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29 Οκτ 2013 (πριν από 3 χρόνια και 7 μήνες)

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Routing Concept

Sirak Kaewjamnong


Computer Network Tech and Security

2

Agenda


Basic concepts



Routing components



Classes of routing protocol


Internet routing protocol

3

What’s Routing

Routing
-

path finding from one end to the
other


Routing occurs at layer 3


Bridging occurs at layer 2


Physical link

Data link layer

Network layer

Transport layer

Session layer

Presentation layer

Application layer

Physical link

Data link layer

Network layer

Transport layer

Session layer

Presentation layer

Application layer

Physical link

Data link layer

Network layer

Network A

Network B

4

IP Routing


IP performs:


search for a matching host address


search for a matching network address


search for a default entry


Routing done by IP router, when it searches the
routing table and decide which interface to end a
packet out
.


incoming

which interface ?

5

Routing Tables


Routing is carried out in a router by consulting
routing table.


No unique format for routing tables, typically
table contains:


address of a destination


IP address of next hop router


network interface to be used


subnet mask for the this interface


distance to the destination


6

Routing Component


Three important routing elements :


algorithm


database


protocol


Algorithm : can be differentiate based on several
key characteristics


Database : table in routers or routing table


Protocol: the way information for routing to be
gathered and distributed


7

Routing Algorithm

Design goals


Optimality

-

compute the best route


Simplicity/low overhead

-

efficient with a
minimum software and utilization overhead


Robustness/stability
-

perform correctly in the face
of unusual circumstances


Rapid convergence
-

responds quickly when the
network changes


Flexibility
-

accurate adapt to a variety of network


8

Routing Protocols


Routing protocol :
protocol to exchange of
information between routers about the current
state of the network


Routing protocol jobs


create routing table entries


keep routing table up
-
to
-
date


compute the best choice for the next hop router


9

Routing Metrics


How do we decide that one route is better than
another?


Solution : using a metric as a measurement to
compare routes


Metrics may be distance, throughput, delay, error
rate, and cost.


Today, IP supports Delay, Throughput, Reliability
and Cost (DTRC)


10

Hop Count


A hop is defined as a passage through one router


R1

R2

R3

1 hop

1 hop

1 hop

1 hop

2 hops

11

Routing Algorithm Types


Static V.S. Dynamic


Source routing V.S. Hop
-
by
-
hop


Centralize V.S. Distributed


Distance vector V.S. Link state

12

Routing Algorithm: S
tatic Route


Manually configuration routing table


Can’t react dynamically to network change such
as router’s crash


Work well with small network or simple topology


Unix hosts use command
route

to add an entry

point to point

connection

route to this

way only, no need

for update

13

Routing Algorithm:
Static Technique

Flooding


Every incoming packet is sent out every outgoing


Retransmit on all outgoing at each node


Simple technique, require no network information


Generate vast numbers of duplicate packet


incoming

flooding

14

Routing Algorithm:
Dynamic Route

Dynamic route


Network protocol adjusts automatically for
topology or traffic changes


Unix hosts run routing daemon
routed

or
gated


15

Routing Algorithm:

Dynamic Route operation


Routing protocol maintains and distributes
routing information


Update Routing Information


Routing

Table



Routing

Table


Routing

Protocol

Routing

Protocol

16

Routing Algorithm: S
ource Routing


Source routing


Source will determine the entire route


Routers only act as store
-
forward devices


Hop
-
by
-
hop


Routers determine the path based on theirs own
calculation


17

Routing Algorithm:
Distance Vector


Distance

means routing metric


Vector

means destination


Flood routing table only to its neighbors


RIP is an example


Also known as
Bellmann
-
Ford algorithm or Ford
-
Fulkerson algorithm



18

Distance Vector Algorithm


Using hop count as a metric


Each router periodically sends a copy of its
routing table to neighbors


send
<network X, hopcount Y>


routing table


W 0


X 0


Y 1


Z 2

routing table


W 1


X 0


Y 0


Z 1

routing table


W 2


X 1


Y 0


Z 0

W

X

Y

Z

R1

R2

R3

19

Distance Vector Routing Update


Step by step from router to router


Slow convergence


R1

R2

R3

topology

change

Œ

recompute R3

s

routing table



R3 sends out

the updated table

Ž

recompute R2

s

routing table



R2 sends out

the updated table



recompute R1

s

routing table

20

Distance Vector: Broadcast (I)


