Abstract:
A communication system includes a first endpoint located behind a first connectivity barrier, such as a firewall or a consumer gateway, and a second endpoint. A service is arranged to respond to a request from the first endpoint to establish communications with the second endpoint by assigning a server to handle a session between the first endpoint and the service. A session initiated by the second endpoint is established with the service if the second endpoint is located behind a second connectivity barrier. If the second endpoint is not located behind a connectivity barrier, a transport level communications connection can be established with the second endpoint. In some cases, the service can instruct the first endpoint to establish a direct session with the second endpoint.

Description:
BACKGROUND 
   The invention relates to connectivity in the presence of barriers. 
   Connectivity between the public Internet and corporate or private home networks can be limited by the presence of firewalls or consumer gateways designed to provide protection of valuable resources on the network. 
   Corporations, for example, typically permit limited incoming access to servers on their internal networks through firewalls, such as an electronic mail (email) gateway or public web site. Outgoing access through firewalls is typically permitted for a few standard protocols such as HyperText Transfer Protocol (HTTP) and File Transfer Protocol (FTP) through a protocol-specific proxy. For example, corporate users can web browse on the Internet through an HTTP proxy. Applications based on alternative protocols, such as buddy lists or Internet Protocol (IP) telephony, may not be able to be employed because of the lack of a suitable firewall proxy. Some applications take advantage of ubiquitously deployed proxies (e.g., an HTTP proxy) by using them to tunnel application data through a firewall. However, application-specific development must be provided to support tunneling. 
   Private home networks face similar connectivity issues. The industry is converging on a centralized access model for sharing Internet access among the personal computers (PCs) and devices in such networks. Centralized access is expected to be provided by consumer gateways based on Network Address Translation (NAT) which allows specific endpoints on the private home network to share outbound access to the Internet, but provides no general mechanism for inbound connections to a specific device on the network. A gateway can be configured manually to map incoming access at a specific network port to a single machine on the private home network. Alternatively, proxies can be installed on a gateway to handle access for a specific application or protocol in a much more flexible way than manually configured port mapping. However, because gateways can be implemented on a variety of operating system platforms, it may be difficult for application developers to provide proxies for each available system. 

   
     DESCRIPTION OF DRAWINGS 
       FIG. 1  illustrates an Internet Protocol (IP) forwarder/relay service supporting a forwarding session. 
       FIG. 2  illustrates an IP forwarder/relay service supporting a relay session. 
       FIG. 3  shows software components associated with endpoints using the IP forwarder/relay service. 
       FIG. 4  is a flow chart of a process for establishing connectivity between source and destination endpoints using the forwarder/relay service. 
       FIG. 5  illustrates hierarchical static mapping that can be used to assign a server to handle a session between an endpoint and the IP forwarder/relay service. 
       FIG. 6  illustrates dynamic mapping that can be used to assign a server to handle a session between an endpoint and the IP forwarder/relay service. 
       FIG. 7  illustrates additional details of exemplary forwarding mode and relay mode firewall traversing sessions. 
       FIG. 8  illustrates a direct session between endpoints. 
   

   DETAILED DESCRIPTION 
   As shown in  FIG. 1 , a client source endpoint  5 , such as a workstation on a corporate network or a private home network, or a computer connected to an Internet Service Provider (ISP), is configured with client software  8  associated with an Internet Protocol (IP) forwarder/relay service  15  described below. 
   The source endpoint  5  is coupled to a firewall  10  which limits inbound and outbound access to and from the source endpoint  5 . The firewall  10  is coupled to a communication medium  12  such as a wide area network or the Internet. The source endpoint  5  establishes communications through the firewall  10  and the communication medium  12  to the IP forwarder/relay service  15 . The service  15  can be implemented, for example, as a cluster of servers or a geographically dispersed set of servers. The number of servers can be increased as needed to partition the load of many clients. 
     FIG. 1  depicts a forwarding session in which the IP forwarder/relay service  15  connects through a communication medium  17  to a destination endpoint  20 . The communication medium  17  can be any public network. The destination endpoint  20  can be any server or workstation that has connectivity with the communication medium  17 . 
