182 strings / 9323 words

Translation Source string Target string
cnp3-ebook/protocols/lan — French
200
 
cnp3-ebook/protocols/lan — French
The `Spanning Tree Protocol` uses its own terminology that we illustrate in the figure above. A switch port can be in three different states : `Root`, `Designated` and `Blocked`. All the ports of the `root` switch are in the `Designated` state. The state of the ports on the other switches is determined based on the `BPDU` received on each port.
 
cnp3-ebook/protocols/lan — French
The `Spanning Tree Protocol` uses the ordering relationship to build the spanning tree. Each switch listens to `BPDUs` on its ports. When `BPDU = <R,c,T,p>` is received on port `q`, the switch computes the port's `root priority vector`: `V[q] = <R,c+cost[q],T,p,q>` , where `cost[q]` is the cost associated to the port over which the `BPDU` was received. The switch stores in a table the last `root priority vector` received on each port. The switch then compares its own `identifier` with the smallest `root identifier` stored in this table. If its own `identifier` is smaller, then the switch is the root of the spanning tree and is, by definition, at a distance `0` of the root. The `BPDU` of the switch is then `<R,0,R,p>`, where `R` is the switch `identifier` and `p` will be set to the port number over which the `BPDU` is sent.
 
cnp3-ebook/protocols/lan — French
Otherwise, the switch chooses the best priority vector from its table, `bv = <R,c+cost[q'],T,p,q'>`. The port `q'`, over which this best root priority vector was learned, is the switch port that is closest to the `root` switch. This port becomes the `Root` port of the switch. There is only one `Root` port per switch (except for the `Root` switches whose ports are all `Designated`). The switch can then compute its own `BPDU` as `BPDU = <R,c',S,p>` , where `R` is the `root identifier`, `c'` the cost of the best root priority vector, `S` the identifier of the switch and `p` will be replaced by the number of the port over which the `BPDU` will be sent.
 
cnp3-ebook/protocols/lan — French
To determine the state of its other ports, the switch compares its own `BPDU` with the last `BPDU` received on each port. Note that the comparison is done by using the `BPDUs` and not the `root priority vectors`. If the switch's `BPDU` is better than the last `BPDU` of this port, the port becomes a `Designated` port. Otherwise, the port becomes a `Blocked` port.
 
cnp3-ebook/protocols/lan — French
The state of each port is important when considering the transmission of `BPDUs`. The root switch regularly sends its own `BPDU` over all of its (`Designated`) ports. This `BPDU` is received on the `Root` port of all the switches that are directly connected to the `root switch`. Each of these switches computes its own `BPDU` and sends this `BPDU` over all its `Designated` ports. These `BPDUs` are then received on the `Root` port of downstream switches, which then compute their own `BPDU`, etc. When the network topology is stable, switches send their own `BPDU` on all their `Designated` ports, once they receive a `BPDU` on their `Root` port. No `BPDU` is sent on a `Blocked` port. Switches listen for `BPDUs` on their `Blocked` and `Designated` ports, but no `BPDU` should be received over these ports when the topology is stable. The utilization of the ports for both `BPDUs` and data frames is summarized in the table below.
 
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Port state
 
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Receives BPDUs
 
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Sends BPDU
 
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Handles data frames
 
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Blocked
 
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yes
 
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no
 
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Root
 
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Designated
 
cnp3-ebook/protocols/lan — French
To illustrate the operation of the `Spanning Tree Protocol`, let us consider the simple network topology in the figure below.
 
cnp3-ebook/protocols/lan — French
A simple Spanning tree computed in a switched Ethernet network
 
cnp3-ebook/protocols/lan — French
Assume that `Switch4` is the first to boot. It sends its own `BPDU = <4,0,4,1>` (resp. `BPDU = <4,0,4,2>`) on port 1 (resp. port 2). When `Switch1` boots, it sends `BPDU = <1,0,1,1>`. This `BPDU` is received by `Switch4`, which updates its `BPDU` and root priority vector tables and computes a new `BPDU = <1,3,4,1>` (resp. `BPDU = <1,3,4,2>`) on port 1 (resp. port 2). Indeed, there is only one root priority vector and hence, it is the best one. Port 1 of `Switch4` becomes the `Root` port while its second port is still in the `Designated` state.
 
