Abstract:
Described is a method for improving both the bandwidth efficiency and bit rate of data streams flowing through binary tree networks. In an embodiment, the invention provides a method for efficiently utilizing end-consumer computers possessing a variety of upstream bandwidth capabilities to deliver online streaming video in a distributed manner.

Description:
[0001]    This application is a continuation of U.S. patent application Ser. No. 12/559,290 filed Sep. 14, 2009, now U.S. Pat. No. 8,103,750, which claims priority to U.S. Provisional Patent Application No. 61/096,688 filed Sep. 12, 2008, the entire disclosures of which are incorporated by reference in their entirety. This application relates to the disclosure of U.S. Pat. No. 7,035,933 issued Apr. 25, 2006 entitled “System of Distributing Content Data over a Computer Network and Method of Arranging Nodes for Distribution of Data over a Computer Network,” which is incorporated herein by reference in its entirety. 
         [0002]    This application includes material which is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent disclosure, as it appears in the Patent and Trademark Office files or records, but otherwise reserves all copyright rights whatsoever. 
     
    
     FIELD OF THE INVENTION 
       [0003]    This invention relates to a system for distributing content data over a computer network and a method for arranging nodes for distribution of data over a computer network. The system and method of the present invention are particularly useful for the distribution of streaming audio and video over the Internet. 
       BACKGROUND OF THE INVENTION 
       [0004]    U.S. Pat. No. 7,035,933 describes a system and method that has proved useful for online distributed broadcasting of audio and video material. The disclosed system and method can reduce the amount of bandwidth consumed by video streaming servers by limiting the server&#39;s number of direct connections over the Internet to only a small number of end-consumer computers, and then utilizing a portion of the end-consumer computers that are tuned to the broadcast over the Internet as “repeater nodes” to help propagate the online broadcast to other viewers&#39; computers. This approach to online broadcasting is in contrast to industry standard content delivery networks wherein each end-consumer&#39;s computer that is to receive an audio/video stream must establish a direct connection to a server. These servers are typically owned or operated on behalf of the broadcaster and thus the broadcaster is responsible for the bandwidth costs associated with each of these direct connections. 
         [0005]    Various embodiments of the system and method disclosed in U.S. Pat. No. 7,035,933 display the following highly desirable traits:
       The network architecture is based on a balanced binary tree and as such is tailored to deliver live broadcasts, not simply pre-recorded material as is the case with peer-to-peer networks, such as BitTorrent and the like.   The network architecture supports the addition of nodes (computers) as end-consumers tune in to the broadcast and the removal of nodes (computers) as end-consumers exit the broadcast. As a result, the invention does not need to run as a “background task” nor continuously tap into end-consumer&#39;s bandwidth as with peer-to-peer file sharing technologies. Instead the invention of U.S. Pat. No. 7,035,933 need only use an end-consumer&#39;s bandwidth while that end-consumer is tuned to a program.   The system and method disclosed in U.S. Pat. No. 7,035,933 does not store audio video content files on end-consumers&#39; computers. This is in contrast to peer-to-peer file sharing systems which to work effectively must cache content throughout the network.   Because the system and method disclosed in U.S. Pat. No. 7,035,933 is based on a balanced binary tree, only one half of the nodes (end-consumer computers) in the network need act as repeater nodes in order for the system to achieve maximum bandwidth savings efficiency. This enables software applications incorporating this invention to be “polite” in the sense that whenever an end-consumer&#39;s computer is acting as a repeater node and that end-consumer needs to use his or her upstream bandwidth for other purposes (such as email, gaming, or Voice Over IP [VOIP] applications) that end-consumer&#39;s computer is moved to the edge of the network where it will no longer be expected to retransmit the broadcast. This reorganization of the network can take place without interrupting the end-consumer&#39;s viewing experience. The end result is a polite system that immediately relinquishes use of the end-consumer&#39;s upstream bandwidth the moment the end-consumer has need of that bandwidth for other purposes. In other words, the invention does not interfere with the end-consumers use of his or her Internet connection since the moment the end-consumer tries to send off a file (or otherwise use the upstream Internet connection) applications incorporating the invention relinquish control of the upstream Internet connection immediately. The bottom line is that end-consumer see no degradation in their Internet service when using applications based on the invention.   The system and method disclosed in U.S. Pat. No. 7,035,933 spreads the bandwidth needs of the system thinly throughout the network—never requiring an end-consumer computer to supply more than two copies of the video stream to other viewers. As a result the system avoids “super-noding” in which nodes (computers) with access to large amounts of bandwidth are expected to provide copies of the broadcast stream to large numbers of viewers. Not only is expecting certain computers to act as super nodes impolite, there are two very important reasons for avoiding it. First, if an application attempts to appropriate a large amount of bandwidth when running on corporate or university LANs the system administrators of those networks will take steps to block the application—which will result in potential viewers on corporate and university LANs from being able to run the application to watch video. The second reason to avoid designating certain computers as super nodes is that systems which depend on a small number of nodes to support a large percentage of the system&#39;s viewers are fragile in the sense that when a super node departs the system a large “hole” is created in the network that must somehow be “filled in” almost immediately or large numbers of viewers will experience signal degradation and loss of video stream.   The system and method of U.S. Pat. No. 7,035,933 delivers an exceptional Quality of Service. In addition to the characteristics described above, the reconfiguration algorithms covered by the patent use the turnover inherent in distributed networks (as viewers tune in and tune out) to promote the most reliable nodes to the most critical regions of the network, driving the network towards highly stable configurations which improve the quality of service delivered by the system.       
 
         [0012]    The system and method of U.S. Pat. No. 7,035,933 also suffers from certain limitations, including the fact that the reconfiguration algorithms are described in detail only for binary trees in which all internal nodes must be capable of supporting two children nodes. This means that for a node (an end-consumer computer) to be used to repeat the broadcast stream to even a single additional computer, the node must have the upstream bandwidth capacity to support two additional computers. As a result, nodes (end-consumer computers) that only have access to Internet connections with upstream bandwidth capacity to send out a single copy of the broadcast stream are underutilized—they are never allowed to repeat the broadcast to anyone. 
