Abstract:
In a multiple element network, a method and network element for providing a constraint based routing system to accommodate non-transitive exceptions. The method and network element receive a connection request for an end to end connection. The network element can select routing information having a pair of adjacent links associated with the multiple elements of the network, where each link of the pair of links has a shared network resource. The shared network resource is compared to a database of exception sets, the exception sets including at least one non-transitive exception predetermined from a plurality of network resources. The network element confirms whether the shared network resource of the selected pair of links is contained within the exception database. A router designs a pathway over the network for the end to end connection using the results of the shared network resource comparison, wherein inclusion in the pathway is inhibited for the shared network resource matching the predetermined exclusion set contained in the database. Accordingly, the list of exception sets is accessible by a constraint based routing algorithm as input to exclude the selected link pair of a pathway for the end to end connection over the network, if the shared network resource of the link pair is contained within the exception sets.

Description:
FIELD OF THE INVENTION 
     The present invention relates to multi-hop distributed networks and, in particular, to constraint based routing between network elements. 
     DESCRIPTION OF THE PRIOR ART 
     Distributed topologies of todays modern networks are applied in a number of different areas, such as optical telecommunication networks, power grids, and routing networks for vehicular traffic. These distributed topologies are commonly referred to as multi-hop networks, in which transport through the network of a package/message, or packet, is done in a series of hops over a sequence of interconnected network nodes. The network resources are typically shared between a number of competing packets, according to established network protocols. 
     As todays networks evolve, new capacities and new routes are added to the existing networks. For example, telecommunication networks continually experience new requirements for transmission from increased traffic volume growth and new types of services having different bandwidth. Quality of service and routing constraints can also change network dynamics. There are a number of well known routing algorithms, such as the Dijkstra Shortest Path and the OSPF algorithm, which are currently applied to networks for configuring a pathway between endpoints of a requested connection. These routing algorithms take into account variables such as cost, distance, and available network element resources, to design an optimum pathway for the connection request using selected network nodes. 
     For example, with telecommunication networks, there exists a class of routing and signalling problems generically known as Constraint Based Routing/Signalling. These telecommunication networks typically consist of a collection of network elements, links between those elements with resource limits, and a control processor (or more) per network element. The constraint based routing/signalling algorithm solves the problem of computing a route for a requested connection through such an arbitrary telecommunication “mesh” network. This route is implemented on a distributed database of network elements associated with the controller at the head end of the connection in question. Normally, Dijkstra&#39;s shortest path algorithm is applied to the set of links in the telecommunication network that have sufficient resources to support the connection. Once the pathway is computed by network element and link selection, the pathway is given to a signalling engine for establishment by taking network resources from the selected links the engine traverses. Once the pathway is established, the network resources, as they are taken (or freed), cause a distributed set of routers adjacent to the resources to trigger periodic floods through the network that inform the set of routers of any resource change. This effectively creates a feed back system, which is used by the network to help in continuously setting up and taking down of connections as required, and making available the updated network resources to all network elements in the network. 
     Typical implementations of constraint based routing/signalling algorithms for telecommunication networks consist of MPLS/GMPLS/PNNI/PORS and numerous other systems. For example, in MPLS and PNNI the network resources are statistically multiplexed bandwidth, which are tracked and flooded by network links. In GMPLS, physical resources are tracked, such as timeslots, wavelengths, fibre ports, or other non-sharable resources. The traditional constraint based routing/signalling algorithms assume that there is no direct relationship of network resources between various respective links in the network. In particular, if resources are available for a requested connection on link A, and also on link B, and link A and B are adjacent, it is assumed that by transitivity that a connection may traverse link A and B sequentially. However, there exist classes of networks where this is not true due to non-transitive constraints, where resource availability between adjacent network elements is discontinuous. 
     A first class of telecommunication network which exhibits non-transitive allocation restrictions for network resources is a pure photonic network. The analog nature of photonic transport causes non-linear behaviours to accumulate as the pathway length/distance grows. For example, a signal can be sent and received unambigously over link A of some distance D 1 , as well as sent and received unambiguously over link B for some distance D 2 . However, this does not mean the signal can be sent over links A and B, over distance D 1 +D 2 , and still be received unambigously. It is understood there are numerous other attributes of photonic transport besides distance that will gang up to create these non-transitive problems, such as but not limited to time slot availability, laser intensity, wavelength, and dispersion characteristics. 
