# Patent

## Title
Protecting multi-segment pseudowires

## Abstract

The protection of multi-segment pseudowires by utilizing pre-computed backup paths is disclosed herein. Disclosed embodiments include methods that establish at least one backup path for multi-segment pseudowires, the establishing being performed prior to detection of failure in the primary path. Upon detecting a path failure, the detected failure is signaled to the head-end, a pre-computed backup path is chosen, and the chosen backup path is signaled to the tail-end. In other disclosed embodiments, apparatus are configured to establish, prior to detection of failure in the primary path, at least one backup path for the multi-segment pseudowire. Networks can be configured to signal a detected failure to the head-end; choose a pre-computed backup path; and signal the backup path to the tail-end.

## Background
FIELD OF THE INVENTION
The present disclosure relates generally to protecting pseudowires. For example, the present disclosure illustrates that multi-segment pseudowires can be protected by utilizing pre-computed backup paths.
BACKGROUND OF THE INVENTION
Computer networks have become ubiquitous. Computer networks include the Internet, Service Provider (SP) networks, private networks, and Local Area Networks (LANs). A network such as an SP network may include peripherally located Provider Edge (PE) routers, each of which couples to one or multiple Customer Edge (CE) routers. The PE routers are used to maintain routing and forwarding context for each customer. The CE routers may couple to private LANs associated with one or multiple customers. The private LANs are frequently referred to as core networks. The PE routers learn local customer routes from the CE routers and distribute remote customer routes to the CE router. The PEs may use Border Gateway Protocol (BGP) to distribute customer routes to each other.
To support operation, the PE routers typically maintain Virtual Routing and Forwarding (VRF) information in a table (a VRF table) dictating how to route and forward traffic through the shared physical network to support corresponding Virtual Private Networks (VPNs) for the different customers. Typically, the SP network selectively couples the LANs to each other through links created between its PE routers. For the core network, an ingress PE may use BGP functions to determine the egress PE. The ingress PE can put the packet in a two-level Multi Protocol Label Switching (MPLS) stack. The top label is used to tunnel packets to the egress PE to accomplish MPLS forwarding through the core network. The bottom label is used by the egress PE to identify the outgoing route for the packet. VPNs provide a secured means for transmitting and receiving data between network nodes even though a corresponding physical network supporting propagation of the data is shared by many users (and VPNs). In a typical networking environment used for routing data, the environment may include a number of Customer Edge (CE) routers, a number of Provider Edge (PE) routers and a packet-switched network (PSN). Data, encapsulated in layer-2 frames, may be forwarded from a first CE router to a first PE router, from the first PE router across the PSN to a second PE router, and from the second PE router to a second CE router.
In particular conventional systems, a PDU (e.g., a frame) traverses the networking environment beginning at a first CE router and ending up at a second CE router. The first CE router sends a layer-2 PDU to an ingress PE router. The ingress PE router receives the PDU and encapsulates the PDU with MPLS labels which are used to identify the individual port/circuit and the egress layer-3 PE router. The encapsulated PDU is then forwarded on the PW, across the packet-switched network, to an egress layer-3 PE router. The egress layer-3 PE router removes the MPLS label that identifies the port/circuit that was added by the ingress PE router and forwards the layer-2 PDU to the second CE router.
Pseudowires (“PWs”) are known in the art of computer networking and telecommunications. A Pseudowire (PW) may be utilized to transfer data across the PSN. In general, a pseudowire is an emulation of the essential attributes of a telecommunications service over a Packet Switched Network (“PSN”). The service may be ATM, T1 leased line, Frame Relay, Ethernet, low-rate TDM, or SONET/SDH, for example. The PSN may be Multiprotocol Label Switching (“MPLS”), IP (either Ipv4 or Ipv6), or Layer 2 Tunnel Protocol Version 3 (“L2TPv3”). The functions provided by a PW includes encapsulating service-specific bit streams, cells, or PDUs arriving at an ingress port and carrying them across a path or tunnel, managing their timing and order, and any other operations required to emulate the behavior and characteristics of the particular service. Pseudowires can be used to carry ingress layer-2 traffic from an ingress PE router to an egress PE router, and the forward the layer-2 traffic out of an egress port of the egress PE router.
Multi-Segment Pseudowires (“MS-PWs”) are also known in the art. MS-PWs may transit more than one PSN domain and more than one PSN tunnel. MS-PWs have become necessary for the extension of network and transport services such as layer-2 and layer-3 technologies across domains. A domain refers to a collection of network elements within a common sphere of address management or path computational responsibility such as an IGP area, Autonomous System (“AS”), service provider area, or area with reduced routing state such as is the case with Route Summarization in effect. Multiple pseudowire segments (i.e., single-hop pseudowires) are stitched together to create a single end-to-end multi-segment pseudowire (i.e., a multi-hop pseudowire) from the source of the pseudowire to the destination of the pseudowire.



