Network protocols are usually tested in operational networks or in simulated environments. With the former approach it is not easy to set and control the various operational parameters such as bandwidth, delays, queue sizes. Simulators are easier to control, but they are often only an approximate model of the desired setting, especially for what regards the various traffic generators (both producers and consumers) and their interaction with the protocol itself. In this paper we show how a simple, yet flexible and accurate network simulator -- dummynet -- can be built with minimal modifications to an existing protocol stack, allowing experiments to be run on a standalone system. dummynet works by intercepting communications of the protocol layer under test and simulating the effects of finite queues, bandwidth limitations and communication delays. It runs in a fully operational system, hence allowing the use of real traffic generators and protocol implementations, while solving the prob...

In this paper we presentacouple of tools developed by theauthor on FreeBSD, and available from the author's Web page in source format. The rst one, called dummynet, is a tool designed for the performance evaluation of network protocols and applications. Despite its original design goal, there has been a lot of interest on using dummynet as a bandwidth manager in network servers. dummynet simulates the e ect of nite queues, bandwidth limitations, and queueing delays, and is embedded in the protocol stack of the host, allowing even complex experiments to be run on a single machine, using existing applications and protocol implementations. The second tool is a software implementation of an erasure code especially suited for use in network protocols. Erasure codes are used in Forward Error Correction (FEC) techniques to reduce or remove the need for retransmissions in presence of communication errors. FEC has been rarely used in network protocols, because of the encoding/decoding overhead, and also because the underlying theory of error correcting codes is generally not well known to network researchers. In this paper we discuss the theory behind a simple erasure code, and provide performance data to show that the encoding/decoding overhead is acceptable for many applications even on low-end machines. 1

We present a new implementation of TCP that is better suited to today's Internet than TCP Reno or Tahoe. Our implementation of TCP, which we call TCP Santa Cruz, is designed to work with path asymmetries, out-of-order packet delivery, and networks with lossy links, limited bandwidth and dynamic changes in delay. The new congestion-control and error-recovery mechanisms in TCP Santa Cruz are based on: using estimates of delay along the forward path, rather than the round-trip delay; reaching a target operating point for the number of packets in the bottleneck of the connection, without congesting the network; and making resilient use of any acknowledgments received over a window, rather than increasing the congestion window by counting the number of returned acknowledgments. We compare TCP Santa Cruz with the Reno and Vegas implementations using the ns2 simulator. The simulation experiments show that TCP Santa Cruz achieves significantly higher throughput, smaller delays, and smaller del...

Recent research has focussed on the problems associated with TCP performance in the presence of wireless links and ways to improve its performance. We present an extension to TCP Santa Cruz which improves TCP performance over lossy wireless links. TCP has no mechanism to differentiate random losses on the wireless link from congestion, and therefore treats all losses as congestive. We present a simple method in which our protocol is able to differentiate these random losses, thereby avoiding the rate-halving approach taken by standard TCP whenever any loss is detected. We compare the performance of our protocol against TCP Reno and demonstrate higher throughput and lower end-to-end delay with our approach.

In this paper, we study the use of continuous-time hidden Markov models (CT-HMMs) for network protocol and application performance evaluation. We develop an algorithm to infer the CT-HMM from a series of end-to-end delay and loss observations of probe packets. This model can then be used to simulate network environments for network performance evaluation. We validate the simulation method through a series of experiments both in ns and over the Internet. Our experimental results show that this simulation method can represent a wide range of real network scenarios. It is easy to use, accurate and time efficient.

This paper presents a new approach to TCP congestion control. The new scheme includes two parts: (1) the Smooth-Start algorithm, which replaces the Slow-Start algorithm at the start of a TCP connection or after a retransmission timeout, and (2) the Dynamic Recovery algorithm, which replaces the Fast Recovery algorithm to recover packet losses when a TCP connection is congested. Both algorithms require modifications only to the sender side of the TCP implementation. Simulation is used to evaluate the performance of the algorithms. The simulation experiments are conducted using the ns simulator, to facilitate comparisons with Tahoe, Reno, New-Reno, SACK, and FACK TCP. The simulation results show that the new scheme performs at least as well as SACK and FACK TCP, which in turn consistently outperform TCP Tahoe and Reno. Furthermore, the implementation of the new scheme is simpler than that of SACK and FACK. 1. Introduction With the rapid growth of the Internet and the widespread use of...

Low priority data transfer across the wide area is useful in several contexts, for example for the dissemination of large files such as OS updates, content distribution or prefetching. Although the design of such a service is reasonably easy when the underlying network supports service differentiation, it becomes more challenging without such network support. We describe an application level approach to designing a low priority service --- one that is `lower than best-effort' in the context of the current Internet. We require neither network support nor changes to TCP. Instead, we use a receive window control to limit the transfer rate of the application, and the optimal rate is determined by detecting a change-point. We motivate this joint control-estimation problem by considering a fluid-based optimisation framework, and describe practical solutions, based on stochastic approximation and binary search techniques. Simulation results demonstrate the effectiveness of the approach.

In this paper, we study the use of continuous-time hidden Markov models for network protocol and application performance evaluation. We develop an algorithm to infer the continuous-time hidden Markov model from a series of end-to-end delay and loss observations of probe packets. This model can then be used to simulate network environments for network performance evaluation. We validate the simulation method through a series of experiments both in ns and over the Internet. Our experimental results show that this simulation method can represent a wide range of real network scenarios. It is easy to use, accurate, and time e#cient.

One of the proposed solutions for increasing the speed of Internet access is to connect the home user to a direct satellite channel, at a speed 20 times faster than that of an average telephone modem. Communication over satellite links is often characterized by sporadic high bit-error rates and burst losses. This is especially true when working in the Ka band, where weather conditions greatly a#ect link availability. Under such conditions, the TCP protocol that is predominantly used by data applications, degrades dramatically in performance. Using simulations, this paper studies the performance of TCP under di#erent network conditions. Several modi#cations, that take advantage of the special properties of the satellite channel, are proposed and a new sender algorithm which can e#ciently handle burst losses is presented. The main attractiveness of the proposed new sender algorithm is that it can be implemented only at the satellite ground station, rather than at every server in the world.

Ongoing research into dynamic, self-organizing, multihop wireless networks, called ad hoc networks, promises to improve the efficiency and coverage of wireless communication. Such networks have a variety of natural civil and military applications. They are particularly useful when networking infrastructure is impossible or too costly to install and when mobility is desired. However, the ability to scale such networks to large numbers of nodes remains an open research problem. For example, routing and transmitting packets efficiently over ad hoc networks becomes difficult as they grow in size. Progress in this area of research fundamentally depends on the capabilities of simulation tools and, more specifically, on the scalability of wireless network simulators. Analytically quantifying the performance and complex behavior of even simple protocols in the aggregate is often imprecise. Furthermore, performing actual experiments is onerous: acquiring hundreds of devices, managing their software and configuration, controlling a distributed experiment and aggregating the data, possibly moving the devices around, finding the physical space for such an experiment, isolating it from interference, and generally ensuring ceteris paribus are but some of the difficulties that make empirical endeavors daunting. Consequently, the vast majority of research in this area is based entirely on simulation, a fact that underscores the critical role of efficient simulators.