TCP Segment format:
The figure below show the TCP segment format. It show the different
field within the TCP header.
bit 0 bit 16 bit 32
------------------------------------------------------------------
Source port (16) Destination port (16)
________________________________________________________________
Sequence number (32)
_________________________________________________________________
Acknowledgement number (32)
_________________________________________________________________
Header Reserved Code Bits Window (16)
Length (6) (6)
(4)
______________________________________________________________
Checksum (16) Urgent (16)
________________________________________________________________
Options (0 or 32 if any)
_________________________________________________________________
Data (varies)
_________________________________________________________________
1 comment:
The TCP header is 20 Bytes long. Here's an explanation of the
different fields.
Source Port - Is the port number of the host sending the data.
Destination Port - Is the port number of the application requested
on the destination host.
Sequence number - Puts the data back in the correct order or
retransmits
missing or damaged data.
Acknowledgement number - Defines which TCP octet is expected next.
Header length (HLEN) - Defines the number of 32 bit words in the
header.
Reserved - Always set to 0
Code bits - Control functions set up and terminate a session.
Window - Is the window size the sender is willing to accept, in octets.
Checksum - Is the CRC.
Urgent Pointer - Indicates the end of urgent data.
Option - Sets the maximum TCP segment size to either 0 or 32 bits, if
any.
Data - The data sent.
It is important to note that TCP and UDP use port numbers for
communication.
Well known port numbers are below 1024.
Here are some well known port numbers:
FTP 21
TELNET 23
DNS 53
TFTP 69
POP3 110
NEWS 144
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