What’s a Network: Key Features

1. 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 Introduction

2. 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.. What’s a Network: “Nuts and Bolts” View“HW/SW that makes the internet” router workstation server mobile local net regional net company net Introduction

3. A Taxonomy of Communication Networks Communication Network • Communication networks can be classified based on the way in which the nodes exchange information: BroadcastCommunication Network (bus) SwitchedCommunication Network 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 Packet-SwitchedCommunication Network Circuit-SwitchedCommunication 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 Datagram Network Virtual Circuit Network • 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 Introduction

4. Broadcast vs. Switched Communication Networks • 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 Introduction

5. 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 3rd world countries Introduction

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

7. 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, Bandwidth division into “pieces” Dedicated allocation Resource reservation Packet Switching 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 Introduction

8. “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 Packet Switching vs Circuit Switching: Why? Introduction

9. 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 Introduction

10. 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, • …… Introduction

11. 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 Introduction

12. 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! Introduction

13. 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, …… Introduction

14. Network components: (edge/core) nodes and links Network Architecture and Structure 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 Introduction

15. 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 What’s a Protocol? protocols define format, order of msgs sent and received among network entities, and actions taken on msg transmission, receipt Introduction

16. a human protocol and a computer network protocol: Hi TCP connection req. Hi TCP connection reply. Get http://gaia.cs.umass.edu/index.htm Got the time? 2:00 <file> time What’s a Protocol? Introduction

17. 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 Introduction

18. 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 Internet Protocol Stack Introduction

19. 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 network link physical application transport network link physical application transport network link physical application transport network link physical application transport network link physical data data data ack Layering: Logical Communication transport transport Introduction

20. network link physical application transport network link physical application transport network link physical application transport network link physical application transport network link physical data data Layering: Physical Communication Introduction

21. network edge: applications and hosts network core: routers network of networks access networks, physical media: communication links A Closer Look at Network Structure: Introduction

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

23. 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 Network Edge: Connection-Oriented Service Introduction

24. 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 (email) App’s using UDP: streaming media, teleconferencing, Internet telephony Network Edge: Connectionless Service Introduction

25. 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? Access Networks and Physical Media Introduction

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

27. 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” The Network Core Introduction

28. Goal: move data among routers from source to dest. Network Core: Routing 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 Introduction