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File “locale/pot/exercises/sockets.pot” was added. 4 years ago
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If you want to observe the packets exchanged over the network, use a packet dissector such as `wireshark`_ or `tcpdump`_, listen the loopback interface (``lo``) and filter UDP packets using port 10000 (``udp.port==10000`` in `wireshark`_, ``udp port 10000`` with `tcpdump`_).
If you want to observe the packets exchanged over the network, use a packet dissector such as `wireshark`_ or `tcpdump`_, listen the loopback interface (``lo``) and filter UDP packets using port 10000 (``udp.port==10000`` in `wireshark`_, ``udp port 10000`` with `tcpdump`_).
4 years ago
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As an example, here is what you could have to invoke your programs.
As an example, here is what you could have to invoke your programs.
4 years ago
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to wrap the ``recv_and_handle_message`` server function in a ``server`` executable, similarly to what you have done with the ``client`` executable.
to wrap the ``recv_and_handle_message`` server function in a ``server`` executable, similarly to what you have done with the ``client`` executable.
4 years ago
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to wrap the ``create_and_send_message`` in a ``client`` executable that can parse user arguments (the ``getopt(3)`` function might help) and appropriately call the wrapped function;
to wrap the ``create_and_send_message`` in a ``client`` executable that can parse user arguments (the ``getopt(3)`` function might help) and appropriately call the wrapped function;
4 years ago
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During this course, you will be asked to implement a transport protocol running on Linux devices. To prepare yourself, try to implement the protocol described in the above tasks on your Linux personal machine. If you did these exercises correctly, most of your answers can be used as it (do not forget to include the required header files). In addition to the previously produced code, you will need
During this course, you will be asked to implement a transport protocol running on Linux devices. To prepare yourself, try to implement the protocol described in the above tasks on your Linux personal machine. If you did these exercises correctly, most of your answers can be used as it (do not forget to include the required header files). In addition to the previously produced code, you will need
4 years ago
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The first possibility was named `big-endian` in a note written by Cohen [Cohen1980]_ while the second was named `little-endian`. Vendors of CPUs that used `big-endian` in memory insisted on using `big-endian` encoding in networked applications while vendors of CPUs that used `little-endian` recommended the opposite. Several studies were written on the relative merits of each type of encoding, but the discussion became almost a religious issue [Cohen1980]_. Eventually, the Internet chose the `big-endian` encoding, i.e. multi-byte fields are always transmitted by sending the most significant byte first, :rfc:`791` refers to this encoding as the :term:`network-byte order`. Most libraries [#fhtonl]_ used to write networked applications contain functions to convert multi-byte fields from memory to the network byte order and the reverse.
The first possibility was named `big-endian` in a note written by Cohen [Cohen1980]_ while the second was named `little-endian`. Vendors of CPUs that used `big-endian` in memory insisted on using `big-endian` encoding in networked applications while vendors of CPUs that used `little-endian` recommended the opposite. Several studies were written on the relative merits of each type of encoding, but the discussion became almost a religious issue [Cohen1980]_. Eventually, the Internet chose the `big-endian` encoding, i.e. multi-byte fields are always transmitted by sending the most significant byte first, :rfc:`791` refers to this encoding as the :term:`network-byte order`. Most libraries [#fhtonl]_ used to write networked applications contain functions to convert multi-byte fields from memory to the network byte order and the reverse.
4 years ago
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Operating systems enable linking a socket to a remote address so that every information sent through the socket will only be sent to this remote address, and the socket will only receive messages sent by this remote address. This can be done using the ``connect`` system call shown below.
Operating systems enable linking a socket to a remote address so that every information sent through the socket will only be sent to this remote address, and the socket will only receive messages sent by this remote address. This can be done using the ``connect`` system call shown below.
4 years ago
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While the provided examples show the usage of a `char` array as the data buffer, implementers should **never** assume that it contains a string. C programs rely on the `char` type to refer to a 8-bit long value, and arbitrary binary values can be exchanged over the network (i.e., the ``\0`` value does not delimit the end of the data).
While the provided examples show the usage of a `char` array as the data buffer, implementers should **never** assume that it contains a string. C programs rely on the `char` type to refer to a 8-bit long value, and arbitrary binary values can be exchanged over the network (i.e., the ``\0`` value does not delimit the end of the data).
4 years ago
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Operating systems allow assigning an address to a socket using the ``bind`` system call. This is useful when you want to receive messages from another program to which you announced your socket address. Once the address is assigned to the socket, the program can receive data from others using system calls such as ``recv`` and ``read``. Note that we can use the ``read`` system call as the operating system provides a socket as a file descriptor.
Operating systems allow assigning an address to a socket using the ``bind`` system call. This is useful when you want to receive messages from another program to which you announced your socket address. Once the address is assigned to the socket, the program can receive data from others using system calls such as ``recv`` and ``read``. Note that we can use the ``read`` system call as the operating system provides a socket as a file descriptor.
