Ccnet Lec 11 Routing

8/6/2019 Ccnet Lec 11 Routing

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Lecture

11Data Communication & Networks

Routing and Forwarding

Muhammad Yousaf


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Routing & Forwarding:

Routing and Forwarding are two different butcomplementary functions

Routing:To acquire topology information of the network

Who is situated where?What is the cost to reach a node?

A complex learning problem

Forwarding:

Already know all information about network topologyWhen a packet is received, then select appropriateoutbound network interface for forwarding the packet

Simply a switching problem


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Routing Table @ my system:

C:\Documents and Settings\yousaf>route print

==================================================================

Interface List

0x1 ........................ MS TCP Loopback interface

0x2 ...00 16 d3 03 0a d4 .. Realtek RTL8139/810x Family Fast Ethernet NIC

0x3 ...00 16 6f 52 f3 8b ..... Intel(R) PRO/Wireless 2200BG Network Connection

==================================================================Active Routes:

Network Destination Netmask Gateway Interface Metric

127.0.0.0 255.0.0.0 127.0.0.1 127.0.0.1 1

255.255.255.255 255.255.255.255 255.255.255.255 2 1

255.255.255.255 255.255.255.255 255.255.255.255 3 1

==================================================================

Persistent Routes:

None


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Forwarding:

Every node (Hosts and routers) maintain aforwarding tableForwarding table maps network address to theoutbound network interfaceMost often there is an entry for Default route too!

Packets with destination address that couldnt match withentries of forwarding table, are forwarded to this defaultroute

For end-systems this table is static and simple

For routers however, this table is dynamicComplex protocols are used to form these tables


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Forwarding example:

dest: 18.26.10.3 mask with 255.0.0.0 matched! send to interface 1

dest: 128.16.14.10 mask with 255.0.0.0 not matched

mask with 255.255.0.0 not matched

mask with 0.0.0.0 matched! send to interface 3

Forwarding Process:

Network # Netmask interface

======== ========= ========

18.0.0.0 255.0.0.0 1

128.32.0.0 255.255.0.0 2

0.0.0.0 0.0.0.0 3

Forwarding Table for a router with three interfaces:


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Masking:

Netmask indicates the number of bits for network partNetmask is bitwise ANDed with destination address ofreceived packet

e.g. for netmask 255.0.0.0

& destination address 18.26.10.311111111.00000000.00000000.00000000

00010010.00011010.00001010.00000011 (AND)

================================

00010010.00000000.00000000.00000000

In decimal notation = 18.0.0.0

It is compared with the entry in forwarding table


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Routing:

?

?

?

??

?

?

?

?

?

B

C

D

E


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Routing schemes:

There are number of schemes to help routers inmaking routing decisions

Flooding

Random routing

Source routingDirectory-Base routing

Static routing

Dynamic routing


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Flooding:

Packet received on one interface is broadcasted onall other network interfaces

Network is flooded with a single message

If there is even a single possible path then it isguaranteed that message will reach the destination

Destination can receive multiple redundant copies

First copy will reach with minimum possible delay

Overhead is high


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Flooding: cont

A

B

C

D

E


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Random routing:

Trying to reduce overhead of floodingDo not forward packet on all interfaces

Rather, randomly choose a subset of interfaces andforward only over them

Reduces flooding overhead

It is not guaranteed that packets will either reachthe destination or not

Even if reached, then delays are likely to be high


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Random routing: cont...

