Sending longer payload -> How to set (request) MTU change from server side

[marcelrv]

I'm trying to send a larger characteristic than the default 20 bytes. I'm trying to understand how I can request a MTU change from the server side.

See below NimBLE_Server example modified to (how I understood) request the larger MTU

added near the NimBLEDevice::init NimBLEDevice::setMTU(48); added in the onConnect pServer->setDataLen(desc->conn_handle, 48); and changed the payload to be longer

Neither seemed to work. If I change the MTU from the client side it works very well.

note: I saw in the NimBLEServer::setDataLen(uint16_t conn_handle, uint16_t tx_octets) that I needed at least ESP_IDF_VERSION 432. I indeed updated it and now I have ESP_IDF_VERSION 441

server: ESP32 client: Android (nRF Connect App)

Any suggestion?

modfied example:

#include <Arduino.h>

/** NimBLE_Server Demo:
 *
 *  Demonstrates many of the available features of the NimBLE server library.
 *
 *  Created: on March 22 2020
 *      Author: H2zero
 *
*/

#include <NimBLEDevice.h>

static NimBLEServer* pServer;

/**  None of these are required as they will be handled by the library with defaults. **
 **                       Remove as you see fit for your needs                        */
class ServerCallbacks: public NimBLEServerCallbacks {
    void onConnect(NimBLEServer* pServer) {
        Serial.println("Client connected");
        Serial.println("Multi-connect support: start advertising");
        NimBLEDevice::startAdvertising();
    };
    /** Alternative onConnect() method to extract details of the connection.
     *  See: src/ble_gap.h for the details of the ble_gap_conn_desc struct.
     */
    void onConnect(NimBLEServer* pServer, ble_gap_conn_desc* desc) {
        Serial.print("Client address: ");
        Serial.println(NimBLEAddress(desc->peer_ota_addr).toString().c_str());
        /** We can use the connection handle here to ask for different connection parameters.
         *  Args: connection handle, min connection interval, max connection interval
         *  latency, supervision timeout.
         *  Units; Min/Max Intervals: 1.25 millisecond increments.
         *  Latency: number of intervals allowed to skip.
         *  Timeout: 10 millisecond increments, try for 5x interval time for best results.
         */
        pServer->updateConnParams(desc->conn_handle, 24, 48, 0, 60);
       // ADDED this line to change data len after the connection is made
        pServer->setDataLen(desc->conn_handle, 48);

    };
    void onDisconnect(NimBLEServer* pServer) {
        Serial.println("Client disconnected - start advertising");
        NimBLEDevice::startAdvertising();
    };
    void onMTUChange(uint16_t MTU, ble_gap_conn_desc* desc) {
        Serial.printf("MTU updated: %u for connection ID: %u\n", MTU, desc->conn_handle);
    };

/********************* Security handled here **********************
****** Note: these are the same return values as defaults ********/
    uint32_t onPassKeyRequest(){
        Serial.println("Server Passkey Request");
        /** This should return a random 6 digit number for security
         *  or make your own static passkey as done here.
         */
        return 123456;
    };

    bool onConfirmPIN(uint32_t pass_key){
        Serial.print("The passkey YES/NO number: ");Serial.println(pass_key);
        /** Return false if passkeys don't match. */
        return true;
    };

    void onAuthenticationComplete(ble_gap_conn_desc* desc){
        /** Check that encryption was successful, if not we disconnect the client */
        if(!desc->sec_state.encrypted) {
            NimBLEDevice::getServer()->disconnect(desc->conn_handle);
            Serial.println("Encrypt connection failed - disconnecting client");
            return;
        }
        Serial.println("Starting BLE work!");
    };
};

/** Handler class for characteristic actions */
class CharacteristicCallbacks: public NimBLECharacteristicCallbacks {
    void onRead(NimBLECharacteristic* pCharacteristic){
        Serial.print(pCharacteristic->getUUID().toString().c_str());
        Serial.print(": onRead(), value: ");
        Serial.println(pCharacteristic->getValue().c_str());
    };

    void onWrite(NimBLECharacteristic* pCharacteristic) {
        Serial.print(pCharacteristic->getUUID().toString().c_str());
        Serial.print(": onWrite(), value: ");
        Serial.println(pCharacteristic->getValue().c_str());
    };
    /** Called before notification or indication is sent,
     *  the value can be changed here before sending if desired.
     */
    void onNotify(NimBLECharacteristic* pCharacteristic) {
        Serial.println("Sending notification to clients");
    };

