Patent Publication Number: US-7219161-B1

Title: Techniques for network address and port translation for network protocols that do not use translated ports when requesting network resources

Description:
FIELD OF INVENTION 
   The present invention generally relates to computer telecommunications networks. The invention relates more specifically to network address and port translation for conserving internet protocol (IP) addresses, with a network protocol that uses registered names for requesting network resources and not translated ports. 
   BACKGROUND OF THE INVENTION 
   A computer network includes computer processors or “hosts” that host software applications that provide or request services, or both. The hosts may be network terminals or end stations that do not perform network traffic routing or forwarding functions. The hosts communicate with each other through network devices, such as switches and routers, sometimes called intermediate devices, which do perform routing and forwarding functions. When a host is added to a network by connecting directly to another host, or indirectly to another host through one or more intermediate devices, the host must be configured for network operations. For example, the newly added host is assigned a logical network address for itself, and a network address for the intermediate device that routes or forwards its traffic, among other configuration information. 
   Many computer networks, including the Internet, employ the Transmission Control Protocol and Internet Protocol (TCP/IP) for sending packets of information between devices on the network. TCP/IP packets include data fields holding data indicating a destination, a destination port, a source, and a source port. The destination field holds data indicating the IP address of the intermediate device or host that is to receive the TCP/IP packet. Routers efficiently transmit TCP/IP packets using hardware configured to interpret the destination address in the destination field. An IP address consists of 32 bits, often represented as four 8-bit numbers separated by periods, e.g., A.B.C.D, where the letters A, B, C, D each represent one 8-bit number. An 8-bit number can represent values from 0 though 255. The source field holds data indicating the IP address of the intermediate device or host that sent the TCP/IP packet. The destination port field holds data indicating the service requested on the destination device, so that the destination device provides the packet to the appropriate server process executing on the destination device. The source port is used to distinguish several client processes operating simultaneously on the same host. 
   Computer networks that employ TCP/IP are proliferating, and as a result, logical network addresses that are used to locate each device on the network have become scarce. In many local networks with many different computers, the number and location of the computers connected to the network may change regularly. In addition, of the many computers physically connected at one time, only a fraction of the computers are on and operating at one time. Thus one or more hosts are being added and dropped frequently. 
   A specific context in which this problem arises involves large-scale cable modem networks in which voice and other services are delivered over an Internet Protocol (IP) network that uses coaxial cable for communications. In this context, cable modems of end users become active and inactive regularly as users initiate and terminate use of service. In this context, it is wasteful to give every computer a unique and permanent network address. A number of addresses sufficient for the simultaneously operating computers can be shared, and re-assigned as one host drops off the network and another host adds onto the network. Configuring each host is a tedious process to perform manually. 
   The Dynamic Host Configuration Protocol (DHCP) provides a mechanism through which computers using TCP/IP can obtain network addresses and other configuration information automatically. DHCP is an open standard protocol for dynamic host configuration described in request for comments (RFC) documents numbered 2131 and 2132 available at the time of this writing as rfc2131.html and rfc2132.html, respectively, on the worldwide web (www) at domain dhcp.org. A DHCP server process operates on a DHCP server host that is conveniently located for several hosts on one or more local networks. One or more DHCP server hosts and processes are set up by a system administrator with information to configure the hosts on one or more local networks to reflect the current architecture of those local networks. A DHCP client process operates on each host of the local networks. As used hereinafter, a server refers to the server process and a client refers to the client process unless otherwise indicated to refer to the host or device on which the process executes. 
   Another mechanism used to conserve scarce IP addresses is the network address and port translation (NAPT) process. NAPT is a process by which many network addresses and their TCP ports are translated into a single network address and its TCP ports. A conventional NAPT is an open standard for address translation described in request for comments (RFC) document number 3022 available at the time of this writing as rfc3022.txt on directory rfc, on the worldwide web (www) at domain ietf.org. 
   NAPT allows a local network with private addresses to connect to an external network with unique registered addresses. For example, assume that private IP addresses 1.1.1.0 through 1.1.1.255 are used on a local network. These addresses may already be registered on the public Internet to other devices on one or more other networks, or reserved for private addresses only, and so not used by the Internet at all. Thus, these addresses cannot be used by hosts on the local network to send or receive messages over the Internet. To communicate with a device on the Internet, data packets from hosts on the local network must go through an intermediate device with an NAPT process. Each data packet has a source port, often randomly generated at the host for each client process sending one or more messages. The intermediate device has an Internet registered address, for example, 125.125.125.130 and a local address, for example, 1.1.1.1. The NAPT process on the intermediate device translates the 256 private addresses 1.1.1.0 through 1.1.1.255, each with one or more ports, to the single address 125.125.125.130 with 256 or more different ports. For example, the NAPT process maps the 256 hosts, each using a single port, to 256 ports ranging from port 3000 to 3255. On messages originating on the private network for the Internet, NAPT substitutes the translated single address (125.125.125.130) for the source addresses and the translated ports (3000 to 3255) for the source ports, and then sends the message over the Internet. On data packets destined for the hosts on the private network from the Internet, NAPT translates the destination address and destination ports to the associated local IP addresses and local ports, and sends the messages onto the local network. 
   NAPT is useful for conserving IP addresses in circumstances interconnecting several local networks using private addresses that are not used outside the local network for privacy reasons or because those addresses are invalid outside the local network. Consider a retailer having dozens or hundreds of store locations, each store having its own local network of hosts running cash register processes. Installing a local network at each store is simplified if every local network can use the same set of IP addresses, e.g., 1.1.1.0 through 1.1.1.255. 
   In addition, if each local network can use the same IP addresses, a limited pool of unique registered IP addresses can be used for all the hosts at a very large number of sites. For example, assume that of 1000 unique registered IP addresses in a pool for the external network, all 1000 are used for 1000 different sites on the corporate wide-area network. At each site, any IP addresses can be used as local addresses, because such addresses are not presented to the wide area network. 
