Distributed Computing Wikiclass | Tutorial / Tutorial

A Distributed Computing Tutorial by Steven Choy

Synopsis TCP/IP networking fundamentals are the subjects that you need to master in the previous lecture. This page contains 20 questions and answers for you to review the following concepts.

protocols and networking stack;
the principles of IP protocol;
the features and differences between TCP and UDP protocols;
the concepts of ports, sockets and datagrams;
the common TCP/IP and UDP/IP application protocols; and
the functions of DNS, firewalls and proxy servers.

Protocol and Networking Stacks

Question 1: The OSI model has seven layers. List them from top to bottom.

The OSI seven-layer model: application, presentation, session, transport, network, datalink, and physical. (I use the sentence ‘All people seems to need data processing’ to help remember the sevenlayer model.)

Question 2: The IP networking stack has only five layers by combining three layers in the OSI networking stacks into the application layer. List the three layers in the OSI stack.

They are application, presentation and session layers.

Question 3: Identify which layer of the IP networking stack the following protocols belong to: IP, TCP, UDP, FTP, HTTP, Ethernet frames.

IP: network layer; TCP: transport layer; UDP: transport layer; FTP: application layer; HTTP: application layer; Ethernet frames: datalink layer.

IP: Internet Protocol

Question 4: What is the netmask number for a non-subnetted Class C network?

255.255.255.0

Question 5: Why there is no network that owns an IP address such as 127.x.x.x?

It is because addresses beginning with 127 always mean the local loopback address. These addresses, which point to the local computer, are usually used for testing purposes.

Question 6: Can you send data to 234.5.6.7? If so, what will happen?

Yes. IP address 234.5.6.7 belongs to the multicast addresses (a Class D address). You can use this IP address to perform some sort of multicast communication with other hosts connected to the same IP address.

TCP and UDP

Question 7: What is meant by connection-oriented and connectionless protocol?

Connection-oriented protocols are those that guarantee that data sent from one end of the connection correctly gets to the other end and in the same order it was sent. Connectionless protocols do not guarantee delivery of data and that packets will not be delivered in the same order in which they were sent.

Question 8: What features of TCP allow it to guarantee quality of service (QoS) in data delivery over the connectionless, non-guaranteed IP layer?

TCP assigns a sequence number to each packet transmitted. On the other end of the connection, a checksum is performed. If data are missing, the receiving end informs the sending end to retransmit. The receiving end sends acknowledgement to the sending end if the data are intact. The receiving end sees each packet only once by automatically removing duplicate packets.

Question 9: What are the features of UDP that make many people happy to use it even though it is regarded as an ‘unreliable’ protocol (i.e. the delivery of data and its order is not guaranteed)?

UDP provides very few error recovery services, offering instead a direct way to send and receive datagrams (independent packets of data) over an IP network. Thus UDP transmits data much faster than TCP. UDP is appropriate when transport layer overheads must be minimized, or the guarantee of reliability is not crucial to the success of the transfer of information.

Question 10: What do we mean by fragmentation of UDP datagram, and how can we avoid fragmentation?

Fragmentation means a UDP datagram must be broken into several small packets for transmission purposes. If any of these fragments is lost, the entire datagram will be discarded. Fragmentation will reduce the chances of data delivery. We should limit UDP datagram size to avoid fragmentation.

The IP family of services

Question 11: What is the underlying protocol of the World Wide Web, and which network port does this protocol use?

HTTP (Hyper Text Transfer Protocol). It uses TCP port 80.

Question 12: Name the protocol that is generally used to send email over the Internet.

SMTP (Simple Mail Transfer Protocol)

Question 13: Name the protocol that is used to transfer accumulated email from the host to sporadically connected clients.

POP3 (Post Office Protocol Version 3)

Question 14: Name the protocol that is used to transfer articles among servers in the USENET Internet news system.

NNTP (Network News Transport Protocol).

Question 15: What is a ‘finger’?

A finger gets information about a user or a group of users.

Question 16: List some IP services that use UDP instead of TCP.

Examples of IP services that use UDP are DNS and SNMP.

Question 17: What is the meaning of 8080 in this URL: http://www.xyz.com:8080/index.html?

The 8080 is the target port number for HTTP in the concerned URL.

Firewalls and proxy servers

Question 18: List the two main types of firewall techniques.

Packet filters and application gateways (proxy server is one type of application gateway).

Question 19: What would you say about the relationship between a firewall and a proxy server?

A proxy server can act as a type of firewall by performing access control to clients’ connection to the outside world.

Question 20: What are the three main purposes of using proxy servers?

To improve access performance, to filter requests, and to hide the details of a private network from the outside world.

Tutorial - Answers to Networking Questions

Protocol and Networking Stacks

IP: Internet Protocol

TCP and UDP

The IP family of services

Firewalls and proxy servers