Algorithms for Supporting Disconnected Write Operations in Wireless Web Access

1. Algorithms for Supporting Disconnected Write Operations for Wireless Web Access in Mobile Client-Server EnvironmentsBy: Dr. Ing-Ray Chen, Ngoc Anh Phan, Dr. I-Ling YenPresented in: IEEE Transactions on Mobile Computing, 2002 CS 5214 Spring 2002, Paper Presentation By: Sapna George Sharath Ramaraj

2. Agenda • Introduction • Problem Definition • Proposed Solution • System Model • Algorithms • Numerical Analysis of algorithms • Proof by simulation • Applications • Conclusion

3. Introduction • The Web, a ubiquitous medium for collaborative creation and sharing of information. • The explosive growth in mobile computing technologies • Problems in merging the two • Communication link problems • Mobile device problems • Lack of support in standard web model for disconnected operations • Synchronous nature of web protocols

4. Introduction • The Web, a ubiquitous medium for sharing information (passive). • The Web, a medium for collaborative creation and sharing of information (active). • Simple to create, easy to deploy, easy to use, uniform UI – the browser (despite the on going browser wars!)

5. Introduction The explosive growth in mobile computing technologies • Wireless technologies provide on-going connection to network based resources. • The wireless Internet – deliver information to and from hand-held device users regardless of where they are and how they are connected. • The Web browser as the end-user interface to resources.

6. Introduction Applications • Distributed calendar tools – BAYOU • Meeting room schedulers – ROVER • Evolving design document/form editing by multiple authors – WEBDAV • Online newspaper/journalism/sports services

7. Introduction Problems in merging the Web and mobile users • Communication Link problems • Low bandwidth – Slower than wired networks • Unreliable – dropped connections, blockage etc • Costly • Security issues • Assumptions of web protocols – computers are connected via high-bandwidth, low latency, inexpensive, reliable links. • On a LAN, server response time is the primary concern. • In wireless, bandwidth and latency are the dominating factors.

8. Introduction Problems in merging the Web and mobile users (contd..) • Mobile device problems • Small displays • Limited memory • Limited processing power • Low battery • Greater vulnerability to network problems and security • Cannot stay connected continuously

9. Introduction Problems in merging the Web and mobile users (contd..) • Problems with HTTP • HTTP uses text format for data transmission • HTTP is very inefficient – numerous TCP/IP connections, redundant headers • Synchronous nature of web protocols – World Wide Wait • No support for disconnected operations. Operations fail when client is disconnected from the server.

10. Introduction Solution - Support for disconnected operations. • Voluntary or involuntary disconnections to save battery and reduce cost of communication. • Pre-fetch into local cache frequently used web pages (hoarding) • Web access during disconnection – read and write operations on only pre-fetched pages. Maintains a log. • Reintegration, using the log, upon reconnection.

11. Introduction Few proposed schemes • eNetwork Express - IBM • Proxies and local request queues • Concept of coherency interval - how often a cached web page should be checked for changes • Caubweb – Open Group Research Institute • Client side proxy to stage updates while disconnected • PUT requests upon reconnection • Server side PUT scripts to accept changes • ARTour Web Express • Client side staging of updates • Asynchronous and incremental flushing of updates

12. Problem Definition Questions to be answered • How should staged updates be propagated to the server? • What performance metric should be used to compare strategies? • When should updates be propagated so as to maximize the performance metric? Objectives of the paper • Design and analyze algorithms to support web write operations in mobile client-server environments • Identify the best reconnection time for propagating updates so as to optimize system performance

13. System Model • Web server is located in the fixed network and is not moved during a web session. • MH communicates with the server via an intelligent gateway on the fixed network, e.g. a base station. • A pre-fetching policy exists. The pages and their version are fetched from the server. After pre-fetch, the MH will voluntarily disconnect itself.

14. System Model • MH obtains from the server during pre-fetch, the update history of each cached page, i.e. • Update rate of the page by all users of the system – λiw • MH knows it’s update frequency of each cached page, i.e. • Update rate of the page by the MH – λi • Reintegration - Staged updates are propagated to the server upon reconnection to resolve update conflicts.

15. System Model • Uses an enhanced HTTP protocol - one TCP/IP connection for all the updated web pages. • For each modified page, MH submits to the server: • Differences between original page pre-fetched from the server and the latest version that it modifies • Version number of the original version

16. System Model • Web server checks if page was modified during the MH’s disconnection period – compares version numbers of page at server and page submitted by MH. • If version numbers are same, update is accepted and server page is modified. • If not, update is rejected.

17. System Model • If update is rejected: • MH stays connected (on-line) • Requests a write lock on the page Differences relative to MH’s original version (before updating) are sent by server • MH performs application specific merge algorithm to resolve update differences • MH propagates final updates and releases lock • If MH is forced to disconnect after locking the page, the server will break the lock after a timeout period.

