[Docs] [txt|pdf|xml|html] [Tracker] [WG] [Email] [Diff1] [Diff2] [Nits]

Versions: (draft-briscoe-tsvwg-ecn-l4s-id) 00 01 02 03 04 05 06 07 08 09 10

Transport Services (tsv)                                  K. De Schepper
Internet-Draft                                           Nokia Bell Labs
Intended status: Experimental                            B. Briscoe, Ed.
Expires: September 10, 2020                                  Independent
                                                           March 9, 2020


 Identifying Modified Explicit Congestion Notification (ECN) Semantics
                   for Ultra-Low Queuing Delay (L4S)
                     draft-ietf-tsvwg-ecn-l4s-id-10

Abstract

   This specification defines the identifier to be used on IP packets
   for a new network service called low latency, low loss and scalable
   throughput (L4S).  It is similar to the original (or 'Classic')
   Explicit Congestion Notification (ECN).  'Classic' ECN marking was
   required to be equivalent to a drop, both when applied in the network
   and when responded to by a transport.  Unlike 'Classic' ECN marking,
   for packets carrying the L4S identifier, the network applies marking
   more immediately and more aggressively than drop, and the transport
   response to each mark is reduced and smoothed relative to that for
   drop.  The two changes counterbalance each other so that the
   throughput of an L4S flow will be roughly the same as a non-L4S flow
   under the same conditions.  Nonetheless, the much more frequent
   control signals and the finer responses to them result in much more
   fine-grained adjustments, so that ultra-low and consistently low
   queuing delay (typically sub-millisecond on average) becomes possible
   for L4S traffic without compromising link utilization.  Thus even
   capacity-seeking (TCP-like) traffic can have high bandwidth and very
   low delay at the same time, even during periods of high traffic load.

   The L4S identifier defined in this document distinguishes L4S from
   'Classic' (e.g.  TCP-Reno-friendly) traffic.  It gives an incremental
   migration path so that suitably modified network bottlenecks can
   distinguish and isolate existing traffic that still follows the
   Classic behaviour, to prevent it degrading the low queuing delay and
   loss of L4S traffic.  This specification defines the rules that L4S
   transports and network elements need to follow to ensure they neither
   harm each other's performance nor that of Classic traffic.  Examples
   of new active queue management (AQM) marking algorithms and examples
   of new transports (whether TCP-like or real-time) are specified
   separately.








De Schepper & Briscoe  Expires September 10, 2020               [Page 1]


Internet-Draft     ECN Semantics for Low Queuing Delay        March 2020


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/.

   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 September 10, 2020.

Copyright Notice

   Copyright (c) 2020 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 and restrictions with respect
   to this document.  Code Components extracted from this document must
   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  . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.1.  Latency, Loss and Scaling Problems  . . . . . . . . . . .   5
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   7
     1.3.  Scope . . . . . . . . . . . . . . . . . . . . . . . . . .   9
   2.  Consensus Choice of L4S Packet Identifier: Requirements . . .   9
   3.  L4S Packet Identification at Run-Time . . . . . . . . . . . .  10
   4.  Prerequisite Transport Layer Behaviour  . . . . . . . . . . .  11
     4.1.  Prerequisite Codepoint Setting  . . . . . . . . . . . . .  11
     4.2.  Prerequisite Transport Feedback . . . . . . . . . . . . .  11
     4.3.  Prerequisite Congestion Response  . . . . . . . . . . . .  12
   5.  Prerequisite Network Node Behaviour . . . . . . . . . . . . .  14
     5.1.  Prerequisite Classification and Re-Marking Behaviour  . .  14
     5.2.  The Meaning of L4S CE Relative to Drop  . . . . . . . . .  15
     5.3.  Exception for L4S Packet Identification by Network Nodes



De Schepper & Briscoe  Expires September 10, 2020               [Page 2]


Internet-Draft     ECN Semantics for Low Queuing Delay        March 2020


           with Transport-Layer Awareness  . . . . . . . . . . . . .  15
     5.4.  Interaction of the L4S Identifier with other Identifiers   16
       5.4.1.  DualQ Examples of Other Identifiers Complementing L4S
               Identifiers . . . . . . . . . . . . . . . . . . . . .  16
         5.4.1.1.  Inclusion of Additional Traffic with L4S  . . . .  16
         5.4.1.2.  Exclusion of Traffic From L4S Treatment . . . . .  18
         5.4.1.3.  Generalized Combination of L4S and Other
                   Identifiers . . . . . . . . . . . . . . . . . . .  18
       5.4.2.  Per-Flow Queuing Examples of Other Identifiers
               Complementing L4S Identifiers . . . . . . . . . . . .  19
   6.  L4S Experiments . . . . . . . . . . . . . . . . . . . . . . .  20
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  20
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  20
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  21
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  21
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  21
     10.2.  Informative References . . . . . . . . . . . . . . . . .  22
   Appendix A.  The 'Prague L4S Requirements'  . . . . . . . . . . .  28
     A.1.  Requirements for Scalable Transport Protocols . . . . . .  29
       A.1.1.  Use of L4S Packet Identifier  . . . . . . . . . . . .  29
       A.1.2.  Accurate ECN Feedback . . . . . . . . . . . . . . . .  29
       A.1.3.  Fall back to Reno-friendly congestion control on
               packet loss . . . . . . . . . . . . . . . . . . . . .  29
       A.1.4.  Fall back to Reno-friendly congestion control on
               classic ECN bottlenecks . . . . . . . . . . . . . . .  30
       A.1.5.  Reduce RTT dependence . . . . . . . . . . . . . . . .  31
       A.1.6.  Scaling down to fractional congestion windows . . . .  31
       A.1.7.  Measuring Reordering Tolerance in Time Units  . . . .  32
     A.2.  Scalable Transport Protocol Optimizations . . . . . . . .  35
       A.2.1.  Setting ECT in TCP Control Packets and
               Retransmissions . . . . . . . . . . . . . . . . . . .  35
       A.2.2.  Faster than Additive Increase . . . . . . . . . . . .  35
       A.2.3.  Faster Convergence at Flow Start  . . . . . . . . . .  36
   Appendix B.  Alternative Identifiers  . . . . . . . . . . . . . .  36
     B.1.  ECT(1) and CE codepoints  . . . . . . . . . . . . . . . .  37
     B.2.  ECN Plus a Diffserv Codepoint (DSCP)  . . . . . . . . . .  39
     B.3.  ECN capability alone  . . . . . . . . . . . . . . . . . .  42
     B.4.  Protocol ID . . . . . . . . . . . . . . . . . . . . . . .  42
     B.5.  Source or destination addressing  . . . . . . . . . . . .  42
     B.6.  Summary: Merits of Alternative Identifiers  . . . . . . .  43
   Appendix C.  Potential Competing Uses for the ECT(1) Codepoint  .  43
     C.1.  Integrity of Congestion Feedback  . . . . . . . . . . . .  43
     C.2.  Notification of Less Severe Congestion than CE  . . . . .  44
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  45







De Schepper & Briscoe  Expires September 10, 2020               [Page 3]


Internet-Draft     ECN Semantics for Low Queuing Delay        March 2020


1.  Introduction

   This specification defines the identifier to be used on IP packets
   for a new network service called low latency, low loss and scalable
   throughput (L4S).  It is similar to the original (or 'Classic')
   Explicit Congestion Notification (ECN [RFC3168]).  RFC 3168 required
   an ECN mark to be equivalent to a drop, both when applied in the
   network and when responded to by a transport.  Unlike Classic ECN
   marking, the network applies L4S marking more immediately and more
   aggressively than drop, and the transport response to each mark is
   reduced and smoothed relative to that for drop.  The two changes
   counterbalance each other so that the throughput of an L4S flow will
   be roughly the same as a non-L4S flow under the same conditions.
   Nonetheless, the much more frequent control signals and the finer
   responses to them result in ultra-low queuing delay without
   compromising link utilization, and this low delay can be maintained
   during high load.  Ultra-low queuing delay means less than 1
   millisecond (ms) on average and less than about 2 ms at the 99th
   percentile.

   An example of a scalable congestion control that would enable the L4S
   service is Data Center TCP (DCTCP), which until now has been
   applicable solely to controlled environments like data centres
   [RFC8257], because it is too aggressive to co-exist with existing
   TCP-Reno-friendly traffic.  The DualQ Coupled AQM, which is defined
   in a complementary experimental specification
   [I-D.ietf-tsvwg-aqm-dualq-coupled], is an AQM framework that enables
   scalable congestion controls like DCTCP to co-exist with existing
   traffic, each getting roughly the same flow rate when they compete
   under similar conditions.  Note that a transport such as DCTCP is
   still not safe to deploy on the Internet unless it satisfies the
   requirements listed in Section 4.

   L4S is not only for elastic (TCP-like) traffic - there are scalable
   congestion controls for real-time media, such as the L4S variant of
   the SCReAM [RFC8298] real-time media congestion avoidance technique
   (RMCAT).  The factor that distinguishes L4S from Classic traffic is
   its behaviour in response to congestion.  The transport wire
   protocol, e.g.  TCP, QUIC, SCTP, DCCP, RTP/RTCP, is orthogonal (and
   therefore not suitable for distinguishing L4S from Classic packets).

   The L4S identifier defined in this document is the key piece that
   distinguishes L4S from 'Classic' (e.g.  Reno-friendly) traffic.  It
   gives an incremental migration path so that suitably modified network
   bottlenecks can distinguish and isolate existing Classic traffic from
   L4S traffic to prevent it from degrading the ultra-low delay and loss
   of the new scalable transports, without harming Classic performance.




De Schepper & Briscoe  Expires September 10, 2020               [Page 4]


Internet-Draft     ECN Semantics for Low Queuing Delay        March 2020


   Initial implementation of the separate parts of the system has been
   motivated by the performance benefits.

1.1.  Latency, Loss and Scaling Problems

   Latency is becoming the critical performance factor for many (most?)
   applications on the public Internet, e.g. interactive Web, Web
   services, voice, conversational video, interactive video, interactive
   remote presence, instant messaging, online gaming, remote desktop,
   cloud-based applications, and video-assisted remote control of
   machinery and industrial processes.  In the 'developed' world,
   further increases in access network bit-rate offer diminishing
   returns, whereas latency is still a multi-faceted problem.  In the
   last decade or so, much has been done to reduce propagation time by
   placing caches or servers closer to users.  However, queuing remains
   a major intermittent component of latency.

   The Diffserv architecture provides Expedited Forwarding [RFC3246], so
   that low latency traffic can jump the queue of other traffic.
   However, on access links dedicated to individual sites (homes, small
   enterprises or mobile devices), often all traffic at any one time
   will be latency-sensitive.  Then, given nothing to differentiate
   from, Diffserv makes no difference.  Instead, we need to remove the
   causes of any unnecessary delay.

   The bufferbloat project has shown that excessively-large buffering
   ('bufferbloat') has been introducing significantly more delay than
   the underlying propagation time.  These delays appear only
   intermittently--only when a capacity-seeking (e.g.  TCP) flow is long
   enough for the queue to fill the buffer, making every packet in other
   flows sharing the buffer sit through the queue.

   Active queue management (AQM) was originally developed to solve this
   problem (and others).  Unlike Diffserv, which gives low latency to
   some traffic at the expense of others, AQM controls latency for _all_
   traffic in a class.  In general, AQM methods introduce an increasing
   level of discard from the buffer the longer the queue persists above
   a shallow threshold.  This gives sufficient signals to capacity-
   seeking (aka. greedy) flows to keep the buffer empty for its intended
   purpose: absorbing bursts.  However, RED [RFC2309] and other
   algorithms from the 1990s were sensitive to their configuration and
   hard to set correctly.  So, this form of AQM was not widely deployed.

   More recent state-of-the-art AQM methods, e.g. fq_CoDel [RFC8290],
   PIE [RFC8033], Adaptive RED [ARED01], are easier to configure,
   because they define the queuing threshold in time not bytes, so it is
   invariant for different link rates.  However, no matter how good the
   AQM, the sawtoothing sending window of a Classic congestion control



De Schepper & Briscoe  Expires September 10, 2020               [Page 5]


Internet-Draft     ECN Semantics for Low Queuing Delay        March 2020


   will either cause queuing delay to vary or cause the link to be
   under-utilized.  Even with a perfectly tuned AQM, the additional
   queuing delay will be of the same order as the underlying speed-of-
   light delay across the network.

   If a sender's own behaviour is introducing queuing delay variation,
   no AQM in the network can "un-vary" the delay without significantly
   compromising link utilization.  Even flow-queuing (e.g.  [RFC8290]),
   which isolates one flow from another, cannot isolate a flow from the
   delay variations it inflicts on itself.  Therefore those applications
   that need to seek out high bandwidth but also need low latency will
   have to migrate to scalable congestion control.