   In a forwarding session, data can be forwarded back and forth between the source endpoint and destination endpoint applications. The source endpoint  5  establishes a session using client software  8  to the service  15 . The service  15  can forward data to other endpoints, such as the destination endpoint  20 , that are not cognizant of the IP forwarder/relay service. In forwarding mode, the service  15  and the destination endpoint  20  use transport level communications (e.g., a TCP/IP connection) to transfer information between them. 
     FIG. 2  illustrates a relay session in which the IP forwarder/relay service  15  establishes a virtual connection between the source endpoint  5  and the destination endpoint  20  to relay data back and forth. As shown in  FIG. 2 , the IP forwarder/relay service  15  and the destination endpoint  20  have connectivity to a common communication medium  17 . Connectivity to the destination endpoint  20  is through a firewall  18 . To conduct a relay session, client software  23  must be installed on the destination endpoint  20  as well so that both endpoints  5 ,  20  can establish a session to the service  15 . 
     FIG. 3  illustrates components of the client software  8  installed on the source endpoint  5  to permit a forwarding or relay session to occur. Similar software components must be installed on the destination endpoint  20  for a relay session to occur. Internet applications  30 ,  32 ,  34 , each of which has a user interface, can include buddy list applications such as AOL&#39;s AIM™ or Microsoft&#39;s MSN Messenger™. Alternatively, the applications  30 ,  32 ,  34  can include Telnet, file transfer, multi-user gaming or other types of network applications. The applications operate in the application layer of the protocol stack. 
   A standard application transport interface  35 , such as Sockets or Winsock2, operates below the applications. The transport interface  35 , also called an application programming interface (API), acts as a bridge between the application and the Transport Control Protocol/Internet Protocol (TCP/IP) suite. 
   The client software  8 ,  23  includes additional elements in the session layer of the protocol stack below the transport interface  35 . A name resolution layer  37  and data layer  39 , which can be combined or implemented separately, examine and process an application&#39;s TCP/IP data and name resolution operations and can perform actions such as header addition/removal and modification of name resolution requests. 
   An optional funneler component  40  in communication with the data layer  39  can be installed to combine the data from several applications into a single data stream to transmit or divide a combined received data stream into individual application streams. Framing information can be used to associate the data with local applications. 
   A security/firewall traversal layer  43  (S/FT layer) performs two main functions. First, the S/FT layer  43  can provide support for privacy and/or authentication between the source endpoint  5  and the IP forwarder/relay service  15 . In a relay session, the S/FT layer  43  also can provide end-to-end privacy and/or authentication support for virtual communications between the source endpoint  5  and the destination endpoint  20 . The security provisions can be based, for example, on standards such as Secure Socket Layer (SSL) or a combination of any known cryptography techniques. 
   In addition, the S/FT layer  43  establishes a firewall traversing session, or tunneling session, that allows data communication between the source endpoint  5  and the IP forwarder/relay service  15 . The S/FT layer  43  automatically determines the appropriate proxied protocol, such as HTTP, FTP or SOCKS4/5, to use to tunnel application data through a firewall. The determination may include operations such as examining the local proxy configuration information and dynamically probing the firewall with test connections to the IP forwarder/relay service  15  or it may involve consulting a Dynamic Host Consulting Protocol (DHCP) server or using a service discovery protocol such as Service Location Protocol (SLP), Jini, or Universal Plug and Play (UpnP). If, for example, HTTP is used as the proxied protocol, request pipelining and multi-part return messages can be used to support two-way symmetric communications. Data in the S/FT layer  43  may be directed to (or from) the funneler  40  if support for multiple internet applications is required. Alternatively, a separate instance of the S/FT layer  43  can reside in each application&#39;s process space and data can be sent over per-application firewall traversal sessions. 
   Firewall traversal sessions are initiated by the endpoints  5 ,  20 . As previously noted, a firewall traversal session  7  is established between the source endpoint  5  and the IP forwarder/relay service  15  in both forwarding and relay modes of operation. In the forwarding mode ( FIG. 1 ), an actual TCP connection or User Datagram Protocol (UDP) association for transporting application data can be made between the IP forwarder/relay service  15  and the destination endpoint  20 . The forwarding mode can enable client/server applications that otherwise would have difficulty traversing firewalls. Exemplary applications include client/server-based buddy lists, multi-user games, and IP telephony conferencing. 
