bill@napier, 2002 bill/nos.html Networking Operating Systems (CO32010) 1. Operating Systems 2....

bill@napier, 2002http://www.soc.napier.ac.uk/~bill/nos.html

Networking Operating Systems (CO32010)

1. OperatingSystems

2. Processesand scheduling

3. Distributedprocessing

4. Distributedfile systems

5. Routingprotocols6. Routers

7. Encryption

8. NT, UNIX and NetWare

5.1 Introduction5.2 Routing fundamentals5.3 Routing protocol techniques5.4 RIP5.5 OSPF5.6 IGRP5.7 EGP/BGP

Objectives:• To outline the fundamental techniques using in routing

protocols.• To define the main problem in routing protocol

techniques, such as routing loops, and count-to-infinity, and how the may be overcome.

• To outline practical protocols, especially RIP and IGRP, and reflect on their strengths and weaknesses.

Objectives:• To outline the fundamental techniques using in routing

protocols.• To define the main problem in routing protocol

techniques, such as routing loops, and count-to-infinity, and how the may be overcome.

• To outline practical protocols, especially RIP and IGRP, and reflect on their strengths and weaknesses.


5.1 Alternative Routes

BB

1

Net1

Net2

Net3

Net4

Net5Net6

Net7

Net8

4

3

6 BBAA 1

2

5

AA 11

22

33

44 66

55 66

BB

BB

55 66

22 44 66 BB


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 = 2.75Route(2,5,6) = 1.1+1.3 = 2.4 [BEST]

Routing based on error probability:

Pe(2 – 5)=0.01 Pe(5 – 6)=0.15Pe(2 – 4)=0.05 Pe(4 – 6)=0.1

Pnoerror(2,5,6) =(1 – 0.01) (1 – 0.15) = 0.8415 Pnoerror(2,4,6) =(1 – 0.05) (1 – 0.1) = 0.855 [BEST]


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.


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.


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.


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.


5.7 Routing protocol types

Bandwidth

Hop count

+

+

+

Event driven v. broadcast

Static .v. dynamic

+

Link-state Distance-vector

Each routertransmits routinginformation toall other routersonlywhen there

are changes(OSPF/BGP/EGP)

Problems:•Initial flooding •Processing/memory

Each router periodically sendsinformation toeach of its neighbors(RIP).

Problems: •Bandwidth•Step-by-step updates

Hybrid (IS-IS)

+

+Routed(IP, IPX,NetBEUI)

+Routing(RIP, OSPF)

+

+

+

Delay

Reliability

Tick

Cost

SessionSession

TransportTransport

NetworkNetwork

Data linkData link

PhysicalPhysical

HTTPHTTP

TCPTCP

IP RIPIP RIP

Ethernet/FDDI

Ethernet/FDDI

RoutingRouting

Layer 3 protocols

Layer 3 protocols TypesTypes

UpdatesUpdates

Distancemetrics

Distancemetrics

Bandwidth

Hop count

+

+

+

Event driven v. broadcast

Static .v. dynamic

+

Link-stateLink-state Distance-vector Distance-vector

Each routertransmits routinginformation toall other routersonlywhen there

are changes(OSPF/BGP/EGP)

Problems:•Initial flooding •Processing/memory

Each router periodically sendsinformation toeach of its neighbors(RIP).

Problems: •Bandwidth•Step-by-step updates

Hybrid (IS-IS)

+

+Routed(IP, IPX,NetBEUI)

+Routing(RIP, OSPF)

+

+

+

Delay

Reliability

Tick

Cost

SessionSession

TransportTransport

NetworkNetwork

Data linkData link

PhysicalPhysical

HTTPHTTP

TCPTCP

IP RIPIP RIP

Ethernet/FDDI

Ethernet/FDDI

SessionSession

TransportTransport

NetworkNetwork

Data linkData link

PhysicalPhysical

HTTPHTTP

TCPTCP

IP RIPIP RIP

Ethernet/FDDI

Ethernet/FDDI

RoutingRouting

Layer 3 protocols

Layer 3 protocols TypesTypes

UpdatesUpdates

Distancemetrics

Distancemetrics


5.8 Example routing

W X

Z Y

1 3

2

4

Network A

Network BNetwork C

Dest Hops

A 1B 2C 1

Dest Hops

A 0B 1C 2

Dest Hops

A 2B 1C 0

Dest Hops

A 1B 0C 1

Next

xzz

Next

Network Ayy

Next

xNetwork Bz

Next

wyNetwork C

W X

Z Y

1 3

2

4

Network A

Network BNetwork C

Dest Hops

A 1B 2C 1

Dest Hops

A 0B 1C 2

Dest Hops

A 2B 1C 0

Dest Hops

A 1B 0C 1

Next

xzz

Next

Network Ayy

Next

xNetwork Bz

Next

wyNetwork C


5.9 Routing loops

W X

Z Y

1 3

2

4

A. Network Aunreachable

A. Network Aunreachable Network

unreachable

Network A

V



B. I can reachNetwork A in

3 hops


3 hops

Router Z thinks it can reach Network A in 4 hops, as Router W says it canreach it in 3 hops, this overrules the information from

Router Y which says it cannotreach Network A

C. Network AReachable via

Router W


Router W

D. Network Areachable


E. Network Areachable


AA

BB

CC

DD

EE

Timing ofevents

W X

Z Y

1 3

2

4


A. Network Aunreachable Network

unreachable

Network A

V




3 hops


3 hops

Router Z thinks it can reach Network A in 4 hops, as Router W says it canreach it in 3 hops, this overrules the information from

Router Y which says it cannotreach Network A


Router W


Router W





AA

BB

CC

DD

EE

Timing ofevents


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.


