Abstract:
To deliver packets to the MAC of a roaming mobile in a seamless manner, a proxy ARP is set up, in addition to the usual ARP, and is informed of the COA and MAC address of the mobile. The proxy ARP then informs the last routing switch in the foreign network of the MAC address of the mobile.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
         [0001]    This application claims priority of European Patent Application No. 0020585.6, which was filed on Aug. 21, 2000.  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    This invention relates to a method of supporting seamless hand-off in a mobile telecommunications network, especially to hand-offs to a roaming mobile terminal in the network.  
           [0004]    2. Description of the Related Art  
           [0005]    In third generation telecommunications networks such as GPRS (General Packet Radio Service) and EDGE (Enhanced Data-rate for GSM Evolution), when a mobile terminal moves into a foreign network, network connectivity is optionally maintained by the use of Mobile Internet Protocol (Mobile IP). In the home network, a Home Agent (HA) is set up which maintains the location information of the mobile by use of Binding Updates, i.e., registration of information sent to the HA by the mobile node.  
           [0006]    Mobile IP has two working modes. The first is illustrated in FIG. 1; a mobile terminal is currently attached as a Mobile Node (MN)  14  in a network different from its home network. The MN  14  is communicating with a Correspondent Node (CN)  12 . A Home Agent  16  is set up in the home metwork by the CN  12 , and a Foreign Agent (FA)  18  is set up in the foreign network. The FA  18  allocates a unique IP address for the visiting mobile, a Care of Address (COA) and this address is sent to the HA  16  in a Binding Update.  
           [0007]    Packets for the mobile are encapsulated by the HA  16  and tunneled along tunnel  20  to the FA  18  for transmission to MN  14 . In such encapsulation, an extra IP header is added to each packet, including the COA of the MN  14 . This is known as FA-COA working mode.  
           [0008]    In the second working mode (not illustrated) there is no FA, the MN  14  is allocated a unique COA and encapsulated packets are tunneled by HA  16  directly to MN  14 ; this is known as Collocated Care of Address mode of working (CO-COA).  
           [0009]    In a conventional packet switched network, when a host A sends a packet to another host B, host A needs to determine the Media Access Control (MAC) address of host B so that the packet can be delivered to the correct physical address in layer  2 . Host A sends a MAC broadcast frame, called an Address Resolution Protocol (ARP) request frame, which contains the host A&#39;s IP and MAC addresses and the IP address of host B. All nodes in the local network receive the broadcast IP request frame, and compare the destination address with their own IP address. Only the node with the correct IP address of the ARP request responds by sending a ARP reply containing its MAC address. On receiving the ARP reply, host A updates its ARP cache, which usually times out periodically. After an ARP cache entry has timed out for a specific host, the ARP request is sent again to discover the MAC address of the destination. The ARP cache is consulted by a host before it sends an ARP request, and if the answer is found in the cache, the host does not need to generate an ARP request.  
           [0010]    It is known to provide a proxy ARP which is an ARP reply sent by one node on behalf of another node which is unwilling or unable to answer its own ARP requests. The sender of a proxy ARP reverses the sender and target protocol address fields and supplies some configured MAC address (generally its own) in the sender hardware address field (the place-holder field). The node receiving the reply then associates this link-layer address with the IP address of the original target node, so that future packets for this target node are transmitted to that MAC address.  
           [0011]    When a mobile roams it maintains connectivity by the use of a unique IP address allocated to it, a Care-of-Address, as explained above. By use of Mobile IP, a packet sent from a correspondent node to a mobile node can still use the home address of the mobile node no matter where the mobile moves to. The packet received in the foreign network needs to bear the current Care of Address of the mobile as the destination address. If applications running on the mobile node still use the home address of the mobile, there is seamless mobility support to those applications; the applications do not have to stop and re-start as the mobile roams.  
           [0012]    However, before the packet can be sent from the last routing switch in the foreign network to the current Care of Address from the mobile, the routing switch needs to know the MAC address of the mobile. In the current functional specifications of ARP and of Mobile IP, this entry cannot be created. Referring again to FIG. 1, in the terms used above, either CN  12  or MN  10  can be host A, the other being host B. In either direction, packets passed through the network from router to router, and in FIG. 1 two routers  22 ,  24 , are shown schematically adjacent the optimized route  20 . Suppose packets are passing from CN  12  to FA  16  via routers  22 ,  24 ; before a packet can be sent from router  24  to the current Care of Address of MN  10 , router  24  needs to know the MAC address of MN  10 , as stated above.  
           [0013]    The routers  22 ,  24  can alternatively be associated with the encapsulated route  18 .  
         SUMMARY OF THE INVENTION  
         [0014]    It is the object of the invention to provide a method of supporting seamless hand-off in a mobile telecommunications network of the type described above.  
           [0015]    According to the invention, in a third generation mobile telecommunications network, a method of delivering packets in layer  2  to a mobile terminal in a foreign network comprises the steps of:  
           [0016]    providing an Address Resolution Protocol (ARP) entity;  
           [0017]    setting up a home agent in the home network; and  
           [0018]    allocating a Care of Address to the mobile node;  
           [0019]    characterized by the further steps of:  
           [0020]    setting up a proxy ARP entity; and  
           [0021]    informing the proxy ARP entity of the Care of Address and the Media Access Control address of the mobile node. 
       