   A limit for the number of local addresses that can use NAPT in this manner is imposed by the number of unique port values that NAPT can use to distinguish the local addresses. Some ports are well-known ports used by certain processes running on a host. For example the port for an HTTP server is 80 and the port for a DHCP server is 67. The number of ports that are not reserved is well over 32,000. Thus, using NAPT, a pool of 1000 IP addresses can be used to access over 32,000 hosts on each of 1000 local networks—a total of over 32 million hosts. 
   However, the hardware and software that forms an NAPT process may fail in some important circumstances. For example, when the computers distributed across more than one network operate the Windows operating system of Microsoft Corp., communications that involve NAPT ultimately fail. The Windows operating system uses a network protocol called network basic input/output system (NetBIOS). NetBIOS is an open standard protocol for providing services over a TCP network described in request for comments (RFC) document number 1001, available at the time of this writing as rfc1001.htm on directory rfc, on the worldwide web (www) at domain ietf.org. 
   Many small offices and home offices use low-cost, personal computers for small local networks. Tens of millions of personal computers already deployed run the Windows operating system. An enterprise that has many sites, each with local networks of personal computers running Windows, is unable to utilize NAPT in attempts to obtain Windows services across those sites over an internetwork. Furthermore, large enterprises installing local networks at large numbers of sites may prefer to install low-cost personal computer networks at each site. Therefore, a failure of NAPT when interconnecting networks of such personal computers prevents data interchange among such enterprises. 
   Unlike NAPT, a simple network address translation (NAT) process has been configured to operate successfully with the NetBIOS protocol. A NAT process translates a local IP address to one of a set of corporate IP addresses associated with the intermediate device on which the NAT process executes. The number of corporate IP addresses used by the NAT process limits the number of local hosts simultaneously communicating with the corporate network, and is often a small subset of the total number of local hosts. The NAT process associates the local IP address with one corporate IP address of the set of corporate IP addresses for the intermediate device on which the NAT process executes. The NAT process is configured to operate successfully with NetBIOS by replacing the local IP address included in a field of the NetBIOS message originating on the local host having that local IP address. The NAT process configured for the NetBIOS protocol places into that field of the NetBIOS message the one corporate IP address associated with that local IP address by the NAT process. A NAT correction to support NetBIOS is described in a document available at the time of this writing as 27.asp on directory support/kb/articles/q172/2, on the worldwide web (www) at domain support.microsoft.com. 
   However a NAT process is not as desirable and powerful as a NAPT process. A NAT process is not as efficient as a NAPT process in saving corporate IP addresses. For example, a NAT process uses several corporate addresses; while an NAPT process may use a single corporate IP address. Furthermore, the NAT process does not allow as many local hosts to communicate with the corporate network as a NAPT process. For example, a NAT process may limit the number of local hosts that communicate with devices on the corporate network to 8 or 32; while a NAPT process may allow tens of thousands of local hosts to simultaneously communicate with devices on the corporate network. 
   Based on the foregoing, there is a clear need for a way to use NAPT with interconnected local networks of computers that use the NetBIOS network protocol. 
   There is a specific need for a way to use NAPT in systems that run the Microsoft Windows operating system. 
   Furthermore, there is a need for an NAPT process that works with other network protocols that fail to function with the conventional NAPT process. 
   SUMMARY OF THE INVENTION 
   The foregoing needs, and other needs and objects that will become apparent from the following description, are achieved in the present invention, which comprises, in one aspect, a method for translating between logical addresses and ports of a first network, and a logical address and ports of a second network connected to the first network at an intermediate device. 
   According to this aspect, a first packet is received at the intermediate device from a first device having a first address on the first network. A second packet is sent to a second device on the second network in response to receiving the first packet. The second packet includes, in a source address field, a particular address of the intermediate device on the second network. It is determined whether the first packet includes a resource registration message that registers a first resource on the first device with a protocol server for a particular protocol. The protocol server is available at the second device on the second network. If the first packet includes the resource registration message, then information in the resource registration message for uniquely requesting the first resource is determined. The first information is stored in a first data structure in association with the first address. 
   In an embodiment of this aspect, the particular protocol uses a well-known port for requesting the first resource. For example, the particular protocol does not use a translated source port provided in the second packet. In an embodiment of this aspect, the particular protocol is the NetBIOS open protocol. 
   According to another aspect of the invention, a method for translating between logical addresses and ports of a first network, and logical addresses and ports of a second network connected to the first network at an intermediate device, includes receiving a first packet at the intermediate device from a first device not on the first network. A second packet is sent to a second device on the first network in response to receiving the first packet, the second packet including, in a destination address field, data indicating a translated address. It is determined whether the first packet includes a resource request message requesting a resource according to a particular protocol. If it is determined that the first packet includes the resource request message, then first information in the first message for uniquely requesting the resource is determined. Before sending the second packet, the translated address on the first network is determined based on a data item in a first data structure. The data item indicates the translated address and the first information for uniquely requesting the resource. 
   In other aspects, the invention encompasses an apparatus, computer apparatus, and a computer readable medium, including a carrier wave, configured to carry out the foregoing steps. 
   These techniques allow an NAPT process to function for the NetBIOS protocol. NAPT fails with the NetBIOS protocol because, in some circumstances, the NetBIOS protocol ignores the source ports generated by the NAPT process, and, instead, uses well-known ports associated with NetBIOS services. By storing in a data structure on the intermediate device with the NAPT process the resource names for NetBIOS servers on the local network in association with their local addresses, as described in this summary, a NetBIOS-enabled NAPT process can associate inbound requests for services on the local network with the local address of the device that hosts the service. NAPT can therefore properly translate the destination address and resource name to the associated local address. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings and in which like reference numerals refer to similar elements and in which: 
