Politecnico di Milano Scuola di Ingegneria Industriale e dell’Informazione

Networking basics

Communication networks

o  N nodes o  Mesh

n  # of links, E, N<E<N(N-1)/2 n  Cost = o(N2-N) n  Special case: Full mesh

o  All direct links o  Cost = #links = α(N(N-1)/2) = o(N2) o  Shortest paths Lmin=1

o  Tree n  Cost = α(N-1) = o(N) n  Paths of variable length

o  Ring n  Cost = αN = o(N)

Communication networks

o  Star n  N links, one per node n  1 switching node n  Cost = αN + βnode = o(N) n  Path length L=2

o  Bus n  Shared communication medium n  Access control to the medium n  Cost = αN + βbus = o(N) n  Path length L=2

o  Mesh + star n  2 layers n  2 class of nodes

Communication networks o  Types of networks

n  LAN: Local Area Network o  Small size networks (building, campus)

n  MAN: Metropolitan Area Network o  Medium size networks up to a few tens of km

n  WAN: Wide Area Network o  Large size networks

102101 103 104 105 106 107

Distanza (m)

Frequenzadicifra(bit/s)

LAN ad altavelocità

105

104

106

107

108

109

1010

WAN a bassaveloacità

MAN ad altavelocità WAN ad alta

velocità

Connessionicon modem

MAN a bassavelocitàLAN a bassa

velocità

Access and Backbone networks

o  Backbone (geographical networks with long distance links)

o  Access network for interconnecting user terminals

Network of networks

o  Internet is designed to interconnect networks of different types

Politecnico di Milano Scuola di Ingegneria Industriale e dell’Informazione

Internet basics

What is the Internet?

o  Millions of computers connected to the network host = terminal

o  On which network applications are executed

o  Links (Fiber, cable, radio, satellite, ...)

o  Network nodes router

local ISP

company network

regional ISP

router workstation server

mobile

What is the Internet? …continue

o  Communication infrastructure allows distributed applications: n  Web, email, games,

e-commerce, file sharing

o  Communication protocols for sending and receiving application messages

What is a protocol?

Human and network protocols

Hello

Hello

What’s the time now?

2:00pm

TCP connection request

TCP connection acknowledgment

Get http://polimi.it!

<file> time

Browser Web Server

Web

10

Example: Email S: 220 hamburger.edu C: HELO crepes.fr S: 250 Hello crepes.fr, pleased to meet you C: MAIL FROM: <alice@crepes.fr> S: 250 alice@crepes.fr... Sender ok C: RCPT TO: <bob@hamburger.edu> S: 250 bob@hamburger.edu ... Recipient ok C: DATA S: 354 Enter mail, end with "." on a line by itself C: Do you like ketchup? C: How about pickles? C: . S: 250 Message accepted for delivery C: QUIT S: 221 hamburger.edu closing connection

Network edges o  Terminals (hosts):

n  Execute application software (Web, email, etc.).

n  Remote processes exchange information

o  Client/server n  Client asks for a service,

server provide it n  Clients asks questions,

servers reply

o  Peer-to-peer: n  All terminals cooperate

without (almost) any differentiation in roles

12

Applications use the network

o  The network provides a communication service to applications for the transport of information among remote processes

o  The transport service offered by the network can be of different types

_________________________

Application process

_________________________

The network transports

information

Application process

Transport of information

o  The network can transport short messages in an unreliable way (examples: DNS, signaling, etc.)

o  It can also transport long sequences of bytes in a reliable way (web, email, file transfer, etc.)

_________________________

Application process

_________________________

The network transports

information

Application process

In the heart of the network

o  Set of interconnected routers

o  The cruciual question: how the information is transferred in the network? n  Circuit switching:

physical circuit dedicated to an entire communication session

n  Packet switching: information cut in pieces delivered one by one

15

Circuit switching

Communication resources reserved for a call

o  Example: old telephone network

16

Circuit switching

1122

3344 1) Want call 3344!

2) Look for a circuit

4) ringing!

