Answer the following questions. Try to provide compact and concise responses.

Describe the process of reassembling an IP datagram at the destination host. What happens if one or more fragments are lost or delayed during transmis$-$ sion?

Explain the difference between routing and forwarding in the context of the network layer. How are routing tables used to determine the next hop for a packet and how are these tables updated?

$$What is the header format of the IPv$4?$

Consider a router R connecting two subnets A and B$,$ the MTU in subnet A is $1400$ while the MTU in subnet B is $512.$ Consider a packet of $1400$ bytes generated from an interface in subnet A to be sent to another interface in subnet B$.$ Show the header of this transmitted packet in both networks.

Consider a network with three routers R$1,$ R$2,$ and R$3$ and two hosts H$1$ and H$2.$ Assume that H$1$ wants to send a packet to H$2$ and that the shortest path from H$1$ to H$2$ is through R$1,$ R$2,$ and R$3$ in that order. Describe the forwarding process at each router and explain how the routing tables are used to forward the packet.