xref: /btstack/src/ble/att_db.c (revision c6d8b8168b34299995423fa16e16b23f3c5e486a)

/*
 * 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 BLUEKITCHEN
 * GMBH 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 <string.h>

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

// check for ENABLE_ATT_DELAYED_READ_RESPONSE -> ENABLE_ATT_DELAYED_RESPONSE,
#ifdef ENABLE_ATT_DELAYED_READ_RESPONSE
    #error "ENABLE_ATT_DELAYED_READ_RESPONSE was replaced by ENABLE_ATT_DELAYED_RESPONSE. Please update btstack_config.h"
#endif

typedef enum {
    ATT_READ,
    ATT_WRITE,
} att_operation_t;


static bool is_Bluetooth_Base_UUID(uint8_t const *uuid){
    // Bluetooth 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 };

    if (memcmp(&uuid[0],  &bluetooth_base_uuid[0], 12) != 0){
        return false;
    }
    if (memcmp(&uuid[14], &bluetooth_base_uuid[14], 2) != 0){
        return false;
    }
    return true;

}

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

// ATT Database

// 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_persistent_ccc_cache(att_iterator_t * it);

static uint8_t const * att_database = NULL;
static att_read_callback_t  att_read_callback  = NULL;
static att_write_callback_t att_write_callback = NULL;
static int      att_prepare_write_error_code   = 0;
static uint16_t att_prepare_write_error_handle = 0x0000;

// single cache for att_is_persistent_ccc - stores flags before write callback
static uint16_t att_persistent_ccc_handle;
static uint16_t att_persistent_ccc_uuid16;

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

static bool 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 == 0u){
        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 & (uint16_t)ATT_PROPERTY_UUID128) != 0u){
        it->value_len = it->size - 22u;
        it->value  = &it->att_ptr[22];
    } else {
        it->value_len = it->size - 8u;
        it->value  = &it->att_ptr[8];
    }
    // advance AFTER setting values
    it->att_ptr += it->size;
}

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

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


static bool att_find_handle(att_iterator_t *it, uint16_t handle){
    if (handle == 0u){
        return false;
    }
    att_iterator_init(it);
    while (att_iterator_has_next(it)){
        att_iterator_fetch_next(it);
        if (it->handle == handle){
            return true;
        }
    }
    return false;
}

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

const uint8_t * gatt_server_get_const_value_for_handle(uint16_t attribute_handle, uint16_t * out_value_len){
    att_iterator_t it;
    bool ok = att_find_handle(&it, attribute_handle);
    if (!ok){
        return NULL;
    }
    if ((it.flags & (uint16_t)ATT_PROPERTY_DYNAMIC) != 0u){
        return NULL;
    }
    *out_value_len = it.value_len;
    return it.value;
}

// end of client API

static void att_update_value_len(att_iterator_t *it, uint16_t offset, hci_con_handle_t con_handle) {
    if ((it->flags & (uint16_t)ATT_PROPERTY_DYNAMIC) == 0u){
        return;
    }
    it->value_len = (*att_read_callback)(con_handle, it->handle, offset, 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 & (uint16_t)ATT_PROPERTY_DYNAMIC) != 0u){
        return (*att_read_callback)(con_handle, it->handle, offset, buffer, buffer_size);
    }

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

void att_set_db(uint8_t const * db){
    // validate db version
    if (db == NULL){
        return;
    }
    if (*db != (uint8_t)ATT_DB_VERSION){
        log_error("ATT DB version differs, please regenerate .h from .gatt file or update att_db_util.c");
        return;
    }
    log_info("att_set_db %p", db);
    // ignore db version
    att_database = &db[1];
}

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];
    log_info("att_dump_attributes, table %p", att_database);
    while (att_iterator_has_next(&it)){
        att_iterator_fetch_next(&it);
        if (it.handle == 0u) {
            log_info("Handle: END");
            return;
        }
        log_info("Handle: 0x%04x, flags: 0x%04x, uuid: ", it.handle, it.flags);
        if ((it.flags & (uint16_t)ATT_PROPERTY_UUID128) != 0u){
            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 == (uint8_t)ATT_ERROR_INVALID_ATTRIBUTE_VALUE_LENGTH) && (error_code != (uint8_t)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 == (uint8_t)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, uint8_t request_opcode, uint16_t handle, uint8_t error_code){
    response_buffer[0] = (uint8_t)ATT_ERROR_RESPONSE;
    response_buffer[1] = request_opcode;
    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, uint8_t request_opcode, uint16_t start_handle){
    return setup_error(response_buffer, request_opcode, start_handle, ATT_ERROR_READ_NOT_PERMITTED);
}

static inline uint16_t setup_error_write_not_permitted(uint8_t * response_buffer, uint8_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, uint8_t request_opcode, uint16_t start_handle){
    return setup_error(response_buffer, request_opcode, start_handle, ATT_ERROR_ATTRIBUTE_NOT_FOUND);
}

