Abstract:
A method and apparatus of establishing a connection between a wireless device and a second device includes maintaining a state of the connection between the wireless device and the second device, and receiving an indication that a wireless link to the wireless device has been lost or may be lost. In response to the receiving the indication that the wireless link to the wireless device has been lost or may be lost, the state of the connection is transitioned from a first state to a second state.

Description:
BACKGROUND 
     Advances in communications technology have enabled for a greater variety of and more convenient communications over data networks. Traditionally, the types of communications over data networks include web browsing, electronic mail, file transfers, and so forth. With the greater bandwidth available on data networks, real-time communications over data networks have also become increasingly popular, including electronic gaming, voice over packet data, streaming communications, and others. 
     A data network typically includes many components, including network terminals (referred to as clients), servers, routers, firewalls, and other network elements. The data network can include a public network (such as the Internet) and/or private networks (such as local area networks or wide area networks). Traditionally, a network terminal has connected to a data network using a wired connection (such as through a modem and telephone line, wired LAN connection, and the like). An increasingly popular form of connection of a network terminal to a data network is a wireless connection. Various standards have provided for such wireless connections, including wireless Ethernet (defined by the 802.11 standards from the Institute of Electrical and Electronics Engineers or IEEE). 
     A network protocol that defines packet-based communications over data networks includes the Internet Protocol (IP). One version of IP is IPv4, as described in Request for Comments (RFC) 791, entitled “Internet Protocol,” dated September 1981. Another version of IP is IPv6, as described in RFC 2460, entitled “Internet Protocol, Version 6 (IPv6) Specification,” dated December 1998. IP provides a network layer that defines packets for communicating data over a data network. Above the network layer is a transport layer to define interconnections between hosts. One example of a transport layer is a Transmission Control Protocol (TCP) layer. TCP is a connection-oriented, end-to-end protocol that provides for reliable inter-process communication between pairs of processes in host computers attached to communication networks. 
     Stateful intermediate devices, such as firewalls or network-address-translation (NAT) routers, are used in many networks to protect one domain from another domain, typically to protect users in a private network from a public network such as the Internet. A stateful intermediate device maintains states (such as TCP states) of the connection between network terminals. A firewall maintains the TCP state of each connection to protect against malicious use of a connection by unauthorized systems to prevent hacking activity such as port scans, topology mapping, and so forth. Also, maintaining states of a connection enables a firewall or other intermediate device to enforce TCP compliance. 
     Typically, a stateful intermediate device, such as a firewall, is designed to handle stationary clients in wireline networks. Normally, because of the reliable nature of wired connections, a client in a wireline environment does not lose a link between the client and an access device to a data network. However, in a wireless network, wireless devices may lose network connectivity at a relatively high rate. As a result, a TCP connection that involves a wireless device may become terminated without the graceful handshaking that is performed to terminate a TCP connection. Although the wireless device has lost its wireless link, any stateful intermediate device in the path of the TCP connection may still think that the connection between the wireless device and another endpoint is still established (albeit idle because no data is being exchanged). When the wireless device re-acquires the wireless link, the wireless device may attempt to establish another connection using the original source TCP port. When the new connection requests reaches the stateful intermediate device (which still thinks that the wireless device is associated with the original source TCP port), the stateful intermediate device considers the new connection request as violating TCP, and as a result, drops the connection request. The dropping of the connection request effectively denies access for the wireless device so that the user at the wireless device will not be able to obtain access of the data network until a timeout (usually on the order of 30 minutes to an hour) occurs in the stateful intermediate device to terminate the connection involving the wireless device. 
     As a result, users of wireless devices may experience unusually long periods of time during which they are unable to access the data network, even though the wireless devices have established wireless links. 
     SUMMARY 
     In general, methods and apparatus are provided to enable a wireless device that has lost its wireless link to re-establish a connection through an intermediate device. For example, a method for establishing a connection between a wireless device and a second device includes maintaining a state of the connection between the wireless device and the second device. The method further includes receiving an indication that a wireless link to the wireless device has been lost or may be lost. In response to receiving the indication that the wireless link to the wireless device has been lost or may be lost, the state of the connection is transitioned from a first state to a second state. 
     Other or alternative features will become apparent from the following description, from the drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a communications network that incorporates an embodiment of the invention. 
