Adaptive Connection Admission Control for Mission Critical Real-Time Communication Networks Abstract 1 Introduction

Transcription

1 Adaptve Connecton Admsson Control for Msson Crtcal Real-Tme Communcaton etwors B. Devalla, A. Sahoo, Y. Guan, C. L, R. Bettat and W. Zhao Department of Computer Scence, Texas A&M Unversty, College Staton, TX Abstract In ths paper, we report our wor on adaptve connecton admsson control n real-tme communcaton networs. Much of the exstng wor on connecton admsson control (CAC specfes the QoS parameters as fxed values and does not explot the dynamc fluctuatons n resource avalablty. We tae an nnovatve approach: Frst, we allow an applcaton to specfy QoS over a range, rather than fxed values. Second and more mportantly, we desgn, analyze, and mplement CAC modules that, based on QoS specfed over a range, adaptvely allocate system resources to connectons. Delay analyss s an ntegral part of connecton admsson control. Our adaptve CAC uses an effcent delay analyss method to derve a closed form soluton for end-to-end delay of messages n a connecton. Our adaptve CAC mproves system performance n terms of the probablty of admttng connectons and the QoS offered to the connectons. Introducton In ths paper, we report our wor on an adaptve approach to connecton admsson control for provdng effectve and effcent connecton admsson control for msson crtcal applcatons dstrbuted over real-tme communcaton networs. These applcatons typcally consst of a set of tass executng on dfferent hosts, exchangng messages over a hgh-speed networ to cooperatvely accomplsh a common msson crtcal objectve. Examples of such applcatons nclude supervsory command and control of defense systems, manufacturng plants, etc. The success of a dstrbuted msson crtcal applcaton thus crucally depends on the ablty of the underlyng networ to guarantee upper bounds on message transfer delay. Connecton-orented communcaton s wellsuted for applcatons that demand performance guarantees [DLSZ97]. A connecton can be vewed as a contract between an applcaton and the connecton management system. A real-tme connecton s addtonally characterzed by strngent deadlne constrants mposed on ts pacet delvery tme. The defnng characterstc of connecton-orented communcaton s the exstence of a connecton establshment phase precedng the actual data transfer. Connecton management s a networ functon that s responsble for settng up, mantanng and tearng down connectons. A crtcal part of connecton management s Connecton Admsson Control (CAC. The CAC determnes whether a connecton request can be admtted or not. Our study focuses on enhancng communcaton support for dstrbuted msson crtcal applcatons through nnovatve connecton admsson control. Wth our adaptve strategy, we can demonstrate dramatc mprovements n both the offered Qualty of Servce (QoS to the applcatons, and the effectve utlzaton of system resources. Much wor has been done on tradtonal connecton admsson control, whch generally requres the QoS parameters to be specfed as fxed values (e.g., traffc wth pea 0 MBPS, and deadlne 30 mllseconds. Once a connecton s admtted, the tradtonal CAC provdes a constant QoS to the connecton throughout ts lfetme. Thus, the tradtonal approach uses a smplstc QoS specfcaton model and the consequent resource management suffers from many shortcomngs that drectly affect the applcatons usng t. Specfcally, ths model s restrctve n that a fxed QoS model s not sutable for many applcatons that may accept admsson at a lower QoS. For example, a vdeo-on-demand applcaton may be wllng to accept a lower QoS (n terms of lesser bandwdth, jtter, etc. to send vdeo frames of poorer qualty rather than send no frame at all. Ths model s also statc, as the QoS offered to a connecton does not change over ts lfetme even though the resource avalablty changes. Consequently, the tradtonal CAC s very neffectve n terms of the number of connecton requests admtted, as t nether explots the dynamsm of the system nor the flexblty n QoS sutable to applcatons. Ths also leads to a gross under-utlzaton of system resources. To address the defcences of the tradtonal method, we tae an adaptve approach. Frst, we allow an applcaton to specfy QoS n a range, rather than fxed values. Second and more mportantly, we ncorporate QoS adaptaton