Abstract:
A telecommunication system comprises a first plurality of virtual circuit label switched paths (VC-LSPs) dedicated to communicate broadcast and unknown frames. The known frames have an encapsulated source site identifier used for media access control (MAC) address learning.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to virtual private networks.  
           [0003]    2. Description of the Related Art  
           [0004]    Various transport and packet-based telecommunication services are provided to interconnect two or more customer locations. Two services provided by SBC Communications are branded as GigaMAN™ and Native LAN Plus™.  
           [0005]    The GigaMAN™ service provides point-to-point 1.25 Gbps Ethernet service over dedicated fiber facilities. The GigaMAN™ service is a local-area network (LAN)/wide-area network (WAN) extension of a customer&#39;s CPE switches. Presently, the GigaMAN™ service does not offer multi-point to multi-point service.  
           [0006]    The Native LAN Plus™ service provides point-to-point and multi-point-to-multi-point service supporting 10/100 Mb Ethernet, 4/16 Mb Token Ring, and Fiber Distributed Data Interface (FDDI). The service is based on existing ATM infrastructure using IP over ATM and RFC 1483/2684 Bridging Mode. Being ATM-based, this service is QoS capable. This service also has good operations and maintenance (OAM) capabilities (e.g. well-defined ATM OAM functionalities), and supports multiple LAN technologies. Presently, this service is neither scaleable (by requiring a large number of permanent virtual circuits), nor flexible (e.g. by requiring upgrades to the network to increase a customer access speed). For example, Gigabit Ethernet speeds are not presently supported, but can be supported by upgrading from an OC3c to an OC48c up-link into the network. A transparent LAN service (TLS), also known as a virtual private LAN service (VPLS), is designed to replace and address issues in the Native LAN Plus™ service.  
           [0007]    In metropolitan area networks (MANs), Ethernet is a promising technology to increase the capacity in a scaleable and flexible manner. Providing Internet access and transparent LAN services over Ethernet platforms has several advantages. Starting from 10 Mbps speed, Ethernet can support 100 Mbps and 1000 Mbps, and soon will reach 10 Gbps. With Ethernet, like other layer 2 services, the carrier is responsible for layer 2 connectivity and does not participate in layer 3 routing. Thus, the carrier avoids the complexities of peering at layer 3 with its customers.  
           [0008]    The simplicity of Ethernet, compared to ATM and SONET, results in challenges to be addressed in order to provide carrier class transparent LAN and dedicated Internet access services. In particular, any platform that is to be widely deployed in carrier networks should address network management and performance guarantees. By not guaranteeing a committed end-to-end rate, Ethernet currently is inferior to SONET and ATM on providing Quality of Service (QoS). Further, Ethernet does not have fast recovery mechanisms to redirect traffic around faults, which is disadvantageous when attempting to deploy high-premium, mission-critical services. In addition, Ethernet does not have built-in monitoring and troubleshooting capabilities. In a sense, the complexity is moved to network management, monitoring, and capacity planning phases.  
           [0009]    To overcome the aforementioned issues, Ethernet platforms are implementing Ethernet over Multi-Protocol Label Switching (EoMPLS). EoMPLS employs a combination of Resource Reservation Protocol with Traffic Engineering (RSVP-TE) extensions and Open Shortest Path First with Traffic Engineering (OSPF-TE) extensions, Connection Admission Control (CAC), and constraint-based routing algorithms to address the aforementioned issues.  
           [0010]    [0010]FIG. 1 is a block diagram of an Ethernet Optical Network (EON). The metro EON comprises Ethernet switches connected by fiber links. The fiber links can be either 10 Gigabit Ethernet links or wave-division multiplexed (WDM) links. The edge of the service provider networks extends to the customer premises and terminates at switch network elements owned by the service provider.  
           [0011]    As defined by the Internet Engineering Task Force (IETF), a VPN or a closed user group has the following network elements: (i) customer edge (CE) devices such as a Multi-Tenant Units (MTUs)  20  associated with the access layer to the network; (ii) provider edge (PE) devices  22  associated with the aggregation layer of the network; and (iii) provider core (P) devices  24  associated with the core layer of the service provider network. The MTU devices  20  reside in the customer premises but is owned by the service provider. The PE devices  22  are at the edge of the service provider network and placed in a service provider&#39;s central office locations. The PE devices  22  possess a significant feature set to support its many tasks. The core MPLS tunnels start and end at the PE switches  22 . Further, an existing Frame Relay/ATM network is connected to the EON at the PE-level. The P devices  24  are high-speed MPLS label swapping devices that reside in the service provider&#39;s central office locations.  
