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Global Internet
Part II
Neil Tang
11/07/2008
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Overview
 Scalability Issues
 CIDR
 Autonomous System (AS)
 Inter-Domain Routing
 Routing Area
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Scalability Issues
 Scalability of Routing: The number of network addresses
carried around by the protocol messages and stored in the
routing table needs to be minimized.
 Address Utilization: The limited IP address space needs to
be used efficiently.
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Classless Inter-Domain Routing (CIDR)
 The subnetting can not completely solve the scaling problem. For
example, suppose there are 16 networks, each of which has 255 hosts.
 CIDR is a technique proposed to reduce routing table sizes by
aggregating routes if the above situation happens.
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Classless Inter-Domain Routing (CIDR)
 Internet will include a large number of class C networks. Based on
CIDR, a set of closely located class C networks will be assigned a set
of continuous addresses. E.g., 192.4.16-192.4.31 share a common 20bit prefix 11000000 00000100 0001.
 A routing message or a routing table usually include a tuple composed
of such a virtual network addresses and a mask which indicates how
many bits in the network prefix.
 A routing table may include “overlapping” entries. E.g., an entry
corresponds to “171.69.0.0/16 ” and another entry corresponds to
“171.69.10.0/24”. In this case, the packet will be forwarded based on
the entry providing longest match.
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Interdomain Routing
 Internet is organized as Autonomous Systems (AS, a.k.a routing
domain), each of which is under control of an administrative entity.
 Intradomain routing protocols deal with routing between routers in a
single domain, e.g., RIP, OSPF.
 Interdomain routing protocols deal with routing between autonomous
systems, e.g., BGP.
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Autonomous System
 Stub AS: An AS has only a single connection to one other AS., e.g.,
small corporation.
 Multihomed AS: An AS has connections to more than one other AS but
refuses to carry transit (relay) traffic, e.g., large corporation.
 Transit AS: An AS has connections to more than one other AS and is
designed to carry local and transit traffic, e.g., backbone provider.
Large corporation
“Consumer”ISP
Peering
point
Backbone service provider
“Consumer”ISP
Large corporation
Peering
point
“Consumer”ISP
Small
corporation
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Border Gateway Protocol (BGP)
 The administrator of each AS picks at least one router to be the BGP
speaker. It will establish sessions with other BGP speakers to exchange
reachability information.
 In addition to a BGP speaker, there are one or more border gateways
which are used to forward packets between ASs.
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Border Gateway Protocol (BGP)
BGP speakers advertise the complete path as an enumerated
list of ASs to reach a particular network. For example, the BGP speaker
AS1 advertises the networks 128.96, 192.4.153, 192.4.32 and 192.4.3 can
be reached along the path (AS1, AS2)
Customer P
(AS 4)
128.96
192.4.153
Customer Q
(AS 5)
192.4.32
192.4.3
Customer R
(AS 6)
192.12.69
Customer S
(AS 7)
192.4.54
192.4.23
Regional provider A
(AS 2)
Backbone netw ork
(AS 1)
Regional provider B
(AS 3)
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Border Gateway Protocol (BGP)
 BGP can use different routing policies to choose “good” paths. E.g.,
policies could be “Use the path that crosses the fewest number of ASs”
or “Use AS x in preference to AS y”.
 The 16-bit AS ID is assigned by a central authority to guarantee
uniqueness.
 The negative advertisement message known as withdrawn route is
used to notify the link/node failures.
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Inter/Intra-Domain Routing
 The complexity of interdomain routing is on the order of the number of
ASs, and the complexity of intradomain routing is on the order of the
number of networks in a single AS.
 A packet destined for a network outside the AS the source belongs to
will be forwarded to a border gateway.
 If an AS is connected to more than one other ASs through multiple
border gateways, a protocol called iBGP is used to redistribute the
reachability information and enable the routers in an AS to learn the
best border gateway for a particular destination network.
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Routing Area
 A routing domain can be further divided to multiple routing areas.
 There is a special routing area called backbone area which is used to
connect other non-backbone areas.
 A router in both a backbone area and a non-backbone area is called an
Area Border Router (ABR)
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Routing in Multiple Areas
 Routing advertisement
information from non-ABR
routers will never leave the area
it is originated.
Area 3
Area 1
Area 0
R9
 An ABR advertises the routing
information it learned from a
routing area, say Area X to
routers in the other areas such
as they think all routers in Area X
are directly connected with R1.
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R8
R3
R1
R4
R2
Area 2
R6
R5
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Optimality and Scalability
 Strength: Scalability is improved because the routing information which
needs to be advertised and maintained, is dramatically reduced.
 Weakness: All packets need to go through ABRs, which may not be the
optimal solution, i.e, two routers in different areas may be able to
directly communicate with each other via a low-cost path not including
an ABR.
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