Abstract:
Techniques for providing layer 4 transparent secure transport for end-to-end application protection are described herein. According to one embodiment, a packet of a network transaction is received from a client over a first network, where the packet is destined to a server of a data center having a plurality of servers over a second network. The packet includes a payload encrypted without encrypting information needed for a layer 4 of OSI (open system interconnection) layers of network processes. The layer 4 process is performed on the packet without having to decrypting the payload to determine whether the packet is eligible to access the destined server over the second network based on the unencrypted layer 4 information. Other methods and apparatuses are also described.

Description:
RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent Application No. 60/966,649, filed Aug. 28, 2007, which is incorporated by reference herein in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to secure transport protocols. More particularly, this invention relates to layer-4 transparent secure transport protocols for end-to-end application protection. 
     BACKGROUND 
     The ability to connect information technology infrastructure reliably, cost-effectively and securely is of high importance for today&#39;s global enterprises. To communicate with customers, clients, business partners, employees, etc., the Internet has proven to be more appropriate compared to private communication networks. However, communication via the Internet, which typically uses TCP/IP (Transmission Control Protocol/Internet Protocol), also increases the requirements for data security. Network firewalls are one of the many examples of solutions for network security. 
     Enterprise Web Application Services build an important foundation for such client, customer, and employee communication. A very common configuration for hosting such enterprise web Application Services is shown in  FIG. 1 . As shown in  FIG. 1 , an enterprise can offer web Application Services to various clients and there are several possibilities for clients to connect to the servers depending on the location of the client relative to the servers&#39; location. The servers which provide the Application Services are typically located in the enterprise&#39;s data center  1016  and are accessible, directly or indirectly, via World-Wide-Web (WWW) servers  1012 . Sometimes enterprises provide access to the Application Services by making the application servers directly accessible by putting those application servers into a Demilitarized Zone (DMZ)  1011 . 
     A client  1003  may connect via a Local Area Network (LAN) through the enterprise&#39;s intranet  1013 . Another client  1004  may connect through a Wireless LAN (WLAN) to the intranet  1013 . Yet another client  1005  may be located inside the enterprise&#39;s campus network  1015 , which connects to the enterprise&#39;s intranet  1013 . An enterprise may have zero or more campuses  1014  and  1015 . Yet another client  1001  may connect through the Internet  1000 , or a client  1002  may have a mobile connection to the Internet  1000 . In any case to prevent illegitimate access to the enterprise&#39;s web Application Services, the “inside” of the enterprise&#39;s network, the intranet  1013 , is protected by having a network perimeter  1010 , which may comprise firewalls, associated network interconnect, and additional resources “within” the perimeter network configured so as to be broadly accessible to users on the “outside” of the enterprise. 
     Behind the perimeter  1010 , access is granted to legitimate client requests only, while illegitimate access is rejected. The fundamentals in determining whether an access request is legitimate or not are based on the network reference model from the International Organization for Standardization (ISO). This ISO network reference model classifies Network Services into seven layers. 
     Traditional security products generally assume the existence of a trusted intranet—locations where enterprises control their own LANs, switches and routers—which can be organized into or placed within some type of security perimeter, to protect its resources from the un-trusted Internet. However, in today&#39;s business environment, enterprises no longer enjoy the same level of trust and control of their intranets, as enterprises increasingly rely on contractors, partners, consultants, vendors, and visitors on-site for daily operation. As a result, enterprises are exposing internal resources to this wide set of clients whose roles are also frequently changing. Thus, the network trust boundary, delineating inside and outside clients, is disappearing—a phenomenon referred to as “de-perimeterization”. In such an environment, protection of an enterprise&#39;s resources—such as its intellectual property, as well as mission-critical and operational systems—becomes of critical importance. Also, most security exploits easily traverse perimeter security, as enterprises typically let through email, web and any encrypted network traffic, such as Secure Sockets Layer (SSL), Simple Mail Transfer Protocol (SMTP) with Transport Layer Security (TLS), and authenticated Virtual Private Network (VPN) traffic, for example via IP Security (IPSec). Traditional perimeter security approaches, for example firewalls, intrusion detection systems and intrusion prevention systems have little or no benefit at the perimeter in providing access control functions to the resources. They have become more attack mitigation mechanisms than access control mechanisms. Enterprises are coming to terms with the fact that a hardened perimeter strategy is un-sustainable. 