that offers the best possble QoS to connectons contngent on the avalable resources. Our approach has many benefts albet offerng determnstc performance guarantees on end-to-end message transfer delay. The probablty of a connecton beng admtted s ncreased as the networ has a choce over the range of QoS to offer. An adaptve resource allocaton cognzant of the dynamc fluctuatons n resource avalablty leads to a better utlzaton of system resources. In addton, at any gven tme, the exstng connectons are offered the best possble QoS allowed by the resources avalable. Ths adaptve connecton management s, nevertheless, a challengng proposton. The most mportant ssue n the desgn s effcency: defned n terms of how qucly an admsson decson can be made. An effcent CAC reduces the tme between a

2 connecton request and the admsson decson. Analyss of end-to-end delay plays an mportant part n determnng the tme taen for connecton admsson. Anytme a new connecton request comes n or when the QoS of exstng connectons s changed, the adaptve CAC has to recompute the delays of exstng connectons. The adaptaton mechansms not only ncrease the complexty of delay analyss n an adaptve CAC but also nvoe delay computaton much more frequently than the tradtonal CAC. An adaptve CAC, therefore, needs an effcent delay analyss method. We address ths ssue by usng a smple yet accurate traffc model and derve a closed form soluton for end-to-end delay. 2 Prevous Wor The U.S. DoD has lad specal emphass on mprovng the responsveness, securty and relablty of communcaton servces that play a crtcal role n current and future mltary operatons [DISA94, ABIS96]. In ths context, connecton admsson control for guaranteed performance (delay, jtter, etc. s a well-researched topc. The communcaton s based on a smplex fxed-route connecton called real-tme channel, varatons of the type defned n [Fer90]. A real-tme channel s essentally a vrtual crcut wth performance guarantees. Varous connecton admsson control approaches for guaranteeng performance of real-tme channels have been suggested n [MZ95, Cru95, MIS96, Rah96, DLSZ97]. Connecton admsson control s addressed n terms of schedulng polces, traffc regulaton methods, and analyss of delay and buffer constrants. The aforementoned studes deal wth QoS specfed as fxed-values. Dynamsm n connecton management s dscussed n [PZF94]. More recently, an archtectural framewor for adaptve resource management s reported n [HTG97]. Our study approprately supplements prevous wor n developng communcatons servce for msson crtcal applcatons. We dentfy and solve the mportant ssues n adaptve connecton admsson control. 3 Overvew of Adaptve Connecton Management In ths secton, we present an overvew of our adaptve connecton management scheme. Connecton management ncludes connecton admsson, mantanng connectons and connecton tear-down. 3. Connectons 3.. Connecton QoS Specfcaton The perodc model s tradtonally used to specfy the QoS of a connecton. The parameters are specfed as the ordered trplet, (C, P, D, where C s the message sze n bts generated perodcally every P seconds, and each message has a deadlne D seconds. The tradtonal model however specfes parameters wth fxed values. We extend ths model to specfy parameters over a range of mnmum and maxmum values. The j-th connecton has ts QoS j gven by, QoS j = [QoS worst j, QoS best j ] ( where QoS worst j = (C mn j, P max j, D max j (2 and, QoS best j = (C max j, P mn j, D mn j (3 If admtted, the connecton s offered an operatng QoS, QoS op j such that, worst QoS j QoS op best j QoS j (4 The objectve of the adaptve CAC s to offer the best QoS possble to an admtted connecton based on resource avalablty.e., the adaptve CAC eeps QoS op j as close to QoS best j as possble Connecton Classfcaton Our scheme dstngushes connectons to be from one of three classes: crtcal, essental and non-essental. Crtcal connectons are of the hghest crtcalty and are always admtted. Reservng resources a pror for crtcal connectons ensures ths. A crtcal connecton, once admtted, cannot be preempted. Essental connectons are of a crtcalty lower than crtcal connectons but hgher than the non-essental ones. An essental connecton may be dened admsson f suffcent resources are not avalable, but once admtted, t cannot be preempted. on-essental connectons are of the lowest crtcalty. They may be dened admsson and be preempted n order to admt other connectons of hgher crtcalty. Such a crtcalty-based classfcaton s approprate for msson crtcal system where tass nherently are of dfferent crtcalty [MZB90, IT96]. In the absence of ths classfcaton, a less crtcal tas would use the same amount of resources as a more crtcal one. Ths clearly results n poor management of resources especally when there s resource contenton. Further, classfyng connectons helps applcatons explot the adaptaton servces durng connecton admsson resultng n better overall performance. 