Abstract:
Tunneling is a mechanism used to aggregate payloads from a network protocol at or below the tunneling layer into a single packet. A user-space application establishes a Transmission Control Protocol (TCP) tunnel and encapsulates an end-to-end TCP payload into a TCP segment for transmission over a TCP connection. This enhanced TCP tunnel eliminates TCP meltdown and can be used over any network which supports TCP. The calling application either identifies the parameters of an existing TCP socket or establishes a new TCP socket. A modified transport layer uses the identified TCP connection in forming the enhanced TCP tunnel. The enhanced TCP tunnel manages the data transmission on the TCP stack to eliminate TCP meltdown.

Description:
FIELD OF THE INVENTION 
       [0001]    The subject matter disclosed herein relates generally to Transmission Control Protocol (TCP) tunneling and, in particular, to a system for and method of encapsulating a TCP segment for communication over a TCP connection. 
       BACKGROUND OF THE INVENTION 
       [0002]    Tunneling is a technique used in computer networks to aggregate and encapsulate a first network protocol inside a second network protocol of a protocol stack, sometimes referred to as a communications stack. Protocol tunnels are commonly used to deliver other protocols across non-compatible networks, bypass firewalls, improve security, or improve fairness in bandwidth consumption. The first network protocol is typically referred to as the “payload protocol.” The payload protocol executes at or below the same layer of the protocol stack as the tunnel protocol. The second network protocol is typically referred to as the “delivery protocol”, the “tunnel protocol”, or simply the “tunnel”. The payload protocol typically contains a packet generated by a higher layer protocol and passed down the protocol stack. The higher layer protocol is referred to as the “end-to-end protocol.” 
         [0003]    A well-known model for describing the protocol stack is the Open Systems Interconnection (OSI) model  50 , shown in  FIG. 2 . The OSI model divides the stack into seven layers: physical layer  52 , data link layer  54 , network layer  56 , transport layer  58 , session layer  60 , presentation layer  62 , and application layer  64 . When a device wishes to communicate using the OSI model  50 , data  66  from one of the higher layers is passed down through each of the subsequently lower layers. At each of the lower layers, a protocol executing on that layer acts on the data from the next higher layer and passes the data to the next lower level. For example, data  66  from one of the higher levels may be passed to the transport layer  58 . The Transmission Control Protocol (TCP) executing on the transport layer adds a header  80  (see  FIG. 3 ) and passes a segment  68  to the network layer  56 . The Internet Protocol (IP) executing on the network layer  56  adds another header and passes a packet  70  to the data link layer  54 . A protocol executing on the data link layer  54  encapsulates the packet  70  into a frame  72  and passes the frame  72  to the physical layer  52 . Finally, the physical layer  52  converts the frame  72  into a bit stream  74  for transmission over a network to another device. When the bit stream is received by the other device or “receiving device”, the data is passed back up through the protocol stack of the receiving device, and each layer strips the header corresponding to that layer, which was added by the sending device, and presents the data  66  to an application executing on the receiving device. It should be noted that, although examples used herein will refer to the OSI model, use of the OSI model and references to specific protocols are not intended to be limiting. It is understood that the present invention may operate using other models of the protocol stack, such as the TCP/IP model, and other protocols executing on each layer. 
         [0004]    A TCP tunnel is used to encapsulate and aggregate the output from an end-to-end protocol from a layer at or below the transport layer  58  into a single TCP segment. The TCP tunnel is created by initializing a first TCP socket using an internet socket application programming interface (API). A TCP socket is one of the endpoints for TCP communication and includes a local IP address and port number. The socket API then initializes a TCP connection with another endpoint that has already created a TCP server socket and bound the TCP server socket to a port. Once the TCP connection between the two sockets is established, data can be read from and written to the TCP socket using the socket API. The output from an end-to-end protocol is provided as the data  66 , or payload, to the tunneling application. The payload is then encapsulated within a TCP segment and transmitted over the TCP connection. For example, a user datagram protocol (UDP) datagram may be provided as the payload to the tunneling application and encapsulated within a TCP segment because UDP and TCP are both protocols that execute on the transport layer  58  in the OSI model  50 . The tunneling application may then send the TCP segment, in which the UDP datagram is encapsulated, over the TCP connection. Traffic over the TCP connection is managed, at least in part, by the TCP protocol executing on the transport layer  58 . 
