Network Layer Functions Overview

Network Layer Functions

• The network layer (OSI Layer 3) enables end devices to exchange data across networks.
• Key network layer protocols include IPv4, IPv6, OSPF (routing protocol), and ICMP (messaging protocol).
• The network layer performs four basic operations: addressing, encapsulation, routing, and de-encapsulation.

Addressing

• End devices must be configured with a unique IP address for identification on the network.

Encapsulation

• The network layer encapsulates the transport layer PDU (protocol data unit) into a packet.
• The source host adds the IP header containing source and destination IP addresses.

Routing

• The network layer directs packets to the destination host on another network.
• Routers examine packet headers to determine the best path and forward packets.
• Each router the packet crosses is called a hop.

De-encapsulation

• The destination host checks the IP header and removes it if the destination IP address matches.
• The transport layer PDU is then passed up to the appropriate service.

IP Characteristics

• Connectionless: No dedicated connection is established before sending data.
• Best Effort: Packet delivery is not guaranteed, and IP does not manage packet flow.
• Media Independent: Operates independently of the physical medium used to carry data (copper, fiber, wireless).

IPv4 Packet Header

• Identifies the packet as IPv4 (version 4).
• Contains fields for differentiated services (prioritization), time to live (packet lifetime), protocol (payload type), header checksum (integrity check), and source and destination IP addresses.

IPv6 Overview

• Introduced to address IPv4 limitations, specifically address depletion.
• Provides a larger address space, simplified header, and eliminates the need for NAT.
• IPv6 uses 128-bit addresses, avoiding the address depletion issue.

IPv6 Packet Header

• Simplified header compared to IPv4.
• Includes fields for version, traffic class, flow label (packet handling), payload length, next header (protocol), hop limit (packet lifetime), and source and destination IPv6 addresses.

IPv4 Address Depletion

• IPv4 has a limited number of unique public addresses.
• Growing number of IP-enabled devices and ever-increasing demand for internet access have led to address depletion.

IPv4 Packet Structure

• Contains a header and a data payload.
• The header contains information about the packet, such as its version, size, and destination.
• The payload contains the actual data being transmitted.

IPv4 Packets

• Packets are broken into fragments when too large for the network's Maximum Transmission Unit (MTU).
• Each fragment has its own header, but they share the same identification number.
• Packet headers include a checksum for integrity verification during transmission.
• IPv4 packets are the foundation of internet data transmission.

IPv6 Header Structure

• Fixed length of 40 bytes regardless of options or extensions.
• Contains critical information for routing packets across networks.
• Key fields:
  • Version (4 bits): Indicates IPv6 (set to 6).
  • Traffic Class (8 bits): Similar to IPv4's Type of Service, used for Quality of Service (QoS).
  • Flow Label (20 bits): Identifies packets needing special handling (e.g., real-time data flows).
  • Payload Length (16 bits): Length of data + any extension headers (excluding the header).
  • Next Header (8 bits): Indicates the next header type (e.g., TCP/UDP or an extension header).
  • Hop Limit (8 bits): Equivalent to IPv4's TTL, decrements at each router hop, discarding the packet at zero.
  • Source Address (128 bits): Sender's IPv6 address.
  • Destination Address (128 bits): Recipient's IPv6 address.

IPv6 Extension Headers

• Used for optional information, streamlining the base header.
• Examples:
  • Hop-by-Hop Options: Processed by every router along the packet's path.
  • Routing Header: Specifies intermediate nodes for routing.
  • Fragment Header: Used when packets need to be fragmented.
  • Destination Options: Information intended only for the destination node.

IPv6 Payload

• Contains the actual data being transmitted.
• Includes upper-layer protocol data (e.g., TCP/UDP segments) or other extension headers.

Key Improvements in IPv6

• Larger Address Space: 128-bit addresses allow for a much larger number of unique IP addresses compared to IPv4's 32-bit addresses.
• No Fragmentation by Routers: Routers do not fragment packets. Fragmentation is handled by the source device using the Fragment extension header.
• Simplified Header: IPv6's fixed-length header simplifies routing and improves processing efficiency.

Key Differences from IPv4

• IPv6 uses 128-bit addresses instead of IPv4's 32-bit addresses.
• IPv6 has a fixed 40-byte header, whereas IPv4 has a variable header size.
• IPv6 uses extension headers for optional functionality, maintaining a streamlined base header.
• No broadcast support in IPv6; it uses multicast instead.

IPv6 Design Goals

• Address the limitations of IPv4, particularly address exhaustion.
• Provide a more scalable and efficient framework for the future of internet communication.