Network Layer: Introduction and Services

Network Layer Services

• The network layer is responsible for host-to-host delivery of datagrams.
• It provides services to the transport layer and receives services from the data-link layer.
• The network layer is involved at the source host, destination host, and all routers in the path.

Packetizing

• The network layer's first duty is packetizing, which is encapsulating the payload (data received from the upper layer) in a network-layer packet at the source and decapsulating the payload from the network-layer packet at the destination.
• The network layer is responsible for carrying a payload from the source to the destination without changing it or using it.

Routing

• The network layer is responsible for routing the packet from its source to the destination.
• The network layer needs to have some specific strategies for defining the best route.
• Routing protocols are used to help the routers coordinate their knowledge about the neighborhood and to come up with consistent tables to be used when a packet arrives.

Forwarding

• Forwarding is the action applied by each router when a packet arrives at one of its interfaces.
• The decision-making table used by a router is called the forwarding table or routing table.
• When a router receives a packet from one of its attached networks, it needs to forward the packet to another attached network (in unicast routing) or to some attached networks (in multicast routing).

Error Control

• Error control means including a mechanism for detecting corrupted, lost, or duplicate datagrams.
• Error control also includes a mechanism for correcting errors after they have been detected.
• The Internet does not directly provide error control at the network layer, but uses an auxiliary protocol, ICMP, which provides some kind of error control.

Switching

• Switching is the process of selecting one of the output ports through which the packet needs to be sent out.
• The two types of switching are:
  • Circuit switching: a physical circuit (or channel) is established between the source and destination of the message before the delivery of the message.
  • Packet switching: the message is divided into manageable packets at the source before being transmitted, and the packets are assembled at the destination.

Packet Switching at Network Layer

• The network layer is designed as a packet-switched network.
• The packet-switched network layer of the Internet was originally designed as a Connectionless service, but recently there is a tendency to change this to a Connection-oriented service.
• The two approaches to packet switching are:
  • Datagram approach: each packet is routed independently, and the packets in a message may or may not travel the same path to their destination.
  • Virtual-circuit approach: a virtual connection is set up to define the path for the datagrams, and the datagrams follow the same path.

Network Layer Performance

• The performance of a network pertains to the measure of service quality of a network as perceived by the user.
• The performance of a network can be measured in terms of:
  • Routing efficiency: latency (delay) and throughput
  • Packet loss: the number of packets lost during transmission

Latency

• Latency is defined as the total time taken for a complete message to arrive at the destination, starting with the time when the first bit of the message is sent out from the source and ending with the time when the last bit of the message is delivered at the destination.
• Latency is measured in milliseconds (ms).

Throughput

• Throughput is the number of messages successfully transmitted per unit time.
• Throughput is controlled by available bandwidth, the available signal-to-noise ratio, and hardware limitations.
• Throughput is measured in terms of bits per second (bps), bytes per second (Bps), kilobytes per second (KBps), megabytes per second (MBps), and gigabytes per second (Gbps).

Packet Loss

• Packet loss is the number of packets lost during transmission.
• Packet loss can be caused by congestion in the network, and can lead to overflow and cause more packet loss.

IPv4 Address

• An IPv4 address is a 32-bit address that uniquely and universally defines the connection of a host or a router to the Internet.
• IPv4 addresses are hierarchical, with a prefix that defines the network and a suffix that defines the node (connection of a device to the Internet).
• The address space of IPv4 is 2^32, or 4,294,967,296.

Classful Addressing

• Classful addressing is a system of addressing that divides the address space into five classes (A, B, C, D, and E).
• Each class has a fixed prefix length, which determines the number of nodes that can be connected to the Internet.
• Classful addressing has become obsolete due to address depletion.

Subnetting and Supernetting

• Subnetting is a technique of dividing a class A or class B block into several subnets.
• Supernetting is a technique of combining several class C blocks into a larger block.
• Both subnetting and supernetting were devised to conserve addresses and make routing more efficient.

Classless Addressing

• Classless addressing is a system of addressing that uses variable-length blocks.
• In classless addressing, the whole address space is divided into variable-length blocks.
• The prefix length in classless addressing is variable, and can range from 0 to 32.
• Classless addressing is more flexible and efficient than classful addressing.### Classless Addressing
• An address in class A, B, C can be thought of as a classless address with a prefix length of 8, 16, and 24 respectively.
• Classful addressing is a special case of classless addressing.

Prefix Length and Slash Notation

• The prefix length, n, is added to the address, separated by a slash (/).
• This notation is informally referred to as slash notation and formally as Classless Interdomain Routing (CIDR).

