Socket Programming. CSIS0234A Computer and Communication Networks. Socket Programming in C

Transcription

1 1 CSIS0234A Computer and Communication Networks Socket Programming in C

2 References Beej's Guide to Network Programming Official homepage: Local mirror W. Stevens, UNIX Network Programming, Volume 1, Prentice Hall Chapter 3 & 4 Socket Programming 2

3 Application Program Interface You get the electricity power from the power socket. Network protocols are mostly built inside OS But Applications are running outside OS Socket API ("System call") to tap into the network world a standard set of functions Permits applications to use network protocols, e.g., TCP, UDP, DCCP, SCTP,... Abstraction for applications to send & receive data Applications create sockets that plug into network Socket Programming 3

4 Berkeley UNIX Socket API Was originally designed For BSD Unix Now Industry standard Available on many operating systems Including MS Windows Although a little bit tweaked OR As socket library if not provide by OS Socket Programming 4

5 5 What is a socket? OS abstraction - a communication endpoint Created dynamically Persists only while application runs Under UNIX, integrated with I/O system Uses the "open-read-write-close" paradigm as the familiar way that we handle a file An open socket is referenced by a file descriptor Small integer that identifies the file Generated by OS when socket created One per active socket Used in all operations on socket Only meaningful to application that owns socket

6 6 Socket Abstraction Socket interface Sender Application 1 Receiver Application 2 Socket interface descriptor User User descriptor Kernel Kernel port number 1 Socket Application references a socket through a descriptor Socket bound to a port number (& IP address) Socket port number 2 Underlying communication protocols Underlying communication protocols Communications network

7 7 Service Mode Connection-oriented Create the local endpoint Get a mobile phone with your SIM card Identify the remote endpoint Find your remote partner's telephone no. Initiate a connection Let's call your partner Send and Receive data Start the chat Terminate a connection First, setup connection between two peer application processes Then, reliable bidirectional insequence transfer of byte stream (boundaries not preserved in transfer) Multiple write/read between peer processes Finally, connection release Uses by TCP

8 8 Service Mode (2) Connectionless Create the local endpoint You have a mailbox available Identify the remote endpoint Write down the destination address (& mailbox no.) of the recipient on the envelop Send out your mail Using the postal service Deliver to the corresponding mailbox Immediate transfer of one block of information No setup overhead & delay Destination address with each block (letter) Send/receive to/from multiple peer processes Best-effort service only Possibly out-of-order Possibly loss Uses by UDP

9 9 Client-Server Communication One application Begins execution first Waits passively at prearranged location Passive program called a server Another application Begins execution later Actively contacts first program Active program called a client Used by most of the network applications (including the peer-to-peer applications)

10 Characteristics of Server & Client Servers Special-purpose program Dedicated to providing some service Can handle multiple remote clients simultaneously Waits passively for clients to contact Accepts requests from arbitrary clients Clients Arbitrary application program Some clients can be the servers for other clients Can also perform other computations Invoked directly by user Runs locally on user s computer Actively initiates contact with a server Contacts one server at a time Socket Programming 10

11 11 Socket Calls for Connection-Oriented Mode Server does Passive Open Program A Program B Client does Active Open

12 12 Socket Calls for Connectionless Mode

13 Create a Socket Application calls socket function int sockfd = socket (int family, int type, int protocol); OS returns descriptor for socket To send data, receive data, or both Descriptor valid until application closes socket or exits For Internet networking family = AF_INET (or PF_INET) type = SOCK_STREAM or SOCK_DGRAM protocol - normally is safe to set to 0 for most cases TCP UDP Socket Programming 13

14 Which Port am I on? I'm talking about your phone no./postal address Socket Programming 14 Bind int bind (int sockfd, struct sockaddr * localaddr, int addrlen); Specify protocol port for a socket Assign a name to an unnamed socket Specify local IP address for a socket Can use INADDR_ANY for any IP address

15 15 Socket Addresses As sockets can be used with arbitrary protocols Each protocol has its own addressing scheme Generic Address Format struct sockaddr { unsigned short sa_family; // address family, AF_xxx 16 bytes char sa_data[14]; // 14 bytes of protocol address }; 16 bytes For TCP/IP protocol, the address format struct sockaddr_in { short int sin_family; // Address family unsigned short int sin_port; // Port number (must in Network Byte Order) struct in_addr sin_addr; // Internet address (must in Network Byte Order) unsigned char sin_zero[8]; // Unused, Pad to keep as same size as struct sockaddr }; // Internet address (a structure for historical reasons) struct in_addr { unsigned long s_addr; // that's a 32-bit long, or 4 bytes };

16 16 Which IP address and Port # am I on? Two uses of bind Server registers it address with the system So the IP address + Port # becomes its phone no., somebody can call it now Client can register a specific address for itself [Optional] Normally, client doesn't need to explicitly call bind

17 A Service Protocol port number used Each service given unique port number, P e.g., HTTP - 80, Telnet - 23, SSH - 22, FTP - 21 (20),... Server Informs OS it is using port P Waits for requests to arrive Client Usually we don't care which port # it uses on its end randomly assigned one (ephemeral port) but must be unique within the host machine Constructs request to the server's port P Send request to port P on server computer Socket Programming 17

