Abstract:
A method and system for managing one or more web requests provided to a web application from a client computer. An application is responsive to a web request to generate verification data. The application sends a reply to the client to calculate a stamp as a function of the generated verification data. The application is responsive to an additional request from the client including the calculated stamp to determine if it corresponds to the generated verification data. If the calculated stamp corresponds to the generated verification data, the additional web request is submitted to the web application for processing. A Web server subject to a DOS attack will be able to distinguish between genuine users(who generate stamps) and malicious users(who will not generate stamps) and over a period of time be able to restore service to the former and deny to the latter.

Description:
TECHNICAL FIELD 
   The present invention relates to managing service web requests between a server and a client of computer network environments. In particular, this invention relates to source throttling of service requests being provided to a web application. 
   BACKGROUND OF THE INVENTION 
   Web sites, or Internet sites, often provide information, products, services, and the like to their users. Those skilled in the art are familiar with various security risks associated with a type of malicious service request known as a denial of service (DOS) attack. Denial of service attacks can be broadly classified into bandwidth attacks and resource attacks. In a bandwidth attack, the attacker disrupts a web service by supplying a plurality of service web requests (web request), from a client computer to a web server to generate a traffic over-load that clogs links or causes routers near the victim to crash. For example, if a web server serving a particular web site is configured with a wideband communication link such as a T 1  link and an attacker pumps dynamically generated service requests to the web server with ten (10) T 1  links, some of packets will be lost. The only way to respond to such an attack is to collaborate with the Internet Service Provider (ISP) to drop the attacker&#39;s packets at the ingress point into the network. 
   In a resource attack, the attacker does not use up more than your maximum bandwidth, but clogs the web server&#39;s resources so that genuine users cannot get through. Application attacks are the worst of the resource attacks because applications are usually designed to consume all the resources for only a fraction of bandwidth the server can support. If a web site is configured with a three-tiered architecture (e.g., presentation layer; a processing layer, and a database layer), the attacker can easily saturate the application server or database server by sending in a few valid fake requests. A valid fake request is a crafted valid request that does not serve the intended purpose for the service (i.e., concocted, bogus, phony). When a particular web resource is accessed more frequently than it is designed to handle, there will be a loss of service to legitimate users. 
   Consider a particular web server designed to handle a maximum of 4,000 content searches per second. Typically, a content search requires the web server to communicate with an application server that searches a database to retrieve the content specified by a search request received from a client computer. Further, consider that the load on the web server peaks at about 3,000 requests per second during the daytime and drops to about 500 in the early hours of morning. Users usually start with the main page and then perform a couple of searches every session. A valid search request for a nonexistent keyword in the database can be the most expensive as it misses all caches. In the worst attack, the attacker creates the search keywords dynamically. Now, consider a distributed attack is launched against the website and it starts receiving 40,000 searches per second. The website will only be able to respond to about ten percent (10%) of the search request and the chance that a valid user gets a response will be 10% and does not improve until the attack ends. 
   Another type of malicious service request involves a request to transmit unsolicited email (SPAM) through e-mail servers to a plurality of email addresses. The originating party (spammer) of such service request typically uses programs called bots to scour the Web and Usenet newsgroups, to harvest e-mail addresses, or may buy them in bulk from other companies. In a single email, spammers may send the same message to tens and thousands of addresses. As a result, SPAM increases the load on email servers, and drives up operational costs for companies operating such email servers, to process, filter and store emails. 
   Public key cryptosystems have been used to enable secure communication between parities over the Internet. For example, public key cryptosystems provide a means for parties communicating over the Internet to transmit encrypted messages to each other while making it nearly impossible for a third party to obtain and decode the transmitted messages. Most cryptosystems are built around two fundamentally hard mathematical problems: the integer factorization problem, or the discrete log problem. Factoring is the act of splitting an integer into a set of smaller integers (factors) which, when multiplied together, form the original integer. For example, the factors of 15 are 3 and 5; the factoring problem is to find 3 and 5 when given 15. Prime factorization requires splitting an integer into factors that are prime numbers; every integer has a unique prime factorization. Multiplying two prime integers together is easy, but factoring the product is much more difficult. The discrete log problem, in its most common formulation, involves solving for the exponent x in the formula a=b x , where x is an integer, and a and b belong to a finite field F. In other words, it seeks to answer the question, to what power (i.e., x) must b be raised in order to obtain a. Like the factoring problem, the discrete log problem is believed to be difficult and also to be the hard direction of a one-way function. There are other, more general, formulations as well. The most common fields used in computer cryptography are: the field of prime numbers: F(p) where p is prime and the integers 1,2, . . . p−1 are closed under multiplication and addition modulo p; the field of characteristic 2: F(2 n ); the field of irreducible polynomials: F(q n ) where arithmetic is with respective to irreducible polynomial p(x); the Elliptic curve field: F(EC) where EC is an elliptic curve and all the elements of the field are point on the elliptic curve. The finite field F(n) is a Galois field if n is prime or the power of a large prime and all the arithmetic is executed with modular exponentiation. Evaluating the expression b x  mod n is less complex than finding x where a=b x  mod n. For example, it is easy compute 3 6  mod 17 is equal to 15 as compared to computing x is equal to 6 given 3 x =15 mod 17. There are well known chaining algorithms to express x as a sum of powers of 2 and reduce the number of multiplications without generating any intermediate results greater than 2x which makes this very suitable for implementation in digital computers. 
   Various solutions have been proposed to handle surges due to malicious service requests when there is a common pattern in those requests. For example, service requests are frequently submitted to validation, authentication, and access controls to protect the resources and reduce the load on the targets. Thus, there is a need for managing malicious service attacks by verifying the authenticity of web request when access control based on usernames is unavailable such as when a web resource is open to the general public via the Internet. 