3.2 Adaptaton Strateges We separate adaptaton strateges n our adaptve connecton management (See Fgure nto two major threads - one for connecton admsson and another for connecton termnaton. There s a QoS Shrnage module for connecton admsson and a QoS Expanson module for connecton termnaton. 2

3 Connecton Admsson Request (j, QoS j, SDS j QoS j op = QoS j best Delay Computaton d D? Yes Resource Allocaton Accept o Success QoS Shrnage Fal Reject Connecton Termnaton Request (j, EDS j Resource Release QoS Expanson Resource Re-allocaton Return d: delay vector D: deadlne vector j: the d of the connecton to be set up/termnated; QoS j : Requested QoS of connecton j SDS: QoS Shrnage Drectve Sequence; EDS: QoS Expanson Drectve Sequence Fgure. Adaptve Connecton Management 3.2. Adaptve Connecton Admsson A Connecton Admsson Request (CAR comes wth the followng parameters: {j, QoS j, SDS j }, where QoS j s the QoS of the j-th connecton as defned n Equaton (. The connecton request also specfes an adaptaton polcy va the Shrnage Drectve Sequence (SDS j. SDS j drects the connecton admsson module as to whch connectons QoS need to be shrun n order to admt the new connecton. For every ncomng connecton request, the QoS j op s set to QoS j best so that the new connecton may be admtted at the best QoS requested f suffcent resources are avalable. The connecton management scheme then computes the upper bound on the delay d due to the new connecton beng admtted at QoS j best. Delay computaton s a very mportant part of the connecton admsson process and s explaned n detal n Secton 4. We then test f all the requested deadlnes can be met.e. d D. The new connecton s admtted only f the deadlnes of ths and the exstng connectons can be guaranteed. If the guarantee test fals, the adaptaton mechansm comes nto play. Based on SDS, QoS Shrnage determnes the level to whch the QoS of a selected set of connectons need to be shrun to successfully admt the new connecton at QoS j op. If QoS Shrnage s successful n admttng a connecton by reducng QoS of some connectons, resource allocaton follows. A connecton s rejected only when adaptaton fals to free up enough resources to admt the new connecton. Once a new If there are -connectons, then d s the vector of upper bounds of end-to-end delays (d, d 2,, d, and D s the correspondng vector of deadlnes (D, D 2,, D, d D ff, d D..e. the worst-case end-to-end delay of every sngle connecton s no greater than ts respectve deadlne. connecton s admtted, the adaptve CAC allocates resources for the new connecton such that the connecton operates at QoS op j. The adaptaton drectve SDS allows user partcpaton n the connecton process. To admt a connecton, the user specfes whch connectons QoS s to be shrun f necessary. Ths ensures adaptaton by the CAC to be consstent wth the user s msson objectves. There s also an added beneft to the effcency of the adaptaton scheme. The adaptve CAC s bereft of the complcatons of a selecton process to determne whch connectons have to be subject to QoS shrnage Adaptve Connecton Termnaton A Connecton Termnaton Request comes wth followng parameters: {j, EDS j }, where the adaptaton polcy of the j-th connecton s specfed by the Expanson Drectve Sequence (EDS j. EDS j drects whch connectons are to get an ncreased QoS as result of resources released by the termnatng connecton. When a vald CTR arrves, the adaptve connecton management module releases all resources that were reserved for the connecton durng admsson tme. The adaptve connecton management module based on EDS then determnes the level to whch the QoS of a selected set of connectons can be ncreased usng the resources released at connecton termnaton. The