Modelling and Performance Evaluation of Router Transparent Web cache Mode

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1 Emad Hassan A-Hemiary IJCSET Juy 2012 Vo 2, Issue 7, Modeing and Performance Evauation of Transparent cache Mode Emad Hassan A-Hemiary Network Engineering Department, Coege of Information Engineering, Nahrain University Baghdad, Iraq Abstract This paper modes and evauates the performance of router transparent web cache mode in oca area networks. A nearby transparent cache server intercepts cient s requests; forwarded by a ayer 3 router or a ayer 4 switch, and ooks for a copy in its oca cache store; if the object requested exits in its oca cache, then a cache resuts and the object is forwarded to the router immediatey which in turns deivers it to the cient. Otherwise, the cache estabishes a one-time TCP connection with the origin server and downoads that object. The anaytica modeing is carried out using state transition diagrams and open queuing network modeing where the average response time is evauated for different parameters ike probabiity of cache, cient requests arriva rate and externa arrivas at the origin server. We aso incude the deay effects of transmission media between the oca network and the Internet where the origin servers reside. Resuts obtained vaidate the mode and show the significance of depoying cache server in a network. Keywords cache mode, performance of transparent cache, cache state transition diagram. I. INTRODUCTION caching has been empoyed to improve the efficiency and reiabiity of data deivery over the Internet. A nearby cache can serve a cached page quicky even if the originating server is swamped or the network path to it is congested [1. Whie this argument provides the sefinterested user with the motivation to expoit caches, it is worth noting that using widespread use of caches aso engenders a genera good: if requests are intercepted by nearby caches, then fewer go to the source server, reducing oad on the server and network traffic to the benefit of a users. Besides the advantage of reducing oad on webservers, caching frequenty visited internet objects reduces ISPs bandwidth effectivey. This fact may change when these objects are dynamic non-cacheabe contents [2. This is due to the cache-contro pragma webservers issue in their headers which in turn cause cache rate percentage number of requested objects fetched from the cache store to the tota number of cients requests to be ow when most of cients requests are for dynamic contents. To overcome this imitation, an vioation shoud be enabed in the cache to modify this cache-contro pragma and have that object cached. The other advantage of using web cache servers is reated to cient speed directy. If we assume that the cient has a imited bandwidth controed by his ISP, then in genera the overa speed is enhanced especiay for oaded webservers. This is done by aowing temporariy high speed for cients requesting objects from the cache storage. The most obvious approach, of providing a group of users with a singe, shared caching server in standaone mode, has severa drawbacks. If the caching machine fais, a users are cut off from the. Even whie running, a singe cache is imited in the number of users it can serve, and may become a botteneck during periods of intense use. This is directy reated to hardware and software used for web caching. The hardware part is the storage media and dynamic memory aocation. The software contros the way web cache reads, writes, swap-out and repace objects. cache depoyment incudes: standaone, router transparent and switch transparent modes [3. In standaone mode, the cache server acts as a gateway between cients and web. In transparent mode, cient requests are redirected using a router or a ayer 4 switch to cache as shown in Fig. 1. Cients Cients Cients Cache Cache Standaone mode Layer 4 Switch First it marks a requests that contain destination port. Then uses destination NAT to cache server transparent mode Cache It examines the destination port of a incoming requests, if it contains port, then it forwards them to the cache Switch transparent mode Fig. 1 cache depoyment methods II. RELATED WORK content deivery using caches has been discussed in the iterature for quite ong time. There have been recent 1316

