Component/Property Presence
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METHOD 1 MUST be "REPLY"
VFREEBUSY 1
ATTENDEE 1 (address of recipient replying)
DTSTAMP 1
DTEND 1 DateTime values must be in UTC
DTSTART 1 DateTime values must be in UTC
FREEBUSY 0+ (values MUST all be of the same data
type. Multiple instances are allowed.
Multiple instances MUST be sorted in
ascending order. Values MAY NOT overlap)
ORGANIZER 1 MUST be the request originator's address
UID 1
COMMENT 0 or 1
CONTACT 0+
REQUEST-STATUS 0+
URL 0 or 1 (specifies busy time URL)
X-PROPERTY 0+
DURATION 0
SEQUENCE 0
X-COMPONENT 0+
VALARM 0
VEVENT 0
VTODO 0
VJOURNAL 0
VTIMEZONE 0