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Networking Operating Systems (CO32010)
Objectives:
2. Processes
• To outline the fundamental techniques
using in routing
and
protocols.
scheduling
• To define the main problem in routing protocol
techniques, such as routing loops, and count-to-infinity,
1. Operating and how the may be overcome.
Systems • To outline practical protocols, especially RIP and IGRP,
and reflect on their strengths and weaknesses.
3. Distributed
processing
7. Encryption
5.1
5.2
5.3
5.4
5.5
5.6
5.7
Introduction
Routing fundamentals8. NT, UNIX
Routing protocol techniques
and NetWare
RIP
OSPF
IGRP
EGP/BGP
6. Routers
5. Routing
protocols
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4. Distributed
file systems
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5.1 Alternative Routes
2
Net5
4
Net6
Net1
A
Net4
11
6
Net8
Net2
3
2
A
1
3
Net3
B
Net7
5
4
6
B
5
6
B
5
6
B
2
4
6
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B
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5.2 Best route?
Routing based on hops:
Route (1,3,5,6) = 4 hops [BEST]
Route (1,3,5,2,4,6) = 6 hops
Routing based on delay (latency):
Route(2,4,6) = 1.5+1.25 = 2.75
Route(2,5,6) = 1.1+1.3 = 2.4 [BEST]
Routing based on error probability:
Pe(2 – 5)=0.01
Pe(2 – 4)=0.05
Pnoerror(2,5,6)
Pnoerror(2,4,6)
Pe(5 – 6)=0.15
Pe(4 – 6)=0.1
=(1 – 0.01)  (1 – 0.15) = 0.8415
=(1 – 0.05)  (1 – 0.1) = 0.855 [BEST]
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5.3 Layer 3 protocols
Routing protocols. A routing protocol provides a mechanism for
routers to share routing information. These protocols allow routers
to pass information between themselves, and update their routing
tables. Examples of routing protocols are Routing Information
Protocol (RIP), Interior Gateway Routing Protocol (IGRP),
Enhanced Interior Gateway Routing Protocol (EIGRP), and Open
Shortest Path First (OSPF).
Routed protocols. These protocols are any network layer
protocol that allows for the addressing of a host and a destination
on a network, such as IP and IPX. Routers are responsible for
passing a data packet onto the next router in, if possible, an
optimal way, based on the destination network address. The
definition of an optimal way depends on many things, especially its
reachability. With IP, routers on the path between a source and a
destination, examine the network part of the IP address to achieve
their routing. Only the last router, which is connected to the
destination node network, examines the host part of the IP
address.
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5.4 Types of Routing
Dynamic routing. In dynamic routing, the routers monitor the
network, and can change their routing tables based on the current
network conditions. The network thus adapts to changing
conditions. Unfortunately, this method tends to reveal everything
known about an internetwork to the rest of the network. This may
be inappropriate for security reasons.
Static routing. In static routing, a system administrator sets up a
manual route when there is only one route to get to a network (a
stub network). This type of configuring reduces the overhead of
dynamic routing. Static routing also allows the internetwork
administrator to specify the information that is advertised about
restricted parts of a network.
Default routing. These are manually defined by the system
administrator and define the path that is taken if there is not a
known route for the destination.
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5.5 Best Route Parameters?
Bandwidth. The data capacity of a link, which is typically
defined in bps.
Delay. The amount of time that is required to send a
packet from the source to a destination.
Load. A measure of the amount of activity on a route.
Reliability. Relates to the error rate of the link.
Hop count. Defined by the number of routers that it takes
between the current router and the destination.
Ticks. Defines the delay of a link by a number of ticks of a
clock.
Cost. An arbitrary value which defines the cost of a link,
such as financial expense, bandwidth, and so on.
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5.6 Type of Update?
Broadcast. In broadcast, routers transmit their information to
other routers at regular intervals. A typical broadcast routing
protocol is RIP, in which routers send their complete routing table
once every few minutes, to all of their neighbors. This technique
tends to be wasteful in bandwidth, as changes in the route do not
vary much over short amounts of time.
Event-driven. In event-driven routing protocols, routing
information is only sent when there is a change in the topology or
state of the network. This technique tends to be more efficient than
broadcast, as it does not use up as much bandwidth.
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5.7 Routing protocol types
Hybrid (IS-IS)
+
Layer
Layer33
protocols
protocols
Routed
(IP, IPX,
NetBEUI)
Types
Types
+
+
Link-state
Distance-vector
+
+
Routing
(RIP, OSPF)
Session
Session
Transport
Transport
Network
Network
Data
Datalink
link
Physical
Physical
Updates
Updates
Routing
Routing
HTTP
HTTP
TCP
TCP
IP
IPRIP
RIP
Ethernet/
Ethernet/
FDDI
FDDI
Each router
transmits routing
information to
all other routers
only when there
are changes
(OSPF/BGP/EGP)
Distance
Distance
metrics
metrics
Each router
periodically sends
information to
each of itsneighbors
(RIP).
