http://sabre.io/dav/building-a-caldav-client/
0. Discovery
PROPFIND / HTTP/1.1
Depth: 0
Prefer: return-minimal
Content-Type: application/xml; charset=utf-8

<d:propfind xmlns:d="DAV:">
  <d:prop>
     <d:current-user-principal />
  </d:prop>
</d:propfind>

Success:
HTTP/1.1 207 Multi-status
Content-Type: application/xml; charset=utf-8

<d:multistatus xmlns:d="DAV:" xmlns:cs="http://calendarserver.org/ns/">
    <d:response>
        <d:href>/</d:href>
        <d:propstat>
            <d:prop>
                <d:current-user-principal>
                    <d:href>/principals/users/johndoe/</d:href>
                </d:current-user-principal>
            </d:prop>
            <d:status>HTTP/1.1 200 OK</d:status>
        </d:propstat>
    </d:response>
</d:multistatus>

Get calendar home:
PROPFIND /principals/users/johndoe/ HTTP/1.1
Depth: 0
Prefer: return-minimal
Content-Type: application/xml; charset=utf-8

<d:propfind xmlns:d="DAV:" xmlns:c="urn:ietf:params:xml:ns:caldav">
  <d:prop>
     <c:calendar-home-set />
  </d:prop>
</d:propfind>

Success:
HTTP/1.1 207 Multi-status
Content-Type: application/xml; charset=utf-8

<d:multistatus xmlns:d="DAV:" xmlns:c="urn:ietf:params:xml:ns:caldav">
    <d:response>
        <d:href>/principals/users/johndoe/</d:href>
        <d:propstat>
            <d:prop>
                <c:calendar-home-set>
                    <d:href>/calendars/johndoe/</d:href>
                </c:calendar-home-set>
            </d:prop>
            <d:status>HTTP/1.1 200 OK</d:status>
        </d:propstat>
    </d:response>
</d:multistatus>

Get all calendars:
PROPFIND /calendars/johndoe/ HTTP/1.1
Depth: 1
Prefer: return-minimal
Content-Type: application/xml; charset=utf-8

<d:propfind xmlns:d="DAV:" xmlns:cs="http://calendarserver.org/ns/" xmlns:c="urn:ietf:params:xml:ns:caldav">
  <d:prop>
     <d:resourcetype />
     <d:displayname />
     <cs:getctag />
     <c:supported-calendar-component-set />
  </d:prop>
</d:propfind>

Success:
HTTP/1.1 207 Multi-status
Content-Type: application/xml; charset=utf-8

<d:multistatus xmlns:d="DAV:" xmlns:cs="http://calendarserver.org/ns/" xmlns:c="urn:ietf:params:xml:ns:caldav">
    <d:response>
        <d:href>/calendars/johndoe/</d:href>
        <d:propstat>
            <d:prop>
                <d:resourcetype>
                    <d:collection/>
                </d:resourcetype>
            </d:prop>
            <d:status>HTTP/1.1 200 OK</d:status>
        </d:propstat>
    </d:response>
    <d:response>
        <d:href>/calendars/johndoe/home/</d:href>
        <d:propstat>
            <d:prop>
                <d:resourcetype>
                    <d:collection/>
                    <c:calendar/>
                </d:resourcetype>
                <d:displayname>Home calendar</d:displayname>
                <cs:getctag>3145</cs:getctag>
                <c:supported-calendar-component-set>
                    <c:comp name="VEVENT" />
                </c:supported-calendar-component-set>
            </d:prop>
            <d:status>HTTP/1.1 200 OK</d:status>
        </d:propstat>
    </d:response>
    <d:response>
        <d:href>/calendars/johndoe/tasks/</d:href>
        <d:propstat>
            <d:prop>
                <d:resourcetype>
                    <d:collection/>
                    <c:calendar/>
                </d:resourcetype>
                <d:displayname>My TODO list</d:displayname>
                <cs:getctag>3345</cs:getctag>
                <c:supported-calendar-component-set>
                    <c:comp name="VTODO" />
                </c:supported-calendar-component-set>
            </d:prop>
            <d:status>HTTP/1.1 200 OK</d:status>
        </d:propstat>
    </d:response>
</d:multistatus>

1. Get display name and ctag
PROPFIND /calendars/johndoe/home/ HTTP/1.1
Depth: 0
Prefer: return-minimal
Content-Type: application/xml; charset=utf-8

<d:propfind xmlns:d="DAV:" xmlns:cs="http://calendarserver.org/ns/">
  <d:prop>
     <d:displayname />
     <cs:getctag />
  </d:prop>
</d:propfind>

Success:
HTTP/1.1 207 Multi-status
Content-Type: application/xml; charset=utf-8

<d:multistatus xmlns:d="DAV:" xmlns:cs="http://calendarserver.org/ns/">
    <d:response>
        <d:href>/calendars/johndoe/home/</d:href>
        <d:propstat>
            <d:prop>
                <d:displayname>Home calendar</d:displayname>
                <cs:getctag>3145</cs:getctag>
            </d:prop>
            <d:status>HTTP/1.1 200 OK</d:status>
        </d:propstat>
    </d:response>
</d:multistatus>

Failure:
HTTP/1.1 207 Multi-status
Content-Type: application/xml; charset=utf-8

<d:multistatus xmlns:d="DAV:" xmlns:cs="http://calendarserver.org/ns/">
    <d:response>
        <d:href>/calendars/johndoe/home/</d:href>
        <d:propstat>
            <d:prop>
                <d:displayname />
                <cs:getctag />
            </d:prop>
            <d:status>HTTP/1.1 403 Forbidden</d:status>
        </d:propstat>
    </d:response>
</d:multistatus>

