Abstract:
A scalable system and method for locating a closest server in response to a client request via an interactive distribution network, such as the Internet, are provided. A closest content server is defined as having the least round trip time for responding to a client request. The system including a plurality of content servers; and a local server in communication with a plurality of clients, the local server acting as a proxy for communicating client requests from clients to a redirection server. Client network distance and load information is periodically collected at each content server in the network from clients communicating with each of the respective content servers. The redirection server periodically aggregates the network distance and load information from each content server to create client clusters from both current and previously aggregated network distance and load information. Each client cluster represents a division or partition of the total IP address space. Each client cluster is then mapped (paired) to one or more content servers in the network. The mapping or pairing is then utilized to respond to client DN requests received from any client in the network. Another aspect of the invention involves considering the respective capacities of the content servers in the network. A selection probability is assigned to each content server/domain index pair to prevent the repeated selection of the content server having lowest round trip time thereby overloading that server&#39;s service capacity. The selection probabilities assigned to each content server effect a load balancing to prevent overloading. Another aspect of the invention involves collecting the distance and load information without incurring any overhead cost by passively collecting TCP information as it is transmitted from clients in communication with content servers in the course of normal communications.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates generally to networks. More particularly, the present invention relates to a system and method for locating a closest server in response to a client domain name request by identifying a best-performing content server considering round trip time and service capacity.  
           [0003]    2. Description of the Related Art  
           [0004]    Methods and systems for resolving DNS requests are well-known. The basic domain name server (DNS) method is described in the paper by Mockapetris, P., entitled “Domain Names—Implementation and Specification”, RFC 1035, USC Information Sciences Institute, November 1987.  
           [0005]    At an early stage in the development of the Internet, the concept of distributing content throughout the network to better serve clients was not considered. Client DNS requests were always resolved to the same IP address (i.e., the same content server) for all clients irrespective of the client&#39;s location in the network. More recently, consideration has been given to the notion of improving the servicing of clients by resolving DNS requests differently depending upon the client&#39;s location in the network. A number of methods have been introduced which attempt to resolve DNS requests in this client-dependent manner.  
           [0006]    One known method in the art that attempts to resolve client DNS requests differently depending upon the client&#39;s location in the network, utilizes pinging mechanisms on the content servers (i.e., caches). Whenever a user makes a request, the DNS sends out a message to all the content servers in the network to ping the requesting client. This method is problematic in that there is a significant latency associated with the DNS instructing the content servers to ping the requesting client. A further disadvantage of the ping approach is that many clients are either behind firewalls, and not pingable as a result, or else are configured to not accept pings.  
           [0007]    Another known method for resolving DNS requests in a client-dependent manner is to download routing table information from the network. Routing tables, however, provide a coarse idea of client locations in a network. That is, the prefixes used in most routing tables cover a widely dispersed geographical area. This is problematic in that determining a client&#39;s position in a network cannot be performed with sufficient accuracy to provide an appropriate best-performing content server in response to a client DNS request.  
           [0008]    Thus, there is a need to provide a method for locating content servers in a network in response to client DNS requests where the method has minimal round trip time.  
         SUMMARY OF THE INVENTION  
         [0009]    The present invention is an improved system and method for responding to client requests. The improved system and method allows clients to obtain content from nearby content servers in a network which includes a plurality of content servers distributed throughout the network. A nearby content server is defined as having the least round trip time for responding to a client request.  
           [0010]    A system according to an embodiment of the present invention for resolving requests in a client-dependent manner includes a plurality of content servers; and a local server in communication with a plurality of clients which serves as a proxy for communicating client requests from clients to a redirection server. The redirection server is coupled to the local server and the plurality of content servers.  
           [0011]    A method consistent with the present invention for determining a best-performing content server in response to a client request, such as, for example a client request for domain “www.cnn.com”, generally comprises the steps of: collecting client network distance and load information at each content server in the network from those clients communicating with the respective content servers; periodically aggregating the network distance and load information from each content server at the redirection server; periodically creating client clusters at the redirection server from current and previously aggregated network distance and load information where each client cluster represents a division or partition of the total IP address space; mapping each created client cluster to one or more content servers/domain index pairs; assigning a selection probability to each content server/domain index pair; and utilizing the mapping to respond to client DN requests received from any client in the network.  
           [0012]    According to one aspect of the invention, consideration is given to the respective capacities of the content servers in the network. In the least sophisticated implementation of the method of the present invention, a content server is always selected which has the lowest round trip time. Problems may arise, however, in that repeatedly selecting the content server having lowest round trip time may overload that server&#39;s service capacity. To circumvent this occurrence selection probabilities are assigned to each content server to effect a load balancing. That is, the selection probabilities will statistically select a server different from the server having the lowest round trip time to ensure that a content server&#39;s maximum service capacity is never exceeded.  
           [0013]    According to another aspect of the invention, the clustering/mapping operation is performed periodically at the redirection server. However, additional means are provided for performing the clustering/mapping operation whenever an unanticipated change in network topology is detected (e.g., failed switches, failed content server).  