     Traditional firewall or router access control lists cannot protect application resources from unauthorized access because network parameters such as Internet Protocol (IP) addresses and IP port numbers no longer deterministically identify resources, nor identify users, clients, or applications accessing these resources. Network firewall technology was invented when enterprises had a limited set of applications such as Telnet, File Transfer Protocol (FTP), and Email, and its primary functions were to limit access to specific applications from the outside and to limit access by systems within the enterprise to specific applications outside the firewall. Network layer parameters such as source, destination IP address and TCP or UDP port numbers were sufficient to identify the client and the operations the clients intended to perform on a particular resource. However, with the proliferation of mobile devices and tunneled applications, the network layer parameters are no longer useful to identify the client, the resource accessed, and the operation. Firewalls have evolved over the time, embracing functions such as deep packet inspection and intrusion detection/prevention, to handle application-level attacks, but the core access control function remains the same. 
     In effect, de-perimeterization demands that access control functions are positioned close to application resources and that a micro-perimeter is established in the heart of the data center by placing an identity-based policy enforcement point in front of any application resource. Enterprise business drivers for such an enforcement point are the need for rich and uniform protection of resources, business agility via attribute-based, policy-driven provisioning, and regulatory compliance. Traditional server-centric authorization solutions providing role-based authorization often require custom code development, extensive cross-vendor testing whenever there is a version change (of the underlying operating system, agent or application), and are costly and difficult to maintain because of their proprietary nature. Also, traditional server-based network appliances—primarily focused on low-bandwidth ISO Layer-4 to ISO Layer-7 perimeter services—are unsuitable for data center deployment, both in functional richness and in ISO Layer-7 performance. 
     SUMMARY OF THE DESCRIPTION 
     Techniques for providing layer 4 transparent secure transport for end-to-end application protection are described herein. According to one embodiment, a packet of a network transaction is received from a client over a first network, where the packet is destined to a server of a data center having a plurality of servers over a second network. The packet includes a payload encrypted without encrypting information needed for a layer 4 of OSI (open system interconnection) layers of network processes. The layer 4 process is performed on the packet without having to decrypting the payload to determine whether the packet is eligible to access the destined server over the second network based on the unencrypted layer 4 information. 
     Other features of the present invention will be apparent from the accompanying drawings and from the detailed description which follows. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention is illustrated by way of example and not limitation in the figures of the accompanying drawings in which like references indicate similar elements. 
         FIG. 1  illustrates a typical corporate computer network connected to the Internet; 
         FIG. 2  illustrates the application of an application network appliance (ANA) as the APS according to one embodiment of the invention; 
         FIG. 3  is a network connected block diagram of an ANA according to one embodiment of the invention; 
         FIG. 4  is a block diagram of a use case of Triangulated Authorization with Transparent Secure Transport in an ANA according to one embodiment of the invention; 
         FIG. 5  is a block diagram which illustrates the various approaches for secure transport, including Transparent Secure Transport according to one embodiment of the invention; 
         FIG. 6  is a block diagram of an ANA deploying Transparent Secure Transport according to one embodiment of the invention; 
         FIG. 7  is a flow diagram of a method for Transparent Secure Transport in an ANA according to one embodiment of the invention; 
         FIG. 8  is a flow diagram of a method for Transparent Secure Transport depending on security zones in an ANA according to one embodiment of the invention; 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, numerous details are set forth to provide a more thorough explanation of embodiments of the present invention. It will be apparent, however, to one skilled in the art, that embodiments of the present invention may be practiced without these specific details. In other instances, well-known structures and devices are shown in block diagram form, rather than in detail, in order to avoid obscuring embodiments of the present invention. 
     Reference in the specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. The appearances of the phrase “in one embodiment” in various places in the specification do not necessarily all refer to the same embodiment. 
     One aspect of the invention provides a Transparent Secure Transport mechanism between client-to-server (or server-to-server) connections which will not break existing ISO Layer-4 networking. While the payload (i.e. the sensitive data) is encrypted for privacy and security, the original TCP and IP headers are kept unchanged. This results in a secure transport method which is transparent to existing ISO Layer-4 network services. 
     One aspect of the invention is a system and method for Transparent Secure Transport for End-to End Application Protection, comprising a method for secure transport in a network environment using data packets which protects the transported data by encrypting the payload of the data packets and which does not alter the ISO Layer-3 and ISO Layer-4 information of said data packets. The described Transparent Secure Transport (TST) may be dynamically installed and enabled in an endpoint by downloading the requisite TST agent software as needed into, for example, a client system, or, the requisite TST capabilities may be pre-installed in an endpoint. 
