Patent Publication Number: US-2011075574-A1

Title: Path protection by sharing continuity check messages

Description:
RELATED APPLICATION/S 
     This application claims the benefit of priority under 35 USC 119(e) of U.S. Provisional Patent Application No. 61/246,591 filed Sep. 29, 2009, the contents of which are incorporated herein by reference in their entirety. 
    
    
     FIELD AND BACKGROUND OF THE INVENTION 
     The present invention, in some embodiments thereof, relates to a point-to-point communication system, and, more particularly, but not exclusively, to a communication network, and, more particularly, but not exclusively, to an Ethernet network. 
     A protected path is a term for a defined communication path between an ingress node I, also termed a head node, and an egress node E, also termed a tail node. The protected path may include one or more intermediate nodes along the path between node I and node E. The protected path includes an active path between node I and node E, over which communication occurs, and at least one passive backup path ready to take over communication if the active path fails. 
     For example, communication standard G.8031—Ethernet Linear Protection defines such a protected path, and a requirement that communication transfer from the active path to the backup path in less than a given time, for example in less than 50 mSec. 
     In order to verify that a path is working, a Continuity Check message (CC message—this is an Ethernet term) is sent every short while. Arrival of the CC message from node I to node E every short while, both through the active path and through the backup path, ensures that both paths are functioning. Node E replies to node I every short while that it received the CC message, signifying that communication along the path is working. 
     It is noted that when several protected paths are set up between the same ingress and egress nodes, each of the several protected paths uses its own CC message, protecting the same path. Because sending CC messages is unproductive overhead, there is a need for improvement and saving of the unproductive overhead. 
     Background art includes: 
     ITU-T G.8031/Y.1342, Ethernet Linear Protection Switching, published June 2006; and 
     ITU-T G.8031/Y.1342 Amendment 1, Ethernet Linear Protection Switching, published October 2007. 
     SUMMARY OF THE INVENTION 
     In some point-to-point communication systems, several protected paths are set up between the same two nodes, say node I and node E. The protected paths are optionally set up for different uses, and optionally have their CC messages sent at different frequencies, based on different requirements. It is noted that the backup paths are not necessarily the same even when the active paths are the same. 
     The present invention, in some embodiments thereof, uses (shares) only one set of CC messages for protected paths set up between the same two nodes. 
     The present invention, in some embodiments thereof, uses (shares) only one set of CC messages for protected paths, if the protected paths have at least one leg in common. If a plurality of protected paths are set up, say a first path between a node A 1  and a node B 1 , and a second path between node A 2  and a node B 2 , and the first path and the second path have a shared leg, say between node I and node E, the shared leg between node I and node E uses only one set of CC messages for protecting the shared paths. 
     Additionally, CC messages are sent every so often, for example every 3.33 mSec, 10 MSec, 100 mSec, and so on, depending on the time allowed for recovery from the active path to the backup path. Each protected path therefore may have a different requirement for a CC message frequency. In some embodiments of the invention, when paths share CC messages, the frequency of CC messages sent will be that of the highest frequency required by the sharing paths, in order to provide adequate protection for the highest protection path. This creates a win-win synergy—a saving of CC messages while at the same time providing a higher level of protection to the lower-requirement paths. 
     According to an aspect of some embodiments of the present invention there is provided a method for managing protection of a plurality of defined paths in a communication system including listing the defined paths, each item in the list including a defined path identifier, a defined path ingress node, a defined path egress node, and a defined path protection period, wherein some of the defined paths share an ingress node and an egress node, managing a sending of Continuity Check Messages (CCMs) along at least some of the defined paths, such that a CCM is sent every protection period, and for at least a plurality of the defined paths which share an ingress node and an egress node, managing a sharing of the protection of the plurality of the defined paths, the sharing including sending one set of CCMs, shared by the plurality of the defined paths, at a frequency based, at least in part, on a shortest one of the protection periods of the plurality of the defined paths. 
     According to some embodiments of the invention, when a message sent along one of the defined paths sharing protection can be detected as sent along one of the defined paths sharing protection, sending a CCM is postponed. 
     According to some embodiments of the invention, the sending a CCM is postponed by the shortest one of the protection periods. 
     According to some embodiments of the invention, the list is included in a database. 
     According to some embodiments of the invention, the method further includes adding managing protection of an additional defined path, the adding including adding the path identifier, the ingress node, the egress node, and the protection period to the list; if the additional defined path shares the ingress node and the egress node with an existing defined path in the list then if the additional defined path has a protection period equal to or longer than the shortest one of the protection periods, then sharing the one set of CCMs, with the shortest one of the protection periods, else changing the shortest one of the protection periods to be the protection period of the additional defined path, and sharing the one set of CCMs, with the protection period of the additional defined path; else managing the protection of the additional defined path by sending an additional set of CCMs at a frequency defined by the protection period of the additional defined path. 
     According to some embodiments of the invention, the method further includes removing protection of a defined path from the managing, the removing including removing the path identifier, the ingress node, the egress node, and the protection period of the defined path from the list. 
