Abstract:
A network device includes a first circuit configured to generate a plurality of packets, and insert, in each of the plurality of packets, a different value for a count. A second circuit receives one or more of the plurality of packets. A third circuit generates a plurality of seeds. Each of the plurality of seeds is based on (i) a predetermined key, (ii) an address of the network device, and (iii) a predetermined value for the count. A fourth circuit encapsulates each of the plurality of packets using one of the plurality of seeds generated based on the value for the count in the respective one of the plurality of packets. A fifth circuit sends a message comprising (i) the address of the network device and (ii) the predetermined value for the count, and sends, subsequent to sending the message, the plurality of encapsulated packets.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a continuation of U.S. patent Ser. No. 13/533,567, filed on Jun. 26, 2012, which is a continuation of U.S. patent application Ser. No. 12/758,865 (now U.S. Pat. No. 8,208,632), filed on Apr. 13, 2010, which is a continuation of U.S. patent application Ser. No. 10/974,388 (now U.S. Pat. No. 7,697,688), filed on Oct. 27, 2004, which is related to U.S. patent application Ser. No. 10/974,458 (now U.S. Pat. No. 7,742,594), filed on Oct. 27, 2004. The entire disclosures of the above applications are incorporated herein by reference. 
    
    
     BACKGROUND 
     The present invention relates generally to encapsulation and decapsulation of data communications packets. More particularly, the present invention relates to pipelining such encapsulation and decapsulation to achieve faster data throughput. 
     Network devices operating in a wireless local-area network (WLAN) may employ the Temporal Key Integrity Protocol (TKIP) specified by the IEEE 802.11i standard (April 2004) to protect the confidentiality and integrity of transmitted data from malicious attacks. 
     According to TKIP, the sender encapsulates packets of data to be sent using a temporal key negotiated with the receiver. The receiver, upon receiving the packets, decapsulates the packets using the temporal key. This scheme works well as long as the transmitted packets are separated by inter-packet gaps that are large enough to allow the receiver to complete the decapsulation of one packet before the next packet arrives. 
     However, in WLAN applications that require very high throughput, data packets are transmitted in succession with no inter-packet gaps. In addition, in IEEE 802.11n Multiple Input Multiple Output (MIMO) systems, data packets are transmitted at even higher data rates, leaving even less time for decapsulation. 
     SUMMARY 
     In general, in one aspect, the invention features an apparatus and corresponding method and computer program for processing N encapsulated Media Access Control (MAC) Payload Data Units (MPDUs), wherein N≧1. It comprises a key mixing circuit to generate N Wired Equivalent Privacy (WEP) seeds each based upon a predetermined temporal key, a transmitter MAC address, and a predetermined start value for a Temporal Key Integrity Protocol (TKIP) Sequence Count (TSC); an input circuit to receive the N encapsulated MPDUs, wherein each of the N encapsulated MPDUs comprises the transmitter MAC address and one of N values for the TSC, wherein each of the N values for the TSC is greater than, or equal to, the predetermined start value for the TSC; and a WEP decapsulation circuit to decapsulate each of the encapsulated MPDUs using the one of the N WEP seeds that was generated based on the value for the TSC in the respective one of the N encapsulated MPDUs; wherein the key mixing circuit generates each of the N WEP seeds before the input circuit receives the respective one of the N encapsulated MPDUs. 
     Particular implementations can include one or more of the following features. The input circuit receives a message comprising the transmitter MAC address and the predetermined start value for the TSC before the key mixing circuit generates the N WEP seeds. The message further comprises the value of N. Particular implementations can include a reassembly circuit to reassemble one or more MAC Service Data Units (MSDUs) based on the N decapsulated MPDUs. Particular implementations can include a verification circuit to verify each of the MSDUs using a Message Integrity Code (MIC) key. Particular implementations can include a countermeasures circuit to employ one or more countermeasures when any of the MSDUs cannot be verified. A network device comprises the apparatus. The wireless network device is otherwise compliant with at least one standard selected from the group consisting of IEEE standards 802.11, 802.11a, 802.11b, 802.11g, 802.11n, 802.16, and 802.20. The wireless network device is compliant with IEEE standard 802.11i. 