   Altering host behaviour is not enough on its own though.  Even if
   hosts adopt low latency behaviour (scalable congestion controls),
   they need to be isolated from the behaviour of existing Classic
   congestion controls that induce large queue variations.  L4S enables
   that migration by providing latency isolation in the network and
   distinguishing the two types of packets that need to be isolated: L4S
   and Classic.  L4S isolation can be achieved with a queue per flow
   (e.g.  [RFC8290]) but a DualQ [I-D.ietf-tsvwg-aqm-dualq-coupled] is
   sufficient, and actually gives better tail latency.  Both approaches
   are addressed in this document.

   The DualQ solution was developed to make ultra-low latency available
   without requiring per-flow queues at every bottleneck.  This was
   because FQ has well-known downsides - not least the need to inspect
   transport layer headers in the network, which makes it incompatible
   with privacy approaches such as IPSec VPN tunnels, and incompatible
   with link layer queue management, where transport layer headers can
   be hidden, e.g. 5G.

   Latency is not the only concern addressed by L4S: It was known when
   TCP congestion avoidance was first developed that it would not scale
   to high bandwidth-delay products (footnote 6 of Jacobson and Karels
   [TCP-CA]).  Given regular broadband bit-rates over WAN distances are
   already [RFC3649] beyond the scaling range of Reno TCP, 'less
   unscalable' Cubic [RFC8312] and Compound [I-D.sridharan-tcpm-ctcp]
   variants of TCP have been successfully deployed.  However, these are
   now approaching their scaling limits.  Unfortunately, fully scalable
   congestion controls such as DCTCP [RFC8257] cause Classic ECN
   congestion controls sharing the same queue to starve themselves,
   which is why they have been confined to private data centres or
   research testbeds (until now).

   It turns out that a congestion control algorithm like DCTCP that
   solves the latency problem also solves the scalability problem of
   Classic congestion controls.  The finer sawteeth in the congestion



De Schepper & Briscoe  Expires September 10, 2020               [Page 6]


Internet-Draft     ECN Semantics for Low Queuing Delay        March 2020


   window have low amplitude, so they cause very little queuing delay
   variation and the average time to recover from one congestion signal
   to the next (the average duration of each sawtooth) remains
   invariant, which maintains constant tight control as flow-rate
   scales.  A background paper [DCttH15] gives the full explanation of
   why the design solves both the latency and the scaling problems, both
   in plain English and in more precise mathematical form.  The
   explanation is summarised without the maths in the L4S architecture
   document [I-D.ietf-tsvwg-l4s-arch].

1.2.  Terminology

   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
   [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 Congestion Control:  A congestion control behaviour that can
      co-exist with standard TCP Reno [RFC5681] without causing
      significantly negative impact on its flow rate [RFC5033].  With
      Classic congestion controls, as flow rate scales, the number of
      round trips between congestion signals (losses or ECN marks) rises
      with the flow rate.  So it takes longer and longer to recover
      after each congestion event.  Therefore control of queuing and
      utilization becomes very slack, and the slightest disturbance
      prevents a high rate from being attained [RFC3649].

      For instance, with 1500 byte packets and an end-to-end round trip
      time (RTT) of 36 ms, over the years, as Reno flow rate scales from
      2 to 100 Mb/s the number of round trips taken to recover from a
      congestion event rises proportionately, from 4 round trips to 200.
      Cubic [RFC8312] was developed to be less unscalable, but it is
      approaching its scaling limit; with the same RTT of 36ms, at
      100Mb/s it takes about 106 round trips to recover, and at 800 Mb/s
      its recovery time triples to over 340 round trips, or still more
      than 12 seconds (Reno would take 57 seconds).  Cubic only becomes
      significantly better than Reno at high delay and rate
      combinations, for example at 90 ms RTT and 800 Mb/s a Reno flow
      takes 4000 RTTs or 6 minutes to recover, whereas Cubic 'only'
      needs 188 RTTs, which is still 17 seconds (double its recovery
      time at 100Mb/s).

   Scalable Congestion Control:  A congestion control where the average
      time from one congestion signal to the next (the recovery time)
      remains invariant as the flow rate scales, all other factors being
      equal.  This maintains the same degree of control over queueing



De Schepper & Briscoe  Expires September 10, 2020               [Page 7]


Internet-Draft     ECN Semantics for Low Queuing Delay        March 2020


      and utilization whatever the flow rate, as well as ensuring that
      high throughput is robust to disturbances.  For instance, DCTCP
      averages 2 congestion signals per round-trip whatever the flow
      rate.  See Section 4.3 for more explanation.

   Classic service:  The Classic service is intended for all the
      congestion control behaviours that co-exist with Reno [RFC5681]
      (e.g.  Reno itself, Cubic [RFC8312], Compound
      [I-D.sridharan-tcpm-ctcp], TFRC [RFC5348]).  The term 'Classic
      queue' means a queue providing the Classic service.

   Low-Latency, Low-Loss Scalable throughput (L4S) service:  The 'L4S'
      service is intended for traffic from scalable congestion control
      algorithms, such as Data Center TCP [RFC8257].  The L4S service is
      for more general traffic than just DCTCP--it allows the set of
      congestion controls with similar scaling properties to DCTCP to
      evolve (e.g.  Relentless TCP [Mathis09], TCP Prague [PragueLinux]
      and the L4S variant of SCREAM for real-time media [RFC8298]).  The
      term 'L4S queue' means a queue providing the L4S service.

      The terms Classic or L4S can also qualify other nouns, such as
      'queue', 'codepoint', 'identifier', 'classification', 'packet',
      'flow'.  For example: an L4S packet means a packet with an L4S
      identifier sent from an L4S congestion control.

      Both Classic and L4S services can cope with a proportion of
      unresponsive or less-responsive traffic as well, as long as it
      does not build a queue (e.g.  DNS, VoIP, game sync datagrams,
      etc).

   Reno-friendly:  The subset of Classic traffic that excludes
      unresponsive traffic and excludes experimental congestion controls
      intended to coexist with Reno but without always being strictly
      friendly to Reno (as allowed by [RFC5033]).  Reno-friendly is used
      in place of 'TCP-friendly', given that the TCP protocol is used
      with many different congestion control behaviours.

   Classic ECN:  The original Explicit Congestion Notification (ECN)
      protocol [RFC3168], which requires ECN signals to be treated the
      same as drops, both when generated in the network and when
      responded to by the sender.  The names used for the four
      codepoints of the 2-bit IP-ECN field are as defined in [RFC3168]:
      Not ECT, ECT(0), ECT(1) and CE, where ECT stands for ECN-Capable
      Transport and CE stands for Congestion Experienced.







De Schepper & Briscoe  Expires September 10, 2020               [Page 8]


Internet-Draft     ECN Semantics for Low Queuing Delay        March 2020


1.3.  Scope

   The new L4S identifier defined in this specification is applicable
   for IPv4 and IPv6 packets (as for Classic ECN [RFC3168]).  It is
   applicable for the unicast, multicast and anycast forwarding modes.

   The L4S identifier is an orthogonal packet classification to the
   Differentiated Services Code Point (DSCP) [RFC2474].  Section 5.4
   explains what this means in practice.

   This document is intended for experimental status, so it does not
   update any standards track RFCs.  Therefore it depends on [RFC8311],
   which is a standards track specification that:

   o  updates the ECN proposed standard [RFC3168] to allow experimental
      track RFCs to relax the requirement that an ECN mark must be
      equivalent to a drop (when the network applies markings and/or
      when the sender responds to them);

   o  changes the status of the experimental ECN nonce [RFC3540] to
      historic;

   o  makes consequent updates to the following additional proposed
      standard RFCs to reflect the above two bullets:

      *  ECN for RTP [RFC6679];

      *  the congestion control specifications of various DCCP
         congestion control identifier (CCID) profiles [RFC4341],
         [RFC4342], [RFC5622].

   This document is about identifiers that are used for interoperation
   between hosts and networks.  So the audience is broad, covering
   developers of host transports and network AQMs, as well as covering
   how operators might wish to combine various identifiers, which would
   require flexibility from equipment developers.

2.  Consensus Choice of L4S Packet Identifier: Requirements

   This subsection briefly records the process that led to a consensus
   choice of L4S identifier, selected from all the alternatives in
   Appendix B.

   The identifier for packets using the Low Latency, Low Loss, Scalable
   throughput (L4S) service needs to meet the following requirements:






De Schepper & Briscoe  Expires September 10, 2020               [Page 9]


Internet-Draft     ECN Semantics for Low Queuing Delay        March 2020


   o  it SHOULD survive end-to-end between source and destination
      applications: across the boundary between host and network,
      between interconnected networks, and through middleboxes;

   o  it SHOULD be visible at the IP layer

   o  it SHOULD be common to IPv4 and IPv6 and transport-agnostic;

   o  it SHOULD be incrementally deployable;

   o  it SHOULD enable an AQM to classify packets encapsulated by outer
      IP or lower-layer headers;

   o  it SHOULD consume minimal extra codepoints;

   o  it SHOULD be consistent on all the packets of a transport layer
      flow, so that some packets of a flow are not served by a different
      queue to others.

   Whether the identifier would be recoverable if the experiment failed
   is a factor that could be taken into account.  However, this has not
   been made a requirement, because that would favour schemes that would
   be easier to fail, rather than those more likely to succeed.

   It is recognised that the chosen identifier is unlikely to satisfy
   all these requirements, particularly given the limited space left in
   the IP header.  Therefore a compromise will be necessary, which is
   why all the above requirements are expressed with the word 'SHOULD'
   not 'MUST'.  Appendix B discusses the pros and cons of the
   compromises made in various competing identification schemes against
   the above requirements.

   On the basis of this analysis, "ECT(1) and CE codepoints" is the best
   compromise.  Therefore this scheme is defined in detail in the
   following sections, while Appendix B records the rationale for this
   decision.

3.  L4S Packet Identification at Run-Time

   The L4S treatment is an experimental track alternative packet marking
   treatment [RFC4774] to the Classic ECN treatment in [RFC3168], which
   has been updated by [RFC8311] to allow experiments such as the one
   defined in the present specification.  Like Classic ECN, L4S ECN
   identifies both network and host behaviour: it identifies the marking
   treatment that network nodes are expected to apply to L4S packets,
   and it identifies packets that have been sent from hosts that are
   expected to comply with a broad type of sending behaviour.




De Schepper & Briscoe  Expires September 10, 2020              [Page 10]


Internet-Draft     ECN Semantics for Low Queuing Delay        March 2020


   For a packet to receive L4S treatment as it is forwarded, the sender
   sets the ECN field in the IP header to the ECT(1) codepoint.  See
   Section 4 for full transport layer behaviour requirements, including
   feedback and congestion response.

   A network node that implements the L4S service normally classifies
   arriving ECT(1) and CE packets for L4S treatment.  See Section 5 for
   full network element behaviour requirements, including
   classification, ECN-marking and interaction of the L4S identifier
   with other identifiers and per-hop behaviours.

4.  Prerequisite Transport Layer Behaviour

4.1.  Prerequisite Codepoint Setting

   A sender that wishes a packet to receive L4S treatment as it is
   forwarded, MUST set the ECN field in the IP header (v4 or v6) to the
   ECT(1) codepoint.

4.2.  Prerequisite Transport Feedback

   For a transport protocol to provide scalable congestion control it
   MUST provide feedback of the extent of CE marking on the forward
   path.  When ECN was added to TCP [RFC3168], the feedback method
   reported no more than one CE mark per round trip.  Some transport
   protocols derived from TCP mimic this behaviour while others report
   the accurate extent of ECN marking.  This means that some transport
   protocols will need to be updated as a prerequisite for scalable
   congestion control.  The position for a few well-known transport
   protocols is given below.

   TCP:  Support for the accurate ECN feedback requirements [RFC7560]
      (such as that provided by AccECN [I-D.ietf-tcpm-accurate-ecn]) by
      both ends is a prerequisite for scalable congestion control in
      TCP.  Therefore, the presence of ECT(1) in the IP headers even in
      one direction of a TCP connection will imply that both ends must
      be supporting accurate ECN feedback.  However, the converse does
      not apply.  So even if both ends support AccECN, either of the two
      ends can choose not to use a scalable congestion control, whatever
      the other end's choice.

   SCTP:  A suitable ECN feedback mechanism for SCTP could add a chunk
      to report the number of received CE marks (e.g.
      [I-D.stewart-tsvwg-sctpecn]), and update the ECN feedback protocol
      sketched out in Appendix A of the standards track specification of
      SCTP [RFC4960].





