cisco certification 2019

cisco certification 2019.Figure 1-81 Format of a TCP HeaderThe following shows the main fields of the TCP header.Source, destination port number field: contains the source, . destination port number. These two fields should be considered the most important fields in the TCP header and can be used to correctly identify TCP connections and TCP sockets.Sequence number (32 bits) field: When the SYN marker of the TCP message segment is in position 1, the value of this field is the initial sequence number (ISN) of this TCP connection, and the sequence number of the first byte of data passed through this TCP connection is ISN+1. When the SYN marker of the TCP message segment is in position 0, the value of this field is the sequence number of the first byte of data of this TCP message segment.The Acknowledgement Number (32-bit) field: Once the TCP connection is properly established, the ACK flag bit of the TCP message segment will be set to 1. The Acknowledgement Number field value is used to indicate the sequence number of the next byte of data that the sender (of this TCP message segment) expects to receive on net.Control tag bits (9 bits, 1 bit for each tag): uses include congestion notification and carrying TCP tags..ECN (Explicit Congestion Notification) tag bits (3 bits): these 3 tag bits (NS, CWR and ECE) relate to the latest definition of congestion notification functionality in RFC3168 and RFC3540 (following the general TCP protocol standard RFC793). This feature supports end-to-end network congestion notification with the purpose of avoiding droppedpackets (packet loss can be considered as a sign of network congestion).. The various TCP marker bits are shown below. > URG: Urgent flag bit, used to indicate urgent pointer data that should be handled specially. > ACK: Acknowledgement marker bit, which should be set to 1 after TCP establishes a connection.PSH: Push flag bit, indicating that data should be pushed directly to the application. RST: reset flag bit, used to reset the TCP connection. >SYN: Synchronization (sequence number) flag bit, used to establish a TCP connection, should only be set to 1 in the header of the first TCP message segment sent by both hosts. >FIN: Termination flag bit, indicating that the TCP data sender has no more data to send. Window (16-bit) field: used to indicate the number of bytes of data that can be received by the sender of a TCP message segment.