     According to some embodiments of the invention, the method further includes checking whether the defined path to be removed is protected by a shared set of CCMs with one or more existing defined paths, and if the defined path to be removed has a protection period shorter than the protection period used by the shared set of CCMs, then changing the protection period used by the shared set of CCMs to be equal to a shortest one of the protection periods of the one or more existing defined paths. 
     According to an aspect of some embodiments of the present invention there is provided a method for protecting a plurality of paths which share at least one leg in a communication network, including sharing, between the paths, Continuity Check Messages (CCMs) for at least the shared leg. 
     According to some embodiments of the invention, the method further includes instructing a communication node to break up a protected path into a plurality of protected sub-paths, and share protection of at least one sub-path. 
     According to some embodiments of the invention, the instructing is performed by a CCM sharing unit which detects shared sub-paths when such shared sub-paths exist among the protected paths. 
     According to some embodiments of the invention, the instructing is performed by a CCM sharing unit which detects shared sub-paths when adding managing protection of an additional defined path. 
     According to some embodiments of the invention, the method further includes a person breaking up a protected path into a plurality of protected sub-paths, and setting up shared protection of at least one sub-path. 
     According to an aspect of some embodiments of the present invention there is provided a node in a communication network with protected paths, including a Continuity Check Message sharing unit. 
     According to some embodiments of the invention, the Continuity Check Message sharing unit includes the methods described above. 
     According to an aspect of some embodiments of the present invention there is provided a Continuity Check Message sharing unit for managing Continuity Check Message sharing for more than one communication node in a communication network. 
     According to an aspect of some embodiments of the present invention there is provided software for implementing the methods described above. 
     According to an aspect of some embodiments of the present invention there is provided a communication network implementing the methods described above. 
     According to an aspect of some embodiments of the present invention there is provided a computer-readable storage medium containing a set of instructions for a device for implementing the methods described above. 
     Unless otherwise defined, all technical and/or scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the invention pertains. Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of embodiments of the invention, exemplary methods and/or materials are described below. In case of conflict, the patent specification, including definitions, will control. In addition, the materials, methods, and examples are illustrative only and are not intended to be necessarily limiting. 
     Implementation of the method and/or system of embodiments of the invention can involve performing or completing selected tasks manually, automatically, or a combination thereof. Moreover, according to actual instrumentation and equipment of embodiments of the method and/or system of the invention, several selected tasks could be implemented by hardware, by software or by firmware or by a combination thereof using an operating system. 
     For example, hardware for performing selected tasks according to embodiments of the invention could be implemented as a chip or a circuit. As software, selected tasks according to embodiments of the invention could be implemented as a plurality of software instructions being executed by a computer using any suitable operating system. In an exemplary embodiment of the invention, one or more tasks according to exemplary embodiments of method and/or system as described herein are performed by a data processor, such as a computing platform for executing a plurality of instructions. Optionally, the data processor includes a volatile memory for storing instructions and/or data and/or a non-volatile storage, for example, a magnetic hard-disk and/or removable media, for storing instructions and/or data. Optionally, a network connection is provided as well. A display and/or a user input device such as a keyboard or mouse are optionally provided as well. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Some embodiments of the invention are herein described, by way of example only, with reference to the accompanying drawings. With specific reference now to the drawings in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of embodiments of the invention. In this regard, the description taken with the drawings makes apparent to those skilled in the art how embodiments of the invention may be practiced. 
       In the drawings: 
         FIG. 1  is a simplified block diagram of a prior art method of protecting a plurality of protected paths between two nodes in a communication network; 
         FIG. 2A  is a simplified block diagram of a method of protecting a plurality of protected paths between two nodes in a communication network according to an example embodiment of the invention; 
         FIG. 2B  is a simplified block diagram of a method of protecting a plurality of protected paths between two nodes in a communication network according to a first alternative example embodiment of the invention; 
         FIG. 2C  is a simplified block diagram of a method of protecting a plurality of protected paths between two nodes in a communication network according to a second alternative example embodiment of the invention; 
         FIG. 3  is a simplified flow chart for a communication node to add a protected path in the example embodiment of  FIG. 2A ; 
         FIG. 4  is a simplified flow chart for a communication node to remove a protected path in the example embodiment of  FIG. 2A ; 
         FIG. 5  is a simplified block diagram illustration of a communication network using the method of the example embodiment of  FIG. 2A ; 
         FIGS. 6A and 6B  are a simplified block diagram illustrations of example protected paths in a communication network, depicting how using alternative example embodiments of the present invention saves overhead in sending Communication Continuity Messages; and 
         FIG. 7  is a simplified block diagram illustration of an example application of the example embodiment of  FIG. 2A . 
     
