Client/Server Computing & Socket Programming

Transcription

1 1 CPSC 826 Intering /Server Computing & Socket Programming Michele Weigle Department of Computer Science Clemson University September 1, Overview Application-layer protocols define:» The types of messages exchanged» The syntax and semantics of messages» The rules for when and how messages are sent Public protocols (defined in RFCs)» HTTP, FTP, SMTP, POP, IMAP, DNS Proprietary protocols» RealAudio, RealVideo» IP telephony» 2 Network Working Group R. Fielding UC Irvine Request for Comments: 2616 J. Gettys Compaq/W3C Obsoletes: 2068 J. Mogul Compaq Category: Standards Track H. Frystyk W3C/MIT L. Masinter Xerox June 1999 P. Leach Microsoft T. Berners-Lee W3C/MIT Abstract Hypertext Transfer Protocol -- HTTP/1.1 The Hypertext Transfer Protocol (HTTP) is an -level protocol for distributed, collaborative, hypermedia information systems. It is a generic, stateless, protocol which can be used for many tasks beyond its use for hypertext, such as name servers and distributed object management systems, through extension of its request methods, error codes and headers [47]. A feature of HTTP is the typing and negotiation of data representation, allowing systems to be built independently of the data being transferred. HTTP has been in use by the World-Wide Web global information initiative since This specification defines the protocol referred to as "HTTP/1.1", and is an update to RFC 2068 [33]. 3 Outline The architecture of distributed systems» /Server computing» P2P computing» Hybrid (/Server and P2P) systems The programming model used in constructing distributed systems» Socket programming Example client/server systems and their -level protocols» The World-Wide Web (HTTP)» Reliable file transfer (FTP)» (SMTP & POP)» Domain Name System (DNS) 4

2 5 Outline Example client/server systems and their -level protocols» The World-Wide Web (HTTP)» Reliable file transfer (FTP)» (SMTP & POP)» Domain Name System (DNS) Protocol design issues:» In-band v. out-of-band control signaling» Push v. pull protocols» Persistent v. non-persistent connections /server service architectures» Contacted server responds versus forwards request Outline Example P2P systems and their -level protocols» Gnutella» Napster (hybrid)» KaZaA 6 -Server Architecture Pure P2P Architecture Server:» always-on host» permanent IP address» server farms for scaling s:» communicate with server» may be intermittently connected» may have dynamic IP addresses» do not communicate directly with each other No always-on server Arbitrary end systems directly communicate Peers are intermittently connected and change IP addresses Example: Gnutella Highly scalable But difficult to manage 7 8

3 9 Hybrid of -Server and P2P Transport Services Napster» File transfer P2P» File search centralized: Peers register content at central server Peers query same central server to locate content Instant messaging» Chatting between two users is P2P» Presence detection/location centralized: User registers its IP address with central server when it comes online User contacts central server to find IP addresses of buddies Data loss Some apps (e.g., audio) can tolerate some loss Other apps (e.g., file transfer, telnet) require 100% reliable data transfer Timing Some apps (e.g., telephony, interactive games) require low delay to be effective Bandwidth Some apps (e.g., multimedia) require minimum amount of bandwidth to be effective Other apps ( elastic apps ) make use of whatever bandwidth they get 10 Applications Transport Service Requirements Transport Protocols Services Provided Application file transfer Web documents real-time audio/video stored audio/video interactive games instant messaging Data loss no loss no loss no loss loss-tolerant loss-tolerant loss-tolerant no loss Bandwidth elastic elastic elastic audio: 5kbps-1Mbps video:10kbps-5mbps same as above few kbps up elastic Time Sensitive no no no yes, 100 s msec yes, few secs yes, 100 s msec yes and no service:» connection-oriented: setup required between client, server» reliable between sending and receiving» flow control: sender won t overwhelm receiver» congestion control: throttle sender when overloaded» does not provide: timing, minimum bandwidth guarantees UDP service:» unreliable data transfer between sending and receiving» does not provide: connection setup, reliability, flow control, congestion control, timing, or minimum bandwidth guarantees Why bother? Why is there a UDP? 11 12

4 13 Applications Application and Transport Protocols Application remote terminal access Web file transfer streaming multimedia telephony Application layer protocol SMTP [RFC 2821] Telnet [RFC 854] HTTP [RFC 2616] FTP [RFC 959] proprietary (e.g. RealNetworks) proprietary (e.g., Dialpad) Underlying protocol or UDP typically UDP The Application Layer The client-server paradigm Typical has two pieces: client and server :» Initiates contact with server ( speaks first )» Requests service from server» For Web, client is implemented in browser; for , in mail er Server:» Provides requested service to client» Always running» May also include a client interface reply request Server 14 /Server Paradigm Socket programming Sockets are the fundamental building block for client/server systems Sockets are created and managed by s» Strong analogies with files a host-local, created/released, OS-controlled interface into which an can both send and receive messages to/from another (remote or local) Two types of services are available via the API:» UDP s: unreliable, datagram-oriented communications» s: reliable, stream-oriented communications 15 /Server Paradigm Socket-programming using A is an created, OS-controlled interface into which an can both send and receive messages to and from another» A door between es and end-to-end protocols controlled by developer controlled by operating system with buffers, variables with buffers, variables controlled by developer controlled by operating system 16

