TCP

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

V4