Add new ROUTER_LATE role

Will always rebroadcast packets, but will do so after all other modes.
Intended for router nodes that are there to provide additional coverage
in areas not already covered by other routers, or to bridge around
problematic terrain, but should not be given priority over other routers
in order to avoid unnecessaraily consuming hops.

By default, this role will rebroadcast during the normal client window.
However, if another node is overheard rebroadcasting the packet, then it
will be moved to a second window *after* the normal client one, with the
same timing behaviour.

src/mesh/FloodingRouter.cpp

@@ -24,11 +24,15 @@ bool FloodingRouter::shouldFilterReceived(const meshtastic_MeshPacket *p)
         printPacket("Ignore dupe incoming msg", p);
         rxDupe++;
         if (config.device.role != meshtastic_Config_DeviceConfig_Role_ROUTER &&
-            config.device.role != meshtastic_Config_DeviceConfig_Role_REPEATER) {
+            config.device.role != meshtastic_Config_DeviceConfig_Role_REPEATER &&
+            config.device.role != meshtastic_Config_DeviceConfig_Role_ROUTER_LATE) {
             // cancel rebroadcast of this message *if* there was already one, unless we're a router/repeater!
             if (Router::cancelSending(p->from, p->id))
                 txRelayCanceled++;
         }
+        if (config.device.role == meshtastic_Config_DeviceConfig_Role_ROUTER_LATE && iface) {
+            iface->clampToLateRebroadcastWindow(getFrom(p), p->id);
+        }
 
         /* If the original transmitter is doing retransmissions (hopStart equals hopLimit) for a reliable transmission, e.g., when
         the ACK got lost, we will handle the packet again to make sure it gets an ACK to its packet. */

src/mesh/MeshPacketQueue.cpp

@@ -16,6 +16,12 @@ inline uint32_t getPriority(const meshtastic_MeshPacket *p)
 bool CompareMeshPacketFunc(const meshtastic_MeshPacket *p1, const meshtastic_MeshPacket *p2)
 {
     assert(p1 && p2);
+
+    // If one packet is in the late transmit window, prefer the other one
+    if ((bool)p1->tx_after != (bool)p2->tx_after) {
+        return !p1->tx_after;
+    }
+
     auto p1p = getPriority(p1), p2p = getPriority(p2);
     // If priorities differ, use that
     // for equal priorities, prefer packets already on mesh.
@@ -94,11 +100,11 @@ meshtastic_MeshPacket *MeshPacketQueue::getFront()
 }
 
 /** Attempt to find and remove a packet from this queue.  Returns a pointer to the removed packet, or NULL if not found */
-meshtastic_MeshPacket *MeshPacketQueue::remove(NodeNum from, PacketId id)
+meshtastic_MeshPacket *MeshPacketQueue::remove(NodeNum from, PacketId id, bool tx_normal, bool tx_late)
 {
     for (auto it = queue.begin(); it != queue.end(); it++) {
         auto p = (*it);
-        if (getFrom(p) == from && p->id == id) {
+        if (getFrom(p) == from && p->id == id && ((tx_normal && !p->tx_after) || (tx_late && p->tx_after))) {
             queue.erase(it);
             return p;
         }
@@ -114,9 +120,10 @@ bool MeshPacketQueue::replaceLowerPriorityPacket(meshtastic_MeshPacket *p)
     if (queue.empty()) {
         return false; // No packets to replace
     }
+
     // Check if the packet at the back has a lower priority than the new packet
     auto &backPacket = queue.back();
-    if (backPacket->priority < p->priority) {
+    if (!backPacket->tx_after && backPacket->priority < p->priority) {
         // Remove the back packet
         packetPool.release(backPacket);
         queue.pop_back();
@@ -125,6 +132,19 @@ bool MeshPacketQueue::replaceLowerPriorityPacket(meshtastic_MeshPacket *p)
         return true;
     }
 
+    if (backPacket->tx_after) {
+        // Check if there's a non-late packet with lower priority
+        auto it = queue.end();
+        auto refPacket = *--it;
+        for (; refPacket->tx_after && it != queue.begin(); refPacket = *--it)
+            ;
+        if (!refPacket->tx_after && refPacket->priority < p->priority) {
+            packetPool.release(refPacket);
+            enqueue(refPacket);
+            return true;
+        }
+    }
+
     // If the back packet's priority is not lower, no replacement occurs
     return false;
 }

src/mesh/MeshPacketQueue.h

@@ -36,5 +36,5 @@ class MeshPacketQueue
     meshtastic_MeshPacket *getFront();
 
