Evolution of Routing Techniques

7/28/2019 Evolution of Routing Techniques

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Delivery, Forwarding, RoutingIP and MaskRouting TableUnicast Routing Protocols

AGENDA

December 7, 2009Tusharadri Sarkar2


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What is routing? Routing is the process of selecting a path in a

network along which the packets shall be sentto a destination

Routing consists of A Router

A set of routing protocols

A routing information base (RIB) One or more routing algorithms



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Why is routing required?

For practical limitation ofphysicalconnections

For efficiently managing the network traffic

For efficient usage of network resources

For catering to different types of services

For congestion control



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At which layer is routing done? Generally routing is done at network layer Multi layer layer routing and Cross layer rout ing is also

prevalent nowadays

Firewalls are often integrated with routers

APPLICATION LAYER

PRESENTATION LAYER

SESSION LAYER

TRANSPORT LAYER

NETWORK LAYER

DATA LAYER

PHYSICAL LAYER

APPLICATION LAYER

SOCKET LAYER

ROUTING LAYER

LINK LAYER

DEVICE DRIVERs



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Router, Switch and Hub The basic difference is varying intelligence

Least expensive and complicated. Nointelligence

J ust directs incoming packets from one portto other

HUB

More expensive and intelligent Knows which port is carrying the traffic from

which host/interfaceSWITCH

Most expensive and intelligent, Mostcomplicated

Learns about its neighboring conditions,manipulates data traffic

ROUTER



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How does a router work?

One

Packet arrives at the router fordelivery

Router reads destination IPaddress

Two

Router searches RoutingTable

Determines next hop of thepacket

Three

Router forwards the packet tonext hop

Packet is said to be routed



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Direct and Indirect Delivery

To rest of Network

Source SourceDestination

DestinationDirect

Indirect

Direct

Direct



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Logical addressing: IP and MASK

Classful addressing1st 2nd 3rd 4th

0

10

110

1110

1111

1st 2nd 3rd 4th

0-127

128-191

192-223

224-239

240-255

Classes

Class A

Class B

Class C

Class D

Class E

Class No. of Blocks Block Size Application

A 128 16,777,216 Unicast

B 16,384 65,536 Unicast

C 2,097,152 256 Unicast

D 1 268,435,456 Multicast

E 1 268,435,456 Reserved



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Logical addressing: IP and MASK Mask: A 32 bit number made ofn contiguous 1s

followed by (32-n) contiguous 0s (n


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Logical addressing: IP and MASK Classless addressing: No more classes but a

block of addresses are assigned, provided thefollowing restrictions are strictly followed The addresses in the block must be contiguous

The number of addresses must be a power of 2 The first address must be evenly divisible by the total

number of addresses allocated

Mask is a better way to define a block

An example: Given an IP address205.16.37.39/28

What are the first, last and the total number ofaddresses assigned?



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Logical addressing: IP and MASK

Binary equivalent of mask /28: 11111111 11111111 11111111 11110000 (255.255.255.240)

Binary equivalent of the address: 11001101 00010000 00100101 00100111 (205.16.37.39)

First address: Set the right most 4 bits to 0: 11001101 00010000 00100101 0010000 (205.16.37.32)

Last address: Set the right most 4 bits to 1: 11001101 00010000 00100101 00101111 (205.16.37.47)

Number of addresses: 232-n = 24 =16

So, in general a address in classless addressing ismentioned as: x.y.z.t/n



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Network Address When a organization is allocated a block of addresses,

normally (not always) the first address is treated as thenetwork address

It is not assigned to any device, it defines the organization

itself to the rest of the world

REST of the WORLD

Network Address:205.16.37.32

All packets with receiveraddress 205.16.37.32 to205.16.37.47 are routed tox.y.z.t/n

205.16.37.32/28

205.16.39.33/28 205.16.39.47/28



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Routing Table

A host or a router maintains a routing table withan entry for each specific destination

The table can be STATIC or DYNAMIC

Static Routing Table: Contains information entered manually by the

administratorat the time ofcreation

Cannot be modified automatically when there is anychange in the Internet

Dynamic Routing Table: Capable ofupdating the table with the help of routing

protocols and algorithms automatically

Only option for managing any large network of today



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A quick look at a system routing table



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Network Configuration of a Systemform the Routing Table

A UNIX server gives the following result with netstatand ifconfig command $ netstat nr

Kernel IP rout ing table

$ ifconfig eth0 Eth0 Link encap:Ethernet Hwaddr 00:B0:D0:DF:09:5D

Inet addr: 153.18.17.11 Bcast: 153.18.31.255 Mask:255.255.240.0

What is the network configuration of the server?

