TCP Header

The TCP Header is 20 bytes in size and consists of many fields of data.The fields are:CWRConnection window reduced. Sent from the sender to the receiver to let it know the ECE was received and so it has slowed down.ECEUsed to signal congestion when ECN is used.URGUrgent pointer. Indicates a byte offset from the current sequence number, Where urgent data is to be found.ACKAcknowledgement number is valid. Set to 1 for nearly all packets, if it is 0 then there is no acknowledgement number set.PSHPUSHed dataRSTReset. Used to abruptly reset the connection.SYNUsed to establish connections by the 3-way handshakeFINUsed to flag a disconnect

Source/Destination portThis identifies the local end points of a connection.The TCP Port + Host IP address = 48bit unique end point.

Sequence numberSpecifies the current order segment.

Acknowledgement numberSpecifies the next in-order byte expected.

TCP Header lengthTells how many 32bit words are in the TCP header. This is because the options field is of variable length. So the receiver needs to know which part is the header!

4 bits unusedReserved for future use

CWRConnection window reduced. Sent from the sender to the receiver to let it know the ECE was received and so it has slowed down.

ECEUsed to signal congestion when ECN is used.

URGUrgent pointer. Indicates a byte offset from the current sequence number, Where urgent data is to be found.

ACKAcknowledgement number is valid. Set to 1 for nearly all packets, if it is 0 then there is no acknowledgement number set.

PSHPUSHed data

RSTReset. Used to abruptly reset the connection.

SYNUsed to establish connections by the 3-way handshake

FINUsed to flag a disconnect

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