AS’s, CIDR, Border Gateway Protocol · 2019. 11. 7. · Analysis Summary-----BGP routing table entries examined: 783546 Prefixes after maximum aggregation (per Origin AS):301812

25 Oct 2020 SE 428: Advanced Computer Networks 1

AS’s, CIDR, Border Gateway

Protocol

25 October 2020

Lecture 2

Lecturer: Dr. Michael J. May

Kinneret College

Topics for Today

• Intra-network routing

• AS Types

• Classless Interdomain Routing (CIDR)

• Inter-Network Routing

– BGP

• Sources: PD 4.1.2, KR 4.6.3


Inter- versus Intra-

• Intra-Network Routing → driving within a single city

• Inter-Network Routing → driving between cities

• Different decisions:

– Traffic lights

– Narrow streets

– Speed bumps

– One way streets

• Fastest versus Shortest

• What shape is the internet?


The old shape (1990s)


A newer picture



Organizations and Backbone


• Key Concept: Autonomous System (AS) (aka Routing

Domain)

Organizations and Backbone


So Far


• AS Types



– BGP


Networks which serve a

company, college, etc.

• Want to get data

directed to them

• Want to send data to

other places

• Pay for bandwidth and

connectivity

• Called Stub Networks


Network Types

Networks which connect

to other networks (ISPs,

etc.)

• Move data from one

place to another

• Offer data transit

services to customers

for a fee

• Called Transit

Networks


Network Types

Challenges of inter-AS

routing:

Scale – things grow

Load balancing

Privacy – don’t let others

see your internals

Policy – Give priorities,

prevent bad things,

respond to changes


Network Types

AS Classes• Stub AS: an AS that has only a single connection to one other

AS– Will only carry local traffic

– Ex. Consumer based Internet Service Provider, small corporation, small academic institution

• Multihomed AS: an AS that has connections to more than one other AS, but refuses to carry transit traffic– Ex. Large corporation, college, etc.

– Complicates routing and allocation (need provider independent address space, ASN, etc.)

• Transit AS: an AS that has connections to more than one other AS, and is designed to carry both transit and local traffic– Ex. Bezeq, Netvision, other international ISPs.


Visualizing It http://thyme.apnic.net/BGP/ME/


Some Numbers (London, 21 Oct 2020)


Analysis Summary----------------BGP routing table entries examined: 816175

Prefixes after maximum aggregation (per Origin AS):314631 Total ASes present in the Internet Routing Table: 69505

Prefixes per ASN: 11.74 Origin-only ASes present in the Internet Routing Table: 59950 Origin ASes announcing only one prefix: 24750 Transit ASes present in the Internet Routing Table: 9555 Transit-only ASes present in the Internet Routing Table: 285 Average AS path length visible in the Internet Routing Table: 4.1

Max AS path length visible: 33 Max AS path prepend of ASN ( 45582) 27

Prefixes from unregistered ASNs in the Routing Table: 950 Number of instances of unregistered ASNs: 967

Number of bogon 32-bit ASNs visible in the Routing Table: 33 Prefixes being announced from unallocated address space: 512 Number of addresses announced to Internet: 2,862,262,784

Equivalent to 170 /8s, 154 /16s and 170 /24s

http://thyme.apnic.net/london/data-summary

So Far


• AS Types



– BGP


Running out of IP Addresses

• We mentioned the problems introduced by the

classes for IP addresses

– Class A: 16,777,214

– Class B: 65,534

– Class C: 254

• Many organizations bought a Class B network

figuring they might eventually go over 254 hosts

– Survey found over half of them had less than 50 hosts

– Millions of addresses have gone to waste

– Can’t recover them easily (already sold!)


