Question

How is it possible for the network layer (L3) to operate in connectionless, best effort manner? Hint: think upper layer

Answer 1

How it is possible for the network layer (L3) to operate in a connectionless, best-effort manner: with respect to the upper layer i.e., the transport layer (L4):
The network layer provides the transport layer with the most important abstraction which is if the network uses a connectionless service called datagram model or connection-oriented service called virtual circuits. The transport layer depends on the services of the network layer. The network layer provides communication services between machines or hosts. Specifically, The network layer moves or transports the transport-layer segments from one host (source) to another (destination). The network layer receives the transport-layer segment from the transport layer. The network layer's responsibility is to send the segment to the destination machine or host and pass the segment up the protocol stack to the transport layer.

The network layer provides a connectionless datagram service instead of a virtual-circuit service. The network layer at the sending or the source host encapsulates the segment within an Internet Protocol (IP) datagram, places the destination host address, and other fields in the datagram. It then sends the datagram to the first router on the path that is on the way to the destination host. When received at the destination host, fragments are reassembled before they are passed on to the transport layer at the destination. The network layer uses User Datagram Protocol (UDP) IP for connectionless transportation of packets. The IP service model is a best-effort delivery service and is considered an unreliable service.

Thus, the Internet network layer uses connectionless service and best-effort to transport packets from the source to the destination host. There is no call that is set up at the network layer. Routers would not have any state about end-to-end connections. There is no network-level concept called "connection". Here, packets that are between the same source-destination pair may take different paths. There is no management of connection required inside the network. There are no tables and there is no state that is needed inside the network. Different packets that are meant for the same destination are routed through a different path or route. The Internet architecture that we currently have provides only one service model which is the datagram service model, which is also called "best-effort service." The "best-effort service" is a substitute for "no service at all".

The best-effort service does not provide a guarantee to be preserved for the timing between packets. It does not provide a guarantee for the packets would be received in the same order they sent. It also does not guarantee the eventual delivery of transmitted packets. Thus, a network responsible for delivering "no" packets to the destination host would satisfy the definition host of the "best-effort" delivery service.

Using connectionless protocols datagrams are cast onto the network with the understanding they will be delivered based on a best-effort manner to the destination host it is addressed to. There will not be any notification of failure. The sender will not receive an acknowledgment of the data packet been received or not. In this connectionless communication, information or data is dropped on the wire or channel and we assume a “best-effort” delivery to send the information to its recipient at the destination host.