diff --git a/quic/api/QuicTransportFunctions.cpp b/quic/api/QuicTransportFunctions.cpp
index 32c8e2c1d..6e35a019b 100644
--- a/quic/api/QuicTransportFunctions.cpp
+++ b/quic/api/QuicTransportFunctions.cpp
@@ -1788,4 +1788,34 @@ bool hasInitialOrHandshakeCiphers(QuicConnectionStateBase& conn) {
       conn.readCodec->getHandshakeReadCipher();
 }
 
+bool setCustomTransportParameter(
+    std::unique_ptr<CustomTransportParameter> customParam,
+    std::vector<TransportParameter>& customTransportParameters) {
+  // Check that the parameter id is in the "private parameter" range, as
+  // described by the spec.
+  if (static_cast<uint16_t>(customParam->getParameterId()) <
+      kCustomTransportParameterThreshold) {
+    LOG(ERROR) << "invalid parameter id";
+    return false;
+  }
+
+  // check to see that we haven't already added in a parameter with the
+  // specified parameter id
+  auto it = std::find_if(
+      customTransportParameters.begin(),
+      customTransportParameters.end(),
+      [&customParam](const TransportParameter& param) {
+        return param.parameter == customParam->getParameterId();
+      });
+
+  // if a match has been found, we return failure
+  if (it != customTransportParameters.end()) {
+    LOG(ERROR) << "transport parameter already present";
+    return false;
+  }
+
+  customTransportParameters.push_back(customParam->encode());
+  return true;
+}
+
 } // namespace quic
diff --git a/quic/api/QuicTransportFunctions.h b/quic/api/QuicTransportFunctions.h
index 8cb322b87..6625c185d 100644
--- a/quic/api/QuicTransportFunctions.h
+++ b/quic/api/QuicTransportFunctions.h
@@ -14,6 +14,7 @@
 #include <quic/api/IoBufQuicBatch.h>
 #include <quic/api/QuicPacketScheduler.h>
 #include <quic/api/QuicSocket.h>
+#include <quic/handshake/TransportParameters.h>
 #include <quic/state/StateData.h>
 
 // Function to schedule writing data to socket. Return number of packets
@@ -333,4 +334,14 @@ bool writeLoopTimeLimit(
     TimePoint loopBeginTime,
     const QuicConnectionStateBase& connection);
 
+/**
+ * Sets private transport parameters that are not in the TransportParameterId
+ * enum. See kCustomTransportParameterThreshold in QuicConstants.h
+ * - checks that the custom param id complies to the spec
+ * - checks that the param is not in the provided vector already
+ */
+bool setCustomTransportParameter(
+    std::unique_ptr<CustomTransportParameter> customParam,
+    std::vector<TransportParameter>& customTransportParameters);
+
 } // namespace quic
diff --git a/quic/api/test/QuicTransportFunctionsTest.cpp b/quic/api/test/QuicTransportFunctionsTest.cpp
index d3617053e..399d8a560 100644
--- a/quic/api/test/QuicTransportFunctionsTest.cpp
+++ b/quic/api/test/QuicTransportFunctionsTest.cpp
@@ -4523,5 +4523,37 @@ TEST_F(QuicTransportFunctionsTest, MissingStreamFrameBytesSingleByteWrite) {
   }
 }
 
+TEST_F(QuicTransportFunctionsTest, CustomTransportParamTest) {
+  std::vector<TransportParameter> customTransportParameters;
+
+  // Add new param.
+  EXPECT_TRUE(setCustomTransportParameter(
+      std::make_unique<CustomIntegralTransportParameter>(
+          kCustomTransportParameterThreshold, 0),
+      customTransportParameters));
+  EXPECT_EQ(customTransportParameters.size(), 1);
+
+  // Existing param not added.
+  EXPECT_FALSE(setCustomTransportParameter(
+      std::make_unique<CustomIntegralTransportParameter>(
+          kCustomTransportParameterThreshold, 1),
+      customTransportParameters));
+  EXPECT_EQ(customTransportParameters.size(), 1);
+
+  // Bad param id is not added.
+  EXPECT_FALSE(setCustomTransportParameter(
+      std::make_unique<CustomIntegralTransportParameter>(
+          kCustomTransportParameterThreshold - 1, 2),
+      customTransportParameters));
+  EXPECT_EQ(customTransportParameters.size(), 1);
+
+  // Another valid param added.
+  EXPECT_TRUE(setCustomTransportParameter(
+      std::make_unique<CustomIntegralTransportParameter>(
+          kCustomTransportParameterThreshold + 1, 0),
+      customTransportParameters));
+  EXPECT_EQ(customTransportParameters.size(), 2);
+}
+
 } // namespace test
 } // namespace quic
diff --git a/quic/client/QuicClientTransport.cpp b/quic/client/QuicClientTransport.cpp
index 787930675..f61aa9de4 100644
--- a/quic/client/QuicClientTransport.cpp
+++ b/quic/client/QuicClientTransport.cpp
@@ -1640,35 +1640,6 @@ void QuicClientTransport::setSelfOwning() {
   selfOwning_ = shared_from_this();
 }
 
