Patent Publication Number: US-9413605-B2

Title: Path protection for ring-based multi-protocol label switched paths

Description:
TECHNICAL FIELD 
     The disclosure relates to computer networks and, more particularly, to engineering traffic flows within computer networks. 
     BACKGROUND 
     Routing devices within a network, often referred to as routers, maintain routing information that describes available routes through the network. Upon receiving an incoming packet, the router examines information within the packet and forwards the packet in accordance with the routing information. In order to maintain an accurate representation of the network, routers exchange routing information in accordance with one or more defined routing protocols, such as the Border Gateway Protocol (BGP). 
     Multi-Protocol Label Switching (MPLS) is a suite of protocols used to engineer traffic patterns within Internet Protocol (IP) networks. By utilizing MPLS, a source device can request a path through a network to a destination device, i.e., a Label Switched Path (LSP). An LSP defines a distinct path through the network to carry MPLS packets from the source device to a destination device. Each router along a LSP allocates a label and propagates the label to the closest upstream router along the path. Routers along the path cooperatively perform MPLS operations to forward the MPLS packets along the established path. A variety of protocols exist for establishing LSPs, such as the Label Distribution Protocol (LDP) and the Resource Reservation Protocol with Traffic Engineering extensions (RSVP-TE). 
     MPLS protocols such as these have been successful in many types of networks including core service provider networks and wide area networks (WAN). However, applying MPLS protocols in access networks can be difficult because many access networks are arranged as a ring topology. Due to ring topology, path protection mechanisms such as RSVP-TE Fast re-route (FRR) and LDP FRR may be inefficient and complex. Moreover, conventional MPLS protocols are typically not sufficiently flexible nor do they scale well for widespread adoption in ring-based access networks. 
     SUMMARY 
     In general, multi-protocol label switching (MPLS) signaling mechanisms are described herein that allow “MPLS rings” to be specified and signaled as a new MPLS construct. As a result, routers may signal membership therein with MPLS rings and automatically establish traffic engineered label switch paths (LSPs) as components of the MPLS rings for packet transport within ring networks. Moreover, as described in further detail below, bandwidth allocation need not be separately provisioned at the time a ring LSP is established, but may be deduced and allocated in real-time based on traffic or services being transported by the routers. Further, the techniques described herein can provide built-in path protection as a natural result of the signaling mechanisms and definitions specified herein for MPLS rings. As a result, separate protection paths, bypass LSPs, or detours need not be signaled. 
     In one example, a method comprises establishing, with a plurality of routers connected to form a ring network, a multi-protocol label switching (MPLS) ring having a plurality of ring label switched paths (LSPs), each of the ring LSPs configured to transport MPLS packets around the ring network to a different one of the routers operating as an egress router for the respective ring LSP, wherein each of the ring LSPs comprises a multipoint-to-point (MP2P) LSP for which any of the routers within the ring network can operate as an ingress to source packet traffic into the ring LSP for transport to the respective egress router for the ring LSP. The method further includes forwarding network traffic as MPLS packets around the ring network in accordance with the ring LSPs. 
     In another example, a router comprises a plurality of physical interfaces interconnecting the router as one of a plurality of routers forming a ring network. The router further includes a processor configured to establish a MPLS ring having a plurality of ring LSPs, each of the ring LSPs configured to transport MPLS packets around the ring network to a different one of the routers operating as an egress router for the respective ring LSP, wherein each of the ring LSPs comprises a MP2P LSP for which any of the routers within the ring network can operate as an ingress to source packet traffic into the ring LSP for transport to the respective egress router for the ring LSP. The router further includes a forwarding component to forward network traffic as MPLS packets around the ring network in accordance with the ring LSPs. 
     In another example, a computer-readable storage medium comprises instructions that cause a network device to output a plurality of messages in accordance with a label distribution protocol to establish, with a plurality of routers connected as a ring network, a MPLS ring having a plurality of ring LSPs around the ring network, wherein each of the ring LSPs is configured to transport MPLS packets around the ring network to a different one of the routers operating as an egress router for the respective ring LSP, and wherein each of the ring LSPs comprises a MP2P LSP for which any of the routers within the ring network can operate as an ingress to source packet traffic into the ring LSP for transport to the respective egress router for the ring LSP. 
     In another example, a method comprises discovering, with an interior gateway routing protocol (IGP), a plurality of routers that are designated as members of a MPLS ring associated with a network in which the routers are interconnected in a ring topology. The method includes, responsive to the discovery, establishing, with the plurality of routers, a plurality of ring LSPs for the MPLS ring, each of the ring LSPs configured to transport MPLS packets around the ring network to a different one of the routers operating as an egress router for the respective ring LSP. Each of the ring LSPs comprises a MP2P LSP for which any of the routers within the ring network can operate as an ingress to source packet traffic into the ring LSP for transport to the respective egress router for the ring LSP. The method further includes forwarding network traffic as MPLS packets around the ring network in accordance with the ring LSPs. 
     In another example, a router comprises a plurality of physical interfaces interconnecting the router as one of a plurality of routers forming a ring network, and a processor executing an IGP. The processor is configured to discover, using the IGP, neighboring ones of the routers that are members of a MPLS ring associated with the ring network. The processor is further configured to, responsive to discovering the neighboring routers that are members of the MPLS ring, establish a plurality of ring LSPs for the MPLS ring. Each of the ring LSPs is configured to transport MPLS packets around the ring network to a different one of the routers that is designated as an egress router for the respective ring LSP. Each of the ring LSPs is configured as a MP2P LSP for which any of the routers within the ring network can operate as an ingress to source packet traffic into the ring LSP for transport to the respective egress router for the ring LSP. The router includes forwarding component to forward network traffic as MPLS packets around the ring network in accordance with the ring LSPs. 
     In another example, a computer-readable storage device stores instructions that cause a processor to receive enhanced IGP messages from neighboring ones of a plurality of routers that are designated as members of a MPLS ring associated with a ring network, wherein each of the enhanced IGP messages specifies an identifier of the MPLS ring to indicate that the respective router is a member of the MPLS ring and specifies an index that indicates the position around the MPLS ring for the respective router. The instructions further cause the processor to select, based on the IGP messages and along the ring network, an upstream neighboring one of the routers and a downstream one of the neighboring routers, and output messages to the upstream neighboring one of the routers and the downstream one of the neighboring routers in accordance with a label distribution protocol to establish a plurality of ring LSPs for the MPLS ring. 
     In another example, a method comprises outputting, with a plurality of routers connected as a ring network, a plurality of messages in accordance with a label distribution protocol to establish a MPLS ring having at least one ring LSP to transport MPLS packets around the ring network to one of the routers operating as an egress router for the ring LSP, wherein the ring LSP comprises a MP2P LSP for which any of the routers within the ring network can operate as an ingress to source packet traffic into the ring LSP for transport to the egress router for the ring LSP. Each of the messages output by the routers specifies bandwidth requirements in a downstream direction to the egress router for any packet traffic to be sourced into the ring LSP by the respective one of the routers around the ring LSP. The method includes forwarding network traffic as MPLS packets around the ring network in accordance with the ring LSP. 
     In another example, a router comprises a plurality of physical interfaces interconnecting the router as one of a plurality of routers forming a ring network. A processor executes a label distribution protocol to communicate with the other routers using messages to establish a ring LSP configured to transport MPLS packets around the ring network to one of the routers operating as an egress router for the ring LSP, wherein the processor is configured to establish the ring LSP as a MP2P LSP for which any of the routers within the ring network can operate as an ingress to source packet traffic into the ring LSP for transport to the egress router for the ring LSP, and wherein the messages specify bandwidth requirements in a downstream direction to the egress router for any packet traffic to be sourced into the ring LSP by the router. A forwarding component to forward network traffic as MPLS packets around the ring network in accordance with the ring LSP. 
     In another example, a method comprises establishing, with a plurality of routers connected to form a ring network, a ring LSP configured to transport MPLS packets around the ring network to one of the routers operating as an egress router for the ring LSP, wherein the ring LSP comprises a bidirectional MP2P LSP for which any of the routers within the ring network can operate as an ingress to source packet traffic into the ring LSP for transport to the egress router for the ring LSP in either an upstream direction or a downstream direction around the ring network. The method further comprises forwarding network traffic as MPLS packets around the ring network in the downstream direction to the egress router in accordance with the ring LSP, detecting, with any one of the routers, a network event that would prevent the MPLS packets from reaching the egress router in the downstream direction, and automatically redirecting the MPLS packets at the one of the routers detecting the network event from the downstream direction of the ring LSP to the upstream direction of the ring LSP to forward the MPLS packets around the ring in the upstream direction to the egress router. Similar techniques may be applied to protect upstream traffic. 
