A data packet header contains a series of zeroes and one, and one digit indicates the PSH flag.
Data travels around networks in packets. A packet carries a segment of data in the packet body and contains a header at the front for addressing, for describing the data and for sending control information about the connection. A packet can contain several headers; one contains a "push flag." In protocol specifications, this flag is always referred to as PSH.
Transmission Control Protocol
One of the protocols involved in moving data around networks is called the Transmission Control Protocol, or TCP. This protocol is responsible for establishing a connection and checking that data packets have arrived in sequence. Some TCP instructions travel in front of data packets, and some travel in a packet that has no data but is intended for instruction. The receiving TCP program usually stores the arriving packets temporarily before sending it to the receiving application. This particularly happens if packets arrive out of sequence. The PSH flag's purpose is to instruct the receiver about buffering.
TCP Header
A TCP header contains 20 bytes of data plus two more bytes for options and padding. The PSH flag is one of six control flags occupying the last six bits of the 14th byte; a byte contains four bits. The flags are TCP's language, enabling the two sides in the transfer to establish a connection and inform each other of their status and the actions each wants the other to perform. Each flag occupies one bit of data. If that bit is set to "1," the flag is active.
Purpose
The PSH flag is an instructor from the sender to the receiver to end buffering. This can have two purposes. It is principally designed for the sender to tell the receiver that the last bit of data is contained in the packet and that the receiver should now empty its buffer and pass all data on. As a computer receiving a PSH packet stops buffering, setting the PSH flag also has the effect of turning buffering off for that packet. Buffering occurs at the points of the packets journey across the network, not just at the receiver. This means that a PSH packet travels faster, as it isn't buffered at any point.
Danger
Hackers can use the PSH flag to prevent a data packet being examined. Firewalls buffer data packets slightly to examine them and perform security functions. If the PSH packet is set, any programs turn off buffering and pass on the packet immediately, which robs the firewall of time to examine the packet.
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