           [0012]    Typically, a number of access/edge network elements have point-to-point (PTP) Gigabit Ethernet links, optionally with protection including diverse routing, to an aggregation network element located at a central office. The access/edge network elements connect to customer CPEs (e.g. routers) via Ethernet or Gigabit Ethernet links.  
           [0013]    The aggregation network elements are interconnected to core network elements in a mesh with PTP fiber Gigabit Ethernet links within the central office. PTP links can be provided over a ring topology where appropriate. Also, where appropriate, WDM links can be used to interconnect the core network elements between central offices. Multiple central offices may reside in a Local Access and Transport Area (LATA). The Ethernet-based services are currently offered within a LATA.  
           [0014]    Typically, a handoff to an Internet Service Provider (ISP) or multiple ISPs  30  occurs at the core network element. This provides access to the global Internet  32 . Other core network elements can couple the network to an ATM/Frame Relay (FR) backbone  34  or an IP-VPN backbone  36 . The network in combination with the backbone  36  can provide a VPN for sites  40  of a corporation C. The network in combination with the backbone  34  can provide a VPN for sites  42  of a corporation D. Sites  44  and  46  for corporations A and B are also served.  
           [0015]    The telecommunication industry is attempting to make MPLS robust and scaleable. IETF draft documents (herein referred to as “Martini specifications”) define PTP transport using Label Distribution Protocol (LDP), and encapsulations for multiple layer 2 services such as FR and ATM. Label stacking is used to improve the scaleability of the network. A tunnel label determines the path to the remote edge, and a Virtual Circuit (VC) label designates the tunnel end point.  
           [0016]    Other IETF draft documents detail multi-point-to-multi-point extensions based on Hierarchical VPLS (HVPLS), Decoupled Transparent LAN Service (DTLS), and Logical Provider Edge (LPE). HVPLS uses LDP to signal the VPLS or TLS. However, an administrator is required to configure the membership explicitly because the LDP does not have a VPN discovery function. In contrast, both DTLS and LPE define a mechanism for automatic VPN discovery in Multi-Protocol Border Gateway Protocol (MP-BGP).  
           [0017]    In HVPLS, a single Virtual Circuit Label-Switched Path (VC-LSP) exists between the MTU edge switch and the PE switch per each closed user group. Both the MTU and the PE perform MAC learning, however only the PE does replication for unknown and broadcast frames. PEs are fully meshed by VC-LSPs for each closed user group. VC-LSPs in HVPLS use Martini specifications.  
           [0018]    In DTLS, MTUs are fully meshed by VC-LSPs per each closed user group. The MTU performs both MAC learning and replication for unknown and broadcast frames. PEs are responsible for closed user group discovery and MPLS label distribution. VC-LSPs in DTLS use Martini specifications.  
           [0019]    In LPE, a single VC-LSP exists between the MTU edge switch and the PE switch per each closed user group. The MTU performs MAC learning, and the PE does replication for unknown and broadcast frames. PEs are fully meshed by VC-LSPs for each closed user group. VC-LSPs in LPE use proprietary encapsulation to perform replication whenever necessary to optimize replication functionality. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    The present invention is pointed out with particularity in the appended claims. However, other features are described in the following detailed description in conjunction with the accompanying drawing in which:  
         [0021]    [0021]FIG. 1 is a block diagram of an Ethernet Optical Network (EON);  
         [0022]    [0022]FIG. 2 is a block diagram of an embodiment of an EoMPLS system in accordance with the present invention;  
         [0023]    [0023]FIG. 3 illustrates how the EoMPLS system in FIG. 2 is adapted to do frame replication and MAC learning; and  
         [0024]    [0024]FIG. 4 is a flow chart of an embodiment of a method of MAC learning in the EoMPLS system. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0025]    Abbreviations used in this patent disclosure are summarized as follows.  
