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2
West
3
East
If this node receives a packet with `label=2`, it forwards the packet on its `West` interface and sets the `label` of the outgoing packet to `2`. If the received packet's `label` is set to `3`, then the packet is forwarded over the `East` interface and the `label` of the outgoing packet is set to `2`. If a packet is received with a label field set to `1`, the packet is discarded since the corresponding `label forwarding table` entry is invalid.
`Label switching` enables a full control over the path followed by packets inside the network. Consider the network below and assume that we want to use two virtual circuits : `R1->R3->R4->R2->R5` and `R2->R1->R3->R4->R5`.
To create these virtual circuits, we need to configure the `label forwarding tables` of all routers. For simplicity, assume that a label forwarding table only contains two entries. Assume that `R5` wants to receive the packets from the virtual circuit created by `R1` (resp. `R2`) with `label=1` (`label=0`). `R4` could use the following `label forwarding table`:
R4's label forwarding table
->R2
->R5
Since a packet received with `label=1` must be forwarded to `R5` with `label=1`, `R2`'s `label forwarding table` could contain :
R2's label forwarding table
Two virtual circuits pass through `R3`. They both need to be forwarded to `R4`, but `R4` expects `label=1` for packets belonging to the virtual circuit originated by `R2` and `label=0` for packets belonging to the other virtual circuit. `R3` could choose to leave the labels unchanged.
R3's label forwarding table
->R4
With the above `label forwarding table`, `R1` needs to originate the packets that belong to the `R1->R3->R4->R2->R5` circuit with `label=0`. The packets received from `R2` and belonging to the `R2->R1->R3->R4->R5` circuit would then use `label=1` on the `R1-R3` link. `R1` 's label forwarding table could be built as follows :
R1's label forwarding table
->R3
The figure below shows the path followed by the packets on the `R1->R3->R4->R2->R5` path in red with on each arrow the label used in the packets.
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`.
Nowadays, most deployed networks rely on distributed algorithms, called routing protocols, to compute the forwarding tables that are installed on the routers. These distributed algorithms are part of the `control plane`. They are usually implemented in software and are executed on the main CPU of the routers. There are two main families of routing protocols : distance vector routing and link state routing. Both are capable of discovering autonomously the network and react dynamically to topology changes.
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`

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cnp3-ebook / principles/network — English

With the above `label forwarding table`, `R1` needs to originate the packets that belong to the `R1->R3->R4->R2->R5` circuit with `label=0`. The packets received from `R2` and belonging to the `R2->R1->R3->R4->R5` circuit would then use `label=1` on the `R1-R3` link. `R1` 's label forwarding table could be built as follows :
With the above `label forwarding table`, `R1` needs to originate the packets that belong to the `R1->R3->R4->R2->R5` circuit with `label=0`. The packets received from `R2` and belonging to the `R2->R1->R3->R4->R5` circuit would then use `label=1` on the `R1-R3` link. `R1` 's label forwarding table could be built as follows :
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read-only
Source string location
../../principles/network.rst:530
String age
2 years ago
Source string age
2 years ago
Translation file
locale/pot/principles/network.pot, string 111