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EECS122 Communications Networks Socket Programming

EECS122 Communications Networks Socket Programming. January 30th, 2003 J örn Altmann. Questions that will be Addressed During the Lecture. What mechanisms are available for a programmer who writes network applications?

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EECS122 Communications Networks Socket Programming

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  1. EECS122Communications NetworksSocket Programming January 30th, 2003 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

  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

  4. Socket ProgrammingTable of Contents • Network Application Programming Interface: Sockets and Internet Sockets • Network Programming Tips • Client-Server Architecture • Example: Client Programming • Example: Server Programming • Network Programmer’s Mistakes

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

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

  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

  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

  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

  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

  11. Network Programming Tips • Byte Ordering • Naming • Addressing

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

  13. Message is: [Hello,512] Processing Processing Message in Memory ofof big-endian Computer Message in Memory oflittle-endian Computer Message is sent across Network 48 45 4C 4C 6F 00 01 48 45 4C 4C 6F 00 01 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]

  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

  15. Network Programming Tips • Byte Ordering • Naming • Addressing

  16. Naming and Addressing • Host name • identifies a single host (see Domain Name System slides) • variable length string (e.g. www.berkeley.edu) • is mapped to one or more IP addresses • IP Address • 32 bits (not a number!) • written as dotted octets (e.g. 10.0.0.1) • Port number • identifies an application on a host • 16 bit number

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

  18. Simple Client-Server Example response Client Server request socket()connect()send()recv()close() socket()bind()listen()accept()recv()send()recv()close() Connectionestablishment Data request Data response End-of-file notification

  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

  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

  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(); }

  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

  23. Receiving Packets:Separating Data in a Stream • Use records (data structures) to partition the data stream Fixed length record Variable length record Variable length record Fixed length record A B C3 C2 0 1 2 3 4 5 6 7 8 9 Variable length record Variable length record D D 0 0 5 6 7 8 9

  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; }

  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() )

  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 • doesnot scale to large number of file descriptors

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

  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()

  29. Literature • Unix Network Programming, volumes 1 and 2 by W. Richard Stevens. Published by Prentice Hall; ISBNs for volumes 1 and 2: 013490012X, 0130810819. • Advanced Programming in the Unix Environment by W. Richard Stevens. Published by Addison Wesley. ISBN 0201563177. • man pages on a Unix computer

  30. The End

  31. EECS 122 - Syllabus • Logistics; Goals; Themes; Outline; Introduction • Examples of Network Applications/Design • Internet Architecture • Socket Programming • Network Performance Metrics; ns • DNS and WWW • Transport Protocols: UDP and TCP • Congestion Control and Avoidance • Congestion Control and Avoidance(cont) • Intradomain Routing: Distance Vector; Link State • Interdomain Routing: BGP • Switching and Forwarding; Router Architecture • Packet Scheduling and Classification • Router Support for Congestion Control: RED and FQ • Midterm Exam

  32. I/O Multiplexing (1) while (1) { if (receive_packets(buffer, buffer_len, &bytes_read) != 0) { break; } if (read_user(user_buffer, user_buffer_len, &user_bytes_read) != 0) { break; } }

  33. I/O Multiplexing (2): Non-blocking int opts = fcntl (chat_sock, F_GETFL); if (opts < 0) { perror ("fcntl(F_GETFL)"); abort (); } opts = (opts | O_NONBLOCK); if (fcntl (chat_sock, F_SETFL, opts) < 0) { perror ("fcntl(F_SETFL)"); abort (); } while (1) { if (receive_packets(buffer, buffer_len, &bytes_read) != 0) { break; } if (read_user(user_buffer, user_buffer_len, &user_bytes_read) != 0) { break; } }

  34. I/O Multiplexing (3): select() // already set descriptors non-blocking fd_set read_set; while (1) { FD_ZERO (read_set); FD_SET (stdin, read_set); FD_SET (chat_sock, read_set); select_retval = select(MAX(stdin, chat_sock) + 1, &read_set, NULL, NULL, &time_out); if (select_retval < 0) { perror ("select"); abort (); } if (select_retval > 0) { if (FD_ISSET(chat_sock, read_set)) { if (receive_packets(buffer, buffer_len, &bytes_read) != 0) { break; } if (FD_ISSET(stdin, read_set)) { if (read_user(user_buffer, user_buffer_len, &user_bytes_read) != 0) { break; } } } }

  35. Other I/O Models • Signal driven • application notified when I/O operation can be initiated • achieves similar CPU efficiency as select() • Asynchronous • application notified when I/O operation is completed • can achieve higher CPU efficiency than select()/signals on architectures that have DMA and available system bus bandwidth • mainly useful for very high bandwidth I/O • Both add significant complexity relative to select() • must use locks to deal with being interrupted at arbitrary code locations • sample complexity cost as threads

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