Topic 22 - Drexel University

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