   In the relay mode, a firewall traversal session also is established from the destination endpoint  20  to the service  15 . Thus, the IP forwarder/relay service  15  acts as an intermediary between two (or more) separate firewall traversal sessions. Virtual TCP connections or virtual UDP associations are set up between the source and destination endpoints  5 ,  20 . In addition to client/server applications, the relay mode can also enable peer-to-peer applications that otherwise would have difficulty traversing firewalls such as peer-based buddy lists, multi-user games, and IP telephones. 
   Destination network addresses, as well as other information used to multiplex or demultiplex application data, are conveyed in headers contained within the transported session data. The IP forwarder/relay service  15  can add, remove and examine session headers and can establish mapping functions to facilitate the forwarding or relaying of data to the intended endpoint(s). In forwarding mode, when an application on the destination endpoint  20  requires that network addressing information be included in its payload, the IP address for the application running on the source endpoint  5  can be made to appear as if it is the IP address of the service  15 . 
   In one implementation, the service  15  uses a Domain Name System (DNS) host naming convention to identify endpoints  5 ,  20 . Other directory systems also can be supported by the service  15 . The IP forwarder/relay service is assigned a domain name, for example “service.com.” Users at the endpoints  5 ,  20  are assigned sub-domain names. In one implementation, the sub-domain names are based upon information readily known by others such as a name. Thus, John Smith might register as “jsmith.service.com.” In some instances, a sub-domain such as “jsmith.service.com” may not be sufficient to identify a unique endpoint  5 ,  20 . For example, a user may use the service  15  from a variety of locations. To avoid naming conflicts, zip codes and/or locations may be added to the sub-domain names. Thus, an endpoint associated with a user&#39;s workplace, “work.jsmith.97211.service.com,” can be distinguished from a mobile endpoint “mobile.jsmith.97211.service.com” that is associated with the same user. 
   The assigned sub-domain name can be used to configure the system software  8 ,  23  for a given endpoint  5 ,  20 . A user at a source endpoint  5  attempting to relay data to a destination endpoint  20  through the IP forwarder/relay service  15  does not necessarily need to know beforehand the full sub-domain name of the destination endpoint. To illustrate, a destination endpoint may be a private home network with several computers. A fully qualified domain name (FQDN) for one computer could be “denpc.home.jsmith.97211.service.com.” If the user at the source endpoint  5  knows only “service.com” or “jsmith.97211.service.com,” the client system software  8  can provide a dialog box with a list of the constituents of the private home network to choose from. Furthermore, the dialog box approach can be extended to allow endpoints to be distinguished by unique identifiers other than sub-domain names. 
   As indicated by  FIG. 4 , a user enters  200  at least the service domain name into the system to request use of the service  15 . For example, the user would enter the domain name “service.com.” The name resolution layer  37  of the client system software  8  intercepts  210  the domain name information that was entered into the system. For requests that involve the service, the name resolution layer  37  returns  220  either a special non-routeable IP address or else an IP address from a local pool associated with the given service  15 . The name resolution layer  37  records  230  a table entry associating the requested name with the returned IP address. That information then is shared  240  with the data layer  39 . The particular application  30 ,  32 ,  34  initiates  250  a transport level communication, for example, a TCP connection or UDP message, using the returned IP address. The initiation request is intercepted  260  by the data layer  39 . The data layer  39  then retrieves the previously-recorded table entry to obtain the complete information needed to determine  270  whether a firewall traversal session to the service  15  should be established and whether the session should use the forwarding or relay mode. 
   Depending upon the domain name entered originally, the data layer  39  may require more information in order to decide whether a forwarding or relay session is necessary. 
