Abstract:
In some illustrative embodiments, an IP-layer based network selection and multihoming method is provided that enables a flexible and secure dynamic selection of one or more serving networks for use by a client node. The method is independent of any link-layer technology. A serving network can be an ISP network, a NAP network exchange facility, a VLAN, or the like. Network information is advertised to a client node, the client node is authenticated and authorized for use of an access router, and a secure tunnel is established between the client node and the access router. The method can be implemented by using standard protocols, and can work over any existing or future link-layer technologies that are able to carry IP datagrams, without any modification.

Description:
BACKGROUND 
     1. Field of the Invention 
     The present invention relates generally to network communications and preferred embodiments relate more particularly to communication network service provider selection at a single client location from among a number of different available providers. In accordance with some preferred embodiments, the invention relates to Internet Service Provider (ISP) selection and multihoming by a user at a client node on an access network. 
     2. Background Discussion 
     Multihoming is the technique of connecting to the Internet via two or more ISPs, either simultaneously or dynamically. Multihoming has a number of advantages, including providing an essential back-up connection to the public Internet if one ISP fails, improved regional and local connectivity, increased bandwidth, and availability of load-sharing which can improve performance. Currently, there are many situations where multiple ISPs are available at a single user location. For example, home users can choose one ISP via a dial-up connection and another ISP via a cable or DSL (Digital Subscriber Line) modem connection. 
     DSL providers that use PPPoE (Point-to-Point Protocol over Ethernet) for IP encapsulation can allow subscribers to choose one of a number of connected ISPs, either statically during the initial sign-up, or dynamically by using NAIs (Network Access Identifiers) provided by the subscribers during the PPP authentication phase or by carrying ISP information in the PPPoE discovery stage. 
     In IEEE 802 LANs (Local Area Networks), a VLAN (Virtual LAN) is used to partition a LAN into multiple smaller LANs. A VLAN is a network of computers that behave as if they are connected to the same wire even though they actually may be physically located on different segments of a LAN. VLANs can be configured through software rather than hardware, which makes them extremely flexible. When a client node is connected to a VLAN through a wired Ethernet connection, the mapping between the Ethernet port of the client node and the VLAN is statically configured in most cases. In public wireless LAN environments, the IEEE 802.11 SSID (Service Set IDentifier) advertised by access points can contain service provider information. SSID also is used for dynamically selecting a VLAN by creating a static mapping between SSID and VLAN, so that stations that are associated with an access point by specifying a particular SSID are connected to a particular VLAN mapped to that SSID. 
     The current methods for selecting an ISP or a VLAN are closely tied to particular link-layer technologies (i.e., PPP and IEEE 802.11) and therefore are difficult to apply across all link-layer technologies. As such, in an environment where access networks are heterogeneous or more flexibility in VLAN assignment to client nodes is needed, it would be desirable to have an IP (Internet Protocol) layer solution that is independent of any link-layer technology. 
     As a simple IP-layer solution, it is possible to place multiple access routers in an access network where each access router is connected to a particular ISP or a VLAN, such that a client node on the access network can select a particular access router to send and receive data packets. However, the simple solution has two problems. First, information leakage could occur in the access network among multiple ISPs or VLANs, especially when the access network uses multi-access technologies. Second, if ingress filtering is performed at the access routers, the simple solution is difficult to implement where a client node with a single physical interface is allowed to connect to two or more ISPs or VLANs simultaneously. Ingress filtering is a technique for preventing attackers from injecting packets with a forged source IP address as if they were generated in a different network than the access network to which the access router attaches. In an access network where ingress filtering is employed, a packet generated in the access network can pass through an access router only when it has a source address with a network prefix that is assigned by the router to the network interface where the packet was received. However, most host implementations do not provide any method to choose an appropriate source address when multiple routable IP addresses with different network prefixes are assigned to a given interface, as would be the case where simultaneous connection to different ISPs or VLANs is enabled. 
     Consequently, there exists a need in the art for, among other things, a solution that prevents any information leakage to occur and also that protects against IP address spoofing attacks. 
     SUMMARY OF THE INVENTION 
     The preferred embodiments of the present invention can significantly improve upon existing methods and/or apparatuses. In some embodiments, the present invention provides substantial improvements over the above-mentioned methods. 
