Handbook of Multimedia for Digital Entertainment and Arts- P7 ppt

7 Countermeasures for Time-Cheat Detection in Multiplayer Online Games 169be accomplished based on estimations of network latencies and by exploiting theinformation contained within tran

7 Countermeasures for Time-Cheat Detection in Multiplayer Online Games 169

be 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 mission latency of events generated at pl for pi pl starts the counterattack bycontinuously computing a measure of the average latency from pito pl Such mea-sure is obtained by taking into account values of •i l

trans-ekifor events coming from pi,

and driftliis the drift between physical clocks of the two peers Such values •i l



ekican 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 mission of each novel event el generated by pl towards pi Such transmission isdelayed for an amount of time œ Concurrently, new latency values •il

trans-

ekiare col-lected at pl, for a given time interval These measurements are averaged to obtain

a novel estimation of the average latency from pi to pl The new measure •il iscompared with the old value •i l 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 until

an upper bound value equal to  for •il+ œ is exceeded, where

  UB C TlW .s/ C maxfgap il; pj 2 …lg: (9)

If such upper bound  is reached while a significant difference between •i l and

•i l has not been noticed, then picannot be considered as a cheater

The use of such a bound on the increment of œ is due to several reasons The

need to reach UB is due to guarantee that eventually plis the peer with the higher(cheated) transmission latency to reach pi i.e •liC œ > UB  •ji, 8 pj 2 …i;l.Moreover, cases may arise where some game events ej, subsequent to el but

1 p i = peer to control

2 assume δδli = δil /*assumption of symmetry*/

3 λ = init value /*init value > 0*/

4 while ((δli + λ ≤ Δ) ^ (p i is not suspected))

6 observe δil* of received game events

7 if (δil* significantly larger than δil)

eikwith simula-tion times higher than el but within a time interval of range s The third term

max˚

gapj l; pj 2 …i l

, instead, accounts for those peers pj with gapj l > 0 Put

in other words, it may happen that pland pj generate event s with same simulationtimes at different real times and pj reaches such simulation times after pl Based

on this consideration, the progressive increment of œ, bounded as mentioned above,

is meant to guarantee that eventually no event in W eik/ is received by pi later than

el Thus, when the considered peer pi is a cheater, performing a look-ahead cheat,

piwill eventually stop to wait for game events generated by pl

The algorithm related to the scheme described above is reported in Figure8 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 •i l) Needless

to 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 •i l Moreover, the increment of œ, mentioned with a call of a hypothetical

function increase() should be implemented using, for instance, a constant increment

of œ or some kind of linear growth Of course, the use of a progressive increment ofthe value of œ allows a smoother and less intrusive counterattack, since the leadercould be able to identify a cheater even before reaching the upper bound for œ [16]

A point worth of mention is that different methods can be devised to suspect

a peer for cheating Probably, a good approach could be to exploit come heuristicsbased on the combination of diverse factors such that, for instance, i) the degradation

of latencies between pl and pi, ii) the observation that other peers in the samegeographical area of pihave latencies smaller than pi, iii) a player that always winsand hence, he is very skilled or he is cheating

An important assumption is the independence of the generation of game events

by a honest peer with those at other peers In other words, even if certain gameevents, generated by some peer, will certainly influence the semantics of subse-quent game events generated by others (by definition of interactivity in MOGs),

in general, the pace of event generation at a given player is mainly influenced by

7 Countermeasures for Time-Cheat Detection in Multiplayer Online Games 171autonomous decisions From a communication point of view, this means that a hon-est player generates its own events mostly regardless of the amount, rate and latency

of the messages he/she receives This assumption is supported by the typical use oftechniques such as dead reckoning and/or optimistic synchronization, exploited tohide latencies on notifications and local losses of availability on updated informa-tion, which provide players with the possibility of independently advance the game[8,19]

