Handbook of Multimedia for Digital Entertainment and Arts- P7

Handbook of Multimedia for Digital Entertainment and Arts- P7: The advances in computer entertainment, multi-player and online games,technology-enabled art, culture and performance have created a new form of entertainmentand art, which attracts and absorbs their participants. The fantastic successof this new field has influenced the development of the new digital entertainmentindustry and related products and services, which has impacted every aspect of ourlives.
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Handbook of Multimedia for Digital Entertainment and Arts- P77 Countermeasures for Time-Cheat Detection in Multiplayer Online Games 169be accomplished based on estimations of network latencies and by exploiting theinformation contained within transmitted messages [17, 18]. The scheme is called Algorithm for Cheating Detection by Cheating (AC/DC)[16, 17]. To briefly outline the idea behind the scheme, AC/DC consists on theexploitation of a counterattack to be performed against a suspected node, in order toverify if such a peer can be recognized as a cheater. More specifically, at a given time only one peer is enabled to perform the coun-terattack. We call such peer the leader peer pl . Of course, mechanisms should beenabled to make sure that eventually a node chosen as a leader is not a cheater.Hence, such role should be passed among peers, as discussed more in detail in thefollowing. Once the leader pl wants to control a suspected node, pl increases the trans-mission latency of events generated at pl for pi . pl starts the counterattack bycontinuously computing a measure of the average latency from pi to pl . Such mea- isure is obtained by taking into account values of •il ek for events coming from pi , ıil ek D WT rec ek C driftil i l i WT i c ek : i (8)In (8), W T rec ek represents the time of reception of ek at pl , WT i c ek is the l i i i i(possibly cheated) wallclock time at which (pi claims that) ek has been generated, iand driftli is the drift between physical clocks of the two peers. Such values •il ekcan be averaged (or manipulated through a low-pass filter to smooth the variablebehaviour of latencies [20, 22, 25, 28]) so as to have a value of the latency from thesuspected node to the leader. The counterattack that pl exploits against pi consists in the delay of the trans-mission of each novel event e l generated by pl towards pi . Such transmission is idelayed for an amount of time œ. Concurrently, new latency values •il ek are col-lected at pl , for a given time interval. These measurements are averaged to obtaina novel estimation of the average latency from pi to pl . The new measure •il iscompared with the old value •il to understand if a statistically significant differenceamong the two values exists. In particular, when •il is significantly larger than •il ,then the hypothesis that the two measured values are equal must be rejected andhence pl suspects pi as a cheater. Conversely, the value of œ is progressively in-creased and the cheating counterattack mentioned above is iteratively repeated untilan upper bound value equal to  for •il + œ is exceeded, where  UB C TlW .s/ C maxfgapil ; pj 2 …l g: (9) If such upper bound  is reached while a significant difference between •il and•il has not been noticed, then pi cannot be considered as a cheater. The use of such a bound on the increment of œ is due to several reasons. Theneed to reach UB is due to guarantee that eventually pl is the peer with the higher(cheated) transmission latency to reach pi i.e. •li C œ > UB •ji , 8 pj 2 …i;l .Moreover, cases may arise where some game events e j , subsequent to e l but170 S. Ferretti 1. pi = peer to control 2. assume δli = δil /*assumption of symmetry*/ 3. λ = init value /*init value > 0*/ 4. while ((δli + λ ≤ Δ) ^ (pi is not suspected)) 5. set additional delaying time = λ 6. observe δil* of received game events 7. if (δil* significantly larger than δil) 8. suspect pi 9. else 10. λ = increase(λ)Fig. 8 AC/DC Pseudocode istill within W ek , can be generated by other peers in …i;l . With this in view,the second term TlW .s/ accounts for those events e j 2 W ek with simula- ition ˚times higher than « l but within a time interval of range s. The third term emax gapj l ; pj 2 …il , instead, accounts for those peers pj with gapj l > 0. Putin other words, it may happen that pl and pj generate event s with same simulationtimes at different real times and pj reaches such simulation times after pl . Basedon this consideration, the progressive increment of œ, bounded as mentioned above, iis meant to guarantee that eventually no event in W .ek / is received by pi later than le . Thus, when the considered peer pi is a cheater, performing a look-ahead cheat,pi will eventually stop to wait for game events generated by pl . The algorithm related to the scheme described above is reported in Figure 8. Suchalgorithm is performed at the leader peer. As mentioned in the pseudo-code, the leader must assume that network latenciesbetween itself and the suspected node are symmetric (i.e., •li D •il ). Needlessto say, to account for a delay jitter that usually occurs in best-effort networks, aproperly tuned number of measurements is needed in order to obtain an accuratevalue of •il . Moreover, the incre ...