Abstract:
A network interface driver embodied in a processor readable medium comprising executable program instructions that, when executed by a processor, independently process internetworking protocols for a host computer (or other network access device) with another computer on a remote network. The driver appears to the operating system as a regular network interface driver, but it can support enhanced kernel-level internetworking protocols by using a state machine to generate, drop, and change incoming and outgoing IP packets in a manner transparent to the operating system.

Description:
FIELD OF THE INVENTION 
   The present invention relates generally to computer networking, and more particularly, to a network interface driver for supporting new or improved internetworking functionality while maintaining backward compatibility with the overlying Internet Protocol (IP) stack of the computer operating system, thereby enabling independent software developers to upgrade kernel-level internetworking functions without the need to rely upon operating system vendors. 
   BACKGROUND 
   It is currently very difficult to add new (or improve legacy) kernel-level internetworking functions, such as IP mobility support, on computers using software-based solutions. This typically requires modifications to the IP stack that is built into an operating system, something that cannot be easily accomplished by independent software developers. 
   In the networking environment, data packets are dispatched across network connections by employing software drivers that control hardware modules. An operating system, such as, for example Microsoft Windows®, requires that drivers be programmed as passive, instantly responsive routines for the purpose of device I/O. A driver does not have active control; it always waits to be called by the operating system. Accordingly, in response to a call, the driver will rapidly complete the specified I/O operation and return control back to the operating system as quickly as possible. If the underlying physical device cannot finish the I/O operation within a specified period of time, the driver will typically start the physical device, instruct the physical device to generate a hardware interrupt to notify the operating system after the I/O operation has finished, and immediately return control back to the operating system with a pending status. 
   This invention proposes using a network interface driver to support new (or improve legacy) internetworking protocols, a task that was previously addressed at the IP layer or above. However, many internetworking protocols that are based on a client-server model require the client (and often the server) to have active, persistent control in order to manage signaling over a network connection. The signaling must account for transmission time over the network and unexpected breaks that can occur in the network connection. Hence, if a network interface driver is used to support internetworking protocols, the requirement for active, persistent control conflicts with the fundamental characteristics of a driver such as passiveness and instant response. 
   SUMMARY OF THE INVENTION 
   In accordance with the present invention, it is an object thereof to provide a network interface driver that “independently” supports internetworking protocols. In this regard, “independently” means that the driver does not require assistance from the operating system or any application program to implement the driver&#39;s functionality. 
   The present invention provides a generic method to solve the problems in the prior art by incorporating state machines into network interface drivers to support new (or improved) functions and to keep backward compatibility with the overlying IP stack of the operating system. 
   The present invention provides a special programming method for network interface drivers. It can be implemented as an intermediate driver independent of the underlying network hardware. It can also be implemented in a network interface driver dedicated to support some type of network hardware. In either case, the internetworking driver is commonly referred to as an “iDriver” hereinafter. As compared to regular network interface drivers that are only responsible for carrying out local network I/O functions for the operating system, an iDriver in accordance with the invention can provide enhanced kernel-level internetworking functions that are not supported by the built-in IP stack of the operating system. In this regard, the iDriver appears to the operating system as a regular network interface driver, but it can support enhanced kernel-level internetworking protocols by using a state machine to generate, drop, and change incoming and outgoing IP packets in a manner transparent to the operating system. 
   The iDriver need not be physically associated with a computer. In addition to use with network interface drivers, the iDriver can also be utilized with other I/O devices that are distributed on the Internet (i.e., a remote camera). 
   The iDriver: (1) enables independent software developers to upgrade kernel-level internetworking functions for networked computers without relying on operating system vendors; (2) allows independent software developers to distribute upgrades as standalone software products because these upgrades are implemented as network interface drivers that are not part of an operating system; and (3) assures compatibility between any upgrades and the operating system because such upgrades are based on standard network driver programming interfaces specified by the operating system. This software solution to many kernel-level internetworking problems previously required changes to be made to the built-in IP stack of the computer&#39;s operating system. 