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

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

static inline uint16_t setup_error_invalid_pdu(uint8_t *response_buffer, uint8_t request_opcode) {
    return setup_error(response_buffer, request_opcode, 0, ATT_ERROR_INVALID_PDU);
}

struct att_security_settings {
    uint8_t required_security_level;
    bool    requires_secure_connection;
};

static void att_validate_security_get_settings(struct att_security_settings * security_settings, att_operation_t operation, att_iterator_t *it){
    security_settings->required_security_level = 0u;
    security_settings->requires_secure_connection = false;
    switch (operation){
        case ATT_READ:
            if ((it->flags & (uint16_t)ATT_PROPERTY_READ_PERMISSION_BIT_0) != 0u){
                security_settings->required_security_level |= 1u;
            }
            if ((it->flags & (uint16_t)ATT_PROPERTY_READ_PERMISSION_BIT_1) != 0u){
                security_settings->required_security_level |= 2u;
            }
            if ((it->flags & (uint16_t)ATT_PROPERTY_READ_PERMISSION_SC) != 0u){
                security_settings->requires_secure_connection = true;
            }
            break;
        case ATT_WRITE:
            if ((it->flags & (uint16_t)ATT_PROPERTY_WRITE_PERMISSION_BIT_0) != 0u){
                security_settings->required_security_level |= 1u;
            }
            if ((it->flags & (uint16_t)ATT_PROPERTY_WRITE_PERMISSION_BIT_1) != 0u){
                security_settings->required_security_level |= 2u;
            }
            if ((it->flags & (uint16_t)ATT_PROPERTY_WRITE_PERMISSION_SC) != 0u){
                security_settings->requires_secure_connection = true;
            }
            break;
        default:
            btstack_assert(false);
            break;
    }
}

static uint8_t att_validate_security(att_connection_t * att_connection, att_operation_t operation, att_iterator_t * it){
    struct att_security_settings security_settings;
    att_validate_security_get_settings(&security_settings, operation, it);

    uint8_t required_encryption_size = (uint8_t)(it->flags >> 12);
    if (required_encryption_size != 0u){
        required_encryption_size++;   // store -1 to fit into 4 bit
    }
    log_debug("att_validate_security. flags 0x%04x (=> security level %u, key size %u) authorized %u, authenticated %u, encryption_key_size %u, secure connection %u",
        it->flags, security_settings.required_security_level, required_encryption_size, att_connection->authorized, att_connection->authenticated, att_connection->encryption_key_size, att_connection->secure_connection);

    bool sc_missing = security_settings.requires_secure_connection && (att_connection->secure_connection == 0u);
    switch (security_settings.required_security_level){
        case ATT_SECURITY_AUTHORIZED:
            if ((att_connection->authorized == 0u) || sc_missing){
                return ATT_ERROR_INSUFFICIENT_AUTHORIZATION;
            }
            /* fall through */
        case ATT_SECURITY_AUTHENTICATED:
            if ((att_connection->authenticated == 0u) || sc_missing){
                return ATT_ERROR_INSUFFICIENT_AUTHENTICATION;
            }
            /* fall through */
        case ATT_SECURITY_ENCRYPTED:
            if ((required_encryption_size > 0u) && ((att_connection->encryption_key_size == 0u) || sc_missing)){
                return ATT_ERROR_INSUFFICIENT_ENCRYPTION;
            }
            if (required_encryption_size > att_connection->encryption_key_size){
                return ATT_ERROR_INSUFFICIENT_ENCRYPTION_KEY_SIZE;
            }
            break;
        default:
            break;
    }
    return ATT_ERROR_SUCCESS;
}

//
// 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){

    if (request_len != 3u){
        return setup_error_invalid_pdu(response_buffer, ATT_EXCHANGE_MTU_REQUEST);
    }

    uint16_t client_rx_mtu = little_endian_read_16(request_buffer, 1);

    // find min(local max mtu, remote mtu) >= ATT_DEFAULT_MTU and use as mtu for this connection
    uint16_t min_mtu = btstack_min(client_rx_mtu, att_connection->max_mtu);
    uint16_t new_mtu = btstack_max(ATT_DEFAULT_MTU, min_mtu);
    att_connection->mtu_exchanged = true;
    att_connection->mtu = new_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 == 0u)){
        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 & (uint16_t)ATT_PROPERTY_UUID128) ? 16u : 2u;

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

        // first
        if (offset == 1u) {
            uuid_len = this_uuid_len;
            // set format field
            response_buffer[offset] = (it.flags & (uint16_t)ATT_PROPERTY_UUID128) ? 0x02u : 0x01u;
            offset++;
        }

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

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

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

    if (offset == 1u){
        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){

    if (request_len != 5u){
        return setup_error_invalid_pdu(response_buffer, ATT_FIND_INFORMATION_REQUEST);
    }

    uint16_t start_handle = little_endian_read_16(request_buffer, 1);
    uint16_t end_handle = little_endian_read_16(request_buffer, 3);
    return handle_find_information_request2(att_connection, response_buffer, response_buffer_size, start_handle, end_handle);
}

//
// 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_request(att_connection_t * att_connection, uint8_t * request_buffer,  uint16_t request_len,
                                           uint8_t * response_buffer, uint16_t response_buffer_size){
    UNUSED(att_connection);

    if (request_len < 7u){
        return setup_error_invalid_pdu(response_buffer, ATT_FIND_BY_TYPE_VALUE_REQUEST);
    }

    // parse request
    uint16_t start_handle = little_endian_read_16(request_buffer, 1);
    uint16_t end_handle = little_endian_read_16(request_buffer, 3);
    uint16_t attribute_type = little_endian_read_16(request_buffer, 5);
    const uint8_t *attribute_value = &request_buffer[7];
    uint16_t attribute_len = request_len - 7u;

    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 == 0u)){
        return setup_error_invalid_handle(response_buffer, request_type, start_handle);
    }

    uint16_t offset      = 1;
    bool in_group        = false;
    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 != 0u) && (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 == 0u) || 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 += 2u;
            in_group = false;

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

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

        // does current attribute match
        if ((it.handle != 0u) && 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 += 2u;
            in_group = true;
        }
    }

