Network Layer Service Models

Network layer provides connectionless, best-effort datagram service without reliability guarantees.

Virtual Circuit Service:

• Guarantees reliable communication through connection establishment
• Network ensures reliability
• Connection setup required
• Uses short virtual circuit identifiers after initial setup
• All packets follow same route
• Node failure affects all circuits through it
• Packet ordering preserved
• Error handling may be network or host responsibility

Datagram Service:

• Best-effort delivery without reliability guarantees
• Host ensures reliability
• No connection setup
• Each packet contains full destination address
• Independent routing per packet
• Node failure may cause packet loss and route changes
• No packet ordering guarantees
• Error handling by host

Internet Protocol Suite

Core IP companion protocols:

• Address Resolution Protocol (ARP)
• Internet Control Message Protocol (ICMP)
• Internet Group Management Protocol (IGMP)

Virtual Internetworks

Logical interconnected networks appearing as unified network at network layer.

Intermediate connecting devices:

• Physical layer: Repeaters
• Data link layer: Bridges
• Network layer: Routers
• Above network layer: Gateways

Classful IP Addressing

IP address classification methods:

• Class-based addressing (fundamental)
• Subnetting (enhancement)
• Supernetting/CIDR (modern classless approach)

Classful Address Structure: Fixed classes with network and host portions. Each IP address is globally unique.

Address classes:

• A, B, C: Unicast addresses
• D: Multicast addresses

Class A:

• Maximum networks: 126 (2⁷-2)
• First network: 1
• Last network: 126
• Maximum hosts per network: 16,777,214 (2²⁴-2)

Class B:

• Maximum networks: 16,383 (2¹⁴-1)
• First network: 128.1
• Last network: 191.255
• Maximum hosts per network: 65,534 (2¹⁶-2)

Class C:

• Maximum networks: 2,097,151 (2²¹-1)
• First network: 192.0.1
• Last network: 223.255.255
• Maximum hosts per network: 254 (2⁸-2)

IP Address Characteristics:

• Hierarchical structure with network and host components
• Identifies interface between host/router and link
• Repeater/bridge-connected networks maintain single network ID
• All networks with assigned network IDs are equal

IP vs Hardware Addressing

Hardware addresses operate at data link/physical layers, while IP addresses function at network layer and above as logical addresses.

Address Resolution Protocol

Maps IP addresses to hardware addresses. Source/destination IP remain constant while MAC addresses change per hop.

ARP Operation:

1. Check ARP cache for target IP
2. Broadcast ARP request if not found
3. Target responds with ARP reply
4. Cache mapping for future use

Entries have time-to-live. ARP resolves only local network mappings.

Common ARP Scenarios:

• Host to local host
• Host to remote network host
• Router to local host
• Router to remote network host

IP Datagram Format

Header Fields:

1. Version (4 bits): IP protocol version
2. Header Length (4 bits): Header size in 4-byte units
3. Differentiated Services (8 bits): Quality of service indicators
4. Total Length (16 bits): Entire datagram size
5. Identification (16 bits): Fragmentation grouping
6. Flags (3 bits): Fragmentation control (MF, DF)
7. Fragment Offset (13 bits): Position in original datagram
8. Time to Live (8 bits): Hop count limit
9. Protocol (8 bits): Upper-layer protocol
10. Header Checksum (16 bits): Header integrity check
11. Source Address (32 bits)
12. Destination Address (32 bits)

Variable options field (0-40 bytes). IPv6 uses fixed-length header.

Packet Forwarding Algorithm

def forward_packet(destination_ip, routing_table):
    network_address = extract_network(destination_ip)
    
    # Direct delivery check
    for local_net in directly_connected_networks:
        if network_address == local_net:
            return direct_delivery(destination_ip)
    
    # Specific host route
    if destination_ip in routing_table.host_routes:
        return routing_table.host_routes[destination_ip]
    
    # Network route
    if network_address in routing_table.network_routes:
        return routing_table.network_routes[network_address]
    
    # Default route
    if routing_table.default_route:
        return routing_table.default_route
    
    raise ForwardingError("No route available")

Subnetting and Supernetting

Subnet Addressing

Classful addressing limitations:

• Poor address space utilizasion
• Large routing tables
• Inflexible allocation

Subnet Concept: Borrow host bits for subnet addressing.