BRIEF DESCRIPTION OF THE DRAWINGS
The above and further advantages of this invention may be better understood by referring to the following description in conjunction with the accompanying drawings, in which like numerals indicate like structural elements and features in the various figures. The drawings are not meant to limit the scope of the invention. For clarity, not every element may be labeled in every figure. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.
FIG. 1 illustrates an example network comprising a plurality of pseudowire elements forming a multi-segment pseudowire.
FIG. 2 illustrates an example method useful for protecting a multi-segment pseudowire.
FIG. 3 illustrates a second example method useful for protecting multi-segment pseudowires.
FIG. 4 illustrates a third example method useful for protecting multi-segment pseudowires.
FIG. 5 illustrates an example computer system architecture for a computer system that performs methods useful in protecting multi-segment pseudowires in accordance with embodiments disclosed herein.



DESCRIPTION OF EXAMPLE EMBODIMENTS
Overview
If a stitching point in a multi-segment pseudowire fails, the pseudowire will eventually recover based on IGP convergence within all segments of the end-to-end path. However, this convergence time can be extensive given the multiple segment nature of the pseudowire. Additionally, the fast restoration of an end-to-end pseudowire path is not possible using current signaling protocols. Multi-level pseudowires suffer from a lack of reachability information because pseudowire segments in multi-segment pseudowires are signaled a priori and only between pseudowire switching routers. Because of this lack of reachability information, rerouting and repair of defects in the pseudowire can occur only at the transport layer (i.e., MPLS or IP), which carries the pseudowire segments between stitching points. Thus, with the current technology, restoration of an end-to-end multi-segment pseudowire path requires the entire end-to-end pseudowire path to be resignalled or requires the use of manual methods, which are slow, prone to error, and insufficiently resilient to additional future failures.
Techniques discussed herein deviate with respect to conventional techniques for protecting pseudowires such as those discussed above and other techniques also known in the prior art. That is, the protection of multi-segment pseudowires by utilizing pre-computed backup paths is disclosed herein. Techniques disclosed herein may be encoded as logic in one or more tangible media for execution and when executed operable to perform the encoded techniques. For example, a first example disclosure embodies logic encoded in one or more tangible media for execution and when executed operable to detect a failure of a pseudowire stitching point in a multi-segment pseudowire containing a head-end and a tail-end; signal the detected failure to the head-end; choose a pre-computed backup path that does not include the failed stitching point; and signal the backup path to the tail-end.
A second disclosure embodies an apparatus comprising logic operable to receive a signal indicating a path failure in a multi-segment pseudowire having a head-end and a tail-end; choose a pre-computed backup path for the pseudowire; and initiate a signal to the tail-end indicating the backup path chosen. A third disclosure embodies an apparatus comprising logic operable to compute a primary path from the apparatus, functioning as a head-end of a multi-segment pseudowire, to a tail-end of a multi-segment pseudowire; and establish, prior to detection of a failure in the primary path, at least one backup path from the head-end of the multi-segment pseudowire to the tail-end of the multi-segment pseudowire.
Description
A Provider Edge (“PE”) is the device that provides the pseudowire to a CE and, as is known in the art, are connected to Customer Edge Equipment (“CE”) via Attachment Circuitry (“AC”). That is, the AC is a physical or virtual circuit attaching a CE to a PE. Attachment Circuitry may be, for example, a Frame Relay DLCI, an ATM VPI/VCI, an Ethernet port, a VLAN, a PPP connection on a physical interface, a PPP session from an L2TP tunnel, or an MPLS LSP. If the ACs at both ends of the pseudowire are of the same technology (e.g., both ATM, both Ethernet, both Frame Relay), the pseudowire is said to provide “homogeneous transport;” otherwise, it is said to provide “heterogeneous transport.” PEs may also be referred to as terminal PEs (“T-PEs”) or user PEs (“uPEs”) for a pseudowire running between two PEs. The T-PE from which traffic originates, also referred to as the source, is referred to herein as the head-end of the pseudowire and the T-PE to which traffic is destined, also referred to as a sink, is referred to herein as the tail-end of the pseudowire. Thus, a CE is a device where one end of a service originates and/or terminates. Generally, the CE is not aware that it is using a pseudowire rather than a native service. From the perspective of the CE, the pseudowire is characterized as an unshared link or circuit of the chosen service.
