Improve TCP performance in Ad Hoc netwoks

Standard TCP misinterpret mobility loss in Ad hoc network as congestion loss, thus, it reduce the TCP performance by invoking unnecessary congestion control action. In this paper, we propose two approaches, simELFN (an variation of TCP-ELFN) and TCP-FSR (an variation of TCP-F). They can distinguish the essence of packet loss and avoid multiple consecutive dupACKs. Analyses and simulations show that they can achieve better TCP performance in Ad hoc network.Keywords: Ad hoc Networks; simELFN; TCP-FSR; TCP performance

A cross-layer middleware architecture for time and safety critical applications in MANETs

Mobile Ad hoc Networks (MANETs) can be deployed instantaneously and adaptively, making them highly suitable to military, medical and disaster-response scenarios. Using real-time applications for provision of instantaneous and dependable communications, media streaming, and device control in these scenarios is a growing research field. Realising timing requirements in packet delivery is essential to safety-critical real-time applications that are both delay- and loss-sensitive. Safety of these applications is compromised by packet loss, both on the network and by the applications themselves that will drop packets exceeding delay bounds. However, the provision of this required Quality of Service (QoS) must overcome issues relating to the lack of reliable existing infrastructure, conservation of safety-certified functionality. It must also overcome issues relating to the layer-2 dynamics with causal factors including hidden transmitters and fading channels. This thesis proposes that bounded maximum delay and safety-critical application support can be achieved by using cross-layer middleware. Such an approach benefits from the use of established protocols without requiring modifications to safety-certified ones. This research proposes ROAM: a novel, adaptive and scalable cross-layer Real-time Optimising Ad hoc Middleware framework for the provision and maintenance of performance guarantees in self-configuring MANETs. The ROAM framework is designed to be scalable to new optimisers and MANET protocols and requires no modifications of protocol functionality. Four original contributions are proposed: (1) ROAM, a middleware entity abstracts information from the protocol stack using application programming interfaces (APIs) and that implements optimisers to monitor and autonomously tune conditions at protocol layers in response to dynamic network conditions. The cross-layer approach is MANET protocol generic, using minimal imposition on the protocol stack, without protocol modification requirements. (2) A horizontal handoff optimiser that responds to time-varying link quality to ensure optimal and most robust channel usage. (3) A distributed contention reduction optimiser that reduces channel contention and related delay, in response to detection of the presence of a hidden transmitter. (4) A feasibility evaluation of the ROAM architecture to bound maximum delay and jitter in a comprehensive range of ns2-MIRACLE simulation scenarios that demonstrate independence from the key causes of network dynamics: application setting and MANET configuration; including mobility or topology. Experimental results show that ROAM can constrain end-to-end delay, jitter and packet loss, to support real-time applications with critical timing requirements

