Abstract:
A scalable protection method for connection oriented networks includes a source, a destination, and primary nodes interconnected by working path segments between the source and destination. A point-to-multipoint connection with multiple destinations and additional primary nodes is also supported. Each of a number of backup nodes is interconnected with an associated primary node by pre-provisioned shunt segments, and the backup nodes are interconnected among themselves by pre-provisioned protection path segments. Upon occurrence of a failure, the primary node upstream of the failure directs input traffic to a shunt segment, and the backup node associated with the upstream primary node directs traffic from the shunt segment to an output protection segment. Downstream of the failure, a backup node directs traffic from an input protection segment to a shunt segment, and the primary node associated with the backup node directs the traffic from the shunt segment to a downstream working segment. In this manner, traffic is routed around failures via local switching operations.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
         [0001]    None  
         STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
         [0002]    Not Applicable  
         BACKGROUND OF THE INVENTION  
         [0003]    The present invention is related to the field of protection switching in data communications networks.  
           [0004]    Many data communications networks employ some form of protection switching to provide better availability of communications services to customers than can be provided by unprotected networks. Generally, protection switching involves the detection of failures within the network, the communication of the failure information to nodes that are affected by a detected failure, and the switching of traffic from one path or connection to another path or connection at the affected nodes as dictated by a predetermined protection switching scheme.  
           [0005]    In so-called connection-oriented networks, which employ pre-established virtual and/or physical connections for carrying user data traffic, one class of protection switching schemes is known as “line-based” protection switching. In contrast to source-based schemes, in which traffic is re-routed at its source upon occurrence of a failure and may take a completely different path to its destination, line-based schemes involve more local or hop-by-hop protection switching decisions. Thus, if an end-to-end connection includes a number of intermediate nodes and connection segments and line-based protection switching is utilized, one or more of the intermediate nodes respond to a failure by taking local actions to re-route the traffic around the failure, without necessarily involving either the source or destination in the protection action. Line-based protection switching can reduce the disruption that can be caused by failures, and under some circumstances may be faster and more efficient than source-based protection switching.  
           [0006]    Common examples of both source-based and line-based protection schemes are found in Synchronous Optical Network (SONET) networks. A SONET ring can employ unidirectional path switched ring (UPSR) protection switching or bidirectional line-switched ring (BLSR) protection switching. In UPSR protection switching, information about a failure must be propagated to the destination node, which responds by switching to accept data already flowing on a protect path separate from the working path. However, the destination node may be many hops away from the failure, potentially resulting in a long switchover delay and concomitant loss of data. In BLSR protection switching, the failure information must be propagated to every node on the ring to enable affected traffic to be re-routed in the opposite direction from source to destination, and then each node must perform the necessary switching. In either case, protection switching may be undesirably slow and/or inefficient. Additionally, these techniques suffer relatively poor scalability due to their reliance on relatively wide-area communication of failures and initiation of protection switching actions.  
           [0007]    A protection switching technique having improved speed, efficiency and scalability is desirable.  
         BRIEF SUMMARY OF THE INVENTION  
         [0008]    In accordance with the present invention, a scalable protection method for connection oriented networks is disclosed in which protection switching actions are generally taken locally based on pre-established protection connection segments, resulting in improved speed and efficiency in protection switching operations.  
           [0009]    The disclosed protected network includes a source and destination, and primary nodes interconnected by working path segments between the source and destination. A number of backup nodes are interconnected by pre-provisioned protection path segments between the source and the destination, and each backup node is also interconnected with an associated primary node by a bidirectional set of pre-provisioned shunt segments.  
           [0010]    Each primary node, under normal working circumstances, directs input traffic from an upstream working path segment to a downstream working path segment. Upon occurrence of a failure on the upstream working path segment, a primary node directs input traffic from an input shunt segment to the downstream working path segment, and upon occurrence of a failure on the downstream working path segment, directs input traffic from the upstream working path segment to an output shunt segment.  
           [0011]    Each backup node, upon occurrence of a failure on the downstream working path segment of the associated primary node, directs input traffic from an input shunt segment to a downstream protection path segment, and upon occurrence of a failure on the upstream working path segment of the associated primary node, directs input traffic from an upstream protection path segment to an output shunt segment. Additionally if the associated primary node itself fails, the backup node directs input traffic from an upstream protection path segment to a downstream protection path segment.  
