Abstract:
A method for selecting a preferred cache for the download of digital data from a plurality of caches is disclosed. The method comprises the steps of requesting an address of the preferred cache and selecting the preferred cache from the plurality of caches. The selection of the preferred cache is derived from a location identifier of a client requesting the download of the digital data.

Description:
FIELD OF THE INVENTION 
     The invention relates to a method and a server for selecting a cache for the download of digital data, in particular the invention relates to the selection of a cache in a peer-to-peer network. 
     BACKGROUND TO THE INVENTION 
     A peer-to-peer (also termed P2P) computer network is a network that relies primarily on the computing power and bandwidth of the participants in the computer network rather than concentrating computing power and bandwidth in a relatively low number of servers. P2P computer networks are typically used for connecting nodes of the computer network via largely ad hoc connections. The P2P computer network is useful for many purposes. Sharing content files containing, for example, audio, video and data is very common. Real time data, such as telephony traffic, is also passed using the P2P network. 
     A pure P2P network does not have the notion of clients or servers, but only equal peer nodes that simultaneously function as both “clients” and “servers” to the other nodes on the network. This model of network arrangement differs from the client-server model in which communication is usually to and from a central server. A typical example for a non P2P file transfer is an FTP server where the client and server programs are quite distinct. In the FTP server clients initiate the download/uploads and the servers react to and satisfy these requests from the clients. 
     Some networks and channels, such as Napster, OpenNAP, or IRC @find, use a client-server structure for some tasks (e.g., searching) and a P2P structure for other tasks. Networks such as Gnutella or Freenet use the P2P structure for all purposes, and are sometimes referred to as true P2P networks, although Gnutella is greatly facilitated by directory servers that inform peers of the network addresses of other peers. 
     One of the most popular file distribution programs used in P2P networks is currently BitTorrent which was created by Bram Cohen. BitTorrent is designed to distribute large amounts of data widely without incurring the corresponding consumption in costly server and bandwidth resources. To share a file or group of files through BitTorrent, clients first create a “torrent file”. This is a small file which contains meta-information about the files to be shared and about the host computer (the “tracker”) that coordinates the file distribution. Torrent files contain an “announce” section, which specifies the URL of a tracker, and an “info” section which contains (suggested) names for the files, their lengths, the piece length used, and a SHA-1 hash code for each piece, which clients should use to verify the integrity of the data they receive. 
     The tracker is a server that keeps track of which seeds (i.e. a node with the complete file or group of files) and peers (i.e. nodes that do not yet have the complete file or group of files) are in a swarm (the expression for all of the seeds and peers involved in the distribution of a single file or group of files). Nodes report information to the tracker periodically and from time-to-time request and receive information about other nodes to which they can connect. The tracker is not directly involved in the data transfer and is not required to have a copy of the file. Nodes that have finished downloading the file may also choose to act as seeds, i.e. the node provides a complete copy of the file. After the torrent file is created, a link to the torrent file is placed on a website or elsewhere, and it is normally registered with the tracker. BitTorrent trackers maintain lists of the nodes currently participating in each torrent. The computer with the initial copy of the file is referred to as the initial seeder. 
     Using a web browser, users navigate to a site listing the torrent, download the torrent, and open the torrent in a BitTorrent client stored on their local machines. After opening the torrent, the BitTorrent client connects to the tracker, which provides the BitTorrent client with a list of clients currently downloading the file or files. 
     Initially, there may be no other peers in the swarm, in which case the client connects directly to the initial seeder and begins to request pieces. The BitTorrent protocol breaks down files into a number of much smaller pieces, typically a quarter of a megabyte (256 KB) in size. Larger file sizes typically have larger pieces. For example, a 4.37 GB file may have a piece size of 4 MB (4096 KB). The pieces are checked as they are received by the BitTorrent client using a hash algorithm to ensure that they are error free. 
     As further peers enter the swarm, all of the peers begin sharing pieces with one another, instead of downloading directly from the initial seeder. Clients incorporate mechanisms to optimize their download and upload rates. Peers may download pieces in a random order and may prefer to download the pieces that are rarest amongst it peers, to increase the opportunity to exchange data. Exchange of data is only possible if two peers have a different subset of the file. It is known, for example, in the BitTorrent protocol that a peer initially joining the swarm will send to other members of the swarm a BitField message which indicates an initial set of pieces of the digital object which the peer has available for download by other ones of the peers. On receipt of further ones of the pieces, the peer will send a Have message to the other peers to indicate that the further ones of the pieces are available for download. 
