Abstract:
Aspects for seamless mobility of mobile terminals in a wireless network are described. The aspects include utilizing a reassociation request from a mobile terminal to identify need for an internetwork handover of the mobile terminal roaming in a wireless local area network (WLAN), and performing a protocol sequence in an access point (AP) for the mobile terminal to handle the internetwork handover to ensure connectivity of the mobile terminal while roaming.

Description:
FIELD OF THE INVENTION 
   The present invention relates to roaming by mobile terminals in wireless networks, and more particularly, to support of L 3  handovers for seamless mobility during the roaming in the wireless networks. 
   BACKGROUND OF THE INVENTION 
   Wireless communication has seen tremendous growth in recent years and is becoming widely applied to personal and business computing. Wireless access is broadening network reach by providing convenient and inexpensive access in hard-to-wire locations. Of major benefit is the increased mobility wireless local area networks (WLANs) allow. Wireless LAN users can roam seemingly without restriction and with access from nearly anywhere without being bounded by conventional wired network connections. 
   One of the most significant issues in the area of wireless and mobile communications technology is the provision of constant IP (Internet protocol)-connectivity to mobile nodes upon roaming. While the IEEE 802.11 standard for WLANs acts as an important milestone in the evolution of wireless networking technology, roaming has not yet gained much coverage in the current IEEE 802.11 standard, resulting in insufficient support of key mobility functions. 
   Referring concurrently to  FIGS. 1 and 2 , a typical IEEE 802.11 infrastructure WLAN environment consists of access points  10   a ,  10   b ,  10   c ,  10   d ,  10   e  (APs) and mobile terminals/stations  12   a ,  12   b  (STAs) communicating over the air via 802.11b specific messages. Neighboring APs are attached to a wired distribution system  14  (DS) and form an extended service set  16   a ,  16   b ,  16   c ,  16   d  (ESS). Upon power up, a STA  12   a  gets associated to an AP  10   a  inside the ESS  16   a  within which it is residing via specific association messages. At the same time, it obtains an IP address (e.g., via DHCP, Dynamic Host Configuration Protocol) so as to be widely reachable at its current location. Furthermore, certain authentication procedures take place (e.g., 802.1x authentication) in order to authenticate the STA  12   a . The IP subnet  18   a  where the STA&#39;s IP address belongs is called the home network (HN). Every time the STA  12   a  powers up inside an ESS  16   a , the IAPP (Inter-Access Point Protocol) is triggered so as to inform the neighboring APs  10   b  about the STA&#39;s  12   a  physical location. This is accomplished via specific layer  2  (L 2 ) message updates sent by the home access point (HAP)  10   a  to the subnet broadcast address. Routing of the IP datagrams is performed via standard IP routing mechanisms. The APs  10   a ,  10   b  are used as L 2  bridges. Any packets sourcing outside the HN and destined to the STA  12   a , arrive at the gateway router  20   a  of the corresponding ESS  16   a . Inside the ESS  16   a , specific L 2  bridging takes place to successfully deliver packets to the STA&#39;s actual location. 
   Within the ESS  16   a , the STAs  12   a ,  12   b  may roam from one AP (e.g.,  10   a ) to another AP (e.g.,  10   b ) via reassociation messages. In an 802.11 WLAN, each time a STA  12   a  is reassociating to a new AP  10   b  inside the ESS  16   a  of its HN, it performs an intra-network handover. The L 2  point of attachment has changed to the MAC address of the new AP  10   b , and the new AP  10   b  becomes the STA&#39;s HAP (home AP). The STA  12   a  preserves its MAC address. The time elapsed between the cut-off of the previous AP-STA and the connection running between the new AP and the STA is called the handover period or handover recovery time. During this period, any active sessions that this STA  12   a  had before its movement get disconnected. The L 2  handover of 802.11 STAs is supported by the IAPP protocol, which provides the necessary means for quick recovery of the interrupted active sessions. Moreover, it assures that the STA is still able to send/receive IP packets from its new location while preserving its home IP address. 
