6. Technical information

6.1. Synchronization

6.1.1. Synchronization logic

These steps are performed when synchronizing a collection:

Prepare synchronization: prepare local collection, settings etc.
Query capabilities with HTTP PROPFIND:
- determine whether the server supports Collection Synchronization
- CardDAV: determine whether the server supports vCard 4
- fetch current CTag and sync-token
Process locally deleted resources: if a local resource is flagged as deleted,
- delete it on the server (HTTP DELETE with If-Match/If-Schedule-Tag-Match set to last known ETag/Schedule-Tag to avoid deleting resources which have been changed on the server in the meanwhile) and
- then remove it locally
Upload locally modified (“dirty”) resources:
- Assign a random UID (if necessary) and resource name to new resources; prepare contact group and recurring events, if necessary
- If no previous ETag/Schedule-Tag of the resource is known (i.e. the resource has not been uploaded yet), use HTTP PUT with If-None-Match: * to avoid overwriting a possibly existing resource with the same name
- If a previous ETag/Schedule-Tag of the resource is known, use HTTP PUT with If-Match/If-Schedule-Tag-Match set to last known ETag/Schedule-Tag to avoid overwriting changes which happend on the server in the meanwhile
- remember returned ETag/Schedule-Tag as last known ETag/Schedule-Tag; otherwise reset last known ETag/Schedule-Tag
Choose sync algorithm (PROPFIND/REPORT vs. Collection Synchronization):
- CardDAV: use Collection Synchronization if supported by server, PROPFIND otherwise
- CalDAV events: use Collection Synchronization if supported by server and past time event limit is disabled, REPORT calendar-query otherwise
- CalDAV tasks: use REPORT calendar-query
Check whether further synchronization is needed. Only continue when:
- modifications (uploads/deletions) have been sent to the server, or
- the sync state (CTag/sync-token) of the collection has changed since last sync, or
- the PROPFIND/REPORT algorithm shall be used and the sync has been initiated manually
Continue with chosen sync algorithm (see below).

6.1.2. Sync algorithm: PROPFIND/REPORT

Unset present remotely flag for all resources.
List and process remote resources (only names and ETag) using PROPFIND or REPORT (see above).
- Download resources which have been added/modified remotely in bunches using REPORT addressbook-multiget/calendar-multiget into the local storage.
- Set present remotely flag for all received resources.
Locally delete all resources which are not flagged as present remotely.
Post-processing: clean up empty contact groups etc.
Save sync state (CTag/sync-token).

6.1.3. Sync algorithm: Collection Synchronization

Was a previous initial sync aborted and is now being continued? → If yes, set initial sync.
Do we have a previous sync-token? → If no, set initial sync.
List and process changes since last sync-token (or all records if no previous sync-token is known) using REPORT sync-collection.
- Download resources which have been added/modified remotely in bunches using REPORT addressbook-multiget/calendar-multiget into the local storage.
- Set present remotely flag for all received resources.
If the requested sync-token was invalid:
- forget the sync-token
- reset present remotely flags of all local resources
- set initial sync and continue with 3.
Save sync-token.
Are there further changes on the server (HTTP 507 on collection URL in multiget response)? → If yes, continue with 3.
Only for initial sync: delete all local resources which are not present remotely.
Post-processing: clean up empty contact groups etc.

6.1.4. Conflict handling

Conflicts occur when different versions of a resource are available and it’s ambigous which one shall be used. For instance:

A contact exists on the server and has been synchronized to a mobile phone with DAVx⁵ and to a desktop PC with Gnome Evolution.
You modify the contact with Evolution, which immediately uploads it to the server.
You modify the same contact on your mobile device, too.
DAVx⁵ wants to upload the modified contact and finds that it has been changed on the server in the meanwhile. Now there’s a conflict of two different versions of this contact.

How DAVx⁵ handles such conflicts:

DAVx⁵ relies on HTTP ETag to determine whether a resource has been changed on the server.
The server always wins. If a local resource can’t be uploaded or deleted safely because it has been modified on the server in the meanwhile, local changes are discarded and the server version is used.
DAVx⁵ doesn’t involve the user in resolving conflicts (like asking which version shall be used) because it’s supposed to run in the background silently.

At the moment, DAVx⁵ doesn’t support automatic vCard merging as suggested in vCard 4.