           [0014]    A distinguishing feature of the system of the present invention over known systems is that the clustering/mapping is created using information provided from only a subset of clients in the network. That is, distance and load information is collected from only those clients in communication with the respective content servers in the network. The mapping created from the information, however, is used to resolve DNS requests from all clients in the network. This is advantageous as it is inefficient to create a separate mapping or assignment for every IP address in the network (e.g., Internet). By creating a mapping from only a small subset of clients an efficiency is realized in that it becomes unnecessary to laboriously map each individual client in the network with a best-performing content server.  
           [0015]    Another distinguishing feature of the system of the present invention over known systems is that the present invention advantageously collects the distance and load information without incurring any overhead cost by passively collecting TCP information as it is transmitted from clients in communication with content servers in the course of normal communications. No active messaging above and beyond the existent TCP protocol messaging is required to collect the required distance and load information.  
           [0016]    A further distinguishing feature of the system of the present invention over known systems is that the system of the present invention automatically adapts to changes in the network as the round trip time of communicating clients change. This occurs as a consequence of collecting the TCP information as described above. Changing network conditions are instantaneously reflected in the TCP information transmitted between clients and content servers.  
           [0017]    A still further distinguishing feature of the system of the present invention over known systems is that the system of the present invention does not make an a-priori estimation or prediction of the network locations of the respective content servers in the network prior to performing the method of the present invention for locating a best-performing content server. Rather, each content server&#39;s location is determined in real-time from the communicated distance and load information as described above.  
           [0018]    Another distinguishing feature of the system of the present invention over known systems is that the mapping result, which associates client clusters with content servers is client-dependent. According to the invention, the mapping depends upon the computation of network distances. As such, the computed network distance may be different for each client in the network.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]    The foregoing features of the present invention will become more readily apparent and may be understood by referring to the following detailed description of an illustrative embodiment of the present invention, taken in conjunction with the accompanying drawings, in which:  
         [0020]    [0020]FIG. 1 is a diagram illustrating a network system according to the present invention;  
         [0021]    [0021]FIG. 2 is a time-line for describing the timing of the data flow from the content servers to the redirection server;  
         [0022]    [0022]FIG. 3 is an illustration of a portion of a binary tree for clustering a plurality of clients according to the present invention; and  
         [0023]    [0023]FIG. 4 is a flow map which characterizes the data flow in the system and for determining the probabilities included in the mapping table.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0024]    General Overview  
         [0025]    The present invention provides a scalable system for resolving DNS queries in a client-dependent manner. In accordance with the invention, software means is provided at a redirection server for responding to client DNS requests by identifying one or more content servers in the network that satisfy the client request with minimum round trip time, while respecting each content server&#39;s service capacity, referred to herein as best-performing content servers. To prevent the situation where a content server&#39;s service capacity may be exceeded, the invention assigns selection probabilities to each content server based on the data flow of the network.  
         [0026]    The system and method achieves the general objective of locating best-performing content servers by periodically performing a clustering/mapping operation in real time at a redirection server. The clustering/mapping operation first performs a clustering operation which creates client clusters (i.e., groups of clients having similar network distance properties), the client clusters are then input to a mapping operation which maps or associates the client clusters with best-performing content servers in the network. The clustering operation generally includes the steps of periodically collecting at the redirection server, load and distance information from each content server in the network and using the collected distance and load information to partition clients into groups or clusters according to CIDR address prefix similarity. Upon completing the clustering operation, the created client clusters are then provided as input to a mapping operation which maps or associates the client clusters to best-performing content servers in the network. A content server is considered to be best-performing with respect to a client cluster by having low round trip time in response to content requests from clients located within the client cluster. The mapping operation further assigns a selection probability to each identified best-performing content server such that the assigned probability ensures that the content server will be selected in a manner which will not exceed the content server&#39;s maximum service (i.e., load) capacity.  
         [0027]    The map which is generated by the mapping operation consists of a plurality of rows of the general form:  
                                                                     CLIENT CLUSTERS INPUT           BEST   CONTENT SERVER       FROM THE CLUSTERING   CLIENT CLUSTER   DOMAIN   PERFORMING   SELECTION       OPERATION   ADDRESS PREFIX   IDENTIFIER   CONTENT SERVERS   PROBABILITY                                Y   135.104.23.XX   X   Z    .3       Y   135.104.23.XX   X   Z2   .7        Y2   135.104.21.XX    X2   Z3   .95                  
 
         [0028]    Column 1 of the table illustrates a few representative client clusters (i.e., Y, Y2) created by the clustering operation which is resident at the redirection server. As shown, the mapping associates the created client clusters with best-performing content servers in the network.  
         [0029]    As one example of how the table is used, when a client request for a particular domain “yahoo.com”, for example, is received at the redirection server, the client&#39;s IP address prefix is used as an index to the map to determine the appropriate map row having a CIDR address prefix similar to the client&#39;s IP address. Upon determining the appropriate map row, a best-performing content server selection must be made from the listed best-performing content servers in that row. That is, each map row may typically include two or more best-performing content servers from which one must be selected. To make such a selection, each best-performing content server will have associated therewith a selection probability indicative of the preference for that content server. For example, a content server with an associated selection probability of 0.90 is a more likely selection candidate as a best-performing content server than a content server whose associated selection probability is 0.05. As a further illustration of the process of selecting a best-performing content server from an identified map row, assume, for example, a map row includes three best-performing content servers whose associated selection probabilities are 0.90, 0.04, and 0.06, respectively. A random number between zero and one is chosen in accordance with the selection method. If the random variable is between 0-0.90, the best-performing content server is selected whose associated selection probability is 0.90. Otherwise if the random variable is between 0.90-0.94, the best-performing content server is selected whose associated selection probability is 0.04. Finally, if the random variable is between 0.94-1, the best-performing content server is selected whose associated selection probability is 0.06.  