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

    response_buffer[0] = ATT_FIND_BY_TYPE_VALUE_RESPONSE;
    return offset;
}

//
// 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 == 0u)){
        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 == 0u ) || (it.handle > end_handle)){
            break;
        }

        // does current attribute match
        if ((it.handle < start_handle) || !att_iterator_match_uuid(&it, attribute_type, attribute_type_len)){
            continue;
        }

        // skip handles that cannot be read but remember that there has been at least one
        if ((it.flags & (uint16_t)ATT_PROPERTY_READ) == 0u) {
            if (first_matching_but_unreadable_handle == 0u) {
                first_matching_but_unreadable_handle = it.handle;
            }
            continue;
        }

        // check security requirements
        error_code = att_validate_security(att_connection, ATT_READ, &it);
        if (error_code != 0u){
            break;
        }

        att_update_value_len(&it, 0, att_connection->con_handle);

#ifdef ENABLE_ATT_DELAYED_RESPONSE
        if (it.value_len == (uint16_t)ATT_READ_RESPONSE_PENDING){
            return ATT_READ_RESPONSE_PENDING;
        }
#endif

        // allow to return ATT Error Code in ATT Read Callback
        if (it.value_len > (uint16_t)ATT_READ_ERROR_CODE_OFFSET){
            error_code =  (uint8_t)(it.value_len - (uint16_t)ATT_READ_ERROR_CODE_OFFSET);
            break;
        }

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

        // first
        if (offset == 1u) {
            pair_len = this_pair_len;
            response_buffer[offset] = (uint8_t) pair_len;
            offset++;
        }

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

        // store
        little_endian_store_16(response_buffer, offset, it.handle);
        offset += 2u;
        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 > 1u){
        response_buffer[0] = ATT_READ_BY_TYPE_RESPONSE;
        return offset;
    }

    // first attribute had an error
    if (error_code != 0u){
        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 != 0u){
        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){

    uint16_t attribute_type_len;
    switch (request_len){
        case 7:
            attribute_type_len = 2;
            break;
        case 21:
            attribute_type_len = 16;
            break;
        default:
            return setup_error_invalid_pdu(response_buffer, ATT_READ_BY_TYPE_REQUEST);
    }

    uint16_t start_handle = little_endian_read_16(request_buffer, 1);
    uint16_t end_handle = little_endian_read_16(request_buffer, 3);
    return handle_read_by_type_request2(att_connection, response_buffer, response_buffer_size, start_handle, end_handle, 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;
    bool 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 & (uint16_t)ATT_PROPERTY_READ) == 0u) {
        return setup_error_read_not_permitted(response_buffer, request_type, handle);
    }

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

    att_update_value_len(&it, 0, att_connection->con_handle);

#ifdef ENABLE_ATT_DELAYED_RESPONSE
    if (it.value_len == (uint16_t)ATT_READ_RESPONSE_PENDING){
        return ATT_READ_RESPONSE_PENDING;
    }
#endif

    // allow to return ATT Error Code in ATT Read Callback
    if (it.value_len > (uint16_t)ATT_READ_ERROR_CODE_OFFSET){
        error_code = (uint8_t)(it.value_len - (uint16_t)ATT_READ_ERROR_CODE_OFFSET);
        return setup_error(response_buffer, request_type, handle, error_code);
    }

    // store
    uint16_t offset   = 1;
    uint16_t bytes_copied = att_copy_value(&it, 0, response_buffer + offset, response_buffer_size - offset, 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){

    if (request_len != 3u){
        return setup_error_invalid_pdu(response_buffer, ATT_READ_REQUEST);
    }

    uint16_t handle = little_endian_read_16(request_buffer, 1);
    return handle_read_request2(att_connection, response_buffer, response_buffer_size, handle);
}

//s
// 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;
    bool 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 & (uint16_t)ATT_PROPERTY_READ) == 0u) {
        return setup_error_read_not_permitted(response_buffer, request_type, handle);
    }

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

    att_update_value_len(&it, value_offset, att_connection->con_handle);

#ifdef ENABLE_ATT_DELAYED_RESPONSE
    if (it.value_len == (uint16_t)ATT_READ_RESPONSE_PENDING){
        return ATT_READ_RESPONSE_PENDING;
    }
#endif

    // allow to return ATT Error Code in ATT Read Callback
    if (it.value_len > (uint16_t)ATT_READ_ERROR_CODE_OFFSET){
        error_code = (uint8_t)(it.value_len - (uint16_t)ATT_READ_ERROR_CODE_OFFSET);
        return setup_error(response_buffer, request_type, handle, error_code);
    }

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

    // prepare response
    response_buffer[0] = ATT_READ_BLOB_RESPONSE;
    uint16_t offset   = 1;

    // fetch more data if available
    if (value_offset < it.value_len){
        uint16_t bytes_copied = att_copy_value(&it, value_offset, &response_buffer[offset], response_buffer_size - offset, att_connection->con_handle);
        offset += bytes_copied;
    }
    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){

    if (request_len != 5u){
        return setup_error_invalid_pdu(response_buffer, ATT_READ_BLOB_REQUEST);
    }

    uint16_t handle = little_endian_read_16(request_buffer, 1);
    uint16_t value_offset = little_endian_read_16(request_buffer, 3);
    return handle_read_blob_request2(att_connection, response_buffer, response_buffer_size, handle, value_offset);
}

