The network layer

Question 1

Why do we need a network layer

Answer 1

• Switches can connect two LANs with each other, but the assumptions is that these two LANs are
  built using the same technology (e.g. Wi‐Fi or Ethernet). Switches do not work across more than
  one link layer technology (cannot translate between different technologies)
• Switches do not scale to a large network (physical addresses are flat, forwarding tables will
  become huge with billions of entries)
• Broadcast technology is mainly used for LANs and does not suit the Internet
• Switches do not perform traffic control (cannot plan routes)

Question 2

What is the role of the network layer?

Answer 2

‐ In summary, the Network Layer provide the following to the Transport Layer:
* Internetworking: Connects heterogeneous subnets (potentially with different datalink
protocols)
* Routing and forwarding: fills in Routing table and forwards packets to the right destination
* Congestion Control: controls the flow of data when there are too many messages in transit

Question 3

Internewtworking

Answer 3

Internetworking joins multiple, different networks into a single larger network
* Internetworking based on a common network layer (mainly IP)
* Tunneling

Question 4

How can networks be connected

Answer 4

• If the source and destination networks use different technologies, Internetworking is done based on a common network layer – IP
• Routers will decapsulate packets and use their headers to decide on destination. Once the destination is identified they will re‐encapsulate the packet according to the protocol of the next network

Question 5

Tunneling

Answer 5

‐ Tunneling connects two networks through a middle one ‐ Packets are encapsulates over
the middle
‐ This approach only works if the source and the destination networks are using the same
technology but there is a network in the middle uses another technology

Question 6

Steps of path MTU discovery

Answer 6

1. Each IP packet is sent with its header bits (Don’t Fragment (DF) bit) set to indicate that no
   fragmentation is allowed
2. If a router receives a packet that is too large, it generates an error packet, returns it to the source,
   and drops the packet
3. When the source receives the error packet, it uses the information inside to re‐fragment the
   packet into pieces that are small enough for the router to handle
4. The process is repeated If a router further down the path has an even smaller MTU (until the MTU
   becomes small enough to traverse the entire path without fragmentation).

Question 7

The internet protocol

Answer 7

‐ The Internet is a (very large) collection of
different kinds of subnets
‐ IP is the “glue” that allows all these subnets to
exchange data – giving the impression of a single,
global net
‐ IP provides delivery of packets from one host in
the Internet to any other host in the Internet,
even if the hosts are on different networks (with
possibly different protocols)