     In another example a router comprises a plurality of physical interfaces interconnecting the router as one of a plurality of routers forming a ring network, and a processor configured to establish, a ring LSP configured to transport MPLS packets around the ring network to one of the routers operating as an egress router for the ring LSP, wherein the ring LSP comprises a bidirectional MP2P LSP for which any of the routers within the ring network can operate as an ingress to source packet traffic into the ring LSP for transport to the egress router for the ring LSP in either an upstream direction or a downstream direction around the ring network. The router further comprises a forwarding component to forward network traffic as MPLS packets around the ring network in accordance with the ring LSP, wherein the forwarding component is programmed by the processor to, in response to a network event, automatically redirect the MPLS packets from the downstream direction of the ring LSP to the upstream direction of the ring LSP to forward the MPLS packets around the ring in the upstream direction to the egress router. Similar techniques may be applied to protect upstream traffic. 
     The techniques may provide certain advantages. For example, the techniques described herein allow a ring LSP to be created as a loop for which the LSP starts and ends with the same node. In one example implementation, each node operates to anchor a respective ring LSP, and each LSP is a MP2P bidirectional LSP that starts and ends at the same node at which traffic for the LSP is egressed (i.e., the node at which traffic exits the LSP). As such, an MPLS ring can be defined and specified for a ring network of n nodes according to the techniques herein and can be constructed to have up to n ring LSPs, each ring LSP having a different anchoring (egress) node of the ring. In this way, only n ring LSPs are used to provide full connectivity for the ring network, rather than n*(n−1) point-to-point LSPs as would be needed for full connectivity using conventional techniques. 
     As another example, unlike traditional RSVP signaling in which RSVP PATH message having an explicit route object (ERO) starts at an ingress for the LSP and is sent along all the nodes of the LSP to the egress, the techniques described herein allow a ring LSP to be formed as a multi-point to point LSP for which any member node of the ring LSP may signal membership of the ring LSP without first needing to receive a RSVP PATH message from an upstream node. That is, once a network device (node) discovers or is otherwise configured with membership of a ring, each member network device may autonomously signal membership in the ring LSP without dependency on receiving MPLS messages, such as label distribution messages, from other member network devices. 
     As another example, according to example implementations described herein, explicit bandwidth for a ring LSP need not be expressly signaled at the time the ring LSP is being established. Instead, bandwidth may be requested and provisioned dynamically for the ring LSP as individual member nodes of the ring network join the MPLS ring and as traffic services are provisioned on those member nodes. Moreover, equal bandwidth need not be provisioned around the entire ring. Instead, the techniques allow any member node to request allocation of bandwidth around the ring, e.g., in a downstream direction, for supporting bandwidth required by the member node and upstream member nodes. As such, the bandwidth allocation can vary to accommodate aggregated bandwidth requirements around the ring, and can be dynamically updated as traffic is provisioned. 
     Further, the techniques described herein provide built-in path protection as a natural result of the signaling mechanisms and definitions specified herein for MPLS rings without requiring additional bypass or standby LSPs or other mechanisms such as loop-free alternatives. In addition, hierarchical LSPs need not be expressly signaled and, instead, access LSPs and other point-to-point LSPs may naturally be configured hierarchically over the ring LSPs that are signaled using the techniques described herein. 
     The details of one or more embodiments are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description and drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram illustrating an example network system that specifies and utilizes MPLS rings in accordance with techniques described herein. 
         FIGS. 2A-2C  are block diagrams illustrating in further detail examples of signaling and construction of an MPLS ring within a network having a ring topology. 
         FIG. 3  is a block diagram illustrating an example ring LSP signaled for the MPLS of the ring network. 
         FIGS. 4A-4D  are block diagrams illustrating the MPLS ring and demonstrating the protection that is automatically built-in to the ring LSPs constructed according to the techniques described herein. 
         FIG. 5  is a block diagram illustrating the MPLS ring and demonstrating the dynamic bandwidth management provided by MPLS rings constructed in accordance with the signaling mechanisms described herein. 
         FIGS. 6A and 6B  are block diagrams illustrating in further detail example control plane signaling messages and forwarding plane configuration when establishing ring LSPs. 
         FIG. 7  is a block diagram illustrating an example format for an RSVP session object that may be used to signal a ring LSP in accordance with the techniques described herein. 
         FIG. 8  is a block diagram illustrating an example format for an RSVP label bindings object that may be used to signal label bindings for a ring LSP in accordance with the techniques described herein. 
         FIGS. 9-12  are block diagrams illustrating example message formats for discovering MPLS rings and member nodes using an interior gateway protocol (IGP), such as the Intermediate System to Intermediate System (ISIS) protocol or the Open Shortest Path First (OSPF) protocol. 
         FIG. 13  is a block diagram illustrating an exemplary device, e.g., a router or gateway, capable of implementing the techniques described herein. 
         FIG. 14  is a flowchart illustrating example operation of a network device in accordance with the techniques described herein. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a block diagram illustrating an example network system  10  that specifies and utilizes MPLS rings in accordance with techniques described herein. As shown in the example of  FIG. 1 , network system  10  includes a service provider network  12  in which a collection of routers (“R”) are arranged to form a plurality of ring networks  14 A- 14 C (collectively, ring networks  14 ). As shown in  FIG. 1 , ring networks  14  operate as access networks for a plurality of customer networks  13 A- 13 D and  15 A- 15 D. That is, ring networks  14  operate to provide fast packet-based transport of communications between customer networks  13 ,  15 , respectively, and provide the customer networks access to public network  11 , such as the Internet or other provider networks. 
     Each customer network  13 ,  15  may comprise a private network and may include local area networks (LANs) or wide area networks (WANs) that comprise a plurality of subscriber devices. The subscriber devices may include personal computers, laptops, workstations, personal digital assistants (PDAs), wireless devices, network-ready appliances, file servers, print servers or other devices. In some cases, the subscriber devices request and communicate multicast streams. 
     In general, the routers within ring networks  14  use the multi-protocol label switching (MPLS) signaling mechanisms described herein to specify and signal “MPLS rings” and membership therein. As a result, the routers automatically establish label switched paths (LSPs) as components of the MPLS rings for packet transport within ring networks  14 . These LSPs may be traffic engineered. Moreover, as described in further detail below, the routers need not separately provision bandwidth allocation, but rather the routers may deduce and allocate bandwidth dynamically based on traffic or services being transported by the routers. Further, the techniques described herein provide built-in path protection as a natural result of the signaling mechanisms and definitions specified herein for MPLS rings. As a result, separate protection paths, bypass LSPs, detours or loop-free alternatives need not be computed or signaled. 
     As described herein, in example implementations an MPLS ring can be defined as follows. Given a graph G=(V, E), where V is a set of vertices (or nodes) and E a set of directed edges (or links), a ring R, such as ring networks  14 A- 14 C, is a sequence of nodes &lt;R 0 , R 1 , R 2 , . . . , Rn&gt;, where n is the number of ring nodes in the ring. A ring node is a router; the links are interfaces between routers. As used herein, R 0  is the hub node for a given MPLS ring, and can also be represented as Rn. That is, the hub node for an MPLS ring is both the zeroth and nth node in the MPLS ring. In the example of  FIG. 1 , MPLS rings for ring networks  14 A,  14 B and  14 C each have a hub node at a respective router R 0  that may also be referred to as router R 6  (denoted “R 0 ( 6 )”), since n=6 in each of the example ring networks. For each node i within the sequence of nodes, link (Ri, R(i+1)) and link (R(i+1), Ri) are both in the set of links E. That is, each node in a given MPLS ring is adjacent to the next and previous node in the ring by a pair of bidirectional links. Indices of adjacent nodes in the MPLS ring, other than the hub nodes, differ in this example by an increment of one and increase in a downstream (DS) direction (clockwise in the example of  FIG. 1 ) from hub node R 0  to node R(n−1). The other direction is, by way of example, represented as the upstream direction (counterclockwise). For example, in the example of  FIG. 1 , communications flowing in the downstream direction for the MPLS ring of ring network  14 A traverse routers R 1 -R 6  in a clockwise direction as this is the direction in which indices of the routers are defined in increasing order. However, communications flowing in the downstream direction for the MPLS ring of ring network  14 C traverse routers R 1 -R 6  in a counterclockwise direction as this is the direction in which indices of the routers are defined in increasing order for the MPLS ring. 
     Each MPLS ring for ring networks  14 A- 14 C is identified by a ring identifier that is unique across service provider (SP) network  12  or an administrative domain. In this example of  FIG. 1 , ring networks  14 A- 14 C, are assigned ring identifiers  22 ,  55  and  33 , respectively. An MPLS ring may be constructed by assigning a ring identifier and index to each of the nodes that form the MPLS ring, and assigning a ring identifier to links between ring nodes that are to be used to carry traffic for the MPLS ring. In some examples herein, ring links between a pair of ring nodes may be treated as a single link, although these links may be bundled at the layer two (2) (e.g., as a Link Aggregation Group), or explicitly bundled at the layer three (L 3 ) as a link bundle, or implicitly bundled by virtue of having the same ring ID. 
