cnp3-ebook/principles/network — English @ Weblate

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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
27 ../../principles/network.rst:199

	English A path may lead to a `black hole`. In a network, a black hole is a router that receives packets for at least one given source/destination pair but does not have an entry inside its forwarding table for this destination. Since it does not know how to reach the destination, the router cannot forward the received packets and must discard them. Any centralized or distributed algorithm that computes forwarding tables must ensure that there are not black holes inside the network. 478/10000 · 478
28 ../../principles/network.rst:203

	English A second type of problem may exist in networks using the datagram organization. Consider a path that contains a cycle. For example, router `R1` sends all packets towards destination `D` via router `R2`. Router `R2` forwards these packets to router `R3` and finally router `R3`'s forwarding table uses router `R1` as its nexthop to reach destination `D`. In this case, if a packet destined to `D` is received by router `R1`, it will loop on the `R1 -> R2 -> R3 -> R1` cycle and will never reach its final destination. As in the black hole case, the destination is not reachable from all sources in the network. In practice the loop problem is more annoying than the black hole problem because when a packet is caught in a forwarding loop, it unnecessarily consumes bandwidth. In the black hole case, the problematic packet is quickly discarded. We will see later that network layer protocols include techniques to minimize the impact of such forwarding loops. 958/10000 · 958
29 ../../principles/network.rst:205

	English Any solution which is used to compute the forwarding tables of a network must ensure that all destinations are reachable from any source. This implies that it must guarantee the absence of black holes and forwarding loops. 222/10000 · 222
30 ../../principles/network.rst:209

	English The `forwarding tables` and the precise format of the packets that are exchanged inside the network are part of the `data plane` of the network. This `data plane` contains all the protocols and algorithms that are used by hosts and routers to create and process the packets that contain user data. On high-end routers, the data plane is often implemented in hardware for performance reasons. 391/10000 · 391

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