           [0012]    As a result of these combined operations of the primary and backup nodes when failures occur, new paths are created including shunt segments and protection segments that bypass the failures. Because the shunt and protection segments are pre-provisioned, protection switching is performed rapidly and efficiently. Additionally, the technique is scalable. As primary nodes are added in a network, additional backup nodes and shunt and protection segments can be added in an incremental fashion. Protection switching can be performed in a relatively small neighborhood of a failure, rather than requiring larger-scale communication and switching responses as in present protection architectures.  
           [0013]    Other aspects, features, and advantages of the present invention will be apparent from the detailed description that follows. 
       
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING  
       [0014]    The invention will be more fully understood by reference to the following Detailed Description of the invention in conjunction with the Drawing, of which:  
         [0015]    [0015]FIG. 1 is a block diagram of a network incorporating line based protection in accordance with the present invention;  
         [0016]    [0016]FIG. 2 is a block diagram of switching circuitry in a primary node in the network of FIG. 1;  
         [0017]    [0017]FIG. 3 is a block diagram of switching circuitry in a backup node in the network of FIG. 1;  
         [0018]    [0018]FIG. 4 is a block diagram of the network of FIG. 1 in the presence of a failure on a working network segment;  
         [0019]    [0019]FIG. 5 is a block diagram of the network of FIG. 1 in the presence of a failure of a primary node;  
         [0020]    [0020]FIG. 6 is a block diagram of a network incorporating line based protection of a point-to-multipoint connection in accordance with the present invention; and  
         [0021]    [0021]FIG. 7 is a block diagram of the network of FIG. 6 in the presence of a failure on a working network segment. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0022]    [0022]FIG. 1 shows a portion of an exemplary network for enabling protected unidirectional communication between a source node (S)  10  and a destination node (D)  12 . The primary or working communication path includes a number of intermediate nodes designated “primary” nodes (P)  14  interconnected by working segments (WS)  16 . Also shown are a set of backup nodes (B)  18 , each being associated with a corresponding one of the primary nodes  14 . The backup nodes  18  are interconnected between the source node  10  and the destination node  12  by a number of protection segments (PS)  20 . Also, each backup node  18  is interconnected with the associated primary node  14  by a corresponding pair of shunt segments (SS)  22 . Each pair  22  includes a first shunt segment  21  for carrying traffic from a primary node  14  to the associated backup node  18 , and a second shunt segment  23  for carrying traffic from a backup node  18  to the associated primary node  14 .  
         [0023]    The various segments  16 ,  20  and  22  are established at the time of connection setup, in advance of carrying any user data traffic from the source node  10  to the destination node  12 . The path consisting of the working segments  16  through the primary nodes  14  is a unidirectional path for carrying working traffic from the source node  10  to the destination node  12 , and the protection segments  20  are designated to carry protection traffic in the same direction. It will be appreciated that the nodes  10  and  12  may exchange data traffic in the other direction as well (i.e. from node  12  to node  10 ), for which a separate set of working and protection segments (not shown) must be established. In general, the segments utilized for traffic in the other direction may flow through a different set of primary nodes, although in practice it is generally advantageous for traffic in both directions to traverse the same set of nodes. Also, a given shunt segment  22  may serve to protect traffic flowing in both directions. In one embodiment, the segments  16 ,  20 , and  22  can be realized as label-switched paths (LSPs) as known in the Multiprotocol Label Switching (MPLS) architecture. They may also be realized as virtual connections (VCs) such as defined in the Asynchronous Transfer Mode (ATM) architecture, or similar pre-established connections.  
         [0024]    [0024]FIG. 2 shows circuitry used for protection switching within the primary nodes  14 . First selection circuit  24  selects the source for traffic sent from the primary node  14  on its downstream or output working segment  16 , shown as “WS-OUT”, and second selection circuit  26  selects the source for traffic sent from the primary node  14  on its output shunt segment  21 , shown as “SS-OUT”. The inputs to the first selection circuit  24  are (1) the upstream or input working segment  16  (“WS-IN”), (2) the input shunt segment  23  (“SS-IN”) and (3) “no connection”  28  (shown as “nc”) When no connection  28  is selected, the output working segment  16  is not being utilized to carry traffic. This case corresponds to the presence of a failure downstream of the primary node  14 , as explained below.  
