Application Programming Interfaces

Transcription

1 Application Programming Interfaces The TCP/IP protocol suite provides only the protocols that can be used by processes to communicate across a network. Though standarized, how these protocols are implemented is system specific Transport protocols and IP are typically implemented in the operating system. Hence, an API is needed to provide user processes with access to these protocols 1

2 Link IP TCP / UDP 3/21/2018 Remember Where We are in the Network Protocol Stack As packets move down the protocol stack header information is added to the front of the packet. Question: Who/What defines the meaning of the bytes in the headers that are added? Who/What defines the meaning of the bytes in the application data portion of the packet? Application Data Application Data Application Data Application Data application Sockets transport network link physical Socket is a Low-level Interface Sockets -developed at Berkeley by Bill Joy and others (1981). Incorporated into BSD Unix and all derivatives, also supported in Windows, Mac OS X, etc. Key concept: make network access look much like file access. 2

3 High-Level APIs Remote Procedure Call (RPC) message passing appears as procedure calling client accesses remote server through procedure call interface. typically implemented as a library atop sockets Object-oriented network programming Java.net class library OO middleware platforms such as CORBA Java Remote Method Invocation (RMI) These are at the Application level of the protocol stack Introduction to Sockets A socket is a kernel data structure, accessed via a file descriptor, that defines a bi-directional endpoint of communication Sockets are used as a basic building block for interprocess communication The kernel data structure contains state information regarding the socket has associated lists of send and receive buffers accesses transport functions directly 3

4 Socket programming goal: learn how to build client/server applications that communicate using sockets socket: door between application process and end-endtransport protocol application process socket application process controlled by app developer transport network link physical Internet transport network link physical controlled by OS Application Layer 2-7 Socket Related System Calls System Call Purpose Takes Returns socket() Creates a socket Protocol family, type of socket, specific protocol getsocketname() Get the port and IP address associated with the socket File descriptor (socket) bind() binds a unique local name to the socket File descriptor (socket) and local address connect() Create a connection with a remote system File descriptor (socket) and remote address listen() Applies only to stream sockets. indicates a readiness to accept client connection requests, and creates a connection request queue of length backlog to queue incoming connection requests. Once full, additional connection requests are rejected. File descriptor (socket) and length of backlog queue File Descriptor Port and IP address Success or Error code Success or Error code Success or Error code accept() Accepts an incoming TCP connection request File descriptor (socket) File descriptor (socket) 4

5 Socket Related System Calls (cont) System Call Purpose Takes Returns send Send a buffer Socket, address of buffer, length of buffer, flags sendto Send a buffer to a remote host Socket, address of buffer, length of buffer, flags, remote address, length of remote address recv Receive a buffer Socket, address of buffer to store bytes in, length of buffer, flags recvfrom Receive a buffer from a remote host Socket, address of buffer, length of buffer, flags, remote address, address of integer containing length of remote address Success or error code Success or error code Success or error Success or error Number of bytes actually received socket() system call A socket is created using the socket() call, which takes as parameters: protocol family type of communication specific protocol (often implicit) socket() returns a file descriptor A socket is created without a name a socket name is a triple: class (a.k.a. family), IP address, port number a name can be affixed to the socket after it is created by invoking the bind() system call 5

6 Protocol Families Internet Domain (AF_INET) interprocess communication among different hosts socket name includes internet address and port some port numbers are reserved for system use (see /etc/services) Unix Domain (AF_UNIX, aka AF_LOCAL) UNIX system internal protocols interprocess communication within same host socket name is a file pathname INET Socket types datagram socket (SOCK_DGRAM) unreliable datagram communication record boundary (fixed maximum) is preserved implemented using the UDP protocol stream socket (SOCK_STREAM) reliable virtual circuit communication record boundary is not preserved implemented using the TCP protocol raw socket (SOCK_RAW) direct interface to the IP protocol super-user (administrator) only, for new protocol development 6

7 Socket programming basics Initial receiver (which we will call the server) must be running before the initial sender (the client) can (successfully) send anything to it. Server must have a socket through which it receives and sends segments Similarly, the client needs a socket The server socket is locally identified with (bound to) a port number Analogous to the mailbox number Client needs to know the server s IP address and port number. Socket programming with UDP UDP: no connection between client and server sending application specifies the IP address and port of the destination the OS places these in the header of each outgoing datagram OS also places IP address and port of the sending socket in the header of each datagram Server can extract IP address, port of sender from received datagram Note: the official terminology for a UDP packet is datagram. Some texts refer to UDP segments, but the term segment usually applies to parts of TCP data streams 7

