Socket Programming. Daniel Zappala. CS 360 Internet Programming Brigham Young University

Transcription

1 Socket Programming Daniel Zappala CS 360 Internet Programming Brigham Young University

2 Sockets, Addresses, Ports

3 Clients and Servers 3/33 clients request a service from a server using a protocol need an API for sending and receiving data need an abstraction for the Internet a reliable connection: TCP an unreliable message service: UDP these are provided by the BSD socket API

4 Processes and Sockets 4/33 inter-process communication uses messages sent on sockets socket API defines how to open, close, read, and write to socket which transport protocol to use various communication parameters

5 Socket API 5/33 BSD Socket API - the dominant socket API on Linux, BSD, Windows Use the man pages! shows C syntax, with include files gives a description of how the system call works lists return values lists errors lists other relevant man pages man man man socket the socket API is in section 2 of the manual, so you will need to specify the section if the same command exists in an earlier section: man 2 bind

6 Addresses and Ports 6/33 to talk to a process on another machine, you need to identify it IP address: identifies the machine port number: identifies the socket the process is using

7 IP Addresses 7/33 IPv4 address identifies an interface/link on a host or router 32 bits dotted-decimal notation: each part is 8 bits

8 Ports 8/33 identifier for a socket on which a process is listening 16 bits a process may listen on more than one socket; each must be on a separate port Operating systems (such as Linux) designate some ports as privileged, meaning only the superuser can listen on them (typically ports less than 1024) To help find common servers, the IANA designates well-known ports for many protocols HTTP: 80 SMTP: 25 SSH: 22 NTP: 123

9 ClientAPI

10 Client API 10/33 1 socket(): create a socket endpoint 2 connect(): connect to a server 3 send(): send data 4 recv(): receive data 5 close(): close the socket

11 Creating a Socket 11/33 1 i n t s o c k e t ( i n t domain, i n t type, i n t p r o t o c o l ) ; 2 3 domain = PF INET 4 t y p e = SOCK STREAM f o r TCP, SOCK DGRAM f o r UDP 5 p r o t o c o l = 0 on success returns a socket descriptor

12 Connecting to a Server 12/33 1 i n t c o n n e c t ( i n t sockfd, c o n s t s t r u c t s o c k a d d r s e r v a d d r, 2 s o c k l e n t a d d r l e n ) ; 3 4 s o c k f d = s o c k e t you c r e a t e d 5 s e r v a d d r = p o i n t e r to s o c k e t a d d r e s s s t r u c t u r e 6 s o c k l e n t = l e n g t h o f s o c k e t a d d r e s s connects to a server uses the socket address structure to pass an IP address and port on success returns zero on error returns -1 and sets errno

13 Sending and Receiving Data 13/33 usually the client sends a request to the server and the server sends a reply socket operations are similar to reading from and writing to a file a socket descriptor acts like a file handle sending writing receiving reading

14 Send and Receive Syntax 14/33 1 s s i z e t send ( i n t s, c o n s t v o i d buf, s i z e t len, i n t f l a g s ) ; 2 s s i z e t r e c v ( i n t s, v o i d buf, s i z e t len, i n t f l a g s ) ; s = socket buf = pointer to buffer len = size of buffer in bytes on success returns the number of bytes actually sent or received if it is less than what you expected, then you must repeat the system call until all data is sent or received recv() will return 0 if the socket has been closed on error returns -1 and sets errno see man pages for more advanced options

15 Closing a Socket 15/33 1 #i n c l u d e <u n i s t d. h> 2 3 i n t c l o s e ( i n t f d ) ; fd = socket on success returns zero on error returns -1 and sets errno releases the socket file descriptor so it can be re-used!

16 Example 16/33 See simple echo client on github. GitHub

17 Socket Addresses

18 Socket Addresses 18/33 to connect a socket, you must supply a socket address structure that includes the IP address and port of the server you are connecting to 1 use DNS to convert host name to IP address 2 initialize a socket address structure

19 Generic Socket Address Structure 19/33 1 s t r u c t s o c k a d d r { 2 s a f a m i l y t s a f a m i l y ; // a d d r e s s f a m i l y 3 c h a r s a d a t a [ 1 4 ] ; // a d d r e s s 4 } used to represent a generic connection between processes potentially provides access to many different addressing standards address family can be AF UNIX : local UNIX socket AF INET : Internet socket

