xref: /btstack/src/ble/att_db.c (revision eb6072adf6784b3d9056e8dc659130816e7ed950)

/*
 * Copyright (C) 2014 BlueKitchen GmbH
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 *
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. Neither the name of the copyright holders nor the names of
 *    contributors may be used to endorse or promote products derived
 *    from this software without specific prior written permission.
 * 4. Any redistribution, use, or modification is done solely for
 *    personal benefit and not for any commercial purpose or for
 *    monetary gain.
 *
 * THIS SOFTWARE IS PROVIDED BY BLUEKITCHEN GMBH AND CONTRIBUTORS
 * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
 * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL MATTHIAS
 * RINGWALD OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
 * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
 * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
 * OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
 * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
 * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF
 * THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 *
 * Please inquire about commercial licensing options at
 * [email protected]
 *
 */

#define __BTSTACK_FILE__ "att_db.c"


#include <stdio.h>
#include <string.h>

#include "ble/att_db.h"
#include "ble/core.h"
#include "bluetooth.h"
#include "btstack_debug.h"
#include "btstack_util.h"

// Buetooth Base UUID 00000000-0000-1000-8000-00805F9B34FB in little endian
static const uint8_t bluetooth_base_uuid[] = { 0xfb, 0x34, 0x9b, 0x5f, 0x80, 0x00, 0x00, 0x80, 0x00, 0x10, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00 };


static int is_Bluetooth_Base_UUID(uint8_t const *uuid){
    if (memcmp(&uuid[0],  &bluetooth_base_uuid[0], 12)) return 0;
    if (memcmp(&uuid[14], &bluetooth_base_uuid[14], 2)) return 0;
    return 1;

}

static uint16_t uuid16_from_uuid(uint16_t uuid_len, uint8_t * uuid){
    if (uuid_len == 2) return little_endian_read_16(uuid, 0);
    if (!is_Bluetooth_Base_UUID(uuid)) return 0;
    return little_endian_read_16(uuid, 12);
}

// ATT Database
static uint8_t const * att_db = NULL;
static att_read_callback_t  att_read_callback  = NULL;
static att_write_callback_t att_write_callback = NULL;
static uint8_t  att_prepare_write_error_code   = 0;
static uint16_t att_prepare_write_error_handle = 0x0000;

static btstack_linked_list_t service_handlers;

// new java-style iterator
typedef struct att_iterator {
    // private
    uint8_t const * att_ptr;
    // public
    uint16_t size;
    uint16_t flags;
    uint16_t handle;
    uint8_t  const * uuid;
    uint16_t value_len;
    uint8_t  const * value;
} att_iterator_t;

static void att_iterator_init(att_iterator_t *it){
    it->att_ptr = att_db;
}

static int att_iterator_has_next(att_iterator_t *it){
    return it->att_ptr != NULL;
}

static void att_iterator_fetch_next(att_iterator_t *it){
    it->size   = little_endian_read_16(it->att_ptr, 0);
    if (it->size == 0){
        it->flags = 0;
        it->handle = 0;
        it->uuid = NULL;
        it->value_len = 0;
        it->value = NULL;
        it->att_ptr = NULL;
        return;
    }
    it->flags  = little_endian_read_16(it->att_ptr, 2);
    it->handle = little_endian_read_16(it->att_ptr, 4);
    it->uuid   = &it->att_ptr[6];
    // handle 128 bit UUIDs
    if (it->flags & ATT_PROPERTY_UUID128){
        it->value_len = it->size - 22;
        it->value  = &it->att_ptr[22];
    } else {
        it->value_len = it->size - 8;
        it->value  = &it->att_ptr[8];
    }
    // advance AFTER setting values
    it->att_ptr += it->size;
}

static int att_iterator_match_uuid16(att_iterator_t *it, uint16_t uuid){
    if (it->handle == 0) return 0;
    if (it->flags & ATT_PROPERTY_UUID128){
        if (!is_Bluetooth_Base_UUID(it->uuid)) return 0;
        return little_endian_read_16(it->uuid, 12) == uuid;
    }
    return little_endian_read_16(it->uuid, 0)  == uuid;
}

static int att_iterator_match_uuid(att_iterator_t *it, uint8_t *uuid, uint16_t uuid_len){
    if (it->handle == 0) return 0;
    // input: UUID16
    if (uuid_len == 2) {
        return att_iterator_match_uuid16(it, little_endian_read_16(uuid, 0));
    }
    // input and db: UUID128
    if (it->flags & ATT_PROPERTY_UUID128){
        return memcmp(it->uuid, uuid, 16) == 0;
    }
    // input: UUID128, db: UUID16
    if (!is_Bluetooth_Base_UUID(uuid)) return 0;
    return little_endian_read_16(uuid, 12) == little_endian_read_16(it->uuid, 0);
}


static int att_find_handle(att_iterator_t *it, uint16_t handle){
    if (handle == 0) return 0;
    att_iterator_init(it);
    while (att_iterator_has_next(it)){
        att_iterator_fetch_next(it);
        if (it->handle != handle) continue;
        return 1;
    }
    return 0;
}

static att_service_handler_t * att_service_handler_for_handle(uint16_t handle){
    btstack_linked_list_iterator_t it;
    btstack_linked_list_iterator_init(&it, &service_handlers);
    while (btstack_linked_list_iterator_has_next(&it)){
        att_service_handler_t * handler = (att_service_handler_t*) btstack_linked_list_iterator_next(&it);
        if (handler->start_handle > handle) continue;
        if (handler->end_handle   < handle) continue;
        return handler;
    }
    return NULL;
}

static att_read_callback_t att_read_callback_for_handle(uint16_t handle){
    att_service_handler_t * handler = att_service_handler_for_handle(handle);
    if (handler) return handler->read_callback;
    return att_read_callback;
}

static att_write_callback_t att_write_callback_for_handle(uint16_t handle){
    att_service_handler_t * handler = att_service_handler_for_handle(handle);
    if (handler) return handler->write_callback;
    return att_write_callback;
}

