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BITS Pilani Pilani | Dubai | Goa | Hyderabad

Computer Networks Lecture-3 January 12, 2012

Rahul Banerjee, PhD (CSE)

Professor, Department of Computer Science & Information Systems

E-mail: [email protected]

BITS Pilani, Deemed to be University under Section 3 of UGC Act, 1956

A recap of the required and desirable features of / services by Computer Networks

About Protocols, Layers, Interfaces, Logical / Virtual Communication & Services

Of Network Architectures & Network Reference Models Examples Architecture of the Internet What is the Internet today? The Internet and the World-Wide Web Who decides about the Internet? Of The Internet, Intranet and Extranet Select References to the literature Summary

Interaction Points

(c) Dr. Rahul Banerjee, BITS Pilani, INDIA


•  Cost-effective and preferably controllable Resource Sharing •  Includes:

•  File-‐sharing, data-‐sharing, printer-‐sharing, display-‐sharing, computaCon-‐capability sharing, storage-‐sharing etc.

•  Support for common services, •  Bandwidth-‐sharing

•  Scalability in terms of both connectivity and performance •  Desired levels of configurable security (computationally

secure variant is what we aim for, where required) •  Manageability

•  In terms of monitoring, resource management, security management etc.

Required & Desirable Features of Computer Networks


4

Protocols, Services and Interfaces •  Protocol: A set of rules and conventions that allow involved

entities to communicate meaningfully –  formatting / encoding / decoding included, as required

•  Channel: A path of communication –  application-level logical / virtual communication path

•  Services: Cohesive set of functionalities provided by a layer / protocol / entity

•  Interfaces: A mechanism that allows communication between a pair of functional building blocks (like layers or modules or entities) using well-defined set of parameters involving functions calls or equivalents –  Peer-to-Peer / Layer-to-Layer / entity-to-entity / module-to-module

•  Service Access Points: defined addresses / ports through which data / parameters are passed

•  Tunneling: Placing content into an enveloped way and passign it through a set of devices / entities without their requiring to know what is inside the envelope –  involves Encapsulation & Decapsulation, may involve encryption and

decryption as well 12/01/12 (c) Dr. Rahul Banerjee, BITS Pilani, INDIA 4

Network Architecture & Reference Models

•  Architecture versus Reference Model: A simplistic perspective: – Architecture: It may be seen as a detailed

generic blueprint with unambiguous definitions of services, interfaces, organization and defined protocols that helps in design and implementation of a set of relevant protocol stack / suite based network / internetwork

– Reference Model: It is the same as the architecture minus the specifically defined readily usable protocols.

Thursday 12 January 12 5 (c) Dr. Rahul Banerjee, BITS-‐Pilani, INDIA

An Example of the Network Architecture

•  Example-1: –  TCP/IP Architecture &

TCP/IP Reference Model

•  Other Examples: –  OSI Reference Model &

OSI Architecture –  ATM Reference Model

& ATM Architecture –  Our own Hypothetical

Reference Model (slide-5)

Thursday 12 January 12 6 (c) Dr. Rahul Banerjee, BITS-‐Pilani, INDIA

LLC Sub-layer

MAC Sub-layer

Physical layer

7

Data Link Layer

Physical Layer

Transport Layer

Network Layer

Applica3on Layer

Presenta3on Layer

Session Layer

The ISO OSI Reference Model

(c) Dr. Rahul Banerjee, BITS-‐Pilani, INDIA

8

The ISO OSI Reference Model


Copyright: Dr. Rahul Banerjee BITS, Pilani (India) 9

Data Link Layer

Physical Layer

Transport Layer

Network Layer

Applica3on Layer

A Hypothetical Network Reference Model for Easy Conceptual Understanding

Layer-‐5

Layer-‐4

Layer-3

Layer-2

Layer-1

Often on the NIC card or chip

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A Simplified Network Reference Model <for Instruction>

Host-1 Host-2

Application Layer Application Layer

Upper Layer-‐to-‐ Lower Layer Interface Upper Layer-‐to-‐ Lower Layer Interface



Same Layer -to- Same Layer Virtual Communication Interface




Same Layer -to- Same Layer Physical Communication Interface



Application Layer •  Application Layer is a layer of the Network Architecture

that is primarily concerned with getting TPDU from the lower layer (usually Transport Layer) and delivering it to the Application and vice-versa (with or without explicit presentation and session management support).