    
    
     DETAILED DESCRIPTION  
       [0022]    In the invention, in addition to the proxy ARP entity, a Dynamic Host Configuration Protocol (DHCP) can also be used. The proxy ARP server can be built into the DHCP server, or it can be in a stand alone node. The proxy ARP is informed of the Care of Address and the MAC address of the MN  10 ; this can be achieved during the mobile node registration process; either the proxy ARP advertises itself and receives a registration reply from the MN  10 , or the MN  10  solicits the proxy ARP by broadcasting a proxy ARP soliciting request, and the proxy ARP replies to confirm the registration.  
         [0023]    The foreign network proxy ARP controls operations in two different cases, when there is transmission from CN  12  to MN  10 , and when there is transmission in the opposite direction.  
         [0024]    In the first case, transmission from CN  12  to MN  10 , when the working mode is COCOA mode, when a packet from the CN  12  arrives at the last routing switch of the foreign network, i.e. switch  24 , the switch broadcasts an ARP request, with the frame format:  
                                                               Sender&#39;s MAC   Sender&#39;s IP   MN&#39;s MAC   MN&#39;s COA               address   (NULL)                      
 
         [0025]    The proxy ARP receives the broadcast message and compares the receiver&#39;s IP address with the registered Care-of-Address of the MN  10 . If a match is found, the proxy ARP sends an ARP reply message having the format:  
                                                               MN&#39;s MAC   MN&#39;s COA   Sender&#39;s MAC   Sender&#39;s IP                       address                      
 
         [0026]    When the switch  24  receives the ARP reply, it updates its ARP cache table by adding an entry with the unique mapping between the Care-of-Address and the MAC address of MN  10 . The switch  24  can then successfully deliver the packet to MN  10  at the correct address in layer  2 .  
         [0027]    When the working mode is FA COA mode, the FA  16  replies to any request for its MAC address, sending the FA COA as the IP address of the mobile node.  
         [0028]    In the second case, transmission from MN  10  to CN  12 , there is a preliminary stage. Before a packet is sent by MN  10 , the ARP module in MN  10  or the daemon running in MN  10  checks whether the destination of the packet (i.e. CN  12 ) is a node which is local to the home network of the MN  10 . If the destination is in the home network, then the MN  10  broadcasts its ARP request using its own home IP address as the sender&#39;s address. The ARP request format is:  
                                                               MN&#39;s MAC   MN&#39;s Home IP   CN&#39;s MAC   CN&#39;s IP Address               Address   (NULL)                      
 
         [0029]    When the proxy ARP receives the ARP request, it responds by sending an ARP reply message to the MN  10  in the format:  
                                               FN Default   CN&#39;s IP Address   MN&#39;s MAC   MN&#39;s IP address       Gateway&#39;s MAC                  
 
         [0030]    Consider now the position when the destination is not in the home network of MN  10 .  
         [0031]    In the foreign network (FN) there is a Default Gateway; this Default Gateway broadcasts an ARP request message and the proxy ARP server learns the IP address of the foreign network Default Gateway by receiving this message. The MN  10  broadcasts an ARP request using the Default Gateway in the foreign network as the destination IP address. The ARP request format is:  
                                                               MN&#39;s MAC   MN&#39;s Home IP   FN&#39;s Default   FN&#39;s Default               Address   Gateway   Gateway&#39;s IP                   MAC(NULL)   address                      
 
         [0032]    When the proxy ARP receives the broadcast, it sends an ARP reply message to the MN  10  in the format:  
                                                               FN Default   FN Default   MN&#39;s MAC   MN&#39;s Home IP           Gateway&#39;s MAC   Gateway IP       Address               Address                      
 
         [0033]    The packet can therefore be successfully delivered to the CN  12 .  
         [0034]    In another variation, the ARP proxy can be built as a daemon running in the MN  10 , care being taken that it does not affect the standard ARP daemon in the MN  10 .  
         [0035]    By application of the method according to the invention, seamless hand-off can be achieved even when a mobile is roaming in a foreign network.