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Versions: 00 01

Transport Area Working Group                                  B. Briscoe
Internet-Draft                                                 CableLabs
Intended status: Informational                              July 2, 2018
Expires: January 3, 2019


 Interactions between Low Latency, Low Loss, Scalable Throughput (L4S)
                      and Differentiated Services
                  draft-briscoe-tsvwg-l4s-diffserv-01

Abstract

   L4S and Diffserv offer somewhat overlapping services (low latency and
   low loss), but bandwidth allocation is out of scope for L4S.
   Therefore there is scope for the two approaches to complement each
   other, but also to conflict.  This informational document explains
   how the two approaches interact, how they can be arranged to
   complement each other and in which cases one can stand alone without
   needing the other.

Status of This Memo

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

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   This Internet-Draft will expire on January 3, 2019.

Copyright Notice

   Copyright (c) 2018 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
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   to this document.  Code Components extracted from this document must



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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Document Roadmap  . . . . . . . . . . . . . . . . . . . .   4
   2.  Architectural Comparison of L4S and Diffserv  . . . . . . . .   4
     2.1.  Overlaps between L4S and Diffserv . . . . . . . . . . . .   4
     2.2.  Differences between L4S and Diffserv  . . . . . . . . . .   4
   3.  Low Latency Diffserv Classes within a DualQ Bandwidth Pool  .   5
   4.  DualQ Bandwidth Pool within a Hierarchy of (Diffserv)
       Bandwidth Queues  . . . . . . . . . . . . . . . . . . . . . .   9
     4.1.  DualQ Complemented by an Assured Bandwidth Service  . . .  10
     4.2.  DualQ Complemented by a Guaranteed Low Latency Service  .  12
     4.3.  DualQ Complemented by a Scavenger Service . . . . . . . .  13
   5.  Coupling More than Two AQMs within a Bandwidth Pool . . . . .  14
   6.  Best Practice for Classification and Marking  . . . . . . . .  14
     6.1.  Never Re-Mark a DSCP  . . . . . . . . . . . . . . . . . .  14
     6.2.  Classification Order  . . . . . . . . . . . . . . . . . .  15
       6.2.1.  Classification Order: Problem . . . . . . . . . . . .  15
       6.2.2.  Classification Order: Solutions . . . . . . . . . . .  15
   7.  Policing and Traffic Conditioning . . . . . . . . . . . . . .  15
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  16
   10. Comments Solicited  . . . . . . . . . . . . . . . . . . . . .  16
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  16
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  16
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  16
     12.2.  Informative References . . . . . . . . . . . . . . . . .  16
   Appendix A.  Open Issues  . . . . . . . . . . . . . . . . . . . .  18
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  18

1.  Introduction

   The Low Latency Low Loss Scalable throughput (L4S) Internet service
   [I-D.ietf-tsvwg-l4s-arch] provides a new Internet service that could
   eventually replace best efforts, but with ultra-low queuing delay and
   loss.  A structure called the Dual-Queue Coupled AQM manages to
   provide the L4S service alongside a second queue for Classic Internet
   traffic, but without prejudging the bandwidth allocations between
   them.  L4S is orthogonal to allocation of bandwidth, so it can be
   complemented by various bandwidth allocation approaches without
   prejudging which one.





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   The Differentiated Services (Diffserv) architecture [RFC2475]
   provides for various service classes, some defined globally, others
   defined locally per network domain.  Certain of these service classes
   offer low latency and low loss, as well as differentiated allocation
   of bandwidth.

   Thus, L4S and Diffserv offer somewhat overlapping services (low
   latency and low loss), but bandwidth allocation is out of scope for
   L4S.  Therefore there is scope for the two approaches to complement
   each other, but also to conflict.  This informational document
   explains how the two approaches interact, how they can be arranged to
   complement each other and in which cases one can stand alone without
   needing the other.

1.1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].  In this
   document, these words will appear with that interpretation only when
   in ALL CAPS.  Lower case uses of these words are not to be
   interpreted as carrying RFC-2119 significance.

   Classic service:  The 'Classic' service is intended for all the
      congestion control behaviours that currently co-exist with TCP
      Reno [RFC5681] (e.g.  TCP Cubic, Compound, SCTP, etc).

