Protocol architectures - Netgroupnetgroup.polito.it/courses/Didattica/CN/osi.pdf · 2019-05-07 · COMPUTER NETWORKS – Protocol architectures - 51 Layer 1: physica • Physical

COMPUTER NETWORKS – Protocol architectures - 1

Protocol architectures

Gruppo Reti TLC [email protected]

http://www.telematica.polito.it/


Architectures and protocols •  CCITT definition

– Communication: transfer of information according to pre-established conventions

•  Communication requires cooperation •  One abstract description of the communication

paradigm between two or more users requires the definition of a reference model

•  At the maximum level of abstraction, the reference model specifies a network architecture


Protocol architectures •  A network architecture defines the objects

used to describe –  the communication process –  the relation among such objects –  the functionalities to support the communication –  the structure of the functions

•  Layered architectures are used because of –  simple design –  simple management – simple standardization – separation among functions


router 1

router 2 router 3

host 1

host 2

host 3

host 4

subnet 1

subnet 2

subnet 4 subnet 3 packet transfer

routing

error control

applications

Separation among functions: Internet


Application

Presentation

Session

Transport

Network

Data link

Physical

7

6

5

4

3

2

1

Open System Interconnection OSI


Layered architectures

OSI

Application

Presentation

Session

Transport

Network

Data Link

Physical

DECNET

User

Netw. Appl.

Session

End to End

Routing

Data Link

Physical

ARPA

Application

Service

Internetwork

Network

SNA

Transaction Service

Presentation Service

Data Flow

Trans. Control

Manag. Service

Virtual Route Explicit Route Transm. Group

Data Link

Physical

path control

half session


Internet Protocol Suite

OSI and Internet

Application

Presentation

Session

Transport

Network

Data link

Physical

OSI

NFS

XDR

RPC

Telnet

FTP

SMTP

SNMP

TCP e UDP

IP

Unspecified

ARP e RARP ICMP Routing

protocols


Management plane

Control plane User plane

High layers

AAL

ATM

Physical

Layer managem

ent

Plane managem

ent

High layers

B - ISDN


Protocols •  CCITT definition

–  formal definition of the procedures adopted to guarantee the communication between two or more objects on the same hierarchical level

•  Protocol definition: – semantics

•  set of commands and answers – syntax

•  structure of commands and answers –  timing

•  temporal sequence of commands and answers


Protocols •  Protocols are set of

– semantic rules • algorithms

– syntactic rules • formats

– timing


ISO/OSI model •  (Open System Interconnection) defined in

the following standards –  ISO IS 7498 – CCITT X.200

•  The fundamental principles defined in the OSI model are universally accepted –  this does not mean that all the protocol

architectures conform to OSI model


System j

Transmission media

System1 System 2

System n

System 3

OSI model


Application process

System A System B System C System D

Transmission media

OSI model


System A

System B

Higher layer

subsystem (N+1) - layer (N) - layer (N-1) - layer

Lower layer transmission media

Layers (or levels)


(N) - layer

System A

System B

(N) - entity

transmission media

Entities •  active elements in a subsystem •  run the functions of the layer •  interact within the same layer


Layering •  Each layer (or level)

– provides services to the higher layer – using

•  the services from the lower layer •  its own functionalities

•  Can be identified: – service provider – service user – SAP (Service Access Point)


(N+1) - layer

(N) - layer (N) - service

uses

provides

Services •  The users at layer N and the (N+1) - entities

cooperate and communicate using the (N) – service offered by the (N) – service provider


Services

•  A service can be: – connection-oriented (CO): a preliminary agreement

(connection) is established between the network and the communication end-points, then the data is transferred and finally the connection is released

– connectionless (CL): data is sent to the network without any preliminary agreement and is treated independently from each other