         [0026]    ATM: Asynchronous Transfer Mode  
         [0027]    BGP: Border Gateway Protocol  
         [0028]    BUF: Broadcast and Unknown Frames  
         [0029]    CE device: Customer Edge device  
         [0030]    CoS: Class of Service  
         [0031]    CPE: Customer Premises Equipment  
         [0032]    DA: Destination Address  
         [0033]    DTLS: Decoupled Transparent LAN Service  
         [0034]    EoMPLS: Ethernet over MPLS  
         [0035]    EON: Ethernet Optical Network  
         [0036]    HVPLS: Hierarchical VPLS  
         [0037]    IETF: Internet Engineering Task Force  
         [0038]    IP: Internet Protocol  
         [0039]    LAN: Local-Area Network  
         [0040]    LDP: Label Distribution Protocol  
         [0041]    LER: Label Edge Router  
         [0042]    LPE: Logical Provider Edge  
         [0043]    LSP: Label Switched Path  
         [0044]    LSR: Label Switched Router  
         [0045]    MAC: Media Access Control  
         [0046]    MAN: Metropolitan Area Network  
         [0047]    MTU: Multi-Tenant Unit  
         [0048]    MP: Multi-protocol  
         [0049]    MPLS: Multi-protocol Label Switching  
         [0050]    OAM: Operations and Maintenance  
         [0051]    OC: Optical Carrier  
         [0052]    OSPF: Open Shortest Path First  
         [0053]    P device: Provider core device  
         [0054]    PE device: Provider Edge device  
         [0055]    PTP: Point-to-point  
         [0056]    PVS: Permanent Virtual Circuit  
         [0057]    QoS: Quality of Service  
         [0058]    RSVP: Resource Reservation Protocol  
         [0059]    RSVP-TE: RSVP with Traffic Engineering capability  
         [0060]    SA: Source Address  
         [0061]    SONET: Synchronous Optical Network  
         [0062]    STP: Spanning Tree Protocol  
         [0063]    TLS: Transparent LAN Service  
         [0064]    VC: Virtual Circuit  
         [0065]    VPLS: Virtual Private LAN Service/Segment  
         [0066]    VPN: Virtual Private Network  
         [0067]    An ideal, scaleable EON architecture has the following eight high-level attributes.  
         [0068]    Attribute 1. MAC learning is performed in the MTUs, but not in the PEs. Since MAC learning is CPU intensive, and a single PE may serve many MTUs, a PE performing MAC learning may be rendered clogged by MAC address explosion.  
         [0069]    Attribute 2. Fully-meshed tunnel LSPs only should be between the PEs and not the MTUs. Since the number of MTUs is much greater than the number of PEs, a large number of tunnel LSPs would be required to logically connect all MTUs.  
         [0070]    Attribute 3. No fully-meshed VC-LSPs should be required between the customer sites. Generally, VC-LSP support per port is about 2k in current platforms. For scaleability concerns, the demand for the number of VC-LSPs should not increase exponentially as the number of VPLSs and MTUs increase.  
         [0071]    Attribute 4. PEs should replicate frames to other PEs, and MTUs should replicate frames to other MTUs. The number of replications will be smaller in the PEs than the MTUs.  
         [0072]    Attribute 5. No fully-meshed targeted LDP sessions between the MTUs should exist. The number of targeted LDP sessions should be kept to a minimum since targeted LDP sessions are CPU intensive. An LDP session can exist between the MTU and PE.  
         [0073]    Attribute 6. Ease of manageability and configuration should be considered. No need for tracking customer VLANs (e.g. true TLS).  
         [0074]    Attribute 7. Ease of troubleshooting should be considered. For this reason, site-to-site granularity should be preserved.  
         [0075]    Attribute 8. A minimum or otherwise low protocol stack in MTUs should exist (e.g. no use of OSPF-TE or RSVP-TE). As a result, the MTUs are kept simple and economical.  
         [0076]    None of the three proposed EoMPLS solutions (DTLS, HVPLS or LPE) has all of the forementioned eight attributes. DTLS does not have attributes 3 and 4, HVPLS does not have attributes 1 and 7, and LPE does not have attribute 7. A main concern of DTLS is replication in the MTUs. Main concerns of HVPLS include MAC learning in the PEs, and no site-to-site granularity. Main concerns of LPE include proprietary and complex states with data encapsulation.  