    
    
     DESCRIPTION OF SPECIFIC EMBODIMENTS OF THE INVENTION 
     The present invention, in some embodiments thereof, relates to a point-to-point communication system, and, more particularly, but not exclusively, to a communication network, and, more particularly, but not exclusively, to an Ethernet network. 
     A protected path is a term for a defined communication path between an ingress node I, also termed a head node, and an egress node E, also termed a tail node. The protected path may include one or more intermediate nodes along the path between node I and node E. The protected path includes an active path between node I and node E, over which communication occurs, and at least one passive backup path ready to take over communication if the active path fails. 
     For example, communication standard G.8031—Ethernet Linear Protection defines such a protected path, and a requirement that communication transfer from the active path to the backup path in less than a given time, for example in less than 50 mSec. 
     In order to verify that a path is working, a Continuity Check message (CC message—this is an Ethernet term) is sent every short while. Arrival of the CC message from node I to node E every short while, both through the active path and through the backup path, ensures that both paths are functioning. Node E replies to node I every short while that it received the CC message, signifying that communication along the path is working. 
     Before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not necessarily limited in its application to the details of construction and the arrangement of the components and/or methods set forth in the following description and/or illustrated in the drawings. The invention is capable of other embodiments or of being practiced or carried out in various ways. 
     For purposes of better understanding some embodiments of the present invention, as illustrated in  FIGS. 2-7  of the drawings, reference is first made to  FIG. 1 , which is a simplified block diagram of a prior art method of protecting a plurality of protected paths between two nodes in a communication network. 
       FIG. 1  depicts a first communication node I  105  and a second communication node E  110 . The first communication node I  105  and the second communication node E  110  are connected by several protected communication paths. 
     For purpose of simplification, only protected communication paths between the first communication node I  105  and the second communication node E  110  are depicted in  FIG. 1 , although: other unprotected communication paths between the first communication node I  105  and the second communication node E  110  may exist; and other protected communication paths between other communication nodes, not necessarily including protection starting at the first communication node I  105  and protection ending at the second communication node E  110 , may exist. 
     A first protected path  115  is depicted, including Continuity Check Messages (CCMs)  120  sent along the first protected path  115 , by way of a non-limiting example, every F 1  milliseconds. 
     A second protected path  125  is depicted, including CCMs  121  sent along the second protected path  125 , by way of a non-limiting example, every F 2  milliseconds. 
     It is noted that for purpose of the present invention, two or more protected paths are described, between the same nodes I and E. Prior art is therefore also described with reference to two or more protected paths. An Nth protected path  130  is depicted, including CCMs  122  sent along the Nth protected path  130 , by way of a non-limiting example, every F N  milliseconds. 
     It is noted that the CCMs  120   121   122  depicted in  FIG. 1  are intended to depict that the frequency F 1  of sending CCMs  120  along the first protected path  115  is higher than the frequency F 2  of sending CCMs  121  along the second protected path  125 , and similarly for the Nth protected path  130 . A message sent along a communication path may take less than a time between sending CCMs, in which case only one CCM may be traveling down each protected path at any time.  FIG. 1  is intended to depict the idea of relatively different frequencies of CCM for different protected paths in a qualitative rather than qualitatively exact fashion. Additional figures in the present application will use the same qualitative convention. 
