multiservice servers (tcp, udp) - ntut.edu.twcmliu/np/ntut_np_f07u/... · initially, it opens one...

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Mobile Computing & Software Engineering Lab

Multiservice Servers (TCP, UDP)

Prof. Chuan-Ming LiuComputer Science and Information Engineering

National Taipei University of TechnologyTaipei, TAIWAN

NTUT, TAIWAN 2


Consolidating ServersSame motivations for multiprotocol serversHigher risk to use a single, multiserviceserver, why?Consolidating many services into a single server process reduces

the number of executing processes dramaticallyThe total code required

Most multiservice servers use a single transport protocol

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Connectionless Multiservice

Server

OS

ApplicationProcess

Master sockets (One for each service

being offered)

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Connection-Oriented Multiservice

Multiservice server can be programmed to handle some services iteratively and other services concurrentlyConcurrency can be implemented with multiple single-threaded processes or a multi-threaded process

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Iterative, Connection-Oriented Multiservice

Master

OS

ApplicationProcess

Master sockets (One for each

service offered)

Socket for an individual

connection

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Concurrent Connection-Oriented Multiservice

Master

OS

ApplicationProcesses(or threads)

Sockets for connection

requests

Sockets for individual

connections

slave 1 slave n

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Single-thread, MultiserviceIt is also possible, but uncommon, to implement multiservice servers with a single thread of executionMainly on connection-oriented style

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Invoking Separate Programs

Main flaw for multiservice servers – inflexibilityChanging small piece of code needs to recompile the entire server – time consumingStopping the server to recompile bothers clients

How to overcome this?Break a server into independent parts Then, handle each part independentlyReplace the original code by execve

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Multiprocess Multiservice (TCP)

Master

OS

ApplicationProcesses

Master sockets (One for each

service offered)

Sockets for individual slave

connections

slave1 slaven

prog1 progn execve( ) used

fork( )

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Invoking Separate ProgramsIn a mulitservice server, the execve( )system call makes it possible to separate the code that handles an individual service from the code that manages initial requests from clients

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Multi-service Multi-protocol

It is possible to design a multi-service server having multi-protocol as described before.Super server: multi-service, multi-protocol server

Initially, it opens one or two sockets for each serviceMaster sockets correspond to UDP or TCPUse select system call to wait for any socket to become readyIf a UDP socket is ready, read, process and replyIf a TCP socket is ready, handle the connection directly (iteratively) or create a process to handle it (concurrently)

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Multiservice Server – Example Example, superd.cThe CHARGEN service

Tests client softwareGenerates an infinite sequence of characters and sends it to the client

For a UDP socket, the server calls the service handler directlyFor a TCP socket, the server calls the service handler indirectly through procedure doTCP

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Server ConfigurationSuper servers are often configurable – set of services can be changed without recompiling source codeUsually, the configuration information is stored in a fileTwo types:

Static: occurs when the server startsDynamic: occurs while a super server is running

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Server ConfigurationHow to inform a server that reconfiguration is needed?

Depends on the OS, for example, in LINUX one can use signal mechanism used for inter-process communication

A well-designed super server handles reconfiguration gracefullyDynamically changing configuration is more flexible

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Most Unix systems provide a “SuperServer”that solves the problem:

executes the startup code required by a bunch of servers.Waits for incoming requests destined for the same bunch of servers.When a request arrives - starts up the right server and gives it the request.

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Super Server, inetdOn most UNIX systems, including LINUX, the super server, inetd, handles a large set of servicesMotivation for inetd – Less consumption on resourcesDynamically configurable

Newer versions of Linux replace the inetd with xinetd

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inetd childreninetd forks a child that executes the real server program and handles the request and exits.The inetd process remains running after the forkThe child process closes all unnecessary sockets.inetd uses the wait status to determine how to proceed after starting a service

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/etc/inetd.conf

inetd reads a configuration file that lists all the services it should handle. inetd creates a socket for each listed service, and adds the socket to a fd_setgiven to select().

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inetd service specificationFor each service, inetd needs to know:

the port number and transport protocolwait/nowait flag.login name the process should run as.pathname of real server program.command line arguments to server program.

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# comments start with #echo stream tcp nowait root internalecho dgram udp wait root internalchargen stream tcp nowait root internalchargen dgram udp wait root internalftp stream tcp nowait root /usr/sbin/ftpd ftpd -ltelnet stream tcp nowait root /usr/sbin/telnetd telnetdfinger stream tcp nowait root /usr/sbin/fingerd fingerd# Authenticationauth stream tcp nowait nobody /usr/sbin/in.identd

in.identd -l -e -o# TFTPtftp dgram udp wait root /usr/sbin/tftpd tftpd -s

/tftpboot

example /etc/inetd.conf

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Wait/nowaitSpecifying WAIT means that inetd should not look for new clients for the service until the child (the real server) has terminated.TCP servers usually specify nowait

this means inetd can start multiple copies of the TCP server program - providing concurrency!

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UDP & wait/nowaitMost UDP services run with inetd told to wait until the child server has died. What would happen if:

inetd did not wait for a UDP server to die.inetd gets a time slice before the real server reads the request datagram?

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Super inetdSome versions of inetd have server code to handle simple services such as

echo server, daytime server, chargen server,…

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xinetdxinetd can be used to

Provide access only to particular hostsDeny access to particular hostsProvide access to a service at a certain timeetc…

The configure file is /etc/xinetd.confNote: xinetd.conf calls separate files in the directory /etc/xinetd.d

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Server Variations

Super server (with a configuration file)Multiple services, multiple protocols

Concurrent, separate program

Multiple services, single protocolConcurrent, multiple threads or processes

(common)Single service, multiple protocolsMultiple services, single protocolMultiple services, multiple protocols

Concurrent, single thread

(unusual)Single service, multiple protocolsMultiple services, single protocol

Iterative

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multiservice servers (tcp, udp) - ntut.edu.twcmliu/np/ntut_np_f07u/... · initially, it opens one...

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