The first round

R1

R2

R3

R4

I

J

K

L

M

I, 1 hop

J, 1 hop

I, 1 hop

K, 1 hop

L, 1 hop

N

J, 1 hop

K, 1 hop

M, 1 hop

N, 1 hop

L, 1 hop

M, 1 hop

O 1 hop

O

R5

N, 1 hop

O, 1 hop

21

Distance Vector: Broadcast (II)


The second round

R1

R2

R3

R4

I

J

K

L

M

I, 1 hop

J, 1 hop

K, 2 hops

L, 2 hops

M, 2 hops

N, 2 hops

I, 1 hop

K, 1 hop

L, 1 hop

J, 2 hops

M, 2 hops

N, 2 hops

O, 2 hops

N

J, 1 hop

K, 1 hop

M, 1 hop

N, 1 hop

I, 2 hops

L, 2 hops

O, 2 hops

L, 1 hop

M, 1 hop

O, 1 hop

I, 2 hops

K, 2 hops

J, 2 hops

N, 2 hops

O

R5

N, 1 hop

O, 1 hop

J, 2 hops

K, 2 hops

M, 1 hop

L, 2 hops

22

Distance Vector: Broadcast (III)


The third round

R1

R2

R3

R4

I

J

K

L

M

I, 1 hop

J, 1 hop

K, 2 hops

L, 2 hops

M, 2 hops

N, 2 hops

O, 3 hops

I, 1 hop

K, 1 hop

L, 1 hop

J, 2 hops

M, 2 hops

N, 2 hops

O, 2 hops

N

J, 1 hop

K, 1 hop

M, 1 hop

N, 1 hop

I, 2 hops

L, 2 hops

O, 2 hops

L, 1 hop

M, 1 hop

O, 1 hop

I, 2 hops

K, 2 hops

J, 2 hops

N, 2 hops

O

R5

N, 1 hop

O, 1 hop

J, 2 hops

K, 2 hops

M, 1 hop

L, 2 hops

I, 3 hops

23

Distance Vector:
Crashed Recovery


R3 crashed


New complete route of R1


R1

R2

R3

R4

I

J

K

L

M

N

O

R5

R1 routing table

hop via

1 N/A

1 N/A

2 R2

2 R2

2 R3

2 R3

3 R5


net

I

J

K

L

M

N

O

hop via

1 N/A

1 N/A

2 R2

2 R2

3 R2

4 R2

3 R2


net

I

J

K

L

M

N

O

24

Count to Infinity


R2 does not hear any thing from R3


R1 says : don’t worry, I can reach R3 in 2 hops,
R2 update hop count to 3


R1 sees R2’s update, then update itself to 4 and so
on……


R1

R2

R3

hop count to R3

2 1 initial

2 3 1st round

4 3 2nd round

4 5 3rd round

I

J

R3 crashed

25

Split Horizon


Solve by set distance “16” as infinity


No destination can be more than 15 hops away
from any other


Distance to X is not reported on the line that
packet for X are sent


Actually, it reports with infinity


R1

R2

R3

I

J

R3 crashed

to R3


to R3


26

Dijkstra’s Shortest Path First Algorithm


Routers send out update messages whenever the
state of a link changes. Hence the name: “Link
State” algorithm.


Each router calculates lowest cost path to all
others, starting from itself.


At each step of the algorithm, router adds the next
shortest (i.e. lowest
-
cost) path to the tree.


Finds spanning tree routed on source router
.

27

Open Shortest Path First (OSPF)


RIP limited in large internets


OSPF preferred interior routing protocol for
TCP/IP based internets


Link state routing used

28

Routing Algorithm:
Link State


Flood routing information to all nodes


Each router finds who is up and flood this
information to the entire routers


Use the link state to build a shortest path map to
everybody


OSPF is an example


Also known as
Shortest Path First (SPF) algorithm


29

Flooding


Packet sent by source router to every
neighbor


Incoming packet resent to all outgoing links
except source link


Duplicate packets already transmitted are
discarded


Prevent incessant retransmission


All possible routes tried so packet will get through
if route exists


Highly robust


At least one packet follows minimum delay route


Reach all routers quickly


All nodes connected to source are visited


All routers get information to build routing table


High traffic load


30

Link State Overview


Using cost as a metric


Exchange its connection and cost to its neighbors


Each router compute the set of optimum path to all
destination (Shortest Path First)


