WO2013120512A1 - Method and apparatus for routing in a multi-area communication network - Google Patents

Method and apparatus for routing in a multi-area communication network Download PDF

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Publication number
WO2013120512A1
WO2013120512A1 PCT/EP2012/052491 EP2012052491W WO2013120512A1 WO 2013120512 A1 WO2013120512 A1 WO 2013120512A1 EP 2012052491 W EP2012052491 W EP 2012052491W WO 2013120512 A1 WO2013120512 A1 WO 2013120512A1
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Prior art keywords
network
area
node
nodes
routing
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PCT/EP2012/052491
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French (fr)
Inventor
Francesco Lazzeri
Giovanni Fiaschi
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Telefonaktiebolaget L M Ericsson (Publ)
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Priority to PCT/EP2012/052491 priority Critical patent/WO2013120512A1/en
Publication of WO2013120512A1 publication Critical patent/WO2013120512A1/en

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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L45/00—Routing or path finding of packets in data switching networks
    • H04L45/02—Topology update or discovery
    • H04L45/04—Interdomain routing, e.g. hierarchical routing

Definitions

  • the present invention relates to a method and apparatus for routing in a multi-area communication network.
  • Routing is a central function in communication networks, and its established practice has been broadly studied.
  • the problem of routing in a communication network is generally solved by representing the network as a graph G(V, E), composed of V nodes (vertices) and E links (edges), and running a shortest path algorithm to compute the route that satisfies a connectivity request whilst seizing minimum resources.
  • Dijkstra's algorithm is currently one of the most efficient shortest path algorithms and its complexity in its basic form is 0(V 2 ).
  • the algorithm is based on link costs for the basic computation, but generally requires modifications for specific purposes. For example it may include variable costs to improve resource usage, diversity information to compute protected paths, optical parameters for wavelength routing, or special multiplexing schemes for Time Division Multiplexing (TDM) networks. This further increases the algorithm complexity.
  • OSPF Open Shortest Path First
  • ISIS Intermediate System to Intermediate System
  • Connection-oriented networks define hierarchical routing, where a sub-network or area is fully known by the switches in it, and is presented as a summarized network to a higher routing level.
  • This approach is used, for example, in Asynchronous Transfer Mode Private Network-to-Network Interface (ATM PNNI) routing, in Optical Interworking Forum External Network-to-Network Interface (OIF E-NNI) routing and described more theoretically in International Telecommunication Union - Telecommunication Standardization Sector (ITU- T) recommendation G.7715.1.
  • ATM PNNI Asynchronous Transfer Mode Private Network-to-Network Interface
  • OF E-NNI Optical Interworking Forum External Network-to-Network Interface
  • ITU- T International Telecommunication Union - Telecommunication Standardization Sector
  • OSPF Open Shortest Path First
  • a good method of summarization should produce a good compromise between speeding-up or simplifying the calculation and producing good results.
  • the existing methods generally do not indicate what information should be visible to the higher level view of the network. Consequently, the existing methods are in some cases (e.g., Optical Interworking Forum External Network-to-Network Interface (OIF E-NNI) routing protocol) only partial solutions, providing a framework, but not a complete method.
  • OFE-NNI Optical Interworking Forum External Network-to-Network Interface
  • Embodiments of the invention seek to alleviate or ameliorate at least some disadvantages of the prior art, and to provide a novel method and apparatus for routing in a multi-area communication network.
  • a method of routing in a multi-area communication network where the communication network comprises border nodes, having a direct link with at least one node in a different area, and terminal nodes, having no direct link with a node in a different area.
  • the method comprises a step of inter area routing from a first node to a second node in a different area using a summary network view.
  • the summary network view comprises all nodes within the network together with links associated with the nodes.
  • a border node in a network area has a respective link to each of the other nodes within the same network area, each respective link representing the total connectivity between the respective nodes, and has a respective link to one or more nodes in a different network area.
  • a terminal node in a network area has a respective link corresponding to each of the border nodes in the same network area, the link representing the potential connectivity between the respective nodes.
  • the step of inter-area routing comprises a step of determining an inter area sequence of nodes from the first node to the second node, in which at a current node the next node in the sequence is determined from a link to that node from the current node, where only links to border nodes or to the second network node are considered in determining the next node.
  • the next node in the sequence is determined by selecting a link from one or more candidate links from the current node to one or more respective candidate nodes.
  • the step of inter-area routing further comprises the step of determining whether the preceding node in the sequence of nodes is in the same network area as the current node. In response to a positive determination, a link is selected as a candidate link only if the respective candidate node is in a different area.
  • the step of inter-area routing further comprises the step of determining whether the preceding node in the sequence of nodes is in one or more of the same network areas as the current node, In response to a positive determination, a link is selected as a candidate link only if the respective candidate node is in a network area that is not in common with the preceding network node.
  • a respective link cost is associated with the links in the summary network view, and inter-area routing is carried out using a least cost routing method.
  • a summary network view of the multi-area communication network is created.
  • the summary network view is updated in response to changes in the communication network.
  • the local area routing within all network areas included in the inter- area sequence of nodes is initiated.
  • the local area routing in each network area included in the inter area route uses a network view comprising all nodes within the network and links associated with the nodes corresponding to the actual communication network.
  • an intra area sequence of nodes is determined between adjacent nodes of the inter area sequence of nodes in each area for which local area routing is initiated.
  • a routing element for routing in a multi-area communication network.
  • the communication network comprises border nodes, having a direct link to at least one node in a different area, and terminal nodes, having no direct link to a node in a different area.
  • the routing element comprises a network summary store containing a summary network view comprising all nodes within the network together with links associated with the nodes.
  • a border node in a network area has a respective link to each of the other nodes within the same network area, each respective link representing the total connectivity between the respective nodes, and has a respective link to one or more nodes in a different network area.
  • the routing element also comprises a routing agent coupled to the network summary store to access link data associated with the nodes in the network in the summary network view and coupled to receive a routing request.
  • the routing agent is operable to carry out inter area routing in response to a routing request for routing between first and second nodes in different areas.
  • the routing agent determines a sequence of nodes from the first node to the second node, in which at a current node the next node in the sequence is determined from a link to that node from the current node, where only links to border nodes or to the second network node are considered in determining the next node.
  • the multi-area communication network has a plurality of communication nodes arranged in a plurality of network areas, each communication node being either a border node, having a direct link with at least one node in a different area, or a terminal node, having no direct link with a node in a different area.
  • the multi-area communication network also has a routing element in accordance with the second aspect of the invention.
  • Figure 1 is a schematic diagram showing an exemplary multi-area network
  • Figure 2 is a schematic diagram illustrating local routing in the multi-area network shown in
  • Figure 3 is a schematic diagram showing network nodes retained in the summary network view
  • Figure 4 is a schematic diagram showing a partial view of links retained in the summary network view
  • Figure 5 is a schematic diagram showing a partial view of links created in the summary network view
  • Figure 6 is a schematic diagram showing a partial view of links created in the summary network view
  • Figure 7 is a schematic diagram showing a complete view of retained network nodes and of links in the summary network view corresponding to Figure 1;
  • Figure 8 is a flow chart showing steps of a method of routing in accordance with
  • Figure 9 is a flow chart showing in more detail steps of inter-area routing in some embodiments.
  • Figure 10 is a flow chart showing in more detail a method of forming a summary network view as described herein in accordance with embodiments of the invention
  • Figure 11 is a schematic block diagram of apparatus elements for routing a communication path in a network implementing embodiments of the invention.
  • Figure 12 is a first exemplary arrangement of apparatus elements shown in Figure 11;
  • Figure 13 is a second exemplary arrangement of apparatus elements shown in Figure 11;
  • Figure 14 is a schematic diagram of the exemplary network shown in Figure 1;
  • Figure 15 is a schematic diagram showing the links of the summary network view considered during an inter-area routing in accordance with embodiments of the invention.
  • Figure 16 is a schematic diagram illustrating the local area routing in respect of the border nodes selected in the intra area routing
  • Figure 17 is a schematic diagram showing the actual link selected in the network resulting from the routing in the communication network
  • Figure 18 is a schematic diagram showing an exemplary multi-area network having network nodes common to two network areas.
