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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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../../glossary.rst:188
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The HyperText Transport Protocol is defined in :rfc:`2616`
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../../exercises/network.rst:366
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D
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../../principles/network.rst:255
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Flat or hierarchical addresses
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../../principles/reliability.rst:223
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0110111111111111111110010
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../../principles/sharing.rst:239
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In datagram networks, this technique cannot be easily used since the basic assumption of such a network is that a host can send any packet towards any destination at any time. A host does not need to request the authorization of the network to send packets towards a particular destination.
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../../principles/transport.rst:185
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Finally, some unreliable connectionless service providers may deliver to a destination a different SDU than the one that was supplied in the `Data.request`. This is illustrated in the figure below.
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../../bibliography.rst:350
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Williams, R. `A painless guide to CRC error detection algorithms`, August 1993, unpublished manuscript, https://web.archive.org/web/20060101004751/http://www.ross.net/crc/download/crc_v3.txt
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../../glossary.rst:180
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hub
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../../exercises/network.rst:366
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3 via B
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../../principles/network.rst:257
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The last, but important, point to discuss about the `data plane` of the networks that rely on the datagram mode is their addressing scheme. In the examples above, we have used letters to represent the addresses of the hosts and network nodes. In practice, all addresses are encoded as a bit string. Most network technologies use a fixed size bit string to represent source and destination address. These addresses can be organized in two different ways.
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../../principles/reliability.rst:223
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01111110011011111011111011111011001001111110
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../../principles/sharing.rst:241
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Based on the feedback received from the network, the hosts can adjust their transmission rate. We discuss in section `Congestion control` some techniques that allow hosts to react to congestion.
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../../principles/transport.rst:201
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As the transport layer is built on top of the network layer, it is important to know the key features of the network layer service. In this book, we only consider the `connectionless network layer service` which is the most widespread. Its main characteristics are :
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../../bibliography.rst:353
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ITU-T, recommendation X.200, `Open Systems Interconnection - Model and Notation <http://www.itu.int/rec/T-REC-X.200-199407-I/en>`_, 1994
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A relay operating in the physical layer.
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1 via D
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../../principles/network.rst:259
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The first organization, which is the one that we have implicitly assumed until now, is the `flat addressing` scheme. Under this scheme, each host and network node has a unique address. The unicity of the addresses is important for the operation of the network. If two hosts have the same address, it can become difficult for the network to forward packets towards this destination. `Flat addresses` are typically used in situations where network nodes and hosts need to be able to communicate immediately with unique addresses. These `flat addresses` are often embedded inside the network interface cards. The network card manufacturer creates one unique address for each interface and this address is stored in the read-only memory of the interface. An advantage of this addressing scheme is that it easily supports unstructured and mobile networks. When a host moves, it can attach to another network and remain confident that its address is unique and enables it to communicate inside the new network.
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01111110
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