A Non-Queue-Building Per-Hop Behavior (NQB PHB) for Differentiated Services
draft-ietf-tsvwg-nqb-02

Document Type Active Internet-Draft (tsvwg WG)
Authors Greg White  , Thomas Fossati 
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Transport Area Working Group                                    G. White
Internet-Draft                                                 CableLabs
Intended status: Standards Track                              T. Fossati
Expires: March 26, 2021                                              ARM
                                                      September 22, 2020

   A Non-Queue-Building Per-Hop Behavior (NQB PHB) for Differentiated
                                Services
                        draft-ietf-tsvwg-nqb-02

Abstract

   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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   This Internet-Draft will expire on March 26, 2021.

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Copyright Notice

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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.  DSCP Marking of NQB Traffic . . . . . . . . . . . . . . . . .   4
     4.1.  End-to-end usage and DSCP Re-marking  . . . . . . . . . .   5
   5.  Non-Queue-Building PHB Requirements . . . . . . . . . . . . .   6
   6.  Impact on Higher Layer Protocols  . . . . . . . . . . . . . .   7
   7.  Relationship to L4S . . . . . . . . . . . . . . . . . . . . .   7
   8.  Configuration and Management  . . . . . . . . . . . . . . . .   8
   9.  Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . .   8
     9.1.  DOCSIS Access Networks  . . . . . . . . . . . . . . . . .   8
     9.2.  Mobile Networks . . . . . . . . . . . . . . . . . . . . .   8
     9.3.  WiFi Networks . . . . . . . . . . . . . . . . . . . . . .   9
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  10
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   12. Security Considerations . . . . . . . . . . . . . . . . . . .  10
   13. Informative References  . . . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

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
   flows that are distinguished from traffic flows managed by an end-to-
   end congestion control algorithm.

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   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
   link.

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

   Active Queue Management (AQM) mechanisms (such as PIE [RFC8033],
   DOCSIS-PIE [RFC8034], or CoDel [RFC8289]) can improve the quality of
   experience for latency sensitive applications, but there are
   practical limits to the amount of improvement that can be achieved
   without impacting the throughput of capacity-seeking applications,
   particularly when only a few of such flows are present.

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

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.  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
   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

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   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 the
   Traditional TCP or QUIC, with BBR or other TCP congestion
   controllers.

4.  DSCP Marking of NQB Traffic

   Applications that align with the above description of NQB behavior
   SHOULD identify themselves to the network using a DiffServ Code Point
   (DSCP) so that their packets can be queued separately from QB flows.

   There are many application flows that fall very neatly into one or
   the other of these categories, 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
   case, the application SHOULD instead implement a congestion control
   mechanism, for example as described in [RFC8085] or
   [I-D.ietf-tsvwg-ecn-l4s-id].

   This document recommends a DSCP of 42 (0x2A) to identify packets of
   NQB flows.

   It is worthwhile to note 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.

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

   In contrast to the existing standard DSCPs, many of which are
   typically only meaningful within a DiffServ Domain (e.g. an AS or an
   enterprise network), this DSCP is expected to be used end-to-end
   across the Internet.  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.  Networks that support the NQB PHB SHOULD preserve the NQB
   DSCP when forwarding via an interconnect from or to another network.
   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 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.  There were no observations
   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.  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 2,
   50, 58), with all of these codepoints getting mapped to DSCP=2, AF11
   or AF21 (depending on the overwrite value used).  Since the
   recommended usage of the standardized codepoints 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

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   currently unassigned DSCPs in that list would be likely to similarly
   be somewhat compatible with NQB 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.

5.  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
   traffic.

   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

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   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.

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.  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
   experienced.

7.  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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8.  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).

9.  Use Cases

9.1.  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.

9.2.  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.

9.3.  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.
   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 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 WiFi networks, as opposed 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
   codepoint 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 codepoint 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 5.  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.

   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.

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   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.  In these
   situations, 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 remap the NQB DSCP to Default.  Such equipment MUST
   support the ability to configure the remapping, so that (when
   appropriate safeguards are in place) traffic can be delivered as NQB-
   marked.

10.  Acknowledgements

   Thanks to 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.

11.  IANA Considerations

   This document assigns the Differentiated Services Field Codepoint
   (DSCP) 42 ('0b101010', 0x2A) from the "Differentiated Services Field
   Codepoints (DSCP)" registry (https://www.iana.org/assignments/dscp-
   registry/) ("DSCP Pool 1 Codepoints", Codepoint Space xxxxx0,
   Standards Action) to denote Non-Queue-Building behavior.

12.  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

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   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
   situations.

   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.

13.  Informative References

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

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

   [I-D.briscoe-docsis-q-protection]
              Briscoe, B. and G. White, "Queue Protection to Preserve
              Low Latency", draft-briscoe-docsis-q-protection-00 (work
              in progress), July 2019.

   [I-D.ietf-tsvwg-aqm-dualq-coupled]
              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-12 (work in
              progress), July 2020.

   [I-D.ietf-tsvwg-ecn-l4s-id]
              Schepper, K. and B. Briscoe, "Identifying Modified
              Explicit Congestion Notification (ECN) Semantics for
              Ultra-Low Queuing Delay (L4S)", draft-ietf-tsvwg-ecn-l4s-
              id-10 (work in progress), March 2020.

   [I-D.ietf-tsvwg-l4s-arch]
              Briscoe, B., Schepper, K., Bagnulo, M., and G. White, "Low
              Latency, Low Loss, Scalable Throughput (L4S) Internet
              Service: Architecture", draft-ietf-tsvwg-l4s-arch-06 (work
              in progress), March 2020.

   [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>.

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   [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>.

   [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>.

   [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>.

   [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>.

   [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>.

   [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>.

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

Authors' Addresses

   Greg White
   CableLabs

   Email: g.white@cablelabs.com

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   Thomas Fossati
   ARM

   Email: Thomas.Fossati@arm.com

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