Abstract:
The invention relates to a computer implemented method of identifying in a network of interconnected devices a path through the network from a source device to a final destination device, the path comprising a connected sequence of devices, the method comprising at a monitor computer connected to the network: identifying a first device connected to the source device; transmitting a first query to the first device, the query including a destination identifier and requesting identification of an egress port for messages addressed to the destination identified by the destination identifier when the query is received at the first device; receiving a result message identifying the egress port and identifying the second device connected to the first device based on a network topology accessible by the monitor computer; and addressing a next query to the second device and receiving a next result message identifying an egress port from the second device; and identifying from the network topology a third device connected to the second device, wherein the path is identified to include the first, second and third devices.

Description:
BACKGROUND 
       [0001]    The present invention relates to identification of a path in a network of interconnected devices. 
         [0002]    Computer networks form the basis for IT (Information Technology) infrastructure in a wide variety of contexts. Such computer networks comprise interconnected devices of various types. The purpose of the network is to support the flow of messages between those devices so as to deliver information, applications and services, etc., over the network. A number of techniques are available for managing a network. In this context, managing a network includes monitoring the network to identify failure points and other problematic areas, such as hotspots, and providing information to administrators and users of the network to allow the problems to be fixed. There are a number of tools available for providing a network topology. The topology of a network identifies how devices in the network are physically or logically connected to each other. Thus, any particular single device may have one or more connections to a neighbour device. Computerised tools which “discover” a network are available, and they create network topologies which define the interconnection of the devices in the network, and the nature of those devices. 
       SUMMARY 
       [0003]    Determining network topology can be done in many ways. Techniques that can all be utilised separately or in combination to give a good representation of network connectivity include, for example:
       Cisco Discovery Protocol (CDP)   Link Layer Discovery Protocol (LLDP)   SynOptics Network Management Protocol (SoNMP)   Spanning Tree Protocol (STP)   IP Traceroute   IPv6 Neighbour Discovery   user additions/modifications/deletions       
 
         [0011]    Knowing the topology of a network is extremely useful, but it does not provide a solution to all of the problems which can occur. Networks are increasingly used to provide the infrastructure for supporting the delivery of applications and services between remote geographical locations, either over long distances or in extremely complex networks with a large number of interconnected devices. Increasingly, network administrators and users are interested to know not necessarily the full details of the network, but to understand the performance of the delivery of applications and services over a network. Thus, so-called “end to end” monitoring is becoming increasingly popular. With “end to end” monitoring, applications involving message flow from a source device to a destination device have their performance monitored as they are delivered between that source and destination device. The performance parameters can be used to estimate or guess at possible failures in the network, although they do not provide any specific information about the location of those failures and therefore do not point directly to a solution. 
         [0012]    Often a source device is a server providing a particular service, and the destination device is a client terminal which is connected to the server via the network and which requires to use that service. The term “device” used herein is intended to cover any devices which can be connected in a network. The term “server” is used to denote a device which is responsible for delivering a service or application, and the term “client” is used to denote a device (either user-based or another dependent machine or server) which depends on that application or service. 
         [0013]    A significant difficulty in guessing where a problem might lie when it can be seen that an application&#39;s performance is deteriorating is a lack of understanding about the path through the network that message flow for that application might have taken. Networks depend on many types of network devices (e.g. routers, switches, firewalls, load balancers, etc.) to connect their endpoint devices, such that it is extremely difficult to say for any given source endpoint how the message from that endpoint will be routed through the network to a given destination endpoint. The complexity of such path determination is exacerbated by the usage of multiple alternating paths, redundant paths, load balancing, etc. 
         [0014]    Attempts have been made to predict how a particular packet will be routed through a network. Such predictions are based on a complex model of the network topology together with indications on a per device basis as to how a particular device will behave in the network. Network devices can be highly sophisticated, and there have been developed a large number of complex algorithms to determine a routing strategy at any particular device. Moreover, that routing strategy can depend on traffic and other environmental considerations which affect the network (such as failure of other devices, etc.). Complex algorithms which can be applied by a device to determine a routing strategy can include for example:
       Ingress interface and ingress interface technology   Packet headers (L2, L3, MPLS, ATM, etc)   Static and directly connected routes   Shared routing tables (full knowledge of BGP, OSPF, RIP, EIGRP, etc—active neighbours, link states, route costs, route weights, etc.)   Learned MAC forwarding tables   Access control lists   Network overlay technologies (e.g. MPLS, 802.1q VLANs), etc.   Loop avoidance technologies—e.g. PVSTP   Tunnelling protocols (MPLS, IPSEC, SSL, GRE)   Load balanced/redundant links   Default gateways       
 
         [0026]    However, even if it is in principle possible to predict where a given packet will be forwarded to next at a particular device, this requires a vast amount of data which is slow to gather, and can be out of date within seconds due to the real time nature of the operation of the routing devices. Furthermore, the mere acquisition of this data can place a significant load on both network devices and networks. 
         [0027]    In addition, models required to simulate modern routing devices are extremely complex and, without the full model, their behaviour cannot be correctly predicted. To keep the model complete, it is necessary for such solutions to be regularly and rapidly updated to include developments in routing technologies. 
         [0028]    With the novel approach detailed here, network devices are queried as to what they would do with a hypothetical packet (as opposed to querying for routing protocol specifics). The routing protocols which feed into the devices&#39; routing and forwarding tables need not be known or maintained. 
         [0029]    According to an aspect of the present invention, there is provided a computer implemented method of identifying in a network of interconnected devices a path through the network from a source device to a destination device, the path comprising a connected sequence of devices, the method comprising at a monitor computer connected to the network: identifying a first device connected to the source device; transmitting a query to the first device, the query including a destination identifier and requesting identification of an egress port for messages addressed to a destination identified by the destination identifier when the query is received at the first device; receiving a result message identifying the egress port and identifying the second device connected to the first device based on a network topology accessible by the monitor computer; and addressing a next query to the second device and receiving a next result message identifying an egress port from the second device; and identifying from the network topology a third device connected to the second device, and so on, wherein the path is identified to include the first, second and subsequent devices. 
