Access Control Method based on Sample Monitoring for Volatile Traffic in Interactive TV Services Hideyuki KOTO†, Haruo HOSHINO‡, Yasuhiko HIEHATA†, Satoshi UEMURA†, and Hajime NAKAMURA† † KDDI R&D Laboratories, Inc. 2-1-15 Ohara, Fujimino-shi, Saitama, 356-8502, JAPAN E-mail: {koto, hiehata, sa-uemura, nakamura}@kddilabs.jp Abstract- This paper proposes an access control method based on sample monitoring for volatile traffic in interactive TV services. The proposed method controls the access of interactive TV users by utilizing the following functions. Firstly, the total volume of access induced by all users is accurately estimated by measuring the volume of access induced by sample users. Then, based on the estimated results, access from other non-sample users is controlled so that they are induced within the system capacity. By applying the proposed method, congestion and overload expected during interactive TV services is effectively avoided, even when the volume of induced access momentarily exceeds the system capacity. The performance of the proposed method is evaluated through experiments on a test bed where a vast number of access is generated. The obtained results quantitatively demonstrate its effectiveness.
I.
INTRODUCTION
Digital terrestrial broadcasting, or Digital TV (DTV), opens up new opportunities for both the viewers and the broadcasters by providing interactivity to TV [1]. For example, the viewers can participate in a TV program by sending their responses, such as sending an answer to a quiz show. In addition, the viewers can enjoy various web services via the Internet that are tied up with the broadcasted contents. As terminals equipped with both Internet connectivity and DTV reception capabilities [2] become more popular in the market [3], the viewers’ use of interactive TV services is expected to increase. One corresponding problem is the characteristics of the traffic induced by the viewers. Since viewer access is triggered by a TV program, a vast number of users could simultaneously access a specific server notified in the program [4]. Moreover, when interactive services are provided in popular programs with millions of viewers, the volume of induced access would increase sufficiently to cause severe overloads of servers and congestion in communication networks. Not only that, since the ratio of viewer response varies depending on various factors such as the contents, audience ratings and response ratio of the program, the potential volume and characteristics of the induced access are volatile or difficult to predict beforehand. Many recent studies have proposed methods to avoid heavy congestion of the Internet and overloads of the servers [4] – [9]. All these methods seem to be effective, but their drawbacks are that some modifications to either software or hardware are needed in order to apply the new methods. For example, [5] needs a gateway in front of the protected server, while [6] changes the OS kernel. Moreover, these methods were evaluated under relative light load conditions. Therefore, scalability and effectiveness under heavy load conditions, which is expected during interactive TV services, are not accounted for. As countermeasure for this problem, we proposed an adaptive transmission control method for interactive TV services in [10]. This method adaptively and randomly restricts and delays access from the viewers depending on the monitored system load, so that
‡ NHK (Japan Broadcasting Corporation) 2-2-1 Jinnan, Shibuya-ku, Tokyo, 150-8001, JAPAN E-mail:
[email protected] severe congestion is effectively avoided. Moreover, the method utilizes the broadcasting channel for feedback control, achieving both reliability and scalability to control access from vast numbers of viewers. The evaluated results showed its effectiveness under extreme traffic conditions. However, the obtained results also showed its limit: The access induced was not always successfully completed (e.g. accepted by the system) even after control. In other words, the volume of access induced by the viewers could exceed the system capacity even when control is applied. In addition, since this method applies random control, fairness of user access is not accounted for. To overcome the limit of the adaptive transmission control method, this paper proposes a new access control method. The proposed method controls the access of interactive TV users by utilizing two distinguishing functions; sample monitoring of access, and dynamic control based on the monitored results. Firstly, the access of sample users is monitored and the volume of total access is accurately estimated. Secondly, based on the monitored results, control information is calculated and dynamic control (specifically suspension and release) is applied to the access of non-sample users. Consequently, the induced accesses are controlled within the system capacity, and congestion/overloads are effectively avoided. The control is applied in sequential order of their induced timings, so that they are accepted on a first-come-first-served basis. The performance of the proposed method is evaluated through experiments on a test bed where up to approximately 250,000 accesses are generated. The obtained results quantitatively demonstrate the effectiveness of the proposed method that accurately estimates and controls the volatile traffic in interactive TV services. II. CHARACTERISTICS OF THE TRAFFIC INDUCED DURING INTERACTIVE TV SERVICES Commercial DTV service in Japan started in December 2003. Since then, terminals with DTV reception capability, such as flatpanel TVs, mobile phones, PCs etc., have become more popular in the market. For example, over 31 million DTVs and over 20 million mobile DTV phones have already been sold in Japan today [3]. With this widespread of DTV, new services that integrate the fields of telecommunication and broadcasting are being provided, and more viewers are expected to use these services. The characteristics of the access induced by the viewers (e.g. web access) during interactive TV services are summarized as follows. Massiveness Vast number of terminals, e.g. in order of millions, could induce traffic. Consequently, massive traffic from a vast number of viewers could be induced. Intensiveness
978-1-4244-2324-8/08/$25.00 © 2008 IEEE. This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the IEEE "GLOBECOM" 2008 proceedings.
