Abstract:
A method, apparatus, system, and non-transitory computer-readable storage medium that in an embodiment dynamically allocate client requests to target servers based on prepare messages sent by the target servers. The addresses of target servers are added to a queue in response to the prepare messages from the target servers. A network interface is then prepared to receive an incoming call request from a client. After the call request arrives from a client, one of the addresses is selected from the queue. The call request is then sent through a tunnel to the target server associated with the selected address.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This is a continuation application of U.S. patent application Ser. No. 10/892,461, filed Jul. 15, 2004,now U.S. Pat. No. 7,480,733, entitled “Routing Incoming Call Requests,” which is herein incorporated by reference. 
    
    
     FIELD 
     An embodiment of the invention generally relates to a computer network. In particular, an embodiment of the invention generally relates to routing incoming call requests to target servers in a network. 
     BACKGROUND 
     The development of the EDVAC computer system of 1948 is often cited as the beginning of the computer era. Since that time, computer systems have evolved into extremely sophisticated devices, and computer systems may be found in many different settings. Computer systems typically include a combination of hardware (such as semiconductors, integrated circuits, programmable logic devices, programmable gate arrays, and circuit boards) and software, also known as computer programs. Years ago, computers were isolated devices that did not communicate with each other. But, today computers are often connected in networks, such as the Internet or World Wide Web, and a user at one computer, often called a client, may wish to access information at multiple other computers, often called target servers, via a network. 
     Various techniques are currently used for communicating between clients and servers. One such technique is called PPP (Point-to-Point Protocol), which is defined in RFC (Request for Comments) 1661. PPP defines an encapsulation mechanism for transporting multi-protocol packets across layer 2 (L2) point-to-point links. Typically, a user obtains an L2 connection to a Network Access Server (NAS) using one of a number of techniques, such as dialup POTS (Plain Old Telephone Service), ISDN (Integrated Services Digital Network), or ADSL (Asymmetric Digital Subscriber Line). Then, the user runs PPP over that connection. In such a configuration, the L2 termination point and PPP session endpoint reside on the same physical device, i.e., the NAS. 
     Another technique for communicating between clients and servers is called L2TP (Layer 2 Tunneling Protocol), which is defined in RFC 2661 and is hereby incorporated by reference. L2TP extends the PPP model by allowing the L2 and PPP endpoints to reside on different devices interconnected via a packet-switched network. With L2TP, a user has an L2 connection to an access concentrator (e.g., a modem bank, ADSL, or DSLAM (Digital Subscriber Line Access Module)), and the concentrator then tunnels individual PPP frames to the NAS. This allows the actual processing of PPP packets to be divorced from the termination of the L2 circuit. 
     A technique for routing client requests to target servers using L2TP is called compulsory tunneling, in which a particular client user id (identification) is routed via a L2TP tunnel after partial identification. For example, an ISP (Internet Service Provider) is connected to a LAN (Local Area Network), which is connected to a particular target server. A variation of compulsory tunneling is called multihop, in which tunnels are chained together after a partial authentication. For example, a client may be connected to the Internet, which is connected to a router, and then the router is connected to a LAN, which is connected to the target server. 
     The main disadvantage of the compulsory tunneling and multihop techniques is that they require static configuration on the ISP, firewall, or router that connects the client to the target server. They also require partial authentication, which involves the following steps. First, the client and the ISP, firewall, or router exchange negotiations (e.g., PPP LCP (Link Control Protocol) negotiations) up to an authentication stage. Second, the ISP, firewall, or router uses the authentication information with some external configuration information to choose a target server. Third, the ISP, firewall, or router starts the next part of the route. Finally, the negotiation may need to restart from the beginning between the client and the target server. Unfortunately, partial identification increases the chance of retry/timeout failures under heavy load, which degrades network performance. 
     The compulsory tunneling and multihop techniques described above also present challenges when a target server is down or overloaded and traffic needs to be re-routed to a different target server. Current techniques for attempting to address these challenges, and the limitations of these techniques include the following. 
     In a first technique, the ISP, firewall, or router is reconfigured to route requests to different servers when a particular target server is down or the network configuration changes. Unfortunately, reconfiguration requires manual intervention by a system administrator, which can causes significant delays while the system administrator diagnoses and addresses the problem. 