         [0005]    The TCP protocol was designed for reliable and ordered delivery of communications. In order to help prevent lost communications, the TCP socket receiving data, or TCP receiver, establishes a receive window, or buffer, in which incoming data is stored. The TCP receiver uses a flow control protocol to adjust the receive window in order to efficiently handle data reception. The TCP socket sending data, or TCP sender, executes a congestion control algorithm. The congestion control algorithm establishes a maximum number of packets that can be sent without receiving an acknowledgement of receiving the packet from the other TCP socket in order to control the rate at which data enters the network. The TCP protocol additionally utilizes error detection and correction techniques to detect, for example, lost or duplicate segments and to retransmit lost segments or discard duplicate segments. 
         [0006]    However, if a TCP tunnel is carrying another TCP segment, known as TCP-over-TCP, a problem commonly known as “TCP meltdown” may occur. TCP meltdown typically occurs when the congestion control and error correction algorithms of both the TCP tunnel protocol and the TCP end-to-end protocol attempt to correct errors in transmission. Each TCP protocol may attempt to correct errors at a different rate, and both TCP protocols may attempt to resend lost packets. The competing error correction algorithms may overwhelm the network bandwidth, hence reducing transmission rates and increasing traffic with retransmitted segments. If, for example, a lost segment is detected by the tunnel TCP protocol, the tunnel TCP protocol will attempt to retransmit the lost segment. However, the TCP payload encapsulated within the tunnel TCP segment is necessarily lost as well. Consequently, the end-to-end protocol, which first generated the TCP payload, will also attempt to retransmit the lost TCP payload, generating additional traffic. 
         [0007]    A TCP tunnel can also increase the round-trip time (RTT) of an end-to-end TCP stream by approximately four times, which causes a decrease in the throughput of a TCP flow by as much as 60%. For example, sample data sent from the TCP sender of the end-to-end TCP connection generates a first segment which gets encapsulated as the payload of the second segment generated by the TCP sender of the TCP tunnel. The second segment generated by the TCP tunnel is first acknowledged (ACK) by the TCP receiver of the TCP tunnel, and the TCP receiver of the TCP tunnel extracts the payload from the second segment. The payload extracted from the second TCP segment is then processed by the TCP receiver of the end-to-end TCP connection, which will generate a second acknowledgement (ACK). The second acknowledgement must be returned to the TCP sender of the end-to-end TCP connection. The TCP receiver of the end-to-end TCP connection now acts as a sender and encapsulates the ACK as data in a return payload. The TCP receiver of the TCP tunnel now also acts as a sender and generates another TCP segment to return the encapsulated ACK to the original TCP sender of the TCP tunnel. This return segment must also be acknowledged by the TCP sender of the TCP tunnel and the payload extracted therefrom. The encapsulated acknowledgment is finally returned to the original TCP sender of the end-to-end TCP connection. As demonstrated, two acknowledgments are required for each end-to-end TCP segment. The requirement for two acknowledgements both reduces bandwidth and increases the RTT for the TCP tunnel. 
         [0008]    Attempts to solve the TCP meltdown and increased RTT problems have not been met without drawbacks. Presently, TCP segments are carried by tunnels created using other protocols, such as UDP, interne protocol security (IPsec), or generic route encapsulation (GRE). However, many networks do not support these protocols and require that the tunnel also be created by TCP protocol. 
         [0009]    Other solutions have proposed modifications to the TCP layer, for example by using selective acknowledgements (SACKs) and explicit congestion notification (ECN) bits of the TCP header. However, such modifications require that both endpoints of the TCP connection support the modified TCP layer, limiting the usefulness of the solution to TCP connections established between endpoints having the modifications. 
         [0010]    Therefore, it would be desirable to provide a method of TCP tunneling that supports TCP-over-TCP tunneling that may be implemented on a single end of the TCP connection. 
       SUMMARY OF THE INVENTION 
       [0011]    In accordance with a first aspect of the invention, the above-referenced and other needs are met by providing an enhanced transport layer executing on the TCP sender. A user-space application establishes a Transmission Control Protocol (TCP) tunnel and encapsulates an end-to-end TCP payload into a TCP segment for transmission over a TCP connection. This enhanced TCP tunnel eliminates TCP meltdown and can be used over any network which supports TCP. The calling application either identifies the parameters of an existing TCP socket or establishes a new TCP socket. A modified transport layer uses the identified TCP connection in forming the enhanced TCP tunnel. The enhanced TCP tunnel manages the data transmission on the TCP stack to eliminate TCP meltdown. 