•  Examples: HTTP, DHCP, DNS, SNMP, FTP (in the context of the TCP/IP Architecture).

•  Web-services, Video-on-Demand over the network, Video/Voice-conferencing over the network etc. are examples of Applications that reside atop the protocols belonging to this layer.


Application Layer Responsibilities

•  It primarily deals with: –  Accepting messages from

the Application Layer through the APIs

–  Processing these messages and generating APDUs

–  Deciding transport connection requirements (for further transmitting this DU after encapsulating it within an APDU)

–  Passing this packet through the SAP to the lower layer (TL)

n  It also deals with ... q  AccepCng APDU from the lower

layer through the SAP q  Processing the APDU q  Removing the encapsulaCon and

passing the messages to the respecCve desCnaCon applicaCon

q  Provide diagnosCc support for network monitoring, configuraCon, management and trouble-‐shooCng at the ApplicaCon Layer or lower layer

(c) Dr. Rahul Banerjee, BITS, Pilani, India 13

Transport Layer: What is it?

•  Transport Layer is a layer of the Network Architecture that is primarily concerned with: – getting TPDU from the upper layer (usually

Application Layer) and – delivering it to the same layer at the intended

destination node (through the underlying Network Layer).

•  Converse is also true of the targeted set of responsibilities of this layer.


Transport Layer Responsibilities <another perspective>

•  It primarily deals with: –  Accepting APDU from the

Application Layer through the Service Access Point (SAP)

–  Processing these APDU –  Deciding transport

connection requirements (for further transmitting this DU after encapsulating it within a TPDU)

–  Passing this packet through the SAP to the lower layer (NL)

n  It also deals with ... q  AccepCng TPDU from the lower

layer through the SAP q  Processing the TPDU q  Removing the encapsulaCon and

passing the messages to the respecCve desCnaCon applicaCon

q  Provide diagnosCc support for network monitoring, configuraCon, management and trouble-‐shooCng at the ApplicaCon Layer or lower layer


Network Layer •  Network Layer is primarily concerned with getting NLDU / Packets from

the source node and delivering it to the intended destination node (through none or many intermediate nodes).

•  Additional responsibilities of this layer include: –  Providing support for connection-oriented / connectionless services as the

case may be (depending upon the protocol stack and need) –  Provide diagnostic support for network monitoring, configuration,

management and trouble-shooting at the Network Layer or higher layer. •  Packet handling, packet management, Routing are its major

responsibilities.

n  In the context of packet rouCng, network layer structural design goals include: q  Ensuring the shortest possible delay and thereby the highest throughput at the least

possible cost q  Ensuring acceptably reliable packet delivery (may be opConal in some cases) q  Ensuring secure packet delivery (may be opConal in some cases)


Data Link Layer

•  Data Link Layer consists of two sub-layers: –  Media Access Control (MAC) sub-layer & –  Logical Link Control (LLC) sub-layer.

•  Major Issues involved in the design of the Data Link Layer include: –  Which services are to be provided to each of the adjacent

layers? –  Exactly when to provide these services? –  How to provide them? –  To whom should they be provided?


Physical Layer

•  Physical Layer deals with transmission of raw digital data using analog or digital signal.

•  This layer is concerned with the logic type (negative or positive), amplitude of the signal, signal representation, bit-length, direction of transmission etc.

•  It deals with connection-establishment and termination.

n  This layer is, in a nutshell, a layer that deals with various electrical and mechanical characterisCcs of every physical component of a computer network.

n  Exact electrical, mechanical and procedural Interface DefiniCon is therefore its responsibility.

n  Choice and use of the physical medium are the Physical Layer Design Issues.