     Caches are used throughout the Internet to provide as data stores. The cache saves a copy of data objects for access by clients. The reason that the caches are used is that they provide for fast access to the data objects at a convenient location for the client. 
     In some instances a plurality of caches are available for the supply of a particular data object. One of the caches has to be selected that is preferred for a particular download of the data object to the client. Caches are generally selected depending upon their availability, data stored on the caches and location of a cache. In many cases, caches are selected based on the location of an internet service provider (ISP) or upon locations of a DNS server of the ISP. 
     SUMMARY OF THE INVENTION 
     This invention provides a method for selecting a preferred cache for the download of digital data from a plurality of caches, the method comprising: a first step of requesting an address of the preferred cache; and a second step of selecting the preferred cache from the plurality of caches, the selection being derived from a location identifier of the client requesting the download of the digital data. 
     This invention furthermore provides a server for selecting a preferred cache for the download of digital data from a plurality of caches, the selection being derived from a location identifier of a client requesting a download of the digital data. The server may be either a proxy for tracker communication, incorporated into a tracker or a dedicated cache location server. 
     It can be advantageous to take a location identifier, or network address, of the client, which can be preferably the IP address of the client to select the cache that is located closest to the client to whom the digital data will be downloaded. In this way network traffic can be reduced and download times for the digital object can be effectively increased. 
     In many applications, the client may be a peer in a P2P network and the client will request the download. The invention is not limited to the use and other elements of a network may request the download of digital data to the client. 
     Digital data may be any data, for example music files, video files or any other type of data files. 
     The server may also return a handle to the client giving the client a cache identification identifier, such as a network address, to connect to the cache or to another data source for download of digital data. The final network address may be provided by a name server that can be a central DNS server. 
     The method may be carried out in a one stage request procedure. In response to the request, a preferred cache will be selected and the address of the preferred cache is returned to the client. 
     The step of requesting the preferred address is a two stage process, wherein a first stage comprises returning a handle. A second stage comprises requesting the address of the preferred cache by name to a name server where the name includes the handle and other information from the meta-information relating to the digital data. The name may be returned to the client and may allow for requesting a cache or data source address via an Internet Service Provider (ISP) DNS server. The ISP DNS or any other DNS server may then directly resolve the name or transfer the request to a central DNS or further name servers. The central DNS can thereby be integrated into the server or be a separate component. The server and central DNS can also be located in the same place or at distance from each other. 
     In case of the two stage processes, the handle may comprise one or more of location, publisher, protocol information or the like. The location, publisher and protocol information may have the form of a data string. The handle may then be used for the selection of the preferred cache and selection may be based on one or more of the location, publisher and protocol information. 
     The server is preferably connected to a database. The database may store information upon availability of the plurality of caches, network costs, location and availability of data and/or network tasks. It may also store data for resolving network addresses, i.e., IP addresses. 
     It is also preferred that a DNS, such as the central DNS is connected to a database for resolving the handle information and for transferring the locations of the preferred cache. 
     Two different databases may be used, a first database for the server and a second database for the central DNS. However, the two databases may also be connected to each other or preferably combined to a single database. 
     The server may also be integrated or connected to other components such as a tracker, tracking peer-to-peer information. Thus, a request for download may be sent to the tracker instead of to a separate server. The tracker may have access to the databases, caches, private trackers, and the like. The tracker may also return a handle in a first stage. The address of the preferred cache may also be derived from peer-to-peer information tracked in the tracker. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a Peer-to Peer network as known in the art. 
         FIG. 2  shows the request for a download of a digital object. 
         FIG. 3  shows an overview of the network in accordance with the invention.  FIG. 4  shows an overview for the distribution of content. 
         FIG. 5  shows a geographical implementation of a content distribution network  FIG. 6  shows an overview of a service point of presence. 
         FIG. 7  shows an overview of a data point of presence. 
         FIGS. 8A and 8B  show a flowchart for a method of selecting a cache for a download of a digital object. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is a block diagram illustrating an environment in which various exemplary embodiments of the invention may be practiced.  FIG. 1  includes a Peer-to-Peer (P2P) network  100 . The P2P network  100  includes a plurality of peers, such as peer  102   a ,  102   b ,  102   c ,  102   d ,  102   e  and  102   f , hereinafter referred to as peers  102 , connected to each other. P2P network  100  may be a Local Area Network (LAN), a Wide Area Network (WAN), a Metropolitan Area Network (MAN), a wireless network, or the like. The peers  102  present in the P2P network  100  include stored digital data. Various examples of the digital data include, but are not limited to, an application file, a video file, a music file, or the like. In P2P network  100  the digital data is shared among the peers  102 . It should be understood that the peers  102  may store multiple copies of the digital data. 