       FIG. 1  is a block diagram that illustrates an overview of a two local networks connected to a corporate network, according to an embodiment. 
       FIG. 2  is a block diagram that illustrates a packet including a message used to communicate among NetBIOS hosts; 
       FIG. 3  is a time line diagram that illustrates a sequence of messages passed between a NetBIOS server, an NAPT process on an intermediate device, a NetBIOS name server, and a NetBIOS service client, according to an embodiment; 
       FIG. 4  is a block diagram that illustrates a router with a NetBIOS-enabled NAPT process, according to an embodiment; 
       FIG. 5A  is a functional decomposition chart that illustrates at a high level a NetBIOS-enabled NAPT process, according to an embodiment; 
       FIG. 5B  is a flowchart that illustrates a method for processing resource name registration messages according to one embodiment of the NetBIOS-enabled NAPT process depicted in  FIG. 5A ; 
       FIG. 5C  is a flowchart that illustrates a method for processing NetBIOS resource requests according to one embodiment of the NetBIOS-enabled NAPT process depicted in  FIG. 5A ; and 
       FIG. 6  is a block diagram that illustrates a router upon which an embodiment may be implemented. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   A method and apparatus for network address and port translation for network protocols that do not use translated ports are described. In the following description, for the purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent, however, to one skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known structures and devices are shown in block diagram form in order to avoid unnecessarily obscuring the present invention. 
   —Operational Context 
   One context in which an enhanced Network Address and Port Translation (NAPT) process (hereinafter called a NetBIOS-enabled NAPT process) may be used is in a corporate wide-area TCP/IP network. In this context, the corporate wide-area network uses corporate IP addresses. The corporate wide-area network is connected to multiple local networks, such as at retail outlets, employing private, local addresses that are not valid corporate IP addresses. Hosts at each site run the Windows operating system and rely on the NetBIOS protocol for accessing resources across the network. A conventional NAPT process fails to support NetBIOS communications in this context. 
   Embodiments of the invention are not limited to the illustrated context. For example, in other embodiments, instead of a corporate network, the public Internet is used to connect multiple local networks. In those embodiments, the NAPT process translates between local IP addresses and IP addresses registered on the public Internet. In some embodiments, the local networks are virtual private networks using private IP addresses. The data is encrypted for transmission over the Internet, and the IP addresses are translated from VPN private IP addresses to the addresses registered on the public Internet. In other embodiments, the connecting network is a not a wide-area network, but is a geographically confined network. For example, the corporate network may be made up of a few nodes, one in each building on a campus. Each building has a local network using independent IP addresses. 
   In some embodiments, a different network protocol from NetBIOS is used for accessing network resources on the corporate network. A NetBIOS enabled NAPT process as disclosed herein supports a class of network protocols that the conventional NAPT does not support. The network protocols in this class include protocols that in some circumstances ignore the source port provided by the NAPT process and use instead a well-known port to communicate with servers across the boundary between the corporate and local networks. The network protocols in this class include protocols that register a name, unique across the entire network, for referencing resources provided by the servers. 
     FIG. 1  is a block diagram that illustrates an overview of a system  100  including two local networks  130 ,  140  connected to a private corporate wide-area network, hereinafter called a corporate network, according to an embodiment. 
   The corporate network includes private network  120 , and routers  132 ,  142 . Private network  120  includes hosts  122   a ,  122   b ,  122   c ,  122   d , collectively called hosts  120 . Private network  120  also includes host  126  and router/concentrator  124 . The devices on the corporate network use corporate IP addresses. DHCP server  127  provides corporate IP addresses to private network  120  and routers  132 ,  142  from a pool of corporate IP addresses. For example, DHCP server  127  uses the pool of IP addresses from 125.125.125.0 to 125.125.125.255. Then, for example, router  132  is given corporate address 125.125.125.130 and host  126  is given corporate address 125.125.125.2. 
   The corporate network is connected to local network  130  at router  132 . The corporate network is connected to local network  140  at router  142 . Local network  130  includes router  132  and hosts  136   a ,  136   b ,  136   c , collectively called hosts  136 . Local network  140  includes router  142  and hosts  146   a ,  146   b ,  146   c , collectively called hosts  146 . To conserve corporate IP addresses, the hosts  136  use a pool of local IP addresses that are not valid corporate IP addresses. DHCP server  134   a  provides local IP addresses to hosts  136  from the pool of local IP addresses. The hosts  146  use the same pool of local IP addresses as hosts  136 . DHCP server  134   b  provides local IP addresses to hosts  146  from the same pool of local IP addresses that DHCP server  134   a  uses. For example, both DHCP servers  134   a ,  134   b  use the pool of IP addresses from 1.1.1.0 to 1.1.128.255. 
   An NAPT process  135   a  runs on router  132  to translate between the ports and local addresses used in data packets traversing local network  130  and the ports and corporate IP addresses used in data packets traversing the corporate network. An NAPT process  135   b  runs on router  142  to translate between the ports and local addresses used in data packets traversing local network  140  and the ports and corporate IP addresses used in data packets traversing the corporate network. 
   In some embodiments, the private network  120  is connected directly to routers  132  and  142 . In the illustrated embodiment, private network  120  is connected to routers  132  and  142  through the network  106  of a third party. In some embodiments, the third party network is made up of secured leased lines in a circuit switched network. In some embodiments, the third party network  106  includes the public Internet of computer networks. If the third party network includes the public Internet, the corporate network may be exposed to the public. The corporate network can be protected from the public by installing firewall processes at the routers  124 ,  132 ,  142 . If desired, data can be encrypted by the firewall processes before transmitting data packets over the Internet. Also, the corporate network can be protected from the public if virtual private network processes are provided by routers  124 ,  132 ,  142 , or by an Internet Service Provider (ISP) to which routers  124 ,  132 ,  142  are connected. 
   According to an embodiment, hosts  122 ,  126 ,  136 ,  146  are NetBIOS hosts running NetBIOS protocol for sharing resources, and NAPT processes  135   a ,  135   b  are NetBIOS-enabled NAPT processes. For example, NetBIOS host  136   c  includes a NetBIOS resource server  133  that provides a resource, such as a service, in response to a request from NetBIOS resource client  123  on NetBIOS host  122   a . The NetBIOS hosts communicate with a NetBIOS name server (NBNS)  128  executing on host  126  on the private network  120 , as is described in more detail below. 
