CPSC 826 Internetworking

Application-Layer Protocols Overview 

Client/Server Computing & Socket Programming

Application-layer protocols define: » The types of messages exchanged » The syntax and semantics of messages » The rules for when and how messages are sent

Michele Weigle 

Department of Computer Science Clemson University [email protected] September 1, 2004

application transport network link physical

local ISP

regional ISP

Public protocols (defined in RFCs) » HTTP, FTP, SMTP, POP, IMAP, DNS

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Proprietary protocols » RealAudio, RealVideo » IP telephony » …

http://www.cs.clemson.edu/~mweigle/courses/cpsc826

company network

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Network Working Group Request for Comments: 2616 Obsoletes: 2068 Category: Standards Track W3C/MIT June 1999 Microsoft

R. Fielding J. Gettys J. Mogul H. Frystyk

UC Irvine Compaq/W3C Compaq

L. Masinter P. Leach

Xerox

T. Berners-Lee

W3C/MIT

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Application-Layer Protocols Outline 

The architecture of distributed systems » Client/Server computing » P2P computing » Hybrid (Client/Server and P2P) systems

Hypertext Transfer Protocol -- HTTP/1.1

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Abstract The Hypertext Transfer Protocol (HTTP) is an application-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.

The programming model used in constructing distributed systems » Socket programming



application transport network link physical

local ISP

regional ISP

Example client/server systems and their application-level protocols » The World-Wide Web (HTTP) » Reliable file transfer (FTP)

HTTP has been in use by the World-Wide Web global information initiative since 1990. This specification defines the protocol referred to as "HTTP/1.1", and is an update to RFC 2068 [33].

» E-mail (SMTP & POP) » Internet Domain Name System (DNS) 3

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Application-Layer Protocols Outline 

Example client/server systems and their application-level protocols » » » »



The World-Wide Web (HTTP) Reliable file transfer (FTP) E-mail (SMTP & POP) Internet Domain Name System (DNS)

Application-Layer Protocols

application transport network link physical

application transport network link physical

Outline 

Example P2P systems and their application-level protocols » Gnutella » Napster (hybrid) » KaZaA

local ISP

local ISP

regional ISP

Protocol design issues:

regional ISP

» In-band v. out-of-band control signaling » Push v. pull protocols » Persistent v. non-persistent connections 

Client/server service architectures » Contacted server responds versus forwards request

company network

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Application-Layer Protocols

Application-Layer Protocols

Client-Server Architecture

Pure P2P Architecture Server:

local ISP

regional ISP

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» always-on host » permanent IP address » server farms for scaling

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Clients:

company network

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» 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

local ISP

regional ISP

Highly scalable But difficult to manage

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company network

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Application-Layer Protocols

Application-Layer Protocols

Hybrid of Client-Server and P2P

Transport Services

Napster

Data loss  Some apps (e.g., audio) can tolerate some loss  Other apps (e.g., file transfer, telnet) require 100% reliable data transfer

» 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:  

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

Timing  Some apps (e.g., Internet telephony, interactive games) require low delay to be “effective”

User registers its IP address with central server when it comes online User contacts central server to find IP addresses of buddies

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Internet Applications

Internet Transport Protocols

Transport Service Requirements

Services Provided 

Application

Data loss

Bandwidth

Time Sensitive

file transfer e-mail Web documents real-time audio/video

no loss no loss no loss loss-tolerant

no no no yes, 100’s msec

stored audio/video interactive games instant messaging

loss-tolerant loss-tolerant no loss

elastic elastic elastic audio: 5kbps-1Mbps video:10kbps-5Mbps same as above few kbps up elastic

yes, few secs yes, 100’s msec yes and no

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TCP service: » connection-oriented: setup required between client, server » reliable transport between sending and receiving process » flow control: sender won’t overwhelm receiver » congestion control: throttle sender when network overloaded » does not provide: timing, minimum bandwidth guarantees

