I²MTC 2008 – IEEE International Instrumentation and Measurement Technology Conference Victoria, Vancouver Island, Canada, May 12–15, 2008 A Hierarchical HMM Model for Online Gaming Traffic Patterns Behnoosh Hariri1,2, Shervin Shirmohammadi1, and Mohammad Reza Pakravan2 1 Distributed Collaborative Virtual Environment Research Laboratory University of Ottawa, Ottawa, Canada [BHariri | Shervin]@discover.uottawa.ca 2 Department of Electrical Engineering Sharif University of Technology, Tehran, Iran [hariri | pakravan]@ee.sharif.edu Abstract Online games seem to become major contributors to Internet traffic. Therefore a great deal of effort is being devoted to the analysis and modeling of network game traffic. However the results from the previous measurements have not been unified into a single frame work while gaming traffic model is of high importance to network designers. Considering the fact that Gaming traffic is generated by the player’s interaction during game sessions, we propose to model the player’s activities and later use this model to extract the traffic pattern. We will propose a model for the gaming player behavior regarding traffic generation based on hierarchical hidden markov model. Player activities are inherently highly complicated. In order to deal with the complicated Player activities, it is necessary to incorporate a hierarchical structure in representation of activities, in which a Player activity is represented by a combination of multiple activity units, or actions. Any action or any combination of actions may occur during a game session. The HHMM hierarchy ends in production states that result in the transmission of update messages to the game server. HHMM Observations are considered to be client packet size and packet interarrival time. Keywords – Massively Multiuser Online Gaming, Traffic Modeling and Measurement, Hierarchical HMM. I. INTRODUCTION Online games are expected to be major contributors to World Wide Web traffic. DFC Intelligence forecasted the worldwide online game market to grow from $4.5 billion in 2006 to over $13 billion in 2011 and the total overall number of online gamers in the twenty leading online game countries is expected to increase 51% from 2006 to 2012. It has been estimated that roughly 9 percent of the traffic sent back and forth over the US backbone was due to online gaming in 2002 [1]. As the performance of gaming hardware platforms get better and number of online gamers increase exponentially, game environment will have much faster connectivity requirements. Therefore, measurement and modeling of online gaming traffic has been the subject of many recent works. The leading online game category is expected to remain high-end massively multiplayer online games (MMOGs). There are several types of massively multiplayer online games. Massively multiplayer online role-playing games (MMORPG) such as World of Warcraft are online games in which a large number of players interact with one another in a virtual world. First-person shooters (FPS) games such as quake are those MMOs that provide large-scale, sometimes team-based combat in real time virtual environment. Real time strategy (RTS) games as age of empire typically combine real-time strategy with a large number of simultaneous army commanders in resource competition. Turn based strategy games (SG) such as Panzer General 3D are games that focus on socialization instead of objectivebased game play. In such games two or more participants make their moves sequentially in turns. Massively multiplayer online racing are online versions of racing games. Simulators such as Grand Prix simulate certain aspects of the real world where the main emphasis is on fast moving environment. There are several other MMO games Such as massively multiplayer online manager games, Online Rhythm Games multiplayer and online tycoon game that are considered to be less popular than the previous ones[2]. The fastest growing MMOG game segments are expected to be First Person-Shooters (FPS/Action) and Sports/Racing In terms of market share growth between 2006 and 2012 according to IDC forecast. Therefore As online games seem to become major contributors to Internet traffic, a great deal of effort is being devoted to the analysis and modeling of network game traffic. Gaming traffic varies over time during the days and over days during a week; it is also considered self-similar with heavy-tailed distribution for several games according to the measurements. This Long-term correlation such as hourly and daily game play trends is good for knowing the busy times at which there is more demand for bandwidth and QoS related parameters. Therefore; In order for the model to be a realistic fit of gaming traffic, it should describe both short range and longrange traffic characteristics. Most of the previous works have been concentrated around the representation of real world traffic measurement and fitting some standard distribution over the measured traffic where the distribution parameters follow several game and player related parameters. However few works addressed the issue of general traffic modeling. Therefore the result of their work only applies to a specific game in a specific state and cannot be extended to the cases other than the one for which the measurement has been performed. The motivation behind this work is to propose a generalized framework to cover all the previous measurements that has been performed for FPS games. The model would have a general description and a number of parameters that can be selected in a way to customize the model for a specific FPS game in a specific case regarding the game and player state. This model can later be used as a traffic synthesizer for FPS games that is necessary as a reference for all gaming related protocol and network design works that require using a realistic traffic model in their designs. Such works are not customized to a specific game and therefore need a prototype model that fits to the wide variety of games under a specified category like FPS. This paper aims at the proposal of a generic model for gaming traffic. Considering the fact that