Structure of TCP/UDP header

Structure of the TCP header

　To understand TCP communication, it is necessary to understand what is contained in the TCP header. TCP establishes a connection prior to communication, and performs acknowledgment and flow control, so the TCP header structure is much more complicated than the UDP packet structure. A typical TCP header size is 20 bytes large.

field	explanation
Source port number (16 bits)	A number that identifies the sending application. Unlike UDP, TCP cannot have a source port number of 0. Available from 1 to 65535.
Destination port number (16 bits)	A number that identifies the destination application. Available from 1 to 65535. *0 is reserved
Sequence number (32 bits)	Number for ordering data to be sent. Increment the sequence number by 1 for each byte of data sent. 2 If it exceeds 32 , repeat the same number again.
Acknowledgment number (32 bits)	A field that indicates how much of the received data has been received by byte position. Returns the sequence number + 1 of the received data position. The ACK number field is valid only when the ACK flag is ON.
Header length (4 bits)	A field that indicates where the TCP data begins. Since the data part immediately follows the TCP header, it can be considered as the size of the TCP header.
URG (1 bit)	URG ... urgent: A flag indicating that urgent urgent data is included. Default value is 0, 1 turns ON. Not used much.
ACK (1 bit)	ACK --- acknowledge A flag indicating that a valid ACK number is included in the TCP header. All other TCP packets have the ACK flag turned ON, except for the very first one during the TCP 3-way handshake.
PSH (1 bit)	PSH ・・・ A flag for requesting that the received push data be delivered to the application immediately. Telnet turns this flag ON because buffering can degrade responsiveness.
RST (1 bit)	RST ・・・ reset A flag that is set when you want to interrupt or reject a TCP connection. By sending a TCP packet with the RST flag ON, the current TCP connection can be forcibly terminated.
SYN (1 bit)	SYN --- synchronize At the start of open processing at the time of TCP 3-way handshake, each side turns ON the SYN flag and synchronizes the ACK number. It is not set in subsequent packets.
FIN (1 bit)	FIN - flag set to terminate the finis TCP connection. A TCP connection is terminated when a FIN is sent from both sides.
Window size (16 bits)	A field used to convey the receiver's window size. The unit is bytes, and the maximum is 65535 bytes. 0 indicates that no data can be received.
Checksum (16 bits)	A field containing test data for checking the integrity of TCP packets.
Urgent pointer (16 bits)	Valid only if the URG flag is 1. Represents the location of urgent data.
option	A variable-length field used to set characteristics of a TCP connection. It is used for exchanging MSS. Padding (0) at the end if necessary to make it a multiple of 32 bits.
data	Data part of TCP. In some cases, packets are sent with only TCP headers and no data to prevent the TCP connection from timing out and being closed.

Structure of the UDP header

　UDP has the advantage of being more real-time than TCP because there are no acknowledgments, retransmissions, or congestion control that impair real-time. UDP headers are very simple compared to TCP headers. It only has 8 bytes.

field	explanation
Source port number (16 bits)	A number that identifies the sending application. For UDP packets that do not request a reply, set the source port number to 0. Available from 0 to 65535.
Destination port number (16 bits)	A number that identifies the destination application. Available from 0 to 65535.
Header length (4 bits)	A field that represents the length of the UDP packet. Stores the number of bytes plus the length of the data part sent by UDP.
Checksum (16 bits)	A field containing inspection data for checking the integrity of UDP packets. The checksum calculation uses 3 parts: UDP pseudo-header (12 bytes), UDP header (8 bytes), and UDP payload. *UDP pseudo-header is a virtual header that is used only when calculating the checksum.
data	Data part of UDP.