Fundamental Interaction Model Synchronous distributed system 8 time to execute each step of computation within a process has known lower and upper bounds 8 message delivery times are bounded to a known value 8 each process has a clock whose drift rate from real time is bounded by a known value Asynchronous distributed system: no bounds on 8 process execution times 8 message delivery times 8 clock drift rate Note 8 synchronous distributed systems are easier to handle, but determining realistic bounds can be hard or impossible 8 asynchronous systems are more abstract and general: a distributed algorithm executing on one system is likely to also work on another one Distributed Systems - Fall 2001 II - 25 Stefan Leue 2001
Fundamental Interaction Model Event ordering 8 as we will see later, in a distributed system it is impossible for any process to have a view on the current global state of the system 8 possible to record timing information locally, and abstract from real time (logical clocks) 8 event ordering rules if e1 and e2 happen in the same process, and e2 happens after e1, then e1 e2 if e1 is the sending of a message m and e2 is the receiving of the same message m, then e1 e2 hence, describes a partial ordering relation on the set of events in the distributed system Distributed Systems - Fall 2001 II - 26 Stefan Leue 2001
Fundamental Interaction Model Event ordering 8 as we will see later, in a distributed system it is impossible for any process to have a view on the current global state of the system 8 possible to record timing information locally, and abstract from real time (logical clocks) 8 event ordering rules if e1 and e2 happen in the same process, and e2 happens after e1, then e1 e2 if e1 is the sending of a message m and e2 is the receiving of the same message m, then e1 e2 hence, describes a partial ordering relation on the set of events in the distributed system Distributed Systems - Fall 2001 II - 27 Stefan Leue 2001
Failures Omission Failures 8 process omission failures: process crashes detection with timeouts crash is fail-stop if other processes can detect with certainty that process has crashed 8 communication omission failures: message is not being delivered (dropping of messages) possible causes: inetwork transmission error ireceiver incomming message buffer overflow Arbitrary failures 8 process: omit intended processing steps or carry out unwanted ones 8 communication channel: e.g., non-delivery, corruption or duplication Distributed Systems - Fall 2001 II - 28 Stefan Leue 2001
Failures Distributed Systems - Fall 2001 II - 29 Stefan Leue 2001
Security Protecting access to objects 8 access rights 8 in client server systems: involves authentication of clients Protecting processes and interactions 8 threats to processes: problem of unauthenticated requests / replies e.g., "man in the middle" 8 threats to communication channels: enemy may copy, alter or inject messages as they travel across network use of secure channels, based on cryptographic methods Denial of service 8 e.g., pings to selected web sites 8 generating debilitating network or server load so that network services become de facto unavailable Mobile code 8 requires executability privileges on target machine 8 code may be malicious (e.g., mail worms) Distributed Systems - Fall 2001 II - 30 Stefan Leue 2001
Computer Networks Computer Networks "interconnected collection of autonomous computers" [Tanenbaum 1996] Types of Networks 8 Local Area Networks (LANs) high-speed communication on proprietary grounds (on-campus) most typical solution: Ethernet with 100 Mbps 8 Metropolitan Area Networks high-speed communication for nodes distributed over medium-range distances, usually belonging to one organization providing "back-bone" to interconnect LAN's technology often based on ATM, FDDI or DSL typical example: the University-network: iatm based i155 Mbit/s itransports data and voice (phony) Distributed Systems - Fall 2001 II - 31 Stefan Leue 2001
Computer Networks Types of Networks 8 Wide Area Networks communication over long distances covers computers of different organizations high degree of heterogeneity of underlying computing infrastructure involves routers speeds up to a few Mbps possible, but around 50-100 Kbps more typical most prominent example: the Internet 8 Wireless Networks end user equipment accesses network through short or mid range radioor infrared signal transmission Wireless WANs igsm (up to about 20 Kbps) iumts (up to Mbps) ipcs Wireless LANs/MANs iwavelan (2-11 Mbps, radio up to 150 metres) Wireless Personal Area Networks ibluetooth (up to 2 Mbps on low power radio signal, < 10 m distance) Distributed Systems - Fall 2001 II - 32 Stefan Leue 2001
Computer Networks Network Type Performance Characteristics Distributed Systems - Fall 2001 II - 33 Stefan Leue 2001
Computer Networks Network topologies for point-to-point networking Star short paths (always 2 hops) robust against leaf node failure but: whole network down if central node fails sometimes physical star used to implement logical ring Prentice-Hall 1996 Ring varying path lengths robust against node failure basis for Token Ring and FDDI LANs Tree varying, relatively long path lengths robust against leaf node failure sensitive to internal node failure suitable topology for multicast / broadcast applications Distributed Systems - Fall 2001 II - 34 Stefan Leue 2001
Computer Networks Network topologies for point-to-point networking Prentice-Hall 1996 Mesh completely connected graph short paths (always 1 hop) robust against node failure expensive point-to-point wireline implementaion inexpensive shared ether implementation Intersecting Rings internetworking for token ring networks sensitive to bridge node failure Irregular most commonly found Wide Area Network topology Distributed Systems - Fall 2001 II - 35 Stefan Leue 2001