18. Parameters

19. Parameters Relationship between parameters T1 = pm so / B T2 = sr / B T = T1 + T2 T – Round-trip communication cost for propagating a web page that has been updated by the MH, but has not been updated by the server at the reconnection time. Normally, T1 >> T2, so T = T1

20. Performance Metric Performance Metric: • Total communication (or reconnection) time between the MH and the web server in the reintegration phase during which all updates are propagated to the web server. Goals of the proposed solution: • Find best disconnection interval so that total communication time for propagating updates is minimized • For real-time applications, this interval should be such that the deadline is not missed

21. Performance Metric (Contd..) Exclusions • Constant Costs (e.g. Connection Setup time, Server processing time). • Intelligent mobile hosts applying heuristics for deciding reconnection time. • Adaptive reconnection times based on external conditions.

22. Update Propagation Algorithms

23. Update Propagation Algorithms ( Non- Forced) Protocol Implemented during reintegration: • MH sends differences between its current version and also the previous version number in a PUT request packet.This step takes time T1 • If web page hasn’t been updated server sends ACK packet to MH. This step takes time T2. The communication cost in this case is T1 + T2

24. Update Propagation Algorithms (Non-Forced) • If web page has been updated, the versions won’t match. Hence the following happens. • Web server sends a rejection packet to MH: Time taken = T2 • On getting reject MH sends lock request o page: Time = T2 • On getting lock request server sends latest version by differencing: Time taken = T1 • MH applies a merge algorithm to resolve update conflict based on latest version received: Time taken = Dm

25. Update Propagation Algorithms (Non-Forced) • MH sends request packet to server containing differences and command to release lock: Time Taken = T1 • Server sends reply packet to complete update process: Time taken = T2 • The total time taken to complete this operation is 3T1 + 3T2 + Dm. = 3T + Dm • Dm stands for average time taken to resolve update conflict by MH

26. Update Propagation Algorithms (Forced) Many Online applications need forced updates when MH is reconnected to server. • Need to propagate updates to the server at a preset interval to avoid missing the deadline • This implies that a special protocol is needed for pages that are not updated • Also implies that reconnection is necessary prior to real time deadline, to account for update propagation time

27. Update Propagation Algorithms (Forced) Protocol Implemented during reintegration: • MH sends inquiry packet with version number for the page: Time taken = T2 • The server responds in the following way • If web page has been updated server sends differences to MH along with a lock on that page. Time taken = T1 • If page is current, server sends reply packet to MH approving validity of the page: Time Taken = T2. A lock on the page is granted in this case also.

28. Update Propagation Algorithms (Forced) • MH modifies the page: Time Taken = Dm • MH Propagates differences to server: Time Taken = T1 • Server sends an ACK to the MH: Time = T2 • If case 1 of step2 was true: Total Cost = 2T1 + 2T2 +Dm = 2T + Dm. • If case 2 of step2 was true: Total Cost = T1 + 3T2 +Dm = T + 2T2 + Dm.

29. Update Propagation Algorithms (Contd..) Suppose Length of Disconnection time is L Probability that server has modified the page during L = ri ri = Prob { Server update time to page i <= L} Fsi(L) Fsi(t) = CDF of time t that server updates page i. Updates arrive as a poisson process with λiw andλi for updates by the world and MH respectively. ri = 1 – e-(λiw - λi )L

30. Update Propagation Algorithms (Contd..) Since we need update propagation time we need the probability that MH has updates to send pi = Prob {MH update time to page i <= L} FMHi(L) FMHi(t) = CDF of time t that MH updates page i. Updates arrive as a poisson process withλi for updates by the MH. pi = 1 – e-λiL Update propagation algorithms are discussed based on these rates and probabilities

31. Single Page Update Propagation Algorithm • Concept of Coherency Interval is applied to determine the best disconnection period for the MH • By this method, access to a cached page is allowed as long as access time is within this interval. Else the MH has to reconnect to server to get a new copy if it is different. • The length of this interval was a heuristic in disconnected read algorithms. It is equal to L in this disconnected write algorithm

32. Single Page Update Propagation Algorithm The Algorithm • If time of modifying the page by the MH is < L, then modifications are allowed to proceed by recording differences between versions. • Subsequent updates are processed locally until time reaches L • When time reaches L, reconnection happens • Updates are propagated by non-forced update protocol • Communication Cost is 3T + Dm with probability ri or T with probability 1- ri

33. Single Page Update Propagation Algorithm (Contd…) The Algorithm • If time of modifying the page by the MH is > L, then modifications are allowed to proceed as follows. • When time reaches L, reconnection happens • Updates are propagated by Forced update protocol • Communication Cost is 2T + Dm with probability ri or T + 2T2 + Dm with probability 1- ri