(N) - service

N+1 N

N+1 N (N) – service

provider

Black-box for the (N+1) - entity

Services


N

N-1

N

N-1

(N-1) - service

(N-1) – service provider

Black-box for the (N) - entity

Services


(N) - entity

(N-1) - entity

(N-1) - SAP

(N) - layer

(N-1) - layer

(N-1) - entity

SAP (Service Access Point)

•  Each (N-1)-SAP is associated with at most one (N)-entity


System A System B

(N) - servizio

(N+1) - protocol

(N) - protocol (N) - layer

(N+1) - entity

(N) - entity

(N) - SAP

(N+1) - entity

(N) - entity

Protocols


(N) - entity

(N-1) - entity

(N) - layer

(N-1) - layer

(N-1) - SAP

(N) - title

(N-1) - address

Addressing


(N) - entity (N-1) - address

(N) - title

Functions for identification

•  Address translation –  (N) directory


(N) - entity

(N) - SAP

(N-1) - SAP

(N) - address

(N-1) - address

Functions for identification

•  Address translation –  (N) - mapping


(N) - layer K K L M E D D C

K L M

E D C B A

F G H J

one-to-one hierarchical tabular

Possible mappings


(N) - entity C

(N) - entity B

(N) - entity A

(N) - layer

(N-1) - layer

(N-1) - connections

(N-1) - SAP

Connections


(N) - entity C

(N) - entity B

(N) - entity A

(N) - layer

(N-1) - layer

(N-1) - SAP

connections one-to-one

connection one-to-many

Connections


(N) - entity A

(N) - entity B

(N) - entity C

(N) - layer

(N-1) - layer

(N-1) - CEP

(N-1) - SAP

Connections


(N) - entity A

(N) - entity B

(N) - entity C

(N) - layer

(N-1) - layer

(N-1) - CEP.id

(N-1) - SAP (N-1) - CEP

Connections

CEP= Connection End Point id=identifier


Agreement among three possible elements

(N+1) - entity (N+1) - entity

(N) – service provider

The three elements

Agreement


The two elements

(N + 1) - entità

(N) - fornitore di servizio



The two elements

The two elements

Agreement

•  In the case of information transfer without connection, it is sufficient an agreement among two elements


Agreement

•  In the case of information transfer with connection, it is necessary an agreement among the three elements

The three elements

(N) - fornitore di servizio




(N+1) - layer

(N) - layer

(N) - SAP (N) - CEP

Connections •  multiplexing of (N) – connections into

one (N-1) - connection


(N) - layer

(N) - SAP

(N) - CEP

Connections •  subdivision of one (N) – connection

in many (N-1) - connections


(N) - layer interface (N-1) - layer

(N-1) - SDU

(N-1) - PCI (N-1) - SDU

(N-1) - PDU

SAP

(N) - PDU

PDU creation


PDU creation •  On data units, there exists the possibility of

– segmentation – concatenation

•  Segmentation can occur either by building many (N) - PDU from one (N) – SDU or by building many (N-1) - SDU from one (N) - PDU

•  Similarly for the concatenation


System A System B System C System D

transmission media

information path

Information transfer


Application Presentation

Session Transport Network Data link Physical

Receiver Transmitter



data APCI ASDU

PPCI PSDU SPCI SSDU

TPCI TSDU NPCI NSDU

DLPCI DLSDU bit or symbols

Information transfer


user (N) - service

user (N) - service

(N) - entity (N) - entity

(N) - SAP (N) - SAP

(N) - protocol

REQ

UES

T

CO

NFI

RM

IND

ICAT

ION

AN

SWER

(N+1) - layer (N) - layer


Primitives


Primitives •  set of interactions on an interface, occurring

in different times and offering a service •  Example: service of mail transfer in the

postal system – Deposit of the letter in the mailbox by the sender – Delivery of the letter into receiver’s mailbox by

