Sockets Final

Sockets Final - Transcript

CS 640 Computer Networking
Ashutosh Shukla Lecture 3 Network Programming

Topics
Client server model Sockets interface Socket primitives Example code for echoclient and echoserver Debugging With GDB Programming Assignment 1 MNS

Client asks request server provides response Typically single server multiple clients The server does not need to know anything about the client even that it exists The client should always know something about the server at least where it is located
1 Client sends request Client process 4 Client handles response 3 Server sends response Server process 2 Server handles request Resource

Client sever model

Note clients and servers are processes running on hosts can be the same or different hosts

Sockets as means for inter process communication IPC

application layer

application layer

Client Process Socket
transportnetwork OS layer TCP UDP network layer IP link layer e g ethernet physical layer

Internet

Server Process Socket
transport layer TCP UDP link layer e g ethernet physical layer

stack

Internet Internet

OS network network layer IP stack

The interface that the OS provides to its networking subsystem

Internet Connections TCP IP
Address the machine on the network Address the process
By IP address By the port number

The pair of IP address port makes up a socket address
Client socket address 128 2 194 242 3479 Server socket address 208 216 181 15 80

Client

Connection socket pair 128 2 194 242 3479 208 216 181 15 80

Server port 80 Server host address 208 216 181 15

Client host address 128 2 194 242 Note 3479 is an ephemeral port allocated by the kernel

Note 80 is a well known port associated with Web servers

Examples of client programs

Clients

How does a client find the server

Web browsers ftp telnet ssh The IP address in the server socket address identifies the host The well known port in the server socket address identifies the service and thus implicitly identifies the server process that performs that service Examples of well known ports
Port 7 Echo server Port 23 Telnet server Port 25 Mail server Port 80 Web server

Using Ports to Identify Services
Server host 128 2 194 242 Client host Service request for 128 2 194 242 80 i e the Web server Web server port 80 Kernel Echo server port 7

Client

Client

Service request for 128 2 194 242 7 i e the echo server

Web server port 80 Kernel Echo server port 7

Servers are long running processes daemons

Servers

Each server waits for requests to arrive on a well known port associated with a particular service
Port 7 echo server Port 23 telnet server See etc services for a Port 25 mail server comprehensive list of the Port 80 HTTP server services available on a Linux machine Other applications should choose between 1024 and 65535

Created at boot time typically by the init process process 1 Run continuously until the machine is turned off

Sockets
What is a socket
To the kernel a socket is an endpoint of communication To an application a socket is a file descriptor that lets the application read write from to the network
Remember All Unix I O devices including networks are modeled as files

Clients and servers communicate with each by reading from and writing to socket descriptors The main distinction between regular file I O and socket I O is how the application opens the socket descriptors

Network Byte Ordering


Socket Programming Cliches

Network is big endian host may be big or little endian Functions work on 16 bit short and 32 bit long values htons htonl convert host byte order to network byte order ntohs ntohl convert network byte order to host byte order Use these to convert network addresses ports

struct sockaddr in serveraddr fill in serveraddr with an address Connect takes struct sockaddr as its second argument connect clientfd struct sockaddr serveraddr sizeof serveraddr

SOCKET int socket int domain int type int protocol
domain AF INET IPv4 protocol type SOCK DGRAM or SOCK STREAM protocol 0 IPPROTO UDP or IPPROTO TCP returned socket descriptor sockfd 1 is an error

Socket primitives

BIND int bind int sockfd struct sockaddr my addr int addrlen
sockfd socket descriptor returned from socket my addr socket address struct sockaddr in is used addrlen sizeof struct sockaddr


struct sockaddr in unsigned short sin family unsigned short sin port struct in addr sin addr unsigned char sin zero 8

address family always AF INET port num in network byte order IP addr in network byte order pad to sizeof struct sockaddr

LISTEN int listen int sockfd int backlog ACCEPT int accept int sockfd void addr int addrlen CONNECT int connect int sockfd struct sockaddr serv addr int addrlen used by TCP client SEND int send int sockfd const void msg int len int flags
msg message you want to send len length of the message flags 0 returned the number of bytes actually sent parameters are same as for bind addr here the socket address of the caller will be written returned a new socket descriptor for the temporal socket backlog how many connections we want to queue

RECEIVE int recv int sockfd void buf int len unsigned int flags
buf buffer to receive the message len length of the buffer don t give me more flags 0 returned the number of bytes received