   One problem of past approaches using NAPT for NetBIOS communications is that NetBIOS utilizes well-known port  138  when a NetBIOS client requests a resource from a NetBIOS server. If the server is on the local network, a request for that resource on that server from a device on the corporate network comes to the NAPT process with port  138 , instead of coming with the translated source port provided by the NAPT process. 
   For example, a request from client  123  comes to the NAPT process  135   a  with a destination address 125.125.125.130 and a destination port of  138 . The NAPT process cannot associate the port value  138  with a particular one of the local addresses of the devices on the local network. For example, the NAPT process associates the local addresses of the devices on the first network with ports 3000 to 3255, and does not associated any local address with port  138 . Thus the conventional NAPT process cannot translate to a local address and port and cannot forward the request for the resource to the correct device on the local network. 
     FIG. 2  is a block diagram that illustrates a packet including a message used to communicate among NetBIOS hosts. 
   TCP/IP packet  200  includes a destination address field  202 , a destination port field  206 , a source address field  204 , and a source port field  208 . The NetBIOS message  210  is included in the payload portion of TCP/IP packet  200 . 
   The fields in the NetBIOS message  210  depend on the function for the message. Two functions relevant to NAPT are registration of a resource name, and a request for a named resource. These functions are described in more detail below. During registration, field  212  holds data indicating the NetBIOS resource name being registered and field  214  holds data indicating the IP address of the NetBIOS host registering the resource name. During requests for a named resource, field  216  includes the registered name of the resource being requested. For example, the resource may be a software application, or server, that performs some service in response to the request. Field  220  holds data provided to the application, if any. Ellipsis  219  indicates that the NetBIOS message includes other fields as well as the fields illustrated. 
   In an alternative approach, the NetBIOS standard could be modified to include a source port in a NetBIOS message requesting resources. However, making such a change available to the tens of millions of devices using NetBIOS that are already deployed is impractical. Therefore, there is a need for a way to enable an NAPT process to interoperate in a system that uses NetBIOS, without changing the NetBIOS standard. 
   —NetBIOS-Enabled NAPT 
     FIG. 5A  is a functional decomposition chart that illustrates at a high level a NetBIOS-enabled NAPT process  500 , according to an embodiment. 
   The NetBIOS-enabled NAPT process  135   a  in  FIG. 1  is an embodiment of the NetBIOS-enabled process  500 . In step  510 , messages for registering a resource name on a remote NetBIOS name server are processed. For example, the NetBIOS-enabled NAPT process  135   a  receives and processes data packets with messages for registering a NetBIOS resource name for a device on the local network at a NetBIOS name server (NBNS) on the corporate network. 
   In step  550 , messages for a resource on a local network are processed. For example, the NetBIOS-enabled NAPT process  135   a  receives and handles data packets received from the corporate network with messages requesting the resource on the device on the local network. 
     FIG. 3  is a time line diagram that illustrates a sequence of data packets passed between a NetBIOS server  133 , a NetBIOS-enabled NAPT process  135   a  on router  132 , a NetBIOS name server  128 , and a client process  123  executing on a NetBIOS host  122   a , according to an embodiment. Time elapses from top to bottom in the diagram. NetBIOS registration involves data packet  322  and data packet  324 . A NetBIOS resource request involves data packets  330 ,  342 ,  352 . In the embodiment of  FIG. 3 , NetBIOS Server  133  is a Windows® NetBIOS Server  310 , and client process  123  is a Windows® NetBIOS Client  312 . 
   ——NetBIOS Registration 
   NetBIOS produces a unique name for each resource on the network within a NetBIOS scope. A resource includes a service provided by a server on a host. A scope is a group of NetBIOS hosts managed together on a network. A NetBIOS Name Server (NBNS) process manages the hosts within a NetBIOS scope by ensuring unique names for all resources in the scope and establishing and maintaining a mapping between the IP address of each host in the NetBIOS scope and the unique name. Each NetBIOS host that provides a resource for the network must register a resource name with the NBNS. Each NetBIOS host may register one or more resource names. 
   When a NetBIOS host joins a network segment, the computer is configured with an IP address for itself and an IP address of a router on the same network segment and an IP address of the NBNS. A network segment consists of all devices not separated by intermediate devices, and the one or more intermediate devices contiguous with those devices. 
   For example, referring again to  FIG. 1 , local networks  130 ,  140  are network segments. When NetBIOS host  136   c  joins the network segment of local network  130 , the configuration information is provided by the DHCP server  134   a  in response to a DHCP message broadcast by the host  136   c . In some systems, this information is provided in a disk file stored on the host computer and automatically read upon start-up. In some systems, a human administrator types in the information. 
   The joining NetBIOS host then generates a name for a resource on the host, such as a NetBIOS file server on the host, and attempts to register the name at the NBNS  128  by sending a data packet  322  to the NBNS. If the generated name is successfully registered as a unique name within the NetBIOS scope, then the generated name is used by other NetBIOS hosts (e.g., NetBIOS clients for the resource) within the scope for subsequent communications with the resource on the joining host (the NetBIOS server for the resource). 
   For example, in  FIG. 1 , assume that host  136   c  joins the local network  130  and is configured with a local IP address of 1.1.1.6 for itself, a local address of 1.1.1.1 for the router  132 , and a corporate IP address of 125.125.125.2 for the host  126  with the NBNS  128 . A NetBIOS resource server  133  on the joining host generates a name to refer to itself that is likely be unique, such as “StationQail.fileserver” 
   The host attempts to register the resource using a NetBIOS message in a TCP/IP data packet  322 . The data packet has a source address of the local IP address for the joining host, a source port generated by the joining host, a destination address of the NBNS server, and a destination port of  137 . The destination port value  137  indicates to the host at the destination address that the payload is a NetBIOS name registration message that is delivered to the NBNS. 
   For example, the NetBIOS resource server  133  generates a data packet  322  with fields as listed in Table 1 below. Data indicating the IP address 125.125.125.2 of the host  126  for the NBNS  128  is inserted in the destination IP address field  202 , and data indicating port  137  is inserted in the destination port field  206 . An arbitrary port number “2345” is generated by the NetBIOS resource server  133 ; and data indicating that value is inserted in the source port field  204 . Data indicating the IP address 1.1.1.6 of the host  136   c  of the resource server  133 , is inserted in the source address field  204  of the data packet and in the NetBIOS message payload of the packet in the Requestor&#39;s IP address field  214 . The NetBIOS message payload also includes data indicating the name “StationQuail.fileserver” being registered with the NBNS  128  in the requestor&#39;s registered name field  212 . 
   
