a feature of TCP stack. i wansnt knowing it, but it falls under sliding window protocol.. now i know it :=}
If a frame is received with a sequence number that results in a gap larger than the size of the receiver’s window, the frame is determined to be outside the window and is discarded. If a frame is received with a sequence number that is lower than or equal to that of the last frame received, this frame is by definition outside the window and must be a duplicate. It is also discarded. Packets with sequence numbers that fall within the window are accepted and put into the buffer.