TCP Segment Structure

TCP segment structure

TCP header is typically 20 bytes
UDP header is typically 8 bytes

1. Source and destination port numbers, which are used for
   multiplexing/demultiplexing data from/to upper-layer applications
2. Checksum field
3. The 32-bit sequence number field and the 32-bit acknowledgment number field are used by the TCP sender and receiver in implementing a reliable data transfer service
4. The 16-bit receive window field is used for flow control. It is used to indicate the number of bytes that a receiver is willing to accept.
5. The 4-bit header length field specifies the length of the TCP header in 32-bit words
6. The optional and variable-length options field is used when a sender and receiver negotiate the maximum segment size (MSS) or as a window scaling factor for use in high-speed networks. A time-stamping option is also defined. (RFC 854 & RFC 1323)
7. The flag field contains 6 bits. The ACK bit is used to indicate that the value carried in the acknowledgment field is valid; that is, the segment contains an acknowledgment for a segment that has been successfully received. The RST, SYN, and FIN bits are used for connection setup and teardown, as we will discuss at the end of this section. Setting the PSH bit indicates that the receiver should pass the data to the upper layer immediately. Finally, the URG bit is used to indicate that there is data in this segment that the sending-side upper-layer entity has marked as “urgent.” The location of the last byte of this urgent data is indicated by the 16-bit urgent data pointer field. TCP must inform the receiving-side upper-layer entity when urgent data exists and pass it a pointer to the
   end of the urgent data. (In practice, the PSH, URG, and the urgent data pointer are not used. However, we mention these fields for completeness.)

Typically, the options field is empty, so that the length of the typical TCP header is 20 bytes.

Reference: Computer Networking: A Top-Down Approach 6th E