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HTTP
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2 via D
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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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01111110000100100100100100100001101111110
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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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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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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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Flat or hierarchical addresses
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0110111111111111111110010
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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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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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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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hub
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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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01111110011011111011111011111011001001111110
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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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