Abstract:
A system comprising a plurality of nodes forming a network, the plurality of nodes comprising source nodes and destination nodes; wherein a propagation limit restricts the travel of link-state information transmitted by the sources nodes to a subset of destination nodes within the network. A network-layer protocol provided at a layer above that of the network facilitates communication between nodes within and outside of this subset of nodes.

Description:
RELATED APPLICATIONS 
     The present application claims priority from U.S. Provisional Patent Application No. 60/555,980, entitled “NO-SIGHT ROUTING FOR AD-HOC NETWORKS”, filed on Mar. 25, 2004. In addition, the present application relates to U.S. patent application Ser. No. 11/087,406, entitled “VARIABLE TRANSLUCENCY NO-SIGHT ROUTING FOR AD-HOC NETWORKS”, filed on the same day as the instant application; U.S. patent application Ser. No. 09/736,827, entitled “NETWORK COMMUNICATION BETWEEN HOSTS”, filed Dec. 14, 2000; U.S. patent application Ser. No. 09/736,807, entitled “DELIVERING MESSAGES TO A NODE AT A FOREIGN NETWORK”, filed Dec. 14, 2000; U.S. patent application Ser. No. 09/737,108, entitled “PUBLISHING NODE INFORMATION”, filed Dec. 14, 2000; and U.S. patent application Ser. No. 09/736,834, entitled “ROUTING MESSAGES BETWEEN NODES AT A FOREIGN SUB-NETWORK”, filed Dec. 14, 2000. The contents of all of these applications are hereby incorporated by reference in their entirety. 
     BACKGROUND INFORMATION 
     In recent years, the use of wireless communication networks as a system for facilitating communication between various types of mobile devices, such as portable computers, personal digital assistants (“PDAs”), cellular telephones and the like, has gained widespread acceptance. In particular, there has been a growing trend at developing infrastructureless network technologies to facilitate direct communication between two or more wireless devices. When two or more devices directly communicate without any infrastructure, they form a type of local area network (“LAN”) known as an ad-hoc network. Due to the mobility of the devices involved, the topology of ad-hoc networks is typically subject to rapid changes, such as when devices are added, removed or moved from one location to another within the network. 
     Wireless devices may form or become part of an ad-hoc network when they are located within the range of at least one other wireless device. Each device (or “node”) in the ad-hoc network may serve as a client, host, or router. Currently, a number of wireless technologies exist for supporting ad-hoc networks, including ones using standard protocols such as Bluetooth, Infrared Data Association (“IrDA”), and IEEE 802.11x. Ad-hoc networks are not limited to wireless devices and some or all of the devices in an ad-hoc network may use temporary wired connections that allow these devices to temporarily be part of the network, such as for the duration of a communications session. 
     Ad-hoc networks face a number of challenges. These challenges can be roughly divided into two main categories: physical layer issues (such as physical connectivity problems due to weak signal strength, etc.) and network layer issues (including network management and routing difficulties). Although physical layer connectivity is, of course, a prerequisite for network connectivity, recent improvements in physical connectivity have not been matched by improvements in network management and routing techniques. 
     For example, according to one conventional routing approach, every node in a network receives, through a process known as “flooding”, enough information to build a complete map of the network. During flooding, each switching node (i.e., nodes that are configured to forward data packets) forwards a link-state packet (“LSP”) to all nodes to which it is directly connected. Typically, link-state packets contain data detailing the ID of the node that created the LSP and a list of directly connected neighbors of that node. 
     Each switching node that receives this LSP then forwards the packet to its directly connected neighbors, which then forwards the same packet to its directly connected neighbors, and so on until the LSP has been forwarded to each node within the network. Once a given node has received an LSP from every other node in the network, it is able to compute a complete map of the topology of the network. Each node in the network is thus able to determine, based on the computed map, the least-cost path to any destination node in the network. Changes in network topology are accounted for by requiring each switching node to transmit a link-state update (“LSU”) upon any perceived change in network topology (i.e., a neighbor of the switching node is disconnected or added). Typically, link-state updates contain data detailing the ID of the node that created the LSU and a list of directly connected neighbors of that node. 
     Although relatively simple in its implementation, this conventional routing approach (commonly known as “link-state” routing) suffers from a number of limitations, particularly when adapted for use in wireless ad-hoc networks. For example, in link-state routing, every node must have and store sufficient information to compute the location of every other node in the network. More particularly, because ad-hoc networks typically have flat address space (i.e., the addresses of each node do not identify a hierarchical relationship due to lack of central administration and constant motion), the routing table for each node in such networks must contain information about each and every other node. As the number of nodes connected to the network increases, the corresponding number of link-state packets and updates that must be transmitted, received and stored by each node also increases. When the amount of link-state traffic exceeds the physical capabilities of the hardware of the network, the network may become unreliable or fail. Thus, unacceptable increases in the amount of link-state data and traffic serve to limit the network&#39;s scalability potential by inhibiting the number of nodes that may feasibly connect to the network. 
