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Assume that the network has started and all port-address and forwarding tables are empty. Host `A` sends a packet towards `B`. Upon reception of this packet, `R1` updates its port-address table. Since `B` is not present in the port-address table, the packet is broadcasted. Both `R2` and `R3` receive a copy of the packet sent by `A`. They both update their port-address table. Unfortunately, they also both broadcast the received packet. `B` receives a first copy of the packet, but `R3` and `R2` receive it again. `R3` will then broadcast this copy of the packet to `B` and `R1` while `R2` will broadcast its copy to `R1`. Although `B` has already received two copies of the packet, it is still inside the network and continues to loop. Due to the presence of the cycle, a single packet towards an unknown destination generates many copies of this packet that loop and will eventually saturate the network. Network operators who are using port-address tables to automatically compute the forwarding tables also use distributed algorithms to ensure that the network topology is always a tree.
Assume that the network has started and all port-address and forwarding tables are empty. Host `A` sends a packet towards `B`. Upon reception of this packet, `R1` updates its port-address table. Since `B` is not present in the port-address table, the packet is broadcasted. Both `R2` and `R3` receive a copy of the packet sent by `A`. They both update their port-address table. Unfortunately, they also both broadcast the received packet. `B` receives a first copy of the packet, but `R3` and `R2` receive it again. `R3` will then broadcast this copy of the packet to `B` and `R1` while `R2` will broadcast its copy to `R1`. Although `B` has already received two copies of the packet, it is still inside the network and continues to loop. Due to the presence of the cycle, a single packet towards an unknown destination generates many copies of this packet that loop and will eventually saturate the network. Network operators who are using port-address tables to automatically compute the forwarding tables also use distributed algorithms to ensure that the network topology is always a tree.
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Host `A` sends a packet towards `B`. When receiving this packet, `R1` learns that `A` is reachable via its `West` interface. Since it does not have an entry for destination `B` in its port-address table, it forwards the packet to both `R2` and `R3`. When `R2` receives the packet, it updates its own forwarding table and forward the packet to `C`. Since `C` is not the intended recipient, it simply discards the received packet. Router `R3` also receives the packet. It learns that `A` is reachable via its `North-West` interface and broadcasts the packet to `R4` and `R5`. `R5` also updates its forwarding table and finally forwards it to destination `B`. Let us now consider what happens when `B` sends a reply to `A`. `R5` first learns that `B` is attached to its `North-East` port. It then consults its port-address table and finds that `A` is reachable via its `North-West` interface. The packet is then forwarded hop-by-hop to `A` without any broadcasting. Later on, if `C` sends a packet to `B`, this packet will reach `R1` that contains a valid forwarding entry in its forwarding table.
Host `A` sends a packet towards `B`. When receiving this packet, `R1` learns that `A` is reachable via its `West` interface. Since it does not have an entry for destination `B` in its port-address table, it forwards the packet to both `R2` and `R3`. When `R2` receives the packet, it updates its own forwarding table and forward the packet to `C`. Since `C` is not the intended recipient, it simply discards the received packet. Router `R3` also receives the packet. It learns that `A` is reachable via its `North-West` interface and broadcasts the packet to `R4` and `R5`. `R5` also updates its forwarding table and finally forwards it to destination `B`. Let us now consider what happens when `B` sends a reply to `A`. `R5` first learns that `B` is attached to its `North-East` port. It then consults its port-address table and finds that `A` is reachable via its `North-West` interface. The packet is then forwarded hop-by-hop to `A` without any broadcasting. Later on, if `C` sends a packet to `B`, this packet will reach `R1` that contains a valid forwarding entry in its forwarding table.
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To understand the operation of the port-address table, let us consider the example network shown in the figure below. This network contains three hosts: `A`, `B` and `C` and five routers, `R1` to `R5`. When the network boots, all the forwarding tables of the nodes are empty.
To understand the operation of the port-address table, let us consider the example network shown in the figure below. This network contains three hosts: `A`, `B` and `C` and five routers, `R1` to `R5`. When the network boots, all the forwarding tables of the nodes are empty.
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Networks deployed several techniques to update the forwarding tables upon the arrival of a packet. In this section, we briefly present the principles that underlie three of these techniques.
Networks deployed several techniques to update the forwarding tables upon the arrival of a packet. In this section, we briefly present the principles that underlie three of these techniques.
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Similarly, routers also fail. There are two types of failures that affect routers. A router may stop forwarding packets due to hardware or software problems (e.g., due to a crash of its operating system). A router may also need to be halted from time to time (e.g., to upgrade its operating system or to install new interface cards). These planned and unplanned events affect the set of links and routers that can be used to forward packets in the network. Still, most network users expect that their network will continue to correctly forward packets despite all these events. With manually computed forwarding tables, it is usually impossible to pre-compute the forwarding tables while taking into account all possible failure scenarios.
Similarly, routers also fail. There are two types of failures that affect routers. A router may stop forwarding packets due to hardware or software problems (e.g., due to a crash of its operating system). A router may also need to be halted from time to time (e.g., to upgrade its operating system or to install new interface cards). These planned and unplanned events affect the set of links and routers that can be used to forward packets in the network. Still, most network users expect that their network will continue to correctly forward packets despite all these events. With manually computed forwarding tables, it is usually impossible to pre-compute the forwarding tables while taking into account all possible failure scenarios.
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In most networks, manual forwarding tables are not a solution for two reasons. First, most networks are too large to enable a manual computation of the forwarding tables. Second, with manually computed forwarding tables, it is very difficult to deal with link and router failures. Networks need to operate 24h a day, 365 days per year. Many events can affect the routers and links that compose a network. Link failures are regular events in deployed networks. Links can fail for various reasons, including electromagnetic interference, fiber cuts, hardware or software problems on the terminating routers,... Some links also need to be either added to or removed from the network because their utilization is too low or their cost is too high.
In most networks, manual forwarding tables are not a solution for two reasons. First, most networks are too large to enable a manual computation of the forwarding tables. Second, with manually computed forwarding tables, it is very difficult to deal with link and router failures. Networks need to operate 24h a day, 365 days per year. Many events can affect the routers and links that compose a network. Link failures are regular events in deployed networks. Links can fail for various reasons, including electromagnetic interference, fiber cuts, hardware or software problems on the terminating routers,... Some links also need to be either added to or removed from the network because their utilization is too low or their cost is too high.
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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.
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.
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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.
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.
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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.
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.
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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`.
L'objectif principal de la couche réseau est de permettre à desaux hôtes, connectés à deifférents réseaux différents, d'échanger des l'informations à travers des systèmes intermédiaires appelés :term:`routeurs`. L'unité d'information danse la couche réseau s'est appellée un :term:`paquet`.
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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`.
L'objectif principal de la couche réseau est de permettre aux hôtes, connectés à différents réseaux, d'échanger des informations à travers des systèmes intermédiaires appelés :term:`routeurs`. L'unité d'information de la couche réseau est appelée un :term:`paquet`.
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locale/en/LC_MESSAGES/principles/network.po
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Footnotes
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