Networks Part 5 (Network Layer)

Question 1

What is the purpose of the network layer?

Answer 1

1. Provides independence from subnet technology (routers translate between different technologies)
2. Provides a uniform numbering scheme for network addresses across LAN and WAN, which allows better scaling as routing tables will have one entry per network.
3. Uses the subnet (which is made up of routers) connected using point-to-point links making network layer tech Point to Point.
4. Is aware of network topology so can make informed decisions about data routes and deal with network congestion.

Question 2

What does the network layer provide to the transport layer?

Answer 2

1. Internetworking - Connect different subnets (even with different datalink protocols.
2. Routing and Forwarding - Fills in routing table and forwards packages to the right destination.
3. Congestion Control - controls the flow of data when there are too many messages in transit.

Question 3

What services does the transport layer provide?

Answer 3

1. Connection Oriented (Virtual Circuit)
2. Connectionless (Datagram)

Question 4

What are the characteristics of a “Virtual Circuit Subnet”?

Answer 4

1. Circuit Setup is required (route known before transmission)
2. Each packet requires a VS (virtual connection) number indicating their order on the connection.
3. Each VC (virtual connection) requires router table space per connection.
4. All packets follow the route created at setup.
5. Any VCs that pass through a failed router are terminated.
6. Quality of service is good but relies on resources allocated at setup.
7. Congestion control is good but relies on resources allocated at setup.

Question 5

What are the characteristics of a “Datagram Subnet”?

Answer 5

1. Circuit setup is not necessary before transmission.
2. Each packet contains the full source and destination address.
3. Routers do not hold information on connections.
4. Each packet is routed independently and can take whatever path is available.
5. Router failure has no effect except for if packets are lost during transmission.
6. Quality of service can be difficult to maintain.
7. Congestion control is difficult to implement.

Question 6

What are the two phases of routing?

Answer 6

1. “Routing Algorithm” phase - Routers exchange information to decide on the best routes and store said routes in tables.
2. “Forwarding” phase - Routers forwards packets according to what the route information in the routers tables say.

Question 7

What two classes of routing algorithms are there?

Answer 7

1. Adaptive Algorithms - Change routing decisions based on changes to traffic or topology. Traffic and topology are monitored during transmission. Can also be called dynamic.
2. Non-Adaptive Algorithms - do not adjust to traffic or topology changes. Can also be called static.

Question 8

What is network flooding?

Answer 8

When a the source sends a packet to all of its neighbours, and then they repeat the process.

For a flood to stop, nodes must remember all the packages they have seen so if it receives another copy it will not pass it on again.
And/or a hop counter must be implemented so a packet stops jumping after it has jumped too many times.

Not practical in most applications, however it is very robust for when any amount of routers can suddenly be destroyed (military scenarios)