project-chip · andy31415 · Mar 20, 2025 · Mar 15, 2025
diff --git a/src/app/CommandSender.cpp b/src/app/CommandSender.cpp
@@ -125,7 +125,8 @@ CHIP_ERROR CommandSender::SendCommandRequestInternal(const SessionHandle & sessi
     mExchangeCtx.Grab(exchange);
     VerifyOrReturnError(!mExchangeCtx->IsGroupExchangeContext(), CHIP_ERROR_INVALID_MESSAGE_TYPE);
 
-    mExchangeCtx->SetResponseTimeout(timeout.ValueOr(session->ComputeRoundTripTimeout(app::kExpectedIMProcessingTime)));
+    mExchangeCtx->SetResponseTimeout(
+        timeout.ValueOr(session->ComputeRoundTripTimeout(app::kExpectedIMProcessingTime, true /*isFirstMessageOnExchange*/)));
 
     if (mTimedInvokeTimeoutMs.HasValue())
     {

diff --git a/src/app/ReadClient.cpp b/src/app/ReadClient.cpp
@@ -987,12 +987,15 @@ CHIP_ERROR ReadClient::ComputeLivenessCheckTimerTimeout(System::Clock::Timeout *
     // So recompute the round-trip timeout directly.  Assume MRP, since in practice that is likely what is happening.
     auto & peerMRPConfig = mReadPrepareParams.mSessionHolder->GetRemoteMRPConfig();
     // Peer will assume we are idle (hence we pass kZero to GetMessageReceiptTimeout()), but will assume we treat it as active
-    // for the response, so to match the retransmission timeout computation for the message back to the peeer, we should treat
-    // it as active.
-    auto roundTripTimeout = mReadPrepareParams.mSessionHolder->GetMessageReceiptTimeout(System::Clock::kZero) +
+    // for the response, so to match the retransmission timeout computation for the message back to the peer, we should treat
+    // it as active and handling non-initial message, isFirstMessageOnExchange needs to be set as false for
+    // GetRetransmissionTimeout.
+    auto roundTripTimeout =
+        mReadPrepareParams.mSessionHolder->GetMessageReceiptTimeout(System::Clock::kZero, true /*isFirstMessageOnExchange*/) +
         kExpectedIMProcessingTime +
         GetRetransmissionTimeout(peerMRPConfig.mActiveRetransTimeout, peerMRPConfig.mIdleRetransTimeout,
-                                 System::SystemClock().GetMonotonicTimestamp(), peerMRPConfig.mActiveThresholdTime);
+                                 System::SystemClock().GetMonotonicTimestamp(), peerMRPConfig.mActiveThresholdTime,
+                                 false /*isFirstMessageOnExchange*/);
     *aTimeout = System::Clock::Seconds16(mMaxInterval) + roundTripTimeout;
     return CHIP_NO_ERROR;
 }

diff --git a/src/app/TimedHandler.cpp b/src/app/TimedHandler.cpp
@@ -127,8 +127,9 @@ CHIP_ERROR TimedHandler::HandleTimedRequestAction(Messaging::ExchangeContext * a
     // will send nothing and the other side will have to time out to realize
     // it's missed its window).
     auto delay = System::Clock::Milliseconds32(timeoutMs);
-    aExchangeContext->SetResponseTimeout(
-        std::max(delay, aExchangeContext->GetSessionHandle()->ComputeRoundTripTimeout(app::kExpectedIMProcessingTime)));
+    aExchangeContext->SetResponseTimeout(std::max(delay,
+                                                  aExchangeContext->GetSessionHandle()->ComputeRoundTripTimeout(
+                                                      app::kExpectedIMProcessingTime, false /*isFirstMessageOnExchange*/)));
     ReturnErrorOnFailure(StatusResponse::Send(Status::Success, aExchangeContext, /* aExpectResponse = */ true));
 
     // Now just wait for the client.

diff --git a/src/controller/AutoCommissioner.cpp b/src/controller/AutoCommissioner.cpp
@@ -649,7 +649,7 @@ Optional<System::Clock::Timeout> AutoCommissioner::GetCommandTimeout(DeviceProxy
     auto sessionHandle = device->GetSecureSession();
     if (sessionHandle.HasValue())
     {
-        timeout = sessionHandle.Value()->ComputeRoundTripTimeout(timeout);
+        timeout = sessionHandle.Value()->ComputeRoundTripTimeout(timeout, true /*isFirstMessageOnExchange*/);
     }
 
