Question

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

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

- 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.

- 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.

- 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.

- 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.

- 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.

- 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.

- Acknowledge the Handoff

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

Key concepts

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.