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Internet Routing Evolution

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Lecture outline

• Evolution of Internet Routing

• Hierarchical Routing

• CIDR (Classless Inter Domain Routing)

• Internet Registrar

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Evolution of Internet Routing

• In 1970- 80s, nobody expected the Internet grow to be 2x

– 10x a year.

• Telephony (Voice) traffic grows 4-10% /yr

• So, only 4 bytes were allocated for addresses

• 32 bits will fit well into the register sizes of many CPUs

(16 bit or 32 bit)

• 64 bits is too wasteful

• Initial thought was

– Few big networks. Class A

– Moderate medium size networks. Class B

– Many Small size networks. Class C

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Evolution of Internet Routing

• Consider this

• PC growth is 2x every 18 months

• Internet user growth is at 30% per year

• Devices (Mobile Device, etc) need to connect via IP - growth

at x% per year

• The network should interconnect all of them

– How fast does the Internet need to grow?

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Sufficient Address

• Imagine in one housing street

• Address Range allocated: from 00 – 99

• What happens if there are > 100 houses in the street?

– Some house will have no address

• Suppose 1 university gets a Class B address but only

uses 20,000 addresses

– 65535 – 20000 = 45535 address wasted

• OK, Throw away the class based addressing

• Now instead of given someone a class B address

– Registrar gives multiple contiguous class C addresses

– 200.1.4.0/23 = 200.1.4.0/24 and 200.1.5.0/24

– Slow down the IP address wastage in terms of usage

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Classless

• Classless Address Assignment gives rise to another problem

• Previously

– 11.0.0.0/8 there is only 1 routing entry on every router

• Now

– All are Class C

– 11.0.0.0/24 … 11.255.255.0/24

– There are 65535 possible routes on each router

– Assume 10% usage

– Initially, the router needs to store 1 entry now needs to

store approximately 6554 entries.

– Routing Table explosion. Increase max 65535x

– Increase RAM for routers (difficult in 1990s and early

2000s)

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Routing Table Explosion

• Assume all networks from 1.x.x.x - 223.x.x.x (whole Internet+intranet)

are assigned with a class C network mask /24

– 256 x 256 x 223= 14,614,528 networks

– Each router will have to store 14.6 million network address if all the

network address is not summarized. Let say 1 network entry takes

up 10 bytes.

– 14.6 million networks x 10 bytes = 146 Mbytes just to store the

routing table.

– If any of the 14.6 million routers or links go down, you may have to

recalculate the path again

– 14.6 million networks = how many routing updates?

– Not scalable

– Routing Table must be summarizable

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Address Summarization CIDR (just read)

• Classful address Waste lot of IP

• Thus, give classless address. Customers get multiple class C.

• Lead to Internet routing table explosion

• IETF introduced CIDR (Classless InterDomain Routing) to address scalability of Internet routing table.

• In CIDR, address class is meaningless

• Each network is determined by 200.10.0.0/X where X is the prefix length

• No longer determined by class

• Top down IPv4 addressing assignment to enhance scalability of Internet routing table

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CIDR

• Eliminate all Class A, B and C addressing

• Allow efficient IPv4 address allocation

• Support route-aggregation (Summarization – Supernetting)

– Few thousand routes represented in 1 route

– Also prevent route flapping

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Net 7 is down

Info is propagated

Other routers calculate the new route

Route Flapping Scenario

RTA

RTB

RTC

RTD ISP

199.1.0.0/25

199.1.0.128/25

199.1.1.0/24

199.1.4.0/24

199.1.5.0/24

199.1.6.0/24

199.1.7.0/24 – flapping route

199.1.2.0/24

199.1.3.0/24

Net 7 down

Net 7 down

Net 7 down

Net 7 down

Calc

Route Calc

Route

Calc

Route

Calc

Route

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Net 7 is up again

All router end up spending lot of time calculate and recalculating route

Route Flapping Scenario: 5 min later

RTA

RTB

RTC

RTD ISP

199.1.0.0/25

199.1.0.128/25

199.1.1.0/24

199.1.4.0/24

199.1.5.0/24

199.1.6.0/24

199.1.7.0/24 – flapping route

199.1.2.0/24

199.1.3.0/24

Net 7 up

Net 7 up

Net 7 up

Net 7 up

reCalc

Route reCalc

Route

reCalc

Route

reCalc

Route

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Upstream router will only store 1 network route 199.1.0.0/21 (first 21 bits are

the same).

CIDR. Example of Route summarization.

RTA

RTB

RTC

RTD ISP

199.1.0.0/25

199.1.0.128/25

199.1.1.0/24

199.1.4.0/24

199.1.5.0/24

199.1.6.0/24

199.1.7.0/24

199.1.2.0/24

199.1.3.0/24

199.1.4.0/22

199.1.0.0/21

199.1.0.0/23

199.1.20/23

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RTD will not know RTC 199.1.7.0/24 is flapping (up and down).

When will someone know 199.1.7.0/24 is down,

=> when the packet reaches RTC.

CIDR.Flapping route

RTA

RTB

RTC

RTD ISP

199.1.0.0/25

199.1.0.128/25

199.1.1.0/24

199.1.4.0/24

199.1.5.0/24

199.1.6.0/24

199.1.7.0/24- flapping route

199.1.2.0/24

199.1.3.0/24

199.1.4.0/22 is still sent

199.1.0.0/21

199.1.0.0/23

199.1.20/23

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Hierarchical Routing

• As the network size grows, routing table size grows as well

• If every network is stored

– Big routing table

– More memory required

– IP Table lookup will be longer/slower

• When a network fails or a router fails, the routers will propagate

this information to all the other routers.

