Chapter1: Conceptual Basispaloalto.unileon.es/cn/lect/CN-Ch1-2018-Section1.pdf · Cooperating host...

Chapter 1: Conceptual BasisSection 1

Leading questions¨ What are the principles guiding the communication between two

parties?

¨ When can a communication be considered fast and efficient?

¨ What are the landmarks about the development of Internet?

¨ Why is networking essential for progress?

¨ What is a network architecture?

Flow of topics

Motivation

Hosts and Applications

Network architecture

Layers and service

interfaces

Core Concepts

Signals

Information and Coding

Patterns

Essential Data Communications

Coremetrics

Performance of networks

Cooperating host applications¨ Applications are computer programs¨ Communicate over the Internet¨ At work, at home and mobile¨ Vastly differing requirements

• Network storage• Streaming Audio/Video• Video conferencing• IP Telephony• e-mail: smtp, pop3, imap

• File Sharing, file transfer: ftp, rcp, scp• Printer sharing• Virtual terminal: telnet, ssh (Secure Shell)• e-commerce• Geolocation• World Wide Web (www)• Social Networks• Instant Messaging (Whatsup, …)

Applications

Elastic Real time

Loss intolerant Loss Tolerant

Non-delay adaptive Delay-adaptive

Speed-adaptvie Delay-adaptive

Essential Internet service: wwwn Web pages are downloaded by the client from

the servern Client and server speak the http protocoln http = Hyper Text Transfer Protocol

n www = World Wide Web:n A distributed, Client/Server application

n Server program (e.g., Apache)n Client program (e.g., Firefox)

n URLn Uniform Resource Locator n http://paloalto.unileon.es/cn/index.html

n HTTP, in turn uses the TCP protocol for reliabilityn TCP = Transmission Control Protocoln TCP provides reliability

n In case of packet loss, duplication, errors, etc

Internet

Client

Server

Units and multipliers

¨ Bandwidth¤ Directly related to the acceptable speed of bit transmission over some medium¤ Number of bits transmitted in one second:

¤ Bps (Bits Per Second = Bits/Sec)

¤ Since bandwidth is a rate, the multipliers take on the following values:¤ K (Kilo = 103)¤ M (Mega = 106)¤ G (Giga = 109)¤ T (Tera = 1012)

¨ Delay¤ Seconds¤ How much time it takes to transport one bit from a source to a destination directly

connected¤ Propagation delay

¨ Jitter¤ The variance of the delay

Manage the complexity of networks

Network Architecture

Logical channels

¨ Applications communicate overthe Internet

¨ The channel between twocommunicating applications islogical

¨ Each channel:¤ Connects two applications

¤ Hosts must be identified:n IP address

¤ Applications must be identified:n Port numbers

Client S-1

Android

Server SServer

Server U

Client S-2

Client U-1

Client T-1

Client T-2

Internet

How to hide network

complexity from application

programmers?

Layering in hosts and network

Internet

Client

Server

Subnetwork

TCP UDP

Application

Subnetwork

NETWORK

Subnetwork

TCP UDP

Application

End host A End host B

Internet Architecture

¨ Network complexity is organized into 4 layers¨ Each layer

¨ Offers a set of services to the upper layers

¨ The mechanism that attains each service is a protocol

¨ An upper layer avails one service from a lower layer bycalling its interface

¨ 1. Subnetwork: Ethernet, Wi-Fi, Bluetooth

¨ 2. Network: Only IP !!!

¨ 3. Transport: TCP and UDP

¨ 4. Application: Whatsup and innumerably others

Internet

Subnetwork

Network

Transport

Application

Internet

Subnetwork

Network

Transport

Application

Subnetwork

Network

Transport

Application

Typical Internet Protocol Stack

UDPTCP

Ethernet

Bluetooth

Subnetwork · 1

Network · 2

Transport · 3

Application · 4

Implementation of protocols

TCP module (software)

IP module

IP input buffer (FIFO)

Network Interface Card (NIC)

Process 1 Process 2 Process n

Linux kernel

Transmission media (Copper wires)Frame (signals)

Frame (bits)

Subnetwork

Network

Transport

Application

¨ Defined by IETF (Internet Engineering Task Force)¨ Three main features

¤ Does not imply strict layering. The application is free to bypass the defined transport layers and to directly use IP or other underlying networks

¤ An hour-glass shape – wide at the top, narrow in the middle and wide at the bottom. IP serves as the focal point for the architecture

¤ In order for a new protocol to be officially included in the architecture, there needs to be both a protocol specification and at least one (and preferably two) representative implementations of the specification