         FIG. 2  is a flow diagram of a process of establishing a connection in the communications network, in accordance with an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, numerous details are set forth to provide an understanding of the present invention. However, it will be understood by those skilled in the art that the present invention may be practiced without these details and that numerous variations or modifications from the described embodiments may be possible. 
       FIG. 1  illustrates an example arrangement of a communications network that includes a wireless client  100 , such as a packet-enabled mobile telephone, a personal digital assistant (PDA), a notebook computer, a tablet computer, a wireless network card, a wireless network peripheral, a wireless appliance, or other wireless devices. The communications network also includes a wireless edge device  102  that communicates with the wireless client  100  over a wireless link  104 . The wireless edge device  102  provides access to the rest of the communications network for the wireless client  100 , as well as for other wireless devices that are able to communicate with the wireless edge device  102 . The wireless edge device  102  is connected to a private network  106  (e.g., a local area network or wide area network), which is in turn connected to a DHCP (Dynamic Host Configuration Protocol) server  108  and a firewall system  110 . DHCP is described in RFC 2131, entitled “Dynamics Host Configuration Protocol,” dated March 1997. The DHCP server  108  contains configuration information regarding network devices connected to the private network  106 . The configuration information includes the Internet Protocol (IP) addresses that can be allocated to the network devices connected to the private network  106 . The DHCP server  108  is also capable of delivering other configuration parameters to network devices. 
     The firewall system  110  is an example of a stateful intermediate device that stores states for communications passing through the firewall system (e.g., between the private network  106  and a public network  112 , such as the Internet). A stateful intermediate device tracks the state of each connection between an endpoint on the private network  106  and an endpoint on the public network  112 . Examples of states of a connection include a state prior to establishment of a connection and a state after the connection has been established. The firewall system  110  also implements a security policy to prevent unauthorized access of network devices and other resources on the private network  106 . 
     The firewall system  110  is coupled to the public network  112  through a network-address-translation (NAT) router  114 . The NAT router  114  performs translations between network addresses (e.g., IP addresses) on the public network  112  and network addresses (e.g., IP addresses) on the private network  106 . Effectively, the NAT router  114  enables network devices connected to the private network  106  to use a set of internal network addresses that are hidden from view on the public network side. A benefit of using a NAT router  114  is that more network addresses are available on the private network  106 . For example, an enterprise that the private network is associated with may be assigned a limited set of public network addresses. The limited set of public network addresses can be mapped to a larger set of internal network addresses on the private network  106  so that a larger number of network devices can be used behind the NAT router  114 . 
     The NAT router  114  is also a stateful intermediate device that maintains a state of a connection between an endpoint coupled to the private network  106  and another endpoint coupled to the public network  112 . For example, the wireless client  100  (an endpoint on the private network  106 ) can establish a connection with a server  116  (an endpoint on the public network), which can be a web server that the wireless client  100  can access to retrieve information. The server  116  can also maintain a state of the connection between the wireless client  100  and the server  116  so that resources can be allocated to the connection between the server  116  and the wireless client  100  by the server  116 . The public network  112  can include other routers that also are stateful intermediate devices. 
     In accordance with some embodiments of the invention, the connection that can be established between the wireless client  100  and the server  116  is a Transmission Control Protocol (TCP) connection. TCP is described in RFC 793, entitled, “Transmission Control Protocol,” dated September 1981. TCP defines a transport layer in each of the network devices to enable such network devices to establish TCP connections over a data network. As used here, the term “data network” refers to one network or a collection of networks (such as the private network  106  and the public network  112  depicted in  FIG. 1 ). The term “data network” also refers to any intermediate devices, such as routers, gateways, and the like, that enable communication between network elements. Also, a “network path” refers to a path through the data network between network elements. A path includes one or more portions of a data network, including routers and other elements that are used for a communications session between network elements. Note that in a packet-switched network, such as an IP network, packets communicated between network elements can travel over multiple different routes. Such multiple routes are considered to be part of a network path between network elements. 
     In other embodiments, instead of establishing TCP connections, other types of connections (according to other transport protocols) can be established. States for such other types of connections are also maintained by stateful intermediate devices. As used here, a “connection” refers to any communications session set up between two or more endpoints. The connection can be established through intermediate network(s) and stateful intermediate devices such as the firewall system  110 , NAT router  114 , and server  116 . 