// experimental client API
uint16_t att_uuid_for_handle(uint16_t attribute_handle){
    att_iterator_t it;
    int ok = att_find_handle(&it, attribute_handle);
    if (!ok) return 0;
    if (it.flags & ATT_PROPERTY_UUID128) return 0;
    return little_endian_read_16(it.uuid, 0);
}
// end of client API

static void att_update_value_len(att_iterator_t *it, hci_con_handle_t con_handle){
    if ((it->flags & ATT_PROPERTY_DYNAMIC) == 0) return;
    att_read_callback_t callback = att_read_callback_for_handle(it->handle);
    if (!callback) return;
    it->value_len = (*callback)(con_handle, it->handle, 0, NULL, 0);
    return;
}

// copy attribute value from offset into buffer with given size
static int att_copy_value(att_iterator_t *it, uint16_t offset, uint8_t * buffer, uint16_t buffer_size, hci_con_handle_t con_handle){

    // DYNAMIC
    if (it->flags & ATT_PROPERTY_DYNAMIC){
        att_read_callback_t callback = att_read_callback_for_handle(it->handle);
        if (!callback) return 0;
        return (*callback)(con_handle, it->handle, offset, buffer, buffer_size);
    }

    // STATIC
    uint16_t bytes_to_copy = it->value_len - offset;
    if (bytes_to_copy > buffer_size){
        bytes_to_copy = buffer_size;
    }
    memcpy(buffer, it->value, bytes_to_copy);
    return bytes_to_copy;
}

void att_set_db(uint8_t const * db){
    att_db = db;
}

void att_set_read_callback(att_read_callback_t callback){
    att_read_callback = callback;
}

void att_set_write_callback(att_write_callback_t callback){
    att_write_callback = callback;
}

void att_dump_attributes(void){
    att_iterator_t it;
    att_iterator_init(&it);
    uint8_t uuid128[16];
    while (att_iterator_has_next(&it)){
        att_iterator_fetch_next(&it);
        if (it.handle == 0) {
            log_info("Handle: END");
            return;
        }
        log_info("Handle: 0x%04x, flags: 0x%04x, uuid: ", it.handle, it.flags);
        if (it.flags & ATT_PROPERTY_UUID128){
            reverse_128(it.uuid, uuid128);
            log_info("%s", uuid128_to_str(uuid128));
        } else {
            log_info("%04x", little_endian_read_16(it.uuid, 0));
        }
        log_info(", value_len: %u, value: ", it.value_len);
        log_info_hexdump(it.value, it.value_len);
    }
}

static void att_prepare_write_reset(void){
    att_prepare_write_error_code = 0;
    att_prepare_write_error_handle = 0x0000;
}

static void att_prepare_write_update_errors(uint8_t error_code, uint16_t handle){
    // first ATT_ERROR_INVALID_ATTRIBUTE_VALUE_LENGTH has highest priority
    if (error_code == ATT_ERROR_INVALID_ATTRIBUTE_VALUE_LENGTH && error_code != att_prepare_write_error_code){
        att_prepare_write_error_code = error_code;
        att_prepare_write_error_handle = handle;
        return;
    }
    // first ATT_ERROR_INVALID_OFFSET is next
    if (error_code == ATT_ERROR_INVALID_OFFSET && att_prepare_write_error_code == 0){
        att_prepare_write_error_code = error_code;
        att_prepare_write_error_handle = handle;
        return;
    }
}

static uint16_t setup_error(uint8_t * response_buffer, uint16_t request, uint16_t handle, uint8_t error_code){
    response_buffer[0] = ATT_ERROR_RESPONSE;
    response_buffer[1] = request;
    little_endian_store_16(response_buffer, 2, handle);
    response_buffer[4] = error_code;
    return 5;
}

static inline uint16_t setup_error_read_not_permitted(uint8_t * response_buffer, uint16_t request, uint16_t start_handle){
    return setup_error(response_buffer, request, start_handle, ATT_ERROR_READ_NOT_PERMITTED);
}

static inline uint16_t setup_error_write_not_permitted(uint8_t * response_buffer, uint16_t request, uint16_t start_handle){
    return setup_error(response_buffer, request, start_handle, ATT_ERROR_WRITE_NOT_PERMITTED);
}

static inline uint16_t setup_error_atribute_not_found(uint8_t * response_buffer, uint16_t request, uint16_t start_handle){
    return setup_error(response_buffer, request, start_handle, ATT_ERROR_ATTRIBUTE_NOT_FOUND);
}

static inline uint16_t setup_error_invalid_handle(uint8_t * response_buffer, uint16_t request, uint16_t handle){
    return setup_error(response_buffer, request, handle, ATT_ERROR_INVALID_HANDLE);
}

static inline uint16_t setup_error_invalid_offset(uint8_t * response_buffer, uint16_t request, uint16_t handle){
    return setup_error(response_buffer, request, handle, ATT_ERROR_INVALID_OFFSET);
}

static uint8_t att_validate_security(att_connection_t * att_connection, att_iterator_t * it){
    int required_encryption_size = it->flags >> 12;
    if (required_encryption_size) required_encryption_size++;   // store -1 to fit into 4 bit
    log_info("att_validate_security. flags 0x%04x - req enc size %u, authorized %u, authenticated %u, encryption_key_size %u",
        it->flags, required_encryption_size, att_connection->authorized, att_connection->authenticated, att_connection->encryption_key_size);
    if ((it->flags & ATT_PROPERTY_AUTHENTICATION_REQUIRED) && att_connection->authenticated == 0) {
        return ATT_ERROR_INSUFFICIENT_AUTHENTICATION;
    }
    if ((it->flags & ATT_PROPERTY_AUTHORIZATION_REQUIRED) && att_connection->authorized == 0) {
        return ATT_ERROR_INSUFFICIENT_AUTHORIZATION;
    }
    if (required_encryption_size > 0 && att_connection->encryption_key_size == 0){
        return ATT_ERROR_INSUFFICIENT_ENCRYPTION;
    }
    if (required_encryption_size > att_connection->encryption_key_size){
        return ATT_ERROR_INSUFFICIENT_ENCRYPTION_KEY_SIZE;
    }
    return 0;
}

//
// MARK: ATT_EXCHANGE_MTU_REQUEST
//
static uint16_t handle_exchange_mtu_request(att_connection_t * att_connection, uint8_t * request_buffer,  uint16_t request_len,
                                         uint8_t * response_buffer){

    UNUSED(request_len);

    uint16_t client_rx_mtu = little_endian_read_16(request_buffer, 1);

