Computer Networks (9.1)  Fundamental terminology  Network

Computer Networks (9.1) Fundamental terminology Network standards organizations Standard network hardware examples – ethernet – token ring – ATM Future issues Protocols OSI layers and internetworking – bridging and routing Web-based commerce

Computer Networks (9.2) Networks are simply a set of computers connected by a communication medium, communicating by established conventions – typically bursty transmission characteristics Network goals – resource sharing » expensive equipment » large, reliable data storage – high reliability » replication of services » although network may be single point of failure – lower overall system costs with better performance » a few shared high-end machines, many individual low-end machines

Computer Networks (continued) (9.3) Network categories – local area networks (LAN) » typically single transmission – wide area networks (WAN) » typically store and forward – interconnected networks (internets)

Fundamental Terminology (9.4) Circuit switched / packet switched – circuit switched telephone » connection route established at start of communication, maintained as long as communication continues » route is dedicated to one communication – packet switched » whole communication is broken down into smaller chunks called packets » each packet may take a different route to the destination » routes may be shared by other communications

(9.5) Fundamental Terminology (continued) Another categorization of packet switched networks is – virtual circuit » packets all take same route, set up in advance – datagram » each packet routed independently, no connection established » typically more robust

(9.6) Fundamental Terminology (continued) Packets typically consist of – header » source and destination addresses » sequence number – data portion – trailer » error detecting and correcting information Packets may be fixed or variable length head data tail

Network Standards Organizations (9.7) International Standards Organization (ISO) – makes standards for many different activities American National Standards Institute (ANSI) – US representative to ISO CCITT – one part of the UN agency International Telecommunications Union – concerned with telephone and data communication services – US representative is the State Department

(9.8) Network Standards Organizations (cont.) National Instituted of Standards and Technology (NIST) – standards body for US government purchases Institute of Electrical and Electronics Engineers (IEEE) – key standards for LANs Internet Engineering Task Force (IETF)

Local Area Network Standards (9.9) Ethernet (IEEE 802.3) – Carrier Sense Multiple Access / Collision Detect (CSMA/CD) » listen to cable, no one transmitting go ahead » if collide, delay random time and retransmit – multiple topologies – multiple media » coaxial cable (thick or thin) » twisted pair wire (10BaseT) – minimum packet size 64 bytes » heavy overhead for one byte of data – fast ethernet technologies allow 100 Mbps on twisted pair » 1000 Mbps now available on optical fiber

(9.10) Local Area Network Standards (continued) Token ring (IEEE 802.5) – token determines which host gets to transmit » special bit pattern used for token » when station wants to transmit watch network for token take token off ring and transmit packet drain packet as it comes around again put token back on ring when done – single topology – one node monitors ring to regenerate token if necessary

(9.11) Local Area Network Standards (continued) Ethernet – faster access if light load – degrades rapidly as load increases » 60% utilization is a problem – no guaranteed access » host can “starve” – most popular LAN technology

(9.12) Local Area Network Standards (continued) Token ring – minimum delay to transmit, since have to wait for token – delivers much better performance under heavy load » can reach close to 100% utilization – fair » host can hold token for 10 msec, then must put back on network » each host gets token at least every 10 * number of hosts msec – “sick” monitor station can cause problems

Frame Relay and ATM Frame Relay – virtual circuit organization – uses leased or private lines » vc set up at time subscribers are connected to the network – “light” protocol » no error checking by network, just delivery mechanism – 64 Kbps to 45 Mbps (9.13)

ATM (9.14) Asynchronous Transfer Mode (ATM) – being standardized by CCITT – switched, virtual circuit approach – allows reserving network capacity – uses fixed size packets » easier for hardware to handle – twisted pair or fiber medium » 100 m to few km – 155 Mbps to 2400 Mbps – also used for LAN interconnect as backbone

Future Issues (9.15) Mobility – mobility within an office complex is possible now » wireless LANs at 10 Mbps » low power radio connecting to wired communication – problem with computing outside the office – several factors force mobile computing outside office » cellular phone popularity » centrality of computing » reliance on networking and communication – problems are » creating enough bandwidth possibly need multiple channels » not enough spectrum current transmission / broadcast activities may swap

Network Protocols (9.16) A protocol is a set of conventions for communicating Concerned with issues such as – addressing and routing – error detection and correction – flow control and congestion control – higher level coordination depending on the purpose of the communication International Standards Organization Open Systems Interconnection model (ISO OSI) – 7 layers of protocols – defined interfaces between each layer – lower layers typically implemented in hardware, upper in software

Network Protocols (continued) (9.17)

Network Protocols (continued) (9.18) ISO layers – application - detailed information about data being exchanged – presentation - conventions for representing data – session - management of connections between programs – transport - delivery (reliable or otherwise) of sequences of packets – network - format of individual data packets – link - access to and control of transmission medium – physical - medium of transmission Internet protocols (IP) are a competing set for lower layers – network and transport – defacto standard

Bridging (9.19) Bridging happens at Link layer – between LANs – provides » filtering - only forward packets destined offnet » repacketization » speed matching

Routing (9.20) Internetworking (in WANs) – happens at network layer – routers connect WANs with very dissimilar characteristics » also called gateways Routing – routers know whole network topology » determines which network to send packet on next from destination address – protocol conversion – fragmentation » when packet too large for next network it must pass through

Routing (continued) (9.21)

Routing (continued) (9.22)

Internet Futures Current IP protocol is running out of addresses (9.23) A – 32 bits 4 billion 7 24 addresses 0 – Class A - 127 large groups with 16M machines » countries B 14 16 – Class B - 16K networks 1 0 with 64K or fewer machines – Class C - 2M networks C 21 8 with 256 machines 1 1 0 – each machine can only have one network address Multica 28 Revised IP protocols st 1 1 1 0 (IPng) currently being devised – 128 bit addresses – also called IPv6

(9.24) Web-based Commerce Conducting business (exchanging money) over the Internet requires several features that aren’t yet available – confidentiality encryptio n » no one else listens to the transaction – authentication » you really are who I think you are digital signatures » no one increases my order enroute and – nonrepudiation certificates – data integrity » I can’t back out after I buy – selective application of services » seller never sees my credit card number » I seal it and only give name of the bank, who authorizes payment

Web-based Commerce (continued) Public-key systems provide confidentiality – I choose 2 keys, one public and one private » publish public key widely – I encode with private key – you decode with public key – you encode with public key – I decode with private key or (9.25)

Web-based Commerce (continued) (9.26) Digital signatures use public key algorithms, usually on digest of message to speed up – encode message digest (hash function) – cleartext message also digested at receiving end – if my decoded digest matches calculated one, I sent it and it hasn’t been changed

Web-based Commerce (continued) (9.27) Authentication provided by authentication entities – I don’t send order directly to Nordstroms – send to authenticator instead, who verifies I’m me and sends the order to a prestored Nordstroms address – other variations exist Several proposals to modify IP protocols at various levels to help this – IP protocol – Secure Sockets Layer session protocol – secure HTTP and MIME messaging