     An issue associated with a connection established with the wireless client  100  is that the wireless link  104  between the wireless client  100  and the wireless edge device  102  may be lost. To address this, the wireless edge device  102  according to some embodiments reports the lost wireless link to the firewall system  110 . The firewall system  110  transitions to a special state that indicates that the connection to the wireless client  100  is potentially terminated. While in this state, the firewall system is able to properly handle subsequent data or connection requests received from the wireless client  100  so that the wireless client  100  is not denied access to the public network  112 . 
     As further shown in  FIG. 1 , the wireless client  100  includes a wireless link layer  120  that enables the wireless client  100  to communicate over the wireless link  104  with a wireless link layer  122  in the wireless edge device  102 . The wireless link layer  120  and the wireless link layer  122  exchange link layer (or layer-1) signaling over the wireless link  104 . 
     The wireless client  100  also includes a TCP/IP stack  124  to enable communication of TCP/IP packets between the wireless client  100  and another endpoint. In some implementations, a simplified TCP/IP stack is used in the wireless client  100  due to the relatively limited resources (such as processing or storage resources) available in the wireless client  100 . Such a simplified TCP/IP stack has a reduced set of TCP ports available that can be employed by the network client  100  in TCP connections established over a data network. 
     The wireless client  100  also includes an application software module  126  that provides the various capabilities of the wireless client  100 . 
     The wireless edge device  102  includes a wireless link monitor module  128  that monitors the wireless link  104  between the wireless client  100  and the wireless edge device  102 . The wireless link monitor module  128  can detect for loss of the wireless link  104  (which can result from weak signaling or the wireless client  100  moving out of range). The wireless link monitor  128  sends reports of wireless link losses to the firewall system  110 . One technique for reporting lost wireless link connections is by use of Simple Network Management Protocol (SNMP) messages, such as an SNMP Trap message. SNMP is described in RFC 1067, entitled “A Simple Network Management Protocol,” dated August 1988. SNMP provides for internetwork management such that various management functions can be provided. In accordance with some embodiments of the invention, one management function that can be provided by use of SNMP messages is the reporting of lost wireless links between the wireless edge device  102  and wireless clients, such as the wireless client  100 . 
     Communication between the wireless edge device  102  and the firewall system  110  is provided through a link layer  130  (which can be an Ethernet layer, for example). 
     The firewall system  110  similarly includes a link layer  132  to communicate over the private network  106 . Above the link layer  132  is a TCP/IP stack  134 . The TCP/IP stack  134  maintains states of connections (TCP connections) between network elements coupled to the private network  106  and network elements coupled to the public network  112 . The states of the various connections are maintained in state table  136 , which can be stored in a storage  138  in the firewall system. The firewall system  110  also includes a firewall module  140  to provide firewall security tasks. 
     Each of the firewall system  110 , wireless edge device  102 , and wireless client  100  includes a processor  142 ,  148 , and  144 , respectively. Each processor  142 ,  148 , and  144  is coupled to a respective storage  138 ,  150 , and  146 . Software modules in each of the firewall system  110 , wireless edge device  102 , and wireless client  100  are executable on a respective processor. 
     The firewall system  110  maintains a state table  136  in the storage  138 . The state table  136  contains states of each connection that passes through the firewall system. State information  154  is also stored in a storage  152  of the NAT router, and state information  158  is stored in a storage  156  in the server  116 . As noted above, routers (not shown) in the public network  112  can also be stateful intermediate devices that store state information. 
       FIG. 2  is a message flow diagram of a process of communicating between the wireless client  100  and the server  116 , in accordance with some embodiments of the invention. The wireless client  100  first establishes a wireless link (at  202 ) with the wireless edge device  102 . The wireless client  100  then sends (at  204 ) a TCP SYN packet to the firewall system  110 . The SYN packet is an IP packet that contains a TCP segment in which the SYN control bit is set. The SYN packet is a connection request (for a connection between the wireless client  100  and the server  116 ) sent by the wireless client  100  to the firewall system  110 . The SYN packet contains a source port (the TCP port number for the wireless client  100 ) and a destination port (a TCP port number for the destination server  116 ). The SYN packet also contains source and destination IP addresses. 
