Network Programming Using Internet Sockets

1. Network Programming Using Internet Sockets Wu Xuanxuan Lai Xinyu 吴璇璇 赖新宇 xuan_tju@126.com 15122211137@163.com

2. Agenda • Basic Socket Programming • Understanding Sockets • Socket API • Structs and Data Handling • A simple example • Assignment • References

3. Basic Socket Programming • What is a Socket • Definition 1: • A door between application process and end-end transport protocol(TCP/UDP) • Definition 2: • A way to speak to other programs using standard UNIX file descriptors. • Definition 3: • An endpoint in communication

4. Basic Socket Programming Understanding Sockets- Definition 1 • Definition 1: a door between application process and end-end transport protocol (TCP/UDP)

5. Basic Socket Programming Understanding Sockets-Definition1 loosely-coupled controlled by app developer Controlled by OS You write sockets programs without knowing how the lower levels‘ protocols works!

6. Basic Socket Programming Understanding Sockets- Definition1 Socket API:(1) choice of transport protocol; (2) ability to fix a few parameters • Two types of transport service via Internet socket API: • Stream Sockets (connection-oriented ) • They use TCP (Transmission Control Protocol) protocol • Reliable stream • Datagram Sockets (connectionless) • They use UDP (User Datagram Protocol) protocol • Unreliable datagram

7. Basic Socket Programming Understanding Sockets- Definition 2 • Definition2: a way to speak to other programs using standard Unix file descriptors.

8. Basic Socket Programming Understanding Sockets- Definition2 • Socket Programming int sockfd; /*socket descriptor*/ sockfd= socket( AF_INET, SOCK_STREAM, 0); read( sockfd, … ); • File Operation char buf[100]; int fd; /*file descriptor*/ fd= open(“foo", O_RDONLY); read( fd, buf, num_bytes);

9. Basic Socket Programming Understanding Sockets- Definition 3 • Definition3: merely an endpoint in communication.

10. Basic Socket Programming Understanding Sockets- Definition 3 • IP：User needs to know the IP ADDRESS of the server. • Port number: On the server, many processes are running. We want to “talk” to the right one. • The knowledge of the IP addresses and the Port Number define uniquely a communication endpoint.

11. Basic Socket ProgrammingSocket API-TCP Model

12. Basic Socket ProgrammingSocket API-UDP Model Server: No bind() and accept(). Client: No connect().

13. Basic Socket ProgrammingSocket API-socket() • socket() -Get the File Descriptor! • The first step is to require the OS to reserve one “descriptor” for the communication channel. intsockfd= socket(AF_INET, socket_type, 0); /*socket_type=SOCK_STREAM|SOCK_DGRAM*/ /*AF_INET means we use IP (version 4)*/ /*0 is for IP (not ICMP or others)*/ /*return a socket descriptor or -1 when error*/ Service Type symbolic name datagram (UDP) SOCK_DGRAM reliable, in order (TCP) SOCK_STREAM raw socket SOCK_RAW Protocol Family Symbolic Name TCP/IP Internet AF_INET Xerox NS AF_NS Intra-host Unix AF_UNIX DEC DNA AF_DECNET If(sockfd< 0) perror(“can’t create socket");

14. Basic Socket ProgrammingSocket API-bind() • bind()-What IP address and port am I on? • struct sockaddr_in servaddr; • bind(sockfd, (struct sockaddr *) &servaddr, sizeof(servaddr)); Note: • Only the server need to call this API; • The type of the second parameter should be sockaddr while we often use sockaddr_in in the program——Type Conversation

15. Basic Socket ProgrammingSocket API-close(),shutdown() • close(sockfd); • frees up any memory associated with the socket • shutdown(sockfd, how); /* how = 0 -> disable receptions */ /* how = 1 -> disable transmission */ /* how = 2 -> the same as close() */

16. Basic Socket ProgrammingTCP API(SERVER)-listen() • listen()-Who will call me? listen(sockfd, req_no); • req_no: specifies the maximum number of client connections that the kernel will queue for this listening descriptor.

17. Basic Socket ProgrammingTCP API(SERVER)-accept() • accept()-Thank you for calling port 3490. int new_sd, sockfd, size; struct sockaddr remote_addr; new_sd = accept(sockfd, &remote_addr, &size); • If the queue is empty, the process will be blocked. • If so, accept() returns a NEW SOCKET DESCRIPTOR ! • the old socket descriptor (sockfd) is still queuing request from the network ! • So, if we want to communicate with the client, we MUST use new_sd. • new_sdis a socket ready for the communication • It is suggested that handle new_sd in a child process or a thread.

18. Basic Socket ProgrammingTCP API(CLIENT)-connect() • connect()-Hey, you! • int sockfd; • connect(sockfd, (sockaddr*) &servaddr, sizeof(servaddr) ); • On the client side, bind is done automatically • The local IP address is the one provided by default • A port “randomly” assigned by the operating system is good. • So, we put in servaddr the information on the server and try to connect to it. • Finish three-way handshake

19. Basic Socket ProgrammingTCP API-send() , recv() • send() and recv() -Talk to me, baby! • int send(int sockfd, const void *msg, int len, int flags);  • int recv(int sockfd, void *buf, int len, unsigned int flags); • char *msg = “Hi, baby!"; • char buffer[SOME_SIZE]; • int len, nset, nrecv; • . • . • len = strlen(msg); • nset= send(sockfd, msg, len, 0); • . • nrecv = recv(sockfd, &buffer, len, flags)

20. Basic Socket ProgrammingTCP API-read() , write() • read() and write() –more choices • read() = recv(*, *, *, 0) • write() = send(*, *, *, 0) • Programmers are familiar with, similar to file operations • recv() and send() • explicit meaning

21. Basic Socket ProgrammingUDP API-sendto() , recvfrom() • sendto() and recvfrom() • int sendto(int sockfd, const void *msg, int len, unsigned int flags, const struct sockaddr *to, int tolen); • int recvfrom(int sockfd, void *buf,int len,unsigned int flags, struct sockaddr *from, int *fromlen) • Send directly without handshake • The first four parameters are the same with send/recv • to/from is a pointer to a struct sockaddr which contains the destination IP address and port. • tolen/fromlen, can simply be set to sizeof(struct sockaddr).

