CS335 Networking &

Network Administration

Tuesday, November 18, 2008

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

EGPs merely finds a path because it cannot compare routing metrics from multiple autonomous systems

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

http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/bgp.htm

http://www.academ.com/nanog/feb1997/BGPTutorial/sld001.htm

http://www.bgp4.as/

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 - http://www.freesoft.org/CIE/RFC/1058/6.htm

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

http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/rip.htm

http://www.faqs.org/rfcs/rfc1058.html http://www.networksorcery.com/enp/protocol/

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

http://www.answers.com/topic/link-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 http://www.cisco.com/warp/public/104/1.html http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito

_doc/ospf.htm

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 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) Core based trees (CBT) Protocol independent multicast – sparse mode (

PIM-SM) Protocol independent multicast – dense mode (PIM-

DM) Multicast extensions to the open shortest path first

protocol (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