Protocols

• Rules for communicating between two entities (e.g., a client and a server)

• “A protocol definition specifies how distributed system elements interact with one another in order to achieve a specified behavior, and the structure of the information exchanged during this interaction.” [Foster et. al, 2001]

Next, a short tutorial on some basic networking concepts…

Brief Network Tutorial

Physical (e.g., cables, etc.)

Data Link (e.g., Ethernet)

Network (e.g., IP)

Transport (e.g., TCP, UDP)

Application (e.g., FTP, HTTP, telnet)

Network Model

When a message is sent

• The application constructs a message

user data

Network Model

When a message is sent

• The message is packaged (encapsulated) with a header from the transport layer (e.g., TCP) and sent to the network layer

user data

TCP

Network Model

When a message is sent

• The network layer adds a header

user data

TCPIP

An IP packet

Network Model

When a message is sent

• The data link layer adds a header, and the frame is sent out on the network

user data

TCPIPEthernet

An Ethernet frame

Network Model

When a message is sent

Application builds message in user memory

Message is copied to kernel and TCP/IP and Ethernet headers are added

Message is sent onto network to receiver

Message is copied to kernel and TCP/IP and Ethernet headers are stripped

Message arrives to user memory and the application is notified

You only see the communication at

THIS LEVEL

Message arrives to user memory and the application is notified

Message arrives to user memory and the application is notified

Network Model

Connectionless or connection-oriented

• If a protocol is connection-oriented then the client and server must connect before communication takes place – like a telephone call

• If a protocol is connectionless then there is no connection, just messaging – like sending a letter in the mail

Client/Server Model

•Starts first

•Waits for contact from a client

•Responds to requests

•Starts second

•Contacts a server with a request

•Waits for response from server

Server Client

Client/Server Model

Types of Servers

A server can be:

Types of Servers

Stateful

Stateless

Iterative

Concurrent

iterative stateful

concurrent stateful

iterative stateless

concurrent

stateless

Stateful Server

• Maintains some information between requests

• Requires smaller messages, since some information is kept between contacts

• May become confused if a connection terminates abnormally (if the design is not fault tolerant)

• Example: FTP

Types of Servers

Stateless Server

• Requires larger messages. That is, the message must contain all information about the request since no state information is kept.

• Example: HTTP

Types of Servers

Iterative Server

while (1) {

accept a connection (or request) from a client service the client

close the connection (if necessary)

}

Types of Servers

Concurrent Server

while (1) {

accept a connection/request from client

start a new thread to handle this client

/* the thread must close the connection! */

}

Types of Servers

Internet Addressing

• Suppose you type:

http://comp.uark.edu/~aapon

• This invokes the HTTP protocol (over TCP/IP), and the computer “comp.uark.edu” is sent a message

Addressing

Internet Addressing

http://comp.uark.edu/~aapon/

• Same as IP address 130.184.252.197

Find the home page of user aaponFind the home page of user aapon

Contact the HTTP server on the Contact the HTTP server on the computer named comp.uark.educomputer named comp.uark.edu

Addressing

Internet Addressing

• A Domain Name Server (DNS) may be called to find the IP address of comp.uark.edu

• Each IP machine is usually configured with the name of a DNS server.

• Some IP names and addresses can also be stored in /etc/hostfile

Addressing

Internet Addressing

“http” says: send the message to port 80• An IP address includes both a host

address and a port number!

• The HTTP server listens to port 80

• The HTTP server responds when a client contacts it

Addressing

Internet Addressing

You can write a server that listens to any port not already in use!

• A port number is a 16-bit integer. Ports below 1024 are reserved for system use.

• Well-known ports include FTP, Telnet, SMTP, etc.

