The most common method involves flooding the victim machine with external communications requests, so much so that it cannot respond to legitimate traffic, or responds so slowly as to be rendered effectively unavailable. 
The other method is of obstructing the communication media between the intended users and the victim so that they can no longer communicate adequately. Even though the technical laws of different countries vary, still denial-of-service attacks are considered as a crime in most of the countries.
The TCP/IP protocols form the basis of today's Internet, but these protocols lack even the basic mechanisms for security, such as validation or encryption. Newer versions of these protocols such as IPV6 do have better security features but currently TCP/IP protocols are the ones most widely used and common protocols and so their lack of built-in security has become increasingly challenging. 
Now in accordance with the question let us look at the working of the protocol to understand the loophole and the problem. In TCP/IP the TCP half uses sequence numbers to keep track of the order or sequence of data packet, (this is to counter high message latency and loss which results in messages usually arriving in non-sequential order). ...