Transmission Control Protocol Three-way handshake TCP Frame
TCP 3 Way Handshake Host 1 starts off with an initial SYN (Synchronize) packet. Host 1 includes the starting sequence number that it has chosen (123) Host 2 Acknowledges (ACKs) that sequence number +1, to indicate that it is expecting the next byte. It sends a SYN of its own with an initial sequence number of “543”. Host 1 Acknowledges the SYN from Host 2, sending an ACK #544, indicating that it is ready to receive the next byte. Connection now established
Transmission Control Protocol
Source/Destination Port Source Port/Destination Port: A port (socket/session) Virtual circuit between two communicating processes on two different computers or devices TCP ports from 0 to 1023, also called "well-known ports," are assigned specific tasks for compatibility. One port may communicate about the network status, while another port communicates about e-mail or file transfers. The source port is the port on the sending device. The destination port is a TCP port on a receiving device that corresponds with the source port on the sending device.
Sequence Number Sequence number: Each frame in a transmission is assigned a 32-bit sequence number, which enables TCP to ensure that all frames are received. Also used to identify duplicate frames, and to place frames back in the correct order
Acknowledgment Number Acknowledgment number: After checking the sequence number, TCP sends back the acknowledgment number, showing that the frame was received If the acknowledgment number is not sent back, the frame is retransmitted.
Offset/Flags Offset or header length: The offset value indicates the length of the header, so that the start of the data portion of the frame can be quickly determined. Flags/control: Two of the flags in this frame area are used to show the beginning (SYN) and the end (FIN) of the complete data stream Other flags are for control information. For example: reset the connection show that the urgent pointer field is in effect
Window Window: This information works in conjunction with flow control. The number of bytes that can be transmitted before the sender receives an acknowledgment of receipt. When the window size is reached, flow control is turned on to stop transmission until acknowledgment is received. For example, if the window size is 64 bytes, then flow control is turned on when 65 bytes have been transmitted without an acknowledgment being returned to the sender. When the network is slow because of heavy traffic, the window size can be increased so that flow control is not turned needlessly.
Checksum Checksum: The checksum is a 16-bit cyclic redundancy check (CRC) that is computed by adding the length of all header fields plus the length of the data payload field (the sum of all fields in the TCP segment). Placed in the frame by the sending station. The recipient also calculates the checksum and compares its calculation with the value in the checksum field. If they are different, the frame is discarded, and the receiving station requests that the frame be sent again.
Urgent Pointer, Options, Payload Urgent pointer: This header field provides a warning to the receiver that urgent data is coming, and points to the end of the urgent data within the sequence of the transmission of frames. Its purpose is to provide advance information about how much data is still to be received in a connected sequence of one or more frames. Options: This area in the frame can hold additional information and flags about a transmission. Padding: The padding area is used when there is too little or no optional data to complete the required header length, which must be divisible by 32.