CS335 Networking & Network Administration Tuesday, May 18, 2010

Internet routing Propagation of routing tables Routing table updates

Static vs. dynamic routing Static routes do not change Routes are loaded when the system starts Dynamic routes – table information changes over time Routes are also loaded when the system starts System also starts route propagation software or routing software Routing software on one computer interacts with routing software on others to learn about optimal routes. Tables are then updated.

Static routing Does not require extra routing software Does not consume bandwidth No CPU cycles used to propagate routing info

Default route

Static routes Most hosts use static routing Host’s routing table contains two entries One for the network to which the host attaches A default entry that directs all other traffic to a specific router

Dynamic routing Each router runs routing software that learns about destinations other routers can reach and informs other routers about destinations that it can reach. Routing tables are updated continuously.

Routing in the global internet To limit routing traffic, the Internet uses a two-level routing hierarchy. Routers and networks are divided into groups All routers in a group exchange routing information. At least one router in each group summarizes information before passing it on to other groups

Autonomous system concept The concept of groups of routers Designers left the concept flexible to accommodate many possibilities of routing groups Each group is an autonomous system Each group shares routing information The group’s information is summarized before being sent to other groups

Internet routing protocols Interior gateway protocols (IGP’s) Routers within an autonomous system use this to exchange routing information Exterior gateway protocols (EGP’s) A router from one system uses these protocols to exchange routing information with another autonomous system

Optimal routes Optimal route is hard to define Depends on the application For an interactive login app a path with least delay is optimal For a browser downloading a large graphic file a path with maximum throughput is optimal For an audio webcast path with least jitter is best

Routing metrics The measure of the path that routing software uses when choosing a route Bandwidth – data capacity of a link Delay – length of time required to move a packet along each link from source to destination Load – amount of activity on a network resource such as a router or a link Reliability – a reference to the error rate of each network link Ticks – the delay on a data link using IBM PC clock ticks (approximately 55 milliseconds or 1/18 second) Cost – an arbitrary value, usually based on bandwidth, monetary expense, or other measurement, that is assigned by a network administrator

Routing metrics IGPs use routing metrics to find optimal routes IGPs EGPs merely finds a path because it cannot compare routing metrics from multiple autonomous systems EGPs

Routes and data traffic A response to a routing advertisement is data Data traffic for a given destination flows in exactly the opposite direction of routing traffic

Border gateway protocol (BGP) Routing among autonomous systems – BGP provides routing information at the autonomous system level Provision for policies – BGP allows senders and receivers to enforce policies, a manager can restrict which routes BGP advertises to outsiders Facilities for transit routing – BGP classifies each autonomous system as a transit system if it agrees to pass traffic to another autonomous system or as a stub system if it does not Reliable transport – BGP uses TCP for all communication for the reliability of data transfer that TCP provides ISPs use BGP to exchange routing information among autonomous systems in the global Internet

BGP resources twk/ito_doc/bgp.htm twk/ito_doc/bgp.htm Tutorial/sld001.htm Tutorial/sld001.htm

RIP RIP Routing information protocol Routing within an autonomous system – RIP is designed as an IGP used to pass information among routers within an autonomous system Hop count metric – RIP measures distance in network hops where each network between source and destination is a single hop. Origin-one counting where a directly connected network is 1 hop away, not zero Unreliable transport – uses UDP Broadcast of multicast delivery – intended for use over LANs that support broadcast

RIP Routing information protocol Support for default route propagation – allows a router to advertise a default route. An organization can use RIP to install a default route in each router such that the default routes all forward traffic to the ISP Uses distance vector algorithm Passive version for hosts – although only a router can propagate routing information, RIP allows a host to listen passively and update its routing table

RIP Chief advantage is simplicity Needs little configuration The routers broadcast messages to each other After a short time all routers in an organization will have all routes to all destinations Also handles propagation of the default route

RIP packet A RIP update message

RIP RIP is an Interior Gateway protocol (IGP) Uses distance vector algorithm to propagate routing information A router running RIP advertises the destinations it can reach along with a distance to each destination Adjacent routers receive the information and update their routing tables

RIP Routing information protocol twk/ito_doc/rip.htm twk/ito_doc/rip.htm rip.htm rip.htm

Open shortest path first protocol (OSPF) OSPF Graph

Open shortest path first protocol (OSPF) Designed to scale better for large organizations OSPF is an IGP to route within an autonomous system Full CIDR support – includes the full 32 bit address mask with each address Authenticated message exchange – OSPF can ensure that messages are only accepted from trusted sources Imported routes – OSPF allows a router to introduce routes learned form other means (e.g., From BGP) Uses link-state algorithm state-protocolhttp:// state-protocol Has support for metrics

OSPF areas Can scale to handle a larger number of routers than other IGPs Does hierarchical routing Allows a manger to divide the routers and networks in an autonomous system into subsets called areas OSPF allows communication between areas _doc/ospf.htm _doc/ospf.htm

Multicast Multicast routing IP multicast allows applications to: Join multicast group at any time Leave the group at any time Membership in a multicast group is dynamic

IGMP IGMP Internet Group Multicast Protocol The protocol uses a host to inform a router when it joins or leaves an Internet multicast group IGMP is only used on the local network Routers must use another multicast routing protocol to inform other routers of group membership

Forwarding and multicast techniques Flood and prune Configuration and tunneling Core-based discovery

Multicast protocols Distance vector multicast routing protocol (DVMRP)DVMRP Core based trees (CBT) Protocol independent multicast – sparse mode (PIM- SM)PIM- SM Protocol independent multicast – dense mode (PIM- DM)PIM- DM Multicast extensions to the open shortest path first protocol (MOSPF)MOSPF The dynamics characteristics of Internet multicast make the problem of multicast route propagation difficult. Internet does not have an Internet-wide multicasting facility

Routing summary Both hosts and routers contain an IP routing table. Hosts use a static table Internet is divided into autonomous systems EGPs and IGPs exchange routing information Multicast route propagation is difficult Many routing protocols