   SUMMARY OF THE INVENTION 
   The invention meets the above needs and overcomes one or more deficiencies in the prior art by providing an improved system, method, and computer readable medium for throttling service requests being provided to a web application from a client via a data communication network. In one embodiment, the invention requires the client to generate a stamp for each service request submitted to the web application. Generating the stamp requires the client&#39;s central processing unit (CPU) to expend processing time (i.e., pay a stamp fee) which is negligible when the client is submitting a few service requests. However, as the amount of service requests submitted by the client increases or as the number of email addresses specified in a service request increases, the cost of generating the stamp in terms of CPU processing time increases, which adversely affects the processing speed of the client. Although the client may expend significant processing time generating stamps, the server uses minimal processing time to verify stamps. By requiring the client computer to expend CPU cycles to generate a valid stamp, the invention discourages malicious service attacks due to the decrease in processing power of the client. Moreover, the decrease in processing power of the client decreases the number of malicious service requests that can be submitted by that particular client. As a result, the invention yields significant reductions in operating costs, and yields significant improvements in security. The cost savings are particularly significant for large-scale interactive web based services. 
   In accordance with one aspect of the invention, a computer-readable media having computer-executable components for managing web requests being received by a server from a client is provided. The web requests each include header data that includes message identification (message ID) data identifying unique message data included in the web request and client identification (client ID) data identifying a particular client sending the web request. A sending component sends a reply message to the client for each received web request having different message ID data. The reply message includes the transformation data and instructions to compute stamp data as a function of the transformation data. A generating component generates verification data as a function of the stamp data included in an additional web request received from the client. A comparing component compares generated verification data to the stamp data included in the additional web request received from the client. A processing component processes the additional web request if the stamp data of the additional web request corresponds to the generated verification data. 
   In accordance with another aspect of the invention, a method for throttling a client sending a plurality of content requests to a server is provided. The content request includes message data specifying content for retrieval. The method includes receiving, at the server, one of a plurality of content request from the client. The method also includes transmitting to a client a response message with instructions to compute a stamp as a function of transformation data for each client. The method also includes receiving, at the server, an additional content request having a computed stamp from the client. The method further includes generating verification data as a function of header data included in the received additional content request, and assigning processing priority to received additional messages from the client which have a computed stamp corresponding to the generated verification data. 
   In accordance with yet another aspect of the invention, a method for throttling a client sending a distribution request with a plurality of addresses to a server. The distribution request includes a message for delivery to a destination mailbox. The method includes receiving, at the server, the distribution request from the client. The method also includes receiving, at the server, a distribution request from intelligent clients with a computed stamp. The method includes determining if the distribution request includes the stamp for each address. The method further includes generating verification data as a function of header data included in the received distribution request when the determining indicates the distribution request includes the stamp, and assigning processing priority to received distribution request which have a stamp corresponding to the generated verification data. 
   Computer-readable media having computer-executable instructions for performing methods of managing application windows embody further aspects of the invention. 
   Alternatively, the invention may comprise various other methods and apparatuses. 
   Other features will be in part apparent and in part pointed out hereinafter. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates in block diagram form an exemplary network environment in which the present invention may be utilized to manage service requests submitted to a server from a client computer. 
       FIG. 2  is an exemplary block diagram illustrating components of a client and a server according to one embodiment of the invention. 
       FIG. 3A  is a block diagram illustrating a server executing a throttling application in response to a content request from a client. 
       FIGS. 3B and 3C  illustrate components of exemplary content requests, before and after the addition of stamp data, respectively. 
       FIG. 4A  is a block diagram illustrating a server executing a throttling layer in response to a distribution request from a client. 
       FIGS. 4B and 4C  illustrate components of an exemplary distribution requests, before and after the addition of stamp data, respectively. 
       FIG. 5  is a flow chart illustrating a method for throttling a plurality of content requests being supplied to a server from one or more clients. 
       FIG. 6  is a flow chart illustrating a method for throttling a plurality of distribution requests being supplied to a server from one or more clients. 
       FIG. 7  is a block diagram illustrating one example of a suitable computing system environment in which the invention may be implemented. 
   

   Corresponding reference characters indicate corresponding parts throughout the drawings. 
   DETAILED DESCRIPTION OF THE INVENTION 
   Referring now to the drawings,  FIG. 1  illustrates an exemplary network environment in which the present invention may be utilized, particularly to combat malicious service request being submitted to a web server. A client computer submitting a service request to a server, is engaged by the server in such a way that it must pay a stamp fee in terms of CPU processing time in order to communicate with the server. This stamp fee is determined as a function of either the client Id and the request content, or the destination email Id and the message included in the service request. The cost of the stamp fee is negligible when the client is submitting a limited number of service requests. However, as the number of service requests received from a particular client, and/or the number delivery addresses included in the service request increases, the greater the cost to the client in terms of processing time. In other words, the invention discourages malicious service requests by imposing an artificial slow down of the processing speed of the clients initiating such requests, thereby minimizing the damaging effects of denial of service (DOS) attacks and spam email. 
   In this instance,  FIG. 1  diagrammatically shows cross-internet collaboration between a server and one or more client computers. Such servers provide a user with the ability to access one or more participating web sites or resources. In the example of  FIG. 1 , the invention is described in the context of preventing service request into web applications associated with dynamic web sites allowing the exchange of data between the client computer and the web site. 
