Network Layer COMPUTER NETWORKS Networking Standards (Network LAYER)

What is the main goal of Network layer? Question to be answered ? What is the main goal of Network layer? How does Network layer achieve its goal? Computer Networks

Network Layer Network Layer - The network layer is concerned with getting packets from the source all the way to the destination. - Therefore, the main function (goal) of network layer is routing packets from the source machine to the destination machine. Computer Networks

To achieve its goal the network layer must: 1- Know about the topology of the communication subnet. 2- Choose appropriate paths through the subnet. 3- Avoid overloaded communication lines and routers (avoid congestion). Computer Networks

Store-and-Forward Packet Switching fig 5-1 The environment of the network layer protocols.

Store-and-Forwarding Packet Switching Network Layer Store-and-Forwarding Packet Switching In Store-and Forwarding Packet Switching mechanism, a host transmits a packet to the nearest router, the packet is stored there until it has fully arrived so the checksum can be verified, then it is forwarded to the next router along the path until it reaches the destination host. Computer Networks

Services provided to the Transport Layer Network Layer Services provided to the Transport Layer There are two services provided by the network layer to the transport layer: Connectionless service. 2) Connection-Oriented Service. Computer Networks

Implementation of Connectionless Service Routing within a diagram subnet.

Implementation of Connection-Oriented Service Routing within a virtual-circuit subnet.

Comparison of Virtual-Circuit and Datagram Subnets 5-4

Network Layer Routing Algorithms In most subnets, packets require multiple hops to reach their destinations. The Routing algorithm is the software part of the network layer responsible for deciding which output line an incoming packet should be transmitted on. Computer Networks

Network Layer Routing Algorithms The decision about the best route either calculated once as the case with connection-oriented paradigm or for every arriving data packet in the case of the subnet uses connectionless paradigm. Computer Networks

Desirable properties in a routing algorithm: Correctness 2) Simplicity Network Layer Routing Algorithms Desirable properties in a routing algorithm: Correctness 2) Simplicity 3) Robustness (hardware or software failures) 4) Stability (to be run for long time) 5) Fairness 6) Optimality Robustness means RA should be able to cope with changes in the topology and traffic without requiring all jobs in all hosts to be aborted and the network to be rebooted every time some router crashes. Computer Networks

Conflict between fairness and optimality. Routing Algorithms Conflict between fairness and optimality.

The Optimality Principle (a) A subnet. (b) A sink tree for router B.

Routing Algorithms can be grouped in two major classes: Network Layer Routing Algorithms Routing Algorithms can be grouped in two major classes: Non-adaptive algorithms (Static Routing) 2) Adaptive algorithms (Dynamic Routing) Computer Networks

Network Layer Routing Algorithms Non-adaptive algorithms, they do not base their routing decision on measurements or estimates of the current traffic and topology. Instead the route is computed in advance, off-line, and downloaded to the routers when the network is booted. Computer Networks

- Adaptive algorithms differ in where they get their information. Network Layer Routing Algorithms Adaptive algorithms, they change their routing decisions to reflect changes in the topology, and usually the traffic as well. - Adaptive algorithms differ in where they get their information. Computer Networks

What metric is used for optimization? - Distance - number of hops Network Layer Routing Algorithms What metric is used for optimization? - Distance - number of hops - estimated transit time - delay - bandwidth Computer Networks

Network Layer Shortest Path Routing - The technique that is used in many routing algorithms is simple and easy to understand. - The idea is to build a graph of the subnet, with each node of the graph representing a router and each arc of the graph representing a communication line (link). To choose a route between a given pair of routers, the algorithm just finds the shortest path between them on the graph. Computer Networks

Shortest Path Routing Network Layer Metric is number of hops D H G 2 7 3 6 1 4 Metric is number of hops The path ABC is equal to path ABE. Metric is the distance in km - the path ABC is longer than the path ABE Different metric are possible with different means faster longer less delay By changing the weighting function, the algorithm would then compute the “shortest” path measured to any one of a number of criteria or to a combination of criteria. Computer Networks

Network Layer Shortest Path Routing There are several algorithms to compute the shortest path between two nodes of a graph. The most popular is Dijkstra algorithm. In this algorithm each node is labeled (in parentheses) with its distance from the source node along the best known path. Computer Networks

It is suitable for military applications. Network Layer Shortest Path Routing - In Flooding Algorithm every packet is sent out on every outgoing line except the one it arrived on. Timer is used to stop (limit) this flooding which can be a hop counter. A practical selective flooding algorithm is used where the packets are sent only to the approximately in the right direction. It is suitable for military applications. Computer Networks

Collection of Subnetworks The Internet is an interconnected collection of many networks.

The IPv4 (Internet Protocol) header. The IP Protocol The IPv4 (Internet Protocol) header.

The IP Protocol (2) 5-54 Some of the IP options.

IP Addresses IP address formats.

IP Addresses (2) Special IP addresses.

A campus network consisting of LANs for various departments. Subnets A campus network consisting of LANs for various departments.

A class B network subnetted into 64 subnets.