De Schepper & Briscoe  Expires September 10, 2020              [Page 11]


Internet-Draft     ECN Semantics for Low Queuing Delay        March 2020


   RTP over UDP:  A prerequisite for scalable congestion control is for
      both (all) ends of one media-level hop to signal ECN support
      [RFC6679] and use the new generic RTCP feedback format of
      [I-D.ietf-avtcore-cc-feedback-message].  The presence of ECT(1)
      implies that both (all) ends of that media-level hop support ECN.
      However, the converse does not apply.  So each end of a media-
      level hop can independently choose not to use a scalable
      congestion control, even if both ends support ECN.

   QUIC:  Support for sufficiently fine-grained ECN feedback is provided
      by the first IETF QUIC transport [I-D.ietf-quic-transport].

   DCCP:  The ACK vector in DCCP [RFC4340] is already sufficient to
      report the extent of CE marking as needed by a scalable congestion
      control.

4.3.  Prerequisite Congestion Response

   As a condition for a host to send packets with the L4S identifier
   (ECT(1)), it SHOULD implement a congestion control behaviour that
   ensures that, in steady state, the average time from one ECN
   congestion signal to the next (the 'recovery time') does not increase
   as flow rate scales, all other factors being equal.  This is termed a
   scalable congestion control.  This is necessary to ensure that queue
   variations remain small as flow rate scales, without having to
   sacrifice utilization.  For instance, for DCTCP, the average recovery
   time is always half a round trip, whatever the flow rate.

   The condition 'all other factors being equal', allows the recovery
   time to be different for different round trip times, as long as it
   does not increase with flow rate for any particular RTT.

   Saying that the recovery time remains roughly invariant is equivalent
   to saying that the number of ECN CE marks per round trip remains
   invariant as flow rate scales, all other factors being equal.  For
   instance, DCTCP's average recovery time of half of 1 RTT is
   equivalent to 2 ECN marks per round trip.  For those who understand
   steady-state congestion response functions, it is also equivalent to
   say that, the congestion window is inversely proportional to the
   proportion of bytes in packets marked with the CE codepoint (see
   section 2 of [PI2]).

   As well as DCTCP, TCP Prague [PragueLinux] and the L4S variant of
   SCReAM [RFC8298] are examples of scalable congestion controls.

   As with all transport behaviours, a detailed specification (probably
   an experimental RFC) will need to be defined for each congestion
   control, following the guidelines for specifying new congestion



De Schepper & Briscoe  Expires September 10, 2020              [Page 12]


Internet-Draft     ECN Semantics for Low Queuing Delay        March 2020


   control algorithms in [RFC5033].  In addition it will need to
   document these L4S-specific matters, specifically the timescale over
   which the proportionality is averaged, and control of burstiness.
   The recovery time requirement above is worded as a 'SHOULD' rather
   than a 'MUST' to allow reasonable flexibility when defining these
   specifications.

   In order to coexist safely with other Internet traffic, a scalable
   congestion control MUST NOT tag its packets with the ECT(1) codepoint
   unless it complies with the following bulleted requirements.  The
   specification of a particular scalable congestion control MUST
   describe in detail how it satisfies each requirement and, for any
   non-mandatory requirements, it MUST justify why it does not comply:

   o  As well as responding to ECN markings, a scalable congestion
      control MUST react to packet loss in a way that will coexist
      safely with a TCP Reno congestion control [RFC5681] (see
      Appendix A.1.3 for rationale).

   o  A scalable congestion control MUST react to ECN marking from a
      non-L4S but ECN-capable bottleneck in a way that will coexist with
      a TCP Reno congestion control [RFC5681] (see Appendix A.1.4 for
      rationale).

      Note that a scalable congestion control is not expected to change
      to setting ECT(0) while it falls back to coexist with Reno.

   o  A scalable congestion control MUST reduce or eliminate RTT bias
      over as wide a range of RTTs as possible, or at least over the
      typical range of RTTs that will interact in the intended
      deployment scenario (see Appendix A.1.5 for rationale).

   o  A scalable congestion control SHOULD remain responsive to
      congestion when typical RTTs over the public Internet are
      significantly smaller because they are no longer inflated by
      queuing delay (see Appendix A.1.6 for rationale).

   o  A scalable congestion control intended for reordering-prone
      networks SHOULD detect loss by counting in time-based units, which
      is scalable, as opposed to counting in units of packets (as in the
      3 DupACK rule of RFC 5681 TCP), which is not scalable (see
      Appendix A.1.7 for rationale).  This requirement is scoped to
      'reordering-prone networks' in order to exclude congestion
      controls that are solely used in controlled environments where the
      network introduces hardly any reordering.

   Each sender in a session can use a scalable congestion control
   independently of the congestion control used by the receiver(s) when



De Schepper & Briscoe  Expires September 10, 2020              [Page 13]


Internet-Draft     ECN Semantics for Low Queuing Delay        March 2020


   they send data.  Therefore there might be ECT(1) packets in one
   direction and ECT(0) or Not-ECT in the other.

   As well as traffic controlled by a scalable congestion control, a
   reasonable level of smooth unresponsive traffic at a low rate
   relative to typical broadband capacities is likely to be acceptable
   (see "'Safe' Unresponsive Traffic" in Section 5.4.1.1.1).

5.  Prerequisite Network Node Behaviour

5.1.  Prerequisite Classification and Re-Marking Behaviour

   A network node that implements the L4S service MUST classify arriving
   ECT(1) packets for L4S treatment and, other than in the exceptional
   case referred to next, it MUST classify arriving CE packets for L4S
   treatment as well.  CE packets might have originated as ECT(1) or
   ECT(0), but the above rule to classify them as if they originated as
   ECT(1) is the safe choice (see Appendix B.1 for rationale).  The
   exception is where some flow-aware in-network mechanism happens to be
   available for distinguishing CE packets that originated as ECT(0), as
   described in Section 5.3, but there is no implication that such a
   mechanism is necessary.

   An L4S AQM treatment follows similar codepoint transition rules to
   those in RFC 3168.  Specifically, the ECT(1) codepoint MUST NOT be
   changed to any other codepoint than CE, and CE MUST NOT be changed to
   any other codepoint.  An ECT(1) packet is classified as ECN-capable
   and, if congestion increases, an L4S AQM algorithm will increasingly
   mark the ECN field as CE, otherwise forwarding packets unchanged as
   ECT(1).  Necessary conditions for an L4S marking treatment are
   defined in Section 5.2.  Under persistent overload an L4S marking
   treatment SHOULD turn off ECN marking, using drop as a congestion
   signal until the overload episode has subsided, as recommended for
   all AQM methods in [RFC7567] (Section 4.2.1), which follows the
   similar advice in RFC 3168 (Section 7).

   For backward compatibility in uncontrolled environments, a network
   node that implements the L4S treatment MUST also implement an AQM
   treatment for the Classic service as defined in Section 1.2.  This
   Classic AQM treatment need not mark ECT(0) packets, but if it does,
   it will do so under the same conditions as it would drop Not-ECT
   packets [RFC3168].  It MUST classify arriving ECT(0) and Not-ECT
   packets for treatment by the Classic AQM (see the discussion of the
   classifier for the dual-queue coupled AQM in
   [I-D.ietf-tsvwg-aqm-dualq-coupled]).






De Schepper & Briscoe  Expires September 10, 2020              [Page 14]


Internet-Draft     ECN Semantics for Low Queuing Delay        March 2020


5.2.  The Meaning of L4S CE Relative to Drop

   The likelihood that an AQM drops a Not-ECT Classic packet (p_C) MUST
   be roughly proportional to the square of the likelihood that it would
   have marked it if it had been an L4S packet (p_L).  That is

      p_C ~= (p_L / k)^2

   The constant of proportionality (k) does not have to be standardised
   for interoperability, but a value of 2 is RECOMMENDED.  The term
   'likelihood' is used above to allow for marking and dropping to be
   either probabilistic or deterministic.

   This formula ensures that Scalable and Classic flows will converge to
   roughly equal congestion windows, for the worst case of Reno
   congestion control.  This is because the congestion windows of
   Scalable and Classic congestion controls are inversely proportional
   to p_L and sqrt(p_C) respectively.  So squaring p_C in the above
   formula counterbalances the square root that characterizes Reno-
   friendly flows.

   [I-D.ietf-tsvwg-aqm-dualq-coupled] specifies the essential aspects of
   an L4S AQM, as well as recommending other aspects.  It gives example
   implementations in appendices.

   Note that, contrary to RFC 3168, a Coupled Dual Queue AQM
   implementing the L4S and Classic treatments does not mark an ECT(1)
   packet under the same conditions that it would have dropped a Not-ECT
   packet, as allowed by [RFC8311], which updates RFC 3168.  However, if
   it marks ECT(0) packets, it does so under the same conditions that it
   would have dropped a Not-ECT packet.

5.3.  Exception for L4S Packet Identification by Network Nodes with
      Transport-Layer Awareness

   To implement the L4S treatment, a network node does not need to
   identify transport-layer flows.  Nonetheless, if an implementer is
   willing to identify transport-layer flows at a network node, and if
   the most recent ECT packet in the same flow was ECT(0), the node MAY
   classify CE packets for Classic ECN [RFC3168] treatment.  In all
   other cases, a network node MUST classify all CE packets for L4S
   treatment.  Examples of such other cases are: i) if no ECT packets
   have yet been identified in a flow; ii) if it is not desirable for a
   network node to identify transport-layer flows; or iii) if the most
   recent ECT packet in a flow was ECT(1).

   If an implementer uses flow-awareness to classify CE packets, to
   determine whether the flow is using ECT(0) or ECT(1) it only uses the



De Schepper & Briscoe  Expires September 10, 2020              [Page 15]


Internet-Draft     ECN Semantics for Low Queuing Delay        March 2020


   most recent ECT packet of a flow (this advice will need to be
   verified as part of L4S experiments).  This is because a sender might
   switch from sending ECT(1) (L4S) packets to sending ECT(0) (Classic
   ECN) packets, or back again, in the middle of a transport-layer flow
   (e.g.  it might manually switch its congestion control module mid-
   connection, or it might be deliberately attempting to confuse the
   network).

5.4.  Interaction of the L4S Identifier with other Identifiers

   The examples in this section concern how additional identifiers might
   complement the L4S identifier to classify packets between class-based
   queues.  Firstly considering two queues, L4S and Classic, as in the
   Coupled DualQ AQM [I-D.ietf-tsvwg-aqm-dualq-coupled], then more
   complex structures within a larger queuing hierarchy.

5.4.1.  DualQ Examples of Other Identifiers Complementing L4S
        Identifiers

5.4.1.1.  Inclusion of Additional Traffic with L4S

   In a typical case for the public Internet a network element that
   implements L4S might want to classify some low-rate but unresponsive
   traffic (e.g.  DNS, LDAP, NTP, voice, game sync packets) into the low
   latency queue to mix with L4S traffic.  Such non-ECN-based packet
   types MUST be safe to mix with L4S traffic without harming the low
   latency service, where 'safe' is explained in Section 5.4.1.1.1
   below.

   In this case it would not be appropriate to call the queue an L4S
   queue, because it is shared by L4S and non-L4S traffic.  Instead it
   will be called the low latency or L queue.  The L queue then offers
   two different treatments:

   o  The L4S treatment, which is a combination of the L4S AQM treatment
      and a priority scheduling treatment;

   o  The low latency treatment, which is solely the priority scheduling
      treatment, without ECN-marking by the AQM.

   To identify packets for just the scheduling treatment, it would be
   inappropriate to use the L4S ECT(1) identifier, because such traffic
   is unresponsive to ECN marking.  Therefore, a network element that
   implements L4S MAY classify additional packets into the L queue if
   they carry certain non-ECN identifiers.  For instance:






De Schepper & Briscoe  Expires September 10, 2020              [Page 16]


Internet-Draft     ECN Semantics for Low Queuing Delay        March 2020


   o  addresses of specific applications or hosts configured to be safe
      (or perhaps they comply with L4S behaviour and can respond to ECN
      feedback, but perhaps cannot set the ECN field for some reason);

   o  certain protocols that are usually lightweight (e.g.  ARP, DNS);

   o  specific Diffserv codepoints that indicate traffic with limited
      burstiness such as the EF (Expedited Forwarding [RFC3246]), Voice-
      Admit [RFC5865] or proposed NQB (Non-Queue-Building
      [I-D.ietf-tsvwg-nqb]) service classes or equivalent local-use
      DSCPs (see [I-D.briscoe-tsvwg-l4s-diffserv]).

   Of course, a packet that carried both the ECT(1) codepoint and a non-
   ECN identifier associated with the L queue would be classified into
   the L queue.