As mentioned, the role of leader should be carefully assigned and managedamong peers First of all, only one leader must be present at a time in the P2Psystem In fact, suppose two peers concurrently elect themselves as leaders, andsuppose they decide to control each other In this case, both peers will delay trans-mission of game events towards the other one Thus, both peers may erroneouslysuspect each other Moreover, each peer should become leader only for a limitedamount of time, then another node should be elected To do this, a token-basedscheme should be utilized to determine which is the leader at a given time, i.e everytime a process receives the token, it becomes the leader A time deadline is set sothat each peer is forced to eventually release the token Thus, after such time dead-line the leader could randomly select another peer to be the next leader, pass thetoken to it and reveal itself to all others This way, other peers know that a leader asdone its job and that it passed the token to another host However, also in this case,additional mechanisms should be employed in order to guarantee that eventually allpeers become leaders, in order to diminish the probability that cheaters collude and,once gained the token, they pass that token only among themselves, thus preventinghonest peers to detect cheaters

Upon identification of a cheater pi, the leader could pass the token to another,randomly selected peer (not pi, of course), informing it of its p0is suspicion Then,the novel peer will in turn control pi Once a majority of peers suspects pi, thensuch peer can be considered as a cheater This solution enables agreement amonghonest peers (on cheaters detection) Moreover, such a cooperative approach makesharder for the cheater to detect if some other node is monitoring his behavior Thus,

it results more difficult for the cheater to dynamically switch off its cheat as soon as

he detects he is being examined

Just to provide the reader with an idea of the efficacy of a detection scheme tocope with time cheats in a P2P scenario, we report in Figure9 the percentage ofcheaters identified based on the use of AC/DC In particular, results were based on asimulation performed to mimic a P2P fully connected gaming architecture built over

a best effort, reliable network Based on the literature [3,13,16,19], transmissionlatencies were based on a lognormal distribution, whose average network latencywas set to 100 msec We varied the value of the delay jitter between 10 and 100 msec,since this parameter may affect the efficacy of our detection schemes

Starting with an initial estimation of the average latency to reach the controlledpeer, the leader was in charge to assess whether, during the counterattack, the newlymeasured average delay (from the cheater to the leader) grew significantly It isclear that this initial estimation plays an important role in AC/DC Indeed, during thegame evolution, measured latencies from the cheater to the leader are affected by the

Look-Ahead Detection with AC/DC

Fig 9 Look-Ahead Detection Through AC/DC

look-ahead cheat Thus, if the cheater alters its initialization protocol (where the firstestimation of the average latency is measured), the leader may start the counterattackwith a delay measure which is higher than the real one [16] To evaluate the impact

of this possible fallacy, we simulated different scenarios with very diverse initialestimations of the average latency from the leader to the peer Here, we report onthe adverse case where the leader starts the counterattack with a false estimation

of the average latency, equal to 2•li, i.e the round trip time from the leader to thecheater Needless to say, with lower values of the initial estimation, it was easierfor our scheme to detect cheaters The value of œ was initialized to 10 msec and letgrow till reaching (if needed) the upper bound  Each curve refers to a differentchoice of 

For each scenario, 30 different simulations have been run During each tion, measurements for different message transmissions were employed to make

simula-a stsimula-atisticsimula-al test Among the possible choices on the stsimula-atisticsimula-al tests to be ployed, due to our interest for measuring average delays and due to the need forviable choices, easily executable on different peers, we decided to employ a classicone-sided t-test ’ D 0:05/ For each test, the number of measurements was set to

em-40 game events

As shown in the Figure, while AC/DC detected all cheaters in most of the sidered scenarios (all the scenarios when  was set greater than 300 msec), somepercentage of false negatives (i.e., cheaters not detected) was obtained when highjitters and relatively low upper bounds were set However, these results suggest that

con-it suffices to augment the upper bound  to detect those cheaters

It is important to notice that no false positives were obtained through AC/DC,even with very high values of the delay jitter (e.g., std.dev set equal to 100 msec)