A

B

C

D

E


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Source routing:

Source node attaches the path to be followed in theheader of packet

Each router reads the header and sees where toforward the packet

Routing-overhead is smallHeader-to-Packet ratio is large

Cant accommodate dynamics of network

Most users dont know the path to be followed bypackets, so they cant give source route

Source routing can be used for QoS or securerouting of packets


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Source routing: cont

A

B

C

D

E

R1-R4-R6-R11-R12-D

R4-R6-R11-R12-D

R6-R11-R12-D R11-R12-D

R12-D

DR1

R2

R3

R4 R6

R8

R9R10

R11

R12


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Directory-Based routing

Lets migrate the intelligence into the routersEach router maintains a table, that helps to makethe routing decision

Routers can have:

Either static tables

Or dynamic tables


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Static routing tables:

At the time of configuring network devices, tablesare established in each router

Entries are static, can not be updated automatically

Addition or removal of routers/nodes is not easy

Require manual updating of each router


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Dynamic routing tables:

Again each node maintain a routing tableBut this time, table is not fix or constant

It can change automatically to accommodate thenetwork changes

Sophisticated routing protocols are designed fordynamically updating these tables

All nodes exchange routing advertisements with

each other periodicallyHence nodes remain aware of the networkdynamics


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Routing Protocols:

Most of the modern protocols are dynamicThey can adapt to the current condition of thenetwork

Well known strategies for adaptive routing are:

Distance Vector routing

Link State routing

These protocols define procedures following which,

routers can acquire information in theirrouting/forwarding tables


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Routing Protocols: cont.

Distance VectorCommunicate information to neighbors only

Exchange information about entire network

Link State

Communicate information to entire network

Exchange information about neighbors only

State of own links is exchanged


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Distance Vector routing:

Distributed Bellman-Ford routing algorithmAlso known as Ford-Fulkerson algorithm

Original ARPANET routing protocol

Used in Internet as RIP

RIP = Routing Information Protocol

First dynamic routing protocol

Automatically compute routing tables

Freeing administrators from statically specifying routes


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Distance Vector routing: cont

Each router maintains a distance vector:Containing one entry for each router in the network

Each entry also contains the distance (cost) and the nexthop to reach that destination

So, each entry is a set of triplet (Destination, Cost, NextHop)Cost = number of hops, time delay, etc

It is assumed that routers know the distance to each of its neighbors

Routing updates are exchanged with directly

connected neighbors by:periodically (from time period of milliseconds to minutes)triggered update (whenever significant change occurs)


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Calculating Cost:

Let cost to reach from me to my neighbor is 1If my neighbor tells me that he can reach the routerD in cost 3

Then I can conclude that the cost to reach from me

to router D is 1+3=4


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Calculating Cost: example

A

B

C

D

E

R1

R2

R3

R4 R6

R8

R9R10

R11

R12R3=(R12, 3, R8)R1=(R12, 4, R3)


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6 2

2 5

1 1 1

source

Destination

A

C E

infinity infinityB

infinity

infinityinfinity

infinity infinity 1 Dest

5 Destinfinity

1 Dest3 Binfinity

5 Dest7 E

1 Dest3 B8 C

4 A7 E

8 C 3 B 1 Dest

4 A6 E

Distance Vector Iterations:


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Distance Vector Iterations: example

Information in routing table of each node:Iteration 1

D

G

A

F

E

B

C

At distance to reach node

node A B C D E F G

A 0 1 1 x 1 1 xB 1 0 1 x x x x

C 1 1 0 1 x x x

D x x 1 0 x x 1

E 1 x x x 0 x xF 1 x x x x 0 1

G x x x 1 x 1 0


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D

G

A

F

E

B

C


node A B C D E F G

A 0 1 1 2 1 1 2B 1 0 1 2 2 2 x

C 1 1 0 1 2 2 2

D 2 2 1 0 x 2 1

E 1 2 2 x 0 2 xF 1 2 2 2 2 0 1

G 2 x 2 1 x 1 0


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D

G

A

F

E

B

C


node A B C D E F G

A 0 1 1 2 1 1 2B 1 0 1 2 2 2 3

C 1 1 0 1 2 2 2

D 2 2 1 0 3 2 1

E 1 2 2 3 0 2 3F 1 2 2 2 2 0 1

G 2 3 2 1 3 1 0


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Distance Vector Routing Table:

Routing table at node B:

D

G

A

F

E

B

C

Destination Cost NextHop

A 1 A

C 1 C

D 2 C

E 2 A

F 2 A

G 3 A


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Distance Vector: (Link Failure)

F detects that link to G has failedF sets distance to G as infinity andsends update to A

A sets distance to G as infinity since it

was using F to reach GA receives periodic update from C with2-hop path to G

A sets distance to G to 3 and sends

update to FF decides it can reach G in 4 hops viaA

D

G

A

F

E

B

C


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Count to Infinity Problem:

A game of mutual deception

Suppose, link from A to E fails

A advertises distance of infinity to E,

B and C advertise a distance of 2 to E

B decides it can reach E in 3 hops;advertises this to all

A decides it can read E in 4 hops;advertises this to all

C decides that it can reach E in 5 hops

We are counting to infinity

D

G

A

F

E

B

C


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Problems with Distance Vector:

One iteration per hop for new routesInformation propagates slowly

Cant scale well for large network

Count to infinity for lost routes


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Link State Routing:

Send to all nodes (not just neighbors) information ofdirectly connected nodes (links)

Most widely used dynamic routing scheme

Best example is OSPF protocol

OSPF = Open Shortest Path First

Work began in 1980s as a replacement for RIP

Uses Dijkstras algorithm for shortest path calculation

OPEN means, it is freely available open standardWhile IGRP & EIGRP are CISCO proprietary protocols


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OSPF:

OSPF is well supported by:CISCOs Internetwork Operating System (IOS)

Routing & Remote Access Service (RRAS) of MicrosoftWindows 2000 & .NET plateforms

So far, there are three versions:OSPF (RFC-1131), experimental, not widely used

OSPFv2 (RFC-2328), most widely deployed routingprotocol since 1990

OSPFv3, recent development, support for IPv6, withenhanced security


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OSPF: cont

Routers discover its neighbors by Hello messagesHello messages are sent every 10 to 15 secondsThese messages indicate the liveliness of the routersIf hello is not received from some router in specified time,then it is assumed that the router is down

Each router then transmit Link State Advertisement(LSA) messages

LSA contains the information about changes in networktopology

Routers maintains a list of all the LSAs from all therouters and then run Dijkstras algorithm on this list


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OSPF message format:

OSPF operates above the network layerThere are five types of OSPF messages

All OSPF messages include the OSPF header


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OSPF header:


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OSPF header: cont

Version:OSPF version, current version is v2

Type:

Type-1 = Hello message

Type-2 = Database Description message

Type-3 = Link State (LS) request message

Type-4 = LS update message

Type-5 = LS Acknowledgement message

Packet Length:

Length of entire packet, including header


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OSPF header: cont

Router ID:ID of the message generating router

Area ID:

Area ID of network on which this packet is being sent

Checksum:Checksum of entire packet, including header


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OSPF header: cont

Au Type:Authentication Type

0 = no authentication

1 = password authentication

2 = MD5 checksum

Authentication Data:

Data that is to be used for authentication


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Dijkstras Algorithm:

Calculates the shortest path between nodes

Relies on the information received in LSAs

For calculation each router maintains two lists:Tentative list

Confirmed listHow algorithm works:

Initialize confirmed list & tentative list with your own entry

Pick the entry of shortest cost from tentative list

Include that entry in confirmed listInclude LSA of most recently picked entry in tentative list

Repeat procedure until tentative list is empty


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Dijkstras Algorithm: (example)

At node D:

Confirmed list Tentative list

1. (D,0,-)

2. (D,0,-) (C,2,C), (B,11,B)

3. (D,0,-), (C,2,C) (B,11,B)

4. (D,0,-), (C,2,C) (B,5,C), (A,12,C)5. (D,0,-), (C,2,C), (B,5,C) (A,12,C)

6. (D,0,-), (C,2,C), (B,5,C) (A,10,C)

7. (D,0,-), (C,2,C), (B,5,C), (A,10,C)

D

A

B

C

5 3

2

11

10


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To be continued