telematica sanitaria e telemedicina

introduzione – il modello ISO/OSI – ilmodello TCP/IP – il modello a tre strati

di Cisco – CCNA (elementi)

Architettura a strati

• La realizzazione di servizi di rete richiede l’interazione tra processi applicativi su due hostp.e.

• Ai processi applicativi non interessa la struttura della rete né i dettagli del trasferimento dei dati.

• Se due calcolatori in rete (host) fossero uniti da un canale ideale, le loro preoccupazioni sarebbero:– • stabilire la connessione– • sincronizzare il dialogo (si parla insieme, uno alla

volta, etc.)– • effettuare conversioni (ASCII, EBCDIC)– • svolgere le azioni necessarie al servizio richiesto

(p.e., trasferimento file)

Architettura a strati

• Come rendere “ideale” il canale di comunicazione che unisce gli host su cui sono ospitate le applicazioni che devono interagire ?

• Occorre:– (1) Annullare l’ effetto degli errori di linea su ogni

tratta host-nodo e nodo-nodo– (2) Provvedere all’indirizzamento e all’instradamento

dei pacchetti nella rete– (3) evitare la congestione e sincronizzare la

produzione ed il consumo di pacchetti su ogni singola tratta e da host a host (controllo di flusso)

– (4) garantire il recupero dei pacchetti perduti, l’eliminazione dei duplicati, il riordinamento (a seconda del servizio offerto)

Architettura a strati

• Alcuni aspetti principali caratterizzano i compiti precedenti:– (a) si tratta di una serie di problemi risolvibili in modo

indipendente, anche se ognuno ha bisogno dei risultati di una serie di problemi precedenti

– (b) alcune funzioni base, quali controllo di errore, controllo di flusso, controllo di sequenza, etc., ricorrono in forma concettualmente simile in diversi livelli gerarchici

– (c) si tratta in ogni caso di problemi di comunicazione e pertanto per risolverli è necessaria una qualche forma di interazione tra i due sistemi comunicanti (le regole che stabiliscono le modalità di tale interazione prendono il nome di protocolli).

Architettura di rete

• Una architettura di rete è dunque una particolare strutturazione in livelli funzionali(o layer) con i relativi protocolli

Protocolli di comunicazione

• a protocol is an “official procedure or system of rules” for a given activity

• a computer communication protocol, therefore, is a set of rules that govern the exchange of information within a network.

• communication protocols define the manner in which peer processes communicate among various computer hardware devices.

• the protocols set the rules for functions such as passing and extraction of messages and handling error conditions.

Protocol layering

• We can organize the modules of protocol sw on each machine into “layers”. Each layer will, then, handle a specific communication sub-problem.

• Many types of problems arise when data in the form of “packets” gets communicated within a network. Some of them are:– host failure– link failure– network congestion– data congestion– packet delay or loss– data corruption– data duplication or packet out of sequence

Protocol layering

• as we can see, when there is such a variety of potential problems in data communication, it is impractical to set a single protocol to handle all of them.

• Therefore, it makes sense to organizethe protocol sw into different layers, so that each layer can handle a particular set of problems

layered protocol characteristics

• each layer has a predefined set of functions

• layers provide services to their immediate upper layers, hiding the details of the service

• peer layers communicate using a peer protocol

• layers are separated from each other with interfaces

layer N layer N

layer N

peer protocol

Advantages of layering

• data hiding and encapsulation (data structures, algorithms, etc... in a layer are not visible to other layers)

• reduced complexity (complex systems can be decompoed into more easily understood pieces and a layer can be omitted if some or all of its sevices are not needed)

• increased evolution (systems can evolve faster, since layers can be changed while still integrating with existing systems)

• easy extension (alternate services can be offered at layer N+1 that share the services of layer N)

• multi vendor integration (alternate implementations of a layer can co-exist)

• easy testing (confidence in correct operation, enhanced by testing each layer independently)

advantages of layering

• a product from one vendor that implements functionality at a particular layer, will be much more likely to interoperate with another vendor's product at another layer because both vendors are following the model

