Lab #4 -- Network programming

CSE 491, Sep 24th, 2009.


It is impossible, by examining any significant piece of completed code, to determine within a factor of two how many man-hours it took to produce that code.

And the corollary:

If you can't tell how long a piece of code would take when you have the finished product available, what chance do you think you have before the first line of code is written?

-- Kyle Wilson,


Blocking vs non-blocking I/0

  • blocking I/O blocks the thread of execution until input/output is available. This means that you must use multiple threads of execution (threading, multiprocess) OR special monitoring functions (select) to handle multiple connections with blocking I/O in a single thread of execution.

  • non-blocking I/O doesn't block, but returns immediately with data OR aborts and signals an error ("no data available, boss").

    (how do you distinguish between "data available" and error? guesses?)

Synchronous vs asynchronous I/O

  • synchronous is like a conversation: you speak, then I speak.
  • asynchronous is like an unfriendly argument: everyone speaking at once.

Synchronous vs asynchronous processing

  • synchronous processing waits for one "conversation" to be over before starting another one.
  • synchronous processing is BAD: inefficient, doesn't scale, resource intensive...
  • asynchronous processing relies on built-in language or OS features to "keep track" of several connections at once. It can be done inefficiently or efficiently (threading/processes, or select); the latter depends more on language/OS features.

(Some diagrams...)

TCP/IP socket stuff

Connecting out via TCP-IP

Creating a socket:

sock = socket.socket()

Connecting that socket to a remote server:

remote_address = ''
port = 25
sock.connect((remote_address, port))

Reading from the socket:

data = sock.recv(4096)

You will get up to bufsize=4096 bytes, and if there is nothing available, 'sock.recv' will block (wait) until either there is something available, or the socket closes, or you hit CTRL-C.

Writing to the socket:

data = sock.sendall('HELO')

Sending mail - example

Here's a sample SMTP (mail) session:

>>> import socket
>>> sock = socket.socket()

Connect to the server:

>>> sock.connect(('', 25))
>>> sock.recv(4096)                   #doctest: +ELLIPSIS
'220 ESMTP Exim 4.63 ...\r\n'

Say hello:

>>> sock.sendall("HELO\n")
>>> sock.recv(4096)                   #doctest: +ELLIPSIS
'250 Hello [...]\r\n'

Identify the sender address:

>>> sock.sendall("MAIL FROM:\n")
>>> sock.recv(4096)
'250 OK\r\n'

Identify the recipient address:

>>> sock.sendall("RCPT TO:\n")
>>> sock.recv(4096)
'250 Accepted\r\n'

OK, greetings are over; set up to send the actual message:

>>> sock.sendall('DATA\n')
>>> sock.recv(4096)
'354 Enter message, ending with "." on a line by itself\r\n'

Aaaaaaaaaaaaand send!

>>> sock.sendall('foo bar baz\n.\n')
>>> sock.recv(4096)       #doctest: +ELLIPSIS
'250 OK...\r\n'

Be nice & 'quit' before closing.

>>> sock.sendall('quit\n')
>>> sock.close()

Receiving connections

Create a socket:

import socket
sock = socket.socket()

If 'interface' is a blank string, connections are allowed in to all IP addresses on this host; if it's non-blank, e.g. e.g. '', then only connections to that network interface are allowed. I would suggest using a blank interface so that you don't have to worry about it:

interface = ''
port = 5555

sock.bind( (interface, port) )

Allow up to 5 "backlogged" connections (unhandled connections in):


Handle incoming connections one at a time until they die:

while 1:
   (client_sock, client_address) = sock.accept()

    while 1:
        data = client_sock.recv(4096)
        if not data:

        print 'got data:', data

For example, if you run this code, you can try using your Web browser to connect to


to see what's printed out on your screen by both sides...

Two final notes -- 'bind' is sometimes a bit sticky, and you may have to wait for a while (10-20 seconds) before rebinding a socket after hitting CTRL-C. One way around this is just to change the port number.

Also, if you can find a 'telnet' program (installed on many UNIXes by default; not available on Windows by default -- enable like this) then you can talk directly to TCP ports; just do

telnet 25

to connect to port 25 on

Non-blocking network I/O

By default, sockets are blocking; see the above 'while' loop. When you try to read from them, your program will hang until some event (data is available; connection closed; etc.)

For the next HW, you must use socket.setblocking(0) on all of your sockets to make a non-blocking echo server. This need to be done on the bound socket as well as on all client connection sockets.

If you set the sockets to non-blocking, then you will get an exception almost every time you try to accept a connection or read from a connection: that is,


will raise a socket.error.