8 Datagram socket passive side Create a datagram socket Bind to socket Get socketname so it can be displayed Use socket to receive and send data Datagram socket active side Create a socket Get server s address and port Use socket to send and receive data 8

9 Client/server socket interaction: UDP server (running on serverip) create socket, port= x: serversocket = socket(af_inet,sock_dgram) read datagram from serversocket write reply to serversocket specifying client address, port number client create socket: clientsocket = socket(af_inet,sock_dgram) Create datagram with server IP and port=x; send datagram via clientsocket read datagram from clientsocket close clientsocket Application Layer Example app: UDP client include Python s socket library create UDP socket for server get user keyboard input Attach server name, port to message; send into socket read reply characters from socket into string print out received string and close socket Python UDPClient from socket import * servername = hostname serverport = clientsocket = socket(af_inet, SOCK_DGRAM) message = raw_input( Input lowercase sentence: ) clientsocket.sendto(message.encode(), (servername, serverport)) modifiedmessage, serveraddress = clientsocket.recvfrom(2048) print modifiedmessage.decode() clientsocket.close() 9

10 Application Layer Example app: UDP server Python UDPServer create UDP socket bind socket to local port number loop forever Read from UDP socket into message, getting client s address (client IP and port) send upper case string back to this client from socket import * serverport = serversocket = socket(af_inet, SOCK_DGRAM) serversocket.bind(('', serverport)) print ( The server is ready to receive ) while True: message, clientaddress = serversocket.recvfrom(2048) modifiedmessage = message.decode().upper() serversocket.sendto(modifiedmessage.encode(), clientaddress) Socket programming with TCP Client (active side) must connect to server (passive side) server process must first be already running server must have created a socket that receives the client s connection request Client contacts server by: creating a local TCP socket issuing a connect() system call, specifying IP address of server socket port number of server socket Server actions: create and bind a name to the receiving socket Await connection request Upon receipt, TCP creates new socket for the server process to communicate with this client allows server to talk with multiple clients on different sockets all have same server port number more on this later 10

11 Stream socket Passive side Create a stream socket Bind to socket Await (listen for) connections using listen() system call to prepare operating system for connection requests Parameters: socket descriptor backlog: defines the maximum length the queue of pending connections (usually 5) Passive side (cont.) Use the accept() system call socket descriptor name (family, host addr, port)? length of name Extracts the first connection on the queue of pending connections, creates a new socket, and allocates/returns a new file descriptor for the socket. Normally blocks, but this can be turned off Returns file descriptor of a new socket. 11

12 Stream socket active side Create a stream socket Initiate a connection on a socket using the connect() system call socket descriptor name (family, host addr, port) length of name Errors ETIMEOUT ECONNREFUSED ENETDOWN or EHOSTDOWN ENETUNREACH or EHOSTREACH Transferring data Transmitting write() as for files send() uses flags to specify such requests as out-of-band transmission (MSG OOB) Receiving read() as for files recv() uses flags to specify such requests as examining data without reading it (MSG PEEK) 12

13 Python Client/server socket interaction: TCP server (running on hostid) client create socket, port=x, for incoming request: serversocket = socket() wait for incoming connection request connectionsocket = serversocket.accept() read request from connectionsocket write reply to connectionsocket close connectionsocket TCP connection setup create socket, connect to hostid, port=x clientsocket = socket() send request using clientsocket read reply from clientsocket close clientsocket Python Example app: TCP client create TCP socket for server, remote port No need to attach server name, port Python TCPClient from socket import * servername = servername serverport = clientsocket = socket(af_inet, SOCK_STREAM) clientsocket.connect((servername,serverport)) sentence = raw_input( Input lowercase sentence: ) clientsocket.send(sentence.encode()) modifiedsentence = clientsocket.recv(1024) print ( From Server:, modifiedsentence.decode()) clientsocket.close() 13

14 Python Example app: TCP server create TCP welcoming socket server begins listening for incoming TCP requests loop forever server waits on accept() for incoming requests, new socket created on return read bytes from socket (but not address as in UDP) close connection to this client (but not welcoming socket) Python TCPServer from socket import * serverport = serversocket = socket(af_inet,sock_stream) serversocket.bind((,serverport)) serversocket.listen(1) print The server is ready to receive while True: connectionsocket, addr = serversocket.accept() sentence = connectionsocket.recv(1024).decode() capitalizedsentence = sentence.upper() connectionsocket.send(capitalizedsentence. encode()) connectionsocket.close() 14