Lecture 5: Routing in the Internet

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29 Οκτ 2013 (πριν από 3 χρόνια και 9 μήνες)

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Lecture 5:

Routing in the Internet






June 2009



Outline


Hierarchical routing


Intra
-
AS Routing in the Internet: RIP
and OSPF


Inter
-
AS Routing in the Internet: BGP

2 problems


Scale
: large number of routers


-
> Memory


-
> Bandwidth


-
> Converge


Administrative autonomy


Autonomous system


All the routers in the
same

AS run the
same

routing algorithm (
intra
-
AS routing protocol)


Gateway router
s


The routing algorithm for gateways is called
inter
-
AS routing protocol.


Gateways:
both

intra and inter routing
protocol


Solve

2 problems above.

Outline


Hierarchical routing


Intra
-
AS Routing in the Internet
:
RIP

and

OSPF


Inter
-
AS Routing in the Internet: BGP


RIP: Routing Information Protocol


Distance vector protocol


Each
link

has cost 1, maximum cost of
the
path

is 15.


30 seconds


RIP response message

or
RIP
advertisement



RIP (cont)


180 seconds wait for
unreachable

neighbor.


RIP ’s
request

message.


UDP, port 520.


RIP is an
application
-
layer

protocol

OSPF: Open shortest path first


Open
: publicly available



A link state protocol + Dijkstra least
-
cost path algorithm.

Complete

topological map

Shortest
-
path tree

Forwarding table

Dijkstra

OSPF (cont)


Individual link
-
costs are configured by
the network admin.


Broadcast

all

other routers in AS.



Periodically ( 30 minutes )

Advances


Security


Multiple same
-
cost paths


Integrated support for unicast and
multicast routing


Support for hierarchy within a single
routing domain.


Hierarchy in a single AS


Areas.


Area border routers.


Only one

OSPF area in AS is configured
to be
the backbone area
.


Backbone = all area border routers +
some nonborder routers.


4 types of OSPF routers

Outline


Hierarchical routing


Intra
-
AS Routing in the Internet: RIP
and OSPF


Inter
-
AS Routing in the Internet
:
BGP

BGP: Border Gateway Protocol


Path

vector protocol


BGP routes to
destination network
rather than to specific hosts or routers


Every AS has a
unique autonomous
system number

(ASN)



3 activities


Receive and filter route advertisements
from a directly attached neighbor(s)


Route selection


Sending route advertisements to
neighbors.

Example


A,B,C: backbone provider network


W,Y: stub network


X: multi
-
home stub network

W

A

B

C

X

Y

BGP (cont)


TCP, port 179


4 types of messages:


OPEN


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