2. Save display name and ctag (if ctag changes the calendar has changes)

3. Download objects
REPORT /calendars/johndoe/home/ HTTP/1.1
Depth: 1
Prefer: return-minimal
Content-Type: application/xml; charset=utf-8

<c:calendar-query xmlns:d="DAV:" xmlns:c="urn:ietf:params:xml:ns:caldav">
    <d:prop>
        <d:getetag />
        <c:calendar-data />
    </d:prop>
    <c:filter>
        <c:comp-filter name="VCALENDAR" />
    </c:filter>
</c:calendar-query>

Success:
HTTP/1.1 207 Multi-status
Content-Type: application/xml; charset=utf-8

<d:multistatus xmlns:d="DAV:" xmlns:cs="http://calendarserver.org/ns/">
    <d:response>
        <d:href>/calendars/johndoe/home/132456762153245.ics</d:href>
        <d:propstat>
            <d:prop>
                <d:getetag>"2134-314"</d:getetag>
                <c:calendar-data>BEGIN:VCALENDAR
                    VERSION:2.0
                    CALSCALE:GREGORIAN
                    BEGIN:VTODO
                    UID:132456762153245
                    SUMMARY:Do the dishes
                    DUE:20121028T115600Z
                    END:VTODO
                    END:VCALENDAR
                </c:calendar-data>
            </d:prop>
            <d:status>HTTP/1.1 200 OK</d:status>
        </d:propstat>
    </d:response>
    <d:response>
        <d:href>/calendars/johndoe/home/132456-34365.ics</d:href>
        <d:propstat>
            <d:prop>
                <d:getetag>"5467-323"</d:getetag>
                <c:calendar-data>BEGIN:VCALENDAR
                    VERSION:2.0
                    CALSCALE:GREGORIAN
                    BEGIN:VEVENT
                    UID:132456-34365
                    SUMMARY:Weekly meeting
                    DTSTART:20120101T120000
                    DURATION:PT1H
                    RRULE:FREQ=WEEKLY
                    END:VEVENT
                    END:VCALENDAR
                </c:calendar-data>
            </d:prop>
            <d:status>HTTP/1.1 200 OK</d:status>
        </d:propstat>
    </d:response>
</d:multistatus>

For each object get:
- Calendar data
- url
- etag

4. Check for changes
- Fetch ctag -> see 1.
- If ctag is changes fetch all etags
REPORT /calendars/johndoe/home/ HTTP/1.1
Depth: 1
Prefer: return-minimal
Content-Type: application/xml; charset=utf-8

<c:calendar-query xmlns:d="DAV:" xmlns:c="urn:ietf:params:xml:ns:caldav">
    <d:prop>
        <d:getetag />
    </d:prop>
    <c:filter>
        <c:comp-filter name="VCALENDAR" />
    </c:filter>
</c:calendar-query>

Success:
HTTP/1.1 207 Multi-status
Content-Type: application/xml; charset=utf-8

<d:multistatus xmlns:d="DAV:" xmlns:cs="http://calendarserver.org/ns/">
    <d:response>
        <d:href>/calendars/johndoe/home/132456762153245.ics</d:href>
        <d:propstat>
            <d:prop>
                <d:getetag>"xxxx-xxx"</d:getetag>
            </d:prop>
            <d:status>HTTP/1.1 200 OK</d:status>
        </d:propstat>
    </d:response>
    <d:response>
        <d:href>/calendars/johndoe/home/fancy-caldav-client-1234253678.ics</d:href>
        <d:propstat>
            <d:prop>
                <d:getetag>"5-12"</d:getetag>
            </d:prop>
            <d:status>HTTP/1.1 200 OK</d:status>
        </d:propstat>
    </d:response>
</d:multistatus>

Compare local url's to request:
 if match compare etag. if etag compares continue else update.
 delete each local url not found
 fetch every url not found local

update/fetch:
REPORT /calendars/johndoe/home/ HTTP/1.1
Depth: 1
Prefer: return-minimal
Content-Type: application/xml; charset=utf-8

<c:calendar-multiget xmlns:d="DAV:" xmlns:c="urn:ietf:params:xml:ns:caldav">
    <d:prop>
        <d:getetag />
        <c:calendar-data />
    </d:prop>
    <d:href>/calendars/johndoe/home/132456762153245.ics</d:href>
    <d:href>/calendars/johndoe/home/fancy-caldav-client-1234253678.ics</d:href>
</c:calendar-multiget>

5. Updating objects
If-Match contains etag to update and You must not change the UID of the original object
PUT /calendars/johndoe/home/132456762153245.ics HTTP/1.1
Content-Type: text/calendar; charset=utf-8
If-Match: "2134-314"

BEGIN:VCALENDAR
....
END:VCALENDAR

Success:
HTTP/1.1 204 No Content
ETag: "2134-315"

If no etag is returned make an immediate GET to get etag

6. Create an object
PUT /calendars/johndoe/home/somerandomstring.ics HTTP/1.1
Content-Type: text/calendar; charset=utf-8

BEGIN:VCALENDAR
....
END:VCALENDAR

Success:
HTTP/1.1 201 Created
ETag: "21345-324"

7. Deleting an object
DELETE /calendars/johndoe/home/132456762153245.ics HTTP/1.1
If-Match: "2134-314"