ACK flag: One of the TCP control flags. ACK is short for acknowledge. A value of one in this field means that the acknowledgment number field should be examined
Acknowledgement number: The number of the next expected segment in a TCP sequence
Application layer: The layer that allows network applications to communicate in a way they understand
Application layer payload: The entire contents of whatever data applications want to send to each other
CLOSE: A connection state that indicates that the connection has been fully terminated, and that no further communication is possible
CLOSE_WAIT: A connection state that indicates that the connection has been closed at the TCP layer, but that the application that opened the socket hasn't released its hold on the socket yet
Connection-oriented protocol: A data-transmission protocol that establishes a connection at the transport layer, and uses this to ensure that all data has been properly transmitted
Connectionless protocol: A data-transmission protocol that allows data to be exchanged without an established connection at the transport layer. The most common of these is known as UDP, or User Datagram Protocol
Data offset field: The number of the next expected segment in a TCP packet/datagram
Demultiplexing: Taking traffic that's all aimed at the same node and delivering it to the proper receiving service
Destination port: The port of the service the TCP packet is intended for
ESTABLISHED: Status indicating that the TCP connection is in working order, and both sides are free to send each other data
FIN: One of the TCP control flags. FIN is short for finish. When this flag is set to one, it means the transmitting computer doesn't have any more data to send and the connection can be closed
FIN_WAIT: A TCP socket state indicating that a FIN has been sent, but the corresponding ACK from the other end hasn't been received yet
Firewall: It is a device that blocks or allows traffic based on established rules
FTP: An older method used for transferring files from one computer to another, but you still see it in use today
Handshake: A way for two devices to ensure that they're speaking the same protocol and will be able to understand each other
Instantiation: The actual implementation of something defined elsewhere
Listen: It means that a TCP socket is ready and listening for incoming connections