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+/*
+ * 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.
+ */
+#ifndef NET_DCSCTP_TX_RETRANSMISSION_TIMEOUT_H_
+#define NET_DCSCTP_TX_RETRANSMISSION_TIMEOUT_H_
+
+#include <cstdint>
+#include <functional>
+
+#include "net/dcsctp/public/dcsctp_options.h"
+
+namespace dcsctp {
+
+// Manages updating of the Retransmission Timeout (RTO) SCTP variable, which is
+// used directly as the base timeout for T3-RTX and for other timers, such as
+// delayed ack.
+//
+// When a round-trip-time (RTT) is calculated (outside this class), `Observe`
+// is called, which calculates the retransmission timeout (RTO) value. The RTO
+// value will become larger if the RTT is high and/or the RTT values are varying
+// a lot, which is an indicator of a bad connection.
+class RetransmissionTimeout {
+ public:
+ static constexpr int kRttShift = 3;
+ static constexpr int kRttVarShift = 2;
+ explicit RetransmissionTimeout(const DcSctpOptions& options);
+
+ // To be called when a RTT has been measured, to update the RTO value.
+ void ObserveRTT(DurationMs measured_rtt);
+
+ // Returns the Retransmission Timeout (RTO) value, in milliseconds.
+ DurationMs rto() const { return DurationMs(rto_); }
+
+ // Returns the smoothed RTT value, in milliseconds.
+ DurationMs srtt() const { return DurationMs(scaled_srtt_ >> kRttShift); }
+
+ private:
+ const int32_t min_rto_;
+ const int32_t max_rto_;
+ const int32_t max_rtt_;
+ const int32_t min_rtt_variance_;
+ // If this is the first measurement
+ bool first_measurement_ = true;
+ // Smoothed Round-Trip Time, shifted by kRttShift
+ int32_t scaled_srtt_;
+ // Round-Trip Time Variation, shifted by kRttVarShift
+ int32_t scaled_rtt_var_ = 0;
+ // Retransmission Timeout
+ int32_t rto_;
+};
+} // namespace dcsctp
+
+#endif // NET_DCSCTP_TX_RETRANSMISSION_TIMEOUT_H_