A Security-Oriented Task Scheduler for Heterogeneous Distributed Systems

Transcription

1 A Securty-Oreted Tas Scheduler for Heterogeeous Dstrbuted Systems Tao Xe 1 ad Xao Q 2 1 Departmet of Computer Scece, Sa Dego State Uversty, Sa Dego, CA 92182, USA xe@cs.sdsu.edu 2 Departmet of Computer Scece, New Mexco Isttute of Mg ad Techology, Socorro, NM 87801, USA x@cs.mt.edu Abstract. Hgh ualty of securty s creasgly crtcal for applcatos rug o heterogeeous dstrbuted systems, where processors operate at dfferet speeds ad commucato chaels have dfferet badwdths. Although there are a few schedulg algorthms the lterature for heterogeeous dstrbuted systems, they geerally do ot tae to accout of securty reuremets of applcatos. I ths paper, we propose a ovel heurstc schedulg algorthm, or SATS, whch s coducve to mprovg securty of heterogeeous dstrbuted systems. Frst, we formalze a cocept of securty heterogeety for our schedulg model the cotext of dstrbuted systems. Next, we devse the SATS algorthm amg at schedulg tass to maxmze the probablty that all tass are executed wthout ay rs of beg attaced. Emprcal results demostrate that wth respect to securty ad performace, the proposed schedulg algorthm outperforms exstg approaches for heterogeeous dstrbuted systems. Keywords: Securty heterogeety, heterogeeous dstrbuted system, schedulg, degree of securty defcecy, rs-free probablty. 1 Itroducto Over the last decade, heterogeeous dstrbuted systems have bee emergg as popular computg platforms for computatoally tesve applcatos wth dverse computg eeds [6]. To date they have bee appled to securty sestve applcatos, such as bag systems ad dgtal govermet, whch reure ew approaches to securty. Iheretly, dstrbuted systems are more vulerable to threats tha cetralzed systems, sce t s dffcult to cotrol processg actvtes of the dstrbuted systems ad formato ca be accessed over etwors. A varety of techues le authetcato [8] ad access cotrol [12] are wdely used to secure dstrbuted systems. Although these techues ca be appled to dstrbuted systems, the covetoal securty techues lac the ablty to express heterogeety securty servces. Our study s teded to troduce a cocept of securty heterogeety, whch provdes a meas of measurg overhead curred by securty servces the cotext of heterogeeous dstrbuted systems. Y. Roberts et al. (Eds.): HPC 2006, LNCS 4297, pp , Sprger-Verlag Berl Hedelberg 2006

2 36 T. Xe ad X. Q Schedulg algorthms play a ey role obtag hgh performace parallel ad dstrbuted systems [10][18]. The obectve of schedulg algorthms s to map tass oto stes ad order ther executo a way to optmze overall performace. I ths wor we cosder the ssue of dyamc tas schedulg. Nowadays, a wde varety of schedulg algorthms for dstrbuted systems have bee reported the lterature [1][3]. Peg ad Sh proposed a ew schedulg algorthm for tass wth precedece costrats dstrbuted systems [9]. Arpac-Dusseau troduced two ey mechasms mplct coschedulg for dstrbuted systems [1]. The above algorthms were desged for homogeeous dstrbuted systems. I recet years, the ssue of schedulg o heterogeeous dstrbuted systems has bee addressed ad reported the lterature [5][16]. Raaweera ad Agrawal developed a scalable schedulg scheme called STDP for heterogeeous systems [11]. Srvasa ad Jha corporated relablty cost, defed to be the product of processor falure rate ad tas executo tme, to schedulg algorthms for tass wth precedece costrats [15]. A. Doga ad F. Özgüer studed relable matchg ad schedulg for tass wth precedece costrats heterogeeous dstrbuted systems. Due to the lac of securty awareess, these algorthms are ot sutable for securty-sestve dstrbuted computg applcatos. O the other had, however, a creasg umber of msso-crtcal applcatos wth hgh demads of ualty of securty servce have bee emergg varous dstrbuted systems [4][7][13][14]. For example, a real-tme stoc uote update ad tradg web servce system, each comg reuest from busess parters ad each outgog respose from a eterprse s bac-ed applcato have deadles ad securty reuremets, whch have to be dealt wth by a server located betwee the busess parters ad eterprse baced applcatos [17]. Furthermore, the server ca udcously select a sutable securty level from the rage of securty servce levels, whch are predefed combatos of trasport ad message securty mechasms. Typcal securty levels a real-tme uote ad tradg system are [17]: Routg + message securty; Routg + SSL; Routg + SSL + message securty; Routg + SSL + clet authetcato ad Routg + SSL + message securty + clet authetcato. I our prevous wor, we proposed a famly of dyamc securty-aware schedulg algorthms for a cluster [18] ad a Grd [19]. Ufortuately, these schedulg algorthms oly support homogeeous computg applcatos, thus lmtg ther applcablty to heterogeeous dstrbuted systems. Hece, we are motvated ths study to formalze the securty heterogeety cocept, ad to propose a schedulg algorthm to mprove securty of heterogeeous dstrbuted systems whle mmzg computatoal overhead. 2 Modelg Tass ad Ther Securty Reuremets I ths study, we cosder a ueug archtecture of a -ste dstrbuted system whch heterogeeous stes are coected va a etwor to process depedet tass submtted by m users. Let M = {M 1, M 2,, M } deote the set of heterogeeous stes.