       FIG. 2  is a block diagram illustrating a user  202  sending a request for download of a digital object through peer  102   a .  FIG. 2  includes the peer  102   a , the user  202 , a server  204  and a tracker  206 . The server  204  may include one or more torrent files, such as torrent file  208   a ,  208   b  and  208   c , hereinafter referred to as the torrent files  208 . The present invention has been described with respect to BitTorrent protocol as an example only. It should be understood by those skilled in the art that present invention is applicable to all P2P protocols. 
     The user  202  makes a request at the peer  102   a  to download the digital object from the peer-to-peer network  100 . The peer  102   a  communicates with the server  204  and provides information for the digital object to be downloaded to the server  204 . Subsequently, the server  204  locates one of the torrent files related to the digital object requested for download by peer  102   a , such as, for example, torrent file  208   a . The torrent files  208  include information related to the name, size, number of pieces and check sum error for the digital object to be downloaded by peer  102   a.    
     Subsequently, the tracker  206  may provide a list of peers  102  present in the P2P network  100  with the pieces of the digital object to be downloaded. The peer  102   a , thereafter, communicates with the available list of peers  102  for downloading the related digital objects. The peer  102   a  communicates with peers  102  by sending a bit field of the pieces of the digital object that peer  102   a  has. After peer  102   a  receives the bitfields from peers  102 , it sends a message to the peers  102  where it finds relevant data and starts downloading the pieces of the requested digital object. 
       FIG. 3  is a block diagram illustrating peer  102   a  in communication with a Cache Location Server (CLS)  302 , in accordance with an example of the present invention.  FIG. 3  includes the peer  102   a , the CLS  302 , a database  304 , an Internet Service Provider Domain Name Server (ISP DNS)  306 , a central Domain Name Server (central DNS)  308 , a cache DNS  310  and one or more caches, such as, cache  312   a ,  312   b  and  312   c , hereinafter referred to as caches  312 . 
     The peer  102   a  communicates with the CLS  302 . The information sent by the peer  102   a  to the CLS  302  may also contain the IP address of the peer  102   a . Based on the received information, the CLS  302  communicates a location string to the peer  102   a . The CLS  302  may get the location string from the database  304 . The database  304  stores information about the IP address ranges of countries, ISPs, regions, towns, etc for the purpose of generating specific location strings with respect to peers  102 . 
     The peer  102   a  then, using the location string and information from the Torrent File  208 , makes communication with the ISP DNS  306 . 
     As illustrative examples only, the information sent by peer  102   a  to ISP DNS 306 may be as follows: 
     Protocol-TruncatedHash.Protocol-Publisher-LocationString.Find-Cache.com 
     An example of the information sent by CLS  302  to peer  102   a  may be as following:
         bt-1234.bt-bigcorp-bigispnyc.find-cache.com
 
where, ‘bt’ represents the BitTorrent protocol used by the peer  102   a , ‘1234’ representing a specific hash value associated with the digital object to be downloaded by the peer  102   a , ‘bigcorp’ representing the publisher (a fictional “Big Corporation”) of the digital object to be downloaded, ‘bigispnyc’ representing the location string for the peer  102   a  (the New York point of presence for a fictional “Big ISP”).
       

     Based on this communication, the ISP DNS  306  redirects the request to the central DNS  308  (which is the name server for the domain contained in the communication). Thereafter, the central DNS  308  provides an address of the cache DNS  310  to the ISP DNS  306 . The cache DNS  310 , thus, receives a DNS request from the ISP DNS  306  for the digital object to be downloaded. Subsequently, the cache DNS  310  allocates one of the caches  312 , such as, for example, cache  312   a . The cache DNS  310  may allocate one of the caches  312  based on the load, availability and content on each of them. The cache DNS  310  communicates this information to the ISP DNS  306 , which in turn communicates the information to the peer  102   a.    
     In an example of the invention, the tracker  206  is able to provide the DNS name or IP address to the peer  102   a . The tracker  206  receives the IP address of the peer  102   a  and uses this to calculate the location string. 
     A proxy for tracker communication may be used which is connected to the peer  102   a . The proxy (not shown) is situated close to the peer  102   a -usually at the same point of access into the Internet. Thus the proxy cache may be provided the relevant DNS name or IP address for the peer  102   a  and insert into responses from the tracker. 
