Dec 1998 NI222: Networks and the Internet (Question 3)

(a)	Explain the structure of the FTP application. Include an explanation of the way it uses two separate connections. You may find it helpful to draw a diagram.	[6]
	Up to (6 marks) for points from the following: User interacts with client ftp program (1 mark), which opens a control channel to the target machine (1 mark). In normal operation, the user is authenticated to the remote machine with a username and a password (1 mark). The actual data is sent over a separate channel (1 mark); this may be a file transfer or a directory listing (1 mark). A number of different file formats are recognised (1 mark), so transfer may be ASCII or binary (1 mark). Allow up to (3 marks) for a good, well-labelled diagram.
(b)	What is anonymous FTP?	[4]
	This is a distribution mechanism for programs and data. (1 mark). Allows outsiders to retrieve files from a restricted area of the system without individual authentication (1 mark). User logs in with name anonymous (1 mark). Some sites request user to send Email address as a password (1 mark). Allow credit for other relevant information.
(c)	Explain the differences between connection-oriented and connectionless transport protocols.	[5]
	Up to (3 marks) for these or other relevant points: Connection-oriented: May or may not guarantee Quality of Services, (1 mark) but it guarantees that message is not lost (1 mark), will arrive in order (1 mark) and will be duplicated (1 mark). Up to (3 marks) for these or other relevant points: Connectionless: moves messages called datagrams along a channel, with no guarantee that the messages will arrive in the order they are sent (1 mark). No acknowledgement of datagrams so messages may be lost (1 mark). Example, UDP-User Datagram Protocol (1 mark). Maximum mark: 5
(d)	FTP uses the TCP service of the transport layer. Is this a connection-oriented, or connections protocol ? What features of FTP make this a sensible choice ? Give an example of an application which might use the other sort of transport protocol.	[5]
	It is connection-oriented (1 mark). Up to (3 marks) for these or other relevant points: FTP must be reliable, so we need to be able to signal if packets are missed, or garbled (1 mark). FTP sessions will often involve transfer of more than one file, or a directory and then a file (1 mark), so it is wise to hold open a connection, rather than setting up separately for each transfer (1 mark). A connectionless protocol is suitable for broadcast voice, or video (1 mark). Allow credit for other suitable examples.