     In addition, because switching nodes in a link-state routing scheme are required to transmit LSUs upon every perceived change in network topology, an inordinate number of LSUs may be propagated within the network due to the relatively high frequency with which topology changes occur in ad-hoc networks. This results in large amounts of routing traffic overhead being transmitted within the network, which may further limit the workable size of the ad-hoc network and lead to degradation of network performance and reliability. Given that wireless communications within a network may often be at lower bandwidths than wired communications, reducing overhead to maximize the total available bandwidth for substantive communications is highly desirable. 
     According to another approach, instead of allowing every link-state update generated by each switching node to fully flood the entire network, global LSUs (i.e., LSUs that are allowed to propagate throughout the entire network) are transmitted only on a periodic basis. Global LSUs typically represent LSUs having a time-to-live value (“TTL”, a value that specifies how far the LSU will propagate prior to expiring) set to infinity to allow the LSU to propagate throughout the entire network. 
     In this approach, known as “hazy-sighted” routing, each switching node transmits a global LSU during initial configuration of the network, thereby providing each node within the network with sufficient information to compute a complete map of the entire network. Thereafter, global LSUs are only transmitted upon the expiration of a period of time specified by a periodic timer. Between global LSU transmissions, non-global LSUs are transmitted. Typically, the TTL value of each non-global LSU is set to a value smaller than that of the size of the network so that they do not propagate throughout the entire network. Upon expiration of the period of time specified by the periodic timer, each switching node again transmits a global LSU. 
     Because global LSUs are only transmitted within this hazy-sighted routing scheme on a periodic basis, during certain periods of time various nodes within a network implementing this routing protocol may lack up-to-date information regarding the exact location of every other node in the network. Thus, although nodes may have received sufficient information to compute an up-to-date map of their surrounding region (as determined by the TTL value of the most recent non-global LSU), their understanding of the location of or best path to distant nodes (i.e., nodes outside of their horizon line) may be based on out-of-date information (as determined by the most recent global LSU). 
     Hazy-sighted routing thus allows information about distant nodes to be inexact, such that a switching node always knows how to get a packet closer to a destination node, but may not always know the details of the best path to this destination node. Once a transmitted packet has been forwarded to a node that is closer to the destination node, more information about this path is provided, and so on at the next closest node until the packet eventually arrives at the destination node. Inasmuch as the number of topological changes that might occur within the time specified by the periodic timer is likely to be greater than one, this periodic timing limitation serves to reduce the number of LSUs generated, thereby limiting the amount of traffic overhead promulgated within the network. Hazy-sighted routing thus sacrifices accuracy in favor of reduced link-state overhead. 
     As with the traditional link-state approach to routing described above, the so-called “hazy-sighted” routing approach suffers from similar scalability, performance and reliability concerns. For example, it is still necessary for the routing table to contain information about each and every node. Global LSUs are essential to providing such information. Thus, as discussed above, the use of global LSUs limits scalability, network performance and reliability. 
     Accordingly, there exists a need for a system and method capable of enabling nodes within an ad-hoc network to seamlessly communicate with adjacent nodes, distant nodes and a wider network (such as the Internet) so long as physical connectivity is maintained with at least one other node. There also exists a need for a system and method capable of scaling beyond the size limitations of traditional ad-hoc networks, while minimizing any potential decreases in network performance and reliability. Preferably, such a system and method would provide significant improvements in scalability, application performance and overall network connectivity. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an exemplary physical topology of a network for implementing a routing protocol. 
         FIG. 2  illustrates an exemplary physical topology of a network for implementing an exemplary no-sight routing protocol. 
         FIG. 3  illustrates an exemplary physical infrastructure of a system for implementing protocols at a network layer and at a sub-network layer. 
         FIG. 4  illustrates an exemplary physical infrastructure of a system for implementing a mobility protocol. 
         FIG. 5  depicts an exemplary physical infrastructure of a system for implementing a dynamic addressing protocol. 
         FIG. 6  depicts an exemplary process flow for performing no-sight routing. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     I. System Overview 
     a. No Sight Routing 
       FIG. 1  is a block diagram illustrating an exemplary physical infrastructure of a system  100  for implementing a no-sight routing protocol.  FIG. 2  illustrates an exemplary physical topology of a sub-network  125  implementing an exemplary no-sight routing protocol with predetermined propagation limits. 
