1. Lecture 10 Overview

2. Network API Application Programming Interface – Services that provide the interface between application and protocol software often by the operating system CPE 401/601 Lecture 10 : Socket Programming 2 Application Network API Protocol A Protocol B Protocol C

3. Network API wish list Generic Programming Interface – Support multiple communication protocol suites (families) – Address (endpoint) representation independence – Provide special services for Client and Server? Support for message oriented and connection oriented communication Work with existing I/O services – when this makes sense Operating System independence 3 CPE 401/601 Lecture 10 : Socket Programming

4. Socket A socket is an abstract representation of a communication endpoint Sockets work with Unix I/O services just like files, pipes & FIFOs Sockets needs to – establishing a connection – specifying communication endpoint addresses 4 CPE 401/601 Lecture 10 : Socket Programming

5. Creating a Socket int socket(int family, int type, int proto); family specifies the protocol family – PF_INET for TCP/IP type specifies the type of service – SOCK_STREAM, SOCK_DGRAM protocol specifies the specific protocol – usually 0, which means the default 5 CPE 401/601 Lecture 10 : Socket Programming

6. socket() The socket() system call returns a socket descriptor (small integer) or -1 on error socket() allocates resources needed for a communication endpoint – but it does not deal with endpoint addressing 6 CPE 401/601 Lecture 10 : Socket Programming

7. Specifying an Endpoint Address Sockets API is generic There must be a generic way to specify endpoint addresses TCP/IP requires an IP address and a port number for each endpoint address Other protocol suites (families) may use other schemes 7 CPE 401/601 Lecture 10 : Socket Programming

8. sockaddr 8 sa_len sa_family sa_data length AF_INET port addr zero sockaddr sockaddr_insockaddr_in6 length AF_INET6 port Flow-label Scope ID addr 28 bytes variable 16 bytes CPE 401/601 Lecture 10 : Socket Programming

9. struct sockaddr_in (IPv4) struct sockaddr_in { uint8_tsin_len; sa_family_tsin_family; in_port_tsin_port; struct in_addrsin_addr; charsin_zero[8]; }; struct in_addr { in_addr_ts_addr; }; 9 Length of structure (16) AF_INET 16 bit Port number 32 bit IPv4 address Make structure 16 bytes CPE 401/601 Lecture 10 : Socket Programming

10. struct sockaddr_in (IPv6) struct sockaddr_in6 { uint8_tsin6_len; sa_family_tsin6_family; in_port_tsin6_port; uint32_tsin6_flowinfo; struct in6_addrsin6_addr; uint32_t sin6_scope_id; }; struct in6_addr { uint8_ts6_addr[16]; }; 10 Length of structure (28) AF_INET6 Port number 128 bit IPv6 address Scope of address Flow label

11. Network Byte Order All values stored in a sockaddr_in must be in network byte order. – sin_port a TCP/IP port number – sin_addr an IP address uint16_t htons(uint16_t); uint16_t ntohs(uint_16_t); uint32_t htonl(uint32_t); uint32_t ntohl(uint32_t); 11 CPE 401/601 Lecture 10 : Socket Programming

12. TCP/IP Addresses We don’t need to deal with sockaddr structures since we will only deal with a real protocol family. We can use sockaddr_in structures BUT: The C functions that make up the sockets API expect structures of type sockaddr 12 CPE 401/601 Lecture 10 : Socket Programming

13. Assigning an address to a socket The bind() system call is used to assign an address to an existing socket. int bind( int sockfd, const struct sockaddr *myaddr, int addrlen); You can give bind() a sockaddr_in structure: int bind( mysock, (struct sockaddr*) &myaddr, sizeof(myaddr) ); 13 CPE 401/601 Lecture 10 : Socket Programming

14. bind() Example int mysock,err; struct sockaddr_in myaddr; mysock = socket(PF_INET,SOCK_STREAM,0); myaddr.sin_family = AF_INET; myaddr.sin_port = htons( portnum ); myaddr.sin_addr = htonl( ipaddress); err=bind(mysock, (sockaddr *) &myaddr, sizeof(myaddr)); 14 CPE 401/601 Lecture 10 : Socket Programming

15. Port schmort - who cares ? Clients typically don’t care what port they are assigned When you call bind you can tell it to assign you any available port: – myaddr.port = htons(0); 15 CPE 401/601 Lecture 10 : Socket Programming

16. What is my IP address ? How can you find out what your IP address is so you can tell bind() ? There is no realistic way for you to know the right IP address to give bind() – what if the computer has multiple network interfaces? specify the IP address as: INADDR_ANY, this tells the OS to take care of things. 16 CPE 401/601 Lecture 10 : Socket Programming