In other words, no honest peers were erroneously identified as cheaters

7 Countermeasures for Time-Cheat Detection in Multiplayer Online Games 173

Conclusions and Future Directions

Several malicious mechanisms can be devised to take unfair advantages in tiplayer online games, by exploiting the inadequacies of best-effort networks Wepresented here a discussion on time cheats, i.e those cheats that consist in assigningfaked timestamps to game events Prevention and detection schemes for cheatingavoidance have been also outlined, together with some countermeasures to copewith the specific look-ahead time cheat

mul-The main open question is when it should be wiser to employ a detection, ratherthan a prevention scheme Certainly, detection schemes should be taken into consid-eration when the considered multiplayer game is a fast-paced one, since cases existwhen game communication protocols, embodying a cheating prevention scheme,fail to provide that level of responsiveness, which is required to ensure compellinggaming experiences to distributed players

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Mul-Chapter 8

Zoning Issues and Area of Interest Management

in Massively Multiplayer Online Games

Dewan Tanvir Ahmed and Shervin Shirmohammadi

Introduction

Game is a way of entertainment, a means for excitement, fun and socialization.Online games have achieved popularity due to increasing broadband adoptionamong consumers Relatively cheap bandwidth Internet connections allow largenumber of players to play together Since the introduction of Network Virtual Envi-ronment (NVE) in 1980s for military simulation, many interesting applications havebeen evolved over the past few decades Massively multiplayer online (role-playing)game, MMOG or MMORPG, is a new genre of online games that has emerged withthe introduction of Ultima since 1997 It is a kind of online computer game with theparticipation of hundreds of thousands of players in a virtual world

Nowadays multiplayer online games are very popular An MMOG could have

millions of subscribers such as World of Warcraft, or Quest Interesting is that all

subscribers do not play with each other in the same space at the same time As aconsequence, the virtual world is divided into realms or kingdoms which are theclones of the original virtual world, each hosting several thousand registered play-ers Technically, the realms are geographically distributed across the world Thus,players from a particular region play together in the same realm To accommodatemillions of subscribers, gaming companies provide many realms across the world asneeded Realms are further divided into separate areas, also known as a zone Zonescan have different themes and different levels of difficulties holding inexperiencedplayers advancing into the next hard level

MMOGs are similar to generic massively multiuser simulations that have existed

for decades, most notably combat training simulations used by Departments of fense (DoD) around the world, and more recently, disaster management applications,

De-emergency planning simulators, etc These have reached their current state because

of their significant impact on virtual training in high-risk situations as well as their

D.T Ahmed and S Shirmohammadi ( )

School of Information Technology and Engineering University of Ottawa, Ottawa,

Ontario, Canada

e-mail: dahmed@discover.uottawa.ca

B Furht (ed.), Handbook of Multimedia for Digital Entertainment and Arts,

DOI 10.1007/978-0-387-89024-1 8, c  Springer Science+Business Media, LLC 2009

175

ability to interpret the real and the simulated results in extraordinary circumstancessuch as natural disasters or terrorist attacks.

Commercial MMOGs use the client-server architecture with a single authenticserver designed for to support the game logic The server pool regulates game trafficusing the zoning concept and makes its implementation more convenient Practi-cally, the communication structure within a zone is similar to the Internet multicaststructure, not client-server, because of the players’ common interest in the gamelogic IP multicast, which was originally proposed for group communication, can

be an ideal solution for this purpose But it is a well-known fact that IP Multicast isnot deployable on the wide-scale Internet, even in future with IPv6 [1] Thus, currentpractices heavily rely on centralized architectures that cause scalability bottlenecks

In addition, the models are costly to adopt and install Current designs (research ented), however, try to incorporate client and server side resources in a peer-to-peerfashion to address its different challenges such as scalability, responsiveness, andpersistence [2][3]

ori-Challenges and Requirements

The development of an MMOG faces many challenges A fundamental requirement

of any real-time application tool is the exchange of regular update messages amongthe participants It is a challenging task while keeping a low data rate without affect-ing the gaming experience Scalability is another important concern when designedfor large-scale simulators or virtual environments and MMOGs, as it is the function

of other gaming components related to the system However, the amount of datathat needs to be exchanged among participants is bounded by server-side resourcesand other technical conditions such as network bandwidth, processing power andnetwork latencies

