Network Programming in Python. Bin Li Assistant Professor Dept. of Electrical, Computer and Biomedical Engineering University of Rhode Island

Transcription

1 Network Programming in Python Bin Li Assistant Professor Dept. of Electrical, Computer and Biomedical Engineering University of Rhode Island

2 Transmission Control Protocol (TCP) Connection-oriented Two endpoints of a virtual circuit Reliable Application needs no error checking Stream-based No predefined blocksize Processes identified by port numbers Services live at specific ports Used by Web Browsers, s, SSH, FTP, etc.

3 User Datagram Protocol (UDP) Datagram-based Connectionless, unreliable, can broadcast, low overhead Applications usually message-based No transport-layer retries Applications handle (or ignore) errors Processes identified by port number Services live at specific ports Usually below 1024, requiring privilege Used for VOIP, Online Games, Streaming

4 Sockets Various sockets Any similarity? Endpoint of a connection Identified by IP address and Port number Primitive to implement high-level networking interfaces e.g., Remote procedure call (RPC)

5 Socket Programming API Application Programming Interface Socket analogous to door sending process shoves message out door sending process assumes transport infrastructure on other side of door which brings message to socket at receiving process host-local, application created/owned, OScontrolled connection between sockets set-up/ managed by OS host or server process socket TCP with buffers, variables controlled by app developer controlled by OS Internet host or server process socket TCP with buffers, variables

6 Socket: Conceptual View socket()

7 Two Essential Types of Sockets SOCK_STREAM a.k.a. TCP reliable delivery in-order guaranteed connection-oriented bidirectional SOCK_DGRAM a.k.a. UDP unreliable delivery no order guarantees no notion of connection app indicates dest. for each packet can send or receive App App D socket Dest socket D2 D3

8 Types of Sockets When sending Hi! and Hope you re well TCP treats them as a single bytes stream Bytes stream l l e w e p o H! i H UDP treats them as separate messages Hope you re well Hi!

9 Types of Sockets (Cont ) Thus, TCP needs application-level message boundary. By carrying length in application-level header Bytes stream l l e w o H 16! i H 3

10 Client/Server Concepts Server opens a specific port The one associated with its service Then just waits for requests Server is the passive opener Clients get ephemeral ports Guaranteed unique, 1024 or greater Uses them to communicate with server Client is the active opener

11 A Socket-eye view of the Internet Each host machine has an IP address cse.unr.edu ( ) newworld.cs.umass.edu ( ) cluster.cs.columbia.edu ( , , , )

12 Ports Each host has 65,536 ports Some ports are reserved for specific apps 20,21: FTP 23: Telnet 80: HTTP see RFC 1700 about 2000 ports are reserved Port 0 Port 1 Port A socket provides an interface to send data to/from the network through a port

13 TCP State Diagram

14 Connection-Oriented Services socket(socket_family, socket_type) The constructer creates a new socket bind(hostname, port) Bind takes a turple of a host address and a port listen() Starts listening for TCP connections accept() accepts a connection when found (returns new socket) connect(hostname, port) Takes a turple of the address and port recv(buffer) Tries to grab data from a TCP connection (waits) send(bytes) attempts to send the bytes given to it. close() closes a socket/connection and frees the port. socket() bind() listen() accept() [blocked] read() [blocked] write() Server When interaction is over, server loops to accept a new connection Client socket() connect() write() read() [blocked]

15 Capitalize Sentence Program Let s create a basic client server program that uses TCP to connect and send text to a server. The server then replies with that text capitalized. We will build the Server first then the Client tcpserver.py and tcpclient.py

16 import socket tcpserver.py import socket tcpclient.py host = port = 5000 s = socket.socket() s.bind((host, port)) s.listen(1) c, addr = s.accept() print Connection from: + str(addr) while True: data = c.recv(1024) if not data break print from connect user: + str(data) data = str(data).upper() print sending: + str(data) c.send(data) s.close() host = port = 5000 s = socket.socket() s.connect((host, port)) message = raw_input( -> ) while message!= q : s.send(message) data = s.recv(1024) print Received from server: + str(data) message = raw_input( -> ) s.close()

17 UDP Capitalize Let s create the same program as before this time using a UDP connection udpserver.py and udpclient.py You will notice we use recvfrom() and sendto() This is because UDP is Connectionless, we need to tell the data where to go when we send it

18 Connectionless Services socket() bind() recvfrom() [blocked] sendto() socket() bind() sendto() recvfrom() [blocked] SERVER CLIENT

19 import socket udpserver.py import socket udpclient.py host = port = 5000 s = socket.socket(socket.af_inet, socket.sock_dgram) s.bind((host, port)) print Server Started while True: data, addr = s.recvfrom(1024) print message from: + str(addr) print from connect user: + str(data) data = str(data).upper() print sending: + str(data) s.sendto(data, addr) s.close() host = port = 5001 server = ( , 5000) s = socket.socket(socket.af_inet, socket.sock_dgram) s.bind((host, port)) message = raw_input( -> ) while message!= q : s.sendto(message, server) data, addr = s.recvfrom(1024) print Received from server: + str(data) message = raw_input( -> ) s.close()

20 Comparison import socket tcpserver.py import socket udpserver.py host = port = 5000 s = socket.socket() s.bind((host, port)) s.listen(1) c, addr = s.accept() print Connection from: + str(addr) while True: data = c.recv(1024) if not data break print from connect user: + str(data) data = str(data).upper() print sending: + str(data) c.send(data) s.close() host = port = 5000 s = socket.socket(socket.af_inet, socket.sock_dgram) s.bind((host, port)) print Server Started while True: data, addr = s.recvfrom(1024) print message from: + str(addr) print from connect user: + str(data) data = str(data).upper() print sending: + str(data) s.sendto(data, addr) s.close()

21 Comparison (Cont ) import socket tcpclient.py import socket udpclient.py host = port = 5000 s = socket.socket() s.connect((host, port)) message = raw_input( -> ) while message!= q : s.send(message) data = s.recv(1024) print Received from server: + str(data) message = raw_input( -> ) s.close() host = port = 5001 server = ( , 5000) s = socket.socket(socket.af_inet, socket.sock_dgram) s.bind((host, port)) message = raw_input( -> ) while message!= q : s.sendto(message, server) data, addr = s.recvfrom(1024) print Received from server: + str(data) message = raw_input( -> ) s.close()

22 Summary of Address Families socket.af_unix Unix named pipe (NOT Windows ) socket.af_inet Internet IP version 4 The basis of this class socket.af_inet6 Internet IP version 6 Rather more complicated

23 Summary of Socket Types socket.sock_stream TCP, connection-oriented socket.sock_dgram UDP, connectionless socket.sock_raw Gives access to subnetwork layer SOCK_RDM, SOCK_SEQPACKET Very rarely used