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Versions: 00 01 02 draft-ietf-tsvwg-ecn-l4s-id

Transport Services (tsv)                                  K. De Schepper
Internet-Draft                                           Nokia Bell Labs
Intended status: Experimental                            B. Briscoe, Ed.
Expires: May 4, 2017                                 Simula Research Lab
                                                                I. Tsang
                                                         Nokia Bell Labs
                                                        October 31, 2016


 Identifying Modified Explicit Congestion Notification (ECN) Semantics
                      for Ultra-Low Queuing Delay
                   draft-briscoe-tsvwg-ecn-l4s-id-02

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 'Classic'
   flow under the same conditions.  However, the much more frequent
   control signals and the finer responses to them result in ultra-low
   queuing delay without compromising link utilization, even during high
   load.  Examples of new active queue management (AQM) marking
   algorithms and examples of new transports (whether TCP-like or real-
   time) are specified separately.  The new L4S identifier is the key
   piece that enables them to interwork and distinguishes them from
   'Classic' traffic.  It gives an incremental migration path so that
   existing 'Classic' TCP traffic will be no worse off, but it can be
   prevented from degrading the ultra-low delay and loss of the new
   scalable transports.

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 http://datatracker.ietf.org/drafts/current/.




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   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
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   This Internet-Draft will expire on May 4, 2017.

Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Problem . . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   5
     1.3.  Scope . . . . . . . . . . . . . . . . . . . . . . . . . .   6
   2.  L4S Packet Identifier . . . . . . . . . . . . . . . . . . . .   6
     2.1.  L4S Packet Identification Requirements  . . . . . . . . .   6
     2.2.  L4S Packet Identification . . . . . . . . . . . . . . . .   7
     2.3.  Pre-Requisite Transport Layer Behaviour . . . . . . . . .   8
     2.4.  L4S Packet Identification by Network Nodes with
           Transport-Layer Awareness . . . . . . . . . . . . . . . .   9
     2.5.  The Meaning of CE Relative to Drop  . . . . . . . . . . .  10
   3.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   5.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  10
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .  11
     6.2.  Informative References  . . . . . . . . . . . . . . . . .  11
   Appendix A.  Alternative Identifiers  . . . . . . . . . . . . . .  15
     A.1.  ECT(1) and CE codepoints  . . . . . . . . . . . . . . . .  16
     A.2.  ECN Plus a Diffserv Codepoint (DSCP)  . . . . . . . . . .  18
     A.3.  ECN capability alone  . . . . . . . . . . . . . . . . . .  20
     A.4.  Protocol ID . . . . . . . . . . . . . . . . . . . . . . .  21
     A.5.  Source or destination addressing  . . . . . . . . . . . .  21
     A.6.  Summary: Merits of Alternative Identifiers  . . . . . . .  22



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   Appendix B.  Potential Competing Uses for the ECT(1) Codepoint  .  23
     B.1.  Integrity of Congestion Feedback  . . . . . . . . . . . .  23
     B.2.  Notification of Less Severe Congestion than CE  . . . . .  24
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  24

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).  '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,
   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 bit-rate of an L4S flow will be
   roughly the same as a 'Classic' flow under the same conditions.
   However, the much more frequent control signals and the finer
   responses to them result in ultra-low queuing delay without
   compromising link utilization, even during high load.

   An example of an active queue management (AQM) marking algorithm that
   enables the L4S service is the DualQ Coupled AQM defined in a
   complementary specification [I-D.briscoe-aqm-dualq-coupled].  An
   example of a scalable transport that would enable the L4S service is
   Data Centre TCP (DCTCP), which until now has been applicable solely
   to controlled environments like data centres [I-D.ietf-tcpm-dctcp],
   because it is too aggressive to co-exist with existing TCP.  However,
   AQMs like DualQ Coupled enable scalable transports like DCTCP to co-
   exist with existing traffic, each getting roughly the same flow rate
   when they compete under similar conditions.  Note that DCTCP will
   still not be safe to deploy on the Internet until it satisfies the
   'Safety Additions' listed in Appendix A of
   [I-D.briscoe-tsvwg-aqm-tcpm-rmcat-l4s-problem].