     A second class of telecommunications network which exhibits non-transitive allocation restrictions is the classic ring network. Examples of which (but not limited to) are BLSR SONET rings, which can be thought of as a subclass of mesh networks. However, implementation of traditional constraint based routing is problematic with ring network segments because segment to segment constraints (such as lack of timeslot interchange) are not, and cannot, easily be reflected in network topology since the constraints grow O(n 2 ) as the intended routing extends along the ring segment. One disadvantage with ring network routing is that the requested connection is usually restricted to using the same TDM time slot on all hops around that ring. In other words, if resource r is used on link A, resource r must also be used on link B (if link A and B are on the same ring). This creates a non-transtitive resource relationship between links on common rings, and hence presents difficult challenges to solve efficiently by normal constraint based routing and signalling algorithms. 
     It is an object of the present invention to provide a constraint based routing system to obviate or mitigate some of the above-presented disadvantages. 
     SUMMARY OF THE INVENTION 
     There exist classes of networks where, due to non-transitive constraints, resource availability between adjacent network elements is discontinuous. According to the present invention there is provided in a multiple element network, a method for providing a constraint based routing system to accommodate non-transitive exceptions. The method comprises the steps of: receiving a connection request for an end to end connection; selecting routing information having a pair of adjacent links associated with the multiple elements of the network, each link of the pair of links having a shared network resource; comparing the shared network resource to a database of exception sets, the exception sets including at least one non-transitive exception predetermined from a plurality of network resources; confirming whether the shared network resource of the selected pair of links is contained within the database; and designing a pathway over the network for the end to end connection using the results of the shared network resource comparison; wherein inclusion in the pathway is inhibited for the shared network resource matching the predetermined exclusion set contained in the database. 
     According to a further aspect of the present invention there is provided in a multiple element network, a network element for providing a constraint based routing system to accommodate non-transitive exceptions. The network element comprises: a first link for connecting the network element to a path layer of the network, the path layer including a plurality of additional network elements with associated network links, the network links having at least a pair of adjacent links having a shared network resource; a topology database including a list of exception sets including a plurality of non-transitive exceptions predetermined from a plurality of network resources of the network links, the topology database accessible by the network element; and a compiler accessible by the network element for assembling exception set data to store in the topology database; wherein the list of exception sets is accessible by a constraint based routing algorithm as input to exclude the selected link pair of a pathway for an end to end connection over the network, if the shared network resource of the link pair is contained within the exception sets. 
     According to a still further aspect of the present invention there is provided in a multiple element network a computer program product for providing a constraint based routing system to accommodate non-transitive exceptions. The product comprises: computer readable medium; a first link module stored on the computer readable medium for connecting the network element to a path layer of the network, the path layer including a plurality of additional network elements with associated network links, the network links having at least a pair of adjacent links having a shared network resource; topology database module stored on the computer readable medium including a list of exception sets including a plurality of non-transitive exceptions predetermined from a plurality of network resources of the network links, the topology database module accessible by the network element; and a compiler module coupled to the topology module and accessible by the network element for assembling exception set data to store in the topology database; wherein the list of exception sets is accessible by a constraint based routing algorithm as input to exclude the selected link pair of a pathway for an end to end connection over the network, if the shared network resource of the link pair is contained within the exception sets. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features of the preferred embodiments of the invention will become more apparent in the following detailed description in which reference is made to the appended drawings wherein: 
         FIG. 1  is a diagram of a data communication network; 
         FIG. 2  is a sub-network of the network of  FIG. 1 ; 
         FIG. 3  shows a connection set-up for the sub-network of  FIG. 2 ; 
         FIG. 4  shows a topology database for the sub-network of  FIG. 3 ; 
         FIG. 5  is an alternative embodiment of the sub-network of  FIG. 3 ; 
         FIG. 6  is an flowchart for the operation of the routing algorithm; and 