   Low-Latency, Low-Loss and Scalable (L4S) service:  The 'L4S' service
      is intended for traffic from scalable congestion control
      algorithms such as Data Centre TCP [RFC8257].  But it is also more
      general--it will allow a set of congestion controls with similar
      scaling properties to DCTCP to evolve.

      Both Classic and L4S services can cope with a proportion of
      unresponsive or less-responsive traffic as well (e.g.  DNS, VoIP,
      etc).

   Pure L4S:  L4S without unresponsive traffic.

   Scalable Congestion Control:  See [I-D.ietf-tsvwg-l4s-arch] for
      definition.

   Classic Congestion Control:  See [I-D.ietf-tsvwg-l4s-arch] for
      definition.

   DualQ:  Abbreviation for Dual-Queue Coupled AQM
      [I-D.ietf-tsvwg-aqm-dualq-coupled], which is not a specific AQM,
      but a framework for coupling two AQMs in order to provide L4S



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      service while doing no harm to 'Classic' traffic from traditional
      sources.

   ECN field:  The Explicit Congestion Notification field [RFC3168] in
      the IP header (v4 or v6).  [RFC8311] has relaxed some of the
      restrictions that RFC 3168 placed on the use of ECN, in order to
      enable experiments like L4S, among others.

   Site:  A home, mobile device, small enterprise or campus, where the
      network bottleneck is typically the access link to the site.  Not
      all network arrangements fit this model but it is a useful, widely
      applicable generalisation.

1.2.  Document Roadmap

   {ToDo}

2.  Architectural Comparison of L4S and Diffserv

   This section compares the L4S architecture [I-D.ietf-tsvwg-l4s-arch]
   with the Diffserv architecture [RFC2475].

   L4S uses an identifier [I-D.ietf-tsvwg-ecn-l4s-id] in the ECN field
   in IP packet headers that is orthogonal to the Diffserv field
   [RFC2474].  This is because the two approaches can either overlap or
   complement each other, as outlined in the following two subsections.

2.1.  Overlaps between L4S and Diffserv

   L4S provides a low queuing latency, low loss Internet Service.
   Specific Diffserv service classes also provide low latency and low
   loss.

   This means that it is possible to mix traffic from certain Diffserv
   classes in the same queue as L4S traffic (see Section 3).

2.2.  Differences between L4S and Diffserv

   Bandwidth allocation:  L4S is orthogonal to allocation of bandwidth,
      so it can be complemented by various bandwidth allocation
      approaches without prejudging which one.  In contrast, with
      Diffserv it was never possible to completely separate control of
      latency and loss from allocation of bandwidth.  The only
      bandwidth-related aspect of L4S is that it ensures that the
      capacity seeking behaviour of end-systems can scale with
      increasing flow rate.





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   Differentiation vs. General improvement:  Diffserv concerns give and
      take of bandwidth, latency and loss between traffic classes.  In
      contrast, the separation of L4S from Classic traffic in separate
      queues concerns incremental deployment of a general improvement in
      latency and loss, without taking from the other queue.

   Open vs. closed loop control:  The Diffserv architecture requires the
      source to keep traffic within a contract and, failing that, it has
      mechanisms to enforce the contract.  In this respect, Diffserv is
      an open-loop control system that is primarily concerned with
      keeping traffic within capacity limits.  Nonetheless, there is an
      element of closed-loop control in Diffserv.  The weighted AQM
      (e.g.  WRED) used for Assured Forwarding [RFC2597] expects traffic
      to seek to fill capacity and exploits the response to feedback of
      congestion controllers at traffic sources (closed-loop).
      Nonetheless, the Diffserv architecture still provides for traffic
      conditioners that tag traffic that is outside the bandwidth
      contract for each AF class (open-loop).  Then out-of-contract
      traffic can be discarded if it would otherwise lead to congestion.

      L4S uses a similar closed-loop mechanism to the weighted AQM used
      in Diffserv AF in order to ensure roughly equal per-flow
      throughput between the L4S and Classic queues.  That is, L4S
      relies on the source's closed-loop response to feedback, not any
      open-loop obligation of each source to keep within a traffic
      contract.  With L4S, any enforcement of per-flow throughput
      (whether open-loop or closed) is set aside as a separate issue
      that may or may not be addressed by separate mechanisms, dependent
      on policy.