A Few More Networking Terms •  Repeaters / Repeater Hubs / Shared Hubs: where usually Physical

layer / level exist with L1-protocol data unit (raw bits) regeneration and onward transmission

•  Managed Hubs / Layer-2 Switching Hubs: where Physical and Data Link layers / levels exist with ability to handle and deliver Layer-2-protocol data unit (frame)

•  Bridges: where Physical and Data Link layers / levels exist with L2-protocol data unit (frame) processing and forwarding

•  Switches: where Physical and Data Link and / or Network (sometimes even higher) layers / levels exist with Layer-2 and / or Layer-3-protocol data unit (frame / packet) processing, switched routing / forwarding

•  Routers: where Physical and Data Link and Network layers / levels exist with L3-protocol data unit (packet) processing, routing and forwarding

•  Gateways: where two or more different networks meet and may require protocol / message translation capabilities

•  Clouds: abstraction of node connectivity in the networking context <details hidden>

What is an Internetwork?

12/01/12 (c) Dr. Rahul Banerjee, SDET Unit, BITS-‐Pilani, INDIA 19 12/01/12 19

(c) Dr. Rahul Banerjee, BITS Pilani, INDIA 20

Of the Internet, Intranet and Extranet

•  The Global Public Internetwork: The Internet •  The Wholly Owned / Private Internetwork:

Intranet •  The Hybrid Internetwork-- private networks /

internetworks connected through the Internet: Extranet In the early stages of development, technologies used for the internetworks of all type were essentially the same, except probably at the lowest level. This situation is rapidly changing.

12/01/12

12/01/12 (c) Dr. Rahul Banerjee, BITS Pilani, INDIA 21

Architecture of the Internet

•  Originally, it was a point-to-point WAN.

•  Original architecture that led to ARPANET has evolved over the years that have passed by.

•  It is loosely hierarchical. •  Currently, Internet

architecture is largely governed by the IAB of the ISoc.

•  Has many sub-organs which facilitate evolution and coordinated maintenance of the Internet.

•  IESG steers the ISoc in a general way the engineering issues are resolved.

•  IETF workgroups do the ground work and by a democratic process helps community in building up engineering solutions through IETF drafts and standards (RFCs) etc.


What is the Internet today? •  Wide Area Network of variety of networks •  Global •  Public •  Not transparent, as yet •  Hybrid topology but largely hierarchical •  No single controller •  Internet Society (ISoc) oversees, assists --- does

not control •  QoS, Security continue to have issues – partly at

least •  Web, mail, commerce, education, entertainment,

sharing continue to dominate its application space


InteracCon Points •  Examples of Types of Applications benefitting from Networking

–  hard real-time, soft real-time, non-real-time / best-effort / delay-tolerant applications / services <with examples>

–  case-study movie –  Constituent networking components of a smart room setup

•  The Internet & its Evolution •  About Internet Architecture •  Who decides about the Internet? •  The Internet versus the World-Wide Web •  Protocols, Layers, Interfaces, Virtual Communication and

Services •  Select References to the literature •  Questions and Answers / Summary


ConvenConal ClassificaCon of Computer Networks

•  Class One: Function-based classification

•  Data Networks •  Voice Networks •  Multimedia Networks ……

•  Class Two: Location-and-Distance-based classification

•  Personal Area Networks (PANs)

•  Local Area Networks (LANs)

•  Metropolitan Area Networks (MANs)

•  Wide Area Networks (WANs) ….

• Class Three: Forwarding-‐based classificaCon

•  Switched Networks •  Circuit-‐Switched Networks •  Packet-‐Switched Networks

•  Shared Networks • Hybrid Networks

• Class Four: Ownership-‐based classificaCon

•  Public Networks •  Private Networks • Virtual Private Networks

12/01/12

25

Comparing Computer Networks with Distributed Systems

•  Terms Computer Network and Distributed System must NOT be used interchangeably since: –  In the former, locations and elements of network

remain visible to the user; –  In the latter, the underlying network remains

transparent to the user who sees the system as a large uni-processor system.

•  Similar differences can be cited in case of Network Operating

Systems and Distributed Operating Systems.

12/01/12 (c) Dr. Rahul Banerjee, BITS Pilani, INDIA


Local Area Internetwork / Intranet •  Traditionally, a Campus Internetwork is a campus-wide

internetwork of individual LANs which may be geographically spread over the part or whole of a single campus. This sometimes called campus intranet.