     Exemplary system  100  generally comprises, among other things, nodes  102 ,  104 ,  106 ,  108 ,  110 ,  112 ,  114 ,  116 ,  118  and  120 . Nodes  102 - 120  are connected to one another via connections  122 , which may include any number of connections recognized in the art, including, for example, wires, wireless communication links, fiber optic cables, etc. Nodes  102 - 120  connected together via connections  122  collectively form sub-network  125 . 
     In general, nodes  102 - 120  represent connection terminals within exemplary sub-network  125 . In some embodiments, a protocol operating on a network above that of sub-network  125  (as will be described in greater detail below) distinguishes between nodes  102 - 120  based on their packet-forwarding capabilities. For example, in some embodiments a protocol operating on a network above that of sub-network  125  recognizes oval-shaped nodes  102 ,  104 ,  106 ,  112 ,  114 ,  118  and  120  as representing “hosts” (i.e., nodes which only forward originating packets, as will be known to those of skill in the art) and rectangular-shaped nodes  108 ,  110 , and  116  as representing “routers” (i.e., nodes which forward/route non-originating packets). This host/router distinction is not, however, made within sub-network  125 . Within sub-network  125 , all nodes are viewed as being directly connected; i.e., any node can send data to any other node. 
     According to certain embodiments, one or more of nodes  102 - 120  collectively forming exemplary sub-network  125  may be a mobile node. Generally speaking, a mobile node is a device whose location and point of attachment to exemplary sub-network  125  may frequently change. Examples of mobile nodes include cellular telephones, handheld devices, PDAs, and portable computers. 
     In many embodiments, the travel of a packet from any source node in sub-network  125  is limited by at least one predetermined limiting value. The factors that contribute to the potential limiting value include, for example, the physical distance the data packet has traveled from the source node, the aggregate bandwidth capacity of the links over which the data packet has traveled, the number of nodes through which the data packet has passed, and the amount of time that has passed since the data packet was transmitted from the source node. 
     For purposes of simplicity, the term “propagation limit” will be used hereinafter to collectively refer to application of one or more of the above-described predetermined limiting values. Further, the time, distance, etc. that a data packet travels or propagates within a network will be referred to as a “propagation parameter”. In addition, generally speaking, the phrase “no-sight routing” will be hereinafter used to refer to systems and methods in which the propagation parameter of a data packet is limited by a propagation limit. 
     According to some embodiments, a propagation limit is a hop limit. Generally, “hop” refers to the trip a data packet takes from one node to the next, while “hop limit” refers to the maximum number of hops that a packet may propagate. A propagation limit may also be, for example, a time-to-live value such as will be known to those of skill in the art, or a variable translucency time-to-live value. Variable translucency time-to-live values are discussed in greater detail in U.S. patent application Ser. No. 11/087,406, entitled “VARIABLE TRANSLUCENCY NO-SIGHT ROUTING FOR AD-HOC NETWORKS”, filed on the same day as the instant application. 
       FIG. 2  illustrates the concept of propagation limit, with horizon lines  130 ,  140  and  150  representing exemplary propagation limit boundaries for nodes  108 ,  114  and  116 , respectively. In this example, an arbitrary hop limit of 2 is chosen as the propagation limit for nodes  108 ,  114  and  116  within sub-network  125 . Accordingly, exemplary horizon line  130  illustrates that packets originating from node  108  may only travel two hops, encompassing nodes  104 ,  106 ,  110 ,  112  and  116 . Similarly, exemplary horizon line  140  illustrates that packets originating from node  114  may only travel to nodes  110  and  112 , while exemplary horizon line  150  illustrates that packets originating from node  116  may only travel to nodes  108 ,  110 ,  112 ,  118  and  120 . 
     Although the propagation limit discussed in connection with  FIGS. 1 and 2  is illustrated as being equal to 2, it is to be understood that the propagation limit may be set to any value or number, or be based on any algorithm, heuristic, etc. According to some embodiments, the propagation limits of all data packets transmitted within sub-network  125  are established irrespective of the type of data packet being transmitted. Alternatively, propagation limits for data packets propagated within sub-network  125  may be established at least in part on the type of data packet being propagated. For example, the value of a specific propagation limit may be set or adjusted based at least in part on whether the data packet in question is an LSP, an LSU, a multicast member LSU (used in multicasting, as will be well known to those of skill in the art), unicast data packet, or a multicast/broadcast data packet. 
