TCP Header 报文信息

Source port (16 bits): Identifies the port number of the sender.

Destination port (16 bits): Identifies the port number of receiver.

Sequence number (32 bits): The sequence number specifies the first byte of attached payload data or is sent when the connection is established or terminated. It is also used for validating and sorting the segments after transmission.

Acknowledgment number (32 bits): This field contains the next sequence number that the sender is expecting. An ACK flag (in the “Flags” field) is a precondition for validity.

Offset (4 bits): The “Offset” field specifies the length of the TCP header in 32-bit words to highlight the starting point of the payload data. This starting point varies from segment to segment due to the variable “Options” field.

Reserved (6 bits): Reserved for future use according to RFC 793 and not yet in use. This field must always be set to 0.
Flags (6 bits): The six possible single bits in the “Flags” field enable various TCP actions for organizing communication and data processing. The following flags are either set or not set for these actions:

URG: The "Urgent" flag signals to the TCP application that the payload data must be processed immediately up to the set Urgent pointer (see above).
ACK: In combination with the acknowledgment number, the ACK flag acknowledges the receipt of TCP packets. If the flag is not set, the confirmation number is also invalid.
PSH: The "Push" flag ensures that a TCP segment is immediately pushed through without first being sent to the buffer of the sender and receiver.
RST: If there is an error during transmission, a TCP packet with the RST flag set can be used to reset the connection.
SYN: Messages that have SYN flag set represent the first step of the three-way handshake, meaning they initiate the connection.
FIN: The "Finish" flag signals to the other party that a sender is ending the transmission.

Window size (16 bits): This field specifies the number of bytes that the sender is willing to receive.

Checksum (16 bits): The Transmission Control Protocol can reliably detect transmission errors. The checksum calculated from the header, the payload data and the pseudo-header is used for this purpose.

Urgent pointer (16 bits): The urgent pointer indicates the position of the first byte after the payload data that is to be processed urgently. As a result, this field is only valid and relevant if the URG flag is set.

Options (0 - 320 bits): Use the Options field if you want to include TCP functions that don’t belong in the general header, for example if you want to define the maximum segment size. The length of the options must always be a multiple of 32, otherwise zero-bit padding is required.