   Per bottleneck vs. per domain:  L4S can be independently and
      incrementally deployed at certain bottlenecks.  In contrast a
      Diffserv system is domain-based consisting of the per-hop
      behaviour of interior nodes and the traffic conditioning behaviour
      of boundary nodes, which have to be deployed as a coordinated
      whole.

   Degree of multiplexing:  Diffserv components such as traffic
      conditioning are less applicable in access networks where
      statistical multiplexing is low, whereas L4S was initially
      designed for access networks, but is also applicable at larger
      pinch-points (e.g. public peerings).

3.  Low Latency Diffserv Classes within a DualQ Bandwidth Pool

   The experimental Dual-Queue Coupled AQM
   [I-D.ietf-tsvwg-aqm-dualq-coupled] consists of a pair of queues.  One
   provides a low latency low loss service but both have full access to



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   the same pool of bandwidth.  When Diffserv was defined no mechanism
   like this was available that could provide low latency without also
   requiring bandwidth controls.  All Diffserv's mechanisms for low
   latency and low loss use some form of priority over bandwidth, then
   apply a bandwidth constraint to prevent the lower priority traffic
   from being starved.

   This Diffserv bandwidth constraint has a flip side - it can also
   provide a bandwidth assurance.  However, in turn, bandwidth assurance
   has both positive and negative aspects.  It certainly prevents other
   traffic encroaching on the bandwidth of the low latency class, but it
   also carves off a partition within which low latency sessions are
   more prone to encroach on each other.

   The DualQ offers an alternative where low latency traffic can access
   the whole pool of bandwidth (in effect, the largest possible
   bandwidth constraint).  This is expected to be preferred by many
   network operators and users who would rather not set a bandwidth
   limit for their low latency traffic - particularly at links in access
   networks where the very low level of flow multiplexing makes the
   bandwidth shares of different traffic classes nearly impossible to
   predict.  Nonetheless, if a bandwidth partition is required for
   bandwidth assurance purposes, it can still be provided separately
   (see Section 4).

   The DualQ classifies packets with the ECN field set to ECT(1) or CE
   into the low latency low loss (L) queue.  The L queue maintains a low
   latency low loss service primarily because an L4S source paces its
   packets and is linearly responsive to ECN markings, which earns it
   the right to set the ECT(1) codepoint [I-D.ietf-tsvwg-ecn-l4s-id]
   [RFC8311].

   Nonetheless, a low level of non-L4S traffic can share the L queue
   without compromising the low latency and low loss of the service.
   Certain existing Diffserv classes are already intended as low latency
   and low loss services.  An operator could use the DualQ instead of
   traditional Diffserv queues to give a few of these classes the
   benefit of low latency and access to the whole pool of bandwidth.

   However, that would only be safe for those Diffserv service classes
   that would not risk ruining the low latency of the service.
   Therefore, an operator must take care to only classify a Diffserv
   traffic class into the L queue if it is expected to send smoothly
   without multi-packet bursts.  Below we give examples of classes that
   should (and should not) be safe to mix into the L queue.

   Table 1 lists the Diffserv service classes that have been allocated
   global use Diffserv codepoints (DSCPs) from Pool 1.  They are



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   described in RFC 4594 [RFC4594] and its updates ([RFC5865] and
   [I-D.ietf-tsvwg-le-phb] so far).  An operator that only deploys a
   DualQ [I-D.ietf-tsvwg-aqm-dualq-coupled] but not the relevant
   Diffserv PHBs could classify those with an 'L' in the 'Coupled Queue'
   column (or local use DSCPs with similar characteristics) into its L
   queue, irrespective of the setting of the ECN field.

   +--------------------+-------------+--------+-------+---------------+
   | Service Class Name | DSCP Name   | DSCP   | AQM?  | Coupled Queue |
   +--------------------+-------------+--------+-------+---------------+
   | Network Control{1} | CS7         | 111000 | Y & N | L if L4S      |
   | Network Control    | CS6         | 110000 | Y & N | L if L4S      |
   | OAM                | CS2         | 010000 | Y & N | L if L4S      |
   | Signalling         | CS5         | 101000 | N     | L if L4S{2}   |
   | Telephony          | EF          | 101110 | N     | L             |
   | RFC 5865           | Voice-Admit | 101100 | N     | L{3}          |
   | R-T Interactive    | CS4         | 100000 | N     | L if L4S{4}   |
   | MM Conferencing    | AF4n        | 100nn0 | Y     | L if L4S      |
   | Broadcast Video    | CS3         | 011000 | N     | L if L4S{4}   |
   | MM Streaming       | AF3n        | 011nn0 | Y     | L if L4S      |
   | Low Latency Data   | AF2n        | 010nn0 | Y     | L if L4S      |
   | High Thru'put Data | AF1n        | 001nn0 | Y     | L if L4S{5}   |
   | Standard           | DF (CS0)    | 000000 | Y     | L if L4S      |
   | Low Priority Data  | LE{6}       | 000001 | Y     | L if L4S{7}   |
   +--------------------+-------------+--------+-------+---------------+