To handle this case, wrap it in a try/except statement body:

except socket.error:
   # handle error

The code in the 'except block ('handle error') can be whatever you want it to be; see below for three examples.

For problem #3 in the homework, you will need to keep track of what connections you are currently processing; you can use a list. Be sure to remove dead connections.

Exception handling

We haven't gone over exception handling in Python yet, but it's fairly similar to exception handling in C++ and I'm hoping most people have seen it. The basic idea is that if you have a statement that could "error out", that statement may throw an exception:

   x = int('foo')
except Exception:
   print 'got an exception'

Consider these three examples:

>>> for i in range(0, 7):
...   try:
...     if i % 3 == 0:
...       raise Exception
...   except Exception:
...     pass                  # silently ignore error
...   print i
>>> for i in range(0, 7):
...   try:
...     if i % 3 == 0:
...       raise Exception
...   except Exception:
...     break                 # exit loop
...   print i
>>> for i in range(0, 7):
...   try:
...     if i % 3 == 0:
...       raise Exception
...   except Exception:
...     continue              # jump to 'for'
...   print i

Why do you get these results?

Network code will throw socket.error exceptions, and you can find out what the complaint is by doing this in the exception handling block:

  # some network code
exception Exception, e:
  print 'Error:', e

Copying code examples

You will cause problems for yourself with carelessness in protocols. Mail servers will hang up on you, in particular.

I run most of code that I hand out; if it's in a doctest format, then I do run it. So if there's a problem, it's probably not in my code.

Second, all of the lecture and lab notes, as well as the homeworks, are available online, under this URL:

They are all available in text format, so you should seriously consider copying and pasting the code from those pages first, to make sure that works. If you copy and paste code and it doesn't work, then the odds are slightly higher that I'm at fault ;).

Print statements

When you want to debug network code, put in print statements! For example, with code like this:

while 1:
  data = socket.recv(...)
  if data == '\n.\n':

you may run into difficulties.

You'll say, "it's never breaking out of the loop." I'll say, "that must be because your if statement is never true." You'll say, "but I'm sure it is! I'm sending \n.\n from the other end!"

Well, prove it!

You need to put in print statements to make sure you know what is going on. In this case, I'd rewrite the code to this:

print '** entering while loop'
while 1:
  print '** asking for data'
  data = socket.recv(...)
  print 'received data:', (data,)

  if data == '\n.\n':
    print 'TRUE! breaking.'

Note that use of (data,)... what does this do? (See 'repr' discussion, lab #2).

Synchronous I/O

If you get a synchronization error from the server, this may be because you either sent wrong data to the server and didn't eyeball what the server returned (an error message...) before sending more data, OR they messed up the conversation (see "Copying code examples", above). Remember, SMTP and HTTP are synchronous protocols so you need to send and receive the right messages in the right order:

server: Hello!
client: Hi!

server: Welcome!
client: Here's my from address.

server: Excellent!  We take that here.
client: Good -- here's an address I'm sending *to*.

server: OK.  Looks good to me.
client: Here's the data, then.

etc. If you omit any step in this conversation the server will justifiably be upset at you for being rude.

Data breakup etc.

I sort of glossed over this in description of TCP, but you need to be aware that clients and servers are under no particular requirement to send their data in the right chunks. Thus, one problem that people will have is in this comparison:

data = sock.recv(4096)
if data == '\n.\n':

If you use telnet to test this code, then you'll find two things: first, telnet uses 'rn' instead of 'n'; and second, telnet sends each line individually, so data would first be


and then


second. Thus the above 'if' statement would never be true. Did you receive a newline followed by a period followed by a newline? Sure! You just didn't keep track of it properly!

Your code needs take this into account.

One way to think about this for planning purposes is that your code should work just as well (if a bit slower) if you change the size of the receive buffer from '4096' to '1'. This makes this statement obviously ridiculous

data = sock.recv(1)
if data == '\n.\n':

since 'data' will at most be one character, this if never be true! Instead you could try:

data_so_far = ''
while 1:
   data = sock.recv(1)
   data_so_far += data
   if '\n.\n' in data_so_far:

HW tips/thoughts, summary

All of these protocols are finicky. Take a known working example and start with that. (Hint: my SMTP example above works.)

You may have trouble seeing your e-mail because of spam filters. And send it to, please, not t.

Put print statements in your network code, especially around commands that might block, or throw exceptions (i.e. network commands).

Blanket exceptions are dangerous -- they catch many kinds of errors -- so be sure to put print statements in them.

Use a list to keep track of open sockets in your HW, problem 3.