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HTML
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Transmitted frame
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Distributing the load across the network
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An `unreliable connectionless` service may suffer from various types of problems compared to a `reliable connectionless service`. First of all, an `unreliable connectionless service` does not guarantee the delivery of all SDUs. This can be expressed graphically by using the time-sequence diagram below.
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../../bibliography.rst:330
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Sklower, K. 1989. `Improving the efficiency of the OSI checksum calculation <http://doi.acm.org/10.1145/74681.74684>`_. SIGCOMM Computer Communication Review 19, 5 (Oct. 1989), 32-43.
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The HyperText Markup Language specifies the structure and the syntax of the documents that are exchanged on the world wide web. HTML is maintained by the `HTML working group <https://www.w3.org/html/wg/>`_ of the :term:`W3C`
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../../exercises/network.rst:365
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1 via C
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../../principles/network.rst:249
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For example, consider again the network topology above. `A` sends a control packet towards `B`. The initial `record route` is empty. When `R1` receives the packet, it adds its own address to the `record route` and forwards a copy to `R2` and another to `R3`. `R2` receives the packet, adds itself to the `record route` and forwards it to `R3`. `R3` receives two copies of the packet. The first contains the `[R1,R2]` `record route` and the second `[R1]`. In the end, `B` will receive two control packets containing `[R1,R2,R3,R4]` and `[R1,R3,R4]` as `record routes`. `B` can keep these two paths or select the best one and discard the second. A popular heuristic is to select the `record route` of the first received packet as being the best one since this likely corresponds to the shortest delay path.
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../../principles/reliability.rst:222
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../../principles/transport.rst:165
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In practice, an `unreliable connectionless service` will usually deliver a large fraction of the SDUs. However, since the delivery of SDUs is not guaranteed, the user must be able to recover from the loss of any SDU.
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../../bibliography.rst:335
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Stevens, R., `UNIX Network Programming, Volume 1, Second Edition: Networking APIs: Sockets and XTI <http://books.google.com/books?id=ptSC4LpwGA0C>`_, Prentice Hall, 1998
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HTTP
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../../principles/network.rst:251
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With the received `record route`, `B` can send a `data packet` to `A`. For this, it simply reverses the chosen `record route`. However, we still need to communicate the chosen path to `A`. This can be done by putting the `record route` inside a control packet which is sent back to `A` over the reverse path. An alternative is to simply send a `data packet` back to `A`. This packet will travel back to `A`. To allow `A` to inspect the entire path followed by the `data packet`, its `source route` must contain all intermediate routers when it is received by `A`. This can be achieved by encoding the `source route` using a data structure that contains an index and the ordered list of node addresses. The index always points to the next address in the `source route`. It is initialized at `0` when a packet is created and incremented by each intermediate node.
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../../principles/sharing.rst:237
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In virtual circuit networks, another way to manage or prevent congestion is to limit the number of circuits that use the network at any time. This technique is usually called `connection admission control`. When a host requests the creation of a new circuit in the network, it specifies the destination and in some networking technologies the required bandwidth. With this information, the network can check whether there are enough resources available to reach this particular destination. If yes, the circuit is established. If not, the request is denied and the host will have to defer the creation of its virtual circuit. `Connection admission control` schemes are widely used in the telephone networks. In these networks, a busy tone corresponds to an unavailable destination or a congested network.
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../../principles/transport.rst:167
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A second imperfection that may affect an `unreliable connectionless service` is that it may duplicate SDUs. Some packets may be duplicated in a network and be delivered twice to their destination. This is illustrated by the time-sequence diagram below.
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../../bibliography.rst:338
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Shreedhar and G. Varghese. `Efficient fair queueing using deficit round robin <http://doi.acm.org/10.1145/217391.217453>`_ SIGCOMM Computer Communication Review 25, 4 (October 1995), 231-242.
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The HyperText Transport Protocol is defined in :rfc:`2616`
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