   For clarity, non-ECN identifiers, such as the examples itemized
   above, might be used by some network operators who believe they
   identify non-L4S traffic that would be safe to mix with L4S traffic.
   They are not alternative ways for a host to indicate that it is
   sending L4S packets.  Only the ECT(1) ECN codepoint indicates to a
   network element that a host is sending L4S packets (and CE indicates
   that it could have originated as ECT(1)).  Specifically ECT(1)
   indicates that the host claims its behaviour satisfies the
   prerequisite transport requirements in Section 4.

   To include additional traffic with L4S, a network element only reads
   identifiers such as those itemized above.  It MUST NOT alter these
   non-ECN identifiers, so that they survive for any potential use later
   on the network path.

5.4.1.1.1.  'Safe' Unresponsive Traffic

   The above section requires unresponsive traffic to be 'safe' to mix
   with L4S traffic.  Ideally this means that the sender never sends any
   sequence of packets at a rate that exceeds the available capacity of
   the bottleneck link.  However, typically an unresponsive transport
   does not even know the bottleneck capacity of the path, let alone its
   available capacity.  Nonetheless, an application can be considered
   safe enough if it paces packets out (not necessarily completely
   regularly) such that its maximum instantaneous rate from packet to
   packet stays well below a typical broadband access rate.

   This is a vague but useful definition, because many low latency
   applications of interest, such as DNS, voice, game sync packets, RPC,
   ACKs, keep-alives, could match this description.





De Schepper & Briscoe  Expires September 10, 2020              [Page 17]


Internet-Draft     ECN Semantics for Low Queuing Delay        March 2020


5.4.1.2.  Exclusion of Traffic From L4S Treatment

   To extend the above example, an operator might want to exclude some
   traffic from the L4S treatment for a policy reason, e.g.  security
   (traffic from malicious sources) or commercial (e.g.  initially the
   operator may wish to confine the benefits of L4S to business
   customers).

   In this exclusion case, the operator MUST classify on the relevant
   locally-used identifiers (e.g. source addresses) before classifying
   the non-matching traffic on the end-to-end L4S ECN identifier.

   The operator MUST NOT alter the end-to-end L4S ECN identifier from
   L4S to Classic, because its decision to exclude certain traffic from
   L4S treatment is local-only.  The end-to-end L4S identifier then
   survives for other operators to use, or indeed, they can apply their
   own policy, independently based on their own choice of locally-used
   identifiers.  This approach also allows any operator to remove its
   locally-applied exclusions in future, e.g.  if it wishes to widen the
   benefit of the L4S treatment to all its customers.

5.4.1.3.  Generalized Combination of L4S and Other Identifiers

   L4S concerns low latency, which it can provide for all traffic
   without differentiation and without affecting bandwidth allocation.
   Diffserv provides for differentiation of both bandwidth and low
   latency, but its control of latency depends on its control of
   bandwidth.  The two can be combined if a network operator wants to
   control bandwidth allocation but it also wants to provide low latency
   - for any amount of traffic within one of these allocations of
   bandwidth (rather than only providing low latency by limiting
   bandwidth) [I-D.briscoe-tsvwg-l4s-diffserv].

   The DualQ examples so far have been framed in the context of
   providing the default Best Efforts Per-Hop Behaviour (PHB) using two
   queues - a Low Latency (L) queue and a Classic (C) Queue.  This
   single DualQ structure is expected to be the most common and useful
   arrangement.  But, more generally, an operator might choose to
   control bandwidth allocation through a hierarchy of Diffserv PHBs at
   a node, and to offer one (or more) of these PHBs with a low latency
   and a Classic variant.

   In the first case, if we assume that there are no other PHBs except
   the DualQ, if a packet carries ECT(1) or CE, a network element would
   classify it for the L4S treatment irrespective of its DSCP.  And, if
   a packet carried (say) the EF DSCP, the network element could
   classify it into the L queue irrespective of its ECN codepoint.
   However, where the DualQ is in a hierarchy of other PHBs, the



De Schepper & Briscoe  Expires September 10, 2020              [Page 18]


Internet-Draft     ECN Semantics for Low Queuing Delay        March 2020


   classifier would classify some traffic into other PHBs based on DSCP
   before classifying between the low latency and Classic queues (based
   on ECT(1), CE and perhaps also the EF DSCP or other identifiers as in
   the above example).  [I-D.briscoe-tsvwg-l4s-diffserv] gives a number
   of examples of such arrangements to address various requirements.

   [I-D.briscoe-tsvwg-l4s-diffserv] describes how an operator might use
   L4S to offer low latency for all L4S traffic as well as using
   Diffserv for bandwidth differentiation.  It identifies two main types
   of approach, which can be combined: the operator might split certain
   Diffserv PHBs between L4S and a corresponding Classic service.  Or it
   might split the L4S and/or the Classic service into multiple Diffserv
   PHBs.  In either of these cases, a packet would have to be classified
   on its Diffserv and ECN codepoints.

   In summary, there are numerous ways in which the L4S ECN identifier
   (ECT(1) and CE) could be combined with other identifiers to achieve
   particular objectives.  The following categorization articulates
   those that are valid, but it is not necessarily exhaustive.  Those
   tagged 'Recommended-standard-use' could be set by the sending host or
   a network.  Those tagged 'Local-use' would only be set by a network:

   1.  Identifiers Complementing the L4S Identifier

       A.  Including More Traffic in the L Queue
           (Could use Recommended-standard-use or Local-use identifiers)

       B.  Excluding Certain Traffic from the L Queue
           (Local-use only)

   2.  Identifiers to place L4S classification in a PHB Hierarchy
       (Could use Recommended-standard-use or Local-use identifiers)

       A.  PHBs Before L4S ECN Classification

       B.  PHBs After L4S ECN Classification

5.4.2.  Per-Flow Queuing Examples of Other Identifiers Complementing L4S
        Identifiers

   At a node with per-flow queueing (e.g.  FQ-CoDel [RFC8290]), the L4S
   identifier could complement the Layer-4 flow ID as a further level of
   flow granularity (i.e.  Not-ECT and ECT(0) queued separately from
   ECT(1) and CE packets).  "Risk of reordering Classic CE packets" in
   Appendix B.1 discusses the resulting ambiguity if packets originally
   marked ECT(0) are marked CE by an upstream AQM before they arrive at
   a node that classifies CE as L4S.  It argues that the risk of re-




De Schepper & Briscoe  Expires September 10, 2020              [Page 19]


Internet-Draft     ECN Semantics for Low Queuing Delay        March 2020


   ordering is vanishingly small and the consequence of such a low level
   of re-ordering is minimal.

   Alternatively, it could be assumed that it is not in a flow's own
   interest to mix Classic and L4S identifiers.  Then the AQM could use
   the ECN field to switch itself between a Classic and an L4S AQM
   behaviour within one per-flow queue.  For instance, for ECN-capable
   packets, the AQM might consist of a simple marking threshold and an
   L4S ECN identifier might simply select a shallower threshold than a
   Classic ECN identifier would.

6.  L4S Experiments

   [I-D.ietf-tsvwg-aqm-dualq-coupled] sets operational and management
   requirements for experiments with DualQ Coupled AQMs.  General
   operational and management requirements for experiments with L4S
   congestion controls are given in Section 4 and Section 5 above, e.g.
   co-existence and scaling requirements, incremental deployment
   arrangements.

   The specification of each scalable congestion control will need to
   include protocol-specific requirements for configuration and
   monitoring performance during experiments.  Appendix A of [RFC5706]
   provides a helpful checklist.

   Monitoring for harm to other traffic, specifically bandwidth
   starvation or excess queuing delay, will need to be conducted
   alongside all early L4S experiments.  It is hard, if not impossible,
   for an individual flow to measure its impact on other traffic.  So
   such monitoring will need to be conducted using bespoke monitoring
   across flows and/or across classes of traffic.

7.  IANA Considerations

   This specification contains no IANA considerations.

8.  Security Considerations

   Approaches to assure the integrity of signals using the new identifer
   are introduced in Appendix C.1.  See the security considerations in
   the L4S architecture [I-D.ietf-tsvwg-l4s-arch] for further discussion
   of mis-use of the identifier.

   The recommendation to detect loss in time units prevents the ACK-
   splitting attacks described in [Savage-TCP].






De Schepper & Briscoe  Expires September 10, 2020              [Page 20]


Internet-Draft     ECN Semantics for Low Queuing Delay        March 2020


9.  Acknowledgements

   Thanks to Richard Scheffenegger, John Leslie, David Taeht, Jonathan
   Morton, Gorry Fairhurst, Michael Welzl, Mikael Abrahamsson and Andrew
   McGregor for the discussions that led to this specification.  Ing-jyh
   (Inton) Tsang was a contributor to the early drafts of this document.
   And thanks to Mikael Abrahamsson, Lloyd Wood, Nicolas Kuhn, Greg
   White, Tom Henderson, David Black, Gorry Fairhurst, Brian Carpenter,
   Jake Holland, Rod Grimes and Richard Scheffenegger for providing help
   and reviewing this draft and to Ingemar Johansson for reviewing and
   providing substantial text.  Appendix A listing the Prague L4S
   Requirements is based on text authored by Marcelo Bagnulo Braun that
   was originally an appendix to [I-D.ietf-tsvwg-l4s-arch].  That text
   was in turn based on the collective output of the attendees listed in
   the minutes of a 'bar BoF' on DCTCP Evolution during IETF-94
   [TCPPrague].

   The authors' contributions were part-funded by the European Community
   under its Seventh Framework Programme through the Reducing Internet
   Transport Latency (RITE) project (ICT-317700).  Bob Briscoe was also
   funded partly by the Research Council of Norway through the TimeIn
   project, partly by CableLabs and partly by the Comcast Innovation
   Fund.  The views expressed here are solely those of the authors.

10.  References

10.1.  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>.

   [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>.

   [RFC4774]  Floyd, S., "Specifying Alternate Semantics for the
              Explicit Congestion Notification (ECN) Field", BCP 124,
              RFC 4774, DOI 10.17487/RFC4774, November 2006,
              <https://www.rfc-editor.org/info/rfc4774>.

   [RFC6679]  Westerlund, M., Johansson, I., Perkins, C., O'Hanlon, P.,
              and K. Carlberg, "Explicit Congestion Notification (ECN)
              for RTP over UDP", RFC 6679, DOI 10.17487/RFC6679, August
              2012, <https://www.rfc-editor.org/info/rfc6679>.




De Schepper & Briscoe  Expires September 10, 2020              [Page 21]


Internet-Draft     ECN Semantics for Low Queuing Delay        March 2020


10.2.  Informative References

   [A2DTCP]   Zhang, T., Wang, J., Huang, J., Huang, Y., Chen, J., and
              Y. Pan, "Adaptive-Acceleration Data Center TCP", IEEE
              Transactions on Computers 64(6):1522-1533, June 2015,
              <http://ieeexplore.ieee.org/xpl/
              articleDetails.jsp?arnumber=6871352>.

   [Ahmed19]  Ahmed, A., "Extending TCP for Low Round Trip Delay",
              Masters Thesis, Uni Oslo , August 2019,
              <https://www.duo.uio.no/handle/10852/70966>.

   [Alizadeh-stability]
              Alizadeh, M., Javanmard, A., and B. Prabhakar, "Analysis
              of DCTCP: Stability, Convergence, and Fairness", ACM
              SIGMETRICS 2011 , June 2011.

   [ARED01]   Floyd, S., Gummadi, R., and S. Shenker, "Adaptive RED: An
              Algorithm for Increasing the Robustness of RED's Active
              Queue Management", ACIRI Technical Report , August 2001,
              <http://www.icir.org/floyd/red.html>.

   [DCttH15]  De Schepper, K., Bondarenko, O., Briscoe, B., and I.
              Tsang, "'Data Centre to the Home': Ultra-Low Latency for
              All", RITE Project Technical Report , 2015,
              <http://riteproject.eu/publications/>.

   [I-D.briscoe-tsvwg-l4s-diffserv]
              Briscoe, B., "Interactions between Low Latency, Low Loss,
              Scalable Throughput (L4S) and Differentiated Services",
              draft-briscoe-tsvwg-l4s-diffserv-02 (work in progress),
              November 2018.

   [I-D.ietf-avtcore-cc-feedback-message]
              Sarker, Z., Perkins, C., Singh, V., and M. Ramalho, "RTP
              Control Protocol (RTCP) Feedback for Congestion Control",
              draft-ietf-avtcore-cc-feedback-message-05 (work in
              progress), November 2019.

   [I-D.ietf-quic-transport]
              Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
              and Secure Transport", draft-ietf-quic-transport-27 (work
              in progress), February 2020.

