Sockets 15H2. Inshik Song

Transcription

1 Sockets 15H2 Inshik Song

2 Internet CAU www server ( Your web browser (Internet Explorer/Safari) Sockets 2

3 How do we find the server? Every computer on the Internet has an Internet address. Called an IP address (Internet Protocol) Internet communications are based on the IP addresses An IP address is 4 8-bit numbers separated by dots. = Sockets 3

4 Domain Name Servers When you enter a URL into your Web browser, your DNS server uses its resources to resolve the name into the IP address for the appropriate Web server 1. Get me the IP addr for 2. That s DNS server 3. Request a web page stored in the server 4. This is the page you requested Sockets 4

5 What is a socket? An interface between application and network The application creates a socket The socket type dictates the style of communication reliable vs. best effort connection-oriented vs. connectionless Once configured, the application can pass data to the socket for network transmission receive data from the socket (transmitted through the network by some other host) Sockets 5

6 Two essential types of sockets SOCK_STREAM TCP sockets reliable delivery in-order guaranteed connection-oriented bidirectional SOCK_DGRAM UDP sockets unreliable delivery no order guarantees no notion of connection can send or receive Sockets 6

7 Socket Communication Process Sockets 7

8 Socket Creation domain: integer, communication domain e.g., AF_INET (IPv4 protocol) type: communication type SOCK_STREAM: reliable, 2-way, connection-based service SOCK_DGRAM: unreliable, connectionless protocol: e.g., TCP or UDP use IPPROTO_TCP or IPPROTO_UDP to send/receive TCP or UDP packets Retuns a socket descriptor, an integer (like a filehandle) Sockets 8

9 Addresses, Ports and Sockets Like apartments and mailboxes You are the application Street address of your apartment building is the IP address Your mailbox is the port The post-office is the network The socket is the key that gives you access to the right mailbox Sockets 9

10 Addresses, Ports and Sockets Choose a port number that is registered for general use, from 1024 to Do not use ports 1 to These ports are reserved for use by the Internet Assigned Numbers Authority (IANA) 20, 21: FTP 23: Telnet 80: HTTP Avoid using ports through These are dynamic ports that operating systems use randomly. If you choose one of these ports, you risk a potential port conflict Sockets 10

11 The bind function Associates a port for use by the socket sockfd: socket being used addr: the (IP) address and port of the machine (address usually set to INADDR_ANY chooses a local address) len: the size (in bytes) of the addr structure bind() is non-blocking: returns immediately Sockets 11

12 Connection Setup (SOCK_STREAM) Recall: no connection setup for SOCK_DGRAM A connection occurs between two kinds of participants passive: waits for an active participant to request connection active: initiates connection request to passive side Once connection is established, passive and active participants are similar both can send & receive data either can terminate the connection Sockets 12

13 Connection setup cont d Passive participant step 1: listen (for incoming requests) step 3: accept (a request) step 4: data transfer The accepted connection is on a new socket The old socket continues to listen for other active participants Active participant step 2: request & establish connection step 4: data transfer Passive Participant a-sock-1 l-sock a-sock-2 socket Active 1 socket Active 2 Sockets 13

14 Connection setup: listen & accept Called by passive participant sockfd: integer, socket descriptor backlog: integer, # of active participants that can wait for a connection listen is non-blocking: returns immediately Sockets 14

15 Connection setup: accept Returns a new socket (used for data-transfer) connected to the client that called connect. sockfd: integer, the orig. socket (being listened on) addr: struct sockaddr, address of the active participant len: sizeof(addr): value/result parameter must be set appropriately before call adjusted by OS upon return accept is blocking: waits for connection before returning Sockets 15

16 connect call Connect to a server sockfd: integer, socket to be used in connection addr: struct sockaddr: address of passive participant len: integer, sizeof(addr) For a connect request to succeed, the machine to which we are trying to connect must be up and running, the server must be bound to the address we are trying to contact, and there must be room in the server s pending connect queue. connect is blocking Sockets 16

17 Sending / Receiving Data With a connection (SOCK_STREAM): int count = send(sock, &buf, len, flags); count: # bytes transmitted (-1 if error) buf: char[], buffer to be transmitted len: integer, length of buffer (in bytes) to transmit flags: integer, special options, usually just 0 int count = recv(sock, &buf, len, flags); count: # bytes received (-1 if error) buf: void[], stores received bytes len: # bytes received flags: integer, special options, usually just 0 Calls are blocking [returns only after data is sent (to socket buf) / received] Sockets 17

18 close When finished using a socket, the socket should be closed: status = close(s); status: 0 if successful, -1 if error s: the file descriptor (socket being closed) Closing a socket closes a connection (for SOCK_STREAM) frees up the port used by the socket Sockets 18

19 Client Sockets 19

20 Sockets 20

21 Server Sockets 21

22 Sockets 22

23 Sockets 23

24 Questions? Sockets 24