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Introduction to
Computer Networks
Routing
University of Ilam
By: Dr. Mozafar Bag-Mohammadi
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Outline
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Routing process
Routing Algorithms
Scalability
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Forwarding vs Routing
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forwarding: to select an output port based on
destination address from the lookup table.
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Minimize the lookup time.
routing: process by which the routing table is
built.
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Optimize the calculating paths in case of changing
topology.
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Routing Process
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Question? How to populate the lookup table?
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Primary solutions:
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Build the lookup table Manually? Is it practical?
The answer is no.
Flooding- Broadcast to all node except the one we
have received the packet.
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Waste the bandwidth
Not scale well- Broadcast storm.
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Overview
A
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Network as a Graph
C
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1
3
4
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6
2
1
B
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E
F
1
D
Problem: Find lowest cost, or shortest, path between
two nodes
The process is distributed and this makes it
complicated, I.e, it may create loop.
Factors
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static: topology
dynamic: load
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Distance Vector
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Each node maintains a set of triples
(Destination, Cost, NextHop)
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Exchange updates with directly connected neighbors
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periodically ( on the order of several seconds)
whenever its table changes (called triggered update)
Each update is a list of pairs:
(Destination, Cost)
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Update local table if receive a “better” route
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smaller cost
came from next-hop
Refresh existing routes; delete if they time out
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Example
B
C
A
D
E
F
Destination
A
C
D
E
F
G
Cost
1
1
2
2
2
3
NextHop
A
C
C
A
A
A
G
•Distance of other nodes from Node B.
•The cost between two nodes has been assumed 1.
•All nodes keep a routing table from themselves.
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The Bellman-Ford Algorithm
•Bellman-Ford algorithm solve the distance
Vector problem in general case.
1. Set: Xo = (  ,  ,  ,…,  ).
2. Send updates of components of Xn to neighbors
3. Calculate: Xn+1 = F(Xn)
4. If Xn+1
 Xn then go to (2)
5. Stop
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Bellman-Ford Algorithm
Example:
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Calculate from R8