   For inter-network handover in IEEE 802.11 WLANs, the STA  10   a  moves inside an ESS  16   b  that belongs to a different IP subnet, i.e., it triggers a layer  3  (L 3 ) handover. This type of handover is performed when a roaming STA  12   a  reassociates to a foreign AP  10   c  of an ESS  16   b  outside of its home network and involves both an L 2  and an L 3  handoff. (Similarly, if a STA already lying in a foreign network roams inside/between foreign networks, it still performs an L 3  handover.) Thus, via specific 802.11 MAC layer mechanisms, the STA  12   a  is now physically attached to a foreign AP  10   c . However, it was not one of the IAPP objectives to provide support for inter-network (L 3  or IP) handover of 802.11 roaming STAs. Accordingly, any packets now destined to the home address of the STA  12   a  are routed to its HN. However, these packets will be dropped due to the fact that the STA  12   a  does not physically belong there anymore. Similarly, any packets originated from the STA  12   a  will be dropped inside the foreign network, because their source IP address does not belong to this subnet. 
   All of these routing issues arising upon an L 3  handover form a problem that is outside the scope of the IAPP. With the increasing deployment of 802.11 networks in both commercial and home environments, the need for inter-network handover increases. The present invention addresses this need, ensuring constant IP-connectivity during any type of handover (IP or MAC layer) to assist in unbounded roaming of 802.11 STAs. 
   SUMMARY OF THE INVENTION 
   Aspects for seamless mobility of mobile terminals in a wireless network are described. The aspects include utilizing a reassociation request from a mobile terminal to identify need for an internetwork handover of the mobile terminal roaming in a wireless local area network (WLAN), and performing a protocol sequence in an access point (AP) for the mobile terminal to handle the internetwork handover to ensure connectivity of the mobile terminal while roaming. 
   Through the present invention, the IP-flows of roaming mobile terminals are preserved even when the mobile terminals move across different sub-networks. Further, the protocol sequence of the present invention requires no changes in the protocol stack of the 802.11 mobile terminals, and the handover is supported in a way that is transparent to the mobile terminals. In addition, real-time and critical sessions are maintained with quick restoration of IP-connectivity via low-loss and low-latency handover that integrates to the existing IEEE 802.11 standard, instead of requiring the additional use of separate handover protocols, such as Mobile IP. These and other advantages will become readily apparent from the following detailed description and accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates a wireless network system in accordance with the prior art. 
       FIG. 2  illustrates inter-network movements in the wireless network of  FIG. 1 . 
       FIG. 3  illustrates a protocol stack in accordance with the present invention. 
       FIGS. 4   a  and  4   b  illustrate a method supporting handover movements in accordance with the present invention. 
       FIG. 5   a  illustrates a general 802.11f IAPP packet of the prior art. 
       FIG. 5   b  illustrates a data field for the packet of  FIG. 5   a.    
       FIGS. 5   c  and  5   d  illustrate data fields for the packet of  FIG. 5   a  in accordance with the present invention. 
       FIGS. 6   a ,  6   b , and  6   c  illustrate pseudo-code for the activities of the APs during handover movements in accordance with the present invention. 
       FIGS. 7 and 8  illustrate routing diagrams and datagrams in accordance with the present invention. 
   

   DETAILED DESCRIPTION 
   The present invention relates to seamless mobility support for mobile terminals in a wireless network. The following description is presented to enable one of ordinary skill in the art to make and use the invention and is provided in the context of a patent application and its requirements. Various modifications to the preferred embodiment and the generic principles and features described herein will be readily apparent to those skilled in the art. Thus, the present invention is not intended to be limited to the embodiment shown but is to be accorded the widest scope consistent with the principles and features described herein. 
   In general, the present invention extends IAPP functionality rather than replacing it and is added in the existing protocol stack of IEEE 802.11 APs, as indicated by the Radius client layer in the protocol stack illustrated in  FIG. 3  . Further, the aspects of the protocol sequence of the present invention are utilized only if an L 3  handover is identified by an AP upon receipt of an IEEE 802.11 Reassociation.Request message by a STA. If no L 3  handover is identified, standard IAPP takes place to handle the L 2  handover. An L 3  handover identification preferably occurs based on IP specific information which is retrieved by the 802.11 Reassociation.Request frame, which is extended in accordance with the present invention to include three new fields that are the only changes required by the STAs for the aspects of the present invention to be applicable to 802.11 L 3  handovers. The fields are: (1) HAP IP address; (2) STA IP address; and (3) Previous AP (PAP) IP address. 