5 17 Socket-programming using programming model A provides a reliable bi-directional communications channel from one to another» A pair of pipes abstraction Socket-programming using Network addressing for s Sockets are addressed using an IP address and port number with buffers, variables End System Local port numbers (e.g., 6500) addresses of hosts (e.g., ) with buffers, variables End System domain name of host e.g., access.cs.clemson.edu DNS 18 Socket-programming using Socket programming in Java Socket-programming using Socket creation in the client-server model creates a local specifying the host and port number of server» Java resolves host names to IP addresses using DNS contacts server» Server must be running» Server must have created that welcomes client s contact Server When the client creates a, the client s establishes connection to server s When contacted by a client, server creates a new for server to communicate with client» This allows the server to talk with multiple clients 19 client host or server 3-way handshake 3-way handshake welcoming connection Server 20

6 21 Socket-programming using Simple client-server example stdin stdout client Server welcoming connection The client s a line of text from standard input and sends the text to the server via a The server receives the line of text from the client and converts the line of characters to all uppercase The server sends the converted line back to the client The client receives the converted text and s it to standard output Socket programming with Example structure s from standard input (infromuser stream), s to server via a (outtoserver stream) Server s line from a Server converts line to uppercase and s back to client s from, (infromserver stream) prints modified line to standard output Standard input Standard output infromuser (4) outtoserver infromserver client (2) (3) (1) 22 Socket programming with Example /server interaction in Java Socket programming with Example Java client Server (running on torpedo1.cs.clemson.edu) create for incoming request (port=6789) welcomesocket = new ServerSocket(...) wait for incoming connection request connection setup = welcomesocket.accept() request from (running on shadow1.cs...) create, connect to torpedo1.cs.clemson.edu, port=6789 = new Socket(...) request using import java.io.*; import java.net.*; class { public static void main(string argv[]) throws Exception { String sentence; String modifiedsentence; // Create (buffered) input stream using standard input BufferedReader infromuser = new BufferedReader( new InputStreamReader(System.in)); System.out.println(" y for input"); reply to reply from program flow // Create client with connection to server at port 6789 data flow 23 Socket = new Socket("torpedo1.cs.clemson.edu", 6789); 24

7 25 Socket programming with Example Java client II Socket programming with Example Java server // Create output stream attached to DataOutputStream outtoserver = new DataOutputStream(.getOutputStream()); // Create (buffered) input stream attached to BufferedReader infromserver = new BufferedReader( new InputStreamReader(.getInputStream())); // Write line to server outtoserver.bytes(sentence + '\n'); import java.io.*; import java.net.*; class Server { public static void main(string argv[]) throws Exception { String clientsentence; String capitalizedsentence; // Create welcoming using port 6789 ServerSocket welcomesocket = new ServerSocket(6789); // Read line from server modifiedsentence = infromserver.line(); System.out.println("FROM SERVER: " + modifiedsentence);.(); } // end main } // end class System.out.println("Server Ready for Connection"); // While loop to handle arbitrary sequence of clients making requests while(true) { // Waits for some client to connect and creates new for connection Socket = welcomesocket.accept(); System.out.println(" Made Connection"); 26 Example Java server II // Create (buffered) input stream attached to connection BufferedReader infrom = new BufferedReader( new InputStreamReader(.getInputStream())); // Create output stream attached to connection DataOutputStream outto = new DataOutputStream(.getOutputStream()); // Read input line from clientsentence = infrom.line(); System.out.println(" sent: " + clientsentence); capitalizedsentence = clientsentence.touppercase() + '\n'; // Write output line to outto.bytes(capitalizedsentence);.(); } // end while; loop back to accept a new client connection } // end main } // end class 27 Socket programming with Example /server interaction in Java Server (running on torpedo1.cs.clemson.edu) create for incoming request (port=6789) welcomesocket = new ServerSocket(...) wait for incoming connection request connection setup = welcomesocket.accept() request from reply to (running on shadow1.cs...) create, connect to torpedo1.cs.clemson.edu, port=6789 = new Socket(...) request using reply from program flow data flow 28

8 29 Socket-programming using UDP UDP programming model A UDP provides an unreliable bi-directional communication channel from one to another» A datagram abstraction Socket programming with UDP Example /server UDP interaction in Java Server (running on torpedo1.cs.clemson.edu) create for incoming request (port=9876) serversocket = new DatagramSocket() create, = new DatagramSocket() request from serversocket reply to serversocket specifying client IP address and port number create address (torpedo1.cs.clemson.edu, port = 9876) and send datagram using reply from program flow data flow 30