     
       
         
             
           
             
               TABLE 1 
             
           
          
             
                 
             
             
               Example Data Packet from NetBIOS Server 
             
          
         
         
             
             
             
          
             
                 
               Data packet field 
               Sample Value 
             
             
                 
                 
             
             
                 
               Destination address 
               125.125.125.2 
             
             
                 
               Destination port 
               137 
             
             
                 
               Source address 
               1.1.1.6 
             
             
                 
               Source port 
               2345 
             
             
                 
               Requestor&#39;s registered name 
               StationQuail.fileserver 
             
             
                 
               Requestor&#39;s IP address 
               1.1.1.6 
             
             
                 
                 
             
          
         
       
     
   
   The data packet  322  is delivered to an NAPT process on the intermediate device between the joining host and the NBNS. As shown in  FIG. 3 , the data packet is delivered to a NetBIOS-enabled NAPT process  135   a  on router  132 . The NAPT process translates the source address and source port of the data packet. 
     FIG. 4  shows structural components of the NetBIOS-enabled NAPT process on a NAPT router  400  serving as the intermediate device. For example, routers  132 ,  142  in  FIG. 1  are NAPT routers like NAPT router  400 . The NAPT router  400  includes NetBIOS enabled NAPT process  410 , which includes a process  420  to associate a local address with a information to uniquely request the resource on the device at that local address. For example, the local address is associated with the unique NetBIOS name for the resource, as described in more detail below. The illustrated NetBIOS-enabled NAPT process uses two data structures  414 ,  424 . The data structures reside in memory. In some embodiments the data structures are also stored in persistent storage. In other embodiments, a single data structure is used for all the data in illustrated data structures  414 ,  424 . 
   When the data packet  322  is received, the local IP address in the source address field is replaced by the corporate address of the intermediate device. For example, NAPT  135   a  translates the source address from the local IP address 1.1.1.6 of host  136   c  to the corporate IP address 125.125.125.130 of the router  132 . The NAPT process also generates a translated source port that is unique for a given combination of local IP address and source port in all data packets received on the local network for delivery over the corporate network. For example, the NAPT  135   a  generates a translated source port of 3000. The local IP address and original source port are stored in the data structure  414  in association with the corporate address and translated port. For example, the values listed in Table 2 are stored in a data item in data structure  414 . 
   In the illustrated embodiment, the translated address is not included in the data item because the translated address is always the address of the router  132  on the corporate network, e.g., 125.125.125.130. Whenever a data packet is subsequently received at router  132  from a device on the local network with a source address of 1.1.1.6 and a port of 2345, the source address is translated to the source address of the router  132  on the corporate network and the source port is translated to source port 3000. Whenever a data packet is subsequently received at router  132  from a device on the corporate network with a destination address of 125.125.125.130 and a destination port of 3000, the destination address is translated to the local address 1.1.1.6 and the port 2345 based on the data item listed in Table 2. 
   
     
       
         
             
           
             
               TABLE 2 
             
           
          
             
                 
             
             
               Example Values of Data Item in One NAPT Data Structure 
             
          
         
         
             
             
             
          
             
                 
               Data Item field 
               Sample Value 
             
             
                 
                 
             
             
                 
               Local IP address 
               1.1.1.6 
             
             
                 
               Original port 
               2345 
             
             
                 
               Translated port 
               3000 
             
             
                 
                 
             
          
         
       
     
   
   When another data packet is received from local source address 1.1.1.6 with a different port, another data item is added to data structure  414  with a unique translated port, e.g., 3001. When another data packet is received from another local source address, e.g., 1.1.1.3, with the same port or another port, another data item is added to data structure  414  with a unique translated port, e.g., 3002. If a data packet is received from a local address and port combination already in a particular data item in the data structure, no new data item is added to the data structure. The translated port in the particular data item is used as the translated port. For example, when the NAPT process receives another data packet from local address 1.1.1.6 and port 2345, a data item is not added to data structure  414 , and the translated port 3000 is used. 
   In many network protocols, subsequent data packets directed to the source of the data packet  322  include the translated source address as the destination address and the translated port as the destination port. Such a data packet is delivered to the intermediate device that stored the data item in the data structure  414 . The NAPT server then finds the translated port in the data structure  414  and retrieves the associated local IP address and original port. 
   A new data packet based on the received data packet is then generated. In the new packet, the local IP address replaces the corporate address of the intermediate device in the destination address field. The original port replaces the translated port in the destination port field. However, some data packets generated by NetBIOS clients do not insert the translated port in the destination port. For such data packets, a conventional NAPT process will not be able to identify the data item in data structure  414  that contains the local IP address to insert in the destination field of the new data packet. 
   Therefore, according to an embodiment, the NetBIOS-enabled NAPT process also stores information in a new data structure  424 . In the new data structure  424 , a data item is stored that associates the local IP address with information that the NetBIOS protocol uses to uniquely identify and request NetBIOS resources. For example, a data item is stored with data indicating the local IP address and the resource name. 
   Table 3 shows the contents of a data item in the new data structure, according to the illustrated example. In some embodiments, data items are added to the new data structure  424  only if the data structure being translated includes a NetBIOS registration message, e.g., a data packet with destination port  137 . 
   
     
       
         
             
           
             
               TABLE 3 
             
           
          
             
                 
             
             
               Example Values of Data Item in New NAPT Data Structure 
             
          
         
         
             
             
             
          
             
                 
               Data Item field 
               Sample Value 
             
             
                 
                 
             
             
                 
               Local IP address 
               1.1.1.6 
             
             
                 
               Information to uniquely request resource 
               StationQuail.fileserver 
             
             
                 
                 
             
          
         
       
     
   
   The NAPT process generates a second data packet based on the first data packet. In the second data packet, the NAPT process replaces the original port in the source port field with the translated port, and replaces the local IP address with the corporate IP address of the intermediate device. 
   While a conventional NAPT process sends the second data packet after replacing the data in the source port and source address fields, the NetBIOS-enabled NAPT process also replaces a field in the NetBIOS message payload before sending the second message. 
   As shown in  FIG. 2 , field  214  in NetBIOS message  210  includes data indicating the address of the requester. The IP address registered by the NBNS with the NetBIOS resource name is based on the data in field  214 . Therefore the NetBIOS-enabled NAPT process also replaces the local IP address in field  214  with the corporate IP address of the intermediate device. 
   For example, the NetBIOS-enabled NAPT process  135   a  generates a second data packet  324  with fields as listed in Table 4, and sends packet  324  to the NetBIOS Name Server at time t 2  after time t 1 . 
   
