From 1b2805118d2e20f2c9ea7689f7bafa4f22a12d09 Mon Sep 17 00:00:00 2001
From: Arvid Norberg <arvid@libtorrent.org>
Date: Sat, 16 Jun 2012 02:16:45 +0000
Subject: [PATCH] make uTP less aggressive resetting cwnd when idle

---
 ChangeLog          |  2 ++
 src/utp_stream.cpp | 14 +++++++++++++-
 2 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/ChangeLog b/ChangeLog
index 8416271c4..ddd1b1307 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,5 @@
+	* make uTP less aggressive resetting cwnd when idle
+
 0.16.1 release
 
 	* fixed crash when providing corrupt resume data
diff --git a/src/utp_stream.cpp b/src/utp_stream.cpp
index 589addff5..5e72c5de0 100644
--- a/src/utp_stream.cpp
+++ b/src/utp_stream.cpp
@@ -2880,7 +2880,19 @@ void utp_socket_impl::tick(ptime const& now)
 		// we can now sent messages again, the send window was opened
 		if ((m_cwnd >> 16) < m_mtu) window_opened = true;
 
-		m_cwnd = boost::int64_t(m_mtu) << 16;
+		if (m_bytes_in_flight == 0 && (m_cwnd >> 16) >= m_mtu)
+		{
+			// this is just a timeout because this direction of
+			// the stream is idle. Don't reset the cwnd, just decay it
+			m_cwnd = m_cwnd * 2 / 3;
+		}
+		else
+		{
+			// we timed out because a packet was not ACKed or because
+			// the cwnd was made smaller than one packet
+			m_cwnd = boost::int64_t(m_mtu) << 16;
+		}
+
 		if (m_outbuf.size()) ++m_num_timeouts;
 
 		if (m_num_timeouts > m_sm->num_resends())