Abstract:
A method of establishing a multi-segment pseudowire across domains executing different pseudowire signaling protocols augments the Label Distribution Protocol (LDP) label message to include a MS-PW TLV. The MS-PW TLV carries pseudowire signal 22 FEC TLV protocol information associated with signaling protocols of other segments of the MS-PW. As a result a multi-segment pseudowire can be extended between domains that execute different pseudowire signaling protocols.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority under 35 U.S.C. §1.11 9 (e) to provisional patent application number 60/711,861 filed on Aug. 26, 2005 by Hamid Ould-Brahim, incorporated herein by reference. 
     
    
     FIELD OF INVENTION 
       [0002]    This invention relates generally to the field of multi-segment pseudowire and more particularly to establishment and maintenance of multi-segment pseudowire networks. 
       BACKGROUND ON THE INVENTION 
       [0003]    A Pseudowire (PW) is an emulation of a native service over a Packet Switched Network (PSN). The native service may be a layer 2 or SONET connection, ATM, Frame Relay, Ethernet, low-rate TDM, or SONET/SDH, while the PSN may be MPLS, IP (either IPv4 or IPv6), or L2TPv3. More particularly, a PW is a tunnel established between two provider-edge (PE) nodes to transport Layer 2 Protocol Data Units (PDUs) across a packet switched network (PSN). 
         [0004]    A multi-segmented PW (MS-PW) is one which traverses multiple PSN domains, i.e., one or more Service Provider (SP) networks, or multiple networks within the same SP network (e.g. an access and core network). More specifically a MS-PW is a static or dynamically configured set of two or more contiguous PW segments that behave and function as a single point-to-point PW. Each end of a MS-PW terminates at an Ultimate Provider Edge (U-PE) device. 
         [0005]    Pseudowires may be established using a variety of processes including by static configuration (using MPLS or L2TPv 
         [0006]      3 ) or by Label Distribution Protocol (LDP). LDP defines two Forwarding Equivalency Class (FEC) elements which may be used to establish PWs; PWid FEC 128 and Generalized PWid FEC 129. As the standards for PW signaling continues to evolve, deployed network devices may execute different LDP PW signaling protocols. It is desirable to identify a method that enables MS-PW to extend between PSN domains that use different PW signaling protocols. 
       SUMMARY OF THE INVENTION 
       [0007]    According to one aspect of the invention, a method of establishing a multi-segment pseudowire (MS-PW) across two domains is provided, where the two domains are coupled by an edge device. The method includes the step of forwarding a label mapping message to the edge device using a first pseudowire signaling protocol, the label mapping message including an MS-PW element including fields associated with a second pseudowire signaling protocol. 
         [0008]    According to another aspect of the invention, an edge device includes a label map for associating labels with pseudowires and logic, coupled to the label map, for generating a label mapping message including a FEC element for storing information related to a first pseudowire signaling protocol and a multi-segment pseudowire element for storing information related to a second pseudowire signaling protocol. The edge device also includes forwarding logic for forwarding the label mapping message to a coupled edge device. 
         [0009]    According to a further aspect of the invention, a method of establishing a multi-segment pseudowire includes the steps of receiving an upstream label message from an upstream edge device using a first pseudowire signaling protocol, the upstream label message including a FEC element including information associated with the first pseudowire signaling protocol and a multi-segment pseudowire (MS-PW) element including information associated with a second pseudowire signaling protocol; determining that a downstream edge device uses the second pseudowire signaling protocol; modifying the upstream label message to provide a downstream label message, including the step of extracting information from the MS-PW element to include in a downstream FEC element of the downstream label; and forwarding the downstream label to the downstream edge device. 
         [0010]    According to another aspect of the invention, a device comprising a computer readable medium storing a label map, the label map includes a label message comprising a label TLV, a FEC TLV for storing information related to a first pseudowire signaling protocol and a multi-segment pseudowire (MS-PW) element for storing information related to a second pseudowire signaling protocol and means for forwarding the label message to a coupled edge device. 