   Some service class names have been abbreviated to fit.  Abbreviations
   are expanded in RFC 4594 or its updates.  For the assured forwarding
        (AF) DSCP names, the digit 'n' represents 1, 2 or 3 and the
   corresponding binary digits 'nn' in the DSCP value represent 01,10 or
         11.  The 'Coupled Queue' column is explained in the text.

   Table 1: Mapping of RFC4594 Diffserv Service Classes in a Coupled AQM

   Notes for Table 1:

   {1}:   Reserved by RFC 2474 [RFC2474].

   {2}:   Superficially, CS5 is a candidate for classification into the
          L queue irrespective of its ECN field, given application
          signalling is bursty but usually lightweight.  However, at
          least one major equipment vendor uses CS5 by default to
          indicate unresponsive broadcast video traffic (to which RFC
          4594 allocates CS3).

   {3}:   Voice-Admit [RFC5865] could be given priority over Expedited
          Forwarding (EF) [RFC3246].




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   {4}:   The Real-Time Interactive and Broadcast Video service classes
          (or any equivalent local-use classes) are intended for
          inelastic traffic.  Therefore they would not be expected to
          mark themselves as ECN-capable.  If they did they would be
          claiming to be elastic and therefore eligible for
          classification into the L queue (subject to any policing).
          These classes should not be classified into the L queue on the
          basis of DSCP alone, because high bandwidth unresponsive
          traffic with potentially variable rate is not compatible with
          the L4S service.

   {5}:   High Throughput Data (or any equivalent local-use class) might
          use the L4S service because of its support for scalable
          congestion control.

   {6}:   [I-D.ietf-tsvwg-le-phb] updates RFC 4594 to deprecate using
          CS1 for Lower Effort (LE).

   {7}:   If a packet is marked LE and ECT(1) and the operator has
          solely provided a DualQ, this recommends that the packet is
          classified into the L queue.  This could result in LE traffic
          competing for bandwidth with other classes of traffic in the L
          queue, but at least it should not harm the latency of other
          traffic.  This is because the ECT(1) marking means the source
          "MUST" use a scalable congestion control
          [I-D.ietf-tsvwg-ecn-l4s-id], but the LE marking only means it
          "SHOULD" use an LBE congestion control
          [I-D.ietf-tsvwg-le-phb].

   Those classes with an 'L' in the 'DualQ-Coupled' column would not be
   expected to have the ECT(1) codepoint set because they are generally
   unresponsive to congestion.  Nonetheless, they could coexist in the
   same queue as L4S traffic because traffic in all of these classes is
   expected to arrive smoothly, not in bursts of more than a few
   packets.  Therefore an operator could configure a DualQ Coupled AQM
   to classify such packets into the L queue solely based on their DSCP,
   irrespective of their ECN codepoint [I-D.ietf-tsvwg-ecn-l4s-id].

   Otherwise, [I-D.ietf-tsvwg-ecn-l4s-id] requires that any other DSCP
   has no effect on classification into the L queue.  Thus a packet of
   any other DSCP will not be classified into the L queue unless it
   carries an ECT(1) or CE codepoint in the ECN field.  This is shown as
   'L if L4S' in in the 'DualQ-Coupled' column of Table 1.








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4.  DualQ Bandwidth Pool within a Hierarchy of (Diffserv) Bandwidth
    Queues

   The DualQ Coupled AQM offers an L queue that provides low latency low
   loss service but it pools bandwidth with the Classic (C) service as
   if they shared a single FIFO.  As explained earlier, unlike previous
   Diffserv low latency mechanisms, the L queue can offer low latency
   without needing to limit its bandwidth.