Addressing

Socket programming

Socket API• introduced in BSD4.1 UNIX,

1981• explicitly created, used, released

by apps • client/server paradigm • two types of transport service via

socket API: – unreliable datagram – reliable, byte stream-oriented

a host-local, application-created,

OS-controlled interface (a “door”) into which

application process can both send and

receive messages to/from another

application process

socket

Goal: learn how to build client/server application that communicate using sockets

Socket-programming using TCP

Socket: a door between application process and end-end-transport protocol (UCP or TCP)

TCP service: reliable transfer of bytes from one process to another

process

TCP withbuffers,

variables

socket

controlled byapplicationdeveloper

controlled byoperating

system

host orserver

process

TCP withbuffers,

variables

socket

controlled byapplicationdeveloper

controlled byoperatingsystem

host orserver

internet

Socket programming with TCPClient must contact server• server process must first be

running• server must have created

socket (door) that welcomes client’s contact

Client contacts server by:• creating client-local TCP

socket• specifying IP address, port

number of server process• When client creates socket:

client TCP establishes connection to server TCP

• When contacted by client, server TCP creates new socket for server process to communicate with client– allows server to talk with

multiple clients– source port numbers used

to distinguish clients (more in Chap 3)

TCP provides reliable, in-order transfer of bytes (“pipe”) between client and server

application viewpoint

Client/server socket interaction: TCP

wait for incomingconnection requestconnectionSocket =welcomeSocket.accept()

create socket,port=x, forincoming request:welcomeSocket =

ServerSocket()

create socket,connect to hostid, port=xclientSocket =

Socket()

closeconnectionSocket

read reply fromclientSocket

closeclientSocket

Server (running on hostid) Client

send request usingclientSocketread request from

connectionSocket

write reply toconnectionSocket

TCP connection setup

outT

oSer

ver

to network from network

inFr

omS

erve

r

inFr

omU

ser

keyboard monitor

Process

clientSocket

inputstream

inputstream

outputstream

TCPsocket

Clientprocess

client TCP socket

Stream jargon

• A stream is a sequence of characters that flow into or out of a process.

• An input stream is attached to some input source for the process, e.g., keyboard or socket.

• An output stream is attached to an output source, e.g., monitor or socket.

Socket programming with TCP

Example client-server app:1) client reads line from

standard input (inFromUser stream) , sends to server via socket (outToServer stream)

2) server reads line from socket

3) server converts line to uppercase, sends back to client

4) client reads, prints modified line from socket (inFromServer stream)

Example: Java client (TCP)

import java.io.*; import java.net.*; class TCPClient {

public static void main(String argv[]) throws Exception { String sentence; String modifiedSentence;

BufferedReader inFromUser = new BufferedReader(new InputStreamReader(System.in));

Socket clientSocket = new Socket("hostname", 6789);

DataOutputStream outToServer = new DataOutputStream(clientSocket.getOutputStream());

Createinput stream

Create client socket,

connect to server

Createoutput stream

attached to socket

Example: Java client (TCP), cont.

BufferedReader inFromServer = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));

sentence = inFromUser.readLine();

outToServer.writeBytes(sentence + '\n');

modifiedSentence = inFromServer.readLine();

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

clientSocket.close(); } }

Createinput stream

attached to socket

Send lineto server

Read linefrom server

Example: Java server (TCP)import java.io.*; import java.net.*;

class TCPServer {

public static void main(String argv[]) throws Exception { String clientSentence; String capitalizedSentence;

ServerSocket welcomeSocket = new ServerSocket(6789); while(true) { Socket connectionSocket = welcomeSocket.accept();

BufferedReader inFromClient = new BufferedReader(new InputStreamReader(connectionSocket.getInputStream()));

Createwelcoming socket

at port 6789

Wait, on welcomingsocket for contact

by client

Create inputstream, attached

to socket

Example: Java server (TCP), cont

DataOutputStream outToClient = new DataOutputStream(connectionSocket.getOutputStream());

clientSentence = inFromClient.readLine();

capitalizedSentence = clientSentence.toUpperCase() + '\n';

outToClient.writeBytes(capitalizedSentence); } } }

Read in linefrom socket

Create outputstream,

attached to socket

Write out lineto socket

End of while loop,loop back and wait foranother client connection

Chapter 2: Application layer

• 2.1 Principles of network applications

• 2.2 Web and HTTP• 2.3 FTP • 2.4 Electronic Mail

– SMTP, POP3, IMAP

• 2.5 DNS

• 2.6 P2P file sharing• 2.7 Socket

programming with TCP

• 2.8 Socket programming with UDP

• 2.9 Building a Web server

Socket programming with UDP

UDP: no “connection” between client and server

• no handshaking

• sender explicitly attaches IP address and port of destination to each packet

• server must extract IP address, port of sender from received packet

UDP: transmitted data may be received out of order, or lost

application viewpoint

UDP provides unreliable transfer of groups of bytes (“datagrams”)

between client and server

Client/server socket interaction: UDP

closeclientSocket

Server (running on hostid)

read reply fromclientSocket

create socket,clientSocket = DatagramSocket()