Network Bandwidth- The bandwidth is the amount of data that can be mitted over a network in a fixed time-slot It is set by the underlying hardware thatconnects to the computers The user’s connection to its Internet Service Provider(ISP) and the ISP’s hardware and software infrastructures also affect available band-width Practically MMOG players have non-uniform bandwidth Thus, the amount

trans-of data that can be exchanged between two computers is restricted by the bandwidth

of the players

Processing Power The processing power expresses the computation capability the amount of computations/instructions executed by a computer in a fixed time-slot.For gaming, higher processing power is required for multiple reasons like physicsengine, collision detection, graphic rendering, artificial intelligence, and to send andreceive network messages for networked games But the processing power of allusers is not homogenous Thus, the amount of information that can be exchangedbetween two computers and the quality of perception also depends on their CPUresources

-8 Zoning Issues and Area of Interest Management 177Network Latency- Latency is the amount of time a message takes to travel over

a network from a source to a destination The physical limitation of the underlyinghardware like routers and switches, and network congestion make it variant fromtime-to-time The interaction details of an MMOG player must be sent to all otheractive participants in time Because of the networking limitations and the trafficconditions, some of these updates can be lost or delayed Thus, latency issue cannot

be ignored The updated game states must be forwarded within a time limit so thatthe responsiveness of game play is maintained Generally, latency acceptance variesfrom game-to-game and is restricted to a value between 100ms to 1000ms for onlinegames [4] The acceptable latency depends on game perspectives (i.e First-person

or Third-person), game genres (i.e racing or role playing game), and the sensitivity

of actions

MMOG Architecture – An Overview

To accommodate a large number of players, the map is logically divided into tiple zones where each zone encompasses the players that are in the same vicinity.Each zone has a master (e.g server) that coordinates the interactions of the zonemembers in a multicast fashion A set of master nodes regulates the operation of theMMOG and provides overlay services in client-server model On the other hand, hy-brid model incorporates the participation of the players The system is hybrid as itcombines the benefits of both centralized and distributed systems To overcome thefunctionality limitations of the IP multicast, application layer multicasting (ALM)can be chosen for intra zonal communication [5]

mul-MMOG Classification

There are many types of massively multiplayer online games Some popular typesare given in the Table1

Communication Architecture

The right communication structure is very important for interest management as

it regulates message transmission and controls network resource usage Differentpacket delivery methods can be used for data communication in multiplayer games.This includes basic unicast communication which is the most popular choice, butbroadcast and multicast communications are sometimes also useful

The proper choice of TCP or UDP protocol for standard unicast tion is important for online games UDP is a simple best-effort procedure thatoffers no reliability and no guaranteed packet ordering On the other hand, it has

communica-Table 1 Types of MMOGs and examples

Types of

MMORPG Massively multiplayer online role

playing games

EverQuest, Star war galaxies MMOFPS Massively multiplayer online first

person shooters

World War II Online World:

Battleground Europe, 10SIX (known

as Project Visitor) MMORTS Massively multiplayer online real-time

games

Hattrick

little overhead, making it appropriate for highly interactive games (e.g., first-personshooter, car racing) where fast packet delivery is more important On the other hand,TCP guarantees ordered packet delivery and simplifies programming with additionaloverhead Most importantly, TCP can work more transparently across firewalls

Thus, many commercial MMOGs (e.g., EVE Online, Lineage II, and World of craft) use TCP for their communication.