the postman •  Similar to a procedure


Service provider

Service user

Service user

Request primitive

Confirm primitive

Indication primitive

Answer primitive

Acknowledged service

Use of primitives


Service provider

Service user

Request primitive Indication

primitive

Use of primitives Un-acknowledged service

Service user


Service provider

Service started from the provider

Use of primitives

Service user

Service user




To open a connection



(N) - CONNECT

(N) - CONNECT

(N) - CONNECT REQUEST

CONFIRM

INDICATION

RESPONSE

(N) - SAP (N) - SAP

(N) - CONNECT

Use of primitives


To transfer data



(N) - DATA (N) - DATA REQUEST

INDICATION

(N) - SAP (N) - SAP

Use of primitives


To close a connection started from the user



(N) - DISCONNECT

(N) - DISCONNECT REQUEST

INDICATION

(N) - SAP (N) - SAP

Use of primitives


application

presentation

session

transport

network

data link

physical

transmission media

Application protocol

Presentation protocol

Session protocol

Transport protocol

Network protocol

Data link protocol

Physical layer protocol

application

presentation

session

transport

network

data link

physical

The seven OSI layers






•  terminal system •  relay system

TERMINAL S. A RELAY SYSTEM TERMINAL S. B

Network

transmission media

Systems

Data link Physical


transfer layers

transfer layers

user layers

terminal equipement

access node

transit node

access network internal network

user protocols

transfer layers

access protocols

network protocols

Public networks


Layer 1: physical •  Physical layer:

– provides the mechanical, physical, functional and procedural means, to activate, maintain and disable the physical connections

– allows to transfer binary digits exchanged among the data link entities

– data units are bits or symbols – defines transmission codes, connectors, voltage

levels, etc.


Layer 2: data link •  Data link layer

– provides the functional and procedural means to transfer data units among network entities

– handle malfunctions and failures at physical level – main functions:

•  error detection and error correction for the transmission

•  flow control •  data unit delimitation


Layer 3: network •  Network layer

– provides the means to setup, maintain and close the connections among the entities at transport level

– provides the functional and procedural means to exchange the information among entities at transport level

– main functions •  routing •  flow control and congestion control •  pricing


Layer 4: transport •  Transport layer

– provides the connections at transport level to the entities at session level

– compensates the possible lack of quality of service in the connections at network level

– optimizes the use of the network layer – main functions

•  error control •  control of sequence •  flow control


Layer 4: transport –  lower layer with end-to-end meaning – provides multiplexing and subdivision of the

connections – allows the fragmentation of messages in packets

and their reassembly


Layer 5: session •  Session layer

– provides one session connection to the entities at presentation layer

– organizes the communication among entities at presentation level

– provides the structure and synchronize the data exchange to allow suspending, recovering and terminating

– masks the interruptions at service level


Layer 6: presentation •  Presentation layer

– solves the compatibility issues regarding the data formats

– solves the issues of data syntax translation – may provide services of data encryption


Layer 7: application •  Application layer

– provides the application processes with the means to access the OSI environment

•  Examples of service –  file transfer - FTAM – virtual terminal - VT – e-mail - X.400


U 1 N 1 U 2

Example •  Trivial network


Example •  Assume that one (4) – entity must

communicate with one remote (4) – entity •  We will follow, step-by-step:

– primitives – SDUs – PDUs


•  layer 3 service with connection •  layer 2 service connectionless •  layer 1 service connectionless

U 1 N 1 U 2

Example

1 2 3

1 2 3

1 2 3 4 4


A B C

H I J

X X’

D E

F G

Example

Z


A B C

H I J

X X’

D E

F G

A, H X, X’ Example

Z

Directory


A B C

H I J

X X’

D E

F G

N-CONNECT.request(H, A, ...)

Z

Example


A B C

H I J

X X’

D E

F G

N-PDU(H, A, VCid’, call request, ...)

Example

Z


A B C

H I J

X X’

D E

F G

A, Z A, H Example

Z

Routing


A B C

H I J

X X’

D E

F G

B, D A, Z Example

Z

Mapping


A B C

H I J

X X’

D E

F G

DL-DATA.request(D, B, DL-SDU, ...)

