Global Internet

1
Global Internet
2
Scalability Issues

• IP hides hosts in address hierarchy, but...
• Inefficient use of address space
• class C network with 2 hosts (2/255 0.78
  efficient)
• class B network with 256 hosts (256/65535 0.39
  efficient)
• Too many networks
• today's Internet has tens of thousands of
  networks
• routing tables do not scale
• route propagation protocols do not scale

3
Internet Structure

• Recent Past
• Today

4
Subnetting

• Add another level to address/routing hierarchy
  subnet
• Subnet masks define variable partition of host
  part of class A and B addresses
• Subnets visible only within site

5
Subnet Example
Forwarding table at router R1
Subnet Number 128.96.34.0 128.96.34.128 128.96.33.
0
Subnet Mask 255.255.255.128 255.255.255.128 255.25
5.255.0
Next Hop interface 0 interface 1 R2
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Forwarding Algorithm

• D destination IP address
• for each entry lt SubnetNum, SubnetMask, NextHopgt
• D1 SubnetMask D
• if D1 SubnetNum
• if NextHop is an interface
• deliver datagram directly to destination
• else
• deliver datagram to NextHop (a router)

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Notes

• Would use a default router if nothing matches
• Not necessary for all ones in subnet mask to be
  contiguous
• Can put multiple subnets on one physical network
• Subnets not visible from the rest of the Internet
• Perform longest prefix match when there are two
  entries in the forwarding table that match
  (128.35.42.1 matches 128.35.0.0-255.255.0.0 and
  128.35.42.0-255.255.255.0, but the second would
  be chosen)

8
Numbers

• www.icann.org Internet Corporation for Assigned
  Names and Numbers
• www.arin.net is our authority and has more
  details
• Names and numbers have been privatized. The US
  government used to allocate them

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The big picture
10
Current
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Destinations
12
Host Counts
13
Top Level Domain Host Count Jan 2003
http//www.isc.org/ds/WWW-200301/dist-bynum.html
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Path Length
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Supernetting

• Assign block of contiguous network numbers to
  near-by networks
• Called CIDR Classless Inter-Domain Routing
• Represent blocks with a single pair
• ltfirst_network_address, countgt
• Restrict block sizes to powers of 2
• Use a bit mask (CIDR mask) to identify block size
• All routers must understand CIDR addressing

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Route Aggregation with CIDR
Corporation X
(11000000000001000001)
Border gateway
Regional network
(advertises path to
11000000000001)
Corporation Y
(11000000000001000000)
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Two Autonomous Systems
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Route Propagation

• Idea Impose a second hierarchy on the network
  that limits what routers talk to each other. (The
  first hierarchy is the address hierarchy that
  governs how packets are forwarded.)
• Autonomous System (AS)
• corresponds to an administrative domain
• examples University, company, backbone network
• assign each AS a 16-bit number
• Two-level route propagation hierarchy
• interior gateway protocol (each AS selects its
  own)
• exterior gateway protocol (Internet-wide standard)

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Popular Interior Gateway Protocols

• RIP Route Information Protocol
• developed at Berkeley
• distributed with Unix
• distance-vector algorithm- neighbors
• based on hop-count
• OSPF Open Shortest Path First
• recent Internet standard
• uses link-state algorithm-bcast
• supports load balancing
• supports authentication

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Multibackboned Internet
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EGP Exterior Gateway Protocol

• Overview
• designed for tree-structured Internet
• concerned with reachability, not optimal routes
• Protocol messages
• neighbor acquisition one router requests that
  another be its peer peers exchange reachability
  information
• neighbor reachability one router periodically
  tests to see if the other router is still
  reachable exchange HELLO/ACK messages uses a
  k-out-of-n rule
• routing updates peers periodically exchange
  their routing tables (distance-vector)

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EGP Example
Exterior Neighbor (Other system)
N1
Source Net N1
G1
G2
G1
1 N2
N3
N2
G3
G2
N4
1 N3
G3
G5
G4
1 N4
N6
N5
2 N5
2 N6
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BGP-4 Border Gateway Protocol

• Assumes the Internet is an arbitrarily
  interconnected set of AS's (Autonomous Systems).
  Define local traffic as traffic that originates
  at or terminates on nodes within an AS, and
  transit traffic as traffic that passes through an
  AS, we can classify AS's into three types
• Stub AS an AS that has only a single connection
  to one other AS such an AS will only carry local
  traffic.
• Multihomed AS an AS that has connections to more
  than one other AS, but refuses to carry transit
  traffic.
• Transit AS an AS that has connections to more
  than one other AS, and is designed to carry both
  transit and local traffic.

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Autonomous System (AS)

• Each AS has
• One or more border routers
• One BGP speaker that advertises
• local networks
• other reachable networks (transit AS only)
• gives path information
• Still pass information about every network

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BGP Example
128.96
Customer P
192.4.153
(AS 4)
Regional Provider A
AS 2
Customer Q
192.4.32
(AS 5)
192.4.3
"Backbone" Network
AS 1
Regional Provider B
Customer R
192.12.69
(AS 6)
AS 3
Customer S
192.4.54
(AS 7)
192.4.23
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BGP Example

• Speaker for AS 2 advertises reachability to P and
  Q
• Network 128.96, 192.4.153, 192.4.32, and 192.4.3,
  can be reached directly from AS 2.
• Speaker for backbone network then advertises
• Networks 128.96, 192.4.153, 192.4.32, and 192.4.3
  can be reached along the path ltAS 1, AS 2gt.
• Speaker can also cancel previously advertised
  paths

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Domain Divided into areas
Area 3
Area 1
Area 0
R9
R7
R3
R8
R1
R4
R2
Area 2
R5
R6
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Routing Basics

• Minimize the size of routing tables
• Create Autonomous routing systems
• Simplify routing
• hierarchical routing
• Optimize within the Autonomous system