War-Local area networks (LANs) can be restrictively configured to allow cast This can make state dissemination much simpler and efficient Unfortunately,MMOGs are not able to take the advantages of broadcast in an Internet setting as it

broad-is not typically supported across the router boundaries Another technique broad-is ticast which provides group-based packet delivery A host can subscribe to one ormultiple multicast addresses and receive all messages sent to those addresses It isusually much more efficient than multiple unicast operations However, multicas-ting is not widely available on the global Internet due to technical, business, andpractical reasons [1] and is therefore not a practical solution for MMOGs

mul-There are two general types of MMOG architectures: client-server and peer There are also hybrid architectures that are between these two main paradigms

peer-to-In client-server architecture, each client has a single connection with the server(Figure1a) The server is responsible to relay the game states between clients Themain advantage of the client-server architecture is the easy controller which is cen-tralized and autonomous As a solution for resource limitations multiple servers aredeployed The architecture also facilitates easy implementation of load balancing,fault tolerance, security, and many other concerns But in client-server architecture,the server is an architectural bottleneck and limits the scalability of the system Still

it is the most popular and practiced approach in the gaming industry

Commercial NVEs use the client-server architecture which is expensive to

de-ploy and cumbersome to maintain For example, the virtual world of Second Life

8 Zoning Issues and Area of Interest Management 179

Fig 1 (a) Client-server architecture (b) Peer-to-peer architecture

has approximately 5000 servers Such expensive deployment issue as well as theneed for regular maintenance stirs us to an alternative solution The P2P architec-ture has a self scalability property, but considering business oriented and practicalissues such as quality, cheating, and distributed game logic, it seems that a pureP2P architecture (Figure1b) is an infeasible and impractical solution Recently, thehybrid P2P architecture is considered as a good solution both for vendors and endusers [6][7] The P2P community strongly believes that online games over hybridpeer-to-peer architecture have promising prospects considering deployment cost andperformance in some sense through reduced latencies

Virtual Space Decomposition - Zoning

The genre of MMOG is relatively new but popular The development of MMOGsencompasses many technical challenges Some of the challenges are

 Ensuring consistency

 Providing fault-tolerance

 Administration of servers and players

 Preventing cheating

 Providing scalability and many others

To achieve these, the concept of zoning is typically used This is what we will

de-scribe next

Zone Definition

For easy state administration, the virtual space is divided into multiple adjacentareas technically called zones But on what perspective a zone is constructed, is

subject to specific implementation From networking perspective, each LAN can

be considered as a zone where several LANs are connected through the Internetforming the whole world A LAN provides high bandwidth, so it could be easy forthe server, i.e the master, responsible for a zone to construct the overlay network ifrequired, maintain its state, and manage new comers and early leavers However, this

is not a sufficient requirement: other factors such as virtual distance and visibility,and logical partitions need to be taken into consideration when defining a zone In anutshell, a zone is a logical partitioning which is usually transparent to the players

in the game space

Multiple Zones and its Space

At its simplest, a zone can be represented by a square or a triangle Multiple zonedefinition can be adopted while defining the map of a game space To accommodatemany players, the map is logically divided into multiple zones Each zone encom-passes the players that are in the same vicinity Henceforth, when a player movesfrom one zone to another, it gets disconnected from one server and joined to anotherserver If this multiple-zone layout is considered, more connections and disconnec-tions can be encountered for the same path traversal scenario (Figure2) Triangularand hexagonal shapes have advantages over other shapes For example, unlike cir-cles, triangular and hexagonal shapes stick together and maintain regular shapes(Figure3)

Fig 2 Two versus multiple zones layout, with a player moving across zones

Fig 3 Hexagonal and circular zone shapes

8 Zoning Issues and Area of Interest Management 181

Area of Interest Management

For online games, Area of Interest Management (AoIM) is a technique to reduce

communication overhead AoIM is a method used to determine useful informationfor a specific player and block all other information For example, the area of interest

of an avatar in an MMOG is the set of avatars and non-playing components (NPC)

with whom it interacts with in its neighborhood Since the virtual world is large,filtering out irrelevant information is a fundament requirement for a scalable system

There are two approaches to model AoIM for MMOGs The first one is static geographical partitioning implemented at the initialization phase of a simulation.