     The peer  102   a , thereafter, makes a communication with the cache  312   a  for downloading the digital object. The communication between the peer  102   a  and cache  312   a  is explained in detail in  FIG. 4 . 
       FIG. 4  is a block diagram illustrating a system  400  for content distribution in the P2P network  100 . The system  400  includes the peer  102   a ,  102   b  and  102   c , the cache  312   a  and  312   b , a content server  402 , a private tracker  404 , a public tracker  406 , a business logic unit  408 , a central database server  410  and a user interface unit  412 . 
     The peer  102   a  sends a request to the cache  312   a  for downloading the digital object. The cache  312   a  is connected to the content server  402  and the private tracker  404 . The content server  402  may include complete copies of a plurality of stored digital objects in the P2P network  100 . In an example of the present invention, the content server  402  is connected to a publisher&#39;s computer network. The content server  402  receives the digital objects, which are to be distributed, from the publisher&#39;s computer network. For example, the publisher wishing to distribute a video file in the P2P network  100  would first upload the video file to the content server  402 . Thereafter, the video file can be subsequently downloaded by the peers  102  from the content server  402 . 
     In an example of the present invention, as soon as the publisher uploads a piece of the digital object on the content server  402 , the digital data becomes available for the peers  102  to be downloaded. Thus, as the publisher progresses with the upload of subsequent pieces of the digital object, the peers  102  are able to download those uploaded pieces in parallel. Therefore, the capability of the system  400  to execute parallel uploads and downloads of the digital object from the content server  402  ensures an efficient real time availability of digital objects in the P2P network  100 . 
     The cache  312   a  downloads the digital objects, based on the request from the peer  102   a , from the content server  402 . If the digital object requested by the peer  102   a  is available on the cache  312   a , the peer  102   a  downloads the digital object from the cache  312   a . If the digital object is not available on the cache  312   a , the cache  312   a  downloads the requested digital object from the content server  402 . Thereafter, the cache  312   a  makes the digital object available to the peer  102   a  for downloading. In an example of the present invention, the peer  102   a  may also download the related digital objects from the other peers  102  available in the P2P network  100 , such as, for example, peer  102   b  and peer  102   c.    
     In another example of the present invention, the cache  312   a  may upload digital objects from the peers  102  available in the P2P network  100 . In such a case, the cache  312   a  acts as one of the peers  102 . 
     The private tracker  404  maintains a track of all the data transferred between the content server  402  and the caches  312 . The tracking of the transferred data by the private tracker  404  eliminates the condition where the cache  312   a  acquires more than one copy of the same digital object. 
     The public tracker  406  is connected to all of the caches  312  and to all of the peers  102  in the P2P network  100 . The public tracker  406  maintains a track of all the data digital objects transferred among the caches  312  and the peers  102 . In particular, the public tracker  406  maintains a list of all of the peers  102  and the caches  312  which hold copies of the digital objects available in the P2P network  100 . 
     The business logic unit  408  is connected to all the caches  312  and the private tracker  404 . The business logic unit  408  authenticates peers  102  before allowing the peers  102  to upload any digital object. Further, the business logic unit  408  is connected to the central database server  410 . The business logic unit  408  acts as an interface between the P2P network  100  and the central database server  410 . Central database server  410  acquires log reports from the private tracker  404  and caches  312 , through the business logic unit  408 , for all the data shared between the caches  312  and the content server  402 . Using the information from the central database server  410  obtained via the business logic unit  408 , such as, the log reports, the user interface unit  412  provides the required information for billing purposes and report generation. 
     The central database server  410  may be connected to the public tracker  406 . In another embodiment of the present invention, the public tracker  406  may be connected to the private tracker  404 . 
     The public tracker  406  may be connected to all the caches  312  available in the P2P network  100 , such as, for example, cache  312   a  and cache  312   b.    
