Wyv9 ?Dy ù ? C Network Application Development Socket Programming 1 Chapter 2: Socket Programming Chapter goal: ? Program interface to protocol ? Socket API ? Socket programming model Wyv9 ?Dy ù ? C Network Application Development Socket Programming 2 Chapter 2: Socket Programming Chapter goal: ? Program interface to protocol ? Socket API ? Socket programming model Wyv9 ?Dy ù ? C Network Application Development Socket Programming 3 Interacting with Protocol Software ? Client or server uses transport protocols ? Protocol software inside OS ? Applications outside OS ? Mechanism needed to bridge the two ? Called Application Program Interface (API) Wyv9 ?Dy ù ? C Network Application Development Socket Programming 4 Loosely specified protocol software interface ? TCP/IP protocol software is part of the computer’s system software. ? TCP/IP was designed to operate in a multi-vendor environment. ? TCP/IP does not define details of API ? The interface between TCP/IP and applications that use it has been loosely specified. ? TCP/IP standards do not specify the details of how application software interfaces with TCP/IP protocol software ? They only suggest the required functionality, and allow system designers to choose the details when creating an API Wyv9 ?Dy ù ? C Network Application Development Socket Programming 5 Advantages and disadvantages of Loose specification ? Advantages: ? Flexibility and tolerance. ? Different OS select own implementation. ? Disadvantages: ? Designers can make the interface details different for each OS. ? Less portable across machines. ? In practice, only a few APIs have been developed for use with TCP/IP. ? Berkeley UNIX: socket interface, or sockets ? AT&T, System V UNIX: TLI ? Windows: windows sockets interface, WINSOCK Wyv9 ?Dy ù ? C Network Application Development Socket Programming 6 API functionality ? API must support: ? Allocate local resources for communication ? Specify local and remote communication endpoints ? Initiate a connection (client side) ? Wait for a connection (server side) ? Send or receive data ? Determine when data arrives ? Generate urgent data ? Terminate a connection gracefully ? Handle connection termination from the remote site ? Abort communication ? Handle error condition or a connection abort ? Release local resources when communication finishes Wyv9 ?Dy ù ? C Network Application Development Socket Programming 7 Conceptual interface specification ? The TCP/IP standards do not leave implementors without any guidance. ? They specify a conceptual interface: a set of procedures and functions. ? The conceptual interface defined by TCP/IP standards does not specify data representations or programming details; ? It merely provides an example of one possible interface that an OS can offer to application programs that use TCP/IP. Wyv9 ?Dy ù ? C Network Application Development Socket Programming 8 Implementation of an API ? All implementations of a particular API appear the same to programmers: the API merely consists of a set of procedures ( or functions) that an application program can call to establish communication or send and receive data. ? In practice, the implementation of the API depends on the underlying system. ? Sharing API code might reside in the computer’s OS, or it might reside in a region of memory devoted to shared libraries. ? Win95/98 uses a shared lib scheme known as Dynamic Linked Library (DLL). Wyv9 ?Dy ù ? C Network Application Development Socket Programming 9 Implementation of an API ? Win95/98 : Operating System Functions DLL containing TCP/IP software DLL containing socket interface procedures App 1 App 2 App n TCP/IP functions …… App programs Socket API I/O functions Wyv9 ?Dy ù ? C Network Application Development Socket Programming 10 Implementation of an API ? Windows NT/2000/XP uses a hybrid scheme that includes both a DLL and resident code. Operating System Functions DLL containing socket interface procedures App 1 App 2 App n …… App programs Socket API TCP/IP functions and I/O functions Wyv9 ?Dy ù ? C Network Application Development Socket Programming 11 Implementation of an API ? Other implementations are possible. ? OS with no DLL support, all the procedures in API are part of OS. ? A programmer can create an application program that uses the socket API without knowing how the API is implemented. ? The programmer makes procedure calls which may invoke procedures that are linked into the application, procedures in DLL, or procedures in OS. Operating System Functions App 1 App 2 App n …… App programs Socket API, TCP/IP functions and I/O functions Wyv9 ?Dy ù ? C Network Application Development Socket Programming 12 Two basic approaches to network communication ? The designer invents entirely new procedures calls that applications use to access TCP/IP. ? Windows uses this method. ? The designer attempts to use conventional I/O calls