rate R, these equations cannot be
   complied with, except for the degenerate cases where R equals 0 and/
   or E_a equals infinity.

   From a practical perspective, EF-marked traffic may be treated with
   high priority in networks where the DSCP is supported, even if the EF
   PHB requirements aren't fully met (e.g. as discussed in [RFC8325]).

Authors' Addresses

   Greg White
   CableLabs
   Email: g.white@cablelabs.com

   Thomas Fossati
   ARM
   Email: Thomas.Fossati@arm.com

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