1. Routing in the Inernet Outcomes: –What are routing protocols used for Intra-ASs Routing in the Internet? –The Working Principle of RIP and OSPF –What is the routing protocols used for Inter- ASs rouging in the Internet? –The Working principle of BGP

2. Routing Information Protocol RIP is a distance vector protocol, it is specified in RFC 1508 In RIP, routing updates are exchanged between neighbours approximately every 30 seconds using a so-called RIP response message. Response message sent by a router or host contains a list of up to 25 destination networks within the AS, as well as the sender’s distance to each of those networks. Response messages are also known as RIP advertisements.

3. Working Principle of RIP RIP routers exchange advertisements approximately every 30 seconds. If a router does not hear from its neighbor at least once every 180 seconds, that neighbor is considered to be no longer reachable. RIP modifies the local forwarding table and then propagates this information by sending advertisements to its neighboring routers. A router can also request information about its neighbor’s cost to a given destination using RIP request message. Router send RIP request and response message each other over UDP using port number 520.

4. Open Shortest Path First OSPF (RFC 2328) is a link state protocol that uses flooding of link state information and a Dijkstra least-cost path algorithm. –With OSPF, a router constructs a complete topological map of the entire AS. –Run the LS algorithm –The administrator configure the cost of the individual link

5. Advances in OSPF Security. All exchanges between OSPF routers (e.x. link state updates) are authenticated. Multiple same-cost paths. When multiple paths to a destination have the same cost, OSPF allows multiple paths to be used. Integrated support for unicast and multicast routing. Support for hierarchy within a single routing domain. OSPF is able to construct an ASs hierarchically –Internal routers –Area border routers –Backbone routers –Boundary routers

6. Inter-AS Routing: BGP The Border Gateway Protocol (BGP4 RFC1771), is the de facto standard interdomain routing protocol in today’s Internet BGP uses distance vector protocol. –It is more appropriately characterized as a path vector protocol. –Neighboring BGP routers, known as BGP peers, exchanged detailed path information (such as the list of ASs on a path to a given destionation) rather than cost information. In BGP, an autonomous system is identified by its globally unique autonomous system number (ASN). AS numbers, like IP addresses, are assigned by ICANN regional registries. At the heart of BGP are route advertisements. An advertisement consists of a CIDRized destination network address and a set of attributes associated with the path to that destination network. Two of the most important attributes are the path attribute (an explicit list of all ASs on the path to the specified destination network) and the identity of the next-hop router along that path to the destination network.

7. BGP activities Receiving and filtering route advertisements from directly attached neighbour's. Route selection. A BGP router may receive several route advertisements to the same destination AS, and must choose which route it will use from among those advertised. –The destination AS and next hop for the chosen path must then be installed in the router’s forwarding tables. –BGP makes a clear distinction between routing mechanism and routing policy. BGP does not specify how an AS should choose a path from among those advertisement. This is a policy decision that is left up to the AS’s network administrator. Sending route advertisements to neighbours.

8. BGP-Border Gateway Protocol BGP peers communicate using the TCP protocol and port number 179. The BGP protocol defines the four types of messages: –OPEN. When a BGP router first wants to establish contact with a BGP peer, an OPEN message is sent to the peer. If the OPEN is acceptable to the peer, it will send back a KEEPALIVE message. –UPDATE. A BGP router uses the UPDATE message to advertise a path to a given destination to the BGP peer. The UPDATE message can also be used to withdraw a path that had previously been advertised. –KEEPAVLIVE. This BGP message is used to let a peer know that the sender is alive but that the sender doesn’t have other information to send –NOTIFICATION. This BGP message is used to inform a peer that an error has been detected or that the sender is about to close the BGP session.