Note that there is a difference between "master keys" and "session keys". Master keys are long-lived and usually work as a shared secret or a means to distribute session keys, which in turn do not outlive the connection for which they were created and usually serve as the functional encryption/decryption mechanism. A specific header field indicates which of the possible keys is to be used. It is also important to note that replay prevention mechanisms require the keeping of counters which are updated on valid receptions. The range of these counters further limits the life of session keys. Other forms of USB over wireless exist, such as those based on the competing direct sequence ultra-wideband technology by Cable-Free USB. The same is also true for other radio frequency based wire replacement systems which can carry USB. The result is that the name "Certified Wireless USB" was adopted to allow consumers to identify which products would be adherent to the standard and would support the correct protocol and data rates. There is also USB over IP, which may use IP-based networking to transfer USB traffic wirelessly. For example, with proper drivers the host side may use 802.11a/b/g/n/ac Wi-Fi (or wired Ethernet) to communicate with the device side. , Media Agnostic USB (MA USB) is a specification being developed by the USB Implementers Forum. It is intended to enable communication using the Universal Serial Bus (USB) protocol to be performed over a wide range of physical communication media, including WiFi and WiGig wireless networks. The protocol is being developed from the base of the Wi-Fi Alliance's previous WiGig Serial Extension specification. Media Agnostic USB is distinct from, and should not be confused with, previous wireless USB protocols such as Certified Wireless USB.