     // Enforce the spec minimal timeout.  Maybe this enforcement should live in

diff --git a/src/controller/python/chip/utils/DeviceProxyUtils.cpp b/src/controller/python/chip/utils/DeviceProxyUtils.cpp
@@ -71,7 +71,7 @@ uint32_t pychip_DeviceProxy_ComputeRoundTripTimeout(DeviceProxy * device, uint32
 
     return deviceProxy->GetSecureSession()
         .Value()
-        ->ComputeRoundTripTimeout(System::Clock::Milliseconds32(upperLayerProcessingTimeoutMs))
+        ->ComputeRoundTripTimeout(System::Clock::Milliseconds32(upperLayerProcessingTimeoutMs), true /*isFirstMessageOnExchange*/)
         .count();
 }
 

diff --git a/src/controller/tests/TestWriteChunking.cpp b/src/controller/tests/TestWriteChunking.cpp
@@ -566,9 +566,10 @@ void TestWriteChunking::RunTest(Instructions instructions)
     err = writeClient->SendWriteRequest(sessionHandle);
     EXPECT_EQ(err, CHIP_NO_ERROR);
 
-    GetIOContext().DriveIOUntil(sessionHandle->ComputeRoundTripTimeout(app::kExpectedIMProcessingTime) +
-                                    System::Clock::Seconds16(1),
-                                [&]() { return GetExchangeManager().GetNumActiveExchanges() == 0; });
+    GetIOContext().DriveIOUntil(
+        sessionHandle->ComputeRoundTripTimeout(app::kExpectedIMProcessingTime, true /*isFirstMessageOnExchange*/) +
+            System::Clock::Seconds16(1),
+        [&]() { return GetExchangeManager().GetNumActiveExchanges() == 0; });
 
     EXPECT_EQ(onGoingPath, app::ConcreteAttributePath());
     EXPECT_EQ(status.size(), instructions.expectedStatus.size());

diff --git a/src/controller/tests/data_model/TestRead.cpp b/src/controller/tests/data_model/TestRead.cpp
@@ -4783,10 +4783,10 @@ System::Clock::Timeout TestRead::ComputeSubscriptionTimeout(System::Clock::Secon
     // Add 1000ms of slack to our max interval to make sure we hit the
     // subscription liveness timer.  100ms was tried in the past and is not
     // sufficient: our process can easily lose the timeslice for 100ms.
-    const auto & ourMrpConfig = GetDefaultMRPConfig();
-    auto publisherTransmissionTimeout =
-        GetRetransmissionTimeout(ourMrpConfig.mActiveRetransTimeout, ourMrpConfig.mIdleRetransTimeout,
-                                 System::SystemClock().GetMonotonicTimestamp(), ourMrpConfig.mActiveThresholdTime);
+    const auto & ourMrpConfig         = GetDefaultMRPConfig();
+    auto publisherTransmissionTimeout = GetRetransmissionTimeout(
+        ourMrpConfig.mActiveRetransTimeout, ourMrpConfig.mIdleRetransTimeout, System::SystemClock().GetMonotonicTimestamp(),
+        ourMrpConfig.mActiveThresholdTime, true /* isFirstMessageOnExchange */);
 
     return publisherTransmissionTimeout + aMaxInterval + System::Clock::Milliseconds32(1000);
 }

@@ -1475,7 +1475,7 @@ - (void)_invokeCommandWithEndpointID:(NSNumber *)endpointID
                 // Clamp to a number of seconds that will not overflow 32-bit
                 // int when converted to ms.
                 auto serverTimeoutInSeconds = System::Clock::Seconds16(serverSideProcessingTimeout.unsignedShortValue);
-                invokeTimeout.SetValue(session->ComputeRoundTripTimeout(serverTimeoutInSeconds));
+                invokeTimeout.SetValue(session->ComputeRoundTripTimeout(serverTimeoutInSeconds, true /*isFirstMessageOnExchange*/));
             }
             ReturnErrorOnFailure(commandSender->SendCommandRequest(session, invokeTimeout));
 

diff --git a/src/messaging/ExchangeContext.cpp b/src/messaging/ExchangeContext.cpp
@@ -77,7 +77,7 @@ void ExchangeContext::SetResponseExpected(bool inResponseExpected)
 
 void ExchangeContext::UseSuggestedResponseTimeout(Timeout applicationProcessingTimeout)
 {
-    SetResponseTimeout(mSession->ComputeRoundTripTimeout(applicationProcessingTimeout));
+    SetResponseTimeout(mSession->ComputeRoundTripTimeout(applicationProcessingTimeout, !HasReceivedAtLeastOneMessage()));
 }
 
 void ExchangeContext::SetResponseTimeout(Timeout timeout)

diff --git a/src/messaging/ReliableMessageProtocolConfig.cpp b/src/messaging/ReliableMessageProtocolConfig.cpp
@@ -126,7 +126,8 @@ Optional<ReliableMessageProtocolConfig> GetLocalMRPConfig()
 }
 
