Distinguishing between link and node failure to facilitate fast reroute

<u style="single">STATEMENT OF RELATED APPLICATIONS</u>

0001The present application is related to the subject matter of the following pending applications: <ul id="ul0001" list-style="none" compact="compact"><li><patcit id="pcit0001" dnum="US080956A" dnum-type="L"><text>U.S. App. No. 10/080,956</text></patcit> entitled "LINEAR PROGRAM-BASED TECHNIQUE FOR PLACING FRR TE TUNNELS WITH BANDWIDTH GUARANTEE," filed on February 22, 2002;</li><li><patcit id="pcit0002" dnum="US038259A" dnum-type="L"><text>U.S. App. No. 10/038,259</text></patcit> entitled "IMPLICIT SHARED BANDWIDTH PROTECTION FOR FAST REROUTE," filed on January 2, 2002;</li><li><patcit id="pcit0003" dnum="US052665A" dnum-type="L"><text>U.S. App. No. 10/052,665</text></patcit> entitled "LOAD BALANCING FOR FAST REROUTE BACKUP TUNNELS," filed on January 17, 2002;</li><li><patcit id="pcit0004" dnum="US166886A" dnum-type="L"><text>U.S. App. No. 10/166,886</text></patcit> entitled "MPLS FAST REROUTE WITHOUT FULL MESH TRAFFIC ENGINEERING," filed on June 11, 2002.</li></ul>

<u style="single">BACKGROUND OF THE INVENTION</u>

0002The present application relates to data networking and more particularly to systems and methods for rerouting around failed links and/or nodes.

0003The Internet and IP networks in general have become key enablers to a broad range of business, government, and personal activities. More and more, the internet being relied upon as a general information appliance, business communication tool, entertainment source, and as a substitute for traditional telephone networks and broadcast media. As the Internet expands its role, users become more and more dependent on uninterrupted access.

0004To assure rapid recovery in the event of failure of a network link or node, so-called "Fast Reroute" techniques have been developed. In a network employing Fast Reroute, traffic flowing through a failed link or node is rerouted through one or more preconfigured backup tunnels. Redirection of the impacted traffic occurs very quickly to minimize impact on the user experience, typically in tens of milliseconds.

0005These Fast Reroute techniques have been developed in the context of MPLS Traffic Engineering where traffic flows through label switched paths (LSPs). Typically, the overall network is configured such that traffic flows through guaranteed bandwidth end-to-end "primary" LSPs. It is also possible to establish short primary LSPs in a non-Traffic Engineering network, only for the purpose of taking advantage of Fast Reroute techniques (see above-referenced patent application entitled "MPLS Reroute Without Full Mesh Traffic Engineering.")

0006In either case, when a link or node failure occurs, traffic affected by the failure is rerouted to the preconfigured backup tunnels. These backup tunnels are used only for a very short time since simultaneously with the rerouting through the backup tunnels, the head ends of all affected primary LSPs are notified of the failure. This causes the head ends to reroute the primary LSPs around the failures so that the backup tunnels are no longer needed. It is generally assumed that the probability of multiple failures in such a short time is small, so each failure may be considered independently.

0007Under the independent failure assumption, link bandwidth available for backup tunnels may be shared between backup tunnels protecting different links or nodes. The techniques disclosed in <patcit id="pcit0005" dnum="US038259A" dnum-type="L"><text>U.S. Patent App. No. 10/038,259</text></patcit> make use of this assumption to allow available backup bandwidth to be shared among links or nodes to be protected while assuring that guaranteed bandwidth requirements continue to be met during Fast Reroute conditions. On the other hand, without taking advantage of the independent failure assumption, it is very difficult to assure guaranteed bandwidth during failure recovery while using bandwidth resources efficiently.

