1 Spring Semester 2008, Dept. of Computer Science, Technion Internet Networking recitation #7 Socket Programming

2 Internet Networking What is socket?  First appeared in 4.1 BSD UNIX  An integer associated with a network connection.  Implemented as file descriptor.  Types of Internet Sockets:  Stream Sockets for TCP connection.  Datagram Socket for UDP communication.  Raw Sockets.

3 Internet Networking Socket Interface IP Stack Socket I/F ND1ND2ND3 Kernel User Space App1App2

4 Internet Networking Structures  ‘Struct sockaddr’ holds socket address information:  struct sockaddr { unsigned short sa_family;// AF_INET for internet char sa_data[14];// 14 bytes of protocol address. contains a destination address and port number for the socket. };  struct sockaddr_in’ is used to deal with ‘struct sockaddr’ for internet family (AF_INET):  struct sockaddr_in { short int sin_family;// AF_INET for internet unsigned short int sin_port;// Port number struct in_addr sin_addr;// Internet address unsigned char sin_zero[8];// padding (must be zero) };

5 Internet Networking Structures (Cont.)  A pointer to a ‘struct sockaddr_in’ can be cast to a pointer to a ‘struct sockaddr’ and vice-versa  they both have the same size.  In order to set the IP address one should use ‘struct in_addr’:  struct in_addr { unsigned long s_addr; // IPv4 address };

6 Internet Networking Bytes Order  The CPU and the NIC may store and read data from the memory in a different order. Memory CPU NIC R/W

7 Internet Networking Two ways to store data in the memory:  Big-Endian  Little-Endian One should convert the order of the bytes (if necessary) in order to transmit them in network in the same way More: On a big endian machine we see Byte: U N I X Location: On a little-endian machine we would see: Byte: N U X I Location:

8 Internet Networking Bytes Order  Accessories Function:  htons(x) – Convert short from host to network order.  htonl(x)  ntohs(x)  ntohl(x)  In order to be portable one should use this function even if not necessary.

9 Internet Networking System Calls - Socket  int socket(int domain, int type, int protocol);  Return a file descriptor (or -1 in case of error).  domain: AF_INET for internet.  type: SOCK_STREAM for TCP, SOCK_DGRAM for UDP and SOCK_RAW for raw socket.  protocol  Example for UDP: // Opening socket if ((sockfd = socket (AF_INET, SOCK_DGRAM, 0)) < 0) error("Could not open socket");

10 Internet Networking System Call - Bind  Associate a socket with a port on the local machine  int bind(int sockfd, struct sockaddr *my_addr, int addrlen);  ‘sockfd’ is the socket file descriptor.  Associate a socket with a port on the local machine (usually used by servers).  ‘my_addr’ is a pointer to a struct sockaddr that contains information about the port.  ‘addrlen’ is set to sizeof(struct sockaddr).  return 0 on success or -1 on failure (when?).

11 Internet Networking Example:  // INADDR_ANY allowed to work without knowing the IP address of the machine running on #define SERVER_PORT 5555 struct sockaddr_in server_addr; server_addr.sin_family = AF_INET; server_addr.sin_addr.s_addr = htonl(INADDR_ANY); server_addr.sin_port = htons(SERVER_PORT); if (bind(sockfd, (struct sockaddr*)&server_addr, sizeof(server_addr)) < 0) error("Could not bind to the socket");

12 Internet Networking Wait for incoming connections, Listen & Accept  int listen(int sockfd, int backlog);  Define the maximum of pending TCP connections.  ‘sockfd’ is the socket file descriptor.  ‘backlog is the number of pending connections incoming connections wait in this queue until accept(). this is the limit on how many can queue up  int accept(int sockfd, void *addr, int *addrlen);  Used by a TCP server to accept pending connection.  Blocked until a connection is arrived.  Return a new socket file descriptor used to receive and send data on this connection by send() and recv() for example.  The original keeps listening on its port  ‘sockfd’ is the new socket file descriptor.  ‘addr’ contains information (port and IP address) of the incoming connection (who is calling).  ‘addrlen’ is a local integer variable set to sizeof(struct sockaddr_in).

13 Internet Networking Connect to a Remote Host  int connect(int sockfd, struct sockaddr *serv_addr, int addrlen);  Used by clients to connect a socket to a server.  ‘sockfd’ is the socket file descriptor.  ‘struct sockaddr’ contains the destination port and IP address  ‘addrlen’ set to sizeof(struct sockaddr).

14 Internet Networking Data Transfer – Send over stream sockets (TCP)  int send(int sockfd, const void *msg, int len, int flags);  Transmit data through a socket.  Can be used only when the socket is connected.  Return the number of bytes that has been sent (or -1 in case of error).  ‘buf’ is a pointer to the buffer to be sent, while ‘len’ is its length.  Flags – mostly zero.  Flag example: MSG_DONTROUTE Don't use a gateway to send out the packet, only send to hosts on directly connected networks.

15 Internet Networking Data Transfer – Receive over stream sockets (TCP)  int recv(int sockfd, void *buf, int len, unsigned int flags);  Obtain data from the socket.  Used only when the socket is in a connected state.  ‘buf’ is a pointer to the buffer for holding the received data and ‘len’ is its length.  Returns the number of bytes actually sent (or -1).

16 Internet Networking Data Transfer – Send over Datagram sockets (UDP)  Datagram sockets are not connected to a remote host  int sendto(int sockfd, const void *msg, int len, unsigned int flags, const struct sockaddr *to, int tolen);  Transmit data to through a socket. The destination (port and IP address) is specified in the call.  Used for UDP communication.

17 Internet Networking Data Transfer – Receive over stream sockets (UDP)  int recvfrom(int sockfd, void *buf, int len, unsigned int flags, struct sockaddr *from, int *fromlen);  Obtain data from the socket.  from is a pointer to a local struct sockaddr that will be filled with the IP address and port of the originating machine.  fromlen is a pointer to a local int that should be initialized to sizeof(struct sockaddr).  When the function returns, fromlen will contain the length of the address actually stored in from.  Returns the number of bytes actually sent.

18 Internet Networking Shut Down and Close  int shutdown(int sockfd, int how);  sockfd - socket file descriptor to shutdown  How:  0 - Further receives are disallowed  1 - Further sends are disallowed  2 - Further sends and receives are disallowed (like close())  close(sockfd);  sockfd - socket file descriptor to shutdown  Only close() frees the socket descriptor

19 Internet Networking Client-Server Model UDP Client sd=socket(…) sendto(sd,…) recvfrom(sd,…) close(sd) UDP Server sd=socket(…) bind(sd,…) recvfrom(sd,…) sendto(sd,…) close(sd)

20 Internet Networking Client-Server Model TCP Client sd=socket(…) connect(sd) send(sd,…) recv(sd,…) close(sd) TCP Server sd=socket(…) bind(sd,…) listen(sd,…) new_sd=accept(sd,…) fork() recv(new_sd,…) send(new_sd,…) close(new_sd)

21 Internet Networking Port Assignment  Ports 0 through 1023 are reserved.  In order to let the system to assign the port number, one should set the port number (e.g. in ‘bind’) to zero.