    // find min(local max mtu, remote mtu) and use as mtu for this connection
    if (client_rx_mtu < att_connection->max_mtu){
        att_connection->mtu = client_rx_mtu;
    } else {
        att_connection->mtu = att_connection->max_mtu;
    }

    response_buffer[0] = ATT_EXCHANGE_MTU_RESPONSE;
    little_endian_store_16(response_buffer, 1, att_connection->mtu);
    return 3;
}


//
// MARK: ATT_FIND_INFORMATION_REQUEST
//
// TODO: handle other types then GATT_PRIMARY_SERVICE_UUID and GATT_SECONDARY_SERVICE_UUID
//
static uint16_t handle_find_information_request2(att_connection_t * att_connection, uint8_t * response_buffer, uint16_t response_buffer_size,
                                           uint16_t start_handle, uint16_t end_handle){

    UNUSED(att_connection);

    log_info("ATT_FIND_INFORMATION_REQUEST: from %04X to %04X", start_handle, end_handle);
    uint8_t request_type = ATT_FIND_INFORMATION_REQUEST;

    if (start_handle > end_handle || start_handle == 0){
        return setup_error_invalid_handle(response_buffer, request_type, start_handle);
    }

    uint16_t offset   = 1;
    uint16_t uuid_len = 0;

    att_iterator_t it;
    att_iterator_init(&it);
    while (att_iterator_has_next(&it)){
        att_iterator_fetch_next(&it);
        if (!it.handle) break;
        if (it.handle > end_handle) break;
        if (it.handle < start_handle) continue;

        // log_info("Handle 0x%04x", it.handle);

        uint16_t this_uuid_len = (it.flags & ATT_PROPERTY_UUID128) ? 16 : 2;

        // check if value has same len as last one if not first result
        if (offset > 1){
            if (this_uuid_len != uuid_len) {
                break;
            }
        }

        // first
        if (offset == 1) {
            uuid_len = this_uuid_len;
            // set format field
            response_buffer[offset] = (it.flags & ATT_PROPERTY_UUID128) ? 0x02 : 0x01;
            offset++;
        }

        // space?
        if (offset + 2 + uuid_len > response_buffer_size) break;

        // store
        little_endian_store_16(response_buffer, offset, it.handle);
        offset += 2;

        memcpy(response_buffer + offset, it.uuid, uuid_len);
        offset += uuid_len;
    }

    if (offset == 1){
        return setup_error_atribute_not_found(response_buffer, request_type, start_handle);
    }

    response_buffer[0] = ATT_FIND_INFORMATION_REPLY;
    return offset;
}

static uint16_t handle_find_information_request(att_connection_t * att_connection, uint8_t * request_buffer,  uint16_t request_len,
                                         uint8_t * response_buffer, uint16_t response_buffer_size){
    UNUSED(request_len);
    return handle_find_information_request2(att_connection, response_buffer, response_buffer_size, little_endian_read_16(request_buffer, 1), little_endian_read_16(request_buffer, 3));
}

//
// MARK: ATT_FIND_BY_TYPE_VALUE
//
// "Only attributes with attribute handles between and including the Starting Handle parameter
// and the Ending Handle parameter that match the requested attri- bute type and the attribute
// value that have sufficient permissions to allow reading will be returned" -> (1)
//
// TODO: handle other types then GATT_PRIMARY_SERVICE_UUID and GATT_SECONDARY_SERVICE_UUID
//
// NOTE: doesn't handle DYNAMIC values
// NOTE: only supports 16 bit UUIDs
//
static uint16_t handle_find_by_type_value_request2(att_connection_t * att_connection, uint8_t * response_buffer, uint16_t response_buffer_size,
                                           uint16_t start_handle, uint16_t end_handle,
                                           uint16_t attribute_type, uint16_t attribute_len, uint8_t* attribute_value){

    UNUSED(att_connection);

    log_info("ATT_FIND_BY_TYPE_VALUE_REQUEST: from %04X to %04X, type %04X, value: ", start_handle, end_handle, attribute_type);
    log_info_hexdump(attribute_value, attribute_len);
    uint8_t request_type = ATT_FIND_BY_TYPE_VALUE_REQUEST;

    if (start_handle > end_handle || start_handle == 0){
        return setup_error_invalid_handle(response_buffer, request_type, start_handle);
    }

    uint16_t offset      = 1;
    uint16_t in_group    = 0;
    uint16_t prev_handle = 0;

    att_iterator_t it;
    att_iterator_init(&it);
    while (att_iterator_has_next(&it)){
        att_iterator_fetch_next(&it);

        if (it.handle && it.handle < start_handle) continue;
        if (it.handle > end_handle) break;  // (1)

        // close current tag, if within a group and a new service definition starts or we reach end of att db
        if (in_group &&
            (it.handle == 0 || att_iterator_match_uuid16(&it, GATT_PRIMARY_SERVICE_UUID) || att_iterator_match_uuid16(&it, GATT_SECONDARY_SERVICE_UUID))){

            log_info("End of group, handle 0x%04x", prev_handle);
            little_endian_store_16(response_buffer, offset, prev_handle);
            offset += 2;
            in_group = 0;

            // check if space for another handle pair available
            if (offset + 4 > response_buffer_size){
                break;
            }
        }

        // keep track of previous handle
        prev_handle = it.handle;

        // does current attribute match
        if (it.handle && att_iterator_match_uuid16(&it, attribute_type) && attribute_len == it.value_len && memcmp(attribute_value, it.value, it.value_len) == 0){
            log_info("Begin of group, handle 0x%04x", it.handle);
            little_endian_store_16(response_buffer, offset, it.handle);
            offset += 2;
            in_group = 1;
        }
    }

    if (offset == 1){
        return setup_error_atribute_not_found(response_buffer, request_type, start_handle);
    }

    response_buffer[0] = ATT_FIND_BY_TYPE_VALUE_RESPONSE;
    return offset;
}

static uint16_t handle_find_by_type_value_request(att_connection_t * att_connection, uint8_t * request_buffer,  uint16_t request_len,
                                           uint8_t * response_buffer, uint16_t response_buffer_size){
    int attribute_len = request_len - 7;
    return handle_find_by_type_value_request2(att_connection, response_buffer, response_buffer_size, little_endian_read_16(request_buffer, 1),
                                              little_endian_read_16(request_buffer, 3), little_endian_read_16(request_buffer, 5), attribute_len, &request_buffer[7]);
}