TCP Performance in Heterogeneous Wireless Networks

The TCP protocol is used by most Internet applications today, including the recent mobile wireless terminals that use TCP for their World-Wide Web, E-mail and other traffic. The recent wireless network technologies, such as GPRS, are known to cause delay spikes in packet transfer. This causes unnecessary TCP retransmission timeouts. This dissertation proposes a mechanism, Forward RTO-Recovery (F-RTO) for detecting the unnecessary TCP retransmission timeouts and thus allow TCP to take appropriate follow-up actions. We analyze a Linux F-RTO implementation in various network scenarios and investigate different alternatives to the basic algorithm. The second part of this dissertation is focused on quickly adapting the TCP's transmission rate when the underlying link characteristics change suddenly. This can happen, for example, due to vertical hand-offs between GPRS and WLAN wireless technologies. We investigate the Quick-Start algorithm that, in collaboration with the network routers, aims to quickly probe the available bandwidth on a network path, and allow TCP's congestion control algorithms to use that information. By extensive simulations we study the different router algorithms and parameters for Quick-Start, and discuss the challenges Quick-Start faces in the current Internet. We also study the performance of Quick-Start when applied to vertical hand-offs between different wireless link technologies.Suurin osa Internet-sovelluksista käyttää TCP-protokollaa turvatakseen luotettavan tiedonvaihdon. Tällaisia sovelluksia ovat esimerkiksi WWW, sähköposti, ja monet pikaviestiohjelmat. TCP-protokollan pääpiirteet on suunniteltu 1970- ja 1980-luvulla, jolloin päätelaitteita ja sovelluksia oli huomattavasti nykyistä vähemmän ja yhteydet pohjautuivat kiinteiden kommunikaatiolinkkien käyttöön. Langattomien päätelaitteiden yleistyessä on huomattu, että TCP-protokollan suorituskyky ei aina ole hyväksyttävällä tasolla, koska monet sen piirteistä on alunperin suunniteltu erilaisessa käyttöympäristössä. Väitöstyö perehtyy langattoman linkin aiheuttamien vaikeasti ennustettavien viiveiden vaikutukseen TCP:n suorituskyvylle. Tällainen käyttäytyminen on ominaista esimerkiksi nykyisin laajalti matkapuhelimissa käytetylle GPRS-teknologialle. Yllättävät viiveet datansiirrossa aiheuttavat TCP:n uudelleenlähetysajastimen tarpeettoman laukeamisen. Tämä aiheuttaa useiden pakettien turhan uudelleenlähetyksen ja vaikeuttaa TCP:n ruuhkanvalvonta-algoritmien toimintaa. Väitöstyössä ehdotetaan F-RTO -nimistä parannusta TCP:n uudelleenlähetysalgoritmeihin, joka pyrkii havaitsemaan turhat uudelleenlähetykset ja välttämään edellä mainitut ongelmat tällaisissa tilanteissa. Väitöstyö analysoi F-RTO:n suorituskykyä erilaisissa kommunikaatioskenaarioissa ja tutkii erilaisia variaatioita perusalgoritmiin. Lisäksi väitöskirjassa tutkitaan TCP:n lähetysnopeuden pikaista sopeuttamista vallitseville siirto-olosuhteille. Normaalisti TCP tarvitsee huomattavan ajan löytääkseen oikean siirtonopeuden yhteyden alussa, mikäli siirtolinkki on erityisen nopea ja siirtoviiveet verraten pitkiä. Tämä on tilanne uusimmissa langattomissa kommunikaatioteknologioissa. Samankaltainen ongelma esiintyy myös, mikäli TCP-yhteys vaihtaa käyttämäänsä siirtoteknologiaa kesken yhteyden esimerkiksi liikkuvuuden seurauksena. Tämä voi tapahtua uusimmissa päätelaitteissa, jotka tukevat useita erityyppisiä radioteknologioita, kuten WLAN ja GPRS. Väitöskirjassa tutkitaan Quick-Start - nimistä mekanismia, joka nopeuttaa huomattavasti TCP:n sopeutumisnopeutta edellä mainitun kaltaisissa tilanteissa. Työssä tarkastellaan erilaisia algoritmeja Quick-Startin käyttöön ja analysoidaan simulointien avulla algoritmien toimintaa erilaisissa ympäristöissä. Väitöstyössä esitetyillä tuloksilla Internet-kommunikaation suorituskykyä ja käytettävyyttä langattomilla laitteilla voidaan parantaa huomattavasti

Limitations of Fixed Timers for Wireless Links

© Springer-Verlag Berlin Heidelberg 2006. Traditional reliable link layer protocols set their fixed retransmission timers under the assumption that they operate in isolation over the link. Emerging wireless networks however allow multiple link layer sessions to dynamically share the link. To assess the impact of this development, we examine the performance of Web Browsing over a Selective Repeat protocol with fixed retransmission timers, showing that the optimal retransmission timer values depend on the level of contention. We therefore propose an adaptive Selective Repeat protocol that modifies its retransmission timers based on prevailing conditions. Our measurements show that this adaptive scheme provides excellent Web Browsing performance regardless of the level of contention, under two very different wireless error models

Limitations of Fixed Timers for Wireless Links

Abstract — Traditional reliable link layer protocols set their fixed retransmission timers under the assumption that they operate in isolation over the link. Emerging wireless networks however allow multiple link layer sessions to dynamically share the link. To assess the impact of this development, we examine the performance of Web Browsing over a Selective Repeat protocol with fixed retransmission timers, showing that the optimal retransmission timer values depend on the level of contention. We therefore propose an adaptive Selective Repeat protocol that modifies its retransmission timers based on prevailing conditions. Our measurements show that this adaptive scheme provides excellent Web Browsing performance regardless of the level of contention, under two very different wireless error models. I

Adaptive timeout policies for wireless links

A considerable body of evidence indicates that the use of reliable link layer protocols over error prone wireless links dramatically improves the performance of Internet protocols and applications. While traditional link layer protocols set their timeout values assuming that they fully control the underlying link, some wireless networks allow multiple link layer sessions to co-exist over the same link. Since the optimal timeout values for a reliable link layer protocol depend on the available bandwidth, with dynamic link sharing such a protocol should ideally adapt its timeout values accordingly. We have thus designed an Adaptive Selective Repeat protocol that modifies its timeout values based on the policy used by TCP. We compare the performance of Web Browsing over Selective Repeat when using our adaptive timeout scheme with a range of parameters, against a manually tuned fixed timeout version. Our measurements show that these adaptive timeout policies outperform the fixed one, regardless of the level of contention, and that the best adaptive timeout policy in this setting is not the one used by TCP. © 2006 IEEE