Architecture des ordinateurs

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23 Οκτ 2013 (πριν από 3 χρόνια και 1 μήνα)

47 εμφανίσεις

Réseaux et Télécommunications

Juri

Luca De Coi

Saint
-
Etienne, France, 03
-
10
-
2011

Outline


The target scenario


The concept of “network”


Network types


Problems and requirements


Protocol hierarchies


The OSI Reference Model


The TCP/IP Reference Model

Outline


The target scenario


The concept of “network”


Network types


Problems and requirements


Protocol hierarchies


The OSI Reference Model


The TCP/IP Reference Model

The name of the game

Interconnection


Ability to exchange information

Computer network


A collection of autonomous computers
interconnected
by a single technology

EX:

RMESS

Computer internetwork


A collection of autonomous networks
interconnected?


A collection of networks maintained by different
organizations?



Gateways

make the interconnection possible

EX:

The Internet

The name of a similar game

Distributed system


A software system


built on top of a(n inter)network


hiding the (inter)network to the users



It is a kind of
middleware

EX:

The World Wide Web

Network types (I)

Broadcast networks


One communication channel shared by all
computers on the network



Every computer receives whatever is sent

EX:

LANs (typically)

Network types (II)

Point
-
to
-
point (
unicast
) networks


Many connections between computer pairs


On the way from source to destination,
messages may have to visit intermediate
computers


Each intermediate computer


receives the (whole) incoming message


stores it until the output line is free


forwards the message

(
store and forward
)

Network types (II)

Hybrid networks


A collection of broadcast and point
-
to
-
point
(sub)networks interconnected

EX:

WAN (typically)

Sub
network

Sub
net

Router

Host

Wide area networks (WANs)

Host


User computer

Router


Special
-
purpose computer transmitting
messages across
subnetworks

Subnet


The technical equipment owned by the
network operator


Typically forming a point
-
to
-
point
(sub)network

The target scenario

Gateway

Network

Internetwork

Outline


The target scenario


The concept of “network”


Network types


Problems and requirements


Protocol hierarchies


The OSI Reference Model


The TCP/IP Reference Model

Scenario
-
related issues

How to identify the receiver?


Typically, processes on some host

Routing


What is (currently) the optimal route between
sender and receiver?

Medium access control


What if ≥2 computers want to transmit
simultaneously in a broadcast networks?

Real world
-
related issues (I)

Errors occur


Transmission errors


Errors/Crashes of physical devices


Software errors


How can the sender know if its messages
have been correctly received?

Performance
asimmetry


How to keep a fast sender from swamping a
slow receiver with data (
flow control
)?

Limited resources


Upper bound on message size

Packet
-
switching


The sending (process on some) host cuts the
message into
packets


The packets are


sent one at a time in quick succession


individually routed



Different packets may follow different routes


The receiving (process on some) host
reassembles the packets into the original
message

Real world
-
related issues (II)

How to reassemble the original message if
packets


get lost


are delivered out of order

?

Requirements (I)

Are messages received in the order they were sent?


Ordered delivery (
connection
-
oriented service
)


Out
-
of
-
order delivery (
connectionless
/
datagram
service
)

Are messages lost?


Reliable delivery (
acknowledged service
)


Unreliable delivery


NOTE:

Unreliable delivery may be desirable if


reliable delivery is not available


its overhead is not acceptable

Requirements (II)

Connection
-
oriented

Reliable

Yes

No

Yes


R敬楡扬攠浥獳慧攠
stream

EX:

Sequence of pages


R敬楡扬攠byt攠s瑲敡m

EX:

Remote login

Acknowledged
datagram

EX:

Registered mail

No

Unreliable connection

EX:

Digitized voice

Unreliable datagram

EX:

Electronic junk mail

Outline


The target scenario


The concept of “network”


Network types


Problems and requirements


Protocol hierarchies


The OSI Reference Model


The TCP/IP Reference Model

Loose coupling

In computer networks, it leads to the definition
of
protocol hierarchies

Protocol hierarchies (I)

Virtual

communication

Physical

communication

Protocol hierarchies (II)

Layer
n


offers services to layer
n+1


(typically) interacts with layer
n

of other
computers


Peer entities


Layer
-
dependent entities carrying out the
interaction


Can be


hardware devices


processes


human beings

Interfaces & protocols (I)

Interfaces


provide service specifications



is an intra
-
host business

Protocols


specify how the communication is to proceed



is an inter
-
host business

Interfaces & protocols (II)

Communication between layers is ensured as
long as interfaces are kept constant



Implementation of the same service might
exploit different protocols


Communication between hosts is ensured as
long as peers entities are available for all
layers



Different hosts might have different
interfaces between layers

Network architectures

Network architectures


A set of layers and protocols (≥1 per layer)


Protocol stack


A set of protocols (exactly 1 per layer)

Implementation of protocol
hierarchies

The layers’


number


name


content


function

differ from network to network

The OSI Reference Model

The TCP/IP Reference Model

Messages


Data link layer:
Frames


Network layer:
Packets


Transport layer:
Transport message
s


No better name is commonly used


Messages contain


data (
payload
)


control information


Header

(if put at the beginning)


Trailer

(if put at the end)

The payload of level
-
n

messages is (typically a
fragment of) level
-
(n+1)

messages