   Typically the DualQ will be able to use all the bandwidth available
   to a customer site, e.g. a household, a campus or a mobile node, as a
   single pool.  However, this section considers scenarios where the
   network operator might want to carve off a fraction of a site's
   bandwidth for other purposes, for instance:

   1.  to ensure that a particularly demanding application (e.g. a
       virtual reality session) survives even if excess traffic
       overloads the remainder of the site's bandwidth;

   2.  to give guaranteed low latency to a particular application (e.g.
       industrial process control), if the statistically assured low
       latency of the L queue is insufficiently stable;

   3.  to provide a bandwidth scavenger service that will have no effect
       on any other applications at the site, but will scavenge any
       unused bandwidth, for instance to transfer backups or large data
       sets.

   In all cases, it is assumed that the DualQ has to be able to borrow
   back any of the carved off bandwidth that is unused by the other
   service.

   The following three subsections present solutions for each of the
   above scenarios.  Depending on the reader's viewpoint, each scenario
   can be seen as:

   o  either taking a queue within an existing Diffserv hierarchy and
      splitting it into L4S and Classic queues;

   o  or building a queuing hierarchy around a pre-existing dual L4S/
      Classic queue.

   In each case, the DualQ remains as an indivisible 'atomic' component
   as if it were a single queue with a single pool of bandwidth (but
   that can either be used for low latency or classic service).

   The three examples represent the three main ways that this queue-like
   'atom' can be included in a hierarchy of other queues.  Without loss



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   of generality only one other queue complements the DualQ in each
   case, but it would be straightforward to extend the examples with
   more queues.

   Although these examples are framed in the context of IP and Diffserv,
   similar queuing hierarchies could be constructed at a lower layer, as
   long as it supported a similar capability to ECN and a similar
   Traffic Class identifier to Diffserv.

4.1.  DualQ Complemented by an Assured Bandwidth Service

   Figure 1 shows a DualQ complemented by an additional queue to add a
   bandwidth assured service.  It is assumed that the operator
   classifies certain packets into the assured bandwidth queue, perhaps
   by class of service, source address or 5-tuple flow ID.

              ---------+--+
     Assured b/w       |  |-----------.
              ---------+--+            \    Weighted
                                       w\.-.scheduler
           ,  -----------++             (   )--->
           |   L      .->||---.         /`-'
     DualQ |  -------/---++   c\.-.    /
     b/w  <         (Coupling  (   )--'
     pool  |  ----+--\----+    /`-'Conditional
           |   C  |   \   |---'    priority
           `  ----+-------+        scheduler


   Figure 1: How to Complement a DualQ with an Assured Bandwidth Service

   The DualQ is used as if it were an indivisible 'atomic' component,
   unchanged from its original description in
   [I-D.ietf-tsvwg-aqm-dualq-coupled]:

   o  The outputs of the AQMs in the two queues (L and C) are coupled
      together so that L4S sources leave enough space for C packets so
      that all 'standard' flows get roughly equal throughput;

   o  A scheduler recombines the outputs of the two queues, giving
      conditional priority to L packets (the condition prevents
      starvation of the C queue if any L traffic misbehaves).

   A weighted scheduler, e.g. weighted round robin (WRR), is used to
   combine the outputs of the assured bandwidth queue and the DualQ.  It
   is configured with weight w for the assured bandwidth queue.  Then,
   packets requesting assured bandwidth will have priority access to
   fraction w of the link capacity.  However, whenever the assured



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   bandwidth queue is idle or under-utilized, the DualQ can borrow the
   balance of the bandwidth.  Likewise the assured bandwidth queue can
   borrow more than fraction w if the DualQ under-utilizes its remaining
   share.

   Note that a weighted scheduler such as WRR can be used to implement
   the conditional priority scheduler between the L and C queues.
   However, the system will not work as intended if the two weighted
   schedulers in series are replaced by a single three-input weighted
   scheduler.  This is because, whenever one queue under-uses its
   weighted share, a weighted scheduler allows the other queue to borrow
   unused capacity.  Whenever traffic is present in the C queue, the
   coupling ensures that L traffic makes space for it by underutiliizing
   its share of the first scheduler.  If the assured bandwidth queue was
   also served by the same scheduler, the assured bandwidth service
   would continually borrow the spare capacity left by the L queue that
   was intended for the C queue.

   The assured bandwidth service could itself also support applications
   using low latency low loss and scalable throughput (L4S).  This would
   be done by serving assured bandwidth traffic with a DualQ (Figure 2)
   and, as usual, confining legacy queue-building traffic to the C
   queue.

