2017 EXAM

Question 1

how does switch learn which port to use, to deliver to a dest. MAC address? What does it do if a given MAC address has not an assigned port?

Answer 1

When a frame arrives at the switchport, the switch checks source mac address, and stores it next to the port number in the TCAM table. If switch is given a frame with a MAC address it does not have a port for, switch multiplicates frame and sends out to all ports.

Question 2

Why ethernet address is not used when organising the internet?

Answer 2

MAC doesn’t support routing, as it is on layer 2. MAC addressing is used to connect two machines on the same network, but IP addresses are needed on layer 3, to identify machines through different networks

Question 3

Why voltage transition is useful in communication?

Answer 3

To transmit a sequence of 0’s and 1’s, a ground voltage of 0 for example might translate to a binary 0. If the voltage changes to a reference value at something like +5V, the receiver can note this as a 1 binary value. Through multiple voltage transitions, the sender is able to communicate effectively over a wire.

Question 4

Write topology from routing table:

DEST. GATEWAY. MASK. INTERFACE

Answer 4

Q.3d) 2019 Exam

Question 5

Given an ip dest. address, choose a route from a routing table

Answer 5

Week 10 IP Routing Slide 10

Question 6

Packet SRC and DST MAC and IP addresses, when Host A sends a packet to Host B through a router. (without ARP).

Answer 6

Source and Destination IP address won’t change when passing through a router, but the MAC addresses will change after passing through the router.

Host A messages Host B
A IP B IP
A MAC Router MAC

Once packet hits router:
A IP B IP
Router MAC B MAC

Question 7

Write a table created when running Dijkstra’s algorithm including DEST. NEXT-HOP. DIST.

Answer 7

Week 10 IP routing

Question 8

When organising external routes, why can operators only use bGP protocol?

Answer 8

Because the internet is already organised with BGP. BGP has a lightweight nature, perfect for organising a worldwide network.

Question 9

How is the ssthresh variable used in TCP slow start

Answer 9

when CWND reaches ssthresh, TCP then switches to congestion avoidance.

Question 10

How is transmission rate increased during congestion avoidance

Answer 10

Increased by one data packet per ack received.

Question 11

Time sequence diagram, where TCP detects packet loss through multiple duplicate acks.

Answer 11

Lecture 10 slide 4

Question 12

What is the rational behind each equation involved in determining the RTO.

Answer 12

(1) adding together the prior estimations with the new sample rtt, with weighting that can be adjusted to prefer new RTT samples (est. RTT adjusts faster) or to prefer previous estimated RTT (est. RTT immune to short lived changes)
(2) caclulates the variation between RTT’s similar to previous equation, where weighting can be set to maximise previous est. delay variation, or the latest sample.
(3) is the Estimated RTT + F * the Estimated Delay. this means we have a high probability of the packet being accepted before RTO kicks in

Question 13

How can you verify A’s digital signature? Which key would you use?

Answer 13

A will hash digital signature, then use private key to encrypt the hash. B then uses A’s public key to decrypt the hash, then computes the hash of the message from A. Then B compares the hash it computed to the hash sent by A and verifies that they are the same.

Question 14

With a host on a different router receiving an ARP request, what will the source and dest IP and MAC addresses be on the ARP packet? Host A sending to Host B

Answer 14

Packet will contain:
A IP B IP
Router MAC FF:FF:FF:FF:FF:FF

Question 15

In which order do: ARP, DNS, DHCP and NAT occur? What do they all do?

Answer 15

DHCP, ARP, NAT, DNS

Question 16

Does BGP pass both IPv4 and IPv6 addresses (if applicable) upstream?

Answer 16

Yes, BGP can announce both routes

Question 17

What characterises an Autonomous System?

Answer 17

is a large network or group of networks that has a unified routing policy.

Question 18

How do we avoid loops in BGP?

Answer 18

If a router receives a route with its own ASN in the AS path, the route will be denied

Question 19

Explain how TCP is Connection Oriented, Reliable and Stream Based.

Answer 19

Connection Oriented: TCP requires a logical connection to be established before sending or receiving any data.
Reliable: TCP does some corruption detection, and will resend lost packets.
Stream Based: client sees a stream of bytes, does not see individual or re-transmitted packets.

Question 20

What are the three steps to signing a Public-Key Certificate?

Answer 20

1. Create public and private key pair, and write CSR (Certificate Signing Request)
2. Pass CSR to CA, for CA to verify that CSR is authentic
3. CA signs the CSR, giving you a certificate

Question 21

How are Public Key Certificates verified?

Answer 21

to validate a Public Key, the receiver must:

• compute a hash of the sender’s message
• Decrypts the digital signature using the sender’s public key
• compares the two hash values

Question 22

If A and B are two peer to peer ASes, why doesn’t B advertise routes from providers and other peers to A?

Answer 22

Providing “free” transit for other traffic is detremental to B, and as they are not paying B any money, he is essentially supplying a service for free, at a cost.

Question 23

How can two parties authenticate each other using Challenge-Response, without disclosing any sensitive information?

Answer 23

A sends encypted message to B
B verifies message
B sends encypted message to A
A verifies message
at this point both parties can be sure the other is who they say they are.

Question 24

How can two parties authenticate each other using Challenge-Response, without disclosing any sensitive information?

Answer 24

A sends large random number to B
B encrypts and sends it back to A
A verifies, B sends large random number to A
A encrypts and sends it back to B
B verifies.
at this point both parties can be sure the other is who they say they are.

Question 25

What is a CA’s role in certificate validation?

Answer 25

CA’s guarantee that you are who you are. It is a third party marked as a trusted authority for systems to use.

Question 26

which java socket method can be used to get the port number a socket is bound to

Answer 26

InetAddress addr = s.getInetAddress();

int port = s.getPort();

Question 27

What is ExecutorService?

Answer 27

Is an interface, (thread is a class) similar to thread, where Executer can execute any number of runnable tasks.

Question 28

why is tcp has two port feilds?

Answer 28

allows client to have multiple connections to a destination application, allowing the fetching of multiple resources at the same time.