2 Emad Hassan A-Hemiary IJCSET Juy 2012 Vo 2, Issue 7, researches on using open network queuing modes for web cache performance evauation. The foowing is a ist of reated works in this area: Bose and Cheng [4 examine the impact of instaing a proxy cache server PCS on overa response time to requests. They anayzed how various factors affect the performance of that server. Their anaysis is based on modeing the proxy cache server using open queuing network approach and the research specificay identifies a crossover probabiity, the minimum cache rate probabiity at which instaing a proxy cache server becomes beneficia. Berczes and Sztrik [5 modified the performance mode in [4 to a more reaistic case when externa visits are aso aowed to the remote servers and the servers have imited buffer. They anayzed how many parameters affect the performance of a Proxy Cache. Numerica resuts for the overa response time with and without a PCS are obtained. These resuts show that the benefit of a PCS depends on various factors. Severa numerica exampes iustrate the effect of visit rates, visit rates for the externa users, and the cache rate probabiity on the mean response times. Berczes [6 evauates the performance of a PCS when the incoming arrivas and service time are generay distributed. The work considers the network mode of [4 as the first step then an expression for the overa system time is evauated using the proposed distributions. For other iterature references, we point to other reated references [7-[10. In this paper, we evauate the performance of a transparent cache depoyed in a oca area network where we first mode the network using state transition diagrams then we give anaytica formuation to the average response time with and without cache server. arrivas from cients denoted by λ. In Fig. 2, the state transition probabiities are given as foows: : the probabiity that the router detects an request port of an arriva and direct it to the cache. The rest are directed to the corresponding non- servers. : the probabiity of cache, in which the cache finds a copy of the requested object in its oca cache and deivered it to the router immediatey. : the probabiity of cache miss, in which the cache does not find a copy of the requested object in its oca cache and forced to downoad it from the origin server. : the probabiity that the cache response back to router with requested object cache-miss. : the probabiity that the non- server responds back to the router for non- requests : the probabiity that the server responds back to cache request for cache-miss requests III. TRANSPARENT WEB CACHE OPERATION In transparent cache mode, a router or a ayer 4 switch is configured to intercept cient s request for object of the type request with destination port and redirect it to a nearby cache caed an interceptor cache sometimes [11. The atter resoves that request and ook for any avaiabe not expired copy of the requested object in its oca cache. If the object exists then a cache is resuted and the object is deivered to that cient with an overa response time dependent on the cache speed and ink speed. On the other and, a cache miss is resuted when the requested object is not found in the oca cache store which forces the cache to contact the origin server and fetches that object, deivered to the cient whie creating a copy in its oca cache for future requests of the same object. The overa response time in the case of cache miss depends on many factors ike cache speed, ink speed between the cache and the origin server, its Network bandwidth and the probabiity of bocking at the origin server itsef. We wi adopt state transition diagrams to mode the network response time with transparent cache server. Fig. 2 shows the state transition diagrams of cache and cache miss for transparent router mode operation with incoming The state transition diagram shown in Fig. 2 can be converted into the equivaent open queuing network mode shown in Fig. 3. The foowing section gives detaied anaytica formuation to the overa response time based on the queue mode given in Fig. 3. To Cient From Cient Non 1-p 1-p From Cient To Cient wr Non 1-p cache cache State transition diagram for cache hw State transition diagram for cache miss Fig. 2 State transition diagrams of router transparent cache mode Network e p 2 p H it 1 3 L1 1-p server TCP handshake - L2 Net Bandwidth Cache Net Bandwidth Non- server Cient Network TCP handshake L2 Miss p L1 Fig. 3 open queuing network mode of router transparent cache mode

3 Emad Hassan A-Hemiary IJCSET Juy 2012 Vo 2, Issue 7, IV. ANALYTICAL MODELLING As shown in Fig. 3, the incoming arrivas λ from cient network are handed by the gateway router which examines the destination port and route the traffic into two directions. traffic with probabiity wi be routed to the cache server and non- traffic with probabiity wi be routed to their destination servers node 1. The cache server upon an arriva of a request searches its oca cache store for a copy of the requested object, if the object exists then represents the probabiity of cache. represents the probabiity of cache Otherwise, misses and the requested object is fetched from the origin server node 2. Since the connection between the router or/and the cache to the origin servers is done through internet, then we incude this ink as a queue mode as shown in Fig. 3. We aso assume that this connection starts with a one-time TCP 3-way handshake which is represented by the two circes in Fig. 3. This handshake depends on the round trip time RTT of the ink between the two ends i.e., ISP ink. Therefore, we