3) Open circuit

5) Conversation

17

Circuit switching

o  Network resources cut in “pieces”

o  Each “piece” (= circuit) is allocated to calls

o  Resources remain inactive if not used (no sharing during call)

o  Link division in “pieces” = multiplexing at physical layer n  Frequency

division n  Time division

18

Packet switching

terminale

router

pacchetto

informazione header

19

Packet switching

Header Data

Destination address: A

Routing table Dest. address Next router

A R2 R3 B

A

B

R1

R2

R3 C

20

Packet switching

Flow of data cut in packets

o  All packets share network resources

o  Each packet uses the full channel during transmission

o  Resources used according to needs

Resource contention o  store and forward:

the network node receive the entire packet before transmitting it to the output link

o  Statistical multiplexing: packet queuing when the output link is busy with another packet transmission

21

Packet vs Circuit switching

o  1 link of 2.048 Mbps o  Each user:

n  Asks for web pages of 50KB every 62.5s on average

o  Circuit switching: n  1 channel of 64 kbps

per user n  Average web page

download delay : 6.25s

32 users

Link 2.048 Mbps

o  Packet switching: n  Average download delay:

0.22s

22

Physical Internet architecture

NAP

NAP

NAP

BSP

ISP

ISP

ISP = Internet Service Provider BSP = Backbone Service Provider NAP = Network (Neutral) Access Point POP = Point of Presence CN = Customer Network

POP

POP

POP

ISP POP

BSP POP

BSP POP

POP

CN

CN

CN

CN CN

CN

CN

CN

23

Internet access: Dialup

o  Dialup via modem n  Up to 56Kbps n  Direct access to ISP router with a telephone

circuit n  Digital transmission in the analog telephone band

Telephone network

Mod

em

Internet access: ADSL o  ADSL: asymmetric digital subscriber line

n  Up to 1 Mbps upstream n  Up to 20 Mbps downstream n  Telephone twisted pair shared with the telephone

network up to the first station n  Direct access to the ISP network or through an

intermediate high speed network of another provider

Internet access: Next Generation Network

o  Fiber optic access network n  Partial of full replacement of the

twisted pair with a fibera

n  Fiber To The Home n  Fiber To The Basement n  Fiber To The Curb n  Fiber To The Neighborhood

Internet access: Next Generation Network

o  Optical access networks n  Point to point n  Passive optical networks

Internet access: Wireless access

o  Wireless networks n  Shared access channel

(wireless) n  Base station or access point

o  Cellular networks n  GPRS ~ 56 kbps n  UMTS ~ 384 kbps n  HSPA ~ 28 Mbps n  LTE ~ 100 Mbps

o  Wireless LAN: n  802.11b/g/n (WiFi): up to 300

Mbps

base station

mobile hosts

router

Politecnico di Milano Scuola di Ingegneria Industriale e dell’Informazione

29

Functional models

Fundamentals of Communication Networks

30

Communication service o  Given two remote entities o  We can describe the communication service

for the exchange of messages as: “the service provider for the information

transport”

Entity A

Entity B

Communication service

conversation

Node A Node B

31

Communication service

o  The communication service manages the exchange of information between two entities

o  It is a service for the exchange of information units that can be: n  messages n  bits n  groups of bits (frames or packets) n  files n  multimedia flows

Entity A

Entity B

32

Service primitives

o  The communication service can be described through service calls that are named service primitives

o  Service primitives are used to describe the service, to request it, and to receive information on the service by the provider

o  Service primitives are characterized by parameters, including: n  Information to transfer n  Destination n  Characteristics of the requested service n  etc.

33

Service primitives

Entity A

Entity B

Bidirectional channel

Service primitives

conversation

Node A Node B

34

Characteristics of the communication service

o Connection mode

n  Connection setup n  Information transfer n  Connection tear down

o Connection-less n  Single phase of communication

35

Connection setup

Information transfer

Connection tear down

Entity A

Entity B

Connection service

36

Entity A

Entity B

trasfer

trasfer

trasfer

Connection-less service

o  Data transfer does not require a preliminary handshake

o  Transfers between same entities of different pieces of information are autonomous and not linked together

37

Layers

o  Two entities that communicate using a communication service can offer a communication service themselves to other entities of “upper layer”