     CC messages are non-productive overhead, used for assuring that a communication path is working. 
     When setting up several defined paths to be protected, between a pair of nodes I and E, or a pair of nodes A and B, each protected path incurs its own overhead, which includes communication bandwidth of the CC messages back and forth. 
     One option which may be used to lower the non-productive overhead is to package the communication of the several defined paths into frames, and send the frames using only one protected path. 
     Another option is to have node A and node B aware whenever there are parallel defined paths to be protected, and use (share) only one set of CC messages for all the parallel protected paths. This saves: (a) the overhead of packing into frames (b) the overhead of more CC messages than needed. 
     Reference is now made to  FIG. 2A , which is a simplified block diagram of a method of protecting a plurality of protected paths between two nodes in a communication network according to an example embodiment of the invention. 
       FIG. 2A  depicts a first communication node I  205  and a second communication node E  210 . The first communication node I  205  and the second communication node E  210  are connected by several communication paths. 
       FIG. 2A  follows a convention of  FIG. 1  of depicting only protected communication paths between the first communication node I  205  and the second communication node E  210 . 
     A first protected path  215  is depicted, including Continuity Check Messages (CCMs)  220  sent along the first protected path  215 , by way of a non-limiting example, every F 1  milliseconds. 
     A second protected path  225  is depicted, and an Nth protected path  230  is also depicted. 
     The second protected path  225  and the Nth protected path  130  are also protected. The second protected path  225  and the Nth protected path  130  do not have CCMs sent, and the CCMs  220  of the first protected path  215  are used to protect the second protected path  225  and the Nth protected path  130 . 
     Since CC messages are sent every so often, by way of a non-limiting example every 3.33 mSec, or every 10 mSec, or every 100 mSec, and so on, depending on a time allowed for recovery from the active path to the backup path, each of the protected paths may require a different CC message frequency. 
     It is noted that when paths share CC messages, the frequency of CC messages sent is optionally that of the highest frequency required by the sharing paths, in order to provide adequate protection for the highest protection path. A win-win synergy is thus produced—a saving of CC messages while at the same time providing a higher level of protection to lower-requirement paths. 
     By way of a non-limiting example, when two protected paths, each of which sends a CCM once every 25 milliseconds, share CCMs, then instead of 2×40=80 CCMs being sent every second, there can be just 1×40=40 CCMs sent each second, for a saving of 50% on overhead for protecting the paths. The saving is greater when more paths share the CCMs. 
     By way of another non-limiting example, when three protected paths, one of which sends a CCM once every 25 milliseconds, and two of which sends a CCM once every 33 milliseconds, which is a lower frequency of protection, share CCMs, then instead of 1×40+2×30=100 CCMs being sent every second, there can be just 1×40=40 CCMs sent each second, for a saving of 60% on overhead for protecting the paths. 
     In contrast with example embodiments of the invention, communication node I  105  of the prior art communication system of  FIG. 1  does not need to manage the sharing of CCMs between the protected paths, because they are not shared. 
     In example embodiments of the invention, the communication node I  205  of  FIG. 2A  automatically manages a sharing of CCMs among protected paths. In the example embodiments the communication node I  205  comprises a CCM sharing management unit  250 . The CCM sharing management unit  250  optionally keeps a list of protected paths, including, for each protected path, at least the following data: 
     
