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Definition

The Transmission Control Protocol (TCP) is one of the main protocols of the Internet protocol suite.

Header Format

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|          Source Port          |       Destination Port        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                        Sequence Number                        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                    Acknowledgment Number                      |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|  Data |           |U|A|P|R|S|F|                               |
| Offset| Reserved  |R|C|S|S|Y|I|            Window             |
|       |           |G|K|H|T|N|N|                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|           Checksum            |         Urgent Pointer        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                    Options                    |    Padding    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                             data                              |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Explanation of Header Fields

Source Port
16 bit source port number.
Destination Port
16 bit destination port number.
Sequence Number
The 32 bit sequence number of the first data octet in this segment
Acknowledgment Number
32 bits. If the ACK control bit is set this field contains the value of the next sequence number the sender of the segment is expecting to receive. Once a connection is established this is always sent.
Data Offset
4 bits. The number of 32 bit words in the TCP Header. This indicates where the data begins. The TCP header (even one including options) is an integral number of 32 bits long.
Reserved
6 bits. Reserved for future use. Must be zero.
Control Bits

6 bits (from left to right):

  • URG: Urgent Pointer field significant
  • ACK: Acknowledgment field significant
  • PSH: Push Function
  • RST: Reset the connection
  • SYN: Synchronize sequence numbers
  • FIN: No more data from sender
Window
16 bits. The number of data octets beginning with the one indicated in the acknowledgment field which the sender of this segment is willing to accept.
Checksum
16 bits. The checksum field is the 16 bit one's complement of the one's complement sum of all 16 bit words in the header and text.
Options
variable. Options may occupy space at the end of the TCP header and are a multiple of 8 bits in length. All options are included in the checksum. An option may begin on any octet boundary.
Padding
variable. The TCP header padding is used to ensure that the TCP header ends and data begins on a 32 bit boundary. The padding is composed of zeros.

Ports

16 bit number represents the port.

Port Service
20 FTP Data
21 FTP Control
23 Telnet
25 SMTP (outgoing mail)
53 DNS
80 HTTP
123 Network Time Protocol
220 IMAP (incoming mail)
443 HTTPS
546 DHCPv6 Client
547 DHCPv6 Server
989 FTP Data over TLS/SSL
990 FTP Control over TLS/SSL

References

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