diff --git a/proxygen/lib/http/codec/FlowControlFilter.cpp b/proxygen/lib/http/codec/FlowControlFilter.cpp
index a09104021c..e49c36ae50 100644
--- a/proxygen/lib/http/codec/FlowControlFilter.cpp
+++ b/proxygen/lib/http/codec/FlowControlFilter.cpp
@@ -22,6 +22,8 @@ HTTPException getException(const std::string& msg) {
 
 }
 
+const uint32_t FlowControlFilter::kDefaultCapacity = spdy::kInitialWindow;
+
 FlowControlFilter::FlowControlFilter(Callback& callback,
                                      folly::IOBufQueue& writeBuf,
                                      HTTPCodec* codec,
diff --git a/proxygen/lib/http/codec/FlowControlFilter.h b/proxygen/lib/http/codec/FlowControlFilter.h
index 00b775cfdc..6430c0f695 100644
--- a/proxygen/lib/http/codec/FlowControlFilter.h
+++ b/proxygen/lib/http/codec/FlowControlFilter.h
@@ -36,6 +36,8 @@ class FlowControlFilter:
     virtual void onConnectionSendWindowClosed() = 0;
   };
 
+  static const uint32_t kDefaultCapacity;
+
   /**
    * Construct a flow control filter.
    * @param callback     A channel to be notified when the window is not
@@ -44,13 +46,15 @@ class FlowControlFilter:
    *                     may generate a window update frame on this buffer.
    * @param codec        The codec implementation.
    * @param recvCapacity The initial size of the conn-level recv window.
-   *                     It must be >= 65536.
+   *                     It must be >= 65536.  If greater than 65536 it
+   *                     will generate an immediate window update into
+   *                     writeBuf.
    */
   explicit
   FlowControlFilter(Callback& callback,
                     folly::IOBufQueue& writeBuf,
                     HTTPCodec* codec,
-                    uint32_t recvCapacity);
+                    uint32_t recvCapacity = kDefaultCapacity);
 
   /**
    * Modify the session receive window
diff --git a/proxygen/lib/http/session/HTTPSession.cpp b/proxygen/lib/http/session/HTTPSession.cpp
index c8273c1b54..659223dafc 100644
--- a/proxygen/lib/http/session/HTTPSession.cpp
+++ b/proxygen/lib/http/session/HTTPSession.cpp
@@ -146,11 +146,11 @@ HTTPSession::HTTPSession(
     settings->setSetting(SettingsId::MAX_CONCURRENT_STREAMS,
                          maxConcurrentIncomingStreams_);
   }
+
+  codec_->generateConnectionPreface(writeBuf_);
+
   if (codec_->supportsSessionFlowControl()) {
-    connFlowControl_ = new FlowControlFilter(*this,
-                                             writeBuf_,
-                                             codec_.call(),
-                                             kDefaultReadBufLimit);
+    connFlowControl_ = new FlowControlFilter(*this, writeBuf_, codec_.call());
     codec_.addFilters(std::unique_ptr<FlowControlFilter>(connFlowControl_));
   }
 
@@ -191,8 +191,11 @@ HTTPSession::~HTTPSession() {
 void HTTPSession::startNow() {
   CHECK(!started_);
   started_ = true;
-  codec_->generateConnectionPreface(writeBuf_);
   codec_->generateSettings(writeBuf_);
+  if (connFlowControl_) {
+    connFlowControl_->setReceiveWindowSize(writeBuf_,
+                                           receiveSessionWindowSize_);
+  }
   scheduleWrite();
   resumeReads();
 }
@@ -214,15 +217,13 @@ void HTTPSession::setFlowControl(size_t initialReceiveWindow,
   CHECK(!started_);
   initialReceiveWindow_ = initialReceiveWindow;
   receiveStreamWindowSize_ = receiveStreamWindowSize;
+  // TODO(t6558522): line up kDefaultReadBufLimit and receiveSessionWindowSize
+  receiveSessionWindowSize_ = receiveSessionWindowSize;
   HTTPSettings* settings = codec_->getEgressSettings();
   if (settings) {
     settings->setSetting(SettingsId::INITIAL_WINDOW_SIZE,
                          initialReceiveWindow_);
   }
-  if (connFlowControl_) {
-    connFlowControl_->setReceiveWindowSize(writeBuf_, receiveSessionWindowSize);
-    scheduleWrite();
-  }
 }
 
 void HTTPSession::setMaxConcurrentOutgoingStreams(uint32_t num) {
diff --git a/proxygen/lib/http/session/HTTPSession.h b/proxygen/lib/http/session/HTTPSession.h
index d482d14b03..9006fd095f 100644
--- a/proxygen/lib/http/session/HTTPSession.h
+++ b/proxygen/lib/http/session/HTTPSession.h
@@ -847,6 +847,7 @@ class HTTPSession:
   // Flow control settings
   size_t initialReceiveWindow_{65536};
   size_t receiveStreamWindowSize_{65536};
+  size_t receiveSessionWindowSize_{65536};
 
   const TransportDirection direction_;
 
diff --git a/proxygen/lib/http/session/test/HTTPDownstreamSessionTest.cpp b/proxygen/lib/http/session/test/HTTPDownstreamSessionTest.cpp
index a2bf70954b..d0c5df7cfb 100644
--- a/proxygen/lib/http/session/test/HTTPDownstreamSessionTest.cpp
+++ b/proxygen/lib/http/session/test/HTTPDownstreamSessionTest.cpp
@@ -1725,6 +1725,8 @@ TEST_F(SPDY31DownstreamTest, testSessionFlowControl) {
   SPDYCodec clientCodec(TransportDirection::UPSTREAM,
                         SPDYVersion::SPDY3_1);
 
+  InSequence sequence;
+  EXPECT_CALL(callbacks, onSettings(_));
   EXPECT_CALL(callbacks, onWindowUpdate(0, spdy::kInitialWindow));
   clientCodec.setCallback(&callbacks);
   parseOutput(clientCodec);