Network Layer Functions & IP

Network Layer Functions

• Network layer crucial for forwarding and routing data.
• Forwarding moves packets from a router's input to the appropriate output link.
• Routing determines the path packets take from source to destination.

IP - Internet Protocol

• Internet Protocol (IP) includes IPv4 & IPv6 datagram formats, addressing, DHCP, ICMP, NAT
• Primary responsibility: Supervises host-to-host packet delivery.
• Transport layer is process-to-process.
• Data link layer is node-to-node.
• Involves transporting segments from a sending host to a receiving host.
• The sender encapsulates segments into datagrams, which are then passed to the link layer.
• The receiver delivers segments to the transport layer protocol.
• Network layer protocols are implemented in every Internet device including hosts and routers.
• Routers examine header fields in IP datagrams and move datagrams from input to output ports.
• Data Plane: Local, per-router function that forwards datagrams based on destination.
• Control Plane: Network-wide logic determining how datagrams are routed end-to-end.
• Routers are specialized computers with CPU, OS and memory that handle packet forwarding.

IP Protocol

• Path selection algorithms are implemented via routing protocols.
• The IP protocol handles datagram formatting, addressing and packet handling conventions.
• ICMP handles error reporting and router signaling.
• It is a connectionless protocol, requiring no dedicated end-to-end connection setup.
• Senders do not get confirmation about the destination's status or reachability.
• IP provides best effort delivery, but does not guarantee packet delivery.
• It is media independent, working across various types of media.

Fragmentation

• Network links have a Maximum Transmission Unit (MTU) which is the largest possible link-level frame.
• Different link types have different MTUs.
• Large IP datagrams are divided into smaller "fragments" within the network.
• One datagram can become several datagrams after fragmentation.
• Fragments are reassembled, at final destination.
• IP header bits are used to identify and order related fragments.

IPv4 Datagrams

• IPv4 datagram contains a 32-bit IP address.
• Consists of fields that indicate version,header length and type of service.
• Holds 16-bit identifier and the fragmentation offset.
• Contains TTL (remaining max hops) & upper layer protocol.
• Has header checksum.
• Includes 32-bit source and destination IP addresses.
• Includes options and payload, variable length.
• Overhead of 20 bytes for TCP and 20 bytes for IP.

IPv4 Addresses

• A 32-bit identifier associated with each host or router interface.
• An interface is the connection between host/router and physical link.
• Routers typically have multiple interfaces.
• Hosts typically have 1-2 interfaces.
• IPv4 addresses are expressed in 32 binary bits, divided into four 8-bit octets.
• IPv4 addresses follow a hierarchical structure with network and host portions.
• Devices on the same network must have identical network portions.

IPv4 Addressing Classes

• Several classes, each defining range of first octet & network/host division
• Class A: 1-126 first octet (0/127 is reserved).
• Class B: 128-191.
• Class C: 192-223.
• Class D: 224-239.
• Class E: 240-254 (255 is reserved).

Subnet Mask

• Identifies the network/host portion of the IPv4 address.

Subnets

• A subnet consists of device interfaces that can reach each other without passing through an intervening router.
• IP addresses have structure, consisting of a subnet part and a host part.
• Devices in the same subnet have common high order bits in their IP addresses.
• The host part consists of the remaining lower order bits in the IP Address

IPv4 Address Types

• Types in network 192.168.10.0/24 include network address, host address, broadcast address.
• Network address has all 0s in host portion.
• First host address has all 0s, ends in a 1.
• Last host address has all 1s, ends in a 0.
• Broadcast address has all 1s in host portion.

IPv4 Addressing: CIDR

• CIDR (Classless Inter-Domain Routing) is based on prefix length and is the subnet portion of the arbitrary length address.
• Shorthand method of expressing the subnet mask.
• Address format is in the form a.b.c.d/x, where x is the number of bits in the subnet portion of address.

Subnetting

• Subnetting or subnet-working
• The process of obtaining additional bits from the left most or the most significant bits of the Host portion of the network to further divide the network into smaller subnetworks
• Used to conserve the number of IP Addresses available and decrease the unused IP Addresses of networks with few hosts in relation to the IP Address range
• Fixed Length Subnet Masking
• Variable Length Subnet Masking

Fixed length subnet mask

• Strategy where every network within infrastructure has same size

FLSM example

• Example of Fixed Lenght Subnet Mask
• MSB = most significant bit LSB= least significant bit

Network and Broadcast Address

• For IP address 192.168.1.182/24, to check network address, perform AND operation to subnet mask.
• Network address would be 192.168.1.0.
• Check its broadcast address, perform OR operation to its subnet mask (flip).
• Broadcast address is 192.168.1.255.

IPv4 Communication

• Unicast is one-to-one communication.
• Broadcast is one-to-all communication.
• Multicast is one-to-select group

DHCP

• DHCP (Dynamic Host Configuration Protocol) dynamically assigns IP addresses from a network server when a host joins a network.
• The joining host can renew its lease on an address in use.
• DHCP allows addresses to be reused.
• Supports mobile users who want to join the network.
• DHCP returns IP address, address of first-hop router for client, Name and IP address of DNS server and Network mask.

Limitations of IPv4

• IPv4 faces issues with IP address depletion, Internet routing table expansion, and lack of end-to-end connectivity.
• Public and Private IPv4 Addresses include private address blocks for internal networks:
  • 10.0.0.0 /8
  • 172.16.0.0 /12
  • 192.168.0.0 /16

Network Address Translation

• NAT enables datagrams that are leaving a local network to have the same source NAT IP address.

ICMP

• ICMP is a messaging protocol used by hosts and routers to communicate network-level information.
• Some functions include reporting errors, echo requests, and route redirection.

IPv6

• IPv6 uses a 128-bit address space, allowing for 340 undecillion addresses.
• It solves limitations associated with IPv4.
• Motivations for IPv6:
  • 32-bit address space soon to be completely allocated
  • Header format helps speed processing and forwarding
  • Header changes to facilitate QoS.
• IPv6 addresses are 128 bits in length.
  • Every 4 bits is represented by a single hexadecimal digit
  • Example IPv6 address: 2001:0DB8:0000:1111:0000:0000:0000:0200
• IPv6 can be reduced or compressed.
  • Omit leading zeros in any segment or Omit All 0 Segments.
• Example compressed IPv6: 2001:0DB8::1111::0200
• IPv6 Datagram format

Datagram

• Priority, Identify priority among datagrams in flow
• Flow Label, Identify datagrams in same “flow.”
• Next header, Identify upper layer protocol for data