Method and Apparatus for Determining a Communications Path Between Two Nodes in an Internet Protocol (IP) Network

This invention relates to computer network communication systems, and in particular to a method and apparatus for determining data paths on an IP network.

In an Internet, several networks are connected together through the use of gateways and an internetworking protocol. The gateways (often called routers), using the protocol, hide the underlying details of the actual networks, in order to provide uniform service across the network.

The leading internetworking technology is the Internet suite of protocols, commonly referred to as TCP/IP, after the two-core protocols in the suite. TCP, the transmission control protocol, is a connection-oriented transport service. IP, the Internet protocol, is a connectionless-mode network service.

IP is called a connectionless-mode network protocol, which means that it is datagram-oriented. When some entity on the network wishes to send data using IP, it sends that data as a series of datagrams. Associated with each datagram is an address indicating where the datagram should be delivered. This address consists of an IP address, an upper-layer protocol number. IP takes the user-data and encapsulates it in an IP datagram, which contains all of the information necessary to deliver the datagram to the IP entity at the destination. The remote IP entity will examine the IP datagram it receives, and then strip off the data and pass it up to the appropriate upper-layer protocol. See, M. Rose, "The Simple Book--An Introduction To Management Of TCP/IP-Based Internets," Prentice Hall, 1991.

SNMP, Simple Network Management Protocol, has become the de facto operational standard for network management of TCP/IP-based internets. A managed network may be considered as consisting of three components: (1) several managed nodes, each containing an agent; (2) at least one network management station (NMS); and (3) a network management protocol, which is used by the station and the agents to exchange management information. The managed node may consist of a host system, e.g., workstation, terminal server, or printer; a gateway system, e.g., a router; or a media device, e.g., a bridge, hub or multiplexor. One activity of the network management system is to compile a topology of the network, defining the connections between various devices on the network. The network management system may query the IP routing table at each gateway, to determine what devices are located at each port on the gateway. This information may be used to construct a data path between any two devices on the internet.

Associated with IP is another protocol providing low-level feedback about how the internet layer is operating. This protocol is termed the "Internet Control Method Protocol" (ICMP). ICMP provides basic control messages for error reporting.

One useful tool in troubleshooting connectivity problems at the internet layer is a program called "traceroute." The traceroute program sends a series of "probe packets" using UDP to an IP address and awaits an ICMP reply. More specifically, IP datagrams carrying the UDP packets are sent with monotonically increasing values in the "time to live" (TTL) field, and the UDP port chosen is one most likely not to be in use. For each TTL value, the traceroute program sends a fixed number of packets (usually three), and reports back the IP addresses of the devices responding. This process continues until an ICMP port unreachable packet is received or some TTL threshold is reached (usually 30).

If a gateway receives an IP datagram and decrements the TTL to zero, then it returns an ICMP time exceeded packet. If the IP datagram eventually reaches the network device in question, an ICMP port unreachable packet will be returned. Combining the information from all the replies, the traceroute program can report on the whole route. See M. Rose, supra, at 66-67.

Unfortunately, traceroute does not provide any information about which ports of the routers are on the path. In addition, not all devices on the network support IP options needed to implement traceroute.

An SNMP query to a router is another method for tracing a route, i.e., by determining the next-hop router on the current router IP routing table. Unfortunately, not all routers can be accessed by

It would thus be desirable to provide a method of tracing a route from any source to any destination, regardless of whether one router is known, and regardless of whether each router on the path can be accessed using SNMP.

The present invention is a method and apparatus for determining a communications path between a source node and a destination node on a network using IP. The method includes compiling a path list of IP addresses for next-hop routers on the path between the source IP address and the destination IP address.

In its broadest sense, the method includes the steps of: (a) sending a series of UDP probe packets out a socket of a first node to find successive next-hop routers on the path; (b) setting the socket of the first node to "loose route" the UDP probe packets through the source IP address; and (c) recording in the path list the next-hop router IP address returned following each one of the series of UDP probe packets. The UDP probe packets have a destination field set with the destination IP address. The time to live (TTL) field of the UDP probe packet is set with an initial value of one, and monotonically increased (i.e., incremented by one) to find each successive next-hop router until the destination is reached.

In a further embodiment, the method includes the step of alternatively sending an SNMP query to a router on the path in order to find the next-hop router on the path. Then, if the SNMP query fails, the method reverts to sending the next UDP probe packet. Thus, if a specific router on the path is discovered and can accept SNMP messages, we can then read its routing table to find out the next router on the way to the destination. The routing table also provides the port which leads to the next router.

