Ticket #13630: 0002-tcp-rfc-5681-implemented-fast-retransmit-and-recover.patch

src/add-ons/kernel/network/protocols/tcp/TCPEndpoint.cpp

@@ -432 +432 @@
     fSendQueue(socket->send.buffer_size),
     fInitialSendSequence(0),
     fDuplicateAcknowledgeCount(0),
+    fPreviousFlightSize(0),
     fRoute(NULL),
     fReceiveNext(0),
     fReceiveMaxAdvertised(0),
@@ -1277 +1278 @@
 void
 TCPEndpoint::_DuplicateAcknowledge(tcp_segment_header &segment)
 {
+    if (fDuplicateAcknowledgeCount == 0)
+        fPreviousFlightSize = (fSendMax - fSendUnacknowledged).Number();
+
     if (++fDuplicateAcknowledgeCount < 3)
         return;
 
     if (fDuplicateAcknowledgeCount == 3) {
-        _ResetSlowStart();
+        fSlowStartThreshold = max_c(fPreviousFlightSize / 2, 2 * fSendMaxSegmentSize);
         fCongestionWindow = fSlowStartThreshold + 3 * fSendMaxSegmentSize;
         fSendNext = segment.acknowledge;
-    } else if (fDuplicateAcknowledgeCount > 3)
-        fCongestionWindow += fSendMaxSegmentSize;
-
-    _SendQueued();
+        _SendQueued();
+        TRACE("_DuplicateAcknowledge(): packet sent under fast restransmit on the receipt of 3rd dup ack");
+
+    } else if (fDuplicateAcknowledgeCount > 3) {
+        uint32 flightSize = (fSendMax - fSendUnacknowledged).Number();
+        if ((fDuplicateAcknowledgeCount - 3) * fSendMaxSegmentSize <= flightSize)
+            fCongestionWindow += fSendMaxSegmentSize;
+        if (fSendQueue.Available(fSendMax) != 0) {
+            fSendNext = fSendMax;
+            _SendQueued();
+        }
+    }
 }
 
 
@@ -1641 +1653 @@
 
     int32 action = KEEP;
 
+    // immediately acknowledge out-of-order segment to trigger fast-retransmit at the sender
+    if (drop != 0)
+        action |= IMMEDIATE_ACKNOWLEDGE;
+
     drop = (int32)(segment.sequence + buffer->size
         - (fReceiveNext + fReceiveWindow)).Number();
     if (drop > 0) {
@@ -1685 +1701 @@
         if (fSendMax < segment.acknowledge)
             return DROP | IMMEDIATE_ACKNOWLEDGE;
 
-        if (segment.acknowledge < fSendUnacknowledged) {
+        if (segment.acknowledge == fSendUnacknowledged) {
             if (buffer->size == 0 && advertisedWindow == fSendWindow
-                && (segment.flags & TCP_FLAG_FINISH) == 0) {
+                && (segment.flags & TCP_FLAG_FINISH) == 0 && fSendUnacknowledged != fSendMax) {
                 TRACE("Receive(): duplicate ack!");
-
                 _DuplicateAcknowledge(segment);
             }
-
+        } else if (segment.acknowledge < fSendUnacknowledged) {
             return DROP;
         } else {
             // this segment acknowledges in flight data
@@ -1702 +1717 @@
                 fCongestionWindow = fSlowStartThreshold;
             }
 
-            fDuplicateAcknowledgeCount = 0;
-
             if (fSendMax == segment.acknowledge)
                 TRACE("Receive(): all inflight data ack'd!");
 
@@ -2044 +2057 @@
     bool shouldStartRetransmitTimer = fSendNext == fSendUnacknowledged;
     bool retransmit = fSendNext < fSendMax;
 
+    if (fDuplicateAcknowledgeCount != 0) {
+        // send at most 1 SMSS of data when under limited transmit, fast transmit/recovery
+        length = min_c(length, fSendMaxSegmentSize);
+    }
+
     do {
         uint32 segmentMaxSize = fSendMaxSegmentSize
                 - tcp_options_length(segment);
@@ -2221 +2239 @@
     ASSERT(fSendUnacknowledged <= segment.acknowledge);
 
     if (fSendUnacknowledged < segment.acknowledge) {
+        fDuplicateAcknowledgeCount = 0;
         fSendQueue.RemoveUntil(segment.acknowledge);
 
         // the acknowledgment of the SYN/ACK MUST NOT increase the size of the congestion window
@@ -2287 +2306 @@
         fCongestionWindow = fSendMaxSegmentSize;
     } else {
         _ResetSlowStart();
-
+        fDuplicateAcknowledgeCount = 0;
         // Do exponential back off of the retransmit timeout
         fRetransmitTimeout *= 2;
         if (fRetransmitTimeout > TCP_MAX_RETRANSMIT_TIMEOUT)

src/add-ons/kernel/network/protocols/tcp/TCPEndpoint.h

@@ -150 +150 @@
     tcp_sequence    fLastAcknowledgeSent;
     tcp_sequence    fInitialSendSequence;
     uint32          fDuplicateAcknowledgeCount;
+    uint32          fPreviousFlightSize;
 
     net_route       *fRoute;
         // TODO: don't use a net_route, but a net_route_info!!!