       
         
           
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Data for each protected path 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
            
               
                   
                 path identifier 
               
               
                   
                 ingress node 
               
               
                   
                 egress node 
               
               
                   
                 protection period 
               
               
                   
                   
               
            
           
         
       
     
     It is noted that the above-mentioned list may be kept as a list in volatile memory, such as in RAM, and/or kept in non-volatile memory such as kept on a hard disk or flash memory. The list may be kept as a list, and/or may be kept in a database. 
     It is noted that the node E  210  of  FIG. 2A  may or may not be aware that the node I  205  is managing a sharing of the protected paths. 
     In some embodiments of the invention the node E  210  is not aware that protected paths are sharing CCMs, but simply returns CCMs every time the node E  210  receives a CCM from the node I  205 . In such embodiments, the node E  210  does not comprise a CCM sharing management unit  250 . 
     In some embodiments of the invention the node E  210  sends CCMs to the node I  205  every protection period, and substantially performs the management function of the node I  205 , sharing CCMs among protected paths, as described above with reference to  FIG. 2A  and with reference to communication node I  205 . In such embodiments, the node E  210  does comprise a CCM sharing management unit (not shown in  FIG. 2A ). 
     In example embodiments of the invention, a technician setting up one or more protected paths between the first communication node I  205  and the second communication node E  210  sets up a sharing of CCMs between the paths. The technician optionally selects a highest frequency of sending the CCMs, to provide suitable protection to the highest requirement. 
     The technician optionally knows when setting up a new protected path whether the new protected path shares input and output nodes with an existing protected path, whether the new protected path may share CCMs with the existing protected path, and at what frequency to send the CCMs. 
     The technician optionally knows when removing an existing protected path whether the existing protected path shares CCMs with another, remaining, existing protected path, and at what frequency to send the CCMs for the remaining protected path. 
     In some embodiments of the invention, the protected path conforms to communication standard G.8031—Ethernet Linear Protection. 
     In some embodiments of the invention, the protected path conforms to communication standard G.8032—Ethernet Ring Protection. 
     In some embodiments of the invention, the first communication node I  205  monitors transmission from a first output port (not shown) of the first communication node I  205 . If the monitoring shows that transmission through the first output port has stopped, also termed “the port falls”, also termed “loss of carrier”, the first communication node I  205  optionally immediately switches to transmitting through a second output port, and/or over a backup path (not shown). The first communication node I  205  optionally does not wait for not receiving a CCM for a full protection period. 
     It is noted that embodiments of the invention are applicable to nodes which are multi-port switches in a communication network. 
     It is noted that detecting failure of a protected path by two methods by monitoring output and by monitoring CCMs, detects various failure modes, some common to both methods of detection, and some detected by just one method. By way of a non-limiting example, when a port “falls”, both methods detect the failure. By way of another non-limiting example, when a path between two nodes fails, monitoring the output port may not detect failure, while monitoring CCMs will detect failure. 
     When a protected path fails, whether detection of failure is by monitoring CCMs or by monitoring output through an output port, the first communication node I  205  optionally transfers transmission to be through a backup path. 
     Reference is now made to  FIG. 2B , which is a simplified block diagram of a method of protecting a plurality of protected paths between two nodes in a communication network according to a first alternative example embodiment of the invention. 
       FIG. 2B  depicts a first communication node I  255  and a second communication node E  256 , connected by several communication paths  215   225   230 . The first communication node I  255  comprises a CCM sharing management unit  257 , and the second communication node E  256  comprises a CCM sharing management unit  258 .  FIG. 2B  follows a convention of  FIG. 1  of depicting only protected communication paths between the first communication node I  255  and the second communication node E  256 . 
     A first protected path  215  is depicted, including Continuity Check Messages (CCMs)  220  and other messages  260 , sent along the first protected path  215 . 
     In order to protect the first protected path  215 , node E  256  should, by way of a non-limiting example, receive CCMs  220  every F 1  milliseconds, and reply to the CCMs  220 . In fact, node E  256  can optionally send reply CCMs to node I  255  based on receiving either a CCM  220  or some other message  260 , as long as node E  256  knows that the other message  260  passed through the protected path  215 . In some cases, node E  256  can tell if the message  260  passed through the protected path  215 . Some such example cases are: 
     1) If the protected path  215  is on a point-to-point connection, any communications arriving at node E  256  over the point-to-point connection may optionally imply that the protected path  215  is also functioning. If the point-to-point connection includes more than one port at each point, any communication arriving at node E  256  at a port to which the protected path  215  is connected may also imply that the protected path  215  is also functioning. 