Problems:
• Bandwidth
• Step-by-step updates
Problems:
• Initial flooding
• Processing/memory
Hop count
+
Delay
Tick
+
Bandwidth
+
Event driven v. broadcast
Cost
+
Reliability
+
Static .v. dynamic
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5.8 Example routing
Dest Hops
A
B
C
1
2
1
Next
Dest
x
z
z
A
B
C
W
Dest
A
B
C
Hops Next
2
1
0
w
y
Network C
0
1
2
Z
Network C
Network A
y
y
Network A
X
2
1
Hops Next
3
Dest
4
Y
A
B
C
Hops Next
1
0
1
x
Network B
z
Network B
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5.9 Routing loops
Timing of
events
AA
E.E.Network
NetworkAA
reachable
reachable
BB
CC
DD
EE
B.B.I Ican
canreach
reach
Network
NetworkAAinin
33hops
hops
W
X
2
1
Z
A.A.Network
NetworkAA
unreachable
unreachable
3
4
D.D.Network
NetworkAA
reachable
reachable
Y
A.A.Network
Router Z thinks it can
NetworkAA
unreachable
unreachable
reach Network A in 4 hops,
as Router W says it can
reach it in 3 hops, this overrules
the information from
C.C.Network
Router Y which says it cannot
NetworkAA
Reachable
Reachablevia
via
reach Network A
Router W
Network
unreachable
V
Network A
Router W
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5.10 Overcoming Distance Vector Problems
Setting infinity values. The count-to-infinity will eventually resolve
itself when the routers have counted to infinity (as infinity will be
constrained with the maximum definable value), but while the network
is counting to this value, the routing information will be incorrect. To
reduce the time that it takes to get to this maximum, a maximum value
is normally defined. In RIP this value is set at 16 hops for hop-count
distance-vectors, thus the maximum number of hops that can occur is
15. This leads to a problem in that a destination which has a distance of
more than 15 hops is unreachable, as a value of 16 or more defines
that the network is unreachable.
Split horizon. This method tries to overcome routing loops. With this
routers do not update their routing table with information on a
destination if they know that the network is already connected to the
router (that is, the router knows more about the state of the network
than any other router, as it connects to it). Thus in Figure X, Router Z
and Router X will not send routing information on Network B to Router
Y, as they know that Network B is connected to Router Y.
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5.11 Overcoming Distance Vector Problems
Hold-Down Timers. This method overcomes the count-to-infinity problem.
With a hold-time time, a router starts a hold-time timer when it receives an
update from a neighbor indicating that a previously accessible network is
now inaccessible. It also marks the route as inaccessible. There are then
three possible situations:
o
If, at any time before the hold-down timer expires, an update is
sent from the same neighbor which alerted the initial problem saying
that it is now accessible, the router marks the network as accessible
and removes the hold-down timer.
o
If an update arrives from a different neighboring router with a
better metric than the original metric, the router marks the network as
accessible and removes the hold-down timer.
o
If, at any time before the hold-down timer expires, an update is
sent from a different neighbor which alerted the initial problem saying
that it is accessible, but has a poorer metric than the previously
recorded metric, the update is ignored. Obviously after the timer has
expired the network will still be prone to looping routes, but the timer
allows for a longer time for the network to settle down and recover the
correct information.
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5.12 Link-state overview
Network 1 becomes unreachable for a short time
LSP:Network
LSP:Network
Reachable
Reachable
Methods
Methods
Problem
Problem
W
LSP:Network
LSP:Network
Unreachable
Unreachable
1
LSP
(Link state
packets)
+
X
2
3
4
Z
Y
Network
unreachable
arrives after
network
reachable
LSP:Network
LSP:Network
Unreachable
Unreachable
+
Topological
database
(for SPF)
Link-state
Link-state
Operation
Operation
OSPF
OSPF(RFC1583)
(RFC1583)
Ver
.
Ver . Type
Type Message
MessageLen.
Len.
Router
ID
Router ID
Area
AreaID
ID
Checksum
Auth.
Checksum
Auth.Type
Type
LSP
A change in
topology causes
updates to all
other routers
Concerns
Concerns
Authentication
Authentication
+
Processing
Increased processing
power required to
build trees
Memory
Increased
amount of
storage memory
for tree
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Each router
builds up a tree
topology of the
subnetworks and find
shortest path
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OSPF header
5.13 OSPF overview
OSPF
OSPF(RFC1583)
(RFC1583)
Ver.
Type
Ver.
Type Message
MessageLen.
Len.
Router
ID
(unique
in
AS)
Router ID (unique in AS)
Area
AreaID
ID(similar
(similarto
tosubnetting)
subnetting)
Checksum
Auth.
Checksum
Auth.Type
Type
+
Hello [1]. Used to establish and maintain aconnection.
Routers agree HelloIntervaland RouterDeadInterval.
• HelloInterval. Number of seconds between Hello
packets. The smaller the value, the fastest the detection
of topological changes. X.25 uses 30 sec, LANs uses
10 sec.
• RouterDeadInterval. Number of seconds before arouter
assumes that a route is down. It should be a multiple
of HelloInterval (such as four times).
+
Database Description[2]. Used to send database
between routers.
+
Link-state Request [3]. Request parts of aneighbor’s
database, which may be more up-to-date.
+
Link-state Update [4]. Used to flood link state advertisements.
+
Link-state Acknowledgement [5]. Used to acknowledge
flooded advertisements.
Authentication
Authentication
Additional
Information
(depends on
packet type)
32 bits
Gateways
OSPF is
an IGP (Interior
Gateway Protocol)
which distributes
routing information between
routers in a single autonomous
system. All routers have the
same database.
Separate
domains
Autonomous
Autonomous
System
System
Autonomous
Autonomous
System
System
Autonomous
Autonomous
System
System
Internet
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EGP used between AS’s
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5.14 Tree-like topology v. Internet-like topology
Single backbone
Org1
Site1
Site2
Org2
Site3
Site1
Site2
Site3
Org 3
LAN1
LAN2
LAN3
LAN1
LAN2
Org1
LAN3
Org2
Site1
Site2
Site3
Site1
LAN1
LAN2
Site3
LAN3
LAN1
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Site2
LAN2
LAN3
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5.15 Autonomously attached networks
Autonomously
Autonomously
Gateway
attached
attached
(G/W)
network
network
(AAN)
(AAN)
G/W
G/W
AAN
AAN
G/W
G/W
AAN
AAN
G/W
G/W
AAN
AAN
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