17. IPv4 Address Conversion int inet_aton(char *, struct in_addr *); – Convert ASCII dotted-decimal IP address to network byte ordered 32 bit value. – Returns 1 on success, 0 on failure. char *inet_ntoa(struct in_addr); – Convert network byte ordered value to ASCII dotted-decimal (a string). 17 CPE 401/601 Lecture 10 : Socket Programming

18. IPv4 & IPv6 Address Conversion int inet_pton(int, const char*, void*); – (family, string_ptr, address_ptr) – Convert IP address string to network byte ordered 32 or 128 bit value – 1 on success, -1 on failure, 0 on invalid input char *inet_ntop(int, const void*, char*, size_t); – (family, address_ptr, string_ptr, length) – Convert network byte ordered value to IP address string x:x:x:x:x:x:x:x or x:x:x:x:x:x:a.b.c.d 18 CPE 401/601 Lecture 10 : Socket Programming

19. Other socket system calls General Use – read() – write() – close() 19 Connection-oriented (TCP) –connect() –listen() –accept() Connectionless (UDP) –send() –recv() CPE 401/601 Lecture 10 : Socket Programming

20. Lecture 11 TCP and UDP Sockets CPE 401 / 601 Computer Network Systems slides are modified from Dave Hollinger

21. TCP Sockets Programming Creating a passive mode (server) socket Establishing an application-level connection send/receive data Terminating a connection 21 CPE 401/601 Lecture 11 : TCP Socket Programming

22. TCP state diagram

23. Creating a TCP socket int socket(int family,int type,int proto); – family: AF_INET, AF_INET6, AF_LOCAL, … – type: SOCK_STREAM, SOCK_DGRAM, SOCK_SEQPACKET, SOCK_RAW – protocol: IPPROTO_TCP, IPPROTO_UDP, IPPROTO_SCTP int sock; sock = socket(PF_INET, SOCK_STREAM, 0); if (sock<0) { /* ERROR */ } 23 CPE 401/601 Lecture 11 : TCP Socket Programming

24. Binding to well known address int bind(int sockfd, const struct sockaddr *myaddr, socklen_t addrlen); int mysock; struct sockaddr_in myaddr; mysock = socket(PF_INET,SOCK_STREAM,0); myaddr.sin_family = AF_INET; myaddr.sin_port = htons( 80 ); myaddr.sin_addr = htonl( INADDR_ANY ); bind(mysock, (sockaddr *) &myaddr, sizeof(myaddr)); 24 CPE 401/601 Lecture 11 : TCP Socket Programming

25. Establishing a passive mode TCP socket Passive mode: – Address already determined Tell the kernel to accept incoming connection requests directed at the socket address – 3-way handshake Tell the kernel to queue incoming connections for us 25 CPE 401/601 Lecture 11 : TCP Socket Programming

26. listen() int listen(int sockfd, int backlog); sockfd is the TCP socket – already bound to an address backlog is the number of incoming connections the kernel should be able to keep track of (queue for us) – Sum of incomplete and completed queues 26 CPE 401/601 Lecture 11 : TCP Socket Programming

27. Accepting an incoming connection Once we call listen(), the O.S. will queue incoming connections – Handles the 3-way handshake – Queues up multiple connections When our application is ready to handle a new connection – we need to ask the O.S. for the next connection 27 CPE 401/601 Lecture 11 : TCP Socket Programming

28. accept() int accept( int sockfd, struct sockaddr* cliaddr, socklen_t *addrlen); sockfd is the passive mode TCP socket – initiated by socket(), bind(), and listen() cliaddr is a pointer to allocated space addrlen is a value-result argument – must be set to the size of cliaddr – on return, will be set to be the number of used bytes in cliaddr 28 CPE 401/601 Lecture 11 : TCP Socket Programming

29. accept() return value accept() returns a new socket descriptor (small positive integer) or -1 on error After accept returns a new socket descriptor, I/O can be done using the read() and write() system calls read() and write() operate a little differently on sockets! – vs. file operation! 29 CPE 401/601 Lecture 11 : TCP Socket Programming

30. Terminating a TCP connection int close(int sockfd); Either end of the connection can call the close() system call What if there is data being sent? If the other end has closed the connection, and there is no buffered data, reading from a TCP socket returns 0 to indicate EOF. 30 CPE 401/601 Lecture 11 : TCP Socket Programming

31. Client Code TCP clients can call connect() which: – takes care of establishing an endpoint address for the client socket – Attempts to establish a connection to the specified server 3-way handshake no need to call bind first, the O.S. will take care of assigning the local endpoint address – TCP port number, IP address 31 CPE 401/601 Lecture 11 : TCP Socket Programming

32. connect() int connect( int sockfd, const struct sockaddr *server, socklen_t addrlen); sockfd is an already created TCP socket server contains the address of the server connect() returns 0 if OK, -1 on error – No response to SYN segment (3 trials) – RST signal – ICMP destination unreachable (3 trials) 32 CPE 401/601 Lecture 11 : TCP Socket Programming