Protocols offer services

Protocols and their Services

7-layer OSI Architecture

¨ A Reference Model¨ Not used today¨ Layering is strict

Session

Transport

Network

Datalink

Physical

Presentation

Application

OSI host 1

NETWORK

Network

Datalink

Physical

OSI host 2

Session

Transport

Network

Datalink

Physical

Presentation

Application

Description of Layers

¨ Physical Layer

¤ Handles the transmission of raw bits over a communication link

¨ Data Link Layer

¤ Collects a stream of bits into a larger aggregate called a frame

¤ Network adaptor along with device driver in OS implement the protocol in this layer

¤ Frames are actually delivered to hosts

¨ Network Layer

¤ Handles routing among nodes within a packet-switched network

¤ Unit of data exchanged between nodes in this layer is called a packet

The lower three layers are implemented on all network nodes

OSI Architecture

Session

Transport

Network

Datalink

Physical

Presentation

Application

Description of Layers

¨ Transport Layer¤ Implements a process-to-process channel¤ Unit of data exchanges in this layer is called a message

¨ Session Layer¤ Provides a name space that is used to tie together the potentially

different transport streams that are part of a single application

¨ Presentation Layer¤ Concerned about the format of data exchanged between peers

¨ Application Layer¤ Standardize common type of exchanges

The transport layer and the higher layers typically run only on end-hosts and not on the intermediate switches and routers

OSI Architecture

Session

Transport

Network

Datalink

Physical

Presentation

Application

Strict layering in OSI

A layer just uses the servicesprovided by the layer below

¨ The internal mechanisms of eachlayer remain hidden¤ Layer N+1 knows nothing about the internal

mechanisms of layer N

¨ Example:¤ Transport layer can only use the Network layer

Transport

Datalink

Physical

Network

Application

Presentation

Session

Layering in Internet

A layer may use the servicesprovided by any layer below

¨ The internal mechanisms of eachlayer remain hidden¤ Layer N+1 knows nothing about the internal

mechanisms of layer N

¨ Example:¤ An Application protocol may use whichever lower

Subnetwork

Network

Transport

Application

Protocol: The foreman of a service

Layer N + 1

Layer N

1 · Layer N+1 uses a service fromLayer N

Layer N + 1

Layer N

2 · Layer N at host A sets protocol Nin motion to fulfil the servicerequested by N + 1

Layer N+1 uses a service at Layer N¨ Layer N

¤ Several services: SVC1, SVC2¤ Each service is accessed through its Service Interface: SIF1, SIF2¤ The protocol N (Host A) fulfils the functionality offered by SVC by

exchanging messages with protocol N at Host B¤ These messages comprise the Peer-to-Peer Interface

Protocol N

SIF 2 SIF 3

SVC2 SVC3

Protocol N + 1

Layer N

Layer N+1

Host A Host B

Protocol N

Protocol N + 1

SIF 2 SIF 3

SVC2 SVC3

Peer-to-peerInterface

Layer N+1 uses a service at Layer N¨ Example: Host A runs Windows, host B runs Linux¨ Services at each host are exactly the same¨ However, Service Interfaces might be vastly different in each

system¨ The Peer-to-peer interface must be honored by each instance

of Protocol N

Protocol N

SIF 2 SIF 3

SVC2 SVC3

Protocol N + 1

Layer N

Layer N+1

Host A Host B

Protocol N

Protocol N + 1

SIF 2 SIF 3

SVC2 SVC3

Peer-to-peerInterface

¨ The syntax and the semantics of the messages exchanged by the two peers must follow a formal specification

¨ Normally, we refer to the peer-to-peer interface with the same word: protocol

¨ Protocols of Internet are specified by the IETF¤ RFC: Request For Comments¤ Example: The ICMP protocol is specified in RFC 792

Peer-to-peer interface

Encapsulation and Multiplexing

¨ What information is sent from N+1 to N through the SIF (ServiceInterface)?¤ Protocol N+1 sends a N+1 Data Unit to Protocol N

¤ Protocol N encapsulates the N+1 Data Unit into a fresh N Data Unit:n Payload(N+1) + Header(N)

n This scheme is reproduced at each service use

Layer N+1 N+1 Data Unit

N Data Unit N+1 Data UnitLayer N

Header Payload

Illustration of encapsulation in OSI

Get myfile.txtTransportheader

Networkheader

Datalinkheader

Physicalheader(Preamble)