     In many embodiments, the propagation limits of all data packets transmitted within sub-network  125  are set to equal values. For example, the propagation limits of all LSPs, LSUs (including multicast member LSUs), unicast data packets, and multicast/broadcast data packets propagated within a network might be set to an arbitrary hop limit of 5. Alternatively, only a portion or none of the data packets propagated within sub-network  125  may be configured to have equal propagation limits. 
     However, it should be noted that various efficiencies are lost when the propagation limits of all data packets are not set to equal values. For example, if the propagation limit of an LSU originating from a source node is set to a hop limit of 5, while the propagation limit of a unicast data packet from this same source node is set to a hop limit of 10, then this source node will be able to send packets to nodes up to 10 hops away, but will only be able to see routes for nodes within 5 hops. Conversely, if the propagation limit of an LSU originating from a source node is set to a hop limit of 10, while the propagation limit of a unicast data packet from this same source node is set to a hop limit of 5, then this source node will be able to see routes for nodes up to 10 hops away, but will only be able to send packets to nodes within 5 hops. 
     Thus, although in many embodiments the propagation limits of all data packet types are set to equal values, in some embodiments the propagation limit of a unicast data packet originating from a source node is preferably at least as large as that of an LSU originating from the same source node to ensure that all routes “seen” by the source node can be reached. In addition, to ensure communication between a source node and a destination node, the propagation limits for both the source node and the destination node are preferably equal to or greater than the propagation parameter between these nodes. 
     With respect to multicast/broadcast data packets, in certain embodiments it may be desirable to allow the propagation limits of a multicast member LSU to differ from the propagation limit of a multicast data packet. For example, the propagation limit of a multicast data packet from a source node may be set to a higher value than the propagation limit of a multicast member LSU for this source node. Setting the propagation limits in this manner enables the source node to send discovery or advertisement messages to a group of nodes, since explicit routes are not generally needed for such discovery messages. Nodes receiving such discovery messages will not, however, be able to reply to the source node. 
     In some embodiments, the propagation limits of data packets within sub-network  125  are dynamically adjusted based on a variety of factors, such as node density, network traffic volumes, and other network conditions. Propagation limits for each node within exemplary sub-network  125  may also be coordinated so as to gradually vary across the network topology, based on a pre-determined heuristic. For example, in certain embodiments the propagation limit of each source node in an exemplary sub-network  125  might be required to differ from the propagation limit of each destination node by no more than PP/x, where PP is equal to the propagation parameter between these nodes (e.g., the number of hops between these nodes, the physical distance between these nodes, the time required to travel between these nodes, etc) and x is equal to a predetermined propagation limit variation value. 
     For example, in a network where the propagation limits of nodes are set to vary by no more than PP/5 (5 being chosen as an exemplary value for propagation limit variation value x), a destination node 5 hops away from a source node in this network would be required to have a propagation limit that differs by no more than 1 (PP/5=5/5=1) from the source node. Defining the relationships between the propagation limits of each node in a network in this exemplary manner thus ensures that propagation limits throughout the network&#39;s topology will vary by no more than a predetermined amount. Generally speaking, while the propagation limit variation value x may be any number or value desired, higher propagation limit variation values x result in smaller amounts of propagation limit variation throughout a network&#39;s topology. 
     Although the propagation limit for each node in exemplary sub-network  125  may be set to any value or number, or set or dynamically adjusted based on any algorithm, heuristic, etc., it should be recognized that lower propagation limits generally improve the scalability of the sub-network (due to reduced sub-network layer overhead), while higher propagation limits result in expanded horizon lines for each node within the sub-network. In certain embodiments, the optimum value for maximizing these competing concerns has been found to fall within a hop limit range of 5-10. 
     According to the exemplary no-sight routing system described above, because the travel of data within exemplary sub-network  125  is limited by a predetermined propagation limit, nodes need not transmit nor store information about distant nodes falling outside of their respective horizon lines. The network overhead price paid by conventional routing protocols (where every node is required to know the location of all other nodes in the network, and where sub-network-layer routing traffic is required to convey that information) is thus advantageously avoided. In particular, because the use of global LSUs (and the sub-network-layer routing traffic associated with these global LSUs) is eliminated in no-sight routing, ad-hoc networks of arbitrarily large physical extent may be created without suffering from the various scalability, reliability and performance limitations discussed in connection with conventional link-state routing schemes. 