//
// MARK: ATT_READ_MULTIPLE_REQUEST 0x0e
// MARK: ATT_READ_MULTIPLE_REQUEST 0x20
//
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, bool store_length){
    log_info("ATT_READ_MULTIPLE_(VARIABLE_)REQUEST: num handles %u", num_handles);
    uint8_t request_type  = store_length ? ATT_READ_MULTIPLE_VARIABLE_REQ : ATT_READ_MULTIPLE_REQUEST;
    uint8_t response_type = store_length ? ATT_READ_MULTIPLE_VARIABLE_RSP : ATT_READ_MULTIPLE_RESPONSE;
    uint16_t offset   = 1;

    uint16_t i;
    uint8_t  error_code = 0;
    uint16_t handle = 0;

#ifdef ENABLE_ATT_DELAYED_RESPONSE
    bool read_request_pending = false;
#endif

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

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

        att_iterator_t it;

        bool 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 & (uint16_t)ATT_PROPERTY_READ) == 0u) {
            error_code = (uint8_t)ATT_ERROR_READ_NOT_PERMITTED;
            break;
        }

        // check security requirements
        error_code = att_validate_security(att_connection, ATT_READ, &it);
        if (error_code != 0u){
            break;
        }

        att_update_value_len(&it, 0, att_connection->con_handle);

#ifdef ENABLE_ATT_DELAYED_RESPONSE
        if (it.value_len == (uint16_t)ATT_READ_RESPONSE_PENDING) {
            read_request_pending = true;
        }
        if (read_request_pending){
            continue;
        }
#endif

        // allow to return ATT Error Code in ATT Read Callback
        if (it.value_len > (uint16_t)ATT_READ_ERROR_CODE_OFFSET){
            error_code = (uint8_t)(it.value_len - (uint16_t)ATT_READ_ERROR_CODE_OFFSET);
            break;
        }

#ifdef ENABLE_GATT_OVER_EATT
        // assert that at least Value Length can be stored
        if (store_length && ((offset + 2) >= response_buffer_size)){
            break;
        }
        // skip length field
        uint16_t offset_value_length = offset;
        if (store_length){
            offset += 2;
        }
#endif
        // store data
        uint16_t bytes_copied = att_copy_value(&it, 0, response_buffer + offset, response_buffer_size - offset, att_connection->con_handle);
        offset += bytes_copied;
#ifdef ENABLE_GATT_OVER_EATT
        // set length field
        if (store_length) {
            little_endian_store_16(response_buffer, offset_value_length, bytes_copied);
        }
#endif
    }

    if (error_code != 0u){
        return setup_error(response_buffer, request_type, handle, error_code);
    }

    response_buffer[0] = (uint8_t)response_type;
    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, bool store_length) {

    uint8_t request_type = store_length ? ATT_READ_MULTIPLE_VARIABLE_REQ : ATT_READ_MULTIPLE_REQUEST;

    // 1 byte opcode + two or more attribute handles (2 bytes each)
    if ( (request_len < 5u) || ((request_len & 1u) == 0u) ){
        return setup_error_invalid_pdu(response_buffer, request_type);
    }

    uint8_t num_handles = (request_len - 1u) >> 1u;
    return handle_read_multiple_request2(att_connection, response_buffer, response_buffer_size, num_handles,
                                         &request_buffer[1], store_length);
}

//
// 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 == 0u)){
        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 != (uint16_t)GATT_PRIMARY_SERVICE_UUID) && (uuid16 != (uint16_t)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;
    bool     in_group = false;
    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 != 0u) && (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 == 0u) || 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 += 2u;
            little_endian_store_16(response_buffer, offset, prev_handle);
            offset += 2u;
            (void)memcpy(response_buffer + offset, group_start_value,
                         pair_len - 4u);
            offset += pair_len - 4u;
            in_group = false;

            // 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 != 0u) && att_iterator_match_uuid(&it, attribute_type, attribute_type_len)) {

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

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

            // first
            if (offset == 1u) {
                pair_len = this_pair_len;
                response_buffer[offset] = (uint8_t) this_pair_len;
                offset++;
            }

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

    if (offset == 1u){
        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){
    uint16_t attribute_type_len;
    switch (request_len){
        case 7:
            attribute_type_len = 2;
            break;
        case 21:
            attribute_type_len = 16;
            break;
        default:
            return setup_error_invalid_pdu(response_buffer, ATT_READ_BY_GROUP_TYPE_REQUEST);
    }

    uint16_t start_handle = little_endian_read_16(request_buffer, 1);
    uint16_t end_handle = little_endian_read_16(request_buffer, 3);
    return handle_read_by_group_type_request2(att_connection, response_buffer, response_buffer_size, start_handle, end_handle, 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);

    if (request_len < 3u){
        return setup_error_invalid_pdu(response_buffer, ATT_WRITE_REQUEST);
    }

    uint8_t request_type = ATT_WRITE_REQUEST;

    uint16_t handle = little_endian_read_16(request_buffer, 1);
    att_iterator_t it;
    bool ok = att_find_handle(&it, handle);
    if (!ok) {
        return setup_error_invalid_handle(response_buffer, request_type, handle);
    }
    if (att_write_callback == NULL) {
        return setup_error_write_not_permitted(response_buffer, request_type, handle);
    }
    if ((it.flags & (uint16_t)ATT_PROPERTY_WRITE) == 0u) {
        return setup_error_write_not_permitted(response_buffer, request_type, handle);
    }
    if ((it.flags & (uint16_t)ATT_PROPERTY_DYNAMIC) == 0u) {
        return setup_error_write_not_permitted(response_buffer, request_type, handle);
    }
    // check security requirements
    int error_code = att_validate_security(att_connection, ATT_WRITE, &it);
    if (error_code != 0) {
        return setup_error(response_buffer, request_type, handle, error_code);
    }
    att_persistent_ccc_cache(&it);
    error_code = (*att_write_callback)(att_connection->con_handle, handle, ATT_TRANSACTION_MODE_NONE, 0u, request_buffer + 3u, request_len - 3u);