       FIG. 5  is a block diagram illustrating an exemplary geographical implementation of a cache distribution network  500 . The cache distribution network  500  includes one or more service points of presence, such as, a service point of presence  502   a  and  502   b , hereinafter referred to as the service points of presence (POPs)  502 . The cache distribution network  500  further includes one or more data points of presence, such as, data point of presence  504   a ,  504   b ,  504   c  and  504   d , hereinafter referred to as data points of presence (POPs)  504 . The service POPs  502  are located at remote geographical locations for, such as, for example London, San Jose and so forth. It should be understood by those skilled in art that the number of the service POPs  502  locations are scalable and may be increased with the increase in network traffic. The service POPs  502 , such as the service POP  502   a  and  502   b , are connected to each other. The connection between the service POPs  502  enables a real time data and information transfer between all of the service POPs  502 , 
     Furthermore, the data POPs  504  are also located in remote geographical locations across the globe, such as, for example, New York, Frankfurt and so forth. It should be understood by those skilled in art that the number of the data POPs  504  locations are scalable and may be increased with the increase in network traffic and digital objects available in the P2P network  100 . The data POPs  504 , such as the data POP  504   a  and  504   b , are connected with all the available service POPs  502  in the P2P network  100 . The connection between the data POPs  504  and service POPs  502  enables a real time data update and information transfer between the data POPs  504  from the service POPs  502 , 
     The geographical location may include both, the service POP  502   a  and the data POP  504   a.    
       FIG. 6  is a block diagram illustrating an arrangement  600  of the components of the service POP  502   a , in accordance with an example of the present invention. The arrangement  600  for the service POP  502   a  includes the cache location server  302 , the central domain name server  308 , the content server  402 , the private tracker  404 , the business logic unit  408  and the central database server  410 . Further, in an example of the present invention, the arrangement  600  for the service POP  502   a  may include the caches  312 , such as, the cache  312   a  and  312   b . The arrangement  600  for the service POP  502   a  may include the public tracker  406 , the business logic unit  408  and the user interface unit  412 . 
     The central database server  410  can be located in each of the service POPs  502 . The central database server  410  of each of the service POPs  502  are connected to each other and act as a central database unit. 
     It should be understood by those skilled in the art that the components illustrated in the arrangement  600  for the service POP  502   a  are scalable and may be increased based on the network traffic and the digital objects available in the P2P network  100 . 
       FIG. 7  is a block diagram illustrating an arrangement  700  of the components of the data POP  504   a , in accordance with an example of the present invention. The arrangement  700  for the data POP  504   a  includes the caches  312 , such as, the caches  312   a ,  312   b ,  312   c  and  312   d  and the cache DNS 310. The data POP  504 a provides digital objects for the peers  102  in the P2P network  100 . The data POPs  504  download data from the service POPs  502 . 
     It should be understood by those skilled in the art that the components illustrated in the arrangement  700  for the data POP  504   a  are scalable and may be increased based on the network traffic and the digital objects available in the P2P network  100 . 
       FIGS. 8   a  and  8   b  illustrate a flowchart for a method of selecting the cache  312   a  for the download of digital objects by the peer  102   a  in one example of the invention. At step  802 , the peer  102   a  communicates the IP address of the client to the CLS  302  when the peer  102   a  requests for downloading a file. At step  804 , the CLS  302  returns a handle including a location string for the peer  102   a . The CLS  302  may get the location string from the database  304 . The CLS  302  can locate the caches  312  closest to the peers  102  based on the generated location strings. The handle and the location string have been explained in  FIG. 3 . 
     In other examples of the invention, the peer  102   a  receives the DNS name or IP address from either the tracker  206  or a proxy for tracker communication as explained above. 
     At step  806 , the peer  102   a  communicates the handle to the ISP DNS  306 . The ISP DNS  306 , thereafter, directs the request to the central DNS  308  at step  808 . At step  810 , the central DNS then communicates a name server to the ISP DNS  306  based on the location string. Subsequently, at step  812 , based on the name server received from the central DNS  308 , the ISP DNS  306  redirects the request for download to the cache DNS  310 . The cache DNS  310  includes one or more caches  312 . Thus, at step  814 , the cache DNS  310  allocates one of the caches  312 , such as, for example cache  312   a . In an embodiment of the present invention, the allocation of one of the caches  312  for downloading is based on the loads of the caches  312 . The cache DNS  310  allocates one of the caches  312  with the minimum load. In an embodiment of the present invention, the load of the caches  312  is based on the number of requests being served for download or the bandwidth availability for downloading digital objects. 
     Thereafter, at step  816 , the ISP DNS  306  communicates the cache  312   a  information to the peer  102   a . The peer  102   a  then establishes a communication with the cache  31 . 2   a , providing details of the digital object to be downloaded, at step  818 . Subsequently, at step  820 , the peer  102   a  downloads the pieces of the digital object from the cache  312   a.    
     The foregoing description is that of the preferred embodiments of the invention and that various changes and modifications may be made thereto without departing from the spirit and scope of the invention.