0008IETF Draft "Framework for MPLS-based Recovery", dated March 2001 describes principles for multi-protocol label switching (MPLS) based recovery and comparison criteria that may be used as a basis for comparing and evaluating different recovery schemes. The document describes local repair which includes link recovery and node recovery. In each case, traffic on a working path is switched over to an alternate path at an upstream LSR that connects to the failed link or failed node.

0009<patcit id="pcit0006" dnum="US20020067693A"><text>US 2002/0067693 (Kodialam et al.</text></patcit><i>)</i> discloses dynamic backup routing of network tunnel paths for local restoration in a packet network. Kodialam <i>et al.</i> use dynamic backup routing to determine the allocation of, and, in operation, to switch between a primary (active) path and a secondary (backup) path upon detection of a network failure. The backup path is thus allocated upon detection of a network failure.

0010Mechanisms currently available for failure detection do not always allow the failure of a link to be distinguished from failure of a node. For example, a network node may lose communication via a particular link without knowing whether only the link itself has failed or the node to which the link has connected has failed. This ambiguity can cause the network to attempt to reroute around simultaneous failures when in fact only a single failure has occurred. The combined backup bandwidth requirements of simultaneous failures may exceed available backup bandwidth on some links leading to a violation of bandwidth guarantees and possible user perception of deteriorated service.

0011In theory it would be possible to correct this ambiguity by centrally determining backup tunnels such that no such clash is possible. However, placing this constraint on backup tunnel placement leads to less efficient use of available bandwidth. Furthermore, computing the correct placement of backup tunnels would also become far more complex and computation-intensive.

0012Furthermore, it is more desirable to compute backup tunnels in a distributed fashion rather than centrally. If backup tunnel computation is to be done in a distributed fashion across the network, the task is made practically impossible due to the need to signal a large amount of backup tunnel information among nodes. If link failures could be distinguished from node failures, the validity of the independent failure assumption would be strengthened, allowing backup tunnels to be computed in a distributed fashion and readily signaled with zero bandwidth in accordance with the techniques disclosed in <patcit id="pcit0007" dnum="US038259A" dnum-type="L"><text>U.S. Patent App 10/038,259</text></patcit> without compromise to bandwidth guarantees.

0013What is needed are systems and methods for determining whether a link or a neighboring node to which the link connects has failed.

<u style="single">SUMMARY OF THE INVENTION</u>

0014Systems and methods for distinguishing a node failure from a link failure are provided by virtue of one embodiment of the present invention. By strengthening the assumption of independent failures, bandwidth sharing among backup tunnels protecting links and nodes of a network is facilitated as well as distributed computation of backup tunnel placement. Thus a backup tunnel overlay network can provide guaranteed bandwidth in the event of a failure.

0015One aspect of the present invention provides a method of operating a selected node to distinguish between failure of a link connected to the selected node and failure of a neighboring node connected to the selected node through the link. The method includes: pre-configuring an alternate path from the selected node to the neighboring node, the alternate path not including the link, and upon detection of a failure of communication via the link, using the alternate path to verify operation of the neighboring node.

0016Further understanding of the nature and advantages of the inventions herein may be realized by reference to the remaining portions of the specification and the attached drawings.

<u style="single">BRIEF DESCRIPTION OF THE DRAWINGS</u>

0017<ul id="ul0002" list-style="none" compact="compact"><li>Fig. 1 depicts a network device according to one embodiment of the present invention.</li><li>Fig. 2 depicts a network configuration useful in illustrating embodiments of the present invention.</li><li>Fig. 3 is a flowchart describing steps of distinguishing a link failure from a node failure according to one embodiment of the present invention.</li></ul>

<u style="single">DESCRIPTION OF SPECIFIC EMBODIMENTS</u>

0018The present invention will be described with reference to a representative network environment that uses a particular representative combination of protocols to move data through the network. Fig. 2 depicts a particular configuration of network nodes in such an environment. The network nodes are interconnected by links may be implemented using any type of physical medium such as e.g., an optical medium, a wireless medium, twisted pair, etc.

0019In one embodiment, the nodes of Fig. 2 interoperate in a manner specified by various protocols including, e.g., TCP/IP as known in the art, suitable link layer protocols such as Link Management Protocol (LMP), and protocols defined by the following documents: <ul id="ul0003" list-style="none" compact="compact"><li><nplcit id="ncit0001" npl-type="s"><text>E. Rosen, et al., "Multiprotocol Label Switching Architecture," RFC 3031, Internet Engineering Task Force, January 2001</text></nplcit>.</li><li><nplcit id="ncit0002" npl-type="s"><text>Braden, et al. "Resource ReSerVation Protocol (RSVP)-Version 1 Functional Specification," RFC 2205, Internet Engineering Task Force, September 1997</text></nplcit>.</li><li><nplcit id="ncit0003" npl-type="s"><text>Awduche, et al., "Requirements for Traffic Engineering Over MPLS," RFC 2702, Internet Engineering Task Force, September 1999</text></nplcit>.</li><li><nplcit id="ncit0004" npl-type="s"><text>Ashwood-Smith, et al., "Generalized MPLS Signaling- RSVP-TE Extensions," Internet Draft, Internet Engineering Task Force, October 2001</text></nplcit>.</li></ul>

0020<nplcit id="ncit0005" npl-type="s"><text>Pan, et al., "Fast Reroute Techniques in RSVP=TE," Internet Draft, Internet Engineering Task Force, October 2001</text></nplcit>.

0021The contents of the above documents are herein incorporated by reference in their entirety for all purposes.

0022In one embodiment, the nodes of Fig. 2 are IP routers that implement multiprotocol label switching (MPLS) and essentially operate as label switched routers (LSRs). The nodes of Fig. 2 may also implement MPLS Traffic Engineering (MPLS TE) and/or Diffserv-Aware Traffic Engineering. Traffic flows through end-to-end LSPs that are configured to offer guaranteed bandwidth, latency, jitter, etc. This type of environment is, however, merely representative. Embodiments of the present invention are applicable regardless of the properties of traffic carried across the network.

0023When a given LSP in a network employing MPLS TE experiences a node or link failure, the head-end, i.e., the ingress, will establish a new LSP as a substitute. However, this process requires far longer than desirable. Accordingly, a local fast reroute capability is provided so that when a node or a link fails, an LSP is temporarily rerouted around the failed link or node while a new end-to-end LSP is being established at the head-end.

0024Each link (a bi-directional link is considered as two links with identical endpoints) is protected by one or more backup tunnels that do not include the protected link. A backup tunnel or tunnels may also collectively protect a shared risk link group (SRLG), i.e., a group of links that are expected to experience simultaneous failure because, e.g., they share a common fiber. Each node is protected by one or more backup tunnels that do not include the protected node.

0025To assure that bandwidth guarantees continue to be met during fast reroute conditions, backup tunnels are preferably configured with sufficient bandwidth to support all primary traffic through the protected link, SRLG, or node. This is impractical unless one assumes that failures will not be simultaneous and that therefore bandwidth available for backup tunnels may be shared among protected elements. With this assumption, however, backup bandwidth may be guaranteed provided that the backup tunnels protecting any one element do not consume more backup bandwidth than is available. Further details of configuring and placing backup tunnels are disclosed in <patcit id="pcit0008" dnum="US080956A" dnum-type="L"><text>U.S. App. No. 10/080,956</text></patcit>, <patcit id="pcit0009" dnum="US038259A" dnum-type="L"><text>U.S. App. No. 10/038,259</text></patcit>, and <patcit id="pcit0010" dnum="US052665A" dnum-type="L"><text>U.S. App. No. 10/052,665</text></patcit>.