-bool QuicClientTransport::setCustomTransportParameter(
-    std::unique_ptr<CustomTransportParameter> customParam) {
-  // check that the parameter id is in the "private parameter" range, as
-  // described by the spec.
-  if (static_cast<uint16_t>(customParam->getParameterId()) <
-      kCustomTransportParameterThreshold) {
-    LOG(ERROR) << "invalid parameter id";
-    return false;
-  }
-
-  // check to see that we haven't already added in a parameter with the
-  // specified parameter id
-  auto it = std::find_if(
-      customTransportParameters_.begin(),
-      customTransportParameters_.end(),
-      [&customParam](const TransportParameter& param) {
-        return param.parameter == customParam->getParameterId();
-      });
-
-  // if a match has been found, we return failure
-  if (it != customTransportParameters_.end()) {
-    LOG(ERROR) << "transport parameter already present";
-    return false;
-  }
-
-  customTransportParameters_.push_back(customParam->encode());
-  return true;
-}
-
 void QuicClientTransport::setD6DBasePMTUTransportParameter() {
   if (!conn_->transportSettings.d6dConfig.enabled) {
     return;
@@ -1687,7 +1658,8 @@ void QuicClientTransport::setD6DBasePMTUTransportParameter() {
   auto basePMTUCustomParam = std::make_unique<CustomIntegralTransportParameter>(
       kD6DBasePMTUParameterId, basePMTUSetting);
 
-  if (!setCustomTransportParameter(std::move(basePMTUCustomParam))) {
+  if (!setCustomTransportParameter(
+          std::move(basePMTUCustomParam), customTransportParameters_)) {
     LOG(ERROR) << "failed to set D6D base PMTU transport parameter";
   }
 }
@@ -1710,7 +1682,8 @@ void QuicClientTransport::setD6DRaiseTimeoutTransportParameter() {
       std::make_unique<CustomIntegralTransportParameter>(
           kD6DRaiseTimeoutParameterId, raiseTimeoutSetting.count());
 
-  if (!setCustomTransportParameter(std::move(raiseTimeoutCustomParam))) {
+  if (!setCustomTransportParameter(
+          std::move(raiseTimeoutCustomParam), customTransportParameters_)) {
     LOG(ERROR) << "failed to set D6D raise timeout transport parameter";
   }
 }
@@ -1733,7 +1706,8 @@ void QuicClientTransport::setD6DProbeTimeoutTransportParameter() {
       std::make_unique<CustomIntegralTransportParameter>(
           kD6DProbeTimeoutParameterId, probeTimeoutSetting.count());
 
-  if (!setCustomTransportParameter(std::move(probeTimeoutCustomParam))) {
+  if (!setCustomTransportParameter(
+          std::move(probeTimeoutCustomParam), customTransportParameters_)) {
     LOG(ERROR) << "failed to set D6D probe timeout transport parameter";
   }
 }
diff --git a/quic/client/QuicClientTransport.h b/quic/client/QuicClientTransport.h
index fce6b3ff4..f1465747c 100644
--- a/quic/client/QuicClientTransport.h
+++ b/quic/client/QuicClientTransport.h
@@ -138,14 +138,6 @@ class QuicClientTransport
    */
   void setSelfOwning();
 
-  /**
-   * Used to set private transport parameters that are not in the
-   * TransportParameterId enum.
-   * See kCustomTransportParameterThreshold in QuicConstants.h
-   */
-  bool setCustomTransportParameter(
-      std::unique_ptr<CustomTransportParameter> customParam);
-
   void onNetworkSwitch(std::unique_ptr<folly::AsyncUDPSocket> newSock) override;
 
   /**
diff --git a/quic/fizz/client/test/QuicClientTransportTest.cpp b/quic/fizz/client/test/QuicClientTransportTest.cpp
index e11eae246..47c067134 100644
--- a/quic/fizz/client/test/QuicClientTransportTest.cpp
+++ b/quic/fizz/client/test/QuicClientTransportTest.cpp
@@ -1053,13 +1053,6 @@ TEST_F(QuicClientTransportTest, FirstPacketProcessedCallback) {
   client->closeNow(folly::none);
 }
 
-TEST_F(QuicClientTransportTest, CustomTransportParam) {
-  EXPECT_TRUE(client->setCustomTransportParameter(
-      std::make_unique<CustomIntegralTransportParameter>(
-          kCustomTransportParameterThreshold, 0)));
-  client->closeNow(folly::none);
-}
-
 TEST_F(QuicClientTransportTest, CloseSocketOnWriteError) {
   client->addNewPeerAddress(serverAddr);
   EXPECT_CALL(*sock, write(_, _)).WillOnce(SetErrnoAndReturn(EBADF, -1));