EECS122 Communications Networks Socket Programming. Jörn Altmann

Transcription

1 EECS122 Communications Networks Socket Programming Jörn Altmann

2 Questions that will be Addressed During the Lecture What mechanisms are available for a programmer who writes network applications? How to write a network application that sends packets between hosts (client and server) across an IP network? IP Network Client Server EECS122 - UCB 2

3 EE122 Projects S03 First assignment: Implement the client side of an client-server architecture (C programming) Handle exceptions in the communication Measure performance of the network and the server EECS122 - UCB 3

4 Socket Programming Table of Contents 1. Network Application Programming Interface: Sockets and Internet Sockets 2. Network Programming Tips 3. Client-Server Architecture 4. Example: Client Programming 5. Example: Server Programming 6. Network Programmer s Mistakes EECS122 - UCB 4

5 Layers of the IP Protocol Suite Application Layer e.g. ftp Application Layer Transport Layer e.g. TCP, UDP Transport Layer Network Layer e.g. IP Network Layer Link Layer Ethernet Link Layer EECS122 - UCB 5

6 Protocol Suite Location Internet Protocol Layer Application Layer Location Applications (e.g. browser, game, ftp) Transport Layer (TCP, UDP) Application Programming Interface (API) (e.g. network API) Network Layer (IP) Operating System (e.g. Unix) Link Layer Interface to the Network Card Network Card & Device Driver (e.g. Ethernet card) EECS122 - UCB 6

7 Network API Operating system provides Application Programming Interface (API) for network application API is defined by a set of function types, data structures, and constants Desirable characteristics of the network interface Simple to use Flexible independent from any application allows program to use all functionality of the network Standardized allows programmer to learn once, write anywhere Application Programming Interface for networks is called socket EECS122 - UCB 7

8 Sockets Sockets provide mechanisms to communicate between computers across a network There are different kind of sockets DARPA Internet addresses (Internet Sockets) Unix interprocess communication (Unix Sockets) CCITT X.25 addresses and many others Berkeley sockets is the most popular Internet Socket runs on Linux, FreeBSD, OS X, Windows fed by the popularity of TCP/IP EECS122 - UCB 8

9 Internet Sockets Support stream and datagram packets (e.g. TCP, UDP, IP) Is Similar to UNIX file I/O API (provides a file descriptor) Based on C, single thread model does not require multiple threads EECS122 - UCB 9

10 Types of Internet Sockets Different types of sockets implement different communication types (stream vs. datagram) Type of socket: stream socket connection-oriented two way communication reliable (error free), in order delivery can use the Transmission Control Protocol (TCP) e.g. telnet, ssh, http Type of socket: datagram socket connectionless, does not maintain an open connection, each packet is independent can use the User Datagram Protocol (UDP) e.g. IP telephony Other types exist: similar to the one above EECS122 - UCB 10

11 Network Programming Tips Byte Ordering Naming Addressing EECS122 - UCB 11

12 Byte Ordering of Integers Different CPU architectures have different byte ordering memory address A +1 memory address A Stored at little-endian computer high-order byte low-order byte Integer representation (2 byte) D3 F2 Stored at big-endian computer low-order byte high-order byte EECS122 - UCB 12

13 Byte Ordering Problem Question: What would happen if two computers with different integer byte ordering communicate? Answer: Nothing if they do not exchange integers! But: If they exchange integers, they would get the wrong order of bytes, therefore, the wrong value! Example: Message is: [Hello,1] Message is: [Hello,512] Message in Memory of little-endian Computer Processing C 4C 6F Message is sent Message in Memory of of big-endian Computer Processing across Network C 4C 6F EECS122 - UCB 13

14 Byte Ordering Solution There are two solutions if computers with different byte ordering system want to communicate They must know the kind of architecture of the sending computer (bad solution, it has not been implemented) Introduction of a network byte order. The functions are: uint16_t htons(uint16_t host16bitvalue) uint32_t htonl(uint32_t host32bitvalue) uint16_t ntohs(uint16_t net16bitvalue) uint32_t ntohs(uint32_t net32bitvalue) Note: use for all integers (short and long), which are sent across the network Including port numbers But not for IP addresses EECS122 - UCB 14

