1. Routing Protocols COSC 541 Data Commun. System & Networks Yue Dou

2. Router Function Routers responsible for receiving and forwarding packets through the interconnected set of networks Each router makes routing decision based on knowledge of the topology and conditions of the internet

3. Routing Protocal Routers need to make dynamic routing decisions to avoid portions of the network that have failed or congested. Routers need the routing protocol to exchange routing information about the topology and delays of the internet

4. Autonomous Systems ( AS ) An AS consists of a group of routers exchanging information via an interior router protocol An AS is a set of routers and networks managed by a single organization ( such as an university ) Except failure, there is a path between any two pair of routers in AS

5. An Example of Autonomous System

6. Internet Construction Internet construct with many autonomous systems Routing protocol divides into interior and exterior router protocol Interior router protocol ( IRP ) passes routing information between routers within an autonomous system Exterior router protocol ( ERP ) passes routing information between routers in different autonomous system

7. Interior & Exterior router protocol An interior router protocol needs to build up a rather detailed model of the interconnection of routers within an AS in order to calculate the least-cost path from a give router to any network within the AS. An exterior router protocol supports the exchange of summary reachability information between separately administered ASs.

8. Border Gateway Protocol ( BGP ) Preferred exterior router protocol for the Internet BGP allow gateway routers in different autonomous systems (ASs ) to cooperate in the exchange of routing information BGP operates in terms of 4 messages: Open, Update, Keepalive, Notification

9. BGP 3 Functional Procedures Neighbor acquisition: one router sends an open message to the other, which may either return a keepalive message or refuse the offer Neighbor reachability: the two routers periodically issue Keepalive messages to each other Network reachability: when a router change its routing information, it will broadcast an Update message

10. BGP Routing Information Exchange Border router in one AS can issue an Update message to the border router in another AS The Update message could include: AS_Path: the identity of AS(s) Next_Hop: the IP address of border router NLRI: a list of all of the networks in AS(s)

11. Open Shortest Path First (OSPF) Protocol Widely used as the interior router protocol in TCP/IP networks Each router maintains a routing table that reflects the known topology of the autonomous system of which it is a part OSPF computes a route through the internet that incurs the least cost based on a user-configurable metric of cost ( like data rate, dollar cost etc.)

12. An Example of Routing Table Destination Next Hop Distance ------------------------------------------------------ N1 R3 10 N7 R10 12 R7 R10 8 ------------------------------------------------------- Destination: LAN or border router Next Hop: Forwarding router Distance: Sum of cost on the whole route