         [0011]    These and other advantages of the invention will be described in more detail with regard to the below figures. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a diagram of a network that includes multiple Packet Switched Network (PSN) domains, and illustrates the extension of a Multi-Segment Pseudowire (MS-PW) across the domains; 
           [0013]      FIG. 2  is a diagram illustrating a modified Label Distribution Protocol (LDP) label message of the present invention, which includes an MS-PW Type-Length-Value (TLV) element; 
           [0014]      FIGS. 3A and 3B  illustrate exemplary fields of a PWid element and a Generalized PWid element, used for PW establishment using LDP; 
           [0015]      FIG. 4  is a diagram illustrating exemplary fields that may by included in a MS-PW TLV in a modified LDP label message such as that shown in  FIG. 2 ; 
           [0016]      FIGS. 5-7  are functional flow diagrams which are used to describe a process for establishing a MS-PW across different pseudowire signaling domains; and 
           [0017]      FIG. 8  is a block diagram illustrating several components that may be included in an edge device implementing the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    Multi-Segmented Pseudowire (MS-PW) is a static or dynamically configured set of two or more contiguous Pseudowire (PW) segments that behave and function as a single point-to-point PW. Thus each MS-PW may be viewed as a single PW that for technical or administrative reasons is segmented into a number of concatenated hops.  FIG. 1  illustrates a typical MS-PW  10 . 
         [0019]    Each end of a MS-PW terminates at an Ultimate Provider Edge (U-PE) device. In  FIG. 1 , MS-PW  17  terminates at U-PE&#39;s  12  and  16 , and includes Single Segment Pseudowires  13  and  15 . Using pseudo-Wire Emulation Edge-to-Edge (PWE3), a MS-PW may traverse any number of Switching Provider Edge devices, such as S-PE  14 . From the perspective of U-PE, a MS-PW is indistinguishable from a Single Segment PW (SS-PW). 
         [0020]    The PEs of  FIG. 1  may each reside in different PSN domains. A first PSN tunnel extends from U-PE  12  to S-PE  14  across PSN domain  1 , and a second PSN tunnel extends from S-PE  14  to U-PE  16 . The tunnels are connected at S-PE to complete the MS-PW  17 . 
         [0021]    A protocol for establishing and maintaining pseudowires (PW) using extensions to a Label Distribution Protocol (LDP) is described in IETF RFC 4447, entitled Pseudowire Setup and Maintenance Using Label distribution Protocol (LDP), by Martini et al, April 2006, incorporated herein by reference. 
         [0022]    IETF RFC 3036, incorporated herein by reference, defines the Label Distribution Protocol. LDP is a set of procedures and messages by which Label Switched Routers (LSRs) establish Label Switched Paths (LSPs) through a network by mapping network-layer routing information directly to data-link layer switched paths. LDP associates a Forwarding Equivalence Class (FEC) with each LSP it creates. The FEC associated with an LSP specifies which packets are “mapped” to that LSP. 
         [0023]    LDP uses a Type-Length-Value (TLV) encoding scheme to encode much of the information carried in LDP messages. An LDP label mapping message  20  of the present invention is shown in  FIG. 2 . The label mapping message  20  includes a Label TLV  24 , FEC TLV  22  and Optional TLVs  26 . According to one aspect of the invention, the label mapping message also includes a MS-PW TLV  30 . 
         [0024]    The Label TLV  24  encodes a label. Label TLVs are carried by the messages used to advertise, request, release and withdraw label mappings. There are several different kinds of Label TLVs which can appear in situations that require a Label TLV. 
         [0025]    The FEC TLV  22  is a list of one or more FEC elements associated with the label. RFC 4777 defines two FEC elements that are used for the emulation of end-to-end pseudowires; a PWid element (FEC element type 0x80, commonly referred to as FEC 128) and Generalized PWid element (FEC element type 0x81, commonly referred to as FEC 128). Each FEC element may have uniquely encoded fields. For example,  FIGS. 3A and 3B , illustrate the encodings for FEC 128 and FEC 129 as described in RFC 4777, incorporated by reference above. 
