Routing A broad Perspective

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28 Οκτ 2013 (πριν από 3 χρόνια και 10 μήνες)

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Routing

A broad Perspective

Presenters

Vasudha Chandrasekaran (Vas)

Prajakta Vaidya (Priya)

Routing is…


What fans do at football games


What pigs do for truffles


Send corps of infantrymen into full scale,
disorganized retreat.


Uh! So what is Routeing?



Something that Brits use, to distinguish
between what happens in Networks from what
happened to them in New Orleans in 1814.


What ISO and CCITT use in their documents


And Yes, it is…



Routeing is…


The process of determining, selecting the
best outgoing path that a packet has to
take in a Internetwork.


But, with due apologies to British Standards

Institute, lets stick to Routing!


OSI Routing Scheme


A set of Routing Protocols


Allows end systems and intermediate systems
to collect and distribute information necessary
to determine routes


Routing Information base


Table containing all routing relevant
connectivity


Routing Algorithms


Uses information in routing information base to
derive routes between end systems
.


OSI Routing Architecture

For the Next 10 min….



Design Goals of Routing Algorithms


Algorithm Types


Routing Metrics


Dynamic Routing Algorithms


Design Goals


Optimality


Simplicity and low Overhead


Robustness and stability


Rapid Convergence


Flexibility

Algorithm types


Static Vs Dynamic


Single Path Vs Multiple Path


Host Intelligent Vs Router Intelligent


Intradomain Vs Interdomain



Routing Metrics


Path Length


Reliability


Delay


Bandwidth


Load


Communication cost


Static Algorithms


Shortest Path Algorithm


Flooding


Flow based Routing


Dynamic Algorithms


Distance Vector Routing


Link State Routing


Hierarchical Routing


Distance Vector Routing


Bellman Ford/ Ford Fulkerson Algorithm
-

Original
ARPANET routing algorithm


AppleTalk and CISCO routers use improves
distance vector protocols


Each router knows the id of every other router in
the network.


Each router maintains a vector with an entry for
every destination that contains:


The cost to reach the destination from this router.


The direct link that is on that least cost path.


Each router, periodically sends its vector to his
direct neighbors.


Upon receiving a vector a router updates the local

vector based

on the direct link’s cost and the
received vector
.



Link State Routing


Discover the neighbors and learn the
network addresses


Measure the delay or cost to each of the
neighbors


Construct a packet telling all it has just
learnt


Send this packet to all other routers


Compute the shortest path to every other
router




Link State Routing contd..


Each router knows the id of every other
router in the network.


Each router maintains a topology map of
the whole network.


Each router, periodically floods its link
state updates (with its direct connectivity
information).


Upon receiving a vector, a router updates
the local topology map and re
-
calculates
shortest paths
.

Increasing Network Size


Router memory requirements increase.


More CPU time for look up and more
bandwidth needed to send status reports.


Routing table size increases with increase in
network size.


Hierarchical Routing


Routers divided into Regions.


Regions > Clusters > Zones > Groups >


Internal structure of a region known only to
routers within that region.


Different networks do not need to know the
topological structure of other ones.

Example of Hierarchical Routing

Penalty for Hierarchical Routing


Path length may increase. But this increase
is sufficiently small and usually acceptable.


The optimum number of levels for an N
router subnet is ln N, with a total of e ln N
entries per router.



Internet Control Protocols


Routing Information Protocol


Open Shortest Path First


Interior Gateway Routing Protocol


Exterior Gateway protocol


Border Gateway Protocol

Routing Information Protocol


The protocol is limited to networks whose longest
path is 15 hops.


Inappropriate for larger networks.


Uses fixed metrics to compare alternative routes.



Not appropriate for routing based on dynamic
parameters.


Cannot operate within a hierarchical structure.


Open Shortest Path First


OSPF is a link
-
state routing protocol that calls for
the sending of link
-
state advertisements to all
other routers within the same hierarchical area.


As OSPF routers accumulate link
-
state
information, they use the SPF algorithm to
calculate the shortest path to each node using
Dijkstra’s Shortest Path Algorithm .


Can operate in a hierarchy.


Supports multi
-
path routing and type of service
based routing.


Interior Gateway Routing Protocol


A distance
-
vector routing protocol.


Replaced RIP



Network Size


Single Metric


IGRP uses five criteria to determine the best path:
the link's speed, delay, packet size, loading and
reliability.


The weighting factors for each of these metrics
can be fixed up by Network Administrators.


Permits multi
-
path routing.

Exterior Gateway Protocol


First exterior routing protocol to gain
widespread accepted on the internet.


Dynamic routing protocol with a very
simple design used for communication of
reachability information.


It does not use metrics and therefore cannot
make intelligent routing decisions


Border Gateway Protocol


Internet standard for inter
-
domain exterior routing.


BGP is used to exchange routing information
between Internet service providers (ISP).


External / Interior BGP


Extremely robust and scalable


Supports classless inter
-
domain routing.


BGP
-
4, the latest version, lets administrators
configure cost metrics based on policy statements.


Routers using BGP, exchange routing information
using TCP connection.



BGP contd.


BGP communicates with autonomous (local)
networks using Internal BGP (IBGP) since it
doesn't work well with IGP. The routers inside the
autonomous network thus maintain two routing
tables: one for the interior gateway protocol and
one for IBGP.



Policies are manually configured into each BGP
router.


Never put Iraq on a route starting at Pentagon.


Traffic starting and ending at IBM should not transit
Microsoft.


Questions ?