gaming traffic is generated by the players interactions during game session, we propose to model players activities and later use this model to extract the traffic pattern. We will use the term activity for representing a sequence of player actions, and an action for representing a component motion, such as “walking”, “sitting”, “going into a room”, “reading a book” and so on. It should be noted that this model would contain a lot more information regarding game states beyond the traffic related statistics. Therefore, the basic idea of our work is to prepare a model for players activities, such that the model represents the total sequence of players actions during a game session. Players activities are inherently highly complicated. In order to deal with such complicated behavior it is necessary to incorporate a hierarchical structure in representation of activities, in which a Player activity is represented by a combination of multiple activity units, or actions. Any action or any combination of actions may occur during a game session. Our proposed approach for the modeling of a player activity is based on Hierarchical Hidden Markov model. The model has been defined in a way to describe the temporal behavior of an online game client and the results of recent gaming traffic have been used to determine the model parameters for a specific game. This general model can then be used to predict various statistical parameters such as player wining chance. The traffic generated by a player is normally a function of player current action and the game state. Therefore the best way to predict the traffic generated by a player is to predict the player action during the game. The player current action at each state depends on the sequence of his previous states. Therefore the player current state can be described as a probabilistic function of his previous states. A number of mathematical models can be used to describe such chained statistical behavior. Markov Model is among these analytical models. In probability theory, a Markov process is a stochastic process in which the probability distribution of the current state is conditionally independent of the path of past states, a characteristic called the Markov property. A hidden Markov model (HMM) is a statistical model in which the system being modeled is assumed to be a Markov process with unknown parameters, and the challenge is to determine the hidden parameters from the observable parameters. In the hierarchical hidden Markov model (HHMM) each state is considered to be a self contained probabilistic model where each state of the HHMM is a markov model by itself. HHMM is Useful in representing “vertical” structure in a time-series model. As for our case, the selection of Markov Model is due to the similarity of a player behavior to a markov process. Therefore our goal is to model a player behavior in the form of a markov chain while the parameters of the markov model for a specific game can be found using the traffic measurement results for that game. In other words, we would like discover the parameters of the markov model given a dataset of observation sequences. However it should be noted that the observation in each state of the markov model is supposed to be traffic characteristics such as packet size and packet interarrival time that are statistically de pendant on the player state in the model. Therefore the model is actually hidden and we want to estimate the hidden model parameters from the traffic measurements that are supposed to be observations. This explains the reason behind the HMM model selection. Moreover we will use the Hierarchical Hidden markov model in order to have to more object oriented representation of a player behavior where the top HMM models the main action while each action can be modeled as a sequence of sub-actions using a separate HMM. In our proposed approach the sequence of player actions in game states is modeled by a hidden markov mixture of first order markov chains. A second hidden markov chain describes the action variations between consecutive states, while state change occurs upon the receiving of a new game state from the server. The HHMM hierarchy ends in production states where production states are defined to be the states that results in the update message generation for the server. The HHMM observations are considered to be client packet size and packet interarrival time that are supposed to be statistically related to the players actions. II. RELATED WORK In the case of first person shouter MMO games, measurements have been reported for Counter-strike [3]-[6], quake [7]-[10], half-life [11], halo[12][13] and strategies tribe [14] for parameters such as interarrival time and packet size in both client and server sides. StarCraft traffic pattern has been analyzed as a representative of Real time Strategy MMO games [15][16] and Lineage has been considered as a representative for massive multiplayer Online Role playing game in [17]. Traffic measurement results have also been reported for Shen Zhu online in [18]. Most of these works have been concentrated around the measurement results for a specific game and tried to fit some standard distribution over the results and the results cannot therefore be used in a generalized case. Only [19] demonstrates the possible use of predictive model for FPS traffic patterns where the distribution for multiple client games proposed to be found by convolving the distribution of lower player games. In this paper, we would propose to model a single player traffic generation pattern using a Hierarchical HMM model. This model may later be used to predict several parameters such as the expected number of players actions, winning probability, game duration and several other useful information. III. BACKGROUND: HIERARCHICAL HIDDEN MARKOV MODELS Hierarchical hidden Markov models (HHMM) are structured multi-level stochastic processes. HHMMs generalize the standard HMMs by making each of the hidden states an “autonomous” probabilistic model on its own. Therefore each state in HHMM model is an HHMM as well. The states of an HHMM emit sequences rather than a single symbol. An HHMM generates sequences by a recursive activation of one of the substates of a state. This substate might also be composed of substates and would thus activate one of its substates. This process of recursive activations ends when we reach a special state that is known as a production state. The production states are the only states that actually emit output symbols through the usual HMM state output mechanism: an output symbol emitted in a production state is chosen according to a probability distribution over the set of output symbols. HHMM can be generally characterized in terms of the following parameters: 1. D that is the number of hierarchy levels where (d ∈ {1,.., D}) . The hierarchy index of the root is 1 and the hierarchy index of the production states is D. An HHMM state of is therefore denoted by q di where i is the state index and d is the hierarchy index. 