4 years ago
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In order to reach a peer, a process must know its :term:`address`. An address is a value that identifies a peer in a given network. There exists many different kinds of address families. For example, some of them allow reaching a peer using the file system on the computer. Some others enable communicating with a remote peer through a network. The socket API provides generic functions: the peer address is taken as a ``struct sockaddr *``, which can point to any family of address. This is partly why sockets are a powerful abstraction.
In order to reach a peer, a process must know its :term:`address`. An address is a value that identifies a peer in a given network. There exists many different kinds of address families. For example, some of them allow reaching a peer using the file system on the computer. Some others enable communicating with a remote peer through a network. The socket API provides generic functions: the peer address is taken as a ``struct sockaddr *``, which can point to any family of address. This is partly why sockets are a powerful abstraction.
4 years ago
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Popular operating systems allow isolating different programs by executing them in separate `processes`. A :term:`socket` is a tool provided by the operating system that enables two separated processes to communicate with each other. A socket takes the form of a file descriptor and can be seen as a communication pipe through which the communicating processes can exchange arbitrary information. In order to receive a message, a process must be attached to a specific :term:`address` that the peer can use to reach it.
Popular operating systems allow isolating different programs by executing them in separate `processes`. A :term:`socket` is a tool provided by the operating system that enables two separated processes to communicate with each other. A socket takes the form of a file descriptor and can be seen as a communication pipe through which the communicating processes can exchange arbitrary information. In order to receive a message, a process must be attached to a specific :term:`address` that the peer can use to reach it.
4 years ago
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File “locale/pot/exercises/reliability.pot” was added. 4 years ago
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A go-back-n sender has sent :math:`2^n` data frames. All the frames have been received correctly and in-order by the receiver, but all the returned acknowledgments have been lost. Show by using a time sequence diagram (e.g. by considering a window of four frames) what happens in this case. Can you fix the problem on the go-back-n sender ?
A go-back-n sender has sent :math:`2^n` data frames. All the frames have been received correctly and in-order by the receiver, but all the returned acknowledgments have been lost. Show by using a time sequence diagram (e.g. by considering a window of four frames) what happens in this case. Can you fix the problem on the go-back-n sender ?
4 years ago
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Checksums and CRCs should not be confused with secure hash functions such as MD5 defined in :rfc:`1321` or SHA-1 described in :rfc:`4634`. Secure hash functions are used to ensure that files or sometimes frames/packets/segments have not been modified. Secure hash functions aim at detecting malicious changes while checksums and CRCs only detect random transmission errors. Use the `shasum <http://linux.die.net/man/1/shasum>`_ or `md5sum <http://linux.die.net/man/1/md5sum>`_ programs on Linux to perform the same tests as above.
Checksums and CRCs should not be confused with secure hash functions such as MD5 defined in :rfc:`1321` or SHA-1 described in :rfc:`4634`. Secure hash functions are used to ensure that files or sometimes frames/packets/segments have not been modified. Secure hash functions aim at detecting malicious changes while checksums and CRCs only detect random transmission errors. Use the `shasum <http://linux.die.net/man/1/shasum>`_ or `md5sum <http://linux.die.net/man/1/md5sum>`_ programs on Linux to perform the same tests as above.
4 years ago
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By using your knowledge of the Internet checksum, can you find a transmission error that will not be detected by these checksums?
By using your knowledge of the Internet checksum, can you find a transmission error that will not be detected by these checksums?
4 years ago
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the Fletcher checksum [Fletcher1982]_, see https://en.wikipedia.org/wiki/Fletcher%27s_checksum for implementation details.
the Fletcher checksum [Fletcher1982]_, see https://en.wikipedia.org/wiki/Fletcher%27s_checksum for implementation details.
4 years ago
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when the third and seventh frames are lost
when the third and seventh frames are lost
4 years ago
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Consider a go-back-n sender and a go-back receiver that are directly connected with a 10 Mbps link that has a propagation delay of 100 milliseconds. Assume that the retransmission timer is set to three seconds. If the window has a length of 4 frames, draw a time-sequence diagram showing the transmission consisting of 10 data frames (each frame contains 10000 bits):
Consider a go-back-n sender and a go-back receiver that are directly connected with a 10 Mbps link that has a propagation delay of 100 milliseconds. Assume that the retransmission timer is set to three seconds. If the window has a length of 4 frames, draw a time-sequence diagram showing the transmission consisting of 10 data frames (each frame contains 10000 bits):
4 years ago
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To speedup the transmission of the frames, a student proposes to compute the CRC over the data part of the frame but not over the header. What do you think of this proposed solution ?
To speedup the transmission of the frames, a student proposes to compute the CRC over the data part of the frame but not over the header. What do you think of this proposed solution ?
4 years ago

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