//
// MARK: ATT_READ_BY_TYPE_REQUEST
//
static uint16_t handle_read_by_type_request2(att_connection_t * att_connection, uint8_t * response_buffer, uint16_t response_buffer_size,
                                      uint16_t start_handle, uint16_t end_handle,
                                      uint16_t attribute_type_len, uint8_t * attribute_type){

    log_info("ATT_READ_BY_TYPE_REQUEST: from %04X to %04X, type: ", start_handle, end_handle);
    log_info_hexdump(attribute_type, attribute_type_len);
    uint8_t request_type = ATT_READ_BY_TYPE_REQUEST;

    if (start_handle > end_handle || start_handle == 0){
        return setup_error_invalid_handle(response_buffer, request_type, start_handle);
    }

    uint16_t offset   = 1;
    uint16_t pair_len = 0;

    att_iterator_t it;
    att_iterator_init(&it);
    uint8_t error_code = 0;
    uint16_t first_matching_but_unreadable_handle = 0;

    while (att_iterator_has_next(&it)){
        att_iterator_fetch_next(&it);

        if (!it.handle) break;
        if (it.handle < start_handle) continue;
        if (it.handle > end_handle) break;  // (1)

        // does current attribute match
        if (!att_iterator_match_uuid(&it, attribute_type, attribute_type_len)) continue;

        // skip handles that cannot be read but rembember that there has been at least one
        if ((it.flags & ATT_PROPERTY_READ) == 0) {
            if (first_matching_but_unreadable_handle == 0) {
                first_matching_but_unreadable_handle = it.handle;
            }
            continue;
        }

        // check security requirements
        if ((it.flags & ATT_DB_FLAGS_READ_WITHOUT_AUTHENTICATION) == 0){
            error_code = att_validate_security(att_connection, &it);
            if (error_code) break;
        }

        att_update_value_len(&it, att_connection->con_handle);

        // check if value has same len as last one
        uint16_t this_pair_len = 2 + it.value_len;
        if (offset > 1){
            if (pair_len != this_pair_len) {
                break;
            }
        }

        // first
        if (offset == 1) {
            pair_len = this_pair_len;
            response_buffer[offset] = pair_len;
            offset++;
        }

        // space?
        if (offset + pair_len > response_buffer_size) {
            if (offset > 2) break;
            it.value_len = response_buffer_size - 4;
            response_buffer[1] = 2 + it.value_len;
        }

        // store
        little_endian_store_16(response_buffer, offset, it.handle);
        offset += 2;
        uint16_t bytes_copied = att_copy_value(&it, 0, response_buffer + offset, it.value_len, att_connection->con_handle);
        offset += bytes_copied;
    }

    // at least one attribute could be read
    if (offset > 1){
        response_buffer[0] = ATT_READ_BY_TYPE_RESPONSE;
        return offset;
    }

    // first attribute had an error
    if (error_code){
        return setup_error(response_buffer, request_type, start_handle, error_code);
    }

    // no other errors, but all found attributes had been non-readable
    if (first_matching_but_unreadable_handle){
        return setup_error_read_not_permitted(response_buffer, request_type, first_matching_but_unreadable_handle);
    }

    // attribute not found
    return setup_error_atribute_not_found(response_buffer, request_type, start_handle);
}

static uint16_t handle_read_by_type_request(att_connection_t * att_connection, uint8_t * request_buffer,  uint16_t request_len,
                                     uint8_t * response_buffer, uint16_t response_buffer_size){
    int attribute_type_len;
    if (request_len <= 7){
        attribute_type_len = 2;
    } else {
        attribute_type_len = 16;
    }
    return handle_read_by_type_request2(att_connection, response_buffer, response_buffer_size, little_endian_read_16(request_buffer, 1), little_endian_read_16(request_buffer, 3), attribute_type_len, &request_buffer[5]);
}

//
// MARK: ATT_READ_BY_TYPE_REQUEST
//
static uint16_t handle_read_request2(att_connection_t * att_connection, uint8_t * response_buffer, uint16_t response_buffer_size, uint16_t handle){

    log_info("ATT_READ_REQUEST: handle %04x", handle);
    uint8_t request_type = ATT_READ_REQUEST;

    att_iterator_t it;
    int ok = att_find_handle(&it, handle);
    if (!ok){
        return setup_error_invalid_handle(response_buffer, request_type, handle);
    }

    // check if handle can be read
    if ((it.flags & ATT_PROPERTY_READ) == 0) {
        return setup_error_read_not_permitted(response_buffer, request_type, handle);
    }

    // check security requirements
    if ((it.flags & ATT_DB_FLAGS_READ_WITHOUT_AUTHENTICATION) == 0){
        uint8_t error_code = att_validate_security(att_connection, &it);
        if (error_code) {
            return setup_error(response_buffer, request_type, handle, error_code);
        }
    }

    att_update_value_len(&it, att_connection->con_handle);

    uint16_t offset   = 1;
    // limit data
    if (offset + it.value_len > response_buffer_size) {
        it.value_len = response_buffer_size - 1;
    }

    // store
    uint16_t bytes_copied = att_copy_value(&it, 0, response_buffer + offset, it.value_len, att_connection->con_handle);
    offset += bytes_copied;

    response_buffer[0] = ATT_READ_RESPONSE;
    return offset;
}

static uint16_t handle_read_request(att_connection_t * att_connection, uint8_t * request_buffer,  uint16_t request_len,
                             uint8_t * response_buffer, uint16_t response_buffer_size){
    UNUSED(request_len);
    return handle_read_request2(att_connection, response_buffer, response_buffer_size, little_endian_read_16(request_buffer, 1));
}

//
// MARK: ATT_READ_BLOB_REQUEST 0x0c
//
static uint16_t handle_read_blob_request2(att_connection_t * att_connection, uint8_t * response_buffer, uint16_t response_buffer_size, uint16_t handle, uint16_t value_offset){
    log_info("ATT_READ_BLOB_REQUEST: handle %04x, offset %u", handle, value_offset);
    uint8_t request_type = ATT_READ_BLOB_REQUEST;

    att_iterator_t it;
    int ok = att_find_handle(&it, handle);
    if (!ok){
        return setup_error_invalid_handle(response_buffer, request_type, handle);
    }