#ifdef ENABLE_ATT_DELAYED_RESPONSE
    if (error_code == ATT_ERROR_WRITE_RESPONSE_PENDING){
        return ATT_INTERNAL_WRITE_RESPONSE_PENDING;
    }
#endif

    if (error_code != 0) {
        return setup_error(response_buffer, request_type, handle, error_code);
    }
    response_buffer[0] = (uint8_t)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){

    uint8_t request_type = ATT_PREPARE_WRITE_REQUEST;

    if (request_len < 5u){
        return setup_error_invalid_pdu(response_buffer, request_type);
    }

    uint16_t handle = little_endian_read_16(request_buffer, 1);
    uint16_t offset = little_endian_read_16(request_buffer, 3);
    if (att_write_callback == NULL) {
        return setup_error_write_not_permitted(response_buffer, request_type, handle);
    }
    att_iterator_t it;
    if (att_find_handle(&it, handle) == false) {
        return setup_error_invalid_handle(response_buffer, request_type, handle);
    }
    if ((it.flags & (uint16_t)ATT_PROPERTY_WRITE) == 0u) {
        return setup_error_write_not_permitted(response_buffer, request_type, handle);
    }
    if ((it.flags & (uint16_t)ATT_PROPERTY_DYNAMIC) == 0u) {
        return setup_error_write_not_permitted(response_buffer, request_type, handle);
    }
    // check security requirements
    int error_code = att_validate_security(att_connection, ATT_WRITE, &it);
    if (error_code != 0) {
        return setup_error(response_buffer, request_type, handle, error_code);
    }

    error_code = (*att_write_callback)(att_connection->con_handle, handle, ATT_TRANSACTION_MODE_ACTIVE, offset, request_buffer + 5u, request_len - 5u);
    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;
#ifdef ENABLE_ATT_DELAYED_RESPONSE
        case ATT_ERROR_WRITE_RESPONSE_PENDING:
            return ATT_INTERNAL_WRITE_RESPONSE_PENDING;
#endif
        default:
            return setup_error(response_buffer, request_type, handle, error_code);
    }

    // response: echo request
    uint16_t bytes_to_echo = btstack_min(request_len, response_buffer_size);
    (void)memcpy(response_buffer, request_buffer, bytes_to_echo);
    response_buffer[0] = ATT_PREPARE_WRITE_RESPONSE;
    return request_len;
}

/*
 * @brief transcation queue of prepared writes, e.g., after disconnect
 */
void att_clear_transaction_queue(att_connection_t * att_connection){
    (*att_write_callback)(att_connection->con_handle, 0, ATT_TRANSACTION_MODE_CANCEL, 0, NULL, 0);
}

// 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(response_buffer_size);

    uint8_t request_type = ATT_EXECUTE_WRITE_REQUEST;

    if (request_len < 2u){
        return setup_error_invalid_pdu(response_buffer, request_type);
    }

    if (att_write_callback == NULL) {
        return setup_error_write_not_permitted(response_buffer, request_type, 0);
    }

    if (request_buffer[1] != 0u) {
        // validate queued write
        if (att_prepare_write_error_code == 0){
            att_prepare_write_error_code = (*att_write_callback)(att_connection->con_handle, 0, ATT_TRANSACTION_MODE_VALIDATE, 0, NULL, 0);
        }
#ifdef ENABLE_ATT_DELAYED_RESPONSE
        if (att_prepare_write_error_code == ATT_ERROR_WRITE_RESPONSE_PENDING){
            return ATT_INTERNAL_WRITE_RESPONSE_PENDING;
        }
#endif
        // deliver queued errors
        if (att_prepare_write_error_code != 0){
            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_write_callback(att_connection->con_handle, 0, ATT_TRANSACTION_MODE_EXECUTE, 0, NULL, 0);
    } 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, uint16_t required_flags){

    if (request_len < 3u){
        return;
    }

    uint16_t handle = little_endian_read_16(request_buffer, 1);
    if (att_write_callback == NULL){
        return;
    }

    att_iterator_t it;
    bool ok = att_find_handle(&it, handle);
    if (!ok){
        return;
    }
    if ((it.flags & (uint16_t)ATT_PROPERTY_DYNAMIC) == 0u){
        return;
    }
    if ((it.flags & required_flags) == 0u){
        return;
    }
    if (att_validate_security(att_connection, ATT_WRITE, &it) != ATT_ERROR_SUCCESS){
        return;
    }
    att_persistent_ccc_cache(&it);
    (*att_write_callback)(att_connection->con_handle, handle, ATT_TRANSACTION_MODE_NONE, 0u, request_buffer + 3u, request_len - 3u);
}

// 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,
                                     const uint8_t *value,
                                     uint16_t value_len,
                                     uint8_t * response_buffer){
    little_endian_store_16(response_buffer, 1, handle);
    uint16_t bytes_to_copy = btstack_min(value_len, att_connection->mtu - 3u);
    (void)memcpy(&response_buffer[3], value, bytes_to_copy);
    return value_len + 3u;
}