    /** The status returned in status is defined in NimBLECharacteristic.h.
     *  The value returned in code is the NimBLE host return code.
     */
    void onStatus(NimBLECharacteristic* pCharacteristic, Status status, int code) {
        String str = ("Notification/Indication status code: ");
        str += status;
        str += ", return code: ";
        str += code;
        str += ", ";
        str += NimBLEUtils::returnCodeToString(code);
        Serial.println(str);
    };

    void onSubscribe(NimBLECharacteristic* pCharacteristic, ble_gap_conn_desc* desc, uint16_t subValue) {
        String str = "Client ID: ";
        str += desc->conn_handle;
        str += " Address: ";
        str += std::string(NimBLEAddress(desc->peer_ota_addr)).c_str();
        if(subValue == 0) {
            str += " Unsubscribed to ";
        }else if(subValue == 1) {
            str += " Subscribed to notfications for ";
        } else if(subValue == 2) {
            str += " Subscribed to indications for ";
        } else if(subValue == 3) {
            str += " Subscribed to notifications and indications for ";
        }
        str += std::string(pCharacteristic->getUUID()).c_str();

        Serial.println(str);
    };
};

/** Handler class for descriptor actions */
class DescriptorCallbacks : public NimBLEDescriptorCallbacks {
    void onWrite(NimBLEDescriptor* pDescriptor) {
        std::string dscVal = pDescriptor->getValue();
        Serial.print("Descriptor witten value:");
        Serial.println(dscVal.c_str());
    };

    void onRead(NimBLEDescriptor* pDescriptor) {
        Serial.print(pDescriptor->getUUID().toString().c_str());
        Serial.println(" Descriptor read");
    };
};

/** Define callback instances globally to use for multiple Charateristics \ Descriptors */
static DescriptorCallbacks dscCallbacks;
static CharacteristicCallbacks chrCallbacks;

void setup() {
    Serial.begin(115200);
    Serial.println("Starting NimBLE Server");

    /** sets device name */
    NimBLEDevice::init("NimBLE-Arduino");
// ADDED to support larger MTU
    NimBLEDevice::setMTU(48);

    /** Optional: set the transmit power, default is 3db */
#ifdef ESP_PLATFORM
    NimBLEDevice::setPower(ESP_PWR_LVL_P9); /** +9db */
#else
    NimBLEDevice::setPower(9); /** +9db */
#endif

    /** Set the IO capabilities of the device, each option will trigger a different pairing method.
     *  BLE_HS_IO_DISPLAY_ONLY    - Passkey pairing
     *  BLE_HS_IO_DISPLAY_YESNO   - Numeric comparison pairing
     *  BLE_HS_IO_NO_INPUT_OUTPUT - DEFAULT setting - just works pairing
     */
    //NimBLEDevice::setSecurityIOCap(BLE_HS_IO_DISPLAY_ONLY); // use passkey
    //NimBLEDevice::setSecurityIOCap(BLE_HS_IO_DISPLAY_YESNO); //use numeric comparison

    /** 2 different ways to set security - both calls achieve the same result.
     *  no bonding, no man in the middle protection, secure connections.
     *
     *  These are the default values, only shown here for demonstration.
     */
    //NimBLEDevice::setSecurityAuth(false, false, true);
    NimBLEDevice::setSecurityAuth(/*BLE_SM_PAIR_AUTHREQ_BOND | BLE_SM_PAIR_AUTHREQ_MITM |*/ BLE_SM_PAIR_AUTHREQ_SC);

    pServer = NimBLEDevice::createServer();
    pServer->setCallbacks(new ServerCallbacks());