   Referring now to  FIGS. 4   a  and  4   b , in accordance with the present invention, the sequence of actions during L 3  handover during inter-network movement and inter/intra-foreign-network movement, respectively, are shown. For inter-network movement, the sequence initiates upon receipt of a reassociation request from a STA  40  to a NAP  42 . The NAP  42  adds a routing entry for the STA  40  and creates a tunnel to the HAP  44 . The NAP  42  further sends an L 3 -MOVE-notify-type packet to the HAP  44 . The HAP  44  in turn registers the FACOA (foreign agent care of address) for the STA and creates a tunnel to the NAP  42 . The HAP  44  then sends an L 3 -MOVE-response-type packet to the NAP  42 . In the inter/intra-foreign network movement, the NAP  46  performs the same sequence with the HAP  44 . However, the sequence also includes a TUNNEL request being sent from the NAP  46  to the PAP  42  when the L 3 -MOVE-notify-type packet is sent to the HAP  44 . In response, the PAP  42  registers the STA new FACOA, creates a tunnel to NAP  46 , and deletes the HAP-PAP tunnel entries. The PAP  42  then sends a TUNNEL response to the NAP  46 . 
   While MOVE-notify- and -response-type packets are presented with reference to  FIGS. 4   a  and  4   b , these packet types, as defined in IEEE 802.11f D3.1, are modified in accordance with the present invention.  FIG. 5   a  illustrates a standard format for these packet types, while  FIG. 5   b  illustrates a standard format for the data field of these packet types. These packets types are modified to provide new IAPP-based packets (TCP/IP and UDP/IP packets) in accordance with the present invention for exchange between the involved parties of a STA&#39;s L 3  handover. These new packets are Roam-request, Roam-response, Create-Tunnel-request, and Create-Tunnel-response. Along with the new packets, new service primitives are generated at the corresponding APs and are analogous to standard IAPP service primitives. These new service primitives are the Roam.Request primitive, Roam.Response primitive, Create-Tunnel.Request primitive, and Create-Tunnel.Response primitive. 
   A Roam.request primitive is generated at a NAP only in the case of a STA&#39;s network handover when the NAP receives an MLME-Reassociate.indication. The NAP then sends a Roam-request packet to the HAP and an L 2  update frame to the subnet broadcast address, which may be useful in situations where the APs support dynamic routing. The Roam-request packet, a TCP/IP packet, causes registration of the FACOA to the HAP and triggers HAP-NAP tunnel establishment. The Roam-request packet is the same as the IAPP Move-notify request packet with an extension to also carry the HAP and STA IP addresses. Since the command values of 0-4 are reserved by the 802.11f IAPP packets, a command value of 5 is suitable for the packet generated, and a preferred data field for the packet is illustrated in  FIG. 5   c.    
   Upon receipt of a Roam-request packet from the NAP of a STA, the Roam.response primitive is generated at the HAP. The HAP then sends a Roam-response packet to the NAP indicating the successful creation of the HA-NAP tunnel at the HAP. The Roam-response packet, a TCP/IP packet, is the same as the IAPP Move-response packet with an extension to also carry the HAP and STA IP addresses. For the packet fields, the status field suitably indicates success or failure of the tunnel creation, while the command field has a distinctive command value, e.g., 6, and the data field is structured as illustrated in  FIG. 5   c.    
   The Create-Tunnel.request primitive is generated at an AP acting as a NAP for a STA in cases of intra/inter-foreign-network movements and causes the sending of a Create-Tunnel-request packet to the PAP of the STA. The Create-Tunnel-request packet, a UDP/IP packet exchanged from the NAP to the PAP, informs the PAP of the new FACOA and triggers NAP-PAP temporary tunnel establishment. The Create-Tunnel-request packet is the same as the IAPP MOVE-notify packet with an extension to also carry the STA IP address. For the packet, the command field has a suggested value of 7. The remote end AP of the tunnel to be created (i.e., RE=NAP) is indicated in the REIP field, and the STA is specified in the MAC address and MNIP fields, as illustrated in the data field of  FIG. 5   d.    