         [0025]    The inputs to the second selection circuit  26  are (1) “no connection”  30  and (2) the upstream or input working segment  16  (“WS-IN”). When no connection  30  is selected, the output shunt segment  21  is not being utilized to carry traffic. This case corresponds to the normal working condition, as explained below.  
         [0026]    [0026]FIG. 3 shows circuitry used for protection switching within the backup nodes  18 . First selection circuit  32  selects the source for traffic sent from the backup node  18  on its downstream or output protection segment  20 , shown as “PS-OUT”, and second selection circuit  34  selects the source for traffic sent from the backup node  18  on its output shunt segment  23 , shown as “SS-OUT”. The inputs to the first selection circuit  32  are (1) “no connection”  36 , (2) the input shunt segment  21  (“SS-IN”), and (3) the input protection segment  20  (“PS-IN”). When no connection  36  is selected, the output protection segment  20  is not being utilized to carry traffic. This case corresponds to the normal working condition, as explained below. The inputs to the second selection circuit  34  are (1) “no connection”  38  and (2) the upstream or input protection segment  20  (“PS-IN”). When no connection  38  is selected, the output shunt segment  23  is not being utilized to carry traffic. This case corresponds to the normal working condition, as explained below.  
         [0027]    The circuitry of FIGS. 2 and 3 is used for protection switching when necessitated by failures within the network. In general, a given backup node  18  and associated protection segments  20  and shunt segments  22  are utilized to route traffic around a failure at or near the primary node  14  with which the given backup node  18  is associated. Specific examples of such failures are given below. From the perspective of a given primary node  14 , failures can be categorized as having occurred at the primary node  14  itself, “upstream” of the primary node  14 , i.e. toward the source node  10 , or “downstream” of the primary node  14 , i.e., toward the destination node  12 . A failure of a primary node  14  itself is considered to be downstream of a working upstream primary node  14 , and upstream of a working downstream primary node  14 . This specific scenario is also described below.  
         [0028]    Tables  1  and  2  summarize the operation of the switch circuits  24 ,  26 ,  32  and  34  at a primary node  14  and associated backup node  18  based on the existence of and relative location of a failure. The contents of these tables are explained below.  
                                 TABLE 1                           Switch Circuits at Primary Node                Case   WS-OUT   SS-OUT                       Working   WS-IN   nc           Failure-upstream   SS-IN   nc           Failure-downstream   nc   WS-IN                      
 
         [0029]    [0029]                                 TABLE 2                           Switch Circuits at Backup Node                Case   PS-OUT   SS-OUT                       Working   nc   nc           Failure-upstream   nc   PS-IN           Failure-downstream   SS-IN   nc           Failure-primary node   PS-IN   nc                        
         [0030]    Tables 1 and 2 are explained as follows. At a primary node  14 , in the absence of a failure, the traffic from WS-IN is passed along to WS-OUT, and no traffic is sent on SS-OUT, because protection is not active due to the absence of a failure. When a failure occurs upstream of the primary node  14 , traffic is still sent on WS-OUT, but the source is the associated backup node  18  via SS-IN. When a failure occurs downstream of the primary node  14 , traffic is still received from WS-IN, but is sent to the associated backup node  18  via SS-OUT rather than being forwarded along the working path via WS-OUT. Recall that from the perspective of a given primary node  14 , the failure of another primary node  14  is either an upstream or downstream failure, depending on its relative location.  
         [0031]    At a backup node  18  (Table 2), in the absence of a failure, no traffic is sent on either PS-OUT or SS-OUT. This is an idle or standby condition. When a failure occurs upstream of the associated primary node  14 , the backup node  18  accepts traffic from PS-IN and directs it to the associated primary node  14  via SS-OUT. When a failure occurs downstream of the associated primary node  14 , the backup node  18  accepts traffic from SS-IN and directs it along the protection path via PS-OUT. When the primary node  14  associated with the backup node  18  fails, then traffic is accepted from PS-IN and directed along the protection path via PS-OUT.  
