Algorithms for Supporting Disconnected Write Operations for Wireless Web Access in Mobile Client-Server Environments By: Dr. Ing-Ray Chen, Ngoc Anh Phan,

Algorithms for Supporting Disconnected Write Algorithms for Supporting Disconnected Write Operations for Wireless Web Access in Mobile Operations for Wireless Web Access in Mobile

Client-Server EnvironmentsClient-Server EnvironmentsBy: Dr. Ing-Ray Chen, Ngoc Anh Phan, Dr. I-Ling YenBy: Dr. Ing-Ray Chen, Ngoc Anh Phan, Dr. I-Ling Yen

Presented in: IEEE Transactions on Mobile Computing, 2002Presented in: IEEE Transactions on Mobile Computing, 2002

CS 5214 Spring 2002, Paper Presentation

By:

Sapna George

Sharath Ramaraj

AgendaAgenda

Introduction Problem Definition Proposed Solution

– System Model– Algorithms– Numerical Analysis of algorithms– Proof by simulation

Applications Conclusion

IntroductionIntroduction

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


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!)


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.


Applications

Distributed calendar tools – BAYOU Meeting room schedulers – ROVER Evolving design document/form editing by

multiple authors – WEBDAV Online newspaper/journalism/sports services


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.


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


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.


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.


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

Problem DefinitionProblem 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

System ModelSystem Model

1. Web server is located in the fixed network and is not moved during a web session.

2. MH communicates with the server via an intelligent gateway on the fixed network, e.g. a base station.

3. A pre-fetching policy exists. The pages and their version are fetched from the server. After pre-fetch, the MH will voluntarily disconnect itself.


4. 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 – λi

w

5. MH knows it’s update frequency of each cached page, i.e.

– Update rate of the page by the MH – λi

6. Reintegration - Staged updates are propagated to the server upon reconnection to resolve update conflicts.


7. Uses an enhanced HTTP protocol - one TCP/IP connection for all the updated web pages.

8. For each modified page, MH submits to the server:

1. Differences between original page pre-fetched from the server and the latest version that it modifies

2. Version number of the original version


9. 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.

10. If version numbers are same, update is accepted and server page is modified.

11. If not, update is rejected.


11. If update is rejected: 1. MH stays connected (on-line)2. Requests a write lock on the page Differences relative

to MH’s original version (before updating) are sent by server

3. MH performs application specific merge algorithm to resolve update differences

4. MH propagates final updates and releases lock5. If MH is forced to disconnect after locking the page,

the server will break the lock after a timeout period.

ParametersParameters

λiw Update rate of a page by all users of the system (world)

λi Update rate of a page by the MH

T1 One-way communication cost over the wireless link for a packet carrying the differences

T2 One-way communication cost over the wireless link for an acknowledgement/reply packet

sr Average size of an ack/reply packet

so Average size of a web page

pm Average fraction of any web page modified

B Bandwidth of the wireless channel

ParametersParameters

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

Performance MetricPerformance 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

Performance Metric (Contd..)Performance Metric (Contd..)

Exclusions1) Constant Costs (e.g. Connection Setup time,

Server processing time).

2) Intelligent mobile hosts applying heuristics for deciding reconnection time.

3) Adaptive reconnection times based on external conditions.

Update Propagation Update Propagation AlgorithmsAlgorithms

Update Propagation Update Propagation Algorithms ( Non- Forced)Algorithms ( Non- Forced)Protocol Implemented during

reintegration:1. MH sends differences between its

current version and also the previous version number in a PUT request packet.This step takes time T1

2. 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

Update Propagation Update Propagation Algorithms (Non-Forced)Algorithms (Non-Forced)

3. If web page has been updated, the versions won’t match. Hence the following happens.

1.Web server sends a rejection packet to MH: Time taken = T2

2.On getting reject MH sends lock request o page: Time = T2

3.On getting lock request server sends latest version by differencing: Time taken = T1

4.MH applies a merge algorithm to resolve update conflict based on latest version received: Time taken = Dm

Update Propagation Update Propagation Algorithms (Non-Forced)Algorithms (Non-Forced)

