number (32 bits) If the ACK flag is set then the value of this field is the next sequence number that the sender of the ACK is expecting. This acknowledges receipt of all prior bytes (if any). The first ACK sent by each end acknowledges the other end's initial sequence number itself, but no data. Data offset (4 bits) Specifies the size of the TCP header in 32-bit words. The minimum size header is 5 words and the maximum is 15 words thus giving the minimum size of 20 bytes and maximum of 60 bytes, allowing for up to 40 bytes of options in the header. This field gets its name from the fact that it is also the offset from the start of the TCP segment to the actual data. Reserved (4 bits)For future use and should be set to zero. The last bit was previously defined by Historic RFC 3540 as NS flag (Nonce Sum). Flags (8 bits)Contains 8 1-bit flags (control bits) as follows: CWR (1 bit): Congestion window reduced (CWR) flag is set by the sending host to indicate that it received a TCP segment with the ECE flag set and had responded in congestion control mechanism. ECE (1 bit): ECN-Echo has a dual role, depending on the value of the SYN flag. It indicates: If the SYN flag is set (1), that the TCP peer is ECN capable. If the SYN flag is clear (0), that a packet with Congestion Experienced flag set (ECN=11) in the IP header was received during normal transmission. This serves as an indication of network congestion (or impending congestion) to the TCP sender. URG (1 bit): Indicates that the Urgent pointer field is significant ACK (1 bit): Indicates that the Acknowledgment field is significant. All packets after the initial SYN packet sent by the client should have this flag set. PSH (1 bit): Push function. Asks to push the buffered data to the receiving application. RST (1 bit): Reset the connection SYN (1 bit): Synchronize sequence numbers. Only the first packet sent from each end should have this flag set. Some other flags and fields change meaning based on this flag, and some are only valid when it is set, and others when it is clear. FIN (1 bit): Last packet from sender Window size (16 bits)The size of the receive window, which specifies the number of window size units