Patent Publication Number: US-7917649-B2

Title: Technique for monitoring source addresses through statistical clustering of packets

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to computer and communications networks and, more particularly, to a technique for monitoring source addresses through statistical clustering of packets. 
     BACKGROUND OF THE INVENTION 
     Denial of Service (DoS) attacks, especially Distributed Denial of Service (DDoS) attacks, pose a serious threat to the availability of internet services. DoS attacks typically consume the resources of a remote host or network, thereby limiting and/or blocking legitimate users&#39; access. Such attacks can result in significant loss of time and money for many organizations. 
     DDoS attacks are among the hardest network security problems because they are simple to implement, difficult to prevent, and very difficult to trace. In order to conceal the origins of attacks and to coax uncompromised hosts into becoming reflectors, DDoS attackers typically spoof their IP packets by randomizing the source address fields. Further, an attacker need not be operating from a single machine; he may be able to coordinate several machines on different networks to launch the attacks. 
     Many solutions have been proposed to prevent and/or trace DDOS attacks. However, these solutions suffer from a number of deficiencies. For example, some solutions propose new protocols or mechanisms to be implemented on all network routers, which is difficult to achieve. Most statistical methods tend to produce a significant number of false positives, therefore are only suitable for tracing instead of prevention of DoS attacks. 
     In view of the foregoing, it would be desirable to provide a technique for preventing and/or tracing DoS attacks which overcomes the above-described inadequacies and shortcomings. 
     SUMMARY OF THE INVENTION 
     According to the present invention, a technique for monitoring source addresses through statistical clustering of packets is provided. In one particular exemplary embodiment, the technique may be realized by/as a method for monitoring source addresses through statistical clustering of packets. The method may comprise identifying at least part of a source address of a packet. The method may also comprise searching at least one recorded source address based on the at least part of the source address, the at least one recorded source address being organized into at least one cluster. The method may further comprise routing the packet if the at least part of the source address falls within one of the at least one cluster, and the one of the at least one cluster contains at least a predetermined number of source addresses. 
     In accordance with other aspects of this particular exemplary embodiment, the method may comprise discarding the packet if the one of the at least one cluster contains less than the predetermined number of source addresses. 
     In accordance with further aspects of this particular exemplary embodiment, the method may comprise creating a new cluster centered around the at least part of the source address and discarding the packet if the at least part of the source address does not fall within any of the at least one cluster. 
     In accordance with additional aspects of this particular exemplary embodiment, the at least one recorded source address may be recorded in at least one hierarchical data structure. 
     In accordance with another aspect of this particular exemplary embodiment, the at least one recorded source address may be linked to the at least one cluster via pointers or references. 
     In accordance with yet another aspect of this particular exemplary embodiment, the at least part of the source address is within a cluster if the at least part of the source address is no more than two standard deviations away from a mean of the cluster. 
     In accordance with still another aspect of this particular exemplary embodiment, the method may further comprise updating the one of the at least one cluster by including the at least part of the source address in the one of the at least one cluster, recalculating a mean and a standard deviation for the one of the at least one cluster, and reorganizing the at least part of the source address and the at least one recorded source addresses based on the recalculated mean and standard deviation. The method may further comprise issuing a warning if the updated cluster has an increased standard deviation. And the method may further comprise limiting the standard deviation to be no less than a predetermined standard deviation value. In addition, the new cluster may be created based on the predetermined standard deviation value. 
     In accordance with a further aspect of this particular exemplary embodiment, the method may comprise discarding at least one of the at least one cluster, where the at least one of the at least one cluster contains less than the predetermined number of source addresses and has not been updated in a predetermined time period. 
     In accordance with a still further aspect of this particular exemplary embodiment, the source address of the packet may be an internet protocol (IP) address. The at least one cluster may have a minimum standard deviation value of 128. And the new cluster may be created based on the minimum standard deviation value. 
     In accordance with another particular exemplary embodiment, the technique may be realized by at least one signal embodied in at least one carrier wave for transmitting a computer program of instructions configured to be readable by at least one processor for instructing the at least one processor to execute a computer process for performing the method as recited above. 
     In accordance with yet another particular exemplary embodiment, the technique may be realized by at least one processor readable carrier for storing a computer program of instructions configured to be readable by at least one processor for instructing the at least one processor to execute a computer process for performing the method as recited above. 
     In accordance with still another particular exemplary embodiment, the technique may be realized by/as a system for monitoring source addresses through statistical clustering of packets. The system may comprise a storage module that records at least one source address, where the at least one source address is organized into at least one cluster. The system may also comprise a processor module that identifies at least part of a source address of a packet, queries the storage module based on the at least part of the source address, and routes the packet if the at least part of the source address falls within one of the at least one cluster and the one of the at least one cluster contains at least a predetermined number of source addresses. 