Tunneling is a method of transparently carrying information over a network. A Packet Switched Network tunnel is a tunnel across a PSN, inside which one or more pseudowires can be carried. PSN tunnel signaling is used to set up, maintain, and tear down the underlying PSN tunnel. A Protocol Data Unit (“PDU”) is the unit of data output to, or received from, the network by a protocol layer. A pseudowire PDU is a PDU sent on the pseudowire that contains all of the data and control information necessary to emulate the desired service.
Two PEs provide one or more PWs on behalf of their client CEs to enable the client CEs to communicate over the PSN. A PSN tunnel is established to provide a data path for the PW. The PW traffic is invisible to the core network, and the core network is transparent to the CEs. Native data units (bits, cells, or packets) arrive via the AC, are encapsulated in a PW-PDU, and are carried across the underlying network via the PSN tunnel. The PEs perform the necessary encapsulation and decapsulation (also sometimes referred to as imposition and disposition) of PW-PDUs and handle any other functions required by the PW service, such as sequencing or timing.
FIG. 1 shows a network 100 comprising a plurality of pseudowire elements forming a multi-segment pseudowire (“MS-PW”). Pseudowire elements are network elements (e.g., routers, switches, etc.) that are capable of determining which network traffic is being transmitted on a pseudowire and will have some capability in handling the pseudowire traffic. Pseudowire elements may be referred to herein as being pseudowire-aware. On the other hand, for network elements that are not pseudowire-aware, the pseudowire traffic is transparent. That is, a network element that is not pseudowire-aware cannot differentiate between the pseudowire traffic and other network traffic that passes through the network element. In FIG. 1, the pseudowire elements are PE1, PE2, PSPa, PSPb, PSPc, and PSPd. The network elements that are not pseudowire-aware (i.e., are not pseudowire elements) are NE1, NE2, NE3, and NE4.
The two PEs (i.e., PE1 and PE2) provide a MS-PW service to the CEs (i.e., CE1 and CE2). PE1 and PE2 reside in different autonomous systems, PSN1 and PSN2, respectively. The network 100 also comprises a plurality of Pseudowire Stitching Points (“PSPs”): PSPa, PSPb, PSPc, and PSPd. A pseudowire-stitching mechanism is a mechanism that permits a service provider to extend an existing PW with another PW. In other words, the stitching mechanism replaces the AC by another PW of the same type or a different type. In FIG. 1, for example, PSPa is stitched to PSPb, allowing a pseudowire segment ending at PSPa to be connected to a pseudowire segment ending at PSPb to form a MS-PW. The path from PE1 through PSPa and PSPb to PE2 is thus a MS-PW having three pseudowire segments: 1) the segment from PE1 to PSPa in PSN1, 2) the segment from PSPa to PSPb, and 3) the segment from PSPb to PE2 in PSN2.
The present invention provides for fast restoration of multi-segment pseudowire connections so as to reduce loss of connectivity during failure of a pseudowire element within the end-to-end path of a given pseudowire. By way of example, the primary pseudowire between PE1 and PE2 takes a path of PE1->NE1->PSPa->PSPb->NE3->PE2 with PE1 being the head-end and PE2 being the tail-end. It is clear from FIG. 1 that other paths may exist, such as the following:
PE1->NE1->PSPa->PSPc->PSPd->NE4->PE2
PE1->NE1->PSPa->PSPc->PSPd->PSPb->NE3->PE2
PE1->NE2->PSPc->PSPd->NE4->PE2
PE1->NE2->PSPc->PSPd->PSPb->NE3->PE2.
If PSPa or PSPb fail, eventually the PE1 to PE2 pseudowire will recover based on IGP convergence within all segments of the end-to-end path. However, this convergence time can be extensive given the multiple segment nature of the pseudowire. Since pseudowire segments in multi-segment pseudowires are signaled a priori and only between pseudowire switching routers, multi-level pseudowires suffer from a lack of reachability information. Because of this lack of reachability information, rerouting and repair of defects in the pseudowire can occur only at the transport layer (i.e., MPLS or IP), which carries the pseudowire segments between the switching points. Thus, the fast restoration of an end-to-end pseudowire path is not possible using conventional signaling protocols. With conventional technology, restoration of an end-to-end pseudowire path requires the entire end-to-end pseudowire path to be resignalled or requires the use of manual methods, which are slow, prone to error, and insufficiently resilient to additional future failures.
Embodiments disclosed herein provide for fast restoration of a multi-segment pseudowire so as to reduce loss of connectivity during failure of a pseudowire element within the end-to-end path of the multi-segment pseudowire. In accordance with embodiments of the present invention, at least one backup pseudowire path is pre-computed. That is, at least one backup path is computed prior to any detection of a failure in the primary pseudowire path. In particular embodiments, a plurality of backup paths are established such that for each PSP in the primary path of a multi-segment pseudowire, there is at least one backup path that does not include that PSP.