         [0026]    The MS-PW TLV of the present invention bridges the information gap between the FEC 128 and FEC 129 protocol in the following manner. 
         [0027]    A pseudowire that is established using FEC 128 is provisioned at both endpoints; i.e., both end-points provision for the pseudowire, and the pseudowire is assigned a PWid such as that shown in  FIG. 3A , which is referenced by both endpoints when the pseudowire is used. The FEC 128 element of  FIG. 3A  illustrates exemplary fields that are included in an FEC 128 element  40 . The PW Type  41  contains a value that represents the type of PW, as specified in “IANA Allocations for Pseudowire Edge to Edge Emulation (PWE3)”, IETF RFC 4446, Martini et al, April 2006. Control word bit C  42  flags the presence of a control word. PW Information Length  43  specifies the length of the PW ID field and the interface parameters sub-TLV in octets. Group ID  44  is an arbitrary value that represents a group of PWs that is used to create groups in the PW space. The group ID is intended to be used as a port index, or a virtual tunnel index. The PW ID  45  is a connection ID that, together with the PW type, identifies a particular PW. For FEC  128 , the PW ID and the PW type must be the same for both endpoints. 
         [0028]    Using the PWid FEC  128 , each of the two endpoints independently initiates the setup of a uni-directional LSP. An outgoing LSP and incoming LSP are bound together in a single pseudowire if they have the same PW ID and PW type. Thus, FEC  128  uses double sided provisioning to establish a pseudowire. 
         [0029]    Pseudowires that are established using FEC 129 elements are provisioned at a single end. Each pseudowire forwarding device is associated with an Attachment Interface, and pseudowires are referenced using AI information such as that shown in  FIG. 3B . The single ended provisioning capabilities provided by FEC 129 facilitate the construction of MS-PW.  FIG. 3B  illustrates several fields that are included in an FEC 129 element. The FEC 129 element  50  includes a Control word bit  51 , a PW type  52  and a PW Information length field  53 . However, rather than a Group ID or PW ID, the FEC 129 element includes a Attachment Group Identifier (AGI) element  53 , a Source Attachment Individual Identifier (SAII) element  55 , and a Target Attachment Individual Identifier (TAII) element  57 . In FEC 129, the association of two unidirectional LSPs into a single bidirectional pseudowire depends on the SAII and the TAII. 
         [0030]    FEC 128 was the first standard proposed for PW, and many edge devices implementing the FEC 128 protocol have been deployed in the network. The IETF PW standards have evolved in the direction of FEC 129, which provides increased network flexibility and allows MS-PW to be more easily implemented. However the protocols as they currently exist do not enable a MS-PW to be established across domains that use different signaling protocols, because any system that implements FEC 128 would not have access to the Attachment Information of FEC 129, and any device that implements FEC 129 would not have readily available the PW ID for supporting FEC 128 pseudowire. 
         [0031]    Referring back again to  FIG. 2 , the present invention overcomes the problems of the prior art by augmented the LDP label message to include the Multi-Segment Pseudowire (MS-PW) TLV  30 . The MS-PW TLV includes certain MS-PW information that allows a MS-PW to be formed of Single Segment pseudowires that are established using different signaling protocols. An example of several fields that may be included in a MS-PW TLV  30  of the present invention is shown in  FIG. 4 . The fields include an Attachment Group Identifier (AGI)  32 , a Source Attachment Individual Identifier (SAII)  34  and a Target Attachment Individual Identifier (TAII)  36 . Although three types of Al are shown in  FIG. 4 , the present invention is not limited to MS PW embodiments that include all or only these Al, but rather encompasses the inclusion of any PW protocol information in a separate TLV to enable MS PW to extend across multiple PEs of different protocols. Thus, all three identifier values need not be present for the purposes of establishing MS-PW. For example, AGI is not required in all scenarios. 