     2) If the protected path  215  is not on a point-to-point connection The CCM sharing management unit  258  in node E  256  optionally detects those messages which come from node I  255 . One example method for such detection is: detecting messages  260  which include a source address, such as an IP address, and in which node I  255  is a source address. The CCM sharing management unit  258  optionally reads source addresses of communication packets, and optionally identifies some messages  260  as coming from node I  255 . 
     Another example method for such detection is optionally by contents of messages  260 . In some cases contents of a message identify where a messages is coming from, and/or through what nodes a message has passed. The CCM sharing management unit  258  optionally scans through contents of messages, and messages containing such identifying content are detected as coming from node I  255 . 
     In light of fact that the CCM sharing management unit  258  is able to detect incoming communications from node I  255  MORE OFTEN than just CCMs sent from node I  255 , the CCM sharing management unit  257  of node I  255  may send CCMs to node E  256  LESS OFTEN. 
     When the CCM sharing management unit  257  of node I  255  detects that node I  255  sends a message  260  over the protected path  215  to node E  256  which will be detected by the CCM sharing management unit  258  of node E  256  as coming through the protected path  215 , the CCM sharing management unit  257  may optionally postpone sending a CCM over the protected path  215 . 
     The CCM sharing management unit  257  optionally waits to send another CCM over the protected path  215  up until a full protection period has passed since sending either a CCM or a message detectable by the CCM sharing management unit  258  of node E  256  as coming over the protected path  215 . 
     In some embodiments of the invention, some of the transmission bandwidth transmitted over a communication network is for Operation, Administration, and Maintenance (OAM). 
     In some embodiments of the invention OAM transmissions over the protected path  215  function similarly to the messages  260  which pass over the protected path  215 , serving to indicate that the protected path  215  is functioning, as are other protected paths which share the protection. 
     One non-limiting example of an OAM message is a CC message. 
     Persons skilled in the art will appreciate that another non-limiting example of an OAM message is an Automatic Protection Switching message (APS message). An APS message serves for forcing a switch between a primary protected path and a backup path. The APS message may be transferred over the backup path, since in many cases the switching is caused by a failure in the primary protected path. 
     Reference is now made to  FIG. 2C , which is a simplified block diagram of a method of protecting a plurality of protected paths between two nodes in a communication network according to a second alternative example embodiment of the invention. 
       FIG. 2C  depicts a first communication node I  265  and a second communication node E  266 , connected by several communication paths  220   225   230 . The first communication node I  265  comprises a CCM sharing management unit  267 , and the second communication node E  266  comprises a CCM sharing management unit  268 .  FIG. 2B  follows a convention of  FIG. 1  of depicting only protected communication paths between the first communication node I  265  and the second communication node E  266 . 
     A first protected path  215  is depicted, including Continuity Check Messages (CCMs)  220  and other messages  260 , sent along the first protected path  215 . 
     According to the method depicted in  FIG. 2B , in order to protect the first protected path  215 , node E  256  should, by way of a non-limiting example, receive CCMs  220  and/or messages  260  which the CCM sharing management unit  168  can identify as coming on the first protected path  215 , every F 1  milliseconds. 
     In fact, messages  260  coming along other paths sharing the protection of the first path  215 , such as a message  260  coming along a shared protection path  225 , also indicate that a path between the first communication node I  265  and the second communication node E  266  is working. 
     The CCM sharing management unit  268  optionally uses any of the methods described above with reference to  FIG. 2B  to detect messages  260  coming along a shared protection path, such as the shared protection path  225 , and/or even a non-protected but shared path  230 . Messages coming along a path which shares the physical medium which is used for the first protected path  215  indicate that the shared path is operational. 
     In light of fact that the CCM sharing management unit  268  is able to detect incoming communications from node I  255  MORE OFTEN than just CCMs and messages sent over the first protected path  215 , the CCM sharing management unit  267  of node I  265  may send CCMs to node E  266  LESS OFTEN. 
     When the CCM sharing management unit  267  of node I  265  detects that node I  265  sends a message  260  over one of the paths sharing communication with the protected path  215 , and that the message  260  will be detected by the CCM sharing management unit  268  of node E  256  as coming through the shared communication path, the CCM sharing management unit  267  may optionally refrain from sending a CCM over the protected path  215 , as described above with reference to  FIG. 2B . 
     Reference is now made to  FIG. 3 , which is a simplified flow chart for a communication node to add a protected path in the example embodiment of  FIG. 2A . 
     When adding a protected path, the communication node I  205  adds data defining the new protected path to a list of protected paths which the communication node I  205  manages ( 300 ). 
     The communication node I  205  also checks whether the new protected path shares both an ingress node and an egress node with an existing protected path ( 305 ). 
     If the new protected path does not share both an ingress node and an egress node with an existing protected path, then the communication node I  205  sets up a new protected path using the ingress node and the egress node of the new protected path, and sends CCMs along the protected path according to a protection period required for the new protected path ( 310 ). 
