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Chapter 5: Problem 34

Propose an extension to TCP by which one end of a connection can hand off its end to a third host; that is, if \(\mathrm{A}\) were connected to \(\mathrm{B}\), and \(\mathrm{A}\) handed off its connection to \(\mathrm{C}\), then afterwards \(\mathrm{C}\) would be connected to \(\mathrm{B}\) and \(\mathrm{A}\) would not. Specify the new states and transitions needed in the TCP state transition diagram, and any new packet types involved. You may assume all parties will understand this new option. What state should A go into immediately after the handoff?

Short Answer

Expert verified
Introduce a HANDOFF packet. A enters HANDOFF_PENDING and then CLOSED, B enters AWAITING_NEW_CONNECTION and then ESTABLISHED, C enters ESTABLISHED.

Step by step solution

01

- Define the Handoff Process

To propose an extension to TCP that allows handoff, first define the process by which one end of a connection (A) can transfer the connection to a third host (C). This involves A notifying B and C of the handoff.
02

- Introduce a New Packet Type

Introduce a new packet type 'HANDOFF' which Host A will send to Host B and Host C. The HANDOFF packet will contain the information about the new host (C) which will take over the connection from Host A.
03

- Modify TCP State Transition Diagram

Modify the TCP state transition diagram to include a new state called 'HANDOFF_PENDING'. When Host A decides to hand off the connection, it transitions to this new 'HANDOFF_PENDING' state and sends the HANDOFF packet to Host B and C.
04

- New States and Transitions for Host A

For Host A, the new state 'HANDOFF_PENDING' should transition to 'CLOSED' once it receives acknowledgment from both Host B and Host C confirming the handoff.
05

- New States and Transitions for Host B

For Host B, upon receiving the HANDOFF packet, it should move to a new state called 'AWAITING_NEW_CONNECTION' and then transition to 'ESTABLISHED' once the connection with Host C is successfully re-established.
06

- New States and Transitions for Host C

For Host C, upon receiving the HANDOFF packet from Host A, it will transition to 'ESTABLISHED' state for managing data communication with Host B.
07

- Acknowledge the Handoff

Host B acknowledges the HANDOFF packet to Host A. Host A performs the necessary cleanup and transitions to 'CLOSED' state.

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Key Concepts

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

TCP state transition diagram
To understand TCP handoff extensions, we first need to dive into the TCP state transition diagram. This diagram maps out the various states a TCP connection can be in during its lifecycle, from initiation to termination. Normally, TCP connections go through states such as 'LISTEN', 'SYN_SENT', 'SYN_RECEIVED', 'ESTABLISHED', 'FIN_WAIT', and 'CLOSED'. Each state represents a specific phase in the connection's lifecycle.

With the new handoff extension, there will be additional states and transitions. When Host A decides to transfer its connection to Host C, it enters a new state called 'HANDOFF_PENDING'. During 'HANDOFF_PENDING', Host A notifies both Host B and Host C about the handoff. Once acknowledgments from both Host B and Host C are received, Host A will transition to 'CLOSED'.

For Host B, upon receiving the HANDOFF packet, it shifts to 'AWAITING_NEW_CONNECTION' and then to 'ESTABLISHED' after the connection with Host C is set up. Similarly, Host C, upon receiving the HANDOFF packet, directly transitions to the 'ESTABLISHED' state to manage data communication with Host B.
connection handoff process
The connection handoff process is crucial for implementing the TCP extension to allow one host to transfer its connection to another. This involves several key steps:
  • Step 1: Host A, which wants to hand off the connection, decides to initiate the handoff process.
  • Step 2: Host A sends a new type of packet called 'HANDOFF' to both Host B (the current communication partner) and Host C (the host to which the connection will be handed off).
  • Step 3: Once Host B receives the HANDOFF packet, it moves to 'AWAITING_NEW_CONNECTION' state, waiting for Host C to establish a new connection.
  • Step 4: Host C, upon receiving the HANDOFF packet, transitions to 'ESTABLISHED' state to interact with Host B.
  • Step 5: Host B and Host C confirm the new connection, and Host B acknowledges receipt of the HANDOFF packet to Host A.
  • Step 6: Upon receiving the acknowledgment from Host B, Host A transitions to the 'CLOSED' state, ending its role in the connection.

The key to a smooth handoff process lies in the correct implementation of new states and ensuring all hosts properly recognize and respond to the 'HANDOFF' packet.
new packet types
A new packet type, 'HANDOFF', is vital for facilitating the handoff process in the extended TCP protocol. This packet contains essential information about the transferring process. Here's how it works:
  • Host A initiates the handoff by crafting a 'HANDOFF' packet, which includes the details of the new host, Host C.
  • The 'HANDOFF' packet is sent to both Host B and Host C, informing them of the impending change.
  • Upon receiving the 'HANDOFF' packet, Host B prepares for the new connection by moving to 'AWAITING_NEW_CONNECTION' state, while Host C moves directly to 'ESTABLISHED' state for the new TCP session.

This 'HANDOFF' packet ensures all parties are aware of the change, facilitating a smooth transition and maintaining the integrity of the connection. The acknowledgment mechanisms built into TCP further ensure that the handoff is confirmed and complete before Host A transitions to the 'CLOSED' state.

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Most popular questions from this chapter

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.

TCP's simultaneous open feature is seldom used. (a) Propose a change to TCP in which this is disallowed. Indicate what changes would be made in the state diagram (and if necessary in the undiagrammed event responses). (b) Could TCP reasonably disallow simultaneous close? (c) Propose a change to TCP in which simultaneous SYNs exchanged by two hosts lead to two separate connections. Indicate what state diagram changes this entails, and also what header changes become necessary. Note that this now means that more than one connection can exist over a given pair of \langlehost, port)s. (You might also look up the first "Discussion" item on page 87 of Request for Comments \(1122 .\) )

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.

Diagnostic programs are commonly available that record the first 100 bytes, say, of every TCP connection to a certain (host, port). Outline what must be done with each received TCP packet, P, in order to determine if it contains data that belongs to the first 100 bytes of a connection to host HOST, port PORT. Assume the IP header is P.IPHEAD, the TCP header is P.TCPHEAD, and header fields are as named in Figures \(4.3\) and 5.4. Hint: To get initial sequence numbers (ISNs) you will have to examine every packet with the SYN bit set. Ignore the fact that sequence numbers will eventually be reused.
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