Figure 2.7 shows
TCP/IP operation of accessing a website. After a user click on the website
link, the application uses DNS client software to request DNS Server to send an
IP address of the destination host. The DNS client software uses UDP and IP to
build a DNS name query message to request the IP address of the host that the
user wants to connect to. Link layer builds an ethernet frame and pass it to the
network operating system to be sent to the DNS Server. DNS Server sends back
the DNS name query response message and a target host IP address. It comes in
link layer, goes through IP layer where the address information is stripped off
and goes back through UDP where the packet integrity is checked. Then it goes
up to the DNS client software and passes the IP address to the application.
The application
then instructs TCP to create a connection. After a TCP connection is
established, the application requests a file from the target host. The target
host uses TCP to segment the file user wanted into packet and send it to the
client. Each packet comes through link layer, through IP and passes it up to
TCP which verify the segment integrity and hold until all files segments had
arrived. It also acknowledges receiving of the packet to the sender. This cycle
repeated until the whole files had been reassembled.
If the packet
arrive does not pass the integrity check, TCP discarded it and not send acknowledgement
to the sender. If the last packet was not acknowledged within a lot of time,
the sender retransmit it. When the last packet arrives, the files are
reassembled and acknowledged to the sender. The client then closed TCP
connection. The completed files passed to the application where users can
access it.
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