Understanding Internet Routing: Static vs Dynamic, BGP, OSPF, RIP

1. Chapter 25 Internet Routing

2. Static Routing • manually configured routes that do not change • Used by hosts whose routing table contains one static route describing the network to with the host is attached • a default route directs all other traffic to a specific router (fig 25.1)

3. Dynamic Routing • routing tables can change automatically over time via information learned via routing messages from other routers. • Each router learns about destinations other routers can reach, and informs other routers about destinations that it can reached (fig 25.2)

4. Routing in the Global Internet • the Internet uses a 2-level routing hierarchy • Routers and networks are divided into groups where each group is known as an autonomous system (a contiguous set of networks and routers all under control of one administrative authority). • All routers within a group(autonomous system) exchange routing information while one router in each group summarizes the information before passing it to other groups.

5. Internet Routing Protocols • Interior Gateway Protocol (IGP) • eg. RIP, OSPF, IGRP: used by routers within an autonomous system • Exterior Gateway Protocols(EGP) • eg.BGP: used by a router in one autonomous system to exchange routing information with a router in another autonomous system (fig 25.3)

6. Routing Metrics • eg. hop count, administrative cost, throughput, delay. • Used by interior gateway protocols but not by exterior gateway protocols due to existence of different metrics. • Within an autonomous system, IGP software uses a routing metric to choose an optimal path to each destination. • EGP software finds a path to each destination, but cannot find an optimal path because it cannot compare routing metrics from multiple autonomous systems.

7. Border Gateway Protocol (BGP) • routing messages among autonomous systems contain routes, each of which is described as a path of autonomous systems ( eg. route to autonomous system 34 is achieve via autonomous systems 17, 2, 56, and 12. • Provision for policies: manager can configure BGP to restrict routes advertised to outsiders • Each autonomous system is classified as a transit system if it agrees to pass traffic through to another autonomous system or as a stub system if it does not • Uses TCP for reliable transport of routing messages • Used by all ISPs to exchange routing information with each other and from an authoritative route server (which has a copy distributed database of all possible destinations in the Internet with information about the ISP that owns each destination)

8. Routing Information Protocol (RIP) • implemented by a program called routed • hop count metric with origin-one counting ( ie. A directly connected network is 1 hop away, not 0) • uses unreliable transport (UDP) • uses broadcast(RIP version 1) or multicast(RIP version2) for message delivery • support for default route propagation

9. Routing Information Protocol (cont.) • uses distance-vector algorithm • RIP advertises the destinations it can reach along with a distance to each destination • adjacent routers receive the information and update their routing tables • allows hosts to be operate in passive listen-only mode ( ie. no advertising) • RIP packet format (fig 25.5) • drawbacks of RIP include large routing messages, slow propagation of route changes (one router at a time), and limited scalability.

10. Open Shortest Path First Protocol (OSPF) • scales well to large organizations • designed as an interior gateway protocol • full CIDR and subnet support • authenticated message exchange • can import routes from BGP • Uses hierarchical routing by dividing routers and networks in an autonomous system into subnets known as areas. • Each router is within a given area exchange link-status messages via broadcasts. • Summarized routing information are exchanged by one router in each area with routers in other areas.

11. Open Shortest Path First Protocol (cont) • uses link-state routing algorithm. • Each router must periodically probe adjacent routers and then broadcast a link-status message • routers that receive the message use Dijkstra’s SPF algorithm to compute the shortest paths using it local copy of network graph(fig 25.6) .

12. Multicasting • an application running on any computer can join a multicast group at any time and begin receiving a copy of all packets sent to the group. • To join or leave a group, the computer informs a nearby router via IGMP (Internet Group Multicast Protocol). • To leave a group, the computer informs the local router that it is no longer participating in the group.

13. Multicasting (cont) • An IP multicast group is anonymous • sender and receiver do not know the identity or the number of group members • routers do not know which applications will send a datagram to a group since any application on any computer can send a datagram to any multicast group at any time. • Membership in a multicast group only defines a set of receivers • sender does not need to join a multicast group before sending a message to the group. • Multicast packets are forwarded using techniques such as flood-and-prune, configuration-and-tunneling, or core-base discovery. 

14. Multicast routing protocols • DVMRP (distance vector multicast routing protocol) • used by Unix program mrouted and Internet Multicast backBONE (MBONE) • CBT (Core Base Trees) • PIM-SM (protocol independent multicast–sparse mode) • PIM-DM (protocol independent multicast–dense mode) • MOSPF(multicast extensions to the open shortest path first protocol)