     In general, in one aspect, the invention features an apparatus and corresponding method and computer program for processing N encapsulated packets of data, wherein N≧1. It comprises a key mixing circuit to generate N decapsulation seeds each based upon a predetermined temporal key, a transmitter address, and a predetermined start value for a count; an input circuit to receive the N encapsulated packets, wherein each of the N encapsulated packets comprises the transmitter address and one of N values for the count, wherein each of the N values for the count is greater than, or equal to, the predetermined start value for the count; and a decapsulation circuit to decapsulate each of the encapsulated packets of data using the one of the N decapsulation seeds that was generated based on the value for the count in the respective one of the N encapsulated packets of data; wherein the key mixing circuit generates each of the N decapsulation seeds before the input circuit receives the respective one of the N encapsulated packets of the data. 
     Particular implementations can include one or more of the following features. The input circuit receives a message comprising the transmitter address and the predetermined start value for the count before the key mixing circuit generates the N decapsulation seeds. The message further comprises the value of N. Particular implementations can include a verification circuit to verify each of the N decapsulated packets using an integrity code key. Particular implementations can include a countermeasures circuit to employ one or more countermeasures when any of the N decapsulated packets cannot be verified. A network device comprises the apparatus. The wireless network device is otherwise compliant with at least one standard selected from the group consisting of IEEE standards 802.11, 802.11a, 802.11b, 802.11g, 802.11n, 802.16, and 802.20. The wireless network device is compliant with IEEE standard 802.11i. 
     In general, in one aspect, the invention features an apparatus and corresponding method and computer program for transmitting N Media Access Control (MAC) Payload Data Units (MPDUs), wherein N≧1. It comprises a key mixing circuit to generate N Wired Equivalent Privacy (WEP) seeds each based upon a predetermined temporal key, a transmitter MAC address, and a predetermined start value for a Temporal Key Integrity Protocol (TKIP) Sequence Count (TSC); an input circuit to receive one or more MAC Service Data Units (MSDUs); a fragmentation circuit to generate the N MPDUs based on the one or more MSDUs, and to insert a different one of N values for the TSC into each of the N MPDUs, wherein each of the N values for the TSC is greater than, or equal to, the predetermined start value for the TSC; a WEP encapsulation circuit to encapsulate each of the N MPDUs using the one of the N WEP seeds that was generated based on the value for the TSC in the respective one of the N MPDUs; and an output circuit to send the N encapsulated MPDUs; wherein the key mixing circuit generates each of the N WEP seeds before the input circuit receives the one or more MSDUs. 
     Particular implementations can include one or more of the following features. The output circuit sends a message comprising the transmitter address and the predetermined start value for the TSC before sending the N encapsulated MPDUs. The message further comprises the value of N. Particular implementations can include an integrity circuit to generate a Message Integrity Code (MIC) for each of the MSDUs using a MIC key, and to insert each MIC into a respective one of the MSDUs before the fragmentation circuit generates the N MPDUs. A network device comprises the apparatus. The wireless network device is otherwise compliant with at least one standard selected from the group consisting of IEEE standards 802.11, 802.11a, 802.11b, 802.11g, 802.11n, 802.16, and 802.20. The wireless network device is compliant with IEEE standard 802.11i. 
     In general, in one aspect, the invention features an apparatus and corresponding method and computer program for transmitting N packets of data, wherein N≧1. It comprises a key mixing circuit to generate N encapsulation seeds each based upon a predetermined temporal key, a transmitter address, and a predetermined start value for a count; an input circuit to receive the N packets of data; a count circuit to insert a different one of N values for the count into each of the N packets of data, wherein each of the N values for the count is greater than, or equal to, the predetermined start value for the count; an encapsulation circuit to encapsulate each of the N packets of data using the one of the N encapsulation seeds that was generated based on the value for the count in the respective one of the N packets of data; and an output circuit to send the N encapsulated packets of data; wherein the encapsulation circuit generates each of the N encapsulation seeds before the input circuit receives the respective one of the N packets of data. 
     Particular implementations can include one or more of the following features. The output circuit sends a message comprising the transmitter address and the predetermined start value for the count before sending the N encapsulated packets of data. The message further comprises the value of N. Particular implementations can include an integrity circuit to generate an integrity code for each of the N packets of data using an integrity key, and to insert each integrity code into a respective one of the N packets of data before the encapsulation circuit encapsulates the respective one of the N packets of data. A network device comprises the apparatus. The wireless network device is otherwise compliant with at least one standard selected from the group consisting of IEEE standards 802.11, 802.11a, 802.11b, 802.11g, 802.11n, 802.16, and 802.20. The wireless network device is compliant with IEEE standard 802.11i. 
