Topic 22 - Drexel University

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Oct 29, 2013 (3 years and 7 months ago)

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Copyright 2002, S.D. Personick. All Rights Reserved.

1

Telecommunications

Networking II

Topic 22

Routing Algorithms

Ref: Tanenbaum pp345
-
374; 424
-
431


Dr. Stewart D. Personick

Drexel University

Copyright 2002, S.D. Personick. All Rights Reserved.

2

Routing Algorithms


An IP network consists of a number
(possibly a large number) of
interconnected routers


As discussed previously, routers contain
routing tables…which are used to
determine which outgoing port each
incoming IP datagram should be
directed to

Copyright 2002, S.D. Personick. All Rights Reserved.

3

Routing Algorithms


The question then arises: how are these
routing tables populated and maintained?


Alternatives:

-
A central entity computes and disseminates
a set of routing tables

-
Each router tries to “discover” what the
network topology is, and creates/maintains its
own routing tables

Copyright 2002, S.D. Personick. All Rights Reserved.

4

Routing Algorithms

there

here

129.25.X.X

Getting from “here” to “there”

Copyright 2002, S.D. Personick. All Rights Reserved.

5

Routing Algorithms


Issues

-
Efficient routing (finding the “best” path)

-
How fast can the routing tables be updated


in the event that something about the


network has changed (e.g., a router is


added, a link “goes down”, …)

Copyright 2002, S.D. Personick. All Rights Reserved.

6


“Least cost” routing

-
Pick routes that have the least number of
hops, or the least delay, or the least expense,
or some other measure of least “cost”

-
Algorithms exist for calculating least cost
routes on a global network basis

-
Iterative methods are used for individual
routers to calculate least cost routes based
on routing costs advertised by their neighbors

Routing Algorithms

Copyright 2002, S.D. Personick. All Rights Reserved.

7


Discovering one’s neighbors

-
send out an “HELLO” message on each
output port…wait for a response

-
send out “ECHO” packets to measure delays
to neighboring routers

-
query neighboring routers regarding the
destinations they can reach, and the
corresponding costs of reaching those
destinations

Routing Algorithms

Copyright 2002, S.D. Personick. All Rights Reserved.

8

Routing Protocols


Interior Gateway Routing Protocol


-
used
within

an “autonomous system”, e.g.,
Drexel’s collection of routers


-
current IETF standard is OSPF: Open
Shortest Path First

Copyright 2002, S.D. Personick. All Rights Reserved.

9

OSPF (overview)


In the public domain (“open”)


Supports a variety of “distance”
metrics: physical distance, delay, …


Dynamic: adapts to changes in network
topology (lost routers, added, routers…)


Supports classes of service


Supports load balancing (splitting loads
among multiple paths)

Copyright 2002, S.D. Personick. All Rights Reserved.

10


Supports hierarchical routing (no one router
needs to know the whole network topology)


Has some security capabilities (e.g., to
prevent hackers from changing routing
tables)


Some details are provided in Tanenbaum, but
a more in
-
depth reference on routing
protocols is needed to really understand this
subject


OSPF (overview)

Copyright 2002, S.D. Personick. All Rights Reserved.

11

Border Gateway Routing Protocol
(BGP)


Allows autonomous systems (AS’s) to:

-

enforce certain rules…such as not
accepting “transit” packets; or carrying transit
packets only from certain foreign AS’s to
other foreign AS’s

-

control routing to avoid certain AS’s


BGP routers inform their neighbors about the
complete paths they use to get to various
destinations