A Simple Example
Figure 3 puts all of these concepts together, showing the interaction between modules to transfer one block of data. For simplicity, the example shows two systems connected to the same network so that no router is involved. Let us say that the file transfer module in computer X is transferring a file one record at a time to computer Y. At X, each record is handed over to TCP. We can picture this action as being in the form of a command or procedure call. The arguments of this procedure call include the destination computer address, the destination port, and the record. TCP appends the destination port and other control information to the record to create a TCP segment. This is then handed down to IP by another procedure call. In this case, the arguments for the command are the destination computer address and the TCP segment. The resulting IP datagram is handed down to the network access layer, which constructs a network-level packet.
Figure 3 Operation of a protocol architecture.
The network accepts the packet from X and delivers it to Y. The network access module in Y receives the packet, strips off the header, and transfers the enclosed transport protocol data unit (PDU) to the X IP module, which strips off the IP header and passes the resulting TCP segment to TCP. TCP examines the segment header and, on the basis of the destination port field in the header, delivers the enclosed record to the appropriate application, in this case the file transfer module in Y.