   An originating client computer system (client)  162  is coupled to a data communication network  164  in  FIG. 1 . In this example, the network  164  is the Internet (or the World Wide Web). However, the teachings of the present invention can be applied to any data communication network. One or more remote, or destination, client computer systems (remote clients)  166  and one or more servers  170 , also referred to as “web servers” or “network servers, are also coupled to network  164 . In turn, the client  162  can communicate with one or more remote clients  166  (e.g., send e-mail) or access the one or more servers  170  via network  164 . 
     FIG. 1  also illustrates a malicious user at a client computer system  168 . Malicious users, for example, such as the one at client computer  168 , attempt to submit service requests to web servers for a variety of undesirable purposes (e.g., disrupting internet commerce, attempting to guess passwords, mass harvesting of public information such as e-mail addresses, abusing mail services for spam, free advertising in chat rooms, and the like). 
   As shown in  FIG. 1 , the server  170  is coupled a content source  172  such as content database or content server. The server is responsive to a service request from the client  162  to retrieve requested content from the content source  172 , and transfer the retrieved content to the client. Alternatively, the server  170  is responsive to a service request from the client  162  to deliver content (e.g., e-mail) to one or more of remote clients  166 , that correspond to address data include in the service request. For example, the server  170  may be component of a communication network in which e-mail messages from a particular originating client computer are received and processed for delivery to one or more destination clients  166 . Thus, a service request may be a content request to retrieve information from a server  170  such as a content server or a distribution request from a client computer to distribute an e-mail to one or more clients via a server  170  such as an e-mail server. 
   The server  170  may be a front end server that allows communication between itself and client computer systems  162  and one or more back-end servers (e.g., content source). In this example, server  170  and client  162  communicate data among themselves using the Hypertext Transfer Protocol (HTTP), a protocol commonly used on the Internet to exchange information between clients and servers, or the Simple Mail Transfer Protocol (SMTP) commonly used for sending e-mail from an originating client to a destination client. Although the content database  172  is shown as a single storage unit separate from server  170 , it is to be understood that in other embodiments of the invention, content source  172  may be one or more memories contained within or separate from server  170 . 
   The present invention involves the use of a throttling layer  174 , which is coupled to the server  170  for throttling (i.e., controlling) malicious service requests being submitted to a web site by one or more by client computers  162 . In this example, the malicious client, represented by client computer system  168 , attempts to submit multiple bogus service requests to a web application  176  operating on server  170 . As explained in more detail in reference to  FIG. 2  below, after receiving a service request from a client (e.g., client  168 ), the throttling layer  174  requests the client  168  to submit an additional service request that includes stamp data, which is calculated by the client. The throttling layer may be implemented via a stand software alone application, as part of another application, as part of an operating system, or by hardware. After receiving the additional web request, the throttling layer  174  assigns processing priority to additional web requests that include stamp data that corresponds to verification data generated by the server  170 . If a web request is assigned processing priority, the throttling layer  174  submits that particular web request to the web application  176  for processing. On the other hand, if a web request is not assigned priority status, either because the retrieved stamp data does not correspond to the verification data or the client  168  is not configured to calculate stamp data, the throttling layer  174  segregates such additional web request for future processing or excludes such additional web request from further processing. As will be explained in greater detail below, as the number of web requests received by the server  170  increases (i.e., load increases), the processing time the client  168  must employ to calculate the stamp data increases. By requiring the client  168  to calculate stamp data and giving low priority or excluding web requests that fail to include stamp data that corresponds to the verification data generated by the server  170 , the invention effectively manages and/or discourages clients  168  from initiating service attacks and SPAM. Notably, clients computer systems  162 ,  166 ,  168  can be personal desktop computers handheld devices, or any other device configured for submitting web request over a communication network such as the Internet. The system is designed in such a way that the client systems  162  effectively get complete service, the malicious systems  168  little service if they behave themselves by computing stamps and no service if they do not compute stamps and dividing the remaining server resources to client systems  166 . 
   Referring next to  FIG. 2 , an exemplary block diagram illustrates components of a client  202  (e.g., client computer system  162 ) and a server  204  (e.g., server  170 ) according to one embodiment of the invention. 
   A client application  206  allows a user  208  to retrieve HTML documents  210  such as a web page or web form via a communication network  212  (e.g., network  164 ). The client application  206  is executable by the client  202  and responsive to user input for initiating the retrieval of such HTML documents (web form)  210 . One skilled in the art will appreciate that the client application  206  may be a web browser such as the Internet Explorer® browser offered by Microsoft Corporation. Frequently, the user  208  uses the client application  206  to contact a web site to retrieve a web form  210  accepting input data from the user  208 . Alternatively, a client application  206  such as Microsoft Outlook® offered by Microsoft Corporation allows a user to transfer message data (e.g., e-mail) to one or more destination client computers (e.g., remote clients  166 ). 
   A user-interface (UI)  214  linked to the client  202  allows the user  208  to interact with the retrieved web form  210 . For example, the UI  214  may include a display  216  such as a computer monitor for viewing the web form and an input device  218  such as a keyboard or a pointing device (e.g., a mouse, trackball, pen, or touch pad) for entering data into the web form  210 . In other words, the UI  214  allows user  208  to interact with the web form  210  to define a web request, as indicated by reference character  220 . The client may be also be configured to execute a malicious application that generates a plurality of web requests (e.g., service attacks or SPAM e-mail) for submission to a web server  204 . 
   A web application  222  (e.g., web application  176 ) is responsive to web requests received from the client  202 , and executable by server  204 , to perform actions requested by the web requests. For example, the requested action may include retrieving content specified by message data included in the web request for return to client  202  via the client application  206 . In one embodiment, client application  206  uses HTTP to transfer the web request from client  202  and to transport data back to client  202 . Alternatively, the requested action may include distributing message data included in the web request to one or more remote client computers located at addresses included in the service request. The web application  222  can be any program executable by server  204  and capable of interfacing with client  202  via the client application  206 . 
