Matakuliah
Tahun
Versi
: H0174/Jaringan Komputer
: 2006
: 1/0
Pertemuan 20
Teknik Routing
1
Learning Outcomes
Pada akhir pertemuan ini, diharapkan mahasiswa
akan mampu :
• Menunjukkan teknik routing
2
Outline Materi
• Routing table
• Routing Algorithm
• Routing Protocol
3
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
4
Routing Techniques Criteria
Performance Criteria
Number of hops
Cost
Delay
Throughput
Decision Time
Packet (datagram)
Session (virtual circuit)
Decision Place
Each node (distributed)
Central node (centralized)
Originating node (source)
Network Information Source
None
Local
Adjacent node
Nodes along route
All nodes
Network Information Update Timing
Continuous
Periodic
Major load change
Topology 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
9
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 State
which 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 nexthop 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
14
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 distancevector
• 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
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