$4.$ TCP sequence numbers and ACK numbers $(20$ points$).$ Hosts A and B are communicating over a TCP connection, and Host B has already received from A all bytes up through byte $2000.$ Suppose Host A then sends two segments back$-$to$-$back to Host B$,$ with $1000$ and $500$ bytes of data in the first and second segments, respectively. In the first segment, the sequence number is $2001($for simplicity, we assume the sequence number starts from $0,$ although it typically starts with a random number in practice$),$ the source port number is $50002,$ and the destination port number is $80.$ Host B sends an ACK to A whenever it receives a segment from A$.$ a$.(6$ points$)$ In the second segment from A to B$,$ what is the sequence number, source port number, and destination port number? b$.(6$ points$)$ Suppose the first segment arrives at B earlier than the second segment. In the ACK for the first arriving segment from B to A$,$ what is the ACK number, source port number, and destination port number? c$.(4$ points$)$ Now suppose the second segment arrives at B earlier than the first segment $($i$.$e$.,$ the two segments arrive out of order$).$ In the ACK for the first arriving segment from B to A$,$ what is the ACK number? d$.(4$ points$)$ Now suppose the first segment is lost and the second segment arrives at B$.$ In the ACK for the arriving segment from B to A$,$ what is the ACK number?