15 Network Programming Tips Byte Ordering Naming Addressing EECS122 - UCB 15

16 Naming and Addressing Host name identifies a single host (see Domain Name System slides) variable length string (e.g. is mapped to one or more IP addresses IP Address 32 bits (not a number!) written as dotted octets (e.g ) Port number identifies an application on a host 16 bit number EECS122 - UCB 16

17 Client-Server Architecture response Client Server request Client requests service from server Server responds with sending service or error message to client Example: Remote Procedure Call EECS122 - UCB 17

18 Simple Client-Server Example Client socket() connect() send() response request Connection establishment Data request Server socket() bind() listen() accept() recv() recv() close() Data response End-of-file notification send() recv() close() EECS122 - UCB 18

19 Example: Client Programming Create stream socket (socket() ) Connect to server (connect() ) While still connected: send message to server (send() ) receive (recv() ) data from server and process it EECS122 - UCB 19

20 Initializing Socket Getting the file descriptor int chat_sock; if ((chat_sock = socket( AF_INET, SOCK_STREAM, IPPROTO_TCP)) < 0) { perror("socket"); printf("failed to create socket\n"); abort (); } 1.parameter specifies protocol/address family 2.parameter specifies the communication type 3.parameter specifies the protocol EECS122 - UCB 20

21 Connecting to Server struct sockaddr_in sin; struct hostent *host = gethostbyname (argv[1]); unsigned int server_address = *(unsigned long *) host->h_addr_list[0]; unsigned short server_port = atoi (argv[2]); memset (&sin, 0, sizeof (sin)); sin.sin_family = AF_INET; sin.sin_addr.s_addr = server_address; sin.sin_port = htons (server_port); if (connect(chat_sock, (struct sockaddr *) &sin, sizeof (sin)) < 0) { perror("connect"); printf("cannot connect to server\n"); abort(); } EECS122 - UCB 21

22 Sending Packets int send_packets(char *buffer, int buffer_len) { } sent_bytes = send(chat_sock, buffer, buffer_len, 0); if (send_bytes < 0) { perror ( send"); } return 0; Needs socket descriptor, Buffer containing the message, and Length of the message EECS122 - UCB 22

23 Receiving Packets: Separating Data in a Stream Fixed length record Fixed length record Variable length record Variable length record A B C3 C Use records (data structures) to partition the data stream Variable length record D Variable length record D 0 9 EECS122 - UCB 23

24 Receiving Packets int receive_packets(char *buffer, int buffer_len, int *bytes_read) { int left = buffer_len - *bytes_read; received = recv(chat_sock, buffer + *bytes_read, left, 0); if (received < 0) { perror ( recv"); } if (received <= 0) { return close_connection(); } *bytes_read += received; while (*bytes_read > RECORD_LEN) { process_packet(buffer, RECORD_LEN); *bytes_read -= RECORD_LEN; memmove(buffer, buffer + RECORD_LEN, *bytes_read); } return 0; } EECS122 - UCB 24

25 Example: Server Programming create stream socket (socket() ) Bind port to socket (bind() ) Listen for new client (listen() ) Wait (select() ) until user connects (accept() ) data arrives from client (recv() ) data has to be send to client (send() ) timeout (select() ) EECS122 - UCB 25

26 I/O Multiplexing using select() waits on multiple file descriptors and timeout returns when any file descriptor is ready to be read or written or indicate an error, or timeout exceeded advantages simple application does not consume CPU cycles while waiting disadvantages does not scale to large number of file descriptors EECS122 - UCB 26

27 Network Programmer s Mistakes byte ordering separating records in streams use of select() misinterpreting the project specification not knowing all available system calls EECS122 - UCB 27

28 There are more System Calls Depends on communication type Datagram sockets use recvfrom() and sendto() for receiving and sending data Closing connection: close(), shutdown() Getting information about a host: gethostbyname() EECS122 - UCB 28

29 Literature Unix Network Programming, volumes 1 and 2 by W. Richard Stevens. Published by Prentice Hall; ISBNs for volumes 1 and 2: X, Advanced Programming in the Unix Environment by W. Richard Stevens. Published by Addison Wesley. ISBN man pages on a Unix computer EECS122 - UCB 29