         [0030]    The present invention utilises a novel approach to identifying the paths taken through a network of interconnected devices for a particular message flow. The concept relies on using a minimum amount of data gathered “up front”—specifically just the static network topology and end-host location (which clients and servers are connected to which access/edge switches), and gathers whatever else is required on the fly and highly selectively as required for such highly dynamic data. For modern dynamic environments, the ability to compute end-to-end path now, that is, in real time, has wide applicability. The gathering of the data and it&#39;s processing needs to be very quick for the algorithm to be of practicable value when used with large scale, real-world networks. 
         [0031]    Behaviour at a particular device is termed “per hop behaviour” (PHB). PHB on its own cannot provide an end-to-end path. However, knowing that a packet leaves the device on a specific interface can be of value if it is not known which device and interface are connected to that interface. By using network topology coupled with PHB, direct computation of an end-to-end path through the network for an application flow can be accomplished. 
         [0032]    Thus, the network topology includes both network device interconnectivity and end host location. In turn, the source and destination devices used to seed the path finding algorithm are determined by the message flow of interest. Finally, the specific path taken by a hypothetical packet between source and destination devices is computed dynamically. 
         [0033]    The query which is transmitted to each device is adapted to query each device to determine the identification of an egress port which represents the egress ports that the device would use for a hypothetical message addressed to a destination identified by the destination identifier. Note that the destination identifier for any given query may or not be the terminal device&#39;s destination identifier depending on the location in the network of the device being queried. This can be achieved when the device is a router by querying what is in its active routing table at the time at which the query is received. 
         [0034]    The query itself can be accommodated in a message or signal transmitted from the monitor computer to the device which is being queried (focus device). The query message or signal does not constitute the message flow for which the path is to be determined. Instead, each query contains a destination identifier which queries the focus device to find out how the focus device would handle a hypothetical message addressed to that destination if it had to make the decision at the time the query was received. Thus, the focus device returns a result which identifies an immediate egress port which would have been used at that time for a real message addressed to that destination. The queries can be transmitted while the network is active and while the message flow is in place. However, they can also be transmitted when the message flow itself is not active—the technique can be used in either context. 
         [0035]    Where the query is in the form of a message or packet, for example, the message can be an SNMP message with a destination IP address, it will carry its own destination address and be delivered over the network from the monitor computer to the focus device. In that case, the destination address of the query message is that of the focus device. This is not the same as the destination identifier which is included in the query itself. In an alternative arrangement, a query signal or signals can be sent from the monitor computer through direct connections to the focus devices, such as through a CLI or XML API mechanism. 
         [0036]    The method described herein enables a number of useful network analysis techniques. It allows for on-demand path determination so that an administrator trying to determine the path for a particular application can more or less instantaneously ask the monitor computer and receive a result of the path. 
         [0037]    It enables multiple path discovery. That is, due to changes in the environment in the network, routing devices can route a message flow differently depending on those changes. Thus, a first set of queries to identify a path might record a first path, whereas a second set of queries might identify a second path, even when the first and second set of queries are very close to each other in time. Information about multiple paths between common endpoints (that is, the same source device and same destination device) can be presented graphically or visually to show a user not only the nature of the path, but the percentage of time each path is adopted for a particular message flow. This can be readily achieved because the queries themselves do not represent a significant overhead to the network, and therefore multiple sets of queries can be despatched without significantly affecting performance. 
         [0038]    The method allows for the detection of rapid legitimate path change. That is, an adjustment to the network can cause the path to change and this can be detected and flagged to a user at a visual graphical user interface. 
         [0039]    Where there are multiple paths between common source and destination devices, the paths can carry different latencies. Sometimes, a routing device performing intelligent routing can cause a phenomenon known as “route flap” where a particular message flow continually switches from path-to-path. It can be useful for a network administrator to identify these occurrences due to the implications of such path changes on end to end latency and the implications of such “jitter” on Voice over IP telephone conversations, for example. 
         [0040]    The method can be used to locate path failure. That is, in the preferred embodiment of the method, queries are despatched and results received and analysed to identify the next device until a device is identified as the destination device. Sometimes, however, there is a failure in the network, such that the network would not deliver the message flow to the destination device. The method allows for identification of that situation by working along an end to end path until the path can traverse no further and this network location can then be notified to an administrator. 
         [0041]    Furthermore, the method can allow for the possibility of restarting at a subsequent device in that path, using estimates based on the network topology. The path identification method can then be adopted again until the destination device is reached from the point of failure. In this way portions of the network for which the monitoring computer has no visibility (e.g. devices which have no appropriate management interface, or which belong to a different organisation) may be circumnavigated and the path analysis continue. 
         [0042]    The method also allows for asymmetric routing identification. It is not uncommon for message flow between a source device and a destination device to adopt different paths depending on its direction. That is, a forwards path can be utilised from the source device to the destination device for the message flow, and a return path from the destination device to the source device which is different. 
         [0043]    The path is recorded in a memory or store at the monitor computer or accessible by the monitor computer. The path record comprises a set of connected devices and interfaces. This can be presented in the form of an ordered inventory of the devices (network components) between the two endpoints. This enables network path availability monitoring, including event notification, reporting, SLA&#39;s (Service Level Agreements); proactive network management including reporting on failing devices, high device CPU, low device memory, port congestion, etc., and impact analysis (capacity planning, “what if” analysis). 
         [0044]    It is a significant advantage of aspects of the present invention that a mapping between the application or service delivered by the network and the network devices or components themselves can be ascertained through the identification of the path. This represents a significant step forwards in the management of networks. 
