MODULE II.pdf
- 2. Network Layer Services 1. Packetizing 2. Routing and Forwarding 3. Other Services Expected services are ■ Error Control ■ Flow Control ■ Congestion Control ■ Quality of Service ■ Security
- 3. 1. Packetizing Packetizing: Encapsulating the payload (data received from upper layer) in a network-layer packet at the source and decapsulating the payload from the network-layer packet at the destination. Carry a payload from the source to the destination without changing it or using it. If the packet is too large, it is fragmented by the intermediate routers. All the fragments have the same header as the original (especially source and destination addresses), with small changes to specify fragments. The fragments are reassembled at the destination.
- 4. 2. Routing and Forwarding Routing: In a large network, there will be a number of routes in between the source and destination devices. Network layer finds the best route based on some specific strategies(load, bandwidth, hops, etc.) The strategies are mostly defined by the Routing protocols. The strategies are used to create a decision-making table, called the routing table for each router. Routing is applying strategies and running some routing protocols to create the decision-making tables for each router.
- 5. Forwarding The action applied by a router when a packet arrives at one of its interfaces. Forwarding is done with the help of Forwarding table or Routing table. On receiving a packet at an interface, the router reads the destination address/label in the incoming packet, find the output interface number from the table and forwards the packet. The packet forwarding can be; ■ to another attached network (in unicast routing) or ■ to some attached networks (in multicast routing).
- 7. 3.Other Services Other services expected from this layer are; Error Control Flow Control Congestion Control Quality of Service Security Error Control: Packets in the network layer may be fragmented at routers, which makes error checking at this layer inefficient. A checksum field in the datagram controls any corruption in the header, but not in the whole datagram
- 8. Flow Control Network layer does not directly provide flow control, because; 1. To make the network layer at the receiver is so simple . 2. The upper layers can implement buffers to receive data from the network layer. 3. Flow control is provided for most of the upper-layer protocols, so another level of flow control makes the network layer more complicated and less efficient.
- 9. Congestion Control ■ Congestion: Too many datagrams are present in an area of the internet. ■ Happens if the number of datagrams sent by source computers is beyond the capacity of the network or routers. ■ Hence, some routers may drop some of the datagrams. ■ Due to the error control mechanism at the upper layers, the sender may send duplicates of the lost packets. ■ If the congestion continues, sometimes the system collapses and no datagrams are delivered.
- 10. Quality of Service (QoS) ■ To keep the network layer simple and untouched, QoS is implemented in upper layers. Security ■ The network layer was created with no security provision. ■ To make the network layer secure, a connection oriented virtual layer service (called IPSec) is created.
- 11. NETWORK LAYER PERFORMANCE ● Can be measured in terms of; ○ Delay ○ Throughput ○ Packet loss ● Congestion control also improves performance.
- 12. Delay A packet, from its source to its destination, encounters delays. Can be subdivided into; i. Transmission delay ii. Propagation delay iii. Processing delay iv. Queuing delay. v. Total Delay
- 13. 1.(i) Transmission Delay ● A sender puts the bits in a packet on the line one by one. ● If the first bit of the packet is put on the line at time t1 and the last bit is put on the line at time t2 , transmission delay of the packet is (t2 − t1). ○ Delaytr = (Packet length) / (Transmission rate). ● The longer the packet, the longer the transmission delay. ● Eg: For a Fast Ethernet LAN (100 million bits/sec) with a packet size of 10,000 bits, the transmission delay is (10,000)/(100,000,000) or 100 microseconds
- 14. 1.(ii) Propagation Delay ● The time taken for a bit to travel from point A to point B in the transmission media. ○ Delaypg = (Distance) / (Propagation speed). ● Eg: If the distance of a cable link in a point-to-point WAN is 2000 meters and the propagation speed of the bits in the cable is 2 × 108 meters/second, then the propagation delay is 10 microseconds.
- 15. 1.(iii) Processing Delay The time required for a router or a destination host to receive a packet from its input port, remove the header, perform an error detectionprocedure, and deliver the packet to the output port (in the case of a router) or deliver the packet to the upper-layer protocol (in the case of the destination host). ● May be different for each packet, but normally is calculated as an average. ○ Delaypr = Time required to process a packet in a router or a destination host.
- 16. 1.(iv) Queuing Delay ● Happen in a router. ● A router has an input queue connected to each of its input ports to store packets waiting to be processed. ● Also an output queue connected to each of its output ports to store packets waiting to be transmitted. ● Queuing delay is the time a packet waits in the input queue and output queue of a router. ○ Delayqu = The time a packet waits in input and output queues in a router
- 17. 1.(v) Total Delay ● The total delay (source-to-destination delay) a packet encounters is the sum of all the above delays in all the devices and routers that a packet transfers between the source and destination, including them. ● If there are n routers in between; ○ Total delay = (n + 1) (Delaytr + Delaypg + Delaypr ) + (n) (Delayqu)