Exit Zen
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../../principles/network.rst:412
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->R2
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104
../../principles/sharing.rst:474
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The second host retransmits immediately and the first defers its retransmission
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../../principles/transport.rst:563
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The figure below shows a typical usage of port numbers. The client process uses port number `1234` while the server process uses port number `5678`. When the client sends a request, it is identified as originating from port number `1234` on the client host and destined to port number `5678` on the server host. When the server process replies to this request, the server's transport layer returns the reply as originating from port `5678` on the server host and destined to port `1234` on the client host.
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105
../../glossary.rst:288
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A telephone line that is permanently available between two endpoints.
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105
../../principles/network.rst:414 • ../../principles/network.rst:424
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->R5
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105
../../principles/reliability.rst:389
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Reliable data transfer on top of an imperfect link
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105
../../principles/sharing.rst:475
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Both hosts defer their retransmission and a new collision occurs
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106
../../glossary.rst:30
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MAN
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106
../../principles/network.rst:417
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Since a packet received with `label=1` must be forwarded to `R5` with `label=1`, `R2`'s `label forwarding table` could contain :
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106
../../principles/reliability.rst:391
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The datalink layer must deal with the transmission errors. In practice, we mainly have to deal with two types of errors in the datalink layer :
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../../principles/sharing.rst:477
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In the second and third cases, both hosts have flipped different coins. The delay chosen by the host that defers its retransmission should be long enough to ensure that its retransmission will not collide with the immediate retransmission of the other host. However the delay should not be longer than the time necessary to avoid the collision, because if both hosts decide to defer their transmission, the network will be idle during this delay. The `slot time` is the optimal delay since it is the shortest delay that ensures that the first host will be able to retransmit its frame completely without any collision.
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../../principles/transport.rst:571
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To support the connection-oriented service, the transport layer needs to include several mechanisms to enrich the connectionless network-layer service. We discuss these mechanisms in the following sections.
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../../glossary.rst:32
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Metropolitan Area Network
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107
../../principles/network.rst:427
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Two virtual circuits pass through `R3`. They both need to be forwarded to `R4`, but `R4` expects `label=1` for packets belonging to the virtual circuit originated by `R2` and `label=0` for packets belonging to the other virtual circuit. `R3` could choose to leave the labels unchanged.
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../../principles/reliability.rst:393
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Frames can be corrupted by transmission errors
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../../principles/sharing.rst:479
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If two hosts are competing, the algorithm above will avoid a second collision 50% of the time. However, if the network is heavily loaded, several hosts may be competing at the same time. In this case, the hosts should be able to automatically adapt their retransmission delay. The `binary exponential back-off` performs this adaptation based on the number of collisions that have affected a frame. After the first collision, the host flips a coin and waits 0 or 1 `slot time`. After the second collision, it generates a random number and waits 0, 1, 2 or 3 `slot times`, etc. The duration of the waiting time is doubled after each collision. The complete pseudo-code for the CSMA/CD algorithm is shown in the figure below.
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../../principles/transport.rst:575
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Connection establishment
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108
../../glossary.rst:253
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MIME
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../../principles/network.rst:432 • ../../principles/network.rst:434
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->R4
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108
../../principles/reliability.rst:394
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Frames can be lost or unexpected frames can appear
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