         FIG. 7  is an alternative embodiment of the sub-network of  FIG. 5 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to  FIG. 1 , one example of a distributed network is a global telecommunication network  10 , which contains a series of sub-networks An, Bn, Cn, Dn, En interconnected by bulk data transmission mediums  12 . These mediums  12  can consist of such as but not limited to optical fibre, wireless, and copper lines, which can be collectively referred to as a Backbone Network. Each sub-network An, Bn, Cn, Dn, En contains a plurality of network elements  14  interconnected by conduits or links  16 , which can be collectively referred to as a path layer  17  (see  FIG. 2 ). These links  16  can consist of fibre optic cables, DSL (Digital Subscriber Loop), cable, and wireless mediums, wherein each link  16  can be capable of providing the transmission of multiple wavelengths  18  as required by the telecommunication network  10 . The telecommunication network  10  can be referred to as a “multi-hop network”, in which transport of the majority of data packets  20  from one network element  14  to another takes multiple “hops” (i.e. node to node transmission), such that the data packets  20  traverse the network  10  across multiple adjacent links  16 . The traffic routing capabilities of the telecommunication network  10  can be used by a variety of different carriers, such as ILECs, CLECs, ISPs, and other large enterprises to monitor and transmit a diverse mixture of data packets  20  in various formats. These formats can include voice, video, and data content transferred over individual SONET, SDH, IP, WDN, ATM, and Ethernet networks associated with the telecommunication network  10 . It is recognised that traffic routing can also be used on network types other than the telecommunication network  10  shown, such as but not limited to power grids, and vehicular traffic routing. 
     Referring to  FIG. 2 , operation of each network element  14  can be monitored by a central integrated management or Operations Support System (OSS)  22 , which for example co-ordinates a plurality of connection requests  24  received from clients  26  connected to the sub-network En. Alternatively, these connection requests  24  can also be communicated directly to any of a series of corresponding Optical Connection Controllers (OCCs)  28 , which make up a control layer  15  of the network  10 . The network  10  also contains a distributed series of routers  34  (a particular type of network element  14 ), which are responsible for allocating the available network  10  resources for each connection request  24 . Traditionally, the OCCs  28  communicate endpoints (head and tail) of the connection requests  24  to the routers  34 , which then design working W and protection P pathways (see  FIG. 3 ) for the connection request  24 , using selected routing algorithms. This pathway information is then given to a signalling engine  36  of the head end of the established connections, such as to network element  1  of connection A-B and network element  6  of connection D-C in  FIG. 3 . The signalling engine  36  then constructs the designed pathways. It is also considered that the OCCs  28  and the OSS  22  could also interact with the routers  34  and signalling engine  36 , if desired. 
     Referring again to  FIG. 2 , the OSS  24  can include a processor  25 . The processor  25  is coupled to a display  27  and to user input devices  23 , such as a keyboard, mouse, or other suitable devices. If the display  27  is touch sensitive, then the display  27  itself can be employed as the user input device  23 . A computer readable storage medium  21  is coupled to the processor  25  for providing instructions to the processor  25  to instruct and/or configure the various OCCs  28 , routers  34 , and other corresponding coupled network elements  14 , to perform steps related to the operation of a constraint based routing algorithm for implementing the connection requests  24  over the subnetwork En. The computer readable medium  21  can include hardware and/or software such as, by way of example only, magnetic disks, magnetic tape, optically readable medium such as CD ROM&#39;s, and semi-conductor memory such as PCMCIA cards. In each case, the medium  21  may take the form of a portable item such as a small disk, floppy diskette, cassette, or it may take the form of a relatively large or immobile item such as hard disk drive, solid state memory card, or RAM provided in the OSS  22 . It should be noted that the above listed example mediums  21  can be used either alone or in combination. Accordingly, the constraint based routing algorithm, as further defined below, can be implemented on the sub-network En in regard to the co-ordination of the plurality of connection requirements  24  submitted by the clients  26 , as well as monitoring the timely transmission of the associated data packets  20 . 
     The clients  26  or other peripheral devices can include such as but not limited to hubs, leased lines, IP, ATM, TDM, PBX, and Framed Relay PVC, which can be connected to the sub-network En by routers  34 . The OCCs  28  are coupled to each network element  14  by link  31 , which transmits connection and data request  30  to each of their corresponding network elements  14 . The association of OCCs  28  can be referred to as the control layer  15 , with each OCC  28  coupled together by links  32 . The OCCs  28  of the control layer  15  have complete information of their corresponding network element  14  interconnections and status identified by the routers  34 . 