6.2. Supported fields

6.2.1. Supported contact properties

6.2.1.1. Name

These vCard properties are mapped to ContactsContract.CommonDataKinds.StructuredName records:

FN ↔ display name

N ↔ prefix, given name, middle name, family name, suffix

X-PHONETIC-FIRST-NAME ↔ phonetic given name

X-PHONETIC-MIDDLE-NAME ↔ phonetic middle name

X-PHONETIC-LAST-NAME ↔ phonetic first name

These vCard properties are mapped to ContactsContract.CommonDataKinds.Nickname records:

NICKNAME ↔ nick name (types are mapped as TYPE x-values)

6.2.1.2. Phone number

vCard TEL properties are mapped to ContactsContract.CommonDataKinds.Phone records (phone number).

Types like private, work etc. are mapped when possible. Not all vCard values have a corresponding Android value and vice versa. Custom labels are supported.

6.2.1.3. Email address

vCard EMAIL properties are mapped to ContactsContract.CommonDataKinds.Email records (email address).

Types like private, work etc. are mapped when possible. Not all vCard values have a corresponding Android value and vice versa. Custom labels are supported.

6.2.1.4. Photo

vCard PHOTO properties are mapped to ContactsContract.CommonDataKinds.Photo records.

Because of Android limitations, contact photos with more than 1 MB can’t be stored in the Android contacts provider, so DAVx⁵ has to resize large vCard photos to the values given by CONTENT_MAX_DIMENSIONS_URI. This limit does not apply in the other direction (Android → vCard).

6.2.1.5. Organization

These vCard properties are mapped to ContactsContract.CommonDataKinds.Organization records:

ORG ↔ company, department
TITLE ↔ (job) title
ROLE ↔ job description

6.2.1.6. Messenger / SIP address

vCard IMPP properties are mapped to ContactsContract.CommonDataKinds.Im (messenger account) and – if the URI scheme is sip: – ContactsContract.CommonDataKinds.SipAddress (SIP address) records.

Types like private, work etc. are mapped when possible. Not all vCard values have a corresponding Android value and vice versa. Custom labels are supported.

When importing a vCard, X-SIP values are treated like IMPP:sip:... and stored as SIP address.

6.2.1.7. Note

vCard NOTE properties are mapped to ContactsContract.CommonDataKinds.Note records (note).

6.2.1.8. Postal address

These vCard properties are mapped to ContactsContract.CommonDataKinds.StructuredPostal records:

ADR ↔ street address, p/o box, extended address, locality, region, postal code, country, vCard 4: formatted address
LABEL ↔ vCard3: formatted address

Types like private, work etc. are mapped when possible. Not all vCard values have a corresponding Android value and vice versa. Custom labels are supported.

If a vCard doesn’t contain a formatted address, it will be generated by DAVx⁵ in this format:

street po.box (extended)
postcode city
region
COUNTRY

6.2.1.9. Web site

vCard URL properties are mapped to ContactsContract.CommonDataKinds.Website records (Web site).

Types like private, work etc. are mapped when possible. Not all vCard values have a corresponding Android value and vice versa. Custom labels are supported.

6.2.1.10. Event/date

These vCard properties are mapped to ContactsContract.CommonDataKinds.Event records:

BDAY ↔ birthday
ANNIVERSARY ↔ anniversary

Partial dates without year are supported.

6.2.1.11. Relation

vCard RELATED properties are mapped to ContactsContract.CommonDataKinds.Relation records (relation).

Not all vCard values have a corresponding Android value and vice versa. Custom relation names are supported.

6.2.1.12. Contact groups

If the Groups are per-contact categories method is set in the account settings, DAVx⁵ will match contact groups to CATEGORIES. For instance, when a contact is in the groups “Friends” and “Family”, this property will be added: CATEGORIES:Friends,Family.

If the Groups are separate vCards method is set in the account settings, DAVx⁵ will use

KIND (or X-ADDRESSBOOKSERVER-KIND if the server doesn’t support VCard 4) to distinguish between contacts and contact groups, and
MEMBER (or X-ADDRESSBOOKSERVER-MEMBER if the server doesn’t support VCard 4) to store contact group members.

6.2.1.13. Custom labels

For some properties, custom labels are supported by vCard property groups. For custom labels, the X-ABLABEL property is used like that:

BEGIN:VCARD
...
davdroid1.TEL:+123456
davdroid1.X-ABLABEL:My Custom Phone
davdroid2.EMAIL:test@example.com
davdroid2.X-ABLABEL:My Custom Email Address
...
END:VCARD

In this example, the phone number +123456 is grouped together with the custom label “My Custom Phone” and the email address test@example.com is labelled “My Custom Email Address”.