 System::Clock::Timeout GetRetransmissionTimeout(System::Clock::Timeout activeInterval, System::Clock::Timeout idleInterval,
-                                                System::Clock::Timeout lastActivityTime, System::Clock::Timeout activityThreshold)
+                                                System::Clock::Timeout lastActivityTime, System::Clock::Timeout activityThreshold,
+                                                bool isFirstMessageOnExchange)
 {
     auto timeSinceLastActivity = (System::SystemClock().GetMonotonicTimestamp() - lastActivityTime);
 
@@ -135,7 +136,14 @@ System::Clock::Timeout GetRetransmissionTimeout(System::Clock::Timeout activeInt
     System::Clock::Timestamp timeout(0);
     for (uint8_t i = 0; i < CHIP_CONFIG_RMP_DEFAULT_MAX_RETRANS + 1; i++)
     {
-        auto baseInterval = ((timeSinceLastActivity + timeout) < activityThreshold) ? activeInterval : idleInterval;
+        auto baseInterval = activeInterval;
+        // If we are calculating the timeout for the initial message, we never know whether the peer is active or not, choose
+        // active/idle interval from PeerActiveMode of session per 4.11.2.1. Retransmissions.
+        // If we are calculating the timeout for response message, we know the peer is active, always choose active interval.
+        if (isFirstMessageOnExchange)
+        {
+            baseInterval = ((timeSinceLastActivity + timeout) < activityThreshold) ? activeInterval : idleInterval;
+        }
         timeout += Messaging::ReliableMessageMgr::GetBackoff(baseInterval, i, /* computeMaxPossible */ true);
     }
 

diff --git a/src/messaging/ReliableMessageProtocolConfig.h b/src/messaging/ReliableMessageProtocolConfig.h
@@ -257,11 +257,12 @@ Optional<ReliableMessageProtocolConfig> GetLocalMRPConfig();
  * @param[in] idleInterval      The idle interval to use for the backoff calculation.
  * @param[in] lastActivityTime  The last time some activity has been recorded.
  * @param[in] activityThreshold The activity threshold for a node to be considered active.
- *
+ * @param[in] isFirstMessageOnExchange Indicates whether this is for the initial message on an exchange.
  * @return The maximum transmission time
  */
 System::Clock::Timeout GetRetransmissionTimeout(System::Clock::Timeout activeInterval, System::Clock::Timeout idleInterval,
-                                                System::Clock::Timeout lastActivityTime, System::Clock::Timeout activityThreshold);
+                                                System::Clock::Timeout lastActivityTime, System::Clock::Timeout activityThreshold,
+                                                bool isFirstMessageOnExchange);
 
 #if CONFIG_BUILD_FOR_HOST_UNIT_TEST
 

diff --git a/src/protocols/secure_channel/CASESession.cpp b/src/protocols/secure_channel/CASESession.cpp
@@ -2418,34 +2418,37 @@ CHIP_ERROR CASESession::OnMessageReceived(ExchangeContext * ec, const PayloadHea
 