     Referring to  FIG. 3 , there is depicted a simplified shared mesh network En structure for clarity purposes only. The shared path protection set-up of the sub-network E consists of a series of network elements  14  indicated as  1 ,  2 ,  3 ,  4 ,  5 ,  6 ,  7 ,  8  with a corresponding number of OCC&#39;s  28  indicated as OCC  1 , OCC  2 , OCC  3 , OCC  4 , OCC  5 , OCC  6 , OCC  7 , and OCC  8 . The elements  1 ,  2 ,  3 ,  4 ,  5 ,  6 ,  7 ,  8  are interconnected by the links  16 , logical and/or physical, with solid line paths A-B and C-D denoting working W paths and the dotted line paths  1 - 3 - 4 - 2 ,  5 - 3 - 4 - 6 , and  1 - 7 - 8 - 2  between the elements  1 ,  2 ,  3 ,  4 ,  5 ,  6 ,  7 ,  8  denoting potential protection P paths. The term “working” refers to the routes and equipment involved in carrying STS-1 frames on the sub-network En during the normal mode of operation, and the term “protection” refers to the routes and equipment involved in carrying the STS-1 frames on the sub-network En during a failure mode of operation. 
     Referring to  FIG. 3 , the routers  34  are responsible for routing the information packets  20  between the clients  26  through sub-network En. For example, the constraint based routing algorithm such as Open Shortest Path First (OSPF), can be used by the routers  34  to define the working W and protection P pathways. The operation of the OSPF algorithm depends on link-state databases  38  that are maintained by each router  34  in the sub-network En for calculating the shortest available pathways between the endpoints of each connection A-B, C-D. Such link-state databases  38  are continually updated by having each router  34  originate one or more Link State Advertisements (LSA) for use as link-state database  38  entries containing information about each link  16  connected to the router  34 . The LSA is a block of data produced by each router  34  specifying such as but not limited to the state of each link  16  attached to the router  34 , as well as link and router identifiers and details for shortest path calculations. All routers  34  in the sub-network En collect the LSAs, check them, and use the link-state data from valid LSAs to build identical link-state databases  38 . Accordingly, if one of the routers  34  fails, the remaining routers  34  can then route the failed router&#39;s  34  packets  20  using their updated link-state databases  38  and coordinate any further routing resulting from the failed router&#39;s  34  connection requests  24 . 
     For example, referring to  FIG. 3 , the link-state databases  38  of every router  34  can be updated every 30 minutes by each of the routers  34  independently flooding the sub-network En with LSAs, whereby every half hour each router  34  produces a fresh LSA. It is noted that each router  34  sends over adjacent links  16  the LSA it prepared as well as any LSAs received from the area network that are attached to the router  34 . LSAs may also be sent on per-need basis, for example, when a new link  16  or a router  34  is connected to the sub-network En. Further, new LSAs can also be sent by the routers  34  when the previous LSAs become too old, that is, as LSAs get routed their age is increased by an aging algorithm. 
     Using the link-state database  38  information, the constraint based routing algorithm of the routers  34  designs the pathways W,P for the connection request  24 . Once determined, routing information  40  on the designed pathways W,P is communicated by the router  34  to the signaling engine  36 , of the head end of the connection A-B, C-D, that will be responsible for implementing the connection request  24 . For example, referring to  FIG. 3 , client  26   a  has made the connection request  24  to communicate data packets  20  to client  26   b . The router  34   a  receives details of the connection request  24  and proceeds to design the connection A-B (typically both the working W and protection P pathways) from the network resources of the network elements  1 , 7 , 8 , 2  (for the working W path), network elements  1 , 3 , 4 , 2  (for the protection P path), and their adjacent links  16 . The pathway routing information  40  is then given to the signaling engine  36  of the network element  1 , which then establishes the designed pathways W,P by allocating the network resources from the selected links  16  and network elements  14  the signalling engine  36  traverses. Once the pathways W,P are established, the data packets  20  are communicated between the clients  26   a,b.    
