:param request: OAuthlib request.
        :type request: oauthlib.common.Request

def validate_token_request(self, request):
        """
        :param request: OAuthlib request.
        :type request: oauthlib.common.Request
        """
        # REQUIRED. Value MUST be set to "refresh_token".
        if request.grant_type != 'refresh_token':
            raise errors.UnsupportedGrantTypeError(request=request)

        for validator in self.custom_validators.pre_token:
            validator(request)

        if request.refresh_token is None:
            raise errors.InvalidRequestError(
                description='Missing refresh token parameter.',
                request=request)

        # Because refresh tokens are typically long-lasting credentials used to
        # request additional access tokens, the refresh token is bound to the
        # client to which it was issued.  If the client type is confidential or
        # the client was issued client credentials (or assigned other
        # authentication requirements), the client MUST authenticate with the
        # authorization server as described in Section 3.2.1.
        # https://tools.ietf.org/html/rfc6749#section-3.2.1
        if self.request_validator.client_authentication_required(request):
            log.debug('Authenticating client, %r.', request)
            if not self.request_validator.authenticate_client(request):
                log.debug('Invalid client (%r), denying access.', request)
                raise errors.InvalidClientError(request=request)
        elif not self.request_validator.authenticate_client_id(request.client_id, request):
            log.debug('Client authentication failed, %r.', request)
            raise errors.InvalidClientError(request=request)

        # Ensure client is authorized use of this grant type
        self.validate_grant_type(request)

        # REQUIRED. The refresh token issued to the client.
        log.debug('Validating refresh token %s for client %r.',
                  request.refresh_token, request.client)
        if not self.request_validator.validate_refresh_token(
                request.refresh_token, request.client, request):
            log.debug('Invalid refresh token, %s, for client %r.',
                      request.refresh_token, request.client)
            raise errors.InvalidGrantError(request=request)

        original_scopes = utils.scope_to_list(
            self.request_validator.get_original_scopes(
                request.refresh_token, request))

        if request.scope:
            request.scopes = utils.scope_to_list(request.scope)
            if (not all((s in original_scopes for s in request.scopes))
                and not self.request_validator.is_within_original_scope(
                    request.scopes, request.refresh_token, request)):
                log.debug('Refresh token %s lack requested scopes, %r.',
                          request.refresh_token, request.scopes)
                raise errors.InvalidScopeError(request=request)
        else:
            request.scopes = original_scopes

        for validator in self.custom_validators.post_token:
            validator(request)