   If a fully qualified user domain name such as “jwblow.23114. service.com” originally were supplied, then the relay mode of operation would be used. On the other hand, if only the domain name “service.com” were originally entered, the data layer  39  would recognize the service host name, but would need additional information to determine whether the session should use the forwarding or relay mode. Specifically, the user would supply either a real destination IP address or physical host name for the forwarding mode, or would select a fully qualified domain name (FQDN) within the service for the relay mode. To obtain the needed information, the data layer  39  can query the user with a dialog box. 
   Once the user has supplied the requested information, the data layer  39  issues  280  a name resolution request so that a server within the service  15  can be assigned for the firewall traversal session. The resolution request, which includes a virtual host name associated with the client endpoint  5 , bypasses the name resolution layer  37  and is issued directly to a domain name resolving server in the IP forwarding/relay service  15 . The service  15  returns  290  an IP address that the physical server uses during the firewall traversal session. 
     FIGS. 5 and 6  illustrate various techniques that the IP forwarding/relay service  15  can employ to assign a physical server to be used for the firewall traversal session. The features are scalable and can be used to map virtual host names to a large number of geographically dispersed servers. 
   In one implementation, shown in  FIG. 5 , a DNS server  80  within the IP forwarder/relay service  15  uses hierarchical partitioning as the basis for selecting the proper physical server (e.g.  82 ,  84 ,  86  or  88 ) to establish a session. The DNS table  90  contains a set of regular expressions to compactly specify a static mapping relationship between the endpoint virtual host names and the physical servers  82  through  88 . According to the table  90 , servers  82  through  84  service requests directed to zip code 97211 and servers  86  through  88  service requests for zip code 99999. Within these two groups, the servers are selected based on the first letter of the user name. 
     FIG. 6  shows a dispatch/switching server model that can be used for dynamic mapping of endpoint virtual host names. The source endpoint  5  sends a virtual host name resolution request to a dispatch server  92  in the service  15 . Based on information received from a load balancing system  94 , the dispatch server  92  returns the IP address of a particular switching server  96 ,  98 ,  100 , that will provide the IP forwarding/relay functionality for the client endpoint session. The load balancing system  94  communicates with the various switching servers  96 ,  98 ,  100  to track the loading of those servers dynamically. In some implementations, the load balancing system  94  can be incorporated into the dispatch server  92 . After a switching server  96 ,  98  or  100  has been assigned, the client endpoint  5  sets up a session to the assigned switching server. 
   An internal dynamic directory can be used in the name resolution process to map an endpoint to a server. In that case, the load balancing system  94  can monitor the dynamic loading of each switching server and assign the least loaded switching server  96 ,  98 ,  100  to handle the session. A corresponding entry can be added to the internal directory to reflect the assignment. The entry contains the mapping from a specific endpoint, such as the endpoint  5 , to the assigned switching server. It allows the service  15  to match client endpoints for relay mode and establish a virtual connection between them. 
   Once the IP address for the session server is obtained, the data layer  39  at the client endpoint  5  establishes  300  a firewall traversal session for the application  30 ,  32  or  34 . Once established, the application&#39;s IP flow can be tagged  310  by the client software  8  with an indication of whether the service  15  should operate in forwarding or relay mode. Alternatively, the service  15  can determine whether forwarding mode or relay mode is to be used based on the destination endpoint&#39;s physical address or virtual host name supplied by the source endpoint  5 . 
   As illustrated in  FIG. 7 , in the forwarding mode, a session server  60  establishes  315  the required TCP connection or UDP association  62  and forwards the data to the IP address for the destination endpoint  20 . 
   In the relay mode, a session server  64  can use its own domain name system or an internal dynamic directory to identify  320  the physical server  66  for the destination endpoint  20 . Assuming that the destination endpoint  20  is listening for TCP/IP requests, a TCP connection or UDP association is established  325  between the source and destination servers  64 ,  66 , creating a virtual connection between the source  5  and destination endpoints  20 . Table entries can be recorded  330  so that future sessions between the endpoints occur over established connections within the service. In some situations, a single server may act as both the source and destination servers  64 ,  66 . 
