Forcing Data Delivery

Normally, TCP waits for the buffer to exceed the maximum segment size before sending any data. This creates serious delays when the two sides of the connection are exchanging short messages and need to receive the response before continuing. For example, the login sequence at the beginning of a session begins with the short message "Login," and the session cannot make any progress until these five characters have been transmitted and the response has been received. This process can be seriously delayed by TCP's normal behavior.

However, an application can force delivery of octets to the output stream using a push operation provided by TCP to the application layer. This operation also causes TCP to set the PSH flag or control bit to ensure that data will be delivered immediately to the application layer by the receiving transport layer.

In the most extreme cases, for example when a user expects each keystroke to be echoed by the receiving application, the push operation can be used each time a keystroke occurs. More generally, application programs use this function to force output to be sent after writing a character or line of characters. By forcing the data to be sent immediately, delays and wait