Exit Zen
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../../principles/network.rst:506
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We will see later a more detailed description of Multiprotocol Label Switching, a networking technology that is capable of using one or more labels.
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../../principles/reliability.rst:407
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`Information theory` defines `coding schemes`. There are different types of coding schemes, but let us focus on coding schemes that operate on binary strings. A coding scheme is a function that maps information encoded as a string of `m` bits into a string of `n` bits. The simplest coding scheme is the (even) parity coding. This coding scheme takes an `m` bits source string and produces an `m+1` bits coded string where the first `m` bits of the coded string are the bits of the source string and the last bit of the coded string is chosen such that the coded string will always contain an even number of bits set to `1`. For example :
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../../principles/sharing.rst:534
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Operation of a CSMA/CA device
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../../principles/transport.rst:652
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the `transport clock` is implemented as a `k` bits counter and its clock cycle is such that :math:`2^k \times cycle >> MSL`. Furthermore, the `transport clock` counter is incremented every clock cycle and after each connection establishment. This clock is illustrated in the figure below.
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../../glossary.rst:123
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MSS
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../../principles/network.rst:511
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The control plane
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../../principles/reliability.rst:409
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`1001` is encoded as `10010`
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../../principles/sharing.rst:540
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The main difficulty with CSMA/CA is when two or more devices transmit at the same time and cause collisions. This is illustrated in the figure below, assuming a fixed timeout after the transmission of a data frame. With CSMA/CA, the timeout after the transmission of a data frame is very small, since it corresponds to the SIFS plus the time required to transmit the acknowledgment frame.
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../../principles/transport.rst:653
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the `transport clock` must continue to be incremented even if the transport entity stops or reboots
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../../glossary.rst:125
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A TCP option used by a TCP entity in SYN segments to indicate the Maximum Segment Size that it is able to receive.
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../../principles/network.rst:513
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One of the objectives of the `control plane` in the network layer is to maintain the routing tables that are used on all routers. As indicated earlier, a routing table is a data structure that contains, for each destination address (or block of addresses) known by the router, the outgoing interface over which the router must forward a packet destined to this address. The routing table may also contain additional information such as the address of the next router on the path towards the destination or an estimation of the cost of this path.
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../../principles/reliability.rst:410
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`1101` is encoded as `11011`
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../../principles/sharing.rst:546
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Collisions with CSMA/CA
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../../principles/transport.rst:659
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Transport clock
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../../glossary.rst:21
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multicast
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../../principles/network.rst:515
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In this section, we discuss the main techniques that can be used to maintain the forwarding tables in a network.
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../../principles/reliability.rst:412
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This parity scheme has been used in some RAMs as well as to encode characters sent over a serial line. It is easy to show that this coding scheme allows the receiver to detect a single transmission error, but it cannot correct it. However, if two or more bits are in error, the receiver may not always be able to detect the error.
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../../principles/sharing.rst:552
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To deal with this problem, CSMA/CA relies on a backoff timer. This backoff timer is a random delay that is chosen by each device in a range that depends on the number of retransmissions for the current frame. The range grows exponentially with the retransmissions as in CSMA/CD. The minimum range for the backoff timer is :math:`[0,7*slotTime]` where the `slotTime` is a parameter that depends on the underlying physical layer. Compared to CSMA/CD's exponential backoff, there are two important differences to notice. First, the initial range for the backoff timer is seven times larger. This is because it is impossible in CSMA/CA to detect collisions as they happen. With CSMA/CA, a collision may affect the entire frame while with CSMA/CD it can only affect the beginning of the frame. Second, a CSMA/CA device must regularly sense the transmission channel during its back off timer. If the channel becomes busy (i.e. because another device is transmitting), then the back off timer must be frozen until the channel becomes free again. Once the channel becomes free, the back off timer is restarted. This is in contrast with CSMA/CD where the back off is recomputed after each collision. This is illustrated in the figure below. Host `A` chooses a smaller backoff than host `C`. When `C` senses the channel to be busy, it freezes its backoff timer and only restarts it once the channel is free again.
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../../principles/transport.rst:662
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It should be noted that `transport clocks` do not need and usually are not synchronized to the real-time clock. Precisely synchronizing real-time clocks is an interesting problem, but it is outside the scope of this document. See [Mills2006]_ for a detailed discussion on synchronizing the real-time clock.
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../../glossary.rst:23
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a transmission mode where an information is sent efficiently to `all` the receivers that belong to a given group
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