•  In common practice, the entire campus internetwork including its communication subnet is wholly owned by a single organization or institution.

•  Usually, the campus internetworks use LAN technology; however, it is possible to use WAN technology, when so desirable.

•  The latter may be desirable in some cases when the campus is very large and comprises of a vast set of buildings spread over it. Protocols used in both of these cases at the lower layers, are, generally, different. 12/01/12

Robert Metcafe’s Ethernet •  The original Ethernet protocol proposed and

implemented by Robert Metcafe was actually based on 1-p CSMA/CD protocol

•  It did have a scheme that allowed greedy access to the channel by a station which led to lower efficiency

•  Schemes like Random Back-off / Exponential Back-off were devised to improve the efficiency to a certain extent

12/01/12 (c) Dr. Rahul Banerjee, SDET Unit, BITS-Pilani, INDIA 27

An Ethernet LAN

28 (c) Dr. Rahul Banerjee, BITS, Pilani, India

Personal Computer

WorkstaCon

WorkstaCon Workstation


Another Form of a Simple LAN


The Shared Ethernet hub

Personal Computer

Network Printer

WorkstaCon

WorkstaCon

Laptop Computer

WorkstaCon

Tablet PC


Frames: Factors that ma^er!

•  Synchronization: Transmitter & Receiver need to be in sync

•  Start Delimiter: Required to mark starting bit •  End Delimiter: Required to mark the end bit •  Control Information: Information suggesting data

handling and interpretation •  Error Detection / Correction / Retransmission •  Flow Control: Required for avoiding data loss

due to overflow at receiving end •  Data Length: Needed if data-field is not of fixed

size 30 (c) Dr. Rahul Banerjee,

BITS, Pilani, India (c) Dr. Rahul Banerjee, BITS Pilani, INDIA

A Sample Frame Format

n-‐Byte Preamble Start-‐of -‐Frame Delimiter DesCnaCon Add. Source Address Length of Data

Data Field Pad Field

Checksum



Network Elements of a Node •  A typical network node has following

hardware elements of relevance, at the least: – Processor (CPU) with / without registers / caches – Optional External Cache(s) – RAM (Main Memory) – ROM / PROM / EPROM / EEPROM / EAPROM – Optional Secondary / Tertiary Memory / Storage

(Flash, Disk, MBM etc.) – Network Adapter / Network Interface Controller – Slots / Ports for connectivity to other node(s) – Power-provisioning – Bus / Lines (Control, Address, Data, Power)


Elements of a Network Interface Controller / Network Adapter

•  A Network Adapter / Interface Controller Unit / Chip / Dongle often comprises of the following elements: –  Host bus / line / link –  Control Status Registers (often called CSR or simply even as Control

Registers) •  logically readable / writable by the CPU

–  --à often, a copy of the contents of the CSR is located in some pre-specified location in memory making it simple for CPU to perform R/W operations, as per need

»  --à Actual writing to the NIC’s CSR is done by the Device Driver though –  Bus Interface Unit –  Internal storage (buffer included) –  Transceivers for transmission and reception at the physical level

•  Data Transfer Methods: DMA (no worry for the CPU) or Programmed I/O (PIO) based Data Transfer (CPU needs to work herein) from the memory of the host node to the NIC / Adapter


A DiagrammaCc View of NIC

h^p://www.plxtech.com/images/about/news/images/image042505.gif

h^p://commons.wikimedia.org/wiki/File:Ethernet_NIC_100Mbit_PCI.jpg

h^p://www.dansdata.com/images/gigabit/gbcard440.jpg

h^p://www.altera.co.jp/products/ip/ampp/morethanip/images/m-‐mCp-‐10g_etherpcs_fig1.gif

Examples of Ethernet Adapters

Source: h^p://www.altera.co.jp/products/ip/ampp/morethanip/images/m-‐mCp-‐10g_etherpcs_fig1.gif (c)


Based on IEEE documents with instructional modifications Copyright: IEEE Inc., N.Y.