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

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

// MARK: ATT_MULTIPLE_HANDLE_VALUE_NTF 0x23u
uint16_t att_prepare_handle_value_multiple_notification(att_connection_t * att_connection,
                                               uint8_t num_attributes,
                                               const uint16_t * attribute_handles,
                                               const uint8_t ** values_data,
                                               const uint16_t * values_len,
                                               uint8_t * response_buffer){

    response_buffer[0] = ATT_MULTIPLE_HANDLE_VALUE_NTF;
    uint8_t i;
    uint16_t offset = 1;
    uint16_t response_buffer_size = att_connection->mtu - 3u;
    for (i = 0; i < num_attributes; i++) {
        uint16_t value_len = values_len[i];
        if ((offset + 4u + value_len) > response_buffer_size){
            break;
        }
        little_endian_store_16(response_buffer, offset, attribute_handles[i]);
        offset += 2u;
        little_endian_store_16(response_buffer, offset, value_len);
        offset += 2u;
        (void) memcpy(&response_buffer[offset], values_data[i], value_len);
        offset += value_len;
    }
    return offset;
}

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

    response_buffer[0] = ATT_HANDLE_VALUE_INDICATION;
    return prepare_handle_value(att_connection, attribute_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;
    const uint16_t response_buffer_size = att_connection->mtu;
    const uint8_t  request_opcode = request_buffer[0];

    switch (request_opcode){
        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, false);
            break;
        case ATT_READ_MULTIPLE_VARIABLE_REQ:
            response_len = handle_read_multiple_request(att_connection, request_buffer, request_len, response_buffer,
                                                        response_buffer_size, true);
            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, ATT_PROPERTY_WRITE_WITHOUT_RESPONSE);
            break;
#ifdef ENABLE_LE_SIGNED_WRITE
        case ATT_SIGNED_WRITE_COMMAND:
            handle_write_command(att_connection, request_buffer, request_len, ATT_PROPERTY_AUTHENTICATED_SIGNED_WRITE);
            break;
#endif
        default:
            response_len = setup_error(response_buffer, request_opcode, 0, ATT_ERROR_REQUEST_NOT_SUPPORTED);
            break;
    }
    return response_len;
}

// returns 1 if service found. only primary service.
bool gatt_server_get_handle_range_for_service_with_uuid16(uint16_t uuid16, uint16_t * start_handle, uint16_t * end_handle){
    bool in_group    = false;
    uint16_t prev_handle = 0;
    uint16_t service_start = 0;

    uint8_t attribute_value[2];
    const uint16_t 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 == 0u) || new_service_started)){
            in_group = false;
            // check range
            if ((service_start >= *start_handle) && (prev_handle <= *end_handle)){
                *start_handle = service_start;
                *end_handle = prev_handle;
                return true;
            }
        }

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

        // check if found
        if ( (it.handle != 0u) && new_service_started && (attribute_len == it.value_len) && (memcmp(attribute_value, it.value, it.value_len) == 0)){
            service_start = it.handle;
            in_group = true;
        }
    }
    return false;
}

// returns false 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 != 0u) && (it.handle < start_handle)){
            continue;
        }
        if (it.handle > end_handle){
            break;  // (1)
        }
        if (it.handle == 0u){
            break;
        }
        if (att_iterator_match_uuid16(&it, uuid16)){
            return it.handle;
        }
    }
    return 0;
}

uint16_t gatt_server_get_descriptor_handle_for_characteristic_with_uuid16(uint16_t start_handle, uint16_t end_handle, uint16_t characteristic_uuid16, uint16_t descriptor_uuid16){
    att_iterator_t it;
    att_iterator_init(&it);
    bool characteristic_found = false;
    while (att_iterator_has_next(&it)){
        att_iterator_fetch_next(&it);
        if ((it.handle != 0u) && (it.handle < start_handle)){
            continue;
        }
        if (it.handle > end_handle){
            break;  // (1)
        }
        if (it.handle == 0u){
            break;
        }
        if (att_iterator_match_uuid16(&it, characteristic_uuid16)){
            characteristic_found = true;
            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 (characteristic_found && att_iterator_match_uuid16(&it, descriptor_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 characteristic_uuid16){
    return gatt_server_get_descriptor_handle_for_characteristic_with_uuid16(start_handle, end_handle, characteristic_uuid16, GATT_CLIENT_CHARACTERISTICS_CONFIGURATION);
}
// returns 0 if not found

uint16_t gatt_server_get_server_configuration_handle_for_characteristic_with_uuid16(uint16_t start_handle, uint16_t end_handle, uint16_t characteristic_uuid16){
    return gatt_server_get_descriptor_handle_for_characteristic_with_uuid16(start_handle, end_handle, characteristic_uuid16, GATT_SERVER_CHARACTERISTICS_CONFIGURATION);
}

// returns true if service found. only primary service.
bool gatt_server_get_handle_range_for_service_with_uuid128(const uint8_t * uuid128, uint16_t * start_handle, uint16_t * end_handle){
    bool in_group    = false;
    uint16_t prev_handle = 0;

    uint8_t attribute_value[16];
    uint16_t attribute_len = (uint16_t)sizeof(attribute_value);
    reverse_128(uuid128, attribute_value);

    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 == 0u) || new_service_started)){
            *end_handle = prev_handle;
            return true;
        }

