Networks and Protocols u networks and protocols definitions

Networks and Protocols u networks and protocols – definitions – motivation – history u protocol hierarchy – reasons for layering – quick tour through the layers u main example: the Internet COS 461 Fall 1997

Networks u. A network is a set of machines connected by communication links. u Machines can be – general-purpose computers – other devices (phones, Coke machines) – specialized network components » routers » switches COS 461 Fall 1997

Motivation u Why connect to a network? – share resources – help people communicate u Sharing resources – from printers to supercomputer centers u Helping people communicate – email, Web, active documents COS 461 Fall 1997

Direct connectivity u point-to-point or multiple access point-to-point network multiple access network COS 461 Fall 1997

Media u links can use many physical media – copper wire – optical fiber – radio – infrared – line-of-sight laser – layer on another network » example: modem connection uses phone network COS 461 Fall 1997

Copper Wires vs. Optical Fibers u advantages of fiber – higher bandwidth – smaller and lighter – less prone to interference – less prone to eavesdropping u advantages of copper – simple – cheap to interface to COS 461 Fall 1997

Topologies ring bus star mesh COS 461 Fall 1997

Network elements u Intranet: many elements in one administrative domain u Internet: collection of interconnected networks, across administrative domains u host: a computer on the net u router: host that routes packets from one link to another – often dedicated, with no applications COS 461 Fall 1997

Circuit Switching u example: telephony u resources reserved during call setup u resources dedicated for duration of call u conservative – guarantee quality of service to all calls – resources dedicated even if call doesn’t always need them – good for constant-bit-rate traffic COS 461 Fall 1997

Circuit Switching COS 461 Fall 1997

Packet Switching u alternative to circuit switching – example: Internet u entering data divided into packets u packets in network share resources – no performance guarantees u queue packets if link contention u statistical multiplexing of resources COS 461 Fall 1997

Packet Switching COS 461 Fall 1997

Packet Switching in the Internet local net COS 461 Fall 1997

Virtual Circuit u cross between circuit switching and packet switching u set up path before data flows u resources along path are shared u example: asynchronous transfer mode (ATM) u cheaper than circuit switching, better guarantees than packet switching COS 461 Fall 1997 u but: complicated

History of Networking u 1870’s – circuit-switched phone network u 1960’s – packet-switched data networks – 4 -node ARPAnet in 1969 u 1970’s – multiple-access nets (Aloha, Ethernet) – commercial nets (DECnet, IBM SNA) – 100 -node ARPAnet in 1979 COS 461 Fall 1997

History of Networking u 1980’s – proliferation of LANs, WANs – 100 k-node Internet in 1989 u 1990’s – 4 M-node Internet in 1995 – commercialization » ISPs – wireless LANs COS 461 Fall 1997

Layering in Networks u simplify complex engineering – layer N relies on services of layer N-1 – layer N provides services to layer N+1 u interfaces between layers define services u hide complexity – separate implementation from interface COS 461 Fall 1997

Layered Protocols layer N+1 abstract view of layer N COS 461 Fall 1997

Protocol ua protocol specifies: – a set of rules for how network elements interact – the format of the messages exchanged – actions to take on receipt of messages u specifications must be exact u interoperability: ability of different implementations to work together COS 461 Fall 1997

Protocol Hierarchy u “official” seven-layer model – usually taught and memorized – seldom used except as terminology u in practice, Internet uses four-layer model – focus on this model in this course u top to bottom: application layer, transport layer, network layer, data link layer COS 461 Fall 1997

Application Layer u process-to-process communication u supports application functionality u examples – file transfer protocol (FTP) – simple mail transfer protocol (SMTP) – hypertext transfer protocol (HTTP) – network news transfer protocol (NNTP) u you can add your own COS 461 Fall 1997

Transport Layer u transmission control protocol (TCP) – provides reliable byte stream service – flow control – congestion control u user datagram protocol (UDP) – provides unreliable unordered datagram service COS 461 Fall 1997

Network Layer u Internet protocol (IP) – the key to the architecture – can use many different data links layers – treats each network in the Internet as a link – no quality of service guarantee – can lose and misorder packets – “best effort” service COS 461 Fall 1997

Data Link/Physical Layer u comes from underlying network – Ethernet – ATM – phone/modem – you can (in theory) build your own COS 461 Fall 1997

Internet Hierarchy FTP HTTP SMTP TCP DNS Finger transport layer UDP network layer IP Ethernet ATM application layer modem SHRIMP data link layer COS 461 Fall 1997