         [0013]    The practical consequences of these limitations are that for a given video streaming bit rate and a collection of nodes (end-consumer computers) possessing a variety of different upstream bandwidth capabilities, some nodes that could have been employed to rebroadcast the video stream will not be asked to do so, reducing the overall bandwidth efficiency that could have been otherwise achieved. Likewise for a given bandwidth efficiency target, a higher bit rate video stream could be supported, resulting in better picture quality and/or audio for the end-consumer. 
         [0014]    Incorporating such a change into the system and method of U.S. Pat. No. 7,035,933 requires addressing the difficult problem of how to build and maintain a binary broadcast tree with repeat capable nodes of differing abilities. In order to create a useful and practical online broadcasting system, algorithms must be designed to build and maintain binary broadcast trees consistent with promoting the most reliable nodes to the most critical regions of the network (to promote stability) while maintaining overall tree balance (to limit propagation delays) and employing nodes that lack the ability to repeat to two children but possess the ability to repeat to a single child (to increase overall network efficiency). 
       SUMMARY OF THE INVENTION 
       [0015]    In an embodiment, the invention provides a method for efficiently utilizing end-consumer computers possessing a variety of upstream bandwidth capabilities to deliver online streaming video in a distributed manner. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    The foregoing and other objects, features, and advantages of the invention will be apparent from the following more particular description of preferred embodiments as illustrated in the accompanying drawings, in which reference characters refer to the same parts throughout the various views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating principles of the invention. 
           [0017]      FIG. 1  is a schematic drawing of an embodiment of a computer information distribution network formed pursuant to an embodiment of the present system and method; 
           [0018]      FIG. 2  is an illustration of the method used to merge two linear chains of nodes upon the departure of a Type 2 node; 
           [0019]      FIG. 3  is an illustration of the general method used to reconfigure the network in the case of the graceful departure of a Type 2 node; 
           [0020]      FIG. 4  is an illustration of the general method used to reconfigure the network in the case of the graceful departure of a Type 2 node with two Type 2 children; 
           [0021]      FIG. 5  is an illustration of the general method used to reconfigure the network in the case of the graceful departure of a Type 2 node with one Type 2 child and one Type 1 Child, where the Type 2 Child is the preferred (green) child; 
           [0022]      FIG. 6  is an illustration of the general method used to reconfigure the network in the case of the graceful departure of a Type 2 node with two Type 1 children; 
           [0023]      FIG. 7  is an illustration of the general method used to reconfigure the network in the case of the graceful departure of a Type 2 node with one Type 1 child and one Type 0 Child, where the Type 1 Child is the preferred (green) child; 
           [0024]      FIG. 8  is an illustration of the general method used to reconfigure the network in the case of the graceful departure of a Type 2 node with two Type 0 children; 
           [0025]      FIG. 9  is an illustration of the general method used to reconfigure the network in the case of the graceful departure of a Type 2 node with one Type 2 child and one Type 0 Child, where the Type 2 Child is the preferred (green) child; 
           [0026]      FIG. 10  is an illustration of the general method used to reconfigure the network in the case of the graceful departure of a Type 1 node where that Type 1 node has a child; 
           [0027]      FIG. 11  is an illustration of the general method used to reconfigure the network in the case of the unexpected (non-graceful) departure of a Type 2 node; 
           [0028]      FIG. 12  is an illustration of the general method used to reconfigure the network in the case of the unexpected (non-graceful) departure of a Type 2 node with two Type 2 children; 
           [0029]      FIG. 13  is an illustration of the general method used to reconfigure the network in the case of the unexpected (non-graceful) departure of a Type 2 node with one Type 2 child and one Type 1 Child, where the Type 2 Child is the preferred (green) child; 
           [0030]      FIG. 14  is an illustration of the general method used to reconfigure the network in the case of the unexpected (non-graceful) departure of a Type 2 node with two Type 1 children; 
           [0031]      FIG. 15  is an illustration of the general method used to reconfigure the network in the case of the unexpected (non-graceful) departure of a Type 2 node with one Type 1 child and one Type 0 Child, where the Type 1 Child is the preferred (green) child; 
           [0032]      FIG. 16  is an illustration of the general method used to reconfigure the network in the case of the unexpected (non-graceful) departure of a Type 2 node with two Type 0 children; 
           [0033]      FIG. 17  is an illustration of the general method used to reconfigure the network in the case of the unexpected (non-graceful) departure of a Type 2 node with one Type 2 child and one Type 0 Child, where the Type 2 Child is the preferred (green) child; 
           [0034]      FIG. 18  is an illustration of the general method used to reconfigure the network in the case of the unexpected (non-graceful) departure of a Type 1 node where that Type 1 node has a child; 
           [0035]      FIG. 19  is an illustration of the general method used to reconfigure the network in the case of a departure of a Type 2 node with two children where the depart message is not received by the non-preferred (red) child; 
           [0036]      FIG. 20  is an illustration of the general method used to reconfigure the network in the case of a departure of a Type 2 node with two children where the depart message is not received by the preferred (green) child; 
           [0037]      FIG. 21  is an illustration of the general method used to reconfigure the network in the case of a departure of a Type 2 node with two children where the depart message is not received by the parent of the departing node; 
           [0038]      FIG. 22  is an illustration of the general method used to reconfigure the network in the case of a departure of a Type 2 node with two children where depart messages are not received by either of the two children; 
           [0039]      FIG. 23  is an illustration of the general method used to reconfigure the network in the case of a departure of a Type 2 node with two children where depart messages are not received by either the parent of the departing node nor the departing node&#39;s non-preferred (red) child; 
           [0040]      FIG. 24  is an illustration of the general method used to reconfigure the network in the case of a departure of a Type 2 node with two children where depart messages are not received by either the parent of the departing node nor the departing node&#39;s preferred (green) child; 
           [0041]      FIG. 25  is an illustration of the general method used to reconfigure the network in the case of a departure of a Type 2 node with two children where depart messages are not received by either the parent of the departing node nor the departing node&#39;s two children; 
           [0042]      FIG. 26  is an illustration of a network in which Node B is departing gracefully; 
           [0043]      FIG. 27  is an illustration of the first two steps in the reconfiguration of the network illustrated in  FIG. 26  following the departure of Node B; 
           [0044]      FIG. 28  is an illustration of the state of the network following the connection of Node C to Node A and Node D to Node C; 
           [0045]      FIG. 29  is an illustration of the final state of the network of  FIG. 26  following the departure of Node B; 
           [0046]      FIG. 30  ( a - f ) illustrates the reconfigurations that result from the graceful departure of a node; 
           [0047]      FIG. 31  ( a - d ) illustrates the reconfigurations that result from the unexpected (non-graceful) departure of a node. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0048]    Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. 