     Overview 
     The approach described herein applies combinations of parallel, multi-processor computing technology with lossless, low-latency, high-bandwidth network fabric technology (also known as Lossless Data Transport Fabric, or LDTF) to form novel methods and systems for high performance, high-reliability, high availability, and secure network applications. The various embodiments of the inventions described herein enable the implementation of highly reliable, highly scalable solutions for enterprise networking such as, for example, the APS  2000  from  FIG. 2 . 
     Multiple network Services are efficiently provided by terminating transport protocols centrally. As can be seen, any transport protocol can be terminated centrally, each PDU&#39;s payload can be collected and converted into a data stream and, vice versa, a data stream can be converted into PDUs for any transport protocol and be transported via the given transport protocol. A simple concatenation of the PDU payload into a byte-stream is not sufficient. Key to the conversion is that state information must be maintained about the meta-data of each connection. Such meta-data includes the session information, for example via a unique connection identification number, the transaction information, as well as the information regarding segments and packets. Finite state machines can be used to track the meta-data. 
     Transport protocols are protocols which are used to transport information via networks. These include, obviously, the ISO Layer-3 protocols such as IPv4, IPv6, IPSec, the ISO Layer-4 protocols such as TCP, UDP, SCTP, the various ISO Layer-5 protocols such as FTP, HTTP, IMAP, SMTP, GTP, L2TP, PPTP, SOAP, SDP, RTSP, RTP, RTCP, RPC, SSH, TLS, DTLS, SSL, IPSec, and VPN protocols. However, other protocols and approaches are contemplated within the scope of the inventions, which serve as transport mechanisms for transmitting information and application data and can also be terminated in a centralized fashion by a protocol proxy and the corresponding PDUs can be transformed into a data stream for application layer processing. Examples of such are, CSIv2, CORBA, IIOP, DCOM and other Object Request Brokers (ORB), MPEG-TS or RTP as a transport for multi-media information, RTSP or SIP as another transport for multi-media information, peer-to-peer transport mechanisms, transport mechanisms based on J2EE such as Java RMI, streaming media protocols such as VoIP, IPTV, etc. 
     For the sake of simplicity we will use the term Centralized Transport Protocol Termination throughout the rest of the description, however, this is for exemplary purposes only and is not intended to be limiting. Centralized Transport Protocol Termination can be performed by dedicated processing units, and different ISO Layer-7 services can be performed in other dedicated processing units. The use of a lossless low-latency high-bandwidth fabric for inter-process communication between such dedicated processing units makes it possible to simultaneously support Centralized Transport Protocol Termination for multiple services. For example, TCP can be terminated once, transformed into a data stream and this data stream is transported from one dedicated processing unit to another using the lossless low-latency high-bandwidth fabric. The low-latency nature of the fabric helps to reduce the overall latency in client-to-server transactions. 
     In one embodiment, the Application Protection System (APS)  2000  is a network appliance that can act as a proxy between the client  2001  and the application server  2005 , and can determine whether a client  2001  shall be granted access to certain applications  2005 . In one example, the client  2001  is one or more of the clients  1001 ,  1002 ,  1003 ,  1004 , or  1005  of  FIG. 1 . In another example, the client  2001  can be a virtual machine or a cluster of computers, or a server (for server-to-server connections, for example). The application server  2005  can be, for example, without limitation, one or more file servers, one or more web servers, one or more database servers, one or more compute servers, one or more storage servers or one or more game servers. The decision whether access is granted or rejected involves an Identity Management Server  2003  to identify the user, client, or application, for example using Lightweight Directory Access Protocol (LDAP) or Active Directory (AD), and is the result of querying a Policy Server  2002  to analyze the access policy for the requested application  2005 . 
     The APS  2000  may use a Triangulated Authorization method which, for example, is based on multiple aspects of a client (such as the client  2001 ), the requested application (such as application  2005 ) and certain network characteristics: Who—a client (a user or a machine) and its associated attributes such as department, role, project association, seniority, citizenship, etc; Where—network and environment attributes such as access methods (wire-line/wireless/VPN), location (e.g., USA, Switzerland, China) and time; What—on-the-wire session attributes, including protocol and content/resource attributes. The outcome of this Triangulated Authorization method can be used to determine whether access to an application is granted or rejected. Optionally, a Single-Sign-On (SSO) server such as server  2004  may be involved that allows the client  2001  to obtain authorization for accessing multiple applications at once. 