     In this way, the techniques described herein may be used to define an MPLS ring, such as by assigning a new, unallocated ring identifier, assigning a hub node (e.g., router  0 ), and assigning the links that belong to the MPLS ring. This may be performed manually, such as by an administrator or network management system, or automatically by the routers of  FIG. 1  by way of discovery using, for example, extensions described herein to an interior gateway protocol (IGP). As one example, as described in further detail below, IGP may be used to discover ring neighbors and ring interfaces. Based on network topology, each node may autonomously elect or may be configured to specify, which of its individual interfaces are coupled to links that form an MPLS ring being established. Using IGP, for example, each router R 1 -R 6  of each of ring networks  14  declares itself as a member of one or more of the MPLS rings and announces its ring identifier for each of the MPLS rings for which it is a member. As further described, each of the hub routers R 0  for each of the MPLS rings also declares, based on discovered neighbors, itself as the largest index in the ring (R 0 →Rn), which allows the MPLS ring to dynamically form as a closed loop. 
     As shown in the example of  FIG. 1 , a ring identifier may be useful when a node is a member of more than one MPLS ring. Further, as illustrated in  FIG. 1 , node indexing and upstream and downstream directions may be different from MPLS ring to MPLS ring. For example, in the MPLS ring of ring network  14 A, the direction from router R 2 / 1  to router R 3 / 2  is downstream while in the MPLS ring of ring network  14 C the direction is upstream. In  FIG. 1 , routers R 2  and R 3  are members of the MPLS ring for ring network  14 A and are also members of the MPLS ring for ring network  14 C as routers R 1  and R 2 , respectively. Router R 4  of the MPLS ring for ring network  14 A is a member of ring  14 B as router R 1 . In this example, the number before the ‘/’ is their index in the MPLS ring for ring network  14 A and after the ‘/’ is their index in ring  14 B or ring  14 C. Some of the links incident on router R 4 / 1 , therefore, are assigned to ring  14 A and some to ring  14 B. Other links, not shown, may not be assigned to any ring. The link shown between router R 2 / 1  and router R 3 / 2  is split physically or logically between the MPLS ring for ring network  14 A and the MPLS ring for ring network  14 C. 
     In many cases, upon defining MPLS rings, a service provider network may include other non-ring nodes, such as service nodes and access nodes, that may be attached to, but not be part of, the MPLS rings. In the example of  FIG. 1 , service provider network  12  includes service node S 1  and service node S 2 , where service node S 2  is dual-homed to hub node R 6  of the MPLS ring for ring network  14 B. In addition, service provider network  12  includes access node AN 1  that is single-homed to router R 3 . Non-ring nodes may be one or more hops away from a MPLS ring. 
     As described herein, in example implementations each ring LSP signaled by the routers starts and ends at the same router (or ring node) Ri. As used herein, Ri is an anchor node for the ring LSP, i.e., where the ring LSP starts and ends. Each ring node is an anchor for one of the n ring LSPs defined on a ring with n nodes. 
     In general, network  12  may utilize two different types of LSPs: ring LSPs described herein and access LSPs. Access LSPs may be conventional point-to-point (P2P) or point-to-multipoint (P2MP) LSPs. One example of a ring LSP described herein is a bidirectional LSP that starts and ends on the same ring node of an MPLS ring, called the ring LSP anchor, and only traverses the routers of the MPLS ring. As one example, the MPLS ring constructed for ring network  14 B includes a ring LSP  17  that starts and ends with router R 0  ( 6 ). That is, router R 0  ( 6 ) is the anchoring node for ring LSP  17 . Each router within a MPLS ring can initiate one or more ring LSPs, each of the ring LSPs being distinguished by a ring LSP identifier. In contrast, an access LSP is a unidirectional or bidirectional LSP that connects non-ring nodes hierarchically over ring LSPs. For example, access LSP  19  may be used to transport traffic between access node AN 1  and service node S 2  hierarchically over ring LSPs of the MPLS ring of ring network  14 B. For example, traffic from access node AN 1  to service node S 2  may flow through access LSP  19  over ring LSP  17  anchored by router R 6  of the MPLS ring of ring network  14 B. Traffic from service node S 2  to access node AN 1  may flow through bidirectional access LSP  19  or a different access LSP over a different ring LSP (not shown) anchored by router R 3  of the MPLS ring of ring network  14 B. 
     Techniques described herein for signaling ring LSPs, such as ring LSP  17 , are designed to naturally automate the creation of access LSPs, such as access LSP  19 , as a hierarchical LSP. For example, suppose the service provider wishes to create unidirectional access LSP  19  from non-ring access node AN 1  connected to R 3  of ring network  14 B to non-ring service node S 2  connected to router R 6 . This may be accomplished by forming access LSP as a hierarchical LSP by creating: (1) an LSP from access node AN 1  to router R 3 , (2) a hierarchical LSP from router R 3  to router R 6  over ring LSP  17 , and (3) an LSP from router R 6  to service node S 2 . Service node S 2  may advertise an LSP to router R 6  using LDP, for example. In accordance with the techniques described herein, as further described in reference to  FIG. 8 , router R 6  advertises not only labels for use with respect to ring LSP  17 , but may also advertise a label to reach service node S 2  in RSVP PATH and RESV messages for ring LSP  17 . These RSVP messages may be propagated to router R 3 , which can in turn advertise reachability for service node N 2  to access node AN 1 , e.g., via label distribution using the Address Resolution Protocol (L-ARP). In this way, the signaling techniques described herein facilitate creation of hierarchical access LSPs over ring LSPs. 
     As further described below, the techniques disclosed herein provide ring LSPs that may have a number of benefits. For example, the techniques described herein may allow for signaling ring LPS that may be highly scalable. Since, in an example implementation, a ring LSP is a bidirectional, multipoint LSP, the ring LSP allows for a packet flow to be directed onto the ring LSP at any ring node and in either direction. The techniques described herein provide scalability by allowing the number of LSPs (k) per node (n) of a ring to require only up to n*k ring LSPs rather than a full mesh of 2*(n^2)*k point-to-point conventional LSPs. This in turn means that the state per node of the ring may only be n*k bidirectional ring LSPs instead of (n^2)*k unidirectional LSPs. 
     As another example, the techniques provided herein allow for auto-configuration by which each node is able to automatically create the ring LSPs for which it is the anchor. Further, the techniques provide for automatic bandwidth management. For example, as described herein the bandwidth reserved on a single ring LSP is not fixed but instead can change hop-by-hop around the ring. A bandwidth-managed connection accounts for the bandwidth requirements of access LSPs that “hop on” to a ring LSP at any ring node. 
     Moreover, the ring LSPs may provide bidirectional, resilient connectivity with fast restoration among ring nodes. The bidirectionality of LSPs is used to provide resilience. If a link or node fault is detected in a given direction, traffic is automatically switched to the other direction; the destination node remains the same. Fast effective restoration may be ensured by a protection switch pre-programmed into the forwarding tables. 
     Further, as described, the techniques may provide for auto-hierarchical access LSPs between service and access nodes attached to the ring, which simplifies access LSPs and makes them more scalable. 
       FIG. 2A  illustrates in further detail an example ring network  20 . In this example, ring network  20  comprises ten (10) routers R 1 -R 10  and has been assigned an identifier  20 . Each router has a plurality of ring network interfaces that couple the router to neighboring routers of ring network  20  via ring links  21 . As shown in  FIG. 2A , the routers R 1 -R 10  may be coupled by additional links  22  that have been determined, e.g., by an administrator or automatically by the routers, as excluded from ring network  20  in favor of links  21 . 
       FIG. 2B  illustrates a portion of ring network  20  in further detail. In this example, routers R 1 -R 10  utilize extensions to an interior routing protocol, such as IGP, to discover MPLS rings and member nodes associated with ring network  20 . For example, as shown in  FIG. 2B , each of routers R 1 -R 10  may output enhanced IGP messages  24  on each interface to announce ring membership and identify neighboring nodes that are members of the same MPLS ring. IGP message may conform to, for example, ISIS or OSPF messages, as further described below. 
     In this example, router R 2  is coupled to both R 1  and R 3  by a respective set of multiple links  21 A and  21 B. Each set of multiple links  21 A,  21 B may be bundled at the layer two (2) (e.g., as a Link Aggregation Group), or explicitly bundled at the layer three (L 3 ) as a link bundle, or implicitly bundled by virtue of having the same ring ID. Initially, router R 2  outputs enhanced IGP messages  24  on each of its physical and/or logical interface, where the IGP messages carry additional data structures (e.g., Type-Length-Value structures, or TLVs) specifying, for each MPLS ring, a ring identifier and an index for router R 2  on the respective ring. In the example of  FIG. 2 , each of the enhanced IGP messages  24  output by router R 2  may carry an identifier of ‘ 20 ’ for MPLS ring of ring network  20  and an index of ‘2’ indicating the position around MPLS ring  20  for router R 2 . Definition of the ring identifier and each node index may be performed manually, such as by an administrator or network management system, or automatically by the routers of  FIG. 1  by way of discovery using, for example, extensions described herein to an interior gateway protocol (IGP). In this way, IGP may be used to discover ring neighbors and ring interfaces. 