     In a second technique, the ISP, firewall, or router looks up the target server by name via DNS (Domain Name System) and receives a list of addresses pointing to different servers. This technique requires special software on the ISP, firewall, or router to take advantage of this list for load balancing of client requests among the target servers. Alternatively, the ISP, firewall, or router can look up the target server every time it needs to route a new connection and the DNS can round robin the first IP address to load balance among the target services. Unfortunately, this technique generates extra traffic on the network and does not handle the target server being down or otherwise inoperative. 
     In a third technique, when a target server goes down, a second server takes over the IP (Internet Protocol) address of the target server. Unfortunately, this technique does not help with load balancing of requests among target servers. When requests are not properly load balanced among the target servers, a subset of the target servers receives a disproportionate number of requests to the exclusion of other target servers, which results in the subset being a bottleneck to performance while the other target servers are underutilized and their performance capacity is wasted. 
     In a fourth technique, called dynamic DNS, the DNS is updated when a server is down due to route traffic to a new target server, which requires additional software and does not help resolve load balancing problems. 
     Without a better technique for routing incoming client requests to target servers, client requests will continue to suffer from degraded performance and target servers will continue to suffer from poor load balancing. 
     SUMMARY 
     A method, apparatus, system, and non-transitory computer-readable storage medium are provided that in an embodiment dynamically allocate client requests to target servers based on prepare messages sent by the target servers. The addresses of target servers are added to a queue in response to the prepare messages from the target servers. A network interface is then prepared to receive an incoming call request from a client. After the call request arrives from a client, one of the addresses is selected from the queue. The call request is then sent through a tunnel to the target server associated with the selected address. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1  depicts a block diagram of an example system for implementing an embodiment of the invention. 
         FIG. 2A  depicts a block diagram of an example data structure for an incoming call queue, according to an embodiment of the invention. 
         FIG. 2B  depicts a block diagram of an example data structure for a control message, according to an embodiment of the invention. 
         FIG. 3  depicts a flowchart of example processing for handling a prepare for incoming call request control message from a target server, according to an embodiment of the invention. 
         FIG. 4  depicts a flowchart of example processing for handling an incoming call request from a client, according to an embodiment of the invention. 
         FIG. 5  depicts a flowchart of example processing for handling a stop control connection control message from a target server, according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to the Drawing, wherein like numbers denote like parts throughout the several views,  FIG. 1  depicts a high-level block diagram representation of a computer system  100  connected to clients  132  and target servers  134  via a network  130 , according to an embodiment of the present invention. The major components of the computer system  100  include one or more processors  101 , a main memory  102 , a terminal interface  111 , a storage interface  112 , an I/O (Input/Output) device interface  113 , and communications/network interfaces  114 , all of which are coupled for inter-component communication via a memory bus  103 , an I/O bus  104 , and an I/O bus interface unit  105 . 
     The computer system  100  contains one or more general-purpose programmable central processing units (CPUs)  101 A,  101 B,  101 C, and  101 D, herein generically referred to as the processor  101 . In an embodiment, the computer system  100  contains multiple processors typical of a relatively large system; however, in another embodiment the computer system  100  may alternatively be a single CPU system. Each processor  101  executes instructions stored in the main memory  102  and may include one or more levels of on-board cache. 
     The main memory  102  is a random-access semiconductor memory for storing data and programs. The main memory  102  is conceptually a single monolithic entity, but in other embodiments the main memory  102  is a more complex arrangement, such as a hierarchy of caches and other memory devices. For example, memory may exist in multiple levels of caches, and these caches may be further divided by function, so that one cache holds instructions while another holds non-instruction data, which is used by the processor or processors. Memory may further be distributed and associated with different CPUs r sets of CPUs, as is known in any of various so-called non-uniform memory access (NUMA) computer architectures. 
     The memory  102  includes an operating system  144 , an incoming call queue  146 , a control message  148 , and a controller  150 . Although the operating system  144 , the incoming call queue  146 , the control message  148 , and the controller  150  are illustrated as being contained within the memory  102  in the computer system  100 , in other embodiments some or all of them may be on different computer systems and may be accessed remotely, e.g., via the network  130 . The computer system  100  may use virtual addressing mechanisms that allow the programs of the computer system  100  to behave as if they only have access to a large, single storage entity instead of access to multiple, smaller storage entities. Thus, while the operating system  144 , the incoming call queue  146 , the control message  148 , and the controller  150  are all illustrated as being contained within the memory  102  in the computer system  100 , these elements are not necessarily all completely contained in the same storage device at the same time. 
