Update minimum RTT for RACK.

We are currently tracking the minimum RTT for RACK as smoothed RTT. As per RFC
minimum RTT can be a global minimum of all RTTs or filtered value of recent
RTT measurements. In this cl minimum RTT is updated to global minimum of all
RTTs for the connection.

PiperOrigin-RevId: 335061518

2 files changed, 15 insertions, 6 deletions

diff --git a/pkg/tcpip/transport/tcp/rack.go b/pkg/tcpip/transport/tcp/rack.go
index d969ca23a..439932595 100644
--- a/pkg/tcpip/transport/tcp/rack.go
+++ b/pkg/tcpip/transport/tcp/rack.go
@@ -39,6 +39,9 @@ type rackControl struct {
 	// sequence.
 	fack seqnum.Value
 
+	// minRTT is the estimated minimum RTT of the connection.
+	minRTT time.Duration
+
 	// rtt is the RTT of the most recently delivered packet on the
 	// connection (either cumulatively acknowledged or selectively
 	// acknowledged) that was not marked invalid as a possible spurious
@@ -48,7 +51,7 @@ type rackControl struct {
 
 // Update will update the RACK related fields when an ACK has been received.
 // See: https://tools.ietf.org/html/draft-ietf-tcpm-rack-08#section-7.2
-func (rc *rackControl) Update(seg *segment, ackSeg *segment, srtt time.Duration, offset uint32) {
+func (rc *rackControl) Update(seg *segment, ackSeg *segment, offset uint32) {
 	rtt := time.Now().Sub(seg.xmitTime)
 
 	// If the ACK is for a retransmitted packet, do not update if it is a
@@ -65,12 +68,21 @@ func (rc *rackControl) Update(seg *segment, ackSeg *segment, srtt time.Duration,
 				return
 			}
 		}
-		if rtt < srtt {
+		if rtt < rc.minRTT {
 			return
 		}
 	}
 
 	rc.rtt = rtt
+
+	// The sender can either track a simple global minimum of all RTT
+	// measurements from the connection, or a windowed min-filtered value
+	// of recent RTT measurements. This implementation keeps track of the
+	// simple global minimum of all RTTs for the connection.
+	if rtt < rc.minRTT || rc.minRTT == 0 {
+		rc.minRTT = rtt
+	}
+
 	// Update rc.xmitTime and rc.endSequence to the transmit time and
 	// ending sequence number of the packet which has been acknowledged
 	// most recently.
diff --git a/pkg/tcpip/transport/tcp/snd.go b/pkg/tcpip/transport/tcp/snd.go
index c55589c45..4c9a86cda 100644
--- a/pkg/tcpip/transport/tcp/snd.go
+++ b/pkg/tcpip/transport/tcp/snd.go
@@ -1365,9 +1365,6 @@ func (s *sender) handleRcvdSegment(rcvdSeg *segment) {
 
 		ackLeft := acked
 		originalOutstanding := s.outstanding
-		s.rtt.Lock()
-		srtt := s.rtt.srtt
-		s.rtt.Unlock()
 		for ackLeft > 0 {
 			// We use logicalLen here because we can have FIN
 			// segments (which are always at the end of list) that
@@ -1389,7 +1386,7 @@ func (s *sender) handleRcvdSegment(rcvdSeg *segment) {
 
 			// Update the RACK fields if SACK is enabled.
 			if s.ep.sackPermitted {
-				s.rc.Update(seg, rcvdSeg, srtt, s.ep.tsOffset)
+				s.rc.Update(seg, rcvdSeg, s.ep.tsOffset)
 			}
 
 			s.writeList.Remove(seg)