     If the new protected path shares an ingress node and an egress node with one or more existing protected paths, then the communication node I  205  checks whether a protection period required for the new protected path is greater than or equal to an existing protection period used for the existing protected paths ( 315 ). 
     If the protection period required for the new protected path is greater or equal to an existing protection period used for the existing protected paths, the communication node I  205  sets up a new protected path using the ingress node and the egress node of the new protected path, and does not need to send additional CCMs, but rather shares the existing CCMs ( 320 ). 
     If the protection period required for the new protected path is less than the existing protection period used for the existing protected path, the communication node I  205  sets a new protection period equal to the protection period required for the new protected path ( 325 ), and sets up the new protected path using the ingress node and the egress node of the new protected path, and sharing the CCMs with the new protection period ( 320 ). 
     In some embodiments of the invention, all it takes to remove a protected path from protection is for the communication node I  205  to remove the path identifier, the ingress node, the egress node, and the protection period which refer to the removed path from the path protection list. If one or more additional paths are still sharing CCMs along the path, the CCMs continue being sent at an existing protection period. If no more protected paths remain to share CCMs, no more CCMs are sent along the path. 
     The above method makes management of shared CCMs simple, and provides protection as good as or better than required. 
     In some embodiments of the invention, protection period of remaining protected paths is adjusted according to need. 
     Reference is now made to  FIG. 4 , which is a simplified flow chart for a communication node to remove a protected path in the example embodiment of  FIG. 2A . 
     As described above with reference to removing a protected path from protection, the communication node I  205  removes the path identifier, the ingress node, the egress node, and the protection period which refer to the removed path, from path protection list ( 400 ). 
     The communication node I  205  checks whether remaining paths share an ingress node and an egress node with the removed path ( 405 ). 
     If no remaining protected paths exist from the communication node I  205  to the communication node E  210 , the communication node I  205  stops sending CCMs to the communication node E  210  ( 410 ). 
     The communication node I  205  checks whether a protection period of any remaining path is equal to the protection period of the removed path ( 415 ). 
     If there is a remaining path with a protection period equal to the protection period of the removed path, there is no need to change protection period for the CCMs, and the communication node I  205  continues to send CCMs to the communication node E  210  at the previous period, for protecting the remaining protected paths ( 420 ). 
     If the protection period of no remaining protected path is equal to the existing protection period, there is no need to continue at such a period, and the communication node I  205  sets a new protection period equal to the shortest protection period required for the remaining protected paths ( 425 ), and continues to send CCMs to the communication node E  210  at the new period, for protecting the remaining protected paths ( 420 ). 
     It is noted that the communication between an ingress node and an egress node need not necessarily be direct, with no intervening nodes. 
     Reference is now made to  FIG. 5 , which is a simplified block diagram illustration of a communication network using the method of the example embodiment of  FIG. 2A . 
     A communication route  505  exists between an ingress communication node I  205  and an egress communication node E  210 . One or more protected paths are established on the communication route  505 . 
       FIG. 5  emphasizes, by depicting additional communication nodes on the route  505 , such as example nodes A  515  and B  520 , that the route  505  is not necessarily a single-hop route. 
       FIG. 5  additionally emphasizes, by depicting additional paths  510 , that any one of the communication nodes I  205 , E  210 , A  515 , and B  520  may optionally be connected to other communication nodes, which are not shown. In fact, in order to optionally provide protection to paths on the route  505 , the ingress communication node I  205  and the egress communication node E  210  are connected by at least one route serving as a backup to the route  505 , and the backup route is not depicted in  FIG. 5 . 
     In some embodiments of the invention, when several protected paths share a common leg between two nodes, one or more of the several protected paths are broken into shorter sub-paths, or legs, including the common leg. The common leg optionally shares CCMs between the several protected paths. 
     Reference is now made to  FIGS. 6A and 6B , which are a simplified block diagram illustrations of example protected paths in a communication network, depicting how using alternative example embodiments of the present invention saves overhead in sending Communication Continuity Messages. 
     By way of a non-limiting example, three communication nodes exist in a communication network: a first communication node A  605 , a second communication node B  610 , and a third communication node C  615 . 
       FIG. 6A  depicts how, in absence of the alternative embodiment of the present invention, path protection would take place. A first protected path  620  would be established between the first communication node A  605  and the third communication node C  615 , and CCMs would be sent between the first communication node A  605  and the third communication node C  615 . A second protected path  625  would be established between the second communication node B  610  and the third communication node C  615 , and CCMs would be sent between the second communication node B  610  and the third communication node C  615 . 
     It is noted that two sets of CCMs are being sent on the communication route between the second communication node B  610  and the third communication node C  615 . There is no need to send two sets of CCMs when one set is enough. 
