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
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Fundamental Interaction Model
© Pearson Education 2001
♦ 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
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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
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Failures
© Pearson Education 2001
♦ 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
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Failures
© Pearson Education 2001
© Pearson Education 2001
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Security
© Pearson Education 2001
♦ 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)
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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 (telephony)
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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)
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Computer Networks
♦ Network Type Performance Characteristics
© Pearson Education 2001
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Computer Networks
♦ Network topologies for point-to-point networking
© Prentice-Hall 1996
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
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Ring
• varying path
lengths
• robust against
node failure
• basis for Token
Ring and FDDI
LANs
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Tree
• varying, relatively long
path lengths
• robust against leaf node
failure
• sensitive to internal node
failure
• suitable topology for
multicast / broadcast
applications
© Stefan Leue 2001
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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
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Intersecting Rings
• internetworking for token
ring networks
• sensitive to bridge node
failure
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Irregular
• most commonly found
Wide Area Network
topology
© Stefan Leue 2001
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