    // check if handle can be read
    if ((it.flags & ATT_PROPERTY_READ) == 0) {
        return setup_error_read_not_permitted(response_buffer, request_type, handle);
    }

    // check security requirements
    if ((it.flags & ATT_DB_FLAGS_READ_WITHOUT_AUTHENTICATION) == 0){
        uint8_t error_code = att_validate_security(att_connection, &it);
        if (error_code) {
            return setup_error(response_buffer, request_type, handle, error_code);
        }
    }

    att_update_value_len(&it, att_connection->con_handle);

    if (value_offset > it.value_len){
        return setup_error_invalid_offset(response_buffer, request_type, handle);
    }

    // limit data
    uint16_t offset   = 1;
    if (offset + it.value_len - value_offset > response_buffer_size) {
        it.value_len = response_buffer_size - 1 + value_offset;
    }

    // store
    uint16_t bytes_copied = att_copy_value(&it, value_offset, response_buffer + offset, it.value_len - value_offset, att_connection->con_handle);
    offset += bytes_copied;

    response_buffer[0] = ATT_READ_BLOB_RESPONSE;
    return offset;
}

static uint16_t handle_read_blob_request(att_connection_t * att_connection, uint8_t * request_buffer,  uint16_t request_len,
                                  uint8_t * response_buffer, uint16_t response_buffer_size){
    UNUSED(request_len);
    return handle_read_blob_request2(att_connection, response_buffer, response_buffer_size, little_endian_read_16(request_buffer, 1), little_endian_read_16(request_buffer, 3));
}

//
// MARK: ATT_READ_MULTIPLE_REQUEST 0x0e
//
static uint16_t handle_read_multiple_request2(att_connection_t * att_connection, uint8_t * response_buffer, uint16_t response_buffer_size, uint16_t num_handles, uint8_t * handles){
    log_info("ATT_READ_MULTIPLE_REQUEST: num handles %u", num_handles);
    uint8_t request_type = ATT_READ_MULTIPLE_REQUEST;

    // TODO: figure out which error to respond with
    // if (num_handles < 2){
    //     return setup_error(response_buffer, ATT_READ_MULTIPLE_REQUEST, handle, ???);
    // }

    uint16_t offset   = 1;

    int i;
    uint8_t error_code = 0;
    uint16_t handle = 0;
    for (i=0;i<num_handles;i++){
        handle = little_endian_read_16(handles, i << 1);

        if (handle == 0){
            return setup_error_invalid_handle(response_buffer, request_type, handle);
        }

        att_iterator_t it;

        int ok = att_find_handle(&it, handle);
        if (!ok){
            return setup_error_invalid_handle(response_buffer, request_type, handle);
        }

        // check if handle can be read
        if ((it.flags & ATT_PROPERTY_READ) == 0) {
            error_code = ATT_ERROR_READ_NOT_PERMITTED;
            break;
        }

        // check security requirements
        if ((it.flags & ATT_DB_FLAGS_READ_WITHOUT_AUTHENTICATION) == 0){
            error_code = att_validate_security(att_connection, &it);
            if (error_code) break;
        }
        att_update_value_len(&it, att_connection->con_handle);

        // limit data
        if (offset + it.value_len > response_buffer_size) {
            it.value_len = response_buffer_size - 1;
        }

        // store
        uint16_t bytes_copied = att_copy_value(&it, 0, response_buffer + offset, it.value_len, att_connection->con_handle);
        offset += bytes_copied;
    }

    if (error_code){
        return setup_error(response_buffer, request_type, handle, error_code);
    }

    response_buffer[0] = ATT_READ_MULTIPLE_RESPONSE;
    return offset;
}
static uint16_t handle_read_multiple_request(att_connection_t * att_connection, uint8_t * request_buffer,  uint16_t request_len,
                                      uint8_t * response_buffer, uint16_t response_buffer_size){
    int num_handles = (request_len - 1) >> 1;
    return handle_read_multiple_request2(att_connection, response_buffer, response_buffer_size, num_handles, &request_buffer[1]);
}

//
// MARK: ATT_READ_BY_GROUP_TYPE_REQUEST 0x10
//
// Only handles GATT_PRIMARY_SERVICE_UUID and GATT_SECONDARY_SERVICE_UUID
// Core v4.0, vol 3, part g, 2.5.3
// "The «Primary Service» and «Secondary Service» grouping types may be used in the Read By Group Type Request.
//  The «Characteristic» grouping type shall not be used in the ATT Read By Group Type Request."
//
// NOTE: doesn't handle DYNAMIC values
//
// NOTE: we don't check for security as PRIMARY and SECONDAY SERVICE definition shouldn't be protected
// Core 4.0, vol 3, part g, 8.1
// "The list of services and characteristics that a device supports is not considered private or
//  confidential information, and therefore the Service and Characteristic Discovery procedures
//  shall always be permitted. "
//
static uint16_t handle_read_by_group_type_request2(att_connection_t * att_connection, uint8_t * response_buffer, uint16_t response_buffer_size,
                                            uint16_t start_handle, uint16_t end_handle,
                                            uint16_t attribute_type_len, uint8_t * attribute_type){

    UNUSED(att_connection);

    log_info("ATT_READ_BY_GROUP_TYPE_REQUEST: from %04X to %04X, buffer size %u, type: ", start_handle, end_handle, response_buffer_size);
    log_info_hexdump(attribute_type, attribute_type_len);
    uint8_t request_type = ATT_READ_BY_GROUP_TYPE_REQUEST;

    if (start_handle > end_handle || start_handle == 0){
        return setup_error_invalid_handle(response_buffer, request_type, start_handle);
    }

