Services of the Network Layer and ICMP

The network layer (Layer 3) is responsible for host-to-host delivery of packets across possibly heterogeneous networks with different topologies, technologies and MTUs.

Services provided by the network layer

1. Packet delivery — the essential service: a packet created at the source must be delivered correctly to the destination host. IPv4 gives a best-effort (unreliable, no guarantee of order/duplication-free) service.
2. Addressing — a hierarchical, technology-independent logical (IP) address identifies every host, so interconnection is possible even when the underlying LANs differ.
3. Forwarding and routing — each router forwards a packet to the next hop by looking up its forwarding table, while routing algorithms (distance vector, link state, path vector) compute the end-to-end paths that populate those tables.
4. Packetization & fragmentation — the layer breaks the transport data into datagrams and, when a datagram must cross a link with a smaller MTU, fragments and reassembles it at the destination.
5. Error reporting, congestion control and QoS — reporting via ICMP, congestion indication/notification, and services such as precedence, delay, throughput and reliability guarantees where the network supports them.
6. Connectionless datagram service — each packet is routed independently, which keeps routers simple and the network robust to failures.

ICMP (Internet Control Message Protocol)

• Purpose: the control and error-reporting companion to IP (RFC 792). It carries diagnostics and network problems back to the sending host; it is not a higher layer above IP — ICMP messages are carried inside ordinary IP datagrams with protocol number 1 (2 for IGMP, 58 for ICMPv6).
• Format: 1-byte Type, 1-byte Code, 2-byte Checksum, 4-byte type-specific "rest of header", followed by as much of the original IP header + first 8 bytes of data as fits — enough for the source to identify the offending packet.
• Common messages:

Type / Code	Meaning
3 / 0,1,3,4	Destination unreachable (net, host, port, protocol)
4	Source quench (congestion, deprecated)
5	Redirect — host told a better route to the destination
8 / 0	Echo request → ping
0	Echo reply
11	Time exceeded — TTL expired (used by traceroute)
12	Parameter problem

• Key points: ICMP reports errors, it never corrects them — recovery (retransmission, congestion control) is the job of TCP/the application. ICMP traffic is rate-limited so that a fault cannot trigger an infinite echo/error storm, and ICMP errors are never generated for ICMP error messages.