Destination Gateway Mask Flags Iface

153.18.16.0 0.0.0.0 255.255.240.0 U eth0

127.0.0.0 0.0.0.0 255.0.0.0 U lo

0.0.0.0 153.18.31.254 0.0.0.0 UG eth0



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Network Configuration from theRouting Table

Rest of the Internet

153.18.16.0/20

eth0

00:B0:D0:DF:09:5D

DefaultRouter



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Forwarding

It means placing the packet in its route to itsdestination

Requires a host or a router to have a routing table

When the host has a packet to send or the routerhas received a packet, it looks up this routingtable to determine route to the final destination

Routing techniques caters to optimizing this table

as maintain a full-fledged look-up table isimpossible to maintain



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Forwarding Techniques Next-hop method Vs Route Method

N1N2 N3

R1 R2Host A

Host B

Routing tables based on routing

Destination Route

Host B R1, R2, host B

Destination Route

Host B R2, host B

Destination Route

Host B Host B

Routing tables based on Next-hop

Destination Route

Host B R1

Destination Route

Host B R2

Destination Route

Host B Host B

ForA

ForR1

ForR2



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December 7, 2009

Forwarding Techniques Network-Specific method Vs Host Specific method

System

DCBA

N2N1

Routing table for host S based onhost-specific method

Destination Next Hop

A R1

B R1

C R1

D R1

Routing table for host S based onnetwork-specific method

Destination Next Hop

N2 R1

R1

Tusharadri Sarkar25


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Forwarding Techniques

Default Method: Using a default router

N1 N2

R1

R2Host A

Rest of the Internet

DefaultRouter

Destinat ion Next Hop

N2 R2

Any other R1

Routingtable forhost A



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Forwarding Process

In classless addressing, at least 4 columns are required

The routing table is searched based on the network addressand mask

Mask NetworkAddress

Next-hop

Address

Interface

ExtractDestinationAddress

SearchTable

Forwarding Module

To ARP

Next hop addressand interface no.



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Managing Routing Table in ClasslessAddressing

Address aggregation: Blocks of addresses of differentinterface and mask are aggregated into one single block inrouting table

Several levels of aggregation are possible

140.24.7.0/26

140.24.7.64/26

140.24.7.128/26

140.24.7.192/26

Org 1

Org 2

Org 3

Org 4

m0

m1

m2

m3

m4 m0 m1

R1 R2



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Managing Routing Table in ClasslessAddressing

Address aggregation: Routing tables for router R1and router R2 For R2, any packet with destination addresses 140.24.7.0 to

140.24.7.255 are sent to interface m0 regardless of any ofthe organizations

Mask NA NHA Iface

/26 140.24.7.0 m0

/26 140.24.7.64 m1

/26 140.24.7.128 m2

/26 140.24.7.192 m3

/0 0.0.0.0 Default m4

Mask NA NHA Iface

/24 140.24.7.0 m0

/0 0.0.0.0 Default m1

Routing table for R1 Routing table for R2



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Longest Mask Matching

What happens if Org. 4 is not geographically close to the other

3 Orgs?

Can we still use Address Aggregation and assign the block140.24.7.192/26 to Org. 4?

R2

R1R3

140.24.7.0/26

140.24.7.64/26

140.24.7.128/26

140.24.7.192/26

Org 1

Org 2

Org 3

Org 4

m0m1

m2

m3

m0 m2

m1

m0m1

m2



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Longest Mask Matching Answer: YES