0026A failure of a neighboring node may be detected by the absence of expected RSVP Hello messages. A failure of a link may be detected by, e.g., alarm conditions reported from the physical and/or link layers. When a node fails, it may or may not bring down the links to which it is connected. If a link has failed, a node to which the link connects will detect the link failure but may not realize that the neighboring node at the other end of the failed link may have failed.

0027To maintain the independent failure assumption, it is desirable that when a link or SRLG fails, traffic is rerouted through the backup tunnel(s) that protect the link or SRLG, and a node fails, traffic is rerouted through the backup tunnel(s) that protect the node. However, to do so, it is necessary to accurately distinguish between link failure and node failure. However, no known prior art mechanism allows distinguishing between different kinds of failures under all circumstances. In particular, if a link failure is detected, it is currently impossible, in most cases, to decide whether it is just this link that failed, or whether the node on the other side of the link failed and caused the failure of its adjacent links as well. As a result, the current equipment simultaneously invokes backup tunnels that are established to protect the link and backup tunnels that are computed to protect the node. If bandwidth guarantees are desired in the presence of failure, then such simultaneous invocation may result in the loss of bandwidth guarantees.

0028Another example of a similar phenomenon will be described with reference to Fig. 2. Assume that traffic flowing from node K through node J to node I has a total bandwidth requirement of 1 Mbps. Assume that the link from node M to node N has only 1 Mbps of available backup bandwidth, and that all other links have 10 Mbps bandwidth available for protection. Under the assumption that nodes J and K will not fail simultaneously, this traffic can be protected against failure of node J by establishing a backup tunnel including nodes K, M, N, and I (because the path K-M-N-I has enough bandwidth to accommodate 1 Mbps of traffic flowing from K to J to I). Assume that traffic flowing from node J through node K to node L also has a total bandwidth requirement of 1 Mbps. This traffic may be protected against failure of node K by establishment of a backup tunnel including nodes J, M, N, and L, because path J, M, N, L has enough bandwidth to accommodate 1 Mbps of traffic flowing from J to K to L. Both nodes J and K can be protected by two different backup tunnels each separately requiring 1 Mbps bandwidth on link MN, which has only 1 Mbps available bandwidth, because the placement of these tunnels assumes that only one of nodes J and K can fail at a time.

0029However, the inability to reliably distinguish node failures from link failures can cause the backup bandwidth to be exceeded. Consider the case where the link between nodes J and K suffers a bi-directional failure but nodes J and K remain operational. Upon detection of the link failure, node J may behave as if node K has failed and invoke the above-described backup tunnel protecting node K. Similarly; node K may detect link failure of the other link direction and behave as if node J has failed, invoking the other backup tunnel described above. Now, both backup tunnels sharing the link from node M to node N are needed and exceed the available backup bandwidth since the independent failure assumption has been implicitly violated even though neither node has actually failed.

0030According to one embodiment of the present invention, scenarios like those described with reference to Fig. 2 may be avoided by accurately distinguishing between link and node failure at the time of failure and then patching in backup tunnels pre-configured for the failed element. Fig. 3 is a flowchart describing steps of distinguishing between link and node failure and reacting appropriately according to one embodiment of the present invention.

0031The mechanism described with reference to Fig. 3 provides at each interface for an alternate path for a node to attempt contact with its neighbor once communication through a direct link has been lost. At step 302, this path is preconfigured. Step 302 may be performed as part of the same process that generally establishes backup tunnels and may be performed centrally or in a distributed fashion. For each bi-directional link, two such contact paths are established between the nodes neighboring the link, one for each direction.

0032The contact paths should preferably not include the direct link, any link in the same SRLG, or any link parallel to this direct link because in case of neighboring node failure, all such parallel links may fail simultaneously, and the alternate contact path would not be available. The contact paths may be backup tunnels such as the ones used to protect nodes and links but requiring only minimal bandwidth. One backup tunnel may provide an alternative contact path for multiple links. It is preferable to establish the contact paths with minimum path length to minimize propagation time. If information is available as to membership in SRLGs, the contact paths should avoid all links in the same SRLG. In one embodiment, the backup tunnels protecting a link are also used to provide the contact path for failure detection purposes.