         [0012]    According to one embodiment of the present invention, a TCP tunnel for transmitting data between a computer and a network includes an interface receiving a payload from a calling application, a packet capture module receiving data from a TCP stack on the computer, and a modified transport layer. The modified transport layer is executable to receive the payload from the interface, encapsulate the payload as a TCP segment, and insert the TCP segment into a TCP stack on the computer. The modified transport layer is also executable to receive the data from the TCP stack, identify a TCP segment encapsulated in the data, and selectively transmit the data to the calling application. If a TCP segment is encapsulated in the data, the modified transport layer transmits the data to the calling application. If no TCP segment is encapsulated in the data, the modified transport layer transmits the data to a standard transport layer. The modified transport layer also periodically passes a stay-alive segment to a standard transport layer. 
         [0013]    As another aspect of the invention, the interface may identify an address and a port number corresponding to an existing TCP socket on the computer and passes the address and the port number to the modified transport layer. The interface may also generate a test message, pass the test message to the TCP socket, and read a sequence number and an acknowledgement number from a return message captured from the TCP socket. Optionally, the interface may establish a TCP connection, store an address and a port number corresponding to the TCP connection, and pass the address and the port number to the modified transport layer. 
         [0014]    As yet another aspect of the invention, the modified transport layer may selectively bypass the transport layer and interface directly with the network layer. The modified transport layer may also insert the TCP segment into any layer of the TCP stack below the transport layer. 
         [0015]    As still another aspect of the invention, the modified transport layer may extract a first TCP sequence number and a first TCP acknowledgment number of an established TCP connection, and computes a second TCP sequence number and a second TCP acknowledgment number corresponding to the first TCP sequence number and the first TCP acknowledgment number respectively. 
         [0016]    According to another embodiment of the present invention, a method of transmitting data between a computer and a network using a Transmission Control Protocol (TCP) tunnel obtains connection parameters for a TCP Socket and initializes a modified transport layer using the connection parameters. The connection parameters may include the source IP address, the destination IP address, the source port number, and the destination port number. The method obtains a sequence number and an acknowledgement number for a TCP connection using the TCP socket and accepts an incoming payload from a calling application. A TCP segment is constructed in the modified transport layer using the sequence number and the acknowledgment number for the established TCP connection and inserted at or below a transport layer of the TCP socket. The modified transport layer may also receive data from the TCP socket and return selected data to a calling application. The sequence and acknowledgement numbers are read from the data, and a next sequence and acknowledgment number are computed corresponding to the sequence and acknowledgment number. The TCP tunnel may obtain an initial sequence and acknowledgement number for the TCP connection by probing the established TCP connection. As another aspect of the invention, probing the established TCP connection may include creating a test segment, sending the test segment on the TCP connection, capturing a return segment from the TCP connection, and reading the sequence number and the acknowledgement number from the return segment. Optionally, the TCP tunnel may establish a TCP connection, and the IP address, the destination IP address, the source port number, and the destination port number may be stored while establishing the TCP connection. 
         [0017]    These and other objects, advantages, and features of the invention will become apparent to those skilled in the art from the detailed description and the accompanying drawings. It should be understood, however, that the detailed description and accompanying drawings, while indicating preferred embodiments of the present invention, are given by way of illustration and not of limitation. Many changes and modifications may be made within the scope of the present invention without departing from the spirit thereof, and the invention includes all such modifications. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    The drawings furnished herewith illustrate a preferred construction of the present invention in which the above advantages and features are clearly disclosed as well as others which will be readily understood from the following description of the illustrated embodiment. 