 namespace {
 System::Clock::Timeout ComputeRoundTripTimeout(ExchangeContext::Timeout serverProcessingTime,
-                                               const ReliableMessageProtocolConfig & remoteMrpConfig)
+                                               const ReliableMessageProtocolConfig & remoteMrpConfig, bool isFirstMessageOnExchange)
 {
     // TODO: This is duplicating logic from Session::ComputeRoundTripTimeout.  Unfortunately, it's called by
     // consumers who do not have a session.
     const auto & maybeLocalMRPConfig = GetLocalMRPConfig();
     const auto & defaultMRRPConfig   = GetDefaultMRPConfig();
     const auto & localMRPConfig      = maybeLocalMRPConfig.ValueOr(defaultMRRPConfig);
     return GetRetransmissionTimeout(remoteMrpConfig.mActiveRetransTimeout, remoteMrpConfig.mIdleRetransTimeout,
-                                    // Assume peer is idle, as a worst-case assumption (probably true for
-                                    // Sigma1, since that will be our initial message on the session, but less
-                                    // so for Sigma2).
-                                    System::Clock::kZero, remoteMrpConfig.mActiveThresholdTime) +
+                                    // The activity time only matters when isFirstMessageOnExchange is false,
+                                    // which only happens for Sigma1.  In that case, assume peer is idle,
+                                    // as a worst-case assumption, and pass System::Clock::kZero.
+                                    System::Clock::kZero, remoteMrpConfig.mActiveThresholdTime, isFirstMessageOnExchange) +
         serverProcessingTime +
         GetRetransmissionTimeout(localMRPConfig.mActiveRetransTimeout, localMRPConfig.mIdleRetransTimeout,
-                                 // Peer will assume we are active, since it's
-                                 // responding to our message.
-                                 System::SystemClock().GetMonotonicTimestamp(), localMRPConfig.mActiveThresholdTime);
+                                 // Peer will be responding to our message, so isFirstMessageOnExchange should be false
+                                 // and the timestamp does not matter.
+                                 System::SystemClock().GetMonotonicTimestamp(), localMRPConfig.mActiveThresholdTime,
+                                 false /*isFirstMessageOnExchange*/);
 }
 } // anonymous namespace
 
 System::Clock::Timeout CASESession::ComputeSigma1ResponseTimeout(const ReliableMessageProtocolConfig & remoteMrpConfig)
 {
-    return ComputeRoundTripTimeout(kExpectedSigma1ProcessingTime, remoteMrpConfig);
+    // Sigma1 is the first message on the exchange.
+    return ComputeRoundTripTimeout(kExpectedSigma1ProcessingTime, remoteMrpConfig, true /*isFirstMessageOnExchange*/);
 }
 
 System::Clock::Timeout CASESession::ComputeSigma2ResponseTimeout(const ReliableMessageProtocolConfig & remoteMrpConfig)
 {
-    return ComputeRoundTripTimeout(kExpectedHighProcessingTime, remoteMrpConfig);
+    // Sigma2 is never the first message on the exchange.
+    return ComputeRoundTripTimeout(kExpectedHighProcessingTime, remoteMrpConfig, false /*isFirstMessageOnExchange*/);
 }
 
 bool CASESession::InvokeBackgroundWorkWatchdog()

diff --git a/src/transport/GroupSession.h b/src/transport/GroupSession.h
@@ -69,13 +69,14 @@ class IncomingGroupSession : public Session, public ReferenceCounted<IncomingGro
 
     System::Clock::Timestamp GetMRPBaseTimeout() const override { return System::Clock::kZero; }
 
-    System::Clock::Milliseconds32 GetAckTimeout() const override
+    System::Clock::Milliseconds32 GetAckTimeout(bool isFirstMessageOnExchange) const override
     {
         VerifyOrDie(false);
         return System::Clock::Timeout();
     }
 
-    System::Clock::Milliseconds32 GetMessageReceiptTimeout(System::Clock::Timestamp ourLastActivity) const override
+    System::Clock::Milliseconds32 GetMessageReceiptTimeout(System::Clock::Timestamp ourLastActivity,
+                                                           bool isFirstMessageOnExchange) const override
     {
         // There are no timeouts for group sessions.
         VerifyOrDie(false);
@@ -128,13 +129,14 @@ class OutgoingGroupSession : public Session, public ReferenceCounted<OutgoingGro
 
     System::Clock::Timestamp GetMRPBaseTimeout() const override { return System::Clock::kZero; }
 
-    System::Clock::Milliseconds32 GetAckTimeout() const override
+    System::Clock::Milliseconds32 GetAckTimeout(bool isFirstMessageOnExchange) const override
     {
         VerifyOrDie(false);
         return System::Clock::Timeout();
     }
 
-    System::Clock::Milliseconds32 GetMessageReceiptTimeout(System::Clock::Timestamp ourLastActivity) const override
+    System::Clock::Milliseconds32 GetMessageReceiptTimeout(System::Clock::Timestamp ourLastActivity,
+                                                           bool isFirstMessageOnExchange) const override
     {
         // There are no timeouts for group sessions.
         VerifyOrDie(false);

diff --git a/src/transport/SecureSession.h b/src/transport/SecureSession.h
@@ -162,14 +162,15 @@ class SecureSession : public Session, public ReferenceCounted<SecureSession, Sec
 
     bool AllowsLargePayload() const override { return GetPeerAddress().GetTransportType() == Transport::Type::kTcp; }
 