     /** Attempt to find and remove a packet from this queue.  Returns the packet which was removed from the queue */
-    meshtastic_MeshPacket *remove(NodeNum from, PacketId id);
-};
+    meshtastic_MeshPacket *remove(NodeNum from, PacketId id, bool tx_normal = true, bool tx_late = true);
+};

src/mesh/RadioInterface.cpp

@@ -254,19 +254,33 @@ uint32_t RadioInterface::getTxDelayMsec()
     return random(0, pow(2, CWsize)) * slotTimeMsec;
 }
 
-/** The delay to use when we want to flood a message */
-uint32_t RadioInterface::getTxDelayMsecWeighted(float snr)
+/** The CW size to use when calculating SNR_based delays */
+uint8_t RadioInterface::getCWsize(float snr)
 {
     // The minimum value for a LoRa SNR
     const uint32_t SNR_MIN = -20;
 
     // The maximum value for a LoRa SNR
     const uint32_t SNR_MAX = 15;
 
+    return map(snr, SNR_MIN, SNR_MAX, CWmin, CWmax);
+}
+
+/** The worst-case SNR_based packet delay */
+uint32_t RadioInterface::getTxDelayMsecWeightedWorst(float snr)
+{
+    uint8_t CWsize = getCWsize(snr);
+    // offset the maximum delay for routers: (2 * CWmax * slotTimeMsec)
+    return (2 * CWmax * slotTimeMsec) + pow(2, CWsize) * slotTimeMsec;
+}
+
+/** The delay to use when we want to flood a message */
+uint32_t RadioInterface::getTxDelayMsecWeighted(float snr)
+{
     //  high SNR = large CW size (Long Delay)
     //  low SNR = small CW size (Short Delay)
     uint32_t delay = 0;
-    uint8_t CWsize = map(snr, SNR_MIN, SNR_MAX, CWmin, CWmax);
+    uint8_t CWsize = getCWsize(snr);
     // LOG_DEBUG("rx_snr of %f so setting CWsize to:%d", snr, CWsize);
     if (config.device.role == meshtastic_Config_DeviceConfig_Role_ROUTER ||
         config.device.role == meshtastic_Config_DeviceConfig_Role_REPEATER) {

src/mesh/RadioInterface.h

@@ -173,9 +173,18 @@ class RadioInterface
     /** The delay to use when we want to send something */
     uint32_t getTxDelayMsec();
 
+    /** The CW to use when calculating SNR_based delays */
+    uint8_t getCWsize(float snr);
+
+    /** The worst-case SNR_based packet delay */
+    uint32_t getTxDelayMsecWeightedWorst(float snr);
+
     /** The delay to use when we want to flood a message. Use a weighted scale based on SNR */
     uint32_t getTxDelayMsecWeighted(float snr);
 
+    /** If the packet is not already in the late rebroadcast window, move it there */
+    virtual void clampToLateRebroadcastWindow(NodeNum from, PacketId id) { return; }
+
     /**
      * Calculate airtime per
      * https://www.rs-online.com/designspark/rel-assets/ds-assets/uploads/knowledge-items/application-notes-for-the-internet-of-things/LoRa%20Design%20Guide.pdf

src/mesh/RadioLibInterface.cpp

@@ -235,12 +235,12 @@ void RadioLibInterface::onNotify(uint32_t notification)
     case ISR_TX:
         handleTransmitInterrupt();
         startReceive();
-        startTransmitTimer();
+        setTransmitDelay();
         break;
     case ISR_RX:
         handleReceiveInterrupt();
         startReceive();
-        startTransmitTimer();
+        setTransmitDelay();
         break;
     case TRANSMIT_DELAY_COMPLETED:
 