  • Figure 19 is a schematic diagram showing a complete view of retained network nodes and of links in the summary network view corresponding to the network shown in Figure 18.
  • the proposed invention assumes the usage of a hierarchical routing model for routing in a network.
  • a hierarchical routing model the entire network is divided into areas or subnetworks.
  • Local route agents perform intra-area routing for each area.
  • a higher level route agent performs inter-area routing on a summarized network view, comprising areas, inter- area links and summarized area information.
  • Embodiments of the invention therefore are applied to routing in a network divided into a number of areas, each of which is locally controlled by local route agents.
  • Embodiments of the invention provide a modified routing algorithm to be used on a summarized view of the network systematically constructed for a higher level route agent.
  • the higher level route agent Upon receipt of a connectivity request for an inter-area communication path, the higher level route agent performs a routing calculation on the summarized network view, and then delegates the computation of intra-area routing segments to the relevant local route agents of the relevant areas.
  • the mechanism of the hierarchical routing may be such that:
  • the local area communication path computations by the local route agents can be carried out in parallel on several concurrent agents;
  • Embodiments of the invention may be implemented using modifications to generally available routing algorithms.
  • some embodiments may be implemented using the Dijkstra shortest path algorithm, and some embodiments may be implemented using the Bellman-Ford shortest path algorithm, or may be implemented using modification and extensions of those algorithms or any other suitable routing algorithm.
  • a border node is a network node that has at least one direct communication link with a network node in a different network area.
  • the border nodes are labelled with a number: thus network area 22 has two border nodes 1 and 2; network area 24 has four border nodes 3, 4, 5 and 6; and network area 26 has one border node 7.
  • a terminal node is a network node that does not have a direct communication link with a network node in a different network area.
  • the terminal nodes are labelled with a letter: thus network area 12 has four terminal nodes a, b, c and d; network area 14 has four terminal nodes e, f, g and h; and network area 16 has four terminal nodes i, j, k, and 1.
  • Each of the network areas 22, 24, 26 is controlled by a respective local route agent 30, 32, 34.
  • Each local route agent 30, 32, 34 performs routing of a communication path within the respective network area 22, 24, 26.
  • FIG 2 is a schematic diagram illustrating the network connections available to local routers in the multi-area network shown in Figure 1.
  • Each local route agent 30, 32, 34 has no view of the external areas or of the inter-area links, as is shown schematically in Figure 2.
  • a global route agent 36 is also provided.
  • the global route agent 36 has access to a summary view of the areas and to information about inter-area links and performs inter-area routing of a communication path between network areas 12, 14, 16.
  • a summary view of the communication network is used for the routing of a communication path in the communication network in response to a communication request.
  • the summary view of the communication network 20 of Figure 1 used in embodiments of the invention will now be explained with reference to Figures 3-6. In these Figures, the same or similar elements have been given the same reference numeral in order to aid understanding.
  • Figure 3 is a schematic diagram showing network nodes retained in the summary network view.
  • all of the network nodes in each network area 22, 24, 26 in the exemplary network 20 as shown in Figure 1 are retained as network nodes in the summary view. This retention of all of the real network nodes in the summary network view enables communication path routing using the summary network information to be carried out for any network node within the
  • FIG 4 is a schematic diagram showing inter-area links in the summary network view.
  • the inter-area links between border nodes of different network areas are also retained in the summary network view.
  • the inter-area link 40 between the border nodes 1 and 3; inter-area link 42 between the border nodes 2 and 4; inter-area link 46 between the border nodes 5 and 7; and inter-area link 48 between the border nodes 6 and 7 are shown in bold lines in Figure 4 for the exemplary network shown in Figure 1.
  • Figure 5 illustrates the intra-area links created between border nodes in the summary network view in accordance with embodiments of the invention.
  • a respective link between a border node and each of any further border nodes within the same network area is created in the summary network view in accordance with embodiments of the invention.
  • Vb border nodes there will be Vb x (Vb-l)/2 border-to-border links in the summary network view of that area.
  • the link created between a border node and each of any further border nodes within the same network area in the summary network view represents the potential connectivity between the two nodes in the network, taking into account all the different potential paths connecting the two border nodes.
  • Methods of calculating the potential connectivity between two nodes in a communication network are known to a skilled person. These methods may be related to solutions for finding a maximum flow, which is a classic graph problem to which solutions are known, for example the Ford-Fulkerson algorithm.
  • WO 2010/063321 Al discloses of calculating the potential connectivity between two nodes in a communication network, although it will be appreciated that any appropriate method of calculating the potential connectivity of two network nodes may be used.
  • network area 22 of the exemplary network shown in Figure 5 there are only two border nodes, namely network nodes 1 and 2, and therefore a single intra-border node link 50 between network nodes 1 and 2 is created in the summary network view.
  • network area 24 of the exemplary network shown in Figure 5 there are four border nodes, namely network nodes 3-6. Therefore six intra-border node links 52, 54, 56, 58, 60, 62 between respective pairs of the border nodes 3-6 in network area 24 are created in the summary network view.
  • network area 26 of the exemplary network shown in Figure 5 there is only a single border node, namely network node 7, and therefore no intra-area links between border nodes are required in network area 26.
  • Figure 6 illustrates for each of the network areas 22, 24, 26 of the exemplary network arrangement 20 shown in Figure 1, the intra-area links created between each terminal node and all border nodes within the same network area, which are created for the summary view in accordance with the exemplary embodiment. For clarity, these links have not been numbered separately.
  • a respective intra-area link between each terminal node of a network area and all border nodes within the same network area is created for the summary network view in accordance with embodiments of the invention, in order to form a complete bipartite graph.
  • Vb border nodes and Vt terminal nodes in a network area there will be Vb x Vt border node- to- terminal node links in the summary view of that network area.
  • the link created between a terminal node and each of the border nodes within the same network area in the summary network view represents the potential connectivity between the two nodes in the network, taking into account all the different potential paths connecting the terminal node and the border node.
  • Methods of calculating the potential connectivity between two nodes in a communication network are known to a skilled person, and will therefore not be discussed in further detail.
  • WO 2010/063321 Al discloses such a method, although it will be appreciated that any appropriate method of calculating the potential connectivity of two network nodes may be used.
  • Figure 7 is a schematic drawing of all nodes and links in a summary network view of the network in accordance with an exemplary embodiment of the invention as shown in Figure 1.
  • the links between the network nodes as shown in Figure 7 are a summation of the links described above with reference to Figures 4-6.
  • the summary view of the communication network with the nodes and links as described above may be stored in different ways selected by a skilled person.
  • the summary network view is not necessarily stored in a single location, but may be distributed across many different network elements.
  • at least some of the data in the summary view may be replicated in a number of different network elements.
  • the summary view may be stored in a database.
  • the summary network view used in embodiments of the invention will be more complex than the original real-world network.
  • the increase in complexity is due to the increased number of links between nodes in the summary network view compared with the links in the original real network.
  • use of the summary network view in accordance with embodiments of the invention during routing of the communication path results in an overall reduction in the complexity of path routing.
  • Figure 8 is a flow chart showing steps of a method of routing in accordance with
  • a summary network view as described above is first created and then maintained in response to changes in the network arrangements. This step will be explained in more detail with reference to Figure 10.
  • a second step 66 inter-area routing using the summary network view using links to the border nodes or to the second network node in the summary network view is carried out. This step will be explained in more detail with reference to Figure 9.
  • a third step 68 local area routing is initiated within each local area through which the sequence of nodes passes. This may mean for example determining a sequence of nodes between identified nodes of the local area network using a network view in which links corresponding to the actual network are associated with nodes in the network area.
  • steps 64 and 68 may be carried out in different elements from the main inter-area routing function of step 66, and have therefore been shown in dashed lines. These elements may be implemented together or may be distributed in a communication network in different embodiments as will be apparent to a skilled person.
  • Figure 9 is a flow chart showing in more detail steps of inter-area routing between a first node and a second node in a different network area in accordance with some embodiments of the invention.