     The operating system  144  may be implemented via OS/400, AIX, or Linux, but in other embodiments any appropriate operating system may be used. The operating system  144  may include low-level code to manage the resources of the computer system  100 . 
     The controller  150  processes the control messages  148  received from the target servers  134  via the network  130  and processes incoming calls received from the clients  132  via the network  130 . The controller  150  further manages the incoming call queue  146 . The incoming call queue  146  is further described below with reference to  FIG. 2A . The control message  148  is further described below with reference to  FIG. 2B . 
     In an embodiment, the controller  150  includes instructions capable of executing on the processor  101  or statements capable of being interpreted by instructions executing on the processor  101  to perform the functions as further described below with reference to  FIGS. 3 ,  4 , and  5 . In another embodiment, the controller  150  may be implemented in microcode. In yet another embodiment, the controller  150  may be implemented in hardware via logic gates and/or other appropriate hardware techniques, in lieu of or in addition to a processor-based system. 
     The memory bus  103  provides a data communication path for transferring data among the processors  101 , the main memory  102 , and the I/O bus interface unit  105 . The I/O bus interface unit  105  is further coupled to the system I/O bus  104  for transferring data to and from the various I/O units. The I/O bus interface unit  105  communicates with multiple I/O interface units  111 ,  112 ,  113 , and  114 , which are also known as I/O processors (IOPs) or I/O adapters (IOAs), through the system I/O bus  104 . The system I/O bus  104  may be, e.g., an industry standard PCI (Peripheral Component Interconnect) bus, or any other appropriate bus technology. The I/O interface units support communication with a variety of storage and I/O devices. For example, the terminal interface unit  111  supports the attachment of one or more user terminals  121 ,  122 ,  123 , and  124 . 
     The storage interface unit  112  supports the attachment of one or more direct access storage devices (DASD)  125 ,  126 , and  127  (which are typically rotating magnetic disk drive storage devices, although they could alternatively be other devices, including arrays of disk drives configured to appear as a single large storage device to a host). The contents of the DASD  125 ,  126 , and  127  may be loaded from and stored to the memory  102  as needed. The storage interface unit  112  may also support other types of devices, such as a tape device  131 , an optical device, or any other type of storage device. 
     The I/O and other device interface  113  provides an interface to any of various other input/output devices or devices of other types. Two such devices, the printer  128  and the fax machine  129 , are shown in the exemplary embodiment of  FIG. 1 , but in other embodiment many other such devices may exist, which may be of differing types. 
     The network interface  114  provides one or more communications paths from the computer system  100  to other digital devices and computer systems; such paths may include, e.g., one or more networks  130 . In various embodiments, the network interface  114  may be implemented via a modem, a LAN (Local Area Network) card, a virtual LAN card, or any other appropriate network interface or combination of network interfaces. 
     Although the memory bus  103  is shown in  FIG. 1  as a relatively simple, single bus structure providing a direct communication path among the processors  101 , the main memory  102 , and the I/O bus interface  105 , in fact the memory bus  103  may comprise multiple different buses or communication paths, which may be arranged in any of various forms, such as point-to-point links in hierarchical, star or web configurations, multiple hierarchical buses, parallel and redundant paths, etc. Furthermore, while the I/O bus interface  105  and the I/O bus  104  are shown as single respective units, the computer system  100  may in fact contain multiple I/O bus interface units  105  and/or multiple I/O buses  104 . While multiple I/O interface units are shown, which separate the system I/O bus  104  from various communications paths running to the various I/O devices, in other embodiments some or all of the I/O devices are connected directly to one or more system I/O buses. 
     The computer system  100  depicted in  FIG. 1  has multiple attached terminals  121 ,  122 ,  123 , and  124 , such as might be typical of a multi-user “mainframe” computer system. Typically, in such a case the actual number of attached devices is greater than those shown in  FIG. 1 , although the present invention is not limited to systems of any particular size. The computer system  100  may alternatively be a single-user system, typically containing only a single user display and keyboard input, or might be a server or similar device which has little or no direct user interface, but receives requests from other computer systems (clients). In other embodiments, the computer system  100  may be implemented as a firewall, router, Internet Service Provider (ISP), personal computer, portable computer, laptop or notebook computer, PDA (Personal Digital Assistant), tablet computer, pocket computer, telephone, pager, automobile, teleconferencing system, appliance, or any other appropriate type of electronic device. 