           ,  -----------++        Conditional
           |   L      .->||---.    priority
   Assured |  -------/---++   c\.-.scheduler
     b/w  <         (Coupling  (   )--.
           |  ----+--\----+    /`-'    \
           |   C  |   \   |---'         \    Weighted
           `  ----+-------+             w\.-.scheduler
                                         (   )--->
           ,  -----------++              /`-'
           |   L      .->||---.         /
     DualQ |  -------/---++   c\.-.    /
     b/w  <         (Coupling  (   )--'
     pool  |  ----+--\----+    /`-'Conditional
           |   C  |   \   |---'    priority
           `  ----+-------+        scheduler


   Figure 2: How to Complement a DualQ with an Assured Bandwidth Service
                          that also Supports L4S

   The symmetry of Figure 2 reveals that both DualQs actually have
   assured bandwidth.  Nonetheless, the label 'Assured bandwidth' is
   only really meaningful from a per-application perspective if the




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   traffic classified into that DualQ is limited to a small number of
   application sessions at any one time.

4.2.  DualQ Complemented by a Guaranteed Low Latency Service

   Figure 3 shows a DualQ complemented by an additional queue to add a
   guaranteed latency service.  It is assumed that the operator
   classifies certain packets into the guaranteed latency queue, perhaps
   by class of service, source address or 5-tuple flow ID.

      o  Token bucket
    | o |rate/burst limiter
    |___|
    |___|     -----------++
   Guaranteed low latency||-----------.
              -----------++            \    Priority
                                       1\.-.scheduler
           ,  -----------++             (   )--->
           |   L      .->||---.         /`-'
     DualQ |  -------/---++   c\.-.    /
     b/w  <         (Coupling  (   )--'
     pool  |  ----+--\----+    /`-'Conditional
           |   C  |   \   |---'    priority
           `  ----+-------+        scheduler

     Figure 3: How to Complement a DualQ with a Guaranteed Low Latency
                                  Service

   As in all the previous example, the DualQ is used as if it were an
   indivisible 'atomic' component.

   A strict priority scheduler is used to combine the outputs of the
   guaranteed latency queue and the DualQ.  Guaranteed low latency
   traffic is shown as subject to a token bucket that limits rate and
   tightly limits burst size, which ensures that:

   o  Excessive guaranteed latency traffic cannot abuse its priority and
      cause the DualQ to starve;

   o  Guaranteed latency traffic cannot ruin its own latency guarantees
      - it has to keep to a the traffic contract enforced by the token
      bucket.

   In a traditional Diffserv architecture, the token bucket would be
   deployed at the ingress network edge, to limit traffic at each entry
   point.  Alternatively, the token bucket could be deployed directly in
   front of the queue, where it would only limit the total traffic from




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   all entry points to the network.  For an access link into a network,
   these two alternative would amount to the same thing.

   Whenever the guaranteed latency queue is idle or under-utilized, the
   DualQ can borrow the balance of the bandwidth.  However, the
   guaranteed latency queue cannot borrow more than the token bucket
   allows, even if the DualQ under-utilizes its remaining share.

4.3.  DualQ Complemented by a Scavenger Service

   Figure 3 shows a DualQ complemented by an additional queue to add a
   bandwidth scavenger service.  It is assumed that the operator
   classifies certain packets into the scavenger queue, probably by
   class of service, e.g. the global-use Lower Effort (LE) Diffserv
   codepoint [I-D.ietf-tsvwg-le-phb].

           ,  -----------++        Conditional
           |   L      .->||---.    priority
     DualQ |  -------/---++   c\.-.scheduler
     b/w  <         (Coupling  (   )--.
     pool  |  ----+--\----+    /`-'    \    Priority
           |   C  |   \   |---'        1\.-.scheduler
           `  ----+-------+             (   )--->
                                        /`-'
             -+-----------+            /
     Bandwidth|scavenger  |-----------'
             -+-----------+

      Figure 4: How to Complement a DualQ with a Bandwidth Scavenger
                                  Service

   As in all the previous example, the DualQ is used as if it were an
   indivisible 'atomic' component.

   A strict priority scheduler is used to combine the outputs of the
   DualQ and the scavenger service.  Section 2 of
   [I-D.ietf-tsvwg-le-phb] suggests alternative mechanisms.