Subnet Masks:

• Identify network/subnet portion of IP address
• Network/subnet bits set to 1, host bits to 0
• IP AND Subnet Mask = Network Address

Subnet count: 2^k - 2 (where k = subnet bits), excluding all-zeros and all-ones.

Subnet Forwarding: Routing tables include destination network, subnet mask, and next hop.

Classless Inter-Domain Routing

Eliminates classful addressing concepts. Uses variable-length network prefixes.

CIDR Features:

• Flexible network/host boundary
• Address blocks with contiguous IP ranges
• Route aggregation reduces routing table size

Longest Prefix Match: Most specific route (longest prefix) takes precedence.

Internet Control Message Protocol

Reports errors and exceptional conditions.

Message Types:

• Error Reporting: Destination unreachable, time exceeded, parameter problem, redirect
• Query: Echo request/reply, timestamp request/reply

ICMP Error Reporting Restrictions:

• No errors about error messages
• No errors for fragment subsequences
• No errors for multicast packets
• No errors for special addresses

Applications:

• Ping: Connectivity testing using echo requests
• Traceroute: Path discovery using TTL expiration

Routing Protocols

Autonomous Systems

Networks under single technical administration using:

• Interior Gateway Protocols (within AS)
• Exterior Gateway Protocols (between AS)

Routing Information Protocol

Distance-vector protocol with maximum 15 hops.

RIP Characteristics:

• Exchange information with immediate neighbors only
• Share entire routing table
• Periodic updates
• Slow convergence for network failures

Distance Vector Algorithm:

def update_routing_table(received_table, neighbor_address):
    updated_table = routing_table.copy()
    
    for route in received_table:
        # Adjust metric for hop to neighbor
        route.metric += 1
        route.next_hop = neighbor_address
        
        if route.destination not in updated_table:
            updated_table.add(route)
        elif route.next_hop == neighbor_address:
            updated_table.replace(route)
        elif route.metric < updated_table[route.destination].metric:
            updated_table.replace(route)
    
    return updated_table

Open Shortest Path First

Link-state protocol using flooding for updates.

OSPF Features:

• Sends link-state information to all routers
• Updates only on topology changes
• Fast convergence
• Hierarchical area structure
• Supports variable-length subnet masks
• Multiple equal-cost paths
• Authentication capabilities

OSPF Packet Types:

1. Hello
2. Database description
3. Link-state request
4. Link-state update
5. Link-state acknowledgment

IPv6 Protocol

IPv6 Header

Key Improvements:

• 128-bit addresses
• Fixed 40-byte header
• Flow labeling for QoS
• Extension headers for options

Header Fields:

• Version (4 bits)
• Traffic Class (8 bits)
• Flow Label (20 bits)
• Payload Length (16 bits)
• Next Header (8 bits)
• Hop Limit (8 bits)
• Source Address (128 bits)
• Destination Address (128 bits)

IPv6 Addressing

Address Notation:

• Colon-hexadecimal format
• Zero compression with :: (once per address)
• IPv4 compatibility: ::192.168.1.1

Address Types:

• Unicast
• Multicast
• Anycast

IPv4 to IPv6 Transition

Dual Stack: Hosts/routers run both protocols simultaneously.

Tunneling: Encapsulate IPv6 packets in IPv4 for traversal through IPv4 networks.

ICMPv6

Combines ARP and IGMP functionality into ICMPv6.

Message categories:

• Error messages
• Informational messages
• Neighbor discovery
• Multicast listener discovery

Virtual Private Networks

Emulate private networks over public infrastructure using encryption.

VPN Types:

• Site-to-sitte: Connect multiple locations
• Remote access: Mobile employee connectivity

Network Address Translation

Maps private addresses to public IP addresses.

NAT Operation:

• Replaces source address with public IP for outgoing traffic
• Replaces destination address with private IP for incoming traffic
• Requires connection initiation from private network

NAPT (Network Address Port Translation): Multiple hosts share single public IP using port differentiation.

Multi-Protocol Label Switching

Uses labels for efficient packet forwarding across various network protocols.

Key Features:

• Connection-oriented label switching
• Protocol independence
• Traffic engineering capabilities