           [0019]    In the drawings: 
           [0020]      FIG. 1  is a block diagram representation of an exemplary environment incorporating one embodiment of the present invention; 
           [0021]      FIG. 2  is a block diagram illustrating the OSI model of a protocol stack and is appropriately labeled “Prior Art”; 
           [0022]      FIG. 3  is a block diagram representation of a TCP segment and is appropriately labeled “Prior Art”; 
           [0023]      FIG. 4  is a block diagram representation of one embodiment of the present invention; 
           [0024]      FIG. 5  is a flow chart illustrating the operation of a TCP tunnel implemented by the embodiment of the present invention illustrated in  FIG. 4 ; and 
           [0025]      FIG. 6  is a flow chart illustrating initialization of the TCP tunnel. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0026]    The various features and advantageous details of the subject matter disclosed herein are explained more fully with reference to the non-limiting embodiments described in detail in the following description. 
         [0027]    Referring to  FIG. 1 , an exemplary environment incorporating the present invention is disclosed. First and second computers  10  and  20  are connected to a network  30 , such as the Internet, by network connections  32 . The network connection  32  may include, but is not limited to, network cables, switches, routers, gateways, or other devices as would be known in the art. Each computer may comprise any network-enabled device including a processor and network interface, including, but not limited to a desktop or laptop computer, network server, or network-enabled mobile phone, personal digital assistant (PDA) or gaming console. Each of the first and second computers,  10  and  20  respectively, includes a processor which executes an operating system that manages a network protocol stack. Preferably, each operating system is configured to communicate via a Transmission Control Protocol (TCP) stack. Either of the first or second computers,  10  and  20  respectively, may establish a TCP connection  35  with the other computer. As illustrated by the dashed lines, a TCP connection  35  appears as a virtual connection between the TCP protocol  16  executing on the first computer  10  and the TCP protocol  26  executing on a second computer  20 . 
         [0028]    In the example of  FIG. 1 , the first computer  10  acts as the sender, establishing a first TCP stack  12 , and the second computer  20  acts as the receiver, establishing a second TCP stack  22 . The first computer  10  may use the TCP connection  35  to establish a TCP tunnel  40  between the first and second computers,  10  and  20  respectively. An application  14  executing on the first computer  10  generates data which is passed to the TCP protocol  16 . The data is encapsulated in a TCP segment and passed to the Internet Protocol (IP) protocol  18 . The operating system managing the TCP sender stack  12  continues to pass the data down through subsequent layers and transmits the data to the physical network connection  32 , for example via a network interface card. The second computer  20  receives the data from its physical network connection  32 , for example via a second network interface card. The operating system managing the TCP receiver stack  22  passes the data up to the IP protocol  28  which, in turn, extracts the TCP segment and passes it to the TCP protocol  26  on the second computer  20 . The TCP protocol  26  extracts the data and passes it to the application  24  executing on the second computer  20 . The application  24  may similarly return a message to the first computer  10  by passing data down the TCP receiver stack  22 , across the network  30 , and back up the TCP sender stack  12 . 
         [0029]    Referring next to  FIG. 4 , the TCP tunnel  40  (shown in  FIG. 1 ) may be used for TCP-over-TCP communications, that is, a first TCP segment is encapsulated in a second TCP segment. An application  100  generates data to be transmitted via an end-to-end TCP connection  102 . The data passes through a first TCP stack  104 , resulting in an end-to-end payload  106  which includes a TCP segment. The end-to-end payload  106  may be captured from any layer of the end-to-end segment and, therefore, may be a segment from a TCP protocol, a packet from a network protocol, or a frame from an Ethernet protocol. A tunneling application  108  calls interface  110  to communicate with a modified transport layer  116 . Preferably, the modified transport layer  116  executes in user space and operates in parallel with a standard transport layer  112  executing on the second TCP stack  114 . The tunneling application  108 , interface  110 , and modified transport layer  116  may execute as separate programs, a single program, or a combination thereof. The structure of the programs may depend, for example, on the operating system, the network, the calling application, the processor on the computer, or a combination thereof. Preferably, each of the tunneling application  108 , interface  110 , and modified transport layer  116  executes in “user space”, such that the “kernel space” of the computer on which they are executing need not be modified. User space is memory allocated to applications executing outside of the operating system. Kernel space is memory allocated to applications executing within the operating system such as device drivers and other system applications. Optionally, a portion or all of the tunneling application  108 , interface  110 , and modified transport layer  116 , may be incorporated into the kernel space of the computer on which they are executing. 