FIG. 2 illustrates a first example method 200 useful for protecting multi-segment pseudowires. As illustrated in FIG. 2, when a pseudowire element detects a failure of a pseudowire stitching point in the primary pseudowire path, the failure is signaled to the head-end of the pseudowire. By failure of a pseudowire stitching point, it is meant herein that the failure can be due to the failure of the PSP itself or the failure can be due to failure in the link between the failed PSP and the upstream PSP that detects the failure. That is, if an upstream PSP is unable to communicate with the next downstream PSP, a failure in the downstream PSP is said to be detected. It should be noted that, because multi-segment pseudowires are bi-directional, each T-PE in a multi-segment pseudowire may serve as both a head-end and a tail-end, depending on which T-PE is the source of the network traffic. That is, the T-PE from which network traffic originates is referred to herein as the head-end of the multi-segment pseudowire with respect to that network traffic and the T-PE that receives network traffic is referred to herein as the tail-end of the multi-segment pseudowire with respect the received network traffic.
In step 202 of FIG. 2, a failure of a pseudowire stitching point in a multi-segment pseudowire is detected. That is, a path failure in a multi-segment pseudowire is detected. In step 204, the detected failure is signaled to the head-end of the multi-segment pseudowire. In step 206, a new path to be used for the pseudowire is chosen for the pseudowire from the at least one backup path that was pre-computed. The backup path does not include the failed stitching point. In step 208, the chosen new path to be used for the pseudowire is signaled to the tail-end of the pseudowire. The signaling of the new path to the tail-end can be conducted along the chosen backup path. Once the tail-end has been signaled of the switch to the backup path, the tail-end can then also switch to the backup path for traffic flowing in the reverse direction on the multi-segment pseudowire. While the present invention can be utilized with any multi-segment pseudowire, it is especially advantageous when the head-end and tail-end of the pseudowire reside in different domains, such as different autonomous systems.
By way of example, if PSPb fails in the network 100 of FIG. 1, the primary pseudowire path will no longer be available. However, the following paths would still be available:
PE1->NE1->PSPa->PSPc->PSPd->NE4->PE2
PE1->NE2->PSPc->PSPd->NE4->PE2.
Given these two available paths, a reroute decision could be taken by several elements in the path. Such a reroute decision needs to be taken in both directions of the traffic path. Since the PEs of the MS-PW are essentially the “owners” of the path selection, they should make the reroute decision on which path to take based on information returned to them by the PSPs.
Each PSP within the end-to-end path of a given MS-PW is responsible for signaling its ability to act as a stitching point for the pseudowire to head-end/tail-end. This is achieved through the advertisement of reachability for endpoints outside of the local autonomous system. Generally, reachability information refers to information required for a device to compute a path to reach the destination. For example, Layer-2 reachability means packets addressed at one end of the PW with a layer-2 address (commonly known as an Ethernet MAC address) can be forwarded correctly to the far end and received/processed by the switches on that side.
FIG. 3 illustrates a second example method 300 useful for protecting multisegment pseudowires. The example method 300 of FIG. 3 can be advantageously encoded in logic that is operable to perform the method. Such logic can be included in an apparatus that functions as a T-PE for a multi-segment pseudowire. In step 302, such an apparatus can receive a signal that a path failure has been detected in a multi-segment pseudowire having a head-end and a tail-end. In step 304, the apparatus can choose a pre-computed backup path for the pseudowire. In particular embodiments, the received signal includes an indication of a failed pseudowire stitching point and the chosen backup path does not include the failed pseudowire stitching point. In step 306, the apparatus can initiate a signal to the tail-end indicating the backup path chosen. In particular embodiments, the apparatus initiates the signal to the tail-end across the chosen backup path. The example method 300 of FIG. 3 can be advantageously utilized if the head-end and tail-end reside in different domains, such as different autonomous systems.
FIG. 4 illustrates a third example method 400 useful for protecting multi-segment pseudowires. The example method 400 of FIG. 4 can be advantageously encoded in logic that is operable to perform the method. Such logic can be included in an apparatus that functions as a T-PE for a multi-segment pseudowire. In step 410, such an apparatus can compute a primary path from the head-end of a multi-segment pseudowire to the tail-end of the multi-segment pseudowire is computed. This primary path can identify all the pseudowire elements in the path.
