ACK A control flag specifying that the Acknowledgment Number field in the TCP header is significant.
Acknowledgment Number field A field in the TCP header specifying the next sequence number the computer is expecting to receive. The acknowledgment number, in effect, acknowledges the receipt of all sequenced bytes prior to the byte specified in the acknowledgment number.
Active open A state in which TCP is attempting to initiate a connection.
Checksum A 16-bit calculated field used to ensure detection of corrupted datagrams.
connection-oriented protocol A protocol that manages communication by establishing a connection between the communicating computers.
connectionless protocol A protocol that transmits data without establishing a connection with the remote computer.
Control flag A 1-bit flag with special information about a TCP segment.
demultiplexing Directing a single input to several outputs.
destination port The TCP or UDP port number of the application on the destination machine that will be the recipient of the data in a TCP segment or UDP datagram.
FIN A control flag used in the process of closing a TCP connection.
firewall A device that protects a network from unauthorized Internet access.
initial sequence number (ISN) A number that marks the beginning of the range of numbers a computer will use for sequencing bytes transmitted through TCP.
multiplexing Combining several inputs into a single output.
Passive open A state in which the TCP port (usually a server application) is ready to receive incoming connections.
port An internal address that provides an interface from an application to a Transport layer protocol.
pseudo-header A structure derived from fields from the IP header that is used to calculate the TCP or UDP checksum and to verify that the datagram has not been delivered to the wrong destination due to alteration of information in the IP header.
resequencing Assembling incoming TCP segments so that they are in the order in which they were actually sent.
segment A package of TCP data and header information.
sequence number A unique number associated with a byte transmitted through TCP.
sliding window A window of sequence numbers that the receiving computer has authorized the sending computer to send. The sliding window flow control method is the method used by TCP.
socket The network address for a particular application on a particular computer, consisting of the computer's IP address followed by the port number of the application.
source port The TCP or UDP port number of the application sending a TCP segment or UDP datagram.
stream-oriented input Continuous (byte-by-byte) input, rather than input in predefined blocks of data.
SYN A control flag signifying that sequence number synchronization is taking place. The SYN flag is used at the beginning of a TCP connection as part of the three-way handshake.
TCP A reliable connection-oriented Transport protocol in the TCP/IP suite.
three-way handshake A three-step procedure that synchronizes sequence numbers and begins a TCP connection.
UDP A nonreliable connectionless transport protocol in the TCP/IP suite.
well-known port Predefined standard port numbers for common applications. Well-known ports are specified by the ICANN.
