Updating the Explicit Congestion Notification (ECN) Congestion Control Response
draft-khademi-tsvwg-ecn-response-00

Abstract

RFC3168 and RFC4774 state that, upon the receipt by an ECN-Capable transport of a single CE packet, the congestion control algorithms followed at the end-systems MUST be essentially the same as the congestion control response to a single dropped packet. This document relaxes this rule in order to encourage experimentation with different backoff strategies. This sender-side update makes it possible to achieve greater benefits with ECN, encouraging wider ECN deployment.

Status of This Memo

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This Internet-Draft will expire on December 2, 2016.

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Table of Contents

• 1. Introduction
• 2. Discussion
• 2.1. Why Use ECN to Vary the Degree of Backoff?
• 2.2. Focus on ECN as Defined in RFC3168
• 3. Updating the Sender-side ECN Reaction
• 3.1. RFC 2119
• 3.2. Update to RFCs 3168 and 4774
• 3.3. ABE: An Experiment That Follows the New Rule
• 4. Acknowledgements
• 5. IANA Considerations
• 6. Security Considerations
• 7. Revision Information
• 8. References
• 8.1. Normative References
• 8.2. Informative References
• Authors' Addresses

1. Introduction

Explicit Congestion Notification (ECN) is specified in [RFC3168]. It allows a network device that uses Active Queue Management (AQM) to set the Congestion Experienced (CE) codepoint in the ECN field of the IP packet header, rather than to drop ECN-capable packets when incipient congestion is detected. When an ECN-capable transport is used over a path that supports ECN, this provides the opportunity for flows to improve their performance in the presence of incipient congestion [I-D.AQM-ECN-benefits].

[RFC3168] not only specifies the router use of the ECN field, it also specifies a TCP procedure for using ECN. This states that a TCP sender should treat the ECN indication of congestion in the same way as that of a non-ECN-Capable TCP flow experiencing loss, by halving the congestion window "cwnd" and by reducing the slow start threshold "ssthresh". [RFC5681] stipulates that TCP congestion control sets "ssthresh" to max(FlightSize / 2, 2*SMSS) in response to packet loss. This corresponds to a backoff multiplier of 0.5 (halving cwnd and sshthresh after packet loss). Consequently, a standard TCP flow using this reaction needs significant network queue space: it can only fully utilise a bottleneck when the length of the link queue (or the AQM dropping threshold) is at least the bandwidth-delay product (BDP) of the flow.

A backoff multiplier of 0.5 is not the only available strategy. As defined in [I-D.CUBIC], CUBIC multiplies the current cwnd by 0.7 in response to loss ( the Linux implementation of CUBIC has used a multiplier of 0.7 since kernel version 2.6.25 released in 2008). Consequently, CUBIC utilises paths well even when the bottleneck queue is shorter than the bandwidth-delay product of the flow. However, in the case of a DropTail (FIFO) queue without AQM, such less-aggressive backoff increases the risk of creating a standing queue [CODEL2012].

Devices implementing AQM are likely to be the dominant (and possibly only) source of ECN CE-marking for packets from ECN-capable senders. AQM mechanisms typically strive to maintain a small average queue length, regardless of the bandwidth-delay product of flows passing through them. Receipt of an ECN CE-mark might therefore reasonably be taken to indicate that a small bottleneck queue exists in the path, and hence the TCP flow would benefit from using a less aggressive backoff multiplier. Such behavior is however prohibited by the rules in [RFC3168].

ECN has seen little deployment so far. Apple recently announced their intention to enable ECN in iOS 9 and OS X 10.11 devices [WWDC2015]. By 2014, server-side ECN negotiation was observed to be provided by the majority of the top million web servers [PAM2015], and only 0.5% of websites incurred additional connection setup latency using RFC3168-compliant ECN-fallback mechanisms. [RFC7567] states that "deployed AQM algorithms SHOULD support Explicit Congestion Notification (ECN) as well as loss to signal congestion to endpoints" and [I-D.AQM-ECN-benefits] encourages this deployment. However, the limitation of [RFC3168] restricts a sender to react to notification of a CE-mark in the same way as if a packet was lost. This prohibits experimentation with ECN mechanisms that could yield greater benefits. This specification therefore relaxes this constraint.

2. Discussion

2.1. Why Use ECN to Vary the Degree of Backoff?

The classic rule-of-thumb dictates that a transport provides a BDP of bottleneck buffering if a TCP connection wishes to optimise path utilisation. A single TCP connection running through such a bottleneck will have opened cwnd up to 2*BDP by the time packet loss occurs. [RFC5681]'s halving of cwnd and ssthresh pushes the TCP connection back to allowing only a BDP of packets in flight -- just sufficient to maintain 100% utilisation of the network path.

AQM schemes like CoDel [I-D.CoDel] and PIE [I-D.PIE] use congestion notifications to constrain the queuing delays experienced by packets, rather than in response to impending or actual bottleneck buffer exhaustion. With current default delay targets, CoDel and PIE both effectively emulate a shallow buffered bottleneck (section II, [ABE2015]) while allowing short traffic bursts into the queue. This interacts acceptably for TCP connections over low BDP paths, or highly multiplexed scenarios (many concurrent TCP connections). However, it interacts badly with lightly-multiplexed cases (few concurrent connections) over a high BDP path. Conventional TCP backoff in such cases leads to gaps in packet transmission and under-utilisation of the path.