         [0045]    According to another aspect of the present invention there is provided a monitoring computer system for identifying in a network of interconnected devices a path through the network from a source device to a destination device, the computer system comprising: an interface connected to the network for transmitting queries and receiving responses; a processor operable to execute a path identification tool which carries out the method as herein defined; a first storage means for storing the path record; and a second storage means storing the network topology. 
         [0046]    A further aspect of the present invention provides a computer program product which comprises computer readable instructions which when executed by a processor implements the method as herein defined. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0047]    For a better understanding of the present invention and to show how the same may be carried into effect, reference will now be made by way of example, to the accompanying drawings, in which: 
           [0048]      FIG. 1  is a schematic diagram of a network; 
           [0049]      FIGS. 2   a  to  2   c  are a diagrammatical illustration of a path discovery algorithm in process; 
           [0050]      FIG. 3  is a flow chart for a path discovery algorithm; 
           [0051]      FIG. 4  shows one discovered path; 
           [0052]      FIG. 5  is the structure of a linear routing table; 
           [0053]      FIG. 6  illustrates a set of results arising from combining a destination address with multiple route masks; 
           [0054]      FIG. 7  shows a structure of an ARP table; 
           [0055]      FIG. 8  is a schematic diagram of a monitor computer; 
           [0056]      FIG. 9  is a schematic diagram of a layer 3 router; 
           [0057]      FIG. 10  is a schematic diagram of a layer 2 switch; and 
           [0058]      FIGS. 11A to 11D  are a flow chart of a utility executed at the monitor computer. 
       
    
    
     DETAILED DESCRIPTION 
       [0059]      FIG. 1  is a schematic diagram of a network. The network extends over a number of different geographical locations. At each end geographical location there are endpoint devices and network devices or nodes. The network devices include routers and switches. The core of the network comprises a plurality of network devices. Considering the geographical location marked London, client terminals  2  can act as endpoint devices. Similarly, a server  4  can act as an endpoint device and the printer  6  can be considered an endpoint device. Similar devices are shown in the Paris and New York geographical locations with different layouts (New York showing a server farm or data centre). Note that at the New York location a plurality of servers  8  represent key application or service endpoint devices. 
         [0060]    It should be appreciated that the network shown in  FIG. 1  is given by way of one example. There is a wide variety of possible networks and the present invention can be used in any network of interconnected devices. In particular, the nature of the endpoint devices and specific network devices or nodes can vary. In the particular network which is disclosed, the network devices can be layer 3 or layer 2 devices. 
         [0061]    The OSI (Open Systems Interconnection) model defines seven layers within which communication systems&#39; protocols can be characterised. The path finding algorithm described here computes network paths using information available at layers  2  and  3 . 
         [0062]    Devices operating at layer 2 (the Data Link Layer) have knowledge of immediately adjacent devices and have responsibility for getting packets from one layer 2 device to the next layer 2 device (based on layer 2 MAC (Media Access Control) address). 
         [0063]    Devices operating at layer 3 (the Network Layer) are responsible for propagating packets from one point in a network to another point in the network—often many tens or hundreds of devices apart. To compute which devices should participate in a given layer 3 path (herein referred to as the layer 3 hops), layer 3 devices exchange routing information and use routing protocols to compute the most desirable path(s). To pass packets between consecutive layer 3 devices in a path, devices operating at layer 2 are used; often with many layer 2 devices (herein referred to as the layer 2 hops) between each layer 3 device. 
         [0064]    Thus large networks are effectively subdivided into multiple segments, each typically containing multiple layer 2 devices, connected by layer 3 devices. 
         [0065]      FIG. 9  is a highly schematic diagram of a layer 3 routing device. The device comprises a controller  90  for example, in the form of a microprocessor running controlling code, firmware or any other suitable implementation. The controller  90  can access a routing table  92  which is discussed in more detail later with reference to  FIG. 5 . The layer 3 routing device has ports Pi/Po. Each port is connected to a physical link as illustrated in the network of  FIG. 1 . In this notation, Pi denotes an “ingress” port and Po denotes an “egress” port. This is for convenience of notation, in practice, devices do not usually have ports which are dedicated as ingress or egress ports—whether they are ingress or egress depends on the data they are transferring at the time. Most ports function as egress and ingress all of the time. 
         [0066]    Packets arriving at an ingress port Pi can have their destination identifiers, e.g. IP (Internet Protocol Addresses) read by the controller  90  via a bus  94 . The controller  90  accesses the routing table  92  and based on information derived therefrom controls a routing switch  96  to which the incoming packet is directed. The routing switch  96  then routes the incoming packet to an appropriate egress port Po depending on the information in the routing table. The routing device includes a mapping table  91  which maps layer 3 to layer 2 addresses for subsequent routing. Operation of such routing devices are known in the art and so will not be described further herein. It is noted in this context that the routing table can be queried by packets from the monitor computer arriving over the links into the ingress port Pi by intercepting such packets at the controller  90 . Such query packets are not supplied to the routing switch  96  for further routing, but instead generate a response which is output from the routing device and returned to the enquiring entity over the network from an egress port. In this case, that enquiring entity is the monitor computer  16 . All packets conveyed over the network (including query packets) contain a source and a destination address—the query packet has a source address corresponding to the monitor computer and a destination address corresponding to the device being queried. When the reply needs to be sent, the source and destination addresses are swapped to make the source address the device being queried and the destination address the monitor computer. 
         [0067]      FIG. 10  is a highly schematised version of a layer 2 switch. Similarly to a layer 3 routing device, the layer 2 switch has ports Pi/Po, each connected to a physical link as shown for example, in the network of  FIG. 1 . As mentioned earlier, ports are not usually dedicated as input or output. Incoming packets at an ingress port Pi are directed to a switch  100  which can access a layer 2 forwarding database (FDB)  102  for determining how to route the packets based on destination identifiers (normally headers) in the packets. A layer 2 forwarding databases maps an incoming packet&#39;s identifier to an egress port on which the packet should be forwarded. As already explained above, according to the OSI model the identifiers for the layer 3 routing devices are IP addresses, whereas the identifiers for the layer 2 devices are MAC addresses. 