• example: a Microsoft application (application layer) sending traffic over a Cisco router (network layer)

common layered models

a)ISO's OSI layered model

b)TCP/IP layered model

c)Cisco's 3 layered model

d)IBM's SNA model

ISO/OSI

Network

Data Link

Application

Presentation

Session

Transport

Physical

TCP / IP

Internet

Application

Transport

Network Access

Physical

SNA vs. OSI

SNA model• Data link control (DLC) - Defines several protocols, including the

Synchronous Data Link Control (SDLC) protocol for hierarchicalcommunication, and the Token Ring Network communicationprotocol for LAN communication between peers. SDLC provided a foundation for ISO HDSL and IEEE 802.2.

• Path control - Performs many OSI network layer functions, includingrouting and datagram segmentation and reassembly (SAR)

• Transmission control - Provides a reliable end-to-end connection service (similar to TCP), as well as encrypting and decryptingservices

• Data flow control - Manages request and response processing, determines whose turn it is to communicate, groups messages, and interrupts data flow on request

• Presentation services - Specifies data-transformation algorithmsthat translate data from one format to another, coordinate resourcesharing, and synchronize transaction operations

• Transaction services - Provides application services in the form ofprograms that implement distributed processing or management services

SNA Model

• SNA was designed around the host-to-terminal communication model that IBM's mainframes use.

• IBM expanded the SNA protocol to supportpeer-to-peer networking.

• SNA is still widely deployed

open and closed systems

• layered models may be open or closed– an open system is a system in which its

architecture is not a secret and is open for everyone who is interested in using, changing and reworking on it, for his or her specific requirement (OSI).

– a closed or proprietary system is a system in which the architecture is kept a secret from the general users by its creator. if a user wants to use it in any manner, he has to obtain the authorization from the vendor, leading to lack of standardization (SNA).

Evolution of OSI model

• In the 1970s', proprietary early network designs evolved. In the early 1980s', manufacturers began attempting standardization.

• Two bodies, which lead the movement for standardization, were:

– ISO (International Organization for Standardization) and

– IEEE (Institute of Electrical and Electronics Engineers)

• IEEE standardized protocols for the Token Ring and Ethernet network types, while ISO developed a networing model known as the Open Systems Interconnect (OSI) reference model.

OSI reference model

• OSI is an ISO standard for worldwide communications that defines a networking framework, for implementing protocols in seven layers.

• The OSI reference model describes the functions involved in data communication.

– it is a conceptual framework for understanding the complex interactions that occurr;

– OSI is not a protocol that performs any specific function in the networking process;

– it is not a code or software;– the OSI model defines data communication functions,

in terms of 7 layers;

OSI reference model

• each layer is responsible for certain basic functions, such as:

– getting data from one device to another or– getting data from an application on one computer

to another• the OSI model does host-to-host networking, and

divides that into layered protocols, sometimes called a stack, consisting of abstractions, called layers.

• communication between peer entities at a given layer is done via one or more protocols; this communication is invoked via the interface with the adjacent layer

OSI reference model

• OSI model allows vendors to implement specific functionality into their networking devices.– a network card is a layer 1 (physical) device– a switch is a layer 2 (data link) device– a router is a layer 3 (network) device

• this helps the vendors to easily interoperate with other vendor's devices that follow the same road map, the OSI model.

Principi base

• L’architettura OSI si basa su una serie di principi progettuali, di seguito elencati:– (1) non creare un numero eccessivo di layer

ai fini della loro definizione ed integrazione

– (2) raccogliere funzioni simili nello stesso layer

– (3) creare layer che possano essere completamente riprogettati (per usufruire di nuove tecnologie) senza influire sui layerprecedenti e seguenti.