   [I-D.ietf-tcpm-accurate-ecn]
              Briscoe, B., Kuehlewind, M., and R. Scheffenegger, "More
              Accurate ECN Feedback in TCP", draft-ietf-tcpm-accurate-
              ecn-11 (work in progress), March 2020.



De Schepper & Briscoe  Expires September 10, 2020              [Page 22]


Internet-Draft     ECN Semantics for Low Queuing Delay        March 2020


   [I-D.ietf-tcpm-generalized-ecn]
              Bagnulo, M. and B. Briscoe, "ECN++: Adding Explicit
              Congestion Notification (ECN) to TCP Control Packets",
              draft-ietf-tcpm-generalized-ecn-05 (work in progress),
              November 2019.

   [I-D.ietf-tcpm-rack]
              Cheng, Y., Cardwell, N., Dukkipati, N., and P. Jha, "RACK:
              a time-based fast loss detection algorithm for TCP",
              draft-ietf-tcpm-rack-07 (work in progress), January 2020.

   [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-10 (work in
              progress), July 2019.

   [I-D.ietf-tsvwg-ecn-encap-guidelines]
              Briscoe, B., Kaippallimalil, J., and P. Thaler,
              "Guidelines for Adding Congestion Notification to
              Protocols that Encapsulate IP", draft-ietf-tsvwg-ecn-
              encap-guidelines-13 (work in progress), May 2019.

   [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-05 (work
              in progress), February 2020.

   [I-D.ietf-tsvwg-nqb]
              White, G. and T. Fossati, "A Non-Queue-Building Per-Hop
              Behavior (NQB PHB) for Differentiated Services", draft-
              ietf-tsvwg-nqb-00 (work in progress), November 2019.

   [I-D.sridharan-tcpm-ctcp]
              Sridharan, M., Tan, K., Bansal, D., and D. Thaler,
              "Compound TCP: A New TCP Congestion Control for High-Speed
              and Long Distance Networks", draft-sridharan-tcpm-ctcp-02
              (work in progress), November 2008.

   [I-D.stewart-tsvwg-sctpecn]
              Stewart, R., Tuexen, M., and X. Dong, "ECN for Stream
              Control Transmission Protocol (SCTP)", draft-stewart-
              tsvwg-sctpecn-05 (work in progress), January 2014.







De Schepper & Briscoe  Expires September 10, 2020              [Page 23]


Internet-Draft     ECN Semantics for Low Queuing Delay        March 2020


   [LinuxPacedChirping]
              Misund, J. and B. Briscoe, "Paced Chirping - Rethinking
              TCP start-up", Proc. Linux Netdev 0x13 , March 2019,
              <https://www.netdevconf.org/0x13/session.html?talk-chirp>.

   [Mathis09]
              Mathis, M., "Relentless Congestion Control", PFLDNeT'09 ,
              May 2009, <http://www.hpcc.jp/pfldnet2009/
              Program_files/1569198525.pdf>.

   [Paced-Chirping]
              Misund, J., "Rapid Acceleration in TCP Prague", Masters
              Thesis , May 2018,
              <https://riteproject.files.wordpress.com/2018/07/
              misundjoakimmastersthesissubmitted180515.pdf>.

   [PI2]      De Schepper, K., Bondarenko, O., Tsang, I., and B.
              Briscoe, "PI^2 : A Linearized AQM for both Classic and
              Scalable TCP", Proc. ACM CoNEXT 2016 pp.105-119, December
              2016,
              <http://dl.acm.org/citation.cfm?doid=2999572.2999578>.

   [PragueLinux]
              Briscoe, B., De Schepper, K., Albisser, O., Misund, J.,
              Tilmans, O., Kuehlewind, M., and A. Ahmed, "Implementing
              the `TCP Prague' Requirements for Low Latency Low Loss
              Scalable Throughput (L4S)", Proc. Linux Netdev 0x13 ,
              March 2019, <https://www.netdevconf.org/0x13/
              session.html?talk-tcp-prague-l4s>.

   [QV]       Briscoe, B. and P. Hurtig, "Up to Speed with Queue View",
              RITE Technical Report D2.3; Appendix C.2, August 2015,
              <https://riteproject.files.wordpress.com/2015/12/rite-
              deliverable-2-3.pdf>.

   [RFC2309]  Braden, B., Clark, D., Crowcroft, J., Davie, B., Deering,
              S., Estrin, D., Floyd, S., Jacobson, V., Minshall, G.,
              Partridge, C., Peterson, L., Ramakrishnan, K., Shenker,
              S., Wroclawski, J., and L. Zhang, "Recommendations on
              Queue Management and Congestion Avoidance in the
              Internet", RFC 2309, DOI 10.17487/RFC2309, April 1998,
              <https://www.rfc-editor.org/info/rfc2309>.

   [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>.



De Schepper & Briscoe  Expires September 10, 2020              [Page 24]


Internet-Draft     ECN Semantics for Low Queuing Delay        March 2020


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

   [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>.

   [RFC3540]  Spring, N., Wetherall, D., and D. Ely, "Robust Explicit
              Congestion Notification (ECN) Signaling with Nonces",
              RFC 3540, DOI 10.17487/RFC3540, June 2003,
              <https://www.rfc-editor.org/info/rfc3540>.

   [RFC3649]  Floyd, S., "HighSpeed TCP for Large Congestion Windows",
              RFC 3649, DOI 10.17487/RFC3649, December 2003,
              <https://www.rfc-editor.org/info/rfc3649>.

   [RFC4340]  Kohler, E., Handley, M., and S. Floyd, "Datagram
              Congestion Control Protocol (DCCP)", RFC 4340,
              DOI 10.17487/RFC4340, March 2006,
              <https://www.rfc-editor.org/info/rfc4340>.

   [RFC4341]  Floyd, S. and E. Kohler, "Profile for Datagram Congestion
              Control Protocol (DCCP) Congestion Control ID 2: TCP-like
              Congestion Control", RFC 4341, DOI 10.17487/RFC4341, March
              2006, <https://www.rfc-editor.org/info/rfc4341>.

   [RFC4342]  Floyd, S., Kohler, E., and J. Padhye, "Profile for
              Datagram Congestion Control Protocol (DCCP) Congestion
              Control ID 3: TCP-Friendly Rate Control (TFRC)", RFC 4342,
              DOI 10.17487/RFC4342, March 2006,
              <https://www.rfc-editor.org/info/rfc4342>.

   [RFC4960]  Stewart, R., Ed., "Stream Control Transmission Protocol",
              RFC 4960, DOI 10.17487/RFC4960, September 2007,
              <https://www.rfc-editor.org/info/rfc4960>.

   [RFC5033]  Floyd, S. and M. Allman, "Specifying New Congestion
              Control Algorithms", BCP 133, RFC 5033,
              DOI 10.17487/RFC5033, August 2007,
              <https://www.rfc-editor.org/info/rfc5033>.

   [RFC5348]  Floyd, S., Handley, M., Padhye, J., and J. Widmer, "TCP
              Friendly Rate Control (TFRC): Protocol Specification",
              RFC 5348, DOI 10.17487/RFC5348, September 2008,
              <https://www.rfc-editor.org/info/rfc5348>.



De Schepper & Briscoe  Expires September 10, 2020              [Page 25]


Internet-Draft     ECN Semantics for Low Queuing Delay        March 2020


   [RFC5562]  Kuzmanovic, A., Mondal, A., Floyd, S., and K.
              Ramakrishnan, "Adding Explicit Congestion Notification
              (ECN) Capability to TCP's SYN/ACK Packets", RFC 5562,
              DOI 10.17487/RFC5562, June 2009,
              <https://www.rfc-editor.org/info/rfc5562>.

   [RFC5622]  Floyd, S. and E. Kohler, "Profile for Datagram Congestion
              Control Protocol (DCCP) Congestion ID 4: TCP-Friendly Rate
              Control for Small Packets (TFRC-SP)", RFC 5622,
              DOI 10.17487/RFC5622, August 2009,
              <https://www.rfc-editor.org/info/rfc5622>.

   [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>.

   [RFC5706]  Harrington, D., "Guidelines for Considering Operations and
              Management of New Protocols and Protocol Extensions",
              RFC 5706, DOI 10.17487/RFC5706, November 2009,
              <https://www.rfc-editor.org/info/rfc5706>.

   [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>.

   [RFC5925]  Touch, J., Mankin, A., and R. Bonica, "The TCP
              Authentication Option", RFC 5925, DOI 10.17487/RFC5925,
              June 2010, <https://www.rfc-editor.org/info/rfc5925>.

   [RFC6077]  Papadimitriou, D., Ed., Welzl, M., Scharf, M., and B.
              Briscoe, "Open Research Issues in Internet Congestion
              Control", RFC 6077, DOI 10.17487/RFC6077, February 2011,
              <https://www.rfc-editor.org/info/rfc6077>.

   [RFC6660]  Briscoe, B., Moncaster, T., and M. Menth, "Encoding Three
              Pre-Congestion Notification (PCN) States in the IP Header
              Using a Single Diffserv Codepoint (DSCP)", RFC 6660,
              DOI 10.17487/RFC6660, July 2012,
              <https://www.rfc-editor.org/info/rfc6660>.

   [RFC7560]  Kuehlewind, M., Ed., Scheffenegger, R., and B. Briscoe,
              "Problem Statement and Requirements for Increased Accuracy
              in Explicit Congestion Notification (ECN) Feedback",
              RFC 7560, DOI 10.17487/RFC7560, August 2015,
              <https://www.rfc-editor.org/info/rfc7560>.





De Schepper & Briscoe  Expires September 10, 2020              [Page 26]


Internet-Draft     ECN Semantics for Low Queuing Delay        March 2020


   [RFC7567]  Baker, F., Ed. and G. Fairhurst, Ed., "IETF
              Recommendations Regarding Active Queue Management",
              BCP 197, RFC 7567, DOI 10.17487/RFC7567, July 2015,
              <https://www.rfc-editor.org/info/rfc7567>.

   [RFC7713]  Mathis, M. and B. Briscoe, "Congestion Exposure (ConEx)
              Concepts, Abstract Mechanism, and Requirements", RFC 7713,
              DOI 10.17487/RFC7713, December 2015,
              <https://www.rfc-editor.org/info/rfc7713>.

   [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>.

   [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>.

   [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>.

   [RFC8298]  Johansson, I. and Z. Sarker, "Self-Clocked Rate Adaptation
              for Multimedia", RFC 8298, DOI 10.17487/RFC8298, December
              2017, <https://www.rfc-editor.org/info/rfc8298>.

   [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>.

   [RFC8312]  Rhee, I., Xu, L., Ha, S., Zimmermann, A., Eggert, L., and
              R. Scheffenegger, "CUBIC for Fast Long-Distance Networks",
              RFC 8312, DOI 10.17487/RFC8312, February 2018,
              <https://www.rfc-editor.org/info/rfc8312>.

   [Savage-TCP]
              Savage, S., Cardwell, N., Wetherall, D., and T. Anderson,
              "TCP Congestion Control with a Misbehaving Receiver", ACM
              SIGCOMM Computer Communication Review 29(5):71--78,
              October 1999.





De Schepper & Briscoe  Expires September 10, 2020              [Page 27]


Internet-Draft     ECN Semantics for Low Queuing Delay        March 2020


   [sub-mss-prob]
              Briscoe, B. and K. De Schepper, "Scaling TCP's Congestion
              Window for Small Round Trip Times", BT Technical Report
              TR-TUB8-2015-002, May 2015,
              <https://arxiv.org/abs/1904.07598>.

   [TCP-CA]   Jacobson, V. and M. Karels, "Congestion Avoidance and
              Control", Laurence Berkeley Labs Technical Report ,
              November 1988, <http://ee.lbl.gov/papers/congavoid.pdf>.

   [TCPPrague]
              Briscoe, B., "Notes: DCTCP evolution 'bar BoF': Tue 21 Jul
              2015, 17:40, Prague", tcpprague mailing list archive ,
              July 2015, <https://www.ietf.org/mail-
              archive/web/tcpprague/current/msg00001.html>.

   [VCP]      Xia, Y., Subramanian, L., Stoica, I., and S. Kalyanaraman,
              "One more bit is enough", Proc. SIGCOMM'05, ACM CCR
              35(4)37--48, 2005,
              <http://doi.acm.org/10.1145/1080091.1080098>.

Appendix A.  The 'Prague L4S Requirements'

   This appendix is informative, not normative.  It gives a list of
   modifications to current scalable congestion controls so that they
   can be deployed over the public Internet and coexist safely with
   existing traffic.  The list complements the normative requirements in
   Section 4 that a sender has to comply with before it can set the L4S
   identifier in packets it sends into the Internet.  As well as
   necessary safety improvements (requirements) this appendix also
   includes preferable performance improvements (optimizations).