-    System::Clock::Milliseconds32 GetAckTimeout() const override
+    System::Clock::Milliseconds32 GetAckTimeout(bool isFirstMessageOnExchange) const override
     {
         switch (mPeerAddress.GetTransportType())
         {
         case Transport::Type::kUdp: {
             const ReliableMessageProtocolConfig & remoteMRPConfig = mRemoteSessionParams.GetMRPConfig();
             return GetRetransmissionTimeout(remoteMRPConfig.mActiveRetransTimeout, remoteMRPConfig.mIdleRetransTimeout,
-                                            GetLastPeerActivityTime(), remoteMRPConfig.mActiveThresholdTime);
+                                            GetLastPeerActivityTime(), remoteMRPConfig.mActiveThresholdTime,
+                                            isFirstMessageOnExchange);
         }
         case Transport::Type::kTcp:
             return System::Clock::Seconds16(30);
@@ -181,7 +182,8 @@ class SecureSession : public Session, public ReferenceCounted<SecureSession, Sec
         return System::Clock::Timeout();
     }
 
-    System::Clock::Milliseconds32 GetMessageReceiptTimeout(System::Clock::Timestamp ourLastActivity) const override
+    System::Clock::Milliseconds32 GetMessageReceiptTimeout(System::Clock::Timestamp ourLastActivity,
+                                                           bool isFirstMessageOnExchange) const override
     {
         switch (mPeerAddress.GetTransportType())
         {
@@ -190,7 +192,7 @@ class SecureSession : public Session, public ReferenceCounted<SecureSession, Sec
             const auto & defaultMRRPConfig   = GetDefaultMRPConfig();
             const auto & localMRPConfig      = maybeLocalMRPConfig.ValueOr(defaultMRRPConfig);
             return GetRetransmissionTimeout(localMRPConfig.mActiveRetransTimeout, localMRPConfig.mIdleRetransTimeout,
-                                            ourLastActivity, localMRPConfig.mActiveThresholdTime);
+                                            ourLastActivity, localMRPConfig.mActiveThresholdTime, isFirstMessageOnExchange);
         }
         case Transport::Type::kTcp:
             return System::Clock::Seconds16(30);

diff --git a/src/transport/Session.cpp b/src/transport/Session.cpp
@@ -64,16 +64,18 @@ const OutgoingGroupSession * Session::AsConstOutgoingGroupSession() const
     return static_cast<const OutgoingGroupSession *>(this);
 }
 
-System::Clock::Timeout Session::ComputeRoundTripTimeout(System::Clock::Timeout upperlayerProcessingTimeout)
+System::Clock::Timeout Session::ComputeRoundTripTimeout(System::Clock::Timeout upperlayerProcessingTimeout,
+                                                        bool isFirstMessageOnExchange)
 {
     if (IsGroupSession())
     {
         return System::Clock::kZero;
     }
 
-    // Treat us as active for purposes of GetMessageReceiptTimeout(), since the
-    // other side would be responding to our message.
-    return GetAckTimeout() + upperlayerProcessingTimeout + GetMessageReceiptTimeout(System::SystemClock().GetMonotonicTimestamp());
+    // Treat us as active for purposes of GetMessageReceiptTimeout(), pass false into GetMessageReceiptTimeout to
+    // indicate we are processing non-initial message since the other side would be responding to our message.
+    return GetAckTimeout(isFirstMessageOnExchange) + upperlayerProcessingTimeout +
+        GetMessageReceiptTimeout(System::SystemClock().GetMonotonicTimestamp(), false /*isFirstMessageOnExchange*/);
 }
 
 uint16_t Session::SessionIdForLogging() const

diff --git a/src/transport/Session.h b/src/transport/Session.h
@@ -254,7 +254,9 @@ class Session
     // GetAckTimeout is the estimate for how long it could take for the other
     // side to receive our message (accounting for our MRP retransmits if it
     // gets lost) and send a response.
-    virtual System::Clock::Milliseconds32 GetAckTimeout() const = 0;
+    //
+    // The MRP retransmit delay computation depends on whether this is the first message on the exchange.
+    virtual System::Clock::Milliseconds32 GetAckTimeout(bool isFirstMessageOnExchange) const = 0;
 
