Abstract:
The present invention provides a protection protocol for fault recovery, such as a ring wrap, for a network, such as a two line bi-directional ring network. An embodiment of the present invention works in conjunction with a ring topology network in which a node in the network can identify a problem with a connection between the node and a first neighbor. The present invention provides a protection protocol which simplifies the coordination required by the nodes in a ring network. The nodes do not need to maintain a topology map of the ring, identifying and locating each node on the ring, for effective protection. Additionally, independently operating ring networks can be merged and the protection protocol will appropriately remove a protection, such as a ring wrap, to allow the formation of a single ring. It also provides for multiple levels of protection priority so that protection for a high priority failure, such as a physical break in a connection, would remove protection for a low priority failure, such as a signal degrade, on another link.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to computer networks. In particular, the present invention relates to a system and method for providing a protection protocol for fault recovery for a two line bi-directional ring network. 
     2. Background of the Invention 
     The need for cooperation among various computers has motivated the growth of efficient architectures of networks. One topology of networks is a ring network, such as FDDI and SONET. 
     FIG. 1 shows an example of a two line bi-directional ring network. The ring network  100  is shown to include nodes  102   a - 102   g.  Each node is typically a computer with embedded processors and at least one network connection. Each node  102   a - 102   g  is shown to be bidirectionally coupled to two neighboring nodes  102   a - 102   g  via an inner connection ring  110   a - 110   g  and an outer connection ring  108   a - 108   g.  For instance, node  102   a  is bidirectionally coupled to nodes  102   b  and  102   g.  The example of FIG. 1 also shows a problem  104  in the connection between node  102   b  and node  102   c . When a problem is detected (such as a bi-directional line cut), the connection between nodes  102   b  and  102   d  wraps back upon itself, as shown by wraps  106   a  and  106   b . In this manner, the connection problem  104  can be avoided. 
     In a conventional SONET Line Switched Network, the nodes on each side of the problem  104  will typically exchange messages with each other over the long path. In this example, node  102   b  would send a message to node  102   c  via connection  110   g,  and vice versa, via connection  108   g.  In a conventional SONET Line Switched Ring Network, the nodes on each side of the problem  104  will typically exchange messages with each other over the long path. The long path is the path that passes the other nodes in the ring network to reach a neighbor of the node that originated the message. For example, a long path from node  102   b  to node  102   c  would pass through nodes  102   a - 102   d  via outer ring  108   a - 108   f.    
     When both the nodes flanking the problem  104  receive the other node&#39;s message, then these nodes will typically perform a wrap. For example, when nodes  102   b  and  102   c  receive each other&#39;s message indicating the detection of the problem  104 , nodes  102   b  and  102   c  will perform wraps  106   a  and  106   b . In order to send these messages, each node that sends a message must typically know the identity and location of the receiving node. If the identity and location of the receiving node is not accurate, then there will typically be a failure to wrap. 
     In a conventional SONET network, each message sent by a sending node to a receiving node typically needs the identification and location of the receiving node to arrive at the proper destination. Accordingly, manual configuration is typically needed in each node to store the identity and location of each other node in the ring network in order to provide for communication between the nodes in the network. 
     A problem can arise when a new node is coupled into the ring network. Each node then typically needs to have its topology map of the ring network reconfigured such that the identity and location of the new node in the ring network can be included. Additionally, the locations of at least some of the original nodes may also change and these changes should also be included in each node&#39;s internal map of the network. Until and unless these new locations and identities can be included in a sending node&#39;s internal map of the network, messages sent by a node indicating that a problem has been detected will typically not arrive at the proper destination node. When these message fail to arrive at the proper destination, the required ring wraps will also fail to occur. 
