2101 Ch08: Networking Layer Overview and Functions

Network Layer Overview

• The network layer (OSI Layer 3) is responsible for communication protocols and routing protocols.
• Routers are used to forward data between networks.
• Key network layer protocols include Internet Protocol version 4 (IPv4), Internet Protocol version 6 (IPv6), Open Shortest Path First (OSPF), and Internet Control Message Protocol (ICMP).

Network Layer Functions

• Addressing end devices: Each device must have a unique IP address for identification.
• Encapsulation: The network layer encapsulates data from the transport layer into a packet by adding an IP header, which includes source and destination IP addresses.
• Routing: Routers use routing tables to determine the best path to forward packets to their destination.
• De-encapsulation: When a packet arrives at the destination host, the IP header is removed.

Network Layer Characteristics

• IPv4 and IPv6 are the primary network layer communication protocols.
• IP is connectionless, meaning no dedicated connection is established before transmitting data.
• IP is best-effort, meaning it does not guarantee delivery of all packets.
• IP is media-independent, meaning it operates independently of the underlying physical network.

IPv4 Packet Structure

• The IPv4 header contains fields with binary values that specify packet settings.
• Key fields include: version, DS, header checksum, TTL, protocol, source IP address, and destination IP address.

IPv6 Packet Structure

• IPv6 was designed to address IPv4 limitations such as address depletion and network complexity.
• Key fields include: version, traffic class, flow label, payload length, next header, hop limit, source IPv6 address, and destination IPv6 address.

Host Routing

• Hosts use routing tables to determine where to send packets.
• The netstat -r (or route print) command displays the host's routing table.
• The routing table lists all known routes, including direct connections, local network, and default routes.

Router Operations

• Routers also contain routing tables, which are used to forward packets based on destination IP addresses.
• The router's routing table includes routes for directly-connected networks, statically configured routes, and routes learned from routing protocols like OSPF.
• Routers forward packets based on the best (longest) matching route entry.

Routing Table Entries

• Directly-connected networks: These routes represent active router interfaces.
• Static routes: These routes are manually configured.
• Dynamic routes: These routes are learned using routing protocols.

Data Encapsulation and De-encapsulation

• The process of encapsulating data at each layer allows for independent development and scaling.
• The transport layer PDU is encapsulated into an IP packet by adding an IP header.
• Routers and Layer 3 switches examine the IP header as the packet travels across the network.
• IP addressing remains unchanged throughout the network, except when NAT is used.

Network Address Translation (NAT)

• NAT is a mechanism for translating network addresses from one format to another.
• NAT is commonly used to conserve IPv4 addresses.
• NAT is discussed in later modules.

Router Operations

• Routers use routing protocols to determine the best path for packets.
• The network layer addressing (IP address) is examined by routers for packet forwarding.
• The data portion of the packet remains unchanged during network layer processing.

IP Protocol Design

• IP was designed for low overhead, providing only the necessary functions for packet delivery.
• IP is not designed to track or manage the flow of individual packets.