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UDP service: » unreliable data transfer between sending and receiving process » does not provide: connection setup, reliability, flow control, congestion control, timing, or minimum bandwidth guarantees

Why bother? Why is there a UDP? 12

Internet Applications

The Application Layer

Application and Transport Protocols

The client-server paradigm

Application e-mail remote terminal access Web file transfer streaming multimedia Internet telephony

Application layer protocol

Underlying transport protocol

SMTP [RFC 2821] Telnet [RFC 854] HTTP [RFC 2616] FTP [RFC 959] proprietary (e.g. RealNetworks) proprietary (e.g., Dialpad)

TCP TCP TCP TCP TCP or UDP

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Typical network application has two pieces: client and server Client:

application transport network link physical

local ISP reply

» Initiates contact with server (“speaks first”) » Requests service from server » For Web, client is implemented in browser; for e-mail, in mail reader

typically UDP

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regional ISP request

Server: » Provides requested service to client » “Always” running » May also include a “client interface”

company network

Server

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Client/Server Paradigm

Socket programming

Socket-programming using TCP socket

Sockets are the fundamental building block for client/server systems

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Sockets are created and managed by applications » Strong analogies with files

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a host-local, application created/released, OS-controlled interface into which an application process can both send and receive messages to/from another (remote or local) application process

application transport network link physical

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Client/Server Paradigm



Client

A socket is an application created, OS-controlled interface into which an application can both send and receive messages to and from another application » A “door” between application processes and end-to-end transport protocols

controlled by application developer

Two types of transport services are available via the socket API:

controlled by operating system

» UDP sockets: unreliable, datagram-oriented communications » TCP sockets: reliable, stream-oriented communications

process socket TCP with buffers, variables

Host (end system) 15

Internet

process socket TCP with buffers, variables

controlled by application developer controlled by operating system

Host (end system) 16

Socket-programming using TCP

Socket-programming using TCP

TCP socket programming model

Network addressing for sockets

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A TCP socket provides a reliable bi-directional communications channel from one process to another

Sockets are addressed using an IP address and port number

» A “pair of pipes” abstraction process

process

Local port numbers (e.g., 6500)

socket Process write read

socket

Internet bytes bytes

read write

socket

End System Host (end system)

socket

TCP with buffers, variables

Process

Host (end system)

TCP with buffers, variables

Internet addresses of hosts (e.g., 130.127.48.4)

Internet domain name of host e.g., access.cs.clemson.edu

End System

DNS

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Socket-programming using TCP

Socket-programming using TCP

Socket programming in Java

Socket creation in the client-server model

Client

Server

Internet

socket

bytes bytes

socket

socket

read write Client

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Client creates a local TCP socket specifying the host and port number of server process

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» Java resolves host names to IP addresses using DNS

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Client contacts server » Server process must be running » Server must have created socket that “welcomes” client’s contact

When the client creates a socket, the client’s TCP establishes connection to server’s TCP When contacted by a client, server creates a new socket for server process to communicate with client » This allows the server to talk with multiple clients

process client socket socket host or server

ay e w ak 3- dsh n ha

write read

ake ay handsh TCP 3-w bytes

process “welcoming”

socket socket

connection

socket socket

bytes

Internet

Server

Client 19

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Socket programming with TCP Example

Simple client-server example

Client structure

Client stdin

client socket socket

stdout

   

Server welcoming

socket socket

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connection

socket socket



The client reads a line of text from standard input and sends the text to the server via a socket 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 writes it to standard output

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Client reads from standard input (inFromUser stream), writes to server via a socket (outToServer stream) Server reads line from a socket Server converts line to uppercase and writes back to client Client reads from socket, (inFromServer stream) prints modified line to standard output Standard

output

Standard input

outToServer

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client socket (3)

(2)

(4)

inFromUser

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Client Process

(1)

Socket programming with TCP Example

Socket programming with TCP Example

Client/server TCP socket interaction in Java

Java client

TCP wait for incoming connection setup connection request connectionSocket = welcomeSocket.accept()

