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...

P2P1

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


Advantages, continued 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


Advantages, continued 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

Date post:	25-Feb-2016
Category:	Documents
Upload:	adsila
View:	47 times
Download:	0 times

Operating Systems CMPSCI 377 Lecture 19: Network Structures

Documents