   The new L4S identifier is the key piece that enables these two parts
   to interwork and distinguishes them from 'Classic' traffic.  It gives
   an incremental migration path so that existing 'Classic' TCP traffic
   will be no worse off, but it can be prevented from degrading the
   ultra-low delay and loss of the new scalable transports.  The
   performance improvement is so great that it is hoped it will motivate
   initial deployment of the separate parts of this system.








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1.1.  Problem

   Latency is becoming the critical performance factor for many (most?)
   applications on the public Internet, e.g.  Web, voice, conversational
   video, gaming, finance apps, remote desktop and cloud-based
   applications.  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 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 Diffserv is of little use.  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, AQMs 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, AQM was not widely deployed.

   More recent state-of-the-art AQMs, e.g.
   fq_CoDel [I-D.ietf-aqm-fq-codel], PIE [I-D.ietf-aqm-pie], 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
   rate of TCP 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.  Flow-queuing can isolate
   one flow from another, but it cannot isolate a TCP flow from the
   delay variations it inflicts on itself, and it has other problems -
   it overrides the flow rate decisions of variable rate video



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   applications, it does not recognise the flows within IPSec VPN
   tunnels and it is relatively expensive to implement.

   Latency is not our only concern: It was known when TCP was first
   developed that it would not scale to high bandwidth-delay products.
   Given regular broadband bit-rates over WAN distances are
   already [RFC3649] beyond the scaling range of 'Classic' TCP Reno,
   'less unscalable' Cubic [I-D.ietf-tcpm-cubic] 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 TCPs such as DCTCP
   [I-D.ietf-tcpm-dctcp] cause 'Classic' TCP to starve itself, which is
   why they have been confined to private data centres or research
   testbeds (until now).

   It turns out that a TCP algorithm like DCTCP that solves TCP's
   scalability problem also solves the latency problem, because the
   finer sawteeth cause very little queuing delay.  A supporting 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 [I-D.briscoe-aqm-dualq-coupled].

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

   Low-Latency, Low-Loss and Scalable (L4S) service:  The 'L4S' service
      is intended for traffic from scalable TCP algorithms such as Data
      Centre TCP.  But it is also more general--it will allow a set of
      congestion controls with similar scaling properties to DCTCP (e.g.
      Relentless [Mathis09]) to evolve.

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

   Classic ECN:  The original Explicit Congestion Notification (ECN)
      protocol [RFC3168].



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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.
   It is an orthogonal packet classification to Differentiated Services
   (Diffserv [RFC2474]), therefore it can be applied to any packet in
   any Diffserv traffic class.  However, as with classic ECN, any
   particular forwarding node might not implement an active queue
   management algorithm in all its Diffserv queues.

   This document is intended for experimental status, so it does not
   update any standards track RFCs.  Therefore it depends on
   [I-D.black-tsvwg-ecn-experimentation], which proposes to:

   o  update the ECN proposed standard [RFC3168] (in certain specified
      cases including the present document) to relax the requirement
      that an ECN mark must be equivalent to a drop, both when applied
      by the network, and when responded to by the sender;

   o  obsolete the experimental ECN nonce [RFC3540] (see Appendix B.1
      for rationale);

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

      *  ECN for RTP [RFC6679];

      *  the congestion control specifications of various DCCP CCIDs
         [RFC4341], [RFC4342], [RFC5622].

2.  L4S Packet Identifier

2.1.  L4S Packet Identification Requirements

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

   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 common to IPv4 and IPv6 and transport agnostic;

   o  it SHOULD be incrementally deployable;





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   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 not lead to some packets of a transport-layer flow being
      served by a different queue from 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 requirements are expressed with the word 'SHOULD' not
   'MUST'.  Appendix A discusses the pros and cons of the compromises
   made in various competing identification schemes against the above
   requirements.  On the basis of this analysis, the "ECT(1) and CE
   codepoints" is the best compromise.  Therefore this scheme is defined
   in detail in the following section (Section 2.2), while Appendix A
   has been left to document the rationale for this decision.

