a potential IndexError

[wudiiv11]

In line 124, since you write

while True:
    ...
    if front == rcvd_ack.ack_num:
        break;

The if statement means the receiver haven't receive the packet with first byte number of ack_num.
However, if the ack_num is not in the window since it always ack the next byte, maybe it will raise an Index Error