34. Single Page Update Propagation Algorithm (Contd…) The Algorithm • Average Communication time for propagating update for a single page on reconnection is Ci = pi x [ri x (3T + Dm) + (1-ri)T ] + (1-pi) x [ri x (2T + Dm) + (1-ri) x (T + 2T + Dm)] • To get the minimum of L, we equate the slope of Ci with respect to L to zero. • ri and pi depend on the inter-arrival time distribution

35. Single Page Update Propagation Algorithm (Contd…) The Algorithm • (2T + Dm)(λiw -λi ) e-(λiw - λi )L • + Tλie-λiL + (T + 2T2 + Dm) λiw e-λiwL = 0 • This means at λiw =λibest L would be at infinity • Thus Ci = T + 2T2 + Dm – pi(2T2 +Dm) = T is at its minimum with no forced update cost • Hence if the MH dominates the page update then λiw ~ λi and reconnection should happen at time Lsubopt such that cost is close to minimum • In case of real time deadline tR the deadline should be upheld

36. Numerical Example Effect of the parameters λw ,λ, Dm, pm, s0 is verified numerically • Effect of λ/λw : Ratio between MH update rate and world update rate • Effect of time to execute a merge algorithm Dm • Effect of size differences between the versions pmso

37. Effect of λ/λw • T1 is fixed at 5 seconds, T2 = 0 and Dm = 100 secs • λis allowed to vary from 0.2λw to λw • For Single page λw is set at 10 updates per hour. • As MH’s update rate gets bigger the curve gets deeper • At λ = λw t there is no minimum point. • As MH’s update rate gets bigger the curve shifts right meaning reconnection time L is shorter when MH’s update rate is far below world update rate

38. Effect of Dm • λ/λw is set to .8, T1 =5 , t2 = 0.1 and Dm varies from 30 to 180 secs • As Dm increases curve gets deeper. If merge operation takes more tme thanit is good to propagate update at the deadline

39. Effect of pms0 • λ/λw is set to .8 and Dm is set to 100. Pm varies in the range of 10% to 50% • As the size difference increases curve gets deeper. This means that if MH modifies a larger portion of the web page, it is good to propagate the update at optimal disconnection time.

40. Simulation • Using smpl. • Generates random update arrivals times at MH and server following exponential distribution – Tic, Tis, L in the range 0 to 3600sec • Accordingly there are 6 possible states for the web page. Since cost is order independent. The number of states reduces to 4 Those states are State1, State3, State4 and state5 in the diagram below. The costs for each of these states is also shown

41. Simulation Batch Means Analysis has been used to obtain average communication cost C for a given L with 95 % accuracy. Sample mean is given by C = Sum(Ci)/k Sample Variance is given by S2 = (Sum(C2i) – k * C2 )/(k-1) Confidence Level is determined as Prob[ C – H <= mu <= C + H] = 1-alpha H = talpha/2;k-1*S/sqrt(k) Accuracy level is determined as H/C <= beta K =10 in the simulation with 2000 simulations in one batch run

42. Simulation

43. Simulation

44. Multiple Page Update Propagation Algorithm 1 System Operation • MH pre-fetches a set of cached web pages • All updates before L are staged and later propagated to the server at reintegration phase one page at a time. • All web pages not updated during L are forcibly updated

45. Multiple Page Update Propagation Algorithm 1 The Algorithm • All the modified pages are propagated to the server one at a time N – Number of pre-fetched pages Ci – Reconnection time needed for propagating updates to page i, as given in equation 7. N Total reconnection time, CN = Σ Ci i = 1

46. Numerical Analysis Effect of λ / λw Ratio N = 5, T1 = 5, T2 = 0, Dm = 100 s λw for the N pages = in the range of [0, 15] updates/hr λ – varied in the range of 0.2 λw to λw • As λ varies from 0.2 times to 1 times λw, the curves get deeper. So,use Lopt • At λ = λw, the Lopt is at infinity. Use Lsubopt • As λ approachesλw, Lopt shifts to the right, I.e., the reconnection interval is shorter when the MH update rate is far below the world update rate.

47. Numerical Analysis Effect of the Time to perform merge operations λ / λw Ratio = 0.8, T1 = 5, T2 = 0 Vary Dm from 3 to 180 sec • As Dm increases from 30 sec to 180 sec, the curves get deeper. • MH can reduce communication cost by propagating updates at the optimal point L.

48. Numerical Analysis Effect of the size of differences between two versions λ / λw Ratio = 0.8, Dm = 100sec Vary pm in the range [10%, 50%] Vary so in the range [50 KB, 250 KB] Then, size of version differences pmso varies in the range [5 KB, 25KB] • As size increases, curves get deeper

49. Simulation Random values generated : Tic – exponential distr at λi per page. Tis – exponential distr at λiwper page. L – 0 to 3600 sec. States – 4N

50. Multiple Page Update Propagation Algorithm 2 System Operation • MH pre-fetches a set of web pages • All updates before L are staged and later propagated to the server at reintegration phase in a batch to shorten reconnection time. • All web pages not updated during L are forcibly updated