In particular embodiments, the head-end and tail-end of a MS-PW initiate signaling for PW setup using directed Label Distribution Protocol (“LDP”) with a given exit point PSP within their local autonomous system. The choice of which exit point to use can be based, for example, on Interior Gateway Protocol (“IGP”) cost to the exit point or based on a dynamic selection criteria utilizing Path Computation Element Protocol (“PCEP”). It should be noted that the selection of best path also can be obtained using something other than IGP cost, such as Quality of Service (“QoS”), for example. The result of this signaling is the establishment of a primary path between the T-PEs (i.e., head-end and tail-end) of the MS-PW.
Numerous methods exist and may be used for setting up and maintaining pseudowires. These methods include LDP, BGP, and manual configuration (static). The PW will utilize an underlying network transport construct to convey packets between the endpoints of the PW. The underlying transport construct can be comprised of an MPLS LSP or MPLS Traffic Engineered Tunnel, an L2TPv3 tunnel, or a GRE tunnel, for example. Some transport mechanisms, such as TE tunnels, inherently are capable of spanning multiple domains, while some transport mechanisms have also been extended to achieve this goal. Some additional extensions are being designed such as Path Computation Element (PCE) to enhance these mechanisms which span domains, such as inter-provider boundaries. The result of signaling (i.e., setting up) a PW is the establishment of a primary path between the T-PEs (i.e., head-end and tail-end) of the MS-PW.
In addition to establishing a primary path for the MS-PW, a T-PE apparatus also may establish at least one backup path, which should be used upon failure of the primary path. This is shown in step 420 of FIG. 4. Such backup paths may be established prior to detection of a failure in the primary path. In fact backup paths may be established immediately after the primary path is established. A backup path should be a diverse path which takes into account failure of any PSP along the primary path of the MS-PW.
In particular embodiments shown as step 422 in FIG. 4, a T-PE apparatus may function as a Path Computation Client (“PCC”) and request a path from a PSP that is functioning as a Path Computation Element (“PCE”). The PSP may compute a path excluding itself as a stitching point. The multi-segment pseudowire can use this computed path in the future when a failure in this PSP is detected. That is, the T-PE may establish a backup path excluding this PSP so that the established backup path can be used later if a failure in this PSP is detected.
A Path Computation Element (“PCE”) is an entity (e.g., component, application, or network node) that is capable of computing a network path or route based on a network graph, and of applying computational constraints during the computation. A Path Computation Client (“PCC”) is an application requesting a path computation to be performed by a PCE. PCEP is a protocol for communications between a PCC and a PCE, or between two PCEs. A PCC can request a path from a PCE by sending a PCReq message to the PCE. A PCE can reply to a requesting PCC by sending a PCRep message to the PCC. A PCRep message can either contain a set of computed path(s) if the request could be satisfied or a negative reply otherwise.
In particular embodiments, a new Path Computation Element (“PCE”) communication Protocol (“PCEP”) object may be defined in order for a Path Communication Client (“PCC”) to request a diverse path from an existing multi-domain path along with various path constraints. One of ordinary skill in the art should be capable of creating such a PCEP object without undue experimentation. In the context of multi-segment pseudowires, a PCC may decide to request the exclusion of one or more PSPs based upon the increased path cost due to the diversity constraint. For example, PE1 may request from PSPa (acting as a PCE) to compute a path excluding itself as a stitching point. Conversely, it may request a fully end-to-end diverse path excluding a list of stitching points. Several combinations may be requested in a single request along with the path costs thus allowing the head-end (i.e., the PCC) to make an appropriate decision after evaluation of the tradeoff between path diversity and cost. It should be noted that the PCRep message returned to the PCC will only comprise the list of traversed PSPs along with the related cost. Such PSPs' address will then be used by signaling to establish the multi-segment pseudowire. It should also be noted that more than one PCE may be involved in such computation should the end-to-end path traverse more than 2 domains. It should also be noted that path segmented diversity should be evaluated by the PCE by performing a Shortest Path First (“SPF”) computation on the path (based on IGP cost for example) from each entry PSP to the tail-end.