         [0049]    The invention is described below with reference to block diagrams and operational illustrations of methods and devices for distributing content data over a computer network. It is understood that each block of the block diagrams or operational illustrations, and combinations of blocks in the block diagrams or operational illustrations, may be implemented by means of analog or digital hardware and computer program instructions. These computer program instructions may be provided to a processor of a general purpose computer, special purpose computer, ASIC, or other programmable data processing apparatus, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, implements the functions/acts specified in the block diagrams or operational block or blocks. In some alternate implementations, the functions/acts noted in the blocks may occur out of the order noted in the operational illustrations. For example, two blocks shown in succession may in fact be executed substantially concurrently or the blocks may sometimes be executed in the reverse order, depending upon the functionality/acts involved. 
         [0050]    Node Types 
         [0051]    In the system and method of U.S. Pat. No. 7,035,933 there were essentially two kinds of nodes: repeat capable nodes and non-repeat capable nodes. A node was deemed “repeat capable” if it possessed the upstream bandwidth capacity to support two outgoing audio/video streams plus some pre-defined additional capacity for overhead and margin. If a node did not have the capacity to support two children it was deemed non-repeat capable. 
         [0052]    As pointed out above, this bifurcation of nodes was somewhat wasteful in that nodes fully capable of supporting a single child were not utilized by the system due to the inability of the reconfiguration algorithms to accommodate nodes capable of supporting only a single child. 
         [0053]    In the present system and method there are three main types of nodes: Type 2 nodes, Type 1 nodes, and Type 0 nodes. As their names imply, Type 2 nodes are capable of supporting two children, Type 1 nodes are capable of supporting a single child, and Type 0 nodes are incapable of supporting any children. 
         [0054]    In addition to these three major types, within a type various priorities may exist. The defined priorities are High, Normal, and Low. Each of these priorities does not necessarily exist for each of the types. 
         [0055]    The type and priority of a node together define that node&#39;s preference level. The defined preference levels for nodes are as follows: 
         [0056]    Type 2, Normal Priority (sometimes written as T2N) is the highest priority node known to the system. These nodes are capable of supporting two children and are roughly equivalent to “repeat capable” nodes in the previous system and method. 
         [0057]    Type 1, High Priority (sometimes written as T1H) are capable of supporting a single child. At some point in their past T1H nodes were categorized as Type 2, but were later “demoted” to Type 1—generally as a result of poor performance when serving as the parent of two children. Type 1 High Priority are given preference over other Type 1 nodes in order to keep the Type 1 High Priority nodes as high in the tree as possible (just below the Type 2). This is due to the fact that we expect these nodes to be “stronger” than normal Type 1&#39;s and also so that if a Type 1 High Priority converts back to a Type 2 Normal node it will already be situated “near” the Type 2 nodes. 
         [0058]    Type 1 Normal Priority is the third preference level of node that the system recognizes Nodes whose upstream bandwidth tested capable of supporting one child but not two are given this rating when they join the network. 
         [0059]    Type 0 High Priority is the fourth preference level of node that the system recognizes. These nodes are currently judged incapable of repeating to even a single child, however they were at one time Type 1. Type 0 High Priority are given preference over Type 0 Normal and Type 0 Low as they have a history of having been a productive node and therefore should be retained in the tree as much as possible in case they reconvert to Type 1 status again. 
         [0060]    Type 0 Normal Priority is the fifth preference level of node that the system recognizes. The only type of node a Type 0 Normal can theoretically bump is a Type 0 Low priority. Type 0 Normal priority nodes correspond to the “non-repeat capable” nodes from the previous system and method. Nodes whose upstream bandwidth test too low to act as a repeater of even a single stream when the node joins the network are given this designation. 
         [0061]    Type 0 Low Priority nodes are the sixth and lowest preference level of node recognized by the system. These nodes are incapable of bumping nodes of any type. A Type 0 node may be assigned “low priority” status for a number of reasons, such as an inability to reliably receive data from its parent node. 
         [0062]    The fundamental rule concerning node type is that a node of a particular preference level may not bump a node at an equal or greater preference level, but may always bump a node at a lesser preference level. (The one exception to this rule is that Type 2 Normal nodes may cause other Type 2 Normal nodes to be bumped when executing priority and system joins, as explained below.) 
         [0063]    Thus, except as stipulated in the previous paragraph, Type 2 Normal nodes may bump all nodes except other Type 2 Normal nodes. A Type 1 High Priority may not bump Type 2 Normal or other Type 1 High Priority nodes, but may bump Type 1 Normal Priority, Type 0 High Priority, Type 0 Normal Priority, and Type 0 Low priority. Likewise, Type 1 Normal Priority nodes cannot bump Type 2 Normal, Type 1 High, or other Type 1 Normal Priority nodes, but are free to bump all Type 0 nodes regardless of their “priority”. Similarly, Type 0 High priority nodes cannot bump Type 2, Type 1, or other Type 0 High Priority nodes, but can bump Type 0 Normal and Type 0 Low priority nodes. Type 0 Normal can only bump Type 0 Low, and Type 0 Low can bump no one. 