     According to one aspect of the invention, a new IP-layer based model for network selection and multihoming is provided that enables a flexible and secure dynamic selection of one or more serving networks to use, where a serving network is an ISP network, a NAP (Network Access Point) network exchange facility, a VLAN, etc. The IP-layer based model according to one preferred embodiment consists of three phases. Network information is advertised to a client node in the first phase, the client node is authenticated and authorized for use of an access router in the second phase, and a secure tunnel is established between the client node and the access router in the third phase. The inventive model can be implemented by using standard protocols, and can work over any existing or future link-layer technologies that are able to carry IP datagrams, without any modification. 
     In particular, according to one preferred embodiment, the present invention provides a method of dynamically connecting a client node to a serving network, including the steps of providing an access network to which a client node has a network connection; providing at least one access router having a network connection to the access network and having a network connection to at least one serving network; sending serving network provider advertising information to the client node in response to a request message from the client node; receiving from the client node serving network provider information specifying a serving network to which the client node desires access; and establishing a secure communication tunnel between the client node and the access router through the access network, such that the client node is able to send and receive data packets to and from the serving network specified by the client node within the secure communication tunnel through the access network. 
     According to a second aspect, the invention provides a method of connecting a client node to multiple Internet service providers, including the steps of providing an access network through which the client node may communicate with the multiple Internet service providers; and establishing a separate secure communication tunnel within the access network for each of the multiple Internet service providers, such that the client node is able to send and receive data packets to and from each of the Internet service providers within the separate secure communication tunnels through the access network. 
     According to a third aspect, the invention provides a method of connecting a client node to a serving network, including the steps of providing an access router having a network connection to at least two serving networks; receiving from the client node serving network information specifying a serving network to which the client node desires to have access; establishing a secure communication tunnel between the client node and the access router through the access network, such that the client node is able to send and receive data packets to and from the serving network specified by the client node within the secure communication tunnel through the access network; and binding the secure communication tunnel to the specified serving network by using serving network information of the specified serving network as a security association identifier of the secure communication tunnel. 
     The above and/or other aspects, features and/or advantages of various embodiments will be further appreciated in view of the following description in conjunction with the accompanying figures. Various embodiments can include and/or exclude different aspects, features and/or advantages where applicable. In addition, various embodiments can combine one or more aspect or feature of other embodiments where applicable. The descriptions of aspects, features and/or advantages of particular embodiments should not be construed as limiting other embodiments or the claims. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The preferred embodiments of the present invention are shown by a way of example, and not limitation, in the accompanying figures, in which: 
         FIG. 1  is a diagram of a physical network topology according to one preferred embodiment of the invention; 
         FIG. 2  is a diagram of a logical network topology according to one preferred embodiment of the invention overlaying the topology of  FIG. 1 ; 
         FIG. 3  is a diagram of a network topology according to one preferred embodiment of the invention using ISPs and NAPs; 
         FIGS. 4 and 5  are diagrams of a network topology according to one preferred embodiment of the invention using access VLANS and serving VLANS; 
         FIGS. 6 and 7  are diagrams of a network topology according to one preferred embodiment of the invention using virtual access points; and 
         FIG. 8  is a diagram of a network topology according to one preferred embodiment of the invention using a remote network. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     While the present invention may be embodied in many different forms, a number of illustrative embodiments are described herein with the understanding that the present disclosure is to be considered as providing examples of the principles of the invention and that such examples are not intended to limit the invention to preferred embodiments described herein and/or illustrated herein. 
     An example physical topology of the proposed IP-layer model according to a preferred embodiment of the invention is illustrated in  FIG. 1 . As shown, an IP access network  101  includes access routers AR 1  and AR 2  and a client node  103 . Additional nodes on the access network are not shown for purposes of simplification. Access router AR 1  is connected to serving network N 1 , and access router AR 2  is connected to serving networks N 2  and N 3 . In the IP access network  101 , client node  103  can communicate with the access routers as well as other nodes (not shown), by using a routable or non-routable IP address that is valid for communication within the access network. When the client node  103  needs to send or receive data packets through a serving network, it establishes a secure tunnel (logical interface) using IP Security protocol (IPSec tunnel) to the access router of that serving network through the IP access network. Tunneling allows one network to send its data through another network&#39;s connections, and works by encapsulating a network protocol within packets carried by the second network. For example, PPTP (Point-to-Point Tunneling Protocol) technology enables organizations to use the Internet to transmit data across a VPN (Virtual Private Network). It does this by embedding its own network protocol within the TCP/IP packets carried by the Internet. 
