Exit Zen
112
../../principles/network.rst:472
English
MultiProtocol Label Switching (MPLS) is the example of a deployed networking technology that relies on label switching. MPLS is more complex than the above description because it has been designed to be easily integrated with datagram technologies. However, the principles remain. `Asynchronous Transfer Mode` (ATM) and Frame Relay are other examples of technologies that rely on `label switching`.
0/3980 · 398
115
../../principles/network.rst:506
English
We will see later a more detailed description of Multiprotocol Label Switching, a networking technology that is capable of using one or more labels.
0/1480 · 148
116
../../principles/network.rst:511
English
The control plane
0/170 · 17
117
../../principles/network.rst:513
English
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.
0/5450 · 545
118
../../principles/network.rst:515
English
In this section, we discuss the main techniques that can be used to maintain the forwarding tables in a network.
0/1120 · 112
119
../../principles/dv.rst:7
English
Distance vector routing
0/230 · 23
120
../../principles/dv.rst:9
English
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.
0/4270 · 427
121
../../principles/dv.rst:11
English
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 :
0/1750 · 175
122
../../principles/dv.rst:13
English
`R[d].link` is the outgoing link that the router uses to forward packets towards destination `d`
0/960 · 96
123
../../principles/dv.rst:14
English
`R[d].cost` is the sum of the metrics of the links that compose the shortest path to reach destination `d`
0/1060 · 106
124
../../principles/dv.rst:15
English
`R[d].time` is the timestamp of the last distance vector containing destination `d`
0/830 · 83
125
../../principles/dv.rst:17
English
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.
0/3190 · 319
126
../../principles/dv.rst:31
English
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.
0/3150 · 315
127
../../principles/dv.rst:54
English
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 :
0/4590 · 459
130
../../principles/dv.rst:59
English
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.
0/2530 · 253
131
../../principles/dv.rst:61
English
To understand the operation of a distance vector protocol, let us consider the network of five routers shown below.
0/1150 · 115
133
../../principles/dv.rst:70
English
Assume that router `A` is the first to send its distance vector `[A=0]`.
0/720 · 72
134
../../principles/dv.rst:72
English
`B` and `D` process the received distance vector and update their routing table with a route towards `A`.
0/1050 · 105
135
../../principles/dv.rst:73
English
`D` sends its distance vector `[D=0,A=1]` to `A` and `E`. `E` can now reach `A` and `D`.
0/880 · 88
136
../../principles/dv.rst:74
English
`C` sends its distance vector `[C=0]` to `B` and `E`
0/520 · 52