   These recommendations have become know as the Prague L4S
   Requirements, because they were originally identified at an ad hoc
   meeting during IETF-94 in Prague [TCPPrague].  The wording has been
   generalized to apply to all scalable congestion controls, not just
   TCP congestion control specifically.  They were originally called the
   'TCP Prague Requirements', but they are not solely applicable to TCP,
   so the name has been generalized, and TCP Prague is now used for a
   specific implementation of the requirements.

   At the time of writing, DCTCP [RFC8257] is the most widely used
   scalable transport protocol.  In its current form, DCTCP is specified
   to be deployable only in controlled environments.  Deploying it in
   the public Internet would lead to a number of issues, both from the
   safety and the performance perspective.  The modifications and
   additional mechanisms listed in this section will be necessary for
   its deployment over the global Internet.  Where an example is needed,



De Schepper & Briscoe  Expires September 10, 2020              [Page 28]


Internet-Draft     ECN Semantics for Low Queuing Delay        March 2020


   DCTCP is used as a base, but it is likely that most of these
   requirements equally apply to other scalable congestion controls.

A.1.  Requirements for Scalable Transport Protocols

A.1.1.  Use of L4S Packet Identifier

   Description: A scalable congestion control needs to distinguish the
   packets it sends from those sent by Classic congestion controls.

   Motivation: It needs to be possible for a network node to classify
   L4S packets without flow state into a queue that applies an L4S ECN
   marking behaviour and isolates L4S packets from the queuing delay of
   Classic packets.

A.1.2.  Accurate ECN Feedback

   Description: The transport protocol for a scalable congestion control
   needs to provide timely, accurate feedback about the extent of ECN
   marking experienced by all packets.

   Motivation: Classic congestion controls only need feedback about the
   existence of a congestion episode within a round trip, not precisely
   how many packets were marked with ECN or dropped.  Therefore, in
   2001, when ECN feedback was added to TCP [RFC3168], it could not
   inform the sender of more than one ECN mark per RTT.  Since then,
   requirements for more accurate ECN feedback in TCP have been defined
   in [RFC7560] and [I-D.ietf-tcpm-accurate-ecn] specifies an
   experimental change to the TCP wire protocol to satisfy these
   requirements.  Most other transport protocols already satisfy this
   requirement.

A.1.3.  Fall back to Reno-friendly congestion control on packet loss

   Description: As well as responding to ECN markings in a scalable way,
   a scalable congestion control needs to react to packet loss in a way
   that will coexist safely with a TCP Reno congestion control
   [RFC5681].

   Motivation: Part of the safety conditions for deploying a scalable
   congestion control on the public Internet is to make sure that it
   behaves properly when it builds a queue at a network bottleneck that
   has not been upgraded to support L4S.  Packet loss can have many
   causes, but it usually has to be conservatively assumed that it is a
   sign of congestion.  Therefore, on detecting packet loss, a scalable
   congestion control will need to fall back to Classic congestion
   control behaviour.  If it does not comply with this requirement it
   could starve Classic traffic.



De Schepper & Briscoe  Expires September 10, 2020              [Page 29]


Internet-Draft     ECN Semantics for Low Queuing Delay        March 2020


   A scalable congestion control can be used for different types of
   transport, e.g. for real-time media or for reliable transport like
   TCP.  Therefore, the particular Classic congestion control behaviour
   to fall back on will need to be part of the congestion control
   specification of the relevant transport.  In the particular case of
   DCTCP, the DCTCP specification [RFC8257] states that "It is
   RECOMMENDED that an implementation deal with loss episodes in the
   same way as conventional TCP."  For safe deployment of a scalable
   congestion control in the public Internet, the above requirement
   would need to be defined as a "MUST".

   Even though a bottleneck is L4S capable, it might still become
   overloaded and have to drop packets.  In this case, the sender may
   receive a high proportion of packets marked with the CE bit set and
   also experience loss.  Current DCTCP implementations react
   differently to this situation.  At least one implementation reacts
   only to the drop signal (e.g. by halving the CWND) and at least
   another DCTCP implementation reacts to both signals (e.g. by halving
   the CWND due to the drop and also further reducing the CWND based on
   the proportion of marked packet).  A third approach for the public
   Internet has been proposed that adjusts the loss response to result
   in a halving when combined with the ECN response.  We believe that
   further experimentation is needed to understand what is the best
   behaviour for the public Internet, which may or not be one of these
   existing approaches.

A.1.4.  Fall back to Reno-friendly congestion control on classic ECN
        bottlenecks

   Description: A scalable congestion control needs to react to ECN
   marking from a non-L4S, but ECN-capable, bottleneck in a way that
   will coexist with a TCP Reno congestion control [RFC5681].

   Motivation: Similarly to the requirement in Appendix A.1.3, this
   requirement is a safety condition to ensure a scalable congestion
   control behaves properly when it builds a queue at a network
   bottleneck that has not been upgraded to support L4S.  On detecting
   Classic ECN marking (see below), a scalable congestion control will
   need to fall back to Classic congestion control behaviour.  If it
   does not comply with this requirement it could starve Classic
   traffic.

   It would take time for endpoints to distinguish Classic and L4S ECN
   marking.  An increase in queuing delay or in delay variation would be
   a tell-tale sign, but it is not yet clear where a line would be drawn
   between the two behaviours.  It might be possible to cache what was
   learned about the path to help subsequent attempts to detect the type
   of marking.



De Schepper & Briscoe  Expires September 10, 2020              [Page 30]


Internet-Draft     ECN Semantics for Low Queuing Delay        March 2020


A.1.5.  Reduce RTT dependence

   Description: A scalable congestion control needs to reduce or
   eliminate RTT bias over as wide a range of RTTs as possible, or at
   least over the typical range of RTTs that will interact in the
   intended deployment scenario.

   Motivation: The throughput of Classic congestion controls is known to
   be inversely proportional to RTT, so one would expect flows over very
   low RTT paths to nearly starve flows over larger RTTs.  However,
   Classic congestion controls have never allowed a very low RTT path to
   exist because they induce a large queue.  For instance, consider two
   paths with base RTT 1ms and 100ms.  If a Classic congestion control
   induces a 100ms queue, it turns these RTTs into 101ms and 200ms
   leading to a throughput ratio of about 2:1.  Whereas if a scalable
   congestion control induces only a 1ms queue, the ratio is 2:101,
   leading to a throughput ratio of about 50:1.

   Therefore, with very small queues, long RTT flows will essentially
   starve, unless scalable congestion controls comply with this
   requirement.

A.1.6.  Scaling down to fractional congestion windows

   Description: A scalable congestion control needs to remain responsive
   to congestion when typical RTTs over the public Internet are
   significantly smaller because they are no longer inflated by queuing
   delay.

   Motivation: As currently specified, the minimum required congestion
   window of TCP (and its derivatives) is set to 2 sender maximum
   segment sizes (SMSS) (see equation (4) in [RFC5681]).  Once the
   congestion window reaches this minimum, all known window-based
   congestion control algorithms become unresponsive to congestion
   signals.  No matter how much drop or ECN marking, the congestion
   window of all these algorithms no longer reduces.  Instead, the
   sender's lack of any further congestion response forces the queue to
   grow, overriding any AQM and increasing queuing delay.

   L4S mechanisms significantly reduce queueing delay so, over the same
   path, the RTT becomes lower.  Then this problem becomes surprisingly
   common [sub-mss-prob].  This is because, for the same link capacity,
   smaller RTT implies a smaller window.  For instance, consider a
   residential setting with an upstream broadband Internet access of 8
   Mb/s, assuming a max segment size of 1500 B.  Two upstream flows will
   each have the minimum window of 2 SMSS if the RTT is 6ms or less,
   which is quite common when accessing a nearby data centre.  So, any




De Schepper & Briscoe  Expires September 10, 2020              [Page 31]


Internet-Draft     ECN Semantics for Low Queuing Delay        March 2020


   more than two such parallel TCP flows will become unresponsive and
   increase queuing delay.

   Unless scalable congestion controls address this requirement from the
   start, they will frequently become unresponsive, negating the low
   latency benefit of L4S, for themselves and for others.

   That would seem to imply that scalable congestion controllers ought
   to be required to be able work with a congestion window less than 2
   SMSS.  For instance, one possible mechanism that can maintain a
   congestion window significantly less than 1 SMSS is described in
   [Ahmed19], and other approaches are likely to be feasible.

   However, the requirement in Section 4.3 is worded as a "SHOULD"
   because the existence of a minimum window is not all bad.  When
   competing with an unresponsive flow, a minimum window naturally
   protects the flow from starvation by at least keeping some data
   flowing.

   By stating this requirement as a "SHOULD", specifications of scalable
   congestion controllers will be able to choose an appropriate minimum
   window, but they will at least have to justify the decision.

A.1.7.  Measuring Reordering Tolerance in Time Units

   Description: A scalable congestion control needs to detect loss by
   counting in time-based units, which is scalable, rather than counting
   in units of packets, which is not.

   Motivation: A primary purpose of L4S is scalable throughput (it's in
   the name).  Scalability in all dimensions is, of course, also a goal
   of all IETF technology.  The inverse linear congestion response in
   Section 4.3 is necessary, but not sufficient, to solve the congestion
   control scalability problem identified in [RFC3649].  As well as
   maintaining frequent ECN signals as rate scales, it is also important
   to ensure that a potentially false perception of loss does not limit
   throughput scaling.

   End-systems cannot know whether a missing packet is due to loss or
   reordering, except in hindsight - if it appears later.  So they can
   only deem that there has been a loss if a gap in the sequence space
   has not been filled, either after a certain number of subsequent
   packets has arrived (e.g. the 3 DupACK rule of standard TCP
   congestion control [RFC5681]) or after a certain amount of time (e.g.
   the experimental RACK approach [I-D.ietf-tcpm-rack]).

   As we attempt to scale packet rate over the years:




De Schepper & Briscoe  Expires September 10, 2020              [Page 32]


Internet-Draft     ECN Semantics for Low Queuing Delay        March 2020


   o  Even if only _some_ sending hosts still deem that loss has
      occurred by counting reordered packets, _all_ networks will have
      to keep reducing the time over which they keep packets in order.
      If some link technologies keep the time within which reordering
      occurs roughly unchanged, then loss over these links, as perceived
      by these hosts, will appear to continually rise over the years.

   o  In contrast, if all senders detect loss in units of time, the time
      over which the network has to keep packets in order stays roughly
      invariant.

   Therefore hosts have an incentive to detect loss in time units (so as
   not to fool themselves too often into detecting losses when there are
   none).  And for hosts that are changing their congestion control
   implementation to L4S, there is no downside to including time-based
   loss detection code in the change (loss recovery implemented in
   hardware is an exception, covered later).  Therefore requiring L4S
   hosts to detect loss in time-based units would not be a burden.

   If this requirement is not placed on L4S hosts, even though it would
   be no burden on them to do so, all networks will face unnecessary
   uncertainty over whether some L4S hosts might be detecting loss by
   counting packets.  Then _all_ link technologies will have to
   unnecessarily keep reducing the time within which reordering occurs.
   That is not a problem for some link technologies, but it becomes
   increasingly challenging for other link technologies to continue to
   scale, particularly those relying on channel bonding for scaling,
   such as LTE, 5G and DOCSIS.

   Given Internet paths traverse many link technologies, any scaling
   limit for these more challenging access link technologies would
   become a scaling limit for the Internet as a whole.

   It might be asked how it helps to place this loss detection
   requirement only on L4S hosts, because networks will still face
   uncertainty over whether non-L4S flows are detecting loss by counting
   DupACKs.  The answer is that those link technologies for which it is
   challenging to keep squeezing the reordering time will only need to
   do so for non-L4S traffic (which they can do because the L4S
   identifier is visible at the IP layer).  Therefore, they can focus
   their processing and memory resources into scaling non-L4S (Classic)
   traffic.  Then, the higher the proportion of L4S traffic, the less of
   a scaling challenge they will have.

   To summarize, there is no reason for L4S hosts not to be part of the
   solution instead of part of the problem.





De Schepper & Briscoe  Expires September 10, 2020              [Page 33]


Internet-Draft     ECN Semantics for Low Queuing Delay        March 2020


   Requirement ("MUST") or recommendation ("SHOULD")?  As explained
   above, this is a subtle interoperability issue between hosts and
   networks, which seems to need a "MUST".  Unless networks can be
   certain that all L4S hosts follow the time-based approach, they still
   have to cater for the worst case - continually squeeze reordering
   into a smaller and smaller duration - just for hosts that might be
   using the counting approach.  However, it was decided to express this
   as a recommendation, using "SHOULD".  The main justification was that
   networks can still be fairly certain that L4S hosts will follow this
   recommendation, because following it offers only gain and no pain.

