Windows 7 / Getting Started

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Key Management Service

KMS activates computers on a local network, eliminating the need for individual computers to connect to Microsoft. To do this, KMS uses a client-server topology. KMS clients can locate KMS hosts by using Domain Name System (DNS) or a static configuration. KMS clients contact the KMS host by using Remote Procedure Call (RPC). KMS can be hosted on computers running the Windows 7, Windows Vista, Windows Server 2008 R2, Windows Server 2008, or Windows Server 2003 operating systems.

Minimum Computer Requirements

If you are planning to use KMS activation, the network must meet or exceed the activation threshold (the minimum number of qualifying computers that KMS requires). IT professionals must also understand how the KMS host tracks the number of computers on the network.

KMS Activation Thresholds

KMS can activate both physical computers and virtual machines (VMs). To qualify for KMS activation, a network must have a minimum number of qualifying computers, called the activation threshold. KMS hosts activate clients only after meeting this threshold. To ensure that the activation threshold is met, a KMS host counts the number of computers requesting activation on the network.

The Windows Server operating systems (starting with Windows Server 2008) and Windows client operating systems (starting with Windows Vista) are activated after meeting different thresholds. The Windows Server activation threshold is 5 computers, and the Windows client activation threshold is 25 computers. The threshold includes Windows client and server operating systems running on physical computers or VMs.

A KMS host responds to each valid activation request from a KMS client with the count of how many computers have contacted the KMS host for activation. Clients that receive a count below their activation threshold are not activated. For example, if the first two computers that contact the KMS host are running Windows 7, the first receives an activation count of 1, and the second receives an activation count of 2. If the next computer is a Windows 7 VM, it receives an activation count of 3, and so on. None of these computers is activated because computers running Windows 7 must receive an activation count greater than or equal to 25 to be activated. KMS clients in the grace state that are not activated because the activation count is too low will connect to the KMS host every two hours to get the current activation count and will be activated when the threshold is met.

If the next computer that contacts the KMS host is running Windows Server 2008 R2, it receives an activation count of 4, because activation counts are a combination of computers running Windows Server 2008 R2 and Windows 7. If a computer running Windows Server 2008 or Windows Server 2008 R2 receives an activation count that is greater than or equal to 5, it is activated. If a computer running Windows 7 receives an activation count greater than or equal to 25, it is activated.

Activation Count Cache

To track the activation threshold, the KMS host keeps a record of the KMS clients that request activation. The KMS host gives each KMS client a client machine identification (CMID) designation, and the KMS host saves each CMID in a table. Each activation request remains in the table for 30 days. When a client renews its activation, the cached CMID is removed from the table, a new record is created, and the 30-day period begins again. If a KMS client does not renew its activation within 30 days, the KMS host removes the corresponding CMID from the table and reduces the activation count by 1.

The KMS host caches twice the number of CMIDs that KMS clients require to help ensure that the CMID count does not drop below the activation threshold. For example, on a network with clients running Windows 7, the KMS activation threshold is 25. The KMS host caches the CMIDs of the most recent 50 activations. The KMS activation threshold for Windows Server 2008 R2 is 5. A KMS host that is contacted only by clients running Windows Server 2008 R2 KMS would cache the 10 most recent CMIDs. If a client running Windows 7 later contacts that KMS host, KMS increases the cache size to 50 to accommodate the higher threshold. KMS never reduces the cache size.