22. Basic Socket ProgrammingStructs and Data Handling-IP address • IP Address • Every host has a unique IP Address • 32bits • Three forms • hostname (string) e.g. localhost • dotted decimal(string) e.g. 192.168.2.1 • binary(u_long) • Dealing with them inet_addr() : dotted decimal to binary gethostbyname() : hostname to binary e.g. servaddr.sin_addr.s_addr =inet_addr(“59.67.33.68");

23. Basic Socket ProgrammingStructs and Data Handling-Port • Port • 1-255 reserved for standard services • 21 ftp • 23 telnet • 25 SMTP • 80 HTTP • 1-1023 Available only to priviledged users • 1024-4999 Usuable by system and user processes • 5000- Usuable by user processes only

24. Basic Socket ProgrammingStructs and Data Handling-struct sock_addr • struct sockaddr ：This structure holds socket address information for many types of sockets: • struct sockaddr { unsigned short sa_family; // address family, AF_xxx char sa_data[14]; // 14 bytes of protocol address }; • sockaddr is the structure with the addresses and the ports. We put IP address and the Port in sa_data[14]. Not convenient to deal with?

25. Basic Socket ProgrammingStructs and Data Handling-struct sockaddr_in • struct sockaddr_in：To deal with sockaddr, programmers created a structure: struct sockaddr_in ("in" for "Internet".) • struct sockaddr_in { short int sin_family; // Address family unsigned short int sin_port; // Port number struct in_addr sin_addr; // Internet address unsigned char sin_zero[8]; // Same size as struct sockaddr }; • struct in_addr{ /* 32-byte IP Address *./ in_addr_t s_addr; }; We can work with sockaddr_in and cast it to sockaddr. • struct sockaddr_in servaddr; • bind(sockfd, (struct sockaddr *) &servaddr, sizeof(servaddr));

26. Basic Socket ProgrammingStructs and Data Handling • Convert the Natives! • Know this: • there are two byte orderings • most significant byte first-“Network Byte Order” (NBO). • or least significant byte first-"Host Byte Order” (HBO). • htons() host to network short • htonl() host to network long • ntohs() network to host short • ntohl() network to host long • e.g. servaddr.sin_addr.s_addr = htonl(INADDR_ANY); /*IP address of localhost*/ servaddr.sin_port = htons(13);

27. Basic Socket ProgrammingStructs and Data Handling • And here's a sample, while packing a struct sockaddr_in /* an Internet endpoint address*/  struct sockaddr_in my_addr; my_addr.sin_family = AF_INET; /* host byte order */  my_addr.sin_port = htons(MYPORT); /* short, NBO*/  my_addr.sin_addr.s_addr = inet_addr("132.241.5.10");  bzero(&(my_addr.sin_zero)); /* zero the rest of the struct */ 

28. Basic Socket Programminga simple example-TCP server

29. Basic Socket Programminga simple example-TCP client

30. Basic Socket Programming a simple example- Result

31. Programming Assignment • P2P Principle login search share logout ping download P2P Server Peer Client Peer Client

32. Assignment • Phase1: Establishing Client-Server Communications (Mandatory; 60 points) • Phase2: Establishing Peer-Peer Communications (Mandatory; 40 points Optional; 20 points)

33. Phase1: Establishing Client-Server Communications -- Mandatory • 1) Authenticate with the server (provide a username and encrypted password) • 2) Send a list of files to the server that you wish to share with other users • 3) Submit a search query to the server for a file you wish to download • 4) Receive the search results, parse them, and output them to the user • 5) Log out

34. Phase1: Authenticate • Protocol packet packet head is 4 bytes!

35. Phase1: Share • Protocol packet:

36. Phase1: Search • Protocol packet:

37. Phase1: Logout • Protocol packet:

38. Phase2: Establishing Peer-Peer Communications • 1) Add functionality to send "ping" messages over UDP and listen for echo responses, in order to estimate the round trip time to another peer. --Mandatory • 2) Add functionality to respond to ping messages from other peers.-- Mandatory • 3) Add functionality to connect to another peer and download a file. • 4) Add functionality to make your peer a file server, so that other peers may connect and download files from you.

39. Phase2: Ping • Send UDP Ping Message. • Protocol packet: Do not guarantee delivery of packets! Use timeout control—select()

40. Phase2: Peer-Peer Communications • Protocol packet: No need password!

41. Phase2: Peer-Peer Communications (cont...) • Protocol packet:

42. Requirement • Program Correctness and Functionality: 60% • Code design & Program Structure: 10% • Documentation: 30%

43. Deadline • Mandatory : April 27 • Optional: May 7 • Submit to: cn_tju@163.com • For more detail: detailed requirements.pdf

44. References • Warren W. Gay , “Linux Socket Programming by Example” • Beej's Guide to Network Programming Using Internet Sockets • Douglas E. Comer, David L. Stevens, “Internetworking With TCP/IP Vol III” Thank you!