         [0068]    As with the layer 3 devices, layer 2 are known in the art and thus will not be discussed further herein. However, it is noted that again like the layer 3 devices they can receive a query in a packet at an ingress port Pi and generate a response to that query to the output from the layer 2 switch at an egress port Po. Thus the query packets themselves are not routed at the switch but instead generate a response which is returned to the querying device, in this case the monitor computer  16 . 
         [0069]    A switch controller  101  at the switch is responsible for forwarding traffic and for generating responses. 
         [0070]    Some more recent devices can perform the function of layer 3 and layer 2. 
         [0071]    The following described embodiments of the present invention provide a method of identifying a path taken by a message flow between a given source device and a given destination device. For example, endpoint X could be considered a source device and endpoint Y could be considered a destination device. Faced with a network of  FIG. 1 , it is, as already discussed above, it is a far from trivial task to establish what path will be adopted through the network between those endpoints at any given time and under any given set of environmental conditions.  FIG. 1  shows a monitor computer  16  which executes a path discovery program which allows such a path to be discovered and recorded.  FIG. 8  is a highly schematic version of a monitor computer  16 . The computer  16  comprises a microprocessor  80  which can access memory  82  in which is stored code for execution by the processor. In the present case, the code includes the path discovery program. The memory  82  also stores a path record  81  as it is created by the path discovery program. The computer has a user interface  84  which can include a user input device such a mouse or keyboard and a display for displaying information to a user. In particular, as discussed in more detail herein alerts following the path discovery program or information concerning the path discovery program can be displayed to a user at the user interface  84 .  FIGS. 2   a  to  2   c  illustrate steps of the path as will now be described. 
         [0072]    At a high level the algorithm uses the notion of a “focus device” which is the device currently being queried as to where it would send a hypothetical packet next (i.e. which interface would it send the hypothetical packet out of). Starting at the source device the algorithm travels towards the terminal device (i.e. the ultimate destination of the packet) by evaluating each focus device in turn—if the device is operating at layer 3, it is queried for which interface (egress port) it would use to send packets bound for the next hop at layer 3 (NHL3); if the device is operating at layer 2, it is queried for which interface (egress port) it would use to send packets bound for the next layer 3 hop&#39;s layer 2 (MAC) address (NHL2). Using the focus device&#39;s response in conjunction with a network topology, the next device in the path can be determined. In this way the algorithm works along the layer 3 path, using layer 2 devices to navigate between the consecutive layer 3 nodes. 
         [0073]    Before commencing the prime algorithm, the source device and terminal device are located. This may not be straight forward and techniques for achieving this are discussed later. 
         [0074]    According to the prime algorithm, the first hop is located. The path is seeded and the loop count is set to zero. The loop limit governs the number of times a path identification loop (discussed later) is executed. 
       Find First Hop at Layer 3 
       [0075]    The first hop is located by finding the initial next hop (the next hop from the source device) at layer 3 (NHL3). In the following explanation, the term “query” is frequently utilised. Queries are generated and structured as described in more detail later. The purpose of a query is to locate a next hop address and egress port from a focus device to which the query is addressed. The initial NHL3 address can be determined by firstly querying a source device X using the destination IP address. That is, an attempt is made to query the routing table on the source device for NHL3 and the egress port. If no route is found, and the source device has a layer 3 access switch, this Layer 3 access switch is queried for NHL3 using the destination IP address. If that is not successful, the default gateway on the source device is queried to ascertain the NHL3. If that is not successful a query is made using the destination IP address to the access switch for the default gateway. If no NHL3 address is found, this is considered as a fail. This does not mean that the algorithm has failed, but that a fail point in the path may have been identified at this point. Alternatively, there may be other reasons why no NHL3 has been found. 
       Seed the Path 
       [0076]    To seed the path, the source device is added to the path when it has been located. The egress interface of the source device is located and added to the path. If NHL3 is found from the routing table on the source device the source device egress interface for this NHL3 address is added to the path. As explained later, the layer 2 address (NHL2) corresponding to the layer 3 address (NHL3) can be ascertained. If no egress port for NHL3 is found from the routing table on the source device, the layer 2 forwarding table on the source device for NHL2 is used to find the egress port. If that is found, then that egress port is added to the path. 
       Overview of Path Discovery Algorithm 
       [0077]    The query dispatched from the monitor computer  16  to the source device X is shown as a direct arrow in  FIG. 2   a , but in fact, could be implemented in the network of  FIG. 1  by the monitor computer  16  issuing a message or packet addressed to the source device X. As explained, the query asks the source device for the next hop IP (and egress port) for the terminal IP (destination IP), which is the layer 3 address of the destination point Y. The aim is to cause the source device X to supply a response which includes NHL3 and the egress port for NHL3 (the terminal IP address). See step S 1  of  FIG. 3  and  FIG. 2   a.    
         [0078]    As explained above, there may be situations when the source device cannot supply the necessary information. Other possibilities mentioned above for obtaining the first “focus” device include querying the connected access switch for layer 3 routing information (in case the access switch is a layer 3 switch), if this fails the algorithm queries the connected access switch for a default gateway and the IP address of the default gateway used as the first NHL3. 
         [0079]    At step S 2 , the next hop layer 2 (MAC) address is resolved from the NHL3 address and NHL2 is set to this MAC address. This can be achieved by querying a mapping table  91  which maps L3 to L2 addresses. One such mapping table is an ARP table (others include “direct mapping” and Neighbour Discovery). This can be the source device ARP, next L3 hop device ARP or global cached ARP using an ARP query described later. The egress port identified in step S 1  is added to the path record S 1 A. At step S 3 , the starting network switch (and port) is found using cached end host location (from switch CAM queries), and set as the focus device. At step S 4 , the terminal network switch is found using the cached end host location (from switch CAM queries). The starting switch is added to the path record. 