Upon failure on any PSP within the primary path of a MS-PW, a PSP that detects the failure signals the failure to the head-end of the MS-PW. Taking the example of FIG. 1, if PSPb fails, PSPa may signal the failure to PE1, telling PE1 to switch to its backup. PE1 should ensure that the backup path does not contain the failed element PSPb. Therefore an indication of the failed segment (i.e., the segment between PSPa-PSPb) should be returned by PSPa to PE1. However, PSPb is unable to signal failure of the MS-PW so PE1 sends a “forward explicit” notification to PE2 across the backup path, telling PE2 that the backup path is now in place. PE2 then switches to the backup path.
In particular embodiments, network devices may comprise logic configured to perform methods disclosed herein. The logic may comprise hardware, such as Attachment Circuitry, for example. The logic may also comprise software configured to implement methods disclosed herein. Such logic embodied in network devices can be produced without undue experimentation by one of ordinary skill in the art of telecommunications upon review of disclosures made herein. For example, in particular embodiments a PE may comprise logic configured to 1) function as a head-end of a multi-segment pseudowire; 2) compute a primary path from the head-end to a tail-end of the multi-segment pseudowire; and 3) establish, prior to detection of failure in the primary path, at least one backup path for the multi-segment pseudowire. In other particular embodiments, a network comprising a plurality of pseudowire elements forming a multi-segment pseudowire containing a head-end, a tail-end, and a plurality of pseudowire stitching points; may also comprise logic configured to 1) detect a failure of a pseudowire stitching point in the multi-segment pseudowire; 2) signal the detected failure to the head-end; 3) choose a pre-computed backup path; and 4) signal the backup path to the tail-end.
FIG. 5 illustrates an example computer system architecture for a computer system 540 that performs methods useful in protecting multi-segment pseudowires in accordance with embodiments disclosed herein. The computer system 540 may be any type of computerized system such as a personal computer, workstation, portable computing device, mainframe, server or the like. In this example, the system includes an interconnection mechanism 511 that couples a memory system 512, a processor 513, and a communications interface 514. The communications interface 514 allows the computer system 540 to communicate with external devices or systems.
The memory system 512 may be any type of computer readable medium that is encoded with an application 555-A that represents software code such as data and/or logic instructions (e.g., stored in the memory or on another computer readable medium such as a disk) that embody the processing functionality of embodiments of the invention as explained above. The processor 513 can access the memory system 512 via the interconnection mechanism 511 in order to launch, run, execute, interpret or otherwise perform the logic instructions of the application 555-A for the host in order to produce a corresponding process 555-B. In other words, the process 555-B represents one or more portions of the application 555-A performing within or upon the processor 513 in the computer system.
It is to be understood that embodiments of the invention include the applications (i.e., the un-executed or non-performing logic instructions and/or data) encoded within a computer readable medium such as a floppy disk, hard disk or in an optical medium, or in a memory type system such as in firmware, read only memory (ROM), or, as in this example, as executable code within the memory system 512 (e.g., within random access memory or RAM). It is also to be understood that other embodiments of the invention can provide the applications operating within the processor 513 as the processes. While not shown in this example, those skilled in the art will understand that the computer system may include other processes and/or software and hardware components, such as an operating system, which have been left out of this illustration for ease of description of the invention.
Having described preferred embodiments of the invention it will now become apparent to those of ordinary skill in the art that other embodiments incorporating these concepts may be used. Additionally, the software included as part of the invention may be embodied in a computer program product that includes a computer useable medium. For example, such a computer usable medium can include a readable memory device, such as a hard drive device, a CD-ROM, a DVD-ROM, or a computer diskette, having computer readable program code segments stored thereon.
The protection of multi-segment pseudowires by utilizing pre-computed backup paths is herein disclosed. While the present invention has been shown and described herein with reference to specific embodiments thereof, it should be understood by those skilled in the art that variations, alterations, changes in form and detail, and equivalents may be made or conceived of without departing from the spirit and scope of the invention. Accordingly, the scope of the present invention should be assessed as that of the appended claims and by equivalents thereto.

