Network Working Group N. Khademi Internet-Draft M. Welzl Intended status: Experimental University of Oslo Expires: December 2, 2016 G. Armitage Swinburne University of Technology G. Fairhurst University of Aberdeen May 31, 2016 TCP Alternative Backoff with ECN (ABE) draft-khademi-tcpm-alternativebackoff-ecn-00 Abstract This memo updates the TCP sender-side reaction to a congestion notification received via Explicit Congestion Notification (ECN). The updated method reduces FlightSize in Congestion Avoidance by a smaller amount than the TCP reaction to loss. The intention is to achieve good throughput when the queue at the bottleneck is smaller than the bandwidth-delay-product of the connection. This is more likely when an Active Queue Management (AQM) mechanism has used ECN to CE-mark a packet, than when a packet was lost. Future versions of this document will also describe a corresponding method for SCTP. 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/. 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 December 2, 2016. Copyright Notice Copyright (c) 2016 IETF Trust and the persons identified as the document authors. All rights reserved. Khademi, et al. Expires December 2, 2016 [Page 1] Internet-Draft ABE May 2016 This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://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. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Discussion: Choice of ABE Multiplier . . . . . . . . . . . . . 3 4. Specification . . . . . . . . . . . . . . . . . . . . . . . . . 4 5. Status of the Update . . . . . . . . . . . . . . . . . . . . . 5 6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 5 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5 8. Security Considerations . . . . . . . . . . . . . . . . . . . . 5 9. Revision Information . . . . . . . . . . . . . . . . . . . . . 6 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6 10.1. Normative References . . . . . . . . . . . . . . . . . . . 6 10.2. Informative References . . . . . . . . . . . . . . . . . . 7 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 7 Khademi, et al. Expires December 2, 2016 [Page 2] Internet-Draft ABE May 2016 1. Definitions The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119]. 2. Introduction Complementing [I-D.AQM-ECN-benefits], [I-D.ECN-response] encourages wider ECN deployment by relaxing a rule that prohibited certain experiments. This rule, from [RFC3168], required the congestion control response to an ECN CE-mark to be similar to the response to packet loss. Without this rule, it is possible to define a new sender reaction to being notified of a CE-mark that differs from the reaction to the detection of loss. [I-D.ECN-response] provides the rationale for such a different behaviour; in brief, a CE-mark is likely to indicate a shorter queue than packet loss, and the standard TCP backoff behaviour defined in [RFC5681] entails reduced link utilisation in situations with short queues and low statistical multiplexing. This memo proposes a concrete sender-side-only congestion control response that remedies this problem. Much of the background to this proposal can be found in [ABE2015]. Using a mix of experiments, theory and simulations with standard NewReno and CUBIC, [ABE2015] recommends enabling ECN and letting individual TCP senders use a larger multiplicative decrease factor as a reaction to the receiver reporting ECN CE-marks from AQM-enabled bottlenecks. Such a change is noted to result in "...significant performance gains in lightly-multiplexed scenarios, without losing the delay-reduction benefits of deploying CoDel or PIE" [I-D.CoDel] [I-D.PIE]. This is achieved when reacting to ECN-Echo in Congestion Avoidance by multiplying cwnd and sstthresh with a value in the range [0.7..0.85]. 3. Discussion: Choice of ABE Multiplier Alternative Backoff with ECN (ABE) decouples a TCP sender's reaction to loss and ECN CE-marks in Congestion Avoidance. The description respectively uses beta_{loss} and beta_{ecn} to refer to the multiplicative decrease factors applied in response to packet loss, and also in response to a receiver indicating that an ECN CE-mark was received on an ECN-enabled TCP connection (based on the terms used in [ABE2015]). For non-ECN-enabled TCP connections, no ECN CE-marks are received and only beta_{loss} applies. In other words, in response to detected loss: Khademi, et al. Expires December 2, 2016 [Page 3] Internet-Draft ABE May 2016 FlightSize_(n+1) = FlightSize_n * beta_{loss} and in response to an indication of a received ECN CE-mark: FlightSize_(n+1) = FlightSize_n * beta_{ecn} where, as in [RFC5681], FlightSize is the amount of outstanding data in the network, upper-bounded by the sender's congestion window (cwnd) and the receiver's advertised window (rwnd). The higher the values of beta_{loss} and beta_{ecn}, the less aggressive the response of any individual backoff event. The appropriate choice for beta_{loss} and beta_{ecn} values is a balancing act between path utilisation and draining the bottleneck queue. More aggressive backoff (smaller beta_*) risks underutilising the path, while less aggressive backoff (larger beta_*) can result in slower draining of the bottleneck queue. The Internet has already been running with at least two different beta_{loss} values for several years: the value in [RFC5681] is 0.5, and Linux CUBIC uses 0.7. ABE proposes no change to beta_{loss} used by any current TCP implementations. beta_{ecn} depends on how the response of a TCP connection to shallow AQM marking thresholds is optimised. beta_{loss} reflects the preferred response of each TCP algorithm when faced with exhaustion of buffers (of unknown depth) signalled by packet loss. Consequently, for any given TCP algorithm the choice of beta_{ecn} is likely to be algorithm-specific, rather than a constant multiple of the algorithm's existing beta_{loss}. A range of experiments (section IV, [ABE2015]) with NewReno and CUBIC over CoDel and PIE in lightly-multiplexed scenarios have explored this choice of parameter. These experiments indicate that CUBIC connections benefit from beta_{ecn} of 0.85 (cf. beta_{loss} = 0.7), and NewReno connections see improvements with beta_{ecn} in the range 0.7 to 0.85 (cf. beta_{loss} = 0.5). 