   In accordance with the present invention, there is provided a network interface driver embodied in a processor readable medium comprising executable program instructions that, when executed by a processor, independently process internetworking protocols for a host computer with another computer on a remote network. The program instructions incorporate a state machine having a plurality of states, each state associated with a plurality of threads and waited events for performing a method on a processor, comprising the steps of: processing outgoing packets from an operating system associated with a host computer, in at least one of the plurality of states; processing incoming packets to an access network, in at least one of the plurality of states; and processing at least one of a plurality of time-out events in at least one of the plurality of states. The processing steps are all instant actions, comprising: generating a new packet and sending it to the operating system or the access network, dropping a packet received from the operating system or the access network, revising the content of a received packet and then sending it to the operating system or the access network, registering a time-out event with the operating system, and changing the state of the driver. 
   The present invention will now be described in detail with particular reference to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a state transition table for an exemplary iDriver that implement mobility management functions using DHCP and Mobile IP with a remote Home Agent; 
       FIG. 2  is a flow diagram of a Wait-Host-DHCP-Discover state (initial state); 
       FIG. 3  is a flow diagram of a Wait-Local-DHCP-Offer state (transient state) 
       FIG. 4  is a flow diagram of a Wait-Local-DHCP-Ack state (transient state) 
       FIG. 5  is a flow diagram of a Wait-Router-ARP-reply state (transient state); 
       FIG. 6  is a flow diagram of a Wait-HA-Registration-Reply state (transient state); 
       FIG. 7  is a flow diagram of a Wait-Host-DHCP-Request state (transient state); and 
       FIG. 8  is a flow diagram of an Operating state (common state); and 
       FIG. 9  is a schematic diagram of tunneling IP packets inside UDP datagrams in order to cross a NAPT box. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   With reference now to the several views of the drawings, the present invention provides a state machine in an iDriver implemented as in an intermediate network driver. The state machine has three standard threads, one processing incoming IP packets (hereinafter “the incoming thread”), one processing outgoing IP packets (the “outgoing thread”), and one processing time-out events (the “time-out thread”). The time-out thread associates with a system timer to obtain periodic control from the operating system, which, in effect, enables the iDriver to obtain active control from the operating system. The other two threads are utilized to process incoming and outgoing IP packets as state-dependent I/O events, each of which can be processed instantly as part of a small step of internetworking signaling utilizing a specified protocol. The state machine may have optional threads to process particular hardware-specific events (such as the Association event for 802.11 WLAN interface cards). These threads can be programmed into NIC (network interface card) drivers, intermediate network drivers, or virtual network interface drivers. A “server-type” iDriver can interact with multiple clients independently. In such an implementation, multiple state machines can be utilized. 
   The iDriver decomposes signaling into state-dependent I/O events that are capable of being processed instantly. An illustrative set of rules is described as follows: 
   (1) To support an enhanced internetworking protocol on behalf of the operating system, the iDriver acts as a proxy to edit all relevant protocols between the operating system and the network. The iDriver also implements the enhanced internetworking protocol between peers on the network and itself. The iDriver processes all relevant signaling with the operating system to keep the enhanced protocol transparent to and backward compatible with the operating system, and to make the network believe the operating system supports the enhanced protocol by itself. Accordingly, all signaling involving the iDriver can be characterized with a message flow chart that depicts the protocol message flow between the operating system and the iDriver, and the iDriver and any peers on the network. Where there is more than one paragraph in a signaling protocol, each can be described using a message flow chart. From all the message flow charts, one can identify the maximum number of states for the iDriver, which is equal to the number of types of protocol messages that the iDriver expects to receive from the network and from the operating system. These protocol messages are embodied in IP packets. If a protocol message must be carried by multiple IP packets, it should be broken into a series of protocol messages, each carried by an IP packet. A counter is associated with the state in which the iDriver is waiting for the entire protocol message. 
   (2) Among all signaling paragraphs, some can and some cannot occur concurrently. After identifying the maximum subset of non-concurrent signaling paragraphs, the actual number of states for the iDriver equals the number of protocol messages that the iDriver expects to receive from the operating system and from the network, if the iDriver engages only in signaling belonging to the subset. 
