Revisiting Old Friends: Is CoDel Really Achieving What RED Cannot? Nicolas Kuhn Emmanuel Lochin Olivier Mehani IMT Telecom Bretagne Université de Toulouse, ISAE, National ICT Australia 2 Rue de la Châtaigneraie TéSA Level 5, 13 Garden St 35510 Cesson-Sévigné, 10 Avenue Édouard Belin Eveleigh NSW 2015, Australia France 31400 Toulouse, France [email protected] nicolas.kuhn@telecom- [email protected] bretagne.eu ABSTRACT in mobile networks [16]. The use of Active Queue Manage- We use ns-2 simulations to compare RED's gentle_ mode ment (AQM) has been proposed as a way to alleviate Buf- to CoDel in terms of their ability to reduce the latency for ferbloat [e.g., 13, 11]: there has been a renewed interest at various TCP variants. We use a common dumbbell topol- IETF is evaluating AQM; a working group has been cre- ogy with Pareto background traffic, and measure the packet ated in 2013 and produced recommandations for AQM [4]. delays and transmission time of a 10 MB FTP transfer. Most notably, CoDel [23] has been proposed specifically to In our scenarios, we find that CoDel reduces the latency by counter Bufferbloat by ensuring that packets do not remain 87 %, but RED still manages to reduce it by 75 %. However, \too long" in the managed queues. CoDel is said to be the use of CoDel results in a transmission time 42 % longer parameter-less, yet it is arbitrarily configured with two hard than when using RED. In light of its maturity, we therefore coded parameters (target delay and interval). argue that RED could be considered as a good candidate to While CoDel has been specifically designed to tackle Buf- tackle Bufferbloat. ferbloat, other AQMs such as RED [10] could also be used to the same effect. RED has the advantage of having been well both studied and tested. Although past studies used to show Categories and Subject Descriptors that RED remains hard to tune [25, 21], RED parameters C.2.6 [COMPUTER-COMMUNICATION NET- found to be fairly well understood [8] and can be now set WORKS]: Internetworking|Routers or managed automatically [15, 20]. Moreover, the authors of [17] mention that there are lessons to be learned from General Terms the old AQMs as their performance can compete with new AQMs proposals. In this article, we perform ns-2 simula- Performance tions to compare the trade-off between latency and through- put proposed by CoDel and RED, by introducing specific Keywords traffic patterns, metrics and transport protocols. AQM; Bufferbloat; CoDel; ns-2; RED As AQM reduces queues by dropping packets, an evalua- tion of the effectiveness of AQMs against Bufferbloat should 1. INTRODUCTION study the trade-off between a reduced packet delay, and an increased data transmission time (due to losses and retrans- Bufferbloat occurs due to oversized buffers at routers missions). Moreover, not all congestion control (CC) mech- where packets arrive at a faster rate than they can be for- anisms react the same to changed loss rates or delays. These warded [11, 5]. As a result, packets are queued until they AQMs should therefore be studied in light of their impact on are either transmitted, or dropped when the buffers are full. the various transport protocols they are expected to man- As the queues fill, it takes longer for each packet to be trans- age. mitted to its destination. Moreover, most congestion control This paper systematically studies this trade-off. We use mechanisms do not recognise an increase of the delay as a ns-2 to simulate a loaded network where Bufferbloat occurs. congestion event and thus, keep increasing their rate on an We then introduce various AQMs, and observe their impact already-saturated path. on flows transported using various TCP's CC mechanisms. While some contention exists as to whether or not Buffer- We find that both AQMs give the same advantages for all bloat really is a widespread problem [1], it has been observed CC algorithms: be they loss-based or delay-based, they all experience relatively similar conditions. Moreover, we show Permission to make digital or hard copies of all or part of this work for personal or classroom use is granted without fee provided that copies are not made or distributed that RED performs quite well, reducing the packet delay by for profit or commercial advantage and that copies bear this notice and the full cita- a sizeable amount|yet not as much as CoDel. More inter- tion on the first page. Copyrights for components of this work owned by others than estingly, we also find that RED allows for shorter transmis- ACM must be honored. Abstracting with credit is permitted. To copy otherwise, or re- sion times than CoDel does, which suggests that RED might publish, to post on servers or to redistribute to lists, requires prior specific permission be closer to achieving the trade-off of efficiency versus Buf- and/or a fee. Request permissions from [email protected]. ferbloat. CSWS’14, August 18, 2014, Chicago, IL, USA. Copyright 2014 ACM 978-1-4503-2991-0/14/08 ...