6.2.1.14. Unknown properties

Contact properties which are not processed by DAVx⁵ (like X- properties) are retained. When importing a vCard, DAVx⁵ saves all unknown properties. When the respective contact is modified and DAVx⁵ generates the vCard again, it starts with all unknown properties and then adds the known ones.

6.2.1.15. Protected properties

These vCard properties are processed/generated by DAVx⁵ and cannot be changed by users:

PRODID is set to the DAVx⁵ identifier
UID is used to identify a vCard (for new vCards, a random UUID will be generated)
REV is set to the current time when generating a vCard
SOURCE is removed because it doesn’t apply anymore as soon as DAVx⁵ generates the vCard
LOGO, SOUND are removed because retaining them might cause out-of-memory errors

6.2.2. Supported event properties

Events are stored as CalendarContract.Events in the Android calendar provider. These iCalendar properties are directly mapped to Android fields:

SUMMARY ↔ title
LOCATION ↔ event location
DESCRIPTION ↔ description
COLOR ↔ event color (only if enabled in DAVx⁵ account settings)
DTSTART ↔ start date/time, event timezone / all-day event
DTEND, DURATION ↔ end date/time, event end timezone / all-day event
CLASS ↔ access level
TRANSP ↔ availability (opaque ↔ busy, transparent ↔ free)
STATUS ↔ status (confirmed/tentative/cancelled)

6.2.2.1. All-day events

Events are considered to be all-day events when DTSTART is a date (and not a time). All-day events

without end date or
with an end date that is not after the start date

are stored with a duration of one day for Android compatibility.

6.2.2.2. Reminders

VALARM components are mapped to CalendarContract.Reminders records and vice versa.

Reminder methods (ACTION) are mapped to Android values as good as possible.

6.2.2.3. Recurring events

RRULE, RDATE, EXRULE and EXDATE values are stored in the respective Android event fields. The Android calendar provider uses these fields to calculcate the instances of a recurring event, which are then saved as CalendarContract.Instances so that calendar apps can access them.

Exceptions of recurring events are identified by RECURRENCE-ID. DAVx⁵ inserts exceptions as separate event records with ORIGINAL_SYNC_ID set to the SYNC_ID of the recurring event and ORIGINAL_TIME set to the RECURRENCE-ID value.

Note

DAVx⁵ is not responsible for calculating the instances of a recurring event. It only provides RRULE, RDATE, EXRULE, EXDATE and a list of exceptions to the Android calendar provider.

However, if a calendar app leaves dirty events with zero instances to be synced, DAVx⁵ will silently delete these events and ask the server to do the same.

6.2.2.4. Group-scheduled events

ATTENDEE properties are mapped to CalendarContract.AttendeesColumns records and vice versa.

Events with at least one attendee are considered to be group-scheduled events. Only for group-scheduled events, the ORGANIZER property

is imported from iCalendars to the Android event so that only the organizer can edit a group-scheduled event,
is exported from the Android event to the iCalendar.

When you add attendees to an event, the server may send invitations to the attendees (for instance, by email). DAVx⁵ doesn’t send invitation emails on its own.

6.2.2.5. Time zones

Thanks to ical4j, DAVx⁵ is able to really process time zone definitions of events (VTIMEZONE). If a certain time zone is referenced by identifier but VTIMEZONE component is provided, DAVx⁵ uses the default time zone definitions from ical4j (Olson DB).

When an iCalendar references a time zone which is not available in Android, DAVx⁵ tries to find an available time zone with (partially) matching name. If no such time zone is found, the system default time zone is used. The original value will be shifted to the available time zone.

For instance, if an event has a start time of 10:00 Custom Time Zone, DAVx⁵ will use the Custom Time Zone VTIMEZONE to calculate the corresponding time in the system default time zone, let’s say 12:00 Europe/Vienna, and then save the event as 12:00 Europe/Vienna.

Warning

Because the Android calendar provider can only process events with time zones which are available in Android, recurring events in time zones which are not available in Android and their exceptions may not be expanded correctly.