         [0030]    It is noted that while the invention is primarily directed to identifying best-performing content servers in accordance with DNS type requests, the invention also contemplates existing and future messaging protocols different from the DNS protocol for communicating requests and responses between clients and content servers.  
         [0031]    Referring now to the drawings, in which like reference numerals identify similar or identical elements. FIG. 1 is a block diagram of a public network system  50  according to an embodiment of the present invention. The public network system  50  is a high-speed, high-bandwidth interactive distribution network. For illustrative purposes, public network system  50  is the Internet, an intranet, an extranet or any other known network. Local DN server  54  is one of a plurality of servers which are accessible by clients, one of which is illustrated by client  52 . The public network system  50  further includes a plurality of redirection servers, one of which is illustrated by redirection server  56 .  
         [0032]    It is assumed that clients make domain name (DNS) requests to local DN servers. FIG. 1 shows a local DN server  54  receiving a DNS request from a client  52 . The local DN server  54  is shown forwarding the client request to a redirection server  56 . The redirection server  56  responds to the client request by returning to the client  52  the IP address of a content server  54   a - e  in the network determined to be a preferred content server.  
         [0033]    It is noted that the present invention makes no a-priori assumptions about the location of the respective content servers  54   a - e  in the network. The locations are discovered by collecting network distance information in accordance with the method of the present invention.  
         [0034]    System Operation  
         [0035]    The system of the present invention for identifying a preferred content server in response to a client DN request includes software resident exclusively in the memory of the redirection server  56 , referred to herein as clustering/mapping software  57 , and software resident in the memory of each content server  54   a - e  in the network, referred to herein as monitoring software.  
         [0036]    Generally, the monitoring software provided at each content server  54   a - e  in the network periodically collects network distance information and load information from communicating clients. The clustering/mapping software  57  pulls the distance and load information collected at each content server  54   a - e  to perform a clustering/mapping operation. The clustering operation involves using past and present collected distance and load information to partition the IP address space into groupings, referred to herein as client clusters. An estimate of the network distance of each client cluster to each content server in the network is also calculated. The mapping operation maps each client cluster to one or more preferred content servers in the network. The map is used by the redirection server  56  to service client DNS requests.  
         [0037]    [0037]FIG. 2 is a time-line for describing the timing associated with performing the data flow processes described above. That is, the process of collecting network distance and load information, and the process of performing the clustering/mapping operation. FIG. 2 illustrates a time-line including discrete time intervals where each discrete time interval represents 10 seconds in one embodiment. Other embodiments may define the data acquisition interval to be other than 10 seconds.  
         [0038]    At each discrete time interval, distance and load data is collected by the redirection server. The data collected from each content server  54   a - e  represents distance and load data from clients communicating with the respective content servers  54   a - e . The data collected by the redirection server  56  at each discrete time interval is combined with exponentially smoothed data collected in all previous time intervals. That is, prior to combining present data with previously collected data, the previously collected data is exponentially smoothed. In one embodiment, exponential smoothing of the previously collected data is performed by multiplying the data by an aging or weighting multiplier at each discrete time interval. In a preferred embodiment, the weighting multiplier is set to 0.95 applied each hour.  
         [0039]    In addition to aggregating data by the redirection server at each discrete time interval a clustering/mapping operation is performed at every third time interval in one embodiment as will be described below in Section II.  
         [0040]    The details concerning the described operations of collecting distance and load information at each content server and aggregating that data at the redirection server in each discrete time interval is discussed with reference to Section I entitled “monitoring software”.  
         [0041]    Monitoring Software  
         [0042]    Distance and load information is continuously collected at each content server  54   a - e  from communicating clients. The distance and load information is collected by monitoring software  59  resident in the memory of each content server  54   a - e  in the network. The distance information is collected at each content server  54   a - e  as distance tuples of the general form  
         [0043]    Eq. 1  
         {Timestamp, content server ID, client IP address, network distance} 
         [0044]    where Timestamp is the time at which the network distance measurement was made, cache ID identifies the particular content server making the measurement, client IP address identifies the particular client accessing the content server.  
         [0045]    Network distance may be measured in a number of ways. A preferred method for measuring network distance is by computing the round trip time (RTT), defined as the time taken for a packet to travel from server to client and back again. Two methods for calculating the RTT are described below. It is noted that other approaches for calculating the RTT are within the scope of the present invention, such as, for example, measuring the bandwidth, jitter or packet loss.  
         [0046]    One method for calculating the RTT as a measure of network distance involves utilizing the tcpdump utility which is a standard utility well known in the art. The tcpdump utility monitors all packets being transmitted and received by a host (e.g., content server). One drawback of using the tcpdump utility as a measure network distance is that it can only be computed for those clients accessing the content server. A corresponding benefit of using the tcpdump utility is that it does not involve any additional overhead. That is, the data is already available because it is a pre-existing process that occurs irrespective of the present invention. A further advantage of the tcpdump utility is that it is highly adaptive in the sense that it instantly reflects the ever-changing state of the Internet.  