necessary resources are then re-allocated to support the ncreased QoS. 4 Delay Analyss Delay analyss s an ntegral part of connecton admsson control. Whenever resources allocated to connectons change, the delays of these connectons have to be recomputed to ensure the new delays are less than ther respectve deadlnes. Only then can the changes to resource allocaton be commtted. In case of adaptve CAC, allocated resources are changed qute frequently durng the adaptaton process. Hence delay computaton s carred out much more often than n tradtonal CAC. Due to ths frequent nvocaton, delay computaton plays a pvotal role n the performance of adaptve CAC. Specfcally, the effcency of adaptve CAC largely depends on how qucly the delay can be calculated. In ths secton, we wll descrbe the delay analyss used n our adaptve CAC. The delay formula derved n ths secton s n a closed form. Thus t avods search for the worst case value and contrbutes to the effcency of the adaptve CAC. In ths analyss, we wll only concentrate on the networ part of the delay. The networ beng analyzed conssts of one ATM swtch. The swtch uses FCFS schedulng to schedule cells at the output port. Hosts are connected to each other va the ATM swtch. 3

4 4. Traffc Descrpton Traffc descrpton s very mportant for delay analyss. Accuracy and effcency of delay analyss s very much dependent on nformaton provded by the traffc descrptor. In msson crtcal systems, we are concerned wth the worst case delays. Hence the traffc descrptor used, should represent the worst case traffc. We had used the maxmum rate functon, Γ(I to represent traffc n some of our prevous wor [SLDZ97, Rah96]. It s defned as the maxmum data arrval rate n a tme nterval of length I. That s number of bts arrved n the nterval (, t t I] ( I max( t 0 I The maxmum rate functon represents the worst case traffc qute accurately. It can be used to descrbe traffc at the ntermedate ponts of the networ. Analyzng a networ wth ths method s much more accurate than when traffc s descrbed only at the source. Furthermore, propertes of maxmum rate functon are well establshed [Rah96]. The maxmum rate functon has certan lmtatons. If a closed form s not avalable, then the value of the functon at dfferent values of tme has to be stored, for the entre duraton of a connecton. Ths may not be practcal n actual systems. Usng maxmum rate functon, computaton of worst case delay nvolves searchng for the worst value n the set of all values. Ths search has a detrmental effect on the effcency of adaptve CAC. Hence, n ths paper we propose mult-segment approxmaton of maxmum rate functon to represent traffc. In ths method, we have the nowledge of the traffc at some nown ponts. Then traffc s represented wth as many lne segments as the number of nown ponts, each correspondng to the gven ponts. Ths representaton has the followng advantages: Storage requrement s very low compared to maxmum rate functon method. Because traffc s represented as multple lne segments, the delay analyss s more tractable usng delay calculus. A closed form delay formulaton s possble n ths representaton of traffc. Hence ths method elmnates the searchng requred to get the worst case delay, whch n turn, ncreases the effcency of CAC. To llustrate the mult-segment approxmaton method, let us assume the nput traffc at the applcaton layer of the connecton M s gven by perodc traffc C and P accordng to the perodc model descrbed n secton 3... Ths s one pont of the traffc descrpton whch s gven by the applcaton. We also measure the pacet length (C pt n bts and the mnmum dstance (5 between two consecutve pacets (P pt n seconds, as the pacets ext the host and enter the networ. Ths gves us the second pont. The thrd pont s obtaned by measurng the cell length (C cell and the mnmum dstance between two consecutve cells (P cell at the entrance of the networ. Further, we assume C C pt C cell and C /P C pt /P pt C cell /P cell. (0 F n (I n bts C C pt B C cell slope = C cell /P cell slope = C pt /P pt Slope = C /P X 0 X I n sec Fgure 2: Plot of nput traffc of a connecton to the multplexer So the three ponts used to construct the threesegment approxmaton of traffc descrpton for connecton M are (C cell, P cell, (C pt, P pt and (C, P. The three-segment approxmaton s shown n Fgure 2. The