Q&A

Q1:	Why are multiplexing and demultiplexing necessary?
A1:	If TCP/IP did not provide multiplexing and demultiplexing, only one application could use the network software at a time, and only one computer could connect to a given application at a time.
Q2:	Why would a software developer use UDP for a transport protocol when TCP offers better quality assurance?
A2:	TCP's quality assurance comes at the price of slower performance. If the extra error control and flow control of TCP are not necessary, UDP is a better choice because it is faster.
Q3:	Why do applications that support interactive sessions, such as Telnet and FTP, tend to use TCP rather than UDP?
A3:	TCP's control and recovery features provide the reliable connection necessary for an interactive session.
Q4:	Why would a network administrator want to use a firewall to intentionally close off Internet access to a TCP or UDP port?
A4:	Internet firewalls close off access to specific ports to deny Internet users access to the applications that use those ports. Firewalls can also close off access to the Internet so that users on the internal LAN cannot make use of certain services available on the Internet.
Q5:	Why don't routers send TCP connection acknowledgments to the computer initiating a connection?
A5:	Routers operate at the Internet layer (below the Transport layer) and therefore do not process TCP information.
Q6:	Would a functioning FTP server most likely be in a passive open, active open, or closed state?
A6:	A working FTP server would most likely be in a passive open state, ready to accept an incoming connection.
Q7:	Why is the third step in the three-way handshake necessary?
A7:	After the first two steps, the two computers have exchanged ISN numbers, so theoretically they have enough information to synchronize the connection. However, the computer that sent its ISN in step 2 of the handshake still hasn't received an acknowledgment. The third step acknowledges the ISN received in the second step.
Q8:	Which field is optional in the UDP header and why?
A8:	The Source Port field. Because UDP is a connectionless protocol, the UDP software on the receiving machine does not have to know the source port. The source port is provided as an option in case the application receiving the data needs the source port for error checking or verification.
Q9:	What happens if the source port is equal to 16 zero bits?
A9:	The application on the destination machine will be unable to send a response.