2.6.4 Version Negotiation Using Exchange ActiveSync

An ActiveSync client can indicate the protocol version that it is using via the MS-ASProtocolVersion header or as part of a query string encoded with base64 encoding in the HTTP request URI, as described in [MS-ASHTTP]. The client can use the HTTP OPTIONS command, as described in [MS-ASHTTP], to obtain the server version and a comprehensive list of supported ActiveSync versions and keywords.