SEND DGRAM style int sendto int sockfd const void msg int len int flags const struct sockaddr to int tolen
msg message you want to send len length of the message flags 0 to socket address of the remote process tolen sizeof struct sockaddr returned the number of bytes actually sent

RECEIVE DGRAM style int recvfrom int sockfd void buf int len unsigned int flags struct sockaddr from int fromlen
buf buffer to receive the message len length of the buffer don t give me more from socket address of the process that sent the data fromlen sizeof struct sockaddr flags 0 returned the number of bytes received

CLOSE close socketfd

Client server connectionless
CREATE BIND

SEND RECEIVE SEND CLOSE

Client server connection oriented
BIND SOCKET CONNECT TCP three way handshake ACCEPT LISTEN

SEND

RECEIVE

SEND RECEIVE CLOSE

Concurrent server

EchoClient c include s
include stdio h for printf and fprintf include sys socket h for socket connect sendto and recvfrom include arpa inet h for sockaddr in and inet addr include stdlib h for atoi and exit include string h for memset include unistd h for close define ECHOMAX 255 Longest string to echo

EchoClient c variable declarations
int main int argc char argv int sock Socket descriptor struct sockaddr in echoServAddr Echo server address struct sockaddr in fromAddr Source address of echo unsigned short echoServPort 7 Echo server port unsigned int fromSize address size for recvfrom char servIP 172 24 23 4 IP address of server char echoString I hope this works String to send to echo server char echoBuffer ECHOMAX 1 Buffer for receiving echoed string int echoStringLen Length of string to echo int respStringLen Length of received response

EchoClient c creating the socket and sending
Create a datagram UDP socket sock socket AF INET SOCK DGRAM 0 Construct the server address structure memset echoServAddr 0 sizeof echoServAddr Zero out structure echoServAddr sin family AF INET Internet addr family echoServAddr sin addr s addr htonl servIP Server IP address echoServAddr sin port htons echoServPort Server port Send the string to the server sendto sock echoString echoStringLen 0 struct sockaddr echoServAddr sizeof echoServAddr Recv a response

EchoClient c receiving and printing
fromSize sizeof fromAddr recvfrom sock echoBuffer ECHOMAX 0 struct sockaddr fromAddr fromSize Error checks like packet is received from the same server null terminate the received data echoBuffer echoStringLen 0 printf Received s n echoBuffer Print the echoed arg close sock exit 0 end of main

EchoServer c
int main int argc char argv int sock Socket struct sockaddr in echoServAddr Local address struct sockaddr in echoClntAddr Client address unsigned int cliAddrLen Length of incoming message char echoBuffer ECHOMAX Buffer for echo string unsigned short echoServPort 7 Server port int recvMsgSize Size of received message Create socket for sending receiving datagrams sock socket AF INET SOCK DGRAM 0 Construct local address structure memset echoServAddr 0 sizeof echoServAddr Zero out structure echoServAddr sin family AF INET Internet address family echoServAddr sin addr s addr htonl 172 24 23 4 echoServAddr sin port htons echoServPort Local port Bind to the local address bind sock struct sockaddr echoServAddr sizeof echoServAddr

for Run forever cliAddrLen sizeof echoClntAddr Block until receive message from a client recvMsgSize recvfrom sock echoBuffer ECHOMAX 0 struct sockaddr echoClntAddr cliAddrLen printf Handling client s n inet ntoa echoClntAddr sin addr Send received datagram back to the client sendto sock echoBuffer recvMsgSize 0 struct sockaddr echoClntAddr sizeof echoClntAddr end of main Error handling is must

man is your friend
man accept man sendto Etc

Socket Programming Help

The manual page will tell you

What include directives you need at the top of your source code The type of each argument The possible return values The possible errors in errno

Prepare program for debugging Two main ways to run gdb
On program directly
run args

Debugging with gdb

Compile with g keep full symbol table Don t use compiler optimization O O2

gdb progname Once gdb is executing we can execute the program with

On a core post mortem

Extensive in program documentation exists

gdb progname core Useful for examining program state at the point of crash

help or help topic or help command

More information
Socket programming
W Richard Stevens UNIX Network Programming Infinite number of online resources http www cs rpi edu courses sysprog sockets sock html

GDB

Official GDB homepage http www gnu org software gdb gdb html GDB primer http www cs pitt edu mosse gdb note html