5. MH sends request packet to server containing differences and command to release lock: Time Taken = T1

6. Server sends reply packet to complete update process: Time taken = T2

7. The total time taken to complete this operation is 3T1 + 3T2 + Dm. = 3T + Dm

8. Dm stands for average time taken to resolve update conflict by MH

Update Propagation Update Propagation Algorithms (Forced)Algorithms (Forced)Many Online applications need forced updates when

MH is reconnected to server. 1. Need to propagate updates to the server at a preset

interval to avoid missing the deadline

2. This implies that a special protocol is needed for pages that are not updated

3. Also implies that reconnection is necessary prior to real time deadline, to account for update propagation time

Update Propagation Update Propagation Algorithms (Forced)Algorithms (Forced)Protocol Implemented during

reintegration:1. MH sends inquiry packet with

version number for the page: Time taken = T2

2. The server responds in the following way

1. If web page has been updated server sends differences to MH along with a lock on that page. Time taken = T1

2. 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.

Update Propagation Update Propagation Algorithms (Forced)Algorithms (Forced)

3. MH modifies the page: Time Taken = Dm

4. MH Propagates differences to server: Time Taken = T1

5. Server sends an ACK to the MH: Time = T2

6. If case 1 of step2 was true: Total Cost = 2T1 + 2T2 +Dm = 2T + Dm.

7. If case 2 of step2 was true: Total Cost = T1 + 3T2 +Dm = T + 2T2 + Dm.

Update Propagation Update Propagation Algorithms (Contd..)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 and λi for updates

by the world and MH respectively.

ri = 1 – e-(λiw - λi )L

Update Propagation Update Propagation Algorithms (Contd..)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 with λi for updates by the MH.

pi = 1 – e-λiL

Update propagation algorithms are discussed based on these rates and probabilities

Single Page Update Single Page Update Propagation AlgorithmPropagation Algorithm1. Concept of Coherency Interval is applied to determine

the best disconnection period for the MH

2. 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.

3. The length of this interval was a heuristic in disconnected read algorithms. It is equal to L in this disconnected write algorithm

Single Page Update Single Page Update Propagation AlgorithmPropagation AlgorithmThe Algorithm

1. If time of modifying the page by the MH is < L, then modifications are allowed to proceed by recording differences between versions.

1. Subsequent updates are processed locally until time reaches L

2. When time reaches L, reconnection happens

3. Updates are propagated by non-forced update protocol

4. Communication Cost is 3T + Dm with probability ri or T with probability 1- ri

Single Page Update Single Page Update Propagation Algorithm (Propagation Algorithm (Contd…)Contd…)

The Algorithm

2. If time of modifying the page by the MH is > L, then modifications are allowed to proceed as follows.

1. When time reaches L, reconnection happens

2. Updates are propagated by Forced update protocol

3. Communication Cost is 2T + Dm with probability ri or T + 2T2 + Dm with probability 1- ri


The Algorithm

2. 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)]

3. To get the minimum of L, we equate the slope of Ci with respect to L to zero.

4. ri and pi depend on the inter-arrival time distribution


The Algorithm1. (2T + Dm)(λi

w - λi ) e-(λiw - λi )L

2. + T λie-λiL + (T + 2T2 + Dm) λiw e-λiwL = 0

3. This means at λiw = λi best L would be at infinity

4. Thus Ci = T + 2T2 + Dm – pi(2T2 +Dm) = T is at its minimum with no forced update cost

5. 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

6. In case of real time deadline tR the deadline should be upheld

Numerical ExampleNumerical Example

Effect of the parameters λw , λ , Dm, pm, s0 is verified numerically

1) Effect of λ / λw : Ratio between MH update rate and world update rate

2) Effect of time to execute a merge algorithm Dm

3) Effect of size differences between the versions pmso

Effect of λEffect of λ // λλww

1. T1 is fixed at 5 seconds, T2 = 0 and Dm = 100 secs

2. λ is allowed to vary from 0.2 λw to λw

3. For Single page λw is set at 10 updates per hour.

4. As MH’s update rate gets bigger the curve gets deeper

5. At λ = λw t there is no minimum point.

6. 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

Effect of DmEffect of Dm1) λ / λw is set to .8, T1 =5 , t2 = 0.1 and Dm varies from 30 to 180 secs

2) As Dm increases curve gets deeper. If merge operation takes more tme thanit is good to propagate update at the deadline

Effect of pEffect of pmmss001) λ / λw is set to .8 and Dm is set to 100. Pm varies in the range of 10% to

50%

2) 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.

