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* further pushing for the in-class socket lib walkthrough
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‎source/presentations/week01.rst‎

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@@ -344,3 +344,227 @@ You can also find out about machines that are located elsewhere, for example::
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['www.rad.washington.edu'], # <- any aliases
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['128.95.247.84']) # <- all active IP addresses
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Sockets in Python
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-----------------
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To create a socket, you use the **socket** method of the ``socket`` library::
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>>> foo = socket.socket()
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>>> foo
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<socket._socketobject object at 0x10046cec0>
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Sockets in Python
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-----------------
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A socket has some properties that are immediately important to us. These
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include the *family*, *type* and *protocol* of the socket::
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>>> foo.family
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2
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>>> foo.type
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1
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>>> foo.proto
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0
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Socket Families
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---------------
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Think back a moment to our discussion of the *Internet* layer of the TCP/IP
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stack. There were a couple of different types of IP addresses:
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.. class:: incremental
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* IPv4 ('192.168.1.100')
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* IPv6 ('2001:0db8:85a3:0042:0000:8a2e:0370:7334')
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.. class:: incremental
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The *family* of a socket corresponds to the type of address you use to make a
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connection to it.
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A quick utility method
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----------------------
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Let's explore these families for a moment. To do so, we're going to define
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a method we can use to read contstants from the ``socket`` library. It will
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take a single argument, the shared prefix for a defined set of constants::
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>>> def get_constants(prefix):
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... """mapping of socket module constants to their names."""
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... return dict( (getattr(socket, n), n)
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... for n in dir(socket)
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... if n.startswith(prefix)
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... )
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...
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>>>
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Socket Families
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---------------
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Families defined in the ``socket`` library are prefixed by ``AF_``::
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>>> families = get_constants('AF_')
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>>> families
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{0: 'AF_UNSPEC', 1: 'AF_UNIX', 2: 'AF_INET',
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11: 'AF_SNA', 12: 'AF_DECnet', 16: 'AF_APPLETALK',
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17: 'AF_ROUTE', 23: 'AF_IPX', 30: 'AF_INET6'}
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.. class:: small incremental
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*Your results may vary*
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.. class:: incremental
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Of all of these, the ones we care most about are ``2`` (IPv4) and ``30`` (IPv6).
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Unix Domain Sockets
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-------------------
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When you are on a machine with an operating system that is Unix-like, you will
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find another generally useful socket family: ``AF_UNIX``, or Unix Domain
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Sockets. Sockets in this family:
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.. class:: incremental
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* connect processes **on the same machine**
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* are generally a bit slower than IPC connnections
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* have the benefit of allowing the same API for programs that might run on one
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machine __or__ across the network
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* use an 'address' that looks like a pathname ('/tmp/foo.sock')
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Socket Families
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---------------
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What is the *default* family for the socket we created just a moment ago?
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.. class:: incremental
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(remember we bound the socket to the symbol ``foo``)
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Socket Types
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------------
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The socket type determines how the socket handles connections. Socket type
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constants defined in the ``socket`` library are prefixed by ``SOCK_``::
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>>> types = get_constants('SOCK_')
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>>> types
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{1: 'SOCK_STREAM', 2: 'SOCK_DGRAM',
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...}
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.. class:: incremental
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In general, the only two of these that are widely useful are ``1``
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(representing TCP type connections) and ``2`` (representing UDP type
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connections).
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Socket Types
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------------
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What is the *default* type for our generic socket, ``foo``?
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Socket Protocols
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----------------
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A socket also has a designated *protocol*. The constants for these are
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prefixed by ``IPPROTO``::
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>>> protocols = get_constants('IPPROTO_')
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>>> protocols
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{0: 'IPPROTO_IP', 1: 'IPPROTO_ICMP',
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...,
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255: 'IPPROTO_RAW'}
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.. class:: incremental
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The choice of which protocol to use for a socket is determined by the type of
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activity the socket is intended to support. What messages are you needing to
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send?
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Socket Protocols
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----------------
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What is the *default* protocol used by our generic socket, ``foo``?
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Address Information
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-------------------
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When creating a socket, you can provide ``family``, ``type`` and ``protocol``
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as arguments to the constructor::
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>>> bar = socket.socket(socket.AF_INET,
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... socket.SOCK_STREAM,
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... socket.IPPROTO_IP)
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...
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>>> bar
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<socket._socketobject object at 0x1005b8b40>
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Address Information
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-------------------
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But how do you find out the *right* values?
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.. class:: incremental
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You ask.
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A quick utility method
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----------------------
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Create the following function::
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>>> def get_address_info(host, port):
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... for response in socket.getaddrinfo(host, port):
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... fam, typ, pro, nam, add = response
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... print 'family: ', families[fam]
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... print 'type: ', types[typ]
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... print 'protocol: ', protocols[pro]
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... print 'canonical name: ', nam
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... print 'socket address: ', add
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... print
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...
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>>>
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On Your Own Machine
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-------------------
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Now, ask your own machine what services are available on 'http'::
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>>> get_address_info(socket.gethostname(), 'http')
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family: AF_INET
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type: SOCK_DGRAM
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protocol: IPPROTO_UDP
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canonical name:
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socket address: ('10.211.55.2', 80)
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family: AF_INET
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...
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>>>
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.. class:: incremental
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What answers do you get?
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On the Internet
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---------------
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>>> get_address_info('www.google.com', 'http')
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>>> get_address_info('www.google.com', 'http')
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family: AF_INET
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type: SOCK_STREAM
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protocol: IPPROTO_TCP
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canonical name:
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socket address: ('74.125.129.105', 80)
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family: AF_INET
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...
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>>>
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.. class:: incremental
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Try a few other servers you know about.
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