2. q di that is considered to be the number of substates of an internal state q id . 3. The HHMM is also characterized by the state transition probability between the internal states. For each internal state q id , there is a state transition probability of making a horizontal transition from the ith state to the jth in level d. This can be represented as a matrix as denoted in (1) d d ( ) A q = {a ijq = P q dj q id } (1) 4. Similarly the initial probability of making a vertical transition to substate q id +1 from its parent state q d can be denoted as described in (2). ∏ qd qd ( ) = {π (q id +1 ) = P q id +1 q d } (2) 5. K is the number of distinct observation symbols per production state. The observation symbols correspond to the physical outputs of the system being modeled. We denote the set of symbols as V ={v1, v2, . . . vK} where Vk{ k = 1, 2, . . .,K} is an individual symbol. 6. Each production state is also parameterized by its output probability vector as described in (3) D D ( ) B qi = {b qi (k ) = P v k q iD } (3) 7. The observation sequence O =O1,O2,O3, . . .OR, where each observation Ot is one of the symbols from V, and R is the number of observations in the sequence. It is evident that a complete specification of an HHMM requires the estimation model parameters, D and q id , and three probability distributions A, B, and Π. There are three canonical problems associated with HHMM. The first problem is called forward-backward algorithm and deals with the computation of the probability of a particular output sequence and the probabilities of the hidden state values given that output sequence. The second problem is Viterbi algorithm and provides the solution to the problem of finding the most likely sequence of hidden states that could have generated a given output sequence. Finally, Baum-Welch algorithm considers the case of finding the most likely set of state transition and output probabilities when an output sequence or a set of such sequences is given. As we aim at discovering the parameters of the HHMM given a set of measurements as observations, the problem that we would address in this paper would be Baum-Welch case. IV. HHMM MODEL FOR GAMING PLAYER ACTIONS Most games utilize a client-server model. Every client’s actions are sent in short messages to the server, and every client is regularly updated with the actions taken by other players. Generally, the game traffic can be modeled by independent traffic streams from each client to the server and a traffic stream from the server to the clients assuming that clients behave independently and client traffic is independent of the server traffic. Players run around shooting and interacting with each other at this time. The game may also be paused as the server changes maps or restarts a previous map. A client transmit cycle consists of reading a server packet, processing it, rendering the client’s current view on the screen, sampling input devices, then transmitting an update packet to the server. The client packet size usually depends on the player action such as walking, running, attacking and state in the game but independent of the number of players, computer hardware or map type. Consider a player that is in the playing process during an online game session. Each local status update is reported in the form of update messages to the game server for global updates. The Game server receives the update and performs the subsequent actions. This in addition to the updates from the other players would result in updates that are transmitted from the game server to the players. From the network side traffic measurements, the types of action that are performed in that session is not included in the traffic log. That is, the states are not observable but some functions, possibly random, of the states are observed. Our goal is to try to find the players actions from the traffic profile. Later In this section we describe our approach to modeling player’s actions using a hierarchical hidden Markov mixture of Markov chains. As explained in the previous section such specification would require the estimation of number of hierarchy levels and states in each hierarchy as well as the probability distributions for both vertical, horizontal and also the output in each state. The first parameter in the HHMM definition is the hierarchy level. The level of hierarchy is defined to be 3. The highest level is related to the game states where the transitions are modeled using a markov model. A markov model is also used to model the player’s actions in each game state. Player activities are arranged into a two-level hierarchy of events ranked by complexity, where the lower level contains shorter motions that chain together to form higherlevel events which are longer and more abstract. Therefore each action is in turn modeled using a Hidden markov model. The states in the lowest hierarchy level are production states. In other words these are the states that result in update message generation. The observation for the production states is assumed to be the packet size and packet interarrival time. Each of these outputs results in one observation sequence and model parameter estimation can be done based on one or both of the observations. The