diff --git a/sylk/applications/webrtcgateway/models/sylkrtc.py b/sylk/applications/webrtcgateway/models/sylkrtc.py
index 057d9c2..6fe8ec1 100644
--- a/sylk/applications/webrtcgateway/models/sylkrtc.py
+++ b/sylk/applications/webrtcgateway/models/sylkrtc.py
@@ -1,407 +1,409 @@
 
 from application.python import subclasses
 from sipsimple.core import SIPURI, SIPCoreError
 
 from .jsonobjects import Validator, CompositeValidator
 from .jsonobjects import JSONObject, AbstractProperty, BooleanProperty, IntegerProperty, StringProperty, ArrayProperty, ObjectProperty
 from .jsonobjects import JSONArray, StringArray
 
 
 # Validators
 
 class URIValidator(Validator):
     def validate(self, value):
         if value.startswith(('sip:', 'sips:')):
             uri = value
         else:
             uri = 'sip:' + value
         try:
             SIPURI.parse(uri)
         except SIPCoreError:
             raise ValueError('invalid SIP URI: {}'.format(value))
         return value
 
 
 class UniqueItemsValidator(Validator):
     def validate(self, sequence):
         seen = set()
         unique = []
         for item in sequence:
             if item not in seen:
                 seen.add(item)
                 unique.append(item)
         return unique
 
 
 class LengthValidator(Validator):
     def __init__(self, minimum=0, maximum=float('inf')):
         self.minimum = minimum
         self.maximum = maximum
 
     def validate(self, value):
         if self.minimum <= len(value) <= self.maximum:
             return value
         else:
             raise ValueError("the value's length must be between {0.minimum} and {0.maximum} inclusive".format(self))
 
 
 # Custom JSONObject properties
 
 class FixedValueProperty(AbstractProperty):
     def __init__(self, value):
         super(FixedValueProperty, self).__init__(optional=True, default=value)
         self.value = value
 
     def _parse(self, value):
         if value != self.value:
             raise ValueError('Invalid value for {property.name!r} property: {value!r} (should be {property.value!r})'.format(property=self, value=value))
         return value
 
 
 class LimitedChoiceProperty(AbstractProperty):
-    def __init__(self, *values):
-        if len(values) == 0:
-            raise ValueError('{.__class__.__name__} needs at least one argument'.format(self))
-        super(LimitedChoiceProperty, self).__init__()
+    def __init__(self, values, optional=False, default=None):
+        if not values:
+            raise ValueError('values needs to be an non-empty sequence of elements')
+        if optional and default is not None and default not in values:
+            raise ValueError('default value needs to be one of the allowed values or None')
+        super(LimitedChoiceProperty, self).__init__(optional=optional, default=default)
         self.values = frozenset(values)
         self.values_string = ' or '.join(', '.join(sorted(values)).rsplit(', ', 1))
 
     def _parse(self, value):
         if value not in self.values:
             raise ValueError('Invalid value for {property.name!r} property: {value!r} (expected: {property.values_string})'.format(property=self, value=value))
         return value
 
 
 # Base models (these are abstract and should not be used directly)
 
 class SylkRTCRequestBase(JSONObject):
     transaction = StringProperty()
 
 
 class SylkRTCResponseBase(JSONObject):
     transaction = StringProperty()
 
 
 class AccountRequestBase(SylkRTCRequestBase):
     account = StringProperty(validator=URIValidator())
 
 
 class SessionRequestBase(SylkRTCRequestBase):
     session = StringProperty()
 
 
 class VideoroomRequestBase(SylkRTCRequestBase):
     session = StringProperty()
 
 
 class AccountEventBase(JSONObject):
     sylkrtc = FixedValueProperty('account-event')
     account = StringProperty(validator=URIValidator())
 
 
 class SessionEventBase(JSONObject):
     sylkrtc = FixedValueProperty('session-event')
     session = StringProperty()
 
 
 class VideoroomEventBase(JSONObject):
     sylkrtc = FixedValueProperty('videoroom-event')
     session = StringProperty()
 
 
 class AccountRegistrationStateEvent(AccountEventBase):
     event = FixedValueProperty('registration-state')
 
 
 class SessionStateEvent(SessionEventBase):
     event = FixedValueProperty('state')
 
 
 class VideoroomSessionStateEvent(VideoroomEventBase):
     event = FixedValueProperty('session-state')
 
