Masterkurs Rechnernetze
IN2097
Advanced computer networking
Internet Protocols
Prof. Dr.-Ing. Georg Carle,
Wolfgang Mühlbauer
Chair for Network Architectures and
Services
Computer Science Department
Technical University Munich
Outline
• Routing hierarchy
• Internet structure
• External BGP (E-BGP)
• Internal BGP (I-BGP)
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Acknowledgements
•
•
•
•
Randy H. Katz
Distinguished professor at UCB
thirteeen best paper awards
Textbook: Contemporary Logic Design, has
sold over 100,000 copies in two editions,
and has been used at over 200 colleges and
universities
• the late 1980s, with colleagues at Berkeley,
he developed Redundant Arrays of
Inexpensive Disks (RAID)
• Prior research interests have included:
database management, VLSI CAD, high
performance multiprocessor (Snoop cache
coherency protocols) and storage (RAID)
architectures, transport (Snoop TCP) and
mobility protocols spanning heterogeneous
wireless networks, and converged data and
telephony network and service architectures.
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Routing Hierarchies
• Flat routing doesn’t scale
– Storage Æ Each node cannot be expected to store
routes to every destination (or destination network)
– Convergence times increase
– Communication Æ Total message count increases
• Key observation
– Need less information with increasing distance to
destination
– Need lower diameters networks
• Solution: area hierarchy
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Areas
• Divide network into areas
– Areas can have nested subareas
– Constraint: no path between
two sub-areas of an area can
exit that area
1
2
2.1
1.1
2.2
2.2.2
2.2.1
1.2
1.2.1
1.2.2
3
• Hierarchically address nodes
in a network
– Sequentially number top-level
areas
– Sub-areas of area are labeled
relative to that area
– Nodes are numbered relative to
the smallest containing area
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3.1
3.2
5
Routing Hierarchy
Area-Border
Router
Backbone Areas
Lower-level Areas
• Partition Network into “Areas”
– Within area
• Each node has routes to every other node
– Outside area
• Each node has routes for other top-level areas only
• Inter-area packets are routed to nearest appropriate border router
• Constraint: no path between two sub-areas of an area can
exit that area
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Routing
• Within area
– Each node has routes to every other node
• Outside area
– Each node has routes for other top-level areas
only
– Inter-area packets are routed to nearest
appropriate border router
• Can result in sub-optimal paths
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Path Sub-optimality
1
2
2.1
1.1
2.2
2.2.1
1.2
1.2.1
start
end
3.2.1
3
3 hop red path
vs.
2 hop green path
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3.1
3.2
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A Logical View of the Internet
• National (Tier 1 ISP)
– “Settlement-free” and
“Default-free” with global
reachability info
Eg: AT & T, UUNET, Sprint
• Regional (Tier 2 ISP)
– Regional or country-wide
Eg: Pacific Bell
Tier 3
Tier 2
Tier 2
Customer
Provider
Tier 1
Tier 1
• Local (Tier 3 ISP)
Eg: Telerama DSL
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Tier 2
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Outline
• Routing hierarchy
• Internet structure
• External BGP (E-BGP)
• Internal BGP (I-BGP)
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Internet’s Area Hierarchy
• Autonomous System (AS)?
– A set of routers under a single technical
administration, using an interior gateway
protocol (IGP) and common metrics to route
packets within the AS and using an exterior
gateway protocol (EGP) to route packets to
other AS’s
• Each AS assigned unique ID
• AS’s peer at network exchanges
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History: IGP
• Most common IGPs:
– RIP: Routing Information Protocol
(v1: 1988, v2: 1994)
– OSPF: Open Shortest Path First
(1988 - 1998)
– IS-IS: Intermediate system to intermediate
system
(mid to end 90s)
– EIGRP: Enhanced Interior Gateway Routing
Protocol (Cisco proprietary, mid-80s)
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History: EGP
• Mid-80s: EGP
– Reachability protocol (no shortest path)
– Did not accommodate cycles (tree topology)
– Evolved when all networks connected to NSF
backbone
• Result: BGP introduced as routing protocol
– Latest version = BGP 4
– BGP-4 supports CIDR
– Primary objective: connectivity not performance
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A Logical View of the Internet
• Tier 1 ISP
– “Default-free” with
global reachability
info
• Tier 2 ISP
– Regional or countrywide
Tier 3
Tier 2
Tier 2
Customer
Provider
Tier 1
Tier 1
• Tier 3 ISP
– Local
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Tier 2
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Internet structure: network of networks
• roughly hierarchical
• at center: “tier-1” ISPs (e.g., Verizon, Sprint,
AT&T, Cable and Wireless),
national/international coverage
– treat each other as equals
Tier-1
providers
interconnect
(peer)
privately
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Tier 1 ISP
Tier 1 ISP
Tier 1 ISP
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Tier-1 ISP: e.g., Sprint
POP: point-of-presence
to/from backbone
peering
…
…
.
…
…
…
to/from customers
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Internet structure: network of networks
• “Tier-2” ISPs: smaller (often regional) ISPs
– Connect to one or more tier-1 ISPs, possibly
other tier-2 ISPs
Tier-2 ISP
Tier-2 ISP pays
tier-1 ISP for
connectivity to
rest of Internet
Tier-2 ISP
Tier 1 ISP
Tier-2 ISP is
customer of
tier-1 provider
Tier 1 ISP
Tier-2 ISP
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Tier 1 ISP
Tier-2 ISPs
also peer
privately with
each other.
Tier-2 ISP
Tier-2 ISP
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Internet structure: network of networks
• “Tier-3” ISPs and local ISPs
– last hop (“access”) network (closest to end systems)
local
ISP
Local and tier3 ISPs are
customers of
higher tier ISPs
connecting
them to rest of
Internet
Tier 3
ISP
local
ISP
local
ISP
Tier-2 ISP
local
ISP
Tier-2 ISP
Tier 1 ISP
Tier 1 ISP
Tier-2 ISP
local
local
ISP
ISP
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Tier 1 ISP
Tier-2 ISP
local
ISP
Tier-2 ISP
local
ISP
18
Internet structure: network of networks
• a packet passes through many networks!
local
ISP
Tier 3
ISP
local
ISP
local
ISP
Tier-2 ISP
local
ISP
Tier-2 ISP
Tier 1 ISP
Tier 1 ISP
Tier-2 ISP
local
local
ISP
ISP
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Tier 1 ISP
Tier-2 ISP
local
ISP
Tier-2 ISP
local
ISP
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Transit vs. Peering
Transit ($$ 1/2)
Transit ($$$)
ISP Y
ISP P
Transit ($)
Transit ($$$)
ISP Z
Transit ($)
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Transit ($$$)
Peering
Transit ($$)
ISP X
Transit ($$)
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