     The firewall system  110  forwards (at  206 ) the SYN packet to the server  116 . To acknowledge the SYN packet, the server  116  responds with a SYN ACK packet (at  208 ), which is received by the firewall system  110 . The firewall system  110  allows the SYN ACK packet to pass through the firewall system  110 , with the SYN ACK packet forwarded (at  210 ) to the wireless client  100 . In response, the wireless client  100  sends (at  212 ) an acknowledgement, in the form of an ACK packet to the firewall system  110 , which forwards the ACK packet (at  214 ) to the server  116 . At this stage, the connection between the client  100  and server  116  has been established, and the firewall system  110  sets (at  216 ) the state of the TCP connection as being the ESTABLISHED state. This state information is kept in the state table  136  ( FIG. 1 ) of the firewall system  110 . The other states of the firewall system include: SYN-SENT, SYN-RECEIVED, FIN-WAIT-1, FIN-WAIT-2, CLOSE-WAIT, CLOSING, LAST-ACK, and TIME-WAIT. The SYN-SENT state indicates that an entity is waiting for a matching connection request after having sent a connection request. The SYN-RECEIVED state indicates that an entity is waiting for a confirming connection request acknowledgement after having both received and sent a connection request. The ESTABLISHED state indicates an open connection exists where data can be received and delivered. The FIN-WAIT-1, FIN-WAIT-2, CLOSE-WAIT, CLOSING, AND LAST-ACK states represent various states associated with terminating a connection. 
     The state table  136  ( FIG. 1 ) in the firewall system  110  maintains the states of multiple connections between different groups of endpoints. The endpoints are identified by source and destination port numbers. 
     While the wireless client  100  and server  116  are exchanging the SYN, SYN-ACK, and ACK packets, the firewall system  110  transitions the state of the connection between the client  100  and server  116  from SYN-SENT to SYN-RECEIVED to ESTABLISHED. After the connection is established, data can be exchanged (at  218 ) between the wireless client  100  and the server  116 . Note that the other stateful intermediate devices in the path from the client  100  to the server  116  also perform similar transitions among the various TCP states. 
     At some point, the wireless link between the wireless client  100  and the wireless edge device  102  may be lost (at  220 ), such as due to weak signal or the wireless client  100  moving out of range. Once the wireless link  104  monitor module  128  in the wireless edge device  102  detects the lost wireless link with the wireless client  100 , the wireless link monitor module  128  sends (at  222 ) a report indicating a lost link to the firewall system  110 . In response to this report, the firewall system  110  transitions (at  224 ) the state of the TCP connection from the ESTABLISHED state to a “POTENTIALLY TERMINATED” state. The POTENTIALLY TERMINATED state refers to a state in which the firewall system  110  indicates that the connection between the wireless client  100  and the server  116  may be terminated, although the firewall system  110  at this stage is not certain. This allows the firewall system  110  to wait for subsequent communications from the wireless client  100  (if any) to determine what further actions are to be taken. 
     A wireless link can be re-established (at  226 ) between the wireless client  100  and the wireless edge device  102  at some later point in time. When this occurs, two scenarios may be presented. A first scenario (scenario 1) involves the wireless client  100  sending data (without issuing a new connection request). This data is sent (at  228 ) by the wireless client  100  to the firewall system  110 . When the firewall system  110  receives this data from the wireless client  100 , the firewall system  110  transitions (at  230 ) the state of the TCP connection from the POTENTIALLY TERMINATED state to the ESTABLISHED state, if the received data is valid data. Valid data includes data packets having sequence numbers within an expected range. If the received data packets are invalid (the sequence numbers of the received packets do not match expected values), then the received data packets are discarded and the firewall system maintains the state of the connection in the POTENTIALLY TERMINATED state. 
     Valid data is then forwarded (at  232 ) from the firewall system  110  to the server  116 , and further communication can occur between the wireless client  100  and the server  116 . In scenario 1, the firewall system  100  is able to transition to the ESTABLISHED state in response to further valid data being sent by the wireless client  100 . No additional messaging is needed in this scenario. The connection is thus treated as if the connection was never lost. 
     In a second scenario (scenario 2), the wireless client  100  sends a new connection request in response to re-establishing the wireless link (at  226 ). This new connection request is in the form of a SYN packet that is sent (at  234 ) to the firewall system  110 . 
     The new connection request indicated by the SYN packet is likely to contain the same source port number as the previously used source port number (for establishing the connection at  218 ). The reusing of the same source port number is likely because the TCP/IP stack  124  ( FIG. 1 ) is a simplified stack (for more efficient use of limited resources of the wireless client  100 ). As a result, the range of available source port numbers is relatively limited. Furthermore, the DHCP server  108  may also assign the same IP address to another wireless client. This other wireless client that is assigned the same IP address may also send the new connection request containing the same port number. 