have represented each one-way ink with a queue to highight the deay amount this ink adds to the overa response time. The performance of the origin servers are characterized by the capacity of its output buffer, the static server time, and the dynamic server rate [7,12. In our mode we assume that the server has a buffer of capacity. Let be the probabiity that a request wi be denied by the server i.e., probabiity of bocking. From basic queuing theory the bocking probabiity for the M/M/1/K queuing system is given by [13: 1 with [12, is the average router system time in seconds, is the average cache system time in seconds and is the average non- system time in seconds The average response time spent in the router queue in Eq. 5 can be represented by an M/M/1 queue. Assuming the processing rate of this router is much more than the arriva rate, then: 6 is the router service rate. Considering a FIFO router queue discipine, incoming requests from cients wi have to pass the router input queue then processed by the NAT chain and redirected to the corresponding destination. The same process wi occur when a response comes from the cache. The second term in Eq. 5 is the average response time for the cache for both cache and cache miss as shown in Fig 3: 7 is the average cache ookup time to check whether the requested object exists in its oca cache store or not. This is derived from the waiting time in an M/M/1 queue where the arrivas form a Poisson process with rate λ and the service rate is. It can be formed as: The second term in Eq. 7 is the response time of a cache. This can be obtained by summing the average response time of the cache object fetch, deivered to the router and finay to the cient network. Thus: 3 server 9 4 Externa arrivas Arrivas from cache Where the subscript wi denote the server and is the sum of arrivas from our cache server and externa arrivas denoted by as shown in Fig. 4. Then: [ 5 8 gives, 2 In order to find the response time for the modeed transparent router mode, we must consider the sum of responses from cache, cache miss and non- requests. Then, the overa response time can be stated as: µ Dropped arrivas Fig. 4 The origin server queuing mode showing externa arrivas Inside the brackets of Eq. 9, the first item is the system time in the cache, where is the cache output buffer in bytes, is the cache static server time in seconds, is the cache dynamic server rate in bytes/seconds and is the average object fie size in bytes. The second item in Eq. 9 corresponds to the required time for the router to route the requested object to cient. Finay, the ast term is the time required to transfer the object over the 1318

4 Emad Hassan A-Hemiary IJCSET Juy 2012 Vo 2, Issue 7, cient network bandwidth where is the cient network bandwidth in bits/seconds. The ast term in Eq. 7 is the average response time when there is a cache miss. Therefore: [ When there is no cache server, then the state transition diagram is as shown in Fig. 5 and the average response time can be formed as: To Cient Non 1-Pb 1-p From Cient Fig. 5 State transition diagram without transparent cache mode The reader can notice that the service rate of the cache server is the same as the server! This is due to the fact that the cache acts as an server reative to cient when deivering objects, and as a cient when connecting to the origin server. Returning back to Eq. 10, the first term represents the internet ink deays where Equation 12 is the same as Eq. 5 except for the second term that corresponds to the response time from the server. It is given by: is the ink service rate which is a function of the ISP s ink speed and object size. When the cache does not find a copy of the requested object in its oca cache store, it estabishes a one-time TCP connection with the origin server to fetch that object. This operation is modeed by the second term in Eq. 10 where is the TCP handshake service ookup rate. The response time of the server for deivering the object is shown in the third term of Eq. 10 and the server network response time is shown in the fourth term, where is the server network bandwidth. The rest terms in Eq. 10 are the same as those in Eq. 9 which modes the response from the cache server when it receives the requested object from the server. If the size of the requested object is greater than the server's output buffer it wi start a ooping process unti the deivery of a requested objects is competed [6. This operation is expressed by the vaue in the third term of Eq. 10 and the first term in Eq. 9. Finay, the time for non- requests can be modeed by: [ [ 11 [ [ 13 V. SIMULATION RESULTS To evauate the performance of the modeed router transparent mode cache network, we adopt the foowing parameters vaue: We conduct severa tests for the modeed network with and without transparent cache to evauate the overa average response time for a cient request. The foowing subsections discuss these