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Chapter 5: Problem 5

When closing a TCP connection, why is the two-segment-lifetime timeout not necessary on the transition from LAST_ACK to CLOSED?

Short Answer

Expert verified
The 2MSL timeout is not needed because both sides have already closed the connection when transitioning from LAST_ACK to CLOSED.

Step by step solution

01

Understand TCP Connection Termination

TCP connection termination involves a sequence of steps that ensure both ends of the connection have successfully received all transmitted data. There are four key TCP states in this sequence: FIN_WAIT_1, FIN_WAIT_2, LAST_ACK, and CLOSED.
02

The Role of LAST_ACK state

In the LAST_ACK state, the side that sent the first FIN is waiting for an acknowledgment of its FIN from the other side. Once it receives this acknowledgment, it transitions to the CLOSED state.
03

Understanding Two-Segment Lifetime (2MSL)

The Two-Segment Lifetime (2MSL) is a timeout period that ensures all remaining segments in the network are invalid before a connection is completely closed.
04

Examine Transition from LAST_ACK to CLOSED

During the transition from LAST_ACK to CLOSED, the first side has already received acknowledgment of its sent FIN. This means both sides have mutually agreed to close the connection, making the 2MSL timeout unnecessary.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

TCP states
The transmission control protocol (TCP) operates in several distinct states during connection establishment, maintenance, and termination. These states ensure reliable communication and orderly connection teardown. When terminating a connection, some of the most critical states include:

* **FIN_WAIT_1:** When a side requests to end the connection by sending a FIN signal. It waits for the ACK of that FIN.
* **FIN_WAIT_2:** After receiving the ACK for its FIN, it moves to this state to wait for the FIN from the other side.
* **LAST_ACK:** After sending the last FIN acknowledgment, it waits for the final acknowledgment of the FIN it sent.
* **CLOSED:** The state where the connection is fully terminated.

Understanding these states helps in grasping the sequence and reason behind each step in the termination process.
Two-Segment Lifetime
The Two-Segment Lifetime (2MSL) is a critical concept in TCP that ensures all remaining data packets in the network are expired before the connection completely closes.

Why is 2MSL important?
* It allows for the retransmission of lost segments.
* It ensures that outdated segments do not cause confusion for future connections.

This timeout is typically calculated to be twice the maximum segment lifetime, which is an estimated time for a TCP segment to be alive in the network.
LAST_ACK state
In the LAST_ACK state, the side that initiated the connection termination waits for the acknowledgment of its FIN packet. Here鈥檚 how it works:

* The sender of the FIN packet waits to receive an ACK from the other side.
* Once the ACK is received, the sender transitions from LAST_ACK to CLOSED state.

This state is crucial as it ensures that both parties are synchronized and agree to close the connection, preventing any loss of data.
CLOSED state
The CLOSED state marks the end of the TCP connection. Once a connection transitions to CLOSED, all resources associated with that connection are released.

Key points about the CLOSED state:
* The connection termination is complete.
* No data transmission occurs.
* Both parties have acknowledged the termination.

It鈥檚 important to note that the transition from LAST_ACK to CLOSED does not require the 2MSL timeout. This is because the acknowledgement (ACK) of the FIN segment has already been received, ensuring the connection is properly synchronized and ready to close without risk of confusion from lingering packets.

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Most popular questions from this chapter

If a UDP datagram is sent from host \(\mathrm{A}\), port \(\mathrm{P}\) to host \(\mathrm{B}\), port \(\mathrm{Q}\), but at host \(\mathrm{B}\) there is no process listening to port \(Q\), then \(B\) is to send back an ICMP Port Unreachable message to A. Like all ICMP messages, this is addressed to A as a whole, not to port \(\mathrm{P}\) on \(\mathrm{A}\). (a) Give an example of when an application might want to receive such ICMP messages. (b) Find out what an application has to do, on the operating system of your choice, to receive such messages. (c) Why might it not be a good idea to send such messages directly back to the originating port \(\mathrm{P}\) on \(\mathrm{A}\) ?

Suppose a TCP connection, with window size 1, loses every other packet. Those that do arrive have RTT \(=1\) second. What happens? What happens to TimeOut? Do this for two cases: (a) After a packet is eventually received, we pick up where we left off, resuming with EstimatedRTT initialized to its pretimeout value and TimeOut double that. (b) After a packet is eventually received, we resume with TimeOut initialized to the last exponentially backed-off value used for the timeout interval. In the following four exercises, the calculations involved are straightforward with a spreadsheet.

Suppose an RPC request is of the form "Increment the value of field X of disk block \(\mathrm{N}\) by \(10 \%\)." Specify a mechanism to be used by the executing server to guarantee that an arriving request is executed exactly once, even if the server crashes while in the middle of the operation. Assume that individual disk block writes are either complete or else the block is unchanged. You may also assume that some designated "undo log" blocks are available. Your mechanism should include how the RPC server is to behave at restart.

If host \(\mathrm{A}\) receives two SYN packets from the same port from remote host \(\mathrm{B}\), the second may be either a retransmission of the original or else, if B has crashed and rebooted, an entirely new connection request. (a) Describe the difference as seen by host A between these two cases. (b) Give an algorithmic description of what the TCP layer needs to do upon receiving a SYN packet. Consider the duplicate/new cases above, and the possibility that nothing is listening to the destination port.

TCP is a very symmetric protocol, but the client/server model is not. Consider an asymmetric TCP-like protocol in which only the server side is assigned a port number visible to the application layers. Client-side sockets would simply be abstractions that can be connected to server ports. (a) Propose header data and connection semantics to support this. What will you use to replace the client port number? (b) What form does TIME_WAIT now take? How would this be seen through the programming interface? Assume that a client socket could now be reconnected arbitrarily many times to a given server port, resources permitting. (c) Look up the \(\mathrm{rsh} / \mathrm{rlogin}\) protocol. How would the above break this?
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