     Likewise, if two ring networks are merged into one, each node will then typically need to have its internal map of the ring network configured to include the identities and locations of each node included in the merged ring network. These reconfigurations typically require the time and effort of a programmer or network administrator. Again, unless and until these new locations and identities can be included in a sending node&#39;s internal map of the network, messages sent by that node will typically not arrive at the proper destination node. In summary, for the protection mechanism to operate, each node needs to know the current ring map (current ring topology). Accordingly, required ring wraps will fail to occur. 
     In summary, for the protection mechanism to operate, each node needs to know the current ring map (current ring topology). What is needed is a system and method for providing fault recovery for two line bi-directional ring network that minimizes the need to keep track of other nodes in the ring network. Preferably, the system would not require reconfiguration of an internal map of the network when a new node is added to, or existing nodes are removed from the network. The present invention addresses such a need. 
     SUMMARY OF THE INVENTION 
     The present invention provides a protection protocol for fault recovery, such as a ring wrap, for a network, such as a two line bi-directional ring network. An embodiment of the present invention works in conjunction with a ring topology network in which a node in the network can identify a problem with a connection between the node and a first neighbor. According to the embodiment, when the problem is identified, the node sends a message identifying the problem to a second neighbor which is located at least one node away from the problem. The second neighbor then forwards the message to a third neighbor, unless the second neighbor is dealing with a situation that is higher in a hierarchy of situations than the problem described in the message by the original node. In general, if the second neighbor&#39;s situation has a higher priority than the situation described by the original node, then the message is ignored and not forwarded. If, however, the message sent by the original node describes a situation with a higher priority than the situation being dealt with by the second neighbor, then, in general, the second neighbor&#39;s situation is ignored, at least for the moment, and the original node&#39;s message is forwarded to the next neighbor. In general, a higher priority request preempts a lower priority request within the ring. Exceptions are noted as rules of the protection protocol. 
     The present invention provides a protection protocol that simplifies the coordination required by the nodes in a ring network. The nodes do not need to maintain a topology map of the ring, identifying and locating each node on the ring, for effective protection. Additionally, independently operating ring networks can be merged and the protection protocol automatically appropriately removes a protection, such as a ring wrap, to allow the formation of a single ring. It also provides for multiple levels of protection priority so that protection for a high priority failure, such as a physical break in a connection, removes protection for a low priority failure, such as a signal degrade, on another link. 
     A method according to an embodiment of the present invention for fault recovery for a ring computer network, the ring network including a plurality of nodes, is presented. The method comprises detecting a situation by a first node, wherein the first node is one of the plurality of nodes; sending a first message via a short path to a second node, wherein the first node is adjacent to the second node; and initiating a fault recovery procedure when the second node receives the first message. 
     In another aspect of an embodiment of the present invention, a method for adding a new node to a ring computer network, the ring network including a plurality of nodes, is presented. The method comprises detecting a situation by a first node, wherein the first node is one of the plurality of nodes; sending a first message via a short path to a second node, wherein the first node is adjacent to the second node prior to an addition of the new node; initiating a fault recovery procedure when the second node receives the first message; receiving a second message from the new node; and entering an idle state when the second message is received. 
     In yet another aspect of an embodiment of the present invention, a system for fault recovery for a ring computer network, the ring network including a plurality of nodes, is presented. The system comprises means for detecting a situation by a first node, wherein the first node is one of the plurality of nodes; means for sending a first message via a short path to a second node, wherein the first node is adjacent to the second node; and means for initiating a fault recovery procedure when the second node receives the first message 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram illustrating a ring network that is utilizing a ring wrap protection. 
     FIG. 2 is block diagram of a ring network utilizing a protection protocol according to an embodiment of the present invention. 
     FIGS. 3 a  and  3   b  illustrate examples of a message format according to an embodiment of the present invention for a message being sent from one node to another. 
     FIGS. 4-6 are flow diagrams illustrating various rules within the protection protocol according to an embodiment of the present invention. 
     FIG. 7 illustrates an example of a priority hierarchy which can be used according to an embodiment of the present invention. 