   An application that is listening for incoming requests for transport level communications connections (e.g., TCP connections or UDP messages) can be handled as follows. The data layer  39  at the destination endpoint  20  can use local policies and configurations to determine whether the applications  30 ,  32 ,  34  require remote listening at the service  15  and a corresponding firewall traversal session. The local policies may indicate that remote listening always is used for certain applications, while for other applications the user should be prompted for further input using, for example, a dialog box. Where remote listening is to be used, the data layer  39  in the destination endpoint  20  establishes a firewall traversal session to the physical server assigned to the local user in the same manner as described above for the source endpoint  5 . Information about an individual listen request is conveyed over the firewall traversal session to the service  15 . Such information can include the fully qualified domain name and application port number for the destination endpoint  20 . 
   As described above, a user can enter the service domain name (e.g., “service.com”) as the destination address to initiate use of the service  15 . In other implementations, instead of entering the service domain name, the user can specify an actual IP address or host name. An automatic determination of whether forwarding mode is appropriate can be made based on the address. For example, network addresses outside an internal domain specified through configuration of the client system software  8 ,  23 , or discovered from standard network configuration parameters such as the user&#39;s subnet, are likely to need forwarding. The software  8 ,  23  can also be configured to recognize specific addresses for which forwarding is required. Alternatively, forwarding mode can be used as a backup after a direct attempt at connection to an external address fails. 
   To increase efficiency, a DNS resolution request for a destination endpoint  20  should resolve successfully only if the destination endpoint is, in fact, listening on at least one port. Also, search directories contain entries for listening endpoints. Such features can increase the likelihood of obtaining a connection in relay mode to a destination endpoint and can reduce the overhead associated with setting up a firewall traversal session for which connections will eventually fail because there is no corresponding listening endpoint. 
   In some implementations, each client endpoint on an internal network can include the software components discussed in connection with  FIG. 3 . Alternatively, a local routing agent can be used. The local routing agent makes it unnecessary for each endpoint located in an internal network to be equipped with system software  8 ,  23 . The local relay agent can act as a virtual router for all inbound communication. The IP forwarder/relay service  15  requires only the address of the local relay agent. The agent then handles the distribution and redirection of communication to particular machines in the internal network, as well as the sessions to the IP forwarder/relay service  15 . 
   A hop component  50  ( FIG. 3 ) also can be included in the client software  8 ,  23  to allow a direct connection to the destination endpoint  20  to be made under certain circumstances. In particular, as shown in  FIG. 8 , when only the source endpoint  5  is located behind a firewall  10  and both the source and destination endpoints include the client software  8 ,  23 , a direct firewall traversal session can be established between the endpoints  5 ,  20  instead of using the relay mode of operation of the service  15 . In such a situation, the service  15  initially can be used to determine whether the relay mode of operation should be used to provide the virtual connection or whether the hop layer  50  in the source endpoint  5  should be instructed to initiate a direct session with the destination endpoint  20  over a communication medium  19 . Alternatively, the hop layer  50  may first attempt a direct session with the destination endpoint  20  and upon failure to establish communications fallback to using the service  15  in the relay mode of operation. 
   Use of virtual host names for identifying parties registered with the service also can facilitate maintaining a connection to a destination endpoint when the source endpoint  5  roams between networks. For example, if the source endpoint  5  is a wireless, mobile device that can roam from one network to another, the service  15  can maintain the connection to the destination endpoint  20  even if the connection to the source endpoint temporarily is lost. In the event that the connection to the source endpoint  5  is lost temporarily, the destination endpoint  20  would not be made aware of that fact because its connection to the service  15  is maintained. To reestablish the session between the source endpoint  5  and the service  15 , the client software  8  can retain information regarding the state of the session. When connectivity to the service  15  subsequently is reestablished, the information regarding the state of the lost session can be used to allow the session to continue from the point when the connection was lost. 
   Various features of the system can be implemented in hardware, software, or a combination of hardware and software. For example, some aspects of the system can be implemented in computer programs executing on programmable computers. Each program can be implemented in a high level procedural or object-oriented programming language to communicate with a computer system. Furthermore, each such computer program can be stored on a storage medium, such as read-only-memory (ROM) readable by a general or special purpose programmable computer, for configuring and operating the computer when the storage medium is read by the computer to perform the functions described above. 
   Other implementations are within the scope of the claims.