Introduction 1-1

COSC6377: Computer Networks

Rong Zhengrzheng@cs.uh.edu

Computer Networking: A Top Down Approach Featuring the Internet, 3rd edition. Jim Kurose, Keith RossAddison-Wesley, July 2004.

Introduction 1-2

Introduction

Overview of the course Basic concepts and structures in

computer networking Network architecture

Introduction 1-3

What’s the Internet: “nuts and bolts” view End systems

Host computer Network applications

Access networks Local area networks communication links

Network core: routers network of networks

local ISP

companynetwork

regional ISP

router workstation

servermobile

Introduction 1-4

What’s the Internet: “nuts and bolts” view Protocols define format, order

of msgs sent and received among network entities, and actions taken on msg transmission, receipt e.g., TCP, IP, HTTP, FTP, PPP

Internet: “network of networks” loosely hierarchical public Internet versus private

intranet Internet standards

RFC: Request for comments IETF: Internet Engineering

Task Force

local ISP

companynetwork

regional ISP

router workstation

servermobile

Introduction 1-5

Network Components (Examples)

Fibers

Coaxial Cable

Links Interfaces Switches/routers

Ethernet card

Wireless card

Large router

Switch

Introduction 1-6

Introduction 1-7

Internet structure: network of networks

roughly hierarchical at center: “tier-1” ISPs (e.g., MCI, Sprint, AT&T,

Cable and Wireless), national/international coverage treat each other as equals

Tier 1 ISP

Tier 1 ISP

Tier 1 ISP

Tier-1 providers interconnect (peer) privately

NAP

Tier-1 providers also interconnect at public network access points (NAPs)

Introduction 1-8

Tier-1 ISP: e.g., Sprint

Sprint US backbone network

Seattle

Atlanta

Chicago

Roachdale

Stockton

San Jose

Anaheim

Fort Worth

Orlando

Kansas City

CheyenneNew York

PennsaukenRelay

Wash. DC

Tacoma

DS3 (45 Mbps)OC3 (155 Mbps)OC12 (622 Mbps)OC48 (2.4 Gbps)

Introduction 1-9

Internet structure: network of networks

“Tier-2” ISPs: smaller (often regional) ISPs Connect to one or more tier-1 ISPs, possibly other tier-2 ISPs

Tier 1 ISP

Tier 1 ISP

Tier 1 ISP

NAP

Tier-2 ISPTier-2 ISP

Tier-2 ISP Tier-2 ISP

Tier-2 ISP

Tier-2 ISP pays tier-1 ISP for connectivity to rest of Internet tier-2 ISP is customer oftier-1 provider

Tier-2 ISPs also peer privately with each other, interconnect at NAP

Introduction 1-10

Internet structure: network of networks

“Tier-3” ISPs and local ISPs last hop (“access”) network (closest to end systems)

Tier 1 ISP

Tier 1 ISP

Tier 1 ISP

NAP

Tier-2 ISPTier-2 ISP

Tier-2 ISP Tier-2 ISP

Tier-2 ISP

localISPlocal

ISPlocalISP

localISP

localISP Tier 3

ISP

localISP

localISP

localISP

Local and tier- 3 ISPs are customers ofhigher tier ISPsconnecting them to rest of Internet

Introduction 1-11

Internet structure: network of networks

a packet passes through many networks!

Tier 1 ISP

Tier 1 ISP

Tier 1 ISP

NAP

Tier-2 ISPTier-2 ISP

Tier-2 ISP Tier-2 ISP

Tier-2 ISP

localISPlocal

ISPlocalISP

localISP

localISP Tier 3

ISP

localISP

localISP

localISP

Introduction 1-12

Protocol “Layers”Networks are

complex! many “pieces”:

hosts routers links of various

media applications protocols hardware,

software

Question: Is there any hope of organizing structure of

network?

Or at least our discussion of networks?

Introduction 1-13

An Imaginary 2-tier

New application has to interface to all existing media adding new application requires O(m) work, m = number of media

New media requires all existing applications be modified adding new media requires O(a) work, a = number of applications

Total work in system O(ma) eventually too much work to add apps/media

Application end points may not be on the same media!