Principi base

• Ancora:– (4) creare un layer ovunque ci sia bisogno di

introdurre un nuovo livello di astrazione nel trattamento dei dati (es: morfologia, sintassi, semantica)

– (5) far si che cambiamenti di funzioni e protocolli all’interno di un layer non influenzino altri layer

– (6) definire interazioni solo tra layer contigui

OSI reference model

• some important data communication concepts related to OSI layers are:1. protocol data units2. data encapsulation/decapsulation

Protocol data units

• data exists at each layer in units called PDU

• the table gives the names of PDU at each layer

Layer PDU Name

Application Data

Presentation Data

Session Data

Transport Segment

Network Packet

Data Link Frame

Physical Bits

Encapsulation / decapsulation• encapsulation

– process by which data moves between different PDU types

– each layer or protocol accepts a message from a layer (or protocol) above it and places its own header

– encapsulation occurrs, as data moves from higher to lower layers

• decapsulation: – reverse of encapsulation– decapsulation occurrs as data

moves from lower to higher layers

Layer PDU Name

Application Data

Presentation Data

Session Data

Transport Segment

Network Packet

Data Link Frame

Physical Bits

encapsulation

decapsulation

Data encapsulation process

• protocol headers are added as data moves from higher to lower layers

application data

transport header application data

network header transport header application data

data link header network header transport header application data data link header

I livelli

• Lo scopo di ciascun livello è quello di fornire servizi ai livelli superiori, mascherando come questi servizi sono implementati

• Ogni livello passa dati e informazioni di controllo al livello sottostante, sino a quando si raggiunge il livello fisico che effettua la trasmissione

Livello fisico

• Trasmissione di sequenze non strutturate di bit sul canale fisico di comunicazione

• Problemi : meccanici, elettrici, funzionali e procedurali delle interfacce di rete (componenti che connettono l'elaboratore al mezzo fisico) e le caratteristiche del mezzo fisico.

• Specifica : codifiche dei bit (lunghezza temporale, forma, etc.) caratteristiche dei cavi e dei connettori

Physical layer

• the physical layer handles all the physical aspects of communication, between two directly connected physical entities

• physical properties include electromechanical characteristics of the medium or links between the communication physical entities, such as

– connectors– voltages– transmission frequencies, etc...

• the physical layer provides an unstructured bit stream, which can be used by higher layers, to provide the basis for higher layer communication services

Physical layer

• main functionality: transmits bits• data units: bits• devices: multiplexer/repeater• examples: LAN e WAN implementations

such as Ethernet, Fast Ethernet, RS-232, etc...

Livello data link

• Trasmissione sequenziale strutturata di dati (frames o trame) tra nodi adiacenti priva di errori non segnalati

• Deve:– • identificare l’inizio e la fine dei pacchetti (framing)– • rilevare e recuperare errori– • eventualmente gestire ritrasmissioni– • gestire il controllo di flusso

• Per le reti locali gestisce il controllo di accesso al mezzo trasmissivo (protocolli di accesso multiplo)

Data Link layer

• the Data Link layer accepts the bit stream provided by the physical layer and provides reliability of transfer of data between two directly connected entities.

• the Data Link layer's functionality includes:

– flow control– error detection and control– topologies (star, bus, ring...) – Access control (MAC)

• the Data Link is divided into two sub-layers:

– Medium Access Control (MAC) and– Logical Link Control (LLC)

LLC sub-layer

• The LLC sub-layer is responsible for reliable transfer of messages-link protocol data units (LPDUs), between two directly connected Layer two entities.

• Its functions include:– framing, indicating where a Layer 2 message

begins and ends– sequence control– error control– flow control

MAC sub-layer

• MAC is closely associated with the physical layer and defines the means by which the physical medium may be accessed

• it coordinates, to seize a shered channel, multiple MAC entities – like a teacher deciding between students conflicting interests to speak

• the Mac layer provides a limited form of error control, especially for any header information that defines the MAC-level destination and higher-layer access mechanism

• Ethernet (IEEE 802.3) is a prime example of a shared medium with a defined MAC sub layer functionality

MAC address

• the Mac sub-layer carries the physical MAC address of each device on the network. The MAC address is a 48-bit address that's encoded on each network device such as router interface or NIC, by its manufacturer

• it's the Mac address that the physical layer uses to move data between nodes of the network.

• no two devices in the worl have same MAC addresses (...)

