Layer 4 TCP Flags

The TCP (Transmission Control Protocol) is layer 4 protocol of OSI stack.
rather than going on to the basics of  TCP like understanding its a connection oriented protocol and else ; Lets discuss the 5 FLAGS :

1.SYN : Initiates a connection
2.ACK : Acknowledges received data
3.URG : Discussed Below
4.PSH : Discussed Below
5.RST :Aborts a connection in response to an error
6.FIN :Closes a connection

"PSH FLAG"

To understand the function of the PSH flag, we first need to understand how TCP buffers data. TCP operates at layer four of the OSI model; it presents to upper layers a simple socket which can be read from and written to, masking the complexities of packet-based communications. To allow applications to read from and write to this socket at any time, buffers are implemented on both sides of a TCP connection in both directions.

Buffers allow for more efficient transfer of data when sending more than one maximum segment size (MSS) worth of data (for example, transferring a large file). However, large buffers do more harm than good when dealing with real-time applications which require that data be transmitted as quickly as possible. Consider what would happen to a Telnet session, for instance, if TCP waited until there was enough data to fill a packet before it would send one: You would have to type over a thousand characters before the first packet would make it to the remote device. Not very useful.

This is where the PSH flag comes in. The socket that TCP makes available at the session level can be written to by the application with the option of "pushing" data out immediately, rather than waiting for additional data to enter the buffer. When this happens, the PSH flag in the outgoing TCP packet is set to 1 (on). Upon receiving a packet with the PSH flag set, the other side of the connection knows to immediately forward the segment up to the application. To summarize, TCP's push capability accomplishes two things:

• The sending application informs TCP that data should be sent immediately.
• The PSH flag in the TCP header informs the receiving host that the data should be pushed up to the receiving application immediately

"URG FLAG"

The URG flag is used to inform a receiving station that certain data within a segment is urgent and should be prioritized. If the URG flag is set, the receiving station evaluates the urgent pointer, a 16-bit field in the TCP header. This pointer indicates how much of the data in the segment, counting from the first byte, is urgent.