2.2.  L4S Packet Identification

   The L4S treatment is an alternative packet marking treatment
   [RFC4774] to the classic ECN treatment [RFC3168].  Like classic ECN,
   it 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 behaviour.

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

   A network node that implements the L4S service MUST classify arriving
   ECT(1) packets for L4S treatment and it SHOULD classify arriving CE
   packets for L4S treatment as well.  Section 2.4 describes a possible
   exception to this latter rule.

   The 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 mark the ECN
   field as CE for an increasing proportion of packets, otherwise



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   forwarding packets unchanged as ECT(1).  The L4S marking treatment is
   defined in Section 2.5.  Under persistent overload conditions, the
   AQM will follow RFC 3168 and turn off ECN marking, using drop as a
   congestion signal until the overload episode has subsided.

   The L4S treatment is the default for ECT(1) packets in all Diffserv
   Classes [RFC4774].

   For backward compatibility in uncontrolled environments, a network
   node that implements the L4S treatment MUST also implement a classic
   AQM treatment.  It MUST classify arriving ECT(0) and Not-ECT packets
   for treatment by the Classic AQM.  Classic treatment means that the
   AQM will mark ECT(0) packets under the same conditions as it would
   drop Not-ECT packets [RFC3168].

2.3.  Pre-Requisite Transport Layer Behaviour

   For a host to send packets with the L4S identifier (ECT(1)), it
   SHOULD implement a congestion control behaviour that ensures the flow
   rate is inversely proportional to the proportion of bytes in packets
   marked with the CE codepoint.  This is termed a scalable congestion
   control, because the number of control signals (ECN marks) per round
   trip remains roughly constant for any flow rate.  As with all
   transport behaviours, a detailed specification will need to be
   defined for each type of transport or application, including the
   timescale over which the proportionality is averaged, and control of
   burstiness.  The inverse proportionality requirement above is worded
   as a 'SHOULD' rather than a 'MUST' to allow reasonable flexibility
   when defining these specifications.

   Data Center TCP (DCTCP [I-D.ietf-tcpm-dctcp]) is an example of a
   scalable congestion control.

   Each sender in a session can use a scalable congestion control
   independently of the congestion control used by the receiver(s) when
   they send data.  Therefore theoretically there might be ECT(1)
   packets in one direction and ECT(0) in the other.

   In general, a scalable congestion control needs feedback of the
   extent of CE marking on the forward path.  Due to the history of TCP
   development, when ECN was added it reported no more than one CE mark
   per round trip.  Some transport protocols derived from TCP mimick
   this behaviour while others report the extent of TCP marking.  This
   means that some transport protocols will need to be updated as a pre-
   requisite for scalable congestion control.  The position for a few
   well-known transport protocols is given below.





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   TCP:  Support for accurate ECN feedback (AccECN
      [I-D.ietf-tcpm-accurate-ecn]) by both ends is a pre-requisite for
      scalable congestion control.  However, the reverse 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.  Nonetheless, the presence of ECT(1) in the IP
      headers even in one direction of a TCP connection will imply that
      both ends support AccECN.

   SCTP:  An ECN feedback protocol such as that specified in
      [I-D.stewart-tsvwg-sctpecn] would be a pre-requisite for scalable
      congestion control.  That draft would update the ECN feedback
      protocol sketched out in Appendix A of the standards track
      specification of SCTP [RFC4960] by adding a field to report the
      number of CE marks.

   RTP over UDP:  A pre-requisite for scalable congestion control is for
      both (all) ends of one media-level hop to signal ECN support using
      the ecn-capable-rtp attribute [RFC6679].  However, the reverse
      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.  Nonetheless, the presence of ECT(1) implies that
      both (all) ends of that hop support ECN.

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

2.4.  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 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
   most recent ECT packet of a flow {ToDo: this advice will need to be
   verified experimentally}. This is because a sender might have to
   switch from sending ECT(1) (L4S) packets to sending ECT(0) (Classic)
   packets, or back again, in the middle of a transport-layer flow.