         [0030]    If a TCP connection has already been established, the tunneling application  108  may identify the TCP socket on the computer and pass the TCP socket to the interface  110 . The interface  110  may be, but is not limited to, an application programming interface (API) executing on the computer. The interface  110  may define multiple routines, such as routines to establish the modified transport layer  116 , pass the TCP socket to the modified transport layer  116 , send a payload from the tunneling application  108  to the modified transport layer  116 , and return data from the modified transport layer  116  to the tunneling application  108 . Optionally, the interface  110  may also include a routine to open a new TCP socket. Having received the TCP socket, the modified transport layer  116  extracts the connection parameters of the TCP socket. The connection parameters may include the source IP address, destination IP address, source port number, and destination port number. The modified transport layer  116  assigns an identifier via the interface  110  for use by the tunneling application  108  for subsequent reads and writes over the TCP connection. Optionally, the tunneling application  108  may be used to establish a TCP connection between the computer on which it is executing and a receiving computer. While the TCP connection is being established, the tunneling application  108  may capture and store each of the IP address, destination IP address, source port number, and destination port number, as well as a current sequence number and acknowledgement number of the TCP socket being created for subsequent transmission to the modified transport layer  116 . 
         [0031]    The modified transport layer  116  interfaces with a second TCP stack  114  to manage data transfer between the tunneling application  108  and the network  30 . End-to-end payloads  106  sent out by the tunneling application  108  are passed directly to the modified transport layer  116  via the interface  110  using the identifier previously assigned by the modified transport layer  116 . The modified transport layer  116  encapsulates the end-to-end payload and injects it into any layer of the TCP stack  114  at or below the network layer. For example, the modified transport layer  116  may encapsulate the end-to-end payload  106  as a TCP segment and inject the TCP segment into the network layer  118  of the TCP stack  114 . Optionally, the modified transport layer  116  may further add an IP and/or an Ethernet header and inject the resulting packet or frame into a lower layer  120  of the TCP stack  114 . 
         [0032]    The modified transport layer preferably also includes a firewall  115  and a packet capture module  117  to manage data transfer from the network  30  to the tunneling application  108 . The packet capture module  117  may capture data from any layer of the TCP stack  114  below the transport layer. For example, the packet capture a frame or a packet from the data link layer or the network layer, respectively, and the TCP segment extracted therefrom. The modified transport layer  116  examines captured segments and determines whether to pass each segment up through the second TCP stack  114  using either the standard transport layer  112  or the modified transport layer  116 . The firewall  115  is a routine executable by the computer to selectively block or transmit segments from the network layer  118  or from lower layers  120  to the transport layer according to a first pre-defined filter. The firewall  115  similarly may selectively block or transmit segments from the transport layer to the network layer  118  or lower layers  120  according to a second pre-defined filter. The first and second pre-defined filters may be configured to selectively block or transmit either the same or a different set of segments between layers. The packet capture module  117  is a routine that is able to read packets matching a pre-defined filter, including, but not limited to, a TCP port, IP address, or TCP header bits, from the transport layer,  112  or  116 , network layer  118 , or a lower layer  120  of the second TCP stack  114 . 
         [0033]    Referring also to  FIGS. 5 and 6 , operation of a TCP tunnel  40  according to one embodiment of the present invention is illustrated. The TCP tunnel  40  is initialized at step  150 . As shown at step  180  in  FIG. 6 , the tunneling application  108  first determines whether a TCP socket is established. If the TCP socket exists, the tunnel initialization may proceed according to steps  184 - 190 . If no TCP socket has been opened, the tunneling application will first open a TCP socket, as shown in step  182 . At step  184 , the tunneling application  108  uses interface  110  to pass the socket to the modified transport layer  116 . The modified transport layer  116  obtains the socket parameters, such as the source IP address, destination IP address, source port, and destination port from the established TCP socket at step  186 . At step  188 , the modified transport layer  116  transmits a test message to the established TCP socket and records the sequence and acknowledgement numbers from the header of the returned TCP segment. Optionally, the modified transport layer  116  may be configured to monitor establishment of the TCP socket. The TCP segments used to establish the TCP socket and subsequent TCP connection may be captured and the socket parameters, including the source IP address, destination IP address, source port, destination port, sequence number, and acknowledgment number may be stored for subsequent use by the modified transport layer  116 . 