   In cases of intra/inter-foreign-network movements, the Create-Tunnel.response primitive is generated at the PAP of a STA and causes the PAP to send a Create-Tunnel-response packet to the NAP of the STA. The Create-Tunnel-response packet, a UDP/IP packet, indicates completion of the PAP&#39;s actions for the PAP-NAP tunnel establishment to the NAP. The Create-Tunnel-response packet is the same as the IAPP Move-response packet with an extension to also carry the STA IP address. Thus, the packet&#39;s data field is the same as that of the Create-Tunnel-request with the reserved field replaced by a status field having a success or failure value. A value of 8 is suggested for the command value. 
   In addition to the packets and primitives, in accordance with the present invention, the management entity architecture of the AP is enhanced. Every AP acting as a HAP preserves a list (RoamingList) for its registered STAs that currently use a FACOA. Further, every AP serving as a FA preserves a list (VisitorList) with the currently connected STAs for which the FA has established tunnels toward the STA&#39;s HAP. For each AP involved in the L 3  handover, it identifies its current role (NAP, HAP, or PAP) and performs the appropriate actions, as described hereinabove and presented in the pseudo-code of  FIGS. 6   a ,  6   b , and  6   c.    
   The concept of IP tunneling is used in the present invention to provide the important functionalities at the involved APs of session re-establishment and routing of IP datagrams after handover completion. These are both achieved via IP encapsulation and decapsulation of the STA&#39;s IP datagrams by the APs. 
   Referring to  FIG. 7 , for inter-network movement, in the forward direction (to the STA), the HAP  50  is able to route packets to the current location of the mobile terminal via IPIP encapsulation. The IP header of any packets in this direction has the IP address of the corresponding node (CN)  52  as a source address (SA) and the STA&#39;s  58  IP address as the destination address (DA). When the packet reaches the HAP  50 , an additional IP header is added to the packet, as shown. The encapsulated datagram is forwarded to the NAP  54  through the existing HAP-NAP tunnel. The NAP  54  (FA) decapsulates/strips the outer IP header off of all packets destined to the STA&#39;s IP address and routes them to the directly connected STA  58  (ARP entry). 
   For inter-network movement in the backward direction (from the STA), the NAP  54  is able to route packets originated at the current location of the mobile terminal via IPIP encapsulation. The IP header of any packets sourced at the STA  58  includes a SA of STA IP and a DA of CN IP. When the NAP  54  has to route such packets, it does so using the HAP-NAP tunnel  56 . The NAP  54  encapsulates the STA&#39;s packets by adding an outer header, as shown. The encapsulated datagram is forwarded to the HAP  50  through the existing HAP-NAP tunnel  56 . There, the HAP  50  decapsulates/strips off the outer IP header of the packet and routes the initial packet to the original DA (which is the CN  52 ). 
   For inter/intra-foreign network movement, the STA  58  becomes associated to a new foreign AP (NAP  60 ,  FIG. 8 ). The NAP  60  may belong to the same FN or another FN. After completion of the protocol of the present invention, a new bi-directional HAP-NAP tunnel is established, which serves the routing of the STA&#39;s IP datagrams, and the previous HAP-PAP tunnel is deleted after successful establishment of the new tunnel. Routing of the IP datagrams after handover completion is supported by the same routing methods presented with reference to  FIG. 7 . For fast and low-loss handoff purposes, a temporary PAP-NAP tunnel  62  is created before the HAP-PAP tunnel deletion. The session re-establishment is supported by the use of the NAP IP address as the STA&#39;s FACOA. Further, any packets that remained at the PAP after movement of the STA are forwarded to the STA&#39;s current AP, the NAP, through the temporary PAP-NAP tunnel. Upon receipt of these packets, the NAP routes them to the DA specified by the IP header of the packets, i.e., the STA IP address (existing STA ARP entry). 
   Thus, the present invention considers APs able to perform IP-in-IP encapsulation in order to utilize the IP tunneling and reverse tunneling methods. In this manner, a feasible and efficient way for supporting all forms of mobility of IEEE 802.11 mobile terminals is provided. Without such a mechanism, IP routing is not feasible in cases of L 3  mobility within 802.11 environments. 
   From the foregoing, it will be observed that numerous variations and modifications may be effected without departing from the spirit and scope of the novel concept of the invention. It is to be understood that no limitation with respect to the specific methods and apparatus illustrated herein is intended or should be inferred. It is, of course, intended to cover by the appended claims all such modifications as fall within the scope of the claims.