Network ProgrammingSeptember 6, 2005A Client-Server ExchangeNetwork ApplicationsInternet Connections (TCP/IP)ClientsUsing Ports to Identify ServicesServersOverview of the Sockets InterfaceSocketsSocket Programming ClichesSocket Programming HelpSocket Address StructuresReliable I/O (RIO) SummaryEcho Client Main RoutineClient-side ProgrammingEcho Client: open_clientfdEcho Client: open_clientfd (socket)Echo Client: open_clientfd (gethostbyname)Echo Client: open_clientfd (connect)Server-side ProgrammingServers and sockets – 1 isn’t enoughConnected vs. Listening DescriptorsEcho Server: accept IllustratedEcho Server: Main LoopEcho Server: open_listenfdEcho Server: open_listenfd (cont)Echo Server: open_listenfd(socket)Echo Server: open_listenfd (initialize socket address)Echo Server: open_listenfd (bind)Echo Server: open_listenfd (listen)Echo Server: acceptEcho Server: Main RoutineEcho Server: Identifying the ClientEcho Server: echoRunning Echo Client/ServerTypes of Server ImplementationsIterative ServersFundamental Flaw of Iterative ServersConcurrent ServersPossible Mechanisms for Creating Concurrent FlowsEvent-Based Concurrent Servers Using I/O MultiplexingThe select FunctionMacros for Manipulating Set DescriptorsEvent-based Concurrent Echo ServerEvent-based Concurrent Server (cont)Event-based Concurrent Server (cont)Event-based Concurrent Server (cont)Event-based Concurrent Server (cont)Pro and Cons of Event-Based DesignsAttack #1Attack #2: Partial LinesFlaky ClientImplementing a Robust ServerRobust ServerRobust Server LoopNOTE (Just to complicate things…)Conceptual ModelFor More InformationFor More InformationGDB and Version Control (RCS/CVS)Debugging with GDBControlling Your Program With GDBExamining the State of Your ProgramVersion Control with RCSCreating RCS FilesChecking out filesVersionsVersion Control with CVSVersion Control continuedMore information…Tips for the pastMore tips for the pastFinally…Network ProgrammingSeptember 6, 2005TopicsProgrammer’s view of the InternetSockets interfaceWriting clients and serversConcurrency with I/O multiplexingDebugging With GDBVersion Control (RCS/CVS)Tips from the trenches: Projects 1 & 2class03.ppt15-441David MurraySlides based on those of Dave Maltz,Randy Bryant, Geoff Langale,and the 15-213 crewA Client-Server ExchangeClientprocessServerprocess1. Client sends request2. Server handlesrequest3. Server sends response4. Client handlesresponseResourceA server process and one or more client processesServer manages some resource.Server provides service by manipulating resource for clients.Note: clients and servers are processes running on hosts (can be the same or different hosts).215-441, Spring 2005Network ApplicationsClient MachineClient MachineServer MachineServer MachineOSNetworkInterfaceClientAppl.SocketOSNetworkInterfaceServerAppl.OS +NetworkAPIsOS +NetworkAPIsSocketInternetAccess to Network via Program InterfaceSockets make network I/O look like filesCall system functions to control and communicateNetwork code handles issues of routing, segmentation.315-441, Spring 2005Internet Connections (TCP/IP)Two common paradigms for clients and servers communicationz Datagrams (UDP protocol SOCK_DGRAM)z Connections (TCP protocol, SOCK_STREAM) Connections are point-to-point, full-duplex (2-way communication), and reliable. (TODAY’S TOPIC!)415-441, Spring 2005Client socket address128.2.194.242:3479Server socket address208.216.181.15:80Connection socket pair(128.2.194.242:3479, 208.216.181.15:80)Server(port 80)ClientClient host address128.2.194.242Server host address208.216.181.15Note: 80 is a well-known portassociated with Web serversNote: 3479 is anephemeral port allocatedby the kernelClients515-441, Spring 2005Examples of client programsWeb browsers, ftp, telnet, sshHow does a client find the server?The IP address in the server socket address identifies the host (more precisely, an adaptor on the host)The (well-known) port in the server socket address identifies the service, and thus implicitly identifies the server process that performs that service.Examples of well known portsz Port 7: Echo serverz Port 23: Telnet serverz Port 25: Mail serverz Port 80: Web serverUsing Ports to Identify ServicesServer host 128.2.194.242Web server(port 80)Client hostService request for128.2.194.242:80(i.e., the Web server)Echo server(port 7)KernelClient615-441, Spring 2005Web server(port 80)Echo server(port 7)Service request for128.2.194.242:7(i.e., the echo server)KernelClientServers715-441, Spring 2005Servers are long-running processes (daemons).Created at boot-time (typically) by the init process (process 1)Run continuously until the machine is turned off.Each server waits for requests to arrive on a well-known port associated with a particular service.Port 7: echo serverPort 23: telnet serverPort 25: mail serverPort 80: HTTP serverA machine that runs a server process is also often referred to as a “server.”See /etc/services for a comprehensive list of the services available on a Linux machine.Overview of the Sockets InterfaceClientServer815-441, Spring 2005Client / ServerSessionsocket socketbindlistenreadwritereadwriteConnectionrequestreadclosecloseEOFopen_listenfdopen_clientfdacceptconnectAwait connectionrequest fromnext clientSockets915-441, Spring 2005What is a socket?To the kernel, a socket is an endpoint of communication.To an application, a socket is a file descriptor that lets the application read/write from/to the network.z Remember: All Unix I/O devices, including networks, are modeled as files.Clients and servers communicate with each by reading from and writing to socket descriptors.The main distinction between regular file I/O and socket I/O is how the application “opens” the socket descriptors.Socket Programming Cliches1015-441, Spring 2005Network Byte OrderingNetwork is big-endian, host may be big- or little-endianFunctions work on 16-bit (short) and 32-bit (long) values htons() / htonl() : convert host byte order to network byte orderntohs() / ntohl(): convert network byte order to host byte orderUse these to convert network addresses, ports, …Structure CastsYou will see a lot of ‘structure casts’struct sockaddr_in serveraddr; /* fill in serveraddr with an address */…/* Connect takes (struct sockaddr *) as its second argument */ connect(clientfd, (struct sockaddr *) &serveraddr,sizeof(serveraddr)); …Socket
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