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Part 2.6
Internetwork Routing
(Static and automatic routing;
route propagation; BGP, RIP, OSPF;
multicast routing)
Robert L. Probert & Os Monkewich
Fall 2004
CSI 4118 R.L.Probert
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Terminology
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Forwarding
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Refers to datagram transfer
Performed by host or router
Uses routing table
Routing
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Refers to propagation of routing information
Performed by routers
Inserts / changes values in routing table
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Two Forms of Internet Routing
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Static routing
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Table initialized when system boots
No further changes unless error is detected
Automatic routing
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Table initialized when system boots
Initialization is started the same way as static
But, rout propagation (routing) software is also
started
Routing software learns optimal routes and
updates routing table
Continuous changes possible
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Static Routing
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No routing software
Does not consume bandwidth or CPU time
It cannot accomodate network failures
It cannot accommodate changes in network
topology
Used when host is on one network and sees
the Internet through a router
Typical routing table has two entries:
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Local network → direct delivery
Default → nearest router
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Example of Static Routing
Routing Table of One Host (E.g. H1)
Direct to H2
To the Internet
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A Note on Addressing
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129.52.18.6/20 means
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This means that some of the bits making up .18.
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belong to network addressing
belong to host addressing
Need to expand back to binary to resolve
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20 leading bits are for network addresses
the remaining 12 bits are for host addresses
expand 10000001 00110100 00010010 00000110
network 10000001 00110100 0001 (address prefix)
host
0010 00000110
Need a subnet mask
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11111111 11111111 11110000 00000000 or 255.255.220.0
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Automatic Routing
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Used by IP routers
Requires special software
Each router communicates with neighbors
Pass routing information
Use route propagation protocol
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Example of Route Propagation
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Router R1 does not know about Net 3
Router R2 does not know about Net 2
If static tables with 100 or more hosts on Net 2 and Net 3
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manual routing table updates become impractical
each time a new host is added, ISP1 needs to inform ISP1 to update
Need to automate - add routing software
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Example of Route Propagation
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Routers R1 and R2 each run routing software
Routing software on R1 installs the route to Net 3 in its routing table
Routing software on R2 installs the route to Net 2 in its routing table
Each router advertises destinations that lie beyond it of its link state
If Net 3 goes down, R1 routing software removes Net 3 from its table
If Net 3 comes back up, R1 routing software places Net 3 into its table
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The Point of Routing
Exchange
Each router runs routing software that learns
about destinations other routers can reach, and
informs other routers about destinations that it
can reach. The routing software uses incoming
information to update the local routing table
continuously.
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Routing in the Global Internet
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The Internet is subdivided into routing areas
to reduce routing traffic
At least one router in an area summarizes the
routing information and passes it to other
areas
Main considerations
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size of routing areas
routing protocol within a routing area
how routing information is represented
what protocol is used between routing areas
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Autonomous System Concept
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Set of networks and routers under one
administrative authority
Flexible, soft definition depending on
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Intuition: a single corporation, university
Needed because
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cost
administrative convenience
capability of routing protocol chosen
no routing protocol can scale to entire Internet
table update traffic would be to great
Each AS chooses a routing protocol
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Classifications of Inernet
Routing Protocols
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Two broad classes
Interior Gateway Protocols (IGPs)
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Used among routers within autonomous system
Destinations lie within IGP
Example of IGP - OSPF
Exterior Gateway Protocols (EGPs)
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Exchange routing information with routers in other
autonomous systems
Destinations lie throughout Internet
Example of EGP - BGP4
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Illustration of IGP / EGP Use
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R1 summarizes routing information from AS1 and
sends the summary to R4
R1 accepts AS2 summary from R4
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Optimal Routes
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Optimal depends on need
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Most Internet routing uses other metrics
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interacitve login - least delay is optimal
browser downloading - max. throughput is optimal
real-time audio - least jitter is optimal
administrative cost (corporate policy to control traffic)
hop count (fewer hops for customer, more for internal)
EGP does not use metrics
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cannot compare routing metrics from different
Autonomous Systems
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The Concept of Route and
Data Traffic
ISP1 advertises routes to its
customers in this direciton
Datagrams to ISP1 customers
flow in this direciton
Each ISP is an autonomous system that uses an