18 Port Number Assignment Valid Range: 0 to Divide into three ranges: Well-known ports: 0 to 1023 Reserved ports Can only use by system processes privileged users Registered ports: 1024 to Available for ordinary user processes Also allow to register with IANA (Internet Assigned Number Authority) Dynamic and/or Private ports: to Socket Programming 18

19 What is Byte Ordering? Problem: Different machines/oss use different word orderings Little-endian lower bytes first Big-endian higher bytes first These machines may communicate with one another over the network Solution: Do the conversion of all data that are sending via the network to Network Byte Order (that is big-endian) Convert it back at the receiving end Socket Programming 19

20 A Simple Example #include <string.h> #include <sys/types.h> #include <sys/socket.h> #include <netinet/in.h> #define MYPORT 3490 main() { int sockfd; struct sockaddr_in my_addr; If you don't care, you can tell the system to automate this // choose an unused port at random my_addr.sin_port = htons(0); // use my IP address my_addr.sin_addr.s_addr = htonl(inaddr_any); sockfd = socket(af_inet, SOCK_STREAM, 0); // do some error checking! my_addr.sin_family = AF_INET; // host byte order my_addr.sin_port = htons(myport); // short, network byte order my_addr.sin_addr.s_addr = inet_addr(" "); memset(&(my_addr.sin_zero), '\0', 8); // zero the rest of the struct // don't forget your error checking for bind(): bind(sockfd, (struct sockaddr *)&my_addr, sizeof(struct sockaddr));. Socket Programming. 20

21 21 Byte Ordering Conversion Routines Byte Ordering u_long htonl (u_long hostlong); host to network long u_short htons (u_short hostshort); u_long ntohl (u_long netlong); u_short ntohs (u_short netshort);

22 22 Address Conversion Routines Address Conversion in_addr_t inet_addr (const char * ip_addr); returns the addr in Network Byte Order in_addr_t struct in_addr int inet_aton (const char * ip_addr, struct in_addr * inp); another way to do the conversion char * inet_ntoa (struct in_addr in); just the reverse

23 Accepting Incoming Calls - TCP Listen Used by server when using Stream socket Prepares socket to accept incoming connections int listen(int sockfd, int req_queuesize); req_queuesize specifies the max. no. of incoming connection requests that can be buffered while waiting for server to accept them Accept Used by server when using Stream socket Waits for next connection and returns new socket The server uses the new socket to communicate with the client instead of using the original (sockfd) int accept(int sockfd, struct sockaddr * addr, int * addrlen); Socket Programming 23

24 24 How to Connect to a Remote Host? Connect Used by client Like pick up the phone and dial the number Either Stream Socket - Performs a TCP connection Establish a connection to a server that has called accept() Datagram Socket - Fully specifies addresses for UDP No connection is setup» Just like you write the address on all envelops Allow the client to send many messages to the same registered address int connect(int sockfd, struct sockaddr * serv_addr, int addrlen);

25 A Typical Scenario - A Server Using Stream Socket #define MYPORT #define BACKLOG 10 main() { int sockfd, new_fd; // listen on sock_fd, new connection on new_fd struct sockaddr_in my_addr; // my address information struct sockaddr_in their_addr; // client's address information int sin_size; sockfd = socket(af_inet, SOCK_STREAM, 0); // do some error checking! my_addr.sin_family = AF_INET; // host byte order my_addr.sin_port = htons(myport); // short, network byte order my_addr.sin_addr.s_addr = htol(inaddr_any); // auto-fill with my IP memset(&(my_addr.sin_zero), '\0', 8); // zero the rest of the struct // don't forget your error checking for these calls: bind(sockfd, (struct sockaddr *)&my_addr, sizeof(struct sockaddr)); listen(sockfd, BACKLOG); for ( ; ; ) { sin_size = sizeof(struct sockaddr_in); new_fd = accept(sockfd, (struct sockaddr *)&their_addr, &sin_size); if (new_fd < 0) fprintf(stderr, "accept error\n"); if (fork() == 0) { close(sockfd); // within child process.... // do some useful work exit(0); } close(new_fd); // within parent process } Socket Programming } 25

26 26 Let's Exchange Messages! Send & Sendto Transmit outgoing data Recv & Recvfrom Receive incoming data int send(int sockfd, const void * msg, int len, int flags); TCP int recv(int sockfd, void * buf, int len, unsigned int flags); int sendto(int sockfd, const void * msg, int len, unsigned int flags, const struct sockaddr * to, int tolen); UDP int recvfrom(int sockfd, void * buf, int len, unsigned int flags, struct sockaddr * from, int * fromlen);

27 27 Terminate Close Close the socket Permanent int close(int sockfd); Shutdown Shut down part of the full-duplex connection Does not close file descriptor int shutdown(int sockfd, int how); 0 - disallow further receives 1 - disallow further sends 2 - no send and receive

28 Other Utility Functions Who are you? getpeername( ) Who am I? getsockname( ) What is my name? gethostname( ) Address looking up gethostbyname( ) Name looking up gethostbyaddr( ) Socket has many parameters and options getsockopt( ) setsockopt( ) Socket Programming 28