Operating Systems CMPSCI 377 Lecture 19: Network Structures

UUNIVERSITY OF NIVERSITY OF MMASSACHUSETTSASSACHUSETTS, A, AMHERST • MHERST • Department of Computer Science Department of Computer Science

Emery BergerUniversity of Massachusetts Amherst

Operating SystemsCMPSCI 377

Lecture 19: Network Structures

UUNIVERSITY OF NIVERSITY OF MMASSACHUSETTSASSACHUSETTS, A, AMHERST • MHERST • Department of Computer Science Department of Computer Science 2

Next Few Classes

Networking basics Distributed services

e-mail, www, telnet Distributed operating systems Distributed file systems


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 e-mail, file servers, network printers, remote

backup, web...

P2

P1

P3P4


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


Advantages of Distributed Systems

Resource sharing Computational speedup Reliability Communication


Advantages of Distributed Systems

Resource sharing Resources need not be replicated

Shared files Expensive (scarce) resources can be

shared Color laser printers

Processors present same environment to user

Keeping files on file server


Advantages, continued

Computational speedup n processors = n times computational

power SETI@home

Problems must be decomposable into subproblems

Trivial = embarrassingly parallel Coordination & communication

required between cooperating processes

Synchronization Exchange of results



Reliability Replication of resources provides fault

tolerance One node crashes, user works on another

one Performance degradation but system

available Must avoid single point of failure

Single, centralized component of system Example: central file servers



Communication Users/processes on different

systems can communicate Mail, transaction processing

systems like airlines & banks, www


Distributed Systems Issues

Operating systems support for distribution Communication & networks Transparency Security Reliability Performance & scalability Programming models


Networks Goal: provide efficient, correct, robust

message passing between two separate nodes

Local area network (LAN) – connects nodes in single building, fast & reliable (Ethernet) Media: twisted-pair, coax, fiber Bandwidth: 10-100MB/s

Wide area network (WAN) – connects nodes across large geographic area (Internet) Media: fiber, microwave links, satellite channels Bandwidth: 1.544MB/s (T1), 45 MB/s (T3)


Network Topologies

Connection of nodes impacts: Maximum & average communication

time Fault tolerance Expense

Two basic topologies: Point-to-point Bus


Point-to-Point Network Topologies

Fully-connected Each message takes one “hop” Node failure – no effect on communication

with others Expensive – impractical for WANs



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



Tree structure: network hierarchy Messages fast between direct descendants

Max message cost? Not failure tolerant

Any interior node fails – network partitioned



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



One-directional ring Given n nodes, max hops? Inexpensive Fault-tolerant?



Bi-directional ring Given n nodes, max hops? Inexpensive Fault-tolerant? One node? Two?



Doubly-connected ring: nodes connected to neighbors & one more distant Given n nodes, max hops? Fault-tolerant? More expensive


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 Ethernet LAN


Bus Network Topologies

Ring bus – single shared circular link Same technology & tradeoffs as

linear bus


Principles ofNetwork Communication Data broken into packets

Basic unit of transfer Packets sent through network Computers & routers at switching

points control packet flow

Road analogy: Packets = cars Network = roads Computer = traffic lights (intersection) Too many packets on shared link/node =

traffic jam


Communication Protocols

Protocol: agreed-upon rules for communication

Protocol stack: layers that comprise networking software Each layer N provides service to

layer N+1


Traditional Layers Application layer – applications that use the net Presentation layer – data format conversion

(big/little endian) Session layer – implements communication

strategy(e.g., RPC)

Transport layer – reliable end-to-end communication

Network layer – routing & congestion control Data link control layer – reliable point-to-point

communication over unreliable channel Physical layer – electrical/optical signaling

across “wire”


TCP/IP Protocol Stack

Internet standard protocol stack TCP: reliable protocol – packets received

in order UDP (user datagram protocol) – unreliable

No guarantee of delivery


Packet Format Contains all info

needed to recreate original message

Packets may arrive out of order = need sequence number

Data segment contains headers for higher protocol layers & application data


Summary

Virtually all computer systems contain distributed components

Networks connect them Key tradeoffs:

Speed Reliability Expense

Operating Systems CMPSCI 377 Lecture 19: Network Structures

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