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../../principles/sharing.rst:197
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In virtual circuit networks, packet marking can be improved if the return packets follow the reverse path of the forward packets. It this case, a network node can detect congestion on the forward path (e.g. due to the size of its buffer), but mark the packets on the return path. Marking the return packets (e.g. the acknowledgments used by reliable protocols) provides a faster feedback to the sending hosts compared to FECN. This technique is usually called `Backward Explicit Congestion Notification (BECN)`.
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../../bibliography.rst:317
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Sechrest, S., `An Introductory 4.4BSD Interprocess Communication Tutorial <http://docs.freebsd.org/44doc/psd/20.ipctut/paper.pdf>`_, 4.4BSD Programmer's Supplementary Documentation
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../../glossary.rst:198
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FTP
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../../exercises/network.rst:364
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B
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../../exercises/sockets.rst:364
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Footnotes
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../../principles/network.rst:216
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the `data packets`
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../../principles/reliability.rst:210
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Bit rate and bandwidth are often used to characterize the transmission capacity of the physical service. The original definition of `bandwidth <https://www.merriam-webster.com/dictionary/bandwidth>`_, as listed in the `Webster dictionary <https://www.merriam-webster.com/dictionary>`_ is `a range of radio frequencies which is occupied by a modulated carrier wave, which is assigned to a service, or over which a device can operate`. This definition corresponds to the characteristics of a given transmission medium or receiver. For example, the human ear is able to decode sounds in roughly the 0-20 KHz frequency range. By extension, bandwidth is also used to represent the capacity of a communication system in bits per second. For example, a Gigabit Ethernet link is theoretically capable of transporting one billion bits per second.
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../../principles/transport.rst:115
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The network layer ensures the delivery of packets on a hop-by-hop basis through intermediate nodes. As such, it provides a service to the upper layer. In practice, this layer is usually the `transport layer` that improves the service provided by the `network layer` to make it usable by applications.
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../../bibliography.rst:318
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Scheifler, R., Gettys, J., `X Window System: The Complete Reference to Xlib <http://h30097.www3.hp.com/docs/base_doc/DOCUMENTATION/V51B_ACRO_SUP/XWINSYS.PDF>`_, X Protocol, ICCCM, XLFD, X Version 11, Release 4, Digital Press
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../../glossary.rst:200
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The File Transfer Protocol is defined in :rfc:`959`
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../../exercises/network.rst:364
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1 via B
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../../exercises/sockets.rst:365
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For example, the ``htonl(3)`` (resp. ``ntohl(3)``) function the standard C library converts a 32-bits unsigned integer from the byte order used by the CPU to the network byte order (resp. from the network byte order to the CPU byte order). Similar functions exist in other programming languages.
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../../principles/network.rst:217
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the `control packets`
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../../principles/reliability.rst:215
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Given that multi-symbol encodings cannot be used by all physical layers, a generic solution which can be used with any physical layer that is able to transmit and receive only bits `0` and `1` is required. This generic solution is called `stuffing` and two variants exist : `bit stuffing` and `character stuffing`. To enable a receiver to easily delineate the frame boundaries, these two techniques reserve special bit strings as frame boundary markers and encode the frames so that these special bit strings do not appear inside the frames.
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../../principles/sharing.rst:203
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Dropping and marking packets is not the only possible reaction of a router that becomes congested. A router could also selectively delay packets belonging to some flows. There are different algorithms that can be used by a router to delay packets. If the objective of the router is to fairly distribute to bandwidth of an output link among competing flows, one possibility is to organize the buffers of the router as a set of queues. For simplicity, let us assume that the router is capable of supporting a fixed number of concurrent flows, say `N`. One of the queues of the router is associated to each flow and when a packet arrives, it is placed at the tail of the corresponding queue. All the queues are controlled by a `scheduler`. A `scheduler` is an algorithm that is run each time there is an opportunity to transmit a packet on the outgoing link. Various schedulers have been proposed in the scientific literature and some are used in real routers.
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../../principles/transport.rst:125
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Most networks use a datagram organization and provide a simple service which is called the `connectionless service`.
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../../bibliography.rst:319
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Stone, J., Greenwald, M., Partridge, C., and Hughes, J., `Performance of checksums and CRC's over real data <https://dx.doi.org/10.1109/90.731187>`_. IEEE/ACM Transactions Networking 6, 5 (Oct. 1998), 529-543.
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../../glossary.rst:313
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hosts.txt
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../../exercises/network.rst:364 • ../../exercises/network.rst:366
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2 via C
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../../principles/reliability.rst:217
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`Bit stuffing` reserves the `01111110` bit string as the frame boundary marker and ensures that there will never be six consecutive `1` symbols transmitted by the physical layer inside a frame. With bit stuffing, a frame is sent as follows. First, the sender transmits the marker, i.e. `01111110`. Then, it sends all the bits of the frame and inserts an additional bit set to `0` after each sequence of five consecutive `1` bits. This ensures that the sent frame never contains a sequence of six consecutive bits set to `1`. As a consequence, the marker pattern cannot appear inside the frame sent. The marker is also sent to mark the end of the frame. The receiver performs the opposite to decode a received frame. It first detects the beginning of the frame thanks to the `01111110` marker. Then, it processes the received bits and counts the number of consecutive bits set to `1`. If a `0` follows five consecutive bits set to `1`, this bit is removed since it was inserted by the sender. If a `1` follows five consecutive bits sets to `1`, it indicates a marker if it is followed by a bit set to `0`. The table below illustrates the application of bit stuffing to some frames.
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