to access TCP/IP. ? Unix uses this method. Wyv9 ?Dy ù ? C Network Application Development Socket Programming 13 The basic I/O functions in ANSI C ? open: prepare a device or file for input or output operations. ? close: terminate use of a previously opened device or file. ? read: obtain data from an input device or file, and place it in the application program’s memory. ? write: transmit data from the application program’s memory to an output device or file. ? lseek: move to a specific position in a file or device. ? ioctl: control a device or software used to access it. Wyv9 ?Dy ù ? C Network Application Development Socket Programming 14 Chapter 2: Socket Programming Chapter goal: ? Program interface to protocol ? Socket API ? Socket programming model Wyv9 ?Dy ù ? C Network Application Development Socket Programming 15 Socket API ? Originally designed ? For BSD UNIX-Berkley Socket Interface ? To use with TCP/IP protocols ? Now ? Industry standard ? Available on many operating systems ? Microsoft chose the socket interface as the primary API. ? In MS Windows, WINSOCK Wyv9 ?Dy ù ? C Network Application Development Socket Programming 16 What is “socket”? ? A socket is a virtual connection between two applications ? Using a socket, two processes can communicate with each other ? The socket is the major communication tool for Internet applications ? A socket is bi-directional (full-duplex) transmission ? A socket can be created dynamically Wyv9 ?Dy ù ? C Network Application Development Socket Programming 17 Socket as a virtual connection between two processes Network (physical connection) Host B Process 2 Host A Process 1 Network Adapter Card Information Hiding Socket connection (virtual connection) Wyv9 ?Dy ù ? C Network Application Development Socket Programming 18 Specifying a protocol interface ? The designers must choose one of two broad approaches: ? Define functions specifically to support TCP/IP communication. ? Define functions that support network communication in general, and use parameters to make TCP/IP communication in special case. ? The socket interface provides generalized functions that support network communication using many possible protocols. ? Socket functions refer to all TCP/IP protocols as a single protocol family. ? The functions allow the programmer to specify the type of service required rather than the name of a specific protocol. Wyv9 ?Dy ù ? C Network Application Development Socket Programming 19 Socket abstraction ? In most OS, an application that needs to perform I/O asks the OS to open a file. ? OS responds by creating a file descriptor. . Internal data structure for file 0 0: . Internal data structure for file 1 1: . Internal data structure for file 2 2: . Internal data structure for file 3 3: . . . Descriptor table (one per process) Operating System Wyv9 ?Dy ù ? C Network Application Development Socket Programming 20 Socket abstraction ? The socket interface adds a new abstraction for network communication-socket. ? OS abstraction (not hardware) ? Created dynamically ? Persists only while application runs ? Referenced by a socket descriptor Wyv9 ?Dy ù ? C Network Application Development Socket Programming 21 Socket Descriptor ? Small integer ? One per active socket ? Used in all operations on socket ? Generated by OS when socket created ? Only meaningful to application/process that owns socket ? In UNIX, integrated with file descriptors ? The Windows OS keeps a separate table of socket descriptors for each process Wyv9 ?Dy ù ? C Network Application Development Socket Programming 22 Creating a Socket ?Application calls socket function ?OS returns descriptor for socket ?Descriptor valid until application closes socket or exits ?Common: protofamily = PF_INET, type = SOCK_STREAM,SOCK_DGRAM or SOCK_RAW desc = socket(protofamily,type,proto); Wyv9 ?Dy ù ? C Network Application Development Socket Programming 23 Socket Functionality ? Socket completely general ? Can be used ?By client ?By server ?With a CO transport protocol ?With a CL transport protocol ?To send data, receive data, or both ? Large set of operations Wyv9 ?Dy ù ? C Network Application Development Socket Programming 24 Using sockets ? A socket used by a server: passive socket ? A socket used by a client: active socket ? The difference lies in how applications use them ? The sockets are created the same way initially Wyv9 ?Dy ù ? C Network Application Development Socket Programming 25 Data structures for socket ? The easiest way to understand the abstraction ? When an application calls socket, the OS allocate a new data structure to hold the information needed for communication ? Fill in a new entry in the process’ socket descriptor table with a pointer to the data structure. ? The system keeps a separate socket descriptor table for each process ? Threads in the process share the table. Wyv9 ?Dy ù ? C Network Application Development Socket Programming 26 Data structures for socket . 