$15.00. http://dx.doi.org/10.1145/2630088.2630094. The rest of this paper is organised as follows. In the next 2.1.4 PIE section we present and discuss the various elements of our determines a dropping probability that is based on the simulations: AQMs, CCs, topology, and parameters caus- average queueing latency and limit the queuing latency ing Bufferbloat. Section 3 presents the impact of AQM on to a target value. various congestion control mechanisms while Section 4 sum- While it is very relevant to this study, at the time we ran marises our results on transmission and packet delays. We the simulations, no implementation of PIE was available. discuss these results in Section 5, before offering a short con- We therefore focused on the comparison between RED and cluding summary in Section 6. CoDel to evaluate if a new generation of AQM is needed. Future work will consider PIE. 2. SIMULATING BUFFERBLOAT IN NS-2 2.2 Congestion Control Algorithms This section presents the AQM schemes, congestion con- Congestion control (CC) mechanisms may be based on trol, topology, traffic patterns and parameters that trigger loss events or measurements of latency. To better under- Bufferbloat. stand the impact of these AQM mechanisms on CC algo- rithms, we need to consider the following transport layer 2.1 Active Queue Management Mechanisms protocols: This article compares the impacts of various AQM and transport layer protocols. We consider the following AQM. TCP NewReno loss-based CC and baseline for our eval- uations; 2.1.1 DropTail TCP Vegas delay-based CC. In the context of reducing In- DropTail is a default queuing mechanism that drops in- ternet latency, there is a renew interest for delay-based coming packets when the queue is full. This default CC, which are less aggressive but which introduce less queue management is the baseline for our evaluations. delay in the network. Hybrid CC (both delay and loss based) are also of interest; 2.1.2 RED RED [10] drops packets depending on the number of TCP Compound hybrid (loss/delay) CC which is imple- packets in the queue. In order to reduce the occupancy in mented in Windows systems; the gateway and manage the size of the queue when the av- TCP CUBIC loss-based CC which is deployed on a large erage queue size exceeds a threshold, packets are randomly scale in Android/Linux systems. dropped to reduce the throughput of different non cooper- ating sources. We use the native Linux implementations for these protocols We consider the default implementation of RED in ns-2 in ns-2. with gentle_ mode activated. This mode consists of a basic improvement that must be considered,1 as it makes RED 2.3 Topology and traffic more suitable for large scale deployment. Exhaustive performance comparisons between RED ver- Pappl pareto applications sions is beyond the scope of this paper. As a result, we do not evaluate Adaptive RED (ARED) [9]. Gentle RED is al- 0 4 ready adaptive, but differs in that max_p is not adjusted to better control the average queue size, as it is in ARED. 2 3 We use the default values for the parameters of RED, such as the targetdelay (not to be confused with CoDel's target 1 5 value for the queue delay), set to 5 ms. Transmission of B bytes with FTP 2.1.3 CoDel CoDel [23] drops packets depending on the sojourn delay Dc, capacity Cc time of each packet in the queue. CoDel switches between delay Dw, capacitiy Cw its \dropping mode" and \non dropping mode" depending on the measured and maximum authorised sojourn times (set Figure 1: Dumbbell topology used for the simulations. by default to 5 ms). The CeroWRT project [7] implements Background Pareto traffic goes from node 0 to 4, while the CoDel as a way to solve the different problems encountered flow under study is sent from 1 to 5. at a home router, particularly Bufferbloat. A variant of CoDel, Flow-Queuing CoDel (FQ- CoDel) [14], which combines Stochastic Fairness Queueing Figure 1 presents the dumbbell topology and correspond- (SFQ) [22] and CoDel, could not be evaluated in this article, ing notations. This network topology is common to evaluate as the description of this scheduling has just been released the performance of TCP [2]. Nodes 0 and 1 (resp. 4 and as an Informational IETF draft. Moreover, it is not fair to 5) are connected to node 2 (resp. 3) by a link of Dw (ms) consider FQ-CoDel, while other tested AQMs do not inte- one way delay and Cw (Mbps) capacity.