  • a first step 70 the routing commences at the first node, and links associated with the first node are found in the summary network view.
  • the links associated with the first node in the summary network view are links to the border nodes of the network area in which the first node is found.
  • a link corresponding to the next node is selected from the candidate links in the summary network view.
  • this next node will be a border node of the network area in which the first node is found.
  • step 74 the selected node is set as the current node and in step 76 it is determined whether the current network node is in the same network area as the second node.
  • step 76-n If the current network node is not in the same network area as the second node, step 76-n, further inter-area routing is required, and so in step 78 the candidate links corresponding to border nodes are established from all links associated with current node, and the method returns to step 72 for further inter-area routing. Since all candidate links relate to links with border nodes, the nodes selected by the inter-area routing will all be border nodes. If the current network node is in the same network area as the second node, step 76-y, it merely remains in step 80 to select the link to the second node from links associated with current node to complete the inter-area routing.
  • Figure 10 is a flow chart showing in more detail a method of forming a summary network view as described herein in accordance with embodiments of the invention.
  • step 82 a current node for processing is selected, and in step 84 it is determined whether the current node is a border node.
  • step 84-n only links with border nodes in the same area are included in the summary network view. Therefore, firstly in step 86 a border node in the same area is selected, and in step 88 a link representing the connectivity between current node and the selected border node is associated with the current node in the summary network view.
  • Steps 86 and 88 are repeated, step 90-y until all links between the current node and all border nodes has been established, step 90-n, whereupon the next node may be selected to establish a summary view in step 82.
  • step 84-y steps 92-96 are carried out to associate a link representing the connectivity between the border node and the all terminal nodes in the same area with the current node in the summary network view. Steps 92-96 broadly correspond to steps 84-88 and therefore will not be described in further detail.
  • FIG 11 is a schematic block diagram of apparatus elements for routing a communication path in a network that can be used to implement embodiments of the invention. As will be apparent to a skilled person from a consideration of the following description, these elements may be distributed within a communication network or may be grouped together in one routing element as determined by a skilled person. Elements that are the same or are similar to elements in Figures 1-7 have been given the same reference numerals.
  • a local network area summarising agent 100 is provided for generating the summary network view of network area 22, which is an exemplary network described above.
  • the local network area summarising agent 100 is arranged to receive at least network configuration information 102 in respect of the network area 22.
  • the local network area summarising agent 100 is arranged to create a summary network view in accordance with the principles and methods described previously.
  • a local network summary store 104 is provided to store information relevant to the routing of a communication path within the network area 22.
  • Local network area summary information 106 created by the local network area summarising agent 100 from received network configuration information 102 is stored in a local network summary store 104.
  • the local network summary information 106 will include network node identification
  • a global network summary store 108 is also provided to store information relevant to the routing of the communication path between the network areas.
  • the global network summary store 108 is coupled to receive at least local network area summary information 106, created by the local network area summarising agent 100, which is relevant to the routing of the communication path between the network areas.
  • the global network summary store 108 will also receive local network area summary information 110 relevant to the other local network areas, for example from the respective local network area summarising agents (not shown in Figure 8) for network areas 24 and 26 of the exemplary network shown in Figure 1 corresponding to local network area summarising agent 100 for local area network 22 shown in Figure 8.
  • a global routing agent 112 is provided and is coupled to the global network summary store 108 to access network summary information 114.
  • the global routing agent 112 is coupled to receive a routing request 116 requesting routing of a communication path within the communication network, and performs global routing of the communication path between network areas as described previously. Once the global routing of the communication path between the network areas has been carried out, routing within all local areas affected can take place in parallel. For simplicity, only a single local routing agent 118, responsible for routing of the communication path within local area network 22, is shown in Figure 8. However, a corresponding local routing agent for other network areas would typically be provided in order to carry out local routing functions in the other areas.
  • the local routing agent 118 routes the communication path in the local area network 22 between the border nodes selected by the global routing agent 112, in local area networks where the communication path is crossing the local area network, or between a border node selected by the global routing agent 112 and a path originating terminal node or a path terminating terminal node where the communication path is originating or terminating within the local area network.
  • the local routing agent 118 accesses the local network summary store 104 to obtain network summary information 120.
  • the global routing agent 112 initiates the routing within the local area network 22 by sending a local routing initiation request 122 to the corresponding local routing agent 108.
  • the local routing agent 108 routes the communication path within the local area network and generates a communication path resource reservation 124 for the nodes and links allocated to the communication path.
  • the nature of the communication between the global routing agent and the relevant local routing agents, and the method by which the resources selected by a routing agent for a communication path are reserved can be varied in different embodiments to suit different circumstances.
  • routing agents implementing local and global routing algorithms in accordance with embodiments of the invention may be implemented in network nodes as part of control plane protocols; in management systems; and in planning and simulation tools.
  • the routing agents implementing local and global routing algorithms may be contained in a single piece of software.
  • the local and global routing algorithms may be distributed across a number of network elements.
  • the routing agents will be implemented in software. Two exemplary arrangements of these elements are shown in Figures 12 and 13, in which the same reference numerals have been used for the same elements shown in Figure 11.
  • a local router 126 is provided for local area network 22 as an exemplary network area, and the local routers for each of the network areas, including local router 126 provided for local area network 22, are coupled to a global router 128.
  • the local router 126 has a local network summarising agent 100; a local network summary store 104; and a local routing agent 118.
  • the global router 128 has a global routing agent 112 and a global network summary store 108.
  • the local router 130 for the network area 22 in the exemplary embodiment shown in Figure 10 thus also has a global routing agent 112 and a global network summary store 108 in addition to local network summarising agent 100; a local network summary store 104; and a local routing agent 118.
  • the local network summary store 104 and the global network summary store 108 are stored in a network summary store 132.
  • network information held by each local router that is relevant to the global routing is shared with other local routers in the network.
  • Figure 14 is a schematic diagram of the exemplary network shown in Figure 1. It is assumed that a summary network view of the network has been maintained as described previously, and that the global routing agent 112 shown in Figure 14 is implemented in the global router 36 and that a local routing agent 118 shown in Figure 14 is implemented in each of the local routers 30, 32 and 34.
  • a global routing step is undertaken using a routing algorithm applied to network summary information relevant to the global routing.
  • the routing algorithm can be a shortest path (minimal cost) algorithm such as Dijkstra and Bellman-Ford.
  • Minimal cost shortest path algorithm
  • the routing algorithm must be modified in accordance with embodiments of the invention so that during the global routing step, the source node and the destination node are the only terminal nodes used, and other than the source terminal node and the destination terminal node, only border nodes and their associated links are considered.
  • Figure 15 shows the links considered during an inter-area routing in accordance with embodiments of the invention as described above.
  • the only links involving terminal nodes are the links connecting terminal node B, acting in this example as the initiating node or first node, with the border nodes 1 and 2, and the link connecting border nodes 7 with terminal node L, acting in this example as the terminating or second network node.
  • the remaining links considered in routing are border node to border node links.
  • border nodes 1, 3, 6 and 7 are selected as the border nodes in the inter-area routing.
  • Figure 16 is a schematic diagram illustrating the local area routing in respect of the border nodes selected in the intra area routing.
  • Figure 17 is a schematic diagram showing the actual link selected in the actual network resulting from the routing in the communication network.
  • the route could cross non-border nodes, but the summary network view only considers border nodes and the intra-area links representing their potential connectivity.
  • a link between two border nodes, if used in the final route, will be expanded in the real intra-area route by the local route agents that operate of the complete accurate network view, not the summary network used for the inter-area routing.
  • Any border node is connected to at least one intra-area link and at least one inter-area link.
  • a split-horizon rule may be used in embodiments of the invention:
  • the split-horizon rule is used because routing between border nodes within a network area is carried out by the local routing agent for that area, which has the complete view of the connectivity of nodes within the network area and not by the global routing agent.
  • any direct path between a pair of border nodes in the same network area by definition represents the summary of all the possible paths between them in the real area. If an indirect path between two border nodes provided a valid alternative path in the real area, it would imply that the initial assumption that the relevant direct path summarizes all the possible real paths is untrue.