Example

Z


A B C

H I J

X X’

D E

F G

DL-PDU(D, B, DL-SDU, DL-PCI)

Example

Z


A B C

H I J

X X’

D E

F G

C, E B, D Example

Z

Mapping


A B C

H I J

X X’

D E

F G

PH-DATA.request(E, C, symbol)

Example

Z


A B C

H I J

X X’

D E

F G

01100111

Example

Z


A B C

H I J

X X’

D E

F G

PH-DATA.indication(E, C, symbol)

Example

Z


A B C

H I J

X X’

D E

F G

DL-PDU(D, B, DL-SDU, DL-PCI)

Example

Z


A B C

H I J

X X’

D E

F G

DL-DATA.indication(D, B, DL-SDU, ...)

Example

Z


N-PDU(H, A, VCid’, call request, ...)

A B C

H I J

X X’

D E

F G

Example

Z


A B C

H I J

X X’

D E

F G

Z, H A, H Example

Z

Routing


A B C

H I J

X X’

D E

F G

F, I Z, H Example

Z

Mapping


A B C

H I J

X X’

D E

F G

N-PDU(H, A, VCid’’, incoming call, ...)

Example

Z


A B C

H I J

X X’

D E

F G

DL-DATA.request(I, F, DL-SDU, ...)

Example

Z


A B C

H I J

X X’

D E

F G

DL-PDU(I, F, DL-SDU, DL-PCI)

Example

Z


A B C

H I J

X X’

D E

F G

G, J F, I Example

Z

Mapping


A B C

H I J

X X’

D E

F G

PH-DATA.request(J, G, symbol)

Example

Z


A B C

H I J

X X’

D E

F G

01100111

Example

Z


A B C

H I J

X X’

D E

F G

PH-DATA.indication(J, G, symbol)

Example

Z


A B C

H I J

X X’

D E

F G

DL-PDU(I, F, DL-SDU, DL-PCI)

Example

Z


A B C

H I J

X X’

D E

F G

DL-DATA.indication(I, F, DL-SDU, ...)

Example

Z


A B C

H I J

X X’

D E

F G

N-PDU(H, A, VCid’’, incoming call, ...)

Example

Z


A B C

H I J

X X’

D E

F G

N-CONNECT.indication(H, A, CEP.id’, ...)

Example

Z


A B C

H I J

X X’

D E

F G

N-CONNECT.response(CEP.id’, ...)

Example

Z


A B C

H I J

X X’

D E

F G

N-PDU(A, H, VCid’’, call accepted, ...)

Example

Z


A B C

H I J

X X’

D E

F G

N-PDU(A, H, VCid’’, call accepted, ...)

Example

Z


A B C

H I J

X X’

D E

F G

N-PDU(A, H, VCid’, call connected, ...)

Example

Z


A B C

H I J

X X’

D E

F G

N-PDU(A, H, VCid’, call connected, ...)

Example

Z


A B C

H I J

X X’

D E

F G

N-CONNECT.confirm(A, H, CEP.id’’, ...)

Example

Z


A B C

H I J

X X’

D E

F G

T-PDU(T-PCI, T-SDU)

Example

Z


A B C

H I J

X X’

D E

F G

N-DATA.request(CEP.id’’, N-SDU, ...)

Example

Z


A B C

H I J

X X’

D E

F G

N-DATA.indication(CEP.id’, N-SDU, ...)

Example

Z


A B C

H I J

X X’

D E

F G

T-PDU(T-PCI, T-SDU)

Example

Z


A B C

H I J

X X’

D E

F G

N-DISCONNECT.request(CEP.id’)

Example

Z


A B C

H I J

X X’

D E

F G

N-DISCONNECT.indication(CEP.id’’)

Example

Z

Documents

Protocol architectures - Netgroupnetgroup.polito.it/courses/Didattica/CN/osi.pdf · 2019-05-07 · COMPUTER NETWORKS – Protocol architectures - 51 Layer 1: physica • Physical