SimulationSimulationUsing smpl. Generates random update arrivals times at MH and server following

exponential distribution – Tic, Ti

s, 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

States Relations b/w times

1 Tis > L > Ti

c

2 L > Tic, Ti

s

3 Tic > L > Ti

s

4 Tic, Ti

s > L

SimulationSimulationBatch 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

SimulationSimulation


Multiple Page Update Multiple Page Update Propagation Algorithm 1Propagation 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


The Algorithm

All the modified pages are propagated to the server one at a timeN – 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

Numerical AnalysisNumerical Analysis

Effect of λ / λw RatioN = 5, T1 = 5, T2 = 0, Dm = 100 s

λw for the N pages = in the range of [0, 15] updates/hrλ – 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 approaches λw, Lopt shifts to the right, I.e., the reconnection interval is shorter when the MH update rate is far below the world update rate.


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.


Effect of the size of differences between two versionsλ / 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


Random values generated :

Tic – exponential distr at λi per page.

Tis – exponential distr at λi

wper page.

L – 0 to 3600 sec.

States – 4N


1 Tis > L > Ti

c

2 L > Tic, Ti

s

3 Tic > L > Ti

s

4 Tic, Ti

s > L


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


1. MH to server – 1 msg– Bit vector A – updated/not

updated pages. – Vector B - original version

numbers.

– Time = T2

2. Server to MH – 1 msg– Bit vector C – accepted

/rejected/not updated– Vector D – current version

numbers.

– Time = T2


3. Server to MH– 1 msg– Differences of rejected and

not updated pages. – Server locks these pages.

N

– Time = Σ riT1

i = 1

4. MH to Server – 1 msg– Differences of updated and

accepted pages

N

– Time = Σ pi(1-ri)T1

i = 1


5. Server to MH– 1 msg– Acknowledgement for updates of

accepted pages.

– Time = T2

6. MH to Server– Forced updates of rejected and not

updated pages

– Differences of these pages

N

– Time = Σ ri(Dm +T1) +

N i = 1

Σ (1-pi)(1-ri)(Dm + T1) i = 1


5. Server to MH– 1 msg– Acknowledgement for updates

of rejected and not updated pages.

– Time = T2

Total reconnection time

CN = 4T2 + Σ [ riT1 +

pi(1-ri) T1+ ri(Dm+T1)+

(1-pi)(1-ri)(Dm + T1)]


Effect of λ / λw RatioN = 5, T1 = 5, T2 = 0, Dm = 100 s

λw for the N pages = in the range of [0, 15] updates/hrλ – 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 approaches λw, Lopt shifts to the right, I.e., the reconnection interval is shorter when the MH update rate is far below the world update rate.


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.


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 Multiple page update Algorithm 2

performs better than Algorithm 1.


Random values generated :

Tic – exponential distr at λi per page.

Tis – exponential distr at λi

wper page.

L – 0 to 3600 sec.

States – 4N


1 Tis > L > Ti

c

2 L > Tic, Ti

s

3 Tic > L > Ti

s

4 Tic, Ti

s > L



ResultsResults

Analytical results were validated by simulation Verified Inter-arrival time probability distributions -

exponential, hyper exponential, erlang, normal Analytical results and simulation produced virtually the

same curves with differences less than 0.1%. Different distributions slightly affect the exact position of

Lopt, but the shape of the curves are same. Lopt, lies in the range of [400, 600] in all cases

Conclusion – Analysis results reported are valid and are not sensitive to underlying probability distribution

ApplicationsApplications

Distributed calendar tools – BAYOU Meeting room schedulers – ROVER Evolving design document/form editing by

multiple authors – WEBDAV Online newspaper/journalism/sports services

Application ModelApplication Model

Web pages are updates by multiple users – stationary and/or mobile

There is a real time application dependent deadline by which pages must be updated – tR

Let (Lopt, Cmin) denote the optimum L point in the graphs at which cost is minimum

For real-time applications with deadline tR, – If Lopt + Cmin < tR choose Lopt

– Else choose largest L < Lopt such that L + C = tR

For non-real-time applications,– Always choose Lopt.