         [0034]    Using the socket parameters, the modified transport layer  116  may dynamically set filters for the firewall  117  that block packets from reaching the TCP layer. For example, the firewall  117  may be configured to block packets from the established TCP connection that contain TCP segments with either the PSH-ACK or PSH bit set. The firewall  117  may permit other segments, for example, those containing TCP segments with other control bits set to enter the TCP layer. The firewall  117  may allow a portion of the segments to pass to the standard transport layer  112  as stay-alive segments. The stay-alive segments are processed by the standard transport layer to maintain the established TCP connection in the operating system and to close the connection when communications are complete. Optionally, the modified transport layer  116  may generate stay-alive segments and pass them to the standard transport layer  112 . When the TCP connection is closed, the modified transport layer  116  may clear the filters for the firewall  117  such that subsequent network traffic is not affected. 
         [0035]    Referring again to  FIG. 5 , once the tunnel is initialized, the modified transport layer  116  waits until it receives data at step  152 . The data may be received either from the tunneling application  108  or from a lower layer  120  of the TCP stack  114 . At step  154 , the modified transport layer  116  handles the data according to whether 1) it was received from the tunneling application  108  and needs to be transmitted to the network layer  118  or a lower layer  120  in the TCP stack  114  or 2) it was received from the network layer  118  or a lower layer  120  in the TCP stack  114  and needs to be sent to the tunneling application  108 . If the data received at step  154  was a payload  106  from the tunneling application  108  and needs to be transmitted, the payload  106  is encapsulated as data in a new TCP segment at step  156 . The modified transport layer  116  utilizes the socket parameters, including the sequence number, acknowledgement number, source port number, and destination port number, to create header information for the TCP segment. At step  158 , the new TCP segment is then inserted into the TCP stack  114 . The TCP segment may be passed directly to the network layer  118  executing the IP protocol or, optionally, additional header information and/or processing may be performed to insert the TCP segment at a lower layer  120  in the stack  114  before the segment is passed to the network layer  118 . 
         [0036]    If the data received at step  154  is from the network layer  118  or a lower layer  120 , a determination as to whether to block the data received is made at step  160 . A first determination of whether to block the data is made by the firewall  115 . The firewall  115  prevents segments from being passed to the standard transport layer  112  according to the rules previously established. If the rules indicate the data is to be allowed through the firewall, the data is passed to the TCP protocol on the standard transport layer  112  at step  162 . If the rules indicate the data is not to be allowed through the firewall, the packet capture module  117  passes a copy of the segment to the modified transport layer  116 , capturing all segments that are part of the established TCP connection. At steps  164  and  166 , the encapsulated payload within the captured TCP segment is extracted and returned to the tunneling application  108 . The tunneling application  108 , in turn, passes the payload to the first TCP stack  104  which extracts the data from the end-to-end TCP segment and passes it to the calling application  100 . 
         [0037]    If a transmission error occurs, the modified transport layer  116  coordinates error correction with the end-to-end TCP connection  102  to prevent TCP meltdown from occurring. Preferably, the congestion control window of the modified transport layer  116  is expanded to a sufficient delay to permit the end-to-end TCP connection  102  to recognize a transmission error and initiate error correction. According to one embodiment, the modified transport layer  116  permits TCP segments to continue to be sent regardless of whether the receiver acknowledges receipt. Thus, the modified transport layer  116  relies on the end-to-end TCP connection  102  to verify proper receipt of all TCP segments. 
         [0038]    Thus, the present invention provides a modified transport layer  116  to manage a TCP connection with another device. The modified transport layer  116  recognizes TCP-over-TCP communications and prevents TCP meltdown, reduces the bandwidth requirements, and improves performance. Further, the modified transport layer  116  of the present invention need only be implemented on the sender side of the TCP connection, enabling TCP-over-TCP connections even with devices executing a standard TCP protocol. 
         [0039]    It should be understood that the invention is not limited in its application to the details of construction and arrangements of the components set forth herein. The invention is capable of other embodiments and of being practiced or carried out in various ways. Variations and modifications of the foregoing are within the scope of the present invention. It also being understood that the invention disclosed and defined herein extends to all alternative combinations of two or more of the individual features mentioned or evident from the text and/or drawings. All of these different combinations constitute various alternative aspects of the present invention. The embodiments described herein explain the best modes known for practicing the invention and will enable others skilled in the art to utilize the invention