         [0077]    Disclosed herein are embodiments of a proposed EoMPLS solution based on DTLS, HVPLS and LPE. The EoMPLS solution satisfies many of the aforementioned attributes, but compromises for conflicting attributes. Additionally, the EoMPLS solution uses a dedicated VPLS VC-LSP for MAC learning using broadcast and unknown frames. A source site identifier is added when encapsulating the broadcast and unknown frames to facilitate the MAC learning. The architecture does not preclude heterogeneous deployment of the proposed EoMPLS solution with end-to-end MPLS.  
         [0078]    [0078]FIG. 2 is a block diagram of an embodiment of an EoMPLS system in accordance with the present invention. For purposes of illustration and example, the EoMPLS system serves to provide a first VPLS to a first closed user group (also identified as closed user group A), and a second VPLS to a second closed user group (also identified as closed user group B). The first closed user group has CEs  100 ,  102 ,  104  and  106  at four different sites. The second closed user group has CEs  110 ,  112  and  114  at three different sites. Those having ordinary skill will appreciate that the EoMPLS system will typically serve more than two closed user groups, and that each closed user group can have a greater number of sites.  
         [0079]    The EoMPLS system comprises MTUs  120 ,  122 ,  124  and  126 . The CE  100  accesses the EoMPLS system via the MTU  120 . The CEs  102  and  110  access the EoMPLS system via the MTU  122 . The CEs  104  and  112  access the EoMPLS system via the MTU  124 . The CE  114  accesses the EoMPLS system via the MTU  126 .  
         [0080]    The EoMPLS system comprises PE devices  130 ,  132  and  134 . The PE device  130  serves the MTUs  120  and  122 . The PE device  132  serves the MTU  124 . The PE device  134  serves the MTU  126  and provides access to the EoMPLS system for the CE  106 .  
         [0081]    The PE devices  130 ,  132  and  134  are interconnected by a fully-meshed set of tunnel LSPs. In particular, the PE devices  130  and  132  communicate via a tunnel LSP  140 , the PE devices  132  and  134  communicate via a tunnel LSP  142 , the PE devices  134  and  130  communicate via a tunnel LSP  144 . Having fully-meshed tunnel LSPs between the PE devices beneficially keeps the number of tunnel LSPs limited and scaleable in the core of the network.  
         [0082]    For ease of troubleshooting, the MTUs  120 ,  122 ,  124  and  126  are fully-meshed by VC-LSPs for each closed user group. Thus, the EoMPLS system comprises VC-LSPs  150 ,  152 ,  154 ,  156 ,  160  and  162  associated with the first closed user group. The CEs  100  and  102  communicate via the VC-LSP  150 . The CEs  100  and  104  communicate via the VC-LSP  152 . The CEs  100  and  106  communicate via the VC-LSP  154 . The CEs  102  and  106  communicate via the VC-LSP  156 . The CEs  102  and  104  communicate via the VC-LSP  160 . The CEs  104  and  106  communicate via the VC-LSP  162 .  
         [0083]    Similarly, the EoMPLS system comprises VC-LSPs  170 ,  172  and  174  associated with the second closed user group. The CEs  110  and  112  communicate via the VC-LSP  170 . The CEs  112  and  114  communicate via the VC-LSP  172 . The CEs  114  and  110  communicate via the VC-LSP  174 .  
         [0084]    The VC-LSPs  152 ,  160  and  170  are supported by the tunnel LSP  140 . The VC-LSPs  162  and  172  are supported by the tunnel LSP  142 . The VC-LSPs  154 ,  156  and  174  are supported by the tunnel LSP  144 .  
         [0085]    The feature of frame replication is performed by the PE devices  130 ,  132  and  134  to avoid an undesirably high number of replications required by the MTUs  120 ,  122 ,  124  and  126 . For example, some customers may have 4000 or more sites that are part of the closed user group, in which case the MTU otherwise would have to replicate a broadcast frame 4000 or more times at line rate.  
         [0086]    Another feature of the EoMPLS system is that MAC learning is performed by the MTUs  120 ,  122  and  124  and not by the PE devices  130 ,  132  and  134 . This avoids MAC explosion and CPU-intensive MAC learning in the PEs. However, a PE device having a directly-connected CE (e.g. the PE device  134  having the CE  106  directly connected thereto) would also perform the herein-disclosed MAC learning acts. Alternatively, if a customer wants to pay extra for dual-homing, an MTU to which the customer connects can have PE functionality. The EoMPLS system performs auto-configuration and auto-discovery similar to DTLS or LPE.  
