Shawn Ostermann

Estimating loss rates with TCP

Estimating loss rates along a network path is a problem that has received much attention within the research community. However, deriving accurate estimates of the loss rate from TCP transfers has been largely unaddressed. In this paper, we first show that using a simple count of the number of retransmissions yields inaccurate estimates of the loss rate in many cases. The mis-estimation stems from flaws in TCPs retransmission schemes that cause the protocol to spuriously retransmit data in a number of cases. Next, we develop techniques for refining the retransmission count to produce a better loss rate estimate for both Reno and SACK variants of TCP. Finally, we explore two SACK-based variants of TCP with an eye towards reducing spurious retransmits, the root cause of the mis-estimation of the loss rate. An additional benefit of reducing the number of needless retransmits is a reduction in the amount of shared network resources used to accomplish no useful work.

An evaluation of TCP with larger initial windows

TCPs slow start algorithm gradually increases the amount of data a sender injects into the network, which prevents the sender from overwhelming the network with an inappropriately large burst of traffic. However, the slow start algorithm can make poor use of the available bandwidth for transfers which are small compared to the bandwidth-delay product of the link, such as file transfers up to few thousand characters over satellite links or even transfers of several hundred bytes over local area networks. This paper evaluates a proposed performance enhancement that raises the initial window used by TCP from 1 MSS-sized segment to roughly 4 KB. The paper evaluates the impact of using larger initial windows on TCP transfers over both the shared Internet and dialup modem links.

Detecting network intrusions via a statistical analysis of network packet characteristics

With the growing threat of abuse of network resources, it becomes increasingly important to be able to detect malformed packets on a network and estimate the damage they fan cause. In this paper, we collect and analyze all of the IP and TCP packets seen on a network that either violate existing standards or should not appear in modern internets. Our goal is to determine what these suspicions packets mean and evaluate what proportion of such packets can cause actual damage. Thus, we divide unusual packets obtained during our experiments into several categories depending on the severity of their consequences, including indirect consequences as a result of information gathering, and show the results. The traces analyzed were gathered at Ohio Universitys main Internet link, providing a massive amount of statistical data.

Statistical analysis of malformed packets and their origins in the modern internet

In this work, we collect and analyze all of the IP and TCP headers of packets seen on a network that either violate existing standards or should not appear in modern internets. Our goal is to determine the reason that these packets appear on the network and evaluate what proportion of such packets could cause actual damage. Thus, we examine and divide the unusual packets obtained during our experiments into several categories based on their type and possible cause and show the results.

A Priority Paradigm for Deep Space Data Communication

Communicating data in deep-space, across interplanetary distances, entails constraints such as signal propagation delays in the order of minutes and hours, high channel error characteristics, meager and asymmetric bandwidth availability, and disruptions due to planetary orbital dynamics and antenna scheduling constraints on Earth. The licklider transmission protocol (LTP) is being designed as a reliable data transmission protocol optimized for this environment. We present a dynamic priority paradigm for LTP jobs that may help improve the volume and value of data communicated in deep-space by quantifying each jobs Intrinsic Value and Immediacy. We study convolutional codes, Reed-Solomon codes, Raptor codes, and some of their combinations, over various channel error rates. We show how the appropriate application of these mechanisms to each job, based on its Immediacy and Intrinsic value, can improve the aggregate value of data transferred over the channel across various job mixes.

Study of a Priority Paradigm for Deep-Space Applications

Deep-space presents a challenging environment for communication by exhibiting constraints such as very high signal propagation delays, high channel error-rates, meager and expensive bandwidth availability, etc. Further, a Spacecraft participating in a deep-space mission typically carries a host of scientific instruments each capable of generating data at different rates coupled with different reliability and timeliness needs for communication. In this paper we present a two-dimensional Priority Paradigm designed for applications operating in this scenario. The first dimension is Immediacy, a measure of how urgently data needs to be delivered; Orthogonal to that is Intrinsic Value, a measure of how reliable the data delivery needs to be. We then study the performance of two candidate mechanisms for implementing the Priority Paradigm policy sought with the two-dimensional priority-paradigm parameters requested: Adapting the FEC mechanism in use and Adaptively varying the packet size in use, for various link error rate characteristics.