     In some example embodiments of the invention a technician setting up the protected paths between node A  605 , node B  610 , and node C  615 , recognizes that there is a shared section between node B  610  and node C  615 , and sets up shared protection for protected paths between node B  610  and node C  615 . 
       FIG. 6B  depicts how, in alternative example embodiments of the present invention, path protection takes place. 
     In some example embodiments of the invention a central CCM sharing management unit  640  determines that a saving is possible in CCMs sent on the leg between the second communication node B  610  and the third communication node C  615 . 
     A non-limiting example of a method by which the central CCM sharing management unit  640  determines whether a saving is possible, that is, whether path protection may be shared, is by reviewing data describing protected paths. An example of such data is described in Table 1, and such data is optionally stored in the central CCM sharing management unit  640 . The central CCM sharing management unit  640  optionally automatically detects that the protected path between node A  605  and node C  615  passes through node B  610 . 
     In some embodiments of the invention some or all of the protected paths are explicitly described in a table, such as Table 1, using all of the nodes through which they pass, and the central CCM sharing management unit  640  detects when a shared section occurs. 
     In some embodiments of the invention some or all of the protected paths are only implicitly described in a table, such as Table 1, using only some of the nodes through which they pass, such as only an ingress node and an egress node. The central CCM sharing management unit  640  optionally detects when a shared section occurs based on having at least a partial topological map of nodes and connections between the nodes. The CCM sharing management unit  640  optionally uses the topological map of the nodes and connections to detect when a path shares a leg with another path. 
     The central CCM sharing management unit  640  communicates with  635  and instructs a CCM sharing unit  650  in the first communication node A  605  to set up a protected path  630  between the first communication node A  605  and the second communication node B  610 , and to be prepared for rerouting to a backup communication route when either the protected path  630  fails or the protected path  625  fails. 
     The central CCM sharing management unit communicates with  635  and instructs a CCM sharing unit  655  in the second communication node B  610  to notify  660  the CCM sharing unit  650  in the first communication node A  605  if the protected path  625  fails. 
     An example application of the invention is now described. Reference is now made to  FIG. 7 , which is a simplified block diagram illustration of an example application of the example embodiment of  FIG. 2A . 
     A wireless communication route  505  is established between antennas  705 , connecting an ingress communication node I  205  and an egress communication node E  210 . 
     A backup route is established between the ingress communication node I  205 , an intermediate communication node B  710 , and the egress communication node E  210 . The backup route includes, by way of a non-limiting example, a wireless route  715  established between an antenna  705  in the ingress communication node I  205  and an antenna  705  in the intermediate communication node B  710 , and a wired communication route  720  between the intermediate communication node B  710  and the egress communication node E  210 . 
     A plurality of protected paths are set up on the wireless communication route  505 , using the above-mentioned backup route. The wireless communication route  505  may deteriorate, at some time, for any of a number of reasons, at which point the protected paths&#39; traffic is rerouted to the backup route, which may comprise one or more wireless sections, and/or one or more wired sections. 
     It is expected that during the life of a patent maturing from this application many relevant communication routes, protected paths, and continuity check messages will be developed and the scope of the terms “communication routes”, “protected paths”, and “continuity check messages”, is intended to include all such new technologies a priori. 
     The terms “comprising”, “including”, “having” and their conjugates mean “including but not limited to”. 
     As used herein, the singular form “a”, “an” and “the” include plural references unless the context clearly dictates otherwise. For example, the term “a unit” or “at least one unit” may include a plurality of units, including combinations thereof. 
     The words “exemplary” and “example” are used herein to mean “serving as an example, instance or illustration”. Any embodiment described as “exemplary” or as an “example” is not necessarily to be construed as preferred or advantageous over other embodiments and/or to exclude the incorporation of features from other embodiments. 
     The word “optionally” is used herein to mean “is provided in some embodiments and not provided in other embodiments”. Any particular embodiment of the invention may include a plurality of “optional” features unless such features conflict. 
     It is appreciated that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention, which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub-combination or as suitable in any other described embodiment of the invention. Certain features described in the context of various embodiments are not to be considered essential features of those embodiments, unless the embodiment is inoperative without those elements. 
     Although the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the spirit and broad scope of the appended claims. 
     All publications, patents and patent applications mentioned in this specification are herein incorporated in their entirety by reference into the specification, to the same extent as if each individual publication, patent or patent application was specifically and individually indicated to be incorporated herein by reference. In addition, citation or identification of any reference in this application shall not be construed as an admission that such reference is available as prior art to the present invention. To the extent that section headings are used, they should not be construed as necessarily limiting.