     The network  130  may be any suitable network or combination of networks and may support any appropriate protocol suitable for communication of data and/or code to/from the computer system  100 . In various embodiments, the network  130  may represent a storage device or a combination of storage devices, either connected directly or indirectly to the computer system  100 . In an embodiment, the network  130  may support Infiniband. In another embodiment, the network  130  may support wireless communications. In another embodiment, the network  130  may support hard-wired communications, such as a telephone line or cable. In another embodiment, the network  130  may support the Ethernet IEEE (Institute of Electrical and Electronics Engineers) 802.3x specification. In another embodiment, the network  130  may be the Internet and may support IP (Internet Protocol). In another embodiment, the network  130  may be a local area network (LAN) or a wide area network (WAN). In another embodiment, the network  130  may be a hotspot service provider network. In another embodiment, the network  130  may be an intranet. In another embodiment, the network  130  may be a GPRS (General Packet Radio Service) network. In another embodiment, the network  130  may be a FRS (Family Radio Service) network. In another embodiment, the network  130  may be any appropriate cellular data network or cell-based radio network technology. In another embodiment, the network  130  may be an IEEE 802.11B wireless network. In still another embodiment, the network  130  may be any suitable network or combination of networks. Although one network  130  is shown, in other embodiments any number of networks (of the same or different types) may be present, and the client  132  and the target server  134  need not be connected to the same network. For example, in an embodiment, the clients  132  are connected to the computer system  100  via the Internet while the target server  134  is connected to the computer system  100  via a LAN. 
     The client  132  may further include some or all of the hardware components previously described above for the computer system  100 . Although only one client  132  is illustrated, in other embodiments any number of clients may be present. The client  132 , or a user of the client  132 , desires to send requests to the target servers  134 . 
     In L2TP terms, the computer system  100  acts as a LAC (L2TP Access Concentrator), which is a node that acts as one side of an L2TP tunnel endpoint and is a peer to the target server  134 , which in L2TP terms is an L2TP Network Server (LNS), but in other embodiments any appropriate protocol may be used. The LAC sits between an LNS and a remote system and forwards packets to and from each of them. The target server  134  may include some or all of the hardware elements previously described above for the computer system  100 . In an embodiment, the target server  134  may be implemented as an LNS in L2TP terms, but in other embodiments any appropriate protocol may be used. 
     It should be understood that  FIG. 1  is intended to depict the representative major components of the computer system  100 , the network  130 , the clients  132 , and the target servers  134  at a high level, that individual components may have greater complexity than represented in  FIG. 1 , that components other than, fewer than, or in addition to those shown in  FIG. 1  may be present, and that the number, type, and configuration of such components may vary. Several particular examples of such additional complexity or additional variations are disclosed herein; it being understood that these are by way of example only and are not necessarily the only such variations. 
     The various software components illustrated in  FIG. 1  and implementing various embodiments of the invention may be implemented in a number of manners, including using various computer software applications, routines, components, programs, objects, modules, data structures, etc., referred to hereinafter as “computer programs,” or simply “programs.” The computer programs typically comprise one or more instructions that are resident at various times in various memory and storage devices in the computer system  100 , and that, when read and executed by one or more processors  101  in the computer system  100 , cause the computer system  100  to perform the steps necessary to execute steps or elements embodying the various aspects of an embodiment of the invention. 
     Moreover, while embodiments of the invention have and hereinafter will be described in the context of fully functioning computer systems, the various embodiments of the invention are capable of being distributed as a program product in a variety of forms, and the invention applies equally regardless of the particular type of non-transitory computer-readable storage medium used to actually carry out the distribution. The programs defining the functions of this embodiment may be delivered to the computer system  100  via a variety of non-transitory computer-readable storage media, which include, but are not limited to: 
     (1) information permanently stored on a non-rewriteable storage medium, e.g., a read-only memory device attached to or within a computer system, such as a CD-ROM readable by a CD-ROM drive; or 
     (2) alterable information stored on a rewriteable storage medium, e.g., a hard disk drive (e.g., DASD  125 ,  126 , or  127 ), CD-RW, or diskette. Such non-transitory computer-readable storage media, when carrying machine-readable instructions that direct the functions of the present invention, represent embodiments of the present invention. 