3 A Securty-Oreted Tas Scheduler for Heterogeeous Dstrbuted Systems System Model The system model, depcted Fgure 1, s composed of a tas schedule ueue, STAS tas scheduler, ad local tas ueues. The fucto of STAS s teded to mae a good tas allocato decso for each arrval tas to satsfy ts securty reuremets ad mata a deal performace covetoal performace metrcs such as average respose tme. User 1 User 2 Securty overhead maager SATS Securty-adaptve wdow cotroller Tas allocato decso maer Local Queue M 1 M 2 Schedule Queue Securty defcecy calculator Executo tme maager User m M Fg. 1. System model of the SATS strategy A schedule ueue s used to accommodate comg tass. SATS scheduler the processes all arrval tass a Frst-Come Frst-Served (FCFS) maer. After beg hadled by SATS, the tass are dspatched to oe of the desgated ste M M for executo. The stes, each of whch matas a local ueue, ca execute tass parallel. The ma compoet of the system model above s SATS, whch s composed of fve modules: (1) Executo tme maager; (2) Securty overhead maager; (3) Degree of securty defcecy (DSD) calculator; (4) Securty-adaptve wdow cotroller; ad (5) Tas allocato decso maer. Sce executo tme of each tas ca be estmated by code proflg ad statstcal predcto [2], we assume that the executo tme of each arrval tas for each ste s a pror ad ths formato s maaged the executo tme maager module. Smlarly, we assume that the securty overhead for each arrval tas o each ste s a pror, ad ths formato s mataed the securty overhead maager module. The DSD calculator s used to calculate dscrepaces betwee a arrval tas s securty level reuremets ad the securty levels that each ste offers. The fucto of securty-adaptve wdow cotroller s to vary sze of the wdow to dscover a sutable ste for the curret arrved tas so that (1) ts securty demads ca be well met; (2) the total executo tme ca be as small as possble. After retrevg formato le executo tme o each ste, securty overhead o each ste, degree of securty defcecy o each ste ad the sze of securty-adaptve wdow for the curret tas from the correspodg modules, the tas allocato decso maer wll decde whch ste wll be assged to the tas.

