The Case for a Hybrid Passive/Active Network Monitoring Scheme in the Wireless Internet

Transcription

1 The Case for a Hybrid Passive/Active Network Moitorig Scheme i the Wireless Iteret Björ Ladfeldt*, Pipat Sookavataa*,** ad Arua Seevirate** Dept. of Electrical Egieerig ad Telecommuicatios The Uiversity of New South Wales, Sydey 2052 Australia **Departmet of Computer Egieerig Mahaakor Uiversity of Techology, Bagkok 10530, Thailad [bjorl, pipats, arua]@ee.usw.edu.au Abstract This paper discusses moitorig withi wired ad wireless etworks ad the type of moitorig iformatio eeded to support differet applicatios. We suggest a hybrid active/passive moitorig approach with a dyamic time widow mechaism ad iterchageable filters to extract requested iformatio. The paper also shows our iitial experimetal results ad presets our coclusios. Keywords-- Network moitorig, wireless liks, dyamic widow geeratio, passive moitorig ad active probig 1. Itroductio The past few years have see a expoetial growth of etworked applicatios based o the Iteret. May of these applicatios are iteractive ad use data with real time characteristics. I parallel, there has bee a rapid growth i cellular ad other mobile commuicatio systems such as wireless LANs, ad ifra red access odes. The wireless Iteret will comprise of a fixed IP based core etwork ad multiple overlaid wireless access etworks. This will provide users with simultaeous access to several differet etworks [14]. I the ideal case, this will provide the capability of matchig the applicatio ad user requiremets to etwork characteristics. However, mappig of user/applicatio requiremets to available etworks is ot trivial. To facilitate this process, we have desiged a reactive quality of service maager called User Services Assistat, USA [15]. A fudametal requiremet for USA to be able to provide the assistace is havig the ability to ascertai the system status, especially the etwork characteristics. Thus, it is of vital importace to USA to have a effective etwork-moitorig tool. I this paper, we discuss the requiremets for performig etwork moitorig i wireless eviromets ad suggest a framework that provides flexibility i statistical processig ad presetatio of moitorig. The rest of this paper is orgaised as follows. Sectio 2 presets related work ad sectio 3 discusses geeral aspects of etwork moitorig. Sectio 4 the examies the relevace of the proposed moitorig framework, sectio 5 describes our experimetal set-up ad results ad fially i sectio 6 we preset our coclusios ad future work. 2. Related work A good survey of etwork moitorig techiques ad the available tools ca be foud i [2]. These moitorig schemes ca be divided ito two categories, amely active ad passive moitorig Active Moitorig Schemes Active moitorig schemes operate by isertig probig traffic ito the etwork. These probes are used to extract iformatio about etwork characteristics such as available ad bottleeck badwidth, roud trip delay etc. Bottleeck badwidth ca be detected by a techique called back-to-back or packet-pair probig [6]. Several moitorig tools use this techique [5,6,7,11]. For example, Pathchar [7] uses this method i combiatio with traceroute [3], i order to derive per-hop etwork iformatio. A more commo way of estimatig the available badwidth is to simulate a bulk data trasfer. Ttcp [8] makes a estimatio of the available badwidth based o

2 data from a TCP bulk data trasfer. Treo [9] uses ICMP echo packets with flow ad cogestio cotrol i order to simulate a TCP bulk data trasfer, ad from that derive the available badwidth. Active moitorig provides a accurate method of determiig etwork characteristics. However, the moitorig traffic competes with applicatio data flows for etwork resources ad is therefore ot scalable, especially for resource scarce eviromets. Furthermore, the delays associated with obtaiig ecessary results may be large, thus makig it usuitable for use with reactive QoS maagers such as USA[15] Passive Moitorig Passive schemes overcome the disadvatages of active schemes associated with overheads ad delay by moitorig streams i progress. From these observatios, they ifer the curret etwork status, rather tha itroducig moitorig traffic. SPAND [1] is a moitorig scheme that is based o the priciples of passive moitorig. It exteds the basic passive moitorig by providig facilities for sharig of measuremet results amog hosts i order to icrease the accuracy. Although the assumptio is valid i the curret fixed Iteret eviromets as show i [4], it is ot directly valid i a wireless Iteret eviromet. The performace of a wireless access etwork ot oly depeds o the curret load, but it also depeds o other factors such as multipath fadig, other forms of iterferece, ad the trasmitted power. These factors i tur deped o the geographic locatio of the mobile host. Cosequetly, sharig measuremet results collected by mobile hosts are likely to lead to ureliable estimates. Therefore, shared passive moitorig caot be directly used i the wireless Iteret eviromet. 