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Questions that will be Addressed
What mechanisms are available for a programmer who writes network applications?How to write a network application that sends packets between hosts (client and server) across an IP network?Answer: socket API
Client ServerIP Network
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Socket ProgrammingTable of Contents1. Network Application Programming Interfa
ce: Sockets and Internet Sockets
2. Network Programming Tips3. Client-Server Architecture4. Example: Client Programming5. Example: Server Programming6. Network Programmer’s Mistakes
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Layers of the IP Protocol Suite
Link Layer
Transport Layer
Network Layer
Application Layer
Link Layer
Transport Layer
Network Layer
Application Layer
Ethernet
e.g. ftp
e.g. TCP, UDP
e.g. IP
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Protocol Suite Location Internet Protocol Layer
Link Layer
Transport Layer (TCP, UDP)
Network Layer (IP)
Application Layer
Network Card &Device Driver
(e.g. Ethernet card)
Operating System(e.g. Unix)
Applications(e.g. browser, game, ftp)
Application ProgrammingInterface (API)
(e.g. network API)
Interface to the Network Card
Location
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Network APIOperating system provides Application Programming Interface (API) for network applicationAPI is defined by a set of function types, data structures, and constantsDesirable characteristics of the network interface
Simple to use Flexible
independent from any application allows program to use all functionality of the network
Standardized allows programmer to learn once, write anywhere
Application Programming Interface for networks is called socket
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Sockets
Sockets provide mechanisms to communicate between computers across a network
There are different kind of sockets DARPA Internet addresses (Internet Sockets) Unix interprocess communication (Unix Sockets) CCITT X.25 addresses and many others
Berkeley sockets is the most popular Internet Socket runs on Linux, FreeBSD, OS X, Windows fed by the popularity of TCP/IP
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Internet Sockets
Support stream and datagram packets (e.g. TCP, UDP, IP)
Is Similar to UNIX file I/O API (provides a file descriptor)
Based on C, single thread model does not require multiple threads
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Types of Internet Sockets
Different types of sockets implement different communication types (stream vs. datagram)Type of socket: stream socket connection-oriented two way communication reliable (error free), in order delivery can use the Transmission Control Protocol (TCP) e.g. telnet, ssh, http
Type of socket: datagram socket connectionless, does not maintain an open
connection, each packet is independent can use the User Datagram Protocol (UDP) e.g. IP telephony
Other types exist: similar to the one above
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Byte Ordering of Integers
Different CPU architectures have different byte ordering
D3
high-order byte low-order byte
memoryaddress A
memoryaddress A
+1
Stored at little-endian computer
Stored at big-endian computer
low-order byte high-order byte
F2Integer representation (2 byte)
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Message in Memory ofof big-endian Computer
Message is sent across Network 48 45 4C 4C 6F 01
00
Byte Ordering Problem
Question: What would happen if two computers with different integer byte ordering communicate?
Message is: [Hello,1]
Message is: [Hello,256]
Pro
cess
ing
48 45 4C 4C 6F 01 00
Message in Memory oflittle-endian Computer P
roce
ssin
g
Answer:Nothing if they do not exchange integers!But: If they exchange integers, they would get the wrong order of bytes, therefore, the wrong value!
Example:
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Byte Ordering SolutionThere are two solutions if computers with different byte ordering system want to communicate
They must know the kind of architecture of the sending computer(bad solution, it has not been implemented)
Introduction of a network byte order. The functions are:
uint16_t htons(uint16_t host16bitvalue)uint32_t htonl(uint32_t host32bitvalue)uint16_t ntohs(uint16_t net16bitvalue)uint32_t ntohs(uint32_t net32bitvalue)
Note: use for all integers (short and long), which are sent across the network
Including port numbers and IP addresses
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Naming and Addressing
Host name identifies a single host (see Domain Name
System slides) variable length string (e.g. www.berkeley.edu) is mapped to one or more IP addresses
IP Address written as dotted octets (e.g. 10.0.0.1) 32 bits. Not a number! But often needs to be
converted to a 32-bit to use.