   Details:

   The speed of loss recovery is much more significant for short flows
   than long, therefore a good compromise is to adapt the reordering
   window; from a small fraction of the RTT at the start of a flow, to a
   larger fraction of the RTT for flows that continue for many round
   trips.

   This is broadly the approach adopted by TCP RACK (Recent
   ACKnowledgements) [I-D.ietf-tcpm-rack].  However, RACK starts with
   the 3 DupACK approach, because the RTT estimate is not necessarily
   stable.  As long as the initial window is paced, such initial use of
   3 DupACK counting would amount to time-based loss detection and
   therefore would satisfy the time-based loss detection recommendation
   of Section 4.3.  This is because pacing of the initial window would
   ensure that 3 DupACKs early in the connection would be spread over a
   small fraction of the round trip.

   As mentioned above, hardware implementations of loss recovery using
   DupACK counting exist (e.g. some implementations of RoCEv2 for RDMA).
   For low latency, these implementations can change their congestion
   control to implement L4S, because the congestion control (as distinct
   from loss recovery) is implemented in software.  But they cannot
   easily satisfy this loss recovery requirement.  However, it is
   believed they do not need to.  It is believed that such
   implementations solely exist in controlled environments, where the
   network technology keeps reordering extremely low anyway.  This is
   why the scope of the normative recommendation in Section 4.3 is
   limited to 'reordering-prone' networks.

   Detecting loss in time units also prevents the ACK-splitting attacks
   described in [Savage-TCP].








De Schepper & Briscoe  Expires September 10, 2020              [Page 34]


Internet-Draft     ECN Semantics for Low Queuing Delay        March 2020


A.2.  Scalable Transport Protocol Optimizations

A.2.1.  Setting ECT in TCP Control Packets and Retransmissions

   Description: This item only concerns TCP and its derivatives (e.g.
   SCTP), because the original specification of ECN for TCP precluded
   the use of ECN on control packets and retransmissions.  To improve
   performance, scalable transport protocols ought to enable ECN at the
   IP layer in TCP control packets (SYN, SYN-ACK, pure ACKs, etc.) and
   in retransmitted packets.  The same is true for derivatives of TCP,
   e.g.  SCTP.

   Motivation: RFC 3168 prohibits the use of ECN on these types of TCP
   packet, based on a number of arguments.  This means these packets are
   not protected from congestion loss by ECN, which considerably harms
   performance, particularly for short flows.
   [I-D.ietf-tcpm-generalized-ecn] counters each argument in RFC 3168 in
   turn, showing it was over-cautious.  Instead it proposes experimental
   use of ECN on all types of TCP packet as long as AccECN feedback
   [I-D.ietf-tcpm-accurate-ecn] is available (which is itself a
   prerequisite for using a scalable congestion control).

A.2.2.  Faster than Additive Increase

   Description: It would improve performance if scalable congestion
   controls did not limit their congestion window increase to the
   standard additive increase of 1 SMSS per round trip [RFC5681] during
   congestion avoidance.  The same is true for derivatives of TCP
   congestion control, including similar approaches used for real-time
   media.

   Motivation: As currently defined [RFC8257], DCTCP uses the
   traditional TCP Reno additive increase in congestion avoidance phase.
   When the available capacity suddenly increases (e.g. when another
   flow finishes, or if radio capacity increases) it can take very many
   round trips to take advantage of the new capacity.  TCP Cubic was
   designed to solve this problem, but as flow rates have continued to
   increase, the delay accelerating into available capacity has become
   prohibitive.  See, for instance, the examples in Section 1.2.  Even
   when out of its Reno-compatibility mode, every 8x scaling of Cubic's
   flow rate leads to 2x more acceleration delay.

   In the steady state, DCTCP induces about 2 ECN marks per round trip,
   so it is possible to quickly detect when these signals have
   disappeared and seek available capacity more rapidly, while
   minimizing the impact on other flows (Classic and scalable)
   [LinuxPacedChirping].  Alternatively, approaches such as Adaptive
   Acceleration (A2DTCP [A2DTCP]) have been proposed to address this



De Schepper & Briscoe  Expires September 10, 2020              [Page 35]


Internet-Draft     ECN Semantics for Low Queuing Delay        March 2020


   problem in data centres, which might be deployable over the public
   Internet.

A.2.3.  Faster Convergence at Flow Start

   Description: Particularly when a flow starts, scalable congestion
   controls need to converge (reach their steady-state share of the
   capacity) at least as fast as Classic congestion controls and
   preferably faster.  This affects the flow start behaviour of any L4S
   congestion control derived from a Classic transport that uses TCP
   slow start, including those for real-time media.

   Motivation: As an example, a new DCTCP flow takes longer than a
   Classic congestion control to obtain its share of the capacity of the
   bottleneck when there are already ongoing flows using the bottleneck
   capacity.  In a data centre environment DCTCP takes about a factor of
   1.5 to 2 longer to converge due to the much higher typical level of
   ECN marking that DCTCP background traffic induces, which causes new
   flows to exit slow start early [Alizadeh-stability].  In testing for
   use over the public Internet the convergence time of DCTCP relative
   to a regular loss-based TCP slow start is even less favourable
   [Paced-Chirping]) due to the shallow ECN marking threshold needed for
   L4S.  It is exacerbated by the typically greater mismatch between the
   link rate of the sending host and typical Internet access
   bottlenecks.  This problem is detrimental in general, but would
   particularly harm the performance of short flows relative to Classic
   congestion controls.

Appendix B.  Alternative Identifiers

   This appendix is informative, not normative.  It records the pros and
   cons of various alternative ways to identify L4S packets to record
   the rationale for the choice of ECT(1) (Appendix B.1) as the L4S
   identifier.  At the end, Appendix B.6 summarises the distinguishing
   features of the leading alternatives.  It is intended to supplement,
   not replace the detailed text.

   The leading solutions all use the ECN field, sometimes in combination
   with the Diffserv field.  This is because L4S traffic has to indicate
   that it is ECN-capable anyway, because ECN is intrinsic to how L4S
   works.  Both the ECN and Diffserv fields have the additional
   advantage that they are no different in either IPv4 or IPv6.  A
   couple of alternatives that use other fields are mentioned at the
   end, but it is quickly explained why they are not serious contenders.







De Schepper & Briscoe  Expires September 10, 2020              [Page 36]


Internet-Draft     ECN Semantics for Low Queuing Delay        March 2020


B.1.  ECT(1) and CE codepoints

   Definition:

      Packets with ECT(1) and conditionally packets with CE would
      signify L4S semantics as an alternative to the semantics of
      Classic ECN [RFC3168], specifically:

      *  The ECT(1) codepoint would signify that the packet was sent by
         an L4S-capable sender.

      *  Given shortage of codepoints, both L4S and Classic ECN sides of
         an AQM would have to use the same CE codepoint to indicate that
         a packet had experienced congestion.  If a packet that had
         already been marked CE in an upstream buffer arrived at a
         subsequent AQM, this AQM would then have to guess whether to
         classify CE packets as L4S or Classic ECN.  Choosing the L4S
         treatment would be a safer choice, because then a few Classic
         packets might arrive early, rather than a few L4S packets
         arriving late.

      *  Additional information might be available if the classifier
         were transport-aware.  Then it could classify a CE packet for
         Classic ECN treatment if the most recent ECT packet in the same
         flow had been marked ECT(0).  However, the L4S service ought
         not to need tranport-layer awareness.

   Cons:

   Consumes the last ECN codepoint:  The L4S service is intended to
      supersede the service provided by Classic ECN, therefore using
      ECT(1) to identify L4S packets could ultimately mean that the
      ECT(0) codepoint was 'wasted' purely to distinguish one form of
      ECN from its successor.

   ECN hard in some lower layers:  It is not always possible to support
      ECN in an AQM acting in a buffer below the IP layer
      [I-D.ietf-tsvwg-ecn-encap-guidelines].  In such cases, the L4S
      service would have to drop rather than mark frames even though
      they might encapsulate an ECN-capable packet.  However, such cases
      would be unusual.

   Risk of reordering Classic CE packets:  Classifying all CE packets
      into the L4S queue risks any CE packets that were originally
      ECT(0) being incorrectly classified as L4S.  If there were delay
      in the Classic queue, these incorrectly classified CE packets
      would arrive early, which is a form of reordering.  Reordering can
      cause TCP senders (and senders of similar transports) to



De Schepper & Briscoe  Expires September 10, 2020              [Page 37]


Internet-Draft     ECN Semantics for Low Queuing Delay        March 2020


      retransmit spuriously.  However, the risk of spurious
      retransmissions would be extremely low for the following reasons:

      1.  It is quite unusual to experience queuing at more than one
          bottleneck on the same path (the available capacities have to
          be identical).

      2.  In only a subset of these unusual cases would the first
          bottleneck support Classic ECN marking while the second
          supported L4S ECN marking, which would be the only scenario
          where some ECT(0) packets could be CE marked by an AQM
          supporting Classic ECN then the remainder experienced further
          delay through the Classic side of a subsequent L4S DualQ AQM.

      3.  Even then, when a few packets are delivered early, it takes
          very unusual conditions to cause a spurious retransmission, in
          contrast to when some packets are delivered late.  The first
          bottleneck has to apply CE-marks to at least N contiguous
          packets and the second bottleneck has to inject an
          uninterrupted sequence of at least N of these packets between
          two packets earlier in the stream (where N is the reordering
          window that the transport protocol allows before it considers
          a packet is lost).

             For example consider N=3, and consider the sequence of
             packets 100, 101, 102, 103,... and imagine that packets
             150,151,152 from later in the flow are injected as follows:
             100, 150, 151, 101, 152, 102, 103...  If this were late
             reordering, even one packet arriving 50 out of sequence
             would trigger a spurious retransmission, but there is no
             spurious retransmission here, with early reordering,
             because packet 101 moves the cumulative ACK counter forward
             before 3 packets have arrived out of order.  Later, when
             packets 148, 149, 153... arrive, even though there is a
             3-packet hole, there will be no problem, because the
             packets to fill the hole are already in the receive buffer.

      4.  Even with the current TCP recommendation of N=3 [RFC5681]
          spurious retransmissions will be unlikely for all the above
          reasons.  As RACK [I-D.ietf-tcpm-rack] is becoming widely
          deployed, it tends to adapt its reordering window to a larger
          value of N, which will make the chance of a contiguous
          sequence of N early arrivals vanishingly small.

      5.  Even a run of 2 CE marks within a Classic ECN flow is
          unlikely, given FQ-CoDel is the only known widely deployed AQM
          that supports Classic ECN marking and it takes great care to




De Schepper & Briscoe  Expires September 10, 2020              [Page 38]


Internet-Draft     ECN Semantics for Low Queuing Delay        March 2020


          separate out flows and to space any markings evenly along each
          flow.

      It is extremely unlikely that the above set of 5 eventualities
      that are each unusual in themselves would all happen
      simultaneously.  But, even if they did, the consequences would
      hardly be dire: the odd spurious fast retransmission.  Admittedly
      TCP (and similar transports) reduce their congestion window when
      they deem there has been a loss, but even this can be recovered
      once the sender detects that the retransmission was spurious.

   Non-L4S service for control packets:  The Classic ECN RFCs [RFC3168]
      and [RFC5562] require a sender to clear the ECN field to Not-ECT
      for retransmissions and certain control packets specifically pure
      ACKs, window probes and SYNs.  When L4S packets are classified by
      the ECN field alone, these control packets would not be classified
      into an L4S queue, and could therefore be delayed relative to the
      other packets in the flow.  This would not cause re-ordering
      (because retransmissions are already out of order, and the control
      packets carry no data).  However, it would make critical control
      packets more vulnerable to loss and delay.  To address this
      problem, [I-D.ietf-tcpm-generalized-ecn] proposes an experiment in
      which all TCP control packets and retransmissions are ECN-capable
      as long as ECN feedback is available.

   Pros:

   Should work e2e:  The ECN field generally works end-to-end across the
      Internet.  Unlike the DSCP, the setting of the ECN field is at
      least forwarded unchanged by networks that do not support ECN, and
      networks rarely clear it to zero.

   Should work in tunnels:  Unlike Diffserv, ECN is defined to always
      work across tunnels.  However, tunnels do not always implement ECN
      processing as they should do, particularly because IPsec tunnels
      were defined differently for a few years.

   Could migrate to one codepoint:  If all Classic ECN senders
      eventually evolve to use the L4S service, the ECT(0) codepoint
      could be reused for some future purpose, but only once use of
      ECT(0) packets had reduced to zero, or near-zero, which might
      never happen.