cnp3-ebook/protocols/lan — French
Assume now that `Switch9` boots and immediately receives `Switch1` 's `BPDU` on port 1. `Switch9` computes its own `BPDU = <1,1,9,1>` (resp. `BPDU = <1,1,9,2>`) on port 1 (resp. port 2) and port 1 becomes the `Root` port of this switch. The `BPDU` is sent on port 2 of `Switch9` and reaches `Switch4`. `Switch4` compares the priority vectors. It notices that the last computed vector (i.e., `V[2] = <1,2,9,2,2>`) is better than `V[1] = <1,3,1,1,1>`. Thus, `Switch4`'s `BPDU` is recomputed and port 2 becomes the `Root` port of `Switch4`. `Switch4` compares its new `BPDU = <1,2,4,p>` with the last `BPDU` received on each port (except for the `Root` port). Port 1 becomes a `Blocked` port on `Switch4` because the `BPDU=<1,0,1,1>` received on this port is better.
 
cnp3-ebook/protocols/lan — French
During the computation of the spanning tree, switches discard all received data frames, as at that time the network topology is not guaranteed to be loop-free. Once that topology has been stable for some time, the switches again start to use the MAC learning algorithm to forward data frames. Only the `Root` and `Designated` ports are used to forward data frames. Switches discard all the data frames received on their `Blocked` ports and never forward frames on these ports.
 
cnp3-ebook/protocols/lan — French
Switches, ports and links can fail in a switched Ethernet network. When a failure occurs, the switches must be able to recompute the spanning tree to recover from the failure. The `Spanning Tree Protocol` relies on regular transmissions of the `BPDUs` to detect these failures. A `BPDU` contains two additional fields : the `Age` of the `BPDU` and the `Maximum Age`. The `Age` contains the amount of time that has passed since the root switch initially originated the `BPDU`. The root switch sends its `BPDU` with an `Age` of zero and each switch that computes its own `BPDU` increments its `Age` by one. The `Age` of the `BPDUs` stored on a switch's table is also incremented every second. A `BPDU` expires when its `Age` reaches the `Maximum Age`. When the network is stable, this does not happen as `BPDU` s are regularly sent by the `root` switch and downstream switches. However, if the `root` fails or the network becomes partitioned, `BPDU` will expire and switches will recompute their own `BPDU` and restart the `Spanning Tree Protocol`. Once a topology change has been detected, the forwarding of the data frames stops as the topology is not guaranteed to be loop-free. Additional details about the reaction to failures may be found in [IEEE802.1d]_.
 
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Virtual LANs
 
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Another important advantage of Ethernet switches is the ability to create Virtual Local Area Networks (VLANs). A virtual LAN can be defined as a `set of ports attached to one or more Ethernet switches`. A switch can support several VLANs and it runs one MAC learning algorithm for each Virtual LAN. When a switch receives a frame with an unknown or a multicast destination, it forwards it over all the ports that belong to the same Virtual LAN but not over the ports that belong to other Virtual LANs. Similarly, when a switch learns a source address on a port, it associates it to the Virtual LAN of this port and uses this information only when forwarding frames on this Virtual LAN.
 
cnp3-ebook/protocols/lan — French
The figure below illustrates a switched Ethernet network with three Virtual LANs. `VLAN2` and `VLAN3` only require a local configuration of switch `S1`. Host `C` can exchange frames with host `D`, but not with hosts that are outside of its VLAN. `VLAN1` is more complex as there are ports of this VLAN on several switches. To support such VLANs, local configuration is not sufficient anymore. When a switch receives a frame from another switch, it must be able to determine the VLAN in which the frame originated to use the correct MAC table to forward the frame. This is done by assigning an identifier to each Virtual LAN and placing this identifier inside the headers of the frames that are exchanged between switches.
 
cnp3-ebook/protocols/lan — French
Virtual Local Area Networks in a switched Ethernet network
 
cnp3-ebook/protocols/lan — French
IEEE defined in the [IEEE802.1q]_ standard a special header to encode the VLAN identifiers. This 32 bit header includes a 12 bit VLAN field that contains the VLAN identifier of each frame. The format of the [IEEE802.1q]_ header is described below.
 
