2.1
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Chapter 2
Application Layer
Chapter 2: Outline
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2.1 INTRODUCTION
2.2 CLIENT-SERVER PARADIGM
2.3 STANDARDAPPLICATIONS
2.4 PEER-TO-PEER PARADIGM
2.5 SOCKET-INTERFACE PROGRAMMING
Chapter 2: Objective
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We introduce the nature of services provided by the Internet: the client-server paradigm and the peer-to-peer paradigm.
We discuss the concept of the client-server paradigm.
We discuss some predefined or standard applications based on the client-server paradigm such as surfing the Web, file transfer, e-mail, and so on.
We show how a new application process can be created in the client-server paradigm by writing two programs in the C language.
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1 INTRODUCTION
• The application layer provides services to the user.
• Communication is provided using a logical
connection.
• Which means that the two application layers assume that there is an imaginary direct connection through which they can send and receive messages.
• Figure 2.1 in the next slide shows the idea behind this logical connection.
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Figure 2.1: Logical connection at the application layer
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2.1.1 Providing Services
• The Internet to provide service to users.
• Application layer as interface and platform to new application & application protocols to be easily added to the Internet
• Growth of Internet and application protocols.
• today Internet and application protocols are being added constantly.
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2.1.1 Providing Services (Cont.)
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Standard and Nonstandard Protocols Standard Application-Layer Protocols
Nonstandard Application-Layer Protocols
2.1.2 Application-Layer Paradigm
• Internet requires two processes on two application programs to interact with each other.
• Source and destination interaction.
• Two programs need to send messages to each
other through the Internet infrastructure.
• Require to discussed relationship between these
programs.
• Should both application programs be able to
request services and provide services, or should the application programs just do one or the other?
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2.1.2 Application-Layer Paradigm (cont)
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Traditional Paradigm: Client-Server New Paradigm: Peer-to-Peer Mixed Paradigm
Figure 2.2: Example of a client-server paradigm
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Figure 2.3: Example of a peer-to-peer paradigm
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2
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2 CLIENT
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SERVER PARADIGM
In this paradigm:
• communication at the application layer is between two running application programs called processes: a client and a server.
• A client is a running program that initializes the communication by sending a request;
• A server is another application program that waits for a request from a client.
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2.2.1 Application Programming Interface
• A computer language has
• a set of instructions for mathematical operations,
• a set of instructions for string manipulation,
• a set of instructions for input/ output access, and so
on.
• If we need a process to be able to communicate
with another process, we need
• a new set of instructions to tell the lowest four layers
of the TCP/IP suite to open the connection.
• Send and Receive data from the other end, and
• Close the connection.
• A set of instructions of this kind is normally referred
to as Application Programming Interface (API).
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2.2.1 Application Programming Interface (continued)
Sockets
Socket Addresses: combination of an IP address and a port number
Finding Socket Addresses Server Site
2.14
Client Site
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Figure 2.4: Position of the socket interface
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Figure 2.5: A Sockets used like any other sources and sinks
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Figure 2.6: Use of sockets in process-to-process communication
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Figure 2.7: A socket address
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• A socket is one endpoint of a two-way communication link between two programs running on the network
• A socket is bound to a port number so that the TCP layer can identify the application that data is destined to be sent to
• An endpoint = Socket is a combination of an IP address and a port number
Example 2.1 Analogy – telephone & socket
• We can find a two-level address in telephone communication.
• A telephone number can define an organization, and an extension can define a specific connection in that organization.
• The telephone number in this case is similar to the IP address, which defines the whole organization;
• the extension is similar to the port number, which defines the particular connection.
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2.2.2 Using Services of Transport Layer
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UDP Protocol
TCP Protocol
SCTP Protocol
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• A pair of processes provide services to the users of the Internet, human or programs.
• A pair of processes, however, need to use the services provided by the transport layer for communication because there is no physical communication at the application layer.
• There are three common transport layer protocols in the TCP/IP suite: UDP, TCP, and SCTP.
–
–
2
APPLICATIONS
3 STANDARD CLIENT
SERVER
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• During the lifetime of the Internet, several application programs have been developed.
• We do not have to redefine them, but we need to understand what they do.
• For each application, we also need to know the options available to us.
• The study of these applications can help us to create customized applications in the future.
