TCP Alternative Backoff with ECN (ABE)

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.

Complementing , enables wider ECN deployment by updating rules in that prohibited certain experiments. Specifically, allows for experiments to specify a congestion control response to a CE-marked packet that differs from the response to a dropped packet. This memo defines such a different congestion control response, called "ABE" (Alternative Backoff with ECN). ABE is thus an Experiment in accordance with . 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 , 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 . The standard TCP backoff behaviour defined in 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. 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. Much of the background to this proposal can be found in . Using a mix of experiments, theory and simulations with standard NewReno and CUBIC, 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" . 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].

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. '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 and 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, ) 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 . 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.

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

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 ). 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: 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 , 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 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, ) 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).

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.

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 ). 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.

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 : 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).

XX RFC ED - PLEASE REMOVE THIS SECTION XXX This memo includes no request to IANA.

ABE is implemented as a patch for Linux and FreeBSD. It is meant for research and available for download from http://heim.ifi.uio.no/naeemk/research/ABE/ This code was used to produce the test results that are reported in .

The described method is a sender-side only transport change, and does not change the protocol messages exchanged. The security considerations of therefore still apply. 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 ). 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.

XX RFC ED - PLEASE REMOVE THIS SECTION XXX -01. This I-D now refers to draft-black-tsvwg-ecn-experimentation-02, which replaces draft-khademi-tsvwg-ecn-response-00 to make a broader update to RFC3168 for the sake of allowing experiments. As a result, some of the motivating and discussing text that was moved from draft-khademi-alternativebackoff-ecn-03 to draft-khademi-tsvwg-ecn-response-00 has now been re-inserted here. -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.