Running out of IP Addresses

14 Sept 2012: RIPE.net (European and Middle East

ISP) announces it’s on its last class A (/8) block

– No more IPv4 allocations to new customers

– Customers with IPv6 can only buy one more /22 (1,024

addresses) block

– 29 Feb 2016: 15.59 million addresses left in /8 block

(93% of it)

• IANA has begun recovering IPv4 address blocks

– 1 March 2016: RIPE.net received a recovered /15

block


25 Oct 2020 19

https://www.ripe.net/manage-ips-and-asns/ipv4/ipv4-available-pool

SE 428: Advanced Computer Networks

No More Classes

Stopgap measure: Stop using classesWorks only for newly granted

networks

Give out addresses in chunks of Class C

sizeNetwork needs 4064 hosts? Give 16 Class C networks

Creates a new problem: Routers designed for class based routing

An organization has 16 Class C networks assigned (4064 hosts)

Each router needs 16 lines for that organization

They all go to the same place!


Aggregate Routes• The answer: Aggregate Routes

– Very similar to Subnetting

– Use addresses and masks

– Requires us to allocate addresses contiguously

– Technique called: Classes Interdomain Routing

• If we give out 16 Class C networks: 192.4.16 – 192.4.31– 192.4.16: 11000000 00000100 00010000 00000000

– 192.4.31: 11000000 00000100 00011111 00000000

– The top 20 bits are the same

• We can write this: 192.4.16/20– Means the top 20 bits matter

– Identical to using the subnet mask: 255.255.240.0

• 11111111.11111111.11110000.00000000


Classless Interdomain Routing

• Routers can now store aggregate routing information

– Address/Length for each organization/network

– Requires us to change the way the routers work

• Good for aggregating networks which are all accesses via

the same main router

– A ISP buys a Class B networks and sells the addresses in Class

C blocks

– All customers are reached via the ISP’s gateway – just advertise

the ISP’s prefix and all traffic for the customers will go via its main

router

– Leads into Border Gateway Protocol (BGP) (next)


ISP Example• ISP sells addresses in the blocks: 192.4.0.0 – 192.4.63.255

• Customers are:

– Customer 1: 192.4.0.0/20

– Customer 2: 192.4.16.0/20

– Customer 3: 192.4.32.0/20

– Left available to sell: 192.4.48.0 – 192.4.63.255


Regional

Network

(ISP)

Customer 2:

192.4.16.0 –192.4.31.255

Customer 1:

192.4.0.0 –192.4.15.255

Customer 3:

192.4.32.0 –192.4.47.255

G

Advertises:

192.4.0.0/14

CIDR

• Now routing packets requires looking at the

address and the prefixes

– Same algorithm as subnetting

• Longest mask wins

– Means it’s more specific

• Advantage: Collapse many areas into a single

routing table line.


So Far


• AS Types



– BGP


Inter-AS Routing


Idea: Provide an additional way to hierarchically aggregate routing information is a large internet.

We need to find routes to destinations

• What are destinations?

• What are nodes?

• What are links?

IP Prefixes (12.X.X.X) (CIDR)

AS (how many are there?)

Connections and Business Relationships

Challenges for Inter-AS Routing

Scale (as of 21 Oct 2020)

• Prefixes: 847,748 (no CIDR) or 460,203 (CIDR

aggregated) and growing

• ASes: 69,971 visible ones, and growing

• Routers: at least in the millions…

• Border routers must know how to get anywhere in the

world!

Coordination with intra-AS protocols (OSPF)

• How to inject external routes to OSPF database


So

urc

e: h

ttp

://w

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.cid

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po

rt.o

rg/a

s2

.0/

Challenges for Inter-AS Routing

Policy

• I want control over where I send traffic

• … and who send traffic through my AS why?

• AS don’t want to expose internal topologies

• … or my business relations with neighbors

Trust:

• Provider A might be unwilling to believe advertisements

from provider B

• See: http://www.cidr-report.org/as2.0/#Bogons


http://www.cidr-report.org/as2.0/#Bogons

Conclusion


• AS Types



– BGP


AS’s, CIDR, Border Gateway Protocol · 2019. 11. 7. · Analysis Summary-----BGP routing table entries examined: 783546 Prefixes after maximum aggregation (per Origin AS):301812

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