Entity A1

Entity B1

Bidirectional channel

Node A Node B

conversation Entity

A2 Entity

B2

conversation

header Dati PDU

38

Functions at layers

o  The communication service offered at upper layer can be richer and more complex thanks to the functions implemented at lower layer

Entity A1

Entity B1

Bidirectional channel

Node A Node B

conversation Entity

A2 Entity

B2

conversation

header Dati PDU

39

Communication protocols

o  The entities that cooperate to offer a communication service at upper layer exchange messages through the service of the lower layer

o  Protocol: n  Set of rules that manages the conversation

(exchange of messages) between entities of the same layer o  Message formats o  Service information o  Transfer algorithms o  etc.

40

Packet Data Units (PDU)

o  A protocol uses information transfer units named PDUs or protocol messages/frames

o  PDUs can include:

header data

■  Data coming from upper layer

■  Service information for the coordination among entities

41

Stack architecture o Complex communication services

are typically based on a number of layers n  From a low layer for bit transfer n  To a top layer for the support of user

applications layer 5

layer 4

layer 3

layer 2

layer 1

layer 5

layer 4

layer 3

layer 2

layer 1

Advantages of stack architectures

o  Reduces complexity

o  Standardizes interfaces

o  Allows interoperability between technologies

o  OSI model (1974)

43

Relation between layers

o  The offered service is represented by a Service Access Point (SAP)

PDU: Packet Data Unit SDU: Service Data Unit PCI: Protocol Control Information

N+1 - PDU

N - SDU N - PCI

N-SAP

Layer N+1

Layer N

N-PDU

44

Relation between layers

2 - PDU

1-SAP

Layer 2

Physical layer

o  At lower layer we have the physical layer o  PDUs are flows of bits

bit

2 - PDU 2 - PDU

1-SAP is a physical port

Upper layers enrich the bit transfer service with more advanced functions

45

Complete architecture

layer 5

layer 4

layer 3

layer 2

layer 1

5 4 3 2

5 4 3

5 4

5 layer 5

layer 4

layer 3

layer 2

layer 1

5

5 4

5 4 3

5 4 3 2

5 4 3 2 1

46

Example: Air transport

ticket (purchase) baggage (check) gates (load) runway takeoff airplane routing

ticket (complain) baggage (claim) gates (unload) runway landing airplane routing

airplane routing

Source: Computer Networking: A Top Down Approach Featuring the Internet, Jim Kurose, Keith Ross, Addison-Wesley, July 2004. All material copyright 1996-2004. J.F Kurose and K.W. Ross, All Rights Reserved

47

ticket (purchase)

baggage (check)

gates (load)

runway (takeoff)

airplane routing

departure airport

arrival airport

intermediate air-traffic control centers

airplane routing airplane routing

ticket (complain)

baggage (claim

gates (unload)

runway (land)

airplane routing

ticket

baggage

gate

takeoff/landing

airplane routing

Example: Air transport

Layer: each layer implements a service n  Through internal operations n  Using lower layer services

Source: Computer Networking: A Top Down Approach Featuring the Internet, Jim Kurose, Keith Ross, Addison-Wesley, July 2004. All material copyright 1996-2004. J.F Kurose and K.W. Ross, All Rights Reserved

48

Why a stack architecture?

Complex systems: o  Easy to identify services (implementation,

discussion) o  Easy management and update

n  Changes in a layer are transparent to others

49

Functions

o Multiple functions can be provided by a layer

o  They can be divided into: n  Adaptation functions

o  examples: n  multiplexing n  segmentation

n  Enriching functions o  Examples:

n  Error control n  Frame reordering

50

Function: segmentation N+1-PDU

N-PDU

N-SDU

51

Functions: Multiplexing

entità liv. N

52

Entity A Selects SAPs Entity C Entity B

Function: network

o  We have a network function when the conversion between more than two entities of the same layer is enabled

o  ROUTING is the main component of the network function (SAP selection)