     FIGS. 8-12 are flow diagrams and a system diagram illustrating further rules of the protection protocol according to an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following description is presented to enable one of ordinary skill in the art to make and to use the invention and is provided in the context of a patent application and its requirements. Various modifications to the preferred embodiments will be readily apparent to those skilled in the art and the generic principles herein may be applied to other embodiments. Thus, the present invention is not intended to be limited to the embodiment shown and is to be accorded the widest scope consistent with the principles and features described herein. 
     For ease of reference, the packet transfer mechanism, the signaling and wrapping mechanism, and the protocol rules according to an embodiment in the present invention are first briefly listed. Further details of the mechanisms and rules according to the present invention will be described in conjunction with FIGS. 2-12. The following rules are numbered simply for ease of reference and should not be inferred that they are to be executed in any particular order. These rules are referenced herein by rule number for simplicity. 
     Spatial Reuse Protocol (SRP) Automatic Protection Switching (APS) Packet Transfer Mechanism 
     1. APS packets can be transferred between nodes. The packets can be stored and forwarded between adjacent nodes. 
     2. All APS messages are sent to the neighboring nodes periodically on both inner and outer rings. The time periods are user configurable. Examples of a time period is 10 msec. when a protection, such as a ring wrap, is taking place, and 1 sec. when a ring wrap is has either completed or the ring is in IDLE state. Providing short and long time periods can reduce APS traffic under steady state conditions. 
     SRP ASP Signaling and Wrapping Mechanism 
     3. APS signaling can be performed using packets carrying information such as a request, a source, a wrap status, and a path indicator. 
     4. A node executing a self detected request signals the protection request, such as a wrap request, on both short (across the failed span) and long (around the ring) paths after performing the wrap. 
     5. The node executing a short path protection request (i.e. receiving node of a short path protection request) sends an idle message with wrapped status across the failed span and a protection request on the long (around the ring) path after performing a wrap. 
     6. A node which is neither executing a self detected request nor executing a short path request sends idle messages to its neighbors on the ring if there is no long path message passing through the node on that ring. 
     7. Protection APS packets are not wrapped. 
     SRP APS Protocol Rules 
     8. A protection request hierarchy is utilized. In general, a higher priority request preempts a lower priority request within the ring. Exceptions are noted as rules. The following is a list of the protection request hierarchy listed in the order of highest priority: 
     Lockout of Protection (LO) 
     Forced Switch (FS) 
     Signal Fail (SF) 
     Signal Degrade (SD) 
     Manual Switch (MS) 
     Wait-to-Restore (WTR) 
     No Request (IDLE) 
     9. Requests which are higher or of equal priority to SF and lower priority to LO can co-exist. 
     10. LO requests can co-exist. 
     11. Requests with lower priority than SF cannot co-exist with other requests. 
     12. A node honors the highest of (short path request, self-detected request) if there is no higher long path message passing through the node. 
     13. When there are multiple requests of the same priority, the priority being lower than SF, the first request to complete long path signaling will take priority. The first request to complete long path signaling refers to the first request that reaches a node determining which request takes priority. 
     14. In case of two equal requests on both inner and outer rings, when their priority is less than SF, the tie is broken by choosing a predetermined ring. For example, the outer ring request can be selected. 
     15. A node does not forward a long APS packet received by it that was originally generated by the node itself. 
     16. Nodes do not forward packets with the path indicator set to short (short path). 
     17. When a node receives a long path request and the request has a priority which is greater or equal to the highest of (short path request, self detected request), the node determines if the same message is coming from its neighbor on a short path. If that is the case, then the node does not unwrap. 
     18. When a node receives a long path request, it terminates the request (does not forward) if the receiving node is a wrapped node and it is in a situation which is of higher or equal priority than the long path request. Otherwise, it forwards the long path request and determines if it needs to unwrap. 
     19. Each node keeps track of the addresses of its immediate neighbors (the neighbor node address can be gleaned from the short path APS messages). 
