Sunday, August 14, 2011

Reviewing the Methods of Communication

It was also noted in the last chapter that connectionless-oriented communication is simpler and more flexible. But you'll see that connection-oriented communication is not really that much more difficult. It does require additional steps, however, and mostly on the server side. A connection is also much more rigid, because after the connection has been established, the socket can only communicate with the connected endpoint.
The selling point in favor of TCP/IP for most application writers is that the communication channel is transparently reliable and that data is delivered in the proper sequence. After the connection has been established, your application can read and write to the socket without worrying about any of the following problems:
  • Lost packets
  • Timeouts and
  • Duplicated packets
  • Packets received out of sequence
  • Flow control

Like opening a file, your program can
  1. Establish a TCP/IP connection with a remote socket
  2. Transmit large amounts of data
  3. Close the socket

These simple steps are all that is necessary to deliver all of your data safely to the remote end. Proven error recovery software will take care of retransmitting lost packets until they can be successfully delivered to the remote host.

TCP/IP Handles Lost Packets
TCP/IP will notice when packets are lost. This does not always happen with UDP. When packet errors are reported, the TCP/IP protocol can immediately respond with retransmissions. However, if an acknowledgement is missing, causing a timeout, the TCP/IP protocol takes steps to ensure that the data is retransmitted to its destination. Carefully crafted algorithms are used to make the
transmission of the data nimble, without taxing the network capacity with retransmitted data.

TCP/IP Handles Duplicated Packets
Whenever a retransmission occurs, there is a slight possibility that more than one identical packet can be received at the remote end. If the retransmission occurs too early, for example, this can easily happen. The receiving end must be able to recognize this and discard extraneous packets. This is automatically performed by the TCP/IP protocol.

TCP/IP Handles Sequencing
When the volume of data requires multiple packets to be sent, there is a race to the finish line. The IP packet can be routed in different ways, according to dynamic routing and buffer congestion. This results in a race to the receiving end, where some packets can arrive ahead of other packets. For this reason, the receiving software must recognize this and sequence the data before presenting it to the application. Again, TCP/IP anticipates and corrects this problem.

TCP/IP Handles Flow Control
The ftp command uses TCP/IP to send and receive files. When you upload a large file to a remote ftp server, using the ftp send command, many data packets are placed on the network. It can
happen that the receiving host can end up receiving packets faster than it can process them. The IP way of dealing with this problem is to discard these extra packets. TCP logically sits on top of the IP protocol like a layer (hence, it is called TCP/IP). It acts as a supervisor of sorts by ensuring that the receiving end is not overloaded with more data than it can handle. When the receiving end feels that it has enough data for the moment, it notifies the sending end not to send more data until further notice. When it catches up, the remote end will signal the sending end to start sending data again. This automatic throttling of data is known as flow control.

Understanding the Advantages of TCP/IP
The purpose of this introduction was to show you the advantage of using a connection-oriented protocol. TCP/IP is one such connection-oriented protocol, which you will explore in this chapter. You have seen the number of services it performs for you behind the scenes. This helps you to focus on your application programming, rather than network communication problems. Furthermore, because the same time-tested algorithms are at work for each program that uses TCP/IP, they perform in the same reliable manner. This allows you to focus on application program bugs instead. Before you have fun working with TCP/IP in this chapter, you need to learn about some additional facilities as TCP/IP pertains to Internet services.

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