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.


5.12 Link-state overview

W X

Z Y

1 3

2

4

LSP:NetworkUnreachable


LSP:NetworkReachable




Networkunreachablearrives afternetwork reachable

Network 1 becomes unreachable for a short time

OSPF (RFC1583)OSPF (RFC1583)

Ver.Ver. TypeType Message Len.Message Len.

Router IDRouter ID

Area IDArea ID

ChecksumChecksum Auth. TypeAuth. Type

AuthenticationAuthentication

+ MemoryIncreased amount of

storage memoryfor tree

ProcessingIncreased processingpower required tobuild trees

+LSP

(Link statepackets)

+Topologicaldatabase(for SPF) A change in

topology causes updates to allother routers

Each routerbuilds up a treetopology of the subnetworksand find shortest path

LSPLink-stateLink-state

MethodsMethodsProblemProblem

OperationOperation

ConcernsConcerns

W X

Z Y

1 3

2

4









W X

Z Y

1 3

2

4











Router IDRouter ID

Area IDArea ID





Router IDRouter ID

Area IDArea ID



+ MemoryIncreased amount of

storage memoryfor tree

ProcessingIncreased processingpower required tobuild trees

+LSP

(Link statepackets)

+Topologicaldatabase(for SPF) A change in

topology causes updates to allother routers


LSPA change intopology causes updates to allother routers


LSPLink-stateLink-state

MethodsMethodsProblemProblem

OperationOperation

ConcernsConcerns


5.13 OSPF overview

OSPFisan IGP(Interior

Gateway Protocol)which distributes

routing information betweenrouters in a single autonomous system. All routers have the same database.

Gateways

Separatedomains



Router ID (unique in AS)Router ID (unique in AS)

Area ID (similar to subnetting)Area ID (similar to subnetting)



Hello [1]. Used to establish and maintain a connection. Routers agree HelloIntervalandRouterDeadInterval.•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 a routerassumes that a routeis down. It should be a multiple

of HelloInterval (such as four times).

Database Description [2]. Used to send databasebetween routers.

Link-state Request [3]. Request parts of a neighbor’sdatabase, which may be more up-to-date.

Link-state Update [4]. Used to flood link state advertisements.

Link-state Acknowledgement[5]. Used to acknowledgeflooded advertisements.

+

+

+

+

+

AdditionalInformation(depends onpacket type)

32 bits

OS

PF

hea

der

Autonomous System

Autonomous System

Autonomous System

Autonomous System

Autonomous System

Autonomous System

EGP used between AS’sInternet

OSPFisan IGP(Interior

Gateway Protocol)which distributes

routing information betweenrouters in a single autonomous system. All routers have the same database.

Gateways

Separatedomains













Hello [1]. Used to establish and maintain a connection. Routers agree HelloIntervalandRouterDeadInterval.•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 a routerassumes that a routeis down. It should be a multiple

of HelloInterval (such as four times).

Database Description [2]. Used to send databasebetween routers.

Link-state Request [3]. Request parts of a neighbor’sdatabase, which may be more up-to-date.

Link-state Update [4]. Used to flood link state advertisements.

Link-state Acknowledgement[5]. Used to acknowledgeflooded advertisements.

+

+

+

+

+

AdditionalInformation(depends onpacket type)

32 bits

OS

PF

hea

der

Autonomous System

Autonomous System

Autonomous System

Autonomous System

Autonomous System

Autonomous System

EGP used between AS’sInternet


5.14 Tree-like topology v. Internet-like topology

Org1

Site1 Site2 Site3

LAN1 LAN2 LAN3

Org2

Site1 Site2 Site3

LAN1 LAN2 LAN3

Single backbone

Org1

Site1 Site2 Site3

LAN1 LAN2 LAN3

Org2

Site1 Site2 Site3

LAN1 LAN2 LAN3

Org 3


5.15 Autonomously attached networks

Autonomously attached network (AAN)


AANAAN

AANAAN

AANAAN

Gateway(G/W)

G/W

G/W

G/W

G/W

G/W

G/W



AANAAN

AANAAN

AANAAN

Gateway(G/W)

G/W

G/W

G/W

G/W

G/W

G/W

bill@napier, 2002 bill/nos.html Networking Operating Systems (CO32010) 1. Operating Systems 2....

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