1
R1
2
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
R1

4
R6
3
2
4
2
4
R4
2
R3
3
R7
2
1
R2
2
step
2
R8

1
R6

3


4
R4
R5
4
R3
2nd
1
R2
2

R5
2
R7
2
3
3
R8
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Bellman-Ford Algorithm
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R1
4
R2
1
1
4
4
R3
5
R1
Result:
R4
R7
2
2
3
5
1
R2
R5
4
R6
3
2
4
2
R4
2
R3
R6
R8
4
1
4
3
2
R5
2
step
2
3
2
3rd
6
2
2
R7 3
R8
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Node Failure
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F detects that link to G has failed
F sets distance to G to infinity and sends update to A
A sets distance to G to infinity since it uses F to reach G
A receives periodic update from C with 2-hop path to G
A sets distance to G to 3 and sends update to F
F decides it can reach G in 4 hops via A
B
C
A
D
E
F
G
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Routing Loops
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link from A to E fails
A advertises distance of infinity to E
B and C advertise a distance of 2 to E
B decides it can reach E in 3 hops; advertises this to A
A decides it can read E in 4 hops; advertises this to C
C decides that it can reach E in 5 hops…
B
C
A
D
E
F
G
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The count-to-infinity problem
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Loop-Breaking Heuristics
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Set infinity to a reasonably small number. For
instance, RIP sets to 16
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Split horizon: Don’t announce the distance to
the node the distance has been gotten from.
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Split horizon with poison reverse: Instead of
not announcing the distance put negative
numbers.
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Link State
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Strategy
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send to all nodes (not just neighbors) information
about directly connected links (not entire routing
table)
Link State Packet (LSP)
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id of the node that created the LSP
cost of the link to each directly connected neighbor
sequence number (SEQNO)
time-to-live (TTL) for this packet
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Link State (cont.)
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Reliable flooding
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store most recent LSP from each node
forward LSP to all nodes but one that sent
it
generate new LSP periodically
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increment SEQNO
start SEQNO at 0 when reboot
decrement TTL of each stored LSP
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discard when TTL=0
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Route Calculation
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Dijkstra’s shortest path algorithm
Let
 N denotes set of nodes in the graph
 l (i, j) denotes non-negative cost (weight) for edge (i, j)
 s denotes this node
 M denotes the set of nodes incorporated so far
 C(n) denotes cost of the path from s to node n
M = {s}
for each n in N - {s}
C(n) = l(s, n)
while (N != M)
M = M union {w} such that C(w)
is the minimum for all w in (N - M)
for each n in (N - M)
C(n) = MIN(C(n), C (w) + l(w, n ))
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Shortest Path Routing: Dijkstra Algorithm
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Metrics
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Original ARPANET metric
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measures number of packets enqueued on each link
took neither latency or bandwidth into consideration
New ARPANET metric
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stamp each incoming packet with its arrival time (AT)
record departure time (DT)
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when link-level ACK arrives, compute
Delay = (DT - AT) + Transmit + Latency
if timeout, reset DT to departure time for retransmission
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link cost = average delay over some time period
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Measuring Line Cost
A subnet in which the East and West parts are
connected by two lines.
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How to Make Routing Scale
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Flat versus Hierarchical Addresses
Inefficient use of Hierarchical Address Space
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class C with 2 hosts (2/255 = 0.78% efficient)
class B with 256 hosts (256/65535 = 0.39%
efficient)
Still Too Many Networks
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routing tables do not scale
route propagation protocols do not scale
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Subnetting
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Add another level to address/routing hierarchy:
subnet
Subnet masks define variable partition of host part
Subnets visible only within site
Network number
Host number
Class B address
1111111111111111111
0000000000000000
Subnet mask (255.255.0.0)
Network number
Subnet ID
Host ID
Subnetted address
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Subnet Example
Subnet
Net
host
Subnet mask: 255.255.255.128.
Subnet number: 128.96.34.0
128.96.34.15
128.96.34.1
111….1.0xxx….x
H1
R1
Subnet mask: 255.255.255.128
Subnet number: 128.96.34.128
128.96.34.130
128.96.34.139
128.96.34.129
H2
R2
H3
128.96.33.14
128.96.33.1
Subnet mask: 255.255.255.0
Subnet number: 128.96.33.0
Forwarding table at router R1
Subnet #
128.96.34.0
128.96.34.128
128.96.33.0
Subnet Mask
255.255.255.128
255.255.255.128
255.255.255.0
Next Hop
interface 0
interface 1
R2
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Forwarding Algorithm
D = destination IP address
for each entry (SubnetNum, SubnetMask, NextHop)
D1 = SubnetMask & D
if D1 = SubnetNum
if NextHop is an interface
deliver datagram directly to D
else
deliver datagram to NextHop
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Use a default router if nothing matches
Can put multiple subnets on one physical network
Subnets not visible from the rest of the Internet
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Supernetting
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Assign block of contiguous network numbers to
nearby networks
Called CIDR: Classless Inter-Domain Routing
Represent blocks with a single pair
(first_network_address, count)
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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 Propagation
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Know a smarter router
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Autonomous System (AS)
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hosts know local router
local routers know site routers
site routers know core router
core routers know everything
corresponds to an administrative domain
examples: University, company, backbone network
assign each AS a 16-bit number
Two-level route propagation hierarchy
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interior gateway protocol (each AS selects its own)
exterior gateway protocol (Internet-wide standard)
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Collection of Subnetworks
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Interior Gateway Protocols
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RIP: Route Information Protocol
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developed for XNS
distributed with Unix
distance-vector algorithm
based on hop-count
OSPF: Open Shortest Path First
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recent Internet standard
uses link-state algorithm
supports load balancing
supports authentication
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EGP: Exterior Gateway Protocol
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Overview
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designed for tree-structured Internet
concerned with reachability, not optimal routes
Protocol messages
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neighbor acquisition: one router requests that another be
its peer; peers exchange reachability information
neighbor reachability: one router periodically tests if the
another 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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BGP-4: Border Gateway Protocol
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AS Types
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stub AS: has a single connection to one other AS
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multihomed AS: has connections to more than one AS
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refuses to carry transit traffic
transit AS: has connections to more than one AS
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carries local traffic only
carries both transit and local traffic
Each AS has:
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one or more border routers
one BGP speaker that advertises:
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local networks
other reachable networks (transit AS only)
gives path information
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BGP Example
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Speaker for AS2 advertises reachability to P and Q
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network 128.96, 192.4.153, 192.4.32, and 192.4.3, can be
reached directly from 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
192.4.54
192.4.23
Regional provider A
(AS 2)
Backbone network
(AS 1)
Regional provider B
(AS 3)
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Speaker for backbone advertises
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(AS 7)
networks 128.96, 192.4.153, 192.4.32, and 192.4.3 can be
reached along the path (AS1, AS2).
Speaker can cancel previously advertised paths
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IP Version 6
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Features
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128-bit addresses (classless)
multicast
real-time service
authentication and security
autoconfiguration
end-to-end fragmentation
protocol extensions
Header
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40-byte “base” header
extension headers (fixed order, mostly fixed length)
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fragmentation
source routing
authentication and security
other options
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Tunneling
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Routing for Mobile Hosts
1- finding location of the mobile host
2- hand-off
3- security
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Routing for Mobile Hosts (2)
Packet routing for mobile users.
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Routing in Ad Hoc Networks
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Military vehicles on battlefield.
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A fleet of ships at sea.
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All moving all the time
Emergency works at earthquake.
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No infrastructure.
The infrastructure destroyed.
A gathering of people with notebook
computers.
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In an area lacking 802.11.
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Ad Hoc Networks
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Two nodes are neighbor if they can
communicate directly using their radio.
danger of asymmetry, limited bandwidth, low
battery life
every thing is variable:
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location,
neighbor-ship,
IP address, topology
when you do not have any information about a
destination, you must discover a route for it
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AODV (Ad hoc On-Demand Distance Vector): Route
Discovery
A wants to know about I. It broadcasts a ROUTE REQUEST
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Arrows show possible reverse routes.
the request is broadcast until it reaches a node that knows
about destination.
then it forwarded along reverse path.
limited broadcast or ring search
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Route Maintenance
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D finds destinations that depend on G (i.e. E, G, I)
D finds active neighbors that use G to reach
somewhere ( i.e. A, B)
D tell them to purge their route that pass through
G,.
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