In a still further embodiment, if both the UDP probe packet and/or SNMP query fail to provide the next-hop router IP address, then an unknown nexthop router IP address is selected and recorded in the path list. We later use a topology information database from a network management system to resolve this unknown router.

In a still further embodiment, the method includes sending a query to a topology information database to determine any unknown next-hop router IP addresses, as well as any intra-router (i.e., layer-2 devices, such as hubs, bridges, etc.) on the path.

In this manner, we can determine a complete route from a source node to a destination node. Apparatus for implementing the method is further provided, including a station with a memory and processor for storing and running the traceroute program and/or a network management station for maintaining a management database and sending SNMP queries to various routers on the network which are SNMP compatible.

FIG. 1 is a schematic diagram of a portion of a network in which there is a source node, a destination node, and a querying node.

FIG. 2 is a flowchart illustrating a mechanism by which the distance, in TTL units, is determined between the querying node and the source node.

FIG. 3 is a flowchart illustrating a mechanism for locating the next-hop router along the path from the source node to the destination node using TTL.

FIG. 4 is a flowchart illustrating the overall mechanism for locating the next router utilizing either an SNMP query or UDP probe packet.

FIG. 5 is a block diagram of a general purpose computer, for implementing the various path determination methods of this invention.

FIG. 1 illustrates a general example, where there is a source node 11, a destination node 12, and a querying node 13. Additionally, there are routers r1 and r2 between the querying node 13 and the source node 11, and routers r3, r4 and r5 between the source node 11 and destination node 12. This representative network will be used to illustrate the method of the present invention.

In a first incremental TTL mechanism, illustrated in FIG. 2, we set a socket in query node 13, used for sending the UDP probe packets, to "loose route" all packets through the source node 11. In this manner, we determine the number of routers between the querying node 13 and the source node 11, i.e., r1 and r2.

Once we know one router on the path, i.e., by the above incremental TTL mechanism, we can send an SNMP query to read its routing table to find out the next router on the path for the destination, along with the port which takes us to the next router. If this fails, we revert to the incremental TTL mechanism to find the next-hop router. If both fail, we still continue, adding an unknown router to the path list. Our subsequent discovery of a network management system topology database, e.g., the Spectrum.TM. program sold by Cabletron Systems, Inc. of Rochester, New Hampshire, may enable us to determine all unknown router nodes as well as identify any intra-router devices on the path.

More specifically, FIG. 2 illustrates the "how.sub.-- far.sub.-- is.sub.-- source(source, dest)" portion of our program. In step 21, we set the "loose-source routing" IP option on the socket through which we are sending the UDP packets out. The source is the loose route we specify to the socket. Thus, all packets going through this socket will be routed through this loose route, i.e., source. See D. Comer, "Internetworking With TCP/IP, Vol. I, Principles, Protocols, And Architecture," Prentice Hall, 2nd ed., pp. 103-104 (1991). In step 22, we initialize by setting ttl=1, and in step 23 we send a UDP probe packet to the destination where TTL=ttl. In step 24 we wait for the TTL.sub.-- EXCEEDED ICMP message. If this message is received from the source, then we return(ttl). If not, in step 25 we increment ttl by one and send another UDP probe packet.

If we are unable to contact a router with an SNMP query, or if we choose to continue using the TTL mechanism, we then utilize the "find.sub.-- next.sub.-- hop using.sub.-- ttl(source, dest, ttl)" portion of our program illustrated in FIG. 3. Again, in step 51 we send a UDP probe packet to the destination with TTL=ttl and in step 52 we wait for one of the following ICMP responses: TTL.sub.-- EXCEEDED, or PORT.sub.-- UNREACHABLE. If the message TTL.sub.-- EXCEEDED is received, this message has come from one of the intermediate hosts and in step 53 we set IP address=sender of the ICMP message and record its IP address in our path list. If in step 54 no response is received within a designated time period, in step 55 we increment the retry.sub.-- count and send another UDP probe packet (return to step 51). If (in step 54) we have reached the maximum period, i.e., MAX.sub.-- RETRY, then we set the IP address to an unknown IP address (step 56) and enter the same in the path list.

If a PORT.sub.-- UNREACHABLE message is received (step 57), this message can only come from the destination and therefore we enter the IP address of the destination in our path list and we are finished.

