1 Next Few Classes Networking basics Protection & Security

2 Distributed Systems distributed system: set of physically separate processors connected by one or more communication links no shared clock or memory Many systems today distributed in some way , p2p system, network printers, remote backup, web... P2 P1 P3 P4

3 Parallel vs. Distributed Systems Tightly-coupled systems: “parallel processing” Processors share clock, memory, run one OS Frequent communication Loosely-coupled systems: “distributed computing” Each processor has own memory, runs independent OS Infrequent communication

4 Advantages of Distributed Systems Resource sharing Computational speedup Reliability Communication

5 Networks Goal: provide efficient, correct, robust message passing between two separate nodes Local area network (LAN) – connects nodes in single building, fast & reliable (Ethernet, WLAN) Media: twisted-pair, coax, fiber, radio Bandwidth: 10Mbps – 1Gbps Wide area network (WAN) – connects nodes across large geographic area (Internet) Media: fiber, microwave links, satellite channels, radio Bandwidth: 1.544MB/s (T1), 45 MB/s (T3),…, 1Gbps

6 LAN Topologies Two basic topologies: Point-to-point Bus Connection of nodes impacts: Speed: maximum & average communication time Reliability: fault tolerance Expense

7 Point-to-Point Topology: Fully- connected Each message takes one “hop” Node failure – no effect on communication with others Expensive – impractical for WANs

8 Point-to-Point Topology: Partially connected Links between some, but not all nodes Less expensive, less tolerant to failures Single node failure can partition network Sending message takes several hops Needs routing algorithms

9 Point-to-Point Topology: Tree Tree structure: network hierarchy Messages past between direct descendants Max message cost? Not failure tolerant Any interior node fails – network partitioned

10 Point-to-Point Topology: Star Star network: all nodes connect to central node Each message takes how many hops? Not failure tolerant Inexpensive – sometimes used for LANs

11 Point-to-Point Topology: One- directional Ring Given n nodes, max hops? Inexpensive Fault-tolerant?

12 Bus Network Topologies Bus nodes connect to common network Linear bus – single shared link Nodes connect directly to each other via bus Inexpensive (linear in # of nodes) Tolerant of node failures Traditional Ethernet LAN

13 Bus Network Topologies Ring bus – single shared circular link Same technology & tradeoffs as linear bus

14 WAN Topology: Internet Graph

15 Principles of Network Communication Data broken into packets Basic unit of transfer Packets sent through network Routers at switching points control packet flow Road analogy: Packets = cars Network = roads router = traffic lights (intersection) Too many packets on shared link/node = traffic jam

16 What’s a protocol? a human protocol and a computer network protocol: Q: Other human protocols? Hi Got the time? 2:00 TCP connection req TCP connection response Get time

17 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, other events

18 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

19 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

20 Protocol Layering and Data Each layer takes data from above adds header information to create new data unit passes new data unit to layer below application transport network link physical application transport network link physical source destination M M M M H t H t H n H t H n H l M M M M H t H t H n H t H n H l message segment datagram frame

21 Summary Virtually all computer systems contain distributed components Networks connect them Key tradeoffs: Speed Reliability Expense