Networking - Chapter Four

Chapter 4Routing

Addis Ababa University Faculty of Informatics Department of Computer Science

What Is Routing?

Routing is the act of moving information across a network from a source to a destination.Occurs at layer 3 the network layerThe router is the device that performs routing, and it connects different LAN segments so that larger networks can be created. Routing involves two basic activities:determining optimal routing paths andtransporting packets through a network.

making a decision and choosing one route whenever there are multiple routesbased on some criteria; how do you choose a route when you drive? may be the condition of your car and the road, the shortest one, the one that is not congested, the one with less traffic lights, avoid forbidden paths,...the job of the network layer routing protocola combination of rules and procedures that lets routers in the internet inform one another of changes (to share whatever they know about the internet or their neighborhood); e.g. a failure of a network can be communicatedat the heart of such protocol is the routing algorithm that determines the path for a packet

routing requires a host or a router to have a routing table which is constructed by the routing algorithm

given big internetworks such as the Internet, the number of entries in the routing table becomes large and table look ups become inefficient; methods for reducing its size required

Routers rely on two types of network protocols to make the routing routable protocols and routing protocols.Large networks need protocols that allow systems to be identified by the address of the network to which they are attached and by an address that uniquely identifies them on that network.Network protocols that provide both of these features are said to be routable.Three routable LAN network protocols are used today: TCP/IP IPX/SPX AppleTalk

Routing protocols are the means by which routers communicate with each other. This communication is necessary so that routers can learn the network topology and changes that occur in it.two kinds of routing algorithms (routing tables): nonadaptive (static) and adaptive (dynamic)nonadaptive (static)routing decisions are not based on measurements or estimates of the current topology or trafficthe choice of a root is computed in advance, off-line, and downloaded to the routers when the network is bootedan administrator enters the route for each destination into the table; not automatically updated when there is a change; may be used in a small internet, but not for big internet like the Internet

Disadvantages of static routing manually entering routes is time-consuming and susceptible to human error. if the topology of the network changes, the routers must be manually reconfigured.

adaptive (dynamic)routing decisions are made periodically (every sec) to reflect changes in the topology, traffic, a shutdown of a router, a break in the link, a better route has been created, ...metrics useda metric is a cost assigned for passing through a networkthe cost could be the level of congestion of a link (mean queue length, transmission delay, average traffic), bandwidth, the geographic distance traversed by the link, number of hops, estimated transit time, communication cost, ...which cost to choose depends on the applicationthe total metric of a particular route is equal to the sum of the metrics of networks that comprise the routea router chooses the route with the shortest (smallest) metric

Two routing protocols distance-vectorlink-stateA router that uses distance-vector protocols need only maintain a small database of the routes accessible by the routers to which it is directly connected.A router that uses link-state protocols must maintain a database of the routers in the entire network.

i.RIP - Routing Information Protocoluses distance vector routing algorithm (dynamic)each router maintains a table (one entry per router) giving the best known distance to each destinationthe tables are updated by exchanging information with neighborsin the general case, the metrics could be: number of hops, time delay in milliseconds, total number of packets queued along the path, ...it is the original ARPANET routing algorithm (using queue length as a metric) and was used until 1979the distance metric used in RIP is the number of hopsall networks are treated as equals, i.e., the cost of passing through each network is the same, it is one hop count

ii. OSPF - Open Shortest Path Firstbecame a standard in 1990 by the IETF (Internet Engineering Task Force)aim: to simplify the implementation at the boundaries between Autonomous Systems by having a standard for the interior routingIETF identified some requirementshas to be published in the open literature - proprietary solutions are not goodhas to support a variety of distance metrics (physical distance, delay, ...)it has to be dynamichas to do load balancing for better performance (the best route is not always the best)

NetworkCostNext RouterOther InformationN15CN27DN310BN411DN515C

has to support routing based on type of servicesupport for hierarchical systems - routers are not expected to know the entire topologyintroduce security so that spoofing routers by sending false routing information is avoideduses link state routing algorithm (dynamic)replaced distance vector routing in ARPANETsince distance vector routing does not consider bandwidth (no need initially since all lines were 56 Kbps; later some lines were upgraded) and other metrics

Another classification of routing protocols is interior and exterior protocols.since an internet can be large, one routing protocol cannot handle the task of updating the routing tables of all routershence, an internet is divided into autonomous systemsan autonomous system (AS) is a group of networks and routers under the authority of a single administrationrouting inside an autonomous system is referred to as interior routing; each AS can choose its own routing protocolrouting between autonomous systems is referred to as exterior routing; one protocol is usually chosen to handle routing between autonomous systems; usually used for routing in the Internet

R1, R2, R3, and R4 use both interior and exterior routing protocolsthe rest use only interior routing protocolssolid lines - communication between routersbroken lines - communication between the routers that use an exterior routing protocol

why an exterior routing protocol apart from size of an internet? - politicspolitical - I hate country X hence I will not handle its trafficsecurity - my information is confidential and should not pass through a hostile countryeconomicit should not pass through a competitors networkI am not paid for it and hence dont want to carry a transit packetsuch policies are typically manually configured into each router and are not part of the protocol itself

Interior and Exterior routing protocols are more commonly referred to as Interior Gateway Protocols (IGP) and Exterior Gateway Protocols (EGP).For the most part, the network administrator is concerned with IGP protocols. RIP and OSPF are the two most implemented IGPs.BGP (Border Gateway Protocol) is distance vector exterior routing protocol.