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Programming with UDP – I

Programming with UDP – I. Covered Subjects : IPv4 Socket Address Structure Byte Ordering Functions Address Access/ Conversion Functions Functions : socket () bind () sendto () recvfrom () close (). Socket Address Structures.

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Programming with UDP – I

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  1. Programming with UDP – I CoveredSubjects: IPv4 SocketAddressStructure ByteOrderingFunctions Address Access/ConversionFunctions Functions: socket() bind() sendto() recvfrom() close()

  2. SocketAddressStructures • Most socket functions require a pointer to a socket address structure as an argument. • Each supported protocol defines its own socket address structure. • Thenames of thesestructuresbeginwithsockaddr_andendwith a uniquesuffixforeachprotocolsuite.

  3. IPv4 SocketAddressStructure • Internet socketaddressstructure is namedsockaddr_inanddefined in <netinet/in.h> • IP address: structin_addr { in_addr_ts_addr; /* 32-bit IP address */ }; /* network byteordered*/ • TCP or UDP Address: structsockaddr_in { uint8_t sin_len; /* length of structure*/ shortsin_family; /* e.g., AF_INET */ ushortsin_port; /* TCP/UDP port */ structin_addr; /* IP address */ charsin_zero[8]; /* unused */ }; Note: Havingsin_lensimplifieshandlingvariablelengthsocketaddressstructures. All but sin_familyin network byteorder

  4. GenericSocketAddressStructures Problem: • A socket address is always passed by reference when passed as an argument to socket functions. • But anysocketcallthattakesone of thesepointers as an argumentmustdealwithsocketaddressstructuresfrom ANY of thesupportedprotocolfamilies. • A problem arises in how todeclarethetype of pointerthat is passed.

  5. continued.. Solution: • is to define a genericsocketstructuredefined in <sys/socket.h> Example: intbind(int, structsockaddr *, socklen_t); structsockaddr_inserv; /* IPv4 address */ bind(sockfd, (structsockaddr *)&serv,sizeof(serv)); Ifweomitthiswarningcomes! Q: Why not do (void *)? structsockaddr{ uint8_t sa_len; sa_family_tsa_family; /* AF_INET, AF_INET6 .. */ charsa_data[14] /* protocolspecificaddr */ };

  6. ByteOrdering • Big Endian vs. Little Endian • LittleEndian (Intel, DEC): • Least significant byte of word is stored in the lowestmemoryaddress • Big Endian (Sun, SGI, HP): • Most significant byte of word is stored in the lowestmemoryaddress • Network Byte Order = Big Endian • Allows both sides to communicate • Must be used for some data (i.e. IP Addresses) • Good form for all binary data

  7. ByteOrderingFunctions • 16- and 32-bit conversion functions (for platformindependence) • Examples: int m, n; shortint s,t; m = ntohl (n) net-to-host long (32-bit) translation s = ntohs (t) net-to-host short (16-bit) translation n = htonl (m) host-to-net long (32-bit) translation t = htons (s) host-to-net short (16-bit) translation

  8. Address Access/ConversionFunctions in_addr_tinet_addr(const char* strptr); • Translate dotted-decimal notation to IP address; returns -1 onfailure, thus cannot handle broadcast value “255.255.255.255” intinet_aton(const char *strptr, struct in_addr *inaddr); • Translate dotted-decimal notation to IP address; returns 1 onsuccess, 0 on failure. char* inet_ntoa(structin_addrinaddr); • Translate IP address to ASCII dotted-decimal notation (e.g., “128.32.36.37”)

  9. UDP Client-Server Interaction Model

  10. socket()Function • Create a socket • intsocket(intfamily, inttype, intprotocol); • Returns file descriptor or -1. • Fields: • intfamily • AF_INET IPv4 protocols • AF_INET6 IPv6 protocols • inttype • SOCK_DGRAM  for UDP datagrams • SOCK_STREAM  for TCP streams • intprotocol(forAF_INET & AF_INET6familysockets) • IPPROTO_UDP  UDP Transport • IPPROTO_TCP  TCP Transport

  11. bind()Function • Bind a socket to a local IP address and port number • intbind (intsockfd, structsockaddr* myaddr, intaddrlen); • Returns 0 if OK or -1 on error. • sockfd:socket file descriptor (returned from socket) • myaddr: includes IP address and port number • IP address: set by kernel if value passed is INADDR_ANY, else set bycaller • port number: set by kernel if value passed is 0, else set bycaller • addrlen: length of address structure • = sizeof (structsockaddr_in)

  12. sendto() & recvfrom()Functions intsendto (intsockfd, char* buf, size_t nbytes, intflags, structsockaddr* destaddr, intaddrlen); • Send a datagram to another UDP socket. • Returns number of bytes written or -1. intrecvfrom (intsockfd, char* buf, size_t nbytes, intflags, structsockaddr* srcaddr, int* addrlen); • Read a datagram from a UDP socket. • Returns number of bytes read or -1.

  13. close()Function intclose (intsockfd); • Close a socket. • Returns 0 on success, -1 on failure. • sockfd:socket file descriptor (returned from socket) • Closes communication on socket in both directions. • All data sent before closeare delivered to other side. • After close, sockfdis not valid for reading orwriting.

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