     One embodiment of the invention acts as a proxy between one or more clients and one or more application servers to control the access of the one or more clients to the one or more applications. This is described, for example, in  FIG. 2 , where the APS  2000  controls access of client  2001  to application server  2005 . Thereby the approach can act as a high-speed, full proxy which terminates both client-side and server-side transport protocol connections, and which behaves as a virtual server to the one or more clients, and as a virtual client to the one or more servers. The proxy function is required because of the need to reassemble PDUs into data streams and (where needed) to decrypt the payload data for inspection such as access control. The proxy function involves ISO Layer-2 to ISO Layer-5 processing such as Centralized Transport Protocol Termination. 
       FIG. 3  is a block diagram illustrating an example of application service appliance system according to one embodiment of the invention. Referring to  FIG. 3 , ANA  2100  acts as a proxy between a client  2104  and an application server  2105 . The client  2104  is connected to the ANA  2100  via a network  2107 . Network  2107  can, for example, be a LAN, a WAN, a WLAN, an intranet, or the Internet. The application server  2105  is connected to the ANA  2100  via network  2106 . Network  2106  can, for example, be a LAN, a WAN, a WLAN, an intranet, or the Internet. Networks  2106 - 2107  may be the same network or different networks. While it is apparent that multiple clients and multiple application servers may be connected to the ANA  2100 , for the sake of simplicity a single client, single application server case is used as a placeholder throughout. Incoming connections, for example, a request from the client  2104  is terminated in the NSM  2103  and is transformed into a data stream. This is done by PDU processing and reassembling the payload of the PDU into a data stream of ISO Layer-7 application data. This data stream is transported via LDTF  2102  to the ASM  2101  for further ISO Layer-7 processing. LDTF  2102  may be an RDMA or IB compatible fabric. The result of ISO Layer-7 processing done by ASM  2101  is then transported back—still as a data stream—via the LDTF  2102  to the NSM  2103 . The NSM  2103  then transforms the data stream into PDUs and sends the PDUs to the application server  2105  via the appropriate transport protocol. Connections which originate from the application server  2105  can be handled similarly. Using this novel approach, both processing domains can be scaled independent of each other and a well-balanced system can be achieved at reasonable costs. 
     Transparent Secure Transport Based on Policies 
     For end-to-end protection, one embodiment of the invention can provide encrypted Transparent Secure Transport for client sessions without breaking existing ISO Layer-2 to ISO Layer-4 services. Because the primary target of this function is to provide data privacy for internal communication, it is important to keep visibility to network headers so that network operators can continue to use traditional traffic monitoring and protocol analysis tools. Also this approach allows the Transparent Secure Transport function to co-exist with existing network layer services such as access control lists (ACL) and Quality of Service (QoS). The Transparent Secure Transport functionality allows creation of resource enclaves with different levels of security. For example, all sessions destined to high-security enclaves would always be encrypted while sessions destined to medium-security enclaves would be cryptographically authenticated only. Like the Triangulated Authorization service support, the Transparent Secure Transport service of our approach is non-invasive to application resources. 
       FIG. 4  illustrates one embodiment of the invention where both the front-end connection between the client  2001  and the APS  2000  can utilize Transparent Secure Transport  2011  and the back-end connection between the APS  2000  and the application server  2005  can use Transparent Secure Transport  2012 . Application resources can be segmented in multiple security zones based on the sensitivity of the data transmitted. 
     Different security zones can be created with different levels of security based on policies. For example, encryption and integrity checks may be used for very sensitive data. In this case the payload in the each packet is encrypted and an integrity code (for example, a Message Authentication Code) is added to make sure there is no tampering with the encrypted data in between. For less sensitive data, only integrity codes may be added to each packet to make sure no one tampers with the data in between; however, the data itself is not encrypted. 
     The Transparent Secure Transport of this approach, for example, Transparent Secure Transport  2011  or Transparent Secure Transport  2012 , are transparent to existing ISO Layer-4 services, unlike other approaches known in the art such as IPSec or SSL-based VPN. For example, a packet, which is transported via IPSec&#39;s Transport Mode, will have its TCP header encrypted. A packet includes an Original IP header, a TCP header and data, which is transported via IPSec&#39;s Tunneling Mode will not only have the TCP header but also have the Original IP header encrypted. In both cases this prevents existing ISO Layer-4 services from analyzing such network traffic because the original IP header and the TCP header are not visible anymore during such secure transport. 