  • Figure 18 is a schematic diagram showing an exemplary multi-area network having network nodes common to two network areas.
  • Network area 140 is provided with border nodes 8 and 9 together with terminal nodes m, n, o and p.
  • Network area 142 is provided with border nodes 8, 9, 10, 11 together with terminal nodes q, r, s and t. As can be seen from Figure 18, the border nodes 8 and 9 are common to the network area 140 and network area 142.
  • Figure 19 is a schematic diagram showing a complete view of retained network nodes and of links in the summary network view corresponding to the network shown in Figure 18 in accordance with embodiments of the invention..
  • border node may be associated with a set of areas rather than with one area described previously.
  • border nodes 8 and 9 shown in Figures 18 and 19 are associated with network areas 140 and 142.
  • each interface of a border node is in any case part of one area only.
  • a link between two interfaces may therefore be:
  • inter area link if its end interfaces are in different areas.
  • the intra-area links between border nodes in the summarized network are added as in the previous description, bearing in mind that an intra-area link must represent potential connectivity which is entirely implemented in the considered area. If two border nodes are shared between the same two areas A and B, it may be necessary to use two intra-area links: these will represent the potential connectivity in area A and in area B respectively. Border-to- terminal links are generated as in the above procedure, with no changes.
  • the split-horizon rule may be changed as follows:
  • the exemplary Dijkstra algorithm has complexity 0(V 2 ) for a network with V nodes.
  • the algorithm on the summarized network operates on S x Vb nodes (only the border nodes), moreover the split-horizon rule adds a further simplification, so the complexity is not higher than 0((S ⁇ Vb) 2 ).
  • the algorithm must be applied locally to the traversed areas, where the complexity is 0((Vb + Vt) 2 ) or 0((V/S) 2 ), and can be performed in parallel by the local route agents.
  • the summarized network has the same nodes as the real network; preserves the links between border nodes in different network areas; and replaces all links within a network areas with Vb (Vb - l) / 2 + Vbx Vt links.
  • D is the average meshing degree of a node
  • Vs Vb + Vt
  • the modified global routing algorithm does not use the terminal nodes and their associated links apart from a source terminal node and a destination terminal node, so that the application of the routing algorithm becomes much simpler during the global routing computation.
  • the local routing in the network areas can be carried out in parallel to reduce the complexity of the calculation, but since summary network information contains summary of all the connectivity information of the original network, accurate routing may be achieved in the network areas.
  • embodiments of the present invention provide a novel method and apparatus for routing in a network having a plurality of network areas.
  • Embodiments of the invention enable network summarisation to be performed in a manner that allows for decisions regarding routing to be made at a higher route computation level using a summarised data set that enables accurate routing between network areas. It is advantageous that the invention in its various embodiments gives precise indications about how to perform network summarization for route scalability and adds important
  • the summary network view preserves much information from the complete network view while using only a limited amount of that information during global routing of a communication path between the network areas.
  • the detailed local area information view preserves much information from the complete network view to enable good routing within a local area so that the algorithmic complexity is reduced and scalability can be obtained.

Abstract

A method of routing in a multi-area communication network is disclosed. The network has border nodes, having a direct link with at least one node in a different area, and terminal nodes, having no direct link with a node in a different area. A summary network view comprises all nodes within the network together with links associated with the nodes. A border node in a network area has a respective link to each of the other nodes within the same network area, each respective link representing the total connectivity between the respective nodes, and has a respective link to one or more nodes in a different network area. A terminal node in a network area has a respective link to each of the border nodes in the same network area, the link representing the potential connectivity between the respective nodes. Inter area routing from a first node to a second node in a different area in a multi-area communication network is carried out using the summary network view. A sequence of nodes from the first node to the second node is determined, in which at a current node the next node in the sequence is determined from a link to that node from the current node, where only links to border nodes or to the second network node are considered in determining the next node.

Description

METHOD AKD APPARATUS FOR ROUTING IN A MULTI-AREA COMMUNICATION NETWORK
TECHNICAL FIELD
The present invention relates to a method and apparatus for routing in a multi-area communication network.
BACKGROUND
Routing is a central function in communication networks, and its established practice has been broadly studied.
The problem of routing in a communication network is generally solved by representing the network as a graph G(V, E), composed of V nodes (vertices) and E links (edges), and running a shortest path algorithm to compute the route that satisfies a connectivity request whilst seizing minimum resources.
Dijkstra's algorithm is currently one of the most efficient shortest path algorithms and its complexity in its basic form is 0(V2). The algorithm is based on link costs for the basic computation, but generally requires modifications for specific purposes. For example it may include variable costs to improve resource usage, diversity information to compute protected paths, optical parameters for wavelength routing, or special multiplexing schemes for Time Division Multiplexing (TDM) networks. This further increases the algorithm complexity.
This complexity results in a need to split a network domain into sub-networks or network areas for performance reasons. Routing protocols widely used in the Internet are able to split a domain into areas. For example, Open Shortest Path First (OSPF) routing protocol uses a backbone area (area 0) and stub areas connected to the backbone area via area border routers, so that the routing information in an area border router is limited to the details of its area and a summary of the external network. The Intermediate System to Intermediate System (ISIS) routing protocol uses a similar approach, replacing the backbone area concept with level 2 routers, used as area representatives and conveying inter-area traffic. Connection-oriented networks define hierarchical routing, where a sub-network or area is fully known by the switches in it, and is presented as a summarized network to a higher routing level. This approach is used, for example, in Asynchronous Transfer Mode Private Network-to-Network Interface (ATM PNNI) routing, in Optical Interworking Forum External Network-to-Network Interface (OIF E-NNI) routing and described more theoretically in International Telecommunication Union - Telecommunication Standardization Sector (ITU- T) recommendation G.7715.1.
Running routing algorithms on multi-area networks, where area information is summarized in higher level views, tends to yield sub-optimal results, as the overall route calculation is performed with less information, hence higher approximation.
Moreover, in the most popular solutions there are restrictions in the routing protocol that force the network architecture leading to sub-optimal results. For example, in the Open Shortest Path First (OSPF) routing protocol, the area 0 must be traversed when connecting two stub areas, even when they could instead possibly be directly connected.
A good method of summarization should produce a good compromise between speeding-up or simplifying the calculation and producing good results.
The existing methods generally do not indicate what information should be visible to the higher level view of the network. Consequently, the existing methods are in some cases (e.g., Optical Interworking Forum External Network-to-Network Interface (OIF E-NNI) routing protocol) only partial solutions, providing a framework, but not a complete method.
Embodiments of the invention seek to alleviate or ameliorate at least some disadvantages of the prior art, and to provide a novel method and apparatus for routing in a multi-area communication network.
SUMMARY
In accordance with a first aspect of the invention, there is provided a method of routing in a multi-area communication network, where the communication network comprises border nodes, having a direct link with at least one node in a different area, and terminal nodes, having no direct link with a node in a different area. The method comprises a step of inter area routing from a first node to a second node in a different area using a summary network view. The summary network view comprises all nodes within the network together with links associated with the nodes. In the summary network view a border node in a network area has a respective link to each of the other nodes within the same network area, each respective link representing the total connectivity between the respective nodes, and has a respective link to one or more nodes in a different network area. In the summary network view a terminal node in a network area has a respective link corresponding to each of the border nodes in the same network area, the link representing the potential connectivity between the respective nodes. The step of inter-area routing comprises a step of determining an inter area sequence of nodes from the first node to the second node, in which at a current node the next node in the sequence is determined from a link to that node from the current node, where only links to border nodes or to the second network node are considered in determining the next node.
In some embodiments, at a current node in a sequence of nodes, the next node in the sequence is determined by selecting a link from one or more candidate links from the current node to one or more respective candidate nodes.
In some embodiments the step of inter-area routing further comprises the step of determining whether the preceding node in the sequence of nodes is in the same network area as the current node. In response to a positive determination, a link is selected as a candidate link only if the respective candidate node is in a different area.
