3) IP and Ethernet offer a connectionless service to the upper layer that makes use of them. (a) If the upper layer sends two small SDUs to IP or Ethernet, would IP or Ethernet aggregate the two small SDUs into one PDU for delivery in order to reduce the header overhead? Explain your answer. (b) Based on your answer in (a). Answer the following question. Suppose an application wants to send a file of 4500 bytes to its peer, using the TCP service. The TCP segment contains the message plus 20 bytes of header. TCP, in turn, uses the IP service. The TCP maximum segment size (MSS) is 1000 bytes. An IP packet has an additional 20 bytes of header. The IP, in turn, makes use of an Ethernet. IP over Ethernet allows maximum transmit unit (MTU) of 1500 bytes. The MAC layer of the Ethernet adds 18 bytes of header and trailer. How many bytes transmitted at the MAC layer are overhead bytes not belonging to the original file at the application layer?