     Transparent Secure Transport for End-to-End Application Protection 
     In one embodiment of the invention described herein, the ANA shown in  FIG. 4  where a client  2001  can access applications  2005  and where the access to such applications  2005  is controlled by the APS  2000  of  FIG. 2 . For security and for privacy reasons the connection between the client  2001  and the APS  2000  and the connection between the APS  2000  and the application server  2005  can be protected by encryption, for example. While the secure transport approaches known in the art are not transparent to ISO Layer-4 networking, because the original TCP/IP header may get encrypted and replaced (see above), in one embodiment of the invention, a novel, Transparent Secure Transport system and method is disclosed. 
       FIG. 5  illustrates the functioning of the novel, Transparent Secure Transport as compared to other secure transport approaches known in the art. Within a Client Host Machine  5020  an application  5021  sends data to transport agent  5022 . The data  5023  transmitted can look like TCP packet  5030  which comprises a header with the destination IP address  5031 , the destination TCP port number  5032  and the payload  5033 , all unencrypted, in clear-text. (This disclosure is relevant for TCP over IP; if another IP-based protocol is used, the disclosure still applies, but some of the parameters may differ. For example, some IP-based protocols do not use TCP and thus do not have a TCP port number available. However, the mechanism can still function in a similar manner.) When agent  5022  sends the data  5024  over an Ethernet network  5025  for privacy and security reasons the data  5024  gets encrypted. In one approach known in the art, IPSec Tunneling, the entire original packet  5030  gets encrypted into portions  5053 ,  5054 ,  5055  and ESP information  5052  and new IP destination information  5051  gets added. In one other approach known in the art, SSL-VPN Tunneling, the entire original packet  5030  gets encrypted as well and SSL header information  5063  gets added together with new IP destination  5061  and TCP port number  5062  information. In both approaches, the original IP information  5031  and  5032  gets encrypted (into  5053  and  5054 , or into  5064  and  5065 ) and thus becomes inaccessible to ISO Layer-4 network analysis. 
     This drawback of encrypting the original IP information is solved by one embodiment of the invention described herein. According to one embodiment of the invention, the original data packet  5030  can be sent by transporting it within the packet  5040 . The original destination IP address  5031  and the original destination TCP port number  5032  are used unencrypted such that ISO Layer-4 network analysis can seamlessly be applied. Therefore the transport mechanism of this approach is transparent to existing networking. And because the original payload  5033  gets encrypted into the encrypted payload  5042  plus an encryption header, for example SSL header  5041 , the transport is also secure. In one embodiment of the invention, SSL is used for encrypting the payload. In another embodiment of the invention, DTLS is used for encrypting the payload. 
       FIG. 6  shows the application of Transparent Secure Transport to perform policy-based access-control and policy-based Transparent Secure Transport, according to one embodiment of the invention. Users and clients, such as  5012 , can use various devices  5013  to access various network-centric applications  5014 . Depending on the current policy which determines access to the application, the Transparent Secure Transport  5011  can be used for communication between the client  5012  and applications  5014 . This communication method can, for example, use a client-side agent as it is illustrated in  FIG. 7 : In step one  5101 , a client connects to the gateway for the first time. This gateway can, for example, be APS  2000  of  FIG. 4 . In a second step  5102 , a security agent transparently gets downloaded to and installed onto the client. This client can, for example, be client  2001  of  FIG. 4 . The security agent can, for example, be agent  5022  of  FIG. 5  and can, for example, be a plug-in for a common web browser such as Mozilla Firefox. In a third step  5103 , the agent establishes a secure control channel to the gateway. In a fourth step  5104 , the agent negotiates the required security parameters with the gateway. In a fifth step  5105 , the agent downloads the policy from the gateway via the secure control channel. In a sixth step  5106 , the agent analyzes the policy to determine the client traffic that requires Transparent Secure Transport. In a seventh step  5107 , the agent transparently traps the client traffic that matches the configured policy. In an eighth step  5108 , the agent proxies connections to provide the required security service by encrypting the traffic&#39;s payload using the negotiated security parameters. In a ninth step  5109 , the client has established Transparent Secure Transport with the applications. This Transparent Secure Transport can, for example, use packets as shown for packet  5040  of  FIG. 5 . The order of the above steps is exemplary only, and is not intended to be limiting. 
