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20 21 22 23 25 80 110 143 443
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The process of establishing a TCP connection involves a precise sequence of steps on both the server and client, as shown in Figure 211
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Server socket() Client socket()
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listen()
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accept()
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wait for connection establish connection connect()
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request
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TCP connection protocol
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For TCP servers, the socket object used to receive connections is not the same socket used to perform subsequent communication with the client In particular, the accept() system call returns a new socket object that s actually used for the connectionThis allows a server to manage connections from a large number of clients simultaneously
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UDP communication is performed in a similar manner, except that clients and servers don t establish a connection with each other, as shown in Figure 212
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Server socket() Client socket()
bind()
bind()
recvfrom()
wait for data request
sendto()
process request
sendto()
response
recvfrom()
UDP connection protocol
The following example illustrates the TCP protocol with a client and server written using the socket module In this case, the server simply returns the current time to the client as a string
# Time server program from socket import * import time s = socket(AF_INET, SOCK_STREAM) sbind(('',8888)) slisten(5) # # # # Create a TCP socket Bind to port 8888 Listen, but allow no more than 5 pending connections
while True: client,addr = saccept() # Get a connection print("Got a connection from %s" % str(addr)) timestr = timectime(timetime()) + "\r\n" clientsend(timestrencode('ascii')) clientclose()
Here s the client program:
# Time client program from socket import * s = socket(AF_INET,SOCK_STREAM) # Create a TCP socket sconnect(('localhost', 8888)) # Connect to the server tm = srecv(1024) # Receive no more than 1024 bytes sclose() print("The time is %s" % tmdecode('ascii'))
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21 Network Programming and Sockets
An example of establishing a UDP connection appears in the socket module section later in this chapter It is common for network protocols to exchange data in the form of text However, great attention needs to be given to text encoding In Python 3, all strings are Unicode Therefore, if any kind of text string is to be sent across the network, it needs to be encodedThis is why the server is using the encode('ascii') method on the data it transmits Likewise, when a client receives network data, that data is first received as raw unencoded bytes If you print it out or try to process it as text, you re unlikely to get what you expected Instead, you need to decode it firstThis is why the client code is using decode('ascii') on the result The remainder of this chapter describes modules that are related to socket programming 22 describes higher-level modules that provide support for various Internet applications such as email and the Web
asynchat
The asynchat module simplifies the implementation of applications that implement asynchronous networking using the asyncore module It does this by wrapping the low-level I/O functionality of asyncore with a higher-level programming interface that is designed for network protocols based on simple request/response mechanisms (for example, HTTP) To use this module, you must define a class that inherits from async_chatWithin this class, you must define two methods: collect_incoming_data() and found_terminator()The first method is invoked whenever data is received on the network connectionTypically, it would simply take the data and store it someplaceThe found_terminator() method is called when the end of a request has been detected For example, in HTTP, requests are terminated by a blank line For data output, async_chat maintains a producer FIFO queue If you need to output data, it is simply added to this queueThen, whenever writes are possible on the network connection, data is transparently taken from this queue
async_chat([sock])
Base class used to define new handlers async_chat inherits from asyncoredispatcher and provides the same methods sock is a socket object that s used for communication An instance, a, of async_chat has the following methods in addition to those already provided by the asyncoredispatcher base class:
aclose_when_done()
Signals an end-of-file on the outgoing data stream by pushing None onto the producer FIFO queueWhen this is reached by the writer, the channel will be closed
acollect_incoming_data(data)
Called whenever data is received on the channel data is the received data and is typically stored for later processingThis method must be implemented by the user
adiscard_buffers()