Question: In TCP, a special segment called a probe is sent by a sending TCP when the _______ timer goes off.

Answer:

• In TCP, a special segment called a probe is sent by a sending TCP when the persist timer goes off.
• The persist timer is a timer used by the sending TCP to handle the situation where the receiver advertises a zero window size. When the sender receives a zero window size advertisement, it must stop sending data until the receiver advertises a non-zero window size. However, the sender must also periodically check whether the receiver's window size has increased, so that it can resume sending data.
• To handle this situation, the sender uses the persist timer. The persist timer is started when the sender receives a zero window size advertisement, and it is reset each time an acknowledgment is received for a previously sent segment. If the timer expires without receiving an acknowledgment, then the sender sends a probe segment to the receiver.
• The probe segment is a special segment that contains no data, and it is used only to elicit a response from the receiver. When the receiver receives the probe segment, it must respond with an acknowledgment, which will update the sender's window size and allow it to resume sending data. If the sender receives no response to the probe segment, it will continue to send probe segments until the receiver advertises a non-zero window size or the maximum number of retransmission attempts is reached.
• Therefore, a special segment called a probe is sent by a sending TCP when the persist timer goes off, and it is used to handle the situation where the receiver advertises a zero window size.