     The established IPSec tunnel is a secure logical interface that provides confidentiality, integrity and replay protection for packets passing through the access router, which also prevents the packets from being leaked to other serving networks. Additionally, the IPSec tunnel establishes a logical tunnel interface overlaying the physical interface of the node. This guarantees that a particular interface address (i.e., the address assigned to a particular logical tunnel interface) is used as the source address of packets forwarded to the corresponding access router. Access routers that employ ingress filtering will never drop packets having such a source address, since it will contain the network prefix assigned to the logical interface by the access router. 
     An example logical topology that overlays the physical topology of  FIG. 1  is illustrated in  FIG. 2 . As shown, the client node  103  has three IPSec tunnels  201 ,  202  and  203 , each associated with a distinct logical interface L 1 , L 2  or L 3 . The logical interfaces L 1 , L 2  and L 3  are used for sending and receiving data packets through particular serving networks N 1 , N 2  and N 3 , respectively. The serving networks N 1 , N 2  and N 3  can be ISPs, NAPs, VLANS, or similar serving networks. The IPSec tunnel  201  corresponding to logical interface L 1  is terminated at access router AR 1 . The IPSec tunnels  202  and  203  corresponding to logical interfaces L 2  and L 3  are terminated at access router AR 2 . 
     The interface address of each logical interface L 1 , L 2  and L 3  of the client node  103  is assigned from the address block of the corresponding serving network. In the example as shown, the client node  103  can send and receive data packets through any of the three serving networks N 1 , and N 2  and N 3 . Of course, it is also possible for the client node to establish connectivity to only one or two of the serving networks instead of all three serving networks. A client node that has on-link connectivity to the access network can be a workstation or a router. 
     The client node  103  can use an IP address that is a routable address or a non-routable address for communicating with other nodes in the access network  101 . A non-routable address is not allowed to be forwarded by a router while a routable address can be forwarded by a router. An example of non-routable address is an IPv4 link-local address or an IPv6 link-local address, for which the client node  103  can autonomously generate the address. Especially when an IPv6 link-local address is used, SEND (SEcure Neighbor Discovery) can be used for protecting IPv6 Neighbor Discovery exchanges. When a routable address is used, it can be either statically configured or dynamically configured, using any method including DHCP (Dynamic Host Configuration Protocol), DHCPv6 or IPv6 address auto-configuration. 
     It is not necessary to use the IPSec key management protocol to establish a tunnel if security is not crucial for the serving network providing the service, such as for a public service access network. In such case, other IP tunneling schemes may be used, such as for example IP-in-IP or GRE (Generic Routing Encapsulation). 
     Serving Network Information Advertising 
     Many wireless LAN hotspot service providers currently use 802.11 SSID that is included in a broadcast beacon frame to advertise service provider information to wireless clients. A technique called virtual access point (VAP) can extend this usage so that a single physical access point can be divided into multiple virtual access points, each of which acts as if it were a distinct physical access point, by advertising a distinct SSID for each VLAN. A disadvantage of VAP is that it is closely tied with a particular access technology and is difficult to apply to other access technologies. Another disadvantage is that more bandwidth is occupied by beacon frames and thus the total data traffic throughput will decrease. For example, if there are 10 virtual access points and each virtual access point generates a beacon frame every 100 msec, a station will receive a beacon frame every 10 msec, in which case more than 30% of the link bandwidth of IEEE 802.11b will be occupied by beacon frames. 
     According to the present invention in contrast, network layer protocols are used for advertising serving network information to the client nodes on the access network  101 . When the routable networks are ISP or NAP networks, a provide identifier and provider name data pair may be advertised per each service provider, where the provider identifier is a unique identifier that is used to identify the provider and the provider name is a character string that represents the name of the provider. When the serving networks are VLANs, a VLAN identifier and VLAN name may be advertised per VLAN, where the VLAN identifier is a unique identifier that is used to identify the VLAN and the VLAN name is a character string that represents the name of the VLAN. The VLAN advertising information may be sent over IP when the access network is not also a VLAN. 
