Question

A sender on a TCP connection that receives a 0 advertised window periodically probes the receiver to discover when the window becomes nonzero. Why would the receiver need an extra timer if it were responsible for reporting that its advertised window had become nonzero (i.e., if the sender did not probe)?

Short answer

The receiver needs an extra timer to periodically check and notify the sender when its advertised window becomes nonzero if the sender does not use probes.

Step-by-step solution

Understanding TCP Probing

In TCP communication, the sender uses probing to check if the receiver's advertised window has become nonzero when it receives a 0 advertised window. This ensures the sender can continue sending data as soon as the receiver can handle it.

Identifying the Role of Probes

Probes are sent periodically by the sender to see if the receiver is ready to accept more data. This is necessary because the receiving window might become nonzero at any point, and the sender needs to be notified.

Receiver's Responsibility Without Probes

If the receiver were responsible for notifying the sender whenever the advertised window becomes nonzero, the sender would not send probes. This shifts the responsibility entirely onto the receiver.

The Need for an Extra Timer

Without sender probes, the receiver must keep track of when the advertised window changes from 0 to nonzero. The receiver would need an extra timer to periodically check and report this change back to the sender, ensuring that any change is promptly communicated.

Conclusion

The extra timer on the receiver is essential if the sender does not probe because it ensures the sender is quickly informed when the receiver can handle more data. This avoids potential delays in communication.

Key concepts

TCP communication

TCP, or Transmission Control Protocol, is a fundamental protocol in the realm of networking.
It ensures reliable, ordered, and error-checked delivery of data between applications running on hosts communicating over an IP network. This protocol is crucial because it provides a connection-oriented communication channel.
A connection is established before data transfer begins and is maintained until all data have been transferred. This stability makes TCP a cornerstone of the Internet. Understanding how TCP works are crucial for grasping concepts like TCP probing, which ensure data can be sent efficiently.
TCP communication involves several processes:

• Establishing a connection using a three-way handshake.
• Reliably transmitting data packets.
• Ensuring data integrity and order.
• Terminating the connection when communication ends.

advertised window

The advertised window is a key concept in TCP communication.
It refers to the amount of data the receiver is willing to accept at any given time, as indicated by the receiver.
When communicating over TCP, the sender and the receiver need to manage data flow to ensure smooth and efficient data transmission. The advertised window plays a crucial role in this process.
The steps involved include:

• The receiver informs the sender about its buffer space (advertised window size) through the TCP header's Window size field.
• If the buffer space is full, the receiver advertises a window size of 0, signaling the sender to stop sending data temporarily.
• When space becomes available, the receiver updates the advertised window size to a nonzero value, indicating readiness to accept more data.

The advertised window ensures that data flow is regulated, preventing buffer overflow and data loss.

sender probes

In scenarios where the advertised window is 0, the sender needs to know when the receiver can accept more data.
This is where sender probes come into play.
The sender periodically sends small data segments or probes to the receiver to check if the advertised window has become nonzero. These probes are essential because they allow the sender to:

• Monitor the availability of buffer space on the receiver's end.
• Ensure that data transmission resumes as soon as possible.
• Maintain efficient communication without waiting for an indefinite period.

Probes are vital components of TCP communication as they help in minimizing delays and ensuring continuous data flow.
Without them, the sender would remain idle, potentially leading to inefficient data transfer and communication breakdowns.

receiver notification

Receiver notification becomes critical when discussing TCP communication without sender probes.
In this model, the receiver is responsible for informing the sender when the advertised window changes from 0 to nonzero.
For this to happen effectively, the receiver must:

• Continuously monitor its buffer space.
• Implement an extra timer to check for changes in the advertised window.
• Send notifications to the sender promptly when buffer space is available.

Adding an extra timer on the receiver ensures that it can track the buffering space precisely and update the sender in a timely manner.
This approach would avoid any delays in the communication process. However, it shifts the responsibility for active monitoring to the receiver, which could add complexity to its operations.
This balanced approach highlights the importance of both sender and receiver roles in efficient TCP communication.