     In general, in one aspect, the invention features an apparatus and corresponding method and computer program for transmitting N Media Access Control (MAC) Payload Data Units (MPDUs), wherein N≧1. It comprises an output circuit to send a message comprising a transmitter MAC address and a predetermined start value for a Temporal Key Integrity Protocol (TKIP) Sequence Count (TSC); a fragmentation circuit to insert a different one of N values for the TSC into each of the N MPDUs, wherein each of the N values for the TSC is greater than, or equal to, the predetermined start value for the TSC; a key mixing circuit to generate N Wired Equivalent Privacy (WEP) seeds each based upon a predetermined temporal key, the transmitter MAC address and one of the N values for the TSC; a WEP encapsulation circuit to encapsulate each of the N MPDUs using the one of the N WEP seeds that was generated based on the value for the TSC in the respective one of the N MPDUs; wherein the output circuit sends the N encapsulated MPDUs after sending the message. 
     Particular implementations can include one or more of the following features. The fragmentation circuit generates the N MPDUs based on one or more MAC Service Data Units (MSDUs). Particular implementations can include an integrity circuit to generate a Message Integrity Code (MIC) for each of the MSDUs using a MIC key and insert each MIC into a respective one of the MSDUs before the fragmentation circuit generates the MPDUs based on the respective one of the MSDUs. The message further comprises the value of N. A network device comprises the apparatus. The wireless network device is otherwise compliant with at least one standard selected from the group consisting of IEEE standards 802.11, 802.11a, 802.11b, 802.11g, 802.11n, 802.16, and 802.20. The wireless network device is compliant with IEEE standard 802.11i. 
     In general, in one aspect, the invention features an apparatus and corresponding method and computer program for transmitting N packets of data, wherein N≧1. It comprises an output circuit to send a message comprising a transmitter address and a predetermined start value for a count; an insertion circuit to insert a different one of N values for the count into each of the N packets of data, wherein each of the N values for the count is greater than, or equal to, the predetermined start value for the count; a key mixing circuit to generate N encapsulation seeds each based upon a predetermined temporal key, the transmitter address and one of the N values for the count; an encapsulation circuit to encapsulate each of the N packets of data using the one of the N encapsulation seeds that was generated based on the value for the count in the respective one of the N packets of data; wherein the output circuit sends the N encapsulated packets of data after sending the message. 
     Particular implementations can include one or more of the following features. Particular implementations can include an integrity circuit to generate a Message Integrity Code (MIC) for each of the N packets of data using a MIC key and insert each MIC into a respective one of the N packets of data before the encapsulation circuit encapsulates the respective one of the N packets of data. The message further comprises the value of N. A network device comprises the apparatus. The wireless network device is otherwise compliant with at least one standard selected from the group consisting of IEEE standards 802.11, 802.11a, 802.11b, 802.11g, 802.11n, 802.16, and 802.20. The wireless network device is compliant with IEEE standard 802.11i. 
     In general, in one aspect, the invention features a packet comprising Temporal Key Integrity Protocol (TKIP) Sequence Count (TSC) information representing a number N of Media Access Control (MAC) Payload Data Units (MPDUs) encapsulated according to TKIP to be transmitted following the packet, wherein N≧1; a transmitter MAC address for a transmitter of the N encapsulated MPDUs; and a receiver MAC address for a receiver of the N encapsulated MPDUs. 
     Particular implementations can include one or more of the following features. The TSC information comprises a predetermined start value for the TSC, wherein each of the N encapsulated MPDUs comprises a value for the TSC that is greater than, or equal to, the predetermined start value for the TSC; and a predetermined range value representing a range of values of the TSC in the N encapsulated MPDUs. 
     In general, in one aspect, the invention features a packet comprising count information representing a number N of encapsulated packets to be transmitted following the packet, wherein N≧1; a transmitter address for a transmitter of the N encapsulated packets; and a receiver address for a receiver of the N encapsulated packets. 
     Particular implementations can include one or more of the following features. The count information comprises a predetermined start value for the count, wherein each of the N encapsulated packets comprises a value for the count that is greater than, or equal to, the predetermined start value for the count; and 
     a predetermined range value representing a range of values of the count in the N encapsulated packets. 