cnp3-ebook/protocols/lan — French
Format of the 802.1q header
 
cnp3-ebook/protocols/lan — French
The [IEEE802.1q]_ header is inserted immediately after the source MAC address in the Ethernet frame (i.e. before the EtherType field). The maximum frame size is increased by 4 bytes. It is encoded in 32 bits and contains four fields. The Tag Protocol Identifier is set to `0x8100` to allow the receiver to detect the presence of this additional header. The `Priority Code Point` (PCP) is a three bit field that is used to support different transmission priorities for the frame. Value `0` is the lowest priority and value `7` the highest. Frames with a higher priority can expect to be forwarded earlier than frames having a lower priority. The `C` bit is used for compatibility between Ethernet and Token Ring networks. The last 12 bits of the 802.1q header contain the VLAN identifier. Value `0` indicates that the frame does not belong to any VLAN while value `0xFFF` is reserved. This implies that 4094 different VLAN identifiers can be used in an Ethernet network.
 
cnp3-ebook/protocols/lan — French
The radio spectrum is a limited resource that must be shared by everyone. During most of the twentieth century, governments and international organizations have regulated most of the radio spectrum. This regulation controls the utilization of the radio spectrum, in order to prevent interference among different users. A company that wants to use a frequency range in a given region must apply for a license from the regulator. Most regulators charge a fee for the utilization of the radio spectrum and some governments have encouraged competition among companies bidding for the same frequency to increase the license fees.
 
cnp3-ebook/protocols/lan — French
In the 1970s, after the first experiments with ALOHANet, interest in wireless networks grew. Many experiments were done on and outside the ARPANet. One of these experiments was the `first mobile phone <http://news.bbc.co.uk/2/hi/programmes/click_online/8639590.stm>`_ , which was developed and tested in 1973. This experimental mobile phone was the starting point for the first generation analog mobile phones. Given the growing demand for mobile phones, it was clear that the analog mobile phone technology was not sufficient to support a large number of users. To support more users and new services, researchers in several countries worked on the development of digital mobile telephones. In 1987, several European countries decided to develop the standards for a common cellular telephone system across Europe : the `Global System for Mobile Communications` (GSM). Since then, the standards have evolved and more than three billion users are connected to GSM networks today.
 
cnp3-ebook/protocols/lan — French
While most of the frequency ranges of the radio spectrum are reserved for specific applications and require a special license, there are a few exceptions. These exceptions are known as the `Industrial, Scientific and Medical <http://en.wikipedia.org/wiki/ISM_band>`_ (ISM) radio bands. These bands can be used for industrial, scientific and medical applications without requiring a license from the regulator. For example, some radio-controlled models use the 27 MHz ISM band and some cordless telephones operate in the 915 MHz ISM. In 1985, the 2.400-2.500 GHz band was added to the list of ISM bands. This frequency range corresponds to the frequencies that are emitted by microwave ovens. Sharing this band with licensed applications would have likely caused interference, given the large number of microwave ovens that are used. Despite the risk of interference with microwave ovens, the opening of the 2.400-2.500 GHz allowed the networking industry to develop several wireless network techniques to allow computers to exchange data without using cables. In this section, we discuss in more detail the most popular one, i.e. the WiFi [IEEE802.11]_ family of wireless networks. Other wireless networking techniques such as `BlueTooth <http://en.wikipedia.org/wiki/BlueTooth>`_ or `HiperLAN <http://en.wikipedia.org/wiki/HiperLAN>`_ use the same frequency range.
 
cnp3-ebook/protocols/lan — French
Today, WiFi is a very popular wireless networking technology. There are more than several hundreds of millions of WiFi devices. The development of this technology started in the late 1980s with the `WaveLAN <http://en.wikipedia.org/wiki/WaveLAN>`_ proprietary wireless network. WaveLAN operated at 2 Mbps and used different frequency bands in different regions of the world. In the early 1990s, the IEEE_ created the `802.11 working group <http://www.ieee802.org/11/>`_ to standardize a family of wireless network technologies. This working group was very prolific and produced several wireless networking standards that use different frequency ranges and different physical layers. The table below provides a summary of the main 802.11 standards.
 
cnp3-ebook/protocols/lan — French
Frequency
 
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Typical throughput
 
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Max bandwidth
 
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Range (m) indoor/outdoor
 
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802.11
 
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2.4 GHz
 
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0.9 Mbps
 
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2 Mbps
 
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20/100
 
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802.11a
 
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5 GHz
 
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23 Mbps
 
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54 Mbps
 
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35/120
 
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802.11b
 
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4.3 Mbps
 
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11 Mbps
 
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38/140
 
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802.11g
 
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19 Mbps
 
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802.11n
 
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2.4/5 GHz
 
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74 Mbps
 
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150 Mbps
 
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70/250
 
cnp3-ebook/protocols/lan — French
When developing its family of standards, the `IEEE 802.11 working group <http://www.ieee802.org/11/>`_ took a similar approach as the `IEEE 802.3 working group <http://www.ieee802.org/3/>`_ that developed various types of physical layers for Ethernet networks. 802.11 networks use the CSMA/CA Medium Access Control technique described earlier and they all assume the same architecture and use the same frame format.
 