     Moreover, because no-sight routing enables the formation of ad-hoc networks of arbitrary physical extent, nodes no longer need to be assigned to particular ad-hoc sub-networks in order to partition the overall network into manageable and workable portions (such as when a conventional network of 10,000 nodes is partitioned into 100 different sub-networks of 100 nodes). This results in a reduction of the configuration operations required and leads to increased ease-of-use and node interoperability, such that all nodes may seamlessly interoperate with one another so long as they are within their respective horizon lines. No-sight routing thus obtains the reachability benefits of an ad-hoc network while addressing traditional scaling and configuration limitations. 
     b. No-Sight Routing &amp; Network-Layer Protocols 
     As detailed above, no-sight routing limits the propagation range of nodes in an ad-hoc sub-network such that these nodes may not “see” or communicate with distant nodes outside of their respective horizon lines. For example, as seen in  FIG. 3 , mobile node  302  may be limited to an arbitrary propagation limit of 2, as represented by horizon line  330 . Similarly, mobile node  320  may be limited to an arbitrary propagation limit of 2, as represented by horizon line  340 . Thus, since the topological location of mobile node  302  falls outside horizon line  340  of mobile node  320  and vice versa, communication between mobile node  302  and mobile node  320  may not be established solely through no-sight routing. 
     To facilitate communication between distant nodes, in many embodiments a network-layer protocol is provided at a layer above that of the sub-network. This network-layer protocol generally represents any protocol capable of enabling communication between distant nodes, including, for example, mobility protocols and dynamic addressing protocols.  FIGS. 3 and 4  illustrate an exemplary physical infrastructure of a system  300  for implementing a no-sight routing protocol at exemplary sub-network  325  and a mobility protocol  360  at network  350 .  FIG. 5  depicts an exemplary physical infrastructure of a system  500  for implementing a dynamic addressing protocol  560 . 
     Generally speaking, mobility protocol  360  is a routing protocol capable of enabling mobile nodes to use an address, such as an Internet Protocol (“IP”) address, at other than their topologically correct location without having to insert host routes into the global routing table of network  350 . Examples of mobility protocol  360  include IEFT-standard Mobile IP, Mobile IP version 6 (MIPv6), Hierarchical Mobile IP version 6 (MHIPv6) and OBS Mobility (on-board switch mobility). OBS Mobility is discussed in detail in U.S. patent application Ser. No. 09/736,827, entitled “NETWORK COMMUNICATION BETWEEN HOSTS”, filed Dec. 14, 2000; U.S. patent application Ser. No. 09/736,807, entitled “DELIVERING MESSAGES TO A NODE AT A FOREIGN NETWORK”, filed Dec. 14, 2000; U.S. patent application Ser. No. 09/737,108, entitled “PUBLISHING NODE INFORMATION”, filed Dec. 14, 2000; and U.S. patent application Ser. No. 09/736,834, entitled “ROUTING MESSAGES BETWEEN NODES AT A FOREIGN SUB-NETWORK”, filed Dec. 14, 2000. 
     Exemplary system  300  for implementing mobility protocol  360  comprises, among other things, nodes  302 ,  304 ,  306 ,  308 ,  310 ,  312 ,  314 ,  316 ,  318 ,  320 ,  322 ,  324 ,  326  and  328 . Nodes  302 - 328  are connected to one another via connections  301 , which generally include any number of connections recognized in the art, such as, for example, wireless communication links. Nodes  302 - 328  connected together via connections  301  collectively form sub-network  325 . Although one or more of nodes  302 - 328  may be a host (stationary or mobile node) or a router, for purposes of simplicity oval-shaped nodes  302 ,  306 ,  310 ,  312  and  320  will be referred to hereinafter as mobile nodes, while rectangular-shaped nodes  304 ,  308 ,  314 ,  316 ,  318 ,  322 ,  324 ,  326  and  328  will be referred to hereinafter as routers. 
     In certain embodiments, sub-network  325  is connected to network  350  via connections  355 , which may include any number of connections recognized in the art, including, for example, wires, wireless communication links, fiber optic cables, etc. Network  350  may represent any number of telecommunications or computer networks known to those skilled in the art, including, for example, an intranet, a wide area network (WAN), or the Internet. In at least one embodiment, mobility protocol  360  operates at network  350 . 
     In order for mobility protocol  360  to enable communication between each mobile node in sub-network  325 , each mobile node must be physically connected, either directly or through another node, to a router. Thus, in some embodiments the propagation limits of each mobile node in exemplary sub-network  325  are chosen to allow each mobile node to transmit to and receive data from (i.e., establish two-way communication) at least one router. More specifically, the propagation limits of each mobile node are preferably chosen such that each mobile node is capable of performing each of the following operations with at least one router: 1) receiving LSPs/LSUs from the router, 2) transmitting LSPs/LSUs to the router; 3) receiving data packets (unicast, multicast, etc.) from the router; and 4) transmitting data packets to the router. 