   A throttling layer  224  (e.g., throttling layer  174 ) is executed by the server  204  to throttle web requests before being provided to the web application  222 . That is, the throttling layer  224  intercepts malicious web request received from a client  202  before they have the opportunity to initiate a denial of service attack or SPAM e-mail distribution, and thus lock up valuable server resources. 
   Referring now to  FIG. 3A , a block diagram illustrates components of a throttling layer  224  being executed in response to a content request received from a client  202 . The UI  214  linked to the client  202  allows a user  208  to submit a content request  302  (see  FIG. 3B ), as indicated by arrow  303 , to the server  204  via the client application  206  and communication network  212 . Alternatively, a malicious application  304  executable by the client  202  generates and submits a plurality of content requests  302  to the server  204 . Referring briefly to  FIG. 3B , the components of an exemplary content request  302  are shown. As known to those skilled in the art, a content request  302  (sometimes called HTTP page request) generally has two parts, a header  306  and a body  308 . The header  306  of the content request  302  includes command data  309  (sometimes called a method) that tells the server  204  a specific action it wants to perform. In this case, a Get command is used to request content (e.g., documents) from the particular content server  204  identified by an assigned URL (e.g., &lt;GET msn.com/news/sports/index.html&gt;). The header  306  may also include a message ID  310  and a client ID  312 . The message ID  310  is a unique identifier used to track particular message data  314  included in the body of that particular content request. In this case, message data  314  included in the body  308  of the content request  302  defines the information or content to be retrieved by the server  204 . Each message ID  310  can have a local portion and a domain portion separated by an @ sign (e.g., 12345.67890@host.example.com). The local portion (e.g., 12345.67890) of the message ID  310  is different for different message data  314 , and the domain part is usually the name of the host (i.e., the computer sending mail or requesting data) that generated the message ID. In other words, although a first content request and a second content request are received from the same particular client  202 , if message data  314  included in the body  308  of the first content request is different from message data  314  included in the body  308  of the second content request, each of the first and second content request will include different message IDs in their headers. The client ID  312  identifies the location (i.e., IP address) from which the content request  302  was initiated. For example, when the server  204  receives a content request  302 , the originating IP address of the request is made available by the simple act of connecting to the server  204  with the TCP/IP or Transmission Control Protocol/Internet Protocol. The term “TCP/IP” refers to the layered combination of TCP and IP protocols, on which many higher-level application protocols (like HTTP) have been built. The following is an example of content request  302  sent by the client  202  to a server  204 :
         GET http://msn.com/news/sports/index.html HTTP/1.0   Host: 101.103.5   User-Agent: Mozilla/4.0(compatible; MSIE 4.0; Windows NT; . . . /1.0)   MessageId: 12345.67890@host.msn.com       

   In this example, “msn.com” identifies the server where the particular document (i.e., content) is stored. The “/news/sports/” is a folder, and identifies the location of the requested content on the server  204 . The “index.html” indicates the particular file or document to retrieve. The http://msn.com/news/sports/index.html is the document requested and the protocol is HTTP version 1.0. The User-Agent header contains information about the client program originating the request, and can be used, for example, to identify the browser software. The HOST 101.103.5 is the server ID, and “2345.67890@host.msn.com” is the message ID. The client ID can be the IP address of the client that the server can find out from the TCP/IP connection. 
   Referring back to  FIG. 3A , the throttling layer  224  is responsive to the content request  302  received from the client  202  to execute a sending component  316  to send a reply message, as indicated by arrow  318 , to the client  202  for each content request  302  having a different message ID  310  or a different client ID  312 . The reply message includes transformation data and instructions for the client  202  to compute stamp data as a function of the transformation data. The client application is responsive to the transformation data included in the reply message to calculate a first stamp data value a s  and a second stamp data value b s  by executing transformations known as hash functions to calculate the following first and second stamp data values:
 
 a   s   =f ( M )  (1),
 
 b   s   =g ( C )  (2),
 
where M is the message ID  310 , C is the client ID  312  or IP address of the client  202  submitting the request, and f and g are inexpensive hash functions. The reply message also includes data defining a finite field F from which a and b must be members. In this embodiment, the first and second stamp data values a s , b s  are determined based on finite-field (i.e., Galois field) arithmetic. As described above, a Galois field is a finite field with p^n elements where p is a prime integer. As will be explained below in more detail, the size (i.e., number of elements) of the Galois field is determined by the 2 n  elements, where n is the number of bits the server uses to calculated the stamp data values a and b and is determined by the load the server  204  is experiencing
 
   In one preferred embodiment, the client application  206  running on the client  202  is configured to generate a stamp by first executing an algorithm in an iterative fashion to identify a mathematical relationship between the two stamp data values a s , b s . For example, the client application  206  is configured to solve for the exponent value x in the following equation:
 
 a   s   x   =b   s   (3).
 
   To calculate the exponent value x, the first verification data value, a s , is iteratively raised to the power of integers (1, x−1) until the equation is satisfied, which requires significant iterations when x is large. After determining the exponent value x that satisfies the above equation, the client  202  generates a stamp which is a tuple of (a, x). Thereafter, the client application  206  uses, for example, JavaScript to prepend the header  306  of the content request  302  to include the generated stamp  324 , and submits a prepended content request  326  (see  FIG. 3C ), as indicated by arrow  327 , to the server  204 . A stamp is generated for each client computer  202  submitting an identical content request (e.g., contains same message ID  310  and client ID  312 ). Alternatively, if content requests  302  originate from the same client  202  but lexical modification are made to message data included in the body of each of the content requests  302  to escape a rate filter (e.g., by adding white spaces), a stamp is generated for each content request including modified message data  314   
   The throttling layer  224  is responsive to the stamp  324  included in the header of the prepended content request  326  to execute a generating component  328  that generates verification data values as a function of header data included in the content request  302 . The generating component  316  executes the same hash functions to calculate the following first and second verification data values:
 
 a   v   =f ( M )  (4),
 
 b   v   =g ( C )  (5),
 
where M is the message ID  310 , C is the client ID  312  or IP address of the client  202  submitting the request, and f and g are inexpensive hash functions. The generating component  320  stores the calculated verification data values in a memory  318 .