Exterior Gateway Protocol to advertise its customers’
networks to other ISPs. After an ISP advertises
destination D, datagrams destined for D can begin to
arrive
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Specific Internet Routing
Protocols
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Border Gateway Protocol (BGP)
Routing Information Protocol (RIP)
Open Shortest Path First Protocol (OSPF)
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Border Gateway Protocol (BGP)
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Provides routing among autonomous systems
(EGP)
Only two routers are involved
A BGP session
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open BGP session over TCP to inform neighbour
 of new routes that are active (in terms of prefix)
 of old routes no longer active
 that this connection is still viable
 of any unusual conditions
Policies to control routes advertised
Gives path of autonomous systems for each
destination
EGP of choice in the Internet is BGP ( BGP-4)
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BGP AS Links and Path Tree
BGP link
AS 2
BGP link
AS 1
AS 3
BGP link
BGP link
BGP link
AS 5
AS 4
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CSI 4118 R.L.Probert
BGP provides routes to other ASs (address prefixes)
BGP builds a graph of ASs
Graph derived from routing information
BGP sees the entire Internet as a graph
BGP can skip intrmediate routers in AS
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BGP Advertising
BGP link
AS 2
AS 1
192.168.0.0/16
BGP link
AS 3
BGP link
BGP link
BGP link
AS 5
AS 4
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AS 1 advertises:
AS 2 advertises:
AS 3 advertises:
AS 5 sees:
AS 5 sees:
CSI 4118 R.L.Probert
192.168.0.0/16 is Reachable throug this AS
192.168.0.0/16 through AS 1
192.168.0.0/16 through AS 1 and AS 2
192.168.0.0/16 through AS 1 and AS 2 and AS 3
192.168.0.0/16 through AS 1 and AS 2
192.168.0.0/16 through AS 1 and AS 2 and AS 3
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The Routing Information
Protocol (RIP)
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Routing within an autonomous system (IGP)
Hop count metric
Unreliable transport (uses UDP)
Broadcast or multicast delivery
Distance vector algorithm
Can propagate a default route
Implemented by Unix program routed
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Illustration of RIP Packet
Format
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The Open Shortest Path First
Protocol (OSPF)
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Routing within an autonomous system (IGP)
Full CIDR address scheme and subnet support
Authenticated message exchange
Allows routes to be imported from outside the
autonomous system
Uses link-status (SPF) algorithm
Support for multi-access networks (e.g.,
Ethernet)
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OSPF Areas and Efficiency
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Allows subdivision of an AS into Areas
Link-status information propagated within
one Area
One router in each Area is designated to
communicate with one or more other Areas
Routing information is summarized before
being propagated to another Area
Reduces overhead (less broadcast traffic)
Able to scale to large or small networks
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Link-Status in the Internet
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Router corresponds to node in graph
Network corresponds to edge
Adjacent pair of routers periodically
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Test connectivity
Broadcast link-status information to area
Each router uses link-status messages to
compute shortest paths
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R1 LSA Data Base
LSA
Flooding
AREA with 2 Routers
2 Links to R1: a, b
LSA1, LSA2,
LSA3, LSA4,
LSA5, LSA6,
LASa, LSAb,
LSAc, LSAd,
LSAe, LSAf,
LSAg, LSAh
R1
1
2
R2 LSA Data Base
R3 LSA Data Base
LSA1, LSA2,
LSA3, LSA4,
LSA5, LSA6,
LASa, LSAb,
LSAc, LSAd,
LSAe, LSAf,
LSAg, LSAh
LSA1, LSA2,
LSA3, LSA4,
LSA5, LSA6,
LASa, LSAb,
LSAc, LSAd,
LSAe, LSAf,
LSAg, LSAh
5
R2
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R3
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Inside the Router-LSA
LSA Header
LS Age
Options
LS Type = 1
Link State ID
Advertising Router
LS Sequence Number
Length
LS Checksum
Router-LSAs
LSA Header
0
V E B
0
# links
Link ID
Link Data
Type
TOS
CSI 4118 R.L.Probert
# TOS
...
0
metric
# TOS
Link ID
Link Fall
Data
2004
...
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Illustration of OSPF Graph
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(a) an interconnect of routers and networks, and
(b) an equivalent OSPF graph
Router corresponds to a node in the graph
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OSPF and Scale
Because it allows a manager to partition the
routers and networks in an autonomous
system into multiple areas, OSPF can scale to
handle a much larger number of routers than
other IGPs
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Internet Multicast Routing
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Difficult because Internet multicast allows
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Arbitrary computer to join multicast group at any
time
Arbitrary member to leave multicast group at any
time
Arbitrary computer to send message to a group
(even if not a member)
Internet Group Multicast Protocol (IGMP)
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Used between computer and local router
Specifies multicast group membership
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Multicast Routing Protocols
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Several protocols exist
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Distance Vector Multicast Routing Protocol (DVMRP)
Core Based Trees (CBT)
Protocol Independent Multicast – Sparse Mode (PIM-SM)
Protocol Independent Multicast – Dense Mode (PIM-DM)
Multicast extensions to the Open Shortest Path First
(MOSPF)
None best in all circumstances
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Summary
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Static routing used by hosts
Routers require automatic routing
Internet divided into autonomous systems
Two broad classes of routing protocols
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Interior Gateway Protocols (IGPs) provide routing
within an autonomous system
Exterior Gateway Protocols (EGPs) provide
routing among autonomous systems
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Summary (continued)
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Border Gateway Protocol (BGP) is current
EGP used in Internet
Interior Gateway Protocols include:
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Routing Information Protocol (RIP)
Open Shortest Path First protocol (OSPF)
Internet multicast routing difficult
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Protocols proposed include: DVMRP, PIM-SM,
PIM-DM, MOSPF
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