Internetwork

Get myfile.txt Application

Presentation

Session

Transport

Network

Datalink

Physical

Presentation

Session

Transport

Network

Datalink

Physical

Get myfile.txt

Presentation

Session

Transport

Network

Datalink

Physical

Get myfile.txt

Presentation

Session

Transport

Network

Datalink

Physical

Get myfile.txt

Get myfile.txtGet myfile.txtPayload

Presentation

Session

Transport

Network

Datalink

Physical

Get myfile.txt

Get myfile.txtGet myfile.txtPayload

Presentation

Session

Transport

Network

Datalink

Physical

Get myfile.txt

Get myfile.txtPayload

Presentation

Session

Transport

Network

Datalink

Physical

Get myfile.txt

Presentation

Session

Transport

Network

Datalink

Physical

Get myfile.txt

Get myfile.txtTransport Header

Presentation

Session

Transport

Network

Datalink

Physical

Get myfile.txt

Presentation

Session

Transport

Network

Datalink

Physical

Get myfile.txt

Get myfile.txt is encapsulated into a Transport Protocol Data Unit (TPDU)=Transport Header + Payload(Get myfile.txt)

Encapsulation at layer 4

Presentation

Session

Transport

Network

Datalink

Physical

Get myfile.txt

Encapsulation at layer 4Get myfile.txt Application

Presentation

Session

Transport

Network

Datalink

Physical

Get myfile.txt

Presentation

Session

Transport

Network

Datalink

Physical

Get myfile.txt

Presentation

Session

Transport

Network

Datalink

Physical

Get myfile.txt

Payload

Presentation

Session

Transport

Network

Datalink

Physical

Get myfile.txt

Payload

Presentation

Session

Transport

Network

Datalink

Physical

Get myfile.txt

Payload

Presentation

Session

Transport

Network

Datalink

Physical

Get myfile.txt

NetworkHeader

Presentation

Session

Transport

Network

Datalink

Physical

Get myfile.txt

NetworkHeader

Presentation

Session

Transport

Network

Datalink

Physical

Get myfile.txt

NetworkHeader

Presentation

Session

Transport

Network

Datalink

Physical

Get myfile.txt

NetworkHeader

Presentation

Session

Transport

Network

Datalink

Physical

Get myfile.txt

NetworkHeader

Payload

Presentation

Session

Transport

Network

Datalink

Physical

Get myfile.txt

NetworkHeader

Payload

Presentation

Session

Transport

Network

Datalink

Physical

Get myfile.txt

NetworkHeader

Payload

Presentation

Session

Transport

Network

Datalink

Physical

Get myfile.txt

NetworkHeader

Payload

Presentation

Session

Transport

Network

Datalink

Physical

Get myfile.txt

NetworkHeader

Presentation

Session

Transport

Network

Datalink

Physical

Get myfile.txt

NetworkHeader

Presentation

Session

Transport

Network

Datalink

Physical

Get myfile.txt

NetworkHeader

DatalinkHeader

Presentation

Session

Transport

Network

Datalink

Physical

Get myfile.txt

NetworkHeader

DatalinkHeader

Presentation

Session

Transport

Network

Datalink

Physical

Get myfile.txt

NetworkHeader

DatalinkHeader

Presentation

Session

Transport

Network

Datalink

Physical

Get myfile.txt

NetworkHeader

DatalinkHeader

Application

Presentation

Session

Transport

Network

Datalink

Physical

NetworkHeader

DatalinkHeader

Payload

Application

Presentation

Session

Transport

Network

Datalink

PhysicalGet myfile.txtTransport

HeaderNetworkHeader

DatalinkHeader

Payload

Application

Presentation

Session

Transport

Network

Datalink

PhysicalGet myfile.txtTransport Header

NetworkHeader

DatalinkHeader

Application

Presentation

Session

Transport

Network

Datalink

NetworkHeader

DatalinkHeader

Bit stream to be transmittedover the wire

Physhdr

Preamble(Phys layer

synchronization)

Application

Presentation

Session

Transport

Network

Datalink

NetworkHeader

DatalinkHeader

Physhdr

Preamble(Phys layer

synchronization)

Application

Presentation

Session

Transport

Network

Datalink

NetworkHeader

DatalinkHeader

Physhdr

Preamble(Phys layer

synchronization)

Presentation

Session

Transport

Network

Datalink

Physical

Get myfile.txt

NetworkHeader

DatalinkHeader

NetworkHeader

DatalinkHeader

Physhdr

Preamble(Phys layer

synchronization)

Presentation

Session

Transport

Network

Datalink

Physical

Get myfile.txt

NetworkHeader

DatalinkHeader

NetworkHeader

DatalinkHeader

Physhdr

Preamble(Phys layer

synchronization)

NetworkHeader

DatalinkHeader

PhysicalHeader

(Preamble)

NetworkHeader

DatalinkHeader

PhysicalHeader

(Preamble)

NetworkHeader

DatalinkHeader

PhysicalHeader

(Preamble)

De-encapsulation at layer 1 (Supress the preamble)

Application

Presentation

Session

Transport

Network

Datalink

NetworkHeader

DatalinkHeader

Bit stream received over the wire

Physhdr

Preamble(Phys layer

synchronization)

De-encapsulation at layer 1 (Supress the preamble)

