Identifying and Handling Non Queue Building Flows in a Bottleneck Link

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Transport Area Working Group                                    G. White
Internet-Draft                                                 CableLabs
Intended status: Standards Track                          March 11, 2019
Expires: September 12, 2019

 Identifying and Handling Non Queue Building Flows in a Bottleneck Link


   This draft proposes the definition of a standardized DSCP to identify
   Non-Queue-Building flows, along with a Per-Hop-Behavior (PHB) that
   provides a separate queue for such flows.

Status of This Memo

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on September 12, 2019.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   3
   3.  Non-Queue Building Flows  . . . . . . . . . . . . . . . . . .   3
   4.  Endpoint Marking and Queue Protection . . . . . . . . . . . .   3
   5.  Non Queue Building PHB and DSCP . . . . . . . . . . . . . . .   4
   6.  End-to-end Support  . . . . . . . . . . . . . . . . . . . . .   5
   7.  Relationship to L4S . . . . . . . . . . . . . . . . . . . . .   6
   8.  Comparison to Existing Approaches . . . . . . . . . . . . . .   6
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   7
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   11. Security Considerations . . . . . . . . . . . . . . . . . . .   7
   12. Informative References  . . . . . . . . . . . . . . . . . . .   8
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   Residential 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.

   The vast majority of packets that are carried by residential
   broadband access networks are managed by an end-to-end congestion
   control algorithm, such as Reno, Cubic or BBR.  These congestion
   control algorithms attempt to seek the available capacity of the end-
   to-end path (which in the case of residential broadband networks, 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 that the application experiences as variable latency, and
   commonly results in packet loss as well.

   In contrast to congestion-controlled applications, there are a
   variety of relatively low data rate applications that do not
   materially contribute to queueing delay, 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

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   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.

   This document considers differentiating between these two classes of
   traffic in bottleneck links in order that both classes can deliver
   exceptional quality of experience for their applications.

2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

3.  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.  Such applications are ideal candidates
   to be queued separately from the capacity-seeking applications that
   cause queue buildups and latency.

   These Non-queue-building (NQB) flows are typically UDP flows, which
   send traffic at a lower data rate and don't seek the capacity of the
   link (examples: online games, voice chat, dns lookups).  Here the
   data rate is essentially limited by the Application itself.  In
   contrast, Queue-building (QB) flows include traffic which uses the
   Traditional TCP, QUIC, BBR or other TCP variants.

   There are a lot of great examples of applications that fall very
   neatly into these two categories, but there are also application
   flows that may be in a gray area in between (e.g. they are NQB on
   high-speed links, but QB on low-speed links).

4.  Endpoint Marking and Queue Protection

   This memo proposes that application endpoints apply a marking,
   utilizing the Diffserv field of the IP header, to packets of NQB
   flows that could then be used by the network to differentiate between
   QB and NQB flows.  It is important for such a marking to be
   universally agreed upon, rather than being locally defined by the
   network operator, such that applications could be written to apply
   the marking without regard to local network policies.

   Some questions that arise when considering endpoint marking are: How
   can an application determine whether it is queue building or not,
   given that the sending application is generally not aware of the

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   available capacity of the path to the receiving endpoint?  Even in
   cases where an application is aware of the capacity of the path, how
   can it be sure that the available capacity (considering other flows
   that may be sharing the path) would be sufficient to result in the
   application's traffic not causing a queue to form?  In an unmanaged
   environment, how can networks trust endpoint marking, why wouldn't
   all applications mark their packets as NQB?