4. Specification This document RECOMMENDS that experimental deployments multiply the FlightSize by 0.8 and reduce the slow start threshold 'ssthresh' in Congestion Avoidance in response to reception of a TCP segment that sets the ECN-Echo flag." Khademi, et al. Expires December 2, 2016 [Page 4] Internet-Draft ABE May 2016 5. Status of the Update This update is a sender-side only change. Like other changes to congestion-control algorithms it does not require any change to the TCP receiver or to network devices (except to enable an ECN-marking algorithm [RFC3168] [RFC7567]). If the method is only deployed by some TCP senders, and not by others, the senders that use this method can gain advantage, possibly at the expense of other flows that do not use this updated method. This advantage applies only to ECN- marked packets and not to loss indications. Hence, the new method can not lead to congestion collapse. The present specification has been assigned an Experimental status, to provide Internet deployment experience before being proposed as a Standards-Track update. 6. Acknowledgements Authors N. Khademi, M. Welzl and G. Fairhurst 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 are solely those of the authors. The authors would like to thank the following people for their contributions to [ABE2015]: Chamil Kulatunga, David Ros, Stein Gjessing, Sebastian Zander. Thanks to (in alphabetical order) Bob Briscoe, Markku Kojo, John Leslie, Dave Taht and the TCPM WG for providing valuable feedback on this document. The authors would like to thank feedback on the congestion control behaviour specified in this update received from the IRTF Internet Congestion Control Research Group (ICCRG). 7. IANA Considerations XX RFC ED - PLEASE REMOVE THIS SECTION XXX This memo includes no request to IANA. 8. Security Considerations The described method is a sender-side only transport change, and does not change the protocol messages exchanged. The security considerations of [RFC3168] therefore still apply. Khademi, et al. Expires December 2, 2016 [Page 5] Internet-Draft ABE May 2016 This document describes a change to TCP congestion control with ECN that will typically lead to a change in the capacity achieved when flows share a network bottleneck. Similar unfairness in the way that capacity is shared is also exhibited by other congestion control mechanisms that have been in use in the Internet for many years (e.g., CUBIC [I-D.CUBIC]). Unfairness may also be a result of other factors, including the round trip time experienced by a flow. This advantage applies only to ECN-marked packets and not to loss indications, and will therefore not lead to congestion collapse. 9. Revision Information XX RFC ED - PLEASE REMOVE THIS SECTION XXX -00. draft-khademi-tsvwg-ecn-response-00 and draft-khademi-tcpm-alternativebackoff-ecn-00 replace draft-khademi-alternativebackoff-ecn-03, following discussion in the TSVWG and TCPM working groups. 10. References 10.1. Normative References [I-D.ECN-response] Khademi, N., Welzl, M., Armitage, G., and G. Fairhurst, "Updating the ECN Congestion Control Response", Internet- draft, IETF work-in-progress draft-khademi-tsvwg-ecn-response-00, May 2016. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/ RFC2119, March 1997, . [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, . [RFC5681] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion Control", RFC 5681, DOI 10.17487/RFC5681, September 2009, . [RFC7567] Baker, F., Ed. and G. Fairhurst, Ed., "IETF Recommendations Regarding Active Queue Management", Khademi, et al. Expires December 2, 2016 [Page 6] Internet-Draft ABE May 2016 BCP 197, RFC 7567, DOI 10.17487/RFC7567, July 2015, . 10.2. Informative References [ABE2015] Khademi, N., Welzl, M., Armitage, G., Kulatunga, C., Ros, D., Fairhurst, G., Gjessing, S., and S. Zander, "Alternative Backoff: Achieving Low Latency and High Throughput with ECN and AQM", CAIA Technical Report CAIA- TR-150710A, Swinburne University of Technology, July 2015, . [I-D.AQM-ECN-benefits] Fairhurst, G. and M. Welzl, "The Benefits of using Explicit Congestion Notification (ECN)", Internet-draft, IETF work-in-progress draft-ietf-aqm-ecn-benefits-08, November 2015. [I-D.CUBIC] Rhee, I., Xu, L., Ha, S., Zimmermann, A., Eggert, L., and R. Scheffenegger, "CUBIC for Fast Long-Distance Networks", Internet-draft, IETF work-in-progress draft-ietf-tcpm-cubic-01, January 2016. [I-D.CoDel] Nichols, K., Jacobson, V., McGregor, V., and J. Iyengar, "The Benefits of using Explicit Congestion Notification (ECN)", Internet-draft, IETF work-in-progress draft-ietf-aqm-codel-02, December 2015. [I-D.PIE] Pan, R., Natarajan, P., Baker, F., White, G., VerSteeg, B., Prabhu, M., Piglione, C., and V. Subramanian, "PIE: A Lightweight Control Scheme To Address the Bufferbloat Problem", Internet-draft, IETF work-in-progress draft-ietf-aqm-pie-03, November 2015. Authors' Addresses Naeem Khademi University of Oslo PO Box 1080 Blindern Oslo, N-0316 Norway Email: naeemk@ifi.uio.no Khademi, et al. Expires December 2, 2016 [Page 7] Internet-Draft ABE May 2016 Michael Welzl University of Oslo PO Box 1080 Blindern Oslo, N-0316 Norway Email: michawe@ifi.uio.no Grenville Armitage Centre for Advanced Internet Architectures Swinburne University of Technology PO Box 218 John Street, Hawthorn Victoria, 3122 Australia Email: garmitage@swin.edu.au Godred Fairhurst University of Aberdeen School of Engineering, Fraser Noble Building Aberdeen, AB24 3UE UK Email: gorry@erg.abdn.ac.uk Khademi, et al. Expires December 2, 2016 [Page 8]