cnp3-ebook/principles/network — English @ Weblate

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7 ../../principles/network.rst:43

	English The second type of datalink layer is the one used in Local Area Networks (LAN). Conceptually, a LAN is a set of communicating devices such that any two devices can directly exchange frames through the datalink layer. Both hosts and routers can be connected to a LAN. Some LANs only connect a few devices, but there are LANs that can connect hundreds or even thousands of devices. In this chapter, we focus on the utilization of point-to-point datalink layers. We describe later the organization and the operation of Local Area Networks and their impact on the network layer. 574/10000 · 574
8 ../../principles/network.rst:47

	English Even if we only consider the point-to-point datalink layers, there is an important characteristic of these layers that we cannot ignore. No datalink layer is able to send frames of unlimited size. Each datalink layer is characterized by a maximum frame size. There are more than dozen different datalink layers and unfortunately most of them use a different maximum frame size. This heterogeneity in the maximum frame sizes will cause problems when we will need to exchange data between hosts attached to different types of datalink layers. 540/10000 · 540
9 ../../principles/network.rst:49

	English As a first step, let us assume that we only need to exchange a small amount of data. In this case, there is no issue with the maximum length of the frames. However, there are other more interesting problems that we need to tackle. To understand these problems, let us consider the network represented in the figure below. 321/10000 · 321
10 ../../principles/network.rst:80

	English This network contains two types of devices. The hosts, represented with circles and the routers, represented as boxes. A host is a device which is able to send and receive data for its own usage in contrast with routers that most of the time simply forward data towards their final destination. Routers have multiple links to neighboring routers or hosts. Hosts are usually attached via a single link to the network. Nowadays, with the growth of wireless networks, more and more hosts are equipped with several physical interfaces. These hosts are often called `multihomed`. Still, using several interfaces at the same time often leads to practical issues that are beyond the scope of this document. For this reason, we only consider `single-homed` hosts in this e-book. 770/10000 · 770
11 ../../principles/network.rst:82

	English To understand the key principles behind the operation of a network, let us analyze all the operations that need to be performed to allow host `A` in the above network to send one byte to host `B`. Thanks to the datalink layer used above the `A-R1` link, host `A` can easily send a byte to router `R1` inside a frame. However, upon reception of this frame, router `R1` needs to understand that this byte is destined to host `B` and not to itself. This is the objective of the network layer. 489/10000 · 489
12 ../../principles/network.rst:86

	English The network layer enables the transmission of information between hosts that are not directly connected through intermediate routers. This transmission is carried out by putting the information to be transmitted inside a data structure which is called a `packet`. As a frame that contains useful data and control information, a packet also contains both data supplied by the user and control information. An important issue in the network layer is the ability to identify a node (host or router) inside the network. This identification is performed by associating an address to each node. An :term:`address` is usually represented as a sequence of bits. Most networks use fixed-length addresses. At this stage, let us simply assume that each of the nodes in the above network has an address which corresponds to the binary representation of its name on the figure. 864/10000 · 864
13 ../../principles/network.rst:88

	English To send one byte of information to host `B`, host `A` needs to place this information inside a `packet`. In addition to the data being transmitted, the packet also contains either the addresses of the source and the destination nodes or information that indicates the path that needs to be followed to reach the destination. 324/10000 · 324
14 ../../principles/network.rst:90

	English There are two possible organizations for the network layer : 60/10000 · 60
15 ../../principles/network.rst:92

	English `datagram` 10/10000 · 10
16 ../../principles/network.rst:93

	English `virtual circuits` 18/10000 · 18
17 ../../principles/network.rst:97

	English The datagram organization 25/10000 · 25
18 ../../principles/network.rst:99

	English The first and most popular organization of the network layer is the datagram organization. This organization is inspired by the organization of the postal service. Each host is identified by a `network layer address`. To send information to a remote host, a host creates a packet that contains: 294/10000 · 294
19 ../../principles/network.rst:101

	English the network layer address of the destination host 49/10000 · 49
20 ../../principles/network.rst:102

	English its own network layer address 29/10000 · 29
21 ../../principles/network.rst:103

	English the information to be sent 26/10000 · 26
22 ../../principles/network.rst:105

	English To understand the datagram organization, let us consider the figure below. A network layer address, represented by a letter, has been assigned to each host and router. To send some information to host `J`, host `A` creates a packet containing its own address, the destination address and the information to be exchanged. 320/10000 · 320
23 ../../principles/network.rst:189

	English With the datagram organization, routers use `hop-by-hop forwarding`. This means that when a router receives a packet that is not destined to itself, it looks up the destination address of the packet in its `forwarding table`. A `forwarding table` is a data structure that maps each destination address (or set of destination addresses) to the outgoing interface over which a packet destined to this address must be forwarded to reach its final destination. The router consults its forwarding table to forward each packet that it handles. 537/10000 · 537
24 ../../principles/network.rst:191

	English The figure illustrates some possible forwarding tables in this network. By inspecting the forwarding tables of the different routers, one can find the path followed by packets sent from a source to a particular destination. In the example above, host `A` sends its packet to router `R1`. `R1` consults its forwarding table and forwards the packet towards `R2`. Based on its own table, `R2` decides to forward the packet to `R5` that can deliver it to its destination. Thus, the path from `A` to `J` is `A -> R1 -> R2 -> R5 -> J`. 529/10000 · 529
25 ../../principles/network.rst:193

	English The computation of the forwarding tables of all the routers inside a network is a key element for the correct operation of the network. This computation can be carried out by using either distributed or centralized algorithms. These algorithms provide different performance, may lead to different types of paths, but their composition must lead to valid paths. 360/10000 · 360
26 ../../principles/network.rst:195

	English In a network, a path can be defined as the list of all intermediate routers for a given source destination pair. For a given source/destination pair, the path can be derived by first consulting the forwarding table of the router attached to the source to determine the next router on the path towards the chosen destination. Then, the forwarding table of this router is queried for the same destination... The queries continue until the destination is reached. In a network that has valid forwarding tables, all the paths between all source/destination pairs contain a finite number of intermediate routers. However, if forwarding tables have not been correctly computed, two types of invalid paths can occur. 709/10000 · 709

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