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

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

// returns 0 if not found
uint16_t gatt_server_get_value_handle_for_characteristic_with_uuid128(uint16_t start_handle, uint16_t end_handle, const uint8_t * uuid128){
    uint8_t attribute_value[16];
    reverse_128(uuid128, attribute_value);
    att_iterator_t it;
    att_iterator_init(&it);
    while (att_iterator_has_next(&it)){
        att_iterator_fetch_next(&it);
        if ((it.handle != 0u) && (it.handle < start_handle)){
            continue;
        }
        if (it.handle > end_handle){
            break;  // (1)
        }
        if (it.handle == 0u){
            break;
        }
        if (att_iterator_match_uuid(&it, attribute_value, 16)){
            return it.handle;
        }
    }
    return 0;
}

// returns 0 if not found
uint16_t gatt_server_get_client_configuration_handle_for_characteristic_with_uuid128(uint16_t start_handle, uint16_t end_handle, const uint8_t * uuid128){
    uint8_t attribute_value[16];
    reverse_128(uuid128, attribute_value);
    att_iterator_t it;
    att_iterator_init(&it);
    bool characteristic_found = false;
    while (att_iterator_has_next(&it)){
        att_iterator_fetch_next(&it);
        if ((it.handle != 0u) && (it.handle < start_handle)){
            continue;
        }
        if (it.handle > end_handle){
            break;  // (1)
        }
        if (it.handle == 0u){
            break;
        }
        if (att_iterator_match_uuid(&it, attribute_value, 16)){
            characteristic_found = true;
            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 (characteristic_found && att_iterator_match_uuid16(&it, GATT_CLIENT_CHARACTERISTICS_CONFIGURATION)){
            return it.handle;
        }
    }
    return 0;
}


bool gatt_server_get_included_service_with_uuid16(uint16_t start_handle, uint16_t end_handle, uint16_t uuid16,
    uint16_t * out_included_service_handle, uint16_t * out_included_service_start_handle, uint16_t * out_included_service_end_handle){

    att_iterator_t it;
    att_iterator_init(&it);
    while (att_iterator_has_next(&it)){
        att_iterator_fetch_next(&it);
        if ((it.handle != 0u) && (it.handle < start_handle)){
            continue;
        }
        if (it.handle > end_handle){
            break;  // (1)
        }
        if (it.handle == 0u){
            break;
        }
        if ((it.value_len == 6u) && (att_iterator_match_uuid16(&it, GATT_INCLUDE_SERVICE_UUID))){
            if (little_endian_read_16(it.value, 4) == uuid16){
                *out_included_service_handle = it.handle;
                *out_included_service_start_handle = little_endian_read_16(it.value, 0);
                *out_included_service_end_handle = little_endian_read_16(it.value, 2);
                return true;
            }
        }
    }
    return false;
}

// 1-item cache to optimize query during write_callback
static void att_persistent_ccc_cache(att_iterator_t * it){
    att_persistent_ccc_handle = it->handle;
    if ((it->flags & (uint16_t)ATT_PROPERTY_UUID128) != 0u){
        att_persistent_ccc_uuid16 = 0u;
    } else {
        att_persistent_ccc_uuid16 = little_endian_read_16(it->uuid, 0);
    }
}

bool att_is_persistent_ccc(uint16_t handle){
    if (handle != att_persistent_ccc_handle){
        att_iterator_t it;
        bool ok = att_find_handle(&it, handle);
        if (!ok){
            return false;
        }
        att_persistent_ccc_cache(&it);
    }
    switch (att_persistent_ccc_uuid16){
        case GATT_CLIENT_CHARACTERISTICS_CONFIGURATION:
        case GATT_CLIENT_SUPPORTED_FEATURES:
            return true;
        default:
            return false;
    }
}

// att_read_callback helpers
uint16_t att_read_callback_handle_blob(const uint8_t * blob, uint16_t blob_size, uint16_t offset, uint8_t * buffer, uint16_t buffer_size){
    btstack_assert(blob != NULL);

    if (buffer != NULL){
        uint16_t bytes_to_copy = 0;
        if (blob_size >= offset){
            bytes_to_copy = btstack_min(blob_size - offset, buffer_size);
            (void)memcpy(buffer, &blob[offset], bytes_to_copy);
        }
        return bytes_to_copy;
    } else {
        return blob_size;
    }
}

uint16_t att_read_callback_handle_little_endian_32(uint32_t value, uint16_t offset, uint8_t * buffer, uint16_t buffer_size){
    uint8_t value_buffer[4];
    little_endian_store_32(value_buffer, 0, value);
    return att_read_callback_handle_blob(value_buffer, sizeof(value_buffer), offset, buffer, buffer_size);
}

uint16_t att_read_callback_handle_little_endian_16(uint16_t value, uint16_t offset, uint8_t * buffer, uint16_t buffer_size){
    uint8_t value_buffer[2];
    little_endian_store_16(value_buffer, 0, value);
    return att_read_callback_handle_blob(value_buffer, sizeof(value_buffer), offset, buffer, buffer_size);
}

uint16_t att_read_callback_handle_byte(uint8_t value, uint16_t offset, uint8_t * buffer, uint16_t buffer_size){
    uint8_t value_buffer[1];
    value_buffer[0] = value;
    return att_read_callback_handle_blob(value_buffer, sizeof(value_buffer), offset, buffer, buffer_size);
}