4 38 T. Xe ad X. Q Each ste the system model above s heretly heterogeeous both computato ad securty. Computatoal heterogeety meas that for each tas the executo tme o dfferet stes s dstctve. Whle each tas has a array of securty servce reuests, each ste offers the securty servces wth dfferet levels. The level of a securty servce provded by a ste s ormalzed the rage from to 1.0. Suppose ste M, offers securty servces, P = ( p, p,, p ), a vector of securty levels, characterzes the securty levels provded by the ste. p s the securty level of the th securty servce provded by M. To meet securty reuremet, securty overhead of the tas wll be cosdered. Securty heterogeety suggests that the securty overhead of a tas vares o each ste. 2.2 Modelg Tass wth Securty Reuremets We cosder a class of heterogeeous dstrbuted systems where a applcato s comprsed of a collecto of tass performed to accomplsh a overall msso. It s assumed that tass are depedet of oe aother. Each tas reures a set of securty servces wth varous securty levels specfed by a user. Values of securty levels are ormalzed the rage from 0.1 to 1.0 as well. For example, a tas specfes ts reuest securty level 0.7 for the authetcato servce, 0.3 for the tegrty servce, ad 0.8 for the ecrypto servce. Note that the same securty level value dfferet securty servces may have varous meags. Suppose there s a tas T submtted by a user, T s modeled as a set of ratoal parameters, e.g., T = (a, E, f, l, S ), where a ad f are the arrval ad fsh tmes, ad l deotes the amout of data (measured MB) to be protected. E s a vector of 1 2 executo tmes for tas T o each ste M, ad E = ( e, e,, e ). Suppose T 1 2 reures securty servces, S =, s,, s ), a vector of securty levels, characterzes the securty reuremets of the tas. s s the securty level of the th securty servce reured by T. Please ote that a way of uattatvely measurg securty s stll a ope uesto to be aswered. Stll, we beleve that the proposed securty reuremet model s of help ths research because of the followg two reasos. Frst, although the measuremets of securty reuremets ad securty levels are ot completely obectve, performace mprovemets of the SATS algorthm compared wth the two exstg approaches terms of securty (.e., rs-free probablty ad degree of securty defcecy) are stll vald because the performace of the three algorthms s evaluated usg a detcal set of crtera uder the same crcumstace. Secod, uattatvely modellg securty reuremets ad securty levels maes t possble for us to compare the securty performace of dfferet algorthms ad perceve the performace dscrepaces amog them. A securty-aware scheduler has to mae use of a fucto to measure the securty beefts gaed by each arrval tas. I partcular, the securty beeft of tas T s uattatvely modeled as a fucto of the dscrepacy betwee securty levels reuested ad the securty levels offered. The securty beeft fucto for tas T o ste M s deoted by DSD: (S, P ) R, where R s the set of o-egatve real umbers:

5 A Securty-Oreted Tas Scheduler for Heterogeeous Dstrbuted Systems 39 where 0 w 1, Note that DSD ) = w g, p ), = 1 w = 1. w s the weght of the th securty servce for tas T. Users specfy ther reuests the weghts to reflect relatve prortes gve to the reured securty servces. Degree of Securty Defcecy, or DSD, s defed to be a weghted sum of dscrepacy values betwee securty levels reuested by a tas ad the securty levels offered by a ste. For each tas, a small DSD value meas a hgh satsfacto degree. Zero DSD value mples that a tas s securty reuremets ca be perfectly met. That s, there exsts at least oe ste M M that ca satsfy the followg codto: [, ] s p 1,. Let X be all possble schedule for tas T, x X be a schedulg decso of T. Gve a tas T, the degree of securty defcecy value (DSDV) of T s expected to be mmzed: DSDV ( x = 1 ) = m { DSD ( x )} = m w x X x X = 1 0,f s p g, p ) =, s p,otherwse ( g where p ( x ) = p f the tas s allocated to ste. A securty-aware scheduler strves to mmze the system s overall DSDV value defed as the sum of the degree of securty defcecy of submtted tass (See Euato 1). Thus, the followg DSDV fucto eeds to be mmzed: SDSD ( x ) =. m DSDV ( x ) x X T T (3) where T s a set of submtted tass. Substtutg Euato 2 to 3 yelds the followg securty obectve fucto. Thus, our proposed SATS schedulg algorthm maes a effort to schedule tass a way to mmze Euato 4: SDSD ( x ) = m m w ( f, p ( x ))). (4) x X x X T T = 1 Sce the degree of securty defcecy for tas T merely reflects the securty servce satsfacto degree expereced by the tas, t s adeuate to measure ualty of securty for T durg ts executo. Therefore, we derve ths secto the probablty Prf ( T, M ) that T remas rs-free durg the course of ts executo. The ualty of securty of a tas T wth respect to the th securty servce s calculated as l l where exp λ e + c ) λ s the tas's rs rate of the l = 1, p ( x ))) (1) (2)