Application

Presentation

Session

Transport

Network

Datalink

NetworkHeader

DatalinkHeader

Physhdr

Preamble(Phys layer

synchronization)

Application

Presentation

Session

Transport

Network

Datalink

NetworkHeader

DatalinkHeader

Application

Presentation

Session

Transport

Network

Datalink

PhysicalGet myfile.txtTransport

HeaderNetworkHeader

DatalinkHeader

Application

Presentation

Session

Transport

Network

Datalink

Physical

NetworkHeader

DatalinkHeader

De-encapsulation at layer 2Application

Presentation

Session

Transport

Network

Datalink

Physical

NetworkHeader

DatalinkHeader

Application

Presentation

Session

Transport

Network

Datalink

Physical

NetworkHeader

Application

Presentation

Session

Transport

Network

Datalink

Physical

NetworkHeader

Application

Presentation

Session

Transport

Network

Datalink

Physical

NetworkHeader

Application

Presentation

Session

Transport

Network

Datalink

Physical

NetworkHeader

De-encapsulation at layer 3

Application

Presentation

Session

Transport

Network

Datalink

Physical

Application

Presentation

Session

Transport

Network

Datalink

Physical

Application

Presentation

Session

Transport

Network

Datalink

Physical

Application

Presentation

Session

Transport

Network

Datalink

Physical

De-encapsulation at layer 4

Application

Presentation

Session

Transport

Network

Datalink

Physical

Get myfile.txt

Application

Presentation

Session

Transport

Network

Datalink

Physical

Get myfile.txt

Application

Presentation

Session

Transport

Network

Datalink

Physical

Get myfile.txt

Application

Presentation

Session

Transport

Network

Datalink

Physical

Get myfile.txt

Application

Presentation

Session

Transport

Network

Datalink

Physical

Get myfile.txt

Application

Presentation

Session

Transport

Network

Datalink

Physical

Get myfile.txt

Application

Presentation

Session

Transport

Network

Datalink

Physical

Get myfile.txt

Application

Presentation

Session

Transport

Network

Datalink

NetworkHeader

DatalinkHeader

Get myfile.txt

NetworkHeader

DatalinkHeader

Get myfile.txt

Physhdr

Preamble(Phys layer

synchronization)

Multiplexing

¨ Transmitter multiplexes several flows by having each layer add its headerwhich contains addressing information

Application

Presentation

Session

Transport

Network

Datalink

Physical Get myfile.txtTransport Header

NetworkHeader

DatalinkHeader

Get myfile.txt

NetworkHeader

DatalinkHeader

Get myfile.txt

PhyshdrGet myfile.txtTransport

HeaderNetworkHeader

DatalinkHeader

Bit stream transmitted over the wire

Get myfile.txt

NetworkHeader

DatalinkHeader

Get myfile.txt

Physhdr

Multiplex Demultiplex

Demultiplexing

¨ Receiver demultiplexes several flows by having each layer analyze its headerwhich contains addressing information about the upper-layer protocol that is toreceive the payload

Application

Presentation

Session

Transport

Network

Datalink

Physical Get myfile.txtTransport Header

NetworkHeader

DatalinkHeader

Get myfile.txt

NetworkHeader

DatalinkHeader

Get myfile.txt

PhyshdrGet myfile.txtTransport

HeaderNetworkHeader

DatalinkHeader

Bit stream transmitted over the wire

Get myfile.txt

NetworkHeader

DatalinkHeader

Get myfile.txt

Physhdr

Multiplex Demultiplex

Computer Networks connect computers; the many more, the better

Connectivity92

Theoretical connectivity

¨ Connectivity is the capacity of connection of a network

¨ If a network has N hosts, its connectivity is: N·(N-1) ≅ N2

¨ Metcalf’s law: The connectivity of a network grows fast as we add more nodes (N2)

Connectivity = 1

Connectivity = 2

Connectivity = 3 x 2 = 6

+ Connectivity = 6 + 6 = 12

6 nodes

Theroretical connectivity is bounded by network technology

¨ Metcalf’s law: Represents the potential connectivity of a network

¨ Nodes communicate by sending/receiving messages through the links which bandwidth is limited

¨ Each node must ‘have some knowledge’ about the topology of the network

¨ These factors limit the use of the potential connectivity

Connectivity = 1

Connectivity = 2

+ Connectivity = 6 + 6 = 12

6 nodes

Scalable connectivity

¨ Not all network technologies use the available connectivity with the same efficiency

¨ Ethernet can function efficiently up to certain network size: we say that Ethernet scales well up to that limit.

¨ Then, how come the Internet has 2000M hosts? How can the Intenet scale to such a huge size so well?¤ IP protocol

Scalable connectivity

End of Ch 1 Section 1

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