B.2.  ECN Plus a Diffserv Codepoint (DSCP)

   Definition:





De Schepper & Briscoe  Expires September 10, 2020              [Page 39]


Internet-Draft     ECN Semantics for Low Queuing Delay        March 2020


      For packets with a defined DSCP, all codepoints of the ECN field
      (except Not-ECT) would signify alternative L4S semantics to those
      for Classic ECN [RFC3168], specifically:

      *  The L4S DSCP would signifiy that the packet came from an L4S-
         capable sender.

      *  ECT(0) and ECT(1) would both signify that the packet was
         travelling between transport endpoints that were both ECN-
         capable.

      *  CE would signify that the packet had been marked by an AQM
         implementing the L4S service.

   Use of a DSCP is the only approach for alternative ECN semantics
   given as an example in [RFC4774].  However, it was perhaps considered
   more for controlled environments than new end-to-end services.

   Cons:

   Consumes DSCP pairs:  A DSCP is obviously not orthogonal to Diffserv.
      Therefore, wherever the L4S service is applied to multiple
      Diffserv scheduling behaviours, it would be necessary to replace
      each DSCP with a pair of DSCPs.

   Uses critical lower-layer header space:  The resulting increased
      number of DSCPs might be hard to support for some lower layer
      technologies, e.g. 802.1p and MPLS both offer only 3-bits for a
      maximum of 8 traffic class identifiers.  Although L4S should
      reduce and possibly remove the need for some DSCPs intended for
      differentiated queuing delay, it will not remove the need for
      Diffserv entirely, because Diffserv is also used to allocate
      bandwidth, e.g. by prioritising some classes of traffic over
      others when traffic exceeds available capacity.

   Not end-to-end (host-network):  Very few networks honour a DSCP set
      by a host.  Typically a network will zero (bleach) the Diffserv
      field from all hosts.  Sometimes networks will attempt to identify
      applications by some form of packet inspection and, based on
      network policy, they will set the DSCP considered appropriate for
      the identified application.  Network-based application
      identification might use some combination of protocol ID, port
      numbers(s), application layer protocol headers, IP address(es),
      VLAN ID(s) and even packet timing.

   Not end-to-end (network-network):  Very few networks honour a DSCP
      received from a neighbouring network.  Typically a network will
      zero (bleach) the Diffserv field from all neighbouring networks at



De Schepper & Briscoe  Expires September 10, 2020              [Page 40]


Internet-Draft     ECN Semantics for Low Queuing Delay        March 2020


      an interconnection point.  Sometimes bilateral arrangements are
      made between networks, such that the receiving network remarks
      some DSCPs to those it uses for roughly equivalent services.  The
      likelihood that a DSCP will be bleached or ignored depends on the
      type of DSCP:

      Local-use DSCP:  These tend to be used to implement application-
         specific network policies, but a bilateral arrangement to
         remark certain DSCPs is often applied to DSCPs in the local-use
         range simply because it is easier not to change all of a
         network's internal configurations when a new arrangement is
         made with a neighbour.

      Recommended standard DSCP:  These do not tend to be honoured
         across network interconnections more than local-use DSCPs.
         However, if two networks decide to honour certain of each
         other's DSCPs, the reconfiguration is a little easier if both
         of their globally recognised services are already represented
         by the relevant recommended standard DSCPs.

         Note that today a recommended standard DSCP gives little more
         assurance of end-to-end service than a local-use DSCP.  In
         future the range recommended as standard might give more
         assurance of end-to-end service than local-use, but it is
         unlikely that either assurance will be high, particularly given
         the hosts are included in the end-to-end path.

   Not all tunnels:  Diffserv codepoints are often not propagated to the
      outer header when a packet is encapsulated by a tunnel header.
      DSCPs are propagated to the outer of uniform mode tunnels, but not
      pipe mode [RFC2983], and pipe mode is fairly common.

   ECN hard in some lower layers::  Because this approach uses both the
      Diffserv and ECN fields, an AQM wil only work at a lower layer if
      both can be supported.  If individual network operators wished to
      deploy an AQM at a lower layer, they would usually propagate an IP
      Diffserv codepoint to the lower layer, using for example IEEE
      802.1p.  However, the ECN capability is harder to propagate down
      to lower layers because few lower layers support it.

   Pros:

   Could migrate to e2e:  If all usage of Classic ECN migrates to usage
      of L4S, the DSCP would become redundant, and the ECN capability
      alone could eventually identify L4S packets without the
      interconnection problems of Diffserv detailed above, and without
      having permanently consumed more than one codepoint in the IP
      header.  Although the DSCP does not generally function as an end-



De Schepper & Briscoe  Expires September 10, 2020              [Page 41]


Internet-Draft     ECN Semantics for Low Queuing Delay        March 2020


      to-end identifier (see above), it could be used initially by
      individual ISPs to introduce the L4S service for their own locally
      generated traffic.

B.3.  ECN capability alone

   This approach uses ECN capability alone as the L4S identifier.  It
   would only have been feasible if RFC 3168 ECN had not been widely
   deployed.  This was the case when the choice of L4S identifier was
   being made and this appendix was first written.  Since then, RFC 3168
   ECN has been widely deployed and L4S did not take this approach
   anyway.  So this approach is not discussed further, because it is no
   longer a feasible option.

B.4.  Protocol ID

   It has been suggested that a new ID in the IPv4 Protocol field or the
   IPv6 Next Header field could identify L4S packets.  However this
   approach is ruled out by numerous problems:

   o  A new protocol ID would need to be paired with the old one for
      each transport (TCP, SCTP, UDP, etc.).

   o  In IPv6, there can be a sequence of Next Header fields, and it
      would not be obvious which one would be expected to identify a
      network service like L4S.

   o  A new protocol ID would rarely provide an end-to-end service,
      because It is well-known that new protocol IDs are often blocked
      by numerous types of middlebox.

   o  The approach is not a solution for AQM methods below the IP layer.

B.5.  Source or destination addressing

   Locally, a network operator could arrange for L4S service to be
   applied based on source or destination addressing, e.g. packets from
   its own data centre and/or CDN hosts, packets to its business
   customers, etc.  It could use addressing at any layer, e.g.  IP
   addresses, MAC addresses, VLAN IDs, etc.  Although addressing might
   be a useful tactical approach for a single ISP, it would not be a
   feasible approach to identify an end-to-end service like L4S.  Even
   for a single ISP, it would require packet classifiers in buffers to
   be dependent on changing topology and address allocation decisions
   elsewhere in the network.  Therefore this approach is not a feasible
   solution.





De Schepper & Briscoe  Expires September 10, 2020              [Page 42]


Internet-Draft     ECN Semantics for Low Queuing Delay        March 2020


B.6.  Summary: Merits of Alternative Identifiers

   Table 1 provides a very high level summary of the pros and cons
   detailed against the schemes described respectively in Appendix B.2
   and Appendix B.1, for six issues that set them apart.

        +--------------+--------------------+--------------------+
        | Issue        |     DSCP + ECN     |    ECT(1) + CE     |
        +--------------+--------------------+--------------------+
        |              | initial   eventual | initial   eventual |
        |              |                    |                    |
        | end-to-end   |  N . .      . ? .  |  . . Y      . . Y  |
        | tunnels      |  . O .      . O .  |  . . ?      . . Y  |
        | lower layers |  N . .      . ? .  |  . O .      . . ?  |
        | codepoints   |  N . .      . . ?  |  N . .      . . ?  |
        | reordering   |  . . Y      . . Y  |  . O .      . . ?  |
        | ctrl pkts    |  . . Y      . . Y  |  . O .      . . ?  |
        |              |                    |                    |
        |              |                    |                    |
        +--------------+--------------------+--------------------+

    Table 1: Comparison of the Merits of Three Alternative Identifiers

   The schemes are scored based on both their capabilities now
   ('initial') and in the long term ('eventual').  The scores are one of
   'N, O, Y', meaning 'Poor', 'Ordinary', 'Good' respectively.  The same
   scores are aligned vertically to aid the eye.  A score of "?" in one
   of the positions means that this approach might optimistically become
   this good, given sufficient effort.  The table summarises the text
   and is not meant to be understandable without having read the text.

Appendix C.  Potential Competing Uses for the ECT(1) Codepoint

   The ECT(1) codepoint of the ECN field has already been assigned once
   for the ECN nonce [RFC3540], which has now been categorized as
   historic [RFC8311].  ECN is probably the only remaining field in the
   Internet Protocol that is common to IPv4 and IPv6 and still has
   potential to work end-to-end, with tunnels and with lower layers.
   Therefore, ECT(1) should not be reassigned to a different
   experimental use (L4S) without carefully assessing competing
   potential uses.  These fall into the following categories:

C.1.  Integrity of Congestion Feedback

   Receiving hosts can fool a sender into downloading faster by
   suppressing feedback of ECN marks (or of losses if retransmissions
   are not necessary or available otherwise).




De Schepper & Briscoe  Expires September 10, 2020              [Page 43]


Internet-Draft     ECN Semantics for Low Queuing Delay        March 2020


   The historic ECN nonce protocol [RFC3540] proposed that a TCP sender
   could set either of ECT(0) or ECT(1) in each packet of a flow and
   remember the sequence it had set.  If any packet was lost or
   congestion marked, the receiver would miss that bit of the sequence.
   An ECN Nonce receiver had to feed back the least significant bit of
   the sum, so it could not suppress feedback of a loss or mark without
   a 50-50 chance of guessing the sum incorrectly.

   It is highly unlikely that ECT(1) will be needed for integrity
   protection in future.  The ECN Nonce RFC [RFC3540] as been
   reclassified as historic, partly because other ways have been
   developed to protect feedback integrity of TCP and other transports
   [RFC8311] that do not consume a codepoint in the IP header.  For
   instance:

   o  the sender can test the integrity of the receiver's feedback by
      occasionally setting the IP-ECN field to a value normally only set
      by the network.  Then it can test whether the receiver's feedback
      faithfully reports what it expects (see para 2 of Section 20.2 of
      [RFC3168].  This works for loss and it will work for the accurate
      ECN feedback [RFC7560] intended for L4S.

   o  A network can enforce a congestion response to its ECN markings
      (or packet losses) by auditing congestion exposure (ConEx)
      [RFC7713].  Whether the receiver or a downstream network is
      suppressing congestion feedback or the sender is unresponsive to
      the feedback, or both, ConEx audit can neutralise any advantage
      that any of these three parties would otherwise gain.

   o  The TCP authentication option (TCP-AO [RFC5925]) can be used to
      detect any tampering with TCP congestion feedback (whether
      malicious or accidental).  TCP's congestion feedback fields are
      immutable end-to-end, so they are amenable to TCP-AO protection,
      which covers the main TCP header and TCP options by default.
      However, TCP-AO is often too brittle to use on many end-to-end
      paths, where middleboxes can make verification fail in their
      attempts to improve performance or security, e.g. by
      resegmentation or shifting the sequence space.

C.2.  Notification of Less Severe Congestion than CE

   Various researchers have proposed to use ECT(1) as a less severe
   congestion notification than CE, particularly to enable flows to fill
   available capacity more quickly after an idle period, when another
   flow departs or when a flow starts, e.g.  VCP [VCP], Queue View (QV)
   [QV].





De Schepper & Briscoe  Expires September 10, 2020              [Page 44]


Internet-Draft     ECN Semantics for Low Queuing Delay        March 2020


   Before assigning ECT(1) as an identifer for L4S, we must carefully
   consider whether it might be better to hold ECT(1) in reserve for
   future standardisation of rapid flow acceleration, which is an
   important and enduring problem [RFC6077].

   Pre-Congestion Notification (PCN) is another scheme that assigns
   alternative semantics to the ECN field.  It uses ECT(1) to signify a
   less severe level of pre-congestion notification than CE [RFC6660].
   However, the ECN field only takes on the PCN semantics if packets
   carry a Diffserv codepoint defined to indicate PCN marking within a
   controlled environment.  PCN is required to be applied solely to the
   outer header of a tunnel across the controlled region in order not to
   interfere with any end-to-end use of the ECN field.  Therefore a PCN
   region on the path would not interfere with any of the L4S service
   identifiers proposed in Appendix B.

Authors' Addresses

   Koen De Schepper
   Nokia Bell Labs
   Antwerp
   Belgium

   Email: koen.de_schepper@nokia.com
   URI:   https://www.bell-labs.com/usr/koen.de_schepper


   Bob Briscoe (editor)
   Independent
   UK

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


















De Schepper & Briscoe  Expires September 10, 2020              [Page 45]


Html markup produced by rfcmarkup 1.129d, available from https://tools.ietf.org/tools/rfcmarkup/