         [0064]    Introduction of Dynamic VTT and Definition of Primary and Secondary Sides of the Broadcast Tree 
         [0065]      FIG. 1  illustrates a complete distributed broadcast tree constructed according to the methods of the present system and method. 
         [0066]    An innovation present in this system and method over that of U.S. Pat. No. 7,035,933 is the ability to form and maintain linear chains of nodes composed of nodes that are capable of broadcasting to only one child. These linear chains may be capped by nodes that are incapable of broadcasting to any nodes 
         [0067]    Another innovation present in this system and method is the demarcation of one side of the binary tree as “Primary” and the other side as “Secondary”. The primary side of the tree, shown as the left, or green, side of the tree in  FIG. 1 , is where the system prefers to place nodes. The secondary side of the tree, shown as the right, or red, side of the tree in  FIG. 1  is used to temporarily hold excess linear chains that cannot be accommodated in the Primary side of the tree. 
         [0068]    A difference between the primary and secondary sides of the tree is that the capacity of the secondary side can be increased on an as-needed basis by the addition of VTT (Virtual Tree Top) nodes. VTT nodes are “virtual” nodes introduced into the binary tree to expand the “capacity” of the tree. VTT nodes map in the physical world to additional connections to the server. Hence, while a classical binary tree can only have two nodes connected directly to the root node/physical server, the use of VTT nodes allow for an arbitrary number of physical connections to the server. While VTT nodes have been described in previous applications related to U.S. Pat. No. 7,035,933, we extend their use to being generated dynamically based on the number and type of nodes present in a broadcast network. Previous applications related to U.S. Pat. No. 7,035,933 include U.S. patent application Ser. Nos. 11/176,956, 11/179,041, 11/179,063, 11/408,169, and 11/746,494, which are incorporated herein by reference. 
         [0069]    An important use of the Secondary side of the tree and use of dynamic VTT is to prevent individual linear chains from growing exceedingly long leading to an unbalanced tree. 
         [0070]    As described elsewhere in this document, it is frequently desirable in the present system and method to merge linear chains of Type 1 nodes. If left unchecked, networks with few Type 2 nodes and many Type 1 nodes could devolve into a small number of very long chains. Long chains should be avoided as they lead to propagation delays. Additionally, large numbers of repeaters between an individual node (such as an edge node or node near the edge of the tree) and the server increase the likelihood of Quality of Service issues—since the “signal” is passing through a large number of “repeaters” each given to failure. 
         [0071]    To limit the length of Type 1 chains in networks with few Type 2 repeaters, we define the concept of “long”. Long, which may be a dynamic definition that is dependent on the total number of nodes, the total number of a particular type of node, the percentage of certain node types, ratios of the various node types, or other dynamic features of the network, is used to limit the total length of Type 1 chains. Whenever a merger of two linear chains is to be performed, if the length of the chain resulting from the merger were to exceed “long” the merge will not be performed and instead one of the linear chains would be moved to the Secondary side of the tree. 
         [0072]    To prevent the secondary side of the tree from becoming saturated, additional VTT nodes are created as necessary to act as docking locations for additional linear Type 1 chains. However, a maximum saturation point may be defined which specifies the maximum number of VTT nodes allowable on the Secondary side of the tree. 
         [0073]    The Basics of Reconfiguration 
         [0074]    Whenever a node with children leaves the broadcast network (or tree), the reconfiguration algorithms decide where to place the children of the departing node as well any node chains associated with those children. 
         [0075]    There are two basic types of ways in which a node may leave the broadcast network (or tree): Graceful Departs, and Unexpected or Non-Graceful Departs. 
         [0076]    The term “Graceful Depart” refers to a normal or procedural shutdown of the client application—the type of departure that is generated when the end-user clicks on the “x” in the upper right hand corner of the client interface in order to exit the application. When a graceful depart is initiated, all shutdown procedures are then completed. The departing node sends a depart message to its children and parent with instructions on how they are to proceed. The children of the departing node send out depart propagate packets to their children. These packets are updated at each level and propagated throughout the sub-trees. 
         [0077]    The term “Unexpected or Non-Graceful Depart” refers to a node leaving the system in an unexpected manner—without the normal shutdown procedures being performed. A non-graceful depart can occur for any number of reasons: the computer that is running the client application may lose its Internet connection, or the machine running the client application may crash, or the end-user abnormally terminates the client application by “killing” it with Task Manager, etc. Generally, when an unexpected depart occurs, the children of the departing node will climb to their grandparent for instructions on how to proceed. 
         [0078]    In addition to nodes leaving the network altogether by way of graceful and non-graceful departs, a node that remains in the network (tree) can change its position by disconnecting from its current parent and reconnecting to some other parent. These reconnections can be voluntary or involuntary in response to changing conditions in the network. 
         [0079]    One type of reconfiguration event is a “Bump”. The term “Bump” refers to a node being forcibly kicked by its parent so that the parent can accept a higher priority node in the bumped node&#39;s place. When the parent kicks the bumped node, it also tells the bumped node where to go next. 
         [0080]    A “Priority Join” is a way to force reconfiguration. When a node registers with the priority join flag set, the node that it registers to kicks its red child and accepts the registering node as its new red child. Priority joins are only allowed for Type 2 nodes. 
         [0081]    A “System Join” is a way to force a priority join in a non-graceful reconfiguration. A system join is triggered when a Type 2 node with children registers with a node that cannot accept it as a child. During a system join, the joining node is told to priority join to the red child of the node with which it is trying to register. 
         [0082]    Two additional terms that are frequently used when discussing reconfiguration are: the “Recommended Parent List” and the “Network Topology Model” or NTM. 