         [0080]    The method is now ready to enter a path identification loop. At step S 5  it is determined whether NHL2 is known. If it is, the loop moves to step S 5 A. If it is not, the process carries out step S 5 B to resolve NHL2 by an ARP query on the focus device or the NHL3 device. Generation of a query to correlate a layer 3 address with a layer 2 address is discussed in more detail later with reference to  FIG. 7 . In brief, for the device being queried, a list of interface indices (ifindices) is obtained from the network topology or by walking if Index from the interface table of device itself. Each if Index for the device is combined with the NHL 3 address to generate a set of keys for including in the query to the device. Thus, a query containing these keys is formulated and transmitted to the focus device. The focus device produces either zero or one successful response. 
         [0081]    If the previous two techniques for resolving NHL2 fail, global ARP is accessed. At step S 5 A, it is determined whether or not the address NHL3 is on the current focus device. 
         [0082]    If NHL3 is not on the current device, at step S 6 , the process dispatches a query to find the layer 2 FDB entry for NHL2 to get the egress port. Generation of a query at layer 2 is discussed later. If it is successful, the egress port is added to the path record (S 6 A), the cached topology  3  is used to find the port and the device on the end of the link (S 7 ), the device is added to the path (S 7 A), and the focus device is set to the device which has just been located at the end of the link (L2 HOP). Steps S 6 A, S 7  and S 7 A can be referred to as an L2 HOP. At this point, refer to  FIG. 2   b . At step S 5 A, the focus device is device A. This receives a query to find the layer 2 FDB entry and returns the egress port. The device which is determined to be at the end of that link is device B ( FIG. 2   c ) which receives a query with NHL3 still set to the destination IP address. 
         [0083]    If a layer 2 FDB entry was not found, or if at S 5 A it was determined that the NHL3 was hosted on the focus device, at step S 8 , a route query is performed to determine if the L3 route is found on the focus device to the destination IP address. The route query can be a single route or recursive route query—these are discussed later. This establishes a next hop IP and an egress interface. If the L3 route is not found, a broken path is indicated and the process stops—S 8 A. At step S 9  (L3 HOP) the routing table egress interface is added to the path, NHL3 is set to the new next hop IP address, and the process queries the device to ascertain the layer 2 address of NHL3. If NHL2 cannot be resolved, NHL2 is set to “unknown”. 
         [0084]    At step S 10 , the current NHL3 address is compared with the destination IP address. If NHL3 is not the destination IP (that is the path identification algorithm is not yet on the final L2 segment), at step S 11  the cached topology is used to find the port and device on the end of link, the device is added to the path record and the focus is set to this device. The process then queries (S 12 ) if the focus device is the terminal device. If the focus device is not the terminal device, the process returns to step S 5 , but using the NHL3 and NHL2 set at step  9 . 
       Termination 
       [0085]    The algorithm ends when the terminal device is reached and the terminal port and destination server are added to the path. Other termination conditions prevent the algorithm looping indefinitely. In each iteration of the path, an iteration begins by setting a switched flag to false and a routed flag to false. When a L2 Hop occurs (S 7 ) the switched flag is set to true; when a L3 Hop occurs (S 9 ) the routed flag is set to true. As already mentioned, the egress port is determined from a focus device and the network topology is used to find the attached device and ingress port of the attached device. For each iteration, the combination of:
       “Focus device, NHL2, NHL3” is stored.       
 
         [0087]    If the focus device, NHL2 or NHL3 have changed and the new combination of “focus device, NHL2, NHL3” has been seen before, a loop detected event is raised and the loop is stopped. If the loop limit has not been reached, and either routing or switching has occurred (i.e. either the routed or switched flags are true) and the focus device is not equal to the terminal device, iterate again. Each time it is assessed if the iteration loop limit has been reached. If it has, the algorithm terminates. 
         [0088]    When iteration ceases, if the focus device is the terminal device, the terminal device is added to the path. If the focus device is not the terminal device, but the algorithm has stopped, an error is reported as the path finding algorithm will have terminated at an unexpected location. If the terminal device is an access switch, the access switch egress port is added from “locate destination” (S 4 ) to the path and the destination device derived from the access switch egress port is added to the path—the algorithm then ends. If the terminal device equals the destination device, the algorithm ends. The detail of the algorithm will now be discussed in more detail. 
       Specific Example 
       [0089]      FIG. 4  shows one result of operation of the path identification algorithm. That is, it provides the route for which a data packet from source device X addressed to destination device Y would take over the network at the time at which the path identification algorithm queries the network. The path is shown to include devices A-J which form part of the path record. The path record includes the ingress and egress ports from each of those devices. 
         [0090]    Looking again at the original network of  FIG. 1 , the first part of the path record shown in  FIG. 4  can be seen to be derived from the network of  FIG. 1 , where corresponding letters have been used to denote the devices selected by the previous switch or routing device. When the path identification algorithm operated, routing device B had determined to send the packet to switch C. However, without use of the present invention, it would have been extremely difficult to work that out in real time. The routing device B similarly had an option to route the packet to router F in the core network. By querying the routing device B in real time (or more or less real time), based on hypothetical packet addressed to destination Y, the routing device B returns the decision that it would have made if a real packet with that address had arrived. By ascertaining that routing device B will despatch the packet to switch C, and then establishing that switch C has connected on the far end of its egress port routing device D, C and D have been added to the path record  81 . In this way, the packet identification algorithm has stepped through the path that the hypothetical packet would have taken at the time that the path identification algorithm queries the devices in the network. The box adjacent routing device D denotes the settings for NHL3 and NHL2, that is, NHL3 is set to the IP address of device E which has been established as the far end device for routing device D based on routing table currently active on D, and the NHL2 has been established as the MAC address for device E by querying device D for its ARP entry for device E. 