   Whenever the DualQ is idle or under-utilized, the scavenger service
   can borrow the balance of the bandwidth.  In contrast to the previous
   guaranteed latency example, no rate limiter is needed on the DualQ
   because, by definition, the scavenger service is expected to starve
   if the higher priority service is using all the capacity.








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5.  Coupling More than Two AQMs within a Bandwidth Pool

   The Diffserv Assured Forwarding (AF) classes of service [RFC2597] use
   an AQM with differently weighted outputs, e.g.  WRED, to provide
   weighted congestion feedback to the transport layer.  Flows
   classified to use a higher weight AQM each take more of the available
   capacity, because the weighted AQM has fooled their congestion
   controller into detecting that the bottleneck is more lightly loaded.

   A similar mechanism can be used to add throughput differentiation to
   either or both of the queues within a DualQ.  Figure 5 illustrates an
   example with an AQM offering three weights within the L queue, where
   L1 gets the highest throughput per flow.  It would be a matter of
   operator policy to choose which of the three L4S AQMs the Classic AQM
   would couple to.  If it were coupled to L3, then C and L3 flows would
   get roughly equal throughput, while L2 and L1 flows would get more.

           ,  -----------++
           |  L1         ||
           |  L2         ||--.
           |  L3    .->  ||   \
     DualQ |  -----/-----++   c\.-.
     b/w  <       ( Coupling   (   )--->
     pool  |  ----+\------+    /`-'Conditional
           |   C  | \     |---'    priority
           `  ----+-------+        scheduler


          Figure 5: Coupling the Classic AQM to Multiple L4S AQMs

   Note: this structure seems straightforward to implement, but the
   authors are not aware of any implementation or evaluation of AQMs
   that are both weighted and coupled to other AQMs.

6.  Best Practice for Classification and Marking

6.1.  Never Re-Mark a DSCP

   It is not a DualQ's job to alter Diffserv codepoints to attempt to
   make other downstream AQMs classify selected packets in certain ways.
   Each DualQ Coupled AQM is independently (but hopefully consistently)
   configured to select certain DSCPs for classification into the L
   queue.  It never alters the DSCP nor the ECN codepoint (except
   setting CE to indicate that congestion was experienced)
   [I-D.ietf-tsvwg-aqm-dualq-coupled].






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6.2.  Classification Order

6.2.1.  Classification Order: Problem

   The above wide range of possible structures raises the question of
   which order it would be more efficient for classifier rules to take:
   DSCP before ECN, ECN before DSCP or some hybrid.

   On the one hand, for a structure like that in Figure 1 it would make
   sense to classify on DSCP first, then ECN.  Otherwise, if packets
   were classified on ECN first, an extra merge stage would be required
   because the assured bandwidth queue handles all ECN codepoints for a
   particular DSCP.

   On the other hand, for a structure like that in Figure 5 it would
   make sense to classify on ECN first, then DSCP.  Otherwise, again an
   extra merge stage would be needed, because the C queue handles all
   DSCPs but only some ECN codepoints.

   A hybrid of these two scenarios would be possible, for instance where
   the L queue in Figure 1 was further broken down into three weighted
   AQMs, as in Figure 5.  In this case, the ideal matching order would
   be DSCP, ECN, DSCP.

6.2.2.  Classification Order: Solutions

   Probably the most straightforward solution would be to classify in a
   single stage over all 8 octets of the IPv6 Traffic Class field or the
   former IPv4 TOS octet, irrespective of the boundary between the 6-bit
   DS field and the 2-bit ECN field [RFC3260].  As long as hardware
   supports this, it will be possible because all the inputs to the
   queues are at the same level of hierarchy, even though the outputs
   form a multi-level hierarchy of schedulers in some cases.

   Pre-existing classifier hardware might consider the 6-bit and 2-bit
   fields as separate.  Then it would seem most efficient for the order
   of the classifiers to depend on the structure of the queues being
   classified (given the structure has to have been designed before the
   classifiers are designed).

7.  Policing and Traffic Conditioning

   {ToDo: L4S latency policing is discussed in the Security
   Considerations section of [I-D.ietf-tsvwg-l4s-arch].  This section
   will compare Diffserv traffic conditioning with L4S latency
   policing.}





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8.  IANA Considerations

   This specification contains no IANA considerations.