6 40 T. Xe ad X. Q l l th securty servce, ad c ) s the securty overhead expereced by the tas o ste. The rs rate s expressed as: λ = 1 exp ( α (1 s )). (5) Note that ths model assumes that rs rate s a fucto of securty levels, ad the dstrbuto of rs-free for ay fxed tme terval s approxmated usg a Posso probablty dstrbuto. The rs rate model s ust for llustrato purpose oly. Thus, the model ca be replaced by ay rs rate model wth a reasoable parameter α. The ualty of securty of tas T o ste M ca be obtaed below by cosderg all securty servces provded to the tas. Coseuetly, we have: P rf l l l l ( T ) = + = +, M exp( λ e c ) ) exp e c ) λ = 1 l = 1 l = 1 = 1. (6) Usg euato 6, we obta the overall ualty of securty of tas T the system as follows, P rf = 1 = 1 l = 1 = 1 l l ( T ) = { [ = ] } = + P x Prf ( T, M ) p exp e c ) λ (7) where p s the probablty that T s allocated to ste M. Gve a tas set T, the probablty that all tass are free from beg attaced durg ther executos s computed based o Euato 7. Thus, P rf ( T ) = P T T rf ( T ). By substtutg the rs rate model to Euato 8, we fally obta P rf (X) as show below: P rf l l ( X ) = p exp ( ) e + c s λ. T T = 1 = 1 = 1 l (9) I summary, DSD values show us securty servce satsfacto degrees expereced by tass, whle rs-free probablty measured by Euato 9 defes ualty of securty provded by a heterogeeous dstrbuted system. I Secto 5 these two metrcs are used to evaluate securty of dstrbuted systems. 2.3 Heterogeety Model The computatoal weght of tas T o ste M (e.g., w ) s defed as a rato betwee ts executo tme o M ad that o the fastest ste the system. That s, we have w = 1 ( e ) = e m. The computatoal heterogeety level of tas T, referred to as (8)

7 A Securty-Oreted Tas Scheduler for Heterogeeous Dstrbuted Systems 41 C H, ca be uattatvely measured by the stadard devato of the computatoal C weghts. Formally, H s expressed as: C 1 2 H =, where avg w w =. (10) = 1 =1 The computatoal heterogeety of a tas set T ca be computed by C 1 C. H = H T T T avg ( w w ) There are three types of securty heterogeetes. () Securty heterogeety of a partcular tas T dcates the dfferece of securty reuremets the securty servces reuested by the tas (see Euato 11). () Securty heterogeety of a gve securty servce provded by each ste a dstrbuted system reflects the dscrepacy of the offered securty levels of the servce the system (see Euato 12). () Securty heterogeety of a partcular ste M shows the devato of the securty levels provded by the ste (see Euato 13). 1 2 Gve a tas T ad ts securty reuremet S =, s,, s ), the heterogeety of securty reuremet for T s measured by the stadard devato of the securty levels the vector. Thus, S 1 2 H =, where avg s s =. (11) = 1 =1 The securty reuremet heterogeety of a tas set T ca be computed by S 1 S. H = H T T T avg s ) The heterogeety level of the th securty servce a dstrbuted system s defed as: V H = 1 2, where p p avg =. (12) = 1 =1 Usg Euato 12, the heterogeety of securty servces ca be wrtte as V 1 V H = H. = 1 Fally, the heterogeety level of securty servces ste M s defed to be: ( p p ) avg H M = 1 2, where avg ( p p ) = 1 p avg = =1 p. (13) 2.4 Securty Overhead Model Now we cosder securty overhead curred by securty servces. The followg securty overhead model cludes three servces, amely, ecrypto, tegrty, ad authetcato [18]. The securty overhead model ca be easly exteded to corporated more securty servces.

