Question

This chapter explains three sequences of state transitions during TCP connection teardown. There is a fourth possible sequence, which traverses an additional arc (not shown in Figure 5.7) from FIN_WAIT_1 to TIME_WAIT and labelled FIN + ACK/ACK. Explain the circumstances that result in this fourth teardown sequence.

Short answer

The fourth sequence happens when a FIN and an ACK are received together, avoiding the FIN_WAIT_2 state by transitioning directly from FIN_WAIT_1 to TIME_WAIT.

Step-by-step solution

- Understand TCP Connection Teardown

A TCP (Transmission Control Protocol) connection is typically terminated through a sequence of four-way handshakes utilizing FIN (Finish) and ACK (Acknowledgment) flags. These ensure that both ends of the connection agree to close the connection.

- Review the Fourth Sequence

The fourth sequence involves a direct transition from FIN_WAIT_1 to TIME_WAIT, bypassing the FIN_WAIT_2 state. This occurs when a FIN and ACK are received together, and an ACK is sent in response.

- Triggering Condition

This sequence happens when the other end of the connection sends its FIN concurrently with its ACK of the previously received FIN. Essentially, one side sends FIN and ACK at the same time, avoiding the intermediate FIN_WAIT_2 state.

- State Transition Explanation

Upon receiving the FIN and acknowledging it, the state changes directly from FIN_WAIT_1 (after sending its own FIN) to TIME_WAIT, indicating that the connection termination process has been accelerated.

Key concepts

TCP state transitions

When two devices communicate over the internet using TCP (Transmission Control Protocol), they go through several states. These states ensure that data is reliably sent and received. Each state corresponds to a step in establishing, maintaining, or terminating a connection. For example:

• **SYN_SENT**: Waiting for a matching connection request after having sent one.
• **ESTABLISHED**: The normal state for data transfer.
• **FIN_WAIT_1**: Waiting for a termination request from the remote TCP.

Think of these states as checkpoints in a well-choreographed dance, ensuring both devices are synchronized and data is safely transferred. When it comes to shutting down a connection, TCP also utilizes specific states to do so cleanly and properly.

FIN_WAIT_1 to TIME_WAIT

A typical TCP connection shutdown consists of several key states.

Under normal circumstances, after the first side initiates the connection termination by sending a FIN (Finish) flag, it transitions to the **FIN_WAIT_1** state. This state means the device is waiting for an acknowledgment of its FIN, and possibly another FIN from the other side.

• **FIN_WAIT_2**: After receiving an ACK for its FIN, the device waits for a FIN from the other side.
• **TIME_WAIT**: After successfully exchanging FINs and ACKs, the connection moves to TIME_WAIT, ensuring that the final acknowledgments are properly handled before fully closing.

However, there's a fourth sequence where the connection skips FIN_WAIT_2. When the other side sends a combined FIN and ACK, the local device, upon acknowledging, directly moves from **FIN_WAIT_1** to **TIME_WAIT**. This expedited transition happens because the combined message confirms both the acknowledgment and the termination request simultaneously.

FIN and ACK flags

In TCP communication, FIN (Finish) and ACK (Acknowledgment) flags are crucial for managing the connection lifecycle.

When a device sends a FIN flag, it indicates its intent to terminate the connection. The receiving device, upon acknowledging this request, sends back an ACK flag. Thereafter, it might send its own FIN flag to indicate its readiness to close the connection.

Consider the following:

• **FIN**: Request to terminate the connection.
• **ACK**: Acknowledgment of receiving a message.

In the case of the fourth sequence discussed earlier, the combined FIN+ACK flag serves a dual purpose. This combination expedites the teardown process by acknowledging the received FIN while simultaneously requesting termination. This efficient use of flags helps quicken the state transition and avoids the additional wait period seen in the standard sequence.

connection termination process

Terminating a TCP connection gracefully involves ensuring both parties agree to and properly close the connection. This four-way handshake process guarantees no data is lost and all acknowledgments are appropriately exchanged.

The steps generally are:

• **Initiator sends FIN**: Moves to FIN_WAIT_1, indicating it's ready to close the connection.
• **Receiver sends ACK**: Acknowledges the FIN, typically moving to CLOSE_WAIT.
• **Receiver sends FIN**: Indicates its intention to close, moving the initiator to FIN_WAIT_2 or, in the fast-track sequence, directly to TIME_WAIT.
• **Initiator sends ACK**: Acknowledges the final FIN, then enters TIME_WAIT.

During **TIME_WAIT**, the connection ensures that any delayed packets are properly received and ensures the connection is fully terminated on both ends. This crucial step in the graceful termination of a TCP connection prevents potential issues, like lost or duplicate packets, from affecting subsequent connections.