     More specifically, initially each router R 1 -R 10  configured as a member of the MPLS ring of ring network  20  sends enhanced IGP messages on all interfaces, where the IGP messages specify the ring identifier and the ring index of the advertising node. Based on topology learned from the enhanced IGP messages  24 , each node may independently select the physical interfaces and links that define the ring. That is, responsive to the enhanced IGP messages  24 , each node dynamically determines its interface by which to reach the upstream neighboring router and the downstream neighboring router of the MPLS ring. 
     To select its downstream facing interface(s), each router R 1 -R 10  listens to incoming enhanced IGP messages  24  and selects, based on the IGP messages, one or more of its interfaces over which an enhanced IPG message was received that identified the MPLS ring (i.e., MPLS ring ‘ 20 ’ for ring network  20  in this example) and that included an index from a neighboring node that was the lowest index received that still exceeds the routers own index for the MPLS ring. In the example of  FIG. 2B , router R 2  receives enhanced IGP messages  24  via links  21 B from router R 3  having an index of ‘3.’ In response, router R 2  having and index of ‘2’ selects one or more of its interfaces to links  21 B as components of MPLS ring  20  for reaching its downstream neighbor router R 3 . As another example, router R 1  receives enhanced IGP messages  24  from router R 2  having an index of ‘2’ via links  21 A. In addition, router R 1  receives enhanced IGP messages  24  from router R 3  having index ‘3.’ In response, router R 1  having an index of ‘1’ selects one or more of its interfaces to links  21 A as components of MPLS ring  20  for reaching its downstream neighbor router R 2  since index ‘2’ is less than index ‘3’ and still exceeds index ‘1’ of router R 1 . Each of the routers R 1 -R 10  perform this process autonomously to select downstream neighbors based on the enhanced IGP messages  24  that each router may continuously and periodically output on its interfaces. 
     To select its upstream facing interface(s), each router R 1 -R 10  listens to all enhanced IGP messages  24  received on its interfaces and selects, based on the IGP messages, one or more of its interfaces over which an IGP message was received that identified MPLS ring ‘ 20 ’ and that included an index from a neighboring node that was the highest index less than its own index. In the example of  FIG. 2B , router R 2  receives enhanced IGP messages  24  via links  21 A from router R 1  having an index of ‘1.’ In response, router R 2  having and index of ‘2’ selects one or more of its interfaces to links  21 A as components of MPLS ring  20  for reaching its upstream neighbor router R 1 . Similarly, router R 3  receives enhanced IGP messages  24  from router R 2  having an index of ‘2’ via links  21 A. In addition, router R 3  receives enhanced IGP messages  24  from router R 1  having index ‘1.’ In response, router R 3  having and index of ‘3’ selects one or more of its interfaces to links  21 B as components of MPLS ring  20  for reaching its upstream neighbor router R 2  since index ‘2’ is greater than index ‘1’ of router R 1  and less than index ‘3’ of router R 3 . Each of the routers R 1 -R 10  perform this process autonomously to select upstream neighbors based on the enhanced IGP messages  24  that each router may continuously and periodically be outputting on its interfaces. 
     Router  10 ( 0 ) is configured with an index of ‘0,’ thereby designated router R 0  as the hub of the MPLS ring. As such, router R 0  announces itself on all of its interfaces as being a member of MPLS ring ‘ 20 ’ for ring network  20  and having and index of zero ‘0.’ In addition, router R 0  listens to all incoming enhanced IGP messages  24  on its interfaces and identifies the neighboring router having the highest index. In this example, router R 0  receives an enhanced IGP message having from router R 9  carrying a ring identifier of ‘ 20 ’ and an index of ‘9,’ an enhanced IGP message having from router R 8  carrying a ring identifier of ‘ 20 ’ and an index of ‘8,’ and an enhanced IGP message having from router R 1  carrying a ring identifier of ‘ 20 ’ and an index of ‘1.’ As such, router R 0  determines router R 1  is its downstream neighbor. In addition, router R 0  determines that index ‘9’ is the highest index of all of its neighboring nodes and, in response, selects router R 9  as its upstream neighbor. Further, based on this selection, router R 0  outputs enhanced IGP messages  24  identifying itself as having both indexes ‘0’ and ‘10,’ where router R 0  computed index ‘10’ as one greater than the index of its upstream neighbor R 9  that advertised the highest index for MPLS ring ‘ 20 ’. This process allows router R 9  to select router  0  ( 10 ) as its downstream neighbor, thereby closing MPLS ring ‘ 20 ’. 
     In this way, the techniques described herein provide for automatically discovering node membership of MPLS rings. Moreover, the techniques allow for member nodes to dynamically and automatically adjust the size and topology of MPLS rings in response to events and network topology changes. 
     As one example,  FIG. 2C  illustrates an example of ring network  20  in which router R 9  has become temporarily offline. As examples, router R 9  may have been expressly taken offline for maintenance or may have stopped operating due an internal error. In any event, the discovery techniques described above with allow the routers to construct MPLS ring ‘ 20 ’ so as to dynamically convergence to a different size based on the currently available topology of ring network  20 . In this example, router R 0  ceases to receive enhanced IGP messages  24  from router R 9  and, instead, received enhanced IGP messages  24  from router R 8  and router R 1 . According to the techniques described herein, after failing to receive IGP messages  24  from router R 9  for a threshold period of time, router R 0  selects router R 8  as its upstream neighbor and announces itself as having indexes ‘0’ and ‘9.’ Similarly, router R 8  receives enhanced IGP messages  24  from routers R 7  and R 0 ( 9 ) and selects its upstream neighbor as R 7  and its downstream neighbor as router R 0 ( 9 ), thereby forming a closed MPLS ring  20  shown in  FIG. 2C  and adapting to the network event of original router R 9  ceasing operation. Similarly, the dynamic discovery techniques allow the routers to expand the size of the MPLS ring ‘ 20 ’ for ring network  20  when supported by the network topology. 
       FIG. 3  further illustrates MPLS ring  20  once established for ring network  20  and, moreover, illustrates an example ring LSP  25  created for the MPLS ring upon completion of the signaling of the ring as described above. As described herein, the MPLS ring topology established for the MPLS ring may be constructed by routers R 1 -R 10  to comprise ten separate ring LSPs, each anchored at a different one of the ten routers R 1 -R 10 . That is, in one example implementation, each router R 1 -R 10  is associated with a respective ring LSP and each ring LSP is a multipoint-to-point (MP2P) bidirectional LSP that starts and ends at that anchoring router. As such, an MPLS ring of n nodes can be defined and specified according to the techniques herein and can be constructed to have n ring LSPs providing full connectivity for the ring, each of the ring LSPs having a different anchoring (egress) node of the ring. As such, the techniques may provide full connectivity within a ring of n routers using n ring LSPs as described herein rather than a full mesh of n*(n−1) point-to-point LSPs using conventional techniques. 
     In the example of  FIG. 3 , an example ring LSP  25  is shown associated with anchoring router R 1 . That is, in this example, router R 1  of the MPLS ring ‘ 20 ’ defined for ring network  20  provides a bidirectional multipoint communication from all of the other routers of the ring network  20 . As shown, ring LSP  25  starts and ends with router R 1 . Every other router R 2 -R 10  is able to source packet traffic, i.e., operate as an ingress, for ring LSP  25 . For all MPLS traffic traversing ring LSP  25 , router R 1  operates as an egress. 
     Although not shown in  FIG. 3 , a similar corresponding ring LSP can be automatically created for each of the other respective routers R 2 -R 10 . Moreover, as each ring LSP associated with the MPLS ring specified for ring network  20  is a bidirectional MP2P LSP, each of routers R 1 -R 10  can send traffic to any of the other routers in either a downstream (clockwise) direction or upstream direction (counterclockwise) using the ring LSP anchored at the router to which the traffic is being sent. 
     In general, every ring node participates in the creation of all ring LSPs for the network ring. In the example of  FIG. 3 , ring LSP  25  is anchored at router R 1  and has two primary data paths: one in the downstream direction going R 1 , R 2 , . . . , R 10  and the other in the upstream direction going R 10 , R 9 , . . . , R 1 . Any ring node, say router R 8 , can send traffic to router R 1  in either the downstream or upstream direction using ring LSP  25 . Typically, traffic will be sent in the direction with fewer hops, but some traffic (e.g., best effort traffic, where latency is not an issue) may be sent the other direction. As another example, traffic may be load balanced across both directions. 