     20. When a wrapped node (which initially detected the failure) discovers disappearance of the failure, it enters WTR (long, user—configurable, WTR time period). 
     21. When a node is in WTR mode, and detects that the new neighbor (as can be identified from the received APS short path message) is not the same as the old neighbor (identity of which can be stored at the time of wrap initiation), it changes the WTR time period to zero (it enters the idle state). 
     22. When a node receives a local protection request of type SD or SF and it cannot be executed (according to protocol rules), then the node keeps the request pending. 
     FIG. 2 is a block diagram showing a ring network system utilizing a method of fault recovery according to an embodiment of the present invention. The ring network  200  is shown to include nodes  202   a - 202   g.  The nodes  202   a - 202   g  are shown to be coupled via an inner ring  210  in which the data flows in one direction, such as a clockwise direction. Additionally, the nodes  202   a - 202   g  are also shown to be coupled by an outer ring  212  in which data can flow in the opposite direction to the inner ring  210 , such as in a counter-clockwise direction. The ring network  200  is shown to have a situation  204   a  that requires protection, such as a ring wrap  206 . 
     Several of the rules of the method according to the embodiment of the present invention can be described in conjunction with FIG. 2 referred to in combination with various other figures. 
     FIGS. 3 a  and  3   b  show examples of Rule (3), which describe an APS packet message format which can be used in conjunction with the system and method according to the embodiment of the present invention. The format  300  shown in FIG. 3 a  is shown to include a source address  302 , a request  304 , and a path  306 . 
     An example of the APS packet message format can be seen in FIG.  2 . In FIG. 2, node  202   b  is shown to send a message to node  202   c . Included in the message is the source address: node B; the request: signal fail (SF); and the path: short (S). The source address  302  indicates which node has sent the message, in this example, node  202   b  is sending the message. The request can indicate the situation  204   a , in this example, a signal fail. The path  306  indicates which path the message is taking. The paths can be either short (S) or long (L). The short path is the shortest route between a first node and its neighbor. The long path is the longer route between a first node and its neighbor. Accordingly, the other message sent by node  202   b  has a source address of B, a request of SF, and a path L being sent along the outer ring  212  in a counterclockwise direction towards its neighbor node  202   c.    
     Another message sent by node  202   a  illustrates Rule (6) which states that a node which is neither executing a self detected request nor executing a short path request signals IDLE messages to its neighbors on the ring if there is no long path message passing through the node on that ring. In the example shown in FIG. 2, the IDLE message sent by node  202   a  is sent prior to receiving the messages regarding the signal fail along the long path. A&#39;s message indicates that the source of the message is A, that the request is IDLE (nothing noteworthy is happening), and that the message is being sent along a short path. 
     An alternative message format  300 ′ is shown in FIG. 3 b . This format is shown to include a source address  302 ′, a request  304 ′, a wrap status  308  and a path  306 ′. The wrap status  308  can be used for debugging by a programmer. The wrap status  308  can indicate whether there is a ring wrap currently located on the ring network. 
     FIG. 4 is a flow diagram of an example of a method according to an embodiment of the present invention implied by Rules 1-22. An APS packet is received via step  400 . It is determined whether the APS packet has been sent along a long path via step  402 . If the packet was not sent via a long path, then the APS packet is not forwarded via step  406 . Accordingly, if the APS packet was sent via the short path, then the packet is not forwarded via step  406 . If, however, the packet was sent through the long path via step  402 , then the APS packet may be forwarded via step  404 . Note that for this example of Rule (1), it is assumed that the long path message does not have to pass through a wrapped connection in order to be forwarded. Otherwise, if the long path message needs to pass through a wrapped connection in order to be forwarded, then the message will simply not be forwarded. 
     An example of the method shown in FIG. 4 can be seen in FIG.  2 . When A receives the message from B (B, SF, L), A forwards the message to its neighbor node  202   g  because it is a message sent via the long path. 