     Conventionally, if the firewall system  110  receives a SYN packet containing a source port/destination port combination that is the same as that for a connection indicated as being ESTABLISHED, such a SYN packet is dropped as not being allowed. In accordance with some embodiments of the invention, rather than drop this new connection request, the firewall system  110  is able to detect that the connection request comes from a wireless client  100  associated with a connection state that is in the POTENTIALLY TERMINATED state. In this case, the firewall system  110  clears the old connection (since the old connection is no longer valid) and establishes a new connection. 
     However, note that downstream network elements (such as the NAT router  114  and the server  116 ) may also contain state information pertaining to the TCP connection between the network client  100  and the server  116 . Before the firewall system  110  can establish a new connection, the firewall system first clears the states in the NAT router  114  and the server  116  (and any other stateful intermediate devices in the network path). This is accomplished by the firewall system  110  sending (at  236 ) an RST packet (which is a reset message) over the path to the server  116 . The RST packet causes the state of the TCP connection to be reset. Following reset, the firewall system  110  forwards (at  238 ) the SYN packet to the server  116 . The TCP state is also changed (at  240 ) from the POTENTIALLY TERMINATED state to the SYN-SENT state. 
     The acts following SYN (at  238 ) performed by the wireless client  100 , firewall system  110 , and server  116  are the same as acts  206 - 218  for establishing a connection. 
     In an alternative embodiment, the wireless edge device  102  and the firewall system  110  may not be configured to allow the wireless edge device  102  to report lost wireless links to the firewall system  110 . In such an alternative embodiment, to detect for a lost wireless link, a SYN-reuse timeout period is set. The SYN-reuse timeout period is smaller than the 30-minute to 1-hour timeout used by a typical stateful intermediate device to drop a TCP connection. After the firewall system  110  detects that a particular TCP connection has been idle for a period that exceeds the SYN-reuse timeout period, the firewall system  110  sets the TCP state of the connection to the POTENTIALLY TERMINATED state if certain other criteria are satisfied. Such other criteria include a predefined IP address range associated with certain endpoints, such as wireless clients that are likely to lose wireless links. Also, the other criteria include IP address identifiers of an ingress/egress interface of the stateful intermediate device for traffic from and to the wireless clients. The ingress/egress interface is the interface used by the wireless clients to establish connections with endpoints on the public network. A further criterion that can be defined is the TCP protocol that is used. Thus, a connection that involves an endpoint in the configured IP address range and/or using the predefined ingress/egress interface and using the predefined TCP protocol will be marked POTENTIALLY TERMINATED after being idle for a period exceeding the SYN-reuse timeout period. The procedure following transitioning of the connection to the POTENTIALLY TERMINATED state includes the same acts  226 - 240  depicted in  FIG. 2 . 
     Instructions of the various software modules discussed herein are loaded for execution on corresponding control units or processors, such as a processor  142 ,  144 , and  148  ( FIG. 1 ). Processors include microprocessors, microcontrollers, processor modules or subsystems (including one or more microprocessors or microcontrollers), or other control or computing devices. As used here, a “controller” refers to hardware, software, or a combination thereof. A “controller” can refer to a single component or to plural components (whether software or hardware). 
     Data and instructions (of the various software modules) are stored in one or more machine-readable storage media, such as storage  138 ,  146 , or  150  ( FIG. 1 ). The storage media include different forms of memory including semiconductor memory devices such as dynamic or static random access memories (DRAMs or SRAMs), erasable and programmable read-only memories (EPROMs), electrically erasable and programmable read-only memories (EEPROMs) and flash memories; magnetic disks such as fixed, floppy and removable disks; other magnetic media including tape; and optical media such as compact disks (CDs) or digital video disks (DVDs). 
     The instructions of the software routines or modules are loaded or transported to a system or device in one of many different ways. For example, code segments including instructions stored on floppy disks, CD or DVD media, a hard disk, or transported through a network interface card, modem, or other interface device are loaded into the system and executed as corresponding software routines or modules. In the loading or transport process, data signals that are embodied in carrier waves (transmitted over telephone lines, network lines, wireless links, cables, and the like) communicate the code segments, including instructions, to the system. Such carrier waves are in the form of electrical, optical, acoustical, electromagnetic, or other types of signals. 
     While the invention has been disclosed with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover such modifications and variations as fall within the true spirit and scope of the invention.