       Network Topology 
       [0091]    As mentioned previously, the network topology includes both network device interconnectivity and end host location. Network topology  3  can be provided by a topology server which provides details of port-to-port connections. Thus, when an egress port is identified at a device, the ingress port of the connected device can be ascertained using port-to-port connection identified in the topology. Both egress and ingress ports can be added to the path record. The topology server also provides a global CAM, a global ARP and device credentials. Furthermore, for each device recorded in the topology there is preferably an interface index (If Index) list, and a VLAN (Virtual Local Area Network) list. VLAN devices have not been discussed yet. They are discussed further herein. When a response is returned to the monitor computer  16 , the monitor computer queries the topology  3  in the following order when handling layer 2 responses. In this context, a layer 2 response is a response which has identified an egress port from a layer 2 switch device. The order of querying is CDP, LLDP, STP and SONMP, IPv6 ND. 
       Locate Source Device 
       [0092]    As mentioned earlier, location of the first device in the path (the device connected to the source device) is not necessarily straight forward. In one embodiment, the monitor computer  16  implements the algorithm to firstly try to find the source as a connected host and if that fails it tries to find the source as a network device. When attempting to find the source as a connected host, it queries the source device for the layer 2 (MAC) address for the source IP. This can be accomplished in the same way as the query on a focus device as described above at step S 5 B. That is, the process dispatches a query to find the ARP entry for the source IP address. 
         [0093]    If there is no layer 2 address from the source device, the global cached ARP table at the topology server is queried. In the described embodiment, these are referred to as ARP tables, but any tables which map layer 3 to layer 2 addresses can be utilised. 
         [0094]    If a MAC address is found which corresponds to the source IP address the topology server is queried for the source IPs MAC location by querying global cached layer 2 forwarding tables on the topology server to find ports which have seen traffic from this MAC address. The topology server is expected to return an unique source MAC location by removing multiple matches (the source MAC seen on many ports), by filtering out ports flagged as trunks, ports with excessive numbers of MACs (access switch ports&#39; FDB entries typically have a single ‘seen’ MAC address), ports with inter-network topology (e.g. if a port has CDP adjacency information it cannot be a port on an access switch), etc. 
         [0095]    If the source cannot be found as a connected host, attempts are made to find the source as a network device. This can be achieved by querying the topology server for all IP addresses found on all managed network devices to see if the IP address is on a network device. If it is, that network device is set as the focus device. 
       Locate Destination Device 
       [0096]    Similar considerations apply to location of the destination device. Firstly attempts are made to find the destination device as a connected host, and if that fails, attempts are made to find the destination as a network device. To find the destination device as a connected host, the destination device is queried for its layer 2 address, or global cached layer 3 to layer 2 mapping tables are queried on the topology server (similarly as for the source device discussed above). Then query global cached layer 2 forwarding tables on the topology server to find ports which have seen traffic from this MAC (again, as described above with reference to the source device location). 
         [0097]    To find the destination as a network device if the above fails, the topology server can be queried for all IP addresses found on all managed devices to see if the IP address is on a network device. The network device can then be set as the terminal device. 
       Per Hop Utility 
       [0098]    In order to implement the path identification algorithm, the monitor computer  16  executes a computer program as discussed. This computer program provides a utility which handles “per hop” queries. That is, the identification algorithm relies on dispatching a query from the monitor computer to a focus device and receiving from the focus device an egress port which can be used to access the topology. This cannot necessarily be achieved by a single query. As described above, the algorithm requires an initial next hop at layer 3 (NHL3). The utility attempts to query a routing table on the source device for NHL3 and the egress port, using the destination IP address. If no route is found, it queries the routing table on the access switch in case it is a layer 3 switch (which is the first device connected to the source device for NHL3). If no route is found the source device is queried for the default gateway for NHL3. If no route is found, the first device is queried for a default gateway. To query a routing table to find NHL3 (as described above), a route is found for the IP address in question (the ‘sought’ IP address) by querying the routing device using a speculative keying technique discussed later. If the route is found but no egress port is specified, the next hop IP address is returned and is used as NHL3. If the route is found with an egress interface if Index greater than zero, the egress port is returned with the NHL3 address and the egress port is added to the path. If the route is found with the egress interface if Index equal to zero, the utility reiterates by setting the sought IP to the next hop IP (from the previous query) and finding the route for the sought IP by querying the device using speculative keying (as discussed later). This is repeated until the if Index returned is non-zero. 
         [0099]    The step of finding the route for the sought IP uses the speculative keying technique to return a route entry. If the route entry is found, the utility polls for the next hop address from ipRouteNextHop.NetworkAddress. The utility also polls for the egress interface from ipRoutelflindex.NetworkAddress and polls for ipRouteType.NetworkAddress. If ipRouteType is ‘direct’, the sought IP is set to the next hop, as an IP route type of direct indicates that it is directly connected to the network segment. 
         [0100]    It is possible that multiple matches will be returned from a routing table on a device. In that case, it is appropriate to determine if multiple routes are being used, for example, when a device is responsible for load balancing traffic. If only a single route is actively being used, the active route should be determined. If multiple routes are being used, the path could be split at this point and the path record could contain the results of the path finder algorithm applied to each and every route found from this point onwards. In many cases, multiple options for routing at a device is indicative of a device which is intelligently routing based on various metrics. These metrics can also be queried and returned for recording at the monitor computer. 
         [0101]    The utility is also responsible for finding the initial next hop at layer 2 by querying the layer 3 to layer 2 mapping table  91  at the focus device. If the layer 2 address is not found, where the focus device is the source device, the utility queries the access switch (if it is a layer 3 switch it should provide a layer 3 to layer 2 mapping). If the layer 2 address is not found the utility queries the global cached ARP tables on the topology server  3 . A query for a layer 2 address on a device is carried out as explained above with reference to step S 5 B. 
