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../../principles/network.rst:13
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In the previous section, we have explained how reliable protocols allow hosts to exchange data reliably even if the underlying physical layer is imperfect and thus unreliable. Connecting two hosts together through a wire is the first step to build a network. However, this is not sufficient. Hosts usually need to interact with other hosts that are not directly connected through a direct physical layer link. This can be achieved by adding one layer above the datalink layer: the `network` layer.
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../../principles/reliability.rst:17
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The first step when building a network, even a worldwide network such as the Internet, is to connect two hosts together. This is illustrated in the figure below.
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../../protocols/ppp.rst:7
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The Point-to-Point Protocol
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../../protocols/routing.rst:14
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Organisation of a small Internet
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../../glossary.rst:183
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AIMD
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../../exercises/intro.rst:12
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A good understanding of the topics covered by this e-book can only be obtained by solving the proposed exercises. Reading the e-book from the first to the last page is not sufficient to get a detailed knowledge of computer networking.
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../../exercises/network.rst:16
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The network below uses port forwarding with flat addresses. The network boots and all hosts start one after the other. Explain at each step how the packets are forwarded and how the port forwarding tables of the network nodes are modified. Host `C` sends a packet to host `B`. Some time later, host `A` sends a packet to host `C`. Finally, host `B` sends a packet to host `A`.
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../../exercises/sockets.rst:98
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Networked applications were usually implemented by using the :term:`socket` :term:`API`. This API was designed when TCP/IP was first implemented in the `Unix BSD`_ operating system [Sechrest]_ [LFJLMT]_, and has served as the model for many APIs between applications and the networking stack in an operating system. Although the socket API is very popular, other APIs have also been developed. For example, the STREAMS API has been added to several Unix System V variants [Rago1993]_. The socket API is supported by most programming languages and several textbooks have been devoted to it. Users of the C language can consult [DC2009]_, [Stevens1998]_, [SFR2004]_ or [Kerrisk2010]_. The Java implementation of the socket API is described in [CD2008]_ and in the `Java tutorial <http://java.sun.com/docs/books/tutorial/networking/sockets/index.html>`_. In this section, we will use the C socket API to illustrate the key concepts.
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../../principles/referencemodels.rst:19
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The five layers reference model
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../../principles/network.rst:15
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The main objective of the network layer is to allow hosts, connected to different networks, to exchange information through intermediate systems called :term:`router`. The unit of information in the network layer is called a :term:`packet`.
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../../principles/reliability.rst:31
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To enable the two hosts to exchange information, they need to be linked together by some kind of physical media. Computer networks have used various types of physical media to exchange information, notably :
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../../principles/security.rst:46
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Alice and Bob
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../../exercises/reliability.rst:19
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How would you set the duration of the retransmission timer in the alternating bit protocol ?
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../../exercises/network.rst:42
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Same question as above, but the network is modified as shown in the figure below.
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../../exercises/sockets.rst:100
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The socket API is quite low-level and should be used only when you need a complete control of the network access. If your application simply needs, for instance, to retrieve data from a web server, there are much simpler and higher-level APIs.
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../../exercises/transport.rst:18
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A reliable connection oriented transport places a 32 bits sequence number inside the segment header to number the segments. This sequence number is incremented for each data segment. The connection starts as shown in the figure below :
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../../principles/naming.rst:15
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Because names are at a higher level than addresses, they allow (both in the example of programming above, and on the Internet) to treat addresses as mere technical identifiers, which can change at will. Only the names are stable.
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../../principles/referencemodels.rst:21
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Our reference model is divided into five layers, as shown in the figure below.
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../../principles/network.rst:18
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Before explaining the operation of the network layer, it is useful to remember the characteristics of the service provided by the `datalink` layer. There are many variants of the datalink layer. Some provide a reliable service while others do not provide any guarantee of delivery. The reliable datalink layer services are popular in environments such as wireless networks where transmission errors are frequent. On the other hand, unreliable services are usually used when the physical layer provides an almost reliable service (i.e. only a negligible fraction of the frames are affected by transmission errors). Such `almost reliable` services are frequently used in wired and optical networks. In this chapter, we will assume that the datalink layer service provides an `almost reliable` service since this is both the most general one and also the most widely deployed one.
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../../principles/reliability.rst:33
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`electrical cable`. Information can be transmitted over different types of electrical cables. The most common ones are the twisted pairs (that are used in the telephone network, but also in enterprise networks) and the coaxial cables (that are still used in cable TV networks, but are no longer used in enterprise networks). Some networking technologies operate over the classical electrical cable.
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