     FIG. 5 is a flow diagram illustrating Rule (2) of the method according to the embodiment of the present invention. An APS packet is sent to a neighboring node via step  500 . It is then determined if there is an incomplete ring wrap via step  502 . An incomplete ring wrap is ring wrap procedure that has initiated but not yet completed. If there is no incomplete ring wrap on the ring network via step  502 , then a pre-determined time is set to an IDLE time, such as one second, via step  506 . The IDLE time is a pre-determined time that is set during a time when there is no incomplete ring wrap. If, however, a problem is detected, for example, a signal for a ring wrap occurs, via step  502 , then a pre-determined time is set to a protection time, such as 10 msec., via step  504 . The protected time is a pre-determined amount of time utilized when the ring network is in the process of being protected, for example by creating a ring wrap. Once a pre-determined time has been set, either via step  506  or  504 , the predetermined time is then measured out via step  508  and another APS packet is sent to the neighboring node via step  500 . Once the ring wrap is completed, then IDLE time is again implemented. 
     An example of the method shown in FIG. 5 can be seen in FIG.  2 . Node  202   a , prior to receiving node  202   b &#39;s long path message, sends an IDLE message to  202   b  regularly at intervals set by the pre-determined time. 
     FIG. 6 is a flow diagram illustrating Rules 4 and 5. A node detects a problem between the node and a first neighbor via step  600 . The node performs a wrap away from the side on which the problem exists via step  602 . A short path message is then sent to the first neighbor informing it of the problem via step  604 . Additionally, a long path message is also sent to a second neighbor informing the second neighbor of the problem via step  604 . The first neighbor then performs a wrap away from the side of the problem via step  606 . The first neighbor also sends an IDLE message, indicating a wrapped status, on a short path to the node that detected the problem via step  608 . This message is sent across the failed span. Note that IDLE messages do not get wrapped and are sent across failed spans if possible. Additionally, the first neighbor also sends a message on a long path toward the side without the problem via step  608 . 
     An example of the method described in FIG. 6 can be seen in FIG.  2 . Node  202   b  has detected a problem  204   a  and performs a wrap  206  on the side on which the problem exists. Node  202   b  also sends a short path message to the neighbor on the other side of the problem  204   a , which is node  202   c . Node  202   b  also sends a long path message to its other neighbor node  202   a  informing it of the problem. Node  202   c  performs a wrap  206  on the side of the problem and sends an IDLE message on a short path to node  202   b . Node  202   c  also sent a message on a long path toward the side without the problem to its neighbor  202   d.    
     FIG. 7 lists the hierarchy of priorities of Rule (8). For ease of reference, the hierarchy is separated into Class I-III. Class I is the highest priority, while Class III is the lowest priority. An example of a highest priority message in Class I is lockout. Lockout is an order stating that the ring network is not to wrap under any circumstances. 
     Examples of the next priority listed in Class II are forced switch and signal fail. Forced switch indicates that the ring network is configured to wrap at the point of the forced switch. Signal fail is a situation where either two nodes cannot communicate with each other, or one node cannot hear the other node. An example of a signal fail is a physical break in the communication lines between two nodes. 
     Examples of a priority hierarchy which can exist in Class III include signal degrade, manual switch, wait-to-restore, and no request (IDLE). A signal degrade indicates that two nodes can communicate, however there are errors in the communication. Manual switch is a situation where the ring network has been configured to wrap at the manual switch point. The difference between a forced switch and a manual switch is that the only command which overrides a forced switch is a lockout, while a manual switch can be overridden by any command which is above it in the hierarchy (signal degrade, signal fail, forced switch, and lockout). Wait-to-restore is a transitional state which waits for a predetermined time after a failure has been resolved prior to entering a no request (IDLE) state. Although the failure has been resolved, the wrap is not unwrapped for a pre-determined time (zero to several minutes such as five minutes). This pre-determined time is user configurable. No request (IDLE) indicates that nothing of significance is occurring and no wrap is being executed. 
