diff --git a/sylk/applications/webrtcgateway/models/sylkrtc.py b/sylk/applications/webrtcgateway/models/sylkrtc.py
index 6f65314..295c7b8 100644
--- a/sylk/applications/webrtcgateway/models/sylkrtc.py
+++ b/sylk/applications/webrtcgateway/models/sylkrtc.py
@@ -1,201 +1,202 @@
 
 from jsonmodels import models, fields, errors, validators
 from sipsimple.core import SIPURI, SIPCoreError
 
 __all__ = ('AccountAddRequest', 'AccountRemoveRequest', 'AccountRegisterRequest', 'AccountUnregisterRequest',
            'SessionCreateRequest', 'SessionAnswerRequest', 'SessionTrickleRequest', 'SessionTerminateRequest',
            'AckResponse', 'ErrorResponse',
            'ReadyEvent')
 
 
 class FixedValueField(fields.BaseField):
     def __init__(self, value):
         super(FixedValueField, self).__init__(required=True)
         self.value = value
 
     def validate(self, value):
         if value != self.value:
             raise errors.ValidationError('field value should be {!r}'.format(self.value))
 
     # noinspection PyMethodOverriding
     def get_default_value(self):
         return self.value
 
 
+class LimitedChoiceField(fields.BaseField):
+    def __init__(self, values):
+        super(LimitedChoiceField, self).__init__(required=True)
+        self.values = set(values)
+
+    def validate(self, value):
+        if value not in self.values:
+            raise errors.ValidationError('field value should be one of: {!s}'.format(', '.join(repr(item) for item in sorted(self.values))))
+
+
 class URIValidator(object):
     @staticmethod
     def validate(value):
         if value.startswith(('sip:', 'sips:')):
             uri = value
         else:
             uri = 'sip:' + value
         try:
             SIPURI.parse(uri)
         except SIPCoreError:
             raise errors.ValidationError('invalid URI: %s' % value)
 
 
 class URIListValidator(object):
     @staticmethod
     def validate(values):
         for item in values:
             URIValidator.validate(item)
 
 
-class OptionsValidator(object):
-    def __init__(self, options):
-        self.options = options
-
-    def validate(self, value):
-        if value not in self.options:
-            raise errors.ValidationError('invalid option: %s' % value)
-
-
 # Base models
 
 class SylkRTCRequestBase(models.Base):
     transaction = fields.StringField(required=True)
 
 
 class SylkRTCResponseBase(models.Base):
     transaction = fields.StringField(required=True)
 
 
 # Miscellaneous models
 
 class AckResponse(SylkRTCResponseBase):
     sylkrtc = FixedValueField('ack')
 
 
 class ErrorResponse(SylkRTCResponseBase):
     sylkrtc = FixedValueField('error')
     error = fields.StringField(required=True)
 
 
 class ICECandidate(models.Base):
     candidate = fields.StringField(required=True)
     sdpMLineIndex = fields.IntField(required=True)
     sdpMid = fields.StringField(required=True)
 
 
 class ReadyEvent(models.Base):
     sylkrtc = FixedValueField('event')
     event = FixedValueField('ready')
 
 
 # Account models
 
 class AccountRequestBase(SylkRTCRequestBase):
     account = fields.StringField(required=True, validators=[URIValidator])
 
 
 class AccountAddRequest(AccountRequestBase):
     sylkrtc = FixedValueField('account-add')
     password = fields.StringField(required=True, validators=[validators.Length(minimum_value=1, maximum_value=9999)])
     display_name = fields.StringField(required=False)
     user_agent = fields.StringField(required=False)
 
 
 class AccountRemoveRequest(AccountRequestBase):
     sylkrtc = FixedValueField('account-remove')
 
 
 class AccountRegisterRequest(AccountRequestBase):
     sylkrtc = FixedValueField('account-register')
 
 
 class AccountUnregisterRequest(AccountRequestBase):
     sylkrtc = FixedValueField('account-unregister')
 
 
 class AccountDeviceTokenRequest(AccountRequestBase):
     sylkrtc = FixedValueField('account-devicetoken')
     old_token = fields.StringField(required=False)
     new_token = fields.StringField(required=False)
 