frst lne segment starts at I=0, the second at I=X 0 and the thrd at I=X. The slopes of the three lne segments are (C cell /P cell, (C pt /P pt and (C /P respectvely. The y- ntercepts of those lnes are C cell, B and C respectvely. Usng smple coordnate geometry prncples, t can be easly establshed that B, X 0 and X are gven by B = C pt C pt (C pt /C cell P cell /P pt, (7 X 0 = (C pt /C cell P cell, (8 and X = [(C C pt + (P cell /P pt C pt (C pt /P pt ]/(C pt /P pt C /P (9 So the three-segment approxmaton of nput traffc to the multplexer for connecton M s gven by C + R. I 0 I < X F n cell cell 0 ( I = B + R pt. I X I < X 0 C + R. I X I < (0 where R cell = C cell /P cell, R pt = C pt /P pt and R = C /P. 4.2 Formulaton of Delay etwor delay s the sum of the delay of a cell n the ATM swtch (d atm and the delay of the cell to 4

5 propagate from sender host to the swtch and from swtch to the recever host (d prop. Thus d networ = d atm + d prop ( d prop s a constant delay and can be measured. ATM swtch can be decomposed nto the nput port demultplexer, the swtchng fabrc and the output port multplexer [Rah96]. So delay n the ATM swtch (d atm can be expressed as the sum of the delays at these three decomposed components. d atm = d demux + d fabrc + d mux, (2 where d demux s the delay n the demultplexer, d fabrc s the delay n the swtchng fabrc and d mux s the delay n the multplexer. d demux and d fabrc are constant delays and can be measured. But d mux s a varable delay snce t depends on the queue length at the output port. In our delay analyss, we wll focus on calculaton of d mux. We use the threesegment approxmaton method to descrbe nput traffc to the multplexer. Let us assume that there are connectons M, M 2,, M that are passng through the multplexer beng analyzed. The nput traffc at the applcaton layer of connecton M s perodc traffc gven by (C, P. The three-segment approxmaton of nput traffc of connecton M s gven by (0. Snce the frst two ponts of all the connectons are the same, the frst bendng pont or flex pont gven by X 0 s same for all the connectons. The second flex pont X wll be, n general, dfferent for dfferent connectons. The total traffc F n (I to the multplexer s the summaton of these ndvdual F n (I for all,. So F n (I wll have at most (+ flex ponts. Wthout loss of generalty, we assume that flex ponts X, 0, of F n (I are n a nondecreasng order.e., X 0 X X 2 X. (3 umber of Bts ( B+ C =.B.C cell F n (I C pt C slope = ( + P = P pt slope=.c cell /P cell A C B S(I X 0 X X + X I n sec D slope = Lne_Speed Fgure 3 : Plot of the total traffc to a multplexer Then F n (I s gven by = (4 The plot of F n (I s shown n Fgure 3. Also shown n Fgure 3 s the servce curve S(I of the multplexer. The multplexer serves at a constant rate of Lne_Speed (n bts/second whenever there are cells to be served. Lne_Speed s 55 Mb/s for ATM swtches wth OC3 ports. Hence S(I s a straght lne wth slope equal to Lne_Speed passng through the orgn. Once the total traffc to the multplexer and the servce functon of the multplexer are determned, basc delay calculus [Cru95] can be appled to fnd the worst case delay. For any I, F n (I represents the number of bts that has arrved at the multplexer and S(I shows the number of bts served by the FCFS server at the multplexer. Hence the dfference F n (I S(I shows the worst case queue length at the multplexer durng any tme nterval of length I. Then the worst case delay at the multplexer occurs when ths dfference F n (I S(I s the maxmum. Snce F n (I conssts of lne segments, the dfference wll be maxmum at one of the flex ponts where the nput data rate (slope of F n (I changes from more than Lne_Speed to less than Lne_Speed. Let ths partcular flex pont be X, 0. Then the worst case delay n the multplexer s gven by d. R + < cell. I. Ccell 0 I X0. R + pt. I. B X I < X [(. R + R]. I+ (. B+ C X I < X [(. R + R]. I+ (. B+ C < X I X mux % K [(. X0. Rcell+. Ccell]/ Rcell = 0 K = & K[( B+ C X{ Lne_ Speed ( Rpt R}] / Lne_ Speed 0< K ' n n F ( I = F ( I pt pt R. I+ C X I < (5 In Fgure 3, the worst case queue length s represented by AC. The correspondng worst case delay s gven by AD. We cannot provde the proof of (5 due to space lmtaton. Complete proof can be found n [L98]. 4.3 Usage of Delay Formula by Adaptve CAC The adaptve CAC recalculates delays as follows: Let us assume that the operatng QoS of a set of connectons G s reduced durng adaptaton to admt a new connecton M j. The set G s determned by the SDS gven by the user n the request