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   Such a switch-over is likely to be very rare, but It could be
   necessary if the path bottleneck moves from a network node that
   supports L4S to one that only supports Classic ECN.  A host ought to
   be able to detect such a change from a change in RTT variation.

2.5.  The Meaning of 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.

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

   The term 'likelihood' is used above to allow for marking and dropping
   to be either probabilistic or deterministic.  The example AQMs in
   [I-D.briscoe-aqm-dualq-coupled] drop and mark probabilistically, so
   the drop probability is arranged to be the square of the marking
   probability.  Nonetheless, an alternative AQM that dropped and marked
   deterministically would be valid, as long as the dropping frequency
   was proportional to the square of the marking frequency.

   Note that, contrary to RFC 3168, an 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.  However, it
   does mark an ECT(0) packet under the same conditions that it would
   have dropped a Not-ECT packet.

3.  IANA Considerations

   This specification contains no IANA considerations.

4.  Security Considerations

   Two approaches to assure the integrity of signals using the new
   identifer are introduced in Appendix B.1.

5.  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.



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   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).  The views expressed
   here are solely those of the authors.

6.  References

6.1.  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,
              <http://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,
              <http://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,
              <http://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, <http://www.rfc-editor.org/info/rfc6679>.

6.2.  Informative References

   [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", 2015, <http://www.bobbriscoe.net/projects/latency/
              dctth_preprint.pdf>.

              (Under submission)

   [I-D.bagnulo-tswg-generalized-ecn]
              Bagnulo, M. and B. Briscoe, "Adding Explicit Congestion
              Notification (ECN) to TCP control packets", draft-bagnulo-
              tswg-generalized-ecn-00 (work in progress), July 2016.




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   [I-D.black-tsvwg-ecn-experimentation]
              Black, D., "Explicit Congestion Notification (ECN)
              Experimentation", draft-black-tsvwg-ecn-experimentation-02
              (work in progress), October 2016.

   [I-D.briscoe-aqm-dualq-coupled]
              Schepper, K., Briscoe, B., Bondarenko, O., and I. Tsang,
              "DualQ Coupled AQM for Low Latency, Low Loss and Scalable
              Throughput", draft-briscoe-aqm-dualq-coupled-01 (work in
              progress), March 2016.

   [I-D.briscoe-tsvwg-aqm-tcpm-rmcat-l4s-problem]
              Briscoe, B., Schepper, K., and M. Bagnulo, "Low Latency,
              Low Loss, Scalable Throughput (L4S) Internet Service:
              Problem Statement", draft-briscoe-tsvwg-aqm-tcpm-rmcat-
              l4s-problem-02 (work in progress), July 2016.

   [I-D.ietf-aqm-fq-codel]
              Hoeiland-Joergensen, T., McKenney, P.,
              dave.taht@gmail.com, d., Gettys, J., and E. Dumazet, "The
              FlowQueue-CoDel Packet Scheduler and Active Queue
              Management Algorithm", draft-ietf-aqm-fq-codel-06 (work in
              progress), March 2016.

   [I-D.ietf-aqm-pie]
              Pan, R., Natarajan, P., Baker, F., and G. White, "PIE: A
              Lightweight Control Scheme To Address the Bufferbloat
              Problem", draft-ietf-aqm-pie-10 (work in progress),
              September 2016.

   [I-D.ietf-tcpm-accurate-ecn]
              Briscoe, B., Kuehlewind, M., and R. Scheffenegger, "More
              Accurate ECN Feedback in TCP", draft-ietf-tcpm-accurate-
              ecn-02 (work in progress), October 2016.

   [I-D.ietf-tcpm-cubic]
              Rhee, I., Xu, L., Ha, S., Zimmermann, A., Eggert, L., and
              R. Scheffenegger, "CUBIC for Fast Long-Distance Networks",
              draft-ietf-tcpm-cubic-02 (work in progress), August 2016.