link state


W 0


X 0

link state


X 0


Y 0

link state


Y 0


Z 0

W

X

Y

Z

31

Link State Concept


Each router initially begins with directly
connected network


Determine full knowledge of distant routers and
theirs connection


R2

R1

R3

R4

exchange link

state packets


Routing

Table


Œ





build topological

database

Ž

compute SPF

update routing


table

32

Link State Routing Update


Send information to other routers


Fast convergence


R2

R1

R3

R4

topology

change

33

OSPF Overview


Router maintains descriptions of state of
local links


Transmits updated state information to all
routers it knows about


Router receiving update must acknowledge


Lots of traffic generated


Each router maintains database


Directed graph

34

Link Costs


Cost of each hop in each direction is called
routing metric


OSPF provides flexible metric scheme
based on type of service (TOS)


Normal (TOS) 0


Minimize monetary cost (TOS 2)


Maximize reliability (TOS 4)


Maximize throughput (TOS 8)


Minimize delay (TOS 16)


35

Areas


Make large internets more manageable


Configure as backbone and multiple areas


Area


Collection of contiguous networks
and hosts plus routers connected to any
included network


Backbone


contiguous collection of
networks not contained in any area, their
attached routers and routers belonging to
multiple areas

36

Operation of Areas


Each are runs a separate copy of the link
state algorithm


Topological database and graph of just that area


Link state information broadcast to other
routers in area


Reduces traffic


Intra
-
area routing relies solely on local link
state information


37

Comparison

Distance Vector

Link State

pass a copy of pass links state update

whole routing table


add metric from calculate the shortest path

router to router to other routers


frequent periodic update: event updated: fast

slow convergence convergence




38

Internet Routing Architecture

Autonomous

System

Autonomous

System

Autonomous


System

IGP

IGP

IGP

BGP4

BGP4

BGP4

IGP

IGP

IGP

IGP

IGP

IGP

EGP/BGP

EGP/BGP

EGP/BGP

EGP/BGP

EGP/BGP

EGP/BGP

Autonomous

System

Autonomous

System

39

Routing in the Internet

The Internet uses hierarchical routing


The Internet is split into Autonomous Systems (AS’s)


Within an AS, the administrator chooses an Interior
Gateway Protocol (IGP)


Examples of IGPs: RIP (rfc 1058), OSPF (rfc 1247).


Between AS’s, the Internet uses an Exterior Gateway
Protocol


AS’s today use the Border Gateway Protocol, BGP
-
4 (rfc
1771)



40

Autonomous System


AS is a collectionof LANs and WANs and the
interconnectting routers which under the control of
one management authority


The same AS runs the same Interior Gateway
Protocol


Why setting up AS?
-

establish a direct link to
each other rather than route through the core
Internet


How to select AS?
-

register and get the AS
number from IAB


41

Gateway Protocol


Interior gateway protocol


exchange routing information between routers
within a single AS


RIP, RIP II, OSPF



Exterior gateway protocol


collect network
-
reachablity information for the
AS


EGP, BGP


42

Interior Routing Protocols


RIP


Uses distributed Bellman
-
Ford algorithm.


Updates sent every 30 seconds.


No authentication.


Originally in BSD UNIX.



OSPF


Link
-
state updates sent (using flooding) as and when
required.


Every router runs Dijkstra’s algorithm.


Authenticated updates.


Autonomous system may be partitioned into “areas”.


43

Exterior Routing Protocols

Problems:


Topology:

The Internet is a complex mesh of different
AS’s with very little structure.


Autonomy of AS’s
: Each AS defines link costs in
different ways, so not possible to find lowest cost paths.


Trust:

Some AS’s can’t trust others to advertise good
routes (e.g. two competing backbone providers), or to
protect the privacy of their traffic (e.g. two warring
nations).


Policies:

Different AS’s have different objectives (e.g.
route over fewest hops; use one provider rather than
another).


44

Border Gateway Protocol (BGP
-
4)


BGP is not a link
-
state or distance
-
vector routing
protocol.


BGP advertises complete paths (a list of AS’s).


Example of path advertisement:


“The network 171.64/16 can be reached via the path
{AS1, AS5, AS13}”.


Paths with loops are detected locally and ignored.


Local policies pick the preferred path among
options.


When a link/router fails, the path is “withdrawn”.


45

References


http://www.cisco.com/en/US/products/hw/routers/p
s274/index.html


http://www.cisco.com/en/US/products/hw/routers/p
s274/products_data_sheet09186a008010fba1.html


Computer Networks with Internet Technology

By William Stallings Chapter 11 Interior Routing Protocols