     In addition, some embodiments of mobility protocol  360  require a connection between each router in sub-network  325  and network  350 . This connection may occur directly (such as via connection  355  between router  322  and network  350 ) or indirectly (such as via connection  301  between router  318  and  322 , with router  322  in turn being connected directly to network  350 ). Examples of mobility protocols requiring this physical connection to network  350  include the various versions of Mobile IP. 
     In other embodiments of mobility protocol  360 , a physical connection between routers in sub-network  325  and network  350  is only required during an initial registration period (to be explained in greater detail below), after which the connection to network  350  may be terminated. Examples of mobility protocols capable of operating without this physical connection to network  350  include OBS Mobility, which is described in detail in U.S. patent application Ser. No. 09/736,827, entitled “NETWORK COMMUNICATION BETWEEN HOSTS”, filed Dec. 14, 2000; U.S. patent application Ser. No. 09/736,807, entitled “DELIVERING MESSAGES TO A NODE AT A FOREIGN NETWORK”, filed Dec. 14, 2000; U.S. patent application Ser. No. 09/737,108, entitled “PUBLISHING NODE INFORMATION”, filed Dec. 14, 2000; and U.S. patent application Ser. No. 09/736,834, entitled “ROUTING MESSAGES BETWEEN NODES AT A FOREIGN SUB-NETWORK”, filed Dec. 14, 2000. 
     An exemplary integration of mobility protocol  360  operating at network  350  for facilitating distant node communication is illustrated in FIG.  4 . System  400  generally comprises, among other things, mobile node  402 , foreign agent  404 , home agent  410 , and correspondent node  420 , each of which is connected to network  350  via connections  430 . In many embodiments, mobile node  402  in  FIG. 4  is the same device as mobile node  302  in  FIG. 3 , foreign agent  404  in  FIG. 4  is the same device as router  304  in  FIG. 3 , and correspondent node  420  in  FIG. 4  is the same device as mobile node  320  in  FIG. 3 . 
     According to one approach, mobility protocol  360  assigns the “home address” of mobile node  402  to home device  425  in a manner well known to those of skill in the art. Generally speaking, the home address of mobile node  402  refers to a semi-permanent IP address to which other nodes in network  350  (such as correspondent node  420 ) may address packets destined for mobile node  402 . By assigning the home address of mobile node  402  to home device  425 , which is connected to home agent  410  via connection  430  and is typically located at a mobile node user&#39;s office or home base facility, nodes desiring to communicate with mobile node  402  need not be updated regarding the topological location of mobile node  402  as it moves to various connection points on network  350 . 
     When mobile node  402  is transported outside of its home network (such as when it connects to exemplary sub-network  325 ), mobile node  402  may obtain packets transmitted to its home address by registering with foreign agent  404  through a process well known to those of skill in the art. During this registration process, mobility protocol  360  assigns, based on the address of foreign agent  404 , a “care-of” address to mobile node  402 . Foreign agent  404  then in turn registers with home agent  410  to exchange information regarding the home and care-of addresses of mobile node  402 . Once this three-way registration process has occurred, home agent  410  acts to forward all packets addressed to the home address of mobile node  402  (such as from correspondent node  420 ) to the care-of address of mobile node  402  provided by foreign agent  404 . 
     In many embodiments, these packets are forwarded from home agent  410  to foreign agent  404  via a tunnel  440  by a process known as “tunneling”. Generally speaking, tunneling involves having home agent  410  encapsulate packets originally addressed to mobile node  402  inside an IP header that addresses the encapsulated packet to the care-of address of mobile node  402 . The encapsulated packets are then transmitted by home agent  410  to foreign agent  404  via network  350 . Upon receipt of this encapsulated packet, foreign agent  404  decapsulates and forwards the tunneled packets received from home agent  410  to mobile node  402 . 
     According to certain embodiments, upon receipt of the tunneled packets, mobile node  402  then either sends packets to other nodes through network  350  via foreign agent  404 , or reverse-tunnels packets to home agent  410  via reverse-tunnel  450 . While reverse-tunnel  450  is often used to avoid egress and ingress firewall filters, it is also useful to avoid disclosing the true location of mobile node  402  to other nodes, such as to correspondent node  420 . 
