1 files changed, 69 insertions, 0 deletions
diff --git a/net/dcsctp/tx/retransmission_timeout.cc b/net/dcsctp/tx/retransmission_timeout.cc
new file mode 100644
index 0000000000..7d545a07d0
--- /dev/null
+++ b/net/dcsctp/tx/retransmission_timeout.cc
@@ -0,0 +1,69 @@
+/*
+ *  Copyright (c) 2021 The WebRTC project authors. All Rights Reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+#include "net/dcsctp/tx/retransmission_timeout.h"
+
+#include <cmath>
+#include <cstdint>
+
+#include "net/dcsctp/public/dcsctp_options.h"
+
+namespace dcsctp {
+namespace {
+// https://tools.ietf.org/html/rfc4960#section-15
+constexpr double kRtoAlpha = 0.125;
+constexpr double kRtoBeta = 0.25;
+}  // namespace
+
+RetransmissionTimeout::RetransmissionTimeout(const DcSctpOptions& options)
+    : min_rto_(*options.rto_min),
+      max_rto_(*options.rto_max),
+      max_rtt_(*options.rtt_max),
+      rto_(*options.rto_initial) {}
+
+void RetransmissionTimeout::ObserveRTT(DurationMs measured_rtt) {
+  double rtt = *measured_rtt;
+
+  // Unrealistic values will be skipped. If a wrongly measured (or otherwise
+  // corrupt) value was processed, it could change the state in a way that would
+  // take a very long time to recover.
+  if (rtt < 0.0 || rtt > max_rtt_) {
+    return;
+  }
+
+  if (first_measurement_) {
+    // https://tools.ietf.org/html/rfc4960#section-6.3.1
+    // "When the first RTT measurement R is made, set
+    //        SRTT <- R,
+    //        RTTVAR <- R/2, and
+    //        RTO <- SRTT + 4 * RTTVAR."
+    srtt_ = rtt;
+    rttvar_ = rtt * 0.5;
+    rto_ = srtt_ + 4 * rttvar_;
+    first_measurement_ = false;
+  } else {
+    // https://tools.ietf.org/html/rfc4960#section-6.3.1
+    // "When a new RTT measurement R' is made, set
+    //        RTTVAR <- (1 - RTO.Beta) * RTTVAR + RTO.Beta * |SRTT - R'|
+    //        SRTT <- (1 - RTO.Alpha) * SRTT + RTO.Alpha * R'
+    //        RTO <- SRTT + 4 * RTTVAR."
+    rttvar_ = (1 - kRtoBeta) * rttvar_ + kRtoBeta * std::abs(srtt_ - rtt);
+    srtt_ = (1 - kRtoAlpha) * srtt_ + kRtoAlpha * rtt;
+    rto_ = srtt_ + 4 * rttvar_;
+  }
+
+  // If the RTO becomes smaller or equal to RTT, expiration timers will be
+  // scheduled at the same time as packets are expected. Only happens in
+  // extremely stable RTTs, i.e. in simulations.
+  rto_ = std::fmax(rto_, rtt + 1);
+
+  // Clamp RTO between min and max.
+  rto_ = std::fmin(std::fmax(rto_, min_rto_), max_rto_);
+}
+}  // namespace dcsctp