Operational Guidance for Deployment of L4S in the Internet
draft-white-tsvwg-l4sops-00

Versions: 00                                                            
Transport Area Working Group                               G. White, Ed.
Internet-Draft                                                 CableLabs
Intended status: Informational                             July 30, 2020
Expires: January 31, 2021


       Operational Guidance for Deployment of L4S in the Internet
                      draft-white-tsvwg-l4sops-00

Abstract

   This is an early, work-in-progress draft - a start at getting some of
   the ideas from the mailing list and email exchanges on paper.

   This draft is intended to provide guidance to operators of end-
   systems, operators of networks, and researchers in order to ensure
   reasonable fairness between L4S and Classic flows sharing a single-
   queue RFC3168 bottleneck link.  This draft identifies opportunites to
   prevent and/or detect and resolve fairness problems in such networks.

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
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   This Internet-Draft will expire on January 31, 2021.

Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Per-Flow Fairness . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Operator of an L4S host . . . . . . . . . . . . . . . . . . .   3
     3.1.  CDN Servers . . . . . . . . . . . . . . . . . . . . . . .   3
     3.2.  Other hosts . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Operator of a Network . . . . . . . . . . . . . . . . . . . .   4
     4.1.  Configure AQM to treat ECT1 as NotECT . . . . . . . . . .   4
     4.2.  Configure Non-Coupled Dual Queue  . . . . . . . . . . . .   5
     4.3.  WRED with ECT1 Differentation . . . . . . . . . . . . . .   5
     4.4.  ECT1 Tunnel Bypass  . . . . . . . . . . . . . . . . . . .   6
     4.5.  Disable RFC3168 ECN Marking . . . . . . . . . . . . . . .   6
     4.6.  Re-mark ECT1 to NotECT Prior to AQM . . . . . . . . . . .   6
   5.  Researchers . . . . . . . . . . . . . . . . . . . . . . . . .   6
     5.1.  Detection of Classic ECN FIFO Bottlenecks . . . . . . . .   6
     5.2.  End-to-end measurement of L4S vs. Classic performance . .   6
   6.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .   6
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   9.  Informative References  . . . . . . . . . . . . . . . . . . .   7
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   In the majority of network paths, including paths where the
   bottleneck link utilizes packet drops (either due to buffer overrun
   or active queue management) in response to congestion, as well as
   paths that implement a 'flow-queuing' scheduler such as fq_codel or
   Cobalt, and those that implement dual-Q-coupled AQM, L4S traffic
   coexists well with classic congestion controlled traffic.

   On network paths where the bottleneck link implements a shared-queue
   (FIFO) with an Active Queue Management algorithm that provides
   Explicit Congestion Notification signaling according to RFC3168, it
   has been demonstrated that when a set of long-running flows
   comprising both "Classic" congestion controlled flows and L4S-
   compliant congestion controlled flows compete for bandwidth, the
   classic congestion controlled flows may achieve lower throughput when
   compared to the L4S congestion controlled flows.  This 'unfairness'
   between the two classes appears to be more pronounced on longer RTT
   paths (e.g. 50ms and above) and/or at higher link rates (e.g. 50 Mbps
   and above).




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   The root cause of this unfairness is that RFC3168 does not
   differentiate between packets marked ECT0 (used by classic senders)
   and those marked ECT1 (used by L4S senders), and provides an
   identical congestion signal (CE marks) to both classes, whereas the
   two classes respond differently to that congestion signal.  The
   classic senders expect that CE marks are sent very rarely (e.g.
   approximately 1 CE mark every 200 round trips on a 50 Mbps x 50ms
   path) while the L4S senders expect very frequent CE marking (e.g.
   approximately 2 CE marks per round trip).  The result is that the
   classic senders respond to the CE marks provided by the bottleneck by
   yielding capacity to the L4S flows.  While this has not been
   demonstrated to cause starvation of the classic flows, the resulting
   rate imbalance can be a cause of concern.

2.  Per-Flow Fairness

   There are a number of factors that influence the relative rates
   achieved by a set of congestion controlled flows sharing a queue in a
   bottleneck link.

   TODO: discuss startup & convergence times, short flows, RTT-
   unfairness, differences in deployed CC algorithms, etc.

   TODO: also mention that flow sharding is commonplace, so per-flow
   fairness does not imply per-application fairness

3.  Operator of an L4S host

   Support for L4S involves both endpoints: ECT1 marking & L4S-
   compatible congestion control on the sender, and ECN feedback on the
   receiver.  Between these two entities, it is incumbent upon the
   sender to evaluate the potential for unfairness and make decisions
   whether or not to use L4S congestion control.  The receiver is not
   expected to perform any testing or monitoring for unfairness, and is
   also not expected to invoke any active response in the case that
   unfairness occurs.

