Exit Zen
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../../exercises/sockets.rst:173
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Depending on the socket address family, the operating system might implicitly assign an address to an unbound socket upon a call to ``write``, ``send`` or ``sendto``. While this is a useful behavior, describing it precisely is out of the scope of this section.
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../../exercises/sockets.rst:175
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Using this code, the program will read and print an arbitrary message received from an arbitrary peer who knows the program's socket address. If we want to know the address of the peer that sent us the message, we can use the ``recvfrom`` system call. This is what a modified version of ``bind_and_receive_from_peer`` is doing below.
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../../exercises/sockets.rst:213
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This function is now using the ``recvfrom`` system call that will also provide the address of the peer who sent the message. As addresses are generic and can have different sizes, ``recvfrom`` also tells us the size of the address that it has written.
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../../exercises/sockets.rst:216
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``connect``: connecting a socket to a remote address
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../../exercises/sockets.rst:224
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This system call will assign the socket ``sockfd`` to the ``addr`` remote socket address. The process can then use the ``send`` and ``write`` system calls that do not to specify the destination socket address. Furthermore, the calls to ``recv`` and ``read`` will only deliver messages sent by this remote address. This is useful when we only care about the other peer messages.
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../../exercises/sockets.rst:227
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The following program connects a socket to a remote address, sends a message and waits for a reply.
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../../exercises/sockets.rst:259
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Creating a new socket to communicate through a network
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../../exercises/sockets.rst:261
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Until now, we learned how to use sockets that were already created. When writing a whole program, you will have to create you own sockets and choose the concrete technology that it will use to communicate with others. In this section, we will create new sockets and allow a program to communicate with processes located on another computer using a network. The most recent standardized technology used to communicate through a network is the :term:`IPv6` network protocol. In the IPv6 protocol, hosts are identified using *IPv6 addresses*. Modern operating systems allow IPv6 network communications between programs to be done using the socket API, just as we did in the previous sections.
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../../exercises/sockets.rst:264
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A program can use the ``socket`` system call to create a new socket.
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../../exercises/sockets.rst:270
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The ``domain`` parameter specifies the address family that we will use to concretely perform the communication. For an IPv6 socket, the ``domain`` parameter will be set to the value ``AF_INET6``, telling the operating system that we plan to communicate using IPv6 addresses. The ``type`` parameter specifies the communication guarantees that we need. For now, we will use the type ``SOCK_DGRAM`` which allows us to send *unreliable messages*. This means that each data that we send at each call of ``sendto`` will either be completely received or not received at all. The last parameter will be set to ``0``. The following line creates a socket, telling the operating system that we want to communicate using IPv6 addresses and that we want to send unreliable messages.
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../../exercises/sockets.rst:280
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Sending a message to a remote peer using its IPv6 address
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../../exercises/sockets.rst:284
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Now that we created an IPv6 socket, we can use it to reach another program if we know its IPv6 address. IPv6 addresses have a human-readable format that can be represented as a string of characters. The details of IPv6 addresses are out of scope of this section but here are some examples :
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../../exercises/sockets.rst:283
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The ``::1`` IPv6 address identifies the computer on which the current program is running.
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../../exercises/sockets.rst:284
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The ``2001:6a8:308f:9:0:82ff:fe68:e520`` IPv6 address identifies the computer serving the ``https://beta.computer-networking.info`` website.
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../../exercises/sockets.rst:288
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An IPv6 address often identifies a computer and not a program running on the computer. In order to identify a specific program running on a specific computer, we use a *port number* in addition to the IPv6 address. A program using an IPv6 socket is this identified using :
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../../exercises/sockets.rst:287
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The IPv6 address of the computer
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../../exercises/sockets.rst:288
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The port number identifying the program running on the computer
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../../exercises/sockets.rst:290
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A program can use the ``struct sockaddr_in6`` to represent IPv6 socket addresses. The following program creates a ``struct sockaddr_in6`` that identifies the program that reserved the port number ``55555`` on the computer identified by the ``::1`` IPv6 address.
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../../exercises/sockets.rst:301
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Now, we have built everything we need to send a message to the remote program. The ``create_socket_and_send_message`` function below assembles all the building blocks we created until now in order to send the message ``"hello"`` to the program running on port ``55555`` on the computer identified by the ``::1`` IPv6 address.
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../../exercises/sockets.rst:322
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Note that we can reuse our ``send_hello_to_peer`` function without any modification as we wrote it to handle any kind of sockets, including sockets using the IPv6 network protocol.
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