#ifdef ENABLE_BTP

// start of auto-PTS testing code, not used in production
// LCOV_EXCL_START
#include "btp.h"

static uint8_t btp_permissions_for_flags(uint16_t flags){

    // see BT_GATT_PERM_*
    // https://docs.zephyrproject.org/latest/reference/bluetooth/gatt.html
    // set bit indicates requirement, e.g. BTP_GATT_PERM_READ_AUTHN requires authenticated connection

    uint8_t permissions = 0;

    uint8_t read_security_level = 0;
    uint8_t write_security_level = 0;
    if (flags & (uint16_t)ATT_PROPERTY_READ){
        if (flags & (uint16_t)ATT_PROPERTY_READ_PERMISSION_BIT_0) {
            read_security_level |= 1;
        }
        if (flags & (uint16_t)ATT_PROPERTY_READ_PERMISSION_BIT_1) {
            read_security_level |= 2;
        }
        if (read_security_level == ATT_SECURITY_AUTHORIZED) {
            permissions |= BTP_GATT_PERM_READ_AUTHZ;
        }
        if (read_security_level == ATT_SECURITY_AUTHENTICATED) {
            permissions |= BTP_GATT_PERM_READ_AUTHN;
        }
        if (read_security_level == ATT_SECURITY_ENCRYPTED) {
            permissions |= BTP_GATT_PERM_READ_ENC;
        }
        if (read_security_level == ATT_SECURITY_NONE) {
            permissions |= BTP_GATT_PERM_READ;
        }
    }
    if (flags & (ATT_PROPERTY_WRITE | ATT_PROPERTY_WRITE_WITHOUT_RESPONSE)){
        if (flags & (uint16_t)ATT_PROPERTY_WRITE_PERMISSION_BIT_0) {
            write_security_level |= 1;
        }
        if (flags & (uint16_t)ATT_PROPERTY_WRITE_PERMISSION_BIT_1) {
            write_security_level |= 2;
        }
        if (write_security_level == ATT_SECURITY_AUTHORIZED) {
            permissions |= BTP_GATT_PERM_WRITE_AUTHZ;
        }
        if (write_security_level == ATT_SECURITY_AUTHENTICATED) {
            permissions |= BTP_GATT_PERM_WRITE_AUTHN;
        }
        if (write_security_level == ATT_SECURITY_ENCRYPTED) {
            permissions |= BTP_GATT_PERM_WRITE_ENC;
        }
        if (write_security_level == ATT_SECURITY_NONE) {
            permissions |= BTP_GATT_PERM_WRITE;
        }
    }
    return permissions;
}

uint16_t btp_att_get_attributes_by_uuid16(uint16_t start_handle, uint16_t end_handle, uint16_t uuid16, uint8_t * response_buffer, uint16_t response_buffer_size){
    log_info("btp_att_get_attributes_by_uuid16 %04x from 0x%04x to 0x%04x, db %p", uuid16, start_handle, end_handle, att_database);
    att_dump_attributes();

    uint8_t num_attributes = 0;
    uint16_t pos = 1;

    att_iterator_t  it;
    att_iterator_init(&it);
    while (att_iterator_has_next(&it) && ((pos + 6) < response_buffer_size)){
        att_iterator_fetch_next(&it);
        log_info("handle %04x", it.handle);
        if (it.handle == 0){
            break;
        }
        if (it.handle < start_handle){
            continue;
        }
        if (it.handle > end_handle){
            break;
        }
        if ((uuid16 == 0) || att_iterator_match_uuid16(&it, uuid16)){
            little_endian_store_16(response_buffer, pos, it.handle);
            pos += 2;
            response_buffer[pos++] = btp_permissions_for_flags(it.flags);
            response_buffer[pos++] = 2;
            little_endian_store_16(response_buffer, pos, uuid16);
            pos += 2;
            num_attributes++;
        }
    }
    response_buffer[0] = num_attributes;
    return pos;
}

uint16_t btp_att_get_attributes_by_uuid128(uint16_t start_handle, uint16_t end_handle, const uint8_t * uuid128, uint8_t * response_buffer, uint16_t response_buffer_size){
    uint8_t num_attributes = 0;
    uint16_t pos = 1;
    att_iterator_t  it;
    att_iterator_init(&it);
    while (att_iterator_has_next(&it) && ((pos + 20) < response_buffer_size)){
        att_iterator_fetch_next(&it);
        if (it.handle == 0){
            break;
        }
        if (it.handle < start_handle){
            continue;
        }
        if (it.handle > end_handle){
            break;
        }
        if (att_iterator_match_uuid(&it, (uint8_t*) uuid128, 16)){
            little_endian_store_16(response_buffer, pos, it.handle);
            pos += 2;
            response_buffer[pos++] = btp_permissions_for_flags(it.flags);
            response_buffer[pos++] = 16;
            reverse_128(uuid128, &response_buffer[pos]);
            pos += 16;
            num_attributes++;
        }
    }
    response_buffer[0] = num_attributes;
    return pos;
}

uint16_t btp_att_get_attribute_value(att_connection_t * att_connection, uint16_t attribute_handle, uint8_t * response_buffer, uint16_t response_buffer_size){
    att_iterator_t it;
    bool ok = att_find_handle(&it, attribute_handle);
    if (!ok){
        return 0;
    }

    uint16_t pos = 0;
    // field: ATT_Response - simulate READ operation on given connection
    response_buffer[pos++] = att_validate_security(att_connection, ATT_READ, &it);
    // fetch len
    // assume: con handle not relevant here, else, it needs to get passed in
    // att_update_value_len(&it, HCI_CON_HANDLE_INVALID);
    uint16_t bytes_to_copy = btstack_min( response_buffer_size - 3, it.value_len);
    little_endian_store_16(response_buffer, pos, bytes_to_copy);
    pos += 2;
    // get value - only works for non-dynamic data
    if (it.value){
        memcpy(&response_buffer[pos], it.value, bytes_to_copy);
        pos += bytes_to_copy;
    }
    return pos;
}
// LCOV_EXCL_STOP
#endif