Naeem Khademi

The New AQM Kids on the Block: An Experimental Evaluation of CoDel and PIE

Active Queue Management (AQM) design has again come into the spotlight of network operators, vendors and OS developers. This reflects the growing concern and sensitivity about the end-to-end latency perceived by todays Internet users. CoDel and PIE are two AQM mechanisms that have recently been presented and discussed in the IRTF and the IETF as solutions for keeping latency low. To the best of our knowledge, they have so far only been evaluated or compared against each other using default parameter settings, which naturally presents a rather limited view of their operational range. We set thus to perform a broader experimental evaluation using real-world implementations in a wired testbed. We have in addition compared them with a decade-old variant of RED called Adaptive RED, which shares with CoDel and PIE the goal of “knob-free” operation. Surprisingly, in several instances results were favorable towards Adaptive RED.

Coupled congestion control for RTP media

Congestion occurs at a bottleneck along an Internet path; multiple flows between the same sender and receiver pairs can benefit from using only a single congestion control instance when they share the same bottleneck. These benefits include the ability to control the rate allocation between flows and reduced overall delay (multiple congestion control instances cause more queuing delay than one since each has no knowledge of the congestion episodes experienced by the others). We present a mechanism for coupling congestion control for real-time media and show its benefits by coupling multiple congestion controlled flows that share the same bottleneck.

Experimental evaluation of TCP performance in multi-rate 802.11 WLANs

The goal of Rate Adaptation (RA) mechanisms in 802.11 WLANs is to provide optimum system throughput under varying channel conditions (e.g. in presence of noise) by carrying out run-time prediction and selection of the most appropriate bit-rate. The cross-layer interaction of TCP, as the major transport protocol in the Internet, with different RA mechanisms and DCF is yet to be thoroughly investigated. Previously reported efforts A) have never included real-life measurements of uplink TCP traffic; B) lack a practical view because they do not consider the RA mechanisms commonly deployed in todays off-the-shelf 802.11 devices; C) miss the study of RA mechanisms in low-noise environments. This paper covers all the above shortages, by conducting real-life measurements in two different test-beds (NDlab and Emulab) alongside with simulations, to study the performance of TCP coupled with different commonly deployed RA mechanisms. Our measurements reveal that 1) most conventional RA mechanisms are unable to distinguish frame errors due to collisions from channel noise/interference, and will respond to them negatively to some extent; 2) different than downlink TCP, uplink TCP can be adversely affected by collision-triggered rate downshifts that some RA schemes exhibit even under perfect channel conditions or in low-noise environments; 3) the relatively recent Minstrel RA mechanism can counter this negative uplink behavior well, yielding almost equal performance as in the downlink case.

Size-based and direction-based TCP fairness issues in IEEE 802.11 WLANs

Cross-layer interaction of Distributed Coordination Function (DCF) of 802.11 MAC protocol and TCP transport protocol leads to two types of unfairness. In a mixed traffic scenario, short-lived TCP flows suffer from poor performance compared to the aggressive long-lived flows. Since the main source of Internet traffic is small file web transfers, this issue forms a major challenge in current WLANs which is called size-based unfairness. In addition, when sharing an access point bottleneck queue, upstream flows impede the performance of downstream flows resulting in direction-based unfairness. Proposed solutions in the literature mostly rely on size-based scheduling policies. However, each proposed method is able to solve any of these two mentioned aspects, none of them can provide both size-based and direction-based fairness in a unique solution. In this paper, we propose a novel queue management policy called Threshold-Based Least Attained Service-Selective Acknowledgment Filtering (TLAS-SAF). We show analytically and by simulation that TLAS-SAF is capable of providing both direction-based and size-based fairness and can be taken into account as a unique solution to be applied at access point buffers.

NEAT: A Platform- and Protocol-Independent Internet Transport API

The sockets API has become the standard way that applications access the transport services offered by the IP stack. This article presents NEAT, a user space library that can provide an alternate transport API. NEAT allows applications to request the service they need using a new design that is agnostic to the specific choice of transport protocol underneath. This not only allows applications to take advantage of common protocol machinery, but also eases introduction of new network mechanisms and transport protocols. The article describes the components of the NEAT library and illustrates the important benefits that can be gained from this new approach. NEAT is a software platform for developing advanced network applications that was designed in accordance with the standardization efforts on transport services in the IETF, but its features exceed the envisioned functionality of a TAPS system.