    NimBLEService* pDeadService = pServer->createService("DEAD");
    NimBLECharacteristic* pBeefCharacteristic = pDeadService->createCharacteristic(
                                               "BEEF",
                                               NIMBLE_PROPERTY::READ |
                                               NIMBLE_PROPERTY::WRITE |
                               /** Require a secure connection for read and write access */
                                               NIMBLE_PROPERTY::READ_ENC |  // only allow reading if paired / encrypted
                                               NIMBLE_PROPERTY::WRITE_ENC   // only allow writing if paired / encrypted
                                              );

    pBeefCharacteristic->setValue("Burger");
    pBeefCharacteristic->setCallbacks(&chrCallbacks);

    /** 2904 descriptors are a special case, when createDescriptor is called with
     *  0x2904 a NimBLE2904 class is created with the correct properties and sizes.
     *  However we must cast the returned reference to the correct type as the method
     *  only returns a pointer to the base NimBLEDescriptor class.
     */
    NimBLE2904* pBeef2904 = (NimBLE2904*)pBeefCharacteristic->createDescriptor("2904");
    pBeef2904->setFormat(NimBLE2904::FORMAT_UTF8);
    pBeef2904->setCallbacks(&dscCallbacks);

    NimBLEService* pBaadService = pServer->createService("BAAD");
    NimBLECharacteristic* pFoodCharacteristic = pBaadService->createCharacteristic(
                                               "F00D",
                                               NIMBLE_PROPERTY::READ |
                                               NIMBLE_PROPERTY::WRITE |
                                               NIMBLE_PROPERTY::NOTIFY
                                              );

    pFoodCharacteristic->setValue("123456789012345678901234567890");
    pFoodCharacteristic->setCallbacks(&chrCallbacks);

    /** Note a 0x2902 descriptor MUST NOT be created as NimBLE will create one automatically
     *  if notification or indication properties are assigned to a characteristic.
     */

    /** Custom descriptor: Arguments are UUID, Properties, max length in bytes of the value */
    NimBLEDescriptor* pC01Ddsc = pFoodCharacteristic->createDescriptor(
                                               "C01D",
                                               NIMBLE_PROPERTY::READ |
                                               NIMBLE_PROPERTY::WRITE|
                                               NIMBLE_PROPERTY::WRITE_ENC, // only allow writing if paired / encrypted
                                               20
                                              );
    pC01Ddsc->setValue("Send it back!");
    pC01Ddsc->setCallbacks(&dscCallbacks);

    /** Start the services when finished creating all Characteristics and Descriptors */
    pDeadService->start();
    pBaadService->start();

    NimBLEAdvertising* pAdvertising = NimBLEDevice::getAdvertising();
    /** Add the services to the advertisment data **/
    pAdvertising->addServiceUUID(pDeadService->getUUID());
    pAdvertising->addServiceUUID(pBaadService->getUUID());
    /** If your device is battery powered you may consider setting scan response
     *  to false as it will extend battery life at the expense of less data sent.
     */
    pAdvertising->setScanResponse(true);
    pAdvertising->start();

    Serial.println("Advertising Started");
}

void loop() {
  /** Do your thing here, this just spams notifications to all connected clients */
    if(pServer->getConnectedCount()) {
        NimBLEService* pSvc = pServer->getServiceByUUID("BAAD");
        if(pSvc) {
            NimBLECharacteristic* pChr = pSvc->getCharacteristic("F00D");
            if(pChr) {
                pChr->notify(true);
            }
        }
    }

  delay(2000);
}

[h2zero]

Normally it's expected that the client requests the MTU exchange. There is no server method for this implemented at this time but it should be possible to use the lower level calls to do this.

[marcelrv]

Thanks.. will see if I can get it to work. thx

[spilz87]

Hello Did you find a way to increase it ? Thanks in advance

[h2zero]

For anyone else that finds this, the BLE specification states the MTU exchange is to be performed by the client:

The default MTU for all BLE devices is 23, 20 usable bytes. The client and server both need to have their own MTU size set higher in order to increase this. After the MTU exchange the MTU will be the smaller of the 2 values that the client and server possess.

The MTU is the max attribute (think characteristic value) size and is used for things like notifications. The data length sets how much of that data can be sent in a single transmission during a connection interval, setting this to a higher value than the MTU will result in greater throughput, if it's smaller, then the data will be chunked and sent slower.