   As an answer the last question, it would be worthwhile to note that
   the NQB designation and marking would be intended to convey
   verifiable traffic behavior, not needs or wants.  Also, it would be
   important that incentives are aligned correctly, i.e. that there is a
   benefit to the application in marking its packets correctly, and no
   benefit for 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 (considering latency, loss and throughput) than
   the QB queue; and for QB flows, the QB queue provides better
   performance (considering latency, loss and throughput) than the NQB

   Even so, it is possible that due to an implementation error or
   misconfiguration, a QB flow would end up getting mismarked as NQB, or
   vice versa.  In the case of an NQB flow that isn't marked as NQB and
   ends up in the QB queue, it would only impact its own quality of
   service, and so it seems to be of lesser concern.  However, a QB flow
   that is mismarked as NQB would cause queuing delays 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, it would likely be valuable to support a "queue
   protection" function that could identify QB flows that are mismarked
   as NQB, and reclassify those flows/packets to the QB queue.  This
   would benefit the reclassified flow by giving it access to a large
   buffer (and thus lower packet loss rate), and would benefit the
   actual NQB flows by preventing harm (increased latency variability)
   to them.  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-level constructs.

5.  Non Queue Building PHB and DSCP

   This section uses the DiffServ nomenclature of per-hop-behavior (PHB)
   to describe how a network node could provide better quality of
   service for NQB flows without reducing performance of QB flows.

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

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   traffic.  This queue SHOULD support a latency-based queue protection
   mechanism that is able to identify queue-building behavior in flows
   that are classified into the queue, and to redirect flows causing
   queue build-up to a different queue.  One example algorithm can be
   found in Annex P of [DOCSIS-MULPIv3.1].

   While there may be some similarities between the characteristics of
   NQB flows and flows marked with the Expedited Forwarding DSCP, the
   NQB PHB would differ from the Expedited Forwarding PHB in several
   important ways.

   o  NQB traffic is not rate limited or rate policed.  Rather, the NQB
      queue would be expected to support a latency-based queue
      protection mechanism that identifies NQB marked flows that are
      beginning to cause latency, and redirects packets from those flows
      to the queue for QB flows.

   o  The node supporting the NQB PHB makes no guarantees on latency or
      data rate for NQB marked flows, but instead aims to provide sub-
      millisecond queuing delays for as many such marked flows as it
      can, and shed load when needed.

   o  EF is commonly used exclusively for voice traffic, for which
      additional functions are applied, such as admission control,
      accounting, prioritized delivery, etc.

   In networks that support the NQB PHB, it may be preferred to also
   include traffic marked EF (101110b) in the NQB queue.  The choice of
   the 0x2A codepoint (101010b) for NQB would conveniently allow a node
   to select these two codepoints using a single mask pattern of

   Additionally, WiFi routers and APs that support WiFi MultiMedia
   commonly use the upper three bits of the DiffServ Field to select the
   WiFi User Priority.  In the case of the 0x2A codepoint, this would
   map to UP_5 which is in the "Video" Access Category (one level above
   "Best Effort").

6.  End-to-end Support

   In contrast to the existing standard DSCPs, which are typically only
   enforced within a DiffServ Domain (e.g. an AS), this DSCP would be
   intended for end-to-end usage across the Internet.  Some access
   network service providers bleach the Diffserv field on ingress into
   their network, and in some cases apply their own DSCP for internal
   usage.  Access networks that support the NQB PHB would need to permit
   the NQB PHB to pass through this bleaching operation such that the
   PHB can be provided at the access network link.

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7.  Relationship to L4S

   The dual-queue mechanism described in this draft is similar to, and
   is intended to be compatible with [I-D.ietf-tsvwg-l4s-arch].

8.  Comparison to Existing Approaches

   Traditional QoS mechanisms focus on prioritization in an attempt to
   achieve two goals, reduced latency for "latency-sensitive" traffic,
   and increased bandwidth availability for "important" applications.
   Applications are generally given priority in proportion to some
   combination of latency-sensitivity and importance.

   Downsides to this approach include the difficulties in sorting out
   what priority level each application should get (making the value
   judgement as to latency-sensitivity and importance), associating
   packets to priority levels (lots of classifier state, or trusting
   endpoint markings and the value judgements that they convey),
   ensuring that high priority traffic doesn't starve lower priority
   traffic (admission control, weighted scheduling, etc. are possible
   solutions).  This solution can work in a managed network, where the
   network operator can control the usage of the QoS mechanisms, but has
   not been adopted end-to-end across the internet.