     
       
         
             
           
             
               TABLE 4  
             
           
          
             
                 
             
             
               Example Data Packet from NAPT Process to NetBIOS Name Server 
             
          
         
         
             
             
             
          
             
                 
               Data packet field 
               Sample Value 
             
             
                 
                 
             
             
                 
               Destination address 
               125.125.125.2 
             
             
                 
               Destination port 
               137 
             
             
                 
               Source address 
               125.125.125.130 
             
             
                 
               Source port 
               3000 
             
             
                 
               Requestor&#39;s registered name 
               StationQuail.fileserver 
             
             
                 
               Requestor&#39;s IP address 
               125.125.125.130 
             
             
                 
                 
             
          
         
       
     
   
   When data packet  324  is received by the NBNS  128 , the resource name in field  212  is compared to other names to determine whether the name is unique for the NetBIOS scope. If so, an acknowledgement message is sent back to the NetBIOS resource server. If not, a negating acknowledgement is sent back. 
   The NBNS sends the acknowledgment or negating acknowledgement in NetBIOS messages inserted into data packets that use, as the destination address and port, the translated source address and translated source port, respectively, from data packet  324 . Therefore the acknowledgement and negating acknowledgment are translated to the local address and port by the NAPT process as in the conventional NAPT process, using the first data structure  414 . 
   If a NetBIOS resource name stored in a data item in the new data structure  424  is not successfully registered, the data item is deleted from the data structure  424 . The resource name is not registered if no acknowledgement message is received within a certain period, or if a negating acknowledgement is received. 
   After successful registration, the resource name becomes unregistered under several circumstances. The resource name becomes unregistered when the name is released explicitly by a subsequent message from the NetBIOS resource server. The resource name becomes unregistered when the name is released implicitly by a failure to renew the registration before a registration period expires. If a NetBIOS resource name stored in a data item in the new data structure  424  becomes unregistered, the data item is deleted from the data structure  424 . 
   ——NAPT Method for NetBIOS Registration 
     FIG. 5B  is a flowchart that illustrates a method for processing resource name registration messages according to one embodiment  510   a  of step  510  of the NetBIOS-enabled NAPT process depicted in  FIG. 5A . Although steps are depicted in  FIG. 5A  and subsequent flowchart  FIG. 5B  in a particular order, in other embodiments the steps occur in a different order or overlapping in time. 
   In step  512 , data packets are received by the NAPT process. The data packets received include some data packets with messages related to registering names for NetBIOS resources. 
   In step  514 , a data packet is received from a local device with a local source address and original, local source port. For example the data packet  322  described in Table 1, above, is received. If a data packet is received from a device on the corporate network, steps  514  through  520  are skipped. 
   In step  516 , a unique translated port is generated based on the source address and the source port. Any technique known at the time the step is implemented may be used to generate the unique port. For example, in one embodiment, a hashing function is used. In another embodiment, a range of ports is used in succession. The ports in use are stored in data structure  414 . If a port already appears in data structure  414  for the local source address and the original source port, that port is used as the translated port. If no data item in the data structure  414  already contains both the local source address and the original port, a different port in the range than any port already in data structure  414  is used as the translated port. 
   In step  518 , a second data packet is generated. The second data packet includes the corporate address of the intermediate device on which the NAPT process executes in the source address field. The second data packet includes the translated port determined in step  516  in the source port field. For example, the data packet  324  with data packet fields described in Table 4, above, is generated. 
   In step  520  the local IP address and translated port are stored in a data item, such as a record, of a data structure. In some embodiments, the original source port is also stored in the data item. For example, the data item described in Table 2, above, is stored in data structure  414 . 
   Step  525  represent a branch point that depends on whether the data packet received relates to registering a NetBIOS resource name. For example, the data packet includes a NetBIOS message in the payload. If not, control passes to step  526  to send the translated data packet. In step  526 , if the data packet is received from a device on the corporate network, control passes to step  552  in  FIG. 5C . 
   If it is determined in step  525  that the data packet received relates to registering a NetBIOS resource name, control passes to step  535 . For example, if the data packet is received from a device on the corporate network with a NetBIOS acknowledgement or negating acknowledgement, control passes to step  535 . 
   In step  535  it is determined whether the data packet includes a NetBIOS message that indicates a resource name is not registered, for example by explicit release or by a negating acknowledgement. In some embodiments, step  535  includes determining whether a registration period has expired without a renewal message. If the resource name is not registered, control passes to step  536  to remove a data item, e.g., a record, from data structure  424  that includes the resource name. 
   If it is determined in step  535  that the resource name is registered, then control passes to step  538 . In step  538 , the resource name is inserted in the data structure  424  with the local IP address, if it is not already there. For example, if the message is a registration request from the local device, the resource name is not already in the data structure  424 , and so the resource name is added in a new record. For example, the data item described in Table 3, above, is inserted in data structure  424 . If the message is an acknowledgement, the resource name is already in a record in data structure  424  and remains there. 
   Although the resource name is used in step  538 , in other embodiments, other information in the message payload that uniquely indicates the resource may be used. 
   Also in step  538 , if the message is a registration request, the received data packet includes a NetBIOS message with the local IP address in field  214 . A translated NetBIOS message, based on the NetBIOS message in the received data packet, is inserted in the second data packet. The translated NetBIOS message includes in the field  214  for the address of the requestor, the corporate address of the intermediate device on which the NAPT process executes. 
   Control then passes to step  526  to send the translated, second data packet. For example the data packet  324  described above with reference to Table 4 is sent. 
   ——NetBIOS Resource Request 
   A NetBIOS resource request involves data packets  330 ,  342 ,  352  shown in  FIG. 3 . 
   Before a NetBIOS resource client requests a NetBIOS resource, the client sends a NetBIOS name query message to the NBNS to obtain the names of available resources. The NetBIOS resource client then requests a named resource. For example, NetBIOS resource client  123  on host  122   a  requests resource named “StationQuail.fileserver.” The resource name is associated with an IP address by the NBNS. 
   In one embodiment, the NBNS response to the name query includes the IP addresses associated with each name. This response is shown as data packet  330 . In another embodiment, only the names are returned in the response; and the request from the resource client for the resource is sent to a NetBIOS Datagram Distribution (NBDD) server. The NBDD server queries the NBNS to obtain the IP address of the named resource, and the NBDD generates and sends the data packet requesting the resource for the NetBIOS resource client. In this embodiment, the NBDD server is the NetBIOS resource client  123  for purposes of the following description, and the NBDD server receives data packet  330 . 
   Data packet  330  is sent from the NBNS to the NetBIOS resource client  123  at time t 3  after time t 2 . The data packet  330  includes a NetBIOS message that provides the IP address stored by the NBNS during registration for the named resource. For example, data packet  330  includes the corporate IP address 125.125.125.130 registered for the NetBIOS resource named StationQuail.fileserver based on data packet  324 . 
   According to the NetBIOS protocol, the source port used by the registration data packet  324  is neither stored by the NBNS nor returned in data packet  330 . For example, the translated source port 3000 used by data packet  324  is neither stored by the NBNS nor returned to NetBIOS resource client  123  in data packet  330 . 
   The NetBIOS resource client  123  then generates and sends data packet  342  at time t 4  after time t 3 . The IP address returned in data packet  330  is used as the destination address of data packet  342 . For example, the data in field  202  in data packet  342  indicates corporate IP address 125.125.125.130. 
   According to the NetBIOS protocol, the data packet requesting a NetBIOS resource includes well-known port  138  in the destination port. The translated port for the NetBIOS resource server is not used. For example, the translated port 3000 is not used by the NetBIOS protocol. 
   It is assumed for purposes of illustration that the corporate IP address is 125.125.125.221 for host  122   a  of NetBIOS resource client  123 , that client  123  has NetBIOS name “StationSparrow.getfile,” and that client  123  generates source port 1234. Table 5 shows example values for some of the fields in data packet  342  generated by the NetBIOS resource client  123 . Table 5 includes the value for the requested application name, field  216  in  FIG. 2 . 
   