     When establishing one or more ring LSPs for MPLS ring ‘ 20 ’ of ring network  20 , each of routers R 1 -R 10  learns, either through auto-discovery described above or configuration, that it is a node of a defined ring. As described above, through a learning process, each of routers R 1 -R 10  is able to learn the index for other ring nodes on the ring and is able to autonomously select the interfaces and links to be used for the ring. See, for example,  FIGS. 2A-2C  above. In addition, each of routers R 1 -R 10  autonomously signals each individual ring LSP of the ring network. For example, each of routers R 1 -R 10  may autonomously output RSVP PATH and RESV messages for each ring LSP of the ring network, even for those ring LSPs for which the router is operating as an intermediate node. In other words, the signaling of each ring LSP may occur at each router R 1 -R 10  of ring network  20  autonomously without being triggered by receipt of a PATH or RESV message from a neighboring router along the ring. In one example, RSVP PATH messages are automatically sent in the downstream direction and contain a corresponding upstream label for the advertising router and any traffic specification (TSPEC) for the data to be carried by the ring LSP. In this way, the RSVP PATH messages automatically create the upstream LSP of the bidirectional ring LSP. RSVP RESV messages are automatically issued by each router of the ring network and are sent in the upstream direction. The RSVP RESV messages advertise a downstream label for the sending router, may include an upstream TSPEC and automatically create the downstream LSP of the bidirectional ring LSP. 
     As further described below with respect to  FIGS. 7-8 , routers R 1 -R 10  may use a new RSVP session object and label bindings object presented herein for signaling ring LSPs, such as ring LSP  25 . In this example, there is no need for a LABEL REQUEST or an ERO to be used in either the RSVP RESV messages or the RSVP PATH message. Moreover, routers R 1 -R 10  may use the new RSVP objects to automatically create hierarchical LSPs, such as when forming access LSP  19  over ring LSP  17  of  FIG. 1 . 
     In general, at each node within an MPLS ring (such as at any of routers R 1 -R 10  of the MPLS ring for ring network  20 ), three types of ring LSP traffic operations may occur within a forwarding plane of the router: “add”, “transit”, or “drop” traffic. Moreover, each type of traffic can flow in the downstream (DS) or upstream (US) directions. Downstream traffic being added to the ring may arrive at any node from non-ring nodes connected to the ring node. Downstream transit traffic arrives at each ring node from its upstream neighbor on one or more interfaces selected as components of the ring. Downstream drop traffic at an anchoring ring node for an LSP is all of the transit traffic that arrives at the node from its upstream neighbors and is sent to non-ring nodes connected to and reachable through the anchoring node. Similar upstream traffic exists for any given ring node. 
       FIGS. 4A-4D  illustrate the MPLS ring described above and demonstrate the protection that is automatically built-in to the ring LSPs constructed according to the techniques described herein. In the example of  FIG. 4A , router R 6  is operating as an ingress for ring LSP  25  and communicating packet traffic  27  in MPLS form to router R 1  in a downstream direction. 
     In the event a link between router R 8  and router R 9  fails, as shown in  FIG. 4B , router R 8  immediately, upon detecting the local link failure, directs downstream forwarded traffic on LSP  25  along the upstream component of the bidirectional ring LSP  25 . As such, the traffic automatically flows around ring network  20  to router R 1  in the upstream direction along LSP  25 . 
     Further, as illustrated in  FIG. 4C , router R 8  outputs a notification message  26  to inform router R 7  of the link failure. As one example, router R 8  may output an RSVP PATH ERR message indicating that that the downstream portion of ring LSP  25  is unavailable. In response, router R 7  updates its forwarding plane and operates to reverse downstream traffic on LSP  25  to forward the traffic in the upstream direction along LSP  25  to router R 1 . Similarly, as shown in  FIG. 4D , router R 7  outputs a notification message  28  to inform router R 6  of the link failure. In response, router R 6  updates its forwarding plane and, since router R 6  is the source of the traffic in this example, injects the traffic directly into LSP  25  in the upstream direction to forward the traffic along LSP  25  to router R 1 . In this way, the ring LSPs described herein naturally responds to link failures along the ring and operate to reverse the direction of traffic along the ring until the traffic is injected and forwarded in the reverse direction. 
     In this way, in case of disruption of one direction of a ring LSP, packets are looped onto the other direction of the LSP within which they are current being forwarded. In some examples, this reroute action is pre-programmed in a forwarding information based (FIB) of a forwarding integrated circuit of the nodes to minimize the time between detection of a fault and the corresponding recovery action. Further, the fast reroute (FRR) actions described herein use active LSPs, and hence do not require computing or signaling additional backup, bypass, detour or loop-free alternative (LFA) paths. 
     Moreover, unlike conventional techniques that make use of a full mesh of P2P LSPs, the techniques allow traffic to flow in the reverse direction around the MPLS ring directly to the egress of the ring LSP  25 , i.e., router R 1 . For example, in response to the link failure shown in  FIG. 4B  using conventional techniques, router R 8  may have redirected traffic from one P2P LSP to a different P2P LSP, causing the traffic to flow entirely around ring network  20  to router R 9  in a counter-clockwise direction only to have router R 9  forward the traffic back to router R 1  in a clockwise direction. 
       FIG. 5  illustrates the MPLS ring described above and demonstrates the dynamic bandwidth management provided by MPLS rings constructed in accordance with the signaling mechanisms described herein. As further described below, explicit bandwidth for ring LSP  25  need not be expressly signaled at the time the ring LSP is being established. Instead, bandwidth may be requested and provisioned dynamically for ring LSP  25  as individual nodes (routers R 1 -R 10 ) of ring network  20  join the MPLS ring and as traffic services are provisioned on those member nodes. Moreover, equal bandwidth need not be provisioned around the entire MPLS ring. Instead, the techniques allow any member node to request allocation of bandwidth around the MPLS ring, e.g., in a downstream direction, for supporting bandwidth required by the member node and upstream member nodes. As such, the bandwidth allocation can vary to accommodate aggregated bandwidth requirements around the ring, and can be dynamically updated as traffic is provisioned. 
     Initially, ring LSP  25  is signaled and provisioned with zero (0) bandwidth. Later, as actual services are provisioned on individual member nodes of the MPLS ring for ring LSP  25 , their bandwidth requirements are added to the bandwidth allocation for ring LSP  25 . Moreover, the allocations are added to and accounted for by ring LSP  25  from the point at which traffic for the services enter the ring LSP  25  and downstream therefrom. 
     In the example of  FIG. 5 , router R 6  is provisioned with a pseudowire to carry inbound traffic  28 , where the pseudowire is configured for 1 Gigabit per second of bandwidth. As such, router R 6  outputs an updated RSVP PATH message  29  to inform downstream routers that 1 Gigabit per second of bandwidth has been reserved for ring LSP  25 . For example, router R 6  may output an updated RSVP PATH message  29  in which the traffic specification (TSPEC) is updated to reflect the new downstream traffic that the downstream routers R 7 -R 10  should expect. Each of the downstream routers performs admission control based on the updated value. If successful, the service is accepted. If not, the rejecting downstream router may issue an RSVP PATH ERR message in the upstream direction with an admission control error. Upon receiving an RSVP PATH ERR message, the upstream router may reject all or portions of the incoming traffic  28  being provisioned until the downstream routers accept the RSVP PATH message  29  having the reduced TSPEC. Alternatively, the upstream router may send an RSVP PATH ERR message in the upstream direction to cause further upstream along the ring to be dropped. 
     Subsequently, router R 5  may be similarly provisioned with a pseudowire to carry inbound traffic  33 , where the pseudowire is configured for 1 Gigabit per second of bandwidth. As a result, router R 5  outputs an updated RSVP PATH message  31  to inform downstream routers that 1 Gigabit per second of bandwidth has been reserved for ring LSP  25 . In response, router R 6  outputs an updated RSVP PATH message  29  to inform downstream routers that 2 Gigabit per second of bandwidth, i.e., the sum of the bandwidth required for traffic  28 ,  33  from both pseudowires, has been reserved for ring LSP  25  within the downstream routers. As such, the resulting signaled bandwidth for ring LSP  25  is zero from routers R 1 -R 5 , 1 Gigabit per second for the portion of the ring LSP from router R 5  to router R 6  and 2 Gigabits per second for the portion of the ring LSP from router R 6  to router R 10 . In this way, the signaled bandwidth of any ring LSP may increase along the downstream direction and may be asymmetric in the upstream direction. 
     Although described for purposes of example as one ring LSP per ring node, each node may operate as an anchor node for multiple ring LSPs. For example, if Quality of Service (QoS)-based bandwidth management with k classes is desired, k ring LSPs of a single class may be created per ring node. Alternatively, a single, k-class, DiffServ-aware ring LSP per ring node may be created. In this way, the techniques allow bandwidth for a ring LSP to be easily and dynamically managed. 