Application ModelApplication Model

For e.g, let tR = 25minλ = 0.5λw - two users updating same

set of pages at the same rate

Then, Lopt = 850s

Since Lopt + Cmin = 850 + 350 = 1200sec < tR

Let tR = 15min, Then, L = 540s,

Since L + C = 540 + 360 = 900 = tR

ConclusionsConclusions

Paper discussed algorithms for propagating updates made by the MH and analyzed their effect on performance in terms of communication time

Developed analytical arguments relating disconnection period and communication time based on system parameters

End result is a simple L-C graph that the MH can apply at run-time to determine how long it should stay disconnected, thus adapting to network changes

ConclusionsConclusions

Results– When MH update rate is a fraction of that of the world, there

exists an optimal L where communication cost is minimum– Optimal disconnection period increases as the MH update rate

approaches the world update rate– When only the MH update a page - cost is minimum at infinity.

Then MH can reconnect at a sub-optimal time where cost is close to minimum

– When world rate is much higher than MH rate - cost is minimum at time 0. World updates the page too often and resolving conflicts nullifies the benefit of disconnection

SMPL CodeSMPL Code

main() {smpl(0,"Single Page Update Propagation");ratios[0] = 1.0; ratios[1] = 0.9;ratios[2] = 0.8; ratios[3] = 0.6;ratios[4] = 0.4; ratios[5] = 0.2;for (i = 0; i <= 5; i++) {printf("\n\n lami = %f * lamw \n\n", ratios[i] );

lami = ratios[i]* lamw; for (L = 0; L <= 3600; L += 300) {

init_bm(200,2000); proceed = TRUE; Y = 0;H = 0; while (proceed) {

Tc = expntl(lami);if (lami == lamw) Ts = L + T1 + T2;

else Ts = expntl(lamw);

if (Ts > L && L > Tc) C = T1 + T2; else if (L > Tc && L > Ts)

C = 3*T1 + 3*T2 + Dm; else if (Tc > L && L > Ts)

C = 2*T1 + 2*T2 + Dm; else if (Tc > L && Ts > L)

C = T1 + 3*T2 + Dm;if (obs(C) == 1) proceed = FALSE;}civals(&Y, &H, &num_batches);printf("L = %d sec C = %f\n", L, Y ); }

} }

SMPL CodeSMPL Code

Main(){smpl(0,"Multiple Page Update Propagation

Algorithm 1");ratios[0] = 1.0; ratios[1] = 0.9;ratios[2] = 0.8; ratios[3] = 0.6;ratios[4] = 0.4; ratios[5] = 0.2;for (i = 0; i <= 5; i++) {for (j = 0; j < 5; j++) { r = uniform(0.0, 15.0);lamw[j] = 3600/r;lami[j] = ratios[i] * lamw[j];}

for (L = 0; L <= 3600; L += 300) {Cn = 0.0;

for (j = 0; j < 5; j++) { init_bm(200,2000); proceed = TRUE; Y = 0; H = 0;

while (proceed) {Tc = expntl(lami[j]);if (lami[j] == lamw[j]) Ts = L + T1 + T2;else Ts = expntl(lamw[j]); if (Ts > L && L > Tc) C = T1 + T2;else if (L > Tc && L > Ts) C = 3*T1 +

3*T2 + Dm;else if (Tc > L && L > Ts) C = 2*T1 + 2*T2 + Dm;else if (Tc > L && Ts > L) C =

T1 + 3*T2 + Dm; if (obs(C) == 1) proceed = FALSE;} Cn += C; }

Documents

Algorithms for Supporting Disconnected Write Operations for Wireless Web Access in Mobile Client-Server Environments By: Dr. Ing-Ray Chen, Ngoc Anh Phan,