   [I-D.ietf-tcpm-dctcp]
              Bensley, S., Eggert, L., Thaler, D., Balasubramanian, P.,
              and G. Judd, "Datacenter TCP (DCTCP): TCP Congestion
              Control for Datacenters", draft-ietf-tcpm-dctcp-02 (work
              in progress), July 2016.






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   [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-07 (work in progress), July 2016.

   [I-D.moncaster-tcpm-rcv-cheat]
              Moncaster, T., Briscoe, B., and A. Jacquet, "A TCP Test to
              Allow Senders to Identify Receiver Non-Compliance", draft-
              moncaster-tcpm-rcv-cheat-03 (work in progress), July 2014.

   [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.

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

   [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,
              <http://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,
              <http://www.rfc-editor.org/info/rfc2474>.






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   [RFC2983]  Black, D., "Differentiated Services and Tunnels",
              RFC 2983, DOI 10.17487/RFC2983, October 2000,
              <http://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,
              <http://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,
              <http://www.rfc-editor.org/info/rfc3540>.

   [RFC3649]  Floyd, S., "HighSpeed TCP for Large Congestion Windows",
              RFC 3649, DOI 10.17487/RFC3649, December 2003,
              <http://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,
              <http://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, <http://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,
              <http://www.rfc-editor.org/info/rfc4342>.

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

   [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,
              <http://www.rfc-editor.org/info/rfc5562>.







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   [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,
              <http://www.rfc-editor.org/info/rfc5622>.

   [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,
              <http://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,
              <http://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,
              <http://www.rfc-editor.org/info/rfc7560>.

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

   [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.  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 A.1) as the L4S
   identifier.  At the end, Appendix A.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.  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.



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A.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 contain an ECN-capable packet.  However, such cases
      would be unusual.

   Risk of reordering classic CE packets:  Having to classify all CE
      packets as L4S risks some classic CE packets arriving early, which
      is a form of reordering.  Reordering can cause the TCP sender to
      retransmit spuriously.  However, one or two packets delivered
      early does not cause any spurious retransmissions because the
      subsequent packets continue to move the cumulative acknowledgement



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      boundary forwards.  Anyway, the risk of reordering would be low,
      because: i) it is quite unusual to experience more than one
      bottleneck queue on a path; ii) even then, reordering would only
      occur if there was simultaneous mixing of classic and L4S traffic,
      which would be more unlikely in an access link, which is where
      most bottlenecks are located; iii) even then, spurious
      retransmissions would only occur if a contiguous sequence of three
      or more classic CE packets from one bottleneck arrived at the
      next, which should in itself happen very rarely with a good AQM.
      The risk would be completely eliminated in AQMs that were
      transport-aware (but they should not need to be);

   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.bagnulo-tswg-generalized-ecn] proposes an experiment
      in which all TCP control packets and retransmissions are ECN-
      capable.

   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.








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

   Definition:

      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.



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   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
      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;

      Global-use 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
         global-use DSCPs.

         Note that today a global-use DSCP gives little more assurance
         of end-to-end service than a local-use DSCP.  In future the
         global-use range 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



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

A.3.  ECN capability alone

   Definition:

      This approach uses ECN capability alone as the L4S identifier.  It
      is only feasible if classic ECN is not widely deployed.  The
      specific definition of codepoints would be:

      *  Any ECN codepoint other than Not-ECT would signify an L4S-
         capable sender;

      *  ECN codepoints would not be used for classic [RFC3168] ECN, and
         the classic network service would only be used for Not-ECT
         packets.

      This approach would only be feasible if

      A.  it was generally agreed that there was little chance of any
          classic [RFC3168] ECN deployment in any network nodes;

      B.  it was generally agreed that there was little chance of any
          client devices being deployed with classic [RFC3168] TCP-ECN
          on by default (note that classic TCP-ECN is already on-by-
          default on many servers);

      C.  for TCP connections, developers of client OSs would all have
          to agree not to encourage further deployment of classic ECN.
          Specifically, at the start of a TCP connection classic ECN
          could be disabled during negotation of the ECN capability:

          +  an L4S-capable host would have to disable ECN if the
             corresponding host did not support accurate ECN feedback
             [RFC7560], which is a prerequisite for the L4S service;

          +  developers of operating systems for user devices would only
             enable ECN by default for TCP once the stack implemented
             L4S and accurate ECN feedback [RFC7560] including
             requesting accurate ECN feedback by default.