     In addition, although not explicitly illustrated as such, correspondent node  420  in  FIG. 4  may also be a mobile node communicating with network  350  via a foreign agent in much the same manner as mobile node  402 . Thus, mobility protocol  360  may serve to provide network-layer connectivity between two mobile nodes located within a no-sight routing ad-hoc sub-network. 
     For example, returning to mobile nodes  302  and  320  in  FIG. 3 , mobility protocol  360  may facilitate communication between these two mobile nodes using IP addressing over network  350 . Specifically, by registering with router  304  (acting as a foreign agent) in the manner described above, mobile node  302  may receive a care-of IP address, enabling mobile node  302  to receive packets addressed to its home address and send packets to other nodes connected to network  350 . Similarly, by registering with router  322  (acting as a foreign agent), mobile node  320  may also receive a care-of IP address, also enabling mobile node  302  to receive packets addressed to its home address. These mobile nodes may then transmit and receive packets to one another over network  350  by addressing these packets to each other&#39;s respective IP home address, either directly or via reverse-tunneling. 
     Accordingly, by integrating a mobility protocol at a network layer above sub-network  325 , communication between two mobile nodes located outside of their respective horizon lines may be established. Such a system and method advantageously retains all of the beneficial properties of no-sight routing while adding the ubiquitous communication ability of mobility protocols. Specifically, because the addition of a mobility protocol still does not require the use of global LSUs, the sub-network-layer routing traffic associated with these global LSUs is eliminated, thereby allowing ad-hoc networks of arbitrarily large physical extent to be created without suffering from the various scalability, reliability and performance limitations discussed in connection with conventional link-state routing schemes. Thus, all nodes within sub-network  325  may be reachable from all other nodes—either through no-sight routing or via a mobility protocol—without having to pay the network overhead costs associated with conventional link-state routing techniques. 
     As described earlier, communication between distant nodes in sub-network  325  may also be facilitated through a dynamic addressing protocol such as DHCP (Dynamic Host Configuration Protocol).  FIG. 5  depicts an exemplary physical infrastructure of a system  500  for implementing a dynamic addressing protocol  560 . 
     Exemplary system  500  for implementing dynamic addressing protocol  560  comprises, among other things, oval-shaped mobile nodes  502 ,  506 ,  510 ,  512  and  520 , and rectangular-shaped routers  504 ,  508 ,  514 ,  516 ,  518 ,  522 ,  524 ,  526  and  528 , connected together via connections  501  to collectively form sub-network  325 . Depending on the desired field of use, sub-network  525  may be connected to a larger network (such as the Internet), or it may not. 
     In the exemplary embodiment illustrated in  FIG. 5 , router  526  acts as a server for dynamic addressing protocol  560 . Exemplary system  500  is not, however, limited to this configuration. For example, in certain embodiments, any one or more of routers  504 ,  508 ,  514 ,  516 ,  518 ,  522 ,  524 ,  526  and  528  may act as servers for dynamic addressing protocol  560 . In addition, any one or more of routers  504 ,  508 ,  514 ,  516 ,  518 ,  522 ,  524 ,  526  and  528  may be configured to forward packets to servers residing inside or outside of exemplary sub-network  525 . 
     Generally speaking, dynamic addressing protocol  560  operating at router  526  automatically assigns an address, such as IP address, to each mobile node within sub-network  525  in a manner well known to those of skill in the art. For example, upon connecting to sub-network  525 , mobile node  502  may initiate a DHCP IP address assignment process by broadcasting a DHCP discovery packet to all nodes to which it is directly connected; namely, router  504 . This discovery packet is then forwarded by router  504  to router  508 , which then forwards this packet to router  516  and so on until it arrives at router  526 . Upon receiving the DHCP discovery packet, router  526  acting as the DHCP server then assigns mobile node  502  an IP address based on various parameters identified by mobile node  502 , such as the hardware address of mobile node  502 . 
     In some embodiments, each router in sub-network  525  is notified of changes in the topology of sub-network  525  by router  526  (acting as the DHCP server). Various approaches for enabling groups of routers to learn about affiliated nodes by communicating with one another in this manner are known to those of skill in the art, including, for example, the use of flooded local bindings according to the OBS Mobility protocol. Although the use of these flooded local bindings in the OBS Mobility protocol results in increased state traffic between these routers, because these flooded local bindings are not part of the routing table of dynamic routing protocol  560 , churn in local bindings will not cause churn in the routing table of dynamic routing protocol  560 , such that these flooded local bindings will not impact the stability and scalability characteristics of system  500 . OBS Mobility and local bindings are discussed in detail in U.S. patent application Ser. No. 09/736,827, entitled “NETWORK COMMUNICATION BETWEEN HOSTS”, filed Dec. 14, 2000; U.S. patent application Ser. No. 09/736,807, entitled “DELIVERING MESSAGES TO A NODE AT A FOREIGN NETWORK”, filed Dec. 14, 2000; U.S. patent application Ser. No. 09/737,108, entitled “PUBLISHING NODE INFORMATION”, filed Dec. 14, 2000; and U.S. patent application Ser. No. 09/736,834, entitled “ROUTING MESSAGES BETWEEN NODES AT A FOREIGN SUB-NETWORK”, filed Dec. 14, 2000. 