0: . 1: . Pointers to other socket structures 2: . 3: . . . Socket descriptor table (one per process) Operating System family:PF_INET service:sock_stream local IP: remote IP: . . . local port: remote port: Data structure for a socket ? The application that creates the socket must make additional procedures calls to fill in information in the socket data structure before the socket can be used. Wyv9 ?Dy ù ? C Network Application Development Socket Programming 27 Specifying an endpoint address ? When a socket is created it does not have addresses ? Before using a socket, must specify one or both end addresses. Network (physical connection) Host B Process 2 Host A Process 1 Network Adapter Card Information Hiding Socket connection (virtual connection) Wyv9 ?Dy ù ? C Network Application Development Socket Programming 28 Specifying an endpoint address ? TCP/IP defines communication endpoint to consists of an IP address and protocol port number. ? Other protocol families define their endpoint addresses in other ways. ? To accommodate multiple families of protocols, socket abstraction does not specify how to define endpoint addresses nor does it define a particular protocol address format. ? It allows each protocol family to specify endpoints however it likes. ? The socket abstraction defines an address family for each type of address. ? A protocol family can use one or more address families to define address representation. ? TCP/IP protocols all use a single address rep, with the address family denoted by the symbolic constant AF_INET. ? Both PF_INET and AF_INET have the same value (2). Wyv9 ?Dy ù ? C Network Application Development Socket Programming 29 Generic Address Structure ? Application may need to manipulate protocol addresses without knowing the details of how every protocol family defines its address representation. ? To accommodate such programs, the socket system defines a generalized format that all endpoint addressed use. (address family, endpoint address in that family) Contains a constant that denotes one of the preassigned address type Contains an endpoint address using the standard representation for the specific address type Wyv9 ?Dy ù ? C Network Application Development Socket Programming 30 Generic Address Structure ? In practice, the socket API provides declarations of predefined data types for address endpoints. ? The most general structure is known as a sockaddr structure. struct sockaddr { u_char sa_len; /*length of address*/ u_char sa_family; /*family of address*/ char sa_data[14]; /*address itself*/ } Wyv9 ?Dy ù ? C Network Application Development Socket Programming 31 Generic Address Structure ? Not all address families define endpoints that fit into the sockaddr structure. ? Confusion often arises in practice because the sockaddr structure accommodates address in the AF_INET family. ? To keep programs portable and maintainable, TCP/IP code should not use the sockaddr structure in declaration. ? Instead, sockaddr should be used only as an overlay, and code should reference only the sa_family field in it. ? Each protocol family that uses sockets defines the exact representation of its endpoint addresses. ? The socket software provides corresponding structure declarations. Wyv9 ?Dy ù ? C Network Application Development Socket Programming 32 Generic Address Structure struct sockaddr_in { u_char sin_len; /*length of address*/ u_char sin_family; /*family of address*/ u_short sin_port; /*protocol port number*/ struct in_addr sin_addr; /*IP address*/ char sin_zero[8]; /*not used(set to zero)*/ } ? Predefined structure sockaddr_in: ? An application that uses TCP/IP protocols exclusively can use structure sockaddr_in exclusively, ? It never needs to use the sockaddr structure. Wyv9 ?Dy ù ? C Network Application Development Socket Programming 33 Generic Address Structure ?Key points: ?When representing a TCP/IP communication endpoint, an application program uses structure sockaddr_in, which contains both an IP address and a protocol port number. ?Programmers must be careful when writing programs that use a mixture of protocols because some non-TCP/IP endpoint addresses require a larger structure. Wyv9 ?Dy ù ? C Network Application Development Socket Programming 34 Functions in the socket API: WSAStartup ?Programs using Windows Sockets must call WSAStartup before using sockets. ?Two arguments: ?First to specify the version of WinSock requested. ? Integer in hexadecimal, e.g. 0x102 means version 2.1 ?Second to return information about actual version of WinSock ? Points to a WSADATA structure into which OS writes version information Wyv9 ?Dy ù ? C Network Application Development Socket Programming 35 Functions in the socket API: WSACleanup ? Once application finishes using and closing sockets, the application calls to deallocate