📡Systems Approach to Computer Networks Unit 10 – Network Layer: Forwarding & Routing

Key Concepts

• Network layer responsible for delivering packets from source to destination across multiple links and through intermediate routers
• Forwarding moves packets from router's input to appropriate output port based on routing table
• Routing determines path taken by packets from source to destination using routing algorithms
• IP addressing assigns unique identifiers to devices on a network enabling packet delivery
• Subnetting divides IP address space into smaller subnetworks for efficient address allocation and network management
  • Subnet mask determines network and host portions of an IP address
  • Classless Inter-Domain Routing (CIDR) notation represents IP address and subnet mask (192.168.1.0/24)
• Routing protocols (OSPF, BGP) exchange information between routers to populate routing tables and determine optimal paths

Network Layer Functions

• Addressing assigns unique identifiers to devices for packet delivery and network configuration
• Encapsulation adds network layer header containing source and destination IP addresses to enable end-to-end delivery
• Fragmentation and reassembly divides packets into smaller fragments when necessary to accommodate link MTU and reassembles at the destination
• Routing determines optimal path for packets from source to destination based on network topology and routing algorithms
• Forwarding moves packets between input and output ports of routers along the determined path
  • Routing table lookup matches destination IP address to appropriate output interface
• Error handling detects and attempts to recover from network layer errors (TTL expiration, checksum mismatch)
• Quality of Service (QoS) prioritizes and manages network traffic to meet performance requirements of different applications

Forwarding Basics

• Forwarding table maps destination IP addresses to output interfaces for packet forwarding
• Longest prefix match determines most specific entry in forwarding table that matches packet's destination IP
  • Example: Packet destined for 192.168.1.100 matches entry for 192.168.1.0/24 rather than 192.168.0.0/16
• Forwarding decision made independently for each packet based on current state of forwarding table
• Forwarding tables populated by routing protocols or manually configured by network administrators
• Hardware-based forwarding uses specialized ASICs for high-speed packet processing and forwarding
• Software-based forwarding implements forwarding logic in router's operating system for flexibility and control
  • Slower than hardware-based forwarding but allows for easier modification and customization

Routing Algorithms

• Link-state algorithms (OSPF) flood network topology information to all routers, allowing each router to independently compute shortest paths
  • Dijkstra's algorithm used to calculate shortest paths based on link costs
  • Converges quickly but requires more memory and processing power
• Distance-vector algorithms (RIP) exchange routing information only with directly connected neighbors
  • Bellman-Ford algorithm used to iteratively update and propagate routing information
  • Slower convergence and prone to routing loops but simpler and less resource-intensive
• Path-vector algorithms (BGP) include full path information with each routing update to prevent loops and allow for policy-based routing
• Hierarchical routing divides network into smaller, more manageable areas or levels to improve scalability and reduce routing table size
  • OSPF uses two-level hierarchy with backbone area (Area 0) and regular areas
• Load balancing distributes traffic across multiple paths of equal cost to improve network performance and reliability

IP Addressing and Subnetting

• IP addresses uniquely identify devices on a network and consist of network and host portions
  • IPv4 addresses are 32 bits long, represented in dotted-decimal notation (192.168.1.1)
  • IPv6 addresses are 128 bits long, represented in hexadecimal notation (2001:0db8:85a3:0000:0000:8a2e:0370:7334)
• Subnetting divides IP address space into smaller subnetworks for efficient address allocation and network management
  • Subnet mask determines the network and host portions of an IP address
  • Classless Inter-Domain Routing (CIDR) notation represents IP address and subnet mask (192.168.1.0/24)
• Private IP addresses (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16) are not globally unique and can be used within an organization
• Public IP addresses are globally unique and assigned by regional internet registries (ARIN, RIPE, APNIC)
• Network Address Translation (NAT) allows multiple devices with private IP addresses to share a single public IP address
  • Enables conservation of public IP address space and provides a level of security by hiding internal network structure

Routing Protocols

• Interior Gateway Protocols (IGPs) operate within a single autonomous system (AS) to exchange routing information between routers
  • Open Shortest Path First (OSPF) is a link-state IGP that floods topology information and uses Dijkstra's algorithm for shortest path calculation
  • Routing Information Protocol (RIP) is a distance-vector IGP that exchanges routing updates with directly connected neighbors
• Exterior Gateway Protocols (EGPs) operate between autonomous systems to exchange routing information and enable inter-domain routing
  • Border Gateway Protocol (BGP) is the de facto standard EGP used for routing between ASes on the Internet
  • BGP uses path-vector algorithm and supports policy-based routing for traffic engineering and business relationships
• Routing protocol configuration includes enabling the protocol on router interfaces, setting router IDs, and defining area or AS boundaries
• Routing protocol authentication ensures the integrity and origin of routing updates to prevent unauthorized changes or attacks
  • MD5 or SHA-based authentication methods commonly used
• Redistribution allows for the exchange of routing information between different routing protocols or instances to enable end-to-end connectivity

Network Layer Protocols

• Internet Protocol (IP) is the primary network layer protocol for the Internet, responsible for addressing, encapsulation, and fragmentation
  • IPv4 is the most widely used version, with a 32-bit address space
  • IPv6 is the next-generation protocol, with a 128-bit address space and improved security and quality of service features
• Internet Control Message Protocol (ICMP) is used for error reporting and diagnostic purposes in IP networks
  • ICMP messages include destination unreachable, time exceeded, and echo request/reply (used by ping utility)
• Internet Group Management Protocol (IGMP) is used for managing multicast group membership in IP networks
  • Allows hosts to join or leave multicast groups and routers to track group membership for efficient multicast forwarding
• Address Resolution Protocol (ARP) maps IP addresses to MAC addresses for communication within a broadcast domain
  • ARP cache stores recently resolved IP-to-MAC mappings for improved performance
• Reverse Address Resolution Protocol (RARP) maps MAC addresses to IP addresses, used by diskless workstations to obtain an IP address during boot

Real-World Applications

• Content Delivery Networks (CDNs) use anycast routing to direct users to the nearest content server for improved performance and scalability
  • Anycast assigns the same IP address to multiple servers, allowing BGP to route users to the closest server based on network topology
• Software-Defined Networking (SDN) separates the control plane and data plane, enabling centralized, programmable network control
  • OpenFlow protocol used for communication between SDN controllers and network devices
  • Enables fine-grained traffic engineering, network virtualization, and rapid deployment of new services
• Virtual Private Networks (VPNs) use network layer tunneling protocols (IPsec, GRE) to create secure, encrypted connections over public networks
  • Allows remote users to securely access corporate resources and enables site-to-site connectivity between branch offices
• Internet of Things (IoT) devices often use 6LoWPAN, an adaptation of IPv6 for low-power wireless networks
  • Enables IP-based communication and interoperability among resource-constrained IoT devices
  • Supports header compression and fragmentation to reduce overhead and accommodate small packet sizes
• Network address translation (NAT) and port forwarding enable communication between private networks and the public Internet
  • NAT translates between private and public IP addresses, allowing multiple devices to share a single public IP
  • Port forwarding maps specific ports on the public IP to devices on the private network, enabling inbound connectivity to servers or services