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CHAPTER 3 / COMMUNICATION NETWORK STRUCTURE  2 1
            The  transport  layer  provides  end-to-end  communication  between  adjacent
        application programs. It decomposes the data transferred from the application layer
        and  creates  the TP  (Transport  Protocol) packet,  attached  with the  control  infor-
        mation, such as a program identifier. Then it transfers the TP packet to the Internet
        layer.
            The  Internet  layer provides  the  communication functions  between a source
        computer  and its  destination  one. It receives  a TP packet  and the destination  IP
        address. Then it constructs the IP datagram using the TP packet and the destina-
        tion IP address.  Using the routing algorithm, it decides  the destination computer
        or  the gateway processor  and transfers the  IP datagram  to the network interface
        layer.
            The network interface layer provides the control  and interface functions  for
        transmitting the IP datagram through the physical layer. To achieve this, it creates
        an HDLC frame or LAN frame,  depending  on the physical network  structure.
            When the physical layer is a LAN structure, it corresponds  to a device driver
        or LAN interfacer. When it is a public network, such as the packet  switched net-
        work, it corresponds to the communication equipment based on the X.25 standard.
            TCP/IP  has  been  implemented  by  clients  independent  of  OSI  activities.
        Therefore,  it does not match the OSI structure. Basically, the application layer cor-
        responds to OSI layers 5 to 7. The transport layer corresponds to OSI layer 4. The
        internet layer is like OSI layer 3. The network layer corresponds to OSI layer 2.
        The physical layer is like OSI layer 1.

        3.3.2  TCP/IP  Subprotocol Structure

        Each layer of the TCP/IP is composed of a set of subprotocols  that correspond  to
        entities  of the OSI reference model, as shown in Table 3.1. The application layer
        provides the protocols that users directly access. In the application  layer, which is
        the top layer, are subprotocols such as the Simple Mail Transfer Protocol  (SMTP),


           TABLE 3.1  TCP/IP Subprotocols
   32   33   34   35   36   37   38   39   40   41   42