 
   The throttling layer  224  then executes a comparing component  329  to verify that the first stamp data value a s , included in the header of prepended content request  326  is equal to the first verification data value a v , and that the first verification data value a v  raised to the exponent value x equals the second verification data value b v . If the first verification data value, a v , raised to the exponent value, x, equals the second verification data value, b v , the throttling layer  224  executes a processing component  330  to submit the prepended content request  326  to the web application  222 . The web application  222  is responsive to the prepended content request  326  to retrieve the requested content from a content source  332  such as a content database or content server and transfers the request content to the client as indicated by arrow  334 . On the other hand, if the first verification data value, a v , raised to determined exponent value x is not equal to the second verification data value, b v , or the client application  206  is not configured to generate a stamp, the processing component  330  transfers the prepended content request  326  to a general data pool  336  for future processing. For example, the prepended content request  326  stored in general data pool  336  are submitted to the web application  222  in a first in first out (FIFO) manner for processing after all prepended content requests having stamp data that corresponds to calculated verification data have been processed. 
   Referring now to  FIG. 4A , a block diagram illustrates components of a throttling layer  224  being executed in response to a distribution request received from client  202 . Although the embodiment of the invention shown in  FIG. 4A  operates substantially the same as the embodiment in  FIG. 3A , the following description relates to an exemplary operation of the invention in response to a distribution request such as an e-mail, received from the client  202 , rather than a content request. In this embodiment, the UI  214  linked to the client  202  allows user  208  to submit a distribution request  402  (see FIG,  4 B), as indicated by arrow  403 , to the server  204  via the client application  206  and communication network  212 . Alternatively, a malicious application  404  executable by the client  202  generates and submits a plurality of distribution request  402  to the server  204 . Referring briefly to  FIG. 4B , the components of an exemplary distribution request  402  are shown. As known to those skilled in the art, a distribution request  402  generally has two parts, a header  406  and a body  408 . The header  406  of the distribution request  402  includes Simple Mail Transfer Protocol (SMTP) command data  409  that instructs the server  204  to perform a specific action. In this case, in addition to a message ID  410  and a client ID  412 , the header  406  of the distribution request  402  includes a “RCPT TO” command followed by one or more addresses specifying a location of one or more destination mailboxes  411 , assessable via the communication network  214 , at which to deliver message data  414  included in the body  408  of the distribution request  402 . 
   Referring back to  FIG. 4A , the throttling layer  224  is responsive to the distribution request  403  received from the client computer  202  to determine if a stamp is included in the request for each address specified in the header  406  of a single distribution request  402 , or for each distribution request  402  with a different message ID  408  in the header  406 . In this embodiment, the client application  206  is responsive to the users instructions to send the distribution request to calculate a first stamp data value a s , a second stamp data value b s , and an exponent x such as described above in reference to  FIG. 3A . (See equation 3 above). However, in this embodiment, the client application includes data defining a finite field F from which a and b must be members. Prior to sending the distribution request, the client generates a stamp which is a tuple of (a, x). In this embodiment, a stamp is required for each destination mailbox  411  corresponding to an address specified in the command data  409  at which the message data  412  is to be delivered. A stamp is also required for each received distribution request that includes different message data (i.e., different message ID). For example, if the distribution request  402  originates from a single source (i.e., same client ID) but lexical modification are made to message data  412  included in the body  408  of the distribution request  402  to escape a rate filter (e.g., by adding white spaces), a stamp is required for each distribution request that includes modified message data. 
   After computing the stamp, the client application  206  uses special program code (e.g., java script) included in the client application  206  to prepend the header  406  of the distribution request  402  to include the generated stamp  422  (i.e., a s , x) and submits the prepended distribution request  424  (see  FIG. 4C ), as indicated by arrow  403  to the server  204 . The throttling layer  224  is responsive to the stamp  422  included in the header  406  of the prepended distribution request  424  to execute a generating component  416  that generates first and second verification data values a v , b v , such as described above in reference to  FIG. 3A  as a function of header data included in the distribution request  302 . The throttling layer  224  executes a verification component  420  to verify that the first stamp data value a s  included in the header of prepended distribution request  424  is equal to the first verification data value a v  respectively. The throttling layer  224  then executes a comparing component  428  that compares the first verification data value a v  raised to the exponent value x, included in the stamp, to the second verification data value b v . If the first verification data value, a v , raised to the exponent value, x, is equal to the second verification data value, b v , the throttling layer  224  executes a processing component  430  to submit the prepended distribution request  422  to a e-mail server application  432 . The e-mail server application  432  delivers the message content to one or more destination computers corresponding to one or more address specified by the RCPT TO command. 
   On the other hand, if the first verification data value, a v , raised to the exponent value, x, is not equal to the second verification data value, b v , or the client application  206  is not configured to generate a stamp, the processing component  430  transfers the prepended content request  424  to a general data pool  434  for future processing. For example, after processing all distribution requests  424  having valid stamp data and the server load (e.g., number of request being supplied to the server) falls below a threshold value, the prepended distribution request  424  stored in the general data pool  434  are submitted to the email application  432  in a first in first out (FIFO) manner for processing. 