Viewer response is triggered by a TV program. Thus, the access timings of viewers tend to concentrate. For example, viewers would send an answer to a live interactive quiz show within a certain period of time. Concentration Viewers that are distributed in the DTV broadcast area access a specific destination (URL) that is advertised in the broadcasted contents. As a result, the induced vast access concentrates on the specific end servers/networks, which could become the bottlenecks of congestion. Volatility Ratio of viewer response varies depending on the content of the program. That is, programs with low audience ratings could induce a vast number of responses, e.g. giveaway programs with luxurious presents, and vice versa. Therefore, the potential volume and characteristics of the traffic are volatile, and difficult to predict beforehand. Transmission via TCP/IP Previous interactive services used phones or faxes to collect responses from the viewers. In DTV, TCP/IP [11] is commonly used as the communication protocol for transmitting and receiving data via the Internet. As a result of all the above characteristics, vast numbers of viewer access (HTTP/TCP connection [12]) would concentrate causing overloads of servers and congestion in communication networks. Let us make a rough estimate of the volume of access expected. For example, let the number of DTV viewers be 20 million, and the audience rating of an interactive quiz show be 10%. Assume that 10% of the viewers send an answer via the Internet. In this example, the total number of user access amounts to 200,000, and this could concentrate in a matter of seconds. III. PROPOSAL OF ACCESS CONTROL METHOD BASED ON SAMPLE MONITORING A. System Outline To effectively control the access induced during interactive TV services, we propose an access control method based on sample monitoring. Figure 1 shows the system architecture of the proposed method. The proposed method performs access control by monitoring access from sample users, and calculating/advertising control information based on the monitored results. All these functions are performed periodically, and the induced accesses are controlled within the system capacity. B. Sample Monitoring of Volatile Traffic In order to perform access control, knowing or being able to estimate the volume and characteristics of the induced access beforehand is preferable. However, since the access induced during interactive services is vast and volatile, this could be difficult. For example, when the total volume of induced access is too vast, it could exceed the measuring capacity causing measurement errors. Additionally, losses and retransmissions could occur making it more difficult to accurately measure its original volume. To overcome this problem, the proposed method utilizes a sample monitoring technique. Firstly, the proposed method classifies users into two types, sample and non-sample users. This classification is performed every time an access is induced using random values, so that no
DTV broadcasting station Advertisement of control information
Calculation of control information
Broadcasting server
Users
Control server
Access control applied Monitoring of sample users
Internet
Base station
Monitoring server
WEB server
Fig. 1. System architecture.
users are fixedly assigned as sample users. Access from sample users are allowed to induce without any control, while access form non-sample users are suspended at the terminal. Then, the volume of access by sample users is monitored. Here, the ratio of sample users is pre-set (i.e. known by the system) and advertised to viewers beforehand. Therefore, the total volume of access induced could be accurately estimated based on this monitored result. Meanwhile, the suspension of non-sample users allows the proposed method to scalably monitor access and perform control. Since access from non-sample users are held at the terminal, the volume of access that reaches the server would be relatively small, even when the potential volume (i.e. volume before control) exceeds the system capacity. In other words, if non-sample users are allowed to access just like sample users, congestion and overload could occur before applying control. Consequently, the proposed method realizes scalable and accurate control against vast and volatile traffic. C. Sequence Flow of Control Figures 2 and 3 illustrate the sequence flow and the image of the induced access when the proposed control is applied, respectively. Here, ∆t is the period of control and τ i is the timeslot between time t + i∆t and t + (i + 1)∆t . The sequence flow of the proposed control method is as follows. (1). Interactive TV users induce λ[τ i ] at timeslot τ i , of which λs [τ i ] by sample users is allowed to induce without any control, while λn [τ i ] by non-sample users is suspended at the terminal from accessing the WEB server. Here, the classification of users is performed at each user terminal by calculating random values and comparing it with the ratio of sample users rs that is advertised beforehand. (2). The monitoring server monitors λs [τ i ] , and reports the monitored results to the control server at the end of τ i . (3). The control server estimates the following values based on the received results. i. Volume of total access induced during τ i : λ E [τ i ]
1 rs ii. Volume of access suspended during τ i : λnE [τ i ]
λ E [τ i ] = λs [τ i ] ×
λnE [τ i ] = λ E [τ i ] − λs [τ i ]
(1)
(2)
iii. Volume of access induced during τ i and continues to be suspended at the end of τ i : ϕτ [τ i ] i
978-1-4244-2324-8/08/$25.00 © 2008 IEEE. This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the IEEE "GLOBECOM" 2008 proceedings.
