Layer 2: send Empty Packet acknowledgements if there’s nothing else to transmit

impl/src/layer2/layer2_rx.c

@@ -47,10 +47,25 @@ result_t layer2_rx_handle_packet(layer2_rx_t *ctx, const uint8_t *buf, size_t bu
 		return ERR_INTEGRITY;
 	}
 
-	LOG(LVL_DEBUG, "Handling packet with rx_seq_nr %u, tx_seq_nr %u.", header.rx_seq_nr, header.tx_seq_nr);
+	LOG(LVL_DEBUG, "Handling %s packet with rx_seq_nr %u, tx_seq_nr %u.",
+			layer2_msg_type_to_string(header.msg_type), header.rx_seq_nr, header.tx_seq_nr);
 
 	ctx->last_acked_seq = header.rx_seq_nr;
 
+	switch(header.msg_type) {
+		case L2_MSG_TYPE_EMPTY:
+			LOG(LVL_DEBUG, "Empty packet: accepted ACK for %u.", ctx->last_acked_seq);
+			return OK; // do not write anything to the TUN device
+
+		case L2_MSG_TYPE_CONN_MGMT:
+		case L2_MSG_TYPE_CONNECTIONLESS:
+			LOG(LVL_WARN, "Message type %s is not implemented yet.", layer2_msg_type_to_string(header.msg_type));
+			return OK;
+
+		case L2_MSG_TYPE_DATA:
+			break;
+	}
+
 	if(ctx->next_expected_seq != header.tx_seq_nr) {
 		LOG(LVL_ERR, "Expected sequence number %u, received %u.", ctx->next_expected_seq, header.tx_seq_nr);
 		return ERR_SEQUENCE;

impl/src/layer2/layer2_tx.c

@@ -94,6 +94,29 @@ result_t layer2_tx_fill_packet_queue(layer2_tx_t *ctx)
 }
 
 
+result_t layer2_tx_add_empty_packet(layer2_tx_t *ctx, bool tx_request)
+{
+	if(packet_queue_get_free_space(&ctx->packet_queue) == 0) {
+		return ERR_NO_MEM;
+	}
+
+	layer2_packet_header_t header;
+
+	header.dst_addr.addr[0] = 0xFFFF;
+	header.dst_addr.length = 1;
+	header.src_addr.addr[0] = 0x0001;
+	header.src_addr.length = 1;
+	header.msg_type = L2_MSG_TYPE_EMPTY;
+	header.rx_seq_nr = 0; // will be filled in layer2_tx_encode_next_packet()
+	header.tx_seq_nr = 0; // not used in empty packets
+	header.tx_request = tx_request;
+
+	packet_queue_add(&ctx->packet_queue, &header, NULL, 0);
+
+	return OK;
+}
+
+
 size_t layer2_tx_encode_next_packet(layer2_tx_t *ctx, uint8_t ack_seq_nr, uint8_t *buf, size_t buf_len)
 {
 	const packet_queue_entry_t *entry = packet_queue_get(&ctx->packet_queue, ctx->next_packet_index);
@@ -116,7 +139,10 @@ size_t layer2_tx_encode_next_packet(layer2_tx_t *ctx, uint8_t ack_seq_nr, uint8_
 	size_t packet_size = layer2_encode_packet_header(&header, buf);
 
 	// add the payload data
-	memcpy(buf + packet_size, entry->data, entry->data_len);
+	if(entry->data) {
+		memcpy(buf + packet_size, entry->data, entry->data_len);
+	}
+
 	packet_size += entry->data_len;
 
 	// calculate CRC of everything and append it to the packet
@@ -155,8 +181,15 @@ void layer2_tx_handle_ack(layer2_tx_t *ctx, uint8_t acked_seq)
 
 	packet_queue_delete(&ctx->packet_queue, packets_to_remove);
 