Client (running on shadow1.cs...) create socket, connect to torpedo1.cs.clemson.edu, port=6789

clientSocket = new Socket(...)

read request from connectionSocket

write request using clientSocket

write reply to connectionSocket

read reply from clientSocket

close connectionSocket

close clientSocket

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import java.io.*; import java.net.*; class TCPClient { public static void main(String argv[]) throws Exception { String sentence; String modifiedSentence;

Server (running on torpedo1.cs.clemson.edu) create socket for incoming request (port=6789) welcomeSocket = new ServerSocket(...)

inFromServer

Socket-programming using TCP

// Create (buffered) input stream using standard input BufferedReader inFromUser = new BufferedReader( new InputStreamReader(System.in)); System.out.println("Client ready for input");

// Create client socket with connection to server at port 6789

program flow

Socket clientSocket = new Socket("torpedo1.cs.clemson.edu", 6789);

data flow 23

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Socket programming with TCP Example

Socket programming with TCP Example

Java client II

Java server

// Create output stream attached to socket DataOutputStream outToServer = new DataOutputStream( clientSocket.getOutputStream());

import java.io.*; import java.net.*; class TCPServer { public static void main(String argv[]) throws Exception { String clientSentence; String capitalizedSentence;

// Create (buffered) input stream attached to socket BufferedReader inFromServer = new BufferedReader( new InputStreamReader( clientSocket.getInputStream())); // Write line to server outToServer.writeBytes(sentence + '\n');

// Create “welcoming” socket using port 6789 ServerSocket welcomeSocket = new ServerSocket(6789);

// Read line from server modifiedSentence = inFromServer.readLine();

System.out.println("Server Ready for Connection");

// While loop to handle arbitrary sequence of clients making requests while(true) {

System.out.println("FROM SERVER: " + modifiedSentence); clientSocket.close();

// Waits for some client to connect and creates new socket for connection Socket connectionSocket = welcomeSocket.accept();

} // end main } // end class

System.out.println("Client Made Connection"); 25

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Socket programming with TCP Example

Socket programming with TCP Example

Java server II

Client/server TCP socket interaction in Java

// Create (buffered) input stream attached to connection socket BufferedReader inFromClient = new BufferedReader( new InputStreamReader( connectionSocket.getInputStream())); // Create output stream attached to connection socket DataOutputStream outToClient = new DataOutputStream( connectionSocket.getOutputStream()); // Read input line from socket clientSentence = inFromClient.readLine(); System.out.println("Client sent: " + clientSentence); capitalizedSentence = clientSentence.toUpperCase() + '\n';

// Write output line to socket outToClient.writeBytes(capitalizedSentence); connectionSocket.close(); } // end while; loop back to accept a new client connection } // end main } // end class

Server (running on torpedo1.cs.clemson.edu) create socket for incoming request (port=6789) welcomeSocket = new ServerSocket(...)

TCP wait for incoming connection setup connection request connectionSocket = welcomeSocket.accept()

create socket, connect to torpedo1.cs.clemson.edu, port=6789

clientSocket = new Socket(...)

read request from connectionSocket

write request using clientSocket

write reply to connectionSocket

read reply from clientSocket

close connectionSocket 27

Client (running on shadow1.cs...)

close clientSocket

program flow

data flow 28

Socket-programming using UDP

Socket programming with UDP Example

UDP socket programming model

Client/server UDP socket interaction in Java



A UDP socket provides an unreliable bi-directional communication channel from one process to another

Server (running on torpedo1.cs.clemson.edu)

» A “datagram” abstraction

create socket for incoming request (port=9876) serverSocket = new DatagramSocket()

read request from serverSocket

Process write read

socket

Host (end system)

Client create socket, clientSocket = new DatagramSocket()

create address (torpedo1.cs.clemson.edu, port = 9876) and send datagram using clientSocket

Process

Internet bytes bytes

socket

read write

write reply to serverSocket specifying client IP address and port number

Host (end system) 29

read reply from clientSocket program flow

close clientSocket

data flow 30