     In addition, various programs described hereinafter may be identified based upon the application for which they are implemented in a specific embodiment of the invention. But, any particular program nomenclature that follows is used merely for convenience, and thus embodiments of the invention should not be limited to use solely in any specific application identified and/or implied by such nomenclature. 
     The exemplary environments illustrated in  FIG. 1  are not intended to limit the present invention. Indeed, other alternative hardware and/or software environments may be used without departing from the scope of the invention. 
       FIG. 2A  depicts a block diagram of an example data structure for the incoming call queue  146 , according to an embodiment of the invention. The incoming call queue  146  includes a FIFO (First In First Out) queue of addresses of the target servers  134 . For a FIFO queue, addresses of the target servers  134  are added to the queue  146 , in order, as control messages are received from the target servers  134  and are removed in the same order that they were received. In another embodiment, the incoming call queue  146  is a round-robin queue of addresses of the target servers  134 . For a round-robin queue, each of the target servers  134  has a separate queue (or a count of how many control messages were received) and the queues are serviced alternately, in turn. 
     Illustrated are entries  205 ,  210 , and  215  in the incoming call queue  146 , but in other embodiments any number of entries with any appropriate type of identifications (IP address or other type of address) of the target servers  134  may be present. Each of the target servers  134  may be identified any appropriate number of times in the incoming call queue  146 . The controller  150  enqueues or adds the entries to the incoming call queue  146  in response to a control message from the target server  134 , as further described below with reference to  FIG. 3 . The controller  150  dequeues or removes the entries from the incoming call queue  146  in response to an incoming call from the client  132 , as further described below with reference to  FIGS. 4 and 5 . 
       FIG. 2B  depicts a block diagram of an example data structure for the control message  148 , according to an embodiment of the invention. In L2TP terms, control messages are exchanged between LAC and LNS pairs, operating in-band within a tunnel protocol. Control messages govern aspects of the tunnel and sessions within the tunnel. A tunnel exists between a LAC-LNS pair. The tunnel consists of a control connection and zero or more L2TP sessions. The tunnel carries encapsulated PPP datagrams and control messages between the LAC and LNS. But, in other embodiments, any appropriate protocol and any appropriate type of control message may be used. 
     The control message  148  includes a type field  250 . In an embodiment, the type  250  is a message type AVP (Attribute Value Pair), which is a variable length concatenation of a unique attribute and a value containing the actual value identified by the attribute. Multiple AVPs make up the control messages  148 , which are used in the establishment, maintenance, and teardown of tunnels and calls. The type field  250  indicates the type of the control message  148 , such as a “prepare for incoming call request” message, a “start control connection request” message, a “stop control connection” message, and a “prepare for incoming call reply” message, but in other embodiments any appropriate types may be used. The processing for the various types of control messages is further described below with reference to  FIGS. 3 ,  4 , and  5 . In an embodiment, the “prepare for incoming call request” type and the “prepare for incoming call reply” type are optional, and the control messages having the “prepare for incoming call request” type and the “prepare for incoming call reply” type may include a mandatory indicator set to off, so that implementations that do not support this type may ignore these control messages. But, in another embodiment, the “prepare for incoming call request” type and the “prepare for incoming call reply type” may be required to be supported. 
       FIG. 3  depicts a flowchart of example processing for handling a prepare for incoming call request control message, according to an embodiment of the invention. Control begins at block  300 . Control then continues to block  305  where the target server  134  creates a tunnel to the computer system  100 . Control then continues to block  310  where the target server  134  sends the control message  148  to the computer system  100 , which is intended to make the target server  134  available to the clients  132 . 
     Control then continues to block  315  where the controller  150  determines whether the control message  148  contains the type  250  that indicates that this control message is intended to prepare the computer system  100  for an incoming call from one of the clients  132 . 
     If the determination at block  315  is true, then the type  250  in the control message  148  indicates that the computer system  100  should prepare for an incoming call, so control continues from block  315  to block  320  where the controller  150  prepares the network interface  114 , such as a modem or LAN card, for the clients  132  to connect to. 