         [0047]    The process by which RTT is measured via the tcpdump utility is as follows. The process measures the TCP three-way handshake for each TCP connection. This handshake consists of a client sending a SYN packet to the host, the content server responds with a SYN packet, and the client responds with an ACK. The RTT is computed as the difference between the transmission time of the host&#39;s SYN packet and the receipt time of the client&#39;s ACK packet. Since both these events occur at the content server, they can be monitored by tcpdump thereby providing one measure of RTT. In addition to measuring RTT, the tcpdump utility can also measure the load on each content server which must be periodically reported to the redirection DN server in addition to the distance information.  
         [0048]    A second method for measuring RTT in accordance with the present invention involves the use of pings. As is well known in the art, a ping is a packet that can be sent from the content server to a specific client in order to determine the network distance to that client. If the client is configured to accept pings it will send back a ping response. In this case, the network distance (i.e., RTT) is determined as the time difference between sending the initial ping and receiving the ping response.  
         [0049]    It is noted that using the ping approach over the tcpdump utility is advantageous in that clients may be actively selected throughout the network to receive the ping, whereas tcpdump only provides information for those clients that are already communicating with the content server. A disadvantage of the ping approach, however, is that it generates additional traffic on the network. A further disadvantage of the ping approach is that many clients are either behind firewalls, and not pingable as a result, or else are configured to not accept pings.  
         [0050]    The present invention is not limited to determining the RTT by employing the approaches discussed above. As an alternative, the present invention contemplates using combinations of the tcpdump and ping approaches as well as alternate methods for determining the network distance.  
         [0051]    In addition to collecting distance information at each content server  54   a - e , load information is collected at each content server  54   a - e  as tuples of the form  
         [0052]    Eq. 2  
         {Timestamp, cache ID, client IP address, number of hits, domain index} 
         [0053]    where Timestamp, cache ID, and client IP address are as defined above, the number of hits is a measure of the number of client accesses in a particular data acquisition interval.  
         [0054]    Domain Indices  
         [0055]    The system of the present invention may conceivably include a large number of domains. Given the large number of domains, the process of identifying the content server having minimum round tip time in response to a client domain name request can be significantly slowed. The present invention addresses this concern by grouping domains which have their content stored on the same subset of content servers  54   a - e  in the network, referred to herein as as domain indices. It is noted that the domains which make up a particular domain index may be wholly unrelated in terms of their subject matter. The sole criterion for determining a domain index is co-location of content on the same subset of content servers  54   a - e.    
         [0056]    The clustering/mapping software, which is described more completely in Section II, maintains a domain list, which is a listing of all domain indices in the system along with those content servers  54   a - e  in the network that store the domain indices. Table I is an exemplary illustration of a representative domain list.  
                       TABLE I                       domain               index   constituent domains in the index   content servers                   0   bell-labs.com, yahoo.com, ebay.com   0, 2, 4       1   yahoo.com, espn.com, nasa.com   2, 3, 4       2   lucent.com, amazon.com   2, 4                  
 
         [0057]    Column 1 of Table I is a domain index identifier. Each domain index is assigned a positive integer value for identifying the domain index. In a practical application the number of domain indices could number in the hundreds. The second column of Table I describes the domain names which make up the domain index. For example, domain index 0 includes three domains, bell-labs.com, yahoo.com and ebay.com. The third column describes the subset of content servers in the network which store the domain index. For example, domain index zero is only stored at content servers 0, 2 and 4 in the network.  
         [0058]    Clustering/mapping Software  
         [0059]    As stated above, the redirection DN server  56  performs a clustering/mapping operation preferably at every third discrete time interval to create a mapping to respond to client DN requests. The clustering/mapping software includes a clustering operation in which client clusters having similar CIDR address prefixes are created and a mapping operation which maps or associates the created client clusters with one or more best-performing content servers in the network. The clustering and mapping operations are described in detail below.  
         [0060]    A. Clustering Operation  
         [0061]    Before the clustering operation can be described it is first necessary to describe a preferred IP addressing scheme. The invention utilizes the classless inter-domain routing (CIDR) style of addressing, which is well known in the art. An example of a CIDR style address is 135.104.0.0/16, which indicates the set of all IP addresses which match the IP address “135.104.0.0” in the first 16 bits (i.e., 135.104). As will be described below, the clustering operation groups clients into client clusters using CIDR style addressing. Thus, the set of client clusters output from the clustering operation will be a list whose elements are of the general form “a.b.c.d/n”. Choosing CIDR style addressing is advantageous in that it allows quick look-ups. In addition, CIDR addressing reflects the structure inherent in the way IP addresses are assigned and managed.  
         [0062]    It is noted that the use of alternative addressing schemes is contemplated by the invention, such as, for example, viewing the IP addresses as integers and forming client clusters consisting of all IP addresses whose integer representation falls between two endpoints.  
         [0063]    The objective of the clustering operation is to partition the total IP address space into smaller clusters or groups where each cluster is defined by a particular CIDR address prefix. Each cluster is made up of clients having IP address prefixes which match the CIDR address prefix of the cluster. In addition to creating client clusters, the clustering operation also provides an estimate of the mean network distance from clients in each client cluster to each content server  54   a - e  in the network.  
