Conventionally, technologies have been developed with respect to bandwidth control in content delivery. For example, Patent Literature 1 discloses a bandwidth control method in content delivery. Patent Literatures 2 and 3 disclose cache servers for preventing a reduction of user QoE due to variations of bands in content delivery through mobile networks. Patent Literature 4 discloses a server device that transmits to a client device media data encrypting contents at a single media rate selected from among multiple media rates. Patent Literature 5 discloses a transmission band delivery method for delivering transmission bands depending on the priority for each connection when distributing bands (e.g. ABR (Available Bit Rate) bands) between multiple connections. Patent Literature 6 discloses an ABR (Available Bit Rate) service quality estimation device.
Recently, according to the advancement of Internet technologies, electronic devices having communication functions have been connected to networks, in particularly, communication traffic has increased due to the spreading of mobile terminal devices and smartphones; this nay increase the burden of loads to network facilities of common carriers. For this reason, various studies have been made to improve client QoE (Quality of Experience) without causing higher traffic than necessary. For example, it is necessary to control traffic for content delivery according to the recently-developed ABR (Adaptive Bit Rate) delivery method.
The ABR delivery method is used for delivering contents such as moving pictures and sounds. Non-Patent Literature 1 teaches HTTP Live Streaming (HLS) as one example of the ABR delivery method. According to the content delivery of HLS, a single content is divided into multiple segments, and therefore their segment files are delivered to a client device. The RLS provides a playlist file (or an index file) describing URLs (Uniform Resource Locators) of segment files for a single content in an order of playback times and multiple types of segment files having different bit rates for a single content, and therefore it uses a variant playlist (Variant Playlist) describing the URL of a playlist corresponding to each bit rate. First, a client device acquires a variant playlist so as to select a playlist file, corresponding to the bit rate suited to viewing environments, from among playlist files described therein. Then, the client device sequentially retrieves segment files described in the selected playlist file so as to reproduce contents.
To delivery contents such as video and audio contents, a pacing (Pacing) delivery technology has been developed as a method for suppressing network traffic without sacrificing reproduction quality at client devices. For example, Patent Literature 1 teaches a pacing delivery control device adapted to a gateway device located between a client device and a content delivery server device according to the ABR delivery method. The pacing delivery control device retrieves bitrate information used for delivering contents from a playlist file corresponding to the contents to be delivered to a client device, thus controlling bandwidths allocated to the client device depending on a target bitrate set to the client device. This makes it possible for the client device to guarantee the target bitrate. In addition, it is possible to prevent the occurrence of burst traffic due to usage of the entire bandwidth for the client device in networks including a content delivery server device, a gateway device, and a client device.