         [0083]    A node&#39;s Recommended Parent list is a list of all nodes known to that node that can act as parent nodes. The Recommended Parent list will vary from node to node as an individual node can only recommend parents that are within its own sub-tree. The Recommended Parent list is usually sorted (or organized in some way) so as to make finding the “best” recommended parent easy to accomplish. The “best” recommended parent will generally be the recommended parent that is closest to the server (the root of the broadcast tree). 
         [0084]    The Network Topology Model (or NTM) is a node&#39;s internal model of the structure of the portion of the network of which the node is aware. A node&#39;s NTM generally consists of a path from the root/server node down to the node in question, together with the sub-tree of the overall network tree that is rooted at the node. Network Topology Models are generally annotated with state information about the nodes contained in the model, such as the recommended parent lists of all nodes in the sub-tree. A node uses its NTM to determine where to place incoming nodes that attempt to connect to the node in question. 
         [0085]    Merging Chains of Type 1 Nodes 
         [0086]    Merging is the process of combining two chains of Type 1 Nodes where each chain may or may not be capped with a Type 0 node. When a node directly supporting two node chains departs the network, either gracefully or non-gracefully, its two chains may be merged as long as their combined length does not exceed the pre-defined value for “long”. 
         [0087]      FIG. 2  illustrates the node chain merger process. Initially Node  2  is the parent of two Type 1 nodes, Node  4 , a Type 1 High Priority, is the green child of the departing node and Node  5 , also a Type 1 High Priority, is the red child of the departing node. Node  4  is the root of a chain of Type 1 nodes (Node  4 , Node  8 , and Node  12 ) capped by a Type 0 node, Node  14 . Similarly, Node  5  is the root of a Type 1 chain (Node  5  and Node  9 ) capped by a Type 0 node, Node  13 . 
         [0088]    Node  2  departs the network leaving behind the two node chains. As Node  4  is the green child of the departing node it will be instructed to connect to its former grandparent, Node  1 . Node  5  will be instructed to connect into the chain rooted by Node  4  immediately below the lowest Type 1 High Priority node. Since there is only one Type 1 High Priority node in the chain rooted at Node  4 , Node  5  will connect to that node, which happens to be Node  4 . 
         [0089]    In order to accept Node  5 , Node  4  was forced to bump its Type 1 Normal child, node  8 . During the bump of Node  8  by Node  4 , Node  4  will have instructed Node  8  to connect to Node  5  for further instructions. Since Node  8  is Type 1 Normal it cannot become a child of Node  5 , instead Node  5  instructs Node  8  to connect as a child of Node  9 . Node  8  connects to Node  9 , causing Node  13  a Type 0 Low Priority node to be bumped. 
         [0090]    Node  13  then attempts to connect to Node  8 , but since Node  13  is Type 0 Low which is incapable of bumping anyone and the chain headed by Node  8  terminates in a Type 0 High (Node  14 ), Node  13  is rejected and must climb its path, beginning at the first Type 2 node and working upwards towards the root server until it can either be placed or rejected completely from this broadcast tree. 
         [0091]    Thus the two linear chains have now been completely merged into a single chain, with the level of nodes respected—Type 1 High Priority nodes situated above Type 1 Normal Priority Nodes and Type 0 High Priority retained in preference to Type 0 Normal or Low priority. 
         [0092]    Handling Graceful Node Departures 
         [0093]    Whenever a departing node completes its shutdown procedure, it sends a depart message to its children and parent. The green child of the departing node is instructed to connect to the parent of the departing node. If there is a red child, a decision has to be made as to what to do with this red child based upon the type of the red child. If the red child is a Type 2, the departing node may tell the red child to priority join to its sibling. If the child is a Type 1 or a Type 0, then the departing node sends the red child a connection path based on the recommended parent list of its green sibling. If there is no place for the Type 1 or Type 0 red child to be placed in the green child&#39;s sub-tree, then the departing node will tell the Type 1 or Type 0 red child to climb its path. 
         [0094]      FIG. 3  through  FIG. 10  illustrate the expected reconfiguration behavior resulting from nodes departing the network in a graceful manner. 
         [0095]    In  FIG. 3 , the end-user whose client application is identified as Node  2  clicks the “X” in the upper right hand corner of the client or otherwise instructs the application to exit. As part of its close out procedure, Node  2  constructs depart messages which contain information instructing the nodes surrounding Node  2  what actions they are to take. Node  3  receives a message from Node  2  instructing it to connect to its former grandparent, Node  1 . Node  1  receives a message from Node  2  instructing it to expect Node  3  to connect to it as a replacement for Node  2 . Node  4  receives a message from Node  2  containing instructions as to where it should connect as described above. Depending on the type and number of children Node  3  already supports as well as the type of Node  4 , Node  4  may connect directly as a child of Node  3 , somewhere in the sub-tree rooted at Node  3 , elsewhere in the network, or even under rare circumstances be rejected completely if there is no available location to which it can dock. 
         [0096]      FIG. 4  illustrates the case in which both children (Node  3  and Node  4 ) of the departing node, Node  2 , are Type 2. In this case Node  3  is instructed to connect to Node  1  and Node  4  is instructed to connect to Node  3 . This type of reconfiguration is similar to standard reconfiguration in the U.S. Pat. No. 7,035,933. 
         [0097]      FIG. 5  illustrates the case in which the preferred (green) child (Node  3 ) of the departing node is Type 2, while the non-preferred (red) child (Node  4 ) of the departing node is Type 1. In this case, Node  3  will connect to its former grandparent Node  1 , while Node  4  will be instructed to connect to a node in Node  3 &#39;s sub-tree based on Node  3 &#39;s recommended parent list. 