    // assert UUID is primary or secondary service uuid
    uint16_t uuid16 = uuid16_from_uuid(attribute_type_len, attribute_type);
    if (uuid16 != GATT_PRIMARY_SERVICE_UUID && uuid16 != GATT_SECONDARY_SERVICE_UUID){
        return setup_error(response_buffer, request_type, start_handle, ATT_ERROR_UNSUPPORTED_GROUP_TYPE);
    }

    uint16_t offset   = 1;
    uint16_t pair_len = 0;
    uint16_t in_group = 0;
    uint16_t group_start_handle = 0;
    uint8_t const * group_start_value = NULL;
    uint16_t prev_handle = 0;

    att_iterator_t it;
    att_iterator_init(&it);
    while (att_iterator_has_next(&it)){
        att_iterator_fetch_next(&it);

        if (it.handle && it.handle < start_handle) continue;
        if (it.handle > end_handle) break;  // (1)

        // log_info("Handle 0x%04x", it.handle);

        // close current tag, if within a group and a new service definition starts or we reach end of att db
        if (in_group &&
            (it.handle == 0 || att_iterator_match_uuid16(&it, GATT_PRIMARY_SERVICE_UUID) || att_iterator_match_uuid16(&it, GATT_SECONDARY_SERVICE_UUID))){
            // log_info("End of group, handle 0x%04x, val_len: %u", prev_handle, pair_len - 4);

            little_endian_store_16(response_buffer, offset, group_start_handle);
            offset += 2;
            little_endian_store_16(response_buffer, offset, prev_handle);
            offset += 2;
            memcpy(response_buffer + offset, group_start_value, pair_len - 4);
            offset += pair_len - 4;
            in_group = 0;

            // check if space for another handle pair available
            if (offset + pair_len > response_buffer_size){
                break;
            }
        }

        // keep track of previous handle
        prev_handle = it.handle;

        // does current attribute match
        // log_info("compare: %04x == %04x", *(uint16_t*) context->attribute_type, *(uint16_t*) uuid);
        if (it.handle && att_iterator_match_uuid(&it, attribute_type, attribute_type_len)) {

            // check if value has same len as last one
            uint16_t this_pair_len = 4 + it.value_len;
            if (offset > 1){
                if (this_pair_len != pair_len) {
                    break;
                }
            }

            // log_info("Begin of group, handle 0x%04x", it.handle);

            // first
            if (offset == 1) {
                pair_len = this_pair_len;
                response_buffer[offset] = this_pair_len;
                offset++;
            }

            group_start_handle = it.handle;
            group_start_value  = it.value;
            in_group = 1;
        }
    }

    if (offset == 1){
        return setup_error_atribute_not_found(response_buffer, request_type, start_handle);
    }

    response_buffer[0] = ATT_READ_BY_GROUP_TYPE_RESPONSE;
    return offset;
}
static uint16_t handle_read_by_group_type_request(att_connection_t * att_connection, uint8_t * request_buffer,  uint16_t request_len,
                                           uint8_t * response_buffer, uint16_t response_buffer_size){
    int attribute_type_len;
    if (request_len <= 7){
        attribute_type_len = 2;
    } else {
        attribute_type_len = 16;
    }
    return handle_read_by_group_type_request2(att_connection, response_buffer, response_buffer_size, little_endian_read_16(request_buffer, 1), little_endian_read_16(request_buffer, 3), attribute_type_len, &request_buffer[5]);
}

//
// MARK: ATT_WRITE_REQUEST 0x12
static uint16_t handle_write_request(att_connection_t * att_connection, uint8_t * request_buffer,  uint16_t request_len,
                              uint8_t * response_buffer, uint16_t response_buffer_size){

    UNUSED(response_buffer_size);

    uint8_t request_type = ATT_WRITE_REQUEST;

    uint16_t handle = little_endian_read_16(request_buffer, 1);
    att_iterator_t it;
    int ok = att_find_handle(&it, handle);
    if (!ok) {
        return setup_error_invalid_handle(response_buffer, request_type, handle);
    }
    att_write_callback_t callback = att_write_callback_for_handle(handle);
    if (!callback) {
        return setup_error_write_not_permitted(response_buffer, request_type, handle);
    }
    if ((it.flags & ATT_PROPERTY_WRITE) == 0) {
        return setup_error_write_not_permitted(response_buffer, request_type, handle);
    }
    if ((it.flags & ATT_PROPERTY_DYNAMIC) == 0) {
        return setup_error_write_not_permitted(response_buffer, request_type, handle);
    }
    // check security requirements
    uint8_t error_code = att_validate_security(att_connection, &it);
    if (error_code) {
        return setup_error(response_buffer, request_type, handle, error_code);
    }
    error_code = (*callback)(att_connection->con_handle, handle, ATT_TRANSACTION_MODE_NONE, 0, request_buffer + 3, request_len - 3);
    if (error_code) {
        return setup_error(response_buffer, request_type, handle, error_code);
    }
    response_buffer[0] = ATT_WRITE_RESPONSE;
    return 1;
}

//
// MARK: ATT_PREPARE_WRITE_REQUEST 0x16
static uint16_t handle_prepare_write_request(att_connection_t * att_connection, uint8_t * request_buffer,  uint16_t request_len,
                                      uint8_t * response_buffer, uint16_t response_buffer_size){

    UNUSED(response_buffer_size);

    uint8_t request_type = ATT_PREPARE_WRITE_REQUEST;

    uint16_t handle = little_endian_read_16(request_buffer, 1);
    uint16_t offset = little_endian_read_16(request_buffer, 3);
    att_write_callback_t callback = att_write_callback_for_handle(handle);
    if (!callback) {
        return setup_error_write_not_permitted(response_buffer, request_type, handle);
    }
    att_iterator_t it;
    int ok = att_find_handle(&it, handle);
    if (!ok) {
        return setup_error_invalid_handle(response_buffer, request_type, handle);
    }
    if ((it.flags & ATT_PROPERTY_WRITE) == 0) {
        return setup_error_write_not_permitted(response_buffer, request_type, handle);
    }
    if ((it.flags & ATT_PROPERTY_DYNAMIC) == 0) {
        return setup_error_write_not_permitted(response_buffer, request_type, handle);
    }
    // check security requirements
    uint8_t error_code = att_validate_security(att_connection, &it);
    if (error_code) {
        return setup_error(response_buffer, request_type, handle, error_code);
    }

    error_code = (*callback)(att_connection->con_handle, handle, ATT_TRANSACTION_MODE_ACTIVE, offset, request_buffer + 5, request_len - 5);
    switch (error_code){
        case 0:
            break;
        case ATT_ERROR_INVALID_OFFSET:
        case ATT_ERROR_INVALID_ATTRIBUTE_VALUE_LENGTH:
            // postpone to execute write request
            att_prepare_write_update_errors(error_code, handle);
            break;
        default:
            return setup_error(response_buffer, request_type, handle, error_code);
    }