     In accordance with other aspects of this particular exemplary embodiment, the processor module may discard the packet if the one of the at least one cluster contains less than the predetermined number of source addresses. 
     In accordance with other aspects of this particular exemplary embodiment, the processor module may create a new cluster centered around the at least part of the source address and discards the packet if the at least part of the source address does not fall within any of the at least one cluster. 
     In accordance with a further particular exemplary embodiment, the technique may be realized by/as a system for monitoring source addresses through statistical clustering of packets. The system may comprise means for identifying at least part of a source address of a packet. The system may also comprise means for querying at least one recorded source address based on the at least part of the source address, where the at least one recorded source address is organized into at least one cluster. The system may further comprise means for routing the packet if the at least part of the source address falls within one of the at least one cluster and the one of the at least one cluster contains at least a predetermined number of source addresses. 
     The present invention will now be described in more detail with reference to exemplary embodiments thereof as shown in the accompanying drawings. While the present invention is described below with reference to exemplary embodiments, it should be understood that the present invention is not limited thereto. Those of ordinary skill in the art having access to the teachings herein will recognize additional implementations, modifications, and embodiments, as well as other fields of use, which are within the scope of the present invention as disclosed and claimed herein, and with respect to which the present invention could be of significant utility. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order to facilitate a fuller understanding of the present invention, reference is now made to the accompanying drawings, in which like elements are referenced with like numerals. These drawings should not be construed as limiting the present invention, but are intended to be exemplary only. 
         FIG. 1  is a flow chart illustrating an exemplary method for monitoring source addresses through statistical clustering of packets in accordance with an embodiment of the present invention. 
         FIG. 2  is a flow chart illustrating a maintenance process for source address clusters in accordance with an embodiment of the present invention. 
         FIG. 3  is a block diagram illustrating an exemplary system for monitoring source addresses through statistical clustering of packets in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENT(S) 
     Referring to  FIG. 1 , there is shown a flow chart illustrating an exemplary method for monitoring source addresses through statistical clustering of packets in accordance with an embodiment of the present invention. 
     The exemplary method starts in step  100 . 
     In step  102 , a signal packet may be received by a network element. The network element may be a computer, a server, or a content-aware switch/router having one or more connections to a computer and/or communications network. For illustration purposes, the exemplary method will be described with reference to Internet Protocol Version 4 (IPv4) though the method should be applicable to other communication protocols and future versions of Internet Protocols including IPv6. Also for illustration purposes, the signal packet is hereinafter referred to as “Packet X.” 
     In step  104 , at least part of a source address of Packet X may be identified. The source address may be an identifier string or number, embedded in the packet, that identifies the origin (or sender) of the packet. For example, the source IP address of an IPv4 packet may be extracted from a 32-bit “source address” field in the packet&#39;s header. However, if the packet has been spoofed by a DDOS attacker, the source address may not reflect the true origin of the packet. Depending on network security requirements and/or available resources, a partial or full source address may be necessary. The partial or full source address of Packet X is hereinafter referred to as “Address X.” 
     In step  106 , a data store of existing clusters of source addresses may be queried. According to embodiments of the invention, the network element may maintain a data store of source addresses of packets it has received within a predetermined time period. The data store may be an in-memory type that accommodates high-speed access. The stored source addresses are typically organized in a hierarchical data structure and grouped into statistical clusters. A statistical cluster may comprise a plurality of source addresses that are within one another&#39;s vicinity. Addresses in a cluster may have a distribution that centers around a mean value and spreads out with a standard deviation. The statistical clusters are created, updated and/or eliminated based on a plurality of rules as set forth below. 
     The hierarchical data structure in which the source addresses are recorded may involve the use of hierarchical trees or tables together with data links or pointers. For example, in IPv4, the recorded source IP addresses may be organized based on their subnet classes. One example of the data store records is shown in Table 1. In Table 1, the first row records the Class A Subnet values; the second row records the Class B Subnet values based on their Class A classification; the third row records the Class C Subnet values based on their Class A and Class B classification; and the fourth row records a pointer that indicates to which cluster the IP address belongs. Table 1 may be built to reflect full IP addresses if desired. For example, if a router is small and/or covers a limited range of traffic, it may be configured to keep track of full IP addresses and their clusters. If that is the case, a pointer value may be assigned to each full address in the data store. 
     