   (3) In each state, the iDriver is programmed as an event handler that can change its state. When an expected event occurs, a corresponding thread of the iDriver is called by the operating system. The iDriver then processes the event, changes the state if needed, and immediately returns control back to the operating system. Specifically, the iDriver defines a set of “waited events” and instant event handlers (callback functions) for every state. A waited event could be a specific incoming IP packet (which may represent an incoming protocol message), a specific outgoing IP packet (which may need to be edited by the iDriver in order to make the network believe the operating system support the protocol), or a specific time-out event (which may be registered by the iDriver to signal that an expected incoming protocol message has not arrived on time). An instant event handler only contains “instant actions” that can be finished immediately, such as reading from and writing variables to memory (including changing the state), registering a time-out event, dropping an incoming/outgoing IP packet, making an incoming/outgoing IP packet, editing an incoming/outgoing IP packet, and passing an original, made-up, or edited incoming/outgoing IP packet to the operating system/network interface card. When a prescribed event occurs, the thread responsible for processing this type of event is called by the operating system. A branch of the thread corresponding to the current state (every thread has multiple branches, each corresponding to a state) is executed and checks whether this event is a waited event in this state. If it is, the branch calls the corresponding instant event handler, then immediately returns. If the event is not a waited event for the state, the event is ignored and the thread returns immediately. 
   (4) A paragraph of a signaling protocol typically consists of one or more pairs of request and reply messages. There may be multiple reply messages responding to a request message. In this instance, all reply messages are considered to be the same type of protocol messages, which accordingly must be processed by different instant event handlers. A pair of request and reply messages, which cannot be finished instantly at the client side, can be decomposed into an instant action in the current state, a state transition, and a waited event in the next state. For example, assume the iDriver is in a first state, “State 1,” when it sends out a request message. As soon as the request message is dispatched, the iDriver changes into “State 2,” registers two time-out events (one for re-sending the request message and one for giving-up the conversation), and returns immediately. The reply message and the two time-out events are defined as waited events in State 2. If the re-sending time-out event occurs, the branch corresponding to State 2 in the time-out thread is executed. The iDriver re-sends the request message, registers a new re-send time-out event, and returns immediately. If the “giving-up” time-out event occurs, the branch corresponding to State 2 in the time-out thread is executed. It removes the re-sending time-out event registration, indicates a networking error to the overlying IP stack, changes the branch back to State 1 (or some initial state, depending on the protocol), and returns immediately. If the reply message arrives, the branch corresponding to State 2 in the incoming thread is executed. It removes both time-out event registrations, processes the reply message (such as sending out the next request message, registering two time-out events for the new reply message, changing into State 3, etc.), and returns immediately. 
   (5) The iDriver starts running in an initial state after some initialization event (e.g., an outgoing DHCP-Discover message sent by the operating system if the operating system uses DHCP to initialize the network interface). The iDriver blocks the network connection until the initialization event occurs. The iDriver can have “intelligence” in the initial state so that it can identify whether the operating system adopts DHCP, static IP address, or APIPA (Automatic Private IP Addressing) as its network interface configuration. 
   (6) The iDriver resides in a “common state” most of time in which it bridges incoming/outgoing IP data packets between the operating system and the network. It waits for an event to trigger the next round of signaling conversations. In this state, a client-type iDriver sends out the first request message. A server-type iDriver waits for the first request message sent by a client. It may be the destination state if a signaling conversation ended because the network connection has been severed or the peer does not respond accordingly. It can also be the state in which the iDriver polls information from the network. 
   In an illustrative embodiment, the iDriver provides IP mobility support to a network access device, such as, for example, a laptop using Microsoft Windows® with the NDIS (Network Device Interface Specification [1]) intermediate driver. The iDriver implements MH (Mobile Host) functions of Mobile IP [2] with security enhancement and routing optimization. It operates in conjunction with a remote HA (Home Agent) of the type well known in the art. These MH functions are transparent to the Windows® operating system. In this regard, Windows views the iDriver as a regular NIC driver, and the iDriver supplies Windows® with a fixed IP address (which belongs to the subnet where the HA is deployed). The iDriver implements the following tasks: (1) obtaining a care-of IP address from the local access network; (2) establishing an IP tunnel with the remote HA using the care-of IP address; (3) encapsulating outgoing IP packets and decapsulating incoming IP packets; and (4) monitoring the local access network and repeating the above steps if any change occurs (due, for example, to motion of the MH). 