Client

Create, address (hostid, port=x,send datagram request using clientSocket

create socket,port=x, forincoming request:serverSocket = DatagramSocket()

read request fromserverSocket

write reply toserverSocketspecifying clienthost address,port number

Example: Java client (UDP)

sendP

ack

et

to network from network

rece

iveP

ack

et

inF

rom

Use

r

keyboard monitor

Process

clientSocket

UDPpacket

inputstream

UDPpacket

UDPsocket

Output: sends packet (recallthat TCP sent “byte stream”)

Input: receives packet (recall thatTCP received “byte stream”)

Clientprocess

client UDP socket

Example: Java client (UDP)

import java.io.*; import java.net.*; class UDPClient { public static void main(String args[]) throws Exception { BufferedReader inFromUser = new BufferedReader(new InputStreamReader(System.in)); DatagramSocket clientSocket = new DatagramSocket(); InetAddress IPAddress = InetAddress.getByName("hostname"); byte[] sendData = new byte[1024]; byte[] receiveData = new byte[1024]; String sentence = inFromUser.readLine();

sendData = sentence.getBytes();

Createinput stream

Create client socket

Translate hostname to IP

address using DNS

Example: Java client (UDP), cont.

DatagramPacket sendPacket = new DatagramPacket(sendData, sendData.length, IPAddress, 9876); clientSocket.send(sendPacket); DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length); clientSocket.receive(receivePacket); String modifiedSentence = new String(receivePacket.getData()); System.out.println("FROM SERVER:" + modifiedSentence); clientSocket.close(); }

}

Create datagram with data-to-send,

length, IP addr, port

Send datagramto server

Read datagramfrom server

Example: Java server (UDP)

import java.io.*; import java.net.*; class UDPServer { public static void main(String args[]) throws Exception { DatagramSocket serverSocket = new DatagramSocket(9876); byte[] receiveData = new byte[1024]; byte[] sendData = new byte[1024]; while(true) { DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length);

serverSocket.receive(receivePacket);

Createdatagram socket

at port 9876

Create space forreceived datagram

Receivedatagra

m

Example: Java server (UDP), cont String sentence = new String(receivePacket.getData()); InetAddress IPAddress = receivePacket.getAddress(); int port = receivePacket.getPort(); String capitalizedSentence = sentence.toUpperCase();

sendData = capitalizedSentence.getBytes(); DatagramPacket sendPacket = new DatagramPacket(sendData, sendData.length, IPAddress, port); serverSocket.send(sendPacket); } }

}

Get IP addrport #, of

sender

Write out datagramto socket

End of while loop,loop back and wait foranother datagram

Create datagramto send to client

Chapter 2: Application layer

• 2.1 Principles of network applications – app architectures– app requirements

• 2.2 Web and HTTP• 2.4 Electronic Mail

– SMTP, POP3, IMAP

• 2.5 DNS

• 2.6 P2P file sharing• 2.7 Socket

programming with TCP

• 2.8 Socket programming with UDP

• 2.9 Building a Web server

Building a simple Web server

• handles one HTTP request

• accepts the request• parses header• obtains requested file

from server’s file system• creates HTTP response

message:– header lines + file

• sends response to client

• after creating server, you can request file using a browser (e.g., IE explorer)

• see text for details

Chapter 2: Summary

• Application architectures– client-server– P2P– hybrid

• application service requirements:– reliability, bandwidth, delay

• Internet transport service model– connection-oriented, reliable: TCP– unreliable, datagrams: UDP

Our study of network apps now complete!

• specific protocols:– HTTP– FTP– SMTP, POP, IMAP– DNS

• socket programming

Chapter 2: Summary

• typical request/reply message exchange:– client requests info or service– server responds with data,

status code

• message formats:– headers: fields giving info

about data– data: info being

communicated

Most importantly: learned about protocols

• control vs. data msgs– in-band, out-of-band

• centralized vs. decentralized

• stateless vs. stateful• reliable vs. unreliable

msg transfer • “complexity at network

edge”