TCP Connection Termination Three-Way Handshake

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This lecture segment explains TCP connection termination using the three-way handshake, focusing on active and passive close. The slide shows a timing diagram with client and server transport layers exchanging FIN, FIN+ACK, and ACK segments. The instructor highlights that a FIN segment consumes one sequence number if it carries no data, using red circles and arrows to emphasize the active close on the client side and passive close on the server side. The sequence numbers progress from seq:x/ack:y to seq:y/ack:x+1 and finally seq:x+1/ack:y+1, illustrating how each side acknowledges the other's FIN and closes its direction of communication.

Chapters

1. 0:00 – 2:00 00:00-02:00

   The slide titled 'Connection Termination (Three-way handshaking)' introduces the termination process with a timing diagram showing Client and Server transport layers. The instructor circles 'Active close' on the client side in red, indicating where the client initiates termination by sending a FIN segment with seq:x/ack:y. The diagram shows the server responding with FIN+ACK (seq:y/ack:x+1), and the client acknowledging with ACK (seq:x+1/ack:y+1). Red arrows point to the seq field and FIN flag box, emphasizing that the client's close command triggers the first FIN segment.

2. 2:00 – 3:53 02:00-03:53

   The instructor underlines the bullet point stating 'The FIN segment consumes one sequence number if it does not carry data,' highlighting this key rule. Red curved arrows are drawn above the diagram to trace the flow of segments between client and server. The 'Passive close' label on the server side is shown in blue, contrasting with the red-circled 'Active close.' Two 'Connection closed' markers appear on the timeline, indicating when each side completes its termination. The instructor uses red ellipses to circle both the 'Active close' label and the FIN flag within segments, reinforcing which side initiates and how the flags propagate through the handshake.

The core teaching point is that TCP termination requires three segments: the initiating side sends FIN, the other responds with FIN+ACK, and the initiator acknowledges. The critical rule emphasized is that a FIN consumes one sequence number only when it carries no data, which affects how seq and ack values increment. Active close refers to the side that initiates termination (client in this example), while passive close is the responding side. The diagram's progression from seq:x to seq:x+1 and ack:y to ack:y+1 demonstrates how each FIN advances the sequence number by one, ensuring both directions of communication are properly closed before the connection is fully terminated.