20 IPv4 Socket Address Structure 20/33 1 s t r u c t s o c k a d d r i n { 2 s a f a m i l y t s i n f a m i l y ; // a d d r e s s f a m i l y 3 u i n t 1 6 t s i n p o r t ; // p o r t 4 s t r u c t i n a d d r s i n a d d r ; // I n t e r n e t a d d r e s s 5 c h a r s i n z e r o [ 8 ] ; // unused 6 } can cast IPv4 socket into a generic socket port is 2 bytes, address is 4 bytes, zero is 8 bytes = 14 bytes IPv4 address structure: 1 s t r u c t i n a d d r { 2 u i n t 3 2 t s a d d r ; // a d d r e s s 3 } ;

21 Network Byte Order 21/33 you must store the port and address in network byte order (most significant byte sent first) provides interoperability among Internet hosts use htons() for the port and inet aton, inet addr() or inet makeaddr() for the address 1 s t r u c t s o c k a d d r i n s e r v e r ; 2 s e r v e r. s i n p o r t = htons ( 8 0 ) ; 3 i f (! i n e t a t o n ( i p a d d r e s s,& s e r v e r. s i n a d d r ) ) 4 p r i n t f ( i n e t a d d r ( ) c o n v e r s i o n e r r o r \n ) ;

22 Example 22/33 See simple echo client on GitHub. GitHub

23 Server API

24 Server API 24/33 1 create a socket 2 bind: associate the socket with an address and port 3 listen: convert socket to listen for incoming connections 4 accept: accept an incoming client connection 5 send: send data 6 recv: receive data

25 Binding the Socket 25/33 1 i n t b i n d ( i n t sockfd, c o n s t s t r u c t s o c k a d d r my addr, 2 s o c k l e n t a d d r l e n ) ; 3 4 s o c k f d = s o c k e t you c r e a t e d 5 my addr = p o i n t e r to s o c k e t a d d r e s s s t r u c t u r e 6 a d d r l e n = l e n g t h o f s o c k e t a d d r e s s associates an address with a socket uses the socket address structure to pass an IP address and port on success returns zero, -1 and errno otherwise

26 Listening on the Socket 26/33 1 i n t l i s t e n ( i n t sockfd, i n t b a c k l o g ) ; 2 3 s o c k f d = s o c k e t you c r e a t e d and bound 4 b a c k l o g = maximum w a i t i n g c o n n e c t i o n s converts a socket to a passive socket (one that accepts connections rather than connecting) backlog is the maximum number of waiting connections the kernel should hold in a queue maximum value in Linux is 128 on success returns zero, -1 and errno otherwise

27 Accepting a Connection 27/33 1 i n t a c c e p t ( i n t sockfd, s t r u c t s o c k a d d r addr, 2 s o c k l e n t a d d r l e n ) ; 3 4 s o c k f d = s o c k e t you c r e a t e d and bound 5 addr = p o i n t e r to empty s o c k e t a d d r e s s s t r u c t u r e 6 a d d r l e n = l e n g t h o f empty s o c k e t a d d r e s s s t r u c t u r e accept a connection from a client; gets the next connection waiting in the queue if there are no pending connections, the process sleeps assuming this is a blocking socket (this is the default) on success returns a new socket descriptor on success, accept() fills in the address of the client

28 Example 28/33 See simple echo server on GitHub. GitHub

29 Parsing Messages

30 Example 30/33 Try simple echo client with slow echo server in the socket example code. GitHub

31 Parsing Messages 31/33 an application protocol reads and write messages, but TCP and the socket API use a byte stream when a client calls recv() there is no guarantee that it will get an entire message the application has to designate where a message starts and ends and then parse the byte stream looking for messages two options for reading a message 1 variable length: read until a sentinel (end-of-message marker) 2 known length: read a length field and then read the listed number of bytes you will need to write a read-until-sentinel() and read-fixed-length() methods for your socket programming labs

32 Sending and Receiving Properly 32/33 results of send() or recv() call 1 less than zero bytes, errno == EINTR the system call was interrupted try again 2 less than zero bytes, any other error fatal error try to recover gracefully 3 zero bytes the socket is closed try to recover gracefully 4 positive bytes the return value is the number of bytes sent or received use a send() or recv() loop!

33 Example 33/33 See echo server and client on GitHub. GitHub