1 Network Layer Lecture 13 Imran Ahmed University of Management & Technology

2 Agenda Introduction & Network layer functions Routing principles Hierarchical routing The Internet protocol (IP) Connectivity device - Router What’s inside a router Summary

3 Network layer Transport segment from sending to receiving host. On sending side encapsulates segments into datagrams. On receiving side, delivers segments to transport layer. Network layer protocols in every host, router. Router examines header fields in all IP datagrams passing through it. network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical application transport network data link physical application transport network data link physical

4 Key Network-Layer Functions Forwarding: Move packets from router’s input to appropriate router output. Routing: Determine route taken by packets from source to destination. –Routing algorithms

value in arriving packet’s header routing algorithm local forwarding table header value output link Interplay between routing and forwarding

6 The Internet Network layer forwarding table Host, router network layer functions: Routing protocols path selection RIP, OSPF, BGP IP protocol addressing conventions datagram format packet handling conventions ICMP protocol error reporting router “signaling” Transport layer: TCP, UDP Link layer physical layer Network layer

7 Agenda Introduction & Network layer functions Routing principles Hierarchical routing The Internet protocol (IP) Connectivity device - Router What’s inside a router

8 IP Packet Delivery Two Processes are required to accomplish IP packet delivery, namely the Routing Process and the Forwarding Process –Routing is the process of discovering and selecting the path to the destination according to some metrics. This is a layer 3 functionality. –Forwarding is the process of inserting the IP packet into a Layer-2 frame and forwarding the frame to the next hop (which could be the destination host or another intermediate router). This is a layer 2/layer 3 functionality.

9 Routing and Forwarding Control plane: run routing protocols: (RIP, OSPF, BGP) Data plane: forwarding packets from incoming to outgoing link

10 Routing Tables All IP packet delivery starts with a router table lookup. Every host/router that is part of the TCP/IP network has a routing table. Routing tables are created either statically or dynamically.

11 Typical Components of Routing Tables Routing Tables are built up by the routing algorithms. They generally consist of: –Destination Network Address: The network portion of the IP address for the destination network. –Subnet Mask: used to distinguish the network address from the host address. –The IP address of the next hop: to which the interface forwards the IP packet for delivery. –The Interface: with which the route is associated.

12 Forwarding Tables After the routing lookup is completed and the next hop is determined, The IP packet is forwarded according to a local or remote delivery models. –Local delivery model is when the destination and the host are on the same local network. In this case, the IP packet is inserted into a MAC-frame which is forwarded directly to the destination. –Remote delivery model is when the destination and the host are on different networks. In this case, the IP packet is inserted into a Layer-2 frame which is forwarded to the next hop router.

13 Static vs. Dynamic Routing Static Routing Tables are entered manually. Strengths of Static Routing: –Ease of use –Reliability –Control –Security –Efficiency Weaknesses of Static Routing: –Not Scalable –Not adaptable to link failures Dynamic Routing Tables are created through the exchange of information between routers on the availability and status of the networks to which an individual router is connected to. Two Types: –Distance Vector Protocols –Link State Protocols

14 Overview Routing Protocols Routing information protocol (RIP) Distance vector Interior Gateway routing protocol (IGRP, Cisco proprietary) Distance vector Open shortest path first (OSPF) Link state Intermediate System-to- Intermediate System (IS-IS) Link state Border gateway protocol (BGP) Path vector Routing protocol Routing Algorithm

15 Routing Metrics Routing metrics are used by dynamic routing protocols to establish preference for a particular route. Goal of routing metrics is to provide the capability to the routing protocol to support Route Diversity and Load Balancing. Most Common routing metrics include: –Hop Count (# of hops) –Bandwidth/Throughput (maximum throughput) –Load (actual usage) –Delay (shortest delay) –Reliability –Cost