all data structures and socket bindings. ? A program usually calls WSACleanup only when it is completely finished and ready to exit. Wyv9 ?Dy ù ? C Network Application Development Socket Programming 36 Functions in the socket API: Creating a Socket desc = socket(protofamily,type,proto); ?Application calls socket function ?OS returns descriptor for socket ?Descriptor valid until application closes socket or exits ?Common: protofamily = PF_INET ?type = SOCK_STREAM,SOCK_DGRAM or SOCK_RAW Wyv9 ?Dy ù ? C Network Application Development Socket Programming 37 Functions in the socket API: closesocket ? closesocket ? Terminate use of socket ? Permanent ? If several processes share a socket, closesocket decrements a reference count and deallocates the socket when the reference count reaches zero. ? There is no socket count for the threads within a process ? If one thread in a process closes a socket, the descriptor is deallocated for all threads in the process. Wyv9 ?Dy ù ? C Network Application Development Socket Programming 38 Functions in the socket API:bind ? bind ? Specify local endpoint address for a socket ? Specify protocol port for a socket ? Specify local IP address for a socket ? Arguments: ? Socket descriptor ? Endpoint address: sockaddr_in ? Can use INADDR_ANY for any IP address bind(socket,localaddr,addrlen); Wyv9 ?Dy ù ? C Network Application Development Socket Programming 39 Functions in the socket API: listen ? listen ? When a socket is created, it is neither active nor passive. ? Used by connection-oriented servers ? Prepares socket to accept incoming connections (in passive mode) ? To ensure that no request is lost, a server must pass listen two arguments: ? Socket ? Size of queue listen(socket,queuesize); Wyv9 ?Dy ù ? C Network Application Development Socket Programming 40 Functions in the socket API: accept ?accept ?For TCP sockets ?Used by server ?Waits for next connection and returns new socket ?Original socket can accept additional connection request ?New socket transfers data for the specific client newsock = accept(socket,caddr,caddrlen); Wyv9 ?Dy ù ? C Network Application Development Socket Programming 41 Functions in the socket API: connect ? connect ? Used by client ? Either ? Performs a TCP connection ? Fully specifies addresses for UDP connect(socket,saddr,saddrlen); Wyv9 ?Dy ù ? C Network Application Development Socket Programming 42 Two Purposes of the Connect Function The connect function, which is called by clients, has two uses. With connection- oriented transport, connect establishes a transport connection to a specified server. With connectionless transport, connect records the server’s address in the socket, allowing the client to send many messages to the same server without specifying the destination address with each message. Wyv9 ?Dy ù ? C Network Application Development Socket Programming 43 Functions in the socket API: send, sendto, sendmsg ? send, sendto, and sendmsg ? Transfer outgoing data from application send(socket,data,length,flags); sendto(socket,data,length,flags, destaddr,addrlen); sendmsg(socket,msgstruct,flags); Wyv9 ?Dy ù ? C Network Application Development Socket Programming 44 Format of msgstruct struct msgstruct { struct sockaddr *m_saddr; /*dest address*/ struct datavec *m_dvec; /*message (vector)*/ int mdvlength; /*size of vector*/ struct access *m_rights; /*access rights*/ int m_alength; /*size of access rights*/ } Wyv9 ?Dy ù ? C Network Application Development Socket Programming 45 Functions in the socket API: recv, recvfrom, and recvmsg ? recv, recvfrom, and recvmsg ? Transfer incoming data to application recv(socket,buffer,length,flags); recvfrom(socket,buffer,length,flags, senderaddr,saddrlen); recvmsg(socket,msgstruct,flags); Wyv9 ?Dy ù ? C Network Application Development Socket Programming 46 Summary of socket calls used with TCP ? WSAStartup: initialize the socket lib (Windows only) ? WSACleanup: terminate use of socket lib (Windows only) ? socket: create a descriptor for use in network communication ? connect: connect to a remote peer (client) ? closesocket: terminate communication and deallocate a descriptor ? bind: bind a local IP address and protocol port number to a socket ? listen: place the socket in passive mode and set the number of incoming TCP connections the system will enqueue (server) ? accept: accept the next incoming connection (server) ? recv: acquire incoming data from a stream connection or the next incoming msg. Wyv9 ?Dy ù ? C Network Application Development Socket Programming 47 Summary of socket calls used with TCP ? recvfrom: receive the next incoming datagram and record its source endpoint address ? select: wait until the first of a specified set of sockets becomes ready for I/O ? send: send outgoing data or a msg. ? sendto: send an outgoing datagram to a specified endpoint ? shutdown: terminate a TCP connection in