Reason: LONGEST MASK MATCHING

The Routing Table is sorted from the longest mask to theshortest mask

Mask NA NHA Iface

/26 140.24.7.0 m0

/26 140.24.7.64 m1

/26 140.24.7.128 m2

/0 0.0.0.0 Default m3

Mask NA NHA Iface

/26 140.24.7.192 m0

/0 0.0.0.0 Default m2

Mask NA NHA Iface/26 140.24.7.192 m1

/24 140.24.7.0 m0

/0 0.0.0.0 Default m2

Routing table for R1Routing table for R2

Routing table for R3December 7, 2009Tusharadri Sarkar31


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Hierarchical Routing

Hierarchical routing can greatly minimize the size ofthe routing tables

For example, a regional ISP is granted a 16,384(214) addresses starting from120.14.64.0/18

It is divided in to 4 sub-blocks each of size 4096 for3 local ISPs. For them the mask is /20

1st local ISP divides its assigns sub-blocks into 8smaller blocks for small ISPs. For them the maskbecomes /23

Each small ISPs divides them into 128 sub-blocksfor households. For them the mask becomes /30,and so on



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Hierarchical Routing The logical representation is displayed here

120.14.64.0/18

Total 16,384

120.14.64.0/20

120.14.64.0/23

120.14.64.0/30

120.14.78.0/30

120.14.78.0/23

120.14.80.0/20

120.14.96.0/22

120.14.112.0/24

120.14.96.0/20

120.14.112.0/20

Total 4096

Total 4096

Total 4096

Total 4096

512

512

ISP 1

ISP 2.1

ISP 3.1

ISP 3.8

ISP 2.2

ISP 2.3

Total 4 Large Orgs.

Total 16 Small Orgs.

128 Each

128 Each



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Geographical Routing

The same concept of hierarchical routing can be

extended in geographical routing To decrease the size of the routing tables further,

segregation is done in geographical level as well

For example, the entire address space is divided

into few large blocks One block is assigned to North America, one to

Asia, one to Africa, one to Europe and so on

So, for all the routers of the ISPs outside Europe,

every router will have one and only entry for all theaddresses assigned to Europe



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Unicast Routing Protocols

Routing protocols are needed to maintain and

update dynamic routing tables A routing protocols is a combination of set ofrules

(algorithms) and procedures

Unicast routing protocols applies where each

incoming packet has to be delivered to one andonly one destination

Router decides the next hope of a packet in aAutonomous Systembased on Optimization

3 most popular and basic Unicast RoutingProtocols are: RIP (Distance Vector routing), OSPF(Path Vector routing) and BGP (Link State routing)



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Autonomous Systems

An Autonomous System or AS is group of

networks and routers under the authority of a singleadministration

Routing inside AS : Intra-domain routing

Routing between AS : Inter-domain routing

AS1

AS4AS3

AS2



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Routing Protocols Scope

RoutingProtocols

Intradomain

Routing

Distance

Vector (RIP)

Link State(OSPF)

Interdomain

Routing

Path Vector

(BGP)



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Distance Vector Routing

In DVR, the least cost route between any two nodes

is the route with minimum distance

Each node maintains a vector (table) of minimumdistance to every node known

There are three steps involved:

Initialization: At the beginning, each node knows thedistance to its immediate neighbors

Sharing: Periodically or in triggered time, the nodesshare their vectors with other nodes

Updating: Based on the shared info, nodes updatestheir vectors about path to indirectly connectednodes



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DVR: Sharing and Updating

Each node will share its routing table on periodic

basis or triggered condition Full routing table needed not be shared. In our

scenario, only column 1 and column 2 will be shared.Next Hop Address (column 3) will be calculated

based on that Receiving a partial table from its neighbor, a node

calculates a temporary updated table

Then each row of the old and new table are

compared based on the next node entry (col. 3) If next node entry is different, the row with smaller cost is

chosen. If there is a tie, old entry is kept

If next node entry is same, the new entry is chosen



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DVR: Updating Table for A

To Cost NextA 0 _

B 5 _

C 2 _

D 3 _

E

To CostA 2

B 4

C 0

D

E 4

To Cost NextA 4 C

B 6 C

C 2 C

D C

E 6 C

To Cost Next

A 0 _

B 5 _

C 2 _

D 3 _

E 6 C

Old Table of ATableReceivedfrom C

ModifiedTable of A

New Table of A

Compare



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DVR: The Finalized TablesTo Cost Next

A 0 _

B 5 _

C 2 _

D 3 _

E 6 C

A

C

B

ED

5

3

2 4

34To Cost Next

A 3 _

B 8 A

C 5 A

D 0 _

E 9 A

To Cost Next

A 5 _

B 0 _

C 4 _

D 8 A

E 3 _

To Cost Next

A 6 C

B 3 _

C 4 _

D 9 C

E 0 _

To Cost Next

A 2 _

B 4 _C 0 _

D 5 A

E 4 _

Table of A Table of B

Table of CTable of D Table of E



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DVR: Two Node Loop Instability

X A B

XX

X

X

A

AA

A

B

B

B

B

2 4

4

4

4

4

.

