Keep-alive like librespot (core/src/session.rs): the server sends a Ping every ~2 min; send
the Pong 60 s later, not at once. With an immediate Pong the server reset the AP connection
(ECONNRESET) on the third Ping, i.e. every 6 minutes, and the Spotify app lost the device.

--- a/cspot/include/MercurySession.h
+++ b/cspot/include/MercurySession.h
@@ -120,6 +120,14 @@
 
   unsigned long long timestampDiff;
   unsigned long long lastPingTimestamp = -1;
+
+  // Keep-alive as librespot does it: Ping -> wait 60 s -> Pong -> PongAck. Answering at once made
+  // the server reset the connection on the third Ping (every 6 minutes).
+  const int PONG_DELAY_MS = 60 * 1000;
+  bool pongPending = false;
+  unsigned long long pongDueTimestamp = 0;
+  std::vector<uint8_t> pongData;
+  void sendPendingPong();
   std::string countryCode = "";
 
   std::mutex isRunningMutex;
--- a/cspot/src/MercurySession.cpp
+++ b/cspot/src/MercurySession.cpp
@@ -45,7 +45,10 @@
         timeProvider->syncWithPingPacket(packet.data);
 
         this->lastPingTimestamp = timeProvider->getSyncedTimestamp();
-        this->shanConn->sendPacket(0x49, packet.data);
+        // Pong 60 s later (sent from triggerTimeout(), called on every 3 s receive timeout)
+        this->pongData = packet.data;
+        this->pongDueTimestamp = this->lastPingTimestamp + PONG_DELAY_MS;
+        this->pongPending = true;
       } else {
         this->packetQueue.push(packet);
       }
@@ -80,6 +83,7 @@
     BELL_SLEEP_MS(100);
 
     lastPingTimestamp = timeProvider->getSyncedTimestamp();
+    pongPending = false;    // a Ping on the old connection needs no Pong on the new one
     isReconnecting = false;
 
     this->executeEstabilishedCallback = true;
@@ -98,9 +102,30 @@
   this->connectionReadyCallback = callback;
 }
 
+void MercurySession::sendPendingPong() {
+  if (!pongPending ||
+      timeProvider->getSyncedTimestamp() < pongDueTimestamp)
+    return;
+  pongPending = false;
+  std::shared_ptr<ShannonConnection> shanConn;
+  {
+    std::scoped_lock lock(connMutex);
+    shanConn = this->shanConn;
+  }
+  if (!shanConn)
+    return;
+  try {
+    shanConn->sendPacket(0x49, pongData);
+    CSPOT_LOG(info, "Sent delayed Pong");
+  } catch (...) {
+    CSPOT_LOG(error, "Failed to send Pong");
+  }
+}
+
 bool MercurySession::triggerTimeout() {
   if (!isRunning)
     return true;
+  sendPendingPong();
   auto currentTimestamp = timeProvider->getSyncedTimestamp();
 
   if (currentTimestamp - this->lastPingTimestamp > PING_TIMEOUT_MS) {