     Referring to  FIGS. 3 and 4 , each head end network element  1  can have access to a topology database  42 , which contains any predetermined exception sets  44 . The exception sets  44  can consist of network resource lists, such as but not limited to path-fragment, size of STS, and priority that cannot be accommodated when accessed by the signaling engine  36 . Other network resources for inclusion in the exception sets could be statistically multiplexed****** bandwidth, timeslots, wavelengths, fibre ports, and other shared physical and logical resources. The exception sets  44  are a result of the construction and feedback of link sequences, which exhibit non-transitive behaviours for resource requests made by the signalling engine  36 . For example, the exception sets  44  are revealed to the network element  1  when the signalling engine  36  attempts to set-up the connection A-B based on the routing information  40 , but is unable to complete the set-up. This routing signal failure can be due to the underlying resource management modules, of the associated network elements  14  in the chosen pathways W,P, which detect an incompatibility between the preceding network element  14 /link  16  resource allocation and the next available resource in sequence along the designed pathway W,P. 
     When this network resource incompatibility is detected, the network element  14  concerned creates the exception set  44 , which consists of a set of hops which will not work for the routing information  40  as specified. The exception set  44  is then transported back to the head end network element  1  through feedback signals  46  by adding to, such as but not limited to a reverse flowing failure message, an exception LSA, a tear message, flooding, or any other means sufficient to communicate the incompatibility. The head end network element  1  maintains in the topology database  42  through a compiler  45  the exception sets  44 , which are avoided in subsequent pathway W,P design and set-up. Accordingly, any subsequent routing information  40  generated by the routers  34  excludes any of the excepted sequences in the exception sets  44 , at the indicated priority, for STS sizes greater than or equal to what is being excluded. It is noted that the head end network element  1  of the requested connection A-B will accumulate these exception sets  44  through the compiler  45  in the topology database  42 , however, the associated OCC 1  and/or the OSS  22  can also have access to and maintain the topology databases  42 , if desired. It is recognised that the router  34  and/or the OCCs  28  and the OSS  22  could have access to the compiler  45 , if desired. 
     Some implementations of the exception set  44  content in the topology databases  42  may choose to hold the exception sets  44  for short periods of time. Others may choose to remove the exception sets  44  when floods are received by the head end network element  1 , which indicate newly available links  16 /network elements  14  contained in the original exception sets  44 . For example, if the head end network element  1  receives new LSAs noting an increase in network resources (such as but not limited to bandwidth), any exception sets  44  containing path fragments associated with the increased network resources are removed. This can facilitate the potential that any change in network resources will now allow use of at least a portion of the now freed-up exception set  44 . Should this not be the case, subsequent exception sets  44  will be encountered by the signalling engine  36  and written into the topology database  42 . 
     This removal of exception sets  44  can also be done when notification of a reallocation of network resources is received by the network element  1 , such that the reallocation does not result in an increase in the network resources (e.g. total available bandwidth remains constant but available channels has changed). For example, when time slot # 1  becomes available but time slot # 4  is taken, which can be a neutral network resource change with respect to LSAs but should be considered for exception set  44  removal. Further, exception sets  44  can also be collapsed. For example, if there is an initial STS-1 exception on a specified path fragment and then a subsequent STS-3 exception occurs, the STS-1 exception can be collapsed into the STS-3 by modifying the exception sets  44  in the topology database  42 . 
     Predefined removal/amendment criteria can be set-up by a network administrator or the OSS  22  to note the conditions for periodic removal and/or amendment of the exception sets  44 , such as but not limited to aging of exception sets  44 . In any event, any exception sets  44  prematurely removed may be reencountered by the signalling engine  36  and rewritten back into the topology database  42 . Accordingly, the predefined criteria can be optimised for various network  10  types and traffic conditions. 
