In the TCP sliding window protocol, the sending application may, for example, send a 1 KB segment to the receiver, which will store the data in a buffer. The received data will remain in the buffer until the receiving application draws the data from it. IF the buffer is 4KB, it will acknowledge the receipt and advertise that it has 3KB of buffer space remaining. The sender transmits another segment of 1KB, and the receiver acknowledges with an advertised 2KB of buffer space left. The sender sends two more segments with 1KB each, at the end of which, the receiver advertises zero bytes of buffer space left. Sometime later, the sender transmits a window probe.

1. What is its purpose?

2. Why is it needed?