Networking Layer

Question 1

What is the size of the IPv4 header? Is it always the same size?

Answer 1

Fixed size 20 Bytes with optional, variable length for options.

Question 2

What is an IPv4 header and for what is it used?

Answer 2

Describes a IPv4 packet and some important meta data.

Question 3

Explain the concept of fragmentation of packets with the information of the IPv4 header.

Answer 3

• DF: Dont’ Fragment bit
• MF: More Fragments bit
• Reserved bit
• Fragment Identification
• Fragment Offset

Question 4

What happens if the packet is too big but the header doesn’t allow fragmentation?

Answer 4

Drop the packet and send a error message with ICMP.

Question 5

Why does the IPv4 packet rely on the ethernet frame in terms of MTU (Maximum Transmission Unit)?

Answer 5

Ethernet is on the lower level and sets the tone in terms of sending a packet over a line.

Question 6

Describe the IPv4 Header and all its fields.

Answer 6

See book page 503

Question 7

You have a network with subnetmask /24. You want to create 4 subnets. What is the subnetmask?

Answer 7

We want to have 4 subnets = 2^2. Subnetmask /26

Question 8

ICMP - Destination Unreachable - Why are the first 64 Bits of the IP header enough to determine the connection?

Answer 8

Probably because of the identification of the IP packet through the fragment identification

Question 9

What were the design targets of IPv6?

Answer 9

• Bigger address space
• Security in protocol (e.g. IPSec)
• Extension Headers are useful
• Reestablishing end-to-end connectivity
• ARP is replaced by neighboor hood discovery protocol

Question 10

How handles the IPv6 packet fragmentation?

Answer 10

By using the extended headers with the value 44 for fragmentation information

Question 11

Describe the IPv6 header and all its fields.

Answer 11

See screenshot in folder 100_exam_papers

Question 12

Why is there no more broadcast in Ipv6?

Answer 12

Broadcast make a lot of traffic. IPv6 replaces broadcasts with specific multicasts by device types (e.g. a group of routers).

Question 13

What are the components of the IPv6 address? Describe them properly

Answer 13

• 8x 16 bits fields, divided by :
• Prefix: Provided by ISP, consists of n bits (common 64)
• Subnet ID: Specific Location, specified by the provider, consists of 64 - n bits
• Interface ID: Derived from Mac, consists of 64 bits

Question 14

How to convert a IPv4 address to be used dual stack in ipv6?

Answer 14

First 80 bits 0s, 16 Bits 0xFFFF, 32 Bits the IPv4 address

Question 15

How to use an IPv6 address as Url with port nr?

Answer 15

https://[IPv6 address]:443

Question 16

What is the problem with privacy and IPv6 and what are some possible fixes?

Answer 16

Interface ID 64 bit is derived from the mac address and therefore globally unique.
Fixes:
- DHCPv6: Random addresses generated (similar Ipv4)
- Privacy extension of IPv6

Question 17

What can you tell me about ICMPv6 in reference to ICMPv4?

Answer 17

• ICMPv6 is mandatory for a correct functioning of IPv6

- ICMPv6 has more message types

Question 18

What is the Neighbourhood discovery protocol and what does it do?

Answer 18

• Does use cases what multiple protocols did for IPv4 (ARP, ICMP, etc)
• Find a neighbouring router
• Find link-layer address of neighbouring device
• Detect duplicate addresses (DAD)
• Detect unreachability of address (NUD)

Question 19

Describe the sequence of a device that is freshly connected to an ipv6 network and needs an ip address. The ip addresses are autoconfigured with SLAAC.

Answer 19

Have a look at the SLAAC diagram.

Question 20

What is the goal of neighbour solicitation and advertisement?

Answer 20

• Get link local address of neighbor
• DAD (Duplicate Address Detection)
• NUD (Neighbor Unreachable Detection)

Question 21

Client 1 wants to communicate with client 2. They never have communicated before. What is the sequence?

Answer 21

1. ) Search in cache for link local address of PC 1
2. ) Not there, send neighbor solicitation message to multicast of all nodes
3. ) PC 1 sends neighbor advertisement with link local address in it back
4. ) They can communicate with eachother

Question 22

What is the problem with ND? What is a solution to this?

Answer 22

• If an attacker has control over the mapping of layer 2/3 he can start a spoofing attack. e.g. PC 1 searches for the router with IP xy and instead of the router PC 2 answers. That is then a man-in-the-middle.
• SEND (Secure Neighbor Discovery) is a solution to this. Crypto approach, the router identifies with a certificate and the client can test it.

Question 23

Describe the sequence of creating a valid globally unique IPv6 address with SLAAC.

Answer 23

1. ) New client creates a “tentative” address with link local + Interface ID
2. ) Neighbor solicitation to multicast all nodes with target his tentative address
3. ) If no response, it is unique in link local scope
4. ) Router solicitation to multicast routers
5. ) Router advertisement with prefix
6. ) Create global address
7. ) Again neighbor solicitation with global address. Response address is a multicast address
8. ) If no response (DAD) then it is (almost) safe to use (No obligation to DAD)

Question 24

What are the two flags on the router advertisement and what do they stand for?

Answer 24

• M: Managed Address Configuration Flag: if it is set, use DHCPv6 (stateful)
• O: Other Configuration: If it is set, use DHCPv6 for more information but still SLAAC for address assignment.

Question 25

When to use DHCPv6 over SLAAC?

Answer 25

If more complex information needed use DHCPv6. SLAAC is only for address assignment