         [0102]    If the NHL 3 address is not on the focus device, the utility polls the focus device for an egress port for the layer 2 address NHL2. The step of polling the focus device for the egress port NHL2 includes VLAN (Virtual Local Area Network) specific polling. That is, it includes the step of establishing which VLANs the device is participating in according to the topology  3  and as recorded at the device. These VLANs are used to help find forwarding table entries for specific VLANs (FDBs are often partitioned according to which VLAN they are related to—e.g. for Per VLAN Spanning Tree Protocol (PVSTP) it is necessary to perform the FDB queries in the context of each VLAN to try to find a match). 
         [0103]    If the egress port is not found from the layer 2 FDB (using either a specific VLAN or the native VLAN) then the utility tries to find which interface heads towards NHL2 from ARP records by polling for ipNetToMediaPhysAddress  71  ( FIG. 7 ). That is, the utility attempts to learn which interface the layer 2 to layer 3 relationship was learned from. 
         [0104]    Once the utility has found an egress port using the layer 2 address, it adds the egress port to the path record and uses the topology server  3  to find the remote port attached to the egress port. This remote port is recorded as the ingress port on the next device. 
       Port Channels/Multiplexed Ports 
       [0105]    If no remote port is found, or the egress port name mandates the use of higher or lower layer ports then the utility checks for lower layer ports or higher layer ports. That is, there can be a scenario where there is a mapping of virtual path outputs to physical ports. In order for the path identification algorithm to be successful, it needs to identify a physical egress port for accessing the topology server. In a scenario where the check for lower layer ports reveals the presence of lower layer ports, these lower layer ports can be used as the egress ports and the topology server is accessed to find the remote ports (ingress ports of next device) attached to the egress ports. At this point, the path is split into multiple separate paths, each of which is traced independently from this point on. 
         [0106]    If higher layer ports are identified, the higher layer port is used for the egress port. The topology server is used to find the remote port attached to this higher layer egress port. 
       Next Hop 
       [0107]    Set the routed and switched flags to false. Using either the topology server or direct queries to the focus device, ascertain whether or not the focus device hosts the NHL3 IP address on any of its ports. If it does host the NHL3 IP address then the utility then moves on to query the focus device routing table for routes to the destination IP by using the speculative keying technique. If the utility locates a candidate route, the next layer 2 address NHL2 is set by querying the focus device (or global cached ARP tables) for layer 3 to layer 2 mapping and the routed flag is set to true. If NHL3 equals the destination IP then that indicates that the utility has reached the last layer 3 device nearest the destination so there is no need to move off this device yet as the next hop would be a layer 2 hop. Therefore, the utility adds the candidate route&#39;s egress ports to the path. If NHL3 does not equal the destination IP, that indicates that it is not on the final layer 2 segment and the candidate route&#39;s egress port is added to the path. 
         [0108]    If no routing occurred during this iteration (the routed flag is still set to false), then the utility polls the focus device for an egress port for the layer 2 address NHL2. The step of polling the focus device for the egress port NHL2 includes VLAN (Virtual Local Area Network) specific polling (as described above). If the egress port is not found from the layer 2 FDB (using either a specific VLAN or the native VLAN) then the utility tries to find which interface heads towards NHL2 from ARP records by polling for ipNetToMediaPhysAddress  71 . That is, the utility attempts to learn which interface the layer 2 to layer 3 relationship was learned from. Once the utility has found an egress port using the layer 2 address, it adds the egress port to the path record and uses the topology server  3  to find the remote port attached to the egress port. This remote port is recorded as the ingress port on the next device. If an egress port is found using either FDB queries or ARP queries, the switched flag is set to true. 
         [0109]    If, when querying the topology server, no remote port is found, or the egress port name mandates the use of higher or lower layer ports, then a check for lower or higher layer ports is conducted as described above. If an egress port is found, it is added to the path, the device containing the port is added to the path and the focus device is set to the remote device. 
         [0110]    This “Next Hop” step is repeated until either a prescribed limit on the number of iterations is reached or the path comes to an end (i.e. neither switching nor routing occurred). 
         [0111]    If the process terminates at the previously identified terminal device and that device is an access switch, the egress port is added from “locate destination” to the path record, and the destination device is added to the path record. If the terminal device is the destination device itself, the utility ends. 
         [0112]      FIGS. 11A to 11D  show a flow chart of the operation of the utility executed at the monitor computer. 
       Load Balancer 
       [0113]    As mentioned above, if the focus device is the terminal device, the terminal device is added with the destination to the path record. If the terminal device is a load balancer then the virtual IP to server pool mapping for the load balancer is obtained. This allows the server to physical server mapping for the load balancer to be identified. The path is retained as far as the “root” path (until the load balancer device). Then, for each physical server IP address, an additional path discovery utility is run from the load balancer to the physical server IP address, with each additional path pre-pended with the “root” path. 
       Routing Table Query 
       [0114]    One of the factors which make the path algorithm particularly efficient is the ability to generate a query to a routing device efficiently, that is to generate a query to which the routing device can respond in a short amount of time without significant overhead.  FIG. 5  illustrates the structure of a linear route table addressable via SNMP. To establish a route to a particular destination, ipRouteDest is the required index into the route table. This is denoted by  48  in  FIG. 5 . The entries of interest in the table are ipRouteIfindex  50  which defines the egress interface, ipRouteNextHop  52  which defines the IP address of the next hop (next hop IP) and ipRouteType  54  which defines the type of routing entry (invalid/direct/indirect). Access into the table normally requires knowledge of the ipRouteMask  56 : this would allow a specific network IP address to be located. However, as can be seen in  FIG. 5 , the IpRouteMask itself is embedded in the ipRouteEntry and therefore is not known to set up in the query. What is required is to find a match for:
       &lt;IP of interest&gt;&amp; &lt;ipRouteMask.X&gt;==&lt;ipRouteDest.X&gt;
 
in order to find the IpRouteDest key  48  which represents the index to the table.