 
 # Miscellaneous models
 
 class SIPIdentity(JSONObject):
     uri = StringProperty(validator=URIValidator())
     display_name = StringProperty(optional=True)
 
 
 class ICECandidate(JSONObject):
     candidate = StringProperty()
     sdpMLineIndex = IntegerProperty()
     sdpMid = StringProperty()
 
 
 class ICECandidates(JSONArray):
     item_type = ICECandidate
 
 
 class URIList(StringArray):
     list_validator = UniqueItemsValidator()
     item_validator = URIValidator()
 
 
 class VideoroomPublisher(JSONObject):
     id = StringProperty()
     uri = StringProperty(validator=URIValidator())
     display_name = StringProperty(optional=True)
 
 
 class VideoroomPublishers(JSONArray):
     item_type = VideoroomPublisher
 
 
 class VideoroomActiveParticipants(StringArray):
     list_validator = CompositeValidator(UniqueItemsValidator(), LengthValidator(maximum=2))
 
 
 class VideoroomSessionOptions(JSONObject):
     audio = BooleanProperty(optional=True)
     video = BooleanProperty(optional=True)
     bitrate = IntegerProperty(optional=True)
 
 
 # Response models
 
 class AckResponse(SylkRTCResponseBase):
     sylkrtc = FixedValueProperty('ack')
 
 
 class ErrorResponse(SylkRTCResponseBase):
     sylkrtc = FixedValueProperty('error')
     error = StringProperty()
 
 
 # Connection events
 
 class ReadyEvent(JSONObject):
     sylkrtc = FixedValueProperty('ready-event')
 
 
 # Account events
 
 class AccountIncomingSessionEvent(AccountEventBase):
     event = FixedValueProperty('incoming-session')
     session = StringProperty()
     originator = ObjectProperty(SIPIdentity)
     sdp = StringProperty()
 
 
 class AccountMissedSessionEvent(AccountEventBase):
     event = FixedValueProperty('missed-session')
     originator = ObjectProperty(SIPIdentity)
 
 
 class AccountConferenceInviteEvent(AccountEventBase):
     event = FixedValueProperty('conference-invite')
     room = StringProperty(validator=URIValidator())
     originator = ObjectProperty(SIPIdentity)
 
 
 class AccountRegisteringEvent(AccountRegistrationStateEvent):
     state = FixedValueProperty('registering')
 
 
 class AccountRegisteredEvent(AccountRegistrationStateEvent):
     state = FixedValueProperty('registered')
 
 
 class AccountRegistrationFailedEvent(AccountRegistrationStateEvent):
     state = FixedValueProperty('failed')
     reason = StringProperty(optional=True)
 
 
 # Session events
 
 class SessionProgressEvent(SessionStateEvent):
     state = FixedValueProperty('progress')
 
 
 class SessionAcceptedEvent(SessionStateEvent):
     state = FixedValueProperty('accepted')
     sdp = StringProperty(optional=True)  # missing for incoming sessions
 
 
 class SessionEstablishedEvent(SessionStateEvent):
     state = FixedValueProperty('established')
 
 
 class SessionTerminatedEvent(SessionStateEvent):
     state = FixedValueProperty('terminated')
     reason = StringProperty(optional=True)
 
 
 # Video room events
 
 class VideoroomConfigureEvent(VideoroomEventBase):
     event = FixedValueProperty('configure')
     originator = StringProperty()
     active_participants = ArrayProperty(VideoroomActiveParticipants)
 
 
 class VideoroomSessionProgressEvent(VideoroomSessionStateEvent):
     state = FixedValueProperty('progress')
 
 
 class VideoroomSessionAcceptedEvent(VideoroomSessionStateEvent):
     state = FixedValueProperty('accepted')
     sdp = StringProperty()
 
 
 class VideoroomSessionEstablishedEvent(VideoroomSessionStateEvent):
     state = FixedValueProperty('established')
 
 
 class VideoroomSessionTerminatedEvent(VideoroomSessionStateEvent):
     state = FixedValueProperty('terminated')
     reason = StringProperty(optional=True)
 
 
 class VideoroomFeedAttachedEvent(VideoroomEventBase):
     event = FixedValueProperty('feed-attached')
     feed = StringProperty()
     sdp = StringProperty()
 
 
 class VideoroomFeedEstablishedEvent(VideoroomEventBase):
     event = FixedValueProperty('feed-established')
     feed = StringProperty()
 
 
 class VideoroomInitialPublishersEvent(VideoroomEventBase):
     event = FixedValueProperty('initial-publishers')
     publishers = ArrayProperty(VideoroomPublishers)
 
 
 class VideoroomPublishersJoinedEvent(VideoroomEventBase):
     event = FixedValueProperty('publishers-joined')
     publishers = ArrayProperty(VideoroomPublishers)
 
 
 class VideoroomPublishersLeftEvent(VideoroomEventBase):
     event = FixedValueProperty('publishers-left')
     publishers = ArrayProperty(StringArray)
 