cnp3-ebook/protocols/lan — French
The architecture of WiFi networks is slightly different from the Local Area Networks that we have discussed until now. There are, in practice, two main types of WiFi networks : `independent` or `adhoc` networks and `infrastructure` networks [#fBSS]_. An `independent` or `adhoc` network is composed of a set of devices that communicate with each other. These devices play the same role and the `adhoc` network is usually not connected to the global Internet. `Adhoc` networks are used when for example a few laptops need to exchange information or to connect a computer with a WiFi printer.
 
cnp3-ebook/protocols/lan — French
An 802.11 independent or adhoc network
 
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Most WiFi networks are `infrastructure` networks. An `infrastructure` network contains one or more `access points` that are attached to a fixed Local Area Network (usually an Ethernet network) that is connected to other networks such as the Internet. The figure below shows such a network with two access points and four WiFi devices. Each WiFi device is associated to one access point and uses this access point as a relay to exchange frames with the devices that are associated to another access point or reachable through the LAN.
 
cnp3-ebook/protocols/lan — French
An 802.11 infrastructure network
 
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An 802.11 access point is a relay that operates in the datalink layer like switches. The figure below represents the layers of the reference model that are involved when a WiFi host communicates with a host attached to an Ethernet network through an access point.
 
cnp3-ebook/protocols/lan — French
802.11 devices exchange variable length frames, which have a slightly different structure than the simple frame format used in Ethernet LANs. We review the key parts of the 802.11 frames. Additional details may be found in [IEEE802.11]_ and [Gast2002]_ . An 802.11 frame contains a fixed length header, a variable length payload that may contain up 2324 bytes of user data and a 32 bits CRC. Although the payload can contain up to 2324 bytes, most 802.11 deployments use a maximum payload size of 1500 bytes as they are used in `infrastructure` networks attached to Ethernet LANs. An 802.11 data frame is shown below.
 
cnp3-ebook/protocols/lan — French
802.11 data frame format
 
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The first part of the 802.11 header is the 16 bit `Frame Control` field. This field contains flags that indicate the type of frame (data frame, RTS/CTS, acknowledgment, management frames, etc), whether the frame is sent to or from a fixed LAN, etc [IEEE802.11]_. The `Duration` is a 16 bit field that is used to reserve the transmission channel. In data frames, the `Duration` field is usually set to the time required to transmit one acknowledgment frame after a SIFS delay. Note that the `Duration` field must be set to zero in multicast and broadcast frames. As these frames are not acknowledged, there is no need to reserve the transmission channel after their transmission. The `Sequence control` field contains a 12 bits sequence number that is incremented for each data frame and a 4 bits fragment number.
 
cnp3-ebook/protocols/lan — French
The astute reader may have noticed that the 802.11 data frames contain three 48-bits address fields [#f4addresses]_ . This is surprising compared to other protocols in the network and datalink layers whose headers only contain a source and a destination address. The need for a third address in the 802.11 header comes from the `infrastructure` networks. In such a network, frames are usually exchanged between routers and servers attached to the LAN and WiFi devices attached to one of the access points. The role of the three address fields is specified by bit flags in the `Frame Control` field.
 
cnp3-ebook/protocols/lan — French
When a frame is sent from a WiFi device to a server attached to the same LAN as the access point, the first address of the frame is set to the MAC address of the access point, the second address is set to the MAC address of the source WiFi device and the third address is the address of the final destination on the LAN. When the server replies, it sends an Ethernet frame whose source address is its MAC address and the destination address is the MAC address of the WiFi device. This frame is captured by the access point that converts the Ethernet header into an 802.11 frame header. The 802.11 frame sent by the access point contains three addresses : the first address is the MAC address of the destination WiFi device, the second address is the MAC address of the access point and the third address the MAC address of the server that sent the frame.
 
