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Chapter 12: Problem 12

Suppose A connects to B via TCP, and sends the message 鈥淎ttack at noon鈥�, followed by FIN. Upon receiving this, B is sure it has received the entire message. (a). What can A be sure of upon receiving B鈥檚 own FIN+ACK? (b). What can B be sure of upon receiving A鈥檚 final ACK? (c). What is A not absolutely sure of after sending its final ACK?

Short Answer

Expert verified
A can be sure that B has received A's data and FIN signal and has no more data to send. B can confirm that A has received B's FIN, completing B鈥檚 closing of the connection. However, A cannot be completely sure that B has received the final ACK.

Step by step solution

01

Answer part (a)

When A receives B鈥檚 own FIN+ACK, it can be sure that B has received A鈥檚 earlier data and the FIN signal, and that B has no more data to send to A.
02

Answer part (b)

Upon receiving A鈥檚 final ACK, B can be sure that A has received B's FIN signal. This completes B鈥檚 closing of the connection.
03

Answer part (c)

After sending its final ACK, A cannot be absolutely sure that B has received the final ACK. There could be a chance that B might not have received the final ACK due to transmission errors or network issues. If B didn鈥檛 receive the final ACK, it might retransmit its FIN signal, waiting for the final ACK from A.

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Key Concepts

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

Understanding FIN+ACK in TCP Termination
When two devices communicate over the Internet using the TCP (Transmission Control Protocol), it's crucial to end the connection smoothly. This ensures that both parties have completed their data transmission and are ready to finish the session. The process of terminating a TCP connection involves several steps, and one of the key messages in this process is the FIN+ACK.

The acronym stands for "Finish" and "Acknowledge," signaling the end of data transmission from one side. When A sends a FIN packet, it indicates that it has completed sending data. B responds with a FIN+ACK to confirm receipt of A's message and to notify that it has no more data to send.
  • The FIN indicates a request to terminate the session.
  • The ACK acknowledges receipt of the FIN.
This handshake ensures that both parties agree to close the connection properly, thus preventing data loss.
Role of ACK in Ensuring Complete Communication
The ACK, short for "Acknowledgement," plays a critical role in both maintaining and closing TCP connections. During the termination phase, after a device (say B) sends its own FIN+ACK, it waits for an ACK from the other side (A). When B receives the ACK, it concludes that A has successfully received its FIN request and is agreeing to terminate the communication.

In TCP communication, every sent packet is followed by an ACK from the receiver to ensure that the data has been successfully delivered. If the ACK is not received back within a certain period, the sender will retransmit the packet. This reliable transmission mechanism is what makes TCP a robust protocol for data communication on the Internet.
Transmission Errors: A Potential Hiccup
In digital communication, transmission errors can occur due to several factors, such as electrical interference, noise, or faulty network equipment. These errors can affect how messages are transmitted and acknowledged within a TCP connection. When dealing with TCP connection termination, it鈥檚 important to understand that even after sending an ACK, the sender cannot be completely sure that it was received on the other side.

Transmission errors may cause an ACK packet to be lost in transit or corrupted, leading to re-transmissions. If one side does not receive an expected acknowledgement, it may keep sending its FIN message, waiting for confirmation. The robust design of TCP handles these errors gracefully by incorporating timeouts and retransmission strategies.
Navigating Network Issues
Beyond errors in transmission, there are broader network issues that can complicate TCP communications. These might include network congestion, routing problems, or disruptions in service. Such issues can lead to delays or even the loss of packets entirely.

When network problems occur during the TCP termination process, they can prevent one side from receiving the final ACK, prompting retransmissions to ensure that the connection is properly closed.
  • Network congestion can slow down packet delivery.
  • Routing issues might direct packets incorrectly, delaying communication.
  • Service disruptions can temporarily interrupt the communication line.
Understanding these potential network issues helps in diagnosing and troubleshooting problems in TCP connections, ensuring effective and smooth communication termination.