       
         
           
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Exemplary Data Store Records of Source IP Addresses. 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 Class A 
                 47 
                 129 
               
               
                 Subnet 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Class B 
                 127 
                 127 
                 128 
                 
                   
                 
                 15 
                 86 
               
               
                 Subnet 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Class C 
                 5 
                 185 
                 32 
                 
                   
                 
                 66 
                 67 
                 96 
               
               
                 Subnet 
               
               
                 Pointer 
                 Cluster 
                 Cluster 
                 Cluster 
                 
                   
                 
                 Cluster 
                 Cluster 
                 Cluster 
               
               
                   
                 II 
                 II 
                 II 
                   
                 V 
                 V 
                 XI 
               
               
                   
               
            
           
         
       
     
     In step  108 , it may be determined whether Address X falls within any existing cluster. According to one embodiment of the invention, an address is considered within a cluster if the address is no more than two standard deviations away from the mean of the cluster. The above-described hierarchical data structure may also be utilized in an efficient determination of where an address belongs. If Address X is within an existing cluster (e.g., Cluster Y), the process may branch to step  110 . Otherwise, the process may branch to step  114 . If the network element has just started operating (e.g., after being reset), no packet may have been recorded and therefore no cluster exists. In such a start-up state, Address X may be considered not within an existing cluster and the process may branch to step  114 . 
     In step  110 , it may be determined whether Cluster Y has a minimum number of addresses. According to embodiments of the invention, it may be mandated that a minimum number of packets must be found in a cluster before any subsequent packets that fall within the cluster can be routed. In other words, at least a predetermined number of packets must have originated from relatively similar source addresses (a cluster) before the network element can ascertain the legitimacy of the cluster. This restriction may effectively block attacks with spoofed addresses since DoS or DDOS attackers typically do not re-use the same source address for multiple packets. On the other hand, this restriction has a minimal impact on legitimate network traffic because most network clients (e.g., IP clients) will re-send the packets a few more times if the first packet fails. In addition, a consistent source address usage would aid tracing of the real source addresses. 
     If it is determined in step  110  that Cluster Y already has a minimum number of addresses, the process may branch to step  112  where Cluster Y may be updated and Packet X may be routed to its destination. Updating Cluster Y may include the steps of (1) recalculating the mean and standard deviation of Cluster Y based on the addition of Address X; (2) setting the pointers or references of all the addresses within the new boundaries of Cluster Y to point to Cluster Y. 
     According to embodiments of the invention, if the standard deviation recalculation increases the size of Cluster Y, a warning may be issued for traceability purposes. The warning issued may be recorded in a read-only medium. If the size of a cluster increases so much that it starts to overlap a neighboring cluster, the two overlapping clusters may be merged to form a new cluster with an updated mean and standard deviation. The pointers of those source addresses that fall within this new cluster may be updated accordingly. 
     According to another embodiment of the invention, if the standard deviation recalculation decreases the size of Cluster Y, those source addresses that happen to fall outside the shrunk cluster may be set to point to no cluster. Optionally, a warning may be issued. A “minimum size” restriction may be imposed on the clusters so that their sizes (or standard deviations) never fall below a predetermined value. For example, in IPv4, a restriction may be imposed that a cluster&#39;s standard deviation be no less than half a subnet (i.e., 128). If a recalculated standard deviation falls below 128, the cluster&#39;s standard deviation may be set as 128. 
     If Cluster Y does not have the required minimum number of addresses, the process may branch to step  116  where Cluster Y is updated in a similar way as described above. However, Packet X is discarded without being routed. 
     If Address X does not fall within any existing cluster, as determined above in step  108 , then, in step  114 , a new cluster may be created to center around Address X. According to one embodiment of the invention, the above-mentioned “minimum size” restriction may be imposed at the initiation of a new cluster. For example, in IPv4, a new cluster may be created with Address X as the mean and 128 as the standard deviation. 