one or both directions ? getpeername: after a connection arrives, obtain the remote machine’s endpoint address from socket ? getsockopt: obtain the current options for a socket ? setsockopt: change the options for a socket Wyv9 ?Dy ù ? C Network Application Development Socket Programming 48 Utility routines for integer conversion ? TCP/IP specifies a standard representation for binary integers used in protocol headers: network byte order ? Some socket routines require arguments to be stored in network byte order. ? Several functions can convert between network byte order and local host’s byte order. ? htons: host to network short (16bits) ? ntohs: network to host short ? htonl: host to network long (32bits) ? ntohl: network to host long ? Doing so makes the source code portable to any machines, regardless of its native byte order. Wyv9 ?Dy ù ? C Network Application Development Socket Programming 49 Symbolic constants for socket call parameters ? A set of predefined symbolic constants and data structure declarations that applications use to declare data and to specify arguments. ? Such as: SOCK_STREAM, SOCK_DGRAM ? In windows, should include statement: #include <winsock.h> Wyv9 ?Dy ù ? C Network Application Development Socket Programming 50 Chapter 2: Socket Programming Chapter goal: ? Program interface to protocol ? Socket API ? Socket programming model Wyv9 ?Dy ù ? C Network Application Development Socket Programming 51 Socket as a client/server model Server Host A The server should always be waiting for requests from the clients. A client makes a request, then the server responds. Client 2 Client 3 Host C socket socket Client 1 Host B Request Reply Socket Wyv9 ?Dy ù ? C Network Application Development Socket Programming 52 Socket Programming Tutorial(1) SERVER bind() listen() accept() recv() send() closesocket() CLIENT socket() connect() send() closesocket() socket() 1: Connection Request 2. Send a command recv() “*” indicates a blocking function call. * R e q u e s t A ck n o w l ed g e WSACleanup WSACleanup WSAStartup WSAStartup * * * 3. Receive the result Wyv9 ?Dy ù ? C Network Application Development Socket Programming 53 Socket Programming Tutorial(2) “*” indicates a blocking function call. SERVER bind() listen() accept() recv() CLIENT socket() connect() send() closesocket() socket() * * * 2. Send a command 3. Receive the result recv() send() We are not doing this... 4. END 1: Connection Request * Wyv9 ?Dy ù ? C Network Application Development Socket Programming 54 Socket Programming Tutorial(3) Step 1: socket(_) call ? Prepare data structure to manage socket ? OS is responsible for this After socket(_) call: Server It declares a socket to be used. Wyv9 ?Dy ù ? C Network Application Development Socket Programming 55 Socket Programming Tutorial(4) Step 2: bind(_) call 6500 Server Port Port = A logical connecting point at a host for two communicating processes using socket After bind(_) call: Port Numbers: 0~1023: System Reserved Port 21: FTP Port 23: telnet Port 80: HTTP It connects a process to a specific port 1024 and above: available to users Wyv9 ?Dy ù ? C Network Application Development Socket Programming 56 Socket Programming Tutorial(5) Step 3: listen(_) call listen( ) system call: prepare memory buffer for incoming connections After listen(_) call: 6500 Server Buffer We need to specify how many connection requests should be held in the buffer when SERVER is busy (can’t accept a request). listen (socket_id, number_of_connection); Wyv9 ?Dy ù ? C Network Application Development Socket Programming 57 Socket Programming Tutorial(6) Step 4 - Part 1: accept(_) call accept ( ) function is a blocking function After accept(_) call: Server Client 6500 The server process accepts a request from a client Wyv9 ?Dy ù ? C Network Application Development Socket Programming 58 Socket Programming Tutorial(7) Step 4 - Part 2: accept(_) call The accept(_) call returns another port number and establish another connection Client Server 7100 6500 d A new port is chosen by OS c OS duplicates the socket connection Wyv9 ?Dy ù ? C Network Application Development Socket Programming 59 Socket Programming Tutorial(8) Step 5: recv(_) and send() call Client Server 7100 6500 Data transmission The server and client communicate using the second socket Wyv9 ?Dy ù ? C Network Application Development Socket Programming 60 Socket Programming Tutorial(9) Step 6: closesocket (_) call Client Server 7100 6500 Close the second socket and leave the first socket for next client Wyv9 ?Dy ù ? C Network Application Development Socket Programming 61 Socket Programming Tutorial(10) Step 7: Go back to accept (_) call Server 6500 Client The server process goes back to the accept call Wyv9 ?Dy ù ? C Network Application Development Socket Programming 62 Summary We have Learned: ? Program interface to protocol ? Socket API ? Socket programming model