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Footnotes
We will see later a more detailed description of Multiprotocol Label Switching, a networking technology that is capable of using one or more labels.
The control plane
One of the objectives of the `control plane` in the network layer is to maintain the routing tables that are used on all routers. As indicated earlier, a routing table is a data structure that contains, for each destination address (or block of addresses) known by the router, the outgoing interface over which the router must forward a packet destined to this address. The routing table may also contain additional information such as the address of the next router on the path towards the destination or an estimation of the cost of this path.
In this section, we discuss the main techniques that can be used to maintain the forwarding tables in a network.
Distance vector routing
Distance vector routing is a simple distributed routing protocol. Distance vector routing allows routers to automatically discover the destinations reachable inside the network as well as the shortest path to reach each of these destinations. The shortest path is computed based on `metrics` or `costs` that are associated to each link. We use `l.cost` to represent the metric that has been configured for link `l` on a router.
Each router maintains a routing table. The routing table `R` can be modeled as a data structure that stores, for each known destination address `d`, the following attributes :
`R[d].link` is the outgoing link that the router uses to forward packets towards destination `d`
`R[d].cost` is the sum of the metrics of the links that compose the shortest path to reach destination `d`
`R[d].time` is the timestamp of the last distance vector containing destination `d`
A router that uses distance vector routing regularly sends its distance vector over all its interfaces. This distance vector is a summary of the router's routing table that indicates the distance towards each known destination. This distance vector can be computed from the routing table by using the pseudo-code below.
When a router boots, it does not know any destination in the network and its routing table only contains its local address(es). It thus sends to all its neighbors a distance vector that contains only its address at a distance of `0`. When a router receives a distance vector on link `l`, it processes it as follows.
The router iterates over all addresses included in the distance vector. If the distance vector contains a destination address that the router does not know, it inserts it inside its routing table via link `l` and at a distance which is the sum between the distance indicated in the distance vector and the cost associated to link `l`. If the destination was already known by the router, it only updates the corresponding entry in its routing table if either :
the cost of the new route is smaller than the cost of the already known route `((V[d].cost + l.cost) < R[d].cost)`
the new route was learned over the same link as the current best route towards this destination `(R[d].link == l)`
The first condition ensures that the router discovers the shortest path towards each destination. The second condition is used to take into account the changes of routes that may occur after a link failure or a change of the metric associated to a link.
To understand the operation of a distance vector protocol, let us consider the network of five routers shown below.
Assume that router `A` is the first to send its distance vector `[A=0]`.
`B` and `D` process the received distance vector and update their routing table with a route towards `A`.
`D` sends its distance vector `[D=0,A=1]` to `A` and `E`. `E` can now reach `A` and `D`.
`C` sends its distance vector `[C=0]` to `B` and `E`
`E` sends its distance vector `[E=0,D=1,A=2,C=1]` to `D`, `B` and `C`. `B` can now reach `A`, `C`, `D` and `E`
`B` sends its distance vector `[B=0,A=1,C=1,D=2,E=1]` to `A`, `C` and `E`. `A`, `B`, `C` and `E` can now reach all five routers of this network.
`A` sends its distance vector `[A=0,B=1,C=2,D=1,E=2]` to `B` and `D`.
At this point, all routers can reach all other routers in the network thanks to the routing tables shown in the figure below.
To deal with link and router failures, routers use the timestamp stored in their routing table. As all routers send their distance vector every `N` seconds, the timestamp of each route should be regularly refreshed. Thus no route should have a timestamp older than `N` seconds, unless the route is not reachable anymore. In practice, to cope with the possible loss of a distance vector due to transmission errors, routers check the timestamp of the routes stored in their routing table every `N` seconds and remove the routes that are older than :math:`3 \times N` seconds.
When a router notices that a route towards a destination has expired, it must first associate an :math:`\infty` cost to this route and send its distance vector to its neighbors to inform them. The route can then be removed from the routing table after some time (e.g. :math:`3 \times N` seconds), to ensure that the neighboring routers have received the bad news, even if some distance vectors do not reach them due to transmission errors.
Consider the example above and assume that the link between routers `A` and `B` fails. Before the failure, `A` used `B` to reach destinations `B`, `C` and `E` while `B` only used the `A-B` link to reach `A`. The two routers detect the failure by the timeouts in the affected entries in their routing tables. Both routers `A` and `B` send their distance vector.
`A` sends its distance vector :math:`[A=0,B=\infty,C=\infty,D=1,E=\infty]`. `D` knows that it cannot reach `B` anymore via `A`
`D` sends its distance vector :math:`[D=0,B=\infty,A=1,C=2,E=1]` to `A` and `E`. `A` recovers routes towards `C` and `E` via `D`.

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cnp3-ebook / principles/network — English

the new route was learned over the same link as the current best route towards this destination `(R[d].link == l)`
the new route was learned over the same link as the current best route towards this destination `(R[d].link == l)`
2 years ago
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../../principles/dv.rst:57
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locale/pot/principles/network.pot, string 132