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../../principles/network.rst:116
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A second type of problem may exist in networks using the datagram organization. Consider a path that contains a cycle. For example, router `R1` sends all packets towards destination `D` via router `R2`. Router `R2` forwards these packets to router `R3` and finally router `R3`'s forwarding table uses router `R1` as its nexthop to reach destination `D`. In this case, if a packet destined to `D` is received by router `R1`, it will loop on the `R1 -> R2 -> R3 -> R1` cycle and will never reach its final destination. As in the black hole case, the destination is not reachable from all sources in the network. In practice the loop problem is more annoying than the black hole problem because when a packet is caught in a forwarding loop, it unnecessarily consumes bandwidth. In the black hole case, the problematic packet is quickly discarded. We will see later that network layer protocols include techniques to minimize the impact of such forwarding loops.
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../../principles/referencemodels.rst:120
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application layer entities exchange *SDUs*
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../../principles/transport.rst:65
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`@` : `1000000b`
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../../bibliography.rst:99
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Kerrisk, M., `The Linux Programming Interface <http://nostarch.com/tlpi>`_, No Starch Press, 2010
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../../exercises/ex-sharing.rst:609
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In this network, a 250 Kbps link is used between the routers. The propagation delays in the network are negligible. Host `A` sends 1000 bits long segments so that it takes one msec to transmit one segment on the `A-R1` link. Neglecting the transmission delays for the acknowledgments, what is the minimum round-trip time measured on host `A` with such segments ?
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../../exercises/network.rst:299
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Some hosts need to be multihomed, i.e. attached to two different network nodes as shown in the figure below.
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../../exercises/reliability.rst:65
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all errors that affect a sequence of bits which is shorter than the length of the CRC
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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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../../principles/naming.rst:139
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Footnotes
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../../principles/network.rst:118
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Any solution which is used to compute the forwarding tables of a network must ensure that all destinations are reachable from any source. This implies that it must guarantee the absence of black holes and forwarding loops.
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../../principles/referencemodels.rst:125
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Reference models
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../../principles/reliability.rst:106
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With the above transmission scheme, a bit is transmitted by setting the voltage on the electrical cable to a specific value during some period of time. We have seen that due to electromagnetic interference, the voltage measured by the receiver can differ from the voltage set by the transmitter. This is the main cause of transmission errors. However, this is not the only type of problem that can occur. Besides defining the voltages for bits `0` and `1`, the above transmission scheme also specifies the duration of each bit. If one million bits are sent every second, then each bit lasts 1 microsecond. On each host, the transmission (resp. the reception) of each bit is triggered by a local clock having a 1 MHz frequency. These clocks are the second source of problems when transmitting bits over a wire. Although the two clocks have the same specification, they run on different hosts, possibly at a different temperature and with a different source of energy. In practice, it is possible that the two clocks do not operate at exactly the same frequency. Assume that the clock of the transmitting host operates at exactly 1000000 Hz while the receiving clock operates at 999999 Hz. This is a very small difference between the two clocks. However, when using the clock to transmit bits, this difference is important. With its 1000000 Hz clock, the transmitting host will generate one million bits during a period of one second. During the same period, the receiving host will sense the wire 999999 times and thus will receive one bit less than the bits originally transmitted. This small difference in clock frequencies implies that bits can "disappear" during their transmission on an electrical cable. This is illustrated in the figure below.
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../../principles/sharing.rst:150
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the node's capacity measured in bits per second
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../../principles/transport.rst:66
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`space` : `0100000b`
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../../bibliography.rst:103
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Karn, P., Price, H., Diersing, R., `Packet radio in amateur service <https://dx.doi.org/10.1109/JSAC.1985.1146214>`_, IEEE Journal on Selected Areas in Communications, 3, May, 1985
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../../exercises/ex-sharing.rst:610
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If host `A` uses a window of two segments and needs to transmit five segments of data. How long does the entire transfer lasts ?
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../../exercises/network.rst:323
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Would this network work correctly with port-forwarding tables if :
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../../exercises/reliability.rst:67
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Implement a small software that computes the CRC-32 for a text file. Then, modify the contents of the file to change an even number of bits or an odd number of bits inside the file. When modifying the file, remember that an ASCII file is composed of 8 bits characters that are encoded by using the ASCII table that you can find at : http://en.wikipedia.org/wiki/ASCII . You can also write a small program that produces binary files that are a small variation of each other.
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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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../../principles/naming.rst:140
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The :term:`hosts.txt` file is not maintained anymore. A historical snapshot from April 1984 is available from http://ftp.univie.ac.at/netinfo/netinfo/hosts.txt
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