• Other router s

– More time spent in processing routing update and

recalculating the network cost/topology again. CPU

Intensive.

– Less time for routing

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Hierarchical Routing. Grouping of routers.

• The network should be interested only in local traffic. Unrelated traffic

should be summarized

– E.g. If you need to know all the roads in Kuala Lumpur before you

can go to KLCC then you may need to know a lot of things before

you can reach there.

• Therefore, a sets of routers should be grouped together so that

– Link/Node Fault is isolated to the particular group of routers only

and not all the routers in the network

– Router can spend more time in routing and less time (CPU) in

processing routing updates, less calculation on the best path in a

large network

• Each group of routers is grouped into an Autonomous System number

in BGP, IGRP, EIGRP and area-ID in OSPF.

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Example. OSPF

Area 1

Area 0

backbone

Area 2

Local Routing update in each area is localized

Link/Router Failure in one area will not affect routers in other area

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Example. IGRP, EIGRP

AS No 10

AS No 20

AS No 30

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Routing Address Planning

• Grouping of routers in areas or AS with the same

address block.

• If possible 1 or few routes per area and per AS

• Grouping + Address Planning comes together

• Few routes is for load balancing purposes

• For Internet to support route summarization

– Top down address assignment

– ISP owned addresses

– If customer change ISP, readdressing is required

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Registry

• How you get your ID?

• Jabatan Pendaftaran Malaysia. Registry

• Similiarly in Internet, there are registry

• A body that assign/manage all the numbers

• IANA (Internet Assigned Numbers Authority)

– APNIC (Asia Pacific Registry)

– MYNIC (Malaysia Registry)

• These IP addresses (SPAM email etc) belong to who, which

network, who is the contact person incharged

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CIDR Allocation Example

Sub-

Allocation

/22

/8

APNIC Allocation

/24

/20

Member Allocation

/25 /26 /27 /26

APNIC Allocates

to APNIC Member

APNIC Member

Customer / End User

Assigns

to end-user

Allocates

to downstream

Downstream

Assigns

to end-user

Source : APNIC = Asia Pacific Network Information Centre

= Regional Internet Registry that allocates IP and AS numbers in the Asia Pacific region www.ciscommu.com

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Allocation vs Assignment

• Allocation

• “A block of address space held by an IR (or

downstream ISP) for subsequent allocation or

assignment”

• Not yet used to address any networks

• Assignment

• “A block of address space used to address an

operational network”

• May be provided to LIR (Local Internet Registry)

customers, or used for an LIR’s infrastructure

(‘self-assignment’)

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Portable vs Non-portable Assignment

• Portable Assignments

– Customer addresses independent from ISP

• Keeps addresses when changing ISP

– Bad for size of routing tables

– Bad for QoS: routes may be filtered, flap-

dampened

– DNS root server

• Non-portable Assignments

– Customer uses ISP’s address space

• Must renumber if changing ISP

– Only way to effectively scale the Internet

– 99% of the address

RFC2439 describes the algorithm and conditions flap damping is applied under www.ciscommu.com

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Aggregation and “portability”

Aggregation

(Non-portable Assignments)

No Aggregation

BGP Announcement (1)

ISP

Allocation

Customer Assignments

(Portable Assignments)

BGP Announcements (4)

Customer Assignments

ISP

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Internet routing Table Growth

last updated 07 July 2004

http://bgp.potaroo.net/as1221/bgp-active.html

Deployment

Period of CIDR

CIDR made it work for a while

Rapid growth due to

Large number of longer

prefixes announced

Projected routing table

growth without CIDR

ISPs

tend to

filter

longer

prefixes

But the routing

table still grows

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2015 BGP Table

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IPv4 address Pool /8. 2015

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What exactly is "IPv4 exhaustion"

• IANA exhausted its IPv4 free pool (3 February 2011)

• RIRs exhaust their unallocated pools

• Expanding networks (ISPs, businesses, etc) exhaust their

pools of unused addresses

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Registar Policy

• “IP addresses not freehold property”

– Assignments & allocations on license basis

• Addresses cannot be bought or sold

• Internet resources are public resources

• ‘Ownership’ is contrary to management goals

• “Confidentiality & security”

– APNIC to observe and protect trust relationship

• Non-disclosure agreement signed by staff

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Allocation Policy

• Aggregation of allocation – Provider responsible for aggregation

– Customer assignments /sub-allocations must be non-portable

• Allocations based on demonstrated need – Detailed documentation required

• All address space held to be declared

– Address space to be obtained from one source • routing considerations may apply

– Stockpiling not permitted

• Transfer of address space – Not automatically recognised

• Return unused address space to appropriate IR (Internet Registry)

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Address Assignment Policy

• Assignments based on requirements

• Demonstrated through detailed documentation

• Assignment should maximise utilisation

– minimize wastage

• Classless assignments

• showing use of VLSM

• Size of allocation

– Sufficient for up to 12 months requirement

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Knowledge Benchmark

• Understand how classless assignment

• => lead to routing table explosion

• How CIDR solve routing table explosion

• Effect on global IP address assignment

• Provider independent address

• Provider dependent address

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Questions

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