     The exemplary method ends in step  118 . This method may be repeated for each incoming packet at a network node. Additional maintenance of the clusters is described below in connection with  FIG. 2 . The additional maintenance process may be implemented in a parallel fashion concurrent with the above-described exemplary method as will be appreciated by those skilled in the art. 
       FIG. 2  is a flow chart illustrating a maintenance process for source address clusters in accordance with an embodiment of the present invention. 
     The maintenance of clusters typically takes place during low-traffic periods of the network. The exemplary process starts in step  200 . 
     In step  202 , it may be determined if there are any partially initiated clusters. A partially initiated cluster is an existing cluster that has not accumulated the required minimum number of packets to become a full cluster. If there is at least one partially initiated cluster, it may be identified in step  204 . All the source addresses within the boundary of the cluster may also be identified. 
     In step  206 , it may be determined whether the identified cluster has been updated in a given time period. If so, the cluster may be marked and retained in the data store. Otherwise, the cluster may be discarded. That is, all the source addresses within its boundary may be erased from the data store. If an hierarchical table is used in recording the source addresses and their cluster pointers, any sub-tables that do not contain any subnets may be deleted. 
     The process may go on until no out-dated clusters (partial or otherwise) are left, if the period of low traffic continues. Then the process may end in step  212 . 
     Referring to  FIG. 3 , there is shown a block diagram illustrating an exemplary system (System  300 ) for monitoring source addresses through statistical clustering of packets in accordance with an embodiment of the present invention. System  300  may be any network element, such as a computer, a server, or a content-aware switch/router, that has packet processing and routing functions. System  300  may typically comprise a processor module  302  and a storage module  304 . A display module  306  and input module  308  may be optional components in System  300 . Processor module  302  may be a central processing unit (CPU), microcontroller, computer or network server that is operatively connected to a network  312  through a network interface  310 . Processor module  302  may be capable of communicating with network  312 , processing signal packets and routing signal packets. Storage module  304  may be a storage device, such as a semiconductor memory, nonvolatile memory, hard drive disk, CD-ROM or similar, that is accessible by processor module  302 . Storage module  304  may hold a data store of source addresses of signal packets that have been received by processor module  302 . Whenever a packet from network  312  arrives at System  300 , processor module  302  may identify the packet&#39;s source address, query the data store in storage module  304 , determine the legitimacy of the packet&#39;s source, and take action based on the above-described clustering algorithm in accordance with the invention. 
     The technique for monitoring source addresses through statistical clustering of packets in accordance with the present invention as described above may be implemented on a variety of network elements as a security feature, on top of which additional tools may be built to integrate with intrusion detection systems (IDS&#39;s) and network monitoring systems. The technique in accordance with the invention allows logging, alerting and/or disposal of suspect network traffic with minimal overhead cost or network disruption. 
     At this point it should be noted that the technique for monitoring source addresses through statistical clustering of packets in accordance with the present invention as described above typically involves the processing of input data and the generation of output data to some extent. This input data processing and output data generation may be implemented in hardware or software. For example, specific electronic components may be employed in a computer and/or communications network or similar or related circuitry for implementing the functions associated with statistical clustering of packets in accordance with the present invention as described above. Alternatively, one or more processors operating in accordance with stored instructions may implement the functions associated with statistical clustering of packets in accordance with the present invention as described above. If such is the case, it is within the scope of the present invention that such instructions may be stored on one or more processor readable carriers (e.g., a magnetic disk), or transmitted to one or more processors via one or more signals. 
     The present invention is not to be limited in scope by the specific embodiments described herein. Indeed, other various embodiments of and modifications to the present invention, in addition to those described herein, will be apparent to those of ordinary skill in the art from the foregoing description and accompanying drawings. Thus, such other embodiments and modifications are intended to fall within the scope of the following appended claims. Further, although the present invention has been described herein in the context of a particular implementation in a particular environment for a particular purpose, those of ordinary skill in the art will recognize that its usefulness is not limited thereto and that the present invention can be beneficially implemented in any number of environments for any number of purposes. Accordingly, the claims set forth below should be construed in view of the full breadth and spirit of the present invention as disclosed herein.