6 Due to change n operatng QoS, second flex pont of connectons n G wll change. So the new second flex ponts X s are calculated for all M G. Let G G be the group representng connectons that pass through the same output port as M j. Let there be connectons n G. Because the second flex pont of connectons n G changed, they are sorted n a nondecreasng order X, X 2,., X. Usng (0 new traffc descrpton F n (I of each ndvdual connectons M G s calculated. F n (I for all M G are added up to fnd the aggregate traffc F n (I usng (4. From F n (I, the flex pont X, 0, s found such that the rate of F n (I changes from more than Lne_Speed to less than Lne_Speed. Ths pont corresponds to the worst case delay. Usng (5 d mux s calculated. Ths delay s same for all the connectons M G. Ths delay s then added up wth other constant delays n (2 to get the delay n the swtch. Thus t s apparent from the steps that the delay calculaton nvolves no costly search. The most costly operaton n ths delay calculaton s sortng the X s whch can be done n a logarthmc complexty. The fnal delay expresson gven by (5 does not need adaptve CAC to search for the worst case delay, the delay s calculated by pluggng the value of X. Thus three-segment approxmaton method of traffc descrpton leads to an effcent delay computaton. 5 Implementaton and Evaluaton 5. Implementaton n etex Our adaptve CAC module has been mplemented n the newer verson of our real-tme toolt, etex [SLDZ97]. etex s a communcaton software pacage developed n the Department of Computer Scence at Texas A&M Unversty. etex s a lbrary of communcaton prmtves that enables user applcatons to partcpate n delay guaranteed communcatons. etex conssts of three man components: user lbrary, Host Traffc Manager (HTM and etwor Traffc Manager (TM. User lbrary s the nterface of etex to the end users. HTM s the module responsble for managng and polcng traffc at the host. TM s prmarly responsble for connecton management of the entre system. For a detaled descrpton of etex archtecture please refer to [SLDZ97]. 5.2 Performance Metrcs and Observatons We use the followng performance metrcs to compare the tradtonal and adaptve approaches to connecton admsson control: Admsson Probablty (AP s defned as umber of connectons admtted AP = Total number of connectons requested for admsson QoS Effectveness (QoSE s defned as QoSE = op mn 2 op max 2 op max 2 ( C C + ( P P + ( D D max mn mn max mn max ( C C + ( P P + ( D D Average Executon Tme (AET of connectons s defned as Sum of executon tme of connectons AET = Table I shows AP, QoSE and AET for the tradtonal and adaptve CAC for low, medum and hgh system utlzaton. System utlzaton s a measure of resources demanded by ncomng connecton requests. Table I Performance of Tradtonal and Adaptve CAC Vs. System Utlzaton Metrc AP(% QoSE AET (mllsecs Utlzaton Trad. Adap. Trad. Adap. Trad. Adap. Low Medum Hgh Trad.- Tradtonal CAC, Adap.- Adaptve CAC As system utlzaton ncreases, AP of both the tradtonal and adaptve CAC decreases. Ths s because the avalablty of system resources decreases as utlzaton ncreases and more connectons are rejected. However the AP for the adaptve approach s always hgher than that for the tradtonal scheme. The tradtonal scheme always accepts connectons at QoS best and hence allocates more resources to each admtted connecton than the adaptve scheme. As a result, the AP of the tradtonal scheme s lower. QoSE s hgh for the tradtonal CAC as t admts connectons at QoS best. For the adaptve CAC, QoSE s closer to tradtonal CAC at low and medum utlzaton. Ths s because at low and medum utlzaton, more resources are avalable, so connectons can be admtted wth operatng QoS equal to or very close to QoS best. But at hgh utlzaton, QoSE for the adaptve CAC drops as t tres to admt more connectons at a lower QoS. AET of both the tradtonal and adaptve schemes ncreases as system utlzaton ncreases. At hgher utlzaton, both the systems have more actve connectons to deal wth, whch maes the executon tme longer. AET of the adaptve CAC s more than tradtonal CAC as the adaptaton expends tme n shrnng and expandng the QoS whle tryng to admt and termnate connectons. evertheless, the overall AET of the adaptve CAC s very low (less than mllsecond, whch s very good for systems n practce. 6