     According to one preferred embodiment of the invention, information concerning the serving networks is advertised by using PANA (Protocol for carrying Authentication information for Network Access). In some scenarios, an IP-based device is required to authenticate itself to the network prior to being authorized to use it. This authentication usually requires a protocol that can support various authentication methods. In the absence of such an authentication protocol on most of the link-layers, architectures have resorted to using a number of inadequate authentication methods. PANA defines a protocol that allows clients to authenticate themselves to the access network using IP protocols that allow a client to interact with a site&#39;s back-end AAA (Authentication, Authorization, and Accounting) infrastructure to gain access without needing to understand the particular AAA infrastructure protocols that are in use at the site. It also allows such interactions to take place without a link-layer specific mechanism. PANA is applicable to both multi-access and point-to-point links. The present invention makes use of the PANA protocols to provide serving network information to client nodes on the access network. 
     PANA is a client-server type protocol where the client and server are referred to as a PaC (PANA Client) and a PAA (PANA Authentication Agent), respectively. In the invention, client node  103  is a PaC. A PAA is placed in the access network and may or may not be co-located with an access router. When a PAA is not co-located with an access router, it uses another protocol such as SNMP (Simple Network Management Protocol) or Diameter to send authorization information on authorized clients to some or all of the access routers connected to the serving network(s) to be advertised by the PAA. 
     The advertising sequence can be performed as follows:
         1. A PaC sends a PANA-PAA-Discover message that may be multicast within the access network or unicast to a particular PAA.   2. Each PAA that received the PANA-PAA-Discover message sends a PANA-Start-Request message back to the PaC. The PANA-Start-Request message contains the information on the serving network(s) associated with the PAA.   3. The PaC that receives the PANA-Start-Request message(s) extracts the serving network information from the received message.       

     It is noted that a PaC may not need to configure an IP address when it uses an unspecified IP address for receiving serving network information using PANA. In such case, the PAA will send information encapsulated in an IP packet to the PaC by using a Layer 2-specific packet delivery mechanism and bypassing the regular IP stack implementation 
     According to an alternate embodiment of the invention, serving network information may be advertised to clients by using the Router Discovery mechanism of IPv4 or IPv6. A client node needs to configure an IP address to obtain serving network information using Router Discovery. The advertising sequence can be performed as follows:
         1. A client node sends a Router Solicitation message that may be multicast within the access network or unicast to a particular router.   2. Each router that receives the Router Solicitation message sends a Router Advertisement message back to the client node. The Router Advertisement message contains information on the serving network(s) connected to the router.   3. The client node that receives the Router Advertisement message(s) extracts the serving network information from the received message.       

     Authentication 
     When PANA is used for advertising the information on the serving networks, it also can be used for its original purpose, i.e., authenticating and authorizing the clients. IKE (Internet Key Exchange) can also be used for authenticating the clients. When IKE is used for client authentication, the client node can immediately establish a secure tunnel. IKE is a key management protocol standard that is used in conjunction with the IPSec standard. IPSec is an IP security feature that provides robust authentication and encryption of IP packets. 
     On the other hand, when a PAA is not co-located with an access router, PANA is always used for authenticating the client. When PANA is used for client authentication, the authentication procedure continues from Step 3 in the previous section:
         4. The PaC sends a PANA-Start-Answer message to a PAA in response to a PANA-Start-Request message. The PaC may specify one or more serving network it wishes to access, by inserting the information on the desired serving network(s) in the PANA-Start-Answer message.   5. The PAA then sends a PANA-Auth-Request message, carrying an EAP (Extensible Authentication Protocol) message and a PANA session identifier. The PANA-Auth-Request message may contain the information on the serving network that is associated with the ongoing authentication. EAP is a general protocol for authentication that also supports multiple authentication methods, such as token cards, one-time passwords, certificates, public key authentication and smart cards. The object of authentication is to confirm the identity of the client or user.   6. The PaC returns a PANA-Auth-Answer message in response to the PANA-Auth-Request message, carrying an EAP message.   7. Steps 5 and 6 are repeated as necessary until the EAP authentication process completes.   8. When the EAP authentication process completes, the PAA sends a PANA-Bind-Request message to the PaC, containing an EAP Success/Failure message. If the EAP authentication completes successfully, a list of IP addresses of the access routers associated with the PAA is additionally contained in the message. If the PAA is not co-located with an access router, a list of access router names associated with the PAA and connected to the serving network(s) is additionally contained in the message. The PANA-Bind-Request may contain the information on the serving network the PAA authorizes access to the PaC. If the client authentication fails, the client node will be denied access to any serving network.   9. The PaC returns a PANA-Bind-Answer message to the PAA.   10. When EAP authentication is needed for more than one serving network, Steps 5 to 9 are repeated for each serving network.       

