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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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../../exercises/sockets.rst:326
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Endianness: exchanging integers between different computers
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../../exercises/sockets.rst:328
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Besides character strings, some applications also need to exchange 16 bits and 32 bits fields such as integers. A naive solution would have been to send the 16- or 32-bits field as it is encoded in the host's memory. Unfortunately, there are different methods to store 16- or 32-bits fields in memory. Some CPUs store the most significant byte of a 16-bits field in the first address of the field while others store the least significant byte at this location. When networked applications running on different CPUs exchange 16 bits fields, there are two possibilities to transfer them over the transport service :
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../../exercises/sockets.rst:330
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send the most significant byte followed by the least significant byte
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../../exercises/sockets.rst:331
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send the least significant byte followed by the most significant byte
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../../exercises/sockets.rst:335
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Besides 16 and 32 bit words, some applications need to exchange data structures containing bit fields of various lengths. For example, a message may be composed of a 16 bits field followed by eight, one bit flags, a 24 bits field and two 8 bits bytes. Internet protocol specifications will define such a message by using a representation such as the one below. In this representation, each line corresponds to 32 bits and the vertical lines are used to delineate fields. The numbers above the lines indicate the bit positions in the 32-bits word, with the high order bit at position `0`.
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../../exercises/sockets.rst:341
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Message format
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../../exercises/sockets.rst:343
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The message mentioned above will be transmitted starting from the upper 32-bits word in network byte order. The first field is encoded in 16 bits. It is followed by eight one bit flags (`A-H`), a 24 bits field whose high order byte is shown in the first line and the two low order bytes appear in the second line followed by two one byte fields. This ASCII representation is frequently used when defining binary protocols. We will use it for all the binary protocols that are discussed in this book.
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../../exercises/sockets.rst:346
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Exercises
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../../exercises/sockets.rst:348
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Here are some exercises that will help you to learn how to use sockets.
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../../exercises/sockets.rst:364
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Footnotes
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