   Notably, as the number of requests received by the server increases (i.e., load increases), the number of bits the server uses to calculate the first and second stamp data values a s , b s  increases. For example, if the load (i.e., number of request being submitted to the server) is less than or equal ten (10) percent of the maximum load capacity of the server, the server may use four bits to determine the first and second verification data values a, b, which indicates 16 (i.e., 2 4 ) different possibilities. However, when the load is eighty (80) percent of the server&#39;s maximum load capacity, the throttling component may use eight bits to determine the first and second verification data values a, b, which indicates 254 (i.e., 2 8 ) different possibilities. As another example, throttling only occurs after the server reaches a predetermined percentage (e.g., 90%) of its maximum load. After the predetermined load is reached, the server uses 16 bit numbers to ward off DOS attacks. If the load does not fall bellow the predetermined percentage within a predetermined period of time, the server doubles the number of bits being used after every interval and so on. Thus, as the load increases the processing time required for the client to calculate x increases because the number of iterations that must be performed by the client to calculate x increases. By requiring the client to calculate verification data and excluding search requests that fail to match the calculated verification data, the invention effectively prioritizes web request and discourages service attacks from a client because of the increased processing time that will be required. 
   In another embodiment, an optional filter component (see reference character  340  in  FIG. 3A  and reference character  440  in  FIG. 4A ) maintains a rate filter for web request (content or distribution request) based on data included in the header of the web request. That is, when the same web request, also know as a replay attack, is submitted to the server from different sources, such web request are identified and dropped at the very beginning of processing. For example, by storing the client ID and first verification data value, a, in memory, and comparing stamp data in the header of subsequently received search requests to the first verification data value data and client ID stored in memory, reoccurring search requests can be identified and dropped from processing or placed in a pool with other non-priority requests. Moreover, as described above, if an attacker makes lexical modification to message data to escape the rate filter (e.g., by adding white spaces) the client(s) providing the request will have to re-compute the stamp, which will slow the processing of the client. 
   Although the arrows in  FIGS. 3A and 4A  show direct data transfer occurring between client and server, it is to be understood data transfer occurs via a communication network such as the Internet. 
   Referring now to  FIG. 5 , a flow diagram illustrates a method for throttling a plurality of content requests being supplied to a server from one or more clients. Each content request includes a header specifying a client ID and a message ID and a body specifying message data. At  502 , a server executes a throttling layer in response to a content request received from a client computer. The throttling layer compares the client ID included in the header of the request to a list of invalid client IDs stored in memory to determine if there is a match at  504 . If the client ID is determined to match an invalid client ID stored in memory at  504 , then the throttling layer drops the request from further processing at  506 . If the client ID does not match any of the invalid client IDs stored in memory at  504 , the throttling layer determines if stamp data (i.e., a s , x) is included in the header data at  508 . If the throttling layer determines that stamp data is included in the header at  508 , the throttling layer determines if the stamp data is valid at  510 . For example, as described above in reference to  FIG. 3A  the throttling layer generates first and second verification data values a v  , b v , and verifies that the first verification data value a v  raised to the exponent value x equals the second verification data value b v . Determining if the stamp is valid includes determining if the service request is a replay message. For example, the throttling layer verifies that the message ID included in the header of the content request does not match any previous message IDs stored in a memory. If the throttling layer determines that the stamp data is valid at  510  (e.g., valid stamp and not a replay message), the server immediately processes the content request to provide the requested content to the client at  511 . In other words, content requests that have a valid stamp are processed with high priority. The throttling layer processes content requests that include stamp data in the header before processing content requests that do not include stamp data. If the throttling layer determines that the stamp data is not valid at  510 , the client ID is added to the list of invalid client IDs stored in memory at  512 . If the throttling layer determines that stamp data is not included in the header at  508 , the throttling layer determines if the request is from a new client at  514 . For example, the throttling layer compares the client ID included in the header of the content request to a list of client IDs stored in a memory that each corresponds to a client computer from which a content request was previously received. If the throttling layer determines that the request is not from a new client at  514 , the throttling layer adds the client ID to the list of invalid client IDs stored in memory at and drops the content request from further processing at  512 . If the throttling layer determines that the request is from a new client at  514 , the throttling layer marks the client ID as old at  518 . At  520 , the throttling layer transfers the content request to a general data pool for future processing. In other words, a content request received from a new client that does not include stamp data is processed with a lower priority. The throttling layer transmits a response message to the client supplying a content request at  522 . The response message includes transformation data and instructions for the client  202  to compute stamp data as a function of the transformation data. For example, as described above in reference to  FIG. 3A , a client application on the client computer is responsive to the transformation data included in the reply message to calculate a first stamp data value a s  and a second stamp data value b s , and identify a mathematical relationship between the two stamp data values a s , b s . More specifically, the client application computes the exponent value x in equation 3. The throttling layer receives an additional content request including computed stamp data from the client and again executes the throttling layer at  502 . 
