COMP211 Chapter 1

Question 1

What is the internet?

Answer 1

Network of networks, governed by protocols

Question 2

What is a protocol?

Answer 2

Protocols define format & order of messages sent and received among network entities, and actions taken on message transmission & receipt

Question 3

What do we mean by “network edge”?

Answer 3

End-systems (the computers and other devices connected to the Internet through a network) are said to be at the network edge, because they are the ends of a line of communication.

Question 4

How does the digital subscriber line (DSL) access network work?

Answer 4

Uses existing telephone line to central office DSL access multiplexer (DSLAM) over dedicated line, where different frequencies that denote whether a message is voice or data would be directed either to the telephone network or to ISPs.

Question 5

What is the cable access network?

Answer 5

Uses frequency division multiplexing (different channels transmitted in different frequency bands) to connect homes to ISP router with hybrid fibre coax cable. Homes share access network to cable headend, unlike DSL which has dedicated access to central office.

Question 6

How does a home access network work?

Answer 6

Devices are connected to a router, which connects to a headend or central office through a cable or DSL modem.

Question 7

How does enterprise access network work?

Answer 7

Devices (including desktop computers, wireless laptops, servers, etc.) connected to ethernet switches, connected to institutional router, connected to ISP through institutional link.

Question 8

What is packet switching?

Answer 8

host breaks message into packets of length L bits, transmits them into access network at transmission rate R, so that (d = packet transmission delay) = (t = time needed to transmit L-bit packet into link) = (L/R)

Question 9

What does store and forward mean?

Answer 9

Entire packet must arrive at router before it can be transmitted on next link

Question 10

Assuming zero propagation delay, how do we calculate end-to-end delay?

Answer 10

2(L/R)

Question 11

What is the difference between routing and forwarding?

Answer 11

Routing determines source-destination route taken by packets, forwarding moves packets from router’s input to appropriate router output

Question 12

What is circuit switching?

Answer 12

Alternative network core, where end-to-end resources are allocated to/reserved for the “call” between soure and destination. There is no resource sharing (-> guaratneed performance, circuit segment idle if not used by call). Commonly used in traditional telephone networks.

Question 13

What are the two approaches to circuit siwtching multiplexing?

Answer 13

FDM (frequency-division multiplexing), where different frequencies are used for different connections; and TDM (time-division multiplexing), where time is divided into frames of fixed duration and each frame is divided into a fixed number of time slots, such that one time slot in every frame is dedicated to each connection.

Question 14

What are the advantages of packet switching over circuit switching?

Answer 14

It allows more users to use the network, works faster with any number of users, and is great for “bursty” data (simpler, shares resources).

Question 15

What are the disadvantages of packet switching over circuit switching?

Answer 15

Excessive congestion possible: packet delay and loss. Protocols needed for reliable data transfer, congestion control.

Question 16

How do end systems connect to the internet?

Answer 16

Through access ISPs (residential, company and university ISPs). Access ISPs must then be interconnected so that any two hosts can send packets to each other.

Question 17

Why shouldn’t we connect each access ISP to all others directly?

Answer 17

Doesn’t scale well: O(n^2)

Question 18

What was the solution to the problem of unscalable direct connection of access ISPs?

Answer 18

Connect all access ISPs to a global ISP, with a financial agreement between clients and supplier.

Question 19

What issues arose from trying to connect access ISPs to a global ISP?

Answer 19

Other ISP providers started competing, resulting in different access ISPs connecting to different larger ISPs. This meant that the larger ISPs needed to be connected, and so internet exchange points (IXPs) were put in place to connect large, competing ISPs. Regional networks then began to appear to connect access nets to ISPs, and content provider networks (e.g. Google, Microsoft, etc.) started running their own networks to bring services & content close to end users.

Question 20

In what did the “global ISP” issues result?

Answer 20

Large Tier 1 ISPs, alongside content provider networks, connected by IXPs. They connected either directly to access ISPs, or to regional ISPs that then connected to access ISPs.

Question 21

What are the four sources of packet delay?

Answer 21

nodal processing (check bit errors), queueing delay (time waiting at output link for transmission), transmission delay (packet length DIV link bandwidth), propagation delay (length of physical link DIV propagation speed).

Question 22

What causes packet loss?

Answer 22

Queue preceding link in buffer has finite capacity, any packets arriving to full queue are dropped (ergo lost).

Question 23

What is throughput?

Answer 23

The rate at which bits transferred between sender/receiver

Question 24

What is instantaneous throughput?

Answer 24

Rate of bit transfer at given point in time

Question 25

What is average throughput?

Answer 25

Rate of bit transfer over long period of time

Question 26

What is a bottleneck link?

Answer 26

A link on the end-end path that constrains end-end throughput.

Question 27

What is protocol layering?

Answer 27

Using a series of layers to implement different services for an interaction

Question 28

Why layering?

Answer 28

Explicit structure allows identification, relationship of complex system’s pieces
Modularisation eases maintenance updating of system

Question 29

What are the layers in the internet protocol stack?

Answer 29

Application layer supports network applications (FTP, SMTP, HTTP)
Transport layer deals with process-process data transfer (TCP, UDP)
Network layer deals with routing of datagrams from source to destination (IP, routing protocols)
Link layer deals with data transfer between neighbouring network elements (Ethernet, 802.111, PPP)
Physical layer deals with bits “on the wire”

Question 30

How does the OSI (open systems interconnectedness) model differ from the protocol stack?

Answer 30

Includes presentation layer (allow applications to interpret meaning of data (e.g. encryption, compression, etc.)) and session layer (synchronisation, checkpointing, recovery of data exchange).

Question 31

How can malware infiltrate a host?

Answer 31

As a virus (self-replicating infection by receiving/executing object), or a worm (self-replicating infection by passively receiving object that gets itself executed

Question 32

What is spyware malware?

Answer 32

Malware that can record keystrokes, web sites visited, upload info to collection site

Question 33

What is DoS (denial of service)?

Answer 33

Attackers make resources unavailable to legitimate traffic by overwhelming resource with bogus traffic.