.

.

X 2 _

X _

X 10 B

X 10 B

X _

X 6 A X 14 A

X 6 A

X 6 A

X _

BeforeFailure

AfterFailure

After Areceivesupdatefrom B

After Breceives

updatefrom A

Finally



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Routing Information Protocol

Routing Information Protocol (RIP) is an

implementation of Distance Vector Algorithm withthe following considerations:

1. In an autonomous system, we are dealing withrouters and networks (links). Only routers have

routing tables, networks not2. The destination in a routing table is a network

always

3. The metric used by RIP is the no of hops

needed to reach the destination4. Infinity is defined as 16

5. The next-node column defines the address ofthe router to which packet is to be sent



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Link State Routing

Domain Topology: Here, each node in the domain has

an entire topology of the domain Link State: For each node, the number of other links

and nodes, their connectivity type, cost (metric) andthe condition of the links (Up or Down) constitutes linkstate

Shortest Path Tree: Based on the link states, a nodecan use Dijkstras Algorithmto create a Shortest Path

Treewhich can used as the routing table

There are four sets of operations required

Creation of Link State Packets (LSPs)Flooding of LSPs

Formation of shortest path treeCalculation of routing based on the tree



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Link State Routing:

A

C

B

ED

5

3

2 4

State ofLinksfor A

Initial Condition:

D

A B

E

5

2

3

3

2 4

4

5

4

3

34

3

State of

Linksfor D

State ofLinksfor B

State ofLinksfor E

State of

Linksfor C



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Link State Routing: Dijkstras Algorithm: Formation of Shortest Path Tree

START

STOPTentative l istis empty?

Set root to local node andmove it to tentative list

Among nodes in tentative list, move theones with shortest path to permanent list

Add each unprocessed neighbor of lastmoved node to tentative list if it is not therealready. If neighbor is in tentative list withlarger cumulative cost, replace with new one

YES

NO



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Link State Routing:

A B

0

1. Set root to A and move A to tentative list

Creation of Shortest Path Tree for node A:

A

Permanent List: Empty Tentative List: A(0)

Root



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Link State Routing:

A B

0

3. Move C to permanent List. Add E tentative list


A

Permanent List : A(0), C(2) Tentative List : B(5), D(3), E(6)

Root

5B

2 C

3 D 6E



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Link State Routing:

A B

0

4. Move D to permanent List.


A

Permanent List : A(0), C(2), D(3) Tentative List : B(5), E(6)

Root

5B

2 C

3 D 6E



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Link State Routing:

A B

0

5. Move B to permanent List.


A

Permanent List : A(0), B(5), C(2), D(3) Tentative List : E(6)

Root

5B

2 C

3 D 6E



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i k S i


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Link State Routing:

Calculation of Routing Table from Shortest Path Tree

Node Cost Next

A 0 _

B 5 _

C 2 _

D 3 _

E 6 C

Routing table for node A

We can see that therouting table of A asdeduced by Link State

Routing is the same asDistance Vector Routing

In real scenario, therouting table is

determined by the costassigned to each node bythe administrator



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Open Shortest Path First (OSPF)

OSPF is based on Link State Routing Protocol

Area:A collection of networks, hosts and routers allcontained within an autonomous system

Area Border Routers: Summarizes all the informationabout an area and shares it across

Backbone: A special area among all areas in an ASwhich all other areas must be connected to. Thebackbone always has area code 0

Backbone Routers: Routers in a backbone. A

backbone router can also be area border router Virtual Link: If the connection between an area and

backbone is broken the administrator can create analternate connection between routers


OSPF I l t ti


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OSPF: Implementation

net net net

net

net

net

net

net netnet

Area 1

Area 2

Area 0 (Backbone)

ABRABR

BR

BR

AS BR

Autonomous System (AS)


P th V t R ti


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Path Vector Routing

Why DVR and LSR are not suitable for inter-domain

routing? Reason: Scalability

DVR becomes instable and intractable for a largenumber of hops (even more than 16)

LSR needs a huge amount of resource to calculateits shortest paths. It also causes heavy traffic in thenetwork because of flooding of LSP

How path vector routing eliminates them?