   Referring now to  FIG. 6 , a flow diagram illustrates a method for throttling distribution requests being supplied to a server. At  602 , a server executes a throttling layer in response to a distribution request received from client computer. Each distribution request includes a header specifying a message ID, a client ID and one or more addresses of destination mailboxes at which to deliver message data included in a body of the distribution request. The throttling layer compares the client ID included in the header of the request to a list of invalid client IDs stored in a memory at  604 . If the client ID is determined to match an invalid client ID stored in memory at  604 , then the throttling layer drops the request from further processing at  606 . If the client ID does not match any of the invalid client IDs stored in memory at  604 , the throttling layer determines if stamp data (i.e., a s , x) is included in the header data at  608 . If the throttling layer determines that stamp data is included in the header at  608 , the throttling layer determines if the stamp data is valid at  610 . For example, as described above in reference to  FIG. 4A . the throttling layer generates a first and second verification data values a v , b v , and verifies that the first verification data value a v  raised to the exponent value x equals the second verification data value b v . Determining if the stamp is valid includes determining if the service request is a replay message. For instance, the throttling layer verifies that the message ID included in the header of the distribution request does not match any previous message IDs stored in memory. If the throttling layer determines that the stamp data is valid at  610  (e.g., valid stamp and not a replay message), the server immediately processes the distribution request to deliver the message data included in the body of the distribution request to the to the one or more addresses of destination mailboxes specified in the header of the distribution request at  611 . In other words, distribution request that include a valid stamp are processed with a higher priority. The throttling layer processes distribution requests that include stamp data in the header before processing distribution requests that do not include stamp data. If the throttling layer determines that the stamp data is not valid at  610 , the client ID is added to the list of invalid client IDs stored in memory and the distribution request is drop from processing at  612 . If the throttling layer determines that stamp data is not included in the header at  608 , the throttling layer transfers the distribution request to a general data pool for future processing by the email application at  614 . In other words, distribution request that do not include a valid stamp are processed with a lower priority. 
     FIG. 7  shows one example of a general purpose computing device in the form of a computer  130 . In one embodiment of the invention, a computer such as the computer  130  is suitable for use in the other figures illustrated and described herein. Computer  130  has one or more processors or processing units  132  and a system memory  134 . In the illustrated embodiment, a system bus  136  couples various system components including the system memory  134  to the processors  132 . The bus  136  represents one or more of any of several types of bus structures, including a memory bus or memory controller, a peripheral bus, an accelerated graphics port, and a processor or local bus using any of a variety of bus architectures. By way of example, and not limitation, such architectures include Industry Standard Architecture (ISA) bus, Micro Channel Architecture (MCA) bus, Enhanced ISA (EISA) bus, Video Electronics Standards Association (VESA) local bus, and Peripheral Component Interconnect (PCI) bus also known as Mezzanine bus. 
   The computer  130  typically has at least some form of computer readable media. Computer readable media, which include both volatile and nonvolatile media, removable and non-removable media, may be any available medium that may be accessed by computer  130 . By way of example and not limitation, computer readable media comprise computer storage media and communication media. Computer storage media include volatile and nonvolatile, removable and non-removable media implemented in any method or technology for storage of information such as computer readable instructions, data structures, program modules or other data. For example, computer storage media include RAM, ROM, EEPROM, flash memory or other memory technology, CD-ROM, digital versatile disks (DVD) or other optical disk storage, magnetic cassettes, magnetic tape, magnetic disk storage or other magnetic storage devices, or any other medium that may be used to store the desired information and that may be accessed by computer  130 . Communication media typically embody computer readable instructions, data structures, program modules, or other data in a modulated data signal such as a carrier wave or other transport mechanism and include any information delivery media. Those skilled in the art are familiar with the modulated data signal, which has one or more of its characteristics set or changed in such a manner as to encode information in the signal. Wired media, such as a wired network or direct-wired connection, and wireless media, such as acoustic, RF, infrared, and other wireless media, are examples of communication media. Combinations of any of the above are also included within the scope of computer readable media. 
   The system memory  134  includes computer storage media in the form of removable and/or non-removable, volatile and/or nonvolatile memory. In the illustrated embodiment, system memory  134  includes read only memory (ROM)  138  and random access memory (RAM)  140 . A basic input/output system  142  (BIOS), containing the basic routines that help to transfer information between components within computer  130 , such as during start-up, is typically stored in ROM  138 . RAM  140  typically contains data and/or program modules that are immediately accessible to and/or presently being operated on by processing unit  132 . By way of example, and not limitation,  FIG. 7  illustrates operating system  144 , application programs  146 , other program modules  148 , and program data  150 . 
   The computer  130  may also include other removable/non-removable, volatile/nonvolatile computer storage media. For example,  FIG. 7  illustrates a hard disk drive  154  that reads from or writes to non-removable, nonvolatile magnetic media.  FIG. 7  also shows a magnetic disk drive  156  that reads from or writes to a removable, nonvolatile magnetic disk  158 , and an optical disk drive  160  that reads from or writes to a removable, nonvolatile optical disk  162  such as a CD-ROM or other optical media. Other removable/non-removable, volatile/nonvolatile computer storage media that may be used in the exemplary operating environment include, but are not limited to, magnetic tape cassettes, flash memory cards, digital versatile disks, digital video tape, solid state RAM, solid state ROM, and the like. The hard disk drive  154 , and magnetic disk drive  156  and optical disk drive  160  are typically connected to the system bus  136  by a non-volatile memory interface, such as interface  166 . 
   The drives or other mass storage devices and their associated computer storage media discussed above and illustrated in  FIG.7 , provide storage of computer readable instructions, data structures, program modules and other data for the computer  130 . In  FIG. 7 , for example, hard disk drive  154  is illustrated as storing operating system  170 , application programs  172 , other program modules  174 , and program data  176 . Note that these components may either be the same as or different from operating system  144 , application programs  146 , other program modules  148 , and program data  150 . Operating system  170 , application programs  172 , other program modules  174 , and program data  176  are given different numbers here to illustrate that, at a minimum, they are different copies. 