3. Hybrid Moitorig Frame Work Neither active, or passive moitorig ca be directly used for moitorig wireless Iterets. However, it is possible to use a hybrid scheme that uses passive moitorig where possible ad resorts to active moitorig whe passive iformatio is uavailable. The scheme also eed be coupled with a mechaism, which accouts for the rapid fluctuatios of performace of wireless eviromets. The proposed scheme is such a hybrid scheme, which provides a uiversal moitorig tool for the emergig wireless Iteret eviromet Hybrid Moitorig The hosts attached to the wireless segmets will oly be able to use passive iformatio if the host has a ope coectio to the wated destiatio due to the volatility of the wireless commuicatio eviromets. If o ope coects are preset, the mobile host eed to resort to sedig active probig. This is realised by treatig the data-path as cosistig of two idepedet segmets, amely the wired ad wireless parts. Ad usig a exteded passive moitorig server. The proposed scheme is show i Figure 1. The exteded passive moitorig server - EXPAND provides support for passive or active moitorig of the wireless access etworks as described below, i additio to the passive or active moitorig of the fixed etwork segmet. Mobile Host Mobile Host Wireless Network Passive/Active Probig Mobile Access Poit Mobile Gateway Etheret Router Iteret SPAND Wired Network Passive Moitorig Figure 1. The hybrid passive ad active moitorig approach I the case there are o ope coectios, the mobile host trasmits a request packet to the EXPAND server. This will requestig iformatio from the EXPAND server about the coectio to the destiatio. The EXPAND server will respod with the requested iformatio about the fixed etwork segmet. The request-respose packets will be used by the requestig mobile host as active probes to determie the characteristics of the wireless access etwork. The requestig host will use the results from the EXPAND server about the fixed segmet of the etwork, ad the iformatio from the request respose packets about the wireless segmet of the etwork, to estimate the ed-toed characteristics of the coectio. A wireless lik exhibit highly varyig characteristics. It is therefore ot possible to make reliable estimates of the characteristics of the lik usig raw data from the active probes. Furthermore, the capture effects reported i [16], will lead to false estimates over CSMA liks such as This is overcome by searchig for the boudaries

3 by makig a sapshot of bottleeck badwidth ad packet loss, ad comparig the results with the fixed lik estimates. Sice the active probig oly takes place o demad, ad oly whe passive iformatio is uavailable, i the above scheme, the probig traffic overhead is miimised Dyamic Widowig As metioed, the raw measuremet data provided by the split-moitorig scheme i itself, is isufficiet to obtai a good estimate of the etwork characteristics. Firstly, i order to obtai a good estimate the raw data eeds to be filtered to suit the applicatio requestig the data. Secodly, as the characteristics of the wireless access segmet will vary sigificatly, it will ot be possible to use simple statistical techiques. This is overcome by usig a dyamic widowig mechaism to filter the raw data. The dyamic widowig scheme exploits the fact that the Iteret displays quasi-stable characteristic [4]. This will be true eve for the wireless etworks as these etworks will be used to access iformatio from statioary locatios, rather tha whilst movig, i.e. from a meetig room, a laboratory or a cliet s premises. This will be eve more true i the emergig wireless LAN eviromets as they reduce the data rate whe the sigal quality drops, thus givig differet lik speeds at differet locatios withi the etwork [12, 13]. Thus, withi these etworks, the data rate will stay stable withi a regio ad the oly differece betwee the wireless ad wired segmets will be the period of stability. W[W W W\W W\ the data set used for the calculatio is betwee t y ad t 1 ad the data betwee t 0 ad t 1 is discarded. Thus, the widowig scheme cosists of two parts. A mechaism for detectig step chages i istataeous measured data ad a statistical filterig mechaism. The step chages are determied by the variace of the data ormalised by the mea value. The variace idicates the variability of the measuremets ad will therefore reflect the step chage. The ormalisatio is doe so that the variability ca be quatified. I order to fid the step, we defie a threshold value δ ad compare it agaist the cumulative variace to mea ratio, i.e. a step chage is foud if 1 1 δ 1 2 ( x x ) i i = 1 i = 1 is true where x is a sample value ad the total umber of samples. 