     In another embodiment of the invention, the Transparent Secure Transport can use a different Transparent Secure Transport depending on a particular security zone configured in a policy. This is described in conjunction with  FIG. 8 . In a first step  5101 , a client connects to the gateway for the first time. In a second step  5102 , a security agent transparently gets downloaded to and installed onto the client. In a third step  5103 , the agent establishes a secure control channel to the gateway. In a fourth step  5104 , the agent negotiates the required security parameters with the gateway. In a fifth step  5105 , the agent downloads the policy from the gateway via the secure control channel. In a sixth step  5106 , the agent analyzes the policy to determine the client traffic that requires Transparent Secure Transport. In a seventh step  5107 , the agent transparently traps the client traffic that matches the configured policy. In an eighth step  5110 , the agent proxies connections to provide the required security service. In a decision  5111 , the agent checks the security zone configured in the downloaded policy. If the security zone only requires medium security, the method continues at step  5113 . However, if the security zone requires high security, the method continues with step  5112  in which the payload is encrypted using the negotiated security parameters. In step  5113 , the agent adds an integrity code (such as a Message Authentication Code (MAC), for example), using the negotiated security parameters. In a last step  5109 , the client has established Transparent Secure Transport with the applications. In yet another embodiment of the invention, if the security zone only requires low security, no encryption may be performed on the payload and no integrity code may be added but just authorization may be performed. The order of the above steps is exemplary only, and is not intended to be limiting. 
     Some portions of the preceding detailed descriptions have been presented in terms of algorithms and symbolic representations of operations on data bits within a computer memory. These algorithmic descriptions and representations are the ways used by those skilled in the data processing arts to most effectively convey the substance of their work to others skilled in the art. An algorithm is here, and generally, conceived to be a self-consistent sequence of operations leading to a desired result. The operations are those requiring physical manipulations of physical quantities. Usually, though not necessarily, these quantities take the form of electrical or magnetic signals capable of being stored, transferred, combined, compared, and otherwise manipulated. It has proven convenient at times, principally for reasons of common usage, to refer to these signals as bits, values, elements, symbols, characters, terms, numbers, or the like. 
     It should be borne in mind, however, that all of these and similar terms are to be associated with the appropriate physical quantities and are merely convenient labels applied to these quantities. Unless specifically stated otherwise as apparent from the above discussion, it is appreciated that throughout the description, discussions utilizing terms such as “processing” or “computing” or “calculating” or “determining” or “displaying” or the like, refer to the action and processes of a computer system, or similar electronic computing device, that manipulates and transforms data represented as physical (electronic) quantities within the computer system&#39;s registers and memories into other data similarly represented as physical quantities within the computer system memories or registers or other such information storage, transmission or display devices. 
     Embodiments of the present invention also relate to an apparatus for performing the operations herein. This apparatus may be specially constructed for the required purposes, or it may comprise a general-purpose computer selectively activated or reconfigured by a computer program stored in the computer. Such a computer program may be stored in a computer readable storage medium, such as, but is not limited to, any type of disk including floppy disks, optical disks, CD-ROMs, and magnetic-optical disks, read-only memories (ROMs), random access memories (RAMs), erasable programmable ROMs (EPROMs), electrically erasable programmable ROMs (EEPROMs), magnetic or optical cards, or any type of media suitable for storing electronic instructions, and each coupled to a computer system bus. 
     The algorithms and displays presented herein are not inherently related to any particular computer or other apparatus. Various general-purpose systems may be used with programs in accordance with the teachings herein, or it may prove convenient to construct more specialized apparatus to perform the required method operations. The required structure for a variety of these systems will appear from the description below. In addition, embodiments of the present invention are not described with reference to any particular programming language. It will be appreciated that a variety of programming languages may be used to implement the teachings of embodiments of the invention as described herein. 
     A machine-readable medium may include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computer). For example, a machine-readable medium includes read only memory (“ROM”); random access memory (“RAM”); magnetic disk storage media; optical storage media; flash memory devices; electrical, optical, acoustical or other form of propagated signals (e.g., carrier waves, infrared signals, digital signals, etc.); etc. 
     In the foregoing specification, embodiments of the invention have been described with reference to specific exemplary embodiments thereof. It will be evident that various modifications may be made thereto without departing from the broader spirit and scope of the invention as set forth in the following claims. The specification and drawings are, accordingly, to be regarded in an illustrative sense rather than a restrictive sense.