The Network Layer

1. Functional Overview

Datagram Service:
Simple, flexible, and connectionless.
Best-effort delivery: The network layer does not guarantee reliability (packets may be dropped, duplicated, delayed, or arrive out of order); these functions are offloaded to the Transport Layer.
Heterogeneous Network Interconnection:
Layer 1: Repeaters/Hubs
Layer 2: Bridges/Switches
Layer 3: Routers
Above Layer 3: Gateways
Routing vs. Forwarding:
Routing (Control Plane): Uses distributed algorithms to determine paths based on network topology.
Forwarding (Data Plane): Uses a forwarding table to move IP datagrams to the appropriate output port.
SDN (Software Defined Networking): Separates the data plane (forwarding) from the control plane (centralized routing logic).
Congestion Control: Managed via open-loop and closed-loop control mechanisms.

Static Routing: Non-adaptive; configured manually by network administrators. Best for small networks.
Dynamic Routing: Adaptive; routers exchange routing tables dynamically.
Distance-Vector Algorithm (e.g., RIP): Nodes periodically broadcast their entire routing table to neighbors.
Link-State Algorithm (e.g., OSPF): Each node maintains a complete network topology, tests neighbors, and floods link-state information when changes occur.
Hierarchical Routing: The internet is divided into smaller Autonomous Systems (AS) to prevent massive, unmanageable routing tables.
IGP (Interior Gateway Protocol): RIP, OSPF
EGP (Exterior Gateway Protocol): BGP

Header Length: 4 bits (Max: 15 $\times$ 4B = 60B).
Total Length: Max $2^{16} = 65,535$ bytes.
Fragmentation Fields:
Identification: Shared by fragments of the same original packet.
Flags:
MF (More Fragments): 1 = more fragments follow; 0 = last fragment.
DF (Don’t Fragment): 1 = forbidden to fragment.
Fragment Offset: 13 bits; indicates position in the original packet (units of 8 bytes).
TTL (Time to Live): Prevents routing loops by limiting the number of routers a packet can traverse.
Protocol: Defines the encapsulated transport protocol (e.g., TCP: 6, UDP: 17).
Header Checksum: Validates the integrity of the header only.

Classful Addressing: Classes A, B, C, D (Multicast), and E (Experimental).
NAT (Network Address Translation): Maps private LAN IPs (Class A: 10.x.x.x, B: 172.16.x.x-172.31.x.x, C: 192.168.x.x) to public WAN IPs.
Subnetting & Subnet Mask: Borrowing host bits to create subnets.
CIDR (Classless Inter-Domain Routing): Replaces classful addressing using prefix length (e.g., /20).
Longest Prefix Matching: When multiple routes match, the most specific one (longest prefix) is chosen.

ARP (Address Resolution Protocol): Resolves IP addresses to MAC addresses for local network delivery.
DHCP (Dynamic Host Configuration Protocol): Dynamically assigns IP addresses (Application Layer protocol using UDP).
ICMP (Internet Control Message Protocol): Reports network errors and diagnostics (used by ping and traceroute).

Features: 128-bit address space, simplified header, no transit fragmentation (only source fragments), and automatic configuration.
Address Types: Unicast, Multicast, and Anycast (delivered to the “nearest” node in a group).
Transition Technologies: Dual-stack devices and tunneling (encapsulating IPv6 packets within IPv4).

RIP: Distance-Vector, hop count limit of 15, uses UDP 520.
OSPF: Link-State, uses Dijkstra’s algorithm for shortest path, fast convergence, uses IP protocol 89.
BGP: Path-Vector, used between ASs, runs on TCP.

Uses D-class addresses (224.0.0.0 – 239.255.255.255).
IGMP (Internet Group Management Protocol): Manages multicast group membership.

Allows roaming between networks while maintaining a permanent IP address.
Entities: Mobile Node, Home Agent, and Foreign Agent.
Mechanism: The Home Agent creates a tunnel to the Mobile Node’s “care-of” address in the visited network.

Collision Domain: Nodes competing for the same medium (Hubs/Repeaters do not divide this; Switches/Routers do).
Broadcast Domain: Nodes receiving the same broadcast (Only Routers divide this).