         [0064]    The clustering operation, which outputs a set of client clusters, is preferably performed at a prescribed fixed interval (e.g., every 30 seconds which is equivalent to every third data acquisition interval). However, it is noted that the clustering and mapping operations may be programmed to be performed at any interval as determined by the system requirements. It is also contemplated that in addition to performing the clustering operation at prescribed intervals, override means are provided to output a set of client clusters whenever a change in network topology or a content server failure is identified. Referring again to FIG. 2, at each data acquisition interval (e.g., 0, 1, 2, . . . ) the clustering/mapping software resident at the redirection server  56  pulls the distance and load tuples from each content server  54   a - e  in the network (See Eqs. 1 and 2). The distance and load tuples are provided as input to the clustering operation.  
         [0065]    The clustering operation first pre-filters the distance and load tuples prior to calculating three summarized data values. Pre-filtering of the tuple data is performed according to one embodiment by taking the logarithm of the received network distance values. It has been experimentally shown that by first taking the logarithm of the network distance values, better client clusters are output from the clustering operation.  
         [0066]    The clustering operation then sorts the distance and load tuples into groupings or client clusters according to the client&#39;s IP address. For each client cluster which defines a particular IP address prefix, three summarized data values are computed:  
         [0067]    Eq. 3  
         1 st  Summarized data value=Sum of filtered network distances for content server i  
         [0068]    Eq. 4  
         2 nd  Summarized data value=Sum of squares of filtered network distances for content server i  
         [0069]    Eq. 5  
         3 rd  Summarized data value=the total number of received tuples in the present discrete time interval and all previous discrete time intervals  
         [0070]    The three summarized data values (Eqs. 3-5) represent the only stored values in the clustering operation. The manner in which the summarized data values are stored will be described below with reference to a binary data tree structure. In addition to storing the three summarized values, the three values are further used to compute a mean and variance for performing a similarity test between clusters. As will be described below, each cluster represents a node in the binary tree. The similarity test therefore compares sibling nodes in the tree and combines nodes when it is determined that there is sufficient similarity. A mean and variance are computed in each cluster for each content server as:  
         [0071]    Eq. 6  
         Sample mean of network distance of content server i=(Sum of filtered network distances for content server i)/total number of received tuples)  
         [0072]    Eq. 7  
         [0073]    Sample variance of network distance of content server i. The sample variance for n numbers  
         x   1     ,   …              ,       x   n                   is                   1     n   -   1              (         ∑   j          x   j   2       -         (       ∑   j          x   j       )     2     n       )     .                             
 
         [0074]    This can be computed since we are storing the number of received tuples n, the sum of filtered network distances  
       (       ∑   j          x   j       )                         
 
         [0075]    and the sum of the squares of the filtered network distances  
         (       ∑   j          x   j   2       )     .                         
 
         [0076]    The clustering operation stores the three summarized data values (i.e., Eqs. 3-5) in a binary tree data structure as input to the clustering operation. Binary tree data structures are well known in the art. It is further noted that non-binary tree data structures may be used in alternate embodiments, for example, hex, octal, or base 10 trees.  
         [0077]    The clustering operation embodied as a binary tree data structure will now be described in the context of a specific example with reference to FIG. 3.  
         [0078]    [0078]FIG. 3 is an exemplary illustration of a binary tree for describing the clustering operation. As shown in FIG. 3, the root node  21  is labeled “0.0.0.0/0” in accordance with the CIDR addressing scheme, and denotes the set of all IP addresses. The root node has two children  22 ,  23  labeled “0.0.0.0/1” and “128.0.0.0/1”. Although IP addresses are received from each content server with 32-bit precision, as a practical matter since addresses which differ only in the last 8 bits typically belong to the same tightly-coupled network, the 32-bit addresses are reduced to 24 bit precision defining the granularity of all addresses supplied to the clustering operation. 24-bit granularity is derived from the received 32-bit granularity by simply truncating the last 8 bits of received IP addresses. Each leaf node  25   a - j  of the tree represents a particular 24-bit IP address prefix.  
         [0079]    To perform the clustering operation using a binary tree structure such as the one illustrated in FIG. 3, the clustering/mapping software first sorts the distance and load tuple information collected in the present discrete time interval into groupings according to the supplied client&#39;s IP address. For each grouping the three summarized data values (See Eqs. 3-5) are computed. That is, the sum of pre-filtered network distances, the sum of the squares of pre-filtered network distances, and the total number of received tuples is computed and stored at each of the respective leaf nodes  25   a - j  of the binary tree, where each leaf node  25   a - j  represents a particular IP address grouping. FIG. 2 illustrates the three stored values for representative node  25   a . Similar summarized values are stored at each of the respective leaf nodes (i.e., nodes  25   a - j ).  
         [0080]    Table II further illustrates exemplary distance tuples which may be received at the redirection server  56  at a particular discrete time interval. The distance tuples are of the general form defined by Eq. 1. 