         [0087]    [0087]FIG. 3 illustrates how the EoMPLS system in FIG. 2 is adapted to do frame replication and MAC learning. MAC learning is accomplished using an additional VC-LSP for each VPLS for broadcast and unknown frames (herein denoted as a BUF-LSP for Broadcast and Unknown Frames LSP). Thus, the tunnel LSP  140  further supports a BUF-LSP  200  for the first VPLS and a BUF-LSP  202  for the second VPLS. The tunnel LSP  142  further supports a BUF-LSP  204  for the first VPLS and a BUF-LSP  206  for the second VPLS. The tunnel LSP  144  further supports a BUF-LSP  210  for the first VPLS and a BUF-LSP  212  for the second VPLS. If required or otherwise desired, the BUF-LSPs  200 ,  202 ,  204 ,  206 ,  210  and  212  can be rate-limited to protect the network against malicious customers.  
         [0088]    The PE devices  130 ,  132  and  134  act as frame replication hubs in the system. Each of the PE devices  130 ,  132  and  134  provides a respective frame replication hub for each closed user group served thereby. Thus, the PE device  130  provides a frame, replication hub  220  for closed user group A and a frame replication hub  222  for closed user group B. Similarly, the PE device  132  provides a frame replication hub  224  for closed user group A and a frame replication hub  226  for closed user group B. Further, the PE device  134  provides a frame replication hub  230  for closed user group A and a frame replication hub  232  for closed user group B.  
         [0089]    The frame replication hubs are fully-meshed by the BUF-LSPs. Thus, the frame replication hubs  220 ,  224  and  230  associated with closed user group A are fully-meshed by the BUF-LSPs  200 ,  204  and  210 . Similarly, the frame replication hubs  222 ,  226  and  232  associated with closed user group B are fully-meshed by the BUF-LSPs  202 ,  206  and  212 .  
         [0090]    The MTUs  120 ,  122 ,  124  and  126  are spokes of the frame replication hubs. The hubs and spokes are directly connected by BUF-LSPs.  
         [0091]    The frame replication hubs interact with other hubs and spokes as follows. A frame replication hub replicates frames coming from a spoke to other hubs and to directly connected spokes. A frame replication hub replicates frames coming from another hub to directly connected spokes. A frame replication hub does not send a copy of a frame coming from another hub to other hubs. The replicated frames include an encapsulated source site identifier as described hereinafter with reference to FIG. 4.  
         [0092]    The EoMPLS system comprises a plurality of media access control (MAC) learning elements responsive to the BUF-LSPs to learn MAC addresses based on the encapsulated source site identifier. In particular, the EoMPLS system comprises MAC learning elements  234 ,  236 ,  238  and  240  to learn MAC addresses for closed user group A, and MAC learning elements  242 ,  244  and  246  to learn MAC addresses for closed user group B. The MAC learning elements  234 ,  236  and  238  are disposed at the MTUs  120 ,  122  and  124 , respectively. The MAC learning elements  242 ,  244  and  246  are disposed at the MTUs  122 ,  124  and  126 , respectively. The MAC learning element  240  is disposed at the PE device  134 .  
         [0093]    [0093]FIG. 4 is a flow chart of an embodiment of a method of MAC learning in the EoMPLS system. The method is described with reference to one VPLS or closed user group provided by the EoMPLS system. Thus, in practice, the method of FIG. 4 is performed for each VPLS provided by the EoMPLS system. For purposes of illustration and example, the method will be described with reference to the second VPLS (i.e. for closed user group B) in FIG. 2. This VPLS has three sites (having the CE devices  110 ,  112  and  114 ), with each site being connected to a different MTU (the MTUs  122 ,  124  and  126 , respectively), and each MTU being connected to a different PE device (the PE devices  130 ,  132  and  134 , respectively).  
         [0094]    As indicated by block  250 , the method comprises sending an unknown frame from one of the sites, which is referred to as a source site. For purposes of illustration and example, consider the unknown frame being sent from a first site served by the MTU  122 .  