Extracting Information from an Address

• Three pieces of information can be extracted from an address:
  • The number of addresses in the block: N = 2^(32-n)
  • The first address in the block: keep the n leftmost bits and set the (32 - n) rightmost bits to 0s
  • The last address in the block: keep the n leftmost bits and set the (32 - n) rightmost bits to 1s

Subnetting

• An organization can divide a range of addresses into several subranges and assign each subrange to a subnetwork (subnet).
• A subnet can be divided into several sub-subnets, and so on.
• Designing subnets requires careful planning to guarantee proper operation.

Designing Subnets

• The number of addresses in each subnetwork should be a power of 2.
• The prefix length for each subnetwork should be found using the formula: nsub = 32 - log2Nsub.
• The starting address in each subnetwork should be divisible by the number of addresses in that subnetwork.

DHCP (Dynamic Host Configuration Protocol)

• DHCP is a network management protocol used to dynamically assign IP addresses and other network configuration parameters to devices on a network.
• DHCP operates on the client-server model, where a DHCP server manages and allocates IP addresses to DHCP clients.

DHCP Server and Client

• The DHCP server is responsible for allocating and managing IP addresses and other configuration parameters.
• The DHCP client requests and obtains network configuration information from a DHCP server.

DHCP Leases

• DHCP leases are temporary assignments of IP addresses and other configuration parameters to DHCP clients.
• Leases have a duration, known as the lease time, after which the client must renew its lease.

DHCP Messages

• DHCP Discover: The DHCP client broadcasts a message to discover available DHCP servers on the network.
• DHCP Offer: DHCP servers respond to DHCP Discover messages with DHCP Offer messages.
• DHCP Request: The DHCP client selects an offered IP address and sends a DHCP Request message to the chosen DHCP server.
• DHCP Acknowledgment: The DHCP server sends a DHCP Acknowledgment message to confirm the assignment of the IP address and provide the client with the configuration parameters.

NAT (Network Address Translation)

• NAT is a technology that provides the mapping between private and universal addresses and supports virtual private networks.
• NAT types: Static NAT, Dynamic NAT, and NAT Overload (PAT - Port Address Translation).

Forwarding of IP Packets

• Forwarding means to deliver the packet to the next hop, which can be the final destination or an intermediate connecting device.
• Forwarding can be based on the destination address of the IP datagram or the label attached to an IP datagram.
• A forwarding table is used to find the next hop to deliver the packet to.

Network Layer Services

• The network layer is responsible for host-to-host delivery of datagrams.
• It provides services to the transport layer and receives services from the data-link layer.
• The network layer is involved at the source host, destination host, and all routers in the path.

Packetizing

• The network layer's first duty is packetizing, which is encapsulating the payload (data received from the upper layer) in a network-layer packet at the source and decapsulating the payload from the network-layer packet at the destination.
• The network layer is responsible for carrying a payload from the source to the destination without changing it or using it.

Routing

• The network layer is responsible for routing the packet from its source to the destination.
• The network layer needs to have some specific strategies for defining the best route.
• Routing protocols are used to help the routers coordinate their knowledge about the neighborhood and to come up with consistent tables to be used when a packet arrives.

Forwarding

• Forwarding is the action applied by each router when a packet arrives at one of its interfaces.
• The decision-making table used by a router is called the forwarding table or routing table.
• When a router receives a packet from one of its attached networks, it needs to forward the packet to another attached network (in unicast routing) or to some attached networks (in multicast routing).

Error Control

• Error control means including a mechanism for detecting corrupted, lost, or duplicate datagrams.
• Error control also includes a mechanism for correcting errors after they have been detected.
• The Internet does not directly provide error control at the network layer, but uses an auxiliary protocol, ICMP, which provides some kind of error control.

Switching

• Switching is the process of selecting one of the output ports through which the packet needs to be sent out.
• The two types of switching are:
  • Circuit switching: a physical circuit (or channel) is established between the source and destination of the message before the delivery of the message.
  • Packet switching: the message is divided into manageable packets at the source before being transmitted, and the packets are assembled at the destination.

Packet Switching at Network Layer

• The network layer is designed as a packet-switched network.
• The packet-switched network layer of the Internet was originally designed as a Connectionless service, but recently there is a tendency to change this to a Connection-oriented service.
• The two approaches to packet switching are:
  • Datagram approach: each packet is routed independently, and the packets in a message may or may not travel the same path to their destination.
  • Virtual-circuit approach: a virtual connection is set up to define the path for the datagrams, and the datagrams follow the same path.