## Claims

1. A non-transitory computer readable storage medium with logic for execution and when executed operable to:
detect a failure of a first pseudowire stitching point network device in a multi-segment pseudowire containing a head-end network device, the first pseudowire stitching point network device, a second pseudowire stitching point network device, and a tail-end network device,
wherein the multi-segment pseudowire is configured to emulate attributes of a plurality of telecommunications services on a packet switched network (PSN),
wherein the head-end network device is a first terminal provider edge and the tail-end network device is a second terminal provider edge; signal an indication of the failure of the first pseudowire stitching point network device to the head-end network device; choose a pre-computed pseudowire backup path that includes a third pseudowire stitching point network device and does not include the failed first pseudowire stitching point network device, wherein the indication of the failed first pseudowire stitching point network device is used to choose the pre-computed pseudowire backup path; and
transmit layer 2 reachability information of the third pseudowire stitching point network device to the tail-end network device across the pre-computed pseudowire backup path before reverse traffic from the tail-end network device to the head-end network device is switched to the pre-computed pseudowire backup path.

2. The computer readable storage medium of claim 1, further operable send a forward explicit notification to the tail-end network device across the pre-computed pseudowire backup path.

3. The computer readable storage medium of claim 1, wherein the head-end network device resides in a different domain than the tail-end network device.

4. The computer readable storage medium of claim 1, wherein the head-end network device resides in a different autonomous system than the tail-end network device.

5. The computer readable storage medium of claim 1, further operable to:
compute a primary path from the head-end network device of the multi-segment pseudowire to the tail-end network device of the multi-segment pseudowire; and
establish, prior to detection of a failure in the primary path, a plurality of backup paths including the pre-computed pseudowire backup path from the head-end network device of the multi-segment pseudowire to the tail-end network device of the multi-segment pseudowire.