     The details of one or more implementations are set forth in the accompanying drawings and the description below. Other features will be apparent from the description and drawings, and from the claims. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1  shows a network device in communication with a network such as a wireless local-area network (WLAN) according to a preferred embodiment. 
         FIG. 2  shows a pipelined two-stage encapsulation and transmission process for network device of  FIG. 1  according to a preferred embodiment. 
         FIG. 3  shows the format of a pipelining setup message according to a preferred embodiment. 
         FIG. 4  shows a network device in communication with a network such as a WLAN according to a preferred embodiment. 
         FIG. 5  shows a pipelined two-stage reception and decapsulation process for network device of  FIG. 4  according to a preferred embodiment. 
     
    
    
     The leading digit(s) of each reference numeral used in this specification indicates the number of the drawing in which the reference numeral first appears. 
     DETAILED DESCRIPTION 
     Embodiments of the present invention employ pipelining strategies that allow encapsulation and decapsulation of packets in two stages, one of which is implemented for each packet before the packet is received. These pipelining approaches significantly reduce the time required to encapsulate and decapsulate a series of packets. 
     The two stages are encapsulation seed generation and encapsulation or decapsulation. When certain parameters for the packets to be received are known or assumed in advance, the encapsulation seeds for the packets can be generated before receiving the packets. Therefore the packets can be encapsulated or decapsulated immediately upon arrival, rather than after the time-consuming encapsulation seed generation process. 
     Embodiments of the present invention are described with respect to the Temporal Key Integrity Protocol (TKIP) and Arcfour algorithm specified by the IEEE 802.11i standard (April 2004). However, as will be apparent to one skilled in the relevant arts after reading this description, the techniques disclosed herein are equally applicable to other sorts of encapsulation and encryption protocols, and to wired networks as well as wireless networks. 
       FIG. 1  shows a network device  102  in communication with a network  104  such as a wireless local-area network (WLAN) according to a preferred embodiment. Network device  102  comprises an input circuit  106 , a controller  108 , a key mixing circuit  110 , a memory  112 , a Message Integrity Code (MIC) circuit  114 , a fragmentation circuit  116 , a WEP encapsulation circuit  118 , and an output circuit  120 . According to some embodiments, network device  102  is compliant with IEEE standards 802.11i (April 2004), and is otherwise compliant with one or more of IEEE standards 802.11 (1999 Edition), 802.11a (1999 Edition; Amended 2000), 802.11b (16 Sep. 1999 Edition), 802.11g (April 2003), 802.11n, 802.16 (2004), and 802.20-PD-06 (Jul. 16, 2004), the disclosures thereof incorporated herein by reference in their entirety. 
       FIG. 2  shows a pipelined two-stage encapsulation and transmission process  200  for network device  102  of  FIG. 1  according to a preferred embodiment. In the second of the two stages, each of N Media Access Control (MAC) Payload Data Units (MPDUs) is encapsulated using one of N Wired Equivalent Privacy (WEP) seeds. The first stage of the process generates the N WEP seeds ahead of time so they are ready when the second stage begins. 
     Referring to  FIG. 2 , controller  108  provides a transmitter MAC address TA, a temporal key TK, and at least a start value for a Temporal Key Integrity Protocol (TKIP) Sequence Count (TSC) to key mixing circuit  110  (step  202 ). The transmitter MAC address TA is a MAC address for network device  102 . The temporal key TK is a key that is negotiated in advance by network device  102  and an intended receiver, as is well-known in the relevant arts. The TSC is a counter for the MPDUs to ensure sequential reception at the intended receiver, as described in detail below. 
     Key mixing circuit  110  generates each of the N WEP seeds based upon temporal key TK, transmitter MAC address TA, and the start value for TSC, and stores the N WEP seeds in memory  112  (step  204 ). Preferably key mixing circuit  110  generates a WEP seed for each of N sequential values of TSC beginning with the start value provided by controller  108 , although other methods could be used. The start value for TSC is preferably initialized to one (TSC=1) during initialization of network device  102 , and is incremented for each WEP seed generated. 
     Input circuit  106  subsequently receives one or more MAC Service Data Units (MSDUs) (step  206 ), for example from a host unit or processor within network device  102 . MIC circuit  114  computes a MIC value for each MSDU based upon a MIC key, and inserts the MIC value into the respective MSDU (step  208 ) according to methods well-known in the relevant arts. 