This is practical as it describes the structure of a virtual world For example, avirtual world consists of multiple cities where each city defines a geographical par-tition: it is the area where most of the interactions take place, and in most cases,

the participants are not captivated on what is happening in other cities Second Life

has adopted such approach [8] The virtual world offers uninteresting items aroundthe borders, like cities separated by empty forests or wilderness where players donot want to stay long Although the static geographic partitioning is good for somecases, it might not be a general solution for all virtual simulators

The second approach for AoIM is behavioral modeling In military simulations,

two different units such as a jeep and an aircraft have different distinctiveness interms of how fast they can move, how far they can see, and the scope of the interac-tion space (a jet launching a missile has a larger area of influence than a jeep patrol)

Lu et al argue that, as the mapping of processing resources to the geographic gionalization is straightforward and uncomplicated, the behavioral approach has notbeen deeply explored [9] One of the limitations of a geographic regionalization isits unintelligence to prevent inter-server communications This is because the ge-ographic regionalization does not give enough importance to players’ interactions.Even though behavioral modeling is the ideal approach to manage interest of theparties, geographic regionalization is not without merits Thus, geographic region-alization can be augmented by behavior-based communications for better interestmanagement

re-Interest Management Models

An MMOG deals with plenty of information keeping each player’s activities andtracking its location Rationally a player does not need state information of the entirevirtual world which is too large Thus, determining information appropriate to eachplayer is a fundamental requirement of online games From this perspective, aninterest management is a way of determining functional details of a player So, theperformance of a virtual world depends on the cost and effectiveness of an AoIMapproach deployed

Publisher-Subscriber Model

Interest management for an MMOG can be abstracted using a publish-subscribe

model The concept is publishers create events and subscribers consume events In

this model, interest management consists of determining when an avatar registers

to or gracefully/ungracefully bows out from a publisher’s (avatar’s) updates erally, interest management for online games can have multiple domains The mostcommon domain is the visibility, although other domains like audible range andradar are also possible Each interest domain has special properties for the transmis-sion and reception of data, so different sets of publisher-subscribers model might beneeded

Gen-Space Model

Interest management can also be categorized into two general groups: space-basedand class-based Space-based interest management can be defined based on the rel-ative position of avatars in a virtual world, while class-based can be determinedfrom an avatar’s attributes Space-based interest management is the most useful forMMOGs because of the relevant information of a player is usually closely related

to its position in the environment and is typically based on proximity which can berealized in terms of the aura-nimbus information model given in Figure4 Aura is

the area that bounds the existence of an avatar in space, while nimbus, i.e area of

interest, is the space in which an object can perceive other objects In the simplest

form, both aura and nimbus are usually represented by circles around the avatar.

This model is more appropriate when a server keeps a connection with each client

The drawback of a pure aura-nimbus model is scalability because of the computing

cost associated with the determination of intersection between the nimbus and theauras of a large number of players

Fig 4 The aura nimbus model

8 Zoning Issues and Area of Interest Management 183

Fig 5 Region-based area of interest

Region Model

Region-based interest management partitions the game space into several fixed gions The interest management scheme then determines the regions which intersectwith the expression of interest of the players Thus, an area of interest becomes theunion of the intersected regions with respect to the expression of interest as shown

re-in Figure5 It is an approximation of the true expression-of-interest and generallycheaper to compute but less precise than a pure aura-nimbus model

Implementation Intelligence

Message Aggregation

Message aggregation is a technique to address MMOG resource limitations [10]

It reduces the overall message update rate by aggregating multiple game messagesinto a single message Thus, it rationalizes bandwidth usage by removing redundantmessage information like the message header Message aggregation introduces somedelay However updates are piggybacked and could limit game performance if notdesigned properly

Message Compression

Simple message compression techniques can be used as a means against strict width requirement [10] Although the method is expensive, considering resourceconstraints it is not without merit