     Control then continues to block  325  where the controller  150  determines whether the preparation at block  320  was successful. If the determination at block  325  is true, then the preparation of the network interface  114  at block  320  was successful, so control continues from block  325  to block  330  where the controller  150  sends a prepare for incoming call reply message in response to the control message  148  to the target server  134  indicating success. Control then continues to block  335  where the controller  150  adds the address of the target server  134  to the incoming call queue  146 , e.g., as entry  205 ,  210 , or  215  as previously described above with reference to  FIG. 2A . Control then continues to block  399  where the logic of  FIG. 3  returns. 
     If the determination at block  325  is false, then the preparation of the network interface  114  was not successful, so control continues from block  325  to block  340  where the controller  150  sends a prepare for incoming call reply message in response to the control message to the target server  134  indicating failure. Control then continues to block  399  where the logic of  FIG. 3  returns. 
     If the determination at block  315  is false, then the type  250  is not prepare for an incoming call, so control continues from block  315  to block  345  where the controller  150  processes other messages. Control then continues to block  399  where the logic of  FIG. 3  returns. 
       FIG. 4  depicts a flowchart of example processing for handling an incoming call request from one of the clients  132 , according to an embodiment of the invention. Control begins at block  400 . Control then continues to block  405  where the client  132  connects to the controller  150  via the network interface  144  via an incoming call request. 
     In an embodiment, a call is a connection or attempted connection between a remote system and LAC. A call, incoming or outgoing, which is successfully established between a remote system and LAC results in a corresponding L2TP session within a previously established tunnel between the LAC and LNS. An incoming call is received at the LAC to be tunneled to the LNS. An outgoing call is a call placed by the LAC on behalf of the LNS. A remote system is an end-system or router attached to a remote access network (i.e., a Public Switched Telephone Network), which is either the initiator or recipient of a call. In another embodiment, a call may be any technique for the clients  132  to connect to the computer system  100  via the network interface  114 . 
     Control then continues to block  410  where the controller  150  dequeues the next entry from the incoming call queue  146 . Control then continues to block  415  where the controller  150  sends the incoming call request to the target server  134  whose address was dequeued from the incoming call queue  146 . Control then continues to block  420  where the client  132  that initiated the incoming call request communicates across the tunnel to the target server  134  whose address was dequeued from the incoming call queue  146 . Control then continues to block  499  where the logic of  FIG. 4  returns. 
       FIG. 5  depicts a flowchart of example processing for handling a stop control connection control message, according to an embodiment of the invention. Control begins at block  500 . Control then continues to block  505  where the target server  134  sends a stop control connection control message to the controller  150  in the computer system  100 , or the controller  150  detects a failure and closes the tunnel. Control then continues to block  510  where the controller  150  removes all entries for the requesting target server  134  from the incoming call queue  146 . Control then continues to block  599  where the logic of  FIG. 5  returns. 
     In this way, a static configuration of the target servers  134  to the clients  132  is not required. Instead, the target servers  134  are made available for incoming calls dynamically as requested by the target servers  134 . Thus, for example, if one of the target servers  134  goes down, it is removed from the queue and its requests may be handled by other target servers  134  as soon as the failure is detected. Also, when the request load from the clients  132  is heavy or at scheduled times, new target servers  134  may be brought online. A target server  134  may also reduced its load dynamically if desired by sending fewer, or none at all, of the prepare control messages to the computer system  100  or by sending a stop control connection control message. Further, because partial authentication is not required, retry/timeout failures are less likely. 
     In the previous detailed description of exemplary embodiments of the invention, reference was made to the accompanying drawings (where like numbers represent like elements), which form a part hereof, and in which is shown by way of illustration specific exemplary embodiments in which the invention may be practiced. These embodiments were described in sufficient detail to enable those skilled in the art to practice the invention, but other embodiments may be utilized and logical, mechanical, electrical, and other changes may be made without departing from the scope of the present invention. Different instances of the word “embodiment” as used within this specification do not necessarily refer to the same embodiment, but they may. The previous detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims. 
     In the previous description, numerous specific details were set forth to provide a thorough understanding of the invention. But, the invention may be practiced without these specific details. In other instances, well-known circuits, structures, and techniques have not been shown in detail in order not to obscure the invention.