         [0098]      FIG. 6  illustrates the case in which both children of the departing node are Type 1 nodes. In this case, Node  3  will connect to Node  1  and Node  4  will be instructed to connect to a node in Node  3 &#39;s sub-tree based on Node  3 &#39;s recommended parent list. Note that if Node  3  and Node  4  were the heads of linear chains, a chain merger would occur using the method illustrated in  FIG. 2  and described above. 
         [0099]      FIG. 7  illustrates the case in which Node  3 , the green child of departing Node  2  is Type 1 while the red child of the departing node (Node  4 ) is Type 0. Node  3  is instructed to connect to Node  1  and Node  4  will be instructed to connect to a node in Node  3 &#39;s sub-tree based on Node  3 &#39;s recommended parent list. 
         [0100]      FIG. 8  illustrates the case in which both children of departing Node  2  are of Type 0. The green child, Node  3 , is instructed to connect to Node  1 . However the red child, Node  4 , must climb its path looking for a home. It first connects to Node  1  and if Node  1  cannot place it, will climb to higher levels of the tree until it reaches the server. If the server cannot place Node  4  it will be rejected from the broadcast tree. 
         [0101]      FIG. 9  illustrates the case where Node  3 , the green child of departing Node  2 , is of Type 2; while Node  4 , the red child of the departing node, is of Type 0. In this case, Node  3  will be instructed by Node  2 &#39;s depart message to connect to Node  1 . Node  1  will expect Node  3  based on its own message from Node  2 . Node  4  will be directed to connect to a location in Node  3 &#39;s sub-tree (assuming a location is available for a Type 0 to dock) based on Node  3 &#39;s recommended parent list. 
         [0102]      FIG. 10  illustrates a graceful depart of a Type 1 node. Node  2 , a Type 1 node, executes a graceful client shutdown. As a result Node  2  transmits depart messages to its parent, Node  1 , and its child, Node  3 . Node  2 &#39;s depart message instructs Node  3  to connect to its former grandparent, Node  1 . 
         [0103]    Handling Unexpected (Non-Graceful) Node Departures 
         [0104]    When an unexpected or non-graceful depart occurs, such as happens when a node loses its connection to its parent, the children of the departing node will climb their path to their former grandparent for placement. The first child that registers with its former grandparent is accepted. The second child that registers with its grandparent will take different actions based on its type. If the slower child is a Type 2 and has children, that node will do a system join to its grandparent. The grandparent will instruct the joining node to priority join to the grandparent&#39;s red child. Otherwise, the second child will follow the normal registration procedure. If the second child is a Type 1 or Type 0, the grandparent places it as if it were just entering the network. The node may not priority join, as priority join functionality is reserved only for Type 2 nodes. Type 1 and Type 0 nodes must register normally at all times. The reason behind this restriction is to prevent Type 1 nodes from bumping Type 2 nodes or higher priority Type 1 nodes. Since nodes that priority join are automatically accepted as a node&#39;s red child and that red child is bumped, Type 1 nodes could bump Type 2 nodes if they were allowed to priority join. Since this would work counter to the notion of placing higher level nodes closer to the server, only Type 2 nodes are allowed to priority join. 
         [0105]      FIG. 11  through  FIG. 18  illustrate the reconfiguration behaviors resulting from nodes departing the network in an unexpected (non-graceful) manner. 
         [0106]      FIG. 11  illustrates the general method for handling unexpected or non-graceful node departures. Node  2 , the parent of two nodes, Node  3  and Node  4 , and the child of Node  1 , departs the network in an unexpected and non-graceful manner. Since no depart messages were generated and sent by Node  2  prior to its departure, Node  3  and Node  4  simply lose their connections to Node  2 . As each of these nodes become aware that Node  2  no longer exists in the network, via connection time outs or some other means, they will climb to their former grandparent, Node  1 . The node that reaches Node  1  first will be connected to Node  1  as its child, replacing the departed Node  2 . The slower of the two nodes will be dealt with as described above. 
         [0107]      FIG. 12  illustrates the case where both children of the unexpectedly departing node are Type 2.  FIG. 13  illustrates the case where the green child of the unexpectedly departing node is Type 2 and the red child is Type 1.  FIG. 14  illustrates the case where both children of the departing node are Type 1.  FIG. 15  illustrates the case where the green child is Type 1 and the red child is Type 0.  FIG. 16  illustrates the case where both child nodes of the departing node are Type 0.  FIG. 17  illustrates the case where the green child is Type 2 and the red child is Type 0. 
         [0108]      FIG. 18  illustrates a unexpected (non-graceful) depart of a Type 1 node. Node  2 , a Type 1 node, departs unexpectedly from the network in a non-graceful manner. Node  3  becomes aware of the departure of Node  2  via the connection between the two nodes timing out or by some other means. Node  3  climbs its path to its former grandparent Node  1  and Node  1  accepts Node  3  as its child. 
         [0109]    Handling Message Loss in Graceful Departs 
         [0110]    When a client is shutdown gracefully, it sends out the depart messages to each of its kids and its parent. However, there is no guarantee that these messages will be received by their intended recipients.  FIG. 19  through  FIG. 25  illustrate the consequences of the failure of various messages to reach their intended recipients. 
         [0111]      FIG. 19  illustrates the case in which the depart message fails to reach the non-preferred (red) child.  FIG. 20  illustrates the case in which the depart message fails to reach the preferred (green) child.  FIG. 21  illustrates the case in which the depart message fails to reach the parent.  FIG. 22  illustrates the case in which the depart messages fail to reach both children.  FIG. 23  illustrates the case in which the depart message fails to reach both the parent and the non-preferred (red) child.  FIG. 24  illustrates the case in which the depart message fails to reach both the parent and the preferred (green) child. 
         [0112]      FIG. 25  illustrates the case in which all three messages fail to reach their intended recipients. In this case the graceful depart reverts to the standard non-graceful depart described above. 