• 00-10-E3-42-A8-BC is an example of a MAC address. The first 6 hexadecimal digits (3 bytes) specify the vendor/manufacturer; the other 6 digits define the host

Data Link layer

• main functionality: MAC address, error control

• data units: frames• devices: switch, bridge, router• examples: Frame Relay, X.25 (also

operates at the Physical and Network layer)

Livello di rete

• Instradamento dei pacchetti ( routing) attraverso i nodi intermedi della sottorete di comunicazione

• Deve:– • conoscere la topologia della rete– • scegliere il cammino migliore per far arrivare

ciascun pacchetto a destinazione– • gestire congestioni della rete– • gestire le incompatibilità di reti eterogenee

(internetworking)

Network layer

• the Network Layer defines the functions necessary to support data communication between indirectly connected entities. It has the ability of forwarding messages from one Layer 3 entity to another, until the final destination is reached.

• Network Layer functions include:

– routing– forwarding of packets to its final destination host– providing logical addressing: IP addresses

Network layer: routing

• forwarding and routing of a packet is accomplished, independent of both the media and transmission types used at any step along the way

• it is like a postal department, where the letter is passed from location to location, until it reaches the destination address on the envelope

• the network layer is the domain of wide area networks (WANs)

• Network layer uses logical addresses to pass packets through communication network of routers, to the final host

Network layer

• main functionality: routing, logical addressing (IP)

• data units: packets• devices: router and layer 3 switches• examples: IP, IPX, NetBEUI, routing

protocols (RIP, OSPF, BGP, IGRP, etc...)

Livello di trasporto

• Fornisce servizi per il trasferimento affidabile dei dati (messaggi) end-to-end controllabili dall’utente

• Deve:– • aprire e chiudere le connessioni– • frammentare e riassemblare i messaggi– • rilevare e correggere gli errori (duplicazione,

perdita o mancata sequenzializzazione dei blocchi di dati)

– • controllare il flusso e le congestioni– • inviare diversi dati (per conto di diversi processi) su

un’unica connessione (multiplexing)– • inviare un unico dato su connessioni diverse (per

aumentare il throughput)

Transport layer

• the Transport Layer is concerned with getting Layer 4 messages - called segments or transport protocol data units (TPDUs) from source to destination, in a reliablemanner.

– Layer 4 is an end-to-end communication, unlike the hop-by-hop communication at layer 3

• Characteristics & services include:

– flow control– error detection and recovery– the support of different flows of data to different

applications on the same host (multiplexing)

Transport layer

• the general idea is that two communicating hosts need not to be concerned with the topology of the internet work which lies between them. They only need to know the state of their communication. If part of the “cloud” suffers a failure, the Layer 3 entities (routers) will deal with it and recover dinamically.

• apart from potential retransmission of any lost segments, the hosts' layer 4 entities do not have to be at all concerned with routing and recovery activities at Layer 3.

Transport layer

• Classification of services: transport services can be classified according to the “quality of service” that they provide to the layers above. There are two types of service quality: reliable and unreliable.

– A reliable service is one that makes sure that data is not lost during a transfer and to provide the data error-free to the service user. It ensures that the receiver acknowledges the receipt of each item of data, and to check that no data is lost in.

– An unreliable service does not have a mechanism to ensure that the receiver did receive the data sent. The receiver has to check for errors on each block of data, and when even a single unknown bit is detected, the complete data block is discarded.

Transport layer

• Classification of services: transport services may also be classified as:– Connection-oriented: the sender of data first establishes a

logical connection with the prospective receiver of the data, sends the data and then terminates the connection. This provides in-sequence delivery; that is, the service user receives packets in the order they were sent. TCP is connection-oriented and uses a 3 way handshake to establish a connection before data is sent.

– Connectionless: the sender does not establish a contact with the receiver first. Whenever there is a data packet ready to be sent, it independently routes the packet to a gateway. As the routers choose the routes dinamically it is possible that different packets from a single session take different routes , and also may arrive out of order to the destination.