tests. A. Effect of existence of transparent cache and its probabiity of cache The first resut obtained by evauating the average response time versus arriva rate λ for different vaues of probabiity of rates as shown in Fig. 6. The dotted ine in Fig. 6 is the average response time for the network without a transparent cache, whie the other curves are the responses when is varied from 0 to 50% cache rates. It is cear that a transparent cache with higher cache rates improve the overa network response time. For exampe, 10% cache rate has cose performance to that without the cache. Whie for λ above 20% the response time dramaticay changes to ower vaues. In practice however, cache rate is reated to the amount of stored frequenty visited objects in 1319

5 Emad Hassan A-Hemiary IJCSET Juy 2012 Vo 2, Issue 7, cache disks and therefore cannot be configured to have fixed vaues. Fig. 6 Average response time versus arriva rate. %, L K=100. the router and the effect of cient network bandwidth. Verification of the anaytica modeing is carried out using numerica anaysis where different outcomes are discussed. The resuts obtained indicate that the network average response time can be enhanced using a transparent cache given that the atter one can produce considerabe amount of cache rates more than of 20% and more. On the other hand, we aowed through anaytica modeing that the origin server can hande externa arriva rates from other users over the Internet. When we increase this rate, we found that the performance of the depoyed transparent router mode cache is satisfactory which confirms one of the advantages of depoying cache in a network; that is, reducing the amount of traffic on the origin servers. REFERENCES =100, [1 B. Effect of Externa Arrivas rate at the Origin One of the advantages of depoying cache server in a network is to improve the performance of interna cients browsing heaviy oaded servers. Fig. 7 shows the resuts obtained when we vary the externa incoming rate to the server L from 90 to 140 req/s and evauate the average response time versus λ with. The upper curves denote performance without cache whie the ower curves are for transparent cache. It is cear that even when the origin server response time is increased due to increase in incoming externa arrivas, the average response time for the network with a transparent web cache remains at ower eves. [2 [3 [4 [5 [6 [7 [8 [9 [10 Fig. 7 Average response time versus arriva rate with variabe externa arriva rates at the origin server. %, L =100, 20% cache rate, K=100 [11 [12 VI. CONCLUSIONS In this paper, we use anaytica modeing to find the average response time of a network with transparent cache using state transition diagrams and open network queuing modeing. To examine this mode, we carried numerica tests by varying different parameters ike the cient arriva rate, probabiity of cache and externa arrivas to the origin server. The modeed network in this work incudes effects of transmission deay between the oca network and the origin servers, the processing deay time of [ D. M. Karger, A. Sherman, A. Berkheimer, B. Bogstad, R. Dhanidina, K. Iwamoto, B. Kim, L. Matkins, and Yoav Yerushami, caching with consistent hashing, Int. Journa of Computer and Teecommunications Networking Vo. 31, Issue 11-16, pp , May C. Amza, G. Soundararajan, E. Cecchet and R. Aes, Transparent caching with strong consistency in dynamic content web sites, Proc. 19th annua int. conf. on Supercomputing, pp , G. Barish and K. Obraczke, Word Wide caching: trends and techniques, IEEE Comm. Magazine, Vo. 38, Issue 5, pp , May I. Bose and H. K. Cheng, Performance modes of a firms proxy cache server, Decision Support Systems and Eectronic Commerce, Vo. 29, Issue 1, pp , Juy T. Berczes and J. Sztrik, Performance Modeing of Proxy Cache s, Journa of Universa Computer Science, vo. 12, no. 9, pp , T. Berczes, Approximation approach to performance evauation of Proxy Cache systems, Annaes Mathematicae et Informaticae, Vo. 36, pp , T. Berczes, G. Guta, G. Kusper, W. Schreiner and J. Sztrik, Anayzing a Proxy Cache Performance Mode with the Probabiistic Mode Checker PRISM, Proc. 5th Int. Workshop on Automated Specification and Verification of Systems WWV 2009, J. Z. Wang, Z. Du and P.K. Srimani, Network cache mode for wireess proxy caching IEEE Int. Symposium on Modeing, Anaysis, and Simuation of Computer and Teecommunication Systems, pp , T. Berczes and J. Sztrik, A queueing network mode to study Proxy Cache s, Proc. of the 7th Int. Conf. on Appied Informatics Eger, Hungary, Vo. 1. pp , Jan R. Srivastava, Estimation of Proxy Cache Size using G/G/1 Queuing Mode, Int. Journa of Appied Research on Information Technoogy and Computing IJARITAC, Vo. 1, No. 1, pp.46-58, Apri K. Saini, Squid Proxy 3.1, Packt Pubishing Ltd, L. P. Sothouber, A Mode of Performance, Proc. 5th Int. Word Wide Conference, C. G. Cassandras, S. Lafortune, Introduction to Discrete Event Systems, Springer, 2008.