Subtitles of the Movie

Another important component of TCP is the TCP Flag. Now, a flag in a TCP segment is basically a bit that can be turned on or off. The flag is said to be set if it's turned on if it's on the one position or off if it's in the zero position. It's just a one-bit field. Now, these flags are used to determine what's going on in a given segment or tell what kind of control the segment is being sent and what condition or state that particular segment is in. Now, there are eight TCP Flags. Six of them are commonly used and we'll talk about those predominantly throughout the course. The SYN Flag is the first flag we'll talk about. It's used to synchronize communications, basically to synchronize sequence numbers when a TCP conversation starts. An ACK is used to acknowledge a segment. So a receiver will send ACK segments back with the ACK Flag sent. Now, a FIN, that flag is set when there's a need to end the communications. Usually the receiver sends that or the sender could send it also, but it's usually sent in one of the segments near the end of the conversation. Now, the URG, or Urgent Flag basically is used to indicate that a particular segment should be processed first in a priority over other segments. It should be resent first or retransmitted first. Basically it has a higher priority. Now, a PUSH Flag is usually sent by the sending computer to indicate that it should flush the TCP buffers and send whatever data is left up to the point of the PUSH Flag was sent. The RST or Reset Flag is usually sent to reset a connection. Oftentimes a connection will be attempted and a reset will be the reply back because a port may not be open. Now, flags can be set individually in each TCP segment or they can be used in combination with each other. Now, what do we use these flags for? We can use them when we're doing packet analysis to determine the state of the communications process at any given moment in the TCP conversation or to trace a session through from the beginning to the end because we can start with the Three-Way Handshake which uses the flags all the way to the FIN segment at the end. Now, unfortunately malicious users can use these flags for ill will. They can be set so that they can be used to do denial of service attacks and other malicious things and we'll talk about how those things can be done later on in the course.