operating systems - networks

UNIVERSITY OF MASSACHUSETTS AMHERST • Department of Computer Science

Operating SystemsCMPSCI 377NetworksEmery Berger

University of Massachusetts Amherst

UNIVERSITY OF MASSACHUSETTS AMHERST • Department of Computer Science 2

Networks

Goal: Efficient, correct, robust message passing between two separate nodes

Local area network (LAN) – nodes in single building, fast & reliable (Ethernet) Media: twisted-pair, coax, fiber Bandwidth: 10-1,000MB/s

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


Principles of Net Communication

Data broken into packets (~ 1Kb) Basic unit of transfer

Computers & routers 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

All info needed to recreate original message

Packets may arrive out of order need sequence

number

Data segment contains headersfor higher protocol layers & application data


Sockets

Standard API for network programming

Duplex communication (2-way)

Can configure sockets to use UDP or TCP

UDP – best for lossy communication

Pings, video, audio, other multimedia

Sends datagrams (packets)

TCP – most stuff

Sends reliable stream of bytes

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Socket communication

Sockets (in TCP) send stream of bytes

Multiple sends may be combinedinto one received message

To send multiple application-level messages, must have application level protocol

E.g., 4-bytes = message type, 4-bytes = message length, plus message

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Client-side: using sockets

Creates socket

int fd = socket (AF_INET, SOCK_STREAM, 0)

Converts hostname to IP address with getaddrinfo

getaddrinfo (name, portStr, NULL, portNum)

Connects to host

connect (fd, addr, len); // from getaddrinfo

Once connected, can send or recv

send (fd, data, amount, 0);

recv (fd, buf, len, 0);

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Server-side: using sockets

As before:

Create socket as before

Convert hostname to IP address with getaddrinfobut in passive mode hint.ai_flags = AI_PASSIVE

getaddrinfo (name, portStr, &hint, &addrinfo)

Binds socket to address: bind (fd, addr, len); // from getaddrinfo

Listens for connection: listen (fd, 1); // number of waiting msgs

Accepts connection ) new socket

int newfd = accept (fd, addr, len);

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Exercise

Lamest web server ever

Wait for connections on localhost port 80

Read message until first newline (‘\n’)

Send <html><body>Hello world</body></html>

Use:

int fd = socket (AF_INET, SOCK_STREAM, 0)

getaddrinfo (name, portStr, &hint, &addrinfo)

bind (fd, addr, addrlen)

int v = listen (fd, 1)

int newfd = accept (fd, addr, len)12


Network Topologies

Connection of nodes impacts:

Maximum & average communication time

Fault tolerance

Expense

Two basic topologies:

Bus (for LANs)

Point-to-point


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


Point-to-Point Network Topologies

Fully-connected

Each message takes one “hop”

Node failure – no effect on communication with others

Expensive – impractical for WANs



Fully-connected






Partially connected Links between some, but not all nodes Less expensive, less tolerant to failures

Single node failure can partition network

Several hops – requires routing

2

3

4

1






2

3

4

1

I want to goto Node 4!



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


The End


Distributed Systems

Distributed system: physically separate processors connected by one or more communication links no shared clock or memory

Most 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

Poster-size 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

operating systems - networks

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