Network Layer

• Network Layer - responsible for the end-to-end communication between hosts - unlike the transport layer, which is responsible for the end-to-end communication between processes;
• Sender: encapsulates the transport layer segment into a datagram;
• Receiver: decapsulates the datagram into the transport layer segment;
• There are two key functions in the network layer:
  • Forwarding - move packets from the router’s input to the appropriate router output;
  • Routing - determine the route taken by packets from the source to the destination;
• Data Plane - determines how packets are forwarded through the network;
  • Forwarding Table - maps destination addresses to output ports;
  • Switching - determines how packets are forwarded from input to output ports;
• Control Plane - determines how to build the forwarding table;
  • Routing Protocols - determine the best path to a destination;
  • Routing Algorithms - determine the best path to a destination;
• Longest Prefix Matching - the router uses the most specific prefix that matches the destination address.

IP - Internet Protocol

• IP - the network layer protocol for the Internet;
• IP Datagram - the unit of data exchanged between the network layer entities;
  • Header - contains control information;
    • Version - the IP version - 4 bits;
    • Header Length - the length of the header - 4 bits;
    • Type of Service - the quality of service - 8 bits;
    • Total Length - the length of the datagram - 16 bits;
    • Identification - the identification of the datagram - 16 bits;
    • Flags - control fragmentation - 3 bits;
    • Fragment Offset - the position of the fragment - 13 bits;
    • Time to Live - the maximum number of hops - 8 bits;
    • Protocol - the transport layer protocol - 8 bits;
    • Header Checksum - the header checksum - 16 bits;
    • Source Address - the source IP address - 32 bits;
    • Destination Address - the destination IP address - 32 bits;
    • Options - additional information - variable length;
  • Payload - contains the data;

IP Addressing

• An address is an identifier for a network interface;
• IPv4 - 32-bit addresses;
• IPv6 - 128-bit addresses - created to solve the address exhaustion problem;
  • Fragmentation is not allowed in IPv6;
• CIDR - Classless Inter-Domain Routing is a subnet addressing scheme: a.b.c.d/n;
  • Address - the IP address in format a.b.c.d;
  • Prefix Length - the number of bits in the prefix in format /n;
• Subnet is a range of IP addresses;
• There are also special addresses:
  • 0.0.0.0/8 - the default route;
  • 127.0.0.0/8 - the loopback address;
  • 100.64.0.0/10 - the shared address space;
  • …
• DHCP - Dynamic Host Configuration Protocol is used to assign IP addresses - from application layer;
  • DHCP Server - assigns IP addresses to clients;
  • DHCP Client - requests an IP address from the server;
  • Client broadcasts a DHCP Discover message;
  • Server responds with a DHCP Offer message;
  • Client requests the IP address with a DHCP Request message;
  • Server acknowledges the request with a DHCP Ack message;

NAT - Network Address Translation

• NAT - Network Address Translation is used to map private addresses to public addresses;
  • Private Addresses - addresses that are not routable on the Internet;
  • Public Addresses - addresses that are routable on the Internet;
• NAT Router - translates the private addresses to public addresses;
  • Static NAT - maps a private address to a public address;
  • Dynamic NAT - maps a private address to a public address from a pool;
  • PAT - Port Address Translation maps a private address to a public address and port;

ICMP - Internet Control Message Protocol

• ICMP - Internet Control Message Protocol is used to send error messages and operational information;
• ICMP Message - contains the Type and Code fields;
  • Type - the type of the message;
  • Code - the code of the message;
• ICMP Echo Request - used to test the reachability of a host;
• ICMP Echo Reply - used to respond to an echo request;
• ICMP Destination Unreachable - used to inform that the destination is unreachable;
• ICMP Time Exceeded - used to inform that the time to live has expired;
• ICMP Redirect - used to inform that a better route is available;
• ICMP Source Quench - used to inform that the router is congested;
• …