device must keep the connection alive by at least confirming its presence within each trust timeout boundary, which is set to four seconds. If it does not succeed at keeping up with this requirement, reauthentication is demanded. Following the natural asymmetry of USB, the host initiates all processes (except signaling), security being no exception. Security requests are made to devices to find their security capabilities, after which the appropriate devices can be chosen. The standard, symmetric encryption method is AES-128 with CCM, though Public key encryption may be used for initial authentication (namely, only the sending of the initial CCM key), provided that the achieved security level is comparable (in practice by using 3072-bit RSA and SHA-256 for hashing). 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. Competitors 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 was also true for other radio frequency based wire replacement systems which could carry USB. The result was 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 was also USB over IP, which may have used IP-based networking to transfer USB traffic wirelessly. For example, with proper drivers the host side may have used 802.11a/b/g/n/ac Wi-Fi (or wired Ethernet) to communicate with the device. Media Agnostic USB , 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