    // response: echo request
    memcpy(response_buffer, request_buffer, request_len);
    response_buffer[0] = ATT_PREPARE_WRITE_RESPONSE;
    return request_len;
}

static void att_notify_write_callbacks(att_connection_t * att_connection, uint16_t transaction_mode){
    // notify all
    btstack_linked_list_iterator_t it;
    btstack_linked_list_iterator_init(&it, &service_handlers);
    while (btstack_linked_list_iterator_has_next(&it)){
        att_service_handler_t * handler = (att_service_handler_t*) btstack_linked_list_iterator_next(&it);
        if (!handler->write_callback) continue;
        (*handler->write_callback)(att_connection->con_handle, 0, transaction_mode, 0, NULL, 0);
    }
    if (!att_write_callback) return;
    (*att_write_callback)(att_connection->con_handle, 0, transaction_mode, 0, NULL, 0);
}

// returns first reported error or 0
static uint8_t att_validate_prepared_write(att_connection_t * att_connection){
    btstack_linked_list_iterator_t it;
    btstack_linked_list_iterator_init(&it, &service_handlers);
    while (btstack_linked_list_iterator_has_next(&it)){
        att_service_handler_t * handler = (att_service_handler_t*) btstack_linked_list_iterator_next(&it);
        if (!handler->write_callback) continue;
        uint8_t error_code = (*handler->write_callback)(att_connection->con_handle, 0, ATT_TRANSACTION_MODE_VALIDATE, 0, NULL, 0);
        if (error_code) return error_code;
    }
    if (!att_write_callback) return 0;
    return (*att_write_callback)(att_connection->con_handle, 0, ATT_TRANSACTION_MODE_VALIDATE, 0, NULL, 0);
}

/*
 * @brief transcation queue of prepared writes, e.g., after disconnect
 */
void att_clear_transaction_queue(att_connection_t * att_connection){
    att_notify_write_callbacks(att_connection, ATT_TRANSACTION_MODE_CANCEL);
}

// MARK: ATT_EXECUTE_WRITE_REQUEST 0x18
// NOTE: security has been verified by handle_prepare_write_request
static uint16_t handle_execute_write_request(att_connection_t * att_connection, uint8_t * request_buffer,  uint16_t request_len,
                                      uint8_t * response_buffer, uint16_t response_buffer_size){

    UNUSED(request_len);
    UNUSED(response_buffer_size);

    uint8_t request_type = ATT_EXECUTE_WRITE_REQUEST;
    if (request_buffer[1]) {
        // validate queued write
        if (att_prepare_write_error_code == 0){
            att_prepare_write_error_code = att_validate_prepared_write(att_connection);
        }
        // deliver queued errors
        if (att_prepare_write_error_code){
            att_clear_transaction_queue(att_connection);
            uint8_t  error_code = att_prepare_write_error_code;
            uint16_t handle     = att_prepare_write_error_handle;
            att_prepare_write_reset();
            return setup_error(response_buffer, request_type, handle, error_code);
        }
        att_notify_write_callbacks(att_connection, ATT_TRANSACTION_MODE_EXECUTE);
    } else {
        att_clear_transaction_queue(att_connection);
    }
    response_buffer[0] = ATT_EXECUTE_WRITE_RESPONSE;
    return 1;
}

// MARK: ATT_WRITE_COMMAND 0x52
// Core 4.0, vol 3, part F, 3.4.5.3
// "No Error Response or Write Response shall be sent in response to this command"
static void handle_write_command(att_connection_t * att_connection, uint8_t * request_buffer,  uint16_t request_len,
                                           uint8_t * response_buffer, uint16_t response_buffer_size){

    UNUSED(response_buffer);
    UNUSED(response_buffer_size);

    uint16_t handle = little_endian_read_16(request_buffer, 1);
    att_write_callback_t callback = att_write_callback_for_handle(handle);
    if (!callback) return;

    att_iterator_t it;
    int ok = att_find_handle(&it, handle);
    if (!ok) return;
    if ((it.flags & ATT_PROPERTY_DYNAMIC) == 0) return;
    if ((it.flags & ATT_PROPERTY_WRITE_WITHOUT_RESPONSE) == 0) return;
    if (att_validate_security(att_connection, &it)) return;
    (*callback)(att_connection->con_handle, handle, ATT_TRANSACTION_MODE_NONE, 0, request_buffer + 3, request_len - 3);
}

// MARK: helper for ATT_HANDLE_VALUE_NOTIFICATION and ATT_HANDLE_VALUE_INDICATION
static uint16_t prepare_handle_value(att_connection_t * att_connection,
                                     uint16_t handle,
                                     uint8_t *value,
                                     uint16_t value_len,
                                     uint8_t * response_buffer){
    little_endian_store_16(response_buffer, 1, handle);
    if (value_len > att_connection->mtu - 3){
        value_len = att_connection->mtu - 3;
    }
    memcpy(&response_buffer[3], value, value_len);
    return value_len + 3;
}

// MARK: ATT_HANDLE_VALUE_NOTIFICATION 0x1b
uint16_t att_prepare_handle_value_notification(att_connection_t * att_connection,
                                               uint16_t handle,
                                               uint8_t *value,
                                               uint16_t value_len,
                                               uint8_t * response_buffer){

    response_buffer[0] = ATT_HANDLE_VALUE_NOTIFICATION;
    return prepare_handle_value(att_connection, handle, value, value_len, response_buffer);
}

// MARK: ATT_HANDLE_VALUE_INDICATION 0x1d
uint16_t att_prepare_handle_value_indication(att_connection_t * att_connection,
                                             uint16_t handle,
                                             uint8_t *value,
                                             uint16_t value_len,
                                             uint8_t * response_buffer){

    response_buffer[0] = ATT_HANDLE_VALUE_INDICATION;
    return prepare_handle_value(att_connection, handle, value, value_len, response_buffer);
}