4. Experimetal Results To verify the viability of the above framework we coducted several tests withi the experimetal test-bed show i Figure 3. RTT measuremets were used for coveiece i these experimets. The WaveLAN etwork covered oe floor of the Electrical Egieerig buildig at the Uiversity of New South Wales, ad provided varyig SNR at differet locatios. The wireless tests cosisted of a perso movig with a laptop withi the wireless LAN, measurig the available badwidth to the PC from fixed locatios. The wired tests were carried out over the Iteret betwee the PC i Australia ad the servers i Swede ad Thailad respectively. x i W W[ Mobile Host 1 Spad Wave LAN Mobile Gateway Mobile Host 2 Etheret Figure 2. Dyamic widow data regios The dyamic widowig scheme thus attempts to determie a step chage i the etwork characteristics, ad the uses a simple statistical aalysis of the data from the step i order to determie the characteristics. This is schematically show i Figure 2. At time t = t x, 0 < t x < t 1, the data set used for calculatio is betwee t x ad t 0. At time t y ; t 1 < t y, i.e. whe the step chage has bee detected, Mobile Host 3 Router Swede Iteret Router Australia (UNSW) Router Thailad (MUTNet) Figure 3. The Experimetal Test-bed

4 4.1. Active versus Passive Moitorig I order to demostrate the advatages of passive moitorig we coducted a series of tests betwee Australia, Swede ad Thailad. Passive moitorig itroduces fewer overheads to the etwork traffic, ad the overhead traffic is oly itroduced locally ad ca therefore be better cotrolled ad accouted for whe dimesioig etworks. We illustrate this i Figure 4. I the example, we compare some of the most widespread tools for active probig with the passive EXPAND. The figures for EXPAND are take from our experimetal implemetatio ad estimated as; 20 reports to the server of 45 bytes each over oe hop, plus active probes of a wireless lik coupled with the iformatio request, of 10 packets of 1500 byte each. Together this itroduces byte over oe hop. The other tools are measured with default values ad the figure compares the itroduced traffic over all liks. The figure shows how the active tools iject sigificatly more traffic ito the etwork tha EXPAND does. $YDLODEOH %DQGZLGWK 7LPHV Figure 5. Available Badwidth Masuremet we could cosistetly see this behaviour. This behavior might occur periodically with i Mobile IP, where may mobiles are roamig form etwork to etwork. Because of the rapid chages, the eed for a dyamic widowig scheme is accetuated. The stable behaviour also meas historical data is useful eve withi a wireless etwork Dyamic widow experimets Total Bytes Number Hops Figure 4. Compariso Active ad Passive traffic 4.2. Wireless experimets EXPAND TRENO Netperf Pathchar Figure 5 shows a typical plot of available badwidth measured over the WaveLAN wireless LAN. Our wireless tests aimed at ivestigatig the traffic behaviour over the wireless lik. I figure, at the iitial state, oly oe cliet accessed to the fixed etwork passig through the mobile gateway while the average available badwidth was 1.4 Mbps. Cosequetly at the time 210-sec., aother cliet came ad joied to the wireless etwork. As show i the figure, the available badwidth dropped sigificatly to 0.6 Mbps. The cliet the moved out from the etwork at time 530-sec ad the available badwidth rapidly icreased to the maximum to value. I our measuremets, TTCP I order to istigate the viability of the dyamic full widow scheme, we used the experimetal result form the pervious path ad also coducted a series of RTT measuremet betwee Australia ad Thailad. The followig example demostrates the performace improvemet of the dyamic widow scheme. As ca be see the measuremets vary sigificatly with time. The Figure 6 shows available badwidth estimatios usig three differet methods, mea value, least square liear approximatio ad least square liear approximatio usig the dyamic widow scheme. I order to fid the edges of the dyamic widows we used the variace to mea formula described above ad compared it to a δ value of Determiig the appropriate δ value is still a ope research issue. For ow, our experieces show that a value betwee 0.02 ad 0.07 makes the scheme perform well. The figure clearly shows that the least square liear approximatio with dyamic widowig estimates the available badwidth better tha the other two methods. To show that dyamic widowig scheme also performs good estimatios i slow chagig behaviors, we measured a roud trip time (RTT) betwee Australia (UNSW) ad Thailad (MUT) durig 10 hours. Agai, although the RTT was ot chage rapidly, the least square liear approximatio with dyamic widowig still performs better tha the other two methods.