2.3.1 World Wide Web and HTTP
• In this section, we first introduce the World Wide Web (abbreviated WWW or Web).
• We then discuss the Hyper Text Transfer Protocol (HTTP), the most common client-server application program used in relation to the Web.
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2.3.1 (continued)
World Wide Web Architecture
Uniform Resource Locator (URL) Web Documents
HyperText Transfer Protocol (HTTP) Nonpersistent versus Persistent Connections
Message Formats Conditional Request Cookies
Web Caching: Proxy Server Proxy Server Location
Cache Update
HTTP Security
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Example 2.2
Assume we need to retrieve a scientific document that contains one reference to another text file and one reference to a large image. Figure 2.8 shows the situation.
• The main document and the image are stored in two separate files in the same site (file A and file B);
• The referenced text file is stored in another site (file C).
• So we are dealing with three different files, we need three
transactions if we want to see/get the whole document.
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Figure 2.8: HTTP Example 2.2 (Retrieving two files and one image)
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Figure 2.9: Browser
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Example 2.3
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The URL http://www.mhhe.com/engcs/compsci/forouzan/ defines the web page related to one of the computer in the McGraw-Hill company (the three letters www are part of the host name and are added to the commercial host).
The path is compsci/forouzan/, which defines Forouzan’s web page under the directory compsci (computer science).
Example 2.4
Figure 2.10 shows an example of a nonpersistent connection
• The client needs to access a file that contains one link to an image. The text file and image are located on the same server.
• Here we need two connections.
• For each connection, TCP requires at least three handshake
messages( SYN, SYN-ACK, ACK) to establish the connection, but the request can be sent with the third one. After the connection is established, the object can be transferred.
• After receiving an object, another three handshake messages( FIN, FIN-ACK, ACK) are needed to terminate the connection.
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Figure 2.10: Example 2.4 (HTTP non-persistent connection)
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Example 2.5
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Figure 2.11 shows the same scenario as in Example 2.4, but using a persistent connection.
• Only one connection establishment and connection termination is used, but the request for the image is sent separately.
Figure 2.11: Example 2.5 (HTTP persistent connection)
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Figure 2.12: Formats of the request and response messages
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Table 2.1: Methods
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Table 2.2: Request Header Names
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Table 2.3: Response Header Names
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Figure 2.13: Example 2.6
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Figure 2.14: Post on the server. We use the PUT method. Example 2.7
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Example 2.8
The following shows how a client imposes/forces the modification data and time condition on a request.
The status line in the response shows the file was not modified after the defined point in time. The body of the response message is also empty.
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Example 2.9: Use of cookies
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Figure 2.15 shows a scenario in which an electronic store
can benefit from the use of cookies.
• Assume a shopper wants to buy a toy from an electronic store
named BestToys.
• The shopper browser (client) sends a request to the BestToys
server.
• The server creates an empty shopping cart (a list) for the client and
assigns an ID to the cart (for example, 12343).
• The server then sends a response message, which contains the
images of all toys available, with a link under each toy that selects
the toy if it is being clicked.
• This response message also includes the Set-Cookie header line
whose value is 12343.
• The client displays the images and stores the cookie value in a file
named BestToys.
Figure 2.15: Example 2.9
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Example 2.10: Use of Proxy Server
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Figure 2.16 shows an example of a use of a proxy server in a local network, such as the network on a campus or in a
company.
• The proxy server is installed in the local network.
• When an HTTP request is created by any of the clients
(browsers), the request is first directed to the proxy server.
• If the proxy server already has the corresponding web page, it
sends the response to the client.
• Otherwise, the proxy server acts as a client and sends the
request to the web server in the Internet.
• When the response is returned, the proxy server makes a copy
and stores it in its cache before sending it to the requesting client.
Figure 2.16: Example of a proxy server
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2.3.2 FTP
File Transfer Protocol (FTP) is the standard protocol provided by TCP/IP for copying a file from one host to another. Although transferring files from one system to another seems simple and straightforward, some problems must be dealt with first. For example, two systems may use different file name conventions. Two systems may have different ways to represent data. All of these problems have been solved by FTP in a very simple and elegant approach.