   For the sake of simplicity, it is assumed that: (1) Windows® runs DHCP to obtain an IP address (the iDriver can be configured so that it automatically identifies whether the Windows runs DHCP or adopts a static IP address based on the type of outgoing IP packets when the network interface is initialized); (2) every local access network runs DHCP to allocate IP addresses (it is possible that a local access network might not run DHCP to allocate IP addresses (the iDriver can be configured to identify the type of the local access network and adopt the appropriate method to apply for a care-of IP address); and (3) there is a high-entropy secret shared between the iDriver and HA, thus enabling the use of CHAP for authentication and a D-H key exchange algorithm [5] signed by a secret key to establish session keys. DHCP is known in the art and described in, for example, R. Droms, “Dynamic Host Configuration Protocol,” IETF Network Working Group, RFC 2131 (March 1997); S. Alexander, R. Droms, “DHCP Options and BOOTP Vendor Extensions,” IETF Network Working Group, RFC 2132 (March 1997); which are incorporated by reference herein. CHAP is described in, for example, W. Simpson, “PPP Challenge Handshake Authentication Protocol (CHAP)”, IETF RFC1994, August 1996, which is incorporated herein by reference. D-H key exchange algorithms are known as described in, for example, W. Diffie and M. E. Hellman, “New Directions in Cryptography”, IEEE Trans. on Information Theory, vol. 22, no. 6, November 1976, pp/644–654, the content of which is incorporated herein by reference. 
   The foregoing is utilized with an iDriver configured as a Mobile IP client. The number of states for the iDriver are determined as described hereinafter. Under the assumption that Windows® runs DHCP to initialize a network interface, the iDriver expects to receive DHCP-Discover, DHCP-Request, DHCP-Decline, and DHCP-Release messages from Windows®. Among these, DHCP-Request and DHCP-Decline are of the same type. These are two different replies that are responsive to each DHCP-Offer message. In connection with the Mobile IP functions implemented by the Mobile IP client, the iDriver expects to receive DHCP-Offer, DHCP-Ack, and DHCP-Nack messages from local DHCP servers, ARP-Reply messages from local gateway routers, and REGR-Reply messages from the HA. Of these, DHCP-Ack and DHCP-Nack are of the same type, because these are two different replies to the DHCP-Request message. In addition, the iDriver expects to receive IP data packets from Windows® and the network. These data packets must be edited (i.e., encapsulation/decapsulation, and encryption/decryption). In total, there are nine types of protocol messages that the iDriver expects to receive from Windows® and the network. Accordingly, the maximum number of states for the iDriver is 9 in accordance with Rule No. 1 described above. However, the signaling conversations for receiving IP data packets from Windows® and the signaling conversations for receiving IP data packets from the network are concurrent with the other conversations (i.e., boot and handoff). Accordingly, the actual number of states is equal to seven, per Rule No. 2 above. 
   With reference to  FIG. 1 , an illustrative iDriver can be configured as a seven-state state machine with three threads: an incoming thread for processing incoming IP packets, an outgoing thread for processing outgoing IP packets, and one associated with a system timer that times out after a prescribed time expires (e.g., every 100 ms). The seven states are characterized as follows: (0) the Wait-Host-DHCP-Discover state (initial state)  100 , (0) the Wait-Local-DHCP-Offer state (transient state)  102 , (3) the Wait-Local-DHCP-Ack state (transient state)  104 , (4) the Wait-Router-ARP-reply state (transient state)  106 , (5) the Wait-HA-Registration-Reply state (transient state)  108 , (6) the Wait-Host-DHCP-Request state (transient state)  110 , and (7) the Operating state (common state)  112 . 
   After the states have been identified, the waited events for each state and the instant actions for their instant event handlers are defined. With reference to  FIG. 2 , initially the iDriver resides in the Wait-Host-DHCP-Discover state at  200  with a Booting-Flag set to “True.” The waited event in this state is an outgoing DHCP-Discover packet from Windows®. In response to a DHCP-Discover packet received from Windows® at  204  (i.e., the thread processing outgoing IP packets is called by Windows), the iDriver generates a DHCP-Discover packet with 0.0.0.0 as the source IP address and 255.255.255.255 as the destination IP address at  206 , and passes the DHCP-Discover packet to the underlying NIC driver at  208 . The NIC driver broadcasts the DHCP-Discover packet on the local access network. The iDriver then changes into the Wait-Local-DHCP-Offer state at  208 , and the outgoing thread returns control to Windows® at  212 . 