   Cons:




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   Near-infeasible deployment constraints:  The constraints for
      deployment above represent a highly unlikely, but not completely
      impossible, set of circumstances.  If, despite the above measures,
      a pair of hosts did negotiate to use classic ECN, their packets
      would be classified into the same queue as L4S traffic, and if
      they had to compete with a long-running L4S flow they would get a
      very small capacity share;

   ECN hard in some lower layers:  See the same issue with "ECT(1) and
      CE codepoints" (Appendix A.1);

   Non-L4S service for control packets:  See the same issue with "ECT(1)
      and CE codepoints" (Appendix A.1).

   Pros:

   Consumes no additional codepoints:  The ECT(1) codepoint and all
      spare Diffserv codepoints would remain available for future use;

   Should work e2e:  As with "ECT(1) and CE codepoints" (Appendix A.1);

   Should work in tunnels:  As with "ECT(1) and CE codepoints"
      (Appendix A.1).

A.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 AQMs below the IP layer;

A.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



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   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.

A.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 A.2,
   Appendix A.3 and Appendix A.1, for six issues that set them apart.

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

             Note 1: Only feasible if classic ECN is obsolete.

    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 'ECN' scheme
   shares the 'eventual' scores of the 'ECT(1) + CE' scheme.  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
   optimisitically become this good, given sufficient effort.  The table
   summarises the text and is not meant to be understandable without
   having read the text.








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

   The ECT(1) codepoint of the ECN field has already been assigned once
   for experimental use as the ECN nonce [RFC3540].  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 without carefully assessing competing
   potential uses.  These fall into the following categories:

B.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).  [RFC3540] proposes 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, termed the ECN nonce.  If
   any packet is lost or congestion marked, the receiver will miss that
   bit of the sequence.  An ECN Nonce receiver has to feed back the
   least significant bit of the sum, so it cannot suppress feedback of a
   loss or mark without a 50-50 chance of guessing the sum incorrectly.

   As far as is known, the ECN Nonce has never been deployed, and it was
   only implemented for a couple of testbed evaluations.  It would be
   nearly impossible to deploy now, because any misbehaving receiver can
   simply opt-out, which would be unremarkable given all receivers
   currently opt-out.

   Other ways to protect TCP feedback integrity have since been
   developed that do not consume any extra codepoints in the base 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 [I-D.moncaster-tcpm-rcv-cheat].
      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.

   ECN in RTP [RFC6679] is defined so that the receiver can ask the
   sender to send all ECT(0); all ECT(1); or both randomly.  It



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   recommends that the receiver asks for ECT(0), which is the default.
   The sender can choose to ignore the receiver's request.  A rather
   complex but optional nonce mechaism was included in early drafts of
   RFC 6679, but it was replaced with a statement that a nonce mechanism
   is not specified, explaining that misbehaving receivers could opt-out
   anyway.  RFC 6679 as published gives no rationale for why ECT(1) or
   'random' might be needed, but it warns that 'random' would make
   header compression highly inefficient.  The possibility of using
   ECT(1) may have been left in the RFC to allow a nonce mechanism to be
   added later.

   Therefore, it seems unlikely that anyone has implemented the optional
   use of ECT(1) for RTP.  Even if they have, it seems even less likely
   that any deployment actually uses it.  However these assumptions will
   need to be verified.

B.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] {ToDo: consider Jonathan Morton's Explicit Load Regulation (ELR)
   if relevant, once the promised write-up appears}.

   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 A.

Authors' Addresses









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   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)
   Simula Research Lab

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


   Ing-jyh Tsang
   Nokia Bell Labs
   Antwerp
   Belgium

   Email: ing-jyh.tsang@nokia.com





























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