Consider two machines A and B, machine A sends 3 packets to B of sequence number 1, 2, 3 during transmission only packets 1 and 3 are received by B. Now B requests for packet 2, and machine A sends the requested packet.

I have a doubt here, does machine store all the packets in some memory as buffer or does it again make packets and send only the requested packets? And if it is based on having buffer memory to store all the packets, where this buffer is stored in the serving machine? and when this buffer is cleared?