In some embodiments the step of inter-area routing further comprises the step of determining whether the preceding node in the sequence of nodes is in one or more of the same network areas as the current node, In response to a positive determination, a link is selected as a candidate link only if the respective candidate node is in a network area that is not in common with the preceding network node.
In some embodiments, a respective link cost is associated with the links in the summary network view, and inter-area routing is carried out using a least cost routing method. In some embodiments, a summary network view of the multi-area communication network is created.
In some embodiments the summary network view is updated in response to changes in the communication network.
In some embodiments the local area routing within all network areas included in the inter- area sequence of nodes is initiated.
In some embodiments the local area routing in each network area included in the inter area route uses a network view comprising all nodes within the network and links associated with the nodes corresponding to the actual communication network.
In some embodiments an intra area sequence of nodes is determined between adjacent nodes of the inter area sequence of nodes in each area for which local area routing is initiated.
In accordance with a second aspect of the invention, there is provided a routing element for routing in a multi-area communication network. The communication network comprises border nodes, having a direct link to at least one node in a different area, and terminal nodes, having no direct link to a node in a different area. The routing element comprises a network summary store containing a summary network view comprising all nodes within the network together with links associated with the nodes. In the summary network records a border node in a network area has a respective link to each of the other nodes within the same network area, each respective link representing the total connectivity between the respective nodes, and has a respective link to one or more nodes in a different network area. In the summary network records a terminal node in a network area is associated with a respective link to each of the border nodes in the same network area, the link representing the total connectivity between the respective nodes. The routing element also comprises a routing agent coupled to the network summary store to access link data associated with the nodes in the network in the summary network view and coupled to receive a routing request. The routing agent is operable to carry out inter area routing in response to a routing request for routing between first and second nodes in different areas. The routing agent determines a sequence of nodes from the first node to the second node, in which at a current node the next node in the sequence is determined from a link to that node from the current node, where only links to border nodes or to the second network node are considered in determining the next node.
In accordance with a third aspect of the invention, there is provided a multi-area
communication network. The multi-area communication network has a plurality of communication nodes arranged in a plurality of network areas, each communication node being either a border node, having a direct link with at least one node in a different area, or a terminal node, having no direct link with a node in a different area. The multi-area communication network also has a routing element in accordance with the second aspect of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be described by way of example with reference to the accompanying drawings illustrating embodiments of the invention, in which:
Figure 1 is a schematic diagram showing an exemplary multi-area network;
Figure 2 is a schematic diagram illustrating local routing in the multi-area network shown in
Figure 1;
Figure 3 is a schematic diagram showing network nodes retained in the summary network view;
Figure 4 is a schematic diagram showing a partial view of links retained in the summary network view;
Figure 5 is a schematic diagram showing a partial view of links created in the summary network view;
Figure 6 is a schematic diagram showing a partial view of links created in the summary network view;
Figure 7 is a schematic diagram showing a complete view of retained network nodes and of links in the summary network view corresponding to Figure 1;
Figure 8 is a flow chart showing steps of a method of routing in accordance with
embodiments of the invention;
Figure 9 is a flow chart showing in more detail steps of inter-area routing in some embodiments;
Figure 10 is a flow chart showing in more detail a method of forming a summary network view as described herein in accordance with embodiments of the invention; Figure 11 is a schematic block diagram of apparatus elements for routing a communication path in a network implementing embodiments of the invention;
Figure 12 is a first exemplary arrangement of apparatus elements shown in Figure 11;
Figure 13 is a second exemplary arrangement of apparatus elements shown in Figure 11;
Figure 14 is a schematic diagram of the exemplary network shown in Figure 1;
Figure 15 is a schematic diagram showing the links of the summary network view considered during an inter-area routing in accordance with embodiments of the invention;
Figure 16 is a schematic diagram illustrating the local area routing in respect of the border nodes selected in the intra area routing;
Figure 17 is a schematic diagram showing the actual link selected in the network resulting from the routing in the communication network;
Figure 18 is a schematic diagram showing an exemplary multi-area network having network nodes common to two network areas; and
Figure 19 is a schematic diagram showing a complete view of retained network nodes and of links in the summary network view corresponding to the network shown in Figure 18.
DETAILED DESCRIPTION
The proposed invention assumes the usage of a hierarchical routing model for routing in a network. In a hierarchical routing model the entire network is divided into areas or subnetworks. Local route agents perform intra-area routing for each area. A higher level route agent performs inter-area routing on a summarized network view, comprising areas, inter- area links and summarized area information.
Embodiments of the invention therefore are applied to routing in a network divided into a number of areas, each of which is locally controlled by local route agents. Embodiments of the invention provide a modified routing algorithm to be used on a summarized view of the network systematically constructed for a higher level route agent.
Upon receipt of a connectivity request for an inter-area communication path, the higher level route agent performs a routing calculation on the summarized network view, and then delegates the computation of intra-area routing segments to the relevant local route agents of the relevant areas. The mechanism of the hierarchical routing may be such that:
- the complexity of the overall computation (summarized network view plus relevant areas) is lower than the direct communication path computation on the whole network, to allow routing scalability;
- the local area communication path computations by the local route agents can be carried out in parallel on several concurrent agents; and
- the resulting communication path routing is reasonably good, or at least not evidently worse than the result that would be achieved had routing of the communication path been carried out on the whole network.
Embodiments of the invention may be implemented using modifications to generally available routing algorithms. In particular, some embodiments may be implemented using the Dijkstra shortest path algorithm, and some embodiments may be implemented using the Bellman-Ford shortest path algorithm, or may be implemented using modification and extensions of those algorithms or any other suitable routing algorithm.
In addition as will be apparent to a skilled person the hierarchical routing model can be iterated on more levels, that is to say, a summarized view can in turn be summarized.
However, for simplicity, a simple hierarchical model is used in the exemplary embodiments, although the skilled person will understand that terms referring to a local or to a global level merely refer to the levels of summarisation at which the embodiment is being applied.
Embodiments of the invention provide a method and apparatus for routing in a
communication network, including a method and apparatus for systematic summarising of network information in a higher level view of a network, which will now be described with reference to the accompanying drawings.
A border node is a network node that has at least one direct communication link with a network node in a different network area. In Figure 1 the border nodes are labelled with a number: thus network area 22 has two border nodes 1 and 2; network area 24 has four border nodes 3, 4, 5 and 6; and network area 26 has one border node 7. A terminal node is a network node that does not have a direct communication link with a network node in a different network area. In Figure 1 the terminal nodes are labelled with a letter: thus network area 12 has four terminal nodes a, b, c and d; network area 14 has four terminal nodes e, f, g and h; and network area 16 has four terminal nodes i, j, k, and 1.
Each of the network areas 22, 24, 26 is controlled by a respective local route agent 30, 32, 34. Each local route agent 30, 32, 34 performs routing of a communication path within the respective network area 22, 24, 26.
Figure 2 is a schematic diagram illustrating the network connections available to local routers in the multi-area network shown in Figure 1. Each local route agent 30, 32, 34 has no view of the external areas or of the inter-area links, as is shown schematically in Figure 2.
A global route agent 36 is also provided. The global route agent 36 has access to a summary view of the areas and to information about inter-area links and performs inter-area routing of a communication path between network areas 12, 14, 16.
In accordance with embodiments of the invention, a summary view of the communication network is used for the routing of a communication path in the communication network in response to a communication request. The summary view of the communication network 20 of Figure 1 used in embodiments of the invention will now be explained with reference to Figures 3-6. In these Figures, the same or similar elements have been given the same reference numeral in order to aid understanding.
Figure 3 is a schematic diagram showing network nodes retained in the summary network view. In the network summary used in embodiments of the present invention, all of the network nodes in each network area 22, 24, 26 in the exemplary network 20 as shown in Figure 1 are retained as network nodes in the summary view. This retention of all of the real network nodes in the summary network view enables communication path routing using the summary network information to be carried out for any network node within the
communication network. Figure 4 is a schematic diagram showing inter-area links in the summary network view. In embodiments of the invention, the inter-area links between border nodes of different network areas are also retained in the summary network view. The inter-area link 40 between the border nodes 1 and 3; inter-area link 42 between the border nodes 2 and 4; inter-area link 46 between the border nodes 5 and 7; and inter-area link 48 between the border nodes 6 and 7 are shown in bold lines in Figure 4 for the exemplary network shown in Figure 1.
Figure 5 illustrates the intra-area links created between border nodes in the summary network view in accordance with embodiments of the invention. A respective link between a border node and each of any further border nodes within the same network area is created in the summary network view in accordance with embodiments of the invention. Generally, it will be appreciated by a skilled person that if there are Vb border nodes in an area, there will be Vb x (Vb-l)/2 border-to-border links in the summary network view of that area.
The link created between a border node and each of any further border nodes within the same network area in the summary network view represents the potential connectivity between the two nodes in the network, taking into account all the different potential paths connecting the two border nodes. Methods of calculating the potential connectivity between two nodes in a communication network are known to a skilled person. These methods may be related to solutions for finding a maximum flow, which is a classic graph problem to which solutions are known, for example the Ford-Fulkerson algorithm. In addition, WO 2010/063321 Al discloses of calculating the potential connectivity between two nodes in a communication network, although it will be appreciated that any appropriate method of calculating the potential connectivity of two network nodes may be used.
In network area 22 of the exemplary network shown in Figure 5, there are only two border nodes, namely network nodes 1 and 2, and therefore a single intra-border node link 50 between network nodes 1 and 2 is created in the summary network view.
In network area 24 of the exemplary network shown in Figure 5, there are four border nodes, namely network nodes 3-6. Therefore six intra-border node links 52, 54, 56, 58, 60, 62 between respective pairs of the border nodes 3-6 in network area 24 are created in the summary network view. In network area 26 of the exemplary network shown in Figure 5 there is only a single border node, namely network node 7, and therefore no intra-area links between border nodes are required in network area 26.
Figure 6 illustrates for each of the network areas 22, 24, 26 of the exemplary network arrangement 20 shown in Figure 1, the intra-area links created between each terminal node and all border nodes within the same network area, which are created for the summary view in accordance with the exemplary embodiment. For clarity, these links have not been numbered separately.
A respective intra-area link between each terminal node of a network area and all border nodes within the same network area is created for the summary network view in accordance with embodiments of the invention, in order to form a complete bipartite graph. As will be apparent to a skilled person, if there are Vb border nodes and Vt terminal nodes in a network area, there will be Vb x Vt border node- to- terminal node links in the summary view of that network area.
The link created between a terminal node and each of the border nodes within the same network area in the summary network view represents the potential connectivity between the two nodes in the network, taking into account all the different potential paths connecting the terminal node and the border node. Methods of calculating the potential connectivity between two nodes in a communication network are known to a skilled person, and will therefore not be discussed in further detail. WO 2010/063321 Al discloses such a method, although it will be appreciated that any appropriate method of calculating the potential connectivity of two network nodes may be used.
Figure 7 is a schematic drawing of all nodes and links in a summary network view of the network in accordance with an exemplary embodiment of the invention as shown in Figure 1. The links between the network nodes as shown in Figure 7 are a summation of the links described above with reference to Figures 4-6. The summary view of the communication network with the nodes and links as described above may be stored in different ways selected by a skilled person. The summary network view is not necessarily stored in a single location, but may be distributed across many different network elements. In addition, at least some of the data in the summary view may be replicated in a number of different network elements. In some embodiments the summary view may be stored in a database.
As will be appreciated from a comparison of the summary network view shown in Figure 7 with the original network shown in Figure 1, in general the summary network view used in embodiments of the invention will be more complex than the original real-world network. The increase in complexity is due to the increased number of links between nodes in the summary network view compared with the links in the original real network. However, as will be appreciated by a skilled person from a consideration of this description, use of the summary network view in accordance with embodiments of the invention during routing of the communication path results in an overall reduction in the complexity of path routing.
A method of routing using the summary network view in accordance with embodiments of the invention will now be described with reference to Figure 8-10.
Figure 8 is a flow chart showing steps of a method of routing in accordance with
embodiments of the invention.
In a first step 64, a summary network view as described above is first created and then maintained in response to changes in the network arrangements. This step will be explained in more detail with reference to Figure 10.
In a second step 66, inter-area routing using the summary network view using links to the border nodes or to the second network node in the summary network view is carried out. This step will be explained in more detail with reference to Figure 9.
In a third step 68, local area routing is initiated within each local area through which the sequence of nodes passes. This may mean for example determining a sequence of nodes between identified nodes of the local area network using a network view in which links corresponding to the actual network are associated with nodes in the network area.
As will be clear from a consideration of the following description, the steps 64 and 68 may be carried out in different elements from the main inter-area routing function of step 66, and have therefore been shown in dashed lines. These elements may be implemented together or may be distributed in a communication network in different embodiments as will be apparent to a skilled person.
Figure 9 is a flow chart showing in more detail steps of inter-area routing between a first node and a second node in a different network area in accordance with some embodiments of the invention.
In a first step 70, the routing commences at the first node, and links associated with the first node are found in the summary network view. As described previously, the links associated with the first node in the summary network view are links to the border nodes of the network area in which the first node is found.
In a second step 72, a link corresponding to the next node is selected from the candidate links in the summary network view. As discussed previously, this next node will be a border node of the network area in which the first node is found.
In step 74, the selected node is set as the current node and in step 76 it is determined whether the current network node is in the same network area as the second node.
If the current network node is not in the same network area as the second node, step 76-n, further inter-area routing is required, and so in step 78 the candidate links corresponding to border nodes are established from all links associated with current node, and the method returns to step 72 for further inter-area routing. Since all candidate links relate to links with border nodes, the nodes selected by the inter-area routing will all be border nodes. If the current network node is in the same network area as the second node, step 76-y, it merely remains in step 80 to select the link to the second node from links associated with current node to complete the inter-area routing.
Figure 10 is a flow chart showing in more detail a method of forming a summary network view as described herein in accordance with embodiments of the invention.
In step 82, a current node for processing is selected, and in step 84 it is determined whether the current node is a border node.
If the current node is a terminal node, step 84-n, only links with border nodes in the same area are included in the summary network view. Therefore, firstly in step 86 a border node in the same area is selected, and in step 88 a link representing the connectivity between current node and the selected border node is associated with the current node in the summary network view.
Steps 86 and 88 are repeated, step 90-y until all links between the current node and all border nodes has been established, step 90-n, whereupon the next node may be selected to establish a summary view in step 82.
If the current node is a border node, step 84-y steps 92-96 are carried out to associate a link representing the connectivity between the border node and the all terminal nodes in the same area with the current node in the summary network view. Steps 92-96 broadly correspond to steps 84-88 and therefore will not be described in further detail.
Figure 11 is a schematic block diagram of apparatus elements for routing a communication path in a network that can be used to implement embodiments of the invention. As will be apparent to a skilled person from a consideration of the following description, these elements may be distributed within a communication network or may be grouped together in one routing element as determined by a skilled person. Elements that are the same or are similar to elements in Figures 1-7 have been given the same reference numerals. A local network area summarising agent 100 is provided for generating the summary network view of network area 22, which is an exemplary network described above. The local network area summarising agent 100 is arranged to receive at least network configuration information 102 in respect of the network area 22. The local network area summarising agent 100 is arranged to create a summary network view in accordance with the principles and methods described previously.
A local network summary store 104 is provided to store information relevant to the routing of a communication path within the network area 22. Local network area summary information 106 created by the local network area summarising agent 100 from received network configuration information 102 is stored in a local network summary store 104. Typically, the local network summary information 106 will include network node identification
information; identification of links associated with a network node; and the weighting given to the link representing the total connectivity between the endpoints, and will be updated by the local network area summarising agent 100 in response to changes in the network.
A global network summary store 108 is also provided to store information relevant to the routing of the communication path between the network areas. The global network summary store 108 is coupled to receive at least local network area summary information 106, created by the local network area summarising agent 100, which is relevant to the routing of the communication path between the network areas. In addition the global network summary store 108 will also receive local network area summary information 110 relevant to the other local network areas, for example from the respective local network area summarising agents (not shown in Figure 8) for network areas 24 and 26 of the exemplary network shown in Figure 1 corresponding to local network area summarising agent 100 for local area network 22 shown in Figure 8.
A global routing agent 112 is provided and is coupled to the global network summary store 108 to access network summary information 114. The global routing agent 112 is coupled to receive a routing request 116 requesting routing of a communication path within the communication network, and performs global routing of the communication path between network areas as described previously. Once the global routing of the communication path between the network areas has been carried out, routing within all local areas affected can take place in parallel. For simplicity, only a single local routing agent 118, responsible for routing of the communication path within local area network 22, is shown in Figure 8. However, a corresponding local routing agent for other network areas would typically be provided in order to carry out local routing functions in the other areas.
The local routing agent 118 routes the communication path in the local area network 22 between the border nodes selected by the global routing agent 112, in local area networks where the communication path is crossing the local area network, or between a border node selected by the global routing agent 112 and a path originating terminal node or a path terminating terminal node where the communication path is originating or terminating within the local area network. The local routing agent 118 accesses the local network summary store 104 to obtain network summary information 120.
In Figure 11 the global routing agent 112 initiates the routing within the local area network 22 by sending a local routing initiation request 122 to the corresponding local routing agent 108. In response the local routing agent 108 routes the communication path within the local area network and generates a communication path resource reservation 124 for the nodes and links allocated to the communication path. Clearly, the nature of the communication between the global routing agent and the relevant local routing agents, and the method by which the resources selected by a routing agent for a communication path are reserved, can be varied in different embodiments to suit different circumstances.
The different routing elements shown in Figure 11 may be implemented in a variety of ways in accordance with different embodiments, as will be apparent to a skilled person. In particular, routing agents implementing local and global routing algorithms in accordance with embodiments of the invention may be implemented in network nodes as part of control plane protocols; in management systems; and in planning and simulation tools. In some of these embodiments, in particular in simulation tools, the routing agents implementing local and global routing algorithms may be contained in a single piece of software. In other embodiments, the local and global routing algorithms may be distributed across a number of network elements. Typically the routing agents will be implemented in software. Two exemplary arrangements of these elements are shown in Figures 12 and 13, in which the same reference numerals have been used for the same elements shown in Figure 11.
In Figure 12, a local router 126 is provided for local area network 22 as an exemplary network area, and the local routers for each of the network areas, including local router 126 provided for local area network 22, are coupled to a global router 128. The local router 126 has a local network summarising agent 100; a local network summary store 104; and a local routing agent 118.
The global router 128 has a global routing agent 112 and a global network summary store 108.
Although the elements described above with reference to Figure 11 are arranged in a number of local routers associated with individual network areas together with a centralised global router 126, the skilled person will understand that the operation of the elements in accordance with embodiments as described herein is not dependent on the location or grouping of the different elements and the operation of the arrangement shown in Figure 12 substantially corresponds with the operation of the arrangement shown in Figure 11.
In Figure 13 a centralised global router is not used, and instead the global routing
functionality is distributed to the local router for each of the network areas. The local router 130 for the network area 22 in the exemplary embodiment shown in Figure 10 thus also has a global routing agent 112 and a global network summary store 108 in addition to local network summarising agent 100; a local network summary store 104; and a local routing agent 118. In some embodiments such as the embodiment as shown in Figure 10 the local network summary store 104 and the global network summary store 108 are stored in a network summary store 132. In such embodiments, network information held by each local router that is relevant to the global routing is shared with other local routers in the network.
Although the elements described above with reference to Figure 11 are arranged in a number of local routers associated with individual network areas in the embodiment shown in Figure 3, the skilled person will understand that the operation of the elements in accordance with embodiments as described herein is not dependent on the location or grouping of the different elements and the operation of the arrangement shown in Figure 13 substantially corresponds with the operation of the arrangement shown in Figure 11.
The summary network view as described above is used during the routing of a
communication path in response to a connectivity request between nodes in different areas network, as will be explained with reference to Figures 14-17.
Figure 14 is a schematic diagram of the exemplary network shown in Figure 1. It is assumed that a summary network view of the network has been maintained as described previously, and that the global routing agent 112 shown in Figure 14 is implemented in the global router 36 and that a local routing agent 118 shown in Figure 14 is implemented in each of the local routers 30, 32 and 34.
The routing of a communication path between the terminal node B in the network area 22 and terminal node L in network area 26 as shown in Figure 14 in response to a connectivity request in an exemplary embodiment will now be described.
First, a global routing step is undertaken using a routing algorithm applied to network summary information relevant to the global routing.
The routing algorithm can be a shortest path (minimal cost) algorithm such as Dijkstra and Bellman-Ford. However, to be used on the network view transformed with the above summarization, as described above the routing algorithm must be modified in accordance with embodiments of the invention so that during the global routing step, the source node and the destination node are the only terminal nodes used, and other than the source terminal node and the destination terminal node, only border nodes and their associated links are considered.
Figure 15 shows the links considered during an inter-area routing in accordance with embodiments of the invention as described above. Thus it can be seem that the only links involving terminal nodes are the links connecting terminal node B, acting in this example as the initiating node or first node, with the border nodes 1 and 2, and the link connecting border nodes 7 with terminal node L, acting in this example as the terminating or second network node. The remaining links considered in routing are border node to border node links.
In this exemplary example, border nodes 1, 3, 6 and 7 are selected as the border nodes in the inter-area routing.
Figure 16 is a schematic diagram illustrating the local area routing in respect of the border nodes selected in the intra area routing.
Figure 17 is a schematic diagram showing the actual link selected in the actual network resulting from the routing in the communication network.
While routing across network areas, the route could cross non-border nodes, but the summary network view only considers border nodes and the intra-area links representing their potential connectivity. A link between two border nodes, if used in the final route, will be expanded in the real intra-area route by the local route agents that operate of the complete accurate network view, not the summary network used for the inter-area routing.
Any border node is connected to at least one intra-area link and at least one inter-area link. A split-horizon rule may be used in embodiments of the invention:
At each relaxation step of a node, if its predecessor is on the same area,
only inter-area links must be considered
The split-horizon rule is used because routing between border nodes within a network area is carried out by the local routing agent for that area, which has the complete view of the connectivity of nodes within the network area and not by the global routing agent.
It should be noted that considering only links between border nodes inside a summarized area does not exclude the selection of any path across the area in the real network. This is because in a summarized area, any direct path between a pair of border nodes in the same network area by definition represents the summary of all the possible paths between them in the real area. If an indirect path between two border nodes provided a valid alternative path in the real area, it would imply that the initial assumption that the relevant direct path summarizes all the possible real paths is untrue.
The situation in which a border node is part of more than one network area will now be described with reference to Figures 18 and 19. This situation is addressed with an approach which is slightly more complex, but consistent.
Figure 18 is a schematic diagram showing an exemplary multi-area network having network nodes common to two network areas.
In Figure 18, two network areas 140 and 142 are shown.
Network area 140 is provided with border nodes 8 and 9 together with terminal nodes m, n, o and p. Network area 142 is provided with border nodes 8, 9, 10, 11 together with terminal nodes q, r, s and t. As can be seen from Figure 18, the border nodes 8 and 9 are common to the network area 140 and network area 142.
Figure 19 is a schematic diagram showing a complete view of retained network nodes and of links in the summary network view corresponding to the network shown in Figure 18 in accordance with embodiments of the invention..
As indicated above a border node may be associated with a set of areas rather than with one area described previously. Thus, border nodes 8 and 9 shown in Figures 18 and 19 are associated with network areas 140 and 142.