         [0095]    As indicated by block  252 , an MTU serving the source site determines that the unknown frame has an unknown destination address (DA). In response thereto, the MTU encapsulates or otherwise adds a site identification (site-ID) for the source site (block  254 ), pushes or otherwise adds a label indicating that the unknown frame is to be communicated on a BUF-LSP (block  256 ), and sends a resulting MPLS packet to the PE device connected to the MTU (block  260 ). Keeping with the example, the MTU  122  would determine that the frame has an unknown destination address, and in response thereto, would add a site-ID for the first site, push a label indicating that the unknown frame is to be communicated on a BUF-LSP, and send a resulting MPLS packet to the PE device  130 .  
         [0096]    The source site-id may be encapsulated in an unknown frame in a variety of ways. In one embodiment, the encapsulated source site-id for an unknown frame is included in an Ethernet control word. The Martini specifications define an optional control word for Ethernet that may be used for this purpose. As presently believed, vendors have not implemented the control word for Ethernet. Thus, the vendors may be asked to include the control word for Ethernet, and the site-id can be carried in the control word by a new definition. In another embodiment, the encapsulated source site-id for an unknown frame is included in a Multi-protocol Label Switching (MPLS) label. This embodiment may require adjustment in MPLS implementation in the MTUs.  
         [0097]    As indicated by block  262 , the PE device receives the MPLS packet from the MTU. The PE device replicates the MPLS packet (block  264 ), and sends a copy to each of the other PE devices via a corresponding BUF-LSP (block  266 ). Continuing with the above example, the PE device  130  receives the MPLS packet from the MTU  122 , replicates the MPLS packet, sends one copy of the MPLS packet to the PE device  132  via the BUF-LSP  202 , and sends another copy of the MPLS packet to the PE device  134  via the BUF-LSP  212 .  
         [0098]    As indicated by block  270 , the PE devices receive the MPLS packets sent thereto. Each receiving PE device forwards its received packet to its associated MTUs (block  272 ). Continuing with the above example, the PE device  132  receives its MPLS packet and forwards same to the MTU  124 , and the PE device  134  receives its MPLS packet and forwards same to the MTU  126 .  
         [0099]    As indicated by blocks  274  and  276 , the MTU pops or otherwise removes the BUF-LSP label from the MPLS packet, and removes the site-ID from the MPLS packet. The MTU correlates a source address to VC-LSP to-Site-ID  and thereby learns the source address (block  280 ). Continuing with the example, the MTU  124  correlates the source address with the VC-LSP  170  and the MTU  126  correlates the source address with the VC-LSP  174 . Thereafter, for frames destined to the source address, the MTU  124  uses the VC-LSP  170 , and the MTU  126  uses the VC-LSP  174 . When the MTU  122  receives frames destined for the source address from the VC-LSP  170 , the MTU  122  learns that frames destined for the site associated with the MTU  124  are to be communicated using the VC-LSP  170 . Similarly, when the MTU  122  receives frames destined to the source address from the VC-LSP  174 , the MTU  122  learns that frames destined to the site associated with the MTU  126  are to be communicated using the VC-LSP  174 . It is noted that no encapsulation changes are used for frames sent through the site-to-site VC-LSPs  150 ,  152 ,  154 ,  156 ,  160 ,  162 ,  170 ,  172  and  174 .  
         [0100]    It is noted that the MAC-learning acts depicted in FIG. 4 may be performed either concurrently or in an alternative order. For example, not all of the MTUs need to perform the act in block  274  before some of the MTUs proceed to blocks  276  and  280 .  
         [0101]    The acts described herein may be directed by computer program code stored by a computer-readable medium. Examples of the computer-readable medium include, but are not limited to, an optical medium such as an optical disk, an electronic medium such as an electronic memory, and a magnetic medium such as a hard disk. Each MTU comprises a processor responsive to computer program code to perform the MTU-related acts. Each PE devices comprises a processor responsive to computer program code to perform the PE-related acts.  
         [0102]    It will be apparent to those skilled in the art that the disclosed inventions may be modified in numerous ways and may assume many embodiments other than the preferred forms specifically set out and described herein.  
         [0103]    Accordingly, it is intended by the appended claims to cover all modifications which fall within the true spirit and scope of the present invention.