diff --git a/stuncore/stunbuilder.cpp b/stuncore/stunbuilder.cpp
index 2cc456e..dbc3651 100644
--- a/stuncore/stunbuilder.cpp
+++ b/stuncore/stunbuilder.cpp
@@ -135,14 +135,31 @@ HRESULT CStunMessageBuilder::AddRandomTransactionId(StunTransactionId* pTransId)
     srand(entropy);
 
 
-    // the first four bytes of the transaction id is always the magic cookie
-    // followed by 12 bytes of the real transaction id
-    memcpy(transid.id, &stun_cookie_nbo, sizeof(stun_cookie_nbo));
-    for (int x = 4; x < (STUN_TRANSACTION_ID_LENGTH-4); x++)
+    int x = 0;
+    if (!_fLegacyMode)
+    {
+        // the first four bytes of the transaction id is always the magic cookie
+        // followed by 12 bytes of the real transaction id
+        memcpy(transid.id, &stun_cookie_nbo, sizeof(stun_cookie_nbo));
+        x = 4;
+    }
+
+    for (; x < STUN_TRANSACTION_ID_LENGTH; x++)
     {
         transid.id[x] = (uint8_t)(rand() % 256);
     }
 
+    if (_fLegacyMode)
+    {
+        // rfc3489 (legacy mode) does not use magic cookie
+        // if the generated txn id happens to start with the magic cookie, keep
+        // re-generating the 1st byte until it doesn't
+        while (memcmp(transid.id, &stun_cookie_nbo, sizeof(stun_cookie_nbo)) == 0)
+        {
+            transid.id[0] = (uint8_t)(rand() % 256);
+        }
+    }
+    
     if (pTransId)
     {
         *pTransId = transid;
diff --git a/testcode/testbuilder.cpp b/testcode/testbuilder.cpp
index f928cf8..3e07224 100644
--- a/testcode/testbuilder.cpp
+++ b/testcode/testbuilder.cpp
@@ -27,6 +27,8 @@ HRESULT CTestBuilder::Run()
     HRESULT hr = S_OK;
     Chk(Test1())
     Chk(Test2());
+    Chk(Test3());
+    Chk(Test4());
 Cleanup:
     return hr;
 }
@@ -138,4 +140,44 @@ HRESULT CTestBuilder::Test2()
     return hr;
 }
 
+// This test validates the non-legacy mode transaction id generated by builder.
+HRESULT CTestBuilder::Test3()
+{
+    HRESULT hr = S_OK;
+    CStunMessageBuilder builder;
+    CStunMessageReader reader;
+    StunTransactionId transid = {};
+    CRefCountedBuffer spBuffer;
+
+    builder.SetLegacyMode(false);
+    ChkA(builder.AddBindingRequestHeader());
+    ChkA(builder.AddRandomTransactionId(&transid));
+    ChkA(builder.GetResult(&spBuffer));
+    
+    ChkIfA(CStunMessageReader::BodyValidated != reader.AddBytes(spBuffer->GetData(), spBuffer->GetSize()), E_FAIL);
+    ChkIf(reader.IsMessageLegacyFormat() == true, E_FAIL);
+
+Cleanup:
+    return hr;
+}
 
+// This test validates the legacy mode transaction id generated by builder.
+HRESULT CTestBuilder::Test4()
+{
+    HRESULT hr = S_OK;
+    CStunMessageBuilder builder;
+    CStunMessageReader reader;
+    StunTransactionId transid = {};
+    CRefCountedBuffer spBuffer;
+
+    builder.SetLegacyMode(true);
+    ChkA(builder.AddBindingRequestHeader());
+    ChkA(builder.AddRandomTransactionId(&transid));
+    ChkA(builder.GetResult(&spBuffer));
+
+    ChkIfA(CStunMessageReader::BodyValidated != reader.AddBytes(spBuffer->GetData(), spBuffer->GetSize()), E_FAIL);
+    ChkIf(reader.IsMessageLegacyFormat() == false, E_FAIL);
+    
+Cleanup:
+    return hr;
+}
\ No newline at end of file
diff --git a/testcode/testbuilder.h b/testcode/testbuilder.h
index 59c216b..2163104 100644
--- a/testcode/testbuilder.h
+++ b/testcode/testbuilder.h
@@ -26,6 +26,8 @@ class CTestBuilder : public IUnitTest
 public:
     HRESULT Test1();
     HRESULT Test2();
+    HRESULT Test3();
+    HRESULT Test4();
 
     virtual HRESULT Run();