ϕτ [τ i ] = λnE [τ i ]
User induces access
(3)
i
Here, ϕ x [ y ] denotes the volume of access induced at timeslot x and continues to be suspended at the end of timeslot y. Equation (3) means that all of λnE [τ i ] is
Sample user ? Yes
Yes
Fig. 2. Sequence flow. Volume of access Total access
λ [τ i ]
System capacity
C max
Access by non-sample users λn [τ i ] Suspended and released after 㱠i+1
λs [τ i ] monitored &
λn [τ i ] and λ [τ i ] estimated Access by sample users λs [τ i ] Transmitted without any control at 㱠i
τ i −1
t 㰱 1) (i t+
τi
k =−∞
i +1
]
Volume of access
Available capacity at 㱠i+1: C [τ i +1 ]
Total access
λ [τ i ]
Suspended access released based on available capacity
System capacity
C max
Released access which was induced at 㱠i
λn [τ i ] Released access which was induced before 㱠i-1
(6)
τ i −1
t 㰱 1) (i t+
ii. Calculate the volume of access that continues to be suspended and that are released at the end of τ i +1 .
ϕτm [τi ]䇭 䇭 䇭䇭 䇭(m ≤ n)䇭 䇭 ° m−1 ° ρτm [τi+1] = ®C[τi+1] − ¦ϕτk [τi ]䇭 (m = n +1) k =−∞ ° ° 0䇭 䇭 䇭 䇭 㩷 㩷 㩷 㩷 䇭 (m > n +1) ¯
iii.
(8)
Here, ρ x [ y ] denotes the volume of access induced at timeslot x and released during timeslot y. Calculate the ratio of access to be released during τ i +1 : rr ρτ [τ i +1 ] (9) rr = n+1 ϕτ n+1 [τ i ]
(6). The control server advertises the calculated control information τ n+1 and rr to the users via the broadcasting channel. Here, τ n+1 is the timeslot where a subset of the
Reserved capacity for sample access induced during 㱠i+1 : C r [τ i +1 ]
λs [τ i ]
Equation (6) calculates the most recent timeslot where all of the accesses still suspended could be released at τ i +1 .
(7)
time
(a). Estimation of total access based on sample monitoring.
where䇭 㩷 n ≤ i
0䇭 䇭 䇭 䇭 䇭 䇭 (m ≤ n)䇭 °m ° ϕτm [τi+1] = ® ¦ϕτk [τi ] −C[τi+1]䇭 (m = n +1) °k=−∞ ° ϕτ [τi ]䇭 䇭 䇭 䇭 䇭 䇭 (m > n +1) ¯ m
τ i +1 τ i + 2 τ i +3
t t t t t 㰱 㰱 㰱 㰱 i㰱 1) 2) 3) 4) t+ (i+ (i+ (i+ (i+ t+ t+ t+ t+
n
i
Added delay elapsed
Access completed
values. Here, suspended access is released in sequential order of their induced timings until available capacity is all used up. i. Find the largest n which satisfies the following equation. k
Apply delay ?
Calculate & add delay
Here, Cmax denotes the maximum system capacity that is pre-set. The proposed method controls the induced access within this capacity. (5). The control server finally calculates the volume of access that could be released during τ i +1 , based on the above calculated
¦ ϕτ [τ ] ≤ C[τ
No
Yes
Here, rf denotes the ratio of reserved capacity. The proposed method assumes that the volume of access by sample users in the next timeslot will be a multiple of λs [τ i ] monitored at the previous timeslot. Some capacity is reserved for sample users to ensure accurate measurement and thus estimation of the induced access. System capacity that is available during τ i +1 : C[τ i +1 ] (5) C[τ i +1 ] = Cmax − Cr [τ i +1 ]
find max n which satisifies
No
Release ?
Connection establishment & data transmission
suspended at the end of τ i . (4). The control server then calculates the following values relating to the system capacity. i. System capacity reserved for access that will be induced by sample users during τ i +1 : Cr [τ i +1 ] (4) Cr [τ i +1 ] = rf