5 $YDLODEOH %DQGZLGWK Figure 6. Available Badwidth Estimatio usig three differet methods Our experimetal results show that usig the dyamic widowig scheme gives cosistetly better result that either usig istataeous measuremets or fixed widows. 577PV 7LPHV $YDLODEOH %DQGZLGWK 0HDQ /HDVW 6TXDUH )XOO :LQGRZ /HDVW 6TXDUH '\QDPLF :LQGRZ 577 0HDQ /HDVW 6TXDUH )XOO :LQGRZ /HDVW 6TXDUH '\QDPLF :LQGRZ Figure 7. RTT Estimatio usig three differet methods. 5. Coclusio ad Future Work I this paper, we discussed how etwork moitorig should be carried out i a mixture of wired ad wireless eviromets. The purpose of this discussio is to form guidelies for a overall framework for etwork moitorig. We have proposed a split etwork moitorig approach with passive/active moitorig. Cetral to this framework is the method of dyamic widowig to fid a regio with a relevat set of data o which to coduct statistical computatios. I the future, we ited to focus o statistical computatios of moitorig results. Firstly, we will ivestigate possible ways of determiig the optimal time widow size for filterig trasiet evets while still reactig o sustaied badwidth shifts. Secodly, we will work o statistical methods for use i differet filters i order to provide accurate ad for applicatios appropriate estimates of the performace parameters. Thirdly, we will work o implemetatio issues such as a protocol betwee a termial ad a EXPAND server for specifyig filter optios etc. Fially, we ited to implemet a full prototype of the framework i order to evaluate its usefuless. 6. Rrefereces [1] S. Sesha, M. Stemm ad R. Katz. SPAND:Shared Passive Network Performace Discovery. I Proc 1st Useix Symposium o Iteret Techologies ad Systems (USITS '97) Moterey, CA December [2] heplibw3.slac.staford.edu/xorg/ mtf/mtf -tools.html. [3] W. R. Steves, TCP/IP Illustrated Volume 1 Addiso-Wesley, Readig MA, Nov [4] V. Paxo. Measuremets ad Aalysis of Ed-to-Ed Iteret Dyamics. PhD thesis, U. C. Berkeley, May [5] R. L. Carter ad M. E. Crovella. Measurig bottleeck-lik speed i packet switched etworks. Techical Report BU-CS , Computer Sciece Departmet, Bosto Uiversity, March [6] S. Keshav. Packet-Pair Flow Cotrol. IEEE/ACM Trasactios o Networkig, Feb [7] Pathchar A Tool to Ifer Characteristics i Iteret Paths. ftp://ee.lbl.gov/pathchar, [8] IEEE RFC 1470 Jue [9] M. Mathis ad J. Mahdavi Diagosig Iteret Cogestio with a Trasport Layer Performace Tool. I Proc. INET 96, Motreal Caada, Jue [10] D. Clark ad D. Taehouse Architectural cosideratios for a ew geeratio of protocols. I SIGCOM 90 Philadelphia Pesyl-vaia September [11] K. Lai ad M. Baker, "Measurig Badwidth." Proceedigs of IEEE Ifocom'99, March [12] [13] [14] R. Katz ad E. Brewer, The Case for Wireless Overlaid Networks, SPIE Multimedia ad Networkig Co-ferece (MMNC 96) Sa Jose Jauary [15] B.Ladfeldt, A.Seevirate, C.Diot, User Services Assistat, a Ed-to-Ed Reactive QoS Architecture, IWQOS98 Napa Califoria May [16] K. K. Ramakrisha ad H. Youg, The Etheret Capture Effect: Aalysis ad Solutio, i Proceedigs of IEEE 19 th Coferece o Local Computer Networks, MN, USA, October 1994.