   Referring now to  FIG. 3 , in the Wait-Local-DHCP-Offer state identified at  300 , the iDriver waits for a DHCP server on the local access network to send back a DHCP-Offer packet at  302  in response to the DHCP-Discover packet received from the MH. If the associated system timer times out in this state at  304  (which means the iDriver has not received the expected DHCP-Offer packet within 100 ms), Windows® calls the time-out thread at  306 . The time out thread carries out the following instant actions: (1) regeneration of the DHCP-Discover packet at  308 ; and (2) passage of the DHCP-Discover packet to the underlying NIC driver at  310 . The NIC driver rebroadcasts the DHCP-Discover packet on the local access network. The time-out thread then returns control back to Windows® at  312 . After a DHCP-Offer packet is received from the local access network in this state, Windows® calls the incoming thread at  314 . The incoming thread then carries out the following instant actions: (1) storing the assigned IP address (as the care-of IP address) and other networking configuration parameters (including the default gateway router&#39;s IP address) in the MH&#39;s memory at  316 ; (2) generating a DHCP-Request packet with the care-of IP address as the source IP address at  318 ; (3) passing the DHCP-Request packet to the underlying NIC driver at  320 , which in turn forwards this DHCP-Request packet to the DHCP server, (4) resetting the Monitoring-Counter to zero at  322 ; and (5) changing into the Wait-Local-DHCP-Ack state at  324 . The incoming thread then returns control back to Windows® at  326 . 
   Referring now to  FIG. 4 , in the Wait-Local-DHCP-Ack state identified at  400 , the iDriver waits for a DHCP server on the local access network to reply with a DHCP-Ack packet or DHCP-Nack packet in response to the DHCP-Request packet. In the interim, the iDriver monitors the local access network based on the value of the Monitoring Counter. The waited events in this state include an incoming DHCP-Ack packet, an incoming DHCP-Nack packet, and a time out event at  402 . If the associated system timer times out in this state at  404  (which means the iDriver has not received the expected DHCP-Ack or DHCP-Nack within 100 ms), Windows® calls the time-out thread at  406 . The time-out thread then implements a series of actions. First, the Monitoring-Counter is increased by one at  408 . If the Monitoring-Counter equals a Changing-Access-Threshold (i.e., where the MH may have moved to another access network) at  410 , the iDriver generates a DHCP-Discover packet at  412  with 0.0.0.0 as the source IP address and 255.255.255.255 as the destination IP address. This packet is passed to the underlying NIC driver at  414 , which in turn broadcasts the DHCP-Discover packet on the local access network. The iDriver then changes back into the Wait-Local-DHCP-Offer state at  416 , and returns control back to Windows® at  418 . If a DHCP-Ack packet is received at  419  from the local access network in this state, Windows® calls the time-out thread at  420  and the iDriver generates an ARP-Request [6] packet with the care-of IP address as the source IP address at  422 . An ARP-Request packet with a care-of IP address as the source IP address is disclosed in D. C. Plummer, “Ethernet Address Resolution Protocol: Or converting network protocol addresses to 48.bit Ethernet address for transmission on Ethernet hardware”, IETF STD37 (RFC826), November 1982, the content of which is incorporated by reference herein. The iDriver then passes the ARP-Request packet to the underlying NIC driver at  424 , which in turn broadcasts the ARP-Request packet on the local access network to resolve the MAC address of the default gateway router. The iDriver “knows” the IP address of the default gateway router from the previously received DHCP-Offer packet. The Monitoring-Counter is then reset to zero at  426 . The iDriver changes into the Wait-Router-ARP-Reply state at  428 , and the incoming thread returns control back to Windows® at  430 . If a DHCP-Nack packet is received from the local access network in this state at  419 , the incoming-thread regenerates a DHCP-Discover packet at  412  with 0.0.0.0 as the source IP address and 255.255.255.255 as the destination IP address and implements the subsequent steps described above. 