Well, it is simply derived from DVR, but does notassign hop count as the metric/cost...


P th V t R ti


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Path Vector Routing

Speaker node: In path vector routing, a special

node acts on behalf on the entire AS. It summarizesall the information of that AS, creates a routing tableand advertizes it to other ASs

What is advertized?

Not the metrics but the paths in an AS Policy: Every AS will have a well defined policy

Paths are decided upon by the speaker nodes byconsulting the policies in neighboring ASs

Reason: Different ASs will have different policies &priorities associated with them



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P th V t R ti


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Path Vector Routing

A1

D1

C1

B1

A2 A3

A4 A5

C2 C3

B2

B3

B4

D2 D3

D4

Dest. Path

A1 AS1

A2 AS1

A3 AS1

A4 AS1

A5 AS1

Dest. PathC1 AS3

C2 AS3

C3 AS3

Dest. Path

B1 AS2

B2 AS2

B3 AS2

B4 AS2

Dest. Path

D1 AS4D2 AS4

D3 AS4

D4 AS4

AS1

AS2

AS3

AS4

Ta

bleofA1

TableofD1

TableofC1

TableofB1


P th V t R ti


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Path Vector Routing Stabilized tables of all AS after update:

Dest Path

A1

A5

AS1

AS1

B1

B4

AS1-AS2

AS1-AS2

C1C3

AS1-AS3

AS1-AS3

D1D4

AS1-AS2-AS4

AS1-AS2-AS4

Dest Path

A1A5

AS2-AS1

AS2-AS1

B1

B4

AS2

AS2

C1C3

AS2-AS3

AS2-AS3

D1D4

AS2-AS3-AS4

AS2-AS3-AS4

Dest Path

A1A5

AS3-AS1

AS3-AS1

B1

B4

AS3-AS2

AS3-AS2

C1C3

AS3

AS3

D1D4

AS3-AS4

AS3-AS4

Dest Path

A1A5

AS4-AS3-AS1

AS4-AS3-AS1

B1

B4

AS4-AS3-AS2

AS4-AS3-AS2

C1C3

AS4-AS3

AS4-AS3

D1D4

AS4

AS4

Table of A1 Table of B1 Table of C1 Table of D1


P th V t R ti


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Path Vector Routing Some important features of updating:

Loop Prevention: The instability of DVR is avoided inPVR; upon receiving a message the router checks tosee if its AS is in the path

Policy Routing: Upon receiving a message a router

checks the path with policy. If an AS in the path isagainst policy it can ignore that

Optimum path: Router find the path that fits theorganization best. A path from AS4 to AS1 can eitherbe AS4->AS3->AS2->AS1 orAS4->AS3->AS1. Herewe will choose the path with less number of ASsinvolvedThis is not a general rule. There are complex criteria which

are always involved in real scenarioDecember 7, 2009Tusharadri Sarkar64

B d G t P t l (BGP)


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Border Gateway Protocol (BGP)

BGP was introduced in 1989

Some features of BGP:

Types of AS

Stub AS: An AS which is connected to another AS. Astub is either a source or a sink

Multihomed AS: An AS which is connected to morethan one AS, but it is only a sink or source. Example:A large corporation which is connected to more thanone regional or national ASs

Transit AS: A multihomed AS that allows flow of datatraffic through it. Example: All national andinternational ISPs



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B d G t P t l (BGP)


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Border Gateway Protocol (BGP) Some features of BGP:

BGP Sessions: A BGP session is a connection setupbetween two BGP routers for the sake of exchangingrouter information

A session in BGP is a connection at the TCP level.

External BGP Session (E-BGP):Takes place when two speakernodes exchange routing information

Internal BGP Session (I-BGP):Takes place when a speakernode collects information from other nodes in the its own As

A1

A2 A3

A4 A5

AS1

C1

C2 C3AS3



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Evolution of Routing Techniques

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