   A user may enter commands and information into computer  130  through input devices or user interface selection devices such as a keyboard  180  and a pointing device  182  (e.g., a mouse, trackball, pen, or touch pad). Other input devices (not shown) may include a microphone, joystick, game pad, satellite dish, scanner, or the like. These and other input devices are connected to processing unit  132  through a user input interface  184  that is coupled to system bus  136 , but may be connected by other interface and bus structures, such as a parallel port, game port, or a Universal Serial Bus (USB). A monitor  188  or other type of display device is also connected to system bus  136  via an interface, such as a video interface  190 . In addition to the monitor  188 , computers often include other peripheral output devices (not shown) such as a printer and speakers, which may be connected through an output peripheral interface (not shown). 
   The computer  130  may operate in a networked environment using logical connections to one or more remote computers, such as a remote computer  194 . The remote computer  194  may be a personal computer, a server, a router, a network PC, a peer device or other common network node, and typically includes many or all of the components described above relative to computer  130 . The logical connections depicted in  FIG. 7  include a local area network (LAN)  196  and a wide area network (WAN)  198 , but may also include other networks. LAN  136  and/or WAN  138  may be a wired network, a wireless network, a combination thereof, and so on. Such networking environments are commonplace in offices, enterprise-wide computer networks, intranets, and global computer networks (e.g., the Internet). 
   When used in a local area networking environment, computer  130  is connected to the LAN  196  through a network interface or adapter  186 . When used in a wide area networking environment, computer  130  typically includes a modem  178  or other means for establishing communications over the WAN  198 , such as the Internet. The modem  178 , which may be internal or external, is connected to system bus  136  via the user input interface  184 , or other appropriate mechanism. In a networked environment, program modules depicted relative to computer  130 , or portions thereof, may be stored in a remote memory storage device (not shown). By way of example, and not limitation,  FIG. 7  illustrates remote application programs  192  as residing on the memory device. The network connections shown are exemplary and other means of establishing a communications link between the computers may be used. 
   Generally, the data processors of computer  130  are programmed by means of instructions stored at different times in the various computer-readable storage media of the computer. Programs and operating systems are typically distributed, for example, on floppy disks or CD-ROMs. From there, they are installed or loaded into the secondary memory of a computer. At execution, they are loaded at least partially into the computer&#39;s primary electronic memory. The invention described herein includes these and other various types of computer-readable storage media when such media contain instructions or programs for implementing the steps described below in conjunction with a microprocessor or other data processor. The invention also includes the computer itself when programmed according to the methods and techniques described herein. 
   For purposes of illustration, programs and other executable program components, such as the operating system, are illustrated herein as discrete blocks. It is recognized, however, that such programs and components reside at various times in different storage components of the computer, and are executed by the data processor(s) of the computer. 
   Although described in connection with an exemplary computing system environment, including computer  130 , the invention is operational with numerous other general purpose or special purpose computing system environments or configurations. The computing system environment is not intended to suggest any limitation as to the scope of use or functionality of the invention. Moreover, the computing system environment should not be interpreted as having any dependency or requirement relating to any one or combination of components illustrated in the exemplary operating environment. Examples of well known computing systems, environments, and/or configurations that may be suitable for use with the invention include, but are not limited to, personal computers, server computers, hand-held or laptop devices, multiprocessor systems, microprocessor-based systems, set top boxes, programmable consumer electronics, mobile telephones, network PCs, minicomputers, mainframe computers, distributed computing environments that include any of the above systems or devices, and the like. 
   The invention may be described in the general context of computer-executable instructions, such as program modules, executed by one or more computers or other devices. Generally, program modules include, but are not limited to, routines, programs, objects, components, and data structures that perform particular tasks or implement particular abstract data types. The invention may also be practiced in distributed computing environments where tasks are performed by remote processing devices that are linked through a communications network. In a distributed computing environment, program modules may be located in both local and remote computer storage media including memory storage devices. 
   An interface in the context of a software architecture includes a software module, component, code portion, or other sequence of computer-executable instructions. The interface includes, for example, a first module accessing a second module to perform computing tasks on behalf of the first module. The first and second modules include, in one example, application programming interfaces (APIs) such as provided by operating systems, component object model (COM) interfaces (e.g., for peer-to-peer application communication), and extensible markup language metadata interchange format (XMI) interfaces (e.g., for communication between web services). 
   The interface may be a tightly coupled, synchronous implementation such as in Java 2 Platform Enterprise Edition (J2EE), COM, or distributed COM (DCOM) examples. Alternatively or in addition, the interface may be a loosely coupled, asynchronous implementation such as in a web service (e.g., using the simple object access protocol). In general, the interface includes any combination of the following characteristics: tightly coupled, loosely coupled, synchronous, and asynchronous. Further, the interface may conform to a standard protocol, a proprietary protocol, or any combination of standard and proprietary protocols. 
   The interfaces described herein may all be part of a single interface or may be implemented as separate interfaces or any combination therein. The interfaces may execute locally or remotely to provide functionality. Further, the interfaces may include additional or less functionality than illustrated or described herein. 
   In operation, computer  130  executes computer-executable instructions such as those illustrated in  FIGS. 5 and 6 . 
   The order of execution or performance of the methods illustrated and described herein is not essential, unless otherwise specified. That is, components of the methods may be performed in any order, unless otherwise specified, and that the methods may include more or less components than those disclosed herein. For example, it is contemplated that executing or performing a particular component before, contemporaneously with, or after another component is within the scope of the invention. 
   When introducing components of the present invention or the embodiment(s) thereof, the articles “a,” “an,” “the,” and “said” are intended to mean that there are one or more of the components. The terms “comprising,” “including,” and “having” are intended to be inclusive and mean that there may be additional components other than the listed components. 
   In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained. 
   As various changes could be made in the above constructions and methods without departing from the scope of the invention, it is intended that all matter contained in the above description and shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.