Port number identifies a process on a host 16 bit number
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Client-Server Architecture
Client requests service from serverServer responds with sending service or error message to client
Client Server
request
response
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Simple Client-Server Example
Client Serverrequest
response
socket()connect()send()
recv()close()
socket()bind()listen()accept()
recv()
send()
recv()close()
Connectionestablishment
Data response
Data request
End-of-file notification
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Example: Client Programming
Create stream socket (socket() )Connect to server (connect() )While still connected: send message to server (send() ) receive (recv() ) data from server and
process it
Close TCP connection and Socket (close())
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socket(): Initializing SocketGetting the file descriptor
int chat_sock;if ((chat_sock = socket(AF_INET, SOCK_STREAM, 0)) < 0) {
perror("socket"); printf("Failed to create socket\n"); abort ();
}
1.parameter specifies protocol/address family2.parameter specifies the socket type
Other possibilities: SOCK_DGRAM
3.parameter specifies the protocol. 0 means protocol is chosen by the OS.
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IP Address Data Structurestruct sockaddr_in {
short int sin_family; // Address family
unsigned short int sin_port; // Port number struct in_addr sin_addr; // Internet address
unsigned char sin_zero[8];
};
struct in_addr {
unsigned long s_addr; // 4 bytes
};
Padding of sin_zeros: struct sockaddr_in has same size as struct sockaddr
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connect(): Making TCP Connection to Serverstruct sockaddr_in sin;
struct hostent *host = gethostbyname (argv[1]);unsigned int server_address = *(unsigned long *) host->h_addr_list[0];unsigned short server_port = atoi (argv[2]);
memset (&sin, 0, sizeof (sin));sin.sin_family = AF_INET;sin.sin_addr.s_addr = server_address;sin.sin_port = htons (server_port);
if (connect(chat_sock, (struct sockaddr *) &sin, sizeof (sin)) < 0) { perror("connect"); printf("Cannot connect to server\n"); abort();}
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send(): Sending Packets
int send_packets(char *buffer, int buffer_len) { sent_bytes = send(chat_sock, buffer, buffer_len, 0); if (send_bytes < 0) { perror (“send"); } return 0;
}
Needs socket descriptor,Buffer containing the message, andLength of the messageCan also use write()
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Receiving Packets:Separating Data in a Stream
Use records (data structures) to partition the data stream
B
Fixed length record
Fixed length record
0 1 3
C
2 94 6 87
D
5
A
receive buffer
slide through
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Receiving Packetsint receive_packets(char *buffer, int buffer_len, int *bytes_read){
int left = buffer_len - *bytes_read; received = recv(chat_sock, buffer + *bytes_read, left, 0); if (received < 0) { perror (“recv"); } if (received <= 0) { return close_connection(); } *bytes_read += received; while (*bytes_read > RECORD_LEN) { process_packet(buffer, RECORD_LEN); *bytes_read -= RECORD_LEN; memmove(buffer, buffer + RECORD_LEN, *bytes_read); } return 0;}
Can also use read()
buffer
*bytes_read
buffer_len
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Server Programming: Simple
Create stream socket (socket() )Bind port to socket (bind() )Listen for new client (listen() )While
accept user connection and create a new socket (accept() )data arrives from client (recv() )data has to be send to client (send() )
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bind(): Assign IP and Portstruct sockaddr_in sin;
struct hostent *host = gethostbyname (argv[1]);unsigned int server_address = *(unsigned long *) host->h_addr_list[0];unsigned short server_port = atoi (argv[2]);
memset (&sin, 0, sizeof (sin));sin.sin_family = AF_INET;sin.sin_addr.s_addr = server_address;sin.sin_port = htons (server_port);
if (bind(chat_sock, (struct sockaddr *) &sin, sizeof (sin)) < 0) { perror("bind"); printf("Cannot bind server application to network\n"); abort();}
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bind():bind() tells the OS to assign a local IP address and local port number to the socket.Many applications let the OS choose an IP address.
Use wildcard INADDR_ANY as local address in this case.
At server, user process must call bind() to assign a portAt client, bind() is not required since OS may assign available port and IP address
The server will get the port number of the client through the UDP/TCP packet header
Note: Each application is represented by a server port number
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listen(): Wait for Connections
int listen(int sockfd, int backlog);
Puts socket in a listening state, willing to handle incoming TCP connection request.Backlog: number of TCP connections that can be queued at the socket.