     Fragmentation circuit  116  fragments the MSDUs to produce the N MPDUs, and inserts a value for TSC into each MPDU so the MPDUs are numbered sequentially (step  210 ). Each of the N values for the TSC is greater than, or equal to, the start value for the TSC. 
     WEP encapsulation circuit  118  encapsulates each of the N MPDUs using the one of the N WEP seeds that was generated based on the value for the TSC in the respective one of the N MPDUs (step  212 ). For example, WEP encapsulation circuit  118  receives a MPDU from fragmentation circuit  116  and the corresponding WEP seed from memory  112 , and performs the encapsulation according to methods well-known in the relevant arts to produce an encapsulated MPDU (EMPDU). 
     Output circuit  120  sends the N EMPDUs (step  214 ). For example, output circuit  120  can comprise a wireless physical-layer device (PHY) and antenna. As another example, output circuit  120  can be a MAC output circuit that provides the EMPDUs to a separate PHY and antenna for transmission to network  104 . 
     According to process  200 , key mixing circuit  110  generates each of the N WEP seeds before input circuit  106  receives the MSDUs. Therefore each WEP seed is available when the corresponding MPDU arrives at WEP encapsulation circuit  118 . This pipelining process ensures that no time is wasted in waiting for a WEP seed to be generated after receiving the corresponding MSDU. 
     In some embodiments, network device  102  generates and sends a pipelining setup message containing information describing the N MPDUs before sending the MPDUs. The pipelining setup message can be used by the intended recipient to pipeline the decapsulation process, as described in detail below. Network device  102  can generate and send the pipelining setup message either with or without performing the pipelining process  200  described above.  FIG. 3  shows the format of a pipelining setup message  300  according to a preferred embodiment. 
     Pipelining setup message  300  preferably comprises a transmitter address  302 , a receiver address  304 , a TSC start value  308 , and an optional TSC range value  310 . Transmitter address  302  is preferably the 6-byte MAC address of network device  102 . Receiver address  304  is preferably the 6-byte MAC address of a wireless receiver, such as a wireless access point, in network  104 . 
     TSC start value  308  is the 6-byte value of TSC for the first of the N MPDUs to be sent. TSC range value  310  is a 1-byte value representing the number N of MPDUs to be sent. Of course, the TSC information can be represented in other ways in pipelining setup message  300 . For example, instead of including the start value and range for TSC, the TSC information could comprise the end value and range, the start and end values, or any other parameters that can be used to calculate the start and range values of TSC. 
       FIG. 4  shows a network device  402  in communication with a network  404  such as a wireless local-area network (WLAN) according to a preferred embodiment. Network device  402  comprises an input circuit  406 , a controller  408 , a key mixing circuit  410 , a memory  412 , a Message Integrity Code (MIC) circuit  414 , a reassembly circuit  416 , a WEP decapsulation circuit  418 , an output circuit  420 , optional comparators  422  and  424 , and an optional countermeasures circuit  426 . According to some embodiments, network device  402  is compliant with IEEE standards 802.11i, and is otherwise compliant with one or more of IEEE standards 802.11 (1999 Edition), 802.11a (1999 Edition; Amended 2000), 802.11b (16 Sep. 1999 Edition), 802.11g (April 2003), 802.11n, 802.16 (2004), and 802.20-PD-06 (Jul. 16, 2004), the disclosures thereof incorporated herein by reference in their entirety. 
       FIG. 5  shows a pipelined two-stage reception and decapsulation process  500  for network device  402  of  FIG. 4  according to a preferred embodiment. In the second of the two stages, each of N encapsulated MPDUs (EMPDUs) are decapsulated using one of N WEP seeds. The first stage of the process generates the N WEP seeds ahead of time so they are ready when the second stage begins. 
     Referring to  FIG. 5 , controller  408  provides a transmitter MAC address TA, a temporal key TK, and at least a start value for a Temporal Key Integrity Protocol (TKIP) Sequence Count (TSC) to key mixing circuit  410  (step  502 ). The transmitter MAC address TA is a MAC address for the network device transmitting the MPDUs. The temporal key TK is a key that is negotiated in advance by network device  402  and the network device transmitting the MPDUs, as is well-known in the relevant arts. The TSC is a counter for the MPDUs to ensure sequential reception, as described in detail below. 