Transport layer

• main functionality: connection and flow control adding reliability with error detection and recovery

• data unit: segments• devices: gateway• examples: TCP, UDP

Livello di sessione

• Organizzazione del dialogo e della sincronizzazione tra due programmi applicativi e del conseguente scambio di dati

• Deve:– • gestire connessioni tra diverse applicazioni– • controllare la sincronizzazione

invio/ricezione inserendo dei checkpoints nel flusso dei dati

– • abilitare il login a macchine remote (anche in modalità time-sharing)

Session layer

• the session layer controls communication or dialogue between applications on hosts

• synchronization of communicating applications comes into play, when coordinated timing of corresponding events at the end points is imperative, such as in financial transactions

Session layer

• main functionality: controls dialogues between applications

• data unit: data • devices: gateway• examples: NFS (network file system), SQL,

RPC, NetBios, XWindows

Livello di presentazione

• Garantisce l’indipendenza ai processi applicazione rispetto alle differenze nella rappresentazione dei dati (sintassi)

• Deve:– • negoziare la sintassi di trasferimento

– • effettuare le conversioni con le rappresentazioni locali dei dati (es., traduzione tra ASCII ed EBCDIC)

Presentation Layer

• this Layer performs any necessary data transformations or formatting required by the end applications.

• Functions of the presentation layer include:– data compression– file formatting– encryption

• Common data formatting is important because it allows the same application file to be accessed by the application, running on different computer platform

Presentation layer

• main functionality: formatting, encryption and compression

• data unit: data • devices: gateway• examples: MPEG, ASCII, QuickTime, GIF,

TIFF, JPEG

Livello di applicazione

• Definizione dei servizi per l’utente e del funzionamento dei programmi applicativi

• Fornisce i servizi utilizzati dall’utente, quali:– • Network Virtual Terminal, che consente il

remote job entry (es. TELNET Protocol)– • File transfer: rende la manipolazione di files

indipendente dal particolare file system in uso sui singoli hosts fra i quali variano le convenzioni sui nomi di files, le codifiche di testo, ecc… (es. FTP)

– • Electronic Mail (es. SMTP)

Application Layer

• the Application Layer provides the services that directly support an application running on a host. This layer is closest to the end user.

• Examples of layer 7 services include:– FTP– Telnet– SNMP (simple network man. protocol)

• most network management activities are based on the services provided by layer 7.

Application layer

• main functionality: provides network services to applications

• data unit: data • devices: gateway• examples: Telnet, SMTP, FTP, SNTP

TCP/IP model - introduction

• the TCP/IP model was creatad by the US Department of Defense to create a network that could survive any conditions

• some of the layers in the TCP/IP model have the same names as layers in the OSI model

• TCP/IP is a “protocol specific” model (where OSI is a protocol independent model)

TCP/IP model layers• Application layer : includes the

OSI Application, Presentation and Session layers

• Transport layer : similar to OSI, with TCP (transmission control protocol) and UDP (user datagram protocol) operating at this layer

• Internet layer : similar to OSI Network layer. IP resides at this layer

• Network Access layer: combines all functionality of Physical and Data Link layers of OSI model

TCP / IP

Internet

Application

Transport

Network Access

Application layer

• Protocolli più diffusi:– TELNET: permette il login a macchine remote,

mettendo a disposizione un terminale virtuale– FTP: File Transfer Protocol– SMTP: Simple Mail Transfer Protocol– DNS: Domain Name Server (gestisce la mappatura

Hosts – Addresses)– NNTP: Network News Transfer Protocol (per la

circolazione di notiziari on line)– HTTP: Hyper Text Transfer Protocol (per il

trasferimento di ipertesti, dotati cioè di links)

Transport layer

• Prevede due protocolli:– TCP: Transmission Control Protocol

• – fornisce collegamenti di tipo connection oriented• – prevede forme di controllo di flusso• – assieme all’Internet Protocol dà il nome al

modello

– UDP: User Datagram Protocol• – fornisce collegamenti di tipo connectionless• – delega il controllo di flusso alle applicazioni• – adatto a comunicazioni in cui il prompt delivery è

più importante di un accurate delivery es. trasmissioni voce e video)

Internet/network layer

• Corrisponde al livello di rete del modello OSI.– • Fornisce esclusivamente comunicazioni

connectionless.