   Flow queueing approaches (such as fq_codel [RFC8290]), on the other
   hand, achieve latency improvements by associating packets into "flow"
   queues and then prioritizing "sparse flows", i.e. packets that arrive
   to an empty flow queue.  Flow queueing does not attempt to
   differentiate between flows on the basis of value (importance or
   latency-sensitivity), it simply gives preference to sparse flows, and
   tries to guarantee that the non-sparse flows all get an equal share
   of the remaining channel capacity and are interleaved with one
   another.  As a result, fq mechanisms could be considered more
   appropriate for unmanaged environments and general internet traffic.

   Downsides to this approach include loss of low latency performance
   due to the possibility of hash collisions (where a sparse flow shares
   a queue with a bulk data flow), complexity in managing a large number
   of queues in certain implementations, and the DRR scheduling, which
   enforces that each non-sparse flow gets an equal fraction of link
   bandwidth, causes problems with VPNs and other tunnels, exhibits poor
   behavior with less-aggressive CA algos, e.g.  LEDBAT, and exhibits
   poor behavior with RMCAT CA algos.  In effect the network element is
   making a decision as to what constitutes a flow, and then forcing all
   such flows to take equal bandwidth at every instant.

   The Dual-queue approach achieves the main benefit of fq_codel:
   latency improvement without value judgements, without the downsides.

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   The distinction between NQB flows and QB flows is similar to the
   distinction made between "sparse flow queues" and "non-sparse flow
   queues" in fq_codel.  In fq_codel, a flow queue is considered sparse
   if it is drained completely by each packet transmission, and remains
   empty for at least one cycle of the round robin over the active flows
   (this is approximately equivalent to saying that it utilizes less
   than its fair share of capacity).  While this definition is
   convenient to implement in fq_codel, it isn't the only useful
   definition of sparse flows.

   The Linux Heavy-Hitter Filter ([HHF]) qdisc and the Cisco Dynamic
   Packet Prioritization ([DPP]) feature both categorize application
   flows into "mice" and "elephants", and provide a separate queue that
   gives high priority to the "mice" flows.  In both of these
   implementations, the definition of a mouse flow is one that falls
   below a defined number of bytes or packets (respectively).  In
   essence, the first N bytes or packets of every new flow are queued
   separately, and given priority over other traffic.  The HHF
   implementation defaults to using 128KB for N, whereas the DPP
   documentation discusses using 120 packets.

   This approach is relatively simple to implement, but it is making the
   wrong distinction between flows.  To illustrate, an hour-long 60 kbps
   multiplayer online gaming flow sending 60 packets per second would be
   classified as an elephant after the first 17 seconds using HFF or 2
   seconds using DPP, whereas it should be considered as NQB for the
   entire duration.

9.  Acknowledgements


10.  IANA Considerations

   This draft proposes the registration of a standardized DSCP = 0x2A to
   denote Non-Queue-Building behavior.

11.  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 queue-protection is not supported by a node) or it could
   receive no benefit (in the case that queue-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.

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   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.

12.  Informative References

              Cable Television Laboratories, Inc., "MAC and Upper Layer
              Protocols Interface Specification, CM-SP-
              MULPIv3.1-I17-190121", January 21, 2019,

   [DPP]      Cisco, "Intelligent Buffer Management on Cisco Nexus 9000
              Series Switches White Paper", June 2017,

   [HHF]      Lam, T., "net-qdisc-hhf: Heavy-Hitter Filter (HHF) qdisc",
              December 2013, <>.

              Briscoe, B., Schepper, K., and M. Bagnulo, "Low Latency,
              Low Loss, Scalable Throughput (L4S) Internet Service:
              Architecture", draft-ietf-tsvwg-l4s-arch-03 (work in
              progress), October 2018.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

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

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

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   [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,

   [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,

Author's Address

   Greg White
   858 Coal Creek Circle
   Louisville, CO  80027


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