 
 # Account request models
 
 class AccountAddRequest(AccountRequestBase):
     sylkrtc = FixedValueProperty('account-add')
     password = StringProperty(validator=LengthValidator(minimum=1, maximum=9999))
     display_name = StringProperty(optional=True)
     user_agent = StringProperty(optional=True)
 
 
 class AccountRemoveRequest(AccountRequestBase):
     sylkrtc = FixedValueProperty('account-remove')
 
 
 class AccountRegisterRequest(AccountRequestBase):
     sylkrtc = FixedValueProperty('account-register')
 
 
 class AccountUnregisterRequest(AccountRequestBase):
     sylkrtc = FixedValueProperty('account-unregister')
 
 
 class AccountDeviceTokenRequest(AccountRequestBase):
     sylkrtc = FixedValueProperty('account-devicetoken')
     old_token = StringProperty(optional=True)
     new_token = StringProperty(optional=True)
 
 
 # Session request models
 
 class SessionCreateRequest(SessionRequestBase):
     sylkrtc = FixedValueProperty('session-create')
     account = StringProperty(validator=URIValidator())
     uri = StringProperty(validator=URIValidator())
     sdp = StringProperty()
 
 
 class SessionAnswerRequest(SessionRequestBase):
     sylkrtc = FixedValueProperty('session-answer')
     sdp = StringProperty()
 
 
 class SessionTrickleRequest(SessionRequestBase):
     sylkrtc = FixedValueProperty('session-trickle')
     candidates = ArrayProperty(ICECandidates)
 
 
 class SessionTerminateRequest(SessionRequestBase):
     sylkrtc = FixedValueProperty('session-terminate')
 
 
 # Videoroom request models
 
 class VideoroomJoinRequest(VideoroomRequestBase):
     sylkrtc = FixedValueProperty('videoroom-join')
     account = StringProperty(validator=URIValidator())
     uri = StringProperty(validator=URIValidator())
     sdp = StringProperty()
 
 
 class VideoroomLeaveRequest(VideoroomRequestBase):
     sylkrtc = FixedValueProperty('videoroom-leave')
 
 
 class VideoroomConfigureRequest(VideoroomRequestBase):
     sylkrtc = FixedValueProperty('videoroom-configure')
     active_participants = ArrayProperty(VideoroomActiveParticipants)
 
 
 class VideoroomFeedAttachRequest(VideoroomRequestBase):
     sylkrtc = FixedValueProperty('videoroom-feed-attach')
     publisher = StringProperty()
     feed = StringProperty()
 
 
 class VideoroomFeedAnswerRequest(VideoroomRequestBase):
     sylkrtc = FixedValueProperty('videoroom-feed-answer')
     feed = StringProperty()
     sdp = StringProperty()
 
 
 class VideoroomFeedDetachRequest(VideoroomRequestBase):
     sylkrtc = FixedValueProperty('videoroom-feed-detach')
     feed = StringProperty()
 
 
 class VideoroomInviteRequest(VideoroomRequestBase):
     sylkrtc = FixedValueProperty('videoroom-invite')
     participants = ArrayProperty(URIList)
 
 
 class VideoroomSessionTrickleRequest(VideoroomRequestBase):
     sylkrtc = FixedValueProperty('videoroom-session-trickle')
     candidates = ArrayProperty(ICECandidates)
 
 
 class VideoroomSessionUpdateRequest(VideoroomRequestBase):
     sylkrtc = FixedValueProperty('videoroom-session-update')
     options = ObjectProperty(VideoroomSessionOptions)
 
 
 # SylkRTC request to model mapping
 
 class ProtocolError(Exception):
     pass
 
 
 class SylkRTCRequest(object):
     __classmap__ = {cls.sylkrtc.value: cls for cls in subclasses(SylkRTCRequestBase) if hasattr(cls, 'sylkrtc')}
 
     @classmethod
     def from_message(cls, message):
         try:
             request_type = message['sylkrtc']
         except KeyError:
             raise ProtocolError('could not get WebSocket message type')
         try:
             request_class = cls.__classmap__[request_type]
         except KeyError:
             raise ProtocolError('unknown WebSocket request: %s' % request_type)
         return request_class(**message)