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U. Glaeser

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Using TCP/IP also makes the network system portable, and program portability is one of the system<br />

design goals.<br />

The network layer uses Internet protocol (IP). IP is responsible for routing individual datagrams and<br />

getting datagrams to their destination. The IP layer provides a connectionless and unreliable delivery<br />

system. It is connectionless because it considers each IP datagram independent of all others and any<br />

association between datagrams must be provided by the upper layers. Every IP datagram contains the<br />

source address and the destination address so that each datagram can be delivered and routed independently.<br />

The IP layer is unreliable because it does not guarantee that IP datagrams ever get delivered or<br />

that they are delivered correctly. Reliability must be provided by the upper layers.<br />

Transport layer uses UDP and transmission control protocol (TCP). TCP is a connection-oriented<br />

protocol that provides a reliable, full-duplex, byte stream for the multimedia communication process.<br />

TCP is responsible for breaking up the message into datagrams, reassembling them at the other end,<br />

resending anything that got lost, and putting everything back in the right order. TCP handles the<br />

establishment and termination of connections between processes, the sequencing of data that might be<br />

received out of order, the end-to-end reliability (checksums, positive acknowledgments, timeouts), and<br />

the end-to-end flow control.<br />

UDP is a connectionless protocol for user processes. Unlike TCP, which is a reliable protocol, there is<br />

no guarantee that UDP datagrams ever reach their intended destination. UDP is less reliable than TCP<br />

but transfers data faster because they are not held up by earlier messages awaiting retransmission. TCP<br />

protocol is used for file transfer that requires reliable, sequenced delivery, where real-time delivery may<br />

not be of utmost importance.<br />

Mobile IP Protocols<br />

In TCP/IP an application is connected with another application through a router. Each host in Internet<br />

has a unique address. An IP address is a 32-bit binary number that can also be used in a dotted notation.<br />

IP addresses contain two parts: network address which identifies the network to which the host is attached,<br />

and local address which identifies the host. Local address can be separated into two parts, subnet address<br />

and local address.<br />

The hierarchical address makes routing simple. A host that wants to send packets to another host only<br />

needs to send packets to the network to which the target host is attached. The host does not need to<br />

know the inside of the network; however, a computer’s IP address cannot be changed during connection<br />

and communication. If the user wants to move the computer to the other area while using it, this will<br />

be difficult because the physical IP address of the computer must be changed in a different subnet. To<br />

solve this problem, a number of protocols have been proposed: virtual IP (VIP), loose source routing IP<br />

(LSRIP), and Internet engineering task force mobile IP (IETF-MIP).<br />

VIP<br />

VIP uses two, 32-bit IP-style addresses to identify mobile hosts: one is named virtual IP (VIP) address,<br />

the other is named temporary IP (TIP) address. VIP address is the IP address that mobile hosts get from<br />

their home network. Mobile hosts always use VIP as their source address inside IP packet. When mobile<br />

hosts move to another network, they get another IP address from the foreign network, it is a TIP address.<br />

Each VIP packet contains information to combine VIP and TIP, so the packet target to VIP can be routed<br />

through general Internet to its temporary network by reading its TIP. VIP uses additional space inside<br />

packet to carry this information: a new IP option to identify VIP while original address fields carry TIP.<br />

When a mobile host moves to a foreign network, the information about mobile host’s current location<br />

is sent to the mobile host’s home network. During the transmission, each intermediate router that<br />

supports VIP protocol can receive this information and update this router’s cache. This cache is a database<br />

that stores information about mobile hosts’ current location.<br />

If a host wants to send a packet to a mobile host, it only knows the VIP address of the mobile host<br />

and does not know its TIP, which is its current location. The packet will be sent to the mobile host’s<br />

home area. If any intermediate router that supports VIP receives this packet, it will modify this packet<br />

© 2002 by CRC Press LLC

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