The Transmission Control Protocol (TCP), defined in the RFC793, prepares a sure transfer layer and virtual connections to application.

4	8	16	32 bits
Source port			Destination port
Sequence number
Acknowledgement number
Offset	Resrvd  |N|C|E|U|A|P|R|S|F		Window
Checksum			Urgent pointer
Options + Padding
Data

Source port	port-number of the sending application
Destination port	port-number of the receiving application
Sequence number	position of the first byte in the data stream
Acknowledgement number	Acknowledgment of all gone ahead bytes (only validly with ACK-Flag)
Offset	Length of the header in units of 32 bits (min. 5)
Resrvd	reserve for future use (must be ZERO)
Flags:	N - NS Nonce Sum C - CWR Congestion Window Reduce E - ECE Explicit Congestion Notification Echo U - URG Urgent pointer is valid A - ACK Acknowledgement number is valid P - PSH push, immediate relegation of the segment at the application layer R - RST reset, connection-reset S - SYN synchronize, connection-call and with it synchronization of the used Sequence number F - FIN final, connection-close
Window	Number of the bytes, that the recipient - beginning with the current Acknowledgement number - is ready to receive
Checksum	Checksum over the header and data plus a so-called pseudo header to the protection against lacking segments (Source IP-Adresse, Destination IP-Adresse, Transport protocol, TCP-length)
Urgent pointer	Pointer at end of urgent data (offset of the first byte behind the urgent data, offset covered on the Sequence number)
Options + Padding	optional parameters: Kind Length Meaning Reference 0 - End of Option List [RFC793] 1 - No-Operation [RFC793] 2 4 Maximum Segment Size [RFC793] 3 3 WSOPT - Window Scale [RFC1323] 4 2 SACK Permitted [RFC1072] 5 N SACK [RFC1072] 6 6 Echo (obsoleted by option 8) [RFC1072] 7 6 Echo Reply (obsoleted by option 8) [RFC1072] 8 10 TSOPT - Time Stamp Option [RFC1323] 9 2 Partial Order Connection Permitted [RFC1693] 10 5 Partial Order Service Profile [RFC1693] 11 CC [Braden] 12 CC.NEWS [Braden] 13 CC.ECHO [Braden] 14 3 TCP Alternate Checksum Request [RFC1146] 15 N TCP Alternate Checksum Data [RFC1146] 16 Skeeter [Knowles] 17 Bubba [Knowles] 18 3 Trailer Checksum Option [Subbu & Monroe] plus filling bytes on 32-bit-borders