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2.3.2 (continued)
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Lifetimes of Two Connections Control Connection
Data Connection
Communication over Data Connection
File Transfer
Figure 2.17: FTP
Control connection
Data connection
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2.3.4 TELNET
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• A server program can provide a specific service to its corresponding client program. However, it is impossible to have a client/server pair for each type of service we need.
• Another solution is to have a specific client/server program for a set of common scenarios, but to have some generic client/server programs that allow a user on the client site to log into the computer at the server site and use the services available there.
• We refer to these generic client/server pairs as remote logging applications. One of the original remote logging protocols is TELNET.
• However, because of serious security concerns when using Telnet over an open network such as the Internet, its use for this purpose has been waned against & significantly in favour of SSH2.
2.3.5 Secure Shell (SSH)
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• Secure Shell (SSH) is a cryptographic network protocol for operating network services securely over an unsecured network.
• SSH provides a secure channel over an unsecured network in a client- server architecture, connecting an SSH client application with an SSH server.
• The most visible application of the protocol is for access to shell accounts on Unix, Linux, windows and other O.S
• SSH was designed as a replacement for TELNET and for unsecured remote shell protocols such as the Berkeley rlogin, rsh, and rexec protocols.
• There are two versions of SSH: SSH-1 and SSH-2, which are totally incompatible. The first version, SSH-1, is now deprecated because of security flaws in it. In this section, we discuss only SSH-2.
Figure 2.33: Port Forwarding
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2.3.6 Domain Name System (DNS)
• To identify an entity, TCP/IP protocols use the IP address, which uniquely identifies the connection of a host to the Internet.
• However, people prefer to use names instead of numeric addresses.
• Therefore, the Internet needs to have a directory system that can map a name to an address.
• This is analogous to the telephone network.
• A telephone network is designed to use telephone numbers, not names. People can either keep a private file to map a name to the corresponding telephone number or can call the telephone directory to do so.
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2.3.6 (continued)
2.50
Name Space
Domain Name Space
Domain
Distribution of Name Space Zone
Root Server
DNS in the Internet Generic Domains
Country Domains
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2.3.1 (continued)
Resolution
Recursive Resolution Iterative Resolution Caching
Resource Records DNS Messages
Encapsulation
Registrars
DDNS Security of DNS
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Figure 2.35: Purpose of DNS Host-name resolution
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Figure 2.36: Domain name space
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Figure 2.37: Domain names and labels
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Figure 2.39: Hierarchy of name servers
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Figure 2.41: Generic domains
Table 2.12: Generic domain labels
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Figure 2.42: Country domains
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Figure 2.43: Recursive Host-name resolution
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1
8
6
4
6
9
170
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3
1
2
4
8
67
5
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6
9
170
Figure 2.44: Iterative Host-name resolution
Table 2.13: DNS types
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Figure 2.45: DNS message
Note: Both queries and responses have the same general format, containing up to five individual sections carrying information.
The query message:
• contains only the Question section !
While the response messages:
• include the Question, Answer section, Authoritative and Additional sections.
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Example 2.14
In UNIX and Windows, the nslookup utility can be used to retrieve address/name mapping. The following shows how we can retrieve an address when the domain name is given.
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2
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5 SOCKET
–
INTERFACE PROGRAMMING
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In this part of lecture, we will see some examples how to write some simple client-server programs using C, a procedural programming language.
• Here in this lecture we have chose the C language;
• In the last lecture (chapter 11), we expand this idea in Java, which provides a more compact version.
2.5.1 Socket Interface in C
• First we shall see how this interface in C language.
• Then understand socket interface and the role of a
socket in communication.
• The socket has no buffer to store data to be sent or received. It is capable of neither sending nor receiving data.
• The socket just acts as a reference or a label. The buffers and necessary variables are created inside the operating system.
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2.65
2.5.1 (continued)
Data Structure for Socket Header Files
Communication Using UDP
Sockets Used for UDP
Communication Flow Diagram Programming Examples
Communication Using TCP Sockets Used in TCP
Communication Flow Diagram
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Figure 2.58: Socket data structure
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Figure 2.59: Sockets for UDP communication
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Figure 2.60: Flow diagram for iterative UDP communication
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UDP ECHO CLIENT SERVER
AIM: To write a program for UDP echo client server.
