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Chapter 5: Problem 33

One of the purposes of TIME_WAIT is to handle the case of a data packet from a first incarnation of a connection arriving very late and being accepted as data for the second incarnation. (a) Explain why, for this to happen (in the absence of TIME_WAIT), the hosts involved would have to exchange several packets in sequence after the delayed packet was sent but before it was delivered. (b) Propose a network scenario that might account for such a late delivery.

Short Answer

Expert verified
TIME_WAIT prevents late packets from a previous connection from being mistaken for packets of a new connection. A network delay, like routing loops, could cause such late delivery.

Step by step solution

01

- Understand TIME_WAIT

TIME_WAIT is a state in TCP that ensures all data packets from a terminated connection are removed from the network before the same port numbers can be re-used.
02

- Delayed Packet Issue

To understand the delayed packet issue, consider that if TIME_WAIT were absent, a data packet could be significantly delayed. If a new connection is established with the same endpoint before the old data packet is delivered, the old packet may be mistakenly processed as part of the new connection.
03

- Sequential Packet Exchange Requirement

For a late packet to be accepted by a new connection, several packets (e.g., SYN, ACK) must be exchanged to fully establish this new connection. These packets are essential for the TCP handshake and to ensure that the new connection is set up before the late packet arrives.
04

- Network Scenario for Late Delivery

A possible network scenario for late delivery could be a packet experiencing significant delay due to network issues like routing loops or congestion, resulting in it taking a longer path through the network and arriving after the new connection has been established.
05

- Conclusion

Without TIME_WAIT, the old packet from the previous connection could interfere with the data of the new connection, leading to data corruption or unexpected behavior.

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Key Concepts

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

TCP connection termination
When a TCP connection is terminated, it doesn't just end abruptly. The termination process ensures that both sides agree to end the communication and that all remaining data is sent and acknowledged. This involves a series of steps known as the four-way handshake:
  • The client sends a FIN (finish) packet.
  • The server responds with an ACK (acknowledgment) packet.
  • The server then sends its own FIN packet.
  • The client sends a final ACK packet.
Once this process is complete, the connection enters the TIME_WAIT state. The TIME_WAIT state ensures that any delayed packets from the old connection are not mistaken for data in a new connection. This state typically lasts for twice the maximum segment lifetime (MSL), a duration defined by the TCP specification to account for delayed packets in the network.
Delayed packet issue
Delayed packets can cause significant issues in TCP connections. If a packet from a previous connection arrives late and there is no TIME_WAIT state, it could be incorrectly interpreted as data for a new connection. This can happen due to several reasons:
  • Network congestion, where packets are stuck in buffers.
  • Routing changes that cause packets to take longer paths.
  • Temporary network failures causing packets to be held and then released.
When TIME_WAIT is in place, it ensures that the old connection's packets have ample time to clear the network before the same port numbers can be reused for a new connection, protecting data integrity and consistency.
Network routing loops
Network routing loops occur when packets are forwarded in the network endlessly because of incorrect routing table entries. These loops can cause delayed delivery of packets and are a common source of issues in networking. Here's how a routing loop may arise:
  • A routing protocol misconfiguration.
  • Temporary network topology changes.
  • Hardware or software failures in routing devices.
When a packet enters a routing loop, it circles the network, consuming bandwidth and increasing delay. The TIME_WAIT state helps mitigate the impact of any delayed packets resulting from routing loops, by ensuring that the connection has fully closed and that any straggling packets are no longer a concern.
TCP handshake process
The TCP handshake process is essential for establishing a reliable connection between two hosts in a network. Known as a three-way handshake, this sequence ensures both parties can communicate before any data is transferred. The process includes:
  • The client sends a SYN (synchronize) packet to the server.
  • The server responds with a SYN/ACK packet, acknowledging the receipt of the SYN.
  • The client sends an ACK packet back to the server, completing the handshake.
This process sets the stage for data transfer, with both hosts agreeing upon initial sequence numbers and readiness to communicate. This handshake is crucial for ensuring any data integrity and avoiding the metaphorical 'crossed wires' where data packets could be misinterpreted, especially in the presence of issues like delayed packets or network routing loops.

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Most popular questions from this chapter

Suppose BLAST runs over a 10-Mbps Ethernet, sending \(32 \mathrm{~K}\) messages. (a) If the Ethernet packets can hold 1500 bytes of data, and optionless IP headers are used as well as BLAST headers, how many Ethernet packets are required per message? (b) Calculate the delay due to sending a \(32 \mathrm{~K}\) message over Ethernet (i) directly (ii) broken into pieces as in (a), with one bridge Ignore propagation delays, headers, collisions, and interpacket gaps.

Suppose a client \(C\) repeatedly connects via TCP to a given port on a server \(S\), and that each time it is \(\mathrm{C}\) that initiates the close. (a) How many TCP connections a second can C make here before it ties up all its available ports in TIME_WAIT state? Assume client ephemeral ports are in the range 1024-5119, and that TIME_WAIT lasts 60 seconds. (b) Berkeley-derived TCP implementations typically allow a socket in TIME WAIT state to be reopened before TIME_WAIT expires, if the highest sequence number used by the old incarnation of the connection is less than the ISN used by the new incarnation. This solves the problem of old data accepted as new; however, TIME_WAIT also serves the purpose of handling late final FINs. What would such an implementation have to do to address this and still achieve strict compliance with the TCP requirement that a FIN sent anytime before or during a connection's TIME_WAIT receive the same response?

When TCP sends a \(\langle\) SYN, SequenceNum \(=x\rangle\) or \(\langle\) FIN, SequenceNum \(=x\rangle\), the consequent ACK has Acknowledgment \(=x+1\); that is, SYNs and FINs each take up one unit in sequence number space. Is this necessary? If so, give an example of an ambiguity that would arise if the corresponding Acknowledgment were \(x\) instead of \(x+1 ;\) if not, explain why.

Write a test program that uses the socket interface to send messages between a pair of Unix workstations connected by some LAN (e.g., Ethernet, ATM, or FDDI). Use this test program to perform the following experiments. (a) Measure the round-trip latency of TCP and UDP for different message sizes (e.g., 1 byte, 100 bytes, 200 bytes, ..., 1000 bytes). (b) Measure the throughput of TCP and UDP for 1-KB, 2-KB, 3-KB, ...,32-KB messages. Plot the measured throughput as a function of message size. (c) Measure the throughput of TCP by sending \(1 \mathrm{MB}\) of data from one host to another. Do this in a loop that sends a message of some size, for example, 1024 iterations of a loop that sends 1-KB messages. Repeat the experiment with different message sizes and plot the results.

Suppose we were to implement remote file system mounting using an unreliable RPC protocol that offers zero-or-more semantics. If a message reply is received, this improves to at-least-once semantics. We define read() to return the specified Nth block, rather than the next block in sequence; this way reading once is the same as reading twice and at-least-once semantics is thus the same as exactly once. (a) For what other file system operations is there no difference between at- leastonce and exactly once semantics? Consider open, create, write, seek, opendir, readdir, mkdir, delete (aka unlink), and rmdir. (b) For the remaining operations, which can have their semantics altered to achieve equivalence of at-least-once and exactly once? What file system operations are irreconcilable with at-least-once semantics? (c) Suppose the semantics of the rmdir system call are now that the given directory is removed if it exists, and nothing is done otherwise. How could you write a program to delete directories that distinguishes between these two cases?
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