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Pertemuan 20Teknik Routing

Matakuliah : H0174/Jaringan Komputer

Tahun : 2006

Versi : 1/0

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Learning Outcomes

Pada akhir pertemuan ini, diharapkan mahasiswa akan mampu :

• Menunjukkan teknik routing

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Outline Materi

• Routing table• Routing Algorithm• Routing Protocol

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Routing

• Routing is the process of deciding what path to take from sender to receiver (packet forwarding)

• Usually there is more than one route possible; devices that perform routing must keep tables to make decisions about which path to send packets on to reach a given destination (routing table)

• Routing decisions on the Internet are usually handled by special purpose devices called Routers, that maintain their own routing tables

• End systems and routers maintain routing tables• Routing table indicates next router to which datagram

should be sent

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Routing Techniques Criteria

Performance CriteriaNumber of hopsCostDelayThroughput

Decision TimePacket (datagram)Session (virtual circuit)

Decision PlaceEach node (distributed)Central node (centralized)Originating node (source)

Network Information SourceNoneLocalAdjacent nodeNodes along routeAll nodes

Network Information Update TimingContinuousPeriodicMajor load changeTopology change

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Routing Strategies

• Fixed• Flooding• Random• Adaptive

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Routing Protocols

• Routing Information– About topology and delays in the internet

• Routing Algorithm– Used to make routing decisions based on

information• Routing algorithms are implemented using

routing protocols

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Autonomous Systems (AS)

• Group of routers– Exchange information – Common routing protocol

• Set of routers and networks managed by single organization

• A connected network– There is at least one route between any

pair of nodes

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Interior Routing Protocol

• Routing protocols that operate within a network (called an autonomous system) are called interior routing protocols.

• Passes routing information between routers within AS

• Routing algorithms and tables may differ between different AS

• IRP needs detailed model

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Exterior Routing Protocol

• Exterior router protocol (ERP) is used for operating outside of or between networks

• Because there are many more possible routes it is far more complex than interior routing

• It cannot maintain tables of every single route and have to concentrate instead on the main routes only.

• There may be more than one AS in internet

• Routing algorithms and tables may differ between different AS

• Routers need some info about networks outside their AS

• ERP supports summary information on reachability

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Interior Routing Protocols - RIP

Routing Information Protocol (RIP)

• Routing protocol commonly used on the Internet. • Computers using RIP broadcast routing tables

every minute or so. • Now used on simpler networks• The original dynamic distance vector protocol

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Interior Routing Protocols - OSPF

Open Shortest Path First (OSPF)• Has overtaken RIP as the most popular interior routing

protocol on the Internet• Has the ability to incorporate traffic and error rate

measures in its routing decisions. • Sends updates state info, not entire routing tables,

and only to other routers (not broadcasting them)• Each router keeps list of state of local links to network• Little traffic (less burdensome to the network since) as

messages are small and not sent often • Route computed on least cost based on user cost

metric using Link State Routing Algorithm• RFC 2328

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Dynamic Routing Algorithms

Routing Algorithms•Distance Vector:

which uses the least number of hops to decide how to route a packet

•Link Statewhich uses a variety of information types and takes into account such factors as congestion and response time to decide how to route a packet.

Because of its more sophisticated approach, link state routing algorithms have become more popular

than distance vector algorithms.

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Routing Distance vector

• Each node (router or host) exchange information with neighboring nodes (both are directly connected to same network)

• First generation routing algorithm for ARPANET• Node maintains vector of link costs for each

directly attached network and distance and next-hop vectors for each destination

• Requires transmission of lots of information by each router– Distance vector to all neighbors– Contains estimated path cost to all networks in

configuration– Changes take long time to propagate

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Bellman-Ford Algorithm

• Find shortest paths from given node subject to constraint that paths contain at most one link

• Find the shortest paths with a constraint of paths of at most two links and so on

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Link-state Routing

• Designed to overcome drawbacks of distance-vector

• When router initialized, it determines link cost on each interface

• Advertises set of link costs to all other routers in topology and not just neighboring routers

• Then monitor link costs, if significantly changes, router advertises new set of link costs

• Each router can construct topology of entire configuration and can calculate shortest path to each destination network

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Least Cost Algorithms

• Basis for routing decisions– Can minimize hop with each link cost 1– Can have link value inversely proportional to

capacity• Given network of nodes connected by bi-directional

links each link has a cost in each direction• Define cost of path between two nodes as sum of

costs of links traversed• For each pair of nodes, find a path with the least

cost• Link costs in different directions may be different

e.g. length of packet queue

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Dijkstra’s Algorithm

• Djikstra's algorithm can accommodate weights on edges in graph

• Shortest path is then the path with lowest total weight (sum of weights of all edges)

• Shortest path not necessarily fewest edges (or hops)

• Find shortest paths from given source node to all other nodes, by developing paths in order of increasing path length

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Bellman vs Dijkstra

• Results from two algorithms agree• Bellman-Ford

•Calculation for node n involves knowledge of link cost to all neighboring nodes plus total cost to each neighbor from•Each node can maintain set of costs and paths for every other node•Can exchange information with direct neighbors, can update costs and paths based on information from neighbors and knowledge of link costs

• Dijkstra•Each node needs complete topology•Must know link costs of all links in network•Must exchange information with all other nodes