6.2.2.6. Event classification

iCalendar event classification is mapped to Android’s ACCESS_LEVEL like that:

no CLASS → ACCESS_LEVEL = ACCESS_DEFAULT (“server default”)
CLASS:PUBLIC → ACCESS_LEVEL = ACCESS_PUBLIC (“public”)
CLASS:PRIVATE → ACCESS_LEVEL = ACCESS_PRIVATE (“private”)
CLASS:CONFIDENTIAL → ACCESS_LEVEL = ACCESS_CONFIDENTIAL (currently not supported by many calendar apps, which will reset the access level to ACCESS_DEFAULT or ACCESS_PRIVATE when the event is edited); additionally, CONFIDENTIAL is stored as original value
other CLASS value (x-name or iana-token) → ACCESS_LEVEL = ACCESS_PRIVATE; additionally, the value is stored as original value

In the other direction, the locally stored access level is mapped to CLASS like that:

ACCESS_LEVEL = ACCESS_PUBLIC (“public”) → CLASS:PUBLIC
ACCESS_LEVEL = ACCESS_PRIVATE (“private”) → CLASS:PRIVATE
ACCESS_LEVEL = ACCESS_CONFIDENTIAL (“confidential”, if available in calendar app) → CLASS:CONFIDENTIAL
ACCESS_LEVEL = ACCESS_DEFAULT (“server default”) →
- if there is an original value: use that value
- no CLASS otherwise (same as PUBLIC)

6.2.2.7. Categories

Added in version 2.6.2: In earlier versions, event categories were treated as unknown properties (see below).

iCalendar CATEGORIES are mapped from/to extended properties with these fields:

name = categories
value = list of category names, separated by backslash (\), for example: Cat A\Cat B\Cat C. If a category name contains a backslash, the backslash will be dropped silenty.

This is the same format as it is used by the AOSP ActiveSync Exchange sync adapter.

6.2.2.8. Extended properties

These properties are not natively supported by Android, but are synchronized as extended properties with a format defined by DAVx⁵:

URL

See extended event properties for more information.

6.2.2.9. Unknown properties

iCalendar properties which are not processed by DAVx⁵ (like X- properties) are retained (unless they’re larger than ≈ 25 kB). When importing an iCalendar, DAVx⁵ saves all unknown event properties as extended property rows. When the respective event is modified and DAVx⁵ generates the iCalendar again, it will include all unknown properties.

6.2.2.10. Protected properties

These iCalendar properties are processed/generated by DAVx⁵ and cannot be changed by users:

PRODID is set to the DAVx⁵ identifier
UID is used to identify an iCalendar (for new iCalendars, a random UUID will be generated)
RECURRENCE-ID is used to identify certain instances of recurring events
SEQUENCE is increased when an iCalendar is modified
DTSTAMP is set to the current time when generating an iCalendar

6.2.3. Supported task properties

DAVx⁵ synchronizes VTODO components (= tasks) with the OpenTasks provider, so OpenTasks or tasks.org must be installed for task synchronization.

To use some features (for instance, to see subtasks as indented task) in the UI, you may need another tasks app that is able to access the OpenTasks provider, like aCalendar+.

These properties are synchronized by DAVx⁵:

UID
SUMMARY, DESCRIPTION
LOCATION
GEO
URL
ORGANIZER
PRIORITY
COMPLETED, PERCENT-COMPLETE
STATUS
CREATED, LAST-MODIFIED
DTSTART, DUE, DURATION
RDATE, EXDATE, RRULE
CATEGORIES
RELATED-TO (used for subtasks)

6.2.3.1. Unknown properties

See unknown properties of events.

6.3. WebDAV mounts

Detection: DAVx⁵ uses OPTIONS to detect whether a URL is WebDAV-capable. It expects at least compliance class 1 (DAV: 1).

Supported features:

Directory listing (PROPFIND)
- WebDAV quota (RFC 4331) of root directory is shown in DAVx⁵ WebDAV mounts
Random-access read (GET with Range headers) for files opened with mode r
- that have a known file size,
- that have an ETag or Last-Modified,
- on Android 8+ only.
This allows things like seeking in videos or extracting data from a file without reading the whole file.

A page cache with a page size of 2 MB is used, so files will be requested in pieces of 2 MB, regardless of the actual read operations.
Atomic (streaming) read (GET) for files opened with mode r that can’t be opened in random-access mode
Atomic (streaming) write (PUT) for files opened with mode w

6.4. TLS stack (protocol versions, ciphers)

Added in version 2.5: DAVx⁵ uses Conscrypt to support modern TLS protocol versions and ciphers even on older devices. Both your client (DAVx⁵) and the CalDAV/CardDAV server must share at least one cipher, otherwise a SSLProtocolException will occur.