COM S 519: Computer Networks Introduction and Overview Jeanna Matthews Spring 2002

Why study computer networks? zThey are engineering marvels! yScalability, layered protocols, lots of subtleties- worthy of study zThey are all around us! yUnderstanding the nuts and bolts behind a technology you use every day is exciting zThey are changing the world! yProfound societal changes taking place in our lifetime

How much do you know already ? zHow much do you know about what happens when you use networking applications? zWhat do you hope to learn in this class?

Internet: Example zClick -> get page zpage from local or remote computer zlink: specifies - protocol (http) - location (

Internet: Locating Resource zwww.cnn.com is the name of a computer (and, implicitly, of a file in that computer) zTo find the address, the application uses a hierarchical directory service called the Domain Name System to translate human readable names to IP addresses

Internet: Connection zThe protocol (http) sets up a connection (another protocol, tcp) between the host and cnn.com to transfer the page zThe connection transfers the page as a byte stream, without errors: flow control + error control

Internet: End-to-end zThe byte stream flows from end to end across many links and switches: routing (+ addressing) zThat stream is regulated and controlled by both ends: retransmission of erroneous or missing bytes; flow control

Internet: Packets zThe network transports bytes grouped into packets zThe packets are “self- contained” and routers handle them one by one zThe end hosts worry about errors and pacing: Destination sends ACKs Source checks losses

Internet: Port Numbers zWhen a packet arrives at its destination, the operating system uses the destination port number to identify which application should receive it. zThis is called demultiplexing.

Internet: Bits zEquipment in each node sends the packets as a string of bits zThat equipment is not aware of the meaning of the bits

Internet: Points to remember zSeparation of tasks ysend bits on a link: transmitter/receiver [clock, modulation,…] ysend packet on each hop [framing, error detection,…] ysend packet end to end [addressing, routing] ypace transmissions [detect congestion] yretransmit erroneous or missing packets [acks, timeout] yfind destination address from name [DNS] zScalability yrouters don’t know full path ynames and addresses are hierarchical

Internet : how does it do: zAddressing ? zRouting ? zReliable transmission ? zInteroperability ? zResource management ? zQuality of service ?

Concepts at the heart of the Internet zProtocol zLayered Architecture zPacket Switching zDistributed Control zOpen System

Protocol zIf two entities are going to communicate, they must agree on the expected order and meaning of messages they exchange. zAsking for the time protocol

Protocol A protocol defines the format and the order of messages exchanged between communicating entities as well as the actions taken on the receipt or the transmission of a message.

Layered Architectures zHuman beings are able to handle lots of complexity in their protocol processing. yAmbiguously defined protocols yMany protocols all at once zHow do computers manage complex protocol processing? ySpecify well defined protocols to enact. yDecompose complicated jobs into layers that each have a well defined task

Layered Architectures zBreak-up design problem intro smaller, more manageable problems. zModular design: easy to extend/modify. zDifficult to implement (careful with interaction of layers for efficiency).

Layered Architecture Applications Middleware Routing Physical Links users network Web, , file transfer,... Reliable/ordered transmission, QOS, security, compression,... End-to-end transmission, resource allocation, routing,... Point-to-point links, LANs, radios,...

Internet protocol stack Application Transport Network Physical users network HTTP, SMTP, FTP, TELNET, DNS, … TCP, UDP. IP Point-to-point links, LANs, radios,...

Air travel Ticket (purchase) Baggage (check) Gates (load) Runway (take off) Passenger Origin Ticket (complain) Baggage (claim) Gates (unload) Runway (landing) Passenger Destination Airplane routing

Protocol stack client TCP server IP server ethernet driver/card user X SMTP TCP IP server TCP server IP server ethernet driver/card user Y IEEE standard electric signals English

Protocol encapsulation client TCP server IP server ethernet driver/card user X server TCP server IP server ethernet driver/card user Y “Hello”

Protocol interfaces client TCP server IP server ethernet driver/card user X server TCP server IP server ethernet driver/card user Y s = open_socket(); socket_write(s, buffer); …

Packet Switching zPackets indicate their destination zNo predetermined path for a packet to take zEach intermediate note routes the packet closer to its destination

A small Internet A V R B W r1,e1 r2,e2 r3 a,e3 w,e5 b,e4 Scenario: A wants to send data to B.

Protocol stack: packet forwarding HTTP TCP IP ethernet Host A IP ethernet Router R link HTTP TCP IP ethernet Router W Host B IP ethernetlink

Distributed Control zWhere ever possible decompose the problem zExamples: yNo one central name to IP address data base - Domain Name System yNo one global routing table - Heirarchical network of networks – handle routing within small autonomous systems zEssential to Scalability

Open System zBasic Internet protocols are published as open standards zStandards freely and readily available zIdeal candidate for study

Internet Standards Process zInternet Draft - anyone zRFC - at discretion of RFC Editor zInternet Standard Maturity Levels yProposed - IESG yDraft - 2 independent, implementations yStandard

How is Internet “governed”? z“We reject kings, presidents, and voting. We believe in rough consensus and running code” zInternet Engineering Task Force (IETF) zWorking groups zInternet Engineering Steering Group (IESG) zInternet Assigned Numbers Authority (IANA)

Backbones, NAPs and ISPs zWould have had time to add this here

Online Resources zISOC zACM zCAIDA zInternet History zW3C