Furuno Felcom 250 Service Manual page 418

TCP (Transmission Control Protocol)
A standard protocol used in the Internet, and it is equivalent to the transport layer for
the OSI reference model. It works as a bridge between the IP for the network layer
and the protocol in the session layer and above (HTTP, FTP, SMTP, POP, etc.). TCP
is used as the protocol for the transport layer in the Internet; the forwarding speed for
UDP is faster but its reliability is low whereas the forwarding speed for TCP is
slower but its reliability is high.
The TCP Header Format is as described below;
0 4
Source Port
Data Offset Reserved
Checksum
Source Port
16 bit; the sender's port number
Destination Port
16 bit; the recipient port number
Sequence Number
32 bit; the sequence number for the divided TCP segment
Acknowledgement Number
32 bit; the sequence number for the data which is to be sent next by the sender is stored in the
ACK segment acknowledgement field for data receipt response.
The sender will be able to see that the data sent previously had been delivered without fail if this
number and the sequence number for the data which is to be sent next match with each other.
Data Offset
4 bit; the TCP header length is stored by the unit of 4 octets.
Reserved
6 bit; currently "0" is being stored. It can be used in the future when the system is expanded.
Code Bit
6 bit; each bit is used as a flag. URG, ACK, PSH, RST, SYN, FIN
URG (Urgent Flag)
When the bit is set to 1, it implies that there is a data requiring urgent processing
contained.
The data requiring urgent processing is shown in the urgent pointer field.
ACK (Acknowledgement Flag)
When the bit is set to 1, it implies that the acknowledgement field is valid. This flag
is to be set to "1" apart from the SYN segment which is required for establishing the
connection.
PSH (Push Flag)
The bit is set to "1": the received data will be immediately delivered to the upper
application.
The bit is set to "0": the received data is not immediately delivered to the upper
application, but is buffered.
RST (Reset Flag)
When the bit is set to 1, the connection will be forcibly disconnected.
10 16
Sequence Number
Acknowledgement Number
Code Bit
Options
AP4-10
Appendix 4. Abbreviations
Destination Port
Window
Urgent Pointer
Padding
31