Скачать или смотреть TCP Header Explained | TCP Header Fields | Transport Layer | TCP Header Flags

We use many application programs like email or any webpage and these applications programs generates data. This data is converted into binary and handed over to the Transport layer.

TCP at transport layer takes that byte-wise and form data part and adds header to it and we call it segment.


In the header part of segment, we have source port address and destination address. These 16 bits fields help differentiate among multiple application programs.


Sequence number: Sequence number is the number assigned to each byte to be transmitted by the sending node
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Acknowledgement number is the number sent by the receiving node calculated by: adding 1 in the the sequence number of the byte received from the sender.
FIN field indicates No more data from sender
SYN if set then sequence number is the initial sequence number
RST if set then it intimates to resent the connection
PSH Used to push the data to the receiving application
ACK indicates that the acknowledgement field is significant
URG indicates that the urgent field is significant
Window size indicates the number of bytes the receiver node is willing to receive. It is used for flow control.
Urgent pointer indicates where exactly is the end of the urgent data being sent.
Checksum field helps in checking the errors occurred during transmission.
Reserved bits are for future use and they are set to 0
HLEN specifies the size of the TCP header in 32 bits word (min is 5 words and maximum is 15 words)
Options:
Maximum segment size: Indicates how large the segment can be during transmission
Timestamps: the maximum time a sending nodes waits before retransmission when no acknowledgement is received.
Padding: Padding are extra zero bits added in the header to make the size of the header an even multiple of 32 bits.
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