                                                     TABLE II                       Distance       Content       network       Tuple   Time-stamp   Server ID   Client IP address   distance                                 1   957197632   1   135.104.23.16   56        2   957197635   1   135.104.23.102   55        3   957197632   1   101.13.12.47   73        4   957197623   2   170.151.22.64   24        5   957197646   2   170.151.22.33   22        6   957197633   3   170.151.22.189   348        7   957197634   3   170.151.22.64   349        8   957197629   3   204.178.224.1   18        9   957197638   3   170.151.22.241   348       10   957197640   4   135.104.23.16   22       11   957197632   4   170.151.22.64   276       12   957197633   4   101.13.12.34   173       13   957197630   4   170.151.22.203   277       14   957197641   5   135.104.23.56   103       15   957197636   5   204.178.224.1   278                  
 
         [0081]    The redirection server sorts the 15 received distance data tuples into groupings defined herein as client clusters characterized by the similarity of their IP address prefixes. For example, Table III. a  illustrates that distance tuples 1, 2, 10 and 14 have respective IP addresses 135.104.23.16, 135.104.23.102, 135.104.23.16 and 135.104.23.56. Each IP address prefix, 135.104.23 is determined to be identical and the distance tuples are therefore grouped as one client cluster identified by the cluster prefix 135.104.23. 
                                                             TABLE III.a                       Distance Tuple   Time-stamp   Content Server ID   Client IP address   network distance   Binary tree node                                1   957197632   1   135.104.23.16   56   135.104.23.0/24       2   957197635   1    135.104.23.102   55   135.104.23.0/24       10   957197640   4   135.104.23.16   22   135.104.23.0/24       14   957197641   5   135.104.23.56   103   135.104.23.0/24                  
 
         [0082]    [0082]                                   TABLE III.b                       Distance Tuple   Time-stamp   Content Server ID   Client IP address   network distance   binary tree node                   13   957197630   4   152.134.72.16   72   152.134.72.0/24                    
         [0083]    [0083]                                                             TABLE III.c                       Distance Tuple   Time-stamp   Content Server ID   Client IP address   network distance   binary tree node                                8   957197629   3   204.178.224.1   18   204.178.224.0/24       15   957197636   5   204.178.224.1   278   204.178.224.0/24                    
         [0084]    [0084]                                                             TABLE III.d                       Distance Tuple   Time-stamp   Content Server ID   Client IP address   network distance   binary tree node                                3   957197632   1   101.13.12.47   73   101.13.12.0/24       12   957197633   4   101.13.12.34   173   101.13.12.0/24                    
         [0085]    [0085]                                                             TABLE III.e                       Distance Tuple   Time-stamp   Content Server ID   Client IP address   network distance   binary tree node                                4   957197623   2   170.151.22.64   24   170.151.22.0/24       5   957197646   2   170.151.22.33   22   170.151.22.0/24       6   957197633   3    170.151.22.189   348   170.151.22.0/24       7   957197634   3   170.151.22.64   349   170.151.22.0/24       9   957197638   3    170.151.22.241   348   170.151.22.0/24       11   957197632   4   170.151.22.64   276   170.151.22.0/24       13   957197630   4    170.151.22.203   277   170.151.22.0/24                    
         [0086]    Each of Tables III a - e  has similar 24 bit IP addresses. Each table represents a client cluster for which the three summarized data values (Eqs. 3-5) will be computed and stored at a corresponding leaf node of the binary data tree. It is further noted that the three summarized data values are computed for each content server in the client cluster. For example, referring to the client cluster represented by Table III. e , three summarized data values will be independently computed for content servers 2, 3 and 4 totaling nine separately computed values.  
         [0087]    Once the binary tree is initially populated with the summarized data at the leaf nodes (i.e., the 24-bit level), the clustering operation utilizes an algorithm whose operation is analagous to performing a pruning operation on the initial binary tree to produce a reduced binary tree. Pruning the binary tree, according to the invention, involves evaluating sibling leaf nodes to determine whether there is sufficient similarity between the nodes. When sufficient similarity is found between two sibling leaf nodes of the tree, those nodes are pruned from the tree and the summarized data associated with each leaf node is aggregated or folded into the parent node. At the end of the clustering operation, when it is determined that leaf nodes can no longer be folded or aggregated into parent nodes, the clustering operation is complete. At this point, the resultant leaf nodes represent the resultant client clusters which are provided as input to the mapping operation. The clustering operation also provides an estimate of the network distance from each client cluster to each of the content servers in the network.  
         [0088]    The test for sufficient similarity for two client clusters (i.e., sibling leaf nodes in the binary tree) involves a two-sample t-test, which is a well known in the art. The statistical t-test determines whether two sets of samples (e.g., two client clusters) have the same mean. This determination is made by checking whether the difference between the means of the two sets of samples is large compared to the variance in the data.  
         [0089]    An example is provided to more fully illustrate the similarity test. Consider two nodes X and Y, where each node represents a client cluster. Let x 1 , . . . , x m  represent m network distances of clients in node X to a selected content server Z in the network, and let y 1 , . . . , y n  represent n network distances of clients in Y to the selected content server Z in the network. The objective of the similarity test is to statistically determine whether the network distance of the two client clusters, X and Y, are sufficiently similar by computing the value T and determining whether T is above or below a predetermined similarity threshold. The value of T is computed using the summarized data values (Eqs. 3-5) computed for each client cluster. The T value is preferably computed as:  
         T   =           X   _     -     Y   _           S   p              1   /   m     +     1   /   m                           where       ,     
            X   _     =         ∑   j          x   j       m                 Y   _     =         ∑   j          y   j       n               S   p     =           (     m   -   1     )          S   X       +       (     n   -   1     )          S   Y           m   +   n   -   2                 S   X     =       1     m   -   1            (         ∑   j          x   j   2       -         (       ∑   j          x   j       )     2     m       )                 S   Y     =       1     n   -   1            (         ∑   j          y   j   2       -         (       ∑   j          y   j       )     2     n       )                             
 
         [0090]    Two client clusters are declared to be sufficiently similar if the value of T is computed to be below some predetermined threshold.  