     In the above sequence, it is assumed that at least one EAP authentication method that is capable of deriving an EAP Master Session Key (MSK) is used. The derived MSK is shared between the PaC and the PAA. Upon successful completion of an EAP authentication process with a derived MSK, the PAA sends at least the following information to each access router associated with the PAA and connected to the serving network(s) that are specified by the PaC in Step 4, so as to authorize the PaC to have access to the serving network:
         PANA session identifier.   IKE pre-shared secret data which is derived from the MSK.       

     Establishing S Cure Tunnel to Access Router 
     Upon successful completion of reception of serving network advertising information and client node authentication (in the case of using PANA for advertising and authentication) or of receiving serving network advertising information (in the case of using Router Discovery), the client knows which access router associated with the PAA is connected to which serving network(s). The client then can perform IKE with any access router to establish an IPSec tunnel. 
     IKE is a hybrid protocol which implements the Oakley key exchange and Skeme key exchange inside the Internet Security Association and Key Management Protocol (ISAKMP) framework. (ISAKMP, Oakley, and Skeme are security protocols implemented by IKE.) IPSec is a framework of open standards that provides data confidentiality, data integrity, and data authentication between participating peers. IPSec provides these security services at the IP layer; it uses IKE to handle negotiation of protocols and algorithms based on local policy, and to generate the encryption and authentication keys to be used by IPSec. IPSec can be used to protect one or more data flows between a pair of hosts, between a pair of security gateways, or between a security gateway and a host. 
     If authentication was performed before entering the secure tunnel establishment phase, the IKE pre-shared key that was derived in the authentication procedure is used for IKE to authenticate the IKE end-points (thus no other client authentication is performed within IKE negotiation). The client can use the PANA session identifier or a valid IP address in the access network as the ISAKMP Security Association (ISAMKP SA) identifier in IKEv1 or the IKE_SA identifier in IKEv2. When an IP address is used as the ISAKMP SA identifier in IKEv1, the IKEv1 Main Mode needs to be used. 
     Otherwise, if client authentication was not performed before entering the secure tunnel establishment phase, an authentication procedure other than using an IKE pre-shared key must be performed within the IKE negotiation. In this case, an identifier that is specific to the authentication procedure is used. 
     When an access router is connected to multiple serving networks (such as access router AR 2  in  FIG. 1 ), a mechanism for binding an IPSec tunnel to a specific serving network is needed so that the access router can (1) assign an IPSec tunnel inner address from the address block of the serving network and (2) forward packets between the client and the serving network. The binding can be created in the IKE negotiation by using the information on the serving network as the IPSec SA identifier credential. In this way, it is possible to establish multiple IPSec tunnels between a client node and an access router, each bound to a distinct serving network, as shown in  FIG. 2 . Multiple schemes can be employed, such as single PANA authentication and session identifier for an entire access network, single PANA authentication and identifier for each access router, or single PANA authentication and identifier for each serving network. Where there is authentication for each serving network, the PANA session identifier may be used as an identifier for IKE, but in other cases the client has to use/generate a unique identifier for IKE for a specific serving network or other information/identifier exchange/negotiation may be required during IKE 
     When IKEv2 is used for establishing an IPSec tunnel, it is also possible to establish multiple IPSec tunnels between a client node and an access router, each bound to a serving network. When an access router is connected to only one serving network, there is only one binding and other identifiers may be used. 
     The inner address of an IPSec tunnel SA may be assigned during IKE negotiation by the access router that terminates the tunnel. For example, IKEv2 defines a Configuration Payload exchange to assign an IPSec tunnel inner address. When an inner address is not assigned in the IKE negotiation, DHCP may be performed through the established IPSec tunnel. In any case, the assigned IPSec inner address must be valid for the serving network bound to the IPSec SA. Other configuration information such as a subnet prefix (or a netmask), a DNS (Domain Name System) server address, or a DHCP server address also may be assigned in the IKE negotiation. In addition, when the client node is an IPv6 router, an IPv6 prefix delegated from the serving network also can be assigned by running DHCPv6 with prefix delegation option through the established IPSec tunnel. In this case, the delegated prefix can be shared among other client nodes for which the client router serves as the client-side gateway to the serving network. 
