winter 2008Introduction1 What’s a Network: Key Features  Providing certain services  transport goods, mail, information or data  Shared resources  used by many users, often concurrently  Basic building blocks  nodes (active entities): process and transferdata  links (passive medium): passive “carrier” of data  Typically “multi- hop”  two “end points” cannot directly reach each other  need other nodes/entities to relay

winter 2008Introduction2 What’s a Network: “Nuts and Bolts” View “ HW/SW that makes the internet” network edge: millions of end-system devices (hosts): –pc’s workstations, servers –PDA’s (personal digital system), mobile computers, toasters, cell-phone running network apps network core: routers, switches forwarding data –packets: packet switching –calls: circuit switching communication links –fiber, copper, radio, … Different links transmit data with different transmission speed measured in bit/sec Nodes are connected indirectly through switching devises (most popular are routers and /or link layer switches) Each packet has a route or path from source to destination Each sys access the internet through internet service provider (ISP) such as local telephone company, etc.. local net company net regional net router workstation server mobile

winter 2008Introduction3 Communication networks can be classified based on the way in which the nodes exchange information : A Taxonomy of Communication Networks Communication Network Switched Communication Network Broadcast Communication Network (bus) Circuit-Switched Communication Network Packet-Switched Communication Network Datagram Network Virtual Circuit Network establishes a fixed bandwidth circuit (or channel) between nodes and terminals before the users may communicate, as if the nodes were physically connected with an electrical circuit is a communications method in which packets (discrete blocks of data) are routed between nodes over data links shared with other traffic. In each network node, packets are queued or buffered, resulting in variable delay Each packet sent independently of the others, No call setup, More reliable (can route around failed nodes or congestion) Fixed route established before any packets sent, No need for routing decision for each packet at each node

winter 2008Introduction4 Broadcast communication networks –Information transmitted by any node is received by every other node in the network E.g., LANs (Ethernet, Wavelan) –Problem: coordinate the access of all nodes to the shared communication medium (Multiple Access Problem) Switched communication networks –Information is transmitted to a sub-set of designated nodes E.g., WANs (Telephony Network, Internet) –Problem: how to forward information to intended node(s) Done by special nodes (e.g., routers, switches) running routing protocols Broadcast vs. Switched Communication Networks

winter 2008Introduction5 Data/Computer Networks  Delivery of information (“data”) among computers of all kinds  servers, desktops, laptop, PDAs, cell phones,  General-Purpose  Not for specific types of data or groups of nodes, or using specific technologies  Utilizing a variety of technologies  “physical/link layer” technologies for connecting nodes  copper wires, optical links, wireless radio, satellite  or even “non-electronic” means: e.g., cars, postal services, humans -- e.g., recent “delay-tolerant networks” efforts for 3 rd world countries

winter 2008Introduction6 Circuit Switching End-end resources reserved for “call” link bandwidth, switch capacity dedicated resources: no sharing circuit-like (guaranteed) performance call setup required

winter 2008Introduction7 Packet Switching each end-end data stream divided into packets user A, B packets share network resources each packet uses full link bandwidth resources used as needed, resource contention: aggregate resource demand can exceed amount available congestion: packets queue, wait for link use store and forward: packets move one hop at a time –transmit over link –wait turn at next link Bandwidth division into “pieces” Dedicated allocation Resource reservation

winter 2008Introduction8 Packet Switching vs Circuit Switching: Why? “reliability” – no congestion, in order data in circuit-switching packet switching: better sharing of bandwidth state, resources: packet switching has less state –advantage less control-plane processing resources along the way –More dataplane (address lookup) processing failure modes (routers/links down): –packet switching routing reconfigures sub-second timescale; –circuit-switching: more complex recovery – need to involve all (downstream) switches on path

winter 2008Introduction9 Fundamental Issues in Networking Networking is more than connecting nodes! Naming/Addressing –How to find name/address of the party (or parties) you would like to communicate with –Address: bit- or byte-string that identifies a node –Types of addresses Unicast: node-specific Broadcast: all nodes in the network Multicast: some subset of nodes in the network Routing/Forwarding: –process of determining how to send packets towards the destination based on its address –Finding out neighbors, building routing tables

winter 2008Introduction10 Other Key Issues in Networking Detecting whether there is an error! Fixing the error if possible Deciding how fast to send, meeting user demands, and managing network resources efficiently Make sure integrity and authenticity of messages, ……

winter 2008Introduction11 Fundamental Problems in Networking … What can go wrong? Bit-level errors: due to electrical interferences Packet-level errors: packet loss due to buffer overflow/congestion Out of order delivery: packets may takes different paths Link/node failures: cable is cut or system crash Others: e.g., malicious attacks

winter 2008Introduction12 Fundamental Problems in Networking What can be done? Add redundancy to detect and correct erroneous packets Acknowledge received packets and retransmit lost packets Assign sequence numbers and reorder packets at the receiver Sense link/node failures and route around failed links/nodes Goal: to fill the gap between what applications expect and what underlying technology provides Key Challenges in Internet: large, complex, decentralized, ever-evolving, distributed network of networks!

