1.7 Versioning and Capability Negotiation

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The HTTP OPTIONS command (section 2.2.3) is used by the client to discover which versions of the ActiveSync protocol are supported by the server. To determine the supported versions, the client examines the MS-ASProtocolVersions header (section 2.2.4.1.2.2), which is returned in the HTTP OPTIONS command response.

The client uses the MS-ASProtocolVersion header (section 2.2.1.1.2.6) of the HTTP POST command (section 2.2.1) to indicate to the server which ActiveSync protocol version it is using.

The latest version of the ActiveSync protocol that the client or server can support is 16.1. Older versions include 16.0, 14.1, 14.0, 12.1, 12.0, and 2.5. Some commands and functionality described in the ActiveSync protocol documentation are not supported by all of the protocol versions. See the command and element descriptions in the ActiveSync protocol documents to determine which commands, elements, and capabilities are supported by the protocol versions.