In determining the links included in the summarised view, it is noted that in the real network, the intra-area links are however entirely contained into a single area. To be more accurate, if a node is connected with a link by means of an interface, each interface of a border node is in any case part of one area only. A link between two interfaces may therefore be:
- an intra area link, if its end interfaces are both in the same area; or
- an inter area link, if its end interfaces are in different areas. The intra-area links between border nodes in the summarized network are added as in the previous description, bearing in mind that an intra-area link must represent potential connectivity which is entirely implemented in the considered area. If two border nodes are shared between the same two areas A and B, it may be necessary to use two intra-area links: these will represent the potential connectivity in area A and in area B respectively. Border-to- terminal links are generated as in the above procedure, with no changes.
In this situation, the split-horizon rule may be changed as follows:
At each relaxation step of a node, if the intersection of the area sets of the
node and its predecessor is not empty, intra-area links belonging in areas
of the intersection must not be considered
The complexity of an exemplary routing algorithm that can be used with a summary network as described above will now be described. For the sake of simplicity, only the Dijkstra algorithm is used as an example. However, as will be appreciated by a skilled person other routing algorithms may benefit from the use of the principles of the present invention and embodiments of the invention may be implemented with other routing algorithms as will be known by a skilled person.
The exemplary Dijkstra algorithm has complexity 0(V2) for a network with V nodes.
Assuming the hierarchical network to be composed of S areas with an average of Vb border nodes and Vt terminal nodes each, we have:
(1) V = S (Vb + Vt)
(2) 0(V2) = 0((S (Vb + Vt))2)
The algorithm on the summarized network operates on S x Vb nodes (only the border nodes), moreover the split-horizon rule adds a further simplification, so the complexity is not higher than 0((S χ Vb)2).
Then the algorithm must be applied locally to the traversed areas, where the complexity is 0((Vb + Vt)2) or 0((V/S)2), and can be performed in parallel by the local route agents. As will be appreciated by a skilled person from a consideration of the present description, the summarized network has the same nodes as the real network; preserves the links between border nodes in different network areas; and replaces all links within a network areas with Vb (Vb - l) / 2 + Vbx Vt links.
Since an actual network may have as a typical and indicative example:
D Vs / 2 links
Where: D is the average meshing degree of a node;
And: Vs = Vb + Vt it is apparent that the summary network will have the same number of nodes as the actual network but will generally contain more links. Therefore, in general, the network information required to represent the summary network will be greater than the network information required to represent the real network.
However, the modified global routing algorithm does not use the terminal nodes and their associated links apart from a source terminal node and a destination terminal node, so that the application of the routing algorithm becomes much simpler during the global routing computation. The local routing in the network areas can be carried out in parallel to reduce the complexity of the calculation, but since summary network information contains summary of all the connectivity information of the original network, accurate routing may be achieved in the network areas.
As described above, embodiments of the present invention provide a novel method and apparatus for routing in a network having a plurality of network areas.
Embodiments of the invention enable network summarisation to be performed in a manner that allows for decisions regarding routing to be made at a higher route computation level using a summarised data set that enables accurate routing between network areas. It is advantageous that the invention in its various embodiments gives precise indications about how to perform network summarization for route scalability and adds important
modifications to be applied to a routing algorithm when used in conjunction with that summarisation. In particular, in embodiments of the invention the summary network view preserves much information from the complete network view while using only a limited amount of that information during global routing of a communication path between the network areas. However the detailed local area information view preserves much information from the complete network view to enable good routing within a local area so that the algorithmic complexity is reduced and scalability can be obtained.

Claims

1. A method of routing in a multi-area communication network, the communication network comprising border nodes, having a direct link to at least one node in a different area, and terminal nodes, having no direct link to a node in a different area, the method comprising:
a step of inter area routing from a first node to a second node in a different area using a summary network view comprising all nodes within the network together with links associated with the nodes,
in which summary network view a border node in a network area has a respective link to each of the other nodes within the same network area, each respective link representing the total connectivity between the respective nodes, and has a respective link to one or more nodes in a different network area, and
in which summary network view a terminal node in a network area has a respective link to each of the border nodes in the same network area, each respective link representing the potential connectivity between the respective nodes,
wherein the step of inter-area routing comprises a step of determining an inter area sequence of nodes from the first node to the second node, in which at a current node the next node in the sequence is determined from a link to that node from the current node, where only links to border nodes or to the second network node are considered in determining the next node.
2. The method of routing in a multi-area communication network as claimed in claim 1 in which, at a current node in a sequence of nodes, the next node in the sequence is determined by selecting a link from one or more candidate links from the current node to one or more respective candidate nodes.
3. The method of routing in a multi-area communication network as claimed in claim 2, in which the step of inter-area routing further comprises the step of determining whether the preceding node in the sequence of nodes is in the same network area as the current node, and in response to a positive determination, comprises the step of selecting a link as a candidate link only if the respective candidate node is in a different area.
4. The method of routing in a multi-area communication network as claimed in claim 2 or 3, in which the step of inter-area routing further comprises the step of determining whether the preceding node in the sequence of nodes is in one or more of the same network areas as the current node, and in response to a positive determination, the step of selecting a link as a candidate link only if the respective candidate node is in a network area that is not in common with the preceding network node.
5. The method of routing in a multi-area communication network as claimed in any preceding claim in which a respective link cost is associated with a link in the summary network view, and inter-area routing is carried out using a least cost routing method.
6. The method of routing in a multi-area communication network as claimed in any preceding claim further comprising the step of creating a summary network view of the multi-area communication network.
7. The method of routing in a multi-area communication network as claimed in claim 6, in which the summary network view is updated in response to changes in the multi- area communication network.
8. The method of routing in a multi-area communication network as claimed in any preceding claim, further comprising the step of initiating the local area routing within all network areas included in the inter area sequence of nodes.
9. The method of routing in a multi-area communication network as claimed in claim 8 further comprising the step of local area routing in each network area included in the inter area route using a network view comprising all nodes within the network and links associated with the nodes corresponding to the actual communication network.
10. The method of routing in a multi-area communication network as claimed in claim 8 or 9 wherein the step of local area routing comprises the step of determining a intra area sequence of nodes, between adjacent nodes of the inter area sequence of nodes in each area for which local area routing is initiated.
11. A routing element for routing in a multi-area communication network, the
communication network comprising border nodes, having a direct link to at least one node in a different area, and terminal nodes, having no direct link to a node in a different area, the routing element comprising:
a network summary store containing a summary network view comprising all nodes within the network together with links associated with the nodes,
in which summary network view a border node in a network area has a respective link to each of the other nodes within the same network area, each respective link representing the total connectivity between the respective nodes, and has a respective link to one or more nodes in a different network area, and
in which summary network view a terminal node in a network area has a respective link to each of the border nodes in the same network area, each respective link representing the total connectivity between the respective nodes;
and a routing agent coupled to the network summary store to access link data associated with the nodes in the network in the summary network view and coupled to receive a routing request
wherein the routing agent is operable to carry out inter area routing in response to a routing request for routing between first and second nodes in different areas by determining a sequence of nodes from the first node to the second node, in which at a current node the next node in the sequence is determined from a link to that node from the current node, where only links to border nodes or to the second network node are considered in determining the next node.
12. A multi-area communication network comprising:
a plurality of communication nodes arranged in a plurality of network areas, each communication node being either a border node, having a direct link with at least one node in a different area, or a terminal node, having no direct link with a node in a different area,
a network summary store containing a summary network view comprising all nodes within the network together with links associated with the nodes,
in which summary network view a border node in a network area has a respective link to each of the other nodes within the same network area, each respective link representing the total connectivity between the respective nodes, and has a respective link to one or more nodes in a different network area, and
in which summary network view a terminal node in a network area has a respective link to each of the border nodes in the same network area, each respective link representing the total connectivity between the respective nodes;
and a routing agent coupled to the network summary store to access link data associated with the nodes in the network in the summary network view and coupled to receive a routing request
wherein the routing agent is operable to carry out inter area routing in response to a routing request for routing between first and second nodes in different areas by determining a sequence of nodes from the first node to the second node, in which at a current node the next node in the sequence is determined from a link to that node from the current node, where only links to border nodes or to the second network node are considered in determining the next node.
PCT/EP2012/052491 2012-02-14 2012-02-14 Method and apparatus for routing in a multi-area communication network WO2013120512A1 (en)

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