3.1.  CDN Servers

   Some hosts (such as CDN leaf nodes and servers internal to an ISP)
   are deployed in environments in which they serve content to a
   constrained set of networks or clients.  The operator of such hosts
   may be able to determine whether there is the possibility of RFC3168
   FIFO bottlenecks being present, and utilize this information to make
   decisions on selectively deploying L4S.

   o  Prior to deploying L4S on servers:




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      *  Consult with network operators on presence of RFC3168 FIFO
         bottlenecks

      *  Perform downstream tests per access network

         +  Tests (TBD) to detect absence of RFC 3168

         +  Enable AccECN feedback, but enable/disable L4S per access
            network

   o  In-band RFC3168 detection and monitoring:

      *  Real-time response (fallback)

      *  Non-real-time response (disable for future connections)

3.2.  Other hosts

   o  In-band RFC3168 detection (and possibly fallback)

   o  Per-dst path test:

      *  For a connection capable of L4S feedback

      *  If CE feedback, perform active test (TBD) for RFC3168 presence

      *  Could cache result per-dst

   o  Query a TBD public whitelist of domains that are participating in
      L4S experiment

4.  Operator of a Network

   While it is, of course, preferred for networks to deploy L4S-capable
   high fidelity congestion signaling, a network operator who has
   deployed equipment in a likely bottleneck link location (i.e. a link
   that is expected to be fully saturated) that is configured with an
   RFC3168 FIFO AQM can take certain steps in order to improve rate
   fairness between classic traffic and L4S traffic.

4.1.  Configure AQM to treat ECT1 as NotECT

   If equipment is configurable in such as way as to only supply CE
   marks to ECT0 packets, and treat ECT1 packets identically to NotECT,
   or is upgradable to support this capability, doing so will eliminate
   the risk of unfairness.





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4.2.  Configure Non-Coupled Dual Queue

   Equipment supporting RFC3168 may be configurable to enable two
   parallel queues for the same traffic class, with classification done
   based on the ECN field.

   Option 1:

   o  Configure 2 queues, both with ECN; 50:50 WRR scheduler

   o  Queue #1: ECT1 & CE packets - Shallow immediate AQM target

   o  Queue #2: ECT0 & NotECT packets - Classic AQM target

   o  Outcome

      *  n L4S flows and m long-running Classic flows

      *  if m & n are non-zero, get 1/2n and 1/2m of the capacity,
         otherwise 1/n or 1/m

      *  never < 1/2 each flow's rate if all had been Classic

   Option 2:

   o  Configure 2 queues, both with AQM; 50:50 WRR scheduler

   o  Queue #1: ECT1 & NotECT packets - ECN disabled

   o  Queue #2: ECT0 & CE packets - ECN enabled

   o  Outcome

      *  ECT1 treated as NotECT

      *  Flow balance for the 2 queues the same as in option 1

4.3.  WRED with ECT1 Differentation

   This configuration is similar to Option 2 in the previous section,
   but uses a single queue with WRED functionality.

   o  Configure the queue with two WRED classes

   o  Class #1: ECT1 & NotECT packets - ECN disabled

   o  Class #2: ECT0 & CE packets - ECN enabled




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4.4.  ECT1 Tunnel Bypass

   Using an RFC6040 compatibility mode tunnel, tunnel ECT1 traffic
   through the RFC3168 bottleneck with the outer header indicating Not-
   ECT.

   Two variants

   1.  per-domain: tunnel ECT1 pkts to domain edge towards dst

   2.  per-dst: tunnel ECT1 pkts to dst

4.5.  Disable RFC3168 ECN Marking

   While not a recommended alternative, disabling RFC3168 ECN marking
   eliminates the fairness issue.  Clearly a downside to this approach
   is that classic senders will no longer get the benefits of Explict
   Congestion Notification.

4.6.  Re-mark ECT1 to NotECT Prior to AQM

   While not a recommended alternative, remarking ECT1 packets as NotECT
   ensures that they are treated identically to classic NotECT senders.
   However, this also eliminates the possibility of downstream L4S
   bottlenecks providing high fidelity congestion signals.

5.  Researchers

5.1.  Detection of Classic ECN FIFO Bottlenecks

   TODO: Describe active testing methods, in-band or out-of-band, that
   can distinguish FIFO from FQ.

5.2.  End-to-end measurement of L4S vs. Classic performance

   TBD

6.  Contributors

   Thanks to Bob Briscoe, Jake Holland, Koen De Schepper, Olivier
   Tilmans, Tom Henderson, Asad Ahmed, and members of the TSVWG mailing
   list for their contributions to this document.

7.  IANA Considerations

   None.





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8.  Security Considerations

   None.

9.  Informative References

   [I-D.ietf-tsvwg-aqm-dualq-coupled]
              Schepper, K., Briscoe, B., and G. White, "DualQ Coupled
              AQMs for Low Latency, Low Loss and Scalable Throughput
              (L4S)", draft-ietf-tsvwg-aqm-dualq-coupled-12 (work in
              progress), July 2020.

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

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

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [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>.

Author's Address

   Greg White (editor)
   CableLabs

   Email: g.white@cablelabs.com






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