     Note that members of Class II can co-exist (Rule 9). For example, multiple forced switches and signal fails can co-exist. Additionally, members of Class I can co-exist (Rule 10). For example, multiple lockouts in a single ring network can co-exist. However, situations in Class III cannot co-exist with other situations (Rule 11). For example, a signal degrade cannot co-exist with a wait-to-restore. 
     When there are multiple requests of the same priority within Class III, the first request to complete a long path signaling will take priority (Rule 13). For example, if there are two signal degrades located on the same ring network, then the first signal degrade which completes the long path signaling will take priority over the other signal degrade. By not allowing members of Class III to co-exist with one another, partitioning of the ring network is avoided. 
     In case of two equal requests within Class III on both inner and outer rings of the ring network, the tie is broken by choosing a request on one of the rings, such as the outer ring request (Rule 14). For example, if a signal degrade occurs both on the inner and outer rings, then a request on a predetermined ring, such as the outer ring, will take priority over the other requests. 
     FIG. 8 is a flow diagram illustrating Rules (9), (10), (11), (13), and (15). Note that the flow diagram described in FIG. 8 is merely an example of one way in which the rules of the method according to the embodiment of the present invention can be executed. For example, the determination of whether the long path message is a Class I request via step  802  or a Class II request via step  810  can be in reverse order. 
     A wrapped node receives a long path message via step  800 . It is then determined if the long path message is a Class I request via step  802 . The classes used in FIG. 8 are meant to correspond with the example of classes defined in FIG.  7 . If the long path message is a Class I request, then it is determined if a local situation also has a Class I request via step  804 . A local situation includes scenarios such as when a node detects a situation or problem, or when a node is made aware of a problem or situation via a short path message from its neighbor. If a local situation is not a Class I request via step  804 , then any existing local wraps are unwrapped and the long path message is forwarded via step  806 . If, however, a local situation is a Class I request via step  804 , then the connections are already unwrapped or was never wrapped, and the long path message is forwarded via step  808 . 
     If the long path message is not a Class I request via step  802 , then it is determined whether the long path message is a Class II request via step  810 . If the long path message is a Class II request via step  810 , it is determined whether a local situation is in a Class II request via step  812 . If a local situation has a Class II request via step  812 , then no local wraps are unwrapped and the long path message is not forwarded via step  816 . 
     If a local situation is not a Class II request via step  812 , then it is determined whether a local situation has higher priority over the long path message via step  814 . If the local situation does have higher priority, then local wraps are not unwrapped and the long path message is not forwarded via step  816 . If, however, the local situation does not have a higher priority than the long path message via step  814 , then the long path message is forwarded and local wraps are unwrapped via step  820 . 
     If the long path message is not a Class II request via step  810 , then it is determined whether a local situation has higher priority than the long path message via step  818 . If a local situation does have higher priority than the long path message, then local wraps are not unwrapped and the long path message is not forwarded via step  816 . If, however, a local situation does not have higher priority over the long path message via step  818 , then it is determined whether a local situation has equal priority with the long path message via step  822 . If the local situation does not have equal priority with the long path message, then the long path message is forwarded and local wraps are unwrapped via step  820 . 
     If, however, the local situation has equal priority with the long path message via step  822 , then it is determined whether a local situation relates to one network ring, such as the inner ring, while the long path message relates to another network ring, such as the outer ring, via step  826 . If both the local situation and the long path message relate to the same ring, then local wraps are not unwrapped and the long path message is forwarded via step  824 . If, however, a local situation is for one ring while the long path message is for another ring via step  826 , then a pre-determined ring, such as the outer ring, is selected for wrapping via step  828 . 
     FIG. 9 is a flow diagram illustrating Rules (15), (16), and (17) of the method according to the present invention. A node receives a message via step  900 . It is then determined if the message was sent through a long path via step  902 . If the message was not sent by a long path, then the message is not forwarded via step  904 . If, however, the message was sent through a long path via step  902 , then it is determined whether the message was generated by the receiving node via step  906 . 