   Referring now to  FIG. 5 , in the Wait-Router-ARP-Reply state identified at  500 , the iDriver waits for the default gateway router on the local access network to send back its MAC address in response to the ARP-Request packet from the MH. In the interim, the iDriver monitors the local access network based on the value of the Monitoring-Counter. The waited events in this state include an incoming ARP-Reply packet and a time-out event at  502 . If the associated system timer times out in this state at  504  (which means the iDriver has not received any ARP-Reply packet within 100 ms), Windows® calls the time-out thread at  506 . The time-out thread then carries out the following instant actions. First, the Monitor-Counter is increased by one at  508 . If the Monitoring-Counter equals a Changing-Access-Threshold at  510  (which means the MH may have moved into another local access network), the iDriver generates a DHCP-Discover packet at  512  with 0.0.0.0 as the source IP address and 255.255.255.255 as the destination IP address, and passes it to the underlying NIC driver at  514 . The NIC driver broadcasts this packet on the local access network. The iDriver then enters the Wait-Local-DHCP-Offer state at  516 . If the Monitoring-Counter does not equal the Changing-Access-Threshold, the iDriver regenerates the ARP-Request packet at  520  and passes it to the underlying NIC driver at  522 . The NIC driver sends the packet to the local access network in order to resolve the MAC address of the default gateway router. The time-out thread then returns control back to Windows® at  518 . If an ARP-Reply packet is received from the local access network in this state at  519 , Windows® calls the incoming thread at  524 , which generates a Registration-Request packet at  526  with the care-of IP address as the source IP address and the HA&#39;s IP address as the destination IP address and passes it to the underlying NIC driver at  528 . The NIC driver sends the Registration-Request packet to the default gateway router using the router&#39;s MAC address as the destination MAC address. The iDriver then resets the Monitoring-Counter to zero at  530 , and changes into the Wait-HA-REGR-Reply state at  532 . The incoming thread then returns control back to Windows® at  534 . The Registration-Request packet includes a MH ID, an Authentication-Challenge message, and a Key-Exchange-Request message. In the Authentication-Challenge message, a nonce is presented to challenge the HA. In the Key-Exchange-Request message, another nonce is presented to initiate the session key establishment process. 
   Referring now to  FIG. 6 , in the Wait-HA-REGR-Reply state identified at  600 , the iDriver waits for the HA on a remote network to send back a Registration-Reply packet in response to a Registration-Request packet from the MH. In the interim, the iDriver monitors the local access network based on the value of the Monitoring-Counter. The waited events in this state include an incoming Registration-Reply packet, an incoming ARP-Reply packet, and a time-out event at  602 . If the associated system timer times out in this state (which means the iDriver has not received the expected Registration-Reply packet within 100 ms), Windows® calls the time-out thread at  606 . The time-out thread then carries out the following instant actions. First, if the Monitoring-Counter equals a Changing-Access-Threshold (which means the MH may have moved into another local access network), the Monitoring-Counter is increased by one at  608 . If the number of time-outs reaches a HA-Down-Threshold at  611  and if the Monitoring Counter has not reached the Changing-Access-Threshold at  610 , the HA is down. In this case, the iDriver either: (1) returns the local IP address to Windows® so that the operating system can continue to initialize the network interface without mobility support; or (2) keeps sending Registration-Request packets to the HA until the HA is up or Windows® times out at  614 . A DHCP-Discover packet is then generated at  616  with 0.0.0.0 as the source IP address and 255.255.255.255 as the destination IP address. This packet is passed to the underlying NIC driver at  618 , which in turn broadcasts the packet on the local access network. The iDriver then changes into the Wait-Local-DHCP-Offer state at  620 . The iDriver then returns control to the operating system at  622 . If the Monitoring-Counter doesn&#39;t reach the Changing-Access-Threshold at  610 , the time-out thread re-generates the Registration-Request packet at  624 , and passes it to the underlying NIC driver at  626 . The NIC driver sends the packet to the default gateway router using the router&#39;s MAC address as the destination MAC address. The time-out thread then generates a unicast ARP-Request packet at  628  with the care-of IP address as the source IP address. This packet is passed to the underlying NIC driver at  630 , which in turn sends it to the default gateway router. As explained above, this allows for monitoring the local access network. If the iDriver does not receive the ARP-Reply packet at  632  from the default gateway router after sending a number of unicast ARP-Request packets, the MH may have moved into the coverage of another local access network. A unicast ARP-Request packet is communicated in lieu of a broadcast ARP-Request packet at  634  to avoid interrupting every node except the default gateway router on the local access network. 