– • Funzionalità principali:• Routing degli IP Packets• Controllo delle congestioni

Network access / host-to-networklayer

• Raggruppa i livelli Data Link e Fisico del modello ISO/OSI

• Il modello TCP/IP non descrive dettagliatamente il suo funzionamento

• Definisce unicamente l’unità di informazione che circola in rete come IP Packet

Confronto fra OSI e TCP-IP

• L’ OSI fa emergere con evidenza tre concetti chiave risultando aderente alla moderna filosofia di programmazione object oriented:– Servizi– Interfacce

– Protocolli (Peers protocols)

• L’oggetto principale è il layer

Confronto fra OSI e TCP-IP

• Nel caso dell’OSI il modello precede la realizzazione dei protocolli. Risulta quindi piùorganico ma anche ridondante. (I livelli Presentation e Session non hanno avuto un particolare sviluppo nella reale evoluzione delle reti).

• Nel caso del TCP/IP il modello è venuto definendosi come semplice descrizione di protocolli già esistenti e risulta inadatto a descrivere sistemi basati su diversi protocolli (minore generalità).

Confronto fra OSI e TCP-IP

• Servizi e comunicazioni:– ISO/OSI: il Network layer prevede protocolli sia per

la comunicazione connection oriented che per quella connectionless. Il Transport layer ha protocolli esclusivamente connection oriented. La scelta del tipo di servizio è dunque invisibile all’utente.

– TCP/IP: la doppia possibilità risiede nel Transportlayer (TCP e UDP), mentre l’Internet layer prevede solo comunicazione connectionless. Il tipo di servizio è una scelta disponibile all’utente, anche se legata alla reliability (TCP: reliable, UDP: unreliable)

Confronto fra OSI e TCP-IP

• ISO/OSI– modello: utile ed adeguato per la descrizione

delle reti

– protocolli: sviluppi praticamente inesistenti

• TCP/IP– modello: del tutto inadatto alla descrizione

generale di altri sistemi

– protocolli: largamente diffusi e utilizzati in tutto il mondo

Cisco 3 layer model -introduction

• Cisco has defined a hierarchical model, known as the hierarchical inter-networking model. This model simplifies the task of building a reliable, scalable, and less expensive hierarchical inter-network.

• it focuses on the 3 layers of a network:– core layer– distribution layer– access layer

Core layer

• this layer is considered the “backbone” of the network, and includes the high-end switches and high-speed cables such as fibre cables

• does not route traffic at the LAN• devices in this layer do no packet

manipulation• speed and reliable delivery of packets are

important at this layer

Core layer• high data transfer rate

– speed and efficiency– load sharing (a particular load can travel through

multiple network connections) • low latency period

– use of high-speed, low latency circuits– packet processing is fast since the device is only

concerned with forwarding packets, and not enforcing policies

• high reliability

– multiple data paths– high fault tolerance

Distribution layer

• this layer includes LAN-based routers and layer 3 switches

• it ensures that packets are properly routed between subnets and VLANs

• it is also called the workgroup layer

Distribution layer

• this layer is responsible for routing. It also provides policy-based network connectivity, including:– packet filtering (firewalling) – QoS (the router or layer 3 switches can read

packets and prioritize delivery, according to policies)

– control broadcast and multicast (boundary for bc and mc domains)

• this layer performs queuing and provides packet manipulation of the network traffic

Access layer

• it includes hubs and switches• it is also called the desktop layer because it

focuses on connecting client nodes, such as workstations to the network

• it ensures that packets are delivered to end users computers

Access layer

• it contains devices that allow workgroups and users to use the services provided by the Distribution and Core layers

• in this layer we have the ability to expand or contract collision domains, using a repeater, hub, or standard switch

• in this layer switch is an advanced version of a hub, rather than a high-powered device found at the core layer

Access layer

• at this layer we can:– enable MAC address filtering– create separate collision domains to improve

performances– share bandwidth– handle switch bandwidth (load balancing)

Cisco 3 layer model

Core layer

Distribution layer

Access layer

Cisco 3 layer model - benefits

• The benefits of the Cisco 3-layer hierarchical model include:– high performance: – efficient management and troubleshooting– policy creation (filters, rules) – scalability– behavior protection of the network