On the uplink performance of TCP in multi-rate 802.11 WLANs

IEEE 802.11 defines several physical layer data rates to provide more robust communication by falling back to a lower rate in the presence of high noise levels. The choice of the current rate can be automatized; e.g., Auto-Rate Fallback (ARF) is a well-known mechanism in which the sender adapts its transmission rate in response to link noise using up/down thresholds. ARF has been criticized for not being able to distinguish MAC collisions from channel noise. It has however been shown that, in the absence of noise and in the face of collisions, ARF does not play a significant role for TCPs downlink performance. The interactions of ARF, DCF and uplink TCP have not yet been deeply investigated. In this paper, we demonstrate our findings on the impact of rate fallback caused by collisions in ARF on the uplink performance of various TCP variants using simulations.

On the Cost of Using Happy Eyeballs for Transport Protocol Selection

Concerns have been raised in the past several years that introducing new transport protocols on the Internet has become increasingly difficult, not least because there is no agreed-upon way for a source end host to find out if a transport protocol is supported all the way to a destination peer. A solution to a similar problem---finding out support for IPv6---has been proposed and is currently being deployed: the Happy Eyeballs (HE) mechanism. HE has also been proposed as an efficient way for an application to select an appropriate transport protocol. Still, there are few, if any, performance evaluations of transport HE. This paper demonstrates that transport HE could indeed be a feasible solution to the transport support problem. The paper evaluates HE between TCP and SCTP using TLS encrypted and unencrypted traffic, and shows that although there is indeed a cost in terms of CPU load to introduce HE, the cost is relatively small, especially in comparison with the cost of using TLS encryption. Moreover, our results suggest that HE has a marginal impact on memory usage. Finally, by introducing caching of previous connection attempts, the additional cost of transport HE could be significantly reduced.

De-Ossifying the Internet Transport Layer: A Survey and Future Perspectives

It is widely recognized that the Internet transport layer has become ossified, where further evolution has become hard or even impossible. This is a direct consequence of the ubiquitous deployment of middleboxes that hamper the deployment of new transports, aggravated further by the limited flexibility of the application programming interface (API) typically presented to applications. To tackle this problem, a wide range of solutions have been proposed in the literature, each aiming to address a particular aspect. Yet, no single proposal has emerged that is able to enable evolution of the transport layer. In this paper, after an overview of the main issues and reasons for transport-layer ossification, we survey proposed solutions and discuss their potential and limitations. The survey is divided into five parts, each covering a set of point solutions for a different facet of the problem space: 1) designing middlebox-proof transports; 2) signaling for facilitating middlebox traversal; 3) enhancing the API between the applications and the transport layer; 4) discovering and exploiting end-to-end capabilities; and 5) enabling user-space protocol stacks. Based on this analysis, we then identify further development needs toward an overall solution. We argue that the development of a comprehensive transport layer framework, able to facilitate the integration and cooperation of specialized solutions in an application-independent and flexible way, is a necessary step toward making the Internet transport architecture truly evolvable. To this end, we identify the requirements for such a framework and provide insights for its development.

Guaranteeing per Station and Per Flow Fairness of Downstream and Upstream Flows over IEEE 802.11 WLAN

Distributed Coordination Function (DCF) in IEEE 802.11 WLANs cannot guarantee both per-station and per-flow fairness among wireless stations. Several solutions to this problem have been proposed. However, per-flow fairness causes per-station unfairness and vice versa due to the fact that each wireless station may transmit different number of flows. In this paper, a novel method has been proposed to allocate fair bandwidth among wireless users and also between flows within each of wireless stations while maintaining maximum available total throughput for different link and wireless delays. Under this method the bandwidth allocation between TCP upstream and downstream flows within each wireless station is equal while all stations enjoy from a higher throughput compare to the previous proposed methods and the total throughput is ensured. In addition, bandwidth is allocated almost fair among all stations providing partial per-station fairness as well.

Less-than-best-effort service for Community Wireless Networks: Challenges at three layers

Community Wireless Networks can be a way to make “Internet access for everyone” possible, by sharing a broadband Internet connection via WLAN. The underlying idea is to freely provide network access to anybody in the vicinity of the wireless access point via a Lower-than-Best-Effort (LBE) service, such that non-paying users interfere as little as possible with the “regular” Internet usage. Such a service faces challenges at various network layers; this paper discusses some of them, focusing on layers 2, 3 and 4.