     // GetReceiptTimeout is the estimate for how long it could take for us to
     // receive a message after the other side sends it, accounting for the MRP
@@ -265,15 +267,19 @@ class Session
     // System::SystemClock().GetMonotonicTimestamp() (to indicate "peer is
     // responding to a message it just received") and System::Clock::kZero (to
     // indicate "peer is reaching out to us, not in response to anything").
-    virtual System::Clock::Milliseconds32 GetMessageReceiptTimeout(System::Clock::Timestamp ourLastActivity) const = 0;
+    // isFirstMessageOnExchange indicates whether the caller is handling the first message on exchange
+    virtual System::Clock::Milliseconds32 GetMessageReceiptTimeout(System::Clock::Timestamp ourLastActivity,
+                                                                   bool isFirstMessageOnExchange) const = 0;
 
     const ReliableMessageProtocolConfig & GetRemoteMRPConfig() const { return GetRemoteSessionParameters().GetMRPConfig(); }
 
     // Returns a suggested timeout value based on the round-trip time it takes for the peer at the other end of the session to
     // receive a message, process it and send it back. This is computed based on the session type, the type of transport, sleepy
     // characteristics of the target and a caller-provided value for the time it takes to process a message at the upper layer on
     // the target For group sessions, this function will always return 0.
-    System::Clock::Timeout ComputeRoundTripTimeout(System::Clock::Timeout upperlayerProcessingTimeout);
+    // isFirstMessageOnExchange parameter indicates whether this ComputeRoundTripTimeout() call is for an initial message or not.
+    System::Clock::Timeout ComputeRoundTripTimeout(System::Clock::Timeout upperlayerProcessingTimeout,
+                                                   bool isFirstMessageOnExchange);
 
     FabricIndex GetFabricIndex() const { return mFabricIndex; }
 

diff --git a/src/transport/UnauthenticatedSessionTable.h b/src/transport/UnauthenticatedSessionTable.h
@@ -88,14 +88,15 @@ class UnauthenticatedSession : public Session, public ReferenceCounted<Unauthent
 
     bool AllowsLargePayload() const override { return GetPeerAddress().GetTransportType() == Transport::Type::kTcp; }
 
-    System::Clock::Milliseconds32 GetAckTimeout() const override
+    System::Clock::Milliseconds32 GetAckTimeout(bool isFirstMessageOnExchange) const override
     {
         switch (mPeerAddress.GetTransportType())
         {
         case Transport::Type::kUdp: {
             const ReliableMessageProtocolConfig & remoteMRPConfig = mRemoteSessionParams.GetMRPConfig();
             return GetRetransmissionTimeout(remoteMRPConfig.mActiveRetransTimeout, remoteMRPConfig.mIdleRetransTimeout,
-                                            GetLastPeerActivityTime(), remoteMRPConfig.mActiveThresholdTime);
+                                            GetLastPeerActivityTime(), remoteMRPConfig.mActiveThresholdTime,
+                                            isFirstMessageOnExchange);
         }
         case Transport::Type::kTcp:
             return System::Clock::Seconds16(30);
@@ -107,7 +108,8 @@ class UnauthenticatedSession : public Session, public ReferenceCounted<Unauthent
         return System::Clock::Timeout();
     }
 
-    System::Clock::Milliseconds32 GetMessageReceiptTimeout(System::Clock::Timestamp ourLastActivity) const override
+    System::Clock::Milliseconds32 GetMessageReceiptTimeout(System::Clock::Timestamp ourLastActivity,
+                                                           bool isFirstMessageOnExchange) const override
     {
         switch (mPeerAddress.GetTransportType())
         {
@@ -116,7 +118,7 @@ class UnauthenticatedSession : public Session, public ReferenceCounted<Unauthent
             const auto & defaultMRRPConfig   = GetDefaultMRPConfig();
             const auto & localMRPConfig      = maybeLocalMRPConfig.ValueOr(defaultMRRPConfig);
             return GetRetransmissionTimeout(localMRPConfig.mActiveRetransTimeout, localMRPConfig.mIdleRetransTimeout,
-                                            ourLastActivity, localMRPConfig.mActiveThresholdTime);
+                                            ourLastActivity, localMRPConfig.mActiveThresholdTime, isFirstMessageOnExchange);
         }
         case Transport::Type::kTcp:
             return System::Clock::Seconds16(30);