SMTP SSH FTP

Packetradio

Coaxial cable

Fiberoptic

Application

TransmissionMedia

HTTP

Introduction 1-14

Solution: Indirection Solution: introduce an intermediate layer that provides

a single abstraction for various network technologies O(1) work to add app/media Indirection is an often used technique in computer science

SMTP SSH NFS

802.11LAN

Coaxial cable

Fiberoptic

Application

TransmissionMedia

HTTP

Intermediate layer

Introduction 1-15

Network Architecture

Architecture is not the implementation itself

Architecture is how to “organize” implementations

what interfaces are supportedwhere functionality is implemented

Architecture is the modular design of the network

Introduction 1-16

Software Modularity

Break system into modules:Well-defined interfaces gives flexibility

can change implementation of modulescan extend functionality of system by

adding new modulesInterfaces hide information

allows for flexibilitybut can hurt performance

Introduction 1-17

Network Modularity

Like software modularity, but with a twist:Implementation distributed across

routers and hostsMust decide both:

how to break system into moduleswhere modules are implemented

Introduction 1-18

Layering

Layering is a particular form of modularization

The system is broken into a vertical hierarchy of logically distinct entities (layers)

The service provided by one layer is based solely on the service provided by layer below

Rigid structure: easy reuse, performance suffers

Introduction 1-19

ISO OSI Reference Model ISO – International Standard Organization OSI – Open System Interconnection Seven layers

Lower two layers are peer-to-peer Network layer involves multiple switches Next four layers are end-to-end

Application

Presentation

Session

Transport

Network

Datalink

Physical

Application

Presentation

Session

Transport

Network

Datalink

Physical

Network

Datalink

Physical

Physical medium A Physical medium B

Host 1 Intermediate switch Host 2

Introduction 1-20

Key Concepts

Service – says what a layer does Ethernet: unreliable subnet

unicast/multicast/broadcast datagram service IP: unreliable end-to-end unicast datagram service TCP: reliable end-to-end bi-directional byte stream

service

Service Interface – says how to access the service E.g. UNIX socket interface

Protocol – says how is the service implemented a set of rules and formats that govern the

communication between two peers

Introduction 1-22

Functions of the Layers Service: Handles details of application

programs. Functions:

Service: Controls delivery of data between hosts.

Functions: Connection establishment/termination,

error control, flow control, congestion control, etc.

Service: Moves packets inside the network. Functions: Routing, addressing, switching, etc.

Service: Reliable transfer of frames over a link. Functions: Synchronization, error control, flow

control, etc.

telnet, ftp, emailwww, NFS

TCP, UDP

IP, ICMP, OSPFRIP, BGP

Ethernet, WiFiT1

ApplicationLayer

TransportLayer

NetworkLayer

(Data) LinkLayer

Introduction 1-23

Internet Protocol Architecture

FTPprogram

TCP

IP

EthernetDriver

EthernetDriver

ATMDriver

IP

FTPprogram

TCP

IP

ATMDriver

FTP protocol

TCP protocol

IP protocol IP protocol

Ethernetprotocol

ATMprotocol

Introduction 1-24

Internet Protocol Architecture

MPEG Servierprogram

UDP

IP

EthernetDriver

EthernetDriver

ATMDriver

IP

MPEG Playerprogram

UDP

IP

ATMDriver

RTP protocol

UDP protocol

IP protocol IP protocol

Ethernetprotocol

ATMprotocol

Introduction 1-25

messagesegment

datagram

frame

sourceapplicatio

ntransportnetwork

linkphysical

HtHnHl M

HtHn M

Ht M

M

destination

application

transportnetwork

linkphysical

HtHnHl M

HtHn M

Ht M

M

networklink

physical

linkphysical

HtHnHl M

HtHn M

HtHnHl M

HtHn M

HtHnHl M HtHnHl M

router

switch

Encapsulation

Introduction 1-26

Hourglass

Note: Additional protocols like routingprotocols (RIP, OSPF) needed to makeIP work

Introduction 1-27

Implications of Hourglass

A single Internet layer module:Allows all networks to interoperate

all networks technologies that support IP can exchange packets

Allows all applications to function on all networks

all applications that can run on IP can use any network

Simultaneous developments above and below IP

Introduction 1-28

Reality

Layering is a convenient way to think about networks

But layering is often violatedFirewallsTransparent cachesNAT boxes

Introduction 1-29

Thinking Exercise – Interplanetary Internet Network entities:

Ground stations Satellites, space stations at

near-Earth orbit 600 kilometers (~ 2s)

Mars rovers take ~ 10 to 30 min

Planets farther away? Objective:

Interconnect with earth-bound networks?

http://www.spectrum.ieee.org/WEBONLY/publicfeature/aug05/0805inte.html