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The Benefits of Using TCP/IP

By using TCP/IP, it enables cross-platform, or heterogeneous, networking. For example, a Windows NT/2000 network could contain Unix and Macintosh workstations or even networks mixed in it. TCP/IP also has the following characteristics that make it unique:

• Good failure recovery
• The ability to add networks without interrupting existing services 
• High error-rate handling
• Platform independence
• Low data overhead

Because TCP/IP was originally designed for Department of Defense–related purposes, what we now call features or characteristics were actually design requirements. The idea behind “Good Failure Recovery” was that if a portion of the network were disabled during an incursion or attack, its remaining pieces would still be able to function fully. Likewise is the capability of adding entire networks without any disruption to the  services already in place. The ability to handle high error rates was built in so that if a packet of information got lost using one route, there would be a mechanism in place to ensure that it would reach its destination using another route. Platform independence means that the networks and clients can be Windows, Unix, Macintosh, or any other platform or combination thereof. The reason TCP/IP is so efficient lies in its low overhead. Performance is key for any network. TCP/IP is unmatched in its speed and simplicity.

TCP Features : Connections

Before application processes can send data by using TCP, they must establish a connection. The connections are made between the port numbers of the sender and the receiver nodes. A TCP connection identifies the end points involved in the connection. The end point on picture below is formally defined as a pair that includes the IP address and port number:

(IP address, port numbers)

The IP address is the internetwork address of the network interface over which the TCP/IP application communicates. The port number is the TCP port number that identifies the application. The end point contains both the IP address and port numbers because port identifiers are selected independently by each TCP, and they may not be unique. By concatenating the unique IP address with port numbers, a unique value for
the end point is created.
A TCP connection is established between two end points. The TCP connection is identified by the parameters of both end points, as follows:
(IP address1, port number1, IP address2, port number2)

These parameters make it possible to have several application processes connect to the same remote end point. Picture above shows that several application processes connect to the same remote end point (199.11.23.1, 2001). The TCP module at 199.11.23.1 can keep the different TCP connections separate because TCP uses both the local and remote end points to identify the connection. In that picture, the end point (199.11.23.1, 2001) is the same, but the end points at the other end of the connection are different. This difference enables TCP to keep these connections separate.
The picture also illustrates that TCP can support multiple connections concurrently. These connections are multiplexed over the same network interface.
A connection is fully specified by the pair of end points. A local end point can participate in many connections to different foreign end points. A TCP connection can carry data in both directions; that is, it is full duplex.

In relationship to TCP connections, it is also helpful to define the notion of a half association and a full association. A half association is an end point that also includes the transport protocol name, as follows:
(TransportProtocol, IP address, port number)

The half associations in picture are, therefore, the following:
(tcp, 199.21.32.2, 1400)
(tcp, 196.62.132.1, 21)
A full association consists of two half associations and is expressed by the following ordered pair:
(TransportProtocol, IP address1, port number1, IP address2, port number2)

The TransportProtocol is listed only once because it has to be the same on both parts of the half association. The concept of half and full associations is useful when dealing with different transport protocols. As an example, the full association in picture is listed as follows:
(tcp, 199.21.32.2, 1400, 196.62.132.1, 21)

A full association consisting of source and destination IP addresses, and source and destination port numbers uniquely identifies a TCP connection.

TCP Features : Multiplexing

TCP enables many processes within a single computer to use the TCP communications services simultaneously; this is called TCP multiplexing. Because these processes may be communicating over the same network interface, they are identified by the IP address of the network interface. However, you need more than the IP address of the network interface to identify a process because all processes that are using the same network interface on a computer have a common IP address.
TCP associates a port number value for applications that use TCP. This association enables several connections to exist between application processes on remote computers because each connection uses a different pair of port numbers.

This picture shows several connections being multiplexed over TCP.
The binding of ports to application processes is handled independently by each computer. In many computer systems, a logger or super daemon process watches over the port numbers that are identified or well known to other computer systems.

TCP Features : Flow Control

Computers that send and receive TCP data segments can operate at different data rates because of differences in CPU and network bandwidth. As a result, it is quite possible for a sender to send data at a much faster rate than the receiver can handle. TCP implements a flow control mechanism that controls the amount of data sent by the sender. TCP uses a sliding window mechanism for implementing flow control.
TCP flow control mechanism exhibits the following properties:
• Octets that are to the left of the window range have already been sent and acknowledged.
• Octets in the window range can be sent without any delay. Some of the octets in
the window range may already have been sent, but they have not been acknowledged. Other octets may be waiting to be sent.
• Octets that are to the right of the window range have not been sent. These octets can be sent only when they fall in the window range.

The left edge of the window is the lowest numbered octet that has not been acknowledged. The window can advance; that is, the left edge of the window can move to the right when an acknowledgment is received for data that has been sent. The TCP packet containing the acknowledgment contains information about the window size that the sender should use.
The window size reflects the amount of buffer space available for new data at the receiver. If this buffer space size shrinks because the receiver is being overrun, the receiver will send back a smaller window size. In the extreme case, it is possible for the receiver to send a window size of only one octet, which means that only one octet can be sent. This situation is referred to as the silly window syndrome, and most TCP implementations take special measures to avoid it.
A TCP module sending back a window size of zero indicates to the sender that its buffers are full and no additional data should be sent. TCP includes mechanisms to shrink window size when the receiver experiences congestion of data and to expand window size as the congestion problem clears.
The goal of the sliding window mechanism is to keep the channel full of data and to reduce to a minimum the delays experienced by waiting for acknowledgments.