The IEEE 802.x Architecture and Specifications revisited

Operation of a Bridge with Two LANs

37

Source Destination

S

H

H H H

H H

H

Hub



An Example of a Switched Intranet

Fixed Lasers

Electronic Switches

GxG MEMS

Group 1

LxM Crossbar

Linecard 1

Linecard 2

Linecard L

Group 2

LxM Crossbar

Linecard 1

Linecard 2

Linecard L

LxM Crossbar

Linecard 1

Linecard 2

Linecard L

Group G

MxL Crossbar

Linecard 1

Linecard 2

Linecard L

Electronic Switches

Optical Receivers

Group 1

MxL Crossbar

Linecard 1

Linecard 2

Linecard L

Group 2

MxL Crossbar

Linecard 1

Linecard 2

Linecard L

Group G

GxG MEMS

GxG MEMS

GxG MEMS

1

2

3

M

Static MEMS

1

2

3

M

1

2

3

M

1

2

3

M

1

2

3

M

1

2

3

M

1

2

3

M

An Example of a Hybrid Switching Fabric

© Dr. Nick McKeown, Stanford University <modified version>

Cisco Catalyst 4500 Series of Switches Cisco Catalyst 4500 Series Switches

(Used in several places in the Institute as Distribution Switches)

© Cisco, Inc.

Cisco Catalyst 6500 Series of Switches Cisco Catalyst 6500 Series

Switches

(One of these is OUR Core Switch located in the IPC

System Room)

© Cisco, Inc.

44

A Bus Topology based Computer Network

SHARED BUS

(c) Dr. Rahul Banerjee, BITS-Pilani, INDIA

N1 N2 N3 N4


45

A Ring Topology based Computer Network

C

C

C

C

C



46

A Ring Topology based Computer Network

C

C

C

C

C



47

A Tree Topology based Computer Network

NC1 NC2

NC11

NR

NC21

NC22

NC12



48

A Star Topology based Computer Network

C

C

C

Switch


S

N1

N2

N3

N4



Summary of Network Topologies •  Bus Topology

–  Shared –  Switched

•  Tree Topology •  Ring Topology

–  Single –  Double

•  Star Topology •  Irregular Topology •  Complete Topology


Summary •  Intranet: Completely private network of networks

•  Wireline •  Wireless

– Fixed – Mobile

•  Hybrid •  The Internet: Global public network of networks

•  Wireline •  Wireless

– Fixed – Mobile

•  Hybrid •  Extranet: Intranets interconnected via the Internet

Thursday 12 January 12 50 (c) Dr. Rahul Banerjee, BITS-‐Pilani, INDIA (c) Dr. Rahul Banerjee, BITS Pilani, INDIA 50


Project BITS-Connect 2.0

References •  Larry L. Peterson & Bruce S. Davie: Computer Networks: A Systems Approach,

Fifth Edition, Morgan Kaufmann / Elsevier, New Delhi, 2011. <System design approach>

•  S. Keshav: Computer Networking: An Engineering Approach, Pearson Education, New Delhi, 1997.

•  A. S. Tanenbaum: Computer Networks, Fifth Edition, Pearson Education, New Delhi, 2012. <Conceptual Approach>

•  Y. Zheng and S. Akhtar: Networks for Computer Scientists and Engineers, Oxford University Press, New York, 2002. <Structural approach>

•  A. Leon Garcia and I. Widjaja: Communication Networks: Fundamental Concepts and Key Architectures, Second Edition, Tata McGraw-Hill, New Delhi, 2004.

•  Mohammed G. Gouda: Elements of Network Protocol Design, Wiley Student Edition, John Wiley & Sons (Pte.) Ltd., Singapore, 2004.

•  Thomas G. Robertazzi: Computer Networks and Systems: Queuing Theory and Performance Evaluation, Third Edition, Springer-Verlag, New York, 2000. <Analytical approach>

© Dr. Rahul Banerjee, BITS, Pilani (India) 12/01/12 (c) Dr. Rahul Banerjee, BITS Pilani, INDIA 52

BITS Pilani Pilani | Dubai | Goa | Hyderabad

Thank you!

Rahul Banerjee

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