// MARK: Dispatcher
uint16_t att_handle_request(att_connection_t * att_connection,
                            uint8_t * request_buffer,
                            uint16_t request_len,
                            uint8_t * response_buffer){
    uint16_t response_len = 0;
    uint16_t response_buffer_size = att_connection->mtu;

    switch (request_buffer[0]){
        case ATT_EXCHANGE_MTU_REQUEST:
            response_len = handle_exchange_mtu_request(att_connection, request_buffer, request_len, response_buffer);
            break;
        case ATT_FIND_INFORMATION_REQUEST:
            response_len = handle_find_information_request(att_connection, request_buffer, request_len,response_buffer, response_buffer_size);
            break;
        case ATT_FIND_BY_TYPE_VALUE_REQUEST:
            response_len = handle_find_by_type_value_request(att_connection, request_buffer, request_len, response_buffer, response_buffer_size);
            break;
        case ATT_READ_BY_TYPE_REQUEST:
            response_len = handle_read_by_type_request(att_connection, request_buffer, request_len, response_buffer, response_buffer_size);
            break;
        case ATT_READ_REQUEST:
            response_len = handle_read_request(att_connection, request_buffer, request_len, response_buffer, response_buffer_size);
            break;
        case ATT_READ_BLOB_REQUEST:
            response_len = handle_read_blob_request(att_connection, request_buffer, request_len, response_buffer, response_buffer_size);
            break;
        case ATT_READ_MULTIPLE_REQUEST:
            response_len = handle_read_multiple_request(att_connection, request_buffer, request_len, response_buffer, response_buffer_size);
            break;
        case ATT_READ_BY_GROUP_TYPE_REQUEST:
            response_len = handle_read_by_group_type_request(att_connection, request_buffer, request_len, response_buffer, response_buffer_size);
            break;
        case ATT_WRITE_REQUEST:
            response_len = handle_write_request(att_connection, request_buffer, request_len, response_buffer, response_buffer_size);
            break;
        case ATT_PREPARE_WRITE_REQUEST:
            response_len = handle_prepare_write_request(att_connection, request_buffer, request_len, response_buffer, response_buffer_size);
            break;
        case ATT_EXECUTE_WRITE_REQUEST:
            response_len = handle_execute_write_request(att_connection, request_buffer, request_len, response_buffer, response_buffer_size);
            break;
        case ATT_WRITE_COMMAND:
            handle_write_command(att_connection, request_buffer, request_len, response_buffer, response_buffer_size);
            break;
#ifdef ENABLE_LE_SIGNED_WRITE
        case ATT_SIGNED_WRITE_COMMAND:
            log_info("handle_signed_write_command preprocessed by att_server.c");
            break;
#endif
        default:
            log_info("Unhandled ATT Command: %02X, DATA: ", request_buffer[0]);
            log_info_hexdump(&request_buffer[9], request_len-9);
            break;
    }
    return response_len;
}

/**
 * @brief register read/write callbacks for specific handle range
 * @param att_service_handler_t
 */
void att_register_service_handler(att_service_handler_t * handler){
    if (att_service_handler_for_handle(handler->start_handle) ||
        att_service_handler_for_handle(handler->end_handle)){
        log_error("att_register_service_handler: handler for range 0x%04x-0x%04x already registered", handler->start_handle, handler->end_handle);
        return;
    }
    btstack_linked_list_add(&service_handlers, (btstack_linked_item_t*) handler);
}

// returns 1 if service found. only primary service.
int gatt_server_get_get_handle_range_for_service_with_uuid16(uint16_t uuid16, uint16_t * start_handle, uint16_t * end_handle){
    uint16_t in_group    = 0;
    uint16_t prev_handle = 0;

    uint8_t attribute_value[2];
    int attribute_len = sizeof(attribute_value);
    little_endian_store_16(attribute_value, 0, uuid16);

    att_iterator_t it;
    att_iterator_init(&it);
    while (att_iterator_has_next(&it)){
        att_iterator_fetch_next(&it);
        int new_service_started = att_iterator_match_uuid16(&it, GATT_PRIMARY_SERVICE_UUID) || att_iterator_match_uuid16(&it, GATT_SECONDARY_SERVICE_UUID);

        // close current tag, if within a group and a new service definition starts or we reach end of att db
        if (in_group &&
            (it.handle == 0 || new_service_started)){
            *end_handle = prev_handle;
            return 1;
        }

        // keep track of previous handle
        prev_handle = it.handle;

        // check if found
        if (it.handle && new_service_started && attribute_len == it.value_len && memcmp(attribute_value, it.value, it.value_len) == 0){
            *start_handle = it.handle;
            in_group = 1;
        }
    }
    return 0;
}

// returns 0 if not found
uint16_t gatt_server_get_value_handle_for_characteristic_with_uuid16(uint16_t start_handle, uint16_t end_handle, uint16_t uuid16){
    att_iterator_t it;
    att_iterator_init(&it);
    while (att_iterator_has_next(&it)){
        att_iterator_fetch_next(&it);
        if (it.handle && it.handle < start_handle) continue;
        if (it.handle > end_handle) break;  // (1)
        if (it.handle == 0) break;
        if (att_iterator_match_uuid16(&it, uuid16)) return it.handle;
    }
    return 0;
}

// returns 0 if not found
uint16_t gatt_server_get_client_configuration_handle_for_characteristic_with_uuid16(uint16_t start_handle, uint16_t end_handle, uint16_t uuid16){
    att_iterator_t it;
    att_iterator_init(&it);
    int characteristic_found = 0;
    while (att_iterator_has_next(&it)){
        att_iterator_fetch_next(&it);
        if (it.handle && it.handle < start_handle) continue;
        if (it.handle > end_handle) break;  // (1)
        if (it.handle == 0) break;
        if (att_iterator_match_uuid16(&it, uuid16)){
            characteristic_found = 1;
            continue;
        }
        if (att_iterator_match_uuid16(&it, GATT_PRIMARY_SERVICE_UUID)
         || att_iterator_match_uuid16(&it, GATT_SECONDARY_SERVICE_UUID)
         || att_iterator_match_uuid16(&it, GATT_CHARACTERISTICS_UUID)){
            if (characteristic_found) break;
            continue;
        }
        if (att_iterator_match_uuid16(&it, GATT_CLIENT_CHARACTERISTICS_CONFIGURATION)){
            return it.handle;
        }
    }
    return 0;
}