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Các thuật toán vector khoảng cách là những ví dụ của giao thức định tuyến động. Các thuật toán này cho phép mỗi thiết bị trong mạng để tự động xây dựng và duy trì một bảng định tuyến IP địa phương. Các nguyên tắc đằng sau khoảng cách vector định tuyến là đơn giản. | 7.1 Mobile IP overview Mobile IP enables a device to maintain the same IP address its home address wherever it attaches to the network. Obviously a device with an IP address plugged into the wrong subnet will normally be unreachable. However the mobile device also has a care-of address which connects to the subnet where it is currently located. The care-of address is managed by a home agent which is a device on the home subnet of the mobile device. Any packet addressed to the IP address of the mobile device is intercepted by the home agent and then forwarded to the care-of address through a tunnel. After it arrives at the end of the tunnel the datagram is delivered to the mobile device. The mobile node generally uses its home address as the source address of all datagrams that it sends. Mobile IP can help resolve address shortage problems and reduce administrative workload because each device that needs to attach to the network at multiple locations only requires a single IP address. The following terminology is used in a mobile IP network configuration Home address The static IP address allocated to a mobile node. It does not change no matter where the node attaches to the network. Home network A subnet with a network prefix matching the home address of the mobile node. Datagrams intended for the home address of the mobile node will always be routed to this network. Tunnel The path followed by an encapsulated datagram. Visited network A network to which the mobile node is connected other than the node s home network . Home agent A router on the home network of the mobile node that maintains current location information for the node and tunnels datagrams for delivery to the node when it is away from home. Foreign agent A router on a visited network that registers the presence of a mobile node and detunnels and forwards datagrams to the node that have been tunneled by the mobile node s home agent. 276 TCP IP Tutorial and Technical Overview 7.1.1 Mobile IP operation