choice of parameter values for the HHMM is an estimation problem and it is desired to find the best parameter values for the model. The usual criterion is maximum likelihood. That is finding the values of parameters which maximize the probability of the observed data. This is the problem that the Baum-Welch computation addresses. BaumWelch training is an expectation-maximization algorithm for training the emission and transition probabilities in an HMM structure. What's important about the Baum-Welch Training Method is that we can feed in observation sequences, and as long as we present enough data, we will get an HMM that is pretty close to optimal for the given training sequences. Thus, if the training sequences are characteristic of the actual distribution of observation sequences in the system we are modeling, our HMM will be able to tell us what state the system is in with a high degree of certainty. Therefore we would use Baum-Welch algorithm to estimate the HHMM parameters for each player. The algorithm starts with an initial estimate of HHMM parameters A, B, and Π and converges to the nearest local maximum of the likelihood function. Initial state probability distribution in each hierarchy level is considered to be uniform, that is, if there are N states, then the initial probability of each state is 1/N. The Baum-Welch algorithm is based on the computation of two functions, known as Forward and Backward probabilities for each state and each frame of an observation sequence where one frame of observation sequence is defined to be a game session for a player. Once Forward and Backward probabilities are computed, they are used to weight the contributions of each observation to the HMM parameters. In the previous steps, we have defined an HHMM model that describes a single player behavior and explained the training method to find the model parameters using the training data. The only remaining issue here is to provide a set of observation data to the training process. Later in the next section we would explain the way that we have extracted a general set of training data from the results of the previous measurements. In order to do that we would refer to a number of recent measurements on the gaming traffic pattern and try to draw a consistent pattern from these measurements. V. HHMM TRAING DATA AQUIZATION Several measurement results have been reported for a number of popular online games during the last few years. The results obtained from the measurements vary from game to game especially when the games are in different categories. This is due to the fact that the gaming traffic pattern is strongly impacted by the game logic. Therefore it is seems reasonable for the traffic pattern of highly dynamic FPS games to be totally different from less dynamic strategy based games. Therefore we would focus on a single class of games known as First person shooter games that are known to be the most challenging games regarding the networking point of view. However it should be noted that the proposed HHMM modeling can be applied to any other class of online games through the model redefinition. Therefore we would concentrate on a general traffic extraction for FPS games that can later be used for HHMM model training. It should be noted that even for the class of FPS games measurements are considered to be dependant on the game map and some other player related parameter such as rendering engine. Therefore using a single model for all FPS games will result in model inaccuracy due to the considerable difference in players actions in different game classes. A more exact HHMM model can still be obtained using only measurement results from single FPS game and even the model can be further customized to the client types to make a better match, however the availability of a general approximate model is still valuable for network design. Therefore we would concentrate on the definition of such a model instead of extracting an exact but specific one. In order to extract such training data for a general model, we would try to draw a consistent approximate result from all FPS traffic measurements. The training data can be both considered to be packet length or packet interarrival time of the traffic. Here we would refer to a number of measurements for these two parameters and choose distributions that best fits the client packet interarrival time and client packet length. Later we also refer to the session time distribution for the clients. The training sequence for each of the observations can then be extracted from the distribution of the related parameter and the session time duration distribution. For counter-strike game, Client interarrival time peak has been measured to be 60 ms with surrounding peaks every 10ms or 20ms [3]. This value has been reported to be 33 ms and 50 for OpenGL clients in quake [11] and 40 ms for Halo players [13]. It has also been measured around 40-50 ms for half-life client while it is independent from the map in half life case and fixed at 50ms for half life clients using Direct3D. Although most of the client packets arrive in regular intervals, the client interarrival distribution still shows a heavy–tailed behavior. Therefore, Exponential or deterministic distribution can be a proper model for client interarrival time. Returning to the case of HHMM modeling, the previous measurements reveal the fact that the transition in client actions is expected performed with a probability p that is near 1 most of the times due to the regular packet transmission. However the heavy tailed behavior implies that the probability is slightly less than one in some of the states. The probability for use in the model can either be selected as a deterministic value or can be selected as a normalized value of the (0-90%) part of the exponential interarrival time distribution. Client packets have been estimated to have an extremely narrow distribution with the mean size of around 40 bytes(without UDP/Ethernet header) for counter strike[3][5], it has also been measured to be distributed between 50 and 70 bytes for quake [10] and between 60 and 90 bytes for half