33. Reading from a TCP socket int read(int fd, char *buf, int max); By default read() will block until data is available reading from a TCP socket may return less than max bytes – whatever is available You must be prepared to read data 1 byte at a time! 33 CPE 401/601 Lecture 11 : TCP Socket Programming

34. Writing to a TCP socket int write(int fd, char *buf, int num); write might not be able to write all num bytes on a nonblocking socket readn(), writen() and readline() functions 34 CPE 401/601 Lecture 11 : TCP Socket Programming

35. Metaphor for Good Relationships To succeed in relationships*: – you need to establish your own identity. – you need to be open & accepting. – you need to establish contacts. – you need to take things as they come, not as you expect them. – you need to handle problems as they arise. 35 bind() accept() connect() check for errors read might return 1 byte *Copyright Dr. Laura’s Network Programming Corp. CPE 401/601 Lecture 11 : TCP Socket Programming

36. UDP Sockets

37. UDP Sockets Programming Creating UDP sockets – Client – Server Sending data Receiving data Connected Mode 37 CPE 401/601 Lecture 11: UDP Socket Programming

38. Creating a UDP socket int socket(int family, int type, int proto); int sock; sock = socket(PF_INET, SOCK_DGRAM,0); if (sock<0) { /* ERROR */ } 38 CPE 401/601 Lecture 11: UDP Socket Programming

39. Binding to well known address typically done by server only int mysock; struct sockaddr_in myaddr; Mysock=socket(PF_INET,SOCK_DGRAM,0); myaddr.sin_family = AF_INET; myaddr.sin_port = htons(1234); myaddr.sin_addr = htonl(INADDR_ANY); bind(mysock, &myaddr, sizeof(myaddr)); 39 CPE 401/601 Lecture 11: UDP Socket Programming

40. Sending UDP Datagrams ssize_t sendto( int sockfd, void *buff, size_t nbytes, int flags, const struct sockaddr* to, socklen_t addrlen); sockfd is a UDP socket buff is the address of the data (nbytes long) to is the destination address Return value is the number of bytes sent, – or -1 on error. 40 CPE 401/601 Lecture 11: UDP Socket Programming

41. sendto() The return value of sendto() indicates how much data was accepted by the O.S. for sending as a datagram – not how much data made it to the destination. There is no error condition that indicates that the destination did not get the data!!! You can send 0 bytes of data! 41 CPE 401/601 Lecture 11: UDP Socket Programming

42. Receiving UDP Datagrams ssize_t recvfrom( int sockfd, void *buff, size_t nbytes, int flags, struct sockaddr* from, socklen_t *fromaddrlen); sockfd is a UDP socket buff is the address of a buffer (nbytes long) from is the address of a sockaddr Return value is the number of bytes received and put into buff, or -1 on error 42 CPE 401/601 Lecture 11: UDP Socket Programming

43. recvfrom() If buff is not large enough, any extra data is lost forever... You can receive 0 bytes of data! The sockaddr at from is filled in with the address of the sender 43 CPE 401/601 Lecture 11: UDP Socket Programming

44. More recvfrom() recvfrom doesn’t return until there is a datagram available, – unless you do something special You should set fromaddrlen before calling If from and fromaddrlen are NULL we don’t find out who sent the data 44 CPE 401/601 Lecture 11: UDP Socket Programming

45. Typical UDP client code Create UDP socket Create sockaddr with address of server Call sendto(), sending request to the server – No call to bind() is necessary! Possibly call recvfrom() – if we need a reply 45 CPE 401/601 Lecture 11: UDP Socket Programming

46. Typical UDP Server code Create UDP socket and bind to well known address Call recvfrom() to get a request, noting the address of the client Process request and send reply back with sendto() 46 CPE 401/601 Lecture 11: UDP Socket Programming

47. Typical UDP Communication 47 CPE 401/601 Lecture 11: UDP Socket Programming

48. UDP Echo Server int mysock; struct sockaddr_in myaddr, cliaddr; char msgbuf[MAXLEN]; socklen_t clilen; int msglen; mysock = socket(PF_INET,SOCK_DGRAM,0); myaddr.sin_family = AF_INET; myaddr.sin_port = htons( S_PORT ); myaddr.sin_addr = htonl( INADDR_ANY ); bind(mysock, &myaddr, sizeof(myaddr)); while (1) { clilen=sizeof(cliaddr); msglen=recvfrom(mysock,msgbuf,MAXLEN,0, cliaddr,&clilen); sendto(mysock,msgbuf,msglen,0,cliaddr,clilen); } 48 NEED TO CHECK FOR ERRORS!!! CPE 401/601 Lecture 11: UDP Socket Programming