         [0032]    [0032]FIG. 4 depicts the operation of the network in the presence of a failure on the working segment  16 - 2  extending between two primary nodes  14 - 1  and  14 - 2 . At the primary node  14 - 1 , the traffic is directed from the input working segment  16 - 1  toward the backup node  18 - 1  along the shunt segment  21 - 1 . The backup node  18 - 1  accepts the traffic from the shunt segment  21 - 1  and directs the traffic toward the downstream backup node  18 - 2  along the protection segment  20 - 2 . From the perspective of the backup node  18 - 2  and the primary node  14 - 2 , the failure is an “upstream” failure. Therefore, the backup node  18 - 2  directs traffic from the protection segment  20 - 2  toward the primary node  14 - 2  via the shunt segment  23 - 2 , and the primary node  14 - 2  accepts the traffic from the shunt segment  23 - 2  and directs it to primary node  14 - 3  via the working segment  16 - 3 .  
         [0033]    [0033]FIG. 5 shows operation when a primary node such as primary node  14 - 2  fails. In this case, operation of nodes  14 - 1  and  18 - 1  is the same as for the situation of FIG. 4, and nodes  14 - 3  and  18 - 3  operate in the same fashion as do nodes  14 - 2  and  18 - 2  in the situation of FIG. 4. Additionally, backup node  18 - 2  forwards traffic from its input protection segment  20 - 2  toward the downstream backup node  18 - 3  via output protection segment  20 - 3 . As a result, traffic is routed around failed primary node  14 - 2 .  
         [0034]    The preceding description has focused on point-to-point connections having one source node  10  and one destination node  12 . The disclosed protection technique can also be utilized in connection with point-to-multipoint connections having a single source and multiple destinations.  
         [0035]    [0035]FIG. 6 shows an example of a point-to-multipoint connection on which the source node  10  sends data to two different destinations  12 A and  12 B. In this simple two-destination connection, the primary node  14 - 2  is responsible for replicating the traffic on two output working segments  16 - 3 A and  16 - 3 B, and likewise the backup node  18 - 2  is responsible for replicating the traffic on two output protection segments  20 - 3 A and  20 - 3 B. The nodes  14 - 2  and  18 - 2  are referred to herein as a “branching primary node” and “branching backup node” respectively. The nodes  14 - 2  and  18 - 2  operate as shown in FIGS. 2 and 3 with respect to both the traffic stream for destination  16 A and the traffic stream for destination  16 B. Generally, it is preferred that the protection switching for these different streams be carried out independently, so that for example a failure of primary node  14 - 3 B would result in protection switching occurring for the traffic for destination  12 B but no protection switching occurring for the traffic for destination  12 A.  
         [0036]    [0036]FIG. 7 shows the existence of a failure on the working segment  16 - 3 A of the “A” branch of the point-to-multipoint connection. In this case, the traffic destined for destination node  12 A is directed along shunt segment  21 - 2  to backup node  18 - 2 , then along protection segment  20 - 3 A to backup node  18 - 3 A, and then along shunt segment  23 - 3 A to primary node  14 - 3 A, which forwards the traffic to destination node  12 A along working segment  14 - 4 . The traffic destined for destination node  12 B is not affected by this failure, and continues to flow along working segments  16 - 3 B and  16 - 4 B.  
         [0037]    While in the illustrated embodiments, there is a different backup node  18  associated with each primary node  14 , in alternative embodiments a node may serve as a backup node  18  for two or more primary nodes  14 , as long as the necessary working segments, protection segments, and shunt segments can be established. It is generally preferred for reliability reasons that a primary node be directly connected to its associated backup node, although it is not strictly required. By “directly connected”, it is meant that there are no intervening nodes that terminate network segments such as LSPs. A lower-level device such as an electrical repeater or hub would generally not qualify as an intervening node. As already mentioned, there may be additional nodes within one or more of the protection segments  20  that do not participate in the protection operation as a backup node  18 . Additionally, it is possible that such additional nodes are also included within the working segments  16 , although such configurations are preferably avoided. Generally, it is preferred that each node along the working path from source  10  to destination  12  be protected.  
         [0038]    It will be apparent to those skilled in the art that modifications to and variations of the disclosed methods and apparatus are possible without departing from the inventive concepts disclosed herein, and therefore the invention should not be viewed as limited except to the full scope and spirit of the appended claims.