life [11]. As for counter-strike, extreme (41,6) was proposed as a good fit for client packet size in the range [20:100] [3]. The client packet size has also been modeled by lognormal (75,3) for quake[7]. The measurement results demonstrate the fact that the client packet size can be best approximated by a number of impulse functions distributed between 50 and 70 bytes for most FPS games [19]. Therefore we would use this model as our training data. The outer markov chain is triggered by receiving the update messages from the server. Therefore we may have a good estimate on the probability transition on the outer chain by knowing the server packet interarrival time. The main server task is to receive the individual client actions, calculate the new game state, and send it to all the clients. Therefore the server data rate increases with the increase in the number of clients; However Traffic rates from server to one client do not vary that much and was mainly designed to saturate the narrowest last-mile link. Packet interarrival time of server is independent on the number of players and in-game behaviors. The mean of server interarrival time for individual client has been measured around 50ms[7][8][4] for counter-strike and quake, 60 ms or 50% at 50 ms and 50% at 70ms depending on the map for half life and around 40 ms for Halo[13]. There are also some high frequency components in server traffic for counter-strike, The frequency component around 30 minutes is due the 30 minute map time of the server during which a dip in traffic occurs when server is doing local tasks to perform the map change over. Other low frequency components also happen due to round changes. According to the measurement results, the server packet interarrival time has been modeled with a burst of 60 ms interarrival time [4] for counter-strike ,deterministic at 50ms [7][11] for quake, 60 ms [10] for half life and a combination of extreme and normal distributions for almost all FPS games [9]-[14]. Therefore it can be concluded that the server packet interarrival time is mostly regular except for a few cases. The outer markov chain will therefore have a transition probability that is near one. This probability can be used as initial values for state transition on that chain. The only remaining issue for HHMM definition is the observation sequence. In other words we should be able to define the duration of a typical observation frame for the training data. In order to have a good estimate on this, we again refer to the recent measurement results on the player session duration. It has been observed that a significant number of players play only for a short time before disconnecting and that the number of players that play for longer periods of time drops sharply as time increases [4]; however the game duration process has a Heavy-tailed distribution due to a number of long-time players [5]. According to these results, the twoparameter form of the Weibull with beta = 0.5, eta = 20, and gamma = 0 was proposed to be a good choice for session time distribution [4][20]. Each observation frame represents a game session for a player. The previously mentioned distribution for session time can therefore be used to determine the observation frame duration. In this section we have explained the way to extract proper training data from the available measurement results for FPS games. The HHMM model can now be trained with this data and the HHMM parameters are therefore estimated. The resulting model can now be used as a traffic synthesizer for the gaming traffic and also as a way to estimate several game related statistics. VI. EXPERIMENTAL EVALUATION The previously described HHMM model has been used to describe a player activity in a generic FPS game whose traffic pattern has been considered to be a typical pattern for the FPS games as described in the previous session. The resulting model have been used to find the dependency of player winning probability to the player session time. The results are illustrated in figure 1. The modeling results show that the chance of winning becomes higher as the session time increases for the players; However it shows a saturation behavior above a specified point to the fact that most players are able to win the game up to that time. Figure 2 shows the probability of players leaving the game after a specific time interval. The simulation results show that there is a high probability that a player leaves the game in the first few minutes that he started. Later the probability of staying in the game increases and finally the probability of staying in sessions longer than a specified value decreases. These results seem to be compatible with the measurement results for session duration. 0.8 0.7 Winning chance 0.6 0.5 level hierarchy of events. A series of measurement data for some popular online FPS games have also been used as the model training data. And the model was trained using the classic Baum-Welch algorithm. The resulting model was then used to estimate a number statistical parameters regarding player behavior such as winning probability and the chance of leaving the game. VIII. REFERENCES 0.4 0.3 0.2 0.1 0 0 20 40 60 Minutes 80 100 120 Figure 1 Player Winning Chance over time 0.6 0.55 0.5 Leaving chance 0.45 0.4 0.35 0.3 0.25 0.2 0.15 0.1 0 20 40 60 80 Minutes 100 120 140 Figure 2 Player leaving Chance over time VII. SUMMARY AND CONCLUSION The motivation behind this work was to propose a generalized framework to cover all the previous measurements that were performed on FPS game traffic. Therefore we aimed at proposing an analytical model for gaming traffic synthesis. Considering the fact that Gaming traffic is generated by the players interactions during game sessions, it would be possible to model the players activities and later use this model to extract the traffic pattern. In order to deal with the complicated Player activities, we proposed to incorporate a hierarchical structure in representation of activities, in which a Player activity is represented by a combination of multiple activity units, or actions. 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