Computer Science 457Networking and the InternetFall 2011Dan MasseyWelcome to:

Chapter 1: FoundationsNetworking is a very broad topicHow does a wireless card talk to a base station?How does a message get from ColoState to Amazon?How does an application (web browser, IM, P2P) work?What about reliability, resource constraints, security?First Goal: Learn Big Picture and TerminologyWhat are the basic network components?What are network layers?What is a protocol?

A Sample ApplicationMy Laptop -Running webbrowserWeb Serverwww.cnn.comMy laptop and the web server are both End Systems = Hosts End systems can also include PDAs, sensors, cell phones, and generally any device using the network to communicateEnd systems are located at the network edge and connected to the network using communication links Use web browser to lookup www.cnn.com

Internet

Clients and ServersMy Laptop -Running webbrowserWeb Serverwww.cnn.comEnd systems may be classified as client, a server, both, or neither. Client - runs some program that requests services: web browser requests a page, email reader requests messages, ftp program requests files, etc.Server - runs some program that listens for requests and provides services web server, email server, ftp server, etc.Client vs. server depends on what programs the end system is running.Simple Example: Use web browser to lookup www.cnn.com

Internet

NetworksPoint-to-PointMultiple AccessWireless

Switched NetworksTwo or more nodes connected by a link, orTwo or more networks connected by two or more nodesA network can be defined recursively as...

Logical Network Viewfixed size pipe from her to him perfect for voice reliable conversations (QoS) provisioning, good engineering dumb end points, smart network evolved for 100 years (analog to digital)Quality of Service

Back in the Old Days...(before packet switching)1920s telephony: circuits---a physical wirefrom one end to the otherthe wirethe router(Aunt Mable)

Then Came TDM...muxdemuxTime Division Multiplexing but keeps the idea of afixed pipe (circuit) the rightsize for a telephone conversation

And FDM and CDM...Frequency Division MultiplexingCode Division Multiplexinga a a a a a a a a a aa a a a a a a a a a a

Packet SwitchingInterleave packets from different sourcesEfficient: resources used on demandStatistical multiplexingMultiple applications, bursty traffic

What is a Packet?Self-contained set of bitsIncludes a header and (in most cases) user data (payload)Header: needed by the network - contains control information needed to deliver the packet to the destinationUser data: can be anything network does not care

Example: IP Packet4-bitVersion4-bitHeaderLength8-bit Type ofService (TOS)16-bit Total Length (Bytes)16-bit Identification3-bitFlags13-bit Fragment Offset8-bit Time to Live (TTL)8-bit Protocol16-bit Header Checksum32-bit Source IP Address32-bit Destination IP AddressOptions (if any)Payload20-byteheader

Packet Switching(Internet is just ONE network to use it)differences: packets as low-level component multiple kinds of traffic smart edges, dumb networkbut: QoS is much harder end-points are more expensive

Statistical Multiplexing GainProblem: Link is 1 Mbps; users require 0.1 mbps when transmitting; users active only 10% of the time. How many users can we support?Answer:Circuit switching: can support 10 users.Packet switching: with 35 users, probability that >=10 are transmitting at the same time = 0.0004.

Networking with LayersSub-divide the problemEach layer relies on services from layer below Each layer exports services to layer aboveInterface between layers defines interactionHides implementation detailsLayers can change without disturbing other layers

The Internet Hourglass Design[Deering98]Application LayerProtocol LayerTransport LayerNetwork LayerLink LayerData CommunicationPhysical Layer

IP Suite: End Hosts vs. RoutersHTTPTCPEthernetinterfaceHTTPTCPIPEthernetinterfaceEthernetinterfaceSONETinterfaceSONETinterfacehosthostrouterrouterHTTP messageTCP segmentIP packetIP packetIP packet

Characteristics of Packet SwitchingStore and forwardPackets are self contained unitsCan use alternate paths - reorderingContentionCongestionDelay

Putting the Network TogetherThe network is composed of:Hosts or endpoints (PCs, PDAs, cell-phones, laptops, etc.)Routers (specialized machines that route packets)The distinction is often blurred (caches, content servers, etc.)

Host ConfigurationHost configuration needs:a physical network cable (Ethernet, etc.)an IP address (often automatic with DHCP)a network maska gatewaya DNS server (and other servers)

A NetworkNetwork needs:wires (from the phone or cable company)Router(s)a firewall?an ISP to connect to the Internetnetwork addresses (e.g., 192.168.1.xxx)servers

An ISPISP needs:a (big?) block of addressesconnections to one or more other ISPs, peeringsmultiple routers, probably at exchange pointsservers for your users: mail, web, etc.servers for you: monitoring, etc.competent network admins (recommended)an AUP (Acceptable Use Policy)a lawyer

Idealized Network StructureBackbones, Tier 1(national, global)Regional, Tier2Campus LANs,Business

How Do Computers Find Each Other?Computer1Computer 2

What Are the Different Kinds of Addresses?Have domain name (e.g., www.colostate.edu)Global, human readable nameDNS translates name to IP address (e.g. 128.82.103.106)Global, understood by all networks Finally, we need local net addresse.g., Ethernet (08-00-2c-19-dc-45)Local, works only on a particular type of link

Domain Naming System (DNS)Local DNS serverWhats the IP address for www.colostate.edu?Computer 1It is 128.82.103.106 DNS address manually configured into OS

Finding Ethernet Address:Address Resolution (ARP)EthernetBroadcast: who knows the Ethernet address for 128.82.138.2?(gateway address)EthernetBroadcast: I do, it is08-00-2c-19-dc-45

Sending a Packet Through the Internet

RRRRRHHHHHRRHRRouters send packet to next closest pointH: HostsR: Routers

How Do the Routers Know Where to Send Data?Forwarding tables at each router populated by routing protocols.Original Internet: manually updated.Routing protocols update tables based on cost.Exchange tables with neighbors or everyone.Use neighbor leading to shortest path.

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