         [0032]    RFC 4777 generally defines attachment identifiers. Every forwarder in a provider edge device is associated with an Attachment Identifier (AI). A set of forwarders may be regarded as being members of a particular group and a PW may be set up among members of the group. An Attachment Group Identifier (AGI)  32  identifies members of a group associated with a PW. An Attachment Identifier may therefore include an Attachment Individual Identifier (All) and an Attachment Group Identifier (AGI). From the perspective of a PE, each pseudowire has a local or “Source All” (SAII)  34  and a remote or Target All (TAII)  36 . In one embodiment, the SAII, TAII and AGI are carried as octect strings. The Length field specifies the size of the Value field. The inclusion of this or similar information in an MS-PW TLV allows SS-PWs that are established using different signaling protocols (FEC 128, FEC 129, L2TP, RSPV-TE) to be included in a MS-PW. 
         [0033]    It should be noted that although the particular examples shown and described herein refer to the use of the MS-PW TLV to bridge the information gap between FEC 128 and FEC 129 protocols, the present invention is not restricted to use with these particular protocols. Rather the basic idea of forwarding bridge information between PSN domains operating different PW signaling protocols to enable MS PW segments to extend through the different PSN domain may be used to forward data for any PW signaling protocol, including but not limited to Layer 2 Tunneling Protocol (L2TP) information, RSVP-TE information, etc. 
         [0034]    In the particular embodiments described in  FIGS. 5-7 , the MS-PW TLV of the present invention overcomes the problem bridging the existing protocols by advantageously including, in the LDP message, a MS-PW TLV that includes PW attachment information. A description of how the MS-PW TLV information may be used to overcome the problems posed by the existing PW protocols will now be described with regards to functional flow diagrams  FIGS. 5-7 . 
         [0035]    In  FIG. 5  illustrates three devices U-PE  72 , S-PE  74  and U-PE  76 . For the purposes of this application, a device is ‘upstream’ to any device to which it forwards a label. A device that receives a label is ‘downstream’ to that device. Thus in  FIG. 5 , S-PE is downstream of device U-PE  72 , and upstream to device U-PE  76 . U-PE  72  is upstream from device S-PE  74 . 
         [0036]    To establish Multi-Segment Pseudowire  71  between U-PE  72  and U-PE  76 , U-PE- 72  generates a label mapping message including FEC TLV element information such as that shown in  FIG. 3A . The LDP label message of the present invention includes an appended MS-PW TLV, including attachment information illustrated in  FIG. 4  (or some variation thereof). When the source U-PE implements FEC 128 signaling, the MS-TLV fields are populated as follows: The SAII=PWID (which is for the source U-PE is the PWID for the MS PW), the TAII=the target identifier of the target U-PE (U-PE  76 ), and the AGI is the group of the target PE (U-PE  76 ). 
         [0037]    S-PE  74  is both FEC 128 and FEC 129 capable. It receives the FEC 128 LDP packet from device  72 , and establishes a FEC 129 PW with U-PE  76  using an FEC 129 LDP label mapping message of the present invention which is augmented to include the MS-PW TLV. The MS-PW TLV forwarded by U-PE  72  is extracted from the FEC 128 LDP message and appended to the FEC 129 LDP message. 
         [0038]    The U-PE  76  receives the LDP message, and generates a reverse label mapping to the Source U-PE  72 . The reverse label mapping message includes the MS-PW TLV of the present invention, populated as follows: The SAII equals the TAII of the received label mapping (i.e., U-PE  72 ), the AGI is the AGI of the target U-PE, and TAII=SAII of the received label mapping (which, in this example is the PWID of the MS-PW). 
         [0039]    With such an arrangement, an FEC 129 device, such as U-PE  76 , obtains PWID information, thereby allowing successful instantiation of a MS-PW with an FEC 128 device. 
         [0040]      FIG. 6  illustrates a network  80  provided to illustrate the use of the present invention to support MS PW which extends from an FEC 128 domain, through a FEC 129 domain and back into a different FEC 128 domain. U-PE  82  forwards a LDP labeling message of the present invention to S-PE  84 . As in  FIG. 5 , the labeling message includes the MS PW TLV information of the present invention, which is populated with the SAII=PWID, the TAII=target U-PE (U-PE  88 ), and the AGI of U-PE  88 . 