0033A step 304 occurs during network operation. At step 304, a node detects loss of communication via an adjoining link. This detection of failure may arise due to a link layer or physical layer alarm message or due to a loss of the RSVP Hello exchange that is expected along an MPLS TE LSP. In the absence of further information as to whether the link itself has failed or a neighboring node reached though the link has failed, traffic is redirected into the backup tunnel(s) protecting the link at step 306. This is done to minimize restoration time if the failure is indeed a link failure. Then at step 308, the node attempts contact with its neighbor via the alternate path established for this purpose. The contact may be in the form of an RSVP Hello message to which a response is expected. A step 310 tests whether contact is successful. If contact was successful, a step 312 determines that the node is operational and that the link has failed. Then at step 314, traffic bound for the neighboring node continues to be rerouted through the backup tunnel(s) protecting the failed link.

0034If contact was unsuccessful, step 316 determines that the neighboring node has failed. The traffic that had previously been shifted to the backup tunnel(s) protecting the link is now shifted to the backup tunnel(s) protecting the neighboring node at step 318.

0035Step 316 may also be reached if there is an indication (e.g., receipt of a Path Error or RSVP Notify message, etc.) that the backup tunnel protecting the failed link has also failed. In this case, the contact attempt is interrupted and step 310 can be skipped since a neighboring node failure can be concluded based on the backup tunnel failure.

0036In an alternative embodiment, the RSVP Hello messages are sent periodically through the alternate path even when no failure has been detected. In this case, step 310 can operate by checking the status of the ongoing exchange. In situations where propagation delay along the alternate contact path is significant, this can accelerate the determination of whether a node failure or a link failure has occurred. This routine RSVP Hello message exchange via the alternate path may supplement or substitute for RSVP Hello exchange in reaction to a detected failure. Where the link failure detection mechanism is expected to react quickly and the propagation delay over the alternate path is small, it may be preferably to initiate the RSVP Hello exchange via the alternate contact path only in the event of a detected failure. By contrast, if the propagation delay is large, it may be preferable to rely on routine exchange of RSVP Hellos.

0037It will be seen that by accurately distinguishing between link and node failure, embodiments of the present invention strengthen the assumption that Fast Reroute need only respond to a single failure at a time. Thus guaranteed bandwidth may readily be provided continuously under failure conditions, even where it is necessary to accurately distinguish between link and node failures.

<u style="single">NETWORK DEVICE DETAILS</u>

0038Fig. 1 depicts a network device 100 that may be used to implement any of the described nodes or a network management workstation. In one embodiment, network device 100 is a programmable machine that may be implemented in hardware, software or any combination thereof. A processor 102 executes code stored in a program memory 104. Program memory 104 is one example of a computer-readable storage medium. Program memory 104 can be a volatile memory. Another form of computer-readable storage medium storing the same codes would be some type of non-volatile storage such as floppy disks, CD-ROMs, DVD-ROMs, hard disks, flash memory, etc. A carrier wave that carries the code across a network is another example of a computer-readable storage medium.

0039Network device 100 interfaces with physical media via a plurality of network interfaces 106. For example, one of network interfaces 106 may couple to an optical fiber and may incorporate appropriate physical and link layer functionality. Other examples of network interfaces include Ethernet interfaces, DSL interfaces, Gigabit Ethernet interfaces, 10-Gigabit Ethernet interfaces, etc. As packets are received, processed, and forwarded by network device 100, they may be stored in a packet memory 108. Network device 100 implements all of the network protocols and extensions thereof described above as well as the data networking features provided by the present invention.

0040It is understood that the examples and embodiments that are described herein are for illustrative purposes only and that various modifications and changes in light thereof will be suggested to persons skilled in the art within the scope of the appended claims.