     
       
         
             
           
             
               TABLE 5 
             
           
          
             
                 
             
             
               Example Data Packet from NetBIOS Resource Client 
             
          
         
         
             
             
             
          
             
                 
               Data packet field 
               Sample Value 
             
             
                 
                 
             
             
                 
               Destination address 
               125.125.125.130 
             
             
                 
               Destination port 
               138 
             
             
                 
               Source address 
               125.125.125.221 
             
             
                 
               Source port 
               1234 
             
             
                 
               Requestor&#39;s registered name 
               StationSparrow.getfile 
             
             
                 
               Requestor&#39;s IP address 
               125.125.125.221 
             
             
                 
               Requested application name 
               StationQuail.fileserver 
             
             
                 
                 
             
          
         
       
     
   
   The data packet  342  is delivered to the NAPT process on the intermediate device between the NetBIOS resource client  123  and the NetBIOS resource server  133 . As shown in  FIG. 3 , the data packet is delivered to a NetBIOS-enabled NAPT process  135   a  on router  132 . The NetBIOS-enabled NAPT process translates the data in the destination address and the destination port of the data packet  324  into data indicating the local address and port for NetBIOS resource server  133 . The translated data is used to generate data packet  352  that is sent to NetBIOS resource server  133  at time t 5  after time t 4 . Table 6 shows example values for some of the fields in data packet  352  generated by the NetBIOS-enabled NAPT process  135   a . 
   
     
       
         
             
           
             
               TABLE 6 
             
           
          
             
                 
             
             
               Example Data Packet Sent to NetBIOS Resource Server 
             
          
         
         
             
             
             
          
             
                 
               Data packet field 
               Sample Value 
             
             
                 
                 
             
             
                 
               Destination address 
               1.1.1.6 
             
             
                 
               Destination port 
               138 
             
             
                 
               Source address 
               125.125.125.221 
             
             
                 
               Source port 
               1234 
             
             
                 
               Requestor&#39;s registered name 
               StationSparrow.getfile 
             
             
                 
               Requestor&#39;s IP address 
               125.125.125.221 
             
             
                 
               Requested application name 
               StationQuail.fileserver 
             
             
                 
                 
             
          
         
       
     