     Accordingly, the constraint based routing algorithm used by the routers  34 , such as but not limited to OSPF, MOSPF, and Dijkstra, is modified to take into account the exception set  44  data contained in any topology databases  42  pertaining to the connection request  24 . For example, when the modified constraint based routing algorithm of the router  34  accesses its link-state database  38  information, the algorithm also has access (by a direct or indirect link  50 ) to the exclusion sets  44  stored in the topology database  42  associated with the head end network element  1  of the connection A-B in question. Therefore, the algorithm is able to recognise when it is about to use one of the network elements  14  that has an exception set  44  associated with it, and then not consider this now excepted network element  14  if the other members of the same exception set  44  are currently in the pathway W,P designed thus far. In the event that initial set-up by the signalling engine  36  of the designed pathway W,P fails, the head end network element  1  first includes the newly determined exception set  44  in its topology database  42  before requesting for a new route selection  48  from the router  34 . Therefore, the modified constraint based routing algorithm operates to transform the sub-network En topology in such a way as to inhibit the routing information  40  containing a sequence or subsequence of hops that matches any of the exception sets  44  learned thus far and stored in the topology database  42 . It is further recognised that the router  34 , OSS  22 , and OCCn may also have the ability to modify the contents of the topology databases  42 . 
     Referring to  FIGS. 5 and 6 , an example operation of the modified constraint based routing algorithm is applied to a ring type sub-network  52 , which has an entry point, network element D, and an exit point, network element K. Accordingly, all other network elements E,F,I,J are considered pass through nodes for any requested connections between network elements D and K around the sub-network  52 . Initially, the router  34  receives the connection request  24  from client  26   a  and proceeds to compute  100  the connection  26   a - 26   b , with the modified constraint based routing algorithm, using a path fragment {D,E,F,K} on the ring network  52 . The router  34  checks  102  the topology database  42  (directly, or indirectly through requesting the network element D) for any exception sets  44  pertaining to the path fragment. If exceptions sets  44  are present, then the router  34  accounts  104  for them in the designed pathway and delivers  106  the routing information  40  to the entry point, network element D. Otherwise, the router  34  delivers the routing information  40  to the network element D without noting any exceptions. 
     For explanation purposes, it is assumed that no exception sets  44  were noted by the router  34 . Accordingly, the signalling engine  36  of the network element D then sends  108  a set-up message  54  with a hop list containing the path fragment {D,E,F,K} towards network element K, the exit point, with such as but not limited to timeslot STS-1 with priority=4. In the event the network resources (in this case STS-1 and priority=4) are available  110  to complete the connection request  24 , the signalling engine  36  receives confirmation of the established  112  connection D-K and the network resources allocated thereto as is known in the art. For example, the set-up message arrived at network element D and an available timeslot set (STS-1, pri=4) was added to the set-up message  54 . Subsequent arrival of the set-up message at the next hops, network elements E, F, and K, resulted in successful intersection of the timeslot set with subsequently available timeslots (i.e {STS-1, pri=4} is ok for path D-E-F-K) and an updated timeslot set for further propagation. Transmission of data packets  20  can then proceed  132  over the established connection D-K, as shown in  FIG. 5 . 
     However, referring to  FIGS. 6 and 7 , a non-transitive exception  60  is encountered at step  110 . For example, when the timeslot set collected up to network element F is propagated by the set-up message  54  to network element K, the timeslot set intersection produces an empty set (i.e. {D,STS-1,pri=4} {E,STS-1,pri=4} {F,STS-1,pri=4} {K,STS-1,pri=4}=Ø). The empty set Ø indicates the first encountered portion of the exception set  44  (i.e. unknown, K, STS-1, pri=4). Accordingly, the feedback signal  46  (such as a tear down message) builds  114  the exception set  44  as a feedback vector, which is eventually propagated  116  back to the ring network  52  entry point, network element D. During building of the feedback vector, the path fragment component E,F, the exit point K, the entry point D, and the type/priority are collected in the feedback message  46  eventually as the new exception set  44  {D,E,F,K, STS-1,pri=4}. 