         [0091]    In addition to evaluating leaf nodes in accordance with the equation above, it is noted that the clustering algorithm further provides capabilities for manually determining apriori that two sibling should either always be combined (i.e. folded into a parent node) or never be combined.  
         [0092]    At the completion of the clustering operation when leaf nodes can no longer be aggregated or folded into parent nodes as determined by the sufficient similarity test, the resulting leaf nodes represent resultant client clusters to be provided as input to the mapping operation. Table IV illustrates an exemplary output of the clustering operation.  
                           TABLE IV                               MeanNetwork                   distance from client                   cluster to content   testing index to       client cluster   content server   server   content server                   X CIDR prefix =   1   10    9       135.104.23.xx       X   2   15   15       X   3   20   14       X   I   23   23       X   I + 1   44   44       :   :       X   N       :       :       Y CIDR prefix =   1    8    8       135.104.21.xx       Y   2   14   12       Y   3   21   21       Y   I   11   11       Y   I + 1   46   46       :   :       Y   N                  
 
         [0093]    Two exemplary client clusters X and Y (i.e., leaf nodes of the reduced binary tree) are shown in Table IV, i.e., as a partial output from the clustering operation. Within each client cluster, data is organized by content server. For each (cluster, content server) pair, a mean network distance is computed between the CIDR prefix address representing that client cluster and the respective content server. For example, referring to the first row of Table IV, client cluster has a CIDR prefix of 135.104.23.xx, the mean network distance between clients in that cluster and content server 1 is determined to be 10. The network distance was previously described as being computed as the round trip time (RTT). This value is further modified prior to being provided as input to the mapping operation. Specifically, a testing index value is computed from the network distance and provided to the mapping operation. The testing index is computed in one embodiment as:  
         Testing                 index     =       (     Network                 distance     )     *     (     1   -     1       #                 distance                 tuples           )                             
 
         [0094]    In the case where the number of hits (i.e., received distance tuples) at a content server is very large, the square root term becomes insignificant, resulting in a testing index value identical to the network distance value. However, in the case where the number of hits at the content server is small, the square root term becomes significant, thereby reducing the testing index to a value less than the network distance value.  
         [0095]    The primary objective of computing a testing index value as a modification to the network distance value is that for a content server receiving less than the average number of distance tuples over time as compared with other content servers in the network, the likelihood of that content server being selected as a best-performing content server is diminished. This inequity arises strictly as a consequence of the content server receiving an insufficient amount of monitoring data (i.e., distance tuples from clients) to make a proper determination as to the server&#39;s candidacy as a best-performing content server. To address this inequity, the testing index value is computed which raises the selection probability of a content server receiving fewer distance tuples over time than average. It is further noted that the testing index equation recited above represents one exemplary approach for addressing the aforementioned inequity. Other computations for addressing the inequity are within the scope of the present disclosure.  
         [0096]    B. Mapping Operation  
         [0097]    As stated above, the clustering/mapping software operates in two stages. The first stage, clustering was previously described. As stated above, the output of the clustering operation (See Table IV) is provided as input to the mapping operation.  
         [0098]    After determining the client clusters, a mapping operation is performed. The mapping operation pairs each identified client cluster with one or more {preferred content server, domain index pairs} in the network where each content server is assigned a selection probability as illustrated in Table IV.  
                           TABLE IV                        client   Preferred Content Server,   Computed   Assignment       cluster I   domain index pair   Testing Index   Probability                   1   1, 12   10   .90       1   2, 31   15   .05       1    3, 222   20   .05       2   1, 14    8   .92       2   4, 42   14   .04       2   6, 36   21   .04                  
 
         [0099]    Table IV is an exemplary table for illustrating the output of the mapping operation. The first column of Table IV identifies the particular client cluster. Two client clusters, clusters 1 and 2 are shown in the table for simplicity. Each client cluster contains a row for each preferred content server, domain index pair identified by the mapping operation as having a low round trip time. For example, row 1 of Table IV identifies content server, domain index pair: (1,12). That is, for client cluster 1, content servers 1 is identified as having a low round trip time with respect to the client cluster and further stores domain index 12. Also shown at column 4 of Table IV is a selection probability which is assigned to each preferred content server in the table to ensure that the respective maximum service capacities of each content server is never exceeded. This is to prevent a situation where a content server is continuously selected as a consequence of having the minimum testing index value. For example, referring to the first three rows of Table V, it is shown that client cluster 1 has associated therewith three preferred content servers 1, 2 and 3. Content server 1 has the smallest computed testing index, i.e., 10. Each client request associated with client cluster 1 would select content server 1 in each case as the content server having the smallest testing index. This is undesirable as it may lead to congestion at content server 1, and may further exceed its service capacity. To circumvent this occurrence, selection probabilities are assigned to each content server to distribute the request load in a manner which ensures that the respective network capacities of each content server will not be exceeded. As shown in the table, while the testing indices associated with content servers 2 and 3 are higher than the testing index of content server 1, they are selected 0.05 percent of the time, respectively based on their respective selection probabilities. The manner in which the selection probabilities are determined will now be described.  