   
   NetBIOS-enabled NAPT process translates the corporate IP address of the intermediate device in the destination address to the local address of the NetBIOS resource server, as described below with reference to  FIG. 5C . For example, the corporate IP address 125.125.125.130 is translated to the local IP address 1.1.1.6. 
   The conventional NAPT process is unable to do this translation. For example, the destination port  138  and the source port 1234 in Table 6 are unrelated to the data item stored in data structure  414  for local address 1.1.1.6, shown in Table 2. 
     FIG. 5C  is a flowchart that illustrates a method for processing NetBIOS resource requests according to embodiment  550   a  of step  550  of the NetBIOS-enabled NAPT process depicted in  FIG. 5A . 
   In step  552  a data packet is received from a device on the corporate network. The data packet includes data indicating a particular value in the destination port field. The particular value is herein called the corporate destination port. For example, the NetBIOS resource request data packet  342  is received from host  122   a  on the corporate network. In the example data packet shown in Table 5, above, the corporate destination port is  138 . 
   It should be noted that the resource request can originate on another local network, such as local network  140 . Requests for the resource that originate on host  146   c , for example, are translated by NetBIOS-enabled NAPT process  135   b  to indicate the corporate address of router  142  in the source address field of the data packet received. 
   In step  555  it is determined whether the corporate destination port can be found in the first data structure  414 . For example it is determined whether the value  138  can be found in the translated port field of any data item in first data structure  414 . 
   If so, control passes to step  556  to obtain the local IP address and local, original port from the data item in the first data structure  414  associated with the corporate destination port, as in the conventional NAPT process. For example, the local IP address and original port are obtained from the data item that contains the corporate destination port in the translated port field. Control then passes to step  570  to generate the data packet for the local network, as described below. 
   If the corporate destination port can not be found in the first data structure  414 , control passes to step  558  to determine whether the corporate destination port is a well-known port for the NetBIOS protocol. If not, the data packet is not requesting a NetBIOS resource and an error is indicated. Control passes to step  559  to handle the error, such as by sending a message to the source address. 
   If the corporate destination port is a well-known port for the NetBIOS protocol then control passes to step  560 . For example if the corporate destination port is well-known port  138  for requesting NetBIOS resources, control passes to step  560 . In the example data packet the corporate destination port is  138 . 
   In step  560 , the resource name is obtained from field  216  for the name of the NetBIOS application requested in the NetBIOS message payload. In the example data of Table 6, the name of the requested application in field  216  is “StationQuail.fileserver.” 
   In step  562 , the local IP address associated with the resource name is obtained from the new data structure  424 . For example, in Table 3, the local IP address 1.1.1.6 is associated with the resource name StationQuail.fileserver as both are in the same data item. 
   Although the resource name is used in steps  560 ,  562 , in other embodiments, other information in the message payload that uniquely indicates the resource for requests may be used. 
   In step  564 , the local destination port is obtained from the corporate destination port, to satisfy the NetBIOS protocol to use well-known port  138  for requests of NetBIOS resources. 
   In step  570 , a translated data packet  352  is generated based on packet  342  by inserting data indicating the local IP address into the destination address field  202  and inserting the local port into destination port field  206 . In embodiments in which the destination port is the same in both data packets  342  and  352 , only the local IP address has to be inserted into data packet  352 . 
   In step  572 , the translated data packet is sent over local network. For example, data packet  352  is sent over local network  130  to host  126   c  having local IP address 1.1.1.6. 
   Thus the data packets with NetBIOS messages are successfully translated with the NetBIOS-enabled NAPT process. Therefore, these techniques allow a network address and port translation (NAPT) process to function for the NetBIOS protocol 
   —Hardware Overview 
     FIG. 6  is a block diagram that illustrates a computer system  600  upon which an embodiment of the Invention may be implemented. The preferred embodiment is implemented using one or more computer programs running on a network element such as a router device. Thus, in this embodiment, the computer system  600  is a router. 
   Computer system  600  includes a bus  602  or other communication mechanism for communicating information, and a processor  604  coupled with bus  602  for processing information. Computer system  600  also includes a main memory  606 , such as a random access memory (RAM), flash memory, or other dynamic storage device, coupled to bus  602  for storing information and instructions to be executed by processor  604 . Main memory  606  also may be used for storing temporary variables or other intermediate information during execution of instructions to be executed by processor  604 . Computer system  600  further includes a read only memory (ROM)  608  or other static storage device coupled to bus  602  for storing static information and instructions for processor  604 . A storage device  610 , such as a magnetic disk, flash memory or optical disk, is provided and coupled to bus  602  for storing information and instructions. 
   A communication interface  618  may be coupled to bus  602  for communicating information and command selections to processor  604 . Communication interface  618  may also be coupled to a network link  620  that is connected to a local network  622 . In some embodiments, interface  618  is a conventional serial interface such as an RS-232 or RS-422 interface. An external terminal  612  or other computer system connects to the computer system  600  and provides commands to it using the interface  618 . Firmware or software running in the computer system  600  provides a terminal interface or character-based command interface so that external commands can be given to the computer system. 
   A switching system  616  is coupled to bus  602  and has a first interface  614  and an second interface  619  to one or more external network elements. The external network elements may include a local network  622  coupled to one or more hosts  624 , or a global network such as Internet  628  having one or more servers  630 . The switching system  616  switches information traffic arriving on first interface  614  to second interface  619  according to pre-determined protocols and conventions that are well known. For example, switching system  616 , in cooperation with processor  604 , can determine a destination of a packet of data arriving on first interface  614  and send it to the correct destination using second interface  619 . The destinations may include host  624 , server  630 , other end stations, or other routing and switching devices in local network  622  or Internet  628 . 
   The invention is related to the use of computer system  600  for network address and port translation. According to one embodiment of the invention, network address and port translation is provided by computer system  600  in response to processor  604  executing one or more sequences of one or more instructions contained in main memory  606 . Such instructions may be read into main memory  606  from another computer-readable medium, such as storage device  610 . Execution of the sequences of instructions contained in main memory  606  causes processor  604  to perform the process steps described herein. One or more processors in a multi-processing arrangement may also be employed to execute the sequences of instructions contained in main memory  606 . In alternative embodiments, hard-wired circuitry may be used in place of or in combination with software instructions to implement the invention. Thus, embodiments of the invention are not limited to any specific combination of hardware circuitry and software. 
   The term “computer-readable medium” as used herein refers to any medium that participates in providing instructions to processor  604  for execution. Such a medium may take many forms, including but not limited to, non-volatile media, volatile media, and transmission media. Non-volatile media includes, for example, optical or magnetic disks, such as storage device  610 . Volatile media includes dynamic memory, such as main memory  606 . Transmission media includes coaxial cables, copper wire and fiber optics, including the wires that comprise bus  602 . Transmission media can also take the form of acoustic or light waves, such as those generated during radio wave and infrared data communications. 
   Common forms of computer-readable media include, for example, a floppy disk, a flexible disk, hard disk, magnetic tape, or any other magnetic medium, a CD-ROM, any other optical medium, punch cards, paper tape, any other physical medium with patterns of holes, a RAM, a PROM, and EPROM, a FLASH-EPROM, any other memory chip or cartridge, a carrier wave as described hereinafter, or any other medium from which a computer can read. 
   Various forms of computer readable media may be involved in carrying one or more sequences of one or more instructions to processor  604  for execution. For example, the instructions may initially be carried on a magnetic disk of a remote computer. The remote computer can load the instructions into its dynamic memory and send the instructions over a telephone line using a modem. A modem local to computer system  600  can receive the data on the telephone line and use an infrared transmitter to convert the data to an infrared signal. An infrared detector coupled to bus  602  can receive the data carried in the infrared signal and place the data on bus  602 . Bus  602  carries the data to main memory  606 , from which processor  604  retrieves and executes the instructions. The instructions received by main memory  606  may optionally be stored on storage device  610  either before or after execution by processor  604 . 
   The second interface  619  of switching system  616  provides a two-way data communication coupling to the local network  622 . For example, interface  619  may be an integrated services digital network (ISDN) card or a modem to provide a data communication connection to a corresponding type of telephone line. As another example, interface  619  may be a local area network (LAN) card to provide a data communication connection to a compatible LAN. Wireless links may also be implemented. In any such implementation, interface  619  sends and receives electrical, electromagnetic or optical signals that carry digital data streams representing various types of information. 
   Interfaces  614  and  619  typically provides data communication through one or more networks to other data devices. For example, interface  619  may provide a connection through local network  622  to a host computer  624 ; while interface  614  provides a connection to data equipment operated by an Internet Service Provider (ISP)  626 . ISP  626  in turn provides data communication services through the world wide packet data communication network now commonly referred to as the “Internet”  628 . Local network  622  and Internet  628  both use electrical, electromagnetic or optical signals that carry digital data streams. The signals through the various networks and the signals on interfaces  614 ,  619  through switching system  616 , which carry the digital data to and from computer system  600 , are exemplary forms of carrier waves transporting the information. 
   Computer system  600  can send messages and receive data, including program code, through the network(s), interfaces  614 ,  619  and switching system  616 . In the Internet example, a server  630  might transmit a requested code for an application program through Internet  628 , ISP  626 , and interface  614 . In accordance with the invention, one such downloaded application provides for network address and port translation as described herein. 
   The received code may be executed by processor  604  as it is received, and/or stored in storage device  610 , or other non-volatile storage for later execution. In this manner, computer system  600  may obtain application code in the form of a carrier wave. 
   —Extensions and Alternatives 
   In the foregoing specification, the invention has been described with reference to specific embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense.