Abstract:
A detection and response system including a set of algorithms for detecting within a stream of normal computer traffic a subset of (should focus on network traffic eliciting a response) TCP or UDP packets with one IP Source Address (SA) value, one or a few Destination Address (DA) values, and a number exceeding a threshold of distinct Destination Port (DP) values. A lookup mechanism such as a Direct Table and Patricia search tree record and trace sets of packets with one SA and one DA as well as the set of DP values observed for the given SA, DA combination. The detection and response system reports the existence of such a subset and the header values including SA, DA, and multiple DPs of the subset. The detection and response system also includes various administrative responses to reports.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application relates to application Ser. No. 10/629,175, filed Jul. 29, 2003, assigned to the assignee of the present invention and incorporated herein in its entirety. 
     The present invention relates to application Ser. No. 10/896,680 assigned to the assignee of the present invention and filed concurrently herewith. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to computer networks in general and in particular to detecting and managing malicious traffic in said computer networks. 
     2. Prior Art 
     The worldwide web (WWW), better known as the Internet, is fast becoming the premier computer network for communicating both private and public information. The Internet is an open network that can be accessed by anyone using primarily a protocol called TCP/IP (Transmission Control Protocol/Internet Protocol) or other protocols. Because of its openness computers on private networks (intranets) are susceptible to malicious attacks by hackers. Computers have become the main instrument of communication for business and government agencies. For example, many business and government agencies use computers and computer networks to link remote offices, share data and other resources among employees within an office or campus, communicate with customers via electronic mail, reach new customers via electronic mail, provide information via web sites, etc. 
     Because businesses, governments and individuals rely heavily on computers and the Internet malicious attacks could result in catastrophic economic loss or embarrassment. As a consequence computer security has become a major concern of business, government and individuals using the computer as a major communication vehicle. 
     There are several ways in which hackers may elect to inflict malicious attacks on computers. One way is to disrupt computers&#39; operation by disseminating programs that take unauthorized control of a computer&#39;s operating system. Another way is to discover confidential information about assets in the computer or a subnet in the network for the purpose of deleting data, modifying date and/or copying data. Any of these actions could adversely affect a home, business, or governmental agency network. 
     There are several techniques used by hackers or attackers to gather information on network devices. Port scanning is one of the techniques used by hackers or attackers to gather information relative to one host or several hosts. When several hosts are involved the technique is referred to as Distributed Port Scanning. With port scanning, the hacker uses legitimate TCP/IP (Transmission Control Protocol/Internet Protocol) commands to gather information. In this regard the hacker issues a SYN (Synchronize) packet masquerading as a station wishing to initiate a TCP session. Alternatively, the hacker may issue an RST packet. In either case the hackers hope a non-suspecting host responds with packets from which information about the host can be gleaned. There are many other combinations that can be used for port scanning. Such combination includes SYN/ACK, FIN, Null Christmas tree packets, etc. In fact, any protocol requiring a response may be used for port scanning. 
     The prior art has recognized the importance of computer networks and has provided intrusion detection systems to protect them from hackers. Examples of prior art detection systems can be found in U.S. Pat. Nos. 6,405,318; 6,363,489; 6,477,651; 6,279,113; 6,282,546; 5,991,881; 6,338,141; 5,414,833; and 5,983,348. Even though the Intrusion Detection System (IDS) described in each of the patents works well for its intended purpose it does not work well in detecting scanning activities. 
     It is believed the inability of prior art IDS to detect scanning of the network is based on the fact that they use a library of known patterns of header values or payload bytes. Packet headers or payloads in received packets are compared to known patterns which can be regular expressions or fixed values. The drawback with this approach is that such libraries cannot fully include reconnaissance or scan patterns not seen before. As a consequence some prior art IDS cannot protect against novel reconnaissance traffic. 
     Even the IDS technologies that boast the ability to detect scanning only mean that they can discover if many packets (some threshold) have been sent to a single destination address. They are unable to correlate scans using fewer packets (than the threshold) and further hampered when multiple destinations and formats are used. 
     In view of the above, prior art techniques and apparatus do not provide the network administrator with sufficient power to defend a computer network against reconnaissance or scan traffic. As a consequence a system and method, such as those described herein, are required to provide the additional protection. 
     SUMMARY OF THE INVENTION 
     It is believed unauthorized entries to network devices, such as hosts (computers), are done in two stages, namely reconnaissance and attack. During the reconnaissance stage information about computers is gathered. The gathered information is used to disrupt computing activities during the attack stage. 
     The present invention detects unauthorized scanning (also called reconnaissance) during the reconnaissance stage and takes corrective action set forth herein. 
     The system includes a programmed general processor, such as the PowerPC, or specialized processors, such as the PowerNP coacting with a lookup and recording structure to detect within high bandwidth traffic TCP/IP SYN (Synchronize) or RST (Reset) packets characterized by one Source Address (SA), one or a few Destination Addresses (DAs) and a few (less than 4) or many (greater than 4) Destination Ports (DP). TCP/IP packets meeting these characteristics are flagged and reported to administrative management for further action. 
     In one embodiment the lookup and recording structure includes a Direct Table (DT) and associated Patricia trees. There are two tiers of Full Match (FM) lookup. The first tier uses a hash of the SA and DA, in the portion of a received packet known as the key or header, to index into a slot of a first Direct Table. Selected bits from the DA and SA are used to walk the tree associated with the slot until a leaf is reached. The SA and DA in the leaf are compared with SA and DA in the key and if match the DP in the key is compared with a link list of DPs for the same SA and DA. If the DP is new (i.e. No match found) it is added to the list. When the number of DPs, in the list, ≧n, the hash of the SA is used to do a second tier lookup in a second Direct Table and associated Patricia tree. The lookup in the second table is similar to the lookup in the first table. If the SA in the leaf of the second table matches the unhash SA the DAs and DPs in the leaf are examined and if DAs≧n and the DPs≧(equal or greater than) n an alarm is sounded declaring the packet with the SA and DA as an unauthorized scanning or reconnaissance packet. In response to the alarm the network administrator takes action to limit action of the SA from which the packets are launched. In one embodiment of the invention n is set to 4. 
     In an alternative embodiment, one could use a ratio of destination addresses to ports searched to determine whether this traffic is malicious or not. This traffic ratio is actually a multiplier that is the number of unique DAs multiplied by the number of unique DPs and ensuring the product is greater than 10 (or another number decided upon). This would allow smaller scans to be discovered, but that are initially crafted to evade even low threshold detectors. This is a technique often employed by attackers, sending recon packets very slowly, as to evade detection. 
     In the previously described alternative embodiment the static number (like 4) would not be used, but rather the multiplier, compared to a static number (like 10). Input to the hash would then consist of the SA only. In each leaf, a list of DAs associated with the SA would be kept. Each DA in the list would be associated with a list of DPs. This larger leaf would allow detection of slow scans to many DA and DP combinations from one or a few SAs. The detection of these scans would be much the same, except instead of comparing the static number (like 4), a count of unique DPs per a DA would be obtained and then multiplied by other DAs (associated with the one SA). The response to such scans with multiple DAs and DPs for a particular SA would continue in the manner described above. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows high level components of a network and in particular some potential logical locations of the present invention as detection mechanism. 
         FIG. 2  shows high level components of hardware and in particular some potential logical locations of the components needed to enable the present invention. 
         FIG. 3  shows a flowchart for the detection portion of the present invention. 
         FIG. 4  shows a flowchart for portion of the present invention that pertains to the insertion of a new pointer and new leaf in the Direct Table (DT). 
         FIG. 5  shows a flowchart for portion of the present invention that pertains to the insertion of a new Patricia tree in the Direct Table (DT). 
         FIG. 6  shows a flowchart for portion of the present invention that pertains to the insertion of a new leaf and new branch within a larger Patricia tree in the Direct Table (DT). 
         FIG. 7  shows a flowchart for comparing the number of distinct Destination Addresses (DA) in a leaf to a threshold, a portion of the present invention. 
         FIG. 8  shows a flowchart for the periodic deletion of all leaves, trees, and pointers to reset the detection apparatus, a portion of the present invention. 
         FIG. 9  shows a block diagram of a Network Processor in which the present invention can be implemented. 
         FIG. 10  represents IP Datagram Header Format. 
         FIG. 11  represents TCP Header Format. 
         FIG. 12  represents look-up search structure according to teachings of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF INVENTION 
     Computer network administration includes defending network assets from malicious reconnaissance traffic. The reconnaissance traffic in some cases entails a set of packets with one Source Address (SA) corresponding to the attacker, one or a few Destination Addresses (DA) values corresponding to a victim machine or machines to be studied by the attacker, and a few or many Destination Ports (DPs) corresponding to applications that are thought to be possibly vulnerable by the attacker. 
     The administrator may rely upon tools, such as the one disclosed in the present invention, to assist in defending the network. As part of network defense, an administrator may wish to detect such sets of packets as they pass through a switch, router, Network Interface Card (NIC), appliance, or, more generally, a machine capable of classifying, switching routing, policing functions, or other security functions based upon classification results. 
       FIG. 1  shows a highly simplified network  100  in which the present invention can be deployed. The Internet or other network  102  connects to Edge devices  104 . Each edge device might or might not be provided with detection assembly  108  containing an instance of the invention. Edge devices also connect subnets  106 . In turn, two subnets might be connected by a Bridge device  110 . A Bridge device might or might not contain an instance of the present invention  112 . The edge devices may include routers, switches, servers, etc. Because Edge devices and Bridge devices are well known in the prior art, only those portions that include the teachings of the present invention or are necessary to understand the present invention will be discussed herein. 
       FIG. 2  shows block diagram  200  of components within detection assembly  108  of Edge devices or Bridge device. Random Access Memory (RAM)  202  stores updates of information as included in the present invention. A Central Processing Unit (CPU)  204  has access to data stored at configuration in Read Only Memory (ROM)  206  to implement the algorithms of the present invention and thereby update RAM. A Bus  208  is provided for communication of signals among the components. An Input/Output (I/O) adapter  210  manages signaling to and from external devices  212  that might include a general purpose computer with monitor (not shown) observed periodically by a human administrator. 
     In an alternate embodiment Network Processor (NET PRO), such as the PowerNP developed and marketed by IBM, is coupled to the I/O adapter  210 . The Network Processor is placed within the data plane and contains all the necessary components and algorithms for routing packets within the network. The reconnaissance algorithms according to the teachings of the present invention are executed on the Network Processor. Turning to  FIG. 9  for the moment a block diagram of the Network Processor is shown.  FIG. 9  is the same as FIG. 18 of U.S. Pat. No. 6,404,752 which describes the Network Processor and is incorporated in its entirety herein. Because of this incorporation detailed description of the Network Processor can be readily obtained. Therefore, only those features of the Network Processor that are directly related to and used by the present invention will be identified and discussed. The Embedded Processor Complex (EPC) contains a plurality of processors that execute algorithms to route data and detection algorithms according to the teachings of the present invention. As stated herein the detection algorithm detects and identifies reconnaissance packets on the network. Another feature of interest is the data structures shown in the incorporated patent. The data structure includes a Direct Table and Patricia Trees coupled thereto. The data structure and associated tree search engine is used as the lookup and tracking mechanism of the present invention. 
     Referring to  FIG. 3 , a flowchart  300  is shown for the DETECTION algorithm included in the present invention. The start Detection  302  of the algorithm is followed by the arrival of the next packet  304 . The Internet Protocol (IP) packet is tested  306  for being TCP. The test is carried out on bits in the header of the packet. If it is not, the algorithm returns to  304  to await the next packet. If the packet is TCP, then the packet is tested  308  for being SYN. The test is done by examining control bits in the header of the packet. If it is not, then the algorithm returns to  304 . If it is SYN, then the concatenation of SA and DA is fed  312  to a hash function. The value of the hash becomes  314  the index into a Direct Table (DT). The DT slot is tested  316  for already having a pointer to a leaf or to a tree with at least two leaves. If there is no pointer, then the algorithm branches  318  to the INSERTION ON DT algorithm ( FIG. 4 ). If there is a pointer, then the algorithm tests  320  for pointing to exactly one leaf. If the pointer is to a tree with at least two leaves, then the SA, DA concatenation is tested  324  by the Patricia tree algorithm. The SA, DA then tested  330  by the Patricia tree algorithm to try to match the label of an existing leaf. If SA, DA does not match, then the algorithm branches  334  to the INSERTION LARGER TREE algorithm ( FIG. 6 ). If SA, DA does match a leaf in the tree, then the DP of the key is compared  332  to the one or more DP values in the leaf. Going again to  320 , if the DT slot points to exactly one leaf, then the algorithm compares  322  the key SA, DA with the label of that leaf. If the SA, DA do not match  326 , then the algorithm branches to INSERTION NEW TREE  328  ( FIG. 5 ). If the SA, DA do match  326 , then the algorithm branches to a comparison  332  of the DA of the key with the one or more DP values in the leaf. If the key DP matches some leaf DP, then the algorithm returns to  304  to await the next packet. If the key DP does not match any leaf DP, then the new DP is stored  338  in the leaf. The algorithm then goes  340  to the THRESHOLD algorithm ( FIG. 7 ). 
     In addition to the flowchart, the detection algorithm may also be described as follows: 
     The 16 bits (or so) of the hash of SA, DA are used as an index in a Direct Table (DT). DT slots are normal FM slots and can point to no leaf, exactly one leaf, or to a tree with branches and two or more leaves. Each leaf compares the full packet SA, DA to an exact pattern of SA, DA. Again, a leaf contains SA, DA as its label and at least one and possibly several DPs. 
     If upon computation of the hash value for an SA, DA combination it happens that the slot in the DT is empty (points to nothing), then the DT slot is revised to point to a new leaf with label SA, DA and content DP. Else, if the SA, DA slot in the DT has a pointer, then follow it to a leaf or tree. If the slot points to a tree, then this entails testing certain bits in SA, DA until at most one previously seen leaf can possibly fit the SA, DA. In either case, the full SA, DA in the key and the full SA, DA in the leaf are compared. If they are not equal, then the tree must be rebuilt to test for bits in SA, DA to distinguish the new key from previously seen keys. If they are equal, then the leaf truly corresponds to the key and the DP value of the key is compared to the one or several DP values stored in the leaf. If the DP value of the key is the same as a previously stored DP, then do nothing. Else, add the DP value of the key to the one or more DP values stored in the leaf. Also compare the new total number of DP values with a threshold. 
     An alternate representation of the Detection Algorithm is set forth in Table I. The steps 0 through 16 are self-explanatory. Therefore, further description is not warranted. 
     
       
         
               
             
               
               
             
           
               
                 TABLE I 
               
               
                   
               
               
                 DETECTION 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 0. 
                 A next packet arrives. 
               
               
                 1. 
                 If the packet is not TCP, then goto 0. 
               
               
                 2. 
                 If the packet is not SYN, then goto DELETION step 1 (TABLE VI). 
               
               
                 3. 
                 The SA, DA combination (key) from the headers of the packet are 
               
               
                   
                 fed to a hash function. 
               
               
                 4. 
                 The hash value is used as an index into a Direct Table slot. 
               
               
                 5. 
                 If the slot has no pointer (there is no current session with SA, 
               
               
                   
                 DA that hash to the slot), then goto INSERTION ON DT step 
               
               
                   
                 0 (TABLE II). 
               
               
                 6. 
                 If the slot points to a tree (with two or more leaves), then goto 12. 
               
               
                 7. 
                 If the slot points to (exactly) one leaf, compare the SA, DA with 
               
               
                   
                 that of the leaf. 
               
               
                 8. 
                 If the SA, DA of the key and the leaf do not match, goto 
               
               
                   
                 INSERTION NEW TREE step 0 (TABLE III). 
               
               
                 9. 
                 Else, compare sequentially the DP of the packet with the one or 
               
               
                   
                 more DP values in the leaf. 
               
               
                 10. 
                 If the DP of the packet and some DP of the leaf are equal, 
               
               
                   
                 then goto 0. 
               
               
                 11. 
                 Else, store the new DP in the leaf. Goto THRESHOLD (TABLE V). 
               
               
                 12. 
                 Perform bit-test branches in the tree on SA, DA of the key until a leaf 
               
               
                   
                 is reached. 
               
               
                 13. 
                 If the SA, DA of the key and leaf do not match, goto INSERTION 
               
               
                   
                 LARGER TREE step 0 (TABLE IV). 
               
               
                 14. 
                 Else, compare sequentially the DP of the packet with the one or 
               
               
                   
                 more DP values in the leaf. 
               
               
                 15. 
                 If the DP of the packet and some DP of the leaf are equal, 
               
               
                   
                 then goto 0. 
               
               
                 16. 
                 Else, store the new DP in the leaf. Goto THRESHOLD step 0. 
               
               
                   
               
             
          
         
       
     
     Referring to  FIG. 4 , a flowchart  400  for the INSERTION algorithm included in the invention is shown. The start  402  of the algorithm is followed by the creation  404  of a new pointer from the Direct Table (DT) slot to a new leaf. The leaf is labeled  406  with the concatenation of SA and DA. The leaf is used to store  408  the value of the DP in the packet IP header. Then the algorithm branches  410  to DETECTION, previously described. 
     TABLE II is a self-explanatory alternate representation of the Insertion Technique. 
     
       
         
               
             
               
               
             
           
               
                 TABLE II 
               
               
                   
               
               
                 INSERTION ON DT 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 0. 
                 Create a pointer from the DT slot to a new leaf which has SA, DA as 
               
               
                   
                 label and stores DP. 
               
               
                 1. 
                 Goto DETECTION step 0. 
               
               
                   
               
             
          
         
       
     
     Referring to  FIG. 5 , a flowchart  500  is shown for the INSERTION NEW TREE algorithm included in the present invention. This algorithm covers the case that one original leaf is already attached to the DT slot and a second leaf with different SA, DA value but the same hash value is to be added. The start  502  of the algorithm is followed by creation of a new tree  504  with root equal to the DT slot. One branch is attached  506  to the root. Two SA, DA value hash to this same slot. In the manner of the well-known Patricia tree algorithm, the first bit is found  508  in which the two SA, DA labels differ. One value of the test bit at the branch points to the original leaf  508 . The other possible value of the same bit points to the newly added leaf  510 . The newly added leaf has its SA, DA as label and stores the DP of the new key  510 . Then the algorithm branches  512  to DETECTION, previously described. 
     Table III sets forth a self-explanatory alternate representation of Insertion New Tree technique. 
     
       
         
               
             
               
               
             
           
               
                 TABLE III 
               
               
                   
               
               
                 INSERTION NEW TREE 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 0. 
                 Create a new tree with root equal the DT slot and one branch. 
               
               
                 1. 
                 Find first bit in the two concatenations of the two SA, DA 
               
               
                   
                 combinations in which they differ. 
               
               
                 2. 
                 Use that bit in a bit-test branch to the two leaves, one leaf is the 
               
               
                   
                 original and the other is added that has label the new SA, DA 
               
               
                   
                 and stores the DP of the new key. 
               
               
                 3. 
                 Goto DETECTION step 0. 
               
               
                   
               
             
          
         
       
     
     Referring to  FIG. 6 , a flowchart  600  is shown for the INSERTION LARGER TREE algorithm included in the invention. The start  602  of the algorithm is followed by well-known Patricia tree insertion  604  of a new branch and a new leaf. The label of the new leaf is designated  606  as the concatenation of the SA, DA of the key. The DP of the packet is stored  608  in the new leaf. Then the algorithm branches  610  to DETECTION, previously described. 
     Table IV sets forth a self-explanatory alternate representation of Insertion Larger Tree Algorithm. 
     
       
         
               
             
               
               
             
           
               
                 TABLE IV 
               
               
                   
               
               
                 INSERTION LARGER TREE 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 0. 
                 Use conventional Patricia tree algorithm to insert a new bit test 
               
               
                   
                 branch, add one leaf. 
               
               
                 1. 
                 The new leaf has the SA, DA of the new key as label and stores 
               
               
                   
                 the DP. 
               
               
                 3. 
                 Goto DETECTION step 0. 
               
               
                   
               
             
          
         
       
     
     Referring to  FIG. 7 , a flowchart  700  is shown for the THRESHOLD algorithm included in the invention. The start  702  of the algorithm is followed by comparing  704  the number of DP values stored in the leaf to a threshold specified at configuration, typically eight. If the number of (distinct) DP values is less than or equal to the threshold, then the algorithm branches  708  to DETECTION. If the number of DP values is greater than the threshold, then a report is sent  706  to the administrator or administration system. Then the algorithm branches  708  to DETECTION. 
     Table V sets forth a self-explanatory alternate representation of Threshold Algorithm. 
     
       
         
               
             
               
               
             
           
               
                 TABLE V 
               
               
                   
               
               
                 THRESHOLD 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 0. 
                 If the new number of DPs in the leaf is above a threshold T, then 
               
               
                   
                 REPORT the possibility of malicious traffic from the SA. 
               
               
                 1. 
                 Goto DETECTION step 0. 
               
               
                   
               
             
          
         
       
     
     Referring to  FIG. 8 , a flowchart  800  is shown for the DELETION algorithm included in the invention. This algorithm deletes stale entries from the Direct Table/Patricia Tree data structure. The start  802  of the algorithm is followed by incrementing  804  the frame count value by one. Then the frame count is compared  806  to a threshold number of frames such as  1024 . If the frame count is not greater than the threshold, then the algorithm branches  812  to DETECTION. If the frame counts is greater than or equal to the threshold, then the algorithm causes the complete deletion  808  of all leaves, trees (branches), and DT pointers. The algorithm next resets  810  the value of frame count to zero. Then the algorithm branches  812  to DETECTION. Table VI sets forth a self-explanatory alternate representation of the Deletion Algorithm. 
     
       
         
               
             
               
               
             
           
               
                 TABLE VI 
               
               
                   
               
               
                 DELETION 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1. 
                 Increment time counter by 1. 
               
               
                 2. 
                 Compare time counter value to a threshold. 
               
               
                 4. 
                 If the time count is less than or equal to a threshold, then goto 
               
               
                   
                 DETECTION step 0. 
               
               
                 5. 
                 If time counter value exceeds the threshold, then delete all leaves 
               
               
                   
                 and trees from the DT. 
               
               
                 6. 
                 Reset the time count to zero. 
               
               
                 7. 
                 Goto DETECTION step 0. 
               
               
                   
               
             
          
         
       
     
     An alternative embodiment, the count used could be time increments, not number of frames. Those practiced in the art will understand this alternative is a simple change in the DELETION algorithm; Table VII shows a self-explanatory representation of Detection Algorithm based upon time increments. 
     
       
         
               
             
               
               
             
           
               
                 TABLE VII 
               
               
                   
               
               
                 DELETION 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1. 
                 Increment time counter by 1. 
               
               
                 2. 
                 Compare time counter value to a threshold. 
               
               
                 4. 
                 If the time count is less than or equal to a threshold, then goto 
               
               
                   
                 DETECTION step 0. 
               
               
                 5. 
                 If time counter value exceeds the threshold, then delete all leaves 
               
               
                   
                 and trees from the DT. 
               
               
                 6. 
                 Reset the time count to zero. 
               
               
                 7. 
                 Goto DETECTION step 0. 
               
               
                   
               
             
          
         
       
     
     As stated herein once reconnaissance packets are detected in the network, they are reported to the administrator or other management function for further action. 
     The invention includes several possible options for a RESPONSE to the reporting mechanism. In one embodiment, the invention can include alerting an administrator who then might investigate the traffic with given SA, DA, DP of the packets causing the branch to the REPORT state for possibly malicious intent. In a second embodiment, the invention could include dropping all subsequent packets with the same SA, DA, DP. In a third embodiment, the invention could include dropping or rate limiting all subsequent packets with the same SA. In a fourth embodiment, the invention could include some combination of the first three possible RESPONSES. The combination of RESPONSES might change with time. 
       FIG. 10  shows a graphical representation for the IP datagram header format. Because this IP datagram header format is well known in the prior art only fields of the header that are relevant to the present invention will be identified and discussed. The fields of interest are the Source IP Address (SA) and Destination IP Address (DA). The SA identifies the origin of the datagram (packet) whereas DA identifies the end (destination) of said packet. 
       FIG. 11  shows a graphical representation for the TCP Header Format which is nested in the payload field of the IP datagram. The TCP Header Format is also well known in the prior art. Therefore, only the fields and bits which are of interest to the present invention will be discussed further. The fields of interest include Source Port (SP) and Destination Port (DP). As is known to those skilled in the art the SP identifies origination port of a packet, whereas DP identifies end port to which packet is to be delivered. The bits of interest include Reset (RST) and Synchronize (SYN). These bits when set are active and when reset are inactive. The packet identity is determined by setting of the bits. For example, if the SYN bit is set, this would include a SYN packet. The use and function of these bits are well documented in the prior art. Therefore, further discussion is not warranted. 
       FIG. 12  shows a graphical representation of the search facility or search look-up structure according to the teachings of the present invention. It is recognized that other tree structures can be devised by those skilled in the art. Therefore, the graphical representation is only exemplary and does not limit the scope of the invention claimed. 
     The data structure  1200  includes the extraction of the key  1202  composed of the concatenation of SA and DA, 64 bits. It further includes applying a hash function  1204  to produce a shorter index such as 16 or 32 bits. The index is used relative to a Direct Table  1208  to identify an entry that is the root of a Patricia tree  1210 . The Patricia tree has one or more leaves such as  1212 , 1214 , 1216 . Each leaf contains the full SA, DA that hashes to the Direct Table entry as well as one or more DP values that have been observed. 
     Operation: 
     In operation packets received in a device on which the present invention is deployed are examined as follows: 
     It is assumed that the lookup mechanism is logically equivalent to a search tree. In one embodiment, the first branch of the tree simultaneously tests many bits, connecting many conventional trees. The first such branch can be called a Direct Table. 
     The first lookup (first tier) hashes the Source Address (SA) and Destination Address (DA) of a TCP SYN or RST packet to a Direct Table (DT). The index of the DT can be a convenient value (such as 16 bits). The full key is SA, DA, and Destination Port (DP). The leaf matches the packet by SA and DA, then in addition compares DP to a linked list of DPs for the same SA and DA. If the DP is new, then it is added to the list. Periodically (such as once per second) some fraction (such as 1/16) of this lookup is completely deleted. 
     If the number of observed DPs in a leaf is greater than a threshold (such as 4), then the SA, DA, DP combinations of the leaf are hashed to a second lookup (the second tier). For the second lookup, the hash is on the SA only. The number of bits in the DT index can be low (such as eight). If one SA has many DA, DP combinations, then the DA, DP combinations are stored in a leaf. If the number of DA, DP combinations in such a leaf exceeds a threshold (such as four), then SA is suspected of taking part in a Distributed Scan. The entire table in the second tier is purged within a medium period (such as four seconds). 
     The suggested purge intervals are based upon reasoning and real traffic statistics. If the purge intervals are too short, then attacks can be overlooked because a full-fledged attack actually spans several periods. If the purge intervals are too long, then the reaction of the invention to a new attack is delayed. 
     In general, purge intervals can be adjusted. If there are few reported attacks, then the interval can be increased up to a limit. If the are too many reported attacks, then the purge intervals can be decreased to a limit. An exponentially weighted function can be used to increase or decrease periods. The function can be described as follows. Let i denote the index=0, 1, 2, . . . of period P, so P(i) is the ith value in a sequence. P( 0 )=minimum limit. The period adjustment can have the form 
     1. If too few reported attacks, then P(i+1)=(1−K)*P(i)+K*maximum limit. 
     2. Else P(i+1)=(1−K)*P(i)+K*minimum limit 
     Here minimum limit&lt;maximum limit and K is some number with 0&lt;K&lt;=1. 
     Even though the present invention has been described in relationship with TCP/IP protocol, this should not be construed as a limitation on the scope of the invention. The present invention has broad application with many different protocols. It is well within the skill of one skilled in the art to apply teachings of the present invention to detect reconnaissance when other protocols are used. Any such usage and/or application are within the scope of the present invention as set forth in the claims. 
     The foregoing is illustrative of the present invention and is not to be construed as limiting thereof. Although exemplary embodiments of this invention have been described, those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiments without materially departing from the novel teaching and advanced use of this invention. Accordingly, all such modifications are intended to be included within the scope of this invention as defined in the claims.