6. A method comprising:
receiving, at a network device, a signal indicating a path failure in a multi-segment pseudowire having a head-end and a tail-end, wherein the multi-segment pseudowire is configured to emulate attributes of a plurality of telecommunications services on a packet switched network (PSN);
choosing, at the network device, a pre-computed backup path for the pseudowire based on layer 2 reachability information returned from each pseudowire stitching point in the multi-segment pseudowire, the layer 2 reachability information indicating an ability of each pseudowire stitching point to act as a stitching point in the multi-segment pseudowire, the pre-computed backup path not including a failed pseudowire stitching point;
switching forward traffic to the pre-computed backup path, wherein the forward traffic originates with the head-end and is destined for the tail-end;
initiating a signal to the tail-end indicating the backup path chosen after forward traffic is switched to the pre-computed backup path, wherein the signal to the tail-end indicating the backup path is sent across the backup path; and
switching reverse traffic to the pre-computed backup path after the signal to the tail-end is received, wherein the reverse traffic originates with the tail-end and is destined for the head-end.

7. The method of claim 6, wherein the head-end resides in a different domain than the tail-end.

8. The method of claim 6, wherein the head-end resides in a different autonomous system than the tail-end.

9. The computer readable storage medium of claim 1, wherein the pre-computed pseudowire backup path is chosen based on information returned from each pseudowire stitching point in the multi-segment pseudowire, the information indicating the ability of each pseudowire stitching point to act as a stitching point in the multi-segment pseudowire.

10. The computer readable storage medium of claim 1, wherein the tail-end network device is operable to switch to the pre-computed pseudowire backup path for network traffic flowing in a reverse direction on the multi-segment pseudowire, the tail-end network device operable to switch to the pre-computed pseudowire backup path when provided with the pseudowire backup path.

11. The computer readable storage medium of claim 1, wherein the plurality of telecommunications services include at least one of frame relay, asynchronous transfer mode, Ethernet, TDM, SONET/SDH, or a T1 leased line.

12. The computer readable storage medium of claim 1, wherein the multi-segment pseudowire is configured to manage the timing and order of service-specific bit streams.

13. An apparatus comprising:
a first terminal provider edge device configured to function as a head-end of a first pseudowire from which traffic originates;
attachment circuitry configured to attach the first terminal provider edge device to a customer edge device using a telecommunication service;
a stitching mechanism configured to connect the first pseudowire to a second pseudowire to form a multi-segment pseudowire terminating at a second terminal provider edge device as a tail-end of the multi-segment pseudowire to which the traffic is destined, wherein the multi-segment pseudowire is configured to emulate attributes of the telecommunications services on a packet switched network;
wherein the first terminal provider edge device is configured to compute a primary path to the tail-end of the multi-segment pseudowire and establish, prior to detection of a failure in the primary path, backup path from the head-end of the multi-segment pseudowire to the tail-end of the multi-segment pseudowire;
wherein the backup path includes a plurality of pseudowire stitching mechanisms, wherein each pseudowire stitching mechanism is selected based on layer 2 reachability of the pseudowire stitching mechanism and a contribution to path cost associated with the pseudowire stitching mechanism;
wherein forward traffic from the head-end to the tail-end is switched to the backup path and the first terminal provider edge device sends an indication of the backup path to the second terminal provider edge device using the backup path, wherein subsequent to receipt of the indication of the backup path reverse traffic from the tail-end to the head-end is switched to the backup path.

14. The apparatus of claim 13, wherein the multi-segment pseudowire is configured to manage a timing and order of bit streams native to the telecommunication service.

15. The apparatus of claim 13, further comprising:
a pseudowire protocol data unit including control information necessary to emulate the telecommunication service using Multiprotocol Label Switching (MPLS).

16. The method of claim 6, wherein the terminal provider edge from which network traffic originates is a first provider edge network device,
wherein a path of the multi-segment pseudowire includes the first provider edge network device, a second provider edge network device, a first pseudowire stitching point network device, a second pseudowire stitching point network device, and at least one non-pseudowire network device;
wherein the first provider edge network device, the second provider edge network device, the first pseudowire stitching point network device, and the second pseudowire stitching point network device are configured to differentiate between pseudowire traffic and other network traffic, and
wherein the at least one non-pseudowire network device cannot differentiate between pseudowire traffic and other network traffic.

17. The computer readable storage medium of claim 1, wherein the layer 2 reachability information of the third pseudowire stitching point network device indicates that the third pseudowire stitching point network device has reachability to the head-end network device in a different autonomous system.