FIG. 4 illustrates generally a preferred method in which we first try an SNMP search 31 (assuming we have a known router), and if it is successful (step 32), we continue to increment TTL (step 34) and then return to conduct an SNMP search (step 31) on the next-hop router. If the SNMP search is not successful, we send (step 33) a UDP probe packet to determine the next-hop router. If this is successful (step 35), we again increment TTL (step 34) and then conduct an SNMP search on the next-hop router. If the UDP probe packet search is not successful, we add (step 36) an unknown router address to our path list and then increment TTL (step 34). Once we have reached the destination, we can then query our management database (i.e., Spectrum.TM.) (step 37) to determine all the intra-router devices, i.e. layer-2 devices including hubs, bridges, etc., between each pair of routers discovered previously. We can also use the management database to try to resolve the unknown router nodes in the path list. Essentially, we use management database's knowledge of how the various device models are connected to each other. For example, Spectrum.TM. acquires this knowledge during an "autodiscovery" process of all the devices on the network. The Spectrum.TM. network management platform is described in U.S. Pat. No. 5,261,044 and in copending and commonly owned U.S. Ser. No. 07/797,121 filed Nov. 22, 1991 by R. Dev et al., which are hereby incorporated by reference in their entirety. Spectrum.TM. implements the Autodiscovery process described in copending and commonly owned U.S. Ser. No. 08/115,232 filed Sep. 1, 1993 by T. Orr et al., which is also incorporated by reference in its entirety. The present invention is not limited to use of the Spectrum.TM. database, but contemplates the use of any such topology database which defines the relative location of devices on the network.

The program may be implemented in a general purpose computer 4I such as shown in FIG. 5. As can be seen, the general purpose computer includes a computer processing unit (CPU) 42, memory 43, a processing bus 44 by which the CPU can access the memory, and access to a network 45.

The following code can be used to illustrate the method of this invention:

Use incremental TTL value program to find out how many hops away is the source from the station running this program.

Note: that all UDP packets used herein originate from the station running this program and are destined for dest. The loose-routing option set above will force these packets to take following path:

This method uses SNMP queries to find out the next node in the path. IP routing table is read from the curr.sub.-- node to find out the next hop for the given destination.

If the dest address is a.b.d.c., we try to read the next hop values for the following addresses (in this order) until one succeeds:

a.b.c.d

a.b.c.0

a.b.0.0

a.0.0.0

If the next hop value is successfully found, we also return the corresponding port information (i.e. port of curr.sub.-- node which connects to the next-hop).

The following is an example of an SNMP routing table:

If a search of the IP routing table fails to find the next hop, it returns an invalid IP address. This causes the "discover.sub.-- ip.sub.-- path ()" method to use the "find.sub.-- next.sub.-- hop.sub.-- using.sub.-- TTL ()" method to find the next hop.

The following example illustrates a method of the invention in accordance with the representative network shown in FIG. 1.

In this example, arrows show the path the UDP probe packets are going to take.

querying: the node running this program

source: given source IP address

dest: given dest IP address

r1, r2: routers between querying node and source

r3, r4, r5: routers between source and dest

The socket used for sending the UDP probe packets from the querying node is set to loose-route all packets through source. Also, all the UDP probe packets are sent to dest on an unused destination port number, so that if the probe reaches the dest, the dest will send us back a PORT.sub.-- UNREACHABLE ICMP message.

how.sub.-- far.sub.-- is.sub.-- source (source, dest)

For ttl=1, r1 will send the TTL.sub.-- EXCEEDED ICMP message,

for ttl=2, r2 will send the TTL.sub.-- EXCEEDED ICMP message

for ttl=3, source will send the TTL.sub.-- EXCEEDED ICMP message

and this method will return 3.

subsequent discovery

The subsequent discovery will depend on whether we have SNMP models for source, r3, r4 and r5 etc. in our Spectrum.TM. database, e.g.,

r3 will be discovered by reading the routing table from the source or using the TTL mechanism with TTL=4

r4 will be discovered by reading the routing table from r3 or using the TTL mechanism with TTL=5

Similarly r5 will be discovered by reading the routing table from r4 or using the TTL mechanism with TTL=6

Finally, we will know that dest is directly connected to r5, either by reading a direct routing entry for dest from r5, or using TTL mechanism (TTL=7) we will receive a PORT UNREACHABLE ICMP message from the dest.

Having thus described a particular embodiment of the invention, various alterations, modifications and improvements will readily occur to those skilled in the art. Accordingly, the foregoing description is by way of example only, and not intended to be limiting. The invention is limited only as defined in the following claims and the equivalents thereto. ##SPC1##