SERVER ALGORITHM:
STEP 1: Start
STEP 2: Declare the variables for the socket
STEP 3: Specify the family, protocol, IP address and port number STEP 4: Create a socket using socket() function
STEP 5: Bind the IP address and Port number
STEP 6: Listen and accept the client’s request for the connection STEP 7: Read and Display the client’s message
STEP 8: Stop
CLIENT ALGORITHM:
STEP 1: Start
STEP 2: Declare the variables for the socket
STEP 3: Specify the family, protocol, IP address and port number STEP 4: Create a socket using socket() function
STEP 5: Call the connect() function
STEP 6: Read the input message
STEP 7: Send the input message to the server
STEP 8: Display the server’s echo
STEP 9: Close the socket
STEP 10: Stop
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SOURCE SERVER CODE:
#include
int main(int argc,char **argv)
{int sockfd;
int n;
socklen_t len; char msg[1024];
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Reference: http://www.sourcecodesolutions.in/2010/09/udp-echo-client-server.html
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Echo server program using UDP
struct sockaddr_in servaddr,cliaddr; sockfd=socket(AF_INET,SOCK_DGRAM,0); bzero(&servaddr,sizeof(servaddr)); servaddr.sin_family=AF_INET; servaddr.sin_addr.s_addr=INADDR_ANY; servaddr.sin_port=htons(5035);
printf(“\n\n Binded”);
bind(sockfd,(struct sockaddr*)&servaddr,sizeof(servaddr)); printf(“\n\n Listening…”);
for(;;)
{
printf(“\n “);
len=sizeof(cliaddr); n=recvfrom(sockfd,msg,MAXLINE,0,(struct
sockaddr*)&cliaddr,&len);
printf(“\n Client’s Message : %s\n”,msg); if(n<6)
perror("send error");
} sendto(sockfd,msg,n,0,(struct sockaddr*)&cliaddr,len);
return 0; }
Echo client program using UDP
SOURCE CLIENT CODE:
#include
#define MAXLINE 1024
int main(int argc,char* argv[])
{int sockfd;
int n;
socklen_t len;
char sendline[1024],recvline[1024]; struct sockaddr_in servaddr; strcpy(sendline,””);
printf(“\n Enter the message : “); scanf(“%s”,sendline);
sockfd=socket(AF_INET,SOCK_DGRAM,0); bzero(&servaddr,sizeof(servaddr)); servaddr.sin_family=AF_INET; servaddr.sin_addr.s_addr=inet_addr(“127.0.0.1”); servaddr.sin_port=htons(5035);
connect(sockfd,(struct sockaddr*)&servaddr,sizeof(servaddr)); len=sizeof(servaddr); sendto(sockfd,sendline,MAXLINE,0,(struct sockaddr*)&servaddr,len); n=recvfrom(sockfd,recvline,MAXLINE,0,NULL,NULL );
recvline[n]=0;
printf(“\n Server’s Echo : %s\n\n”,recvline); return 0;
}
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Reference: http://www.sourcecodesolutions.in/2010/09/udp-echo-client-server.html
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Figure 2.61: Sockets used in TCP communication
2
Create
5
Create
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Please check out the other program: Socket program written in C using TCP transport protocol to chat between tcpchatserver.c & tcpchatclient.c
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http://mcalabprogram.blogspot.com.au/2012/01/tcp-sockets-chat-applicationserver.html
Please check out the other program: Socket program written in C using TCP transport protocol to get Date & Timestamp from server TStmpserver.c & TStmpclient.c
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Reference: https://www.thegeekstuff.com/2011/12/c-socket-programming/
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Chapter 2: Summary
Applications in the Internet are designed using either a client-server paradigm or a peer-to-peer paradigm.
The World Wide Web (WWW) is a repository of information linked together from points all over the world. Hypertext and hypermedia documents are linked to one another through pointers. The HyperText Transfer Protocol (HTTP) is the main protocol used to access data on the World Wide Web (WWW).
File Transfer Protocol (FTP) is a TCP/IP client-server application for copying files from one host to another. FTP requires two connections for data transfer: a control connection and a data connection.
TELNET is a client-server application that allows a user to log into a remote machine, giving the user access to the remote system.
The Domain Name System (DNS) is a client-server application that identifies each host on the Internet with a unique name. DNS organizes the name space in a hierarchical structure to decentralize the responsibilities involved in naming.
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