       
 
         [0116]    As observed by the inventors, there are only 33 possibilities for the IpRouteMask (/32 . . . /0), that is 255.255.255.255, 255.255.255.254, 255.255.255.252, . . . 0.0.0.0. A number of these produce duplicate network ID&#39;s for the same IP address, because of the number of zeros in the IP address.  FIG. 6  shows the application of all 33 netmasks to the IP address 10.44.1.213=OA.2C.01.D5=0000 1010 0010 1100 0000 0001 1101 0101. 
         [0117]    This generates 12 unique values (labelled 32, 31, 29, 27, 25, 24, 23, 13, 12, 10, 6, 4). Thus, it is now only necessary to make 12 SNMP queries (which can be presented in a single query packet) to find the route. That is, the 12 results are matched into the route table of the focus device and when a match is found the required elements ipRouteIfIndex, ipRouteNextHop and ipRouteType are retrieved and returned in a response to the monitor computer  16 . 
         [0118]    The reduction in the number of queries required to find the route is termed herein “speculative keying” and enables the undertaking of real time route table querying in a very efficient manner. 
         [0119]    When examining real routing tables, it is not uncommon for the route found for a given IP address not to have a valid egress interface and to only provide a next-hop address. In these cases the next hop address is used for a subsequent query of the routing table to try to obtain an egress interface for that next hop address. This re-using of the next-hop address is repeated until an egress interface is obtained. 
         [0120]    According to this approach, in a first step a find single route query uses speculative keying to find a routing entry for the specified IP address (IP X ) as just outlined. If the associated ipRouteType is “direct”, IP X  (and ipRouteIfIndex X ) are returned in a response to monitor computer as the next hop. That is, it is directly connected and therefore has no layer 3 next hop. 
         [0121]    If the associated ipRouteType is not direct, ipRouteNextHop and ipRouteIfIndex are returned in response to the monitor computer. 
         [0122]    In the FindRecursiveRoute expansion, the FindSingleRoute step is taken for the required IP address (IP X ). If no route is found, a failure is returned. If a route is found, but there is no egress interface, ipRouteNextHop is returned. If the route is found and ipRouteIfIndex is greater than zero, ipRouteNextHop and ipRouteIfIndex are returned. If the route is found and the ipRouteIfIndex equals zero, then a subsequent FindRecursiveRoute step is taken for the IP address of ipRouteNextHop, with the same four possible outcomes. 
         [0123]    While speculative keying is a particularly good technique for efficient querying of large data sets, its main applicability is when querying data which is indexed with a derived key for which a partial key is already known. That is why it is particularly useful in the context of SNMP route table analysis and SNMP ARP table querying. However, rapid per network device forwarding behaviour can also be ascertained using other querying techniques, for example, CLI access and XML API. 
       ARP Query 
       [0124]    Reference will now be made to  FIG. 7  to describe an efficient technique to query an ARP table using speculative keying. Generation of a query is discussed in more detail later with reference to  FIG. 7 . For the device being queried, a list of interface indices (If Indices) is obtained from the network topology or by walking IfIndex from the device itself. Each if Index for the device is combined with the NHL 3 address to generate a set of keys for including in the query to the device. Thus, a query containing these keys is formulated and transmitted to the focus device. The focus device produces either zero or one successful response.  FIG. 7  illustrates an ipNetToMediaEntry table format which would in principle allow the MAC address to be determined for any given IP address. Since a unique entry cannot be found for a specific IP address unless it is known which interface the ARP entry was learned from, speculative keying is used by combining the IP address with each and every if Index on the device. That is, each query key can be created by combining the IP address with an ifIndex. In this way the number of SNMP queries is the number of interfaces on the device which is typically far fewer than the number of ARP entries on the device and thus is significantly more efficient. 
         [0125]    In speculative keying, multiple query keys can be contained in a single query message. 
       Additional Technologies/Protocols 
       [0126]    The path identification algorithm when utilised above provides an effective way of identifying a particular path which a particular packet or message is likely to take through the network of interconnected devices operating according to generally known network protocols. Situations do arise where for one reason or another, the path identification algorithm meets a particular challenge. Some of these challenges are discussed in the following. 
         [0127]    In some cases, the utility executed in the algorithm has to traverse a Multi-Protocol Label Switched (MPLS) network segment. It accomplishes this by finding the initial label assignment (at the point where the traffic enters the MPLS segment) and tracking across the MPLS network by hopping using per hop details of label popping, pushing and forwarding until the traffic has its final label popped and leaves the MPLS segment. 
         [0128]    Another challenge is traversing NAT boundaries which can be accomplished by polling NAT tables of the NAT device. This may require real time speculative polling for dynamic NAT, but it might be possible to use background polling for static NAT. 
         [0129]    For tunnel protocols such as IPSEC/GRE/SSL, etc., the utility checks for a direct route from one end of the tunnel to the other (typically with one unknown layer 3 hop representing all of the nodes in-between). The utility further checks for protocol specific topological information and checks in the routing tables/interfaces for the presence of crypto/tunnelling hops. 
         [0130]    Another challenge is virtualisation. It is important that the algorithm identifies physical egress ports so that a physical device connected to the egress port can be accessed from the topology. Many networks operate at various different layers of virtualisation. Virtual switches can be queried using additional APIs and to ensure that the topology server has timely information on the end host location, it might be necessary for the topology server to integrate with virtualisation management platforms to get updates regarding virtual machine relocation to allow a proactive polling of end host location on affected virtual switches. 
         [0131]    The utility negotiates virtualised routing and forwarding tables (VRF) by querying the appropriate IP forward (routing table) required for a specific VRF identifier. In SNMP, for example, this can be done using VRF contextualised community strings.