     An access router can perform Quality of Service (QoS) control on the IPSec tunnels it terminates, to provide differentiated services among IPSec tunnels from different client nodes and/or among IPSec tunnels from the same client nodes. The advertising information on the serving network can also contain the QoS information so that a client node can specify QoS information during IPSec SA negotiation in IKE. 
     The present invention also allows multiple access routers on the same access network to connect to the same serving network. Thus, load balancing among access routers is possible. When PANA is used in serving network advertising and authentication, a list of access routers contained in a PANA-Bind-Request message during the authentication phase can be used for identifying which access router is connected to which serving network. 
     Broadcast and/or multicast traffic also may be transmitted through an IPSec tunnel. An access router may have a configuration option for allowing and prohibiting transmission of broadcast/multicast traffic through an IPSec tunnel. 
     A client node that has an IPSec tunnel to an access router may not use the IPSec tunnel to send or receive packets to other nodes in the access network. Such packets include application traffic such as printing data to a local printer in the access network. 
     Usage Scenarios 
     When the present invention is used in DSL or Wireless LAN hotspots, a serving network can be an ISP network or a NAP exchange network. The access network is typically owned by a single NAP, but it is also possible for multiple NAPs to share the same access network. An example physical topology when a single NAP  301  owns the access network  101  is shown in  FIG. 3 . 
     In this example, the client node  103  is able to selectively establish connectivity to one, some or all of the serving networks owned by ISP 1 , ISP 2  ISP 3  or NAP  301 . When PANA is used to authenticate the client node, it is possible to perform two EAP authentications in a single PANA session, one for the ISP and the other for the NAP, possibly with using different client identifiers. When the client node  103  creates multiple IPSec tunnels to different ISPs, multihoming is achieved. For each provider (either ISP or NAP), the identifier and name of the provider may be used as the serving network information. 
     Example physical topologies for VLAN usage are shown in  FIGS. 4-8 . VLAN topologies are used mainly for enterprise network configurations. In the example shown in  FIG. 4 , there are four VLANs configured in the network. The Access VLAN  401  is used as the access network for the client node  103 . The Serving VLANs  402 - 404  are VLANs that are used as the serving networks. Connectivity to the Serving VLANs is made only through IPSec tunnels established between the client node  103  and access routers AR 1  and AR 2  (the access routers may be virtual routers where the VLANS are configured in the same physical network). For each Serving VLAN, the identifier and name of the VLAN is used as the serving network information. 
     It is also possible for the access network to be composed of multiple Access VLANs  501  and  502  as shown in  FIG. 5 . This configuration is useful for partitioning traffic in the access network (as a legacy VLAN network does) such that client node  103  may establish connections to the Serving VLANs  402 - 404  through Access VLAN  501 , and client node  104  may establish connections to the Serving VLANs  402 - 404  through Access VLAN  502 , while still allowing the client nodes to create dynamic binding to Serving VLANs. 
     It will now be explained with reference to  FIGS. 6 and 7  how the present invention can be used with a layer-2 Dynamic VLAN model based on a Virtual Access Point (VAP). A Virtual Access Point is a logical entity that exists within a physical Access Point (AP). When a single physical AP supports multiple Virtual APs, each Virtual AP appears to client stations to be an independent physical AP, even though only a single physical AP is present. For example, multiple Virtual APs might exist within a single physical AP, each advertising a distinct SSID and capability set. It is assumed that IEEE 802.11 SSID is used as the information to identify a VLAN. There are two alternate configurations for the present invention as applied to VAPs, as explained below. The two configurations also can be combined. 
     If an AP supports both a secure access mechanism as strong as IPSec (or IEEE 802.11i) and dynamic VLAN functionality (i.e. the ability to handle multiple VLANs), it is possible to directly (virtually) connect the AP to the Serving VLANs where a distinct SSID is associated with each Serving VLAN as shown in  FIG. 6 . Client node  103  uses the present invention through a wired Ethernet connection  601  to connect to the Serving VLANs  1 - 3 , while wireless client node  104  (which supports IEEE 802.11i) can connect directly to a Serving VLAN through a wireless connection  602  to Virtual AP  3 . However, the wireless client will not be able to connect to multiple Serving VLANs at the same time unless it has multiple wireless LAN cards (or supports some sort of “virtual station” interface on a single physical layer). 