7 6 Concluson and Future Wor In ths paper we have ntroduced adaptve connecton admsson control that addresses the shortcomngs n tradtonal connecton admsson control. We have demonstrated that by tang an adaptve approach to connecton admsson control, we can enhance communcaton support for msson crtcal real-tme applcatons. The hghlghts of ths study are: A Flexble QoS model: We have extended the tradtonal fxed-value QoS model to one that accepts QoS specfed over a range. Ths gves the applcatons and the admsson control flexblty n resource allocaton. Crtcalty-based connecton model: Our adaptve CAC dstngushes between connectons based on crtcalty. Ths s partcularly sutable to msson crtcal applcatons that need crtcalty-based connecton admsson. Effcent and effectve QoS adaptaton modules: Wth QoS Shrnage and QoS Expanson modules, the resources are dynamcally re-allocated n order to meet the needs of ncomng connectons. Our performance data shows that the addtonal cost (n terms of executon tme of adaptaton s low whle the benefts are hgh. User-level adaptaton drectves: These allow user partcpaton n the connecton admsson control. Ths further enhances connecton-level flexblty, whch s useful for msson crtcal applcatons. Practcal and compatble technology: The proposed adaptve connecton admsson control scheme has been mplemented n etex. etex s realzed wth networ products that are currently commercally avalable and does not requre any changes to them. Several extensons to adaptve connecton admsson control are possble. We are currently developng admsson control algorthms for heterogenous networs and mang the entre scheme fault-tolerant. 7 Acnowledgements Ths wor was partally sponsored by AFOSR, AFMC, USAF under a grant (number F and by Texas Hgher Educaton Coordnatng Board under ts Advanced Technology Program and by DARPA va Honeywell Technology Center by contract B Reference [ABIS96] ABIS Tas Force, DoD Advanced Battlespace Informaton Systems Tas Force Report, 996. [ATM95] ATM Forum, ATM Forum- ATM User- etwor Interface Specfcaton Verson 3., 995. [BKHDP96] G. v. Bochman, B. Kerherve, A. Hafd, P. Dn, A. Pons, Archtectural Desgn of Adaptve Dstrbuted Multmeda Systems, IEEE Internatonal Worshop on Multmeda Software Development, 996. [Cru95] R. L. Cruz, Qualty of Servce Guarantees n Vrtual Crcut Swtched etwors, IEEE Journal of Selected Areas n Communcaton, Aug [DISA94] Defense Informaton Systems Agency, Baselne Common Operatng Envronment, 994. [DLSZ97] B. Devalla, C. L, A. Sahoo, W. Zhao, Connecton-Orented Real-Tme Communcaton for Msson Crtcal Applcatons: An Introducton to etex, Proc. of AECO 97, July 997. [Fer90] D. Ferrar, Clent requrements for real-tme communcaton servces, IEEE Communcatons Magazne, ov [FHBM97] S. Fscher, A. Hafd, G. Bochmann, H. De Meer, Co-operatve QoS Management for Multmeda Applcatons, Proc. of IEEE Intnl. Conf. on Multmeda Computng and Systems 997. [HTG97] J. Huang et al, RT-ARM: Real-Tme Adaptve Resource Management System for Dstrbuted Msson- Crtcal Applcatons, IEEE Worshop on Mddleware for Dstrbuted Real-Tme Systems & Servces 997. [IT96] Informaton Technology - QoS Framewor, DIS 3236, ISO/IEC JTC/SC2 0339, June 996. [L98] C. L, Desgn of Adaptve Connecton Admsson Control for Hard Real-tme ATM etwors, M.S. Thess, Texas A&M Unversty, May 998. [MIS96] A. Mehra, A. Indresan, and K. Shn, Resource management for real-tme communcaton: Mang theory meet practce. Proc. of RTAS, 996. [MZ95]. Malcolm and W. Zhao, Hard Real-Tme Communcaton n Multple- Access etwors, Journal of Real-Tme Systems, January 995. [MZB90]. Malcolm, W. Zhao, C. Barter, Guarantee Protocols for Communcaton n Dstrbuted Hard Real- Tme Systems, Proc. of IEEE IFOCOM 90, San Francsco, USA 990. [PZF94] C.J. Parrs, H. Zhang, D. Ferrar, A Dynamc Management Scheme for Real-Tme Connectons, Proc. IEEE IFOCOM 94, Toronto, Ontaro, Canada 994. [Rah96] A. Raha. Real-Tme Communcaton n ATM etwors. Ph.D. Thess. Texas A&M Unversty, 996. [SLDZ97] A. Sahoo, C. L, B Devalla, W. Zhao, Desgn and Implementaton of etex: A Toolt for Delay Guaranteed Communcatons, Proc. of MlCom 97. 7