8 42 T. Xe ad X. Q Suppose tas T reures securty servces, whch are provded seuetal order. Let s ad c ) be the securty level ad overhead of the th securty servce, the securty overhead c expereced by T o ste M ca be computed usg Euato 14. I partcular, the securty overhead of T wth securty reuremets for the three servces above s modelled by Euato 15. c = = 1 c { a, e, g} ), where s S. (14) c = c ), where s S. e e g g a a where c ), c ), ad c ) are overheads caused by the authetcato, ecrypto, ad tegrty servces [18]. Our securty level assgmet s reasoable because a securty mechasm provdg hgher ualty of securty mposes hgher overhead tha mechasms offerg lower securty. The ecrypto overhead c e of T o M s computed usg Euato 16, where e π s the CPU tme spet ecryptg securty sestve data. e e e e e c ) = π s, where s S. (16) The tegrty overhead ca be calculated usg the followg euato, where l s g g the amout of securty sestve data, ad μ ) s a fucto mappg a securty level to ts correspodg tegrty servce performace. g g g g g c ) = l μ ), where s S. (17) The securty level of a securty mechasm ca be uattatvely measured by the amout of cost eeded to successfully brea the mechasm. However, uattatvely measurg the securty level of a securty mechasm s a otrval research ssue, ad t s out of the scope of ths wor. (15) 3 The SATS Algorthm For tas T, the earlest start tme o ste M s es (T ), whch ca be computed by Euato 18: es ( T ) = r + e l + c l l ) (18) T l W = 1 where r represets the remag overall executo tme of a tas curretly rug o the th ste, ad e + c ) s the overall executo tme (securty overhead l = 1 l l s factored ) of watg tas T l assged to ste M pror to the arrval of T. Thus, the earlest start tme of T l s a sum of the remag overall executo tme of the rug

9 A Securty-Oreted Tas Scheduler for Heterogeeous Dstrbuted Systems 43 tas ad the overall executo tmes of the tass wth earler arrval o ste M. Therefore, the earlest completo tme for tas T o ste M ca be calculated as: e c ( T ) = es ( T ) + e + c ) = r + el + c l l ) + e + c ) (19) = 1 Tl W = 1 = 1 1.for each tas T submtted to the schedule ueue do 2. for each ste M the system do 3. Use E.18 to compute es (T ), the earlest start tme of T o ste M ; 4. Use E.19 to compute ec (T ), the earlest completo tme of T o M ; 5. ed for 6. Sort all stes earlest completo tme a o-decrease order 7. for each ste the securty-adaptve wdow do 8. Use E.1 to compute DSD ) /* Compute DSD for each ste*/ 9. ed for 10. Select the ste M r that ca offer the smallest DSD value ad assg T o t 11. Update ste M r s earlest avalable tme es 12. Use E.6 to compute rs-free probablty for tas T 13. Record start tme ad completo tme for tas T 14.ed for Fg. 2. The SATS algorthm The SATS algorthm s outled Fgure 2. The goal of the algorthm s to delver optmal ualty of securty whle matag hgh performace for tass rug o heterogeeous systems. To acheve the goal, SATS maages to mmze degree of securty defcecy (see Euato 1) of each tas (see Step 10 Fg. 2) wthout performace deterorato. Before optmzg the degree of securty defcecy of tas T, SATS sorts all the stes a o-decrease order T s total executo tme based o the formato retreved from the executo tme maager ad the securty overhead maager (see Fgure 1). Step 7 computes the degree of securty defceces for the tas o each ste the lght of the securty defcecy calculator. Combg the put from the securty-adaptve wdow cotroller, the tas allocato decso maer decdes a ste to whch the tas s allocated. 4 Smulatos Usg extesve smulato expermets based o Sa Dego Supercomputer Ceter (SDSC) SP2 log, we evaluate ths secto the potetal beefts of the SATS algorthm. The real trace was sampled o a 128-ode (66MHz) IBM SP2 from May 1998 through Aprl To smplfy our expermets, we utlzed the frst three moths data wth 6400 parallel tass smulato. Sce the trace was sampled from a homogeous evromet, to reflect the heterogeety of the smulated dstrbuted system, we traslated the executo tme of each tas from a sgle value to a vector wth (umber of stes) elemets based o the heterogeety model descrbed Secto 2.3. I purpose of revealg the stregth of SATS, we compared t wth two well-ow schedulg algorthms, amely, M-M ad Sufferage [14]. M-M ad Sufferage are o-preemptve tas schedulg algorthms, whch schedule a