         [0113]    Determining Placement During Reconfiguration &amp; Maintaining Accurate Network Topology Models 
         [0114]    When Node A sends a message to Node B requesting that Node A be placed in the broadcast tree, it is necessary that Node B have access to an internal Network Topology Model (NTM) that reflects the overall state of the network with enough fidelity to assign Node A to a parent node consistent with maintaining the key properties of the network, such as maintaining approximate tree balance and placing stronger nodes as high in the tree (as close to the server) as practical. 
         [0115]    This NTM must be maintained even during times of high turnover when nodes are frequently joining and departing the network. To that end, the parent of a departing node must be able to effectively predict the likely effect on the overall topology of the network its departing child will have. In other words, the parent node must update its internal NTM to account for connections that will likely be broken and new connections that will likely be formed as a result of the departure of its child from the network. These effects are not necessarily localized and may propagate throughout the sub-tree rooted at the parent all the way to the edge of the network. 
         [0116]    In the system and method of U.S. Pat. No. 7,035,933 reconfigurations to a node&#39;s local copy of the NTM for the parent of a departing node involved recursively following the green path of the parent&#39;s sub-tree and having red children connect to their green siblings. This elegant approach emerged from the system requirement that all nodes could either repeat to two children or not at all—and that, aside from greenness/redness, all repeaters were essentially equal. Thus, a red child (Node C) was assumed able to connect to its green sibling (Node B), and the green child (Node B) was assumed able to accept its red sibling (Node C) as a child, provided the green child (Node B) did not already have two children of its own (Node D and Node E) or if it (Node B) did it (Node B) would be required to bump its existing red child (either Node D or Node E) making room for its (Node B&#39;s) incoming red sibling (Node C). In cases where the red sibling was not capable of repeating to two children and the red child of the green sibling was also a not capable of repeating to two children, reconfigurations were allowed to continue as described above with the understanding that the green sibling in reality would simply reject the connection request from the red sibling as a non-repeat capable node was not allowed to bump another non repeat capable node. The difference between the outcomes of the NTM reconfiguration and the reality of the network-level request logic was allowed to persist, because the difference did not affect the structure of the NTM as far as placement of new nodes was concerned. 
         [0117]    With the complexity of the new placement logic for the present system and method, reconfiguration must solve issues that didn&#39;t exist before. In the present system and method, all repeat capable nodes are not essentially equal as they were in U.S. Pat. No. 7,035,933. Some repeat capable nodes can support two children (Type 2 nodes) while other repeat capable nodes can support only one child (Type 1 nodes). Of the repeat capable nodes that can only support one child, there is the additional consideration of its priority. A Type 1 node of normal priority (e.g., a T1N) cannot be the parent of a Type 1 of a higher priority (e.g., a T1H), and no Type 1 node, regardless of its priority, may become the parent of a Type 2 node. 
         [0118]    Consider the situation illustrated in  FIG. 26 . Node B is departing the network in a graceful manner. The parent of Node B, Node A, needs to update its internal Network Topology Model to account for the reconfigurations that are likely to occur as a result of Node B&#39;s departure. 
         [0119]    In one embodiment, the method of U.S. Pat. No. 7,035,933 would be: (1) the green child of the departing node connects to its former grandparent, in this particular case Node C connects to Node A, (2) the red child of the departing node cross connects to its green sibling, in this particular case Node D cross connects to Node C, (3) the green child bumps its own red child to make room for its incoming red sibling, in this particular case Node C bumps Node F to accept Node D, (4) the bumped red grandchild of the departing node cross connects to its own green sibling, in this particular case Node F would cross connect to Node E, (5) the green-green grandchild of the departing node would bump its own red child to make room for its incoming red sibling, in this particular case Node E bumps Node J to accept Node F, and finally (6) the bumped red great grandchild of the departing node cross connects to its own green sibling, in this case Node J cross connects to Node I. 
         [0120]    The problem with the method of U.S. Pat. No. 7,035,933 is that it does not account for the fact that Node F cannot connect to Node E since Node F is a Type 1 node and both of Node E&#39;s children are Type 2. In the system of the present system and method Type 1 nodes cannot bump Type 2 nodes. 
         [0121]    Thus the present system and method includes the following method that allows higher level nodes (such as Node A) to predict the reconfiguration behavior of lower level nodes (such as Node E) in order for the higher level node to update its internal Network Topology Model to reflect the most likely result of a reconfiguration caused by the departure of its child (Node B). 
         [0122]    The method consists of having each node periodically propagate up to its parent a description of that node&#39;s state (including that node&#39;s recommended parent list) together with the state of all nodes located beneath that node and the structure of their relationship to the node in question. Thus for example referring to  FIG. 26 , Node&#39;s I and J would propagate their information up to Node E; Node K its information to Node F; Nodes L and M up to Node G; Nodes N and O up to Node H. Node E would propagate its information and its children&#39;s information (including the fact that Node I is the green child of Node E and Node J is the Red Child of E) to Node C. Likewise Node F would propagate its information and its child&#39;s information to Node C; Nodes G and H their information and their children&#39;s information to Node D. Nodes C and D would propagate their information and the information of all of their descendants to Node B, and likewise Node B its information and its descendants information to Node A. 
         [0123]    In this manner, Node A will have received information from all of the nodes in its entire sub-tree with which to populate its internal network topology model. Thus, when Node B departs the network, Node A will be able to determine the likely result of this departure by consulting the recommended parent lists of the various nodes in its sub-tree. 
         [0124]    It should be noted at this point, that a particular node&#39;s internal model of the network may not be completely accurate as configuration changes may have taken place at lower levels and those changes may not have yet propagated up to the node in question. Regardless of this limitation, the information provided is judged “good enough” for making placement decisions and predictions as to network state following child departure. 
         [0125]    Continuing with the example begun in  FIG. 26 ,  FIG. 27  illustrates the first two steps of the reconfiguration begun by the graceful departure of Node B. Node C connects to Node A, corresponding to the green child connecting to the parent of the departing node; and Node D connects to Node C, corresponding to the red child of the departing node cross connecting to its green sibling. In point of fact, as illustrated in  FIG. 4 , Node B instructs Node C to connect to Node A, and Node D to connect to Node C. 
