TCP Header 报文信息

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TCP协议头包含源端口、目标端口、序列号、确认号、偏移量、保留位、标志位、窗口大小、校验和、紧急指针和选项等字段。这些字段用于标识发送者和接收者、管理数据传输顺序、确认接收、重置连接、建立和结束连接以及控制数据紧迫性。此外，还涉及TCP段的缓冲、错误检测及最大段尺寸的定义。

TCP Header 报文信息

The individual components or fields of the header of the TCP protocol have the following meaning:

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.