El HIP HOP: UN MODO DE VIDA.

These two frames are the same, with the only difference identifiable through the reason code. This code is embedded in the body of the frame and is used to identify why a client was removed from a wireless network. The reason code is a small 2-byte portion of the frame. This reason code is capable of identifying more than 50,000 reasons; luckily, there are only ten currently specified reasons for this field. The reasons used inside this code are laid out from least significant to most significant, running from 0 bytes as the least significant to 65,535 bytes as the most significant.

Both of these frames are used to remove a wireless device from the network. The main differences between these two frames are the reason for why they removed a device and how or what piece of equipment made the determination to remove the wireless device. The Disassociation message is used by the access points to remove clients that have not talked for a particular amount of time. This allows the access point to clean up after it- self and remove old clients that have disconnected without notice. The De-Authentication frame is sent by an administrator is remove a wireless device from the network.

5.8 CSMA/CA

Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) is the technology that operates at the data-link layer. This second layer of the OSI model responsible for moving frames across its known view of the world or its network segment. This is similar to Ethernet’s Carrier Sense Multiple Access with Collision Detection CSMA/CD, which is a second OSI layer protocol to move frames in an Ethernet environment. The difference between the two is more than what media the frames are moving across. With wireless networking having bandwidth at such a premium, a scheme with an acceptance for bandwidth loss was not exactly the best approach. On the wire, losing a small amount of bandwidth to speed up the movement of network frames was considered acceptable. This was where the last two words in the CSMA/CD acronym stand for “collision detection.” In CSMA/CD, it detects that a collision took place and resends the frame. On a wireless network losing bandwidth along

with time to detect that a collision took place was not acceptable, so another mechanism (called collision avoidance) was utilized. The process of how CSMA/CA works is detailed below, including each of the four critical steps involved in a transmission and how they work together to move frames across a wireless network.

5.8.1 RTS

The first step in this process is the initial request. This is called the Request to Send or, for short, RTS. The client computer initiates the RTS when it needs to send data to the access point or through the access point to other network resources. As part of the initial request, the client determines the amount of time he or she needs to complete the transmission. This is set with the NAV bit. The access point will use this to make a decision on how long the client can talk before it has to ask permission again.

Looking at Figure 5.3, one can see that this 20-byte frame has five different parts. First is the frame control section, which is a field identifying the frame as RTS. Next is the duration portion, which signals to the access point the amount of time that the client wishes to have the airwaves for to send information. Next is a section called the receiver address; this address is the MAC of the sending station. Next is the transmitting address section, which is also composed of a MAC address. The final portion consists of the frame check sequence. This is the wireless version of an Ethernet checksum. With the wireless airwaves being less reliable media than standard Ethernet, a different technique was used to send a checksum. This technique works by sending a checksum for the previous frame.

Figure 5.3 Request to send frame detail.

Figure 5.4 Clear to send frame detail.

Frame

Control Duration Receiver Address Transmitter Address Frame Check Sequence

2 Bytes 2 Bytes 6 Bytes 6 Bytes 4 Bytes

Duration Frame

Control Receiver Address

2 Bytes 2 Bytes 6 Bytes

Frame Check Sequence 4 Bytes

68 Wireless Security Handbook

5.8.2 CTS

Next, one sees the request as it is returned by the access point with a clear to send frame (CTS). This frame indicates how the long the client can talk for before having to close the CSMA/CA process again. This CTS frame consists of segments to the RTS frame. Looking at Figure 5.4, one can see the frame control segment, which announces its existence as a CTS frame. Next is a duration segment, which makes up the amount of time the wireless device has to transmit data. Next is the receiver’s MAC address, which is copied from the previous frame’s transmitting address, and finally the frame check sequence.

5.8.3 DATA

This section does not deal with any management per se, however, the data section is needed to fully understand what sequence of events takes place when a wireless device wishes to transfer data. Looking at how this plays a role in the entire data transmitting sequence, one can see that the data frame is sent and subsequent data frames continue to be sent until the time allotted by the CTS frame has been used. Once this happens, the data is finished transmitting until a new CTS frame is received with a new period for which the data frame process can resume. To get this new CTS frame, the entire process must be restarted with an RTS. As one can see from Figure 5.5, this frame looks a little different because it is actually a data frame and not a management frame like the RTS and CTS frames.

5.8.4 ACK

Acknowledgment frames are sent by many different networking protocols to identify to the receiver that the data sent was received and was not corrupted en route. This acknowledgment frame is made up of similar sections such as the Request to Send and Clear to Send frames. Looking at Figure 5.6, one can see that, like the other frames, the acknowledgment has a frame control section that identifies it as an acknowledgment frame. It has a duration portion that is used to inform the access point if this acknowl- edgment is the last one in the current data transfer process. Finally, there is the receiver address section, which is copied from the transmitter’s MAC.