         [0126]      FIG. 28  illustrates the state of the network immediately following the connection of Node C to Node A and Node D to Node C. Note that Node F, the green-red grandchild of the departing node, has been bumped by Node C in order to make room for Node D to connect. 
         [0127]    Node F wishes to connect to Node E, but unfortunately since Node F is Type 1 and both of Node E&#39;s children are Type 2, Node F cannot connect directly to Node E. This issue can be resolved in the following way. When Node F is bumped by Node C, Node C can consult its Network Topology Model and determine that Node E&#39;s recommend parent for incoming nodes will be Node I (as Node I is the green child of Node E) and pass this recommended parent directly to Node F as part of the depart/bump message Node C sends to Node F. 
         [0128]    Node A, when updating its own internal Network Topology Model, can predict that Node C will instruct Node F to connect to Node I, as Node A can determine the recommended parents associated with every node in its sub-tree. This allows Node A to update its internal model in accordance with what is likely to happen in the actual network during the reconfiguration events. 
         [0129]      FIG. 29  illustrates the final state of the network of  FIG. 26  following the departure of Node B. Assuming all reconfiguration events resulting from the departure of Node B proceed as planned and there are no other node additions, deletions, or reconfigurations that take place during this time period  FIG. 29  should be an accurate reflection of both the physical network and Node A&#39;s internal Network Topology Model. 
         [0130]    A Complete Example of Network Reconfiguration Following Graceful Depart 
         [0131]      FIG. 30 , parts ( a )-( f ), provide a complete example of network reconfiguration following the graceful departure of a node. 
         [0132]      FIG. 30  ( a ) shows the complete network (full tree) prior to the departure of a node.  FIG. 30  ( b ) illustrates the graceful departure of Node  2  from the network. Node  4  is instructed by the departing node to connect to Node  1  and the result can be seen in  FIG. 30  ( c ). Node  5  is instructed by the departing node to connect to Node  4 . The particular type of join Node  5  is instructed to perform with Node  4  is a “priority join” meaning that Node  4  must accept Node  5  (and therefore Node  4  must bump its red child to make room for Node  4 ).  FIG. 30  ( d ) illustrates the state of the network after Node  5  is accepted by Node  4 . Note that Node  9 , the former red child of Node  4 , has been bumped. When Node  4  bumped Node  9  it provided Node  9  with a connection path. That connection path included the recommended parent that Node  9  should attempt to connect to. In this case Node  9  is told by Node  4  to connect to Node  25 . The reason Node  4  chose Node  25  for Node  9  is that Node  25  is the highest node (closest node to the server) that Node  4  is aware of that can accept a node of Node  9 &#39;s type. Note that being of Type 1N, all that can be bumped by Node  9  are Type 0 nodes—resulting in the candidate parents for Node  9  being Node  35 , Node  28 , Node  29 , and Node  25 , of which Node  25  is the closest to the server.  FIG. 30  ( e ) illustrates the state of the network after Node  9  connects to Node  25 . Note that in order for Node  25  to accept Node  9  as a child, it was necessary for node  25  to bump Node  34 , a Type 0 Normal Priority node. As part of the bump message, Node  25  instructs Node  34  to attempt to connect to Node  9  (either for a direct connection or for instructions as to where it can connect). Node  34  attempts to connect to Node  9 , the root of a Type 1 chain that terminates in a Type 0 Low Priority node (Node  26 ). Since Node  34 &#39;s Type 0 Normal Priority out ranks Node  26 &#39;s Type 0 Low Priority, Node  9  instructs Node  34  to connect to Node  26 &#39;s parent, Node  17 . Since Node  26  cannot connect to Node  34  or Node  17 , Node  26  must climb it parent path, beginning with the first Type 2 node in the path, Node  4 , and, if necessary, working back up the parent path towards the server. In this case, however, Node  4  is able to recommend connection to Node  29  and the reconfiguration cascade concludes.  FIG. 30  ( f ) illustrates the final configuration of the network. 
         [0133]    A Complete Example of Network Reconfiguration Following Unexpected (Non-Graceful) Depart 
         [0134]      FIG. 31 , parts ( a )-( d ), provide a complete example of network reconfiguration following the unexpected (non-graceful) departure of a node. 
         [0135]      FIG. 31  ( a ) illustrates a complete network prior to an unexpected departure of Node  6 , a Type 2 Normal Priority node. After the departure of Node  6 , its children, Nodes  12  and  13 , note its absence. Both Node  12  and Node  13  connect to their grandparent, Node  3 . At this point a “race condition” exists between Node  12  and Node  13 . Whichever node first connects to Node  3  will be allowed to join as its child. In this case we assume that Node  12  arrives first and is added as a child of Node  3 .  FIG. 31  ( b ) illustrates the state of affairs at this point. Node  13  then attempts to connect to Node  3 . Since Node  13  is a Type 1 High Priority node, Node  3  has three potential slots at which it could place Node  13 . These are: (1) as a child of Node  12 , (2) as a child of Node  23 , or (3) as a child of Node  15 . Since Node  12  is the closest potential parent to the root of the tree, Node  3  tells Node  13  to connect to Node  12 . Node  13  then connects to Node  12  causing it to bump Node  21 , a Type 1 Normal Priority Node. As Node  12  bumps Node  21 , it tells the node to connect to Node  13  for a connection path. The situation at this point is illustrated in  FIG. 31  ( c ). Node  13  tells Node  21  to connect to Node  22 . Node  21  does so, causing Node  22  to bump its Type 0 Normal Priority child, Node  31 . Node  31  attempts to connect to Node  21  but is rejected and must climb its path. Eventually, Node  31  finds a home as a child of Node  32 . 
         [0136]    While the invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.