         [0041]    S-PE establishes a PW connection with S-PE  86 , using FEC 129 protocol, passing the MS-PW TLV received from U-PE  82  onto S-PE  86 . S-PE  86  extracts the information from the MS-PW TLV to populate an FEC 128 labeling message to U-PE  88 . 
         [0042]      FIG. 7  illustrates how the MS-PW TLV of the present invention may be used to establish a MS-PW in a network with a FEC 128 domain sandwiched by two FEC 129 domains. The U-PE  92  generates a FEC 129 SS PW message as in the prior art. The SAI, TAI and AGI in the MS-PW TLV are populated using the corresponding fields of the FEC 129 element. S-PE  94  establishes an FEC 128 SS PW with S-PE  96  using FEC 128 protocol, and passing the MS-PW TLV information unchanged to the S-PE  96 . The S-PE  96  uses the MS-PW TLV information to establish the FEC 129 SS PW to U-PE  98 , while also performing a reverse mapping back to S-PE  94  using the FEC 128 information. 
         [0043]      FIG. 8  illustrates several exemplary components that may be included at an edge device  100  implementing the present invention. The components may be implemented in hardware, software or a combination thereof. The embodiment of  FIG. 8  is exemplary only; it is realized that similar functions that are differently delineated are also within the scope of the present invention. 
         [0044]    Device  100  is shown to include a label map  102  and label modification and forwarding logic  104 . As labels are received by at the device  100 , the label modification logic analyzes the FEC TLV to determine the signaling protocol of the PW. As labels are generated for downstream domains, the MS-PW element may be adapted by the logic  104  based on the pseudowire signaling domains at either end of the MS-PW that traverses the device  100 . The adaptation will vary depending upon the protocols of the domains that are bridged, as discussed in  FIGS. 5-7 . 
         [0045]    Accordingly a suggested architecture and use of a Multi-Segment PW TLV has been shown and described. The MS-PW TLV generally includes information that assists in bridging the signaling information gap between PW PSN domains. 
         [0046]    Having described various embodiments of the invention, it will be appreciated that many of the above figures are flowchart illustrations of methods, apparatus (systems) and computer program products according to an embodiment of the invention. It will be understood that each block of the flowchart illustrations, and combinations of blocks in the flowchart illustrations, can be implemented by computer program instructions. These computer program instructions may be loaded onto a computer or other programmable data processing apparatus to produce a machine, such that the instructions which execute on the computer or other programmable data processing apparatus create means for implementing the functions specified in the flowchart block or blocks. These computer program instructions may also be stored in a computer-readable memory that can direct a computer or other programmable data processing apparatus to function in a particular manner, such that the instructions stored in the computer-readable memory produce an article of manufacture including instruction means which implement the function specified in the flowchart block or blocks. The computer program instructions may also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer implemented process such that the instructions which execute on the computer or other programmable apparatus provide steps for implementing the functions specified in the flowchart block or blocks. 
         [0047]    Those skilled in the art should readily appreciate that programs defining the functions of the present invention can be delivered to a computer in many forms; including, but not limited to: (a) information permanently stored on non-writable storage media (e.g. read only memory devices within a computer such as ROM or CD-ROM disks readable by a computer I/O attachment); (b) information alterably stored on writable storage media (e.g. floppy disks and hard drives); or (c) information conveyed to a computer through communication media for example using baseband signaling or broadband signaling techniques, including carrier wave signaling techniques, such as over computer or telephone networks via a modem. 
         [0048]    The above description and figures have included various process steps and components that are illustrative of operations that are performed by the present invention. However, although certain components and steps have been described, it is understood that the descriptions are representative only, other functional delineations or additional steps and components can be added by one of skill in the art, and thus the present invention should not be limited to the specific embodiments disclosed. In addition it is understood that the various representational elements may be implemented in hardware, software running on a computer, or a combination thereof. 
         [0049]    While the invention is described through the above exemplary embodiments, it will be understood by those of ordinary skill in the art that modification to and variation of the illustrated embodiments may be made without departing from the inventive concepts herein disclosed. Accordingly, the invention should not be viewed as limited except by the scope and spirit of the appended claims.