     In some embodiments, the network device transmitting the MPDUs first transmits a pipelining setup message such as the pipelining setup message described above with reference to  FIG. 3  before transmitting the MPDUs. Network device  402  then extracts TA and the start value for TSC from the pipelining setup message. 
     In other embodiments, network device  402  generates TA and the start value for TSC without the use of a pipelining setup message. For example, network device  402  can generate a set of WEP seeds for one or more of the known transmitters in network  404  using TSC values that are assumed or tracked by monitoring network traffic. In some embodiments, each network device maintains a separate TSC counter for each of the other network devices. The network devices can set TSC=1 on network initialization, association with the corresponding network device, and the like, to ensure that their TSC counters remain synchronized. In such embodiments, no pipelining setup message is needed. 
     Key mixing circuit  410  generates each of the N WEP seeds based upon temporal key TK, transmitter MAC address TA, and the start value for TSC, and stores the N WEP seeds in memory  412  (step  504 ). Preferably key mixing circuit  410  generates a WEP seed for N sequential values of TSC beginning with the start value provided by controller  408 , although other methods could be used. The start value for TSC is preferably initialized to one (TSC=1) during initialization of network device  402 , and is incremented for each WEP seed generated. 
     Input circuit  406  subsequently receives N EMPDUs (step  506 ). For example, input circuit  406  can comprise a wireless physical-layer device (PHY) and antenna. As another example, input circuit  406  can be a MAC input circuit that receives the EMPDUs from a separate PHY and antenna in communication with network  404 . 
     Optional comparator  322  checks the value of TSC in each EMPDU against the expected value. Out-of-sequence EMPDUs are discarded. In-sequence EMPDUs are provided to WEP decapsulation circuit  418 . 
     WEP decapsulation circuit  418  decapsulates each of the N EMPDUs using the one of the N WEP seeds that was generated based on the value for the TSC in the respective one of the N EMPDUs (step  508 ). For example, WEP decapsulation circuit  418  receives an EMPDU and receives the corresponding WEP seed from memory  412 , and performs the decapsulation according to methods well-known in the relevant arts to produce a decapsulated MPDU. Reassembly circuit  416  reassembles the N MPDUs to produce the original MSDUs (step  510 ). 
     MIC circuit  414  computes a MIC value for each MSDU based upon a MIC key (step  512 ). Optional comparator  324  checks the value of TSC in each MSDU against the computed value (step  513 ). If the MIC values are equal, output circuit  420  sends the MSDU (step  514 ), for example to a host unit or processor within network device  402 . Otherwise, optional countermeasures circuit  326  employs one or more countermeasures (step  516 ) such as those described in IEEE standard 802.11i (April 2004). 
     According to process  500 , key mixing circuit  410  generates each of the N WEP seeds before input circuit  406  receives the MSDUs. Therefore each WEP seed is available when the corresponding EMPDU arrives at WEP decapsulation circuit  418 . This pipelining process ensures that no time is wasted in waiting for a WEP seed to be generated after receiving the corresponding EMPDU. 
     The invention can be implemented in digital electronic circuitry, or in computer hardware, firmware, software, or in combinations of them. Apparatus of the invention can be implemented in a computer program product tangibly embodied in a machine-readable storage device for execution by a programmable processor; and method steps of the invention can be performed by a programmable processor executing a program of instructions to perform functions of the invention by operating on input data and generating output. The invention can be implemented advantageously in one or more computer programs that are executable on a programmable system including at least one programmable processor coupled to receive data and instructions from, and to transmit data and instructions to, a data storage system, at least one input device, and at least one output device. Each computer program can be implemented in a high-level procedural or object-oriented programming language, or in assembly or machine language if desired; and in any case, the language can be a compiled or interpreted language. Suitable processors include, by way of example, both general and special purpose microprocessors. Generally, a processor will receive instructions and data from a read-only memory and/or a random access memory. Generally, a computer will include one or more mass storage devices for storing data files; such devices include magnetic disks, such as internal hard disks and removable disks; magneto-optical disks; and optical disks. Storage devices suitable for tangibly embodying computer program instructions and data include all forms of non-volatile memory, including by way of example semiconductor memory devices, such as EPROM, EEPROM, and flash memory devices; magnetic disks such as internal hard disks and removable disks; magneto-optical disks; and CD-ROM disks. Any of the foregoing can be supplemented by, or incorporated in, ASICs (application-specific integrated circuits). 
     A number of implementations of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other implementations are within the scope of the following claims.