cnp3-ebook/protocols/lan — French
802.11 control frames are simpler than data frames. They contain a `Frame Control`, a `Duration` field and one or two addresses. The acknowledgment frames are very small. They only contain the address of the destination of the acknowledgment. There is no source address and no `Sequence Control` field in the acknowledgment frames. This is because the acknowledgment frame can easily be associated to the previous frame that it acknowledges. Indeed, each unicast data frame contains a `Duration` field that is used to reserve the transmission channel to ensure that no collision will affect the acknowledgment frame. The `Sequence Control` field is mainly used by the receiver to remove duplicate frames. Duplicate frames are detected as follows. Each data frame contains a 12 bits sequence number in the `Sequence Control` field and the `Frame Control` field contains the `Retry` bit flag that is set when a frame is transmitted. Each 802.11 receiver stores the most recent sequence number received from each source address in frames whose `Retry` bit is reset. Upon reception of a frame with the `Retry` bit set, the receiver verifies its sequence number to determine whether it is a duplicated frame or not.
 
cnp3-ebook/protocols/lan — French
IEEE 802.11 ACK and CTS frames
 
cnp3-ebook/protocols/lan — French
802.11 RTS/CTS frames are used to reserve the transmission channel, in order to transmit one data frame and its acknowledgment. The RTS frames contain a `Duration` and the transmitter and receiver addresses. The `Duration` field of the RTS frame indicates the duration of the entire reservation (i.e. the time required to transmit the CTS, the data frame, the acknowledgments and the required SIFS delays). The CTS frame has the same format as the acknowledgment frame.
 
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IEEE 802.11 RTS frame format
 
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The 802.11 service
 
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Despite the utilization of acknowledgments, the 802.11 layer only provides an unreliable connectionless service like Ethernet networks that do not use acknowledgments. The 802.11 acknowledgments are used to minimize the probability of frame duplication. They do not guarantee that all frames will be correctly received by their recipients. Like Ethernet, 802.11 networks provide a high probability of successful delivery of the frames, not a guarantee. Furthermore, it should be noted that 802.11 networks do not use acknowledgments for multicast and broadcast frames. This implies that in practice such frames are more likely to suffer from transmission errors than unicast frames.
 
cnp3-ebook/protocols/lan — French
In addition to the data and control frames that we have briefly described above, 802.11 networks use several types of management frames. These management frames are used for various purposes. We briefly describe some of these frames below. A detailed discussion may be found in [IEEE802.11]_ and [Gast2002]_.
 
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A first type of management frames are the `beacon` frames. These frames are broadcasted regularly by access points. Each `beacon frame` contains information about the capabilities of the access point (e.g. the supported 802.11 transmission rates) and a `Service Set Identity` (SSID). The SSID is a null-terminated ASCII string that can contain up to 32 characters. An access point may support several SSIDs and announce them in beacon frames. An access point may also choose to remain silent and not advertise beacon frames. In this case, WiFi stations may send `Probe request` frames to force the available access points to return a `Probe response` frame.
 
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IP over 802.11
 
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Two types of encapsulation schemes were defined to support IP in Ethernet networks : the original encapsulation scheme, built above the Ethernet DIX format is defined in :rfc:`894` and a second encapsulation :rfc:`1042` scheme, built above the LLC/SNAP protocol [IEEE802.2]_. In 802.11 networks, the situation is simpler and only the :rfc:`1042` encapsulation is used. In practice, this encapsulation adds 6 bytes to the 802.11 header. The first four bytes correspond to the LLC/SNAP header. They are followed by the two bytes Ethernet Type field (`0x800` for IP and `0x806` for ARP). The figure below shows an IP packet encapsulated in an 802.11 frame.
 
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IP over IEEE 802.11
 
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The second important utilization of the management frames is to allow a WiFi station to be associated with an access point. When a WiFi station starts, it listens to beacon frames to find the available SSIDs. To be allowed to send and receive frames via an access point, a WiFi station must be associated to this access point. If the access point does not use any security mechanism to secure the wireless transmission, the WiFi station simply sends an `Association request` frame to its preferred access point (usually the access point that it receives with the strongest radio signal). This frame contains some parameters chosen by the WiFi station and the SSID that it requests to join. The access point replies with an `Association response frame` if it accepts the WiFI station.
 
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The 802.11 working group defined the `basic service set (BSS)` as a group of devices that communicate with each other. We continue to use `network` when referring to a set of devices that communicate.
 
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In fact, the [IEEE802.11]_ frame format contains a fourth optional address field. This fourth address is only used when an 802.11 wireless network is used to interconnect bridges attached to two classical LAN networks.