   The time-out thread then returns control back to Windows® at  622 . If an ARP-Reply packet is received from the default gateway router in this state (which is the response to the ARP-Request packet that was previously sent out for the purpose of monitoring the local access network), at  636  Windows® calls the incoming thread, which then resets the Monitoring Counter to zero at  638 . The iDriver returns control to Windows® at  640 . If a Registration-Reply packet is received from the HA in this state at  642 , Windows® calls the incoming thread at  644 . The iDriver then validates the Registration-Reply packet received from the access network. This packet contains an Authentication-Response message, an Authentication-Challenge message, and a Key-Exchange-Response message. If the HA correctly responds to the iDriver&#39;s Authentication-Challenge message that was previously sent to the HA in the Registration-Request packet, the iDriver stores the IP address assigned by the HA as the IP address used by Windows® at  648 . The iDriver then generates a Registration-Complete packet at  650  with its care-of IP address as the source IP address and the HA&#39;s IP address as the destination IP address. This packet is passed to the underlying NIC driver at  652 , which in turn sends it to the default gateway router using the router&#39;s MAC address as the destination MAC address. If the Booting-Flag is True at  654 , the iDriver also generates a DHCP-Offer packet at  656  with the HA&#39;s IP address as the source IP address and the IP address assigned by the HA as the destination address. This packet is passed to the overlying IP stack in Windows® at  658 . The iDriver then resets the Monitoring-Counter to zero at  660 , and changes into the Wait-Host-DHCP-Request state at  662 . If the Booting-Flag is False at  654 , the iDriver resets the Monitoring-Counter to zero at  664 , and changes into the Operating state at  666 . If the HA replies with an incorrect response to the iDriver&#39;s Authentication-Challenge message at  646 , the iDriver regenerates the Registration-Request packet at  668  and passes it to the underlying NIC driver at  670 . The NIC driver then sends the packet to the default gateway router. The incoming thread then returns control back to Windows®. The Registration-Complete packet includes a MH ID and an Authentication-Response message containing the iDriver&#39;s response to the HA&#39;s Authentication-Challenge message received by the MH from the HA in the Registration-Reply packet. [stop Mar. 26, 2002] 
   Referring now to  FIG. 7 , in the Wait-Host-DHCP-Request state  700 , the iDriver waits for Windows® to send out a DHCP-Request message or DHCP-Decline message. In the interim, the iDriver monitors the local access network. The waited events in this state include at least one of an outgoing DHCP-Request packet, an outgoing DHCP-Decline packet, an incoming ARP-Reply packet, and a time-out event at  702 . If the associated system timer times out in this state at  704 , the operating system calls the time-out thread at  706 . The time-out thread then carries out the following instant actions. Initially, the Monitoring-Counter is increased by one at  708  (even if the Monitoring-Counter reaches the Changing-Access-Threshold, the time-out thread does not change the state). The iDriver then generates a unicast ARP-Request packet at  710  with the care-of IP address as the source IP address. This packet is passed to the NIC driver at  712 , which in turn sends it to the default gateway router on the local access network. The time-out thread then returns control back to Windows® at  714 . If an ARP-Reply packet is received from the default gateway router in this state at  716  (which is the response to the ARP-Request packet that was sent out for the purpose of monitoring the local access network) Windows® calls the incoming thread at  718 , which resets the Monitoring-Counter to zero at  720 , and returns control back to Windows® at  722 . If a DHCP-Request packet is received from Windows® in this state at  724 , Windows® calls the outgoing thread at  726 , and the outgoing thread generates a DHCP-Ack packet at  728  with the HA&#39;s IP address as the source IP address and the IP address assigned by the HA as the destination address, passes it to the overlying IP stack of Windows at  730 , sets the Booting-Flag to False at  732 , and changes into Operating state at  734 . The outgoing thread then returns control back to Windows® at  736 . If at  738  a DHCP-Decline packet is received from Windows® (i.e., Windows® rejected the IP address assigned by the HA and wants to restart the DHCP process), Windows® calls the outgoing thread at  740 . The outgoing thread then generates a DHCP-Release packet at  742  with the care-of IP address as the source IP address, passes the packet to the underlying NIC driver at  744  (which in turn sends it to the DHCP server on the local access network to release the care-of IP address), and changes into the Wait-Host-DHCP-Discover state at  746 . The outgoing thread then returns control back to Windows® at  748 . 