         [0100]    The selection probabilities are assigned by performing a min-cost network flow optimization routine which attempts to route network traffic in a more globally optimal manner to prevent or minimize network congestion at each content server in the network. The min-cost network flow optimization routine first computes a flow map which characterizes the data flow in the network. The flow map considers system constraints including bounds on the content servers to not receive more traffic than they can handle. It is further noted that such server capacity constraints can be manually altered to achieve desired effects in traffic flow. Other system constraints which may be incorporated include setting threshold capacities such that traffic originating from a domain/cluster (e.g., domain X/cluster Y), will never be sent to a content server whose round trip time is much worse than (X,Y)&#39;s preferred content server. For example, a threshold for much worse can be quantitatively determined to be no more than 2 times the round trip time (RTT).  
         [0101]    Other system constraints include forcing traffic for a domain/cluster to be split among at least three or more content servers.  
         [0102]    Other contemplated system constraints include artificially adjusting the maximum service capacities of the content servers to lower values to ensure that each content server will always have a residual capacity to respond to high-traffic bursts.  
         [0103]    [0103]FIG. 4 is a flow map which characterizes the data flow in the system. FIG. 4 also determines the probabilities shown in the mapping table, as illustrated by column 4 of Table V. The flow map of FIG. 4 includes a first column of nodes  32   a - f  and a second column of nodes  34   a - g . Nodes  32   a - f  represent (client cluster, domain index) pairs. These nodes can be considered demand nodes in that each pair defines a requestor (i.e., client cluster) and the requested content (i.e., domain index). Nodes  34   a - g  represent all content servers in the network and can be considered resources for satisfying the requesting nodes  32   a - f . The directed arrows represent the flow from demand nodes  32   a - f  to resource nodes  34   a - g . Each directed arrow defines a distance from a demand node (client cluster) to a resource node (cache). These distances may be obtained directly from the output of the clustering algorithm. For example, directed arrow  36  is assigned a distance of 20 from the clustering algorithm.  
         [0104]    It is an objective of the assignment operation to push flow from the demand nodes to the resource nodes in such a way that the majority of flow is conducted along directed arrows whose distance values are small thereby promoting minimum round trip time. However, as an additional consideration, it must be ensured that the resource nodes on the right  34   a - g  are not overloaded.  
         [0105]    Promoting minimum round trip time while preventing overloading at the content servers is achieved in accordance with the assignment operation by assigning a capacity value to each resource node  34   a - g  on the right defining each node&#39;s service capacity. Second, each node on the left  32   a - g  is assigned a demand value which defines the amount of demand from each client cluster for each domain. This value is determined from the aggregation of the number of hits emanating from that client cluster for each domain over time. The hit total provides a measure for estimating future demand. The demand, capacity and directed arrow distance measurements comprise the inputs required by the flow map to assign content servers which reduce round trip time while preventing congestion.  
         [0106]    More particularly, the problem to be solved may be characterized as one of assigning content servers in the network such that the total number of hits assigned to any of the content servers on the right is less than their capacity, while ensuring that most of the assignments minimize the network distance. We solve the problem using a conventional minimum-cost flow algorithm. For a more detailed description of a minimum cost flow algorithm, see A. V. Goldberg, “An efficient implementation of a scaling minimum-cost flow algorithm”, Journal of Algorithms, volume 22, pages 1-29, 1997. The solution of the minimum-cost flow algorithm is a set of probabilities for choosing content servers from the mapping as illustrated in Table IV above.  
         [0107]    Time-to-live Calculation  
         [0108]    The embodiments described above all share as a primary objective the goal of returning to a client the IP address of a content server considered closest as defined by the invention. Generally, client DN requests generally involve accessing websites having multiple pages where each page access requires a new DNS request for a new connection. Given the fact that the answer returned from the redirection server will in all likelihood be the same for each request in a given user session, it is apparent that this request process is inefficient. Accordingly, the invention attempts to obviate this inefficiency by returning the first result to the client with a corresponding time-to-live field which informs the client&#39;s browser the time for which the initial response remains valid. In other words, an efficiency is realized in that the browser need not make redundant DN requests for the same content for a stipulated time as defined by the Time-to-live calculation:  
         TTL   ′              10   6     *     (     DNS                 server                 load     )             (     m      s     )     2          N                   m   ′                     sample mean of network distance to chosen server               s2   ′                     sample variance of network distance to chosen server                           
 
         [0109]    The time-to-live calculation effectively outputs larger time-to-live values in the case where the confidence that a content server is a preferred content server is high. Alternatively, the time-to-live calculation outputs a smaller time-to-live result in the case where there is less confidence in a content server being preferred.  
         [0110]    It will be understood that various modifications may be made to the embodiments disclosed herein, and that the above descriptions should not be construed as limiting, but merely as exemplifications of preferred embodiments. Those skilled in the art will envision other modifications within the scope and spirit of the claims appended hereto.