     As with mobility protocol  360 , in order for a mobile node in sub-network  525  to receive an IP address from a DHCP server, this mobile node must be able to communicate with at least one router. Accordingly, in certain embodiments the propagation limits of each mobile node in exemplary sub-network  525  are chosen such that each mobile node can transmit and receive (i.e., establish two-way communication) with at least one router. 
     By assigning addresses to each node in sub-network  525  in the manner described above and further by enabling groups of routers to learn about affiliated nodes by communicating with one another, dynamic addressing protocol  560  thus enables communication between distant nodes in sub-network  525 . For example, once mobile node  502  and mobile node  520  have each received an IP address from router  526  acting as a DHCP server, and once the details of these assigned addresses have been distributed to each router in sub-network  525 , these mobile nodes may then transmit and receive packets to and from one another via the various routers distributed throughout the topology of sub-network  525 . 
     Accordingly, by integrating a dynamic addressing protocol with a no-sight routing scheme, communication between two mobile nodes located outside of their respective horizon lines may be established. Such a system and method advantageously retains all of the beneficial properties of no-sight routing while adding the ubiquitous communication ability of dynamic addressing protocols. Specifically, because the addition of a dynamic routing protocol still does not require the use of global LSUs, the sub-network-layer routing traffic associated with these global LSUs is eliminated, thereby allowing ad-hoc networks of arbitrarily large physical extent to be created without suffering from the various scalability, reliability and performance limitations discussed in connection with conventional link-state routing schemes. 
     II. Exemplary Process Flow 
       FIG. 6  depicts an exemplary process flow  600  for performing no-sight routing. Process flow  600  is meant to serve merely as an example for how no-sight routing may be implemented, such that the various steps recited therein should not be deemed to limit the scope of its application. In addition, various other intermediary steps not explicitly illustrated in  FIG. 6  may be performed as needed, regardless of their exclusion from  FIG. 6 . 
     At step  602 , the various parameters of each mobile node are configured. In many embodiments, these parameters are configured by system administrators of the network and/or device manufacturers of the mobile nodes. According to certain embodiments, a propagation limit and/or an algorithm or heuristic for computing a propagation limit, as discussed above, is stored in the mobile node during step  602 . As described in greater detail above, a mobile node may be assigned one or more propagation limits during this step for use in connection with all types of data to be transmitted by the mobile node. A mobile node may also be assigned a different propagation limit for each data packet type to be transmitted by this mobile node. In addition, although all mobile nodes within a network may be configured to have the same propagation limit, the propagation limits of each mobile node may be modified as needed. 
     Step  602  may occur in an automated fashion, such as by a computer, so that manual configuration is unnecessary. In addition, step  602  may be omitted from exemplary process flow  500  as needed, such as when propagation limits are to be dynamically assigned or adjusted based on network properties such as node density, as discussed in greater detail above. 
     Upon completion of step  602 , control proceeds to step  604  where a mobile node transmits an LSP to all neighboring nodes. As described above, link-state packets typically contain data detailing the ID of the node that created the LSP and a list of directly connected neighbors of that node. In many embodiments, the travel of this LSP throughout the network is determined, as explained in greater detail above, by a propagation limit. 
     At step  606 , LSPs from neighboring nodes are received. Again, due to the propagation limits placed on these LSPs, not all nodes connected to the network will receive all other nodes&#39; LSPs. Upon receipt of these LSPs from neighboring nodes, at step  608  each node computes a limited network map based on the information received from each LSP received. As various topology changes to the network are perceived by a mobile node (i.e., a neighboring mobile node is added or removed), the mobile node generates and transmits to its neighboring devices LSUs detailing the changes that have been observed. 
     III. Alternative Embodiments 
     In accordance with the provisions of the patent statutes, the principles and modes of operation have been explained and illustrated. However, it should be understood that embodiments described herein may be practiced otherwise than is specifically explained and illustrated without departing from the spirit or scope thereof, and that the invention described herein is intended to be limited only by the following claims.