IPC

Question 1

What is Inter-process communication?

Answer 1

• Allows for the exchange of data between multiple processes (or threads within processes)
• The processes can reside and run on different computers (but connected by a network)

Question 2

List 4 Reasons for using IPC

Answer 2

1. Sharing data
2. Accessing remote services
3. Computational efficiency
4. Distributed applications
   e. g. iCloud, iTunes, mobile games, social networking, Skype, Facetime, shared whiteboards, etc.

Question 3

List some characteristics of inter-process communication

Answer 3

• Message passing between two processes via send and receive
• May involve the synchronisation of the two processes
• Synchronous and Asynchronous communication

Question 4

What is synchronous communication?

Answer 4

Sender and receiver synchronise (send and receive are blocking operations)

Question 5

What is asynchronous communication?

Answer 5

Send is non-blocking; sender is allowed to continue

Question 6

What is meant by reliability?

Answer 6

• Guaranteed delivery despite a ‘reasonable’ number of packets being dropped or lost
• Integrity - messages must arrive uncorrupted and without duplication

Question 7

What is the difference between unicast and multicast communication?

Answer 7

• Unicast communication is from one process to a single other process (e.g. socket communication)
• Multicast is communication from one process to a group of processes (e.g. publish / subscribe message model)

Question 8

List 4 characteristics of Internet Protocol (IP)

Answer 8

1. Delivers data packets to network devices
2. Uses logical IP addresses for devices (rather than physical MAC, Media Access Control, addresses)
3. IP address consists of a network part and a host part and is a ‘routable’ protocol – can forward packets to another network (i.e. inter-networking)
4. An internet - two or more connected TCP/IP networks that can be reached by routing

Question 9

List 6 characteristics of Transmission Control Protocol (TCP)

Answer 9

1. Connection-oriented ‘Transport layer’ protocol.
2. Allows data packets to be sent reliably between devices (on same or different network).
3. Ensures packet delivery if at all possible by establishing a connection with the receiving device and then sending the packets.
4. Resends packets if they don’t arrive.
5. Connection closed only after packet has been successfully delivered or an unrecoverable error has occurred.
6. Used for one-to-one communications. (User Datagram Protocol (UDP) is used for multicast / broadcast).

Question 10

Give examples of applications that rely on TCP

Answer 10

• Web browser / server uses HTTP to send request / replies via TCP
• Telnet (terminal emulation)
• FTP (file transfer protocol – used for file exchange)
• SMTP (simple mail transfer protocol).

Question 11

List 4 characteristics of User Datagram Protocol (UDP)

Answer 11

1. Connectionless Transport layer protocol
2. Used when the overhead of a connection isn’t required.
3. Doesn’t guarantee delivery - UDP places a packet on the network (via the IP protocol) and forgets about it.
4. Programmer may have to deal with confirming delivery

Question 12

Give examples of applications that rely on UDP

Answer 12

• DNS, the Domain Name System; used to lookup domain names / ip addresses. DNS sends a UDP packet to a DNS server to look up the domain and the domain’s IP address is returned in another UDP packet.
• SNMP (Simple Network Management Protocol)
• DHCP (Dynamic Host Configuration Protocol)
• Voice and video applications normally use UDP

Question 13

What is a socket address?

Answer 13

A socket address is a communication identifier that consists of a port number and an Internet address

Question 14

Describe datagram communication?

Answer 14

• Connectionless protocol – message-passing abstraction
• Enables a sending process to transmit a single message to a receiving process
• Each time you send datagrams, you also need to send local socket descriptor and the receiving socket’s address. - Additional data must be sent each time a communication is made

Question 15

Describe Stream Communication

Answer 15

• Connection-oriented protocol – stream abstraction
• A connection must first be established between pair of sockets. Two-way stream between sender and receiver.
• One socket (the server) listens for a connection request, the other (a client) asks for a connection.
• Once the sockets have been connected, data can be transmitted in one or both directions.
• Information communication consists of a stream of data items with no message boundaries.
• Used to implement some services such as remote login and file transfer.
• Streams provide building blocks for producer-consumer communication - items ‘produced’, ‘queued’ and ‘consumed’.

Question 16

Is UDP reliable?

Answer 16

No, there is no guarantee that the datagrams sent will be received in the same order by the receiving socket.

Question 17

Is TCP reliable?

Answer 17

Yes, it is guaranteed that packets will be received in the order in which they were sent.

Question 18

What are the size limits of data packages that can be sent via UDP or TCP?

Answer 18

UDP - 64KB

TCP - Unlimited

Question 19

What is the Java API for internet addresses?

Answer 19

InetAddress aComputer = InetAddress.getByName(“php.infc.ulst.ac.uk”);

Question 20

Describe an I/O stream?

Answer 20

1. An I/O Stream represents an input source or an output destination.
2. Supports different kinds of data:
   - bytes, primitive data types, localized characters, and objects.
   - some streams simply pass on data; others might modify the data.
3. All streams have the same simple model for programs that use them: a stream is a sequence of data.
4. A program uses an input stream to read data from a source, one item at a time
5. A program uses an output stream to write data to a destination, one item at time