10 44 T. Xe ad X. Q stream of depedet tass oto a heterogeeous dstrbuted computg system. The two algorthms are brefly descrbed below. (1) MINMIN: For each submtted tas, the ste that offers the earlest completo tme s tagged. Amog all the mapped tass, the oe that has the mmal earlest completo tme s chose ad the allocate to the tagged ste. (2) SUFFERAGE: Allocatg a ste to a submtted tas that would suffer most terms of completo tme f that ste s ot allocated to t. Table 1. Characterstcs of system parameters Parameter Number of tass (6400) Value (Fxed) - (Vared) Number of stes (16) (8, 16, 32,64,128) Tas arrval rate Decded by the trace Sze of securty-adaptve wdow (8) (1, 2, 4, 8, 16) Ste securty level (uform dst.) ( ) Tas securty level (uform dst.) ( ) Weghts of securty servces Authetcato=0.2, Ecrypto=0.5, Itegrty = Smulator ad Smulato Parameters The parameters of stes the smulated dstrbuted system are chose to resemble real-world worstatos le IBM SP2 odes. Table 1 summarzes the ey cofgurato parameters of the smulated dstrbuted system used our expermets. We modfed the trace by addg a bloc of data to be secured for each tas the trace. The sze of the securty-reured data assged to each tas s cotrolled by a uform dstrbuto (see Table 1). Although tas umber, submt tme, ad executo tme of tass submtted to the system are tae drectly from the trace, sze of data to be secured, umber of stes, computatoal heterogeety, ad securty heterogeety are sythetcally geerated accordace wth the above model sce they are ot avalable the trace. The performace metrcs we used clude: rs-free probablty (see Euato 9), degree of securty defcecy (see Euato 1), ste utlzato (defed as the percetage of total tas rug tme out of total avalable tme of a gve ste), maespa (the latest tas completo tme the tas set), average respose tme (the respose tme of a tas s the tme perod betwee the tas s arrval ad ts completo ad the average respose tme s the average value of all tass respose tme), slowdow rato (the slowdow of a tas s the rato of the tas s respose tme to ts servce tme ad the slowdow rato s the average value of all tass slowdows). 4.2 Overall Performace Comparsos The goal of ths expermet s two fold: (1) to compare the proposed SATS algorthm agast the two heurstcs, ad (2) to uderstad the sestvty of SATS to the sze of securty-adaptve wdow.

11 A Securty-Oreted Tas Scheduler for Heterogeeous Dstrbuted Systems 45 (a) Rs-free probablty (b) Degree of securty defcecy (c) Ste utlzato (d) Maespa (e) Average respose tme (f) Slowdow rato Fg. 3. Performace mpact of sze of securty-adaptve wdow Fgure 3 shows the smulato results for the three algorthms o a dstrbuted system wth 16 stes. Sce MINMIN ad SUFFERAGE do ot have a securtyadaptve wdow, ther performace all sx metrcs eeps costat. We observe from Fgure 3a that SATS sgfcatly outperforms the two heurstcs terms of rs-free probablty, whereas MINMIN ad SUFFERAGE algorthms exhbt smlar performace. 5 Summary ad Future Wor I ths paper, we cosdered the securty reuremets of applcatos the cotext of tas schedulg heterogeeous dstrbuted systems because a creasg umber of applcatos rug o heterogeeous dstrbuted systems reures ot oly descet schedulg performace but also hgh ualty of securty. To solve ths problem, we proposed a securty-adaptve schedulg heurstc that s based o the cocept of securty heterogeety. Expermetal results demostrate that our strategy outperforms exstg approaches both securty ad performace. I future research, the heurstc wll be exteded to schedule parallel applcatos.