     If the message was generated by the receiving node, then the message is not forwarded via step  904 . If, however, the message was not generated by the receiving node via step  906 , then it is determined whether the message has a lower priority than a local situation via step  908 . If the message does have a lower priority than a local situation, then the message is not forwarded via step  904 . If, however, the message does not have a lower priority than a local situation via step  908 , then the message is forwarded and it is determined if the receiving node should unwrap via step  910 . The determination of whether the receiving node should unwrap can be determined by using the method illustrated in FIG.  8 . 
     FIGS. 10 and 11 illustrate Rule (17) of the method according to the present invention. A long path message is received via step  1000 . For this example, the situation is such that it is determined that the message is the same or of higher priority than a local situation via step  1002 . As previously stated, a local situation is intended to include a situation which has been detected by the receiving node as well as a situation which was notified to the receiving node via a short path message. 
     It is then determined if the same message is coming from a neighboring node on a short path via step  1004 . If the same message is not coming from a neighboring node on a short path, it is determined whether the receiving node should unwrap any existing local wraps via step  1006 . If, however, the same message is coming from a neighboring node on a short path via step  1004 , then the receiving node does not unwrap any existing local wraps via step  1008 . 
     An example of the method illustrated in FIG. 10 can be seen in the block diagram of FIG.  11 . Assume that the receiving node is node  202   a ′. Problems  204   a ′ and  204   b ′ have been detected by node  202   b ′. Originally, the problems  204   a ′ and  204   b ′ were signal degrades. Accordingly, node  202   b ′ sends a short path message to its neighbor node  202   a ′ indicating that there is a signal degrade, as well as a long path message towards its other neighbor node  202   c ′ indicating that there is a signal degrade. Assume that the signal degrade turns into a signal fail, for example, a technician has disconnected the wiring. Node  202   b ′ will then send a short path message to its neighbor, node  202   a ′ indicating that a signal fail has occurred, while also sending a long path message to its other neighbor node  202   c ′ indicating that a signal fail has occurred. 
     Assume that the long path message indicating the signal fail arrives prior to the short path message indicating the signal fail. Examples of when this can occur can include when a node is down or comes back up or two separate ring networks are merged together. When node  202   a ′ receives the long path message with the higher priority (SF) from its neighbor node  202   b ′ then the short path message it received from its neighbor  202   b ′, then node  202   a ′ does not unwrap if it does not receive a corresponding short path message from its neighbor node  202   b ′. Accordingly, this long path message from a neighbor does not cause a wrap or an unwrap. 
     FIG. 12 is a flow diagram illustrating rules (20) and (21) of the method according to the embodiment of the present invention. A wrapped node determines that a problem has been cleared via step  1200 . It then enters a wait-to-restore state via step  1202 . It is then determined if its neighbor is the same neighbor as previously noted via step  1204 . The node can save the source of a short path message at the time of wrap initiation to note the identity of its neighbor. If the current neighbor is not the same as the previous neighbor via step  1204 , then an IDLE state is entered via step  1206 . If, however, the current neighbor is the same as the previous neighbor via step  1204 , then it is determined whether a pre-determined wait-to-restore time has expired via step  1208 . Once the pre-determined wait-to-restore time has expired, then the node enters an IDLE state via step  1206 . 
     A method and system for fault recovery for a two line bi-directional network has been disclosed. Software written according to the present invention may be stored in some form of computer-readable medium, such as memory or CD-ROM, or transmitted over a network, and executed by a processor. 
     Although the present invention has been described in accordance with the embodiment shown, one of ordinary skill in the art will readily recognize that there may be variations to the embodiment and that such variations are within the spirit and scope of the present invention. Accordingly, many modifications may be made by one of ordinary skill in the art without departing from the spirit and scope of the appended claims.