   Referring now to  FIG. 8 , in the Operating state  800  the iDriver receives outgoing IP packets from Windows®, encapsulates the IP packets, and tunnels them to the HA. The iDriver also receives tunneled IP packets from the HA, decapsulates them into regular IP packets, and sends them to Windows®. In the interim, the iDriver monitors the local access network and implements handoff functions as required. The waited events in this state include at least one of an outgoing DHCP-Release packet, an outgoing DHCP-Request message, any other outgoing IP packet, an incoming ARP-Reply message, any other incoming IP packet, and a time-out event at  802 . If the associated system timer times out in this state at  804 , Windows® calls the time-out thread at  806 , which increases the Monitoring-Counter by one at  808  if the Monitoring-Counter equals a Changing-Access-Threshold (which means the MH may have moved into another local access network). The incoming thread then generates a DHCP-Discover packet at  810  with 0.0.0.0 as the source IP address and 255.255.255.255 as the destination IP address, passes the packet to the underlying NIC driver at  812  (which in turn broadcasts this packet on the local access network), and changes into the Wait-Local-DHCP-Offer state at  814 . If the system timer has not timed out at  804 , the time-out thread regenerates a unicast ARP-Request packet at  816  with the care-of IP address as the source IP address, and passes this packet to the underlying NIC driver at  818  (which in turn sends it to the default gateway router). The time-out thread then returns control back to Windows® at  820 . If at  822  an ARP-Reply packet is received from the default gateway router in this state (in response to the ARP-Request packet sent by the iDriver for the purpose of monitoring the local access network), Windows® then calls the incoming thread at  824 . The incoming thread resets the Monitoring Counter to zero at  826 , and returns control back to Windows® at  828 . If a DHCP-Release packet is received from Windows® in this state at  830  (which means that Windows® wants to shut down the network interface), Windows® calls the outgoing thread at  832 . The outgoing thread then generates a DHCP-Release packet at  834  with the care-of IP address as the source IP address, passes the packet to the underlying NIC driver at  836 , (which in turn sends it to the DHCP server on the local access network to release the care-of IP address), sets the Booting-Flag to True at  838 , and changes into the Wait-Host-DHCP-Discover state at  840 . If a DHCP-Request packet is received from Windows® in this state at  842 (which means Windows wants to renew the lease of the assigned IP address), Windows® calls the outgoing thread at  844 . The outgoing thread then generates a DHCP-Ack packet at  846  with the HA&#39;s IP address as the source IP address and the IP address assigned by the HA as the destination address, passes the packet to the overlying IP stack of Windows® at  848 , and then returns control back to Windows® at  850 . If a regular IP packet is received from Windows® in this state at  852 , Windows® calls the outgoing thread at  854 . The outgoing thread then encrypts the IP packet using the session key at  856 , encapsulates the packet into a UDP datagram with the care-of IP address as the source IP address at  858 , and passes it to the underlying NIC driver at  860 , which in turn tunnels it to the HA&#39;s UDP port  434 . The outgoing thread then returns control back to Windows® at  861 . When encapsulated incoming IP packets are received from the HA at  862 , Windows® calls the incoming thread at  864 , which decapsulates the IP packet at  866 , decrypts it using the session key at  868 , and passes it to the overlying IP stack of Windows® at  870 . The incoming thread then returns control back to Windows. IP packets are encapsulated in UDP datagrams to enable the packets to pass NAPT boxes that are popular in home network environments, such as illustrated in  FIG. 9 . NAPT boxes are known in the art as shown and described in, for example, P. Srisuresh and K. Egevang, “Traditional IP Network Address Translator (Traditional NAT)”, IETF RFC3022, January 2001, the content of which is incorporated by reference herein. 
   The present invention has been shown and described in what are considered to be the most practical and preferred embodiments. It is anticipated, however, that obvious modifications may be implemented by those skilled in the art. 
   A program code listing is attached in text format as Appendix A, and on a CD-ROM as Appendix B. 
   This specification is being submitted with program code contained on an appended CD-ROM which contains the file Code.txt, 69 Kb, created on Apr. 22, 2002.