     If an AP supports dynamic VLAN functionality but does not support a secure access mechanism as strong as IPsec, the network administrator will not allow the AP to be directly connected to a Serving VLAN, but it may be connected to the Access VLANs as shown in  FIG. 7 . In this case, a client node  103  ( 104 ) first connects to an Access VLAN  501  ( 502 ) through a Virtual AP  701  ( 702 ), and then uses the present invention to establish connectivity to the Serving VLANs  402 - 404 . 
     As shown in  FIG. 8 , client node  103  may want to connect to the Serving VLANs  402 - 404  from a remote network site  801 . The present invention can support such a situation provided that the client node  103  knows the IP address of the PAA or the access routers AR 1  or AR 2  so as to receive service provider advertising information. This is because neither the access routers nor the PAA can broadcast provider information to the remote network. 
     When the client node is connected to the internal network from an external network through a firewall in the DMZ (DeMilitarized Zone, which is a computer or small subnetwork that sits between a trusted internal network, such as a corporate private LAN, and an untrusted external network, such as the public Internet), the following scenarios are presented:
         Where internal network management policy mandates that all access from the external network is to be protected with IPSec through an IPSec gateway in the DMZ (such as where an access router of a Serving VLAN is placed in the internal network)—In this case, packets transmitted through an IPSec tunnel established between the client node and an access router of a Serving VLAN will be protected with another IPSec tunnel established between the client node and the IPSec VPN gateway (double IPsec).   Where internal network management policy mandates that all access from the external network is to be protected with IPsec, but the IPsec gateway does not have to be in the DMZ—In this case, an additional IPsec tunnel is not needed.       

     The present invention also can be used with Mobile IP (such as MIPv4) in the following ways. First, since the present invention allows a client node to dynamically switch from one serving network to another, a stable IP address is needed for an application that needs a persistent connectivity to its corresponding node when switching occurs (it will be noted that the switching can occur on a mobile client node that does not physically move). By using Mobile IP, a home address can be used as such a stable IP address. 
     Second, a mobile client node that is connected to a serving network may physically move from an area covered by one access network to an area covered by another access network, where the access network may be an access network of a serving network or an access network in a remote network. By using Mobile IP, the client node can seamlessly move among different access networks without losing application connection. 
     In both cases, the IP header that contains the home address in a packet appears inside the IPSec tunnel header. In the case where the client node is connected from an external site to an internal serving network through a DMZ, an additional Mobile IP may be used to support external mobility. 
     In VLAN scenarios, if a mobile client node establishes connectivity to multiple Serving VLANs and each Serving VLAN uses its own home agent, the client node may run dual Mobile IP in parallel using multiple home addresses. The above discussion applies also to the case where Mobile IPv6 is used instead of Mobile IP. 
     Broad Scope of the Invention 
     While illustrative embodiments of the invention have been described herein, the present invention is not limited to the various preferred embodiments described herein, but includes any and all embodiments having modifications, omissions, combinations (e.g., of aspects across various embodiments), adaptations and/or alterations as would be appreciated by those in the art based on the present disclosure. The limitations in the claims are to be interpreted broadly based on the language employed in the claims and not limited to examples described in the present specification or during the prosecution of the application, which examples are to be construed as non-exclusive. For example, in the present disclosure, the term “preferably” is non-exclusive and means “preferably, but not limited to.” Means-plus-function or step-plus-function limitations will only be employed where for a specific claim limitation all of the following conditions are present in that limitation: a) “means for” or “step for” (i.e., not step on is expressly recited; b) a corresponding function is expressly recited; and c) structure, material or acts that support that structure are not recited. In this disclosure and during the prosecution of this application, the terminology “present invention” or “invention” may be used as a reference to one or more aspect within the present disclosure. The language present invention or invention should not be improperly interpreted as an identification of criticality, should not be improperly interpreted as applying across all aspects or embodiments (i.e., it should be understood that the present invention has a number of aspects and embodiments), and should not be improperly interpreted as limiting the scope of the application or claims. In this disclosure and during the prosecution of this application, the terminology “embodiment” can be used to describe any aspect, feature, process or step, any combination thereof, and/or any portion thereof, etc. In some examples, various embodiments may include overlapping features.