Network Layer — Functions and Design Issues

The network layer (Layer 3) is responsible for host-to-host delivery of packets across possibly heterogeneous networks, i.e., moving packets from the source host to the destination host through intermediate routers, independent of the underlying physical/link technologies.

Functions of the Network Layer

1. Inter-networking / packet delivery: Transfers data from source to destination across multiple networks using routers; each packet is forwarded hop-by-hop.
2. Addressing: Uses a hierarchical logical address (IP address) that identifies the host irrespective of physical (MAC) addresses; supports subnetting and CIDR for scalable address allocation.
3. Routing: Determines the best path for each packet using routing protocols (RIP, OSPF, BGP) and the routing table built by the forwarding process.
4. Forwarding: At each router, moves the packet from the correct input interface to the appropriate output interface based on the destination address.
5. Fragmentation and Reassembly: Splits a packet that exceeds the MTU of the outgoing link into fragments (sets offset, MF flag, ID) and reassembles them at the destination.
6. Error control, flow control and congestion control: Signalled through ICMP; congestion handling via source quench/RED (IP itself is best-effort).
7. Datagram services: Connectionless (datagram) or connection-oriented (virtual circuit) delivery, plus quality-of-service support (TOS field, diffserv).

Design Issues of the Network Layer

Design issue	What must be decided
1. Services provided to the transport layer	Should the network offer connection-oriented (guaranteed, ordered) or connectionless (datagram) service to the transport layer — and should that choice be visible to the sender? (IP chose connectionless; TCP/UDP add reliability above.)
2. Addressing & service model	How hosts and interfaces are named (hierarchical IP addressing) so packets can be located anywhere; how addressing interacts with subnetting/CIDR/NAT.
3. Packet delivery semantics	Datagrams vs virtual circuits: independent routing of each packet vs a pre-established path; implications for ordering, duplicate control and QoS.
4. Fragmentation & MTU handling	How to handle packets larger than a link's MTU — fragmentation and reassembly, and the per-hop overhead it creates.
5. Routing algorithms & forwarding	Choosing shortest/best paths (optimality, correctness, robustness, simplicity, stability, fairness, efficiency) and building forwarding tables.
6. Congestion control	Detecting and relieving congestion — buffering vs pacing (leaky bucket), credit schemes, choke packets; where control should sit (network vs hosts).
7. Inter-networking & heterogeneity	How to hide differences among networks (addressing, MTU, frame size, maximum packet size) and connect them via routers/gateways.
8. Security & service differentiation	Authentication, confidentiality (IPsec), and providing differentiated services (TOS/DiffServ) over a best-effort core.

Summary: The network layer must (i) define what service it gives to transport, (ii) how packets are identified and addressed, (iii) how the best path is chosen and packets forwarded, (iv) how large packets are fragmented, and (v) how congestion, heterogeneity and inter-networking are handled — these are the classic design issues, with IP choosing connectionless datagram service and best-effort delivery.