-	LOG(LVL_DEBUG, "handling ack for seq_nr %u, removing %zu packets, %zu packets remaining.", acked_seq, packets_to_remove, packet_queue_get_used_space(&ctx->packet_queue));
+	packets_available = packet_queue_get_used_space(&ctx->packet_queue);
 
+	LOG(LVL_DEBUG, "handling ack for seq_nr %u, removing %zu packets, %zu packets remaining.", acked_seq, packets_to_remove, packets_available);
+
+	if(packets_available == 0) {
+		// no packets left in queue, but an acknowledgement must be
+		// transmitted. Add an empty packet to do that.
+		layer2_tx_add_empty_packet(ctx, false);
+	}
 }
 
 

impl/src/layer2/layer2_tx.h

@@ -32,6 +32,12 @@ void layer2_tx_destroy(layer2_tx_t *ctx);
  */
 result_t layer2_tx_fill_packet_queue(layer2_tx_t *ctx);
 
+/*!\brief Add an empty packet to ensure an acknowledgement is sent.
+ * \param ctx         The transmitter context.
+ * \param tx_request  Value of the TX Request field in the packet.
+ */
+result_t layer2_tx_add_empty_packet(layer2_tx_t *ctx, bool tx_request);
+
 /*!\brief Encode the next packet for transmission.
  *
  * \note

impl/src/layer2/packet_queue.c

@@ -61,7 +61,11 @@ void packet_queue_delete(packet_queue_t *q, size_t count)
 
 		packet_queue_entry_t *entry = &q->entries[q->read_idx];
 
-		free(entry->data);
+		if(entry->data) {
+			free(entry->data);
+			entry->data = NULL;
+		}
+
 		entry->data_len = 0;
 
 		q->read_idx++;

impl/src/layer2/packet_structs.c

@@ -72,20 +72,23 @@ bool layer2_decode_packet_header(const uint8_t *encoded, size_t encoded_len, lay
 }
 
 
-static const char* msg_type_to_string(layer2_message_type_t type)
+const char* layer2_msg_type_to_string(layer2_message_type_t type)
 {
 	switch(type) {
 		case L2_MSG_TYPE_CONNECTIONLESS: return "Connectionless";
 		case L2_MSG_TYPE_DATA:           return "Data";
 		case L2_MSG_TYPE_CONN_MGMT:      return "Connection Management";
+		case L2_MSG_TYPE_EMPTY:          return "Empty";
 	}
+
+	return "!UNHANDLED TYPE!";
 }
 
 
 void layer2_dump_packet_header(int level, const layer2_packet_header_t *header)
 {
 	LOG(level, "Packet header dump:");
-	LOG(level, "  Packet type: %s", msg_type_to_string(header->msg_type));
+	LOG(level, "  Packet type: %s", layer2_msg_type_to_string(header->msg_type));
 	LOG(level, "  TX Request:  %s", header->tx_request ? "true" : "false");
 	LOG(level, "  RX Seq. Nr:  %2u", header->rx_seq_nr);
 	LOG(level, "  TX Seq. Nr:  %2u", header->tx_seq_nr);

impl/src/layer2/packet_structs.h

@@ -12,6 +12,7 @@
 typedef enum {
 	L2_MSG_TYPE_DATA           = 0x0,
 	L2_MSG_TYPE_CONN_MGMT      = 0x1,
+	L2_MSG_TYPE_EMPTY          = 0x2,
 	L2_MSG_TYPE_CONNECTIONLESS = 0x4
 } layer2_message_type_t;
 
@@ -67,6 +68,10 @@ bool layer2_decode_packet_header(const uint8_t *encoded, size_t encoded_len, lay
  */
 void layer2_dump_packet_header(int level, const layer2_packet_header_t *header);
 
+/*!\brief Get a string representation of the given message type.
+ */
+const char* layer2_msg_type_to_string(layer2_message_type_t type);
+
 /* Data Packet Structs */
 
 typedef enum {