12 46 T. Xe ad X. Q Refereces 1. Arpac-Dusseau, A.C.: Implct Coschedulg: Coordated Schedulg wth Implct Iformato Dstrbuted Systems. ACM Tras. o Computer Systems, Vol.19, No.3, (2001) Brau, T. D. et al.: A Comparso Study of Statc Mappg Heurstcs for a Class of Meta-tass o Heterogeeous Computg Systems. Proc. Worshop Heterogeeous Computg, (1999) Casavat, T.L, Kuhl, J.G.: A Taxoomy of Schedulg Geeral-purpose Dstrbuted Computg Systems. IEEE Tras. Software Egeerg, Vol.14, No.2, (1988) Coelly, K., Che, A.A.: Breag the barrers: hgh performace securty for hgh performace computg. Proc. Wor-shop o New securty paradgms, Sept. (2002) 5. Doga, A., Özgüer, F.: Relable matchg ad schedulg of precedece-costraed tass heterogeeous dstrbuted computg. Proc. It l Cof. Parallel Processg, (2000) Doga, A., Özgüer, F.: LDBS: A Duplcato Based Schedulg Algorthm for Heterogeeous Computg Systems. Proc. It l Cof. Parallel Processg, (2002) Dooho, G.: Buldg a Web Servce to Provde Real-Tme Stoc Quotes. MCAD.Net, Feb. (2004) 8. Lampso, B., Abad, M., Burrows, M., Wobber, E.: Authetcato dstrbuted systems: Theory ad practce. ACM Tras. Computer Systems, Vol.10, No.4, Nov. (1992) Peg, D.-T., Sh, K.G.: Optmal schedulg of cooperatve tass a dstrbuted system usg a eumeratve method. IEEE Tras. Software Egeerg, Vol.19, No.3, Mar. (1993) Petr F., Feg, W.-C.: Schedulg wth Global Iformato Dstrbuted Systems. Proc. 20th It l Cof. Dstrbuted Computg Systems, Aprl (2000) Raaweera, S., Agrawal, D.P.: Schedulg of Perodc Tme Crtcal Applcatos for Ppeled Executo o Heterogeeous Systems. Proc. It l Cof. Parallel Processg, Sept. (2001) Sadhu, R.S. et al.: Role-Based Access Cotrol Models. IEEE Computer, Vol.29,No.2, (1996) So, S.H., Muamala, R., Davd, R.: Itegratg securty ad real-tme reuremets usg covert chael capacty. IEEE Tras. Kowledge ad Data Egeerg. Vol.12, No.6, (2000) Sog, S., Kwo, Y.-K., Hwag, K.: Trusted Job Schedulg Ope Computatoal Grds: Securty-Drve Heurstcs ad A Fast Geetc Algorthms. Proc. It l Symp. Parallel ad Dstrbuted Processg, (2005) 15. Srvas, S., Jha, N.K.: Safety ad Relablty Drve Tas Allocato Dstrbuted Systems. IEEE Tras. Parallel ad Dstrbuted Systems, Vol.10, No.3, Mar. (1999) Topcuoglu, H., Harr, S., Wu, M.-Y.: Performace-effectve ad Low-complexty Tas Schedulg for Heterogeeous Computg. IEEE Tras. Parallel ad Dstrbuted Sys., Vol.13, No.3, Mar. (2002) 17. VerSg Corp.: Smplfyg Applcato ad Web Servces Securty - VerSg Trust Gateway. (2003) 18. Xe, T., Q, X.: Schedulg Securty-Crtcal Real-Tme Applcatos o Clusters. IEEE Trasactos o Computers, vol. 55, o. 7, July (2006) Xe, T., Q, X.: Ehacg Securty of Real-Tme Applcatos o Grds through Dyamc Schedulg. Proc. 11th Worshop Job Schedulg Strateges for Parallel Processg, MA, Jue (2005)