Network Layer Performance

• The performance of a network pertains to the measure of service quality of a network as perceived by the user.
• The performance of a network can be measured in terms of:
  • Routing efficiency: latency (delay) and throughput
  • Packet loss: the number of packets lost during transmission

Latency

• Latency is defined as the total time taken for a complete message to arrive at the destination, starting with the time when the first bit of the message is sent out from the source and ending with the time when the last bit of the message is delivered at the destination.
• Latency is measured in milliseconds (ms).

Throughput

• Throughput is the number of messages successfully transmitted per unit time.
• Throughput is controlled by available bandwidth, the available signal-to-noise ratio, and hardware limitations.
• Throughput is measured in terms of bits per second (bps), bytes per second (Bps), kilobytes per second (KBps), megabytes per second (MBps), and gigabytes per second (Gbps).

Packet Loss

• Packet loss is the number of packets lost during transmission.
• Packet loss can be caused by congestion in the network, and can lead to overflow and cause more packet loss.

IPv4 Address

• An IPv4 address is a 32-bit address that uniquely and universally defines the connection of a host or a router to the Internet.
• IPv4 addresses are hierarchical, with a prefix that defines the network and a suffix that defines the node (connection of a device to the Internet).
• The address space of IPv4 is 2^32, or 4,294,967,296.

Classful Addressing

• Classful addressing is a system of addressing that divides the address space into five classes (A, B, C, D, and E).
• Each class has a fixed prefix length, which determines the number of nodes that can be connected to the Internet.
• Classful addressing has become obsolete due to address depletion.

Subnetting and Supernetting

• Subnetting is a technique of dividing a class A or class B block into several subnets.
• Supernetting is a technique of combining several class C blocks into a larger block.
• Both subnetting and supernetting were devised to conserve addresses and make routing more efficient.

Classless Addressing

• Classless addressing is a system of addressing that uses variable-length blocks.
• In classless addressing, the whole address space is divided into variable-length blocks.
• The prefix length in classless addressing is variable, and can range from 0 to 32.
• Classless addressing is more flexible and efficient than classful addressing.### Classless Addressing
• An address in class A, B, C can be thought of as a classless address with a prefix length of 8, 16, and 24 respectively.
• Classful addressing is a special case of classless addressing.

Prefix Length and Slash Notation

• The prefix length, n, is added to the address, separated by a slash (/).
• This notation is informally referred to as slash notation and formally as Classless Interdomain Routing (CIDR).

Extracting Information from an Address

• Three pieces of information can be extracted from an address:
  • The number of addresses in the block: N = 2^(32-n)
  • The first address in the block: keep the n leftmost bits and set the (32 - n) rightmost bits to 0s
  • The last address in the block: keep the n leftmost bits and set the (32 - n) rightmost bits to 1s

Subnetting

• An organization can divide a range of addresses into several subranges and assign each subrange to a subnetwork (subnet).
• A subnet can be divided into several sub-subnets, and so on.
• Designing subnets requires careful planning to guarantee proper operation.

Designing Subnets

• The number of addresses in each subnetwork should be a power of 2.
• The prefix length for each subnetwork should be found using the formula: nsub = 32 - log2Nsub.
• The starting address in each subnetwork should be divisible by the number of addresses in that subnetwork.

DHCP (Dynamic Host Configuration Protocol)

• DHCP is a network management protocol used to dynamically assign IP addresses and other network configuration parameters to devices on a network.
• DHCP operates on the client-server model, where a DHCP server manages and allocates IP addresses to DHCP clients.

DHCP Server and Client

• The DHCP server is responsible for allocating and managing IP addresses and other configuration parameters.
• The DHCP client requests and obtains network configuration information from a DHCP server.

DHCP Leases

• DHCP leases are temporary assignments of IP addresses and other configuration parameters to DHCP clients.
• Leases have a duration, known as the lease time, after which the client must renew its lease.

DHCP Messages

• DHCP Discover: The DHCP client broadcasts a message to discover available DHCP servers on the network.
• DHCP Offer: DHCP servers respond to DHCP Discover messages with DHCP Offer messages.
• DHCP Request: The DHCP client selects an offered IP address and sends a DHCP Request message to the chosen DHCP server.
• DHCP Acknowledgment: The DHCP server sends a DHCP Acknowledgment message to confirm the assignment of the IP address and provide the client with the configuration parameters.

NAT (Network Address Translation)

• NAT is a technology that provides the mapping between private and universal addresses and supports virtual private networks.
• NAT types: Static NAT, Dynamic NAT, and NAT Overload (PAT - Port Address Translation).

Forwarding of IP Packets

• Forwarding means to deliver the packet to the next hop, which can be the final destination or an intermediate connecting device.
• Forwarding can be based on the destination address of the IP datagram or the label attached to an IP datagram.
• A forwarding table is used to find the next hop to deliver the packet to.