       FIG. 6A  illustrates in further detail the control plane signaling messages utilized when establishing a ring LSP, such as ring LSP  25 . In this example, routers R 7 -R 9  ( FIG. 2 ) output control plane message in accordance with a label distribution protocol, such as RSVP. In the example of  FIG. 6A , router R 8  allocates an upstream label UL  85  and a downstream label DL  85  for ring LSP  25 . Router R 8  sends upstream label UL  85  to downstream neighbor router R 9  via RSVP PATH message  52  and sends downstream label  85  to upstream neighbor router R 7  via RSVP RESV message  54 . As described herein, router R 8 , which is intermediate router along ring LSP  25 , need not wait for RSVP messages from neighboring routers to trigger RSVP messages  52 ,  54 . 
     Other routers along ring LSP  25  operate in a similar manner. For example, router R 7  allocates an upstream label UL  75  and a downstream label DL  75  for ring LSP  25  and sends upstream label UL  75  to downstream neighbor router R 8  via RSVP PATH message  50  and sends downstream label  75  to upstream neighbor router R 6  (not shown in  FIG. 6 ) via RSVP RESV message  58 . Similarly, router R 9  allocates an upstream label UL  95  and a downstream label DL  95  for ring LSP  25  and sends upstream label UL  95  to downstream neighbor router R 10  (not shown in  FIG. 6A ) via RSVP PATH message  60  and sends downstream label  95  to upstream neighbor router R 8  via RSVP RESV message  56 . 
     In response to receiving the RSVP messages, the routers install primary forwarding entries within internal forwarding integrated circuits that routes packets within the network including along ring LSP  25 . For example, upon receiving RSVP PATH message  50 , router R 8  installs upstream label UL  75  within the forwarding integrated circuit. That is, router R 8  may program the forwarding integrated circuit with a primary forwarding entry for ring LSP  25  as follows:
         US Primary Entry: swap UL  85  with UL  75 ; output packet to Router R 7 .
 
Similarly, upon receiving RSVP RESV message  56 , router R 8  installs upstream label DL  95  within the forwarding integrated circuit. That is, router R 8  may program the forwarding integrated circuit with a primary forwarding entry for ring LSP  25  as follows:
   DS Primary Entry: swap DL  85  with DL  95 ; output packet to Router R 9 .
 
Note that each of these actions can occur independently. That is, ring node does not need to have received both an RSVP PATH message and an RSVP RESV message to install primary forwarding entries within the forwarding information of its forwarding integrated circuit. Further, as updated RSVP PATH and RSVP RESV messages are received with a LABEL_BINDINGS object carrying updated labels, the corresponding forwarding state is installed. Similarly, as updated RSVP PATH and RSVP RESV messages are received without the LABEL_BINDINGS object, the corresponding LFIB state is removed.
       

     Routers along a ring LSP may also similarly install fast reroute entries in the case of a link failure or other event that prevents traffic flow. For example, upon receiving RSVP PATH message  50 , router R 8  installs fast reroute (FRR) forwarding entry as follows:
         DS FRR: swap DL  85  with UL  75 ; output packet to Router R 7 .
 
The DS primary forwarding entry is used when the downstream direction of ring LSP  25  is operational. The DS FSS forwarding entry is automatically used when a fault is detected on the downstream path and has the effect of switching packets to the upstream path. That is, in the event of a link fail in the downstream direction, as shown in  FIG. 6B , router R 8  is able to immediately U-turn downstream traffic  62  and direct the traffic in the upstream direction along ring LSP  25 . Router R 8  may also propagate notification of the error upstream, as described above, such as by sending an RSVP PATH ERR message for ring LSP  25  to router R 7 .
       

     The following illustrates an example format of an enhanced RSVP PATH message that has been configured to establish ring LSPs as described herein. 
                                                &lt;Path Message&gt; ::=   &lt;Common Header&gt; [ &lt;INTEGRITY&gt; ]               &lt;SESSION&gt; &lt;RSVP_HOP&gt;               &lt;TIME_VALUES&gt;               [ &lt;SESSION_ATTRIBUTE&gt; ]               &lt;SENDER_TSPEC&gt;               &lt;UPSTREAM_LABEL&gt;,                        
where &lt;SESSION&gt; represents a session object for specifying the configuration information described herein for a ring LSP, &lt;SENDER_TSPEC&gt; represents the traffic specification of the sender, and &lt;UPSTREAM_LABEL&gt; specifies the upstream label allocated by the router sending the RSVP PATH message and may conform to the label binding object of  FIG. 8 . An RSVP RESV message may be similarly modified such that RSVP enabled routers can specify MPLS rings and ring LSP in accordance with the techniques described herein.
 
       FIG. 7  illustrates an example format for an RSVP session object  70  that may be used to signal a ring LSP in accordance with the techniques described here. For example, RSVP session object  70  may be used in RSVP RESV message or an RSVP PATH message when signaling a ring LSP. 
     In one example, RSVP session object  80  may be of C-Type of either RING_IPv4 or RING_IPv6, which are new RSVP session types defined herein. In the example session object  80 , the RING LSP ANCHOR IP ADDRESS may be the loopback address of the anchor node. The RING ANCHOR INDEX is the index of the anchor node in the ring LSP being constructed. The RING LSP ID is the unique identifier of the MPLS ring LSP, which may be different from the index of the anchor node. The RING LSP ID may, for example, be used when an anchor node has multiple ring LSPs, for example, one per QoS class. The RING ID is the unique identifier of the MPLS ring. 
       FIG. 8  illustrates an example format for an RSVP label bindings object  72  that may be used to signal label bindings for a ring LSP in accordance with the techniques described here. RSVP label bindings object  72  may be inserted in the RSVP PATH and RSVP RESV messages initiated by the ring LSP anchor. A ring node that receives a RSVP PATH or RSVP RESV messages for a ring LSP with the label bindings object  72  updates its own RSVP PATH and RSVP RESV messages for that ring LSP to include the label bindings object as received. 
     Inclusion of RSVP label bindings object  72  within a RSVP PATH and RSVP RESV messages indicates continuity of signaling from the ring anchor. An anchoring node (e.g., router R 1  for ring LSP  25  of  FIG. 3 ) automatically inserts RSVP label bindings object  72  in its PATH message for its ring LSP. Other nodes around the ring LSP, such as routers R 2 -R 10  do not insert RSVP label bindings object  72  in the initial RSVP PATH messages they each autonomously originate. However, if an intermediate node along a given ring LSP receives an RSVP PATH message for the ring LSP that includes RSVP label bindings object  72 , the intermediate router copies RSVP label bindings object  72  to its own RSVP PATH message and outputs and updated RSVP PATH message. As such, any node receiving an RSVP PATH message with RSVP label bindings object  72  knows that upstream labels are present and have been successfully allocated all the way to the anchor for the particular ring LSP, i.e., that continuity exists. If a ring node receives an updated RSVP PATH message without RSVP label bindings object  72 , the node updates its own RSVP PATH message for the ring LSP to remove RSVP label bindings object  72  and outputs the updated RSVP PATH message, thereby providing an indication to downstream nodes that continuity does not exist. Similarly, RSVP RESV messages may make use of RSVP label bindings object  72  to provide an indication of continuity in the downstream direction. 
     Further, RSVP label bindings object  72  can be used to facilitate automatic creation of hierarchical LSPs over ring LSPs. For example, an auto-hierarchy LSP to a non-ring node, such as service node S 2  of  FIG. 1 , can easily be created when a ring node through which the non-ring node is reachable (e.g., router R 0  ( 6 ) of ring network  14 B of  FIG. 1 ) receives an RSVP message containing a label binding L to the non-ring node S 2 . In response, router R 0  of ring network  14 B allocates a label M to reach service node N 2  and installs a label forwarding entry within its forwarding plane to swap M with L and forward the traffic to service node S 2 . Next, router R 0  advertises service node S 2 &#39;s loopback address and label M for reaching service node S 2  to all other ring nodes using the new label bindings object  72  of  FIG. 8 . This enables other ring nodes (e.g., routers R 1 -R 5 ) to send traffic to the non-ring node using a two-label stack specified in label bindings object  72 : an outer label to the ring node (router R 0 ) through which the non-ring node (S 2 ) is reachable (either DS or US), and the inner label M. In the example of  FIG. 1 , router R 3  can in turn allocate a label S, install an LFIB entry to swap S with the two-label stack to service node S 2 , and announce S to another non-ring node, such as access node AN 1 . In this way, access LSP  19  may easily be created as an auto-hierarchical tunnel that allows AN 1  to send traffic to S 2 . Other mechanisms may be used for creating a tunnel, such as targeted LDP. 
