libiso15118: Add response message interval control

There are EV implementations that can configure request messages within a few milliseconds, which can currently trigger a flood of internal communication. To avoid performance issues, we should therefore wait a certain amount of time after receiving a request message before sending the next response message. The delay of maximum 100 ms should not cause any problems, as the most critical time of the ISO 15118-20 (V2G_SECC_Msg_Performance_Time of the charging loop) is 250 ms.

Changes:
- introduce MIN_RESPONSE_INTERVAL_MS to manage time between responses
- add last_response_tx_time to track last response timestamp
- implement delay to ensure minimum interval between response messages

Signed-off-by: Fabian Hartung <fabian.hartung@chargebyte.com>

Author: FaHaGit

lib/everest/iso15118/include/iso15118/session/iso.hpp

@@ -72,6 +72,8 @@ class Session {
 
     d20::Timeouts timeouts;
 
+    std::optional<TimePoint> last_response_tx_time; // timestamp of the last response message sent
+
     void handle_connection_event(io::ConnectionEvent event);
 };
 

lib/everest/iso15118/src/iso15118/session/iso.cpp

@@ -16,6 +16,7 @@
 namespace iso15118 {
 
 static constexpr auto SESSION_IDLE_TIMEOUT_MS = 5000;
+static constexpr auto MIN_RESPONSE_INTERVAL_MS = 100; // minimum time between two response messages
 
 static void log_sdp_packet(const iso15118::io::SdpPacket& sdp) {
     static constexpr auto ESCAPED_BYTE_CHAR_COUNT = 4;
@@ -226,8 +227,21 @@ TimePoint const& Session::poll() {
     const auto [got_response, payload_size, payload_type, response_type] = message_exchange.check_and_clear_response();
 
     if (got_response) {
+        // Before we send back the response message, we check the time between two response messages
+        // sent out. If this is less than MIN_RESPONSE_INTERVAL_MS, we delay the response message to
+        // avoid potential performance issues.
+        if (last_response_tx_time.has_value()) {
+            const auto elapsed = std::chrono::duration_cast<std::chrono::milliseconds>(get_current_time_point() -
+                                                                                       last_response_tx_time.value());
+        const auto min_delay = std::chrono::milliseconds(MIN_RESPONSE_INTERVAL_MS);
+            if (elapsed < min_delay) {
+                std::this_thread::sleep_for(min_delay - elapsed);
+            }
+        }
+
         const auto response_size = setup_response_header(response_buffer, payload_type, payload_size);
         connection->write(response_buffer, response_size);
+        last_response_tx_time = get_current_time_point();
 
         timeouts.start_timeout(d20::TimeoutType::SEQUENCE, d20::TIMEOUT_SEQUENCE);