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Most popular questions from this chapter

A user moves the computer mouse and sees the mouse-cursor鈥檚 position updated on the screen. Suppose the mouseposition updates are being transmitted over a TCP connection with a relatively long RTT. The user attempts to move the cursor to a specific point. How will the user perceive the mouse鈥檚 motion (a). with the Nagle algorithm (b). without the Nagle algorithm

Exercises are given fractional (floating point) numbers, to allow for interpolation of new exercises. Exercise 4.5 is distinct, for example, from exercises 4.0 and 5.0. (a) Suppose you see multiple connections on your workstation in state FIN_WAIT_1. What is likely going on? Whose fault is it? | (b). What might be going on if you see connections languishing in state FIN_WAIT_2?

Host A connects to the Internet via Wi-Fi, receiving IPv4 address \(10.0 .0 .2\), and then opens a TCP connection conn 1 to remote host B. After conn 1 is established, A's Ethernet cable is plugged in. A's Ethernet interface receives IP address \(10.0 .0 .3\), and A automatically selects this new Ethernet connection as its default route. Assume that A now starts using \(10.0 .0 .3\) as the source address of packets it sends as part of conn. (contrary to \(\underline{\text { RFC } 1122}\) ). Assume also that A's TCP implementation is such that when a packet arrives from \(\left(\mathrm{B}_{\mathrm{IP}}, \mathrm{B}_{\text {pon }}\right\rangle\) to \(\left\langle\mathrm{A}_{\mathrm{IP}}, \mathrm{A}_{\text {port }}\right\rangle\) and this socketpair is to be matched to an existing TCP connection, the field \(\mathrm{A}_{\mathrm{IP}}\) is allowed to be any of A's IP addresses (that is, either \(10.0 .0 .2\) or \(10.0 .0 .3\) ); it does not have to match the IP address with which the connection was originally negotiated. (a). Explain why conn 1 will now fail, as soon as any packet is sent from \(\mathrm{A}\). Hint: the packet will be sent from \(10.0 .0 .3 .\) What will B send in response? In light of the second assumption, how will A react to B's response packet? (The author regularly sees connections appear to fail this way. Perhaps some justification for this behavior is that, at the time of establishment of \(\operatorname{con} n 1\), A was not yet multihomed.) (b). Now suppose all four fields of the socketpair \(\left(\left(B_{I P}, B_{\text {port }}\right\rangle,\left\langle A_{I P}, A_{\text {port }}\right\rangle\right)\) are used to match an incoming packet to its corresponding TCP connection. The connection conn1still fails, though not as immediately. Explain what happens. See also 7.9.5 ARP and multihomed hosts, 7 IP version 4 exercise 11.0, and \(\underline{9 \text { Routing-Update Algorithms exercise 13.0. }}\)

Suppose you are creating software for a streaming-video site. You want to limit the video read-ahead 鈥� the gap between how much has been downloaded and how much the viewer has actually watched 鈥� to approximately 1 MB; the server should pause in sending when necessary to enforce this. On the other hand, you do want the receiver to be able to read ahead by up to this much. You should assume that the TCP connection throughput will be higher than the actual video-data-consumption rate. (a). Suppose the TCP window size happens to be exactly 1 MB. If the receiver simply reads each video frame from the TCP connection, displays it, and then pauses briefly before reading the next frame in accordance with the frame rate, explain how the flow-control mechanism of 12.17 TCP Flow Control will achieve the desired effect. (b). Applications, however, cannot control their TCP window size. What support would you have to add to the video-transfer application to allow it to read ahead by 1 MB but not to exceed this? Hint: both client and server sides of the application will have to implement something to enable this feature.

Suppose you have fallen in with a group that wants to add to TCP a feature so that, if A and B1 are connected, then B1 can hand off its connection to a different host B2; the end result is that A and B2 are connected and A has received an uninterrupted stream of data. Either A or B1 can initiate the handoff. (a). Suppose B1 is the host to send the final FIN (or HANDOFF) packet to A. How would you handle appropriate analogues of the TIMEWAIT state for host B1? Does the fact that A is continuing the connection, just not with B1, matter? (b). Now suppose A is the party to send the final FIN/HANDOFF, to B1. What changes to TIMEWAIT would have to be made at A鈥檚 end? Note that A may potentially hand off the connection again and again, eg to B3, B4 and then B5.
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