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verifying that packets can reach a given destination
We will discuss these functions in more details when we will describe the protocols that are used in the network layer of the TCP/IP protocol suite.
Virtual circuit organization
The second organization of the network layer, called `virtual circuits`, has been inspired by the organization of telephone networks. Telephone networks have been designed to carry phone calls that usually last a few minutes. Each phone is identified by a telephone number and is attached to a telephone switch. To initiate a phone call, a telephone first needs to send the destination's phone number to its local switch. The switch cooperates with the other switches in the network to create a bi-directional channel between the two telephones through the network. This channel will be used by the two telephones during the lifetime of the call and will be released at the end of the call. Until the 1960s, most of these channels were created manually, by telephone operators, upon request of the caller. Today's telephone networks use automated switches and allow several channels to be carried over the same physical link, but the principles roughly remain the same.
In a network using virtual circuits, all hosts are also identified with a network layer address. However, packet forwarding is not performed by looking at the destination address of each packet. With the `virtual circuit` organization, each data packet contains one label [#flabels]_. A label is an integer which is part of the packet header. Routers implement `label switching` to forward `labelled data packet`. Upon reception of a packet, a router consults its `label forwarding table` to find the outgoing interface for this packet. In contrast with the datagram mode, this lookup is very simple. The `label forwarding table` is an array stored in memory and the label of the incoming packet is the index to access this array. This implies that the lookup operation has an `O(1)` complexity in contrast with other packet forwarding techniques. To ensure that on each node the packet label is an index in the `label forwarding table`, each router that forwards a packet replaces the label of the forwarded packet with the label found in the `label forwarding table`. Each entry of the `label forwarding table` contains two pieces of information :
the outgoing interface for the packet
the label for the outgoing packet
For example, consider the `label forwarding table` of a network node below.
index
outgoing interface
label
0
South
7
1
none
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

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../../principles/network.rst:461 • ../../principles/network.rst:461 • ../../principles/network.rst:513 • ../../principles/network.rst:513
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4 years ago
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locale/pot/principles/network.pot, string 95