 
 # Session models
 
 class SessionRequestBase(SylkRTCRequestBase):
     session = fields.StringField(required=True)
 
 
 class SessionCreateRequest(SessionRequestBase):
     sylkrtc = FixedValueField('session-create')
     account = fields.StringField(required=True, validators=[URIValidator])
     uri = fields.StringField(required=True, validators=[URIValidator])
     sdp = fields.StringField(required=True)
 
 
 class SessionAnswerRequest(SessionRequestBase):
     sylkrtc = FixedValueField('session-answer')
     sdp = fields.StringField(required=True)
 
 
 class SessionTrickleRequest(SessionRequestBase):
     sylkrtc = FixedValueField('session-trickle')
     candidates = fields.ListField([ICECandidate])
 
 
 class SessionTerminateRequest(SessionRequestBase):
     sylkrtc = FixedValueField('session-terminate')
 
 
 # VideoRoom models
 
 class VideoRoomRequestBase(SylkRTCRequestBase):
     session = fields.StringField(required=True)
 
 
 class VideoRoomJoinRequest(VideoRoomRequestBase):
     sylkrtc = FixedValueField('videoroom-join')
     account = fields.StringField(required=True, validators=[URIValidator])
     uri = fields.StringField(required=True, validators=[URIValidator])
     sdp = fields.StringField(required=True)
 
 
 class VideoRoomControlTrickleOptions(models.Base):
     # ID for the subscriber session, if specified, otherwise the publisher is considered
     session = fields.StringField(required=False)
     candidates = fields.ListField([ICECandidate])
 
 
 class VideoRoomControlUpdateOptions(models.Base):
     audio = fields.BoolField(required=False)
     video = fields.BoolField(required=False)
     bitrate = fields.IntField(required=False)
 
 
 class VideoRoomControlFeedAttachOptions(models.Base):
     session = fields.StringField(required=True)
     publisher = fields.StringField(required=True)
 
 
 class VideoRoomControlFeedAnswerOptions(models.Base):
     session = fields.StringField(required=True)
     sdp = fields.StringField(required=True)
 
 
 class VideoRoomControlFeedDetachOptions(models.Base):
     session = fields.StringField(required=True)
 
 
 class VideoRoomControlInviteParticipantsOptions(models.Base):
     participants = fields.ListField([str, unicode], validators=[URIListValidator])
 
 
 class VideoRoomControlRequest(VideoRoomRequestBase):
     sylkrtc = FixedValueField('videoroom-ctl')
-    option = fields.StringField(required=True, validators=[OptionsValidator(['trickle', 'update', 'feed-attach', 'feed-answer', 'feed-detach', 'invite-participants'])])
+    option = LimitedChoiceField({'feed-attach', 'feed-answer', 'feed-detach', 'invite-participants', 'trickle', 'update'})
 
     # all other options should have optional fields below, and the application needs to do a little validation
     trickle = fields.EmbeddedField(VideoRoomControlTrickleOptions, required=False)
     update = fields.EmbeddedField(VideoRoomControlUpdateOptions, required=False)
     feed_attach = fields.EmbeddedField(VideoRoomControlFeedAttachOptions, required=False)
     feed_answer = fields.EmbeddedField(VideoRoomControlFeedAnswerOptions, required=False)
     feed_detach = fields.EmbeddedField(VideoRoomControlFeedDetachOptions, required=False)
     invite_participants = fields.EmbeddedField(VideoRoomControlInviteParticipantsOptions, required=False)
 
 
 class VideoRoomTerminateRequest(VideoRoomRequestBase):
     sylkrtc = FixedValueField('videoroom-terminate')