TCP Packet Format:

TCP Packet Field Descriptions

• Source Port and Destination Port---Identifies points at which upper-layer source and destination processes receive TCP services.

• Sequence Number---Usually specifies the number assigned to the first byte of data in the current message. In the connection-establishment phase, this field also can be used to identify an initial sequence number to be used in an upcoming transmission.

• Acknowledgment Number---Contains the sequence number of the next byte of data the sender of the packet expects to receive.

• Header Length (hlen)---Indicates the number of 32-bit words in the TCP header. Also referred to as the Data Offset (4 bits).

• Reserved---Remains reserved for future use (6 bits).

• Flags---Carries a variety of control information, including the SYN and ACK bits used for connection establishment, and the FIN bit used for connection termination (6 bits).

• Window---Specifies the size of the sender's receive window (that is, the buffer space available for incoming data).

• Checksum---Indicates whether the header was damaged in transit.

• Urgent Pointer---Points to the first urgent data byte in the packet.

• Options---Specifies various TCP options.

• Data---Contains upper-layer information.