@@ -250,23 +250,32 @@ void RadioLibInterface::onNotify(uint32_t notification)
             if (!canSendImmediately()) {
                 setTransmitDelay(); // currently Rx/Tx-ing: reset random delay
             } else {
-                if (isChannelActive()) { // check if there is currently a LoRa packet on the channel
-                    startReceive();      // try receiving this packet, afterwards we'll be trying to transmit again
-                    setTransmitDelay();
+                meshtastic_MeshPacket *txp = txQueue.getFront();
+                assert(txp);
+                long delay_remaining = txp->tx_after ? txp->tx_after - millis() : 0;
+                if (delay_remaining > 0) {
+                    // There's still some delay pending on this packet, so resume waiting for it to elapse
+                    notifyLater(delay_remaining, TRANSMIT_DELAY_COMPLETED, false);
                 } else {
-                    // Send any outgoing packets we have ready as fast as possible to keep the time between channel scan and
-                    // actual transmission as short as possible
-                    meshtastic_MeshPacket *txp = txQueue.dequeue();
-                    assert(txp);
-                    bool sent = startSend(txp);
-                    if (sent) {
-                        // Packet has been sent, count it toward our TX airtime utilization.
-                        uint32_t xmitMsec = getPacketTime(txp);
-                        airTime->logAirtime(TX_LOG, xmitMsec);
+                    if (isChannelActive()) { // check if there is currently a LoRa packet on the channel
+                        startReceive();      // try receiving this packet, afterwards we'll be trying to transmit again
+                        setTransmitDelay();
+                    } else {
+                        // Send any outgoing packets we have ready as fast as possible to keep the time between channel scan and
+                        // actual transmission as short as possible
+                        txp = txQueue.dequeue();
+                        assert(txp);
+                        bool sent = startSend(txp);
+                        if (sent) {
+                            // Packet has been sent, count it toward our TX airtime utilization.
+                            uint32_t xmitMsec = getPacketTime(txp);
+                            airTime->logAirtime(TX_LOG, xmitMsec);
+                        }
                     }
                 }
             }
         } else {
+            // Do nothing, because the queue is empty
         }
         break;
     default:
@@ -277,15 +286,24 @@ void RadioLibInterface::onNotify(uint32_t notification)
 void RadioLibInterface::setTransmitDelay()
 {
     meshtastic_MeshPacket *p = txQueue.getFront();
+    if (!p) {
+        return; // noop if there's nothing in the queue
+    }
+
     // We want all sending/receiving to be done by our daemon thread.
     // We use a delay here because this packet might have been sent in response to a packet we just received.
     // So we want to make sure the other side has had a chance to reconfigure its radio.
 
-    /* We assume if rx_snr = 0 and rx_rssi = 0, the packet was generated locally.
-     *   This assumption is valid because of the offset generated by the radio to account for the noise
-     *   floor.
-     */
-    if (p->rx_snr == 0 && p->rx_rssi == 0) {
+    if (p->tx_after) {
+        unsigned long add_delay = p->rx_rssi ? getTxDelayMsecWeighted(p->rx_snr) : getTxDelayMsec();
+        unsigned long now = millis();
+        p->tx_after = max(p->tx_after + add_delay, now + add_delay);
+        notifyLater(now - p->tx_after, TRANSMIT_DELAY_COMPLETED, false);
+    } else if (p->rx_snr == 0 && p->rx_rssi == 0) {
+        /* We assume if rx_snr = 0 and rx_rssi = 0, the packet was generated locally.
+         *   This assumption is valid because of the offset generated by the radio to account for the noise
+         *   floor.
+         */
         startTransmitTimer(true);
     } else {
         // If there is a SNR, start a timer scaled based on that SNR.
@@ -312,6 +330,20 @@ void RadioLibInterface::startTransmitTimerSNR(float snr)
     }
 }
 
+/**
+ * If the packet is not already in the late rebroadcast window, move it there
+ */
+void RadioLibInterface::clampToLateRebroadcastWindow(NodeNum from, PacketId id)
+{
+    // Look for non-late packets only, so we don't do this twice!
+    meshtastic_MeshPacket *p = txQueue.remove(from, id, true, false);
+    if (p) {
+        p->tx_after = millis() + getTxDelayMsecWeightedWorst(p->rx_snr);
+        txQueue.enqueue(p);
+        LOG_DEBUG("Move existing queued packet to the late rebroadcast window %dms from now", p->tx_after - millis());
+    }
+}
+
 void RadioLibInterface::handleTransmitInterrupt()
 {
     // This can be null if we forced the device to enter standby mode.  In that case

src/mesh/RadioLibInterface.h

@@ -140,10 +140,16 @@ class RadioLibInterface : public RadioInterface, protected concurrency::Notified
      * doing the transmit */
     void setTransmitDelay();
 
-    /** random timer with certain min. and max. settings */
+    /**
+     * random timer with certain min. and max. settings
+     * @return Timestamp after which the packet may be sent
+     */
     void startTransmitTimer(bool withDelay = true);
 
-    /** timer scaled to SNR of to be flooded packet */
+    /**
+     * timer scaled to SNR of to be flooded packet
+     * @return Timestamp after which the packet may be sent
+     */
     void startTransmitTimerSNR(float snr);
 
     void handleTransmitInterrupt();
@@ -193,4 +199,9 @@ class RadioLibInterface : public RadioInterface, protected concurrency::Notified
     virtual void setStandby();
 
     const char *radioLibErr = "RadioLib err=";
+
+    /**
+     * If the packet is not already in the late rebroadcast window, move it there
+     */
+    void clampToLateRebroadcastWindow(NodeNum from, PacketId id);
 };