53

SAP connecting different entities

Entity N+1

Entity routing

o  Problem: identify conversation partner

o  Routing can be implemented by the lower layer if we introduce ADDRESSING

Routing

54

Entity N+1

Routing Entity

Routing Entity

Routing Entity

Identify destination N-SAP

Addressing

55

PDU is passed to lower layer with the ADDRESS as parameter

The ADDRESS is used to route PDU (select output SAP) And it is encapsulated in the header so as to be used by other entities

Entity N+1

Routing entity

PDU

SDU

Addressing & routing

56

Addressing

o Address: identified of the SAP through which the destination entity can be reached; must be unique among all SAPs of the same layer

o Addressing types o unicast: single SAP o multicast: group of SAPs o broadcast: all SAPs

57

Forwarding or switching

o  It’s the information forwarding service that an entity provides to others of the same layer

o  The SAP is already selected and the service operates the information forwarding

C B A

58

Routing tables

o Selection of the output SAP based on a table stored in the node

Routing table destination Output SAP

o  To write routing tables it is possible to gather information from other nodes n  Routing protocols

59

Path to destination o  Many nodes can be crossed before reaching

destination o  Intermediate nodes just implement network

function and not upper layer

Application

Transport

Network

Data Link

Physical

Network

Data Link

Physical

Application

Transport

Network

Data Link

Physical Network

Data Link

Physical

Example IP Router IP: Network function at layer 3!!!

Router Host

60

Path to destination (2) o  There are devices that implement network

function at different layers o  We’ll see later on: LAN Switch

Application

Transport

Network

Data Link

Physical

Data Link

Physical

Application

Transport

Network

Data Link

Physical Data Link

Physical

Example Switch: Network function at layer 2!!!

Host Switch

61

Path to destination (3) o  There are even nodes (proxy) that implement

network function at application layer

Application

Transport

Network

Data Link

Physical

Data Link

Physical

Application

Transport

Network

Data Link

Physical

Example Proxy: Network function at layer 5!!!

Application

Transport

Network

Host

Proxy

OSI model

o  Even if the assignment of functions to layers depends on technologies, there is a standard description of the layers

62

Physical layer

Functions: o  Modulation and Transmission of bits o  Coding and error control o  Multiplexing (in physical channels) o  Multiple access (in physical channels) o  Synchronization

63

Link Layer Functions: o  Aggregation of bits in groups (frame) o  Error control and retrasmission o  Flow control o  Multiplexing (logical among information flows)

o  Point-to-point links:

64

Physical

Data Link

Physical

Data Link

bits

frame

Link layer o  In case of shared transmission medium

n  Multiple access (logical among multiple transmitting stations)

n  Link layer divided into two sub layers

65

MAC

LLC Medium Access Control

Logical Link Control

Link layer

Physical

MAC

LLC

Physical

MAC

LLC

Physical

MAC

LLC

Network layer Functions: o  Addressing o  Forwarding o  Routing o  Fragmentation/reassembly o  Based on technology also:

n  Congestion control n  Quality of service management

66

Physical

Data Link

Network

Physical

Data Link

Interface 1 Interface 2

Note: we’ll focus on the Internet network layer

Physical

Data Link

Network

Physical

Data Link

Interface 1 Interface 2

Physical

Data Link

Network

Physical

Data Link

Interface 1 Interface 2

packets

Transport layer

Functions: o  Flow control o  Congestion control o  Multiplexing (between multiple applications) Implemented only in terminals and not in

intermediate routing nodes

67

Physical

Data Link

Network

Transport

Physical

Data Link

Network

Transport

Physical

Data Link

Network

Physical

Data Link

Interface 1 Interface 2

segments

Data layers Session: o  Organizes dialogue among applications coordinating the

exchange of sets of data Presentation: o  Manages the data formats to facilitate the dialogue between

applications Application: o  Defines the format of messages exchanged by applications

68 Physical

Data Link

Network

Transport

Data Application

Presentation

Session

Physical

Data Link

Network

Transport

Application

Presentation

Session

69

Suite TCP/IP and OSI model

SMTP, FTP, TFTP, Telnet, Rlogin SNMP, DNS TCP, UDP

IP, ARP, RARP, ICMP, IGMP

Reference

o  Tanenbaum book: Chapter 1 o Kurose-Ross: Chapter 1