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Server Example#define MYPORT 3490 // the port users will be connecting to #define BACKLOG 10 // how many pending connections queue will hold
int main(void) { int sockfd, new_fd; // listen on sockfd, new connection on new_fd struct sockaddr_in my_addr; // my address information struct sockaddr_in their_addr; // connector's address information int sin_size;
if ((sockfd = socket(AF_INET, SOCK_STREAM, 0)) == -1) { perror("socket");
exit(1); }
my_addr.sin_family = AF_INET; // host byte ordermy_addr.sin_port = htons(MYPORT); // short, network byte order my_addr.sin_addr.s_addr = INADDR_ANY; // auto. filled with local IP memset(&(my_addr.sin_zero), '\0', 8); // zero the rest of the struct
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if (bind(sockfd, (struct sockaddr *)&my_addr, sizeof(struct sockaddr)) == -1) { perror("bind"); exit(1);
}
if (listen(sockfd, BACKLOG) == -1) { perror("listen"); exit(1);
}
while(1) { // main accept() loop sin_size = sizeof(struct sockaddr_in); if ((new_fd = accept(sockfd, (struct sockaddr *)&their_addr, &sin_size)) == -1)
{ perror("accept"); continue;
}
printf("server: got connection from %s\n", inet_ntoa(their_addr.sin_addr));
if (send(new_fd, "Hello, world!\n", 14, 0) == -1) perror("send");
close(new_fd); }return 0;
}
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Client Example#include <netinet/in.h> #include <sys/socket.h>
#define PORT 3490 // the port client will be connecting to #define MAXDATASIZE 100 // max number of bytes we can get
// at once int main(int argc, char *argv[]) {
int sockfd, numbytes; char buf[MAXDATASIZE]; struct hostent *he; struct sockaddr_in their_addr; // server's address information
if (argc != 2) { fprintf(stderr,"usage: client hostname\n"); exit(1);
}
if ((he=gethostbyname(argv[1])) == NULL) { // get the host info perror("gethostbyname"); exit(1);
}
if ((sockfd = socket(AF_INET, SOCK_STREAM, 0)) == -1) { perror("socket"); exit(1);
}
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their_addr.sin_family = AF_INET; // host byte order their_addr.sin_port = htons(PORT); // short, network byte order their_addr.sin_addr = *((struct in_addr *)he->h_addr); // already network byte order memset(&(their_addr.sin_zero), '\0', 8); // zero the rest of the struct
if (connect(sockfd, (struct sockaddr *)&their_addr, sizeof(struct sockaddr)) == -1){ perror("connect"); exit(1);
}
if ((numbytes=recv(sockfd, buf, MAXDATASIZE-1, 0)) == -1) { perror("recv"); exit(1);
}
buf[numbytes] = '\0'; printf("Received: %s",buf); close(sockfd); return 0;
}
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I/O Blocking
socket();bind() ;listen();
whileaccept();recv() ;send() ;
Simple server has blocking problem Suppose 5 connections accepted. Suppose next accept() blocks. Other connections cannot send and receive. Cannot get keyboard input either.
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select() :I/O Multiplexingwaits on multiple file descriptors and timeoutreturns when any file descriptor is ready to be read or written or indicate an error, or timeout exceeded
advantages simple application does not consume CPU cycles while
waiting
disadvantages does not scale to large number of file
descriptors
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Example: Server Programming
create stream socket (socket() )Bind port to socket (bind() )Listen for new client (listen() )While Wait for (select() )
(depending on which file descriptors are ready)accept user connection and create a new socket (accept() )data arrives from client (recv() )data has to be send to client (send() )
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Server: Alternative Ways of Handling Many Clients
Forking a new process for each client: fork()But, creating new process is expensive.Multithreaded implementation: have one thread handling each client.Thread is like a process but light-weighted.
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Network Programmer’s Mistakes
byte orderingseparating records in streamsuse of select()misinterpreting the project specificationnot knowing all available system calls
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There are more System Calls
Depends on communication type Datagram sockets use recvfrom() and
sendto() for receiving and sending data
Closing connection: close(), shutdown()Convenient functions (on UNIX) inet_aton, inet_ntoa inet_pton, inet_ntop
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Literatureshort tutorial: Beej's Guide to Network Programming http://www.ecst.csuchico.edu/~beej/guide/net/
Unix Network Programming, volumes 1 and 2 by W. Richard Stevens. Published by Prentice Hall; ISBNs for volumes 1 and 2: 013490012X, 0130810819.Advanced Programming in the Unix Environment by W. Richard Stevens. Published by Addison Wesley. ISBN 0201563177. man pages on a Unix computer
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