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
Enter the password to open this PDF file:
File name:
-
File size:
-
Title:
-
Author:
-
Subject:
-
Keywords:
-
Creation Date:
-
Modification Date:
-
Creator:
-
PDF Producer:
-
PDF Version:
-
Page Count:
-
Preparing document for printing…
0%
Σχόλια 0
Συνδεθείτε για να κοινοποιήσετε σχόλιο