     The techniques described herein for signaling MPLS rings and ring LSPs are described in reference to RSVP for purposes of example. Other label distribution protocols may be used. As one example, the Label Distribution Protocol (LDP) can be used to signal ring LSPs. In this example, a ring node (router R i ) may announce a ring LSP for which it is operating as an anchor by sending an LDP label mapping message to its downstream and upstream neighbors in the ring in accordance with the LDP protocol. The ring node specifies, within a FEC TLV of the LDP label mapping message, the loopback address of the ring node. In the Label TLV of the LDP label mapping message, the ring node places labels for the upstream and downstream directions, respectively. In addition, the member node inserts a new Ring TLV in the message that specifies the ring identifier, the node index of the ring anchor, and an indication whether this announcement should be propagated downstream or upstream. The former indication would be sent to its downstream neighbor, the latter to its upstream. As these messages propagate around the ring, the receiving nodes can create upstream or downstream (respectively) ring LSPs anchored at router A router Rj sending traffic downstream to Ri, on learning of a fault in the ring in the downstream direction, can, as described herein, loop traffic back in the upstream direction to Ri, thus effecting traffic protection without the need of loop-free alternatives or other such means. 
       FIGS. 9-12  illustrate example message formats for discovering MPLS rings and member nodes using an interior gateway protocol (IGP), such as the Intermediate System to Intermediate System (ISIS) protocol or the Open Shortest Path First (OSPF) protocol. 
     For example,  FIG. 9  illustrates a new top-level Type-Length-Value (TLV)  74  that may be included in an ISIS hello message that may be sent by a ring node to announce all of the MPLS rings of which the ring node is a member. In this example, TLV  74  first includes a Type value of ‘MPLS Ring Type,’ which would be a newly allocated TLV type consuming the first octet. The second octet stores the Length value, where the value would be would be 6*N octets, where N is the number of rings in which this node is participating. As shown in  FIG. 9 , each ring is specified by 6 octets including a ring identifier (4 octets) and a node index (2 octets), where the ring identifier specifies the identifier of an MPLS ring and the node index specifies the position of the node around the MPLS ring as described herein. The advertising node may, for example, have a different index for each of the MPLS rings specified in TLV  74 . In IS-IS, TLV  74  represents a sub-TLV of an Extended IS Reachability TLV, as defined in  IS - IS Extensions for Traffic Engineering, RFC  5305, section 3, the entire contents of which are incorporated herein. 
       FIG. 10  illustrates an example top-level TLV  76  for use within a new OSPF link-local LSA for announcing membership within one or more MPLS rings and the position within each ring for the announcing node. As with TLV  74  for ISIS, TLV  76  for OSPF has a TLV length value of 6*N octets, where N is the number of rings in which this node is participating. In OSPF, TLV  79  represents a sub-TLV of a link TLV as specified in Traffic Engineering (TE) Extensions to OSPF Version 2, RFC 3630, section 2.4, the entire contents of which are incorporated herein. 
       FIGS. 11 and 12  illustrate two new sub-TLVs for the IGP LSAs that are used on each interface of the node and just contain ring identifiers. As such, the new TLVs have TLV length values of 4*N octets since the ring index is not included.  FIG. 11 , for example, shows a new sub-TLV  78  for use in an IS-IS LSA.  FIG. 12  shows a new sub-TLV  79  for use in an OSPF LSA. 
       FIG. 13  is a block diagram illustrating an exemplary device, e.g., a router or gateway, capable of implementing the techniques described herein. Router  80  may comprise any router in a network, such as SP network  12 . For example, router  80  may comprise a gateway, an edge or access router, a core router or other device illustrated herein that forwards MPLS traffic using ring LSPs in accordance with the techniques described herein. 
     In the example of  FIG. 13 , router  80  includes control unit  82  in which routing component  86  provides control plane functionality for router  80 . Router  80  also includes a plurality of packet-forwarding engines  114 A- 114 N (“PFEs  114 ”) and a switch fabric  118  that collectively provide a data plane for forwarding network traffic. PFEs  114  receive and send data packets via interface cards  112  (“IFCs  112 ”). In other embodiments, each of PFEs  114  may comprise more or fewer IFCs. Although not shown, PFEs  114  may each comprise a central processing unit (CPU) and a memory. In this example, routing component  86  is connected to each of PFEs  114  by a dedicated internal communication link  120 . For example, dedicated link  34  may comprise a Gigabit Ethernet connection. Switch fabric  118  provides a high-speed interconnect for forwarding incoming data packets between PFEs  114  for transmission over a network. U.S. Pat. No. 8,050,559, entitled MULTI-CHASSIS ROUTER WITH MULTIPLEXED OPTICAL INTERCONNECTS, describes a multi-chassis router in which a multi-stage switch fabric, such as a 3-stage Clos switch fabric, is used as a high-end forwarding plane to relay packets between multiple routing nodes of the multi-chassis router. The entire contents of U.S. Patent Application 2008/0044181 are incorporated herein by reference. 
     Routing component  86  provides an operating environment for execution of various protocols  89  that may comprise software processes having instructions executed by a computing environment. As described in further detail below, protocols  89  provide control plane functions for storing network topology in the form of routing tables or other structures, executing routing protocols to communicate with peer routing devices and maintain and update the routing tables, and providing management interface(s) to allow user access and configuration of router  80 . Control unit  82  provides an operating environment for routing component  86  and may be implemented solely in software, or hardware, or may be implemented as a combination of software, hardware or firmware. For example, control unit  82  may include one or more processors which execute software instructions. In that case, routing component  86  may include various software modules or daemons (e.g., one or more routing protocol processes, user interfaces and the like), and control unit  82  may include a computer-readable storage medium, such as computer memory or hard disk, for storing executable instructions. 
     Command line interface daemon  92  (“CLI  92 ”) provides an interface by which an administrator or other management entity may modify the configuration of router  80  using text-based commands. Simple Network Management Protocol daemon  99  (“SNMP  99 ”) comprises an SNMP agent that receives SNMP commands from a management entity to set and retrieve configuration and management information for router  80 . Using CLI  92  and SNMP  99 , management entities may enable/disable and configure services, install routes, enable/disable and configure rate limiters, and configure interfaces, for example. 
     One or more routing protocols, such as IGP  94 , maintains routing information in the form of routing information base (RIB)  104  that describes a topology of a network, and derives a forwarding information base (FIB)  106  in accordance with the routing information. In general, the routing information represents the overall topology of the network. IGP  94  interacts with kernel  101  (e.g., by way of API calls) to update RIB  104  based on routing protocol messages received by router  80 . RIB  104  may include information defining a topology of a network, including one or more routing tables and/or link-state databases. Typically, the routing information defines routes (i.e., series of next hops) through a network to destinations/prefixes within the network learned via a distance-vector routing protocol (e.g., BGP) or defines the network topology with interconnected links learned using a link state routing protocol (e.g., IS-IS or OSPF). In contrast, FIB  106  is generated based on selection of certain routes within the network and maps packet key information (e.g., destination information and other select information from a packet header) to one or more specific next hops and ultimately to one or more specific output interface ports of IFCs  112 . Routing component  86  may generate the FIB in the form of a radix tree having leaf nodes that represent destinations within the network. Example details of an example embodiment of a router that utilizes a radix tree for route resolution are provided in U.S. Pat. No. 7,184,437, the contents of which being incorporated herein by reference in its entirety. 
     Routing component  86  also provides an operating environment of one or more traffic engineering protocols to establish tunnels for forwarding subscriber packets through the ordered set of service nodes associated with different service chains. For example, RSVP-TE  96  may execute the Resource Reservation Protocol with Traffic Engineering extensions to exchange traffic engineering (TE) information, such as MPLS labels for enabling label-based packet forwarding. Routing component  86  may, as another example, execute the Label Distribution Protocol  97  to signal MPLS rings and ring LSPs as described herein. As another example, routing component  86  may use GRE or IP-based tunneling protocols to establish traffic engineered tunnels. Routing component  86  may maintain, for example, a traffic engineering database (TED)  109  to store the traffic engineering data. 
     Routing component  86  communicates data representative of a software copy of the FIB  106  into each of PFEs  114  to control forwarding of traffic within the data plane. This allows the software FIB stored in memory (e.g., RAM) in each of PFEs  114  to be updated without degrading packet-forwarding performance of router  80 . In some instances, routing component  86  may derive separate and different software FIBs for each respective PFEs  114 . In addition, one or more of PFEs  114  include application-specific integrated circuits (ASICs  116 ) that PFEs  114  program with a hardware-copy of the FIB based on the software FIBs (i.e., hardware versions of the software FIBs) copied to each respective PFE  114 . 
     For example, kernel  101  executes on master microprocessor  102  and may comprise, for example, a UNIX operating system derivative such as Linux or Berkeley Software Distribution (BSD). Kernel  101  processes kernel calls from IGP  94  and RSVP-TE  96  to generate forwarding information in the form of FIB  106  based on the network topology represented in RIB  104 , i.e., performs route resolution and path selection. Typically, kernel  101  generates FIB  106  in the form of radix or other lookup trees to map packet information (e.g., header information having destination information and/or a label stack) to next hops and ultimately to interface ports of interface cards associated with respective PFEs  114 . FIB  106  may associate, for example, network destinations with specific next hops and corresponding IFCs  112 . For MPLS-related traffic forwarding, FIB  106  stores, for a given FEC, label information that includes an incoming label, an outgoing label, and a next hop for a packet. 