The idea to react differently to loss upon detecting an ECN CE-mark pre-dates [ABE2015]. [ICC2002] also proposed using ECN CE-marks to modify TCP congestion control behaviour, using a larger multiplicative decrease factor in conjunction with a smaller additive increase factor to work with RED-based bottlenecks that were not necessarily configured to emulate a shallow queue.

2.2. Focus on ECN as Defined in RFC3168

Some mechanisms rely on ECN semantics that differ from the definitions in [RFC3168] -- for example, Congestion Exposure (ConEx) [RFC7713] and DCTCP [I-D.ietf-tcpm-dctcp] need more accurate ECN information than the feedback mechanism in [RFC3168] offers (defined in [I-D.ietf-tcpm-accurate-ecn]). Such mechanisms allow a sending rate adjustment more frequent than each RTT. These mechanisms are out of the scope of the current document.

3. Updating the Sender-side ECN Reaction

This section specifies an update to [RFC3168] (and corresponding text in [RFC4774]) and refers to an experiment that is possible within the framework provided by the update.

3.1. RFC 2119

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].

3.2. Update to RFCs 3168 and 4774

This document specifies an update to the TCP sender reaction that follows when the TCP receiver signals that ECN CE-marked packets have been received.

[RFC3168] and [RFC4774] contain the following text:

"Upon the receipt by an ECN-Capable transport of a single CE packet, the congestion control algorithms followed at the end-systems MUST be essentially the same as the congestion control response to a *single* dropped packet. For example, for ECN-Capable TCP the source TCP is required to halve its congestion window for any window of data containing either a packet drop or an ECN indication."

This memo updates the preceding text by replacing it with the following text:

"Upon the receipt by an ECN-Capable transport of a single CE packet, the congestion control algorithms followed at the end-systems MUST make a congestion control response as specified in [RFC3168] or its updates. For example, for ECN-Capable TCP the source TCP could halve its congestion window for any window of data containing either a packet drop or an ECN indication."

The first paragraph of Section 6.1.2, "The TCP Sender", in [RFC3168] contains the following text:

"If the sender receives an ECN-Echo (ECE) ACK packet (that is, an ACK packet with the ECN-Echo flag set in the TCP header), then the sender knows that congestion was encountered in the network on the path from the sender to the receiver. The indication of congestion should be treated just as a congestion loss in non-ECN-Capable TCP. That is, the TCP source halves the congestion window "cwnd" and reduces the slow start threshold "ssthresh"."

This memo updates the preceding text by replacing it with the following text:

"If the sender receives an ECN-Echo (ECE) ACK packet (that is, an ACK packet with the ECN-Echo flag set in the TCP header), then the sender knows that congestion was encountered in the network on the path from the sender to the receiver. An indication of congestion, signalled by reception of the ECN-Echo flag (with the semantics defined in [RFC3168]) MUST produce a rate reduction of at least 15%, so that flows sharing the same bottleneck can increase their share of the capacity. The indication of congestion could be treated in the same way as if the flow had experienced loss, but future congestion control methods are allowed to specify a reduction that is less than the reduction for congestion loss.

An ECN-capable network device cannot eliminate the possibility of packet loss. A drop may still occur due to a traffic burst exceeding the instantaneous available capacity of a network buffer or as a result of the AQM algorithm (overload protection mechanisms, etc [RFC7567]). Whatever the cause of loss, detection of a missing packet needs to trigger the standard loss-based congestion control response". This update explicitly does not change the use of standard TCP mechanisms following loss, as required in [RFC3168].

3.3. ABE: An Experiment That Follows the New Rule

This update to [RFC3168] enables experimentation with a different backoff behavior in response to a CE-mark than in response to packet loss. One experiment, called "Alternative Backoff with ECN" (ABE), is based upon [ABE2015] and defined in [I-D.ABE].

4. Acknowledgements

The 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.

5. IANA Considerations

XX RFC ED - PLEASE REMOVE THIS SECTION XXX

This memo includes no request to IANA.

6. 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.

A congestion control backoff that is less in response to ECN than the response to a packet loss can lead to a change in the capacity achieved when flows share a network bottleneck. This can result in redistribution of capacity between sharing flows, potentially resulting in unfairness in the way that capacity is shared. This potential gain applies only to ECN-marked packets using the updated method (and not to detected packet loss). Similar unfairness can be exhibited by congestion control mechanisms that have been used 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.

Packet loss can be expected from an AQM algorithm experiencing persistent queuing, but could also imply the presence of faulty equipment or media in a path, or it may imply the presence of congestion [RFC7567]. The update does not change the congestion control response to packet loss, and will therefore not lead to congestion collapse.

7. 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.

8. References

8.1. Normative References

8.2. Informative References

Authors' Addresses