9.  Security Considerations

   {ToDo}

10.  Comments Solicited

   Comments and questions are encouraged and very welcome.  They can be
   addressed to the IETF Transport Area working group mailing list
   <tsvwg@ietf.org>, and/or to the authors.

11.  Acknowledgements

   Thanks to Greg White, David Black, Wes Eddy and Gorry Fairhurst for
   their useful discussions prior to this -00 draft.

12.  References

12.1.  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>.

12.2.  Informative References

   [I-D.ietf-tsvwg-aqm-dualq-coupled]
              Schepper, K., Briscoe, B., Bondarenko, O., and I. Tsang,
              "DualQ Coupled AQMs for Low Latency, Low Loss and Scalable
              Throughput (L4S)", draft-ietf-tsvwg-aqm-dualq-coupled-04
              (work in progress), March 2018.

   [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-02 (work in progress), March 2018.

   [I-D.ietf-tsvwg-l4s-arch]
              Briscoe, B., Schepper, K., and M. Bagnulo, "Low Latency,
              Low Loss, Scalable Throughput (L4S) Internet Service:
              Architecture", draft-ietf-tsvwg-l4s-arch-02 (work in
              progress), March 2018.




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   [I-D.ietf-tsvwg-le-phb]
              Bless, R., "A Lower Effort Per-Hop Behavior (LE PHB)",
              draft-ietf-tsvwg-le-phb-04 (work in progress), March 2018.

   [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>.

   [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>.

   [RFC2597]  Heinanen, J., Baker, F., Weiss, W., and J. Wroclawski,
              "Assured Forwarding PHB Group", RFC 2597,
              DOI 10.17487/RFC2597, June 1999,
              <https://www.rfc-editor.org/info/rfc2597>.

   [RFC3168]  Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
              of Explicit Congestion Notification (ECN) to IP",
              RFC 3168, DOI 10.17487/RFC3168, September 2001,
              <https://www.rfc-editor.org/info/rfc3168>.

   [RFC3246]  Davie, B., Charny, A., Bennet, J., Benson, K., Le Boudec,
              J., 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>.

   [RFC3260]  Grossman, D., "New Terminology and Clarifications for
              Diffserv", RFC 3260, DOI 10.17487/RFC3260, April 2002,
              <https://www.rfc-editor.org/info/rfc3260>.

   [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>.

   [RFC5681]  Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
              Control", RFC 5681, DOI 10.17487/RFC5681, September 2009,
              <https://www.rfc-editor.org/info/rfc5681>.

   [RFC5865]  Baker, F., Polk, J., and M. Dolly, "A Differentiated
              Services Code Point (DSCP) for Capacity-Admitted Traffic",
              RFC 5865, DOI 10.17487/RFC5865, May 2010,
              <https://www.rfc-editor.org/info/rfc5865>.



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   [RFC8257]  Bensley, S., Thaler, D., Balasubramanian, P., Eggert, L.,
              and G. Judd, "Data Center TCP (DCTCP): TCP Congestion
              Control for Data Centers", RFC 8257, DOI 10.17487/RFC8257,
              October 2017, <https://www.rfc-editor.org/info/rfc8257>.

   [RFC8311]  Black, D., "Relaxing Restrictions on Explicit Congestion
              Notification (ECN) Experimentation", RFC 8311,
              DOI 10.17487/RFC8311, January 2018,
              <https://www.rfc-editor.org/info/rfc8311>.

Appendix A.  Open Issues

   o  The Abstract promises "in which cases one can stand alone without
      needing the other", but that's TBA in the text.

   o  Answer the interaction question between Diffserv and L4S the other
      way round as well: Which global PHBs is a DualQ applicable to?

   o  Document Roadmap TBA

   o  Mapping to 802.11 user priorities (or LTE QCIs)?  Not strictly
      within the scope, but perhaps desirable to add, or at least to
      mention how L4S (experimental) would affect RFC8325 which gives
      (standards track) mappings between Diffserv and 802.11.

   o  Identify L4S-friendly rate policers

   o  Comparison between L4S policing and Diffserv traffic conditioning
      is TBA

   o  Security Considerations are TBA (largely depends on the previous
      bullet)

Author's Address

   Bob Briscoe
   CableLabs
   UK

   Email: ietf@bobbriscoe.net
   URI:   http://bobbriscoe.net/










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