     Master microprocessor  102  executing kernel  101  programs PFEs  114  to install copies of the FIB  106 . Microprocessor  102  may comprise one or more general- or special-purpose processors such as a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA), or any other equivalent logic device. Accordingly, the terms “processor” or “controller,” as used herein, may refer to any one or more of the foregoing structures or any other structure operable to perform techniques described herein. Example details of an example embodiment of a router in which a processor programs one or more forwarding components are provided in U.S. Pat. No. 7,990,993, the contents of which being incorporated herein by reference in its entirety. 
     In this example, ASICs  116  are microcode-controlled chipsets (i.e., forwarding circuits) programmably configured by a slave microprocessor executing on each of PFEs  114 . When forwarding packets, control logic with each ASIC  116  traverses the forwarding information (FIB  106 ) received from routing component  86  and, upon reaching a FIB entry for the packet (e.g., a leaf node), microcode-implemented control logic  56  automatically selects a forwarding next hop and processes the packets in accordance with the operations defined within the next hop. In this way, ASICs  116  of PFEs  114  process packets by performing a series of operations on each packet over respective internal packet forwarding paths as the packets traverse the internal architecture of router  80 . Operations may be performed, for example, on each packet based on any of a corresponding ingress interface, an ingress PFE  114 , an egress PFE  30 , an egress interface or other components of router  80  to which the packet is directed prior to egress, such as one or more service cards. PFEs  114  each include forwarding structures that, when executed, examine the contents of each packet (or another packet property, e.g., incoming interface) and on that basis make forwarding decisions, apply filters, and/or perform accounting, management, traffic analysis, and load balancing, for example. 
     In one example, each of PFEs  114  arranges forwarding structures as next hop data that can be chained together as a series of “hops” along an internal packet forwarding path for the network device. In many instances, the forwarding structures perform lookup operations within internal memory of ASICs  116 , where the lookup may be performed against a tree (or trie) search, a table (or index) search. Other example operations that may be specified with the next hops include filter determination and application, or a rate limiter determination and application. Lookup operations locate, within a lookup data structure (e.g., a lookup tree), an item that matches packet contents or another property of the packet or packet flow, such as the inbound interface of the packet. The result of packet processing in accordance with the operations defined by the next hop forwarding structure within ASICs  116  determines the manner in which a packet is forwarded or otherwise processed by PFEs  114  from its input interface on one of IFCs  112  to its output interface on one of IFCs  112 . 
     In general, kernel  101  may generate FIB  106  and thereby program ASICs  116  to store forwarding structures associated with each service chain. For example, ASICs  116  may be configured with forwarding information that specifies traffic engineering information, such as IP header information or MPLS labels, as well as operations for causing programmable ASICs  116  to encapsulate subscriber packets in accordance with the forwarding information. In this way, ASICs  116  may process subscriber packets to select particular service paths for each packet and encapsulate the subscriber packets in accordance with the selected service paths. Routing component  86  may generate RIB  104  and FIB  106  to associate subscriber packet flows with particular service paths based on one or more service profiles associated with each subscriber, as may be received from an Authentication, Authorization and Accounting (AAA) server, a policy controller, Software-Defined Network (SDN) controller or other network element. 
     In general, RSVP-TE  96 , LDP  97  and/or IGP  94  operates in accordance with the techniques described herein. For example, RSVP-TE  96  may maintain routing communication sessions with peer routers and output control-plane messages, e.g., RSVP PATH messages and RSVP RESV messages as described herein to signal membership of ring LSPs. Moreover, RSVP-TE  96  may program RIB  104  with forwarding labels for LSP rings as learned from the peer router, which in turn causes kernel  101  to program forwarding ASICS  116  with primary and backup forward entries for forwarding MPLS traffic as described herein. Further, as described herein, RSVP-TE  96  may implement the automatic bandwidth management functions for LSP rings based on provisioning information received via CLI  92  or SNMP  99 , for example. In addition, or alternatively, LDP  97  may perform these functions. IGP  94  may be extended as described herein for auto-discovery of MPLS rings and ring LSPs. 
     The architecture of router  80  illustrated in  FIG. 13  is shown for example purposes only. This disclosure is not limited to this architecture. In other examples, router  80  may be configured in a variety of ways. In one example, some of the functionally of control unit  82  may be distributed within IFCs  112 . Control unit  82  may be implemented solely in software, or hardware, or may be implemented as a combination of software, hardware, or firmware. For example, control unit  82  may comprise one or more of a processor, a programmable processor, a general purpose processor, an integrated circuit, an Application Specific Integrated Circuit (ASIC), a Field Programmable Gate Array (FPGA), or any type of hardware unit capable of implementing the techniques described herein. Control unit  82  may further include one or more processors which execute software instructions stored on a computer readable storage medium, such as random access memory (RAM), read only memory (ROM), programmable read only memory (PROM), erasable programmable read only memory (EPROM), electronically erasable programmable read only memory (EEPROM), non-volatile random access memory (NVRAM), flash memory, a hard disk, a CD-ROM, a floppy disk, a cassette, magnetic media, optical media, or other computer-readable storage media. In some instances, the computer-readable storage medium may include instructions that cause a programmable processor to perform the techniques described herein. 
       FIG. 14  is a flowchart illustrating example operation of a network device, such as a router, in accordance with the techniques described herein. 
     As shown in  FIG. 14 , the router executes a routing protocol, such as an interior gateway protocol (IGP), to discover, using the IGP, neighboring routers that are also designated as members of a multi-protocol label switching (MPLS) ring associated with a ring network ( 200 ). For example, the router may receive enhanced IGP messages from two or more of the routers and select, based on the indexes within the enhanced IGP messages, an upstream neighboring one of the routers and a downstream neighboring one of the routers for the ring LSPs of the MPLS ring. 
     Responsive to discovering the neighboring routers that are members of the MPLS ring, the router establishes a plurality of ring label switched paths (LSPs) for the MPLS ring ( 202 ). Each of the ring LSPs are configured to transport MPLS packets around the ring network to a different one of the routers that is designated as an egress router for the respective ring LSP. Moreover, each of the ring LSPs is configured as a multipoint-to-point (MP2P) LSP for which any of the routers within the ring network can operate as an ingress to source packet traffic into the ring LSP for transport to the respective egress router for the ring LSP. For example, a processor of the router may be configured to output, for each of the ring LSPs, one or more messages in accordance with a label distribution protocol to signal membership in the MPLS ring and the ring LSPs. For each of the ring LSPs, the messages output by the processor includes an identifier of the MPLS ring, an identifier for the respective one of the plurality of routers that operates as the egress for the ring LSP and label bindings specifying one or more MPLS labels associated with the ring LSP and, for each of the MPLS labels, a network address to which network traffic having the respective MPLS label will be forwarded by the forwarding component upon receipt of the network traffic. 
     When establishing the LSPs, the router may specify bandwidth requirements in a downstream direction to the egress router for any packet traffic to be sourced into the ring LSP by the router ( 204 ). For example, each of the messages output by the routers specifies bandwidth requirements in a downstream direction to the egress router for any packet traffic to be sourced into the ring LSP by the respective one of the routers around the ring LSP. Moreover, after establishing the ring LSPs and responsive to receiving configuration data provisioning additional network traffic to be sourced by the router into the ring LSP, the router may output an updated message to reserve bandwidth in the downstream direction around the ring LSP for the additional network traffic. Further, as services are provisioned upstream from the router, the router may reserve bandwidth in the downstream direction around the ring LSP to the egress for the sum of the network traffic to be sourced by the router and any network traffic to be sourced by any of the routers that are positioned in the upstream direction from the router. 
     The router forwards network traffic as MPLS packets around the ring network in accordance with the ring LSPs ( 204 ). Further, in response to a network event that would prevent the MPLS packets from reaching the egress router in the downstream direction, the router may automatically redirect the MPLS packets from a downstream direction of the ring LSP to the upstream direction of the ring LSP to forward the MPLS packets around the ring in the upstream direction to the egress router ( 206 ). For example, when forwarding the MPLS packets in the downstream direction, the router may receive the MPLS packets from an upstream neighboring one of the routers, swap the downstream label on each of the MPLS packets with a downstream label advertised by a downstream neighboring one of the router and forward the MPLS packet in the downstream direction around the ring LSP. When automatically redirecting the MPLS packets in response to a network event, the router may receive the MPLS packets from the upstream neighboring one of the routers, swap the downstream label on each of the MPLS packets with the upstream label advertised by the upstream neighboring one of the router and forward the MPLS packet back in the upstream direction around the ring LSP. 
     Various embodiments of the invention have been described. These and other embodiments are within the scope of the following claims.