     Accordingly, the head end network element D now records  118  through the compiler  45  the exception set  44  in the topology database  42  as part of the failed set-up. The head end network element D then requests  120  a new route selection  48  from the router  34  starting back at step  100 . For example, a new route computation starts again with network element D, but now does not include  102  the {STS-1,pri=4} condition in the network element K direction, as present in the topology database  42 . Therefore, as the considered path set collected by the router  34  grows, it will still include D even though D has an exception associated with it, but only if K is not yet part of the set of network elements  14  with D required to complete the pathway on the {STS-1,pri=4} designation. In situations where the pathway computation by the router  34  is about to consider K, the router  34  notes in the exception set  44  of K in combination with the other network elements  14 , and therefore excludes  104  all pathways satisfying the exception set  44 . Therefore, the modified constraint based routing algorithm, noting non-transitive exceptions, of the router  34  could consider network elements  14  adjacent to F, E, or D, which could lead to the alternative D,I,J,K route information  40  delivered  106  to the head end network element D. The rest of the connection set-up procedure would continue from step  108  as described above. It is recognised a time out  122  procedure could be used, in the event that the requested connection cannot be completed. At this stage, the OCCs  28  of the control layer  15  (see  FIG. 2 ) could be consulted  124  for further instructions. 
     Further, as the network  10  is operated  132 , the head end network element D is continually polling  126  for, or receiving, updates to the network resources of the ring network  52 . In the event parameters of a resource change  128  satisfies the predefined criteria, any affected exception sets  44  are removed  130  from the topology database  42 . For instance, in the above ring network  52  example, an LSA received by the head end network element D could indicate growth or change in the status of timeslots on the network element E,F path fragment, which could result in the removal of all exception sets  44  from the topology database  42  having the (E,F) designation in their exclusion sub-path. This removal would allow rerouting over the D,E,F,K path fragment for all timeslots. It is also noted also that the topology database  42  can be populated with exception sets  44  by testing the result of the path computations against network simulators, rather than actually setting up and testing the path or sub paths by the signalling engine  36 . It is also recognised that the topology database  42  could be updated to reflect the complete failure of certain network elements  14  and links  16 , or other such failures simultaneously affecting large blocks of network resources. These network failures could be communicated by the LSAs or any other failure indication method for use by the head end network element D. Accordingly, the head end network element  1  and/or the associated router  34  acknowledges whether any further connection requests are received  134  for set-up of the connection starting at step  100 , as described above. 
     It is recognised that a unit of allocation for the non-transitive allocation restrictions can be represented by a generalised label, such as but not limited to a GMPLS label that covers time/space and frequency switching. The use of labels can be applied to such as but not limited to the communication of transport routing information  40 , set-up message  54 , and feedback signals  46 . It is recognised that the label can include implicit values defined by a particular link  16  medium that is being provisioned, for example a wavelength for a DWDM system or a timeslot for a SONET device. This enables label switched paths (LSPs) of the network  10  to pass through different types of label switch routers (LSRs)  34 , for example SONET ADMs and Optical Cross Connects (OXCs). 
     Basic attributes of labels are as follows; forwarding information (label) is separate from a content of an IP header, the ability of using a single forwarding paradigm (label swapping), multiple routing paradigms, multiple link-specific realizations of the label swapping forwarding paradigm: “shim,” virtual connection/path identifier (VCI/VPI), frequency slot (wavelength) and time slot, and the flexibility to form forwarding equivalence classes (FECs). The generalized label contains information to allow the receiving device to program its switch and forward data regardless of its construction (packet, TDM, lambda, etc.). A generalized label can represent a single wavelength, a single fiber, or a single time-slot. Traditional MPLS labels—e.g., ATM, VCC, or IP shim—are also included. The information that is embedded in a generalized label includes the following: LSP encoding type that indicates what type of label is being carried (e.g., packet, lambda, SONET, etc.); switching type that indicates whether the node is capable of switching packets, time-slot, wavelength, or fiber; a general payload identifier to indicates what payload is being carried by the LSP (e.g., virtual tributary [VT], DS-3, ATM, Ethernet, etc.). 
     In operation of the network  10  using labels for the transport routing information  40 , set-up message  54 , and feedback signals  46 , separation of forwarding information from the content of the IP header allows use with devices such as OXCs, whose data plane cannot recognize the IP header. Label switch routers (LSRs)  34  forward data using the label carried by the data. This label, combined with the port on which the data was received, is used to determine the output port and outgoing label for the data. For example, a wavelength could be viewed as an implicit label. The concept of a forwarding hierarchy via label stacking enables interaction with devices that can support only a small label space. 
     Although the invention has been described with reference to certain specific embodiments, various modifications thereof will be apparent to those skilled in the art without departing from the spirit and scope of the invention as outlined in the claims appended hereto.