winter 2008Introduction13 Key Performance Metrics Bandwidth (throughput) –data transmitted per time unit –link versus end-to-end( provide connection directly from an application on one computer to an application on the remote computer) Latency (delay) –time to send message from point A to point B –one-way versus round-trip time (RTT) –components Latency = Propagation + Transmit + Queue Propagation = Distance / c Transmit = Size / Bandwidth Delay Bandwidth Product: # of bits that can be carried in transit Reliability, availability, … Efficiency/overhead of implementation, ……

winter 2008Introduction14 Network Architecture and Structure Network components: (edge/core) nodes and links How do we talk about “structure” of network and its architecture? layered architecture –structure allows identification, relationship of complex system’s pieces: layered reference model for discussion –layer N builds on services provided by layer N-1 –Layer N provides service to layer N+1 Notions of protocol, service and peer interfaces physical topology, interconnection

winter 2008Introduction15 What’s a Protocol? human protocols: “what’s the time?” “I have a question” introductions … specific msgs sent … specific actions taken when msgs received, or other events network protocols: machines rather than humans all communication activity in Internet governed by protocols protocols define format, order of msgs sent and received among network entities, and actions taken on msg transmission, receipt

winter 2008Introduction16 What’s a Protocol? a human protocol and a computer network protocol: Hi Got the time? 2:00 TCP connection req. TCP connection reply. Get time

winter 2008Introduction17 Protocols and Interfaces Protocols: specification/implementation of a “service” or “functionality” Each protocol object has two different interfaces –service interface: operations on this protocol –peer-to-peer interface: messages exchanged with peer

winter 2008Introduction18 Internet Protocol Stack application: supporting network applications –ftp, smtp, http transport: host-host data transfer –tcp, udp network: routing of datagrams from source to destination –ip, routing protocols link: data transfer between neighboring network elements –ppp, ethernet physical: bits “on the wire” application transport network link physical

winter 2008Introduction19 Layering: Logical Communication application transport network link physical application transport network link physical application transport network link physical application transport network link physical network link physical data E.g.: transport take data from app add addressing, reliability check info to form “datagram” send datagram to peer wait for peer to ack receipt analogy: post office data transport ack

winter 2008Introduction20 Layering: Physical Communication application transport network link physical application transport network link physical application transport network link physical application transport network link physical network link physical data

winter 2008Introduction21 A Closer Look at Network Structure: network edge: applications and hosts network core: –routers –network of networks access networks, physical media: communication links

winter 2008Introduction22 The Network Edge: end systems (hosts): –run application programs –e.g., WWW, –at “edge of network” client/server model –client host requests, receives service from server –e.g., WWW client (browser)/ server; client/server peer-peer model: – host interaction symmetric –e.g.: Gnutella, KaZaA

winter 2008Introduction23 Network Edge: Connection-Oriented Service Goal: data transfer between end sys. handshaking: setup (prepare for) data transfer ahead of time –Hello, hello back human protocol –set up “state” in two communicating hosts TCP - Transmission Control Protocol –Internet’s connection- oriented service TCP service [RFC 793] reliable, in-order byte- stream data transfer –loss: acknowledgements and retransmissions flow control: –sender won’t overwhelm receiver congestion control: –senders “slow down sending rate” when network congested

winter 2008Introduction24 Network Edge: Connectionless Service Goal: data transfer between end systems –same as before! UDP - User Datagram Protocol [RFC 768]: Internet’s connectionless service –unreliable data transfer –no flow control –no congestion control App’s using TCP: HTTP (WWW), FTP (file transfer), Telnet (remote login), SMTP ( ) App’s using UDP: streaming media, teleconferencing, Internet telephony

winter 2008Introduction25 Access Networks and Physical Media Q: How to connect end systems to edge router? residential access nets institutional access networks (school, company) mobile access networks Keep in mind: bandwidth (bits per second) of access network? shared or dedicated?

winter 2008Introduction26 Example Access Net: Home Network Typical home network components: ADSL or cable modem router/firewall Ethernet wireless access point wireless access point wireless laptops router/ firewall cable modem to/from cable headend Ethernet (switched)

winter 2008Introduction27 The Network Core mesh of interconnected routers the fundamental question: how is data transferred through net? –circuit switching: dedicated circuit per call: telephone net –packet-switching: data sent thru net in discrete “chunks”

winter 2008Introduction28 Network Core: Routing Goal: move data among routers from source to dest. datagram packet network: –destination address determines next hop –routes may change during session –analogy: driving, asking directions –No notion of call state virtual circuit network: –packet carries tag, tag determines next hop –fixed path (for call) determined at call setup time –routers maintain little per-call state; resources not allocated circuit-switched network: –call allocated time slots of bandwidth at each link –fixed path (for call) determined at call setup –switches maintain lots of per call state (what?): resource allocation