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../../principles/sharing.rst:420
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Now that we have looked at how a frame is actually transmitted as an electrical signal on a shared bus, it is interesting to look in more detail at what happens when two hosts transmit a frame at almost the same time. This is illustrated in the figure below, where hosts A and B start their transmission at the same time (point `(1)`). At this time, if host C senses the channel, it will consider it to be free. This will not last a long time and at point `(2)` the electrical signals from both host A and host B reach host C. The combined electrical signal (shown graphically as the superposition of the two curves in the figure) cannot be decoded by host C. Host C detects a collision, as it receives a signal that it cannot decode. Since host C cannot decode the frames, it cannot determine which hosts are sending the colliding frames. Note that host A (and host B) will detect the collision after host C (point `(3)` in the figure below).
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../../principles/security.rst:958
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Alice and Bob have agreed on the secret information :math:`3` without having sent it explicitly through the network. If the integers used are large enough and have good properties, then even Eve who can capture all the messages sent by Alice and Bob cannot recover the secret key that they have exchanged. There is no formal proof of the security of the algorithm, but mathematicians have tried to solve similar problems with integers during centuries without finding an efficient algorithm. As long as the integers that are used are random and large enough, the only possible attack for Eve is to test all possible integers that could have been chosen by Alice and Bob. This is computationally very expensive. This algorithm is widely used in security protocols to agree on a secret key.
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../../exercises/ipv6.rst:851
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The problem can be detected by using :manpage:`traceroute6(8)`.
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../../exercises/routing-protocols.rst:1351
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What is the path that packets follow from a host attached to ``AS8`` to a host attached to ``AS2`` ?
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../../protocols/bgp.rst:365
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Another important utilization of the `local-pref` attribute is to support the `customer->provider` and `shared-cost` peering relationships. From an economic point of view, there is an important difference between these three types of peering relationships. A domain usually earns money when it sends packets over a `provider->customer` relationship. On the other hand, it must pay its provider when it sends packets over a `customer->provider` relationship. Using a `shared-cost` peering to send packets is usually neutral from an economic perspective. To take into account these economic issues, domains usually configure the import filters on their routers as follows :
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../../protocols/email.rst:242
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../../protocols/ethernet.rst:362
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The `Spanning Tree Protocol` uses its own terminology that we illustrate in the figure above. A switch port can be in three different states : `Root`, `Designated` and `Blocked`. All the ports of the `root` switch are in the `Designated` state. The state of the ports on the other switches is determined based on the `BPDU` received on each port.
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../../protocols/ipv6.rst:345
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``TCP`` uses `Next Header` number ``6``
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../../protocols/ethernet.rst:337
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the number of the switch port over which the `BPDU` was sent (`p`)
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../../protocols/tcp.rst:267
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`snd.wnd` : the current size of the sending window (in bytes)
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../../protocols/tls.rst:516
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See http://www.iana.org/assignments/tls-parameters/tls-parameters.xhtml#tls-parameters-4
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../../glossary.rst:38
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The International Standardization Organization is an agency of the United Nations that is based in Geneva and develop standards on various topics. Within ISO, country representatives vote to approve or reject standards. Most of the work on the development of ISO standards is done in expert working groups. Additional information about ISO may be obtained from https://www.iso.org
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../../principles/network.rst:371 • ../../principles/network.rst:412 • ../../principles/network.rst:414 • ../../principles/network.rst:422 • ../../principles/network.rst:432 • ../../principles/network.rst:442
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../../principles/sharing.rst:427
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Frame collision on a shared bus
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../../principles/security.rst:970
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Unfortunately, the Diffie-Hellman key exchange alone cannot cope with man-in-the middle attacks. Consider Mallory who sits in the middle between Alice and Bob and can easily capture and modify their messages. The modulus and the base are public. They are thus known by Mallory as well. He could then operate as follows :
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../../exercises/ipv6.rst:861
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In the output of :manpage:`traceroute6(8)`, a ``!N`` indicates that host ``a`` received from ``2001:db8:1341:12::2``, i.e. router ``r2``, a Network unreachable ICMPv6 message. The forwarding table of ``r2`` confirms the root cause of this problem.
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../../exercises/routing-protocols.rst:1364
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What is the path that packets follow from a host attached to ``AS2`` to a host attached to ``AS7`` ?
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../../protocols/bgp.rst:367
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insert a high `local-pref` attribute in the routes learned from a customer
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../../protocols/email.rst:242
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n
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../../protocols/ethernet.rst:364
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The `Spanning Tree Protocol` uses the ordering relationship to build the spanning tree. Each switch listens to `BPDUs` on its ports. When `BPDU = <R,c,T,p>` is received on port `q`, the switch computes the port's `root priority vector`: `V[q] = <R,c+cost[q],T,p,q>` , where `cost[q]` is the cost associated to the port over which the `BPDU` was received. The switch stores in a table the last `root priority vector` received on each port. The switch then compares its own `identifier` with the smallest `root identifier` stored in this table. If its own `identifier` is smaller, then the switch is the root of the spanning tree and is, by definition, at a distance `0` of the root. The `BPDU` of the switch is then `<R,0,R,p>`, where `R` is the switch `identifier` and `p` will be set to the port number over which the `BPDU` is sent.
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